[Federal Register: September 21, 2007 (Volume 72, Number 183)]
[Proposed Rules]
[Page 54111-54156]
[DOCID:fr21se07-26]

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

Part II

Environmental Protection Agency

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

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);
Proposed Rule

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

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 51 and 52

[EPA-HQ-OAR-2006-0605; FRL-8470-1]
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: Proposed rule.

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

SUMMARY: The Clean Air Act (Act) authorizes EPA to establish
regulations to prevent significant deterioration of air quality due to
emissions of any pollutant for which a national ambient air quality
standard (NAAQS) has been promulgated. The NAAQS for particulate matter
using the PM2.5 indicator were promulgated in 1997. The EPA
is proposing to facilitate implementation of a PM2.5
Prevention of Significant Deterioration (PSD) program in areas
attaining the particulate matter less than 2.5 micrometers
(PM2.5) NAAQS by developing PM2.5 increments,
Significant Impact Levels (SILs), and a Significant Monitoring
Concentration (SMC). In addition, EPA is proposing to revoke the annual
PM10 increments.
    ``Increments'' are maximum increases in ambient PM2.5
concentrations (PM2.5 increments) allowed in an area above
the baseline concentration. The SILs and SMCs are numerical values that
represent thresholds of insignificant, i.e., de minimis, modeled source
impacts or monitored (ambient) concentrations, respectively. The EPA is
proposing such values for PM2.5 that will be used as
screening tools by a major source subject to PSD to determine the
subsequent level of analysis and data gathering required for a PSD
permit application for emissions of PM2.5.

DATES: Comments must be received on or before November 20, 2007. Under
the Paperwork Reduction Act, comments on the information collection
provisions must be received by the Office of Management and Budget
(OMB) on or before October 22, 2007.
    Public Hearing. If anyone contacts us requesting to speak at a
public hearing by October 11, 2007, we will hold a public hearing.
Additional information about the hearing would be published in a
subsequent Federal Register notice.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2006-0605, by one of the following methods:
     http://www.regulations.gov. Follow the on-line instructions for

submitting comments.
     E-mail<EMAIL_ADDRESS>     Mail: Air and Radiation Docket and Information Center,
Environmental Protection Agency, Mailcode: 2822T, 1200 Pennsylvania
Avenue, NW., Washington, DC 20460. Please include a total of two
copies. In addition, please mail a copy of your comments on the
information collection provisions to the Office of Information and
Regulatory Affairs, Office of Management and Budget (OMB), Attn: Desk
Officer for EPA, 725 17th Street, Northwest, Washington, DC 20503.
     Hand Delivery: Air and Radiation Docket and Information
Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Avenue, NW.,
Washington, DC 20004. Such deliveries are only accepted during the
Docket Center's normal hours of operation, and special arrangements
should be made for deliveries of boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2006-0605. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at http://www.regulations.gov, including any personal information

provided, unless the comment includes information claimed to be
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Do not submit information that you
consider to be CBI or otherwise protected through http://www.regulations.gov

or e-mail. The http://www.regulations.gov Web site is an ``anonymous access''

system, which means EPA will not know your identity or contact
information unless you provide it in the body of your comment. If you
send an e-mail comment directly to EPA without going through
http://www.regulations.gov your e-mail address will be automatically captured

and included as part of the comment that is placed in the public docket
and made available on the Internet. If you submit an electronic
comment, EPA recommends that you include your name and other contact
information in the body of your comment and with any disk or CD-ROM you
submit. If EPA cannot read your comment due to technical difficulties
and cannot contact you for clarification, EPA may not be able to
consider your comment. Electronic files should avoid the use of special
characters, any form of encryption, and be free of any defects or
viruses. For additional instructions on submitting comments, go to
section I.B of the SUPPLEMENTARY INFORMATION section of this document.
    Docket: All documents in the docket are listed in the
http://www.regulations.gov index. Although listed in the index, some

information is not publicly available, e.g., CBI or other information
whose disclosure is restricted by statute. Certain other material, such
as copyrighted material, will be publicly available only in hard copy.
Publicly available docket materials are available either electronically
in http://www.regulations.gov or in hard copy at the Air and Radiation Docket

and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution
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 (202)
566-1744, and the telephone number for the Air and Radiation Docket and
Information Center is<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT: Mr. Raghavendra (Raj) Rao, Air Quality
Policy Division, Office of Air Quality Planning and Standards (C504-
03), Environmental Protection Agency, Research Triangle Park, North
Carolina 27711; telephone number<PHONE_NUMBER>; fax number (919) 541-
5509; e-mail address<EMAIL_ADDRESS>or Dan deRoeck, Air Quality
Policy Division, Office of Air Quality Planning and Standards (C504-
03), Environmental Protection Agency, Research Triangle Park, North
Carolina 27711; telephone number<PHONE_NUMBER>; fax number (919) 541-
5509; e-mail address<EMAIL_ADDRESS>To request a public hearing
or information pertaining to a public hearing on this document, contact
Ms. Pamela S. Long, Air Quality Policy Division, Office of Air Quality
Planning and Standards (C504-03), Environmental Protection Agency,
Research Triangle Park, North Carolina 27711; telephone number (919)
541-0641; fax number<PHONE_NUMBER>; e-mail address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

I. General Information

A. Does this action apply to me?

    Entities potentially affected by this proposed action include
owners and operators of emission sources in all industry groups, as
well as the EPA and State, local, and tribal governments that are
delegated authority to implement these regulations. The majority of
sources potentially affected are expected to be in the following
groups:

----------------------------------------------------------------------------------------------------------------
             Category                    NAICS\a\                             Industry group
----------------------------------------------------------------------------------------------------------------
Industry.........................  221111, 221112,       Electric services.
                                    221113, 221119,
                                    221121, 221122.
                                   32411...............  Petroleum refining.
                                   325181, 32512,        Industrial inorganic chemicals.
                                    325131, 325182,
                                    211112, 325998,
                                    331311, 325188.
                                   32511, 325132,        Industrial organic chemicals.
                                    325192, 325188,
                                    325193, 32512,
                                    325199.
                                   32552, 32592, 32591,  Miscellaneous chemical products.
                                    325182, 32551.
                                   211112..............  Natural gas liquids.
                                   48621, 22121........  Natural gas transport.
                                   32211, 322121,        Pulp and paper mills.
                                    322122, 32213.
                                   322121, 322122......  Paper mills.
                                   336111, 336112,       Automobile manufacturing.
                                    336712, 336211,
                                    336992, 336322,
                                    336312, 33633,
                                    33634, 33635,
                                    336399, 336212,
                                    336213.
                                   325411, 325412,       Pharmaceuticals.
                                    325413, 325414.
Federal government...............  924110..............  Administration of Air and Water Resources and Solid
                                                          Waste Management Programs.
State/local/tribal Government....  924110..............  Administration of Air and Water Resources and Solid
                                                          Waste Management Programs.
----------------------------------------------------------------------------------------------------------------
\a\ North American Industry Classification System.

    This table is not intended to be exhaustive, but rather provides a
guide for readers regarding entities likely to be regulated by this
action. To determine whether your facility is regulated by this action,
you should examine the applicability criteria in the PSD rules for
attainment areas (40 CFR 52.21). If you have any questions regarding
the applicability of this action to a particular entity, contact the
person listed in the preceding FOR FURTHER INFORMATION CONTACT section.

B. What should I consider as I prepare my comments for EPA?

    1. Submitting CBI. Do not submit this information to EPA through
http://www.regulations.gov or e-mail. Clearly mark the part or all of the

information that you claim to be CBI. For CBI information in a disk or
CD ROM that you mail to EPA, mark the outside of the disk or CD ROM as
CBI and then identify electronically within the disk or CD ROM the
specific information that is claimed as CBI. In addition to one
complete version of the comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2. Send or deliver information
identified as CBI only to the following address: Roberto Morales, OAQPS
Document Control Officer (C404-02), Environmental Protection Agency,
Research Triangle Park, NC 27711, Attention Docket ID No. EPA-HQ-OAR-
2006-0605.
    2. Tips for Preparing Your Comments. When submitting comments,
remember to:
     Identify the rulemaking by docket number and other
identifying information (subject heading, Federal Register date and
page number).
     Follow directions--The agency may ask you to respond to
specific questions or organize comments by referencing a Code of
Federal Regulations (CFR) part or section number.
     Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
     Describe any assumptions and provide any technical
information and/or data that you used.
     If you estimate potential costs or burdens, explain how
you arrived at your estimate in sufficient detail to allow for it to be
reproduced.
     Provide specific examples to illustrate your concerns, and
suggest alternatives.
     Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
     Make sure to submit your comments by the comment period
deadline identified.

