6560-50-P
ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 51, and 93
[EPA-HQ-OAR-2013-0691; FRL -      ]
RIN 2060-AQ48
Fine Particulate Matter National Ambient Air Quality Standards: State Implementation Plan Requirements
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
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 notice 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 notice 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 [INSERT DATE 60 DAYS FROM DATE OF PUBLICATION IN THE FEDERAL REGISTER].
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
um	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). 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:
Note: This linked Table of Contents will be removed prior to printing in the Federal Register.
SUPPLEMENTARY INFORMATION:	2
I. General Information	2
A. Preamble Glossary of Terms and Acronyms	2
B. Entities affected by this rule	4
C. Obtaining a copy of this document and other related information	4
D. Organization of this Federal Register document	5
II. Background	10
A. Introduction	10
B. Overview of PM2.5 NAAQS and Implementation	12
C. Atmospheric Chemistry of PM2.5 and Its Precursors	19
III. Requirements with Respect to the Treatment of PM2.5 Precursors in Attainment Plans and the NNSR Program	30
A. Background	30
B. Summary of Proposal	38
C. Final Rule	40
IV. Requirements for PM2.5 Moderate Nonattainment Area Plans	63
A. Plan Due Dates	64
B. Emissions Inventory Requirements	67
C. Pollutants to be Addressed in the Plan	93
D. Attainment Plan Control Strategy	93
E. Modeling for Attainment Demonstrations	148
F. RFP Requirements	177
G. Quantitative Milestones	196
H. Contingency Measures	209
I. Attainment Date	218
J. Attainment Date Extensions	222
V. Reclassification of a PM2.5 Moderate Nonattainment Area to Serious	232
A. Discretionary Authority	233
B. Mandatory Duty	238
VI. Requirements for PM2.5 Serious Nonattainment Area Plans	241
A. Plan Due Dates	242
B. Emissions Inventory Requirements	254
C. Pollutants to be Addressed in the Plan	260
D. Attainment Plan Control Strategy	261
E. Modeling for Attainment Demonstrations	290
F. RFP Requirements	302
G. Quantitative Milestones	306
H. Contingency Measures	311
I. Attainment Dates	315
J. Attainment Date Extensions	316
VII. Requirements under CAA Section 189(d) for PM2.5 Serious Areas that Fail to Attain the NAAQS by the Applicable Attainment Date	331
A. Plan Due Dates	332
B. Emissions Inventory Requirements	334
C. Pollutants to be Addressed in the Plan	337
D. Attainment Plan Control Strategy	338
E. Modeling for Attainment Demonstrations	348
F. RFP Requirements	353
G. Quantitative Milestones	357
H. Contingency Measures	359
I. Attainment Dates	364
VIII. NNSR Requirements for PM2.5 Nonattainment Areas	366
A. Background	366
B. Final NNSR Requirements for PM2.5 Nonattainment Areas	374
C. Transition Provisions for Major Source Permitting in PM2.5 Nonattainment Areas	420
IX. Other Requirements and Considerations for PM2.5 Nonattainment Areas	424
A. Waivers Under CAA Section 188(f)	424
B. Conformity Requirements	430
C. Clean Data Policy	441
D. CAA Section 179B/International Border Areas	450
E. Enforcement and Compliance	463
F. Multi-pollutant Considerations	473
G. Measures to Ensure Appropriate Protections for Overburdened Populations	477
H. Tribal Issues	485
I. Voluntary Programs for Reducing Ambient PM2.5	489
J. Improved Stationary Source Emissions Monitoring	491
K. Stationary Source Test Methods for Emissions of Condensable PM2.5	493
X. Revocation of the 1997 Primary Annual PM2.5 NAAQS	498
A. Summary of the Proposal	498
B. Final Rule	501
C. Comments and Responses	510
XI. Environmental Justice Considerations	519
XII. Statutory and Executive Order Reviews	520
A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review	520
B. Paperwork Reduction Act (PRA)	520
C. Regulatory Flexibility Act (RFA)	522
D. Unfunded Mandates Reform Act (UMRA)	523
E. Executive Order 13132: Federalism	523
G. Executive Order 13045: Protection of Children from Environmental Health and Safety Risks	524
H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use	524
I. National Technology Transfer and Advancement Act	524
J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations	524
K. Congressional Review Act (CRA)	525
L. Determination Under Section 307(d)	525
M. Judicial Review	525
XIII. Statutory Authority	526
PART 50  -  NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS	528
PART 51  -  REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF IMPLEMENTATION PLANS	529

