[Federal Register Volume 81, Number 164 (Wednesday, August 24, 2016)]
[Rules and Regulations]
[Pages 58009-58162]
[FR Doc No: 2016-18768]

Vol. 81

Wednesday,

No. 164

August 24, 2016

Part II

Environmental Protection Agency

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40 CFR Parts 50, 51, and 93

 Fine Particulate Matter National Ambient Air Quality Standards: State
Implementation Plan Requirements; Final Rule

Federal Register / Vol. 81 , No. 164 / Wednesday, August 24, 2016 /
Rules and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 51, and 93

[EPA-HQ-OAR-2013-0691; FRL-9946-36-OAR]
RIN 2060-AQ48

Fine Particulate Matter National Ambient Air Quality Standards:
State Implementation Plan Requirements

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The Environmental Protection Agency (EPA) is finalizing
requirements that state, local and tribal air agencies would have to
meet as they implement the current and future national ambient air
quality standards (NAAQS) for fine particulate matter
(PM2.5). Specifically, this document provides details on
meeting the statutory state implementation plan (SIP) requirements that
apply to areas designated nonattainment for any PM2.5 NAAQS,
such as: General requirements for attainment plan due dates and
attainment dates; emissions inventories; attainment demonstrations;
provisions for demonstrating reasonable further progress; quantitative
milestones; contingency measures; and nonattainment New Source Review
(NNSR) permitting programs, among other things. This rule clarifies the
specific attainment planning requirements that apply to
PM2.5 NAAQS nonattainment areas based on their
classification (either Moderate or Serious), and the process for
reclassifying Moderate areas to Serious. Additionally, in this document
the EPA is revoking the 1997 primary annual standard for areas
designated as attainment for that standard because the EPA revised the
primary annual standard in 2012. The EPA first established the
PM2.5 NAAQS in 1997, completed a review and revision of
those standards in 2006, and most recently completed a review and
revision of the PM2.5 NAAQS on December 14, 2012.

DATES: This final rule is effective on October 24, 2016.

ADDRESSES: The EPA has established a docket for this action, identified
by Docket ID No. EPA-HQ-OAR-2013-0691. All documents in the docket are
listed in the http://www.regulations.gov Web site. Although listed in
the index, some information is not publicly available, e.g.,
Confidential Business Information or other information whose disclosure
is restricted by statute. Certain other material, such as copyrighted
material, is not placed on the Internet and will be publicly available
only in hard copy. Publicly available docket materials are available
either electronically in http://www.regulations.gov.

FOR FURTHER INFORMATION CONTACT: For general information on this rule,
contact Mr. Rich Damberg, Office of Air Quality Planning and Standards,
U.S. EPA, by phone at<PHONE_NUMBER> or by email at
<EMAIL_ADDRESS>or Mr. Patrick Lessard, Office of Air Quality
Planning and Standards, U.S. EPA, by phone at<PHONE_NUMBER> or by
email at<EMAIL_ADDRESS>For information on the Information
Collection Request (ICR), contact Mr. Butch Stackhouse, Office of Air
Quality Planning and Standards, U.S. EPA, by phone at<PHONE_NUMBER> or
by email at<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

I. General Information

A. Preamble Glossary of Terms and Acronyms

    The following are abbreviations of terms used in the preamble.

AERR Air Emissions Reporting Requirements
BACM Best Available Control Measures
BACT Best Available Control Technology
BART Best Available Retrofit Technology
BC Black Carbon
CAA Clean Air Act
CAIR Clean Air Interstate Rule
CAMx Comprehensive Air Quality Model with Extensions
CBI Confidential Business Information
CBSA Core-based Statistical Area
CDD Clean Data Determination
CFR Code of Federal Regulations
CMAQ Community Multi-Scale Air Quality Model
CSAPR Cross-State Air Pollution Rule
CSN Chemical Speciation Network
DOD Department of Defense
DOT Department of Transportation
EC Elemental Carbon
EGU Electric Generating Unit
EPA Environmental Protection Agency
Fe Iron
FEM Federal Equivalent Method
FIP Federal Implementation Plan
FRM Federal Reference Method
HCl Hydrogen Chloride
ICR Information Collection Request
LAER Lowest Achievable Emission Rate
MACT Maximum Achievable Control Technology
MATS Mercury and Air Toxics Standards
MSM Most Stringent Measures
MPO Metropolitan Planning Organization
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NAPAP National Acid Precipitation Assessment Program
NEI National Emissions Inventory
NESHAP National Emissions Standard for Hazardous Air Pollutants
NH3 Ammonia
NH4 Ammonium
NH4NO3 Ammonium Nitrate
NH4HSO4 Ammonium Bi-Sulfate
(NH4)2SO4 Ammonium Sulfate
NNSR Nonattainment New Source Review
NOX Nitrogen Oxides
NO3 Nitrate
NSPS New Source Performance Standards
O3 Ozone
OM Organic Mass
OMB Office of Management and Budget
PM Particulate Matter
PM2.5 Particulate Matter Equal to or Less than 2.5
Microns in Diameter (Fine Particulate Matter)
PM10 Particulate Matter Equal to or Less than 10 Microns
in Diameter
PRA Paperwork Reduction Act
PSD Prevention of Significant Deterioration
RACM Reasonably Available Control Measures
RACT Reasonably Available Control Technology
RFP Reasonable Further Progress
RICE Reciprocating Internal Combustion Engines
SIP State Implementation Plan
SOA Secondary Organic Aerosols
SO2 Sulfur Dioxide
SO4 Sulfate
TAR Tribal Authority Rule
TIP Tribal Implementation Plan
TIP Transportation Improvement Program
TSP Total Suspended Particles
[mu]m Micrometer (Micron)
VMT Vehicle Miles Traveled
VOC Volatile Organic Compounds

B. Entities Affected by This Rule

    Entities potentially affected directly by this final rule include
state, local and tribal governments and air pollution control agencies
responsible for attainment and maintenance of the NAAQS. Entities
potentially affected indirectly by this final rule as regulated sources
include owners and operators of sources that emit PM2.5,
sulfur dioxide (SO2), oxides of nitrogen (NOX),
volatile organic compounds (VOC) and/or ammonia (NH3).
Parties affected by the conformity-related elements include state and
local transportation and air quality agencies, metropolitan planning
organizations (MPOs), and all federal agencies including the U.S.
Department of Transportation, the U.S. Department of Defense, the U.S.
Department of Interior and the U.S. Department of Agriculture. Others
potentially affected indirectly by this final rule include members of
the general public who live, work, or recreate in areas affected by
elevated ambient PM2.5 levels in areas designated
nonattainment for a PM2.5 NAAQS.

C. Obtaining a Copy of This Document and Other Related Information

    In addition to being available in the docket, an electronic copy of
this Federal Register document will be posted at http://www3.epa.gov/airquality/particlepollution/actions.html.

D. Organization of This Federal Register Document

    The information presented in this document is organized as follows:

I. General Information
    A. Preamble Glossary of Terms and Acronyms
    B. Entities Affected by This Rule
    C. Obtaining a Copy of This Document and Other Related
Information
    D. Organization of This Federal Register Document
II. Background
    A. Introduction
    B. Overview of PM2.5 NAAQS and Implementation
    C. Atmospheric Chemistry of PM2.5 and Its Precursors
III. Requirements With Respect to the Treatment of PM2.5
Precursors in Attainment Plans and the NNSR Program
    A. Background
    B. Summary of Proposal
    C. Final Rule
IV. Requirements for PM2.5 Moderate Nonattainment Area
Plans
    A. Plan Due Dates
    B. Emissions Inventory Requirements
    C. Pollutants To Be Addressed in the Plan
    D. Attainment Plan Control Strategy
    E. Modeling for Attainment Demonstrations
    F. RFP Requirements
    G. Quantitative Milestones
    H. Contingency Measures
    I. Attainment Dates
    J. Attainment Date Extensions
V. Reclassification of a PM2.5 Moderate Nonattainment
Area to Serious
    A. Discretionary Authority
    B. Mandatory Duty
VI. Requirements for PM2.5 Serious Nonattainment Area
Plans
    A. Plan Due Dates
    B. Emissions Inventory Requirements
    C. Pollutants To Be Addressed in the Plan
    D. Attainment Plan Control Strategy
    E. Modeling for Attainment Demonstrations
    F. RFP Requirements
    G. Quantitative Milestones
    H. Contingency Measures
    I. Attainment Dates
    J. Attainment Date Extensions
VII. Requirements Under CAA Section 189(d) for PM2.5
Serious Areas That Fail To Attain the NAAQS by the Applicable
Attainment Date
    A. Plan Due Dates
    B. Emissions Inventory Requirements
    C. Pollutants To Be Addressed in the Plan
    D. Attainment Plan Control Strategy
    E. Modeling for Attainment Demonstrations
    F. RFP Requirements
    G. Quantitative Milestones
    H. Contingency Measures
    I. Attainment Dates
VIII. NNSR Requirements for PM2.5 Nonattainment Areas
    A. Background
    B. What are the final NNSR requirements for PM2.5?
    C. Transition Provisions for Major Source Permitting in
PM2.5 Nonattainment Areas
IX. Other Requirements and Considerations for PM2.5
Nonattainment Areas
    A. Waivers Under Section 188(f)
    B. Conformity Requirements
    C. Clean Data Policy
    D. Section 179B/International Border Areas
    E. Enforcement and Compliance
    F. Multi-Pollutant Considerations
    G. Measures To Ensure Appropriate Protections for Overburdened
Populations
    H. Tribal Issues
    I. Voluntary Programs for Reducing Ambient PM2.5
    J. Improved Stationary Source Emissions Monitoring
    K. Stationary Source Test Methods for Emissions of Condensable
PM2.5
X. Revocation of the 1997 Primary Annual PM2.5 NAAQS
    A. Background
    B. History of Revocation of Other NAAQS
    C. Requirements for Revocation and Related Anti-Backsliding
Requirements for the 1997 Primary Annual PM2.5 NAAQS
XI. Environmental Justice Considerations
XII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
    K. Congressional Review Act (CRA)
    L. Determination Under Section 307(d)
    M. Judicial Review
XIII. Statutory Authority

II. Background

A. Introduction

    Ambient, or outdoor, air can contain a variety of pollutants,
including particulate matter (PM). Airborne PM can be comprised of
either solid or liquid particles, and can be a complex mixture of
particles in both solid and liquid form. The most common constituents
of airborne PM include the following: Sulfate (SO4); nitrate
(NO3); ammonium (NH4); elemental carbon (EC);
organic mass (OM); and inorganic material, generally referred to as
``crustal'' material, which can include metals, dust, sea salt and
other trace elements. Airborne PM can be of different sizes, commonly
referred to as ``coarse'' and ``fine'' particles. Fine particles, in
general terms, are PM with an aerodynamic diameter less than or equal
to a nominal 2.5 micrometers ([micro]m). For this reason, particles of
this size are referred to as PM2.5. PM2.5
particles commonly include ``primary'' particles and ``secondary''
particles. Primary particles, or direct PM2.5, are emitted
by sources directly into the air as solid or liquid particles (e.g.,
elemental carbon from diesel engines or wildfires, or condensable
organic particles from gasoline engines). Secondary particles are
formed in the atmosphere as a result of chemical reactions between
specific pollutants known as PM2.5 precursors (e.g.,
reactions between NOX and SO2 emissions from
mobile and stationary sources combined with ammonia to form ammonium
nitrate and ammonium sulfate).
    The human health effects associated with long or short-term
exposure to PM2.5 are significant and include premature
mortality, aggravation of respiratory and cardiovascular disease (as
indicated by increased hospital admissions and emergency room visits)
and development of chronic respiratory disease. In addition, welfare
effects associated with elevated PM2.5 levels include
visibility impairment as well as effects on sensitive ecosystems,
materials damage and soiling and climatic and radiative processes.\1\
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    \1\ For a complete discussion of the human health and welfare
effects associated with exposure to elevated concentrations of
particulate matter, see generally ``Integrated Science Assessment
for Particulate Matter.'' U.S. Environmental Protection Agency,
Office of Research and Development, National Center for
Environmental Assessment--RTP Division, February 10, 2010. EPA/600/
R-08/139F. Available at: http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html. See Chapter 2.
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    On December 14, 2012, the EPA made revisions to the suite of the
NAAQS for PM to provide requisite protection of public health and
welfare with an adequate margin of safety. The EPA also made
corresponding revisions to the data handling conventions for PM and the
ambient air monitoring, reporting and network design requirements for
PM. Specifically, the agency revised the primary annual
PM2.5 standard by lowering the level from 15.0 to 12.0
[mu]g/