C. 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 proposal will also be available on the World Wide Web. Following
signature by the EPA Administrator, a copy of this notice will be
posted in the regulations and standards section of our NSR home page
located at http://www.epa.gov/nsr.

D. How can I find information about a possible Public Hearing?

    Persons interested in presenting oral testimony should contact Ms.
Pamela Long, New Source Review Group, Air Quality Policy Division
(C504-03), Environmental Protection Agency, Research Triangle Park, NC
27711; telephone number<PHONE_NUMBER> or e-mail<EMAIL_ADDRESS>at
least 2 days in advance of the public hearing. Persons interested in
attending the public hearing should also contact Ms. Long to verify the
time, date, and location of the hearing. The public hearing will
provide interested parties the opportunity to present data, views, or
arguments concerning these proposed rules.
    The information presented in this preamble is organized as follows:

I. General Information
    A. Does this action apply to me?
    B. What should I consider as I prepare my comments for EPA?
    C. Where can I get a copy of this document and other related
information?
    D. How can I find information about a possible Public Hearing?
II. Overview of Proposed Regulations
    A. Summary of Proposed Options for Increments
    B. Summary of Proposed Options for SILs
    C. Summary of Proposed Options for the PM2.5 SMC
III. Background
    A. PSD Program
    B. History of PM NAAQS
    1. TSP and PM10 NAAQS
    2. PM2.5 NAAQS
    3. Revised PM2.5 and PM10 NAAQS
    C. Implementation of NSR for PM2.5
    D. Background on Implementation of PSD Increments

    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 NOX 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)
IV. EPA's Interpretation of Section 166 of the Clean Air Act
    A. Which Criteria In Section 166 Should EPA Use to Develop
Increments for PM2.5?
    1. Support for ``Contingent Safe Harbor'' Approach for
PM2.5 Under Section 166(a)
    2. Support of ``Equivalent Substitution'' Approach for
PM2.5 Under Section 166(f)
    B. Requirements of Sections 166(a)-(d) of the Clean Air 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
    C. Requirements of Section 166(f) of the Clean Air Act
V. Increments and Other Measures to Prevent Significant
Deterioration
    A. Option 1--Contingent Safe Harbor Approach for Annual and
Short-Term Increments--Section 166(a)
    1. Proposed Framework for Pollutant Specific PSD Regulations for
PM2.5
    a. Increment System
    b. Area Classifications
    c. Permitting Procedures
    d. Air Quality Related Values Review by Federal Land Manager and
Reviewing Authority
    e. Additional Impacts Analysis
    f. Installation of Best Available Control Technology
    2. Proposed Increments
    a. Identification of Safe Harbor Increments
    b. Data Utilized by EPA for the Evaluation of the Safe Harbor
Increments for PM2.5
    c. Scope of Effects Considered
    d. Evaluation of the Health and Welfare Effects of
PM2.5
    e. Fundamental Elements of Increments
    f. Evaluation of the Safe Harbor Increments
    3. Proposed Baseline Dates for PM2.5 Increments Under
Option 1
    4. Revocation of PM10 Annual Increments
    B. Option 2--Equivalent Substitution Approach for Annual
Increments--Section 166(f)
    1. Development of Equivalent Increments
    2. Proposed Annual Increments for PM2.5
    a. Option 2A
    b. Option 2B
    3. Baseline dates
VI. Significant Impact Levels (SILs)
    A. EPA's Guidance on SILs in the PSD Program
    B. Legal Basis for SILs
    C. Relationship of SILs to AQRVs
    D. Proposed Options for PM2.5 SILs (for PSD and NA-
NSR)
    1. Option 1. Propose SILs using the approach we proposed for
PM10 in 1996
    2. Option 2. PM2.5 to PM10 Emissions Ratio
    3. Option 3. PM2.5 to PM10 NAAQS Ratio
VII. Significant Monitoring Concentrations (SMCs)
    A. Background on SMCs
    1. Preconstruction Monitoring and Its Role in NSR Program
    2. History of SMC Rules Adopted by EPA
    B. Legal Basis for SMCs
    C. Proposed Options for PM2.5 SMC
    1. Option 1. Lowest Detectable Concentration
    2. Option 2. PM2.5 to PM10 Emissions Ratio
    3. Option 3. PM2.5 to PM10 NAAQS Ratio
    D. Correction of Cross References
VIII. Effective Date of the Final Rule, SIP Submittal/Approval
Deadlines and PM10 Revocation Deadline
    A. Option 1: Increments promulgated pursuant to section 166(a)
of the Act.
    1. Effective Date of Final Rule
    2. State Program
    3. Federal Program
    B. Option 2: Increments Promulgated Pursuant to Section 166(f)
of the Act.
    1. Effective date of Final Rule
    2. State Program
    3. Federal Program
    C. Revocation of the PM10 Increment
    D. Transition Period
    E. Effective Date for SILs and SMCs
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From
Environmental Health & Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions to Address
Environmental Justice in Minority Populations and Low-Income
Populations
X. Statutory Authority

II. Overview of Proposed Regulations

    This proposal is the first step in the rulemaking process for
promulgating PM2.5 increments, SILs, and a SMC. The purpose
of this proposed rulemaking is to develop the final elements that will
aid implementation of the PSD program for PM2.5. When final,
these elements will supplement the final NSR implementation rule for
PM2.5. Following final action on this proposal and the
PM2.5 implementation rule for NSR, the Federal
PM2.5 NSR programs will no longer have to rely on the
PM10 program as a surrogate, as has been the practice under
our existing guidance. A State implementing a NSR program in an EPA
approved State Implementation Plan (SIP) may continue to rely on the
interim surrogate policy until we approve a revised SIP addressing
these requirements. In this rulemaking, we \1\ are proposing several
options for increments, SILs and the SMC, respectively.
---------------------------------------------------------------------------

    \1\ In this proposal, the terms ``we,'' ``us,'' and ``our''
refer to the EPA and the terms ``you'' and ``your'' refer to the
owners or operators of stationary sources of air pollution.
---------------------------------------------------------------------------

A. Summary of Proposed Options for Increments

    We are proposing three sets of PM2.5 increments, based
on several approaches that are described in greater detail later in
this preamble. For the first set (option 1), we are relying on an
approach that treats PM2.5 as a new pollutant. This option
follows our statutory authority section 166(a) of the Act to develop
increments for ``pollutants for which national ambient air quality
standards are promulgated after the date of enactment of this part * *
* '' This is the same approach that we used to establish NOX
increment regulations on October 12, 2005 (70 FR at 59586). The second
and third options (options 2A and 2B) rely on an approach that we used
in 1993 to promulgate PM10 increments in lieu of the
statutory increments for particulate matter (PM) following our
replacement of the then existing indicator for the PM NAAQS based on
total suspended particulate with a new indicator based on
PM10. (58 FR 31622, June 3, 1993.) These two options
represent variations of the approach used under the authority of
section 166(f) of the Act to ``substitute'' PM10 increments
for TSP increments. The increment values resulting from each of these
three options are:

--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Proposed increments  ([mu]g/m\3\)                  NAAQS  ([mu]g/m\3\)
                                                                 ---------------------------------------------------------------------------------------
                             Option                                      Class I              Class II              Class III
                                                                 ------------------------------------------------------------------   Annual     24-hr
                                                                    Annual     24-hr      Annual     24-hr      Annual     24-hr
--------------------------------------------------------------------------------------------------------------------------------------------------------
1...............................................................          1          2          4          9          8         18  .........  .........
2A..............................................................          1          2          4          9          8         18         15         35
2B..............................................................          1          2          5          9         10         18  .........  .........
--------------------------------------------------------------------------------------------------------------------------------------------------------

