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6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 51, and 93

[EPA-HQ-OAR-2013-0691; FRL-9916-08-OAR]

RIN 2060-AQ48

Fine Particulate Matter National Ambient Air Quality Standards: State 

Implementation Plan Requirements

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

SUMMARY: The Environmental Protection Agency (EPA) is proposing 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 how the EPA proposes that air 

agencies meet 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 proposed rule clarifies the specific 

attainment planning requirements that would 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 proposing 

to revoke the 1997 primary annual standard because the EPA revised the primary annual 

standard in 2012. The EPA first established the PM2.5 NAAQS in 1997, completed a
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review of those standards in 2006, and most recently completed a review of the PM2.5 

NAAQS on December 14, 2012.

DATES: Comments. Comments must be received on or before [INSERT DATE 60 

DAYS FROM DATE OF PUBLICATION IN THE FEDERAL REGISTER]. Public 

Hearing. The EPA will hold a public hearing related to this proposed rule on [INSERT 

DATE]. Information Collection Request. Under the Paperwork Reduction Act (PRA), 

comments on the information collection provisions are best assured of having full effect 

if the Office of Management and Budget (OMB) receives a copy of your comments on or 

before [INSERT DATE 30 DAYS FROM DATE OF PUBLICATION IN THE 

FEDERAL REGISTER].

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-OAR2013-0691, by one of the following methods:

 http://www.regulations.gov. Follow the on-line instructions for submitting 

comments.

 Email: a-and-r-docket@epa.gov.

 Mail: Air and Radiation Docket and Information Center, Attention Docket ID No. 

EPA-HQ-OAR-2013-0691, Environmental Protection Agency, Mailcode:

28221T, 1200 Pennsylvania Avenue, NW, Washington, D.C. 20460. In addition, 

please mail a copy of your comments on the information collection (ICR) 

provisions to the Office of Information and Regulatory Affairs, Office of 

Management and Budget (OMB), Attn: Desk Officer for EPA, 725 17th Street, 

NW, Washington, D.C. 20503.
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 Hand Delivery: Air and Radiation Docket and Information Center, Attention 

Docket ID No. EPA-HQ-OAR-2013-0691, Environmental Protection Agency in 

the EPA Headquarters Library, Room No. 3334 in the EPA Docket Center, 

located at William Jefferson Clinton Building West, 1301 Constitution Avenue, 

NW, Washington, D.C. 20004. Such deliveries are only accepted during the 

Docket’s normal hours of operation, and special arrangements should be made for 

delivery of boxed information.

Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2013-0691. The 

EPA's policy is that all comments received will be included in the public docket without 

change and may be made available online at http://www.regulations.gov, including any 

personal information provided, unless the comment includes information claimed to be 

Confidential Business Information (CBI) or other information whose disclosure is 

restricted by statute. Do not submit information that you consider to be CBI or otherwise 

protected through http://www.regulations.gov or email. The http://www.regulations.gov

website is an “anonymous access” system, which means the EPA will not know your 

identity or contact information unless you provide it in the body of your comment. If you 

send an email comment directly to the EPA without going through 

http://www.regulations.gov, your email address will be automatically captured and 

included as part of the comment that is placed in the public docket and made available on 

the Internet. If you submit an electronic comment, the EPA recommends that you include 

your name and other contact information in the body of your comment and with any disk 

or CD-ROM you submit. If the EPA cannot read your comment due to technical 

difficulties and cannot contact you for clarification, the EPA may not be able to consider 
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your comment. Electronic files should avoid the use of special characters, any form of 

encryption and be free of any defects or viruses. For additional information about the 

EPA’s public docket visit the EPA Docket Center homepage at 

http://www.epa.gov/epahome/dockets.htm. For additional instructions on submitting 

comments, go to the SUPPLEMENTARY INFORMATION section of this document.

Docket: All documents in the docket are listed in the http://www.regulations.gov index. 

Although listed in the index, some information is not publicly available, e.g., CBI or 

other information whose disclosure is restricted by statute. Certain other material, such as 

copyrighted material, will be publicly available only in hard copy. Publicly available 

docket materials are available either electronically in http://www.regulations.gov or in 

hard copy at the Air and Radiation Docket and Information Center in the EPA 

Headquarters Library, Room No. 3334 in the William Jefferson Clinton Building West, 

located at 1301 Constitution Avenue, NW, Washington, D.C. 20460. The Public Reading 

Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal 

holidays. The phone number for the Public Reading Room is (202) 566-1744.

FOR FURTHER INFORMATION CONTACT: For general information on this 

proposed rule, contact Mr. Rich Damberg, Office of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, by phone at (919) 541-5592 or by email at 

damberg.rich@epa.gov; or Ms. Megan Brachtl, Office of Air Quality Planning and 

Standards, U.S. Environmental Protection Agency, by phone at (919) 541-2648 or by 

email at brachtl.megan@epa.gov. For information on the public hearing, contact Ms. 

Pamela Long, Office of Air Quality Planning and Standards, U.S. Environmental 

Protection Agency, by phone at (919) 541-0641 or by email at long.pamela@epa.gov. 
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For information on the ICR, contact Mr. Butch Stackhouse, Office of Air Quality 

Planning and Standards, U.S. Environmental Protection Agency, by phone at (919) 541-

5208 or by email at stackhouse.butch@epa.gov.

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 Rule

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
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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 Plan

TSP Total Suspended Particles

μm Micrometer (Micron)

VMT Vehicle Miles Traveled

VOC Volatile Organic Compounds

B. Does this action apply to me?

Entities potentially affected directly by this proposed 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 proposed rule 
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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 proposed 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. What should I consider as I prepare my comments for the EPA?

1. Submitting CBI. Do not submit this information to the EPA through 

www.regulations.gov or email. Clearly mark the specific information that you claim to be 

CBI. For CBI in a disk or CD-ROM that you mail to the EPA, mark the outside of the 

disk or CD-ROM as CBI and then identify electronically within the disk or CD-ROM the 

specific information that is claimed as CBI. In addition to one complete version of the 

comment that includes information claimed as CBI, a copy of the comment that does not 

contain the information claimed as CBI must be submitted for inclusion in the public 

docket. Information so marked will not be disclosed except in accordance with 

procedures set forth in 40 CFR part 2.

2. Tips for preparing comments. When submitting comments, remember to:

• Identify the rulemaking by docket number and other identifying 

information (subject heading, Federal Register date and page number).

• Follow directions. The proposed rule may ask you to respond to specific 

questions or organize comments by referencing a Code of Federal 

Regulations (CFR) part or section number.

• Explain why you agree or disagree, suggest alternatives and substitute 

language for your requested changes.
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• Describe any assumptions and provide any technical information and/or 

data that you used to support your comment.

• If you estimate potential costs or burdens, explain how you arrived at your 

estimate in sufficient detail to allow for it to be reproduced.

• Provide specific examples to illustrate your concerns wherever possible, 

and suggest alternatives.

• Explain your views as clearly as possible, avoiding the use of profanity or 

personal threats.

• Make sure to submit your comments by the comment period deadline 

identified.

D. What information should I know about possible public hearings?

The EPA intends to hold one public hearing on this proposal. Further details 

concerning the public hearing for this proposed rule will be published in a separate 

Federal Register document. For updates and additional information on the public 

hearings, please check the EPA’s website for this rulemaking at

http://www.epa.gov/airquality/particlepollution/actions.html.

E. Where can I obtain 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://www.epa.gov/airquality/particlepollution/actions.html.

F. How is this Federal Register notice organized?

The information presented in this document is organized as follows:

I. General Information

A. Preamble Glossary of Terms and Acronyms
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B. Does this action apply to me?

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

D. What information should I know about possible public hearings?

E. Where can I obtain a copy of this document and other related information?

F. How is this Federal Register notice organized?

II. Background for Proposal

A. Introduction

B. Atmospheric Chemistry of PM2.5 and Its Precursors

C. Historical Overview of PM2.5 NAAQS Setting and Implementation

D. State Implementation Planning Process for PM2.5 NAAQS

III. What is the EPA proposing with respect to the treatment of PM2.5 precursors in 

nonattainment area planning and permitting?

A. Background

B. Proposed Precursor Policy Options

C. Technical Approaches for Demonstrating that a Precursor Does Not Need to 

Be Subject to Control Requirements

IV. What are the EPA’s proposed requirements for Moderate area attainment 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. How would a PM2.5 Moderate nonattainment area be reclassified to Serious?

A. Discretionary Authority

B. Mandatory Duty

VI. What are the EPA’s proposed requirements for Serious area attainment 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. What are the EPA’s proposed requirements for attainment plans under CAA section 

189(d) for 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
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D. Attainment Plan Control Strategy

E. Modeling for Attainment Demonstrations

F. RFP Requirements

G. Quantitative Milestones

H. Contingency Measures

I. Attainment Dates

VIII. What are the EPA’s proposed NNSR permitting requirements?

A. Statutory Requirements for NSR

B. Federal NNSR Regulations

C. What changes is the EPA proposing for NNSR for PM2.5 nonattainment areas?

D. Plan Due Dates

E. Avoidance of Dual Review for PSD and NNSR for PM2.5

IX. What other proposed requirements would apply in 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. Efforts to Encourage a Multi-Pollutant Approach When Developing PM2.5

Attainment Plans

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. What is the EPA proposing with respect to revoking the 1997 primary annual PM2.5

NAAQS?

A. Background

B. History of Revocation of Other NAAQS

C. Proposed Options for Revocation and Related Anti-Backsliding Requirements 

for the 1997 Primary Annual PM2.5 NAAQS

D. Discussion of Options

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 

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments

G. Executive Order 13045: Protection of Children from Environmental Health 

and Safety Risks

H. Executive Order 13211: Actions Concerning Regulations That Significantly 

Affect Energy Supply, Distribution, or Use
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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. Determination Under Section 307(d)

Statutory Authority

List of Subjects

II. Background for Proposal

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: 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 particulate matter with an aerodynamic 

diameter less than or equal to a nominal 2.5 micrometers (μm). 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 NO3 and SO4).

The human health effects associated with long- or short-term exposure to PM2.5

are significant and include premature mortality, aggravation of respiratory and 
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cardiovascular disease (as indicated by increased hospital admissions and emergency 

room visits) and development of chronic respiratory disease. In addition, welfare effects 

associated with elevated PM2.5 levels include visibility impairment as well as effects on 

sensitive ecosystems, materials damage and soiling and climatic and radiative processes.1

On December 14, 2012, the EPA made revisions to the suite of 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 μg/m3to 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 μg/m3.

2 The EPA eliminated spatial averaging as 

part of the form of the PM2.5 annual standards to avoid potential disproportionate impacts 

on at-risk populations. In addition, the EPA retained the level and form of the primary 

and secondary 24-hour PM2.5 standards to continue to provide supplemental protection 

against health effects associated with short-term PM2.5 exposures. Although not directly 

relevant to this rulemaking with respect to the PM2.5 NAAQS, it should be noted that in 

 

1 For a complete discussion of the human health and welfare effects associated with 

exposure to elevated concentrations of particulate matter, see generally “Integrated 

Science Assessment for Particulate Matter.” U.S. Environmental Protection Agency, 

Office of Research and Development, National Center for Environmental AssessmentRTP Division, February 10, 2010. EPA/600/R-08/139F. Available at: 

http://www.epa.gov/ncea/pdfs/partmatt/Dec2009/PM_ISA_full.pdf. See Chapter 2.

2

78 FR 3086 (January 15, 2013).
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December 2012, the EPA also did not revise the level or form of the primary and 

secondary 24-hour PM10 standards, which remain at 150 μg/m3.

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.

4 Attainment of the 

primary PM2.5 standards will have welfare co-benefits in addition to direct human health 

benefits. The term welfare co-benefits covers both environmental and societal benefits of 

reducing pollution, such as reductions in visibility impairment, materials damage and 

ecosystem damage.5

B. 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 

 

3 This proposed rulemaking is to develop implementation regulations with respect to the

PM2.5 NAAQS. For the PM10 NAAQS, states and the EPA will continue to implement 

those NAAQS in accordance with the applicable statutory requirements of the Clean Air 

Act (CAA) and the EPA’s existing guidance in the General Preamble and the Addendum. 

57 FR 13498 (April 16, 1992); 59 FR 41998 (August 16, 1994).

4

“Regulatory Impact Analysis for the Final Revisions to the National Ambient Air 

Quality Standards for Particulate Matter.” U.S. Environmental Protection Agency, Office 

of Air Quality and Planning Standards, Health and Environmental Impacts Division, 

February 28, 2013. EPA-452/R-12-005.

5

Ibid.
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regulations for implementation of the PM2.5 NAAQS. Properly designed regulatory 

requirements will help to assure that the PM2.5 NAAQS are attained effectively and 

expeditiously in all areas.

As noted earlier, the term PM2.5 refers to particles of solid and liquid material less 

than 2.5 microns in aerodynamic diameter.

6

“Primary” PM2.5 is emitted directly from 

emissions sources or activities, such as from diesel fuel combustion, wood burning, 

construction activities or unpaved roads, and it includes both filterable and condensable 

particles.7“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. Table 1 provides National 

Emissions Inventory (NEI) data for 2011 that represent nationwide anthropogenic 

 

6 The regulatory definition of PM2.5 includes particles with an upper 50 percent cut-point 

of 2.5μm aerodynamic diameter (the 50 percent cut-point diameter is the diameter at 

which the sample collects 50 percent of the particles and rejects 50 percent of the 

particles). PM2.5 particles have a penetration curve as measured by a reference method 

based on Appendix L of 40 CFR part 50 and designated in accordance with 40 CFR part 

53, by an equivalent method designed in accordance with 40 CFR part 53, or by an 

approved regional method designated in accordance with Appendix C of 40 CFR part 58.

7 Certain commercial or industrial activities involving high temperature processes (e.g., 

fuel combustion, metal processing, cooking operations) emit gaseous pollutants into the 

ambient air which rapidly condense into particle form. These “condensable” PM 

emissions exist almost entirely in the 2.5 or less micron range and can consist of organic 

material, sulfuric acid and metals.
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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 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

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.

It should be noted that the “Miscellaneous” category has the highest level of 

estimated PM2.5 emissions of any category primarily due to emissions from wildfire,

which can significantly contribute to periodic high PM2.5 levels. Besides their effect on 

air quality, wildfires pose a direct threat to public safety – a threat that can be mitigated 

through management of wildland vegetation. Attempts to suppress wildfires have resulted 

in unintended consequences, including increased risks to both humans and ecosystems. 

The use of wildland prescribed fire can influence the occurrence, behavior and effects of 

catastrophic wildfires, which may help manage the contribution of wildfires to PM2.5

Commented [A1]: Wildfire, prescribed fire and RACM 

discussion is now in “RACM for managing fire emissions” section 

around page 134
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concentrations. Additionally, prescribed fires can have benefits to those plant and animal 

species that depend upon natural fires for propagation, habitat restoration and 

reproduction, as well as myriad ecosystem functions (e.g., carbon sequestration). The 

EPA understands the importance of prescribed fire which mimics a natural process 

necessary to manage and maintain fire-adapted ecosystems and climate change 

adaptation, while reducing risk of uncontrolled emissions from catastrophic wildfires, and 

is committed to working with federal land managers, tribes, and states to effectively 

manage prescribed fire use to reduce the impact of wildfire related emissions.

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).

8

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.9 Section II.B.3 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 

 

8 Seinfeld J.H. and Pandis S.N., 2006. Atmospheric Chemistry and Physics: From Air 

Pollution to Climate Change. 2nd edition, J. Wiley, New York. 

9

“The Particle Pollution Report: Current Understanding of Air Quality and Emissions 

through 2003.” U.S. Environmental Protection Agency, Office of Air Quality Planning 

and Standards, Emissions, Monitoring, and Analysis Division, December 2004. Available

at: http://www.epa.gov/airtrends/aqtrnd04/pm.html.
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PM2.5 levels in excess of the NAAQS. Section II.B.4 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 includes 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, 
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calcium, titanium, magnesium and potassium, as well as oxygen, are major components.10

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 

 

10 Appel, K.W., Pouliot, G.A., Simon, H., Sarwar, G., Pye, H.O.T., Napelenok, S.L., 

Akhtar, F., and Roselle, S.J., 2013. Evaluation of dust and trace metal estimates from the 

Community Multiscale Air Quality (CMAQ) model version 5.0, Geoscientific Model 

Development Discussions 61859-1899; Sorooshian, A., Shingler, T., Harpold, A., 

Feagles, C.W., Meixner, T., and Brooks, P.D., 2013. Aerosol and precipitation chemistry 

in the southwestern United States: spatiotemporal trends and interrelationships, 

Atmospheric Chemistry and Physics 13, 7361-7379.
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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 coming from other industrial sources, such as oil refining and pulp 

and paper production.

11 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 electricity generating units, and from meteorological conditions that are 

conducive to sulfate formation.

 

11

“2008 National Emissions Inventory: Review Analysis and Highlights.” U.S.

Environmental Protection Agency, Office of Air Quality Planning and Standards, Air 

Quality Assessment Division, May 2013. EPA-454/R-005. Available at: 

http://www.epa.gov/ttn/chief/net/2008report.pdf.
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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 electric 

generating units (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.

12

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 can be 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 

 

12 Carlton, A.G., Pinder, R.W., Bhave, P.B., Pouliout, G.A., 2010. To what extent can 

Biogenic SOA be Controlled, Environmental Science and Technology 44(9), 3376-80.
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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.13

Anthropogenic sources of VOC include mobile sources, petrochemical 

manufacturing, oil and gas emissions and solvents.

14 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.

Much of those emissions comes from catalytic converters installed on light-duty gasoline 

 

13 Pandis S.N., Harley R.A., Cass G.R., and Seinfeld J.H., 1992. Secondary Organic 

Aerosol Formation and Transport, Atmospheric Environment, 26, 2266-82.

14 Carlton, A.G., Bhave, P.B., Napelenok, S.L., Edney, E.O., Sarwar, G., Pinder, R.W., 

Pouliout, G.A., and Houyoux, M. (2010), Model Representation of Secondary Organic 

Aerosol in CMAQ4.7, Environmental Science and Technology 44(22), 8553–60.
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vehicles, which are designed to convert NOx to nitrogen (N2); however, some ammonia is 

formed as a secondary product 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.

15 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.

16

f. Role of NOx in sulfate, 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 that have an impact on secondary formation of 

organic compounds that make up SOA. NOx is thus involved in all secondary PM

chemistry, not just in particulate nitrate formation.17

4. Fine Particulate Composition By Location

 

15 Seinfeld, J.H. and Pandis, S.N. (1998), Atmospheric Chemistry and Physics: From Air 

Pollution to Climate Change, 1st edition, J. Wiley, New York.

16 NARSTO, 2003. Particulate Matter Science for Policy Makers. A NARSTO 

Assessment. Parts 1 and 2. NARSTO. Management Office (Envair), Pasco, Washington. 

Available at: http://narsto.org/pm_science_assessment.

17 Carlton, A.G., Pinder, R.W., Bhave, P.B., and Pouliout, G.A., 2010. To what extent 

can Biogenic SOA be Controlled, Environmental Science and Technology 44(9), 3376-

3380.
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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).

18 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 that of 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 proposal.

19

Table 2. PM2.5 Chemical Composition Data at 2009-2011 Nonattainment Sites

Source: EPA Speciation Trends Network

Region Statistic Concentration (μg/m3)

 

18 The organic matter (OM) values in Table 2 were calculated by multiplying the 

measured organic carbon (OC) concentrations by 1.6 (Turpin and Lim (2001), Aerosol 

Science and Technology, 35, 602-610). PM2.5 concentrations come from measurements 

of the Federal Reference/Equivalance Methods (FRM/FEM) rather than from the CSN 

PM2.5 measurement.

19 Reff and Rao, Memo to the docket, 2013.
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SOe NO3 OM EC CrM PM2.5

Central Min (μg/m3) 1.46 0.3 2.73 0.31 0.01 8.92

Mean (μg/m3) 2.69 1.49 3.57 0.68 0.26 11.63

Max (μg/m3) 4.19 3.34 4.81 1.1 1.0 13.51

N 61 61 50 50 61 42

East North Central Min (μg/m3) 0.83 0.38 1.97 0.19 0.01 6.03

Mean (μg/m3) 1.68 1.8 2.84 0.48 0.19 9.86

Max (μg/m3) 2.51 3.57 3.69 0.79 0.61 11.87

N 29 28 20 20 28 23

North East Min (μg/m3) 0.58 0.12 1.74 0.14 0 4.42

Mean (μg/m3) 2.06 0.97 3.14 0.69 0.17 9.33

Max (μg/m3) 5.12 2.26 5.05 1.69 0.52 15.05

N 59 59 39 39 59 46

North West Min (μg/m3) 0.24 0.05 2.91 0.42 0.01 6.06

Mean (μg/m3) 0.54 0.4 5.02 0.81 0.15 8.33

Max (μg/m3) 1.09 1.79 8.44 1.25 0.53 10.96

N 33 33 13 13 33 14

South Min (μg/m3) 0.88 0.18 1.36 0.12 0.02 5.22

Mean (μg/m3) 2.06 0.8 3.32 0.57 0.5 10.05

Max (μg/m3) 3.08 1.67 5.1 1.48 2.38 14.27

N 36 27 23 23 36 23

South East Min (μg/m3) 1.6 0.2 1.75 0.37 0.01 6.76

Mean (μg/m3) 2.39 0.53 4.12 0.63 0.26 10.77

Max (μg/m3) 4.33 1.51 5.71 1.2 0.85 13.38

N 44 43 30 30 43 29

South West Min (μg/m3) 0.34 0.07 2.34 0.46 0.02 5.3

Mean (μg/m3) 0.63 0.49 3.01 0.7 0.5 7.93

Max (μg/m3) 1.13 2.65 4.39 1.04 1.96 9.73

N 46 46 11 11 46 12

West Min (μg/m3) 0.33 0.08 1.79 0.52 0.01 6.84

Mean (μg/m3) 0.9 1.4 5.22 0.85 0.32 11.49

Max (μg/m3) 2.08 5.14 10.27 1.56 1.05 16.57

N 44 44 20 20 44 21

West North Central Min (μg/m3) 0.29 0.06 1.22 0.09 0 3.23

Mean (μg/m3) 0.67 0.48 3.16 0.44 0.22 7.25

Max (μg/m3) 1.79 2.02 8.28 1.21 0.53 13.72

N 30 30 7 7 30 10

C. Historical Overview of PM2.5 NAAQS Setting and Implementation

Sections 108 and 109 of the CAA govern the establishment, review and revision, 

as appropriate, of 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, 
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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 EPA first promulgated annual and 24-hour NAAQS for PM2.5 in July 1997.

20

Prior to that time, the EPA had addressed ambient particulate matter 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 2002.21 The EPA 

subsequently promulgated designations for the 1997 PM2.5 NAAQS nationwide, and 

designated a number of areas as nonattainment for the 1997 PM2.5 NAAQS, effective 

April 2005.22 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”).23 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”).

24 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 

 

20 62 FR 38652 (July 18, 1997).

21 For a complete summary of legal challenges and related court decisions on the PM 

NAAQS, see generally 78 FR 3086 (January 15, 2013).

22 70 FR 944 (January 5, 2005).

23 72 FR 20583 (April 25, 2007).

24 73 FR 28231 (May 16, 2008).
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areas for the PM2.5 NAAQS were subject solely to the general nonattainment 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 NAAQS for PM, and in particular the EPA revised the 24-hour PM2.5 standards.25

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 2009.26

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.

27 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 areview of the PM2.5 NAAQS in June 2007, proposing

revisions to the primary and secondary PM2.5 NAAQS on June 29, 2012.

28 The EPA 

issued its final rule on December 14, 2012, in which it lowered the primary annual PM2.5

 

25 71 FR 61144 (October 17, 2006).

26 74 FR 58688 (November 13, 2009).

27 Memorandum of March 2, 2012 (withdrawn June 6, 2013), from Stephen D. Page, 

Director, Office of Air Quality Planning and Standards, to EPA Regional Air Directors, 

Regions I-X re: “Implementation Guidance for the 2006 24-Hour Fine Particle (PM2.5) 

National Ambient Air Quality Standards (NAAQS).” Available at: 

http://epa.gov/ttn/naaqs/pm/pm25_guide.html.

28 77 FR 38890 (June 29, 2012).
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standard from 15.0 μg/m3to 12.0 μg/m3to provide increased protection against health 

effects associated with long- and short-term fine particle exposures.29 The EPA also 

eliminated spatial averaging as part of the form of the annual standard to avoid potential 

disproportionate impacts on at-risk populations.30 The EPA retained the level (35 μg/m3) 

and form (98th percentile, averaged over 3 years) of the primary 24-hour PM2.5 standard, 

as revised in 2006, to provide supplemental protection against health effects associated 

with short-term PM2.5 exposures, especially in areas with high peak PM2.5

concentrations.31 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.32 With regard to the secondary (welfarebased) standards, the EPA retained the existing annual PM2.5 standard of 15.0 μg/m3

and 

the existing 24-hour PM2.5 standard of 35 μg/m3to protect against PM-related nonvisibility welfare effects including ecological effects, effects on materials and climate 

 

29 78 FR 3086 (January 15, 2013).

30 Spatial averaging of monitored ambient air quality data was a feature of the prior PM2.5

NAAQS monitoring regulations which had the potential for masking particularly high 

PM2.5 concentrations at certain monitored locations within nonattainment areas.

31 71 FR 61144 (October 17, 2006).

32 General information regarding the health effects associated with PM2.5 exposures is 

available at: http://www.epa.gov/airquality/particlepollution/health.html. Additional 

information, such as the EPA’s technical documents supporting the latest review of the 

standards, is available at: http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html.
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impacts. In addition, the secondary 24-hour PM2.5 standard provides protection for PMrelated 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”).33 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 are thus subject to the same statutory 

requirements. 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. 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 

 

33 Sierra Club v. EPA, 706 F.3d 428 (D.C. Cir. 2013).
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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 2012 guidance for attainment plans for the 2006 

PM2.5 NAAQS premised solely upon subpart 1 requirements was no longer appropriate.

The EPA intends to use this current rulemaking to accomplish multiple objectives. 

First, the EPA is taking this action to clarify how air agencies should meet the statutory 

SIP requirements that apply to areas designated nonattainment for any PM2.5 NAAQS 

under subparts 1 and 4. To this end, the EPA is proposing 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. Second, the EPA is taking this action to provide 

guidance, in addition to regulatory requirements, to assist air agencies in developing 

attainment plans for the 2012 PM2.5 NAAQS and any future revisions of the PM2.5

NAAQS. Finally, the EPA is taking this action in response to the D.C. Circuit’s remand 

of the 2007 PM2.5 Implementation Rule and the 2008 PM2.5 NSR Rule. Through this 

rulemaking, the EPA intends to address requirements associated with states’ ongoing 

implementation efforts for the 1997 and 2006 PM2.5 NAAQS. In the interim, the EPA 

will rely on the statutory attainment planning requirements34 contained in subparts 1 and 

4 and on the EPA’s General Preamble and Addendum for guidance on how to apply those 

requirements to current PM2.5 NAAQS nonattainment areas.

 

34 See “The General Preamble for Implementation of Title I of the Clean Air Act (CAA) 

Amendments,” 57 FR 13498 (April 16, 1992).
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D. State Implementation Planning Process for PM2.5 NAAQS

1. Overview

The CAA 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 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 each nonattainment area, consistent with the requirements of the CAA. The EPA often 

assists air agencies by promulgating regulations or providing guidance for meeting 

implementation requirements and technical tools, including information on 

control measures.35 For example, the EPA intends this rulemaking to clarify the specific 

statutory requirements, and schedule for meeting those requirements, that state and tribal 

air pollution control agencies (“air agencies”) must address as they prepare SIP 

submissions for the PM2.5 standards in future.

36

 

35 It is important to note that the EPA does not have a mandatory duty to promulgate an 

implementation rule for the PM2.5 NAAQS, and the obligations of state and tribal air 

agencies to develop and submit an attainment plan are independent obligations and not 

conditioned upon the EPA promulgating an implementation rule for the PM2.5 NAAQS.

36 When the term “state” is used hereafter, it will refer generically to states, local air 

agencies, and tribal governments electing to be treated as states for the purposes of 

implementing the CAA. Of additional note is that the 1998 Tribal Authority Rule (TAR), 

which is found in 40 CFR part 49, which implements section 301(d) of the CAA, 

provides that tribes be treated in the same manner as a state when implementing certain 

sections of the CAA. It gives tribes the option of developing tribal implementation plans 
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The EPA also promulgates nationally applicable control requirements and

emission limits for many sources such as new motor vehicles, certain categories of new 

and modified major stationary sources and existing stationary sources of toxic air 

pollutants. These federal actions assist state and tribal air agencies by achieving emission 

reductions from certain categories of sources nationwide, which can help with local 

attainment needs in a given nonattainment area. 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.37 In 

addition, the EPA has the authority and responsibility to review and take action to 

approve or disapprove submitted attainment plans based upon whether they meet 

applicable statutory and regulatory requirements, to provide funding and technical 

assistance to states and to initiate the process for imposition of sanctions and/or issue 

federal implementation plans (FIPs) when states fail to fulfill their CAA obligations.

More information on area designations, the role of ambient air monitoring, the SIP 

development process and the role of federal measures in bringing an area into attainment 

is presented below.

2. Initial Area Designations and Classifications

The NAAQS implementation planning process begins with initial area 

designations, through which states and the EPA identify areas of the country that either 

 

(TIPs), but unlike states, tribes are not required to develop implementation plans. Section 

IX.I of this preamble provides further discussion of tribal issues.

37 See 70 FR 25162 (May 12, 2005) and 76 FR 48208 (August 8, 2011). 
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meet or do not meet the new or revised NAAQS, along with identifying the nearby areas 

contributing to violations of the NAAQS. Section 107(d)(1) of the CAA requires that:

“By such date as the Administrator may reasonably require, but not later than 1 year after 

promulgation of a new or revised national ambient air quality standard for any pollutant 

under section 109, the Governor of each state shall ... submit to the Administrator a list 

of all areas (or portions thereof) in the State” that designates those areas as 

nonattainment, attainment, or unclassifiable.38 Thus, states are required to make their 

initial designation recommendations to the EPA by no later than 1 year after the 

promulgation of new or revised NAAQS. Section 107(d)(1)(B)(i) further provides:

“Upon promulgation or revision of a NAAQS, the Administrator shall promulgate the 

designations of all areas (or portions thereof) ... as expeditiously as practicable, but in no 

case later than 2 years from the date of promulgation. Such period may be extended for 

up to 1 year in the event the Administrator has insufficient information to promulgate the 

designations.” Thus, the EPA is required to promulgate the actual designations for all 

areas across the U.S. by no later than 2 years after the promulgation of any new or 

revised NAAQS, unless the EPA elects to take up to one additional year in situations 

where there is insufficient information. Under section 107(d)(1)(B)(ii), the EPA is 

authorized to modify the designations recommendations from the states, with respect to 

the designation of an area and the boundaries of an area, if the EPA deems that necessary. 

 

38 While the CAA provides for “designating” with respect to the Governor’s list, in the 

full context of the CAA section it is clear that the Governor actually makes a 

recommendation, to which the EPA must respond using a specified process if the EPA 

does not accept the recommendation.
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By no later than 120 days prior to promulgating final designations, the EPA is required to 

notify states of any intended modifications to their recommendations. States then have an 

opportunity to demonstrate to the EPA why the EPA's intended modification is 

inappropriate. Regardless of whether a state provides an initial designation

recommendation for any area, the EPA must timely promulgate the designations it deems 

appropriate.39

Under subpart 4, the CAA provides for classification of PM2.5 nonattainment 

areas as either “Moderate” or “Serious.” As provided in section 188(a) and reiterated in 

the General Preamble, all PM10 nonattainment areas and by extension all PM2.5

nonattainment areas are initially classified as Moderate by operation of law at the time of 

designation. Initial classifications are not subject to public notice-and-comment pursuant 

to section 107(d)(2)(B), although the EPA may elect to take comment on designations 

and classifications and its recent practice has been to do so. 

All areas designated as nonattainment for the 2012 PM2.5 NAAQS and any future 

revised PM2.5 NAAQS will be initially classified as Moderate nonattainment areas upon 

designation, regardless of the severity of the PM2.5 problem in the area. This statutory 

 

39 While section 107 of the CAA specifically addresses states, the EPA is following the 

same process for tribes that choose to make a recommendation to the extent practicable, 

pursuant to section 301(d) of the CAA regarding tribal authority, and the TAR. 63 FR 

7254 (February 12, 1998). To provide for clarity and consistency, the EPA issued a 2011 

guidance memorandum for working with tribes during the designations process. 

Memorandum of December 20, 2011 from Stephen D. Page, Director, Office of Air 

Quality Planning and Standards, to EPA Regional Administrators, Regions I-X re: 

“Guidance to Regions for Working with Tribes during the National Ambient Air Quality 

Standards (NAAQS) Designations Process.” Available at: 

http://www.epa.gov/ttn/oarpg/t1/memoranda/20120117naaqsguidance.pdf.
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approach to classifications for nonattainment areas under subpart 4 for the PM2.5 NAAQS

is notably different from the approach for ozone NAAQS nonattainment areas under 

subpart 2 (of part D, title I of the CAA), wherein the statute includes several area 

classifications, and initial classifications are based on monitored ozone levels. Thus, 

unlike for ozone nonattainment areas, all PM2.5 nonattainment areas initially receive the 

same classification – Moderate – and the EPA only reclassifies such areas to Serious 

upon a showing by the state or a determination by the agency that the area cannot 

practicably attain by the statutory attainment date, or upon a finding that the area in fact 

failed to attain the NAAQS by the applicable Moderate area attainment date. The statute 

requires that Moderate nonattainment areas attain the NAAQS as expeditiously as 

practicable, but not later than the end of the sixth calendar year following designation. 

States have an incentive to avoid having a Moderate area reclassified to Serious because, 

as discussed later in this preamble, the specific subpart 4 requirements for areas classified 

as Serious include, among other things, a more stringent level of control for sources of 

direct PM2.5 and PM2.5 precursors than for Moderate areas. 

As of the date of this proposal, the first round of initial designations for areas for 

the 2012 primary annual PM2.5 NAAQS has been completed, and those designations will 

become effective on April 15, 2015. All areas designated as nonattainment for the 2012 

PM2.5 NAAQS were classified as Moderate nonattainment areas.

40

3. Ambient Air Monitoring for PM2.5

 

40 See the Federal Register notice for the first round of designations for the 2012 PM2.5

NAAQS at 80 FR 2206 (January 15, 2015).
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Ambient air quality monitoring for PM2.5 plays an integral role in implementation 

of a NAAQS, including identifying areas violating the NAAQS, control strategy 

development and tracking progress toward attainment. States are required to monitor 

PM2.5 mass concentrations using approved methods to determine compliance with the 

NAAQS.41 The locations of monitors are identified in states’ Annual Monitoring 

Network Plans, which are required to be submitted to the EPA by July 1 of each year.42

The EPA in turn reviews these annual plans for compliance with applicable regulations 

and consistency with relevant guidance. Currently there are more than 900 monitoring 

locations across the country eligible for comparison to the PM2.5 NAAQS. States are 

required to maintain monitors in designated nonattainment areas in order to track 

progress toward attainment and ultimately determine whether the area has attained the 

PM2.5 standards. In addition to the approved monitors for comparison to the NAAQS, the 

EPA and states also maintain a chemical speciation network (CSN) of about 200 stations 

around the country to support analyses of chemical composition of PM2.5 (e.g. sulfate, 

nitrate and organic carbon). The data provided by the CSN help states identify 

contributing source categories and develop control strategies to reach attainment.

In conjunction with the promulgation of the 2012 PM2.5 NAAQS, the EPA 

finalized a schedule for deployment of PM2.5 monitors at near-road monitoring locations. 

Under revised monitoring requirements, states are required to locate a minimum of one 

 

41 The ambient air monitoring requirements that apply to the PM2.5 NAAQS are detailed 

in 40 CFR part 58. These monitoring requirements are applicable to state and local air 

agencies.

42 See 40 CFR 58.10.
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PM2.5 monitor in each core-based statistical area (CBSA) with a population of 1 million 

or more, to be phased-in between January 2015 and January 2017.43

For initial area designations for any PM2.5 NAAQS, the EPA relies on monitoring 

data to identify areas to be designated nonattainment due to violations of the standard(s). 

The EPA uses other information to identify areas contributing to the monitored violations 

in those areas.44 The agency’s protocol for designating areas and determining whether an 

area has attained the PM2.5 NAAQS is based on monitored air quality data collected over 

a period of 3 calendar years. Data from the new PM2.5 near-road monitors was not 

available for the EPA to consider within the timeframe for initial area designations

provided by the CAA for the 2012 PM2.5 NAAQS; the agency will not be able to consider 

data from a near-road monitor in the implementation process until 3 years of data are 

available. The initial set of near-roadway PM2.5 monitors are to be fully deployed by 

January 2015, with the first 3 years of air quality data (2015-2017) available beginning in 

2018; the second set of near-roadway monitors are to have the first 3 years of data 

available beginning in 2020.

4. SIP Development Process

In general terms, a SIP is the compilation of EPA-approved state statutes, 

regulations and programs that a state develops and relies upon to carry out its NAAQS 

 

43 Near-road monitors for CBSAs larger than 2.5 million in population are to be 

operational by 1/1/2015; and monitors for CBSAs with population larger than 1 million 

but less than 2.5 million are to be operational by 1/1/2017. CBSA is defined by OMB as a 

statistical geographic entity consisting of the county or counties associated with at least 

one urbanized area/urban cluster of at least 10,000 population, plus adjacent counties 

having a high degree of social and economic integration. 

44 See Catawba County v. EPA, 571 F.3d 20 (D.C. Cir. 2009).
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implementation responsibilities under the CAA, including the attainment, maintenance 

and enforcement of NAAQS. States use the SIP development process to identify the 

emissions sources that contribute to the nonattainment problem in a particular area, and to 

select the required emissions reduction measures most appropriate for that area, 

considering factors such as technological and economic feasibility. As part of developing 

an attainment plan, the states must meet specific requirements of the CAA to attain the 

NAAQS, e.g., a state with a Moderate PM2.5 nonattainment area must impose RACM 

(including RACT) and additional reasonable measures on sources located in the 

nonattainment area. Under the CAA, states must develop attainment plans that ensure that 

areas reach attainment as expeditiously as practicable, but no later than the applicable

statutory attainment date. In these attainment plans, states may take into consideration 

emission reductions resulting from federally applicable national programs (such as 

mobile source regulations, the national acid rain program, or maximum achievable 

control technology (MACT) standards for air toxics), as well as from state or local 

programs not directly mandated, but authorized, under the CAA, if such measures are 

incorporated into the SIP and thus are made federally enforceable.

5. Geographic Extent of PM2.5 Problem

The EPA recognizes the significant variability in the nature and sources of PM2.5

in different nonattainment areas and believes it is important to keep this variability in 

mind when providing guidance to states as they develop control strategies to bring their 

PM2.5 nonattainment areas into attainment with the relevant NAAQS. The variability of 

PM2.5 concentrations across the country has a substantial regional component because the 

formation and transport of secondarily formed particles, such as sulfates and nitrates, can 
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extend over hundreds of miles. As a result, monitored violations of the PM2.5 NAAQS 

can often reflect the impact of the combination of “local” sources of emissions located 

within the designated nonattainment area and “regional” sources of emissions that may be 

located much farther away.

In addition, data suggest that ambient PM2.5 concentrations tend to rise and fall in 

a consistent manner across very large geographic areas. The transport phenomenon

associated with PM2.5 and its precursors has been well documented for many years. For 

example, one significant source of information on long-range transport is the National 

Acid Precipitation Assessment Program (NAPAP) research from the 1980s and its 

associated reports published in 1991.45 Additional studies and air quality modeling 

analyses since that time have added to the body of information documenting the regional 

nature of PM2.5.

46

6. Strategies for Reducing Ambient PM2.5

The control measures identified and adopted by a state through the SIP 

development process for bringing nonattainment areas into attainment constitute an 

important component of the CAA’s overall strategy for meeting the PM2.5 standards, but 

 

45 National Acid Precipitation Assessment Program. Acid Deposition: State of the 

Science and Technology. Washington, DC. 1991. See also Environmental Protection 

Agency. (2004) Air Quality Criteria for Particulate Matter. Research Triangle Park, NC: 

Office of Research and Development; report no. EPA/600/P-99/002a,bF. Available at: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html.

46 NARSTO (2004) Particulate Matter Assessment for Policy Makers: A NARSTO 

Assessment. P. McMurry, M. Shepherd, and J. Vickery, eds. Cambridge University Press, 

Cambridge, England. ISBN 0 52 184287 5. For more information, see generally 

http://narsto.org/pm_science_assessment. See also supporting technical information for 

the Clear Skies Act, http://www.epa.gov/clearskies/, and for the CAIR, 

http://www.epa.gov/cleanairinterstaterule.
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they are not the only component. The CAA also includes requirements for national rules 

or programs that will reduce emissions and help achieve cleaner air. Specifically, the 

EPA has adopted a number of national rules over the past few years that require or will 

require emission reductions from sources of both direct PM2.5 and PM2.5 precursors, 

especially of SO2 and NOx. The national rules that will help states meet their attainment 

dates include, but are not limited to: the Tier 2 Light-Duty Vehicle Rule; the Tier 3 

Tailpipe and Evaporative Emission and Vehicle Fuel Standards; the Heavy-Duty Engine 

and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements; the Clean

Air Nonroad Diesel Rule; the Regional Haze Regulations and Guidelines for Best 

Available Retrofit Technology Determinations; the NOx Emission Standard for New 

Commercial Aircraft Engines; the CSAPR; the Emissions Standards for Locomotives and 

Marine Compression-Ignition Engines; the Control of Emissions for Nonroad Spark 

Ignition Engines and Equipment; the C3 Oceangoing Vessels rule; area and major source

Boilers NESHAPs, New Source Performance Standards and Emission Guidelines for 

Hospital/Medical/Infectious Waste Incinerators; the Reciprocating Internal Combustion 

Engines (RICE) NESHAPs; and the Mercury and Air Toxics Standards (MATS).

47

 

47 Compliance with the MATS emission standard for acid gas hazardous air pollutants 

(HAP) is demonstrated by direct measurement of either hydrogen chloride (HCl) or SO2

as surrogates for all acid gas HAP. See CAA section 112(b) for a list of statutory 

hazardous air pollutants. Thus, compliance with MATS is expected to result in a 

substantial amount of new pollution controls (scrubbers and dry sorbent injection) and 

upgrading of existing acid gas controls that will significantly reduce acid gas emissions, 

including SO2 emissions, from power plants. MATS implementation is projected to 

reduce nationwide SO2 emissions from power plants to a level more than 40 percent 

lower than the SO2 emissions projected under CSAPR without MATS in place. States 

that want to rely on SO2 reductions from MATS controls as part of their attainment 
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Additionally, there are PM2.5 reductions that will be achieved as a result of 

previously adopted state and local agency regulations and voluntary programs to the 

extent they can be relied on under the EPA’s voluntary measures policies, such as the use 

of low sulfur fuel for home heating and industrial purposes, curtailment of residential 

wood burning and burn bans. Furthermore, under the voluntary PM Advance program, 

the EPA works with states, tribes and local governments to ensure they are aware of the 

advantages of early action and to provide assistance in taking steps to achieve emission 

reductions in areas currently attaining the PM2.5 NAAQS but approaching levels that 

could lead to nonattainment in the future. Early reductions may help these areas maintain 

the annual and 24-hour PM2.5 NAAQS over the long-term. Furthermore, there may be 

emissions controls that can be implemented to meet NAAQS for ozone (O3) or SO2 that 

may have co-benefits for meeting and continuing to meet the current PM2.5 NAAQS and 

any future revised PM2.5 NAAQS. 

 

strategy for the PM2.5 NAAQS must adopt and have approved into their SIPs specific SO2

emissions limits that equate with SO2 reductions attributable to compliance with the 

MATS rule since coal- and oil-fired power plants can choose to comply with either the 

HCl or alternative equivalent SO2 surrogate standard. Thus, absent a SIP requirement 

requiring future compliance with the SO2 surrogate standard, the power plant could at any 

time revise its MATS compliance approach and comply instead with the HCl surrogate 

standard. This approach is similar to that recommended in the EPA’s guidance for 

nonattainment areas for the 2010 1-hour SO2 NAAQS. See "Guidance for 1-Hour SO2

Nonattainment Area SIP Submissions." Memo from Stephen D. Page, Director, Office of 

Air Quality Planning and Standards, U.S. Environmental Protection Agency, to Regional 

Air Division Directors, April 23, 2014. Available at: 

http://www.epa.gov/airquality/sulfurdioxide/pdfs/20140423guidance.pdf. 
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The EPA will continue to work closely with air agencies as they develop and use 

an appropriate combination of national, regional and local pollution reduction measures 

to meet the standards as expeditiously as practicable, as required by the CAA.

III. What is the EPA proposing with respect to the treatment of PM2.5 precursors in 

nonattainment area planning and permitting?

A. Background

The EPA recognizes that a threshold question in developing PM2.5 attainment 

plans and implementing NNSR programs is the question of which precursors must be 

regulated in a given nonattainment area in order to attain the relevant NAAQS and to 

meet the statutory requirements of part D, including subpart 4, of the CAA. Before 

discussing the specific CAA attainment plan and NNSR requirements in detail in 

Sections IV through IX of this preamble, the EPA discusses in this section how a state

should evaluate PM2.5 precursors in order to identify the specific precursors to which the

PM2.5 attainment plan and NNSR requirements will apply in a given nonattainment area. 

This section first provides a brief overview of the precursor policies that the agency 

included in the 2007 PM2.5 Implementation Rule and in the 2008 PM2.5 NSR Rule for the 

1997 PM2.5 NAAQS that were remanded by the court. It then describes the EPA’s three 

proposed options for addressing PM2.5 precursors under the attainment planning and 

NNSR programs to meet the statutory requirements of subpart 4. Lastly, this section 

discusses possible approaches for states to develop an adequate technical demonstration 

showing whether emissions of a given PM2.5 precursor significantly contribute to ambient 

concentrations that exceed the standard. The EPA requests public comment on the 

options and information presented below. 
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The EPA’s 2007 PM2.5 Implementation Rule included regulatory presumptions 

concerning the need to address certain PM2.5 precursors in attainment plans and through 

control measures related to those plans.48 The EPA has long recognized the scientific 

basis for concluding that there are multiple scientific precursors to PM10, and in particular 

to PM2.5.

49 As described in Section II of this preamble (on technical background issues 

associated with PM2.5 and PM2.5 precursors), appropriate control of precursors is 

especially important because secondarily formed particles comprise a large fraction of 

ambient PM2.5 concentrations in many nonattainment areas.

Section 302(g) of the CAA indicates that the term “air pollutant” 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.” In the 2007 PM2.5 Implementation Rule and the 2008 PM2.5 NSR 

Rule, the EPA recognized that the main scientific precursors of fine particle formation are 

SO2, NOx, VOC, and ammonia. Pursuant to the discretionary authority provided under 

section 302(g) to identify PM2.5 precursors for a particular program, the EPA also 

included requirements describing which precursor gases states were to evaluate for 

potential emission reductions as part of the state’s analysis of control measures to bring 

the area into attainment as expeditiously as practicable. 

 

48 See 2007 PM2.5 Implementation Rule, 72 FR 20586 at 20589-97 (April 25, 2007). 

49 See, e.g., the EPA’s 2007 PM2.5 Implementation Rule which discussed the fact that 

emissions of SO2, NOx, VOC and ammonia are factual and scientific precursors to PM2.5. 

Ibid. That rule was at issue in NRDC v. EPA.
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To facilitate the evaluation and identification of reasonable control measures, the 

2007 PM2.5 Implementation Rule included nationally applicable presumptions regarding 

the need to evaluate and potentially control emissions of certain precursors. Specifically, 

in 40 CFR 51.1002, the EPA provided that a state must evaluate sources of direct PM2.5

and SO2 for potential control measures; a state presumptively was required to evaluate 

sources of NOx for potential control measures; and, a state was presumptively not

required to evaluate sources of VOC and ammonia emissions for potential control 

measures. The EPA established these presumptions concerning VOC and ammonia in the 

2007 PM2.5 Implementation Rule because of factors such as uncertainties regarding the 

emissions inventories for ammonia, uncertainties concerning the role of some VOC in the 

formation of particles, and uncertainties regarding the effectiveness of specific precursor 

control measures in various regions of the country in reducing PM2.5 concentrations. For 

example, in some areas of the U.S., emission reductions of a particular precursor may 

lead to large changes in PM2.5 concentrations because there are relatively few tons of 

such precursor emissions in the area in the first place. In other areas, the opposite may be 

true, where emission reductions of a particular precursor may lead to small changes in 

PM2.5 concentrations because the area has an abundance of emissions of that particular 

precursor.

The rule also included provisions for potentially reversing the EPA’s initial 

presumptions for certain precursors in a nonattainment area where the state or the EPA 

had information demonstrating that the presumption was not valid for that area. The EPA 

left open the possibility in the 2007 PM2.5 Implementation Rule for regulation of VOC 

and ammonia emissions as PM2.5 precursors in any nonattainment area where regulation
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was necessary for purposes of attaining the 1997 PM2.5 NAAQS. Similarly, the EPA left 

open the possibility for not regulating NOx where NOx sources from within the state did 

not have a significant contribution to PM2.5 concentrations in the nonattainment area. The 

preamble to the 2007 PM2.5 Implementation Rule discussed that to “reverse” the 

presumptions in the rule for NOx, VOC or ammonia, the state would need to provide an 

appropriate technical demonstration, and it provided examples of the types of analyses 

that could be included in such a demonstration. The EPA intended these to be rebuttable 

presumptions that either the state or the EPA might reverse through notice and comment 

rulemaking. These presumptions were not limited to precursor emissions only from major

stationary sources, but rather were presumptions applicable to precursor emissions from 

all sources of such emissions within the area.50

The 2008 PM2.5 NSR Rule included similar policies for precursor presumptions in 

connection with the NSR requirements for nonattainment areas (the NNSR program).51

That rule provided a discussion of the possibility for the state or the EPA to provide a 

technical demonstration to reverse the presumptions for NOx, VOC or ammonia.52 The 

one significant difference between the two rules was the geographic scope of the 

requirements. The 2008 PM2.5 NSR Rule indicated that a precursor presumption could be 

rebutted if the emissions of that precursor from sources within the nonattainment area

(emphasis added) did not significantly contribute to PM2.5 concentrations in the 

nonattainment area. This distinction is logical because the requirements of the NNSR 

 

50 Ibid. 

51 See 73 FR 28321 (May 16, 2008) at 38326-27.

52 Ibid. 
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program apply only to sources located within a designated nonattainment area. 

Conversely, the 2007 PM2.5 Implementation Rule indicated that the evaluation of whether 

a given precursor should be regulated should be based on emissions from sources 

throughout the entire state (emphasis added), because the state air agency has jurisdiction 

over sources throughout the entire state in developing strategies to improve air quality 

specifically in nonattainment areas. A more complete discussion of the 2008 NNSR 

program requirements for the PM2.5 NAAQS and the proposed changes concerning the 

regulation of PM2.5 precursors from new or modified major stationary sources of PM2.5

precursors in PM2.5 nonattainment areas is provided in Section VIII of this preamble. 

The EPA’s approach to the evaluation and regulation of PM2.5 precursors in both 

the 2007 and 2008 rules for implementing the 1997 PM2.5 NAAQS was called into 

question 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, 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 decision specifically discussed both the approach to 

precursors in the 2007 PM2.5 Implementation Rule and the 2008 PM2.5 NSR Rule and 

compared those 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) [section 189(e)]. 

But 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 
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“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.53

...

In light of our disposition, we need not address the petitioners’ challenge to the 

presumptions in [40 CFR 51.1002] that volatile organic compounds and ammonia 

are not PM2.5 precursors, as subpart 4 expressly governs precursor presumptions.54

Section 189(e) for PM10 precursors (which the court concluded expressly includes 

PM2.5) 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.55

The provisions of subpart 4 do not define the term “precursor” for purposes of 

PM10, nor do they explicitly require the control of any specifically identified particulate 

matter precursor. However, as stated above, 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 

 

53 NRDC v. EPA, 706 F.3d 428, 437, n.7.

54 NRDC v. EPA, 706 F.3d 428, 437, n.10.

55 Ibid. 
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PM, and thus the attainment plan requirements of subpart 4 initially apply equally to 

emissions of direct PM2.5 and all of its identified precursors, except as otherwise provided 

in the statute (e.g. CAA section 189(e)). Section 189(e) explicitly requires the control of 

precursors from all major stationary sources, 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 area.56 Section 189(e) contains 

the only express exception to control requirements under subpart 4. The control 

requirements for major sources referred to in this exception include requirements for

RACM and RACT, additional reasonable measures, BACM and BACT, most stringent 

measures (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 section 

189(e) to both existing and new major stationary sources, including new and modified 

sources subject to NNSR permitting requirements. Even though section 189(e) only 

explicitly contemplates exceptions to control requirements for PM2.5 precursors from 

major stationary sources, 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 source categories as well, under appropriate circumstances.

When Congress adopted the 1990 CAA Amendments, a NAAQS for PM10 was in 

effect, but no standard for PM2.5 had yet been established. At that time, it was understood 

 

56 The EPA notes that it has already addressed the requirements of subpart 4 for 

precursors, specifically within the context of the requirements of section 189(e), in the 

General Preamble. See 57 FR 13498 at 13539 and 13541- 2. 
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that the interaction of PM precursors in the atmosphere led to the formation of particulate 

matter 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, section 189(e) provided a possible 

exception to the requirement to control all PM2.5 precursors from major sources in all 

nonattainment areas.

While section 189(e) expressly requires control of precursors from major 

stationary sources where direct PM from major sources is to be controlled unless certain 

conditions are met, as stated above, it is clear that subpart 4 and other CAA provisions 

collectively require the control of direct PM 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 

area.57 Long-standing EPA guidance for RACM has stated that the state should inventory 

all emissions of the relevant pollutants and precursors in the nonattainment area and 

evaluate all economically and technologically feasible control measures for the relevant 

pollutant and precursors, and that the state should adopt those measures that are deemed 

reasonably available and necessary in order to attain the NAAQS 

 

57 See CAA requirements for states to demonstrate attainment “as expeditiously as 

practicable” (section 188(c)(1); section 172(a) (2)).
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as expeditiously as practicable.58 The state also must ensure that there is no other 

collection of available control measures that if adopted would advance the attainment 

date by at least one year.59 Section IV.D of this preamble provides additional discussion 

on the development of emissions inventories and the identification, adoption and 

implementation of reasonable control measures for Moderate PM2.5 nonattainment

areas.

60

B. Proposed Precursor Policy Options

The EPA is proposing this rule to address the attainment plan and certain NNSR 

requirements for PM2.5 under subpart 4. In light of the court’s decision in NRDC v. EPA, 

the EPA considers it necessary to address in this implementation rule how states must

address regulation of PM2.5 precursor gases in attainment plans and NNSR programs for 

the PM2.5 NAAQS. As noted earlier, the court’s decision made clear that appropriate 

regulation of all precursors is initially presumptively required under the CAA, and the 

regulation of precursors is a critical issue for attainment of the PM2.5 NAAQS because 

 

58 57 FR 13498.

59 In the context of the PM10 NAAQS, the EPA has concluded that “advancement of the 

attainment date” should mean an advancement of at least one calendar year. See State 

Implementation Plans; General Preamble for the Implementation of Title I of the CAA 

Amendments of 1990, 57 FR 13498 (April 16, 1992). See also Sierra Club v. EPA, 294 

F.3d 155 (D.C. Cir. 2002).

60See Section IV of this preamble for a thorough discussion of past reasonably available 

control measures (RACM) and reasonably available control technology (RACT) policy 

and guidance. Section IV discusses the EPA’s proposed policy that under subpart 4, for 

Moderate areas that demonstrate that attainment by the statutory attainment date is 

impracticable, RACM and RACT would constitute all those technologically and 

economically feasible measures available for sources in the area that can be implemented 

within 4 years of designation, but they would not constitute the complete set of measures 

required to demonstrate attainment as expeditiously as practicable.
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secondarily formed particles are a substantial component of the PM2.5 nonattainment 

problem in most areas of the U.S.

For the purposes of this implementation rule, the EPA considers that for all 

nonattainment areas, the PM2.5 precursors for regulatory purposes are SO2, NOx, VOC 

and ammonia. This rule does not propose any national presumption that would simply 

allow a state to exclude sources of emissions of a particular precursor from further 

analysis for control requirements. However, the EPA’s existing 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 area, i.e., states may determine that only certain precursors need to be 

regulated for attainment purposes.61 Courts have upheld this approach to the requirements 

of subpart 4 for PM10.

62

The EPA believes that application of a similar approach to PM2.5 precursors under 

subpart 4 is appropriate and reasonable. Thus, this proposal describes three proposed 

precursor options that provide for the possibility that, with appropriate justification

provided by the state, further evaluation and implementation of control strategies for one 

or more PM2.5 precursors in a given nonattainment area may not be needed or required.

Under each option, a state may provide a technical demonstration and reasoned 

 

61 See 57 FR 13498 at 13540-41.

62 See, e.g., Assoc. of Irritated Residents v. EPA, et al., 423 F.3d 989 (9th Cir. 2005).
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justification for the exclusion of a PM2.5 precursor or precursors from control 

requirements for a particular nonattainment area.

As explained above, the EPA interprets the CAA to require states to inventory and 

regulate all sources of PM2.5 precursors from all sources in the area, including area 

sources, mobile sources and stationary sources. This interpretation is based on CAA 

provisions requiring adoption of all RACM needed to attain the standard as expeditiously 

as practicable; section 302(g), which defines an air pollutant as including all precursors 

contributing to the formation of that pollutant; and, the EPA’s identification of the four 

main PM2.5 precursors. For major stationary sources, section 189(e) requires that the 

control requirements applicable for major stationary sources of PM2.5 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 which exceed the standard in the area. Thus, the statute generally 

requires control of all PM2.5 precursors, but it provides an express exception applicable to 

major stationary sources. Because the statutory provisions of subparts 1 and 4 are not 

explicit with respect to how states should address PM2.5 precursors from non-major

sources, the EPA is proposing regulations to assure proper evaluation and regulation of 

PM2.5precursor emissions in PM2.5 nonattainment areas. Moreover, even with respect to 

regulation of precursor emissions from major stationary sources, section 189(e) contains 

ambiguities that require interpretation. For example, section 189(e) does not specify the 

method by which the EPA should determine whether precursor emissions from major 

stationary sources contribute significantly to levels which exceed the standard in a given 

nonattainment area. Given that the provisions of subpart 4 are ambiguous with respect to 
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these issues, the EPA believes that it is necessary to interpret those requirements in this 

rulemaking.

The EPA is thus seeking comment on three potential approaches to address PM2.5

precursors pursuant to the specific statutory requirements of subpart 4 and the 

overarching requirements of the CAA. In these proposed options, particular emphasis is 

given to the situations and circumstances under which the state would or would not be 

required to evaluate emission controls for a particular precursor and to adopt those 

controls that are necessary to demonstrate attainment of the NAAQS as expeditiously as 

practicable. Note that these options describe analyses that the state may choose to pursue 

to demonstrate that control requirements should not apply to a particular precursor. 

However, the state also may choose to require controls for all PM2.5 precursors in 

attainment plans and in its NNSR permitting program, and choose not to conduct any

analyses to eliminate one or more precursors from consideration for controls. 

The descriptions of the three precursor policy options being proposed in this 

section discuss how PM2.5 precursors would need to be addressed by the state with regard 

to three specific implementation situations: (1) a Moderate area for which attainment of 

the relevant NAAQS by the end of the sixth calendar year after designation can be 

demonstrated; (2) a Moderate area for which it can be demonstrated that the relevant 

NAAQS cannot practicably be attained by the end of the sixth calendar year after 

designation; and (3) an area that is reclassified to Serious and is obligated to develop a 

Serious area attainment plan to attain the relevant NAAQS. Additionally, the EPA 

describes how each of the proposed precursor policy options would apply to the 

implementation of NNSR in a Moderate or Serious PM2.5 nonattainment area. Later in 
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this section, the EPA discusses specific issues related to the technical “precursor 

demonstrations” that states could choose to develop. The technical demonstration section 

includes a discussion of several types of analyses that a state could provide to the EPA to 

show that control measures for a specific PM2.5 precursor would not be needed for 

attainment or to expedite attainment, or to show that major stationary sources of a given 

precursor collectively do not significantly contribute to PM2.5 levels that exceed the 

relevant NAAQS in a given area.

Before discussing the three precursor options, it is important to introduce a new

term that is used throughout this section and other sections of the notice. Under subpart 4, 

RACM (including RACT) are those measures that can and must be implemented within 4

years of the area’s designation as nonattainment (pursuant to section 189(a)(1)(C)). The 

EPA recognizes, however, that other, similarly reasonable emissions reduction measures 

could be implemented after this 4 year period, and as late as the end of the sixth calendar 

year following designation, to help an area attain as expeditiously as practicable. 

Therefore, in this proposal the EPA is proposing to define the term “additional reasonable 

measures” to describe those technologically and economically feasible control measures 

that could not be implemented within the 4 year period after designation, but could be 

implemented starting any time after that 4 year period through the end of the sixth 

calendar year after designation (note that this period could extend almost 3 additional 

years, depending on when during the year area designations are finalized). See proposed 

40 CFR 51.1000. The EPA proposes to require implementation of these “other” control 

measures to the extent necessary to demonstrate attainment by the applicable attainment 

date pursuant to section 172(c)(6) of the CAA. That provision provides that 
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nonattainment “plan provisions shall include enforceable emissions limitations, and such 

other control measures...as may be necessary or appropriate to provide for attainment of 

such standard in such area by the applicable attainment date...” Together, RACM and 

RACT and “additional reasonable measures” make up the set of control strategies 

referred to in this proposed rule as “reasonable control measures.”63 (Section IV.D of this 

preamble provides a detailed discussion of how a state must determine reasonable control 

measures for a Moderate PM2.5 nonattainment area.) The EPA requests comment on each 

of the three proposed options discussed below which describe how a state may 

demonstrate that additional emissions reductions of a particular precursor would not be 

needed or appropriate for an area’s attainment plan, and how it could demonstrate that 

emissions control requirements for a particular precursor would not be needed in NNSR 

permits for new or modified sources in the area. In particular, the EPA requests comment 

on whether only one of these approaches should be included in the final rule, or whether 

it would be appropriate to include multiple approaches (e.g., both Options 1 and 2), or 

only specific elements from the different options. The three proposed options are 

summarized as follows:

 Option 1: Two independent analyses: (a) an attainment planning analysis 

demonstrating that control measures for a particular precursor are not needed for 

expeditious attainment, meaning that the precursor can be excluded from measures 

needed to attain as expeditiously as practicable for all types of sources; and (b) a 

 

63 In Section VI.D, the EPA describes a parallel approach for distinguishing control 

measures required under sections 172(c)(6) and 189(b)(1)(B) for Serious nonattainment 

areas.
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section 189(e) technical demonstration showing that major stationary sources of a 

particular precursor do not contribute significantly to levels that exceed the PM2.5

standard, meaning that the precursor can be excluded from control requirements for 

major sources including NNSR permitting;

 Option 2: Single analysis demonstrating that all emissions of a particular precursor 

from within the area do not significantly contribute to PM2.5 levels that exceed the 

standard, meaning that control requirements for emissions of the precursor from 

major stationary and area sources, as well as mobile sources, would not be required 

for expeditious attainment, control requirements for major sources, or for NNSR 

permitting;

 Option 3: An attainment planning analysis demonstrating that control measures for all 

types of sources of a particular precursor are not needed for expeditious attainment 

also would be deemed to meet the section 189(e) technical demonstration 

requirement, meaning that the state would not need to regulate emissions of the 

particular precursor from major stationary sources under the NNSR permitting 

program or other control requirements for major stationary sources.

Each of these proposed options is presented in greater detail below. 

1. Option 1: Two independent analyses: a) an attainment planning analysis 

demonstrating that control measures for a particular precursor are not needed for 

expeditious attainment, meaning that the precursor can be excluded from measures 

needed to attain as expeditiously as practicable for all types of sources; and b) a 

section 189(e) technical demonstration showing that major stationary sources of a 

particular precursor do not contribute significantly to levels that exceed the PM2.5
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standard, meaning that the precursor can be excluded from control requirements for 

major sources and from NNSR permitting.

As with the other options discussed below, 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 be unable to reasonably determine whether emissions 

of a given PM2.5 precursor contribute significantly to the nonattainment problem in an 

area if the state has failed to account adequately for all such emissions in the area in its 

emissions inventory. 

In general terms, Option 1 would require separate analyses for purposes of 

attainment planning and for NNSR. Section 189(a) of the CAA describes the 

requirements for Moderate nonattainment areas. Within 18 months of designation as 

nonattainment, the state is required to submit a Moderate area plan that either 

demonstrates attainment as expeditiously as practicable but by no later than the end of the 

sixth year following designation, or demonstrates that attainment by such date would be 

impracticable. 

Under Option 1, the state would determine the precursors for which new control 

measures need to be adopted for a given nonattainment area through its determination of 

reasonable control measures needed for attainment. The state’s analysis of reasonable 

measures for a given PM2.5 NAAQS nonattainment area should begin by identifying 

potential control measures (and factors related to technological feasibility, economic 

feasibility, and time needed for implementation) for all precursors from all types of 

sources in the area (i.e., stationary, area, mobile) included in the emissions inventory. The 

analysis of reasonable measures and selection by the state of those emissions reduction 
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measures that would provide for attainment as expeditiously as practicable (but no later 

than the end of the sixth calendar year after designation) would determine which 

precursors must be regulated in the nonattainment area for purposes of attainment. Except 

for the requirement to determine whether implementation of all remaining reasonable 

measures could collectively advance attainment by a year, there would be no additional 

demonstration needed by the state to justify that attainment planning control requirements 

should not apply to a particular precursor. Therefore, the analysis of reasonable measures 

may result in the state controlling only a subset of the four main PM2.5 precursors as part 

of the attainment demonstration. 

a. Moderate area for which the state can demonstrate attainment by the statutory 

attainment date. For certain nonattainment areas, the state may be able to demonstrate 

that attainment of the standard “as expeditiously as practicable” is possible by the end of 

the sixth year after designation (the statutory Moderate area attainment date) or sooner, 

and could be achieved by adopting regulations to reduce emissions of only a subset of the 

four PM2.5 precursors. Under this scenario, the state would be expected to provide 

analytical information showing that, even though new economically and technically 

feasible control measures may be available for one or more precursors, the reductions in 

emissions of the precursor(s) that could be achieved are not necessary for expeditious 

attainment and would not advance the attainment date by at least a year. Under Option 1, 

if the state determined that new emissions reductions of a particular precursor are not 

necessary for attainment and would not accelerate the attainment date by at least 1 year, 

then for the purposes of this particular PM2.5 Moderate area attainment plan, the state 

would not need to adopt additional control measures for that PM2.5 precursor. Given that 
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additional regulation of that PM2.5 precursor would not be necessary for attaining the 

standard as expeditiously as practicable, the EPA would be able to approve the attainment 

plan for the area as meeting the requirements of subpart 4.

b. Moderate area for which the state can demonstrate that attainment by the 

statutory attainment date is impracticable. Section 189(a)(1)(B) provides that for certain 

nonattainment areas, the state may demonstrate that, even with implementation of all 

reasonable control measures available for reducing emissions of all direct PM and PM2.5

precursors, it would be impracticable to attain the standard by the end of the sixth

calendar year after designation. In other words, the analysis would need to demonstrate 

that implementing all economically and technically feasible control measures that are 

available in the area, and the expected air quality change from such measures, would not 

be able to provide for attainment by the end of the sixth year after designation. 

For states that can make the showing that they cannot attain the NAAQS by the 

end of the sixth calendar year after designation, the question arises as to whether the state 

should be required to adopt all reasonable measures (i.e. measures that represent RACM 

and RACT because they are technologically and economically feasible and can be 

implemented in 4 years and all additional reasonable measures that can be implemented 

within 6 years) through regulation as part of the Moderate area plan, even if a subset of 

these measures collectively would have a minimal effect on reducing ambient PM2.5

concentrations. The EPA proposes two sub-options for areas that cannot demonstrate 

attainment during the Moderate area timeframe even with the implementation of all

reasonable measures in the area. Under the first sub-option, the state would be required to 

adopt all available control measures for precursors through regulation as part of the 
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Moderate area plan. The rationale supporting this approach would be that adopting all 

technologically and economically feasible measures would bring the area as close to 

attainment as possible during the timeframe prescribed for Moderate areas. Under this 

approach, if a measure can be implemented by the end of the sixth calendar year after the 

nonattainment designation and it meets the criteria for being considered “reasonable,” 

then the state must adopt and implement the measure. 

Under the second sub-option, the state would be able to elect not to impose those 

technologically and economically feasible measures that collectively have minimal effect 

on ambient PM2.5 levels in the area, based on the premise that such measures would be 

unreasonable to implement. To support this conclusion, the state would need to submit a 

technical demonstration showing that implementing available emissions controls for a 

particular precursor and/or a specific set of sources would provide only minimal changes 

in PM2.5 concentrations in the area, and therefore such control measures should not be 

required during the timeframe prescribed for Moderate areas. The EPA requests comment 

on these two sub-options, including any technical information that would help support the 

commenter’s position. Regarding the second sub-option, the EPA requests comment on 

what degree of air quality change should be considered minimal for purposes of this 

analysis.64

 

64 Note that under either sub-option, the state would be able to show that control of

precursor emissions from major stationary sources would not be required if it could be 

demonstrated that such emissions do not contribute significantly to PM2.5 levels that 

exceed the standard, consistent with section 189(e).
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c. Area reclassified to Serious. A Moderate area can be reclassified to a Serious 

area under two scenarios. Under the first scenario, if a Moderate area fails to attain the 

standard by the applicable attainment date, it would then be reclassified by the EPA as a 

Serious area and the state would be required to develop and submit a Serious area 

attainment plan within 18 months of reclassification. Under the second scenario, the EPA 

could reclassify an area to Serious prior to the Moderate area attainment date if the EPA 

determines that it would be impracticable for the area to attain by the Moderate area 

attainment date. (Section V of this preamble provides additional detail on reclassifying a 

Moderate area to Serious under subpart 4.)

After an area has been reclassified to Serious, subpart 4 requires a state’s Serious 

area attainment plan to include the imposition of more stringent control measures (best 

available control measures (BACM) and best available control technology (BACT)) 

intended to bring the area into attainment as expeditiously as practicable but no later than 

the end of the tenth calendar year after designation. Given that the CAA requires a more 

stringent new attainment plan for Serious areas, under Option 1 the state would be 

required to identify the best available measures for all sources of direct PM2.5 emissions 

and emissions of PM2.5 precursors and adopt those measures to attain the standard as 

expeditiously as practicable.

65

The BACM and BACT determination requires a more rigorous analysis than the 

RACM and RACT analysis, and such measures collectively should lead to a greater 

 

65 The EPA’s two proposed options for determining BACM and BACT are discussed in 

detail in Section VI.D of this preamble.
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degree of emission reduction in the area than the analysis of reasonable control measures 

for the Moderate area plan. For this reason, under Option 1, if the state’s previous 

Moderate area attainment plan had indicated that new emissions reduction measures from 

sources of one or more precursors were not needed to attain by the end of the sixth 

calendar year after designation, then for the Serious area plan the state would need to 

reevaluate the best control measures addressing all PM2.5 precursors (i.e. SO2, NOx, 

VOC, and ammonia) and require implementation of those “best” available control 

measures for all precursors in order to bring the area into attainment as expeditiously as 

practicable, but no later than the end of the tenth year after designation. Under Option 1, 

any precursor demonstration that excluded one or more precursors from regulation in the 

Moderate area plan would not by itself also be sufficient to exclude the precursors from 

regulation in the Serious area plan. Further analysis would be needed to determine if 

control measures for those precursors qualify as “best” control measures. The EPA has 

interpreted the starting point for considering “best” control measures as including those 

control measures to reduce emissions of direct PM2.5 or PM2.5 precursors that have been 

adopted by any state, particularly those states with the most severe PM2.5 air quality 

problems. (Note that in Section VI.D of this preamble, more details are provided on 

BACM and BACT determination criteria. The EPA is taking comment on two options for 

BACM and BACT determinations – one that expresses it as a requirement independent of 

the attainment demonstration, and one that expresses it as only those “best” measures that 

are needed for expeditious attainment no later than the end of the tenth calendar year after 

designation. The BACM and BACT determination approach adopted in the final rule 
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accordingly will determine whether all best available emission controls for a particular 

precursor must be adopted or not in a Serious area).

d. NNSR. Under Option 1, the initial expectation is that the state will need to 

address all four PM2.5 precursors under the NNSR program pursuant to the CAA and as 

reinforced by the January 2013 NRDC v. EPA court decision. Pursuant to section 189(e), 

however, the state may provide a demonstration showing that emissions of a particular 

precursor from existing major stationary sources located in the nonattainment area do not 

contribute significantly to PM2.5 levels that exceed the standard in the area. Under Option 

1, this analysis under section 189(e) for major sources would be completed independently 

from the analysis of reasonable control measures conducted for attainment planning 

purposes. Such an analysis would involve assessing the potential addition of precursor 

emissions in the area due to potential new major stationary sources, and would likely 

involve air quality modeling and other technical analyses by the state, developed in 

consultation with the EPA (see Section III.C. of this preamble for further discussion on 

such technical demonstrations). Note that under this provision of the CAA, it might be 

possible that a precursor would be considered important for attainment planning 

purposes, but would not be regulated as a PM2.5 precursor in NNSR permitting actions

which, by definition, only apply to major sources of the nonattainment pollutant. For 

example, it might be possible that in a particular area the principal source of emissions of 

a certain precursor could be from mobile and area sources but not from major stationary 

sources of that precursor. The EPA requests comment on all aspects of proposed Option 1 

as discussed above.
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2. Option 2: Single analysis demonstrating that all emissions of a particular precursor 

from within the area do not significantly contribute to PM2.5 levels that exceed the 

standard, meaning that control requirements for emissions of the precursor from 

stationary major and area sources, as well as mobile sources, would not be required 

for expeditious attainment, control requirements for major sources, or for NNSR 

permitting.

Option 2 would provide the state the opportunity to provide the EPA with a 

scientifically credible technical analysis that would demonstrate that one or more 

precursors do not contribute significantly to the PM2.5 levels that exceed the standard, 

therefore controls on those emissions would not be effective in reducing PM2.5 levels in 

the area. As noted earlier in this section of the preamble, section 302(g) of the CAA 

includes “precursors” in the definition of “air pollutant,” but provides the EPA with some 

discretion in defining how these terms should be interpreted. In subpart 4, the CAA does 

not explicitly address control of precursors, except with regard to major stationary 

sources in section 189(e). The EPA interprets subpart 4 to require states to address PM2.5

precursors from all source categories in the evaluation of controls needed for attainment 

in a given area, e.g., in the evaluation of RACM and RACT level controls. By analogy to 

section 189(e), the EPA also believes that there may be circumstances in which states 

may validly demonstrate that control of one or more PM2.5 precursors is not needed to 

attain the relevant NAAQS expeditiously.

Section 189(e) provides that precursor control requirements apply to major 

stationary sources of precursors of PM2.5 if major sources of PM are regulated under the 

attainment plan, unless it can be shown that such precursor emissions do not contribute 
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significantly to exceedances of the relevant NAAQS in the area. Under Option 2, the 

EPA relies on the discretion provided in section 302(g) and the section 189(e) concept of 

precursor emissions in an area having a significant or insignificant effect on PM2.5

concentrations that exceed the standard to propose two precursor technical demonstration 

suboptions. Option 2A would allow the state to provide a technical demonstration 

showing that all emissions (i.e., from area, mobile and stationary sources in the area) of a 

particular precursor collectively do not provide a significant contribution to PM2.5 levels 

that exceed the standard in the area. The kinds of analytical approaches that could be 

appropriate for this type of “contribution demonstration” are described later in this 

section.

For Option 2B, the EPA proposes to allow states to provide a technical 

demonstration showing that PM2.5 concentrations in the area are not sensitive to potential 

reductions or increases in emissions of a particular precursor in the nonattainment area

(e.g. because the particular precursor is not the limiting factor in secondary PM2.5

formation). More information is provided later in this section about possible analytical 

approaches to assess precursor “sensitivities” in an area (the optional technical 

demonstration described for Options 2A and 2B hereafter will be referred to as a 

“precursor demonstration”). The EPA requests comment on which of the two options 

(Option 2A or Option 2B) would be more preferable, and why. The EPA encourages 

commenters to provide examples of specific situations and areas in support of their 

recommendations.

These proposed options are consistent with the EPA’s past practice for 

determining which technologically and economically feasible controls are necessary for 
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expeditious attainment of the NAAQS. Specifically, the EPA has interpreted the RACM 

requirement in the CAA as requiring imposition of all reasonable controls as needed for 

expeditious attainment or to advance the attainment date by at least one year. The statute 

does not require imposition of additional controls if collectively such measures would not 

advance the attainment date. The EPA maintains it is reasonable to treat regulation of 

PM2.5 precursors in a manner similar to the agency’s treatment of direct pollutants and 

therefore concludes that states should not be required to implement control measures for 

a particular precursor or precursors if such measures will have little or no impact on 

PM2.5 concentrations in the area or if the state demonstrates that all emissions of a given 

precursor or precursors do not contribute significantly to the PM2.5 NAAQS exceedances 

in the area. 

a. Moderate area for which the state can demonstrate attainment by the statutory 

attainment date or for which the state can demonstrate that attainment by the statutory 

attainment date is impracticable. An approved precursor demonstration under Option 2A 

would show that emissions of the particular precursor from all types of sources do not 

contribute significantly to PM2.5 levels that exceed the standard. As proposed, this type of 

demonstration therefore by definition would also satisfy the section 189(e) provision 

(which allows the state to demonstrate that emissions from just major stationary sources 

are not significant and therefore should not be subject to control requirements, such as 

NNSR, that apply to major stationary sources of direct PM2.5). Thus, the state could 

possibly develop one precursor demonstration analysis that would serve the purposes of 

both attainment planning and the section 189(e) insignificant major source contribution

demonstration.
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The sensitivity analyses required under Option 2B would need to assess a series 

of precursor emissions reductions and increases to determine the sensitivity to air quality 

in the area. For example, the analysis should evaluate the effect on PM2.5 concentrations 

of various precursor emissions reduction scenarios appropriate to determine the 

sensitivity of precursors for the area (as would be relevant for an attainment plan); the 

analysis should also evaluate the effect on PM2.5 concentrations of various precursor 

emissions increase scenarios appropriate to determine the sensitivity of precursors for the 

area, simulating the potential effect of the addition of potential new major stationary 

sources (or major modifications) to the nonattainment area under the NNSR program.

The EPA would evaluate the relevant analyses and other supporting information 

provided by the state. By submitting a “precursor demonstration” of this type, the state

would not need to compile additional information on precursor control measures, or to 

proceed with actions to adopt and implement local or state regulations for the precursor. 

Precursor demonstrations as described in Options 2A or 2B could be conducted for

Moderate areas for which the state can show that it can attain the standard by the end of 

the sixth calendar year after designation and for Moderate areas where the state’s plan 

demonstrates that attainment by such date would be impracticable.

The EPA believes that general legal authorities under the CAA support the 

proposal of the overall precursor demonstration concept described above, and that 

requesting comment on these proposed options is appropriate from both a technical and a 

legal standpoint. This case specific approach is technically appropriate because the mix 

of PM2.5 precursor emissions and other relevant technical factors varies from area to area. 

For example, in some areas, one precursor may be abundant while the main precursor 
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with which it reacts may be less abundant. In such cases, reducing emissions of the less 

abundant precursor (the “limiting” precursor) is generally more effective for reducing 

PM2.5 concentrations. 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. The emissions of the particular precursor from sources in the 

nonattainment area could be found to have an insignificant contribution to PM2.5 levels

that exceed the standard, and the potential air quality improvement from reducing 

emissions of the precursor in the area may be limited.

The EPA believes that proposing Options 2A and 2B is appropriate from a legal 

standpoint based on authority provided the Administrator in sections 302(g) and 

301(a)(1) of the CAA. Section 302(g) includes in the definition of “air pollutant” all the 

precursors to that pollutant, and it allows the EPA Administrator to regulate precursors 

for “the particular purpose for which the term ‘air pollutant’ is used.” Under section 

301(a)(1), “[t]he Administrator is authorized to prescribe such regulations as are 

necessary to carry out his functions under this Act.” Thus, with Option 2, the EPA 

proposes a framework by which the regulation of PM2.5 precursors for a specific 

nonattainment area can be modified if the state provides the EPA with a credible 

technical demonstration for exempting a particular precursor which meets certain criteria 

and can be approved by the EPA. In addition, as noted earlier the set of analyses 

described under Option 2A could also satisfy the section 189(e) provision allowing the 

state to demonstrate that major stationary source emissions of a particular precursor do 

not significantly contribute to levels that exceed the standard. While this approach is not 

explicitly described in the statute, the EPA believes that the proposed Option 2 approach 
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to precursor regulation is reasonable and allowed under the statutory authority provided 

in sections 302(g) and 301(a)(1) noted above. 

The EPA anticipates that development of an approvable PM2.5 precursor 

demonstration by the state at the beginning of the attainment plan development process 

will require a substantial level of effort and consultation with the EPA. Such a 

demonstration by the state would likely involve a combination of technically rigorous and 

complex analyses, such as air quality modeling and ambient data analyses. The extensive 

nature of this type of a technical demonstration early in the attainment plan development

process is necessary because the demonstration serves as the basis for limiting the 

applicability and associated control strategy decisions only to specific precursors for both 

the attainment plan and for the NNSR permitting program. 

b. Area reclassified to serious. As noted earlier in this section, a Moderate area 

can be reclassified to Serious under two scenarios. Under the first scenario, if a 

Moderate area fails to attain the standard by the end of the sixth calendar year 

after designation, it would then be reclassified by the EPA as a Serious area, and 

the state would be required to develop and submit a Serious area attainment plan 

within 18 months of reclassification. Under the second scenario, EPA could 

reclassify an area to Serious prior to the Moderate area attainment date if it can be 

shown that it would be impracticable for the area to attain by the Moderate area 

attainment date. 

Proposed Option 2 would allow a “precursor demonstration” approach for Serious 

area plans in the same manner as for Moderate area plans. However, if the state had 

previously submitted a precursor demonstration that the EPA approved for the Moderate 
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area attainment plan, under either proposed Option 2A or 2B the state would be required 

to review and update the precursor demonstration, taking into account any changes in the 

emissions inventory and any other relevant information or advances in technical tools 

developed since the initial demonstration was approved. Examples of such information 

would be improved emission estimation methods or emission factors for key source 

categories; changes in precursor emissions inventories due to emissions control programs 

or new source growth; the development of more advanced technical tools to assess the 

effectiveness of precursor reductions; and, updated information about new or more 

effective control technologies or emission reduction techniques. Any precursor 

demonstration that is approved as part of the Serious area attainment plan would need to 

be revised and updated if the area cannot attain the standard by the end of the tenth

calendar year after designation and seeks an extension under section 188(e) or does not 

attain the standard by the applicable Serious area attainment date and is subsequently 

subject to 5 percent annual emission reductions under section 189(d).

One other important factor to consider is the substantial amount of time that can 

elapse between the submission of a Moderate area attainment plan for a particular 

nonattainment area, and submission of a Serious area attainment plan. The plan for a 

Moderate area is due within 18 months of designation. Under the EPA’s overall proposed

approach to attainment plan development, the state would be required to evaluate control 

measures for all types of sources and for all PM2.5 precursors in order to ensure 

attainment of the standard as expeditiously as practicable. The full assessment to identify 

reasonable control measures would involve a thorough compilation and analysis of 

information on control technologies and the technological feasibility of implementation 
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of such measures for sources in the area; the assessment of associated control costs and 

economic feasibility of implementation; information on the time needed for deployment 

and implementation of such control measures; and, the resulting timeline for achieving 

emissions reductions. 

If the Moderate area does not attain the standard by the end of the sixth calendar 

year after designation, then as required by to the CAA, the EPA would have 6 months to 

make a determination to that effect, and the area would be reclassified to Serious. The 

state would then have 18 months to submit, at a minimum, a new attainment 

demonstration and control strategy comprising BACM and BACT. Thus, under these 

circumstances, these key Serious area plan elements would be due at least 8 years after 

the EPA designated the area nonattainment, and more than 6 years after the state 

submitted the original Moderate area plan. Because of the potentially protracted timeline 

for developing, implementing and revising as necessary the SIP for a given PM2.5

nonattainment area under subpart 4, the EPA believes it is reasonable for the state to be 

required to update any precursor demonstration it had previously developed for the area if 

the area is reclassified as Serious. 

The EPA requests comment on the requirement for the state to review and update 

any previously approved “precursor demonstration” if the area fails to attain the standard 

by the applicable Moderate area attainment date. The EPA also requests comment on the 

requirement for the state to review and update any previously approved “precursor 

demonstration” if the area fails to attain the standard by the applicable Serious area 

attainment date. 
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c. NNSR. An approvable precursor demonstration under either Option 2A or 

Option 2B would evaluate emissions of a particular precursor from all types of 

sources. Accordingly, if the state provides an approvable precursor demonstration 

for all types of sources of a particular precursor as described above, then under 

Option 2A, the state would also be able to rely on the same technical 

demonstration to conclude that emissions of that precursor just from major 

stationary sources in the area do not provide a “significant contribution” to PM2.5

concentrations in the area pursuant to section 189(e). Thus, under Option 2A, the 

state would not need to apply the NNSR control requirements for PM2.5 to that 

precursor in the particular PM2.5 nonattainment area(s) for which the EPA 

approves the demonstration.

Under Option 2B, the state would conduct analyses to determine the sensitivity of 

PM2.5 levels in the area (that exceed the standard) to potential increases in emissions 

(relevant for NNSR) and decreases (relevant for attainment demonstrations). If the state 

provided an approvable precursor demonstration showing that PM2.5 concentrations are 

insensitive to potential increases in emissions of a particular precursor in the area, then 

under Option 2B the state would be able to rely on this technical demonstration as the 

basis for not regulating that precursor for major stationary sources under NNSR. 

Additionally, there could be a situation where the state finds that emissions of 

another precursor (i.e., a precursor that was not the subject of the initial precursor 

demonstration) from only major stationary sources located in the nonattainment area 

could be considered to have an insignificant contribution to PM2.5 levels that exceed the 

standard in the area (under Option 2A). For example, mobile and area source emissions 
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of a PM2.5 precursor could be determined to provide a larger contribution to PM2.5 levels 

than major stationary sources in a given nonattainment area and would be the focus of the 

attainment strategy, and the major stationary source emissions of that same precursor 

might have only a minimal contribution to PM2.5 levels. In this situation, the state could 

develop a separate demonstration under section 189(e) to support the exclusion of the 

additional precursor from implementation requirements applicable to all major stationary 

sources, including NNSR program requirements (assuming the state analysis includes 

appropriate consideration of potential new sources of the relevant precursor). With an 

approved demonstration under section 189(e), major stationary sources of that precursor 

could also be excluded from the NNSR control requirements for PM2.5. The EPA seeks 

comment on all aspects of proposed Option 2.

3. Option 3: An attainment planning analysis demonstrating that control measures 

for all types of sources of a particular precursor are not needed for expeditious 

attainment also would be deemed to meet the section 189(e) technical 

demonstration requirement, meaning that the state would not need to regulate 

emissions of the particular precursor from major stationary sources under the 

NNSR permitting program or other control requirements for major stationary 

sources.

Under proposed Option 3, the consideration of precursors in the attainment 

planning process for Moderate and Serious areas would closely follow the approach 

described for Option 1 (see Sections III.B.1.a-c of this preamble). As described for 

Option 1, after developing a comprehensive emissions inventory, the state would conduct 

an analysis to identify the new reasonable control measures that need to be adopted and 
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implemented in order for the Moderate area to attain the standard as expeditiously as 

practicable, but no later than by the end of the sixth calendar year after designation (this 

analysis is described in greater detail in Section III.B.1.a in this preamble, above). If the 

state determines that adoption of additional economically and technically feasible 

emission reduction measures for a particular precursor are not necessary for expeditious 

attainment by the end of the sixth calendar year after designation, and that such measures 

collectively would not accelerate the attainment date by at least a year, then for the 

purposes of this Moderate area attainment plan, the state would not need to adopt such 

additional measures because they would not be considered reasonable. (Note that the 

need for additional emissions reductions of the particular precursor would have to be reevaluated if the area is reclassified to Serious, or if the area submitted a SIP revision 

requesting an extension of the Serious area attainment date under section 188(e)).

To clarify the intent of Option 3, unlike under Option 1, a separate analysis to 

show that major stationary sources of a particular precursor do not contribute 

significantly to PM2.5 levels in a given PM2.5 nonattainment area for purposes of section 

189(e) would not be needed. If the state’s single analysis shows that emission reduction 

measures are not needed from sources of a particular precursor in order to demonstrate 

expeditious attainment, then under proposed Option 3 the same analysis would also be 

considered adequate to meet the requirements of section 189(e). In effect, the attainment 
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planning analysis would define the set of precursors that would be subject to control 

under both the attainment plan and the NNSR permitting program for the area.

66

The rationale supporting the Option 3 approach focuses on the section 189(e) 

emphasis on precursor control requirements. If control measures are not needed in a 

Moderate nonattainment area to reduce emissions of a particular precursor from all types 

of sources in order to demonstrate attainment or to advance the attainment date, then 

under the rationale of proposed Option 3, it would follow that the state would not need to 

include any other control requirements that apply to major stationary sources of that 

precursor, including control requirements for PM2.5 under the NNSR program. The theory 

for this option would be that if the state determines that new control requirements for 

emissions of the particular precursor are not needed for purposes of attainment planning

because they would not contribute to reducing PM2.5 levels that exceed the standard, then 

other control requirements to address emissions of that precursor also would not be 

needed. Note that under this option, the state also would not be required to analyze the 

potential effect of increases in emissions of the particular precursor (e.g., from the 

possible permitting of new sources) on PM2.5 concentrations in the area. The EPA 

requests comment on the rationale supporting Option 3.

Additionally, under Option 3, as was the case with Option 2, there could be a 

situation where the state determines that control measures for a particular precursor are 

 

66 Note that while the NNSR program needs to be implemented from the effective date of 

an area’s nonattainment designation, in some situations the state would implement either 

its existing NNSR program for PM2.5 or, in the absence of such program, 40 CFR part 51 

Appendix S, the default NNSR program, until the EPA approves the state’s PM2.5

attainment plan and revised NNSR regulations for PM2.5.
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generally needed in order to demonstrate attainment as expeditiously as practicable, but 

that the major stationary sources of that precursor that are located in the nonattainment 

area have an insignificant contribution to PM2.5 levels that exceed the standard in the 

area. Under this Option 3, the EPA believes that section 189(e) provides the state with the 

authority to develop a separate demonstration to show that, even though control measures 

for a specific precursor emitted by sources other than major stationary sources are 

necessary to demonstrate expeditious attainment in an area, major stationary sources of 

that precursor have an insignificant contribution to PM2.5 concentrations that exceed the 

standard in the area. Thus, controls from major stationary sources of that precursor would 

not be required for either the attainment plan or the NNSR program. More discussion on 

the potential options for precursor technical demonstrations is included in Section III.C of 

this preamble. The EPA seeks comment on all aspects of proposed Option 3.

The EPA also seeks comment on whether only one of these approaches should be 

included in the final rule, or whether it would be appropriate to include multiple 

approaches (e.g., both Options 1 and 2) or a hybrid of two approaches by which a state 

could demonstrate that a particular precursor would not need to be addressed in the 

attainment plan or NNSR permitting program for a specific area.

C. Technical Approaches for Demonstrating that a Precursor Does Not Need to Be 

Subject to Control Requirements

As noted earlier, in the preamble to the 2007 PM2.5 Implementation Rule, the EPA 

included a discussion allowing for the state to submit a technical demonstration to show 

to the satisfaction of the EPA that emissions of a particular precursor do not significantly 

contribute to PM2.5 concentrations in the area. In that preamble discussion, the EPA 
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indicated that such a demonstration should be based on the weight of evidence of 

available information, and that any such demonstration by the state must be approved by 

the EPA. The 2007 PM2.5 Implementation Rule also discussed a number of types of 

analyses that could inform this precursor demonstration, such as speciation data analyses, 

air quality modeling studies, chemical tracer studies, emissions inventories, or special 

intensive measurement studies to evaluate specific atmospheric chemistry in an area. In 

the 2007 PM2.5 Implementation Rule, the EPA intended to provide states with the 

flexibility to provide a range of different supporting analyses that would be appropriate 

for the area, recognizing that nonattainment areas differed in terms of such factors as: (i) 

the mix of emissions sources located in the nonattainment area and outside the area that 

are contributing to PM2.5 concentrations in the area; (ii) the levels of PM2.5 species 

measured in the area; (iii) the times of year when highest PM2.5 concentrations are 

observed; (iv) the topography of the area; (v) the severity of the nonattainment problem;

and, (vi) the patterns of emissions and population growth in and around the 

nonattainment area. Under the 2007 PM2.5 Implementation Rule, an important criterion 

for any technical precursor demonstration provided by a state, however, was that it had to

fairly represent the information available to the state and the information made available 

to it by the public. 

For this proposed implementation rule, the EPA similarly proposes that the state 

should have the flexibility to present multiple types of analyses to support any 

demonstration for exempting a precursor from control requirements as long as they fairly 

represent the available information, and accordingly proposes that the EPA should review 

any such demonstration based on the weight of evidence. Unlike in the prior 
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implementation rule, however, later in this section the EPA raises the question of whether 

certain specific types of analyses should be included as minimum required components of

any precursor demonstration that a state chooses to submit to the EPA for approval.

The preamble to the 2007 PM2.5 Implementation Rule indicated that if a state 

developed a precursor demonstration as part of its draft SIP, then in accordance with the 

state rulemaking process, the demonstration would be subject to public review at the state 

level. It 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 insuring that this important issue was explicitly addressed and supported in 

the attainment plan 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 regulations being 

proposed as part of this rule include similar language providing for public review of any 

proposed precursor demonstration. 

The 2007 PM2.5 Implementation Rule did not provide a specific due date for 

submittal of any precursor demonstration, although it was assumed that if a state were to 

pursue such a demonstration, it would need to be done early in the attainment plan

development process and submitted to the EPA no later than the date of the attainment 

plan submission itself. It was recommended that the state develop any such 

demonstration in consultation with the appropriate EPA Regional Office. In this proposal, 

the EPA is proposing that if a state is interested in developing a PM2.5 precursor 

demonstration to support not regulating one or more PM2.5 precursors in the attainment 
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plan for an area, it should consult with the EPA Regional Office as early as possible to 

discuss appropriate analyses to be included. In its review of any precursor demonstration

provided by a state, the EPA will consider all currently available information. 

Under all three proposed precursor policy options described above, the state

would have the opportunity to provide a precursor demonstration to meet the 

requirements of section 189(e) of the CAA. Precursor demonstrations pursuant to section 

189(e) should evaluate the significance of the contribution of emissions of a particular 

precursor from existing major stationary sources to fine particle concentrations that 

exceed the standard. However, Options 2A and 2B differ from the others in that they

would provide the state with the ability to conduct a precursor demonstration that 

comprehensively assesses the contribution of a particular precursor from all types of 

sources in the nonattainment area (not just from major stationary sources as specifically 

addressed by section 189(e)) for the purposes of informing which precursors must be 

addressed in both the attainment plan and in the NNSR program for a particular PM2.5

nonattainment area. (Note that Option 2 would not prevent the state from also conducting 

an additional analysis under section 189(e), if warranted, to further demonstrate that 

while all emissions of a particular precursor make a significant contribution to PM2.5

levels that exceed the standard, the emissions from just the major stationary sources of 

that precursor collectively do not contribute significantly to PM2.5 levels that exceed the 

NAAQS in the area.) The EPA has considered three important questions regarding the

scope and the potential requirements associated with precursor demonstrations, and

requests comment on the questions and technical analysis options presented below. 
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1. What is the geographic area from which precursor emissions should be 

assessed?

In the 2007 PM2.5 Implementation Rule, the preamble indicated that a precursor 

demonstration analysis addressing all source types covered by the attainment plan should 

evaluate the impact of emissions from sources located throughout the entire state. In 

contrast, the 2008 PM2.5 NSR Rule suggested that a precursor demonstration for NNSR

purposes should evaluate emissions from major stationary sources of a particular 

precursor located within the nonattainment area only. 

In determining which approach to include in the present proposal, the EPA 

believes that it continues to be reasonable that any precursor demonstration conducted to 

assess precursor significance for NNSR purposes should evaluate emissions from major 

stationary sources of the precursor from within the nonattainment area only. Section 

189(e) is included in a part of the CAA that specifically sets forth nonattainment area 

requirements. For attainment planning purposes it is less clear that the evaluation of 

emissions should be limited only to sources from within the nonattainment area, because 

the state has jurisdiction over emissions sources located throughout the state, and can 

impose emission reduction requirements on contributing sources outside of 

nonattainment areas if necessary to help bring areas with violating monitors into 

attainment. At the same time, that argument would suggest that section 189(e) should be 

interpreted as requiring two different analyses of the impacts of precursors emitted from 

two different geographic scales (from within the nonattainment area, as well as from a 

broader area that influences air quality within the nonattainment area, which could 

include the entire state). The EPA does not believe such an interpretation is required, nor 
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does it believe that such multiple analyses are warranted. The statute simply refers in 

general terms to precursor emissions from major stationary sources and does not 

differentiate between control requirements for attainment planning and control 

requirements for other purposes, such as NNSR permitting. The statute also does not 

indicate that multiple analyses must be done to assess major stationary source impacts 

from multiple geographic scales. For these reasons, the EPA is proposing that any 

precursor demonstration must include an evaluation of emissions from sources located in 

the nonattainment area only. The EPA requests comment on this proposed approach. 

2. Should the EPA’s guidance provide a specific list of analyses as “minimum 

requirements” that must be included in any proposed precursor demonstration?

As noted above, the EPA encourages states to provide a range of analyses to 

thoroughly understand the effect of precursor emissions on PM2.5 concentrations in an 

area. In past discussions with state representatives regarding potential approaches to 

regulating PM2.5 precursors, some representatives have suggested that this PM2.5

implementation rulemaking should include more specificity about the minimum 

requirements for technical demonstrations to support exclusion of PM2.5 precursors from 

regulatory requirements in attainment plans, while others have recommended a less 

prescriptive approach. One overarching issue is how detailed the EPA’s guidance should 

be with regard to the analytical requirements for any proposed precursor demonstration.

