Prevention of Significant Deterioration (PSD) 

for Particulate Matter Less Than 

2.5 Micrometers (PM2.5)—Increments, 

Significant Impact Levels (SILs) and Significant 

Monitoring Concentration (SMC)

Response to Comments 
(This page intentionally left blank.)
Prevention of Significant Deterioration (PSD) 

for Particulate Matter Less Than 

2.5 Micrometers (PM2.5)—Increments, 

Significant Impact Levels (SILs) and 

Significant Monitoring Concentration (SMC)

Response to Comments

New Source Review Group

Air Quality Policy Division

Office of Air Quality Policy and Standards

U. S. Environmental Protection Agency

Research Triangle Park, NC 27711

August 2010
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i

Table of Contents

Acronyms and Abbreviations ........................................................................................................iii

Units of Measure............................................................................................................................ iv

1 Introduction............................................................................................................................... 1

1.1 Background........................................................................................................................ 1

1.2 The Commenters................................................................................................................ 1

1.3 Organization of This Document ........................................................................................ 1

2 Comments on PM2.5 Increments ............................................................................................... 6

2.1 Comments on the Legal Basis for PM2.5 Increments......................................................... 7

2.1.1 Comments Supporting and Opposing Sections 166(a) and 166(f) of the Act as the 

Basis for PM2.5 Increments ..................................................................................... 8

2.1.2 Comments on EPA’s Interpretation of the Requirements under the Section 166(a) 

Contingent Safe Harbor Approach – Sections 166(a)-(d) of the Act.................... 15

2.1.3 Comments on EPA’s Interpretation of the Requirements under the Section 166(f) 

Equivalent Substitution Approach ........................................................................ 18

2.2 Comments on Option 1 for PM2.5 Increments – Contingent Safe Harbor Approach for 

Annual and Short-Term Increments ................................................................................ 21

2.2.1 Comments Supporting and Opposing Finalizing Option 1 at the Proposed Levels

............................................................................................................................... 21

2.2.2 Comments on the Proposed Framework for PSD Regulations for PM2.5 and on 

EPA’s Evaluation of the Statutory Factors for Increments................................... 24

2.3 Comments on Option 2 for PM2.5 Increments – Equivalent Substitution Approach for 

Annual Increments........................................................................................................... 38

2.3.1 General Comments on Option 2 ........................................................................... 39

2.3.2 Comments on Option 2A ...................................................................................... 42

2.3.3 Comments on Option 2B ...................................................................................... 43

2.4 Other Comments on Increments...................................................................................... 44

2.4.1 Comments on Baseline Areas............................................................................... 44

2.4.2 Comments on Baseline Dates for the PSD Increments for PM2.5 ......................... 47

2.4.3 Miscellaneous Comments on Increments for PM................................................. 51

3 Comments on the PM10 Increments........................................................................................ 53

4 Comments on Significant Impact Levels (SILs) for PM2.5 ..................................................... 59

4.1 Comments on the Legal Basis for SILs........................................................................... 59

4.2 General Comments on SILs for PM2.5 ............................................................................. 60

4.3 Comments on the Relationship of SILs to AQRVs......................................................... 64

4.4 Comments on the Proposed Levels for SILs ................................................................... 65

4.4.1 General Comments on Proposed SIL Levels........................................................ 65

4.4.2 Comments on Proposed Levels for Class I SILs .................................................. 69

4.4.3 Comments on Proposed Levels for Class II and III SILs ..................................... 74

4.5 Other Comments on SILs ................................................................................................ 83

4.5.1 Comments on SILs for Other Pollutants............................................................... 83

4.5.2 Miscellaneous Comments on SILs........................................................................ 84
ii

5 Comments on the Significant Monitoring Concentration (SMC) for PM2.5 ........................... 87

5.1 General Comments on an SMC for PM2.5 ....................................................................... 87

5.2 Comments on the Legal Basis for the SMC .................................................................... 89

5.3 Comments on the Proposed Level of the SMC for PM2.5................................................ 93

5.4 Other Comments on SMCs.............................................................................................. 97

6 Comments on Transition Issues.............................................................................................. 99

6.1 Comments on the Effective Date for PM2.5 Increments................................................... 99

6.2 Comments on the Implementation Date for PM2.5 Increments...................................... 102

6.2.1 Comments on the Schedule for Submitting SIP Revisions................................. 102

6.2.2 Comments on Coordinating Implementation of PM2.5 Increments in the State and 

Federal Programs................................................................................................ 105