m\3\ to provide increased protection against health effects associated
with long- and short-term PM2.5 exposures. The EPA did not
revise the secondary annual PM2.5 standard, which remains at
15.0 [mu]g/m\3\.\2\ The EPA eliminated spatial averaging as part of the
form of the PM2.5 annual standards to avoid potential
disproportionate impacts on at-risk populations. In addition, the EPA
retained the level and form of the primary and secondary 24-hour
PM2.5 standards to continue to provide supplemental
protection against health effects associated with short-term
PM2.5 exposures. Although not directly relevant to this
rulemaking with respect to implementation of the PM2.5
NAAQS, it should be noted that in December 2012, the EPA also did not
revise the level or form of the primary and secondary 24-hour
PM10 NAAQS, which remain at 150 [mu]g/m\3\.\3\
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    \2\ 78 FR 3086 (January 15, 2013).
    \3\ This final rulemaking applies to implementation of the
PM2.5 NAAQS. For the PM10 NAAQS, states and
the EPA will continue to implement those NAAQS in accordance with
the applicable statutory requirements of the CAA and the EPA's
existing guidance in the ``The General Preamble for Implementation
of Title I of the Clean Air Act (CAA) Amendments,'' 57 FR 13498
(April 16, 1992); and ``State Implementation Plans for Serious PM-10
Nonattainment Areas: Addendum to the General Preamble for the
Implementation of Title I of the Clean Air Act (CAA) Amendments,''
59 FR 41998 (August 16, 1994). Throughout this preamble, these
documents will be referred to as the ``General Preamble'' and the
``Addendum,'' respectively.
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    Estimates show that attainment of the primary PM2.5
standards will result in hundreds fewer premature deaths each year,
prevent tens of thousands of hospital admissions each year and prevent
hundreds of thousands of doctor visits, absences from work and school
and respiratory illnesses in children annually.\4\ Attainment of the
primary PM2.5 standards will have welfare co-benefits in
addition to direct human health benefits. The term ``welfare co-
benefits'' covers both environmental and societal benefits of reducing
pollution, such as reductions in visibility impairment, materials
damage and ecosystem damage.\5\
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    \4\ ``Regulatory Impact Analysis for the Final Revisions to the
National Ambient Air Quality Standards for Particulate Matter.''
U.S. Environmental Protection Agency, Office of Air Quality and
Planning Standards, Health and Environmental Impacts Division,
February 28, 2013. EPA-452/R-12-005. See http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_ria.html.
    \5\ Ibid.
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B. Overview of PM2.5 NAAQS and Implementation

1. General Background
    Sections 108 and 109 of the Clean Air Act (CAA or Act) govern the
establishment, review and revision, as appropriate, of the NAAQS for
widespread pollutants emitted from numerous and diverse sources
considered harmful to public health and the environment. The CAA
requires two types of NAAQS: (i) Primary standards, which set limits to
protect public health, including the health of at-risk populations; and
(ii) secondary standards, which set limits to protect public welfare,
including protection against visibility impairment, damage to animals,
crops, vegetation and buildings.
    The CAA also establishes important roles both for state and tribal
governments and for the EPA in implementing the NAAQS. In accordance
with the principle of cooperative federalism, both state and tribal
governments and the EPA have respective authorities and
responsibilities under the CAA. At the outset, the EPA has the
authority and responsibility to promulgate the NAAQS. In turn, state,
local and tribal air pollution control agencies (``air agencies'') have
the authority and primary responsibility for developing and
implementing attainment plans that contain emission control measures
needed to achieve the air quality standards in a timely manner in each
nonattainment area, consistent with the requirements of the CAA. The
EPA often assists states by promulgating regulations or providing
guidance for meeting implementation requirements and by providing
technical tools, including information on control
measures.6 7
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    \6\ It is important to note that the EPA does not have a
mandatory duty to promulgate an implementation rule for the
PM2.5 NAAQS, and the obligations of state and tribal air
agencies to develop and submit an attainment plan are independent
obligations and not conditioned upon the EPA promulgating an
implementation rule for the PM2.5 NAAQS.
    \7\ When the term ``state'' is used hereafter, it will refer
generically to states, local air agencies, and tribal governments
electing to be treated as states for the purposes of implementing
the CAA. Of additional note is that the 1998 Tribal Authority Rule
(TAR), which is found in 40 CFR part 49, which implements section
301(d) of the CAA, provides that tribes be treated in the same
manner as a state when implementing certain sections of the CAA. It
gives tribes the option of developing tribal implementation plans
(TIPs), but unlike states, tribes are not required to develop
implementation plans. Section IX.I of this preamble provides further
discussion of tribal issues.
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    The EPA also promulgates nationally applicable control requirements
and emission limits for many sources such as new motor vehicles,
certain categories of new and modified major stationary sources and
existing stationary sources of toxic air pollutants. These federal
actions assist states by achieving emissions reductions from certain
categories of sources nationwide, which can help with local attainment
needs in a given nonattainment area. The EPA also has authority to
provide funding, technical assistance, and guidance to states to
support implementation of the NAAQS. In addition, the EPA has authority
to address interstate transport of pollutants, in the event that states
fail to do so. Through this authority, the EPA has addressed regional
transport of pollutants from upwind states to downwind states, and has
previously done so for purposes of the PM2.5 NAAQS.\8\ In
addition, the EPA has the authority and responsibility to review and
take action to approve or disapprove attainment plans submitted by
states based upon whether they meet applicable statutory and regulatory
requirements and to initiate the process for imposition of sanctions
and/or issue federal implementation plans (FIPs) when states fail to
fulfill their CAA obligations.
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    \8\ See 76 FR 48208 (August 8, 2011).
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2. History of PM2.5 NAAQS Implementation
    The EPA first promulgated annual and 24-hour NAAQS for
PM2.5 in July 1997.\9\ Prior to that time, the EPA had
addressed ambient PM through other means, first by regulating ``total
suspended particles'' (TSP) and then later by regulating
PM10. After protracted litigation, the 1997 NAAQS for
PM2.5 were upheld by the U.S. Court of Appeals for the
District of Columbia Circuit in March 2002.\10\ The EPA subsequently
promulgated designations for the 1997 PM2.5 NAAQS
nationwide, designating a number of areas as nonattainment for the 1997
PM2.5 NAAQS, effective April 2005.\11\ In April 2007, the
EPA issued a detailed implementation rule to assist states with the
development of SIP submissions to meet attainment plan requirements for
the 1997 NAAQS (the ``2007 PM2.5 Implementation Rule'').\12\
In May 2008, the EPA issued another rule to assist states with SIP
submissions to meet the specific requirements for permitting programs
for NNSR purposes in designated nonattainment areas (the ``2008
PM2.5 NSR Rule'').\13\ The EPA premised both the 2007
PM2.5 Implementation Rule and the 2008 PM2.5 NSR
Rule on the EPA's interpretation of the statute that

nonattainment areas for the PM2.5 NAAQS were subject solely
to the general attainment plan requirements of subpart 1, part D of
title I of the CAA (``subpart 1'').
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    \9\ 62 FR 38652 (July 18, 1997).
    \10\ For a complete summary of legal challenges and related
court decisions on the PM NAAQS, see generally 78 FR 3086 (January
15, 2013).
    \11\ 70 FR 944 (January 5, 2005).
    \12\ 72 FR 20583 (April 25, 2007).
    \13\ 73 FR 28231 (May 16, 2008).
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    Section 109(d)(1) of the CAA requires the EPA periodically to
review the science upon which the standards are based and the standards
themselves, and to revise the standards as may be appropriate. In
October 2006, the EPA promulgated revisions to the suite of the NAAQS
for PM, and in particular the EPA revised the 24-hour PM2.5
standards.\14\ In accordance with section 107(d), the EPA subsequently
designated a number of areas as nonattainment for the revised 2006 24-
hour PM2.5 standards, effective December 2009.\15\ In March
2012, the EPA issued a guidance document specifically to aid states in
preparing their SIP submissions to meet attainment plan requirements
for the 2006 24-hour PM2.5 NAAQS in designated nonattainment
areas.\16\ The EPA's guidance for the 2006 PM2.5 NAAQS was
based, in large part, on the requirements finalized in the 2007
PM2.5 Implementation Rule, which the EPA based solely upon
the statutory requirements of subpart 1.
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    \14\ 71 FR 61144 (October 17, 2006).
    \15\ 74 FR 58688 (November 13, 2009).
    \16\ Memorandum of March 2, 2012 (withdrawn June 6, 2013), from
Stephen D. Page, Director, Office of Air Quality Planning and
Standards, to the EPA Regional Air Directors, Regions I-X,
``Implementation Guidance for the 2006 24-Hour Fine Particle
(PM2.5) National Ambient Air Quality Standards (NAAQS).''
Available at: http://www3.epa.gov/ttn/naaqs/aqmguide/collection/cp2/20120302_page_implement_guidance_2006-24-hr_pm2.5_naaqs.pdf.
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    The EPA initiated a review of the PM2.5 NAAQS in June
2007, proposing revisions to the primary and secondary PM2.5
NAAQS on June 29, 2012.\17\ The EPA issued its final rule on December
14, 2012, in which it lowered the primary annual PM2.5
standard from 15.0 [mu]g/m\3\ to 12.0 [mu]g/m\3\ to provide increased
protection against health effects associated with long- and short-term
fine particle exposures.\18\ The EPA also eliminated spatial averaging
as part of the form of the annual standard to avoid potential
disproportionate impacts on at-risk populations.\19\ The EPA retained
the level (35 [mu]g/m\3\) and form (98th percentile, averaged over 3
years) of the primary 24-hour PM2.5 standard, as revised in
2006, to provide supplemental protection against health effects
associated with short-term PM2.5 exposures, especially in
areas with high peak PM2.5 concentrations.\20\ This suite of
primary PM2.5 standards provides increased public health
protection, including the health of at-risk populations which include
children, older adults, persons with pre-existing health and lung
disease and persons of lower socioeconomic status, against a broad
range of PM2.5-related effects that include premature
mortality, increased hospital admissions and emergency department
visits and development of chronic respiratory disease.\21\ With regard
to the secondary (welfare-based) standards, the EPA retained the
existing annual PM2.5 standard of 15.0 [mu]g/m\3\ and the
existing 24-hour PM2.5 standard of 35 [mu]g/m\3\ to protect
against PM-related non-visibility welfare effects including ecological
effects, effects on materials and climate impacts. In addition, the
secondary 24-hour PM2.5 standard provides protection for PM-
related visibility impairment.
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    \17\ 77 FR 38890 (June 29, 2012).
    \18\ 78 FR 3086 (January 15, 2013).
    \19\ Spatial averaging of monitored ambient air quality data was
a feature of the prior PM2.5 NAAQS monitoring regulations
which had the potential for masking particularly high
PM2.5 concentrations at certain monitored locations
within nonattainment areas.
    \20\ 71 FR 61144 (October 17, 2006).
    \21\ General information regarding the health effects associated
with PM2.5 exposures is available at: http://www3.epa.gov/airquality/particlepollution/health.html. Additional
information, such as the EPA's technical documents supporting the
latest review of the standards, is available at: http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html.
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    On January 4, 2013, shortly after the EPA promulgated the 2012
revisions to the suite of PM NAAQS, the D.C. Circuit issued its
decision in a challenge to the 2007 PM2.5 Implementation
Rule and the 2008 PM2.5 NSR Rule. In NRDC v. EPA, the court
held that the EPA erred in implementing the 1997 PM2.5 NAAQS
pursuant only to the general implementation requirements of subpart 1,
rather than also to the implementation requirements specific to
particulate matter (PM10) in subpart 4, part D of title I of
the CAA (``subpart 4'').\22\ The court reasoned that the plain meaning
of the CAA requires implementation of the 1997 PM2.5 NAAQS
under subpart 4 because PM2.5 particles fall within the
statutory definition of PM10 and thus implementation of the
PM2.5 NAAQS is subject to the same statutory requirements as
the PM10 NAAQS. In addition, although the court stated that
its decision that the EPA must implement the PM2.5 NAAQS
pursuant to subpart 4 requirements meant that it did not have to reach
decisions on other issues concerning the regulation of precursors to
PM2.5, the court nonetheless noted that subpart 4 has
specific requirements with respect to regulation of such precursors. As
a result, the court remanded to the EPA both the 2007 PM2.5
Implementation Rule and the 2008 PM2.5 NSR Rule, both of
which were premised on the EPA's interpretation of the statute that
subpart 1 was the only applicable subpart for the implementation of the
1997 PM2.5 NAAQS in nonattainment areas. The court
instructed the EPA ``to repromulgate these rules pursuant to subpart 4
consistent with this opinion.'' Given the D.C. Circuit's opinion in
NRDC v. EPA, the EPA withdrew its 2012 guidance document for the 2006
24-hour PM2.5 NAAQS in June 2013. Because the court had
concluded that the EPA and states must implement the PM2.5
NAAQS consistent with the statutory requirements of subpart 4, the
EPA's 2012 guidance for attainment plans for the 2006 PM2.5
NAAQS premised solely upon subpart 1 requirements was no longer
appropriate.
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    \22\ NRDC v. EPA, 706 F.3d 428 (D.C. Cir. 2013).
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    The EPA issued a notice of proposed rulemaking (NPRM) on March 23,
2015 (80 FR 15340) titled, ``Fine Particulate Matter National Ambient
Air Quality Standards: State Implementation Plan Requirements''
(PM2.5 SIP Requirements Rule) to meet a number of
objectives. This final rule accomplishes those objectives. It clarifies
how states should meet the statutory SIP requirements that apply to
areas designated nonattainment for any PM2.5 NAAQS under
subparts 1 and 4. It does so by establishing regulatory requirements
and providing guidance that will be applicable to attainment plans for
the 2012 PM2.5 NAAQS and any future revisions of the
PM2.5 NAAQS, subject to revisions that may be necessary for
implementation purposes in the future. In addition, this action
responds to the D.C. Circuit's remand of the 2007 PM2.5
Implementation Rule and the 2008 PM2.5 NSR Rule. As a
result, the requirements of the rule will also govern future actions
associated with states' ongoing implementation efforts for the 1997 and
2006 PM2.5 NAAQS.
    The public comment period for the proposed PM2.5 SIP
Requirements Rule closed on May 29, 2015, and the EPA received 56
comments during that period. The preamble to this final rule includes
discussion of the most significant comments received on the proposal
and how the EPA considered them in developing the agency's final action
concerning the specific nonattainment planning requirements. The
Response to Comments document that accompanies this final rule provides
more detailed responses to the significant comments received. The
public comments received on the NPRM and the EPA's Response to Comment