As noted earlier, technical demonstrations can include data such as ambient speciation 

data analyses, air quality modeling studies, chemical tracer studies, emissions inventories, 

and/or special intensive measurement studies. Air quality modeling analyses are 

discussed in more detail below.
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a. Contribution analysis. Based on the statutory language of section 189(e), it 

appears that, at a minimum, any precursor demonstration conducted specifically pursuant 

to section 189(e) must evaluate the contribution of current emissions of the relevant 

precursor from existing major stationary sources to current (or most recent) PM2.5

concentrations observed in the nonattainment area. (Note that this type of analysis is 

possible under Option 1 and Option 3.) In addition, as described above, any precursor 

demonstration under Option 2A must evaluate the contribution of emissions of the 

relevant precursor from all sources (not just major stationary sources) to current (or 

recent) PM2.5 concentrations observed in the nonattainment area.

In light of the statutory language and the capabilities of existing technical tools, 

the EPA proposes to require that the state conduct such a contribution analysis at a 

minimum as part of any proposed precursor demonstration, and that the state conduct an 

analysis using an air quality modeling system that adequately accounts for the PM2.5

pollution problem within the nonattainment area. 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.

67 For example, 

states could compare base case conditions (at current precursor emissions levels) with a 

separate model simulation in which the relevant precursor emissions are reduced by a 

large percentage. The difference in the estimated PM2.5 concentrations provides one 

 

67 For more information on CMAQ, see 

http://www.epa.gov/AMD/Research/RIA/cmaq.html. For more information on CAMx, see 

http://www.camx.com/.
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indication of the relative significance of the precursor emissions to PM2.5 concentrations 

in the area. This type of contribution analysis can also be accomplished by using existing 

advanced tools within photochemical air quality models, such as “source apportionment” 

capabilities which allow one to track precursor emissions as they “form” PM2.5 (in the 

model) and then report their contributions separately. The EPA requests comment on 

including a contribution analysis as a minimum requirement in any proposed precursor 

demonstration under Option 2A.

b. Sensitivity analysis. The EPA notes that changes in PM2.5 concentrations from 

current conditions in any area will not necessarily be linear with respect to changes in 

PM2.5 precursor emissions. Therefore, another important question is whether any 

precursor demonstration should be required to include an assessment of how “sensitive” 

the area will be to potential reductions or increases in emissions of the relevant precursor. 

Sensitivity analyses of potential reductions in emissions would be most appropriate for 

attainment planning (and relevant to Option 2B), whereas sensitivity analyses of potential 

increases in emissions (e.g., relevant to NNSR permitting) would be appropriate for all 

section 189(e) technical evaluations (possible under Options 1, 2B, and 3). Sensitivity 

analyses are important because of the complexity and variability of the atmospheric 

chemistry affecting PM2.5 concentrations in different areas across the country. 

The principal PM2.5 components that are secondarily formed in the atmosphere are 

the result of chemical reactions between various PM2.5 precursors (see Section II of this 

preamble for more information on specific precursor reactions). 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 
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or limiting PM2.5 formation in the particular area. Likewise, in evaluating which 

precursors would be appropriate to exclude from regulation for NNSR in an area, it is 

important to understand the current sensitivity of the atmosphere to potential increases in 

precursor emissions that could result from the addition of new sources to the 

nonattainment area. 

One approach to assessing precursor sensitivities would be to conduct a model 

simulation that evaluates the effect on PM2.5 concentrations in the area resulting from a 

given set of precursor emission reductions and emission increases. Simulations could be 

conducted to assess a set of emission reduction and emission increase scenarios deemed 

appropriate to determine the sensitivity of a particular precursor in a specific area. 

Another approach that could be used is a scientific technique called the “decoupled direct 

method” (DDM), which efficiently estimates the impacts on PM2.5 concentrations as a 

result of reducing or increasing precursor emissions in the model.

68

For the reasons discussed above, the EPA also proposes that any precursor 

demonstration conducted under proposed Option 2B must provide a set of sensitivity 

analyses that evaluate the effect of a range of emissions changes associated with 

measures considered economically and technically feasible in a particular nonattainment 

area. Analyses that reduce emissions of a particular precursor will help the state and the 

EPA to understand how “responsive” the atmosphere would be to control measures and 

how effective such reductions would be relative to other precursor reductions. Although 

 

68 See Simon et al., Memorandum to ozone NAAQS docket EPA-HQ-OAR-2008-0699, 

“Model-based Rollback Using the Higher Order Direct Decoupled Method (HDDM),” 

August 14, 2012.
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not specifically required for other options under this proposed rule, precursor sensitivity 

analyses evaluating the effect of varying degrees of potential precursor reductions would 

provide meaningful information for any precursor demonstration intended to show that a 

particular precursor does not need to be addressed for attainment planning. Conversely, 

sensitivity analyses that consider the effect of a range of potential emissions increases in 

the nonattainment area will help the state and the EPA to understand the potential 

response of PM2.5 concentrations to projected growth in the area, including potential 

increases in emissions associated with potential newly permitted sources that emit the 

precursor in question. Any precursor demonstration intended to show that a particular 

precursor does not need to be addressed for NNSR should include sensitivity analyses 

evaluating the effect of varying degrees of precursor emission increases in the area. The 

EPA recommends that the state conduct these analyses using air quality modeling tools, 

but the state could provide additional relevant analyses as well. The EPA requests 

comment on the proposed requirement for inclusion of sensitivity analyses in any 

precursor demonstration.

3. Should there be a “bright line” value to indicate that any estimated 

contribution to annual average or 98th percentile PM2.5 concentrations in the 

nonattainment area that exceeds this value would be considered “significant”?

In considering this question, it is helpful to first look to how the concept of a 

significant, or insignificant, contribution has been interpreted with regard to particulate 

matter in past PM10 guidance (Addendum to the General Preamble) and in other PM2.5-

related regulations, such as the CAIR. In the Addendum, the EPA introduced the concept 

of a “de minimis” impact from a source category for the purposes of the identification and 
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evaluation of BACM.

69 While a later discussion in this proposal addresses whether or not 

to maintain a similar de minimis source category-based policy approach for future BACM 

and BACT source category analyses, what is relevant for this precursor discussion is the 

EPA’s guidance in the Addendum on what could be considered a “de minimis,” or 

“insignificant,” ambient impact for purposes of PM10. In the Addendum, the EPA 

indicated that a 1 μg/m3contribution to the annual PM10 standard of 50 μg/m3(equal to 2 

percent of the applicable NAAQS at the time), or a 5 μg/m3contribution to the 24-hour 

PM10 standard of 150 μg/m3(equal to 3.3 percent of the applicable NAAQS at the time) 

presumptively would be considered “de minimis.” The EPA set forth these levels in a 

Federal Register document, citing the discretionary authority of an administrative agency 

to exempt from regulation emissions (from source categories) “which contribute only 

negligibly to ambient concentrations which exceed the NAAQS.”

Developed pursuant to subpart 4, this past guidance on what could be considered 

to be a de minimis or insignificant level of PM10 contribution from a source category can 

potentially inform this proposed rule for implementing the PM2.5 NAAQS. Accordingly, 

this proposal includes two options: (i) a “no-threshold” option, and (ii) a proposed 

threshold option derived from the ambient levels relied on for the PM10 source category 

de minimis thresholds, but adjusted to account for the 2012 PM2.5 NAAQS. 

 

69 State Implementation Plans for Serious PM10 Nonattainment Areas, and Attainment 

Date Waivers for PM10 Nonattainment Areas Generally; Addendum to the General 

Preamble for the Implementation of Title I of the CAA Amendments of 1990, 59 FR 

41998 at 42011 (August 16, 1994).
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The concept of “significant contribution” also has been a central one with regard 

to interstate transport and the interpretation of section 110(a)(2)(D) of the CAA. In past 

programs to address interstate transport, such as the CAIR, an “upwind” state was 

identified as potentially subject to additional emission control requirements if the impact 

of SO2 and NOx emissions from the upwind state to any nonattainment area in a 

downwind state exceeded 1 percent of the relevant PM2.5 standard at a violating monitor 

in another state. This was merely the first step of the analysis, but it provided an initial 

threshold for determining whether further analysis was warranted. In this proposal, the 

concept of a significant contribution refers to the effect of emissions of a particular 

precursor from sources within the state or nonattainment area to local PM2.5

concentrations in the nonattainment area. The specific purpose and context for which the 

phrase “contribute significantly” is used in section 189(e) is very different from the 

purpose and context for which it is used in section 110(a)(2)(D). Thus, while a previous 

interstate transport rule under section 110(a)(2)(D) considered the combined impact of 

SO2 and NOx emissions from an upwind state on ambient PM2.5 at a violating monitor to 

be insignificant if it was less than 1 percent (i.e., 0.15 μg/m3on an annual average basis), 

it would not necessarily be appropriate to also consider the contribution from emissions 

of a specific precursor within a nonattainment area to be “insignificant” if it does not 

exceed a similar 1 percent ambient concentration level.70

 

70 See Rule To Reduce Interstate Transport of Fine Particulate Matter and Ozone (Clean 

Air Interstate Rule); Revisions to Acid Rain Program; Revisions to the NOx SIP Call, 70 

FR 25162 (May 12, 2005).
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There are a number of important distinctions between the section 110(a)(2)(D) 

interstate transport provision and the section 189(e) provision addressing contributions of 

major stationary sources in a nonattainment area which would indicate that the 1 percent

of the NAAQS significant contribution thresholds that have been included in section 

110(a)(2)(D) rulemakings may not be relevant for purposes of section 189(e) precursor 

demonstrations. Section 110(a)(2)(D) was designed to address the collective contribution 

of interstate transport of pollution from multiple upwind states, while section 189(e) 

addresses contributions from major stationary sources in a single nonattainment area. In 

addition, section 110(a)(2)(D) requires that SIPs contain provisions to eliminate the 

contributions that are deemed significant, whereas section 189(e) merely requires that the 

emissions be controlled. Given the differences in purpose, scale, and scope, the EPA does 

not believe it is necessary for a threshold for “significant contribution” to be the same for 

the two programs. 

Based on the considerations discussed above regarding inclusion of a potential 

significance “threshold” for purposes of this PM2.5 implementation rulemaking, the EPA

proposes and seeks comment on two options. The first option would not specify a 

threshold for what is a significant contribution to levels that exceed the relevant NAAQS 

in a given area. Rather, the state would be required to conduct a contribution analysis and 

sensitivity analyses as described above to determine the estimated level of ambient 

impact from the relevant precursor, and to provide the analyses to the EPA as part of its 

precursor demonstration. The EPA would then consider these analyses in addition to the 

other analyses provided by the state in determining whether to approve the precursor 

demonstration. This option would provide greatest flexibility for the state and the EPA to 
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consider the contribution analysis in combination with other information relevant to the 

unique PM2.5 composition, source mix, and attainment needs of each individual 

nonattainment area. See proposed 40 CFR 51.1006.

The second option would specify a “significance” threshold of 3 percent, such 

that if contribution modeling indicated that base year emissions of the precursor from the 

relevant sources in the nonattainment area (i.e. from major stationary sources for all 

analyses pursuant to section 189(e); from all types of sources for the upfront analysis in 

Option 2) leads to an ambient impact that exceeds 3 percent of the PM2.5 NAAQS (e.g., ,

0.36 μg/m3on an annual average basis for the 2012 primary annual PM2.5 NAAQS) at 

monitors in the nonattainment area, then the precursor demonstration would not be 

approvable. The threshold equivalent to 3 percent of the relevant PM2.5 NAAQS is

proposed as reasonable because it is between the two de minimis ambient contribution 

levels included in previous PM10 guidance issued under subpart 4 to identify a de minimis

level of ambient contribution from a group of emissions sources. The EPA acknowledges 

that the context in which the proposed threshold is used here is different from the context 

in which it was used in previous guidance. Absent any explicit language provided in the 

statute to define significant contribution in the context of section 189(e), however, the 

only other existing guidance that in some way addresses the concept of significant 

contribution for PM10 is the de minimis source category threshold values from the 

Addendum. One benefit of having a specific threshold in the rule is that states will have 

more concrete guidance on what could potentially be approvable in a precursor 

demonstration. 
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The EPA therefore seeks comment on: (1) whether a specific significant 

contribution threshold should be included in the final rule or not; (2) if the commenter 

considers inclusion of a specific threshold to be appropriate, whether the proposed 3 

percent of the relevant NAAQS threshold and its basis would be appropriate, and why; 

and (3) whether a threshold with an alternative level and supporting rationale would be 

more appropriate. 

IV. What requirements apply to Moderate area attainment 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 (section 189(a)(1)(A)); (ii) a demonstration that the 

plan provides for attainment by no later than the applicable Moderate area deadline or a 

demonstration that attainment by that deadline is impracticable (section 189(a)(1)(B)); 

(iii) provisions for the implementation of RACM and RACT no later than 4 years after 

designation (section 189(a)(1)(C)); (iv) quantitative milestones that will be used to 

evaluate compliance with the requirement to demonstrate reasonable further progress

(RFP) (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 required for major stationary sources by section 189(e)). Other subpart 1 

requirements for attainment plans continue to apply to PM2.5 nonattainment areas subject 

to subpart 4 and include the following: (i) a description of the expected annual 

incremental reductions in emissions that will demonstrate RFP (section 172(c)(2)); (ii) 

emissions inventories (section 172(c)(3)); (iii) other control measures (besides RACM 
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and RACT) needed for attainment (section 172(c)(6); and, (iv) contingency measures 

(section 172(c)(9)).

Each of these statutory requirements is described more fully below. In certain 

cases, the EPA is proposing options for implementing a statutory requirement for 

purposes of the PM2.5 NAAQS. Based on comments the agency receives, the EPA will 

then promulgate regulations to implement the statutory requirements in the final action on 

this proposal, as appropriate. The EPA notes that its longstanding guidance on these 

statutory requirements is embodied in the General Preamble and the Addendum.71 Where 

appropriate, this proposal notes options that may vary from past EPA guidance and 

explains the EPA’s reasons for considering an amended approach.

A. Plan Due Dates

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.72 To 

be consistent with this subpart 4 deadline for the attainment plan submission, the EPA is 

 

71 57 FR 13498 at 13536-45; 59 FR 41988.

72 The EPA notes that Congress provided different statutory deadlines for submission of 

attainment plans under subpart 1 and subpart 4. Under section 172(b), the EPA is directed 

to establish the date for the attainment plan submission, but it can extend no later than 

three years from the date of a nonattainment designation. By contrast, under section 

189(a)(2)(B), the statute provides that states must make the attainment plan submissions 

within 18 months after designation. Due to the court decision in NRDC v.EPA, however, 

the EPA has proposed to promulgate an alternative submission date for attainment plans 

for the 1997 PM2.5 and 2006 PM2.5 NAAQS in order to provide a reasonable, prospective 

due date for attainment plans that must comply with subpart 4 requirements and to clarify 
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proposing that states must also submit those elements of the attainment plan required 

under subpart 1 (i.e., emissions inventories and contingency measures) no later than 18 

months from the date of designation of the area. The provisions of subpart 4 do not 

explicitly specify when states must submit these attainment plan elements that carry over 

from subpart 1, so the EPA needs to interpret the requirements of the CAA to meet the 

objectives of the attainment plan requirements. The EPA believes that requiring states to 

submit the necessary emissions inventory (or inventories) either before or at the same 

time as the other attainment plan elements due under subpart 4 is necessary, given that a 

state will need information contained in the emissions inventory for other elements of its 

Moderate area attainment plan, such as its precursor analysis, analysis of RACM and 

RACT and additional reasonable measures, and attainment demonstration modeling. The 

EPA also believes it is reasonable to require the state to submit contingency measures, 

which need to be adopted and ready for immediate 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, simultaneous with the other elements of the attainment 

plan. 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 addressing the proper pollutants for control in a given area, the appropriate 

sources for controls beyond those already required for RACM and RACT for the area, 

 

the requirements that a state must meet prior to redesignation of a PM2.5 nonattainment 

area. See 78 FR 69806-69812 (November 21, 2013).
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and the amount of emission reductions that the contingency measures should achieve, 

based upon the facts and circumstances of the attainment plan for the area.

The EPA believes that the statutory deadline for submission of a Moderate area 

attainment plan for the PM2.5 NAAQS is straightforward and, absent unusual 

circumstances, the statute requires states to make such attainment plan submissions 

within 18 months after the effective date of a nonattainment designation for an area. See

proposed 40 CFR 51.1003(a). Although nothing in the CAA prohibits states from making 

separate attainment plan submissions to meet the individual statutory requirements 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 effective, 

both for the state in the first instance and for the EPA in reviewing the state’s submission. 

For example, the EPA designated areas as nonattainment for the 2012 PM2.5 NAAQS 

with an effective date of April 15, 2015; states will thus be required by statute to submit 

Moderate area attainment plans for any nonattainment areas to the EPA no later than

October 15, 2016.

B. Emissions Inventory Requirements

Pursuant to its authority under section 110 of title I of the CAA, the EPA has long 

required states to submit inventories of the emissions of criteria pollutants and their 

precursors. The EPA codified these requirements in 40 CFR part 51, subpart Q in 1979 

and amended them in 1987. Additionally, the 1990 CAA Amendments revised many of 

the provisions of the CAA related to attainment of the NAAQS and the protection of 

visibility in mandatory Class I federal areas (certain national parks and wilderness areas). 

These revisions established new emissions inventory requirements applicable to areas 
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that were designated nonattainment for certain pollutants. In the case of particulate 

matter, Congress did not create a specific emissions inventory requirement in subpart 4 

that would supersede the emissions inventory requirement under subpart 1. Thus, the 

section 172 (c)(3) emissions inventory requirements continue to apply, and that provision

explicitly requires “a comprehensive, accurate, and current inventory of actual emissions 

of the relevant pollutants” in the nonattainment area. In addition, the specific attainment 

plan requirements for the PM2.5 NAAQS set forth in section 189(a) and associated 

modeling requirements make an accurate and up-to-date emissions inventory a critical 

element of any viable attainment plan. Because of the nature of PM2.5, the EPA concludes 

that the statutory requirements for emissions inventories need further elaboration through 

additional regulatory requirements as described below.

Emissions inventory data serve as the foundation for various types of analyses 

that enable states to evaluate the degree to which different emissions sources contribute 

to the nonattainment problem in a given nonattainment area and enable states to estimate 

the air quality improvement that can be achieved through different control measures. 

States should use the best available, current emissions inventory information for 

attainment plan development, because high quality emissions inventory data are essential 

for the development of an effective control strategy. To assist states in preparing 

complete, high quality inventories, the EPA provides guidance for developing emissions 

inventories called “Emissions Inventory Guidance for Implementation of Ozone and 

Particulate Matter National Ambient Air Quality Standards (NAAQS) and Regional 

Haze,” which is available from http://www.epa.gov/ttn/chief/eidocs/eiguid/index.html. 

This guidance is commonly called the “SIP Emissions Inventory Guidance.” The EPA 
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recommends that states consult this guidance while developing the emissions inventories 

to meet statutory and regulatory requirements.

1. How do states meet the inventory requirements for the PM2.5 NAAQS?

Neither section 172(c)(3), nor the provisions specifically applicable to attainment 

plans for the PM2.5 NAAQS in subpart 4, specify how states should meet statutory 

emissions inventory requirements. Although section 172(c)(3) explicitly requires that 

states submit only “an” emissions inventory in conjunction with other elements of an 

attainment plan, that term is ambiguous in the context of the PM2.5 NAAQS, and the EPA 

is authorized to interpret that term and to impose additional requirements as necessary 

and appropriate. In addition, pursuant to section 301, the EPA has additional authority to 

promulgate regulations as necessary for the implementation of the PM2.5 NAAQS, 

including requirements pertaining to emissions inventories. Accordingly, the EPA is 

proposing specific emissions inventory requirements it considers necessary to effectuate 

the attainment plan requirements of the CAA for the PM2.5 NAAQS. 

There are three key facets of the EPA’s proposed emissions inventory 

requirements, as laid out below: (i) the type of inventories required; (ii) the timing of 

submittal of these inventories; and, (iii) the content of these inventories. These inventory 

requirements are being proposed to provide all of the requirements in a concise and direct 

way. In some cases, the EPA’s rationale for the content requirements needs additional 

supporting description, which is provided in the subsequent text related to the use of 

seasonal inventories, required pollutants, etc.

First, the EPA believes that in order to implement the PM2.5 NAAQS effectively, 

states will be required to submit at least two separate and distinct nonattainment area 
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emissions inventories as elements of an attainment plan. The first emissions inventory is 

relevant for assessing the current or base year emissions in the nonattainment area; the 

second emissions inventory is a projected inventory relevant for assessing emissions in 

the target attainment year in the nonattainment area. The first type of inventory is 

expressly required by section 172(c)(3), and is called the “base year inventory for the 

nonattainment area.” The second type of inventory the EPA is proposing to require under 

section 301(a)(1) as necessary to implement the attainment demonstration requirement of 

section 189(a)(1)(B), and is called the “attainment projected inventory for the

nonattainment area.” See proposed 40 CFR 51.1000. The need for this latter inventory 

stems from the need for both the EPA and the public to be able to compare, during their 

reviews of the plan, the base year inventory against the attainment projected inventory for 

the nonattainment area. For these reasons, the EPA is proposing to establish the 

regulatory requirement that attainment plans must include a base year inventory for the 

nonattainment area and an attainment projected inventory for the nonattainment area.

Second, as noted above, to meet the statutory requirements for submission of 

attainment plans under subpart 4, the EPA believes that states must meet the same 

submission schedule for these emissions inventories as for the other elements of an 

attainment plan, i.e., within 18 months after the effective dates of the designation of the 

nonattainment area. This schedule must apply to both of these emissions inventories 

because they are necessary for effective evaluation of the attainment plan as a whole. 

Consequently, under the authority of section 172(b), the EPA is proposing to establish the 

regulatory requirement that emissions inventories be submitted by 18 months after 

designation.
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Third, the EPA proposes to establish specific requirements for both the base year 

inventory for the nonattainment area and for the attainment projected inventory for the 

nonattainment area in order to implement the PM2.5 NAAQS most effectively. 

Accordingly, the EPA proposes that the base year inventory for the nonattainment area 

must meet the following minimum criteria (a) through (g): 

(a) The inventory year must be one of the 3 years used for designations or another 

technically appropriate inventory year. Another inventory year may be chosen 

under specific circumstances (e.g., to account for a change in sources in the 

nonattainment area, changes in nonattainment area boundaries, or significant 

time lag between designations and preparation of the inventory) with 

consultation from the appropriate EPA Regional Office. This requirement is 

intended to ensure that the inventory will represent the emissions sources

whose contributions resulted in a nonattainment designation for the area. 

(b) The inventory must include actual emissions of all sources within the 

nonattainment area. This requirement stems directly from the wording of 

section 172(c)(3). Sources outside of the nonattainment area are explicitly not 

included in the section 172(c)(3) requirement with the words “in such area.”

Furthermore, the EPA interprets the Act requirement for “actual emissions 

from all sources” in section 172(c)(3) as intending to include all emissions 

that may contribute to the formation of PM2.5 within the nonattainment area.

(c) The emissions values must either be annual total emissions or average-seasonday emissions, as appropriate for the nonattainment problem. The rationale for 

providing annual or seasonal emissions must be included as part of the plan. A 
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discussion of the EPA’s rationale for proposing the option of seasonal or 

annual inventories is provided in Section IV.B.4 of this preamble.

(d) As discussed above and consistent with past implementation rule 

requirements, the inventory must include emissions of direct PM2.5 (both 

filterable PM2.5 and condensable PM2.5), as well as all scientific PM2.5

precursors (SO2, NOx, VOC and ammonia). A discussion of the EPA’s 

rationale for proposing this requirement is provided in Section IV.B.5 of this 

preamble.

(e) The emissions thresholds for which emissions sources must be reported as 

point sources must be followed from the Air Emissions Reporting Rule 

(AERR), 40 CFR part 51, subpart A. This requirement is consistent with past 

implementation rules and is needed to define the data structure (as opposed to 

the emissions values themselves) of the emissions submitted to the EPA. A 

discussion of the use of 40 CFR part 51, Subpart A, for the emissions 

thresholds and data reporting elements is provided in Section IV.B.6 of this 

preamble.

(f) The detail of the emissions included in the inventory must be consistent with 

the detail required by 40 CFR part 51, subpart A. For example, all emissions 

must be subdivided to individual emissions processes within a facility or 

county. While these details should underlie the inventory, the emissions 

included in the attainment plan can be summarized. This requirement is 

consistent with the 2007 PM2.5 Implementation Rule and is needed to define 
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the data structure (as opposed to the emissions values themselves) of the 

emissions submitted to the EPA.

(g) If the base year inventory for the nonattainment area is submitted to the EPA 

as a separate plan submission (i.e., severed from the overall attainment plan 

and provided separately), the inventory must still meet all public review 

requirements associated with that plan. See proposed 40 CFR 51.1008(a)(1).

For the attainment projected inventory for the nonattainment area, the EPA also 

proposes to promulgate more specific requirements in order to implement the PM2.5

NAAQS most effectively. Accordingly, the EPA proposes that the attainment projected 

inventory must meet the following minimum criteria (a) through (g):

(a) The year of the projected inventory must be the first year for which attainment 

is demonstrated by the modeled attainment plan.

(b) The emissions values must be projected emissions of the same sources 

included in the base year inventory for the nonattainment area (i.e., only those 

located within the nonattainment area) and any new sources. The projected 

emissions values should be the best available representation of expected 

emissions, and thus should take into account emissions growth and 

contraction, facility closures, new facilities, new controls and other factors 

forecast to occur between the base year and the attainment year. In deciding 

what factors are relevant, states should consider factors affecting projected 

emissions that could significantly alter the conclusions of the attainment 

demonstration.
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(c) The temporal period of emissions must be the same temporal period (annual 

or average-season-day) as the base year inventory for the nonattainment area. 

(d) Consistent with the base year inventory for the nonattainment area, the 

inventory must include all emissions of direct PM2.5 (both filterable and 

condensable PM2.5), as well as all emissions of all scientific precursors (SO2, 

NOx, VOC and ammonia).

(e) The same sources reported as point sources in the base year inventory for the 

nonattainment area must also be provided as point sources in the attainment 

projected inventory for the nonattainment area. Likewise, nonpoint and 

mobile source projected emissions must also be provided using the same 

detail (e.g., state, county and process codes) as the base year inventory. 

(f) The detail of the emissions included must be consistent with the level of detail 

in the base year inventory (i.e., as required by 40 CFR part 41, subpart A).

(g) If the attainment projected inventory for the nonattainment area is submitted 

to the EPA as a separate plan submission (e.g., severed from the overall 

attainment plan and provided separately), the inventory must still meet all 

public review requirements associated with that SIP submission. See proposed 

40 CFR 51.1008(a)(2).

2. Are there new inventory requirements in this proposed rule that have not been 

included in previous rules?

This proposed rule includes more specific requirements for emissions inventories 

than past implementation rules. First, the EPA proposes to require the attainment 

projected inventory for the nonattainment area. In practice, some states were providing
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this information at the request of their respective EPA Regional Offices, but it was not a 

specific requirement. The EPA believes that a specific requirement is necessary to ensure 

that the EPA and the public can reasonably assess the changes in emissions in the 

nonattainment area that the state maintains demonstrate that the area will attain the 

standard or that it is impracticable to attain the standard by the attainment date. Without 

such information, there is no way for the EPA to assess the projected emissions changes 

in the nonattainment area that the state asserts contribute to attainment. In addition, this 

proposed requirement would support the EPA’s first proposed approach for conducting 

an RFP analysis as described in Section IV.F of this preamble.

This proposed rule also is more specific about the requirements for the emissions 

inventories submitted. While the various criteria (a) through (g) listed above have been 

implicit in prior rules and associated guidance, the EPA believes that not having these 

specific requirements has caused confusion and inconsistencies across attainment plan

inventories in the past. Thus, the EPA is proposing to require these minimum criteria in 

this proposed rule. Furthermore, the option for using only seasonal inventories in some

attainment plans is a new facet of this rule, further described in Section IV.B.5 of this 

proposal.

3. Are there other inventory requirements from earlier PM2.5 implementation 

rules that the EPA is proposing to retain or change?

The 2007 PM2.5 Implementation Rule required states to submit specific emissions 

inventories in connection with the RFP requirements of section 172 (c)(2) under subpart 

1. The EPA believes that a separate emissions inventory will be important to illustrate 

how a nonattainment area may achieve incremental emissions reductions toward 
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attainment, and would be appropriate in light of the agency’s proposed approaches for 

states to meet the statutory RFP requirements. Past EPA guidance with respect to RFP 

requirements under subpart 4 has not required any explicit, separate emissions inventory 

for this purpose for PM10 NAAQS. For this reason, the EPA describes this issue and 

proposed approaches more fully in Section IV.F of this preamble.

The 2007 PM2.5 Implementation Rule also required states to submit a statewide 

base year emissions inventory as part of the attainment plan. The EPA proposes not to 

include this statewide emissions inventory requirement in this rule. Subpart 4 does not 

expressly require such an inventory, and the EPA does not believe that it is needed for 

successful attainment of the PM2.5 NAAQS. Furthermore, statewide inventories are 

already required as part of the AERR (40 CFR part 51, subpart A) on a triennial basis. 

While these inventories do not receive the same level of scrutiny as inventories 

associated with attainment plans, the EPA believes that this existing statewide 

requirement is sufficient for understanding the PM2.5 nonattainment problems nationally 

and assessing the quality of inventories proposed to be required by this rule.

4. Why is the EPA proposing to permit seasonal inventories to meet the inventory 

reporting requirements?

The statute does not explicitly address whether the emissions inventory required 

under section 172(c)(3) should include emissions throughout an entire calendar year or 

emissions during some shorter portion of the year that may be appropriate for 

implementation of a particular NAAQS. In the case of the PM2.5 NAAQS, the standards 

currently include both annual NAAQS and 24-hour NAAQS. With respect to the annual 

NAAQS, the form of the NAAQS includes monitored ambient PM2.5 values at all times 
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throughout the course of the year and thus an annual emissions inventory is necessarily 

required for development of an appropriate attainment plan for a given area. In the case 

of the 24-hour NAAQS, however, the form of the NAAQS is based upon monitored 

ambient PM2.5 values on particular days with high levels of PM2.5, and in some 

nonattainment areas those days may occur only during a distinct and definable season of 

the year. The EPA considers it appropriate to interpret the emissions inventory 

requirements of the CAA in light of the specific inventory needs that are relevant for the 

NAAQS in question, and in the case of the PM2.5 NAAQS, the inventory requirement 

may thus include both an annual emissions inventory for the attainment area, and a 

seasonal emissions inventory for the area as appropriate for the attainment plan at issue.

In contrast with the annual PM2.5 NAAQS, the 24-hour PM2.5 NAAQS are 

designed to protect against peak exposures. Thus, for the 24-hour PM2.5 NAAQS, there 

are circumstances in which the EPA believes that only seasonal emissions inventories 

may be required for attainment planning purposes. The EPA proposes to allow states to 

use only seasonal inventories for attainment plan development for attaining the 24-hour 

PM2.5 standard in areas that are nonattainment for only the 24-hour standard. In the event 

that it is appropriate to rely on a seasonal emissions inventory, the state should confer 

with the EPA concerning the exact length of the season and the start and stop dates of the 

season. The duration and start and stop dates of the season will be an important 

component of the attainment plan and must be approved by the EPA along with other 

elements of the attainment plan for a given nonattainment area. The EPA further proposes 

to require that seasonal inventories must use average-season-day emissions values for this 

purpose. The average-season-day is defined as the sum of all emissions during the 
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applicable season divided by the number of days in that season. The nature of some 

seasonal PM2.5 emissions sources (e.g., residential wood combustion) does not allow for 

only weekday emissions to be included in the inventory, therefore all days must be 

included. The state would need to explain the rationale for the duration of the season used 

for the inventory as part of the attainment plan submission. To justify the use of a 

seasonal inventory, the state must demonstrate why a seasonal attainment plan is 

appropriate for the particular PM2.5 nonattainment area in question.

5. Why is the EPA requiring certain pollutants be included in the inventories?

The EPA is proposing that states must submit emissions inventories that include 

all emissions of direct PM2.5 and all emissions of all PM2.5 precursors: SO2, NOx, VOC 

and ammonia. Furthermore, the inventory must differentiate between the condensable and 

filterable portions of direct PM2.5 emissions. Section II.B of this proposal describes the 

background needed to understand the importance of including these precursors in 

emissions inventories for attainment plan purposes for the PM2.5 NAAQS. Emissions 

information about PM2.5 and its precursors is a necessary precondition to meeting other 

core attainment plan requirements, such as effective evaluation of control measures and 

adequate demonstration of projected future attainment of the NAAQS through modeling. 

The EPA notes that with respect to requiring states to include emissions of direct PM2.5

and PM2.5 precursors in emissions inventories, the agency is following the requirements it 

established for the PM2.5 NAAQS in the past. Section 172(c)(3) explicitly requires states 

to submit a “comprehensive, accurate, and current inventory of actual emissions of the 

relevant pollutants” and the EPA concludes that in order to meet these basic statutory 
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requirements for the PM2.5 NAAQS, states must address PM2.5 and all PM2.5 precursors in 

their emissions inventories.

The EPA requires air agencies to use the best available methodologies for 

estimating emissions of PM2.5 and its precursors. It should be noted that for ammonia, in 

particular, updated emissions estimating methodologies for animal feeding operations are 

under development using data collected during the period 2007-2009 from representative 

operations pursuant to the National Air Emissions Monitoring Study.

73 The EPA is 

hopeful that such updated methodologies will help to reduce uncertainties in current 

ammonia inventories and will improve the quality of future emissions inventories needed 

for implementing the PM2.5 NAAQS.

6. Why is the AERR used to define data elements and data methods that are 

required for the emissions inventories required by this rule?

Because the provisions of the CAA do not specifically state the form of the 

emissions information to be reported to the EPA for meeting their attainment plan 

inventory requirement, it is necessary for the EPA to prescribe specifically the data 

elements of those emissions inventories. Distinct from the emissions values (i.e., how 

much emissions derive from each source or source category), the emissions elements (i.e., 

how they are reported) refer to the reporting definitions, data codes and required data 

fields. The EPA proposes that states must use the emissions elements from 40 CFR part 

51, subpart A, in preparing their inventories submitted to the EPA for implementing the 

 

73 For more information on the NAEMS study, see: 

http://www.epa.gov/agriculture/airmonitoringstudy.html
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PM2.5 NAAQS. This is consistent with past requirements for the form of emissions 

inventories.

In addition to defining the data elements, 40 CFR part 51, subpart A also requires 

states to submit emissions information to the EPA. The EPA is not referring to those 

emissions submission requirements here, but rather the emissions elements – the 

definitions, data codes and required data fields. Below, the EPA addresses the issue of 

whether the emissions values submitted through the AERR are relevant to the inventory 

requirements of this proposed rule (see Section IV.B.8 of this proposal).

As noted earlier, the EPA recommends that states consult the SIP Emissions Inventory 

Guidance in preparing the inventories needed for this rule. In addition to the AERR, this 

guidance includes definitions for data fields that are not required by the AERR, such as 

seasonal emissions values and other fields that are optional in the data system that 

collects data submitted for the AERR. The EPA is updating the SIP Emissions Inventory 

Guidance in coordination with this proposal. It provides specific guidance to air agencies 

on how to develop base year inventories for the nonattainment area and attainment 

projected inventories for 8-hour ozone, PM2.5, and regional haze SIPs. While the AERR 

sets forth requirements for data elements and definitions, the guidance complements these 

requirements, defines all data elements (even those that are voluntary AERR elements), 

and indicates how the data should be prepared, documented and publicly reviewed for 

attainment plan submissions.

7. How do emissions inventories support modeling for attainment 

demonstrations?
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This section attempts to clarify the difference between the inventories required to 

be a part of a state’s Moderate area attainment plan submission (as described earlier) and 

other modeling inventories that are also relevant for attainment planning. While the EPA 

is not proposing additional modeling inventory requirements in this rule (i.e., for which a 

state must submit an emissions inventory to the EPA), to meet the attainment 

demonstration requirements of CAA sections 189(a)(1) and 189(b)(1), states will need to 

submit an attainment demonstration (which includes air quality modeling) to show how 

the area will either attain the NAAQS by the applicable attainment date or that the area 

cannot attain by the attainment date. The modeled attainment demonstration requirements 

for Moderate areas are described fully in Section IV.E of this preamble.

As part of this demonstration, the EPA presumes that states will need to prepare 

attainment demonstration modeling inventories for both a modeled base year and 

projected attainment year. Respectively, these are called the “base year (baseline) 

inventory for modeling” and the “attainment projected inventory for modeling.” These 

inventories contain emissions for all regions (i.e., not just the nonattainment area) within 

the modeling domain being used for the attainment plan modeling demonstration, which 

typically includes counties and even states outside of the nonattainment area. They 

include detailed spatial and temporal elements needed to support air quality modeling. 

States should follow the requirements laid out in Section IV.E of this preamble and the 

procedures described in the SIP Emissions Inventory Guidance and the Air Quality 

Modeling Guidance to meet the minimum requirements for documentation and emissions 

summaries supporting modeling demonstrations.
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The base year inventory and projected attainment year inventory include 

emissions from only within the nonattainment area. The EPA expects that modeling 

inventories will be consistent with those nonattainment area inventories; however, some 

exceptions may exist. Where possible, the nonattainment area base year and projected 

attainment year inventories can be a sum (for annual data) or average (for PM2.5 seasonday data) of day-specific or hour-specific data used for modeling. In some cases, 

however, this approach may not be sufficient for modeling purposes. For example, 

greater spatial and temporal detail are needed for on-road mobile modeling inventories as 

compared to the base year inventory for the nonattainment area. For the nonattainment 

area base year inventory, one goal is to allow for the repeatability of the approach in 

order to create average, seasonal or annual inventories for use in rule requirements, such 

as reasonable further progress or conformity demonstrations. That goal is not necessarily 

compatible with the modeling need for greater spatial and temporal detail. In cases where 

some differences are unavoidable, air agencies should attempt to promote consistency

where feasible.

The AERR includes both triennial and annual statewide reporting requirements, 

with more extensive reporting requirements for triennial inventory years. For the interim 

annual inventories, reporting is limited to emissions data from only the larger point 

sources (Type “A” sources), as defined by Appendix A of 40 CFR part 51, subpart A. For 

the triennial inventories, lower point source thresholds are given in Appendix A, 

consistent with the definition of major sources in 40 CFR part 70, and all other sources of 

emissions must be reported as nonpoint or mobile sources on a county basis. 
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In the past, some states have incorrectly asserted that their AERR submission 

meets the requirements for base year inventories required by past implementation rules. 

To avoid confusion, the EPA provides here the limited circumstances in which the AERR 

emissions inventories can meet the base year inventory for the nonattainment area 

requirement for Moderate areas. The following conditions must be met to use AERR 

inventories for attainment planning:

(a) The AERR emissions inventory must have gone through the public review 

process required for attainment plans.

(b) The AERR emissions inventory needs to include all sources of emissions and all 

pollutants required for the base year inventory for the nonattainment area. This is 

only possible if the inventory year for the base year inventory for the 

nonattainment area aligns with a triennial AERR year, because the data system 

implementing the AERR only accepts emissions from point sources and not other 

source categories in non-triennial years.

(c) The EPA must be accepting data for the inventory year. Inventories are allowed to 

be submitted to the AERR for a given year for only a limited time during the 

development cycle of the National Emissions Inventory.

(d) The AERR submission must include emissions from all relevant sources as 

described for the base year inventory for the nonattainment area requirements. In 

some cases, the AERR requirement can be met without electronically 

“submitting” emissions, which would not meet the requirements for the base year 

inventory for the nonattainment area. For example, states may elect to accept the 

EPA estimates for some nonpoint emissions sectors, but this would not meet the 
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requirements of section 172(c)(3). In addition, the AERR revision finalized in 

February 2015 replaces the prior requirement of reporting onroad mobile and 

nonroad mobile source emissions with a requirement for reporting the input 

parameters that can be used to run the EPA models to generate the emissions. If 

choosing to use an AERR submission to meet the base year inventory for the 

nonattainment area requirement, the state should submit the nonattainment area 

emissions, irrespective of the options provided to meet the AERR requirements. 

Since the “statewide” emissions are actually provided for individual point sources 

and counties, the EPA believes that these resolutions can be sufficient for most 

PM2.5 nonattainment areas. 

8. What models should be used for mobile source emissions?

A key part of emissions inventory development includes estimating mobile source 

emissions. For all of the mobile source inventories used for PM2.5 NAAQS 

implementation, states should use the latest emissions models available at the time the 

attainment plan inventory is developed.74 In general, the latest approved version of the 

MOVES model should be used by states other than California to estimate emissions from 

onroad transportation sources. States should use the latest available planning emission 

inputs including, but not limited to, vehicle miles traveled (VMT), speeds, fleet mix, SIP 

control measures and fuels. The current version of MOVES is available at 

 

74 Section 172(c)(3) requires that SIP inventories and control measures be based on the 

most current information and applicable models that are available when a SIP is 

developed. 
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http://www.epa.gov/otaq/models/moves/index.htm. The appropriate EPA-approved 

model(s) should similarly be used for California onroad source emissions.

75 When using 

MOVES, states should follow the most current version of the MOVES Technical 

Guidance, available at http://www.epa.gov/otaq/models/moves/index.htm#sip. MOVES 

includes multiple options for estimating and processing emissions that could result in 

different emissions inventories. The EPA recommends that states use the same approach 

in any analysis that compares two or more emissions cases (e.g., different control 

scenarios, different years). If different approaches are taken for inventories that serve 

different purposes (for example between inventories developed for air quality modeling, 

which may require greater temporal and spatial detail, and inventories used as the motor 

vehicle emissions budget), states should seek to understand and minimize any differences 

in results. For example, an approach may be used for the modeled attainment 

demonstration that uses gridded temperatures and other meteorological data, but this 

approach could be too burdensome for use in the base year inventory for the 

nonattainment area. This is because emissions inventories created for purposes of RFP 

and motor vehicle conformity analysis must use the same MOVES approach used in the 

base year inventory for the nonattainment area, and using a straightforward MOVES 

approach without gridded meteorology is more reasonable for that purpose.

The most current version of the NONROAD model should be used for estimates 

of nonroad mobile source emissions, preferably with state-supplied model input data. 

States can alternatively develop technologically equivalent or superior state-specific 

 

75 At this time, the California onroad mobile model is called EMFAC.
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nonroad emissions estimates, but should explain why their approach gives a better 

estimate than the EPA model. For nonroad sources not estimated by the NONROAD 

model, the best available methods should be used, and the EPA recommends that states 

refer to the SIP Emissions Inventory Guidance for more information on emissions from 

these sources. Links to Federal Register documents and policy guidance memos on the 

latest approved versions of MOVES and NONROAD can be found at 

http://www.epa.gov/otaq/models.htm.

9. What special considerations exist for tribal areas? 

In the past, there have been instances where portions of tribal areas have been 

included in designated nonattainment areas, but when the base year inventory for the 

nonattainment area was prepared, emissions from the tribal lands were not included. This 

has had the effect of preventing tribes from generating emissions reductions from existing 

sources to develop emissions offsets, as well as impairing the ability of the state to 

prepare as accurate a modeling demonstration as possible. It could also cause sources in 

tribal areas to remain uncontrolled even though they are contributing to violations in a 

given nonattainment area. The EPA encourages states and tribes to work together to 

ensure that the information used in developing the baseline emissions inventory is 

inclusive of all emissions from a designated nonattainment area, including emissions 

from sources in tribal areas located therein.

C. Pollutants to be Addressed in the Plan

Under subpart 4 of the CAA, air agencies are initially required to analyze and 

evaluate emissions reduction measures for all sources of direct PM2.5 and PM2.5

precursors (i.e., SO2, NOx, VOC and ammonia) in developing PM2.5 attainment plans. As 
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described in Section II of this preamble, and reiterated in the proposed emissions 

inventory requirements for Moderate area attainment plans under Section IV.B of this 

preamble, direct PM2.5 includes both filterable and condensable PM2.5 emissions. Thus, a 

state must evaluate control measures for sources of filterable and condensable PM2.5

emissions as part of an approvable control strategy for a Moderate PM2.5 nonattainment 

area.

In addition, while evaluating sources of direct PM2.5 for reasonably available 

controls is an implicit requirement in the context of implementing the PM2.5 NAAQS 

under any scenario, the EPA is proposing and seeking comment on several options for 

evaluating PM2.5 precursors under the PM2.5 NAAQS implementation program. The EPA 

interprets the requirements of the CAA to allow the air agency to provide a “precursor 

demonstration” to the EPA that supports a state’s finding that one or more PM2.5

precursors need not be subject to control requirements in a given nonattainment area. 

Section III of this preamble presents a complete discussion of the EPA’s proposed 

options for states to address PM2.5 precursors in attainment plans and in the NNSR 

permitting program. Specifically, the EPA is proposing and seeking comment on three 

options describing different approaches to such precursor demonstrations, and requests 

comment on each. In general terms, the three options can be summarized as follows: 

 Option 1: Two independent analyses: (a) an attainment planning analysis 

demonstrating that control measures for a particular precursor are not needed for 

expeditious attainment, meaning that the precursor can be excluded from measures 

needed to attain as expeditiously as practicable for all types of sources; and, (b) a 

section 189(e) technical demonstration showing that major stationary sources of a 
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particular precursor do not contribute significantly to levels that exceed the PM2.5

standard, meaning that the precursor can be excluded from control requirements for 

major sources and from NNSR permitting;

 Option 2: Single analysis demonstrating that all emissions of a particular precursor 

from within the area do not significantly contribute to PM2.5 levels that exceed the 

standard, meaning that control requirements for emissions of the precursor from 

major stationary and area sources, as well as mobile sources, would not be required 

for expeditious attainment, control requirements for major sources, or for NNSR 

permitting;

 Option 3: An attainment planning analysis demonstrating that control measures for all 

types of sources of a particular precursor are not needed for expeditious attainment 

also would be deemed to meet the section 189(e) technical demonstration 

requirement, meaning that the state would not need to regulate emissions of the 

particular precursor from major stationary sources under the NNSR permitting 

program or other control requirements for major stationary sources.

The EPA will finalize its approach to PM2.5 precursors and clarify the 

implications for states conducting analyses to identify required control measures after 

considering public comment received on this proposal.

D. Attainment Plan Control Strategy

1. General approach to designing a control strategy for a Moderate 

nonattainment area

The statutory attainment planning requirements of subparts 1 and 4 were 

established to ensure that the following goals of the CAA are met: (i) that states 
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implement measures that provide for attainment of the PM2.5 NAAQS as expeditiously as 

practicable; and, (ii) that states adopt emissions reduction strategies that will be the most 

effective, and the most cost effective, at reducing PM2.5 levels in nonattainment areas. In 

addition to having an obligation to meet the statutory requirements for specific control 

measures on sources located within a nonattainment area (e.g., RACM and RACT), a 

state has discretion to require reductions from any source inside or outside of a PM2.5

nonattainment area (but within the state’s boundaries) in order to fulfill its obligation to 

demonstrate attainment in a PM2.5 nonattainment area as expeditiously as practicable. A

state may need to require emissions reductions on sources located outside of a PM2.5

nonattainment area if such reductions are needed in order to provide for expeditious

attainment of the PM2.5 NAAQS.

With this in mind, the following sections describe the EPA’s proposed approach 

for a state to follow in order to identify and select the complete suite of measures needed 

for an attainment plan submission for a Moderate PM2.5 nonattainment area. The 

proposed process consists of identifying all technologically and economically feasible 

control measures, including control technologies, for all sources of direct PM2.5 and PM2.5

precursors in the emissions inventory for the nonattainment area which are not otherwise 

exempted from consideration for controls.

76 From that list of measures, the state must 

identify those that it can implement within 4 years of designation of the area (and which 

 

76 Such exemptions could be due to a demonstrated lack of significant contribution of a 

certain PM2.5 precursor to the area’s elevated PM2.5 concentrations or due to a 

presumptive determination that a certain source category contributes only a de minimis

amount toward PM2.5 levels in a nonattainment area.
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would thus meet the statutory requirements for RACM and RACT) and any “additional 

reasonable measures,” which the EPA proposes to define as those technologically and 

economically feasible measures that the state can only implement on sources in the 

nonattainment area after the 4 year deadline for RACM and RACT has passed. See

proposed 40 CFR 51.1000. The state must also assess whether there are other measures 

that it can implement to control sources within the state but outside the nonattainment 

area that contribute to the PM2.5 nonattainment status of the area in order to bring the area 

into attainment as expeditiously as practicable.

As discussed in Section II.D.6 of this preamble, one important component of a 

state’s control strategy for a PM2.5 nonattainment area is the suite of control measures that 

a state is already implementing or will be implementing to comply with national, 

regional, or state and local regulations already adopted or promulgated, as long as such 

measures will lead to permanent and enforceable reductions in emissions after the area is 

designated nonattainment. Such “existing” measures could apply to sources inside the 

nonattainment area, in which case the state must include them in the RACM and RACT

and additional reasonable measures analysis for the area. The measures may also apply to 

sources located outside the nonattainment area but would achieve reductions in direct 

PM2.5 emissions or emissions of PM2.5 precursors to help bring the area into attainment. 

A state must evaluate the potential effects of all of these measures as part of its modeled 

attainment demonstration for the area, and must clearly indicate which of these measures 

will contribute toward timely attainment for the area in the attainment plan submission.

2. Identification and selection of RACM and RACT and additional reasonable 

measures
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a. Statutory requirements and existing guidance. CAA section 172(c) under 

subpart 1 describes the general attainment plan requirement for RACM and RACT, 

requiring that attainment plan submissions “provide for the implementation of all 

reasonably available control measures as expeditiously as practicable (including such 

reductions in emissions from existing sources in the area as may be obtained through the 

adoption, at a minimum, of reasonably available control technology) and shall provide for 

attainment” of the NAAQS. The attainment planning requirements specific to PM10, 

including PM2.5, under subpart 4 likewise impose upon states an obligation to develop 

attainment plans that impose RACM on sources of direct PM2.5 and PM2.5 precursors 

within a Moderate nonattainment area. CAA section 189(a)(1)(C) requires that states with 

areas classified as Moderate have attainment plan provisions to assure that RACM are 

implemented by no later than 4 years after designation of the area.77 The EPA reads CAA 

sections 172(c)(1) and 189(a)(1)(C) together to require that attainment plans for 

Moderate nonattainment areas must provide for the implementation of RACM and RACT 

for existing sources of PM2.5 and PM2.5 precursors in the nonattainment area as 

expeditiously as practicable but no later than 4 years after designation.78

The terms RACM and RACT are not defined within subpart 4, nor do the 

provisions of subpart 4 specify how states are to meet the RACM and RACT 

requirements. However, the EPA’s longstanding guidance in the General Preamble 

 

77 States with areas later reclassified as “Serious” nonattainment areas under subpart 4 

must also develop and submit later plans to meet additional requirements for Serious 

areas.

78 This interpretation is consistent with guidance described in the General Preamble. See

57 FR 13540 (April 16, 1992).
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described in detail considerations for determining what control measures constitute 

RACM and RACT for purposes of subpart 4. The EPA’s guidance for RACM for sources 

of PM10 and PM10 precursors under subpart 4 in the General Preamble included: (i) a list 

of some potential measures for states to consider; (ii) a statement of the EPA’s 

expectation that the state will provide a reasoned explanation for a decision not to adopt a 

particular control measure; (iii) recognition that some control measures might be 

unreasonable because the emissions from the sources that would be affected by the 

measure in the area are de minimis (i.e., aggregate emissions from all sources in a 

particular source category do not contribute significantly to PM2.5 concentrations in the 

area); (iv) an emphasis on state evaluation of potential control measures for 

reasonableness, considering factors such as technological and economic feasibility; and,

(v) encouragement to states evaluating potential control measures imposed upon 

municipal or other governmental entities to include consideration of the impacts on such 

entities, and the possibility of partial implementation when full implementation would be 

infeasible (e.g., phased implementation of measures such as road paving).79 Thus, the 

RACM requirement under subpart 4 applies to all types of sources and is not necessarily 

focused upon forms of control that are strictly technology-based.

With respect to RACT requirements, the EPA’s guidance in the General 

Preamble: (i) noted that RACT has historically been defined as “the lowest emission limit 

that a source is capable of meeting by the application of control technology that is 

reasonably available considering technological and economic feasibility”; (ii) noted that 

 

79 See 57 FR 13498 at 13540-41. 
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RACT generally applies to stationary sources, both stack and fugitive emissions; (iii) 

suggested that major stationary sources be the minimum starting point for a state’s RACT 

analysis; and, (iv) recommended that states evaluate RACT not only for major stationary 

sources, but for other source categories as needed for attainment and considering the 

feasibility of controls.80 Thus, the RACT requirement under subpart 4 is primarily 

focused on stationary sources and forms of emissions control that are technology-based.

In addition to the statutory requirements under sections 172(c)(1) and 

189(a)(1)(C) for RACM and RACT, section 172(c)(6) requires that a state’s attainment 

plan for a nonattainment area “include enforceable emission limitations, and such other 

control measures, means or techniques (including economic incentives such as fees, 

marketable permits, and auctions of emission rights), as well as schedules and timetables 

for compliance, as may be necessary or appropriate to provide for attainment of such 

standard in such area by the applicable attainment date specified in this part.” The EPA 

interprets this statutory provision to require a state to identify, select and implement 

additional measures to those identified as RACM and RACT for the area if needed to 

provide for timely attainment of the area. In the EPA’s proposed approach detailed in this 

section, the EPA describes criteria for identifying and selecting “additional reasonable 

measures” for sources of direct PM2.5 and PM2.5 precursors in a Moderate nonattainment 

area which may be necessary in order to bring the area into expeditious attainment.

b. Proposed approach. This section describes the EPA’s proposed approach for 

determining what measures qualify as RACM and RACT or as additional reasonable 

 

80 Ibid.
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measures for controlling sources contributing to nonattainment in a Moderate PM2.5

nonattainment area. Under the proposed approach, the specific determination of RACM 

and RACT would be made within the broader context of assessing control measures for 

all stationary, area and mobile sources of direct PM2.5 and PM2.5 precursors that would 

collectively contribute to meeting the statutory Moderate area attainment date as 

expeditiously as practicable.81 The proposed approach is designed to ensure that states 

consider and adopt control measures for sources in a way that is consistent with the 

statute’s overarching requirement to attain the standards as expeditiously as practicable, 

yet to provide flexibility for states to focus regulatory resources on those sources of 

emissions whose control will most effectively and expeditiously contribute to attainment 

in a given area.

Specifically, the EPA proposes that a state must follow a process by which it 

would: (i) identify all sources of emissions of direct PM2.5 and all PM2.5 precursors in the 

nonattainment area and all potential control measures to reduce emissions from those 

source categories not otherwise deemed de minimis; (ii) determine if any of the identified 

potential control measures are technologically infeasible; (iii) determine if any of the 

identified technologically feasible control measures are economically infeasible; (iv) 

determine which technologically and economically feasible measures can be 

implemented, in whole or in part, within 4 years from the date of designation of the area 

and which can be implemented, in whole or in part, by the end of the sixth calendar year 

 

81 In Sierra Club v. EPA, 294 F.3d 155 (D.C. Cir. 2002), the court stated in upholding the 

EPA’s statutory interpretation of RACM that the CAA does not compel a state to 

consider a measure without regard to whether it would expedite attainment.
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following designation; and, (v) perform an analysis to determine the earliest practicable 

attainment date for the area and identify the control measures and control technologies 

that will be needed to achieve attainment by the demonstrated attainment date and to 

meet statutory control requirements.

The statutory attainment date for Moderate nonattainment areas is as 

expeditiously as practicable, but not later than the end of the sixth calendar year after 

designation of the area as nonattainment. In the case of Moderate areas that can reach 

attainment by the statutory attainment date, and consistent with existing policies, states 

would be required to evaluate the combined effect of reasonably available control 

measures that are not necessary to demonstrate attainment within the maximum statutory 

timeframe to determine whether implementation of the remaining measures could 

advance the attainment date by at least 1 year. The EPA has long applied this particular 

test – whether reasonably available control measures that were not necessary to 

demonstrate attainment within the maximum statutory timeframe, collectively can 

advance an area’s applicable attainment date by at least 1 year -- to satisfy the statutory 

provision related to an area demonstrating attainment “as expeditiously as practicable.”82

The EPA continues to believe that this approach provides an appropriate degree of 

flexibility to a state to tailor its attainment plan control strategy to the needs of a 

particular PM2.5 nonattainment area. In the case of Moderate areas that cannot practicably 

attain by the statutory attainment date, states would be required to implement all RACM 

 

82 The term “expeditious attainment” is used throughout this proposal to describe the 

ability of a nonattainment area to attain “as expeditiously as practicable” based on the test 

described here.
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and RACT, together with any additional reasonable measures on sources in the 

nonattainment area. In either case, the statute requires that a state’s attainment plan 

provide for implementation of RACM and RACT within 4 years of designation.

The following discussion provides further detail on the specific steps and criteria 

that the EPA proposes states must apply when making their determinations for RACM 

and RACT and additional reasonable measures. The EPA seeks comment on the 

proposed steps, criteria and considerations described below. See proposed 40 CFR 

51.1009(a).

Step 1: Identify sources to be controlled and existing and potential control 

measures

i. Identify sources to be controlled. As described more fully in Section IV.B of 

this preamble, section 172(c)(3) of the CAA requires that attainment plans for 

PM2.5 nonattainment areas include a “comprehensive, accurate, current inventory 

of actual emissions from all sources of the relevant pollutant or pollutants.” As 

proposed, the inventory must include emissions information for all major 

stationary sources, nonpoint or area sources, and mobile sources of direct PM2.5

and PM2.5 precursors in the nonattainment area.

The EPA proposes to require that a state must look at all of the sources reflected 

in the nonattainment area’s base year inventory as part of the first step in identifying 

reasonable control measures for the area, as each of these sources may play a role in the 

area’s PM2.5 problem and thus may be controlled currently or may need to be controlled 

to bring the area into attainment as expeditiously as practicable. Under this proposed 

approach, a state would need to consider all inventoried sources of direct PM2.5 emissions 
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and sources of all four scientific PM2.5 precursors as it conducts its determination of 

reasonable control measures for an area. A possible exception to this comprehensive 

review requirement for all inventoried sources could arise if the EPA finalizes a precursor 

approach that would allow a state to demonstrate that one or more precursors in a 

nonattainment area do not significantly contribute to the PM2.5 problem in the area and/or 

that reducing emissions of one or more precursors in an area would not be effective in 

reducing PM2.5 concentrations in the area.83 In such a case, a state could exempt sources 

of any precursor for which the state has made such a demonstration from further 

consideration for measures to control emissions of that precursor. Independent of whether 

or not the EPA finalizes such an approach to precursors, however, a state could still 

determine that it is not necessary to control emissions of direct PM2.5 or any of the PM2.5

precursors in order to attain the PM2.5 NAAQS in a given area, or to advance the 

attainment date for that area, at a later point in this proposed process for determining 

RACM and RACT and additional reasonable measures.

It was noted in Section II of this preamble that national PM2.5 emissions from 

wildfire are substantial. In some areas, wildfire impacts can be significant and contribute 

to exceedances of the PM2.5 standard and adverse health effects. Fires also play an 

important ecological role across the globe, benefiting those plant and animal species that 

depend upon natural fires for propagation, habitat restoration and reproduction. Fires are 

one tool that can be used to reduce fuel load, unnatural understory, and tree density, 

 

83 See Section III of this preamble for further details on the agency’s proposed options for 

how to handle precursors in attainment planning.

Commented [A2]: Moved to RACM for managing fire 

emissions section around page 134
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helping to reduce the risk of catastrophic wildfires. Some wildfires and the use of 

prescribed fire can influence the occurrence of catastrophic wildfires which may reduce 

the probability of fire-induced smoke impacts and subsequent health effects.

In light of these wide-ranging effects of fire and the uniqueness of this category of 

emissions, the EPA requests comment on the concept of RACM to reduce the impacts of 

emissions from catastrophic wildfire and prescribed fire. Wildland fuels can be reduced 

through fuels management, including the use of prescribed fire and allowing some 

wildfire to occur naturally in systems that are ecologically fire dependent. Where 

appropriate, states, land managers and landowners may consider developing plans to 

ensure that fuel accumulations are addressed and fuels management efforts, including 

prescribed fire, are not delayed. RACM for managing emissions from prescribed fires 

should also be considered by air agencies. Information is available from the DOI and the 

USDA Forest Service on smoke management programs and basic smoke management 

practices. The EPA requests comments and ideas about the concept of RACM for 

wildfire and prescribed fire, and input on practical considerations associated with how 

such measures can be characterized in the emissions inventory and attainment 

demonstration, and made federally enforceable for adoption in a nonattainment area SIP. 

ii. De minimis source category exemptions. The concept of exempting certain 

source categories from consideration for control measures due to their minimal (i.e., de 

minimis) contribution is discussed at length in the Addendum for sources located in 

Serious PM10 nonattainment areas that would otherwise be subject to BACM and BACT 

requirements. The EPA’s guidance in the General Preamble on Moderate PM10

nonattainment area requirements also provided support for exempting de minimis source 
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categories from RACM and RACT requirements: “If it can be shown that one or more 

measures are unreasonable because emissions from the sources affected are insignificant 

(i.e., de minimis), those measures may be excluded from further consideration as they 

would not represent RACM for that area.”84, 85

As with RACM for PM10, the EPA proposes to allow states to exempt de minimis

source categories from further consideration as they determine reasonable control 

measures for bringing a Moderate PM2.5 nonattainment area into attainment with the 

relevant NAAQS. The EPA proposes that if a state can demonstrate that a particular 

source category does not contribute significantly to nonattainment of the PM2.5 NAAQS

in a Moderate nonattainment area, then the state may eliminate the source category from 

further consideration for control measures.

86 A state would be required to evaluate all 

other sources in the nonattainment area in source categories that do not qualify as de 

minimis for reasonable control measures.

 

84 57 FR 13498 at 13540.

85 Where the sources at issue contribute only negligibly to ambient concentrations that 

exceed the NAAQS, the EPA’s policy is that it would be unreasonable to regulate those 

sources, and, therefore, the sources would not be subject to RACM or other control 

requirements, unless it is determined that even sources identified as de minimis must be 

controlled in order for the area to attain the NAAQS. In this regard, it is worth noting that 

the inherent authority of administrative agencies to exempt de minimis situations from 

regulation has been recognized by courts as “a tool to be used in implementing the 

legislative design” (see Alabama Power Co. v. Costle, 636 F.2d 323, 360 (D.C. Cir. 

1979)).

86 Ibid. See Alabama Power v. Costle, 636 F.2d 323, 360–61 (D.C. Cir. 1979). 
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The EPA notes that there are some challenges in establishing de minimis source 

categories for PM2.5 sources in the same manner as was performed for PM10 sources and

seeks comment on the following proposed options.

(1) Defining source categories. Source categories, in particular for stationary

sources, can be defined very broadly or narrowly, and the definition could determine 

which sources are able to meet the thresholds for de minimis exemptions. The North 

American Industry Classification System (NAICS) is the standard industrial classification 

system used by federal agencies. NAICS codes are between 2 and 6 digits, with greater 

industrial source specificity with increased digits.87 Each digit in the code is part of a 

series of progressively narrower categories, and the more digits in the code signify 

greater classification detail. The first two digits designate the economic sector, the third 

digit designates the subsector, the fourth digit designates the industry group, the fifth 

digit designates the NAICS industry, and the sixth digit designates the national industry. 

The 5-digit NAICS code is the level at which there is comparability in code and 

definitions for most of the NAICS sectors across the three countries participating in 

NAICS (the United States, Canada, and Mexico). The 6-digit level allows for the United 

States, Canada, and Mexico each to have country-specific detail. A complete and valid 

NAICS code contains six digits.

Defining source categories by NAICS codes would still require a determination of 

how broadly to set the source category boundaries as NAICS codes with fewer digits 

represent larger source categories (e.g., sector ‘21’ is for mining processes, while a 

 

87 http://www.census.gov/eos/www/naics (last accessed August 12, 2013).
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further specification of ‘2122’ is for metal mining processes, and ‘212210’ is for iron ore 

mining). If source categories are defined in a very narrow or specific way, it is possible 

that many source categories will be below a set de minimis threshold, and therefore 

potentially inappropriately exempted from consideration for reasonable control measures. 

For this reason, the EPA proposes and seeks comment on a requirement that a state would 

need to define any source category for which a NAICS code exists at the four-digit 

industry group level. The EPA believes that relying on the four-digit industry group level 

to define “source category” for this purpose would provide an appropriate degree of 

distinction between industrial processes, while not making the source category definition 

overly broad. The EPA also seeks comment on two other alternative approaches for 

defining source category for this purpose, at the six-digit level, and the two-digit level. 

The EPA notes that not all source categories have NAICS codes, and for these other 

categories, states would need to use the appropriate recognized categories, e.g., on-road 

mobile sources. The EPA also seeks comment on alternative source categorization 

approaches that would ensure that sources that could be controlled with reasonable

control measures to achieve meaningful reductions are not inappropriately excluded from 

consideration for such control measures as de minimis. 

(2) Determining the appropriate threshold for de minimis emissions. For the PM10

NAAQS, the EPA’s guidance in the Addendum recommended that a source category is 

presumed not to be de minimis if the aggregate emissions from such source category have 

an impact that exceeds 5 μg/m3 with respect to the then-applicable 24-hour PM10

NAAQS or an impact that exceeds 1 μg/m3 with respect to the then-applicable annual 

PM10 NAAQS. The EPA designed these presumptive thresholds for de minimis source 
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categories to apply to PM10 NAAQS nonattainment areas and to the level and form of the 

PM10 NAAQS at the time the Addendum was written. However, because of the 

differences in level and form of the PM10 and PM2.5 NAAQS, the agency finds that those 

levels are not appropriate for current or future PM2.5 NAAQS implementation.

The EPA therefore proposes two options regarding the threshold for de minimis

emissions. Under the first proposed option, the EPA would not establish a nationally

applicable “bright line” threshold for defining a de minimis source category for purposes 

of implementing the PM2.5 NAAQS in a Moderate nonattainment area. Rather, under this

option, the EPA proposes to allow a state to determine whether a particular source 

category should be considered de minimis given the particular facts and circumstances of 

a specific PM2.5 nonattainment area and subject to approval by the EPA. See proposed 40 

CFR 51.1007. 

Under the second option, the EPA proposes to establish a nationally applicable de 

minimis source category threshold that would be a specific percentage of the level of the 

relevant PM2.5 NAAQS. The EPA seeks comment on what value within the range of 1

and 3 percent of the relevant NAAQS would represent an appropriate threshold level. The 

3 percent upper end of the proposed range is generally derived from the de minimis

source category contribution levels for PM10 as described in the General Preamble. The 

EPA defined these PM10 de minimis levels as follows: i) for the annual standard of 50 

μg/m3, a source category contribution of 1 μg/m3or less to the annual average design 

value (e.g., a contribution of about 2 percent or less); and, ii) for the 24-hour standard of 

150 μg/m3, a source category contribution of 5 μg/m3or less to the 24-hour design value 

(e.g., a contribution of about 3 percent or less). The 1 percent lower end of the proposed 
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range is consistent with the value that the EPA established in the CAIR as a preliminary 

threshold for further evaluation of a state’s contribution to interstate transport. That is, 

under the CAIR, a state was identified as potentially subject to additional emission 

control requirements if the impact of SO2 and NOx emissions from sources in that state to 

any nonattainment or maintenance area in another state exceeded 1 percent of the 

relevant PM2.5 standard at a receptor monitor in the other state. This value was merely the 

first step of the analysis, but it provided an initial threshold for determining whether 

further analysis was warranted. 

The EPA is requesting comment on the appropriateness of including de minimis

threshold options for exempting certain source categories from consideration for 

reasonable control measure determinations, and seeks input on several key questions: 

First, if a de minimis threshold is included, what is the appropriate definition for source 

categories? In addition, what are the appropriate thresholds for impacts on ambient PM2.5

concentrations that would adequately reflect presumptive de minimis concentrations from 

a given source category comparable to those recommended for purpose of the PM10 

NAAQS? Also, should the de minimis source category thresholds be a percentage of the 

relevant NAAQS (i.e., similar to what was recommended for PM10, but set at a level that 

is more appropriate for the level and form of the relevant NAAQS)? The EPA requests 

that commenters submit any relevant data or analyses to support their comments with 

respect to these issues. Furthermore, the EPA notes that even in the event the agency 

finalizes this rulemaking with a de minimis source category policy of any kind, states are 

obligated under the CAA to demonstrate how their PM2.5 nonattainment area(s) will 

attain the standard as expeditiously as practicable. Accordingly, a state could not elect to 
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treat source categories as de minimis if doing so would prevent the state from being able 

to demonstrate attainment for an area by the statutory attainment date. 

iii. Identify existing and potential control measures and technologies. The state’s 

compilation of existing and potential control measures must be sufficiently broad to 

provide a basis for identifying all technologically and economically feasible controls that 

may be RACM or RACT for sources of direct PM2.5 and PM2.5 precursor emissions in the 

nonattainment area at issue. Because RACM applies to area and mobile sources as well 

as stationary sources, the EPA proposes to require that states consider a variety of types 

of measures in conducting their control strategy analysis. As stated earlier, inherent to the 

concept of RACM and RACT is the basic premise that the measure be “reasonable,” thus 

the EPA believes that a state may decline to evaluate control measures that are plainly 

“absurd, unenforceable, or impractical,” for example, measures that would cause 

“severely disruptive socioeconomic impacts, (e.g. gas rationing and mandatory source 

shutdowns).” It is the agency’s interpretation that evaluation of such measures is not 

required by the CAA.88

Furthermore, the EPA believes that reducing air emissions may not justify 

adversely affecting other resources, for example, by increasing pollution in bodies of 

water, creating additional solid waste disposal problems or creating excessive energy 

demands. An otherwise available control technology may not be reasonable if these other 

environmental impacts are sufficiently adverse and cannot reasonably be mitigated. The 

EPA proposes that a state may consider a control measure for direct PM2.5 or a PM2.5

 

88 55 FR 38327 and 57 FR 13560.
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precursor not reasonable if, considering the availability of mitigating adverse impacts of 

that control on pollution of other media, the control would not, in the state's reasoned 

judgment, provide a net benefit to public health and the environment. It should be noted 

that, in many past situations, states and owners of existing sources have adopted control 

technologies for direct PM2.5 and/or PM2.5 precursors with known energy penalties and 

some adverse effects on other media, based on the reasoned judgment that installation of 

such technology would result in a net benefit to public health and the environment. States 

should consider this before determining that a control technology is not reasonable 

because it may have other, negative environmental impacts that are, on balance, marginal.

Generally, this proposed approach allows states to apply reasoned judgment as 

they identify potential control measures for sources of direct PM2.5 and PM2.5 precursors 

in their respective nonattainment areas, and the EPA expects that a state will provide a 

complete and reasoned explanation to support its selection of potential control measures 

and control technologies as part of the attainment plan submission for any Moderate 

nonattainment area. The proposed regulations include language to require the inclusion of 

this explanation in a state’s attainment plan submission.

(1) Existing control measures. The EPA proposes that, as a starting point, a state

must include in its initial list of control measures those measures and technologies that 

are being implemented or will be implemented due to promulgated and/or adopted (i.e., 

“on the books”) regulations for sources of direct PM2.5 and PM2.5 precursors in its 

Moderate PM2.5 nonattainment area. The EPA expects that the state will incorporate 

current or anticipated emissions reductions from these “existing” control measures (such 

as expected SO2 reductions from the MATS; reductions of NOx and direct PM2.5 from 
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engine and fuel standards to reduce emissions from on-road and nonroad mobile sources) 

into its attainment demonstration modeling for the nonattainment area, and therefore the 

EPA believes it is appropriate for the state to clearly indicate such measures in the 

attainment plan for the area.

The EPA recognizes that for some sources located in a Moderate PM2.5

nonattainment area, a state may have previously conducted RACM and RACT analyses

to address emissions for other statutory purposes. Some of the RACM and RACT 

determinations could be relatively recent, while other determinations may be 15 years old 

or older. The EPA proposes that a state may not simply rely on a previous RACM or 

RACT determination for a particular source or source category when developing the 

attainment plan for a PM2.5 NAAQS, but rather that the state must consider all existing 

and potential new measures together as part of a comprehensive RACM and RACT 

analysis. In this way, the state’s new RACM and RACT analysis will represent the most 

thorough, up-to-date review of control measures for its PM2.5 nonattainment area. For 

example, the state would still need to provide a RACT analysis for a stationary source 

that has installed new emissions controls recently (e.g., within the last 3 years), but the 

state’s determination may consider that recent installation when determining whether 

additional control is technologically and economically feasible.

(2) Potential control measures. In addition to identifying existing control 

measures for sources in a Moderate PM2.5 nonattainment area, a state must develop a 

comprehensive list of potential control measures for sources in the area. There are a 

number of resources available to assist states in identifying additional, potential control 

measures and control technologies for their RACM and RACT and additional reasonable 
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measures determinations for their Moderate PM2.5 nonattainment areas. First, the EPA’s 

Office of Air Quality Planning and Standards maintains a Menu of Control Measures 

document, available online at http://www.epa.gov/air/criteria.html. This document was 

developed to provide information useful in the development of local emissions reduction 

and NAAQS SIP scenarios, and identifying and evaluating potential control measures. It 

provides a broad, though not comprehensive, listing of potential emissions reduction 

measures for direct PM2.5 and precursors of ozone and PM2.5 from stationary, area and 

mobile sources. More complete information on mobile source control measures can be 

found on the EPA’s Office of Transportation and Air Quality website at

http://www.epa.gov/otaq.

The RACT/BACT/LAER Clearinghouse (RBLC) provides a central database of 

air pollution technology information (including past RACT, BACT and LAER decisions

contained in NSR permits) to promote the sharing of information among permitting 

agencies and to aid in future case-by-case control measure determinations. The RBLC 

permit database contains over 5,000 determinations that can help a state identify 

appropriate technologies to mitigate most air pollutant emission streams. The RBLC 

includes data submitted by several U.S. territories and all 50 states on over 200 different 

air pollutants and 1,000 industrial processes. The RBLC can be found at: 

http://cfpub.epa.gov/rblc/.

Another resource for states in identifying potential control technologies for 

stationary sources is the EPA guidance document “Stationary Source Control Techniques 
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Document for Fine Particulate Matter.”89 This 1998 guidance presents information on 

topics such as the design, operation and maintenance of general PM control systems such 

as electrostatic precipitators, fabric filters and wet scrubbers. The filterable PM collection 

efficiency of each system is discussed as a function of particle size. The guidance 

document also provides information concerning other relevant considerations such as 

energy and environmental considerations, procedures for estimating costs of particulate 

matter control equipment, and evaluation of secondary environmental impacts. Because 

control technologies and monitoring approaches are constantly being improved, the state 

must also consider more updated or advanced technologies not referenced in this 1998 

guidance when determining reasonable control measures for a PM2.5 nonattainment area. 

Emissions reductions may also be achieved through the application of monitoring and 

maintenance programs that use critical process and control parameters to verify that 

emission controls are operated and maintained so that they more continuously achieve the 

level of control that they were designed to achieve.90

Additionally, the EPA maintains a website with links to other online sources of 

information on control measures for states to consider.91 Again, the EPA recognizes that 

 

89 “Stationary Source Control Techniques Document for Fine Particulate Matter” (EPA452/R-97-001), EPA Office of Air Quality Planning and Standards, October 1998. 

Available at http://www.epa.gov/ttn/catc/dir1/finepmtech.pdf (last accessed on 02/20/15). 

“Controlling SO2 Emissions: A Review of Technologies” (EPA/600/R-00/093), EPA 

Office of Research and Development, November 2000. Available at 

http://nepis.epa.gov/Adobe/PDF/P1007IQM.pdf (last accessed on 01/14/14).

90 See the EPA’s website for more information: http://www.epa.gov/ttn/emc/monitor.html.

91 Links are provided to a number of national, state and local air quality agency sites from 

the EPA’s PM2.5 website: http://www.epa.gov/pm/measures.html.
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some control technology guidance for certain source categories has not been updated for 

many years, and, for this reason, the agency expects states to identify and consider new 

and updated information in their RACM and RACT determinations as it becomes 

available.

(3) RACM for managing emissions from fire. Wildfire emissions account for a 

large portion of direct PM2.5 emissions nationally and can significantly contribute to 

periodic high PM2.5 levels.92 Besides their effect on air quality, wildfires pose a direct 

threat to public safety – a threat that can be mitigated through management of wildland 

vegetation. Attempts to suppress wildfires have resulted in unintended consequences, 

including increased risks to both humans and ecosystems.93 The use of wildland 

prescribed fire can influence the occurrence, behavior, and effects of catastrophic 

wildfires which may help manage the contribution of wildfires to background PM2.5

levels and periodic peak PM2.5 events. Additionally prescribed fires can have benefits to 

those plant and animal species that depend upon natural fires for propagation, habitat 

restoration, and reproduction, as well as myriad ecosystem functions (e.g., carbon 

sequestration). The EPA understands the importance of prescribed fire which mimics a 

natural process necessary to manage and maintain fire-adapted ecosystems and climate 

change adaptation, while reducing risk of uncontrolled emissions from catastrophic 

wildfires, and is committed to working with federal land managers, tribes, and states to 

 

92 For example, see “miscellaneous” category of direct PM2.5 emissions in Table 1.

93 Indeed, “fFire policy that focuses on [wildfire] suppression only, delays the inevitable, 

promising more dangerous and destructive future...fires.” Stephens, SL; Agee, JK; Fule, 

PZ; North, MP; Romme, WH; Swetnam, TW. (2013). Managing Forests and Fire in 

Changing Climates. Science 342: 41-42.

Commented [A3]: Revised wildfire and RACM discussion per 

interagency discussion on 2/23/15.
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effectively manage prescribed fire use to reduce the impact of wildfire related emissions 

on PM2.5.

If wildfire impacts are significant, contributing to exceedances of the standard, the 

EPA proposes that air agencies should consider RACM for this source and seeks 

comment on this guidance. Fires play an important ecological role across the globe, 

benefiting those plant and animal species that depend upon natural fires for propagation, 

habitat restoration, and reproduction. Fires are one tool that can be used to reduce fuel 

load, unnatural understory, and tree density, helping to reduce the risk of catastrophic 

wildfires. Some wildfires and the use of prescribed fire can influence the occurrence of 

catastrophic wildfires which may reduce the probability of fire-induced smoke impacts 

and subsequent health effects. RACM must be determined for each area on a case-bycase basis. Possible RACM for wildfire may include measures that reduce wildland fuels 

through fuels management, including the use of prescribed fire and possibly allowing 

some wildfire to occur naturally in systems that are ecologically fire dependent. Where 

appropriate, states, land managers, and landowners may consider developing plans to 

ensure that fuel accumulations are addressed and fuels management efforts, including 

prescribed fire, are not delayed. The EPA also proposes that air agencies should consider 

RACM for managing emissions from prescribed fires (including those prescribed fires 

conducted to reduce future wildfire emissions) and seeks comment on this approach. 

Information is available from the DOI and the USDA Forest Service on smoke 

management programs and basic smoke management practices. The EPA requests input 

on practical considerations associated with RACM for wildfire and prescribed fire, 

including such issues as how such measures can be characterized in the emissions
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inventory and attainment demonstration and made federally enforceable for adoption in a 

SIP.

(43) RACT for EGUs. Through guidance in the preamble to the 2007 PM2.5

Implementation Rule, the EPA established a rebuttable presumption that compliance with 

the CAIR would satisfy RACM and RACT requirements for SO2 and NOx emissions 

from EGUs in states participating in the CAIR cap-and-trade program for such 

emissions.94 The EPA indicated that states could presume that EGUs located within a 

given nonattainment area were meeting the RACM and RACT requirements, based solely 

upon a regional program that imposed controls for SO2 and NOx emissions from sources 

both within and outside designated nonattainment areas.

In June 2007, the EPA received a petition for reconsideration questioning the 

legality of this presumption, which the D.C. Circuit later found to be unlawful in the 

context of a similar presumption in the Phase 2 Ozone (NAAQS) Implementation Rule.95

The agency granted the petition for reconsideration in 2011 and proposed to withdraw 

 

94 72 FR 20623–5.

95 See “Petition for Reconsideration,” filed by Paul Cort, Earthjustice, on behalf of the 

American Lung Association, Medical Advocates for Healthy Air, Natural Resources 

Defense Council, and the Sierra Club (June 25, 2007). A copy of the petition is in the 

docket for this action. The EPA’s decision to grant the petition for reconsideration on the 

issue of the CAIR being presumptively equal to RACT for EGUs was in part based on a 

D.C. Circuit decision related to a similar issue. Specifically, the Court decided that the 

provisions in the Phase 2 Ozone Implementation Rule indicating that a state need not 

perform (or submit) a NOx RACM/RACT analysis for EGU sources subject to a cap-andtrade program that meets the requirements of the NOx SIP Call are inconsistent with the 

statutory requirements of section 172(c)(1). The Court concluded that the phrase “in the 

area” means that reductions must occur from sources within the area and “reductions 

from outside the nonattainment area do not satisfy the requirement.” See NRDC v. EPA, 

571 F.3d 1245 (D.C. Cir. 2009).
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from the 2007 PM2.5 Implementation Rule any presumption that compliance with the 

CAIR automatically satisfies RACM and RACT requirements for SO2 and NOx

emissions from EGUs located in nonattainment areas for the 1997 PM2.5 NAAQS.

96, 97 In 

that proposal, the EPA explained that given the explicit wording of section 172(c)(1) that 

sources “in the area” (i.e., in the nonattainment area) must at a minimum adopt RACT 

controls for that area, the agency believes that it is no longer appropriate to presume that 

this requirement is satisfied merely based upon the participation of a source in a regional 

cap-and-trade program. Indeed, implicit in a regional cap-and-trade program is that some 

sources, including those located within nonattainment areas, may elect to buy allowances 

in lieu of controlling emissions in order to meet the regional emissions reductions 

requirements. 

Accordingly, the EPA is not proposing any rebuttable presumption that the CAIR 

or any other regional control strategy constitutes RACM or RACT for EGUs or any other 

source category. Instead, the EPA is clarifying that in order to meet the RACM and 

RACT requirements for the PM2.5 NAAQS, states should evaluate EGU sources for 

RACM and RACT level controls just like any other source category, and not merely 

presume for EGUs located in a nonattainment area that compliance with a cap-and-trade 

program, including the CAIR or any other program, could satisfy their obligation to 

implement RACM and RACT. As required by the CAA, states are required to analyze 

what constitutes RACM and RACT for EGUs in each nonattainment area. 

 

96 Letter dated April 25, 2011, from former Administrator Lisa Jackson to Paul Cort, 

Earthjustice. A copy of this letter is located in the docket for this action.

97 79 FR 32892 (June 9, 2013).
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Step 2: Determine whether an available control measure or technology is 

technologically feasible. Once a state has identified existing and potential control 

measures and technologies for sources of direct PM2.5 and PM2.5 precursors in the 

nonattainment area(s), it must evaluate these controls to determine if any of those 

controls would be technologically infeasible in the particular nonattainment area.

i. Stationary sources. With respect to the technological feasibility of control

technologies for stationary sources, the EPA has a longstanding approach to evaluating 

facts relevant to this criterion under subpart 4.98 The EPA interprets the term 

technological feasibility to include consideration of factors such as a source’s processes 

and operating procedures, raw materials, physical plant layout, and potential 

environmental impacts such as increased water pollution, waste disposal and energy 

requirements. For example, the EPA recognizes that the process, operating procedures 

and raw materials used by a source can affect the feasibility of implementing process 

changes that reduce emissions and can also affect the selection of add-on emissions

control equipment. The feasibility of modifying processes or applying control equipment 

also can be influenced by the physical layout of the particular plant, if the physical space 

available in which to implement such changes limits the choices. The EPA proposes to 

retain its longstanding practice that a state should be allowed to consider such factors in 

 

98 57 FR 13540-41.
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order to eliminate from consideration control measures that are not technologically 

feasible to implement.99

ii. Area and mobile sources. With respect to determining whether a given control

measure might not be technologically feasible for an area or mobile source, the EPA also 

proposes to retain its longstanding practice that a state may consider relevant factors in 

conducting its analysis, such as the social acceptability of the measure (e.g., residential 

woodstove change-out programs rely in large part on the willingness of individual 

citizens to participate in such a program) and local circumstances, such as the condition 

and extent of needed infrastructure, population size, or workforce type and habits, which 

may prohibit certain potential control measures from being implementable.

The EPA seeks comment on the factors described above for states to consider 

when determining whether a control technology or measure is technologically feasible.

Step 3: Determine whether an available control measure or technology is 

economically feasible. The EPA has a longstanding interpretation of the term 

“economic feasibility” in the context of evaluating potential RACM and RACT 

which involves considering the cost of reducing emissions and the difference 

between the cost of an emissions reduction measure at a particular source and the 

cost of emissions reduction measures that have been implemented at other similar 

sources in the same or other areas.100 Absent other indications, the EPA presumes 

 

99 59 FR 42013. Guidance is provided in the context of Serious area BACM 

determination, but the EPA is proposing to apply it here for Moderate area RACM 

determinations.

100 57 FR 13540-41.
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that it is reasonable for similar sources to bear similar costs of emissions 

reductions. Economic feasibility of RACM and RACT is thus largely informed by 

evidence that other sources in a source category have in fact applied the control 

technology, process change or measure in question in similar circumstances.

In the preamble to the 2007 PM2.5 Implementation Rule, the EPA provided 

guidance on how to interpret the term “economic feasibility” which deviated from the 

agency’s longstanding interpretation of the term. After promulgating the final rule, the 

EPA received and granted a petition for reconsideration on issues related to the agency’s 

revised approach to interpreting the term “economically feasible.”101, 102 Consistent with 

the EPA’s granting of that petition for reconsideration, the EPA is proposing in this 

action an interpretation of economic feasibility that is consistent with the EPA’s 

longstanding interpretation of what factors are appropriate for consideration of economic 

feasibility in a RACM and RACT analysis, instead of that adopted in the 2007 PM2.5

Implementation Rule.

Specifically, the EPA proposes that for each technologically feasible control 

measure or technology, a state must evaluate the economic feasibility of the measure or 

control, through consideration of the capital costs, operating and maintenance costs, and 

cost effectiveness (i.e., cost per ton of pollutant reduced by that measure or technology) 

 

101 “Petition for Reconsideration,” filed by Paul Cort, Earthjustice, on behalf of the 

American Lung Association, Medical Advocates for Healthy Air, Natural Resources 

Defense Council, and the Sierra Club (June 25, 2007). A copy of the petition is in the 

docket for this action.

102 Letter dated April 25, 2011 from former Administrator Lisa Jackson to Paul Cort, 

Earthjustice. A copy of this letter is located in the docket for this action.
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associated with such measure or control. Furthermore, the EPA proposes that a state may 

not reject a technologically feasible control measure or technology as being economically 

infeasible if such a measure or technology has been implemented at other similar sources

(i.e., at sources that would be included in the same source category in the emissions

inventory data collection process), unless the state provides an adequate justification that 

clearly explains the specific circumstances of the source or sources in the nonattainment 

area that make such a measure or technology economically infeasible in that particular 

area.

The EPA believes that it is appropriate for states to give substantial weight to cost 

effectiveness in evaluating the economic feasibility of an emission reduction measure or 

technology. The cost effectiveness of a measure is its annualized cost ($/year) divided by 

the emissions reduced (tons/year) which yields a cost per amount of emission reduction 

($/ton). Cost effectiveness provides a relative value for each emissions reduction option 

that is comparable with other options and, in the case of control technologies, other 

facilities. 

The EPA also seeks comment on an alternative cost effectiveness metric that 

would allow a state to take into account the effect of controlling a particular precursor on 

reducing PM2.5 concentrations in the area. Such a cost effectiveness metric would be the 

annualized cost ($/year) of a control measure divided by the emissions reduced 

(tons/year) multiplied by the amount of reductions needed in the precursor emissions to 

yield 1 μg/m3reduction in PM2.5 ($/(μg/m3)). Such a metric would allow a state to 

compare the relative cost effectiveness associated with each measure toward the 

attainment goal for the nonattainment area. The EPA notes however the difficulty in 
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determining the appropriate value relating precursor reductions with reductions in 

ambient PM2.5 concentrations and therefore seeks comment on the appropriateness of this 

approach and how a state might demonstrate the validity of the input values it chooses to 

use.

In considering what level of control is reasonable, the EPA is not proposing a 

fixed dollar per ton cost threshold for economic feasibility of controls identified as 

potential RACM and RACT. In addition, if a state contends that a source-specific 

control-level should not be established because the source(s) cannot afford the control 

measure or technology that is demonstrated to be economically feasible for other sources 

in its source category, the EPA proposes that the state must support the claim with 

information regarding the impact of imposing the identified control measure or 

technology on the following financial indicators, to the extent applicable:

1. Fixed and variable production costs ($/unit)

2. Product supply and demand elasticity

3. Product prices (cost absorption vs. cost pass-through)

4. Expected costs incurred by competitors

5. Company profits

6. Employment costs

7. Other costs (e.g., for RACM implemented by public sector entities).103

 

103 These long-standing factors were established in EPA guidance in 1992 and are 

applicable to implementation programs for all NAAQS pollutants. See the appendices to 

the General Preamble, 57 FR 18070 (April 28, 1992).
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The EPA seeks comment on the factors described above for states to consider 

when determining whether a control technology or measure is economically feasible.

Step 4: Determine the earliest date by which a control measure or technology can 

be implemented in whole or in part. CAA section 189(a)(1)(C) requires that the 

attainment plan for a Moderate PM2.5 nonattainment area provide for the 

implementation of RACM and RACT no later than 4 years after designation. The 

agency has long interpreted the term “implement” to mean that a control measure 

or technology has not only been submitted to the EPA for approval as part of a 

SIP but has also been built, installed and/or otherwise physically manifested, and 

is achieving the intended emissions reductions, and the EPA proposes to retain 

such a definition in this rule. See proposed 40 CFR 51.1000. However, the EPA 

recognizes that a state may be able to implement a given control measure only 

partially within 4 years after designation. The EPA addressed this situation in the 

General Preamble, stating: “It is important to note that a State should consider the 

feasibility of implementing measures in part when full implementation would be 

infeasible.”104 This guidance endorses the notion that a state should not reject an 

otherwise technologically and economically feasible control measure or 

technology as RACM or RACT even if it can be only partially implemented 

within the statutory 4-year timeframe following designation of the area. Instead, 

the EPA interprets the statute to require states to adopt as RACM and RACT that 

portion of a control measure or technology that can feasibly be implemented 

 

104 57 FR 13498 at 13541.
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within 4 years of the effective date of designation. For instance, if paving unpaved 

roads is a control measure that is technologically and economically feasible in a 

nonattainment area but a state cannot pave all roads within 4 years of designation, 

the state must adopt as RACM a measure that requires paving of that portion of 

roads that the state could feasibly accomplish within 4 years if such a measure is 

needed for timely attainment of the PM2.5 NAAQS in the area.

The EPA thus proposes that a state must identify those technologically and 

economically feasible control measures and technologies that it can implement fully or 

partially within 4 years of designation of its Moderate PM2.5 nonattainment area. 

Depending on the severity of the PM2.5 nonattainment problem in the area, some or all of 

these measures identified as implementable within 4 years may be needed in order to

bring the area into attainment as expeditiously as practicable. These measures will satisfy 

the EPA’s criteria for RACM and RACT if the state determines, through its attainment 

demonstration that it needs to implement them to achieve timely attainment for the area.

In addition, the EPA proposes that a state must separately identify those 

technologically and economically feasible control measures that can only be implemented 

after the statutory window for implementing RACM and RACT. The statutory 4-year 

timing requirement for implementing RACM and RACT under section 189(a)(1)(C) 

limits the control measures and technologies that can qualify as RACM and RACT for a 

Moderate PM2.5 nonattainment area. However, the statutory requirement of CAA 

172(c)(6) also requires states to implement “other measures” necessary to provide for 

timely attainment in an area. The EPA proposes that among such other measures should 

be “additional reasonable measures,” which would be those measures and technologies 
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that are otherwise technologically and economically feasible but can only be 

implemented in whole or in part later than 4 years after designation and initiated no later 

than the 

beginning of the sixth calendar year following designation of the area.105 Such additional 

reasonable measures would necessarily be implemented on sources in the nonattainment 

area. However, the EPA interprets the “other measures” required under section 172(c)(6) 

to apply to stationary, area and mobile sources located outside of the nonattainment area 

but within the state if the application of reasonable control measures on such sources 

would facilitate attainment of the PM2.5 NAAQS in the nonattainment area. See proposed 

40 CFR 51.1009(b).

Step 5: Model to determine the attainment date that is as expeditious as 

practicable and select the control measures necessary to achieve attainment and 

meet statutory requirements for control measures. Section 189(a)(1) of the CAA 

establishes a requirement that the attainment plan for a Moderate PM2.5

nonattainment area must demonstrate either that an area can attain the relevant 

NAAQS by the applicable attainment date or that it is impracticable for the area to 

do so. As noted previously, for Moderate PM2.5 nonattainment areas, the 

“applicable attainment date” is as expeditious as practicable, but no later than the 

 

105 With respect to “partial measures” under this proposed approach, the EPA would 

require that a state implement as RACM that portion of any control measure determined 

to be technologically and economically feasible and implementable within 4 years after 

designation of a nonattainment area. The state would then be required to implement as an 

additional reasonable measure that portion of the same control measure that can be 

implemented starting 4 years from designation through the sixth calendar year following 

designation.
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end of the sixth calendar year after designation as nonattainment. A complete 

discussion of the EPA’s proposed requirements for attainment demonstration 

modeling is presented in Section IV.E of this preamble. However, one of the key 

features of attainment demonstration modeling is that it provides a means of 

synthesizing the effects of emissions reductions from all existing and potential 

new control measures identified for sources in a given nonattainment area on 

overall air quality in that area. States will be required to use the results of their 

attainment demonstration modeling to identify the appropriate combination of 

reasonable control measures for sources in their Moderate PM2.5 nonattainment 

area and any other control measures needed on sources outside the nonattainment 

area to ensure expeditious attainment of the relevant NAAQS in the area and to 

meet the statutory requirements of sections 189(a)(1)(B) and 172(c)(6) as 

explained below.

106

Step 5a: If the state can demonstrate attainment in the area by the statutory 

attainment date for a Moderate area, then the state must implement those control 

measures needed for expeditious attainment of the NAAQS in the area. If a state 

determines that a Moderate nonattainment area can attain the PM2.5 NAAQS by 

the statutory attainment date, the state must adopt and implement any 

technologically and economically feasible control measures that are necessary to 

 

106 Note that under section 110(l) of the CAA, after a state has adopted a control measure 

into the SIP for an attainment demonstration, it may remove or modify a measure if the 

state demonstrates to the satisfaction of the EPA that such removal or modification will 

not interfere with any applicable requirement of the CAA, such as attainment of the PM2.5

NAAQS or meeting RFP requirements.

Commented [A4]: Reviewer requested adding a footnote noting 

that if a state wishes to remove a control measure that had been 

adopted into its SIP, it can do so under existing CAA mechanisms. 
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ensure that the area will attain the NAAQS as expeditiously as practicable. The 

EPA will consider any such measures that can be implemented within 4 years of 

designation of the area to fulfill the RACM and RACT requirements for the area. 

In addition, the EPA will consider any such measures that can only be 

implemented between 4 years and the sixth calendar year after designation to 

meet the requirements of section 172(c)(6) as “additional reasonable measures”

for the area and necessary to demonstrate timely attainment under section 

189(a)(1)(B). 

Under this approach, the state may reject any otherwise technologically or 

economically feasible measures that are not needed to demonstrate attainment or that will 

not advance the attainment date by at least 1 year. That is, for a Moderate area that can 

demonstrate attainment by the statutory Moderate area attainment date, the EPA proposes 

to define as “reasonable” only those technologically and economically feasible measures 

that are necessary for expeditious attainment of the NAAQS, as the CAA does not require 

a state to adopt measures that are not needed for expeditious attainment in a Moderate 

PM2.5 nonattainment area. Thus, a state may exclude those otherwise reasonably available 

measures that, if adopted and considered collectively, would not advance the attainment 

date for the area by at least 1 year, so long as the state can demonstrate attainment as 

expeditiously as practicable and no later than the statutory Moderate area attainment date.

See proposed 40 CFR 51.1009(a)(4)(i).

The EPA recognizes that identifying which measures could not collectively 

advance the attainment date for a Moderate area by at least 1 year may be an iterative 

process that requires additional analysis and/or modeling. The agency believes that such 
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effort is reasonable for a state seeking to demonstrate the lack of need for certain controls 

that are determined to be technologically and economically feasible in light of the 

requirement for expeditious attainment in a given Moderate nonattainment area. The 

basis for deciding that it would be reasonable not to require imposition of otherwise 

available and appropriate controls because they would not be needed for attainment, or 

would not advance attainment, requires a suitably robust analysis and explanation.

Step 5b: If the state cannot demonstrate attainment by the statutory attainment 

date for a Moderate area, then the state must adopt all reasonable control 

measures. As noted elsewhere in this section, section 189(a)(1)(B) of the CAA 

requires a state to submit as part of the attainment plan either a demonstration that 

the plan will provide for attainment of the relevant NAAQS by the applicable 

attainment date, or a demonstration that attainment by such date is 

“impracticable.” This subpart 4 requirement anticipates that not all nonattainment 

areas initially classified as Moderate will necessarily be able to attain by the latest 

statutory attainment date for Moderate areas, and it incorporates the concept of an 

“impracticability demonstration” for such areas.107 The CAA is thus structured to 

provide that Moderate areas that cannot timely attain the NAAQS through the 

required elements of a Moderate area attainment plan will be reclassified to 

 

107 The concept of an “impracticability demonstration” is established in section 188(b), 

which addresses reclassifying Moderate PM2.5 areas to Serious. Section 188(b)(1) 

describes the EPA’s discretionary authority to reclassify an area upon a determination 

that an area cannot practicably attain by the Moderate area attainment date. More relevant 

to this determination, however, section 189(a)(1)(B) specifically provides for submission 

of a demonstration addressing this concept in the case of Moderate areas that cannot 

attain the NAAQS by the applicable attainment date.
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Serious and will have to meet additional control requirements beyond those that 

are “reasonable” to assure attainment of the NAAQS by a later date that is as 

expeditious as practicable. 