B. Summary of Proposed Options for SILs

    We are also proposing three options for SILs. The first option
utilizes the same approach we proposed for PM10 in the 1996
NSR Reform proposal. For option 2, we are proposing to scale the
PM10 SIL values by the ratio of direct PM2.5 to
direct PM10 emissions. The PM2.5/PM10
emissions ratio is the national average derived from the 2001
extrapolation of the 1999 National Emissions Inventory. For option 3,
we are proposing to scale the PM10 SIL values by the ratio
of the PM2.5 NAAQS to the PM10 NAAQS. The SIL
values resulting from each of these options are:

----------------------------------------------------------------------------------------------------------------
                                                                   Proposed SILs  ([mu]g/m\3\)
                                               -----------------------------------------------------------------
                    Option                             Class I              Class II              Class III
                                               -----------------------------------------------------------------
                                                  Annual     24-hr      Annual     24-hr      Annual     24-hr
----------------------------------------------------------------------------------------------------------------
1.............................................       0.04       0.08        1.0        5.0        1.0        5.0
2.............................................       0.16       0.24        0.8        4.0        0.8        4.0
3.............................................       0.06       0.07        0.3        1.2        0.3        1.2
----------------------------------------------------------------------------------------------------------------

C. Summary of Proposed Options for the PM2.5 SMC

    The first option we are proposing for the SMC is the ``Lowest
Detection Concentration'' or LDC approach that we used for establishing
the SMC for TSP and PM10. For option 2, we are proposing to
scale the PM10 SMC value by the ratio of direct
PM2.5 to direct PM10 emissions. The
PM2.5/PM10 emissions ratio is the national
average derived from the 2001 extrapolation of the 1999 National
Emissions Inventory. For option 3, we are proposing to scale the
PM10 SMC value by the ratio of the PM2.5 NAAQS to
the PM10 NAAQS. The proposed SMC values for each of these
options for the 24-hour averaging period are:

Option 1--10 [mu]g/m\3\
Option 2--7.9 [mu]g/m\3\
Option 3--2.3 [mu]g/m\3\

III. 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 new source
review (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.\2\ 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
incremental ambient pollutant concentration increase (increment). 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.''
---------------------------------------------------------------------------

    \2\ We have delegated authority to some States to implement the
Federal PSD program. The EPA remains the reviewing authority in non-
delegated States and in Indian country.
---------------------------------------------------------------------------

    As part of the analysis of air quality impacts to determine
compliance with the NAAQS and increment, the permit applicant and
reviewing authority may compare the source's impacts for a pollutant
with the corresponding SIL for that pollutant to show that a cumulative
air quality impacts analysis is not necessary. Similarly, the permit
applicant and reviewing authority may use the corresponding SMC for
that pollutant to determine if pre-application site-specific ambient
monitoring data is needed to conduct the air quality analysis.
    When the reviewing authority reaches a preliminary decision to
authorize construction of each 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 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 PM NAAQS

1. 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 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 promulgated new NAAQS for PM in which we
changed the indicator from TSP to PM10, the latter including
particles with a mean aerodynamic diameter less than or equal to 10
micrometers. These smaller 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
for 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. Our 1997 rules 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 action 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 micrograms per cubic meter ([mu]g/m\3\) to 35
[mu]g/m\3\, and retained the existing annual PM2.5 NAAQS of
15 [mu]g/m\3\. In addition, we retained the existing PM10
24-hour NAAQS of 150 [mu]g/m\3\, and revoked the annual PM10
NAAQS (previously set at 50 [mu]g/m\3\).

C. Implementation of NSR for PM2.5

    After we established new annual and 24-hour NAAQS for
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 in July 1997, we issued a guidance
document ``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 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 provides 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 recommends that until we promulgate 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 indicate in that proposal that we will develop increments, SILs,
and SMC in a separate rulemaking--i.e. this proposed rulemaking. Since
there was an interim surrogate NSR program in place, EPA decided to
first promulgate the non-NSR part of the implementation rule (including
attainment demonstrations, designations, control measures etc.)--which
was promulgated on April 25, 2007. The NSR part of the implementation
rule is anticipated to be promulgated in September 2007. Additionally,
once this proposed rulemaking is finalized, States will be able to
fully implement a PM2.5 NSR program.

D. Background on Implementation of PSD Increments

    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. * * *'' 42 U.S.C. 7475(a)(3). 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.''
    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 an
ambient pollutant increase of at least 1 [mu]g/m\3\ (annual average).
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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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) minor source baseline date; and (3)
major source baseline date. Chronologically, 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, e.g., 40 CFR 52.21(b)(14)(ii). For PM 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 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 action, as described later, we are proposing to add a
new trigger date for purposes of calculating the new PM2.5
increments.
    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 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 changes in
general (i.e., from both major and minor sources) affect the increment
and the date when emissions resulting from the construction at a major
stationary source consume the increment, we established the terms
``minor source baseline date'' and ``major source baseline date,''
respectively. See 40 CFR 51.166(b)(14) and 52.21(b)(14). Accordingly,
the ``minor source baseline date'' is the date on which the first
complete application for a PSD permit is filed in a particular area.
Any change in actual emissions after that date affects the PSD
increment for that area.
    The ``major source baseline date'' is the date after which actual
emissions increases associated with construction at any major
stationary source affect 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 is February 8, 1988--the date on which we
proposed increments for NO2. 53 FR 40656. In this action, as
described later, we are proposing to add a new major source baseline
date for PM2.5.
    The PSD regulations set out the third date that is relevant to the
PSD baseline concept. These regulations provide that the earliest date
on which the minor source baseline date can be established is the date
immediately following the ``trigger date'' for the pollutant-specific
increment. See, e.g., 40 CFR 52.21(b)(14)(ii). For PM 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 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.
    Once the minor source baseline date associated with the first PSD
permit application for a proposed 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 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 via 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 has not been consumed. Therefore, new or modified sources
located in areas where the air pollutant concentration is 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.\4\ We have also
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.\5\ In
addition, we established significant impact levels for each pollutant
under the nonattainment major NSR program that have also been used
under the PSD program to identify levels below which the source's
modeled impact is regarded as de minimis. See 40 CFR 51.165(b) and part
51, appendix S, section III.A. In the event that a source's modeled
impacts of a particular pollutant are below the applicable significant
impact level at all ambient air locations modeled, i.e., de minimis
everywhere, EPA policy provides that no further modeling analysis is
required for that pollutant. Our policy is that when a preliminary
screening analysis based on the significant impact level is sufficient
to demonstrate that the source's

emissions will not cause or contribute to a violation of the increment,
there is no need for a full impacts analysis involving a cumulative
evaluation of the emissions from the proposed source and other sources
affecting the area.
---------------------------------------------------------------------------

    \4\ We note that on June 6, 2007, we published a notice of
proposed rulemaking proposing to refine several aspects of the
increment calculation process to clarify how States and regulated
sources may calculate increases in pollutant concentrations for
purposes of determining compliance with the PSD increments. See 72
FR at 31372. When final, these revisions will amend the PSD
regulations at 40 CFR 51.166 and 52.21.
    \5\ See EPA's ``Guideline on Air Quality Models'' at 40 CFR part
51, appendix W.
---------------------------------------------------------------------------