document are posted in the docket at http://www.regulations.gov (Docket
ID No. EPA-HQ-OAR-2013-0691).

C. Atmospheric Chemistry of PM2.5 and Its Precursors

1. Overview
    In order to determine how to regulate sources of direct
PM2.5 and PM2.5 precursors to attain the
PM2.5 NAAQS in a given nonattainment area, it is necessary
to understand the basic chemical processes that cause or contribute to
the formation of ambient PM2.5. Accordingly, an
understanding of these processes is necessary to design appropriate
regulations for implementation of the PM2.5 NAAQS.
    As noted earlier, the term PM2.5 refers to particles of
solid and liquid material less than 2.5 microns in aerodynamic
diameter.\23\ ``Primary'' PM2.5 is emitted directly from
emissions sources or activities, such as from diesel fuel combustion,
wood burning, construction activities, and unpaved roads, and it
includes both filterable and condensable particles.\24\ ``Secondary''
PM2.5 is formed as a result of emissions of certain
precursor gases that undergo chemical reactions in the atmosphere. The
principal precursor gases that contribute to secondary PM2.5
formation are SO2, from the combustion of coal or other high
sulfur fuels; NOX, from many types of fossil fuel
combustion; VOC, from certain fuels, solvents and industrial processes;
and ammonia, from sources such as animal feeding operations, wastewater
treatment and fertilizer. To illustrate the types of sources that emit
relevant pollutants, Table 1 provides National Emissions Inventory
(NEI) data for 2011 that represent nonattainment area anthropogenic and
wildfire emissions estimates for direct PM2.5 and the four
main PM2.5 precursor gases from major source sectors.
---------------------------------------------------------------------------

    \23\ The regulatory definition of PM2.5 includes
particles with an upper 50 percent cut-point of 2.5[mu]m aerodynamic
diameter (the 50 percent cut-point diameter is the diameter at which
the sample collects 50 percent of the particles and rejects 50
percent of the particles). PM2.5 particles have a
penetration curve as measured by a reference method based on
Appendix L of 40 CFR part 50 and designated in accordance with 40
CFR part 53, by an equivalent method designed in accordance with 40
CFR part 53, or by an approved regional method designated in
accordance with Appendix C of 40 CFR part 58.
    \24\ Certain commercial or industrial activities involving high
temperature processes (e.g., fuel combustion, metal processing,
cooking operations) emit gaseous pollutants into the ambient air
that rapidly condense into particle form. These ``condensable'' PM
emissions exist almost entirely in the 2.5 or less micron range and
can consist of organic material, sulfuric acid and metals.

       Table 1--Total Emissions of PM2.5 and Precursors for Major Sectors in PM2.5 Nonattainment Areas \a\
                                                 [In tons/year]
                            Source: 2011 National Emissions Inventory (Version 2) \b\
----------------------------------------------------------------------------------------------------------------
            Category               Direct PM2.5         SO2             NOX             VOC             NH3
----------------------------------------------------------------------------------------------------------------
Fuel combustion, electric                 11,339         324,658          82,509           3,001           3,572
 generating utilities (EGUs)....
Fuel combustion, industrial.....          10,286          23,762          57,690           6,251             892
Fuel combustion, other..........          29,582           8,224          60,636          32,320           8,819
Chemical and allied products....           1,504           1,329           1,056           2,828             685
Metals processing...............           4,037          19,490           4,543           4,586             130
Petroleum and related industries           1,534           7,273           3,775          18,830             215
Other industrial processes......          24,168           8,466          22,599          24,928           1,094
Solvent utilization.............           1,089              39              56         242,022              68
Storage and transport...........           3,420             628           7,067          55,410           3,684
Waste disposal and recycling....           4,143             830           4,130          16,492          19,389
Onroad mobile...................          21,073           2,598         540,800         234,136          17,525
Offroad mobile..................          13,660           5,874         239,169         152,504             150
Miscellaneous (includes                  158,565           7,368          13,734         248,835         236,577
 emissions from fire,\c\ dust
 and some agricultural
 operations)....................
                                 -------------------------------------------------------------------------------
    Total.......................         284,401         410,540       1,037,764       1,042,144         292,800
----------------------------------------------------------------------------------------------------------------
\a\ There were 33 areas designated as nonattainment for the 1997, 2006, or 2012 PM2.5 NAAQS as of June 6, 2016.
  These areas were comprised of 67 whole or partial counties. The emissions data in this table represents whole
  county emissions for the 67 counties because such data is readily available in EPA databases. Actual emissions
  totals for the 33 nonattainment areas in aggregate would be somewhat lower because some nonattainment areas
  include partial counties.
\b\ For more details on the definitions of the emission categories listed in Table 1, see Sector/Tier crosswalk
  table for the 2011 NEI, available at: ftp://ftp.epa.gov/EmisInventory/2011/doc/scc_eis_crosswalk_2011neiv1.xlsx.
\c\ Emissions from fire include wildfire, prescribed fire, and agricultural burning.

2. Composition and Sources of PM2.5 Constituents
    PM2.5 is a complex and highly variable mixture of
particles, but the majority of PM2.5 by mass is often
comprised of five constituents: (i) OM; (ii) EC; (iii) crustal
material; (iv) ammonium sulfate
((NH4)2SO4); and (v) ammonium nitrate
(NH4NO3).\25\ The discussion that follows
provides an overview of each of the five major components of
PM2.5, all of which are known to contribute to ambient
PM2.5 levels in areas throughout the U.S.\26\ Section
II.C.3.d of this preamble provides more details on the atmospheric
chemistry involved in the formation of sulfate, nitrate and OM, to
illustrate the importance of controlling emissions of PM2.5
precursors as part of any comprehensive strategy to reduce ambient
PM2.5 levels in excess of the NAAQS. Section II.C.4 of this
preamble presents a brief overview of PM2.5 composition by
region of the U.S.
---------------------------------------------------------------------------

    \25\ Seinfeld J.H. and Pandis S.N., 2006. Atmospheric Chemistry
and Physics: From Air Pollution to Climate Change. 2nd edition, J.
Wiley, New York.
    \26\ U.S. Environmental Protection Agency, 2004. ``The Particle
Pollution Report: Current Understanding of Air Quality and Emissions
through 2003.'' Office of Air Quality Planning and Standards,
Emissions, Monitoring, and Analysis Division, December 2004.
Available at: http://www.epa.gov/airtrends/reports.html.
---------------------------------------------------------------------------

    OM is the fraction of ambient PM2.5 with the most
diverse chemical composition, containing potentially thousands of
different organic compounds (i.e., those compounds containing carbon)
composed primarily of carbon, hydrogen, oxygen and nitrogen. Both
primary particles and secondary particles contribute to ambient OM
concentrations, with

combustion sources being the dominant type of emissions sources.
Another portion of primary OM particles results from direct emissions
of organic compounds from sources of incomplete combustion, such as gas
and diesel engines. Secondary OM particle formation involves oxidation
of both anthropogenic and biogenic (plant-derived) VOC, and can involve
other, more complex chemical reactions. Further details of the
chemistry behind the formation of secondary OM, known more commonly as
secondary organic aerosols (SOA), are described in Section II.C of this
preamble.
    EC refers to particulate carbon that has a graphitic molecular
structure, and is sometimes referred to as ``black carbon'' (BC). It is
emitted directly from emission sources and does not undergo any
significant reactions with other gases in the atmosphere. EC particles
result from primary emissions involving combustion, especially from
diesel-fueled vehicles, but also from other processes involving the
burning of fossil fuels. The latter include anthropogenic sources such
as boilers and waste disposal. In addition, some EC particles originate
from biomass combustion such as from prescribed fires, wildfires and
residential wood combustion.
    Crustal PM is comprised of particles of soil and oxides of metals
from some industrial processes. Compounds comprised of elements such as
silicon, aluminum, iron, calcium, titanium, magnesium and potassium, as
well as oxygen, are major components.\27\ Sources of crustal
PM2.5 include windblown dust, dust from mechanical
resuspension (e.g. dust from construction activities or vehicles
driving on unpaved roads) and some forms of combustion, especially of
coal. Crustal PM2.5 comprised of elements, like iron (Fe),
and their oxides can also be emitted from industrial sources.
---------------------------------------------------------------------------

    \27\ Appel, K.W., Pouliot, G.A., Simon, H., Sarwar, G., Pye,
H.O.T., Napelenok, S.L., Akhtar, F., and Roselle, S.J., 2013.
Evaluation of dust and trace metal estimates from the Community
Multiscale Air Quality (CMAQ) model version 5.0, Geoscientific Model
Development Discussions 61859-1899; Sorooshian, A., Shingler, T.,
Harpold, A., Feagles, C.W., Meixner, T., and Brooks, P.D., 2013.
Aerosol and precipitation chemistry in the southwestern United
States: spatiotemporal trends and interrelationships, Atmospheric
Chemistry and Physics 13, 7361-7379.
---------------------------------------------------------------------------

    The remaining portion of ambient PM2.5 is mostly
composed of SO4, NO3 and NH4, which
react in the ambient air to form ammonium sulfate
((NH4)2SO4) and ammonium nitrate
(NH4NO3). Another common PM2.5
particle is ammonium bi-sulfate (NH4HSO4). In
some areas, less common ions such as chloride are also found in
PM2.5 samples in the form of particles that include sodium
chloride and ammonium chloride. Particle-bound water is often also
associated with this fraction of PM2.5. Sulfate, nitrate and
ammonium particles originate through both primary and secondary
mechanisms, although the vast majority of these PM2.5
particles are formed through secondary formation, as described in the
following section.
3. Secondary Formation of PM2.5 From Gaseous Precursors
    a. Overview. The composition of PM2.5 is complex and
highly variable due in part to the large contribution of secondary
PM2.5 to total fine particle mass in most locations, and to
the complexity of secondary particle formation processes. A large
number of possible chemical reactions, often non-linear in nature, can
convert the gases SO2, NOX, VOC and ammonia to
PM2.5. Thus, these gases are precursors to PM2.5.
A brief discussion of SO4, NO3 and SOA formation,
as well as the role of ammonia in their formation, follows.
    b. SO4 Formation. SO2 is emitted mostly from the
combustion of fossil fuels in boilers operated by electric utilities
and other industries, with less than 10 percent of SO2
emissions nationwide currently coming from other industrial sources,
such as oil refining and pulp and paper production.\28\ When
SO2 oxidizes it forms sulfuric acid, a highly corrosive
compound toxic to humans and to ecosystems that contributes to acid
deposition (acid rain). In the presence of ammonia, however, sulfuric
acid will react to form (NH4)2SO4, a
less acidic compound and one of the five major components of
PM2.5. If there is not enough ammonia present to fully
neutralize the sulfuric acid, part of it may convert to
NH4HSO4, which is more acidic than
(NH4)2SO4, but less so than sulfuric
acid. There is a large amount of emerging scientific evidence that
SO2 may also contribute to the formation of SOA from
biogenic VOC emissions (see section later on SOA). Sulfate levels in
the ambient air peak in summer months due to increased SO2
emissions, generally from electric generating units (EGUs), and from
meteorological conditions that are conducive to sulfate formation.
---------------------------------------------------------------------------