Existing guidance in the General Preamble on implementing this section of the 

CAA states that “the EPA believes it is reasonable for all available control measures that 

are technologically and economically feasible to be adopted for areas that do not 

demonstrate attainment [by the applicable attainment date].”108 The EPA maintains that it 

is reasonable to require a state to model the effects of emissions reductions from all 

technologically and economically feasible controls identified by the state for sources in a 

nonattainment area before asserting a claim that the area cannot practicably attain the 

relevant NAAQS by the Moderate area attainment date. However, the magnitude of 

certain PM2.5 precursor emissions and/or local atmospheric conditions of some PM2.5

nonattainment areas may render certain technologically and economically feasible control 

measures ineffective in reducing ambient PM2.5 levels. Therefore, even in a Moderate 

PM2.5 nonattainment area that cannot practicably attain the relevant NAAQS by the

statutory attainment date, the EPA believes that it may not be reasonable in all cases to 

require that a state implement all technologically and economically feasible control 

measures identified for sources in the area.

Consistent with the EPA’s long-standing interpretation that subpart 4 Moderate 

area control requirements must be reasonable, the EPA proposes that for a Moderate 

PM2.5 nonattainment area that cannot practicably attain the NAAQS by the statutory 

 

108 Ibid. at 13544.
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attainment date, a state must adopt and implement all technologically and economically 

feasible measures identified for sources in the area, except for any such measures that 

collectively will not effectively reduce ambient PM2.5 concentrations. See proposed 40 

CFR 51.1009(a)(4)(ii). The EPA views this approach as similar to the agency’s approach 

of allowing states to reject any otherwise technologically or economically feasible 

measures that are not needed to demonstrate attainment and that will not advance the 

attainment date by at least 1 year for nonattainment areas for which states can 

demonstrate attainment by the statutory attainment date. Once again, the EPA recognizes 

that identifying which measures collectively will not effectively reduce ambient PM2.5

concentrations will likely be an iterative process that requires specific analysis, 

potentially including modeling. However, the agency believes that such effort is 

appropriate for a state seeking to demonstrate the lack of need for certain controls that are 

determined to be technologically and economically feasible in a Moderate nonattainment 

area that cannot practicably attain the relevant PM2.5 NAAQS by the latest statutory 

Moderate area attainment date. The basis for establishing that it would not be reasonable 

to require imposition of otherwise available and appropriate controls because they would 

not be effective in reducing ambient PM2.5 concentrations requires an adequately robust 

analysis and explanation.

The EPA also proposes an alternative approach to identifying all reasonable 

control measures for a Moderate nonattainment area that cannot practicably attain the 

PM2.5 NAAQS by the end of the sixth calendar year following designation. Under this 

alternative, states would be required to implement all technologically and economically 

feasible control measures that they have identified for sources of direct PM2.5 emissions 
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and sources of emissions of significant PM2.5 precursors in the area. The EPA believes 

that this interpretation would be consistent with the agency’s previous guidance in the 

General Preamble and is compelled by the language of section 189(a)(1)(C), which 

separately requires a state to submit a Moderate area attainment plan and meet the RACM 

and RACT requirement, even if the state submits a demonstration that it cannot attain the 

NAAQS through those measures by the applicable attainment date. In addition, as with a 

Moderate PM2.5 nonattainment area which a state demonstrates can attain the NAAQS by 

the end of the sixth calendar year following designation, the EPA interprets the 

provisions of section 172(c)(6) to require that such an area must implement all additional 

reasonable measures that it can implement through the sixth calendar year following 

designation of the area, in addition to those measures meeting the definition of RACM 

and RACT, in order to make progress toward attainment after the end of the fourth year 

following designation.

As described in Section III of this preamble, the EPA is proposing three options 

for implementing CAA requirements applicable to PM2.5 precursors in the context of 

attainment planning and NNSR permitting. Proposed precursor Options 2A and 2B would 

provide an opportunity for a state to demonstrate that emissions of a particular precursor 

from all sources located in a Moderate PM2.5 nonattainment area do not contribute 

significantly to ambient PM2.5 levels that exceed the standard in the area, or reductions of 

which will not be effective in reducing ambient PM2.5 concentrations, in which case the 

state would not be required to identify or otherwise evaluate control measures for the 

particular precursor. Under proposed precursor Options 1 and 3, on the other hand, states 

would rely on their control strategy analyses (e.g., for Moderate nonattainment areas, 
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analyses to determine RACM and RACT and additional reasonable measures) to identify 

whether and/or which controls on sources of PM2.5 precursors are “reasonable.” The EPA 

believes that if proposed precursor Option 1 or 3 is finalized, it would be most 

appropriate to finalize the first approach to identifying reasonable control measures for 

Moderate areas that cannot practicably attain the NAAQS by the statutory attainment 

date, since states would not have an opportunity prior to evaluating the specific control 

measures for sources of PM2.5 precursors in the nonattainment area to demonstrate that 

controlling all sources of a particular precursor would not be effective in reducing 

ambient PM2.5 levels in the area. Likewise, if the agency finalizes proposed precursor 

Options 2A or 2B, the EPA believes that it would be most appropriate to finalize the 

alternative proposed approach of requiring a state to implement all technologically and 

economically feasible measures identified by the state for sources in the area that can be 

implemented by the end of the sixth calendar year following designation if the state 

demonstrates that the area cannot practicably attain the NAAQS by the statutory 

attainment date, since the “measures identified by the state” would already implicitly 

exclude control measures on sources of any “insignificant” precursor. The EPA seeks 

comment on the two proposed approaches to selecting RACM and RACT and additional 

reasonable measures for Moderate nonattainment areas that cannot practicably attain the 

NAAQS by the statutory attainment date, and on the EPA’s evaluation of the 

compatibility of these proposed approaches with the agency’s proposed precursor 

options.

The EPA’s proposed analytical process for determining RACM and RACT is 

intended to result in a comprehensive list of such technologically and economically 
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feasible controls that would include local and state measures that could achieve emissions 

reductions from sources within the area, beyond those that could or would be achieved 

through regional or national measures. Furthermore, the EPA is proposing to require that 

the Moderate area attainment plan must include modeling of all RACM and RACT and 

additional reasonable measures, and other state, regional and federal measures, to 

demonstrate that a state will not be able to attain the NAAQS by the end of the sixth 

calendar year after designation due to the severity of nonattainment in the area and/or due 

to the lack of availability or feasibility of implementing controls in the area by such date. 

Subpart 4 requires that Moderate areas that cannot or do not meet the Moderate 

area attainment date be reclassified as Serious nonattainment areas, in which case sources 

in the areas are then subject to BACM and BACT requirements. In the General Preamble, 

the EPA indicated that “it may be reasonable, in some limited circumstances, for States to 

consider the compatibility of RACM and RACT with the BACM and BACT that will 

ultimately be implemented under the Serious area plans for those areas.”

109 Furthermore, 

for such areas that do not meet the Moderate area attainment date, the EPA indicated that 

“in the case of RACM for area sources, EPA anticipates that any future implementation 

of BACM for these sources will be additive to, and hence compatible with, RACM. This 

is because BACM will generally consist of a more extensive implementation of the 

RACM measures... Since EPA anticipates that RACM and BACM for these sources will 

be compatible, the SIP’s (sic) for these areas should reflect the application of available 

 

109 Ibid. at 13544.
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control measures to existing sources in moderate nonattainment areas as determined by 

the analysis described...for RACM.”110 The EPA believes that a state should consider 

selecting and implementing controls that may qualify as BACM or BACT in a Moderate 

nonattainment area as part of their RACM and RACT analysis if they have reason to 

suspect that the area may not be able to attain the NAAQS by the applicable Moderate 

area attainment date as long as the control can be implemented by the statutory Moderate 

area attainment date. Early adoption of controls that would constitute BACM or BACT 

could be more efficient and could further the objectives of attaining the NAAQS 

expeditiously to protect public health and the environment.

3. RACM and RACT and additional reasonable measures submission 

requirements

To ensure that attainment plan submissions contain the necessary supporting 

information to enable the EPA to review and approve a state’s evaluation and selection of 

measures that constitute RACM and RACT in a given nonattainment area, the EPA 

proposes to require under the authority of section 301(a) that a state must submit the 

following information as part of its submission: 

 A list of all source categories, sources and activities in the nonattainment area that 

emit direct PM2.5 or any PM2.5 precursor (for multi-state nonattainment areas, this 

would include source categories, sources and activities from all states which make 

up the area);

 For each source category, source or activity in the nonattainment area, an 

 

110 Ibid.
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inventory of direct PM2.5 emissions and emissions of all PM2.5 precursors;

 For each non-de minimis source category, source or activity in the nonattainment 

area, a comprehensive list of potential control measures considered by the state 

for the nonattainment area;111, 112

 For each potential control measure considered by the state but eliminated from 

further consideration due to a determination by the state that the control measure 

or technology was not technologically feasible, a narrative explanation and 

quantitative or qualitative supporting documentation to justify the state’s 

conclusion;

 For each technologically feasible emission control measure or technology, the 

state must provide the following information relevant to economic feasibility: (1) 

the control efficiency by pollutant; (2) the possible emissions reductions by 

pollutant; (3) the estimated cost per ton of pollutant reduced; and, (4) a 

determination of whether the measure is economically feasible, with narrative 

explanation and quantitative supporting documentation to justify the state’s 

conclusion.

 For each technologically and economically feasible emission control measure or 

technology, the date by which the technology or measure could reasonably be

 

111 If the EPA finalizes proposed precursor Option 2A or 2B, which would effectively 

allow a state to demonstrate that a given precursor does not contribute significantly to 

PM2.5 concentrations in a nonattainment area, then this step would require potential 

control measures only for sources of direct PM2.5 and precursors not exempted from 

further analysis.

112 Menu of Control Measures document available at http://www.epa.gov/air/criteria.html

(last accessed 01/14/14).
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implemented.

Each of these elements will provide information needed by the EPA to evaluate 

correctly and efficiently whether the state is meeting the statutory requirements for an 

attainment plan, and in particular meeting the statutory requirement for states to 

implement RACM and RACT on sources within the nonattainment area. The EPA 

recognizes that the base year emissions inventory for the area that the state submits in 

conjunction with its attainment plan will likely contain some of the information proposed 

to be required under the first two items in this list. However, the EPA believes that it is 

incumbent on the state to ensure that the information needed for the EPA to evaluate the 

state’s RACM and RACT analysis is presented more specifically as part of the RACM 

and RACT analysis and in a format that provides transparency, consistency and the 

ability for another party to evaluate the state’s analysis effectively. For this reason, the 

EPA is including emissions inventory information specifically relevant to the RACM and 

RACT element of the state’s attainment plan.

4. Criteria for effective regulations to implement RACM and RACT and additional 

reasonable measures

After a state has identified a particular control measure as RACM or RACT or 

additional reasonable measure for a particular nonattainment area, it must then implement 

that measure through a legally enforceable mechanism that will be included in the SIP 

(e.g., a state rule that the EPA will approve as a part of the federally enforceable SIP for 

the state). The EPA is proposing that in order for the EPA to be able to approve any such 

measure as part of the SIP, the state would have to provide information to meet the 
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following four criteria. These criteria are similar to the criteria finalized as part of the 

remanded 2007 PM2.5 Implementation Rule.

First, the base year emissions from the source or group of sources to which the 

control measure applies and the future year projected emissions from those sources once 

controlled must be quantifiable so that the projected emissions reductions from the 

sources can be attributed to the specific measures being implemented. It is important that 

the emissions from the source category in question are accurately represented in the base 

year inventory so that emissions reductions are properly calculated. In particular, it is 

especially important to ensure that both the filterable and condensable components of 

direct PM2.5 emissions are accurately represented in the base year.

Second, the control measures must be enforceable. This means that they must 

specify clear, unambiguous and measurable requirements. The measurable requirements 

for larger emitting facilities must include periodic source testing, monitoring or other 

viable means to establish whether the affected source meets the applicable emission limit. 

Additionally, to verify the continued performance of the control measure, specific 

emissions monitoring programs appropriate for the type of control measure employed and 

the level of emissions must be included to verify the continued performance of the 

control measure. The control measures and monitoring program must also have been 

adopted according to proper legal procedures.

Third, the results of application of the control measures must be replicable. This 

means that where a rule contains procedures for interpreting, changing or determining 

compliance with the rule, the procedures are sufficiently specific and objective so that 

two independent entities applying the procedures would obtain the same result.
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Fourth, the control measures must be accountable. This means, for example, that 

source-specific emission limits must be permanent and must reflect the assumptions used 

in the attainment plan for the area, including the modeling conducted in conjunction with 

the attainment demonstration. It also means that the attainment plan must establish 

requirements to track emissions changes at sources and provide for corrective action if 

emissions reductions are not achieved according to the plan.

The EPA seeks comment on these criteria for approval of any control measures 

adopted by a state for a Moderate area to assure that such measures are legally 

enforceable.

5. Determination of RACM and RACT and additional reasonable measures in 

multi-state nonattainment areas

States in multi-state nonattainment areas will need to consult with each other on 

appropriate control measures for the shared nonattainment area. The agency anticipates 

that states could decide upon RACM and RACT and additional reasonable measures that 

differ from state to state in a shared nonattainment area, based upon each state’s 

determination of the most effective strategies given the relevant mixture of sources and 

potential controls in the respective states’ portions of a shared nonattainment area. As 

long as each state can adequately demonstrate that its chosen attainment strategy, 

including its selection and adoption of RACM and RACT and additional reasonable 

measures, will provide for meeting RFP requirements and for attainment of the NAAQS 

as expeditiously as practicable for the nonattainment area at issue, the EPA anticipates 

being able to approve individual state plans that may elect to control a different mix of 

sources or to implement different controls, under the proper circumstances. Nevertheless, 
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in evaluating RACM and RACT and additional reasonable measures for a particular 

nonattainment area, states must consider potential reasonable control measures developed 

for other areas or other states, and particularly for other portions of an interstate 

nonattainment area. In addition, states in multi-state nonattainment areas must evaluate 

whether the reasonable measures each state may have identified as not being necessary 

for attainment could collectively advance the attainment date for the area by at least 1 

year. The EPA may consider such measures in assessing the approvability of each state’s 

individual attainment plan for a multistate nonattainment area.

6. Environmental justice considerations in developing the attainment plan control 

strategy for a Moderate PM2.5 nonattainment area

The EPA strongly urges states to consider environmental justice concerns with 

respect to any control measures they have identified as potential RACM or RACT or 

additional reasonable measures in an area, particularly to the extent that control measures

that a state may be considering are otherwise approximately equal (in terms of 

technological and economic feasibility) but unequal with respect to their direct or indirect 

impacts on overburdened populations.113 In such cases, the EPA encourages the state to 

prioritize imposition of the control measures that will result in the least possible burden 

and greatest degree of health protection for overburdened populations in the 

 

113 The term “overburdened populations” is defined in the EPA’s “Plan EJ 2014” to 

describe the minority, low-income, tribal, and indigenous populations or communities in 

the U.S. that potentially experience disproportionate environmental harms and risks as a 

result of greater vulnerability to environmental hazards. This increased vulnerability may 

be attributable to an accumulation of both negative and lack of positive environmental, 

health, economic or social conditions within these populations or communities.
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nonattainment area. Section IX of this preamble discusses this and other possible 

approaches for states to incorporate ways to address environmental justice concerns 

associated with implementation of the PM2.5 NAAQS in their attainment plans and SIP 

development process, and the EPA seeks comment on ways to more fully address such 

concerns.

E. Modeling for Attainment Demonstrations

1. Statutory requirements

Section 189(a) generally requires a state with a designated Moderate 

nonattainment area to submit an attainment plan for such area. As discussed earlier, 

section 189(a)(1)(B) more specifically requires the state to submit an attainment 

demonstration including air quality modeling to establish either: (i) that the area will 

attain the relevant NAAQS by the applicable attainment date; or, (ii) that it is 

impracticable for the area to attain the relevant NAAQS by the applicable attainment 

date. For Moderate nonattainment areas, the attainment date is as expeditiously as 

practicable, but no later than the end of the sixth calendar year after designation as 

nonattainment. Section 189(a)(2)(B) of the CAA requires states with designated 

nonattainment areas to submit attainment plans no later than 18 months after designation.

2. What is an attainment demonstration?

Section 189(a)(2)(B) does not define the term “demonstration” and does not 

specify precisely how a state should make the required demonstration. Thus, the EPA 

believes it is necessary to provide more specific parameters for such demonstrations in 

order to assure that they contain the requisite information to allow for meaningful 

evaluation of the issues that the demonstrations are intended to address. An attainment 
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demonstration is a set of analyses that provide an explanation of how a state will attain 

the PM2.5 NAAQS by the applicable attainment date in a particular nonattainment area.114

The EPA is proposing that the demonstration must contain: (i) technical analyses such as 

base year and future year modeling of emissions which identify sources and quantify

emissions that are contributing to violations of the PM2.5 NAAQS; and, (ii) analyses of 

future year emissions reductions and air quality improvement resulting from existing (i.e., 

already-adopted or “on the books”) national, regional and local programs, and potential 

new local measures needed for attainment, including RACM and RACT controls for the 

area. Each state with a Moderate nonattainment area must submit an attainment plan with 

an attainment demonstration that includes analyses supporting the state’s determination 

of its proposed attainment date. In all cases, the state must show that the Moderate area 

will attain the NAAQS as expeditiously as practicable, but not later than the end of the 

sixth calendar year after designation. In order to establish that the attainment date is as 

expeditious as practicable, the state must explain why any control measures adopted in 

the attainment plan provide for the most expeditious attainment and, specifically, must 

demonstrate that collectively the reasonable measures that were not adopted as RACM or 

RACT or additional reasonable measures will not advance the attainment date by at least 

1 year if implemented. See proposed 40 CFR 51.1011(a).

 

114 An area is designated nonattainment for either the annual PM2.5 NAAQS or the 24-hr 

PM2.5 NAAQS or both. The attainment demonstration should show that the area is 

attaining the form of the NAAQS for which they have been designated nonattainment.
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A state may alternatively submit a demonstration that shows that attainment by 

the statutory attainment date for a Moderate area is impracticable.115 The statute does not 

define the term “impracticable” in this context, so it is necessary for the EPA to interpret 

this term in the context of a submission from the state for this purpose. In order to support 

this type of demonstration, the EPA proposes to require that the state must show that, 

even if all technologically and economically feasible controls that can be implemented 

within 6 years were implemented, the state could not attain the NAAQS within the 

statutory timeframe for a Moderate area. A state could do this by performing a modeling 

analysis which projects emissions to the sixth year after designations in order to predict 

future year PM2.5 design values in the area. The projected emissions would account for all 

existing federal and state SIP-adopted regulations on sources outside the nonattainment 

area that were in place at the time, plus all measures that were identified as 

technologically and economically feasible controls that can be implemented in the 

nonattainment area within six years of designation (i.e. all measures that would qualify as

RACM or RACT or as additional reasonable measures), as well as any other reasonable 

measures available in the state that could aid in achieving timely attainment. If the 

modeling shows that attainment cannot be reached by the end of the sixth calendar year 

following designation, then the analysis could be used to demonstrate that it is 

impracticable for the area to attain the relevant NAAQS by the statutory attainment date. 

Other information can also be used to support the demonstration, including ambient data 

 

115 Pursuant to section 188(b)(1)(B), upon an EPA determination that attainment by the 

Moderate date is impracticable, the EPA shall reclassify the area as Serious within 18 

months after the Moderate area attainment plan due date. 
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and emissions trends data. States are encouraged to work with their respective EPA 

Regional Office to identify appropriate information that could be used to support an 

impracticability demonstration. The EPA emphasizes that states that can make the 

required showing that a Moderate nonattainment area cannot attain the NAAQS by the 

statutory attainment date are nonetheless required to meet the substantive requirements 

for a Moderate area attainment plan, including the implementation of control measures 

that are RACM and RACT and additional reasonable measures in that area.

3. What modeling is required?

States are required to submit air quality modeling in support of an attainment 

demonstration for a Moderate PM2.5 nonattainment area. Although air quality modeling is 

not expressly required for a Moderate area demonstration showing that attainment by the 

attainment date is impracticable (per section 189(a)(1)(B)(ii)), the EPA proposes to 

interpret the CAA to require air quality modeling similar to that required for an 

attainment demonstration in order to demonstrate that attainment of the relevant PM2.5

NAAQS by the statutory attainment date is impracticable. Because air quality modeling 

is a required element of the attainment demonstration in section 189(a)(1)(B), the EPA 

believes that it logically follows that similar modeling should also be required to show 

that an area will not be able to attain by the attainment date contemplated by the statute. 

There may be limited cases in which a state may be able to demonstrate through a 

rigorous technical analysis with supporting documentation that attainment by the 

statutory Moderate area attainment date is impracticable. Given that the statute may be 

interpreted as not requiring air quality modeling for an impracticability demonstration, 

the EPA proposes and seeks comment on an alternative option under which air quality 
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modeling would not be a requirement for a Moderate area impracticability demonstration. 

The EPA would recommend that a state submit modeling as part of any Moderate area 

impracticability demonstration, but under this alternative option such modeling would not 

be a regulatory requirement. 

Given that secondarily formed PM2.5 (e.g. ammonium sulfate, ammonium nitrate 

and SOA) is a large fraction of the total measured PM2.5 in most PM2.5 nonattainment 

areas, the EPA assumes that photochemical grid modeling (which considers secondary 

PM2.5 formation) will be needed for a state to demonstrate attainment with the NAAQS. 

Most previous PM2.5 attainment demonstrations for both the 1997 and 2006 PM2.5

NAAQS have utilized photochemical grid models. However, in some nonattainment 

areas that are dominated by primary PM2.5 emissions (e.g. residential wood smoke), more 

simplistic dispersion models, such as a combination of dispersion, receptor and box 

airshed models, may suffice to demonstrate that the area will attain the NAAQS. 

Regardless of the modeling approach selected to support the attainment demonstration, 

the analyses must be based on technically credible methods and provide for the timely 

submittal of the attainment demonstration and implementation of control measures. States 

should consult with their respective EPA Regional Office to determine the appropriate 

type of modeling demonstration for the particular nonattainment area. 

4. Do states need to develop new modeling for their attainment demonstrations?

The EPA believes that the statutory provision requiring attainment demonstrations 

for Moderate PM2.5 nonattainment areas to include air quality modeling can be fulfilled in 

a variety of ways. Thus the EPA proposes to allow states to fulfill the statutory modeling 

requirement through either locally generated photochemical and/or dispersion modeling 
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or, with proper justification, through appropriate regional or national modeling. The EPA 

seeks comment on what types of modeling demonstrations should be required to fulfill 

the CAA requirement to “include air quality modeling” as part of the attainment 

demonstrations for Moderate nonattainment areas.

New modeling analyses that follow the EPA modeling guidance, conducted by the 

state for implementing the PM2.5 NAAQS, will presumably satisfy the attainment 

demonstration modeling requirement. However, many areas that were designated as 

nonattainment for the 1997 and/or 2006 PM2.5 NAAQS have already invested 

considerable resources in local and/or regional PM2.5 modeling analyses. Most states with 

potential PM2.5 nonattainment areas are already participating in regional modeling 

analyses through multi-jurisdictional organizations (MJOs). These MJOs (e.g. SESARM, 

LADCO and WRAP) represent most states with PM2.5 nonattainment areas in the 

country. There is ongoing PM2.5 modeling that may provide useful information for state 

PM2.5 NAAQS attainment demonstrations.

In addition to local and regional modeling, the EPA conducts nationwide 

modeling (generally limited to the contiguous 48 states) in support of various national 

rulemakings. The base and future modeling year for national rule modeling varies 

depending on compliance dates for the rule being analyzed and on when the modeling 

was conducted. For example, there are several analyses of recent and ongoing rules 

which may provide useful PM2.5 modeling information for state attainment 

demonstrations. Among them are modeling to support the 2012 PM2.5 NAAQS review, 

the final Tier-3 mobile source emissions standards, and the current ozone NAAQS 
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review.116 While the analyses in these rulemaking actions may not be precisely relevant

for the purposes of a PM2.5 attainment plan, they may nevertheless provide useful 

information or input relevant to states developing attainment plans for the PM2.5 NAAQS.

Similar nationwide modeling efforts may be helpful for purposes of future PM2.5

NAAQS.

States may be able to use regional and/or EPA modeling to demonstrate that 

specific nonattainment areas will attain the relevant PM2.5 NAAQS by the applicable 

attainment date. But states must evaluate the relevant modeling information to show that 

it is suitable for that purpose. For example, the modeling should be evaluated to show 

that it is performing adequately for the area; that the future modeling year is appropriate 

for the particular attainment demonstration; and, that the base year emissions and 

projected emissions and controls adequately represent the base year conditions and 

emissions expected to occur in the area in the future. States should work closely with the 

appropriate EPA Regional Office to determine what (if any) existing modeling may be 

suitable for use in an attainment demonstration (or an impracticability demonstration) for 

a Moderate PM2.5 nonattainment area.

The EPA requests comment on how states can use existing regional and/or 

national modeling to meet their attainment demonstration requirements. The agency also 

notes that even when regional or EPA modeling is available to show that an area is 

expected to attain the PM2.5 NAAQS by the applicable attainment date, other CAA 

 

116 Even though the ozone NAAQS modeling will be focused on ozone, PM2.5 modeling 

results will likely be generated from the analysis in order to inform health benefits 

calculations.
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requirements may be difficult to satisfy through the use of regional or EPA modeling. For 

example, states may or may not be able to satisfy their CAA requirements for emissions 

inventory submittals or RFP demonstrations by using data derived from MJO or EPA 

modeling. The available regional/national modeling may not include an appropriate base 

year or future year, and the level of detail or how the emissions were derived may not be 

appropriate or compatible with inventories needed to satisfy specific CAA requirements. 

States may have to derive more local specific inventory data, for the appropriate years, to 

adequately satisfy these CAA requirements.

Because it will be challenging for states to prepare new modeling analyses to 

meet the submission deadline for the Moderate area attainment plans, the EPA 

encourages states to start work on modeling analyses as soon as possible, in order to 

ensure that adequate time is devoted to developing a technically credible attainment 

demonstration. States that have the most challenging PM2.5 problems will likely need to 

develop new and/or updated photochemical modeling analyses for their nonattainment 

areas, with emissions (including potential new controls) projected to the appropriate 

future attainment year.

5. What guidance is available for using models to demonstrate attainment?

The procedures for modeling PM2.5 as part of an attainment demonstration are 

described in the EPA’s “Guidance on the Use of Models and Other Analyses for 

Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze.”117

 

117 The modeling guidance can be found at the following website: 

http://www.epa.gov/scram001/guidance/guide/final-03-pm-rh-guidance.pdf.
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All modeling in support of an attainment demonstration should be consistent with the 

EPA’s PM2.5 photochemical modeling guidance (referenced above) as well as the 

Guideline on Air Quality Models (40 CFR part 51, Appendix W). 

The PM2.5 attainment demonstration modeling guidance describes how states can 

apply air quality models to generate results needed to demonstrate attainment. These 

recommendations include developing a conceptual description of the problem to be 

addressed; developing a modeling/analysis protocol; selecting an appropriate model to 

support the demonstration; selecting appropriate meteorological episodes or time periods 

to model; choosing an appropriate area to model with appropriate horizontal/vertical 

resolution; generating meteorological and air quality inputs to the air quality model; 

generating emissions inputs to the air quality model; and, evaluating performance of the 

air quality model. After these steps are completed, the state can apply a model to simulate 

effects of future year emissions and candidate control strategies.

The EPA is not requiring a specific model for use in the attainment demonstration 

for the PM2.5 NAAQS. At present, there is no single model which has been extensively 

tested and shown to be clearly superior to other available models. The current modeling 

guideline, 40 CFR part 51, Appendix W, does not identify a preferred model for use in 

attainment demonstrations of the PM2.5 NAAQS. Thus, states may choose from several 

alternatives so long as the alternative is appropriate for the nonattainment area under 

evaluation.

In some cases, a state may need to apply multiple models in the attainment 

demonstration. In most cases, a photochemical grid model is needed to predict base and 

future year concentrations of secondary PM2.5. Photochemical grid models can also be 
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used to predict concentrations of primary particulate and are useful in assessing steep 

concentration gradients arising from area sources. However, in areas with high 

concentrations of primary PM2.5, or strongly stratified air at the surface, a Gaussian 

plume model or puff model may also be needed to more accurately represent steep 

concentration gradients (or lack of mixing to the surface) in locations with a large 

contribution from a single or multiple primary PM2.5 point sources or locations in nearroad areas. The EPA’s attainment demonstration modeling guidance provides details and 

recommendations on using multiple models.

Models are used to test whether control measures in an attainment plan are likely 

to result in attainment of the relevant standard(s). The attainment demonstration modeling 

guidance recommends a modeled attainment test for the annual and 24-hour PM2.5

NAAQS that uses a combination of ambient PM2.5 and PM2.5 species data and modeled 

PM2.5 concentrations to estimate future year air quality. In the recommended attainment 

test, the state applies the test at each PM2.5 ambient monitor location within or near a 

designated nonattainment area. Models are used in a relative sense to estimate the 

response of measured air quality to future changes in emissions. Future air quality is 

estimated by multiplying recent monitored PM2.5 values by the modeled relative response 

(percent change) to projected future changes in emissions. If the future design value at all 

monitoring locations in the nonattainment area does not exceed the concentration of 

PM2.5 specified in the NAAQS, the area is projected to attain the NAAQS.

Because PM2.5 is a mixture of chemical components, states should use recent 

observations and modeled responses of major components of PM2.5 (i.e. sulfate, nitrate, 
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organic carbon, etc.) to estimate future concentrations of each component. 118 The 

predicted future concentration of PM2.5 is the sum of the future year predicted component 

concentrations.

The attainment demonstration modeling guidance contains additional details 

regarding the treatment of PM2.5 and speciation monitoring data. Because PM species 

data are not available at each PM2.5 FRM site, the EPA recommends a methodology 

which interpolates species data to each FRM site in order to estimate the species 

concentrations in the area. This information, combined with modeling results, may be 

used to calculate future air quality at each FRM monitoring site. The EPA has developed 

software to perform both the annual and 24-hour PM2.5 attainment test (including 

interpolating PM species data). The software is called the Modeled Attainment Test 

Software (MATS) and is available for no cost at: 

http://www.epa.gov/scram001/modelingapps_mats.htm. The software is provided to make 

it relatively easy for states to apply the recommended modeled attainment test. However, 

states are not required to use MATS and can develop their own post-processing software.

The modeling guidance also describes the opportunity for states to supplement 

their modeling with a “weight of evidence” demonstration. States may use other 

information and analyses, in addition to the modeled attainment test, to estimate whether 

future attainment of the NAAQS in an area is likely. Other analyses may include, but are 

not limited to, emissions trends, ambient data trends and analyses, other modeling 

 

118 The exact years of the “recent” ambient data are defined by the base year selected for 

the modeling. The guidance recommends using 5 years of ambient data, centered about 

the base modeling year.
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analyses, and documentation of other non-modeled emissions control strategies, 

including voluntary programs.

The reliability of tests for estimating future attainment depends upon having 

reliable databases for inputs to those tests. The modeling guidance identifies and 

prioritizes key data-gathering activities and analytical capabilities that will increase 

credibility of analyses used to estimate if the NAAQS will be attained in the area by the 

statutory attainment date. 

The EPA is considering updates to the modeling guidance to address PM2.5

modeling for the 2012 PM2.5 NAAQS. The agency released a revised draft modeling 

guidance for developing demonstrations to meet PM2.5, ozone, and regional haze air 

quality goals in December 2014, and intends to revise the guidance after considering 

public comments received.119

The application of air quality models requires a substantial effort by state and 

local agencies. Therefore, states should work closely with their respective EPA Regional 

Office in executing each step of the modeling process. Doing so will ensure that states 

know what EPA analyses they can rely on, if they wish, to simplify this task, and it will 

increase the likelihood of the EPA’s approval of a state’s demonstration submitted at the 

end of the modeling and overall attainment plan development process. 

 

119 See “Draft Modeling Guidance for Demonstrating Attainment of Air Quality Goals for

Ozone, PM2s, and Regional Haze,” issued by Richard Wayland, Director of Air Quality 

Assessment Division, EPA Office of Air Quality Planning and Standards, to EPA 

Regional Air Division Directors, Regions I-X, December 3, 2014. Available at: 

http://www.epa.gov/ttn/scram/guidance/guide/Draft_O3-PM-RH_Modeling_Guidance2014.pdf
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6. Demonstrating attainment at near-road monitors

The 2012 PM2.5 NAAQS final rule contains new requirements for operating nearroad monitors in the largest metropolitan areas.120 The first set of monitors were required 

to be in place as of January 1, 2015 (see Section II of this preamble for more details). 

These monitors will not have the requisite 3 years of monitoring data necessary to 

calculate a PM2.5 design value until 2018 at the earliest. Therefore, these data were not 

available to inform the first round of initial designations for the 2012 PM2.5 NAAQS and 

there will be less than 3 years’ worth of data available when the initial attainment 

demonstrations for Moderate areas are due in October 2016. As a result of this timing, the 

agency is proposing that the initial set of Moderate area attainment demonstrations will 

not need to include projected design values for near-road monitor locations. However, 

subsequent attainment demonstrations for the PM2.5 NAAQS (after 2018, when 3 or more 

years of complete ambient data are available at near-road monitors) will need to address 

those monitor locations in attainment plans and will need to include a demonstration that 

those monitor locations will show attainment of the NAAQS by the applicable statutory 

attainment date. The revised modeling guidance document for the PM2.5 NAAQS

includes procedures for applying a dispersion model or a combination of photochemical 

grid models and dispersion modeling to demonstrate attainment at near-road monitor

locations.

7. Demonstrating attainment in unmonitored areas

 

120 78 FR 3283.
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As explained in the 2012 PM2.5 NAAQS final rule and summarized in Section II 

of this preamble, the EPA’s monitoring requirements for PM2.5 are designed to ensure a 

robust nationwide monitoring network in both nonattainment and attainment areas. Air 

agencies have achieved this by maintaining their PM2.5 networks in accordance with 

EPA’s network design criteria. Historically, these criteria provided that CBSAs have at 

least one PM2.5 monitoring site located in an “area-wide” location of expected maximum 

concentration (within the CBSA).

121 Thus, by assuring compliance with the NAAQS at 

the location of the expected highest area-wide concentration in the CBSA, air quality is 

protected throughout each CBSA. However, due to limited resources, there are limits to 

the number of air quality monitors that can be deployed and it therefore may be useful to 

consider what, if any, additional monitoring needs there may be as agencies prepare their

attainment plans. 122

Under the 2007 PM2.5 Implementation Rule, the EPA required states to follow 

existing modeling guidance, which suggested that a state’s PM2.5 attainment plan could be 

 

121 As explained in the final 2012 PM NAAQS rule, the EPA expects that each CBSA 

will maintain its existing highest concentration area-wide monitoring site (referred to as 

the design value site), See 78 FR 3240. These sites were set up during the period of time 

when the network design criteria required having at least one site in an area-wide location 

of expected maximum concentration. The EPA intends to maintain the highest priority 

sites in the existing network, which are often at the neighborhood scale, as the largest part 

of the PM2.5 monitoring network to continue to support a number of monitoring 

objectives, while also allowing lower value sites to move to near-road locations as that 

part of the network is phased in.

122Annual monitoring network plans and 5 year assessments are required by regulation in 

40 CFR 58.10. The 5 year monitoring network assessment is a comprehensive evaluation 

of a monitoring agency’s ambient air monitoring network, while the annual plan 

describes the existing network and changes being proposed to support implementing 

recommendations from the most recent 5 year assessment as well as any applicable 

changes finalized in association with NAAQS revisions.
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approved if it demonstrated attainment, through the modeled attainment test, at monitored

locations only. But the guidance also recommended that states conduct further analyses 

based on the modeling results to determine whether there were unmonitored areas that 

merited additional analysis or investigation. The guidance further recommended that 

states either reduce emissions that, based on these recommended additional analyses, 

could cause violations in unmonitored areas, or that they place a new monitor in such an 

area. The EPA found that the minimum requirements for the unmonitored area analysis in 

the 2007 modeling guidance (and the 2007 PM2.5 Implementation Rule) were not 

sufficiently clear. The EPA is therefore proposing several alternative options in order to 

clarify the appropriate treatment of model results in unmonitored areas for purposes of 

implementing current and future PM2.5 NAAQS. 

The EPA is proposing four possible approaches to demonstrating attainment in 

unmonitored areas. Option 1 would require states to perform the attainment test at 

locations that have current or recent FRM and/or FEM monitoring data. The EPA would 

not require states to analyze areas that have no monitoring data with which to anchor the 

attainment demonstration modeling results. The EPA is proposing this approach to 

evaluating monitored and unmonitored areas in order to be consistent with how 

attainment of the PM2.5 NAAQS is determined for purposes of designations and 

redesignations, and due to uncertainty in modeled projections in locations where there are

no monitoring data to anchor the future year model results. As discussed in Section II of 

this preamble, the EPA promulgates designations for PM2.5 NAAQS nonattainment areas 
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based primarily on ambient data measured at FRM and FEM monitors.

123 Although the 

EPA considers other forms of information for purposes of evaluating areas with sources 

that contribute to those monitored violations for inclusion within the nonattainment area 

boundaries, the fundamental basis for designating an area as nonattainment for a NAAQS 

is the presence of one or more FRM or FEM monitors with data showing violations of the 

NAAQS in question. Similarly, determinations of attainment of the NAAQS for purposes 

of redesignation actions are based primarily on monitored data. When all FRM and FEM 

monitors in a nonattainment area measure attainment of the NAAQS, the state is eligible 

to submit a redesignation request for the area, assuming that it has complied with all other 

applicable requirements for purposes of redesignation. Specifically, the EPA’s approval 

of a redesignation request is subject to meeting the requirements of CAA section 

107(d)(3)(E). Among those requirements is that the area has attained the NAAQS. For 

the PM2.5 NAAQS, this determination is based on ambient data measured at the FRM and 

FEM monitors in the area in question. Thus, neither PM2.5 designations nor 

redesignations currently take into account information regarding potential violations of 

the NAAQS at unmonitored locations throughout a given area. Therefore, consistent with 

how PM2.5 areas are designated and redesignated, the EPA is first proposing to require 

that states only show attainment at PM2.5 FRM and FEM monitoring locations as an 

element of their attainment demonstrations for the PM2.5 NAAQS.

 

123 A monitor must have 3 years of ambient data available to be used to calculate a PM2.5

design value and determine compliance with the NAAQS.
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In addition, the “relative” attainment test for PM2.5 uses FRM or FEM ambient 

monitoring data, combined with future year modeled percentage changes in PM2.5 

concentrations, to project future year design values. Since the attainment test relies on 

ambient monitoring data, an analysis of future year concentrations in unmonitored areas 

can only be accomplished by interpolating ambient data to a particular location where 

there is no existing monitor or recent monitoring data. Therefore, in the context of an 

attainment demonstration, the projection of future year PM2.5 concentrations in 

unmonitored locations is inherently more uncertain than projections in monitored 

locations due to the fact that the ambient concentrations from which these projections are 

developed are unknown in the unmonitored locations. 

Proposed Option 2 for unmonitored area analyses would require the state to 

conduct an unmonitored area analysis as part of all attainment demonstrations (for 

Moderate and Serious areas) and require the state to eliminate potential violations in 

unmonitored areas through enforceable emissions reductions in the SIP. The requirement 

would be based on a premise that states must demonstrate attainment of the NAAQS in 

all locations of a nonattainment area, and models can and should be used for that purpose. 

Modeled attainment demonstrations using photochemical grid models provide modeling 

results for all grid cells in the nonattainment area. Therefore, notwithstanding the 

uncertainty that is inherent to this approach as discussed above, model outputs (optionally 

combined with interpolated ambient data) could be used to derive estimates of PM2.5

concentrations in unmonitored areas. 

Proposed Option 3 would require states to show attainment at all current and 

recent monitoring locations. In addition, states would be required to provide an 
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unmonitored area analysis as part of all attainment demonstrations (for Moderate and 

Serious areas). However, rather than requiring states to impose additional enforceable 

emissions reductions in the SIP to address potential violations in these locations, states 

would be required to use the unmonitored area analysis results to develop an assessment 

of the likelihood of violations in unmonitored areas. This assessment may be especially

important in areas with a relatively sparse PM2.5 monitoring network or in locations 

where information such as modeling data, emissions inventories or non-FEM monitoring 

data (such as from special purpose monitors or saturation monitoring studies) may 

indicate potential high PM2.5 concentrations in areas that are currently unmonitored. 

The nature of the assessment depends on local area modeling, but could include,

as appropriate, elements such as an evaluation of the emissions inventory (particularly for 

local direct PM2.5 sources), the existing ambient data for the area, and meteorological 

model inputs to determine if the modeled violations in unmonitored areas appear to be 

credible. If potential violations are found to be credible, additional steps may include 

imposition of enforceable emissions reductions at nearby emission sources or a 

commitment to deploy special purpose monitors and/or saturation monitors in the area (in 

order to further evaluate the problem). The state would be required to document the 

assessment, including analyses of emissions, meteorological inputs and ambient data

and/or make a commitment to establish special purpose monitors as part of the attainment 

demonstration. Special purpose ambient air monitoring data that is collected after the 

attainment demonstration is submitted should be summarized for use in the area’s 5-year 
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monitoring assessment and, where appropriate, annual monitoring network plans.

124

Additionally, monitoring data that is collected as a result of the unmonitored area analysis 

assessment (after the attainment demonstration is submitted) must be reported as a 

quantitative milestone required under section 189(c)(1) (see Section IV.G of this 

preamble). 

In summary, Option 3 would clarify that an unmonitored area analysis would be

required in all attainment demonstrations, and an assessment of the unmonitored area 

analysis results would be required as part of the attainment demonstration documentation. 

But the unmonitored area analysis results would not be used as part of the specific 

analytical approach for determining whether a particular control strategy will result in the 

area attaining the NAAQS. 

Finally, proposed Option 4 would require states to show attainment at all current 

and recent monitoring locations. States would not be required to provide an unmonitored 

area analysis as part of the attainment demonstration. However, the EPA would 

encourage states to use information available to them to consider what, if any, impacts 

may be occurring in unmonitored areas. States could consider information such as 

modeling data, emissions inventories or non-FEM monitoring data (such as from special 

purpose monitors or saturation monitoring studies) which may indicate potential high 

PM2.5 concentrations in areas that are currently unmonitored. Under this approach, states 

could consider model results to develop an assessment of the likelihood of violations in 

 

124 All states are required to have an annual monitoring plan (see Section II of this 

preamble) which meets the siting criteria for PM2.5 monitors (40 CFR 58.10). 
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unmonitored areas. This proposed option differs from Option 3 in that it would not 

require an unmonitored area analysis. Rather, under proposed Option 4, an unmonitored 

area analysis would be recommended where the state and/or the EPA has reason to 

believe that potential violations may be occurring in unmonitored areas, or other 

available information indicates that further analysis is warranted. States would be 

expected to consult with the appropriate EPA Regional Office to evaluate available 

information to determine if an unmonitored area analysis is needed for a particular area. 

The four options presented above would lead to a range of potential analysis costs

by requiring attainment demonstrations at more locations and with varying degrees of 

specificity. To the extent that these analyses reveal additional locations with potential 

violations, the effort needed to address these violations could also be higher, and may 

ultimately lead to additional reductions with their associated costs and benefits. In terms 

of analysis costs, Option 1 would be expected to be the least costly option, whereas

Option 2 would be expected to be the most resource intensive. Option 3 is similar to 

Option 2, except that if a potential violation is indicated in an unmonitored area, there 

would not be a regulatory requirement for the air agency to identify enforceable controls 

to eliminate the potential violation. For example, the air agency could instead elect to site 

a new monitor to further characterize air quality in the area. The analysis costs associated 

with Option 3 would thus be similar to Option 2.

Option 4 most closely describes the current policy for the PM2.5 NAAQS 

implementation program. Currently, the EPA recommends that air agencies conduct an 

unmonitored area analysis, but there is no regulatory requirement for the air agency to 

either perform an unmonitored area analysis or to impose control requirements if the 
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analysis indicates potential violations. Thus, under Option 4, if an unmonitored area 

analysis is performed, the analysis costs associated with this option would be the same as 

for Options 2 and 3.Under Option 4, if it is determined by the EPA and the air agency to 

be unnecessary to perform an unmonitored area analysis, there would be no additional 

analysis costs beyond the monitor-only approach of Option 1. Regarding the costs and 

benefits of reductions resulting from additional efforts to address unmonitored locations

(i.e., to the extent that efforts necessary to address monitored locations do not also 

address unmonitored locations), the EPA does not have enough information to determine 

the extent of such areas or the measures that would be needed to address them, nor can 

the agency predict the extent to which such measures would be adopted under one option 

but not another.

The EPA requests comment on whether an unmonitored area analysis should be a 

required component of an attainment demonstration for a PM2.5 nonattainment area and,

if required, how the results of an unmonitored area analysis should be used. The EPA’s 

four proposed options reflect various combinations with respect to whether such an 

analysis is required and the purposes for which the state and the EPA might use the 

results of the analysis. The EPA requests comment on whether an unmonitored area 

analysis should be a required component of an attainment demonstration for a PM2.5

nonattainment area and, if required, how the results of an unmonitored area analysis 

should be used. The EPA also requests comment on the potential costs and benefits of 

each of the four specific options, and on which of the options the commenter believes 

should be included in the final rule, and why. 

8. What future year(s) should states model in attainment demonstrations?

Commented [A5]: Reviewer requested adding request for 

comment on costs and benefits of unmonitored area options.
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A state performing a modeling analysis for an attainment demonstration or 

impracticability analysis must select a future year for the analysis. For an attainment 

demonstration, a state should select the future modeling year such that all control 

measures relied on for attainment will have been fully implemented by the beginning of 

that year. To demonstrate attainment, the modeling results for the nonattainment area 

must predict that emissions controls implemented no later than the beginning of the last 

calendar year preceding the attainment date will result in PM2.5 concentrations that meet 

the level of the standard.125

While states should choose the future modeling year based on a number of 

factors, the EPA recommends the last possible year permitted under the statute as a 

starting point for modeling. There are several reasons for this. First, states with Moderate 

areas that submit an impracticability demonstration must show that the area cannot attain 

the NAAQS by the end of the sixth calendar year following designation of the area. 

Therefore, the appropriate future modeling year for such a demonstration is the sixth 

calendar year after designation. Even if a state does not submit (or does not intend to 

submit) an impracticability demonstration, modeling the sixth calendar year is a logical 

starting point to determine if attainment by that year is likely. Second, even though 

attainment is determined by averaging 3 years’ worth of ambient data, states do not have 

to model 2 years before the attainment date to show modeled attainment. Since the design 

 

125 Note that for purposes of the PM2.5 NAAQS, a determination of attainment (or failure 

to attain), which the EPA is required to make after the attainment date has passed, is 

based on an average of the most recent 3 years of ambient data prior to the area’s 

attainment date.
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value is an average of 3 years’ worth of data, attainment can still be shown even if 

concentrations exceed the NAAQS in one or more of the 3 years used to determine 

attainment (as long as the 3 year average is below the level of the NAAQS). Therefore, it

can be appropriate to model any of the 3 years used to determine attainment. Third, if 

ambient data show attainment level concentrations in the final statutory attainment year, a 

state may be eligible for up to two 1-year extensions of the attainment date, if the area

meets the criteria for such extensions under CAA section 188(d). Therefore, modeling 

attainment level concentrations for the last year permitted by statute is acceptable.

For all of the reasons stated above, it is both acceptable, and will in fact be most 

efficient, for a state to begin the attainment demonstration process by modeling the last 

year permitted under the statute to determine future year modeled PM2.5 concentrations in 

the sixth year after designations. Thus, in the attainment demonstrations for areas 

designated nonattainment in the first round of designations for the 2012 PM2.5 NAAQS,

it would be appropriate for states to model air quality for 2021.

Because an area must attain “as expeditiously as practicable” according to the 

CAA, additional considerations are necessary before an attainment date can be 

established for a Moderate PM2.5 nonattainment area. For purposes of determining the 

attainment date that is as expeditious as practicable, the state must conduct future year 

modeling which takes into account expected growth and known controls. For example, 

for a Moderate nonattainment area for the 2012 PM2.5 NAAQS, a future base case 

scenario for the year 2021 (6 years after designations) would project future air quality 

given implementation of existing federal, state and local measures. If this base case 

scenario demonstrates attainment, then the state must demonstrate whether attainment 
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could be achieved in an earlier year. Therefore, the state needs to conduct an analysis to 

determine if, collectively, all technologically and economically feasible measures 

identified by the state for which the state can initiate implementation by the beginning of 

the sixth calendar year following designations, can advance the attainment date by at least 

1 year. Results of this analysis may indicate attainment can be achieved earlier, through 

implementation of all reasonable control measures (i.e., RACM and RACT and additional 

reasonable measures).

If the future base case scenario does not demonstrate attainment, then a control 

case scenario is needed to examine whether the implementation of all technnologically 

and economically feasible measures identified by the state would result in attainment in 

2021 (for purposes of this example based on the 2012 PM2.5 NAAQS). The control case 

scenario would add to the model potential control measures (i.e., RACM and RACT and 

additional reasonable measures, plus any additional intrastate transport measures or other 

measures on sources outside of the nonattainment area that the state has identified as 

feasible to implement by the attainment date). This modeling, along with other relevant 

information, would inform a judgment as to whether attainment of the relevant NAAQS 

is practicable by the end of the sixth year after designation or earlier. In the case of areas 

designated nonattainment for the 2012 PM2.5 NAAQS in the first round of designations, if 

the analysis does not demonstrate attainment by December 31, 2021, then the analysis 

could serve as the technical basis for the state to submit a demonstration that attainment 

by the latest statutory attainment date for Moderate areas is impracticable. This 
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demonstration in turn could serve as the technical basis for the Administrator to reclassify 

the area to Serious.126

The EPA believes that it is not reasonable to require states to model each and 

every calendar year to determine the appropriate attainment date for a nonattainment 

area. Developing and modeling future year inventories is a time-consuming and resource 

intensive process. Multiple emissions models are needed in order to generate yearspecific emissions for the various emissions sectors (e.g. mobile, non-road, non-EGU 

point and EGU point). In some cases it may be reasonable to model one additional 

interim year before the maximum statutory attainment date.

127 However, in most cases, 

the air quality benefits of an identified set of RACM and RACT and additional 

reasonable measures can be estimated through model sensitivity analyses and the 

development of transfer factors (factors to relate tons of emissions reductions in the area 

to PM2.5 concentration changes in the area). For example, states can model across-theboard percentage reductions in direct PM2.5 and/or precursor emissions (in separate model 

runs) to determine the impact of emissions reductions on PM2.5 concentrations in the area. 

This modeling can be performed with a single attainment year modeling platform, which 

is much less resource intensive than modeling additional future years. The identified 

potential emissions reductions available from RACM and RACT and additional 

 

126 A demonstration that the area cannot practicably attain by the Moderate area 

attainment date would not be the only trigger for a discretionary reclassification to 

Serious. The Administrator maintains wide discretion in making such a determination, 

with an impracticability demonstration serving as one potential source of analysis to 

inform such a determination.

127 If several future modeling years are available, in some cases it may be appropriate for 

states to interpolate PM2.5 concentrations between years.
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reasonable measures can be compared to the magnitude of the modeled PM2.5 reductions 

from the sensitivity analyses to determine if all such controls will advance attainment by 

a year. The EPA strongly recommends that states discuss the selection of the future 

year(s) to model with their respective EPA Regional Office as part of the modeling 

protocol development process and before embarking on running the model(s).

9. Modeling analysis of controls that have a de minimis impact on ambient PM2.5

concentrations

In Section IV.D of this preamble, the EPA is proposing that if a state determines 

that a Moderate nonattainment area can attain the PM2.5 NAAQS by the statutory 

attainment date, the state must adopt and implement as reasonable control measures (i.e., 

as RACM and RACT and additional reasonable measures) only those technologically and 

economically feasible control measures that are necessary to ensure that the area will 

attain the NAAQS as expeditiously as practicable. In a Moderate PM2.5 nonattainment 

area that cannot practicably attain the relevant NAAQS by the statutory attainment date, 

the EPA similarly believes that it may not be reasonable in all cases to require that a state 

implement all technologically and economically feasible control measures. The EPA is 

thus proposing an option under which the state may evaluate the air quality impact of 

technologically and economically feasible control measures to determine if there is a 

subset of such measures that collectively will only achieve negligible reductions in 

ambient PM2.5 concentrations in the area. Similar to the EPA’s proposed approach, 

described earlier in this section, to determine if a set of technologically and economically 

feasible control measures can collectively advance the attainment date by a year for a 

Moderate nonattainment area for which a state can demonstrate attainment by the 
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statutory attainment date, the state would be required under this proposed option (for a 

Moderate area that cannot practicably attain the NAAQS by the statutory attainment date) 

to use an air quality model to determine the impact on ambient PM2.5 levels of the set of 

otherwise “reasonable” controls that it believes will not collectively reduce ambient 

PM2.5 concentrations in the area. For this analysis, the state would have to show that the 

collective set of controls will have little to no effect on reducing PM2.5 concentrations in 

the area. 

10. Attainment year motor vehicle emissions budgets

The transportation conformity rule requires that attainment plans establish motor 

vehicle emissions budgets for the area’s attainment year. Therefore, once an area’s 

attainment date has been established, the state would establish motor vehicle emissions 

budgets for direct PM2.5 and any relevant PM2.5 precursor for the attainment year.

128 A 

motor vehicle emissions budget for the purposes of a PM2.5 attainment plan is that portion 

of the total allowable emissions within the nonattainment area allocated to on-road 

sources as defined in the submitted attainment plan.

129 Such motor vehicle emissions 

budgets would be calculated using the latest planning assumptions and the latest 

 

128 For more information on PM2.5 precursor requirements, see section 93.102(b)(2)(iv) 

and (v) of the transportation conformity rule. See also the May 6, 2005, final 

transportation conformity rule that addressed requirements for PM2.5 precursors. (70 FR 

24280).

129 A state would also establish motor vehicle emissions budgets for an area’s attainment 

year. Those budgets would be the motor vehicle emissions that the SIP establishes as 

being necessary to attain the NAAQS.
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approved motor vehicle emissions model available at the time that the attainment plan is 

developed.130

F. RFP Requirements

1. Statutory requirements and existing guidance

“Reasonable further progress” (RFP) is a concept included in the CAA under part 

D, title I to assure that states make steady, incremental progress toward attaining air 

quality standards in the years prior to the attainment date for a nonattainment area, rather 

than merely deferring implementation of control measures and therefore emissions 

reductions until the date by which the standards are to be attained. As discussed 

elsewhere in this preamble, section 172 of the CAA addresses nonattainment plan 

provisions in general. Section 172(c)(2) requires attainment plans to provide for RFP, 

which is defined in section 171(l) as “such annual incremental reductions in emissions of 

the relevant air pollutant as are required by [part D of title I] or may reasonably be 

required by the Administrator for the purpose of ensuring attainment of the applicable 

national ambient air quality standard by the applicable date." Section 172(c)(3) requires 

the state plan to include “a comprehensive, accurate, current inventory of actual 

emissions from all sources of the relevant pollutant or pollutants in such area...” Section 

172(c)(1) requires the state plan to include “all reasonably available control measures as 

expeditiously as practicable (including such reductions in emissions from existing 

 

130 If an area includes re-entrained road dust in the motor vehicle emissions budget, the 

latest approved version of AP-42 should be used unless the EPA has approved an 

alternative model for the area.
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sources in the area as may be obtained through the adoption, at a minimum, of reasonably 

available control technology)...”

In general terms, the EPA interprets that the purpose of requiring RFP is to ensure 

that states with nonattainment areas develop attainment plans that achieve generally 

linear progress toward attainment, rather than deferring emissions reductions until the 

applicable attainment date for the area. In the context of implementing the PM2.5

NAAQS, “generally linear progress” means that emissions of direct PM2.5 and PM2.5

precursors from controlled sources generally decrease year by year such that the area 

ultimately attains the relevant NAAQS by the applicable attainment date. In the 

Addendum, the EPA provided guidance and identified four specific situations in which 

“linear progress” in emissions reductions to meet RFP may be appropriate: 

1. When pollutants are emitted by numerous and diverse sources.

2. Where the relationship between any individual source and the overall 

air quality is not explicitly quantified.

3. Where a chemical transformation is involved.

4. Where the emission reductions necessary to attain the standard are 

inventory-wide.131

For example, a state with an area whose nonattainment problem is caused 

primarily by area sources, such as residential wood combustion, should be able to 

demonstrate generally linear progress toward attainment in that area. In such an area, the 

 

131 59 FR 41998 at 42015.
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state might be able to require the replacement of a specified percentage of the residential 

woodstoves on an annual basis for each year to assure RFP on an annual basis. 

The EPA’s guidance in the Addendum also provided examples of situations in 

nonattainment areas in which it might be less appropriate to expect RFP to be linear, 

including:

1. Where there are a limited number of sources.

2. Where the relationships between individual sources and air quality are 

relatively well defined.

3. Where the emission control systems utilized (e.g., at major point 

sources) will result in swift and dramatic emission reductions.132

In nonattainment areas characterized by any of these circumstances, the EPA 

understands that RFP may be better represented as step-wise progress as controls are 

implemented and achieve significant reductions soon thereafter. For example, if an area’s 

nonattainment problem can be attributed to a few major stationary sources, the EPA’s 

guidance indicates that “RFP should be met by ‘adherence to an ambitious compliance 

schedule’ which is likely to periodically yield significant emission reductions.” 133, 134

While the EPA noted in the Addendum that adherence to such a schedule does not 

necessarily mean it would be unreasonable to achieve generally linear progress, the 

agency has long interpreted the language of section 171(1) not to require some specific 

 

132 Ibid.

133 USEPA, Office of Air Quality Planning and Standards, “Guidance Document for 

Correction of Part D SIP’s for Nonattainment Areas,” Research Triangle Park, NC, 

January 24, 1984, page 25.

134 59 FR 41998 at 42015.
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level of emissions reductions in any given year. Unlike certain provisions under subpart 2 

governing ozone NAAQS implementation, subpart 4 does not specify a set percentage of 

emissions reductions to be achieved over a certain period of time. Accordingly, the EPA 

believes that the facts and circumstances of each specific area will be relevant to whether 

the emissions reductions meet the agency’s expectations for “generally linear progress.”

With respect to implementation schedules, the EPA recommended in the 

Addendum that to meet the statutory RFP requirements, attainment plans must include 

“detailed schedules for compliance with emission regulations in the [nonattainment] areas 

and accurately indicate the corresponding annual emission reductions to be realized from 

each milestone in the schedule. In reviewing the SIP, the EPA will determine whether the 

annual incremental emission reductions to be achieved are reasonable in light of the 

statutory objective to ensure timely attainment of the PM10 NAAQS. Additionally, the 

EPA believes that it is appropriate to require early implementation of the most costeffective control measures... while phasing in the more expensive control measures.”135

The EPA believes that these prior interpretations of the Act’s provisions for RFP 

continue to be appropriate for the PM2.5 NAAQS. Accordingly, the following section 

describes the EPA’s proposal for requirements to ensure that states meet the statutory 

provisions for RFP for Moderate PM2.5 nonattainment areas.

2. General proposed approach to RFP

To satisfy the statutory requirements for RFP at section 172(c)(2), the EPA 

proposes that a state must submit an RFP plan as part of its Moderate area attainment 

 

135 Ibid. at 42016.
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plan submission. The RFP plan must contain appropriate information to demonstrate that 

adequate emissions reductions will be achieved through control measures in the 

attainment plan in order to meet the statutory definition of RFP. The plan must include an 

implementation schedule for control measures on sources in the nonattainment area and 

an analysis that demonstrates when -- and through what control measures -- emissions 

will decline from the applicable baseline year to the attainment year. As part of the 

analysis, the RFP plan must include a projected inventory for sources in the area for one 

(or more) interim year(s). The EPA is proposing and seeking comment on two options for 

developing an RFP plan, as well as on related requirements, as described below. See

proposed 40 CFR 51.1012. The EPA also notes that quantitative milestones required 

under section 189(c) are directly linked to the RFP plan, as interim quantifiable indicators

intended to demonstrate that an area is making progress toward attaining the PM2.5

NAAQS, and are therefore related to the implementation schedule of control measures 

for a PM2.5 nonattainment area. Quantitative milestones are more fully discussed in 

Section IV.G of this preamble.

a. Proposed option #1. Under the first option, the EPA proposes that the RFP 

analysis for any Moderate PM2.5 nonattainment area that can demonstrate attainment by 

the statutory attainment date must demonstrate either: (i) generally linear progress toward 

attainment by the applicable attainment date through emissions reductions to be achieved 

annually between a baseline year and the projected attainment date for the area; or, (ii)

step-wise progress toward attainment by the applicable attainment date that will be 

achieved through adherence to an ambitious compliance schedule that would not 

necessarily achieve reductions on an annual basis. In the second case, the state would be 
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required to submit a clear rationale and supporting information to explain why generally 

linear progress during the attainment period is not reasonable on an annual basis (e.g., 

due to the nature of the nonattainment problem and the types of sources contributing to 

PM2.5 levels in the area as discussed in Section IV.F.1 of this preamble). The EPA also 

proposes to require that RFP analyses need to show progress in achieving emissions 

reductions only for direct PM2.5 and any precursors that are controlled in the attainment 

plan for the nonattainment area.

Note that the two approaches presented in Option 1 for demonstrating RFP within 

the nonattainment area are consistent with the pattern of emissions reductions of many 

nationally-applicable federal emissions reduction measures. For example, new emission

standards for mobile sources may achieve reductions in a generally linear manner over 

time, as a portion of the existing vehicle fleet is replaced each year with new vehicles

meeting the more stringent standards. On the other hand, regulations to reduce emissions 

from certain stationary source sectors often have a single compliance date by which 

controls must be in place, which typically result in a significant drop in emissions over a 

relatively short period (i.e., yield step-wise reductions). 

Because the statute does not clearly establish the applicable baseline year from 

which to begin calculating annual emissions reductions for purposes of demonstrating 

RFP, the EPA is proposing to require and seeks comment on a requirement that states use 

the same year as the base year inventory chosen for the area, as this inventory will serve 

as the basis for developing the control strategy necessary to bring the area into 

expeditious attainment. Furthermore, in developing their RFP analyses for specific 

nonattainment areas, the EPA expects that states will use the emissions inventories 
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developed for those areas and air quality modeling they have completed for attainment 

planning purposes. This approach is consistent with the EPA’s proposed approach, 

described later in this section, not to interpret the CAA as allowing states to take credit 

for emissions reductions from sources outside a nonattainment area when developing 

their plan to meet the statutory RFP requirements for PM2.5 nonattainment areas.

For states with Moderate areas that cannot demonstrate attainment by the 

statutory Moderate area attainment date, the statutory RFP requirements still apply.

However, the EPA proposes to require that, for such areas, the state must provide an 

analysis of the anticipated emissions reductions associated with implementing the control 

measures identified as RACM and RACT and additional reasonable measures for the 

area. The EPA notes that even if a state adequately demonstrates that it cannot attain the 

NAAQS in a given area by the statutory attainment date, the CAA still requires the state 

to submit a Moderate area attainment plan meeting the requirements for such attainment 

plans, including for RFP. An additional RFP analysis will be required as part of the 

Serious attainment plan for the area once the EPA reclassifies it to Serious.

Similar to the approach taken for RFP in the remanded 2007 PM2.5

Implementation Rule, the EPA is proposing under this option that all states must follow 

one primary approach for conducting the RFP analysis, but that they also have an option 

to conduct a secondary analysis that will provide greater flexibility in setting RFP goals 

with alternative emissions reductions and air quality improvement scenarios. The primary 

approach would be to benchmark emissions reductions on a pollutant-by-pollutant basis 

starting from the pollutant’s baseline emissions level. The state would then be required to
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calculate reductions in emissions of each pollutant on an annual basis that would be 

needed to bring the area into attainment by the projected attainment date.

The EPA recognizes that different control measures address different pollutants, 

and that states may be able to implement some measures more quickly than others. Thus, 

in the optional secondary analysis, the state could present a different combination of 

emissions reductions at similar time intervals that would provide an equivalent or better

result in terms of net air quality improvement. This “equivalency determination” would 

allow states flexibility to address different pollutants (i.e., direct PM2.5 and PM2.5

precursors regulated under the control strategy for the area) according to different 

schedules so long as the EPA finds the projected net air quality improvements to be 

achieved through this alternative combination of emissions reductions to be equivalent to 

or better than those that would be achieved through generally linear emissions reductions 

across all pollutants in the area. This proposed approach recognizes that an important 

element of establishing appropriate emissions reductions targets for meeting RFP 

requirements for PM2.5 is quantifying the relative degrees of control of various pollutants.

As discussed above, the primary approach for ensuring that RFP is met in a PM2.5

nonattainment area is to require that the state reduce each pollutant – that is, direct PM2.5

and all precursors not otherwise eliminated from control requirements – by some amount 

on an annual basis. The EPA’s primary proposed RFP analysis, an emissions benchmark 

analysis, would reflect generally linear progress (or step-wise progress if more 

appropriate and adequately justified) to reduce those pollutants that the state intends to 

control to attain the PM2.5 NAAQS by the applicable attainment date. See proposed 40 

CFR 51.1012(b). For example, a state that can demonstrate that their Moderate 
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nonattainment area can attain the 2012 PM2.5 NAAQS by an attainment date of December 

31, 2021 would also need to achieve emissions levels that represent attainment in 2021. If 

the attainment plan requires a 10 percent reduction in NOx emissions and a 14 percent 

reduction in PM2.5 direct emissions from 2011 levels in order for the area to demonstrate 

attainment in 2021, then the RFP benchmark for NOx would reflect roughly a 1 percent 

reduction in NOx emissions per year, and the benchmark level for PM2.5 would be 

roughly a 1.4 percent reduction per year.

The EPA proposes that states must provide an implementation schedule for 

control measures that would achieve emissions reductions consistent with those 

calculated as part of the RFP benchmark analysis. However, a state could choose to 

submit an “equivalency” analysis in addition to the RFP benchmark analysis and 

associated implementation schedule that presents an alternative combination of pollutant 

emission reductions (i.e., alternative implementation schedule for control measures) that 

achieves air quality improvements that are equivalent to or better than the RFP 

benchmark analysis. In such a case, the state would need to make an adequate showing 

that the alternative schedule for implementing control measures will provide estimated air 

quality improvements that are roughly the same as, if not better than, those that the 

emissions reductions determined through the RFP benchmark analysis would provide. If 

a state elects to follow this approach, it must provide in its RFP plan the information 

necessary to assess whether an alternative schedule of emissions reductions is generally 

equivalent, in air quality terms, to the RFP benchmark analysis reduction levels, such as 

attainment demonstration modeling results that link emissions reductions of various 

precursor emissions with air quality improvements. Under this proposed approach, the 
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EPA would require states to use this information to evaluate the equivalence of 

alternative combinations of pollutant emissions reductions. The EPA would recommend 

that states estimate air quality improvements associated with intermediate emissions

control levels (i.e., air quality improvement targets) by assuming that the same 

relationship between emissions and air quality applies at intermediate levels as would 

apply at attainment levels. 

The EPA continues to recognize that because atmospheric processes are quite 

complex, a specific percent change in emissions of PM2.5 precursors does not lead to an 

equivalent percent change in air quality, potentially creating uncertainty as to whether 

alternate emissions control scenarios will achieve equivalent benefits. Nevertheless, the 

EPA believes that it is important to provide the flexibility to address different pollutants 

on different timetables so long as the plan can reasonably be expected to achieve the 

intended air quality benefits represented by the RFP benchmark analysis. In general, the 

EPA would not expect a state to conduct dispersion modeling specifically to assess 

whether an alternative approach to meeting RFP will provide equivalent air quality 

benefits as the benchmark approach. Instead, the attainment plan modeling addresses the 

nonlinearities at attainment levels, and the EPA believes for RFP analysis purposes that 

the relationship between emissions and air quality at attainment levels provides an 

adequate approximation of the relationship at interim RFP levels.

b. Proposed option #2. Under the second option, the EPA proposes a simplified 

approach to developing an RFP plan that focuses on the emissions reductions anticipated 

from each of the particular control measures identified by the state as part of the analysis 

to identify RACM and RACT and additional reasonable measures for sources in the 
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nonattainment area. Under this option, the first step in developing the RFP plan would be 

for the state to establish the implementation schedule on a year-by-year basis for all 

control measures contained in the control strategy for sources in the area beginning with 

the date of designation of the area and ending with the projected attainment date of the 

area. The schedule would need to comply with the statutory requirement that all RACM 

and RACT must be implemented within the first 4 years following designation, but the 

state would have discretion beyond that requirement to schedule the implementation of 

any other measures necessary for expeditious attainment. Overall, the implementation 

schedule would need to demonstrate that control measures to bring the area into 

attainment will be implemented as expeditiously as practicable.

The second step in developing an RFP plan under this second proposed option 

would be for the state to calculate the emissions reductions that would be achieved by all 

measures implemented on sources in the area corresponding with quantitative milestone 

dates (i.e., by 4.5 years and 7.5 years after designation of the area). These are the dates by 

which milestones for the area must be met, after which a report is due to the EPA from 

the state to verify that the area has met the milestones identified for the area and thereby 

has also met the RFP requirements for the area. The EPA proposes that the state must 

calculate the emissions reductions to be achieved at each milestone year on a pollutantby-pollutant basis.

The third step under this proposed option would be for the state to conduct 

modeling or employ another quantitative method to predict the overall PM2.5

concentrations in the nonattainment area in each milestone year. This air quality target 

could simply be interpolated between the design value at the time of the area’s
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designation and the design value in the projected attainment year. These air quality target 

values would serve as a points of comparison for the monitored ambient air data that the 

EPA is proposing that the state must submit as part of the milestone report due after the 

area reaches each milestone date.

This simplified approach to determining RFP for a Moderate nonattainment area 

could apply equally well to areas that can demonstrate attainment with the relevant 

NAAQS by the statutory attainment date and those that cannot. See proposed 40 CFR 

51.1012(c). In addition, the EPA believes it offers a reasonable approach to ensure that 

RFP is generally being met in the area without requiring extensive quantitative analysis

so long as it is generally linear for purposes of achieving annual emissions reductions. 

The EPA seeks comment on these two options proposed for states to meet the statutory 

RFP requirements.

3. RFP inventories for RFP analyses

The EPA proposes that a state with a Moderate PM2.5 nonattainment area must 

submit one or more emissions projections as part of the RFP plan (the “RFP inventory”) 

for the area that, at a minimum, includes projected emissions by different source types 

corresponding to the quantitative milestone date(s) for the area, described in greater detail 

in Section IV.H of this preamble. Specifically, the EPA proposes that the RFP plan for 

any Moderate area must contain a projected RFP inventory for each calendar year in 

which quantitative milestones for a Moderate nonattainment area must be met. For 

example, as explained in Section IV.H of this preamble, a state must identify as part of 

the attainment plan submission for a Moderate nonattainment area quantitative milestones 

to be achieved every 3 years from the Moderate area attainment plan due date, or 4.5 
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years from the effective date of designation of the area.136 For example, the first round of 

designations for the 2012 PM2.5 NAAQS become effective in April 2015; Moderate area 

attainment plans for these areas will thus be due 18 months later, or in October 2016. The 

first quantitative milestones for each of these areas would then have to be met in October 

2019; the second quantitative milestones, in October 2022; and so on, until the area 

attains the NAAQS. Under the EPA’s proposed approach for projected emissions 

inventories for RFP analyses, the state would be required to submit such inventories as 

part of the Moderate area attainment plan due in October 2016 that project emissions 

from sources in the nonattainment area for the same calendar years as those for which 

quantitative milestones would be due.

The transportation conformity rule requires that attainment plans establish motor 

vehicle emissions budgets. RFP plans submitted as part of an attainment plan submission 

would therefore be required to establish motor vehicle emissions budgets for direct PM2.5

and any relevant PM2.5 precursor.137 A motor vehicle emissions budget for the purposes 

of a PM2.5 RFP plan is that portion of the total allowable emissions allocated to on-road 

sources as defined in the submitted RFP plan for the relevant years as described above.138

 

136 According to section 189(a)(2)(B), Moderate area attainment plans are due to the EPA 

18 months after designation.

137 For more information on PM2.5 precursor requirements, see section 93.102(b)(2)(iv) 

and (v) of the transportation conformity rule. See also the May 6, 2005, final 

transportation conformity rule that addressed requirements for PM2.5 precursors. (70 FR 

24280).

138 A state would also establish motor vehicle emissions budgets for an area’s attainment 

year. Those budgets would be the motor vehicle emissions that the SIP establishes as 

being necessary to attain the NAAQS.
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Such motor vehicle emissions budgets would be calculated using the latest planning 

assumptions and the latest approved motor vehicle emissions model available at the time 

that the attainment plan is developed.139

4. Geographic coverage of emission sources for RFP

The EPA is proposing that the RFP demonstration to be included with a state’s 

PM2.5 nonattainment area plan must include emissions only for sources located in the 

nonattainment area, and not from an area larger than the nonattainment area. This policy 

approach differs from the remanded 2007 PM2.5 implementation rule. This section 

describes the evolution of policy on a similar RFP issue in the ozone NAAQS 

implementation program, and it discusses the reasoning behind this revised approach for 

PM2.5.

In the preamble to the remanded 2007 PM2.5 Implementation Rule, the EPA 

allowed states to incorporate reductions of NOx and SO2 emissions up to 200 km from 

outside the nonattainment area (and potentially for reductions of VOC or ammonia) into 

their RFP plan when certain conditions were met. This policy was included in the 2007 

PM2.5 Implementation Rule in part to be consistent with a similar RFP policy for NOx and 

VOC that was included in the November 2005 Phase 2 ozone NAAQS implementation 

rule which provided guidance for states on implementing the 1997 ozone NAAQS.140

 

139 If an area includes re-entrained road dust in the motor vehicle emissions budget, the 

latest approved version of AP-42 should be used unless the EPA has approved an 

alternative model for the area.

140 See Phase 2 Ozone Implementation rule, 70 FR 71612, November 29, 2005.
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Under the policy in the 2007 PM2.5 NAAQS implementation rule, if a state 

intended to include emissions reductions from outside the nonattainment area in the RFP 

plan, the state would need to take on the additional work associated with developing: (i) 

an expanded baseline emissions inventory for the entire geographic area (i.e., the 

nonattainment area plus the additional area outside the nonattainment area) that 

characterizes emissions for all stationary, area and mobile sources (rather than for just a 

select few stationary sources) in the overall area; and, (ii) a projected attainment year 

inventory for this expanded area outside the boundaries of the designated nonattainment 

area. By requiring inclusion of all types of sources in these “expanded area” emissions

inventories, the EPA intended for this approach to reflect the projected net emissions 

reductions in this area (the difference between the “expanded area” base year inventory 

and the projected attainment year inventory). However, it should be noted that for a state 

to develop these more extensive inventories, it would likely have involved a substantial 

amount of additional time and resources. In addition, the state would have needed to have 

provided information supporting its decision regarding how far outside the nonattainment 

area the RFP inventory should extend. While this “outside the nonattainment area” RFP 

approach was theoretically available to states in developing their PM2.5 attainment plans 

due in 2008, there were no states to the agency’s knowledge that elected to follow this 

approach. 

Both the 2005 Phase 2 ozone implementation rule and the 2007 PM2.5

Implementation Rule were challenged on several issues. With regard to the Phase 2 ozone 

implementation rule, the EPA granted a petition for reconsideration and ultimately issued 

a final notice of reconsideration in June 2007. In November 2008, the U.S. Court of 
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Appeals for the D.C. Circuit heard oral argument concerning multiple petitions for 

judicial review of the Phase 2 ozone rule and the notice of reconsideration. One of the 

issues in this case involved whether compliance by EGUs with a regional emissions 

trading program could be considered to meet the RACT requirement for those sources 

located in a nonattainment area. In its July 2009 decision, the court emphasized that: ‘‘the 

RACT requirement calls for reductions in emissions from sources in the area; reductions 

from sources outside the nonattainment area do not satisfy the requirement . . . 

Accordingly, participation in the NOx SIP call would constitute RACT only if 

participation entailed at least RACT-level reductions in emissions from sources within 

the nonattainment area.”

In light of this court decision, the EPA has determined that the best reading of the 

statute would be to interpret the term “sources in the area” in the same manner where it 

appears in different nonattainment provisions for ozone. The term appears in CAA 

section 182 (requirements for ozone nonattainment areas) with regard to RFP as well as 

RACT. The decision on the Phase 2 ozone rule found that section 182(b)(2) requires that 

a SIP must provide for implementation of RACT (under section 172(c)) for emissions 

sources “in the area,” meaning in the nonattainment area. Similarly, the EPA believes that 

when section 182(b)(1)(A)-(B) defines baseline emissions for RFP as “the total amount 

of actual VOC or NOx emissions from all anthropogenic sources in the area,” this also 

means sources in the nonattainment area.

With regard to the 2007 PM2.5 Implementation Rule, the EPA received a petition 

for reconsideration in June 2007 that raised objections on several issues. One such issue 

dealt with the EPA’s interpretation of the statutory RFP requirements to allow a state to 
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take “credit” for emissions reductions from outside the nonattainment area when 

addressing RFP in its attainment plan.141 The EPA granted the petition for 

reconsideration on this issue in 2010, after the D.C. Circuit issued its decision on the 

Phase 2 Ozone Implementation Rule.142, 143

Specifically, the EPA believes that the D.C. Circuit’s interpretation of the phrase 

“sources in the area” applies to RACT and RFP requirements for both the ozone NAAQS 

and the PM2.5 NAAQS. In particular, for PM2.5, the statutory language at section 171(1) 

defines RFP in terms of “reductions in emissions” required in an attainment plan, which 

the EPA interprets as being directly linked to the baseline emissions inventory for sources 

located in a PM2.5 nonattainment area. The baseline emissions inventory is the foundation 

for the attainment plan. The emissions inventory requirement of section 172(c)(3) 

explicitly requires that the attainment plan inventory include all sources of the relevant 

pollutants “in such area,” which is a clear reference to the designated nonattainment area. 

Given that the baseline inventory must reflect the emissions “in such area,” and that this 

inventory provides the starting point for a state’s RFP analysis, in which the state must 

calculate generally linear progress in emissions reductions that will lead to attainment of 

the NAAQS in the area, the EPA believes it is appropriate that a state should focus on 

 

141This same petition raised concerns regarding the criteria used to determine the 

economic feasibility of controls being considered for RACT for the 1997 PM2.5 NAAQS. 

See “Petition for Reconsideration,” filed by Paul Cort, Earthjustice, on behalf of the 

American Lung Association, Medical Advocates for Healthy Air, Natural Resources 

Defense Council, and the Sierra Club (June 25, 2007). A copy of the petition is in the 

docket for this action.

142Letter dated May 13, 2010, from Gina McCarthy to David S. Baron and Paul Cort, 

Earthjustice. A copy of the letter is located in the docket for this action.

143 See NRDC v. EPA, 571 F.3d 1245 (D.C. Cir. 2009).
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sources located within the nonattainment area when conducting its analysis to determine 

the annual emissions reductions necessary for demonstrating RFP.

The EPA believes that the most appropriate approach with regard to the 

geographic area required to be covered for demonstrating RFP in a PM2.5 attainment plan 

also should be limited to the nonattainment area for two other reasons. First, EPA 

believes that it makes policy sense for the PM2.5 implementation rule approach to be 

consistent with the approach in the ozone implementation rule. In the past, a number of 

areas have been designated as nonattainment for both standards, and the nonattainment 

area boundaries often are the same. For such areas, a common policy approach for the 

geographic area covered by the RFP plan will be more efficient to implement and would 

be expected to be less burdensome for the air agency than if the geographic areas covered 

by RFP plans for the two pollutants differed. 

Second, a policy allowing the geographic area of the RFP plan to be larger than 

the nonattainment area would conflict with a key provision of subpart 4 which requires 

annual incremental reductions in emissions from sources within the nonattainment area. 

Under subpart 4, an area that fails to attain the standard by the Serious area attainment 

date is then subject to the provisions of section 189(d). Section 189(d) specifies that the 

state must submit a plan revision within 12 months which provides for “an annual 

reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent 

of the amount of such emissions as reported in the most recent inventory prepared for 

such area” (emphasis added). The EPA does not believe the rule should include an RFP 

policy approach which would not be consistent with section 189(d).
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After reconsideration of the approach to RFP that was opposed in the petition for 

reconsideration of the 2007 PM2.5 Implementation Rule, and in light of the D.C. Circuit 

decision on the Ozone Phase 2 Implementation Rule, the EPA believes the best reading 

of the statute is that the CAA does not allow for a state to include emissions reductions 

from sources outside a nonattainment area when developing the plan to meet the CAA 

section 172(c)(2) RFP requirements for a PM2.5 nonattainment area. The EPA seeks 

comment on this proposed approach. 

5. Other RFP considerations

In general, the EPA seeks to ensure that PM2.5 nonattainment areas that are shared 

by more than one state or tribe meet RFP requirements as a whole. States and tribes that 

share a nonattainment area should therefore consult with one another to develop the RFP 

analysis and control strategy implementation schedule for the area as a whole. Such states 

and tribes should work with the EPA region or regions that oversee them to confirm that 

their collective approach is appropriate for RFP.

The EPA’s proposed approach for states to meet the RFP requirement is designed 

to ensure emissions reductions will yield incremental improvements in air quality on the 

path to attainment, while being sufficiently flexible to accommodate the range of control 

strategies necessary to address the complex mixtures of pollutants comprising PM2.5 in 

different areas. The EPA seeks comment on all of its proposed requirements and options 

for RFP plans and analyses for Moderate PM2.5 attainment plans.

G. Quantitative Milestones

1. Statutory requirements and existing guidance
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Section 189(c)(1) requires that a PM10 NAAQS attainment plan submission has 

“quantitative milestones which are to be achieved every 3 years until the area is 

redesignated to attainment and which demonstrate reasonable further progress...toward 

attainment by the applicable date.” Section 189(c)(2) further requires that, within 90 days 

of each milestone, each affected state must submit a demonstration that all measures to 

assure RFP have been implemented and that the quantitative milestone has been met. 

Thus, the CAA imposes requirements upon states not only to make “reasonable further 

progress” toward attainment, but also to identify objective means (i.e., quantitative 

milestones) by which to measure this reasonable further progress every 3 years, and to 

submit them as part of the attainment plan for the nonattainment area. In addition, 

according to section 189(c)(2), states must, within 90 days of the passage of each such 

milestone, submit to the EPA a demonstration that control measures have been 

implemented according to the approved RFP plan schedule and the milestone has been 

met.

The EPA has previously described its interpretation of the requirements under 

section 189(c) for the PM10 NAAQS in the General Preamble and the Addendum and 

believes that these interpretations should also apply both in developing plans that 

demonstrate RFP and include appropriate quantitative milestones, and in demonstrating 

that those milestones have been met for the PM2.5 NAAQS.

144, 145 The EPA’s guidance in 

the Addendum also noted that: “Section 189(c) provides that the quantitative milestones 

 

144 57 FR 13498 at 13539.

145 59 FR 41998 at 42015-17.
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submitted by a State for an area also must be consistent with RFP for the area. Thus, EPA 

will determine an area’s compliance with RFP in conjunction with determining its 

compliance with the quantitative milestone requirement. Because RFP is an annual 

emission reduction requirement and the quantitative milestones are to be achieved every 

3 years, when a state demonstrates an area’s compliance with the quantitative milestone 

requirement, it should also demonstrate that RFP has been achieved during each of the 

relevant 3 years.”146

The EPA’s existing guidance in the Addendum with respect to the quantitative 

milestone requirements of CAA section 189(c) thus includes several important features: 

(i) that the control measures comprising the RFP plan should be implemented and in 

place to meet the statutory quantitative emissions reductions milestone requirement; (ii) 

that it is reasonable for the 3-year periods for quantitative milestones to run from the 

statutory due date for the Moderate area attainment plan submission; and, (iii) that the 

precise form that the quantitative milestones should take is not specified, but the state 

must choose milestones that will allow it to quantify or measure, track and report 

progress adequately and objectively. 

The EPA’s proposed approach to identifying quantitative milestones for any 

Moderate PM2.5 nonattainment area and demonstrating compliance with the milestones is 

generally consistent with the existing guidance, as described in the following sections.

2. Proposed approach

 

146 Ibid.
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The statute at section 189(c) is clear that quantitative milestones must be achieved 

every 3 years, however it does not make clear the starting date for counting the 3 year

periods. In the General Preamble, the agency proposed that quantitative milestones must 

be achieved every 3 years starting from the attainment plan submission due date (i.e., 

because the Moderate area attainment plan is due no later than 18 months after 

designation of the area, the first set of milestones would need to be achieved 4.5 years 

after the area’s designation) until the attainment date.147 The EPA proposes to maintain 

this approach for the PM2.5 NAAQS. Specifically, the EPA proposes that the attainment 

plan for a Moderate area that can demonstrate attainment by the statutory Moderate area 

attainment date must identify appropriate quantitative milestones to be achieved by 4.5 

years following designation of the area. For a Moderate area that cannot practicably attain 

the relevant PM2.5 NAAQS within the statutory timeframe for a Moderate area, the EPA 

proposes that a state must submit two sets of quantitative milestones – one set to be 

achieved at year 4.5 from designation and the second set to be achieved at year 7.5 from 

designation. The EPA believes that this proposed requirement will help to ensure that the 

state maintains progress toward bringing the area into attainment during the period in 

which such area is reclassified to Serious, the state works to develop a Serious area 

attainment plan for the area, and the EPA approves it. Pursuant to the statute, the EPA 

must reclassify a Moderate area for which a state submits an attainment impracticability 

demonstration within 18 months after the Moderate area attainment plan due date, or no 

later than 3 years after the date of designation of the area. Even under a scenario in which

 

147 57 FR 13498 at 13539.
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the state develops and submits a Serious area attainment plan 18 months after being 

reclassified to Serious, the milestone date of 4.5 years after designation would likely 

come and go before the area had a new set of approved quantitative milestones with 

which to demonstrate compliance. Similarly, the milestone date of 7.5 years after 

designation could also come and go before the EPA is able to fully approve the Serious 

area plan and any quantitative milestones contained therein. Because of the timing of the 

various steps involved in reclassifying a Moderate area to Serious and a state developing 

a new Serious area plan, the EPA believes that requiring a state to identify quantitative 

milestones that the area must achieve 4.5 years and 7.5 years after designation as 

elements of its Moderate area attainment plan is reasonable and seeks comment on this 

proposed requirement.

The EPA is also proposing that the quantitative milestones contained in the 

attainment plan for a Moderate nonattainment area must be constructed such that they can 

be tracked, quantified and/or measured adequately in order for the state to meet its 

milestone reporting obligations, which come due 90 days after a given milestone date. In 

the Addendum, the EPA suggested some possible metrics that “support and demonstrate 

how the overall quantitative milestones identified for an area may be met,” such as 

percent implementation of control strategies, percent compliance with implemented 

control measures, and adherence to a compliance schedule. This list was not exclusive or 

exhaustive but reflected the EPA’s view that the purpose of the quantitative milestone 

requirement is to provide an objective way to assess that the state is making the necessary 
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progress towards attainment in the area by the applicable attainment date.148 The EPA 

continues to believe that the quantitative milestone requirement should be interpreted to 

allow states to devise milestones that are suitable for the specific facts and circumstances 

of the attainment plan for a particular area, so long as they provide an objective means to 

measure RFP.

The EPA therefore proposes to require that states select the quantitative 

milestones that are appropriate and quantifiable and that will provide for objective 

evaluation of progress toward attainment in their Moderate PM2.5 nonattainment area, 

whether the area can practicably attain the PM2.5 NAAQS by the statutory attainment date 

or not. For this approach, the EPA is not proposing to require that such quantitative 

milestones must take any particular form, merely that they provide a means to evaluate 

progress (i.e., demonstrate RFP) meaningfully. The EPA, in its attainment plan approval 

process, will determine if the specific quantitative milestones developed by the state for a 

specific nonattainment area satisfy the statutory requirements. The EPA recommends that 

states confer with their respective EPA regional office to develop appropriate quantitative 

milestones. See proposed 40 CFR 51.1013(a)(1).

In addition to this general proposed approach for selecting quantitative milestones 

for a Moderate nonattainment area, the EPA is proposing and seeks comment on a 

requirement that, at a minimum, states must include in all attainment plans for Moderate 

PM2.5 nonattainment areas a metric to confirm that all control measures identified and 

adopted as RACM and RACT for the area have been fully implemented within 4 years of 

 

148 59 FR 41998 at 42016.
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designation. This metric specifically derives from the statutory provision that applies to 

all Moderate areas and thus represents a milestone that all Moderate areas must meet 

regardless of whether it is listed explicitly as an individual milestone. The EPA believes 

it would be appropriate to include it as a metric that any state with a Moderate 

nonattainment area would need to demonstrate compliance with when they submit their 

milestone report as described below, and thus seeks comment on this proposal.

3. Milestone report submittal

Under the quantitative milestone requirement of section 189(c)(2), a state must 

demonstrate to the EPA that the RFP plan for the area and its approved milestones are 

being met within 90 days after the milestone due date. The EPA then has 90 days to 

determine whether or not a state’s demonstration is adequate. Specifically, section 

189(c)(2) requires that: “Not later than 90 days after the date on which a milestone 

applicable to the area occurs, each State in which all or part of such [nonattainment] area 

is located shall submit to the Administrator a demonstration that all measures in the plan 

approved under this section have been implemented and that the milestone has been met. 

A demonstration under this subsection shall be submitted in such form and manner, and 

shall contain such information and analysis, as the Administrator shall require.”

In the event a state fails to submit a milestone demonstration report by the due 

date or the EPA determines that a milestone was not met, the state must submit a SIP 

revision within 9 months of either the missed reporting deadline or the EPA’s 

determination of the state’s failure to meet a milestone. According to the statutory 

requirements of section 189(c)(3), the new SIP revision must assure “that the State will 

achieve the next milestone (or attain the national ambient air quality standard ..., if there 
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is no next milestone) by the applicable date.” If a state fails to make a SIP submission to 

correct a failure to meet RFP expeditiously, sanctions under sections 110(m) and 179(b) 

may apply. If a state is unable to correct a failure to meet RFP, this may be evidence that 

the state cannot practicably attain the NAAQS by the applicable attainment date and may 

serve as a basis for reclassification of the area to Serious under the agency’s discretionary 

authority. See proposed 40 CFR 51.1013(c).

Because the statute does not define the parameters of these demonstrations, the 

statute grants the EPA discretion to determine the components of the required 

demonstration and the form and manner for submission. In the Addendum, the EPA 

offered guidance about what the milestone report should contain: “This report must 

contain technical support sufficient to document completion statistics for appropriate 

milestones. For example, the demonstration should graphically display RFP over the 

course of the relevant 3 years and indicate how the emission reductions achieved to date 

compare to those required or scheduled to meet RFP and the required [quantitative] 

milestones. The calculations (and any assumptions made) necessary to determine the 

emission reductions to date should also be submitted. The demonstration should also 

contain an evaluation of whether the PM10 NAAQS will be attained by the projected 

attainment date.”149 The EPA believes this guidance is still appropriate for states 

demonstrating compliance with RFP and quantitative milestones for PM2.5 NAAQS and 

hereby proposes under the authority of section 301(a) to require that the milestone report 

submission must include the following four components:

 

149 Ibid. at 42017.
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First, the report must include a certification by the Governor or Governor’s 

designee that the state’s attainment plan control strategy, including the RFP plan, is being 

implemented as described in the applicable attainment plan. Second, as described in the 

Addendum, the report must contain technical support, including calculations, sufficient to 

document completion statistics for appropriate milestones and to demonstrate that the 

quantitative milestones have been satisfied and how the emissions reductions achieved to 

date compare to those required or scheduled to meet RFP. Third, the state must submit an 

air quality screening analysis to determine if measured air quality progress is consistent 

with the expected air quality improvement target correlated with the RFP emissions 

reductions for the previous 3-year period. Fourth, the report must contain an evaluation of 

whether the PM2.5 NAAQS will be attained by the projected attainment date for the area.

In addition, the EPA proposes that the milestone report must include a description and 

schedule for any remedial actions the state has taken or will take to address any failure to 

meet a quantitative milestone, including the implementation status of contingency 

measures for failing to meet RFP in the area. See proposed 40 CFR 51.1013(b). The EPA 

seeks comment on these proposed components to a milestone report.

The EPA stated in the Addendum that the milestone report must be submitted 

from the Governor or Governor’s designee to the Regional Administrator of the 

respective EPA Regional Office serving the submitting state, and that the EPA will notify 

the state of its determination (regarding whether or not the state’s report is adequate) by 

sending a letter to the appropriate Governor or Governor’s designee. The EPA believes 

that it would be appropriate for states to submit milestone reports, including supporting 

documents, through the agency’s electronic SIP (eSIP) submission system in order to 
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simplify the process and reduce resource burden on all sides. The EPA seeks comment on 

how electronic reporting could facilitate a state’s submittal of the required milestone 

report, how it could accommodate the various narrative and data-dependent components 

that the EPA is proposing be part of such a submittal, and what particular system features 

might be desirable to accommodate milestone report submissions through the eSIP 

system. 

H. Contingency Measures

States with PM2.5 nonattainment areas must include contingency measures in their 

attainment plans consistent with section 172(c)(9). Contingency measures are additional 

control measures to be implemented in the event that an area fails to meet RFP 

requirements or fails to attain the PM2.5 standard by the applicable attainment date. These 

measures must be fully adopted rules or control measures that are ready to be 

implemented quickly upon failure to meet RFP or failure of the area to meet the standard 

by its attainment date, and such measures are required to take effect without further 

action by the state or the EPA. The EPA provided extensive guidance on contingency 

measures in the General Preamble and Addendum, including the following: “States must

show that their contingency measures can be implemented with minimal further action on 

their part and with no additional rulemaking actions such as public hearings or legislative 

review. After the EPA determines that a moderate PM10 nonattainment area has failed to 

attain the PM10 NAAQS, the EPA generally expects all actions needed to effect full 

implementation of the measures to occur within 60 days after the EPA notifies the state 
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of the area’s failure. The state should ensure that the measures are fully implemented as 

expeditiously as practicable after they take effect.”150

The EPA does not believe that the D.C. Circuit’s decision in NRDC v. EPA

affects the overall contingency measure requirements that were finalized in the remanded 

2007 PM2.5 Implementation Rule, because section 172(c)(9) imposes the contingency 

measure requirement for attainment plans for the PM2.5 NAAQS and it is not superseded 

or subsumed by any specific contingency measure requirements under subpart 4. 

Although section 172(c)(9) requires contingency measures, the provision does not specify 

exactly what parameters such measures must meet. The EPA has longstanding 

interpretations of the statute with respect to the contingency measure requirement, both 

for PM and for other pollutants, in the General Preamble and Addendum. The EPA 

proposes to adopt an approach to contingency measures for the PM2.5 NAAQS similar to 

that recommended in earlier EPA guidance, but seeks comment on particular proposed 

approaches that differ in important ways from earlier guidance on contingency measures 

for the PM2.5 NAAQS. The EPA believes that it may be necessary to adopt a different 

approach to contingency measures for PM2.5 attainment plans due to proposed changes in 

determining RFP for a PM2.5 nonattainment area and in order to accommodate Moderate 

PM2.5 nonattainment areas that cannot practicably attain the standard by the statutory 

Moderate area attainment date. 

 

150 Ibid. at 42015.
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The EPA is proposing and seeking comment on the following general 

requirements for contingency measures to be approvable as part of a state’s Moderate 

area attainment plan submission for the PM2.5 NAAQS:

1. Contingency measures must be fully adopted rules or control measures that are 

ready to be implemented quickly upon a determination by the Administrator of 

the nonattainment area’s failure to meet RFP or failure to meet the standard by its 

attainment date. 

2. The state’s attainment plan submission must contain trigger mechanisms for the 

contingency measures, specify a schedule for implementation, and indicate that 

the measures will be implemented with minimal further action by the state or by 

the EPA.

3. Contingency measures must consist of control measures that are not otherwise 

included in the control strategy for the attainment plan.

4. Contingency measures must provide for emissions reductions approximately 

equivalent to 1 year’s worth of reductions needed for RFP, based on the overall 

level of reductions needed to demonstrate attainment divided by the number of 

years from the base year to the attainment year, or approximately equivalent to 1 

year’s worth of air quality improvement or emissions reductions proportional to 

the overall amount of air quality improvement or emissions reductions to be 

achieved by the area’s attainment plan. See proposed 40 CFR 51.1014.

The EPA interprets the contingency measure requirement of section 172(c)(9) to 

require control measures that are not already included in the attainment plan for other 

purposes, such as to meet RACM and RACT requirements. However, suitable 
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contingency measures may be measures that were technologically and economically 

feasible for the area, but did not qualify as RACM or RACT or additional reasonable 

measures for one or more reasons. For example, a candidate contingency measure may 

have been deemed technologically and economically feasible, but it was not needed to 

achieve expeditious attainment in a Moderate area for which the state could demonstrate 

attainment by the statutory attainment date and therefore was not included as part of the 

attainment demonstration for the area. The agency believes it is important that states 

make decisions concerning contingency measures in conjunction with their determination 

of the overall control strategy for bringing the area into expeditious attainment, and that 

states first must identify those control measures needed in order to demonstrate 

expeditious attainment of the standards; any remaining measures should then be 

considered as candidates for contingency measures. 

For Moderate areas that cannot practicably attain the NAAQS by the statutory 

attainment date, the EPA is proposing that states must implement all control measures 

that they determine to be reasonable for sources in the area. In such cases, the EPA 

expects that contingency measures for such nonattainment areas would necessarily 

exceed the criteria for determining whether a measure is reasonable (i.e., technologically 

and economically feasible) as described in Section IV.D of this preamble. Such 

contingency measures would only be triggered in the event the area fails to meet RFP; the 

EPA does not interpret the requirement for contingency measures for failing to attain the 

NAAQS by the applicable attainment date to apply to a Moderate area that a state 

demonstrates cannot practicably attain the NAAQS by the statutory attainment date. 

Rather, the EPA believes it is appropriate for the state to identify and adopt contingency 
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measures for failing to attain the NAAQS in a timely way as part of the Serious area 

attainment plan that it will develop once the EPA reclassifies such an area.