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 (um). 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.
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 ug/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 ug/m[3]. 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 ug/m[3].
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. 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.
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.,
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. 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.
2. History of PM2.5 NAAQS Implementation.
The EPA first promulgated annual and 24-hour NAAQS for PM2.5 in July 1997. 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. 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. 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"). 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"). 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").
      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. 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. 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. 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.
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. The EPA issued its final rule on December 14, 2012, in which it lowered the primary annual PM2.5 standard from 15.0 μg/m[3] to 12.0 μg/m[3] to provide increased protection against health effects associated with long- and short-term fine particle exposures. The EPA also eliminated spatial averaging as part of the form of the annual standard to avoid potential disproportionate impacts on at-risk populations. The EPA retained the level (35 μg/m[3]) and form (98[th] 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. 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. With regard to the secondary (welfare-based) standards, the EPA retained the existing annual PM2.5 standard of 15.0 μg/m[3] and the existing 24-hour PM2.5 standard of 35 μg/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.
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"). 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.
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. First, 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 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 to establishing these regulatory requirements, the EPA is also providing, in this preamble and in associated guidance documents accompanying it, additional guidance and recommendations for states to use in applying these requirements to the development of attainment plans for the 2012 PM2.5 NAAQS and for any future revisions of the PM2.5 NAAQS. Finally, 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. "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. "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 nationwide anthropogenic emissions estimates for direct PM2.5 and the four main PM2.5 precursor gases from major source sectors.

Table 1. Total Emissions of PM2.5 and Precursors for Major Sectors (in tons/year)
           Source: 2011 National Emissions Inventory (Version 1)[a]
Category
                                    Direct
                                     PM2.5
                                      SO2
                                      NOx
                                      VOC
                                      NH3
Chemical and allied products
                                                                         16,464
                                                                        125,768
                                                                         49,867
                                                                         79,236
                                                                         23,044
Fuel combustion---electric generating utilities (EGUs)
                                                                        196,685
                                                                      4,612,641
                                                                      2,031,855
                                                                         40,597
                                                                         24,968
Fuel combustion ---other
                                                                        628,199
                                                                        987,552
                                                                      1,856,716
                                                                        588,346
                                                                         79,679
Other industrial
                                                                        273,857
                                                                        185,859
                                                                        348,561
                                                                        328,222
                                                                         53,039
Onroad mobile
                                                                        208,629
                                                                         28,969
                                                                      5,785,570
                                                                      2,413,026
                                                                        119,654
Metals processing
                                                                         48,451
                                                                        144,630
                                                                         70,655
                                                                         34,277
                                                                          1,140
Miscellaneous (mainly fire emissions, dust and some agricultural operations)
                                                                      4,489,694
                                                                        219,318
                                                                        434,547
                                                                      5,810,566
                                                                      3,934,405
Offroad mobile
                                                                        207,543
                                                                         92,036
                                                                      3,133,798
                                                                      2,159,368
                                                                          3,270
Petroleum & related industries
                                                                         31,738
                                                                        116,317
                                                                        684,808
                                                                      2,488,123
                                                                          1,643
Solvent utilization
                                                                          3,810
                                                                            107
                                                                            893
                                                                      2,814,551
                                                                            577
Storage and transport
                                                                         20,098
                                                                          9,109
                                                                         19,079
                                                                      1,221,185
                                                                          5,734
Waste disposal and recycling
                                                                        172,144
                                                                         16,842
                                                                         83,469
                                                                        131,777
                                                                         68,281
TOTAL
                                                                      6,297,312
                                                                      6,539,148
                                                                     14,499,818
                                                                     18,109,274
                                                                      4,315,434
[a] 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.