6.2.3 Other Comments on Implementation.................................................................. 107

6.3 Comments on Timing for the Revocation of the PM10 Increments............................... 108

6.4 Comments on a Transition Period for Permit Applicants.............................................. 108

6.5 Comments on Whether SILs and SMCs Should Be Mandatory Program Elements..... 109

7 Comments on Statutory and Executive Order Reviews........................................................ 111

7.1 Comments on Review under the Unfunded Mandates Reform Act .............................. 111

7.2 Comments on Review under Executive Order 13211: Actions that Significantly Affect 

Energy Supply, Distribution, and Use ........................................................................... 112

8 Other Comments on the Proposed Rulemaking.................................................................... 113

8.1 Comments on Secondary PM2.5 ..................................................................................... 113

8.1.1 Comments on Inclusion of Secondary PM2.5 in PSD Modeling Analyses ......... 113

8.1.2 Comments on Models for Secondary PM2.5........................................................ 115

8.2 Comments on Inclusion of the Condensable Fraction of PM2.5 in PSD Modeling 

Analyses......................................................................................................................... 116

8.3 General Comments on the Proposed Rulemaking......................................................... 121
iii

Acronyms and Abbreviations

Act Clean Air Act (42 U.S.C. 7401 - 7671q)

AQRV Air Quality Related Value 

BACT Best Achievable Control Technology

CAA Clean Air Act (42 U.S.C. 7401 - 7671q)

CAIR Clean Air Interstate Rule

CFR Code of Federal Regulations

EGU Electric generating unit 

EPA U.S. Environmental Protection Agency

FLM Federal Land Manager

FR Federal Register

LAER Lowest Achievable Emission Rate

LDC Lowest detectable concentration 

MDL Minimum detection limit

NAAQS National Ambient Air Quality Standards

NAICS North American Industry Classification System

NEI National Emissions Inventory

NESCAUM Northeast States for Coordinated Air Use Management

NO Nitric oxide

NO2 Nitrogen dioxide

NOx Nitrogen oxides

NSR New Source Review

PM Particulate matter

PM10 Particulate matter with aerodynamic diameter less than or equal to a nominal 

10 μm

PM10-2.5 Particulate matter with aerodynamic diameter less than or equal to a nominal 

10 μm but greater than a nominal 2.5 μm

PM2.5 Particulate matter with aerodynamic diameter less than or equal to a nominal 

2.5 μm

PSD Prevention of Significant Deterioration

SIL Significant impact level

SIP State Implementation Plan
iv

Acronyms and Abbreviations

SMC Significant monitoring concentration

SO2 Sulfur dioxide

SOx Sulfur oxides

TSP Total suspended particulate matter

U.S.C. United States Code

Units of Measure

MW Megawatt 

μm Micrometer (one-millionth of a meter)

μg/m3 Microgram per cubic meter

km Kilometer 
1

1 Introduction

1.1 Background 

On September 21, 2007, the U.S. Environmental Protection Agency (EPA or ―we‖)

proposed to amend the requirements for particulate matter (PM) with aerodynamic diameter less 

than or equal to a nominal 2.5 micrometers (μm) (PM2.5) under the Prevention of Significant 

Deterioration (PSD) program by adding maximum allowable increases in ambient pollutant 

concentrations (―increments‖) and two screening tools, known as the Significant Impact Levels 

(SILs) and a Significant Monitoring Concentration (SMC) for PM2.5.

1

 The SILs for PM2.5 were 

also proposed to be added to two other New Source Review (NSR) rules that regulate the 

construction and modification of major stationary sources locating in attainment and 

unclassifiable areas, where the source’s emissions may cause or contribute to a violation of the 

national ambient air quality standards (NAAQS).2 The EPA also proposed to revoke the annual 

increments for PM with aerodynamic diameter less than or equal to a nominal 10 μm (PM10) in 

the PSD rules.

1.2 The Commenters 

The public comment period was originally scheduled to end on November 20, 2007, but 

was extended to January 21, 2008 as the result of two timely requests for an extension. We 

received a total of 35 sets of comments on the proposed rulemaking (not counting duplicates and 

requests for extension of the comment period). These commenters can be classified by general 

type as follows: 9 state/local regulatory agencies and regulatory agency associations, 21 industry 

and industry association commenters, 1 environmental group commenter, 1 federal agency, 

2 tribal commenters, and 1 private citizen. The commenters are listed in Table 1, which appears 

at the end of this section. 