The EPA proposes that for any Moderate PM2.5 nonattainment area, contingency 

measures can include measures that achieve emissions reductions on sources located 

outside the nonattainment area as well as from sources within the nonattainment area, 

provided that the measures are factually demonstrated to produce the appropriate air 

quality impact within the nonattainment area. The EPA continues to believe it appropriate 

that a state might choose to rely on federal measures (e.g. federal mobile source measures 

based on the incremental turnover of the motor vehicle fleet each year) and local 

measures already scheduled for implementation for purposes other than meeting 

attainment plan requirements, such as RACM and RACT, as meeting part or all of the 

contingency measure requirements, as the purpose of the contingency measures is to 

provide a cushion while the attainment plan for the area is being revised to meet the 

missed attainment milestone. The EPA has approved numerous attainment plans under an 

interpretation that one or more federal or local measures that are in place and provide 

reductions in the year following a failure to attain the relevant NAAQS or meet RFP in 

excess of the reductions required by the attainment demonstration or RFP plan can meet 

the contingency measure requirements.151, 152

 

151 See, e.g., 62 FR 15844 (April 3, 1997); 62 FR 66279 (December 18, 1997); 66 FR 

30811 (June 8, 2001); 66 FR 586 and 66 FR 634 (January 3, 2001).

152 A court ruling upheld contingency measures for ozone attainment plans that were 

previously required and implemented where they were in excess of the attainment 

demonstration and RFP SIP. See LEAN v. EPA, 382 F.3d 575, 5th Circuit, 2004.
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The EPA recognizes that some states have historically relied on emissions 

reductions achieved through the implementation of control measures in excess of what 

was determined to be necessary to meet RFP in certain PM2.5 nonattainment areas in 

order to satisfy the contingency measure requirement in such areas. The EPA believes 

that this approach is reasonable for Moderate PM2.5 nonattainment areas that can 

demonstrate attainment by the statutory attainment date, as the state would calculate the 

emissions reductions needed for RFP separately from the control strategy determination 

for such an area. However, crediting an area for “excess” emissions reductions to satisfy 

the contingency measure requirement would not be possible for a Moderate area that 

cannot practicably attain by the statutory attainment date under the EPA’s proposed 

approach for calculating RFP for such areas, as RFP would be calculated directly from 

the projected emissions reductions from all control measures identified for the area (as 

RACM and RACT or additional reasonable measures), such that there would be no 

difference between emissions reductions estimated from control measures and those 

estimated for demonstrating RFP. 

As mentioned earlier, contingency measures should represent a portion of the 

actual emissions reductions necessary to bring about attainment in the area. Consistent 

with the EPA’s past approach for contingency measures for PM2.5 nonattainment areas, 

the EPA proposes to require that the emissions reductions anticipated by imposition of 

the contingency measures must be equal to approximately 1 year’s worth of emissions 

reductions while the state is revising its attainment plan for the area. The EPA has 

historically applied a policy of equating 1 year’s worth of emissions reductions for 

contingency measures with those annual reductions determined to be necessary to 
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achieve RFP for the area, unless the state demonstrates that some smaller reduction is 

appropriate. As described in Section IV.F of this preamble, the EPA is proposing an 

approach for interpreting the statutory RFP requirement that would require demonstrating 

RFP based on reductions from sources located inside the nonattainment area. Keeping 

with the historic linkage between RFP and contingency measures, the EPA is also 

proposing and seeking comment on a similar approach for calculating 1 year’s worth of 

emissions reductions for purposes of adopting appropriate contingency measures. That is, 

the EPA’s proposed approach for determining the level of emissions reductions for 

contingency measure purposes is to calculate the annual reductions in emissions of direct 

PM2.5 and PM2.5 precursors needed from sources located inside the nonattainment area. 

The EPA seeks comment on this proposed approach.

The CAA requires that states must implement contingency measures after the 

EPA determines that the area has either failed to meet RFP requirements, or failed to 

attain the standards by the applicable attainment date. The purpose of the contingency 

measure provision is to ensure that corrective measures are put in place automatically at 

the time that the EPA makes its determination that an area has either failed to meet RFP 

or failed to meet the standard by its attainment date. The EPA is required to determine 

within 90 days after receiving a state’s milestone demonstration, and within 6 months 

after the attainment date for an area, whether these requirements have been met. The 

consequences for states with areas that fail to attain the NAAQS or to meet RFP are 

described in section 179(d) of the CAA and discussed in Section V of this preamble. 

As noted earlier in this section, the EPA proposes to require that states must 

submit contingency measures at the same time as the rest of the Moderate area attainment 
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plan elements, i.e., within 18 months after designation. Section 172(b) requires the 

Administrator to “establish a schedule according to which the State containing such 

[nonattainment] area shall submit a plan or plan revision (including the plan items) 

meeting the applicable [subpart 1 nonattainment plan] requirements... Such schedule 

shall, at a minimum, include a date or dates, extending no later than 3 years from the date 

of the nonattainment designation...” The EPA believes it is reasonable to require the 

submittal of contingency measures for Moderate PM2.5 nonattainment areas on the same 

schedule as the other Moderate area attainment plan requirements because of the close 

relationship between an area’s control strategy, RFP analysis and selection of quantitative 

milestones, and contingency measures. The EPA seeks comment on this proposed due 

date for submission of contingency measures.

I. Attainment Dates

1. Statutory requirements

Section 188 establishes the attainment dates for Moderate and Serious PM10

nonattainment areas, which also apply to Moderate and Serious PM2.5 nonattainment 

areas. Section 188(c)(1) provides that for a Moderate area, “the attainment date shall be 

as expeditiously as practicable but no later than the end of the sixth calendar year after 

the area’s designation as nonattainment.” The EPA has the responsibility for determining 

whether a nonattainment area has attained the standard by its applicable attainment date. 

Section 179(c)(1) requires the EPA to make determinations of attainment no later than 6 

months following the attainment date for the area. Under section 179(c)(2), the EPA must 

publish a notice in the Federal Register identifying those areas which failed to attain by 

the applicable attainment date. The statute further provides that the EPA may revise or 
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supplement its determination of attainment for the affected areas based upon more 

complete information or analysis concerning the air quality for the area as of the area’s 

attainment date.

Section 179(c)(1) provides that the EPA is to base the attainment determination 

for an area upon an area’s “air quality data as of the attainment date.” The EPA will make 

the determination of whether an area’s air quality is meeting the PM2.5 NAAQS by the 

applicable attainment date based upon data gathered from the air quality monitoring sites 

which have been entered into the EPA’s Air Quality System (AQS) database. No special 

or additional attainment plan submission will be required from the state for this 

determination.

A Moderate PM2.5 nonattainment area’s air quality status is determined in 

accordance with Appendix N of 40 CFR part 50. To show attainment of the current 24-

hour and annual standards for PM2.5, the most recent 3 consecutive years’ data prior to 

the area’s attainment date must show that PM2.5 concentrations over the prior 3-year 

period are at or below the levels of the standards. A complete year of air quality data, as 

described in part 50, Appendix N, is comprised of all 4 calendar quarters with each 

quarter containing data from at least 75 percent of the scheduled sampling days. 

The EPA will begin processing and analyzing data related to the attainment of 

Moderate PM2.5 nonattainment areas after the applicable attainment date for the affected 

areas. Current EPA regulations, under 40 CFR part 58, set the deadline for the state to 

submit air quality data into the AQS database as no later than 90 days after the end of the 

calendar year.
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While the EPA may determine that an area’s air quality data indicates that an area 

may be meeting the PM2.5 NAAQS for a specified period of time, this does not eliminate 

the state’s responsibility under the Act to adopt and implement an approvable attainment 

plan. If the area’s monitored data indicates that the area is factually attaining the 

NAAQS, however, the EPA may issue a “clean data determination” which will suspend 

the obligation of the state to submit the elements of the attainment plan for the area that 

are related to planning requirements, as discussed in Section IX.C of this preamble. If the 

EPA determines that an area has attained the standard as of its attainment date, the area 

will remain classified as nonattainment until the state has requested, and the EPA has 

approved, redesignation to attainment for the area.

In order for an area to be redesignated as attainment, the state must comply with 

the five requirements listed under section 107(d)(3)(E) of the CAA. Briefly, this section 

requires that:

 the EPA has determined that the area has met the PM2.5 NAAQS; 

 the EPA has fully approved the applicable state implementation plan; 

 the improvement in air quality is due to permanent and enforceable reductions in 

emissions;

 the EPA has fully approved a maintenance plan for the area; and,

 the state(s) containing the area or portions of the area have met all applicable 

requirements under section 110 and part D.

2. Proposed approach

As noted earlier, section 188(c)(1) states that for a Moderate area, “the attainment 

date shall be as expeditiously as practicable but no later than the end of the sixth calendar 
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year after the area’s designation as nonattainment.” For purposes of clarity, the EPA 

proposes to interpret the reference to “the area’s designation” in this provision as 

meaning “the area’s effective date of designation,” consistent with the agency’s approach 

for implementing the 1997 and 2006 PM2.5 NAAQS and with its approach for 

implementing NAAQS for other criteria pollutants under part D, title I of the CAA. See

proposed 40 CFR 51.1000. As discussed elsewhere in this preamble, the effective date of 

designation is April 15, 2015, for areas designated nonattainment in the first round of 

designations for the 2012 PM2.5 NAAQS. For these areas, the Moderate area attainment 

date would be as expeditious as practicable, but no later than December 31, 2021 (i.e., the 

end of the sixth calendar year after designation). The EPA seeks comment on this 

proposed interpretation of the date of designation of a PM2.5 NAAQS nonattainment area

and the resulting attainment date for such areas.

As described in Sections IV.D and IV.E of this preamble, in the case of a 

Moderate PM2.5 nonattainment area for which a state can demonstrate attainment by the 

end of the sixth calendar year following designation, the state must follow a two-step 

process for determining the appropriate attainment date for the area. First, the state must 

demonstrate through air quality modeling that the area can attain the relevant NAAQS by 

the latest statutory attainment date and determine which control measures and 

technologies are needed for the area to attain by that date. Second, the state must 

determine whether implementing other reasonable controls (i.e., those not needed for 

attainment by the latest possible date but that are technologically and economically 

feasible) can cumulatively advance the attainment date for the area by at least 1 year. In 

the event that a state determines that the area can attain the relevant NAAQS earlier 
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through the application of other measures, the state must propose the earlier date as part 

of the attainment plan submission for the area. When the EPA takes action to approve the 

different elements of the attainment plan for the area, one of the elements that the agency 

will take action on will be the state’s proposed attainment date for the area. If the EPA 

approves an attainment date for the area that is earlier than the latest date allowed by 

statute, then the applicable attainment date for the area will be the approved date. See

proposed 40 CFR 51.1004(a)(1)(i). If the area ultimately needs additional time to attain 

the relevant NAAQS, the state may request an attainment date extension for the Moderate 

nonattainment area under section 188 as long as certain conditions are met, as described 

in Section IV.J.

The EPA’s approach to approving an attainment date for a PM2.5 nonattainment 

area will be different for a Moderate area that cannot practicably attain the relevant PM2.5

NAAQS by the end of the sixth calendar year after designation. Given that the agency 

will reclassify any such area to Serious and thereby trigger additional Serious area 

requirements for the area, the EPA will approve an attainment date for the area when it 

takes action on the Serious area attainment plan submitted for the area. In the interim, 

before the EPA takes action to reclassify the area, the statutory Moderate area attainment 

date will continue to apply to such an area. See proposed 40 CFR 51.1000 and 

51.1004(a)(1)(ii). When the EPA reclassifies the area, then the presumptive attainment 

date for the area will be as expeditious as practicable, but no later than the end of the 

tenth calendar year following designation. A complete discussion of Serious area 

attainment dates is provided in Section VI.H of this preamble.

J. Attainment date extensions
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1. Statutory requirements

The CAA under subpart 4 provides the EPA with authority to grant extensions of 

the attainment date for a Moderate area that otherwise could be found to have failed to 

attain the relevant PM2.5 NAAQS, if the area can meet specific statutory criteria related to 

the implementation of measures contained in the attainment plan for the area, and to 

monitored air quality in the area. Specifically, under section 188(d), a state may apply to 

the EPA for an extension of a Moderate area’s attainment date of one additional year (the 

“Extension Year”) if “(1) the state has complied with all requirements and commitments 

pertaining to the area in the applicable implementation plan; and (2) no more than one 

exceedance of the 24-hour [NAAQS] level for PM10 has occurred in the area in the year 

preceding the Extension Year, and the annual mean concentration of PM10 in the area for 

such year is less than or equal to the standard level.” Section 188(d) limits the number of 

1-year extensions that the EPA may grant for a Moderate nonattainment area to two.

The provisions of section 188(d) thus allow a state an opportunity to demonstrate 

that a Moderate area should continue to be classified as Moderate and not be reclassified 

to Serious even if the area exceeded the level of the applicable PM2.5 NAAQS in one or 

both of the 2 calendar years preceding the year in which the area is otherwise required to 

attain the NAAQS. Although section 188(d) provides the criteria for such an extension, 

the EPA believes that there are some ambiguities in the statutory language that warrant 

interpretation and clarification through regulations for the PM2.5 NAAQS. The EPA is 

thus proposing a preferred interpretation of section 188(d) to provide clarity to states 

about how and when they may qualify for a Moderate area attainment date extension for 

purposes of the PM2.5 NAAQS.
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2. Proposed interpretations of attainment date extension criteria

With respect to the criterion in section 188(d)(1) that requires that “the state has 

complied with all requirements and commitments pertaining to the area in the applicable 

implementation plan,” the EPA proposes to interpret this provision to mean that the state 

has implemented the control measures in the SIP submission it made to address the 

attainment plan requirements for the applicable PM2.5 NAAQS, and not to require the 

area to have a fully approved attainment plan that meets all of the CAA’s requirements 

for Moderate areas. This proposed interpretation is based on the plain language of section 

188(d) that does not explicitly require that the state comply with all requirements 

pertaining to the area in the CAA, but merely requires that the state comply with all 

requirements in the applicable SIP.

153 In other words, the EPA believes that section 

188(d)(1) should be interpreted to mean that so long as the state has submitted the 

necessary attainment plan for the area for the applicable PM2.5 NAAQs and is 

implementing the control measures in the submission, the fact that the EPA has not yet 

acted on such submission to make it an approved part of the applicable SIP should not be 

a barrier to the state obtaining an extension of the attainment date under section 

188(d)(1). For the same reason, the EPA also proposes to read this provision not to bar an 

extension if all or part of an area’s Moderate area plan is disapproved or has been 

promulgated by the EPA as a federal implementation plan (FIP). In the case that the 

“applicable implementation plan” is a FIP (or combination of SIP and FIP), then the EPA 

 

153 This interpretation as applied to section 188(e) for Serious area attainment date 

extensions was upheld by the Ninth Circuit Court of Appeals in Vigil v. Leavitt, 366 F.3d 

1025, amended at 381 F.3d 826 (9th Cir. 2004).
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proposes that the state must have implemented the control measures contained therein in 

order to meet the statutory criteria at section 188(d)(1) for a Moderate area attainment 

date extension. The EPA seeks comment on this proposed interpretation of section 

188(d)(1). See proposed 40 CFR 51.1005(a)(2).

The EPA also proposes and seeks comment on an alternative interpretation of 

section 188(d)(1) that would require a state to have a Moderate area attainment plan fully 

approved by the EPA as meeting the applicable attainment plan requirements under 

sections 172 and 189 for a Moderate PM2.5 nonattainment area before the state obtains an 

extension. Given that Moderate area attainment plans are due 18 months from the date of 

designation, and that RACM and RACT must be implemented within 4 years after 

designation, states should have sufficient time under the statutory schedule to satisfy all 

applicable requirements in advance of seeking a Moderate area attainment date extension. 

Under this alternative approach, the EPA proposes that a state subject to a FIP (or SIP 

and FIP) for a Moderate PM2.5 nonattainment area could qualify for an attainment date 

extension for the area if it had implemented all requirements and commitments of the FIP

(or SIP and FIP), as the FIP (or SIP and FIP) would be the “applicable implementation 

plan” for the area. Although this alternative interpretation could also be a reasonable 

reading of this criterion of section 188(d)(1), the EPA considers it less appropriate than

the preferred interpretation because this approach could foreclose states from obtaining 

an otherwise appropriate extension merely because of logistical and timing considerations 

that might have prevented the EPA from acting on the state’s attainment plan by the 

requisite point in time. Nevertheless, the EPA seeks comment on this alternative 

interpretation of section 188(d)(1).
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The second criterion that states must meet to qualify for an extension relates to the 

monitored ambient air in a nonattainment area in the year prior to the attainment date for 

the area. If a state has met the requirements of section 188(d)(1), the EPA may grant an 

extension of a Moderate area’s attainment date if the state also satisfies the requirements 

of section 188(d)(2) that “no more than one exceedance of the 24-hour national ambient 

air quality standard level for PM10 has occurred in the area in the year preceding the 

Extension Year, and the annual mean concentration of PM10 in the area for such year is 

less than or equal to the standard level.” Again, the EPA may grant up to two such 1-year 

extensions and thus this criterion would apply to the calendar year prior to the applicable 

attainment date and to the Extension Year, in the case of a second extension.

The EPA believes that the references to the ambient air quality standards in 

section 188(d)(2) are ambiguous in two significant ways in the context of the PM2.5

NAAQS implementation. First, the statutory language explicitly sets ambient air quality 

conditions for an attainment date extension in terms that relate factually to the 24-hour 

PM10 NAAQS that was in effect at the time of the 1990 Amendments of the CAA, which 

has a statistical form that is substantially different from the 24-hour PM2.5 NAAQS. 

Specifically, the form of the 24-hour PM10 NAAQS allows for no more than one 

“exceedance” of the standard per year on average over 3 years, and if there is more than 

one such exceedance on average over 3 years the area is violating the NAAQS. Thus, as a 

means of limiting extensions to areas that are close to attaining the NAAQS in the 

calendar year prior to the applicable attainment date, section 188(d)(2) imposes the 

criterion of having “no more than one exceedance of the 24-hour... standard level” as a 

way of demonstrating that a nonattainment area has “clean data” for the year prior to the 
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attainment date.154 This statutory language does not translate readily to the PM2.5

NAAQS, which postdate the creation of section 188(d) and are not structured with the 

same mathematical form. For example, the 2006 24-hour PM2.5 NAAQS incorporates a 3-

year average of the 98th percentile form, which means that an area with valid monitored 

ambient readings every day (or almost every day) could have seven readings above the 

numerical level of the standard (i.e., “exceedances”) in any given year and still have 

“clean data” for that year. A literal interpretation of section 188(d)(2) to permit only one 

exceedance of the 24-hour PM2.5 NAAQS, rather than the number of exceedances that is 

relevant for purposes of determining attainment of such NAAQS, is illogical. In light of 

the different form of the PM2.5 NAAQS, the statutory language of section 188(d)(2) is 

thus ambiguous in how it should apply to implementation of the 24-hour PM2.5 NAAQS.

Additionally, the language of section 188(d)(2) may be considered ambiguous 

vis-à-vis how it should apply to the PM2.5 NAAQS to the extent that it does not specify 

whether the air quality criteria for an attainment date extension apply equally for a 

Moderate area designated nonattainment for both the 24-hour and annual standards, or for 

just one of the standards. In practice, most areas designated nonattainment for the PM10

NAAQS following passage of the 1990 CAA Amendments were designated 

nonattainment only for the 24-hour PM10 NAAQS, with a few designated for only the 

annual PM10 NAAQS or for both the 24-hour and the annual PM10 NAAQS. The 24-hour 

NAAQS has served as the “controlling” (i.e., functionally more stringent) PM10 standard, 

 

154 The 24-hour PM10 NAAQS, set at150 μg/m3

, cannot be exceeded more than once per 

year on average over 3 years.
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such that the agency’s experience to date in granting PM10 Moderate area attainment date 

extension requests has been limited to extending the attainment date for the 24-hour PM10

NAAQS.155

The situation is distinctly different for PM2.5 nonattainment areas, as the specific 

facts and circumstances of a particular area may warrant a nonattainment designation for 

either the 24-hour standard or the annual standard, but often not both. In most cases, for 

instance, the current nonattainment areas for PM2.5 are designated either for the 1997 

annual NAAQS or for the 2006 24-hour NAAQS, but not both.156 For example, the EPA 

recently promulgated designations for areas violating only the annual PM2.5 NAAQS

revised in 2012, not the 24-hour NAAQS which was retained at the level established 

during the 2006 PM NAAQS review. If a PM2.5 nonattainment area is designated only for 

the 24-hour or only for the annual PM2.5 NAAQS, this situation raises the question of 

how section 188(d)(2) air quality criteria for both standards should apply to such a PM2.5

NAAQS nonattainment area if the state seeks an extension of the applicable attainment 

date for such area.

Due to the ambiguities associated with applying this subpart 4 requirement to 

current and future PM2.5 NAAQS, the agency believes it is important to propose a 

reasonable interpretation of the statutory requirement and seek public comment on this 

preferred interpretation as well as two alternative interpretations specifying the PM2.5

 

155 For examples of the EPA actions to extend attainment dates for Moderate PM10 areas, 

see 61 FR 20730, 61 FR 66602 and 66 FR 32752.

156 Nonattainment areas designated for both the 24-hour and annual PM2.5 NAAQS are 

located in central and southern CA.
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standard or standards for which a state would need to demonstrate a Moderate 

nonattainment area met the air quality criteria of section 188(d)(2) in order to qualify for 

an attainment date extension. The agency also believes it is important to clarify how the 

air quality criteria of section 188(d)(2) apply specifically for the 24-hour PM2.5 NAAQS. 

For this reason, the EPA is proposing a preferred interpretation of section 188(d)(2) for 

application to current and future PM2.5 NAAQS, and is seeking comment on two 

alternative interpretations that the agency considers less appropriate. 

The preferred proposed approach would only require a state to demonstrate that in 

the year prior to the applicable attainment date for the area, a Moderate area did not 

exceed the level of (i.e., had clean data for) the specific PM2.5 NAAQS for which the area 

is designated nonattainment (the “applicable NAAQS”) and for which the state is seeking 

the extension of the attainment date. The second approach, on which the EPA seeks 

comment, would require that a state demonstrate that in the year prior to the applicable 

attainment date for an area, the Moderate area did not exceed the level of the specific 

PM2.5 NAAQS for which the area is designated nonattainment (the applicable NAAQS), 

and did not exceed the most stringent level of any other PM2.5 NAAQS in effect 

nationally at the time the area was designated for the applicable NAAQS. The third 

approach, on which the EPA also seeks comment, would require that a state demonstrate 

that in the year prior to the applicable attainment date for an area, the Moderate area did 

not have more than one exceedance of the level of the 24-hour PM2.5 standard, and that 

the annual mean concentration of PM2.5 in the area for the attainment year was less than 

or equal to the annual standard, regardless of the NAAQS for which the state is seeking 

an attainment date extension.
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The EPA prefers the proposed interpretation (described in more detail later in this 

section) for implementing the Moderate area attainment date extension criteria of section 

188(d)(2) considering the fact that, due to the specific atmospheric conditions and sourcedependent nature of PM2.5 problems in different areas around the country, the EPA has 

historically and may continue its practice of designating PM2.5 nonattainment areas for 

either the annual or the 24-hour NAAQS. As discussed earlier, the agency’s designations 

processes for the 2006 revised 24-hour PM2.5 NAAQS and the 2012 revised annual PM2.5

NAAQS have each been conducted to address only one standard individually. In addition, 

the current 24-hour PM2.5 NAAQS does not have a “one exceedance” form of the 

standard, as cited in section 188(d)(2). Nevertheless, the EPA requests comment on the 

second and third interpretations of section 188(d)(2) described later in this section 

because they more closely reflect the specific statutory wording.

a. Proposed approach: the EPA preferred option. The EPA’s proposed 

interpretation of section 188(d)(2) would simply require that a state demonstrate that in 

the year prior to the applicable attainment date for the area, a Moderate nonattainment 

area had clean data for the specific PM2.5 NAAQS for which the state was seeking an 

attainment date extension (the applicable NAAQS). Under this proposed approach, a state 

seeking an attainment date extension for a Moderate nonattainment area for a 24-hour 

PM2.5 NAAQS would be required to demonstrate that the area had clean data for that 

particular standard in the calendar year prior to the applicable attainment date for the 

area, rather than demonstrating that the area necessarily had no more than one 

exceedance of the 24-hour PM2.5 NAAQS. 
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For example, under this proposed interpretation of section 188(d)(2), in the case 

of a state seeking an extension of the attainment date for a Moderate area designated 

nonattainment for the 2006 24-hour PM2.5 NAAQS, the state would need to demonstrate 

that the area had no more than the allowable number of valid monitored readings 

exceeding 35μg/m3to meet the 98th percentile statistical form of the standard in the year 

prior to the area’s attainment date. The state would not have to demonstrate that the area 

also had clean data for any other PM2.5 NAAQS, including any annual PM2.5 NAAQS or 

later revision of the 24-hour PM2.5 NAAQS.

Likewise under the EPA’s preferred approach, a state seeking an attainment date 

extension for a Moderate nonattainment area for an annual PM2.5 NAAQS would be 

required to demonstrate that the area had clean data for that particular standard in the 

calendar year prior to the applicable attainment date for the area. For example, in the case 

of a state seeking an extension of the attainment date for a Moderate area designated 

nonattainment for the 2012 annual PM2.5 NAAQS, the state would need to demonstrate 

that the annual mean concentration of PM2.5 at each monitor in the area as analyzed in 

accordance with Appendix N to 40 CFR part 50 for the year prior to the area’s attainment 

date was less than or equal to 12.0 μg/m3. Again, under this proposed approach, the state 

would not have to demonstrate that the area had clean data for any other PM2.5 NAAQS. 

Under the EPA’s preferred approach, if a state were to have an area that is 

designated nonattainment for both the 24-hour and the annual PM2.5 NAAQS, with the 

same applicable attainment date, then a state seeking attainment date extensions for both 

NAAQS would need to meet the ambient air quality criterion of both NAAQS. The EPA 

notes that this would not be a common occurrence, but these are the limited 
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circumstances under which a state should be required to have clean data for both NAAQS 

in order to qualify for an extension of the applicable attainment date under section 

188(d)(2). If a state has a nonattainment area that is only designated for either the 24-hour

or the annual PM2.5 NAAQS, the EPA believes that the state need only meet the air 

quality criterion of section 188(d)(2) for the NAAQS relevant to the attainment date at 

issue. See proposed 40 CFR 51.1005(a)(1)(ii) and (iii).

The EPA believes this preferred interpretation of section 188(d)(2) is appropriate 

for two reasons. First, as discussed above, while most PM10 nonattainment areas were 

designated nonattainment for either just the 24-hour PM10 NAAQS or for both the 24-

hour and annual PM10 NAAQS, the majority of current PM2.5 nonattainment areas are 

designated for either the 24-hour or the annual PM2.5 NAAQS, and should arguably only 

need to demonstrate clean data for the NAAQS for which the area is designated 

nonattainment. For those few PM2.5 nonattainment areas designated for 24-hour and 

annual PM2.5 NAAQS, the EPA believes it may also be appropriate that a state must only 

demonstrate clean data for the specific NAAQS for which the state is seeking an 

attainment date extension because such an approach is consistent with the statute’s 

overall approach to designating nonattainment areas and implementing control strategies 

for each separate PM2.5 NAAQS.

Second, as discussed earlier, the statutory language that requires that a 

nonattainment area have “no more than one exceedance of the 24-hour” NAAQS level 

reflects a statistical form for the 24-hour PM10 standard that is different from the current 

form of the 24-hour PM2.5 NAAQS. This difference, and the fact that the form could be 

subject to further revision in the future, leads the EPA to conclude that it is appropriate to 
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describe this particular criterion more broadly so that it can apply to any 24-hour PM2.5

NAAQS, now or in the future regardless of the specific statistical form any such NAAQS 

may take. The EPA seeks comment on this preferred proposed approach.

b. Alternative approach #1. The EPA also seeks comment on two alternative 

interpretations of section 188(d)(2). The EPA’s first alternative interpretation of section 

188(d)(2) would require that a state seeking an attainment date extension for a Moderate 

PM2.5 nonattainment area would have to demonstrate that the area met the level of the 

PM2.5 NAAQS for which it is seeking the attainment date extension, as well as met the 

numerical level of the most stringent PM2.5 NAAQS in effect at the time the area was 

designated nonattainment. That is, under this approach, the area would need to have clean 

data for the year preceding the attainment date for the PM2.5 NAAQS for which the state 

is seeking an attainment date extension and for the other PM2.5 NAAQS that were part of 

the same suite of PM2.5 standards (i.e., both the 24-hour and the annual PM2.5 NAAQS) in 

effect at the time the EPA designated the area nonattainment.

For example, if a state seeks an extension of the attainment date for an area 

designated nonattainment only for the 2012 annual PM2.5 NAAQS, it would have to 

demonstrate that the annual mean concentration of PM2.5 at each monitor in the Moderate 

area as analyzed in accordance with Appendix N to 40 CFR part 50 in the attainment year 

was less than or equal to 12.0 μg/m3. Additionally, the state would have to demonstrate 

that the 98th percentile of valid 24-hour monitored readings in the area for the year 

preceding the attainment date did not exceed 35 μg/m3, the level of the 24-hour PM2.5

NAAQS set in 2006 and retained with the 2012 PM NAAQS review as part of the suite 

of PM NAAQS, even if the area was not designated nonattainment for the 2006 24-hour 
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PM2.5 NAAQS.157 As with the agency’s preferred approach, a state seeking an attainment 

date extension for a Moderate nonattainment area for a 24-hour PM2.5 NAAQS would be 

required to demonstrate that the area had clean data for that particular standard in the 

calendar year prior to the applicable attainment date for the area in accordance with the 

statistical form of the 24-hour PM2.5 NAAQS, rather than demonstrating that the area had 

no more than one exceedance of the 24-hour PM2.5 NAAQS.

The EPA presents this first alternative interpretation of the statute for two reasons. 

First, as noted earlier, the statute at section 188(b)(2) does not specify whether the air 

quality criteria for an attainment date extension apply for Moderate areas designated 

nonattainment for both the 24-hour and annual PM10 standards, or for just one of the 

standards. Read literally, however, the statute seems to require that an area seeking an 

extension of a Moderate area attainment date for any PM10 NAAQS must be meeting the 

level of both the 24-hour standard and the annual standard, even if it was only designated 

for just one of the standards. Under this interpretation of the statute for purposes of 

implementing the PM2.5 NAAQS, even though an area may be designated nonattainment 

for only one PM2.5 NAAQS and therefore seeking an attainment date extension only for 

that particular NAAQS, it would also have to meet the level of the other PM2.5 standards. 

As explained above, the EPA does not consider this the most appropriate interpretation of 

section 188(d). However, under this alternative interpretation the agency would take the 

position that the other PM2.5 standards whose level the state must show the Moderate 

 

157 Given the rounding provisions specified in 40 CFR part 50, Appendix N, these criteria 

would be satisfied if the concentrations before final rounding are less than an annual 

average of 12.05 μg/m3and a 24-hour value of 35.5 μg/m3.
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nonattainment area met in the year preceding its attainment date would be the most 

stringent PM2.5 NAAQS in effect nationally at the time the area was designated 

nonattainment. For example, if the EPA were to strengthen the 24-hour PM2.5 standard 

below the current 35 μg/m3prior to December 31, 2021 (the anticipated statutory 

Moderate area attainment date for the 2012 PM2.5 NAAQS), then an area seeking an 

extension of the Moderate area attainment date for the 2012 PM2.5 NAAQS would have 

to demonstrate that the area met the most stringent 24-hour PM2.5 NAAQS that applied at 

the time it was designated (35 μg/m3), and not the less stringent 24-hour NAAQS set in 

1997 (65 μg/m3) or any more stringent standard set after designation but before the 

attainment date. 

Second, as with the proposed approach to interpreting section 188(d)(2), the EPA 

believes it is appropriate to interpret the statutory language regarding “no more than one 

exceedance of the 24-hour” NAAQS level broadly to mean that the area had clean data 

for the 24-hour PM2.5 NAAQS, consistent with the form of the NAAQS at issue, so that 

the requirement can apply to any 24-hour PM2.5 NAAQS, now or in the future. Even if it 

were appropriate to interpret section 188(d)(2) to require that a state meet the air quality 

criterion for both the 24-hour and the annual PM2.5 NAAQS, the EPA believes that the 

statutory provision concerning the number of exceedances must still be read in light of 

the different form of the 24-hour PM2.5 NAAQS. The EPA seeks comment on this first 

alternative interpretation of section 188(d)(2).

c. Alternative approach #2. The EPA’s second alternative interpretation of section 

188(d)(2) would require that a state demonstrate that a Moderate area did not have more 

than one exceedance of the applicable 24-hour PM2.5 standard level, and the annual mean 
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concentration of PM2.5 in the area was less than or equal to the applicable annual PM2.5

standard level, in the year preceding the applicable attainment date for the area. In other 

words, the EPA would not interpret the air quality criterion with respect to the 24-hour 

PM2.5 NAAQS in light of the significantly different form of the PM2.5 NAAQS. 

Furthermore, as with the first alternative interpretation, the “applicable” PM2.5 standards 

would be those that applied at the time the Moderate area was designated for a given 

PM2.5 NAAQS, even if the area was not designated nonattainment for all of them. This 

interpretation would mean that regardless of the form of the applicable 24-hour PM2.5

standard, the Moderate area seeking an attainment date extension could not have more 

than one exceedance of the numerical level of the applicable 24-hour standard in order to 

qualify for a Moderate area attainment date extension. This requirement would be more 

stringent – and in some cases considerably so – than under the preferred proposed and 

first alternative interpretations, given the current statistical form of the 24-hour PM2.5

NAAQS. Additionally, under this reading of section 188(d)(2), any future changes to the 

PM2.5 NAAQS in terms of form or averaging time would also not be addressed, 

potentially creating confusion with respect to how a PM2.5 Moderate area could qualify 

for an attainment date extension in the future.

The EPA believes that while this interpretation of section 188(d)(2) may appear to 

be a straightforward reading of the statutory language, it does not reasonably account for 

the important differences between the statistical form of the PM10 and PM2.5 NAAQS or 

between the EPA’s longstanding convention for designating PM10 and PM2.5

nonattainment areas generally. The EPA therefore seeks comment on its preferred 

proposed approach and two alternative approaches for interpreting the air quality criteria 
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of section 188(d)(2) that a state would need to demonstrate compliance with in order for 

the EPA to consider granting an extension of a Moderate PM2.5 area attainment date.

3. Proposed process for attainment date extension request submittals

Regardless of which interpretation of section 188(d)(1) the EPA finalizes as part 

of this rulemaking, the EPA proposes to require states to submit sufficient information to 

demonstrate that they have complied with applicable requirements and commitments in 

the applicable implementation plan. This information would be needed in order for the 

EPA to make a decision on whether to grant a 1-year attainment date extension. The EPA 

would not be authorized to grant an attainment date extension to an area unless the state 

can demonstrate that it has met all of the requirements and commitments contained in the 

state’s applicable implementation plan for the area. Under the EPA’s first proposed 

approach for interpreting section 188(d)(1), a state would have to demonstrate that 

control measures have been submitted in the form of a SIP revision and that RACM and 

RACT and additional reasonable measures for sources in the area have been 

implemented. Under the agency’s alternative proposal for interpreting section 188(b)(1), 

the attainment plan submitted by the state would have to have been fully approved by the 

EPA and the state would have to be in compliance with any elements required under any 

applicable FIP for the area. In addition, under the EPA’s second proposed approach, the 

state would have to demonstrate that: (i) RACM and RACT and additional reasonable 

measures for sources in the area have been implemented, and (ii) the area has made 

emissions reductions progress that represents RFP toward attainment of the NAAQS and 

has met its quantitative milestones, and the state has submitted a milestone compliance 

demonstration (milestone report) to that effect if due. Any decision made by the EPA to 
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extend the attainment date for an area would be based on facts specific to the 

nonattainment area at issue.

Section 188(d) does not specify the process by which the EPA should evaluate 

and act upon requests from states for an extension of the Moderate area attainment date. 

However, the EPA proposes that an attainment date extension would only be granted 

after the agency provides notice in the Federal Register and an opportunity for the public 

to comment. This notice and comment process would allow for appropriate evaluation of 

the relevant criteria and facts in order to assure that the extension is granted or denied 

after full evaluation. This process also is consistent with past practice by the EPA in 

granting attainment date extensions, most recently for ozone nonattainment areas. In 

addition, for ease of implementation, the EPA proposes to interpret section 188(d) to 

authorize the EPA to stipulate that any extension would begin on January 1 and end on 

December 31 of the extension year and these dates would not depend on when the state 

submitted its request for an extension or was granted the extension by the EPA. The EPA 

believes this is a reasonable approach as the applicable attainment date for the area will 

either be the end of the sixth calendar year following designation of the area, or the end 

of an earlier calendar year if the state could advance attainment of the area by at least 1 

year through the implementation of extra control measures. In addition, compliance with 

the relevant NAAQS will be evaluated based on monitored data collected over a full 

calendar year (i.e., over the period beginning January 1 and ending December 31), so 

starting the extension year on January 1 is logical.

Because air quality criteria are part of the conditions that must be met in order for 

the EPA to grant a Moderate area attainment date extension, the EPA proposes to require 
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that a state seeking such an extension must submit its complete attainment date extension 

request, including any available preliminary data for the year preceding the area’s 

applicable Moderate attainment date, on or before the area’s attainment date. The EPA 

also proposes to require that the state requesting such an extension must submit to the 

respective EPA Regional Office certified ambient PM2.5 monitoring data for the year 

preceding the attainment date for the area in question by no later than February 28 of the 

year following the area’s attainment date. Submission of the necessary data by this date 

will allow the EPA to review the state’s request and take appropriate action on the 

request prior to the date by which the EPA is required to make a determination that the 

area failed to attain by its Moderate area attainment date, i.e., within 6 months of the 

applicable attainment date (see the discussion of reclassification in Section V of this 

preamble). The EPA seeks comment on these proposed deadlines for a state to request an 

extension of a Moderate area’s attainment date and submit certified air quality data as 

required under CAA section 188(d)(2).

As noted earlier in this discussion of Moderate area attainment date extensions, 

the statute at section 188(d) provides that a state may seek up to two 1-year extensions of 

the Moderate area attainment date if it meets the applicable criteria of sections 188(d)(1) 

and 188(d)(2). The statute makes no distinction between the criteria that must be met for 

the first 1-year extension and the criteria for the second 1-year extension, therefore the 

EPA plans to apply the same interpretations of the statutory criteria proposed throughout 

this section, including the proposed deadlines for the state to submit the extension request 

and the certified air quality data, for purposes of a state seeking a second 1-year 

attainment date extension for a Moderate nonattainment area.
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The EPA seeks comment on the proposed approaches described above for 

interpreting the criteria of section 188(d)(1) and 188(d)(2) and establishing a process for 

states to request attainment date extensions for Moderate areas.

V. How would a PM2.5 Moderate nonattainment area be reclassified to serious?

As discussed elsewhere in this preamble, subpart 4, part D of title I of the CAA 

establishes a two-tier classification system for areas designated nonattainment for the 

PM2.5 NAAQS. While all areas designated nonattainment are initially classified as 

Moderate, section 188(b) describes two pathways by which the EPA has the authority or 

the duty to reclassify a Moderate nonattainment area to a Serious nonattainment area. 

Pursuant to section 188 (b)(1), the EPA has general discretionary authority to reclassify 

from Moderate to Serious any area that the Administrator determines cannot practicably 

attain the NAAQS by the applicable Moderate area attainment date. Pursuant to section 

188(b)(2), the EPA has a mandatory duty to reclassify from Moderate to Serious any area 

that fails to attain the NAAQS by the applicable Moderate area attainment date. Both of 

these pathways are more fully described below.

A. Discretionary Authority

The EPA’s discretionary authority to reclassify a Moderate area to Serious derives 

from language in section 188(b)(1) of the CAA which provides that: “The Administrator 

may reclassify as a Serious PM10 nonattainment area... any area that the Administrator 

determines cannot practicably attain the [NAAQS]... by the attainment date... for 

Moderate Areas.” The use of this discretionary authority thus would be triggered by the 
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EPA making a determination that the Moderate area in question could not practicably 

attain by its statutory attainment date. 

The CAA does not specify the basis on which the EPA may make the 

determination that the area cannot practicably attain by the applicable attainment date. In 

the General Preamble, the EPA explained that the agency could base this determination 

upon whatever factors are pertinent and do so whether or not the state in question has 

submitted a Moderate area attainment plan, and whether or not the state has made the 

demonstration contemplated in section 189(a)(1)(B).158 The EPA may make such a 

determination based on evaluation of the attainment plan for the Moderate area in 

question or other facts known to the agency. As discussed earlier in this preamble, the 

attainment plan that a state would submit for a Moderate nonattainment area must include 

either a demonstration that the area will attain the NAAQS by the statutory Moderate area 

attainment date or a demonstration that attaining by the statutory Moderate area 

attainment date is impracticable. If the state makes and the EPA concurs with an

impracticability demonstration submitted as part of the attainment plan, then the 

demonstration could serve as the basis for the EPA initiating a notice-and-comment 

rulemaking to reclassify the area to Serious. However, the CAA does not specify the 

basis for the EPA’s exercise of its discretionary authority and does not require the EPA to 

make its determination based on a submission from the state. Indeed, such a prerequisite 

would be illogical in the case of a state that fails to make any attainment plan submission 

or fails to address the issue of the need for reclassification in such submission.

 

158 57 FR 13498 at 13537–38.
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Section 188(b)(1)(B) does establish mandatory timeframes by which EPA must

act if it intends to exercise its discretionary authority to reclassify areas as appropriate 

following the Moderate area attainment plan due date, stating that “the Administrator 

shall reclassify appropriate areas within 18 months after the required date for the state’s 

submission of a SIP for the Moderate Area.” In the case of areas designated 

nonattainment for the 2012 PM2.5 NAAQS in the first round of designations, states will 

be required by statute to submit a Moderate area attainment plan within 18 months of the 

date of designation (April 2015), or no later than October 2016. Pursuant to section 

188(b)(1)(B), the EPA would then have until April 2018 (18 months following the 

Moderate area attainment plan submission deadline) to use its discretionary authority to 

reclassify any area that the EPA determines at that time cannot practicably attain by the 

Moderate area attainment date of December 2021.

As noted above, the EPA believes that while a Moderate area impracticability 

demonstration as contemplated in section 189(a)(1)(B) is desirable in order to help the 

agency make a determination that the area cannot practicably attain by its attainment 

date, such a demonstration is not necessary to trigger action by the EPA to reclassify a 

Moderate area to Serious. The statute does not prohibit the EPA from using the weight of 

available evidence, including information available in the public record of a state, to 

make such a determination, even in the absence of a complete attainment plan 

submission. Thus, the EPA expressed in the General Preamble that:

...under the plain meaning of the terms of section 188(b)(1) EPA has 

general discretion to reclassify at any time before the applicable 

attainment date any area EPA determines cannot practically attain the 

standards by such date. Accordingly, CAA section 188(b)(1) is a general 

expression of delegated rulemaking authority. In addition, subparagraphs 

(A) and (B) of CAA section 188(b)(1) mandate that the EPA reclassify at 
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specified timeframes any areas it determines appropriate for 

reclassification at those dates. These subparagraphs do not restrict the 

general authority but simply specify that, at a minimum, it must be 

exercised at certain times.159

The EPA continues to consider this the correct interpretation of the statutory 

requirements concerning its authority to reclassify a Moderate nonattainment area to 

Serious at any time prior to the area’s Moderate area attainment date, if the agency 

determines that the area cannot practicably attain the relevant PM2.5 NAAQS by that date.

The EPA emphasizes that states with an area designated as nonattainment for the 

PM2.5 NAAQS are required to meet all Moderate area attainment plan requirements, even 

after the EPA reclassifies the area to Serious. Section 189(b)(1) states clearly that “in 

addition to” the Moderate area attainment plan requirements, states with areas reclassified 

to Serious must also meet Serious area attainment plan requirements, i.e., the 

reclassification does not eliminate the statutory obligation to meet Moderate area 

attainment plan requirements.160 Thus, the EPA believes that reclassifying Moderate 

areas to Serious at any time under its discretionary authority does not reward areas who 

delay development and implementation of control measures by excusing states from 

meeting substantive Moderate area attainment plan requirements or by extending the 

applicable attainment date. The EPA articulated this position in the General Preamble, 

explaining that this interpretation:

... creates an incentive for the timely submittal and effective 

implementation of moderate area SIP requirements and facilitates 

the PM10 attainment objective. For example, if an area that fails to 

submit a timely moderate area SIP is reclassified, this does not 

 

159 Ibid. at 13537.

160 See, Vigil v. Leavitt, 366 F.3d 1025, amended at 381 F.3d 826 (9th Cir. 2004).
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obviate the requirement that the area submit and implement 

RACM consistent with the moderate area schedule. Accordingly, 

the area could be subject to sanctions for its delay in submitting the 

RACM SIP requirement... Further, reclassification before the 

applicable attainment date will ensure that additional control 

measures (i.e., in addition to RACM, serious areas must implement 

best available control measures (BACM)), are implemented sooner 

and will expedite the application of more stringent new source 

review requirements to the area... Similarly, where an area submits 

a timely moderate area SIP, EPA may not discover that the area 

cannot practicably attain until sometime after it begins 

implementing its moderate area control measures. The EPA then 

may want to reclassify the area in order to facilitate the 

development and implementation of BACM.

161

The EPA considers this the correct interpretation of the statutory requirements 

and proposes to apply this longstanding interpretation of section 188(b)(1) to 

nonattainment areas for the PM2.5 NAAQS.

B. Mandatory Duty

In addition to the EPA’s discretionary authority to reclassify a Moderate area to 

Serious under certain circumstances, the CAA also directs the EPA to do so under other 

circumstances. The alternative circumstances under which the EPA will reclassify an area 

from Moderate to Serious are if that area fails to attain the relevant NAAQS by the 

applicable Moderate area attainment date, including any extension of that date under 

section 188(d) for which the area qualifies. Under such circumstances, the EPA has a 

mandatory duty to identify any area that fails to attain the PM2.5 NAAQS by the 

applicable Moderate area attainment date. Reclassification under such circumstances

would happen by operation of law when the EPA determines that the area failed to attain 

 

161 57 FR 13498 at 13537.
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the NAAQS by the applicable attainment date, in accordance with section 188(b)(2)(A). 

Section 188(b)(2) requires that “within six months following the applicable attainment 

date for a PM10 nonattainment area, the Administrator shall determine whether the area 

attained the standard by that date” and publish its determination in the Federal Register. 

The EPA proposes that the date of reclassification for an area reclassified under the 

EPA’s mandatory duty to reclassify an area would be the effective date of the Federal 

Register document announcing that the area had not attained the relevant PM2.5 NAAQS 

and is therefore reclassified by operation of law. Thus, for example in the case of the 

2012 PM2.5 NAAQS, assuming a Moderate PM2.5 nonattainment area fails to attain the 

standard by its approved attainment date of December 31, 2021, the EPA would be 

required to publish in the Federal Register no later than June 30, 2022 its determination 

that the area failed to attain the NAAQS and is therefore reclassified as Serious by 

operation of law. The date of reclassification for the area would be the effective date of 

the Federal Register document, or sometime after June 30, 2022. To meet the 

requirements of section 189(b)(2), the Serious area attainment plan for the area would be 

due within 18 months thereafter, or no later than December 2023.

An alternative approach for setting the date of reclassification for an area 

reclassified to Serious under the EPA’s mandatory authority could be to make it the same 

date as the missed attainment date for the area. Applying this approach in the example 

above would yield an earlier date of reclassification of December 31, 2021, and an earlier 

Serious area attainment plan due date of June 30, 2023.

Although section 188(b)(2) does not explicitly address this issue, the EPA 

believes that its proposed approach is a reasonable interpretation of statutory ambiguity in 
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section 188(b)(2) and preferable over the alternative approach for two reasons. First, the 

statute at section 189(b)(2) gives a state 18 months from the date of reclassification of an 

area to submit for the EPA’s approval an attainment demonstration with air quality 

modeling and provisions to assure timely implementation of BACM and BACT on 

sources in the nonattainment area. The EPA believes that it is reasonable for a state with 

a Serious PM2.5 nonattainment area to have 18 months plus the additional time needed by 

the EPA to issue a Federal Register document announcing the area’s failure to attain by 

the applicable Moderate area attainment date and subsequent reclassification (up to 6 

additional months) to ensure that the state has time to develop and submit a thorough, 

complete and accurate Serious area attainment plan that will provide for timely 

attainment of the NAAQS. Second, the statutory attainment date for a Serious area 

reclassified under any circumstances is as expeditious as practicable but no later than the 

end of the tenth year following designation of the area, and is thus independent of the 

date of reclassification of the area. Allowing a state some additional amount of time 

beyond 18 months from the missed attainment date to develop and submit a complete 

Serious area attainment plan, including adopting BACM and BACT, will not change the 

statutory obligation on the state for the area to attain the relevant NAAQS by the 

applicable attainment date. On the contrary, the EPA believes that the extra time may in 

fact help the area timely attain the relevant NAAQS by allowing the state to develop a 

more effective attainment plan for the area.

The EPA seeks comment on its proposed approach of basing the date of 

reclassification for an area reclassified under the agency’s mandatory duty in section 

188(b)(2) on the effective date for the Federal Register document in which the EPA 
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announces that the area failed to attain the PM2.5 NAAQS by the applicable Moderate 

area attainment date and is reclassified by operation of law. The EPA intends to make 

determinations of whether or not an area attained the relevant NAAQS pursuant to 

section 188(b)(2) via notice and comment rulemaking.

VI. What requirements would apply to Serious area attainment plans?

Sections 189(b) and (c) of the CAA include the following requirements for 

Serious area attainment plan submissions: (i) an attainment demonstration (section 

189(b)(1)(A)); (ii) provisions for the implementation of best available control measures 

(BACM) no later than 4 years after reclassification of the area to Serious (section 

189(b)(1)(B)); (iii) quantitative milestones that will be used to evaluate compliance with 

the requirement to demonstrate RFP (section 189(c)); and, (iv) regulation of PM2.5

precursors (in general to meet attainment and control strategy requirements and as 

specifically required for major stationary sources by section 189(e)). Other subpart 1 

requirements for attainment plans not otherwise superseded under subpart 4 also apply to 

Serious areas for the PM2.5 NAAQS, including: (i) a description of the expected annual 

incremental reductions in emissions that will demonstrate RFP (section 172(c)(2)); (ii) 

emissions inventories (section 172(c)(3)); (iii) other control measures (besides BACM 

and BACT) needed for attainment (section 172(c)(6)); and, (iv) contingency measures 

(section 172(c)(9)).

Additionally, section 189(b)(1) requires that “in addition” to the attainment plan 

requirements specific to Serious areas, states must also meet all Moderate area attainment 

plan requirements. The EPA interprets the statutory language of section 189(b)(1) to 

require states with areas that are reclassified to Serious to meet Moderate area attainment 
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plan requirements, including any areas that the EPA reclassifies through rulemaking 

under its discretionary authority, even if that occurs before the area has met all of its 

Moderate area attainment plan requirements.162

The remainder of this section presents the EPA’s proposed regulatory approaches 

to implement the requirements for attainment plan submissions for Serious areas.

A. Plan Due Dates

The timing of Serious area attainment plan elements is dictated by two provisions 

of the CAA: section 189(b)(2) for certain subpart 4 elements and section 172(b) for 

subpart 1 elements not superseded by subpart 4 requirements. Section 189(b)(2) 

addresses the due dates for Serious area attainment demonstrations due under section 

189(b)(1)(A) and provisions for BACM and BACT implementation under section 

189(b)(1)(B). Specifically, section 189(b)(2) stipulates two alternative schedules for 

states to submit Serious area attainment demonstrations, depending upon the statutory 

authority invoked by the EPA to reclassify the area from Moderate to Serious. For an area 

reclassified to Serious by operation of law under section 188(b)(2) upon a determination 

by the EPA that the area failed to attain the relevant NAAQS by the applicable Moderate 

area attainment date, a state must submit a new attainment demonstration for the area no 

later than 18 months after reclassification. For an area reclassified to Serious pursuant to 

the agency’s discretionary authority provided under section 188(b)(1), a state must 

submit a new attainment demonstration no later than 4 years after reclassification of the 

 

162 See Vigil v. Leavitt, 366 F.3d 1025, amended at 381 F.3d 826 (9th Cir. 2004).
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area.163 For all Serious nonattainment areas, section 189(b)(2) requires a state to submit 

within 18 months of an area’s reclassification “provisions to assure that the best available 

control measures [BACM] for the control of PM10 shall be implemented no later than 4 

years after the date the area is classified (or reclassified) as a Serious Area.”

By contrast, section 172(b) provides the EPA discretion to set a due date for 

subpart 1 attainment plan elements that is no later than 3 years after designation of the 

area. In the Addendum, the EPA interpreted the date of reclassification of an area to 

Serious to be analogous to the date of designation of the area to nonattainment 

generally.164 If the EPA selects the proposed option, discussed later in this section, to 

adopt this convention, the subpart 1 attainment plan elements of provisions to 

demonstrate RFP, emissions inventories, additional control measures beyond BACM and 

BACT needed for expeditious attainment of the PM2.5 NAAQS, and contingency 

measures could in theory be due as late as 3 years after reclassification of an area to 

Serious. For the reasons discussed below, the EPA believes that it is necessary to 

harmonize the submission dates of the various elements of a Serious are attainment plan 

for the PM2.5 NAAQS to provide for more effective evaluation of such attainment plan 

submissions by states, the EPA and members of the general public.

As with Moderate area attainment plans consisting of both subpart 1 and 4 

elements, the EPA presumes that simultaneous development and submission of most, if 

not all, of the Serious area attainment plan elements will be most effective, both for the 

 

163 Section V of this preamble provides a more detailed discussion of the process for 

reclassifying areas with severe nonattainment problems to Serious.

164 59 FR 41998 at 42015.
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state in developing the plan and for the EPA in reviewing the state’s submission, given 

the interplay between all plan elements in the formation of a successful control strategy 

for the area. Just as importantly, a complete attainment plan submission facilitates the 

general public’s review of the entire control strategy adopted by the state. Therefore 

where there is ambiguity in the statutory provisions, the EPA is proposing one or more 

approaches to schedule submission of the various elements of Serious area attainment 

plans in a way that will facilitate better development and evaluation of such attainment 

plan submissions. The EPA’s proposed options for due dates for specific elements of a 

Serious area attainment plan are described below.

1. Area reclassified to Serious after failing to attain the PM2.5 NAAQS

If the EPA reclassifies a Moderate area to Serious because of a failure to attain the 

relevant NAAQS by the applicable attainment date, section 189(b)(2) requires that the 

state must submit both the attainment demonstration for the area and provisions to ensure 

timely BACM and BACT implementation to the EPA within 18 months after 

reclassification. Because an up-to-date base year emissions inventory, required under 

section 172(c)(3), will serve as the foundation of a state’s BACM and BACT 

determination, and additional control measures (beyond BACM and BACT) that are 

necessary for expeditious attainment of the PM2.5 NAAQS as required under section 

172(c)(6) will need to be identified in order to complete the control strategy for the area, 

the EPA proposes that both the base year inventory and additional control measures 

(beyond BACM and BACT) needed for expeditious attainment must also be submitted 

within 18 months after reclassification of the area to Serious by operation of law.
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The EPA also proposes and seeks comment on two possible due dates for the 

remaining Serious area attainment plan elements for areas that failed to attain the 

NAAQS by the applicable Moderate area attainment date. Those plan elements are 

provisions for RFP, quantitative milestones and contingency measures. The first proposed 

due date for these remaining Serious area attainment plan elements would be no later than 

18 months after reclassification of the area, consistent with the due date for the plan 

elements already described above. As noted above, the EPA maintains that requiring 

states to submit all elements of an attainment plan by the same date is reasonable because 

it allows for a complete review of the state submission by the EPA, regulated entities, and 

the general public, and it also may prove most efficient for states. See proposed 40 CFR 

51.1003(b)(2)(ii).

The other proposed due date for the remaining elements would be 3 years 

following reclassification to Serious, which would be consistent with guidance the EPA 

provided in the Addendum specific to the due date for contingency measures for Serious 

areas.165 This guidance references the EPA’s discretion under section 172(b) to establish 

due dates up to 3 years after designation for attainment plan elements required under 

section 172(c), which also include RFP provisions. Subpart 4 meanwhile requires 

quantitative milestones to demonstrate RFP but does not specify a due date for submitting 

such milestones as part of the attainment plan for the area (as separate and distinct from 

the clear statutory requirements related to demonstrating compliance with those 

milestones established in the attainment plan). When taken together, the EPA believes 

 

165 Ibid. at 42015.
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that these statutory provisions may be read to permit a state to submit these three 

elements of the plan as late as 3 years after reclassification of the area. While the EPA 

does not believe that such a reading is as logical as the agency’s first proposed approach, 

the EPA seeks comment on this alternative proposed approach to setting due dates for a 

state to submit an RFP plan, quantitative milestones and contingency measures for a 

Serious area reclassified under the EPA’s mandatory authority.

2. Area reclassified to Serious due to an inability to practicably attain the NAAQS

by the statutory Moderate area attainment date

If the EPA determines that a Moderate area cannot practicably attain the relevant 

NAAQS by the applicable attainment date and reclassifies the area to Serious pursuant to 

its discretionary authority under section 188(b)(1), section 189(b)(2) requires the state to 

submit provisions to ensure timely implementation of BACM and BACT to the EPA 

within 18 months after reclassification. As stated earlier, because an up-to-date emissions 

inventory serves as the foundation for a state’s BACM and BACT determination and 

pursuant to the authority granted to the EPA under section 172(b), the EPA proposes that 

the state must meet the emissions inventory requirement under section 172(c)(3) also 

within 18 months after reclassification of the area by submission of an up-to-date 

emissions inventory.

With respect to the attainment demonstration requirement for Serious areas 

reclassified pursuant to section 188(b)(1), section 189(b)(2) allows the state up to 4 years 

after reclassification to submit a new attainment demonstration for an area reclassified to 

Serious because it cannot practicably attain the PM2.5 NAAQS by the applicable 

Moderate area attainment date. This due date could generally be appropriate, 
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notwithstanding the related issues discussed in the following paragraphs, if the EPA 

finalizes an approach for determining the overall control strategy for the area in which 

BACM and BACT are identified independent of the attainment demonstration for the 

area (see proposed option #1 for BACM and BACT determinations described in Section 

VI.D of this preamble). 

However, the EPA is also proposing an alternative approach for determining the 

control strategy for a Serious area, under which BACM and BACT and additional 

feasible measures would be identified in conjunction with the attainment demonstration 

for the area (see proposed option #2 for BACM and BACT determinations described in 

Section VI.D of this preamble). Under such an approach, the EPA proposes that the due 

date for the Serious area attainment demonstration would be no later than 18 months after 

reclassification if the EPA finalizes its proposed option #2 for determining BACM and 

BACT for the area, as the attainment demonstration would be necessary in order for the 

EPA and the public to determine whether the control strategy identified for the area is 

adequate, and the statute requires that a state submit its BACM provisions within 18 

months after reclassification of an area.

With respect to other elements of a Serious area attainment plan, under the EPA’s 

prior interpretation as described in the Addendum, the EPA had suggested that states 

could submit contingency measures no later than 3 years after reclassification of an area 

to Serious because of the language of section 172(b).

166 The EPA believes it may be 

appropriate to extend a similar approach to establishing due dates for some other 

 

166 Ibid. at 42015.
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attainment plan elements required under subpart 1. Therefore, the EPA proposes to 

provide a state with the maximum time permitted under section 172(b) – 3 years from the 

date of reclassification of the area – to submit the following plan elements: provisions to 

demonstrate RFP, other control measures (beyond BACM and BACT) needed to bring 

the area into expeditious attainment, and contingency measures. The EPA proposes that 

quantitative milestones, required under subpart 4 but linked to RFP which is required 

under subpart 1, would also be included with the plan elements due 3 years following 

reclassification. 

The EPA believes that this proposed due date for certain attainment plan elements 

required under subparts 1 and 4 would be most appropriate if finalized in conjunction

with proposed option #2 for BACM and BACT, which would require the state to submit 

the attainment demonstration for the area within 18 months after reclassification of the 

area to Serious. However, in the event the EPA finalizes proposed option #1 for 

determining BACM and BACT for a Serious nonattainment area independent of the 

attainment demonstration for the area, the attainment demonstration for the area would be 

due no later than 4 years after the date of reclassification of the area to Serious. Given the 

integral role that the attainment demonstration plays in helping to identify additional 

feasible measures (beyond BACM and BACT) that an area may need to attain the 

relevant standard expeditiously (and which are required under section 172(c)(6)), to 

calculate emissions reductions needed on an annual basis to demonstrate RFP, and to 

calculate the emissions reductions that contingency measures need to achieve and 

identify what controls could constitute such measures, the EPA is proposing and seeking 

comment on an alternative submittal deadline for provisions for RFP and quantitative 
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milestones, additional control measures needed for expeditious attainment, and 

contingency measures that would align their due date with the statutory Serious area 

attainment demonstration due date, no later than 4 years from the date of reclassification.

See proposed 40 CFR 51.1003(b)(2)(i). The EPA believes that coordinating submission 

of attainment plan elements so that they may be developed and reviewed together can 

prove most efficient for the submitting state, the EPA, and the general public, and 

therefore this proposed alternative is the agency’s preferred approach. However, the EPA 

seeks comment on all of its proposed due date options for the various elements of a 

Serious area attainment plan.

B. Emissions Inventory Requirements

1. What emissions inventory requirements apply to Serious area attainment 

plans?

As with PM2.5 nonattainment areas classified as Moderate, Congress did not 

create a specific emissions inventory requirement in subpart 4 that would supersede the 

emissions inventory requirement under subpart 1 for Serious areas. Thus, the statutory 

emissions inventory requirements that apply for Serious area attainment plans continue to 

be those of section 172(c)(3), which explicitly requires “a comprehensive, accurate, and 

current inventory of actual emissions of the relevant pollutants” in the nonattainment 

area. In addition, the specific attainment plan requirements for the PM2.5 NAAQS set 

forth in section 189(a) and associated modeling requirements make an accurate and upto-date emissions inventory a critical element of any viable attainment plan. Finally, the 

additional attainment plan requirements for the PM2.5 NAAQS for Serious areas 
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contained in subpart 4 at section 189(b) have additional requirements that affect the 

emissions inventory requirements for Serious areas.167

 As noted earlier in this preamble, states must use the best available, current 

emissions inventory information for attainment plan development, because complete, 

high quality emissions inventory data are essential for the development of an effective 

control strategy. To assist states in preparing complete, high quality inventories, the EPA 

provides guidance for developing emissions inventories in its SIP Emissions Inventory 

Guidance, available at http://www.epa.gov/ttn/chief/eidocs/eiguid/index.html. The EPA 

recommends that states consult this guidance while developing emissions inventories to 

meet requirements for Serious area attainment plans.

2. How do states meet the inventory requirements for the PM2.5 NAAQS for areas 

classified as Serious?

As with Moderate PM2.5 nonattainment areas, neither section 172(c)(3) nor the 

provisions specifically applicable to attainment plans for the PM2.5 NAAQS in subpart 4 

specify how states should meet statutory emissions inventory requirements for Serious 

PM2.5 nonattainment areas. Section 172(c)(3) requires that states submit “a 

comprehensive, accurate, current inventory of actual emissions from all sources of the 

relevant pollutant or pollutants in such area, including such periodic revisions as the 

Administrator may determine necessary to assure that the requirements of this part are 

met” (emphasis added). The EPA interprets this provision to authorize the agency to 

 

167 All definitions described in Section IV.B of this preamble for areas classified as 

Moderate apply in this section.
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require states to revise their base year emissions inventories whenever the state is 

required to submit a new attainment plan because of a change in the nonattainment area’s

status (e.g. failure to attain by the applicable attainment date resulting in reclassification). 

In addition, pursuant to CAA section 301, the EPA has additional authority to promulgate 

regulations as necessary for the implementation of the PM2.5 NAAQS, including 

requirements pertaining to emissions inventories. Accordingly, the EPA is proposing 

specific emissions inventory requirements it considers necessary to effectuate the 

attainment plan requirements of the CAA for the PM2.5 NAAQS. 

Like Moderate areas, there are three key facets of the EPA’s proposed emissions 

inventory requirements, as laid out below: (i) the types of inventories required; (ii) the 

content of these inventories; and, (iii) the timing of submittal of these inventories. The

three facets are addressed in the following paragraphs.

First, the EPA proposes that the same two types of inventories required for 

Moderate areas are also required for Serious areas. While these inventories are the same 

types and names of inventories as for Moderate areas, they must be created specifically 

for Serious area attainment plans in accordance with the applicable Serious area 

requirements. The first type of inventory, the “base year inventory for the nonattainment 

area,” is expressly required by section 172(c)(3). The second type of inventory the EPA is 

proposing to require under section 301(a)(1) is necessary to implement the attainment 

demonstration requirement of section 189(a)(1)(B). This second inventory is called the 

“attainment projected inventory for the nonattainment area.” See proposed 40 CFR 

51.1008(b)(1) and (2).
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Second, the EPA proposes that the content of the inventories will follow the 

content requirements for Moderate area inventories, with one exception needed to meet 

the requirements of section 189(b)(3). For Serious areas, section 189(b)(3) defines a 

separate emissions threshold for major sources in Serious nonattainment areas (70 tpy 

potential to emit of PM10), and this major source threshold is used in 40 CFR part 51, 

subpart A (the AERR) to define which sources must be reported as point sources for 

PM10. This threshold is lower than the 100 tpy potential to emit general requirement for 

major sources of PM10, PM2.5 or one of its precursors that is used for Moderate area 

emissions inventories. Inventories for Serious area attainment plans must include these 

smaller sources as point sources (rather than the nonpoint source category that would 

apply for these in Moderate area plans) using the lower threshold specified in the CAA 

and codified in 40 CFR part 51, subpart A. Also as described above and in 40 CFR part 

51, subpart A, this means that all other smaller stationary sources must be included in the 

inventory as nonpoint sources.

Third, Section VI.A of this preamble describes the EPA’s proposal to require that 

a state submit the base year inventory for a Serious nonattainment area at the same time 

that it submits provisions to implement BACM and BACT on sources in the area (due no 

later than 18 months from reclassification of the area pursuant to section 189(b)(2)) as the 

base year inventory serves as the starting point for conducting a BACM and BACT 

determination. On the other hand, because the attainment projected inventory is more 

closely related to the Serious area attainment demonstration, the EPA believes that a state 

should be required to submit its attainment projected inventory with the attainment 

demonstration for a given Serious area in order to allow effective evaluation of the 
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attainment plan as a whole. Consequently, the EPA is proposing to establish the 

regulatory requirement that attainment projected emissions inventories be submitted at 

the same time as the Serious area attainment demonstration, which would mean no later 

than 18 months after reclassification for areas reclassified after failing to attain the 

NAAQS by the applicable Moderate area attainment date, or no later than 4 years after 

reclassification for areas reclassified by the EPA because the area cannot practicably 

attain the NAAQS by the statutory attainment date if the EPA finalizes proposed option 

#1 for determining BACM and BACT for area. See proposed 40 CFR 51.1008(b)(3) and 

(4). If the EPA finalizes an approach for determining BACM and BACT that links the 

control strategy analysis to the attainment demonstration, then the attainment 

demonstration including the attainment projected emissions inventory would be due no 

later than 18 months after reclassification (i.e., at the same time BACM provisions are 

due under the statute).

The EPA seeks comment on these proposed requirements and due dates for 

emissions inventories for Serious area attainment plans.

C. Pollutants to be Addressed in the Plan

Section III of this preamble includes a detailed discussion about how states should 

address PM2.5 precursors in attainment plans and in the NNSR program for purposes of 

implementing current and future PM2.5 NAAQS. While evaluating sources of direct PM2.5

for BACM and BACT is an implicit requirement in the context of implementing the 

PM2.5 NAAQS under any scenario, the EPA is proposing and seeking comment on 

several options for addressing PM2.5 precursors under the PM2.5 NAAQS implementation 

program. The EPA interprets the requirements of the CAA to allow an air agency to 
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provide a “precursor demonstration” that can seek to make a technical case to the EPA 

that one or more PM2.5 precursors need not be subject to control requirements in a given 

nonattainment area, whether from sources in general or from major stationary sources. 

Section III presented three options describing different proposed approaches to such 

precursor demonstrations, and requested comment on each. The discussion for each 

option described how precursors would be addressed for Moderate areas and for Serious 

areas.

In general terms, the three options can be summarized as follows: 

 Option 1: Two independent analyses: (a) an attainment planning analysis 

demonstrating that control measures for a particular precursor are not needed for 

expeditious attainment, meaning that the precursor can be excluded from measures 

needed to attain as expeditiously as practicable for all types of sources; and, (b) a 

section 189(e) technical demonstration showing that major stationary sources of a 

particular precursor do not contribute significantly to levels that exceed the PM2.5

standard, meaning that the precursor can be excluded from control requirements for 

major sources and from NNSR permitting. For an area reclassified to Serious, the 

state would once again need to evaluate potential control measures for all sources of 

direct PM2.5 and all PM2.5 precursor emissions as part of the control strategy 

determination process (described more fully in Section VI.D of this preamble).

 Option 2: Single analysis demonstrating that all emissions of a particular precursor 

from within the area do not significantly contribute to PM2.5 levels that exceed the 

standard, meaning that control requirements for emissions of the precursor from 

major stationary and area sources, as well as mobile sources, would not be required 
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for expeditious attainment, control requirements for major sources, or for NNSR 

permitting. For an area reclassified to Serious for which a precursor had previously 

been demonstrated to not significantly contribute to PM2.5 levels that exceed the 

standard, the air agency would be required to update the precursor demonstration 

taking into account any relevant information or technical tools that had been 

developed since the initial demonstration was approved, but could still conclude that 

control requirements are not required for Serious area attainment planning if the 

updated demonstration still shows that all source emissions of a precursor do not 

significantly contribute to PM2.5 levels that exceed the standard.

 Option 3: An attainment planning analysis demonstrating that control measures for all 

types of sources of a particular precursor are not needed for expeditious attainment 

also would be deemed to meet the section 189(e) technical demonstration 

requirement, meaning that the state would not need to regulate emissions of the 

particular precursor from major stationary sources under the NNSR permitting 

program or other control requirements for major stationary sources. As under 

proposed precursor Option 1, for an area reclassified to Serious, the state would once 

again need to evaluate potential control measures for all sources of direct PM2.5 and 

all PM2.5 precursor emissions as part of the control strategy determination process 

(see Section VI.D of this preamble). 

The EPA will finalize its approach to PM2.5 precursors and clarify the implications for 

states conducting analyses to determine the appropriate control strategy for a Serious area 

after considering public comment received on this proposal.

D. Attainment Plan Control Strategy
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1. General approach to designing a control strategy for a Serious nonattainment 

area

As noted in Section IV.D of this preamble, the statutory attainment planning 

requirements of subparts 1 and 4 were established to ensure that states meet the following 

goals of the CAA: (i) implement measures that provide for attainment of the PM2.5

NAAQS as expeditiously as practicable, and (ii) adopt emission reduction strategies that 

will be the most effective, and the most cost effective, at reducing PM2.5 levels in 

nonattainment areas. A state has discretion to require reductions from any source inside 

or outside of a PM2.5 nonattainment area (but within the state’s boundaries) in order to 

fulfill its obligation to demonstrate attainment in a PM2.5 nonattainment area as 

expeditiously as practicable, in addition to having an obligation to meet the statutory 

requirements for specific control measures on sources located within a nonattainment 

area (e.g., BACM and BACT). A state may need to require emissions reductions on 

sources located outside of a PM2.5 nonattainment area if such reductions are needed in 

order to provide for expeditious attainment of the PM2.5 NAAQS.

The following sections describe the EPA’s proposed approach for a state to follow 

in order to identify and select the complete suite of measures needed for an approvable 

attainment plan submission for a Serious PM2.5 nonattainment area. 

2. Identification and selection of BACM and BACT and additional feasible 

measures

a. Statutory requirements and existing guidance. As discussed earlier, a state must 

prepare a new attainment plan for any Moderate area reclassified to Serious. Such a plan 

must include provisions to implement BACM on sources in a Serious nonattainment area, 
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as provided by section 189(b)(1)(B), no later than 4 years after reclassification. Under 

section 189(b)(2), a state has 18 months following reclassification to submit these BACM 

provisions.

Section 189(b)(1)(B) refers only to BACM, but the EPA has long interpreted this 

term to include BACT, just as the analogous term for RACM includes RACT for 

Moderate areas. The legislative history for the 1990 Amendments to the CAA supports 

this interpretation, as the EPA has explained in past guidance.168 Additionally, the 

requirement for BACT in the context of PM2.5 NAAQS implementation in nonattainment 

areas is separate and distinct from the requirement for BACT under the Prevention of 

Significant Deterioration (PSD) permitting program for new stationary sources in areas 

designated as attainment or unclassifiable for the PM2.5 NAAQS. As described later in 

this section, the process and criteria that states have historically used to determine BACT 

for PSD have been applied to determine BACT for PM10 NAAQS implementation, but 

these requirements are otherwise unrelated.

Longstanding guidance in the General Preamble and Addendum, together with 

past practice associated with implementing the PM10 NAAQS under subpart 4, have 

helped to establish a general approach for states and the EPA to determine BACM and 

BACT for Serious PM10 nonattainment areas. This approach has served as the basis for 

developing a more stringent control strategy for a Serious PM10 nonattainment area than 

that developed for such area when it was classified as Moderate. Indeed, as BACM and 

BACT are required to be implemented when a Moderate nonattainment area is 

 

168 Ibid. at 42008-09.
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reclassified as Serious due to its actual or projected inability to attain the relevant 

NAAQS by the Moderate area attainment date through the implementation of

“reasonable” measures, it is logical that “best” control measures should represent a more 

stringent and potentially more costly level of control.169 The level of stringency generally 

refers to the overall level of emissions reductions of a control measure or technology, or 

of such measures and technologies combined.

Congress first defined BACT in CAA section 169(3) for the PSD permitting 

program as: “an emission limitation based on the maximum degree of reduction of each 

pollutant... which the permitting authority, on a case-by-case basis, taking into account 

energy, environmental, and economic impacts and other costs, determines is achievable 

for such facility through application of production processes and available methods, 

systems, and techniques...”

In the Addendum, the EPA provided guidance concerning the requirements for 

BACM and BACT for Serious area attainment plan requirements for the PM10

NAAQS.170 The EPA discussed in the Addendum that when Congress amended the CAA, 

Congress selected the same “best” terminology for PM10 nonattainment areas as they did 

for the language selected for the PSD program in 1977. The EPA interpreted this word 

choice at the time to mean that PSD BACT and PM10 nonattainment area BACM should 

be generally analogous in definition and implementation, but with some differences due 

 

169 Ibid. at 42009.

170 Ibid. at 42009.
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to different end policy goals between the PSD and nonattainment area programs.171 The 

EPA thus defined BACM for PM10 Serious nonattainment area planning to be the 

maximum degree of emission reduction achievable from a source or source category 

which is determined on a case-by-case basis, considering energy, economic and 

environmental impacts and other costs.172

The EPA has described BACM as a generally independent requirement, to be 

determined without regard to the specific attainment analysis (i.e., attainment 

demonstration) for the area.173 The EPA established that such an interpretation is in 

accordance with the structural scheme of the CAA, which by its definition requires that 

when an area is classified as Serious, BACM are implemented in addition to RACM. 

Because of the two types of measures employed, the EPA found it reasonable in the past 

to interpret the statute as requiring a different analysis for determining BACM, i.e., that 

while RACM has been interpreted as those reasonable measures necessary to bring a 

nonattainment area into expeditious attainment, BACM has been interpreted as those 

measures that best control sources’ emissions without regard to whether such measures 

 

171 Ibid. at 42010. “EPA will interpret PSD BACT and PM–10 BACM as generally 

similar because, despite the similarity in terminology, certain key differences exist 

between control measures applicable in the PSD and PM–10 serious nonattainment area 

programs. The BACT under the PSD program applies only in areas already meeting the 

NAAQS, while PM–10 applies in areas which are seriously violating the NAAQS. The 

difference in policy goals, arguably, suggests that the PM–10 BACM control standard 

should be more stringent than that for PSD BACT. . . . EPA considers it reasonable to use 

the approach adopted in the PSD BACT program as defined in section 169(3) of the Act 

as an analogue for determining appropriate PM–10 nonattainment control measures in 

serious areas, while at the same time retaining the discretion to depart from that approach 

on a case-by-case basis as particular circumstances warrant.”

172 Ibid.

173 Ibid. at 42011.
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are needed for purposes of attainment of the relevant NAAQS in the area. The view that 

BACM and BACT measures are generally independent of the attainment needs of the 

area is also consistent with the statutorily specified submission date for BACM and 

BACT control measures, versus the statutorily specified submission date for the 

attainment demonstration for Serious areas. Specifically, states with Serious 

nonattainment areas must submit BACM and BACT measures within 18 months of 

reclassification of areas to Serious, whereas they are given up to 4 years from 

reclassification to submit the attainment demonstration for such areas.

In addition, the EPA has historically provided an exemption from BACM and

BACT for source categories that contribute only de minimis levels to ambient PM10

concentrations in a Serious nonattainment area. In the Addendum, the EPA proposed that 

all sources in a Serious area are subject to BACM unless “the state adequately 

demonstrates that a particular source category does not contribute significantly to 

nonattainment of the NAAQS.”174 Because the language regarding BACM 

implementation in section 189(b)(1)(B) of the CAA requires “provisions to assure that 

best available control measures (BACM) for the control of PM10 shall be 

implemented...” without stating that “all” BACM must be implemented, the EPA has 

interpreted this language as providing the EPA discretion to exclude from BACM 

requirements source categories that do not contribute significantly to an area’s 

nonattainment status. Additionally, in the Addendum, the EPA argued that based on the 

 

174 Ibid. at 42011.
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decision in Alabama Power Co. v. Costle, the courts have supported the interpretation 

that sources that contribute negligibly to an area’s nonattainment status can be excluded 

from regulation.175 The EPA further indicated that the same criteria used in the NSR 

permitting program at the time to determine if a source category contributes significantly 

to an area’s nonattainment status should apply, such that a source category would be 

considered a significant contributor to an area’s nonattainment status if its emission 

contribution was expected to exceed 5 μg/m3for the 24-hour PM10 NAAQS (150 μg/m3

at the time), or 1 μg/m3for the annual PM10 NAAQS (50 μg/m3at the time).

A discussion of the EPA’s existing process and criteria for determining BACM 

and BACT for Serious PM10 nonattainment areas and the agency’s proposed options for 

defining the criteria by which a state must determine BACM and BACT and additional

feasible measures for a Serious PM2.5 nonattainment area are presented in the sections 

that follow.

In accordance with the PM10 guidance in the Addendum, the EPA has applied a 

four-step process for states to use to identify measures that constitute BACM or BACT 

for sources located in PM10 Serious areas. The four-step BACM selection process was 

designed to take into account the local facts and circumstances and the nature of the air 

pollution problem in a given nonattainment area. The BACM determination process for 

PM10 Serious nonattainment areas has historically entailed: (i) developing a

comprehensive inventory of sources and source categories of directly emitted PM10 and 

PM10 precursors; (ii) evaluating source category impact and determining if any source 

 

175 See Alabama Power Co. v. Costle, 636 F.2d 323, 360–61 (D.C. Cir. 1979).
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categories are de minimis and thus do not need further evaluation for emission controls; 

(iii) evaluating alternative control measures available for significant source categories for 

technological feasibility; and, (iv) evaluating costs (i.e., economic feasibility) of the

technologically feasible control measures.176, 177 These steps are described more fully 

below. 

Step 1: Inventory sources and precursors. As with any control strategy analysis 

for a nonattainment area, the EPA recommended that a state begin with a current 

emissions inventory as the first step toward determining what constitutes BACM or 

BACT for a particular Serious PM10 nonattainment area. The EPA expected that a state 

would start with the base year emissions inventory submitted with the Moderate area 

attainment plan for the area as required under section 172(c)(3), and update it as 

necessary to reflect new source construction, facility shutdowns, growth in certain source 

categories, and any other relevant changes. The EPA reiterated in the Addendum that the 

 

176 For additional information, see ibid. at 42012–13.

177 For examples of how states have applied these steps and criteria for Serious PM10

nonattainment areas and how the EPA has evaluated them, see generally Approval and 

Promulgation of Implementation Plans for California—San Joaquin Valley PM–10 

Nonattainment Area; Serious Area Plan for Attainment of the 24-Hour and Annual PM–

10 Standards, 69 FR 5412 (February 4, 2004); Approval and Promulgation of 

Implementation Plans for California—San Joaquin Valley PM–10 Nonattainment Area; 

Serious Area Plan for Attainment of the 24-Hour and Annual PM–10 Standards, 69 FR 

30006 (May 26, 2004); Approval and Promulgation of Implementation Plans for Arizona; 

Maricopa County PM – 10 Nonattainment Area; Serious Area Plan for Attainment of the 

24-Hour and Annual PM–10 Standards, 73 FR 45542 (August 14, 2008); Approval and 

Promulgation of Implementation Plans; Arizona—Maricopa County PM–10 

Nonattainment Area; Serious Area Plan for Attainment of the Annual PM–10 Standard, 

65 FR19964 at 19972 (April 13, 2000). 
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emissions inventory for the area must identify both nonanthropogenic and anthropogenic 

emissions sources.178

Step 2: Evaluate source category impact. The next step in the BACM analysis for 

PM10 Serious areas was for the state to identify source categories having significant (i.e., 

non-de minimis) impacts on air quality in the Serious area. The EPA suggested in the 

Addendum that receptor modeling, screening modeling, or refined dispersion modeling 

would likely be necessary to identify key source categories, which the state may have 

performed during the development of the Moderate area attainment plan.179

Step 3: Evaluate alternative control techniques. Once the significant source 

categories were identified for a PM10 Serious nonattainment area, the state was expected 

to evaluate the technological and economic feasibility of control measures “discussed in 

the BACM guidance documents and other relevant materials for all source categories 

impacting the nonattainment area except those with a de minimis impact considering 

emission reductions achieved with RACM.”180 Control measures were supposed to be 

expanded to include options not previously considered RACM as well as consider 

additional measures not previously evaluated in the RACM analysis.

Under the Addendum, the test for determining technological feasibility could 

differ depending on the type of source category evaluated. For area sources, the EPA’s 

 

178 59 FR 41998 at 42012.

179 See ibid.

180 Ibid. at 42012. At the time of publication of the Addendum, the EPA had already 

issued BACM guidance documents pursuant to section 190 for residential wood 

combustion, prescribed burning, and fugitive dust. The agency referred to these 

documents as establishing the control measures that a state should consider, at a 

minimum, as BACM for those PM10 sources in Serious PM10 nonattainment areas.
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guidance suggested that technological feasibility depended on the ability to alter the 

characteristics that affect emissions from the sources, such as the size or extent of the 

area sources and operation procedures. The EPA’s guidance suggested that for specific 

point sources, technological feasibility should consider factors such as layout of the plant, 

space available to make changes in the plant, energy requirements, operating procedures, 

and materials used, among others.181

Step 4: Evaluate costs of control. The EPA’s previous guidance recommended 

that a control should be considered economically feasible by the state when “the control 

technology in question has previously been implemented at other sources in a similar 

source category without unreasonable economic impacts.”182 Feasibility of public funding 

for BACM could have been a consideration that states evaluated for all of the 

technologically feasible control measures determined in Step 3. Other costs that could be 

considered included capital costs, operating and maintenance costs, and the cost 

effectiveness of a particular control measure or technology.183

The EPA believes that the difference between RACM and BACM primarily lies 

in the extent of the actual emissions reductions achieved through the application of a

given suite of candidate measures. For example, a state may have deemed a candidate 

RACM or RACT measure economically infeasible because its cost effectiveness (dollar 

per ton of pollutant reduced) was high relative to other measures, but the same measure 

 

181 Ibid. at 42013 (discussing in detail factors which affect the selection of mobile, area, 

and point source alternative control techniques for particulate matter).

182 Ibid. at 42013.

183 EPA Air Pollution Control Cost Manual, Sixth Edition, (EPA/452/B–02–001), 

January 2002 (explaining how to determine costs under a BACT analysis).
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could qualify as BACM if, for the increased cost, it would ultimately provide substantial 

PM2.5 attainment benefits. An example of RACM might be to implement a particular 

control in a limited way, while BACM could mean a more widespread implementation of 

that same measure, even though wider implementation would incur greater cost. In the 

PM10 context, states and the EPA have determined that BACM have sometimes been 

measures that were first implemented as RACM, but were then later implemented on a 

broader scale as BACM in the nonattainment area after it was reclassified as Serious.184

While the proposed approaches and criteria for identifying appropriate control 

measures for a Serious area are necessarily different than for a Moderate area, it is 

important to note two similarities: first, that the EPA interprets the requirement under 

section 172(c)(6) for a state to adopt “other measures” needed for attainment to apply to 

sources located inside and outside of any PM2.5 nonattainment area (but within the state’s 

boundaries), whether the area is classified as Moderate or Serious; and, second, similar to 

the RACM requirement for Moderate nonattainment areas under subpart 4, section 

189(b)(1)(B) requires that BACM must be implemented no later than 4 years after a 

Moderate area is reclassified to Serious. 

Taking these two statutory provisions together, the EPA proposes that the other 

measures required under section 172(c)(6) must include “additional feasible measures,” 

 

184 59 FR 41998 at 42014. The Addendum provides one example of RACM to reduce 

PM10, to “[p]ave 4 miles of unpaved city streets.” Ibid. BACM for PM10 for the same 

nonattainment area could later mean to “[p]ave 10 miles of the most heavily-traveled, 

unpaved county roads.” Ibid. Therefore, the measure itself was not necessarily changed, 

but the extent to which the measure was implemented was significantly expanded. Such a 

measure would also contribute to more expeditious attainment of the NAAQS. 
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which would be those measures and technologies that otherwise meet the criteria for 

BACM and BACT but that can only be implemented in whole or in part beginning 4 

years after reclassification of an area, but no later than the statutory attainment date for 

the area. See proposed 40 CFR 51.1000. Such measures would necessarily be 

implemented on sources in the nonattainment area, and a state would only be required to 

implement them if they were needed in addition to BACM and BACT to bring the area 

into expeditious attainment. The state must also assess whether there are other control 

measures that it can implement to control sources within the state but outside the 

nonattainment area that contribute to the PM2.5 nonattainment status of the area in order 

to bring the area into attainment as expeditiously as practicable, and may consider 

existing measures that, applied more extensively, could meet the more stringent criteria 

for control measures that must be adopted to bring a Serious nonattainment area into 

expeditious attainment.

These “additional feasible measures” would be analogous to the “additional 

reasonable measures” in the proposed RACM and RACT analysis process, which are 

technologically and economically feasible measures that cannot qualify as RACM or 

RACT because they cannot be implemented within 4 years of designation of a Moderate 

nonattainment area. Under either of the two proposed approaches for determining BACM 

and BACT for sources in a Serious nonattainment area descrfibed later in this section, a 

state would identify additional feasible measures as part of the BACM and BACT 

determination process, just as additional reasonable measures would be identified as part 

of the state’s RACM and RACT determination process.
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The EPA recognizes that only a nonattainment area that is reclassified under the 

agency’s discretionary authority might have sufficient time between the required date for 

implementing BACM and BACT and the statutory Serious area attainment date to 

implement additional measures beyond BACM and BACT. BACM and BACT must be 

implemented no later than 4 years after reclassification of the area; areas reclassified to 

Serious because they cannot practicably attain the relevant NAAQS by the applicable 

attainment date could potentially have significantly more than 4 years between the date of 

reclassification and the statutory Serious area attainment date, during which time the area 

could continue to implement additional measures to bring the area into attainment. By 

way of illustration, for areas designated in the first round of designations for the 2012 

PM2.5 NAAQS, the statutory Moderate area attainment date will be no later than 

December 31, 2021. If a state submits a Moderate area attainment plan by the statutory 

attainment plan due date (18 months after designation, or in this example, October 2016) 

and the plan demonstrates that the area cannot practicably attain the NAAQS by 

December 31, 2021, then the EPA has a statutory duty to reclassify such an area within 

18 months of the attainment plan due date (i.e., by April 2018). The statutory Serious 

area attainment date would be the end of the tenth year following designation, or 

December 31, 2025. In such a case, the state would need to implement BACM for the 

area within 4 years of reclassification, or by April 2022, leaving over 3.5 years between 

the statutory deadline for implementing BACM and the statutory attainment date for the 

area. The EPA’s proposal to require the state to identify and adopt additional feasible 

measures for the area would mean that the state would need to identify those control 

measures and technologies that are feasible (according to the proposed BACM and 
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BACT criteria described later in this section) and that can be implemented between April 

2022 and December 2025. The EPA expects that while such a long span of time may be 

available only to a very few Serious nonattainment areas, it would be appropriate to 

require such areas to implement measures in addition to BACM and BACT if, taken 

together, they can advance the attainment date for the area by at least 1 year. The EPA 

seeks comment on its proposal to require additional feasible measures for Serious 

nonattainment areas as described here.

b. Proposed approaches for determining BACM and BACT and additional 

feasible measures for Serious PM2.5 nonattainment areas

The EPA proposes and seeks comment on two approaches for a state to meet the 

statutory control requirements that apply for Serious nonattainment areas. The EPA is 

first proposing an approach consistent with prior guidance summarized in the preceding 

section of this preamble which would center on determining BACM and BACT and 

additional feasible measures “generally independent” of whether such measures are 

needed for expeditious attainment of the relevant NAAQS in a Serious PM2.5

nonattainment area. Under this first proposed approach, states would have the option, 

with the proper evidence and justification, to eliminate de minimis source categories from 

consideration for controls. 

The EPA’s second proposed approach would require states to identify BACM and 

BACT and additional feasible measures simply within the context of what is necessary to 

bring an area into attainment as expeditiously as practicable. In other words, the second 

proposed option would take a different approach to determining Serious area control 

measures from the approach included in prior EPA guidance, in that it would allow states 
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not to impose specific measures that would otherwise be BACM or BACT (or additional

feasible measures) in the area, if those measures would not be necessary to bring the area 

into attainment with the relevant NAAQS by the statutory attainment date, and the 

collective emissions reductions from such measures would not be sufficient to advance 

the attainment date by at least 1 year in the area. A discussion of the proposed options 

follows.

i. Proposed option #1. The EPA seeks comment on a proposed approach to 

maintain, with some modifications, the existing approach to determining BACM and 

BACT for Serious PM10 nonattainment areas to BACM and BACT determinations for

Serious PM2.5 nonattainment areas. Under this approach, a state would be required to 

determine BACM and BACT and additional feasible measures for a Serious PM2.5

nonattainment area independent of an analysis of the specific attainment needs of the 

Serious area; in other words, the BACM and BACT analysis would need to be conducted 

without regard to whether all such controls are needed to bring the area into expeditious 

attainment. Keeping in mind that the overall objective of the implementation of BACM 

and BACT and additional feasible measures is to bring a Serious PM2.5 nonattainment 

area into attainment as expeditiously as practicable, this option would continue to provide 

that the test for BACM puts a “greater emphasis on the merits of the measure or 

technology alone,” rather than on “flexibility in considering other factors,” in contrast to 

the approach for determining RACM and RACT described in both the EPA’s past 

guidance and in this proposal in Section IV.D.185 This option #1 is consistent with the 

 

185 Ibid.
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statutory provisions governing the timing for submission of BACM and BACT measures 

versus the timing for attainment demonstrations for Serious areas. By interpreting the 

statutory requirement for BACM and BACT for Serious PM2.5 nonattainment areas as a 

requirement that a state must meet independent of the attainment planning needs of the 

area, the EPA would not consider such requirement to be a “planning” requirement tied 

to the actual attainment status of the area, and thus would not suspend such a requirement 

in the event the agency determines that a Serious area is attaining the relevant PM2.5

NAAQS and in turn grants a clean data determination for the area.186

Under the EPA’s first proposed approach, a state would be required to follow a 

multi-step process similar to the existing BACM process for PM10 (outlined earlier in this 

section) to identify and select control measures and technologies more stringent than 

RACM and RACT and additional reasonable measures for non-de minimis source 

categories in the nonattainment area. This process would involve analyzing the impact of 

the different source categories identified in the up-to-date base year emissions inventory 

for the area to identify those with a significant contribution to the area’s PM2.5

concentrations. Any source categories found not to have such an impact would be 

considered de minimis and therefore exempt from further consideration. The specific 

steps the EPA is proposing for this approach are explained below. See proposed 40 CFR 

51.1010(a) for proposed option #1.

Step 1: Update base year emissions inventory for the area.

 

186 For a complete discussion of the EPA’s Clean Data Policy and the EPA’s proposal for 

applying this policy for purposes of implementing the PM2.5 NAAQS, see Section IX.B 

of this preamble.
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The first step under this proposed approach would be for the state to develop a 

detailed emissions inventory of the various sources and source categories that emit direct 

PM2.5 and PM2.5 precursors in the Serious area. This inventory should be the most 

comprehensive and accurate inventory available. The EPA expects that the work for this 

step would be completed in order to meet the emissions inventory requirements for 

Serious area plans as described in Section VI.B, and would start with reviewing and 

updating the emissions inventory submitted as part of the Moderate area attainment plan 

for the area.

Step 2: Evaluate source category impacts. As with BACM for PM10, the EPA 

proposes to allow states to exempt from further consideration de minimis source 

categories in Step 2 of the agency’s first proposed approach for determining BACM and 

BACT for a Serious PM2.5 nonattainment area. The EPA proposes to apply the same 

overarching test for identifying de minimis source categories as that described in the 

Addendum.187 That is, if a state can demonstrate that a particular source category does 

not contribute significantly to nonattainment of the PM2.5 NAAQS after the application of 

any RACM or RACT controls on the sources in the source category, then the state may 

eliminate the source category from further consideration for BACM or BACT.188 A state 

would be required to evaluate for BACM and BACT controls all other sources in the 

nonattainment area in source categories that do not qualify as de minimis.

 

187 59 FR 42011.

188 Ibid. See also Alabama Power v. Costle, 636 F.2d 323, 360–61 (D.C. Cir. 1979). 
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This option could be beneficial for some states that may already exclude de 

minimis PM10 source categories from BACM in Serious PM10 nonattainment areas or that 

may exclude de minimis PM2.5 source categories from RACM and RACT and additional 

reasonable measures in Moderate PM2.5 nonattainment areas. As discussed earlier, a state 

may rely on receptor or dispersion modeling conducted for the area as part of its 

Moderate area attainment plan. Alternative or additional modeling, including screening 

modeling, or filter analysis may also be necessary to identify significant contributors to 

PM2.5 levels in the area. More discussion on the EPA’s proposal regarding how to 

evaluate source category impacts and identify those that are de minimis can be found in 

Section IV.D of this preamble. The EPA notes that a state may face the same challenges 

in establishing de minimis source categories for PM2.5 sources in a Serious nonattainment 

area as it did in establishing de minimis source categories for PM2.5 sources when the area 

was classified as Moderate. Therefore, the EPA seeks comment on its proposed options, 

described in Section IV.D, for defining source categories and determining the appropriate 

threshold for de minimis emissions. The EPA requests that commenters submit any 

relevant data or analyses to support their comments. In the absence of compelling 

evidence to support establishing a nationally-applicable “bright line” threshold for 

defining a de minimis source category for purposes of implementing the PM2.5 NAAQS 

in a Serious nonattainment area, the EPA would apply a presumptive approach allowing a 

state to apply its own reasoned judgment to determine whether a particular source 

category should be considered de minimis in the event the EPA finalizes proposed option 

#1 for BACM and BACT determinations.
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Step 3: Identify existing and potential control measures. After evaluating source 

category impacts to eliminate de minimis source categories from further consideration, 

the state would identify all existing and potential measures (including those measures that 

were rejected in the RACM and RACT determination and additional new potential 

measures) for reducing emissions from the remaining (i.e., non-de minimis) source 

categories listed in the latest base year emissions inventory for the area. For purposes of 

identifying new measures to consider in its BACM and BACT analysis, the EPA 

proposes to require that the state conduct a survey of other nonattainment areas for the 

PM2.5 NAAQS and other NAAQS (i.e., PM10, ozone, SO2 and NOx) both in the same 

state and in other states to identify potential control measures that other air agencies are 

implementing, and the state must incorporate such measures into the list of potential 

control measures for the source categories in the Serious nonattainment area. The EPA 

would expect the state to identify an array of existing and potential new measures at least 

as broad as that identified for the same area as part of the RACM and RACT analysis, in 

order to ensure that the state has a sufficiently expansive and comprehensive set of 

potential measures to evaluate. Therefore, at a minimum the EPA proposes that the list of 

potential measures must include all measures identified as potential control measures for 

the nonattainment area when it was classified as Moderate or, for a given source 

category, one or more alternative control measures or technologies that would control 

emissions even more stringently than the measures and technologies included in the 

RACM and RACT analysis. In this way, the state will begin its BACM and BACT 

determination with a list of potential control options that is as complete and up-to-date as 

possible.
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In addition to identifying existing control measures for sources in a Serious PM2.5

nonattainment area, a state must develop a comprehensive list of potential control 

measures for sources in the area. The EPA’s RACT/BACT/LAER Clearinghouse 

provides a central data base of air pollution technology information that may be highly 

relevant to states seeking information on stationary source control technology that may 

qualify as BACT for PM2.5 NAAQS implementation, and is available online at 

http://cfpub.epa.gov/RBLC/. There are also other resources available to assist states in 

identifying other potential control measures and control technologies for their BACM and 

BACT determinations. The EPA encourages states with Serious PM2.5 nonattainment 

areas to visit the agency’s website to find links to other online sources of information on 

potential control measures for states to consider.189

Specific to potential control measures for mobile source emissions, the EPA’s 

past guidance has indicated that where mobile sources contribute significantly to PM2.5

violations, “the state must, at a minimum, address the transportation control measures 

listed in CAA section 108(f) to determine whether such measures are achievable in the 

area considering energy, environmental and economic impacts and other costs.”190 The 

EPA proposes to retain this guidance and require that a state include for evaluation as 

BACM for mobile sources those measures listed in section 108(f), and the agency seeks 

comment on this specific requirement.

 

189 Links are provided to a number of national, state and local air quality agency sites 

from the EPA’s PM2.5 website: http://www.epa.gov/pm/measures.html.