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). 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. Section II.B.3 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.B.4 of this preamble presents a brief overview of PM2.5 composition by region of the U.S.
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.B.3 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.
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.
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. 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.
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.
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.
Anthropogenic sources of VOC include mobile sources, petrochemical manufacturing, oil and gas emissions and solvents. 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.
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. 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.
      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.
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). 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.
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.
   Table 2. PM2.5 Chemical Composition Data at 2009-2011 Nonattainment Sites
                                    Region
                                   Statistic
                           Concentration (ug/m[3])

                                      SOe
                                      NO3
                                      OM
                                      EC
                                      CrM
                                     PM2.5
Central
Min (ug/m[3])
                                     1.46
                                      0.3
                                     2.73
                                     0.31
                                     0.01
                                     8.92

Mean (ug/m[3])
                                     2.69
                                     1.49
                                     3.57
                                     0.68
                                     0.26
                                     11.63

Max (ug/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 (ug/m[3])
                                     0.83
                                     0.38
                                     1.97
                                     0.19
                                     0.01
                                     6.03

Mean (ug/m[3])
                                     1.68
                                      1.8
                                     2.84
                                     0.48
                                     0.19
                                     9.86

Max (ug/m[3])
                                     2.51
                                     3.57
                                     3.69
                                     0.79
                                     0.61
                                     11.87

N
                                      29
                                      28
                                      20
                                      20
                                      28
                                      23
North East
Min (ug/m[3])
                                     0.58
                                     0.12
                                     1.74
                                     0.14
                                       0
                                     4.42

Mean (ug/m[3])
                                     2.06
                                     0.97
                                     3.14
                                     0.69
                                     0.17
                                     9.33

Max (ug/m[3])
                                     5.12
                                     2.26
                                     5.05
                                     1.69
                                     0.52
                                     15.05

N
                                      59
                                      59
                                      39
                                      39
                                      59
                                      46
North West
Min (ug/m[3])
                                     0.24
                                     0.05
                                     2.91
                                     0.42
                                     0.01
                                     6.06

Mean (ug/m[3])
                                     0.54
                                      0.4
                                     5.02
                                     0.81
                                     0.15
                                     8.33

Max (ug/m[3])
                                     1.09
                                     1.79
                                     8.44
                                     1.25
                                     0.53
                                     10.96

N
                                      33
                                      33
                                      13
                                      13
                                      33
                                      14
South
Min (ug/m[3])
                                     0.88
                                     0.18
                                     1.36
                                     0.12
                                     0.02
                                     5.22

Mean (ug/m[3])
                                     2.06
                                      0.8
                                     3.32
                                     0.57
                                      0.5
                                     10.05

Max (ug/m[3])
                                     3.08
                                     1.67
                                      5.1
                                     1.48
                                     2.38
                                     14.27

N
                                      36
                                      27
                                      23
                                      23
                                      36
                                      23
South East
Min (ug/m[3])
                                      1.6
                                      0.2
                                     1.75
                                     0.37
                                     0.01
                                     6.76

Mean (ug/m[3])
                                     2.39
                                     0.53
                                     4.12
                                     0.63
                                     0.26
                                     10.77

Max (ug/m[3])
                                     4.33
                                     1.51
                                     5.71
                                      1.2
                                     0.85
                                     13.38

N
                                      44
                                      43
                                      30
                                      30
                                      43
                                      29
South West
Min (ug/m[3])
                                     0.34
                                     0.07
                                     2.34
                                     0.46
                                     0.02
                                      5.3

Mean (ug/m[3])
                                     0.63
                                     0.49
                                     3.01
                                      0.7
                                      0.5
                                     7.93

Max (ug/m[3])
                                     1.13
                                     2.65
                                     4.39
                                     1.04
                                     1.96
                                     9.73

N
                                      46
                                      46
                                      11
                                      11
                                      46
                                      12
West
Min (ug/m[3])
                                     0.33
                                     0.08
                                     1.79
                                     0.52
                                     0.01
                                     6.84

Mean (ug/m[3])
                                      0.9
                                      1.4
                                     5.22
                                     0.85
                                     0.32
                                     11.49

Max (ug/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 (ug/m[3])
                                     0.29
                                     0.06
                                     1.22
                                     0.09
                                       0
                                     3.23

Mean (ug/m[3])
                                     0.67
                                     0.48
                                     3.16
                                     0.44
                                     0.22
                                     7.25

Max (ug/m[3])
                                     1.79
                                     2.02
                                     8.28
                                     1.21
                                     0.53
                                     13.72

N
                                      30
                                      30
                                       7
                                       7
                                      30
                                      10
Source: EPA Speciation Trends Network
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 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 PM10and PM2.5. 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 particular precursors need not be regulated for purposes of attainment planning or in the NNSR permitting program for a specific nonattainment area.
	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 EPA has also provided more detailed technical guidance on conducting such demonstrations in a separate document that accompanies this 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 certain 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. While each of these issues is discussed in this preamble to some degree, all of them are addressed more fully in the  separate technical guidance document noted above.
	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.