    \28\ U.S. Environmental Protection Agency, 2013. ``2008 National
Emissions Inventory: Review Analysis and Highlights.'' Office of Air
Quality Planning and Standards, Air Quality Assessment Division, May
2013. EPA-454/R-005. Available at: https://www.epa.gov/sites/production/files/2015-07/documents/2008report.pdf.
---------------------------------------------------------------------------

    c. NO3 Formation. The main sources of NOX emissions are
combustion of fossil fuel in boilers and mobile sources, accounting for
more than 80 percent of national anthropogenic NOX emissions
(based on the 2011 NEI), with boilers and EGUs contributing about 27
percent and mobile sources contributing 56 percent. Oxides of nitrogen
react in the atmosphere to form nitric acid, another prime contributor
to acid deposition in the environment. Nitric acid converts to ammonium
nitrate, one of the five main components of PM2.5, in the
presence of ammonia. Low temperatures and high relative humidity create
ideal conditions for the formation of ammonium nitrate, typically
leading to higher atmospheric levels in winter months and lower levels
in summer months.\29\
---------------------------------------------------------------------------

    \29\ Carlton, A.G., Pinder, R.W., Bhave, P.B., Pouliout, G.A.,
2010. To What Extent Can Biogenic SOA Be Controlled, Environmental
Science and Technology 44(9), 3376-80.
---------------------------------------------------------------------------

    d. SOA Formation. As discussed earlier, the OM component of ambient
PM2.5 is a complex mixture of hundreds or even thousands of
anthropogenic and biogenic organic compounds. These compounds are
either emitted directly from sources (i.e., as ``primary''
PM2.5) or formed by reactions in the ambient air to make SOA
(i.e., as ``secondary'' PM2.5).
    VOC (both anthropogenic and biogenic) are key precursors to the SOA
component of PM2.5. The relative importance of these
compounds in the formation of organic particles varies between
geographic areas, depending upon local emission sources, atmospheric
chemistry and season of the year. It should be further noted that not
all inventoried VOC may be contributing to the formation of organic
particles. For example, chemical reactions involving VOC are generally
accelerated in warmer temperatures, and for this reason studies show
that SOA typically comprises a higher percentage of PM2.5 in
the summer than in the winter.\30\
---------------------------------------------------------------------------

    \30\ Pandis S.N., Harley R.A., Cass G.R., and Seinfeld J.H.,
1992. Secondary Organic Aerosol Formation and Transport, Atmospheric
Environment, 26, 2266-82.
---------------------------------------------------------------------------

    Anthropogenic sources of VOC include mobile sources, petrochemical
manufacturing, oil and gas emissions, fire emissions, and solvents.\31\
In addition, some biogenic VOC, emitted by vegetation such as trees,
can also contribute significantly to SOA formation, especially in
heavily forested areas, such as the southeastern U.S. It should be
noted, however, that

anthropogenic contributions to SOA are likely highest in the wintertime
when biogenic SOA levels are lower; conversely, in the summertime,
biogenic contributions to SOA are likely higher. Despite significant
progress that has been made in understanding the origins and properties
of SOA, it remains the least understood component of PM2.5
and continues to be a significant topic of research and investigation.
---------------------------------------------------------------------------

    \31\ Carlton, A.G., Bhave, P.B., Napelenok, S.L., Edney, E.O.,
Sarwar, G., Pinder, R.W., Pouliout, G.A., and Houyoux, M. (2010),
Model Representation of Secondary Organic Aerosol in CMAQ4.7,
Environmental Science and Technology 44(22), 8553-60.
---------------------------------------------------------------------------

    e. Role of Ammonia in Sulfate, Nitrate and SOA Formation. Ammonia
is a gaseous pollutant emitted by natural and anthropogenic sources.
The EPA's 2011 NEI shows that the two main sources of ammonia emissions
are fertilizer application (27 percent) and livestock raising (54
percent). It should be noted that the 2011 NEI indicates that mobile
sources in the aggregate contribute about 3 percent of nationwide
ammonia emissions. Catalytic converters installed on light-duty
gasoline vehicles are designed to convert NOX to nitrogen
(N2); however, some ammonia is formed as a secondary product
and emitted during this process.
    As indicated earlier, ammonia plays an important role in
neutralizing acids, such as sulfuric acid and nitric acid, in clouds,
precipitation and particles. On the other hand, deposited ammonia can
contribute to problems of eutrophication in water bodies due to its
nutritive properties.\32\ Ammonia would not exist in particles if not
for the presence of acidic species with which it can combine to form a
particle. In the eastern U.S., sulfate, nitrate and the ammonium
associated with them can together account for between roughly 30
percent and 75 percent of the total PM2.5 mass in a given
area. The ammonium portion by itself roughly accounts for between 5
percent and 20 percent of the total PM2.5 mass in the
East.\33\
---------------------------------------------------------------------------

    \32\ Seinfeld, J.H. and Pandis, S.N. (1998), Atmospheric
Chemistry and Physics: From Air Pollution to Climate Change, 1st
edition, J. Wiley, New York.
    \33\ NARSTO, 2003. Particulate Matter Science for Policy Makers.
A NARSTO Assessment. Parts 1 and 2. NARSTO. Management Office
(Envair), Pasco, Washington. Available at: http://narsto.org/pm_science_assessment.
---------------------------------------------------------------------------

    f. Role of NOX in Nitrate and SOA Formation. In addition to the
contribution of NOX emissions to secondary particulate
nitrate formation, NOX also reacts with anthropogenic and
biogenic VOC to enhance the secondary formation of organic compounds
that make up SOA. NOX is thus involved in all secondary PM
chemistry, not just in particulate nitrate formation.\34\
---------------------------------------------------------------------------

    \34\ Carlton, A.G., Pinder, R.W., Bhave, P.B., and Pouliout,
G.A., 2010. To what extent can Biogenic SOA be Controlled,
Environmental Science and Technology 44(9), 3376-3380.
---------------------------------------------------------------------------

4. Fine Particulate Composition by Location.
    Table 2 shows regional 3-year mean concentrations (2009-2011) of
PM2.5 and its main components at sites in the Chemical
Speciation Network (CSN).\35\ In addition to the mean values for all
sites in each region, the table includes the minimum and maximum
observed PM2.5 and species concentrations for sites within
each region. These data illustrate broad observed spatial patterns
across the U.S. in PM2.5 concentrations and its composition.
For example, PM2.5 concentrations are highest on average in
the Central and West regions. Sulfate mass comprises a larger fraction
of PM2.5 than nitrate mass in the northeastern U.S., whereas
nitrate has a greater contribution than sulfate in the West. OM is the
dominant component in all regions, with the highest concentrations of
OM on average found in the West, Northwest and Southeast. On a
percentage basis, the concentrations of EC and crustal material are
relatively low throughout all regions of the U.S. compared to the other
major PM2.5 components.
---------------------------------------------------------------------------

    \35\ The organic matter (OM) values in Table 2 were calculated
by multiplying the measured organic carbon (OC) concentrations by
1.6 (Turpin and Lim (2001), Aerosol Science and Technology, 35, 602-
610). PM2.5 concentrations come from measurements of the
Federal Reference/Equivalance Methods (FRM/FEM) rather than from the
CSN PM2.5 measurement.
---------------------------------------------------------------------------

    The composition of PM2.5 also varies between urban and
rural areas. This is reflective of the distribution of urban and
regional emission sources, atmospheric reactions and transport of fine
particles. More details about the spatial distribution and origins of
PM2.5 components can be found in the docket for this
action.\36\
---------------------------------------------------------------------------

    \36\ Reff and Rao, Memo to the docket, 2013.

                                        Table 2--PM2.5 Chemical Composition Data at 2009-2011 Nonattainment Sites
                                                         [Source: EPA Speciation Trends Network]
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Concentration  ([mu]g/m\3\)
                   Region                              Statistic           -----------------------------------------------------------------------------
                                                                              Sulfate      Nitrate         OM           EC        Crustal       PM2.5
--------------------------------------------------------------------------------------------------------------------------------------------------------
Central....................................  Min ([mu]g/m\3\).............         1.46          0.3         2.73         0.31         0.01         8.92
                                             Mean ([mu]g/m\3\)............         2.69         1.49         3.57         0.68         0.26        11.63
                                             Max ([mu]g/m\3\).............         4.19         3.34         4.81          1.1          1.0        13.51
                                             N............................           61           61           50           50           61           42
East North Central.........................  Min ([mu]g/m\3\).............         0.83         0.38         1.97         0.19         0.01         6.03
                                             Mean ([mu]g/m\3\)............         1.68          1.8         2.84         0.48         0.19         9.86
                                             Max ([mu]g/m\3\).............         2.51         3.57         3.69         0.79         0.61        11.87
                                             N............................           29           28           20           20           28           23
North East.................................  Min ([mu]g/m\3\).............         0.58         0.12         1.74         0.14            0         4.42
                                             Mean ([mu]g/m\3\)............         2.06         0.97         3.14         0.69         0.17         9.33
                                             Max ([mu]g/m\3\).............         5.12         2.26         5.05         1.69         0.52        15.05
                                             N............................           59           59           39           39           59           46
North West.................................  Min ([mu]g/m\3\).............         0.24         0.05         2.91         0.42         0.01         6.06
                                             Mean ([mu]g/m\3\)............         0.54          0.4         5.02         0.81         0.15         8.33
                                             Max ([mu]g/m\3\).............         1.09         1.79         8.44         1.25         0.53        10.96
                                             N............................           33           33           13           13           33           14
South......................................  Min ([mu]g/m\3\).............         0.88         0.18         1.36         0.12         0.02         5.22
                                             Mean ([mu]g/m\3\)............         2.06          0.8         3.32         0.57          0.5        10.05
                                             Max ([mu]g/m\3\).............         3.08         1.67          5.1         1.48         2.38        14.27
                                             N............................           36           27           23           23           36           23
South East.................................  Min ([mu]g/m\3\).............          1.6          0.2         1.75         0.37         0.01         6.76
                                             Mean ([mu]g/m\3\)............         2.39         0.53         4.12         0.63         0.26        10.77

                                             Max ([mu]g/m\3\).............         4.33         1.51         5.71          1.2         0.85        13.38
                                             N............................           44           43           30           30           43           29
South West.................................  Min ([mu]g/m\3\).............         0.34         0.07         2.34         0.46         0.02          5.3
                                             Mean ([mu]g/m\3\)............         0.63         0.49         3.01          0.7          0.5         7.93
                                             Max ([mu]g/m\3\).............         1.13         2.65         4.39         1.04         1.96         9.73
                                             N............................           46           46           11           11           46           12
West.......................................  Min ([mu]g/m\3\).............         0.33         0.08         1.79         0.52         0.01         6.84
                                             Mean ([mu]g/m\3\)............          0.9          1.4         5.22         0.85         0.32        11.49
                                             Max ([mu]g/m\3\).............         2.08         5.14        10.27         1.56         1.05        16.57
                                             N............................           44           44           20           20           44           21
West North Central.........................  Min ([mu]g/m\3\).............         0.29         0.06         1.22         0.09            0         3.23
                                             Mean ([mu]g/m\3\)............         0.67         0.48         3.16         0.44         0.22         7.25
                                             Max ([mu]g/m\3\).............         1.79         2.02         8.28         1.21         0.53        13.72
                                             N............................           30           30            7            7           30           10
--------------------------------------------------------------------------------------------------------------------------------------------------------

III. Requirements With Respect to the Treatment of PM2.5
Precursors in Attainment Plans and the NNSR Program

A. Background

    The EPA recognizes that the treatment of PM2.5
precursors is an important issue in developing a PM2.5
attainment plan \37\ or implementing the NNSR program in a
nonattainment area. The EPA has long recognized the scientific basis
for concluding that there are multiple scientific precursors to
PM10\38\and PM2.5.\39\ Appropriate control of
precursors is especially important for attaining the PM2.5
NAAQS because secondarily formed particles (such as ammonium nitrate,
ammonium sulfate, and some portion of organic carbon) comprise a large
fraction of ambient PM2.5 concentrations in many
nonattainment areas. However, in some PM2.5 nonattainment
areas, a particular precursor or precursors may not contribute
significantly to PM2.5 levels that exceed the relevant
NAAQS. This section of the preamble describes optional precursor
demonstrations that a state may choose to submit to the EPA in order to
establish that sources of particular precursors need not be regulated
for purposes of attainment planning or in the NNSR permitting program
for a specific nonattainment area.
---------------------------------------------------------------------------