1.3 Organization of This Document

After this introductory section, this document is divided into seven additional sections as 

follows:

 Comments on PM2.5 Increments

 Comments on PM10 Increments

 Comments on Significant Impact Levels (SILs) for PM2.5

 Comments on the Significant Monitoring Concentration (SMC) for PM2.5

 Comments on Transition Issues

 Comments on Statutory and Executive Order Reviews

 Other Comments on the Proposed Rulemaking

 

1 The rulemaking was proposed in the Federal Register, Volume 72, pages 54112 through 54156 (72 FR 

54112). The PSD rules that were proposed for revision are in the Code of Federal Regulations, Title 40, Part 51, 

section 51.166, and Part 52, section 52.21 (40 CFR 51.166 and 52.21).

2 These additional NSR rules are 40 CFR 51.165(b) and 40 CFR part 51, Appendix S.
2

Table 1. List of Commenters

Document Id a b

EPA-HQ-OAR2006-0605-XXXX Title Type of Commenter Notes

0007 Comment and attachment submitted by Julie C. Becker, Assistant General Counsel, Alliance 

of Automobile Manufacturers 

Industry Ass'n Request for 

extension

0008 Comment and attachment submitted by Ted Steichen, Senior Regulatory Analyst, American 

Petroleum Institute (API) 

Industry Ass'n Request for 

extension

0011 Comment and attachment submitted by Joseph S. Casper, Vice President, Environmental, 

Health & Safety, The Brick Industry Association (BIA) 

Industry Ass'n

0012 Comment submitted by B. Sachau Private Citizen 

0013 Comment and attachment submitted by Leon Brasowski, Senior Environmental Manager, 

Covanta Energy Corporation 

Industry

0014 Comment submitted by Marion Loomis, Executive Director, Wyoming Mining Association 

(WMA) 

Industry Ass'n

0015 Comment and attachment submitted by Glenn Shankle, Executive Director, Texas 

Commission on Environmental Quality (TCEQ) 

S/L Agency

0016 Comment and attachment submitted by Howard L. Gilberg, Guida, Slavich & Flores, P.C. on 

behalf of Monarch Ceramic Tile, Inc. and American Marazzi Tile, Inc. 

Industry

0017 Comment submitted by Glenn Shankle, Executive Director, Texas Commission on 

Environmental Quality (TCEQ) 

S/L Agency Duplicate of 

0015

0018 Comment and attachment submitted by Arthur N. Marin, Executive Director, Northeast 

States for Coordinated Air Use Management (NESCAUM) 

S/L Agency Ass’n

0019 Comment and attachment submitted by David J. Shaw, Director, Division of Air Resources, 

New York State Department of Environmental Conservation (NYSDEC) 

S/L Agency

0020 Comment submitted by Peter Manousos, FirstEnergy Corp. (FE) Industry

0021 Comment and attachment submitted by John Paul, Co-Chair, Bill O'Sullivan, Co-Chair, et al., 

National Association of Clean Air Agencies (NACAA) 

S/L Agency Ass’n

0022 Comment and attachment submitted by David A. Finley, Administrator, Air Quality Division, 

Wyoming Department of Environmental Quality (WDEQ) 

S/L Agency 
3

Table 1. List of Commenters

Document Id a b

EPA-HQ-OAR2006-0605-XXXX Title Type of Commenter Notes

0023 Comment submitted by John W. Fainter, Jr., Chief Executive Officer (CEO) and President, 

Association of Electric Companies of Texas (AECT) 

Industry Ass'n

0024 Comment and attachment submitted by Craig Wysong, Environmental Health and Safety 

(EHS) Supervisor, CARBO Ceramics Inc. 

Industry Support 

0046

0025 Comment and attachment submitted by William Skrabak, Chief, Department of 

Transportation and Environmental Services, Division of Environmental Quality, City of 

Alexandria 

S/L Agency 

0026 Comment and attachment submitted by Pamela F. Faggert, Vice President and Chief 

Environmental Officer, Dominion Resources Services, Inc.

