[Federal Register: October 20, 2010 (Volume 75, Number 202)]
[Rules and Regulations]
[Page 64863-64907]
[DOCID:fr20oc10-14]

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Part III

Environmental Protection Agency

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40 CFR Parts 51 and 52

Prevention of Significant Deterioration (PSD) for Particulate Matter
Less Than 2.5 Micrometers (PM2.5)--Increments, Significant
Impact Levels (SILs) and Significant Monitoring Concentration (SMC);
Final Rule

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 51 and 52

[EPA-HQ-OAR-2006-0605; FRL-9210-9]
RIN 2060-AO24

Prevention of Significant Deterioration (PSD) for Particulate
Matter Less Than 2.5 Micrometers (PM2.5)--Increments,
Significant Impact Levels (SILs) and Significant Monitoring
Concentration (SMC)

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The EPA is amending the requirements for particulate matter
less than 2.5 micrometers (PM2.5) under the Prevention of
Significant Deterioration (PSD) program by adding maximum allowable
increases in ambient pollutant concentrations (``increments'') and two
screening tools, known as the Significant Impact Levels (SILs) and a
Significant Monitoring Concentration (SMC) for PM2.5. The
SILs for PM2.5 are also being added to two other New Source
Review (NSR) rules that regulate the construction and modification of
any major stationary source locating in an attainment or unclassifiable
area, where the source's emissions may cause or contribute to a
violation of the national ambient air quality standards (NAAQS).

DATES: This final rule is effective on December 20, 2010.

ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2006-0605. All documents in the docket are
listed on the http://www.regulations.gov Web Site. Although listed in
the index, some information may not be publicly available, e.g.,
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Certain other material, such as
copyrighted material, is not placed on the Internet and 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 Docket, EPA/DC, EPA
West, Room 3334, 1301 Constitution Avenue, Northwest, 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<PHONE_NUMBER>, and the telephone number for the
Air Docket is<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT: Mr. Dan deRoeck, Air Quality Policy
Division, Office of Air Quality Planning and Standards (C504-03), U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711, telephone number:<PHONE_NUMBER>, facsimile number: (919) 541-
5509, e-mail address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION: The information in this Supplementary
Information section of this preamble is organized as follows:

I. General Information
    A. Does this action apply to me?
    B. Where can I get a copy of this document and other related
information?
II. Purpose
III. Overview of Final PM2.5 PSD Regulations
    A. Increments
    B. Significant Impact Levels
    C. Significant Monitoring Concentration
IV. Background
    A. PSD Program
    B. History of Particulate Matter (PM) NAAQS
    1. Total Suspended Particulate (TSP) and PM10 NAAQS
    2. PM2.5 NAAQS
    3. Revised PM2.5 and PM10 NAAQS
    C. Implementation of NSR for PM2.5
    D. Increments Under the PSD Program
    E. Historical Approaches for Developing Increments
    1. Congressional Enactment of Increments for PM and
SO2
    2. EPA's Promulgation of Increments for NO2 and
PM10
    a. Increments for NO2 Using the ``Contingent Safe
Harbor'' Approach Under Section 166(a) of the Act
    b. Increments for PM10 Using ``Equivalent
Substitution'' Approach Under Section 166(f) of the Act
V. Final Action on PM2.5 Increments
    A. Decision To Establish PM2.5 Increments Using
``Contingent Safe Harbor Approach'' Under Section 166(a)
    B. Rationale for the Applicability of Section 166(a)
    C. EPA's Interpretation of the Requirements Under Sections
166(a)-(d) of the Act
    1. Regulations as a Whole Should Fulfill Statutory Requirements
    2. Contingent Safe Harbor Approach
    3. The Statutory Factors Applicable Under Section 166(c)
    4. Balancing the Factors Applicable Under Section 166(c)
    5. Authority for States To Adopt Alternatives to Increments
    D. Framework for Pollutant-Specific PSD Regulations for
PM2.5
    1. Increment System
    2. Area Classifications
    3. Permitting Procedures
    4. AQRV Review by Federal Land Manager (FLM) and Reviewing
Authority
    5. Additional Impacts Analysis
    6. Installation of BACT
    E. Final PM2.5 Increments
    1. Identification of Safe Harbor Increments
    2. Data Used by EPA for the Evaluation of the Safe Harbor
Increments for PM2.5
    3. Scope of Effects Considered
    4. Evaluation of the Health and Welfare Effects of
PM2.5
    a. Health Effects
    b. Welfare Effects
    5. Fundamental Elements of Increments
    6. Evaluation of the Safe Harbor Increments
    7. Compliance Determinations for the PM2.5 Increments
    a. Modeling Compliance With PM2.5 Increments
    b. Condensable PM
    c. PM2.5 Precursors
    F. Final Action on Trigger and Baseline Dates for
PM2.5 Increments
    G. Definition of ``Baseline Area'' for PM2.5
    H. No Final Action With Respect to the Proposed Revocation of
PM10 Annual Increments
    I. Other Comments on Increments
VI. Final Action on PM2.5 SILs
    A. EPA's Determination on SILs for PM2.5
    B. Response to Comments Concerning the SILs
    1. Legal Basis for SILs
    2. Levels of the SILs
    a. Class I SILs
    b. Class II and III SILs
    3. Relationship Between SILs and AQRVs
    4. Form of the SILs
    5. SILs for Other Pollutants
VII. Final Action on the PM2.5 SMC
    A. EPA's Determination on the PM2.5 SMC
    B. Response to Comments Concerning the SMC
    1. Legal Issues
    2. Level of the SMC
    C. Correction of Cross Reference in PSD Ambient Monitoring
Requirements
VIII. Dates Associated With Implementation of the Final Rule
    A. Effective Date of the Final Rule
    1. State PSD Programs
    2. Federal PSD Program
    B. Transition Period
    C. SILs and SMC for PM2.5
IX. Other Regulatory Changes
X. 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 and 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
    K. Congressional Review Act
XI. Judicial Review
XII. Statutory Authority

I. General Information

A. Does this action apply to me?

    Entities affected by this rule include sources in all industry
groups. The majority of sources potentially affected are expected to be
in the following groups:

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                  Industry group                                              NAICS \a\
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Electric services.................................  221111, 221112, 221113, 221119, 221121, 221122
Petroleum refining................................  32411
Industrial inorganic chemicals....................  325181, 32512, 325131, 325182, 211112, 325998, 331311,
                                                     325188
Industrial organic chemicals......................  32511, 325132, 325192, 325188, 325193, 32512, 325199
Miscellaneous chemical products...................  32552, 32592, 32591, 325182, 32551
Natural gas liquids...............................  211112
Natural gas transport.............................  48621, 22121
Pulp and paper mills..............................  32211, 322121, 322122, 32213
Paper mills.......................................  322121, 322122
Automobile manufacturing..........................  336111, 336112, 336712, 336211, 336992, 336322, 336312,
                                                     33633, 33634, 33635, 336399, 336212, 336213
Pharmaceuticals...................................  325411, 325412, 325413, 325414
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\a\ North American Industry Classification System.

    Entities affected by this rule also include State and local
permitting authorities, and tribal authorities that implement these
regulations.

B. Where can I get a copy of this document and other related
information?

    In addition to being available in the docket, an electronic copy of
this final rule will also be available on the World Wide Web. Following
signature by the EPA Administrator, a copy of this final rule will be
posted in the regulations and standards section of our NSR home page
located at http://www.epa.gov/nsr.

II. Purpose

    The purpose of this rulemaking is to finalize certain program
provisions under the regulations to prevent significant deterioration
of air quality due to emissions of PM2.5 (i.e., under the
PM2.5 PSD regulations). This final rule supplements the
final implementation rule for PM2.5, known as the Clean Air
Fine Particle Implementation Rule (CAFPIR) that we promulgated on April
25, 2007 (72 FR 20586), and the PM2.5 NSR Implementation
Rule that we promulgated on May 16, 2008 (73 FR 28321). Together, these
three rules encompass the elements necessary for implementation of a
PM2.5 program in any area. This final rule is important
because it establishes increments, SILs, and an SMC for
PM2.5 to facilitate ambient air quality monitoring and
modeling under the PSD regulations for areas designated attainment or
unclassifiable for PM2.5.

III. Overview of Final PM2.5 PSD Regulations

A. Increments

    This rulemaking establishes increments for PM2.5
pursuant to the legal authority contained in section 166(a) of the
Clean Air Act (CAA or Act) for pollutants for which NAAQS are
promulgated after 1977. The final PM2.5 increments were
identified as Option 1 in the 2007 Notice of Proposed Rulemaking (NPRM)
for this action, and are as follows:

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                                                                 NAAQS            Increments ([mu]g/m\3\)
                      Averaging period                          ([mu]g/   --------------------------------------
                                                                 m\3\)       Class I      Class II    Class III
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Annual......................................................           15            1            4            8
24-hour.....................................................           35            2            9           18
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    As discussed in more detail in sections V.F and VIII, the
increments for PM2.5 will become applicable on October 20,
2011 in order to comply with section 166(b) of the Act (providing that
regulations under section 166(a) ``shall become effective one year
after the date of promulgation'').
    This final rule does not revoke the annual increments for
particulate matter less than 10 micrometers (PM10) as
proposed under Option 1 in the 2007 NPRM. Thus, we are retaining the
24-hour and annual PM10 increments in addition to adding
PM2.5 increments. This outcome is discussed in greater
detail in section V.H of this preamble.

B. Significant Impact Levels

    This rule establishes SILs for PM2.5 for evaluating the
impact a proposed new source or modification may have on the NAAQS and
PSD increments for PM2.5. The SILs for PM2.5 were
developed by scaling the existing PM10 SILs using a
PM2.5-to-PM10 NAAQS ratio. The final SILs were
identified as Option 3 in the 2007 NPRM, and are as follows:

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                                             SILs ([mu]g/m\3\)
         Averaging period         --------------------------------------
                                     Class I      Class II    Class III
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Annual...........................         0.06          0.3          0.3
24-hour..........................         0.07          1.2          1.2
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    These values will be added to the State implementation plan (SIP)
provisions for PSD at 40 CFR 51.166 (as an optional screening tool) and
the Federal PSD program at 40 CFR 52.21, as well as under the
preconstruction review permit requirements at 40 CFR 51.165(b) and part
51, Appendix S. See a more detailed discussion of the SILs, as well as
the relevant comments and our responses to them, in section VI of this
preamble. The SILs for PM2.5 are incorporated into the
Federal PSD program as well as into the regulations for State-
implemented PSD programs, although they are regarded as optional for
State programs. The effective date for implementing the SILs under the
Federal PSD program is the effective date of this final rule. See
section VIII of this preamble for further discussion of the effective
date.

C. Significant Monitoring Concentration

    This final rule establishes the SMC for PM2.5 as 4
[mu]g/m\3\ PM2.5 (24-hour average). This value has been
developed pursuant to proposed Option 1; however, it should be noted
that the value being established in this final rule is lower than the
proposed value of 10 [mu]g/m\3\ that was originally developed under
Option 1. A more detailed discussion of the proposed SMC is presented
in section VII of this preamble, describing the rationale for altering
the proposed SMC, and the relevant comments on the proposed SMC and our
responses to them. The SMC for PM2.5 is incorporated into
the Federal PSD program as well as into the regulations for State-
implemented PSD programs, although they are regarded as optional for
State programs. As with the SILs for PM2.5, the effective
date for implementing the SMC under the Federal PSD program is the
effective date of this final rule. See section VIII of this preamble
for further discussion of the effective date.

