[Federal Register Volume 75, Number 119 (Tuesday, June 22, 2010)]
[Rules and Regulations]
[Pages 35519-35603]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2010-13947]



[[Page 35519]]

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Part II





Environmental Protection Agency





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40 CFR Parts 50, 53, and 58



Primary National Ambient Air Quality Standard for Sulfur Dioxide; Final 
Rule

Federal Register / Vol. 75 , No. 119 / Tuesday, June 22, 2010 / Rules 
and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

[EPA-HQ-OAR-2007-0352; 9160-4]
RIN 2060-A048


Primary National Ambient Air Quality Standard for Sulfur Dioxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Based on its review of the air quality criteria for oxides of 
sulfur and the primary national ambient air quality standard (NAAQS) 
for oxides of sulfur as measured by sulfur dioxide (SO2), 
EPA is revising the primary SO2 NAAQS to provide requisite 
protection of public health with an adequate margin of safety. 
Specifically, EPA is establishing a new 1-hour SO2 standard 
at a level of 75 parts per billion (ppb), based on the 3-year average 
of the annual 99th percentile of 1-hour daily maximum concentrations. 
The EPA is also revoking both the existing 24-hour and annual primary 
SO2 standards.

DATES: This final rule is effective on August 23, 2010.

ADDRESSES: EPA has established a docket for this action under Docket ID 
No. EPA-HQ-OAR-2007-0352. All documents in the docket are listed on the 
http://www.regulations.gov Web site. Although listed in the index, some 
information is not publicly available, e.g., confidential business 
information or other information whose disclosure is restricted by 
statute. Certain other material, such as copyrighted material, will be 
publicly available only in hard copy form. Publicly available docket 
materials are available either electronically through http://www.regulations.gov or in hard copy at the Air and Radiation Docket and 
Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution 
Ave., NW., Washington, DC. The Public Reading Room is open from 8:30 
a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The 
telephone number for the Public Reading Room is (202) 566-1744 and the 
telephone number for the Air and Radiation Docket and Information 
Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Dr. Michael J. Stewart, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail code C504-06, 
Research Triangle Park, NC 27711; telephone: 919-541-7524; fax: 919-
541-0237; e-mail: stewart.michael@epa.gov.

SUPPLEMENTARY INFORMATION:

Table of Contents

    The following topics are discussed in this preamble:

I. Background
    A. Summary of Revisions to the SO2 Primary NAAQS
    B. Statutory Requirements
    C. Related SO2 Control Programs
    D. History of Reviews of the Primary NAAQS for Sulfur Oxides
    E. Summary of Proposed Revisions to the SO2 Primary 
NAAQS
    F. Organization and Approach to Final SO2 Primary 
NAAQS Decisions
II. Rationale for Decisions on the Primary Standards
    A. Characterization of SO2 Air Quality
    1. Anthropogenic Sources and Current Patterns of SO2 
Air Quality
    2. SO2 Monitoring
    B. Health Effects Information
    1. Short-Term (5-Minute to 24-Hour) SO2 Exposure and 
Respiratory Morbidity Effects
    a. Adversity of Short-Term Respiratory Morbidity Effects
    2. Health Effects and Long-Term Exposures to SO2
    3. SO2-Related Impacts on Public Health
    C. Human Exposure and Health Risk Characterization
    D. Approach for Determining Whether To Retain or Revise the 
Current Standards
    E. Adequacy of the Current Standards
    1. Rationale for Proposed Decision
    2. Comments on the Adequacy of the Current Standards
    a. Comments on EPA's Interpretation of the Epidemiologic 
Evidence
    b. Comments on EPA's Interpretation of the Controlled Human 
Exposure Evidence
    c. Comments on EPA's Characterization of SO2-
Associated Exposures and Health Risks
    3. Conclusions Regarding the Adequacy of the Current 24-Hour and 
Annual Standards
    F. Conclusions on the Elements of a New Short-Term Standard
    1. Indicator
    a. Rationale for Proposed Decision
    b. Comments on Indicator
    c. Conclusions on Indicator
    2. Averaging Time
    a. Rationale for Proposed Decision
    b. Comments on Averaging Time
    c. Conclusions on Averaging Time
    3. Form
    a. Rationale for Proposed Decision
    b. Comments on Form
    c. Conclusions on Form
    4. Level
    a. Rationale for Proposed Decision
    b. Comments on Level
    c. Conclusions on Level
    5. Retaining or Revoking the Current 24-Hour and Annual 
Standards
    a. Rationale for Proposed Decision
    b. Comments on Retaining or Revoking the Current 24-Hour and 
Annual Standards
    c. Conclusions on Retaining or Revoking the Current 24-Hour and 
Annual Standards
    G. Summary of Decisions on Primary Standards
III. Overview of the Approach for Monitoring and Implementation
IV. Amendments to Ambient Monitoring and Reporting Requirements
    A. Monitoring Methods
    1. Requirements for SO2 Federal Reference Method 
(FRM)
    a. Proposed Ultraviolet Fluorescence SO2 FRM and 
Implementation
    b. Public Comments
    c. Conclusions on Ultraviolet Fluorescence SO2 FRM 
and Implementation
    2. Requirements for Automated SO2 Methods
    a. Proposed Performance Specifications for Automated Methods
    b. Public Comments
    c. Conclusions for Performance Specifications for SO2 
Automated Methods
    B. Network Design
    1. Approach for Network Design
    a. Proposed Approach for Network Design
    b. Alternative Network Design
    c. Public Comments
    2. Modeling Ambient SO2 Concentrations
    3. Monitoring Objectives
    a. Proposed Monitoring Objectives
    b. Public Comments
    c. Conclusions on Monitoring Objectives
    4. Final Monitoring Network Design
    5. Population Weighted Emissions Index
    a. Proposed Use of the Population Weighted Emissions Index
    b. Public Comments
    c. Conclusions on the Use of the Population Weighted Emissions 
Index
    6. Regional Administrator Authority
    a. Proposed Regional Administrator Authority
    b. Public Comments
    c. Conclusions on Regional Administrator Authority
    7. Monitoring Network Implementation
    a. Proposed Monitoring Network Implementation
    b. Public Comments
    c. Conclusions on Monitoring Network Implementation
    C. Data Reporting
    1. Proposed Data Reporting
    2. Public Comments
    3. Conclusions on Data Reporting
V. Initial Designation of Areas for the 1-Hour SO2 NAAQS
    A. Clean Air Act Requirements
    1. Approach Described in Proposal
    2. Public Comments
    B. Expected Designations Process
VI. Clean Air Act Implementation Requirements
    A. How This Rule Applies to Tribes
    B. Nonattainment Area Attainment Dates
    1. Attaining the NAAQS
    2. Consequences of a Nonattainment Area Failing To Attain by the 
Statutory Attainment Date
    C. Section 110(a)(1) and (2) NAAQS Maintenance/Infrastructure 
Requirements
    1. Section 110(a)(1)-(2) Submission

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    D. Attainment Planning Requirements
    1. SO2 Nonattainment Area SIP Requirements
    2. New Source Review and Prevention of Significant Deterioration 
Requirements
    3. General Conformity
    E. Transition From the Existing SO2 NAAQS to a 
Revised SO2 NAAQS
VII. Appendix T--Interpretation of the Primary NAAQS for Oxides of 
Sulfur and Revisions to the Exceptional Events Rule
    A. Interpretation of the NAAQS for Oxides of Sulfur
    1. Proposed Interpretation of the Standard
    2. Comments on Interpretation of the Standard
    3. Conclusions on Interpretation of the Standard
    B. Exceptional Events Information Submission Schedule
VIII. Communication of Public Health Information
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health & Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect 
Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
References

I. Background

A. Summary of Revisions to the SO2 Primary NAAQS

    Based on its review of the air quality criteria for oxides of 
sulfur and the primary national ambient air quality standard (NAAQS) 
for oxides of sulfur as measured by sulfur dioxide (SO2), 
EPA is making revisions to the primary SO2 NAAQS so the 
standards are requisite to protect public health with an adequate 
margin of safety, as appropriate under section 109 of the Clean Air Act 
(Act or CAA). Specifically, EPA is replacing the current 24-hour and 
annual standards with a new short-term standard based on the 3-year 
average of the 99th percentile of the yearly distribution of 1-hour 
daily maximum SO2 concentrations. EPA is setting the level 
of this new standard at 75 ppb. EPA is adding data handling conventions 
for SO2 by adding provisions for this new 1-hour primary 
standard. EPA is also establishing requirements for an SO2 
monitoring network. These new provisions require monitors in areas 
where there is an increased coincidence of population and 
SO2 emissions. EPA is also making conforming changes to the 
Air Quality Index (AQI).

B. Statutory Requirements

    Two sections of the Clean Air Act (Act or CAA) govern the 
establishment and revision of National Ambient Air Quality Standards 
NAAQS. Section 108 of the Act directs the Administrator to identify and 
list air pollutants that meet certain criteria, including that the air 
pollutant ``in his judgment, cause[s] or contribute[s] to air pollution 
which may reasonably be anticipated to endanger public health and 
welfare'' and ``the presence of which in the ambient air results from 
numerous or diverse mobile or stationary sources.'' CAA section 
108(a)(1)(A) and (B). For those air pollutants listed, section 108 
requires the Administrator to issue air quality criteria that 
``accurately reflect the latest scientific knowledge useful in 
indicating the kind and extent of all identifiable effects on public 
health or welfare which may be expected from the presence of [a] 
pollutant in ambient air * * *'' Section 108(a)(2).
    Section 109(a) of the Act directs the Administrator to promulgate 
``primary'' and ``secondary'' NAAQS for pollutants for which air 
quality criteria have been issued. Section 109(b)(1) defines a primary 
standard as one ``the attainment and maintenance of which in the 
judgment of the Administrator, based on [the air quality] criteria and 
allowing an adequate margin of safety, are requisite to protect the 
public health.'' \1\ Section 109(b)(1). A secondary standard, in turn, 
must ``specify a level of air quality the attainment and maintenance of 
which, in the judgment of the Administrator, based on [the air quality] 
criteria, is requisite to protect the public welfare from any known or 
anticipated adverse effects associated with the presence of such 
pollutant in the ambient air.'' \2\ Section 109(b)(2) This rule 
concerns exclusively the primary NAAQS for oxides of sulfur.
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    \1\ The legislative history of section 109 indicates that a 
primary standard is to be set at ``the maximum permissible ambient 
air level * * * which will protect the health of any [sensitive] 
group of the population,'' and that for this purpose ``reference 
should be made to a representative sample of persons comprising the 
sensitive group rather than to a single person in such a group.'' S. 
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970). See also American 
Lung Ass'n v. EPA, 134 F. 3d 388, 389 (DC Cir. 1998) (``NAAQS must 
protect not only average healthy individuals, but also `sensitive 
citizens'--children, for example, or people with asthma, emphysema, 
or other conditions rendering them particularly vulnerable to air 
pollution. If a pollutant adversely affects the health of these 
sensitive individuals, EPA must strengthen the entire national 
standard.''); Coalition of Battery Recyclers Ass'n v. EPA, No. 09-
1011 (DC Cir. May 14, 2010) slip op. at 7 (same).
    \2\ EPA is currently conducting a separate review of the 
secondary SO2 NAAQS jointly with a review of the 
secondary NO2 NAAQS (see http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html for more information).
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    The requirement that primary standards include an adequate margin 
of safety is intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It is also intended to provide a reasonable degree 
of protection against hazards that research has not yet identified. 
Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (DC Cir 1980), 
cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v. 
Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied, 455 U.S. 1034 
(1982). Both kinds of uncertainties are components of the risk 
associated with pollution at levels below those at which human health 
effects can be said to occur with reasonable scientific certainty. 
Thus, in selecting primary standards that include an adequate margin of 
safety, the Administrator is seeking not only to prevent pollution 
levels that have been demonstrated to be harmful but also to prevent 
lower pollutant levels that may pose an unacceptable risk of harm, even 
if the risk is not precisely identified as to nature or degree. The CAA 
does not require the Administrator to establish a primary NAAQS at a 
zero-risk level or at background concentration levels, see Lead 
Industries Association v. EPA, 647 F.2d at 1156 n. 51, but rather at a 
level that reduces risk sufficiently so as to protect public health 
with an adequate margin of safety.
    In addressing the requirement for a margin of safety, EPA considers 
such factors as the nature and severity of the health effects involved, 
the size of the at-risk population(s), and the kind and degree of the 
uncertainties that must be addressed. The selection of any particular 
approach to providing an adequate margin of safety is a policy choice 
left specifically to the Administrator's judgment. Lead Industries 
Association v. EPA, 647 F.2d at 1161-62.
    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. In so doing, EPA may not consider the 
costs of implementing the standards. Whitman v. American Trucking

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Associations, 531 U.S. 457, 471, 475-76 (2001).
    Section 109(d)(1) of the Act requires the Administrator to 
periodically undertake a thorough review of the air quality criteria 
published under section 108 and the NAAQS and to revise the criteria 
and standards as may be appropriate. The Act also requires the 
Administrator to appoint an independent scientific review committee 
composed of seven members, including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies, to review the air 
quality criteria and NAAQS and to ``recommend to the Administrator any 
new * * * standards and revisions of existing criteria and standards as 
may be appropriate under section 108 and subsection (b) of this 
section.'' CAA section 109(d)(2). This independent review function is 
performed by the Clean Air Scientific Advisory Committee (CASAC) of 
EPA's Science Advisory Board.

C. Related SO2 Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once EPA has established 
them. Under section 110 of the Act, and related provisions, States are 
to submit, for EPA approval, State implementation plans (SIPs) that 
provide for the attainment and maintenance of such standards through 
control programs directed to sources of the pollutants involved. The 
States, in conjunction with EPA, also administer the prevention of 
significant deterioration program that covers these pollutants. See CAA 
sections 160-169. In addition, Federal programs provide for nationwide 
reductions in emissions of these and other air pollutants through the 
Federal motor vehicle and motor vehicle fuel control program under 
title II of the Act (CAA sections 202-250) which involves controls for 
emissions from all moving sources and controls for the fuels used by 
these sources; new source performance standards under section 111; and 
title IV of the Act (CAA sections 402-416), which specifically provides 
for major reductions in SO2 emissions. EPA has also 
promulgated the Clean Air Interstate Rule (CAIR) to require additional 
SO2 emission reductions needed in the eastern half of the 
United States to address emissions which contribute significantly to 
nonattainment with, or interfere with maintenance of, the PM NAAQS by 
downwind States in the CAIR region. This rule was remanded by the DC 
Circuit, and although it remains in effect, EPA is reevaluating it 
pursuant to the court remand.
    Currently, there are several areas designated as being in 
nonattainment of the primary SO2 NAAQS (see section VI). 
Moreover, as a result of this final rule, additional areas could be 
classified as non-attainment. Certain States would then be required to 
develop SIPs that identify and implement specific air pollution control 
measures to reduce ambient SO2 concentrations to attain and 
maintain the revised SO2 NAAQS, most likely by requiring air 
pollution controls on sources that emit oxides of sulfur 
(SOx).

D. History of Reviews of the Primary NAAQS for Sulfur Oxides

    On April 30, 1971, the EPA promulgated primary SO2 NAAQS 
(36 FR 8187). These primary standards, which were based on the findings 
outlined in the original 1969 Air Quality Criteria for Sulfur Oxides, 
were set at 0.14 parts per million (ppm) averaged over a 24-hour 
period, not to be exceeded more than once per year, and 0.030 ppm 
annual arithmetic mean. In 1982, EPA published the Air Quality Criteria 
for Particulate Matter and Sulfur Oxides (EPA, 1982) along with an 
addendum of newly published controlled human exposure studies, which 
updated the scientific criteria upon which the initial standards were 
based (EPA, 1982). In 1986, EPA published a second addendum presenting 
newly available evidence from epidemiologic and controlled human 
exposure studies (EPA, 1986). In 1988, EPA published a proposed 
decision not to revise the existing standards (53 FR 14926) (April 26, 
1988). However, EPA specifically requested public comment on the 
alternative of revising the current standards and adding a new 1-hour 
primary standard of 0.4 ppm (400 ppb) to protect asthmatics against 5-
10 minute peak SO2 concentrations.
    As a result of public comments on the 1988 proposal and other post-
proposal developments, EPA published a second proposal on November 15, 
1994 (59 FR 58958). The 1994 re-proposal was based in part on a 
supplement to the second addendum of the criteria document, which 
evaluated new findings on 5-10 minute SO2 exposures in 
asthmatics (EPA, 1994a; EPA, 1994b). As in the 1988 proposal, EPA 
proposed to retain the existing 24-hour and annual standards. EPA also 
solicited comment on three regulatory alternatives to further reduce 
the health risk posed by exposure to high 5-minute peaks of 
SO2 if additional protection were judged to be necessary. 
The three alternatives were: (1) Revising the existing primary 
SO2 NAAQS by adding a new 5-minute standard of 0.6 ppm (600 
ppb) SO2; (2) establishing a new regulatory program under 
section 303 of the Act to supplement protection provided by the 
existing NAAQS, with a trigger level of 0.6 ppm (600 ppb) 
SO2, one expected exceedance; and (3) augmenting 
implementation of existing standards by focusing on those sources or 
source types likely to produce high 5-minute peak concentrations of 
SO2.
    On May 22, 1996, EPA announced its final decision not to revise the 
NAAQS for SOx (61 FR 25566). EPA found that asthmatics--a 
susceptible population group--could be exposed to short-term 
SO2 bursts resulting in repeated `exposure events' such that 
tens or hundreds of thousands of asthmatics could be exposed annually 
to lung function effects ``distinctly exceeding * * * [the] typical 
daily variation in lung function'' that asthmatics routinely 
experience, and found further that repeated occurrences should be 
regarded as significant from a public health standpoint. 61 FR at 
25572, 25573. Nonetheless, the agency concluded that ``the likelihood 
that asthmatic individuals will be exposed * * * is very low when 
viewed from a national perspective'', that ``5-minute peak 
SO[2] levels do not pose a broad public health problem when 
viewed from a national perspective'', and that ``short-term peak 
concentrations of SO[2] do not constitute the type of 
ubiquitous public health problem for which establishing a NAAQS would 
be appropriate.'' Id. at 25575. EPA concluded, therefore, that it would 
not revise the existing standards or add a standard to specifically 
address 5-minute exposures. EPA also announced an intention to propose 
guidance, under section 303 of the Act, to assist States in responding 
to short-term peaks of SO2 and later initiated a rulemaking 
to do so (62 FR 210 (Jan. 2, 1997).
    The American Lung Association and the Environmental Defense Fund 
challenged EPA's decision not to establish a 5-minute standard. On 
January 30, 1998, the Court of Appeals for the District of Columbia 
Circuit found that EPA had failed to adequately explain its 
determination that no revision to the SO2 NAAQS was 
appropriate and remanded the determination back to EPA for further 
explanation. American Lung Ass'n v. EPA, 134 F. 3d 388 (DC Cir. 1998). 
Specifically, the court held that EPA had failed to adequately explain 
the basis for its conclusion that short-term SO2 exposures 
to asthmatics do not constitute a public health problem,

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noting that the agency had failed to explain the link between its 
finding that repeated short-term exposures were significant, and that 
there would be tens to hundreds of thousands of such exposures annually 
to a susceptible subpopulation. 134 F. 3d at 392. The court also 
rejected the explanation that short-term SO2 bursts were 
``localized, infrequent, and site-specific'' as a rational basis for 
the conclusion that no public health problem existed for purposes of 
section 109: ``[N]othing in the Final Decision explains why 
`localized', `site-specific', or even `infrequent' events might 
nevertheless create a public health problem, particularly since, in 
some sense, all pollution is local and site-specific * * *''. Id. The 
court accordingly remanded the case to EPA to adequately explain its 
determination or otherwise take action in accordance with the opinion. 
In response, EPA has collected and analyzed additional air quality data 
focused on 5-minute concentrations of SO2. These air quality 
analyses conducted since the last review helped inform the current 
review, which (among other things) address the issues raised in the 
court's remand of the Agency's last decision.
    EPA formally initiated the current review of the air quality 
criteria for oxides of sulfur and the SO2 primary NAAQS on 
May 15, 2006 (71 FR 28023) with a general call for information. EPA's 
draft Integrated Review Plan for the Primary National Ambient Air 
Quality Standards for Sulfur Dioxide (EPA, 2007a) was made available in 
April 2007 for public comment and was discussed by the CASAC via a 
publicly accessible teleconference on May 11, 2007. As noted in that 
plan, SOX includes multiple gaseous (e.g., SO3) 
and particulate (e.g., sulfate) species. Because the health effects 
associated with particulate species of SOX have been 
considered within the context of the health effects of ambient 
particles in the Agency's review of the NAAQS for particulate matter 
(PM), the current review of the primary SO2 NAAQS is focused 
on the gaseous species of SOX and does not consider health 
effects directly associated with particulate species.
    The first draft of the Integrated Science Assessment for Oxides of 
Sulfur-Health Criteria (ISA) and the Sulfur Dioxide Health Assessment 
Plan: Scope and Methods for Exposure and Risk Assessment (EPA, 2007b) 
were reviewed by CASAC at a public meeting held on December 5-6, 2007. 
Based on comments received from CASAC and from the public, EPA 
developed the second draft of the ISA and the first draft of the Risk 
and Exposure Assessment to Support the Review of the SO2 
Primary National Ambient Air Quality Standard (Risk and Exposure 
Assessment (REA)). These documents were reviewed by CASAC at a public 
meeting held on July 30-31, 2008. Based on comments received from CASAC 
and the public at this meeting, EPA released the final ISA in September 
of 2008 (EPA, 2008a; henceforth referred to as ISA). In addition, 
comments received were considered in developing the second draft of the 
REA. Importantly, the second draft of the REA contained a draft staff 
policy assessment that considered the evidence presented in the final 
ISA and the air quality, exposure, and risk characterization results 
presented in the second draft REA, as they related to the adequacy of 
the current SO2 NAAQS and potential alternative primary 
SO2 standards. This document was reviewed by CASAC at a 
public meeting held on April 16-17, 2009. In preparing the final REA 
report, which included the final staff policy assessment, EPA 
considered comments received from CASAC and the public at and 
subsequent to that meeting. The final REA containing the final staff 
policy assessment was completed in August 2009 (EPA 2009a; henceforth 
referred to as REA)).
    On December 8, 2009 EPA published its proposed revisions to the 
primary SO2 NAAQS. 74 FR 64810 presented a number of 
conclusions, findings, and determinations proposed by the 
Administrator. EPA invited general, specific, and/or technical comments 
on all issues involved with this proposal, including all such proposed 
judgments, conclusions, findings, and determinations. EPA invited 
specific comment on the level, or range of levels, appropriate for such 
a standard, as well as on the rationale that would support that level 
or range of levels. These comments were carefully considered by the 
Administrator as she made her final decisions, as described in this 
notice, on the primary SO2 NAAQS
    The schedule for completion of this review is governed by a 
judicial order resolving a lawsuit filed in September 2005, concerning 
the timing of the current review. Center for Biologic Diversity v. 
Johnson (Civ. No. 05-1814) (D.D.C. 2007). The order that now governs 
this review, entered by the court in August 2007 and amended in 
December 2008, provides that the Administrator will sign, for 
publication, a final rulemaking concerning the review of the primary 
SO2 NAAQS no later than June 2, 2010.

E. Summary of Proposed Revisions to the SO2 Primary NAAQS

    For the reasons discussed in the preamble of the proposal for the 
SO2 primary NAAQS, EPA proposed to make revisions to the 
primary SO2 NAAQS (and to add SO2 data handling 
conventions) so the standards provide requisite protection of public 
health with an adequate margin of safety. Specifically, EPA proposed to 
replace the current 24-hour and annual standards with a new short-term 
SO2 standard. EPA proposed that this new short-term standard 
would be based on the 3-year average of the 99th percentile (or 4th 
highest) of the yearly distribution of 1-hour daily maximum 
SO2 concentrations. EPA proposed to set the level of this 
new 1-hour standard within the range of 50 to 100 ppb and solicited 
comment on standard levels as high as 150 ppb. EPA also proposed to 
establish requirements for an SO2 monitoring network at 
locations where maximum SO2 concentrations are expected to 
occur and to add a new Federal Reference Method (FRM) for measuring 
SO2 in the ambient air. Finally, EPA proposed to make 
corresponding changes to the Air Quality Index for SO2.

F. Organization and Approach to Final SO2 Primary NAAQS Decisions

    This action presents the Administrator's final decisions regarding 
the need to revise the current SO2 primary NAAQS, and what 
those revisions should be. Revisions to the primary NAAQS for 
SO2, and the rationale supporting those revisions, are 
described below in section II.
    An overview of the approach for monitoring and implementation is 
presented in section III. Requirements for the SO2 ambient 
monitoring network and for a new, additional FRM for measuring 
SO2 in the ambient air are described in section IV. EPA's 
current plans for designations and for implementing the revised 
SO2 primary NAAQS are discussed in sections V and VI 
respectively. Related requirements for data completeness, data 
handling, data reporting, rounding conventions, and exceptional events 
are described in section VII. Communication of public health 
information through the AQI is discussed in section VIII. A recitation 
of statutory authority and a discussion of those executive order 
reviews which are relevant are provided in section IX.
    Today's final decisions are based on a thorough review in the ISA 
of scientific information on known and potential human health effects 
associated with exposure to SO2 in the

[[Page 35524]]

air. These final decisions also take into account: (1) Assessments in 
the REA of the most policy-relevant information in the ISA as well as 
quantitative exposure and risk analyses based on that information; (2) 
CASAC Panel advice and recommendations, as reflected in its letters to 
the Administrator and its public discussions of the ISA and REA; (3) 
public comments received during the development of the ISA and REA; and 
(4) public comments received on EPA's notice of proposed rulemaking.

II. Rationale for Decisions on the Primary Standards

    This section presents the rationale for the Administrator's 
decision to revise the existing SO2 primary standards by 
replacing the current 24-hour and annual standards with a new 1-hour 
SO2 standard at a level of 75 ppb, based on the 3-year 
average of the annual 99th percentile of 1-hour daily maximum 
concentrations. As discussed more fully below, this rationale takes 
into account: (1) Judgments and conclusions presented in the ISA and 
the REA; (2) CASAC advice and recommendations as reflected in the CASAC 
panel's discussions of drafts of the ISA and REA at public meetings, in 
separate written comments, and in letters to the Administrator 
(Henderson 2008a; Henderson 2008b; Samet, 2009); (3) public comments 
received at CASAC meetings during the development of the ISA and the 
REA; and (4) public comments received on the notice of proposed 
rulemaking.
    In reaching this decision, EPA has drawn upon an integrative 
synthesis of the entire body of evidence on human health effects 
associated with the presence of SO2 in the ambient air, and 
upon the results of the quantitative exposure and risk assessments 
reflecting this evidence. As discussed below, this body of evidence 
addresses a broad range of health endpoints associated with exposure to 
SO2 in the ambient air. In considering this entire body of 
evidence, EPA chose to focus most on those health endpoints for which 
the ISA found the strongest evidence of an association with 
SO2 (see section II.B below). Thus, the rationale for this 
final decision on the SO2 NAAQS focused primarily on 
respiratory morbidity following short-term (5-minutes to 24-hours) 
exposure to SO2, for which the ISA found a causal 
relationship.
    As discussed below, a substantial amount of new research has been 
conducted since EPA's last review of the SO2 NAAQS, with 
important new information coming from epidemiologic studies in 
particular. In addition to the substantial amount of new epidemiologic 
research, the ISA considered a limited number of new controlled human 
exposure studies and re-evaluated key older controlled human exposure 
studies. In evaluating both the new and key older controlled human 
exposure studies, the ISA utilized updated guidelines published by the 
American Thoracic Society (ATS) on what constitutes an adverse effect 
of air pollution (see ISA, section 3.1.3; p. 3-4). Importantly, all 
controlled human exposure and epidemiologic studies evaluated in the 
ISA have undergone intensive scrutiny through multiple layers of peer 
review and opportunities for public review and comment. Thus, the 
review of this information has been extensive and deliberate.
    After a background discussion of the principal emitting sources and 
current patterns of SO2 air quality and a description of the 
current SO2 monitoring network from which those air quality 
patterns are obtained (section II.A), the remainder of this section 
discusses the Administrator's rationale for her final decisions on the 
primary standards. Section II.B includes an overview of the scientific 
evidence related to the respiratory effects associated with ambient 
SO2 exposure. This overview includes a discussion of the at-
risk populations considered in the ISA. Section II.C summarizes the key 
approaches taken by EPA to assess exposures and health risks associated 
with exposure to ambient SO2. Section II.D summarizes the 
approach that was used in the current review of the SO2 
NAAQS with regard to consideration of the scientific evidence and the 
air quality, exposure, and risk-based results related to the adequacy 
of the current standards and potential alternative standards. Sections 
II.E and II.F discuss, respectively, the Administrator's decisions 
regarding the adequacy of the current standards and the elements of a 
new short-term standard, taking into consideration public comments on 
the proposed decisions. Section II.G summarizes the Administrator's 
decisions with regard to the SO2 primary NAAQS.

A. Characterization of SO2 Air Quality

1. Anthropogenic Sources and Current Patterns of SO2 Air 
Quality
    Anthropogenic SO2 emissions originate chiefly from point 
sources, with fossil fuel combustion at electric utilities (~66%) and 
other industrial facilities (~29%) accounting for the majority of total 
emissions (ISA, section 2.1). Other anthropogenic sources of 
SO2 include both the extraction of metal from ore as well as 
the burning of high sulfur-containing fuels by locomotives, large 
ships, and equipment utilizing diesel engines. SO2 emissions 
and ambient concentrations follow a strong east to west gradient due to 
the large numbers of coal-fired electric generating units in the Ohio 
River Valley and upper Southeast regions. In the 12 Consolidated 
Metropolitan Statistical Areas (CMSAs) that had at least four 
SO2 regulatory monitors from 2003-2005, 24-hour average 
concentrations in the continental U.S. ranged from a reported low of ~1 
ppb in Riverside, CA and San Francisco, CA to a high of ~12 ppb in 
Pittsburgh, PA and Steubenville, OH (ISA, section 2.5.1). In addition, 
outside or inside all CMSAs from 2003-2005, the annual average 
SO2 concentration was 4 ppb (ISA, Table 2-8). However, 
spikes in hourly concentrations occurred. The mean 1-hour maximum 
concentration outside or inside CMSAs was 13 ppb, with a maximum value 
of greater than 600 ppb outside CMSAs and greater than 700 ppb inside 
CMSAs (ISA, Table 2-8).
    Temporal and spatial patterns of 5-minute peaks of SO2 
are also important given that controlled human exposure studies have 
demonstrated that exposure to these peaks can result in adverse 
respiratory effects in exercising asthmatics (see section II.B below). 
For those monitors which voluntarily reported 5-minute block average 
data,\3\ when maximum 5-minute concentrations were reported, the 
absolute highest concentration over the ten-year period exceeded 4000 
ppb, but for all individual monitors, the 99th percentile was below 200 
ppb (ISA, section 2.5.2 Table 2-10). Median concentrations from these 
monitors reporting 5-minute data ranged from 1 ppb to 8 ppb, and the 
average for each maximum 5-minute level ranged from 3 ppb to 17 ppb. 
Delaware, Pennsylvania, Louisiana, and West Virginia had mean values 
for maximum 5-minute data exceeding 10 ppb. Among aggregated within-
State data for the 16 monitors from which all 5-minute average 
intervals were reported, the median values ranged from 1 ppb to 5 ppb, 
and the means ranged from 3 ppb to 11 ppb (ISA, section 2.5.2 at 2-43). 
The highest reported concentration was 921 ppb, but the 99th percentile 
values

[[Page 35525]]

for aggregated within-State data were all below 90 ppb (id).
---------------------------------------------------------------------------

    \3\ A small number of sites, 98 total from 1997 to 2007 of the 
approximately 500 SO2 monitors, and not the same sites in 
all years, voluntarily reported 5-minute block average data to AQS 
(ISA, section 2.5.2). Of these, 16 reported all twelve 5-minute 
averages in each hour for at least part of the time between 1997 and 
2007. The remainder reported only the maximum 5-minute average in 
each hour.
---------------------------------------------------------------------------

2. SO2 Monitoring
    Although EPA established the SO2 standards in 1971, 
uniform minimum monitoring network requirements for SO2 
monitoring were only adopted in May 1979. From the time of the 
implementation of the 1979 monitoring rule through 2008, the 
SO2 monitoring network has steadily decreased in size from 
approximately 1496 sites in 1980 to the approximately 488 sites 
operating in 2008. At present, except for SO2 monitoring 
required at National Core Monitoring Stations (NCore stations), there 
are no minimum monitoring requirements for SO2 in 40 CFR 
part 58 Appendix D, other than a requirement for EPA Regional 
Administrator approval before removing any existing monitors and a 
requirement that any ongoing SO2 monitoring must have at 
least one monitor sited to measure the maximum concentration of 
SO2 in that area. EPA removed the specific minimum 
monitoring requirements for SO2 in the 2006 monitoring rule 
revisions, except for monitoring at NCore stations, based on the fact 
that there were no SO2 nonattainment areas at that time, 
coupled with trends showing an increasing gap between national average 
SO2 concentrations and the current 24-hour and annual 
standards. The rule was also intended to provide State, local, and 
Tribal air monitoring agencies flexibility in meeting perceived higher 
priority monitoring needs for other pollutants, or to implement the new 
multi-pollutant sites (NCore network) required by the 2006 rule 
revisions (71 FR 61236, (October 6, 2006)). More information on 
SO2 monitoring can be found in section IV.

B. Health Effects Information

    The ISA concluded that there was sufficient evidence to infer a 
``causal relationship'' between respiratory morbidity and short-term 
(5-minutes to 24-hours) exposure to SO2 (ISA, section 5.2). 
Importantly, we note that a ``causal relationship'' is the strongest 
finding the ISA can make.\4\ This conclusion was based on the 
consistency, coherence, and plausibility of findings observed in 
controlled human exposure studies of 5-10 minutes, epidemiologic 
studies mostly using 1-hour daily maximum and 24-hour average 
SO2 concentrations, and animal toxicological studies using 
exposures of minutes to hours (ISA, section 5.2). This evidence is 
briefly summarized below and discussed in more detail in the proposal 
(see sections II.B.1 to II.B.5, see 74 FR at 64815-821). We also note 
that the ISA judged evidence of an association between SO2 
exposure and other health categories to be less convincing; other 
associations were judged to be suggestive but not sufficient to infer a 
causal relationship (i.e., short-term exposure to SO2 and 
mortality) or inadequate to infer the presence or absence of a causal 
relationship (i.e., short-term exposure to SO2 and 
cardiovascular morbidity, and long-term exposure to SO2 and 
respiratory morbidity, other morbidity, and mortality). Key conclusions 
from the ISA are described in greater detail in Table 5-3 of the ISA.
---------------------------------------------------------------------------

    \4\ A causal relationship is based on ``[e]vidence [that] is 
sufficient to conclude that there is a causal relationship between 
relevant pollutant exposures and the health outcome. That is, a 
positive association has been observed between the pollutant and the 
outcome in studies in which chance, bias, and confounding could be 
ruled out with reasonable confidence. Evidence includes, for 
example, controlled human exposure studies; or observational studies 
that cannot be explain by plausible alternatives or are supported by 
other lines of evidence (e.g. animal studies or mechanism of action 
information). Evidence includes replicated and consistent high-
quality studies by multiple investigators.'' ISA Table 1-2, at 1-11.
---------------------------------------------------------------------------

1. Short-Term (5-minute to 24-hour) SO2 Exposure and 
Respiratory Morbidity Effects
    The ISA examined numerous controlled human exposure studies and 
found that moderate or greater decrements in lung function (i.e., 
[gteqt] 15% decline in Forced Expiratory Volume (FEV1) and/
or [gteqt] 100% increase in specific airway resistance (sRaw)) occur in 
some exercising asthmatics exposed to SO2 concentrations as 
low as 200-300 ppb for 5-10 minutes. The ISA also found that among 
asthmatics, both the percentage of individuals affected, and the 
severity of the response increased with increasing SO2 
concentrations. That is, at 5-10 minute concentrations ranging from 
200-300 ppb, the lowest levels tested in free breathing chamber 
studies, approximately 5-30% percent of exercising asthmatics 
experienced moderate or greater decrements in lung function (ISA, Table 
3-1). At concentrations of 400-600 ppb, moderate or greater decrements 
in lung function occurred in approximately 20-60% of exercising 
asthmatics, and compared to exposures at 200-300 ppb, a larger 
percentage of asthmatics experienced severe decrements in lung function 
(i.e., [gteqt] 20% decrease in FEV1 and/or [gteqt] 200% 
increase in sRaw; ISA, Table 3-1). Moreover, at SO2 
concentrations [gteqt] 400 ppb (5-10 minute exposures), moderate or 
greater decrements in lung function were often statistically 
significant at the group mean level and frequently accompanied by 
respiratory symptoms. Id.
    The ISA also found that in locations meeting the current 
SO2 NAAQS, numerous epidemiologic studies reported positive 
associations between ambient SO2 concentrations and 
respiratory symptoms in children, as well as emergency department 
visits and hospitalizations for all respiratory causes and asthma 
across multiple age groups. Moreover, the ISA concluded that these 
epidemiologic studies were consistent and coherent. This evidence was 
consistent in that associations were reported in studies conducted in 
numerous locations and with a variety of methodological approaches 
(ISA, section 5.2; p. 5-5). It was coherent in that respiratory symptom 
results from epidemiologic studies of short-term (predominantly 1-hour 
daily maximum or 24-hour average) SO2 concentrations were 
generally in agreement with respiratory symptom results from controlled 
human exposure studies of 5-10 minutes. These results were also 
coherent in that the respiratory effects observed in controlled human 
exposure studies of 5-10 minutes further provided a basis for a 
progression of respiratory morbidity that could lead to the increased 
emergency department visits and hospital admissions observed in 
epidemiologic studies (ISA, section 5.2; p. 5-5). In addition, the ISA 
found that when evaluated as a whole, SO2 effect estimates 
in multi-pollutant models generally remained positive and relatively 
unchanged when co-pollutants were included. Therefore, although 
recognizing the uncertainties associated with separating the effects of 
SO2 from those of co-occurring pollutants, the ISA concluded 
that ``the limited available evidence indicates that the effect of 
SO2 on respiratory health outcomes appears to be generally 
robust and independent of the effects of gaseous co-pollutants, 
including NO2 and O3, as well as particulate co-
pollutants, particularly PM2.5'' (ISA, section 5.3; p. 5-9).
    The ISA also found that the respiratory effects of SO2 
were consistent with the mode of action as it is currently understood 
from animal toxicological and controlled human exposure studies (ISA, 
section 5.2; p. 5-2). The immediate effect of SO2 on the 
respiratory system is bronchoconstriction. This response is mediated by 
chemosensitive receptors in the tracheobronchial tree. Activation of 
these receptors triggers central nervous system reflexes that result in

[[Page 35526]]

bronchoconstriction and respiratory symptoms that are often followed by 
rapid shallow breathing (id). The ISA noted that asthmatics are likely 
more sensitive to the respiratory effects of SO2 due to pre-
existing inflammation associated with the disease. For example, pre-
existing inflammation may lead to enhanced release of inflammatory 
mediators, and/or enhanced sensitization of the chemosensitive 
receptors (id).
    Taken together, the ISA concluded that the controlled human 
exposure, epidemiologic, and toxicological evidence supported its 
determination of a causal relationship between respiratory morbidity 
and short-term (5-minutes to 24-hours) exposure to SO2.
a. Adversity of Short-Term Respiratory Morbidity Effects
    As discussed more fully in the proposal (section II.B.1.c, 74 FR at 
64817) and in section II.E.2.b below, based on: (1) American Thoracic 
Society (ATS) guidelines; (2) advice and recommendations from CASAC 
(see specific consensus CASAC comments in sections II.E.2.b and 
II.F.4.b below); and (3) conclusions from previous NAAQS reviews, EPA 
found that 5-10 minute exposures to SO2 concentrations at 
least as low as 200 ppb can result in adverse health effects in some 
asthmatics (i.e., 5-30% of the tested individuals in controlled human 
exposure studies of 200-300 ppb). As just mentioned, at SO2 
concentrations >= 400 ppb, controlled human exposure studies have 
reported decrements in lung function that are often statistically 
significant at the group mean level, and that are frequently 
accompanied by respiratory symptoms. Being mindful that the ATS 
guidelines specifically indicate decrements in lung function with 
accompanying respiratory symptoms as being adverse (see proposal 
section II.B.1.c, 74 FR at 64817 and section II.E.2.b below), exposure 
to 5-10 minute SO2 concentrations >= 400 ppb can result in 
health effects that are clearly adverse.
    The ATS also indicated that exposure to air pollution that 
increases the risk of an adverse effect to a population is adverse, 
even though it may not increase the risk of any individual to an 
unacceptable level (ATS 2000; see proposal section II.B.1.c, 74 FR at 
64817). As an example, ATS states:

    A population of children with asthma could have a distribution 
of lung function such that no individual child has a level 
associated with significant impairment. Exposure to air pollution 
could shift the distribution toward lower levels without bringing 
any individual child to a level that is associated with clinically 
relevant consequences. Individuals within the population would, 
however, have diminished reserve function and are at potentially 
increased risk if affected by another agent, e.g., a viral 
infection. Assuming that the relationship between the risk factor 
and the disease is causal, the committee considered that such a 
shift in the risk factor distribution, and hence the risk profile of 
the exposed population, should be considered adverse, even in the 
absence of the immediate occurrence of frank illness (ATS 2000, p. 
668).

    As mentioned above, the ISA reported that exposure to 
SO2 concentrations as low as 200-300 ppb for 5-10 minutes 
results in approximately 5-30% of exercising asthmatics experiencing 
moderate or greater decrements in lung function (defined in terms of a 
>= 15% decline in FEV1 or 100% increase in sRaw; ISA, Table 
3-1). Even though these results were not statistically significant at 
the group mean level, in light of EPA's interpretation of how to apply 
the ATS guidelines for defining an adverse effect, as described above, 
the REA found that these results could reasonably indicate an 
SO2-induced shift in these lung function measurements for 
this subset of the population. As a result, an appreciable percentage 
of exercising asthmatics exposed to SO2 concentrations as 
low as 200 ppb would be expected to have diminished reserve lung 
function and would be expected to be at greater risk if affected by 
another respiratory agent, for example, viral infection. Importantly, 
as explained immediately above, diminished reserve lung function in a 
population that is attributable to air pollution is considered an 
adverse effect under ATS guidance. In addition to the 2000 ATS 
guidelines, the REA was also mindful of previous CASAC recommendations 
(Henderson 2006) and NAAQS review conclusions (EPA 2006, EPA 2007d) 
indicating that moderate decrements in lung function can be clinically 
significant in some asthmatics (discussed in detail below, see section 
II.E.2.b). The REA further considered that subjects participating in 
these controlled human exposure studies do not include severe 
asthmatics and that it was reasonable to presume that persons with more 
severe asthma than the study participants would have a more serious 
health effect from short-term exposure to 200 ppb SO2.\5\ 
Taken together, the REA concluded that exposure to SO2 
concentrations at least as low as 200 ppb can result in adverse health 
effects in asthmatics and that this conclusion was in agreement with 
consensus CASAC comments and recommendations expressed during the 
current SO2 NAAQS review (see sections II.E.2.b and II.F.4.b 
below).
---------------------------------------------------------------------------

    \5\ We also note that very young children were not included in 
the controlled human exposure studies and this absence of data on 
what is likely to be a sensitive life stage is a source of 
uncertainty for children's susceptibility to SO2.
---------------------------------------------------------------------------

    In addition to the controlled human exposure evidence, 
epidemiologic studies also indicate that adverse respiratory morbidity 
effects are associated with SO2 (REA, section 4.3). As 
mentioned above, in reaching the conclusion of a causal relationship 
between respiratory morbidity and short-term SO2 exposure, 
the ISA generally found positive associations between ambient 
SO2 concentrations and emergency department visits and 
hospitalizations for all respiratory causes and asthma. Notably, 
emergency department visits, hospitalizations, episodic respiratory 
illness, and aggravation of respiratory diseases (e.g. asthma) 
attributable to air pollution are considered adverse health effects 
under ATS guidelines.
2. Health Effects and Long-Term Exposures to SO2
    There were numerous studies published since the last review 
examining possible associations between long-term SO2 
exposure and mortality and morbidity (respiratory morbidity, 
carcinogenesis, adverse prenatal and neonatal outcomes) endpoints. 
However, the ISA concluded that the evidence relating long-term (weeks 
to years) SO2 exposure to adverse health effects was 
``inadequate to infer the presence or absence of a causal 
relationship'' (ISA, Table 5-3). That is, the ISA found the long-term 
health evidence to be of insufficient quantity, quality, consistency, 
or statistical power to make a determination as to whether 
SO2 was truly associated with these health outcomes (ISA, 
Table 1-2).
3. SO2-Related Impacts on Public Health
    Interindividual variation in human responses to air pollutants 
indicates that some populations are at increased risk for the 
detrimental effects of ambient exposure to SO2. The NAAQS 
are intended to provide an adequate margin of safety for both the 
general population and susceptible populations that are potentially at 
increased risk for health effects in response to exposure to ambient 
air pollution (see footnote 1 above). To facilitate the identification 
of populations at increased risk for SO2-related health 
effects, studies have identified factors that contribute to the 
susceptibility of individuals to SO2. Susceptible 
individuals are broadly defined as those with a greater

[[Page 35527]]

likelihood of an adverse outcome given a specific exposure in 
comparison with the general population (American Lung Association, 
2001). The susceptibility of an individual to SO2 can 
encompass a multitude of factors which represent normal developmental 
phases or life stages (e.g., age) or biologic attributes (e.g., 
gender); however, other factors (e.g., socioeconomic status (SES)) may 
influence the manifestation of disease and also increase an 
individual's susceptibility (American Lung Association, 2001). In 
addition, populations may be at increased risk to SO2 due to 
an increase in their exposure during certain life stages (e.g., 
childhood or old age) or as a result of external factors (e.g., SES) 
that contribute to an individual being disproportionately exposed to 
higher concentrations than the general population.\6\ It should be 
noted that in some cases specific populations may be affected by 
multiple susceptibility factors. For example, a population that is 
characterized as having low SES may have less access to healthcare 
resulting in the manifestation of a disease, which increases their 
susceptibility to SO2, while they may also reside in a 
location that results in disproportionately high exposure to 
SO2.
---------------------------------------------------------------------------

    \6\ This aspect of susceptibility is referred to as 
vulnerability in the proposal and in the ISA.
---------------------------------------------------------------------------

    To examine whether SO2 differentially affects certain 
populations, stratified analyses are often conducted in epidemiologic 
investigations to identify the presence or absence of effect 
modification. A thorough evaluation of potential effect modifiers may 
help identify susceptible populations that are at increased risk to 
SO2 exposure. These analyses are based on the proper 
identification of confounders and subsequent adjustment for them in 
statistical models, which helps separate a spurious from a true causal 
association. Although the design of toxicological and human clinical 
studies does not allow for an extensive examination of effect 
modifiers, the use of animal models of disease and the study of 
individuals with underlying disease or genetic polymorphisms do allow 
for comparisons between subgroups. Therefore, the results from these 
studies, combined with those results obtained through stratified 
analyses in epidemiologic studies, contribute to the overall weight of 
evidence for the increased susceptibility of specific populations to 
SO2. Those populations identified in the ISA to be 
potentially at greater risk of experiencing an adverse health effect 
from SO2 were described in detail in the proposal (section 
II.B.5) and include: (1) Those with pre-existing respiratory disease; 
(2) children and older adults; (3) persons who spend increased time 
outdoors or at elevated ventilation rates; (4) persons with lower SES; 
and (5) persons with certain genetic factors.
    As discussed in the proposal (section II.B.5.g, 74 FR at 64821), 
large proportions of the U.S. population are likely to be at increased 
risk of experiencing SO2-related health effects. In the 
United States, approximately 7% of adults and 9% of children have been 
diagnosed with asthma. Notably, the prevalence and severity of asthma 
is higher among certain ethnic or racial groups such as Puerto Ricans, 
American Indians, Alaskan Natives, and African Americans (EPA 2008b). 
Furthermore, a higher prevalence of asthma among persons of lower SES 
and an excess burden of asthma hospitalizations and mortality in 
minority and inner-city communities have been observed (EPA, 2008b). In 
addition, population groups based on age comprise substantial segments 
of individuals that may be potentially at risk for SO2-
related health impacts. Based on U.S. census data from 2000, about 72.3 
million (26%) of the U.S. population are under 18 years of age, 18.3 
million (7.4%) are under 5 years of age, and 35 million (12%) are 65 
years of age or older. There is also concern for the large segment of 
the population that is potentially at risk to SO2-related 
health effects because of increased time spent outdoors at elevated 
ventilation rates (those who work or play outdoors). Overall, the 
considerable size of the population groups at risk indicates that 
exposure to ambient SO2 could have a significant impact on 
public health in the United States.

C. Human Exposure and Health Risk Characterization

    To put judgments about SO2-associated health effects 
into a broader public health context, EPA has drawn upon the results of 
the quantitative exposure and risk assessments. Judgments reflecting 
the nature of the evidence and the overall weight of the evidence are 
taken into consideration in these quantitative exposure and risk 
assessments. These assessments include estimates of the likelihood that 
asthmatic children at moderate or greater exertion (e.g. while 
exercising) in St. Louis or Greene County, Missouri would experience 
SO2 exposures of potential concern. In addition, these 
analyses include an estimate of the number and percent of exposed 
asthmatic children in these locations likely to experience 
SO2-induced lung function responses (i.e., moderate or 
greater decrements in lung function defined in terms of sRaw or 
FEV1) under varying air quality scenarios (i.e., current air 
quality and air quality simulated to just meet the current or potential 
alternative standards). These assessments also characterize the kind 
and degree of uncertainties inherent in such estimates.
    As previously mentioned, the ISA concluded that the evidence for an 
association between respiratory morbidity and short-term SO2 
exposure was ``sufficient to infer a causal relationship'' (ISA, 
section 5.2) and that the ``definitive evidence'' for this conclusion 
was from the results of 5-10 minute controlled human exposure studies 
demonstrating decrements in lung function and/or respiratory symptoms 
in exercising asthmatics (ISA, section 5.2). Accordingly, the air 
quality and exposure analyses and their associated risk 
characterizations focused on 5-minute concentrations of SO2 
in excess of potential health effect benchmark values derived from the 
controlled human exposure literature (see proposal section II.C.1, 74 
FR at 64821, and REA, section 6.2). These benchmark levels are not 
potential standards, but rather are SO2 exposure 
concentrations which represent ``exposures of potential concern'' which 
are used in these analyses to estimate potential exposures and risks 
associated with 5-minute concentrations of SO2. The REA 
considered 5-minute benchmark levels of 100, 200, 300, and 400 ppb in 
these analyses, but especially noted exceedances or exposures with 
respect to the 200 and 400 ppb 5-minute benchmark levels. These 
benchmark levels were highlighted because (1) 400 ppb represents the 
lowest concentration in free-breathing controlled human exposure 
studies where moderate or greater lung function decrements occurred 
which were often statistically significant at the group mean level and 
were frequently accompanied by respiratory symptoms; and (2) 200 ppb is 
the lowest level at which moderate or greater decrements in lung 
function in free-breathing controlled human exposure studies were found 
in some individuals, although these lung function changes were not 
statistically significant at the group mean level. Notably, 200 ppb is 
also the lowest level that has been tested in free-breathing controlled 
human exposure studies (REA, section 4.2.2).\7\
---------------------------------------------------------------------------

    \7\ The ISA cites one chamber study with intermittent exercise 
where healthy and asthmatic children were exposed to 100 ppb 
SO2 in a mixture with ozone and sulfuric acid. The ISA 
notes that compared to exposure to filtered air, exposure to the 
pollutant mix did not result in statistically significant changes in 
lung function or respiratory symptoms (ISA, section 3.1.3.4).

---------------------------------------------------------------------------

[[Page 35528]]

    The REA utilized three approaches to characterize health risks. In 
the first approach, for each air quality scenario, statistically 
estimated 5-minute SO2 concentrations \8\ and measured 
ambient 5-minute SO2 concentrations were compared to the 5-
minute potential health effect benchmark levels discussed above (REA, 
chapter 7). This air quality analysis included all available ambient 
monitoring data as well as a more detailed analysis in 40 counties. The 
air quality analysis was considered a broad characterization of 
national air quality and human exposures that might be associated with 
these 5-minute SO2 concentrations. An advantage of the air 
quality analysis is its relative simplicity; however, there is 
uncertainty associated with the assumption that SO2 air 
quality can serve as an adequate surrogate for total exposure to 
ambient SO2. Actual exposures might be influenced by factors 
not considered by this approach, including small-scale spatial 
variability in ambient SO2 concentrations (which might not 
be represented by the current fixed-site ambient monitoring network) 
and spatial/temporal variability in human activity patterns. A more 
detailed overview of the air quality analysis and its associated 
limitations and uncertainties is provided in the proposal (see sections 
II.C.2, 74 FR at 64822 and II.C.3, 74 FR at 64823, respectively) and 
the air quality analysis is thoroughly described in the REA (chapter 
7).
---------------------------------------------------------------------------

    \8\ Benchmark values derived from the controlled human exposure 
literature were associated with a 5-minute averaging time. However, 
as noted in footnote 3 above, only 98 ambient monitors located in 13 
States from 1997-2007 reported measured 5-minute SO2 
concentrations since such monitoring is not required (see section 
II.A.2 and section IV). In contrast, 809 monitors in 48 States, DC, 
Puerto Rico, and the Virgin Islands reported 1-hour SO2 
concentrations over a similar time period. Therefore, to broaden 
analyses to areas where measured 5-minute SO2 
concentrations were not available, the REA utilized a statistical 
relationship to estimate the highest 5-minute level in an hour, 
given a reported 1-hour average SO2 concentration (REA, 
section 6.4). Then, similar to measured 5-minute SO2 
concentrations, statistically estimated 5-minute SO2 
concentrations were compared to 5-minute potential health effect 
benchmark values (REA, chapters 7 and 8, respectively).
---------------------------------------------------------------------------

    In the second approach, an inhalation exposure model was used to 
generate more realistic estimates of personal exposures in asthmatics 
(REA, chapter 8). This analysis estimated temporally and spatially 
variable microenvironmental 5-minute SO2 concentrations and 
simulated asthmatics' contact with these pollutant concentrations while 
at moderate or greater exertion (i.e., while at elevated ventilation 
rates). The approach was designed to estimate exposures that are not 
necessarily represented by the existing ambient monitoring data and to 
better represent the physiological conditions corresponding with the 
respiratory effects reported in controlled human exposure studies. 
AERMOD, an EPA dispersion model, was used to estimate 1-hour ambient 
SO2 concentrations using emissions estimates from 
stationary, non-point, and where applicable, port sources. The Air 
Pollutants Exposure (APEX) model, an EPA human exposure model, was then 
used to estimate population exposures using the estimated hourly census 
block level SO2 concentrations. From the 1-hour census block 
concentrations, 5-minute maximum SO2 concentrations within 
each hour were estimated by APEX (REA, section 8.7.1) using the 
statistical relationship mentioned above in footnote 8. Estimated 
exposures to 5-minute SO2 levels were then compared to the 
5-minute potential health effect benchmark levels discussed above. This 
approach to assessing exposures was more resource intensive than using 
ambient levels as an indicator of exposure; therefore, the final REA 
included the analysis of two locations: St. Louis and Greene County, 
MO. Although the geographic scope of this analysis was limited, the 
approach provided estimates of SO2 exposures in asthmatics 
and asthmatic children in St. Louis and Greene Counties, and thus 
served to complement the broader air quality characterization. A more 
detailed overview of this exposure analysis and its associated 
limitations and uncertainties is provided in the proposal (see sections 
II.C.2, 74 FR at 64822 and II.C.3, 74 FR at 64823, respectively) and 
the exposure analysis is thoroughly described in the REA (chapter 8).
    The third approach was a quantitative risk assessment. This 
approach combined results from the exposure analysis (i.e., the number 
of exposed total asthmatics or asthmatic children while at moderate or 
greater exertion) with exposure-response functions derived from 
individual level data from controlled human exposure studies (see ISA, 
Table 3-1 and Johns (2009) \9\) to estimate the percentage and number 
of exposed asthmatics and asthmatic children in St. Louis and Greene 
County likely to experience a moderate or greater lung function 
response (i.e., decrements in lung function defined in terms of 
FEV1 and sRaw) under the air quality scenarios mentioned 
above (REA, chapter 9). A more detailed overview of this analysis and 
its associated limitations and uncertainties is provided in the 
proposal (see sections II.C.2, 74 FR at 64822 and II.C.3, 74 FR at 
64823, respectively) and the quantitative risk analysis is thoroughly 
described in the REA (chapter 9).
---------------------------------------------------------------------------

    \9\ EPA recently conducted a complete quality assurance review 
of all individual subject data. The results of this review did not 
substantively change any of the entries in ISA, Table 3-1, and did 
not in any way affect the conclusions of the ISA (see Johns and 
Simmons, 2009).
---------------------------------------------------------------------------

    Notably, for the reasons described in the REA (REA, section 10.3.3) 
and the proposal (see section II.E.1.b, 74 FR at 64827), when 
considering the St. Louis and Greene County exposure and risk results 
as they relate to the adequacy of the current standards, the REA 
concluded that the St. Louis results were more informative in terms of 
ascertaining the extent to which the current standards protect against 
health effects linked to the various benchmarks (linked in turn to 5-
minute SO2 exposures). The results in fact suggested that 
the current standards may not adequately protect public health (REA, 
section 10.3.3, p. 364). Moreover, the REA judged that the exposure and 
risk estimates for the St. Louis study area provided useful insights 
into exposures and risks for other urban areas in the U.S. with similar 
population and SO2 emissions densities (id.). For similar 
reasons, the St. Louis results were more informative for ascertaining 
the adequacy of the potential alternative standards under 
consideration.
    Key results of the air quality, exposure, and risk analyses were 
presented in the policy assessment chapter of the REA (chapter 10) and 
summarized in the proposal (see Tables 2-4 in the preamble to the 
proposed rule). In considering these results, the proposal noted that 
these analyses support that 5-minute SO2 exposures, 
reasonably judged important from a public health perspective, were 
associated with air quality adjusted upward to simulate just meeting 
the current standards (see proposal, section II.E.1.c, 74 FR at 64828). 
Moreover, these results indicated that 99th percentile 1-hour daily 
maximum standard levels in the range of 50-100 ppb could substantially 
limit exposures of asthmatic children at moderate or greater exertion 
from 5-minute SO2 concentrations >=400 ppb, and appreciably 
limit exposures of these children from 5-minute SO2 
concentrations >=200 ppb (REA, p. 392-393). Results of these analyses 
also indicated that a 1-hour standard at 150

[[Page 35529]]

ppb could still substantially limit exposures of asthmatic children at 
moderate or greater exertion from 5-minute SO2 
concentrations >=400 ppb, but would provide these children appreciably 
less protection from exposure to 5-minute SO2 concentrations 
>=200 ppb (REA, p. 395-396).

D. Approach for Determining Whether To Retain or Revise the Current 
Standards

    EPA notes that the final decision on retaining or revising the 
current primary SO2 standards is a public health policy 
judgment to be made by the Administrator. This judgment has been 
informed by a recognition that the available health effects evidence 
reflects a continuum consisting of ambient levels of SO2 at 
which scientists generally agree that health effects are likely to 
occur, through lower levels at which the likelihood and magnitude of 
the response become increasingly uncertain. The Administrator's final 
decisions draw upon scientific information and analyses related to 
health effects, population exposures and risks; judgments about the 
appropriate response to the range of uncertainties that are inherent in 
the scientific evidence and analyses; and comments received from CASAC 
and the public.
    To evaluate whether the current primary SO2 standards 
are adequate or whether revisions are appropriate, EPA has used an 
approach in this review described in chapter 10 of the REA which builds 
upon the approaches used in reviews of other criteria pollutants, 
including the most recent reviews of the NO2, Pb, 
O3, and PM NAAQS (EPA, 2008c; EPA, 2007c; EPA, 2007d; EPA, 
2005), and reflects the latest body of evidence and information that is 
currently available, as reflected by the ISA. As in other recent 
reviews, EPA considered the implications of placing more or less weight 
or emphasis on different aspects of the scientific evidence and the 
exposure-/risk-based information, recognizing that the weight to be 
given to various elements of the evidence and exposure/risk information 
is part of the public health policy judgments that the Administrator 
will make in reaching decisions on the standard.
    A series of general questions framed this approach to considering 
the scientific evidence and exposure-/risk-based information. First, 
EPA's consideration of the scientific evidence and exposure/risk 
information with regard to the adequacy of the current standards has 
been framed by the following questions:

     To what extent does evidence that has become available 
since the last review reinforce or call into question evidence for 
SO2-associated effects that were identified in the last 
review?
     To what extent has evidence for different health 
effects and/or susceptible populations become available since the 
last review?
     To what extent have uncertainties identified in the 
last review been reduced and/or have new uncertainties emerged?
     To what extent does evidence and exposure-/risk-based 
information that has become available since the last review 
reinforce or call into question any of the basic elements 
(indicator, averaging time, form, and level) of the current 
standard?

    To the extent that the available evidence and exposure-/risk-based 
information suggests it may be appropriate to consider revision of the 
current standards, EPA considers that evidence and information with 
regard to its support for consideration of a standard that is either 
more or less stringent than the current standards. This evaluation is 
framed by the following questions:

     Is there evidence that associations, especially causal 
or likely causal associations, extend to ambient SO2 
concentrations as low as, or lower than, the concentrations that 
have previously been associated with health effects? If so, what are 
the important uncertainties associated with that evidence?
     Are exposures above benchmark levels and/or health 
risks estimated to occur in areas that meet the current standard? If 
so, are the estimated exposures and health risks important from a 
public health perspective? What are the important uncertainties 
associated with the estimated risks?

    To the extent that there is support for consideration of a revised 
standard, EPA then considers the specific elements of the standard 
(indicator, averaging time, form, and level) within the context of the 
currently available information. In so doing, the Agency addresses the 
following questions regarding the elements of the standard:

     Does the evidence provide support for considering a 
different indicator for gaseous SOX?
     Does the evidence provide support for considering 
different, or additional averaging times?
     What ranges of levels and forms of alternative 
standards are supported by the evidence, and what are the associated 
uncertainties and limitations?
     To what extent do specific averaging times, levels, and 
forms of alternative standards reduce the estimated exposures above 
benchmark levels and risks attributable to exposure to ambient 
SO2, and what are the uncertainties associated with the 
estimated exposure and risk reductions?

    The questions outlined above have been addressed in the REA. The 
following sections present considerations regarding the adequacy of the 
current standards and conclusions on the elements of a new short-term 
standard in terms of indicator, averaging time, form, and level.