190 59 FR 41998 at 42013.
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Step 4: Determine whether an available control measure or technology is 

technologically feasible. After developing a list of existing and potential new 

measures to evaluate for BACM and BACT, the state would then need to 

determine the technological feasibility of each identified control measure in light 

of a number of considerations, including each measure’s individual energy and 

environmental impacts.191

(1) Stationary sources. As described under the technological feasibility criteria for 

the control measures analysis for Moderate area attainment plans in Section IV.D, the 

EPA’s prior guidance on factors to consider for judging whether a particular control 

technology is technologically feasible should include a source’s processes and operating 

procedures, raw materials, physical plant layout and potential environmental impacts 

such as increased water pollution, waste disposal and energy requirements. For example, 

the EPA recognizes that the process, operating procedures and raw materials used by a 

source can affect the feasibility of implementing process changes that reduce emissions 

and can also affect the selection of add-on emission control equipment. The feasibility of 

modifying processes or applying control equipment also can be influenced by the 

physical layout of the particular plant, if the physical space available in which to 

implement such changes limits the choices.192

(2) Area and mobile sources. With respect to determining whether a given control 

measure might not be technologically feasible as BACM for an area or mobile source, the 

 

191 Ibid. at 42012. 

192 Ibid. at 42013.
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EPA proposes that a state may consider factors in conducting its analysis that are similar 

to factors the state may have considered during the RACM and RACT determination 

process, such as the social acceptability of the measure, and local circumstances, such as

the condition and extent of needed infrastructure, population size or workforce type and 

habits, which may prohibit certain potential control measures from being implementable. 

However, in the instance where a given control measure has been applied in another 

NAAQS nonattainment area (for PM2.5 or other pollutant), the EPA proposes that the 

state will need to provide a detailed justification for rejecting any potential BACM 

measure as technologically infeasible. Furthermore, if the state identifies a certain control 

measure for area or mobile sources that has been implemented in another nonattainment 

area and may qualify as BACM or BACT for the state’s Serious nonattainment area, the 

state must provide a reasoned justification if it deems it technologically infeasible to 

implement the same control measure to the same extent or magnitude as it was applied in 

the other nonattainment area.

The EPA seeks comment on the factors described above for states to consider 

when evaluating the technological feasibility of a control measure or technology for 

BACM and BACT.

Step 5: Determine whether an available control technology or measure is 

economically feasible. The fifth step under this proposed approach is to evaluate 

the costs of implementing each of the technologically feasible control measures 

and technologies in order to eliminate from further consideration any measures 

determined to be economically infeasible. As discussed elsewhere in this 

proposal, in assessing “best” control measures and technologies, states with 
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Serious PM2.5 nonattainment areas must identify a control strategy for the area 

that overall is more stringent than that identified for the area when the state 

considered only the “reasonableness” of potential control measures. Thus the EPA

is proposing to require states to consider emission reduction measures with higher 

costs per ton when assessing the economic feasibility of BACM and BACT 

controls (and, where applicable, additional feasible measures) as compared to the 

economic feasibility criteria applied in their RACM and RACT analysis (and 

analysis for additional reasonable measures) for the same nonattainment area. 

Indeed, consistent with prior guidance on evaluating costs of a potential BACM 

or BACT control, the EPA maintains that while the economic feasibility of a control 

measure is as important as its technological feasibility under the RACM and RACT 

determination process, economic feasibility is a less significant factor in the BACM and 

BACT determination process. In other words, a state must apply a higher standard for 

eliminating a technologically feasible control measure from further consideration as 

BACM due to cost alone.

In the Addendum, the EPA stated that “for PM10 BACM purposes, it is reasonable 

for similar sources to bear similar costs of emission reduction.”193 Additionally, the EPA 

indicated that “economic feasibility for PM10 BACM purposes should focus upon 

evidence that the control technology in question has previously been implemented at 

other sources in a similar source category without unreasonable economic impacts.”194

 

193 Ibid.

194 Ibid.
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Thus, a state may not eliminate a particular control measure from further consideration as 

potential BACM if similar sources have successfully implemented such a measure. That 

is, a state must at a minimum continue to consider as potential BACM any 

technologically feasible control measures or technologies implemented by similar 

sources.

In addition, the EPA seeks to clarify that a state may not automatically eliminate a 

particular control measure merely because other sources have not implemented the 

measure. In other words, a state must continue to consider technologically feasible 

measures that have not been implemented by similar sources but that can nonetheless 

effectively reduce emissions from the source category in question at a cost that is not 

wholly cost prohibitive. 

As with the EPA’s proposed approach for evaluating economic feasibility of 

potential reasonable measures for Moderate area attainment plans, the EPA proposes that 

for each technologically feasible control measure or technology, a state must evaluate the 

economic feasibility of the measure or control through consideration of the capital costs, 

operating and maintenance costs, and cost effectiveness (i.e., cost per ton of pollutant 

reduced by that measure or technology) associated with such measure or control. While 

the EPA is not proposing a fixed dollar per ton cost threshold for economic feasibility of 

controls identified as potential BACM and BACT, the EPA proposes that the threshold 

should be higher for the BACM and BACT analysis than it was for the RACM and 

RACT analysis for the same nonattainment area. In addition, if a state contends that a 

source-specific control-level should not be established because the source(s) cannot 

afford the control measure or technology that is demonstrated to be economically feasible 
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for purposes of BACM for other sources in its source category, the state must support the 

claim with information regarding the impact of imposing the identified control measure 

or technology on the following financial indicators, to the extent applicable: 

1. Fixed and variable production costs ($/unit);

2. Product supply and demand elasticity;

3. Product prices (cost absorption vs. cost pass-through);

4. Expected costs incurred by competitors;

5. Company profits

6. Employment costs;

7. Other costs (e.g., for BACM implemented by public sector entities).

The EPA seeks comment on the factors described above for states to consider 

when determining whether a control measure or technology is economically feasible as 

BACM or BACT.195

Step 6: Determine the earliest date by which a control measure or technology can 

be implemented in whole or in part. Section 189(b)(1)(B) requires that Serious 

area attainment plans provide for the implementation of BACM no later than 4 

years after reclassification of the area to Serious. As with the EPA’s proposed 

approach to RACM and RACT, the EPA proposes the term “implement” to mean 

that the control measure or technology has not only been adopted into the SIP for 

the area but has also been built, installed and/or otherwise physically manifested

 

195 These long-standing factors were established in EPA guidance in 1992 and are 

applicable to implementation programs for all NAAQS pollutants.See the appendices to 

the General Preamble, 57 FR 18070 (April 28, 1992).
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and the affected sources are required to comply. See proposed 40 CFR 51.1000. 

The EPA thus expects a state with a Serious nonattainment area to take deliberate 

and timely action to implement BACM and BACT in the area. The EPA proposes 

that if a state evaluates a potential BACM or BACT measure and determines that 

it can be implemented only partially within 4 years after reclassification, the state 

must adopt the partial measure as BACM. 

The EPA proposes that a state must identify those technologically and 

economically feasible control measures and technologies that it can implement fully or 

partially within 4 years of reclassification of its Serious PM2.5 nonattainment area. These 

measures will be considered BACM and BACT for the area. “Additional feasible 

measures” would be “best”-level, feasible measures that a state could implement in whole 

or in part on sources in the area sometime after the fourth year following reclassification 

and prior to the statutory attainment date for the area.

ii. Proposed option #2. The second proposed approach for evaluating control 

measures and technologies and determining which qualify as BACM or BACT or 

additional feasible measures for a Serious PM2.5 nonattainment area would directly link

the control strategy determination process with the attainment demonstration for the area, 

allowing a state to eliminate potential measures that are not necessary to demonstrate 

attainment of the relevant NAAQS in the area and would not collectively advance the 

attainment date for the area by at least 1 year. For this second proposed approach, the 

EPA proposes a process similar to the one proposed for Moderate area control strategy 

determinations. However, the specific potential control measures to be evaluated as 

BACM and BACT and additional feasible measures would continue to be distinguished 
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by stricter criteria to yield a set of control measures that reflects an overall higher level of 

stringency in the control strategy for the nonattainment area than that provided by the 

implementation of reasonable control measures (i.e., RACM and RACT and additional 

reasonable measures).

Under the EPA’s second proposed approach for determining which measures 

must be part of the control strategy for a Serious PM2.5 nonattainment area, a state would 

follow many of the same steps as described under the EPA’s first proposed approach for 

the such determinations, with two important differences. First, Step 2 as described above 

would be eliminated from the process. That is, after a state updates the baseline emissions 

inventory for sources located in the area, the state would be required to identify existing 

and potential new measures for all sources in the inventory for evaluation as potential 

BACM and BACT and additional feasible measures without exempting any source 

categories as de minimis. Second, Step 6 as described above would not be the last step in 

the control strategy determination process, but rather would serve as another interim step 

in the process prior to making a final determination of what constitutes BACM and 

BACT and additional feasible measures for the area through modeling for the attainment 

demonstration. The EPA’s proposed requirements for what the state would need to 

evaluate during this step under this second proposed approach are described in greater 

detail in the following section.

The EPA emphasizes that proposed option #2 for determining BACM and BACT 

and additional feasible measures depends on the state submitting its attainment 

demonstration earlier than may otherwise be required under the statute so that it can be
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contemporaneous with the submission of BACM and BACT measures, due 18 months 

after the date of reclassification of a PM2.5 nonattainment area to Serious.

Given all of the above, the EPA is proposing and seeking comment on a second 

approach for determining BACM and BACT and additional feasible measures for a 

Serious PM2.5 nonattainment area comprised of the following steps. See proposed 40 CFR 

51.1010(a) for proposed option #2. Note that Steps 1 through 5 would incorporate the 

same considerations and requirements as those in the equivalent steps described in the 

EPA’s first proposed approach with the two important exceptions discussed in the 

preceding section:

Step 1: Update base year emissions inventory for the area.

Step 2: Identify existing and potential control measures for all emissions sources 

in the emissions inventory for the area.

Step 3: Determine whether an available control measure or technology is 

technologically feasible.

Step 4: Determine whether an available control measure or technology is 

economically feasible.

Step 5: Determine the earliest date by which a control measure or technology can 

be implemented in whole or in part.

During this step in the process, the state would be required to identify two groups 

of measures. The first group of measures would be potential BACM and BACT; that is, 

“best”-level, feasible measures that the state could implement in whole or in part within 4 

years of reclassification. The second group of measures would be additional feasible 

measures, defined as “best”-level, feasible measures that a state could implement in 
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whole or in part on sources in the area sometime after the fourth year following 

reclassification and prior to the statutory attainment date for the area. 

Step 6: Model to determine the attainment date that is as expeditious as 

practicable. As with the proposed Moderate area attainment plan control strategy 

analysis, the EPA proposes that states would need to model air quality impacts to 

determine the Serious area attainment date that is as expeditious as practicable for 

the area. After developing an inventory, identifying potential measures, 

determining economic and technological feasibility, and determining whether a 

measure would be able to be implemented in 4 years or between 4 years from 

reclassification and the statutory attainment date for the area, the state would 

conduct modeling that shows the combined air quality impact of all BACM and 

BACT measures and additional feasible measures as applicable. The purpose of 

this modeling would be to determine the attainment date that is as expeditious as 

practicable and to identify whether there are certain control measures that a state 

could eliminate from the Serious area attainment plan because they cannot 

collectively expedite attainment of the area by 1 year or more. A complete 

discussion of the EPA’s proposed modeling requirements for Serious area 

attainment demonstration is presented in Section VI.E below.

Step 6a: If area can demonstrate attainment by the statutory attainment date, then 

select only those control measures needed for expeditious attainment as BACM or 

BACT or additional feasible measures. Under this second proposed approach to 

BACM and BACT determinations, the EPA proposes that if a Serious area will be 

able to demonstrate attainment by the statutory Serious area attainment date, then 
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the state must adopt all measures identified as potential BACM and BACT, and 

additional feasible measures if applicable, that will ensure that the attainment date 

is as expeditious as practicable. The state may, however, reject those potential 

BACM and BACT and additional feasible measures that would not collectively 

contribute to emissions reductions that could advance the attainment date for the 

area by at least 1 year. 

The EPA recognizes that identifying the measures that would not collectively 

advance the attainment date for a Serious area by at least 1 year will likely be an iterative 

process that requires additional modeling. As with modeling for Moderate area 

attainment demonstrations, the EPA believes that such extra effort is reasonable for a 

state seeking to reject certain potential BACM or BACT or additional feasible measures 

from implementation in a given Serious nonattainment area.

One notable point of discussion in the Addendum indicates that short-term 

BACM measures are not preferred by the EPA unless such a measure is the only way to 

implement BACM within 4 years.196 This is because the ultimate goal of selection of 

BACM controls is that those measures will prevent future emissions, rather than a 

temporary reduction of emissions. Therefore, consistent with this previous guidance, the 

EPA proposes that those measures that a state must reject first under this proposed 

approach would be those that offer only short-term emissions reductions.

Step 6b: If an area cannot demonstrate attainment by the statutory attainment 

date, then submit request for Serious area attainment date extension including 

 

196 Ibid.
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adopting MSM. Section 189(b)(1)(A) of the CAA requires a state to submit as part 

of its Serious area attainment plan either a demonstration that the plan will 

provide for attainment by the statutory Serious area attainment date, or a 

demonstration that attainment by such date is “impracticable.” If the state cannot 

demonstrate attainment based on the implementation of all BACM and BACT and 

additional feasible measures by the end of the tenth calendar year following 

designation of the area, then under sections 189(b)(1)(A)(ii) and 188(e), the state 

must submit as part of its Serious area attainment plan a complete request to 

extend the attainment date for the area that meets the statutory provisions of 

section 188(e) and meets all of the regulatory criteria proposed under Section VII 

in this preamble, including the evaluation and adoption of MSM.

The EPA acknowledges that this second proposed approach for determining 

BACM and BACT and additional feasible measures for a Serious area, which would 

authorize states to link the attainment control strategy to the attainment needs for an area,

is different from the approach the agency has historically applied to BACM 

determinations for PM10. The EPA believes that effectively eliminating the step of 

exempting de minimis source categories the beginning of the control strategy 

determination process and linking the determination of BACM and BACT and additional 

feasible measures with the attainment analysis for a Serious area would not be a 

relaxation of the statutory requirement for implementation of “best” measures in Serious 

PM2.5 nonattainment areas as Congress required in section 189(b)(1)(B), however. 

Rather, the agency believes that in order to ensure that a state develops an appropriately 

stringent control strategy for a Serious PM2.5 nonattainment area, it is appropriate to 
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require that state to identify and evaluate potential control measures for all sources of 

direct PM2.5 emissions and emissions of any PM2.5 precursors not otherwise found to 

contribute insignificantly to PM2.5 levels in the area. Eliminating the possibility for de 

minimis source category exemptions means that a state’s evaluation of potential control 

measures and technologies will be more thorough and comprehensive and potentially 

lead to the implementation of controls on a wider variety of source categories.

Additionally, the test of whether the potential BACM and BACT and additional feasible 

measures not needed for an area to attain the NAAQS by the outside statutory attainment 

date could collectively advance the attainment date for the area by at least 1 year could 

result in a state implementing such measures on source categories which, if they had each 

been evaluated separately for purposes of a de minimis source category analysis, might 

have been exempted from control. Furthermore, as noted earlier in this section, in order 

for the state, the EPA, and the general public to be able to fully evaluate whether the 

selected control strategy (i.e., BACM and BACT and additional feasible measures) will 

provide for expeditious attainment of the NAAQS in a Serious PM2.5 nonattainment area, 

the state would be required to submit the attainment demonstration for the area at the 

same time as it submits provisions to meet the BACM and BACT requirement under 

section 189(b)(1)(B), 18 months after reclassification of the area to Serious. This date 

would be stricter than the statutory due date for a Serious area attainment demonstration 

for areas reclassified to Serious under the EPA’s discretionary authority of section 

188(b)(1), which is no later than 4 years from the date of reclassification of the area.

By defining a process for determining BACM and BACT and additional feasible 

measures in a way that is similar to the process for determining RACM and RACT and 
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additional reasonable measures for the same area, the EPA believes that a state with a 

Serious PM2.5 nonattainment area may be able to conserve resources by relying in part on 

the analytical work performed for the RACM and RACT analysis for the area when it 

was classified as Moderate. Furthermore, the challenges associated with properly 

identifying de minimis source categories as described earlier in this section may be 

avoided. Finally, the EPA believes that tying the final selection of BACM and BACT and 

additional feasible measures to the specific attainment needs of a nonattainment area 

could help to focus limited air agency resources on control measures that are most needed 

to bring a Serious area into expeditious attainment for the PM2.5 NAAQS.

The EPA seeks comment on all aspects of both proposed approaches and criteria 

for determining BACM and BACT and additional feasible measures for a Serious 

nonattainment area. The agency may finalize either of the proposed approaches or 

various elements of each after analyzing submitted comments.

3. BACM and BACT submittal requirements

To ensure that attainment plan submissions contain the necessary supporting 

information for EPA review and approval of the state’s selected BACM and BACT and 

additional feasible measures as applicable, the EPA proposes to require under the 

authority of section 301(a) that a state must submit the following information as part of 

its Serious area attainment plan submission: 

 A list of all emissions source categories, sources and activities in the 

nonattainment area that emit direct PM2.5 or any PM2.5 precursor (for multi-state 

nonattainment areas, this would include source categories, sources and activities 

from all states which make up the area);
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 For each source category, source or activity in the nonattainment area, an 

inventory of direct PM2.5 and all PM2.5 precursor emissions;

 For each source category, source or activity in the nonattainment area, a 

comprehensive list of potential control measures considered by the state for the 

nonattainment area;197, 198

 For each potential control measure considered by the state but eliminated from 

further consideration due to a determination by the state that the control measure 

or technology was not technologically feasible, a narrative explanation and 

quantitative or qualitative supporting documentation to justify the state’s 

conclusion;

 For each technologically feasible emission control measure or technology, the 

state must provide the following information relevant to economic feasibility: (i) 

the control efficiency by pollutant; (ii) the possible emission reductions by 

pollutant; (iii) the estimated cost per ton of pollutant reduced; and, (iv) a 

determination of whether the measure is economically feasible, with narrative 

 

197 The EPA believes that it is not necessary to identify every possible variation of every 

type of control measure, or all possible combinations of technologies and measures that 

would apply to a given source or activity, as long as the state has properly characterized 

the potentially available emissions reductions and their costs. For example, the EPA 

believes that the state can conduct a thorough analysis of VMT reduction measures 

without including every possible level or stringency of implementation of certain possible 

measures or combinations of measures for reducing VMT, so long as those measures 

would not affect the overall assessment of VMT reduction capabilities and the associated 

costs.

198 Menu of Control Measures document available at http://www.epa.gov/air/criteria.html

(last accessed 01/14/14).
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explanation and quantitative supporting documentation to justify the state’s 

conclusion;

 For each technologically and economically feasible emission control measure or 

technology, the date by which the technology or measure could be implemented.

As with a Moderate area attainment plan submission, the EPA recognizes that the 

base year emissions inventory for the area that the state submits in conjunction with its 

Serious area attainment plan will likely contain the information proposed to be required 

under the first two items in this list. However, the EPA believes that it is incumbent on 

the state to ensure that the information needed for the EPA to evaluate the state’s BACM 

and BACT and additional feasible measures analysis is presented as part of that analysis 

and in a format that provides transparency, consistency and the ability for another party 

to evaluate the state’s analysis effectively and to duplicate the state’s results. For this 

reason, the EPA is proposing to require the state to include the base year emissions

inventory information with the BACM and BACT submittal and as one element of the 

state’s attainment plan due 18 months after reclassification of the area to Serious.

4. Criteria for effective regulations to implement BACM and BACT and additional 

feasible measures

As with control measures identified as part of a Moderate area’s attainment 

control strategy, after a state has identified its BACM and BACT and additional feasible 

measures for a particular nonattainment area, it must implement those measures through a 

legally enforceable mechanism to be included in the SIP. As with Moderate area control 

measures, the EPA is proposing that in order for the agency to be able to approve any
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Serious area control measure and approve it as part of the SIP, the state will have to 

provide information to meet the following four criteria.

First, the base year emissions from the source or group of sources to which the 

control measure applies and the future year projected emissions from those sources once 

controlled must be quantifiable so that the projected emissions reductions from the 

sources can be attributed to the specific measures being implemented. Once again, it is 

important that the emissions from the source category in question are accurately 

represented in the base year inventory so that emissions reductions are properly 

calculated. In particular, it is especially important to ensure that both the filterable and 

condensable components of PM2.5 are accurately represented in the base year.

Second, the control measures must be enforceable, meaning that they must 

specify clear, unambiguous and measurable requirements. The measurable requirements 

for larger emitting facilities must include periodic source testing to establish the 

capability of such facilities to achieve the required emission level. Additionally, to verify 

the continued performance of the control measure, specific emissions monitoring 

programs appropriate for the type of control measure employed and the level of 

emissions must be included to verify the continued performance of the control measure. 

The control measures and monitoring program must also have been adopted according to 

proper legal procedures.

Third, the results of application of the control measures must be replicable. This 

means that where a rule contains procedures for interpreting, changing or determining 

compliance with the rule, the procedures are sufficiently specific and objective so that 

two independent entities applying the procedures would obtain the same result.
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Fourth, the control measures must be accountable. For example, source-specific 

emission limits must be permanent and must reflect the assumptions used in the 

attainment plan for the area, including the modeling conducted in conjunction with the 

attainment demonstration. The attainment plan must establish requirements to track 

emissions changes at sources and provide for corrective action if emissions reductions are 

not achieved according to the plan.

The EPA seeks comment on these criteria for approval of any control measures 

adopted by a state for a Serious area to assure that such measures are legally enforceable.

5. Relevance of prior BACT, BART and LAER determinations

The EPA believes that BACT, best available retrofit technology (BART) or 

lowest achievable emission rate (LAER) provisions for new sources (as distinct from 

BACT for existing sources) could qualify as BACM or BACT for purposes of meeting 

the Serious area attainment plan requirements. However, the EPA does not believe it is 

appropriate for a state to assume that just because a certain control technology was 

determined to meet BACT, BART or LAER criteria for a new source, such a control will 

also automatically meet the criteria for BACM or BACT or additional feasible measures 

for attainment planning purposes because the regulated pollutant or source applicability 

may differ and the analyses may be conducted many years apart. Thus, a state may not 

simply rely on prior BACT, BART or LAER analyses for the purposes of showing that a 

source has also met BACT for the relevant PM2.5 NAAQS. Rather, the EPA expects that

in Step 2 of either of the agency’s proposed approaches to the BACM and BACT 

determination process, the state would identify such measures as “existing measures” that 

should be further evaluated as potential BACM or BACT or additional feasible measures.
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6. Multi-state nonattainment areas

States that share a multi-state Serious PM2.5 nonattainment area must consult with 

one another on BACM and BACT and additional feasible measures that will be required 

for the nonattainment area in the different states. This requirement would be consistent 

with the overall requirements for BACM and BACT and additional feasible measures 

determinations, as all states with Serious areas need to consider implementing BACM 

and BACT-level measures that have been implemented in other states, even if those 

measures incur higher costs. The EPA anticipates that states may potentially adopt 

controls that differ from state to state, based upon each state’s determination of what 

qualifies as “best” given the mixture of sources and potential controls in the state portions 

of relevant nonattainment areas, subject to EPA approval. If the state can adequately 

demonstrate that its chosen BACM and BACT and additional feasible measures fully 

meet the EPA’s proposed criteria for such measures, then the agency may consider 

approving individual state plans that differ in implementation of control measures.

7. Environmental justice considerations for developing the attainment plan 

control strategy for a Serious PM2.5 nonattainment area

The EPA strongly urges states to consider the environmental justice aspect of any 

control measures they have identified as BACM and BACT or additional feasible 

measures. Because the criteria for determining BACM and BACT will lead in most cases 

to the selection of an overall more stringent control strategy in a Serious area than what 

RACM and RACT could provide, an appropriate control strategy for a Serious 

nonattainment area will likely implicitly include the best measures for ensuring that 

overburdened populations are appropriately protected. Nonetheless, the EPA encourages 
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states when possible to select BACM and BACT measures that will result in the least 

possible burden and greatest degree of health protection for overburdened populations in 

the nonattainment area.

E. Modeling for Attainment Demonstrations

Section IV.E. describes the EPA’s proposed attainment demonstration and 

modeling requirements for Moderate area plans, and the EPA is proposing that the same 

general requirements should apply to Serious area attainment demonstrations. However, 

Serious area plans have additional statutory requirements, which the EPA proposes to 

address as described below.

1. Statutory requirements

Section 189(b) generally requires a state with a designated Serious nonattainment 

area to submit an attainment plan for such area. As discussed earlier, section 

189(b)(1)(A) more specifically requires the state to submit an attainment demonstration 

including air quality modeling to establish either: (i) that the area will attain the relevant 

NAAQS by the applicable attainment date, or (ii) if the state is seeking an extension of 

the attainment date, that it is impracticable for the area to attain the relevant NAAQS by 

the statutory Serious area attainment date. For Serious nonattainment areas, the 

attainment date is as expeditiously as practicable, but no later than the end of the tenth 

calendar year after designation as nonattainment. An attainment demonstration that 

shows that it is impracticable for the area to attain within this timeframe must also 

provide for attainment of the NAAQS by the most expeditious alternative date 

practicable, but no later than 5 years after the maximum statutory Serious area attainment 

date (based on the criteria specified in section 188(e)).
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Attainment demonstrations are due 18 months after reclassification if the EPA 

reclassifies the area to Serious after failure of the area to attain the applicable Moderate 

area deadline. Alternatively, section 189(b)(2) requires states with designated Serious 

nonattainment areas to submit attainment demonstrations no later than 4 years after 

reclassification of the area to Serious if the reclassification occurs before the Moderate 

area attainment deadline. However, the EPA is proposing an approach for determining an 

appropriate attainment plan control strategy for a Serious PM2.5 nonattainment area that 

would require the state to submit the attainment demonstration for the area within 18 

months after reclassification regardless of when or the authority under which an area was 

reclassified to Serious. Sections VI.A and VI.D of this preamble describe more fully the 

EPA’s proposed approach for control strategy analyses and due dates for all elements of a 

Serious area attainment plan. Section VI.J of this preamble provides a complete 

discussion of the EPA’s proposed criteria for granting a Serious area attainment date 

extension.

2. Attainment demonstrations for Serious areas

As described in Section IV.E of this preamble, an attainment demonstration is a 

plan that demonstrates how a state will attain the PM2.5 NAAQS by the applicable 

attainment date. The EPA is proposing that the demonstration for Serious areas must 

consist of: (i) technical analyses such as base year and future year modeling of emissions 

which identify sources and quantify emissions that are contributing to violations of the 

PM2.5 NAAQS; and, (ii) analyses of future year projected emissions reductions and air 

quality improvement resulting from existing (i.e. already-adopted or “on the books”) 

national, regional and local programs, and potential new local measures needed for 
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attainment, including RACM and RACT and BACM and BACT controls for the area, as 

well as other measures either inside the nonattainment area or outside the nonattainment 

area but within the state that could potentially accelerate attainment. Each state with a 

Serious nonattainment area must submit an attainment plan with an attainment 

demonstration that includes analyses supporting the state’s determination of its proposed 

attainment date. In all cases, the state must show that the area will attain the NAAQS as 

expeditiously as practicable, but not later than the tenth calendar year after designation. 

In order to establish that the attainment date is as expeditious as practicable, the state 

must explain why the control measures adopted in the attainment plan provide for the 

most expeditious attainment and must either: (i) under proposed option #1 for the BACM 

and BACT determination include all BACM and BACT controls in the analysis, or (ii) 

under proposed option #2 for BACM and BACT, provide the requisite analysis to show 

that implementation of additional emissions controls, including any potential BACM and 

BACT, would not advance the attainment date for the area by at least 1 year if considered 

collectively.

A state with a Serious nonattainment area can also submit an impracticability 

demonstration (under section 189(b)(1)(A)(ii)) as part of seeking an extension of the 

attainment date under section 188(e). The impracticability demonstration for a Serious 

area would be functionally the same as an impracticability demonstration for Moderate 

areas because it must include air quality modeling which shows that the area will not be 

able to attain the PM2.5 NAAQS by the outside statutory attainment date, which in this 

case is by the end of the tenth calendar year following designation. However, in order to 

support a Serious area impracticability demonstration, the state must also show (through 
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modeling) that attainment cannot be reached by the statutory Serious area attainment 

date, even if all RACM and RACT and BACM and BACT controls, as well as other 

measures either inside the nonattainment area or outside the nonattainment area but 

within the state, were implemented before the attainment date. Moreover, in addition to 

the Serious area impracticability demonstration, to support an extension of the attainment 

date, the Serious area plan must demonstrate (again, using air quality modeling) that it 

provides for attainment by the most expeditious alternative date practicable employing 

MSM, as specified in section 188(e). As a result, the required plan is both an 

impracticability demonstration (to justify an extension beyond the statutory attainment 

date) and an attainment demonstration which serves as the basis for proposing an 

appropriate alternative attainment date. 

3. What modeling is required?

States are required to submit air quality modeling in support of an attainment 

demonstration for a Serious PM2.5 nonattainment area. Unlike the impracticability 

demonstration for Moderate areas described in section 189(a)(1)(B)(ii), the 

impracticability demonstration for Serious areas in section 189(b)(1)(A)(ii) also requires 

air quality modeling establishing the most expeditious alternative attainment date 

practicable. Therefore, air quality modeling is a required element in all attainment 

demonstrations for Serious areas.

Other than the timing of plan submissions and additional required elements of a 

Serious area plan (such as BACM and BACT), the relevant air quality modeling 

procedures and guidance for Moderate and Serious area plans are the same. See Section 
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IV.E. of this preamble for more details on proposed modeling requirements and guidance 

for all PM2.5 nonattainment areas. 

4. Will areas reclassified to Serious need to submit two separate attainment 

demonstrations?

Under section 189(a)(1)(B), a state is required to submit as part of an area’s 

Moderate area attainment plan a demonstration that the area either will attain or cannot 

practicably attain the NAAQS by the statutory Moderate area attainment date. Regardless 

of whether the state submits an attainment demonstration or an impracticability 

demonstration for a Moderate area, if such area is reclassified to Serious prior to or after 

failing to attain the applicable NAAQS, the state is required under section 189(b)(1)(A) 

to submit a new attainment demonstration as part of an area’s Serious area attainment 

plan. The separate statutory requirements for Moderate and Serious nonattainment areas 

anticipate two separate attainment plan submissions, and the EPA’s existing guidance in 

the General Preamble and Addendum further support this expectation. While the state 

would be required to submit a separate Serious area attainment plan, the EPA anticipates 

that certain control strategies may build upon those previously adopted and implemented 

as part of the Moderate area plan. For example, it could be the case that an area 

dominated by woodsmoke emissions could not attain the standard by the statutory 

Moderate area attainment date because all necessary woodstove change-outs could not 

occur in that timeframe, but additional woodstove change-outs could occur by the 

statutory Serious area attainment date.

5. What future year(s) should be modeled in attainment demonstrations?
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A state performing a modeling analysis for an attainment demonstration or a 

Serious area impracticability analysis must select a future year for the analysis. For an 

attainment demonstration, a state should select the future modeling year such that all 

emissions control measures relied on for attainment will have been implemented by the 

beginning of that year. To demonstrate attainment, the modeling results for the 

nonattainment area must predict that emissions reductions implemented by the beginning 

of the last calendar year preceding the attainment date will result in PM2.5 concentrations 

that meet the level of the standard.199

While states should choose the future modeling year based on a number of 

factors, the EPA recommends the last year of the statutory attainment date as a starting 

point for modeling for two reasons. First, a state with a Serious area for which it submits

an attainment date extension request under section 188(e) must show that the area cannot 

practicably attain the NAAQS by the end of the tenth calendar year following designation 

of the area. Therefore, the appropriate future modeling year for making such a 

demonstration would be the tenth year after designations. Even if a state does not submit 

(or does not intend to submit) a Serious area attainment date extension request, modeling 

the tenth year is a logical starting point to determine if attainment by year 10 is likely. If 

attainment-level concentrations of PM2.5 are not expected in the tenth calendar year after 

designations, then the area must also, as a requirement to receive an extension of the 

 

199 Note that for purposes of the PM2.5 NAAQS, a determination of attainment (or failure 

to attain), which the EPA is required to make after the attainment date has passed, is 

based on an average of the most recent 3 years’ worth of ambient data prior to the area’s 

attainment date.
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Serious area attainment date, submit a demonstration (using air quality modeling) that 

provides for attainment by the most expeditious alternative date practicable, but no later 

than the end of the fifteenth year after designation, with the implementation of MSM (see

Section VI.J of this preamble for details about MSM determinations).

Second, even though attainment of any PM2.5 NAAQS is determined by averaging 

3 years’ worth of ambient data, states do not have to model 2 years before the attainment 

date to show modeled attainment. Since the design value is an average of 3 years’ worth 

of data, attainment can still be shown even if concentrations exceed the NAAQS in one or 

more of the 3 years used to determine attainment (as long as the 3 year average is less 

than the NAAQS). Therefore, it can be appropriate to model any of the 3 years used to 

determine attainment. For these reasons, it is acceptable, and may in fact be most 

efficient, for a state to begin the Serious area attainment demonstration process by 

modeling the final year of the statutory attainment date to determine future year modeled 

PM2.5 concentrations in the tenth year after designations.

Because an area must attain “as expeditiously as practicable,” additional 

considerations are necessary before an attainment date can be established. For purposes 

of determining the attainment date that is as expeditious as practicable, the state must 

conduct future year modeling which takes into account growth and known controls 

(including any controls that were previously determined to be RACM and RACT for the 

area). For example, for an area designated nonattainment for the 2012 PM2.5 NAAQS 

during the first round of designations and subsequently reclassified to Serious, a future 

case scenario for the year 2025 (10 years after the initial nonattainment designation) 

would be needed to examine whether the the BACM and BACT identified by the state 
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would result in attainment. Under the proposed BACM and BACT determination option 

#1 (where BACM and BACT must be determined independent of the attainment

demonstration for the area), the future case scenario must include BACM and BACT 

controls in the analysis plus any additional measures on sources inside and outside of the 

nonattainment area (but within the state) that the state has identified as feasible to 

implement by the attainment date. Under proposed option #2 for determining BACM and 

BACT (where BACM and BACT is determined according to what is needed to 

expeditiously attain the NAAQS), the future case scenario must show whether 

implementation of emissions controls, including all BACM and BACT and additional

feasible measures on sources inside and outside of the nonattainment area (but within the 

state), collectively would advance the attainment date by at least 1 year. Note that similar 

to RACM and RACT, BACM and BACT controls must be implemented within 4 years 

after reclassification to Serious nonattainment. In order to justify an extension of the 

attainment date beyond the end of the tenth year after designation, the state must show 

that attainment by that date (including the anticipated emissions reductions from RACM 

and RACT and additional reasonable measures, and BACM and BACT and additional 

feasible measures) would be impracticable. Any proposed attainment date after the 10 

year period must include modeling of BACM and BACT controls plus the most stringent 

measures that are included in the implementation plan of any state and can be feasibly 

implemented in the area. The attainment date extension beyond 10 years can be for up to 

5 additional years, but the proposed attainment date must also be shown to be as 

expeditious as practicable. Section VI.J of this preamble provides a complete discussion 
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of the EPA’s proposed interpretation of the statutory requirements for a Serious area 

attainment date extension under section 188(e).

As with Moderate area attainment demonstrations, the EPA believes that it is not 

necessary or reasonable to require states to model each and every year to determine the 

appropriate attainment date for a Serious PM2.5 nonattainment area given the resource

demands associated with modeling.

200 In some cases it may be reasonable to model one 

additional interim year before the maximum statutory attainment date. However, in most 

cases, the air quality benefits of an identified set of reasonable control measures, BACM

and BACT and additional feasible control measures can be estimated through model 

sensitivity analyses and the development of transfer factors (factors to relate tons of 

emissions reductions in the area to PM2.5 concentration changes in the area). The EPA 

strongly recommends that states discuss the selection of the future year(s) to model with 

their respective EPA Regional Office as part of the modeling protocol development 

process prior to embarking on the modeling.

6. Attainment year motor vehicle emissions budgets

As with Moderate areas, the transportation conformity rule requires that Serious 

area attainment plans establish motor vehicle emissions budgets for the area’s attainment 

year. Therefore, once a Serious area’s attainment date has been established, the state is 

required to establish motor vehicle emissions budgets for direct PM2.5 and any relevant 

 

200 States with Serious areas that request an attainment date extension beyond 10 years 

must model the tenth year after designation of the area as part of an impracticability 

demonstration, plus an additional year beyond that which represents the attainment date.
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PM2.5 precursor for the attainment year.

201 A motor vehicle emissions budget for the 

purposes of a Serious area PM2.5 attainment plan is that portion of the total allowable 

emissions within the nonattainment area allocated to on-road sources as defined in the 

submitted attainment plan.

202 Such motor vehicle emissions budgets would be calculated 

using the latest planning assumptions and the latest approved motor vehicle emissions 

model available at the time that the attainment plan is developed.203

F. RFP Requirements

1. Statutory requirements

As with Moderate area attainment plans, Serious PM2.5 nonattainment area plans 

must provide for RFP as required under CAA section 172(c)(2). Section IV.F of this 

preamble fully describes the statutory requirements and overall proposed approaches for 

states to fulfill the RFP requirement in the context of Moderate area attainment plans. 

The EPA believes that the proposed approaches described for RFP for Moderate area 

plans can apply to Serious area attainment plans as well. The following section offers 

additional detail about how the EPA proposes that the approach to RFP should apply 

specifically to Serious area attainment plans.

 

201 For more information on PM2.5 precursor requirements, see section 93.102(b)(2)(iv) 

and (v) of the transportation conformity rule. See also the May 6, 2005, final 

transportation conformity rule that addressed requirements for PM2.5 precursors. (70 FR 

24280).

202 A state would also establish motor vehicle emissions budgets for an area’s attainment 

year. Those budgets would be the motor vehicle emissions that the SIP establishes as 

being necessary to attain the NAAQS.

203 If an area includes re-entrained road dust in the motor vehicle emissions budget, the 

latest approved version of AP-42 should be used unless the EPA has approved an 

alternative model for the area.
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2. Proposed approach

As with a Moderate area attainment plan, the EPA is generally proposing that a 

state must submit an RFP plan as part of any attainment plan submission for a Serious 

nonattainment area in order to satisfy the statutory requirements for RFP. The plan must 

include a schedule and an analysis that collectively demonstrate when and through what 

control measures emissions from sources in the nonattainment area will decline from the 

applicable baseline year to the projected attainment year. The EPA is proposing that the 

applicable baseline year must be the same year as that represented by the latest base year 

inventory for the Serious area. The projected attainment year may be up to the end of the 

tenth year following designation of the area for a Serious area that can demonstrate 

attainment pursuant to section 189(b)(1)(A), or up to the end of the fifteenth year 

following designation for a Serious area that is seeking an extension to the statutory 

attainment date pursuant to section 188(e).204 The RFP analysis must clearly convey how 

the schedule for implementing BACM and BACT and any additional control measures 

will provide for generally linear progress towards attainment or, if step-wise progress is 

more appropriate for the specific nonattainment area in question, the analysis must 

convey an appropriate implementation schedule and must explain why generally linear 

progress towards emissions reductions in the area is not appropriate (e.g., due to the 

nature of the nonattainment problem and the types of sources contributing to PM2.5 levels 

in the area). For a Serious area that cannot demonstrate attainment by the statutory 

 

204 As noted in Section VI.B of this preamble, depending upon when the area is 

reclassified from Moderate to Serious, this base year inventory may need to be more 

recent than that submitted with the Moderate area attainment plan.
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Serious area attainment date, the EPA proposes that the state must include in its RFP 

analysis the anticipated emissions reductions expected to be achieved through the 

implementation of BACM and BACT and MSM on sources in the nonattainment area. As 

with RFP plans for Moderate areas, the EPA proposes that a state must submit one or 

more projected emissions inventories as part of the RFP plan for any Serious PM2.5

nonattainment area following the same guidance that applies to emissions inventories for 

attainment plans (see Section VI.B of this preamble for a complete discussion of 

emissions inventories for Serious area attainment plans). These projected inventories 

must correspond with the quantitative milestone date(s) for the area as described in 

Section VI.H of this preamble. The EPA proposes that motor vehicle emissions budgets 

must also be established for direct PM2.5 and any relevant PM2.5 precursor using the latest 

planning assumptions and the latest approved motor vehicle emissions model available at 

the time that the Serious area attainment plan is developed.205

The EPA seeks comment on all aspects of the agency’s proposal for meeting the 

statutory RFP requirements as they apply to Serious nonattainment areas. Furthermore, 

the EPA seeks comment on the proposed options described in Section IV.F of this 

preamble regarding how to prepare an RFP plan, geographic coverage of emission 

sources for RFP, and RFP requirements for multi-state nonattainment areas, which would 

also apply to Serious area attainment plans.

G. Quantitative Milestones

 

205 If an area includes re-entrained road dust in the motor vehicle emissions budget, the 

latest approved version of AP-42 should be used unless the EPA has approved an 

alternative model for the area.
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The attainment plan for any Serious nonattainment area must include quantitative 

milestones pursuant to section 189(c). These quantitative milestones would be in addition 

to those identified in the area’s Moderate area attainment plan, and would need to 

continue to be achieved every 3 years until the area attains the NAAQS. Specifically, the 

Serious area plan for an area that can demonstrate attainment by the statutory Serious 

area attainment date would have to contain quantitative milestones to be achieved by 7.5 

years from the area’s date of designation as nonattainment. This date would be 3 years 

after the first quantitative milestones for the area, to be met 4.5 years from designation of 

the area and 3 years after the Moderate area attainment plan was due to the EPA. The 

EPA also proposes and seeks comment on a requirement that a Serious area plan for an 

area that can demonstrate attainment by the statutory Serious area attainment date must 

also include quantitative milestones to be reached 10.5 years from designation, to help 

assess the state’s progress toward attaining the PM2.5 NAAQS in the event the area fails 

to attain by the applicable attainment date. For a Serious area that cannot demonstrate 

attainment by the statutory Serious area attainment date, the EPA proposes that the state 

must include in the Serious area attainment plan quantitative milestones to be achieved at 

years 7.5, 10.5 and 13.5 from the area’s date of designation. 

The Addendum included guidance that recommended milestones “should be 

addressed by quantifying and comparing the annual incremental emission reductions 

which result from implementation of BACM and BACT (required within 4 years after the 

area is reclassified as serious) and from additional measures included in the final serious 

area SIP to those reductions which were identified in the SIP as quantitative milestones 
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necessary to achieve the NAAQS by the applicable attainment date.”206

The EPA continues to agree with the fundamental concept conveyed in the 

existing guidance, but believes that it is impractical to expect that a state will always be 

able to quantify and compare real and projected emissions reductions, and submit a report 

to the EPA within 90 days of a given milestone, as required under section 189(c)(2). 

Therefore, the EPA proposes that the general proposed approach to selecting quantitative 

milestones, described in Section IV.G, should apply to any attainment plan for a PM2.5

nonattainment area, independent of its classification. Specifically, the EPA proposes that 

states be allowed to select the quantitative milestones that they identify as appropriate 

and quantifiable and that will provide for objective evaluation of progress toward 

attainment in their Serious PM2.5 nonattainment area, and that the EPA, in its attainment 

plan approval process, will determine if they satisfy the statutory requirements of section 

189(c).

In addition to this general proposed approach for selecting quantitative milestones 

and similar to an option proposed for Moderate area attainment plans, the EPA proposes 

to require that, at a minimum, states must include in all attainment plans for Serious 

PM2.5 nonattainment areas a measure to confirm that some specific portion of BACM and 

BACT for the area has been implemented as appropriate in order to comply with the 

statutory requirement at section 189(b)(1)(B). The EPA acknowledges that the precise 

quantifiable metric (e.g., 50 percent of BACM and BACT measures implemented by 

milestone date 7.5 years from designation) would need to be determined on a case-by-

 

206 59 FR 41998 at 42016.
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case basis, as it would depend upon the date of reclassification of the area, whether the 

metric is to be achieved at year 7.5 or year 10.5 from designation, and the anticipated 

implementation timing and nature of the BACM and BACT controls themselves. 

Nonetheless, the EPA believes it would be appropriate to include it as a metric that any 

state with a Serious nonattainment area must adopt as a quantitative milestone to 

demonstrate RFP (and thus must demonstrate compliance with when they submit their 

milestone report), as it derives from a statutory provision that applies to all Serious areas 

and thus represents a milestone that all Serious nonattainment areas must meet.

The EPA seeks comment on these proposed options for interpreting the statutory 

quantitative milestone requirements for Serious areas.

H. Contingency Measures

As noted in Section IV.G of this preamble, all PM2.5 nonattainment areas must 

include in their attainment plans contingency measures consistent with section 172(c)(9). 

Contingency measures are additional control measures to be implemented in the event 

that an area fails to meet RFP requirements or fails to attain the PM2.5 standard by the 

applicable attainment date. These measures must be fully adopted rules or control 

measures that are ready to be implemented quickly upon a determination by the EPA that 

the area failed to meet RFP or failed to meet the standard by the applicable attainment 

date, and such measures are required to take effect without significant further action by 

the state or the EPA.

The statutory contingency measure requirement at section 172(c)(9) is not 

superseded or subsumed by any requirement under subpart 4, nor does it apply only to 

Moderate area attainment plans. Thus, contingency measures are required for Serious 
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PM2.5 nonattainment areas as part of a state’s Serious area attainment plan submission. 

The EPA proposes that the criteria for identifying and selecting contingency measures for 

a Serious area attainment plan should be the same as those for Moderate area plans. 

Specifically, the EPA proposes that the following requirements must be met in order for 

contingency measures to be approvable as part of a state’s Serious area attainment plan 

submission:

1. Contingency measures must be fully adopted rules or control measures that are 

ready to be implemented quickly upon a determination by the Administrator of 

the nonattainment area’s failure to meet RFP or failure to meet the standard by its 

attainment date.

2. The SIP must contain trigger mechanisms for the contingency measures, specify a 

schedule for implementation, and indicate that the measures will be implemented 

without significant further action by the state or by the EPA.

3. Contingency measures must consist of control measures that are not otherwise 

included in the control strategy for the SIP, or must require further 

implementation of partial measures already included in the SIP as BACM or 

BACT, additional feasible measures, or MSM.

4. Contingency measures must provide for emissions reductions equivalent to 1 

year’s share of reductions needed to demonstrate attainment (i.e., the overall 

needed reductions divided by the number of years from the base year to the 

attainment year), or equivalent to 1 year’s worth of air quality improvement or 

emissions reductions proportional to the overall amount of air quality 

improvement or emissions reductions to be achieved by the area’s attainment 
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plan.

The EPA further proposes that a state may elect to rely on contingency measures 

that achieve emissions reductions on sources located outside the nonattainment area, but 

within the state, as well as from within the nonattainment area, provided that the 

measures on sources outside the designated nonattainment area are demonstrated to 

produce the appropriate air quality impact within the nonattainment area. 

As with contingency measures for Moderate nonattainment areas, the EPA 

believes it appropriate that a state might rely on additional reductions in the years 

following a failure to meet RFP requirements or a failure to attain the NAAQS by the 

applicable attainment date from federal or local measures already scheduled for 

implementation as part or all of their contingency measures. The EPA could potentially 

consider such measures as meeting the contingency measure requirement as long as they 

produce emissions reductions in excess of those required to meet other statutory 

provisions, such as to meet BACM and BACT and RFP requirements, and they can be 

relied on to achieve a sufficient portion of the actual emissions reductions necessary to 

reduce emissions in the area while the state develops a new plan to bring the area into 

attainment.207 As with contingency measures for Moderate area attainment plans, the 

EPA proposes that the emissions reductions associated with contingency measures for 

Serious area plans must be equal to approximately 1 year’s worth of emissions reductions 

necessary to achieve RFP for the area, unless the state adequately demonstrates that some 

smaller amount of reductions is appropriate while the state is revising its attainment plan 

 

207 See LEAN v. EPA, 382 F.3d 575 (5th Cir. 2004).
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for the area. The EPA seeks comment on this requirement.

The Addendum provided guidance related specifically to the selection and 

implementation of contingency measures for Serious nonattainment areas. First, the EPA 

guidance indicated that “for those moderate areas reclassified as serious, if all or part of 

the moderate area plan contingency measures become part of the required serious area 

control measures (i.e., BACM), then additional contingency measures must be submitted 

whether or not the previously submitted contingency measures had already been 

implemented. Further, the affected states must ensure that serious areas have adequate 

contingency measures considering, among other things, new information about the 

potential attainment shortfall for the newly reclassified serious area.”208 The EPA 

continues to believe that this approach to the statutory contingency measure requirement 

is appropriate and proposes to adopt it for purposes of implementing the PM2.5 NAAQS 

in Serious nonattainment areas.

With regard to the timing for implementing contingency measures, the EPA 

reiterates that the purpose of contingency measures is to ensure that corrective measures 

are put in place automatically at the time that the EPA makes a determination that an area 

has failed to meet RFP or failed to meet the NAAQS by the applicable attainment date. 

For any nonattainment area, the EPA is required to determine within 90 days after 

receiving a state’s RFP demonstration, and within 6 months after the attainment date for 

an area, whether the state has met their statutory obligations for demonstrating RFP or 

attaining the standard, as appropriate. As with Moderate areas, the EPA believes that 

 

208 59 FR 41988 at 42015.
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contingency measures should become effective for Serious areas within 60 days of the 

EPA making its determination that the area failed to meet RFP or attain the NAAQS and 

proposes to require this for purposes of PM2.5 NAAQS implementation in Serious 

nonattainment areas. 

Finally, while section 172(b) gives discretion to the Administrator to establish a 

deadline for submitting contingency measures up to 3 years from designation of the area, 

it does not explicitly address the appropriate submittal date for contingency measures for 

areas reclassified to Serious. In the Addendum, the EPA indicated that “states must 

submit contingency measures for serious areas or otherwise demonstrate that adequate 

measures are in place within 3 years of reclassification.”209 The EPA proposes and seeks 

comment on applying this guidance to Serious nonattainment areas for current and future 

PM2.5 NAAQS. 

In addition, as described in Section VI.A, the EPA proposes an alternative 

submission deadline for Serious area contingency measures that would align the 

contingency measure due date with the Serious area attainment demonstration due date. If 

an area is reclassified under the EPA’s discretionary authority, the Serious area 

attainment demonstration is due 4 years from the date of reclassification; under this 

alternative proposed approach, contingency measures would also be due 4 years from the 

date of reclassification for such areas. If an area is reclassified under the EPA’s 

mandatory duty upon failure of the area to attain the NAAQS by the Moderate area 

attainment date, then the Serious area attainment demonstration is due 18 months from 

 

209 Id.
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the date of reclassification; accordingly, under this alternative proposed approach, 

contingency measures would also be due 18 months from the date of reclassification for 

such an area. In either case, the BACM and BACT provisions for the Serious area would 

be due at or before the time contingency measures would be due, which is appropriate 

given that the EPA expects a state to consider its BACM and BACT measures as it 

develops its contingency measures. The state may ascertain that measures not otherwise 

required or necessary for BACM or BACT may nevertheless be suitable for purposes of 

contingency measures. The EPA seeks comment on this alternative approach to setting 

Serious area contingency measure due dates.

I. Attainment Dates

As explained earlier, section 188 establishes the attainment dates for both 

Moderate and Serious areas. For a Serious area, section 188(c)(2) provides that “the 

attainment date shall be as expeditiously as practicable but no later than the end of the 

tenth calendar year beginning after the area’s designation as nonattainment.” 210 For 

example, for an area initially designated as nonattainment effective in early 2015 is 

reclassified to Serious at some future date, the Serious area attainment date, absent any 

approved Serious area attainment date extension, would be no later than December 31, 

2025 (the end of the tenth calendar year after designation). As discussed in Section IV.I, 

the EPA proposes to interpret the references to “designation” in CAA section 188(c) as 

 

210 The EPA believes that there is no real effect on attainment date determinations due to 

the small difference in statutory language in section 188(c) basing the Moderate area 

attainment date on the “sixth calendar year after the area’s designation” and the Serious 

area attainment date on the “tenth calendar year beginning after the area’s designation,” 

(emphasis added).
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meaning “effective date of designation,” consistent with the agency’s prior approach for 

implementing the previous PM2.5 NAAQS under subpart 1 and other NAAQS.

The process for a state to determine the most expeditious attainment date 

practicable for a Serious area will depend upon the final approach selected for 

determining BACM and BACT for the area. Therefore the EPA is proposing two 

approaches for determining the appropriate attainment date for a Serious area. Under the 

first approach, which would correspond to the agency’s proposed option #1 for 

determining BACM and BACT – independent of the attainment demonstration for the 

area – the state would simply include the control measures determined to be BACM and 

BACT for the area in its air quality modeling, and would report the results of the 

modeling, including the earliest projected attainment date. 

Under the second proposed approach, which would correspond to the EPA’s 

proposed option #2 for determining BACM and BACT – tied to the attainment needs of 

the particular nonattainment area – the state would be required to follow a two-step 

process for determining the appropriate attainment date for the area. First, the state would 

be required to demonstrate through air quality modeling that the area can attain the 

relevant NAAQS by the latest statutory attainment date and determine which control 

measures and technologies are needed for the area to attain by that date. Second, the state 

would be required to determine whether implementing any remaining BACM or BACT 

controls (i.e., those not needed for attainment by the latest date) or any other additional 

controls can cumulatively advance the attainment date for the area by at least 1 year. In 

the event that a state determines that the area can attain the relevant NAAQS earlier 

through the application of these other measures, the state must propose the earlier date as 
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part of the attainment plan submission for the area. This second approach is similar to the 

proposed approach for determining the most expeditious attainment date for a Moderate 

area. 

As with Moderate area attainment dates, when the EPA takes action to approve 

the different elements of the attainment plan for the Serious area, one of the elements that 

the agency will take action on will be the state’s proposed attainment date for the area. If 

the EPA approves an attainment date for the area that is earlier than the latest date 

allowed by statute, then the applicable attainment date for the area will be the approved 

date. If the state demonstrates that the Serious area cannot practicably attain the NAAQS 

by the end of the tenth calendar year following designation, the state may request a 

Serious area attainment date extension as long as certain conditions are met, as described 

next in Section VI.J.

J. Attainment Date Extensions

1. Statutory requirements

As with Moderate areas, the EPA may grant an extension of the attainment date 

for a Serious PM2.5 nonattainment area if certain statutory criteria are met. Specifically, 

section 188(e) provides that the EPA may allow one attainment date extension of no more 

than 5 years “upon application by any state . . . if attainment by the [original Serious area 

attainment date] would be impracticable, the state has complied with all requirements and 

commitments pertaining to that area in the implementation plan, and the state 

demonstrates to the satisfaction of the Administrator that the plan for that area includes 

the most stringent measures that are included in the implementation plan of any state or 

are achieved in practice in any state, and can feasibly be implemented in the area.” In 
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addition to the required preconditions for such an extension, the statute also includes 

factors which the Administrator may use as she considers whether to grant the extension 

and the length of the extension, including “the nature and extent of nonattainment, the 

types and numbers of sources or other emitting activities in the area (including the

influence of uncontrollable natural sources and transboundary emissions from foreign 

countries), the population exposed to concentrations in excess of the standard, the 

presence and concentrations of potentially toxic substances in the mix of particulate 

emissions in the area, and the technological and economic feasibility of various control 

measures.”211

2. Proposed approach

In the Addendum, the EPA generally described the statutory requirements listed 

above and expressed an intent to issue guidance on applying for an extension of the 

Serious area attainment date, if appropriate. While the ultimately EPA did not deem it 

necessary to issue such guidance, the EPA has interpreted these statutory requirements 

through actual exercise of its authority under section 188(e) in past rulemakings for 

specific PM10 nonattainment areas. For example, the EPA interpreted section 188(e) in 

approving an extension of a Serious area attainment date for purposes of the PM10

NAAQS for the Maricopa area (AZ).212 The EPA believes that the steps finalized in the 

Maricopa County PM10 Serious area SIP approval notice provide an appropriate starting 

 

211 Notably, these statutory criteria do not include specific ambient air quality criteria like 

the criteria that need to be met in the year prior to a Moderate area attainment date in 

order for the area to qualify for an attainment date extension under section 188(d).

212 65 FR 19964 and 67 FR 48718.
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point for a proposed regulatory approach, with some potential modification, for states to 

meet the statutory requirements that could apply nationally. The EPA is thus proposing to 

require that states adhere to the following steps when preparing and submitting a request 

for a Serious area attainment date extension:

Step 1: Demonstrate that attainment by the statutory Serious area attainment date 

is impracticable. In order to demonstrate impracticability, the state would have to 

show that the implementation of all BACM and BACT and all additional feasible

measures required under section 172(c)(6) will not bring the area into attainment 

by the statutory Serious area attainment date (i.e., by no later than the end of the 

tenth calendar year after designation).213 The statutory provision for 

demonstrating impracticability requires that the demonstration be based on air 

quality modeling (see section 189(b)(1)(A)). Additional guidance on this 

demonstration is provided in Section VI.E of this preamble.

Step 2: Comply with all requirements and commitments in the applicable

implementation plan. Similar to the proposed approach described in Section IV.J 

of this preamble for Moderate area attainment date extensions, the EPA proposes 

to interpret the criterion under section 188(e) that requires a state to have 

“complied with all requirements and commitments pertaining to that area in the 

 

213 This proposed approach parallels the EPA’s proposed approach, described earlier in 

this preamble, for the impracticability option for Moderate areas under CAA section 

189(a)(1)(B) in which all measures that qualify as RACM and RACT and all additional 

reasonable measures are required before a Moderate area plan could show 

impracticability of attainment by the statutory Moderate area attainment date (the end of 

the sixth calendar year after designation).
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implementation plan” simply to mean that the state has implemented the control 

measures in the SIP revisions it has submitted to address the applicable 

requirements in sections 172 and 189. For a Serious area attainment date 

extension request being submitted contemporaneously with the “original” Serious 

area attainment plan for the area, the EPA proposes to read section 188(e) not to 

require the area to have a fully approved attainment plan that meets the CAA’s 

requirements for Moderate areas. The agency proposes to base this reading on the 

plain language of section 188(e) which requires the state to comply with all 

requirements and commitments pertaining to that area in the implementation plan 

but does not require that the state comply with all requirements pertaining to the 

area in the CAA.214 For the same reason, the EPA also proposes to read this 

provision not to bar an extension if all or part of an area’s Moderate area plan is 

disapproved or has been promulgated as a FIP, provided the area has complied 

with all of the requirements in the applicable FIP, or in the applicable SIP and 

FIP. 

However, for a Serious area attainment date extension request being submitted 

sometime after submission of an “original” Serious area attainment plan that contained an 

attainment demonstration meeting the requirements of section 189(b)(1)(A)(i), the EPA 

proposes to read section 188(e) not to require the area to have a fully approved attainment 

plan that meets the CAA’s requirements for Serious areas, but to have a fully approved 

 

214 This interpretation as applied to section 188(e) for Serious area attainment date 

extensions was upheld by the Ninth Circuit Court of Appeals in Vigil v. Leavitt, 366 F.3d 

1025, amended at 381 F.3d 826 (9th Cir. 2004).
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Moderate area attainment plan. The rationale for this distinction is due to the timing of 

the Serious area attainment date extension request under these circumstances, which is 

discussed in greater detail later in this section. The EPA believes that this proposed 

interpretation of this criterion would apply whether the area was reclassified to Serious 

under the EPA’s discretionary authority (section 188(b)(1)) or by operation of law upon 

failing to attain by the Moderate area attainment date (section 188(b)(2)).

The EPA also seeks comment on an alternative interpretation of the 

implementation plan compliance criterion that would require a state to have a Moderate 

area attainment plan fully approved by the EPA, not just fully implemented by the state, 

at the time of the Serious area attainment date extension request, regardless of when such 

a request is submitted to the EPA. The EPA believes that one may reasonably argue that a 

state seeking an extension of the Serious area attainment date should have fully 

implemented all elements of an approved Moderate area attainment plan. The EPA 

believes that while such a condition may be reasonable generally speaking, there may be 

circumstances in which a state submits a Moderate area attainment plan that the EPA is 

unable to approve in a timely way, potentially creating a situation in which the state 

cannot qualify for a Serious area attainment date extension (due to its unapproved 

Moderate area plan) even if the area is reclassified to Serious and cannot practicably 

attain by the statutory attainment date for a Serious area. The EPA seeks comment on this 

alternate proposed interpretation of the applicable implementation plan compliance 

criterion under section 188(e). Recognizing that a situation such as that described above 

may be rare, the agency also seeks comment on what remedy might be available under 
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the statute if such a situation comes to pass if the EPA were to finalize this alternative

proposed interpretation of the applicable implementation plan criterion.

Step 3: Demonstrate the inclusion of MSM. To qualify for any extension of a 

Serious area attainment date, section 188(e) requires a state to “demonstrate to the 

satisfaction of the Administrator that the plan for the area includes the most stringent 

measures that are included in the implementation plan of any state, or are achieved in 

practice in any state, and can feasibly be implemented in the area.” In its prior guidance 

in the Addendum, the EPA interpreted the term ‘‘most stringent measure’’ (MSM) to 

mean the maximum degree of emission reduction that has been required or achieved from 

a source or source category in any other attainment plans or in practice in any other states 

and that can feasibly be implemented in the area seeking the extension, such as what 

LAER represents for new or modified sources under the NNSR permit program.215

The agency proposes that a state would need to follow a process for determining 

MSM for a Serious nonattainment area that is generally similar to proposed option #2 for 

BACM and BACT described in Section VI.D of this preamble, which would include

exemptions from MSM for sources in de minimis source categories if such measures did 

not collectively advance the attainment date for the area by at least 1 year. The EPA is 

also proposing an alternative approach for determining MSM for a Serious nonattainment 

area that would provide for de minimis source category exemptions for MSM only for 

those source categories that do not contribute significantly to ambient PM2.5

 

215 59 FR 41998 at 42010.
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concentrations in the Serious nonattainment area, an approach more closely aligned with

proposed option #1 for determining BACM and BACT.

Under proposed approach #1 for MSM, the EPA would prescribe a five-step 

process for states to follow when selecting and implementing MSM. This proposed 

approach is similar to that used in practice for approving the PM10 Serious area 

attainment plan and Serious area attainment date extension request submitted for 

Maricopa County (AZ) in 2000.216

The first step of this proposed approach would be for the state to update as needed 

the emissions inventory of direct PM2.5 and PM2.5 precursor sources and source categories 

in the Serious nonattainment area required under section 172(c)(3) for any attainment 

plan submission. The EPA expects that the state would meet this inventory requirement 

as part of its Serious area attainment plan submittal without any additional work if the 

state submits the Serious area attainment date extension request simultaneously with the 

plan itself. However, in the event the attainment date extension request is submitted after 

the “original” Serious area attainment plan for the area (i.e., toward the end of the Serious 

area attainment period), then the EPA proposes to require that the state must submit a 

more recent, complete and accurate emissions inventory that meets the same emissions 

inventory requirements for Moderate and Serious PM2.5 nonattainment areas pursuant to 

section 172(c)(3), as well as an attainment projected inventory as part of the new Serious 

area attainment plan for the area. The inventories submitted to support a Serious area 

 

216 67 FR 48718 (July 25, 2002).
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attainment plan must also include point sources meeting the lower major stationary 

source threshold in 40 CFR part 51, Subpart A.

The second step in this proposed MSM determination process would require the 

state to perform air quality modeling in order to evaluate, for each of the various source 

categories included in the emissions inventory for the area, the impact on PM2.5

concentrations in excess of the applicable NAAQS in order to determine which categories 

are significant for the purposes of adopting MSM. Those source categories for which 

such modeling indicates potential control measures collectively would have only a de 

minimis effect on advancing the attainment date for the area could be eliminated from 

further consideration. In the context of the EPA’s action to approve the Maricopa County 

PM10 Serious area attainment plan and attainment date extension request, the agency 

finalized an approach for judging what constitutes a de minimis source category for MSM 

by applying a test of whether MSM controls on the allegedly de minimis sources would 

result in more expeditious attainment, rather than applying a test of whether or not 

requiring the application of controls for such sources would make the difference between 

attainment and nonattainment by the statutory Serious area deadline, as the latter test 

implicitly would be met through the controls chosen for demonstrating attainment by the 

alternate attainment date for the area. In the agency’s explanation of the proposed 

approach, the EPA explained that “Our responsibility under section 188(e)... is to grant 

the shortest practicable extension of the attainment date by assuring the plan provides for 

attainment as expeditiously as practicable. Thus, one means of determining an 

appropriate de minimis level is to determine if applying MSM to the proposed de minimis

source categories would meaningfully expedite attainment. If it did, then the de minimis
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level is too high, and if it did not, then the de minimis level is appropriate.”217 The EPA 

thus proposes to determine whether any source categories should be eliminated from 

MSM controls through a de minimis exemption based on a demonstration that 

collectively applying MSM controls to such source categories would not advance 

attainment of the NAAQS in the area by at least 1 year. This test would presumably result 

in a more stringent threshold for what is considered a de minimis source category for 

MSM as compared to the threshold for de minimis source categories for BACM and 

BACT as described in the EPA’s proposed option #1 for BACM and BACT 

determination criteria (see Section VI.D of this preamble). The EPA proposes and seeks 

comment on this test for determining whether any source categories could be found to be 

de minimis and thus not subject to MSM controls.

The third step in the EPA’s first proposed approach to determining MSM for a 

Serious nonattainment area would involve identifying the potentially most stringent 

measures in other implementation plans for PM2.5 or other NAAQS, or used in practice in 

other states for controlling emissions from each of the remaining source categories listed 

in the emissions inventory that were not determined to be de minimis. For each measure, 

the state would be required to determine its technological and economic feasibility for 

sources in the area. The EPA proposes generally to apply more stringent criteria for 

determining the feasibility of potential MSM than that described for BACM and BACT 

in Section VI.D. In some situations, MSM could involve increasing the coverage of 

measures that were already adopted and implemented as BACM and BACT (for example, 

 

217 65 FR 19969.
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changing out an even greater percentage of woodstoves in an area, or paving even more 

roads, if such source categories were major contributors to the air quality problem in the 

nonattainment area).

However, because BACM and BACT represent the “best” level of control feasible 

for an area, it would be possible for the MSM requirement to result in no more controls 

and no more emissions reductions in an area than result from the implementation of 

BACM and BACT. Stated another way, there may be sources or categories for which no 

other feasible controls exist beyond what a state has already adopted as BACM or BACT. 

Given the strategy in the nonattainment provisions of the CAA to offset longer attainment 

timeframes with more stringent control requirements, the EPA therefore proposes to 

interpret the MSM provision in order to increase the potential that it will result in 

additional controls beyond the set of measures adopted as BACM and BACT by 

requiring a state to reanalyze any measures that were rejected during the state’s BACM 

and BACT analysis for the area to see if they are now feasible for the area given the 

potentially longer attainment date (up to 5 years after the statutory Serious area 

attainment date) or given the changes that have occurred in the interim that improve the 

feasibility of previously rejected measures.

The fourth step of this first proposed approach would require the state to compare 

the potential MSM for each non-de minimis source category against the measures, if any, 

already adopted for that source category in the Serious nonattainment area to determine if 

such MSM would provide any additional reductions. 

The fifth step would then require that the plan provide for the adoption and 

expeditious implementation of any MSM that is more stringent than existing measures or, 
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in lieu of adoption, provide a reasoned justification for rejecting the potential MSM, i.e., 

provide an explanation as to why such measures cannot be feasibly implemented in the 

area.

As noted earlier, the EPA expects that this first proposed approach to determining 

MSM would be most compatible with the agency’s proposed option #2 for determining 

BACM and BACT, described in Section VI.D. Under proposed option #2 for BACM and 

BACT determinations, a state would be required to implement only those “best” control 

measures necessary to bring a Serious nonattainment area into attainment expeditiously. 

Such an approach to BACM and BACT determinations would not incorporate an explicit 

step in the process for a state to exempt de minimis source categories from consideration 

for potential control measures. However, it would allow a state to eliminate any potential 

BACM or BACT or additional feasible measures that are not needed to bring a Serious 

area into attainment by the statutory attainment date and that cannot, collectively, 

advance the attainment date for the area by at least 1 year. Proposed option #2 for 

determining BACM and BACT for an area is thus similar to the proposed approach to 

MSM described above, in which a state could eliminate from further consideration those 

source categories for which potential control measures collectively would have only a de 

minimis effect on advancing the attainment date for the area (see proposed step 2).

The EPA’s proposed option #1 for BACM and BACT determinations would 

include an explicit step in the process for exempting de minimis source categories from 

further consideration for potential control measures. However, under such approach, a 

state would need to assess whether emissions of a particular pollutant from a given source 

category contributed significantly to PM2.5 concentrations in the nonattainment area. If 
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the state determined that the source category contributed only a de minimis amount of 

emissions, then the state could exempt the source category from further consideration for 

potential control measures. Thus, while it incorporates a step to identify de minimis

source categories, the EPA’s proposed option #1 for BACM and BACT determinations is 

not wholly consistent with the agency’s proposed approach #1 for determining MSM.

Therefore, the EPA is also proposing an alternative approach for determining 

MSM for a Serious nonattainment area that would be more compatible with the EPA’s 

proposed approach #1 for determining BACM and BACT. Under this alternative 

proposed approach for determining MSM, a state could exempt de minimis source 

categories from further consideration, but de minimis source categories would be 

identified by virtue of their lack of significant contribution to PM2.5 levels in the area, not 

by virtue of whether controlling such sources categories collectively could expedite 

attainment of the relevant NAAQS. In this way, de minimis source categories for MSM 

would be defined in a similar way, or subject to a similar “significant contribution” test, 

as de minimis source categories for BACM and BACT determinations under proposed 

option #1. Thus under proposed approach #2 for MSM, the steps described for 

determining MSM would generally be the same as under proposed approach #1, with the 

exception of step 2. Rather, the EPA proposes an alternative step 2 in the MSM 

determination process in which a state could identify de minimis source categories to 

exempt from further control based on an analysis of the particular contribution made by a 

given source category to ambient PM2.5 levels in the nonattainment area. The EPA 

believes that defining de minimis source categories and “significant contribution” for 

determining de minimis source categories would be equally challenging in the context of 
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MSM determinations as in the context of BACM and BACT determinations.218 However, 

in the event the agency finalizes proposed option #1 for BACM and BACT 

determinations, the EPA believes it would be appropriate to finalize proposed approach 

#2 for MSM, and would require that a state seeking to exempt from MSM sources in a 

given source category apply more stringent criteria for evaluating whether a certain 

source category’s contributions to the area’s PM2.5 concentrations are indeed de minimis.

The EPA believes that either of these proposed approaches for determining MSM 

for a Serious nonattainment area would be consistent with the EPA’s guidance in the 

Addendum to define MSM as those measures that can “feasibly be implemented in the 

relevant area from among those which are either included in any other SIP or have been 

achieved in practice by any other state.” One of the key features of this guidance relates 

to identifying control measures implemented elsewhere, which is also a key feature of the 

EPA’s proposed process for identifying RACM and RACT and additional reasonable 

measures (and BACM and BACT and additional feasible measures, if necessary) for a 

PM2.5 nonattainment area. For these processes, the EPA is proposing that a state identify 

potential measures for consideration as RACM or RACT or additional reasonable 

measures (or BACM or BACT or additional feasible measures) by looking at measures 

implemented by other states to meet PM2.5 NAAQS or other NAAQS. Thus, a state 

seeking to identify MSM should be able to start its process using with the work already 

undertaken for the nonattainment area’s RACM and BACM determinations and to make 

updates to the list of potential control measures accordingly.

 

218 See the discussion of de minimis source categories in Section VI.D in this preamble.
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The EPA notes that section 188(e) does not identify a deadline for a state to 

implement MSM, while elsewhere the statute establishes a deadline for implementing 

RACM and RACT and BACM and BACT (see CAA sections 189(a)(1)(C) and 

189(b)(1)(A), respectively). However, because the clear intent of section 188(e) is to 

minimize the length of a Serious area attainment date extension, the EPA proposes that 

the implementation of MSM must be as expeditious as practicable but no later than 1 year 

prior to the alternative Serious area attainment date identified by the state in its extension 

request.

The EPA seeks comment on whether the two proposed approaches to determine 

MSM are sufficiently consistent with the agency’s respective proposed approaches to 

BACM and BACT determination. The agency also seeks comment on whether 

considerations regarding its MSM approach should influence the final selection of a 

BACM and BACT approach.

Step 4: Demonstrate attainment by the most expeditious alternative date 

practicable. Section 189(b)(1)(A) requires that a Serious area plan demonstrate 

attainment, using air quality modeling, by the most expeditious date practicable 

after the statutory Serious area attainment date. This demonstration is the final 

criterion that must be met before the EPA may consider granting an extension. 

The agency’s determination of whether the plan provides for attainment by the 

most expeditious date practicable would depend on whether the plan provides for 

implementation of BACM and BACT by the statutory implementation deadline 

and MSM as expeditiously as practicable. In no case would a state be able to seek 

an extension of a Serious area attainment date to a date more than 5 years past the 
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statutory attainment date for Serious areas. Section VI.E of this preamble 

describes the EPA’s proposed requirements for attainment demonstration 

modeling for Serious area attainment plans.

Step 5: Apply for an attainment date extension. The state would have to apply to 

the EPA for any extension of a Serious area attainment date. The request would have to 

accompany an attainment plan submission containing an attainment demonstration 

showing attainment by the most expeditious alternative date practicable, and the state 

would need to submit modeling as part of the attainment demonstration in accordance 

with Section VI.E. Furthermore, the state would have to provide the public reasonable 

notice and a public hearing on the attainment date extension request before submitting it 

to the EPA, as the EPA would consider it an integral part of the attainment demonstration 

and part of the revised SIP submission which is subject to the requirements of the CAA 

and federal regulations for public notice and hearing on SIP revisions.

3. Timing of extension request submittal

The EPA believes that a state may submit a request for an extension of the 

Serious area attainment date either at the time the original Serious area attainment plan is 

submitted following reclassification of the area or at a point in time closer to the Serious 

area attainment date. In the first case, when taken together with language under section 

189(b)(1)(A)(ii) which describes the possibility of including an impracticability 

demonstration in a Serious area attainment plan that parallels the impracticability 

demonstration for a Moderate area attainment plan, section 188(e) appears to set an 

expectation that a state may request an extension of the attainment date for a Serious area 
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when the state initially submits its Serious area plan. Therefore, the EPA would deem 

such a request as timely and appropriate.

On the other hand, the EPA also recognizes that a state may prepare and fully 

implement a timely Serious area plan that includes modeling demonstrating attainment no 

later than the statutory Serious area attainment date (the end of the tenth calendar year 

following designation), and yet may see as the attainment date nears that the Serious area 

will in fact fail to attain by its projected attainment date. While the statute provides a 

remedy to be instituted immediately upon failure of a Serious area to attain the standard 

(through contingency measures and other measures stipulated in section 189(d)), the EPA 

also believes that the criteria of section 188(e) could be applied after a state submits a 

Serious area attainment plan but prior to the area failing to attain, as long as the area had 

not already been granted a prior Serious area attainment date extension under section 

188(e). In such a case, the EPA believes that it would be acceptable for a state to submit a 

Serious area attainment date extension request similar to that described above (for 

submissions made simultaneous with initial Serious area attainment plans) together with 

a new Serious area attainment plan meeting all of the statutory requirements that apply to 

such plans, however the complete submission would have to be made in a timely way 

such that the EPA could fully review the new attainment plan for the area and the 

accompanying attainment date extension request, including the status of compliance with 

all requirements and commitments in the Moderate area attainment plan for the area, the 

justification for the selection of the alternate attainment date, and provisions for the 

implementation of MSM, prior to making its determination of failure of the area to timely 

attain the relevant NAAQS.
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The EPA seeks comment on this option, particularly with respect to whether the 

criteria proposed above are appropriate in a situation in which a state seeks a Serious area 

attainment date extension after submitting a Serious area attainment plan that initially 

demonstrated attainment by the statutory Serious area attainment date. For example, the 

EPA seeks comment in particular on whether it would be appropriate to interpret the 

section 188(e) requirement for a state to have “complied with all requirements and 

commitments pertaining to that area in the implementation plan” as referencing those 

requirements and commitments contained in the area’s Moderate area plan (as proposed 

above for areas seeking a Serious area attainment date extension simultaneous with 

submittal of their Serious area plan) or whether, for areas that already submitted Serious 

area plans demonstrating attainment, it is more appropriate that the state must have 

complied with all requirements and commitments pertaining to the area in the area’s 

original Serious area attainment plan. The EPA believes this second interpretation is the 

more appropriate interpretation as it pertains to Serious areas seeking an extension of 

their attainment date as they approach their statutory Serious area attainment date, and 

therefore the agency is proposing and seeking comment on this approach. The EPA 

believes that this second interpretation is especially preferable if the EPA finalizes its 

proposal that interprets the SIP compliance requirement for areas seeking an attainment 

date extension simultaneous with their Serious area attainment plan submittal to mean 

that the state need only have implemented the control measures in the SIP revisions it has 

submitted to the EPA to address the CAA requirements in section 189 (i.e., to mean that 

the area need not have a fully approved attainment plan that meets the CAA’s 

requirements for Serious areas).
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The EPA seeks comment on these proposed options for interpreting and 

implementing the statutory language at section 188(e) for Serious area attainment date 

extensions.

VII. What requirements would apply to attainment plans under section 189(d) of 

the CAA for Serious areas that fail to attain the NAAQS by the applicable 

attainment date?

In the event that a Serious area fails to attain the PM2.5 NAAQS by the applicable 

attainment date, section 189(d) requires that “the state in which such area is located shall, 

after notice and opportunity for public comment, submit within 12 months after the 

applicable attainment date, plan revisions which provide for attainment of the ...standard 

and, from the date of such submission until attainment, for an annual reduction in PM10

or PM10 precursor emissions within the area of not less than 5 percent of the amount of 

such emissions as reported in the most recent inventory prepared for such area.”

A state must submit to the EPA its SIP submission to meet the requirements of 

section 189(d) in the form of a complete attainment plan submission that contains the 

following elements: (i) an attainment demonstration and provisions for the 

implementation of measures that will achieve annual emissions reductions of not less 

than 5 percent from the most recent emissions inventory for the area for each year until 

attainment (section 189(d)); (ii) quantitative milestones that will be used to measure 

compliance with the RFP requirement (section 189(c)); and, (iii) regulation of PM2.5

precursors (in general to meet attainment and control strategy requirements and as 

specifically required for major stationary sources by section 189(e)). Subpart 1 

requirements that apply to Serious PM2.5 nonattainment areas also subject to the 
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requirements of section 189(d) include the following: (i) a description of the expected 

annual incremental reductions in emissions that will demonstrate RFP (section 

172(c)(2)); (ii) emissions inventories (section 172(c)(3)); and, (iii) contingency measures 

(section 172(c)(9)). A state with a Serious PM2.5 nonattainment area that fails to attain the 

NAAQS by the applicable Serious area attainment date must also address any statutory 

requirements relevant to Moderate nonattainment areas and Serious nonattainment areas 

under sections 172 and 189 of the CAA that have not already been satisfied. In addition, 

the EPA must approve a new attainment date for the area under sections 172(a)(2) and 

179(d)(3).

The remainder of this section presents the EPA’s proposed requirements for 

attainment plan submissions under section 189(d).

A. Plan Due Dates

Section 189(d) requires a state with a Serious PM10 nonattainment area that failed 

to attain the NAAQS by the applicable Serious area attainment date to submit a new

attainment plan submission for the area within 12 months after the missed attainment 

date. Therefore a state with a nonattainment area subject to section 189(d) must submit a 

new attainment plan for the area – with all required elements of the attainment plan –

within 12 months after the missed attainment date.

B. Emissions Inventory Requirements

As with all other attainment plan submissions required for Moderate and Serious 

PM2.5 nonattainment areas, a state must develop its submission to meet section 189(d) 

based on “the most recent emissions inventory prepared for such [nonattainment] area.” 

This inventory must meet the same requirements that would apply to any other emissions 
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inventory submitted for a PM2.5 nonattainment area to meet the requirements of section 

172(c)(3), which requires “a comprehensive, accurate, and current inventory of actual 

emissions of the relevant pollutants” in the nonattainment area. Therefore the EPA 

proposes that the inventory submitted with an attainment plan to meet section 189(d) 

requirements must also meet the EPA’s proposed regulatory requirements for such 

emissions inventories as described earlier in this preamble under Section IV.B (for 

Moderate area attainment plans) and Section VI.B (for Serious area attainment plans).

One important aspect of the emissions inventory required to be submitted with an

attainment plan under section 189(d) is its role as the basis for calculating the emissions 

reductions of direct PM2.5 and PM2.5 precursors necessary to satisfy the 5 percent annual 

reduction criteria of section 189(d). For this reason, the EPA proposes that the “most 

recent inventory” for the area must not only meet the criteria as that described for a base 

year inventory submitted pursuant to section 172(c)(3) and in Section VI.B of this 

preamble, but also fully account for emissions reductions achieved to date through the 

implementation of all RACM and RACT, BACM and BACT and additional reasonable 

and feasible measures submitted with the Moderate and original Serious area attainment 

plans for the area. In this way, the state will calculate the additional reductions that the 

nonattainment area will need beyond those already required in order to fulfill the 

requirements of section 189(d) and bring the area into attainment as expeditiously as 

practicable. 

In order to ensure that the “most recent inventory” is representative of the 

nonattainment problem in the area current at the time of the section 189(d) submission, 

the EPA proposes that the inventory year must be one of the 3 years from which 
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monitored data was used to determine that the area failed to attain the PM2.5 NAAQS by 

the applicable Serious area attainment date. The EPA believes that associating the 

inventory with one of these 3 years is reasonable in light of the fact that some BACM and 

BACT controls and additional feasible controls (required under section 172(c)(6)) for 

sources in the area may not be implemented until the beginning of the attainment year. 

Thus, requiring that a state use an emissions inventory for one of those 3 years will help 

ensure that the inventory adequately captures the emissions reductions already achieved 

through the prior implementation of BACM and BACT and additional feasible measures.

The EPA recognizes the additional level of effort that may be needed to produce 

an up-to-date emissions inventory for a nonattainment area, and therefore is proposing 

and seeking comment on an alternative approach that would allow a state to select an 

inventory year earlier than one of the 3 years from which monitored data were used to 

determine that the area failed to attain the NAAQS by the applicable attainment date. 

Under this alternative proposed approach, another inventory year may be included in the 

plan under specific circumstances with the submission of a written justification for 

selecting the earlier year and in consultation with the appropriate EPA Regional Office. 

At a minimum, the state would need to demonstrate that the inventory for the alternative 

year adequately incorporates emissions reductions projected to be achieved through the 

implementation of BACM and BACT and additional feasible control measures submitted 

with the original Serious area attainment plan for the area. Modification of an older 

inventory to incorporate those emissions reductions would be an acceptable way to meet 

this requirement. In considering use of this option, states could be obligated to achieve a 
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larger annual reduction than 5 percent if the older inventory has higher emissions levels

than the “most recent inventory” for the area.

The EPA seeks comment on these proposed criteria and options for emissions 

inventories to be submitted as part of the attainment plan due for a Serious area under 

section 189(d).

C. Pollutants to be Addressed in the Plan

Section 189(d) requires states to develop a new attainment plan for an area that 

failed to attain by the applicable Serious area attainment date that provides for “an annual 

reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent 

of the amount of such emissions” reported in the latest emissions inventory for the area. 

In Section III of this preamble, the EPA is proposing several options on how a state may 

evaluate which PM2.5 precursors to control for purposes of attaining the NAAQS in a 

particular nonattainment area. The EPA interprets the requirements of the CAA generally 

to allow an air agency to provide a “precursor demonstration” that can support a 

determination that one or more precursors need not be subject to control requirements in 

a given nonattainment area, even if the area has failed to attain the relevant NAAQS by 

the applicable Serious area attainment date. 

Section III presents three options describing different proposed approaches to 

such precursor demonstrations, and requests comment on each. The discussion for each 

option describes how states and the EPA should address precursors for Moderate areas 

and for Serious areas, including Serious areas that fail to attain the PM2.5 NAAQS by the 

applicable attainment date. This section describes, for each of the three options, how the 
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given precursor approach would apply to plans required to be submitted where the area 

has failed to attain by the Serious area attainment date. 

 Option 1: Two independent analyses: (a) an attainment planning analysis 

demonstrating that control measures for a particular precursor are not needed for 

expeditious attainment, meaning that the precursor can be excluded from measures 

needed to attain as expeditiously as practicable for all types of sources; and (b) a 

section 189(e) technical demonstration showing that major stationary sources of a 

particular precursor do not contribute significantly to levels that exceed the PM2.5

standard, meaning that the precursor can be excluded from control requirements for 

major sources and from NNSR permitting. Consistent with this approach, for an area 

subject to the requirements of section 189(d), the state would need to evaluate control 

measures to identify those needed to achieve a minimum 5 percent reduction in 

emissions of direct PM2.5 or precursors on an annual basis, and identify those control 

measures for direct PM2.5 and all precursors that would bring the area into attainment 

as expeditiously as practicable.

 Option 2: Single analysis demonstrating that all emissions of a particular precursor 

from within the area do not significantly contribute to PM2.5 levels that exceed the 

standard, meaning that control requirements for emissions of the precursor from 

major stationary and area sources, as well as mobile sources, would not be required 

for expeditious attainment, control requirements for major sources, or for NNSR 

permitting. For an area subject to section 189(d) requirements for which a precursor 

had previously been demonstrated not to significantly contribute to PM2.5 levels that 

exceed the standard, the air agency would be required to update the precursor 
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demonstration taking into account any relevant information or technical tools that had 

been developed since the demonstration was approved. Consistent with this approach, 

if, upon failure to attain, the state continued to demonstrate that the precursor did not 

contribute significantly to PM2.5 concentrations in the area, then the state would not 

need to identify or implement any measures to control that precursor’s emissions.

 Option 3: An attainment planning analysis demonstrating that control measures for all 

types of sources of a particular precursor are not needed for expeditious attainment 

also would be deemed to meet the section 189(e) technical demonstration 

requirement, meaning that the state would not need to regulate emissions of the 

particular precursor from major stationary sources under the NNSR permitting 

program or other control requirements for major stationary sources. Consistent with 

this approach, for an area subject to the requirements of section 189(d), the state 

would need to evaluate control measures to identify those needed to achieve a 

minimum 5 percent reduction in emissions of direct PM2.5 or precursors on an annual 

basis, and identify those control measures for direct PM2.5 and all precursors that 

would bring the area into attainment as expeditiously as practicable.

The EPA will finalize its approach to PM2.5 precursors and clarify the 

implications for states conducting analyses to identify measures to satisfy the 

requirements of section 189(d) after considering public comment received on this 

proposal.

D. Attainment Plan Control Strategy

The control strategy to be developed for the attainment plan submission for a 

Serious area subject to section 189(d) should place particular emphasis on control 
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measures that can be implemented quickly, in order to ensure that the area attains the 

PM2.5 NAAQS as expeditiously as practicable. The control strategy would need to 

include any additional measures that are beyond those already adopted for the area as 

RACM and RACT and additional reasonable measures, or BACM and BACT and 

additional feasible measures, and that are necessary to achieve annual reductions in 

emissions of direct PM2.5 and PM2.5 precursors from sources in the area of at least 5 

percent of the amount of such emissions reported in the most recent emissions inventory 

for the area. The EPA is proposing to interpret section 189(d) in this way to address the 

ambiguity of how the statutory language should apply to the PM2.5 NAAQS, as section 

189(d) requires “an annual reduction in PM10 or PM10 precursor emissions... as reported 

in the most recent inventory prepared for such area.” 

1. Proposed approach

The EPA believes that in light of the important role that PM2.5 precursors play in 

the formation of PM2.5, it is appropriate to require a state to implement control measures 

for all types of sources in a Serious nonattainment area subject to section 189(d) to 

achieve the requisite 5 percent annual reduction in emissions of both direct PM2.5 and

PM2.5 precursors from sources in that area. Accordingly, the EPA is proposing that, for 

direct PM2.5 and for PM2.5 precursors that the state and the EPA have determined are 

necessary to be controlled for purposes of attainment in the area, the attainment plan 

required by section 189(d) would have to include control measures that will achieve at 

least 5 percent reductions from the latest emissions inventory of each such pollutant on an 

annual basis until the area attains the relevant PM2.5 NAAQS. The EPA believes this is an 
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appropriate interpretation of the 5 percent requirement of section 189(d) and seeks 

comment on this proposed approach. 

The EPA also proposes and seeks comment on an alternative reading of the 

statute that would require a state to achieve 5 percent reductions of inventoried emissions 

of either direct PM2.5 or of any relevant PM2.5 precursors. This approach, while consistent 

with past guidance on how to interpret section 189(d) requirements for PM10 NAAQS 

implementation, could potentially allow a state to delay the implementation of measures 

to control the relevant pollutants. However, paired with the requirement for the area to 

reach attainment of the NAAQS as expeditiously as practicable, the EPA believes that 

such an interpretation may be reasonable and seeks comment on this approach.

It is important to note that under implementation of either of the options presented 

above, and as described more fully in Section III of this preamble, the EPA is proposing 

that in the event that a state has demonstrated and can continue to demonstrate that 

emissions of a given precursor from all sources in a nonattainment area do not contribute 

significantly to PM2.5 concentrations in the area, then the state would not need to achieve 

5 percent reductions in emissions of that precursor even if the nonattainment area 

becomes subject to the requirements of section 189(d).

The statute requires that the requisite minimum 5 percent emissions reductions 

must be calculated from the total emissions for each precursor and for direct PM2.5

contained in the most recent inventory for the area, as described earlier in this section. In 

addition, the EPA proposes that these required reductions must then be achieved every 

year between the section 189(d) plan submission date and the new projected attainment 

date for the area. For example, assume it is 2025, and a Serious area has failed to attain 
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the 2012 PM2.5 NAAQS within 10 years of designation. Assume also that the most recent 

inventory available for an area subject to section 189(d) is for the year 2023. This 

inventory would serve as the base inventory for determining the emissions reduction 

requirement under section 189(d). If the most recent inventory indicates that emissions of 

direct PM2.5 from all sources in the area are 100 tons/day, then the area would need to 

reduce emissions of direct PM2.5 by 5 percent of the base inventory (in this example, 5 

percent of the 2023 base inventory, or 5 tons/day) each year until the area attains the 

NAAQS. Thus, in the first year following submission of the section 189(d) plan for the 

area, emissions of direct PM2.5 could not exceed 95 tons/day; in the second year, 

emissions could not exceed 90 tons/day, and so forth.

Although section 189(d) requires that a state develop measures that will obtain 

annual emissions reductions of “not less than 5 percent” from the most recent inventory, 

the EPA interprets this language to authorize states to elect to front-load emissions 

reductions in earlier years and still meet the 5 percent per year requirement. The EPA 

notes that interpreting the statute in this way will encourage states to implement measures 

earlier, where possible, rather than delay implementation of measures merely to assure 

that the 5 percent requirement can be met in later years. Thus, using the example 

described above, the annual reduction requirement for the area would be 5 tons/day from 

a base year emissions level of 100 tons/day. The required level after year 1 would be 95 

tons/day, after year 2 the level would be 90 tons/day, and so on. If the air agency reached 

a level of 81 tons/day by the end of year 3, then by the end of year 4 it would only need 

to reduce emissions by 1 ton/day to yield an emissions level of 80 tons/day. Consistent 

with its past action to approve a Serious area nonattainment plan for the San Joaquin 
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Valley (CA) PM10 nonattainment area under section 189(d), the EPA therefore proposes 

and seeks comment on an approach to allow states to carry forward any emissions 

reductions beyond the required minimum 5 percent in a given year to the next year as a 

means to encourage states to achieve emissions reductions as quickly as possible.219

The EPA also proposes to clarify its interpretation of the statutory language under 

section 189(d) that requires a state to submit a new attainment plan to achieve annual 

reductions “from the date of such submission until attainment,” to mean annual 

reductions beginning from the due date of such submission until the new projected 

attainment date for the area based on the new or additional control measures identified to 

achieve at least 5 percent emissions reductions annually. This proposed clarification is 

intended to make clear that even if a state is late in submitting its section 189(d) plan, the 

area must still achieve its annual 5 percent emissions reductions beginning from the past 

due date for the section 189(d) plan submission. Because attainment dates for PM2.5

nonattainment areas established under subpart 4 occur at the end of the calendar year, any 

section 189(d) plan, which is required within 12 months of the missed attainment date for 

the area, would also be due by the end of the calendar year.

2. Additional guidance on section 189(d) control measures

The EPA believes that an appropriate starting point for a state to identify 

measures to achieve the requisite minimum 5 percent annual emissions reductions of 

direct PM2.5 and PM2.5 precursors is the list of potential control measures initially 

required as part of the RACM and RACT determination for the area, then updated as part 

 

219 69 FR 30006 (May 26, 2004).
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of the required BACM and BACT determination for the area. The EPA anticipates that a 

state should be able to rely on much of the work it previously undertook to develop this 

list of potential control measures and analyze their technological and economic 

feasibility, and the time required to implement them. However, for purposes of meeting 

the requirements of section 189(d), the EPA recommends that the state first identify any 

additional potential measures not previously identified for the area, and then analyze any 

new or additional measures that the state has not already adopted in a previous attainment 

plan for the area. The EPA expects that such an analysis to identify new control measures 

would necessarily take into account recent technological advances in control 

technologies, the possibility of a greater availability of funding to expand implementation 

of control measures for area sources, and the additional time the area will have to attain 

the PM2.5 NAAQS under sections 189(d) and 179(d)(3). 

In addition, a state may include in the section 189(d) plan control strategy for the 

area any control measures triggered as contingency measures upon the EPA’s 

determination that area failed to attain the PM2.5 NAAQS by the applicable attainment 

date. In order to be included as control measures that will help the area meet its requisite 

minimum 5 percent reductions in direct PM2.5 and PM2.5 precursor emissions, such 

measures would have to meet the same requirements as all other approvable control 

measures for being quantifiable, enforceable, replicable and accountable. The EPA 

believes that reliance on such measures is appropriate given the short timeline provided 

for in the statute for states to revise and submit their SIP revisions (12 months from the 

missed attainment date) and the fact that the contingency measures included in the prior 

attainment plan for the area under section 172(c)(9) must be activated once the EPA 
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publishes its finding of the area’s failure to attain the NAAQS by the applicable 

attainment date. If contingency measures from the Serious area attainment plan are relied 

on in the new attainment demonstration as part of the control strategy, the state will need 

to submit additional contingency measures for the section 189(d) attainment plan 

submission.

3. Control strategy submission requirements

To ensure that attainment plan submissions contain the necessary supporting 

information for the EPA to review and approve the state’s new control strategy to achieve 

at least 5 percent reductions in emissions of direct PM2.5 and significant PM2.5 precursors, 

the EPA proposes to require under the authority of section 301(a) that a state must submit 

the following information as part of its section 189(d) plan submission: 

 A list of all emissions source categories, sources and activities in the 

nonattainment area (for multi-state nonattainment areas, this would include source 

categories, sources and activities from all states which make up the area);

 For each source category, source or activity in the nonattainment area, an 

inventory of direct PM2.5 and all PM2.5 precursor emissions;

 For each source category, source or activity in the nonattainment area, a 

comprehensive list of potential control measures considered by the state for those 

sources in the nonattainment area; 220

 

220 Menu of Control Measures document available at http://www.epa.gov/air/criteria.html

(last accessed 01/14/14).
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 For each potential control measure considered by the state but eliminated from 

further consideration due to a determination by the state that the control measure 

or technology was not technologically feasible, a narrative explanation and 

quantitative or qualitative supporting documentation to justify the state’s 

conclusion;

 For each technologically feasible emission control measure or technology, the 

state must provide the following information relevant to economic feasibility: (i) 

the control efficiency by pollutant; (ii) the possible emission reductions by 

pollutant; (iii) the estimated cost per ton of pollutant reduced; and, (iv) a 

determination of whether the measure is economically feasible, with narrative 

explanation and quantitative supporting documentation to justify the state’s 

conclusion;

 For each technologically and economically feasible emission control measure or 

technology, the date by which the technology or measure could be implemented.

As with other PM2.5 attainment plan submissions, the EPA believes that it is 

incumbent on the state to ensure that the information needed for the EPA to evaluate the 

state’s analysis of new control measures needed to achieve annual 5 percent reductions is 

presented separately as part of the control strategy analysis and in a format that provides 

transparency, consistency and the ability for another party to evaluate the state’s analysis 

effectively and to duplicate the state’s results. For this reason, the EPA is including the

section 189(d) plan base year emissions inventory information as a necessary part of the 

control strategy submittal and as one element of the state’s section 189(d) plan due 12 

months after the missed attainment date for the area. In addition, the EPA proposes that 
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the state must provide information as part of any attainment plan submitted to meet the 

requirements of section 189(d) consistent with the criteria described in Section VI.D.5 of 

this preamble to ensure that a state adopts effective regulations to implement the control 

measures identified as being needed to meet those requirements. Specifically, all control 

measures must be quantifiable, enforceable, replicable and accountable.

The section 189(d) requirement to reduce emissions by 5 percent per year is in 

effect a fixed level of RFP to be achieved annually. Accordingly, just as quantitative 

milestones are used to track progress with RFP requirements, the EPA proposes that the 

state would be required to submit quantitative milestone reports to describe the area’s 

progress in meeting the 5 percent annual emission reduction requirement under section 

189(d). See Section VII.G of this preamble for more details.

E. Modeling for Attainment Demonstrations

Section 189(d) requires a state with a Serious nonattainment area that failed to 

attain the relevant NAAQS by the applicable Serious area attainment date to submit a 

new attainment plan for such area within 12 months after the missed attainment date. The 

EPA is proposing that the same general requirements for attainment demonstrations and 

modeling that apply to Moderate area plans and Serious area plans due under sections 

189(a) and 189(b) should also apply to section 189(d) attainment plans. However, the 

EPA is proposing additional requirements specific to plans states submitted pursuant to

section 189(d) as described below.

1. Attainment demonstrations for Serious areas that fail to attain the NAAQS by 

the applicable attainment date
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The EPA is proposing that the attainment demonstration for Serious areas subject 

to section 189(d) requirements must consist of: (i) technical analyses such as base year 

and future year modeling of emissions which identifies sources and quantifies their 

emissions that are contributing to violations of the PM2.5 NAAQS; and, (ii) analyses of 

future year projected emissions reductions and air quality improvement resulting from 

national, regional and local programs already implemented as part of previous Moderate 

and/or Serious area attainment plans for the area (including reasonable control measures, 

BACM and BACT and additional feasible measures), and additional measures needed for 

expeditious attainment, including measures needed to achieve 5 percent emissions 

reductions on an annual basis. Each state with a nonattainment area subject to the 

requirements of section 189(d) must submit an attainment plan with an attainment 

demonstration that includes analyses supporting the state’s determination of its proposed 

new attainment date. In all cases, the state must show that the area will attain the NAAQS 

as expeditiously as practicable.