IV. Background

A. PSD Program

    The NSR provisions of the Act are a combination of air quality
planning and air pollution control technology program requirements for
new and modified stationary sources of air pollution. In brief, section
109 of the Act requires us to promulgate primary NAAQS to protect
public health and secondary NAAQS to protect public welfare. Once we
have set these standards, states must develop, adopt, and submit to us
for approval SIPs that contain emission limitations and other control
measures to attain and maintain the NAAQS and to meet the other
requirements of section 110(a) of the Act. Part C of title I of the Act
contains the requirements for a component of the major NSR program
known as the PSD program. This program sets forth procedures for the
preconstruction review and permitting of new and modified major
stationary sources of air pollution locating in areas meeting the NAAQS
(``attainment'' areas) and areas for which there is insufficient
information to classify an area as either attainment or nonattainment
(``unclassifiable'' areas). Most states have SIP-approved
preconstruction permit (major NSR) programs. The Federal PSD program at
40 CFR 52.21 applies in some states that lack a SIP-approved permit
program, and in Indian country.\1\ The applicability of the PSD program
to a major stationary source must be determined in advance of
construction and is a pollutant-specific determination. Once a major
source is determined to be subject to the PSD program (PSD source),
among other requirements, it must undertake a series of analyses to
demonstrate that it will use the best available control technology
(BACT) and will not cause or contribute to a violation of any NAAQS or
increment. For the latter demonstration, the PSD regulations generally
require sources to submit for review and approval a source impact
analysis and an air quality analysis.
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    \1\ We have delegated our authority to some states to implement
the Federal PSD program. The EPA remains the reviewing authority in
non-delegated states lacking SIP-approved programs and in Indian
country.
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    The source impact analysis is primarily a modeling analysis
designed to show that the allowable emissions increase from the
proposed project, in conjunction with other emissions increases from
existing sources, will not result in a violation of either the NAAQS or
increments. In cases where the source's emissions may adversely affect
an area classified as a Class I area, additional review is conducted to
protect the increments and special attributes of such an area defined
as ``air quality related values'' (AQRVs).
    The air quality analysis must assess the ambient air quality in the
area that the proposed project would affect. For this analysis, the
owner or operator of the proposed project must submit as part of a
complete permit application air quality monitoring data that represent
the air quality in the area affected by the proposed source for the 1-
year period preceding receipt of the application. Where data may
already exist to represent existing air quality, it may be used by the
applicant; otherwise, the source owner or operator is responsible for
the installation and operation of monitors to collect the necessary
data.
    Historically, EPA has allowed the use of several types of screening
tools to facilitate implementation of the preconstruction review
process to reduce the permit applicant's burden and streamline the
permitting process for de minimis circumstances. These tools include a
significant emissions rate (SER), SILs, and a SMC. The SER, defined in
tons per year (tpy) for each regulated pollutant, is used to determine
whether the emissions increase from any proposed source or modification
can be excluded from review on the grounds that the increase of any
particular pollutant is de minimis. An emission increase for a
particular pollutant that is greater than the SER defined in the NSR
regulations for that pollutant is considered to be a significant
increase.
    The SIL, expressed as an ambient pollutant concentration
(micrograms per cubic meter ([mu]g/m \3\)), is used to determine
whether the ambient impact of a particular pollutant (once it is
determined to be emitted in significant amounts) is significant enough
to warrant a complete source impact analysis involving modeling the
collective impacts of the proposed project and emissions from other
existing sources.
    The PSD regulations generally require each PSD applicant to collect
1 year of continuous air quality monitoring data for any pollutant
determined to be subject to preconstruction review as part of complete
PSD permit application. Using the SMC as a screening tool, expressed as
an ambient pollutant concentration ([mu]g/m\3\), sources may be able to
demonstrate that the modeled air

quality impact of emissions from the new source or modification, or the
existing air quality level in the area where the source would
construct, is less than the SMC, i.e., de minimis, and may be allowed
to forego the preconstruction monitoring requirement for a particular
pollutant at the discretion of the reviewing authority.\2\ See 40 CFR
51.166(i)(5) and 52.21(i)(5).
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    \2\ The basic monitoring exemption provision is part of the
original monitoring requirements adopted in the 1980 PSD rulemaking.
45 FR 52676, 52710, August 7, 1980.
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    When the reviewing authority reaches a preliminary decision to
authorize construction of a proposed major new source or major
modification, it must provide notice of the preliminary decision and an
opportunity for comment by the general public, industry, and other
persons that may be affected by the emissions of the proposed major
source or major modification. After considering these comments, the
reviewing authority may issue a final determination on the construction
permit in accordance with the PSD regulations.

B. History of Particulate Matter (PM) NAAQS

1. Total Suspended Particulate (TSP) and PM10 NAAQS
    The EPA initially established NAAQS for PM in 1971, measured by the
TSP indicator. Based on the size of the particles collected by the
``high-volume sampler,'' which at that time was the reference method
for determining ambient concentrations, TSP included all PM up to a
nominal size of 25 to 45 micrometers. We established both annual and
24-hour NAAQS for TSP.
    On July 1, 1987, we revised the NAAQS for PM and changed the
indicator from TSP to PM10; the latter indicator includes
particles with a mean aerodynamic diameter less than or equal to 10
micrometers. The PM10 particles are the subset of inhalable
particles small enough to penetrate to the thoracic region (including
the tracheobronchial and alveolar regions) of the respiratory tract
(referred to as thoracic particles). We established annual and 24-hour
NAAQS for PM10, and revoked the NAAQS for TSP. (52 FR
24634).
2. PM2.5 NAAQS
    On July 18, 1997, we again revised the NAAQS for PM in several
respects. While we determined that the NAAQS should continue to focus
on particles less than or equal to 10 micrometers in diameter, we also
determined that the fine and coarse fractions of PM10 should
be considered separately. We established new annual and 24-hour NAAQS
using PM2.5 (referring to particles with a nominal mean
aerodynamic diameter less than or equal to 2.5 micrometers) as the
indicator for fine particles. The 1997 NAAQS rule also modified the
PM10 NAAQS for the purpose of regulating the coarse fraction
of PM10 (referred to as thoracic coarse particles or coarse-
fraction particles; generally including particles with a nominal mean
aerodynamic diameter greater than 2.5 micrometers and less than or
equal to 10 micrometers, or PM10-2.5); however, this part of
the rulemaking was vacated during subsequent litigation, leaving the
pre-existing 1987 PM10 NAAQS in place (62 FR 38652).
3. Revised PM2.5 and PM10 NAAQS
    On October 17, 2006, we promulgated revisions to the NAAQS for
PM2.5 and PM10 with an effective date of December
18, 2006 (71 FR 61144). We lowered the 24-hour NAAQS for
PM2.5 from 65 [micro]g/m\3\ to 35 [micro]g/m\3\, and
retained the existing annual PM2.5 NAAQS of 15 [micro]g/
m\3\. In addition, we retained the existing PM10 24-hour
NAAQS of 150 [micro]g/m\3\, and revoked the annual PM10
NAAQS (set at 50 [micro]g/m\3\).

C. Implementation of NSR for PM2.5

    After we established new annual and 24-hour NAAQS based on
PM2.5 as the indicator for fine particles in July 1997, we
issued a guidance document titled ``Interim Implementation for the New
Source Review Requirements for PM2.5,'' John S. Seitz,
Director, Office of Air Quality Planning and Standards, EPA, October
23, 1997. As noted in that guidance, section 165 of the Act implies
that certain PSD requirements become effective for a new NAAQS upon the
effective date of the NAAQS. Section 165(a)(1) of the Act provides that
no new or modified major source may be constructed without a PSD permit
that meets all of the section 165(a) requirements with respect to the
regulated pollutant. Moreover, section 165(a)(3) provides that the
emissions from any such source may not cause or contribute to a
violation of any increment or NAAQS. Also, section 165(a)(4) requires
BACT for each pollutant subject to PSD regulation. The 1997 guidance
stated that sources would be allowed to use implementation of a
PM10 program as a surrogate for meeting PM2.5 NSR
requirements until certain difficulties were resolved. These
difficulties included the lack of necessary tools to calculate the
emissions of PM2.5 and related precursors, the lack of
adequate modeling techniques to project ambient impacts, and the lack
of PM2.5 monitoring sites.
    On April 5, 2005, we issued a guidance document entitled
``Implementation of New Source Review Requirements in PM-2.5
Nonattainment Areas,'' Stephen D. Page, Director, Office of Air Quality
Planning and Standards, EPA. This memorandum provided guidance on the
implementation of the nonattainment major NSR provisions in
PM2.5 nonattainment areas in the interim period between the
effective date of the PM2.5 NAAQS designations (April 5,
2005) and when we promulgate regulations to implement nonattainment
major NSR for the PM2.5 NAAQS. In addition to affirming the
continued use of the John S. Seitz guidance memo in PM2.5
attainment areas, this memo recommended that, until we promulgated the
PM2.5 major NSR regulations, states should use a
PM10 nonattainment major NSR program as a surrogate to
address the requirements of nonattainment major NSR for the
PM2.5 NAAQS.
    On November 1, 2005, we proposed a rule to implement the
PM2.5 NAAQS, including proposed revisions to the NSR
program. For those states with EPA-approved PSD programs, we proposed
to continue the 1997 NSR guidance to use PM10 as a surrogate
for PM2.5, but only during the SIP development period. We
also indicated in that proposal that we would be developing increments,
SILs, and an SMC in a separate rulemaking, i.e., this final rule. Since
there was an interim surrogate NSR program in place, i.e., the
PM10 Surrogate Policy, EPA decided to first promulgate the
non-NSR part of the implementation rule (including attainment
demonstrations, designations, control measures, etc.). This rule was
promulgated as the CAFPIR on April 25, 2007 (72 FR 20586).
    The NSR part of the implementation rule was issued separately as a
final rule on May 16, 2008 (73 FR 28321), and included sets of NSR
regulations for both attainment (PSD) and nonattainment areas
(nonattainment NSR) for PM2.5. In the May 16, 2008 rule we
added one of the important screening tools--the SER--for
PM2.5. The SER for PM2.5 is defined as an
emissions rate of 10 tpy for direct PM2.5 emissions. We also
listed sulfur dioxide (SO2) and nitrogen oxides
(NOX) as precursors of ambient PM2.5 and defined
``significant'' as 40 tpy or more of either precursor pollutant. States
were allowed up to 3 years from the date of publication in the Federal
Register to

revise their SIPs and submit their revised NSR programs to EPA for
approval.

D. Increments Under the PSD Program

    Under section 165(a)(3) of the Act, a PSD permit applicant must
demonstrate that emissions from the proposed construction and operation
of a facility ``will not cause, or contribute to, air pollution in
excess of any (A) maximum allowable increase or maximum allowable
concentration for any pollutant * * *.'' The ``maximum allowable
increase'' of an air pollutant that is allowed to occur above the
applicable baseline concentration for that pollutant is known as the
PSD increment. By establishing the maximum allowable level of ambient
pollutant concentration increase in a particular area, an increment
defines ``significant deterioration'' of air quality in that area.
    For PSD baseline purposes, a baseline area for a particular
pollutant emitted from a source includes the attainment or
unclassifiable area in which the source is located, as well as any
other attainment or unclassifiable area in which the source's emissions
of that pollutant are projected (by air quality modeling) to result in
a significant ambient pollutant increase. See, e.g., 40 CFR
52.21(b)(15)(i). Once the baseline area is established, subsequent PSD
sources locating in that area need to consider that a portion of the
available increment may have already been consumed by previous
emissions increases.
    In general, the submittal date of the first complete PSD permit
application in a particular area is the operative ``baseline date.''
\3\ On or before the date of the first complete PSD application,
emissions generally are considered to be part of the baseline
concentration, except for certain emissions from major stationary
sources, as explained in the following discussion of baseline dates.
Most emissions increases that occur after the baseline date will be
counted toward the amount of increment consumed. Similarly, emissions
decreases after the baseline date restore or expand the amount of
increment that is available.
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    \3\ Baseline dates are pollutant specific. That is, a complete
PSD application establishes the baseline date only for those
regulated NSR pollutants that are projected to be emitted in
significant amounts (as defined in the regulations) by the
applicant's new source or modification. Thus, an area may have
different baseline dates for different pollutants.
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    In practice, three dates related to the PSD baseline concept are
important in understanding how to calculate the amount of increment
consumed-- (1) Trigger date; (2) major source baseline date; and (3)
minor source baseline date. The first relevant date is the trigger
date. The trigger date, as the name implies, triggers the overall
increment consumption process nationwide. Specifically, this is a fixed
date, which must occur before the minor source baseline date can be
established for the pollutant-specific increment in a particular
attainment area. See, 40 CFR 51.166(b)(14)(ii) and 52.21(b)(14)(ii).
For PM (regulated as TSP) and SO2, Congress defined the
applicable trigger date as August 7, 1977--the date of the 1977
amendments to the Act when the original statutory increments were
established by Congress. For nitrogen dioxide (NO2), we
selected the trigger date as February 8, 1988--the date on which we
proposed increments for NO2. See 53 FR 40656, 40658; October
17, 1988. In this final rule, as described later, we are establishing a
separate trigger date for purposes of implementing the PM2.5
increments. See section V.F of this preamble for additional discussion
of the trigger date for PM2.5.
    The two remaining dates--``minor source baseline date'' and ``major
source baseline date''--as described later, are necessary to properly
account for the emissions that are to be counted toward the amount of
increment consumed following the national trigger date, in accordance
with the statutory definition of ``baseline concentration'' in section
169(4) of the Act. The statutory definition provides that the baseline
concentration of a pollutant for a particular baseline area is
generally the air quality at the time of the first application for a
PSD permit in the area. Consequently, any increases in actual emissions
occurring after that date (with some possible exceptions that we will
discuss later) would be considered to consume the applicable PSD
increment. However, the statutory definition in section 169(4) also
provides that ``[e]missions of sulfur oxides and particulate matter
from any major emitting facility on which construction commenced after
January 6, 1975, shall not be included in the baseline and shall be
counted in pollutant concentrations established under this part.''
    To make this distinction between the date when emissions resulting
from the construction at a major stationary source consume the
increment and the date when emissions changes in general (i.e., from
both major and minor sources) begin to consume the increment, we
established the terms ``major source baseline date'' and ``minor source
baseline date,'' respectively. See 40 CFR 51.166(b)(14) and
52.21(b)(14). Accordingly, the ``major source baseline date,'' which
precedes the trigger date, is the date after which actual emissions
increases associated with construction at any major stationary source
consume the PSD increment. In accordance with the statutory definition
of ``baseline concentration,'' the PSD regulations define a fixed date
to represent the major source baseline date for each pollutant for
which an increment exists. Congress defined the major source baseline
date for the statutory increments for PM and SO2 as January
6, 1975. For the NO2 increments, which we promulgated in
1988 under our authority to establish an increment system under section
166(a) of the Act, the major source baseline date we selected was
February 8, 1988--the date on which we proposed increments for
NO2. 53 FR 40656. In both instances, the major source
baseline date for the individual increments was set as a date which
preceded the date on which the regulations pertaining to those
increments were issued. In this final rule, as described later, we are
establishing a separate major source baseline date for implementing the
PM2.5 increments. See section V.F of this preamble for
further discussion of the major source baseline date for
PM2.5.
    The ``minor source baseline date'' is the earliest date after the
trigger date on which a source or modification submits the first
complete application for a PSD permit in a particular area. After the
minor source baseline date, any increase in actual emissions (from both
major and minor sources) consumes the PSD increment for that area.
    Once the minor source baseline date is established, the new
emissions increase from that major source consumes a portion of the
increment in that area, as do any subsequent actual emissions increases
that occur from any new or existing source in the area. When the
maximum pollutant concentration increase defined by the increment has
been reached, additional PSD permits cannot be issued until sufficient
amounts of the increment are ``freed up'' via emissions reductions that
may occur voluntarily, (e.g., via source shutdowns) or by mandatory
control requirements imposed by the reviewing authority. Moreover, the
air quality in a region cannot deteriorate to a level in excess of the
applicable NAAQS, even if all the increment in the area has not been
consumed. Therefore, new or modified sources located in areas where the
air pollutant concentrations are near the level allowed by the NAAQS
may not have full use of the amount of