E. Adequacy of the Current Standards

    This section discusses considerations related to the decision as to 
whether the current 24-hour and annual SO2 primary NAAQS are 
requisite to protect public health with an adequate margin of safety. 
Specifically, section II.E.1 provides an overview of the rationale 
supporting the Administrator's proposal that the current standards do 
not provide adequate public health protection; section II.E.2 discusses 
public comments received on the adequacy of the current standards; and 
section II.E.3 discusses the Administrator's final decision on whether 
the current SO2 primary NAAQS is requisite to protect public 
health with an adequate margin of safety, as required by sections 
109(d) and (b) of the Act.
1. Rationale for Proposed Decision
    In the proposal, the Administrator initially concluded that the 
current 24-hour and annual SO2 NAAQS were not adequate to 
protect public health with an adequate margin of safety (see section 
II.E.4, 74 FR at 64829). In reaching this conclusion, she considered 
the: (1) Scientific evidence and conclusions in the ISA; (2) exposure 
and risk information presented in the REA; (3) conclusions of the 
policy assessment chapter of the REA; and (4) views expressed by CASAC. 
These considerations are discussed in detail in the proposal (see 
section II.E., 74 FR at 64826) and are summarized in this section.
    In the proposal the Administrator noted the following in 
considering the adequacy of the current 24-hour and annual primary 
SO2 standards:
     The conclusion of the ISA that the results of controlled 
human exposure and epidemiologic studies form a plausible and coherent 
data set that supports a causal relationship between short-term (5-
minutes to 24-hours) SO2 exposures and adverse respiratory 
effects, and that the epidemiologic evidence (buttressed by the 
clinical evidence) indicates that the effects seen in the epidemiologic 
studies are attributable to exposure to SO2 (ISA, section 
5.2).
     The conclusion of the ISA that ``[i]n the epidemiologic 
studies, respiratory effects were observed in areas where the maximum 
ambient 24-h avg SO2 concentration was below the current 24-

[[Page 35530]]

h avg NAAQS level * * *.'' (ISA, section 5.2, p. 5-2.) and so would 
occur at ambient SO2 concentrations that are present in 
locations meeting the current 24-hour NAAQS.
     These respiratory effects also occurred in areas with 
annual air quality levels considerably lower than those allowed by the 
current annual standard, indicating that the current annual standard is 
also not providing protection against short-term health effects 
reported in epidemiologic studies (ISA, section 5.2).
     Analyses in the REA supporting that 5-minute exposures, 
reasonably judged important from a public health perspective (i.e., 
respiratory effects judged to be adverse to the health of asthmatics, 
see sections II.B.1.c above, and II.E.2.b below), were associated with 
air quality adjusted upward to simulate just meeting the current 24-
hour and annual standards.
     CASAC advice ``that the current 24-hour and annual 
standards are not adequate to protect public health, especially in 
relation to short term exposures to SO2 (5-10 minutes) by 
exercising asthmatics'' (Samet, 2009, p. 15).
    Based on these considerations (discussed in more detail in the 
proposal, see sections II.E.1 and II.E.2), the Administrator proposed 
that the current 24-hour and annual SO2 standards are not 
requisite to protect public health with an adequate margin of safety 
against adverse respiratory effects associated with short-term (5-
minute to 24-hour) SO2 exposures. In considering approaches 
to revising the current standards, the Administrator initially 
concluded it appropriate to consider setting a new 1-hour standard. The 
Administrator noted that a 1-hour standard would likely provide 
increased public health protection, especially for members of at-risk 
groups, from the respiratory effects described in both epidemiologic 
and controlled human exposure studies.
2. Comments on the Adequacy of the Current Standards
    This section discusses public comments on the proposal that either 
supported or opposed the Administrator's proposed decision to revise 
the current SO2 primary NAAQS. Comments on the adequacy of 
the current standards that focused on the scientific and/or the 
exposure/risk basis for the Administrator's proposed conclusions are 
discussed in sections II.E.2.a-II.E.2.c. Comments on the epidemiologic 
evidence are considered in section II.E.2.a. Comments on the controlled 
human exposure evidence are considered in section II.E.2.b. Comments on 
human exposure and health risk assessments are considered in section 
II.E.2.c. To the extent these comments on the evidence and information 
are also used to justify commenters' conclusions on decisions related 
to indicator, averaging time, form, or level, they are noted as well in 
the appropriate sections below (II.F.1-II.F.4, respectively). The 
summaries of comments, and responses thereto, presented below are not 
exclusive: other comments and responses are being included in the 
Response to Comment (RTC) Document which is part of the record for this 
rulemaking (EPA, 2010).
    Many public commenters agreed with the proposal that based on the 
available information, the current SO2 standards are not 
requisite to protect public health with an adequate margin of safety 
and that revisions to the standards are therefore appropriate. Among 
those calling for revisions to the standards were environmental groups 
(e.g., Sierra Club, WEACT for Environmental Justice, Center for 
Biological Diversity, (CBD) Environmental Defense Fund (EDF), Natural 
Resources Defense Council (NRDC)); medical/public health organizations 
(e.g., American Lung Association (ALA), American Thoracic Society 
(ATS)); State environmental organizations (e.g., National Association 
of Clean Air Agencies (NACAA), Northeast States for Coordinated Air Use 
Management (NESCAUM); State environmental agencies (e.g., such agencies 
in DE, IA, IL, MI, NY, NM, OH, PA, TX, VT); the Fond du Lac Band of 
Lake Superior Chippewa (Fond du Lac) Tribe, local groups (e.g., 
Houston-Galveston Area Council, Alexandria Department of Transportation 
and Environmental Services) and most individual commenters (~13,000). 
These commenters generally concluded that the current SO2 
standards need to be revised and that a more stringent standard is 
needed to protect the health of susceptible population groups. In 
supporting the need to adopt a more stringent NAAQS for SO2, 
these commenters often referenced the conclusions of CASAC, as well as 
evidence and information presented in the proposal. As such, the 
rationale offered by these commenters was consistent with that 
presented in the proposal to support the Administrator's proposed 
decision to revise the current SO2 NAAQS.
    Most industry commenters (e.g., Utility Air Regulatory Group 
(UARG), American Petroleum Institute (API), Arizona Public Service, 
National Petrochemical & Refiners Association (NPRA), Montana-Dakota 
Utilities Co., Dominion Resources, Council of Industrial Boiler Owners 
(CIBO), Edison Electric Institute (EEI), Duke Energy, National Mining 
Association (NMA)); and some organizations (e.g., Texas Association of 
Business, The Annapolis Center for Science-Based Public Policy 
(ACSBPP), South Carolina Chamber of Commerce) opposed the proposed 
revisions to the SO2 primary NAAQS. In supporting their 
views, industry commenters generally concluded that EPA did not 
appropriately consider uncertainties associated with the epidemiologic 
and controlled human exposure evidence.
    More specifically, with respect to the epidemiologic studies, many 
of these commenters concluded that results of these studies are 
confounded by co-pollutants and thus too uncertain to determine whether 
SO2 is truly associated with the health outcomes being 
measured (e.g., hospital admissions; Federal Register see below). With 
respect to the controlled human exposure studies, many commenters were 
critical of the 5-minute benchmark levels that were derived from these 
studies and subsequently used by EPA in the air quality, exposure, and 
risk analyses. These groups were particularly concerned about the 
Administrator's reliance on the 200 ppb 5-minute benchmark level in 
assessing the adequacy of the current and potential alternative 
standards. In general, many industry groups maintained that adverse 
respiratory effects did not occur following 5-10 minute SO2 
exposures < 400 ppb (e.g., API, EEI, CIBO) and some groups stated that 
even at SO2 concentrations >= 400 ppb, reported effects may 
not be of clinical concern, and thus are likely not adverse (e.g., 
UARG). Many industry groups (e.g., API, UARG) also disagreed with EPA's 
(and CASAC's) conclusions that severe asthmatics were not included in 
these controlled human exposure studies, and that severe asthmatics 
would likely have a more pronounced response to SO2 
exposures at a given level, or would respond to even lower levels of 
SO2.
    In responding to these specific comments, we note that the 
Administrator relied in the proposal on the evidence, information, and 
judgments contained in the ISA and the REA (including the policy 
assessment chapter), as well as on the advice of CASAC. In considering 
the evidence, information, and judgments of the ISA and the REA, the 
Agency notes that these documents have been reviewed and discussed 
extensively by CASAC at multiple public meetings (see above, section 
I.D) and in their letters to the

[[Page 35531]]

EPA Administrator. Thus, it is important to note that CASAC generally 
accepted the key findings and conclusions presented in both the ISA and 
REA (see Henderson 2008a, Henderson 2008b, and Samet, 2009).
a. Comments on EPA's Interpretation of the Epidemiologic Evidence
    Many industry groups (e.g., API, UARG, American Chemistry Council 
(ACC), Dominion Resources, ExxonMobil, Progress Energy, CIBO, The 
Fertilizer Institute, EEI, Dow Chemical Company (Dow), MeadWestvaco 
Corporation (MWV), (NMA) and some organizations (e.g., ACSBPP) 
commented that, given the presence of numerous co-pollutants in the 
air, the epidemiologic studies do not support the contention that 
SO2 itself is causing health effects. For example, UARG 
stated: ``The epidemiological evidence cannot determine that 
SO2 is a cause of or a contributor to hospital admissions 
(``HA''), emergency department (``ED'') visits or respiratory symptoms, 
the effects cited in the Proposed Rule.''
    Although EPA has recognized that multiple factors can contribute to 
the etiology of respiratory disease and that more than one air 
pollutant could independently impact respiratory health, we continue to 
judge, as discussed in the ISA, that the available evidence supports 
the conclusion that there is an independent effect of SO2 on 
respiratory morbidity. In reaching this judgment, we recognize that a 
major methodological issue affecting SO2 epidemiologic 
studies concerns the evaluation of the extent to which other air 
pollutants, particular PM2.5,\10\ may confound or modify 
SO2-related effect estimates. The use of multi-pollutant 
regression models is a common approach for evaluating potential 
confounding by co-pollutants in epidemiologic studies. It is therefore 
important to note that when the ISA evaluated U.S. and international 
epidemiologic studies employing multi-pollutant models, SO2 
effect estimates generally remained positive and relatively unchanged 
when co-pollutants, including PM, were included (see ISA, p. 5-5). 
Therefore, although recognizing the uncertainties associated with 
separating the effects of SO2 from those of co-occurring 
pollutants, the ISA concluded that the limited available evidence 
indicates that the effect of SO2 on respiratory health 
outcomes appears to be generally robust and independent of the effects 
of gaseous co-pollutants, including NO2 and O3, 
as well as particulate co-pollutants, particularly PM2.5 
(ISA, section 5.2; p. 5-9).
---------------------------------------------------------------------------

    \10\ As noted in the proposal (see sections II.D.1, 74 FR at 
64824-64825 and II.F.4.a, 74 FR at 64835), there is special 
sensitivity in this review in disentangling SO2-related 
effects from PM-related effects (especially sulfate PM).
---------------------------------------------------------------------------

    In considering questions of confounding and causation, the 
epidemiologic studies should not be considered in a vacuum. As 
emphasized by the ISA, and endorsed by CASAC, controlled human exposure 
studies provide support for the plausibility of the associations 
reported in epidemiologic studies (ISA, section 5-5; Henderson 2008a; 
Henderson 2008b). These controlled human exposure studies exposed 
exercising asthmatics to 5-10 minute peaks of SO2 and 
reported decrements in lung function and/or respiratory symptoms in up 
to 60% of these individuals (depending on exposure concentration; see 
ISA, Table 5-3; p. 5-11). Thus, these experimental study results 
provide strong support for an independent contribution of 
SO2 to the respiratory health effects reported in 
epidemiologic studies: ``The effects of SO2 on respiratory 
symptoms, lung function, and airway inflammation observed in the human 
clinical studies using peak exposures further provides a basis for a 
progression of respiratory morbidity resulting in increased emergency 
department visits and hospital admissions. Collectively, these findings 
provide biological plausibility for the observed association between 
ambient SO2 levels and emergency department visits and 
hospitalizations for all respiratory diseases and asthma, notably in 
children and older adults. * * *'' (ISA, section 5.2 at p. 5-5). Thus, 
EPA is not relying solely on the epidemiologic studies to evaluate 
whether associations reported in these studies (e.g., associations with 
emergency department visits) are likely the result of ambient 
SO2 exposure.
b. Comments on EPA's Interpretation of the Controlled Human Exposure 
Evidence
    Many industry groups (e.g., API, ACC, Progress Energy, EEI, CIBO) 
commented that adverse health effects do not occur following 5-10 
minute SO2 exposures < 400 ppb. In addition, some groups 
(e.g., UARG) commented that adverse respiratory effects do not occur in 
exercising asthmatics following SO2 exposures below 600 ppb. 
The disagreement is not whether effects occur in exercising asthmatics 
at these exposure levels and exposure durations. Rather, the issue is 
whether the effects experienced can properly be regarded as adverse. In 
general, these groups conclude that EPA's judgment of adverse health 
effects at SO2 exposure levels below 600 or 400 ppb is 
inappropriately based on an unsound interpretation of ATS guidelines. 
More specifically, these groups generally contend that decrements in 
lung function without accompanying respiratory symptoms are not adverse 
effects of SO2 exposure, and that decrements in lung 
function in a percentage of exercising asthmatics does not represent a 
shift in lung function at the population level. Some of these groups 
also contend that EPA followed the advice of individual CASAC members, 
rather than consensus CASAC written comments on the ISA and REA when 
concluding respiratory effects associated with SO2 exposures 
below 600 or 400 ppb are adverse. Furthermore, some groups contend that 
effects below 400 ppb should not be considered adverse because compared 
to the number of asthmatics experiencing decrements in lung function, 
there were similar numbers of asthmatics experiencing increases in lung 
function. EPA disagrees with these comments, and believes that the 
clinical evidence also supports the conclusion that the current 
standards are not requisite to protect public health with and adequate 
margin of safety.
    The Agency disagrees that adverse respiratory effects do not occur 
in exercising asthmatics following 5-10 minute SO2 exposures 
ranging from 400-600 ppb. As previously mentioned, at SO2 
concentrations ranging from 400-600 ppb, moderate or greater decrements 
in lung function occur in approximately 20-60% of exercising asthmatics 
(again, defined in terms of a >= 15% decline in FEV1 or 100% 
increase in sRaw; ISA, Table 3-1). Moreover, at concentrations >= 400 
ppb, decrements in lung function are often statistically significant at 
the group mean level, and are frequently accompanied by respiratory 
symptoms (ISA, Table 5-1). ATS guidelines on what constitutes an 
adverse health effect of air pollution clearly state that reversible 
loss of lung function in combination with the presence of symptoms 
should be considered adverse (ATS 1985, 2000). Moderate or greater 
decrements in lung function accompanied by respiratory symptoms fit 
this description. Thus, the Agency's conclusion of adverse health 
effects associated with SO2 concentrations >= 400 ppb is 
consistent with ATS guidelines.
    The Agency also disagrees with industry commenters regarding the 
adversity of the respiratory effects seen in exercising asthmatics 
following 5-10 minute SO2 exposures ranging from 200-300 
ppb. As mentioned above (section II.B.1), and discussed more

[[Page 35532]]

fully in the proposal (see section II.B.3, 74 FR at 64819), the ISA 
reported that exposure to SO2 concentrations as low as 200-
300 ppb for 5-10 minutes results in approximately 5-30% of exercising 
asthmatics experiencing moderate or greater decrements in lung 
function. In 2000, the ATS updated its guidelines on ``what constitutes 
an adverse health effect of air pollution.'' These guidelines indicated 
that exposure to air pollution that increases the risk of an adverse 
effect to the entire population is adverse, even though it may not 
increase the risk of any individual to an unacceptable level (ATS 
2000). For example, ATS notes that a population of asthmatics could 
have a distribution of lung function such that no individual has a 
level associated with significant impairment. Exposure to air pollution 
could shift the distribution to lower levels that still do not bring 
any individual to a level that is associated with clinically relevant 
effects. However, this would be considered adverse because individuals 
within the population would have diminished reserve function, and 
therefore would be at increased risk if affected by another agent (ATS 
2000).
    Considering the 2000 ATS guidelines, the results of the clinical 
studies conducted at 200-300 ppb were reasonably interpreted by EPA to 
indicate an SO2-induced shift in these lung function 
measurements for a subset of this population. That is, an appreciable 
percentage of this population of exercising asthmatics would be 
expected to experience moderate or greater decrements in lung function 
in response to SO2 concentrations as low as 200 ppb, and 
thus would be expected to have diminished reserve lung function. As a 
result, this sub-population would be at greater risk of a more severe 
response if affected by another respiratory agent (e.g., viral 
infection, or O3).
    EPA is also mindful of CASAC comments on this issue following the 
second draft ISA. The second draft ISA placed relatively little weight 
on health effects associated with SO2 exposures at 200-300 
ppb. CASAC strongly disagreed with this characterization of the health 
evidence. Their consensus letter following the second draft ISA states:

    Our major concern is the conclusions in the ISA regarding the 
weight of the evidence for health effects for short-term exposure to 
low levels of SO2. Although the ISA presents evidence 
from both clinical and epidemiological studies that indicate health 
effects occur at 0.2 ppm or lower, the final chapter emphasizes 
health effects at 0.4 ppm and above * * * CASAC believes the 
clinical and epidemiological evidence warrants stronger conclusions 
in the ISA regarding the available evidence of health effects at 0.2 
ppm or lower concentrations of SO2. The selection of a 
lower bound concentration for health effects is very important 
because the ISA sets the stage for EPA's risk assessment decisions. 
In its draft Risk and Exposure Assessment (REA) to Support the 
Review of the SO2 Primary National Ambient Air Quality Standards 
(July 2008), EPA chose a range of 0.4 ppm-0.6 ppm SO2 
concentrations for its benchmark analysis. As CASAC will emphasize 
in a forthcoming letter on the REA, we recommend that a lower bound 
be set at least as low as 0.2 ppm. (Henderson 2008a)

    EPA also notes the similar CASAC comments on the first draft of the 
REA. The consensus CASAC letter following the 1st draft REA states:

    The CASAC believes strongly that the weight of clinical and 
epidemiology evidence indicates there are detectable clinically 
relevant health effects in sensitive subpopulations down to a level 
at least as low as 0.2 ppm SO2. These sensitive 
subpopulations represent a substantial segment of the at-risk 
population. (Henderson 2008b; p. 1)

See Coalition of Battery Recyclers Association v. EPA, No. 09-1011 (DC 
Cir., May 14, 2010), slip opinion at 9, holding that it was reasonable 
for EPA to conclude that a two IQ point mean population loss is an 
adverse effect based in part on CASAC advice that such a decrement is 
significant. CASAC's strong advice regarding the adversity of effects 
at the 200 ppb level similarly supports EPA's conclusion that the 
observed lung decrements are adverse.
    In addition to the considerations described above, we also note the 
following key points:
     In the current SO2 NAAQS review, clinicians on 
the CASAC Panel advised that moderate or greater decrements in lung 
function can be clinically significant in some individuals with 
respiratory disease.\11\
---------------------------------------------------------------------------

    \11\ See hearing transcripts from EPA Clean Air Scientific 
Advisory Committee (CASAC), July 30-31 2008, Sulfur Oxides-Health 
Criteria (part 3 of 4) pages 211-213). These transcripts can be 
found in Docket ID No. EPA-HQ-ORD-2006-0260. Available at http://www.regulations.gov.
---------------------------------------------------------------------------

     In the last O3 NAAQS review, CASAC indicated 
that moderate decrements in lung function can be clinically significant 
in some asthmatics (Henderson 2006), and that in the context of 
standard setting, a focus on the lower end of the range of moderate 
functional responses is most appropriate for estimating potentially 
adverse lung function decrements in people with lung disease (e.g., 
asthma; see 73 FR at 16463).
     In the last O3 NAAQS review, the Criteria 
Document and the Staff Paper indicated that for many people with lung 
disease (e.g., asthma), even moderate decrements in lung function or 
respiratory symptoms would likely interfere with normal activities and 
result in additional and more frequent use of medication (EPA 2006, EPA 
2007d).
     Subjects participating controlled human exposure studies 
do not include severe asthmatics, and it is reasonable to presume that 
persons with more severe asthma than the study participants would have 
a more serious health effect from short-term exposure to 200 ppb 
SO2.
    Considering these key points along with the ATS guidelines and 
consensus CASAC comments on the draft ISA and REA described above, we 
reasonably conclude that 5-10 minute exposures to SO2 
concentrations at least as low as 200 ppb can result in adverse health 
effects in exercising asthmatics.
    In addition, as noted above some groups (e.g., API) contend that 
effects below 400 ppb should not be considered adverse because compared 
to the number of asthmatics experiencing decrements in lung function, 
there were similar numbers of asthmatics experiencing increases in lung 
function.
    The commenters correctly point out that at the lowest concentration 
tested in free-breathing chamber studies (200 ppb), there are a similar 
number of asthmatics experiencing a moderate or greater decrease in 
lung function (i.e., >= 100 increase in sRaw or >= 15 decrease in 
FEV1) and experiencing what might be called a moderate 
improvement in lung function (i.e., >= 100 decrease in sRaw or >= 15 
increase in FEV1). This observation is consistent with data 
presented in Figures 4-2 and 4-3 of the ISA showing essentially no 
SO2 -induced change in lung function at 200 ppb when 
averaged across asthmatics participating in the three Lin et al., 
controlled human exposure studies. However, these figures also 
demonstrate that asthmatics who are sensitive to SO2 at a 
higher concentration (600 ppb) experience, on average, a greater 
decrement in lung function at lower concentrations, including 200 ppb, 
when compared with all subjects combined. Therefore, while some 
asthmatics are relatively insensitive to SO2-induced 
respiratory effects even at concentrations >= 600 ppb, there is clear 
empirical evidence that others experience significant 
bronchoconstriction following exposures to both relatively high (600 
ppb) and low (200 ppb) SO2 concentrations. Among these 
SO2-sensitive asthmatics, Figures 4-2 and 4-3 of the ISA 
show a clear increase in

[[Page 35533]]

bronchoconstriction with increasing SO2 concentrations from 
200-400 ppb. Given this clear relationship of exposure and effect at 
all levels in the sensitive asthmatics (i.e. those who experienced 
significant decrements in lung function at the highest exposure 
concentration used (600 ppb)), EPA does not accept the commenter's 
premise that controlled human exposure studies do not demonstrate 
adverse effects in some asthmatics at 5-10 minute levels below 400 ppb.
    In addition to disagreeing with EPA's proposed finding of adverse 
health effects following 5- 10 minute SO2 exposures as low 
as 200 ppb, many industry groups (e.g., API, UARG, ACC, ExxonMobil) 
also disagreed with EPA that severe asthmatics were not included in 
controlled human exposure studies. That is, these groups contend that 
EPA is incorrect in assuming that severe asthmatics would likely have a 
more pronounced response to SO2 exposures at a given level, 
or would respond to even lower levels of SO2 and that this 
should be taken into account when judging the adequacy of the current 
standards. As support for their assertion, multiple industry groups 
cite controlled human exposure studies in the ISA stating that they 
included ``severe asthmatics'' and also cite a study by Linn et al. 
(1987) which concluded that among asthmatics, responses to 
SO2 exposure are not dependent on the clinical severity of 
asthma and that ``the subjects with the highest risk [of temporary 
respiratory disturbances from ambient SO2] can be identified 
only by actually measuring their responses to SO2''.
    We disagree with the assertion that severe asthmatics have been 
evaluated in 5-10 minute controlled human exposure studies. Although 
studies cited in the ISA referred to a group of subjects as ``moderate/
severe'' asthmatics, these individuals had well-controlled asthma, were 
able to withhold medication, were not dependent on corticosteroids, and 
were able to engage in moderate to heavy levels of exercise. By today's 
standards, these individuals would clearly be classified as moderate 
asthmatics. EPA therefore concludes that persons with asthma that is 
more severe than moderate asthma, as that term is currently understood, 
were not included in the controlled human exposure studies (and 
understandably so, for ethical reasons).
    In addition, EPA agrees with the commenters that there is little 
evidence from controlled human exposure studies to suggest that the 
respiratory effects of SO2 differ between mild and moderate 
asthmatics (see Linn et al., 1987). However, this may very well be due, 
at least in part, to persistence of medication among the moderate 
asthmatic subjects. More importantly, the moderate asthmatics began the 
exposure with compromised lung function relative to the mild 
asthmatics. Therefore, similar functional declines from different 
baselines between mild and moderate asthmatics would clearly not have 
the same physiological importance. CASAC specifically addressed the 
issue of asthma severity in a letter to the Administrator: ``For 
ethical reasons severe asthmatics were not part of these clinical 
studies, but it is not unreasonable to presume that they would have 
responded to even a greater degree (Henderson 2008a; p. v).'' It is 
also important to note that in addition to the strict health-specific 
inclusion and exclusion criteria for a given controlled human exposure 
study, many asthmatics who might otherwise be able to participate 
choose not to participate because of anxiety related to what they 
viewed as potential adverse health risks. EPA concludes that it is 
appropriate to assume, as CASAC suggested, that persons with more 
severe asthma would respond to an even greater degree than the moderate 
asthmatics in the clinical studies.
c. Comments on EPA's Characterization of SO2-Associated 
Exposures and Health Risks
    Several commenters discussed the analyses of SO2-
associated exposures and health risks presented in the REA. As in past 
reviews (EPA 2005, 2007c, 2007d), EPA has estimated risks associated 
with the current standards to inform judgments on the public health 
risks that could exist under different standard options. Some industry 
commenters (e.g., API, UARG, Lignite Energy Council (LEC), Jackson 
Walker, ASARCO, the National Rural Electric Cooperative Association) 
concluded that when considering the adequacy of the current standards, 
the Administrator should consider exposures and risks associated with 
actual SO2 air quality rather than air quality allowed by 
the current NAAQS. They consequently challenged the relevance and 
appropriateness of EPA's use of SO2 concentrations that have 
been simulated to just meet the current standards in assessing the 
adequacy of the current standards.
    In addition to the objections noted above, we note that UARG 
generally concluded that the results of EPA's quantitative risk 
assessment are fundamentally flawed in that they substantially 
overestimate risks associated with the various air quality scenarios. 
UARG contends that this is because EPA did not use proper exposure-
response functions in estimating risks associated with SO2 
exposure. Moreover, UARG contends EPA further overestimates risk 
because of the use of 50 ppb exposure bins in estimating the number of 
occurrences of an adverse lung function response (see below).
    With respect to comments that when considering the adequacy of the 
current standards, the Administrator should consider exposures and 
risks associated with actual SO2 air quality rather than 
that simulated to just meet the current standards, these commenters 
generally concluded: (1) It is more relevant to assess exposures and 
risks associated with actual SO2 air quality since adjusting 
air quality to just meet the current standards require large 
adjustments to air quality that are highly uncertain; and (2) NAAQS are 
intended to address actual, rather than highly improbable, risks to 
human health. In addition, these groups generally concluded that 
exposure and risk estimates presented in the REA suggest relatively 
little health risk associated with current levels of SO2, 
and thus, there is no need to revise the current SO2 
standards.
    We disagree with these commenters that exposure- and risk-related 
considerations in the NAAQS reviews should rely only on actual air 
quality, and that EPA therefore improperly adjusted air quality in its 
risk and exposure analyses to simulate air quality allowed by the 
current primary SO2 NAAQS. EPA is required to review whether 
the present standards--not present air quality--are requisite to 
protect public health with an adequate margin of safety. Section 
109(b)(1). In making this determination it is relevant to consider 
exposures and risks which could be permissible under the current 
standards. See American Trucking Associations v. EPA, 283 F.3d 355, 370 
(DC Cir. 2002) (existence of evidence showing adverse effects occurring 
at levels allowed by the current standards justifies finding that it is 
appropriate to revise the existing NAAQS). Consequently, it is at the 
very least reasonable for EPA, in its REA, to make air quality 
adjustments to estimate SO2-related exposures and health 
risks that could exist in areas that just meet the present standards. 
Thus, although we acknowledge that exposure and health risk estimates 
associated with current ambient concentrations are substantially 
smaller than those associated with air quality adjusted to just meet 
the current standards, we also note that this is

[[Page 35534]]

irrelevant to the question of whether the current standards are 
requisite to protect public health with an margin of safety.
    In both of these cases, EPA is not trying to evaluate whether areas 
would or would not be in attainment of the current standards. Those are 
issues that are addressed during the implementation of the NAAQS. 
Instead, in this rulemaking EPA is evaluating what NAAQS would be 
appropriate under section 109(b)(1), by evaluating the impact on or 
risks to public health from air quality that is at the level of the 
current standards, as well as evaluating air quality that is at the 
level of various alternative standards. EPA uses this information to 
inform the decision on what NAAQS would be requisite to protect public 
health with an adequate margin of safety.
    If EPA determines that the current standards require revision, EPA 
is further required to determine what revisions are appropriate in 
light of the requirement that primary NAAQS be requisite to protect 
public health with an adequate margin of safety. Section 109(d)(1). It 
is thus similarly reasonable for EPA to make air quality adjustments to 
simulate different potential alternative standards to provide 
information on exposures and risks under these potential alternative 
standards.\12\
---------------------------------------------------------------------------

    \12\ In conducting these analyses, EPA is not trying to evaluate 
whether areas would or would not be in attainment of the current 
standards. Again, those issues are addressed during the 
implementation of the NAAQS.
---------------------------------------------------------------------------

    We agree that there are uncertainties inherent in making air 
quality adjustments. These uncertainties are discussed thoroughly in 
the REA (REA, sections 6.5 and 7.4.2.5). For example, the REA noted the 
following regarding adjustment of SO2 concentrations:

    This procedure for adjusting either the ambient concentrations 
(i.e., in the air quality characterization) or health effect 
benchmark levels (i.e., in the exposure assessment) was necessary to 
provide insight into the degree of exposure and risk which would be 
associated with an increase in ambient SO2 levels such 
that the levels were just at the current standards in the areas 
analyzed. Staff recognizes that it is extremely unlikely that 
SO2 concentrations in any of the selected areas where 
concentrations have been adjusted would rise to meet the current 
NAAQS and that there is considerable uncertainty associated with the 
simulation of conditions that would just meet the current standards. 
Nevertheless, this procedure was necessary to assess the ability of 
the current standards, not current ambient SO2 
concentrations, to protect public health (REA, section 6.5; p. 64)

    These air quality adjustments are not meant to imply an expectation 
that SO2 concentrations will increase broadly across the 
United States or in any given area. Rather, as just noted above, they 
are meant to estimate SO2-related exposures and health risks 
if air quality were at the level of the current and potential 
alternative standards. Such estimates can inform decisions on whether 
the current standards, or particular potential alternative standards, 
provide the requisite protection of public health.
    As mentioned above, UARG generally concluded that under all air 
quality scenarios, the results of EPA's quantitative risk assessment 
(the third of the analyses conducted in the REA (chapter 9), see 
section II.C above) are substantially overestimated because EPA did not 
use proper methods to estimate the parameters of the exposure-response 
functions used in its analyses. UARG contends this is because many of 
the subjects in the controlled human exposure studies from which EPA's 
exposure-response functions were derived (see REA, Table 9-3) were 
exposed to more than one SO2 concentration, yet EPA treated 
each exposure event as being independent (e.g., if the same subject was 
exposed to 200 and 300 ppb SO2, EPA considered these as 
representing two independent exposure events). UARG contends that 
observations from the same subject exposed to different SO2 
concentrations are not independent observations and should not be 
treated as such. Notably, when UARG derived their own exposure-response 
functions taking into account that observations from the same subject 
exposed to different SO2 concentrations are not independent 
of each other, they estimated appreciably less risk than that estimated 
by EPA.
    There are a variety of techniques and/or assumptions that can be 
used to fit individual subject data from the controlled human exposure 
studies (see REA, Table 9-3) to exposure-response curves. Moreover, any 
technique or assumption utilized will have inherent uncertainties. EPA 
discussed the uncertainties associated with our quantitative risk 
assessment in detail in the REA (REA, section 9.4); we also gave an 
overview of key uncertainties in the proposal (see section II.C.3, 74 
FR at 64824). The approach used to estimate the exposure-response 
functions was not first introduced in the SO2 risk 
assessment, it was previously recommended to EPA by an applied 
statistician serving on the O3 CASAC Panel and used in the 
O3 risk assessment (which had individual controlled human 
exposure data similar to that in the current SO2 NAAQS 
review; see EPA 2007d and EPA 2007e). Importantly, this approach 
allowed EPA to use all the available individual subject data. Moreover, 
an inspection of the estimated exposure-response curve and the 
underlying data suggest that any biases in the parameter estimates are 
likely to be slight (see EPA 2010, section II.C). Consequently, EPA 
does not accept UARG's view that the methodology used in EPA's 
quantitative risk assessment was inappropriate.
    We further note that UARG's exposure-response functions do not fit 
the underlying controlled human exposure data (the proportions of 
subjects who responded at each exposure level) nearly as well as the 
exposure-response functions estimated using EPA's approach. We believe 
this could be due to the methodology used in UARG's reanalysis of the 
individual-level data from the controlled human exposure studies used 
in the quantitative risk assessment. UARG attempted to estimate 
subject-specific exposure-response functions, and to use the results of 
these estimates to obtain estimates of the two parameters in the 
population-level exposure-response functions. As described in more 
detail in section II.C of the RTC document (EPA 2010), EPA does not 
believe there are sufficient data to properly estimate the parameters 
of subject-specific exposure-response functions. More specifically, 
UARG chose a three-parameter quadratic function for the subject-
specific exposure-response functions. However, none of the subjects had 
more than three exposures, and many had only one or two. EPA believes 
that this information is particularly limited for estimating these 
subject-specific exposure-response functions, especially given that a 
large percentage of the total number of subjects had fewer exposures 
than the number of parameters UARG was attempting to estimate (i.e., 
UARG estimated three parameters in its exposure-response functions, but 
over fifty percent of subjects only had one or two exposures). It 
appears that UARG's population-level exposure-response function 
estimates depended on these subject-specific exposure-response function 
estimates and thus could explain why UARG's estimated population-level 
exposure-response functions do not fit the underlying controlled human 
exposure data nearly as well as the approach used by EPA. A more 
detailed response to this comment can be found in section II.C of the 
RTC document (EPA 2010).
    As mentioned above, UARG also concluded that EPA further 
overestimates the total number of occurrences of an adverse lung 
function response (i.e., total number of

[[Page 35535]]

occurrences of increases in sRaw >= 100 or 200% and/or declines in 
FEV1 >= 15 or 20%) in its quantitative risk assessment. More 
specifically, UARG concluded that the use of 50 ppb bins, combined with 
assigning all exposures within a bin the probability of an adverse lung 
function response at the midpoint of that bin (e.g., all exposures from 
0-50 ppb were assigned the probability of an adverse lung function 
response occurring at 25 ppb), resulted in a substantial overestimate 
of the total number of occurrences of lung function responses in 
asthmatics at moderate or greater exertion. UARG generally concludes 
that this is because the vast majority of exposures of asthmatics at 
moderate or greater exertion are occurring below the midpoint of the 0-
50 ppb exposure bin (i.e., most exposures are occurring below 25 ppb), 
yet EPA is assigning these very low SO2 exposures the higher 
probability of a lung function response associated with the midpoint of 
the 0-50 ppb exposure bin. UARG contends that this results in a 
substantial overestimation of the total number of occurrences of lung 
function response in asthmatics and asthmatic children at moderate or 
greater exertion. UARG further notes that this methodological concern 
was raised in its comments on the second draft REA, but EPA failed to 
address this issue and relied heavily on this metric in the proposal 
with respect to the adequacy of the current and potential alternative 
standards. EPA's response to this comment is discussed below and in 
more detail in section II.C of the RTC document (EPA 2010).
    EPA generally agrees with UARG's technical comments that there is a 
substantial overestimation of the total occurrences of lung function 
responses because of the binning issues described above. However, we 
strongly disagree that: (1) This issue was not acknowledged in the 
final REA; and (2) the metric of total occurrences was relied on 
heavily in the policy assessment chapter of the REA (REA, chapter 10) 
and in the Administrator's rationale with respect to the adequacy of 
the current and potential alternative standards. First, EPA did respond 
to this concern in the final REA. More specifically, page 344 of the 
final REA states:

    As noted in public comments on the 2nd draft SO2 REA, 
the assignment of response probability to the midpoint of the 
exposure bin combined with the lack of more finely divided intervals 
in this range can lead to significant overestimation of risks based 
on total occurrences of a defined lung function response. This is 
because the distribution of population exposures for occurrences is 
not evenly distributed across the bin, but rather is more heavily 
weighted toward the lower range of the bin. Thus, combining all 
exposures estimated to occur in the lowest bin with a response 
probability assigned to the midpoint of the bin results in a 
significant overestimate of the risk. Therefore, staff places less 
weight on the estimated number of occurrences of lung function 
responses.

    Thus, as noted in the final REA, less weight was placed on this 
metric in the quantitative risk assessment chapter (REA, chapter 9), 
and importantly, no weight was placed on this metric in either the 
policy assessment chapter of the REA (REA, chapter 10) or in the 
Administrator's rationale sections of the proposal preamble. Rather, 
the policy assessment chapter of the REA and the Administrator's 
rationale at the proposal considered the percent of exposed asthmatic 
children at moderate or greater exertion estimated to have at least one 
defined lung function response per year in St. Louis. Importantly, this 
metric is not appreciably affected by the binning issue raised in 
UARG's comments. As stated on page 344-345 of the final REA:

    This overestimation of total occurrences does not impact the 
risk metric expressed as incidence or percent incidence of a defined 
lung function response 1 or more times per year because the bulk of 
the exposures contributing to these risk metrics are not skewed 
toward the lower range of the reported exposure bins.\13\
---------------------------------------------------------------------------

    \13\ Although in St. Louis, the percent of exposed asthmatic 
children at moderate or greater exertion estimated to have at least 
one defined lung function response per year was not appreciably 
affected, it was found that for this same metric, the already very 
low risk estimates in Greene County became appreciably lower when 
the binning issue discussed above was considered. However, as noted 
above in section II.C and discussed in more detail in the REA (REA, 
section 10.3.3) and the proposal (see section II.E.b, 74 FR at 
64827), the St. Louis exposure and risk results were found to be 
more informative in addressing the adequacy of the current and 
potential alternative standards. Moreover, while the Administrator's 
rationale in the proposal relied minimally on the St. Louis 
quantitative risk results (see above), she importantly placed no 
weight on any metric from the Greene County quantitative risk 
assessment.

    Finally, it is important to note that the Administrator's rationale 
in the proposal regarding the adequacy of the current and potential 
alternative standards in general placed only limited reliance on the 
results of the quantitative risk assessment in St. Louis, with no 
reliance on the estimates of total occurrences. Rather, in addition to 
the substantial weight that she placed on the scientific evidence as 
described in the ISA, the Administrator placed relatively more weight 
on the results of the St. Louis exposure analysis. For example, in 
discussing the adequacy of the current standards, the proposal states: 
``The Administrator especially notes the results of the St. Louis 
exposure analysis which, as summarized above, indicates that 
substantial percentages of asthmatic children at moderate or greater 
exertion would be exposed, at least once annually, to air quality 
exceeding the 400 and 200 ppb benchmarks'' (see 74 FR at 64829). We 
note that results of the quantitative risk assessment in St. Louis, 
with respect to the percent of asthmatic children estimated to have at 
least one lung function response per year (using EPA's exposure-
response functions), supports the Administrator's overall conclusions 
in the proposal regarding the adequacy of the current and potential 
alternative standards.
3. Conclusions Regarding the Adequacy of the Current 24-Hour and Annual 
Standards
    In reviewing the adequacy of the current standards, the 
Administrator has considered the scientific evidence assessed in the 
ISA, the exposure and risk results presented in the REA, the 
conclusions of the policy assessment chapter of the REA, and comments 
from CASAC and the public. These considerations are described below.
    As in the proposal, the Administrator accepts and agrees with the 
ISA's conclusion that the results of controlled human exposure and 
epidemiologic studies form a plausible and coherent data set that 
supports a causal relationship between short-term (5 minutes to 24 
hours) SO2 exposures and adverse respiratory effects. The 
Administrator acknowledges that there are uncertainties associated with 
the epidemiologic evidence (e.g., potential confounding by co-
pollutants). However, she agrees that the epidemiologic evidence, 
supported by the controlled human exposure evidence, generally 
indicates that the effects seen in these studies are attributable to 
exposure to SO2, rather than co-pollutants, most notably 
PM2.5. She also accepts and agrees with the conclusion of 
the ISA that ``[i]n the epidemiologic studies, respiratory effects were 
observed in areas where the maximum ambient 24-h avg SO2 
concentration was below the current 24-h avg NAAQS level. * * *'' (ISA, 
section 5.2, p. 5-2) and so would occur at ambient SO2 
concentrations that are present in locations meeting the current 24-
hour NAAQS. The Administrator also notes that these effects occurred in 
areas with annual air quality levels considerably lower than those 
allowed by the current annual standard, indicating that the annual 
standard also

[[Page 35536]]

is not providing protection against such effects. Existence of 
epidemiologic studies showing adverse effects occurring at levels 
allowed by the current standards is an accepted justification for 
finding that it is appropriate to revise the existing standards. See, 
e.g. American Trucking Associations v. EPA, 283 F. 3d at 370; see also 
American Farm Bureau v. EPA, 559 F. 3d.512, 521-23 (DC Cir. 2009) 
(effects associated with short-term exposure seen in areas with ambient 
concentrations lower than long-term standard, so that without further 
explanation, standard does not adequately protect against short-term 
exposures).
    With respect to the controlled human exposure studies, the 
Administrator judges that effects following 5-10 minute SO2 
exposures >= 400 ppb and >= 200 ppb can result in adverse health 
effects to asthmatics. This judgment is based on ATS guidelines, 
explicit CASAC consensus written advice and recommendations, and 
judgments made by EPA in previous NAAQS reviews. Thus, similar to the 
proposal, she notes analyses in the REA supporting that 5-minute 
exposures >= 400 ppb and >= 200 ppb were associated with air quality 
adjusted upward to simulate just meeting the current standards. The 
Administrator especially notes the results of the St. Louis exposure 
analysis which, as summarized in the proposal (see section II.E.1.b and 
Table 3, see 74 FR at 64841), indicates that substantial percentages of 
asthmatic children at moderate or greater exertion would be exposed, at 
least once annually, to air quality exceeding the 400 and 200 ppb 5-
minute benchmarks given air quality simulated to just meet the current 
standards. The Administrator judged these 5-minute exposures to be 
significant from a public health perspective due to their estimated 
frequency: Approximately 24% of child asthmatics at moderate or greater 
exertion in St. Louis are estimated to be exposed at least once per 
year to air quality exceeding the 5-minute 400 ppb benchmark, a level 
associated with lung function decrements in the presence of respiratory 
symptoms. Additionally, approximately 73% of child asthmatics in St. 
Louis at moderate or greater exertion would be expected to be exposed 
at least once per year to air quality exceeding the 5-minute 200 ppb 
benchmark. This health evidence and risk-based information underlie 
CASAC's conclusion that the current SO2 standards do not 
adequately protect public health. As discussed in the proposal, CASAC 
stated: ``the current 24-hour and annual standards are not adequate to 
protect public health, especially in relation to short-term exposures 
to SO2 (5-10 minutes) by exercising asthmatics'' (Samet, 
2009, p. 15). The Administrator agrees with this conclusion.
    In considering approaches to revising the current standards, the 
Administrator concludes that it is appropriate to set a new standard, 
that such standard must provide requisite protection with an adequate 
margin of safety to a susceptible population (i.e., asthmatics at 
elevated ventilation), and that the standard must afford protection 
from short-term exposures to SO2 in order to prevent the 
adverse health effects reported in both the controlled human exposure 
and epidemiologic studies. The Administrator notes that a 1-hour 
standard could provide increased public health protection, especially 
for members of at-risk groups, from health effects described in both 
controlled human exposure and epidemiologic studies, and hence, health 
effects associated with 5-minute to 24-hour exposures to 
SO2.\14\ As discussed in section II.F.5 below, given the 
degree of protection afforded by such a standard, it may be appropriate 
to replace, and not retain, the current 24-hour and annual standards in 
conjunction with setting a new short-term standard.
---------------------------------------------------------------------------

    \14\ We also note that such a standard would, among other 
things, address the deficiency in the current NAAQS which occasioned 
the remand of that standard for failing to adequately explain the 
absence of protection from short-term SO2 bursts which 
could cause adverse health effects in hundreds of thousands of 
heavily breathing asthmatics. American Lung Ass'n v. EPA, 134 F. 3d 
at 392-93.
---------------------------------------------------------------------------

F. Conclusions on the Elements of a New Short-Term Standard

    In considering a revised SO2 primary NAAQS, the 
Administrator notes the need to protect at-risk populations from: (1) 
1-hour daily maximum and 24-hour average exposures to SO2 
that could cause the types of respiratory morbidity effects reported in 
epidemiologic studies; and (2) 5-10 minute SO2 exposure 
concentrations reported in controlled human exposure studies to result 
in moderate or greater decrements in lung function and/or respiratory 
symptoms. Considerations with regard to potential alternative standards 
and the specific conclusions of the Administrator are discussed in the 
following sections in terms of indicator, averaging time, form, and 
level (sections II.F.1 to II.F.4 below).
1. Indicator
a. Rationale for Proposed Decision
    In the last review, EPA focused on SO2 as the most 
appropriate indicator for ambient SOX. In making a decision 
in the current review on the most appropriate indicator, the 
Administrator has considered the conclusions of the ISA and REA as well 
as the views expressed by CASAC and the public. The REA noted that, 
although the presence of gaseous SOX species other than 
SO2 has been recognized, no alternative to SO2 
has been advanced as being a more appropriate surrogate for ambient 
gaseous SOX. Controlled human exposure studies and animal 
toxicology studies provide specific evidence for health effects 
following exposure to SO2. Epidemiologic studies also 
typically report levels of SO2, as opposed to other gaseous 
SOX. Because emissions that lead to the formation of 
SO2 generally also lead to the formation of other 
SOX oxidation products, measures leading to reductions in 
population exposures to SO2 can generally be expected to 
lead to reductions in population exposures to other gaseous 
SOX. Therefore, as noted in the proposal, meeting an 
SO2 standard that protects the public health can also be 
expected to provide protection against potential health effects that 
may be independently associated with other gaseous SOX even 
though such effects are not discernable from currently available 
studies indexed by SO2 alone. See American Petroleum 
Institute v. EPA, 665 F, 2d 1176, 1186 (DC Cir. 1981) (reasonable for 
EPA to use ozone as the indicator for all photochemical oxidants even 
though health information on the other photochemical oxidants is 
unknown; regulating ozone alone is reasonable since it presents a 
``predictable danger'' and in doing so EPA did not abandon its 
responsibility to regulate other photochemical oxidants encompassed by 
the determination that photochemical oxidants as a class may be 
reasonably anticipated to endanger public health or welfare). Given 
these key points, the REA concluded that the available evidence 
supports the retention of SO2 as the indicator in the 
current review (REA, section 10.5.1). Consistent with this conclusion, 
CASAC stated in a letter to the EPA Administrator that: ``for 
indicator, SO2 is clearly the preferred choice'' (Samet 
2009, p. 14).
b. Comments on Indicator
    A small number of commenters directly addressed the issue of the 
indicator for the standard. These

[[Page 35537]]

commenters generally endorsed the proposal to continue to use 
SO2 as the indicator for ambient SOX.
c. Conclusions on Indicator
    Based on the available information discussed above, and consistent 
with the views of CASAC and other commenters, the Administrator 
concludes that it is appropriate to continue to use SO2 as 
the indicator for a standard that is intended to address effects 
associated with exposure to SO2, alone or in combination 
with other gaseous SOX. In so doing, the Administrator 
recognizes that measures leading to reductions in population exposures 
to SO2 will also reduce population exposures to other oxides 
of sulfur.
2. Averaging Time
    This section discusses considerations related to the averaging time 
of the SO2 primary NAAQS. Specifically, this section 
summarizes the rationale for the Administrator's proposed decision 
regarding averaging time (II.F.2.a below; see section II.F.2 of the 
proposal for more detail at 74 FR 64832-64833), discusses public 
comments and EPA responses related to averaging time (II.F.2.b), and 
presents the Administrator's final conclusions regarding averaging time 
(II.F.2.c). Notably, public comments and the Administrator's 
conclusions on whether to retain or revoke the current 24-hour and/or 
annual standards given a new 1-hour standard are discussed in section 
II.F.5.
a. Rationale for Proposed Decision
    In considering the most appropriate averaging time for the 
SO2 primary NAAQS, the Administrator noted in the proposal 
the conclusions and judgments made in the ISA about the available 
scientific evidence, air quality correlations discussed in the REA, 
conclusions of the policy assessment chapter of the REA, and CASAC 
recommendations (section II.F.2 in the proposal). Specifically, she 
noted the following:
     The REA conclusion that an appropriate averaging time 
should focus protection on SO2 exposures from 5-minutes to 
24-hours (REA, section, 10.5.2).
     Air quality, exposure, and risk analyses from the REA 
indicating it is likely a 1-hour standard--with the appropriate form 
and level--can substantially reduce 5-10 minute peaks of SO2 
shown in controlled human exposure studies to result in respiratory 
symptoms and/or decrements in lung function in exercising asthmatics 
(i.e. 5-minute SO2 concentrations >= 200 and 400 ppb).
     Air quality analyses indicating that a 1-hour standard--
with the appropriate form and level--can substantially reduce the upper 
end of the distribution of SO2 levels more likely to be 
associated with adverse respiratory effects (see section II.F.3 below); 
that is: (1) 99th percentile 1-hour daily maximum air quality 
concentrations in U.S. cities where positive effect estimates in 
epidemiologic studies of hospital admissions and emergency department 
visits for all respiratory causes and asthma were observed; and (2) 
99th percentile 24-hour average air quality concentrations found in 
U.S. cities where emergency department visit and hospitalization 
studies (for all respiratory causes and asthma) reported statistically 
significant associations in multi-pollutant models with PM.
     The REA conclusion that a 5-minute averaging time is 
undesirable because it would result in significant and unnecessary 
instability due to the likelihood that locations would frequently shift 
in and out of attainment--thereby reducing public health protection by 
disrupting an area's ongoing implementation plans and associated 
control programs.
     CASAC statement addressing whether a 1-hour averaging time 
can adequately control 5-10 minute peak exposures and whether there 
should be a 5-minute averaging time. CASAC stated that the REA's 
rationale for a one-hour standard was ``convincing'' (Samet 2009, p. 
16), and that ``a one-hour standard is the preferred averaging time'' 
(Samet 2009, p. 15).
     CASAC's statement that they were ``in agreement with 
having a short-term standard and finds that the REA supports a 1-hour 
standard as protective of public health'' (Samet 2009, p. 1).
b. Comments on Averaging Time
    A large number of public commenters also endorsed the establishment 
of a new standard with a 1-hour averaging time (although some groups' 
support hinged on the accompanying level). These included a number of 
State organizations (e.g., NACAA, NESCAUM); State environmental 
agencies (e.g., such agencies in IA, IL, NY, MI, NM, OH, PA, TX, VT); 
public health and environmental organizations (e.g., ALA, ATS, New York 
Department of Health (NYDOH), Sierra Club, EDF); the Fond du Lac Tribe; 
local groups (e.g., Houston-Galveston Area Council, New York City); and 
almost all of the individual commenters (13,000). The supporting 
rationales offered by these commenters often acknowledged the 
recommendations of CASAC and the Administrator's rationale as discussed 
in the proposal.
    Though many industry commenters did not support the proposed 
revisions to the SO2 primary NAAQS (as discussed above in 
section II.E.2), a few of these groups did express that if a short-term 
standard were to be set, a 1-hour averaging time could be appropriate, 
depending on the level and form selected (e.g., ExxonMobil, Kean 
Miller). Other industry commenters (e.g., ASARCO, RIO Tinto Alcan, 
Association of Battery Recyclers (ABR)) and the South Dakota Department 
of Environment and Natural Resources (SD DENR) expressed that EPA 
should have considered longer averaging times (e.g., 3 hours). In 
addition, although health and environmental groups were supportive of 
setting a new 1-hour standard to protect against short-term exposures 
to SO2 (again, depending on the level of the 1-hour standard 
selected), these groups also commented that a 5-minute standard to 
protect susceptible populations from health effects associated with 5-
minute peaks of SO2 would be optimal (e.g., ALA, ATS, Sierra 
Club, EDF). These comments, and EPA's responses, are discussed in more 
detail below.
    As discussed above, industry commenters who disagreed with setting 
a new 1-hour standard generally based this conclusion on their 
interpretation of the scientific evidence and their conclusion that 
this evidence does not support the proposed revisions to the current 
SO2 NAAQS. EPA's responses to these commenters were 
presented above in section II.E.2.a and II.E.2.b.
    Also noted above, some industry commenters (e.g., ASARCO, RIO Tinto 
Alcan, ABR) and the SD DENR expressed that EPA should have considered 
longer averaging times (e.g., 3-hour, 8-hour, 24-hour). In general, 
these groups concluded that a standard with a longer averaging time 
could potentially provide the same public health protection as a 1-hour 
standard, while also providing a more stable regulatory target. For 
example, in its comments, the SD DENR states: ``DENR recommends EPA 
evaluate a 3-hour or 8-hour standard to determine if these averaging 
periods are also protective of the public health. If they are, EPA 
should propose a 3-hour or 8-hour sulfur dioxide standard instead of a 
1-hour standard. A longer averaging period would smooth out the 
variability of the upper range measurements and provide a more stable 
standard.'' Similarly, Rio Tinto Alcan stated in its comments: ``the 
short-term averaging period defined by EPA (i.e., 5 minutes

[[Page 35538]]

to 24 hours) is not limited to only 5-minute, 1-hour and 24-hour 
averaging periods. EPA could explain in more detail why these three 
averaging periods were examined when considering appropriate averaging 
periods to limit short-term peaks of SO2 * * * a longer term 
average could provide additional stability to the standard while at the 
same time effectively protecting public health.''
    Although we agree that alternative averaging times could 
potentially provide similar public health protection (assuming an 
appropriate form and level), we believe that a 1-hour averaging time is 
reasonably justified by the scientific evidence presented in the ISA 
and by the air quality information presented in the REA. As described 
in detail in the proposal (see section II.F.2), the controlled human 
exposure evidence presented in the ISA provided support for an 
averaging time that protects against 5-10 minute peak SO2 
exposures (REA, section 10.5.2, pp. 371-372), and results from 
epidemiologic studies most directly provided support for both 1-hour 
and 24-hour averaging times (REA, section 10.5.2, p. 372). Thus, we 
found it most reasonable to consider these averaging times for a 
revised SO2 NAAQS given that there is very little basis in 
the health evidence presented in the ISA to consider other averaging 
times (e.g., 3-hour or 8-hour). In so doing, we first noted the 
likelihood that averaging times of 1 and 24 hours could provide 
protection against 5-minute peak SO2 exposures. As described 
in detail in the proposal (see section II.F.2, 74 FR at 64830-64833), 
it was initially concluded that a 1-hour averaging time, rather than a 
24-hour averaging time, would be more appropriate for limiting 5-minute 
peaks of SO2. Similarly, we concluded that a 1-hour 
standard, given the appropriate form and level, could likely limit 99th 
percentile 24-hour average air quality concentrations found in U.S. 
locations where emergency department visit and hospitalization studies 
(for all respiratory causes and asthma) observed statistically 
significant associations in multi-pollutant models with PM (i.e., 99th 
percentile 24-hour average SO2 concentration >= 36 ppb). 
Taken together, we reasonably concluded that a 1-hour standard, with an 
appropriate form and level, can provide adequate protection against the 
range of health outcomes associated with averaging times from 5 minutes 
to 24 hours (proposal section II.F.2 and REA, section 10.5.2.3). We 
also note that our conclusion is in agreement with CASAC comments on 
the second draft REA. CASAC stated that they were ``in agreement with 
having a short-term standard and finds that the REA supports a one-hour 
standard as protective of public health'' (Samet 2009, p. 1). In 
addition, as discussed in more detail below in section II.F.3, we found 
that a 1-hour standard in combination with the selected form, will 
provide a stable regulatory target.
    As noted above, although health and environmental groups were 
supportive of setting a new 1-hour standard to protect against short-
term exposures to SO2 (again, depending on the level of the 
1-hour standard selected), these groups generally commented that a 5-
minute standard to protect against health effects associated with 5-
minute peaks would be optimal (e.g., ALA, Sierra Club, EDF). For 
example, in their combined comments ALA, EDF, NRDC, and Sierra Club 
(ALA et al.,) stated: ``We need a short-term SO2 standard, 
optimally a 5-minute standard, to protect against bursts of pollution 
that can result from start-up, shutdown, upset, malfunction, downwash, 
complex terrain, atmospheric inversion conditions, and other 
situations'' and that ``EPA has over emphasized a concern about the 
stability of a 5-minute standard * * * The record does not show that 
any alleged instability of a 5-minute standard has any relevance to 
whether such a standard is requisite to protect public health.''
    We agree that there needs to be a short-term standard to protect 
against 5-minute peaks of SO2. However, we do not believe 
setting a 5-minute standard to be the best way of accomplishing that 
objective. As in past NAAQS reviews, EPA properly considered the 
stability of the design of pollution control programs in its review of 
the elements of a NAAQS, since more stable programs are more effective, 
and hence result in enhanced public safety. American Trucking 
Associations v. EPA, 283 F. 3d at 375 (choice of 98th percentile form 
for 24-hour PM NAAQS, which allows a number of high exposure days per 
year to escape regulation under the NAAQS, justifiable as ``promot[ing] 
development of more `effective [pollution] control programs' '', since 
such programs would otherwise be ``less `stable'--and hence * * * less 
effective--than programs designed to address longer-term average 
conditions'', and there are other means (viz. emergency episode plans) 
to control those high exposure days). In this review, there were 
legitimate concerns about the stability of a standard using a 5-minute 
averaging time. Specifically, there was concern that compared to longer 
averaging times (e.g., 1-hour, 24-hour), year-to-year variation in 5-
minute SO2 concentrations were likely to be substantially 
more temporally and spatially diverse. Thus, it is more likely that 
locations would frequently shift in and out of attainment thereby 
reducing public health protection by disrupting an area's ongoing 
implementation plans and associated control programs. Consequently, the 
REA concluded that a 5-minute averaging time would not provide a stable 
regulatory target and therefore would not be the preferred approach to 
provide adequate public health protection. A 1-hour averaging time does 
not have these drawbacks. As noted in the REA and the proposal (see 
proposal sections II.F.2.a and II.F.2.c), air quality, exposure, and 
risk analyses support that a 1-hour averaging time, given an 
appropriate form and level can adequately limit 5-minute SO2 
exposures and provide a more stable regulatory target than setting a 5-
minute standard. More specifically, based on the air quality and 
exposure analyses presented in chapters 7 and 8 of the REA, there is 
also a strong likelihood that a 99th percentile 1-hour daily maximum 
standard will limit 5-10 minute peaks of SO2 shown in 
controlled human exposure studies to result in decrements in lung 
function and/or respiratory symptoms in exercising asthmatics (see 
especially REA Tables 7-11 to 7-14 and Figure 8-19).
    We also note that a 1-hour standard to protect against 5-minute 
exposures is in agreement with CASAC advice and recommendations. That 
is, CASAC stated that they were ``in agreement with having a short-term 
standard and finds that the REA supports a 1-hour standard as 
protective of public health'' (Samet 2009, p. 1). Similarly, in a CASAC 
statement addressing whether a 1-hour averaging time can adequately 
control 5-10 minute peak exposures and whether there should be a 5-
minute averaging time, CASAC stated that the REA had presented a 
``convincing rationale'' (Samet 2009, p. 16) for a 1-hour standard, and 
that ``a one-hour standard is the preferred averaging time'' (Samet 
2009, p. 15).
c. Conclusions on Averaging Time
    In considering the most appropriate averaging time(s) for the 
SO2 primary NAAQS, the Administrator notes the conclusions 
and judgments made in the ISA about the available scientific evidence, 
air quality considerations from the REA, CASAC advice and 
recommendations, and public comments received. Based on these 
considerations, the Administrator concludes that a new standard based 
on

[[Page 35539]]

1-hour daily maximum SO2 concentrations will provide 
increased protection against effects associated with short-term (5 
minutes to 24 hours) exposures. The rationale for this decision is 
described below.
    Similar to the proposal (see section II.F.2.c), the Administrator 
first agrees with the REA's conclusion that the standard should focus 
protection on short-term SO2 exposures from 5 minutes to 24 
hours. As noted above, CASAC's strong recommendation supports this 
approach as well.\15\ The Administrator further agrees that the 
standard must provide requisite protection from 5-10 minute exposure 
events, but believes that this can be provided without having a 
standard with a 5-minute averaging time. The Administrator agrees with 
the REA conclusion that it is likely a 1-hour standard--with the 
appropriate form and level--can substantially reduce 5-10 minute peaks 
of SO2 shown in controlled human exposure studies to result 
in respiratory symptoms and/or decrements in lung function in 
exercising asthmatics. The Administrator further believes that a 5-
minute averaging time would result in significant and unnecessary 
instability and is undesirable for that reason. The Administrator also 
notes the statements from CASAC mentioned above addressing whether a 1-
hour averaging time can adequately control 5-10 minute peak exposures 
and whether there should be a 5-minute averaging time. As noted above, 
addressing this question, CASAC stated that the REA had presented a 
``convincing rationale'' (Samet 2009, p. 16) for a 1-hour standard, and 
that ``a one-hour standard is the preferred averaging time'' (Samet 
2009, p. 15).
---------------------------------------------------------------------------

    \15\ As noted above, such a standard also satisfactorily 
addresses the issue raised by the reviewing court in the litigation 
that followed the last review of the SO2 NAAQS: Why was 
no protection afforded in the standard for a susceptible 
subpopulation known to experience repeated adverse effects from 
exposure to 5-10 minute SO2 bursts. American Lung Ass'n, 
134 F. 3d at 392-93.
---------------------------------------------------------------------------