Industry 

0027 Comment submitted by Russell A. Wozniak, Environmental Health & Safety (EH&S) 

Operations Regulatory Management, The Dow Chemical Company 

Industry 

0028 Comment and attachment submitted by John A. Paul, Administrator, Regional Air Pollution 

Control Agency (RAPCA) 

S/L Agency Concur with 

0021

0029 Comment and attachment submitted by Andrea Bear Field, Hunton & Williams LLP on behalf 

of Utility Air Regulatory Group (UARG) 

Industry Ass’n

0030 Comment and attachment submitted by Smith Aldridge, Inc. on behalf of Craig Wysong, EHS 

Supervisor, Carbo Ceramics, Inc. 

Industry Duplicate of 

0024

0031 Comment and attachment submitted by Britt D. Banks, Executive Vice President, Newmont 

Mining Corporation and Newmont USA Limited (collectively referred to as "Newmont") 

Industry Support 

0034

0032 Comment and attachment submitted by Ted Steichen, Advisor, American Petroleum Institute 

(API) 

Industry Ass'n 

0033 Comment submitted by Ali Mirzakhalili, P.E., Administrator, State of Delaware Department of 

Natural Resources and Environmental Control (DNREC) 

S/L Agency 

0034 Comment and attachment submitted by Harold P. Quinn, Jr., Executive Vice President and 

General Counsel, National Mining Association (NMA) 

Industry Ass'n

0035 Comment and attachment submitted by Leslie Sue Ritts, Counsel, National Environmental 

Development Association's Clean Air Project (NEDA/CAP) 

Industry Ass'n
4

Table 1. List of Commenters

Document Id a b

EPA-HQ-OAR2006-0605-XXXX Title Type of Commenter Notes

0036 Comment and attachment submitted by David A. Buff, Principal Engineer, Golder Associates 

Inc. (Golder) on behalf of Florida Sugar Industry (FSI) 

Industry Ass’n

0037 Comment and attachment submitted by Terry Behrman, Manager, Alliance of Automobile 

Manufacturers 

Industry Ass'n

0038 Comment and attachment submitted by Frank W. Rambo, Baker Botts, LLP on behalf of 

Class of '85 Regulatory Response Group 

Industry Ass'n

0039 Comment and attachment submitted by C.V. Mathai, President, WEST Associates (Western 

Energy Supply and Transmission Associates, or “WEST”) 

Industry Ass'n

0040 Comment and attachment submitted by David S. Baron, et al, Counsel, Earthjustice, on 

behalf of Environmental Defense and Sierra Club 

Environmental Group

0041 Comment submitted by Brandy Toft, Air Quality Specialist, Leech Lake Band of Ojibwe Tribe

0042 Comment and attachment submitted by William C. Scott, Modrall, Sperling, Roehl, Harris 

and Sisk, PA., on behalf of BHP Navajo Coal Company 

Industry 

0043 Comment and attachment submitted by Barry H. Christensen, Manager Air Quality, 

Occidental Chemical Corporation (OCC) 

Industry 

0044 Comment submitted by John Bunyak, Chief, Policy, Planning and Permit Review Branch, Air 

Resources Division, National Park Service (NPS), U. S. Department of the Interior (DOI) 

Federal Agency

0045 Comment submitted by Barry H. Christensen, Manager Air Quality, Occidental Chemical 

Corporation (OCC) 

Industry Duplicate of 

0043

0046 Comment and attachment submitted by Lee R. Lemke, Executive Vice President, Georgia 

Mining Association (GMA) 

Industry Ass'n

0047 Comment submitted by David A. Finley, Administrator, Air Quality Division, Wyoming 

Department of Environmental Quality (WDEQ) 

S/L Agency Duplicate of 

0022

0048 Comment submitted by John Bunyak, Chief, Policy, Planning and Permit Review Branch, 

National Park Service (NPS), Air Resources Division, United States Department of the 

Interior 

Federal Agency Duplicate of 

0044
5

Table 1. List of Commenters

Document Id a b

EPA-HQ-OAR2006-0605-XXXX Title Type of Commenter Notes

0049 Comment submitted by Karen R. Diver, Chairwoman, Fond du Lac Band of Lake Superior 

Chippewa (the Band) 

Tribe 

0050 Comment submitted by John A. Paul, Administrator, Regional Air Pollution Control Agency 

(RAPCA) 