pollutant concentration increase allowed by the increment.
    Under EPA guidance, the actual increment analysis that a proposed
new or modified source undergoing PSD review must complete depends on
the area impacted by the source's new emissions. We have provided
approved air quality models and guidelines for sources to use to
project the air quality impact of each pollutant (over each averaging
period) for which an increment analysis must be done.\4\ In addition,
we established SILs for each pollutant under the permit requirements
applicable to new and modified major stationary sources locating in
attainment areas that would cause or contribute to a violation of any
NAAQS. See 40 CFR 51.165(b) and part 51, Appendix S, section III.A.
These SILs have also been used for implementing the PSD program to
identify levels below which the source's modeled impact of a particular
pollutant is regarded as de minimis. In this final rule, we are
establishing SILs (24-hour and annual) for PM2.5 that are
being added to the aforementioned regulations containing SILs for other
pollutants, as well as to the PSD regulations in 40 CFR 51.166 and
52.21. See further discussion of the SILs for PM2.5 in
section VI of this preamble.
---------------------------------------------------------------------------

    \4\ See EPA's ``Guideline on Air Quality Models'' at 40 CFR part
51, Appendix W.
---------------------------------------------------------------------------

    In the event that a source's modeled impacts of a particular
pollutant are below the applicable SIL at all ambient air locations
modeled, i.e., de minimis everywhere, EPA's policy for PSD provides
that no further modeling analysis is required for that pollutant. Our
longstanding policy under the PSD program is that when a preliminary
screening analysis based on the SIL is sufficient to demonstrate that
the source's emissions throughout the area modeled will not cause or
contribute to a violation of the increment, there is no need for a
comprehensive source impact analysis involving a cumulative evaluation
of the emissions from the proposed source and other sources affecting
the area.
    Within the impact area of a source subject to PSD, that is, the
area within which the proposed project's emissions increase does have a
significant impact, increment consumption is calculated using the
source's proposed emissions increase, along with other actual emissions
increases or decreases of the particular pollutant from any sources in
the area, which have occurred since the minor source baseline date
established for that area. In addition, the emissions increases or
decreases from any major source that has commenced construction since
the major source baseline date (which precedes the minor source
baseline date) will consume or expand increment. Thus, an emissions
inventory of sources whose emissions, in whole or in part, of a
particular pollutant consume or expand the available increment in the
area must be compiled. The inventory of increment-consuming emissions
includes not only sources located directly in the impact area, but
sources outside the impact area that affect the air quality for the
particular pollutant within the impact area.
    The inventory of increment-consuming emissions includes emissions
from increment-affecting sources at two separate time periods--the
baseline date and the current period of time. For each source that was
in existence on the relevant baseline date (major source or minor
source), the inventory includes the source's actual emissions on the
baseline date and its current actual emissions. The change in emissions
over these time periods represents the emissions that consume increment
(or, if emissions have gone down, expand the available increment). For
sources constructed since the relevant baseline date, all their current
actual emissions consume increment and are included in the inventory.
    When the inventory of increment-consuming emissions has been
compiled, computer modeling is used to determine the change in ambient
concentration that will result from these emissions when combined with
the proposed emissions increase from the new major source or major
modification that is undergoing PSD review. The modeling has generally
been guided by the ``Guideline on Air Quality Models'' (40 CFR part 51,
Appendix W), which includes provisions on air quality models and the
meteorological data input into these models. The model output
(expressed as a change in concentration) for each relevant averaging
period is then compared to the corresponding allowable PSD increment.

E. Historical Approaches for Developing Increments

1. Congressional Enactment of Increments for PM and SO2
    Congress established the first increments defining significant
deterioration of air quality in the 1977 Amendments to the Act. These
amendments, among other things, added part C to title I, setting out
the requirements for PSD. In section 163, Congress included numerical
increments for PM and SO2 for Class I, II, and III areas.
    The three area classes are part of the increment system originally
established by Congress. Congress designated Class I areas (including
certain national parks and wilderness areas) as areas of special
national concern, where the need to prevent deterioration of air
quality is the greatest. Consequently, the allowable level of
incremental change is the smallest relative to the other area classes,
i.e., most stringent, in Class I areas. The increments of Class II
areas are larger than those of Class I areas and allow for a moderate
degree of emissions growth. For future redesignation purposes, Congress
defined a ``Class III'' classification to allow the redesignation of
any existing Class II area for which a State may desire to promote a
higher level of industrial development (and emissions growth). Thus,
Class III areas are allowed to have the greatest amount of pollutant
increase of the three area classes while still achieving the NAAQS. To
date, there have been no redesignations made to establish a Class III
area.
    In establishing these PSD increments, Congress used the then-
existing NAAQS for those pollutants as the benchmark for determining
what constitutes ``significant deterioration.'' Congress established
the increments for PM as a percentage of the then-existing PM NAAQS. At
the time the Act was amended in 1977, the NAAQS for PM were expressed
in terms of ambient concentrations of TSP. Thus, EPA interpreted the
statutory increments for PM using the same ambient TSP ``indicator.''
2. EPA's Promulgation of Increments for NO2 and
PM10
    Congress also provided authority for EPA to promulgate additional
increments and to update the original PM increments created by statute.
The EPA has promulgated two regulations pursuant to this authority.
a. Increments for NO2 Using the ``Contingent Safe Harbor''
Approach Under Section 166(a) of the Act
    Based on section 166(a) of the Act, on October 17, 1988, EPA
promulgated increments for NO2 to prevent significant
deterioration of air quality due to emissions of NOX (53 FR
40656). The EPA based these increments on percentages of the NAAQS in
the same way that Congress derived the statutory increments for PM and
SO2. Those NO2 increments were challenged in 1988
by the Environmental Defense Fund (EDF) when EDF filed suit in the U.S.
Court of

Appeals for the District of Columbia Circuit against the Administrator
(Environmental Defense Fund, Inc. v. Reilly, No. 88-1882). The EDF
successfully argued that we failed to sufficiently consider certain
provisions in section 166 of the Act. The court remanded the case to
EPA ``to develop an interpretation of section 166 that considers both
subsections (c) and (d), and if necessary to take new evidence and
modify the regulations.'' See Environmental Defense Fund v. EPA, 898
F.2d 183, 190 (D.C. Cir. 1990) (EDF v. EPA). Section 166(c) of the Act
requires the PSD regulations to, among other things, meet the goals and
purposes set forth in sections 101 and 160 of the Act. Section 166(d)
requires these regulations be at least as effective as the increments
established for PM (in the form of TSP) and SO2 in section
163 of the Act. The court considered the NO2 increment
values determined using the percentage-of-NAAQS approach as ``safe
harbor'' increments which met the requirements of section 166(d) of the
Act. However, the court also determined that EPA's reliance on such
increment levels was contingent upon our completing the analyses
required under section 166(c), which provided that the final increment
values must address the goals of sections 101 and 160 of the Act to
protect public health and welfare, parks, and AQRVs \5\ and to insure
economic growth.
---------------------------------------------------------------------------

    \5\ The term ``air quality related values'' is not defined in
the Act, but the legislative history provides language saying that
``The term `air quality related values' of Federal lands designated
as Class I includes the fundamental purposes for which such lands
have been established and preserved by the Congress and the
responsible Federal agency. For example, under the 1916 Organic Act
to establish the National Park Service (16 U.S.C. 1), the purpose of
such national park lands `is to conserve the scenery and the natural
and historic objects and the wildlife therein and to provide for the
enjoyment of the same in such manner and by such means as will leave
them unimpaired for the enjoyment of future generations.' '' S. Rep.
No. 95-127 at 36 (1977).
---------------------------------------------------------------------------

    In response to the court's decision, we proposed rulemaking on
increments for NO2 on February 23, 2005 (70 FR 8880) and
finalized the rule on October 12, 2005 (70 FR 59582). In the final
rule, we established our policy on how to interpret and apply the
requirements of sections 166(c) and (d) of the Act. In accordance with
the court ruling, we conducted further analyses (considering the health
and welfare effects of NOX) and concluded that the existing
NO2 increments were adequate to fulfill the requirements of
section 166(c). See 70 FR 59586 for our detailed analysis of how
pollutant regulations satisfy the requirements of section 166 of the
Act. Hence, we retained the existing NO2 increments along
with other parts of the existing framework of pollutant-specific
NO2 increment regulations. We also amended the PSD
regulations under 40 CFR 51.166 to make it clear that states may seek
EPA approval of SIPs that utilize a different approach than EPA used to
establish these NO2 increments. To receive our approval of
an alternative program, a State must demonstrate that its program
satisfies the requirements of sections 166(c) and 166(d) of the Act and
prevents significant deterioration of air quality from emissions of
NOX.\6\
---------------------------------------------------------------------------

    \6\ Under the 2005 NOX regulation, states can adopt
measures other than increments as long as they can demonstrate that
the measures selected comply with the same criteria and goals of
sections 166(c) and (d) of the Act that must be met for increments.
---------------------------------------------------------------------------

b. Increments for PM10 Using ``Equivalent Substitution''
Approach Under Section 166(f) of the Act
    On October 5, 1989, we proposed PM10 increments. See 54
FR 41218. Although section 163 did not expressly define the existing
statutory increments for PM in terms of a specific indicator, EPA
reasoned that Congress' knowledge that TSP was the indicator for the PM
NAAQS, and that the TSP standards were the starting point for the
increments levels when the increments were established in 1977, meant
that TSP was also the appropriate measure for the PM increments in
section 163. As a consequence, EPA believed that the statutory PM
increments could not simply be administratively redefined as
PM10 increments, retaining the same numerical values,
following the revision of the PM NAAQS. Rather, we stated our belief
that with the promulgation of the PM10 NAAQS, EPA had both
the responsibility and the authority under sections 166 and 301 of the
Act to promulgate new increments for PM to be measured in terms of
PM10. We further concluded that promulgating PM10
increments to replace, rather than supplement, the statutory TSP
increments under section 163 represented the most sensible approach for
preventing significant deterioration with respect to PM. See 54 FR
41220-41221.
    We promulgated PM10 increments to replace the then-
existing TSP increments on June 3, 1993 (58 FR 31622). In the interim
between proposal and promulgation, Congress enacted the 1990 CAA
Amendments. As part of these amendments, Congress amended section 166
to add a new section 166(f). This section specifically authorized EPA
to substitute PM10 increments for the existing section 163
PM increments based on TSP, provided that the substituted increments
are ``of equal stringency in effect'' as the section 163 increments.
    Thus, we were able to replace the TSP increments under section 163
of the Act using PM10 increments based directly on the newly
enacted authority under section 166(f) of the Act. In the
PM10 rule, we maintained the existing baseline dates and
baseline areas for PM that had been previously established using the
TSP indicator. Also, as proposed, we promulgated PM10
increments based on an approach we called the ``equivalent to statutory
increments'' approach. Under this approach, we used the original TSP
increments as a benchmark for calculating the PM10
increments, thereby retaining roughly the same limitations on future
deterioration of air quality as was allowed under the TSP increments.
    In using this approach, we considered the historical consumption of
TSP increment by a sample population of permitted PSD sources, and then
determined the PM10 increments for each area classification
and averaging time that would provide approximately the same percentage
of PM10 increment consumption, on average, by the same
population of sources. Then, all future calculations of increment
consumption after the PM10 implementation date would be
based on PM10 emissions. See 58 FR 31622 and 31625.