    \37\ Note that in this document the term ``attainment plan''
refers to a state's required SIP submittal elements other than those
elements related to the NNSR program.
    \38\ See the General Preamble, 57 FR 13498, (April 16, 1992).
    \39\ See 2007 PM2.5 Implementation Rule, 72 FR 20586
(April 25, 2007). The rule discussed the fact that emissions of
SO2, NOX, VOC and ammonia are factual and
scientific precursors to PM2.5.
---------------------------------------------------------------------------

    Section III.A of this preamble provides background on the January
2013 NRDC v. EPA court decision, in which the court found that subpart
4 of part D of the CAA presumptively requires regulation of all
PM2.5 precursors, except under certain circumstances.
Section III.A of this preamble also provides information on the
requirements of the subpart 4 provisions applicable to attainment plans
for PM NAAQS. Section III.B of this preamble provides a summary of the
precursor demonstration options in the proposed rule and comments
received. Section III.C of this preamble provides a discussion of the
optional precursor demonstrations provided in the final rule.
    The final rule describes how in some cases a state may demonstrate
that the adoption of additional emission reduction measures for a
particular precursor is not needed for purposes of achieving
expeditious attainment nor for advancing the attainment date by at
least a year in a nonattainment area. (This is referred in the preamble
as an ``expeditious attainment demonstration.'') The rule also
describes three optional approaches for demonstrating that a particular
precursor is not a significant contributor to ambient PM2.5
levels that exceed the standard in a particular nonattainment area.
These three precursor demonstrations are: (a) Comprehensive precursor
demonstration; (b) major stationary source precursor demonstration; and
(c) NNSR precursor demonstration. If a state chooses to submit a
precursor demonstration, it must do so in accordance with provisions in
the final rule. A state may use this type of demonstration to justify
that sources of the given precursor may be excluded from certain
PM2.5 attainment plan requirements and/or NNSR requirements,
although the particular sources and requirements eligible for exclusion
will depend on the type of demonstration submitted.
    Section III.C of this preamble also outlines certain technical
issues, such as the appropriate geographic scope of a precursor
demonstration, recommended significance thresholds, and recommended
analytical approaches for evaluating precursor contributions to ambient
PM2.5 levels and the sensitivity of PM2.5 levels
in an area to decreases or increases of emissions.
    January 2013 court decision in NRDC v. EPA. As explained in the
proposed rule, the EPA's approach to the evaluation and regulation of
PM2.5 precursors pursuant to subpart 1 in both the 2007
PM2.5 Implementation Rule and the 2008 PM2.5 NSR
Rule was invalidated in the court's 2013 decision in NRDC v. EPA. As an
example of the distinction between the divergent substantive
requirements of subpart 1 and subpart 4 of part D of the CAA, the court
noted that subpart 4 has specific provisions related to regulation of
precursors not present in subpart 1. Although the court stated that it
was not reaching a decision on the issue of regulation of precursors,
the court's opinion specifically discussed the approach to precursors
in both the 2007 PM2.5 Implementation Rule and the 2008
PM2.5 NSR Rule and compared that approach to section 189(e)
of the CAA, which contains the sole explicit reference to the
regulation of precursors in subpart 4. The court decision included the
following statements with regard to precursors:

    Ammonia is a precursor to fine particulate matter, making it a
precursor to both PM2.5

and PM10. For a PM10 nonattainment area
governed by subpart 4, a precursor is presumptively regulated. See
42 U.S.C. 7513a(e) [CAA section 189(e)]. Under the PM rules
challenged here, the EPA established a rebuttable presumption
against regulating ammonia unless a state or the EPA ``provides an
appropriate technical demonstration'' that shows emissions from
ammonia ``significantly contribute to PM concentration in the
nonattainment area.'' 40 CFR 51.1002(c)(4)(i). When Congress enacted
subpart 4, it sought to end this administrative gamesmanship.\40\
---------------------------------------------------------------------------

    \40\ NRDC v. EPA, 706 F.3d 428, 437, n.7 (D.C. Cir. 2013).

    The court continued to hold that ``[i]n light of our disposition,
we need not address the petitioners' challenge to the presumptions in
40 CFR 51.1002(c)(3)-(4) that volatile organic compounds and ammonia
are not PM2.5 precursors, as subpart 4 expressly governs
precursor presumptions.'' \41\
---------------------------------------------------------------------------

    \41\ NRDC v. EPA, 706 F.3d 428, 437, n.10 (D.C. Cir. 2013).
---------------------------------------------------------------------------

    Section 189(e) of the CAA establishes requirements for precursors
to PM10 (which the court concluded expressly includes
PM2.5) and provides that: ``The control requirements
applicable under plans in effect under this part for major stationary
sources of PM10 shall also apply to major stationary sources
of PM10 precursors, except where the Administrator
determines that such sources do not contribute significantly to
PM10 levels which exceed the standard in the area.'' The
court reasoned that the EPA's approach to precursors in the 2007
PM2.5 Implementation Rule and 2008 PM2.5 NSR Rule
had the effect of reversing the presumption embodied within subpart 4
that a state should address all PM10 precursors unless the
state has made a specific showing why regulation of a particular
precursor is not necessary.
    Subpart 4 of part D of the CAA. The provisions of subpart 4 (CAA
sections 188-190) do not define the term ``precursor'' for purposes of
PM10, nor do they explicitly require the control of any
specifically identified PM precursor. However, the statutory definition
of ``air pollutant'' provides that the term ``includes any precursors
to the formation of any air pollutant, to the extent the Administrator
has identified such precursor or precursors for the particular purpose
for which the term `air pollutant' is used.'' See CAA section 302(g).
The EPA has determined that SO2, NOX, VOC and
ammonia are factual and scientific precursors to PM and, thus, the
attainment plan requirements of subpart 4 apply equally to emissions of
direct PM2.5 and these precursors in PM2.5
nonattainment areas, except as otherwise provided in the statute.
Section 189(e) of the CAA explicitly requires the control of precursors
from all major stationary sources in PM2.5 nonattainment
areas unless there is a demonstration to the satisfaction of the
Administrator that such major stationary sources do not contribute
significantly to PM levels that exceed the standards in the
nonattainment area.\42\ Section 189(e) of the CAA contains the only
express exception to control requirements for PM precursors under
subpart 4.
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    \42\ The EPA notes that it previously had addressed the
requirements of subpart 4 for precursors, specifically within the
context of the requirements of CAA section 189(e), in the General
Preamble. See the Federal Register published on April 16, 1992 (57
FR 13498, 13539, 13541 and 13542).
---------------------------------------------------------------------------

    When Congress adopted the 1990 CAA Amendments, the NAAQS for
PM10 was in effect, but no standard for PM2.5 had
yet been established. At that time, it was understood that the
interaction of PM10 precursors in the atmosphere led to the
formation of PM10 in many areas. However, in some of the
PM10 nonattainment areas, air quality problems were caused
primarily by area sources emitting direct PM emissions (e.g., a
nonattainment area with numerous wood burning devices, or with
substantial sources of windblown coarse particles from construction
sites), and precursor emissions from major stationary sources were not
considered to make a significant contribution to the local
nonattainment problem. For cases such as these, CAA section 189(e)
provided a possible exception to the requirement to control all
PM10 precursors from major sources in a particular
nonattainment area.
    Consistent with past practice for implementation of the
PM10 NAAQS, the EPA proposed to interpret the control
requirements addressed by CAA section 189(e) to include RACM/RACT (and
additional reasonable measures) for Moderate nonattainment areas, BACM/
BACT (and additional feasible measures) for Serious nonattainment
areas, most stringent measures (MSM) (for Serious areas as applicable)
and NNSR on all major sources of precursors in the nonattainment areas.
The General Preamble indicates that consideration of precursors is
necessary for attainment plans, and it recognizes the specific
applicability of CAA section 189(e) to both existing and new major
stationary sources, including new and modified sources subject to NNSR
permitting requirements. Even though CAA section 189(e) only explicitly
contemplates exceptions to control requirements for PM2.5
precursors from major stationary sources in nonattainment areas, the
EPA believes that by analogy it has authority to promulgate regulations
that allow states to determine that it is not necessary to regulate
PM2.5 precursors from other sources in nonattainment areas
as well, under appropriate circumstances.
    While CAA section 189(e) expressly requires control of precursors
from major stationary sources, it is clear that subpart 4 and other CAA
provisions collectively require the control of direct PM2.5
and all PM2.5 precursors from all types of sources (i.e.,
stationary sources, area sources, and mobile sources) as may be needed
for the purposes of demonstrating attainment as expeditiously as
practicable in a given nonattainment area.\43\ Longstanding EPA
guidance for RACM has indicated that the state should inventory all
emissions of the relevant pollutants and precursors in the
nonattainment area, evaluate the available control measures for the
relevant pollutant and precursors to determine if such controls are
economically and technologically feasible, and then adopt those
measures that are deemed reasonably available and necessary in order to
attain the NAAQS as expeditiously as practicable.\44\ The EPA guidance
has also long indicated that the state must ensure that there is no
other collection of available control measures that if adopted would
advance the attainment date by at least 1 year.\45\ Section IV.D of
this preamble provides additional discussion on the development of
emissions inventories and the identification, adoption and
implementation of reasonably available control measures for
PM2.5 nonattainment areas, including a discussion particular
to wildfire and wildland prescribed fire found in Section IV.D.3.b.\46\
---------------------------------------------------------------------------

    \43\ See CAA requirements for states to demonstrate attainment
``as expeditiously as practicable'' (CAA section 188(c)(1); CAA
section 172(a)(2)).
    \44\ 57 FR 13498 (April 16, 1992).
    \45\ In the context of the PM10 NAAQS, the EPA has
concluded that ``advancement of the attainment date'' should mean an
advancement of at least 1 calendar year. See State Implementation
Plans; General Preamble for the Implementation of Title I of the CAA
Amendments of 1990, 57 FR 13498 (April 16, 1992). See also Sierra
Club v. EPA, 294 F.3d 155 (D.C. Cir. 2002).
    \46\ See Section IV of this preamble for a thorough discussion
of past policy and guidance on reasonably available control measures
(RACM) and reasonably available control technology (RACT). Section
IV of this preamble discusses the EPA's final policy that under
subpart 4, for Moderate areas that demonstrate that attainment by
the statutory attainment date is impracticable, RACM and RACT would
constitute all those technologically and economically feasible
measures available for sources in the area that can be implemented
within 4 years of designation, but they would not constitute the
complete set of measures required to demonstrate attainment as
expeditiously as practicable.

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

    In light of the court's decision in NRDC v. EPA, the EPA considers
it necessary to describe how states must address regulation of
PM2.5 precursors in attainment plans and NNSR programs for
the PM2.5 NAAQS. The court's decision made clear that
appropriate regulation of all precursors in designated nonattainment
areas is presumptively required under the CAA, and the regulation of
precursors in general is a critical issue for attainment of the
PM2.5 NAAQS because secondarily formed particles are a
substantial component of PM2.5 concentrations in most
nonattainment areas of the United States.
    For the purposes of this rule, the EPA considers that for all
PM2.5 nonattainment areas, the PM2.5 precursors
for regulatory purposes are the four scientific precursors that the EPA
has previously identified: SO2, NOX, VOC and
ammonia. This rule does not include any national presumption that would
allow a state to exclude, without a demonstration, sources of emissions
of a particular precursor from further analysis for attainment plan or
NNSR control requirements in a PM2.5 nonattainment area.
However, the EPA's interpretation of subpart 4 requirements with
respect to precursors in attainment plans for PM10, as set
out in the General Preamble, contemplates that the state may develop an
attainment plan that regulates only those precursors that are necessary
to control for purposes of timely attainment in the nonattainment area,
i.e., states may determine that only certain precursors need to be
regulated in a particular PM2.5 nonattainment area for
attainment purposes.\47\ Courts have upheld this approach to the
requirements of subpart 4 for PM10.\48\ The EPA believes
that application of a similar approach to PM2.5 precursors
under subpart 4 is appropriate and reasonable.
---------------------------------------------------------------------------

    \47\ See the Federal Register published on April 16, 1992 (57 FR
13498, 13540 and 13541).
    \48\ See, e.g., Assoc. of Irritated Residents v. EPA, et al.,
423 F.3d 989 (9th Cir. 2005).
---------------------------------------------------------------------------

    The EPA interprets the CAA to require states to inventory emissions
and adopt control measures as appropriate for direct PM2.5
and all PM2.5 precursors. This interpretation is based on
CAA section 302(g), which defines an air pollutant as including
precursors contributing to the formation of that pollutant; the EPA's
identification of the four main scientific PM2.5 precursors;
and the CAA provisions requiring adoption of all control measures
(i.e., RACM and RACT) needed in order to attain the standard as
expeditiously as practicable. CAA section 189(e) explicitly requires
that the control requirements applicable for major stationary sources
of direct PM2.5 emissions must also apply to major
stationary sources of PM2.5 precursors, unless the state
provides a showing that emissions of a particular precursor from major
stationary sources do not contribute significantly to levels that
exceed the standard in the nonattainment area of concern. Thus, the
statute generally requires control of all PM2.5 precursors
in a nonattainment area, but it provides an express exception
applicable to major stationary sources in such areas if an appropriate
demonstration is made.
    The EPA also notes that CAA section 189(e) contains certain
ambiguities that require interpretation. For example, CAA section
189(e) does not specify the precise method by which a state or the EPA
should determine whether precursor emissions from major stationary
sources do not ``contribute significantly'' to levels which exceed the
standard in a given nonattainment area. Subpart 4 also does not
explicitly address whether it would be appropriate to include a
potential exemption from precursor controls for all source categories
under certain circumstances, because a specific exemption from
precursor controls is expressly made available in the statute only for
major stationary sources. These issues are addressed in this final
rule.