S/L Agency Duplicate of 

0028

0051 Comment submitted by Maya Rao, P.E., DEE, Chief, Air Division, Mississippi Department of 

Environmental Quality (MDEQ), State of Mississippi

S/L Agency 

0052 Comment submitted by Ali Mirzakhalili, P.E., Administrator, Division of Air & Waste 

Management, Delaware Department of Natural Resources & Environmental Control 

(DNREC)

S/L Agency Duplicate of 

0033

0053 Comment submitted by John W. Fainter , Jr., CEO and President, Association of Electric 

Companies of Texas (AECT)

Industry Ass'n Duplicate of 

0023

a Document ID identifies comments found in Docket No. EPA-HQ-OAR-2006-0605 by document number.

b Documents submitted to the docket together appear in the docket as multiple documents with the same 4-digit "root number" followed by a unique 

extension. For example, comments received via email are generally assigned two numbers. First, the email message itself is assigned the root number 

(e.g., 0015). Second, the attached comment document is assigned an extended number based on the root number (e.g., 0015.1). Additional 

attachments, if any, would follow the same pattern (e.g., 0015.2, 0015.3, etc.). For the purposes of this table and the summary text, we listed all related 

documents under the root number.
6

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7

2 Comments on PM2.5 Increments

2.1 Comments on the Legal Basis for PM2.5 Increments

Comment:

One industry commenter (0016) asserts that EPA failed to expressly state and support its 

legal authority for the PM2.5 increments, offering two possible sources of authority (―contingent 

safe harbor,‖ ―equivalent substitution,‖ or possibly a combination of the two) but never stating its 

legal position with clarity. The commenter agrees with EPA’s assessment that the PM2.5

increments should and must fulfill the legal requirements of the Clean Air Act (CAA or the Act)

(citing 72 FR 54121, column 3), and added that it is the government’s burden of proof to 

establish its legal authority for action. The commenter states that it would be arbitrary and 

capricious to promulgate these regulations for which EPA has no stated legal authority.

The commenter states that EPA discussed in the proposal preamble its belief that the 

NSR permitting process is sufficient to satisfy the requirements of section 165 of the Act (citing 

72 FR 54125, column 1, and 54140-54141). The commenter expresses concern that EPA’s sole 

focus is on major PSD sources, and that EPA failed to consider the impact of NSR permitting for 

non-major (PSD) sources or their consumption of a portion of the allowable increment. The 

commenter believes that this is arbitrary and capricious.

Response:

We acknowledge that the proposal described two possible legal authorities for the two 

different options for establishing increments, but we disagree that these discussions did not 

clearly present the alternative legal bases that EPA was considering for taking action in this 

rule. We also disagree with the comment that our presentation of two sets of legal authority for 

developing the PM2.5 increments was arbitrary and capricious. We clearly described our legal 

authority for developing the 24-hour and annual PM2.5 increments under section 166(a) of the 

Act, which is the basis on which we decided to finalize the rule. First, we expressly stated that 

Option 1 was based on the statutory authority of section 166(a) of the Act. See 72 FR 54123 

(Under the first option, “we would use the authority of section 166(a) of the Act to develop new 

increments for PM2.5”). Second, we provided a discussion of this authority both in general (see

72 FR 54118 – 54119 and 54120 – 54123), and how it would be applied to establish increments 

for PM2.5 (see 72 FR 54119 – 54120 and 54123 – 54136). 

In addition, we described how section 166(f) of the Act might provide alternative 

authority for establishing increments for PM2.5 (see, e.g., 72 FR 54120 - 54121), but will not 

repeat that information in detail here because we did not base the increments in the final rule on 

section 166(f) authority. Comments, and our responses to those comments, regarding support 

for or opposition to the two legal authorities presented in the proposal, and on our interpretation 

of the requirements for PM2.5 increments under each, are presented in the following subsections

contained in this section 2.1.
8

With regard to the commenter‟s concern that EPA focuses only on “major (PSD) 

sources” to satisfy the requirements of section 165 of the Act, we respond that section 165 

clearly applies to major stationary sources and major modifications. The section begins by 

saying, “No major emitting facility...may be constructed...unless....” Section 165 of the Act is 

one component of the statutory requirements for Prevention of Significant Deterioration of Air 

Quality, which provides for a preconstruction review and permitting program for major emitting 

facilities (major stationary sources). We do not deny that emissions from minor sources also 

contribute to the PSD increments and air quality levels associated with the NAAQS, but such 

sources are not the concern of the specific statutory program prerequisites for obtaining a PSD 

construction permit. The commenter has not provided any basis for a conclusion that section 

165 of the Act also pertains to permitting minor sources of air pollution.