V. Final Action on PM2.5 Increments

    In this section of the preamble, we will summarize the
considerations that went into our proposed action and describe the
final action being taken regarding new regulations for preventing
significant deterioration of PM2.5 air quality--including
PM2.5 increments (sections V.A through V.E, baseline dates
and other permit requirements for PM2.5 (section V.F),
baseline areas for PM2.5 (section V.G), and PM10
increments (section V.H).

A. Decision To Establish PM2.5 Increments Using ``Contingent
Safe Harbor Approach'' Under Section 166(a)

    The EPA's 2007 NPRM contained three options for developing
numerical PM2.5 increments. Option 1 used the authority of
section 166(a) of the Act to establish increments for PM2.5
as a new pollutant for which NAAQS were established after August 7,
1977, and established 24-hour and annual PM2.5 increments
(Class I, II, and III) based on the ``contingent safe harbor''
approach. Options 2 and 3 used the contingent safe harbor approach
under section 166(a) to only develop 24-hour PM2.5
increments (Class I, II, and III), while using the ``equivalent
substitution''

approach under section 166(f) of the Act to develop annual
PM2.5 increments. Each of these options is discussed in
detail in the 2007 NPRM. 72 FR 54123-54138. In addition, significant
comments on each of the three options, and our responses to them, are
provided in this section V of this preamble.
    In this final rule, after considering the available information and
comments from interested parties, EPA has decided to select Option 1
and establish increments for PM2.5 using the ``contingent
safe harbor'' approach in accordance with the authority provided in
section 166(a) of the Act.
    This final rule establishes increments for PM2.5 at the
following levels:

----------------------------------------------------------------------------------------------------------------
                                                                 NAAQS           Increments ([micro]g/m\3\)
                      Averaging period                         ([micro]g/ --------------------------------------
                                                                 m\3\)       Class I      Class II    Class III
----------------------------------------------------------------------------------------------------------------
Annual......................................................           15            1            4            8
24-hour.....................................................           35            2            9           18
----------------------------------------------------------------------------------------------------------------

B. Rationale for the Applicability of Section 166(a)

    In the 2007 NPRM, we expressed our belief that it is permissible to
interpret section 166(a) to apply to PM2.5. Section 166(a)
requires EPA to develop regulations to prevent the significant
deterioration of air quality due to emissions of certain named
pollutants, and to develop such regulations for any pollutants for
which NAAQS are subsequently promulgated. Although EPA has generally
characterized the NAAQS for PM2.5 as a NAAQS for a new
indicator of PM, EPA did not replace the PM10 NAAQS with the
NAAQS for PM2.5 when the latter NAAQS were promulgated in
1997. Rather, EPA retained the annual and 24-hour PM10 NAAQS
(retaining PM10 as an indicator of coarse particulate
matter), and established new annual and 24-hour NAAQS for
PM2.5 as if PM2.5 was a new pollutant, even
though EPA had already developed air quality criteria for PM generally.
Thus, for purposes of section 166(a), the promulgation of a NAAQS for
PM2.5 established a NAAQS for an additional pollutant after
1977.
    Nine commenters supported our proposed Option 1, although only
three of these explicitly expressed support for the use of section
166(a) authority to promulgate PM2.5 increments. Ten other
commenters specifically opposed the use of section 166(a) authority
and/or supported the use of section 166(f) authority (on which the
annual increments under Options 2A and 2B were based).
    One of the commenters who explicitly agreed with our proposed use
of section 166(a) authority stated that it is the only option that is
legally available. This commenter asserted that section 166(a) plainly
applies to PM2.5 because PM2.5 is a pollutant for
which NAAQS were promulgated after August 7, 1977. This commenter held
that EPA's rulemaking duty under section 166(a) is not confined to
``new pollutants,'' but is triggered by post-1977 NAAQS promulgations,
regardless of whether for new or previously regulated pollutants. On
the other hand, this commenter noted that by its terms, section 166(f)
is limited to authorizing the adoption of PM10 increments as
a substitute for the statutory TSP increments and does not provide for
substitution of PM2.5 increments for TSP or PM10
increments.
    The opposing commenters did not believe that section 166(a)
provides a legal basis for EPA to promulgate PM2.5
increments. One of these commenters stated that section 166(a) can only
be used for a new pollutant, and PM2.5 is not a new
pollutant.
    Another commenter who opposed the use of section 166(a) authority
argued that nothing in section 166(a) of the Act can be interpreted to
allow it to be used as the basis of increments when EPA revises an
existing NAAQS. The commenter explained that, on its face, section
166(a) can only be interpreted to apply to pollutants other than PM and
SO2 since increments for these pollutants were enacted by
Congress in section 163 of the Act. The commenter added that it can be
argued that Congress intended to have section 166(a) apply to the four
other pollutants specifically listed there.
    This commenter found unpersuasive our argument that we are not
``substituting'' increments (as section 166(f) requires for
PM10) but rather adding PM2.5 increments to the
existing PM10 increments, and that only section 166(a)
allows such an approach (72 FR 54121). The commenter asserted that if
EPA had defined a coarse fraction to the particulate matter standards,
then that fraction, together with the PM2.5 standards, would
form the set of ``substituted'' new standards for the existing
PM10 standards, and, thus, the increments.
    The commenter also disagreed with EPA's argument that it can treat
PM2.5 as a new pollutant under section 166(a) of the Act
since it has been demonstrated that sub-PM2.5 particles have
distinctly different health and welfare effects than the other forms of
PM (i.e., coarse or PM10). The commenter indicated that just
as EPA replaced the TSP standards by PM10 as a better
indicator of health effects, ongoing research has led to establishment
of the PM2.5 standards as a better indicator of certain
health effects, and it is the natural outcome of such research that has
enabled EPA to separate the effect of total particulate matter into two
fractions with distinct effects. The commenter added that given that
the definition of particulate matter includes a vast conglomeration of
solids and liquids, the finding of differing effects should not come as
a surprise. The commenter explained that as is the case of different
pollutants having similar effects that are, nonetheless, treated as
separate pollutants, the same concept should apply to a range or
fraction of particulate matter found to have different effects in
establishing it as another indicator and not a different pollutant.
    The commenter did not disagree with the specific numerical
increments proposed by EPA under Option 1, but did have concerns with
the potential consequences of the section 166(a) approach. The
commenter's primary concern was the proposal to allow states to
substitute other measures in the place of uniform national increments
for PM2.5. (This is discussed further in section V.C.5 of
this preamble.) Another commenter also expressed this concern.
    Another commenter who opposed the section 166(a) approach believes
that the legal and congressional history regarding the establishment of
PM increments shows that Congress added section 166(f) to the Act based
on the conviction that without it, EPA had no authority to revise the
PM increments for PM10 (citing and quoting from S. Rep. No.
228, 101st Cong., 2nd Sess. 75 (1990), reprinted in 1990 U.S.C.C.A.N.
3385, 3461). The commenter concluded that EPA did not have authority in
1987 under section 166(a) to adopt PM10 increments, and does
not have authority now under section 166(a) to adopt PM2.5
increments.
    We read section 166(a) to authorize EPA to promulgate pollutant-
specific PSD regulations meeting the

requirements of sections 166(c) and 166(d) for any pollutant for which
EPA promulgates a NAAQS after 1977. Most of the pollutants identified
in section 166(a) (NOX, photochemical oxidants, carbon
monoxide) are pollutants for which EPA had established NAAQS in 1977
when Congress adopted section 166 of the Act. There was no need for
Congress to list other criteria pollutants, SO2 and PM, in
section 166(a) because Congress had already established increments for
these pollutants in section 163 of the Act. In addition to requiring
regulations for the enumerated pollutants, we conclude that under
section 166 of the Act Congress intended to authorize EPA to establish
additional pollutant-specific PSD regulations, potentially containing
increments, for any additional pollutants for which EPA promulgated a
NAAQS under section 109 of the Act. Furthermore, because the Act refers
to pollutants for which EPA promulgates NAAQS after 1977, and does not
use the phrase ``additional pollutants,'' section 166(a) provides
authority for EPA to promulgate new increments after revising an
existing NAAQS (including NAAQS first promulgated before 1977), when we
find that such action is appropriate.
    Moreover, any new increments developed pursuant to section 166(a)
have no effect on existing increments, as there is no indication
therein that an existing increment should be revoked or replaced when
additional increments are promulgated. This was the situation following
the promulgation of new NAAQS for PM in 1987 when EPA replaced the old
NAAQS based on TSP with new ones based on PM10. Had Congress
not added new section 166(f) in 1990, increments for PM10
could have been developed pursuant to section 166(a) of the Act, but
such increments would have had no effect on the original statutory
increments for PM (based on TSP). Consequently, seeing no basis for
retaining the original increments, Congress added section 166(f) which
explicitly provides for the replacement of the existing increments with
PM10 increments.
    One commenter asserted that if EPA establishes increments for
PM2.5 under the authority of section 166(a) on the basis
that PM2.5 is a new pollutant, then it must also establish
PM10 increments under section 166(a) because (according to
the commenter's analysis) PM10 is also a new pollutant. In
the same analysis, the commenter concluded that EPA must adopt new
measures to prevent significant deterioration from coarse PM based on
section 166(a).
    In this final rule, EPA is not setting or amending any increments
for PM10 or otherwise taking action with respect to
PM10 increments. The preexisting annual and 24-hour
increments for PM10 are being retained. See section V.H.
Similarly, EPA is not taking any action with respect to coarse PM in
this rule. For these reasons, the commenter's arguments on what
authority must be used to set increments for PM10 and/or
coarse PM, and that EPA has some obligation to take action with respect
to coarse PM, are not on point for this rule. Thus, no substantive
response to this comment is needed. Nevertheless, as mentioned earlier,
Congress provided explicit authority under section 166(f) of the Act to
address increments for PM10, because it intended for such
increments to be substitute increments for the original statutory
increments for PM measured as TSP. Thus, the PM10 increments
legally supersede the original statutory increments for PM. Had the
PM10 increments been developed under section 166(a), which
prior to the 1990 Act Amendments was the only authority available for
developing new increments, then the original statutory PM increments
would have remained in effect in addition to the PM10
increments.
    One commenter expressed general objections to EPA's legal rationale
for the PM2.5 increments proposal, asserting that we failed
to expressly state and support our legal authority for the
PM2.5 increments, offering two possible sources of authority
(``contingent safe harbor,'' ``equivalent substitution,'' or possibly a
combination of the two) but never stating our legal position with
clarity. The commenter agreed with EPA's assessment that the
PM2.5 increments should and must fulfill the legal
requirements of the Act (72 FR 54121), and added that it is the
government's burden of proof to establish its legal authority for
action. The commenter stated that it would be arbitrary and capricious
to promulgate these regulations for which EPA has not stated legal
authority.
    We do not disagree that the 2007 NPRM described two different legal
authorities for the two different options for establishing increments,
but we disagree that these discussions did not clearly present the
alternative legal bases that the Agency was considering for taking
action in this rule. In particular, we clearly described our legal
authority for developing the 24-hour and annual PM2.5
increments under section 166(a) of the Act, which is the basis on which
we are taking final action in this rule.\7\ First, we expressly stated
that Option 1 was based on the statutory authority of section 166(a) of
the Act. See 72 FR 54123 (Under the first option, ``we would use the
authority of section 166(a) of the Act to develop new increments for
PM2.5''). Second, we provided a discussion of this authority
both in general (see 72 FR 54118-54119 and 54120-54123), and how it
would be applied to establish increments for PM2.5 (see 72
FR 54119-120 and 54123-136).
---------------------------------------------------------------------------

    \7\ We also believe that we sufficiently described how section
166(f) might provide alternative authority for establishing
increments for PM2.5 (see, e.g., 72 FR 54120-54121), but
will not address that in detail here because the increments in this
rule are not based on section 166(f) authority.
---------------------------------------------------------------------------

    We now believe that section 166(a) provides the most
straightforward approach for developing increments for a pollutant or
pollutant indicator for which no increments have yet been established.
Our position is also consistent with the comments we received which
supported the delay in implementation of the PM2.5
increments, opposed the potential for two sets of definitions for
``major source baseline date'' and ``trigger date'' for the
PM2.5 increment system, and highlighted the complexities
involved with having to establish and maintain two sets of emissions
inventories for the 24-hour and annual PM2.5 increments.
(See further description of relevant comments in section VIII of this
section.)