    Within the impact area of a source that does have a significant
impact, increment consumption is calculated using the source's proposed
emissions increase, along with other emissions increases or decreases
of the particular pollutant from sources in the area, which have
occurred since the minor source baseline date established for that
area. (For major sources, emissions increases or decreases that have
occurred since the major source baseline date consume or expand
increment.) Thus, an emissions inventory of sources whose emissions
consume or expand the available increment in the area must be compiled.
The inventory includes not only sources located directly in the impact
area, but sources outside the impact area that affect the air quality
within the impact area.
    The inventory of 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 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 to the Act, among other things, added subpart C to title I,
setting out the requirements for PSD. In section 163, Congress included
numerical increments for PM and sulfur dioxide (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 as Class III 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. There have been no Class III redesignations
to date.
    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 total suspended particulate
(TSP). Thus, EPA interpreted the statutory increments for PM using the
same ambient ``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 NOX Using the ``Contingent Safe Harbor''
Approach Under Section 166(a) of the Act
    As enacted in 1977, subpart C of the Act also included sections
166(a) through 166(e), which set out requirements related to increments
for other pollutants. 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. Section 166(b) prescribes timelines for the effective date
of such regulations, and for corresponding SIP submittals and EPA
approvals. Specifically, regulations, including increments, developed
pursuant to section 166(a) become effective 1 year after the date of
promulgation, and State plan revisions containing the new regulations
are to be submitted to EPA for review within 21 months of promulgation.
The same provision then calls for EPA's approval or disapproval of the
revised plan within 25 months of promulgation. The legislative history
indicates that this 1-year delay before the new PSD requirements,
including the new increments, become effective is to allow Congress an
opportunity to review them before States are required to implement
them. H.R. Conf. Rep. 95-564, at 151 (1977), 1977 U.S.C.C.A.N. 1502,
1532. Section 166(c) and (d) set forth criteria and goals that such
regulations must meet.
    Based on section 166 of the Act, on October 17, 1988, EPA
promulgated increments for nitrogen dioxide (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
(now Environmental Defense, or ``ED'') when ED 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). Environmental Defense 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).
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 air quality related
values (AQRVs) \6\ and to ensure economic growth.
---------------------------------------------------------------------------

    \6\ The term ``air quality related values'' is not defined in
the Act, but the legislative history provides 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 NOX 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 PSD
regulations for NOX. We also amended the requirements of 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.\7\
---------------------------------------------------------------------------

    \7\ 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 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)
    On October 5, 1989, we proposed new 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's 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 existing
TSP increments on June 3, 1993 (58 FR 31622). In the interim between
proposal and promulgation, Congress enacted the 1990 Act Amendments. As
part of these Act 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 developed 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.

IV. EPA'S Interpretation of Section 166 of the Clean Air Act

A. Which Criteria in Section 166 Should EPA Use to Develop Increments
for PM2.5?

    The EPA interprets section 166 of the Act to give the Administrator
the discretion to use either the ``contingent safe harbor'' approach or
the ``equivalent substitution'' approach to establish increments for
PM2.5. Since sections 166(a) and section 166(f) contain or
incorporate different criteria for establishing PSD regulations
containing increments or other measures, the interpretation that EPA
chooses to follow could have an impact on the increments or other
measures that EPA adopts. Regulations promulgated under section 166(a)
must be based on the criteria in section 166(c) and 166(d). 42 U.S.C.
7476(c)-(d). Regulations promulgated under section 166(f) must ``be of
equal stringency in effect as those specific in the provisions for
which they are substituted.'' 42 U.S.C. 7476(f). Furthermore, section
166(a) calls broadly for regulations, which may include increments,
whereas section 166(f) addresses only increments.
    Section 166(a) provides authority for EPA to promulgate additional
pollutant-specific PSD regulations, which may include increments, for
the pollutants specifically identified in that provision plus
additional pollutants for which EPA may promulgate a NAAQS after a
specific date 42 U.S.C. 7476(a). The last sentence of section 166(a)
provides the following:

    In the case of pollutants for which national ambient air quality
standards are promulgated after August 7, 1977, [the Administrator]
shall promulgate such regulations not more than 2 years after the
date of promulgation of such standards.

Since EPA promulgated an additional NAAQS for PM, based on the
PM2.5 indicator, in 1997, one potential

approach for developing increments for PM2.5 is for EPA to
promulgate these increments under the authority of section 166(a).
Under this approach, EPA would promulgate increments or other measures
for PM2.5 that satisfy the standards set forth in
subsections (c) and (d) of section 166, as interpreted by EPA in our
recent rulemaking for nitrogen oxides.
    However, in light of the provisions in section 163 and 166(f) of
the Act that address increments for TSP and PM10,
respectively, there is some ambiguity on the question of the legal
authority EPA should rely upon to establish increments for
PM2.5. In 1993, EPA construed section 166(f) to establish
the sole criteria for promulgation of a new PM increment and thus did
not base our final PM10 increment on section 166(a) of the
Act. Considering sections 163, 166(a), and 166(f) together, an
alternative interpretation of these provisions might be that Congress
intended that section 163 and 166(f) alone cover PM. Under this
reading, EPA would promulgate additional increments for particular
matter based on the section 163 increments and 166(f) of the Act, which
are the only provisions that specifically mention PM and PSD
increments. However, as discussed later, it may also be possible to
read sections 166(a) and 166(f) in harmony. Thus, we propose to adopt
one of the following legal theories to support promulgation of
increments for PM2.5 using either of the two methods that
EPA used in prior rules to develop PSD increments.
1. Support for ``Contingent Safe Harbor'' Approach for PM2.5
Under Section 166(a)
    The EPA believes it is permissible to interpret section 166(a) to
apply to PM2.5. 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 in 1997. Rather, EPA established an additional NAAQS
for PM2.5 as if it were a new pollutant, even though EPA had
already developed air quality criteria for PM generally. Thus, for
purposes of section 166(a), the addition of a NAAQS for
PM2.5 is functionally the same as establishing a NAAQS for
an additional pollutant after 1977.
    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) (nitrogen
oxides, 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, sulfur dioxide and particular matter, 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, Congress clearly 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'' we believe that Section 166(a) provides authority for EPA
to promulgate new increments after revising an existing NAAQS
(including one first promulgated before 1977), when we find that such
action is appropriate.
    In our 1989 proposal on the PM10 increments, EPA
construed section 166(a) to apply to PM10, even though EPA
regarded PM10 to be a new indicator for PM. 58 FR 31623-24.
Thus, before the adoption of section 166(f), EPA read the language of
section 166(a) to apply to the promulgation of increments using a new
indicator for PM and did not limit the application of section 166(a) to
wholly new criteria pollutants. Similarly, in the current proposal, EPA
believes it can continue to interpret section 166(a) to apply to the
promulgation of an additional increment for a new indicator of an
existing criteria pollutant since EPA promulgated a NAAQS for a new
indicator of that pollutant after 1977.
    Although EPA ultimately applied the standard in section 166(f) as
the sole basis for our PM10 increments in 1993, that
provision does not necessarily govern the situation EPA currently faces
with PM2.5. One could read section 166(f) to address only
EPA's authority to substitute new PM increments for the
congressionally-established increments for TSP rather than the distinct
issue now faced by EPA concerning the promulgation of additional PM
increments for PM2.5 without necessarily revoking existing
increments. Furthermore, the language in section 166(f) could be read
to limit the scope of this provision to only increments using the
PM10 indicator. Thus, section 166(f) may not necessarily be
applicable to the substitution of PM10 increments with
PM2.5 increments.
    The EPA believes that section 166(a) could apply to the adoption of
new increments, without the revocation of existing increments. As
reflected in the 2005 increments rule for NOX and the court
decision in EDF v. EPA, when sections 166(a)-(d) apply, EPA is
obligated to evaluate which indicator or form should be used in our
pollutant-specific PSD regulations to meet these requirements in the
Act. Based on this interpretation, we are proposing to use a contingent
safe harbor approach (option 1) that involves first deriving increment
values based on percentage of the NAAQS and then evaluating whether
alternative increments or additional measures are necessary to meet the
criteria in section 166(c).
2. Support of ``Equivalent Substitution'' Approach for PM2.5
Under Section 166(f)
    The EPA believes it is also permissible for the Agency to construe
section 166(f) as a continuing grant of authority for the Administrator
to update the increments for particular matter whenever the
Administrator decides to adopt a new form of particular matter as the
indicator for the NAAQS. Although the terms of section 166(f) of the
Act appear to address PM10 alone, the overall intent of this
provision was to clarify that EPA had the authority to update the
original TSP increments to reflect changes in the NAAQS indicator.
Language describing the PM10 indicator was used in the Act
because this was the indicator for PM that EPA was seeking to
incorporate into the PSD program at the time of the 1990 Amendments
when section 166(f) was adopted. However, we believe it is reasonable
to conclude that Congress intended to authorize EPA to continue
updating the particular matter increments contained in section 163 if
EPA promulgated a NAAQS for another appropriate indicator for
particular matter.
    We believe EPA is authorized to promulgate increments for
PM2.5 as a substitute for the PM10 increments, as
well as the original TSP increments, so long as the new increments for
PM2.5 are of ``equal stringency in effect as those specified
in the provisions for which they are substituted.'' 42 U.S.C. 7476(f).
Based on this interpretation, we propose two approaches (options 2A and
2B discussed later) for developing PM2.5 increments that
would meet the ``equal stringency in effect'' standard contained in
section 166(f).
    While we believe section 166(f) may be construed to provide
continuing authority to ``update'' the increments for PM to conform to
the NAAQS, section