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."
	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." 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. Section 189(e) of the CAA contains the only express exception to control requirements for PM precursors under subpart 4.
      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. 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. 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. 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.
	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. Courts have upheld this approach to the requirements of subpart 4 for PM10. The EPA believes that application of a similar approach to PM2.5 precursors under subpart 4 is appropriate and reasonable.
	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 from all sources in the nonattainment area, including stationary sources, area sources, and mobile sources. 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 in a particular nonattainment area. The proposed "precursor demonstration" options outlined procedures and technical analyses a state could elect to perform to demonstrate that control requirements for 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 evaluate control requirements for 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.
	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 impact should not be considered to be significant, thereby establishing that control 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.
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 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 impact on 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.
	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 that EPA is addressing more extensively in the guidance being released at the same time as this rule, including the appropriate geographic scope of the analyses, recommended 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 6th 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 a given precursor. 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.
	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 evaluate potential 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 demonstration does not include 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.c of this preamble). If the contribution of current source emissions to the PM2.5 concentration in the nonattainment area is significant, 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.d of this preamble.
	If a comprehensive precursor demonstration is deemed adequate by the EPA, then in developing the attainment plan for the area, the state would not be required to evaluate or adopt potential 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 evaluating potential 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 level of contribution exceeds the recommended contribution threshold in EPA guidance, 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 deemed adequate by the EPA, then in developing the attainment plan for the area, the state would not be required to evaluate or adopt potential 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 evaluating potential 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 evaluate and potentially 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 deems a major stationary source demonstration to be adequate, the attainment plan would still need to evaluate and potentially impose control requirements for the relevant precursor for 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 evaluating the significance of the precursor to the PM nonattainment problem 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 would exempt 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). The precursor demonstration technical guidance accompanying this rule describes the recommendations for conducting such a sensitivity analysis in more detail. 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 impacts from the precursor), but the introduction of a new major stationary source of emissions of that particular precursor could in some cases significantly impact the ambient PM2.5 levels in the area because other pollutants with which the precursor reacts in the atmosphere could be relatively abundant. These issues are addressed in more detail in the EPA guidance document on precursor demonstrations.
      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 impact PM2.5 levels in the area. Note, however, that the NNSR precursor demonstration is used only to justify an exclusion of the precursor from the NNSR requirements in the area. A state would need to pair the NNSR precursor demonstration with another type of precursor demonstration to address 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 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 impact was found to exceed the threshold amount, then it would 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 a significance 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 accompanying this final rule. This technical guidance document on PM2.5 precursor demonstrations includes recommended insignificance thresholds based on analyses of recent air quality data designed to identify the level of air quality change that would be considered "insignificant" for the current annual and 24-hour PM2.5 standards.
	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 major sources) in the nonattainment area significantly contribute to PM2.5 concentrations that exceed the standard in the area. The state should use technically credible approaches for estimating the ambient impact of emissions of a particular precursor to total PM2.5 concentration in the nonattainment area. The technical guidance discusses 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.
	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 impact of precursor emissions reductions could be used in the event the state cannot demonstrate via concentration analysis that sources of a particular precursor have an insignificant impact on 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 using a sensitivity analysis evaluating the impact of precursor emissions increases  -  without the need for a separate evaluation of existing source contribution to PM2.5 concentration. 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 for 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 find that the emissions of a particular precursor from sources 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. The accompanying precursor demonstration guidance provides recommendations for appropriate levels of emissions changes to evaluate when conducting sensitivity analyses to facilitate the development of approvable precursor demonstrations.
      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. If a concentration-based contribution analysis conducted for either a comprehensive precursor demonstration or a major stationary source precursor demonstration shows that the impact from a precursor is less than the relevant significance threshold at each PM2.5 monitor in the area, then the concentration-based contribution analysis alone would be adequate. However, if a concentration-based contribution analysis shows that the impact from a precursor to PM2.5 levels in the area is greater than the recommended significance threshold at one or more monitors, then the precursor demonstration still could be deemed adequate, 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 consistent with recommended guidance would not exceed the relevant threshold, then it would be adequate for the EPA to determine the precursor as insignificant for attainment planning purposes other than for NNSR permitting.
      In evaluating whether any precursors would be appropriate to exclude 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. The precursor demonstration guidance provides recommendations for assessing the sensitivity of the nonattainment area to increases in precursor emissions, in order to facilitate the development of approvable NNSR precursor demonstrations where appropriate. In contrast to the emissions decrease 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 growth in the area in order to assess whether the contribution from such increases is not significant under CAA section 189(e).