C. EPA's Interpretation of the Requirements Under Sections 166(a)-(d)
of the Act

    In section 166(a) of the Act, Congress directed EPA to develop
pollutant-specific regulations to prevent significant deterioration of
air quality. Congress further specified that such regulations meet
specific requirements set forth in sections 166(c) and 166(d) of the
Act. We stated in the 2007 NPRM that because we believed that section
166(a) could be applied to the development of increments for
PM2.5, we would follow the interpretation of sections
166(a)-(d) that the Agency adopted in its most recent NO2
increments rule. 70 FR 59582, October 12, 2005. That particular
interpretation and application was upheld in Environmental Defense v.
EPA, 489 F.3d 1320 (D.C. Cir. 2007).
    The EPA's interpretation of these provisions is grounded on five
principles and conclusions. First, we read section 166 of the Act to
direct EPA to conduct a holistic analysis that considers how a complete
system of regulations will collectively satisfy the

applicable criteria, rather than evaluating one individual part of a
regulatory scheme in isolation. Second, we use a ``contingent safe
harbor'' approach which calls for EPA to first determine an increment
that is at least as effective as the increments in section 163 of the
Act, as required under section 166(d) and then to conduct further
analysis to determine if additional measures are necessary to fulfill
the requirements of section 166(c). Third, we interpret section 166(c)
of the Act to identify eight statutory factors that EPA must apply when
promulgating pollutant-specific regulations to prevent significant
deterioration of air quality. Fourth, where these factors are at odds
with each other, we interpret the statute to require EPA to use its
judgment to balance the conflicting factors. Fifth, we recognize that
the requirements of section 166 may be satisfied by adopting other
measures besides an increment and that EPA may allow states to
demonstrate that alternatives to increments contained in a SIP meet the
requirements of sections 166(c) and 166(d). Below is a brief discussion
of each of these five principles and conclusions. A more detailed
description of each of these is contained in the 2007 NPRM at 72 FR
54121-54123.
1. Regulations as a Whole Should Fulfill Statutory Requirements
    Section 166(a) of the Act directs EPA to develop pollutant-specific
regulations to prevent the significant deterioration of air quality.
Sections 166(c) and 166(d) provide detail on the contents of those
regulations, but do not necessarily require the same type of increment
system Congress created in section 163 of the Act. The EPA interprets
section 166 to require that the entire system of PSD regulations (the
framework and details, as described in section V.D of this preamble)
for a particular pollutant must, as a whole, satisfy the criteria in
sections 166(c) and 166(d) of the Act.
2. Contingent Safe Harbor Approach
    Section 166(c) of the Act describes the kinds of measures to be
contained in the regulations to prevent significant deterioration of
air quality called for in section 166(a) and specifies that these
regulations are to ``fulfill the goals and purposes'' set forth in
sections 160 and 101 of the Act. Section 166(d) of the Act directs EPA
to ``fulfill such goals and purposes'' by providing ``specific measures
at least as effective as the increments established in section 163 * *
*.'' Thus, EPA reads section 166(d) to require that the Agency identify
``safe harbor'' pollutant-specific PSD regulations adopted under
section 166.
    The EPA reads section 166(c) to require that the Agency conduct
further review to determine whether, based on the criteria in section
166(c), EPA's pollutant-specific PSD regulations under section 166
should contain measures that are different from the ``safe harbor''
identified under section 166(d). The EPA construes section 166(d) to
require that the measures be ``at least as effective'' as the statutory
increments set forth in section 163.
    To apply the ``contingent safe harbor'' approach for
PM2.5, we first identified ``safe harbor'' increments for
each area classification (Class I, II, or III), using: (1) Equivalent
percentages of the NAAQS as the percentages used for developing the
statutory increments; (2) the same pollutant as the NAAQS, i.e.,
PM2.5, and (3) the same time (averaging) periods as were
used for the PM2.5 NAAQS. We concluded that this approach
would ensure that the increments would be ``at least as effective as
the increments established in section 163,'' as required by section
166(d). Second, EPA conducted further review to determine whether the
``safe harbor'' increments, in conjunction with existing elements of
the PSD program or additional measures proposed under section 166 to
augment the increments, sufficiently fulfill the criteria in subsection
(c) of section 166.
    In this review, we weighed and balanced the criteria set forth in
subsection (c) (and, as provided in subsection (c), the incorporated
goals and purposes of the Act in section 101 and the PSD program in
section 160) to determine whether additional measures might be needed
to satisfy the criteria in subsection (c). See section V.E.6 of this
preamble for further discussion of our evaluation, comments on the
evaluation, and our response to them.
3. The Statutory Factors Applicable Under Section 166(c)
    The EPA interprets section 166(c) of the Act to establish eight
factors to be considered in the development of PSD regulations for the
pollutants covered by this provision. These eight factors included the
three criteria stated in section 166(c) and the five goals and purposes
identified in section 160 of the Act (which, as noted below, also cover
the goals and purposes set forth in section 101). The three stated
criteria in section 166(c) indicate that PSD regulations for specific
pollutants should provide: (1) Specific numerical measures for
evaluating permit applications; (2) a framework for stimulating
improved control technology, and (3) protection of air quality values.
The five goals and purposes in section 160 are incorporated into the
analysis by virtue of the fourth criterion in section 166(c), which
directs that EPA's pollutant-specific PSD regulations ``fulfill the
goals and purposes'' set forth in sections 160 and 101 of the Act. We
construed the term ``fulfill the goals and purposes,'' as used in
section 166(c), to mean that EPA should apply the goals and purposes
listed in section 160 as factors applicable to pollutant-specific PSD
regulations established under section 166. The Agency's view is that
PSD measures that satisfy the specific goals and purposes of section
160 also satisfy the more general purposes and goals identified in
section 101 of the Act. See 72 FR 54122.
    One commenter disagreed with our interpretation that the goals and
purposes of section 160 also satisfy all of those in section 101. This
commenter asserted that although there is some overlap between the two
sections, they are not identical. As an example, the commenter noted
that section 101 expressly states that a primary goal of the Act is to
promote pollution prevention--a goal not stated in section 160. The
commenter asserted that, although the proposed increments would limit
some pollution increases, there was no provision in the proposal that
would require or promote pollution prevention.
    We disagree with the commenter and continue to believe that
measures that satisfy the specific goals and purposes of section 160
also satisfy the more general purposes and goals identified in section
101 of the Act. As we stated in the 2005 NO2 increment
rulemaking, the overall goals and purposes of the Act listed in
sections 101(b) and 101(c) are general goals regarding protecting and
enhancing the nation's air resources and controlling and preventing
pollution. Because these broad goals are given more specific meaning in
section 160, EPA does not believe it is necessary to consider them in
detail when evaluating whether PSD regulations satisfy the criteria in
section 166(c). 70 FR 59587 FN 3.
    Regarding pollution prevention specifically, we believe that this
general goal is encompassed in, and given more specific meaning by,
sections 160(1), 160(2), and 160(4) of the Act. These sections spell
out the specific purposes under the PSD program for the general section
101 goals of controlling and preventing pollution. We believe that any
requirement to limit or reduce emissions serves to promote pollution
prevention, which is often the most cost

effective means of lowering pollutant emissions.
    In addition to citing the purposes set out in section 160, section
166(c) includes the criterion that pollutant-specific PSD regulations
should provide a framework for stimulating improved control technology.
As discussed subsequently in sections V.D.1 and V.D.6 of this preamble,
we believe that this criterion is fulfilled by the system of increments
for PM2.5 and by the requirement for PSD permittees to apply
BACT to minimize PM2.5 emissions. In stimulating improved
control technology generally, these elements of the PSD program also
promote pollution prevention. As noted previously, pollution prevention
is often the most cost effective means of control, particularly for new
sources and new process lines at existing sources. In addition, because
BACT is a case-by-case determination that considers cost and collateral
environmental impacts, pollution prevention, where technically
feasible, often fairs well in BACT analyses because it is typically
free from the negative environmental impacts that result from the use
of add-on air pollution control devices.
4. Balancing the Factors Applicable Under Section 166(c)
    While the eight factors in section 166(c) are generally
complementary, there are circumstances where some of the objectives may
be in conflict with each other. In these situations, some degree of
balance or accommodation is inherent in the requirement to establish
regulations that satisfy all of these factors. As first discussed in
our 2005 NO2 increments rulemaking (70 FR 59582 at 59587),
we believe this balancing test derives primarily from the third goal
and purpose set forth in section 160: To insure economic growth
consistent with the preservation of existing clean air resources. A
more detailed discussion of how the balancing of factors should be
interpreted is contained in the 2007 NPRM at 72 FR 54122-54123.
    One commenter claimed that EPA ``incorrectly and repeatedly
asserts'' that a goal of section 160 of the Act is to insure economic
growth. The commenter claimed that neither section 160 nor section 101
of the Act uses language to support a goal of promoting or maximizing
opportunities for economic growth. Instead, the commenter asserted that
both sections state only that any growth that does occur must be
consistent with protection of air quality. The commenter concluded that
``EPA's notion that the need to satisfy the other requirements of
Section 166 and other goals and purposes in Sections 101 and 160 can
never preclude additional emissions from economic growth unlawfully
elevates such growth over all other statutory factors.''
    The language in section 160(3) provides that one of the purposes of
the PSD program is ``to insure that economic growth will occur in a
manner consistent with the preservation of existing clean air
resources.'' The commenter suggests that this language can only be read
as if the statutory phrase ``economic growth'' actually said ``any
economic growth that does occur'' such that section 160(3) says ``to
insure that any economic growth that does occur will occur in a manner
consistent with the preservation of existing clean air resources.'' We
disagree; the phrasing used by Congress is ``to insure that economic
growth will occur.'' Thus, we believe the plain language of the statute
supports EPA's reading that section 160(3) requires a balancing of the
goals of (1) economic growth and (2) preservation of existing clean air
resources. At a minimum, if the language were to be considered
ambiguous enough to allow the commenter's reading, then the Agency's
interpretation is also a reasonable reading of the statutory language.
5. Authority for States To Adopt Alternatives to Increments
    While section 166 of the Act authorizes EPA to promulgate
increments for pollutants listed under section 166(a), we have also
interpreted the section to allow states to employ approaches other than
increments to prevent significant deterioration of air quality, so long
as such an approach otherwise meets the requirements of sections 166(c)
and 166(d). This interpretation was explained in the 2005
NO2 increment rulemaking (70 FR 59611-59612), in which we
amended the PSD regulations at 40 CFR 51.166 by adding new paragraph
(c)(2) to codify this statutory authority. Under the existing provision
in 40 CFR 51.166(c)(2), states may seek EPA approval of SIPs that use
an alternative approach to increments if the State can demonstrate that
the alternative program satisfies the requirements of sections 166(c)
and 166(d). However, the current language at paragraph (c)(2) states
the authority for states to adopt alternative measures only with
respect to increments for NO2. To clarify our interpretation
that the authority to adopt alternative measures covers any pollutant
listed in section 166(a), we are revising 40 CFR 51.166(c)(2) to make
it inclusive to all applicable pollutants rather than just
NO2.
    Two commenters supported our proposal to revise paragraph (c)(2) to
include PM2.5, while four State/local agency commenters
expressed opposition. An environmental commenter agreed that the Act
allows for other approaches, but believes that such approaches must be
in addition to the national increments. Specifically, this commenter
stated that ``although EPA can provide for states to adopt approaches
in addition to increments in order to fulfill the statutory purposes,
the agency must make clear that states cannot adopt approaches that are
less protective that the national increments.'' This commenter further
stated that ``to the extent that EPA is suggesting that it can allow
states to adopt PSD programs that do not include the minimum Federal
increments, that position is contrary to the statute.''
    As in the 2005 NO2 increment rulemaking, we are
codifying the basic principle that states can seek to use alternative
measures without defining any specific type of alternative program that
would be approved or otherwise creating standards beyond the
requirements of sections 166(c) and 166(d). Instead, we plan to make
determinations on a case-by-case basis when a State submits a specific
alternative approach for EPA to approve as part of a SIP. In making
those determinations, we will address the specific alternative measures
as states propose them to the Agency in light of the requirements of
sections 166(c) and 166(d), including whether the alternative program
is ``at least as effective as the increments established in section
163,'' as required in section 166(d).
    The four State/local agency commenters opposing the revision to 40
CFR 51.166(c)(2) expressed the importance of using uniform national
increments for PM2.5. One commenter argued that a nationally
inconsistent approach to PM2.5 in attainment areas could
result in a patchwork of State PSD regulations--and the exact kinds of
economic repercussions that Congress wished to avoid. The same
commenter argued that varying increment-equivalent measures could also
result in an uneven playing field for industry and could exacerbate
difficulties between states experiencing transport problems.
    Another opposing commenter was concerned that allowing states to
adopt alternatives to increments would likely lead to a ``mish-mash''
of State approaches which defeats the intention of Congress that there
be uniformity in PSD rules to avoid economic