166(f) describes a process in which EPA would ``substitute'' one PM
increment for another. The language in section 166(f) does not address
whether EPA may adopt additional increments for other PM indicators
while retaining the existing PM increments. In contrast, section 166(a)
does contain language addressing the promulgation of PSD regulations
when EPA adds to the suite of NAAQS. Thus, we construe section 166(a)
to have the closest connection to the task of adding, rather than the
substituting or replacing, PSD increments for PM. As a result, for
purposes of establishing the proposed 24-hour PM2.5
increments, we propose only one option--using the contingent safe
harbor approach described in option 1--because we are not proposing to
replace the existing 24-hour PM10 increment with a new 24-
hour PM2.5 increment, since we have retained the 24-hour
PM10 NAAQS. However, we also seek comment on whether we
could rely on section 166(f) to promulgate the 24-hour PM2.5
increments using the same methodology as for the annual PM2.5
increments described later, even though the 24-hour PM10
NAAQS is not being revoked.

B. Requirements of Sections 166(a)-(d) of the Clean Air Act

    If we determine that section 166(a) applies to PM2.5, we
propose to follow the interpretation of sections 166(a)-(d) that we
adopted in our most recent increments rule for NOX. This
interpretation was upheld in a recent court decision E.D. v. EPA, No.
05-1446 (June 19, 2007 DC Cir.). We summarize the key elements of this
interpretation later, but a more detailed discussion can be found in
our October 2005 final rule for NOX. 70 FR 59582.
    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 the
following requirements set forth in sections 166(c) and 166(d):

    (c) Such regulations shall provide specific numerical measures
against which permit applications may be evaluated, a framework for
stimulating improved control technology, protection of air quality
values, and fulfill the goals and purposes set forth in section 101
and section 160.
    (d) The regulations * * * shall provide specific measures at
least as effective as the increments established in section 163 [for
SO2 and PM] to fulfill such goals and purposes, and may
contain air quality increments, emission density requirements, or
other measures.

    The goals and purposes of the PSD program set forth in section 160
are as follows:

    (1) To protect public health and welfare from any actual or
potential adverse effect which in the Administrator's judgment may
reasonably be anticipate[d] to occur from air pollution or from
exposures to pollutants in other media, which pollutants originate
as emissions to the ambient air, notwithstanding attainment and
maintenance of all national ambient air quality standards;
    (2) To preserve, protect, and enhance the air quality in
national parks, national wilderness areas, national monuments,
national seashores, and other areas of special national or regional
natural, recreational, scenic, or historic value;
    (3) To insure that economic growth will occur in a manner
consistent with the preservation of existing clean air resources;
    (4) To assure that emissions from any source in any State will
not interfere with any portion of the applicable implementation plan
to prevent significant deterioration of air quality for any other
State; and
    (5) To assure that any decision to permit increased air
pollution in any area to which this section applies is made only
after careful evaluation of all the consequences of such a decision
and after adequate procedural opportunities for informed public
participation in the decisionmaking process.

    As described in our 2005 rule for NOX, EPA's
interpretation of these provisions is grounded on five central
elements. 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 establish the minimum level of effectiveness
necessary to satisfy 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 interpreted 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, we interpreted the
requirements to simultaneously satisfy each of these factors to
establish a balancing test in cases where certain objectives may be at
odds with each other. Fifth, we recognized 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 an increment contained in a SIP meet the requirements
of sections 166(c) and 166(d).
1. Regulations as a Whole Should Fulfill Statutory Requirements
    Section 166(a) 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. Thus, in order to
develop pollutant-specific regulations under subsection (a), EPA must
establish both the overall regulatory framework for those regulations
(such as system of increments) and fill details around that framework
(such as the level of the increments). Thus, EPA interprets section 166
to require that the entire system of PSD regulations (the framework and
details) for a particular pollutant must, as a whole, satisfy the
criteria in sections 166(c) and 166(d). We propose to use the same
approach to establish pollutant-specific regulations for PM2.5
under option 1 of this proposal.
    When we propose a framework involving numerical increments under
section 166(a) of the Act, we do not look at increments in isolation,
but we also consider how these increments work in conjunction with
other measures to satisfy the statutory criteria. The other measures
that EPA may consider include new measures proposed by EPA for that
pollutant or measures applicable to other pollutants that EPA proposes
to apply to additional pollutants. Examples of other measures are an
area classification system, AQRV review in Class I areas, additional
impacts analysis, and control technology requirements. This approach is
consistent with section 166(d), which says that pollutant-specific PSD
regulations ``may contain'' increments or ``other measures.''
2. Contingent Safe Harbor Approach
    The EPA continues to view the ``contingent safe harbor'' approach
to be an appropriate methodology for ensuring that our pollutant-
specific PSD regulations meet the requirements of sections 166(c) and
166(d). Subsection (c) of section 166 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. Then, under subsection (d), to
``fulfill such goals and purposes,'' EPA must promulgate ``specific
measures at least as effective as the increments established in section
7473