      The technical guidance document accompanying this rule provides additional guidelines on how to conduct an analysis of potential emissions increases from new or modified sources in a nonattainment area. The EPA recommends that the state conduct these analyses using air quality modeling tools, but the state could provide additional or different analyses, if appropriate. The guidance document contains recommended methodologies for conducting these sensitivity-based analyses, and provides EPA recommendations for appropriate ways to characterize the increases that would be evaluated. If the sensitivity-based analysis conducted for a NNSR precursor demonstration shows that the expected impact from increasing precursor emissions is less than the recommended significance threshold, then the NNSR precursor demonstration could be deemed adequate by the EPA.
4. Procedural Considerations.
	a. Consultation and Public Review.
	The EPA anticipates that a state's development of an adequate 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 the EPA's technical guidance.
	If a state chooses to develop a precursor demonstration, it must be submitted to the EPA no later than the date of submission of the relevant attainment plan or NNSR program revision; an earlier submission is preferable. 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 such emissions are a significant contributor 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 develops a comprehensive precursor demonstration or major stationary source precursor demonstration for a nonattainment area, it must submit any concentration-based contribution analysis and, if applicable, any 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, even though it may be the case that the concentration-based contribution analysis indicates an air quality impact that exceeds the recommended threshold. 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 adequate 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 10th year after designation. For a state wishing to continue a precursor exclusion in a subsequent attainment plan or NNSR program submission, the final rule generally 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 deemed adequate as part of the Serious area plan. As noted in the discussion of the provisions for excluding 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 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 10th 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 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 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 significant 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 new control requirements for a particular precursor, but it would not address the potential impact of increased emissions of the precursor in the area due to new or modified sources, as is reasonably needed under the NNSR precursor demonstration. The evaluation of controls required for expeditious attainment does not consider what happens if new sources move into an area. Thus, while a state might be able to show that controlling existing sources of a precursor does not advance attainment, the analysis would not determine whether a new major source of that precursor might have a significant impact on air quality. The EPA believes it is important for purposes of CAA section 189(e) and our overall environmental goal under the NNSR program to evaluate emissions increases. Consequently, the EPA has revised the details of the specific types of demonstrations to include a specific standalone demonstration for purposes of exempting major stationary sources of a precursor from regulation under the NNSR permitting program.
IV. Requirements for PM2.5 Moderate Nonattainment Area Plans
	Sections 189(a), (c), and (e) of the CAA require that Moderate area attainment plans contain the following: (i) an approved permit program for construction of new and modified major stationary sources (CAA section 189(a)(1)(A)); (ii) a demonstration that the plan provides for attainment by no later than the applicable Moderate area attainment date or a demonstration that attainment by that date is impracticable CAA (section 189(a)(1)(B)); (iii) provisions for the implementation of RACM and RACT no later than 4 years after designation (CAA section 189(a)(1)(C)); (iv) quantitative milestones that will be used to evaluate compliance with the requirement to demonstrate reasonable further progress (RFP) (CAA section 189(c)); and, (v) evaluation and regulation of PM2.5 precursors (in general to meet RACM and RACT and other attainment planning requirements, and as specifically provided for major stationary sources under CAA section 189(e)). In addition, subpart 1 requirements for attainment plans continue to apply to PM2.5 nonattainment areas unless they are superseded by subpart 4 provisions and include the following: (i) a description of the expected annual incremental reductions in emissions that will demonstrate RFP (CAA section 172(c)(2)); (ii) emissions inventories, as necessary (CAA section 172(c)(3)); (iii) other control measures (besides RACM and RACT) needed for attainment (CAA section 172(c)(6)); and, (iv) contingency measures (CAA section 172(c)(9)). The EPA notes that its longstanding guidance on interpreting these statutory requirements is embodied in the General Preamble and the Addendum. The preamble for the proposed rule presented several interpretations of these provisions, and further explained where its proposal varies from past EPA guidance and the reasons for the variance. The following sections of this preamble explain the EPA's final approach and, where different from the proposal, also explain EPA's reasons for finalizing an amended approach. This final rule reflects our careful consideration of the numerous thoughtful comments we received from air agencies, who are responsible under the CAA for these implementation activities, and a variety of other stakeholders.
A. Plan Due Dates
1. Summary of Proposal.
      The EPA proposed to require that all Moderate area plan elements for a nonattainment area be submitted by the state no later than 18 months from the effective date of designation. The attainment plan submission would thus include all necessary plan elements required under CAA subparts 1 and 4.
2. Final Rule.
      The final regulations at 51.1003(a) require all Moderate nonattainment area elements to be submitted by no later than 18 months from the date of designation, as proposed. Section 189 of the CAA specifies the schedule by which states must submit attainment plans for the PM2.5 NAAQS. Specifically, CAA section 189(a)(2)(B) requires states to submit an attainment plan that meets Moderate area attainment plan requirements no later than 18 months from the date of
a nonattainment designation. While subpart 1 of the CAA could potentially be interpreted to authorize the EPA to provide up to 3 years after designation for states to submit certain attainment plan elements, the EPA believes that such an interpretation would be inconsistent with the specific deadlines that Congress imposed in subpart 4. The EPA concludes that all subpart 1 and subpart 4 nonattainment area requirements should be considered together in order to facilitate state development, and EPA review, of a comprehensive plan to attain the PM2.5 NAAQS in a given nonattainment area. In fact, the EPA finds that meeting key subpart 1 requirements within the 18-month timeframe of subpart 4 is fundamentally necessary for the state to develop an approvable plan. For example, the state must develop an emissions inventory (or inventories) either before or at the same time as the other attainment plan elements due under subpart 4 because the information contained in the emissions inventory is critical for development of other elements of the Moderate area attainment plan, such as its precursor analysis, analysis of RACM and RACT and additional reasonable measures, and attainment demonstration modeling. The EPA's ability to evaluate the submitted attainment plan therefore will be impaired if the state does not submit all the required plan elements at the same time.