dissimilarities from State to State that could allow interstate
competition for industry based upon which State offers the best (least
expensive) environmental compliance regulations. Another commenter
objected to allowing the use of alternatives to increments by stating
that such alternative allowances undermine the desired national
consistency, and EPA has failed to even identify any Act programs which
would benefit from this approach.
    While we acknowledge the potential problems identified by the
commenters associated with allowing states to adopt alternative
approaches to the numerical increments that we are establishing, we
also note that section 166(d) expressly gives EPA some latitude in
promulgating regulations that will be at least as effective as the
increments in section 163, by stating that such regulations ``may
contain air quality increments, emission density requirements, or other
measures.'' Thus, EPA is authorized to provide that states may consider
alternatives to the increments established in this rule. That said, the
statutory authority is not a blank check for states to do as they
please, but enables states to consider options that may provide a
meaningful way for them to manage their air resources within the
framework allowed by the statutory PSD requirements.

D. Framework for Pollutant-Specific PSD Regulations for PM2.5

    In the 2007 NPRM, we proposed to apply the same basic framework for
pollutant-specific PSD regulations for PM2.5 that we used in
our 2005 NO2 increments regulations. Specifically, we
proposed adopting an increment and area classification system for
PM2.5 and applying the statutory AQRV review process to
PM2.5 as well. We also indicated that while some of the
factors applicable under section 166(c) are fulfilled by using this
type of framework for pollutant-specific PSD regulations under section
166(a) of the Act, this framework of regulations also needs to satisfy
the other applicable factors. Thus, the details of our regulations
(such as the characteristics of the increments themselves) are
important, and we evaluated the effectiveness of the framework in
conjunction with more detailed elements of our regulations. As
discussed in the following subsections, we believe our obligations
under section 166(c) of the Act are satisfied when the PSD regulations
collectively satisfy the factors applicable under 166(c) of the Act.
1. Increment System
    An increment-based program satisfies the requirements under 166(c)
to provide ``specific numerical measures against which permit
applications may be evaluated.'' An increment is the maximum allowable
level of ambient pollutant concentration increase that is allowed to
occur above the applicable baseline concentration in a particular area.
As such, an increment defines ``significant deterioration.''
Establishing an increment system for PM2.5 will fulfill two
of the factors applicable under section 166(c): (1) Providing specific
numerical measures to evaluate permit applications, and (2) stimulating
improved control technology.
    First, under section 165(a)(3) of the Act, a permit applicant must
demonstrate that emissions from the proposed construction and operation
of a facility ``will not cause, or contribute to, air pollution in
excess of any (A) maximum allowable increase or maximum allowable
concentration for any pollutant * * *.'' Once the baseline date
associated with the application for the first new major stationary
source or major modification in an area is established, the new
emissions from that source consume a portion of the increment in that
area, as do any subsequent emissions increases that occur from any
source in the area. When the maximum pollutant concentration increase
defined by the increment has been reached, additional PSD permits
cannot be issued until sufficient amounts of the increment are ``freed
up'' via emissions reductions that may be required by the reviewing
authority. Thus, an increment is a quantitative value that establishes
a ``maximum allowable increase'' for a particular pollutant. It
functions, therefore, as a specific numerical measure that can be used
to evaluate whether an applicant's proposed project will cause or
contribute to air pollution in excess of allowable levels.
    Increments also satisfy the second factor in section 166(c) by
providing ``a framework for stimulating improved control technology.''
Increments establish an incentive to apply improved control
technologies in order to avoid violating the increment and to ``free
up'' available increment to promote continued economic growth. These
control technologies may become the basis of BACT determinations
elsewhere, as the technologies become more commonplace and the costs
tend to decline.
    One commenter stated that, although increments may encourage the
use of existing control technologies, EPA has not cited any evidence
that increments actually stimulate the development of improved
technologies. Moreover, the commenter asserted that even if increments
provide the incentive asserted by EPA, any encouragement of improved
control technology is wholly incidental and hardly amounts to a
``framework'' whose purpose is to stimulate such technology.
    We continue to believe that the total program, encompassing
increments and BACT, does provide an appropriate framework to stimulate
BACT in such a way that it is not simply ``wholly incidental,'' as the
commenter claims. The fact that economic growth in an area must occur
within a defined amount of allowable air quality deterioration should
logically lead to the application of improved pollution control
technology as the amount of deterioration increases, and should not be
regarded as an incidental consequence. As stated in the 2007 NPRM,
Congress envisioned that the increments they originally established
would serve as an incentive: ``The incremental ceiling should serve as
an incentive to technology, as a potential source may wish to push the
frontiers of technology in a particular case to obtain greater
productive capacity with the limits of the increments.'' S. Rep. 95-127
at 18, 30 (3 LH at 1392, 1404). We, too, believe that as the available
increment in an area becomes smaller, and as states try to preserve
some of the remaining increments for future growth, it will be
necessary to require sources to install more stringent controls in that
area. Such levels of control ultimately must be considered in
subsequent BACT evaluations in other PSD areas throughout the country.
Admittedly, the increasing stringency of control technologies over
time, as observed in EPA's BACT/Lowest Achievable Emission Rate (LAER)
Clearinghouse, supports but cannot in itself conclusively demonstrate
that the PSD program has already stimulated development of improved
control technology; there are undoubtedly a number of factors that
could cause such trends. Nevertheless, even the need to require a more
stringent BACT determination in only a few PSD areas (due to dwindling
increment availability) necessitates consideration of that level of
control for all other PSD sources wherever they may decide to locate.
In any event, while the commenter generally questions the effectiveness
of the increments as an incentive for tightening BACT, they provided no
evidence that more stringent BACT is not related to the

increment system established as an integral part of the PSD program.
2. Area Classifications
    In this final rule, EPA is establishing the same three-tiered area
classification system for PM2.5 that is applicable to the
increments for NO2 and other pollutants under the PSD
program and the Act. Accordingly, areas that are currently Class I for
other pollutants will also be Class I for PM2.5 and all
other areas will be Class II for PM2.5 unless we redesignate
the area based on a request by a State or tribe pursuant to the process
in section 164 of the Act and EPA's regulations at 40 CFR 51.166(g) and
52.21(g).
    As explained earlier in section IV.E.1, Class I areas are areas
where very clean air is most desirable. In contrast, Class III areas
are designed as those areas in which a State wishes to permit the
highest relative level of industrial development, and thus allow the
largest incremental increase in pollution. Areas that are not
especially sensitive and where states have not provided for a higher
level of industrial growth are classified as Class II. When Congress
established this three-tiered scheme for SO2 and PM, it
intended that Class II areas be subject to an increment that allows
``moderately large increases over existing pollution.'' H.R. Rep. 95-
294, 4 LH at 2609.
    Establishing increments at different levels for each of the three
area classifications helps to fulfill two of the factors applicable
under section 166(c) of the Act. First, establishing the smallest
increments in Class I areas helps fulfill EPA's obligation to establish
regulations that ``preserve, protect, and enhance the air quality'' in
parks and special areas. Class I areas are primarily the kinds of parks
and special areas covered by section 160(2) of the Act. Second, by
providing for two additional area classifications with increment levels
that are higher but still protective, the area classification system
helps satisfy the goal in section 160(3) of the Act that EPA ``insure
that economic growth will occur in a manner consistent with
preservation of clean air resources.'' In those areas where clean air
resources may not require as much protection, more growth is allowed.
By employing an intermediate level (Class II areas) and higher level
(Class III areas), this classification scheme helps ensure that growth
can occur where it is needed (Class III areas) without putting as much
pressure on existing clean air resources in other areas where some
growth is still desired (Class II areas).
    By requesting that EPA redesignate an existing Class II area to
Class III, states may accommodate economic growth and air quality in
areas where the Class II increment is too small to allow the siting of
new or modified sources. The procedures specified by the Act for such a
redesignation require a commitment by the State government to create
such an area, extensive public review, local government participation
in the SIP area redesignation process, and a finding that the
redesignation will not result in the applicable increment being
exceeded in a nearby Class I or Class II area. See sections 164(a) and
(b) of the Act. (No State has yet requested a Class III redesignation.)
The EPA believes that the three-tiered classification system has
allowed for economic growth, consistent with the preservation of clean
air resources.
    However, an area classification system alone may not completely
satisfy the factors applicable under section 166(c) of the Act. The
increment that is employed for each class of area is also relevant to
an evaluation of whether the area classification system achieves the
goals of the PSD program. We briefly discuss the characteristics of
increments in section V.E.5.
    One commenter took issue with our assessment of the two factors
that we believe a classification system helps to fulfill. As discussed
previously in section V.C.4, the commenter asserted that EPA has
unlawfully interpreted section 160(3) of the Act to elevate economic
growth over all other statutory factors. As explained in greater detail
in section V.C.4, we disagree that our interpretation elevates economic
growth over other factors, and believe that the plain language of the
statute supports EPA's reading that section 160(3) requires a balancing
of the goals of (1) economic growth and (2) preservation of existing
clean air resources.
    The commenter also stated that EPA has failed to demonstrate that
the classification system and safe harbor increments, in combination
with the other elements of the regulatory framework, will ``preserve,
protect, and enhance the air quality'' in parks and special areas as
required under section 160(2) of the Act. These comments and our
response to them are found in section V.E.6 of this preamble where we
discuss our evaluation of the safe harbor increments.
3. Permitting Procedures
    Two of the factors applicable under section 166(c) are fulfilled by
the case-by-case permit review procedures that are built into our
existing PSD regulations. The framework of our existing PSD regulations
employs the preconstruction permitting system and procedures required
under section 165 of the Act. These requirements are generally
reflected in 40 CFR 51.166 and 52.21 of EPA's PSD regulations. These
permitting and review procedures, which apply to construction of new
major sources and to major modifications, fulfill the goals set forth
in sections 160(4) and 160(5) of the Act. These goals require that PSD
programs in one State not interfere with the PSD programs in other
states and that PSD programs assure that any decision to permit
increased air pollution is made after careful evaluation and public
participation in the decision-making process. For the same reasons
discussed in our proposal for the pollutant-specific NO2
increments regulations (70 FR 8896, February 23, 2005), we believe
these factors are also fulfilled for PM2.5 by employing the
permit review procedures.
4. AQRV Review by Federal Land Manager and Reviewing Authority
    In this final rule, we apply the existing requirements to evaluate
impacts on AQRVs in Class I areas (see existing 40 CFR 51.166(p) and
52.21(p)) to PM2.5. The existing requirements for an AQRV
review, which Congress applied to SO2 and TSP, provide
Federal land managers (FLMs) with the responsibility to review source
impacts on site-specific AQRVs in Class I areas and to bring any
alleged adverse impacts to the attention of the reviewing authority.
Under an increment approach, we consider this review to be an
additional measure that helps satisfy the factors in sections 166(c)
and 160(2) which require EPA's pollutant-specific PSD regulations to
protect (1) air quality values, and (2) parks and other special areas,
respectively.
    Two State/local agency commenters supported our proposal to apply
the requirements to evaluate impacts on AQRV in Class I areas to
PM2.5 review. However, one commenter indicated that FLM
review does not and cannot assure the prevention of all significant
PM2.5-related deterioration because it applies only to the
construction or modification of very large stationary sources (e.g.,
factories and power plants) affecting Class I areas. This commenter
pointed out that Class I areas do not include Bureau of Land Management
wilderness and wilderness study areas (encompassing more than 15
million acres), 341 of the nation's 390 national park units (only 49
national parks are Class I), and many U.S. Forest Service lands
(including a number of wilderness areas). The commenter added that FLM