B. Summary of Proposal

    In the proposal, the EPA sought comment on how states could focus
regulatory efforts on the appropriate PM2.5 precursors in
each area. Rather than simply requiring each state to regulate direct
PM2.5 and all PM2.5 precursors without regard to
whether that would be appropriate and necessary for expeditious
attainment of the NAAQS, EPA took comment on different approaches for
states to focus regulatory efforts on the appropriate pollutants. Thus,
in the proposal, the EPA sought comment on three options by which a
state could demonstrate that emission control requirements for a
particular PM2.5 precursor or precursors would not be
required for sources in a particular nonattainment area.\49\ The
proposed ``precursor demonstration'' options outlined procedures and
technical analyses a state could elect to perform to demonstrate that
control requirements for sources of a particular precursor are not
needed for expeditious attainment, or that a particular
PM2.5 precursor does not significantly contribute to
PM2.5 concentrations in the area. The proposal indicated
that if the EPA were to approve such a precursor demonstration, then it
would not be necessary for the state to adopt control requirements for
sources of the precursor or precursors in the PM2.5
attainment planning process generally and/or in the NNSR permitting
process for that particular area. The EPA requested comment on whether
the final rule should include one or more precursor demonstration
approaches, and whether it would be appropriate to combine specific
elements from different options.
---------------------------------------------------------------------------

    \49\ The three proposed options were: (1) Option 1--two
independent analyses consisting of an attainment planning analysis
showing that control measures for a particular precursor are not
needed for expeditious attainment and an optional NNSR analysis
showing that major stationary sources of a particular precursor do
not contribute significantly to levels that exceed the
PM2.5 standard, (2) Option 2--a single analysis (for
purposes of attainment planning and NNSR) showing that all emissions
of a particular precursor do not contribute significantly to levels
that exceed the PM2.5 standard, and (3) Option 3--a
single analysis (for purposes of attainment planning and NNSR)
showing that control measures for all sources for a particular
precursor are not needed for expeditious attainment.
---------------------------------------------------------------------------

    The EPA also described three technical issues associated with any
such precursor demonstration and sought comment on the following: (1)
The appropriate geographic scope of the analysis; (2) whether specific
types of technical analyses (such as evaluating the contribution of the
precursor to total PM2.5 concentrations, or evaluating the
sensitivity of the area to decreases or increases of the precursor)
should be required for a precursor demonstration; and (3) whether the
EPA should establish a bright-line ambient air quality threshold (e.g.,
3 percent of the level of the relevant NAAQS in the area) to define an
air quality change below which a precursor contribution should not be
considered to be significant, thereby establishing that control of
sources of the precursor is unnecessary in the area.
    Lastly, the EPA indicated in the proposal that if a state had an
approved precursor demonstration for a particular precursor in a
Moderate area and the EPA later reclassifies the area to Serious, then
the state would be required to develop an updated precursor
demonstration if the state were again interested in having the
precursor treated as insignificant for purposes of the Serious area
plan. An updated precursor demonstration is necessary because many
factors (e.g., emissions, air quality and fine particle concentrations)
could have changed substantially since the original

demonstration for the Moderate area attainment plan.\50\
---------------------------------------------------------------------------

    \50\ For more information on the proposed precursor
demonstration options, see 80 FR 15340, at 15350-15362.
---------------------------------------------------------------------------

C. Final Rule

    The EPA received many comments on the three proposed precursor
demonstration approaches. Most commenters supported the inclusion of
some kind of optional precursor demonstration in the final rule. Some
commenters suggested that states should have the flexibility to develop
any of the types of demonstrations that the EPA described in the three
proposed options. One group of commenters opposed any option that would
exempt a particular precursor from control measures even if the state
could demonstrate it could expeditiously attain the standard by the
attainment date without controls on sources of the precursor. Another
group of commenters suggested that if only one option is finalized, it
should allow a state to rely on a sensitivity analysis to show that
changes in emissions of a particular precursor would not have a
substantial contribution to PM2.5 concentrations in the
area.
    The EPA agrees with commenters who suggested that states should
have the flexibility to conduct different types of precursor
demonstrations appropriate to the area in question. Regardless of the
type of precursor demonstration, the state will still need to provide
adequate technical support and that demonstration will be subject to
EPA approval. Thus, the EPA concludes that the specific form of the
demonstration is not as crucial as its content and adequacy, in light
of the facts and circumstances in the area. The EPA disagrees with
commenters who argued that a state should not be able to determine
insignificance for a precursor based on an attainment planning analysis
showing expeditious attainment in the area without adopting new
emissions reduction measures for the precursor in question. This
approach has been upheld under subpart 4 with respect to implementation
of the PM10 NAAQS, and the EPA finds that it is reasonable
to allow for a similar policy when implementing the PM2.5
NAAQS.\51\
---------------------------------------------------------------------------

    \51\ See, e.g., Assoc. of Irritated Residents v. EPA, et al.,
423 F.3d 989 (9th Cir. 2005).
---------------------------------------------------------------------------

    After consideration of the numerous comments received on this
issue, the EPA has decided to adopt a final approach that allows
exclusion of certain precursor sources from certain SIP requirements,
provided that states make the appropriate demonstrations. However, the
EPA has revised the details of the specific types of demonstrations
based on further evaluation of the comments received. Section III.C.1
of this preamble describes the expeditious attainment demonstration, in
which a state shows that control requirements for a particular
precursor are not needed for expeditious attainment by the Moderate
area attainment date. Section III.C.2 of this preamble describes the
three types of optional precursor demonstrations a state may submit to
the EPA to establish that emissions of a precursor do not contribute
significantly to PM2.5 levels in a particular nonattainment
area: (a) Comprehensive precursor demonstration; (b) major stationary
source precursor demonstration; and (c) NNSR precursor demonstration.
Each option is described in detail in the following subsections.
    Section III.C.3 of this preamble highlights various technical
issues associated with precursor demonstrations, including the
appropriate geographic scope of the analyses, thresholds for
characterizing an insignificant air quality change, and different
analytical methods for assessing precursor contributions. Section
III.C.4 of this preamble discusses certain procedural issues associated
with precursor demonstrations. Section III.C.5 of this preamble
addresses other relevant comments and responses.
1. Expeditious Attainment Demonstration
    As noted earlier, the EPA's interpretation of subpart 1 and 4
requirements with respect to precursors in attainment plans for
PM10 has been that a state may develop an attainment plan
that regulates only those precursors that are necessary to control for
purposes of timely attainment in the area. The EPA believes that a
similar policy approach for PM2.5 precursors is also
appropriate.
    Under the expeditious attainment demonstration, a state may be able
to determine through its identification of RACM/RACT for existing
sources in an area whether expeditious attainment could be achieved
without new control measures for a particular PM2.5
precursor. It is important to note that this approach is available to a
state only if the demonstration for the area (1) ensures attainment by
the Moderate area attainment date (i.e., the end of the sixth calendar
year after designation), and (2) ensures that the area could not
advance the attainment date by at least 1 year if it were to adopt
reasonable control measures for the precursor in question. If the state
determines that the area cannot practicably attain by the relevant
Moderate area attainment date, then the state still would have the
option of developing one of the precursor demonstrations described in
Sections III.C.2.a-c of this preamble for showing that the precursor
contribution is not significant. The expeditious attainment option is
not available for Serious nonattainment areas because BACM/BACT
measures for Serious areas are not solely limited to those measures
needed for expeditious attainment under this final rule. (See further
discussion of this issue in Section VI.D of this preamble, Serious Area
Attainment Plan Control Strategies.)
    For the expeditious attainment demonstration, the required analysis
is what is already needed for a Moderate area attainment demonstration:
The identification of reasonably available control measures that
provide for expeditious attainment by the attainment date, and a
determination that attainment cannot be advanced through the imposition
of other reasonable measures (i.e. RACM/RACT and other reasonable
measures that are identified for the area but not necessary for the
area to attain within 6 years). See 40 CFR 51.1006(a). After a
comprehensive emissions inventory has been developed, the state should
then identify potential control measures and assess factors related to
technological feasibility, economic feasibility, and time needed for
implementation for all types of sources in the area (i.e., stationary,
area, mobile) and all precursors emitted by such sources as included in
the emissions inventory.
    After identifying the set of control measures that are economically
and technologically feasible for all precursors, the state may be able
to show (using best available information on emissions, control
options, technologies, and costs, along with appropriate air quality
modeling) that those measures that could be identified as RACM/RACT and
additional reasonable measures would not need to include new control
measures for sources of a given precursor.\52\ The state could show
this by demonstrating that one set of control measures to be adopted
into the plan would provide for attainment by the statutory attainment
date; and that an additional set of potential control measures
(including measures for the precursor in question, and remaining
measures for all other contributing pollutants) collectively

would not advance the attainment date by at least 1 year (i.e., enable
the area to attain 1 year earlier). Under these circumstances, the
state would not need to adopt the second set of measures (including
measures for the particular precursor) because they would not expedite
attainment of the NAAQS in the area.
---------------------------------------------------------------------------

    \52\ See Section IV.D.1 of this preamble, Background for
Attainment Plan Control Strategy, for further discussion of
``additional reasonable measures.''
---------------------------------------------------------------------------