2.1.1 Comments Supporting and Opposing Sections 166(a) and 166(f) of 

the Act as the Basis for PM2.5 Increments

2.1.1.1 Comments on Statutory Considerations

Comments Supporting Section 166(a) and/or Opposing Section 166(f):

Two tribal commenters (0041, 0049) and one environmental group commenter (0040)

expressly support our proposed reliance on section 166(a) authority to promulgate PM2.5

increments. Another eight commenters support proposed Option 1, which relies on authority 

under section 166(a) of the Act for developing the PM2.5 increments, but do not explicitly 

express support for the use of section 166(a) authority. The latter comments are summarized in 

Section 2.2.1 of this document.

The two supportive tribal commenters (0041, 0049) support utilizing section 166(a) of 

the Act for developing and setting the PM2.5 increments because they believe that the advantages 

of this approach better suit the tribal and environmental perspectives.

The supportive environmental group commenter (0040) argues that adoption of the rules 

under section 166(a) of the Act is the only legally available option. The commenter asserts that 

section 166(a) plainly applies to PM2.5, because PM2.5 is a pollutant for which NAAQS were

promulgated after August 7, 1977. The commenter indicates that EPA’s rulemaking duty under 

section 166 is not confined to ―new pollutants,‖ but is triggered by post-1977 NAAQS 

promulgations, regardless of whether for new or previously regulated pollutants. The commenter 

adds that limitation of the section 166(a) rulemaking duty to new pollutants would conflict with 

sections 166(c) and (d), which require post-1977 PSD rules to (among other things) provide 

specific measures at least as effective as the statutory increments in section 163, and to fulfill the 

goals and purposes of the PSD program and the Act. The commenter contends that where EPA 

strengthens the NAAQS for a pollutant after 1977, the Agency must adopt stronger increments to 

maintain the same percentage of the NAAQS as the statutory increments, and to ensure that the 

other requirements of sections 166(c) and (d) are met. The commenter notes that in proposing 

increments for PM10 in 1989, EPA read section 166(a) as not being limited to wholly new criteria 

pollutants (54 FR 41218, 41220, October 5, 1989).
9

This environmental group commenter also argues that EPA does not have the authority to 

adopt PM2.5 increments pursuant to section 166(f). The commenter explains that by its terms, 

section 166(f) is limited to authorizing the adoption of PM10 increments as a substitute for the 

statutory PM increments based on total suspended PM (TSP) and does not provide for 

substitution of PM2.5 increments for TSP or PM10 increments. The commenter believes that there 

is no ambiguity in the statute that would allow EPA to infer otherwise.

One industry association commenter (0029) believes that EPA’s analysis of the approach 

based on section 166(a) of the Act (i.e., Option 1) is correct and indicates support for this 

approach. However, this commenter believes that EPA also is authorized to use the equivalent 

substitution approach for developing PM2.5 increments based on section 166(f) (i.e., Options 2A 

and 2B). 

Comments Supporting Section 166(f) and/or Opposing Section 166(a):

Ten commenters (0018, 0019, 0021, 0025, 0028, 0031, 0033, 0034, 0039, 0042) support 

section 166(f) of the Act as the basis for PM2.5 increments. These commenters either explicitly

express support and provide reasons for their support of section 166(f) or indicate their

opposition to the use of section 166(a) of the Act as the basis for PM2.5 increments. Four of the

commenters (0019, 0033, 0034, 0042) assert that section 166(f) provides the only legitimate 

basis for PM2.5 increments under the Act, and three of these (0019, 0033, 0034) specifically 

include the 24-hour PM2.5 increments in this assertion. Another four of the commenters (0018, 

0021, 0031, 0039) believe section 166(f) provides a better basis than does section 166(a), and 

two of the commenters (0025, 0028) simply recommend that EPA use the section 166(f) 

approach.

One state/local agency commenter (0019) who believes that the section 166(f) approach 

is the only legitimate approach under the Act agrees with EPA’s proposed interpretation that 

Congress intended to authorize EPA to update PM increments when another indicator is defined,

and that section 166(f) allows EPA to do so as long as these increments are of equal stringency to 

the prior increments. 