review does not help to fulfill section 160(2)'s goal of preserving and
protecting air quality in ``other areas of special national or regional
natural, recreational, scenic, or historic value,'' such as State and
local parks, wildlife refuges, recreation areas, lakes, and historic
areas, none of which are Class I areas. In addition, the commenter
noted that FLM review does not apply to emissions increases from
sources of PM2.5 and precursor pollution other than major
stationary sources, such as motor vehicles and non-major industrial
sources (which are sources that emit substantial amounts of
PM2.5 and precursors). Alabama Power v. Costle, 636 F.2d
323, 362 (D.C. Cir. 1979) (Alabama Power) (expressly recognizing that
``[s]ignificant deterioration may occur due to increased emissions from
unregulated minor sources.'').
    The commenter also asserted that FLM review is of limited reach
even where it does apply. Under the current PSD regulations, a State
must consider an FLM's objections and must justify its decision in
writing when it disagrees with those objections, but the State can
still issue a PSD permit over those objections unless emissions are
predicted to cause an exceedance of the applicable increment. The
commenter believes that, given these limitations, EPA cannot plausibly
claim that the existing provision for FLM review ensures the
preservation, protection, and enhancement of air quality for parks and
natural areas throughout the nation as required by section 160(2) of
the Act.
    In our rulemakings addressing PSD for NOX, EPA extended
the AQRV review procedures set forth in 40 CFR 51.166(p) and 52.21(p)
to cover NO2. These AQRV review procedures were established
based on section 165(d) of the Act, and they were originally applied
only in the context of the statutory increments for PM and
SO2. However, because they also address many of the factors
applicable under section 166(c) of the Act, EPA also applied them to
NOX through regulation. In this final rule, we are amending
the existing PSD regulations to extend, as proposed, the AQRV review
procedures to include PM2.5 by explicitly including
PM2.5 in the regulatory text that now simply references
``particulate matter.'' See new 40 CFR 51.166(p)(4) and 52.21(p)(5).
    Section 165(d) creates a scheme in which the FLM and reviewing
authority must review the impacts of a proposed new or modified
source's emissions on AQRVs. The Act assigns to the FLM an
``affirmative responsibility'' to protect the AQRVs in Class I areas.
This is in notable contrast to the reviewing authority's responsibility
for protecting the increments--including Class I increments. The FLM
may object to or concur in the issuance of a PSD permit based on the
impact, or lack thereof, that new emissions may have on any affected
AQRV that the FLM has identified and for which information is available
to the general public. If the proposed source's emissions are shown not
to cause or contribute to a violation of a Class I increment, the FLM
may still prevent issuance of the permit by demonstrating to the
satisfaction of the reviewing authority that the source or modification
will have an adverse impact on AQRVs. Section 165(d)(2)(C). On the
other hand, if the proposed source is shown to cause or contribute to a
violation of a Class I increment, the reviewing authority (State or
EPA) shall not issue the permit unless the owner or operator
demonstrates to the satisfaction of the FLM that there will be no
adverse impact on AQRVs.\8\ Thus, the showing of compliance with the
increment determines whether the FLM or the permit applicant has the
burden of satisfactorily demonstrating whether or not the proposed
source's emissions would have an adverse impact on AQRVs.\9\ In any
event, the FLM plays an important and material role by raising these
issues for consideration by the reviewing authority, which in the
majority of cases will be the State.
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    \8\ Even if such a waiver of the Class I increment is allowed
upon a finding of no adverse impact, the source must comply with
such emissions limitations as may be necessary to ensure that
alternative increments specified in the rules for SO2 or
PM are not exceeded. The alternative increments are generally at the
level of the Class II increments, with the lone exception being a
more restrictive 3-hour increment for SO2. Section
165(d)(2)(C)(iv). The EPA made this provision applicable to the PSD
provisions for NOX at the level of the NO2
Class II increment (53 FR 3704; 53 FR 40656) and substituted the
PM10 Class II increments for the statutory alternative PM
increments, which were based on TSP (58 FR 31622). This final rule
expands this provision to include the PM2.5 Class II
increments as well. See 40 CFR 51.166(p)(4) and 52.21(p)(5).
    \9\ In response to concerns that Class I increment would hinder
growth in areas surrounding the Class I area, Congress established
Class I increments as a means of determining where the burden of
proof should lie for a demonstration of adverse effects on AQRVs.
See Senate Debate, June 8, 1977 (3 LH at 725).
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    Extending the AQRV review procedures of the PSD regulations to
PM2.5 helps to provide protection with respect to potential
adverse effects from PM2.5 for parks and special areas
(which are generally the Class I areas subject to this review) not
afforded by the increment system alone. As discussed later, we believe
the factors applicable under section 166(c) of the Act can be fulfilled
when the review of AQRVs is applied in conjunction with increments and
other aspects of our PSD regulations. In those cases where the
increment is not violated and the reviewing authority agrees that a
proposed project will adversely affect AQRVs, the parks and other
special areas will be protected by denying issuance of the permit or by
requiring the applicant to modify the project to alleviate the adverse
impact.
    We read the legislative history to show that Congress intended the
AQRV review provisions of section 165(d) to provide a special layer of
protection, beyond that provided by increments. The Senate committee
report stated the following:

    A second test of protection is provided in specified Federal
land areas (Class I areas), such as national parks and wilderness
areas; these areas are also subjected to a review process based on
the effect of pollution on the area's air quality related values.''

S. Rep. 95-127, at 17, 4 LH at 1401.

    As we stated in the NO2 increment rule, we believe the
term ``air quality values'' should be given the same meaning as ``air
quality related values.'' Legislative history indicates that the term
``air quality value'' was used interchangeably with the term ``air
quality related value'' (AQRV) regarding Class I lands.\10\
---------------------------------------------------------------------------

    \10\ See S. Rep. 95-127, at 12, reprinted at 3 LH at 1386, 1410
(describing the goal of protecting ``air quality values'' in
``Federal lands--such as national parks and wilderness areas and
international parks,'' and in the next paragraph and subsequent text
using the term ``air quality related values'' to describe the same
goal); id. at 35, 36 (``The bill charges the Federal land manager
and the supervisor with a positive role to protect air quality
values associated with the land areas under the jurisdiction of the
[FLM]'' and then describing the statutory term as ``air quality
related values''). H.R. Report 95-564 at 532 (describing duty of
Administrator to consider ``air quality values'' of the tribal and
State lands in resolving an appeal of a tribal or State
redesignation, which is described in the final bill as ``air quality
related values'').
---------------------------------------------------------------------------

    The commenter is correct that the FLM (or AQRV) review applies only
to Class I areas, and not to other ``special'' areas such as the
numerous State and local parks and some other areas that could be seen
as being covered by the protective purposes of section 160(2) of the
Act. This level of coverage by FLM review to protect AQRVs was
established by Congress when it enacted the PSD program, including the
purposes set out in section 160(2). Thus, we conclude that Congress
believed that the special areas not designated as Class I areas were
properly addressed by the other elements of the PSD program. As
discussed further in the next section, one such element is the
requirement for sources to conduct an ``additional impacts analysis,''
which includes an

analysis of the impacts on visibility, soils, and vegetation of the
proposed source and associated growth, regardless of the classification
of the area impacted by the source. Note also that states have the
option under the Act of designating additional areas as Class I areas
and providing for AQRV review for these State Class I areas if they
believe that there are areas within their borders that merit such
protection.
    The commenter is not correct in saying that the review to protect
AQRVs does not apply to emissions increases from sources other than
major stationary sources. While it is generally true that a major
stationary source may trigger the analysis as part of the required PSD
review for new major stationary sources and major modifications where
such source's emissions increase may affect a Class I area, the review
itself includes the impacts on an AQRV of other emissions in the area,
including emissions from non-major sources. In addition, states may
adopt requirements in their State implementation plans to require
certain minor sources seeking a permit to undergo an AQRV analysis if
they choose to do so.
    We agree with the commenter that the AQRV review has certain
limitations in that a State can, under some circumstances, issue a PSD
permit over the objection of the FLM. Here again, Congress enabled this
outcome when it provided that a permit would not be issued when the FLM
demonstrates ``to the satisfaction of the State'' that the source will
have an adverse impact on AQRVs in a Class I area. Section
165(d)(2)(C)(ii). We read this provision to reflect Congress's judgment
on the appropriate balance between State and FLM discretion in the
reach of AQRV review. That said, when a reviewing authority declines to
follow a determination of adverse impact by the FLM, the reviewing
authority is expected to provide a rational basis for doing so, and a
reviewing authority's rejection of an FLM's finding may not be
arbitrary and capricious. As stated by EPA's Environmental Appeals
Board in In the Matter of: Hadson Power 14--Buena Vista, 4 E.A.D. 258,
1992 WL 345661 (October 5, 1992)(in Section II.A):

    States do not have unfettered discretion to reject an FLM's
adverse impact determination. If a State determines that an FLM has
not satisfactorily demonstrated an adverse impact on AQRVs from the
proposed facility, the State must provide a ``rational basis'' for
such a conclusion, ``given the FLMs' affirmative responsibility and
expertise regarding the Class I areas within their jurisdiction.''
50 FR 28549, July 12, 1985. Arbitrary and capricious rejections of
adverse impact determinations are not sustainable. (citations
omitted).

    In sum, the commenter correctly enumerated some of the limitations
of the AQRV review under the Act. However, such review is only one
element of the full PSD program, which must be evaluated against the
statutory requirements in their entirety. We continue to believe, as
previously stated, that under an increment approach, FLM review for
AQRV impacts is an additional measure that helps satisfy the factors in
sections 166(c) and 160(2) of the Act (which require EPA's pollutant-
specific PSD regulations to protect (1) air quality values, and (2)
parks and other special areas, respectively) in balance with the other
statutory factors. We add that the AQRV review requirements of the
existing regulations mirror these requirements in the Act, which
reflect Congress' judgment of how AQRV review should properly be used
to promote the purposes of the program as set out in section 160 of the
Act.
5. Additional Impacts Analysis
    The ``additional impacts analysis'' requirements set forth in our
part 51 and 52 PSD regulations also help fulfill the criteria and goals
and purposes in sections 166(c) and 160. The additional impacts
analysis involves a case-by-case review of potential harm to
visibility, soils, and vegetation in Class II and III areas that could
occur from the construction or modification of a PSD source.
    Sections 51.166(o)(1) and 52.21(o)(1) of the PSD regulations
require that a permit provide the following analysis:

An analysis of the impairment to visibility, soils and vegetation
that would occur as a result of the source or modification and
general commercial, residential, industrial and other growth
associated with the source or modification. The owner or operator
need not provide an analysis of the impact on vegetation having no
significant commercial or recreational value.

    This requirement was based on section 165(e)(3)(B) of the Act,
which provides that EPA establish regulations that require ``an
analysis of the ambient air quality, climate and meteorology, terrain,
soils and vegetation, and visibility at the site of the proposed major
emitting facility and in the area potentially affected by emissions
from such facility * * *.''
    As mentioned in the previous section, one commenter argued that the
provisions for protection of Class I areas are of no help in fulfilling
the goal set forth in section 160(2) of the Act to preserve and protect
air quality in the countless ``other areas of special national or
regional natural, recreational, scenic, or historic value'' such as
State and local parks, wildlife refuges, recreation areas, lakes and
historic areas, none of which were originally defined by Congress as
Class I areas.
    We acknowledge that the special provisions for protecting Class I
areas are not applicable for protecting areas that are not designated
as ``Class I.'' However, we believe that the ``additional impacts
analysis'' provisions are especially helpful for satisfying the
requirements of section 166(c) in Class II and Class III areas,
including the types of areas described by the commenter, that are not
Class I areas but are worthy of special protection beyond what might be
provided by the NAAQS and increments. 40 CFR 51.166(o) and 52.21(o).
These areas are not subject to the special AQRV review that applies
only in Class I areas. While the additional impacts analysis is not as
intensive a review as the AQRV analysis required in Class I areas, the
requirement to consider impairments to visibility, soils, and
vegetation through the additional impacts analysis contributes to
satisfying the factors applicable under section 166(c) of the Act in
all areas, including Class II and Class III areas.
6. Installation of BACT
    The requirement that new sources and modified sources subject to
PSD apply BACT is an additional measure that helps to satisfy the
factors in sections 166(c), 160(1), and 160(2) of the Act. This
requirement, based on section 165(a)(4) of the Act, is already included
in EPA's PSD regulations for all pollutants generally and thus, in the
2007 NPRM we considered it to be a part of the regulatory framework for
the Agency's pollutant-specific regulations for PM2.5. 40
CFR 51.166(j) and 52.21(j). Our existing regulations define ``best
available control technology'' as ``an emission limitation * * * based
on the maximum degree of reduction for each pollutant subject to
regulation under the Act * * * which the Administrator, on a case-by-
case basis, taking into account energy, environmental, and economic
impacts and other costs, determines is achievable for such source
through application of production processes or available methods,
systems, and techniques * * *.'' 40 CFR 51.166(b)(12) and 52.21(b)(12).
This pollutant control technology requirement, in practice, has
required significant reductions in the pollutant emissions increases
from new and modified sources while also stimulating the on-going
improvement of control

technology. The control of PM2.5 emissions through the
application of BACT helps to protect air quality values, public health
and welfare, and parks and other special areas.