    If the attainment planning demonstration shows that the area can
attain the NAAQS expeditiously without new emission reduction measures
for a particular precursor, the state would be required to adopt
control measures for only a subset of the four PM2.5
precursors as part of the attainment plan for the area, and existing
sources in the nonattainment area would not be required to adopt new
control measures for the particular precursor. Accordingly, the state
would not need to address the precursor in the reasonable further
progress plan, in quantitative milestones and associated reports, or be
required to adopt contingency measures to reduce the precursor. See 40
CFR 51.1009(a)(4)(i). (Note that for purposes of meeting the
contingency measure requirement, however, the state would still have
the discretion to adopt control measures as contingency measures for a
precursor that would otherwise not be subject to RACM/RACT
requirements.)
    It also should be noted that development of an approvable
attainment plan that does not include new control measures for a
particular precursor would not exempt the state from the requirements
to address the same precursor with respect to the NNSR program, nor
would it excuse the state from reconsidering the significance of the
precursor to the PM nonattainment problem in any subsequent Serious
area SIPs that could be required for the nonattainment area.
2. Optional Precursor Demonstrations
    a. Comprehensive Precursor Demonstration. In line with the EPA's
proposal for precursor insignificance demonstrations, the EPA is
finalizing an option whereby a state may submit a comprehensive
precursor demonstration as part of any Moderate or Serious area
attainment plan. The use of the term ``comprehensive'' here refers to
the fact that the demonstration covers all existing stationary, area,
and mobile sources, rather than major sources alone. Note, however,
that the comprehensive precursor demonstration does not affect
precursor requirements for future new sources. Under this comprehensive
precursor demonstration, the state would need to show that emissions of
a particular precursor from all existing stationary, area, and mobile
sources located in the nonattainment area do not contribute
significantly to PM2.5 levels that exceed the standard in
the area. The state would first need to evaluate the contribution of
all existing source emissions of the particular precursor to
PM2.5 concentrations that exceed the PM2.5
standard (described in Section III.C.2 of this preamble). If the state
cannot demonstrate via the concentration-based precursor demonstration
that sources of a particular precursor have an insignificant
contribution to PM2.5 levels in an area,, then the state
could still demonstrate that the precursor's contribution is
insignificant by conducting a sensitivity analysis to evaluate the
sensitivity of ambient PM2.5 concentrations in the
nonattainment area to decreases in the precursor emissions in the area
(e.g., whether a given decrease is insignificant) as discussed further
in Section III.C.2.c of this preamble.
    If a comprehensive precursor demonstration for a precursor is
approved, the state would not establish a motor vehicle emissions
budget for the relevant precursor, and regional emissions analyses for
the precursor would not be required to be included in transportation
conformity determinations. This is consistent with the transportation
conformity rule's provisions concerning PM2.5 precursors.
(See 40 CFR 93.102(b)(2)(iv) and (v)). Separately, states may continue
to determine that on-road emissions of PM2.5 precursors are
insignificant even if emissions of a given precursor from other sources
are significant. (See 40 CFR 93.102(b)(2)(iv) and (v) and 93.109(f)).
With regard to general conformity, if a state precursor demonstration
is approved for one or more precursors, federal agencies would not be
required to address the affected precursor(s) in general conformity
determinations.
    If a comprehensive precursor demonstration is approved by the EPA,
then in developing the attainment plan for the area, the state would
not be required to adopt control measures (e.g., RACM/RACT) for the
precursor for any existing stationary, area, or mobile sources in the
nonattainment area. The attainment plan also would not be required to
address the relevant precursor in meeting the RFP or quantitative
milestone requirements, or in adopting contingency measures because
these requirements commonly apply to pollutants that are the subject of
emission reduction measures in the attainment plan. (Note that for
purposes of meeting the contingency measure requirement, however, the
state would still have the discretion to adopt emission reduction
requirements on the precursor in question, in conjunction with emission
reduction requirements on other pollutants.) The state would still need
to include the precursor in all nonattainment area emission inventory
submissions.
    It also should be noted that development of an approvable
attainment plan that does not include new control measures for a
particular precursor would not exempt the state from the requirements
to address that precursor with respect to the NNSR program, nor would
it excuse the state from reevaluating the significance of the precursor
to the PM nonattainment problem in any subsequent Serious area SIPs
that could be required for the nonattainment area.
    b. Major Stationary Source Precursor Demonstration. The state has
the option of submitting a major stationary source precursor
demonstration as part of any Moderate or Serious area plan, consistent
with CAA section 189(e). This demonstration differs from the
comprehensive demonstration in that it only evaluates existing major
sources, and therefore may only be used to justify the exclusion of
existing major sources from the control requirements for the applicable
precursor. Although the EPA expects that most states making precursor
demonstrations will opt for comprehensive demonstrations, this option
is provided to offer additional flexibility. The requirements for a
stationary source precursor demonstration are nearly identical to those
of the comprehensive precursor demonstration, except the state would
only need to show that a particular precursor from all existing major
stationary sources located in the nonattainment area do not contribute
significantly to PM2.5 levels that exceed the standard in
the area. Similar to the comprehensive demonstration, the state must
first evaluate the contribution of major stationary source emissions of
the particular precursor to PM2.5 levels that exceed the
PM2.5 standard (pursuant to section III.C.3.c of this
preamble). If the state cannot demonstrate via the concentration-based
precursor demonstration that sources of a particular precursor have an
insignificant contribution to PM2.5 levels in an area, then
the state could still try to demonstrate that the precursor is
insignificant by conducting a sensitivity analysis to evaluate the
sensitivity of PM2.5 levels in the nonattainment area to a
reduction in major stationary source

emissions in the area (pursuant to Section III.C.3.d of this preamble).
    If such a demonstration is approved by the EPA, then in developing
the attainment plan for the area, the state would not be required to
adopt control measures for the precursor for existing major stationary
sources in the nonattainment area. The attainment plan also would not
be required to address the emissions of the relevant precursor from
major stationary sources in meeting the RFP or quantitative milestone
requirements, or in adopting contingency measures. (Note that for
purposes of meeting the contingency measure requirement, however, the
state would still have the discretion to adopt emission reduction
requirements on the precursor in question, in conjunction with emission
reduction requirements on other pollutants.) The state would still need
to include stationary source emissions of the precursor in all
nonattainment area emission inventory submissions.
    Note that a state might consider developing a major stationary
source demonstration to avoid the requirement to adopt nonattainment
planning control measures for a particular precursor emitted from
existing major stationary sources in the area if the state does not
believe that it could comprehensively demonstrate that the precursor
does not have a significant contribution, and if major stationary
source emissions of the precursor do not make up a very large
percentage of the emissions inventory in the area. For example, it
might be possible that in a particular area the overwhelming amount of
emissions of a certain precursor could originate from mobile or area
sources, or both, but not from existing major stationary sources. If
the EPA approves a major stationary source precursor demonstration, the
attainment plan would still need to evaluate and potentially impose
control requirements for the relevant precursor for existing non-major
stationary sources, area sources and mobile sources in order to
demonstrate expeditious attainment.
    It also should be noted that development of an approvable
attainment plan that does not include new control measures for a
particular precursor would not exempt the state from the requirements
to address that precursor with respect to the NNSR program, nor would
it excuse the state from the requirement to evaluate and adopt control
measures for the precursor in any subsequent Serious area SIPs that
could be required for the nonattainment area.
    c. NNSR Precursor Demonstration. The state also has the option of
submitting a NNSR precursor demonstration as part of any Moderate or
Serious area plan. This specific type of precursor demonstration is the
only one of the three demonstrations described in this section that if
approved would exempt new and modified major stationary sources of a
precursor from regulation under the NNSR permitting program.
    Under the NNSR precursor demonstration, the state would need to
conduct an analysis to evaluate the sensitivity of PM2.5
levels in the nonattainment area to an increase in emissions of a
particular precursor in the area, simulating the response of the
atmosphere (and associated PM2.5 concentrations) to the
addition of one or more new or modified stationary sources in the
nonattainment area (see Section III.C.3.d of this preamble). Section
III.C.3 of this preamble addresses additional issues related to
technical analyses for precursor demonstrations.
    The EPA believes that this approach to interpreting CAA section
189(e) of the statute as it applies to control requirements for the
NNSR program is appropriate because (1) an analysis that evaluates the
sensitivity of the atmosphere in an area to increases in emissions
would most closely replicate the scenario of concern, where precursor
emissions from new major stationary sources or major modifications are
added to the existing inventory for the area; and (2) this approach
would take into consideration the specific atmospheric chemistry and
emissions profile that varies from area to area. For example, one
nonattainment area may have low emissions of a particular precursor
from all existing sources (and corresponding low current ambient
contributions from the precursor), but the introduction of a new major
stationary source of emissions of that particular precursor could in
some cases significantly contribute to the ambient PM2.5
levels in the area because other pollutants with which the precursor
reacts in the atmosphere could be relatively abundant.
    For purposes of the NNSR precursor demonstration, the state is not
required to first evaluate the contribution of existing major sources
to PM2.5 levels that exceed the standard in the area, as
would be required by the comprehensive and major stationary source
demonstrations. Since NNSR permitting requirements do not apply to
existing sources (unless such sources engage in a major modification),
the EPA does not believe it is necessary or reasonable to require
evaluation of current emissions from existing major stationary sources
as it would not inform the question of whether increases in emissions
would significantly contribute to PM2.5 levels in the area.
Note, however, that the NNSR precursor demonstration is used only to
justify an exclusion of sources of the precursor from the NNSR control
requirements in the area. A state would need to pair the NNSR precursor
demonstration with another type of precursor demonstration to address
control requirements beyond NNSR, as described previously for each type
of demonstration.
3. Technical Issues Associated With Precursor Demonstrations
    a. Geographic Area. The proposal indicated that the emissions
inventory to be used as the starting point for the comprehensive, major
stationary source, and NNSR precursor demonstrations should represent
emissions from sources located in the nonattainment area, and the final
rule remains unchanged. The EPA believes that limiting the emissions
inventory for these analyses to sources in the nonattainment area is
appropriate based on the statutory construction of CAA section 189(e),
in which the relevant test is whether ``such sources contribute
significantly to [PM2.5] levels which exceed the standard in
the area.'' The EPA believes that a reasonable interpretation is that
this provision applies to sources in the nonattainment area.
    b. Significance Threshold. The proposal described the concept of
including a bright-line threshold of 3 percent of the relevant NAAQS in
the rule for precursor demonstrations other than the expeditious
attainment approach, such that if an air quality contribution was found
not to exceed the threshold amount, then it would not be considered
significant. The proposal also included an option for no bright-line
threshold in the final rule, based on the recognition that all
nonattainment area situations are different.
    Some commenters supported the bright-line threshold concept, but
they suggested thresholds across a broad range, from less than 1
percent of the relevant NAAQS, to up to 5 percent. Some commenters
stated that inclusion of a bright-line threshold of 3 percent of the
relevant NAAQS was preferred because without such a threshold, states
would be unsure about whether their proposed precursor assessment would
be acceptable. Other commenters supported having no bright line
threshold because the circumstances of each area are unique, and for
that reason

each area should be considered on a case-by-case basis.
    The EPA found merit in comments supporting both proposed options.
The EPA agrees that an insignificance threshold can help avoid
situations where lack of clarity may lead to delays in the EPA
assessment of precursor demonstrations. At the same time, the EPA
understands that PM2.5 nonattainment problems are complex
and vary greatly based on the facts and circumstances of each area.
    After considering the range of comments on this issue and the
complexity of the types of analyses that may be conducted for precursor
demonstrations, the EPA has decided that the best approach is for the
final rule to codify the availability and basic requirements for
precursor demonstrations, but to provide technical details (such as a
recommended approach for assessing whether a particular air quality
concentration threshold can be considered to be insignificant in a
given area) in guidance supporting this final rule.
    c. Concentration-based Contribution Analysis. The first type of
analysis required for the comprehensive precursor demonstration (or,
less commonly, the major stationary source precursor demonstration) is
an existing source contribution analysis that would demonstrate whether
emissions of a particular precursor from all existing sources (or, for
a major source precursor demonstration, emissions from existing major
sources) in the nonattainment area do not significantly contribute to
PM2.5 concentrations that exceed the standard in the area.
The state should use technically credible approaches for estimating the
ambient contribution of emissions of a particular precursor to total
PM2.5 concentration in the nonattainment area. The EPA
anticipates that the forthcoming technical guidance will discuss the
possible use of advanced air quality modeling tools to estimate
precursor contributions to total PM2.5 concentrations in an
area. For example, several photochemical air quality models (e.g.,
Community Multi-Scale Air Quality Model (CMAQ) and the Comprehensive
Air Quality Model with Extensions (CAMx)) can be used to quantify the
contributions of precursor emissions to PM2.5 concentrations
in the area.\53\
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    \53\ For more information on CMAQ, see http://www.epa.gov/air-research/community-multi-scale-air-quality-cmaq-modeling-system-air-quality-management. For more information on CAMx, see http://www.camx.com/.
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    Other techniques such as the analysis of chemical speciation data
and emissions inventories also may be appropriate for determining the
contribution of a particular precursor to PM2.5
concentrations. For example, SO2 emissions and measured
sulfate concentrations (in the form of ammonium sulfate or other forms)
may be small in a particular nonattainment area. A simple analysis of
measured species concentrations (attributable to a particular
precursor) combined with nonattainment area emissions and other
relevant data analyses may be sufficient to show that a precursor does
not contribute significantly to PM2.5 concentrations in the
area.
    d. Sensitivity-based Contribution Analysis. A second type of
analysis may also be used in developing comprehensive precursor
demonstrations (or, less commonly, major source precursor
demonstrations). This type of analysis is a sensitivity-based
contribution analysis that would demonstrate the degree to which
concentrations in the nonattainment area are sensitive to decreases of
a precursor. Changes in PM2.5 concentrations at a particular
location often will not be linear with respect to changes in
PM2.5 precursor emissions; therefore, sensitivity analyses
are useful for better understanding the complexity and variability of
the atmospheric chemistry affecting PM2.5 concentrations in
different areas across the country. A sensitivity-based contribution
analysis evaluating the effect of precursor emissions reductions could
be used in the event the state cannot demonstrate via the
concentration-based analysis that sources of a particular precursor
have an insignificant contribution to PM2.5 levels in an
area.
    The EPA also requires a sensitivity-based analysis as the means for
conducting the NNSR precursor demonstration. In this case, in contrast
to the assessment of decreases described for the comprehensive (or
major source) precursor demonstration for existing sources, the
appropriate sensitivity analysis is one that evaluates the impact of
precursor emissions increases--without the need for a separate
evaluation of existing source contribution to PM2.5
concentrations. This analysis is clearly most appropriate for NNSR,
which is a program that governs emissions increases. Thus, the final
rule requires that such an analysis must be used if a state chooses to
submit a NNSR precursor demonstration.
    The EPA states in the final rule that a sensitivity-based analysis
is an appropriate approach for understanding whether emissions of a
precursor make an insignificant contribution to PM2.5 levels
in an area. Several main components of PM2.5 are secondarily
formed in the atmosphere and are the result of chemical reactions
between various PM2.5 precursors. In some areas, one
precursor may be abundant while a second precursor, with which it
primarily reacts, may be less abundant. In such cases, a sensitivity
analysis may find that reducing emissions of the second, less abundant
precursor (the ``limiting'' precursor) may be generally more effective
for reducing PM2.5 concentrations. It may also find that
increasing emissions of the less abundant precursor may be more
effective at increasing PM2.5 concentrations than a
comparable tonnage increase of a more abundant precursor.
    In another type of area, the PM2.5 concentrations that
exceed the standard may be commonly dominated by primary
PM2.5 emissions rather than by secondarily formed
PM2.5. In such an area, a sensitivity analysis may be able
to demonstrate that sources of a particular precursor in the
nonattainment area do not contribute significantly to PM2.5
levels that exceed the standard, and that the potential air quality
improvement from reducing emissions of the precursor in the area may be
limited.
    Thus, the most effective precursor strategies for reducing
PM2.5 concentrations as part of attainment planning will
vary from area to area, depending upon which specific precursors play a
role in forming or limiting PM2.5 formation in the
particular area. The EPA therefore believes that it is a reasonable
interpretation of the statute to allow a precursor to be excluded from
control requirements if the PM2.5 concentration in the area
is insensitive to decreases of that precursor.
    For states that choose to develop an optional precursor
demonstration, the final rule provides that in addition to the basic
requirement to do a concentration-based contribution analysis, the
state may choose to develop a sensitivity-based contribution analysis
evaluating potential emissions reductions for either a comprehensive
precursor demonstration or a major stationary source demonstration
intended to show that emissions reductions of the particular precursor
are not effective in reducing PM2.5 levels that exceed the
standard in the area. As noted previously, the EPA expects to recommend
approaches for assessing whether a particular air quality concentration
threshold can be considered to be insignificant in a given area. If a
concentration-based contribution analysis conducted for