    Second, as in the proposal the Administrator agrees that a 1-hour 
averaging time (again, with the appropriate form and level) would 
provide protection against the range of health outcomes associated with 
averaging times of 1 hour to 24 hours. Specifically, the Administrator 
finds that a 1-hour standard can substantially reduce the upper end of 
the distribution of SO2 levels more likely to be associated 
with adverse respiratory effects (see discussion on Form, section 
II.F.3); that is: (1) 99th percentile 1-hour daily maximum 
SO2 air quality concentrations in U.S. locations where 
positive SO2 effect estimates were reported in epidemiologic 
studies of emergency department visits and hospital admissions for all 
respiratory causes and asthma; and (2) 99th percentile 24-hour average 
SO2 air quality concentrations found in U.S. locations where 
emergency department visit and hospital admission studies using multi-
pollutant models with PM reported statistically significant 
associations (for all respiratory causes or asthma) with ambient 
SO2 (see REA, section 10.5.2.2 and proposal section II.F.2, 
74 FR at 64831). Finally, the Administrator again notes that 
establishing a new 1-hour averaging time is in agreement with CASAC 
recommendations. As noted above, CASAC stated that they were ``in 
agreement with having a short-term standard and finds that the REA 
supports a one-hour standard as protective of public health'' (Samet 
2009, p. 1). Moreover, CASAC agreed with the REA that a ``one-hour 
standard is the preferred averaging time'' (Samet 2009, p.15).
3. Form
    This section discusses considerations related to the form of the 1-
hour SO2 primary NAAQS. Specifically, this section 
summarizes the rationale for the Administrator's proposed decision 
regarding form (II.F.3.a; see proposal section II.F.3, 74 FR at 64833-
64834 of the proposal for more detail), discusses comments related to 
form (II.F.3.b), and presents the Administrator's final conclusions 
regarding form (II.F.3.c).
a. Rationale for Proposed Decision
    In considering the most appropriate form for the SO2 
primary NAAQS, the Administrator noted in the proposal the conclusions 
and judgments made in the ISA about available scientific evidence, air 
quality information discussed in the REA, conclusions of the policy 
assessment chapter of the REA, and CASAC recommendations (see section 
II.F.3, 74 FR at 64833-64834 in the proposal). Specifically, the 
proposal referenced the following:
     Information in the ISA that suggested that adverse 
respiratory effects are more likely to occur at the upper end of the 
distribution of ambient SO2 concentrations. That is, the ISA 
describes a few studies that reported an increase in SO2-
related respiratory health effects at the upper end of the distribution 
of SO2 concentrations (ISA, section 5.3, p. 5-9).
     The REA conclusion that a concentration-based form 
averaged over three years would better reflect the continuum of health 
risks posed by increasing SO2 concentrations (i.e. the 
percentage of asthmatics affected and the severity of the response 
increases with increasing SO2 concentrations; REA, section 
10.5.3) by giving proportionally greater weight to years when 1-hour 
daily maximum SO2 concentrations are well above the level of 
the standard, than just above the level of the standard.
     Analyses in the REA that suggested for a given 
SO2 standard level, a 99th percentile form is appreciably 
more effective at limiting 5-minute peak SO2 concentrations 
than a 98th percentile form (REA, section 10.5.3 and REA, Figures 7-27 
and 7-28).
     Analyses in the REA indicating that over the last 10 years 
and for the vast majority of the sites examined, there appears to be 
little difference in 98th and 99th percentile design value stability 
(REA, section 10.5.3).
     The REA conclusion that taken together, the evidence and 
air quality information indicate that consideration should be given 
primarily to a 1-hour daily maximum standard with a 99th percentile or 
4th highest daily maximum form (REA, section 10.5.3.3).
     CASAC indications that: ``there is adequate information to 
justify the use of a concentration-based form averaged over 3 years'' 
(Samet 2009, p. 16).
     CASAC recommendations that when evaluating 98th vs. 99th 
percentile forms, EPA should consider the number of days per year 98th 
vs. 99th percentile forms would allow SO2 concentrations to 
exceed the selected standard level. Similarly, CASAC recommendations to 
consider the number of exceedences of 5-minute benchmarks given 98th 
vs. 99th percentile forms at a given standard level (Samet 2009).
b. Comments on Form
    Most all State organizations and agencies (e.g., NAACA, NESCAUM and 
agencies in FL, NM, PA, SC, TX, VT) supported a 99th percentile or 4th 
highest form. Similarly, public health (e.g., ALA, ATS) and 
environmental organizations (e.g., CBD, WEACT for Environmental 
Justice) and the Alexandria Department of Transportation and 
Environmental Services preferred either a 99th percentile or a more 
stringent form (e.g., no exceedence) to further limit the occurrence of 
SO2 concentrations that exceed the standard level in 
locations that attain the standard. In contrast, many industry groups 
(e.g., UARG, NAM, LEC, RRI Energy, AirQuality Research & Logistics 
(AQRL)), and the SD DENR conditionally supported a

[[Page 35540]]

98th percentile form if EPA were to set a 1-hour standard.\16\ EPA 
responses to specific comments on the form of the standard can be found 
below and in the RTC document (EPA 2010).
---------------------------------------------------------------------------

    \16\ EPA did not propose or seek comment on a 98th percentile 
form or a more restrictive form (e.g., an exceedence based form). 
EPA also considered a 4th highest form, which is generally 
equivalent to the 99th percentile. However, a percentile based form 
is preferred since it results in a sampling from the same part of 
the annual distribution of 1-hour daily maximum SO2 
concentrations regardless of the number of 1-hour daily maximum 
concentrations reported in a given year for a particular location.
---------------------------------------------------------------------------

    As mentioned above, a number of industry groups and the SD DENR 
preferred a 98th percentile form. In general, their preference for a 
98th percentile form was based on their conclusion that a form based on 
the 98th percentile would be more stable than a form based on the 99th 
percentile, and that a 98th percentile form is consistent with the 
forms selected in recent NAAQS reviews (i.e. PM2.5 and 
NO2). For example AQRL stated: ``The Administrator should 
reconsider her proposal and choose instead the 98th percentile (or 
equivalent nth highest value) form of the standard for the added 
reliability and stability it offers in determining compliance or 
progress towards attainment. This approach has been promulgated for 
recent revisions of the PM2.5 and NO2 standards 
and this consistency should be maintained with SO2.''
    We agree with the commenters that it is important that a 1-hour 
standard have a form that is reasonably stable, but we disagree that a 
98th percentile form is significantly more stable than a 99th 
percentile form. We note that the REA discussed analyses (also briefly 
described in the proposal; see section II.F.3, 74 FR at 64834) 
comparing trends in 98th and 99th percentile design values from 54 
sites located in the 40 counties selected for the detailed air quality 
analysis (REA section 10.5.3 and Thompson, 2009). These results 
suggested that at the vast majority of sites, there would have been 
similar changes in 98th and 99th percentile design values over the last 
ten years (i.e. based on evaluating overlapping three year intervals 
over the last ten years; see REA, Figure 10-1 and Thompson, 2009). As 
part of this analysis, all of the design values over this ten year 
period for all 54 sites were aggregated and the standard deviation 
calculated (REA, Figure 10-2 and Thompson, 2009). Results demonstrated 
similar standard deviations--i.e. similar stability--based on 
aggregated 98th or aggregated 99th percentile design values over the 
ten year period (see REA, Figure 10-2 and Thompson 2009). Thus, we 
believe that in most locations, there will not be a substantial 
difference in stability between 98th and 99th percentile forms.
    We also disagree with the commenters that the forms of NAAQS 
standards should be consistent across different NAAQS pollutants. This 
is almost like advocating consistent levels or averaging times for 
different NAAQS pollutants. Each pollutant is manifestly different from 
another, and the decision as to an appropriate standard for each, and 
appropriate elements (including form) of each standard and the 
interaction of these elements, necessarily is fact-specific. Cf. Sierra 
Club v. EPA, 353 F. 3d 976, 986 (DC Cir. 2004) (``This court has 
adopted an `every tub on its own bottom' approach to EPA's setting of 
standards pursuant to the CAA, under which the adequacy of the 
underlying justification offered by the agency is the pertinent 
factor--not what the agency did on a different record concerning a 
different industry'') (Roberts J.). There is thus no basis to say a 
priori that any element of one NAAQS should be consistent with another, 
although if all other things are equal, selecting stable forms for each 
NAAQS is a legitimate objective.
    A 99th percentile form, rather than a 98th percentile form, is also 
needed for the standard to provide requisite public health protection. 
In this review of the primary SO2 NAAQS, we considered 
information in the ISA suggesting that adverse respiratory effects are 
more likely to occur at the upper end of the distribution of ambient 
SO2 concentrations. That is, the ISA described a few studies 
that reported an increase in SO2-related respiratory health 
effects at the upper end of the distribution of SO2 
concentrations (i.e., above 90th percentile SO2 
concentrations; ISA, section 5.3, p. 5-9). Moreover, we considered the 
extent to which different percentile forms, given the same standard 
level, limit 5-minute concentrations of SO2 above benchmark 
levels. As noted above in section II.F.3.a, and in more detail in the 
proposal (see section II.F.3.a, 74 FR at 64834), air quality analyses 
presented in the REA suggested that at a given SO2 standard 
level, a 99th percentile form is appreciably more effective at limiting 
5-minute peak SO2 concentrations than a 98th percentile form 
(REA, section 10.5.3, and REA, Figures 7-27 and 7-28). Taken together 
with the analyses suggesting that 98th and 99th percentile forms have 
similar stabilities, we reasonably concluded that a 99th percentile 
form was most appropriate for a 1-hour SO2 standard.
    As mentioned above, a number of health and environmental groups 
supported a 99th percentile form, but expressed that they would prefer 
a more restrictive form, such as a no-exceedence based form. In 
addition, the Alexandria Department of Transportation and Environmental 
Services only recommended a no, or one exceedence based form. In 
general, these groups concluded that a more restrictive form would 
further limit the: (1) Number of days an area could exceed the standard 
level and still attain the standard; and (2) the occurrence of 5-minute 
peaks of SO2 above benchmark levels.
    It is important that the particular form selected for a 1-hour 
daily maximum standard reflect the nature of the health risks posed by 
increasing SO2 concentrations. The REA and proposal (see 
section II.F.3, 74 FR at 64833) noted that the form of the standard 
should reflect results from controlled human exposure studies 
demonstrating that the percentage of asthmatics affected, and the 
severity of the respiratory response (i.e. decrements in lung function, 
respiratory symptoms) increases as SO2 concentrations 
increase. Taking this into consideration, EPA staff concluded that a 
concentration-based form, averaged over three years, is more 
appropriate than an exceedance-based form (REA, section 10.5.3). This 
is because a concentration-based form averaged over three years gives 
proportionally greater weight to years when 1-hour daily maximum 
SO2 concentrations are well above the level of the standard, 
as it gives to years when 1-hour daily maximum SO2 
concentrations are just above the level of the standard. In contrast, 
an expected exceedance form gives the same weight to years when 1-hour 
daily maximum SO2 concentrations are just above the level of 
the standard as it gives to years when 1-hour daily maximum 
SO2 concentrations are well above the level of the standard. 
Therefore, we concluded that a concentration-based form, averaged over 
three years (which also increases the stability of the standard) better 
reflects the continuum of health risks posed by increasing 
SO2 concentrations (i.e. the percentage of asthmatics 
affected and the severity of the response increases with increasing 
SO2 concentrations; REA, section 10.5.3). Moreover, we note 
that analyses in the REA indicate that in most locations analyzed, a 
99th percentile form would correspond to the 4th highest daily maximum 
concentration in a year, and that the 99th percentile, combined with 
the standard level

[[Page 35541]]

selected, will substantially limit 5-minute peaks of SO2 
above the 200 ppb and higher benchmark levels (see below, section 
II.F.4). Finally, we note that a concentration based form is in 
agreement with CASAC advice that: ``there is adequate information to 
justify the use of a concentration-based form averaged over 3 years'' 
(Samet 2009, p. 16).
c. Conclusions on Form
    The Administrator agrees that the form of the standard should 
reflect the health evidence presented in the ISA indicating that the 
percentage of asthmatics affected and the severity of the response 
increases with increasing SO2 concentrations. The 
Administrator also agrees that it is reasonable to consider the 
standard's stability as part of consideration of the form of the 
standard. Thus, the Administrator agrees that the standard should use a 
concentration-based form averaged over three years in order to give due 
weight to years when 1-hour SO2 concentrations are well 
above the level of the standard, than to years when 1-hour 
SO2 concentrations are just above the level of the standard. 
She also notes that a concentration-based form averaged over 3 years 
would likely be appreciably more stable than a no-exceedence based 
form.
    In selecting a specific concentration based form, the Administrator 
first notes that a few epidemiologic studies described in the ISA 
reported an increase in SO2-related respiratory health 
effects at the upper end of the distribution of ambient SO2 
concentrations (i.e., above 90th percentile SO2 
concentrations; see ISA, section 5.3, p. 5-9). The Administrator notes 
further that numerous controlled human exposure studies have reported 
decrements in lung function and/or respiratory symptoms in exercising 
asthmatics exposed to peak 5-10 minute SO2 concentrations. 
The Administrator therefore concludes that the form of a new 1-hour 
standard should be especially focused on limiting the upper end of the 
distribution of ambient SO2 concentrations (i.e., above 90th 
percentile SO2 concentrations) in order to provide 
protection with an adequate margin of safety against effects reported 
in both epidemiologic and controlled human exposure studies.
    In further considering specific concentration based forms, the 
Administrator notes as outlined above in section II.F.3.b, and 
discussed in more detail in the REA (REA, section 10.5.3) and proposal 
(see section II.F.3, 74 FR at 64834), that a 99th percentile form is 
likely to be appreciably more effective at limiting 5-minute benchmark 
exposures of concern compared to a 98th percentile form. Taken together 
with the considerations just discussed above, the Administrator has 
selected a 99th percentile form, averaged over 3 years. The 
Administrator concludes that a 99th percentile form, given the level 
selected (see section II.F.4 immediately below), will limit both the 
upper end of the distribution of ambient SO2 concentrations 
reported in some epidemiologic studies to be associated with increased 
risk of SO2-related respiratory morbidity effects (e.g., 
emergency department visits), as well as 5-minute peak SO2 
concentrations resulting in decrements in lung function and/or 
respiratory symptoms in exercising asthmatics participating in 
controlled human exposure studies.
4. Level
    As discussed below and in more detail in the proposal (section 
II.F.4, 74 FR at 64834), the Administrator proposed to set a 1-hour 
standard with a 99th percentile form (averaged over three years), with 
a level in the range of 50 to 100 ppb. The Administrator also solicited 
comment on standard levels greater than 100 ppb up to 150 ppb. This 
section summarizes the rationale for the Administrator's proposed range 
of standard levels (II.F.3.a), discusses comments related to the range 
of standard levels (II.F.3.b), and presents the Administrator's final 
conclusions regarding the level of a new 1-hour SO2 standard 
(II.F.3.c).
a. Rationale for Proposed Decision
    In assessing the level of a 1-hour standard with a 99th percentile 
form (averaged over three years), the Administrator considered the 
broad range of scientific evidence assessed in the ISA, including the 
epidemiologic studies and controlled human exposure studies, as well as 
the results of air quality, exposure, and risk analyses presented in 
the REA. In light of this body of evidence and analyses, the 
Administrator found it is necessary to provide increased public health 
protection for at-risk populations against an array of adverse 
respiratory health effects related to short-term (i.e., 5 minutes to 24 
hours) exposures to ambient SO2. In considering the most 
appropriate way to provide this protection, the Administrator was 
mindful of the extent to which the available evidence and analyses 
could inform a decision on the level of a standard. The Administrator's 
proposed decisions on level, as discussed in detail in the proposal 
(see section II.F.4.e), are outlined below.
    Given the above considerations, the Administrator proposed to set a 
level for a new 99th percentile 1-hour daily maximum primary 
SO2 standard within the range from 50 to 100 ppb and took 
comment on levels above 100 ppb, up to 150 ppb. In reaching this 
proposed decision, the Administrator considered: (1) The evidence-based 
considerations from the final ISA and the final REA; (2) the results of 
the air quality, exposure, and risk assessments discussed above and in 
the final REA; (3) CASAC advice and recommendations on both the ISA and 
REA discussed above and provided in CASAC's letters to the 
Administrator; and (4) public comments received on the first and second 
drafts of the ISA and REA. In considering what level of a 1-hour 
SO2 standard is requisite to protect public health with an 
adequate margin of safety, the Administrator was mindful that this 
choice requires judgments based on an interpretation of the evidence 
and other information that neither overstates nor understates the 
strength and limitations of that evidence and information.
    As noted above, the Administrator selected an upper end of a range 
of levels to propose at 100 ppb. The selection of this level focused on 
the results of the controlled human exposure studies and is primarily 
based on the results of the air quality and exposure analyses which 
suggest that a 1-hour standard should be at or below 100 ppb to 
appreciably limit 5-minute SO2 benchmark concentrations >= 
200 ppb (see proposal Tables 2-4, and proposal sections II.F.4.a and 
II.F.4.b). That is, as described in the proposal (see section 
II.F.4.e), the 40-county air quality analysis estimates that a 100 ppb 
1-hour standard would allow at most 2 days per year on average when 
estimated 5-minute daily maximum SO2 concentrations exceed 
the 400 ppb benchmark, and at most 13 days per year on average when 5-
minute daily maximum SO2 concentrations exceed the 200 ppb 
benchmark (see proposal Table 2). Furthermore, given a simulated 1-hour 
100 ppb standard level, most counties in the air quality analysis were 
estimated to experience 0 days per year on average when 5-minute daily 
maximum SO2 concentrations exceed the 400 ppb benchmark and 
<= 3 days per year on average when 5-minute daily maximum 
SO2 concentrations were estimated to exceed the 200 ppb 
benchmark (see REA, Tables 7-14 and 7-12). The Administrator also noted 
that the St. Louis exposure analysis indicated that a 1-hour standard 
at

[[Page 35542]]

100 ppb would still be estimated to protect > 99% of asthmatic children 
at moderate or greater exertion from experiencing at least one 5-minute 
SO2 exposure >= 400 ppb per year, and about 97% of these 
children from exposures >= 200 ppb. In contrast, as described in the 
proposal (see section II.F.4.b), the St. Louis exposure analysis 
estimated that a 1-hour standard at 150 ppb would likely only protect 
about 88% of asthmatic children at moderate or greater exertion from 
experiencing at least one 5-minute SO2 exposure >= 200 ppb 
per year.
    As noted above and described in detail in the proposal (see section 
II.F.4.e), the Administrator selected 50 ppb as the lower end of a 
range of levels to propose, which is consistent with CASAC's advice. 
The selection of this level focused in part on the U.S. epidemiologic 
evidence described in detail in the proposal (see sections II.B.2, 
II.F.4.a, and II.F.4.e). With respect to these epidemiologic studies, 
seven of ten U.S. emergency department visit and hospital admission 
studies reporting generally positive associations with ambient 
SO2 were conducted in locations where 99th percentile 1-hour 
daily maximum SO2 levels were about 75-150 ppb, and three of 
these studies observed statistically significant positive associations 
between ambient SO2 and respiratory-related emergency 
department visits and hospitalizations in multi-pollutant models with 
PM (NYDOH (2006), Ito et al., (2007), and Schwartz et. al, (1995)). 
Thus, the Administrator noted that a 99th percentile 1-hour daily 
maximum standard set at a level of 50 ppb is well below the 99th 
percentile 1-hour daily maximum SO2 concentrations reported 
in locations where these three studies were conducted (i.e. well below 
99th percentile 1-hour daily maximum SO2 levels of 78-150 
ppb seen in NYDOH (2006), Ito et al., (2007), and Schwartz et. al, 
(1995)). Finally, the Administrator noted that two epidemiologic 
studies reported generally positive associations between ambient 
SO2 and emergency department visits in cities when 99th 
percentile 1-hour daily maximum SO2 concentrations were 
approximately 50 ppb, but did not consider that evidence strong enough 
to propose setting a standard level lower than 50 ppb.
    In considering the results of the air quality and exposure 
analyses, the Administrator also noted that the 40-county air quality 
analysis estimates that a 99th percentile 1-hour daily maximum standard 
set at a level of 50 ppb would result in zero days per year when 
estimated 5-minute SO2 concentrations exceed the 400 ppb 5-
minute benchmark level and at most 2 days per year when modeled 5-
minute SO2 concentrations exceed the 200 ppb 5-minute 
benchmark level (see proposal section II.F.4.b and proposal Table 2). 
In addition, the St. Louis exposure analysis estimates that a 99th 
percentile 1-hour daily maximum standard set at a level of 50 ppb would 
likely protect > 99% of asthmatic children at moderate or greater 
exertion from experiencing at least one 5-minute exposure both >= 400 
and  200 ppb per year (see proposal section II.F.4.b and 
Table 3). In addition, although not directly analyzed in the REA, the 
proposal (section II.F.4.b) noted that a 1-hour daily maximum standard 
at a level of 75 ppb would be bound by the exposure estimates from air 
quality adjusted to just meet 99th percentile 1-hour daily maximum 
standards at 50 and 100 ppb. Thus, a 1-hour daily maximum standard at a 
level of 75 ppb would be estimated to protect > 99% of asthmatic 
children at moderate or greater exertion in St. Louis from experiencing 
at least one exposure >= 400 ppb per year, and about 97% to > 99% of 
these children from experiencing at least one exposure >= 200 ppb per 
year.
    The Administrator thus proposed to set the level of a new 1-hour 
standard that would protect public health with an adequate margin of 
safety between 50 ppb and 100 ppb. In so doing, the Administrator 
relied on reported findings from both epidemiologic and controlled 
human exposure studies, as well as the results of air quality and 
exposure analyses. The Administrator noted that the lower end of the 
proposed range was consistent with CASAC advice that there is clearly 
sufficient evidence for consideration of standard levels starting at 50 
ppb (Samet 2009, p. 16). With respect to the upper end of the proposed 
range, the Administrator noted that CASAC concluded that standards up 
to 150 ppb ``could be justified under some interpretations of weight of 
evidence, uncertainties, and policy choices regarding margin of 
safety'' (id.), although the letter did not provide any indication of 
what interpretations, uncertainties, or policy choices might support 
selection of a level as high as 150 ppb.
    In light of the range of levels included in CASAC's advice, the 
Administrator also solicited comment on setting a standard level above 
100 ppb and up to 150 ppb. In so doing, the Administrator recognized 
that there are uncertainties with the scientific evidence, such as 
attributing effects reported in epidemiologic studies specifically to 
SO2 given the presence of co-occurring pollutants, 
especially PM, and the uncertainties associated with using ambient 
SO2 concentrations as a surrogate for exposure. However, the 
Administrator noted that compared to the proposed range of 50-100 ppb, 
a standard level as high as 150 ppb would not comparably limit 5-minute 
SO2 exposures >= 200 ppb. That is, she noted that the St. 
Louis exposure analysis estimated that a 150 ppb standard would protect 
approximately 88% of asthmatic children at moderate or greater exertion 
from experiencing at least one SO2 exposure >= 200 ppb per 
year (compared to > 99% and approximately 97% given standards at 50 and 
100 ppb respectively; see proposal Table 3 at 74FR at 64841).
b. Comments on Level
    Most State and local agencies and organizations that commented on 
this issue expressed support for setting the level of a 1-hour 
SO2 standard somewhere within the proposed range of 50 to 
100 ppb. More specifically, State environmental organizations (i.e., 
NACAA and NESCAUM); State environmental agencies (e.g., such agencies 
in DE, IL, MI, NY, NM, PA, VT), the Fond du Lac Tribe, and local groups 
(e.g., NYDOH, City of Houston, New York City, Houston-Galveston Area 
Council) supported a level of a 1-hour SO2 standard in the 
range of 50 to 100 ppb. In addition, State environmental agencies in IA 
and TX specifically supported a standard level of 100 ppb. In general, 
these groups cited the conclusions of CASAC and the Administrator's 
rationale as stated in the proposal as a basis for their 
recommendations, though State environmental agencies in IA and TX 
generally recommended placing more weight on the controlled human 
exposure evidence rather than on the epidemiology.
    A number of environmental and medical/public health organizations 
(e.g., ALA, ATS, EDF, Sierra Club, WEACT for Environmental Justice, 
NRDC, CBD) and some local organizations (e.g., Alexandria Department of 
Transportation and Environmental Services, and Harris County (TX) 
Public Health & Environmental Services) supported setting a standard 
level at or near 50 ppb. This recommendation was typically based on the 
commenters' interpretation of the controlled human exposure and 
epidemiologic evidence, as described below.
    With regard to the controlled human exposure evidence, health and 
environmental groups generally concluded that a 1-hour SO2 
standard

[[Page 35543]]

no higher than 50 ppb is needed to protect against 5-minute 
SO2 benchmark exposures as low as 100 ppb identified from 
mouthpiece exposure studies, rather than the 200 ppb 5-minute 
SO2 benchmark identified from ``free breathing'' controlled 
human exposure studies. More specifically, ALA et al., stated:

    In its analysis of data from chamber studies in the ISA and in 
the REA, EPA focuses on studies of ``free breathing'' exposure. In 
doing so, EPA improperly and arbitrarily downplays important 
evidence that reported increased airway resistance, a measure of 
bronchoconstriction, in subjects with mild asthma at concentrations 
of 100 ppb. Regrettably, EPA does not rely on the mouthpiece studies 
in formulating its proposed standards * * * In downplaying the 
mouthpiece studies, EPA ignores the large segment of people who rely 
on oral or oronasal breathing some or all of the time.

    The Administrator disagrees with the assertion that results from 
mouthpiece studies were improperly downplayed. These studies are 
discussed in the ISA, REA, and proposed rule as demonstrating 
respiratory effects of SO2 at concentrations of 100 ppb, the 
lowest concentration tested using a mouthpiece exposure system. 
Nonetheless, these mouthpiece studies are not a reasonable proxy for 
actual exposure. In these studies, SO2 is delivered directly 
through the mouth, typically in conjunction with nasal occlusion. This 
allows a greater fraction of the inhaled SO2 to reach the 
tracheobronchial airways. Although we agree with commenters that some 
individuals do breathe oronasally both while at rest and during 
exercise, nasal ventilation still constitutes a significant percentage 
of total ventilation. The consequence is that individuals exposed to 
SO2 through a mouthpiece are likely to experience greater 
respiratory effects from a given SO2 exposure than they 
would in real life. Thus, as noted in the REA (REA, section 6.2) and in 
the proposal preamble (see section II.B.1.b), these mouthpiece studies 
only provide very limited evidence of decrements in lung function 
following exposure to 100 ppb SO2. Therefore, the 
Administrator did not place great weight on these mouthpiece studies 
when considering the appropriate level of a 1-hour SO2 
standard.
    In addition to their interpretation of the controlled human 
exposure evidence, health and environmental groups (e.g., ALA, ATS, 
EDF, NRDC, Sierra Club, CBD) and the Alexandria Department of 
Transportation and Environmental Services generally concluded that the 
epidemiologic evidence indicates that a standard no higher than 50 ppb 
is required to protect public health. For example, it its comments the 
CBD stated:

    Epidemiologic studies referenced in the Proposed Rule showed 
positive, and in many cases statistically significant, relationships 
between ambient SO2 concentrations and hospital 
admissions where 99th percentile 1-hour concentrations ranged from 
50-460 ppb. Of these studies, two showed positive and sometimes 
statistically significant relationships in single-pollutant models 
at 50 ppb, and three studies showed statistically significant 
correlations at 78-150 ppb in multi-pollutant models. These three 
multipollutant studies, moreover, ``lend[] strong support * * * to 
the conclusion that SO2 effects are generally 
independent'' of those of co-pollutants like particulate matter. 
Giving these studies their proper weight, and allowing for an 
adequate margin of safety, EPA should set a one-hour NAAQS at a 
level no higher than the lowest concentration at which positive, 
adverse relationships have been demonstrated: 50 ppb (note that 
footnotes were omitted).

    The Administrator agrees that the epidemiologic studies referenced 
in the proposal need to be considered in judging the appropriate level 
for a new 99th percentile 1-hour SO2 standard. However, she 
disagrees that when considered in total, these studies strongly support 
an SO2 standard no higher than 50 ppb. The Administrator 
notes that selecting a standard level of 50 ppb would place 
considerable weight on the two U.S. emergency department visit studies 
conducted in locations where 99th percentile 1-hour SO2 
concentrations were approximately 50 ppb (i.e., Wilson et al., (2005) 
in Portland, ME and Jaffe et al., (2003) in Columbus, OH). However, the 
Administrator does not find this appropriate given that, importantly, 
neither of these studies evaluated the potential for confounding by co-
pollutants through the use of multipollutant models and thus, left 
unaddressed the issue of whether the effects seen in the studies were 
partially or totally attributable to exposure to sulfate PM. In 
addition, the Administrator notes that the overall results reported in 
these studies are mixed. It is important to note that mixed results do 
not automatically disqualify studies from being used as part of the 
evidence base for setting levels in NAAQS reviews. However, in this 
review the Administrator judges that the lack of mutipollutant model 
evaluation for potential confounding by PM in two locations with the 
lowest SO2 levels combined with the presence of mixed 
emergency department visit results renders these two studies 
inappropriate to serve as the primary basis for the selection of the 
level of the SO2 NAAQS. As an additional matter, the 
suggestion in some of the comments that EPA should necessarily base the 
level of a NAAQS on the lowest level seen in epidemiologic studies has 
been rejected repeatedly. See, e.g. American Petroleum Inst. v. EPA, 
665 F. 2d at 1187 (``In so arguing NRDC essentially ignores the mixed 
results of the medical studies evident in the record, choosing instead 
to rely only on the studies that favor its position. The Administrator, 
however, was required to take into account all the relevant studies 
revealed in the record. Because he did so in a rational manner, we will 
not overrule his judgment as to the margin of safety.'') Thus, although 
the Administrator finds that these two studies provide limited evidence 
of emergency department visits in cities where 99th percentile 1-hour 
daily maximum SO2 concentrations are approximately 50 ppb, 
she also concludes that these studies do not provide enough evidence to 
warrant a standard at this level.
    As discussed above in section, II.E.2, a number of industry groups 
(e.g., ACC, UARG) did not support setting a new 1-hour SO2 
standard. However, several of these groups (e.g., UARG, API) and the SC 
Chamber of Commerce concluded that, if EPA does choose to set a new 1-
hour standard, the level of that standard should be >= 150 ppb. In 
addition, State environmental agencies in SD (SD DENR) and OH 
recommended standard levels at 150 ppb. As a basis for this 
recommendation, these groups generally emphasized uncertainties in the 
scientific evidence. Specifically, as discussed in more detail above 
(section II.E.2.a), these commenters typically concluded that the 
available epidemiologic studies do not support the conclusion that 
SO2 causes the reported health effects. This was based on 
their assertion that the presence of co-pollutants in the ambient air 
precludes the identification of a specific SO2 contribution 
to reported effects. Thus, these groups generally concluded that weight 
should not be placed on the cluster of three epidemiologic studies 
reporting statistically significant effects in multipollutant models 
with PM (i.e., NYDOH 2006; Ito 2007; and Schwartz 1995). That is, these 
groups contend that these studies do not demonstrate an independent 
effect of SO2. In addition, as noted in section II.E.2.b, 
many of these groups also disagreed with the Agency's judgment that 
adverse respiratory effects occur following 5-minute exposures to 
SO2 concentrations as low as 200 ppb. These comments and 
EPA's responses are discussed below

[[Page 35544]]

and in section II of the RTC document (EPA 2010).
    As described in more detail in section II.E.2.a, we agree that the 
interpretation of SO2 epidemiologic studies is complicated 
by the fact that SO2 is but one component of a complex 
mixture of pollutants present in the ambient air. However, the ISA 
concluded that when U.S. and international epidemiologic literature is 
evaluated as a whole, SO2 effect estimates generally 
remained positive and relatively unchanged in multi-pollutant models 
with gaseous or particulate co-pollutants. Thus, although recognizing 
the uncertainties associated with separating the effects of 
SO2 from those of co-occurring pollutants, the ISA concluded 
that the limited available evidence from studies employing multi-
pollutant models indicates that the effect of SO2 on 
respiratory health outcomes appears to be generally robust and 
independent of the effects of gaseous co-pollutants, including 
NO2 and O3, as well as particulate co-pollutants, 
particularly PM2.5 (ISA, section 5.2; p. 5-9).
    In addition, as described in detail above in section II.E.2.a, the 
ISA emphasized that controlled human exposure studies provide support 
for the plausibility of the associations reported in epidemiologic 
studies. The ISA noted that the results of controlled human exposure 
and epidemiologic studies form a plausible and coherent data set that 
supports a causal relationship between short-term (5-minutes to 24-
hours) SO2 exposures and adverse respiratory effects, and 
that the epidemiologic evidence (buttressed by the clinical evidence) 
indicates that the effects seen in the epidemiologic studies are 
attributable to exposure to SO2 (ISA, section 5.2). The ISA 
in fact made the strongest finding possible regarding causality: 
``[e]valuation of the health evidence, with consideration of issues 
related to atmospheric sciences, exposure assessment, and dosimetry, 
led to the conclusion that there is a causal relationship between 
respiratory morbidity and short-term exposure to SO2. This 
conclusion is supported by the consistency, coherence, and plausibility 
of findings observed in the human clinical, epidemiologic, and animal 
toxicological studies.'' ISA p. 5-2 (emphasis original).
    As mentioned above, many groups dispute the ISA conclusion that 
taken together, results from U.S. and international epidemiologic 
studies employing multipollutant models indicate that SO2 
has an independent effect on the respiratory health outcomes reported 
in these studies. Thus, these groups contend that the Administrator 
should not place weight on epidemiologic studies and their associated 
air quality information in general, and more specifically, the 
Administrator should not place weight on air quality information from 
the three U.S. epidemiologic studies reporting statistically 
significant effects in multipollutant models with PM (i.e., NYDOH 2006; 
Ito 2007; and Schwartz 1995). Specific comments on these three 
epidemiologic studies reporting statistically significant effects in 
multi-pollutant models with PM, and EPA responses are presented below 
and in the RTC document (EPA 2010).
    Industry groups (e.g., API) had several comments with respect to 
the study conducted by the NYDOH (NYDOH, 2006). First, these groups 
generally concluded that the results of this study are mixed. That is, 
while SO2 effect estimates were positive and statistically 
significant even in multipollutant models with PM2.5 or 
NO2 in the Bronx, SO2 effect estimates were 
actually negative in Manhattan in both single and multipollutant 
models. These groups also contend that this report was not peer-
reviewed and that the authors of this study indicated that high 
correlations among pollutants in the Bronx made it difficult to 
confidently identify which pollutants are actually increasing risks. 
For these reasons, industry groups generally concluded that this study 
should not be relied upon by the Administrator.
    We acknowledge that the results of the NYDOH analysis are mixed 
when comparing the Bronx and Manhattan study areas. However, we 
disagree that the presence of mixed results renders this study 
unreliable. We note that the mixed results reported in this study are 
likely to reflect greater statistical power for identifying effects in 
the Bronx, where the average daily emergency department visits differed 
substantially from those in Manhattan. Specifically, daily asthma 
emergency department visits were six times higher in the Bronx study 
area (43 per day) than in the Manhattan study area (7.2 per day). Thus, 
the more prominent effects in the Bronx likely at least partially 
reflect greater statistical power for identifying effects there. To put 
these numbers in perspective, the crude daily rates of asthma emergency 
department visits can be estimated by dividing the daily asthma counts 
by the population. The mean daily crude rates of asthma emergency 
department visits were over eight-fold higher in the Bronx study area 
(16.9 per 100,000 persons) than in the Manhattan area (2.02 per 100,000 
persons). Population age structures were quite different in the two 
communities, with larger proportions of younger persons in the Bronx 
versus Manhattan. There are likely additional differences in population 
structures of the two communities, including differences in SES, race/
ethnicity, and access to primary asthma care. These differences in the 
two communities may explain the differences in the results, and do not 
prevent EPA from legitimately relying on this study.
    As mentioned above, these groups also contend that the NYDOH 
epidemiologic study should not be relied upon because it was not peer-
reviewed. We disagree with this assertion. The NYDOH study was subject 
to multiple peer-review processes. This included reviews by the Agency 
for Toxic Substances and Disease Registry (ATSDR), EPA, and CASAC.
    Finally, as also mentioned above, these groups contend that the 
NYDOH epidemiologic study is unreliable because the study authors 
indicated that high correlations among pollutants in the Bronx make it 
difficult to confidently identify which pollutants are actually 
increasing risks. In response, we note that high correlations among 
ambient air pollutant concentrations are not specific to the NYDOH 
study, and may contribute to uncertainty in the interpretation of many 
epidemiologic studies of air pollution. The approach most commonly 
utilized to disentangle the effects of correlated pollutants in air 
pollution epidemiology is the copollutant model. The NYDOH uses 
copollutant models and finds that the results for SO2 remain 
significant in models considering the simultaneous effects of 
NO2, O3, and PM2.5. This indicates an 
independent effect of SO2 on the asthma emergency department 
visits reported in this study.
    With respect to Ito et al., (2007), industry groups generally 
commented that since the SO2 effect estimate did not remain 
statistically significant in multipollutant models with NO2, 
this study does not indicate an independent effect of SO2 on 
emergency department visits in the NYC study area. API specifically 
commented:

    The RR for an increase of 6 ppb SO2 was statistically 
significant (1.20; 95% CI: 1.13, 1.28) and remained so when 
PM2.5, O3, or CO was included in the model, 
but became nonsignificant when NO2 was included in the 
model (RR not provided, 95% CI: 0.9, 1.1). Because associations with 
SO2 could be attributable to NO2, this study 
cannot be used to assess the effects of SO2 on health 
effects with small incremental increases in exposure.


[[Page 35545]]


    We disagree with the commenters. We believe that this study does 
demonstrate an independent effect of SO2 on emergency 
department visits in NYC. We note that evidence from controlled human 
exposure studies has demonstrated effects of NO2 (EPA, 
2008b) and SO2 independently on respiratory morbidity. Since 
each of these criteria pollutants has an independent effect on the 
respiratory system, it is logical that each may be responsible for an 
increase in emergency department visits for asthma in epidemiologic 
studies. In addition, the authors note that the attenuation of the 
SO2 effect estimate when NO2 is included in the 
model is ``consistent with the result of monitor-to-monitor 
correlations, suggesting that NO2 has less exposure error 
than CO or SO2 in this data set.'' Thus, it appears as 
though the high spatial heterogeneity of SO2 compared to 
NO2, leading to increased exposure error, may be causing the 
attenuation of the SO2 effect estimate when NO2 
is included in the model in this study--not that the effects seen in 
the study are attributable to NO2. Overall, the results from 
this study are consistent with the SO2 effect on respiratory 
emergency department visits and hospital admissions across studies and 
are coherent with the respiratory effects observed in controlled human 
exposure studies. This study thus provides persuasive evidence of an 
independent effect of short-term SO2 exposure on respiratory 
morbidity.
    With respect to Schwartz et al., (1995), industry groups generally 
commented that the results of this study are mixed, and therefore 
should not be considered by the Administrator. More specifically, these 
commenters noted that although the results in New Haven remained 
statistically significant in the presence of PM10, the 
SO2 effect estimate in Tacoma was reduced and no longer 
statistically significant in the presence of PM10. 
Commenters also noted that in both cities, the SO2 effect 
estimate was reduced and no longer statistically significant in the 
presence of O3.
    We disagree that the results of this study of hospital admissions 
should not be considered by the Administrator. As noted by the 
commenters, this study was conducted in two cities, New Haven, CT and 
Tacoma, WA. These cities were chosen because they differ in several 
important aspects and the author expected the results from the two 
cities to be different due to the inherent nature of the study design 
and study locations. ``New Haven has almost twice the mean 
SO2 concentration of Tacoma, almost two and a half times the 
SO2 concentration in the peak winter season, and a much 
larger summer ozone peak than Tacoma (Schwartz 1995).'' Since the study 
was designed to examine the differences in these two cities, the fact 
that the results differed in the two cities does not invalidate those 
results. In addition, EPA considers the SO2 effect to be 
robust to inclusion of O3 in New Haven. The central effect 
estimate for SO2 changed from 1.03 to 1.02 after the 
addition of O3 as a copollutant and likely lost statistical 
significance due to a greater than 40% reduction in the number of days 
included because O3 was only measured during the warm 
months. This reduction likely led to model instability and a loss of 
statistical significance. To be consistent with how results of other 
studies were interpreted in the ISA, and as supported by the CASAC, the 
effect of SO2 is considered robust to the inclusion of 
O3 in New Haven.
    In addition to generally concluding that the epidemiology is too 
uncertain to demonstrate that SO2 has an independent effect 
on the respiratory effects reported in those studies, many industry 
groups (e.g., API, ACC, Progress Energy, EEI, CIBO) also commented that 
adverse health effects do not occur following 5-10 minute 
SO2 exposures < 400 ppb in controlled human exposure studies 
(an issue also discussed above in section II.E.2.b). Thus, these groups 
generally maintained that the level of a 1-hour standard should not 
take into account limiting 5-minute peaks as low as 200 ppb. From this 
argument, many of these groups further maintained that 1-hour standard 
levels >= 150 ppb are requisite to protect public health with an 
adequate margin of safety.
    As first discussed in section II.E.2.b above, we disagree with the 
commenters that adverse respiratory effects do not occur following 5-
minute SO2 exposures as low as 200 ppb. The ISA reported 
that exposure to SO2 concentrations as low as 200-300 ppb 
for 5-10 minutes results in approximately 5-30% of exercising 
asthmatics experiencing moderate or greater decrements in lung function 
(defined in terms of a >= 15% decline in FEV1 or 100% 
increase in sRaw; ISA, Table 3-1). Considering the 2000 ATS guidelines 
described in section II.E.2.b, we determined that these results could 
reasonably indicate an SO2-induced shift in these lung 
function measurements for this sub-population. Under this scenario, an 
appreciable percentage of exercising asthmatics exposed to 
SO2 concentrations as low as 200 ppb would likely have 
diminished reserve lung function and thus would likely be at greater 
risk if affected by another respiratory agent (e.g., viral infection). 
Importantly, diminished reserve lung function in a population that is 
attributable to air pollution is considered an adverse effect under ATS 
guidance.\17\ Also noted in section II.E.2.b, we were mindful of 
CASAC's pointed comments. The second draft ISA placed relatively little 
weight on health effects associated with SO2 exposures at 
200-300 ppb. CASAC strongly disagreed with this characterization of the 
health evidence. Their consensus letter following the second draft ISA 
states:
---------------------------------------------------------------------------

    \17\ See Coalition of Battery Recyclers Association v. EPA, No. 
09-1011 (DC Cir., May 14, 2010), slip opinion at 9, holding that it 
was reasonable for EPA to conclude that a two IQ point mean 
population loss is an adverse effect based in part on consideration 
of comments from the American Academy of Pediatrics that such a loss 
should be prevented.

    Our major concern is the conclusions in the ISA regarding the 
weight of the evidence for health effects for short-term exposure to 
low levels of SO2. Although the ISA presents evidence 
from both clinical and epidemiological studies that indicate health 
effects occur at 0.2 ppm or lower, the final chapter emphasizes 
health effects at 0.4 ppm and above * * * CASAC believes the 
clinical and epidemiological evidence warrants stronger conclusions 
in the ISA regarding the available evidence of health effects at 0.2 
ppm or lower concentrations of SO2. The selection of a 
lower bound concentration for health effects is very important 
because the ISA sets the stage for EPA's risk assessment decisions. 
In its draft Risk and Exposure Assessment (REA) to Support the 
Review of the SO2 Primary National Ambient Air Quality Standards 
(July 2008), EPA chose a range of 0.4 ppm--0.6 ppm SO2 
concentrations for its benchmark analysis. As CASAC will emphasize 
in a forthcoming letter on the REA, we recommend that a lower bound 
---------------------------------------------------------------------------
be set at least as low as 0.2 ppm (Henderson 2008a).

    Similarly, we were also mindful of CASAC comments on the first 
draft of the REA. The consensus CASAC letter following the 1st draft 
REA states:

    The CASAC believes strongly that the weight of clinical and 
epidemiology evidence indicates there are detectable clinically 
relevant health effects in sensitive subpopulations down to a level 
at least as low as 0.2 ppm SO2. These sensitive 
subpopulations represent a substantial segment of the at-risk 
population (Henderson 2008b).

    As noted in section II.E.2.b, we were also mindful of: (1) Previous 
CASAC recommendations (Henderson 2006) and NAAQS review conclusions 
(EPA 2006, EPA 2007d) indicating that moderate decrements in lung 
function can be clinically significant in some asthmatics (see section 
II.E.2.b for more detail) and

[[Page 35546]]

(2) controlled human exposure studies not including severe asthmatics 
and thus, that it is reasonable to assume that persons with more severe 
asthma than the study participants would have a more serious health 
effect from short-term exposure to 200 ppb SO2. CASAC echoed 
this concern in its comments on the policy assessment chapter of the 
REA:

    Chapter 10 should better address uncertainty in identifying 
alternative NAAQS for SO2. In particular, the 
uncertainties discussed in the health risk characterization should 
be considered in specifying a NAAQS that provides adequate margin of 
safety. One particular source of uncertainty needing acknowledgment 
is the characteristics of persons included in the clinical studies. 
The draft REA acknowledges that clinical studies are unlikely to 
have included severe asthmatics that are likely to be potentially at 
greater risk than those persons included in the clinical studies 
(Samet 2009; p. 15).

    Taken together, the Administrator concluded that exposure to 
SO2 concentrations as low as 200 ppb can result in adverse 
health effects in asthmatics. Consequently the Administrator also 
concluded that a 1-hour standard of 150 ppb is not requisite to protect 
public health with an adequate margin of safety, even with a 99th 
percentile form. This conclusion takes into account the St. Louis 
exposure analysis estimating that only 88% of asthmatic children at 
moderate or greater exertion would be protected from at least one 5-
minute SO2 exposure >= 200 ppb per year at a 1-hour standard 
level of 150 ppb, and appropriate weight placed on the epidemiologic 
evidence (see section II.F.4.c for a discussion of the epidemiologic 
evidence with respect to level).
c. Conclusions on Standard Level
    Having carefully considered the public comments on the appropriate 
level for a 1-hour SO2 standard, as discussed above, the 
Administrator believes the fundamental conclusions reached in the ISA 
and REA remain valid. In considering the level at which the 1-hour 
primary SO2 standard should be set, the Administrator 
continues to place primary emphasis on the body of controlled human 
exposure and epidemiologic evidence assessed in the ISA, as summarized 
above in section II.B. In addition, the Administrator continues to view 
the results of exposure and risk analyses, discussed above in section 
II.C, as providing supporting information for her decision.
    In considering the level of a 1-hour SO2 standard, the 
Administrator notes that there is no bright line clearly mandating the 
choice of level within the reasonable range proposed. Rather, the 
choice of what is appropriate within this reasonable range is a public 
health policy judgment entrusted to the Administrator. This judgment 
must include consideration of the strengths and limitations of the 
evidence and the appropriate inferences to be drawn from the evidence 
and the exposure and risk assessments. These considerations and the 
Administrator's final decision with regard to the level of a new 1-hour 
SO2 standard are discussed below.
    In considering the controlled human exposure studies, the 
Administrator notes that these studies provide the most direct evidence 
of respiratory effects from exposure to SO2. These studies 
exposed groups of exercising asthmatics to defined concentrations of 
SO2 for 5-10 minutes and found adverse respiratory effects. 
As noted above (see section II.C), SO2 exposure levels which 
resulted in respiratory effects in these studies were considered 5-
minute benchmark exposures of potential concern in the analyses found 
in the REA. With respect to this evidence, the Administrator notes the 
following key points:
     Exposure of exercising asthmatics to 5-10 minute 
SO2 concentrations >= 400 ppb results in moderate or greater 
decrements in lung function (in terms of FEV1 or sRaw) in 
20-60% of tested individuals in these studies. Moreover, these 
decrements in lung function are often statistically significant at the 
group mean level and are frequently accompanied by respiratory 
symptoms.\18\ Based on ATS guidelines, exposure to SO2 
concentrations >= 400 ppb clearly result in adverse respiratory effects 
(i.e., decrements in lung function in the presence of respiratory 
symptoms). Therefore, the Administrator has concluded it appropriate to 
place weight on the 400 ppb 5-minute SO2 benchmark 
concentration of concern.
---------------------------------------------------------------------------

    \18\ The ISA concluded that collective evidence from key 
controlled human exposure studies considered in the previous review, 
along with a limited number of new controlled human exposure 
studies, consistently indicates that with elevated ventilation rates 
a large percentage of asthmatic individuals tested in a given 
chamber study (up to 60%, depending on the study) experience 
moderate or greater decrements in lung function, frequently 
accompanied by respiratory symptoms, following peak exposures to 
SO2 at concentrations of 0.4-0.6 ppm. (ISA, p. 3-9).
---------------------------------------------------------------------------

     Exposure of exercising asthmatics to 5-10 minute 
SO2 concentrations at 200-300 ppb results in moderate or 
greater decrements in lung function in 5-30% of the tested individuals 
in these studies. The Administrator notes that although these 
decrements in lung function have not been shown to be statistically 
significant at the group mean level, or to be frequently accompanied by 
respiratory symptoms, she considers effects associated with exposures 
as low as 200 ppb to be adverse in light of CASAC advice, similar 
conclusions in prior NAAQS reviews, and the ATS guidelines described in 
detail above (see section II.E.2.b and II.F.4.b). Therefore, she has 
concluded it appropriate to place weight on the 200 ppb 5-minute 
benchmark concentration.
     There is very limited evidence from two mouthpiece 
exposure studies suggesting respiratory effects in exercising 
asthmatics following SO2 exposures at 100 ppb. However, 
given the uncertainties and potential unrepresentativeness associated 
with mouthpiece studies (see section II.F.4.b above), the Administrator 
found it appropriate not to place weight on this 5-minute 
SO2 benchmark concentration.
    The Administrator also considered the results of the air quality, 
exposure, and risk analyses, as they serve to estimate the extent to 
which a given 1-hour standard limits the 5-minute benchmark 
concentrations of concern identified from controlled human exposure 
studies (see REA chapters 7-9, proposal section II.F.4.b, and proposal 
Tables 2-4). In considering these results as they relate to limiting 5-
minute SO2 benchmark concentrations >= 200 and 400 ppb, the 
Administrator notes the following key points:
     The 40-county air quality analysis estimates that a 100 
ppb 1-hour daily maximum standard would allow at most 2 days per year 
on average in any county when estimated 5-minute daily maximum 
SO2 concentrations exceed the 400 ppb benchmark, and at most 
13 days per year on average when 5-minute daily maximum SO2 
concentrations exceed the 200 ppb benchmark (see proposal, Table 2, 74 
FR at 64840). Furthermore, given a simulated 1-hour 100 ppb standard 
level, most of the counties in that air quality analysis were estimated 
to experience 0 days per year on average when 5-minute daily maximum 
SO2 concentrations exceed the 400 ppb benchmark and <= 3 
days per year on average when 5-minute daily maximum SO2 
concentrations were estimated to exceed the 200 ppb benchmark (see REA, 
Tables 7-14 and 7-12).
     The St. Louis exposure analysis estimates that a 99th 
percentile 1-hour daily maximum standard at a level of 100 ppb would 
likely protect > 99% of asthmatic children in that city at moderate or 
greater exertion from experiencing at least one 5-minute exposure >= 
400 ppb per year, and

[[Page 35547]]

approximately 97% of those asthmatic children at moderate or greater 
exertion from experiencing at least one exposure >= 200 ppb per year 
(see proposal, section II.F.4.b).
     The St. Louis risk assessment estimates that a 99th 
percentile 1-hour standard level at 100 ppb would likely protect about 
97-98% of exposed asthmatic children in that city at moderate or 
greater exertion from experiencing at least one moderate or greater 
lung function response (defined as a >= 100% increase in sRaw; see 
proposal, section II.F.4.b).
    Given the above considerations, the Administrator concludes that a 
1-hour standard at a level of 100 ppb would appropriately limit 5-
minute SO2 benchmark concentrations >= 200 or 400 ppb. 
Moreover, although the Administrator acknowledges that the air quality 
and exposure analyses mentioned above suggest that a 50 ppb standard 
may somewhat further limit 5-minute SO2 concentrations/
exposures in excess of the 200 ppb benchmark (see proposal section 
II.F.4.b), she does not believe this information alone warrants a 
standard level lower than 100 ppb. More specifically, although she 
considers the health effects resulting from 5-minute SO2 
exposures as low as 200 ppb to be adverse, she also recognizes that 
such effects are appreciably less severe than those at SO2 
concentrations >= 400 ppb. Thus, she concludes that there is little 
difference in limiting 5-minute concentrations/exposures >= 400 ppb 
given 1-hour standard levels in the range of 50 to 100 ppb.
    In considering the epidemiologic evidence with regard to level, the 
Administrator notes that there have been more than 50 peer reviewed 
epidemiologic studies published worldwide evaluating SO2 
(ISA, Tables 5-4 and 5-5). These studies have generally reported 
positive, although not always statistically significant associations 
between more serious health outcomes (i.e. respiratory-related 
emergency department visits and hospitalizations) and ambient 
SO2 concentrations and have generally included populations 
potentially at increased risk for SO2-related respiratory 
effects (e.g, children, older adults, and those with pre-existing 
respiratory disease). The Administrator finds that in assessing the 
extent to which these studies and their associated air quality 
information can inform the level of a new 99th percentile 1-hour daily 
maximum standard for the U.S., air quality information from the U.S. 
and Canada is most relevant since these areas have similar monitor 
network designs and patterns of air quality. However, as described in 
proposal section II.F.4.a, SO2 concentrations reported for 
Canadian studies were not directly comparable to those reported for 
U.S. studies due to use of different monitoring protocols in those 
studies. Thus, the Administrator focused on 99th percentile air quality 
information from U.S. studies for informing potential 1-hour standard 
levels. She concludes that this information provides evidence of 
associations between ambient SO2 and emergency department 
visits and hospital admissions in U.S. cities with particular 99th 
percentile 1-hour SO2 levels, and thus provides information 
that is particularly relevant for setting the level of a 1-hour 
SO2 standard. With regard to these studies she notes the 
following key points:
     Ten studies (some conducted in multiple locations) 
reported mostly positive, and sometimes statistically significant, 
associations between ambient SO2 concentrations and 
emergency department visit and hospital admissions in locations where 
99th percentile 1-hour daily maximum SO2 levels ranged from 
approximately 50-460 ppb.
     Within this broader range of SO2 
concentrations, there is a cluster of three epidemiologic studies 
between 78-150 ppb (for the 99th percentile of the 1-hour 
SO2 concentrations) where the SO2 effect estimate 
remained positive and statistically significant in multi-pollutant 
models with PM (NYDOH (2006), Ito et al., (2007), and Schwartz et al., 
(1995)). Notably, although statistical significance in multi-pollutant 
models is an important consideration, it is not the only consideration 
when relying on such epidemiologic evidence.\19\ However, as noted 
earlier, there is special sensitivity in this review in disentangling 
PM-related effects (especially sulfate PM) from SO2-related 
effects in interpreting the epidemiologic studies; thus, these studies 
are of particular relevance here, lending strong support both to the 
conclusion that SO2 effects are generally independent of PM 
(ISA, section 5.2) and that these independent adverse effects of 
SO2 have occurred in cities with 1-hour daily maximum, 99th 
percentile concentrations in the range of 78-150 ppb. Nor did EPA find 
the comments criticizing these studies persuasive, as explained above 
in section II.F.4.b and in the RTC document (EPA 2010). The 
Administrator therefore judges it appropriate to place substantial 
weight on this cluster of three U.S. epidemiologic studies in selecting 
a standard level, as they are a group of studies that reported positive 
and statistically significant associations between ambient 
SO2 and emergency department visits or hospital admissions 
even when potential confounding by PM was considered.
---------------------------------------------------------------------------

    \19\ For example, as noted in the proposal (proposal, section 
II.F.4, 74 FR at 64835) evidence of a pattern of results from a 
group of studies that find effect estimates similar in direction and 
magnitude would warrant consideration of and reliance on such 
studies even if the studies did not all report statistically 
significant associations in single- or multi-pollutant models. The 
SO2 epidemiologic studies fit this pattern, and are 
buttressed further by the results of the clinical studies. ISA, 
section 5.2.
---------------------------------------------------------------------------

     The Administrator agrees with the finding in the ISA that 
the controlled human exposure evidence lends biological plausibility to 
the effects reported in epidemiologic studies (ISA, p. 5-9).
     There is limited evidence from two epidemiologic studies 
employing single pollutant models that found generally positive 
associations between ambient SO2 and emergency department 
visits in locations where 99th percentile 1-hour SO2 
concentrations were approximately 50 ppb (see proposal, Figures 1 and 
2). However, considering that the results of these studies were mixed, 
and importantly, that neither of these two studies evaluated the 
potential for confounding by co-pollutants through the use of 
multipollutant models (particularly with PM), the Administrator judges 
it appropriate to place limited weight on these studies.
     With regard to the cluster of three studies conducted in 
the Bronx (NYDOH 2006), NYC (Ito et al., 2007), and New Haven (Scwartz 
et al., 1995), there is a degree of uncertainty as to whether the 99th 
percentile 1-hour daily maximum SO2 concentrations reported 
from monitors in these three study areas reflect the highest 99th 
percentile 1-hour daily maximum SO2 concentration. Our 
limited qualitative analysis suggests that 99th percentile 1-hour daily 
maximum SO2 concentrations reported by monitors in these 
study areas are reasonable approximations for the absolute highest 99th 
percentile 1-hour daily maximum SO2 concentration that can 
occur across the entire area in these studies (including the areas 
where monitors were not located) (see Brode, 2010). However, although a 
reasonable approximation, it is still likely that these monitored 
concentrations are somewhat lower than the absolute highest 99th 
percentile 1-hour daily maximum SO2 concentrations occurring 
across these epidemiologic study areas.

[[Page 35548]]

    Weighing all of this evidence, the Administrator concludes that the 
epidemiologic studies provide strong support for setting a standard 
that limits the 99th percentile of the distribution of 1-hour daily 
maximum SO2 concentrations to 75 ppb. This judgment takes 
into account the strong determinations in the ISA (and endorsed by 
CASAC), based on a much broader body of evidence, that there is a 
causal association between exposure to SO2 and the types of 
respiratory morbidity effects reported in these studies. The 
Administrator further judges that it is not necessary based on existing 
epidemiologic evidence, to set a standard below 75 ppb. That is, the 
Administrator concludes that a standard level of 75 ppb is sufficiently 
below the SO2 levels in three cities where epidemiologic 
studies found statistically significant effects in multipollutant 
models with PM (i.e., 78, 82, and 150 ppb) to provide an adequate 
margin of safety given the uncertainty as to whether monitors in these 
study locations reflected the highest 1-hour daily maximum 
SO2 concentration across the entire study area. Thus, a 
standard set at a level of 75 ppb is likely further below the 99th 
percentile 1-hour daily maximum concentrations in these three study 
areas than the bare comparison of levels would otherwise indicate. 
Finally, the Administrator again notes that epidemiologic evidence 
below 75 ppb is more uncertain because studies below 75 ppb did not 
evaluate potential confounding of results in multipollutant models, and 
because these studies reported mixed results.
    Given the above considerations and the comments received on the 
proposal, the Administrator determines that the appropriate judgment, 
based on the entire body of evidence and information available in this 
review, and the related uncertainties, is a standard level of 75 ppb. 
She concludes that such a standard, with a 1-hour averaging time and 
99th percentile form, will provide a significant increase in public 
health protection compared to the current standards and would be 
expected to protect against the respiratory effects that have been 
linked with SO2 exposures in both controlled human exposure 
and epidemiologic studies. Specifically, she concludes that such a 
standard will limit 1-hour exposures at and above 75 ppb for those in 
susceptible populations that are at-risk of experiencing adverse health 
effects from short-term exposure to SO2. Such a standard 
will also maintain SO2 concentrations below those in 
locations where key U.S. epidemiologic studies have reported that 
ambient SO2 is associated with clearly adverse respiratory 
health effects, as indicated by increased hospital admissions and 
emergency department visits. She also notes that a 1-hour standard at a 
level of 75 ppb is expected to substantially limit asthmatics' exposure 
to 5-10 minute SO2 concentrations >= 200 ppb, thereby 
substantially limiting the adverse health effects associated with such 
exposures. Finally, the Administrator notes that a standard level of 75 
ppb is consistent with the consensus recommendation of CASAC.
    In setting the standard level at 75 ppb rather than at a lower 
level, the Administrator notes that a 1-hour standard with a level 
lower than 75 ppb would only result in significant further public 
health protection if, in fact, there is a continuum of serious, adverse 
health risks caused by exposure to SO2 concentrations below 
75 ppb. Based on the available evidence, the Administrator does not 
believe that such assumptions are warranted. Taking into account the 
uncertainties that remain in interpreting the evidence from available 
controlled human exposure and epidemiologic studies, the Administrator 
notes that the likelihood of obtaining benefits to public health with a 
standard set below 75 ppb decreases, while the likelihood of requiring 
reductions in ambient concentrations that go beyond those that are 
needed to protect public health increases.
    Therefore, the Administrator judges that a 1-hour SO2 
standard at 75 ppb is sufficient to protect public health with an 
adequate margin of safety. This includes protection with an adequate 
margin of safety for susceptible populations at increased risk for 
adverse respiratory effects from short-term exposures to SO2 
for which the evidence supports a causal relationship with 
SO2 exposures. The Administrator does not believe that a 
lower standard level is needed to provide this degree of protection. 
These conclusions by the Administrator appropriately consider the 
requirement for a standard that is neither more nor less stringent than 
necessary for this purpose and recognizes that the CAA does not require 
that primary NAAQS be set at a zero-risk level or to protect the most 
susceptible individual, but rather at a level that reduces risk 
sufficiently so as to protect the public health with an adequate margin 
of safety.
5. Retaining or Revoking the Current 24-Hour and Annual Standards
    This section discusses considerations related to retaining or 
revoking the current 24-hour and annual SO2 primary NAAQS. 
Specifically, this section summarizes the rationale for the 
Administrator's proposed decision regarding whether to retain or revoke 
the current standards (section II.F.5.a), discusses public comments 
related to whether to retain or revoke the current standards 
(II.F.5.b), and presents the Administrator's final conclusions 
regarding whether to retain or revoke the current standards (II.F.5.c).
a. Rationale for Proposed Decision
    As noted in the proposal (see section II.F.5), the REA recognized 
that the particular level selected for a new 99th percentile 1-hour 
daily maximum standard would have implications for deciding whether to 
retain or revoke the current 24-hour and annual standards. That is, 
with respect to SO2-induced respiratory morbidity, the lower 
the level selected for a 99th percentile 1-hour daily maximum standard, 
the less additional public health protection the current standards 
would be expected to provide. CASAC expressed a similar view following 
their review of the 2nd draft REA: ``Assuming that EPA adopts a one 
hour standard in the range suggested, and if there is evidence showing 
that the short-term standard provides equivalent protection of public 
health in the long-term as the annual standard, the panel is supportive 
of the REA discussion of discontinuing the annual standard'' (Samet 
2009, p. 15). With regard to the current 24-hour standard, CASAC was 
generally supportive of using the air quality analyses in the REA as a 
means of determining whether the current 24-hour standard was needed in 
addition to a new 1-hour standard to protect public health. CASAC 
stated: ``The evidence presented [in REA Table 10-3] was convincing 
that some of the alternative one-hour standards could also adequately 
protect against exceedances of the current 24-hour standard'' (Samet 
2009, p. 15).
    In accordance with the REA findings and CASAC recommendations 
mentioned above, the Administrator noted that 1-hour standards in the 
range of 50-100 ppb would have the effect of maintaining 24-hour and 
annual SO2 concentrations generally well below the levels of 
the current 24-hour and annual NAAQS (see REA Tables 10-3 and 10-4 and 
REA Appendix Tables D-3 to D-6). Thus, if a new 99th percentile 1-hour 
daily maximum standard was set in the proposed range of 50-100 ppb, 
then the Administrator proposed to revoke the current 24-hour and 
annual standards. However, as noted in the proposal, if a standard was 
set at a level >100 ppb and

[[Page 35549]]

up to 150 ppb, then the Administrator indicated that she would retain 
the existing 24-hour standard, recognizing that a 99th percentile 1-
hour daily maximum standard at 150 ppb would not have the effect of 
maintaining 24-hour average SO2 concentrations below the 
level of the current 24-hour standard in all locations analyzed (see 
REA Appendix Table D-4). Under this scenario, the Administrator would 
still revoke the current annual standard recognizing: (1) 99th 
percentile 1-hour daily maximum standards in the range of 50-150 ppb 
would maintain annual average SO2 concentrations below the 
level of the current annual standard (see REA Table 10-4 and REA 
Appendix tables D-5 and D-6); and (2) the lack of sufficient evidence 
linking long-term SO2 exposure to adverse health effects.
b. Comments on Retaining or Revoking the Current 24-Hour and Annual 
Standards
    As noted above, most industry groups were opposed to the proposed 
revisions to the SO2 NAAQS. However, some of these groups 
noted that if a 1-hour standard was adopted, then they would support 
revoking the current 24-hour and annual standards. State agencies 
generally supported revoking the current standards if a 1-hour standard 
was set in the proposed range, although NAACA, NESCAUM, and VT, while 
supportive of revoking the existing standards, also suggested that EPA 
explore setting a new 24-hour standard to minimize the potential that 
multiple hours within a day would exceed a 1-hour standard (see RTC 
document (EPA 2010), section IV). Groups which supported revoking the 
current 24-hour and annual standards (if a 1-hour standard was set in 
the proposed ranged) generally referenced the Administrator's rationale 
and CASAC advice described in the proposal (see section II.F.5).
    Public health (e.g., ALA, ATS) and environmental organizations 
(e.g., CBD, WEACT for Environmental Justice) were generally opposed to 
revoking the current 24-hour and annual standards. These groups 
generally concluded that the 24-hour standard should be revised while 
the annual standard should be retained. In support of this position, 
ALA et al., cited air quality information from the REA indicating that 
if air quality was simulated to just meet a 99th percentile 1-hour 
daily maximum standard in the proposed range of 50-100 ppb, then in 
some locations analyzed, 99th percentile 24-hour average SO2 
concentrations would be above concentrations (i.e., above 99th 
percentile 24-hour average concentrations) in cities where U.S. 
emergency department visit and hospital admission studies reported 
positive associations with SO2. In addition, many of these 
groups were opposed to revoking the current annual standard. In 
general, these groups concluded that given the uncertainties associated 
with SO2 exposure and long-term health effects, EPA should 
err on the side of being health protective and retain the existing 
annual standard. EPA responses to comments on whether the current 
standards should be retained or revoked are presented below as well as 
in section IV of the RTC document (EPA 2010).
    As stated in the REA and proposal, 99th percentile 24-hour average 
SO2 concentrations in cities where U.S. emergency department 
visit and hospital admission studies (for all respiratory causes and 
asthma; identified from Table 5-5 of the ISA) were conducted ranged 
from 16 ppb to 115 ppb (Thompson and Stewart, 2009). Moreover, as 
stated in the REA and proposal (see section II.F.2), effect estimates 
that remained statistically significant in multi-pollutant models with 
PM were found in cities with 99th percentile 24-hour average 
SO2 concentrations ranging from approximately 36 ppb to 64 
ppb. In its comments, ALA et al., stated (based on the air quality 
information in REA Appendix Table D-2) ``with a 1-hour 50 ppb 99th 
percentile standard, 7 counties would experience a 99th percentile 24-
hour concentration of 16 ppb or greater, the range found to be harmful 
in epidemiological studies. With an hourly standard of 100 ppb, 24 of 
30 counties would have 99th percentile 24-hour concentrations above 16 
ppb, with 1 county exceeding 36 ppb.'' Thus, these commenters generally 
maintained that a lowered 24-hour standard is needed to protect against 
these 24-hour SO2 concentrations.
    We disagree that a lowered 24-hour standard is needed to protect 
against 24-hour average SO2 concentrations of concern 
identified from cities where U.S. emergency department visit and 
hospital admission studies were conducted. As noted in detail in the 
REA, there is uncertainty as to whether the health effects reported in 
epidemiologic studies using 24-hour average SO2 
concentrations are in fact due to 24-hour average SO2 
exposures (REA, section 10.5.2). That is, when describing epidemiologic 
studies observing positive associations between ambient SO2 
and respiratory symptoms, the ISA stated ``that it is possible that 
these associations are determined in large part by peak exposures 
within a 24-hour period'' (ISA, section 5.2 at p. 5-5). Similarly, the 
ISA stated that: ``The effects of SO2 on respiratory 
symptoms, lung function, and airway inflammation observed in the human 
clinical studies using peak exposures further provides a basis for a 
progression of respiratory morbidity resulting in increased emergency 
department visits and hospital admissions'' and makes the associations 
observed in the epidemiologic studies ``biologica[lly] plausib[le]'' 
(id.). In contrast, evidence from controlled human exposure studies of 
5-10 minutes and epidemiologic studies using 1-hour daily maximum 
SO2 concentrations provided appreciably stronger evidence of 
respiratory morbidity effects following SO2 exposures <= 1-
hour.
    Given that respiratory morbidity effects following SO2 
exposure may be most related to averaging times <=1-hour, EPA found it 
most reasonable to consider the extent to which a 1-hour averaging 
time, given an appropriate form and level (which as discussed above, 
also substantially limits 5-minute benchmark exposures of concern; see 
sections II.F.2 and II.F.4), limited 99th percentile 24-hour average 
concentrations of SO2 in locations where emergency 
department visit/hospitalization studies reported that the 
SO2 effect estimate remained statistically significant in 
multi-pollutant models with PM (i.e., locations with 99th percentile 
24-hour average SO2 concentrations >=36 ppb). Considering 
this, we note that ALA et al., identified only one county with 99th 
percentile 24-hour average SO2 concentrations >=36 ppb given 
a 99th percentile 1-hour daily maximum standard at 100 ppb, and no 
counties >=36 ppb given a 99th percentile 1-hour daily maximum standard 
at 50 ppb. Thus, given a 99th percentile 1-hour daily maximum standard 
level at 75 ppb (i.e., the form and level selected for a new 1-hour 
SO2 standard), it is possible that no county in the ALA et 
al., analysis would have had a 99th percentile 24-hour average 
SO2 concentration >=36 ppb.
    With regard to the annual standard, we also disagree that this 
standard needs to be retained. First, the ISA found that ``[t]he 
evidence linking short-term SO2 exposure and cardiovascular 
effects, and morbidity and mortality with long-term exposures to 
SO2 is inadequate to infer a causal relationship.'' ISA, p. 
5-10. Thus, an annual standard is unnecessary to prevent long-term 
health effects. The remaining issue is whether such a standard provides 
further protection

[[Page 35550]]

against short-term effects, given the new one hour standard. We 
conclude that it does not. As noted in the proposal, our air quality 
information indicates that 1-hour standard levels in the range of 50-
100 ppb are estimated to generally keep annual SO2 
concentrations well below the level of the current annual standard. 
CASAC agreed. The panel stated: ``Assuming that EPA adopts a one hour 
standard in the range suggested, and if there is evidence showing that 
the short-term standard provides equivalent protection of public health 
in the long-term as the annual standard, the panel is supportive of the 
REA discussion of discontinuing the annual standard'' (Samet 2009, p. 
15). Taken together, this information indicates that retaining the 
annual standard would add no additional public health protection.
c. Administrator's Conclusions on Retaining or Revoking the Current 24-
Hour and Annual Standards
    In accordance with the REA findings and CASAC recommendations 
mentioned above, the Administrator concludes that a 1-hour standard at 
level of 75 ppb would have the effect of maintaining 24-hour and annual 
SO2 concentrations generally well below the levels of the 
current 24-hour and annual NAAQS (see REA Tables 10-3 and 10-4 and REA 
Appendix Tables D-3 to D-6). She also concludes that, as noted above in 
section II.F.2, a 1-hour standard at 75 ppb will likely limit 99th 
percentile 24-hour SO2 concentrations in U.S. locations 
where emergency department visit and hospital admission studies 
reported statistically significant associations in multi-pollutant 
models with PM. Finally, she notes the lack of sufficient health 
evidence to support an annual standard to protect against health 
effects associated with long-term SO2 exposure. Taken 
together, the Administrator concludes it appropriate to revoke the 
current 24-hour and annual standards.