3. Comments and Responses.
	Comment: Commenters suggested that the EPA should interpret the statute to allow more time for states to develop and submit contingency measures.
	Response: As discussed earlier in this section, the EPA believes that it would be inconsistent with the specific deadlines that Congress imposed in subpart 4 to allow contingency measures to be submitted later than the other elements of the attainment plan. Contingency measures need to be adopted and ready for rapid and timely implementation in the event a nonattainment area fails to meet RFP requirements or fails to attain the PM2.5 NAAQS by the applicable attainment date. The state's evaluation of what emissions controls are appropriate to meet the contingency measure requirement is closely related to other aspects of the attainment plan, such as the pollutants and sources to be addressed in meeting the RACM/RACT requirements, and the amount of emissions reductions that the contingency measures should achieve, based upon the facts and circumstances of the attainment plan for the area. The same types of facts and analyses that are necessary for the other elements of an attainment plan are directly relevant to the development of contingency measures.
      Although nothing in the CAA prohibits states from making separate attainment plan submissions to meet the individual required elements for attainment plans in advance of the required date, the EPA presumes that development and submission of all of the attainment plan elements simultaneously will be most efficient, both for the state and for the EPA in reviewing the state's submission. A Moderate area implementation plan with a single SIP submission due date will be less administratively burdensome than a program with two SIP submission due dates. Under an approach with two submissions, the state would likely need to issue two sets of proposed regulations, hold two sets of public hearings, and respond to two sets of public comments, rather than dealing with all of these requirements in one comprehensive action. Likewise, the EPA would have two separate submissions to review and two sets of proposed and final actions to publish in the Federal Register for every Moderate nonattainment area. Thus, for the reasons outlined earlier, the final rule includes a single Moderate area attainment plan submission deadline of 18 months after designation. Accordingly, the areas designated as nonattainment for the 2012 PM2.5 NAAQS (with an effective date of April 15, 2015) are required to submit Moderate area attainment plans to the EPA no later than October 15, 2016. See 40 CFR 51.1003(a).
B. Emissions Inventory Requirements
1. Summary of Proposal.