either a comprehensive precursor demonstration or a major stationary
source precursor demonstration shows that the contribution from a
precursor is less than a particular threshold which may be considered
insignificant at each PM2.5 monitor in the area, then the
EPA could approve the concentration-based contribution analysis.
However, if a concentration-based contribution analysis cannot be
approved (e.g., shows that the contribution of a precursor to
PM2.5 levels in the area is not less than such a threshold
at one or more monitors), then the overall precursor demonstration
still could be approved, but only if the state also provides an
appropriate sensitivity-based contribution analysis. If the
sensitivity-based contribution analysis shows that the reduction in
PM2.5 concentration at each PM2.5 monitor
resulting from an emission reduction level that would not exceed such a
threshold, then the EPA could approve the overall precursor
demonstration, and the state would not be required to adopt control
requirements for the precursor or address the precursor for attainment
planning purposes.
    In evaluating whether it would be appropriate to exclude sources of
any precursors from NNSR regulation in a nonattainment area, it is
important to understand the sensitivity of the atmosphere to potential
increases in precursor emissions that could result from major source
growth (from both new sources and major modifications at existing major
sources) in the nonattainment area. For example, in some circumstances,
adding a few hundred tons of a ``less abundant'' precursor to an area
could result in a significant increase in PM2.5
concentrations even if there are currently very few existing major
sources of the precursor in the area. In contrast to the emissions
reduction analyses described for attainment planning purposes,
sensitivity analyses that consider the effect of potential emissions
increases of a particular precursor in the nonattainment area will help
the state and the EPA to understand the potential response of
PM2.5 concentrations to increased emissions in the area in
order to assess whether the contribution from such increases is not
significant under CAA section 189(e). In assessing whether a state
precursor demonstration (i.e., for attainment planning or for NNSR) can
be approved, the EPA will consider the air quality changes estimated in
the state's technical sensitivity analyses, their relationship to
thresholds developed under any EPA-recommended approaches (including
any thresholds that EPA may recommend), and any other information
presented by the state.
4. Procedural Considerations
    a. Consultation and Public Review. The EPA anticipates that a
state's development of an approvable PM2.5 precursor
demonstration will require a substantial level of effort and
consultation with the EPA. Such a demonstration by the state would
likely involve technically rigorous and complex analyses, such as air
quality modeling and ambient data analyses. Accordingly, the EPA
strongly recommends that any state that is considering limiting the
applicability and associated control strategy decisions only to
specific precursors, either for the attainment plan, for the NNSR
permitting program, or for both, should develop a precursor
demonstration early in the attainment plan development process. The EPA
is committed to consulting with states on designing technically
appropriate precursor demonstrations consistent with EPA technical
guidance. If a state chooses to develop a precursor demonstration, it
must be submitted to the appropriate EPA regional office no later than
the date of submission of the relevant attainment plan or NNSR program
revision; an earlier submission is preferable. For example, if a state
submits the Moderate area plan elements no later than 18 months from
the date of designation (as discussed in Section IV.A of the preamble),
it should submit any precursor demonstration no later than this same
date. In its review of any precursor demonstration provided by a state,
the EPA will consider all relevant information.
    The critical first step in any precursor analysis is the
development of a comprehensive inventory of all precursor emissions in
the nonattainment area. A state will not be able to reasonably
determine whether reductions of a given PM2.5 precursor are
needed for expeditious attainment, or whether sources of such precursor
are insignificant contributors to PM2.5 levels above the
standard in an area, unless the state has adequately accounted for all
nonattainment area emissions in its emissions inventory. (See section
IV.B of this preamble for more details on emission inventory
requirements.)
    In the preamble to proposed rule, we indicated that if a state
developed a precursor demonstration as part of its draft attainment
plan or NNSR program submission, then in accordance with the state
rulemaking process, the demonstration would be subject to public review
at the state level. We also stated that, as required under any
rulemaking process, the state had to consider and provide a response in
the rulemaking record to any information or evidence brought forward by
commenters during the state's SIP planning, development and review
process. By ensuring that this important issue was explicitly addressed
and supported in any attainment plan or NNSR program revision submitted
to the EPA, the EPA could better evaluate the precursor demonstration
in accordance with its obligations under the CAA. The EPA believes
these are sound procedural steps for a state rulemaking process, and
the final rule includes similar language requiring public review of any
proposed precursor demonstration.
    If a state chooses to develop a comprehensive precursor
demonstration or major stationary source precursor demonstration for a
nonattainment area, it must submit a concentration-based contribution
analysis and, if applicable, a sensitivity-based contribution analysis
conducted for the area. In cases where a sensitivity-based analysis was
developed the concentration-based analysis must also still be
submitted. Although the rule clearly provides that the precursor
demonstration requirement may still be satisfied in such cases, the
information in the concentration-based analysis will help inform review
of the overall demonstration by the EPA. Similarly, the data from the
concentration-based analysis should be available in the public record
because it will help inform the review of the overall precursor
demonstration by the public. See 40 CFR 51.1006(b).
    b. Precursor Demonstration to be Reevaluated for Each New State
Implementation Plan. There may be situations where the EPA approved a
Moderate area plan that excluded a precursor from regulation from one
or more requirements based on an approvable precursor demonstration,
and then the area is reclassified as a Serious area, triggering an
additional plan submission requirement. (Section V of this preamble
provides additional detail on reclassification of areas from Moderate
to Serious under subpart 4.) In addition, an area that had been
reclassified as Serious later may be required to submit one or more
additional SIPs if it obtains an extension of the Serious area
attainment date, or if it fails to attain the standard by the end of
the tenth year after designation. For a state seeking to continue a
precursor exclusion in a subsequent attainment plan or NNSR program

submission, the final rule requires the state to assess the
appropriateness of continuing the exclusion by providing a new
precursor demonstration updated to reflect the type of plan and the
conditions in effect when the new plan is submitted.
    When an area is reclassified to Serious, existing sources of all
PM2.5 precursors in the area are again presumptively subject
to evaluation for BACM/BACT control measures and potential future
control requirements, unless a new precursor demonstration is developed
and approved as part of the Serious area plan. As noted in the
discussion of the provisions for excluding sources of precursors from
certain Moderate area requirements based on an expeditious attainment
demonstration, this option is not available for Serious areas.
Accordingly, if the state seeks to submit an updated precursor
demonstration for a Serious area, at this stage it must submit a
comprehensive, major stationary source, or NNSR precursor
demonstration. Regardless of the type of demonstration(s) provided in
the Moderate area plan, the final rule requires that the state must
submit a reevaluated and updated precursor demonstration for the
Serious area plan. The reason for this is that the Serious area plan
would be due several years after the submission of a state's original
precursor demonstration, and over that period, substantial emissions
changes could have occurred that might call into question the basis of
the previous precursor demonstration. In addition, because the area
failed to attain by the Moderate area attainment date, it is reasonable
and appropriate to require the state to reconsider and update its prior
precursor demonstration. The final rule also requires similar updates
for each successive plan beyond the initial Serious area plan (such as
a revised Serious area plan for an area that fails to attain by the end
of the tenth calendar year after designation). The EPA recommends that
in developing a revised precursor demonstration, the state should
consider changes in a number of factors, including: Changes in
emissions inventory levels due to implementation of control programs,
growth in emissions, and changes in emission estimation methodologies;
recent ambient air quality concentrations; fine particle composition
and the sensitivity of the atmosphere to increases and decreases of
different precursors; advances in technical tools and modeling
techniques to assess the effectiveness of precursor reductions; and
advances in control technologies and emission reduction programs.
5. Comments and Responses.
    Comment: With regard to whether the existing source contribution
analysis or the sensitivity-based contribution analyses should be
required if a state opts to submit a precursor demonstration, a number
of commenters supported only the sensitivity analysis because they
believed the analysis would help identify the control measures that are
most effective at reducing PM2.5 concentrations. Some
commenters noted that conducting a ``zero-out'' analysis (i.e.,
simulating the change in atmospheric chemistry and PM2.5
concentrations due to a hypothetical removal of 100 percent of the
emissions of a precursor from the inventory) is not appropriate for a
sensitivity analysis because the response of the photochemical grid
model is highly non-linear under such circumstances.
    Another group of commenters supported requiring only the
concentration-based existing source contribution analysis because only
that analysis would address the question alluded to in the statute,
which is whether sources of the precursor contribute significantly to
levels which exceed the standard in the area. These commenters stated
that sensitivity-based analyses reflect localized conditions and do not
represent a consistent effect across an air basin. The commenters
suggested that sensitivity analyses might be considered to inform what
pollutants are most cost-effective to control, but believed that this
is dubious because the fact that certain pollutants are very abundant
is likely the result of a history of under-regulation. They suggested
that it actually may be cheaper to control the more abundant pollutant
than the less abundant pollutant in order to achieve an equal amount of
air quality improvement.
    Response: The EPA agrees with commenters who suggested that the
rule should closely align with the statutory language in CAA section
189(e) of subpart 4 and include provisions for evaluating the
contribution of existing sources to PM2.5 levels which
exceed the standard in the area. For this reason, the final rule states
that the existing source contribution analysis should be required for
any comprehensive precursor demonstration or major stationary source
precursor demonstration seeking to exempt a precursor from attainment
planning requirements.
    The EPA also believes that a sensitivity-based contribution
analysis is consistent with the language and intent of CAA section
189(e). As applied to attainment plans, CAA section 189(e) allows
states to evaluate whether PM2.5 precursors significantly
contribute to levels which exceed the standard in the area. The intent
of CAA section 189(e) in applying control requirements to
PM2.5 precursors is to ensure expeditious attainment of the
standard. However, if conditions in a particular area are such that
control of sources of one or more precursors does not reduce
PM2.5 concentrations in the area, then those controls will
not help the area attain (expeditiously or otherwise). Therefore, the
EPA disagrees with commenters who argue that sensitivity-based
contribution analyses are not appropriate for determining if precursors
do not significantly contribute to PM2.5 levels in the area.
The EPA believes that sensitivity-based contribution analyses can be
useful for determining whether adoption of control requirements for
sources of a particular precursor would be effective in reducing
PM2.5 concentrations, and can be useful for determining
whether potential emissions increases under the NNSR program would lead
to insignificant air quality changes. For this reason, the final rule
allows states to conduct sensitivity-based contribution analyses for
the comprehensive, major stationary source, and NNSR precursor
demonstrations.
    Comment: Some commenters expressed support for the precursor option
from the proposal (i.e., Option 3) that would have allowed for an
expeditious attainment precursor demonstration to be deemed to
demonstrate under CAA section 189(e) that emissions of the precursor do
not need to be addressed for all major stationary source requirements,
such as the NNSR program.
    Response: Upon further consideration of this potential approach,
the EPA decided that it would not be appropriate to include such an
approach in the final rule. The reason for this is that an expeditious
attainment planning analysis on its own would determine that the area
could attain the standard by the Moderate area attainment date without