G. Summary of Decisions on the Primary Standards

    For the reasons discussed above, and taking into account 
information and assessments presented in the ISA and REA as well as the 
advice and recommendations of CASAC, the Administrator concludes that 
the current 24-hour and annual primary standards are not requisite to 
protect public health with an adequate margin of safety. The 
Administrator also concludes that establishing a new 1-hour standard 
will appropriately protect public health with an adequate margin of 
safety, and specifically will afford requisite increased protection for 
asthmatics and other at-risk populations against an array of adverse 
respiratory health effects related to short-term (5 minutes to 24 
hours) SO2 exposure. These effects include decrements in 
lung function (defined in terms of sRaw and FEV1), increases 
in respiratory symptoms, and related serious indicators of respiratory 
morbidity including emergency department visits and hospital admissions 
for respiratory causes.
    Specifically, the Administrator is establishing a new short-term 
primary SO2 standard with a 1-hour (daily maximum) averaging 
time and a form defined as the 3-year average of the 99th percentile of 
the yearly distribution of 1-hour daily maximum SO2 
concentrations, and a level of 75 ppb. In addition to setting a new 1-
hour standard at 75 ppb, the Administrator is revoking the current 24-
hour and annual standards recognizing that a 1-hour standard set at 75 
ppb will have the effect of generally maintaining 24-hour and annual 
SO2 concentrations well below the levels of the current 24-
hour and annual standards.

III. Overview of the Approach for Monitoring and Implementation

    We received several comments regarding the approaches discussed in 
the proposal for monitoring and modeling for comparison to the proposed 
new 1-hour SO2 NAAQS, designations of areas as either 
attaining or not attaining the NAAQS, and implementation of the new 
NAAQS in State implementation plans (SIPs) that would ensure ultimate 
attainment of the new NAAQS in transitioning from the annual and 24-
hour NAAQS in a timely manner. These comments raised fundamental 
questions regarding our contemplated approaches in all three areas, and 
caused us to re-examine them and review their consistency with past 
practice under the SO2 NAAQS implementation program. After 
conducting that review, and in response to the public comments we are 
revising our general anticipated approach toward implementation of the 
new 1-hour NAAQS. This revised approach would better address: (1) The 
unique source-specific impacts of SO2 emissions; (2) the 
special challenges SO2 emissions present in terms of 
monitoring short-term SO2 levels for comparison with the 
NAAQS in many situations; (3) the superior utility that modeling offers 
for assessing SO2 concentrations; and (4) the most 
appropriate method for ensuring that areas attain and maintain the new 
1-hour SO2 NAAQS in a manner that is as expeditious as 
practicable, taking into account the potential for substantial 
SO2 emissions reductions from forthcoming national and 
regional rules that are currently underway.
    Below, we provide an overview of our revised approach to 
monitoring, and of our expected approaches to designations of areas, 
and implementation of the NAAQS. Due to the unique challenges presented 
by SO2, we do not expect that the anticipated approaches 
discussed below would be necessarily transferable to other NAAQS 
pollutant situations. For NAAQS pollutants other than SO2, 
air quality monitoring is more appropriate for determining whether all 
areas are attaining the NAAQS, and there is comparatively less 
dependence upon conducting refined modeling. Each of these subjects 
(i.e., our revised approach to monitoring, and our expected approaches 
to designations of areas, and implementation of the NAAQS) is further 
addressed later in the preamble, in sections IV, V and VI, 
respectively. Where specific public comments on the proposal are 
addressed and responded to, further details of the specific revised 
approaches are explained. In many respects, both the overview 
discussion below and the subsequent more detailed discussions explain 
our expected and intended future action in implementing the new 1-hour 
NAAQS--in other words, they constitute guidance, rather than final 
agency action--and it is possible that our approaches may continue to 
evolve as we, States, and other stakeholders proceed with actual 
implementation. In other respects, such as in the final regulatory 
provisions regarding the promulgated monitoring network, we are 
explaining EPA's final conclusions regarding what is required by this 
rule. We expect to issue further guidance regarding implementation, 
particularly concerning issues that may arise regarding the application 
of refined dispersion modeling under this revised approach for 
monitoring and implementation, and issues that States and other 
stakeholders may also ask us to address as we proceed toward various 
stages of ensuring attainment. EPA intends to solicit public comment 
prior to finalizing this guidance.
    The main necessary elements of implementing the new 1-hour NAAQS 
are: (1) An approach for assessing ambient concentrations to determine 
compliance with the NAAQS; (2) a process for using these assessments to 
designate areas relative to the new standard; and (3) the development 
of State plans that include control measures sufficient for ensuring 
the NAAQS is attained everywhere as expeditiously as possible, which we

[[Page 35551]]

believe should be no later than 2017. EPA's revised anticipated 
approach to determining compliance with the new SO2 NAAQS is 
consistent with our historical approach to SO2 designations 
and implementation through permits and emissions limitations, which 
involves the combined use of monitoring and modeling. The emphasis we 
would place on monitoring and modeling, compared with each other, under 
the revised expected approach is therefore significantly different than 
that in the approach discussed in the proposal, which was less in line 
with our historical practice for SO2, as the public comments 
highlighted.
    In the SO2 NAAQS proposal, we recommended a monitoring-
focused approach for comparison to the new NAAQS, featuring a two-
pronged monitoring network design. This included monitors in certain 
CBSAs based on a combination of population and SO2 emissions 
coupled with additional monitors within a State based on that State's 
contribution to national SO2 emissions. The resulting 
proposed network would have required approximately 348 monitors 
nationwide to be sited at the locations of maximum concentration. 
Numerous State and local government commenters expressed concerns 
regarding the burdens of implementing the proposed monitoring network 
and the sufficiency of its scope for purposes of identifying 
violations. These commenters contended that our proposed monitoring 
network was too small and insufficient to cover the range of 
SO2 sources, and yet too burdensome and expensive to expand 
to an adequate scale. Some of these commenters (the City of Alexandria, 
and the States of Delaware, North Carolina and Pennsylvania) suggested 
using modeling to determine the scope of monitoring requirements, or 
favored modeling over monitoring to determine compliance with the 
NAAQS.
    Partly in response to these comments, and after reconsidering the 
proposal's monitoring-focused approach in light of EPA's historical 
approach to SO2 NAAQS implementation and area designations 
decisions, we intend to use a hybrid analytic approach that would 
combine the use of monitoring and modeling to assess compliance with 
the new 1-hour SO2 NAAQS. We believe that some type of 
hybrid approach is more consistent with our historical approach and 
longstanding guidance toward SO2 than what we originally 
proposed. In addition, we believe that for a short-term 1-hour standard 
it is more technically appropriate, efficient, and effective to use 
modeling as the principle means of assessing compliance for medium to 
larger sources, and to rely more on monitoring for groups of smaller 
sources and sources not as conducive to modeling. We discuss the 
details of the final revised monitoring network requirements in section 
IV later in the preamble, but note here the relationship that the 
revised approach toward monitoring and modeling--taken partly in 
response to the public comments mentioned above--has to the other two 
general subject areas in implementation for which we are providing 
guidance, namely initial area designations and development of 
substantive implementation plans that ensure timely attainment and 
maintenance of the NAAQS. Our ultimate intention is to place greater 
emphasis on modeling than did the proposed rule as the most technically 
appropriate, efficient, and readily available method for assessing 
short-term ambient SO2 concentrations in areas with large 
point sources. This projected change in approach would necessarily 
result in a lesser emphasis on the less appropriate, more expensive, 
and slower to establish monitoring tool than did the proposed rule. 
Therefore, the minimum requirements for the SO2 monitoring 
network in this final rule are of a smaller scale than proposed, and we 
do not expect monitoring to become the primary method by which ambient 
concentrations are compared to the new 1-hour SO2 NAAQS.
    Instead, in areas without currently operating monitors but with 
sources that might have the potential to cause or contribute to 
violations of the NAAQS, we anticipate that the identification of NAAQS 
violations and compliance with the 1-hour SO2 NAAQS would 
primarily be done through refined, source-oriented air quality 
dispersion modeling analyses, supplemented with a new, limited network 
of ambient air quality monitors. Historically, we have favored 
dispersion modeling to support SO2 NAAQS compliance 
determinations for areas with sources that have the potential to cause 
an SO2 NAAQS violation, and we have explained that for an 
area to be designated as ``attainment,'' dispersion modeling regarding 
such sources needs to show the absence of violations even if monitoring 
does not show a violation. This has been our general position 
throughout the history of implementation of the SO2 NAAQS 
program. See, e.g., ``Air Quality Control Regions, Criteria, and 
Control techniques; Attainment Status Designations,'' 43 FR 40412, 
40415-16 (Sept. 11, 1978); ``Air Quality Control Regions, Criteria, and 
Control Techniques,'' 43 FR 45993, 46000-02 (Oct. 5, 1978); ``Air 
Quality Implementation Plans: State Implementation Plans; General 
Preamble,'' 57 FR 13498, 13545, 13547-48 (Apr. 16, 1992); ``Approval 
and Promulgation of State Implementation Plans; Call for Sulfur Dioxide 
SIP Revisions for Billings/Laurel, MT,'' 58 FR 41430 (Aug. 4, 1993); 
``Designation of Areas for Air Quality Planning Purposes; Ohio,'' 59 FR 
12886, 12887 (Mar. 18, 1994); ``Ambient Air Quality Standards, National 
and Implementation Plans for Sulfur Oxides (Sulfur Dioxide),'' 60 FR 
12492, 12494-95 (Mar. 7, 1995); ``Air Quality Implementation Plans; 
Approval and Promulgation: Various States: Montana,'' 67 FR 22167, 
22170-71, 22183-887 (May 2, 2002).
    Compared to other NAAQS pollutants, we would not consider ambient 
air quality monitoring alone to be the most appropriate means of 
determining whether all areas are attaining a short-term SO2 
NAAQS. Due to the generally localized impacts of SO2, we 
have not historically considered monitoring alone to be an adequate, 
nor the most appropriate, tool to identify all maximum concentrations 
of SO2. In the case of SO2, we further believe 
that monitoring is not the most cost-efficient method for identifying 
all areas of maximum concentrations. However, for some situations 
monitoring is well suited, and we therefore will require it to some 
extent, as further explained in section IV of the preamble. For 
example, monitoring may appropriately be relied upon to assess 
compliance with the NAAQS by groups of smaller sources and sources that 
may not be as conducive to modeling as are larger SO2 
sources.
    States will need to make any adjustments to the existing monitoring 
network to ensure that monitors meeting today's network design 
regulations for the new 1-hour NAAQS are sited and operational by 
January 1, 2013. We also expect to provide additional guidance 
regarding the application of refined dispersion modeling under this 
revised expected approach for implementation of the new SO2 
standard. Appendix A to the Guideline on Air Quality Models (Appendix W 
of 40 CFR part 51), Summaries of Preferred Air Quality Models, provides 
``key features of refined air quality models preferred for specific 
regulatory applications'' (see Appendix A to Appendix W of Part 51 at 
A.0(1)). Refined dispersion modeling, following our current Guideline 
on Air Quality Models with appropriate flexibility for use in 
implementation, is anticipated to better reflect and account

[[Page 35552]]

for source-specific SO2 impacts than the more limited 
monitoring-focused proposal. As noted above, EPA intends to solicit 
public comment prior to finalizing this guidance.
    Based on a revised, hybrid approach, we expect to implement the new 
SO2 standard in the following manner. In accordance with CAA 
section 107(d), EPA must designate areas as ``attainment,'' 
``nonattainment'' or ``unclassifiable'' for the new 1-hour 
SO2 NAAQS by June 2012 (i.e., two years following 
promulgation of the new NAAQS).\20\ State Governors are required to 
submit their initial area designation recommendations to EPA no later 
than June 2011. We expect that EPA's final area designation decisions 
in 2012 would be based principally on data reported from SO2 
monitors currently in place today, and any refined modeling the State 
chooses to conduct specifically for initial area designations.\21\ For 
these initial designations, we would expect to designate an area 
``nonattainment'' if either monitoring data or appropriate refined 
modeling results show a violation. Any area that has monitoring and 
appropriate modeling data showing no violations we would expect to 
designate as ``attainment.'' \22\ All other areas, absent monitoring 
data and air quality modeling results showing no violations, we would 
expect to initially designate as ``unclassifiable,'' as required by the 
Clean Air Act. The expected presumptive boundary for any area 
designated ``nonattainment'' would be the county boundary associated 
with the violation unless additional information provided to EPA 
demonstrates otherwise, as has been our general approach for other 
NAAQS pollutants. Any area initially designated ``nonattainment'' or 
``unclassifiable'' could request redesignation to ``attainment'' after 
an assessment based on air quality modeling, conducted in accordance 
with the new guidance, and available monitoring data indicates that the 
standard has been met, as well as meeting all other requirements of the 
CAA for redesignation to attainment.
---------------------------------------------------------------------------

    \20\ EPA is authorized by the Clean Air Act to take up to 3 
years to complete the initial area designations in the event that 
insufficient information is available to complete the designations 
within 2 years.
    \21\ Since three complete years of data from any newly sited 
monitors meeting the new monitoring network design criteria are not 
expected to be obtained until the end of 2015, any newly sited 
monitors will not play a role in EPA's initial area designations.
    \22\ EPA anticipates making the determination of when monitoring 
alone is ``appropriate'' for a specific area on a case-by-case 
basis, informed by that area's factual record, as part of the 
designations process. EPA would expect to address this issue for 
such areas by examining the historic treatment of the area with 
respect to prior SO2 designations as well as whether the 
area is one in which monitoring would be the more technically 
appropriate tool for determining compliance with the new 
SO2 NAAQS. An example of a situation in which monitoring 
may be the more preferred approach is a shipping port (non-point 
source or ``area'' source) that is not in close proximity to other 
significant stationary SO2 sources.
---------------------------------------------------------------------------

    This anticipated approach toward initial area designations is a 
change from the approach discussed in the proposal, and logically 
follows from our general change in approach to the use and utility of 
monitoring versus modeling for determining short-term SO2 
ambient concentrations. As public commenters pointed out, establishment 
and implementation of the proposed monitoring network would have been 
both too limited and too late to inform initial area designations, and 
the expense and burden of accelerating it and expanding it would have 
been severe for State implementing agencies. Given the time needed to 
establish monitors, it is not realistic to expect either such an 
expanded monitoring network or even the more reasonable limited network 
of the final rule to be the chief tool for informing initial 
designations.
    That means that some other approach is needed to inform initial 
designations of areas and other implementation decisions under the new 
SO2 NAAQS. In addition to using any valid data generated by 
existing monitors, refined dispersion modeling may inform designation 
and implementation decisions regarding sources that may have the 
potential to cause or contribute to a NAAQS violation. In order for 
modeling to be done on the scale sufficient to identify all areas that 
might violate the new 1-hour standard, EPA anticipates issuing guidance 
that addresses a variety of issues, such as how to identify and 
appropriately assess the air quality impacts of small SO2 
sources (e.g., those emitting less than 100 tons of SO2 per 
year) that may potentially cause or contribute to a violation of the 
new SO2 NAAQS. EPA expects that it will take more time for 
EPA to issue that guidance than is available in order to use it for the 
initial round of attainment designations. In addition to any smaller 
sources that might cause or contribute to NAAQS violations, States 
would need to model approximately 2000 larger sources across the 
country (i.e., sources that emit greater than 100 tons per year and are 
collectively responsible for about 99% of all SO2 emissions 
from point sources in the U.S.) to determine whether areas are 
attaining or not attaining the 1-hour standard. While these sources 
emitting 100 or more tons of SO2 per year represent the 
significant fraction of the total emissions from point sources in the 
U.S., smaller sources also have the potential to violate the new 
SO2 NAAQS.
    After receiving EPA's forthcoming modeling guidance, States might 
initially focus modeling assessments on these larger sources that have 
been subject to permitting requirements and are generally better 
characterized than smaller sources. But even this effort would entail a 
substantial burden on States, under a compressed timeline following 
EPA's issuance of further modeling guidance. Consequently, EPA does not 
believe that for this new 1-hour SO2 NAAQS it would be 
realistic or appropriate to expect States to complete such modeling and 
incorporate the results in initial designation recommendations, which 
under CAA section 107(d)(1)(A) must be submitted to EPA within 1 year 
of the promulgation of the 1-hour standard.
    The remaining issue, then, is how to most appropriately use a 
modified hybrid approach, and its constituent modeling and monitoring 
tools, in the implementation plan development process in order to 
ensure expeditious attainment and maintenance of the NAAQS. Under the 
CAA, all States must develop and submit to EPA State implementation 
plans (SIPs) to attain and maintain the new 1-hour SO2 
NAAQS. CAA section 110(a)(1) requires States, regardless of designation 
status, to adopt SIPs that provide for implementation, maintenance and 
enforcement of each primary NAAQS. Traditionally, for areas that were 
designated ``attainment'' or ``unclassifiable'', we accepted State 
submissions of prevention of significant deterioration (PSD) permitting 
programs and other ``infrastructure'' SIP elements contained in CAA 
section 110(a)(2) as being sufficient to satisfy the section 110(a)(1) 
SIP submission requirement. However, due to our recognition here that 
monitoring is not generally the most appropriate or effective tool for 
assessing compliance with the new 1-hour SO2 NAAQS, that 
additional guidance from EPA on conducting refined modeling for the new 
1-hour NAAQS is anticipated to support our expected implementation 
approach, and that considerable time and resources may be needed to 
fully identify and properly characterize all SO2 sources 
(including those emitting less than 100 tons of SO2 per 
year) that may potentially cause or contribute to a violation of the 
new SO2 NAAQS, we also had to assess how and when to best 
use modeling as the primary method in implementation.

[[Page 35553]]

    The approach that EPA expects to take, which is described in 
sections V and VI of the preamble, is consistent with the language of 
the Clean Air Act and would accommodate the time needed for an accurate 
assessment of ambient air quality levels for the 1-hour SO2 
standard. Section 107(d)(1) requires areas to be designated 
``attainment'' if they meet the standard, ``nonattainment'' if they do 
not meet the standard or contribute to a nearby violation, or 
``unclassifiable'' if they cannot be designated on the basis of 
available information. EPA's expected approach would enable us to make 
the appropriate designation decision required by the CAA, based on the 
record of information that will be before EPA regarding each area. 
Areas would be designated ``nonattainment'' if either available 
monitoring data or modeling shows that a violation exists, or 
``attainment'' if both available monitoring data and modeling indicate 
the area is attaining. All other areas would be designated 
``unclassifiable,'' as required by section 107(d)(1)(A).
    We currently anticipate that our projected post-designation 
implementation approach would look to robust CAA section 110(a)(1) 
SIPs, which have sometimes been previously referred to as 
``maintenance'' or ``infrastructure'' SIPs but for the new 
SO2 NAAQS would serve as substantive ``attainment'' SIPs. 
Our current thinking is that, to be approved by EPA, such plans would 
need to provide for attainment and maintenance of the new 1-hour 
SO2 NAAQS as expeditiously as practicable, which we expect 
to be no later than five years after initial designation (or 
approximately August 2017) in all areas of the State, including any 
area initially designated ``nonattainment,'' and also including any 
area designated ``unclassifiable'' that has SO2 sources with 
the potential to cause or contribute to a violation of the NAAQS. The 
CAA establishes deadlines for States to submit these plans to EPA.\23\ 
State plans that address areas designated as ``nonattainment'' (i.e., 
``nonattainment area SIPs'') are due within 18 months from the 
effective date of the designation, under CAA section 192. EPA 
anticipates that this deadline would be February 2014. State plans 
addressing all other areas (i.e., ``maintenance SIPs'') are due within 
3 years following the promulgation of the new NAAQS, or June 2013, 
under CAA section 110(a)(1).
---------------------------------------------------------------------------

    \23\ The schedule for State plans addressing areas designated 
``nonattainment'' is governed by CAA section 191. The schedule for 
State plans for all other areas, including areas designated 
``unclassifiable'' and ``attainment,'' is governed by CAA section 
110(a)(1).
---------------------------------------------------------------------------

    Section 110(a)(1), unlike section 192, does not specify a maximum 
deadline by which States are required to show they have met the 
requirements to implement, maintain, and enforce a NAAQS. EPA believes, 
however, that August 2017 is the latest date by which areas should show 
they have achieved attainment and maintenance of the standard because 
this deadline is the same as would be required for areas designated 
nonattainment in June 2012. It is therefore presumptively reasonable as 
it is identical to the period Congress provided for nonattainment areas 
to reach attainment. Moreover, EPA notes that the maintenance SIPs will 
be due in June 2013, rather than in February 2014, giving States and 
sources at least as much time between SIP development and submission 
and the date by which attainment should be achieved as they would have 
had the area been designated nonattainment in 2012. These section 
110(a)(1) SIPs would be able to rely on modeling reflecting any 
SO2 reductions that we expect to result before the 
attainment date from compliance with the rules EPA expects to 
promulgate before 2013, (including technology-based standards under CAA 
section 112(d) for certain source categories emitting large amounts of 
SO2 such as Electric Generating Units and industrial 
boilers, and revised rules establishing further limits on 
SO2 emitted by sources in upwind States which contribute 
significantly to downstream States' inability to attain or maintain the 
PM2.5 NAAS (the so-called Clean Air Interstate Replacement rule)). 
Thus, we intend that a State's section 110(a)(1) SIP may account for 
projected emissions reductions, including any from national and 
regional rules that are promulgated before these SIP submissions, 
provided that those reductions occur under a schedule that ensures 
attainment as expeditiously as practicable. We expect that date to be 
no later than 5 years from the date of initial designation or August 
2017.
    Under this anticipated approach, attainment SIPs for nonattainment 
areas would have to include enforceable emissions limitations, 
timetables for compliance, and appropriate testing/reporting to assure 
compliance, and demonstrate attainment through air quality modeling for 
all sources contributing to monitored and modeled violations, or that 
have the potential to cause or contribute to a violation of the NAAQS. 
The SIPs under section 110(a)(1) would need to demonstrate through 
refined air quality modeling that any source or group of sources that 
have the potential to cause or contribute to a violation of the NAAQS 
are, or will be, sufficiently controlled to ensure timely attainment 
and maintenance of the NAAQS. We would expect this to include any 
individual sources with the potential to emit 100 or more tons per year 
of SO2, and other sources that may also cause or contribute 
to violations of the new SO2 NAAQS. We expect to develop 
guidance for the States' use on how best to identify and assess the 
impact of sources that may have this potential. As mentioned 
previously, we intend to provide an opportunity for notice and comment 
on this guidance before finalizing it.
    EPA again notes that it anticipates several forthcoming national 
and regional rules, such as the pending Industrial Boilers MACT 
standard under CAA section 112(d), that are likely to require 
significant reductions in SO2 emissions over the next 
several years. A limited qualitative assessment based on the results of 
preliminary modeling of some sample facilities indicates that well 
controlled sources should meet the new SO2 NAAQS (see Brode 
2010b). Exceptions could include unique sources with specific 
characteristics that contribute to higher ambient impacts (short stack 
heights, complex terrain, etc.). These national and regional rules are 
expected to lead to SO2 reductions that will help achieve 
compliance with the new SO2 NAAQS prior to 2017. If, upon 
EPA review of submitted SIPs that rely upon those reductions or other 
local controls, it appears that States will nevertheless fail to attain 
the NAAQS as expeditiously as practicable (and no later than August 
2017), the Clean Air Act provides authorities for EPA to solve such 
failure, including, as appropriate, disapproving submitted SIPs, re-
designating unclassifiable areas to nonattainment, issuing SIP calls, 
and promulgating FIPs.
    For the reasons discussed above, EPA has determined that it is 
appropriate and efficient to principally use modeling to assess 
compliance for medium to larger sources, and to rely more on monitoring 
for groups of smaller sources and sources not as conducive to modeling. 
EPA's revised monitoring network requirements have been developed to be 
consistent with this approach. However, EPA is still considering how 
monitoring and modeling data would be used together in specific 
situations to define attainment and nonattainment boundaries and under 
what circumstances it may be appropriate to rely on monitoring data 
alone to make attainment determinations. EPA intends

[[Page 35554]]

to address these issues as it develops implementation guidance.
    In light of the new approach that EPA intends to take with respect 
to implementation of the SO2 NAAQS, EPA intends to solicit 
public comment on guidance regarding modeling, and also solicit public 
comment on additional implementation planning guidance, including the 
content of the maintenance plans required under section 110(a)(1) of 
the Clean Air Act. EPA also notes that State monitoring plans and the 
SIP submissions that States will make will also be subject to public 
notice and comment.

IV. Amendments to Ambient Monitoring and Reporting Requirements

    In this section of the preamble, we describe the proposal, the 
public comments that we received on the proposed monitoring and 
reporting requirements, and the final requirements for the 
SO2 monitoring network. We are modifying our proposed 
approach to the amount of monitoring to require following consideration 
of public comments and a review of our historic practice in assessing 
compliance with the SO2 NAAQS. As we explain above in 
section III, we will use a hybrid approach that combines monitoring and 
modeling, using each of these analytic tools where they are most 
appropriate and effective. This approach and its requirements are 
intended to support the revised SO2 NAAQS, described in 
section II above. For a short-term 1-hour standard, dispersion modeling 
of stationary sources will generally be more technically appropriate, 
efficient, and effective because it takes into account fairly 
infrequent combinations of meteorological and source operating 
conditions that can contribute to peak ground-level concentrations of 
SO2. Even an expansive monitoring network could fail to 
identify all such locations. Consequently, we have revised the scope of 
the monitoring network, reflecting a modified and expanded set of 
objectives. This section also describes and explains the final 
requirements for the new SO2 Federal Reference Method (FRM), 
and the SO2 network design, monitoring objectives, data 
reporting, and data quality objectives that support the revised primary 
SO2 NAAQS.

A. Monitoring Methods

1. Requirements for SO2 Federal Reference Method (FRM)
    The proposal to promulgate an automated SO2 FRM was 
based on a need to update the cumbersome existing manual wet-chemistry 
(pararosaniline) method to a continuous-type automated method that can 
readily provide 1-hour SO2 measurement capability. See 74 FR 
at 64846-849. The following paragraphs provide background, rationale, 
and the final changes to the automated SO2 Federal Reference 
Method (FRM) and to the associated performance specifications for 
automated SO2 analyzers.
a. Proposed Ultraviolet Fluorescence SO2 FRM and Its 
Implementation
    FRMs, set forth in several appendices to 40 CFR Part 50, serve (1) 
To provide a specified methodology for definitively measuring 
concentrations of ambient air pollutants for comparison to the NAAQS in 
Part 50, and (2) to provide a standard of comparison for determining 
equivalency of alternative pollutant measurement methods that can be 
used in lieu of the FRM for such monitoring.
    The FRM for measuring SO2 in the ambient air was 
promulgated on April 30, 1971 in conjunction with the first primary 
SO2 NAAQS (36 FR 8196). This SO2 FRM is specified 
in Appendix A of Part 50 and identified as the pararosaniline manual 
method. See generally 74 FR at 64846. In the interim, EPA has 
designated many SO2 methods as equivalent methods (FEMs), 
most of which are based on the ultraviolet fluorescence (UVF) measuring 
technique. Id. In fact, virtually all SO2 monitoring data 
are now obtained with FEMs that use the UVF technique.
    In light of this, EPA proposed to establish a new automated 
SO2 FRM based on UVF--the same measurement technique 
employed by FEM analyzers now in widespread use by most State and local 
monitoring agencies and having the measurement capability needed to 
implement the proposed 1-hour SO2 NAAQS. FRM analyzers using 
this UVF technique can provide the needed detection limits, precision, 
and accuracy and fulfill other purposes of an FRM, including use as an 
appropriate standard of reference for testing and designation of new 
FEM analyzers. At proposal, EPA specified the new method in 
performance-based form, describing a generic reference measurement 
principle and associated calibration procedure in a new Appendix A-1 to 
40 CFR Part 50. Associated performance requirements applicable to 
candidate automated SO2 analyzers (both FRMs and FEMs) were 
proposed in 40 CFR Part 53.
    EPA also proposed retaining the existing manual pararosaniline FRM 
for SO2. Although EPA recognized that the existing method is 
cumbersome for one-hour measurements, it is capable of making 
measurements of 1 hour or even 30 minute periods. 74 FR at 64846; see 
also Part 50 Appendix A at 1.1 (``[t]he method is applicable to the 
measurement of ambient SO2 concentrations using sampling 
periods ranging from 30 minutes to 24 hours''). Supersession of the 
existing manual FRM, as defined in Sec.  53.16, would require not only 
withdrawal of that existing FRM but also the cancellation of the 
designations of all existing SO2 FEMs. Loss of the use of 
these FEM analyzers would leave State and local monitoring agencies 
with no approved SO2 monitors until new FRM and FEM 
analyzers could be designated under the new FRM. The resulting costs 
and disruptions to monitoring agencies is unnecessary because the 
current SO2 FEMs readily and accurately measure (and report) 
one-hour ambient measurements. See 74 FR at 64847. Accordingly, EPA 
concluded that supersession of the existing FRM was not warranted, 
given the costs and disruptions which would occur to State monitoring 
programs and the limited benefits from such an action given the 
suitability of the in-use FEMs. Id. at 68646; see also section 
53.16(b)(1) stating that in exercising its discretion as to whether to 
proceed with supersession of an FRM, EPA will consider the benefits (in 
terms of requirements and purposes of the Act) from specifying a new 
reference method, potential economic consequences of such supersession 
for State and local monitoring agencies, and disruption to State and 
local air quality monitoring programs. Instead, EPA proposed to add the 
new UVF FRM while retaining the existing FRM for some period of time to 
support the continued approval of existing SO2 FEM 
analyzers.
b. Public Comments on the Proposed FRM and Implementation
    EPA received comments from State and local groups (e.g., City of 
Houston, Houston-Galveston Area Council, KY, NC, NY, PA, SC, SD, and 
WI) and industry (e.g., AirQuality Research and Logistics (AQRL), 
Consumers Energy, ExxonMobil, Montana Sulfur and Chemical Company, Inc. 
(MSCC), and the Utility Air Regulatory Group (UARG)), all generally 
supporting EPA's proposal to adopt the proposed automated UVF as an 
FRM. For example, South Dakota supported adding the UVF SO2 
method as an additional FRM and stated that this method is currently 
being used in the network and will reduce the cost of implementing the 
new monitoring

[[Page 35555]]

requirements for this rule. The UARG stated that the proposal to 
specify a different FRM to judge compliance is entirely reasonable, and 
UARG generally supported the proposed specifications for a new FRM but 
maintained that the current FRM could not be used along with a new FRM. 
ExxonMobil stated that it supports ``* * * EPA allowing monitoring 
agencies to choose mobile monitoring that meets monitoring quality 
requirements.'' AQRL stated that ``EPA is correct in choosing to 
designate [promulgate] a new (automated) FRM for measurement of 
SO2.''
    EPA did not receive any public comments opposing the proposed 
automated UVF SO2 FRM but did receive a few technical 
comments on specific provisions of the method. EPA proposed use of an 
inlet line particle filter as a requirement for new UVF SO2 
FRM analyzers, believing that use of a particle filter is advantageous 
to prevent interference, malfunction, or damage to the analyzer from 
particles in the sampled air. The State of Missouri questioned this 
requirement, noting that such a filter can sometimes cause problems and 
that filter requirements for other FRM and FEM analyzers have been 
analyzer-specific depending on the manufacturer's stipulation. EPA 
believes, however, that for new SO2 FRM analyzers, the 
benefits and uniformity provided by a mandatory filter requirement 
outweigh possible disadvantages of such a filter.
    Missouri also suggested that the language of proposed Sections 
4.1.1 and 4.1.2 regarding calibration system flow rate requirements 
were somewhat confusing, and that the high (50-100 ppm) concentration 
requirement for the calibration standard specified in Section 4.1.6.1 
is sometimes a problem. In response to these comments, the language of 
Sections 4.1.1 and 4.1.2 has been clarified, and the concentration of 
the standard specified in Section 4.1.6.1 has been reduced to 10 ppm.
    EPA received a number of comments from States (e.g., NC, NYSDEC, 
PA, SC, and SD) that supported the EPA proposed plan of temporary 
retention of the existing wet-chemistry pararosaniline FRM and for FEMs 
approved based on that method. For example, Pennsylvania stated 
``[t]his methodology should enable State and local agencies to continue 
using their existing monitoring equipment and [thereby] avoid large 
capital fund outlays for samplers and ultimately avoid any delays in 
collecting data that would be comparable to the proposed new primary 
sulfur dioxide NAAQS.'' North Carolina requested ``* * * that the EPA 
maintain the current reference method for at least an additional 10 
years.'' Wisconsin and the Center for Biological Diversity (CBD) 
suggested expeditiously phasing out the existing manual SO2 
FRM.
    In contrast, however, EPA also received comments from industry that 
opposed the retention of the existing pararosaniline FRM while 
promulgating a new automated UVF FRM. In particular, UARG stated ``* * 
* having two FRMs specified for a given NAAQS--is not viable,'' 
pointing out that there is only one FRM for each NAAQS under the 
present standards, a result UARG appears to believe is legally 
mandated.
    EPA disagrees with this comment. First, there is nothing in the Act 
that mandates a single FRM for each NAAQS. Section 109 of the Act, in 
fact, does not address this issue at all. Second, as noted previously, 
there are sound policy reasons for not withdrawing the existing FRM at 
this time. Therefore, EPA sees no legal or other obstacle in adding a 
new automated UVF FRM while retaining the existing manual FRM.
    UARG further maintained that EPA provided no support for its 
statement that the existing FEMs, which constitute the bulk of the 
existing SO2 monitoring network, are adequate for the 
current and proposed new SO2 NAAQS. UARG also stated that 
``although the FEMs may be adequate for many other purposes, they may 
only be used to judge compliance with the 1-hour NAAQS if they are 
shown to qualify as FRMs or FEMs under the new FRM definition.''
    EPA disagrees with this comment also. In answer to UARG's second 
point, it is not necessary that these existing FEMs be re-designated as 
FRMs pursuant to the new automated FRM to continue their approved use. 
There is no legal impediment to such continued use, since they are (and 
will continue to be) FEMs approved based on an FRM that adequately 
measures one-hour ambient SO2 concentrations. Nor is there 
any technical impediment to the continued use of these FEMs, given that 
they are automated continuous monitoring methods capable of measuring 
SO2 concentrations ranging from a few minutes to a 1-hour 
period. The existing FEMs in the network use the same UVF technology as 
the proposed (and now final) automated FRM and have been reporting 1-
hour monitoring data for decades. These FRMs have been tested against 
the test and performance requirements of Part 53, which are designed 
specifically to test such continuous methods. Further, the proposed 
SO2 method performance specifications for the standard 
measurement range were derived from data submitted in FEM applications 
for analyzers that were subsequently designated as FEMs. Therefore, 
these FEMs are technically and legally sound to judge compliance with 
the one-hour NAAQS.
    EPA has clarified the regulatory text so that the rules state 
unambiguously that both SO2 FRMs apply to the new one-hour 
standard (as well as to the 24-hour and annual standards so long as 
they are retained), as do all presently-designated FEMs.
c. Conclusions on Ultraviolet Fluorescence SO2 FRM and 
Implementation
    We are finalizing the proposed new automated SO2 FRM, 
which is based on UVF technology, with the following minor technical 
changes: The language of Sections 4.1.1 and 4.1.2 has been clarified, 
and the minimum concentration of the calibration standard specified in 
Section 4.1.6.1 has been reduced to 10 ppm. The new FRM is codified as 
Appendix A-1 to 40 CFR Part 50 and titled ``Reference Measurement 
Principle and Calibration Procedure for the Measurement of Sulfur 
Dioxide in the Atmosphere (Ultraviolet Fluorescence Method).'' EPA is 
retaining the previously existing manual pararosaniline SO2 
FRM for the time being and re-codifying it as Appendix A-2 to 40 CFR 
Part 50. However, EPA plans to rescind this manual FRM at a future time 
when new SO2 FRM analyzers have adequately permeated State 
monitoring networks.
2. Requirements for Automated SO2 Methods
a. Performance Specifications for Automated Methods
    In association with the proposal to adopt a new automated FRM, EPA 
proposed to update the performance-based designation requirements for 
FEM SO2 analyzers currently specified in 40 CFR Part 53. As 
noted in the proposal preamble (74 at 64846), these requirements were 
established in the 1970's, based primarily on the wet-chemical 
measurement technology available at that time. Those initial 
requirements have become significantly outdated and need to be modified 
to match current technology, particularly because they would apply to 
new SO2 FRM analyzers under the proposed new FRM. The better 
instrumental performance available with the proposed new UVF FRM 
technique allows the performance requirements in Part 53 to be made 
more stringent for

[[Page 35556]]

both FRM and FEM SO2 analyzers. Updating these performance 
requirements is needed to ensure that, going forward, all new 
SO2 monitors will have improved performance.
    EPA solicited comments on the proposed new performance requirements 
for automated SO2 methods that were included in Table B-1 
(Performance Specifications for Automated Methods) of Part 53. We 
proposed revised performance specifications for noise, lower detectable 
limit, interference equivalent, zero drift, span drift, lag time, rise 
time, fall time, and precision. EPA proposed to reduce the allowable 
noise limit from 5 to 1 ppb, the lower detectable limit from 10 to 2 
ppb, the interference equivalent limits from 20 ppb to 
5 ppb for each interferent, and from 60 ppb to 20 ppb for 
the total of all interferents, the zero drift limit from 20 
to 4 ppb, the lag time limit from 20 to 2 minutes, both 
rise and fall time limits from 15 to 2 minutes, and the precision 
limits from 15 ppb to 2 percent of the upper range limit. EPA further 
proposed to eliminate the requirements for span drift at 20% of the 
upper range limit. In addition, to address the need for more sensitive, 
lower measurement ranges for SO2 analyzers, EPA proposed a 
separate set of performance requirements that would apply specifically 
to narrower measurement ranges, i.e. ranges extending from zero to 
concentrations less than 0.5 ppm. Other minor changes were proposed in 
the wording of a few sections of Part 53 Subparts A and B, including 
provision for alternate data recording devices in Sec.  53.21 to 
supplement the older language relating specifically to strip chart 
recorders.
b. Public Comments
    EPA received a number of comments from industry (AQRL and UARG) and 
from the multi-State organization NESCAUM regarding the proposed 
interferent limit requirements listed in Table B-1. UARG submitted 
comments supportive of all the proposed requirements for the new UVF 
SO2 FRM, except for the proposed total interferent limits of 
20 ppb. UARG acknowledged that EPA proposed to reduce the total 
interferent level substantially from 60 ppb to 20 ppb, but maintained 
that the proposed level of 20 ppb is still too high because it amounts 
to 20%-40% of the levels being considered for the NAAQS (50-150 ppb). 
AQRL recommended limiting ``* * * each interferent to no more than 
3 ppb and total interference to no more than 12 ppb.'' 
NESCAUM recommended tightening the nitric oxide (NO) interference limit 
from 100:1 to 300:1 (i.e., one third of the proposed value of 5 ppb). NESCAUM states that the proposed interferent value of 
5 ppb results in substantial NO interference at sites with 
low SO2 levels in urban areas.
    EPA revisited the issue of the interferent equivalent limit for 
SO2 analyzers in context of the above comments and 
reconsidered what is reasonably feasible with current technology. We 
reviewed the current instrument specifications and test data submitted 
for numerous SO2 FEM applications. We also took into account 
that the test concentrations of most of these interferents are 
substantially higher than the concentrations normally observed in 
ambient air. EPA considered lowering the testing concentrations of 
these interferents, which would have correspondingly lowered the 
interferent equivalent for each analyte. However, EPA took a more 
conservative approach and retained the existing test concentrations for 
H2S, NO2, NO, O3, m-xylene, and water 
vapor. Based on this review, we found that it is not feasible to 
further lower the limit requirement for these interferents below 5 ppb. However, in response to the NESCAUM comment, EPA 
determined that the interferent equivalent limit requirement for NO 
interference could be reduced to 3 ppb (166:1) for the new, 
lower measurement range to reduce possible NO interference at sites 
with low SO2 levels in urban area.
    In regard to the total limit for all interferent equivalents for 
SO2 analyzers, EPA notes that many of the interferents for 
which testing is required (specified in Table B-3 of Part 53) would 
likely react with each other and would thus not co-exist in ambient air 
at the specified test concentrations. Therefore, EPA determined that 
the limit requirement for total interference equivalent can be 
eliminated, and Table B-1 now reflects this change.
    EPA received comment from AQRL on the existing span drift 
requirement for SO2 analyzers specified in Table B-1. AQRL 
recommended lowering the span drift requirement at 80% URL to 2.5%, 
stating that ``ambient air monitors in the 21st century should be able 
to hold span drift to no more than 2.5% under the 
conditions specified in EPA testing * * *.'' Based on information from 
FEM testing laboratories and manufacturers' data (EPA, 2009c), EPA 
largely agrees with this comment and concludes that the span drift 
requirement at 80% can be lowered to 3%. Table B-1 has been 
changed to include this revised limit.
    EPA received comment from the State of Wisconsin suggesting that 
the proposed revised provisions of section 53.21 (Test conditions) be 
further changed to more specifically recognize use of digital recorders 
for obtaining test results rather than maintaining the tie to analog 
strip chart recorder technology. EPA acknowledges that industry has 
moved away from strip chart recording technology to digital data 
recording. However, the proposed language of Sec.  53.21 calls for a 
graphic representation of analyzer responses to test concentrations to 
facilitate visual examination of test results and allows any 
``alternative measurement data recording device'' as long as it can 
provide such a graphic representation. Describing the analog strip 
chart recorder in this section provides an appropriate model to help 
define the type of graphic representation needed for the Part 53 tests. 
EPA believes that the proposed language of Sec.  53.21 is adequately 
broad to permit digital or other types of data recording devices.
c. Conclusions for Performance Specifications for SO2 
Automated Methods
    Based on typical performance capabilities of current UVF analyzers 
and manufacturers' actual testing data, we are keeping the limit for 
each interference equivalent for SO2 analyzers at 5 ppb. However, we are lowering the interference equivalent 
requirement for NO to 3 ppb for the lower measurement 
range. A footnote denoting this specific requirement is being added to 
Table B-1. We are eliminating the total interference equivalent 
requirement for SO2 analyzers, and Table B-1 is being 
revised to incorporate this change.
    The 24-hour span drift at 80% of the upper range limit for 
SO2 analyzers is being lowered to 3% in Table B-
1 to be in line with current technology. Also, unrelated to 
SO2, a typographical error for the noise requirement for CO 
analyzers is being corrected to 0.5 ppm in Table B-1.
    Finally, information on generation and verification of test 
concentrations for naphthalene was inadvertently omitted from Table B-
2, Test Atmospheres, even though it was added as a required interferent 
test in our proposal. Therefore, we are adding that information for 
naphthalene. Also in Table B-2, we are correcting the verification 
information for nitric oxide.

B. Network Design

    Ambient SO2 monitoring data are collected by State, 
local, and Tribal monitoring agencies (``monitoring agencies'') in 
accordance with the monitoring requirements contained in

[[Page 35557]]

40 CFR parts 50, 53, and 58. A monitoring network is generally designed 
to measure, report, and provide related information on air quality data 
as described in 40 CFR Part 58. To ensure that the data from the 
network is accurate and reliable, the monitors in the network must meet 
a number of requirements including the use of monitoring methods that 
EPA has approved as Federal Reference Methods (FRMs) or Federal 
Equivalent Methods (FEMs) (discussed in some detail above in section 
IV.A), focusing on particular monitoring objectives, and following 
specific siting criteria, data reporting, quality assurance and data 
handling rules or procedures.
    With the revision to the SO2 NAAQS, which establishes a 
new 1-hour averaging period intended to limit short-term exposures that 
may occur anywhere in an area, EPA evaluated the existing network to 
determine if it was adequate to support the revised SO2 
NAAQS. A significant fact for ambient SO2 concentrations is 
that stationary sources are the predominant emission sources of 
SO2 and the peak, maximum SO2 concentrations that 
may occur are most likely to occur nearer the parent stationary source, 
as noted in the ISA (ISA, 2-1), section II.A.1 above, and in section 
IV.B.1 below. According to the 2005 National Emissions Inventory, there 
are 32,288 sources (facilities) emitting SO2, of which 1,928 
are emitting 100 tons per year (tpy) or more. In the proposal (74 FR 
64851), EPA had anticipated requiring 348 source-oriented monitors in 
the network design based on a population and emissions metric and a 
State's emissions contribution to the National Emissions Inventory 
(NEI). In response to this proposal, EPA received numerous comments 
arguing that the required number of monitors in the network would be 
too small. Other commenters argued that expanding the monitoring to an 
adequate scale would impose a large burden and expense on the States. 
Some commenters referred to SO2 modeling in their 
submissions as an addition or alternative to monitoring. Consequently, 
as part of developing a balanced response to these comments, we 
revisited how we had historically dealt with SO2 for various 
purposes including designations and implementation through permitting 
and emissions limitations. As explained in section III, this has been 
realized through a combined monitoring and modeling approach. As set 
out below, and in sections III, VI, and VII, our ultimate intention is 
to utilize a combined monitoring and modeling approach, a hybrid 
analytic approach, to assess compliance with the revised SO2 
NAAQS.
    As a result of this contemplated hybrid analytic approach, the 
minimum number of monitors required in the network through this 
rulemaking is reduced to approximately 163 monitors from the 
approximated 348 monitors that were proposed. This section of the 
preamble includes a discussion of the proposal, the comments received, 
and the details of and the rationale for the final changes to the 
SO2 network design requirements.
1. Approach for Network Design
a. Proposed Approach for Network Design
    To fully support the proposed revision to the SO2 NAAQS, 
EPA indicated the need to identify where short-term, peak ground-level 
concentrations--i.e., concentrations from 5 minutes to one hour (or 
potentially up to 24 hours)--may occur. Given that large stationary 
sources are the predominant source of emissions, monitoring short-term, 
peak ground-level concentrations would require monitors to be sited to 
assess impacts of individual or groups of sources and therefore be 
source-oriented in nature. As a result, under a monitoring-focused 
approach, EPA proposed a two-pronged monitoring network of all source-
oriented monitors. However, due to the multiple variables that affect 
ground level SO2 concentrations from individual or groups of 
sources, including stack heights, emission velocities, stack diameters, 
terrain, and meteorology, EPA could not specify a source specific 
threshold, algorithm, or metric by which to require monitoring. The 
design of the proposed network represented a primarily monitoring-
focused approach to assess compliance with the primary SO2 
NAAQS.
    In preparation for the SO2 NAAQS proposal, EPA conducted 
an analysis of the approximately 488 SO2 monitoring sites 
operating during calendar year 2008 (Watkins and Thompson, 2009). This 
analysis indicated that approximately ~ 35% of the monitoring network 
was addressing locations of maximum (highest) concentrations, likely 
linked to a specific source or group of sources. Meanwhile, just under 
half (~ 46%) of the sites were reported to be for the assessment of 
concentrations for general population exposure. These data allowed EPA 
to conclude that the network \24\ was not properly focused to support 
the revised NAAQS (under the assumption that source-oriented monitoring 
data would be the primary tool for assessing compliance with the 
NAAQS). As a result, EPA proposed a two-pronged monitoring network (74 
FR 64850), based on the premise of a monitoring-focused approach, with 
minimum requirements for: (1) Monitors in urban areas where there is a 
higher coincidence of population and emissions, utilizing a Population 
Weighted Emissions Index (PWEI), and (2) monitors in States based on 
each State's contributions to the national SO2 emissions 
inventory. In addition, all the monitors in the network would be sited 
at locations of expected maximum hourly concentrations and therefore 
likely be source-oriented. This two-pronged network would have resulted 
in a minimum of approximately 348 monitors nationwide \25\ providing 
data for comparison with the 1-hour standard and supporting its 
implementation.
---------------------------------------------------------------------------

    \24\ Prior to this rulemaking there were no minimum monitoring 
requirements, except for those required at the multi-pollutant 
National Core (NCore) monitoring sites. The monitoring rule 
promulgated in 2006 (71 FR 61236) removed minimum monitoring 
requirements (except for those NCore stations). This change was 
largely driven by the fact that there was no longer an 
SO2 nonattainment problem under the then-existing 
standards. However, this logic does not apply to the revised primary 
SO2 NAAQS.
    \25\ Required monitor estimates were based on 2008 Census 
estimates and the 2005 National Emissions Inventory.
---------------------------------------------------------------------------

    Under the first prong of the network design, EPA proposed that the 
ambient SO2 monitoring network account for SO2 
exposure by requiring monitors in locations where population and 
emissions may lead to higher potential for population exposure to peak 
hourly SO2 concentrations. In order to do this, EPA 
developed a Population Weighted Emissions Index (PWEI) that uses 
population and emissions inventory data at the CBSA \26\ level to 
assign required monitoring for a given CBSA (with population and 
emissions being obvious relevant factors in prioritizing numbers of 
required monitors). The PWEI for a particular CBSA was proposed to be 
calculated by multiplying the population (using the latest Census 
Bureau estimates) of a CBSA by the total amount of SO2 
emissions in that CBSA. The CBSA SO2 emission value would be 
in tons per year, and calculated by aggregating the county level 
emissions for each county in a CBSA. We would then divide the resulting 
product of CBSA population and CBSA SO2 emissions by 
1,000,000 to provide a PWEI value, the units of

[[Page 35558]]

which would be millions of people-tons per year.
---------------------------------------------------------------------------

    \26\ CBSAs are defined by the U.S. Census Bureau, and are 
comprised of both Metropolitan Statistical Areas and Micropolitan 
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------

    We proposed that the first prong of the SO2 network 
design require monitors in CBSAs, according to the following criteria. 
For any CBSA with a calculated PWEI value equal to or greater than 
1,000,000, a minimum of three SO2 monitors would be required 
within that CBSA. For any CBSA with a calculated PWEI value equal to or 
greater than 10,000, but less than 1,000,000, a minimum of two 
SO2 monitors would be required within that CBSA. For any 
CBSA with a calculated PWEI value equal to or greater than 5,000, but 
less than 10,000, a minimum of one SO2 monitor would be 
required within that CBSA. EPA estimated that the proposed criteria 
would have resulted in 231 required sites in 131 CBSAs.
    Under the second prong of the network design, EPA proposed to 
require a monitor or monitors in each State, allocated by State-level 
SO2 emissions. This prong of the network design was intended 
to allow a portion of the overall required monitors to be placed where 
needed, independent of the first prong of the network design, inside or 
outside of CBSAs. EPA proposed to require monitors, using State 
boundaries as the geographic unit for allocation purposes, in 
proportion to a State's SO2 emissions, i.e., a State with 
higher emissions would have been required to have a proportionally 
higher number of monitors. The proposed percent contribution of 
individual States would have been based on the most recent NEI, with 
SO2 emissions being aggregated by State. The number of 
required monitors per State would correspond to every one percent 
(after rounding) of each State's contribution to the national 
SO2 inventory. EPA also proposed that each State have at 
least one monitor required as part of this second prong, even if a 
particular State contributes less than 0.5% of the total anthropogenic 
national emissions inventory. As a result, the proposed second prong 
would have required approximately 117 monitoring sites based on State-
level SO2 emissions in the most recent NEI, which at the 
time of the proposal, was the 2005 NEI.
    EPA also stated in the proposal that the multi-pollutant National 
Core (NCore) monitoring sites would not have counted towards meeting 
the proposed monitoring requirements. However, data from the NCore 
would be compared to the NAAQS even though NAAQS comparisons are not 
the sole objective of NCore monitors. The monitoring rule promulgated 
in 2006 (71 FR 61236) and codified at 40 CFR Part 58 and its Appendices 
established the NCore multi-pollutant network requirement to support 
integrated air quality management data needs. In particular, NCore 
sites are intended to provide long-term data for air quality trends 
analysis, model evaluation, and, for urban sites, tracking metropolitan 
air quality statistics. To do this, NCore sites are required to measure 
various pollutants, including SO2, but they are not source 
oriented monitoring sites, and therefore are not likely to be the 
location of maximum expected concentration in an area. NCore sites are 
intended to provide data representing concentrations at the broader 
neighborhood and urban spatial scales. These reasons were the rationale 
justifying why SO2 monitors at NCore stations would not have 
been part of the minimum monitors required under the proposed network.
b. Alternative Network Design
    EPA also solicited comment on an alternative network design, 
including alternative methods to determine the minimum number of 
monitors per State (74 FR 64854). EPA requested comment on whether a 
screening approach for assessing the likelihood of a NAAQS exceedance 
could be developed and serve as a basis for determining the number and 
location of required monitors. In particular, EPA requested comment on 
whether it should utilize existing screening tools such as AERSCREEN or 
SCREEN3, which use parameters such as effective stack height and 
emissions levels to identify facilities with the potential to cause an 
exceedance of the proposed standard. For that set of sources, EPA could 
then require States to conduct more refined modeling (using the 
American Meteorological Society (AMS)/EPA Regulatory Model (AERMOD)) to 
determine locations where monitoring should be conducted. Any screening 
or refined modeling would likely be carried out by States by using EPA 
recommended models and techniques referenced by 40 CFR Part 51, 
Appendix W, which provides guidance on air quality modeling. Such 
screening or refined modeling uses facility emission tonnage, stack 
heights, stack diameters, emission temperatures, emission velocities, 
and accounts for local terrain and meteorology in determining where 
expected maximum hourly concentrations may occur. In using this 
approach, EPA would then require States to locate monitors at the point 
of maximum concentration around sources identified as likely causing 
NAAQS exceedances. EPA also noted that this alternative approach would 
not distinctly use population as a factor for where monitors should be 
placed.
c. Public Comments
    EPA received many comments on the proposed network design and the 
alternative network design approaches. Based on comments that were 
clear enough on the issue, EPA believes the commenters' positions on 
the network design approach generally fell into one of three 
categories: (1) Those who supported the two-prong approach, but 
suggested some modification to it, (2) those who supported the 
alternative network design, and (3) those who suggested other concepts 
for the network instead of the two approaches EPA presented in the 
proposal.
    The commenters who generally supported the two-prong network 
design, but suggested some modification included some State and local 
air agencies (e.g. NACAA and nine other State groups or agency 
commenters) and industry groups (e.g. AQRL, ACC, and eight other 
commenters). Of this group, some of the State and local air agencies 
specifically commented on how EPA should modify one or both of the 
prongs of the proposed network design. Some particular individual 
suggestions will be addressed here and those comments not addressed 
here will be addressed in the response to comment document. However, 
one recurring suggestion from the State and local agency commenters in 
this group was that the network design leads to some duplicative and/or 
unneeded monitoring, and therefore they requested that EPA include a 
provision to ``waive'' the monitoring network design requirements in 
situations where minimum monitoring requirements appear duplicative or 
unnecessary. In particular, NACAA stated that it ``* * * is concerned 
that the two pronged approach in the proposed regulation will lead to 
duplicative monitoring in some areas and require monitors in areas 
where monitors are not needed. EPA recognizes the potential for 
duplicative monitoring, but the proposal does not permit the removal of 
duplicative monitors.'' This NACAA comment was echoed by some of the 
other States who commented on the proposed approach (e.g. AK, FL, IL, 
NC, SC, and WI). The industry commenters were also generally supportive 
of the two-prong approach, with some making general suggestions to 
modify the network design. For example, AQRL stated that the ``* * * 
network design proposal seems to provide the flexibility for States and 
the EPA regions to work together to arrive at the adequate monitoring 
network.'' AQRL also