of this title [section 163 of the Act].'' 42 U.S.C. 7476. Thus,
subsection (d) can be construed to require that EPA identify a minimum
level of effectiveness, or safe harbor, for the body of pollutant-
specific PSD regulations adopted under section 166. Subsection (c) may
then be read to require that EPA conduct further review to determine
whether, based on the criteria in subsection (c), EPA's pollutant-
specific PSD regulations under section 166 should contain measures that
deviate from the minimum ``safe harbor'' identified under subsection
(d). EPA construes subsection (d) to require that the measures be ``at
least as stringent'' as the statutory increments set forth in section
163.
    When EPA employs an increment and area classification system in
regulations promulgated under section 166 of the Act, we interpret the
Act to require that EPA, at minimum, establish increments that are
consistent with the statutory increments established by Congress in
section 163 of the Act. Thus, we start by identifying ``safe harbor''
increments for each area classification (Class I, II, or III) that are
established (1) Using an equivalent percentage of the NAAQS as the
statutory increments; (2) for the same pollutants as the NAAQS; and (3)
for the same time period as the NAAQS. We then conduct further review
to determine whether these ``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 weigh and balance the criteria set forth in subsection (c) (and the
incorporated goals and purposes of the Act in section 101 and the PSD
program in section 160) to determine whether additional measures are
needed to satisfy the criteria in subsection (c).
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 factors are three of the
four criteria listed in section 166(c) and the five goals and purposes
identified in section 160 of the Act. The three stand-alone 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. 42 U.S.C.
7476(c). 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 construe 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.
4. Balancing the Factors Applicable Under Section 166(c)
    The EPA interprets the Act to establish a balancing test among the
eight factors. Since, as discussed further later, many of the factors
can be satisfied by using an increment framework, when determining the
characteristics of numerical increments themselves within that
framework, EPA focuses on balancing the goal to promote economic growth
with the factors that direct us to protect: (1) AQRVs; (2) the public
health and welfare from reasonably anticipated foreseeable adverse
effects; and (3) the air quality in parks and special areas. Section
166 of the Act authorizes EPA to promulgate pollutant-specific PSD
regulations that satisfy each of the eight factors. While these
objectives are generally complementary, there are circumstances where
some of the objectives may be in conflict. In these situations, some
degree of balance or accommodation is inherent in the requirement to
establish regulations that satisfy all of these factors.
    As discussed in our PSD regulations for NOX, we believe
this balancing test derives primarily from the third goal and purpose
set forth in section 160. Section 160(3) directs us to ``insure that
economic growth will occur in a manner consistent with the preservation
of existing clean air resources.'' To some extent, this goal of the PSD
program in section 160(3) more specifically articulates the broader
purpose of the Act, described in section 101(b)(1) of the Act, to
``protect and enhance the quality of the Nation's air resources so as
to promote the public health and welfare and the productive capacity of
its population.'' 42 U.S.C. 7401(b)(1). Sections 160(3) and 101(b)(1)
are similar in that both sections reflect the goal to protect air
quality and maximize opportunities for economic growth. Thus, in
interpreting the meaning of section 160(3) when used as a factor
applicable under section 166(c), we also consider the broader purpose
of the Act set forth in section 101(b)(1).
    The need to balance the applicable factors to achieve these
objectives is also supported by our interpretation of the second goal
in section 160(2) of the Act to ``protect public health and welfare.''
The precise meaning of this goal in the context of the PSD program is
somewhat ambiguous because it appears to mirror the legal standards
applicable to the promulgation of the primary and secondary NAAQS.
Under section 109(b) of the Act, the primary NAAQS must ``protect the
public health'' with an adequate margin of safety (section 109(b)(1))
and the secondary NAAQS must ``protect the public welfare from any
known or anticipated adverse effects'' associated with ambient
concentrations of the pollutant (section 109(b)(2)). The term
``welfare'' is defined in the Act to include ``effects on soils, water,
crops, vegetation, man-made materials, animals, wildlife, weather,
visibility, and climate.'' Section 302(h) of the Act.
    When applied as one of the factors applicable to pollutant-specific
PSD regulations under section 166(c) of the Act, we construe the goal
in section 160(3) of the to ``protect public health and welfare'' to
mean EPA should evaluate whether reasonably anticipated adverse effects
may occur as a result of increases in ambient pollutant concentrations
to levels below the NAAQS. If such effects may occur in some areas of
the country, then EPA would establish PSD regulations that protect
public health and welfare against those effects where they may occur.
However, we do not interpret the PSD program to require regulations
that eliminate all negative effects that may result from increases in
pollution in attainment areas.
    The PSD program is, as its title indicates, designed to prevent
``significant deterioration'' from a baseline concentration. See S.
Rep. 95-127 at 11 (3 LH at 1385) (``This legislation defines
`significant deterioration' in all clean air areas as a specified
amount of additional pollution.* * * This definition is intended to
prevent any major decline in air quality currently existing in clean
air areas.'' (emphasis added)). Thus, some decline in air quality
(relative to the baseline air quality concentration) is permissible for
any particular area of the country that is currently achieving the
NAAQS, as long as it is not ``significant.''
    When EPA employs an area classification system in its section 166

regulations, we generally weigh these factors in each type of area
(Class I, Class II, and Class III). However, the weight given to each
factor may be more or less, depending on the area involved and the
amount of deterioration deemed ``significant'' for that type of area.
For example, economic growth may be the most important factor in a
Class III area, but our PSD regulations for such areas should offer
some level of protection for existing clean air resources. In a Class I
area, our PSD regulations should allow some level of economic growth,
even though preservation of existing clean air resources may be the
dominant factor for these areas.
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 also
interpret the section to authorize 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). As described earlier, we explained this
interpretation in the 2005 NOX increment rulemaking
whereupon we amended the PSD regulations at 40 CFR 51.166 by adding new
paragraph (c)(2) to codify this statutory authority (70 FR 59582,
October 12, 2005). However, in establishing the new provision, the
language at paragraph (c)(2) reflected the authority for States to
adopt alternative measures only with respect to increments for
NOX. In order to clarify our interpretation that the
authority to adopt alternative measures covers any pollutant listed in
section 166(a), we are proposing in this action to revise existing 40
CFR 51.166(c)(2) to make it inclusive to applicable pollutants rather
than just NOX.

C. Requirements of Section 166(f) of the Clean Air Act

    If we decide to use the equivalent substitution options in this
proposal for PM2.5, EPA proposes to interpret section 166(f)
of the Act in the same manner that the Agency interpreted that
provision in our 1993 rule for PM10. In 1993, EPA construed
section 166(f) as authorizing EPA to follow the path that EPA laid out
in our 1989 proposal for developing equivalent increments for PM
measured as PM10. 58 FR 31626. Thus, in our 1993 rulemaking,
EPA developed our PM10 increments using the ``equivalent to
statutory increments'' option that EPA described in our notice of
proposed rulemaking. The EPA did not interpret the ``equivalent
stringency in effect'' standard in section 166(f) to require EPA to use
the second approach from the proposal, the ``percentage of NAAQS''
approach that Congress had originally used to establish TSP increments.
The Agency observed that if Congress intended to require EPA to update
the TSP increments using a straight percentage, Congress could have
easily revised the increments in section 163 instead of providing EPA
discretion to establish increments following the standard provided in
section 166(f). 58 FR 31626. The EPA thus construed section 166(f) as
providing EPA discretion to determine appropriate equivalent levels of
PM10. Id. The EPA identified equivalent levels by developing
a ratio based on a comparison of the TSP and PM10 impacts of
stationary sources. 58 FR 31627.
    In this rulemaking, EPA proposes to apply the same type of ratio
approach to establish equivalent increments for PM10 under
section 166(f) of the Act. Since this ratio approach was the foundation
of EPA's equivalency method in the 1989 proposal, we believe it is
permissible, as we did in 1993, to construe section 166(f) as
authorizing EPA to continue utilizing this approach to establish
equivalent increments for PM.
    In 1993, EPA disagreed with commenters who recommended that EPA
consider welfare effects and visibility impairments when establishing
PM10 increments under section 166(f) for class I areas. The
EPA observed that there was no evidence that Congress itself adopted
increments that would ensure specific levels of welfare and visibility
protection at each Class I area throughout the nation. The increments
established by Congress did not establish an absolute ceiling on air
quality, but rather limited the marginal amount of deterioration in air
quality above a baseline concentration that varies for each area, and
thus permitted each area with the same classification to deteriorate in
the same amount without regard to its particular sensitivities as
compared to other areas with the same classification. 58 FR 31625. The
EPA concluded that the PM10 increments should be designed to
protect each area from large adverse changes in air quality while the
air quality related values analysis was the main tool for protecting
specific ecologically-based attributes in particular class I areas. Id.
    In this rulemaking for PM2.5, we maintain the view that
the ``equal stringency in effect'' language in section 166(f) does not
require EPA to consider welfare effects and visibility when
promulgating replacement increments under that provision. However, as
reflected in our recent increments rule for NOX, when
promulgating PSD increments under section 166(a), welfare effects and
visibility impacts are factors in the contingent safe harbor analysis
under the criteria in sections 166(c) and 160 of the Act. Consistent
with our recent PSD regulations for NOX, we continue to
believe that increments (whether promulgated under section 166(a) or
166(f)) should be designed to provide each area with a basic level of
protection from large adverse changes in air quality without
necessarily reflecting the unique air pollution sensitivities in each
class I area. The EPA considers welfare and visibility impacts across
the nation when establishing increments under section 166(a), but we
continue to believe that the Air Quality Related Values (AQRV) review
is the preferred tool for identification and protection of specific
ecologically-based attributes within particular class I areas. See 58
FR 31625.

V. Increments and Other Measures To Prevent Significant Deterioration

    In this action, EPA is proposing three options for establishing
increments for PM2.5. The first option described uses the
``contingent safe harbor'' approach (using percentages of the NAAQS as
our initial basis) following section 166(a) of the Act. The other two
options are variations of the section 166(f) ``Equivalent Increment''
approach. The EPA is proposing option 1 as our preferred option and
seeking comments on the other two options.