2. What modeling is required?

States are required to submit air quality modeling in support of an attainment 

demonstration for a nonattainment area subject to the requirements of section 189(d). The 

modeling demonstration must show how and when the area will attain the NAAQS. Other 

than the timing of plan submissions and requirement to achieve 5 percent emissions 

reductions in direct PM2.5 and PM2.5 precursors, the relevant air quality modeling 

procedures and guidance for all PM2.5 nonattainment area plans are the same. See

Sections IV.E. and VI.E of this preamble for more details on proposed modeling 
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requirements and guidance for Moderate and Serious PM2.5 nonattainment areas, 

respectively. 

3. What future year(s) should be modeled in attainment demonstrations?

As discussed more fully in Section VII.I of this preamble, the EPA must establish 

a new attainment date for a PM2.5 nonattainment area subject to section 189(d) and must 

do so according to the provisions of sections 179(d)(3) and 172(a)(2), which require that 

the new attainment date must be as expeditious as practicable, but no later than 5 years 

from the date of publication in the Federal Register of the EPA’s determination that the 

area failed to attain the relevant NAAQS. The EPA may extend the attainment date by up 

to 5 additional years (thus to 10 years from the date of publication of the notice of finding 

of failure to attain by the applicable attainment date for the area) if the agency deems it 

appropriate “considering the severity of nonattainment and the availability and feasibility 

of pollution control measures.”

For purposes of determining the attainment date that is as expeditious as 

practicable, the state must conduct future year modeling which takes into account 

emissions growth, known controls (including any controls that were previously 

determined to be RACM or RACT or additional reasonable measures, or BACM or

BACT or additional feasible measures for the area), the 5 percent per year emissions 

reductions required by section 189(d), plus any other emissions controls that are needed 

for expeditious attainment of the NAAQS. A state performing a modeling analysis for a 

plan submitted under section 189(d) must select a future modeling year such that all 

emissions control measures relied on for attainment will have been implemented by the 

beginning of that year. To demonstrate attainment, the modeling results for the 
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nonattainment area must predict that emissions reductions implemented by the beginning 

of the last calendar year preceding the attainment date will result in PM2.5 concentrations 

that meet the level of the standard.221

For a PM2.5 nonattainment area subject to section 189(d), the EPA expects that the 

state will adopt any control measures necessary to demonstrate expeditious attainment 

within 5 years of the area failing to attain the NAAQS by the applicable Serious area 

attainment date.

4. Attainment year motor vehicle emissions budgets

As with all other PM2.5 NAAQS attainment plans, the transportation conformity 

rule requires that attainment plans for areas subject to section 189(d) establish motor 

vehicle emissions budgets for the area’s attainment year. Therefore, for such an area, the 

state would first determine the new attainment date as described in Section VII.I of this 

preamble. Once an area’s attainment date has been established, the state would establish 

motor vehicle emissions budgets for direct PM2.5 and any relevant PM2.5 precursor for the 

attainment year.

222 A motor vehicle emissions budget for the purposes of a PM2.5

attainment plan is that portion of the total allowable emissions within the nonattainment 

 

221 Note that for purposes of the PM2.5 NAAQS, a determination of attainment (or failure 

to attain), which the EPA is required to make after the attainment date has passed, is 

based on an average of the most recent 3 years’ worth of ambient data prior to the area’s 

attainment date.

222 For more information on PM2.5 precursor requirements, see section 93.102(b)(2)(iv) 

and (v) of the transportation conformity rule. See also the May 6, 2005, final 

transportation conformity rule that addressed requirements for PM2.5 precursors. (70 FR 

24280).
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area allocated to on-road sources as defined in the submitted attainment plan.

223 Such 

motor vehicle emissions budgets would be calculated using the latest planning 

assumptions and the latest approved motor vehicle emissions model available at the time 

that the attainment plan is developed.224

The EPA seeks comment on these proposed attainment demonstration and 

modeling requirements for new Serious area attainment plans due under section 189(d).

F. RFP Requirements

As with other PM2.5 attainment plans, a plan submitted to meet the requirements 

of section 189(d) must provide for RFP as required under sections 172(c)(2) and 

189(c)(1). Section 171(1) defines RFP as “such annual incremental reductions in 

emissions of the relevant air pollution as are required by this part or may reasonably be 

required by the Administrator for the purpose of ensuring attainment of the applicable 

[NAAQS] by the applicable attainment date.” The purpose of RFP requirements is to 

assure that a state is making progress towards attainment on an annual basis through the 

attainment plan, rather than deferring emissions reductions until just before the 

attainment date for the area. This requirement is similar to, though less prescriptive than, 

the requirement under section 189(d) for 5 percent emissions reductions of direct PM2.5

or PM2.5 precursors from the most recent emissions inventory on an annual basis until the 

 

223 A state would also establish motor vehicle emissions budgets for an area’s attainment 

year. Those budgets would be the motor vehicle emissions that the SIP establishes as 

being necessary to attain the NAAQS.

224 If an area includes re-entrained road dust in the motor vehicle emissions budget, the 

latest approved version of AP-42 should be used unless the EPA has approved an 

alternative model for the area.
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area attains. Therefore, the EPA proposes to determine that a state has satisfied the RFP 

requirement if the state submits an approvable control strategy under section 189(d) that 

demonstrates that the state will achieve at least 5 percent reductions in direct PM2.5 and 

PM2.5 precursor emissions from sources in the area annually until attainment.

The EPA proposes that motor vehicle emissions budgets must also be established 

as part of any RFP plan for direct PM2.5 and for any relevant PM2.5 precursor using the 

latest planning assumptions and the latest approved motor vehicle emissions model 

available at the time that the Serious area plan due under section 189(d) is developed.225

The EPA seeks comment on this proposed approach related to RFP requirements 

for new attainment plans due under section 189(d).

G. Quantitative Milestones

The revised attainment plan for any Serious nonattainment area that fails to attain 

the relevant PM2.5 NAAQS by the applicable attainment date must include quantitative 

milestones pursuant to section 189(c). These quantitative milestones would be additional 

to those previously identified in the Moderate area and original Serious area attainment 

plans, and would need to reflect the projected emissions reductions or air quality 

improvements expected through the implementation of specific control measures 

identified to achieve the minimum 5 percent annual reductions required under section 

189(d). Such milestones would need to be achieved every 3 years until the area attains the 

relevant NAAQS, such that the EPA proposes that, at a minimum, quantitative milestones 

 

225 If an area includes re-entrained road dust in the motor vehicle emissions budget, the 

latest approved version of AP-42 should be used unless the EPA has approved an 

alternative model for the area.
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selected for an attainment plan submitted under section 189(d) would need to 

demonstrate a reduction of at least 15 percent in emissions of direct PM2.5 and significant 

precursors below those emissions reported in the most recent inventory for the area. 

The section 189(d) plan for an area that failed to attain the standard by the 

applicable Serious area attainment date would have to contain quantitative milestones to 

be achieved by 13.5 years from the area’s date of designation and every 3 years thereafter 

until the area’s new projected attainment date. In the event a state is developing a revised 

attainment plan pursuant to section 189(d) that will be due sometime after 13.5 years

following designation of the area, the EPA proposes to allow the state to submit 

quantitative milestones beginning for the year 16.5 from designation and every 3 years 

thereafter until the area’s projected attainment date.

The EPA believes that its proposed requirements for quantitative milestones, 

described in Sections IV.G and VI.G of this preamble, should also apply to quantitative 

milestones submitted with any revised attainment plan pursuant to section 189(d), and 

thus proposes and seeks comment on the agency’s proposed milestone requirements for 

application to attainment plans due under section 189(d).

H. Contingency Measures

All PM2.5 attainment plans, including plans for areas subject to section 189(d), 

must contain contingency measures that are consistent with section 172(c)(9). Section 

VI.H of this preamble describes the EPA’s proposed criteria for contingency measures 

for a Serious area attainment plan, and the agency proposes that contingency measures 

for a section 189(d) plan must meet the same criteria. The EPA proposes that the 

emissions reductions associated with contingency measures for section 189(d) plans must 
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be at least 5 percent of direct PM2.5 and significant PM2.5 precursor emissions as reported 

in the most recent inventory for the area. The EPA believes this requirement would 

appropriately align the proposed requirement for selecting contingency measures with the 

agency’s proposed approach to RFP for these areas. In other words, if RFP for an area is 

about 1 year’s worth of emissions reductions, then the adopted contingency measures 

should likewise achieve about 1 year’s worth of emissions reductions.

The EPA recognizes that identifying contingency measures for a Serious PM2.5

nonattainment area that failed to attain the relevant NAAQS by the applicable attainment 

date may be challenging for a state that should already have fully implemented all control 

measures identified as “reasonable” and “best,” and potentially “most stringent,” in 

addition to identifying new control measures to achieve the requisite minimum 5 percent 

reductions in direct PM2.5 and significant PM2.5 precursor emissions necessary for 

expeditious attainment. Nonetheless, given the statutory language of section 172(c)(9), 

the EPA seeks comment on applying the same proposed requirements for contingency 

measures for section 189(d) plans, and on the agency’s proposed approach for calculating 

the emissions reductions that such measures must be able to achieve.

I. Attainment Dates

As previously discussed, section 189(d) requires a minimum 5 percent annual 

reduction in emissions of direct PM2.5 and PM2.5 precursors until the area attains the 

relevant NAAQS. However, neither section 189(d) nor other sections in subpart 4 

explicitly establish or provide the authority to establish a new attainment date for the 

area; other Subpart 4 attainment date provisions for Moderate or Serious areas are 

likewise not applicable to areas in this situation. Therefore, once an area is beyond the 
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attainment dates that Congress specified in subpart 4 for the PM10 NAAQS, the EPA 

must look to the existing provisions of the CAA to provide authority for a new attainment 

date. Sections 179(d)(3) and 172(a)(2) provide generally applicable attainment dates that 

fill the gap in the statute left for areas subject to the requirements of section 189(d). Thus, 

for a PM2.5 nonattainment area subject to section 189(d) requirements, the EPA must 

establish a new attainment date, and must do so according to the provisions of section 

179(d)(3) and 172(a)(2). The EPA has followed this same approach in the past for PM10

nonattainment areas governed by subpart 4 nonattainment requirements.226

The new attainment date must be as expeditious as practicable, but no later than 5 

years from the date of publication in the Federal Register of the EPA’s determination that 

the area failed to attain the relevant NAAQS. The EPA may extend the attainment date by 

up to 5 additional years (thus to 10 years from the date of publication of the notice of 

finding of failure to attain by the applicable attainment date for the area) if the agency 

deems it appropriate “considering the severity of nonattainment and the availability and 

feasibility of pollution control measures.” For a PM2.5 nonattainment area subject to 

section 189(d), the EPA expects that the state will adopt any control measures necessary 

to demonstrate expeditious attainment within 5 years of the area failing to attain the 

NAAQS by the applicable Serious area attainment date.

As discussed earlier in this section, the EPA will consider the state’s attainment 

demonstration and proposed attainment date for the area, in addition to the state’s revised 

 

226 For example, see the Federal Register notice from June 6, 2007 (72 FR 31183) in 

which the EPA found that the Phoenix PM10 Serious nonattainment area failed to attain 

the standard by the 2006 attainment date.
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control strategy and the relevant facts and circumstances, in order to identify the most 

expeditious attainment date practicable for the area.

The EPA seeks comment on this proposal for interpreting the statutory 

requirements under section 189(d) for a Serious area that fails to attain the PM2.5 NAAQS 

by the applicable attainment date.

VIII. What NNSR permitting requirements would apply in nonattainment areas for 

the PM2.5 NAAQS?

A. Statutory Requirements for NSR

Section 110(a)(2)(C) of the CAA requires states to include in their SIPs a 

preconstruction review permitting program that regulates the construction and 

modification of stationary sources as necessary to ensure that NAAQS are achieved. To 

address the regulation of the larger pollutant-emitting sources (defined as major 

stationary sources), Congress provided specific permitting requirements in the CAA in

parts C and D of title I. The requirements for preconstruction permits under parts C and D 

of the CAA are commonly known collectively as the major NSR program because they 

apply specifically to the preconstruction review and permitting of new major stationary 

sources, and major modifications at existing sources. As explained in Sections VIII.A.1 

and 2 of this preamble, the preconstruction review of each new and modified major 

stationary source generally is carried out on a pollutant-specific basis and the 

requirements with regard to each pollutant apply based on whether the area in which the 

proposed major source or major modification would locate is designated attainment (or 

unclassifiable) or nonattainment for that pollutant at the time the permit is issued. 

1. PSD
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Part C of title I of the CAA (hereafter referred to simply as Part C) contains 

implementation plan requirements that apply to new major stationary sources and major 

modifications in areas designated attainment or unclassifiable for any NAAQS. These 

requirements constitute the PSD program. Pursuant to Part C, the EPA has adopted PSD 

regulations at 40 CFR 51.166 (minimum requirements for an approvable state PSD 

program in the SIP) and 40 CFR 52.21 (the federal PSD program, applicable in areas 

where the state does not have an EPA-approved PSD program in its SIP). The EPA last 

amended the PSD regulations for PM2.5 on January 15, 2013, in the final rule revising the 

PM2.5 NAAQS.227 The present notice does not relate to the PSD program, nor does it 

propose further changes to the PSD regulations. Any future revisions to the PSD 

regulations for PM2.5 would be done through a separate notice-and-comment rulemaking.

2. NNSR

Part D of title I of the CAA (hereafter referred to as Part D) contains 

implementation plan requirements for nonattainment areas, which include the 

requirements for permitting new major stationary sources and major modifications in 

designated nonattainment areas, referred to as the NNSR program. As noted earlier, part 

D contains several subparts that include various requirements for addressing 

nonattainment areas. Subpart 1 addresses plan requirements for nonattainment areas 

generally, including section 172(c)(5) which requires preconstruction and operating 

 

227 More information on the PSD requirements for PM2.5 as well as the public comments 

and the EPA’s responses to those comments and the related issues for which comments 

were received is contained in the January 15, 2013 Federal Register document (78 FR 

3086, beginning at page 3251).
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permits for new major stationary sources and major modifications in nonattainment areas. 

Section 173 outlines the minimum statutory requirements for a state’s NNSR permit 

program and serves as the basis for the EPA’s NNSR regulations for PM2.5 as 

promulgated in the 2008 PM2.5 NSR Rule. Subpart 4 was added to part D as part of the 

1990 CAA Amendments and includes additional plan provisions for designated PM10

nonattainment areas. Relevant here, section 189(a)(1)(A) of subpart 4 requires states to 

include in their implementation plan a permit program addressing major stationary 

sources of PM10 that meets the requirements under section 173 of subpart 1. Subpart 4 

also includes some additional preconstruction review requirements for which, to date, the 

EPA has promulgated NSR regulations applying only to major stationary sources of PM10

in PM10 nonattainment areas. The specific NNSR requirements contained in both subparts 

1 and 4 are described below including the changes to the NNSR regulations needed to 

address PM2.5 specifically that the EPA is proposing in this notice. 

B. Federal NNSR Regulations

Federal regulations pertaining to the preconstruction permitting of new major 

stationary sources and major modifications in areas designated nonattainment are 

contained at 40 CFR 51.165; part 51, Appendix S; and §52.24. An approved NNSR 

program in a state’s implementation plan must, at a minimum, meet the program 

requirements set forth in the federal NNSR requirements at 40 CFR 51.165, which for 

PM2.5 are currently based on changes made under the 2008 PM2.5 NSR Implementation 

Rule. States are required to adopt regulations consistent with those plan requirements and 

submit them to the EPA for approval as part of their SIP within a period of time 

consistent with the schedule prescribed by the CAA. 
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The EPA interprets the requirement established under section 110(a)(2)(C) of the 

CAA for states to regulate the construction and modification of sources to apply in 

nonattainment areas as of the effective date of a new nonattainment area designation.228

Although section 110(a)(2)(C) does not contain specific requirements a state must follow 

for issuing major source permits during the interim period between effective date of

designation and the date when a state has an EPA-approved NNSR program, the EPA 

regulation at 40 CFR 52.24(k) authorizes states to apply 40 CFR part 51, Appendix S, 

known as the Emission Offset Interpretative Ruling, during the interim period. 229, 230

Accordingly, states with newly designated nonattainment areas for the revised 

primary PM2.5 NAAQS have two possible means by which they can implement NNSR 

requirements for PM2.5 following the effective date of the designations and until the EPA 

approves a SIP submission meeting the NNSR requirements for PM2.5 promulgated in 

 

228 70 FR 71612 at 71677-78 (November 29, 2005).

229 States with designated PM2.5 nonattainment areas were required to submit SIPs 

satisfying the requirements of the 2008 PM2.5 NSR Implementation Rule by May 16, 

2011, 3 years from the date of publication of that rule. See 73 FR 28342. Such approved 

state programs can continue to be implemented to issue permits to new major stationary 

sources and major modifications until the state’s revised program containing the subpart 

4 NNSR provisions promulgated in this rulemaking is approved under the applicable SIP.

230 Appendix S was originally promulgated in 1976 to address whether, and to what 

extent, new and modified sources would be allowed to construct in nonattainment areas 

whose attainment deadlines had already passed, in light of the regulatory requirement that 

new or modified sources be disapproved where the source would interfere with 

attainment of the NAAQS (41 FR 55524 (December 21 1976)). When Congress added 

the part D provisions in the 1977 CAA Amendments, it also added the requirement that 

SIPs contain NNSR provisions as set forth in Part D. Additionally, Congress provided 

that Appendix S would govern preconstruction permitting in nonattainment areas lacking 

approved part D SIPs before a construction ban went into effect. When Congress 

removed the construction ban via the 1990 CAA Amendments (except as provided for in 

section 110(n)(3)) it left in place the use of the interim NNSR program under Appendix 

S.
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this rule under subpart 4. First, any state that has an approved NNSR program for PM2.5

can continue to apply those permitting requirements in the interim. Second, states that 

lack any approved NNSR program for PM2.5 may rely upon the NNSR provisions in 

Appendix S until the EPA approves a SIP submission from the state to address PM2.5 in 

order to ensure that proposed new major stationary sources and major modifications for 

PM2.5 in newly designated PM2.5 nonattainment areas undergo the appropriate type of 

preconstruction review in the interim.

1. General applicability

New major stationary sources are subject to the NNSR requirements when they 

are major for the pollutant for which an area is designated nonattainment. See 40 CFR 

51.165(a)(2)(i). With regard to major modifications, NNSR applies to proposed physical 

changes or changes in the method of operation of an existing stationary source that (1) is 

major for the nonattainment pollutant (or a precursor for that pollutant) and (2) results in 

both a significant emissions increase and a significant net emissions increase of that 

nonattainment pollutant (or a precursor for that pollutant).231

For each proposed major new source and major modification, the general NNSR 

requirements that are required to be included in a state’s SIP include (1) the installation 

and continuous operation of pollution control technology that complies with the LAER; 

 

231 As will be explained in ensuing discussions, the nonattainment pollutant and any 

applicable precursors for that pollutant are considered separately for NNSR applicability 

purposes. See 40 CFR 51.165(a)(1)(v)(A), (a)(2)(ii)A).
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(2) the acquisition of creditable emissions reductions to adequately offset the proposed 

emissions increase of the nonattainment pollutant; and (3) a demonstration of compliance

with other analyses as required under section 173 of the CAA.232 These NNSR 

requirements must be satisfied by a major new source or major modification as a 

prerequisite for receipt of a construction permit and apply as of the effective date of the 

designation of an area as nonattainment for the pollutant.

2. Historical overview of NNSR for PM10 and PM2.5 NAAQS

Following the adoption of new PM NAAQS based on the PM10 indicator in 1987 

(replacing the original Total Suspended Particulate indicator), the EPA announced that it 

did not intend to designate areas as nonattainment for PM10. As a result, the EPA initially 

determined that Part D, which at that point consisted only of generally applicable 

requirements, did not apply to the PM10 NAAQS.233 Thus, nonattainment area 

requirements, including the NNSR program, did not initially apply with respect to PM10. 

Consequently, all new major stationary sources and major modifications of PM10 were 

required to undergo PSD review as a prerequisite for construction or modification. 

The approach for implementing the NNSR program for PM changed when in 

1990 Congress established a new subpart 4 specifically to address implementation plan 

 

232 The basic NNSR requirements are set forth in section 173 of subpart 1. Subpart 4 adds 

a more stringent definition of “major source” for PM10 sources in PM10 nonattainment 

areas classified as Serious and sets forth provisions for the regulation and potential 

exemption of major sources of PM10 precursors in PM10 nonattainment areas. Until the 

decision in NRDC v. EPA was issued, the additional Subpart 4 requirements had not been 

directly applied with regard to PM2.5.

233 At the time the EPA promulgated the new PM10 NAAQS, part D of the CAA did not 

include subpart 4. See 52 FR 24672 (July 1, 1987).
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requirements for PM10 nonattainment areas, including new preconstruction permit 

requirements for major stationary sources and major modifications with respect to PM10

and PM10 precursors. Moreover, Congress created new PM10 nonattainment areas through 

designations that became effective upon enactment of the 1990 Amendments on 

November 15, 1990.234 In section 189(a)(2)(A) of subpart 4, Congress also required states 

to submit the necessary NNSR permit program SIP revisions for these areas to the EPA 

by June 30, 1992.

In a letter to its Regional Offices dated March 11, 1991,235 the EPA initially 

indicated that states should implement such new requirements by operation of law, 

without the need for formal rulemaking by the EPA to establish the necessary 

requirements for states to adopt. In the General Preamble, the EPA offered states 

additional guidance and described the EPA’s preliminary views on how the states and the 

EPA should interpret various provisions of the 1990 Amendments, primarily those 

provisions concerning planning and control measure requirements for the attainment of 

the NAAQS in nonattainment areas. In a 2005 final rule, the EPA formally amended the 

NNSR regulations to incorporate the requirements contained in subpart 4 of Part D of the 

1990 CAA Amendments concerning PM10 nonattainment areas. 236

 

234 See section 107(d)(4)(B) of the CAA. The EPA subsequently published a list of the 

statutorily created PM10 areas in a Federal Register document at 55 FR 45799 (October 

31, 1990).

235 The EPA memorandum titled “New Source Review (NSR) Program Transition 

Guidance,” signed by John S. Seitz, Director, Office of Air Quality Planning & 

Standards.

236 See “Final Rule to Implement Certain Aspects of the 1990 Amendments Relating to 

New Source Review and Prevention of Significant Deterioration as They Apply in 
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The EPA revised the PM NAAQS in 1997, establishing new annual and 24-hour 

NAAQS using PM2.5 particles as a new indicator, while retaining the NAAQS for 

PM10.

237 In 2006, the EPA again revised the suite of PM NAAQS by tightening the 24-

hour PM2.5 standards and retaining the level of the annual PM2.5 standards.

238 In 2008, the 

EPA issued the PM2.5 NSR Rule that established various provisions ensuring that 

proposed new major stationary sources or major modifications of sources of direct PM2.5

emissions or emissions of applicable PM2.5 precursors would be required to undergo 

preconstruction review.239 The EPA included specific provisions in the 2008 PM2.5 NSR 

Rule to apply when such sources are located in a designated PM2.5 nonattainment area. 

Unlike the NNSR requirements for PM10 developed under subpart 4, the EPA determined 

that the applicable implementation requirements for the PM2.5 NAAQS were contained in 

the general nonattainment provisions under subpart 1. 

With regard to NSR applicability for PM2.5 precursors in the 2008 PM2.5 NSR 

Rule, the EPA recognized NOx, SO2, VOC, and ammonia as precursors of PM2.5 in the 

scientific sense (because those pollutants under the appropriate conditions can contribute 

to the formation of PM2.5 in the ambient air) but did not require that states subject all of 

these precursors to control as part of the attainment plan or NSR permitting requirements 

applicable to a given nonattainment area.

240 Instead, based on the authority in section 

 

Carbon Monoxide, Particulate Matter and Ozone NAAQS.” 70 FR 71611 (November 29, 

2005).

237 See 62 FR 38652 (July 18, 1997).

238 See 71 FR 61144 (October 17, 2006).

239 See 73 FR 28321 (May 16, 2008).

240 See 72 FR 20589.
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302(g) of the CAA, the EPA established the initial presumptions for nonattainment areas 

that SO2 and NOx should be regulated precursors for PM2.5, but VOC and ammonia need 

not be regulated precursors. The EPA or the states could rebut the initial presumptions 

regarding NOx, VOC or ammonia on an area-by-area basis with a demonstration 

approved by the Administrator and thus reverse any of those presumptions in the state’s 

implementation plan for that area.

241

As described above in Section II.C of this preamble, the court in NRDC v. EPA

held that the EPA erred in implementing the PM2.5 NAAQS pursuant only to the general 

implementation requirements in subpart 1, rather than also to the implementation 

requirements specific to particulate matter in subpart 4. Accordingly, the court directed 

the EPA to comply with the requirements of subpart 4 when developing implementing 

regulations for PM2.5 nonattainment areas. 

The court decision, requiring that the EPA implement the PM2.5 NAAQS

consistent with the requirements of subpart 4, clearly has specific implications for 

implementing the NNSR program for PM2.5. Two provisions of subpart 4 impose 

additional requirements on the existing NNSR program requirements for PM2.5. The first 

relates to the definition of “major stationary source.” Section 188(b) of subpart 4 provides 

that some areas initially designated as Moderate areas for PM10 subsequently may be 

reclassified as Serious areas. For any PM10 nonattainment area classified as a Serious 

area, section 189(b)(3) provides that a major stationary source of PM10 be defined to 

 

241 In the 2008 PM2.5 NSR Rule, the EPA concluded that SO2 should be regulated as a 

precursor for PM2.5 in all areas. See 73 FR 28327. 
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include any stationary source or group of stationary sources located within a contiguous 

area and under common control that emits or has the potential to emit at least 70 tpy of 

PM10. In accordance with the statute, the EPA is proposing to establish a major source 

emissions threshold for stationary sources of PM2.5 that satisfies the intent of section 

189(b)(3). 

The second relevant subpart 4 provision governs the treatment of major sources of 

PM10 precursors. As previously explained in Section III.A of this preamble, the court 

specifically criticized the EPA’s prior establishment of the rebuttable presumptions for 

addressing PM2.5 precursors, specifically citing the requirement of section 189(e). Section 

189(e) requires that the control requirements in the plan applicable to major stationary 

sources of PM10 must also apply to major stationary sources of PM10 precursors. Section 

189(e) also provides that states may elect not to impose control requirements major 

stationary sources of PM10 precursors if such emissions do not contribute significantly to 

ambient PM10 concentrations that exceed the standard in the PM10 nonattainment area.

Section 189(e) requires that the EPA must make this determination, and thus the EPA 

must approve the decisions of a state that elects to use this provision to exempt any major 

stationary sources of PM2.5 precursors from controls in its attainment plan or NNSR 

program. 

The court’s observation that the EPA’s prior presumptions regarding precursors 

were inconsistent with the explicit requirements of section 189(e) that major sources of 

all PM2.5 precursors are subject to control requirements thus necessitates that the agency 

revise the NNSR regulations governing precursors for PM2.5. As explained in greater 

detail later in this section, the EPA is proposing different potential options to make the 
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necessary changes to the NNSR regulations in order to address the precursor 

requirements contained in subpart 4.

C. What are the changes the EPA is proposing for NNSR for PM2.5 nonattainment areas?

In this section, the EPA presents for comment certain proposed revisions to the 

NNSR regulations as well as alternative approaches for incorporating the subpart 4 

requirements into the NNSR regulations for PM2.5. The proposed changes would affect 

the existing regulations at 40 CFR 51.165 and part 51 Appendix S. The agency does not 

intend to propose any changes to the regulations at 40 CFR 52.24, which provide the 

authorization for states to issue NNSR permits to major new sources and major 

modifications “during the period between the date of designation as nonattainment and 

the date the NSR permit program meeting the requirements of part D is approved.” 

1. What are the changes the EPA is proposing for the NNSR requirements for 

PM2.5 at 40 CFR 51.165?

As explained above, the existing NNSR regulations applicable to PM2.5 are based 

solely on the permit requirements contained in section 173 of subpart 1. In subpart 4, 

section 189(a)(1)(A) requires states to include in their SIPs for PM10 nonattainment areas 

a permit program meeting the requirements of section 173; however, other provisions in 

subpart 4 add additional requirements for the NNSR permit program. Those additional 

provisions concern (1) the definition of “major stationary source” in nonattainment areas 

classified as Serious areas, and (2) control requirements for applicable major stationary 

sources of PM10 precursors. While those particular requirements in subpart 4 refer 

specifically to PM10, the EPA is proposing to add similar requirements for PM2.5 in 
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accordance with the court’s holding in NRDC v. EPA that subpart 4 also governs 

implementation of the PM2.5 NAAQS. 

a. Definition of “major stationary source” in Serious PM2.5 nonattainment areas. 

In Section III.A of this preamble, the EPA indicated its intention to propose new 

provisions based on the requirements in subpart 4 for classifying certain PM2.5

nonattainment areas as Serious areas. Because the existing NNSR regulations for PM2.5, 

as set forth in the 2008 PM2.5 NSR Rule, were developed pursuant to subpart 1, which 

does not provide for the classification of designated nonattainment areas, the EPA has not 

yet developed regulations to address subpart 4 requirements concerning nonattainment 

areas classified as Serious. With respect to NNSR, section 189(b)(3) in subpart 4 

provides that, for any PM10 nonattainment area classified as Serious, the major source 

threshold with regard to the terms “major source” and “major stationary source” shall be 

70 tpy of PM10. Accordingly, the EPA is proposing to amend the NNSR regulations at 40 

CFR 51.165 consistent with this provision to establish a major source threshold for new 

major stationary sources and major modifications in PM2.5 nonattainment areas classified 

as Serious consistent with subpart 4. The EPA is proposing to set the major source 

threshold for direct PM2.5 emissions at 70 tpy. See proposed 40 CFR 

51.165(a)(1)(iv)(A)(1)(vii).

While the court decision did not mandate that the EPA define “major source” and 

“major stationary source” for PM2.5 at a threshold of 70 tpy of PM2.5 emissions for areas 

classified as Serious, the most straightforward and consistent application of section 

189(b)(3) to PM2.5 nonattainment areas is to establish the same numerical threshold for 

serious PM2.5 nonattainment areas as applies to serious PM10 nonattainment areas. 
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Moderate nonattainment areas for both PM10 and PM2.5 are already subject to the same 

major source thresholds by statute, so the EPA believes that it is also reasonable to 

establish the threshold for PM2.5 in Serious areas at the same level as the threshold that 

applies to PM10 in Serious areas. For the reasons explained below, the EPA believes that 

potential alternative approaches to setting the major source threshold for serious PM2.5

nonattainment areas could have significant drawbacks. Nevertheless, the EPA is 

proposing and requesting comments on other possible thresholds for Serious areas. 

A possible alternative approach is to promulgate a PM2.5 major source threshold 

lower than 70 tpy of PM2.5 emissions, recognizing that PM2.5 is a subset of PM10. 

Generally, any source’s PM2.5 emissions will be a fraction of that source’s PM10 

emissions. However, determining the appropriate major source emissions threshold for 

PM2.5 that would be equivalent to 70 tpy of PM10 on a national basis is problematic 

because, while PM2.5 is generally a subset of PM10, there is not a consistent ratio of PM2.5

to PM10 emissions for all stationary sources. Combustion sources, such as industrial and 

commercial boilers that burn fossil fuels, and selected industrial processes, emit primarily 

finer particles within the PM2.5 size range, while other industrial processes—typically 

involving crushing and grinding operations—tend to emit more coarse particles in the 

PM10 size range. While the PM10:PM2.5 ratio for most sources decreases when the overall 

emissions of PM are controlled, the quantitative difference between PM2.5 emissions and 

PM10 emissions from specific sources can still be significant, thus making a national 

PM2.5 major source threshold based on a single ratio difficult to define. The EPA seeks 

comments on possible ways in which a PM2.5 emissions rate different from the statutory 
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70 tpy rate for PM10 emissions can be established, taking into account variations in the 

PM10:PM2.5 ratio for different source categories and activities. 

Accordingly, while the EPA seeks comment on this alternative approach, because 

of the associated limitations just described, the first option (i.e., a major source threshold 

of 70 tpy of PM2.5 emissions for stationary sources proposing to construct or modify in 

PM2.5 nonattainment areas classified as Serious) represents the agency’s preferred 

approach.

b. Control requirements for new major stationary sources and major 

modifications of PM2.5 precursors. The second key provision contained in subpart 4 that 

is not contained in subpart 1 relates to the control of major stationary sources and major 

modifications of precursor pollutants. Section 189(e) of subpart 4 provides that, with 

respect to NNSR, the control requirements applicable to major stationary sources of PM10 

also apply to major stationary sources of PM10 precursors, except that major stationary 

sources of a particular precursor may be exempt from the control requirements that apply 

to major stationary sources of PM10 if the state can demonstrate (based on guidance 

provided by the EPA) that the precursor emissions from those sources do not contribute 

significantly to ambient PM10 concentrations that exceed the standard in the 

nonattainment area. 

The specific “control requirements” for new or modified major stationary sources 

of PM2.5 are contained in section 173 of the CAA (outlining requirements for the state 

permit program required to be submitted in a state plan under section 189(a)(1)(A)) and 

189(b)(3) (establishing a major source threshold for sources in Serious areas). Consistent 

with these requirements, the EPA is proposing a series of revisions to address PM2.5
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precursors in the NNSR regulations at 40 CFR 51.165, including: revision of the 

definition of “regulated NSR pollutant” to require regulation under the permitting 

program of all PM2.5 precursors; the establishment of major stationary source thresholds 

(for both Moderate areas and Serious areas) for all PM2.5 precursors; and a provision for 

an exemption from the NNSR requirements, pursuant to section 189(e) of the CAA, for 

major stationary sources of any PM2.5 precursor where such sources do not contribute 

significantly to ambient concentrations of PM2.5 that exceed the standard in a particular 

nonattainment area. As described in greater detail below, the EPA is not at this time 

proposing any new significant emissions rates for the PM2.5 precursors.

As described in Section VIII.A.2.b of this preamble, the NNSR regulations at 40 

CFR 51.165 currently require states to regulate new major stationary sources and major 

modifications of SO2 and NOx as precursors under the NNSR requirements for PM2.5.

242

Optionally, a state may avoid regulating new major stationary sources and major 

modifications of NOx under the NNSR requirements for PM2.5 if that state demonstrates 

to the satisfaction of the EPA that NOx is not a significant contributor to PM2.5

concentrations in a particular PM2.5 nonattainment area. Similarly, the existing 

regulations provide that a state may opt to regulate new major stationary sources and 

major modifications of VOC or ammonia under the NNSR requirements for PM2.5 if that 

state demonstrates to the satisfaction of the EPA that VOC or ammonia are precursors for 

PM2.5 that need to be controlled in a particular PM2.5 nonattainment area.243 In accordance 

 

242 See the definition of “regulated NSR pollutant” at existing 40 CFR 

51.165(a)(1)(xxxvii)(C)(2) and (3).

243 Ibid at (a)(1)(xxxvii)(C)(3) and (4).
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with the court’s statement that section 189(e) requires all PM2.5 precursors to be 

addressed, the EPA is proposing to revise the NNSR regulations to require that new 

major stationary sources and major modifications of SO2, NOx, VOC and ammonia meet 

the NNSR requirements for PM2.5 in PM2.5 nonattainment areas. In doing so, the EPA 

believes that it is necessary to propose several revisions to 40 CFR 51.165 to ensure that 

the NNSR requirements for PM2.5 adequately address the regulated precursors consistent 

with the requirements of subpart 4.

First, the EPA is proposing to revise the regulations at 40 CFR 51.165 to ensure 

that new major stationary sources and major modifications of the four scientific 

precursors for PM2.5 are subject to the same requirements under the NNSR regulations 

that apply to new major stationary sources and major modifications of direct PM2.5

emissions. As explained earlier in this preamble, the court decision in NRDC vs. EPA

concluded that section 189(e) under subpart 4 “expressly governs precursor 

presumptions” and this necessitates that the EPA revise its existing provisions in the 

NNSR rules that indicate that VOC and ammonia are not regulated PM2.5 precursors. The 

EPA is thus proposing to revise the NNSR definition of “regulated NSR pollutant” to 

ensure that the NNSR regulations are consistent with the subpart 4 of the CAA) in 

establishing that SO2, NOx, VOC and ammonia are all regulated PM2.5 precursors for 

purposes of NNSR requirements, except under certain conditions explained below. See

proposed 40 CFR 51.165(a)(1)(xxxvii)(C)(2).

While section 189(e) generally requires that major stationary sources of PM2.5

precursors must apply the control requirements (including those for NNSR) for major 

stationary sources of direct PM2.5 emissions, the section also provides for an exemption 
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from such requirements for any precursor for which “the Administrator determines that 

such sources do not contribute significantly” to the levels of PM2.5 in the nonattainment 

area that exceed the standard. Section 189(e) further authorizes the EPA to issue 

guidelines concerning the application of the exemption process. 

In Section III of this preamble, the EPA described the agency’s proposed 

approaches for interpreting requirements for states to control PM2.5 precursors in their 

attainment plans for the PM2.5 NAAQS, which includes several proposed options to 

enable states to exempt a precursor from the attainment plan control requirements 

(including NNSR) for a particular PM2.5 nonattainment area with the appropriate factual 

and analytical basis. In summary, the options included (1) separate analyses to determine 

which precursors are subject to the control requirements for attainment plans and which 

precursors are subject to the control requirements for NNSR for PM2.5; (2) an up-front 

technical demonstration showing that all sources of a particular precursor do not 

significantly contribute to the PM2.5 levels that exceed the standards in an area, thus 

exempting the precursor from control under both the attainment plan and NNSR 

programs; and (3) one analysis to determine whether control measures for a precursor are 

not needed for expeditious attainment for purposes of the attainment plan, which also 

defines the precursors that should be addressed for NNSR for PM2.5. Accompanying the 

description of each of the above options, Section III.C of this preamble discusses the 

potential analytical requirements for any proposed demonstration that any particular 

precursor should be exempted from the control requirements for PM2.5 in a given 

nonattainment area. The EPA is requesting comments on the three precursor options and 

the technical approaches for requesting a precursor exemption. Any comments received 
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will be considered in developing the agency’s final policy for addressing each PM2.5

precursor under the NNSR program for PM2.5. 

The second proposed change with regard to the nonattainment area control 

requirements for PM2.5 precursors involves the definition of “major stationary source” as 

it relates specifically to precursors. The EPA is proposing to revise the definition of 

“major stationary source” contained in the NNSR regulations to ensure that new sources 

that emit major amounts of any PM2.5 precursor that the state is regulating in the 

attainment plan for the area are appropriately considered major stationary sources subject 

to the NNSR requirements for PM2.5. See proposed 40 CFR 51.165(a)(1)(iv)(A)(1). The 

proposed change concerning the regulation of precursors for PM2.5 is being accomplished 

by adding to the term “regulated NSR pollutant” the phrase “(as defined in paragraph 

(a)(1)(xxxvii) of this section).” It should be noted that the definition of “major 

modification” already contains this phrase. As described above, the EPA is also 

proposing to revise the definition of “regulated NSR pollutant” to clarify that four 

precursors are being regulated for PM2.5 in nonattainment areas for PM2.5. The EPA is 

proposing to set the major source threshold for each PM2.5 precursor (SO2, NOx, VOC 

and ammonia) at 100 tpy of each precursor for sources locating in Moderate areas, and 70 

tpy of any precursor for sources locating in Serious areas. See proposed 40 CFR 

51.165(a)(1)(iv)(A)(1) and (a)(1)(viii), respectively. For example, in order to be a major 

source for purposes of the PM2.5 NAAQS, the source would need to emit at least 100 tpy 

of PM2.5 emissions or at least 100 tpy of any individual PM2.5 precursor that is a regulated 

precursor in a Moderate PM2.5 nonattainment area. The individual treatment of pollutants 
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and precursors for applicability purposes is consistent with the EPA’s policy as explained 

in previous rulemakings.244

In proposing to set the major source threshold for each PM2.5 precursor at 100 tpy 

for Moderate areas, the EPA is following the precedent established in the 2008 PM2.5

NSR Rule in which the agency set the same 100 tpy major source threshold for PM2.5 and 

each of its precursors (at that time SO2 and NOx).245 As the EPA stated in that 2008 

notice, sections 169 and 302(j) of the CAA contain definitions of “major emitting 

facility” and “major stationary source” that apply to programs implemented under subpart 

1, which contain the PSD and NNSR program requirements, respectively.246 Those 

definitions also apply to programs implemented under subpart 4 to the extent that they 

regulate areas classified as moderate PM2.5 nonattainment areas, as Subpart 4 does not 

establish a different threshold for such areas. This proposal to set the same 100 tpy major 

source thresholds for sources of PM2.5 emissions and applicable PM2.5 precursor 

emissions is also consistent with the requirements of section 189(e), which make the 

control requirements applicable to major stationary sources of PM10 also applicable to 

major stationary sources of applicable PM10 precursors.247

As noted above, section 189(b)(3) in subpart 4 sets a lower major source threshold 

of 70 tpy of PM10 emissions for sources locating in PM10 nonattainment areas classified 

as Serious. Because subpart 4 NNSR requirements must be applied to PM2.5, the EPA 

 

244 “Different pollutants, including precursors, are not summed to determine 

applicability.” See 73 FR 28231 at 28331 (May 16, 2008).

245 See ibid.

246 See ibid.

247 See 57 FR 13498 at 13538.
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must set a lower major source threshold for PM2.5, pursuant to section 189(b)(3), in PM2.5

nonattainment areas that will be classified as Serious areas. Thus, the EPA’s preferred 

approach proposed above is to set a major source threshold of 70 tpy of PM2.5 emissions 

for sources in PM2.5 nonattainment areas classified as Serious. 

Consistent with this proposal, the EPA is also proposing to set the major source 

threshold for Serious areas for each precursor at 70 tpy of that particular precursor. As 

noted above, section 189(e) makes the control requirements for major stationary sources 

of PM10 also applicable to major stationary sources of PM10 precursors; thus, in 

accordance with the provision of the statute, the control requirements applicable to major 

stationary sources of PM2.5 emissions are also applicable to major stationary sources of 

PM2.5 precursors. Accordingly, the EPA must develop a major source threshold for PM2.5

precursors that is consistent with the threshold for direct PM2.5 that will apply in PM2.5

nonattainment areas classified as Serious. See proposed 40 CFR 51.165(a)(1)(iv)(1)(viii).

The EPA’s proposal to set a major source threshold of 70 tpy for Serious areas is 

also consistent with the approach the EPA has taken for establishing a major source 

threshold for precursors of PM10 under subpart 4. In an Addendum to the General 

Preamble offering guidance as to how to apply the new subpart 4 requirements in Serious 

areas, the EPA indicated that it interpreted the statute as applying the 70 tpy threshold to 

sources of PM2.5 precursors.248

 

248 See 59 FR 41998 at 42012 (defining major point sources in Serious areas as “sources 

with the potential to emit at least 70 tons per year of PM10 (or PM10 precursors) as 

required in sections 189(b)(3) and 189(e) of the Act”).
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The EPA also solicits comments on the appropriateness of setting the precursor 

thresholds at a different rate, particularly if, as alternatively proposed above, the agency 

defines “major stationary source” for sources of direct PM2.5 in Serious PM2.5

nonattainment areas at a rate lower than 70 tpy of PM2.5 emissions. For example, if the 

agency sets the major source threshold at 60 tpy of PM2.5 emissions in PM2.5 Serious 

areas, the agency would also consider setting the major source threshold for each PM2.5

precursor at 60 tpy of that particular precursor. 

Moreover, the EPA believes that a reasonable argument can be made that 

whatever threshold is set for PM2.5 emissions, the same level would be too low to be 

regarded as “major” for each precursor when considering the effects that such precursor 

sources could have on ambient PM2.5 concentrations. The EPA previously analyzed the 

relationship between emissions of SO2 and NOx and the formation of secondary PM2.5 in 

the ambient air expressly for purposes of determining an appropriate ratio for allowing 

interprecursor offsets for PM2.5. Those studies resulted in the EPA providing in the 2008 

PM2.5 NSR Rule “preferred” ratios for both SO2 and NOx, whereby a source could obtain 

reductions of a PM2.5 precursor to offset an increase of direct PM2.5 emissions or another 

PM2.5 precursor based on the “preferred” offset ratios.249 In brief, the preferred ratios 

were as follows: for NOx-to-primary PM2.5: 200 to 1 (NOx tons to PM2.5 tons) for areas in 

the eastern U.S, and 100 to 1 for areas in the western U.S.; and for SO2-to-primary PM2.5: 

 

249 The technical assessment, with details on data and modeling inputs, was fully 

described in a technical memo titled “Details on Technical Assessment to Develop 

Interpollutant Trading Ratios for PM2.5 Offsets,” which was placed in the docket to the 

2008 final rule. See also 73 FR 28339.
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40 to 1 (SO2 tons for PM2.5 tons). In each case, the ratio illustrates that it requires 

considerably more precursor emissions than direct PM2.5 emissions to result in a 

particular ambient concentration of PM2.5. It should be noted that at that time the EPA did 

not consider using the preferred ratios for the purpose of adjusting the major source 

thresholds or significant emissions rates for SO2 and NOx when regulating as PM2.5

precursors.

The preferred ratios as presented in the 2008 notice were later challenged in a 

petition for reconsideration and the EPA withdrew them via an EPA memorandum issued 

in 2011.250 In withdrawing the preferred ratios, the EPA cited several concerns. First, it 

was determined that the preferred ratios were not sufficiently conservative to be 

representative of conditions in all areas of the country. Second, the EPA determined that 

the preferred ratios were not adequate for addressing the precursor relationship ambient 

PM2.5 concentrations for the short-term (daily) averaging period.251 In addition, the EPA 

believes that the overall analysis conducted for the 2008 notice generally illustrates that 

the threshold for defining “major” for either SO2 or NOx as a precursors for PM2.5 could 

reasonably be set at an emissions rate considerably higher than 70 tpy of that particular 

precursor and be equally protective of air quality as the 70 tpy threshold applied to PM2.5

emissions. 

 

250 Memorandum from Gina McCarthy, then Assistant Administrator, dated July 21, 

2011, titled “Revised Policy to Address Reconsideration of Interpollutant Trading 

Provisions for Fine Particles (PM2.5)” and sent to Regional Air Division Directors.

251 Nevertheless, while the ratios are no longer considered appropriate to use 

presumptively to meet the NNSR requirements for emissions offsets, a state may still 

conduct its own analysis and propose area-specific ratios for EPA approval on a case-bycase basis for interpollutant offset trading.
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Although the statutory definition at section 189(b)(3) applicable to PM10 does not 

explicitly apply to other pollutants, the EPA is considering the possibility that it may not 

have the legal authority to set a higher major source threshold for PM2.5 precursors, even 

if it were technically justified. As previously noted, section 189(e), as interpreted in light 

of the court decision in NRDC v. EPA, requires that the same control requirements 

applicable to major stationary sources of PM2.5 also apply to major stationary sources of 

PM2.5 precursors. Courts have determined in other contexts that the term “controls” under 

the Act includes NSR requirements, and in particular includes major source thresholds as 

specified in the statute.252 Thus, if the holding of South Coast directs the EPA’s actions, 

section 189(e) must be read to require the same major source threshold be applied to 

PM2.5 precursors as it applied to direct emissions of PM2.5.

This conclusion is also consistent with the limited legislative history on this issue. 

A House Report accompanying the 1990 amendments to the CAA described the effects 

of adding section 189(b)(3) to the CAA to include the requirement that “new or modified 

sources emitting 70 tons or more per year of VOC will be subject to new source review 

requirements.”253 Thus, Congress seems to have contemplated that the same major source 

threshold would apply to sources of PM2.5 emissions and PM2.5 precursors in Serious 

areas.

 

252 See South Coast Air Quality Management District v. EPA, 472 F.3d 882, 900-902 

(D.C. Cir. 2006) (holding that “controls” in section 172(e) anti-backsliding provision 

include NSR requirements such as LAER, offset ratios, and major source thresholds).

253 H.R. Rep. 101-490.
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The EPA does not believe that a sufficient technical basis exists at this time to 

enable the agency to propose specific higher major source thresholds for any of the four 

PM2.5 precursors presumptively regulated in PM2.5 nonattainment areas. The EPA intends 

to continue its analysis of the relationship between each precursor and ambient PM2.5

concentrations with the possibility that higher major source thresholds for specific 

precursors could be established in the future. In the meantime, the agency solicits 

comments on the general appropriateness of setting generally higher major source 

thresholds for one or more PM2.5 precursors in PM2.5 nonattainment areas, as well as legal 

and technical considerations that should be made as part the EPA’s future analysis of 

NNSR requirements with respect to PM2.5 precursors.

c. Significant emissions rates for PM2.5 precursors. As explained above, a 

modification to an existing major stationary source of a nonattainment pollutant such as 

PM2.5 is a major modification and subject to the NNSR requirements for that pollutant 

when the sources proposes to make a physical or operational change that results in both a 

significant emissions increase and a significant net emissions increase of that 

nonattainment pollutant. With regard to PM2.5 precursors, a modification to a major 

stationary source of any such precursor is likewise a major modification subject to the 

NNSR requirements for PM2.5 when the source proposes a physical or operational change 

resulting in a significant net emissions increase of that precursor. The EPA defined 

“significant” for SO2 and NOx as PM2.5 precursors in the 2008 PM2.5 NSR 

Implementation Rule. For both precursors, the EPA set the significant emissions rate for 

each pollutant when it is regulated as a precursor to PM2.5 at 40 tpy, the same level as the 

existing significant emissions rate for the pollutant as independently regulated as a 
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criteria pollutant for purposes of the SO2 and NO2 NAAQS.

254 Also, in the preamble to 

the 2008 PM2.5 NSR Rule, the EPA indicated that it would consider 40 tpy for VOC as a 

PM2.5 precursor; however, that rate was not codified in any of the NSR regulations 

because the regulations provided that VOC was generally presumed not to be a precursor 

to PM2.5. Instead, the agency explained that any state making a demonstration that VOC 

should be treated as a PM2.5 precursor in a particular nonattainment area “would be 

required to adopt the 40-tpy significant emissions rate unless it demonstrated that a more 

stringent significant emissions rate (lower rate) is more appropriate.”255

The 2008 PM2.5 NSR Rule codified the presumption that ammonia, like VOC, 

need not be regulated as a PM2.5 precursor and the EPA did not set a significant emissions 

rate for ammonia. Instead, the agency indicated that it was allowing states that determine 

that ammonia significantly contributes to PM2.5 concentrations in a given PM2.5

nonattainment area to set the significant emissions rate for ammonia based on 

information developed for each individual attainment plan.

256

As explained in the 2008 PM2.5 NSR Rule, the EPA set the significant emissions 

rates for the presumed PM2.5 precursors at the levels for those pollutants already included 

in the NSR programs. The EPA explained that the use of the existing rates where the 

PM2.5 precursor is also regulated as a separate criteria pollutant harmonizes the NSR 

program for PM2.5 with the NSR programs for those other criteria pollutants. The agency 

 

254 See 73 FR 28321 at 28333-34 and existing 40 CFR 51.165(a)(1)(x)(A).

255 See 73 FR 28333.

256 Ibid.
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further explained that this approach for setting the significant emissions rates for PM2.5

precursors follows the precedent for setting the significant emissions rate for NOx as a 

precursor to ozone, where the same 40 tpy threshold was used for NOx emissions as both 

a criteria pollutant (NO2) and a precursor for ozone.257

Nevertheless, the EPA gave some consideration in the development of the 2008 

PM2.5 NSR Rule to setting the significant emissions rates for the individual PM2.5

precursors at different levels based on the effect of each precursor on ambient PM2.5

concentrations. The EPA concluded that it did not have adequate data on the impacts of 

precursor emissions from individual sources to override the administrative advantages of 

setting the significant rates for SO2, NOx and VOC for purposes of the PM2.5 NSR 

program at the same levels that are already used for other purposes in the major NSR 

program for other NAAQS. The EPA continues to believe, however, that when more data 

are available, it could provide a reasonable basis for considering subsequent changes to 

the significant emissions rates for each PM2.5 precursor for purposes of the PM2.5

NAAQS, whereby the significant emissions rates for the individual PM2.5 precursors 

could more realistically reflect the effect that each precursors has on ambient PM2.5

concentrations. 

The EPA is currently undertaking a separate rulemaking for both NNSR and PSD 

in which it intends to include a technical analysis of each PM2.5 precursor to better 

understand the relationship of emissions of each precursor to ambient PM2.5

 

257 See 73 FR 38334.
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concentrations. The agency intends to consider the results of that analysis and other 

factors and may propose new significant emissions rates accordingly for SO2 and NOx as

PM2.5 precursors. The EPA also intends to propose individual significant emissions rates 

for VOC and ammonia as PM2.5 precursors at that time. Thus, the EPA is not proposing 

any changes to the existing significant emissions rates for SO2 and NOx as PM2.5

precursors in this proposal document. 

It is the EPA’s expectation that any new or revised SERs for the individual PM2.5

precursors will become effective in that separate rulemaking not long after the date of 

that final rule, allowing states to adopt and use them in their own NNSR regulations once 

the EPA approves their individual SIPs. However, in the event that the timing of that rule 

does not allow ample time for states to rely on it to adopt any new or revised SERs in 

their rules, it was explained earlier that individual SERs already exist for SO2 and NOx at

40 tpy. Additionally, the SER for VOC was identified as 40 tpy in the 2008 PM2.5 NSR 

Implementation Rule notice (but not in the final regulations), but the EPA is proposing to 

add that precursor and emissions rate to the list of PM2.5 precursors. See proposed 40 

CFR 51.165(a)(1)(x)(A). Hence, only the ammonia SER would remain to be defined by 

each state that needs to control major stationary sources of ammonia as part of their 

NNSR program. 

d. Transition provisions for PM2.5. The CAA requires proposed major stationary 

sources and major modifications to meet major NSR permitting requirements that apply 
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on the basis of the area’s attainment designation.258 Accordingly, the EPA’s longstanding 

interpretation of the CAA is that a proposed new major stationary source or major 

modification must satisfy the appropriate major NSR requirements (PSD vs. NNSR) for a 

particular pollutant that are in effect on the date that a permit is issued to the source, 

rather than the requirements that may have been applicable when the permit application 

was submitted.259

In the final 2012 PM NAAQS rule, the EPA established a grandfathering 

provision that would enable some proposed new and modified sources that had already 

submitted a PSD application prior to the effective date of the revised primary annual 

PM2.5 NAAQS to continue being reviewed under the pre-existing PSD requirements for 

PM2.5. This provision applies where the PSD program continues to be the applicable set 

of major NSR requirements for the area of concern. In response to the EPA’s proposal to 

add this grandfathering provision for certain PSD permit applications pending upon the 

effective date of the new NAAQS, the EPA received comments concerning the need for a 

transition period for implementing the NNSR requirements in newly designated PM2.5

nonattainment areas as a result of the tightening of the primary annual PM2.5 NAAQS.260

The commenters recommended that the EPA establish a grandfathering provision to 

enable pending permit applications to continue under review for the pre-existing 

 

258 Compare CAA section 165(a) (permitting requirements for sources locating in 

attainment and unclassifiable areas) with sections 172(c)(5) and 173 (permitting 

requirements for sources locating in nonattainment areas).

259 See Memorandum from John S. Seitz, Director, Office of Air Quality Planning and 

Standards, on March 11, 1991, titled “New Source Review (NSR) Transitional 

Guidance,” Attachment p. 6, sent to Regional Air Division Directors.

260 See 78 FR 3086 at 3263.
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requirements. A subset of the commenters recommended that grandfathering be 

accomplished by establishing an effective date for designations 1 year after initial 

publication in the Federal Register. Presumably, these commenters believe that by 

delaying the effective date of any new nonattainment designations for the primary annual 

PM2.5 NAAQS sources with pending PSD permit applications could continue to be

reviewed under the PSD permitting requirements rather than the NNSR requirements for 

PM2.5. 

In the final 2012 PM NAAQS rule, the EPA expressed its disagreement with 

those commenters, explaining that the obligation to adopt new provisions under a state’s 

NNSR program will not apply with regard to the revised NAAQS until such time that an 

area is designated nonattainment, and beginning on the effective date of the new area 

designations for PM2.5 proposed new and modified major sources would be required to 

meet the applicable NNSR requirements for PM2.5.

261 However, the EPA further 

indicated that it would continue to consider the need to establish a grandfathering 

provision under the NNSR program for PM2.5, and would propose such provision, if 

appropriate, as part of a subsequent NSR implementation rulemaking with additional 

opportunity for public comment.262

 

261 The applicable NNSR requirements would be either the NNSR requirements for PM2.5

in the state’s existing SIP or the requirements found at 40 CFR part 51 Appendix S, 

where a state’s SIP does not currently include NNSR requirements for PM2.5. States will 

be required to submit to the EPA for approval SIP revisions containing the amended 

NNSR program requirements for PM2.5 contained in the final PM NAAQS 

Implementation Rule being proposed in this notice, but those additional requirements will 

not apply in states with SIPs that include NNSR requirements for PM2.5 until the EPA 

approves the SIP revision. See ibid.

262 Ibid.
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After further considering the issue in this rulemaking, the EPA is not now 

proposing a grandfathering provision that would apply to pending PSD permit 

applications that were submitted but not approved prior to the effective date of the new 

nonattainment designations for the primary annual PM2.5 NAAQS. The EPA does not 

believe it would be acceptable for the EPA or a state to issue a PSD permit, instead of a 

NNSR permit, with regard to a particular pollutant for which an area is designated 

nonattainment on the date the permit is to be issued. Instead, if the PSD permit has not 

been issued by the effective date of the new nonattainment designation, then the applicant 

should be required to withdraw that part of the permit application that addresses the 

nonattainment pollutant and submit an application that satisfies the applicable NNSR or 

minor NSR requirements in effect in the implementations plan on the date the permit will 

be issued. Given adverse conditions that already exist in a nonattainment area and the 

Congressional directive to reach attainment as expeditiously as practicable, construction 

at a major stationary source that significantly increases emissions of a nonattainment 

pollutant in such an area should be expected to address NNSR requirements, even if this 

could cause delay to the permit applicant. 

As explained in Section VIII.D of this preamble, states will have 18 months from 

the date of the new nonattainment designations to revise their existing NNSR programs 

or establish new programs in accordance with the applicable requirements under subpart 

4. Where the area was already designated nonattainment for any prior PM2.5 NAAQS 

before the effective date of designations for the new NAAQS, the state should continue to 

apply the NNSR requirements contained in the approved SIP to issue the final permit 

addressing all PM2.5 NAAQS until the new SIP revisions required by this rule are 
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approved. In areas already designated nonattainment for any PM2.5 NAAQS but lacking 

an approved NNSR program that applies to PM2.5, the requirements of Appendix S may 

continue to be applied for issuing permits in that area. However, any changes to the 

Appendix S requirements that the EPA may make in this rulemaking must be 

implemented in any area that applies Appendix S once these revisions become effective

in Appendix S. Section VIII.C.2 that follows discusses the possible changes to the NNSR 

requirements in Appendix S that the agency is proposing in this action. 

The EPA is not proposing to add any grandfathering provisions that would apply 

to changes in NNSR permitting requirements in areas that the EPA may already have 

designated nonattainment for PM2.5 at the time the source submitted a permit application. 

For reasons similar to those identified above in cases where an area designation changes, 

the EPA generally believes that major sources that would contribute to the air quality in 

an area that is not meeting the NAAQS for a particular pollutant should be expected to 

address the most current requirements that apply in the nonattainment area. The agency

acknowledges it is possible that a proposed new or modified source may need to address 

additional precursor control requirements that did not apply when a permit application 

was submitted once the EPA’s final rule is promulgated and the appropriate revisions are 

approved into a state’s NNSR SIP. However, based on the terms of section 189(e) of the 

Act, the EPA generally believes that those requirements should be addressed in pending 

permit applications unless the air agency has determined, and the EPA has approved such 

demonstration, that major stationary sources of that precursor do not contribute 

significantly to PM2.5 levels in the nonattainment area. Nevertheless, the agency

recognizes that there may be certain circumstances where proposed construction might be 
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delayed and an applicant may feel fundamental fairness would support exempting a 

particular pending permit from newly established requirements; therefore, the EPA seeks

comments on what circumstances, if any, would justify a grandfathering provision for 

pending nonattainment NSR permits similar to the grandfathering provision promulgated 

in the 2012 PM2.5 NAAQS Rule for PSD permitting purposes. See 40 CFR 51.166(i)(10) 

52.21(i)(11). In addition, the EPA requests comment on how such a grandfathering 

provision would be consistent with the relevant provisions of the CAA. The EPA does 

not believe the statutory deadline in section 165(c) of the Act that forms part of the 

EPA’s basis for grandfathering in the PSD context is applicable to NNSR permit 

decisions.

2. What are the changes the EPA is proposing in Appendix S?

As described above, 40 CFR 52.24(k) provides that the Emission Offset 

Interpretive Ruling, 50 CFR part 51, Appendix S, shall govern permits to construct and 

operate for which a NNSR permit application is submitted between the effective date of 

designation as nonattainment and the date a state’s NSR permit program meeting the 

requirements part D is approved and effective. The EPA is considering a range of options 

concerning how and whether to address the proposed subpart 4 requirements in the 

interim NNSR program requirements contained in Appendix S. 

Permitting requirements for new major stationary sources and major 

modifications in PM2.5 nonattainment areas were originally added to Appendix S in the 

2008 PM2.5 NSR Rule. The amendments generally followed the NNSR requirement 

contained in subpart 1 of part D. However, in the 2008 PM2.5 NSR Rule, the EPA 

determined that, in light of the transitional function of Appendix S, it would be 
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appropriate to regulate PM2.5 precursors under Appendix S in a manner that differed 

slightly from the regulatory approach taken in 40 CFR 51.165. 

As explained in section VIII.B.2 of this preamble, under the existing requirements 

for NNSR plans at 40 CFR 51.165, SO2 is regulated as a PM2.5 precursor, NOx is 

presumed to be a regulated PM2.5 precursor, and VOC and ammonia are presumed not to 

be regulated precursors (with either states or the EPA having authority to rebut any such 

presumption for a particular nonattainment area). However, in developing Appendix S, 

the EPA determined that it would be premature to presume that NOx is a regulated PM2.5

precursor in all PM2.5 nonattainment areas that proposed new major sources and major 

modifications in a PM2.5 nonattainment area should be required to address as a 

prerequisite to obtaining a NNSR permit, while at the same time the states were in the 

process of determining whether in fact NOx emissions contribute significantly to ambient 

PM2.5 concentrations in areas that are exceeding the PM2.5 NAAQS. Accordingly, the 

EPA decided to delay implementing any control requirements for NOx as a PM2.5

precursor until the states completed the necessary analyses to determine the need for NOx

controls as part of their SIP revisions addressing the revised PM2.5 NAAQS. Thus, the 

existing NNSR requirements for PM2.5 under Appendix S do not contain a requirement 

for proposed sources to consider the control of NOx emissions as a PM2.5 precursor. 

Moreover, as states presumptively did not need to regulate VOC and ammonia in 

accordance with the 2008 PM2.5 NSR Rule in 40 CFR 51.165, the EPA similarly did not 

require sources seeking permits pursuant to the Appendix S requirements to address those 

precursors. 
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As an interim measure to facilitate permitting while states develop NNSR rules 

for PM2.5, the EPA believes that the NNSR requirements under Appendix S need not be 

identical to those governing states’ development of approvable programs pursuant to 

subpart 4, which requires regulation of all PM2.5 precursors unless a state provides, and 

the EPA approves, a demonstration that such control is not necessary for major stationary 

sources in the area under section 189(e). This is reasonable because the EPA anticipates 

that many states may be able to demonstrate to the EPA that there is not a need to 

regulate one or more PM2.5 precursors from major stationary sources in a given 

nonattainment area, as described in Section III of this preamble. 

Accordingly, the EPA is proposing to revise the definition of regulated NSR 

pollutant as contained in Appendix S to provide for the regulation of some precursors 

during the transition period, but not others. Specifically, for reasons explained below, the 

EPA is proposing to require that both SO2 and NOx be considered regulated PM2.5

precursors in Appendix S and is proposing a SER of 40 tpy for NOx as PM2.5 precursor. 

See proposed Sections II.A.31(iii)(b) and II.A.10(i) of Appendix S, respectively.

However, this proposal would not provide states the option of submitting a demonstration 

that could relieve them of the obligation to regulate SO2 and NOx as PM2.5 precursors 

during the transition period. The EPA believes that it is not necessary or efficient to 

expend effort on such a demonstration for the transitional program, when states are 

developing the demonstration for submittal with the NNSR SIP submission that, when 

approved, would replace the Appendix S transitional program for that area.

The EPA is proposing to include SO2 and NOx in Appendix S based on the 

principle that the national application of a transition program should correspond to the 
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general expectation of what the prevailing regulation of precursors will ultimately be 

when SIPs are submitted. Although such expectations are uncertain at this time, it is 

nonetheless appropriate to base the transition program on them. The EPA believes it is 

likely in many cases that states will determine that emissions of VOC and/or ammonia do 

not contribute significantly to PM2.5 concentrations in the affected PM2.5 nonattainment 

area, although such determinations should be made on a case-by-case basis for individual 

PM2.5 nonattainment areas. 

On the other hand, the EPA expects that the cases where NOx does not contribute 

significantly to PM2.5 concentrations in the affected PM2.5 nonattainment area will be few 

in number. Accordingly, given this likelihood, the EPA believes that it is reasonable to 

require the regulation of SO2 and NOx as PM2.5 precursors during the interim period when 

states are developing their PM2.5 attainment plans for newly designated areas (including 

the necessary revisions to the NNSR programs based on subpart 4). An added benefit of 

this proposed approach is that it will also ensure that states using the permitting 

requirements contained in Appendix S will regulate the same precursors that are required 

to be regulated in states that have already adopted NNSR for PM2.5 based on the 2008 

PM2.5 NSR Rule. The EPA seeks comments on this approach as part of this proposal. 

As one alternative approach that the EPA is presenting for public comment, the 

agency is proposing to amend Appendix S to regulate not only SO2 and NOx, but also 

VOC and ammonia as PM2.5 precursors that must be controlled during this interim period. 

This alternative would more closely match the basic NNSR program requirements of 

subpart 4, which indicate that states should regulate precursors from major stationary 

sources in the nonattainment area unless the EPA has determined that such emissions do 
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not significantly contribute to violations of the NAAQS in the area. However, it would 

require states to control new major stationary sources and major modifications of each 

PM2.5 precursor during the interim period prior to submission of the required SIP 

revisions without the benefit of first allowing states to determine whether the control of 

each precursor is warranted. The EPA does not prefer this option for amending Appendix 

S as an interim NNSR program; however, the EPA is seeking comments on the approach 

to address the policy and legal implications associated with it. This alternative, while 

being proposed for comment, is not shown in the proposed regulatory text.

Another alternative that the agency is proposing for comment is for the EPA to 

establish a phased in process for regulating PM2.5 precursors in the NNSR program 

whereby states would initially require sources issued a permit to control only SO2 and 

NOx as PM2.5 precursors (as under the preferred option), with a second requirement to 

later require sources issued a permit after the prescribed date (e.g., the date on which SIP 

revisions based on subpart 4 requirements are due) to control emissions of VOC and 

ammonia as well. For each precursor, the requirement to control would apply to major 

stationary sources of that particular precursor. The EPA believes that by phasing in the 

requirement to address all precursors, states that are ultimately able to demonstrate to the 

EPA’s satisfaction that VOC and/or ammonia do not need to be subjected to control 

under the NNSR requirements for PM2.5, but have not yet submitted such demonstration, 

will have ample time to make the necessary demonstration and will not have to control 

such precursors even temporarily. At the same time, the phase-in provision could address 

concerns about delays in SIP submittal or approval in states with PM2.5 nonattainment 

areas in which VOC and ammonia need to be regulated. Such delays could result in 
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prolonged exclusion of these precursors from control requirements beyond the time when 

an EPA-approved state NNSR program is expected to be in place. This alternative, while 

being proposed for comment, is not shown in the proposed regulatory text.

Separately, the EPA is proposing to amend Appendix S by revising the definition 

of “major stationary source” to include a separate PM2.5 major source threshold 

applicable to new major stationary sources and major modifications in PM2.5

nonattainment areas classified as Serious areas. See proposed section II.A.4(i)(a)(7). 

Inclusion of the new definition is not an immediate concern for the revised primary 

annual PM2.5 NAAQS or any future revision to the PM2.5 NAAQS because the possible 

reclassification of any Moderate area to a Serious area will not occur for several years 

and states are required to submit their SIP revisions addressing NNSR requirements prior 

to such time. There is a possibility, however, that existing PM2.5 nonattainment areas 

based on the PM2.5 NAAQS prior to the 2012 NAAQS revision could be reclassified as 

Serious areas sooner. States that still do not have approved NNSR programs addressing 

PM2.5 would be without the appropriate NNSR provisions to address new major 

stationary sources and major modifications in those Serious areas until they submit 

revisions to their existing programs and the EPA approves those revisions. The EPA 

solicits comments on this proposal to incorporate a definition of “major stationary 

source” for PM2.5 nonattainment areas classified as Serious areas. 

The EPA is not proposing any Appendix S provisions for grandfathering proposed 

new and modified sources from newly established permit requirements applicable to 

PM2.5 nonattainment areas. The EPA generally believes that it would not be appropriate 

to grandfather sources from requirements that apply in areas that are not meeting the 
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NAAQS. Nevertheless, the EPA seeks comments on possible circumstances where 

grandfathering, similar to the grandfathering provision established for pending PSD 

permits under the 2012 PM2.5 NAAQS Rule, may be appropriate with respect to changes 

made to Appendix S. 

D. Plan Due Dates

For Moderate areas, section 189(a)(2)(B) requires that states make a SIP 

submission satisfying the requirements contained therein, including applicable NNSR 

programs for PM10 (and PM2.5), to the EPA for approval within 18 months of an area 

being designated nonattainment. The agency recognizes that this submittal date 

represents a considerably earlier date than anticipated when it issued the 2012 PM 

NAAQS final rule.263 However as the CAA requires, the EPA will apply the 18 month 

deadline from the effective date of designation of a Moderate PM2.5 nonattainment area 

for the submission of any applicable NNSR program revisions for PM2.5 as included in 

any final implementation rule. 

In the event a Moderate area is reclassified as a Serious PM2.5 nonattainment area, 

it will be required to implement the NNSR program with a “major stationary source” 

threshold of 70 tons per year (per CAA section 189(b)(3)). However, the Act does not 

specify a deadline for the state’s submittal of any NNSR program revisions (e.g., to lower 

the major stationary source threshold from 100 tons per year (tpy) to 70 tpy) that would 

be needed to implement the program in a Serious area. Pursuant to EPA’s gap-filling 

authority in CAA section 301(a), and to effectuate the statutory control requirements in 

 

263 See ibid.
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section 189 of the Act, EPA proposes to require the state to submit these NNSR SIP 

revisions no later than 18 months from the effective date of final reclassification of the 

area as a Serious nonattainment area. This timeframe is consistent with the 18 month 

timeframe required for submittal of the Serious area attainment demonstration and other 

Serious area plan elements, and it is consistent with the 18 month time for submittal of 

Moderate area plan revisions. We also request comment on a 12-month time frame for 

submittal of the NNSR revisions for Serious areas. An approach that requires the NNSR 

revisions to be submitted on the same 18-month schedule as other Serious area plan 

elements is expected to be more administratively efficient than one that would require the 

NNSR revisions on a different schedule. On the other hand, this type of revision to the 

NNSR regulations may be relatively straightforward and potentially could be completed

within 12 months of the reclassification date, thereby assuring that new major sources or 

modified major sources in the area will be subject to the lower statutory major source 

thresholds expeditiously. We request comment on the proposed 18-month timeframe for 

submission of the NNSR SIP revisions for Serious areas. We also request comment on an 

alternative 12-month option.

E. Avoidance of Dual Review for PSD and NNSR for PM2.5

Because the EPA designates nonattainment areas for the primary annual and 24-

hour PM2.5 NAAQS independently, some areas ultimately may be designated 

nonattainment for one of these standards and unclassifiable/attainment or attainment for 

another. This may raise concerns that the sources locating in such an area may be subject 

to both PSD and NSSR for the same pollutant. In the preamble to the final PM NAAQS 
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rule, the EPA explained that the existing PSD regulations resolved this issue.264

Specifically, the PSD regulations, at 40 CFR 51.166(i)(2) and 52.21(i)(2) provide that the 

PSD requirements do not apply to a major stationary source or major modification with 

respect to a pollutant when “as to that pollutant, the source or modification is located in 

an area designated as nonattainment....”265 [emphasis added]. This policy was explained 

in the preamble to the final rule promulgating the revised primary annual PM2.5

NAAQS.266 The EPA is simply reiterating in this action the agency’s policy for 

addressing NSR applicability for areas that may be designated nonattainment for one 

averaging period and attainment or unclassifiable for another averaging period. Thus, for 

PM2.5 only the NNSR requirements would apply with regard to major stationary sources 

of PM2.5 locating in that nonattainment area. 

IX. What other proposed requirements would apply in PM2.5 NAAQS 

nonattainment areas?

A. Section 188(f) Waiver Provisions

1. Statutory requirements and existing guidance

Section 188(f) of the CAA provides a means for the EPA to waive a specific date 

for attainment and certain control and planning requirements for PM2.5 nonattainment 

areas if certain conditions are met in the nonattainment area. Specifically, the statute 

 

264 See ibid.

265 The policy for applying the PSD exemption is clear with regard to the federal PSD 

program at 40 CFR 52.21; however, the requirements for a SIP-approved PSD program 

state that “[t]he plan may provide...” Accordingly, a state may choose to apply a 

different applicability strategy if it so wishes. 

266 See ibid.
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provides that: “the Administrator may, on a case-by-case basis, waive any requirement 

applicable to any Serious Area... where the Administrator determines that anthropogenic 

sources of PM10 do not contribute significantly to the violation of the PM10 standard in 

the area.” In addition, “the Administrator may also waive a specific date for attainment of 

the [PM10] standard where the Administrator determines that nonanthropogenic sources 

of PM10 contribute significantly to the violation of the PM10 standard in the area.” In the 

Addendum, the EPA provided extensive guidance on how the Agency interpreted section 

188(f) and how it intended to apply the statutory waiver provisions for purposes of 

implementing the PM10 NAAQS.267 At this time, the EPA is not revising the guidance 

presented in the Addendum with respect to section 188(f), but the agency requests 

comment on whether the existing guidance in the Addendum is appropriate when

implementing the current and any future PM2.5 NAAQS.

2. Relationship between the CAA section 188(f) waiver provisions and the EPA’s 

Exceptional Events Rule

On March 22, 2007, the EPA promulgated the “Treatment of Data Influenced by 

Exceptional Events; Final Rule” (72 FR 13560), known as the Exceptional Events Rule, 

pursuant to the 2005 amendment of CAA section 319.268 The Exceptional Events Rule 

provides a mechanism by which the EPA can concur with an air agency’s request to 

 

267 59 FR 42004.

268 Section 319 of the CAA, as amended by section 6013 of the Safe Accountable 

Flexible Efficient-Transportation Equity Act: A Legacy for Users (SAFE-TEA-LU) of 

2005, required the EPA to propose and promulgate regulations governing the review and 

handling of air quality monitoring data influenced by exceptional events.
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exclude from regulatory decisions air quality monitoring data determined by EPA to have 

been affected by exceptional events.269 The Exceptional Events Rule applies to all 

NAAQS pollutants, including PM2.5. Section 188(f) and the Exceptional Events Rule 

provide separate mechanisms by which states and/or other air agencies can seek to have 

event-influenced monitoring data excluded from certain regulatory requirements or 

decisions associated with the PM implementation process, under appropriate 

circumstances. This section explains the EPA’s views on how these two mechanisms can 

operate.

The Exceptional Events Rule addresses elevated emissions from specific events 

that influence monitored air quality concentrations. The EPA’s regulations at 40 CFR 

50.1(j), defines an “exceptional event” as one that “affects air quality, is not reasonably 

controllable or preventable, is an event caused by human activity that is unlikely to recur 

at a particular location or a natural event, and is determined by the Administrator in 

accordance with 40 CFR 50.14 to be an exceptional event.” Further, 40 CFR 50.1(j)

explicitly provides that exceptional events do “...not include stagnation of air masses or 

meteorological inversions, a meteorological event involving high temperatures or lack of 

precipitation, or air pollution relating to source noncompliance.” At 40 CFR 50.1(k), the 

EPA’s regulations define a “natural event” as an event in which human activity plays 

little or no direct causal role to the event in question.270 The Exceptional Events Rule 

 

269 References to “air agencies” are meant to include state, local and tribal air agencies 

responsible for implementing the Exceptional Events Rule.

270 The EPA will generally consider human activity to have played little or no direct role 

in causing emissions of the dust generated by high wind for purposes of the regulatory 
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allows the EPA to exclude from regulatory decisions air quality monitoring data that it 

determines to have been influenced by emissions that result from exceptional events. Air 

quality monitoring data that the EPA determines to have been influenced by an 

exceptional event under the procedural steps, substantive criteria, and schedule specified 

in the Exceptional Events Rule may be excluded from regulatory decisions such as initial 

area designations decisions and decisions associated with implementing the PM2.5

NAAQS such as clean data determinations, evaluation of attainment demonstrations, and 

discretionary or mandatory reclassifications of nonattainment areas from Moderate to 

Serious. While the EPA may agree with an air agency’s request to exclude eventinfluenced air quality monitoring data from regulatory decisions, these regulatory actions 

require the EPA to provide an opportunity for public comment on the claimed 

exceptional event and all supporting data prior to the EPA taking final Aagency action.

If wildfire is a potential contributor to these exceedances of the NAAQS and 

exceptional events, the EPA urges state and local agencies to coordinate with the land 

management agencies, as appropriate, in developing plans and appropriate public 

communications regarding public safety and reducing exposure. This action can directly 

 

definition of “natural event” if contributing anthropogenic sources of the dust are 

reasonably controlled at the time of the event, regardless of the amount of dust coming 

from these reasonably controlled anthropogenic sources, and thus the event could be 

considered a natural event. In such cases, the EPA believes that it would generally be a 

reasonable interpretation of its regulations to find that the anthropogenic source had 

“little” direct causal role. If anthropogenic sources of windblown dust that are reasonably 

controllable but that did not have those reasonable controls applied at the time of the high 

wind event have contributed significantly to a measured concentration, then the event 

would not be considered a natural event. See preamble to the Exceptional Events Rule at 

72 FR 13566, f.n. 11. 

Commented [A6]: Reviewer suggested adding this text (similar 

to text in ozone implementation rule).
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help states meet their Exceptional Events Rule EER obligation whereby “Sstates must 

provide public notice, public education, and must provide for implementation of 

reasonable measures to protect public health when an event occurs.” When wildfire 

impacts are significant in a particular area, air agencies and communities may be able to 

lessen the impacts of wildfires by working collaboratively with land managers and land 

owners to employ various mitigation measures including taking steps to minimize fuel 

loading in areas vulnerable to fire.

The EPA notes that there could be some potential overlap between the application 

of the Exceptional Events Rule and section 188(f). The EPA believes that this potential 

for overlap can best be addressed by considering the applicability of the Exceptional 

Events Rule and section 188(f) in sequence. Thus, the EPA recommends that air agencies 

first consider whether the monitored air quality data on specific days were influenced by 

an exceptional event. If the air agency requests and the EPA agrees with this request and 

determines that the monitored air quality data should be excluded from consideration in 

regulatory decisions, then using the provisions in the Exceptional Events Rule could 

address the situation adequately. Thereafter, if the air agency determines that the waiver 

provisions of section 188(f) may also be applicable, then the EPA can evaluate that 

question based on the remaining data that are representative for the area in question.

B. Conformity Requirements

1. What requirements apply to both transportation and general conformity?

a. What are transportation and general conformity? Conformity is required under 

CAA section 176(c) to ensure that federal actions are consistent with (“conform to”) the 

purpose of the SIP. Conformity to the purpose of the SIP means that federal activities will 
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not cause new air quality violations, worsen existing violations, or delay timely 

attainment of the relevant NAAQS or interim reductions and milestones. Conformity 

applies to areas that are designated nonattainment, and those nonattainment areas 

redesignated to attainment with a CAA section 175A maintenance plan after 1990 

(“maintenance areas”). 

The EPA’s Transportation Conformity Rule (40 CFR 51.390 and part 93, subpart 

A) establishes the criteria and procedures for determining whether transportation 

activities conform to the SIP. These activities include adopting, funding or approving 

transportation plans, transportation improvement programs (TIPs) and federally 

supported highway and transit projects. The EPA first promulgated the Transportation 

Conformity Rule on November 24, 1993 (58 FR 62188), and subsequently published 

several amendments. For example, the EPA published a final rule on July 1, 2004 (69 FR 

40004) that provided conformity procedures for state and local agencies under the 1997 

PM2.5 NAAQS, among other things. On May 6, 2005 (70 FR 24280) the EPA published a 

final rule that addressed transportation conformity requirements for PM2.5 precursors. 271

The EPA published another final rule on March 24, 2010 (75 FR 14260) that addressed 

additional requirements for the 2006 PM2.5 NAAQS. Finally, the EPA published a final 

rule on March 14, 2012 (77 FR 14979) that restructured portions of the transportation 

conformity rule so that they would clearly apply to nonattainment and maintenance areas 

for new and revised NAAQS, including the 2012 PM2.5 NAAQS. All of these rules apply

 

271This final rule was not challenged or affected in any way by the January 2013 DC 

Circuit Court decision requiring the EPA to implement the PM2.5 NAAQS pursuant to 

subpart 4 of the CAA.
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to the current PM2.5 NAAQS including the 1997 PM2.5 NAAQS, the 2006 24-hour PM2.5

NAAQS and the 2012 primary annual PM2.5 NAAQS and will apply to future PM2.5

NAAQS. For further information on conformity rulemakings, policy guidance and 

outreach materials, see the EPA’s website at 

http://www.epa.gov/otaq/stateresources/transconf/index.htm. The EPA may issue future 

transportation conformity guidance as needed to implement the 2012 primary annual 

PM2.5 NAAQS.

With regard to general conformity, the EPA first promulgated general conformity 

regulations in November 1993. (40 CFR part 51, subpart W, 40 CFR part 93, subpart B) 

Subsequently the EPA finalized revisions to the general conformity regulations on April 

5, 2010. (75 FR 17254-17279) Besides ensuring that federal actions not covered by the 

transportation conformity rule will not interfere with the SIP, the general conformity 

program also fosters communications between federal agencies and state/local air quality 

agencies, provides for public notification of and access to federal agency conformity 

determinations and allows for air quality review of individual federal actions. More 

information on the general conformity program is available at 

http://www.epa.gov/air/genconform/.

b. Why is the EPA discussing transportation and general conformity in this 

proposed rulemaking? The EPA is discussing transportation and general conformity in 

this proposed rulemaking in order to provide affected parties with information on when 

conformity must be implemented after nonattainment areas are designated for a new or 

revised PM2.5 NAAQS. At this time the EPA is using the 2012 PM2.5 NAAQS as an 

example. The agency is also discussing how it plans to make the transition from 
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demonstrating conformity for the 1997 annual PM2.5 NAAQS to the 2012 primary annual 

PM2.5 NAAQS because this transition is unique in that the 1997 annual PM2.5 NAAQS 

was retained as a secondary NAAQS. The information presented here is consistent with 

existing conformity regulations and statutory provisions that are not addressed by this 

PM2.5 implementation rulemaking. Affected parties would include state and local 

transportation and air quality agencies, metropolitan planning organizations (MPOs), and 

federal agencies including the U.S. Department of Transportation (DOT), the U.S. 

Department of Defense, the U.S. Department of Interior and the U.S. Department of 

Agriculture. 

c. When would transportation and general conformity apply to areas designated 

nonattainment for the 2012 primary annual PM2.5 NAAQS? Transportation and general 

conformity apply 1 year after the effective date of nonattainment designations for a new 

or revised PM2.5 NAAQS including the 2012 primary annual PM2.5 NAAQS. This is 

because CAA section 176(c)(6) provides a 1-year grace period from the effective date of 

initial designations for any new NAAQS before transportation and general conformity 

apply in areas newly designated nonattainment for a specific pollutant and NAAQS. With 

regard to general conformity, the EPA’s April 2010 revisions to its general conformity 

regulations (see 75 FR 17277, April 5, 2010) apply the same 1-year grace period for the 

purposes of general conformity.

With regard to transportation conformity, the conformity grace period applies to 

all areas designated nonattainment for a new or revised PM2.5 NAAQS including the 

2012 primary annual PM2.5 NAAQS. The requirements differ depending on whether the 

nonattainment area is considered a metropolitan area. Metropolitan areas are urbanized 
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areas that have a population greater than 50,000 and a designated MPO responsible for 

transportation planning per 23 U.S.C. 134. In general, within 1 year after the effective 

date of the initial nonattainment designation for a given pollutant and NAAQS, the 

metropolitan area’s MPO and DOT must make a conformity determination with regard to 

that pollutant and standard for the area’s transportation plan and TIP. The conformity 

requirements for surrounding “donut areas,” including the application of the 1-year 

conformity grace period, are generally the same as those for metropolitan areas. 272 For 

the purposes of the implementation of the 2012 PM2.5 NAAQS MPOs and any adjacent 

donut areas must continue to meet conformity requirements in nonattainment and 

maintenance areas for the 1997 annual PM2.5 NAAQS during the grace period, in addition 

to any other applicable NAAQS including the 2006 24-hour PM2.5 NAAQS. If, at the end 

of the grace period for the 2012 annual PM2.5 NAAQS, the MPO and DOT have not 

made a transportation plan and TIP conformity determination for that NAAQS, the area 

would be in a conformity “lapse.” During a conformity lapse, only certain projects can 

receive additional federal funding or approvals to proceed. The practical impact of a 

conformity lapse will vary from area to area. 

Isolated rural nonattainment and maintenance areas are areas that do not contain 

or are not part of an MPO (40 CFR 93.101). Conformity requirements for isolated rural 

nonattainment and maintenance areas can be found at 40 CFR 93.109(g). An isolated 

rural area would be required to make a transportation conformity determination only at 

 

272 For the purposes of transportation conformity, a “donut” area is the geographic area 

outside a metropolitan planning area boundary, but inside a designated nonattainment or 

maintenance area boundary that includes an MPO (40 CFR 93.101).
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the point when a transportation project needs funding or approval. This point may occur 

significantly after the 1-year grace period has ended. See the EPA’s July 1, 2004 final 

rule for further background on how the EPA has implemented this conformity grace 

period in metropolitan, donut and isolated rural areas (69 FR 40008-40014). 

d. How will transportation and general conformity apply with regard to the 1997 

annual PM2.5 NAAQS, which was retained as a secondary NAAQS? Transportation and 

general conformity for the 1997 secondary annual PM2.5 NAAQS continue to apply in 

existing nonattainment areas until the EPA approves a redesignation for such NAAQS. 

The current CAA and conformity regulations already address this transition, and nothing 

in this proposal affects transportation and general conformity requirements for the 1997 

secondary annual PM2.5 NAAQS.

In the December 2012 PM NAAQS final rule the EPA established a new healthbased primary annual PM2.5 NAAQS of 12.0 μg/m3

. In that same action the EPA retained 

the 1997 annual PM2.5 NAAQS of 15.0 μg/m3as a secondary NAAQS to protect against 

certain welfare effects. A discussion of how transportation and general conformity apply 

in this situation follows.

Section 176(c)(5) establishes that conformity applies to “(A) a nonattainment area 

and each pollutant for which the area is designated as a nonattainment area; and (B) an 

area that was designated as a nonattainment area but that was later redesignated by the 

Administrator as an attainment area and that is required to develop a maintenance plan 

under section 7505a of this title with respect to the specific pollutant for which the area 

was designated nonattainment.” Section 176(c)(5) is clear that transportation and general 
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conformity apply in nonattainment areas and in areas that have been redesignated to 

attainment and are required to develop a maintenance plan under section 175A.

Section 175A(a) establishes the requirements for areas that are required to submit 

a maintenance plan as one of the requirements that must be fulfilled in order to be 

redesignated to attainment. Section 175A(a) states:

Each State which submits a request under section 7407(d) of this title for 

redesignation of a nonattainment area for any air pollutant as an area which has 

attained the national primary ambient air quality standard for that air pollutant 

shall also submit a revision of the applicable State implementation plan to provide 

for the maintenance of the national primary ambient air quality standard for such 

air pollutant in the area concerned for at least 10 years after the redesignation. The 

plan shall contain such additional measures, if any, as may be necessary to ensure 

such maintenance.

Section 175A(a) requires nonattainment areas for primary NAAQS to submit 

maintenance plans in order to be redesignated. Section 175A(a) does not require 

nonattainment areas for secondary NAAQS to submit maintenance plans in order to be 

designated to attainment. Therefore, the EPA concludes that transportation and general 

conformity do not apply in areas that have been redesignated for any secondary NAAQS, 

such as the 1997 secondary annual PM2.5 NAAQS, since conformity does not apply in 

areas that have been redesignated without maintenance plans.

Elsewhere in this notice, the EPA is proposing several options for revoking the 

1997 primary annual PM2.5 NAAQS, which has been replaced by the more health 

protective 2012 primary annual PM2.5 NAAQS. If the EPA finalizes any option that 

results in the revocation of the 1997 primary annual PM2.5 NAAQS, nonattainment and 

maintenance areas would not be required to make transportation or general conformity 

determinations for the 1997 primary annual PM2.5 NAAQS after the effective date of the 

revocation of the 1997 primary annual NAAQS. The revocation would leave designations 
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in place for the 1997 secondary annual NAAQS. Any area that is designated as 

nonattainment for the 1997 secondary annual NAAQS would have to continue to make 

transportation and general conformity determinations for that NAAQS as conformity 

applies in nonattainment areas for secondary NAAQS. 

However, for any area that has been redesignated to attainment for the 1997 

secondary NAAQS and is not designated nonattainment for the 2012 primary annual 

PM2.5 NAAQS, the relevant planning organization will not have to make conformity

determinations for any annual PM2.5 NAAQS after the effective date of the revocation of 

the 1997 primary annual PM2.5 NAAQS because, as discussed above, the CAA does not 

require maintenance areas for secondary NAAQS to make conformity determinations. 

This means that if the EPA finalizes any of the options for revoking the 1997 primary 

annual PM2.5 NAAQS, after the effective date of the revocation, areas redesignated to 

attainment for the 1997 secondary annual PM2.5 NAAQS will no longer be required to 

make transportation plan, TIP, or project-level transportation conformity determinations 

for that NAAQS. In addition, federal agencies will no longer be required to make general 

conformity determinations for that NAAQS. Areas that remain designated nonattainment 

for the 1997 secondary annual PM2.5 NAAQS will continue to make transportation plan, 

TIP, and project-level conformity determinations for that NAAQS and federal agencies 

will be required to continue to make general conformity determinations for that NAAQS 

in these areas until such time as they attain that NAAQS and are redesignated to 

attainment. 

e. What impact will the implementation of a new or revised PM2.5 NAAQS such as 

the 2012 PM2.5 NAAQS have on a state’s transportation and/or general conformity SIP?
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As long as the EPA does not make specific changes to its transportation or general 

conformity regulations states should not need to revise their transportation and/or general 

conformity SIPs. The EPA is not proposing any changes to its transportation conformity 

regulations. The EPA is proposing to change the de minimis levels in its general 

conformity regulations as discussed in section IX.B.2.b. of this proposal. States with a 

general conformity SIP should evaluate the need to revise those SIPs if this change is 

finalized. States with new nonattainment areas may also need to revise conformity SIPs 

in order to ensure the state regulations apply in any newly designated areas. 

However, if this is the first time that transportation conformity will apply in a 

state, such a state is required by the statute and EPA regulations to submit a SIP revision 

that addresses three specific transportation conformity requirements that address 

consultation procedures and written commitments to control or mitigation measures 

associated with conformity determinations for transportation plans, TIPs or projects. (40 

CFR 51.390) Additional information and guidance can be found in the EPA’s “Guidance 

for Developing Transportation Conformity State Implementation Plans” 

(http://www.epa.gov/otaq/stateresources/transconf/policy/420b09001.pdf).

2. What additional requirements apply to general conformity?

a. What de minimis emission levels will apply for direct PM2.5 and its precursors? 

Federal actions estimated to have an annual net emissions increase less than the de 

minimis levels established in the general conformity regulations are not required to 

demonstrate conformity under those regulations. For direct PM2.5 and its precursors (SO2, 

NOx, VOC and ammonia), the existing de minimis emission levels are set forth in the 

EPA’s general conformity regulations at 40 CFR 93.153(b)(1). Those levels were based 
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on the definition of a major stationary source for nonattainment NSR programs as 

established by sections 182, 183 and 302 of the CAA. The EPA believes it is appropriate 

to continue this practice for implementing the current and any future PM2.5 NAAQS. 

However, because the definition of precursors currently in the general conformity 

regulations at 40 CFR 93.152(b)(1) does not reflect the elimination of rebuttable 

presumptions for certain PM2.5 precursors, the EPA is proposing changes to these 

conformity provisions to make them consistent with the agency’s revised precursor 

requirements. Specifically, the current definition of precursors for PM2.5 in the general 

conformity regulations reflects the rebuttable presumptions for VOC and ammonia

finalized in the 2007 PM2.5 Implementation Rule. It also does not reflect the subpart 4 

definitions for “major source” and “major stationary source” that apply for Serious PM2.5

nonattainment areas. Therefore, through this proposal the EPA proposes to change the 

PM2.5 precursor de minimis levels currently in 40 CFR 93.153(b)(1) to be consistent with 

the statutory requirements for major stationary source thresholds under subpart 4 and any 

relevant changes being proposed in Section III of this preamble. The EPA proposes to set 

the de minimis levels that apply to direct PM2.5 and PM2.5 precursors for PM2.5

nonattainment areas for purposes of general conformity as identified in Table 3 below.

Table 3 -- General Conformity De Minimis Emission Levels for PM2.5 Precursors

Type of Emission Tons/Year

in Moderate PM2.5

nonattainment areas and all 

maintenance areas

Tons/Year

in Serious PM2.5

nonattainment areas

Direct emissions 100 70

SO2 100 70

NOx 100 70

VOC 100 70

Ammonia 100 70
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b. Are there any other impacts related to general conformity based on 

implementation of the 2012 PM2.5 NAAQS? The EPA is not proposing any other 

revisions to the general conformity regulations at this time. However, as areas develop 

SIPs for the 2012 and future PM2.5 NAAQS, the agency recommends that state and local 

air quality agencies work with federal agencies with large facilities (e.g., commercial 

airports, ports and large military bases) that are subject to the general conformity 

regulations to establish an emissions budget for those facilities in order to facilitate future 

conformity determinations under the conformity regulations. Such a budget could be used 

by federal agencies in determining conformity or identifying mitigation measures if the 

budget level is included and identified in the SIP.

Significant tracts of land under federal management may also be included in 

nonattainment area boundaries. The role of fire in these areas should be assessed and 

emissions budgets developed in concert with those federal land management agencies. In 

such areas the EPA encourages states to consider in any baseline, modeling and SIP 

attainment inventory used and/or submitted to include emissions expected from projects 

subject to general conformity, including emissions from wildland fire that may be 

reasonably expected in the area.Where appropriate, states may consider developing plans 

for addressing wildland fuels in collaboration with land managers and owners. 

Information is available from DOI and USDA Forest Service on the ecological role of 

fire and on smoke management programs and basic smoke management practices.

273

 

273 USDA Forest Service and Natural Resources Conservation Service, Basic Smoke 

Management Practices Tech Note, October 2011, 

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1046311.pdf.

Commented [A7]: Reviewer suggested adding two inserts which 

have been combined into one here. 

(The second insert below has been folded into this paragraph: 

“In such areas the EPA encourages states to consider in any baseline, 

modeling and SIP attainment inventory used and/or submitted to 

include emissions expected from projects subject to general 

conformity, including emissions from wildland fire that may be 

reasonably expected in the area.”
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C. Clean Data Policy

This section describes the ongoing status of the EPA’s Clean Data Policy and 

proposes provisions applicable to any determinations of attainment under current and 

future PM2.5 NAAQS. This section also sets forth the regulatory consequences of an EPA 

determination, made after notice-and-comment rulemaking, that an area designated 

nonattainment for a particulate matter standard has air quality attaining that standard. 

Upon such a determination by the EPA, the state’s requirement for the area to submit the 

separate required elements of an attainment plan (including an attainment demonstration, 

but not the emissions inventory requirement), shall be suspended until such time as the 

area is redesignated to attainment at which time the requirements no longer apply. If the 

EPA determines that the area, after reaching attainment, has again violated that 

particulate matter NAAQS, the requirements are again applicable. The following 

discussion of this interpretation, known as the EPA’s Clean Data Policy, explains the 

basis for the EPA’s interpretation and is relevant to all particulate matter NAAQS under 

subpart 4.

1. What is a clean data determination?

The EPA’s interpretation of the CAA applies when the agency, after notice and 

comment rulemaking, issues a “clean data determination” (CDD), in which it determines 

that a specific nonattainment area has attained the relevant standard. For such areas, the 

EPA interprets the CAA as suspending the state requirements to submit to the EPA the 

planning elements of an attainment plan related to attaining the NAAQS for as long as 
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the area continues to attain the standard.274 These planning elements generally include 

reasonable further progress (RFP) requirements, attainment demonstrations, 

RACM/RACT, nonattainment area contingency measures, and other state planning 

requirements related to the attainment of the NAAQS.275 The suspension of the obligation 

to submit applies regardless of when the plan submissions are due. The CDD does not 

suspend CAA requirements that are independent of helping the area achieve attainment, 

such as the requirements to submit an emissions inventory and nonattainment new source 

review requirements.

The emissions inventory is a basic compilation of information used to 

characterize the sources of emissions of the nonattainment area. Section 172(c)(3), the 

statutory provision requiring submission of an emissions inventory, is not tied to 

attainment of the NAAQS, unlike the attainment planning provisions which are 

suspended by a CDD. A base year inventory continues to be relevant to a nonattainment 

area that is attaining the NAAQS and has obtained a CDD because, for example, the 

inventory is a necessary component to an approvable redesignation request. In addition, 

in the event the air quality in the area exceeds the standard in a subsequent year, the state 

 

274 In the context of CDDs, the EPA distinguishes between attainment planning 

requirements of the CAA, which relate to the attainment demonstration for an area and 

related control measures for bringing an area into attainment for a given NAAQS as 

expeditiously as practicable, and other types of requirements, such as permitting 

requirements under the NNSR program, and any specific control requirements 

independent of those strictly needed to ensure timely attainment of a given NAAQS.