[[Page 35559]]

suggests that ``a State/local area should have the option to shutdown 
or relocate any site mandated [by monitoring requirements] if measured 
design values at the site are less than 75% of the selected standard 
level.'' Multiple industry commenters (e.g. API, LEC, and RRI Energy) 
expressed concern that the proposed network design had no monitoring 
required specifically to measure background concentrations of 
SO2. Dow Chemical suggested that EPA maintain some of the 
existing monitors that characterize population exposure and other non-
source oriented sites for trends analysis.
    Those commenters who did not support the proposed network design, 
and instead generally supported the concepts of the alternative network 
design, include public health and environmental groups (e.g. ALA, CBD, 
EDF, EJ, NRDC, and SC) and the States of Delaware and Iowa. In 
particular, ALA, EDF, NRDC, and SC stated ``* * * the proposed 348 
monitors are a grossly inadequate number to detect peak concentrations 
from the nearly 2,000 major sources that emit more than 100 tons per 
year of sulfur dioxide * * *'' and that ``it is most appropriate to use 
screening tools to site all the monitors in the areas of highest 
expected concentration * * *'' The Center for Biological Diversity, 
with regard to the proposed network design, stated that ``* * * a 
number of communities with very significant SO2 emissions 
will not have any monitoring stations at all * * *'' Further, the State 
of Iowa claimed that ``the proposed design of the SO2 
ambient monitoring network provides insufficient assurances that the 
public is protected from the health effects of SO2 
exposure,'' and suggested that ``* * * the final rule contain 
provisions that require monitors to be sited only at locations where 
dispersion modeling indicates that the NAAQS is violated.''
    Commenters also suggested other concepts for the monitoring network 
design in lieu of the approaches discussed in the proposal. NESCAUM, 
NYSDEC, and PADEP, all suggested using an emissions-only approach to 
trigger required monitoring instead of using the PWEI to require 
monitors in an area. For example, NYSDEC suggests that the proposed 
approach, using the PWEI, is ``* * * not more predictive than using 
emissions data alone.'' NYSDEC went on to recommend that monitors be 
required in CBSAs with aggregated emissions of 50,000 tons per year or 
more and that ambient monitoring be considered for point sources with 
20,000 tons per year. PADEP made several suggestions on network design, 
including monitoring in any CBSA ``where there is a sulfur dioxide 
source or combination of sources within 50 miles emitting a total of at 
least 20,000 tons of SO2 per year * * *''
    Among all three groups of commenters discussed above, there was a 
subset of commenters who specifically mentioned using modeling in some 
form. Modeling was a component of the alternative network design, where 
monitors would be required based on screening models and possibly 
refined modeling of individual sources. EPA also expected that under 
the proposed approach, many States would use modeling as a quantitative 
analysis tool to site required monitors. Finally, source modeling is a 
critical element for PSD and facility permitting. In their comments, 
NESCAUM recommended that EPA allow modeling to be used in conjunction 
with monitoring data to better determine nonattainment areas. North 
Carolina advocated that EPA require SO2 sources, without 
specifying a threshold size for sources, to perform modeling to 
demonstrate that fence-line (ambient) air does not exceed the NAAQS due 
to that particular source's emissions. North Carolina went on to 
suggest that if a source's modeling showed an exceedance of the NAAQS, 
the source could ``then be required to reduce emissions from the stack, 
install continuous emissions monitoring (CEM) in the stack itself, or 
require a fence-line monitor at the target facility.'' North Carolina 
also stated, in the context of discussing its own PSD program, that 
``the costs for modeling are small compared to the costs for 
monitoring.'' Sierra Club stated that EPA should ``* * * employ modern 
computer models to determine whether areas should be designated 
nonattainment because they do not meet the NAAQS in areas where there 
is no monitor.'' From these comments, EPA gathers that some public 
commenters find modeling a useful tool and support the use of modeling 
to ascertain ambient concentrations of SO2.
2. Modeling Ambient SO2 Concentrations
    EPA considered the various and sometimes competing concerns raised 
by the commenters including duplicative monitoring, lack of adequate 
number of monitors, insufficient flexibility, the monitoring burden, 
and the modeling suggestions. EPA considered its historic practice and 
the analytic tools available to arrive at a balanced approach that took 
into account these concerns. In the past, EPA used a combination of 
modeling and monitoring for SO2 during permitting, 
designations, and re-designations in recognition of the fact that a 
single monitoring site is generally not adequate to fully characterize 
ambient concentrations, including the maximum ground level 
concentrations, which exist around stationary SO2 sources. 
With representative and appropriate meteorological and other input 
data, refined dispersion models are able to characterize air quality 
impacts from the modeled sources across the domain of interest on an 
hourly basis with a high degree of spatial resolution, overcoming the 
limitations of an approach based solely on monitoring. By simulating 
plume dispersion on an hourly basis across a grid of receptor 
locations, dispersion models are able to estimate the detailed spatial 
gradients of ambient concentrations resulting from SO2 
emission sources across a full range of meteorological and source 
operating conditions. The 1-hour NAAQS is intended to provide 
protection against short-term (5 minute to 24 hour) peak exposures, 
whether they result from typical meteorological conditions or not. 
Because ambient monitors are in fixed locations and a single monitor 
can only represent impacts which occur at the location of the monitor, 
a single monitor cannot identify all instances of peak ground-level 
concentrations if, for example, different wind directions on various 
days cause peak ground-level concentrations in different areas that do 
not overlap. The uncertainty associated with this limitation is much 
higher for an hourly standard than a long-term standard due to the 
higher degree of spatial and temporal variability associated with peak 
hourly impacts (discussed in ISA chapters 2.4 and 2.5). This limitation 
of ambient monitoring may be true even if the source-oriented ambient 
monitor was sited with the aid of modeling data, since the model is 
less reliable at predicting the precise location of maximum impacts 
than at predicting the distribution of impacts across the full modeling 
domain, and no single monitor can be sited in a way to always measure 
the peak ground-level SO2 concentrations that may be 
occurring in the area around a source.
    EPA's Guideline on Air Quality Models, Appendix W to 40 CFR Part 
51, provides recommendations on modeling techniques and guidance for 
estimating pollutant concentrations in order to assess control 
strategies and determine emission limits. These recommendations were 
originally published in April 1978 and were incorporated by reference 
in the PSD regulations, 40 CFR sections 51.166 and

[[Page 35560]]

52.21 in June 1978 (43 FR 26382). The purpose of Appendix is to promote 
consistency in the use of modeling within the air quality management 
process. Appendix W is periodically revised to ensure that new model 
developments or expanded regulatory requirements are incorporated. The 
most recent revision to Appendix W was published on November 9, 2005 
(70 FR 68218), wherein EPA adopted AERMOD as the preferred dispersion 
model for a wide range of regulatory applications in all types of 
terrain. AERMOD is a steady-state plume dispersion model that employs 
hourly sequential preprocessed meteorological data to simulate 
transport and dispersion from multiple point, area, or volume sources 
for averaging times from one hour to multiple years, based on an 
advanced characterization of the atmospheric boundary layer. AERMOD 
also accounts for building wake effects (i.e., downwash) on plume 
dispersion. To support the promulgation of AERMOD as the preferred 
model for near-field dispersion (50 km or less), EPA evaluated the 
performance of the model across a total of 17 field study data bases 
(Perry, et al., 2005; EPA, 2003), including several field studies based 
on model-to-monitor comparisons of SO2 concentrations from 
operating power plants.
    EPA anticipates that additional guidance for States may be needed 
to clarify how to conduct dispersion modeling under Appendix W to 
support the implementation of the new 1-hour SO2 NAAQS. 
Although AERMOD is identified as the preferred model under Appendix W 
for a wide range of applications and will be appropriate for most 
modeling applications to support the new SO2 NAAQS, Appendix 
W allows flexibility to consider the use of alternative models on a 
case-by-case basis when an adequate demonstration can be made that the 
alternative model performs better than, or is more appropriate than, 
the preferred model for a particular application.
    In conclusion, EPA believes that a hybrid analytic approach that 
uses a combination of modeling and monitoring information addresses the 
varying and competing concerns expressed by the commenters. Modeling 
large emission sources, along with smaller sources with the potential 
to violate the NAAQS, deals effectively with the concern that the 
monitoring network is not large enough to account for all sources that 
could have high ambient SO2 concentrations. EPA believes 
that more SO2 sources will ultimately be directly addressed 
through modeling alone versus the number of sources which would have 
been monitored under the proposed network design (which proposed a 
minimum of 348 monitors). Because modeling provides a technically 
appropriate and efficient method to identify locations of maximum 
concentrations attributable to the major stationary SO2 
sources, in the final network design (discussed below in section 
IV.B.4), EPA is not requiring that monitors must be in locations of 
expected maximum concentration, and thus, typically source-oriented. 
Instead, monitors required under the final network design now can 
address multiple monitoring objectives (discussed in IV.B.3 below), 
with fewer number of monitors required overall than the number 
estimated in the proposal. The flexibility that States now have, where 
relatively fewer required monitors may be sited to meet multiple 
objectives, effectively addresses concerns about duplicative monitoring 
and the need for waivers, the need for measuring background 
concentrations, and that emissions data rather than the PWEI could be 
more predictive of high ambient SO2 concentrations as a 
basis on which to require monitoring. The comments that suggested the 
use of modeling, along with an examination of past practice, resulted 
in the change to a hybrid approach where we use both modeling and 
monitoring to assess ambient SO2 concentrations.
3. Monitoring Objectives
    Because EPA contemplates an ultimate approach that combines both 
monitoring and modeling, the monitor objectives of the final network 
design are now broadened to include assessment of source impacts, 
highest concentration, population exposure, general background 
concentrations, SO2 transport, and long-term trends. The 
following paragraphs provide background, rationale, and details for the 
final changes to monitoring objectives.
a. Proposed Monitoring Objectives
    EPA proposed that all minimally required monitoring sites in the 
proposed two-prong network design be sited at locations of expected 
maximum 1-hour concentrations, which would also likely discern 5-minute 
peaks. EPA noted that in general, such locations would be close to 
larger emitting sources (in tons per year) and/or areas of relatively 
high emissions densities where multiple sources may be contributing to 
peak ground-level concentrations. As a result, the proposed monitoring 
network would have been comprised primarily of source-oriented 
monitors. EPA also proposed that when selecting monitoring sites from 
among a pool of candidate locations (which would be source-oriented 
under the proposed network design), States prioritize these sites based 
on where the maximum expected hourly concentrations would occur in 
greater proximity to populations. EPA solicited general comments on the 
role of population exposure in the site selection process.
b. Public Comments
    Commenters discussed a variety of issues on the subject of 
monitoring objectives including the importance of considering 
population exposure, the need for flexibility in monitor placement, 
monitoring for background concentrations, monitoring for long term 
trends analysis, and characterizing potential long-range transport of 
SO2.
    EPA received many comments from States (e.g., NACAA, DE, IL, IN, 
MO, SD, WI), the public health group ATS, and industry (e.g., AQRL, 
Consumers Energy, Dominion, Dow, EPRI, ExxonMobil, Montana Sulfur and 
Chemical, NPRA, Portland Cement, Rio Tinto, and UARG) suggesting that 
required monitors account for, or be focused on, population exposure. 
EPA also received many comments from States (e.g., NACAA, NESCAUM, FL, 
IL, IN, IA, MI, OH, SC, and WI) and industry (e.g., API, Dow, and 
TxOGA) asking for more flexibility in (source-oriented) monitor 
placement with regard to both the target source and the physical 
location of a monitor relative to that source. For example NACAA stated 
that ``for source oriented monitors, placement at the point of 1-hour 
maximum concentration must be realistic and flexible. EPA must allow 
agencies to determine the most scientifically defensible location, 
while taking into account potential exposures and access to locations 
with adequate siting.'' Wisconsin stated that ``* * * monitor siting 
should be balanced toward population-based monitors with a preference 
toward maximum exposure.'' Wisconsin added that ``* * * placing 
monitors at the maximum downwind location does not necessarily result 
in effective protection of public health.''
    EPA received a number of comments on background monitoring \27\ 
from industry (API, LEC, and RRI Energy) and from the State of South 
Carolina. API stated that ``because the monitors provide background 
concentrations

[[Page 35561]]

needed to model impacts of new sources or sources undergoing major 
modification in addition to providing data for judging compliance with 
the NAAQS, it is important that some monitors be sited in a manner 
suitable for assessing this background.'' API went on to state that ``* 
* * EPA should encourage States to site an appropriate number of area-
wide monitors for use in establishing ambient background levels of 
SO2.'' South Carolina states that ``to better support the 
monitoring objectives, in particular those improving our understanding 
and context for the source oriented monitoring data, the monitoring 
requirements must include the ability for States to address the needs 
for area and regional background concentration measurements.''
---------------------------------------------------------------------------

    \27\ Background monitoring can be considered to be 
representative of ambient concentrations upwind of (and therefore 
not typically influenced by) a geographic area such as an urban 
area, or of an individual or group of emission sources.
---------------------------------------------------------------------------

    A number of commenters, including States (e.g., Missouri, NESCAUM, 
Ohio, and South Carolina), citizens (Valley Watch at the Atlanta public 
hearing), the CBD, and Dow, commented on SO2 transport and 
related cross-boundary monitoring. Dow stated that ``SO2 
distribution has long been known as an interstate issue with the vast 
majority of SO2 sources being power plants and other fossil 
fuel combustion facilities. These facilities are more likely to impact 
distant areas than local areas and the resultant ground-level 
concentrations are often minimal.'' Ohio stated that, under the 
proposed approach, ``* * * it is likely that OH, WV, KY, and IN will 
find sources along the Ohio River which could result in monitors being 
located across the river from each other.'' In such situations, Ohio 
asserts that ``States are capable of working with our neighbors to 
determine which State would be in the best position to site and operate 
a monitor.''
c. Conclusions on Monitoring Objectives
    A hybrid analytical approach, as noted above in section III and 
IV.B.1 would ultimately make the most appropriate use of available 
tools such as modeling and monitoring. Thus, unlike under the proposal, 
the monitoring network will not have to be focused solely at locations 
of expected maximum concentration relative to an SO2 source 
given the anticipated adoption of a hybrid analytical approach. The 
final network design is intended to be flexible to meet multiple 
monitoring objectives, most of which were identified in the public 
comments. Ambient monitoring networks are generally designed to meet 
three primary monitoring objectives, as listed in 40 CFR Part 58 
Appendix D, Section 1, including: (1) Providing air pollution data to 
the general public in a timely manner, (2) support compliance with 
ambient air quality standards and emissions strategy development, and 
(3) support air pollution research studies (which includes health 
studies and research). In order to support these air quality management 
objectives, monitoring networks can have a variety of monitoring sites 
that can be sited, as necessary, to characterize (a) emission sources 
(i.e., source-oriented monitoring), (b) the highest concentration in an 
area, (c) population exposure, (d) general background concentrations, 
(e) regional transport, and (f) welfare-based impact.
    In light of the approach described in section III and further in 
IV.B.1 above, EPA is finalizing an SO2 network design, with 
broadened objectives, which EPA believes will address the concerns 
noted in the public comments above, particularly those regarding siting 
flexibility, population exposure, cross-boundary impacts, and the need 
for the network to address multiple monitoring objectives. The final 
network design requires that any SO2 monitors required in a 
particular CBSA as determined based on PWEI values, discussed below in 
section IV.B.4, shall satisfy the minimum monitoring requirements if 
they are sited at locations where they can meet any one or more of the 
following objectives (see Part 58 Appendix D section 4.4.2 as added by 
today's final rule):
    (1) Source-Oriented Monitoring: This is accomplished with a monitor 
sited to determine the impact of significant sources or source 
categories on air quality. In some situations, such monitoring sites 
may also be classified as high concentration sites (discussed below). 
Examples of source-oriented monitors include those sited to capture or 
assess peak ground-level concentrations from one or more major 
SO2 sources, or those sited in an area with multiple smaller 
sources with overlapping plumes.
    (2) Highest Concentration: This is assessed by a monitor sited to 
measure the highest concentrations expected to occur in the area 
covered by the network. Such a location may, or may not, also be 
considered a source-oriented location (discussed above). Depending on 
the case, this location is representative of the highest concentration 
occurring across a relatively homogeneous area with spatial scales 
typically ranging from tens of meters up to four kilometers.\28\
---------------------------------------------------------------------------

    \28\ Spatial scales are defined in 40 CFR Part 58 Appendix D, 
section 1. Each scale is a description of the physical dimensions of 
an air parcel nearest a monitoring site throughout which pollutant 
concentrations are reasonably similar.
---------------------------------------------------------------------------

    (3) Population Exposure: This is assessed by a monitor sited to 
measure typical concentrations in areas of (relatively) high population 
density. Some examples are a monitor placed in an area of elevated or 
high SO2 concentrations that also has a high population 
density, an area that might be included in public health studies, or in 
areas with vulnerable and susceptible populations.
    (4) General Background: This is assessed by placing a monitor in an 
area to determine general background concentrations. Such locations 
might be considered to be representative of ambient concentrations 
upwind of (and therefore not typically influenced by) a geographic area 
such as an urban area, or of an individual or group of emission 
sources. EPA notes that although a required monitor is allowed to be 
sited to assess background concentrations, the required monitor is not 
allowed to be sited outside of the parent CBSA (whose PWEI value 
triggered required monitoring, discussed in section IV.B.4 and IV.B.5). 
If a State believes that there is a need to conduct background 
monitoring outside of CBSAs with required monitoring, EPA notes that 
States always have the prerogative to conduct monitoring above the 
minimum requirements in any location the State believes is appropriate.
    (5) Regional Transport: This is assessed by placing a monitor in a 
location to determine the extent of regional pollutant transport. Such 
locations could be either upwind or downwind of urban areas, 
characterizing the entry or exit of the pollutant in a region, 
respectively. EPA notes that although a required monitor is allowed to 
be sited to assess regional transport, the required monitor is not 
allowed to be sited outside of the parent CBSA (whose PWEI value 
triggered required monitoring, discussed in section IV.B.4 and IV.B.5). 
If a State believes that there is a need to conduct background 
monitoring outside of CBSAs with required monitoring, EPA notes that 
States always have the prerogative to conduct monitoring above the 
minimum requirements in any location the State believes is appropriate.
    In regard to the public comments expressing concerns on the issue 
of cross-boundary transport, i.e., a source on one side of a political 
boundary contributes to peak ground-level concentrations on the other 
side of that boundary, EPA will allow a required monitor to be placed 
outside of the parent CBSA (whose PWEI value triggered monitoring, 
discussed in section IV.B.4 and IV.B.5) under one

[[Page 35562]]

particular condition. A source-oriented monitor may be sited outside of 
the parent CBSA, whose PWEI value triggered required monitoring, if 
that monitor is characterizing the location of expected maximum 
concentration of a source inside that parent CBSA. If a State chooses 
to exercise this flexibility in source-oriented monitor siting, the 
State must provide clear rationale for their choice in their annual 
monitoring plan, which is subject to EPA regional approval. If the 
source-oriented monitor is to be placed in another State, such as the 
example provided by the State of Ohio in the public comments above, the 
two States are responsible for collaboration on the location and 
operation of that monitoring site.
    Further, due to the broadened objectives of the final network 
design, EPA also is finalizing the provision that an NCore 
SO2 monitor within a CBSA (where a CBSAs PWEI value 
triggered required monitoring) can be counted towards meeting the 
minimum monitoring requirements in this rulemaking (discussed in 
section IV.B.4) because they can meet some of the expanded objectives 
of the network. NCore sites are intended to provide long-term data for 
air quality trends analysis, model evaluation, and, for urban sites, 
tracking metropolitan air quality statistics, and therefore are 
appropriate to allow to count towards minimum monitoring requirements 
under the revised monitoring scheme.
    Finally, EPA strongly encourages State and local air agencies to 
consider using required monitoring, as appropriate, to characterize 
those sources which are not as conducive to dispersion modeling and to 
assess population exposure. Sources that are not conducive to 
dispersion modeling include (1) sources classified as non-point sources 
(a.k.a. ``area-sources'') such as shipping ports, (2) a source situated 
in an area of complex terrain and/or situated in a complex 
meteorological regime, and (3) locations that have multiple, relatively 
small sources with overlapping plumes.
4. Final Monitoring Network Design
    The use of a hybrid analytic approach (discussed above in section 
III and IV.B.1) makes it unnecessary for the final monitoring network 
design to be distinctly focused on monitoring locations of expected 
maximum concentration (and thus be primarily source-oriented), as 
discussed in section IV.B.3 above. Instead, with the dual use of 
modeling and monitoring for designations, the final monitoring network 
is designed to provide flexibility for required monitors to address the 
multiple monitoring objectives just discussed in the preceding section. 
This flexibility in monitoring objectives is in response, in part, to 
the many public comments received from States (e.g., NACAA and six 
other States), industry (API, EPRI, UARG, and eight other groups), and 
from the American Thoracic Society (ATS), urging EPA to ensure that 
some or all of the required monitors be sited and suited to 
characterize population exposure and, from many of these same 
commenters, to allow flexibility in implementing the siting 
requirements for the monitors. Under a hybrid approach, and the 
different monitoring objectives resulting thereof, the final monitoring 
network design also does not need to be a two-prong approach like the 
one proposed. Therefore, EPA is adopting a modified version of the 
first prong of the proposed network design, which will use PWEI values 
to require monitors in certain CBSAs where there is increased 
coincidence of population and SO2 emissions. There is no 
second prong in the final network design by which monitors are required 
based on a State's individual contribution to the national 
anthropogenic SO2 inventory, as was proposed.
    The final monitoring network design requires monitoring in CBSAs 
based on calculated PWEI values, where a PWEI shall be calculated (as 
discussed in section IV.B.5 below) for each CBSA. For any CBSA with a 
calculated PWEI value equal to or greater than 1,000,000, a minimum of 
three SO2 monitors are required within that CBSA. This 
requirement remains the same as proposed. For any CBSA with a 
calculated PWEI value equal to or greater than 100,000, but less than 
1,000,000, a minimum of two SO2 monitors are required within 
that CBSA. For any CBSA with a calculated PWEI value equal to or 
greater than 5,000, but less than 100,000, a minimum of one 
SO2 monitor is required within that CBSA. EPA has adjusted 
the thresholds for requiring one or two monitors in a CBSA and the 
rationale for this adjustment is explained more fully below in section 
IV.B.5. As just explained in section III.B.3, these monitors shall be 
sited to meet one or more of a number of monitoring site objectives, 
including the assessment of source impacts, highest concentrations, 
population exposure, general background, and regional transport. EPA 
believes that the monitors required within these PWEI breakpoints 
provide a reasonable minimum number of monitors in a CBSA, where there 
is a relatively increased coincidence of population and SO2 
emissions and therefore increased potential for exposures, because we 
are directly accounting for both population and emissions that exist in 
individual CBSAs.\29\ EPA estimates that these minimum monitoring 
criteria (based on 2008 population and 2005 NEI data) require 163 
monitors within 131 CBSAs. EPA also intends for SO2 monitors 
at NCore stations to satisfy these minimum monitoring requirements. 
Based on analysis of proposed and approved NCore sites (as of April 
2010), all of which are scheduled to be operational no later than 
January 1, 2011, EPA estimates that 52 of the total 80 SO2 
monitors at NCore stations are within the 131 CBSAs that have required 
monitors based on their PWEI values. As a result, EPA estimates that 
between these minimum monitoring requirements and the NCore network, 
there will be at least 191 SO2 monitors operating across the 
country.
---------------------------------------------------------------------------

    \29\ The rationale for finalizing the use of the PWEI and the 
number of monitors required through its application are discussed in 
section III.B.4.
---------------------------------------------------------------------------

5. Population Weighted Emissions Index
    In the proposal, EPA had introduced a metric based on population 
and emissions as a basis for locating monitors in the network. EPA 
anticipated that this metric would characterize the potential for 
exposure based on the proximity of source emissions to populations. The 
following paragraphs provide background, rationale, and details for the 
final changes of the calculation and use of the Population Weighted 
Emissions Index in determining minimum monitoring requirements.
a. Proposed Use of the Population Weighted Emissions Index
    In the proposed network design approach, which utilized a two-prong 
network design, EPA created the Population Weighted Emissions Index 
(PWEI) in an attempt to focus monitoring resource where there was a 
higher proximity of population and SO2 emissions. In effect, 
areas with higher PWEI values have higher potential for population 
exposure to short-term SO2 emissions. EPA proposed that the 
PWEI be calculated using population and emissions inventory data at the 
Core Based Statistical Area (CBSA) \30\ level to assign required 
monitoring for a given CBSA, with population and emissions being the 
relevant factors. To calculate the PWEI for a particular CBSA, using

[[Page 35563]]

the latest Census Bureau estimates, the population of a CBSA must be 
multiplied by the total amount of SO2 emissions in that 
CBSA. The CBSA emission value is in tons per year (using the latest 
available National Emissions Inventory [NEI] data), and is calculated 
by aggregating the county level emissions for each county in a CBSA. We 
then divide the resulting product of CBSA population and CBSA 
SO2 emissions by 1,000,000 to provide a PWEI value in more 
manageable units of millions of people-tons per year.
---------------------------------------------------------------------------

    \30\ CBSAs are defined by the U.S. Census Bureau, and are 
comprised of both Metropolitan Statistical Areas and Micropolitan 
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------

    With the change in the approach discussed in section III and 
section IV.B.1 above, and considering the final monitoring network 
design discussed in IV.B.4 above, the use of the PWEI from that which 
was proposed also changes. The following paragraphs discuss some of the 
public comments received on the general use and calculation of the 
PWEI; other comments that focused on the detailed application of the 
PWEI as proposed will be addressed in the response to comments document 
since our approach in applying the PWEI has changed.
b. Public Comments
    EPA received a number of comments from State and local groups 
(e.g., NACAA and eight others) and industry (e.g., AQRL, ACC, and eight 
others) who generally agreed with the two-pronged network design 
concept which had the PWEI as a component. More specifically, some 
State commenters (e.g. NACAA, AK, FL, IL, NC, SC, and WI) expressed 
concern that the PWEI (along with the second prong of the proposed 
network design) created monitoring requirements that were 
``duplicative'' and also called for monitors in areas where they were 
not needed. Even amongst some of the commenters who generally agreed 
with the PWEI concept, some provided examples of where the PWEI 
appeared to be duplicative in its proposed application. One example was 
provided by the State of Florida, ``in the case of Homosassa Springs, 
the [proposed network design] requires two monitors [in that CBSA as a 
result of the proposed use of the PWEI]. The driving source is the 
Crystal River Power Plant, with emissions in 2008 of over 85,000 tons 
per year of SO2. The next largest source in the CBSA has 
emissions of roughly two tons per year.'' EPA believes that Florida is 
asserting that the one large source disproportionately drove the PWEI 
too high for that particular CBSA and only one monitor was actually 
needed. EPA notes that these particular comments on duplicative 
monitoring were made under the premise that all proposed required 
monitors would be sited in locations of expected maximum concentration, 
and therefore would be source-oriented in nature. As a result, these 
commenters believed it was necessary that a waiver provision be 
included if they could show that the required number of monitors was 
too many, as in Florida's example.
    As discussed in section IV.B.4 above, a hybrid approach results in 
a final network design with a reduced number of required monitors from 
the number proposed, a different application of the PWEI, and provides 
flexibility in meeting additional monitoring objectives for the 
required monitors, making the need for a waiver from the minimally 
required monitors unnecessary. If a CBSA is required to have multiple 
monitors now, those monitors are not specifically required to be 
located near sources where maximum concentrations of SO2 are 
expected to occur. Instead, they can be sited at different locations to 
fulfill a variety of objectives, although, as noted in secion IV.B.3 
above, EPA is strongly encouraging States to consider monitoring near 
sources not conducive to dispersion modeling and for characterization 
of population exposures.
    EPA received comments from Michigan, South Carolina, and CBD 
requesting clarification on the logic behind the proposed PWEI 
thresholds, or breakpoints, by which three, two, one, or no monitors 
would be required in a given CBSA. In addition, some States (e.g., MI, 
MO, SC, and WI) and industry (e.g., LCA, LMOGA, and LPPA) suggested 
specific adjustments to the proposed application of the PWEI. For 
example, Michigan suggested that the required monitor breakpoint values 
be adjusted to the ``natural breakpoints in the overall distribution''. 
South Carolina suggested EPA identify a way to normalize the PWEI 
stating the PWEI would be more appropriate ``* * * if it used a value 
that better addressed difference in area, population distribution, land 
use, number, types of sources, etc.''
    In the proposed network design, EPA selected the PWEI values, or 
breakpoints, to require one or more monitors based on the overall 
distribution of PWEI values across all CBSAs. Based on U.S. Census 
Bureau data (http://www.census.gov), there are approximately 939 CBSAs 
in the country. EPA proposed and now requires that a PWEI value be 
calculated for each of these CBSAs to determine if monitoring is 
required in that CBSA. Based on 2008 census estimates and the 2005 NEI, 
the average CBSA PWEI value is 21,900 while the median value is only 
121. This indicates that a relatively small number of CBSAs with high 
PWEI values are driving the very upper end of the PWEI distribution. 
The proposed breakpoint where one monitor was required in a CBSA was a 
PWEI value of 5,000. EPA estimated that 131 out of 939 CBSAs (~14%) 
have a PWEI value of 5,000 or more. Further, these 131 CBSAs occupy 
~98% of the sum of PWEI values across all 939 CBSAs, where high PWEI 
values indicate increased coincidence in population and SO2 
emissions. Within this group of CBSAs with PWEI values of 5,000 or 
more, EPA considered the relative amounts of population, emissions, and 
general frequency of occurrence of relatively larger SO2 
sources (such as those that emit 100 tons per year or more) in 
selecting the breakpoints to require two and three monitors in a CBSA 
for the proposed network design. These considerations were made in an 
effort to apply a nationally applicable process by which to require a 
minimum number of monitors for an area, which all were to be sited in 
locations of expected maximum concentration, and therefore likely 
source-oriented monitors. In regard to the comments suggesting 
modification to the calculation or to normalize the PWEI, EPA believes 
that the proposed calculation, under a hybrid analytical approach, is 
still most appropriate. Under a hybrid analytical approach, States have 
the flexibility to move monitoring resources where needed within CBSAs 
that have a high coincidence of population and emissions instead of 
only being able to site monitors to characterize sources. States have 
the option to consider additional factors such as those listed in South 
Carolina's comments above in further identifying where required 
monitoring may be most appropriate in their areas with required 
monitoring.
    Several States (e.g. NESCAUM, NYSDEC, and PADEP) suggested 
abandoning the PWEI concept altogether and instead using some form of 
emissions-only approach to require monitors. For example, NESCAUM, who 
generally supported a ``hot-spot'' monitoring approach, suggested that 
the PWEI be abandoned and EPA instead ``* * * adopt an emissions-only 
approach, resulting in fewer CBSA monitors. We [NESCAUM] suggest a 
threshold of 50,000 tpy CBSA SO2 emissions to trigger the 
first CBSA monitor and a second CBSA monitor required when emissions 
exceed 200,000 tpy.'' NESCAUM states that the proposed use of the PWEI 
``* * * can

[[Page 35564]]

result in multiple monitors in large cities that have relatively small 
CBSA SO2 emissions, or no monitor in a CBSA with large 
emissions.'' NYSDEC suggests that the proposed approach, using the 
PWEI, is ``* * * not more predictive than using emissions data alone.'' 
NYSDEC went on to suggest that monitors be required in CBSAs with 
aggregated emissions of 50,000 tons per year or more and that ambient 
monitoring be considered for point sources with 20,000 tons per year. 
PADEP made several suggestions on network design, with one that 
suggested monitoring in any CBSA ``where there is a sulfur dioxide 
source or combination of sources within 50 miles emitting a total of at 
least 20,000 tons of SO2 per year * * *''
    EPA reviewed emissions and 2005 NEI data and compared the 
suggestions provided by NESCAUM and NYSDEC to the requirement of the 
final network design. Under NESCAUM's suggested design, EPA estimates 
there would be 75 required monitors in 65 CBSAs. Of these 65 CBSAs, 6 
CBSAs that are not covered by the final network design would be 
included; however, 72 CBSAs that will have monitors under the final 
network design would otherwise not have monitors under NESCAUM's 
design. EPA believes that the exclusion of those 72 CBSAs would lead to 
too sparse a network to adequately meet the monitoring objectives of 
the network. Under NYSDEC's suggested network design, EPA estimates 
that there would be a minimum of 65 monitors in the same 65 CBSAs of 
the NESCAUM suggested design. Further, if States ensured that monitors 
were placed near all sources emitting 20,000 tons per year (as NYSDEC 
suggested should be ``considered'' for monitoring), there could be an 
additional 69 monitors.\31\ EPA believes that the final network design 
as discussed above in section IV.B.4, with the increased flexibility 
for monitors to meet multiple monitoring objectives (discussed in 
IV.B.3 above) including, among others, characterization of source 
impacts or population exposure, is better served using PWEI values to 
require monitors because it explicitly accounts for population to 
require and distribute monitors as compared to an emissions-only 
approach. If there is reason for concern that other CBSAs or areas not 
included in the final network design, such as the six CBSAs that were 
included in the NESCAUM and NYSDEC suggested network designs noted 
above, warrant monitoring resources, States or the EPA Regional 
Administrator may take action to require monitoring in such areas. The 
authority of an EPA Regional Administrator to require additional 
monitoring above the minimum requirements is discussed in section 
IV.B.6 below.
---------------------------------------------------------------------------

    \31\ In simulating NYSDEC's suggested network design, EPA 
assumed that no CBSA would have more than one monitor. According to 
the 2005 NEI, there are 162 sources emitting 20,000 tpy or more a 
year. 93 of those sources are estimated to be inside CBSAs that have 
emissions of 50,000 tpy, leaving approximately 62 sources that would 
need a monitor to satisfy NYSDEC's suggested network design.
---------------------------------------------------------------------------

    EPA received a number of comments from States (e.g., IA, NESCAUM, 
NC, NYSDEC, SC, and WI) and industry (e.g., CE, Dominion, EEI, LCA, 
LMOGA, LPPA, and UARG) raising concern over the way the PWEI is 
calculated. Specifically, many commenters in this group indicated that 
they believed that the 2005 NEI would be used in an exclusive or 
permanent fashion to calculate the PWEI, and that updated NEI data 
would not be used. For example, NESCAUM states that ``EPA should not 
require States to rely solely on EPA's inventories [for calculating the 
PWEI], such as the National Emissions Inventory (NEI), as they do not 
always have the updated information that is necessary for such 
regulatory decisions.'' Wisconsin ``* * * believes that States should 
be allowed to use their own annual point source inventories instead of 
EPA's National Emissions Inventory (NEI) for evaluating emission 
sources. Wisconsin's point inventory is updated annually and has a 
reporting threshold of five tons per year for SO2, making it 
more sensitive to changes in facility operations than the NEI, which is 
updated triennially.'' UARG stated that their ``primary concern with 
this network design is its reliance on old emissions data. For electric 
utilities which report their SO2 emissions to EPA annually, 
the use of more recent data would be appropriate.''
    EPA does not intend for relatively old emissions data to be used in 
calculating the PWEI values for individual CBSAs. As was detailed in 
the proposed regulatory text for 40 CFR Part 58 Appendix D (74 FR 
64880), EPA stated that ``The PWEI shall be calculated by multiplying 
the population of each CBSA, using the most current census data, by the 
total amount of SO2 in tons per year emitted within the CBSA 
area, using an aggregate of the most recent county level emissions data 
available in the National Emissions Inventory for each county in each 
CBSA.'' Although commenters suggested that there may be other resources 
from which emissions data may be obtained, particularly at the 
individual State level, the NEI is comprised of emissions data which is 
collected by EPA from the States themselves. The Air Emissions 
Reporting Requirements (40 CFR Part 51), by which EPA sets out how 
States are to report their emission inventories, was recently revised 
in December of 2008. That rulemaking was intended to provide enhanced 
options to States for emissions data collection and exchange and unify 
reporting dates for various categories of inventories. EPA notes that 
the NEI is updated in full every three years and the 2008 NEI is 
scheduled to be available by January 2011. States will have submitted 
their data by May 31, 2010, before this rule is promulgated and 
published, and EPA will provide comment on these submittals during the 
summer of 2010. States will have an opportunity to revise their 2008 
data submissions in the fall of 2010. In the triennial update, both 
point and nonpoint data are required to be submitted by States and are 
included in the inventory. Further, States are required to submit 
emissions data annually for all sources emitting 2,500 tons per year or 
more of SO2 as well as for sources emitting other pollutants 
in excess of thresholds set for those pollutants. In all point source 
submittals to the NEI, States are also allowed to submit emissions data 
for sources of any emissions level, but are not required to do so. 
Starting with the 2009 NEI, the annual and triennial State NEI 
submittals will be due one year after the end of the emissions year. 
States have an additional opportunity to revise their submittals based 
on EPA comment in the spring of the following year, with EPA publishing 
the inventory no later than 6 months after the inventory submittal 
dates (18 months after the end of the emissions year). This approach 
and schedule is accelerated over past NEI schedules and has been 
designed as part of the development of the new Emission Inventory 
System (EIS). Rather than representing old emissions data, the NEI 
available through EIS represents a timely and appropriate source of 
emissions data.
    EPA believes that the process by which the NEI will be updated 
(through use of the EIS) will be adjusted in a manner that will allow 
for more frequent insertion of State supplied emissions data, allowing 
for a more up-to-date inventory. EPA takes this opportunity to 
encourage States to supply all of their available emissions information 
to the NEI as soon as practicable. Therefore, EPA believes that the NEI 
is an appropriate and nationally representative source of emissions 
data by which PWEI calculations may be

[[Page 35565]]

made. PWEI calculations for all CBSAs will use the same year of data at 
any given time, and States, local agencies, and Tribes will have 
uniform opportunity for revising their emissions data for this purpose. 
EPA again encourages States to view the NEI submittals as their 
opportunity to submit their best available SO2 and other 
inventory data with the knowledge that it will be used for the purpose 
of PWEI values.
c. Conclusions on the Use of the Population Weighted Emissions Index
    In the final network design, EPA has determined that it is 
appropriate to use PWEI values as the mechanism by which to require 
monitors in certain CBSAs, similar to its use in the first prong of the 
proposed two-prong network design. EPA believes that using the PWEI 
metric to inform where monitoring is required is more appropriate for 
the SO2 network design than utilizing a population-only or 
emissions-only type of approach, because it takes into account not just 
one factor, i.e., only population or only emissions, but instead takes 
into account the exposure from SO2 emissions to groups of 
people who are in greater proximity to such emissions.
    In the final rule, EPA is retaining the requirement to calculate 
the PWEI by multiplying the population of each CBSA, using the most 
current census data/estimates from the U.S. Census bureau, by the total 
amount of SO2 in tons per year emitted within the CBSA area, 
using an aggregate of county level emissions data available in the most 
recent published version of the National Emissions Inventory for each 
county in each CBSA. The resulting product shall be divided by one 
million, providing a PWEI value, the units of which are million 
persons-tons per year. For any CBSA with a calculated PWEI value equal 
to or greater than 1,000,000, a minimum of three SO2 
monitors are required within that CBSA. For any CBSA with a calculated 
PWEI value equal to or greater than 100,000, but less than 1,000,000, a 
minimum of two SO2 monitors are required within that CBSA. 
For any CBSA with a calculated PWEI value equal to or greater than 
5,000, but less than 100,000, a minimum of one SO2 monitor 
is required within that CBSA. EPA believes that the monitors required 
within these breakpoints provide a reasonable minimum number of 
monitors in a CBSA that considers the combination of population and 
emissions that exist in a CBSA. These criteria (based on 2008 
population and 2005 NEI data) are estimated to require 163 monitors 
within 131 CBSAs.
    EPA has changed the PWEI breakpoint in the final rule at which two 
monitors are required in a CBSA to 100,000 from the breakpoint of 
10,000 in the proposed network design based on multiple considerations. 
First, EPA changed the breakpoint because of a hybrid analytic approach 
and attendant changes in monitoring objectives (see section IV.B.3), 
with the result being that the monitoring network is no longer intended 
to be comprised primarily of source-oriented monitors that are sited at 
locations of expected maximum concentration. This change in objective 
of the network design allows fewer monitors to provide the necessary 
amount of ambient monitoring data EPA to meet the multiple monitoring 
objectives. Second, the breakpoint of 100,000 occurs near a ``natural'' 
breakpoint in the PWEI distribution, a consideration that Michigan 
suggested, where the estimated 28 CBSAs with PWEI values of 100,000 or 
more occupy ~87% of the sum of PWEI values across all 939 CBSAs. 
Finally, EPA considered commenters' assertion that the first prong of 
the proposed network design created duplicative monitoring in certain 
CBSAs. This duplicative monitoring is especially recognized in some 
CBSAs with relatively small populations and somewhat large emissions 
which are dominated by a single source (such as the Homosassa Springs, 
FL example discussed above). Raising the second breakpoint helps to 
alleviate some of the duplicative monitoring that many of the State 
commenters noted.
    EPA therefore is keeping the first and third breakpoints, which 
require one monitor in a CBSA having a PWEI value of 5,000 and three 
monitors in a CBSA having a PWEI value of 1,000,000. EPA believes 
maintaining these breakpoints along with the revised 100,000 PWEI 
breakpoint, will (1) ensure that highly populated areas will be 
monitored for ambient SO2 concentrations even if the 
emissions in that area are moderate, which is appropriate given the 
fact that the greater population creates increased potential for 
exposure to those moderate emissions, and (2) that those areas with 
higher emissions or emission densities, with moderate or modest 
populations will be monitored because those increased emissions are 
likely to have a significant impact on nearby populations.
6. Regional Administrator Authority
    The following paragraphs provide background, rationale, and details 
for the final changes to Regional Administrator authority to use 
discretion in requiring additional SO2 monitors beyond the 
minimum network requirements.
a. Proposed Regional Administrator Authority
    EPA proposed that the Regional Administrators will have discretion 
to require monitoring above the minimum requirements, as necessary, to 
address situations where the minimum monitoring requirements are not 
sufficient to meet monitoring objectives. EPA recognized that the 
minimum required monitors in the proposed two-pronged network design 
were based on indicators that may not have always provided spatial 
coverage for all the areas that have SO2 sources. Although 
the network design and the objectives of the network design have 
changed from those that were proposed because of our contemplated use 
of a hybrid analytical approach, EPA believes it is still important for 
Regional Administrators to have the discretion, and authority, to 
require monitoring above the minimum requirements. Providing the RAs 
with this discretion will allow them to fill any identified gaps in 
meeting the monitoring objectives of the network.
b. Public Comments
    Some commenters (e.g., LCA, LMOGA, LPPA, and South Carolina) 
expressed concerns with the proposed provision authorizing the Regional 
Administrator to require additional monitoring above the minimum 
requirements. The LCA, LMOGA, and LPPA stated that ``the EPA's proposal 
to allow the Regional Administrator discretion to require a State to 
add additional monitors is flawed in that it provides unfettered 
discretion. Criteria should be added * * * that limit such discretion 
and require the Regional Administrator to consider certain objective 
factors when determining whether to require any additional ambient 
SO2 monitors to the network.'' South Carolina stated that 
``the Regional Administrators should not have the discretion to require 
monitoring above the requirements described in [the proposal for] Part 
58 and its Appendices. State monitoring organizations must be given 
discretion to decide the appropriate use of resources to meet uniform 
monitoring requirements. Additional monitoring requirements should not 
be imposed without concurrence of the monitoring organization and 
additional funding that completely supports the additional costs.''

[[Page 35566]]

c. Conclusions on Regional Administrator Authority
    The authority of Regional Administrators to require additional 
monitoring above the minimum required is not unique to the 
SO2 NAAQS. For example, Regional Administrators have the 
authority to use their discretion to require additional NO2 
or Pb monitors (40 CFR Part 58 Appendix D section 4.3.4 and 4.5, 
respectively) and to work with State and local air agencies in 
designing and/or maintaining an appropriate ozone monitoring network 
(40 CFR Part 58 Appendix D section 4.1). EPA believes that the 
nationally applicable final network design, although somewhat dictated 
by local factors (population and emissions), may not account for all 
locations where monitors should be sited, including where potentially 
high concentrations of SO2 may be occurring. Examples 
include locations that have the potential to violate or contribute to 
violations of the NAAQS, areas that might have high concentrations of 
SO2 that are not characterized by modeling or have sources 
that are not conducive to modeling, and locations with susceptible and 
vulnerable populations. As a result, EPA believes it is important for 
Regional Administrators to have the authority to address possible gaps 
in the minimally required monitoring network, especially near sources 
or areas that are not conducive to modeling by granting them authority 
to require monitoring above the minimum requirements. However, in 
response to public comments, EPA notes that Regional Administrators 
would use this authority in collaboration with State agencies to design 
and/or maintain the most appropriate SO2 monitoring network 
to meet the needs of a given area. For all the situations where the 
Regional Administrators may require additional monitoring, it is 
expected that the Regional Administrators will work on a case-by-case 
basis with State or local air agencies. Further, any monitor required 
through the Regional Administrator and selected by the State agency, or 
any new monitor proposed by the State itself, is not done so with 
unfettered discretion, since any such action would be included in the 
Annual Monitoring Network Plan per Sec.  58.10, which must be made 
available for public inspection or comment, and approval by the EPA 
Regional Administrator.
    Therefore, EPA is finalizing the proposal that Regional 
Administrators may use their authority to require monitoring above the 
minimum requirements, as necessary, in any area, to address situations 
where the minimally required monitoring network is not sufficient to 
meet monitoring objectives. In all cases in which a Regional 
Administrator may consider the need for additional monitoring, it is 
expected that the Regional Administrators will work with the State or 
local air agencies to evaluate evidence or needs to determine if a 
particular area may warrant additional monitoring.
7. Monitoring Network Implementation
    The following paragraphs provide background, rationale, and details 
for the final approach for the monitoring network implementation.
a. Proposed Monitoring Network Implementation
    EPA proposed that State and, where appropriate, local air 
monitoring agencies submit a plan for deploying SO2 monitors 
in accordance with the proposed requirements discussed above by July 1, 
2011. EPA also proposed that the SO2 network be physically 
established no later than January 1, 2013. EPA also proposed that the 
number of sites required to operate as a result of the Population 
Weighted Emissions Index (PWEI) values calculated for each CBSA be 
reviewed and revised for each CBSA through the 5-year network 
assessment cycle required in Sec.  58.10.
b. Public Comments
    EPA received comments from the ALA, EDF, NRDC, and SC that 
supported ``* * * a more accelerated deployment of new monitoring than 
the 2013 target date proposed by EPA. The sooner monitors are in place, 
the sooner the public will experience the health benefits of the new 
standard.'' However, EPA received comment from States (e.g., IA, MI, 
NC, SC and WI), industry (e.g., LCA, LMOGA, and LPPA) and public health 
and environmental groups (e.g., ALA, EDF, NRDC, and SC) expressing 
concern with the proposed deployment schedule of the proposed 
SO2 network in that it was too fast or needed to be phased 
in. The States of Iowa, South Carolina, and Wisconsin suggested that 
EPA allow the proposed network to deploy on a phased schedule. For 
example, South Carolina recommended a ``phased implementation with 
largest source/highest probability population exposure areas designated 
for implementation in 2013 (some proportion of the highest PWEI 
monitors) and establishment of the remaining PWEI and the State level 
emissions triggered monitoring required the following year.'' 
Meanwhile, the States of Michigan and North Carolina, along with the 
industry commenters LCA, LMOGA, and LPPA, suggested EPA reconsider 
implementation dates in light of the multiple rulemakings that impose 
mandates on States that have and will be occurring in the future. For 
example, North Carolina stated that ``EPA must keep in mind that it is 
simultaneously revising numerous ambient standards and associated 
monitoring requirements. EPA seems to view each of these proposals as 
independent actions; but the State and local agencies must consider the 
cumulative impact of EPA's various regulatory actions on their ability 
to comply.'' North Carolina goes on to say that ``EPA must allow States 
the flexibility to prioritize among the new requirements to get 
community based monitors in place first and to establish the others as 
funding and personnel resources allow.''
    EPA believes that with the use of a hybrid analytical approach, the 
concerns raised by States and industry commenters suggesting a phased 
or delayed implementation are addressed because the final network 
minimum design requirements result in fewer monitors being required 
than in the proposed network design. EPA's analysis of the existing 
network had indicated that a substantial number of monitors were not 
sited at locations of maximum concentrations. These monitors would have 
had to be re-located to count towards minimum monitoring requirements 
under the proposed monitoring-focused approach. Under a combined 
modeling and monitoring approach, the required monitors can be used to 
satisfy multiple monitoring objectives and therefore, many of the 
monitors in the existing network will satisfy the requirements in the 
final network design, eliminating any need for a phased or delayed 
network implementation. In regard to the suggestion by public health 
and environmental groups to speed up implementation, EPA notes that 
under a hybrid analytical approach much of the existing network will 
fulfill minimum monitoring requirements, and an accelerated schedule is 
not necessary; the network implementation date provides a balance 
between ensuring the minimally required network is fully in place in a 
reasonable amount of time and providing States adequate time to fulfill 
all the requirements in this rulemaking.\32\
---------------------------------------------------------------------------

    \32\ Moreover, as explained in section IV.A, the existing FEM 
monitors in operation may continue to be used to monitor compliance 
with the NAAQS.
---------------------------------------------------------------------------

    EPA received comment on the frequency by which the minimally

[[Page 35567]]

required network will be reviewed and possibly adjusted based on 
updated population and emissions inventories. The State commenters 
listed above, and some others including NACAA, indicated that they 
believed that the proposal for reviewing the SO2 network 
every five years was intended to be a separate review from the required 
5-year network assessments required in Sec.  58.10(d). NACAA stated 
``EPA proposes that the SO2 monitoring network be evaluated 
every five years. This is an unnecessary duplication of effort in light 
of the current requirements for the annual network plan and five year 
network review.'' NACAA went on to say that ``the current requirements 
[in Sec.  58.10] should be regarded as the primary source of monitoring 
network information for all NAAQS pollutant monitoring, regardless of 
the pollutant.''
    EPA concurs with NACAA's statements that the existing requirements 
for network assessment are an appropriate primary source of monitoring 
network information. In the proposal, EPA did not intend for a required 
5-year review of the SO2 network to be an additional effort 
on top of the existing required network assessments but instead to be 
included as part of the 5-year assessment in Sec.  58.10(d). EPA notes 
that CBSA populations and emissions inventories change over time, 
suggesting a need for periodic review of the monitoring network. At the 
same time, EPA recognizes the advantages of a stable monitoring 
network. However, after considering comments, EPA is not finalizing the 
proposed language for 40 CFR Part 58 Appendix D, section 4.4.3(2) which 
simply referenced back to Sec.  58.10. This proposed text it is not 
needed and appears to simply cause confusion. EPA asserts that the 
existing requirements in Sec.  58.10 provide a sufficient and 
appropriate mechanism for network updates and assessment.
c. Conclusions on Monitoring Network Implementation
    Based on the public comments, and due to the contemplated use of a 
hybrid analytical approach, EPA is finalizing, as was proposed, that 
State and, where appropriate, local air monitoring agencies submit a 
plan for deploying SO2 monitors in accordance with the 
proposed requirements presented below by July 1, 2011. Minimally 
required SO2 monitors shall be physically established no 
later than January 1, 2013.

C. Data Reporting

    The following paragraphs provide background, rationale, and details 
for monitor data reporting requirements.
a. Proposed Data Reporting
    Controlled human exposure studies indicate that exposures to peaks 
of SO2 on the order of 5 to 10 minutes result in moderate or 
greater decrements in lung function and/or respiratory symptoms in 
exercising asthmatics (section II.B.1 above, ISA section 5.2, REA 
section 7.2.3, and REA section 10.3.3.2). As a result, the 1-hour 
standard is intended to protect against short term exposures, including 
exposures on the order of 5 minutes up to 24 hours, as is discussed in 
section II.F.2 above. Therefore, in support of the revised NAAQS and 
its intent, EPA proposed that State and local agencies shall report to 
AQS the maximum 5-minute block average of the twelve 5-minute block 
averages of SO2 for each hour. This 5-minute block reporting 
requirement is in addition to the existing requirement to report the 1-
hour average. In addition, EPA solicited comment on the advantages and 
disadvantages (including associated resource burdens) of alternatively 
requiring State and local agencies to report all twelve 5-minute 
SO2 values for each hour or the maximum 5-minute 
concentration in an hour based on a moving 5-minute averaging period 
rather than time block averaging.
    EPA also proposed Data Quality Objectives (DQOs) for the 
SO2 network. DQOs generally specify the tolerable levels for 
potential decision error used as a basis for establishing the quality 
and quantity of data needed to support the objectives of the monitors. 
EPA proposed the goal for acceptable measurement uncertainty for 
SO2 methods to be defined as an upper 90 percent confidence 
limit for the coefficient of variation (CV) of 15 percent for precision 
and as an upper 95 percent confidence limit for the absolute bias of 15 
percent for bias.
b. Public Comments
    EPA received many comments on the reporting of 5-minute data 
values. The comments generally fell into one of the following 
categories: \33\ (1) Those State, public health, and environmental 
groups who supported the proposed requirement to report the maximum 5-
minute block average of the twelve 5-minute block averages of 
SO2 for each hour (e.g., Missouri, NESCAUM, North Carolina, 
ALA, EJ, EDF, NRDC, and SC), (2) those State, public health, and 
environmental groups who supported the reporting of all twelve 5-minute 
averages of each hour (e.g., Kentucky, NYSDEC, AQRL, ALA, ATS, CBD, EJ, 
EDF, NRDC, and SC), (3) those State, public health, and environmental 
groups who supported reporting the maximum 5-minute concentration in an 
hour based on a moving 5-minute average (e.g., South Dakota, ALA, CBD, 
EJ, EDF, NRDC, and SC), and (4) those State and industry groups who did 
not support the reporting of any 5-minute data (e.g., Iowa, South 
Carolina, LEC, and RRI Energy).
---------------------------------------------------------------------------

    \33\ Note that some commenters supported more than one form of 
reported 5-minute data.
---------------------------------------------------------------------------

    Public health and environmental groups (e.g. ALA, CBD, EJ, EDF, 
NRDC, and SC) supported an approach where 5-minute data must be 
reported. However, these commenters were flexible in their position and 
supported multiple forms or types of 5-minute data reporting. The ALA, 
EJ, EDF, NRDC, and SC stated that ``we support the proposed requirement 
for State and local monitoring agencies to report both hourly average 
and maximum 5-minute averages out of the twelve 5-minute block averages 
of SO2 for each hour.'' They also expressed a preference for 
alternative 5-minute data reporting stating that they ``strongly prefer 
that States be required to report the peak 5-minute concentrations of 
SO2 based on a rolling average.'' Similarly, CBD stated that 
``* * * EPA should require that State and local agencies report all 12 
five-minute SO2 values for each hour in addition to 1-hour 
averages. Where possible, EPA also should require reporting of rolling 
five-minute averages rather than block data * * *''
    Missouri generally supported the proposed requirement to report the 
maximum 5-minute average in the hour, saying ``it is not a problem to 
report both the hourly average and the maximum 5-minute block 
average.'' Nevertheless, Missouri went on to note constraints, stating 
that ``* * * [their] data logger and associated software do not have 
the capability to report all twelve 5-minute SO2 values for 
each hour'' and that they ``* * * could not do this without software 
being developed for this purpose and it could be time intensive to 
validate this data.''
    Kentucky did not support the proposal to report the maximum 5-
minute data block in the hour because of the limitations in their data 
acquisition systems. They explained that ``the data acquisition system 
used by the [State] does not have the capability to automatically 
report the maximum 5-minute block of data from an hour concentration. 
[State] personnel would have to manually determine that

[[Page 35568]]

value and then manually enter that data into AQS.'' Kentucky goes on to 
suggest that ``the only feasible option for the [State] to submit 5-
minute data to AQS would be to submit all twelve 5-minute blocks of 
data for each hour to AQS.''
    South Dakota stated that its ``* * * preference would be to report 
the maximum 5-minute average for each hour calculated using a 5-minute 
rolling average.'' South Dakota goes on to state that ``* * * while 
doubling the work required to validate data and load the data into AQS, 
the additional data should help determine if the selected standard 
concentration level has achieved the necessary reduction in high 
concentration 5-minute levels and provide the necessary data for 
further study of health impacts * * *''
    South Carolina stated that it ``* * * does not support mandatory 
reporting of 5-minute averages in addition to the 1-hour average 
required for comparison to the standard. The validation and reporting 
of 5-minute averages imposes a significant additional burden on the 
reporting organization and its Quality System.'' Iowa, who also did not 
support any form of 5-minute data reporting stated that ``the five-
minute data is not used to determine compliance with the NAAQS, and 
represents ancillary data,'' and that ``validating and uploading the 
five-minute data will take at least as much staff time as generating 
the hourly data used for compliance.'' As a result, Iowa states that 
``if EPA determines that five-minute data is needed, we recommend that 
EPA require the maximum five-minute average in each hour, rather than 
all twelve five-minute averages, in order to reduce the burden 
associated with generation of the ancillary data set.''
    With regard to the proposed DQOs, EPA received comments from some 
States (e.g., Kentucky, North Carolina, NYSDEC, and South Carolina) 
providing general support for the goals for acceptable measurement 
uncertainty for precision and bias. North Carolina stated that the ``* 
* * precision and bias measurement uncertainty criteria should emulate 
those that have been established for other recent NAAQS and NCore 
pollutants.'' NYSDEC stated that ``the proposal does not seem 
unreasonable, however these statistics are now expressed in terms of 
confidence limits: Precision--90% confidence of a CV of 15% and Bias--
95% confidence of a CV of 15%.'' NYSDEC raises concern that ``* * * the 
results are now dependent on the number of audits performed. This is 
highly variable because some agencies run automatic audits every night, 
[while] others use the old standard of once every 2 weeks.''
    In regard to comments on the proposed DQOs, EPA notes that the 
precision and bias estimation technique on which NYSDEC comments were 
focused were proposed and adopted in the monitoring rule promulgated on 
October 6, 2006 and EPA did not intend to reopen those requirements for 
comment. Moreover, SO2 precision and bias estimates have 
been performed in this manner for the past four years and there have 
been no adverse effects on data quality at the minimum required level 
of performance checks every two weeks. The statistics for the precision 
and bias estimates and the DQO goals are based on the accumulation of 
the one-point precision checks aggregated at the frequencies required 
in CFR which is every two weeks. Any organization performing more 
frequent checks (such as every night) would accumulate more data for 
the precision and bias estimates, have higher confidence in the data, 
and would have less potential for outliers or higher than normal values 
effecting the precision and bias estimate. In addition, monitoring 
organizations running precision checks every 24 hours would be more 
able to control data quality to meet the DQO goals than organizations 
running the check every two weeks.
c. Conclusions on Data Reporting
    EPA received a fairly diverse set of comments on the 
appropriateness of reporting 5-minute data and in what particular 
format it may be provided in. EPA has considered the comments by the 
States regarding validation of potentially 13 data values per hour 
(instead of 1 or 2) and some States' lack of data acquisition capacity 
or processing capability to report any particular type of 5-minute 
value. EPA believes that in light of these comments, adopting a 
requirement for continuous SO2 analyzers to report all 
twelve 5-minute values or a rolling 5-minute value does not appear to 
provide enough added value for the potential increased burden on 
States, such as increased staff time dedicated to data processing and 
QA, or in improving or adjusting data acquisition capabilities. 
However, EPA also believes that obtaining some form of 5-minute data is 
appropriate because such data have been critical to this NAAQS review, 
and are anticipated to be of high value to inform future health studies 
and, subsequently, future SO2 NAAQS reviews.\34\ Indeed, as 
noted earlier, it was EPA's failure to adequately explain the absence 
of protection from elevated short-term (5- to 10-minute exposure) 
SO2 concentrations for heavily breathing asthmatics that 
occasioned the remand of the 1996 SO2 primary NAAQS 
(American Lung Association, 134 F.3d at 392). This belief is supported 
further by the expectation that a significant portion of the monitors 
operating to satisfy the final monitoring network design will likely be 
sited for population exposures, which have traditionally provided 
ambient data that is often utilized by epidemiologic health studies. 
Therefore, EPA is finalizing the requirement that State and local air 
agencies operating continuous SO2 analyzers shall report the 
maximum 5-minute block average out of the twelve 5-minute block 
averages in each hour, for each hour of the day, and that State and 
local air agencies operating any type of SO2 analyzer shall 
report the integrated 1-hour average value, as was proposed. EPA 
encourages States capable of reporting all twelve 5-minute data blocks 
in an hour to report such data to AQS. AQS is currently set-up to take 
the 5-minute maximum value in an hour under parameter code 42406 and 
can take all twelve 5-minute values under parameter code 42401 (with a 
duration code of H). EPA notes that if a State were to choose to submit 
all twelve 5-minute blocks in the hour, by default, they would be 
submitting the maximum 5-minute data block within that hour, although 
they have not singled out that particular value. Since the 5-minute 
data is not directly being used for comparison to the NAAQS, EPA 
believes that any State electing to submit all twelve 5-minute values 
is still satisfying the intent of having the maximum 5-minute value 
reported. Therefore, if a State chooses to submit all twelve 5-minute 
values in an hour, they will be considered to be satisfying the data 
reporting requirement of submitting the maximum 5-minute value in an 
hour, and they do not have to separately report the maximum 5-minute 
value from within that set of data values to AQS under parameter code 
42406.
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    \34\ The REA assessed exposure and risks associated with 5-
minute SO2 concentrations above 5-minute health effect 
benchmark levels derived from controlled human exposure studies. In 
the analyses, the REA noted that very few State and local agencies 
report ambient 5-minute SO2 data (REA, section 10.3.3.2) 
and that the lack of 5-minute data necessitated the use of 
statistically estimated 5-minute SO2 data in order to 
expand the geographic scope of the exposure and risk analyses (REA, 
section 7.2.3).
---------------------------------------------------------------------------

    EPA proposed new regulation text for 40 CFR Part 58 Appendix C, 
which would have added section 2.1.2 that would have required any 
SO2 FRM or

[[Page 35569]]

FEM used for making NAAQS decisions to be capable of providing both 1-
hour and 5-minute averaged concentration data. EPA is not finalizing 
this proposed language, as the manual wet-chemistry pararosaniline 
reference method cannot provide 5-minute data. Therefore, the proposed 
language is inappropriate. However, both the UVF FEM and the new UVF 
FRM continuous methods are capable of providing 5-minute averaged data. 
As a result, the language in 58.12(g) and 58.16(g) requiring 5-minute 
SO2 data has been adjusted to appropriately specify that 
only those States operating continuous FRM or FEMs are required to 
report the maximum 5-minute data value for each hour.
    With regard to acceptable measurement uncertainties, EPA reviewed 
summary data for each Primary Quality Assurance Organization (PQAO) in 
the 2008 Data Quality Indicator Report on SO2 data within 
the 2008 Criteria Pollutant Quality Indicator Summary Report for AQS 
Data (http://www.epa.gov/ttn/amtic/qareport.html). Of the 100 PQAOs in 
the report, none of those organizations had summary CV or bias values 
exceeding 10 percent. Thus, EPA believes that the SO2 
network can and does easily attain measurement uncertainty criteria 
more stringent than the finalized goal values and the monitoring 
required under the final network design should be able to maintain this 
level of performance. Therefore, in consideration of comments and 
existing quality assurance data, EPA is changing the final goals from 
those which were proposed for acceptable measurement uncertainty for 
SO2 methods to be defined for precision as an upper 90 
percent confidence limit for the coefficient of variation (CV) of 10 
percent and for bias as an upper 95 percent confidence limit for the 
absolute bias of 10 percent.