A. Option 1--Contingent Safe Harbor Approach for Annual and Short-Term
Increments--Section 166(a)

    Under the first option, we would consider PM2.5 to be a
new pollutant \8\ for which a NAAQS was promulgated after the date of
enactment of subpart C, and we would use the authority of section
166(a) of the Act to develop new increments for PM2.5. Using
this option, we are proposing to establish a system of increments at
the safe harbor level in conjunction with the other measures described
as follows:
---------------------------------------------------------------------------

    \8\ In our review of the PM NAAQS, we concluded that, because
the fine and thoracic coarse components of PM10 generally
have different sources, composition and formation processes, they
should be treated as separate pollutants. (OAQPS SP, December 2005,
page 3-1.)
---------------------------------------------------------------------------

1. Proposed Framework for Pollutant Specific PSD Regulations for
PM2.5
    Under this option, EPA proposes to apply the same basic
framework reflected in our regulation for NOX in

pollutant-specific PSD regulations for PM2.5. Thus, we
propose to adopt an increment and area classification system for
PM2.5 and to apply an AQRV review process to PM2.5
as well. As discussed further later, EPA believes that many 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. For other factors, this framework of regulations
partially contributes to the fulfillment of an applicable factor but
may not fully satisfy that factor. In these instances, the details of
our regulations (such as the characteristics of the increments
themselves) are also important and we evaluate the effectiveness of the
framework in conjunction with more detailed elements of our
regulations. The EPA believes 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.
a. Increment System
    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).
    An increment-based program satisfies the requirements under 166(c)
to provide ``specific numerical measures against which permit
applications may be evaluated.'' 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.'' 42
U.S.C. 7475(a)(3). Once the baseline date associated with the first
proposed 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. See also S. Rep. 95-127 at 18, 30 (3 LH at 1392, 1404)
(``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 within the limits
of the increments'').
    However, we recognize that an increment system is not the only way
to fulfill the requirements of section 166 of the Act. Congress did not
require EPA to utilize increments in our PSD regulations for
NOX but gave EPA the discretion to employ increments if
appropriate to meet the criteria and goals and purposes set forth in
sections 166 and 160 of the Act. 42 U.S.C. 7474(d); EDF v. EPA, 898
F.2d at 185 (``Congress contemplated that EPA might use increments'').
Thus, in this action, we are also proposing to allow States to develop
alternatives to an increment system at their discretion, and to submit
any such alternative program to EPA so that we can determine whether it
satisfies the requirements of section 166.
b. Area Classifications
    The EPA proposes to establish the same three-tiered area
classification system for PM2.5 that is applicable to
NOX and other pollutants under the PSD program and the Clean
Air Act. Accordingly, areas that are currently Class I for other
pollutants would also be Class I for PM2.5 and all other
areas would be Class II for PM2.5 unless we redesignated 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 III.E.1, Class I areas are areas
where especially clean air is most desirable. In contrast, Class III
areas, which are those areas in which a State wishes to permit the
highest relative level of industrial development, have the largest
increment level. Areas that are not especially sensitive or that do not
wish to allow 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. 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. With the air
quality in Class I areas subject to the greatest protection, this
scheme then provides two additional area classifications with higher
increment levels to help 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 42 U.S.C. 7474(a)-
(b) (Section 164(a)-(b) of the Act). 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 scheme achieves the goals of the PSD
program. We discuss the characteristics of increments later.
c. 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 regulations. The framework of our existing PSD regulations
employs the preconstruction permitting system and procedures required
under section 165 of the Act. 42 U.S.C. 7475. These requirements are
generally reflected in 40 CFR 51.166 and 52.21 of EPA's PSD regulations
in Title 40 of the Code of Federal Regulations. These permitting and
review procedures, which we interpret to apply to construction of new
major sources and to major modifications at existing sources, 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 decisionmaking process. For the same
reasons discussed in our proposal for the pollutant-specific PSD
regulations for NOX regulations, 70 FR 8896, we believe
these factors are also fulfilled for PM2.5 by employing the
permit review procedures.
d. Air Quality Related Values Review by Federal Land Manager and
Reviewing Authority
    The EPA also proposes to apply the requirement to evaluate impacts
on AQRVs in Class I areas to PM2.5. The AQRV review provides
the Federal Land Managers (FLM) the opportunity to review source
impacts on site-specific AQRVs in Class I areas and to bring any
adverse impacts to the attention of the reviewing authority. Under an
increment approach, we consider this review to be an additional measure
that helps to satisfy the factors in sections 166(c) and 160(2) which
require that EPA's pollutant-specific PSD regulations protect air
quality values, and parks and other special areas, respectively.
    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. We propose the same approach for PM
2.5 in this rulemaking.
    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.
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 do
not 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 will 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.\9\ Thus, the 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.\10\ 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.
---------------------------------------------------------------------------

    \9\ 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 the
Class II increment for SO2 or PM is not exceeded. Section
165(d)(2)(C)(iv). The EPA made this provision applicable to the PSD
provisions for NOX, with a cap of 25 g/m \3\--the NO
2 Class II increment. 53 FR 3704; 40 CFR 51.166(p)(4) and
52.21(p)(5).
    \10\ 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).
---------------------------------------------------------------------------

    Incorporating these AQRV review procedures into the PSD regulations
for PM2.5 helps to provide protection for parks and special
areas (which are generally the Class I areas subject to this review)
and air quality values (which are factors considered in the review). 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. Legislative history suggests that the
AQRV review provisions of section 165(d) were intended to provide
another 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 NOX 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.\11\
---------------------------------------------------------------------------

    \11\ 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'').
---------------------------------------------------------------------------

e. Additional Impacts Analysis.
    The additional impacts analysis set forth in our regulations also
helps 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 that could occur
from the construction or modification of a 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 * * * ''

42 U.S.C. 7475(e)(3)(B).

    This portion of the additional impacts analysis is especially
helpful for satisfying the requirements of section 166(c) in Class II
and Class III areas. These areas are not subject to the additional AQRV
review that applies only in Class I areas. While not as intensive a
review as AQRV analysis required in Class I areas, considering
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.
f. Installation of Best Available Control Technology
    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 and thus we consider it to be a part of the
regulatory framework for the Agency's pollutant-specific regulations
for PM2.5. 40 CFR 52.21(j); 40 CFR 51.166(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
52.21(b)(12); 40 CFR 52.166(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.
2. Proposed 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 in section 101 and the PSD
program in section 160), EPA proposes to find that the ``safe harbor''
increments for PM2.5 (which meet the minimum requirements in
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 propose to 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
the proposed 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) ensuring economic
growth.\12\ In accordance with the contingent safe harbor approach, to
determine the specific characteristics of the proposed increments, we
first established the minimum level of effectiveness necessary to
satisfy section 166(d) and then conducted further analysis to determine
if additional measures are necessary to fulfill the requirements of
section 166(c).
---------------------------------------------------------------------------

    \12\ 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.
---------------------------------------------------------------------------

a. Identification of Safe Harbor Increments
    Using the percentage-of-NAAQS approach, we derived the following
safe harbor increments for PM2.5:

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

    The PM2.5 levels of both the primary and secondary NAAQS
are 15 [mu]g/m \3\ for the annual averaging time and 35 [mu]g/m \3\ for
the 24-hour averaging time. See 40 CFR 50.7. 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. Increments with these characteristics are
sufficient to satisfy the requirement in section 166(d) requirement
that we adopt increments (or other PSD regulations) that are ``at least
as effective as'' the increments established in section 163 of the Act.
42 U.S.C. 7476(d); See EDF v. EPA, 898 F.2d at 188, 190.
b. Data Utilized 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 (SP) used for the periodic
review of the 2004 PM air quality Criteria Document (CD).\13\
---------------------------------------------------------------------------

    \13\ This periodic review of the PM NAAQS updates the last
review, which began in 1994 and resulted in revised standards for PM
in 1997.
---------------------------------------------------------------------------