275 See December 14, 2004 memorandum from Steven D. Page, Director, EPA Office of 

Air Quality Planning and Standards, entitled “Clean Data Policy for the Fine Particle 

National Ambient Air Quality Standards.” Available at: 

http://www.epa.gov/airquality/urbanair/sipstatus/docs/pm25_clean_data_policy_14dec20

04.pdf
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would be obligated to submit an attainment demonstration and other planning elements 

for the area, and a base year inventory would need to be available immediately in order 

for the state to submit an approvable attainment plan expeditiously. Similarly, the new 

source review requirement is not suspended because section 172(c)(5) is not tied to 

attainment of the NAAQS, andan area with a CDD is still designated nonattainment. 

NNSR permitting is required in each nonattainment area until the area is redesignated to 

attainment.” For the past two decades, and for many NAAQS, the EPA has consistently 

applied its Clean Data Policy interpretation to attainment-related provisions of subparts 1, 

2 and 4 of Part D, Title I of the CAA. The Clean Data Policy is the subject of several 

EPA memoranda and regulations and numerous individual rulemakings published in the 

Federal Register. These rulemakings have applied the interpretation to a broad spectrum 

of NAAQS, including the 1-hour and 1997 ozone standards, PM10, 1997 and 2006 PM2.5 

standards and the carbon monoxide (CO) and lead standards. The D.C. Circuit has upheld 

the Clean Data Policy interpretation as embodied in the EPA’s 8-hour ozone

Implementation Rule, 40 CFR 51.918.276 NRDC v. EPA, 571 F. 3d 1245 (DC Cir. 2009). 

Other U.S. Circuit Courts of Appeals that have considered and reviewed the EPA’s Clean 

Data Policy interpretation have upheld it and the rulemakings applying the EPA’s 

interpretation. Sierra Club v. EPA, 99 F.3d 1551 (10th Cir. 1996); Sierra Club v. EPA, 

375 F. 3d 537 (7th Cir. 2004); Our Children's Earth Foundation v. EPA, N. 04-73032 

 

276 “The EPA’s Final Rule to implement the 8-hour Ozone National Ambient Air Quality 

Standard--Phase 2 (Phase 2 Final Rule).” 70 FR 71612 at 71645-46 (November 29, 

2005).
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(9th Cir. June 28, 2005) (memorandum opinion); Latino Issues Forum, v. EPA, Nos. 06-

75831 and 08-71238 (9th Cir. March 2, 2009) (memorandum opinion). The EPA 

incorporated its Clean Data Policy interpretation in both its 1997 8-hour ozone 

implementation rule and in its remanded 2007 PM2.5 Implementation Rule in 40 CFR 

51.1004(c). 72 FR 20585, 20665 (April 25, 2007). The D.C. Circuit, in its January 4, 

2013 decision remanding the PM 2.5 implementation rule, did not address the merits of 

that regulation or the EPA’s existing interpretation of the statutory provisions as they 

pertained to the EPA’s Clean Data Policy.

The EPA has previously articulated its Clean Data Policy interpretation under 

subpart 4 in implementing the PM10 standard. See, e.g., 75 FR 27944 (May 19, 2010) 

(determination of attainment of the PM10 standard in Coso Junction, California); 71 FR 

13021 (March 14, 2006) (Yuma, Arizona area); 71 FR 40023 (July 14, 2006) (Weirton, 

West Virginia area); 71 FR 44920 (August 8, 2006) (Rillito, Arizona area); 71 FR 63642 

(October 30, 2006) (San Joaquin Valley, California area) 72 FR 14422 (March 28, 2007) 

(Miami, Arizona area). In the EPA’s proposed and final rulemakings determining that the 

San Joaquin Valley nonattainment area attained the PM10 standard, the EPA set forth at 

length its rationale for applying the Clean Data Policy to PM10 under subpart 4. 71 FR at 

63643-45. The Ninth Circuit upheld the EPA’s final rulemaking, and specifically the 

EPA’s Clean Data Policy, in the context of subpart 4. Latino Issues Forum v. EPA, supra. 

Nos. 06-75831 and 08-71238 (9th Cir. March 2, 2009) (memorandum opinion). In 

rejecting the petitioner’s challenge to the Clean Data Policy under subpart 4 for PM10, the 

Ninth Circuit stated, “As the EPA explained, if an area is in compliance with PM10

standards, then further progress for the purpose of ensuring attainment is not necessary.” 
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Thus the EPA has previously established its interpretation that, under subpart 4, a clean 

data determination suspends the obligations to submit an attainment demonstration, 

RACM/RACT, RFP and quantitative milestones, contingency measures, and other 

measures related to attainment. The EPA is proposing to codify this interpretation in this 

implementation rule for the PM2.5 NAAQS.

As with its Clean Data Policy interpretation for 8-hour ozone, which the EPA 

embodied in a regulation that was upheld by the D.C. Circuit in NRDC v. EPA, 571 F. 3d 

1245 (D.C. Cir. 2009), the EPA intends to embody its interpretation for the Clean Data 

Policy for current and future PM2.5 NAAQS in a regulation as part of this proposed 

rulemaking. This interpretation complies with the D.C. Circuit’s ruling (Sierra Club v. 

EPA, 706 F.3d 428 (D.C. Cir. 2013) that both subparts 1 and subpart 4 apply to 

implementation, and reflects the interpretation upheld by the Latino Issues Forum Court. 

Latino Issues Forum v. EPA, supra. Nos. 06-75831 and 08-71238 (9th Cir. March 2, 

2009)(memorandum opinion). Under this proposed regulation, if the EPA determines, 

after notice and comment rulemaking, that an area has attained the applicable PM2.5

NAAQS based on the most recent 3-years’ worth of complete, quality-assured data 

meeting the requirements of 40 CFR part 50, Appendix N, the area’s obligation to submit 

the following Moderate or Serious area attainment-related planning requirements is 

suspended for so long as the area continues to attain the PM2.5 standard: (i) the part D, 

subpart 4 and subpart 1 obligation to provide an attainment demonstration pursuant to 

section 189(a)(1)(B); (ii) the RACM and RACT provisions of section 189(a)(1)(C); (iii) 

the RFP and quantitative milestones provisions of section 189(c); and, (iv) related 
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attainment demonstration, RACM and RACT, RFP and contingency measure provisions 

requirements of subpart 1, section 172. 

A final determination of attainment, also known as a clean data determination, 

would not constitute a redesignation to attainment under CAA section 107(d)(3). The 

state would still have to meet the statutory requirements for redesignation in order to be 

redesignated to attainment. A determination of attainment for purposes of the Clean Data 

Policy is also not linked to any particular attainment deadline, and is not necessarily 

equivalent to a determination that an area has attained the standard by its applicable 

attainment deadline, e.g., under section 189(c).

2. Planning requirements suspended with a CDD

a. Control measure requirements for Moderate areas. Both sections 172(c)(1) and 

189(a)(1)(C) require “provisions to assure that reasonably available control 

measures'' (i.e., RACM) are implemented in a nonattainment area. Reasonably 

available control technology (i.e., RACT) is a subset of RACM. The General 

Preamble states that the EPA interprets section 172(c)(1) so that RACM 

requirements are a “component'' of an area's attainment demonstration.277 Thus, 

for the same reason the obligation to submit an attainment demonstration is 

suspended, the requirement for a state to submit RACM measures is suspended if 

the nonattainment area reaches attainment. For PM2.5 , the EPA has consistently 

interpreted this provision to require only implementation of potential RACM 

measures that could contribute to reasonable further progress or to attainment 

 

277 57 FR 13498 at 13560.
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within six years only. General Preamble, 57 FR at 13498. Thus, where an area is 

already attaining the standard, no additional RACM measures are required, but all 

measures adopted into the SIP prior to attainment would remain.278 The EPA is 

interpreting section 189(a)(1)(C) consistent with its interpretation of section 

172(c)(1). 

b. RFP and quantitative milestones. The EPA has long interpreted the provisions 

of part D, subpart 1 of the Act (sections 171 and 172) as not requiring the 

submission of RFP for an area already attaining the PM10 NAAQS. For an area 

that is attaining, showing that the state will make RFP towards attainment “will, 

therefore, have no meaning at that point.”279

Section 189(c)(1) of subpart 4 states that:

Plan revisions demonstrating attainment submitted to the Administrator for 

approval under this subpart shall contain quantitative milestones which are to be 

achieved every 3 years until the area is redesignated attainment and which 

demonstrate reasonable further progress, as defined in section [171(1)] of this 

title, toward attainment by the applicable date.

With respect to RFP, section 171(1) states that, for purposes of part D, RFP 

“means such annual incremental reductions in emissions of the relevant air pollutant as 

 

278 The EPA's interpretation that the statute requires implementation only of RACM 

measures that would advance attainment was upheld by the Fifth Circuit Court (Sierra 

Club v. EPA, 314 F.3d 735, 743-745 (5th Cir. 2002), and by the D.C. Circuit Court 

(Sierra Club v. EPA, 294 F.3d 155, 162-163 (D.C. Cir. 2002)).

279 57 FR 13498 at 13564. See 71 FR 40952 and 71 FR 63642 (proposed and final 

determination of attainment for San Joaquin Valley); 75 FR 13710 and 75 FR 27944 

(proposed and final determination of attainment for Coso Junction).
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are required by this part or may reasonably be required by the Administrator for the 

purpose of ensuring attainment of the applicable NAAQS by the applicable date.” 42 

U.S.C. 7501(1). Thus, whether dealing with the general RFP requirement of section 

172(c)(2), the ozone-specific RFP requirements of sections 182(b) and (c), or the specific 

RFP requirements for PM10 areas of part D, subpart 4, section 189(c)(1), the stated 

purpose of RFP is to ensure attainment by the applicable attainment date. Although 

section 189(c) states that revisions shall contain milestones which are to be achieved until 

the area is redesignated to attainment, such milestones are designed to show reasonable 

further progress “toward attainment by the applicable attainment date,” as defined by 

section 171. Thus, it is clear that once the area has attained the standard, no further 

milestones are necessary or meaningful. This interpretation is supported by language in 

section 189(c)(3), which mandates that a state that fails to achieve a milestone must 

submit a plan that assures that the state will achieve the next milestone or attain the 

NAAQS if there is no next milestone. Thus, section 189(c)(3) itself assumes that the 

requirement to submit and achieve milestones does not continue after attainment of the 

NAAQS.

In the General Preamble, the EPA noted with respect to section 189(c) that the 

purpose of the milestone requirement is “to provide for emission reductions adequate to 

achieve the standards by the applicable attainment date (H.R. Rep. No. 490 101st Cong., 

2d Sess. 267 (1990)).” 57 FR 13539 (April 16, 1992). If an area has in fact attained the 
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standard, the stated purpose of the RFP requirement will have already been fulfilled.280

Similarly, the requirements of section 189(c)(2) with respect to milestones no longer 

apply so long as an area has attained the standard. Section 189(c)(2) provides in relevant 

part that:

Not later than 90 days after the date on which a milestone applicable to the area 

occurs, each State in which all or part of such area is located shall submit to the 

Administrator a demonstration * * * that the milestone has been met.

Where the area has attained the standard and there are no further milestones, there 

is no further requirement to make a submission showing that such milestones have been 

met. This is consistent with the position that the EPA took with respect to the general 

RFP requirement of section 172(c)(2) in the General Preamble and in the May 10, 1995 

Seitz memorandum281 with respect to the requirements of sections 182(b) and (c). In the 

 

280 Thus, the EPA believes that it is a distinction without a difference that section 

189(c)(1) speaks of the RFP requirement as one to be achieved until an area is 

“redesignated attainment,'' as opposed to section 172(c)(2), which is silent on the period 

to which the requirement pertains, or the ozone nonattainment area RFP requirements in 

sections 182(b)(1) or 182(c)(2), which refer to the RFP requirements as applying until the 

“attainment date,'' since section 189(c)(1) defines RFP by reference to section 171(1) of 

the Act. Reference to section 171(1) clarifies that, as with the general RFP requirements 

in section 172(c)(2) and the ozone-specific requirements of section 182(b)(1) and 

182(c)(2), the PM-specific requirements may only be required “for the purpose of 

ensuring attainment of the applicable national ambient air quality standard by the 

applicable date.” 42 U.S.C. § 7501(1). As discussed in the text of this rulemaking, the 

EPA interprets the subpart 4 RFP requirements, in light of the definition of RFP in 

section 171(1), and its incorporation into section 189(c)(1), to no longer apply once the 

EPA makes a determination that the standard has been attained.

281 Memorandum from John S. Seitz, titled "Reasonable Further Progress, Attainment 

Demonstration, and Related Requirements for Ozone Nonattainment Areas Meeting the 

Ozone National Ambient Air Quality Standard,” (Seitz Memo). May 10, 1995.
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Seitz memorandum, the EPA also noted that section 182(g), the milestone requirement of 

subpart 2, which is analogous to provisions in section 189(c), is suspended upon a 

determination that an area has attained. The memorandum, citing additional provisions 

related to attainment demonstration and RFP requirements, stated:

Inasmuch as each of these requirements is linked with the attainment 

demonstration or RFP requirements of section 182(b)(1) or 182(c)(2), if an area is 

not subject to the requirement to submit the underlying attainment demonstration 

or RFP plan, it need not submit the related SIP submission either. (Seitz memo, 

page 4).

c. Contingency measures. Other SIP submission requirements are linked with 

these attainment demonstration and RFP requirements, and similar reasoning 

applies to them. These requirements include the contingency measure 

requirements of sections 172(c)(9). The EPA has interpreted the obligation to 

submit contingency measure requirements of sections 172(c)(9) as suspended 

when an area has attained the standard because those “contingency measures are 

directed at ensuring RFP and attainment by the applicable date.” 57 FR at 13564; 

see also Seitz memo at pgs. 5-6.

Section 172(c)(9) provides that:

SIPs in nonattainment areas shall provide for the implementation of specific 

measures to be undertaken if the area fails to make reasonable further progress, or 

to attain the [NAAQS] by the attainment date applicable under this part. Such 

measures shall be included in the plan revision as contingency measures to take 

effect in any such case without further action by the state or the EPA. 
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The contingency measure requirement is inextricably tied to the reasonable 

further progress and attainment demonstration requirements. Contingency measures are 

implemented if reasonable further progress targets are not achieved, or if attainment is 

not realized by the attainment date. Where an area has already achieved attainment and 

continues to do so it has no need to rely on contingency measures to come into attainment 

or to make further progress to attainment. As the EPA stated in the General Preamble: 

“The section 172(c)(9) requirements for contingency measures are directed at ensuring 

RFP and attainment by the applicable date.” See 57 FR 13564.

d. Attainment demonstrations. With respect to the attainment demonstration 

requirements of section 172(c) and section 189(a)(1)(B), the EPA proposes to find 

that, as with the RFP requirements, if an area is already monitoring attainment of 

the standard, there is no need for an area to make a further submission containing 

additional measures to achieve attainment. The plain language of section 

189(a)(1)(B) requires that the attainment plan provide for “a demonstration 

(including air quality modeling) that the [SIP] will provide for attainment by the 

applicable attainment date * * *.” Where the area has attained the standard, such a 

demonstration no longer serves a purpose. This interpretation is consistent with 

the interpretation of the section 172(c) requirements provided by the EPA in the 

General Preamble, the Page memo282, and the section 182(b) and (c) requirements 

set forth in the Seitz memo. As the EPA stated in the General Preamble, no other 

 

282 Memorandum from Stephen D. Page titled "Clean Data Policy for the Fine Particle 

National Ambient Air Quality Standards" are equally pertinent to all NAAQS. December 

14, 2004.
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measures to provide for attainment would be needed by areas seeking 

redesignation to attainment since “attainment will have been reached.” 57 FR at 

13564. See also Latino Issues Forum, v. EPA, Nos. 06-75831 and 08-71238 (9th 

Cir.), Memorandum Opinion, March 2, 2009.

e. Control measure requirements for Serious areas. Under proposed option #1 for 

BACM and BACT determinations, described in Section VI.D of this preamble, 

BACM and BACT for sources in the nonattainment area would be determined 

independent of the attainment needs of the area, and thus the requirement for 

BACM and BACT would not be considered an attainment planning requirement. 

Therefore, under such an approach, a determination of attainment would not 

suspend the obligation to submit any applicable outstanding BACM and BACT 

requirements. Under proposed option #2 for BACM and BACT determinations, 

BACM and BACT would be identified based on the specific attainment needs of 

the area, thus tying the BACM and BACT requirement directly to attainment 

planning for the area. Consistent with this second proposed approach for 

determining BACM and BACT, issuance of a CDD would therefore also suspend 

BACM and BACT requirements.

In addition, for a Serious area that failed to attain the relevant PM2.5 NAAQS by 

the applicable attainment date and that is therefore subject to the annual 5 percent 

emissions reduction requirement under section 189(d), but is nevertheless now attaining 

the relevant NAAQS, the EPA believes that the clean data policy may apply to the 

obligations of the state to make a SIP submission to meet the requirements of section 
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189(d). Once such an area is attaining the relevant NAAQS, a clean data determination 

would suspend the section 189(d) submission requirement.

3. Planning requirements not suspended with a CDD

For Moderate nonattainment areas the planning elements that are not suspended 

with a clean data determination are: emissions inventories, nonattainment new source 

review including 189(e) control requirements for major stationary source precursors, and 

conformity. For Serious nonattainment areas, the planning elements not suspended with a 

clean data determination are: emissions inventories, nonattainment NSR including section 

189(e) control requirements for major stationary sources of PM2.5 precursors, the Most 

Stringent Measures (MSM) requirements (if the area has elected to seek an extension of 

the attainment date under section 188(e)), and conformity. In addition, for a Serious 

PM2.5 nonattainment area, if the EPA finalizes proposed option #1 for BACM and BACT 

determinations, in which BACM and BACT would be determined independent of the 

attainment needs for the area, then the requirement for implementation of BACM and 

BACT would not be considered an attainment planning requirement and would thus not 

be suspended with a Clean Data Determination for the area.

4. Violations of the NAAQS after a CDD

The suspension of the state’s obligations to submit attainment plan elements such 

as the RACM and RACT measures, RFP and quantitative milestones, contingency 

measures, an attainment demonstration and other related attainment planning 

requirements exists only for as long as the area continues to monitor attainment of the 

relevant NAAQS prior to redesignation. If the EPA determines, after notice-andcomment rulemaking but prior to redesignation, that the area has monitored a violation of 
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the relevant NAAQS, the basis for the suspension of the requirements no longer exists. In 

that case, the area would again be subject to the requirement to submit the pertinent 

attainment plan elements or SIP revisions and would need to address those requirements. 

Thus, a final determination that the area need not currently submit one of the required 

attainment plan elements amounts to no more than a suspension of the obligation to make 

the submission for so long as the area continues to attain the standard. Only if and when 

the EPA redesignates the area to attainment under section 107(d)(3) would the area be 

permanently relieved of these attainment plan submission obligations.

Upon the EPA’s determination that an area is currently attaining the applicable 

PM2.5 standard, the EPA proposes that the obligations to submit attainment planning 

provisions to meet the requirements for an attainment plan for the PM2.5 NAAQS, 

including RFP plans, RACM and RACT measures, quantitative milestones, contingency 

measures and an attainment demonstration are suspended for as long as the area 

continues to monitor attainment of the applicable PM2.5 standards. If in the future, prior to 

redesignation of the nonattainment area to attainment, the EPA determines after noticeand-comment rulemaking that the area again violates the applicable PM2.5 standard, then 

the basis for suspending the obligation of the state to make one or more of these 

submissions would no longer exist and these attainment plan elements would again be 

due. Since all attainment planning requirements had been suspended for this area and the 

area attained by its attainment date, the CAA attainment plan contingency measures 

would not apply at the time of the NAAQS violation. In addition, because the area did not 

have a maintenance plan, the CAA section 175A maintenance plan contingency measures 

would also not apply. When an area violates after a CDD, and the statutory submission 
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date has passed, CAA section 110(k)(5) applies, requiring that if the EPA finds that the 

applicable implementation plan is substantially inadequate to attain or maintain the 

NAAQS, the Administrator shall establish a reasonable deadline (not to exceed 18 

months) for a state to submit a SIP plan revision. 

D. Section 179B/International Border Areas

The EPA recognizes that some states are affected not only by local and regional 

sources of PM2.5 and PM2.5 precursors, but also international sources that can contribute 

to an area’s PM2.5 NAAQS nonattainment status. As discussed in Section II of this 

preamble, direct PM2.5 and more importantly PM2.5 precursors can be transported long 

distances and can be found in the air thousands of miles from where the emissions 

occurred and the particles were formed. Nitrates and sulfates formed from NOx and SO2

emissions are generally transported over wide areas leading to substantial background 

contributions to NAAQS violations in urban areas. Organic carbon, which has both a 

primary and secondary component, can also be transported but to a far lesser degree. In 

general, higher concentrations of elemental carbon and crustal matter are found closer to 

the sources of these emissions.

Section 179B of the CAA, entitled “International Border Areas,” applies to areas 

that could attain the relevant NAAQS by the statutory attainment date “but for” emissions 

emanating from outside the U.S. Specifically, section 179B(a) provides that the EPA 

shall approve an attainment plan for such an area if : (1) the attainment plan meets all 

other applicable requirements of the CAA, and (2) the submitting state can satisfactorily 

demonstrate that “but for emissions emanating from outside of the United States,” the 

area would attain and maintain the relevant NAAQS. In addition, section 179B(d) applies 
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specifically to PM10 NAAQS (which would include the PM2.5 NAAQS) and provides that 

if a state demonstrates that an area would have timely attained the NAAQS but for 

emissions emanating from outside the U.S., then the area is not subject to the mandatory 

reclassification element of section 188(b)(2) for Moderate areas that fails to attain the 

PM10 NAAQS by the applicable attainment date. 

Under section 179B, areas affected by emissions from outside the U.S. continue 

to have attainment plan obligations. First, even if the area is impacted by emissions from 

outside the U.S., that fact does not affect the designation of the area. An area that is 

violating the relevant NAAQS, even if emissions from outside the U.S. contribute to that 

violation, will be designated nonattainment. Section 179B does not affect designation. 

Second, as a result of that designation, the state is required to meet the applicable 

attainment plan requirements for the relevant NAAQS. Section 179B does not negate the 

attainment plan requirements, it only eliminates the obligation for an attainment 

demonstration that demonstrates attainment and maintenance of the NAAQS, and 

elimination of that obligation is conditioned upon the state meeting all other attainment 

plan requirements.

Under section 179B, states remain obligated to meet the attainment plan 

requirements other than the requirement to demonstrate timely attainment. The applicable 

requirements for an attainment plan for PM2.5 include those requirements that apply to a 

Moderate area attainment plan, including an emissions inventory, RACM and RACT 

measures, RFP and quantitative milestones, and contingency measures. The Addendum 

includes a discussion of the applicable attainment plan requirements in the context of 

developing a SIP subject to section 179B. In it, the EPA clarified that “RACM/RACT 
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must be implemented to the extent necessary to demonstrate attainment by the applicable 

attainment date if emissions emanating from outside the U.S. were not included in the 

analysis.”283 The EPA further encouraged states “to reduce emissions beyond the 

minimum necessary to satisfy the ‘but for’ test in order to reduce the PM concentrations 

to which their populations are exposed”.”

284 However, the EPA acknowledged that 

“if...States...were also required, because of contributions to PM10 violations caused by 

foreign emissions, to shoulder more of a regulatory and economic burden than States not 

similarly affected...such a requirement would unfairly penalize States containing 

international border areas and effectively undermine the purpose of section 179B. Indeed, 

to the extent an affected State can satisfactorily demonstrate that implementation of such 

measures clearly would not advance the attainment date, EPA and the state could 

conclude they are unreasonable and hence do not constitute RACM.”285

The EPA has considered this past interpretation of RACM and RACT 

requirements in the context of section 179B attainment plans for PM2.5 NAAQS and no 

longer views it as appropriate or consistent with the agency’s guidance that encourages 

states “to reduce emissions beyond the minimum necessary to satisfy the ‘but for’ test in 

order to reduce the PM10 concentrations to which their populations are exposed.”286 That 

is, given that the primary purpose of an attainment plan is to achieve emission reductions 

so that people living in a nonattainment area receive the public health protection intended 

 

283 59 FR 41998 at 42001.

284 Ibid.

285 Ibid.

286 Ibid.
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by the NAAQS, adopting an interpretation that would allow those people to continue to 

be subjected to levels of PM2.5 above the NAAQS that the state could reasonably reduce 

– in this case not to attainment level, but to a level below the current level -- would be 

antithetical to the objectives of the CAA. In addition, as with all other Moderate PM2.5

nonattainment areas, the EPA interprets the provisions of section 172(c)(6) to require that 

such areas must implement all additional reasonable measures that it can implement 

through the sixth calendar year following designation of the area, in addition to those 

measures meeting the definition of RACM and RACT, in order to make progress toward 

attainment after the end of the fourth year following designation.

Therefore, the EPA is proposing and seeking comment on two proposed 

approaches that would give greater clarity to the agency’s existing interpretation of 

control strategy requirements for Moderate area attainment plans to be approved under 

section 179B. The first proposed interpretation would clarify that the control strategy for 

an area that could attain by the Moderate area attainment date, “but for” foreign 

emissions of direct PM2.5 or its precursors, must include all control measures identified 

by the state to be technologically and economically feasible and implementable on 

sources in the area by the end of the sixth calendar year following designation of the area,

thus satisfying requirements for RACM and RACT and additional reasonable measures,

with a possible exception for any such measures that collectively would not be effective 

in reducing ambient PM2.5 levels in the area. This interpretation would closely align the 

EPA’s interpretation of what constitutes a reasonable control strategy for a Moderate area 

attainment plan submitted pursuant to section 179B with the EPA’s proposed 

interpretation of what constitutes a reasonable control strategy for a Moderate area 
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attainment plan submitted pursuant to section 189(a)(1) for an area that cannot 

practicably attain by the statutory Moderate area attainment date. 

More specifically, under the first proposed approach for identifying appropriate 

control measures on sources in a Moderate PM2.5 nonattainment area that could attain the 

NAAQS “but for” foreign emissions, the EPA is proposing that the state would be 

required to implement all technologically and economically feasible measures that can be 

implemented on sources in the area by the end of the sixth calendar year following 

designation of the area in order to ensure that the area makes reasonable progress toward 

attaining the standard even if such measures are not expected to yield attainment by the 

statutory Moderate area attainment date. However, because the EPA recognizes that it 

may not be reasonable to require that a state implement those technologically and 

economically feasible control measures that collectively will not effectively reduce 

ambient PM2.5 concentrations, the agency is proposing to allow the state not to implement 

such measures if it can demonstrate that collectively they will not be effective in reducing 

PM2.5 levels in the area. The EPA seeks comment on this proposed approach for 

Moderate PM2.5 nonattainment areas potentially subject to an attainment demonstration 

waiver under section 179B, and seeks comment on an alternative proposed approach that 

would not allow such an exception based on the collective effectiveness of otherwise 

“reasonable” measures. This alternative proposed option parallels a similar option 

described in Section IV.D in this preamble for Moderate PM2.5 nonattainment areas that 

cannot practicably attain the NAAQS by the latest statutory attainment date for the area.

The EPA also seeks comment on a distinct, second proposed approach for 

interpreting what would constitute an acceptable control strategy for sources in an area 
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for which a state is seeking an attainment plan approval under section 179B. Under this 

second option, a state would need to demonstrate that its selected control measures for a 

Moderate nonattainment area would achieve reductions in PM2.5 levels that exceeded the 

applicable NAAQS in proportion to their contribution to overall PM2.5 levels. For 

example, if monitors in a Moderate nonattainment area reveal that the area is exceeding 

the 2012 PM2.5 NAAQS of 12 μg/m3by 2 μg/m3, for a total of 14 μg/m3, and the state 

concludes that foreign sources are contributing 3 μg/m3, then the state would be 

responsible for the remaining 11 μg/m3and would need to implement enough reasonable 

control measures to achieve reductions in monitored ambient PM2.5 concentrations equal 

to ((14-3)/14)*2 or 1.6 μg/m3. The EPA recognizes that this approach could require a 

high level of precision to be able to quantify accurately contributions from sources inside 

and outside the nonattainment area as well as projected emission reductions to be 

achieved with the implementation of each potential control measure for sources inside the 

area. However, the agency believes that such precision may be justified to support any 

“but for” demonstration submitted to the EPA and to support any claims that a state 

should only be required to implement a subset of otherwise “reasonable” control 

measures on sources of direct PM2.5 emissions or emissions of PM2.5 precursors located 

in the nonattainment area.

The EPA seeks comment on these two approaches to clarify what constitutes a 

reasonable control strategy in the context of a SIP submitted pursuant to section 179B.

The EPA is also proposing regulations for the PM2.5 NAAQS consistent with the existing 

guidance with respect to requirements for RFP and quantitative milestones and 
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contingency measures for areas seeking Moderate area attainment plan approval under 

section 179B. The General Preamble states that:

In international border areas, EPA will not require the contingency measures for 

PM10 to be implemented after the area fails to attain if EPA determines that the 

area would have attained the NAAQS, but for emissions emanating from outside 

the U.S. However, the EPA will require contingency measures to be implemented 

if it determines that the area failed to make RFP in achieving the required 

reductions in PM10 emissions from sources within the U.S., or if the area does not, 

in fact, obtain the emission reductions that were necessary to demonstrate timely 

attainment of the NAAQS, but for emissions emanating from outside the U.S.287

The EPA is proposing that this interpretation of section 179B(a)(1) with respect to 

contingency measures and RFP requirements should apply to Moderate nonattainment 

areas for the PM2.5 NAAQS. Specifically, the EPA proposes that as part of any Moderate 

area attainment plan submitted under section 179B, a state must include an RFP plan 

developed consistent with proposed option #2 for RFP analyses for Moderate 

nonattainment areas that cannot practicably attain the relevant NAAQS by the statutory 

attainment date, described in Section IV.F of this preamble. Furthermore, the state must 

include as part of any attainment plan submission made for such an area contingency 

measures that can be implemented without significant effort in the event the EPA finds 

that such area failed to meet RFP requirements. The contingency measures should 

achieve approximately 1 year’s worth of emissions reductions as calculated by the state 

for purposes of the RFP analysis. In addition, the EPA proposes that the state must 

identify quantitative milestones for the area to be achieved 4.5 years and 7.5 years from 

the date of designation of the area. The EPA proposes to apply the same proposed 

 

287 Ibid.
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requirements for establishing and reporting on quantitative milestones on Moderate 

nonattainment areas seeking attainment date waivers under section 179B as on all other 

Moderate nonattainment areas, described fully in Section IV.G of this preamble.

The agency seeks comment on these proposed requirements for Moderate area 

plans submitted pursuant to section 179B.

The EPA has historically evaluated section 179B “but for” demonstrations on a 

case-by-case basis, based on the individual circumstances and data provided by the 

submitting state. These demonstrations have included information such as ambient air 

quality monitoring data, modeling scenarios, emissions inventory data and 

meteorological or satellite data.288 The Moderate area attainment demonstration modeling 

and other elements of the attainment demonstration must show timely attainment of the 

NAAQS but for the emissions from outside of the U.S. Section 179B does not, however, 

provide authority to exclude monitoring data influenced by international transport from 

regulatory determinations related to attainment and nonattainment. Thus, even if the EPA 

approves a section 179B “but for” demonstration for an area, the area would continue to 

be designated as nonattainment and subject to the applicable requirements, including 

nonattainment new source review, conformity and other measures prescribed for 

nonattainment areas by the CAA. Section 179B requires states to continue to meet 

attainment plan requirements, notwithstanding the contribution of emissions from sources 

 

288 Ibid. The Addendum includes further examples of information a state may present for 

the EPA to consider as part of the “but for” demonstration, including additional monitors 

in international border areas, more detailed emissions inventories, and speciation data 

that identifies PM2.5 components from foreign sources. 
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outside the U.S. in order to provide the public health protection intended by the NAAQS. 

However, if the EPA approves a “but for” demonstration for a Moderate nonattainment 

area, the area would not be subject to reclassification for failure to attain by the 

applicable attainment date as explained earlier.

Although monitor data cannot be excluded for a determination of whether an area 

has attained based solely on the fact the data are affected by emissions from outside the 

U.S., such data may be excluded from consideration if they were significantly influenced 

by exceptional events under section 319(b)(3) of the CAA. Where international transport 

of emissions contributes to an exceedance or violation and comes from natural sources 

such as wildfires, and otherwise meets the criteria contained in the EPA’s Exceptional 

Events Rule, it can be addressed by that rule.289 Specifically, ifWhereo interstate 

transported emissions contribute to an exceedance or violation and come from natural 

sources such as wildfires, air agencies can use the provisions in the EPA’s Exceptional 

Events Rule (40 CFR 50.14) to request exclusion of affected data. Once the EPA concurs 

with an air agency’s request to exclude affected data, the event-influenced data are 

officially noted and removed from the data set used to calculate official design values.

Because of previously expressed stakeholder feedback regarding implementation of the 

Exceptional Events Rule and specific stakeholder concerns regarding the analyses that 

can be used to support wildfire-related exceptional event demonstrations, the EPA 

intends to propose revisions to the Exceptional Events Rule in a future notice and 

 

289 See 40 CFR 50.14.

Commented [A8]: Adding text from interagency reviewer, with 

editing to make it read good.
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comment rulemaking effort and will solicit public comment at that time. Additionally, 

tThe EPA has approved PM2.5 wildfire influenced exceptional events demonstrations in 

the past, which are posted on the agency’s Exceptional Events Rule webpagesite.290

(http://www.epa.gov/ttn/analysis/exevents.htm ). Depending on the nature and 

scope of international emissions events affecting air quality in the U.S., the EPA may be 

able to assist states in developing approvable exceptional events demonstrations. More 

generally, 

Tthe EPA believes that the best approach for evaluating the potential 

impacts of international transport on nonattainment is for states to work with the EPA on 

a case-by-case basis to determine the most appropriate information and analytical 

methods for each area’s unique situation. The EPA will work with states that are 

developing attainment plans for which section 179B is relevant, and ensure the states 

have the benefit of the EPA's understanding of international transport of PM2.5 and PM2.5

precursors.

E. Enforcement and Compliance

Section 172(c)(6) in subpart 1 of the CAA requires nonattainment SIPs to 

“include enforceable emission limitations, and such other control measures, means or 

techniques... as well as schedules and timetables for compliance, as may be necessary or 

appropriate to provide for attainment.” In the remanded 2007 PM2.5 Implementation Rule, 

the EPA described the general elements that characterize an enforceable SIP regulation, 

 

290 The EPA’s Exceptional Events Rule website is located at 

http://www.epa.gov/ttn/analysis/exevents.htm.
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recognizing that enforceable SIP regulations may address the elements in different ways 

depending on the type of source category being regulated. The agency continues to 

believe and hereby proposes that in general, in order for a SIP regulation to be 

enforceable, it must clearly spell out which sources or source types are subject to its 

requirements and what its requirements (e.g., emission limits or work practices) are. An 

enforceable regulation would also specify the timeframes within which these 

requirements must be met, and definitively state the recordkeeping and monitoring 

requirements appropriate to the type of sources being regulated. The recordkeeping and 

monitoring requirements would have to be sufficient to enable the state or the EPA to 

determine whether the source is complying with the emission limit on a continuous basis. 

An enforceable regulation would also contain test procedures in order to determine 

whether sources are in compliance.

The EPA continues to believe that complete and effective regulations that ensure 

compliance with an applicable emissions limit would have to include requirements for 

both performance testing of emissions and ongoing monitoring of the compliance 

performance of control measures, and the agency proposes to require that SIP regulations 

that establish emission limits include the following:

(a) Indicator(s) of compliance - the pollutant or pollutants of interest (e.g., filterable 

and condensable PM2.5) and the applicable units of measurement for expressing 

compliance (e.g., ng/J of heat input, lb/hr); 

(b) Test method - reference to a specific EPA or other published set of sample 

collection and analytical procedures, equipment design and performance criteria, 

and the calculations providing data in units of the indicator of compliance 
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(Section IX.K of this preamble presents a discussion of specific test methods for 

condensable PM2.5 emissions); 

(c) Averaging time - the minimum length of each required test run and the 

requirement to average the results of the test runs (e.g., three runs) representing a 

specified period of time (e.g., 8 hours); and, 

(d) Frequency - the maximum time between emissions or performance tests (e.g., 

within 30 days of facility start-up and once each successive quarter, every 6-

month period, or yearly). 

In order to be complete with regard to compliance monitoring provisions, the 

EPA proposes that regulations adopted into the SIP must include the following critical 

elements:

(a) Indicator(s) of performance - the parameter or parameters measured or observed 

for demonstrating proper operation of the pollution control measure or 

compliance with the applicable emissions limitation or standard. Indicators of 

performance could include direct or predicted emissions measurements, process 

or control device (and capture system) operational parametric values that 

correspond to compliance with efficiency or emissions limits, and recorded 

findings of verification of work practice activities, raw material or fuels pollutant 

content, or design characteristics. Indicators could be expressed as a single 

maximum or minimum value, a function of process variables (e.g., within a range 

of pressure drops), a particular operational or work practice status (e.g., a damper 

position, completion of a waste recovery task), raw material or fuel pollutant 

content, or an interdependency between two or more variables;
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(b) Measurement technique - the means used to gather and record information of or 

about the indicators of performance. The components of the measurement 

technique include the detector type or analytical method, location and installation 

specifications, inspection procedures, and quality assurance and quality control 

measures. Examples of measurement approaches include continuous emissions 

monitoring systems (CEMS), continuous opacity monitoring systems (COMS), 

continuous parametric monitoring systems (CPMS), performance testing, vendor 

or laboratory analytical data, and manual inspections and data collection that 

include making records of process conditions, raw materials or fuel specifications, 

or work practices. Directly enforceable emission measurements, such as PM 

CEMs, are preferred wherever feasible. Where COMS are feasible, it should be 

clear that opacity is a directly enforceable standard, not merely an indicator of 

compliance;

(c) Averaging time - the period over which to average data to verify compliance with 

the emissions limitation or standard or proper operation of the pollution control 

measure. Examples of averaging time include a 3-hour average in units of the 

emissions limitation, a 30-day rolling average emissions value, a daily average of 

a control device operational parametric range, periodic (e.g., monthly, annual) 

average of raw materials or fuel pollutant content, and an instantaneous alarm;

(d) Monitoring frequency - the number of monitoring data values recorded over a 

specified time interval. Examples of monitoring frequencies include at least one 

data value every 15 minutes for CEMS or CPMS, at least every 10 seconds for 

COMS, upon receipt or application of raw materials or fuel to the process, or at 
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least once per operating day (or week, month, etc.) for performance testing, work 

practice verification, or equipment design inspections; and,

(e) Reporting and record retention requirements – criteria for retaining monitoring 

and test data in an electronic form and periodic electronic reporting of information 

as needed to the compliance office. Electronic record retention and submission 

have been widely adopted, and the EPA believes that such readily accessible 

documentation could be used by state, federal and other analysts to spot trends 

and non-compliance more easily than if these entities conducted reviews of paper 

documents. The EPA also recommends that compliance reports be made available 

online so that the general public can readily access the information without the 

need to submit Freedom of Information Act (FOIA) requests to the EPA. The 

EPA is in the process of revising federal rules to make similar requirements apply.

The EPA continues to believe that approval of regulations adopted into SIPs 

would have to ensure that these critical elements are present and clearly defined to be 

approvable. In particular, the compliance obligations, including emissions limits and 

other applicable requirements, would need to be representative of and accountable to the 

assumptions used in a state’s attainment demonstration. This accountability would

include the ability to transfer the applicable regulatory requirements to a title V operating 

permit subject to the EPA and public review.291

 

291 Under the Title V regulations, sources have an obligation to include in their Title V 

permit applications, among other components, all emissions of pollutants for which the 

source is major, and all emissions of regulated air pollutants. See, e.g., 40 CFR 

70.5(c)(3). The definition of regulated air pollutant in 40 CFR 70.2 includes any pollutant 

for which a NAAQS has been promulgated, including PM2.5.
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The EPA seeks comment on the elements proposed to be required to ensure that 

regulations adopted into a SIP are enforceable.

F. Efforts to Encourage a Multi-Pollutant Approach When Developing PM2.5 Attainment 

Plans 

1. General Guidance

From a planning and resource perspective, the EPA believes that it can be 

efficient for states to develop integrated control strategies that address multiple pollutants 

rather than separate strategies for each pollutant or NAAQS individually. An integrated

air quality control strategy that reduces multiple pollutants can help ensure that 

reductions are efficiently achieved and produce the greatest overall air quality benefits. 

For example, it is widely known that certain control measures that reduce emissions of 

NOx and VOC, and thus reduce ambient PM2.5 levels, can also result in reduced ambient 

concentrations of ground-level ozone.292 Many VOC are also hazardous air pollutants 

(HAP), so a control strategy for a PM2.5 nonattainment area that reduces VOC emissions 

may provide the additional benefit of reducing air toxics. It is also widely known that 

many sources of PM2.5 also emit toxic metals as particulates, so controlling directly 

emitted PM2.5 emissions from these sources would also reduce the emissions of toxic 

metals. In addition, due to expected changes in meteorology resulting from climate 

change, the EPA encourages states to assess climate change and air pollution together and 

 

292 For a list of potential control measures for PM2.5 precursors, see 

http://www.epa.gov/airquality/particlepollution/measures/pm_control_measures_tables_

ver1.pdf. 
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account for the potential effects of climate change in their multi-pollutant planning 

efforts.

In June 2007, the EPA’s CAA Advisory Committee (CAAAC) recommended that 

the agency allow states to integrate SIP requirements and other air quality goals into a 

comprehensive plan.293 The recommended plan would demonstrate 

attainment/maintenance of multiple NAAQS, accomplish sector-based reductions, realize 

risk reductions of HAPs and make improvements in visibility. It could also be structured 

to integrate programs addressing land use, transportation, energy and climate. 

The EPA has encouraged states to take a multi-pollutant approach to managing air 

quality.294 Specifically, the agency has encouraged states to involve all stakeholders when 

planning to meet air quality standards and to provide a basic outline for how local 

jurisdiction(s) could address air pollutants in an integrated manner.

While the agency encourages states to develop multi-pollutant plans, it recognizes

that the requirement for the agency to review and, as necessary, revise NAAQS every 5 

years, which can trigger new statutory attainment plan submission and attainment dates, 

as well as the ever-evolving understanding of pollutants and many control programs that 

may be available to reduce emissions, can sometimes make such efforts challenging. For 

example, under the current law, the 2007 submission date for Regional Haze SIPs has 

already passed while the proposed submittal date for attainment demonstrations for the 

 

293 Recommendations to the Clean Air Act Advisory Committee: Phase II, June 2007, 

http://epa.gov/air/caaac/aqm/phase2finalrept2007.pdf.

294 Memorandum from Stephen D. Page to Regional Air Division Directors, 

“Consideration of Multiple Pollutants in Control Strategy Development.” August 10, 

2005. http://epa.gov/air/caaac/aqm/aqm-page-memo.pdf.
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2008 ozone NAAQS is more than 2 years before the anticipated due date for Moderate 

area attainment plans for areas ultimately designated nonattainment for the 2012 annual 

PM2.5 NAAQS. Although it is thus not feasible to integrate fully these planning 

requirements, states could potentially use common databases and modeling tools for all 

three SIP submissions for these different requirements and rely on similar control 

measures as appropriate. Furthermore, as states develop plans to meet any current or 

future PM2.5 NAAQS, they may wish to modify existing plans for implementing the 

ozone NAAQS or other NAAQS, or for regional haze, as they consider strategies more 

comprehensively. However, it is important to note that states and the EPA must continue 

to meet all the CAA mandated planning and program elements for the individual

NAAQS. The EPA seeks comment on alternative approaches to integrate the planning 

requirements for multiple NAAQS and other CAA programs that are promulgated at 

different times. 

2. What is the EPA doing beyond encouraging states to integrate their air quality 

planning activities to the extent feasible? 

Ideally, an air quality management plan (AQMP) is a set of pollution reduction 

strategies/planning activities for an area demonstrating: attainment/maintenance of one or 

more NAAQS; risk reductions from HAPs; improvements in visibility and ecosystem 

health; and integration of land use, transportation, energy and climate activities in the 

area. Three areas in the country – North Carolina, New York and the city of St. Louis 

(involving both Missouri and Illinois) – participated in an EPA-led pilot effort to develop 

multi-pollutant AQMPs. The pilot projects provided lessons regarding AQMP 

development that should prove useful to other areas interested in better integrating their 
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air quality planning. The areas’ initial AQMPs and other materials are available on the 

EPA’s website.295

Implementation of the 2012 PM2.5 NAAQS provides an opportunity for states to 

consider how to use a multi-pollutant approach from the beginning of their planning 

process. The EPA recommends that states and tribes wishing to take a comprehensive 

approach consider the following activities:

 Develop models for the attainment demonstration that include previously 

implemented or planned measures to reduce PM2.5 precursors and secondary fine 

particles, ozone precursors, pollutants that contribute to regional haze and, where 

appropriate, air toxics and any potential negative impacts on ecosystems. 

 Conduct an integrated assessment of the impact controls have on ambient levels of 

PM2.5, ozone, regional haze, and, where applicable, air toxics, greenhouse gases, 

ecosystem protection and environmental justice to identify those controls with the 

greatest potential co-benefits. 

 Use common data bases and analytical tools, where possible. 

The EPA is requesting comment on what incentives or assistance the agency might be 

able to provide to encourage states to integrate their planning activities.

3. Multi-pollutant assessments/one-atmosphere modeling 

A multi-pollutant assessment, or one-atmosphere modeling, is conducted with a 

single air quality model that is capable of simulating transport and formation of multiple 

 

295 http://www.epa.gov/air/aqmp/.
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pollutants simultaneously.296 For example, this type of model can simulate formation and 

deposition involving pollutants associated with PM2.5, ozone and regional haze, and it can 

include algorithms simulating gas phase chemistry, aqueous phase chemistry, aerosol 

formation and acid deposition. This type of model could also include the formation and 

deposition of key air toxics and the chemical interactions that occur with these individual 

toxic species to produce PM2.5 and ozone.

Multi-pollutant assessments are recommended for PM2.5 attainment 

demonstrations because the formation and transport of VOC and NOx are closely related 

to the formation of both ozone and regional haze. There is often a positive correlation 

between measured secondary particulate matter and ozone. Many of the same factors 

affecting PM2.5 concentrations also affect ozone concentrations because similarities exist 

in sources of precursors for both pollutants. For example, emissions of NOx may lead to 

formation of nitrates, which affect both ambient PM2.5 and ozone levels and impair 

visibility. Many VOC (such as toluene) are air toxics and may also be sources or 

precursors for both organic particles and ozone. In addition, the presence of ozone itself 

may be an important factor affecting secondary particle formation.

Because of these relationships, models and data analysis intended to address 

PM2.5 could be beneficial for use in addressing ozone and visibility impairment. When

performing a multi-pollutant assessment, the modeling should take into account 

 

296 Depending on the context, “multi-pollutant” can be defined in different ways. In this 

context the agency is defining multi-pollutant modeling as simultaneous modeling of 

PM2.5, ozone, key air toxics, and regional haze. Future multi-pollutant models may 

include the ability to model a broader array of air toxics as well as greenhouse gases.
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previously implemented or planned measures to reduce PM2.5, ozone, and regional haze. 

States that undertake multi-pollutant assessments as part of their attainment 

demonstration should consider assessing the impact of their PM2.5 strategies on ozone and 

visibility impairment to ensure that optimal emission reduction strategies are developed 

for the three programs to the extent possible. This could facilitate addressing all of these 

pollutants in a more cost effective manner.

States may also find it desirable to assess the impact of PM2.5, ozone, and/or 

regional haze control strategies on toxic air pollutants regulated under the CAA or under 

state air toxic initiatives. Given the relationships that exist between air toxics and the 

formation of PM2.5 and ozone, states may find that controls can be selected to meet goals 

for PM2.5 and/or ozone attainment as well as those of specific air toxic programs.

G. Measures to Ensure Appropriate Protections for Overburdened Populations

1. Review of PM NAAQS and at-risk populations

As discussed in Section II of this preamble, when the EPA sets a primary 

NAAQS, the CAA directs the Administrator to establish a standard that is “requisite” to 
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protect public health with “an adequate margin of safety.”297 In setting the NAAQS, the 

EPA considers available, relevant scientific information on the health effects that may 

occur in the general population, as well as specific groups within the general population 

that are at increased risk for experiencing adverse pollutant-related health effects (i.e., atrisk populations).298 These groups could exhibit a greater risk of pollutant-related health 

effects than the general population for a number of reasons including being affected by 

lower pollutant concentrations, experiencing a larger health impact at a given pollutant 

concentration, and/or being exposure to higher pollutant concentrations than the general 

population. Thus, the NAAQS review process inherently takes into consideration certain 

environmental justice factors as part of the standard-setting process. In setting a 

secondary standard, the CAA directs the Administrator to establish a standard that “is 

requisite to protect the public welfare from any known or anticipated adverse effects.”

 

297 The requirement that primary standards provide an adequate margin of safety was 

intended to address uncertainties associated with inconclusive scientific and technical 

information available at the time of standard setting. It was also intended to provide a 

reasonable degree of protection against hazards that research has not yet identified. Both 

kinds of uncertainties are components of the risk associated with pollution at 

concentrations below those at which human health effects can be said to occur with 

reasonable scientific certainty. Thus, in selecting primary standards that provide an 

adequate margin of safety, the EPA Administrator is seeking not only to prevent pollution 

levels that have been demonstrated to be harmful but also to prevent lower pollutant 

levels that may pose an unacceptable risk of harm, even if the risk is not precisely 

identified as to nature or degree. The CAA does not require the Administrator to establish 

a primary NAAQS at a zero-risk level or at background concentration levels, but rather at 

a level that reduces risk sufficiently so as to protect public health with an adequate 

margin of safety. 

298 The legislative history of section 109 of the CAA indicates that a primary standard is 

to be set at the “maximum permissible ambient air level... which will protect the health 

of any [sensitive] group of the population” and that for this purpose “reference should be 

made to a representative sample of persons comprising the sensitive group rather than to 

a single person in such a group.”
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Section 109(d) of the CAA requires the EPA to periodically review (every 5 years) the 

science upon which the standards are based and the standards themselves. The agency 

recently completed a review of the NAAQS for particulate matter (PM) and revised the 

primary standards for fine particles (generally referring to particles less than or equal to 

2.5 micrometers (μm) in diameter, PM2.5) (78 FR 3086, January 15, 2013). Specifically, 

the EPA revised the primary annual PM2.5 standard by lowering the level to 12.0 

micrograms per cubic meter (μg/m3) so as to provide increased protection against health 

effects associated with long- and short-term PM2.5 exposures. The agency also revised the 

form of the primary annual PM2.5 standard to eliminate the spatial averaging provisions to 

avoid potential disproportionate impacts on at-risk populations. In conjunction with these 

revisions, the EPA retained the primary 24-hour PM2.5 standard, as revised in 2006 (71 

FR 61144, October 17, 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 annual 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 status299, against a broad range of PM2.5-related effects that include 

 

299 In the final NAAQS rule, based on information presented in the Integrated Science 

Assessment for Particulate Matter (U.S. EPA, 2009, sections 2.2.1 and 8.1.7), the EPA 

made a finding that persons with lower socioeconomic status are at increased risk for 

experiencing adverse health effects related to PM exposures (78 FR 3104). Persons with 

lower socioeconomic status (SES) have been generally found to have a higher prevalence 

of pre-existing diseases, limited access to medical treatment, and increased nutritional 

deficiencies, which can increase this population’s risk to PM-related effects (77 FR 

38911, June 29, 2012).
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premature mortality, increased hospital admissions and emergency department visits, and 

development of chronic respiratory disease.

In addition, the Policy Assessment (U.S. EPA, 2011a, p. 2-60) observed that the 

highest concentrations of PM2.5 in an area tend to be measured at monitors located in 

areas where the surrounding populations are more likely to live below the poverty line 

and to have higher percentages of minorities. The EPA directed states to relocate a 

limited number of existing monitors to near-roadway sites in large urban areas. Both of 

these revisions were informed by scientific evidence that underscored the potentially 

disproportionate exposure to high PM2.5 concentrations and therefore disproportionate 

risk to low-income and minority populations.

2. Relationship between direct PM2.5 emissions and PM2.5 precursor emissions

reductions and at-risk populations

Sources of direct PM emissions have their greatest impact on PM2.5

concentrations and public health in the general vicinity of the source (e.g., within 10 

miles), while sources of precursor emissions can contribute to PM2.5 concentrations more 

than 100 miles away and are considered to have a more regional impact. To date, state 

PM2.5 attainment plans have generally relied to a greater extent on reductions of precursor 

pollutants rather than on reductions of direct PM2.5 emissions. Studies show, however, 

that on a per ton basis, the reduction of a ton of direct PM2.5 emissions leads to greater 

health benefits than the reduction of a ton of SO2 or NOx.

300

 

300 See Fann, N., Fulcher, C., and B. Hubbell, 2009. The Influence of location, source, 

and emission type in estimates of the human health benefits of reducing a ton of air 
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The process for developing attainment plans for the current and future PM2.5

NAAQS presents a potential opportunity to target the health protections afforded by the 

NAAQS, as the EPA expects that attainment for the 2012 PM2.5 NAAQS and future PM2.5

NAAQS in nonattainment areas with the most severe pollution problems may need to 

give greater emphasis to reducing direct PM2.5 emissions in combination with efforts 

already underway to further reduce precursor emissions. Placing greater emphasis on 

reducing emissions from sources of direct PM2.5 (e.g., certain industrial facilities located 

in more densely populated areas; areas with high motor vehicle and other diesel engine 

emissions, such as rail yards and near major roadways; and areas with high wood smoke 

emissions) could provide the added benefit of reducing exposure to PM2.5 in low-income 

and minority communities.

With this in mind, the EPA is seeking comment on additional ways that air 

agencies can provide public health protection specifically for overburdened populations 

when preparing attainment plans for the PM2.5 NAAQS. The discussion that follows 

provides some examples of points in the attainment plan development process at which a 

state could assess opportunities for providing such additional protections, and examples 

of what those additional protections might look like.

3. Options for states to consider to ensure appropriate protections from PM2.5

exposure for overburdened populations

 

pollution. Air Quality, Atmosphere & Health. Volume 2, Number 3, 169-176, June 2009. 

See also Fann et. al., 2011. Maximizing health benefits and minimizing inequality: 

incorporating local-scale data in the design and evaluation of air quality policies. Society 

for Risk Analysis, vol. 31, no. 6, p. 908–922, June 2011.
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The EPA believes that states have sufficient flexibility and discretion under the 

CAA in implementing their attainment strategies to focus resources on controlling those 

sources of emissions that directly and adversely affect low-income and other at risk 

populations. By reducing impacts on at-risk populations, states can maximize health 

benefits, thereby creating greater net benefits for the state in a cost-effective manner.301

In addition, reducing adverse impacts to low-income and minority populations advances 

the environmental justice goal of fair treatment for these populations.

There are a number of actions that states could take to focus resources in this way. 

Some of these actions can help identify areas where additional ambient monitoring may 

be needed in low income and overburdened communities. Such information can be used 

to support updates to the state’s annual monitoring plan. Examples of actions to support 

updates to the annual monitoring plan include: 

 Develop databases and online mapping tools that enable users (including state staff, 

public, and the regulated community) to understand where sources of direct PM2.5

emissions are located and where new or modified sources of emissions could have 

potential impacts on low income and other overburdened communities. 

 Incorporate existing mapping tools which identify target areas in the attainment plan 

development process and related actions.

 

301 Wesson, K., Fann, N., Morris, M, Fox, T., Hubbell, T., 2010. A multipollutant, riskbased approach to air quality management. Case study for Detroit. Atmospheric Pollution 

Research, 1, 296-304. The study compared air quality control strategies and concluded 

that the multi-pollutant, risk-based approach was able to produce approximately two 

times greater monetized benefits through avoided health impacts and was more cost 

effective than a pollutant-by-pollutant approach.
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 Analyze emissions data, ambient data, and available modeling to identify potential 

unmonitored PM2.5 hotspots in areas with a high percentage of low income, minority, 

or indigenous persons (see Section IV.E in this preamble for further discussion of this 

option).

Once target areas for addressing these sensitive population needs within a 

nonattainment area have been identified, the state could consider taking any of the 

following actions which help target emissions reductions that may be needed to attain the 

PM2.5 NAAQS:

 Prioritize the selection of control measures that target reductions of direct PM2.5, 

particularly from sources located in “at-risk” areas as part of the state’s RACM

and RACT analysis (for Moderate nonattainment areas) or BACM and BACT 

analysis (for Serious nonattainment areas), as well as other measures needed to 

demonstrate attainment (see Sections IV.D and VI.D, respectively, in this 

preamble for further discussion of this option).

 Improve the understanding of the potential impact of minor sources by improving 

or generating an emissions inventory for such minor sources, including sources 

that are not currently required to report emissions, to generate options on how 

emissions can be reduced in the target area.

 Design voluntary programs to reduce VMT and mobile source-related PM2.5 

emissions (e.g., diesel retrofits).

 Incorporate environmental justice criteria into the alternatives analysis to ensure 

appropriate siting and require cumulative impact studies for proposed projects.

 Eliminate exemptions from and/or raise thresholds for minor source permitting.
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 Develop a list of potential supplemental environmental projects (SEPs)302 that 

could be applied in the target area.

 Prioritize targeted enforcement strategies.

In addition to the above, states could increase opportunities for meaningful 

involvement of community groups in attainment plan development, annual monitoring 

network plan reviews, and permitting processes303 for at-risk and minority populations by 

taking the following steps:

 Develop advisory boards and/or develop enhanced notice and comment 

requirements for low income and minority communities to assure meaningful 

involvement relative to projects that impact their communities;

 Provide special notice of important actions affecting target areas in appropriate 

languages and with attention to cultural barriers;

 Provide advance notification for low income and minority communities of 

upcoming opportunities for public comment on SIPs, ambient air monitoring 

plans, and other relevant actions;

 Maintain multi-lingual websites and offer translators for public meetings and 

hearings.

 Coordinate with the state’s EJ coordinator to assist with outreach efforts.

 

302 For more information on SEPs, go to www2.epa.gov/enforcement/supplementalenvironmental-projects-sep.

303 See May 2013 FR document on Actions that EPA Regional Offices Are Taking to 

Promote Meaningful Engagement in the Permitting Process by Overburdened

Communities and Promising Practices for Permit Applicants Seeking EPA-Issued

Permits: Ways to Engage Neighboring Communities.
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 Provide states with appropriate federal EJ guidance tools.

The EPA is seeking comment on these examples and whether and how the EPA 

might provide recommendations to states preparing SIPs for the 2012 and any future 

PM2.5 NAAQS on additional ways to provide equal protections for overburdened 

populations.

H. Tribal Issues

The 1998 Tribal Air Rule (TAR) (40 CFR part 49), which implements section 

301(d) of the CAA, gives tribes the option of developing TIPs. Specifically, the TAR 

provides for the tribes to be treated in the same manner as a state in implementing certain 

sections of the CAA. However, tribes are not required to develop implementation plans. 

The EPA determined in the TAR that it was inappropriate to treat tribes in a manner 

similar to a state with regard to specific plan submittal and implementation deadlines for 

NAAQS-related requirements, including, but not limited to, such deadlines in CAA 

sections 110(a)(1), 172(a)(2), 182 187, and 191. See 40 CFR 49.4(a). In addition, the 

EPA determined it was not appropriate to treat tribes similarly to states with respect to 

provisions of the CAA requiring as a condition of program approval the demonstration of 

criminal enforcement authority or providing for the delegation of such criminal 

enforcement authority. See 40 CFR 49.4(g). To the extent a tribe is precluded from 

asserting criminal enforcement authority, the federal government will exercise primary 

criminal enforcement responsibility. See 40 CFR 49.8. In such circumstances, tribes 

seeking approval for CAA programs provide potential investigative leads to an 

appropriate federal enforcement agency.
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If a tribe elects to do a TIP, the agency will work with the tribe to develop an 

appropriate schedule which meets the needs of the tribe, and which does not interfere 

with the attainment of the NAAQS in other jurisdictions. The tribe developing a TIP can 

work with the EPA Regional Office on the appropriateness of addressing RFP and other 

substantive SIP requirements that may or may not be appropriate for the tribe’s situation.

The CAA and the TAR provide tribes opportunities and flexibility for the tribe in 

the preparation of a TIP to address the NAAQS. If a tribe elects to develop a TIP, the 

TAR offers flexibility for the tribe to identify and implement on a case-by-case basis only 

those CAA programs or reasonably severable program elements needed to address their 

specific air quality problems. In the TAR, the EPA described this flexible implementation 

approach as a modular approach. Each tribe may evaluate the particular activities, 

including potential sources of air pollution within the exterior boundaries of its 

reservation (or within non-reservation areas for which it has demonstrated jurisdiction), 

which cause or contribute to its air pollution problem. A tribe may adopt measures for 

controlling those sources of PM2.5-related emissions, as long as the elements of the TIP 

are reasonably severable from the package of elements that can be included in a whole 

TIP. A TIP must include regulations designed to solve specific air quality problems for 

which the tribe is seeking the EPA’s approval, as well as a demonstration that the tribal 

air agency has the authority from the tribal government to develop and run their program, 

the capability to enforce their rules, and the resources to implement the program they 

adopt. In addition, the tribe must receive an eligibility determination from the EPA to be 

treated in the same manner as a state for the particular matter at issue and to receive 

authorization from the EPA to run a CAA program.
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The EPA would review and approve, where appropriate, these partial TIPs as one 

step of an overall air quality plan to attain the NAAQS. A tribe may step in later to add 

other elements to the plan, or the EPA may step in to fill gaps in the air quality plan as 

necessary or appropriate. In approving a TIP, the agency would evaluate whether the plan 

appropriately coordinates with the overall air quality plan for an area when tribal lands 

are part of a multi-jurisdictional area.

Because many PM2.5 nonattainment areas will include multiple jurisdictions, and 

in some cases both Indian country and state lands, it is particularly important for the 

tribes and the states to work together to coordinate their planning efforts. States need to 

incorporate Indian country emissions in their base emissions inventories if Indian country 

is part of an attainment or nonattainment area.304 Tribes and states should coordinate their 

planning activities as appropriate to ensure that neither is adversely affecting attainment 

of the NAAQS in the area as a whole. Coordinated planning in these areas will help 

ensure that the planning decisions made by the states and tribes complement each other 

and that the nonattainment area makes reasonable progress toward attainment and 

ultimately attains the applicable PM2.5 NAAQS. In reviewing and approving individual 

 

304 On January 17, 2014, the United States Court of Appeals for the District of Columbia 

Circuit issued a decision vacating the EPA’s 2011 rule entitled “Review of New Sources 

and Modifications in Indian Country” (76 FR 38748) with respect to non-reservation 

areas of Indian country (See, Oklahoma Department of Environmental Quality v. EPA, 

740 F.3d 185 (D.C. Cir. 2014)). Under the court’s reasoning, with respect to CAA state 

implementation plans, a state has primary regulatory jurisdiction in non-reservation areas 

of Indian country (i.e., Indian allotments located outside of reservations and dependent 

Indian communities) within its geographic boundaries unless the EPA or a tribe has 

demonstrated that a tribe has jurisdiction over a particular area of non-reservation Indian 

country within the state.
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TIPs and SIPs, the EPA will determine if together they are consistent with the overall air 

quality needs of an area.

To date, very few tribes have submitted for the EPA’s approval TIPs covering 

areas over which they have jurisdiction. In the absence of a TIP, the EPA is authorized 

under the TAR to implement CAA programs in such areas as necessary or appropriate.

For example, an unhealthy air quality situation on an Indian reservation may require the 

EPA to develop a FIP to reduce emissions from sources on the reservation. Likewise, if 

the agency determines that sources in an area under tribal jurisdiction could interfere with 

a larger nonattainment area meeting the NAAQS by its attainment date, it would develop 

a FIP for those sources in consultation with the tribe, as necessary or appropriate.

States have an obligation to notify other states in advance of any public hearing(s) 

on their state plans if such plans will significantly impact such other states. 40 CFR 

51.102(d)(5). Under section 301(d) of the CAA and the TAR, tribes may become eligible 

to be treated in a manner similar to states (TAS) for this purpose. Affected tribes with this 

status must also be informed of the contents of such state plans and given access to the 

documentation supporting these plans. In addition to this mandated process, the EPA

encourages states to extend the same notice to all affected tribes, regardless of their TAS 

status. 

Executive Orders and the EPA’s Indian policies generally call for the EPA to 

coordinate and consult with tribes on matters that affect tribes. Executive Order 13175, 

titled, “Consultation and Coordination with Indian Tribal Governments” requires the 

EPA to develop a process to ensure “meaningful and timely input by tribal officials in the 

development of regulatory policies that have Tribal implications.” In addition, the EPA’s 
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policies include the agency’s 1984 Indian Policy relating to Indian tribes and 

implementation of federal environmental programs, the April 10, 2009, Office of Air 

Quality Planning and Standards guidance “Consulting with Indian Tribal Governments,”

and the “EPA Policy on Consultation and Coordination With Indian Tribes.”

Consistent with these policies, the EPA intends to meet with tribes on activities 

potentially affecting the attainment and maintenance of the current and future PM2.5

NAAQS in Indian country, including agency actions on SIPs. As such, it would be 

helpful for states to work with tribes with land that is part of the same air quality area 

during the SIP development process and to coordinate with tribes as they develop their 

SIPs.

I. Voluntary Programs for Reducing Ambient PM2.5

1. PM Advance program

The EPA believes there are significant advantages for states, tribes and local 

agencies to take steps to reduce direct PM2.5 emissions and emissions of PM2.5 precursors 

as early as possible. First and foremost, early reductions help to achieve cleaner air 

sooner, and help to ensure continued health protection. Secondly, early steps could help 

an area avoid a nonattainment designation in the first place, or for an area eventually 

designated as nonattainment, early reductions could help bring the area back into 

attainment sooner, which may lead to qualifying for a CDD and subsequent suspension of 

attainment planning requirements as described in Section IX.C of this preamble. In 

addition, early action to improve air quality can help an eventual nonattainment area, 

particularly an area that has never been designated nonattainment before, to establish 

working relationships between key stakeholders. The EPA’s expectation is that early 
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actions to reduce emissions in such areas would be less resource-intensive than actions 

taken once a nonattainment designation has been made, since at that point the 

implementation of controls would need to occur in conjunction with actions to comply 

with other requirements such as nonattainment NSR and transportation conformity.

In January 2013, the EPA began a new early emission reduction program for 

attainment areas called “PM Advance,” which is much like the related “Ozone Advance” 

program that began in April 2012. Additional information about the PM Advance 

program for the annual and 24-hour PM2.5 NAAQS is provided in a separate guidance 

document that is available at http://www.epa.gov/ozonepmadvance.

2. Residential wood smoke programs

The EPA recognizes that residential wood smoke is a concern for many 

nonattainment areas. The EPA estimates that wood stoves, hydronic heaters and 

fireplaces emit more than 345,000 tons of PM2.5 into the air throughout the country each 

year – mostly during the winter months. Residential wood smoke can increase fine 

particle pollution to levels that cause significant health concerns (e.g., asthma attacks, 

heart attacks, premature death). Wood smoke causes many counties throughout the U.S. 

to either exceed the national health-based standards for fine particles, or places them on 

the cusp of exceeding the standards. Because wood stoves, hydronic heaters and other 

similar appliances can be used around the clock in residential areas, they can cause 

significant and varying health and quality of life issues. 

To reduce fine particle pollution, many PM2.5 nonattainment areas will need to 

address residential wood smoke. The EPA has developed the Strategies for Reducing 

Residential Wood Smoke document that provides education and outreach tools, 
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information on regulatory approaches to reduce wood smoke, as well as information 

about voluntary programs that communities around the country have used.305 In addition, 

it includes methods for calculating emission reductions, funding ideas and the basic 

components of a wood smoke reduction plan that can be adopted into a SIP as an 

enforceable control measure.306 To access the document, go to 

http://epa.gov/burnwise/pdfs/strategies.pdf. For more information on the EPA’s wood 

smoke reduction program, visit http://www.epa.gov/burnwise.

J. Improved Stationary Source Emissions Monitoring

1. Background

For purposes of demonstrating compliance with the EPA's air quality regulatory 

requirements, the EPA, air agencies, and sources rely on two basic types of monitoring: 

ambient air quality monitoring and stationary source emissions monitoring. Ambient air 

quality monitoring, as discussed in Section II of this preamble, entails collecting and 

measuring samples of criteria pollutants in ambient air to evaluate air quality as 

compared to clean air standards and historical information. Stationary source emissions 

monitoring, on the other hand, entails collecting and using measurement data (or other 

 

305 On January 3, 2014, the EPA proposed to revise its New Source Performance 

Standards (NSPS) for new residential wood heaters and to establish NSPS for other new 

wood heaters, including outdoor and indoor wood-fired boilers (also known as hydronic 

heaters). The proposed standards would reduce emissions of direct PM2.5 as well as 

carbon monoxide, VOC, air toxics (including formaldehyde, benzene and polycyclic 

organic matter), and black carbon. See 79 FR 6330-6416.

306 For further guidance on incorporating voluntary measures into a SIP, see

“Incorporating Emerging and Voluntary Measures in a State Implementation Plan (SIP).” 

U.S. EPA. Office of Air and Radiation. September 2004. Available at 

http://www.epa.gov/ttn/caaa/t1/memoranda/evm_ievm_g.pdf.
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information) from individual stationary sources to demonstrate compliance with 

emissions standards, to assess process or control device performance, or to verify work 

practices. While ambient air quality monitoring is used to assess compliance with the 

NAAQS, stationary source emissions monitoring is used to assess compliance with 

source-specific regulations under programs like the New Source Performance Standards 

(NSPS), the National Emissions Standards for Hazardous Air Pollutants (NESHAP), the 

compliance assurance monitoring (CAM) program, the title V air operating permits 

program, and the acid deposition control program, as well as specific SIP control 

measures.

307

Accurate stationary source emissions monitoring is critical for purposes of 

developing accurate emissions inventories and in order to identify appropriate control 

measures to reduce emissions from stationary sources. In addition, after control measures 

are in place, stationary source emissions monitoring provides process and control device 

performance information to the facility operator so that appropriate corrective action can 

be taken if emission levels exceed applicable thresholds. Thus, appropriate stationary 

source emissions monitoring requirements, like the control measures with which they are 

associated, are a fundamental element of an approvable attainment plan.

By way of example, in a limited study on improving stationary source emissions 

monitoring, the EPA found that revising the measurement technique at a stationary 

source could provide information to the facility operator to take corrective action that 

 

307 Regulations governing the implementation of these programs are located at 40 CFR 

parts 60, 61, 63, 64, 70, 71 and 75.
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could potentially reduce emissions up to15 percent, and that increasing monitoring 

frequency at the facility could provide information that could be used to inform 

corrective actions that could yield potential stationary source emission reductions of up to 

13 percent.308,309 Implementation of stationary source emissions monitoring 

improvements could thus lead to actions to achieve additional emissions reductions not 

only at individual sources but also in the nonattainment areas where these sources are 

located.

2. Guidance to help improve stationary source emissions monitoring

Because of the important role that effective stationary source emissions 

monitoring can play in informing the development of attainment strategies for PM2.5 

NAAQS nonattainment areas, the EPA is interested in applied best practices for 

stationary source emissions monitoring that could be included in guidance for other 

stationary sources and air agencies. The EPA seeks to gather information about ways to 

make the source emissions monitoring data collection process easier and more 

transparent. The EPA therefore seeks appropriate examples and supporting data from 

individual sources and air agencies with experience in this area to inform such future 

guidance. The EPA also seeks comment on the specific topics and questions that follow, 

which the agency may address in future guidance related to improved source monitoring. 

 

308 Impact of Improved Monitoring on PM2.5 Emissions, memorandum from L. Barr and 

K. Schaffner, RTI International, to B. Parker, U.S. Environmental Protection Agency. 

December 2003.

309 As discussed in Section IX.E, emissions monitoring has four essential components: (i) 

indicator(s) of performance; (ii) measurement technique(s); (iii) monitoring frequency; 

and, (iv) averaging time. 
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Specifically: 

(1) Based on your experience, in which cases do you believe improved 

monitoring techniques are more appropriate than visual emissions (VE) techniques for 

monitoring compliance with PM2.5 (or PM, in general) emissions limits? Please identify 

monitoring techniques that you would you recommend in lieu of VE, and describe the 

instances in which VE remains appropriate.

(2) Based on your experience, are bag leak detection systems, PM continuous 

parameter monitoring systems (CPMS), or PM continuous emissions monitoring systems 

(CEMS) reliable, cost-effective methods for monitoring compliance with PM emissions? 

Please provide additional information on reliability and cost to support your position.

(3) Will increasing the frequency of VE observations resolve the issue of 

applicability of VE techniques for monitoring compliance with PM2.5 emissions? In other 

words, are there situations in which increased VE frequency (i.e., daily versus weekly) 

would be expected to have no impact on compliance with PM2.5 emission limits? If so, 

please provide relevant data and explanation of such situations.

(4) Should the EPA consider mandating through rulemaking the use of 

alternatives to VE techniques for monitoring compliance with PM2.5 and PM emissions 

limits in certain situations and applications? If so, in what cases?

(5) Should the EPA’s effort with regard to the use of improved monitoring

techniques in lieu of VE monitoring be focused on applicable requirements 

established/relied upon for compliance with the PM2.5 NAAQS, or should the agency 

more broadly address other applicable requirements where VE techniques are commonly 

used (e.g., to estimate TSP and PM10 emissions)?
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(6) Should the EPA consider mandating through rulemaking the use of 

alternatives to continuous opacity monitoring systems (COMS) for monitoring 

compliance with PM2.5 and PM emissions limits in certain situations and applications? If 

so, in what cases?

(7) In its study published in 2003, the EPA identified stationary source emission 

reduction techniques that air agencies should consider when developing their potential 

list of control measures for a PM2.5 NAAQS nonattainment area.310 Specifically, the EPA 

identified improved measurement techniques and increased monitoring frequency as 

practices that could better inform sources and air agencies of excess emissions from 

individual sources which, if responded to more quickly, could yield significant reductions 

and assist in bringing the area into attainment for the NAAQS. Please comment on 

whether these techniques remain appropriate, given that they were based on the best 

technical information available at the time. Are there ways to improve the methodologies 

described in the study? 

(8) Please submit any examples of improved stationary source emissions 

monitoring, including a description of the measure, monitoring data, etc.

(9) Please submit any other methodologies - complete with equations and 

explanations - for estimating emissions reductions due to improved monitoring.

The EPA will continue to explore and implement innovative, cost-effective ideas 

that offer tangible incentives for improved source monitoring to be adopted as part of the 

 

310 Ibid.
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associated emissions limitations that will help achieve additional reductions from 

stationary sources and bring areas into attainment for the PM2.5 NAAQS in a timely way.

K. Stationary Source Test Methods for Emissions of Condensable PM2.5

1. Background

As discussed in Section II of this preamble, direct PM2.5 comprises of two 

components: filterable PM2.5 and condensable PM2.5 emissions. Accurate test methods for 

quantifying filterable PM emissions have been available for air agencies and states to 

apply since the early 1970s. In addition, controls have improved over the past 40 years 

and most sources now achieve substantially lower emissions than required by state and 

federal emissions limits. With the filterable portion of PM2.5 emissions being relatively 

well controlled, the condensable portion of PM2.5 emissions now represents a larger share 

of overall PM2.5 emissions for several categories of stationary sources. However, accurate 

test methods for condensable PM2.5 emissions have only been recently developed and 

approved by the EPA. Thus, many states may have stationary source emission limits 

adopted into their existing SIPs based only on filterable PM2.5 emissions or based on 

outdated methods for measuring or estimating condensable PM2.5 emissions.

The following discussion focuses on current test methods for quantifying 

condensable PM2.5 emissions and the EPA’s proposed requirements for states developing 

control strategies for PM2.5 nonattainment areas.

2. Test methods for condensable PM from stationary sources

Since January 1, 2011, the EPA has required that states take into consideration 

condensable PM2.5 emissions when establishing emission limits for stationary sources as 
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part of any control strategy for PM2.5 NAAQS nonattainment areas.311 This date 

coincided with the effective date of the agency’s revisions to test methods for measuring 

filterable PM10 emissions from stationary sources (Method 201A) and for measuring 

condensable PM emissions from stationary sources (Method 202).

312 The revisions 

increased the precision of Method 202 and improved the consistency in the measurements 

obtained between source tests performed under different regulatory authorities.

In the preamble to the 2007 PM2.5 Implementation Rule, the EPA explained that 

the use of the (then anticipated) revisions to the EPA Method 201A combined with 

Method 202 to obtain measured source specific emissions of PM2.5 would improve the 

quality of emissions inventories for stationary sources and would aid in the development 

of a more reliable attainment strategy, as sources that may have a considerable amount of 

condensable PM2.5 emissions could be better characterized with the new methods. The 

EPA continues to believe that using these improved test methods can help identify 

sources of primary PM2.5 emissions which, if better controlled, can help to bring a PM2.5

nonattainment area into attainment. Likewise, use of these test methods may help a state 

identify sources whose condensable emissions may have been incorrectly estimated and 

therefore may not provide meaningful PM2.5 control opportunities. 

3. Proposed SIP requirements for test methods for condensable PM2.5 emissions

The EPA proposes to require that a state that needs to adopt control measures for

direct PM2.5 from sources in a nonattainment area must specify PM2.5 emission limits in 

 

311 72 FR 20586 (April 25, 2007).

312 75 FR 80118 (December 21, 2010).
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its SIP that include both filterable and condensable emissions. In addition, compliance 

testing of those sources must include measurement of condensable emissions (such as 

through the use of Method 202). Under this proposal, any new or revised emission limit 

used as a control measure to bring an area into attainment for any current or future PM2.5

NAAQS must use methods that measure PM2.5 or total PM including both filterable and 

condensable particulate matter. Existing emission limitations that are not being revised as 

part of a Moderate area or Serious area attainment plan can remain as filterable PM or 

whatever test method is used by the state for compliance determination. In these cases, 

the acceptability of existing stationary source test methods for PM2.5 attainment plans will 

depend upon what is required under the state’s current test methods for PM emissions.

The EPA believes that this proposed requirement is appropriate because the addition of 

the condensable portion of PM2.5 to filterable PM2.5 may increase direct PM2.5 emissions 

by a factor of five or more, and the use of test methods that only measure filterable 

emissions potentially limit the control measures available for developing cost effective 

strategies to achieve attainment of the PM2.5 NAAQS. 

The EPA seeks comment on this proposed requirement for states to quantify 

condensable PM2.5 emissions in their attainment plans for PM2.5 nonattainment areas.

X. What is the EPA proposing with respect to revoking the 1997 primary annual 

PM2.5 NAAQS?

A. Background

If the 1997 primary annual PM2.5 NAAQS were to remain in place after 

conformity requirements begin to apply for the 2012 primary annual PM2.5 NAAQS (1 

year after the effective date of designations), a number of federal agencies, metropolitan 
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planning organizations (MPOs) and other state, local, and federal transportation and air 

quality agencies in areas that are currently designated nonattainment or maintenance for 

the 1997 annual PM2.5 NAAQS and will be designated nonattainment for the 2012 

primary annual NAAQS would be required to implement conformity requirements for 

both annual PM2.5 NAAQS concurrently. Additionally, some areas would also be 

implementing conformity requirements for the 2006 24-hour PM2.5 NAAQS, and two 

areas remain subject to conformity requirements for the 1997 24-hour PM2.5 NAAQS. 

This could lead to unnecessary complexity for transportation conformity determinations, 

especially if an area’s boundaries for the various PM2.5 NAAQS differ from one another 

and the same test of conformity cannot be used for all the PM2.5 NAAQS. Even where an 

area’s boundaries are unchanged, different analysis years under the conformity rules may 

be required for each PM2.5 NAAQS. It could also lead to general conformity 

determinations being made in areas that are attainment for the 2012 primary annual PM2.5

NAAQS. Finally, state and local air quality agencies would be required to continue 

attainment planning activities for the 1997 primary annual PM2.5 NAAQS even if they 

had air quality data that resulted in their being designated attainment for the 2012 primary 

annual PM2.5 NAAQS.

The EPA believes that it is more important and consistent with CAA requirements 

to determine conformity for the new 2012 primary annual PM2.5 NAAQS, which is more 

stringent and thus more protective of health than the 1997 PM2.5 NAAQS. This section 

therefore describes the EPA’s proposed approaches for transitioning from the 1997 

primary annual PM2.5 NAAQS to the 2012 primary annual PM2.5 NAAQS. This section 

discusses a number of options for revoking the 1997 primary annual PM2.5 NAAQS and 
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addresses anti-backsliding requirements that would apply, as appropriate, under each of 

the revocation options. The EPA is not proposing to revoke the 1997 secondary annual 

PM2.5 NAAQS in this action because that NAAQS has been retained in order to prevent 

certain welfare effects associated with PM2.5.

313

The proposed options are framed in the context of the CAA requirements that 

apply to NAAQS transitions to ensure that states and nonattainment areas continue to 

make progress and do not reverse progress, or backslide, from improvements already 

made in air quality. The CAA contains several provisions indicating congressional intent 

not to allow a state to alter or remove provisions from an approved attainment plan if the 

revision would reduce air quality protection. Section 193 of the CAA prohibits 

modification of a control requirement in effect or required to be adopted as of November 

15, 1990 (the date of enactment of the 1990 CAA Amendments), unless such a 

modification would ensure equivalent or greater emissions reductions. Section 172(e), 

which addresses relaxations of a NAAQS, requires protections for areas that have not 

attained a NAAQS prior to a relaxation by requiring controls which are at least as 

stringent as the controls applicable in nonattainment areas prior to any such relaxation. 

Section 110(l) provides that a SIP revision cannot be approved if it will interfere with 

attainment or other CAA requirements. Under section 175A(d), an area that is 

redesignated to attainment may, with an appropriate showing of no interference, cease to

 

313 78 FR 3086 (January 15, 2013).
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implement a measure that is contained in the SIP at the time of redesignation, but only if 

that measure is retained as a contingency measure in the area’s maintenance plan.314, 315

The transition from the 1997 to the 2012 primary annual PM2.5 NAAQS is a 

straightforward tightening of the level with little change in the form of the standard, so it 

is unambiguous that the 2012 primary annual PM2.5 NAAQS is more stringent than the 

1997 primary annual PM2.5 NAAQS. In the 2012 PM NAAQS rulemaking the EPA 

eliminated the provisions that allowed for an area to use spatial averaging of monitoring 

data to determine whether or not it is attaining the 1997, 2012 and any future annual 

PM2.5 NAAQS.316 Eliminating spatial averaging provides additional protection for 

populations that may be at a greater risk to exposures of elevated levels of PM2.5. In these 

circumstances where the annual PM2.5 NAAQS has clearly been strengthened, section 

172(e) on its face does not apply. The EPA’s interpretation that anti-backsliding 

provisions consistent with the purposes of section 172(e) by analogy should apply as 

 

314 Nonattainment areas that were redesignated to attainment with an approved section 

175A maintenance plan are referred to throughout this document as “maintenance areas.”

315 Unimplemented requirements in the SIP or those shown to be unnecessary for 

maintenance can be shifted to the contingency measures portion of the SIP upon 

redesignation. See “Procedures for Processing Requests to Redesignate Areas to 

Attainment,” Memorandum from John Calcagni, Director, Air Quality Management 

Division, September 4, 1992; “State Implementation Plan (SIP) Requirements for Areas 

Submitting Requests for Redesignation to Attainment of the Ozone and Carbon 

Monoxide (CO) National Ambient Air Quality Standards (NAAQS) On or After 

November 15, 1992,” Memorandum from Michael H. Shapiro, Acting Assistant 

Administrator for Air and Radiation, September 17, 1993. As discussed elsewhere in this 

document, an exception is made for NNSR, which can be removed from the SIP 

completely and need not be retained as a contingency measure after redesignation to 

attainment.

316 78 FR 3124-7 and 78 FR 3229 (January 15, 2013).
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upheld by the court in South Coast as appropriate in the absence of statutory provisions 

addressing tightened air quality standards. In proposing anti-backsliding requirements 

that would apply as appropriate to the options that are being considered, the EPA seeks to 

apply the principles of section 172(e).317

B. History of Revocation of Other NAAQS

The EPA has either adopted or has proposed to adopt transition policies for other 

NAAQS, including the policies for the transitions from:

 the 1-hour ozone NAAQS to the 1997 ozone NAAQS; 

 the 1997 ozone NAAQS to the 2008 ozone NAAQS; 

 the prior lead NAAQS to the 2008 lead NAAQS; and,

 the prior sulfur dioxide (SO2) NAAQS to the 2010 SO2 NAAQS. 

It is important to note that for all previous NAAQS transitions, the EPA has used 

revocation to reduce the burden associated with implementing a NAAQS that has been 

replaced with a more stringent NAAQS.

In its Phase 1 Rule for the transition from the 1-hour ozone NAAQS to the 1997 

ozone NAAQS, the EPA stated that the 1-hour ozone NAAQS would be revoked (i.e., no 

longer apply) 1 year after the effective date of initial area designations for the 1997 ozone 

NAAQS. The EPA also included anti-backsliding requirements in the Phase 1 Rule to 

address the transition between the two standards. 

 

317 South Coast Air Quality Management District v. EPA, 472 F.3d 882 (D.C. Cir. 2006).
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The Phase 1 Rule for implementation of the 1997 ozone NAAQS was the subject 

of legal challenges, and the resulting court decision in South Coast upheld the EPA’s 

authority to revoke the 1-hour ozone NAAQS as long as adequate anti-backsliding 

measures were retained to prevent backsliding.318 The decision directed the EPA to 

provide additional 1-hour ozone NAAQS anti-backsliding requirements for NNSR, 

section 185 fees and section 172(c)(9)and 182(c)(9) contingency measures for failure to 

attain the 1-hour ozone NAAQS by the applicable attainment date or to make reasonable 

further progress toward attainment of that standard, in addition to the anti-backsliding 

measures contained in the Phase 1 rule.319

As part of its final SIP requirements rule for the 2008 ozone NAAQS, the EPA 

included requirements for the transition from the 1997 ozone NAAQS to the 2008 ozone 

NAAQS.320 In developing that rulemaking, the EPA built upon its experience in 

implementing the Phase 1 rule for the transition from the 1-hour ozone NAAQS to the 

1997 ozone NAAQS and the decision in the South Coast litigation. The EPA revoked the 

1997 ozone NAAQS on the effective date of the final SIP requirements rule and finalized

anti-backsliding requirements consistent with the implementation of the court decision 

 

318 South Coast Air Quality Management District v. EPA, 472 F.3d 882 (D.C. Cir. 2006)

319 For a more complete discussion of the requirements for the transition from the 1-hour 

ozone NAAQS to the 1997 ozone NAAQS, see 69 FR 23969-89 (April 30, 2004).

320 See the published proposal at 78 FR 34178 (June 6, 2013) and the pre-publication 

version of the final rule at 

http://www.epa.gov/airquality/ozonepollution/pdfs/20150213fr.pdf.
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for the previous ozone transition that would apply in areas designated nonattainment for 

the 1997 ozone NAAQS at the time of revocation.321

It should be noted that as part of the transition from the 1997 ozone NAAQS to 

the 2008 ozone NAAQS, the EPA revoked the 1997 ozone NAAQS for transportation 

conformity purposes only in a separate action related to classifications for the 2008 ozone 

NAAQS that was finalized prior to the time that the full implementation rule had been 

proposed.322 The EPA took this action because the D.C. Circuit Court in litigation on the 

transportation conformity rule and in its decision in the South Coast litigation affirmed 

that the use of motor vehicle emissions budgets that have been approved or found 

adequate for use in transportation conformity determinations for the prior NAAQS must 

be used in transportation conformity determinations for the new NAAQS until a state 

submits motor vehicle emissions budgets for the new NAAQS and those budgets are 

either found adequate or are approved. 323, 324 These cases seemed to indicate that the use 

of these existing budgets until new budgets are available is the appropriate antibacksliding measure with respect to transportation conformity to support revocation for 

 

321 Ibid.

322 77 FR 30160.

323 See South Coast Air Quality Management District v. EPA, 472 F.3d 882 (D.C. Cir. 

2006)

324 40 CFR 93.101 defines “motor vehicle emissions budget” as that portion of the total 

allowable emissions defined in the submitted or approved control strategy 

implementation plan revision or maintenance plan for a certain date for the purpose of 

meeting reasonable further progress milestones or demonstrating attainment or 

maintenance of the NAAQS, for any criteria pollutant or its precursors, allocated to 

highway and transit vehicle use and emissions.”
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that purpose.325 It should be noted, however, that the revocation of the 1997 ozone 

NAAQS for transportation conformity purposes was the subject of litigation in the D.C.

Circuit Court.326 The court issued its decision on December 23, 2014, and held that the 

EPA lacked authority to revoke the 1997 ozone NAAQS only for transportation 

conformity purposes because for areas that remain designated as nonattainment or 

maintenance for the 1997 ozone NAAQS, CAA section 176(c) requires transportation 

conformity determinations in nonattainment and maintenance areas. 

Following promulgation of the 2008 lead NAAQS and 2010 SO2 NAAQS, the 

EPA revoked the prior lead and SO2 NAAQS for all purposes in areas that had attained 

the prior NAAQS and had been redesignated to attainment, as well as in areas that had 

initially been designated as attainment for those NAAQS. The EPA retained the prior 

NAAQS in areas that had not yet attained those NAAQS until those areas had an 

approved attainment plan for the revised NAAQS. Because the EPA revoked the prior 

lead and SO2 NAAQS in areas that had been redesignated to attainment for those 

NAAQS, the EPA primarily relied on the CAA’s anti-backsliding provisions found in 

sections 110(l) and 193 in order to provide anti-backsliding protection.327

 

325 In addition, the Court affirmed that conformity determinations need not be made for a 

revoked standard. 

326 NRDC v. EPA, No. 12-1321 (D.C. Cir.) (challenging EPA actions taken at 77 FR 

30160 (May 21, 2012)).

327 For details on the requirements for the lead and SO2 NAAQS, respectively, see 73 FR 

67043 and 75 FR 35580.
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In developing the options for revoking the 1997 primary annual PM2.5 NAAQS 

contained in this proposal, the EPA has drawn from these prior anti-backsliding 

approaches.

C. Proposed Options for Revocation and Related Anti-Backsliding Requirements for the 

1997 Primary Annual PM2.5 NAAQS

The EPA is proposing and seeking comment on two options for revoking the 1997 

primary annual PM2.5 NAAQS and is seeking comment on whether to revoke the 

NAAQS at the current time. Under any of the options to revoke the 1997 NAAQS, 

revocation would take effect no sooner than 1 year after the effective date of designations 

for the 2012 primary annual PM2.5 NAAQS. One of these options would provide for 

revocation at a later date for some areas. 

After revocation of the 1997 primary annual PM2.5 NAAQS, the designations (and 

the classifications associated with those designations) for that standard would no longer 

be in effect, and the designations that would remain in effect would be those for the 1997 

secondary annual PM2.5 NAAQS, the 2006 primary and secondary 24-hour PM2.5

NAAQS and the 2012 primary annual PM2.5 NAAQS. However, the EPA would retain

the listing of the designated nonattainment areas for the revoked 1997 primary annual 

PM2.5 NAAQS in 40 CFR part 81, for the sole purpose of identifying the anti-backsliding 

requirements that may apply to the areas at the time of revocation. Accordingly, such 

references to historical designations for the revoked standard should not be viewed as 

current designations under CAA section 107(d).

For any proposed option that allows for revocation in nonattainment areas for the 

1997 primary annual PM2.5 NAAQS, the EPA is also proposing anti-backsliding 
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provisions to ensure that in these areas: (i) there is protection against degradation of air 

quality (e.g., the areas do not backslide in terms of air quality improvements); (ii) the 

areas continue to make progress toward attainment of the new, more stringent 2012 

primary annual PM2.5 NAAQS; and, (iii) there is consistency with the PM2.5 NAAQS 

implementation framework outlined in subpart 4 of part D, title I of the CAA. At the 

current time, there are 14 areas that continue to be designated as nonattainment for the 

1997 annual PM2.5 NAAQS; however all but 2 of these areas have 2011-2013 air quality 

data showing that they are attaining that NAAQS. Therefore, the EPA expects many of 

these current nonattainment areas will be eligible to seek redesignation to attainment 

prior to any revocation. The EPA is proposing and seeking comment on the following

two options:

 Option 1: Revoke the 1997 primary annual PM2.5 NAAQS for all purposes in 

attainment areas for that NAAQS 1 year after the effective date of the 

designations for the 2012 primary annual PM2.5 NAAQS; and,

 Option 2: Revoke the 1997 primary annual PM2.5 NAAQS for all purposes in all 

nonattainment and attainment areas for that NAAQS 1 year after the effective date 

of the designations for the 2012 primary annual PM2.5 NAAQS.

More details on the proposed options and associated rationale are included below. 

1. Option 1: Revoke the 1997 primary annual PM2.5 NAAQS for all purposes in 

attainment areas for that NAAQS 1 year after the effective date of the 

designations for the 2012 primary annual PM2.5 NAAQS

The EPA’s first proposed option would revoke the 1997 primary annual PM2.5

NAAQS for all purposes in areas that are designated as attainment for that NAAQS 1 
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year after the effective date of designations for the 2012 primary annual PM2.5 NAAQS, 

as well as in future areas that are redesignated as attainment areas after the initial 

revocation. The areas addressed by this option are those that were originally designated 

as attainment areas for the 1997 primary PM2.5 NAAQS and those that were originally 

designated as nonattainment but have since or will in the future be redesignated to 

attainment for that NAAQS. Under this option, the EPA would not revoke the 1997 

primary annual PM2.5 NAAQS in any area that is designated nonattainment for that

NAAQS. 

Areas that are designated nonattainment for the 1997 annual PM2.5 NAAQS at the 

time of the initial revocation would be required to continue to meet all applicable 

requirements for such NAAQS, and could continue to seek redesignation to attainment

for the 1997 primary annual PM2.5 NAAQS. For example, even if the revocation were to 

become effective in April 2016, redesignations could continue to be approved after that 

date. For such areas, the effective date of the revocation would be the effective date of the 

area’s redesignation to attainment for the 1997 annual PM2.5 NAAQS. 

The EPA notes that under proposed Option 1 it is unnecessary to propose antibacksliding requirements for the 1997 primary annual PM2.5 NAAQS, since Option 1 

would only revoke this NAAQS in attainment areas. Anti-backsliding requirements are 

not applicable to attainment areas (i.e., for former nonattainment areas that have been 

redesignated to attainment the EPA has already determined through the redesignation 

process and approval of maintenance plans that all applicable requirements for the 1997 

primary annual PM2.5 NAAQS-- including anti-backsliding requirements -- have been 

fulfilled and areas that have always been designated attainment for this NAAQS). 
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For areas that were initially designated as attainment for both the 1997 and 2012 

annual PM2.5 NAAQS the EPA is proposing that the approved PSD SIPs for these areas 

satisfy the obligation to submit an approvable maintenance plan for the 2012 primary 

annual PM2.5 NAAQS under section 110(a)(1). 

The EPA also notes that areas designated nonattainment for the 2012 primary 

annual PM2.5 NAAQS would be required to comply with applicable conformity 

requirements beginning 1 year after the effective date of designations for that NAAQS.

For transportation conformity purposes these requirements would include using adequate 

or approved motor vehicle emissions budgets for the 1997 annual PM2.5 NAAQS where 

they exist until the area has approved or adequate budgets for the 2012 primary annual

PM2.5 NAAQS.328 The use of such budgets serves as the appropriate anti-backsliding 

measure for transportation conformity purposes. 

In general, Option 1 builds upon the EPA’s practice in the transition from the 1-

hour to the 1997 ozone NAAQS in that areas will not only be able to redesignate to 

attainment up to the date of the initial revocation, but any remaining nonattainment areas 

will be able to redesignate after the initial revocations occur one-year after the effective 

date of designations.

329 This approach is also consistent with the approach established for 

the transition from the prior lead and SO2 NAAQS to the current lead and SO2 NAAQS. 

 

328 Areas that do not have adequate or approved motor vehicle emissions budgets for the 

1997 annual PM2.5 NAAQS or the 2006 24-hour PM2.5 NAAQS would use one of the two 

interim emissions tests required by 40 CFR 93.109(c)(3) and 40 CFR 93.119(b).

329 Although section 51.905(a) specified that the anti-backsliding requirements “attached” 

at the time of designation for the 1997 ozone NAAQS, areas were still able to redesignate 

to attainment for the 1-hour ozone NAAQS up to the date of revocation of that standard. 
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2. Option 2: Revoke the 1997 primary annual PM2.5 NAAQS for all purposes in 

all nonattainment and attainment areas for that NAAQS 1 year after the effective 

date of designations for the 2012 primary annual PM2.5 NAAQS

Under this second proposed option, the EPA would revoke the 1997 primary 

annual PM2.5 NAAQS for all purposes in all nonattainment and attainment areas 1 year 

after the effective date of designations for the 2012 primary annual PM2.5 NAAQS. The 

requirements for revoking the 1997 primary annual NAAQS in attainment areas for that 

NAAQS are discussed under proposed Option 1. However, revoking the 1997 primary 

annual PM2.5 NAAQS in nonattainment areas for that NAAQS would require antibacksliding measures. Therefore, the EPA is proposing the following anti-backsliding 

measures for any designated nonattainment areas that exist for the 1997 primary annual 

PM2.5 NAAQS upon the effective date of the proposed revocation:

 For areas designated attainment for the 2012 primary annual PM2.5 NAAQS and 

nonattainment for the 1997 primary annual PM2.5 NAAQS, the EPA’s preferred 

proposed option is not to require these areas to adopt any outstanding applicable 

requirements for the revoked 1997 primary annual PM2.5 standard. However, the 

EPA proposes that the approved PSD SIPs for these areas satisfy the obligation to 

submit an approvable maintenance plan for the 2012 primary annual PM2.5

NAAQS under section 110(a)(1).

 For these same areas (i.e., those designated attainment for the 2012 primary 

annual PM2.5 NAAQS and nonattainment for the 1997 primary annual PM2.5

NAAQS), the EPA is also proposing an alternative anti-backsliding option where 

these areas would be required to show maintenance for the 2012 primary annual 
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PM2.5 NAAQS. This maintenance showing would be due 3 years after the 

effective date of designations for the 2012 primary annual PM2.5 NAAQS. The 

maintenance showing would contain a demonstration of continued maintenance of 

the 2012 primary annual PM2.5 NAAQS in the area for 10 years from the effective 

date of the area’s designation as attainment for the 2012 primary annual PM2.5

NAAQS. The EPA would provide guidance regarding the specific elements of 

such a maintenance showing should the agency require it in the final rule.