V. Initial Designation of Areas for the 1-Hour SO2 NAAQS

    This section of the preamble further addresses the process under 
which EPA intends to identify whether areas of the country attain or do 
not attain or are ``unclassifiable'' regarding the new 1-hour 
SO2 NAAQS. After EPA establishes a new NAAQS, the CAA 
directs States and EPA to take this first step, known as the ``initial 
area designations,'' in ensuring that the NAAQS is ultimately attained.
    We are revising our discussion of an expected approach toward 
issuing initial area designations in response to comments we received 
on the proposed rule's treatment of monitoring and modeling (both 
generally and in the specific context of designations), and to make the 
expected process more consistent with our historical approach to 
implementing the SO2 NAAQS. A revised anticipated approach 
for issuing designations logically follows from our revised hybrid 
approach to monitoring and modeling as discussed above in sections III 
and IV. It would also affect a revised expected implementation approach 
that we later discuss in section VI. 1. Designations.

a. Clean Air Act Requirements

    The CAA requires EPA and the States to take steps to ensure that 
the new NAAQS are met following promulgation. The first step is for EPA 
to identify whether areas of the country meet, do not meet, or cannot 
yet be classified as either meeting or not meeting the new NAAQS. 
Section 107(d)(1)(A) provides that, ``By such date as the Administrator 
may reasonably require, but not later than 1 year after promulgation of 
a new or revised NAAQS for any pollutant under section 109, the 
Governor of each State shall * * * submit to the Administrator a list 
of all areas (or portions thereof) in the State'' that should be 
designated as nonattainment, attainment, or unclassifiable for the new 
NAAQS. 42 U.S.C. 7407(d)(1)(A)(i)-(iii). Section 107(d)(1)(B)(i) 
further provides, ``Upon promulgation or revision of a NAAQS, the 
Administrator shall promulgate the designations of all areas (or 
portions thereof) * * * as expeditiously as practicable, but in no case 
later than 2 years from the date of promulgation. Such period may be 
extended for up to one year in the event the Administrator has 
insufficient information to promulgate the designations within 2 
years.'' 42 U.S.C. 7407(d)(1)(B)(i).
    Under CAA section 107(d)(1)(B)(ii), no later than 120 days prior to 
promulgating designations, EPA is required to notify States of any 
intended modifications to their boundaries as EPA may deem necessary, 
and States will have an opportunity to comment on EPA's tentative 
decision. Whether or not a State provides a recommendation, the EPA 
must promulgate the designation that it deems appropriate. 42 U.S.C. 
7407(d)(1)(B)(ii).
    Accordingly, since the new 1-hour SO2 NAAQS is being 
promulgated today, Governors should submit their initial SO2 
designation recommendations to EPA no later than June 2, 2011. If the 
Administrator intends to modify any State's boundary recommendation, 
the EPA will notify the Governor no later than 120 days prior to 
designations or, February 2012. States that believe the Administrator's 
modification is inappropriate will have an opportunity to demonstrate 
why they believe their recommendation is more appropriate before 
designations are finalized in June 2012.
    For initial designations that will be finalized in June 2012, 
States should use monitoring data from the existing SO2 
network for the years 2008-2010, as well as any refined SO2 
dispersion modeling (see Appendix W to 40 CFR Part 51) for sources that 
may have the potential to cause or contribute to a NAAQS violation, 
provided that it is recent and available. EPA will then issue 
designations based on the record of information for that area. Under 
our anticipated approach, an area that has monitoring data or refined 
modeling results showing a violation of the NAAQS would be designated 
as ``nonattainment.'' An area that has both monitoring data and 
appropriate modeling results showing no violations would be designated 
as ``attainment.'' All other areas, including those with SO2 
monitors showing no violations but without modeling showing no 
violations, would be designated as ``unclassifiable.'' Areas with no 
SO2 monitors at all i.e., ``rest of State,'' would be 
designated as ``unclassifiable'' as well.

b. Approach Described in Proposal

    In the proposed rule's preamble, we explained that we had proposed 
a new SO2 ambient monitoring network, with new monitors 
expected to be deployed no later than January 2013. We also explained 
that we expected compliance with the new NAAQS to be determined based 
on 3 years of complete, quality assured, certified monitoring data. We 
further explained that we did not expect newly-cited monitors for the 
proposed network to generate sufficient monitoring data for us to use 
in determining whether areas complied with the new NAAQS by the 
statutory deadline to complete initial designations. Therefore, we 
explained, we intended to complete designations by June 2012 based on 3 
years of complete, quality assured, certified air quality monitoring 
data as generated from the current monitoring network.
    Consequently, we discussed our expectations to base initial 
designations on air quality data from the years 2008-2010 or 2009-2011, 
from SO2 monitors operating at current locations, which we 
expected to continue through 2011. While those monitors are generally 
sited to measure 24-hour and annual average SO2 
concentrations, we noted that they all report hourly data, and we 
estimated that at least one third of those monitors might meet the 
proposed network

[[Page 35570]]

design requirements and not need to be moved. We explained that if any 
monitor in the current network indicated a violation of the new 1-hour 
NAAQS, we would intend to designate the area as ``nonattainment.'' We 
further explained that if a monitor did not indicate a violation, our 
designation decision for the area would be made on a case-by-case 
basis, with one possibility being a designation of ``unclassifiable.''
    We also explained that while the CAA section 107 designation 
provisions specifically address States, we intended to follow the same 
process for Tribes to the extent practicable, pursuant to CAA section 
301(d), 42 U.S.C. 7601(d), and the Tribal Authority Rule, 40 CFR part 
49.

c. Comments

    Several commenters stated that the EPA did not provide 
nonattainment boundary guidance in the proposed rule and argued that 
guidance should be developed. Commenters also stated that EPA should 
consider boundaries that are less than the Core Based Statistical Area 
(CBSA), and perhaps even smaller than the county boundary (State of 
Michigan, Sierra Club).
    In response, we note that the CAA requires that the EPA designate 
as ``nonattainment'' any area that does not meet (or contributes to an 
area that does not meet) the NAAQS. 42 U.S.C. 7407(d)(1)(A)(i). States 
with monitored or modeled SO2 violations will need to 
recommend an appropriate nonattainment boundary that both includes 
sources contributing to that violation, as well as informs the public 
of the extent of the violation. For purposes of determining 
nonattainment boundaries, the EPA expects to consider the county line 
as the presumptive boundary for SO2. This would be 
consistent with our approach under other NAAQS. States recommending 
less-than-countywide nonattainment boundaries should provide additional 
information along with their recommendation, demonstrating why a 
smaller area is more appropriate, as we have advised for other NAAQS. 
If States request it, EPA may develop additional guidance on the 
factors that States should consider when determining nonattainment 
boundaries.
    In addition, as further discussed in section IV.B above, in the 
SO2 NAAQS proposal, we proposed a monitoring-focused 
approach for comparison to the new NAAQS. The proposed network would 
have required approximately 348 monitors nationwide to be sited at the 
locations of maximum concentration. Numerous State and local government 
commenters expressed concerns regarding the perceived burdens of 
implementing the proposed monitoring network and the sufficiency of its 
scope for purposes of identifying violations. Some of these commenters 
(the City of Alexandria, and the States of Delaware, North Carolina and 
Pennsylvania) suggested using modeling to determine the scope of 
monitoring requirements, or favored modeling over monitoring to 
determine compliance with the NAAQS. Partly in response to these 
comments, and after reconsidering the proposal's monitoring-focused 
approach, specifically regarding how we have historically implemented 
SO2 designations, we now anticipate taking a revised 
approach toward designations, using a hybrid analytic approach that 
combines the use of monitoring and available modeling to assess 
compliance with the new 1-hour SO2 NAAQS. We discuss a 
revised expected approach toward designations below, and further 
discuss in section VI how we expect a hybrid approach to affect other 
implementation activities.

d. Expected Designations Process

    As discussed in sections III and IV of this preamble, in response 
to the comments and after reviewing our historical SO2 
implementation practice, we intend to use a hybrid analytic approach 
for assessing compliance with the new 1-hour SO2 NAAQS for 
initial designations. We also believe that a hybrid approach is more 
consistent with our historical approach and longstanding guidance 
toward SO2 NAAQS designations and implementation than what 
we originally proposed. Technically, for a short-term 1-hour standard, 
it is more appropriate and efficient to principally use modeling to 
assess compliance for medium to larger sources, and to rely more on 
monitoring for groups of smaller sources and sources not as conducive 
to modeling.
    In cases where there is complete air quality data from FRM and FEM 
SO2 monitors, that data would be considered by EPA in 
designating areas as either ``attainment'' or ``nonattainment'' for the 
new SO2 NAAQS. See Appendix T to Part 50 section 3b. In 
addition, in cases where a State submits air quality modeling data that 
are consistent with our current guidance or our expected revisions 
thereto, and which indicates that an area is attaining the standard or 
violating the standard, these data may support recommendations of 
``attainment'' or ``nonattainment.'' As explained in section IV above, 
we would not consider monitoring alone to be an adequate, nor the most 
accurate, tool to identify all areas of maximum concentrations of 
SO2. In the case of SO2, we further believe that 
monitoring is not the most cost-efficient method for identifying all 
areas of maximum concentrations.
    Due to the necessarily limited spatial coverage provided by any 
monitoring regime, and the strong source-oriented nature of 
SO2 ambient impacts, we recognize that using this more 
traditional approach in designations, would be more likely to identify 
a greater number of potential instances of nonattainment, if areas were 
to immediately conduct modeling of current source emissions, as 
compared to the approach we discussed in the proposed rule. As 
discussed in section III, forthcoming national and regional rules, such 
as the pending Industrial Boilers ``Maximum Achievable Control 
Technology'' (MACT) standard under CAA section 112(d), are likely to 
result in significant SO2 emissions reductions in the next 
three to four years. A limited qualitative assessment of preliminary 
modeling of some sample facilities that would be covered by those rules 
indicates that well-controlled facilities should meet the new 
SO2 NAAQS. However, there are some exceptions. These 
exceptions include unique sources with specific source characteristics 
that contribute to higher ambient impacts (short stack heights, complex 
terrain, etc.).
    Again as described in section III, in order for States to conduct 
modeling on a large scale for the new 1-hour NAAQS, EPA expects 
additional guidance would be needed to clarify how to conduct 
dispersion modeling under Appendix W to support the implementation of 
the new 1-hour SO2 NAAQS, and how to identify and 
appropriately assess the air quality impacts of sources that 
potentially may cause or contribute to violations of the NAAQS. Our 
anticipated modeling guidance will provide for refined modeling that 
will better reflect and account for source-specific impacts by 
following our current Guideline on Air Quality Models, Appendix W to 40 
CFR Part 51, with appropriate flexibility for use in implementation. 
EPA intends to solicit public comment on this modeling guidance. We 
expect it will take some time for EPA to issue this guidance, and 
believe that given the timing and substantial burden of having to model 
several hundred sources, it would not be realistic or appropriate to 
expect States to complete such modeling and incorporate the results in 
designation recommendations for the new 1-hour SO2 NAAQS 
that, under CAA section

[[Page 35571]]

107(d), are due to EPA within 1 year of the promulgation of the NAAQS.
    Consequently, we expect that in most instances, Governors will 
submit designation recommendations of ``unclassifiable'' rather than 
conduct large-scale refined modeling of sources in advance of receiving 
our anticipated guidance. The absence of monitoring data showing 
violations for most areas, combined with the paucity of refined 
modeling of sources that have the potential to cause or contribute to 
violations of the NAAQS, will likely result in informational records 
that are insufficient to support initial designations of either 
``attainment'' or ``nonattainment.'' Under the Clean Air Act, in such a 
situation EPA is required to issue a designation for the area as 
``unclassifiable.'' However, we do not expect this result to delay 
expeditious attainment and maintenance of the new NAAQS, or to cause 
inappropriate, indefinite uncertainty regarding whether or not sources 
cause or contribute to NAAQS violations.
    As described more fully in section III above and in section VI 
below, EPA's expected implementation approach would rely on the CAA 
section 110(a)(1) SIP obligation to ensure that all areas of the 
country attain and maintain the NAAQS on a timely basis even if they 
are designated ``unclassifiable'' initially. This SIP is due under CAA 
section 110(a)(1) within 3 years after promulgation of the new NAAQS, 
and does not depend upon EPA designating an area ``nonattainment'' 
based on recently monitored or modeled SO2 levels. This 
period of time would allow States to use EPA's anticipated guidance on 
modeling for the new 1-hour SO2 NAAQS, as well as account 
for SO2 reduction levels at individual sources that are 
anticipated to result from promulgated national and regional rules to 
show attainment.
    Once areas have both appropriate monitoring data (if required) and 
modeling data as appropriate, consistent with the new guidance, showing 
no violations of the SO2 NAAQS, and have met other 
applicable requirements of CAA section 107(d)(3), the Agency would 
consider re-designating them from ``unclassifiable'' or 
``nonattainment'' to ``attainment'' under CAA section 107(d)(3).

VI. Clean Air Act Implementation Requirements

    This section of the preamble discusses the CAA requirements that 
States and emissions sources would need to address when implementing 
the new 1-hour SO2 NAAQS based on the structure outlined in 
the CAA and existing rules. The EPA believes that existing guidance 
documents and regulations will be useful in helping States and sources 
to implement the new SO2 NAAQS, but we also expect to 
develop additional guidance on modeling for the new one-hour standard 
and on developing SIPs under Section 110(a)(1) of the CAA.\35\ In light 
of the new approach that EPA intends to take with respect to 
implementation of the SO2 NAAQS, EPA intends to solicit 
public comment on guidance regarding modeling, and also solicit public 
comment on additional implementation planning guidance, including the 
content of the maintenance plans required under section 110(a)(1) of 
the Clean Air Act. EPA also notes that State monitoring plans and the 
SIP submissions that States will make will also be subject to public 
notice and comment.''
---------------------------------------------------------------------------

    \35\ See SO2 Guideline Document, Office of Air 
Quality Planning and Standards, Research Triangle Park, NC 27711, 
EPA-452/R-94-008, February 1994.
---------------------------------------------------------------------------

    In this section, we also further discuss how EPA's modified 
expected approaches toward monitoring and modeling and toward initial 
designations under the new SO2 NAAQS (compared to how the 
proposed rule discussed addressing these issues) are anticipated to 
affect the types of SIP submissions States will need to provide to EPA 
and the timing of EPA's actions on those submissions leading up to 
attainment and maintenance of the new SO2 NAAQS. In section 
IV above, we discuss the final amendments to the ambient monitoring and 
reporting requirements, and explain how in response to comments 
received on the proposal and after revisiting our historical practice 
in assessing compliance with prior SO2 NAAQS, we have 
revised both the scope of the revised monitoring network and our 
expectations on how monitoring will be used in conjunction with 
modeling in assessing compliance and designating areas. In section V 
above, we discuss how we have revised our expected approach for issuing 
designations for the new 1-hour SO2 NAAQS, and similarly 
explain how, in response to comments and after reviewing our historical 
approach, we have modified our expectations as discussed in the 
proposal for how and when monitoring and modeling will be used for 
designations. In this section VI, we describe in more detail how and 
when we expect States to demonstrate attainment, implementation, 
maintenance and enforcement of the new one-hour SO2 NAAQS.
    The CAA assigns important roles to EPA, States and Tribal 
governments to achieve the NAAQS. States have the primary 
responsibility for developing and implementing State implementation 
plans (SIPs) that contain State measures necessary to achieve the air 
quality standards in each area once EPA has established the NAAQS. EPA 
provides assistance to States and Tribes by providing technical tools, 
assistance, and guidance, including information on the potential 
control measures that may assist in helping areas attain the standards.
    Under section 110 of the CAA, 42 U.S.C. 7410, and related 
provisions, States are directed to submit, for EPA approval, SIPs that 
provide for the attainment, implementation, maintenance, and 
enforcement of such standards through control programs directed at 
sources of SO2 emissions. See CAA sections 110(a), and 191-
192, 42 U.S.C. 7410(a) and 7514-7514a. If a State fails to adopt and 
implement the required SIPs by the time periods provided in the CAA, 
EPA has the responsibility under the CAA to adopt a Federal 
implementation plan (FIP) to ensure that areas attain the NAAQS in an 
expeditious manner. The States, in conjunction with EPA, also 
administer the prevention of significant deterioration (PSD) program 
for SO2. See sections 160-169 of the CAA, 42 U.S.C. 7470-
7479. In addition, Federal programs provide for nationwide reductions 
in emissions of SO2 and other air pollutants under Title II 
of the Act, 42 U.S.C. 7521-7574. These programs involve limits on the 
sulfur content of the fuel used by automobiles, trucks, buses, 
motorcycles, non-road engines and equipment, marine vessels and 
locomotives. Emissions reductions for SO2 are also obtained 
from implementation of the new source performance standards (NSPS) for 
stationary sources under sections 111 and 129 of the CAA, 42 U.S.C. 
7411 and 7429; and the national emission standards for hazardous air 
pollutants (NESHAP) for stationary sources under section 112 of the 
CAA, 42 U.S.C. 7412 (such reductions resulting due to control of 
hazardous air pollutants (HAP) such as hydrogen chloride (HCl) under 
those rules). Title IV of the CAA, sections 402-416, 42 U.S.C. 7651a-
7651o, specifically provides for major reductions in SO2 
emissions. EPA has also promulgated the Clean Air Interstate Rule 
(CAIR) to define additional SO2 emission reductions needed 
in the Eastern United States to eliminate significant contribution of 
upwind States to downwind States'

[[Page 35572]]

nonattainment, or inability to maintain, the PM2.5 NAAQS 
pursuant to CAA section 110(a)(2)(D), 42 U.S.C. 7410(a)(2)(D), a rule 
which EPA is reevaluating pursuant to court remand.

A. How This Rule Applies to Tribes

    CAA section 301(d) authorizes EPA to treat eligible Indian Tribes 
in the same manner as States under the CAA and requires EPA to 
promulgate regulations specifying the provisions of the statute for 
which such treatment is appropriate. EPA has promulgated these 
regulations--known as the Tribal Authority Rule or TAR--at 40 CFR Part 
49. See 63 FR 7254 (February 12, 1998). The TAR establishes the process 
for Indian Tribes to seek treatment-as-a-State eligibility and sets 
forth the CAA functions for which such treatment will be available. 
Under the TAR, eligible Tribes may seek approval for all CAA and 
regulatory purposes other than a small number of functions enumerated 
at section 49.4. Implementation plans under section 110 are included 
within the scope of CAA functions for which eligible Tribes may obtain 
approval. Section 110(o) also specifically describes Tribal roles in 
submitting implementation plans. Eligible Indian Tribes may thus submit 
implementation plans covering their reservations and other areas under 
their jurisdiction.
    The CAA and TAR do not, however, direct Tribes to apply for 
treatment as a State or implement any CAA program. In promulgating the 
TAR EPA explicitly determined that it was not appropriate to treat 
Tribes similarly to States for purposes of, among other things, 
specific plan submittal and implementation deadlines for NAAQS-related 
requirements. 40 CFR 49.4(a). In addition, where Tribes do seek 
approval of CAA programs, including section 110 implementation plans, 
the TAR provides flexibility and allows them to submit partial program 
elements, so long as such elements are reasonably severable--i.e., 
``not integrally related to program elements that are not included in 
the plan submittal, and are consistent with applicable statutory and 
regulatory requirements.'' 40 CFR 49.7.
    To date, very few Tribes have sought treatment as a State for 
purposes of section 110 implementation plans. However, some Tribes may 
be interested in pursuing such plans to implement today's proposed 
standard, once it is promulgated.
1. Approach Described in the Proposal
    In the proposed rule preamble, EPA described the various roles and 
requirements States would address in implementing the proposed NAAQS. 
Such references to States generally included eligible Indian Tribes to 
the extent consistent with the flexibility provided to Tribes under the 
TAR. Where Tribes do not seek treatment as a State for section 110 
implementation plans, we explained that EPA under its discretionary 
authority will promulgate FIPs as ``necessary or appropriate to protect 
air quality.'' 40 CFR 49.11(a). EPA also noted that some Tribes operate 
air quality monitoring networks in their areas. We explained that for 
such monitors to be used to measure attainment with the proposed 
revised primary NAAQS for SO2, the criteria and procedures 
identified in the proposed rule would apply.
2. Current Approach
    EPA did not receive any comments on this issue. However, as 
discussed elsewhere in this preamble, the final rule reflects in 
several respects modified expected approaches regarding the use of 
monitoring and modeling, the manner in which we expect to issue 
designations under the new SO2 NAAQS, and the types of SIP 
submissions we expect would be needed to show attainment, 
implementation, maintenance and enforcement of the new NAAQS. Those 
changes in expected approach would, as appropriate, also apply to how 
we address data and any other submissions from Tribes for purposes of 
the new SO2 NAAQS.

B. Nonattainment Area Attainment Dates

    The latest date by which an area designated as nonattainment is 
required to attain the SO2 NAAQS is determined from the 
effective date of the nonattainment designation for the affected area. 
For areas designated nonattainment for the revised SO2 
NAAQS, SIPs must provide for attainment of the NAAQS as expeditiously 
as practicable, but no later than 5 years from the effective date of 
the nonattainment designation for the area. See section 192(a) of the 
CAA, 42 U.S.C. 7651a(a). The EPA expects to determine whether an area 
has demonstrated attainment of the new SO2 NAAQS by 
evaluating air quality monitoring and modeling data consistent with 40 
CFR part 50, Appendix T and 40 CFR part 51, Appendix W. (Note that this 
differs from how we explained we would expect to make such 
determinations in the proposed rule, where we only mentioned monitoring 
as supplying the data we would evaluate. This expanded and changed 
discussion reflects the contemplated changes in our overall approaches 
to using monitoring and modeling, expectations for issuing 
designations, and expectations for reviewing SIP submissions showing 
attainment, implementation, maintenance, and enforcement of the new 
SO2 NAAQS.)
1. Attaining the NAAQS
a. Approach Described in the Proposal
    In the proposal preamble, we set forth the basic five conditions 
provided under section 107(d)(3)(E) of the CAA, 42 U.S.C. 7407(d)(3)(E) 
that a nonattainment area must meet in order to be redesignated as 
attainment:
     EPA must have determined that the area has met the 
SO2 NAAQS;
     EPA has fully approved the State's implementation plan;
     The improvement in air quality in the affected area is due 
to permanent and enforceable reductions in emissions;
     EPA has fully approved a maintenance plan for the area; 
and
     The State(s) containing the area have met all applicable 
requirements under section 110 and part D.
b. Current Approach
    EPA did not receive any comments on this aspect of the preamble of 
the proposal. However, in light of the fact that in the final rule, in 
response to other comments and consistent with historic practice, we 
are revising our proposed anticipated approaches to the overall use of 
monitoring and modeling and our expected approaches to issuing initial 
designations and reviewing SIP submissions, it follows that the way in 
which a nonattainment area seeks redesignation as an attainment area 
would also be affected by the final rule's overall changed approaches. 
For example, for EPA to determine that a nonattainment area has met the 
SO2 NAAQS, we anticipate that the area would need to not 
only provide any monitoring data showing such compliance (and there 
would need to be an absence of monitoring data showing otherwise), but 
modeling where appropriate, consistent with modeling guidance that we 
plan to issue, would also need to show that the area is attaining and 
maintaining the NAAQS.
2. Consequences of a Nonattainment Area Failing To Attain by the 
Statutory Attainment Date
a. Approach Described in the Proposal
    We explained in the proposal that any SO2 nonattainment 
area that fails to attain by its statutory attainment date would be 
subject to the requirements of sections 179(c) and (d) of the CAA, 42

[[Page 35573]]

U.S.C. 7509(c) and (d). EPA is required to make a finding of failure to 
attain no later than 6 months after the specified attainment date and 
publish a notice in the Federal Register. The State would then need to 
submit an implementation plan revision no later than one year following 
the effective date of the Federal Register notice making the 
determination of the area's failure to attain. This submission must 
demonstrate that the standard will be attained as expeditiously as 
practicable, but no later than 5 years from the effective date of EPA's 
finding that the area failed to attain. In addition, section 179(d)(2) 
provides that the SIP revision must include any specific additional 
measures as may be reasonably prescribed by EPA, including ``all 
measures that can be feasibly implemented in the area in light of 
technological achievability, costs, and any nonair quality and other 
air quality-related health and environmental impacts.''
b. Current Approach
    EPA did not receive any comments on this aspect of the discussion 
in the preamble to the proposal. However, due to the changes in the 
final rule's discussion of the overall expected approaches to 
monitoring and modeling, designations and EPA review of SIP 
submissions, it follows that the implementation of CAA sections 179(c) 
and (d) would also be affected by those changes. For example, under the 
anticipated approach, a nonattainment area's initial demonstration of 
attainment would need to show through modeling consistent with modeling 
guidance that we plan to issue, that the area attains and maintains the 
new SO2 NAAQS. If the area fails to attain on time, any 
remedial implementation plan submission would also need to show, where 
appropriate, through modeling consistent with modeling guidance that we 
plan to issue, that the area attains and maintains the new 
SO2 NAAQS.

C. Section 110(a)(1) and (2) NAAQS Maintenance/Infrastructure 
Requirements

    We are significantly revising our expected approaches to the use of 
monitoring and modeling, expected issuance of initial designations, and 
EPA review of SIP submissions. This change in anticipated approach has 
particular relevance for how States would meet their statutory 
obligations under CAA section 110(a) to implement, maintain and enforce 
the new SO2 NAAQS. In short, under such an approach, all 
areas, whether designated as attainment, nonattainment, or 
unclassifiable, would need to submit SIPs under CAA section 110(a) that 
show that they are attaining and maintaining the 1-hour SO2 
NAAQS as expeditiously as practicable through permanent and enforceable 
measures. In other words, the duty to show maintenance of the 
SO2 NAAQS would not be limited to areas that are initially 
designated as nonattainment, but instead would apply regardless of 
designation. As has been expected historically, areas initially 
designated attainment for SO2 are expected to submit to EPA 
the infrastructure elements of the 110(a) SIP, including the PSD 
program. Historically, EPA has determined this to be sufficient to 
demonstrate maintenance absent other available information to suggest 
the area would have difficulty maintaining the NAAQS.
    As required by CAA section 192, nonattainment areas must 
demonstrate attainment as expeditiously as practicable, and no later 
than 5 years after designation (which would be August 2017). Under a 
hybrid approach as we have discussed earlier in sections III, IV, and V 
of this preamble, EPA believes that August 2017 would be the latest 
point that could be as expeditiously as practicable for attainment and 
unclassifiable areas as well, and EPA anticipates establishing this 
date through future rulemaking actions on individual SIPs.
    As noted in earlier sections of this preamble, in the 
SO2 NAAQS proposal, we recommended a monitoring-focused 
approach for comparison to the NAAQS. We received public comments that 
contended our proposed monitoring network was too small and 
insufficient to assess the hundreds of areas that might violate the new 
SO2 NAAQS and yet too burdensome and expensive to expand to 
an adequate scale. Some commenters, especially State air agencies, 
recommended the use of modeling either to determine potential 
nonattainment areas or to identify areas subject to monitoring 
requirements. Because SO2 is primarily a localized 
pollutant, modeling is the the most appropriate tool to accurately 
predict SO2 impacts from large sources, EPA has used it in 
the past to determine SO2 attainment status, and it can be 
performed more quickly and less costly than monitoring. Consequently, 
as part of developing a balanced response to the numerous comments we 
received on modeling and monitoring, we expect to use a hybrid analytic 
approach that combines the use of monitoring and modeling to assess 
compliance with respect to the new SO2 NAAQS.
    A hybrid analytic approach for assessing compliance with the new 
SO2 NAAQS would make the most appropriate use of available 
tools and be more consistent with our historical approach than was what 
we originally proposed. For a short-term 1-hour standard, it is more 
accurate and efficient to use modeling to assess medium to larger 
sources and to rely on monitoring for groups of smaller sources and 
sources not as conducive to modeling.
    We expect that States would initially focus performance of 
attainment demonstration modeling on larger sources (e.g., those 
= 100 tons per year (tpy) of SO2), and that 
States would also identify and eventually conduct refined modeling of 
any other sources that may be anticipated to cause or contribute to a 
violation to determine compliance with the new SO2 NAAQS. As 
discussed in Section III, EPA anticipates providing additional guidance 
to States to clarify how to conduct dispersion modeling under Appendix 
W to support the implementation of the new 1-hour SO2 NAAQS. 
Prior to issuing this guidance, EPA intends to solicit public comment.
    Since determining compliance with the SO2 NAAQS will 
likely be a uniquely source-driven analysis, EPA explored options to 
ensure that the SO2 designations process realistically 
accounts for anticipated SO2 reductions at those sources 
that we expect will be achieved by current and pending national and 
regional rules. To ensure that all areas of the country attain the 
NAAQS on a timely basis, while accommodating modeling that is both 
informed by anticipated modeling guidance and accounts for those 
anticipated SO2 reductions, EPA's intention is to emphasize 
the CAA section 110(a)(1) requirement that all States submit a SIP that 
shows implementation, maintenance and enforcement of the NAAQS. This 
SIP would be due under CAA section 110(a)(1) within 3 years after 
promulgation of the new NAAQS, and would not depend upon EPA 
designating an area nonattainment based on recently monitored or 
modeled SO2 levels. In addition, like an attainment SIP 
required for a designated nonattainment area under CAA section 192, to 
show attainment this SIP can account for controlled SO2 
levels at individual sources that will be achieved after submission of 
the SIP but before the demonstrated attainment date. EPA intends to 
implement this approach in a way that ensures expeditious attainment of 
the NAAQS, under a schedule that we explain more fully below.

[[Page 35574]]

1. Section 110(a)(1)-(2) Submission
a. Approach Described in the Proposal
    In the preamble to the proposal, we explained that section 
110(a)(2) of the CAA directs all States to develop and maintain a solid 
air quality management infrastructure, including enforceable emission 
limitations, an ambient monitoring program, an enforcement program, air 
quality modeling capabilities, and adequate personnel, resources, and 
legal authority. Section 110(a)(2)(D) also requires State plans to 
prohibit emissions from within the State which contribute significantly 
to nonattainment or maintenance areas in any other State, or which 
interfere with programs under part C of the CAA to prevent significant 
deterioration of air quality or to achieve reasonable progress toward 
the national visibility goal for Federal class I areas (national parks 
and wilderness areas).
    Under sections 110(a)(1) and (2) of the CAA, all States are 
directed to submit SIPs to EPA which demonstrate that basic program 
elements have been addressed within 3 years of the promulgation of any 
new or revised NAAQS. Subsections (A) through (M) of section 110(a)(2) 
set forth the elements that a State's program must contain in the 
SIP.\36\ The proposed rule listed section 110(a)(2) NAAQS 
implementation requirements as the following:
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    \36\ In the proposed rule preamble, we explained that two 
elements identified in section 110(a)(2) were not listed in our 
summary because, as EPA interprets the CAA, SIPs incorporating any 
necessary local nonattainment area controls would not be due within 
3 years, but rather are generally due at the time the nonattainment 
area planning requirements are due. See 74 FR 64860 at n. 39. These 
elements are: (1) Emission limits and other control measures, 
section 110(a)(2)(A), and (2) Provisions for meeting part D, section 
110(a)(2)(I), which requires areas designated as nonattainment to 
meet the applicable nonattainment planning requirements of part D, 
title I of the CAA. To implement our revised intended approach in 
the final rule, however, it would be necessary for States to 
include, if relied upon to show attainment and maintenance of the 
new SO2 NAAQS, any necessary emission limits and other 
control measures under section 110(a)(2)(A).
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     Ambient air quality monitoring/data system: Section 
110(a)(2)(B) requires SIPs to provide for setting up and operating 
ambient air quality monitors, collecting and analyzing data and making 
these data available to EPA upon request.
     Program for enforcement of control measures: Section 
110(a)(2)(C) requires SIPs to include a program providing for 
enforcement of SIP measures and the regulation and permitting of new/
modified sources.
     Interstate transport: Section 110(a)(2)(D) requires SIPs 
to include provisions prohibiting any source or other type of emissions 
activity in the State from contributing significantly to nonattainment 
or interfering with maintenance of the NAAQS in another State, or from 
interfering with measures required to prevent significant deterioration 
of air quality or to protect visibility.
     Adequate resources: Section 110(a)(2)(E) directs States to 
provide assurances of adequate funding, personnel and legal authority 
to implement their SIPs.
     Stationary source monitoring system: Section 110(a)(2)(F) 
directs States to establish a system to monitor emissions from 
stationary sources and to submit periodic emissions reports to EPA.
     Emergency power: Section 110(a)(2)(G) directs States to 
include contingency plans, and adequate authority to implement them, 
for emergency episodes in their SIPs.
     Provisions for SIP revision due to NAAQS changes or 
findings of inadequacies: Section 110(a)(2)(H) directs States to 
provide for revisions of their SIPs in response to changes in the 
NAAQS, availability of improved methods for attaining the NAAQS, or in 
response to an EPA finding that the SIP is inadequate.
     Consultation with local and Federal government officials: 
Section 110(a)(2)(J) directs States to meet applicable local and 
Federal government consultation requirements when developing SIPs and 
reviewing preconstruction permits.
     Public notification of NAAQS exceedances: Section 
110(a)(2)(J) directs States to adopt measures to notify the public of 
instances or areas in which a NAAQS is exceeded.
     PSD and visibility protection: Section 110(a)(2)(J) also 
directs States to adopt emissions imitations, and such other measures, 
as may be necessary to prevent significant deterioration of air quality 
in attainment areas and protect visibility in Federal Class I areas in 
accordance with the requirements of CAA Title I, part C.
     Air quality modeling/data: Section 110(a)(2)(K) requires 
that SIPs provide for performing air quality modeling for predicting 
effects on air quality of emissions of any NAAQS pollutant and 
submission of data to EPA upon request.
     Permitting fees: Section 110(a)(2)(L) requires the SIP to 
include requirements for each major stationary source to pay permitting 
fees to cover the cost of reviewing, approving, implementing and 
enforcing a permit.
     Consultation/participation by affected local government: 
Section 110(a)(2)(M) directs States to provide for consultation and 
participation by local political subdivisions affected by the SIP.
b. Final
    EPA did not receive any comments on this aspect of the approached 
explained in the proposal preamble. However, in light of the modified 
approach discussed above, EPA is providing additional guidance 
concerning the CAA section 110(a)(1) maintenance plan requirement as a 
part of this discussion so that States will have sufficient information 
to meet this requirement with a SIP submittal three years after 
promulgation of the NAAQS. Section 110(a)(1) of the CAA states that 
each State, after reasonable notice and public hearing, is required to 
adopt and to submit to EPA, within 3 years after promulgation of any 
new or revised NAAQS for any pollutant, a SIP which provides for the 
implementation, maintenance, and enforcement of any new or revised 
NAAQS in each area of the State. As stated previously, in light of the 
new approach that EPA intends to take with respect to implementation of 
the SO2 NAAQS, EPA intends to solicit public comment on 
guidance regarding modeling, and also solicit public comment on 
additional implementation planning guidance, including the content of 
the maintenance plans required under section 110(a)(1) of the Clean Air 
Act.
    EPA expects that most areas of the country would be designated as 
unclassifiable for the 1-hour NAAQS for SO2, due to a lack 
of both monitoring and modeling information concerning the attainment 
status of areas, in advance of States conducting further refined 
modeling according to our anticipated guidance. For areas that are 
designated unclassifiable, States are required to submit section 
110(a)(1) plans to demonstrate implementation, maintenance and 
enforcement of the new SO2 NAAQS. As previously explained in 
section III of the preamble, in order to meet the requirements of 
section 110(a)(1) and to ensure timely attainment of the NAAQS on a 
schedule that is as expeditious as would be required if an area had 
been designated nonattainment, EPA's current expectation is that States 
would submit SIPs which provide for attainment, implementation, 
maintenance, and enforcement of the 1-hour SO2 NAAQS in all 
areas as expeditiously as practicable, which EPA believes in these 
cases would be no later than 5 years from the effective date of the 
area's designation. The section 110(a)(1) maintenance plan would also 
need to contain the following elements: (1) An

[[Page 35575]]

attainment emissions inventory, (2) a control strategy, as appropriate, 
(3) a maintenance demonstration, using an EPA approved air quality 
model as appropriate, (4) a contingency plan, and (5) a plan for 
verification of continued attainment of the standard. Attainment areas 
that appear to have difficulty maintaining attainment may also have to 
submit some of these elements. These elements are now explained in 
detail.
(1) Attainment Emissions Inventory
    The State should develop an accurate attainment emissions inventory 
to identify the level of emissions in the area which is sufficient to 
attain the 1-hour SO2 NAAQS. This inventory should be 
consistent with EPA's most recent guidance on emissions inventories 
currently available, and should include the emissions for the time 
period associated with the modeling and monitoring data showing 
attainment. Major source size thresholds for SO2 are 
currently listed as 100 ton/yr, however, in cases where sources, 
individually, or collectively, that are below this level may 
potentially cause or contribute to a violation of the standard, these 
sources should also be included in the emissions inventory for the 
affected area. EPA notes that, unlike any monitoring or modeling data 
used in the initial designations context, which would be limited to 
current emissions levels, this estimate under a hybrid approach we 
expect to use for the new SO2 NAAQS would be able to rely on 
modeled controlled emissions levels at sources achieved by enforceable 
national, regional or local rules that will be in place within the 
timeframe for demonstrating attainment. This is because demonstrations 
of attainment and maintenance of a NAAQS, unlike designations, are 
necessarily projections regarding future and continuing levels of 
ambient air pollution concentrations given that the statutory deadlines 
for their submission are in advance of the required achievement of 
attainment and maintenance. See, e.g., CAA sections 191(a) and 192(a).
(2) Maintenance Demonstration
    The key element of a section 110(a)(1) maintenance plan is a 
demonstration using, as appropriate, refined SO2 dispersion 
modeling (see Appendix W to 40 CFR Part 51) which provides an 
indication of how the area will attain and maintain the 1-hour 
SO2 NAAQS as expeditiously as practicable, which EPA 
believes would be within the 5 year period following the designation of 
the area. For SO2 the State may generally demonstrate 
maintenance of the NAAQS by using refined dispersion modeling to show 
that the future mix of sources and emission rates in an area will not 
cause a violation of the 1-hour SO2 NAAQS. As a result of 
applying the control strategy, EPA anticipates that additional guidance 
for States may be needed to clarify how to conduct dispersion modeling 
under Appendix W to support the implementation of the new 1-hour 
SO2 NAAQS.
    As explained above in IV.B, EPA believes that for SO2 
attainment and maintenance demonstrations, monitoring data alone is 
generally not adequate to characterize fully short-term ambient 
concentrations around major stationary sources of SO2, and 
as a result may not capture the maximum SO2 impacts. With 
representative and appropriate meteorological and other input data, 
refined dispersion models are able to characterize air quality impacts 
from the modeled sources across the domain of interest on an hourly 
basis with a high degree of spatial resolution, overcoming the 
limitations of an approach based solely on monitoring. By simulating 
plume dispersion on an hourly basis across a grid of receptor 
locations, dispersion models are able to estimate the detailed spatial 
gradients of ambient concentrations resulting from SO2 
emission sources across a full range of meteorological and source 
operating conditions. To capture such results on a monitor would 
normally require a prohibitively expansive air quality monitoring 
network. Further, as we have observed in prior actions (see., e.g., 43 
FR 45993, 45997, 46000-03 (Oct. 5, 1978)), monitoring data would not be 
adequate to demonstrate attainment if sources are using stacks with 
heights that are greater than good engineering practice (GEP), or other 
prohibited dispersion techniques, as section 123 prohibits credit in an 
attainment demonstration for any such practices.
    Refined dispersion modeling for the section 110(a)(1) maintenance 
plan is expected to follow EPA's Guideline on Air Quality Models, 
Appendix W to 40 CFR Part 51, which provides recommendations on 
modeling techniques and guidance for estimating pollutant 
concentrations in order to assess control strategies and determine 
emission limits. These recommendations were originally published in 
April 1978 and were incorporated by reference in the PSD regulations, 
40 CFR sections 51.166 and 52.21 in June 1978 (43 FR 26382-26388). The 
purpose of Appendix W is to promote consistency in the use of modeling 
within the air quality management process. Appendix W is periodically 
revised to ensure that new model developments or expanded regulatory 
requirements are incorporated. The most recent revision to Appendix W 
was published on November 9, 2005 (70 FR 68218), wherein EPA adopted 
AERMOD as the preferred dispersion model for a wide range of regulatory 
applications in all types of terrain. To support the promulgation of 
AERMOD as the preferred model, EPA evaluated the performance of the 
model across a total of 17 field study data bases (Perry, et al., 2005; 
EPA, 2003), including several field studies based on model-to-monitor 
comparisons of SO2 concentrations from operating power 
plants. AERMOD is a steady-state plume dispersion model that employs 
hourly sequential preprocessed meteorological data to simulate 
transport and dispersion from multiple point, area, or volume sources 
for averaging times from one hour to multiple years, based on an 
advanced characterization of the atmospheric boundary layer. AERMOD 
also accounts for building wake effects (i.e., downwash) on plume 
dispersion.
    As stated previously, EPA anticipates that additional guidance for 
States, Tribal, and local governments is needed to clarify how to 
conduct refined dispersion modeling under Appendix W to support the 
implementation of the new 1-hour SO2 NAAQS. EPA intends to 
solicit public comment on guidance regarding modeling. Although AERMOD 
is identified as the preferred model under Appendix W for a wide range 
of applications and will be appropriate for most modeling applications 
to support the new SO2 NAAQS, Appendix W allows flexibility 
to consider the use of alternative models on a case-by-case basis when 
an adequate demonstration can be made that the alternative model 
performs better than, or is more appropriate than, the preferred model 
for a particular application.
(3) Control Strategy
    The EPA believes that in order to meet the implementation, 
maintenance and enforcement plan requirements of section 110(a)(1) for 
the new SO2 NAAQS, States should consider all control 
measures that are reasonable to implement in light of the attainment 
and maintenance needs for the affected area(s). The EPA believes that 
where additional controls are necessary it would be appropriate for the 
level of controls in these areas to be similar to that required in 
areas that are designated as nonattainment for SO2. These 
controls would provide for the attainment and maintenance of the 
SO2 1-hour standard as expeditiously as

[[Page 35576]]

practicable. EPA believes that expeditious attainment in these areas 
will be within 5 years of the effective date of designation of an area. 
This approach would allow States to take into consideration emission 
reductions that we expect to be achieved from the implementation of 
future controls from national control measures as well as regional and 
local control measures that will be in place by the anticipated 
attainment date and are projected to help achieve attainment and 
maintenance of the standard. It would also reduce the risk of such 
areas failing to meet the NAAQS as expeditiously as nonattainment areas 
must meet it.
(4) Contingency Plan
    The contingency plan is considered to be an enforceable part of the 
section 110(a)(1) plan and should ensure that there are appropriate 
contingency measures which can be implemented as expeditiously as 
practicable once they are triggered. The contingency plan should 
clearly identify the measures to be adopted, provide a schedule and 
procedures for adoption and implementation, and provide a specific time 
limit for actions by the State.
    The EPA believes that in this case the contingency measures 
implemented under the contingency plan requirement for the section 
110(a)(1) plan in unclassifiable areas under a revised approach for 
SO2 should closely resemble the contingency measures 
required under section 172(c)(9) of the CAA. Section 172(c)(9) of the 
CAA defines contingency measures as measures in the SIP which are to be 
implemented in the event that an area fails to attain the NAAQS, or 
fails to meet the reasonable further progress (RFP) requirement, by the 
applicable attainment date for the area. Contingency measures become 
effective without further action by the State or EPA, upon 
determination by EPA that the area (1) failed to attain the NAAQS by 
the applicable attainment date, or (2) fail to meet RFP. These 
contingency measures should consist of other available control measures 
that are not included in the control strategy for the SIP.
    The EPA interprets the contingency measure provision as primarily 
directed at general control programs which can be undertaken on an 
area-wide basis. Since SO2 control measures are based on 
what is directly and quantifiably necessary to attain the 
SO2 NAAQS, it would be unlikely for an area to implement the 
necessary emissions control yet fail to attain the NAAQS. Therefore, 
for SO2 programs, EPA believes that State agencies should 
have a comprehensive program to identify sources of violations of the 
SO2 NAAQS and undertake an aggressive follow-up for 
compliance and enforcement, including expedited procedures for 
establishing enforceable consent agreements pending the adoption of 
revised SIPs.
    Such an approach toward minimum contingency measures for 
SO2 would not preclude a State from requiring additional 
contingency measures that are enforceable and appropriate for a 
particular source or source category. A contingency measure for an 
SO2 SIP might be a consent agreement between the State and 
EPA to reduce emissions from a source further in the event that the 
contingency measures are triggered. Alternatively, a source might adopt 
a contingency measure such as switching to low sulfur coal or reducing 
load until more permanent measures can be put into place to correct the 
problem. In either case, the contingency measure should be a fully 
adopted provision in the SIP in order for it to become effective at the 
time that EPA determines that the area either fails to attain the NAAQS 
or fails to meet RFP.
    As a necessary part of the section 110(a)(1) plan, the State should 
also identify specific indicators, or triggers, which will be used to 
determine when the contingency measures need to be implemented. The 
identification of triggers would allow a State an opportunity to take 
early action to address potential violations of the NAAQS before they 
occur. By taking early action, States may be able to prevent any actual 
violations of the NAAQS, and therefore, reduce the need on the part of 
EPA to start the process to re-designate the areas as nonattainment. An 
example of a trigger would be monitored or modeled violations of the 
NAAQS. The EPA will review what constitutes an approvable contingency 
plan on a case-by-case basis.
(5) Verification of Continued Attainment
    The submittal should provide an indication of how the State will 
track the progress of the section 110(a)(1) plan. This is necessary due 
to the fact that the emissions projections made for the attainment and 
maintenance demonstrations depend on assumptions of point, area, and 
mobile source growth. One option for tracking the progress of the 
attainment and maintenance demonstrations, provided here as an example, 
would be for the State to update periodically the emissions inventory. 
The attainment and maintenance demonstration should project maintenance 
during the five year period following the designations for the 1-hour 
SO2 NAAQS, not simply that the area will be in attainment in 
the fifth year.
    States should develop interim emission projection years to show a 
trend analysis for attainment and maintenance of the standard. These 
emission projections can also be used as triggers for implementing 
contingency measures. The EPA recognizes that it would be difficult and 
time consuming to develop projections for each year of the 5 year 
period. Therefore, the number of interim projection years should 
reflect whatever information exists regarding the potential for 
increases in emissions in the intervening years. For instance, if there 
is a high probability that emissions will increase to such an extent as 
to jeopardize continued maintenance of the standard even temporarily 
over the intervening years, the number of interim projection periods 
should be sufficient to document that such increases will not interfere 
with maintenance of the 1-hour SO2 NAAQS.
    When modeling for the attainment and maintenance demonstrations, 
one option for tracking progress would also be for the State to 
reevaluate periodically the modeling assumptions and data input. Such 
reevaluation, for example, could address any delays in source 
compliance with national, regional or local rules for which the State 
had previously modeled timely SO2 reductions. In any event, 
the State should monitor the indicators for triggering the contingency 
measures on a regular basis.
    EPA recognizes that the approach discussed above for SO2 
SIPs submitted under CAA section 110(a)(1)-(2) is significantly 
different from the one outlined in the proposal, and from what we have 
applied in the context of other criteria pollutants. However, EPA 
anticipates using a revised approach under section 110(a)(1)-(2) as 
part of an overall revised hybrid monitoring and modeling approach in 
response to comments on the proposed monitoring-focused approach to 
implementation of the new SO2 NAAQS. We believe that such an 
approach would best account for the unique source-specific and 
localized impacts inherent to SO2, and would be the most 
reasonable way to ensure that all areas of the United States timely 
attain and maintain the new NAAQS, while at the same time avoiding 
inappropriately requiring immediate refined modeling of all sources 
without appropriate EPA guidance. This would also allow attainment 
demonstrations to account

[[Page 35577]]

for expected substantial SO2 reductions that will occur well 
in advance of the attainment deadline. Of course, for such a unique 
SO2 approach to work, it would be imperative for all areas 
to timely submit, and for EPA to able to approve, adequate attainment, 
implementation, maintenance and enforcement SIPs that show attainment 
as expeditiously as practicable, and no later than 5 years following 
initial designations. Only by applying such a timeframe to the section 
110(a)(1) SIP approach we are adopting for SO2 could the 
approach be a reasonable one. To that end, EPA would not intend to 
approve SIPs that do not meet this schedule, and would take necessary 
and appropriate actions in response to any submission that would result 
in unacceptable delay of attainment. Such actions may include, but are 
not limited to, any combination of SIP disapproval, redesignation to 
nonattainment, and promulgation of a Federal implementation plan (FIP). 
Any future action establishing an attainment deadline will be completed 
through notice-and-comment rulemaking on individual SIP submissions.
    The timeline below shows how we expect the several steps from 
promulgation of the new NAAQS through attainment should proceed, 
whether areas are designated nonattainment or unclassifiable, assuming 
timely action at each step:
     June 2010: EPA issues new SO2 NAAQS, which 
starts periods within which CAA section 107 initial area designations 
must occur and CAA section 110(a)(1)-(2) SIPs must be submitted.
     June 2011: States submit initial area designations 
recommendations, based on available monitoring data, and on any refined 
modeling performed in advance of submitting CAA section 110(a)(1)-(2) 
SIPs.
     June 2012: EPA issues initial area designations. Any 
monitored or modeled violations would trigger nonattainment 
designations. (Per below, States designated nonattainment would submit 
nonattainment SIPs by February 2014, relying on refined modeling that 
demonstrates attainment by no later than August 2017.) States would be 
designated attainment if they submit both monitoring and modeling 
showing adequate evidence of no violations. All other cases would be 
initially designated as unclassifiable.
     June 2013: States submit CAA section 110(a)(1)-(2) SIPs. 
SIPs would rely on refined modeling and any required monitoring that 
demonstrates attainment and maintenance of the new SO2 NAAQS 
as expeditiously as practicable, and no later than August 2017. For 
areas within the State designated attainment and unclassifiable, the 
section 110(a) SIP must contain any additional Federally enforceable 
control measures necessary to ensure attainment and maintenance of the 
NAAQS. (Control measures to be implemented in designated nonattainment 
areas are due later as part of the nonattainment SIP in February 2014.)
     February 2014: Any initially designated nonattainment 
areas submit CAA section 191-192 SIPs showing attainment no later than 
August 2017.
     June 2014: EPA approves or disapproves submitted CAA 
section 110(a)(1)-(2) SIPs. For attainment and unclassifiable areas, 
EPA's action would be based on adequacy of States' modeling (and any 
required monitoring) showing attainment as expeditiously as 
practicable, and no later than August 2017, in partial reliance on 
SO2 reductions from national and regional standards that are 
achieved by the attainment date. EPA would also have discretion to re-
designate areas based on these SIPs, including to nonattainment if SIPs 
are inadequate, as well as promulgate FIPs.
     February 2015: EPA approves or disapproves CAA section 
191-192 attainment SIPs submitted by areas initially designated as 
nonattainment, with similar remedies as discussed above if SIPs are 
deficient.
     June 2016: CAA section 110(c) deadline by which EPA must 
issue a FIP for any area whose section 110(a)(1) SIP is disapproved in 
June 2014.
     February 2017: CAA section 110(c) deadline by which EPA 
must issue a FIP for a nonattainment area whose section 192 SIP is 
disapproved in February 2015.
    August 2017: Expected date by which all areas, regardless of 
classification, achieve attainment, implementation, maintenance and 
enforcement of the new SO2 NAAQS.

D. Attainment Planning Requirements

1. SO2 Nonattainment Area SIP Requirements
a. Approach Described in the Proposal
    We explained in the preamble to the proposal that any State 
containing an area designated as nonattainment with respect to the 
SO2 NAAQS would need to develop for submission to EPA a SIP 
meeting the requirements of part D, Title I, of the CAA, providing for 
attainment by the applicable statutory attainment date. See sections 
191(a) and 192(a) of the CAA. As indicated in section 191(a), all 
components of the SO2 part D SIP must be submitted within 18 
months of the effective date of an area's designation as nonattainment.
    Section 172 of the CAA addresses the general requirements for areas 
designated as nonattainment. Section 172(c) directs States with 
nonattainment areas to submit a SIP which contains an attainment 
demonstration showing that the affected area will attain the standard 
by the applicable statutory attainment date. The SIP must show that the 
area will attain the standard as expeditiously as practicable, and must 
``provide for the implementation of all Reasonably Available Control 
Measures (RACM) as expeditiously as practicable (including such 
reductions in emissions from existing sources in the area as may be 
obtained through the adoption, at a minimum, of Reasonably Available 
Control Technology (RACT)).''
    SIPs required under Part D of the CAA must also provide for 
reasonable further progress (RFP). See section 172(c)(2) of the CAA. 
The CAA defines RFP as ``such annual incremental reductions in 
emissions of the relevant air pollution as are required by part D, or 
may reasonably be required by the Administrator for the purpose of 
ensuring attainment of the applicable NAAQS by the applicable 
attainment date.'' See section 171 of the CAA. Historically, for some 
pollutants, RFP has been met by showing annual incremental emission 
reductions sufficient to maintain generally linear progress toward 
attainment by the applicable attainment date.
    All SO2 nonattainment area SIPs must include contingency 
measures which must be implemented in the event that an area fails to 
meet RFP or fails to attain the standards by its attainment date. See 
section 172(c)(9) of the CAA. These contingency measures must be fully 
adopted rules or control measures that take effect without further 
action by the State or the Administrator. The EPA interprets this 
requirement to mean that the contingency measures must be implemented 
with only minimal further action by the State or the affected sources 
with no additional rulemaking actions such as public hearings or 
legislative review.
    Emission inventories are also critical for the efforts of State, 
local, and Federal agencies to attain and maintain the NAAQS that EPA 
has established for criteria pollutants including SO2. 
Section 191(a) in conjunction with section 172(c) requires that areas 
designated as nonattainment for SO2 submit an emission 
inventory to EPA no later than 18 months after designation as 
nonattainment. In the case of SO2,

[[Page 35578]]

sections 191(a) and 172(c) also direct States to submit periodic 
emission inventories for nonattainment areas. The periodic inventory 
must include emissions of SO2 for point, nonpoint, mobile, 
and area sources.
b. Current Approach
    EPA did not receive any comments on this issue. Thus, EPA has no 
changes to make to this discussion.
2. New Source Review and Prevention of Significant Deterioration 
Requirements
a. Approach Described in the Proposal
    We provided a discussion of the new source review and prevention of 
significant deterioration programs in the preamble to the proposed 
rule. The Prevention of Significant Deterioration (PSD) and 
nonattainment New Source Review (NSR) programs contained in parts C and 
D of Title I of the CAA govern preconstruction review of any new or 
modified major stationary sources of air pollutants regulated under the 
CAA as well as any precursors to the formation of that pollutant when 
identified for regulation by the Administrator.\37\ The EPA rules 
addressing these programs can be found at 40 CFR 51.165, 51.166, 52.21, 
52.24, and Part 51, appendix S.
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    \37\ The terms ``major'' and ``minor'' define the size of a 
stationary source, for applicability purposes, in terms of an annual 
emissions rate (tons per year, tpy) for a pollutant. Generally, a 
minor source is any source that is not ``major.'' ``Major'' is 
defined by the applicable regulations--PSD or nonattainment NSR.
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    The PSD program applies when a major source located in an area that 
is designated as attainment or unclassifiable for any criteria 
pollutant is constructed or undergoes a major modification.\38\ The 
nonattainment NSR program applies on a pollutant-specific basis when a 
major source constructs or modifies in an area that is designated as 
nonattainment for that pollutant. The minor NSR program addresses major 
and minor sources that undergo construction or modification activities 
that do not qualify as major, and it applies, as necessary to assure 
attainment, regardless of the designation of the area in which a source 
is located.
---------------------------------------------------------------------------

    \38\ In addition, the PSD program applies to non-criteria 
pollutants subject to regulation under the Act, except those 
pollutants regulated under section 112 and pollutants subject to 
regulation only under section 211(o).
---------------------------------------------------------------------------

    The PSD requirements include but are not limited to the following:
     Installation of Best Available Control Technology (BACT);
     Air quality monitoring and modeling analyses to ensure 
that a project's emissions will not cause or contribute to a violation 
of any NAAQS or maximum allowable pollutant increase (PSD increment);
     Notification of Federal Land Manager of nearby Class I 
areas; and public comment on the permit.
    To the extent necessary to address these PSD requirements for the 
new 1-hour SO2 NAAQS, SIPs are due no later than 3 years 
after the promulgation date. Generally, however, the owner or operator 
of any major stationary source or major modification obtaining a final 
PSD permit on or after the effective date of the new 1-hour 
SO2 NAAQS will be required, as a prerequisite for the PSD 
permit, to demonstrate that the emissions increases from the new or 
modified source will not cause or contribute to a violation of that new 
NAAQS. The EPA anticipates that individual sources will be able to 
complete this demonstration under the PSD regulations based on current 
guidance in EPA's Guideline on Air Quality Models, Appendix W of 40 CFR 
Part 51.
    The owner or operator of a new or modified source will still be 
required to demonstrate compliance with the annual and 24-hour 
SO2 increments, even when their counterpart NAAQS are 
revoked. The annual and 24-hour increments are established in the CAA 
and will need to remain in the PSD regulations because EPA does not 
interpret the CAA to authorize EPA to remove them. It appears necessary 
for Congress to amend the CAA to make appropriate changes to the 
statutory SO2 increments. In 1990, the CAA was amended to 
accommodate PM10 increments in lieu of the statutory TSP 
increments.
    In association with the requirement to demonstrate compliance with 
the NAAQS and increments, the owner or operator of a new or modified 
source must submit for review and approval a source impact analysis and 
an air quality analysis. The source impact analysis, primarily a 
modeling analysis, must demonstrate that allowable emissions increases 
from the proposed source or modification, in conjunction with emissions 
from other existing sources will not cause or contribute to either a 
NAAQS or increment violation. The air quality analysis must assess the 
ambient air quality in the area that the proposed source or 
modification would affect.
    For the air quality analysis, the owner or operator must submit in 
its permit application air quality monitoring data that shall have been 
gathered over a period of one year and is representative of air quality 
in the area of the proposed project. If existing data representative of 
the area of the proposed project is not available, new data may need to 
be collected by the owner or operator of the source or modification. 
Where data is already available, it might be necessary to evaluate the 
location of the monitoring sites from which the SO2 data 
were collected in comparison to any new siting requirements associated 
with the 1-hour SO2 NAAQS. If existing sites are 
inappropriate for providing the necessary representative data, then new 
monitoring data will need to be collected by the owner or operator of 
the proposed project.
    Historically, EPA has allowed the use of several screening tools to 
help facilitate the implementation of the new source review program by 
reducing the permit applicant's burden, and streamlining the permitting 
process for de minimis circumstances. These screening tools include a 
significant emissions rate (SER), significant impact levels (SILs), and 
a significant monitoring concentration (SMC). The SER, as defined in 
tons per year for each regulated pollutant, is used to determine 
whether any proposed source or modification will emit sufficient 
amounts of a particular pollutant to require the review of that 
pollutant under the NSR permit program. EPA will consider whether to 
evaluate the existing SER for SO2 to see if it would change 
substantially based on the NAAQS levels for the 1-hour averaging 
period. Historically, for purposes of defining the SER, we have defined 
a de minimis pollutant impact as one that results in a modeled ambient 
impact of less than approximately 4% of the short-term NAAQS. The 
current SER for SO2 (40 tpy) is based on the impact on the 
24-hour SO2 NAAQS. See 45 FR 52676, 52707 (August 7, 1980). 
We have typically used the most sensitive averaging period to calculate 
the SER, and we may want to evaluate the new 1-hour period for 
SO2 because it is likely to represent the most sensitive 
averaging period for SO2.
    The SIL, expressed as an ambient pollutant concentration (ug/m3), 
is used to determine whether the impact of a particular pollutant is 
significant enough to warrant a complete air quality impact analysis 
for any applicable NAAQS and increments. EPA has promulgated 
regulations under 40 CFR 51.165(b) which include SILs for 
SO2 to determine whether a source's impact would be 
considered to cause or contribute to a NAAQS violation for the 3-hour 
(the secondary NAAQS), 24-hour or annual averaging periods. These SILs 
were originally developed in 1978 to limit the application of air 
quality dispersion models to a downwind

[[Page 35579]]

distance of no more than 50 kilometers or to ``insignificant levels.'' 
See 43 FR 26398, June 19, 1978. Through guidance, EPA has also allowed 
the use of SILs to determine whether or not it is necessary for a 
source to carry out a comprehensive source impact analysis and to 
determine the extent of the impact area in which the analysis will be 
carried out. The existing SILs for SO2 were not developed on 
the basis of specific SO2 NAAQS levels, so there may be no 
need to revise the existing SILs. Even upon revocation of the annual 
and 24-hour NAAQS, the corresponding SIL should still be useful for 
increment assessment. A SIL for the 1-hour averaging period does not 
exist, and would need to be developed for use with modeling for 1-hour 
SO2 NAAQS and any 1-hour increments.
    Finally, the SMC, also measured as an ambient pollutant 
concentration ([mu]g/m\3\), is used to determine whether it may be 
appropriate to exempt a proposed project from the requirement to 
collect ambient monitoring data for a particular pollutant as part of a 
complete permit application. EPA first defined SMCs for regulated 
pollutants under the PSD program in 1980. See 45 FR 52676, 52709-10 
(August 7, 1980). The existing SMC for SO2, based on a 24-
hour averaging period, may need to be re-evaluated to consider the 
effect of basing the SMC on the 1-hour averaging period, especially in 
light of revocation of the NAAQS for the 24-hour averaging period. 
Third, even if the 1-hour averaging period does not indicate the need 
for a revised SMC for SO2, the fact that the original SMC 
for SO2 is based on 1980 monitoring data (Lowest Detectable 
Level, correction factor of ``5''), could be a basis for revising the 
existing value. More up-to-date monitoring data and statistical 
analyses of monitoring accuracy may yield a different--possibly lower--
correction factor today. The new 1-hour NAAQS will not necessarily 
cause this result, but may provide a ``window of opportunity'' to re-
evaluate the SMC for SO2.
    States which have areas designated as nonattainment for the 
SO2 NAAQS are directed to submit, as a part of the SIP due 
18 months after an area is designated as nonattainment, provisions 
requiring permits for the construction and operation of new or modified 
stationary sources anywhere in the nonattainment area. Prior to 
adoption of the SIP revision addressing major source nonattainment NSR 
for SO2 nonattainment areas, the requirements of 40 CFR part 
51, appendix S will apply. Nonattainment NSR requirements include but 
are not limited to:
     Installation of Lowest Achievable Emissions Rate (LAER) 
control technology;
     Offsetting new emissions with creditable emissions 
reductions;
     A certification that all major sources owned and operated 
in the State by the same owner are in compliance with all applicable 
requirements under the CAA;
     An alternatives and siting analysis demonstrating that the 
benefits of a proposed source significantly outweigh the environmental 
and social costs imposed as a result of its location, construction, or 
modification; and
     Public comment on the permit.
    Minor NSR programs must meet the statutory requirements in section 
110(a)(2)(C) of the CAA which requires ``* * * regulation of the 
modification and construction of any stationary source * * * as 
necessary to assure that the [NAAQS] are achieved.'' These programs 
must be established in each State within 3 years of the promulgation of 
a new or revised NAAQS.
b. Comments and Responses
    Several commenters stated that in order to avoid confusion and lag 
time as it relates to PSD/NSR and permitting activities, which must be 
taken by States following the promulgation of the revised NAAQS, EPA 
must provide guidance as soon as possible related to these issues. 
Commenters also stated that EPA must develop guidance as soon as 
possible to address the screening tools for PSD/NSR such as SILs, SERs, 
SMCs, and the development of increments. Several commenters also stated 
that guidance should be provided as it relates to the use of AERMOD to 
address PSD issues.
    The EPA acknowledges that a decision to promulgate a new short-term 
SO2 NAAQS will have implications for the air permitting 
process. The full extent of how a new short-term SO2 NAAQS 
will affect the NSR process will need to be carefully evaluated. First, 
major new and modified sources applying for NSR/PSD permits will 
initially be required to demonstrate that their proposed emissions 
increases of SO2 will not cause or contribute to a violation 
of any NAAQS or PSD increments for SO2, including the new 1-
hour SO2 NAAQS. In addition, we believe that section 166(c) 
of the CAA authorizes EPA to consider the need to promulgate a new 1-
hour increment. Historically, EPA has developed increments for each 
applicable averaging period for which a NAAQS has been promulgated. 
However, increments for a particular pollutant do not necessarily need 
to match the averaging periods that have been established for NAAQS for 
the same pollutant. Environmental Defense Fund, Inc. v. EPA, 898 F.2d 
183, 189-190 (DC Cir. 1990) (``* * * the `goals and purposes' of the 
PSD program, set forth in Sec.  160, are not identical to the criteria 
on which the ambient standards are based.'') Thus, we would need to 
evaluate the need for a new 1-hour SO2 increment in 
association with the goals and purposes of the statutory PSD program 
requirements.
    We agree with the commenters that there may be a need for EPA to 
provide additional screening tools or to revise existing screening 
tools that are frequently used under the NSR/PSD program for reducing 
the burden of completing SO2 ambient air impact analyses. 
These screening tools include the SILs, as mentioned by the commenter, 
but also include the SER for emissions of SO2 and the SMC 
for SO2. The existing sceening tools apply to the averaging 
periods used to define the existing NAAQS for SO2, including 
the annual, 24-hour, and 3-hour averaging periods. EPA intends to 
evaluate the need for possible changes or additions to each of these 
useful screening tools for SO2 due to the revision of the 
SO2 NAAQS to provide for a 1-hour standard. We believe it is 
highly likely that in order to be most useful for implementing the new 
1-hour averaging period for NSR purposes, new 1-hour screening values 
will be appropriate.
    Finally, in response to the comment concerning the need for 
additional guidance as it relates to the use of AERMOD to address PSD 
issues, EPA anticipates providing additional technical guidance on 
modeling and analysis as a part of the SIP demonstration process. As 
stated previously, EPA intends to solicit public comment on guidance 
regarding modeling, and also solicit public comment on additional 
implementation planning guidance. However, EPA believes that the air 
quality models currently required for NSR/PSD permitting as provided in 
the EPA's Guideline on Air Quality Models, Appendix W of CFR 40 Part 51 
would be appropriate for demonstrating compliance with the revised 
SO2 NAAQS under these programs. At this time, EPA is not 
considering modifying the AERMOD dispersion model and its underlying 
science for predicting SO2 concentrations to accommodate the 
revised NAAQS for SO2.