    Section 166 provides that EPA is to establish pollutant-specific
PSD regulations, including increments, after the establishment of a
NAAQS for the applicable pollutants. 42 U.S.C. 7476(a). Under normal
circumstances, the Act provides that EPA promulgate new PSD regulations
under section 166, including new increments if appropriate, within 2
years from the promulgation of any NAAQS after 1977. 42 U.S.C. 7476(a).
In such instances, 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 use that same information as the basis for
our pollutant-specific PSD regulations, we must evaluate 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.
    Since we just completed a review of the PM 2.5 NAAQS,
the information used in that review is current and timely for purposes
of this proposal to establish pollutant-specific PSD regulations for
PM2.5. As discussed earlier, 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 the 2004 CD and
2005 SP that we also consider for purposes of this proposed rule was
used for this latest review of the PM NAAQS.
    The 2004 CD and 2005 SP 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's presence in the ambient air. 42
U.S.C. 7408(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 38654/1-2.
c. Scope of Effects Considered
    The effects of ambient PM2.5 concentrations may include
secondarily-formed PM2.5. Hence, in this analysis we have
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 is consistent with the approach we described
for addressing these effects in the recently completed review of our
pollutant-specific PSD regulations for NOX. 70 FR 59590.
d. 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
different sizes, compositions and properties. Particle size
distributions show that atmospheric particles exist in two classes:
fine particles and coarse particles. PM2.5 is an indicator
for fine particles and represents particles that are mostly less than
2.5 micrometers in size. PM10-2.5 is an indicator for
thoracic coarse particles and represents particles sized between 2.5
and 10 micrometers. In the last two reviews of the PM NAAQS, EPA
concluded that these two indicators, because of their different
sources, composition, and formation processes should be treated as
separate subclasses of PM pollution for purposes of setting ambient air
quality standards.
    Coarse particles are generally primary particles, emitted directly
from their source as particles. These particles result from mechanical
disruption of large particles by crushing or grinding, from evaporation
of sprays, or from dust resuspension. In addition, some combustion-
generated particles may be found as coarse particles. By comparison,
fine PM is derived directly from combustion material that has
volatilized and then condensed to form primary PM or from precursor
gases, such as SO2 and NOX, reacting in the
atmosphere to form secondary PM. Because of the complexity of the
composition of ambient PM2.5 and PM10-2.5,
sources are best discussed in terms of individual constituents of both
primary and secondary PM2.5 and PM10-2.5. Each of
these constituents can have anthropogenic and natural sources. Major
components of fine particles are sulfates, strong acid, ammonium
nitrate, organic compounds, trace elements (including metals),
elemental carbon, and water. Primary and secondary fine particles have
long lifetimes in the atmosphere (days to weeks) and travel long
distances (hundreds to thousands of kilometers). They tend to be
uniformly distributed over urban areas and larger regions, especially
in the eastern United States. As a result, they are not easily traced
back to their individual sources. By contrast, coarse particles are not
readily transported across urban or broader areas. These particles can
settle rapidly from the atmosphere with lifetimes ranging from minutes
to days depending on their specific size, atmospheric conditions, and
altitude.
(1) Health Effects
    The EPA reported important progress since the last PM NAAQS review
in advancing our understanding of potential mechanisms by which ambient
PM2.5, alone and in combination with other pollutants, is
causally linked to a number of key health effects. The more extensive
and stronger body of evidence used by EPA to study the health effects
of PM2.5 in our latest review identified a broader range of
effects than those previously documented, involving premature mortality
and indices of morbidity (including respiratory hospital admissions and
emergency room visits, school absences, work loss days, restricted
activity days, effects on lung function and symptoms, morphological
changes, and altered host defense mechanisms) associated with both
long- and short-term exposure to PM2.5.
    An overview of the scientific and technical evidence considered in
the 2004 CD and 2005 SP can be found in our proposed rule for revising
the NAAQS for PM published at 70 FR 2619, January 17, 2006, beginning
at page 2626. The discussion which follows is only a brief summary of
those

effects, with an explanation of the range of PM2.5
concentrations that we examined in considering revisions to the primary
PM2.5 NAAQS.
    While most epidemiological studies continue to be indexed by
PM2.5, some studies also implicate various components within
the mix of fine particles that have been more commonly studied (e.g.,
sulfates, nitrates, carbon, organic compounds, and metals) as being
associated with adverse effects. Furthermore, the available information
suggests that many different chemical components of fine particles and
a variety of different types of source categories are all associated
with, and probably contribute to, effects associated with
PM2.5. While there remains uncertainty about the role and
relative toxicity of various components of fine PM, the current
evidence continues to support the view that fine particles should be
addressed as a group for purposes of public health protection.
    Short-term exposure (from less than 1 day up to several days) to
PM2.5 is likely causally associated with mortality from
cardiopulmonary diseases, increased hospitalization and emergency
department visits for cardiopulmonary diseases, increased respiratory
symptoms, decreased lung function, and changes in physiological
indicators for cardiovascular health. Effects associated with short-
term exposure identified since the last NAAQS review include increased
non-hospital medical visits (physician visits) and aggravation of
asthma associated with short-term exposure to PM2.5.
Although a growing body of studies provided evidence of effects
associated with exposure periods shorter than 24-hours (e.g., one to
several hours), EPA concluded in our 2004 SP that this information was
too limited to serve as a basis for establishing a primary fine
particle standard with less than a 24-hour averaging time. However, it
was concluded that this information added weight to the importance of a
24-hour standard. In addition, some studies suggested consideration of
a multiple-day averaging time, but EPA concluded that a multiple-day
averaging time would add complexity without providing more effective
protection than a 24-hour averaging time.
    For setting the level of the short-term PM standard, EPA focused on
a range of 24-hour 98th percentile PM2.5 concentrations of
about 30 to 35 [mu]g/m\3\. Some new short-term mortality studies
considered for the last NAAQS review provided evidence of statistically
significant associations with PM2.5 in areas with air
quality levels below the level of the then-current primary 24-hour
PM2.5 NAAQS (65 [mu]g/m\3\). The EPA observed a strong
predominance of studies with 24-hour 98th percentile values down to
about 39 [mu]g/m\3\ showing statistically significant association with
mortality, hospital admissions, and respiratory symptoms. Within the
range of 24-hour average 98th percentile PM2.5
concentrations of about 30 to 35 [mu]g/m\3\, EPA no longer observed
this strong predominance of statistically significant results. Below 35
[mu]g/m\3\, EPA found increasing variation in the short-term exposure
studies, which indicated an increase in the uncertainty as to whether
likely causal associations could be extended. In considering what level
would be appropriate for the primary 24-hour PM2.5 standard,
the Administrator indicated that in the absence of evidence of any
clear effects thresholds, EPA had discretion to select a specific
standard level from within this range of values. In ultimately deciding
to set the level of the primary 24-hour PM2.5 standard at 35
[mu]g/m\3\, the Administrator concluded that a standard set at a higher
level would not likely result in improvement in air quality in areas
across the country in which short-term exposure to PM2.5 can
reasonably be expected to be associated with serious health effects.
Similarly, a standard set at a lower level was rejected because of
uncertainties in interpreting the available epidemiologic studies that
could causally relate the reported associations of health risks to
PM2.5 at those lower levels.
    New epidemiologic studies have built upon earlier limited evidence
to provide fairly strong evidence that long-term exposure to
PM2.5 is likely causally associated with mortality from
cardiopulmonary disease, as well as development of chronic respiratory
disease and reduced lung function growth. The new studies also provide
evidence suggesting that long-term exposure to fine particles is
associated with lung cancer mortality. The 2004 CD placed the greatest
weight on re-analyses and extensions of two mortality studies (Six
Cities and American Cancer Society (ACS) studies) originally considered
in the previous NAAQS review. In the Six Cities study, the long-term
mean PM2.5 concentration was 18 [mu]g/m\3\, within an
overall range of 11 to 30 [mu]g/m\3\. In the extended ACS study, the
mean for the more recent time period used in the analysis was 14 [mu]g/
m\3\, while the confidence intervals around the relative risk functions
start to become appreciably wider (more uncertain) below approximately
12 to 13 [mu]g/m\3\. Based on this and other sets of evidence, EPA
decided to consider, for setting the level of the annual
PM2.5 standard, a range of annual PM2.5
concentrations beginning somewhat below 15 [mu]g/m\3\ (the then-
existing primary annual PM2.5 NAAQS) down to about 12 [mu]g/
m\3\. However, after carefully considering public comments and relevant
studies, including the uncertainties in interpreting the available
long-term exposure epidemiologic studies, the Administrator decided to
retain the level of the primary annual PM2.5 standard at 15
[mu]g/m\3\ to protect public health with an adequate margin of safety