 For areas designated nonattainment for the 2012 primary annual PM2.5 NAAQS 

and also designated nonattainment for the 1997 annual PM2.5 NAAQS, the EPA is 

proposing that these areas continue to implement their approved SIPs for the 1997 

annual PM2.5 NAAQS and fulfill any outstanding requirements, and that they 

comply with the applicable requirements for the current 2012 primary annual 

PM2.5 NAAQS. For example, at some time in the future there may be an area that 

is reclassified as Serious for the 1997 PM2.5 NAAQS while also classified as 

Moderate for the 2012 PM2.5 NAAQS. In such an area, the lower Serious area 

major source threshold of 70 tpy (PTE) would apply.

In addition to these proposed requirements, if a state seeks to revise any measure already 

approved into its SIP for a nonattainment area for the 1997 annual PM2.5 NAAQS, the 

state must meet the requirements of sections 110(l) and 193, if applicable.

The EPA notes that Option 2 for 2012 attainment/1997 nonattainment would be 

similar to the approach to revocation of the 1-hour ozone NAAQS consistent with court 

decisions and the approach to revocation of the 1997 ozone NAAQS as proposed in the 
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June 6, 2013.330 The EPA also notes that areas designated nonattainment for the 2012 

primary annual PM2.5 NAAQS would be required to comply with applicable conformity 

requirements beginning 1 year after the effective date of designations for that NAAQS. 

For transportation conformity purposes these requirements would include using adequate 

or approved motor vehicle emissions budgets for the 1997 annual PM2.5 NAAQS where 

they exist until the area has approved or adequate budgets for the 2012 primary annual 

PM2.5 NAAQS.331 The use of such budgets serves as the appropriate anti-backsliding 

measure for transportation conformity purposes. Further details regarding this option and 

associated rationale are described below.

Lastly, the EPA requests comment on the possible approach of not revoking the 

1997 primary annual PM2.5 NAAQS at this time. Under this concept, the EPA would not 

revoke the 1997 primary annual PM2.5 NAAQS for any purpose at this time. As a result, 

all nonattainment and maintenance areas would be required to continue planning 

activities associated with the 1997 annual PM2.5 NAAQS such as submitting attainment 

SIPs and maintenance plans, NSR, and transportation and general conformity 

requirements for the 1997 primary annual PM2.5 NAAQS, in addition to any new 

requirements associated with the more health-protective 2012 primary annual PM2.5

NAAQS. Under this approach the EPA would not have to establish any anti-backsliding 

requirements. 

 

330 78 FR 34178.

331 Areas that do not have adequate or approved motor vehicle emissions budgets for the 

1997 annual PM2.5 NAAQS or the 2006 24-hour PM2.5 NAAQS would use one of the two 

interim emissions tests required by 40 CFR 93.109(c)(3) and 40 CFR 93.119(b).
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The EPA again notes that if this approach were finalized it would be the first time 

that the EPA has not taken some action to reduce the burden associated with 

implementing a NAAQS that has been replaced with a more stringent NAAQS.332 If the 

EPA were to finalize this approach it would result in state and local agencies being 

required to implement the requirements associated with two primary annual PM2.5

NAAQS. These agencies would be required to continue attainment planning activities for 

the 1997 primary annual PM2.5 NAAQS even if they had air quality data that resulted in 

their being designated attainment for the 2012 primary annual PM2.5 NAAQS. State, local 

and federal agencies would be required to continue to make transportation and general 

conformity determinations for the less protective 1997 primary annual PM2.5 NAAQS. 

D. Discussion of Options

Until the 1997 primary annual PM2.5 NAAQS is revoked, that NAAQS remains in 

effect, in parallel with the 2012 primary annual PM2.5 NAAQS, and continues to apply 

independently and by its own terms. The EPA believes that all of the proposed options to 

revoke the 1997 primary annual PM2.5 NAAQS are consistent with the CAA and previous 

precedent in transitioning from a previous NAAQS to a new, more stringent NAAQS, 

and would ensure that attainment areas continue to attain the revoked NAAQS into the 

future. If the 1997 primary annual PM2.5 NAAQS is revoked, the EPA is proposing that 

the anti-backsliding requirements for the 1997 primary annual PM2.5 NAAQS, as 

proposed in this rulemaking, will become applicable. However, the EPA notes that most 

 

332As discussed in Section IX.F.2 of this preamble, the EPA has taken action to revoke 

previous ozone, SO2 and lead NAAQS when the previous NAAQS has been revised. 
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of the areas that were initially designated as nonattainment for the 1997 primary annual 

PM2.5 NAAQS where the NAAQS would be revoked have already been redesignated to 

attainment (i.e., they are maintenance areas) or could qualify for redesignation based on 

current air quality data, and in such cases their approved maintenance plan for the 1997 

primary annual PM2.5 would prevent backsliding.333 Under Option 2 there would be a 

limited number of nonattainment areas where the 1997 primary annual NAAQS would be 

revoked and where anti-backsliding measures would be required. Under all of the 

proposed options, conformity would apply in areas that are designated nonattainment for 

the more health protective 2012 primary annual PM2.5 NAAQS. In the case of 

transportation conformity, adequate or approved motor vehicle emissions budgets for the 

1997 primary annual PM2.5 NAAQS would be used in conformity determinations until 

motor vehicle emissions budgets for the 2012 primary annual PM2.5 NAAQS are found 

adequate or are approved. Once a NAAQS is revoked in a nonattainment area, the EPA 

would not designate or redesignate areas for that NAAQS after the revocation of that 

NAAQS except as described in Option 1. The extent of continued implementation of a 

revoked standard derives from administration of anti-backsliding requirements for that 

standard. 

 

333 Based on 2010-12 air quality data, many of the areas that were initially designated 

nonattainment for the 1997 annual PM2.5 NAAQS will have already met the 1997 annual 

PM2.5 NAAQS and will have been redesignated to attainment by the time it is revoked 

(projected to be in or around April 2016), and thus after revocation of the 1997 primary 

annual PM2.5 NAAQS, the number of areas with 1997 anti-backsliding requirements will 

be correspondingly reduced.
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Under Option 1, the 1997 primary annual PM2.5 NAAQS would be revoked only 

in areas that have attained the 1997 annual PM2.5 NAAQS and have been redesignated to 

attainment with an approved section 175A maintenance plan for the 1997 primary annual 

PM2.5 NAAQS; and under Option 2, many of the areas where the 1997 primary annual 

NAAQS would be revoked would have been redesignated to attainment with an approved 

maintenance plan. The EPA also anticipates that states will continue to request that areas 

be redesignated to attainment and the EPA will continue to act on those requests under 

Option 2. As a result the EPA anticipates that a number of such requests will be approved 

prior to the point in time that the EPA has proposed for the revocations to become 

effective (i.e., 1 year after the effective date of designations for the 2012 primary annual 

PM2.5 NAAQS, approximately April 2016). Therefore, the number of nonattainment 

areas for the 1997 primary annual PM2.5 NAAQS will continue to decrease and fewer 

areas will be required to comply with anti-backsliding requirements and correspondingly 

larger number of areas will be required to continue to implement their approved section 

175A maintenance plan for the 1997 primary annual PM2.5 NAAQS.

It should also be noted that for either proposed option after the effective date of 

any revocation of the 1997 primary annual PM2.5 NAAQS, transportation and general 

conformity determinations would continue to be required in areas that are designated 

nonattainment for the 1997 secondary annual PM2.5 NAAQS until they are redesignated 

to attainment pursuant to the requirements of section 107(d)(3). Areas that are initially 

designated as nonattainment for the 2012 primary annual NAAQS are subject to 

transportation and general conformity requirements after the end of the grace period that 

ends 1 year after the effective date of designations for the 2012 primary annual PM2.5
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NAAQS. See further information for how conformity will be implemented for the 2012 

PM2.5 NAAQS in Section IX.B of this preamble. Under Options 1 and 2 the timing that 

the EPA is proposing means that any area that was previously a 1997 annual PM2.5

NAAQS nonattainment area, but has been redesignated to attainment for the 1997 annual 

PM2.5 NAAQS by the time of revocation of the 1997 primary annual PM2.5 NAAQS (e.g., 

approximately April 2016 for most areas), will not be subject to the anti-backsliding 

requirements for the 1997 annual PM2.5 NAAQS. This is because when an area has been 

redesignated to attainment for a PM2.5 NAAQS while that NAAQS is in effect, it has 

fulfilled all applicable requirements for that NAAQS, including applicable antibacksliding requirements for the 1997 annual PM2.5 NAAQS. The area is, therefore, not 

subject to anti-backsliding requirements for the revoked 1997 primary annual PM2.5

NAAQS. These areas are required instead to implement their approved CAA section 

175A maintenance plan for the 1997 primary annual PM2.5 NAAQS and implement a 

PSD program for this NAAQS, if they are designated attainment for the 2012 primary 

annual PM2.5 NAAQS.334,335 Revisions to the approved maintenance plan can only be 

made if the revisions meet the requirements of section 110(l) and, if applicable, section 

193. The EPA proposes that these areas not be required to submit a second 10-year 

maintenance plan for the 1997 primary annual PM2.5 NAAQS because there is no 

 

334 Areas initially designated as attainment for the 1997 annual PM2.5 NAAQS would also 

be required to continue to implement a PSD program unless an area was designated 

nonattainment for the 2012 primary annual PM2.5 NAAQS. Such an area would be 

required to implement a NNSR program for that NAAQS.

335 Areas designated nonattainment for the 2012 primary annual PM2.5 NAAQS would 

implement a NNSR program for that NAAQS.
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justification for additional maintenance plan burdens to be imposed on these areas solely 

because at one time they were designated nonattainment under the revoked 1997 primary 

annual PM2.5 NAAQS. Not requiring a second 10-year maintenance plan for these areas 

would help to minimize the burden associated with preparing SIPs for a succession of 

NAAQS of increasing stringency. 

As explained previously, for areas redesignated to attainment under Options 1 and 

2, the section 175A maintenance plan for the 1997 primary annual PM2.5 NAAQS 

satisfies the anti-backsliding requirements of these areas. The EPA believes that for these 

areas any further 110(a)(1) maintenance plan requirement under the 2012 primary annual 

PM2.5 NAAQS for areas designated attainment for that NAAQS would be unnecessarily 

burdensome. 

For Option 2, the EPA is applying a general principle to apply transition 

requirements depending on how the area is designated -- attainment or nonattainment --

for the 2012 primary annual PM2.5 NAAQS, while taking into account the area’s status 

with respect to the 1997 primary annual PM2.5 NAAQS. For those areas which have 

already incorporated measures into their approved SIPs that satisfy the nonattainment 

requirements for that standard, section 110(l) functions to require continued 

implementation of such measures unless revised in accordance with its provisions. 

Under Option 2, the EPA is proposing as one alternative that areas designated 

attainment for the 2012 primary annual PM2.5 NAAQS and nonattainment for the 1997 

annual PM2.5 NAAQS (as of revocation of the 1997 primary annual PM2.5 NAAQS) not 

be required to adopt any outstanding applicable requirements for the revoked 1997 

primary annual standard. This approach is similar to the approach followed in the 
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transition from the 1-hour ozone NAAQS to the 1997 ozone NAAQS. However, instead 

of submitting a maintenance plan the EPA is also proposing that the approved PSD SIPs 

for these areas satisfy the obligation to submit an approvable maintenance plan for the 

2012 primary annual PM2.5 NAAQS under section 110(a)(1). This is similar to what the 

EPA has proposed for the transition from the 1997 ozone NAAQS to the 2008 ozone 

NAAQS.336 The EPA’s rationale for this approach is as follows: areas designated 

attainment for the 2012 primary annual PM2.5 NAAQS and nonattainment for the 1997 

primary annual PM2.5 NAAQS (as of revocation of the 1997 primary annual PM2.5

NAAQS) have already attained the most stringent existing standard. These areas thus 

have developed nonattainment plans that in combination with federal measures and 

emissions controls in upwind areas have produced sufficient emissions reductions to 

achieve the more protective 2012 primary annual PM2.5 NAAQS. They remain subject to 

the 1997 nonattainment area requirements already approved into the SIP, which can be 

revised only upon a showing that such revision is consistent with sections 110(l) and 193, 

if applicable. At this time, and given the succession of NAAQS of increasing stringency 

that has occurred, the EPA believes that the burden of developing an approvable 

maintenance plan for the 2012 primary annual PM2.5 NAAQS would outweigh any 

compensating benefit for an area that is already attaining that more stringent NAAQS and 

that is subject to prior nonattainment requirements which are already incorporated into 

the SIP.

 

336 78 FR 34178.
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Under Option 2, the EPA is also proposing, for areas that are attainment for the 

2012 primary annual PM2.5 NAAQS, that the nonattainment NSR anti-backsliding 

requirement(s) for the 1997 annual PM2.5 NAAQS cease to apply, since PSD will then be 

in effect the state may request that the corresponding NSR requirements be removed 

entirely, rather than be retained in the SIP as a maintenance plan contingency measure.337

Areas that are designated nonattainment for the more stringent 2012 primary annual 

PM2.5 NAAQS will be subject to nonattainment NSR and other nonattainment 

requirements for their classification under the more stringent 2012 primary annual PM2.5

NAAQS. 

The revocation of the 1-hour ozone NAAQS and the associated anti-backsliding 

provisions were the subject of litigation. In its December 2006 decision on that challenge, 

as modified following rehearing, the Court held with respect to the anti-backsliding 

approach for conformity that 1-hour ozone motor vehicle emissions budgets must be used 

in transportation conformity determinations for the more protective 1997 ozone NAAQS 

where such SIP motor vehicle emissions budgets have been found adequate or approved, 

until SIP motor vehicle emissions budgets for the 1997 ozone NAAQS are available.338 In 

addition, the Court affirmed more broadly that in order for transportation conformity 

determinations to fulfill the requirements of CAA section 176(c)(1), motor vehicle 

emissions budgets for a prior NAAQS must be used in transportation conformity 

 

337 See 40 CFR 51.905(a)(3), the comparable provision for transitions from the 1-hour 

NAAQS to the 1997 ozone NAAQS, which allows such areas to request that the 1-hour 

NNSR provisions be removed from the SIP.

338 See South Coast Air Quality Management District v. EPA, 472 F.3d at 882.
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determinations under a revised NAAQS until emissions budgets for the revised NAAQS 

are either found adequate or are approved, but that conformity determinations need not be 

made for a revoked standard. Therefore, areas designated nonattainment for the 2012 

primary annual PM2.5 NAAQS that have adequate or approved SIP budgets for the 1997 

annual PM2.5 NAAQS must continue to use such budgets in transportation conformity 

determinations until budgets for the 2012 primary annual PM2.5 NAAQS are found 

adequate or are approved.339

With regard to general conformity, the D.C. Circuit Court did not address the 

need for specific anti-backsliding measures in its initial decision or in the modified 

decision on the South Coast litigation. Therefore, if the EPA finalizes either Option 1 or 2

and revokes the 1997 primary annual PM2.5 NAAQS, general conformity determinations 

will be required in nonattainment areas for the 2012 primary annual NAAQS as required 

by section 176(c)(5) to ensure that the action of federal agencies do not cause a violation 

of that NAAQS, make an existing violation worse or delay timely attainment of the 

NAAQS or an interim milestone.340 The EPA believes that revoking the 1997 primary 

annual PM2.5 NAAQS under Options 1 or 2 is logical because it would result in only one 

primary annual PM2.5 NAAQS – the 2012 primary annual PM2.5 NAAQS – applying for 

 

339Such areas without adequate or approved SIP budgets for either the 1997 annual PM2.5

NAAQS or the 2006 24-hour PM2.5 NAAQS are required to demonstrate transportation 

conformity using one of the interim emissions tests depending on their classification as 

required by 40 CFR 93.119. 

340 It should be noted that some areas will remain designated nonattainment for 1997 

secondary annual PM2.5 NAAQS. Such areas will remain subject to transportation and 

general conformity for that NAAQS until such time that they are redesignated to 

attainment for that NAAQS pursuant to the requirements of section 107(d)(3).
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purposes of transportation and general conformity in most areas, after the end of the 1-

year conformity grace period that applies to newly designated nonattainment areas. (CAA 

section 176(c)(6)). 

Areas that are attaining the more health protective 2012 primary annual PM2.5

NAAQS would no longer have to expend resources to make conformity determinations 

for any of the current primary annual PM2.5 NAAQS after the 1997 primary annual PM2.5

NAAQS is revoked and the area is redesignated as attainment for the 1997 secondary 

annual PM2.5 NAAQS. Some of these areas would be required to continue to make 

conformity determinations for the 2006 24-hour PM2.5 NAAQS and based on 2010-12 air 

quality data two areas would be required to make conformity determinations for the 1997 

24-hour PM2.5 NAAQS. It should be noted that any areas that are attaining the more 

health protective 2012 primary annual NAAQS are also necessarily attaining the less 

stringent 1997 annual PM2.5 NAAQS by a wide margin. Therefore, the options of this 

proposal would provide a seamless transition from demonstrating conformity for the 

1997 annual PM2.5 NAAQS to demonstrating conformity for the more stringent 2012 

primary annual PM2.5 NAAQS. 

Areas designated nonattainment for the 2012 primary annual PM2.5 NAAQS will 

likely need the full 1-year grace period provided in section 176(c)(6) to complete the 

required initial transportation conformity determination. Those areas that were designated 

as either nonattainment or maintenance for the 1997 annual PM2.5 NAAQS at the time of 

designation as nonattainment for the 2012 primary annual PM2.5 NAAQS, will need 

certainty as to the specific requirements for that conformity determination. For example 

they need to know what analysis years must be addressed and, if the boundaries for the 
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PM2.5 NAAQS are different, they need to know whether to address conformity for both 

areas and which test or tests would apply.

The EPA seeks comment on all of the options proposed in the preceding 

discussion regarding revoking the 1997 primary annual PM2.5 standard, as well as on 

whether the agency should take no action to revoke the standard as this time.

XI. Environmental Justice Considerations

The EPA believes the human health or environmental risk addressed by this 

action will not have disproportionately high and adverse human health or environmental 

effects on minority, low-income, or indigenous populations because it would not 

negatively affect the level of protection provided to human health or the environment

under the PM2.5 NAAQS. When promulgated, these proposed regulations will clarify the 

state implementation plan requirements and the NNSR permitting requirements to be met 

by states in order to attain the PM2.5 NAAQS as expeditiously as practicable. These 

requirements are designed to protect all segments of the general population. The EPA 

included specific discussion in this preamble about actions that could be considered for 

the protection of minority, low-income or indigenous populations in Section IV.D.6 on 

Moderate area attainment plan control strategies; Section VI.D.7 on Serious area 

attainment plan control strategies; and Section IX.G, measures to ensure appropriate 

protections for overburdened populations. In addition, as part of the consultation 

activities conducted in developing this rule, the EPA participated in training and outreach 

activities with representatives from environmental justice organizations in a March 2014 

conference held in Research Triangle Park, NC entitled, “Clean Air Act Rulemaking and 

Permitting Training for EJ Communities.” These proposed regulations are designed to 
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protect and enhance the health and safety of these and other populations, and they will 

not adversely affect the health or safety of minority, low-income or indigenous 

populations. 

XII. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at 

http://www2.epa.gov/laws-regulations/laws-and-executive-orders. 

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 

13563: Improving Regulation and Regulatory Review

This action is a significant regulatory action that was submitted to the Office of 

Management and Budget (OMB) for review because it raises novel policy issues. Any 

changes made in response to OMB recommendations have been documented in the 

docket. 341

B. Paperwork Reduction Act (PRA)

The information collection activities in this proposed rule have been submitted for 

approval to the Office of Management and Budget (OMB) under the PRA. The 

Information Collection Request (ICR) document prepared by the EPA has been assigned 

 

341 Note that a regulatory impact analysis evaluating the costs and benefits associated 

with attaining the 2012 PM2.5 NAAQS was released at the time the NAAQS review was 

finalized. See “Regulatory Impact Analysis for the Final Revisions to the National 

Ambient Air Quality Standards for Particulate Matter.” U.S. Environmental Protection 

Agency, Office of Air Quality and Planning Standards, Health and Environmental 

Impacts Division, February 28, 2013. EPA-452/R-12-005.
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the EPA ICR number 2258.03, OMB Control No. 2060-0611. You can find a copy of the 

ICR in the docket for this rule, and it is briefly summarized here.

The EPA is proposing this PM2.5 NAAQS SIP Requirements Rule to describe the 

CAA requirements that must be met by states with nonattainment areas required to 

develop attainment plans for attaining and maintaining the NAAQS. The intended effect 

of the SIP Requirements Rule is to provide certainty to states regarding their planning 

obligations such that states may begin SIP development. Only states with nonattainment 

areas are required to submit SIPs under this rule.

For purposes of analysis of the estimated paperwork burden, the EPA assumed 

there were 21 existing nonattainment areas for the 1997 and 2006 PM2.5 NAAQS, and 15 

hypothetical, newly-designated nonattainment areas.

342 The attainment demonstration 

requirement would appear as 40 CFR 51.1007 which implements CAA subsections 

172(c)(1) and (2), and 189(a)(1)(B) and (C), 189(b)(1)(A) and (B) and 189(c). Some 

states have new nonattainment areas and some states should already have information 

from emission sources, as facilities should have provided this information to meet 1997 

and 2006 PM2.5 NAAQS SIP requirements, operating permits and/or emissions reporting 

requirements. Such information does not generally reveal the details of production 

processes. But, to the extent it may, confidential business information for the affected 

facilities is protected. Specifically, submissions of emissions and control efficiency 

information that is confidential, proprietary and trade secret and is not emission data is 

 

342 These hypothetical nonattainment areas were developed based on 2010-12 air quality 

data and state recommendations. Actual nonattainment designations and boundaries will 

be based on the most recent, complete air quality data available.
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protected from disclosure under the requirements of subsections 503(e) and 114(c) of the 

CAA.

The annual state burden for this information collection for the 15 hypothetical 

newly designated 2012 PM2.5 nonattainment areas, averaged over the first 3 years of this 

ICR, is estimated to be a total of 54,000 labor hours per year at an annual labor cost of 

$3.2 million (present value) over the 3-year period, or approximately $649,000 per state 

for the 5 state respondents. The average annual reporting burden is approximately 3,600 

hours per response, with approximately 3 responses per state for 15 state responses. 

There are no capital or operating and maintenance costs associated with the proposed rule 

requirements. Burden is defined at 5 CFR 1320.3(b).

The annual state burden for this information collection for the 21 existing

nonattainment areas for the 1997 and 2006 PM2.5 NAAQS, averaged over the first 3 years 

of this ICR, is estimated to be a total of 43,400 labor hours per year at an annual labor 

cost of $2.6 million (present value) over the 3-year period, or approximately $370,000 

per state for the 7 state respondents. The average annual reporting burden is 

approximately 2,000 hours per response, with approximately 3 responses per state for 21 

state responses. There are no capital or operating and maintenance costs associated with 

the proposed rule requirements. Burden is defined at 5 CFR 1320.3(b).

An agency may not conduct or sponsor, and a person is not required to respond to, 

a collection of information unless it displays a currently valid OMB control number. The 

OMB control numbers for the EPA's regulations in 40 CFR are listed in 40 CFR part 9. 
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Submit your comments on the Agency’s need for this information, the accuracy of the 

provided burden estimates and any suggested methods for minimizing respondent burden 

to the EPA using the docket identified at the beginning of this rule. You may also send 

your ICR-related comments to OMB’s Office of Information and Regulatory Affairs via 

email to oria_submissions@omb.eop.gov, Attention: Desk Officer for the EPA. Since 

OMB is required to make a decision concerning the ICR between 30 and 60 days after 

receipt, OMB must receive comments no later than [insert date 30 days after publication 

in the Federal Register]. The EPA will respond to any ICR-related comments in the final 

rule.

C. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) generally requires an agency to prepare a 

regulatory flexibility analysis of any regulation subject to notice and comment 

rulemaking requirements under the Administrative Procedures Act or any other statute 

unless the agency certifies the rule will not have a significant economic impact on a 

substantial number of small entities. Small entities include small businesses, small 

organizations and small governmental jurisdictions.

For purposes of assessing the impacts of today’s rule on small entities, small 

entity is defined as: (1) a small business as defined in the Small Business 

Administration’s (SBA) regulations at 13 CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a city, county, town, school district or special district 

with a population of less than 50,000; and (3) a small organization that is any not-forprofit enterprise which is independently owned and operated and is not dominant in its 

field.
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After considering the economic impacts of today’s proposed rule on small 

entities, I certify that this action will not have a significant economic impact on a 

substantial number of small entities. This proposed rule will not impose any requirements 

directly on small entities. Entities potentially affected directly by this proposal include 

state, local and tribal governments and none of these governments are small governments. 

Other types of small entities are not directly subject to the requirements of this rule. The 

EPA continues to be interested in the potential impacts of the proposed rule on small 

entities and welcomes comments on issues related to such impacts.

D. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 

U.S.C. 1531-1538, and does not significantly or uniquely affect small governments. This 

action imposes no enforceable duty on any state, local or tribal governments or the 

private sector. The CAA imposes the obligation for states to submit attainment plans to 

implement the PM2.5 NAAQS. In this rule, the EPA is clarifying those requirements.

Therefore, this action is not subject to the requirements of sections 202, 203, and 205 of 

the UMRA.

E. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial 

direct effects on the states, on the relationship between the national government and the 

states, or on the distribution of power and responsibilities among the various levels of 

government, as specified in Executive Order 13132. The requirement to submit 

attainment plans to meet a PM2.5 NAAQS is imposed by the CAA. This proposed rule, if 
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made final, would interpret those requirements as they apply to current and future PM2.5

NAAQS. Thus, Executive Order 13132 does not apply to these proposed regulations.

In the spirit of Executive Order 13132 and consistent with the EPA policy to 

promote communications between the EPA and state and local governments, the EPA 

specifically solicits comments on this proposed action from state and local officials. In 

addition, the EPA intends to meet with organizations representing state and local officials 

during the comment period for this action.

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments

This proposed action does not have tribal implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 2000). It would not have a substantial direct 

effect on one or more Indian tribes. Furthermore, these proposed regulation revisions do 

not affect the relationship or distribution of power and responsibilities between the 

federal government and Indian tribes. The CAA and the TAR establish the relationship of 

the federal government and tribes in characterizing air quality and developing plans to 

attain the NAAQS, and these revisions to the regulations do nothing to modify that 

relationship. Thus, Executive Order 13175 does not apply to this action. 

Although Executive Order 13175 does not apply to this action, the EPA solicits 

comment on this proposed action from tribal officials. The EPA also intends to offer to 

consult with any tribal government to discuss this proposal.

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

Risks
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The EPA interprets Executive Order 13045 as applying only to those regulatory 

actions that concern environmental health or safety risks that the EPA has reason to 

believe may disproportionately affect children, per the definition of “covered regulatory 

action” in section 2-202 of the Executive Order. This action is not subject to Executive 

Order 13045 because it implements a previously promulgated health or safety-based 

federal standard established pursuant to the CAA.

These proposed regulatory provisions are designed to help implement the current 

and future PM2.5 NAAQS, promulgated to protect the health and welfare of individuals, 

including children, who are susceptible to the adverse effects of exposure to unhealthy 

levels of ambient PM2.5.

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect 

Energy Supply, Distribution, or Use

This action is not a “significant energy action” as defined in Executive Order 

13211 (66 FR 28355 (May 22, 2001)), because it is not likely to have a significant 

adverse effect on the supply, distribution, or use of energy.

I. National Technology Transfer and Advancement Act

Section 12(d) of the National Technology Transfer and Advancement Act of 1995 

(NTTAA), Public Law No. 104-113, section 12(d) (15 U.S.C. 272 note) directs the EPA 

to use voluntary consensus standards in its regulatory activities unless to do so would be 

inconsistent with applicable law or otherwise impractical. Voluntary consensus standards 

are technical standards (e.g., materials specifications, test methods, sampling procedures 

and business practices) that are developed or adopted by voluntary consensus standards 
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bodies. NTTAA directs the EPA to provide Congress, through OMB, explanations when 

the agency decides not to use available and applicable voluntary consensus standards.

This proposed rulemaking does not involve technical standards. Therefore, the 

EPA is not considering the use of any voluntary consensus standards. 

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations

The EPA believes the human health or environmental risk addressed by this 

action will not have disproportionately high and adverse human health or environmental 

effects on minority, low-income, or indigenous populations. The results of this evaluation 

are contained in Section XI of this preamble.

K. Determination Under Section 307(d)

Pursuant to sections 307(d)(1)(E) and 307(d)(1)(V) of the CAA, the

Administrator proposes to determine that this action is subject to the provisions of section 

307(d). Under section 307(d)(1)(V), the provisions of section 307(d) apply to “such other

actions as the Administrator may determine.”

Statutory Authority

The statutory authority for this action is provided by 42 U.S.C. 7403, 7407, 7410 

and 7601.
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Page 494 of 551 - Fine Particulate Matter National Ambient Air Quality Standards:

State Implementation Plan Requirements

List of Subjects 

40 CFR Part 50

Environmental protection, Air pollution control, Intergovernmental relations, Particulate 

matter.

40 CFR Part 51

Environmental protection, Air pollution control, Intergovernmental relations, Particulate 

matter.

40 CFR Part 93

Environmental protection, Air pollution control, Intergovernmental relations, Particulate 

matter.

__________________________________

Dated:

__________________________________

Gina McCarthy,

Administrator.
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For the reasons stated in the preamble, Title 40, Chapter I of the Code of Federal 

Regulations is proposed to be amended as follows:

PART 50 – NATIONAL PRIMARY AND SECONDARY AMBIENT AIR 

QUALITY STANDARDS

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

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

2. In §50.13, add paragraph (d) to read as follows:

§50.13 National primary and secondary ambient air quality standards for PM2.5.

* * * * *

(d) The standards set forth in this section will remain applicable to all areas 

notwithstanding the promulgation of the 2012 primary annual PM2.5 national ambient air 

quality standards (NAAQS) in §50.18. The 1997 primary annual PM2.5 NAAQS set forth 

in this section will no longer apply to an area 1 year after the effective date of the 

designation of that area, pursuant to section 107 of the Clean Air Act, for the primary 

annual PM2.5 NAAQS set forth in §50.18; except that for areas designated nonattainment 

for the 1997 annual PM2.5 NAAQS set forth in this section as of 1 year after the effective 

date of the designations for the primary annual PM2.5 NAAQS established in § 50.18, the 

requirements applicable to the 1997 annual PM2.5 NAAQS set forth in this section will 

apply until the effective date of an area’s redesignation to attainment for the 1997 annual 

NAAQS pursuant to the requirements of section 107 of the Clean Air Act. The 1997 
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secondary annual PM2.5 NAAQS and the 1997 24-hour PM2.5 NAAQS shall remain in 

effect.

* * * * *

PART 51 – REQUIREMENTS FOR PREPARATION, ADOPTION, AND 

SUBMITTAL OF IMPLEMENTATION PLANS

3. The authority citation for part 51 continues to read as follows:

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

Subpart I–[Amended] 

4. In §51.165:

a. Revise paragraph (a)(1)(iv)(A)(1).

b. Revise paragraph (a)(1)(x)(A).

c. Remove paragraphs (a)(1)(xxxvii)(C)(2), (3), and (4), and add a new paragraph 

(a)(1)(xxxvii)(C)(2).

d. Revise paragraphs (a)(2)(i) and (a)(2)(ii)(A).

§51.165 Permit requirements.

(a)(1) * * *

(iv)(A) Major stationary source means:
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(1) Any stationary source of air pollutants that emits, or has the potential to emit, 

100 tons per year or more of any regulated NSR pollutant (as defined in paragraph 

(a)(1)(xxxvii) of this section), except that lower emissions thresholds shall apply in areas 

subject to subpart 2, subpart 3, or subpart 4 of part D, title I of the Act, according to 

paragraphs (a)(1)(iv)(A)(1)(i) through (viii) of this section.

(i) 50 tons per year of volatile organic compounds in any serious ozone 

nonattainment area.

(ii) 50 tons per year of volatile organic compounds in an area within an ozone 

transport region, except for any severe or extreme ozone nonattainment area.

(iii) 25 tons per year of volatile organic compounds in any severe ozone 

nonattainment area.

(iv) 10 tons per year of volatile organic compounds in any extreme ozone 

nonattainment area.

(v) 50 tons per year of carbon monoxide in any serious nonattainment area for 

carbon monoxide, where stationary sources contribute significantly to carbon monoxide 

levels in the area (as determined under rules issued by the Administrator).

(vi) 70 tons per year of PM10 in any serious nonattainment area for PM10.

(vii) 70 tons per year of PM2.5 in any serious nonattainment area for PM2.5.
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(viii) 70 tons per year of any precursor for PM2.5 in any serious nonattainment area 

for PM2.5.

* * * * *

(x)(A) Significant means, in reference to a net emissions increase or the potential of 

a source to emit any of the following pollutants, a rate of emissions that would equal or 

exceed any of the following rates:

POLLUTANT EMISSION RATE

Carbon monoxide: 100 tons per year (tpy)

Nitrogen oxides: 40 tpy

Sulfur dioxide: 40 tpy

Ozone: 40 tpy of volatile organic compounds or nitrogen oxides

Lead: 0.6 tpy

PM10: 15 tpy

PM2.5: 10 tpy of direct PM2.5 emissions; 40 tpy of sulfur dioxide emissions; 40 tpy of 

nitrogen oxide emissions; 40 tpy of VOC emissions; 

* * * * *

(xxxvii) * * *
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(2) Sulfur dioxide, nitrogen oxides, volatile organic compounds and ammonia are 

precursors to PM2.5 in any PM2.5 nonattainment area, unless the State demonstrates to the 

Administrator’s satisfaction or the EPA demonstrates that major stationary sources of a 

particular precursor do not contribute significantly to PM2.5 levels that exceed the PM2.5

ambient standards in a particular area.

* * * * *

(a)(2) Applicability procedures. (i) Each plan shall adopt a preconstruction review 

program to satisfy the requirements of sections 172(c)(5) and 173 of the Act for any area 

designated nonattainment for any national ambient air quality standard under subpart C of 

40 CFR part 81. Such a program shall apply to any new major stationary source or major 

modification that is major for the pollutant (as defined in paragraph (a)(1)(xxxvii) of this 

section) for which the area is designated nonattainment under section 107(d)(1)(A)(i) of 

the Act, if the stationary source or modification would locate anywhere in the designated 

nonattainment area. Different pollutants, including individual precursors, are not summed 

to determine applicability of a major stationary source or major modification.

(ii) * * *

(A) Except as otherwise provided in paragraphs (a)(2)(iii) and (iv) of this section, 

and consistent with the definition of major modification contained in paragraph 

(a)(1)(v)(A) of this section, a project is a major modification for a regulated NSR 

pollutant (as defined in paragraph (a)(1)(xxxvii) of this section), if it causes two types of 

emissions increases—a significant emissions increase (as defined in paragraph 
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(a)(1)(xxvii) of this section), and a significant net emissions increase (as defined in 

paragraphs (a)(1)(vi) and (x) of this section). The project is not a major modification if it 

does not cause a significant emissions increase. If the project causes a significant 

emissions increase, then the project is a major modification only if it also results in a 

significant net emissions increase.

* * * * *

5. In Appendix S to part 51:

a. Revise paragraph II.A.4(i)(a).

b. Add paragraphs II.A.4.(a)(7) and (8).

c. Revise paragraph II.A.10.(i).

d. Revise paragraph II.A.31.(ii)(b).

Appendix S to Part 51 – Emission Offset Interpretative Ruling

* * * * *

II. * * *

A. * * *

4. (i) * * * 
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(a) Any stationary source of air pollutants which emits, or has the potential to emit, 

100 tons per year or more of a regulated NSR pollutant(as defined in paragraph II.A.31 of 

this Ruling), subject to regulation under the Act, except that lower emissions thresholds 

shall apply in areas subject to subpart 2, subpart 3, or subpart 4 of part D, title I of the 

Act, according to paragraphs II.A.4(i)( a )( 1 ) through ( 6 ) of this ruling. 

* * * * *

(7) 70 tons per year of PM2.5 in any serious nonattainment area for PM2.5.

(8) 70 tons per year of any PM2.5 precursor (as defined in paragraph II.A.31 of this 

Ruling) in any Serious nonattainment area for PM2.5.

* * * * *

10. (i) Significant means, in reference to a net emissions increase or the potential of 

a source to emit any of the following pollutants, a rate of emissions that would equal or 

exceed any of the following rates:

POLLUTANT AND EMISSIONS RATE

Carbon monoxide: 100 tons per year (tpy)

Nitrogen oxides: 40 tpy

Sulfur dioxide: 40 tpy

Ozone: 40 tpy of volatile organic compounds or nitrogen oxides

Lead: 0.6 tpy
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Particulate matter: 25 tpy of particulate matter emissions

PM10: 15 tpy

PM2.5: 10 tpy of direct PM2.5 emissions; 40 tpy of sulfur dioxide emissions; 40 tpy of 

nitrogen oxides emissions

* * * * *

31. Regulated NSR pollutant, * * *

* * * * * 

(ii) * * *

(b) Sulfur dioxide and nitrogen oxides are precursors to PM2.5 in all PM2.5

nonattainment areas; or

* * * * *

6. Revise subpart Z to read as follows:

Subpart Z - Provisions for Implementation of PM2.5 National Ambient Air Quality 

Standards

Sec.

51.1000 Definitions.

51.1001 Applicability of part 51.
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51.1002 Classifications.

51.1003 Attainment plan submittals and due dates.

51.1004 Attainment dates.

51.1005 Attainment date extensions.

51.1006 Requirements for demonstrating insignificant contribution of PM2.5

precursors.

51.1007 Requirements for de minimis source category determinations for direct 

PM2.5 and PM2.5 precursors.

51.1008 Emissions inventory requirements.

51.1009 Moderate area attainment plan control strategy requirements.

51.1010 Serious area attainment plan control strategy requirements.

51.1011 Attainment demonstration and modeling requirements.

51.1012 Reasonable further progress (RFP) requirements. 

51.1013 Quantitative milestone requirements.

51.1014 Contingency measures requirements.

51.1015 Clean data requirements.

§51.1000 Definitions.
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The following definitions apply for purposes of this subpart. Any term not defined herein 

shall have the meaning as defined in 40 CFR 51.100 or Clean Air Act section 302.

Act means the Clean Air Act as codified at 42 U.S.C. 7401-7671q (2003).

Additional feasible measure is any control measure that otherwise meets the definition of 

“best available control measure” (BACM) but can only be implemented in whole or in 

part beginning 4 years after the date of reclassification of an area as Serious and no later 

than the statutory attainment date for the area.

Additional reasonable measure is any control measure that otherwise meets the definition 

of “reasonably available control measure” (RACM) but can only be implemented in 

whole or in part during the period beginning 4 years after the date of designation of a 

nonattainment area and the following implementation end date:

(a) For a Moderate nonattainment area for which the state projects attainment on or 

before the end of the sixth calendar year following the date of designation of the area, the 

implementation end date shall be no later than the beginning of the calendar year of the 

projected attainment date for the area;

(b) For a Moderate nonattainment area for which the state demonstrates that attainment of 

the relevant PM2.5 NAAQS by the end of the sixth calendar year following the date of 

designation of the area is impracticable, the implementation end date shall be no later 

than the end of the sixth calendar year following the date of designation of the area.

Applicable annual standard is the annual PM2.5 NAAQS established, revised, or retained 

as a result of a particular PM2.5 NAAQS review.
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Applicable attainment date means the latest statutory date by which an area is required to 

attain a particular PM2.5 NAAQS, unless EPA has approved an attainment plan for the 

area to attain such NAAQS, in which case the applicable attainment date is the date 

approved under such attainment plan. If EPA grants an extension of an approved 

attainment date, then the applicable attainment date for the area shall be the extended 

date.

Applicable 24-hour standard is the 24-hour PM2.5 NAAQS established, revised, or 

retained as a result of a particular PM2.5 NAAQS review.

Attainment projected inventory means the projected emissions of direct PM2.5 and all 

PM2.5 precursors from sources included in the base year inventory, and from any 

additional sources of such emissions expected within the boundaries of the nonattainment 

area by the projected attainment date for the area. 

Base year inventory means the actual emissions of direct PM2.5 and all PM2.5 precursors 

from all sources within the boundaries of a nonattainment area in one of the 3 years used 

for purposes of designations or another technically appropriate year. 

Benchmark RFP analysis means the analysis submitted as part of the RFP plan for a 

PM2.5 nonattainment area that requires generally linear emissions reductions in direct 

PM2.5 and in each PM2.5 precursor from the base year through the projected attainment 

year.

Best available control measure (BACM) is any technologically and economically feasible 

control measure that can be implemented in whole or in part within 4 years after the date 

of reclassification of a PM2.5 nonattainment area and that generally can achieve greater 
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permanent and enforceable emissions reductions in direct PM2.5 emissions and/or 

emissions of PM2.5 precursors from sources in the area than can be achieved through the 

implementation of RACM on the same source(s). BACM includes best available control 

technology (BACT).

Date of designation means the effective date of a PM2.5 area designation as promulgated 

by the Administrator. 

Date of reclassification means the effective date of a PM2.5 area reclassification from 

Moderate to Serious as promulgated by the Administrator.

Direct PM2.5 emissions means solid particles emitted directly from an air emissions 

source or activity, or gaseous emissions or liquid droplets from an air emissions source or 

activity which condense to form particulate matter at ambient temperatures. Direct PM2.5

emissions include filterable and condensable PM2.5 emissions composed of elemental 

carbon, directly emitted organic carbon, directly emitted sulfate, directly emitted nitrate, 

and other inorganic particles (including but not limited to crustal material, metals, and sea 

salt). 

Existing control measure means any federally enforceable national, state, or local control 

measure that results in reductions in direct PM2.5 emissions or emissions of PM2.5

precursors in a nonattainment area in that state. 

Implemented means adopted by the state and fully approved into the SIP by EPA for the 

nonattainment area; built, installed, and/or otherwise physically manifested; and, fully 

complied with by the affected sources.
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Most stringent measure (MSM) is any permanent and enforceable control measure that 

achieves the most stringent emissions reductions in direct PM2.5 emissions and/or 

emissions of PM2.5 precursors from among those control measures which are either 

included in any other SIP for any NAAQS or have been achieved in practice by any other 

state and that can feasibly be implemented in the relevant PM2.5 NAAQS nonattainment 

area.

PM2.5 design value (DV) for a PM2.5 nonattainment area is the highest of the three-year 

average concentrations calculated for the ambient air quality monitors in the area, in 

accordance with 40 CFR part 50, appendix N.

PM2.5 NAAQS are the fine particulate matter National Ambient Air Quality Standards 

codified at 40 CFR part 50.

PM2.5 precursors are sulfur dioxide (SO2), oxides of nitrogen (NOx), volatile organic 

compounds (VOC), and ammonia (NH3). 

Reasonable further progress (RFP) means such annual incremental reductions in 

emissions of direct PM2.5 and PM2.5 precursors regulated in the attainment plan as are 

required for the purpose of ensuring attainment of the applicable PM2.5 NAAQS in a 

nonattainment area by the applicable attainment date.

Reasonably available control measure (RACM) is any technologically and economically 

feasible measure that can be implemented in whole or in part within 4 years after the date 

of designation of a PM2.5 nonattainment area and that achieves permanent and 
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enforceable reductions in direct PM2.5 emissions and/or PM2.5 precursor emissions from 

sources in the area. RACM includes reasonably available control technology (RACT).

Subpart 1 means subpart 1 of part D of title I of the Act.

Subpart 4 means subpart 4 of part D of title I of the Act.

§51.1001 Applicability of part 51. 

The provisions in subparts A through X of this part apply to areas for purposes of the 

PM2.5 NAAQS to the extent they are not inconsistent with the provisions of this subpart.

§51.1002 Classifications.

(a) Initial classification as Moderate PM2.5 nonattainment area. Any area designated 

nonattainment for a PM2.5 NAAQS shall be classified at the time of such designation, by 

operation of law, as a Moderate PM2.5 nonattainment area.

(b) Reclassification as Serious PM2.5 nonattainment area. A Moderate nonattainment area 

shall be reclassified to Serious under the following circumstances:

(1) The EPA shall reclassify as Serious through notice-and-comment rulemaking any 

Moderate PM2.5 nonattainment area that the EPA determines cannot practicably attain a 

particular PM2.5 NAAQS by the applicable Moderate area attainment date.

(2) A Moderate PM2.5 nonattainment area shall be reclassified by operation of law as a 

Serious nonattainment area if the EPA finds through notice-and-comment rulemaking 

that the area failed to attain a particular PM2.5 NAAQS by the applicable Moderate area 

attainment date.
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§51.1003 Attainment plan submittals and due dates.

(a) Nonattainment areas initially classified as Moderate.

(1) For any area designated as nonattainment and initially classified as Moderate for a 

PM2.5 NAAQS, the state(s) shall submit a Moderate area attainment plan that meets all of 

the following requirements:

(i) Emissions inventory requirements set forth at §51.1008(a)(1);

(ii) Emissions inventory requirements set forth at §51.1008(a)(2);

(iii) Moderate area attainment plan control strategy requirements set forth at §51.1009;

(iv) Attainment demonstration and modeling requirements set forth at §51.1011;

(v) Reasonable Further Progress (RFP) requirements set forth at §51.1012;

(vi) Quantitative milestone requirements set forth at §51.1013; and,

(vii) Contingency measure requirements set forth at §51.1014.

(viii) Any Nonattainment New Source Review plan requirements pursuant to section 

189(a)(1)(A) and section 172(c)(5) of the CAA

(2) The state(s) shall submit its Moderate area attainment plan to EPA no later than 18 

months from the date of designation of the area.

(b) Nonattainment areas reclassified to Serious.

(1) For any nonattainment area reclassified to Serious for a PM2.5 NAAQS under 

§51.1002(b), in addition to meeting the Moderate area attainment plan submittal 
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requirements set forth at §51.1003(a), the state(s) shall submit a Serious area attainment 

plan that meets all of the following requirements:

(i) Emissions inventory requirements set forth at §51.1008(b)(1);

(ii) Emissions inventory requirements set forth at §51.1008(b)(2);

(iii) Serious area attainment plan control strategy requirements set forth at §51.1010;

(iv) Attainment demonstration and modeling requirements set forth at §51.1011;

(v) Reasonable Further Progress (RFP) requirements set forth at §51.1012;

(vi) Quantitative milestone requirements set forth at §51.1013; and,

(vii) Contingency measure requirements set forth at §51.1014.

(viii) Any Nonattainment New Source Review plan requirements pursuant to section 

189(b)(3) and section 172(c)(5) of the CAA

(2) The state(s) shall submit its Serious area attainment plan to EPA according to the 

following schedule:

(i) For any nonattainment area reclassified to Serious for a particular PM2.5 NAAQS 

under §51.1002(b)(1), the state(s) shall submit to EPA the portion of the Serious area 

attainment plan that meets the requirements set forth at §51.1003(b)(1)(i) and (b)(1)(iii)

no later than 18 months from the date of reclassification. The state(s) shall submit to EPA 

the portion of the Serious area attainment plan that meets the requirements set forth at 

§51.1003(b)(1)(ii) and (b)(1)(iv)-(vii) to EPA no later than 4 years from the date of 

reclassification.
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(ii) For any nonattainment area reclassified to Serious for a particular PM2.5 NAAQS 

under §51.1002(b)(2), the state(s) shall submit to EPA a Serious area attainment plan 

meeting the requirements set forth at §51.1003(b)(1)(i)-(vii) no later than 18 months from 

the date of reclassification. 

(iii) If the state(s) submits to EPA a request for a Serious area attainment date extension 

simultaneous with the Serious area attainment plan due under §51.1003(b)(1), such a plan 

shall meet the most stringent measure (MSM) requirements set forth at §51.1010(b) in 

addition to the BACM and BACT and additional feasible measure requirements set forth 

at §51.1010(a).

(c) Serious nonattainment areas subject to CAA section 189(d) for failing to attain the 

PM2.5 NAAQS by the applicable Serious area attainment date.

(1) For any Serious nonattainment area that fails to attain a particular PM2.5 NAAQS by 

the applicable Serious area attainment date, the state(s) shall submit a revised Serious 

area attainment plan that demonstrates that the area annually will achieve at least 5 

percent reductions in emissions of direct PM2.5 and PM2.5 precursors based on the most 

recent emissions inventory for the area and that meets the following requirements:

(i) Emissions inventory requirements set forth at §51.1008(c)(1);

(ii) Emissions inventory requirements set forth at §51.1008(c)(2);

(iii) Demonstration of attainment and modeling requirements set forth at §51.1011;

(iv) Reasonable Further Progress (RFP) requirements set forth at §51.1012;

(v) Quantitative milestone requirements set forth at §51.1013; and,
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(vi) Contingency measure requirements set forth at §51.1014.

(2) The state(s) shall submit to EPA the revised attainment plan meeting the requirements 

set forth at §51.1003(c)(1)(i)-(v) no later than 12 months from the missed applicable 

Serious area attainment date.

(d) Any attainment plan submitted to EPA under this section shall establish motor vehicle 

emissions budgets for the projected attainment year for the area, if applicable. The state 

shall develop such budgets according to the requirements of the transportation conformity 

rule as they apply to PM2.5 nonattainment areas (40 CFR part 93).

§51.1004 Attainment dates.

(a) The state shall submit a projected attainment date as part of its attainment plan 

submission under §51.1003 for any PM2.5 NAAQS nonattainment area located in whole 

or in part within its boundaries. The state shall justify the projected attainment date for 

each such nonattainment area (or portion of a nonattainment area) as part of the 

demonstration of attainment developed and submitted according to the requirements set 

forth at §51.1011 and according to the following:

(1) Nonattainment areas initially classified as Moderate.

(i) Except for nonattainment areas that meet the criterion under (a)(1)(ii) of this section, 

the projected attainment date for a Moderate PM2.5 nonattainment area shall be as 

expeditious as practicable with the implementation of all control measures required under 

§51.1009 and may be as late as the end of the sixth calendar year after the date of 

designation if the state demonstrates that the implementation of certain control measures 
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that qualify as RACM or RACT or additional reasonable measures, but that are not 

necessary for demonstrating attainment by the end of the sixth calendar year after the date 

of designation, will not collectively advance the attainment date by at least 1 year.

(ii) The projected attainment date for a Moderate PM2.5 nonattainment area which the 

state demonstrates cannot practicably attain the applicable PM2.5 NAAQS by the end of 

the sixth calendar year after the date of designation of the area with the implementation 

of all control measures required under §51.1009 shall be such date unless and until the 

area is reclassified as Serious according to §51.1002. 

(2) Nonattainment areas reclassified to Serious.

(i) Except for nonattainment areas that meet the criterion under (a)(2)(ii) of this section, 

the projected attainment date for a Serious PM2.5 nonattainment area shall be as 

expeditious as practicable with the implementation of all control measures required under 

§51.1010 but no later than the end of the tenth calendar year after the date of designation.

[ALTERNATIVE PROPOSED REGULATORY TEXT:

(i) Except for nonattainment areas that meet the criterion under (a)(2)(ii) of this section, 

the projected attainment date for a Serious PM2.5 nonattainment area shall be as 

expeditious as practicable with the implementation of all control measures required under 

§51.1010 and may be as late as the end of the tenth calendar year after the date of 

designation if the state demonstrates that the implementation of certain control measures 

that qualify as BACM or BACT or additional feasible measures, but that are not 

necessary for demonstrating attainment by the end of the tenth calendar year after the 

date of designation, will not collectively advance the attainment date by at least 1 year.]
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(ii) A state that submits an attainment plan that demonstrates that a Serious PM2.5

nonattainment area cannot practicably attain the PM2.5 NAAQS by the end of the tenth 

calendar year following the date of designation of the area with the implementation of all 

control measures required under §51.1010(a) must request an extension of the Serious 

area attainment date consistent with §51.1005(b). The request must propose a projected 

attainment date for the nonattainment area that is as expeditious as practicable, but no 

later than the end of the fifteenth calendar year from the date of designation of the area. 

(3) Serious nonattainment areas subject to CAA section 189(d) for failing to attain by the 

applicable Serious area attainment date. The projected attainment date for a Serious PM2.5

nonattainment area that failed to attain the PM2.5 NAAQS by the applicable Serious area 

attainment date shall be as expeditious as practicable based on annual reductions in direct 

PM2.5 and significant PM2.5 precursor emissions within the area of not less than 5 percent 

of the amount of such emissions as reported in the most recent emissions inventory 

prepared for the area, but no later than 5 years following the missed Serious area 

attainment date.

(b) Except for attainment plans that meet the conditions of §51.1004(a)(1)(ii) or 

§51.1004(a)(3), the Administrator shall approve an attainment date at the same time and 

in the same manner in which the Administrator approves the attainment plan for the area. 

(1) In accordance with §51.1004(a)(1)(ii), if a state demonstrates that a Moderate PM2.5

nonattainment area cannot practicably attain the PM2.5 NAAQS by the end of the sixth 

calendar year following the date of designation of the area, EPA shall proceed under the 
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provisions of §51.1002(b)(1) to reclassify the area to Serious through notice-andcomment rulemaking.

(2) In accordance with §51.1004(a)(3), if a Serious PM2.5 nonattainment area fails to 

attain the PM2.5 NAAQS by the applicable Serious area attainment date, EPA will 

proceed to establish a new attainment date through a direct final action published in the 

Federal Register.

§51.1005 Attainment date extensions.

(a) Nonattainment areas initially classified as Moderate.

(1) A state with a Moderate PM2.5 nonattainment area may apply for a 1-year attainment 

date extension for the area if the following conditions are met in the year preceding the 

applicable attainment date for the area:

(i) The state has complied with all requirements and commitments pertaining to the area 

in the applicable implementation plan; 

(ii) For an area designated nonattainment for a particular 24-hour PM2.5 NAAQS for 

which the state seeks an attainment date extension, the 98th percentile concentration at 

each monitor in the area for the calendar year prior to the applicable attainment date is 

less than or equal to the level of the applicable 24-hour standard (calculated according to 

the data analysis requirements in 40 CFR part 50, appendix N);

(iii) For an area designated nonattainment for a particular annual PM2.5 NAAQS for 

which the state seeks an attainment date extension, the annual average concentration at 

each monitor in the area for the calendar year prior to the applicable attainment date is 
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less than or equal to the level of the applicable annual standard (calculated according to 

the data analysis requirements in 40 CFR part 50, appendix N).

(2) The applicable implementation plan for a Moderate PM2.5 nonattainment area for 

which a state seeks an attainment date extension is the plan submitted to EPA to meet the 

requirements of §51.1003(a).

(3) For a Moderate PM2.5 nonattainment area, the requesting state (or states) shall submit 

a written request by February 28 of the year following the applicable attainment date for 

the area.

(4) A state with a Moderate area that received an initial 1-year attainment date extension 

may apply for a second 1-year attainment date extension for the area if the state meets the 

conditions described in §51.1005(a)(1) in the year preceding the approved attainment 

date.

(b) Nonattainment areas reclassified as Serious.

(1) A state may apply for one attainment date extension not to exceed 5 years for a 

Serious nonattainment area if the following conditions are met:

(i) The state demonstrates that attainment of the applicable PM2.5 NAAQS by the 

approved attainment date for the area would be impracticable or, in the absence of an 

approved attainment date, attainment of the applicable PM2.5 NAAQS by the applicable 

statutory attainment date for the area would be impracticable;

(ii) The state has complied with all requirements and commitments pertaining to the area 

in the applicable implementation plan; and,
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(iii) The state demonstrates that the attainment plan for the area includes the most 

stringent measures (MSM) that are included in the attainment plan of any state or are 

achieved in practice in any state, and can feasibly be implemented in the area consistent 

with §51.1010(b).

(2) At the time of application for an attainment date extension, the state shall submit to 

EPA a Serious area attainment plan that meets the following requirements:

(i) Emissions inventory requirements set forth at §51.1008(b);

(ii) Most stringent measures (MSM) requirement described under paragraph (b)(1)(iii) of 

this section and §51.1010(b);

(iii) Attainment demonstration and modeling requirements set forth at §51.1011 that 

justify the state’s conclusion under paragraph (b)(1)(i) of this section;

(iv) Reasonable Further Progress (RFP) requirements set forth at §51.1012;

(v) Quantitative milestone requirements set forth at §51.1013; and,

(vi) Contingency measure requirements set forth at §51.1014.

(3) The applicable implementation plan for a Serious PM2.5 nonattainment area for which 

a state seeks an attainment date extension under §51.1004(a)(2)(ii) is the plan submitted 

to EPA to meet the requirements set forth at §51.1003(a).

(4) The applicable implementation plan for a Serious PM2.5 nonattainment area for which 

a state seeks an attainment date extension under §51.1004(a)(2)(i) is the plan submitted to 

EPA to meet the requirements set forth at §51.1003(b)(1).
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(5) A state applying for an attainment date extension for a Serious nonattainment area 

under §51.1004(a)(2)(ii) shall submit to EPA a request for an extension at the same time 

as it submits the Serious area attainment plan due under §51.1003(b)(1).

(6) A state applying for an attainment date extension for a Serious nonattainment area 

subsequent to submitting an initial Serious area attainment plan that demonstrated 

attainment of the NAAQS by the applicable attainment date consistent with 

§51.1004(a)(2)(i) at the time of submittal may apply for such an extension no later than 

60 calendar days prior to the approved attainment date for the area or, in the absence of 

an approved attainment date, no later than 60 calendar days prior to the applicable 

statutory attainment date for the area.

(c) Serious nonattainment areas subject to CAA section 189(d) for failing to attain by the 

applicable Serious area attainment date. If a Serious area fails to attain a particular PM2.5

NAAQS by the applicable Serious area attainment date, the area is then subject to the 

requirements of section 189(d) of the Act, and, for this reason, the state is prohibited from 

requesting an extension of the applicable Serious area attainment date for such area.

(d) For any attainment date extension request submitted pursuant to this section, the 

requesting state (or states) shall submit a written request and evidence of compliance with 

these regulations which includes both of the following:

(i) Evidence that all control measures submitted in the applicable attainment plan have 

been implemented, and

(ii) Evidence that the area has made emission reduction progress that represents 

reasonable further progress toward timely attainment of the applicable PM2.5 NAAQS.
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(e) For a PM2.5 nonattainment area located in two or more states or jurisdictions, all states 

and/or jurisdictions in which such area is located shall submit separate attainment date 

extension requests for the area consistent with the requirements set forth at §51.1005(d).

§51.1006 Requirements for demonstrating insignificant contribution of PM2.5

precursors.

(a) For purposes of determining that a particular PM2.5 precursor does not contribute 

significantly to ambient PM2.5 concentrations in a PM2.5 nonattainment area, the state 

shall conduct a technical analysis that accounts for all emissions of such PM2.5 precursor 

from all sources located within the area. 

(b) The state shall submit results and supporting documentation for any technical 

analyses conducted pursuant to §51.1006(a) as part of any attainment plan for the area.

§51.1007 Requirements for de minimis source category determinations for 

direct PM2.5 and PM2.5 precursors. 

(a) All categories of sources of direct PM2.5 emissions and of emissions of PM2.5

precursors in a PM2.5 nonattainment area shall be considered non-de minimis unless and 

until the state conducts a technical analysis to determine whether a particular source 

category may qualify for a presumptive de minimis source category exemption from 

evaluation for potential control measures due to its minimal contribution to the ambient 

PM2.5 concentrations in the area.
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(b) The state shall define source categories for stationary sources classified under the 

North American Industry Classification System (NAICS) at the level represented by four 

(4) digits or fewer.

(c) The state shall define a single source category for on-road mobile sources, including

on-road vehicles and engines, and a single source category for nonroad mobile sources, 

including nonroad engines, equipment, and vehicles, or may define a single source 

category for all mobile sources in the aggregate.

§51.1008 Emissions inventory requirements. 

(a) For any nonattainment area initially classified as Moderate, the state shall submit to 

EPA all of the following:

(1) A base year inventory for the nonattainment area for all emissions sources that meets 

the following minimum criteria:

(i) The inventory year shall be one of the 3 years used for designations or another 

technically appropriate inventory year if justified by the state in the plan submission. 

(ii) The inventory shall include actual emissions of all sources within the nonattainment 

area. 

(iii) The emissions values shall be either annual total emissions or average-season-day 

emissions. The state shall include as part of the plan a rationale for providing annual or 

seasonal emissions.

(iv) The inventory shall include direct PM2.5 emissions and emissions of all PM2.5

precursors.
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(v) The state shall report emissions as point sources according to the point source 

emissions thresholds of the Air Emissions Reporting Rule (AERR), 40 CFR part 51, 

subpart A. 

(vi) The detail of the emissions inventory shall be consistent with the data elements 

required by 40 CFR part 51, subpart A. 

(2) An attainment projected inventory for the nonattainment area that meets the following 

minimum criteria:

(i) The year of the projected inventory shall be the most expeditious year for which 

attainment is demonstrated by the modeled attainment plan.

(ii) The emissions values shall be projected emissions of the same sources included in the 

base year inventory for the nonattainment area (i.e., those only within the nonattainment 

area). The state shall include in this inventory projected emissions growth and contraction 

from both controls and other causes during the relevant period.

(iii) The temporal period of emissions shall be the same temporal period (annual or 

average-season-day) as the base year inventory for the nonattainment area.

(iv) Consistent with the base year inventory for the nonattainment area, the inventory 

shall include direct PM2.5 emissions and emissions of all PM2.5 precursors.

(v) The same sources reported as point sources in the base year inventory for the 

nonattainment area shall be provided as point sources in the attainment projected 

inventory for the nonattainment area. Nonpoint and mobile source projected emissions 
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shall be provided using the same detail (e.g., state, county, and process codes) as the base 

year inventory.

(vi) The same detail of the emissions included shall be consistent with the level of detail 

in the base year inventory (i.e., as required by 40 CFR part 41, subpart A).

(b) For any nonattainment area reclassified as Serious, the state shall submit to EPA all of 

the following:

(1) For purposes of meeting the emissions inventory requirements of section 172(c)(3) of 

the Act, a base year inventory for the nonattainment area for all emissions sources that 

meets the requirements listed under §51.1008(a)(1), and in addition, uses the Serious area 

definition of a major source listed under §51.165(a)(1)(iv)(A)(vii) and (viii) in 

determining sources to include as point sources.

(2) An attainment projected inventory for the nonattainment area that meets the criteria 

listed under §51.1008(a)(2).

(c) Serious nonattainment areas subject to CAA section 189(d) for failing to attain a 

PM2.5 NAAQS by the applicable Serious area attainment date. No later than 12 months 

after EPA finds through notice and comment rulemaking that a Serious nonattainment 

area, or portion thereof contained within a state’s borders, fails to attain a PM2.5 NAAQS 

by the applicable attainment date and thus becomes subject to the requirements under 

section 189(d) of the Act, the state shall submit to EPA all of the following:

(1) For purposes of meeting the emissions inventory requirements of section 172(c)(3) of 

the Act, a base year inventory for the nonattainment area for all emissions sources that 
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meets the requirements listed under §51.1008(a)(1), and in addition, uses the Serious area 

definition of a major source listed under §51.165(a)(1)(iv)(A)(vii) and (viii) in 

determining sources to include as point sources.

(2) An attainment projected inventory for the nonattainment area as defined by 

§51.1000(e) and that meets the criteria listed under §51.1008(a)(2).

§51.1009 Moderate area attainment plan control strategy requirements.

(a) The state shall identify, adopt, and implement control measures, including control 

technologies, on sources of direct PM2.5 emissions and sources of emissions of PM2.5

precursors located in any Moderate PM2.5 nonattainment area or portion thereof located 

within the state consistent with the following:

(1) The state shall identify all sources of direct PM2.5 emissions and all sources of 

emissions of PM2.5 precursors in the nonattainment area in accordance with the emissions 

inventory requirements of §51.1008(a);

(2) The state shall identify all potential control measures to reduce emissions from all 

sources of direct PM2.5 emissions and all sources of emissions of PM2.5 precursors in the 

nonattainment area identified under §51.1009(a)(1).

(i) The state may elect not to identify potential control measures to reduce emissions from 

any sources of a particular PM2.5 precursor if the state demonstrates that all sources of 

such PM2.5 precursor contribute insignificantly to ambient PM2.5 concentrations in the 

area under §51.1006.
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(ii) The state may elect not to identify potential control measures to reduce emissions 

from sources in any source category of direct PM2.5 emissions or emissions of PM2.5

precursors determined to be a de minimis source category under §51.1007.

(3) For any potential control measure identified under §51.1009(a)(2), the state may make 

a demonstration that such measure is not technologically or economically feasible to 

implement in whole or in part by the end of the sixth calendar year following the date of 

designation of the area, and the state may eliminate such whole or partial measure from 

further consideration under this paragraph. 

(i) For purposes of evaluating the technological feasibility of a potential control measure, 

the state may consider factors including but not limited to a source’s processes and 

operating procedures, raw materials, physical plant layout, and potential environmental 

impacts such as increased water pollution, waste disposal, and energy requirements.

(ii) For purposes of evaluating the economic feasibility of a potential control measure, the 

state may consider factors including but not limited to capital costs, operating and 

maintenance costs, and cost effectiveness of the measure.

(iii) The state must submit to EPA as part of its Moderate area attainment plan a detailed 

written justification for eliminating from further consideration any potential control 

measure identified under §51.1009(a)(2) on the basis of technological or economic 

infeasibility.

(4) The state shall use air quality modeling that meets the requirements of §51.1011(a) 

and that accounts for emissions reductions estimated due to all technologically and 

economically feasible control measures identified for sources of direct PM2.5 emissions 
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and sources of emissions of PM2.5 precursors in the Moderate PM2.5 nonattainment area 

to demonstrate that the area can attain the applicable PM2.5 NAAQS as expeditiously as 

practicable but no later than the end of the sixth year following the date of designation of 

the area, or to demonstrate that the Moderate PM2.5 nonattainment area cannot practicably 

attain the applicable PM2.5 NAAQS by such date.

(i) If the state demonstrates through air quality modeling that the area can attain the 

applicable PM2.5 NAAQS by the end of the sixth calendar year following the date of 

designation of the area, the state shall adopt and implement all technologically and 

economically feasible control measures identified under §51.1009(a)(3) that are 

necessary to bring the area into attainment by such date. The state shall also adopt and 

implement all other technologically and economically feasible measures identified under 

§51.1009(a)(3) that, when considered collectively, would advance the attainment date for 

the area by at least 1 year.

(A) Any control measure identified for adoption and implementation under this paragraph 

that can be implemented in whole or in part by 4 years after the date of designation of the 

Moderate PM2.5 nonattainment area shall be considered RACM for the area. Any such 

control measure that is also a control technology shall be considered RACT for the area.

(B) Any control measure identified for adoption and implementation under this paragraph 

that can only be implemented in whole or in part during the period beginning 4 years 

after the date of designation of the Moderate PM2.5 nonattainment area and the beginning 

of the calendar year containing the applicable attainment date for the area shall be 

considered an additional reasonable measure for the area.
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(ii) If the state demonstrates through air quality modeling that the area cannot practicably 

attain the applicable PM2.5 NAAQS by the end of the sixth calendar year following the 

date of designation of the area, the state shall adopt all technologically and economically 

feasible control measures identified under §51.1009(a)(3) unless the state makes a 

demonstration that one or more such measures, when considered collectively, would have 

minimal effect on reducing ambient PM2.5 concentrations in the area.

(A) Any control measure identified for adoption and implementation under this paragraph 

that can be implemented in whole or in part by 4 years after the date of designation of the 

Moderate PM2.5 nonattainment area shall be considered RACM for the area. Any such 

control measure that is also a control technology shall be considered RACT for the area. 

(B) Any control measure identified for adoption and implementation under this paragraph 

that can only be implemented in whole or in part during the period beginning 4 years 

after the date of designation of the Moderate PM2.5 nonattainment area through the end of 

the sixth calendar year following the date of designation of the area shall be considered 

an additional reasonable measure for the area.

(b) The state shall identify, adopt, and implement control measures, including control 

technologies, on sources of direct PM2.5 emissions and sources of emissions of PM2.5

precursors located outside the moderate PM2.5 nonattainment area, or portion thereof, 

located within the state if doing so is necessary to provide for attainment or will expedite 

attainment of the applicable PM2.5 NAAQS in such area.
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(c) For control measures on sources of direct PM2.5 emissions in the form of source 

emissions limitations, the state shall establish such limitations taking into account the 

filterable and condensable fractions of such emissions. 

§51.1010 Serious area attainment plan control strategy requirements.

[PROPOSED REGULATORY TEXT FOR OPTION #1:

(a) The state shall identify, adopt, and implement control measures, including control 

technologies, on sources of direct PM2.5 emissions and sources of emissions of PM2.5

precursors located in any Serious PM2.5 nonattainment area or portion thereof located 

within the state to yield a control strategy for the area that is more stringent than that 

developed for the area when it was classified as Moderate, and consistent with the 

following:

(1) The state shall identify all sources of direct PM2.5 emissions and all sources of 

emissions of PM2.5 precursors in the nonattainment area in accordance with the emissions 

inventory requirements of §51.1008(b);

(2) The state shall identify all potential control measures to reduce emissions from all 

sources of direct PM2.5 emissions and sources of emissions of PM2.5 precursors in the 

nonattainment area identified under §51.1010(a)(1).

(i) The state shall survey other NAAQS nonattainment areas in the U.S. and identify any 

measures not previously identified by the state during the development of the Moderate 

area attainment plan for the area.
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(ii) The state may elect not to identify potential control measures to reduce emissions 

from any sources of a particular PM2.5 precursor if the state demonstrates that all sources 

of such PM2.5 precursor contribute insignificantly to ambient PM2.5 concentrations in the 

area under §51.1006.

(iii) The state may elect not to identify potential control measures to reduce emissions 

from sources in any source category of direct PM2.5 emissions or emissions of PM2.5

precursors determined to be a de minimis source category under §51.1007.

(3) The state may make a demonstration that any measure identified under 

§51.1010(a)(2) is not technologically or economically feasible to implement in whole or 

in part by the end of the tenth calendar year following the date of designation of the area, 

and may eliminate such whole or partial measure from further consideration under this 

paragraph. 

(i) For purposes of evaluating the technological feasibility of a potential control measure, 

the state may consider factors including but not limited to a source’s processes and 

operating procedures, raw materials, physical plant layout, and potential environmental 

impacts such as increased water pollution, waste disposal, and energy requirements. 

(ii) For purposes of evaluating the economic feasibility of a potential control measure, the 

state may consider capital costs, operating and maintenance costs, and cost effectiveness 

of the measure.

(iii) The state shall submit to EPA as part of its Serious area attainment plan submission a 

detailed written justification for eliminating from further consideration any potential 

control measure identified under §51.1010(a)(2) on the basis of technological or 
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economic infeasibility. The state shall provide as part of its written justification an 

explanation of how its criteria for determining the technological and economic feasibility 

of potential control measures under §51.1010(a)(3)(i)-(ii) are more stringent than its 

criteria for determining the technological and economic feasibility of potential control 

measures under §51.1009(a)(3)(i)-(ii) for the same sources in the PM2.5 nonattainment 

area. 

(4) Except as provided under §51.1010(a)(3), the state shall adopt and implement all 

potential control measures identified under §51.1010(a)(2).

(i) Any control measure that can be implemented in whole or in part by the end of the 

fourth year following the date of reclassification of the area to Serious shall be considered 

a best available control measure for the area. Any such control measure that is also a 

control technology for a stationary source in the area shall be considered a best available 

control technology for the area.

(ii) Any control measure that can be implemented in whole or in part between the end of 

the fourth year following the date of reclassification of the area to Serious and the 

applicable attainment date for the area shall be considered an additional feasible measure.

(5) The state shall use air quality modeling that meets the requirements of §51.1011(b) 

and that accounts for emissions reductions estimated due to all best available control 

measures, including best available control technologies, and additional feasible measures 

identified for sources of direct PM2.5 emissions and sources of emissions of PM2.5

precursors in the area to demonstrate that the area can attain the PM2.5 NAAQS as 

expeditiously as practicable but no later than the end of the tenth calendar year following 
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the date of designation of the area, or to demonstrate that the Serious PM2.5

nonattainment area cannot practicably attain the applicable PM2.5 NAAQS by such date.]

[PROPOSED REGULATORY TEXT FOR OPTION #2:

(a) The state shall identify, adopt, and implement control measures, including control 

technologies, on sources of direct PM2.5 emissions and sources of emissions of PM2.5

precursors located in any Serious PM2.5 nonattainment area or portion thereof located 

within the state to yield a control strategy for the area that is more stringent than that 

developed for the area when it was classified as Moderate, and consistent with the 

following:

(1) The state shall identify all sources of direct PM2.5 emissions and sources of emissions 

of PM2.5 precursors in the nonattainment area in accordance with the emissions inventory 

requirements of §51.1008;

(2) The state shall identify all potential control measures to reduce emissions from all 

sources of direct PM2.5 emissions and sources of emissions of PM2.5 precursors in the 

nonattainment area identified under §51.1010(a)(1).

(i) The state shall survey other NAAQS nonattainment areas in the U.S. and identify any 

measures not previously identified by the state during the development of the Moderate 

area attainment plan for the area.

(ii) The state may elect not to identify potential control measures to reduce emissions 

from any sources of a particular PM2.5 precursor if the state demonstrates that all sources 
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of such PM2.5 precursor contribute insignificantly to ambient PM2.5 concentrations in the 

area under §51.1006.

(3) The state may make a demonstration that a measure identified under §51.1010(a)(2) is 

not technologically or economically feasible to implement in whole or in part by the end 

of the tenth calendar year following the date of designation of the area, and may eliminate 

such whole or partial measure from further consideration under this paragraph. 

(i) For purposes of evaluating the technological feasibility of a potential control measure, 

the state may consider factors including but not limited to a source’s processes and 

operating procedures, raw materials, physical plant layout, and potential environmental 

impacts such as increased water pollution, waste disposal, and energy requirements. 

(ii) For purposes of evaluating the economic feasibility of a potential control measure, the 

state may consider capital costs, operating and maintenance costs, and cost effectiveness 

of the measure.

(iii) The state shall submit to EPA as part of its Serious area attainment plan submission a 

detailed written justification for eliminating from further consideration any potential 

control measure identified under §51.1010(a)(2) on the basis of technological or 

economic infeasibility. The state shall provide as part of its written justification an 

explanation of how its criteria for determining the technological and economic feasibility 

of potential control measures under §51.1010(a)(3)(i)-(ii) are more stringent than its 

criteria for determining the technological and economic feasibility of potential control 

measures under §51.1009(a)(3)(i)-(ii) for the same sources in the PM2.5 nonattainment 

area. 
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(4) The state shall use air quality modeling that meets the requirements of §51.1011(b) 

and that accounts for emissions reductions estimated due to all technologically and 

economically feasible control measures identified for sources of direct PM2.5 emissions 

and sources of emissions of PM2.5 precursors in the area to demonstrate that the area can 

attain the applicable PM2.5 NAAQS as expeditiously as practicable but no later than the 

end of the tenth calendar year following the date of designation of the area, or to 

demonstrate that the Serious PM2.5 nonattainment area cannot practicably attain the 

applicable PM2.5 NAAQS by such date. 

(5) For a Serious PM2.5 nonattainment area which air quality modeling demonstrates that 

the area can attain the applicable PM2.5 NAAQS by the end of the tenth calendar year 

following the date of designation of the area, the state shall adopt and implement all 

technologically and economically feasible control measures needed to bring the area into 

attainment by such date and additionally any other such measures that, when considered 

collectively, would advance the attainment date for the area by at least 1 year..

(i) Any control measure that can be implemented in whole or in part by the end of the 

fourth year following the date of reclassification of the area to Serious shall be considered 

a best available control measure for the area. Any such control measure that is also a 

control technology for a stationary source in the area shall be considered a best available 

control technology for the area.

(ii) Any control measure that can only be implemented in whole or in part between the 

end of the fourth year following the date of reclassification of the area to Serious and the 
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applicable attainment date for the area shall be considered an additional feasible 

measure.]

(b) For a Serious PM2.5 nonattainment area which air quality modeling demonstrates 

cannot practicably attain the applicable PM2.5 NAAQS by the end of the tenth calendar 

year following the date of designation of the area, the state shall identify, adopt, and 

implement the most stringent control measures that are included in the attainment plan 

for any state or are achieved in practice in any state, and can be feasibly implemented in 

the area, consistent with the following:

(1) The state shall identify all sources of direct PM2.5 emissions and sources of emissions 

of PM2.5 precursors in the nonattainment area in accordance with the emissions inventory 

requirements of §51.1008(b).

(2) The state shall identify all potential control measures to reduce emissions from all 

sources of direct PM2.5 emissions and sources of emissions of PM2.5 precursors in the 

nonattainment area identified under §51.1010(a)(1) and not otherwise determined to 

contribute insignificantly to ambient PM2.5 concentrations in the area according to 

§51.1006 or to be de minimis according to §51.1007.

(i) The state shall survey other NAAQS nonattainment areas in the U.S. and identify the 

most stringent measures adopted into any SIP for any NAAQS or used in practice to

control emissions from any non-de minimis source categories.

(ii) The state shall reanalyze any measures previously rejected by the state during the 

development of any Moderate area or Serious area attainment plan control strategy for 

the area, unless the extension request is made at the same time as the Serious area 
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attainment plan required after the area is initially reclassified in accordance with 

§51.1005(b)(5).

(3) The state may make a demonstration that a measure identified under §51.1010(b)(2) is 

not technologically or economically feasible to implement in whole or in part by 5 years 

after the applicable attainment date for the area, and may eliminate such whole or partial 

measure from further consideration under this paragraph. 

(i) For purposes of evaluating the technological feasibility of a potential control measure, 

the state may consider factors including but not limited to a source’s processes and 

operating procedures, raw materials, physical plant layout, and potential environmental 

impacts such as increased water pollution, waste disposal, and energy requirements. 

(ii) For purposes of evaluating the economic feasibility of a potential control measure, the 

state may consider capital costs, operating and maintenance costs, and cost effectiveness 

of the measure.

(iii) The state shall submit to EPA as part of its Serious area attainment plan submittal a 

detailed written justification for eliminating from further consideration any potential 

control measure identified under §51.1010(b)(2) on the basis of technological or 

economic infeasibility.

(iv) The state shall submit to EPA as part of its Serious area attainment plan submission a 

detailed written justification for eliminating from further consideration any potential 

control measure identified under §51.1010(b)(2) on the basis of technological or 

economic infeasibility. The state shall provide as part of its written justification an 

explanation of how its criteria for determining the technological and economic feasibility 
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of potential control measures under §51.1010(b)(3)(i)-(ii) are more stringent than its 

criteria for determining the technological and economic feasibility of potential control 

measures under §51.1010(a)(3)(i)-(ii) and under §51.1009(a)(3)(i)-(ii) for the same 

sources in the PM2.5 nonattainment area. 

(4) Except as provided under §51.1010(b)(3), the state shall adopt and implement all 

control measures identified under §51.1010(b)(2) that may achieve greater emissions 

reductions from any non-de minimis sources of direct PM2.5 emissions or sources of 

emissions of PM2.5 precursors in the area than previously adopted measures have 

achieved and shall achieve attainment as expeditiously as practicable but no later than 5 

years after the applicable attainment date for the area.

(c) The state shall identify, adopt, and implement control measures, including control 

technologies, on sources of direct PM2.5 emissions and sources of emissions of PM2.5

precursors located outside the Serious PM2.5 nonattainment area and within the state if 

doing so will expedite attainment of the applicable PM2.5 NAAQS within the area.

(d) For control measures on sources of direct PM2.5 emissions in the form of source 

emissions limitations, the state shall establish such limitations taking into account the 

filterable and condensable fractions of such emissions. 

§51.1011 Attainment demonstration and modeling requirements.

(a) Nonattainment areas initially classified as Moderate. The attainment demonstration 

due to EPA as part of any Moderate area attainment plan required under §51.1003(a)

shall meet all of the following criteria:
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(1) The attainment demonstration shall show the projected attainment date for the 

Moderate nonattainment area that is as expeditious as practicable in accordance with the 

requirements of §51.1004(a)(1).

(2) The attainment demonstration shall meet the requirements of Appendix W of this part 

and shall include inventory data, modeling results, and emission reduction analyses on 

which the state has based its projected attainment date.

(3) The base year for the emissions inventory required for an attainment demonstration 

under §51.1011(a) shall be one of the 3 years used for designations or another technically 

appropriate inventory year if justified by the state in the plan submission.

(4) The control strategies modeled as part of the attainment demonstration shall be 

consistent with the following as applicable:

(i) For a Moderate area that can demonstrate attainment of the applicable PM2.5 NAAQS 

no later than the end of the sixth calendar year following the date of designation of the 

area with the implementation of RACM and RACT and additional reasonable measures, 

the control strategies modeled as part of the attainment demonstration shall be consistent 

with control strategy requirements under §51.1009(a).

(ii) For a Moderate area that cannot practicably attain the applicable PM2.5 NAAQS by 

the end of the sixth calendar year following the date of designation of the area with the 

implementation of RACM and RACT and additional reasonable measures, the control 

strategies modeled as part of the attainment demonstration shall be consistent with 

control strategy requirements under §51.1009(b).
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(5) The attainment demonstration and supporting air quality modeling should be 

consistent with the most current version of EPA’s PM2.5 attainment demonstration 

modeling guidance.

(6) Required time frame for obtaining emissions reductions. For each Moderate 

nonattainment area, the attainment plan must provide for implementation of all control 

measures needed for attainment as expeditiously as practicable. All control measures in 

the attainment demonstration must be implemented no later than the beginning of the 

year prior to the attainment date, notwithstanding RACM implementation deadline 

requirements in §51.1009.

(b) Nonattainment areas reclassified as Serious. The attainment demonstration due to 

EPA as part of a Serious area attainment plan required under §51.1003(b) shall meet all 

of the following criteria:

(1) The attainment demonstration shall show the projected attainment date for the Serious 

nonattainment area that is as expeditious as practicable in accordance with the 

requirements of §51.1004(a)(2);

(2) The attainment demonstration shall meet the requirements of Appendix W of this part 

and shall include inventory data, modeling results, and emission reduction analyses on 

which the state has based its projected attainment date.

(3) The base year for the emissions inventories required for attainment demonstrations 

under §51.1011(b) shall be one of the 3 years used for designations or another technically 

appropriate inventory year if justified by the state in the plan submission.
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(4) The control strategies modeled as part of the attainment demonstration shall be 

consistent with the following as applicable:

(i) For a Serious area that can demonstrate attainment of the applicable PM2.5 NAAQS no 

later than the end of the tenth calendar year following the date of designation of the area 

with the implementation of best available control measures (BACM), including best 

available control technologies (BACT), and additional feasible measures, the control 

strategies modeled as part of the attainment demonstration shall be consistent with 

control strategy requirements under §51.1010(a).

(ii) For a Serious area that cannot practicably attain the applicable PM2.5 NAAQS by the 

end of the tenth calendar year following the date of designation of the area with the 

implementation of best available control measures (BACM), including best available 

control technologies (BACT), and additional feasible measures, the control strategies 

modeled as part of the attainment demonstration shall be consistent with control strategy 

requirements under §51.1010(b).

(5) The attainment demonstration and supporting air quality modeling should be 

consistent with the most current version of EPA’s PM2.5 attainment demonstration 

modeling guidance.

(6) Required timeframe for obtaining emissions reductions. For each serious 

nonattainment area, the State implementation plan must provide for implementation of all 

control measures needed for attainment as expeditiously as practicable. All control 

measures must be implemented no later than the beginning of the year prior to the 
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attainment date, notwithstanding BACM implementation deadline requirements in 

§51.1010. 

§51.1012 Reasonable further progress (RFP) requirements. 

(a) Consistent with section 172(c)(2) of the Act, the state shall submit in each attainment 

plan for a PM2.5 nonattainment area a plan that demonstrates that the area will achieve, on 

an annual basis, reasonable further progress (RFP) in reducing emissions of direct PM2.5

and any PM2.5 precursors from sources in the area that the state has determined are 

necessary to be controlled in order for the area to attain the applicable PM2.5 NAAQS as 

expeditiously as practicable. The RFP plan shall include all of the following:

(1) A description of each control measure adopted by the state to satisfy the control 

strategy requirements of §51.1009 (for Moderate area attainment plans) or §51.1010 (for 

Serious area attainment plans), as appropriate, and the projected reductions in direct 

PM2.5 emissions and emissions of PM2.5 precursors that each control measure will achieve 

by the projected attainment date for the area.

(2) A schedule for implementing the measures described in paragraph (a)(1) of this 

section. 

(3) An analysis that demonstrates that by the end of the calendar year for each milestone 

date for the area determined in accordance with §51.1013(a), emissions will be at a level 

that reflects generally linear progress in reducing emissions on an annual basis between 

the base year and the attainment year.
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(b) Except as provided under paragraph (c), the RFP analysis required under paragraph 

(a)(3) shall include, at a minimum, a benchmark RFP analysis, and may include an 

alternative RFP analysis, consistent with the following: 

(1) The base year for the RFP emissions inventory shall be one of the 3 years used for 

designations or another technically appropriate inventory year if justified by the state in 

the plan submission.

(2) In the benchmark RFP analysis, the state must identify direct PM2.5 emissions and 

PM2.5 precursors regulated in the control strategy for the area and specify target emission 

reduction levels to be achieved during the milestone years. In developing the benchmark 

RFP analysis, the state must develop emissions inventory information for the area and 

calculate the following: 

(i) For direct PM2.5 emissions and each PM2.5 precursor addressed in the control strategy, 

the full implementation reduction is calculated by subtracting the full implementation 

inventory from the base year inventory.

(ii) The "milestone date fraction" is the ratio of the number of years from the base year to 

the milestone year divided by the number of years from the baseline year to the full 

implementation year.

(iii) For direct PM2.5 emissions and each PM2.5 attainment plan precursor addressed in the 

attainment strategy, a benchmark emission reduction is calculated by multiplying the full 

implementation reduction by the milestone date fraction.
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(iv) The benchmark emission level in the milestone year is calculated for direct PM2.5

emissions and each PM2.5 precursor by subtracting the benchmark emission reduction 

from the base year emission level. 

(v) In comparing inventories between the base year and future years for direct PM2.5

emissions and emissions of PM2.5 precursors, the inventories must be derived for sources 

located within the nonattainment area.

(vi) For purposes of establishing motor vehicle emissions budgets for transportation 

conformity purposes (as required in 40 CFR part 93) for a PM2.5 nonattainment area, the 

state shall include in its RFP submittal an inventory of on-road mobile source emissions 

in the nonattainment area for each milestone year.

(3) The RFP analysis must demonstrate that emissions for the milestone year are either:

(i) At levels that are roughly equivalent to the benchmark emission levels for direct PM2.5

emissions and emissions of PM2.5 precursors addressed in the attainment plan; or 

(ii) At levels included in an alternative RFP analysis that projects generally equivalent 

improvement in air quality by the milestone year as would be achieved under the 

benchmark RFP plan.

(iii) The equivalence of an alternative RFP analysis to the corresponding benchmark 

analysis must be determined by comparing the expected air quality changes from the two 

analyses at the design value monitor location. This comparison must use the information 

developed for the attainment plan to assess the relationship between emissions reductions 
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of the direct PM2.5 emissions and emissions of PM2.5 precursors addressed in the control 

strategy for the area and the ambient air quality improvement.

(c) For an attainment plan submittal that demonstrates that a Moderate PM2.5

nonattainment area cannot practicably attain the applicable PM2.5 NAAQS by the end of 

the sixth calendar year following the effective date of designation of the area with the 

implementation of control measures as required under §51.1009, the RFP analysis 

required under subparagraph (a)(3) of this section shall demonstrate generally linear 

emissions reductions in direct PM2.5 emissions and emissions of PM2.5 precursors 

projected from the Moderate area control strategy determined according to §51.1008 for 

each milestone year.

(d) For a multi-state or multi-jurisdictional nonattainment area, the RFP plans for each 

state represented in the nonattainment area shall demonstrate RFP on the basis of 

common multi-state inventories. The states or jurisdictions within which the area is 

located must provide a coordinated RFP plan. Each state in a multi-state nonattainment 

area must ensure that the sources within its boundaries comply with enforceable emission 

levels and other requirements that in combination with the reductions planned in other 

state(s) within the nonattainment area will provide for attainment as expeditiously as 

practicable and demonstrate RFP consistent with these regulations. 

§51.1013 Quantitative milestone requirements.

(a) Consistent with section 189(c)(1) of the Act, the state must submit in each attainment 

plan for a PM2.5 nonattainment area specific quantitative milestones that demonstrate 
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reasonable further progress toward attainment of the applicable PM2.5 NAAQS in the area 

and that meet the following requirements:

(1) Nonattainment areas initially classified as Moderate.

(i) For an attainment plan submittal that demonstrates that a Moderate PM2.5

nonattainment area can attain the applicable PM2.5 NAAQS by the end of the sixth 

calendar year following the date of designation of the area or earlier with the 

implementation of control measures as required under §51.1009, the state shall submit 

quantitative milestones to be achieved no later than a milestone date of 4.5 years from the 

date of designation of the area.

(ii) For an attainment plan submittal that demonstrates that a Moderate PM2.5

nonattainment area cannot practicably attain the applicable PM2.5 NAAQS by the end of 

the sixth calendar year following the effective date of designation of the area with the 

implementation of control measures as required under §51.1009, the state shall submit 

quantitative milestones to be achieved no later than milestone dates of 4.5 years and 7.5 

years, respectively, from the date of designation of the area.

(iii) The state shall select quantitative milestones that coincide with the milestone due 

dates specified in §51.1013(a)(1)(i) and (ii), as applicable, and that provide for objective 

evaluation of emissions reductions and/or air quality improvements representing progress 

toward attainment of the applicable PM2.5 NAAQS in the area, including, at a minimum, 

a milestone that all control measures identified and adopted as RACM and RACT for the 

area will be fully implemented within 4 years after the date of designation.

(2) Nonattainment areas reclassified to Serious.
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(i) For an attainment plan submittal that demonstrates that a Serious PM2.5 nonattainment 

area can attain a particular PM2.5 NAAQS by the end of the tenth calendar year following 

the effective date of designation of the area with the implementation of control measures 

as required under §51.1010(a), the state shall submit quantitative milestones to be 

achieved no later than milestone dates of 7.5 years and 10.5 years, respectively, from the 

date of designation of the area.

(ii) For an attainment plan submittal that demonstrates that a Serious PM2.5 nonattainment 

area cannot practicably attain a particular PM2.5 NAAQS by the end of the tenth calendar 

year following the date of designation of the area with the implementation of control 

measures required under §51.1010(a), the state shall submit quantitative milestones to be 

achieved no later than milestone dates of 7.5 years, 10.5 years, and 13.5 years, 

respectively, from the date of designation of the area.

(iii) The state shall select quantitative milestones that coincide with the milestone due 

dates specified in §51.1013(a)(2)(i) and (ii), as applicable, and that provide for objective 

evaluation of emissions reductions and/or air quality improvements representing progress 

toward attainment of the applicable PM2.5 NAAQS in the area, including, at a minimum, 

a milestone that all control measures identified and adopted as BACM and BACT for the 

area will be fully implemented within 4 years of reclassification of the area to Serious.

(3) Serious areas that fail to attain by the applicable Serious area attainment date. For an 

attainment plan submittal for a Serious area that failed to attain a particular PM2.5

NAAQS by the applicable Serious area attainment date and is therefore subject to the 

requirements of section 189(d) of the Act and §51.1003(c), the state shall submit 
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quantitative milestones to be achieved no later than a milestone date of 13.5 years from 

the date of designation of the area and every 3 years thereafter until the projected 

attainment date for the area. The state shall select quantitative milestones that coincide 

with the milestone due dates for the area, and that provide for objective evaluation of 

emissions reductions and/or air quality improvements representing progress toward 

attainment of the applicable PM2.5 NAAQS in the area.

(b) Not later than 90 days after the date on which a milestone applicable to a PM2.5

nonattainment area occurs, each state in which all or part of such area is located shall 

submit to the Administrator a milestone report that contains all of the following:

(1) A certification by the Governor or Governor’s designee that the state’s attainment 

plan control strategy, including the RFP plan, is being implemented as described in the 

applicable attainment plan;

(2) A technical demonstration, including calculations, to document completion statistics 

for appropriate milestones and to demonstrate that the quantitative milestones have been 

satisfied and how the emission reductions achieved to date compare to those required or 

scheduled to meet RFP;

(3) An air quality screening analysis to determine if measured air quality progress is 

consistent with the expected air quality improvement target correlated with the RFP 

emissions reductions for the previous 3-year period calculated in accordance with 

§51.1012;

(4) An evaluation of whether the area will attain the applicable PM2.5 NAAQS by the 

projected attainment date for the area; and,
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(5) A description and schedule for any remedial actions the state has taken or will take to 

address any failure to meet a quantitative milestone, including the implementation status 

of contingency measures required under §51.1014(a)(1)(i) for failing to meet RFP.

(c) In the event a state fails to submit a milestone report that meets the requirements of 

§51.1013(b) by the due date or the Administrator determines that the state failed to meet 

a milestone by the milestone date, the state shall submit an attainment plan revision 

within 9 months of the missed due date or the Administrator’s determination of the state’s 

failure to meet a milestone that assures that the state will achieve the next milestone or 

attain the applicable NAAQS by the applicable date, whichever is earlier.

§51.1014 Contingency measure requirements.

(a) The state must include as part of each attainment plan submitted under this subpart for 

a PM2.5 nonattainment area specific contingency measures that shall take effect with 

minimal further action by the state or EPA within 60 days of the Administrator making a 

determination that the area has failed to meet either of the following conditions:

(1) The area failed to meet the RFP requirements of §51.1012 or to submit a milestone 

report due to EPA in accordance with §51.1013(b); or, 

(2) The area failed to attain the applicable PM2.5 NAAQS by the applicable attainment 

date.

(b) The contingency measures adopted as part of a PM2.5 attainment plan shall meet all of 

the following requirements:
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(1) The contingency measures shall consist of control measures that are not otherwise 

included in the control strategy for the area.

(2) The contingency measures shall provide for emissions reductions approximately 

equivalent to 1 year’s worth of reductions needed for RFP, based on the overall level of 

reductions needed to demonstrate attainment divided by the number of years from the 

base year to the attainment year, or approximately equivalent to 1 year’s worth of air 

quality improvement or emissions reductions proportional to the overall amount of air 

quality improvement or emissions reductions to be achieved by the area’s attainment 

plan.

(c) The attainment plan submission shall contain a description of the specific trigger 

mechanisms for the contingency measures and specify a schedule for implementation.

§51.1015 Clean data determinations.

(a) Nonattainment areas initially classified as Moderate. Upon a determination by EPA 

that a Moderate PM2.5 nonattainment area has attained the PM2.5 NAAQS, the 

requirements for the state to submit an attainment demonstration, provisions 

demonstrating that reasonably available control measures, including reasonably available 

control technology for stationary sources, shall be implemented no later than 4 years 

following the date of designation of the area, reasonable further progress plan, and 

contingency measures for the area shall be suspended until such time as:

(1) The area is redesignated to attainment, after which such requirements are permanently 

discharged; or,
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(2) EPA determines that the area has re-violated the PM2.5 NAAQS, at which time the 

state shall submit such attainment plan elements for the Moderate nonattainment area by 

a future date to be determined by EPA and announced through publication in the Federal 

Register at the time EPA determines the area is violating the PM2.5 NAAQS.

(b) Nonattainment areas reclassified as Serious. Upon a determination by EPA that a 

Serious PM2.5 nonattainment area has attained the PM2.5 NAAQS, the requirements for 

the state to submit an attainment demonstration, reasonable further progress plan, and 

contingency measures for the area shall be suspended until such time as:

(1) The area is redesignated to attainment, after which such requirements are permanently 

discharged; or,

(2) EPA determines that the area has re-violated the PM2.5 NAAQS, at which time the 

state shall submit such attainment plan elements for the Moderate nonattainment area by 

a future date to be determined by EPA and announced through publication in the Federal 

Register at the time EPA determines the area is violating the PM2.5 NAAQS.

[ALTERNATIVE PROPOSED REGULATORY TEXT: 

(b) Nonattainment areas reclassified as Serious. Upon a determination by EPA that a 

Serious PM2.5 nonattainment area has attained the PM2.5 NAAQS, the requirements for 

the state to submit an attainment demonstration, provisions demonstrating that best 

available control measures, including best available control technology for stationary 

sources, shall be implemented no later than 4 years following the date of reclassification 

of the area to Serious, reasonable further progress plan, and contingency measures for the 

area shall be suspended until such time as:
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(1) The area is redesignated to attainment, after which such requirements are permanently 

discharged; or,

(2) EPA determines that the area has re-violated the PM2.5 NAAQS, at which time the 

state shall submit such attainment plan elements for the Serious nonattainment area by a 

future date to be determined by EPA and announced through publication in the Federal 

Register at the time EPA determines the area is violating the PM2.5 NAAQS.]
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PART 93--DETERMINING CONFORMITY OF FEDERAL ACTIONS TO 

STATE OR FEDERAL IMPLEMENTATION PLANS

7. The authority citation for part 93 continues to read as follows: 

Authority: 42 U.S.C. 7401-7671q.

Subpart B--[Amended]

8. In §93.153, revise paragraph (b)(1) to read as follows: 

§93.153 Applicability.

* * * * *

(b) * * *

(1) For purposes of paragraph (b) of this section the following rates apply in 

nonattainment areas (NAA’s):

Tons/year

Ozone (VOC’s or NOx):

 Serious NAA’s 50

 Severe NAA’s 25

 Extreme NAA’s 10

 Other ozone NAA’s outside an ozone transport region 100

Other ozone NAA’s inside an ozone transport region:

 VOC 50

 NOx 100

Carbon Monoxide: All maintenance areas 100

SO2 or NO2: All NAA's 100

PM10:

 Moderate NAA's 100

 Serious NAA's 70
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PM2.5 (direct emissions, SO2, NOx, VOC, and ammonia):

 Moderate NAA’s 100

 Serious NAA's 70

Pb: All NAA's 25

(2) For purposes of paragraph (b) of this section the following rates apply in 

maintenance areas:

Tons/year

Ozone (NOx, SO2 or NO2):

All maintenance areas 100

Ozone (VOC's):

Maintenance areas inside an ozone transport region 50

Maintenance areas outside an ozone transport region 100

Carbon monoxide: All maintenance areas 100

PM10: All maintenance areas 100

PM2.5 (direct emissions, SO2, NOx, VOC, and ammonia) 100

All maintenance areas 100

Pb: All maintenance areas 25

* * * * *