[[Page 35580]]

c. Current Approach
    In the preamble to the proposed regulation, EPA noted that ``PSD 
permit requirements are effective on the promulgation date of a new or 
revised standard.'' However, this statement did not reflect an 
important distinction that needs to be clarified here. Under section 
51.166(b)(49)(i) and 52.21(b)(50)(i) of EPA's regulations, a pollutant 
that has not been regulated previously would become a ``regulated NSR 
pollutant'' upon promulgation of a NAAQS. See, 75 FR 17004, 17018-19. 
However, in the case of pollutants that are already ``regulated NSR 
pollutants,'' at the time a new NAAQS is promulgated or an existing 
NAAQS is revised, EPA interprets the CAA and EPA regulations to require 
implementation of the new or revised standard in the Federal PSD 
permitting process upon the effective date of any new or revised 
standards. Section 165(a)(3) of the CAA and section 52.21(k) of EPA's 
regulations require that a permit applicant demonstrate that it will 
not cause or contribute to a violation of ``any'' NAAQS. See, 
Memorandum from Stephen D. Page, Director of EPA Office of Air Quality 
Planning and Standards, ``Applicability of the Federal Prevention of 
Significant Deterioration Permit Requirements to New and Revised 
National Ambient Air Quality Standards'' (April 1, 2010).
    Amendments to the existing PSD requirements set forth in EPA 
regulations concerning SILs, SERs and SMCs may involve notice and 
comment rulemaking which could take at least one year to complete. For 
PM2.5, EPA developed SERs under the initial NSR 
implementation requirements for PM2.5. See 73 FR 28321, May 
16, 2008. The SILs and SMC for PM2.5 are being developed 
under a subsequent rulemaking simultaneously with the promulgation of 
PM2.5 increments, pursuant to a CAA schedule that allows EPA 
2 years from the promulgation of new and revised NAAQS to promulgate 
increments. Under such an approach, SILs and SMC are not available 
until the increments are promulgated. States and industry have 
criticized that approach because it has left State permitting 
authorities without an EPA-approved de minimis value that could be used 
in determining the level of analysis that individual PSD sources must 
undergo, and could result in more detailed analyses for sources that 
will have only have de miminis impacts on the NAAQS.
    To address this concern, we believe it is appropriate to proceed 
with development of the PSD screening tools in advance of an increment 
rulemaking to hasten their availability. In addition, we are assessing 
the possibility of developing interim screening tools that can be used 
by States prior to the completion of the SIP-development process if the 
States establish an appropriate record for individual permitting 
actions based on the supporting technical information provided by EPA. 
It is our expectation, that if such interim screening tools are 
appropriate, we would make the interim SIL and the supporting record 
for EPA's assessment available before the effective date of the new 1-
hour SO2 NAAQS to facilitate more efficient PSD permit 
reviews once the new standard becomes effective.
3. General Conformity
a. Approach Described in the Proposal
    Section 176(c) of the CAA requires that all Federal actions conform 
to an applicable implementation plan developed pursuant to section 110 
and part D of the CAA. The EPA rules developed under section 176(c) 
prescribe the criteria and procedures for demonstrating and assuring 
conformity of Federal actions to a SIP. Each Federal agency must 
determine that any actions covered by the general conformity rule 
conform to the applicable SIP before the action is taken. The criteria 
and procedures for conformity apply only in nonattainment areas and 
those nonattainment areas redesignated to attainment since 1990 
(``maintenance areas'') with respect to the criteria pollutants under 
the CAA: \39\ carbon monoxide (CO), lead (Pb), nitrogen dioxide 
(NO2), ozone (O3), particulate matter (PM2.5 and 
PM10), and sulfur dioxide (SO2). The general 
conformity rules apply one year following the effective date of 
designations for any new or revised NAAQS.\40\
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    \39\ Criteria pollutants are those pollutants for which EPA has 
established a NAAQS under section 109 of the CAA.
    \40\ Transportation conformity is required under CAA section 
176(c) (42 U.S.C. 7506(c) to ensure that Federally supported highway 
and transit project activities are consistent with (``conform to'') 
the purpose of the SIP. Transportation conformity applies to areas 
that are designated nonattainment, and those areas redesignated to 
attainment after 1990 (``maintenance areas'' with plans developed 
under CAA section 175A) for transportation-related criteria 
pollutants. Due to the relatively small amounts of sulfur in 
gasoline and on-road diesel fuel, transportation conformity does not 
apply to the SO2 NAAQS. 40 CFR 93.102(b)(1).
---------------------------------------------------------------------------

    The general conformity determination examines the impacts of direct 
and indirect emissions related to Federal actions. The general 
conformity rule provides several options to satisfy air quality 
criteria, such as modeling or offsets, and requires the Federal action 
to also meet any applicable SIP requirements and emissions milestones. 
The general conformity rule also requires that notices of draft and 
final general conformity determinations be provided directly to air 
quality regulatory agencies and to the public by publication in a local 
newspaper.
b. Current Approach
    EPA did not receive any comments on this aspect of the discussion 
in the proposal and expects to follow that approach.

E. Transition From the Existing SO2 NAAQS to a Revised 
SO2 NAAQS

a. Proposal
    In addition to proposing a short-term 1-hour SO2 NAAQS, 
EPA proposed to revoke the annual and 24-hour standards (annual 0.03 
ppm and 24-hour 0.14 ppm). Specifically, EPA proposed that the level 
for the 1-hour standard for SO2 be a range between 50-100 
ppb, and took comment on setting the level of the standard up to 150 
ppb. We explained that if the Administrator sets the 1-hour standard at 
100 ppb or lower, EPA proposed to revoke the 24-hour standard. If the 
Administrator set the level of the 1-hour standard between a range of 
100-150 ppb, then EPA proposed to retain the 24-hour standard.
    We explained that if EPA revised the SO2 NAAQS and 
revoked either the annual or 24-hour standard, EPA would need to 
promulgate adequate anti-backsliding provisions. The CAA establishes 
anti-backsliding requirements where EPA relaxes a NAAQS. Here, in EPA 
replacing the annual and 24-hour standards with a short term 1-hour 
standard, EPA must address the section 172(e) anti-backsliding 
provision of the CAA and determine whether it applies on its face or by 
analogy, and what provisions are appropriate to provide for transition 
to the new standard. States will need to insure that the health 
protection provided under the prior SO2 NAAQS continues to 
be achieved as well as maintained as States begin to implement the new 
NAAQS. This means that States are directed to continue implementing 
attainment and maintenance SIPs associated with the prior 
SO2 NAAQS until such time as they are subsumed by any new 
planning and control requirements associated with the new NAAQS.
    Whether or not section 172(e) directly applies to EPA's final 
action on the SO2 NAAQS, EPA has previously looked to other 
provisions of the CAA to determine how to address anti-

[[Page 35581]]

backsliding. The CAA contains a number of provisions that indicate 
Congress's intent to not allow provisions from implementation plans to 
be altered or removed if the plan revision would jeopardize the air 
quality protection being provided by the existing plan when EPA revises 
a NAAQS to make it more stringent. For example, section 110(l) provides 
that EPA may not approve a SIP revision if it interferes with any 
applicable requirement concerning attainment and RFP, or any other 
applicable requirement under the CAA. In addition, section 193 of the 
CAA prohibits the modification of a control, or a control requirement, 
in effect or required to be adopted as of November 15, 1990 (i.e., 
prior to the promulgation of the Clean Air Act Amendments of 1990), 
unless such a modification would ensure equivalent or greater emissions 
reductions. Further, section 172(e) of the CAA specifies that if EPA 
revises a NAAQS to make it less stringent than a previous NAAQS, 
control obligations no less stringent than those that apply in 
nonattainment area SIPs may not be relaxed, and adopting those controls 
that have not yet been adopted as needed may not be avoided. The intent 
of Congress, concerning the aforementioned sections of the CAA, was 
confirmed in a recent DC Circuit Court opinion on the Phase I ozone 
implementation rule. See South Coast Air Quality Management Dist. v. 
EPA, 472 F.3d 882 (DC Cir. 2006).
    To ensure that the anti-backsliding provisions and principles of 
section 172(e) are met and applied upon EPA revocation of the annual 
and 24-hour standards, EPA is providing that those SO2 NAAQS 
will remain in effect for one year following the effective date of the 
initial designations under section 107(d)(1) for the new SO2 
NAAQS before the current NAAQS are revoked in most attainment areas. 
However, any existing SIP provisions under CAA sections 110, 191 and 
192 associated with the annual and 24-hour SO2 NAAQS will 
remain in effect, including all currently implemented planning and 
emissions control obligations, including both those in the State's SIP 
and that have been promulgated by EPA in FIPs. This will ensure that 
both the new nonattainment NSR requirements and the general conformity 
requirements for a revised standard are in place so that there will be 
no gap in the public health protections provided by these two programs. 
It will also ensure that all nonattainment areas under the annual and/
or 24-hour NAAQS and all areas for which SIP calls have been issued 
will continue to be protected by currently required control measures.
    EPA is also providing that the annual and 24-hour NAAQS remain in 
place for any current nonattainment area, or any area for which a State 
has not fulfilled the requirements of a SIP call, until the affected 
area submits, and EPA approves, a SIP with an attainment, 
implementation, maintenance and enforcement SIP which fully addresses 
the attainment and maintenance requirements of the new SO2 
NAAQS. This, in combination with the CAA mechanisms provided in 
sections 110(l), 193, and 172(e) will help to ensure that continued 
progress is made toward timely attainment of the SO2 NAAQS. 
Also, in light of the nature of the new SO2 NAAQS, the lack 
of classifications (and mandatory controls associated with such 
classifications pursuant to the CAA), and the small number of current 
nonattainment areas, and areas subject to SIP calls, EPA believes that 
retaining the current standard for a limited period of time until 
attainment and maintenance SIPs are approved for the new standard in 
current nonattainment areas and SIP call areas, and one year after 
designations in other areas, will adequately serve the anti-backsliding 
requirements and goals of the CAA.\41\
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    \41\ The areas that are currently designated as nonattainment 
for the pre-existing SO2 primary NAAQS are Hayden, AZ; 
Armstrong, PA; Laurel, MT; Piti, GU; and Tanguisson, GU. The areas 
that are designated nonattainment for both the primary and the 
secondary standards are East Helena, MT, Salt Lake Co, MT, Toole Co, 
UT, and Warren Co, NJ. (See http://www.epa.gov/oar/oaqps/greenbk/lnc.html). The Billings/Laurel, MT, area is the only area currently 
subject to a SIP call.
---------------------------------------------------------------------------

b. Comments and Responses
    Several commenters stated that they support EPA's proposal stating 
that the annual and 24-hour SO2 NAAQS EPA would remain in 
effect for one year following the effective date of the initial 
designations under section 107(d)(1) for the revised SO2 
NAAQS before the current NAAQS are revoked in most attainment areas. 
The commenters also support EPA's proposal that any existing SIP 
provisions under CAA sections 110, 191 and 192 associated with the 
annual and 24-hour SO2 NAAQS would remain in effect, 
including all currently implemented planning and emissions control 
obligations, including both those in the State's SIP and that have been 
promulgated by EPA in FIPs. Several commenters also stated that they 
support EPA's proposal that an area's nonattainment designation and the 
subsequent CAA requirements under the current SO2 NAAQS will 
remain in effect until the affected State submits, and EPA approves a 
SIP which meets all of the relevant CAA requirements for the affected 
nonattainment area. EPA appreciates the support of the commenters on 
its strategy for addressing the anti-backsliding requirements related 
to the current and revised SO2 standard, pursuant to section 
172(e) of the CAA.
    One commenter, however, stated that while they support EPA's 
proposal to address the anti-backsliding provisions of section 172(e) 
of the CAA, they believe that EPA's proposal is deficient in several 
respects. The commenter stated that EPA's proposal to not terminate the 
annual and 24-hour standards for SO2 in any nonattainment 
area, or any area for which a State has not fulfilled the requirements 
of a SIP call, until after the affected area submits and EPA approves a 
SIP with an attainment demonstration which fully ``addresses'' the 
attainment requirements of the revised SO2 NAAQS is flawed. 
The commenter states that EPA's use of the term ``addresses'' is 
impermissibly and arbitrarily ambiguous and that the agency needs to 
clarify that ``fully addressing'' the attainment requirements of the 
revised NAAQS actually means providing for timely attainment of the 
NAAQS, and the submittal of a SIP that fully meets all of the 
requirements of section 110 and part D of Title I of the CAA, including 
sections 172, 173, and 191-193 of the CAA.
    Another commenter stated that the 24-hour SO2 standard 
should not be revoked in attainment areas until EPA approves section 
110(a)(2) ``infrastructure'' SIPs under the new 1-hour standard for 
such areas, in order to avoid delays in between attainment designation 
and such SIP approvals resulting in leaving the public unprotected or 
creating inter-state conflict that triggers section 126 petitions. This 
commenter further stated that the annual SO2 standard should 
not be revoked until EPA approves SIPs in attainment areas under the 
future SO2 secondary standard, which may also be based on an 
annual averaging time.
    EPA agrees with the comment made by the commenter regarding the 
need to approve SIPs in nonattainment areas (and in SIP call areas) 
before revoking the 24-hour and annual NAAQS for such areas. EPA 
clarifies that for those areas designated as nonattainment for the 
current NAAQS, or areas which have not met the requirements of a SIP 
call, that the State must submit a SIP that meets all of the applicable 
CAA requirements as they relate to section 110 and part D of Title I of 
the CAA, including sections 110(a), 172, 173, and 191-193 of the CAA. 
In addition to the

[[Page 35582]]

submittal of the SIP related to these requirements, EPA must approve 
the submittal for the area before the current standard can be revoked 
for the affected area.
    EPA disagrees with the comment. This rulemaking concerns only the 
primary standards for SO2. 74 FR at 64812 n. 2. The annual 
SO2 standard is a primary standard, not a secondary 
standard. See 40 CFR section 50.4 (a). The exclusive secondary standard 
for SO2 is the 3-hour standard codified in 40 CFR section 
50.5. EPA is not determining the adequacy of this secondary standard in 
this review or this rulemaking, as just noted. The commenter's request 
to retain the annual primary standard until SIPs reflecting a new 
secondary standard are approved is effectively a request to amend the 
present secondary standard, and is therefore inappropriate given the 
scope of this review. In any case, in the event that any substantive 
responsive to this comment is required, air quality information 
indicates that a 1-hour standard of 75 ppb is estimated to generally 
keep annual SO2 concentrations well below the level of the 
current annual standard. 74 FR at 64845. Thus, there would be no loss 
of protection to public welfare due to revocation of the annual primary 
standard.
    EPA further disagrees with the commenter's request that we not 
revoke the 24-hour standard in attainment areas before section 
110(a)(2) ``infrastructure'' SIPs are approved under the new 1-hour 
SO2 standard. An area that has shown it has attained the 24-
hour standard and that is not the subject of a SIP call, even after 
revocation of the 24-hour standard, will still have in its SIP its 
prior ``infrastucture'' SIP elements. There is no need to delay 
revocation when that will not cause the area to become subject to a new 
SIP under the new 1-hour NAAQS any faster than the statute already 
requires (i.e., three years from the date of promulgation of the new 
NAAQS). Furthermore, as we have explained in sections III, IV, V and VI 
of this preamble, all areas are required by section 110(a)(1) of the 
Clean Air Act to submit such SIPs by June 2013, and we expect that to 
be approved they will all need to show attainment, implementation, 
maintenance and enforcement of the new NAAQS as expeditiously as 
practicable, which we believe is no later than August 2017. EPA 
believes this anticipated approach would more than sufficiently address 
the backsliding concerns raised by the commenter.
c. Final
    EPA is making no changes to the proposed rule's discussion of the 
transition strategy discussion for SO2 with the exception of 
the clarifications noted above.

VII. Appendix T--Interpretation of the Primary NAAQS for Oxides of 
Sulfur and Revisions to the Exceptional Events Rule

    EPA proposed to add Appendix T, Interpretation of the Primary 
National Ambient Air Quality Standards for Oxides of Sulfur, to 40 CFR 
Part 50 in order to provide monitoring data handling procedures for the 
proposed SO2 1-hour primary standard. The proposed section 
50.17 which sets the averaging period, level, indicator, and form of 
the NAAQS referred to this Appendix T. The proposed Appendix T detailed 
the computations necessary for determining when the proposed 1-hour 
primary SO2 NAAQS is met based on data from ambient 
monitoring and also addressed monitoring data reporting, data 
completeness considerations, and rounding conventions.
    EPA proposed two versions of Appendix T. The first applied to a 1-
hour primary standard based on the annual 4th high value form, while 
the second applied to a 1-hour primary standard based on the 99th 
percentile daily value form. The final version of the Appendix reflects 
our choice to adopt the 99th percentile daily form (see section II. E.3 
above).
    For the 1-hour primary standard, EPA proposed monitoring data 
handling procedures, a cross-reference to the Exceptional Events Rule, 
a grant of discretion for the Administrator to consider otherwise 
incomplete monitoring data to be complete, and a provision addressing 
the possibility of there being multiple SO2 monitors at one 
site. EPA is finalizing these proposals, with one change from the 
proposal with regard to the multiple monitor provision.
    EPA is also making certain drafting changes to the proposed 
regulatory text to clarify certain points and to assure that the 
regulatory text conforms with EPA's intentions as stated in the 
preamble. Specifically, EPA has slightly edited the text of the rule 
from that proposed by adding the phrase ``at an ambient air monitoring 
site'' to section 50.17 (b) and to section 1.1 of Appendix T to part 
50, and also by adding a section 50.17 (c) stating that the level of 
the standard is to be measured by an FRM found in Appendix A or A-1 to 
Part 50, or by a properly designated FEM. Both of these provisions are 
being added to conform the text of the new 1-hour standard to the 
language of other NAAQS. See. e.g. the text of the 8-hour primary 
standard for ozone in section 50.10 (a) and (b). The reference to ``at 
an ambient monitoring site'' makes clear that the regulatory text 
refers to situations where compliance with a NAAQS is measured by means 
of monitoring. This text does not restrict or otherwise address 
approaches which EPA or States may use to implement the new 1-hour 
NAAQS, which may include, for example, use of modeling (see sections 
III--VI above). See CAA sections 107 (d) (3) (A) (any ``air quality 
data'' may be used for redesignations); 110 (a) (1) (which does not 
address the issue of the types of data States may use in devising plans 
for implementation, maintenance, and enforcement of a primary NAAQS); 
192 (a) (which does not specify the types of data that may support a 
demonstration that a non-attainment area has attained a NAAQS). 
Similarly, EPA notes that Appendix T applies when ambient monitoring 
data is gathered and utilized in support of the new 1-hour 
SO2 NAAQS. As noted in sections III, IV, V, and VI above, 
there are circumstances when EPA is considering use of modeling in the 
SO2 NAAQS implementation effort, and other considerations 
would apply if and to the extent modeling is utilized.
    The EPA is also making SO2-specific changes to the 
deadlines in 40 CFR 50.14, by which States must flag ambient air data 
that they believe have been affected by exceptional events and submit 
initial descriptions of those events, and to the deadlines by which 
States must submit detailed justifications to support the exclusion of 
those data from EPA monitoring-based determinations of attainment or 
nonattainment with the NAAQS.

A. Interpretation of the Primary NAAQS for Oxides of Sulfur

    The purpose of a monitoring data interpretation rule for the 
SO2 NAAQS is to give effect to the form, level, averaging 
time, and indicator specified in the regulatory text at 40 CFR 50.17, 
anticipating and resolving in advance various future ambiguities that 
could otherwise occur regarding use of ambient monitoring data. The new 
Appendix T provides definitions and requirements that apply to the new 
1-hour primary standard for SO2. The requirements concern 
how ambient monitoring data are to be reported, what ambient monitoring 
data are to be considered (including the issue of which of multiple 
monitors' data sets will be used when more than one monitor has 
operated at a site), and the

[[Page 35583]]

applicability of the Exceptional Events Rule to the primary 
SO2 NAAQS.
1. Proposed Interpretation of the Standard Based on Data From Ambient 
Monitoring
    With regard to monitoring data completeness for the proposed 1-hour 
primary standard, the proposed Appendix T followed past EPA practice 
for other NAAQS pollutants by requiring that in general at least 75% of 
the monitoring data that should have resulted from following the 
planned monitoring schedule in a period must be available for the key 
air quality statistic from that period to be considered valid. For the 
1-hour primary SO2 NAAQS, the key air quality statistics are 
the daily maximum 1-hour concentrations in three successive years. It 
is important that sampling within a day encompass the period when 
concentrations are likely to be highest and that all seasons of the 
year are well represented. Hence, the 75% requirement was proposed to 
be applied at the daily and quarterly levels.
    Recognizing that there may be years with incomplete data, the 
proposed Appendix T for the 99th percentile form provided that a design 
value derived from incomplete monitoring data will nevertheless be 
considered valid if the relevant one of two diagnostic substitution 
tests validated such a design value as being either above the NAAQS 
level or equal to or below the NAAQS level.
    The first proposed diagnostic data substitution test, relevant when 
the design value derived from incomplete data was equal to or below the 
NAAQS level, was intended to identify those cases with incomplete 
monitoring data in which it nevertheless is very likely, if not 
virtually certain, that the daily 1-hour design value would have been 
observed to be less than or equal to the level of the NAAQS if 
monitoring data had been minimally complete. This test involved the 
substitution of a high historical concentration for any missing data. 
The second proposed diagnostic data substitution test, relevant when 
the design value derived from incomplete data was above the NAAQS 
level, was intended to identify those cases with incomplete monitoring 
data in which it nevertheless is very likely, if not virtually certain, 
that the daily 1-hour design value would have been observed to be above 
the level of the NAAQS if monitoring data had been minimally complete. 
This test involved the substitution of a low historical concentration 
for any missing data.
    It should be noted that one possible outcome of applying the 
relevant proposed substitution test is that a 3-year period with 
incomplete monitoring data may nevertheless be determined to not have a 
valid design value and thus to be unusable in making 1-hour primary 
NAAQS compliance determinations based on monitoring for that 3-year 
period.
    Also, we proposed that the Administrator have general discretion to 
use incomplete monitoring data based on case specific factors, either 
at the request of a State or at her own initiative. Similar provisions 
existed already for some other NAAQS.
    The 99th percentile version of the proposed Appendix T provided a 
table for determining which day's maximum 1-hour concentration will be 
used as the 99th percentile concentration for the year. The proposed 
table is similar to one used now for the 24-hour PM2.5 NAAQS 
and the new 1-hour NO2 NAAQS, which are both based on a 98th 
percentile form, but adjusted to reflect a 99th percentile form for the 
1-hour primary SO2 standard. The proposed Appendix T also 
provided instructions for rounding (not truncating) the average of 
three annual 99th percentile hourly concentrations before comparison to 
the level of the primary NAAQS.
2. Comments on Interpretation of the Standard
    Several commenters expressed support for EPA's proposed 75% 
completeness requirement for daily and quarterly monitoring data. A 
comment was received that the substitution test should not be used to 
make attainment or non-attainment designations. This commenter also 
said that the same completeness requirement as used for nonattainment 
should be used for attainment. Another commenter agreed that there 
should be completeness criteria, but thought that monitoring data 
should be substituted to make the set only 75% complete. We received 
one comment that the computation of design values where multiple 
monitors are present at a site should be averaged and not taken from a 
designated primary monitor. We received no comment on the provision 
which would afford the Administrator (or her delegee) discretion to use 
incomplete monitoring data based on specified factors and accordingly 
are adopting that provision as proposed.
3. Conclusions on Interpretation of the Standard
    Consistent with the Administrator's decision to adopt a 99th 
percentile form for the 1-hour NAAQS, the final version of Appendix T 
is based on that form.
    We agree with the three comments expressing the view that the 
requirement for 75% monitoring data completeness per quarter should 
apply with respect to the 1-hour standard. The final rule includes this 
requirement.
    We agree that nonattainment based on data from ambient monitoring 
should not be declared without a very high confidence that actual air 
quality did not meet the NAAQS, but we believe the proposed (and final) 
substitution test provides this irrefutable proof. In the relevant 
substitution test (Appendix T section 3.c.iii), the lowest daily 
maximum concentration observed in the same calendar quarter within the 
3-year period is the value used in the substitution. Moreover, to guard 
against the possibility that even this lowest observed value is 
unrepresentative because only a small number of days that happened to 
have had poor air quality have valid monitoring data, substitution is 
permitted only if there are at least 200 days across the three matching 
quarters of the three years under consideration for which 75 percent of 
the hours in the day have reported concentrations. (If less than 200 
days are available, the outcome is that no conclusion can be reached 
based on data from monitoring as to whether the NAAQS is met, an 
outcome which satisfies the concern expressed by the commenter.) While 
it is conceivable that the actual daily maximum concentration on the 
day(s) without sufficiently complete data could have been even lower 
than the value selected as the substitute value, the value that is 
selected for substitution will be quite low, and therefore it is 
extremely unlikely to be a candidate for selection as the annual 99th 
percentile daily maximum concentration. The actual effect of the data 
substitution, if any, is to change which of the actually observed and 
ranked daily maximum concentrations during the year is identified as 
the 99th percentile; the direction of the change, if any, will always 
be towards a lower design value. For example, if the substitution test 
of section 3.c.iii is used because there is one quarter of 92 days is 
missing 70 of its 92 daily maximum concentration values; causing there 
to be only 295 days with valid daily values for the whole year, it 
would be necessary to substitute 47 values to make that quarter 75 
percent complete. This would result in 343 days of actual or 
substituted monitoring data for the year. The increase from 292 days to 
342 days would cause the annual 99th percentile value to shift from the 
3rd highest value to the 4th highest. Since a low

[[Page 35584]]

concentration is being used for the substitution, it is impossible for 
the 4th highest value to itself be a substituted value. If this shift 
results in the 3-year design value remaining above the NAAQS, the 
failure to meet the NAAQS is confirmed. If this shift results in the 3-
year design value changing to be equal to or below the NAAQS, under the 
terms of the substitution test the outcome is that no conclusion could 
be reached based on this ambient monitoring data as to whether the 
NAAQS is met. Since either the same or a lower ranking actually 
measured concentration will always be identified, it is impossible for 
the outcome of the substitution test of section 3.c.iii to be that an 
area truly meeting the NAAQS based on ambient monitoring data is 
determined to not meet it based on ambient monitoring data.
    The commenter who said that the same completeness requirement 
should be used for nonattainment as for attainment appears to have been 
referring to a particular feature of the proposed diagnostic 
substitution test rather than to the basic completeness requirement of 
75%, which in both the proposal and the final rule applies equally to 
both attainment and nonattainment situations. This particular feature 
is discussed in the next paragraph.
    The commenter who said that it is appropriate to substitute data to 
make the set only 75% complete appears to have taken note that in the 
proposed substitution test relevant in the case of an incomplete design 
value equal to or below the NAAQS (section 3.c.ii), data are 
substituted until 100% completeness is reached for the affected 
quarter, while in the test relevant in the case of an incomplete design 
value above the NAAQS (section 3.c.iii) data are substituted only until 
75% completeness is reached. EPA believes this distinction is 
appropriate, and we have retained the 100% substitution limit in the 
final rule. In the case of an incomplete design value that is equal to 
or below the NAAQS, the concern is that the actual concentrations on 
the days without a valid daily maximum 1-hour concentration may have 
been quite high such that the concentration on one of those days would 
have been selected as the annual 99th percentile value. To be selected 
as the annual 99th percentile value, a daily maximum must be ranked no 
lower than the 4th highest daily value for the year. If substitution 
stopped when 75% of the days in a quarter had an actual or substituted 
value, there could be a situation in which only one, two, or three 
historical high values would need to be substituted to reach the 75% 
limit. It would therefore be possible for one of the actually measured 
concentrations (for the same or another quarter) to be identified as 
the annual 99th percentile value even if the substitution value is 
higher than any value actually measured, defeating the very purpose of 
the diagnostic test for an incomplete design value below the NAAQS, 
which is to essentially rule out the possibility of not meeting the 
NAAQS (when making monitoring-based determinations). The simplest way 
to ensure that at least four values are substituted (when there are at 
least four missing daily values) is to require substitution up to the 
100% limit.
    With regard to situations with multiple monitors operating at one 
site, we note that there are few cases of this situation for 
SO2 monitoring. Of over 500 SO2 monitoring sites 
in operation any time during 2007-2009, for example, only seven 
stations reported 1-hour data to the Air Quality System under two or 
more distinct Pollutant Occurrence Codes (POC). In the same period, 
collocated monitors reported data to AQS under distinct POCs for only 
one of over 400 nitrogen dioxide sites, for only two of almost 400 
carbon monoxide sites, and for only eight of almost 1300 ozone sites. 
Even so, we believe is it important to have a well defined monitor data 
handling procedure for such situations. Also, there is a practical 
advantage in implementation if the same or similar procedure is used 
across NAAQS pollutants especially for these four gaseous pollutants 
that are measured on a 1-hour basis. A procedure that is simple to 
implement also has advantages in implementation. Finally, the procedure 
should not introduce any upward or downward bias in the determination 
of the design value for the monitoring site.\42\
---------------------------------------------------------------------------

    \42\ Selecting the maximum or minimum observed concentration for 
an hour, the maximum or minimum annual 99th percentile, or the 
maximum or minimum three-year design value would introduce such a 
bias. Averaging multiple 1-hour measurements when available, 
designating one monitor as primary and using a second monitor's 
measurement only when the primary monitor fails to give a valid 
measurement, or simply choosing to use the data record from only one 
of the monitors (on some basis that is independent of the 
concentration values obtained) would not introduce such a bias.
---------------------------------------------------------------------------

    The proposed procedure for multiple SO2 monitors was the 
same as EPA recently proposed and finalized for the new 1-hour NAAQS 
for nitrogen dioxide, where there were no adverse comments received on 
the proposal (75 FR 6474, February 9, 2010). It is also the same as 
recently proposed in the reconsideration of the 8-hour ozone NAAQS (75 
FR 2938, January 19, 2010). In the proposed procedure, in general, data 
from two monitors would never be mixed within a year but data from 
different monitors in different years could be used to calculate the 3-
year design value. As noted above, one commenter on the SO2 
proposal suggested that instead of designating a primary monitor when 
there are two monitors at a site, the measurements for an hour from 
multiple monitors should be averaged instead. EPA has also received at 
least one comment disagreeing with the recent proposal regarding 
multiple ozone monitors. The comment in the ozone rulemaking favored 
hour-by-hour substitution of data from a secondary monitor when the 
designated primary monitor has not given a value measurement, as 
opposed to the proposed restriction against mixing data within a year. 
These comments have caused us to rethink the direction set in the final 
NO2 rule and in the proposals for SO2 and ozone. 
We now believe that substitution of monitoring data hour-by-hour is an 
acceptable and in some ways superior approach to the other possible 
approaches, while averaging hour-by-hour would be unduly complex. Also, 
averaging hour-by-hour might not be transparent depending on whether 
the averaging is done at the monitoring agency before submission to EPA 
or by EPA as part of calculating a design value. However, in light of 
the rarity of collocated monitors, it would be an unwarranted demand on 
limited EPA resources to develop and maintain software for hour-by-hour 
data substitution. Also, an hour-by-hour data substitution approach 
depends on the advance designation of a primary monitor, which itself 
could introduce confusion and would require software changes to EPA's 
data system. Therefore, EPA believes that the most practical, and still 
a technically valid approach, is to allow monitoring agencies the 
option of hour-by-hour substitution between secondary and primary 
monitors before submission of data to EPA, and for EPA to select for 
use in calculating design values the one monitoring data record which 
has the highest degree of completeness for a given year. The final rule 
is based on this approach. EPA will also consider this approach when 
finalizing the ozone NAAQS reconsideration rule, and when proposing 
data interpretation provisions for a planned rulemaking to review the 
carbon monoxide NAAQS. The already finalized procedures for nitrogen 
dioxide data interpretation will be

[[Page 35585]]

implemented as promulgated, but will affect only an extremely small 
number of collocated SO2 monitoring situations.
    Finally, as proposed, the final version of Appendix T has a cross 
reference to the Exceptional Events Rule (40 CFR 50.14) with regard to 
the exclusion of monitoring data affected by exceptional events. In 
addition, the specific steps for including such data in completeness 
calculations while excluding such data from actual design value 
calculations is clarified in Appendix T.

B. Exceptional Events Information Submission Schedule

    The Exceptional Events Rule at 40 CFR 50.14 contains generic 
deadlines for a State to submit to EPA specified information about 
exceptional events and associated air pollutant concentration data. A 
State must initially notify EPA that data have been affected by an 
event by July 1 of the calendar year following the year in which the 
event occurred; this is done by flagging the data in AQS and providing 
an initial event description. The State must also, after notice and 
opportunity for public comment, submit a demonstration to justify any 
claim within 3 years after the quarter in which the data were 
collected. However, if a regulatory decision based on the data (for 
example, a designation action) is anticipated, the schedule to flag 
data in AQS and submit complete documentation to EPA for review is 
shortened, and all information must be submitted to EPA no later than 
one year before the decision is to be made.
    These generic deadlines are suitable for the period after initial 
designations have been made under a NAAQS, when the decision that may 
depend on data exclusion is a redesignation from attainment to 
nonattainment or from nonattainment to attainment. However, these 
deadlines present problems with respect to initial designations under a 
newly revised NAAQS. One problem is that some of the deadlines, 
especially the deadlines for flagging some relevant data, may have 
already passed by the time the revised NAAQS is promulgated. Until the 
level and form of the NAAQS have been promulgated a State does not know 
whether the criteria for excluding data (which are tied to the level 
and form of the NAAQS) were met on a given day. Another problem is that 
it may not be feasible for information on some exceptional events that 
may affect final designations to be collected and submitted to EPA at 
least one year in advance of the final designation decision. This could 
have the unintended consequence of EPA designating an area 
nonattainment because of uncontrollable natural or other qualified 
exceptional events.
    The Exceptional Events Rule at section 50.14(c)(2)(v) indicates 
``when EPA sets a NAAQS for a new pollutant, or revises the NAAQS for 
an existing pollutant, it may revise or set a new schedule for flagging 
data for initial designation of areas for those NAAQS.''
    For the specific case of SO2, the signature date for the 
revised SO2 NAAQS is June 2, 2010. State/Tribal area 
designations recommendations will be due by June 2, 2011, and EPA will 
make initial area designations under the revised NAAQS by June 1, 2012 
(since June 2, 2012 would be on a Saturday) and will be informed by air 
quality data from the years 2008-2010 or 2009-2011 if there is 
sufficient data for these data years and by any refined modeling that 
is conducted. (See Sections III, V and VI above for more detailed 
discussions of the designation schedule and what data EPA expects to 
use.) Because final designations would be made by June 1, 2012, all 
events to be considered during the designations process would have to 
be flagged and fully documented by States one year prior to 
designations, by June 1, 2011. A State would not be able to flag and 
submit documentation regarding events that occurred between June to 
December 2011 by one year before designations are made in June 2012.
    EPA is adopting revisions to 40 CFR 50.14 only to change submission 
dates for information supporting claimed exceptional events affecting 
SO2 data. The rule text at the end of this notice shows the 
changes that will apply to the new 1-hour SO2 NAAQS. For air 
quality data collected in 2008, we are extending the generic July 1, 
2009 deadline for flagging data (and providing a brief initial 
description of the event) to October 1, 2010. EPA believes this 
extension will provide adequate time for States to review the impact of 
exceptional events from 2008 on the revised standard and notify EPA by 
flagging the relevant data in AQS. EPA is not changing the 
foreshortened deadline of June 1, 2011 for submitting documentation to 
justify an SO2-related exceptional event from 2008. We 
believe the generic deadline provides adequate time for States to 
develop and submit proper documentation.
    For data collected in 2009, EPA is extending the generic deadline 
of July 1, 2010 for flagging data and providing initial event 
descriptions to October 1, 2010. EPA is retaining the deadline of June 
1, 2011 for States to submit documentation to justify an 
SO2-related exceptional event from 2009. For data collected 
in 2010, EPA is promulgating a deadline of June 1, 2011 for flagging 
data and providing initial event descriptions and for submitting 
documentation to justify exclusion of the flagged data. EPA believes 
that this deadline provides States with adequate time to review and 
identify potential exceptional events that occur in calendar year 2010, 
even for those events that might occur late in the year. EPA believes 
these deadlines will be feasible because experience suggests that 
exceptional events affecting SO2 data are few in number and 
easily assessed, so no State is likely to have a large workload.
    If a State intends 2011 data to be considered in SO2 
designations, 2011 data must be flagged and detailed event 
documentation submitted 60 days after the end of the calendar quarter 
in which the event occurred or by March 31, 2012, whichever date occurs 
first. Again, EPA believes these deadlines will be feasible because 
experience suggest that exceptional events affecting SO2 
data are few in number and easily assessed, so no State is likely to 
have a large workload.
    Table 1 summarizes the designation deadlines discussed in this 
section and provides designation schedule information from recent, 
pending or prior NAAQS revisions for other pollutants. EPA is revising 
the final SO2 exceptional event flagging and documentation 
submission deadlines accordingly to provide States with reasonably 
adequate opportunity to review, identify, and document exceptional 
events that may affect an area designation under a revised NAAQS.

[[Page 35586]]



      Table 1--Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
                                 Designations Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
                                        Air quality
  NAAQS pollutant/standard/(level)/   data collected    Event flagging & initial       Detailed documentation
          promulgation date            for calendar       description deadline           submission deadline
                                           year
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m\3\)       2004-2006  October 1, 2007 \a\.........  April 15, 2008 \a\.
 Promulgated October 17, 2006.
Ozone/8-Hr Standard (0.075 ppm)            2005-2007  June 18, 2009 \a\...........  June 18, 2009 \a\.
 Promulgated March 12, 2008.                    2008  June 18, 2009 \a\...........  June 18, 2009 \a\.
                                                2009  60 Days after the end of the  60 Days after the end of the
                                                       calendar quarter in which     calendar quarter in which
                                                       the event occurred or         the event occurred or
                                                       February 5, 2010, whichever   February 5, 2010, whichever
                                                       date occurs first \b\.        date occurs first \b\.
NO2/1-Hour Standard (80-100 PPB,                2008  July 1, 2010 \a\............  January 22, 2011 \a\.
 final level TBD).                              2009  July 1, 2010 \a\............  January 22, 2011 \a\.
                                                2010  April 1, 2011 \a\...........  July 1, 2011 \a\.
SO2/1-Hour Standard (50-100 PPB,                2008  October 1, 2010 \b\.........  June 1, 2011 \b\.
 final level TBD).                              2009  October 1, 2010 \b\.........  June 1, 2011 \b\.
                                                2010  June 1, 2011 \b\............  June 1, 2011 \b\.
                                                2011  60 Days after the end of the  60 Days after the end of the
                                                       calendar quarter in which     calendar quarter in which
                                                       the event occurred or March   the event occurred or March
                                                       31, 2012, whichever date      31, 2012, whichever date
                                                       occurs first \b\.             occurs first \b\.
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, and are shown in this table for
  informational purposes--the Agency is not opening these dates for comment under this rulemaking.
\b\ Indicates change from general schedule in 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
  initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
  notably, for data used by EPA for redesignations to attainment.

    Note further that EPA is reprinting portions of this Table in 
section 5014 but, with respect to the pollutants other than 
SO2, is doing so only for readers' convenience and is not 
reopening or otherwise reconsidering any aspect of the rules related to 
these other pollutants.

VIII. Communication of Public Health Information

    Information on the public health implications of ambient 
concentrations of criteria pollutants is currently made available 
primarily through EPA's Air Quality Index (AQI) program. The current 
AQI has been in use since its inception in 1999 (64 FR 42530). It 
provides accurate, timely, and easily understandable information about 
daily levels of pollution (40 CFR 58.50). The AQI establishes a 
nationally uniform system of indexing pollution levels for nitrogen 
dioxide, carbon monoxide, ozone, particulate matter and SO2. 
The AQI converts pollutant concentrations in a community's air to a 
number on a scale from 0 to 500. Reported AQI values enable the public 
to know whether air pollution levels in a particular location are 
characterized as good (0-50), moderate (51-100), unhealthy for 
sensitive groups (101-150), unhealthy (151-200), very unhealthy (201-
300), or hazardous (300-500). The AQI index value of 100 typically 
corresponds to the level of the short-term primary NAAQS for each 
pollutant. An AQI value greater than 100 means that a pollutant is in 
one of the unhealthy categories (i.e., unhealthy for sensitive groups, 
unhealthy, very unhealthy, or hazardous) on a given day; an AQI value 
at or below 100 means that a pollutant concentration is in one of the 
satisfactory categories (i.e., moderate or good). Decisions about the 
pollutant concentrations at which to set the various AQI breakpoints, 
that delineate the various AQI categories, draw directly from the 
underlying health information that supports the review of the primary 
NAAQS.
    The Agency recognizes the importance of revising the AQI in a 
timely manner to be consistent with any revisions to the primary NAAQS. 
Therefore, EPA proposed to finalize conforming changes to the AQI in 
connection with the Agency's final decision on the SO2 
NAAQS. Conforming changes that were proposed include setting the 100 
level of the AQI at the same level as the revised primary 
SO2 standard if a short-term primary standard was 
promulgated, and revising the other AQI breakpoints at the lower end of 
the AQI scale (i.e., AQI values of 50 and 150). EPA did not propose to 
change breakpoints at the higher end of the AQI scale (from 200 to 
500), which would apply to State contingency plans or the Significant 
Harm Level (40 CFR 51.16), because the information from this review 
does not inform decisions about breakpoints at those higher levels.
    With regard to an AQI value of 50, the breakpoint between the good 
and moderate categories, historically this value is set at the level of 
the annual NAAQS, if there is one, or one-half the level of the short-
term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 12, 
1998). Taking into consideration this practice, EPA proposed to set the 
AQI value of 50 to be between 25 and 50 ppb SO2, 1-hour 
average; stating that concentrations toward the lower end of this range 
would be appropriate if the standard was set at the lower end of the 
range of proposed standard levels, while concentrations toward the 
higher end of this range would be more appropriate if the standard was 
set at the higher end of the range of proposed standard levels. EPA 
solicited comments on this range for an AQI value of 50 and the 
appropriate basis for selecting an AQI value of 50.
    With regard to an AQI value of 150, the breakpoint between the 
unhealthy for sensitive groups and unhealthy categories, historically 
values between the short-term standard and an AQI value of 500 are set 
at levels that are approximately equidistant between the AQI values of 
100 and 500 unless there is health evidence that suggests a specific 
level would be appropriate (63 FR 67829, Dec. 12, 1998). For an AQI 
value of 150, EPA proposed to set the breakpoint within the range from 
175 to 200 ppb SO2, 1-hour average, since it represents the 
midpoint between the proposed range for the short-term

[[Page 35587]]

standard and the level of an AQI value of 200 (300 ppb SO2, 
1-hour average).
    EPA received few comments on the proposed breakpoints. Consistent 
with the level of the short-term primary SO2 standard 
promulgated in this rule, EPA is setting the AQI value of 100, the 
breakpoint between the moderate and unhealthy for sensitive groups 
category, at 75 ppb, 1-hour average. EPA is setting the AQI value of 
50, the breakpoint between the good and moderate categories, at 35 ppb 
SO2, 1-hour average, which is approximately one-half the 
level of the new short-term standard, since the annual SO2 
standard is being revoked. EPA is setting the AQI value of 150, the 
breakpoint between the unhealthy for sensitive groups and unhealthy 
categories, at 185 ppb SO2, 1-hour average, which represents 
the approximate midpoint between the level of the new short-term 
standard (75 ppb SO2, 1-hour average) and the level of an 
AQI value of 200 (300 ppb SO2, 1-hour average).
    EPA received comments from several State environmental 
organizations and organizations of State and local air agencies about 
forecasting and reporting the AQI for SO2. These commenters 
expressed the view that forecasting hourly SO2 
concentrations would be difficult. One commenter requested that EPA 
delay the forecasting requirement for one year and other agencies 
requested that EPA provide assistance in developing a forecast model. 
Another commenter expressed the view that it is impractical to 
incorporate SO2 into its forecasting and public health 
notification program because SO2 does not behave like a 
regional pollutant, and that exceedances may occur with little or no 
warning and for two hours or less. This commenter requested EPA 
consider the resources necessary for public communications at the State 
and local levels, particularly in areas where other air quality 
exceedances are relatively rare.
    EPA recommends and encourages air quality forecasting but it is not 
required (64 FR 42548; August 4, 1999). We agree that there will be new 
challenges associated with creating and communicating an SO2 
forecast, and will work with State and local agencies that want to 
develop an SO2 forecasting program on issues including, but 
not limited to, forecasting air quality for short time periods. We plan 
to work with State and local air agencies to figure out the best way to 
present this information to the public using the AQI.
    With respect to the comment that it is impractical to incorporate 
SO2 into a forecasting and public health notification 
program because SO2 does not behave like a regional 
pollutant, this final rule departs from the proposed rule in that it 
allows for a combined monitoring and modeling approach. Because of 
this, the monitoring network is not required to be wholly source-
oriented in nature. States have flexibility to allow required 
monitoring sites to serve multiple monitoring objectives including 
characterizing source impacts, highest concentrations, population 
exposure, background, and regional transport. Further, EPA expects that 
much of the existing network will be retained by States to satisfy the 
minimum monitoring requirements. This means that it is unlikely that 
AQI reporting and forecasting will be heavily driven by source-oriented 
monitors. Rather, many of the existing monitors (a majority of which 
are community-wide monitors) will remain in place, which prevents the 
need for new geographic regions to be delineated. With respect to 
concerns expressed about the resources required to report the AQI in 
areas were exceedances of the standard are very rare, Appendix G to 
Part 58 specifies that if the index value for a particular pollutant 
remains below 50 for a season or year, then a State or local agency may 
exclude the pollutant from the calculation of the AQI.

IX. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under section 3(f)(1) of Executive Order 12866 (58 FR 51735, 
October 4, 1993), this action is an ``economically significant 
regulatory action'' because it is likely to have an annual effect on 
the economy of $100 million or more. Accordingly, EPA submitted this 
action to the Office of Management and Budget (OMB) for review under EO 
12866 and any changes made in response to OMB recommendations have been 
documented in the docket for this action. In addition, EPA prepared a 
Regulatory Impact Analysis (RIA) of the potential costs and benefits 
associated with this action. However, the CAA and judicial decisions 
make clear that the economic and technical feasibility of attaining the 
national ambient standards cannot be considered in setting or revising 
NAAQS, although such factors may be considered in the development of 
State implementation plans to implement the standards. Accordingly, 
although an RIA has been prepared, the results of the RIA have not been 
considered by EPA in developing this final rule.
    When estimating the SO2- and PM2.5-related 
human health benefits and compliance costs in Table 2 below, EPA 
applied methods and assumptions consistent with the state-of-the-
science for human health impact assessment, economics and air quality 
analysis. EPA applied its best professional judgment in performing this 
analysis and believes that these estimates provide a reasonable 
indication of the expected benefits and costs to the nation of the 
selected SO2 standard and alternatives considered by the 
Agency. The Regulatory Impacts Analysis (RIA) available in the docket 
describes in detail the empirical basis for EPA's assumptions and 
characterizes the various sources of uncertainties affecting the 
estimates below.
    EPA's 2009 Integrated Science Assessment for Particulate Matter 
concluded, based on the scientific literature, that a no-threshold log-
linear model most adequately portrays the PM-mortality concentration-
response relationship. Nonetheless, consistent with historical practice 
and our commitment to characterizing the uncertainty in our benefits 
estimates, EPA has included a sensitivity analysis with an assumed 
threshold in the PM-mortality health impact function in the RIA. EPA 
has included a sensitivity analysis in the RIA to help inform our 
understanding of the health benefits which can be achieved at lower air 
quality concentration levels. While the primary estimate and the 
sensitivity analysis are not directly comparable, due to differences in 
population data and use of different analysis years, as well as the 
difference in the assumption of a threshold in the sensitivity 
analysis, comparison of the two results provide a rough sense of the 
proportion of the health benefits that occur at lower PM2.5 
air quality levels. Using a threshold of 10 [mu]g/m\3\ is an arbitrary 
choice (EPA could have assumed 6, 8, or 12 [mu]g/m\3\ for the 
sensitivity analysis). Assuming a threshold of 10 [mu]g/m\3\, the 
sensitivity analysis shows that roughly one-third of the benefits occur 
at air quality levels below that threshold. Because the primary 
estimates reflect EPA's current methods and data, EPA notes that 
caution should be exercised when comparing the results of the primary 
and sensitivity analyses. EPA appreciates the value of sensitivity 
analyses in highlighting the uncertainty in the benefits estimates and 
will continue to work to refine these analyses, particularly in those 
instances in which air quality modeling data are available.
    Table 2 shows the results of the cost and benefits analysis for 
each standard alternative. As indicated above, implementation of the 
SO2 control

[[Page 35588]]

measures identified from AirControlNET and other sources does not 
result in attainment with the all target NAAQS levels in several areas. 
In these areas, additional unspecified emission reductions might be 
necessary to reach some alternative standard levels. The first part of 
the table, labeled Partial attainment (identified controls), shows only 
those benefits and costs from control measures we were able to 
identify. The second part of the table, labeled Unidentified Controls, 
shows only additional benefits and costs resulting from unidentified 
controls. The third part of the table, labeled Full attainment, shows 
total benefits and costs resulting from both identified and 
unidentified controls. It is important to emphasize that we were able 
to identify control measures for a significant portion of attainment 
for many of those counties that would not fully attain the target NAAQS 
level with identified controls. Note also that in addition to 
separating full and partial attainment, the table also separates the 
portion of benefits associated with reduced SO2 exposure 
(i.e., SO2 benefits) from the additional benefits associated 
with reducing SO2 emissions, which are precursors to 
PM2.5 formation--(i.e., the PM2.5 co-benefits). 
For instance, for the selected standard of 75 ppb, $2.2 million in 
benefits are associated with reduced SO2 exposure while $15 
billion to $37 billion are associated with reduced PM2.5 
exposure.

                                    Table 2--Monetized Benefits and Costs To Attain Alternate Standard Levels in 2020
                                                                 [Millions of 2006$] \a\
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                      Number of
                                   counties fully   Discount rate   Monetized SO2   Monetized PM2.5 co-       Costs                Net benefits
                                     controlled       (percent)       benefits         benefits,c,d
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                        Partial Attainment (identified controls)
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb...........................              40               3             \b\  $30,000 to $74,000..          $2,600  $27,000 to $71,000.
                                   ..............               7  ..............  $28,000 to $67,000..  ..............  $25,000 to $64,000.
75 ppb...........................              20               3             \b\  $14,000 to $35,000..            $960  $13,000 to $34,000.
                                   ..............               7  ..............  $13,000 to $31,000..  ..............  $12,000 to $30,000.
100 ppb..........................               6               3             \b\  $6,900 to $17,000...            $470  $6,400 to $17,000.
                                   ..............               7  ..............  $6,200 to $15,000...  ..............  $5,700 to $15,000.
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                  Unidentified Controls
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb...........................              16               3             \b\  $4,000 to $9,000....          $1,800  $2,200 to $7,200.
                                   ..............               7  ..............  $3,000 to $8,000....  ..............  $1,200 to $6,200.
75 ppb...........................               4               3             \b\  $1,000 to $3,000....            $500  $500 to $1,500.
                                   ..............               7  ..............  $1,000 to $3,000....  ..............  $500 to $2,500.
100 ppb..........................               3               3             \b\  $500 to $1,000......            $260  $240 to $740.
                                   ..............               7  ..............  $500 to $1,000......  ..............  $240 to $740.
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                     Full Attainment
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb...........................              56               3           $8.50  $34,000 to $83,000..          $4,400  $30,000 to $79,000.
                                   ..............               7  ..............  $31,000 to $75,000..  ..............  $27,000 to $71,000.
75 ppb...........................              24               3           $2.20  $15,000 to $37,000..          $1,500  $14,000 to $36,000
                                   ..............               7  ..............  $14,000 to $34,000..  ..............  $13,000 to $33,000.
100 ppb..........................               9               3           $0.60  $7,400 to $18,000...            $730  $6,700 to $17,000.
                                   ..............               7  ..............  $6,700 to $16,000...  ..............  $6,000 to $15,000.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Estimates have been rounded to two significant figures and therefore summation may not match table estimates.
\b\ The approach used to simulate air quality changes for SO2 did not provide the data needed to distinguish partial attainment benefits from full
  attainment benefits from reduced SO2 exposure. Therefore, a portion of the SO2 benefits is attributable to the known controls and a portion of the SO2
  benefits are attributable to the unidentified controls. Because all SO2-related benefits are short-term effects, the results are identical for all
  discount rates.
\c\ Benefits are shown as a range from Pope et al. (2002) to Laden et al. (2006). Monetized benefits do not include unquantified benefits, such as other
  health effects, reduced sulfur deposition, or improvements in visibility.
\d\ These models assume that all fine particles, regardless of their chemical composition, are equally potent in causing premature mortality because
  there is no clear scientific evidence that would support the development of differential effects estimates by particle type. Reductions in SO2
  emissions from multiple sectors to meet the SO2 NAAQS would primarily reduce the sulfate fraction of PM2.5. Because this rule targets a specific
  particle precursor (i.e., SO2), this introduces some uncertainty into the results of the analysis.

B. Paperwork Reduction Act

    The information collection requirements in this final rule have 
been submitted for approval to the Office of Management and Budget 
(OMB) under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The 
Information Collection Request (ICR) document prepared by EPA for these 
revisions to part 58 has been assigned EPA ICR number 2370.02. The 
information collected under 40 CFR part 53 (e.g., test results, 
monitoring records, instruction manual, and other associated 
information) is needed to determine whether a candidate method intended 
for use in determining attainment of the NAAQS in 40 CFR part 50 will 
meet the design, performance, and/or comparability requirements for 
designation as a Federal reference method (FRM) or Federal equivalent 
method (FEM). We do not expect the number of FRM or FEM determinations 
to increase over the number that is currently used to estimate burden 
associated with SO2 FRM/FEM determinations provided in the 
current ICR for 40 CFR part 53 (EPA ICR numbers 2370.01). As such, no 
change in the burden estimate for 40 CFR part 53 has been made as part 
of this rulemaking.
    The information collected and reported under 40 CFR part 58 is 
needed to determine compliance with the NAAQS, to characterize air 
quality and

[[Page 35589]]

associated health impacts, to develop emissions control strategies, and 
to measure progress for the air pollution program. The amendments would 
revise the technical requirements for SO2 monitoring sites, 
require the siting and operation of additional SO2 ambient 
air monitors, and the reporting of the collected ambient SO2 
monitoring data to EPA's Air Quality System (AQS). The ICR is estimated 
to involve 102 respondents for a total approximate cost of $15,203,762 
(total capital, and labor and non-labor operation and maintenance) and 
a total burden of 207,662 hours. The labor costs associated with these 
hours is $11,130,409. Included in the $15,203,762 total are other costs 
of other non-labor operations and maintenance of $1,104,377 and 
equipment and contract costs of $2,968,975. In addition to the costs at 
the State and local air quality management agencies, there is a burden 
to EPA for a total of 14,749 hours and $1,060,621. Burden is defined at 
5 CFR 1320.3(b). State, local, and Tribal entities are eligible for 
State assistance grants provided by the Federal government under the 
CAA which can be used for monitors and related activities. An agency 
may not conduct or sponsor, and a person is not required to respond to, 
a collection of information unless it displays a currently valid OMB 
control number. The OMB control numbers for EPA's regulations in 40 CFR 
are listed in 40 CFR part 9.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency 
to prepare a regulatory flexibility analysis of any rule subject to 
notice and comment rulemaking requirements under the Administrative 
Procedure Act or any other statute unless the agency certifies that the 
rule will not have a significant economic impact on a substantial 
number of small entities. Small entities include small businesses, 
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of this rule on small 
entities, small entity is defined as: (1) A small business that is a 
small industrial entity as defined by the Small Business 
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small 
governmental jurisdiction that is a government of a city, county, town, 
school district or special district with a population of less than 
50,000; and (3) a small organization that is any not-for-profit 
enterprise which is independently owned and operated and is not 
dominant in its field.
    After considering the economic impacts of this final rule on small 
entities, I certify that this action will not have a significant 
economic impact on a substantial number of small entities. This final 
rule will not impose any requirements on small entities. Rather, this 
rule establishes national standards for allowable concentrations of 
SO2 in ambient air as required by section 109 of the CAA. 
American Trucking Ass'ns v. EPA, 175 F.3d 1027, 1044-45 (DC Cir. 1999) 
(NAAQS do not have significant impacts upon small entities because 
NAAQS themselves impose no regulations upon small entities). Similarly, 
the amendments to 40 CFR Part 58 address the requirements for States to 
collect information and report compliance with the NAAQS and will not 
impose any requirements on small entities.

D. Unfunded Mandates Reform Act

    This action is not subject to the requirements of sections 202 and 
205 of the UMRA. EPA has determined that this final rule does not 
contain a Federal mandate that may result in expenditures of $100 
million or more for State, local, and Tribal governments, in the 
aggregate, or the private sector in any one year. The revisions to the 
SO2 NAAQS impose no enforceable duty on any State, local or 
Tribal governments or the private sector. The expected costs associated 
with the monitoring requirements are described in EPA's ICR document, 
but those costs are not expected to exceed $100 million in the 
aggregate for any year. Furthermore, as indicated previously, in 
setting a NAAQS, EPA cannot consider the economic or technological 
feasibility of attaining ambient air quality standards. Because the CAA 
prohibits EPA from considering the types of estimates and assessments 
described in section 202 when setting the NAAQS, the UMRA does not 
require EPA to prepare a written statement under section 202 for the 
revisions to the SO2 NAAQS.
    With regard to implementation guidance, the CAA imposes the 
obligation for States to submit SIPs to implement the SO2 
NAAQS. In this final rule, EPA is merely providing an interpretation of 
those requirements. However, even if this rule did establish an 
independent obligation for States to submit SIPs, it is questionable 
whether an obligation to submit a SIP revision would constitute a 
Federal mandate in any case. The obligation for a State to submit a SIP 
that arises out of section 110 and section 191 of the CAA is not 
legally enforceable by a court of law, and at most is a condition for 
continued receipt of highway funds. Therefore, it is possible to view 
an action requiring such a submittal as not creating any enforceable 
duty within the meaning of U.S.C. 658 for purposes of the UMRA. Even if 
it did, the duty could be viewed as falling within the exception for a 
condition of Federal assistance under U.S.C. 658.
    EPA has determined that this final rule contains no regulatory 
requirements that might significantly or uniquely affect small 
governments because it imposes no enforceable duty on any small 
governments. Therefore, this rule is not subject to the requirements of 
section 203 of the UMRA.

E. Executive Order 13132: Federalism

    This final rule does not have federalism implications. It will not 
have substantial direct effects on the States, on the relationship 
between the national government and the States, or on the distribution 
of power and responsibilities among the various levels of government, 
as specified in Executive Order 13132. The rule does not alter the 
relationship between the Federal government and the States regarding 
the establishment and implementation of air quality improvement 
programs as codified in the CAA. Under section 109 of the CAA, EPA is 
mandated to establish NAAQS; however, CAA section 116 preserves the 
rights of States to establish more stringent requirements if deemed 
necessary by a State. Furthermore, this rule does not impact CAA 
section 107 which establishes that the States have primary 
responsibility for implementation of the NAAQS. Finally, as noted in 
section E (above) on UMRA, this rule does not impose significant costs 
on State, local, or Tribal governments or the private sector. Thus, 
Executive Order 13132 does not apply to this rule.

F. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    Executive Order 13175, entitled ``Consultation and Coordination 
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000), 
requires EPA to develop an accountable process to ensure ``meaningful 
and timely input by Tribal officials in the development of regulatory 
policies that have Tribal implications.'' This final rule does not have 
Tribal implications, as specified in Executive Order 13175. It does not 
have a substantial direct effect on one or more Indian Tribes, on the 
relationship between the Federal government and Indian Tribes, or on 
the distribution of power and responsibilities between the

[[Page 35590]]

Federal government and Tribes. The rule does not alter the relationship 
between the Federal government and Tribes as established in the CAA and 
the TAR. Under section 109 of the CAA, EPA is mandated to establish 
NAAQS; however, this rule does not infringe existing Tribal authorities 
to regulate air quality under their own programs or under programs 
submitted to EPA for approval. Furthermore, this rule does not affect 
the flexibility afforded to Tribes in seeking to implement CAA programs 
consistent with the TAR, nor does it impose any new obligation on 
Tribes to adopt or implement any NAAQS. Finally, as noted in section E 
(above) on UMRA, this rule does not impose significant costs on Tribal 
governments. Thus, Executive Order 13175 does not apply to this rule.

G. Executive Order 13045: Protection of Children From Environmental 
Health and Safety Risks

    This action is subject to Executive Order (62 FR 19885, April 23, 
1997) because it is an economically significant regulatory action as 
defined by Executive Order 12866, and we believe that the environmental 
health risk addressed by this action has a disproportionate effect on 
children. The final rule will establish uniform national ambient air 
quality standards for SO2; these standards are designed to 
protect public health with an adequate margin of safety, as required by 
CAA section 109. The protection offered by these standards may be 
especially important for asthmatics, including asthmatic children, 
because respiratory effects in asthmatics are among the most sensitive 
health endpoints for SO2 exposure. Because asthmatic 
children are considered a sensitive population, we have evaluated the 
potential health effects of exposure to SO2 pollution among 
asthmatic children. These effects and the size of the population 
affected are discussed in chapters 3 and 4 of the ISA; chapters 3, 4, 
7, 8, 9 of the REA, and sections II.A through II.E of this preamble.