E. Final PM2.5 Increments

    Based on our evaluation of the effects of PM2.5 and a
balancing of the criteria in section 166(c) of the Act (and the
incorporated goals and purposes of the Act contained in section 101 and
the statutory PSD program in section 160 of the Act), EPA has concluded
that the ``safe harbor'' increments for PM2.5 (which satisfy
section 166(d) of the Act) are sufficient to fulfill the criteria in
section 166(c) when combined with the other measures described earlier
that we apply to PM2.5. Since several of the eight factors
applicable under section 166(c) are satisfied by adopting the framework
and other measures described earlier, our development of these
increments for PM2.5 was guided by the four remaining
factors that may not be fully satisfied by the framework and other
measures: (1) Protecting AQRVs; (2) protecting the public health and
welfare from reasonably-anticipated adverse effects; (3) protecting the
air quality in parks and special areas, and (4) insuring economic
growth.\11\ In accordance with the ``contingent safe harbor'' approach,
to determine the specific characteristics of the proposed increments,
we first established safe harbor increments representing the level of
effectiveness necessary to satisfy the ``at least as effective as''
requirement in section 166(d) of the Act and then conducted further
analysis to determine if additional measures are necessary to fulfill
the requirements of section 166(c).
---------------------------------------------------------------------------

    \11\ We have paraphrased these factors here and in other
sections to facilitate the explanation of our reasoning. However, we
recognize, as we did in our regulation for NOX, that the
statutory language is broader than the shorthand we use here for
convenience.
---------------------------------------------------------------------------

1. Identification of Safe Harbor Increments
    Using the percentage-of-NAAQS approach under proposed Option 1, as
explained in section V.C.2 of this preamble, we derived the following
safe harbor increments for PM2.5:

----------------------------------------------------------------------------------------------------------------
                                                                 NAAQS           Increments ([micro]g/m\3\)
                      Averaging period                         ([micro]g/ --------------------------------------
                                                                 m\3\)       Class I      Class II    Class III
----------------------------------------------------------------------------------------------------------------
Annual......................................................           15            1            4            8
24-hour.....................................................           35            2            9           18
----------------------------------------------------------------------------------------------------------------

    The table shows PM2.5 NAAQS levels (primary and
secondary NAAQS) at 15 [micro]g/m\3\ for the annual averaging time and
35 [micro]g/m\3\ for the 24-hour averaging time. See 40 CFR 50.7. From
these NAAQS levels, we calculated the safe harbor increments based on
the same percentages that were used by Congress to establish the
original PM increments (measured as TSP) in section 163 of the Act,
i.e., 6.6 percent of the NAAQS for Class I areas, 25 percent of the
NAAQS for Class II areas, and 50 percent of the NAAQS for Class III
areas. We have concluded that increments with these characteristics are
sufficient to satisfy the requirement in section 166(d) that we adopt
increments (or other PSD regulations) that are ``at least as effective
as'' the increments established in section 163 of the Act. See EDF v.
EPA, 898 F.2d at 188, 190.
    Nine commenters supported proposed Option 1, either explicitly or
implicitly supporting our method of calculating the safe harbor
increments used to develop increments for PM2.5. One of
these commenters, while agreeing with the safe harbor increment
approach under Option 1, disagreed with our analysis of the adequacy of
the safe harbor increments, as discussed in other sections of this
preamble. One commenter who opposed Option 1 (based on the belief that
section 166(a) of the Act is not the appropriate basis for
PM2.5 increments) nevertheless supported the percentage-of-
NAAQS approach for developing PM2.5 increments under the
statutory authority at section 166(f).
    A commenter who opposed our proposal to calculate increments using
percentages of the NAAQS argued that this approach for setting the
PM2.5 increments is not scientifically supported. This
commenter indicated that basing the PM2.5 increments on the
same percentage of the NAAQS that were used to set PM10
increments based on the TSP NAAQS ignores the relationship between
PM10 and PM2.5 emissions, which may be much
different than the relationship between TSP and PM10
emissions. The commenter argued that, because the ratio of
PM2.5 to PM10 emissions is 0.8, it appears that
using the percentages proposed by EPA would indirectly restrict
PM10/TSP emissions and air quality impacts to proportionally
lower levels than the PM10 increments in order to avoid
exceeding the PM2.5 increments. The commenter conceded that
using the 0.8 factor to set PM2.5 increments may seem too
high, but asserted that using the safe harbor approach would set
increments for PM2.5 that are too low.
    We conclude that the commenter is mistaken in saying that the
PM2.5 increments use the same percentage of the NAAQS that
were used to set the PM10 NAAQS. We adopted the
PM10 increments using the ``equivalent substitution''
approach set forth under section 166(f) of the Act. Under that
approach, rather than calculating the PM10 increments as
specific percentages of the PM10 NAAQS (using the same
percentages that Congress used for setting the statutory increments for
PM and SO2), EPA determined the levels of the
PM10 increments that could represent an equivalent amount of
increment consumed, as if the TSP increments were still in effect. See
58 FR 31622, June 3, 1993, at 31626-31627. Nevertheless, the commenter
is correct that, in cases where the ratio of PM2.5 to
PM10 emissions is 0.8 for an individual source, the source
may have to reduce its PM10 emissions more than would
otherwise be necessary to meet the PM10 increments in order
to control its PM2.5 emissions sufficiently to meet the safe
harbor PM2.5 increments.\12\ This is because the safe harbor
PM2.5 increments are less than 80 percent of the
PM10 increments. For example, the Class II 24-hour
PM2.5 safe harbor increment (9 [micro]g/m\3\) is only 30
percent of the corresponding PM10 increment (30 [micro]g/
m\3\).
---------------------------------------------------------------------------

    \12\ Note that the PM10 increment may still be more
limiting in areas where much of that increment has already been
consumed.
---------------------------------------------------------------------------

    The underlying reason that the safe harbor PM2.5
increments are so much less than the PM10 increments is that
the PM2.5 NAAQS are much less than the PM10
NAAQS.\13\ This is the result of the evolution in our knowledge about
the health and welfare effects of PM, in particular the effects of the
fine PM

represented by PM2.5. We believe that it is fitting for
PM2.5 increments to reflect our greater knowledge about
PM2.5 effects (as embodied in the NAAQS), rather than to
simply maintain the control level required by the PM10
increments as suggested by the commenter. If this results in
PM2.5 increments that are more limiting than PM10
increments, we believe that this outcome is appropriate in light of our
statutory requirement to prevent significant deterioration of air
quality as it relates to PM2.5.
---------------------------------------------------------------------------

    \13\ The 24-hour PM2.5 NAAQS (35 [micro]g/m\3\) is
about 23 percent of the 24-hour PM10 NAAQS (150 [micro]g/
m\3\).
---------------------------------------------------------------------------

2. Data Used by EPA for the Evaluation of the Safe Harbor Increments
for PM2.5
    We evaluated whether measures other than the safe harbor increments
are necessary by analyzing primarily the scientific and technical
information on the health and welfare effects of PM2.5
contained in the June 2005 OAQPS Staff Paper which accompanied the last
full review of the PM NAAQS completed in 2006.\14\
---------------------------------------------------------------------------

    \14\ The review completed in 2006 updated the previous review,
which began in 1994 and resulted in revised standards for PM in
1997.
---------------------------------------------------------------------------

    Section 166(a) of the Act provides that EPA establish pollutant-
specific PSD regulations, such as increments, after the establishment
of a NAAQS for the applicable pollutants. The Act provides that EPA
will promulgate new PSD regulations under section 166, including new
increments if appropriate, within 2 years from the promulgation of any
NAAQS after 1977. Within that time frame, the health and welfare
information used for the setting of the NAAQS would also be ``current''
for purposes of establishing pollutant-specific PSD regulations. We
believe this timing reflects congressional intent that EPA consider the
same body of information concerning a pollutant's health and welfare
effects when it promulgates the NAAQS and subsequent PSD increments (or
other measures) defining significant air quality deterioration for the
same pollutant. However, when we used that same information as the
basis for our proposed pollutant-specific PSD regulations, we evaluated
that information under the legal criteria in section 166 of the Act
rather than the criteria in section 109 applicable to the promulgation
of NAAQS. See EDF v. EPA, 898 F.2d at 190.
    At the time of our proposal of PM2.5 increments, we had
just completed a review of the PM2.5 NAAQS. Thus, the
information used in the NAAQS review was current and timely for
purposes of establishing pollutant-specific PSD regulations for
PM2.5. On October 17, 2006, based primarily on considerable
new data on the air quality and human health effects for
PM2.5 directly, EPA revised the primary and secondary NAAQS
to provide increased protection of public health and welfare by
retaining the level of the annual standard and tightening the level of
the 24-hour standard from 65 to 35 [mu]g/m\3\ while retaining the 24-
hour PM10 NAAQS and revoking the annual PM10
NAAQS. The information contained in both the 2004 Criteria Document and
2005 Staff Paper that was used for the latest review of the PM NAAQS
was also considered for the purpose of evaluating the PM2.5
increments that we have established in this final rule.
    The 2004 Criteria Document and 2005 Staff Paper are the products of
a rigorous process that is followed to validate and interpret the
available scientific and technical information, and provided the basis
for recommending the PM2.5 NAAQS. In accordance with the
Act, the NAAQS process begins with the development of ``air quality
criteria'' under section 108 for air pollutants that ``may reasonably
be anticipated to endanger public health or welfare'' and that come
from ``numerous or diverse'' sources. Section 108(a)(1). For each NAAQS
review, the Administrator must appoint ``an independent scientific
review committee composed of seven members of the National Academy of
Sciences, one physician, and one person representing State air
pollution control agencies,'' known as the Clean Air Scientific
Advisory Committee (CASAC). Section 109(d)(2)(A). The CASAC is charged
with recommending revisions to the criteria document and NAAQS, and
advising the Administrator on several issues, including areas in which
additional knowledge is required to appraise the adequacy and basis of
existing, new, or revised NAAQS. Section 109(d)(2)(B),(C).
    ``Air quality criteria'' must reflect the latest scientific
knowledge on ``all identifiable effects on public health or welfare''
that may result from a pollutant presence in the ambient air. Section
108(a)(2). The scientific assessments constituting air quality criteria
generally take the form of a ``criteria document,'' a rigorous review
of all pertinent scientific studies and related information. The EPA
also develops a ``staff paper'' to ``bridge the gap'' between the
scientific review and the judgments the Administrator must make to set
standards. See Natural Resources Defense Council v. EPA (``NRDC ''),
902 F.2d 962, 967 (D.C. Cir. 1990). Both documents undergo extensive
scientific peer review as well as public notice and comment. See, e.g.,
62 FR 386542.
3. Scope of Effects Considered
    The effects of ambient PM2.5 concentrations may include
effects from secondarily-formed PM2.5. Thus, when we
analyzed the data in this rulemaking, we evaluated the health and
welfare effects of both direct PM2.5 and secondarily-formed
PM2.5 that may result from the transformation of other
pollutants such as SO2 and NOX. This was
consistent with the approach we described for addressing these effects
in the review of our pollutant-specific NO2 increments
regulations. 70 FR 59590.
4. Evaluation of the Health and Welfare Effects of PM2.5
    Airborne PM is not a specific chemical entity, but rather is a
mixture of liquid and solid particles from different sources and of