H. Executive Order 13211: Actions That Significantly Affect Energy 
Supply, Distribution or Use

    This rule is not a ``significant energy action'' as defined in 
Executive Order 13211, ``Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 
28355; May 22, 2001) because it is not likely to have a significant 
adverse effect on the supply, distribution, or use of energy. The 
purpose of this rule is to establish revised NAAQS for SO2. 
The rule does not prescribe specific control strategies by which these 
ambient standards will be met. Such strategies will be developed by 
States on a case-by-case basis, and EPA cannot predict whether the 
control options selected by States will include regulations on energy 
suppliers, distributors, or users. Thus, EPA concludes that this rule 
is not likely to have any adverse energy effects.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement 
Act of 1995 (NTTAA), Public Law 104-113, section 12(d) (15 U.S.C. 27) 
directs EPA to use voluntary consensus standards in its regulatory 
activities unless to do so would be inconsistent with applicable law or 
otherwise impractical. Voluntary consensus standards are technical 
standards (e.g., materials specifications, test methods, sampling 
procedures, and business practices) that are developed or adopted by 
voluntary consensus standards bodies. The NTTAA directs EPA to provide 
Congress, through OMB, explanations when the Agency decides not to use 
available and applicable voluntary consensus standards.
    This final rulemaking involves technical standards with regard to 
ambient monitoring of SO2. The use of this voluntary 
consensus standard would be impractical because the analysis method 
does not provide for the method detection limits necessary to 
adequately characterize ambient SO2 concentrations for the 
purpose of determining compliance with the final revisions to the 
SO2 NAAQS.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations

    Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes 
Federal executive policy on environmental justice. Its main provision 
directs Federal agencies, to the greatest extent practicable and 
permitted by law, to make environmental justice part of their mission 
by identifying and addressing, as appropriate, disproportionately high 
and adverse human health or environmental effects of their programs, 
policies, and activities on minority populations and low-income 
populations in the United States.
    EPA has determined that this final rule will not have 
disproportionately high and adverse human health or environmental 
effects on minority or low-income populations because it increases the 
level of environmental protection for all affected populations without 
having any disproportionately high and adverse human health effects on 
any population, including any minority or low-income population. The 
final rule will establish uniform national standards for SO2 
in ambient air.

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List of Subjects

40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

40 CFR Part 53

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental

[[Page 35592]]

relations, Reporting and recordkeeping requirements.

40 CFR Part 58

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Reporting and 
recordkeeping requirements.

    Dated: June 2, 2010.
Lisa P. Jackson,
Administrator.

0
For the reasons stated in the preamble, title 40, chapter I of the Code 
of Federal Regulations is amended as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

0
1. The authority citation for part 50 continues to read as follows:

    Authority: 42 U.S.C. 7401, et seq.


0
2. Section 50.4 is amended by adding paragraph (e) to read as follows:


Sec.  50.4  National primary ambient air quality standards for sulfur 
oxides (sulfur dioxide).

* * * * *
    (e) The standards set forth in this section will remain applicable 
to all areas notwithstanding the promulgation of SO2 
national ambient air quality standards (NAAQS) in Sec.  50.17. The 
SO2 NAAQS set forth in this section will no longer apply to 
an area one year after the effective date of the designation of that 
area, pursuant to section 107 of the Clean Air Act, for the 
SO2 NAAQS set forth in Sec.  50. 17; except that for areas 
designated nonattainment for the SO2 NAAQS set forth in this 
section as of the effective date of Sec.  50. 17, and areas not meeting 
the requirements of a SIP call with respect to requirements for the 
SO2 NAAQS set forth in this section, the SO2 
NAAQS set forth in this section will apply until that area submits, 
pursuant to section 191 of the Clean Air Act, and EPA approves, an 
implementation plan providing for attainment of the SO2 
NAAQS set forth in Sec.  50.17.

0
3. Section 50.14 is amended by revising paragraph (c)(2)(vi) to read as 
follows:


Sec.  50.14  Treatment of air quality monitoring data influenced by 
exceptional events.

* * * * *
    (c) * * *
    (2) * * *
    (vi) When EPA sets a NAAQS for a new pollutant or revises the NAAQS 
for an existing pollutant, it may revise or set a new schedule for 
flagging exceptional event data, providing initial data descriptions 
and providing detailed data documentation in AQS for the initial 
designations of areas for those NAAQS. Table 1 provides the schedule 
for submission of flags with initial descriptions in AQS and detailed 
documentation. These schedules shall apply for those data which will or 
may influence the initial designation of areas for those NAAQS. EPA 
anticipates revising Table 1 as necessary to accommodate revised data 
submission schedules for new or revised NAAQS.

Table 1--Schedule of Exceptional Event Flagging and Documentation Submission for Data To Be Used in Designations
                                       Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
                                            Air quality
    NAAQS Pollutant/standard/(level)/     data collected   Event flagging & initial     Detailed documentation
            promulgation date              for calendar      description deadline         submission deadline
                                               year
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m3)             2004-2006  October 1, 2007 \a\.......  April 15, 2008. \a\
 Promulgated October 17, 2006.
----------------------------------------------------------------------------------------------------------------
Ozone/8-Hr Standard (0.075 ppm)                2005-2007  June 18, 2009 \a\.........  June 18, 2009 \a\
 Promulgated March 12, 2008.                        2008  June 18, 2009 \a\.........  June 18, 2009 \1\
                                                    2009  60 days after the end of    60 days after the end of
                                                           the calendar quarter in     the calendar quarter in
                                                           which the event occurred    which the event occurred
                                                           or February 5, 2010,        or February 5, 2010,
                                                           whichever date occurs       whichever date occurs
                                                           first \b\.                  first.\b\
----------------------------------------------------------------------------------------------------------------
NO2/1-Hour Standard (80-100 PPB, final              2008  July 1, 2010 \a\..........  January 22, 2011. \a\
 level TBD).                                        2009  July 1, 2010 \a\..........  January 22, 2011. \a\
                                                    2010  April 1, 2011 \a\.........  July 1, 2010. \a\
----------------------------------------------------------------------------------------------------------------
SO 2/1-Hour Standard (50-100 PPB, final             2008  October 1, 2010 \b\.......  June 1, 2011. \b\
 level TBD).                                        2009  October 1, 2010 \b\.......  June 1, 2011. \b\
                                                    2010  June 1, 2011. \b\.........  June 1, 2011. \b\
                                                    2011  60 days after the end of    60 days after the end of
                                                           the calendar quarter in     the calendar quarter in
                                                           which the event occurred    which the event occurred
                                                           or March 31, 2012,          or March 31, 2012,
                                                           whichever date occurs       whichever date occurs
                                                           first \b\.                  first. \b\
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, or are being proposed elsewhere
  and are shown in this table for informational purposes--the Agency is not opening these dates for comment
  under this rulemaking.
\b\ Indicates change from general schedule in 40 CFR 50.14.


    Note: EPA notes that the table of revised deadlines only applies 
to data EPA will use to establish the final initial designations for 
new or revised NAAQS. The general schedule applies for all other 
purposes, most notably, for data used by EPA for redesignations to 
attainment.

* * * * *

0
4. A new 50.17 is added to read as follows:


Sec.  50.17  National primary ambient air quality standards for sulfur 
oxides (sulfur dioxide).

    (a) The level of the national primary 1-hour annual ambient air 
quality standard for oxides of sulfur is 75 parts per billion (ppb, 
which is 1 part in 1,000,000,000), measured in the ambient air as 
sulfur dioxide (SO2).
    (b) The 1-hour primary standard is met at an ambient air quality 
monitoring site when the three-year average of the annual (99th 
percentile) of the daily maximum 1-hour average concentrations is less 
than or equal to 75 ppb, as determined in accordance with Appendix T of 
this part.
    (c) The level of the standard shall be measured by a reference 
method based on Appendix A or A-1 of this part, or by a Federal 
Equivalent Method (FEM)

[[Page 35593]]

designated in accordance with part 53 of this chapter.

0
5. Add Appendix A-1 to Part 50 to read as follows:

Appendix A-1 to Part 50--Reference Measurement Principle and 
Calibration Procedure for the Measurement of Sulfur Dioxide in the 
Atmosphere (Ultraviolet Fluorescence Method)

1.0 Applicability

    1.1 This ultraviolet fluorescence (UVF) method provides a 
measurement of the concentration of sulfur dioxide (SO2) 
in ambient air for determining compliance with the national primary 
and secondary ambient air quality standards for sulfur oxides 
(sulfur dioxide) as specified in Sec.  50.4, Sec.  50.5, and Sec.  
50.17 of this chapter. The method is applicable to the measurement 
of ambient SO2 concentrations using continuous (real-
time) sampling. Additional quality assurance procedures and guidance 
are provided in part 58, Appendix A, of this chapter and in 
Reference 3.

2.0 Principle

    2.1 This reference method is based on automated measurement of 
the intensity of the characteristic fluorescence released by 
SO2 in an ambient air sample contained in a measurement 
cell of an analyzer when the air sample is irradiated by ultraviolet 
(UV) light passed through the cell. The fluorescent light released 
by the SO2 is also in the ultraviolet region, but at 
longer wavelengths than the excitation light. Typically, optimum 
instrumental measurement of SO2 concentrations is 
obtained with an excitation wavelength in a band between 
approximately 190 to 230 nm, and measurement of the SO2 
fluorescence in a broad band around 320 nm, but these wavelengths 
are not necessarily constraints of this reference method. Generally, 
the measurement system (analyzer) also requires means to reduce the 
effects of aromatic hydrocarbon species, and possibly other 
compounds, in the air sample to control measurement interferences 
from these compounds, which may be present in the ambient air. 
References 1 and 2 describe UVF method.
    2.2 The measurement system is calibrated by referencing the 
instrumental fluorescence measurements to SO2 standard 
concentrations traceable to a National Institute of Standards and 
Technology (NIST) primary standard for SO2 (see 
Calibration Procedure below).
    2.3 An analyzer implementing this measurement principle is shown 
schematically in Figure 1. Designs should include a measurement 
cell, a UV light source of appropriate wavelength, a UV detector 
system with appropriate wave length sensitivity, a pump and flow 
control system for sampling the ambient air and moving it into the 
measurement cell, sample air conditioning components as necessary to 
minimize measurement interferences, suitable control and measurement 
processing capability, and other apparatus as may be necessary. The 
analyzer must be designed to provide accurate, repeatable, and 
continuous measurements of SO2 concentrations in ambient 
air, with measurement performance as specified in Subpart B of Part 
53 of this chapter.
    2.4 Sampling considerations: The use of a particle filter on the 
sample inlet line of a UVF SO2 analyzer is required to 
prevent interference, malfunction, or damage due to particles in the 
sampled air.

3.0 Interferences

    3.1 The effects of the principal potential interferences may 
need to be mitigated to meet the interference equivalent 
requirements of part 53 of this chapter. Aromatic hydrocarbons such 
as xylene and naphthalene can fluoresce and act as strong positive 
interferences. These gases can be removed by using a permeation type 
scrubber (hydrocarbon ``kicker''). Nitrogen oxide (NO) in high 
concentrations can also fluoresce and cause positive interference. 
Optical filtering can be employed to improve the rejection of 
interference from high NO. Ozone can absorb UV light given off by 
the SO2 molecule and cause a measurement offset. This 
effect can be reduced by minimizing the measurement path length 
between the area where SO2 fluorescence occurs and the 
photomultiplier tube detector (e.g. <5 cm). A hydrocarbon scrubber, 
optical filter and appropriate distancing of the measurement path 
length may be required method components to reduce interference.

4.0 Calibration Procedure

    Atmospheres containing accurately known concentrations of sulfur 
dioxide are prepared using a compressed gas transfer standard 
diluted with accurately metered clean air flow rates.
    4.1 Apparatus: Figure 2 shows a typical generic system suitable 
for diluting a SO2 gas cylinder concentration standard 
with clean air through a mixing chamber to produce the desired 
calibration concentration standards. A valve may be used to 
conveniently divert the SO2 from the sampling manifold to 
provide clean zero air at the output manifold for zero adjustment. 
The system may be made up using common laboratory components, or it 
may be a commercially manufactured system. In either case, the 
principle components are as follows:
    4.1.1 SO2 standard gas flow control and measurement 
devices (or a combined device) capable of regulating and maintaining 
the standard gas flow rate constant to within 2 percent 
and measuring the gas flow rate accurate to within 2, 
properly calibrated to a NIST-traceable standard.
    4.1.2 Dilution air flow control and measurement devices (or a 
combined device) capable of regulating and maintaining the air flow 
rate constant to within 2 percent and measuring the air 
flow rate accurate to within 2, properly calibrated to a 
NIST-traceable standard.
    4.1.3 Mixing chamber, of an inert material such as glass and of 
proper design to provide thorough mixing of pollutant gas and 
diluent air streams.
    4.1.4 Sampling manifold, constructed of glass, 
polytetrafluoroethylene (PTFE TeflonTM), or other 
suitably inert material and of sufficient diameter to insure a 
minimum pressure drop at the analyzer connection, with a vent 
designed to insure a minimum over-pressure (relative to ambient air 
pressure) at the analyzer connection and to prevent ambient air from 
entering the manifold.
    4.1.5 Standard gas pressure regulator, of clean stainless steel 
with a stainless steel diaphragm, suitable for use with a high 
pressure SO2 gas cylinder.

4.1.6 Reagents

    4.1.6.1 SO2 gas concentration transfer standard 
having a certified SO2 concentration of not less than 10 
ppm, in N2, traceable to a NIST Standard Reference 
Material (SRM).
    4.1.6.2 Clean zero air, free of contaminants that could cause a 
detectable response or a change in sensitivity of the analyzer. 
Since ultraviolet fluorescence analyzers may be sensitive to 
aromatic hydrocarbons and O2-to-N2 ratios, it 
is important that the clean zero air contains less than 0.1 ppm 
aromatic hydrocarbons and O2 and N2 
percentages approximately the same as in ambient air. A procedure 
for generating zero air is given in reference 1.

4.2 Procedure

    4.2.1 Obtain a suitable calibration apparatus, such as the one 
shown schematically in Figure 1, and verify that all materials in 
contact with the pollutant are of glass, TeflonTM, or 
other suitably inert material and completely clean.
    4.2.2 Purge the SO2 standard gas lines and pressure 
regulator to remove any residual air.
    4.2.3 Ensure that there are no leaks in the system and that the 
flow measuring devices are properly and accurately calibrated under 
the conditions of use against a reliable volume or flow rate 
standard such as a soap-bubble meter or a wet-test meter traceable 
to a NIST standard. All volumetric flow rates should be corrected to 
the same reference temperature and pressure by using the formula 
below:
[GRAPHIC] [TIFF OMITTED] TR22JN10.000

Where:

Fc = corrected flow rate (L/min at 25 [deg]C and 760 mm Hg),
Fm = measured flow rate, (at temperature, Tm and 
pressure, Pm),
Pm = measured pressure in mm Hg, (absolute), and
Tm = measured temperature in degrees Celsius.

    4.2.4 Allow the SO2 analyzer under calibration to 
sample zero air until a stable response is obtained, then make the 
proper zero adjustment.
    4.2.5 Adjust the airflow to provide an SO2 
concentration of approximately 80 percent of the upper measurement 
range limit of the SO2 instrument and verify that the 
total air flow of the calibration system exceeds the demand of all 
analyzers sampling from the output manifold (with the excess 
vented).
    4.2.6 Calculate the actual SO2 calibration 
concentration standard as:

[[Page 35594]]

[GRAPHIC] [TIFF OMITTED] TR22JN10.001

Where:

C = the concentration of the SO2 gas standard
Fp = the flow rate of SO2 gas standard
Ft = the total air flow rate of pollutant and diluent gases

    4.2.7 When the analyzer response has stabilized, adjust the 
SO2 span control to obtain the desired response 
equivalent to the calculated standard concentration. If substantial 
adjustment of the span control is needed, it may be necessary to re-
check the zero and span adjustments by repeating steps 4.2.4 through 
4.2.7 until no further adjustments are needed.
    4.2.8 Adjust the flow rate(s) to provide several other 
SO2 calibration concentrations over the analyzer's 
measurement range. At least five different concentrations evenly 
spaced throughout the analyzer's range are suggested.
    4.2.9 Plot the analyzer response (vertical or Y-axis) versus 
SO2 concentration (horizontal or X-axis). Compute the 
linear regression slope and intercept and plot the regression line 
to verify that no point deviates from this line by more than 2 
percent of the maximum concentration tested.

    Note: Additional information on calibration and pollutant 
standards is provided in Section 12 of Reference 3.

5.0 Frequency of Calibration

    The frequency of calibration, as well as the number of points 
necessary to establish the calibration curve and the frequency of 
other performance checking will vary by analyzer; however, the 
minimum frequency, acceptance criteria, and subsequent actions are 
specified in Reference 3, Appendix D: Measurement Quality Objectives 
and Validation Template for SO2 (page 9 of 30). The 
user's quality control program should provide guidelines for initial 
establishment of these variables and for subsequent alteration as 
operational experience is accumulated. Manufacturers of analyzers 
should include in their instruction/operation manuals information 
and guidance as to these variables and on other matters of 
operation, calibration, routine maintenance, and quality control.

6.0 References for SO2 Method

1. H. Okabe, P. L. Splitstone, and J. J. Ball, ``Ambient and Source 
SO2 Detector Based on a Fluorescence Method'', Journal of 
the Air Control Pollution Association, vol. 23, p. 514-516 (1973).
2. F. P. Schwarz, H. Okabe, and J. K. Whittaker, ``Fluorescence 
Detection of Sulfur Dioxide in Air at the Parts per Billion Level,'' 
Analytical Chemistry, vol. 46, pp. 1024-1028 (1974).
3. QA Handbook for Air Pollution Measurement Systems--Volume II. 
Ambient Air Quality Monitoring Programs. U.S. EPA. EPA-454/B-08-003 
(2008).
BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TR22JN10.002


[[Page 35595]]


[GRAPHIC] [TIFF OMITTED] TR22JN10.003

BILLING CODE 6560-50-C


0
6. Appendix A to Part 50 is redesignated as Appendix A-2 to Part 50.


0
7. Appendix T to Part 50 is added to read as follows:

Appendix T to Part 50--Interpretation of the Primary National Ambient 
Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide)

1. General

    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the primary national 
ambient air quality standards for Oxides of Sulfur as measured by 
Sulfur Dioxide (``SO2 NAAQS'') specified in Sec.  50.17 
are met at an ambient air quality monitoring site. Sulfur Dioxide 
(SO2) is measured in the ambient air by a Federal 
reference method (FRM) based on appendix A or A-1 to this part or by 
a Federal equivalent method (FEM) designated in accordance with part 
53 of this chapter. Data handling and computation procedures to be 
used in making comparisons between reported SO2 
concentrations and the levels of the SO2 NAAQS are 
specified in the following sections.
    (b) Decisions to exclude, retain, or make adjustments to the 
data affected by exceptional events, including natural events, are 
made according to the requirements and process deadlines specified 
in Sec. Sec.  50.1, 50.14 and 51.930 of this chapter.
    (c) The terms used in this appendix are defined as follows:
    Daily maximum 1-hour values for SO2 refers to the 
maximum 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    Design values are the metrics (i.e., statistics) that are 
compared to the NAAQS levels to determine compliance, calculated as 
specified in section 5 of this appendix. The design value for the 
primary 1-hour NAAQS is the 3-year average of annual 99th percentile 
daily maximum 1-hour values for a monitoring site (referred to as 
the ``1-hour primary standard design value'').
    99th percentile daily maximum 1-hour value is the value below 
which nominally 99 percent of all daily maximum 1-hour concentration 
values fall, using the ranking and selection method specified in 
section 5 of this appendix.
    Pollutant Occurrence Code (POC) refers to a numerical code (1, 
2, 3, etc.) used to distinguish the data from two or more monitors 
for the same parameter at a single monitoring site.
    Quarter refers to a calendar quarter.
    Year refers to a calendar year.

2. Requirements for Data Used for Comparisons With the SO2 
NAAQS and Data Reporting Considerations

    (a) All valid FRM/FEM SO2 hourly data required to be 
submitted to EPA's Air Quality System (AQS), or otherwise available 
to EPA, meeting the requirements of part 58 of this chapter 
including appendices A, C, and E shall be used in design value 
calculations. Multi-hour average concentration values collected by 
wet chemistry methods shall not be used.
    (b) Data from two or more monitors from the same year at the 
same site reported to EPA under distinct Pollutant Occurrence Codes 
shall not be combined in an attempt to meet data completeness 
requirements. The Administrator will combine annual 99th percentile 
daily maximum concentration values from different monitors in 
different years, selected as described here, for the purpose of 
developing a valid 1-hour primary standard design value. If more 
than one of the monitors meets the completeness requirement for all 
four quarters of a year, the steps specified in section 5(a) of this 
appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values to derive 
a valid annual 99th percentile daily maximum concentration. If no 
monitor is complete for all four quarters in a year, the steps 
specified in section 3(c) and 5(a) of this appendix shall be applied 
to the data from the monitor with the highest average of the four 
quarterly completeness values in an attempt to derive a valid annual 
99th percentile daily maximum concentration. This paragraph does not 
prohibit a monitoring agency from making a local designation of one 
physical monitor as the primary monitor for a Pollutant Occurrence 
Code and substituting the 1-hour data from a second physical monitor 
whenever a valid concentration value is not obtained from the 
primary monitor; if a monitoring agency substitutes data in this 
manner, each substituted value must be accompanied by an

[[Page 35596]]

AQS qualifier code indicating that substitution with a value from a 
second physical monitor has taken place.
    (c) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.

3. Comparisons With the 1-Hour Primary SO2 NAAQS

    (a) The 1-hour primary SO2 NAAQS is met at an ambient 
air quality monitoring site when the valid 1-hour primary standard 
design value is less than or equal to 75 parts per billion (ppb).
    (b) An SO2 1-hour primary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all 4 
quarters are complete. A quarter is complete when at least 75 
percent of the sampling days for each quarter have complete data. A 
sampling day has complete data if 75 percent of the hourly 
concentration values, including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator, are reported.
    (c) In the case of one, two, or three years that do not meet the 
completeness requirements of section 3(b) of this appendix and thus 
would normally not be useable for the calculation of a valid 3-year 
1-hour primary standard design value, the 3-year 1-hour primary 
standard design value shall nevertheless be considered valid if one 
of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of 
three consecutive years have at least one reported hourly value, and 
the design value calculated according to the procedures specified in 
section 5 is above the level of the primary 1-hour standard.
    (ii) (A) A 1-hour primary standard design value that is equal to 
or below the level of the NAAQS can be validated if the substitution 
test in section 3(c)(ii)(B) results in a ``test design value'' that 
is below the level of the NAAQS. The test substitutes actual 
``high'' reported daily maximum 1-hour values from the same site at 
about the same time of the year (specifically, in the same calendar 
quarter) for unknown values that were not successfully measured. 
Note that the test is merely diagnostic in nature, intended to 
confirm that there is a very high likelihood that the original 
design value (the one with less than 75 percent data capture of 
hours by day and of days by quarter) reflects the true under-NAAQS-
level status for that 3-year period; the result of this data 
substitution test (the ``test design value'', as defined in section 
3(c)(ii)(B)) is not considered the actual design value. For this 
test, substitution is permitted only if there are at least 200 days 
across the three matching quarters of the three years under 
consideration (which is about 75 percent of all possible daily 
values in those three quarters) for which 75 percent of the hours in 
the day, including State-flagged data affected by exceptional events 
which have been approved for exclusion by the Administrator, have 
reported concentrations. However, maximum 1-hour values from days 
with less than 75 percent of the hours reported shall also be 
considered in identifying the high value to be used for 
substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved 
for exclusion by the Administrator; if any quarter has less than 50 
percent data capture then this substitution test cannot be used. 
Identify for each quarter (e.g., January-March) the highest reported 
daily maximum 1-hour value for that quarter, excluding State-flagged 
data affected by exceptional events which have been approved for 
exclusion by the Administrator, looking across those three months of 
all three years under consideration. All daily maximum 1-hour values 
from all days in the quarter period shall be considered when 
identifying this highest value, including days with less than 75 
percent data capture. If after substituting the highest reported 
daily maximum 1-hour value for a quarter for as much of the missing 
daily data in the matching deficient quarter(s) as is needed to make 
them 100 percent complete, the procedure in section 5 yields a 
recalculated 3-year 1-hour standard ``test design value'' less than 
or equal to the level of the standard, then the 1-hour primary 
standard design value is deemed to have passed the diagnostic test 
and is valid, and the level of the standard is deemed to have been 
met in that 3-year period. As noted in section 3(c)(i), in such a 
case, the 3-year design value based on the data actually reported, 
not the ``test design value'', shall be used as the valid design 
value.
    (iii) (A) A 1-hour primary standard design value that is above 
the level of the NAAQS can be validated if the substitution test in 
section 3(c)(iii)(B) results in a ``test design value'' that is 
above the level of the NAAQS. The test substitutes actual ``low'' 
reported daily maximum 1-hour values from the same site at about the 
same time of the year (specifically, in the same three months of the 
calendar) for unknown hourly values that were not successfully 
measured. Note that the test is merely diagnostic in nature, 
intended to confirm that there is a very high likelihood that the 
original design value (the one with less than 75 percent data 
capture of hours by day and of days by quarter) reflects the true 
above-NAAQS-level status for that 3-year period; the result of this 
data substitution test (the ``test design value'', as defined in 
section 3(c)(iii)(B)) is not considered the actual design value. For 
this test, substitution is permitted only if there are a minimum 
number of available daily data points from which to identify the low 
quarter-specific daily maximum 1-hour values, specifically if there 
are at least 200 days across the three matching quarters of the 
three years under consideration (which is about 75 percent of all 
possible daily values in those three quarters) for which 75 percent 
of the hours in the day have reported concentrations. Only days with 
at least 75 percent of the hours reported shall be considered in 
identifying the low value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture. Identify for each quarter (e.g., January-March) the 
lowest reported daily maximum 1-hour value for that quarter, looking 
across those three months of all three years under consideration. 
All daily maximum 1-hour values from all days with at least 75 
percent capture in the quarter period shall be considered when 
identifying this lowest value. If after substituting the lowest 
reported daily maximum 1-hour value for a quarter for as much of the 
missing daily data in the matching deficient quarter(s) as is needed 
to make them 75 percent complete, the procedure in section 5 yields 
a recalculated 3-year 1-hour standard ``test design value'' above 
the level of the standard, then the 1-hour primary standard design 
value is deemed to have passed the diagnostic test and is valid, and 
the level of the standard is deemed to have been exceeded in that 3-
year period. As noted in section 3(c)(i), in such a case, the 3-year 
design value based on the data actually reported, not the ``test 
design value'', shall be used as the valid design value.
    (d) A 1-hour primary standard design value based on data that do 
not meet the completeness criteria stated in 3(b) and also do not 
satisfy section 3(c), may also be considered valid with the approval 
of, or at the initiative of, the Administrator, who may consider 
factors such as monitoring site closures/moves, monitoring 
diligence, the consistency and levels of the valid concentration 
measurements that are available, and nearby concentrations in 
determining whether to use such data.
    (e) The procedures for calculating the 1-hour primary standard 
design values are given in section 5 of this appendix.

4. Rounding Conventions for the 1-Hour Primary SO2 NAAQS

    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    (b) Daily maximum 1-hour values and therefore the annual 99th 
percentile of those daily values are not rounded.
    (c) The 1-hour primary standard design value is calculated 
pursuant to section 5 and then rounded to the nearest whole number 
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest 
whole number, and any decimal lower than 0.5 is rounded down to the 
nearest whole number).

5. Calculation Procedures for the 1-Hour Primary SO2 NAAQS

    (a) Procedure for identifying annual 99th percentile values. 
When the data for a particular ambient air quality monitoring site 
and year meet the data completeness requirements in section 3(b), or 
if one of the conditions of section 3(c) is met, or if the 
Administrator exercises the discretionary authority in section 3(d), 
identification of annual 99th percentile value is accomplished as 
follows.
    (i) The annual 99th percentile value for a year is the higher of 
the two values resulting from the following two procedures.

[[Page 35597]]

    (1) Procedure 1. For the year, determine the number of days with 
at least 75 percent of the hourly values reported.
    (A) For the year, determine the number of days with at least 75 
percent of the hourly values reported including State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator.
    (B) For the year, from only the days with at least 75 percent of 
the hourly values reported, select from each day the maximum hourly 
value excluding State-flagged data affected by exceptional events 
which have been approved for exclusion by the Administrator.
    (C) Sort all these daily maximum hourly values from a particular 
site and year by descending value. (For example: (x[1], x[2], x[3], 
* * *, x[n]). In this case, x[1] is the largest number and x[n] is 
the smallest value.) The 99th percentile is determined from this 
sorted series of daily values which is ordered from the highest to 
the lowest number. Using the left column of Table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99, y = the nth largest value.
    (2) Procedure 2. For the year, determine the number of days with 
at least one hourly value reported.
    (A) For the year, determine the number of days with at least one 
hourly value reported including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator.
    (B) For the year, from all the days with at least one hourly 
value reported, select from each day the maximum hourly value 
excluding State-flagged data affected by exceptional events which 
have been approved for exclusion by the Administrator.
    (C) Sort all these daily maximum values from a particular site 
and year by descending value. (For example: (x[1], x[2], x[3], * * 
*, x[n]). In this case, x[1] is the largest number and x[n] is the 
smallest value.) The 99th percentile is determined from this sorted 
series of daily values which is ordered from the highest to the 
lowest number. Using the left column of Table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99,y = the nth largest value.
    (b) The 1-hour primary standard design value for an ambient air 
quality monitoring site is mean of the three annual 99th percentile 
values, rounded according to the conventions in section 4.

                                 Table 1
------------------------------------------------------------------------
                                                     P0.99,y is the nth
  Annual number of days with valid data for year    maximum value of the
                    ``y'' (cny)                     year, where n is the
                                                        listed number
------------------------------------------------------------------------
1-100.............................................                     1
101-200...........................................                     2
201-300...........................................                     3
301-366...........................................                     4
------------------------------------------------------------------------

PART 53-AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS

0
8. The authority citation for part 53 continues to read as follows:

    Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec. 
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat. 
1713, unless otherwise noted.

Subpart A--[Amended]

0
9. Section 53.2 is amended by revising paragraphs (a)(1) and (b) to 
read as follows:


Sec.  53.2  General requirements for a reference method determination.

* * * * *
    (a) Manual methods--(1) Sulfur dioxide (SO2) and Lead. 
For measuring SO2 and lead, appendixes A-2 and G of part 50 
of this chapter specify unique manual FRM for measuring those 
pollutants. Except as provided in Sec.  53.16, other manual methods for 
lead will not be considered for a reference method determination under 
this part.
* * * * *
    (b) Automated methods. An automated FRM for measuring 
SO2, CO, O3, or NO2 must utilize the 
measurement principle and calibration procedure specified in the 
appropriate appendix to part 50 of this chapter (appendix A-1 only for 
SO2 methods) and must have been shown in accordance with 
this part to meet the requirements specified in this subpart A and 
subpart B of this part.

0
10. Section 53.8 is amended by revising paragraph (c) to read as 
follows:


Sec.  53.8  Designation of reference and equivalent methods.

* * * * *
    (c) The Administrator will maintain a current list of methods 
designated as FRM or FEM in accordance with this part and will send a 
copy of the list to any person or group upon request. A copy of the 
list will be available via the Internet and may be available from other 
sources.

0
11. Table A-1 to Subpart A is revised to read as follows:

  Table A-1 to Subpart A of Part 53--Summary of Applicable Requirements for Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                            Applicable subparts of part 53
     Pollutant          Reference or       Manual or automated   Applicable part 50 appendix -----------------------------------------------------------
                         equivalent                                                               A         B         C         D         E         F
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...............  Reference...........  Manual..............  A-2
                                          Automated...........  A-1                           [check]   [check]
                    Equivalent..........  Manual..............  A-1                           [check]             [check]
                                          Automated...........  A-1                           [check]   [check]   [check]
CO................  Reference...........  Automated...........  C                             [check]   [check]
                    Equivalent..........  Manual..............  C                             [check]             [check]
                                          Automated...........  C                             [check]   [check]   [check]
O3................  Reference...........  Automated...........  D                             [check]   [check]
                    Equivalent..........  Manual..............  D                             [check]             [check]
                                          Automated...........  D                             [check]   [check]   [check]
NO2...............  Reference...........  Automated...........  F                             [check]   [check]
                    Equivalent..........  Manual..............  F                             [check]             [check]
                                          Automated...........  F                             [check]   [check]   [check]
Pb................  Reference...........  Manual..............  G
                    Equivalent..........  Manual..............  G                             [check]             [check]
                                          Automated...........  G                             [check]             [check]
PM10-Pb...........  Reference...........  Manual..............  Q
                    Equivalent..........  Manual..............  Q                             [check]             [check]

[[Page 35598]]

 
                                          Automated...........  Q                             [check]             [check]
PM10..............  Reference...........  Manual..............  J                             [check]                       [check]
                    Equivalent..........  Manual..............  J                             [check]             [check]   [check]
                                          Automated...........  J                             [check]             [check]   [check]
PM2.5.............  Reference...........  Manual..............  L                             [check]                                 [check]
                    Equivalent Class I..  Manual..............  L                             [check]             [check]             [check]
                    Equivalent Class II.  Manual..............  L\1\                          [check]             [check]\            [check]   [check]\
                                                                                                                       2\                          1 2\
                    Equivalent Class III  Automated...........  L\1\                          [check]             [check]             [check]   [check]\
                                                                                                                                                     1\
PM10	2.5..........  Reference...........  Manual..............  L, O                          [check]                                 [check]
                    Equivalent Class I..  Manual..............  L, O                          [check]             [check]             [check]
                    Equivalent Class II.  Manual..............  L, O                          [check]             [check]\            [check]   [check]\
                                                                                                                       2\                          1 2\
                    Equivalent Class III  Automated...........  L\1\, O\1\                    [check]             [check]             [check]   [check]\
                                                                                                                                                     1\
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator.
2. Alternative Class III requirements may be substituted.

Subpart B--[Amended]

0
12. Section 53.20 is amended by revising paragraph (b) and Table B-1 in 
paragraph (c) to read as follows:


Sec.  53.20  General provisions.

* * * * *
    (b) For a candidate method having more than one selectable 
measurement range, one range must be that specified in table B-1 
(standard range for SO2), and a test analyzer representative 
of the method must pass the tests required by this subpart while 
operated in that range. The tests may be repeated for one or more 
broader ranges (i.e., ones extending to higher concentrations) than the 
range specified in table B-1, provided that the range does not extend 
to concentrations more than four times the upper range limit specified 
in table B-1. For broader ranges, only the tests for range 
(calibration), noise at 80% of the upper range limit, and lag, rise and 
fall time are required to be repeated. The tests may be repeated for 
one or more narrower ranges (ones extending to lower concentrations) 
than that specified in table B-1. For SO2 methods, table B-1 
specifies special performance requirements for narrower (lower) ranges. 
For methods other than SO2, only the tests for range 
(calibration), noise at 0% of the measurement range, and lower 
detectable limit are required to be repeated. If the tests are 
conducted or passed only for the specified range (standard range for 
SO2), any FRM or FEM method determination with respect to 
the method will be limited to that range. If the tests are passed for 
both the specified range and one or more broader ranges, any such 
determination will include the additional range(s) as well as the 
specified range, provided that the tests required by subpart C of this 
part (if applicable) are met for the broader range(s). If the tests are 
passed for both the specified range and one or more narrower ranges, 
any FRM or FEM method determination for the method will include the 
narrower range(s) as well as the specified range. Appropriate test data 
shall be submitted for each range sought to be included in a FRM or FEM 
method determination under this paragraph (b).
    (c) * * *

                                               Table B-1--Performance Specifications for Automated Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                        SO 2
                                                            ----------------------------                                         Definitions and test
       Performance parameter                Units \1\                           Lower        O 3          CO         NO 2             procedures
                                                             Std. range \3\   range 2 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range...........................  ppm...................         0-0.5       <0.5        0-0.5           0-50     0-0.5    Sec. 53.23(a).
2. Noise...........................  ppm...................           0.001      0.0005       0.005          0.5       0.005  Sec. 53.23(b).
3. Lower detectable limit..........  ppm...................           0.002      0.001        0.010          1.0       0.010  Sec. 53.23(c).
4. Interference equivalent
    Each interferent...............  ppm...................      0.005  minus>0.00  minus>0.02    minus>1.0  minus>0.02
                                                                                 5
    Total, all interferents........  ppm...................          --         --            0.06           1.5       0.04   Sec. 53.23(d).
5. Zero drift, 12 and 24 hour......  ppm...................      0.004  minus>0.00  minus>0.02    minus>1.0  minus>0.02
                                                                                 2
6. Span drift, 24 hour
    20% of upper range limit.......  Percent...............          --         --       20.0   minus>10.0  minus>20.0
    80% of upper range limit.......  Percent...............      3.0    minus>3.0   minus>5.0     minus>2.5  minus>5.0
7. Lag time........................  Minutes...............           2          2           20               10      20      Sec. 53.23(e).
8. Rise time.......................  Minutes...............           2          2           15                5      15      Sec. 53.23(e).
9. Fall time.......................  Minutes...............           2          2           15                5      15      Sec. 53.23(e).
10. Precision
    20% of upper range limit.......  ppm...................          --         --            0.010          0.5       0.020  Sec. 53.23(e).
                                     Percent...............           2          2       ..........  ...........  ..........  Sec. 53.23(e).
    80% of upper range limit.......  ppm...................          --         --            0.010          0.5       0.030  Sec. 53.23(e).
                                     Percent...............           2          2           --               --      --      Sec. 53.23(e).
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas.
  Percent means percent of the upper range limit.
2. Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range shows that
  the lower range specification is met for each of these test parameters.

[[Page 35599]]

 
3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct
  suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are
  within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct
  calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM
  designation.
4. For nitric oxide interference for the SO2 UVF method, interference equivalent is 0.003 ppm for the lower range.

* * * * *

0
13. Section 53.21 is amended by revising paragraph (a) to read as 
follows:


Sec.  53.21  Test conditions.

    (a) Set-up and start-up of the test analyzer shall be in strict 
accordance with the operating instructions specified in the manual 
referred to in Sec.  53.4(b)(3). Allow adequate warm-up or 
stabilization time as indicated in the operating instructions before 
beginning the tests. The test procedures assume that the test analyzer 
has an analog measurement signal output that is connected to a suitable 
strip chart recorder of the servo, null-balance type. This recorder 
shall have a chart width of a least 25 centimeters, chart speeds up to 
10 cm per hour, a response time of 1 second or less, a deadband of not 
more than 0.25 percent of full scale, and capability either of reading 
measurements at least 5 percent below zero or of offsetting the zero by 
at least 5 percent. If the test analyzer does not have an analog signal 
output, or if other types of measurement data output are used, an 
alternative measurement data recording device (or devices) may be used 
for the tests, provided it is reasonably suited to the nature and 
purposes of the tests and an analog representation of the analyzer 
measurements for each test can be plotted or otherwise generated that 
is reasonably similar to the analog measurement recordings that would 
be produced by a conventional chart recorder.
* * * * *

0
14. Section 53.22(d) is amended by revising Table B-2 to read as 
follows:


Sec.  53.22  Generation of test atmospheres.

* * * * *
    (d) * * *

                       Table B-2--Test Atmospheres
------------------------------------------------------------------------
           Test gas                  Generation          Verification
------------------------------------------------------------------------
Ammonia.......................  Permeation device.   Indophenol method,
                                 Similar to system    reference 3.
                                 described in
                                 references 1 and 2.
Carbon dioxide................  Cylinder of zero     Use NIST-certified
                                 air or nitrogen      standards whenever
                                 containing CO2 as    possible. If NIST
                                 required to obtain   standards are not
                                 the concentration    available, obtain
                                 specified in Table   2 standards from
                                 B-3.                 independent
                                                      sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Carbon monoxide...............  Cylinder of zero     Use a FRM CO
                                 air or nitrogen      analyzer as
                                 containing CO as     described in
                                 required to obtain   reference 8.
                                 the concentration
                                 specified in Table
                                 B-3.
Ethane........................  Cylinder of zero     Gas chromatography,
                                 air or nitrogen      ASTM D2820,
                                 containing ethane    reference 10. Use
                                 as required to       NIST-traceable
                                 obtain the           gaseous methane or
                                 concentration        propane standards
                                 specified in Table   for calibration.
                                 B-3.
Ethylene......................  Cylinder of pre-     Do.
                                 purified nitrogen
                                 containing
                                 ethylene as
                                 required to obtain
                                 the concentration
                                 specified in Table
                                 B-3.
Hydrogen chloride.............  Cylinder\1\ of pre-  Collect samples in
                                 purified nitrogen    bubbler containing
                                 containing           distilled water
                                 approximately 100    and analyze by the
                                 ppm of gaseous       mercuric
                                 HCL. Dilute with     thiocyante method,
                                 zero air to          ASTM (D612), p.
                                 concentration        29, reference 4.
                                 specified in Table
                                 B-3.
Hydrogen sulfide..............  Permeation device    Tentative method of
                                 system described     analysis for H2S
                                 in references 1      content of the
                                 and 2.               atmosphere, p.
                                                      426, reference 5.
Methane.......................  Cylinder of zero     Gas chromatography
                                 air containing       ASTM D2820,
                                 methane as           reference 10. Use
                                 required to obtain   NIST-traceable
                                 the concentration    methane standards
                                 specified in Table   for calibration.
                                 B-3.
Naphthalene...................  1. Permeation        Use NIST-certified
                                 device as            standards whenever
                                 described in         possible. If NIST
                                 references 1 and 2.  standards are not
                                2. Cylinder of pre-   available, obtain
                                 purified nitrogen    2 standards from
                                 containing 100 ppm   independent
                                 naphthalene.         sources which
                                 Dilute with zero     agree within 2
                                 air to               percent, or obtain
                                 concentration        one standard and
                                 specified in Table   submit it to an
                                 B-3..                independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Nitric oxide..................  Cylinder\1\ of pre-  Use NIST-certified
                                 purified nitrogen    standards whenever
                                 containing           possible. If NIST
                                 approximately 100    standards are not
                                 ppm NO. Dilute       available, obtain
                                 with zero air to     2 standards from
                                 required             independent
                                 concentration.       sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Nitrogen dioxide..............  1. Gas phase         1. Use an FRM NO2
                                 titration as         analyzer
                                 described in         calibrated with a
                                 reference 6.         gravimetrically
                                2. Permeation         calibrated
                                 device, similar to   permeation device.
                                 system described    2. Use an FRM NO2
                                 in reference 6..     analyzer
                                                      calibrated by gas-
                                                      phase titration as
                                                      described in
                                                      reference 6.
Ozone.........................  Calibrated ozone     Use an FEM ozone
                                 generator as         analyzer
                                 described in         calibrated as
                                 reference 9.         described in
                                                      reference 9.
Sulfur dioxide................  1. Permeation        Use an SO2 FRM or
                                 device as            FEM analyzer as
                                 described in         described in
                                 references 1 and 2.  reference 7.
                                2. Dynamic dilution
                                 of a cylinder
                                 containing
                                 approximately 100
                                 ppm SO2 as
                                 described in
                                 Reference 7..

[[Page 35600]]

 
Water.........................  Pass zero air        Measure relative
                                 through distilled    humidity by means
                                 water at a fixed     of a dew-point
                                 known temperature    indicator,
                                 between 20[deg]      calibrated
                                 and 30[deg] C such   electrolytic or
                                 that the air         piezo electric
                                 stream becomes       hygrometer, or wet/
                                 saturated. Dilute    dry bulb
                                 with zero air to     thermometer.
                                 concentration
                                 specified in Table
                                 B-3.
Xylene........................  Cylinder of pre-     Use NIST-certified
                                 purified nitrogen    standards whenever
                                 containing 100 ppm   possible. If NIST
                                 xylene. Dilute       standards are not
                                 with zero air to     available, obtain
                                 concentration        2 standards from
                                 specified in Table   independent
                                 B-3.                 sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Zero air......................  1. Ambient air
                                 purified by
                                 appropriate
                                 scrubbers or other
                                 devices such that
                                 it is free of
                                 contaminants
                                 likely to cause a
                                 detectable
                                 response on the
                                 analyzer.
                                2. Cylinder of       ...................
                                 compressed zero
                                 air certified by
                                 the supplier or an
                                 independent
                                 laboratory to be
                                 free of
                                 contaminants
                                 likely to cause a
                                 detectable
                                 response on the
                                 analyzer.
------------------------------------------------------------------------
\1\ Use stainless steel pressure regulator dedicated to the pollutant
  measured.
Reference 1. O'Keefe, A. E., and Ortaman, G. C. ``Primary Standards for
  Trace Gas Analysis,'' Anal. Chem. 38, 760 (1966).
Reference 2. Scaringelli, F. P., A. E. Rosenberg, E., and Bell, J. P.,
  ``Primary Standards for Trace Gas Analysis.'' Anal. Chem. 42, 871
  (1970).
Reference 3. ``Tentative Method of Analysis for Ammonia in the
  Atmosphere (Indophenol Method)'', Health Lab Sciences, vol. 10, No. 2,
  115-118, April 1973.
Reference 4. 1973 Annual Book of ASTM Standards, American Society for
  Testing and Materials, 1916 Race St., Philadelphia, PA.
Reference 5. Methods for Air Sampling and Analysis, Intersociety
  Committee, 1972, American Public Health Association, 1015.
Reference 6. 40 CFR 50 Appendix F, ``Measurement Principle and
  Calibration Principle for the Measurement of Nitrogen Dioxide in the
  Atmosphere (Gas Phase Chemiluminescence).''
Reference 7. 40 CFR 50 Appendix A-1, ``Measurement Principle and
  Calibration Procedure for the Measurement of Sulfur Dioxide in the
  Atmosphere (Ultraviolet FIuorscence).''
Reference 8. 40 CFR 50 Appendix C, ``Measurement Principle and
  Calibration Procedure for the Measurement of Carbon Monoxide in the
  Atmosphere'' (Non-Dispersive Infrared Photometry)''.
Reference 9. 40 CFR 50 Appendix D, ``Measurement Principle and
  Calibration Procedure for the Measurement of Ozone in the
  Atmosphere''.
Reference 10. ``Standard Test Method for C, through C5 Hydrocarbons in
  the Atmosphere by Gas Chromatography'', D 2820, 1987 Annual Book of
  Aston Standards, vol 11.03, American Society for Testing and
  Materials, 1916 Race St., Philadelphia, PA 19103.


0
15. Section 53.23(d) is amended by revising Table B-3 to read as 
follows:


Sec.  53.23  Test procedures.

* * * * *
    (d) * * *

                                                                 Table B-3--Interferent Test Concentration,\1\ Parts per Million
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                   Hydro-            Hydro-                                                                                Carbon
      Pollu- tant             Analyzer type       chloric   Ammo-     gen     Sulfur  Nitrogen   Nitric   Carbon   Ethy-    Ozone      M-    Water vapor    mon-    Meth-    Ethane  Naphthalene
                                                    acid     nia    sulfide  dioxide   dioxide   oxide   dioxide    lene             xylene                oxide     ane
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...................  Ultraviolet fluorescence  .......  .......  \5\ 0.1      \4\       0.5      0.5  .......  .......      0.5      0.2       20,000  .......  .......  .......   \6\ 0.05
                                                                                0.14
SO2...................  Flame photometric.......  .......  .......     0.01      \4\  ........  .......      750  .......  .......  .......   \3\ 20,000       50  .......  .......  ...........
                                                                                0.14
SO2...................  Gas chromatography......  .......  .......      0.1      \4\  ........  .......      750  .......  .......  .......   \3\ 20,000       50  .......  .......  ...........
                                                                                0.14
SO2...................  Spectrophotometric-wet        0.2      0.1      0.1      \4\       0.5  .......      750  .......      0.5  .......  ...........  .......  .......  .......  ...........
                         chemical                                               0.14
                         (pararosanaline).
SO2...................  Electrochemical.........      0.2      0.1      0.1      \4\       0.5      0.5  .......      0.2      0.5  .......   \3\ 20,000  .......  .......  .......  ...........
                                                                                0.14
SO2...................  Conductivity............      0.2      0.1  .......      \4\       0.5  .......      750  .......  .......  .......  ...........  .......  .......  .......  ...........
                                                                                0.14
SO2...................  Spectrophotometric-gas    .......  .......  .......      \4\       0.5  .......  .......  .......      0.5      0.2  ...........  .......  .......  .......  ...........
                         phase, including DOAS.                                 0.14
O3....................  Chemiluminescent........  .......  .......  \3\ 0.1  .......  ........  .......      750  .......      \4\  .......   \3\ 20,000  .......  .......  .......  ...........
                                                                                                                              0.08
O3....................  Electrochemical.........  .......  \3\ 0.1  .......      0.5       0.5  .......  .......  .......      \4\  .......  ...........  .......  .......  .......  ...........
                                                                                                                              0.08
O3....................  Spectrophotometric-wet    .......  \3\ 0.1  .......      0.5       0.5  \3\ 0.5  .......  .......      \4\  .......  ...........  .......  .......  .......  ...........
                         chemical (potassium                                                                                  0.08
                         iodide).
O3....................  Spectrophotometric-gas    .......  .......  .......      0.5       0.5      0.5  .......  .......      \4\     0.02       20,000  .......  .......  .......  ...........
                         phase, including                                                                                     0.08
                         ultraviolet absorption
                         and DOAS.
CO....................  Infrared................  .......  .......  .......  .......  ........  .......      750  .......  .......  .......       20,000   \4\ 10  .......  .......  ...........
CO....................  Gas chromatography with   .......  .......  .......  .......  ........  .......  .......  .......  .......  .......       20,000   \4\ 10  .......      0.5  ...........
                         flame ionization
                         detector.
CO....................  Electrochemical.........  .......  .......  .......  .......  ........      0.5  .......      0.2  .......  .......       20,000   \4\ 10  .......  .......  ...........
CO....................  Catalytic combustion-     .......      0.1  .......  .......  ........  .......      750      0.2  .......  .......       20,000   \4\ 10      5.0      0.5  ...........
                         thermal detection.
CO....................  IR fluorescence.........  .......  .......  .......  .......  ........  .......      750  .......  .......  .......       20,000   \4\ 10  .......      0.5  ...........
CO....................  Mercury replacement-UV    .......  .......  .......  .......  ........  .......  .......      0.2  .......  .......  ...........   \4\ 10  .......      0.5  ...........
                         photometric.
NO2...................  Chemiluminescent........  .......  \3\ 0.1  .......      0.5   \4\ 0.1      0.5  .......  .......  .......  .......       20,000  .......  .......  .......  ...........
NO2...................  Spectrophotometric-wet    .......  .......  .......      0.5   \4\ 0.1      0.5      750  .......      0.5  .......  ...........  .......  .......  .......  ...........
                         chemical (azo-dye
                         reaction).
NO2...................  Electrochemical.........      0.2  \3\ 0.1  .......      0.5   \4\ 0.1      0.5      750  .......      0.5  .......       20,000       50  .......  .......  ...........
NO2...................  Spectrophotometric-gas    .......  \3\ 0.1  .......      0.5   \4\ 0.1      0.5  .......  .......      0.5  .......       20,000       50  .......  .......  ...........
                         phase.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Concentrations of interferent listed must be prepared and controlled to 10 percent of the stated value.
2. Analyzer types not listed will be considered by the Administrator as special cases.
3. Do not mix with the pollutant.

[[Page 35601]]

 
4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.
6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate
  response for interference

* * * * *

Subpart C [Amended]

0
16. Section 53.32 is amended by revising paragraph (e)(2) to read as 
follows:


Sec.  53.32  Test procedures for methods for SO2, CO, 
O3, and NO2.

* * * * *
    (e) * * *
    (2) For a candidate method having more than one selectable range, 
one range must be that specified in table B-1 of subpart B of this 
part, and a test analyzer representative of the method must pass the 
tests required by this subpart while operated on that range. The tests 
may be repeated for one or more broader ranges (i.e., ones extending to 
higher concentrations) than the one specified in table B-1 of subpart B 
of this part, provided that such a range does not extend to 
concentrations more than four times the upper range limit specified in 
table B-1 of subpart B of this part and that the test analyzer has 
passed the tests required by subpart B of this part (if applicable) for 
the broader range. If the tests required by this subpart are conducted 
or passed only for the range specified in table B-1 of subpart B of 
this part, any equivalent method determination with respect to the 
method will be limited to that range. If the tests are passed for both 
the specified range and a broader range (or ranges), any such 
determination will include the broader range(s) as well as the 
specified range. Appropriate test data shall be submitted for each 
range sought to be included in such a determination.
* * * * *

0
17. Table C-1 to Subpart C is revised to read as follows:

          Table C-1 to Subpart C of Part 53--Test Concentration Ranges, Number of Measurements Required, and Maximum Discrepancy Specifications
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                            Simultaneous measurements required               Maximum
                                                                                   ----------------------------------------------------    discrepancy
                   Pollutant                       Concentration range, parts per            1-hour                    24-hour           specification,
                                                           million (ppm)           ----------------------------------------------------     parts per
                                                                                     First set    Second set   First set    Second set       million
--------------------------------------------------------------------------------------------------------------------------------------------------------
Ozone..........................................  Low 0.06 to 0.10.................            5            6  ...........  ...........              0.02
                                                 Med. 0.15 to 0.25................            5            6  ...........  ...........              0.03
                                                 High 0.35 to 0.46................            4            6  ...........  ...........              0.04
������������������������������������������������
                                                    Total.........................           14           18  ...........  ...........  ................
������������������������������������������������
Carbon monoxide................................  Low 7 to 11......................            5            6  ...........  ...........               1.5
                                                 Med. 20 to 30....................            5            6  ...........  ...........               2.0
                                                 High 25 to 45....................            4            6  ...........  ...........               3.0
������������������������������������������������
                                                    Total.........................           14           18  ...........  ...........  ................
������������������������������������������������
Sulfur dioxide.................................  Low 0.02 to 0.05.................            5            6            3            3              0.02
                                                 Med. 0.10 to 0.15................            5            6            2            3              0.03
                                                 High 0.30 to 0.50................            4            6            2            2              0.04
������������������������������������������������
                                                    Total.........................           14           18            7            8  ................
������������������������������������������������
Nitrogen dioxide...............................  Low 0.02 to 0.08.................  ...........  ...........            3            3              0.02
                                                 Med. 0.10 to 0.20................  ...........  ...........            2            2              0.02
                                                 High 0.25........................  ...........  ...........            2            2              0.03
������������������������������������������������
                                                    Total.........................  ...........  ...........            7            8  ................
--------------------------------------------------------------------------------------------------------------------------------------------------------

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

0
The authority citation for part 58 continues to read as follows:

    Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.

Subpart B [AMENDED]


0
19. Section 58.10, is amended by adding paragraph (a)(6) to read as 
follows:


Sec.  58.10  Annual monitoring network plan and periodic network 
assessment.

* * * * *
    (a) * * *
    (6) A plan for establishing SO2 monitoring sites in 
accordance with the requirements of appendix D to this part shall be 
submitted to the EPA Regional Administrator by July 1, 2011 as part of 
the annual network plan required in paragraph (a) (1). The plan shall 
provide for all required SO2 monitoring sites to be 
operational by January 1, 2013.
* * * * *


0
20. Section 58.12 is amended by adding paragraph (g) to read as 
follows:


Sec.  58.12  Operating Schedules

* * * * *
    (g) For continuous SO2 analyzers, the maximum 5-minute 
block average concentration of the twelve 5-minute blocks in each hour 
must be collected except as noted in Sec.  58.12 (a).
* * * * *


0
21. Section 58.13 is amended by adding paragraph (d) to read as 
follows:


Sec.  58.13  Monitoring network completion.

* * * * *
    (d) The network of SO2 monitors must be physically 
established no later than January 1, 2013, and at that time, must be 
operating under all of the requirements of this part, including the

[[Page 35602]]

requirements of appendices A, C, D, and E to this part.


0
22. Section 58.16 is amended by adding paragraph (g) to read as 
follows:


Sec.  58.16  Data submittal and archiving requirements.

* * * * *
    (g) Any State or, where applicable, local agency operating a 
continuous SO2 analyzer shall report the maximum 5-minute 
SO2 block average of the twelve 5-minute block averages in 
each hour, in addition to the hourly SO2 average.


0
23. Appendix A to Part 58 is amended as by adding paragraph 2.3.1.6 to 
read as follows:

Appendix A to Part 58--Quality Assurance Requirements for SLAMS, SPMs 
and PSD Air Monitoring

* * * * *
    2.3.1.6 Measurement Uncertainty for SO2. The goal for 
acceptable measurement uncertainty for precision is defined as an 
upper 90 percent confidence limit for the coefficient of variation 
(CV) of 10 percent and for bias as an upper 95 percent confidence 
limit for the absolute bias of 10 percent.
* * * * *


0
24. Appendix D to Part 58 is amended as by revising paragraph 4.4 to 
read as follows:

Appendix D to Part 58--Network Design Criteria for Ambient Air Quality 
Monitoring

* * * * *

4.4 Sulfur Dioxide (SO2) Design Criteria.

    4.4.1 General Requirements. (a) State and, where appropriate, 
local agencies must operate a minimum number of required 
SO2 monitoring sites as described below.
    4.4.2 Requirement for Monitoring by the Population Weighted 
Emissions Index. (a) The population weighted emissions index (PWEI) 
shall be calculated by States for each core based statistical area 
(CBSA) they contain or share with another State or States for use in 
the implementation of or adjustment to the SO2 monitoring 
network. The PWEI shall be calculated by multiplying the population 
of each CBSA, using the most current census data or estimates, and 
the total amount of SO2 in tons per year emitted within 
the CBSA area, using an aggregate of the most recent county level 
emissions data available in the National Emissions Inventory for 
each county in each CBSA. The resulting product shall be divided by 
one million, providing a PWEI value, the units of which are million 
persons-tons per year. For any CBSA with a calculated PWEI value 
equal to or greater than 1,000,000, a minimum of three 
SO2 monitors are required within that CBSA. For any CBSA 
with a calculated PWEI value equal to or greater than 100,000, but 
less than 1,000,000, a minimum of two SO2 monitors are 
required within that CBSA. For any CBSA with a calculated PWEI value 
equal to or greater than 5,000, but less than 100,000, a minimum of 
one SO2 monitor is required within that CBSA.
    (1) The SO2 monitoring site(s) required as a result 
of the calculated PWEI in each CBSA shall satisfy minimum monitoring 
requirements if the monitor is sited within the boundaries of the 
parent CBSA and is one of the following site types (as defined in 
section 1.1.1 of this appendix): population exposure, highest 
concentration, source impacts, general background, or regional 
transport. SO2 monitors at NCore stations may satisfy 
minimum monitoring requirements if that monitor is located within a 
CBSA with minimally required monitors under this part. Any monitor 
that is sited outside of a CBSA with minimum monitoring requirements 
to assess the highest concentration resulting from the impact of 
significant sources or source categories existing within that CBSA 
shall be allowed to count towards minimum monitoring requirements 
for that CBSA.
    4.4.3 Regional Administrator Required Monitoring. (a) The 
Regional Administrator may require additional SO2 
monitoring stations above the minimum number of monitors required in 
4.4.2 of this part, where the minimum monitoring requirements are 
not sufficient to meet monitoring objectives. The Regional 
Administrator may require, at his/her discretion, additional 
monitors in situations where an area has the potential to have 
concentrations that may violate or contribute to the violation of 
the NAAQS, in areas impacted by sources which are not conducive to 
modeling, or in locations with susceptible and vulnerable 
populations, which are not monitored under the minimum monitoring 
provisions described above. The Regional Administrator and the 
responsible State or local air monitoring agency shall work together 
to design and/or maintain the most appropriate SO2 
network to provide sufficient data to meet monitoring objectives.
    4.4.4 SO2 Monitoring Spatial Scales. (a) The 
appropriate spatial scales for SO2 SLAMS monitors are the 
microscale, middle, neighborhood, and urban scales. Monitors sited 
at the microscale, middle, and neighborhood scales are suitable for 
determining maximum hourly concentrations for SO2. 
Monitors sited at urban scales are useful for identifying 
SO2 transport, trends, and, if sited upwind of local 
sources, background concentrations.
    (1) Microscale--This scale would typify areas in close proximity 
to SO2 point and area sources. Emissions from stationary 
point and area sources, and non-road sources may, under certain 
plume conditions, result in high ground level concentrations at the 
microscale. The microscale typically represents an area impacted by 
the plume with dimensions extending up to approximately 100 meters.
    (2) Middle scale--This scale generally represents air quality 
levels in areas up to several city blocks in size with dimensions on 
the order of approximately 100 meters to 500 meters. The middle 
scale may include locations of expected maximum short-term 
concentrations due to proximity to major SO2 point, area, 
and/or non-road sources.
    (3) Neighborhood scale--The neighborhood scale would 
characterize air quality conditions throughout some relatively 
uniform land use areas with dimensions in the 0.5 to 4.0 kilometer 
range. Emissions from stationary point and area sources may, under 
certain plume conditions, result in high SO2 
concentrations at the neighborhood scale. Where a neighborhood site 
is located away from immediate SO2 sources, the site may 
be useful in representing typical air quality values for a larger 
residential area, and therefore suitable for population exposure and 
trends analyses.
    (4) Urban scale--Measurements in this scale would be used to 
estimate concentrations over large portions of an urban area with 
dimensions from 4 to 50 kilometers. Such measurements would be 
useful for assessing trends in area-wide air quality, and hence, the 
effectiveness of large scale air pollution control strategies. Urban 
scale sites may also support other monitoring objectives of the 
SO2 monitoring network such as identifying trends, and 
when monitors are sited upwind of local sources, background 
concentrations.
    4.4.5 NCore Monitoring. (a) SO2 measurements are 
included within the NCore multipollutant site requirements as 
described in paragraph (3)(b) of this appendix. NCore-based 
SO2 measurements are primarily used to characterize 
SO2 trends and assist in understanding SO2 
transport across representative areas in urban or rural locations 
and are also used for comparison with the SO2 NAAQS. 
SO2 monitors at NCore sites that exist in CBSAs with 
minimum monitoring requirements per section 4.4.2 above shall be 
allowed to count towards those minimum monitoring requirements.
* * * * *


0
25. Appendix G to Part 58 is amended as by revising Table 2 to read as 
follows:

Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily 
Reporting

* * * * *

[[Page 35603]]



                                                            Table 2--Breakpoints for the AQI
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                              These breakpoints                                                            Equal these AQI's
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                            PM2.5      PM10
      O3 (ppm)  8-hour        O3 (ppm)    ([mu]g/m   ([mu]g/  CO (ppm)  SO2 (ppm)  1-hour  NO2 (ppm)  1-hour    AQI                 Category
                             1-hour \1\     \3\)      m \3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.000-0.059................  ..........    0.0-15.4     0-54   0.0-4.4            0-0.035            0-0.053     0-50  Good.
0.060-0.075................  ..........   15.5-40.4   55-154   4.5-9.4        0.036-0.075        0.054-0.100   51-100  Moderate.
0.076-0.095................  0.125-0.16   40.5-65.4  155-254  9.5-12.4        0.076-0.185        0.101-0.360  101-150  Unhealthy for Sensitive Groups.
                                      4
0.096-0.115................  0.165-0.20   \3\ 65.5-  255-354  12.5-15.    \4\ 0.186-0.304         0.361-0.64  151-200  Unhealthy.
                                      4       150.4                  4
0.116-0.374................  0.205-0.40  \3\ 150.5-  355-424  15.5-30.    \4\ 0.305-0.604          0.65-1.24  201-300  Very Unhealthy.
                                      4       250.4                  4
(\2\)......................  0.405-0.50  \3\ 250.5-  425-504  30.5-40.    \4\ 0.605-0.804          1.25-1.64  301-400  .................................
                                      4       350.4                  4
(\2\)......................  0.505-0.60  \3\ 350.5-  505-604  40.5-50.    \4\ 0.805-1.004          1.65-2.04  401-500  Hazardous.
                                      4       500.4                  4
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour
  ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be
  calculated, and the maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>=301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations.
\3\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
\4\ 1-hr SO2 values do not define higher AQI values (>=200). AQI values of 200 or greater are calculated with 24-hour SO2 concentrations.

* * * * *

[FR Doc. 2010-13947 Filed 6-21-10; 8:45 am]
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