An industry association commenter (0034) similarly concurs with EPA’s position in the 

proposal that it is permissible for the Agency to construe section 166(f) as a continuing grant of 

authority for the Administrator to update the increments for PM whenever a new form of PM is 

adopted as the indicator for the NAAQS. This commenter believes that section 166(f) contains 

principles for updating PM increments, which are summarized in Section 2.1.3 of this document. 

Another state/local agency commenter (0033) does not share EPA’s concern that because 

the 24-hour PM10 NAAQS has not been revoked, section 166(f) is inappropriate to use. The

commenter believes that section 166(f) was included in the 1990 CAA Amendments because 

Congress knew that EPA was struggling with methods for replacing the then-existing TSP 

increments with PM10 increments. The commenter explains that section 166(f) was written to 

facilitate the move to PM10 and that it is broad enough to cover further extension to cover the 

PM2.5 fraction of particulate.
10

An industry commenter (0042) concludes that section 166(f) – not section 166(a) – of the 

Act provides EPA authority to promulgate increments for PM2.5, noting that Congress added 

section 166(f) to the Act to define EPA’s authority to issue a PM increment and to establish the 

framework for EPA’s development of such an increment. The commenter believes that the 

annual PM2.5 increment should be substituted for the annual PM10 increment, but indicates that

EPA should first properly calculate the PM2.5 increment to satisfy the equal stringency 

requirement of section 166(f). The commenter’s comments on the methodology for calculating 

the increments are summarized in Section 2.3 of this document.

A state/local agency association commenter (0021) asserts that section 166(f) is a better 

statutory fit for PM2.5 increments, and states that because EPA retained the 24-hour PM10

NAAQS in its PM NAAQS revision, and has specifically included regulation of PM2.5 in that 

standard, this section provides a more appropriate statutory basis for PM2.5 increments. The 

commenter adds that EPA has continued the PM10 24-hour standard and included the fine PM2.5 

fraction within it; by logical extension, section 166(f), which addresses PM10 increments, can and 

should provide the statutory basis for PM2.5 increments.

Another industry association commenter (0039) also believes that EPA may rely on the

statutory authority of section 166(f) when adopting PM2.5 increments. The commenter notes 

that, from a physical standpoint, PM10 is merely a fraction (subset) of total PM, and PM2.5 is 

merely a fraction of PM10. The commenter argued that it is reasonable to conclude that Congress 

intended to authorize EPA to continue updating the particular matter increments contained in 

section 163 if EPA promulgated a NAAQS for another indicator for particular matter. The 

commenter believes that this statutory grant of authority extends naturally to developing another 

indicator of the same PM air quality standard – that is, merely a smaller fraction of the same 

ambient PM pollutant in the atmosphere – not a new or entirely different air pollutant.

One state/local agency commenter (0019) finds unpersuasive EPA’s argument that it is

not ―substituting‖ increments (as section 166(f) requires for PM10) but rather adding PM2.5

increments to the existing PM10 increments, and that only section 166(a) allows such an 

approach (citing 72 FR 54121). The commenter indicates that if EPA had defined a coarse 

fraction to the particulate standards, then that fraction, together with the PM2.5 standards, would 

form the set of substituted new standards for the existing PM10 standards, and, thus, the 

increments. The commenter asserts that EPA’s argument that there is lack of language in 

section 166(f) for adding other PM indicators while retaining the existing PM10 indicators 

misrepresents the purpose of what Congress intended when it added section 166(f), and also 

conveys an unreasonable expectation of finding such language in a 1990 revision to the Act for 

future prediction of how exactly the PM indicators would progress since that time. The 

commenter adds that the term ―substitution‖ need not mean replacement in total, but rather, 

replacement of portions determined not to serve the originally intended purpose, and this is what 

the PM2.5 increments, in addition to the PM10 increments, will achieve. 

Two state/local agency commenters (0019, 0033) contend that section 166(a) cannot be 

used as the basis for PM2.5 increments because section 166(a) can only be used for a new 

pollutant and PM2.5 is not a new pollutant. One of the commenters (0019) argues that nothing in 

section 166(a) of the Act can be interpreted to allow it to be used as the basis of increments when 
11

EPA revises an existing NAAQS. The commenter explains that, on its face, section 166(a) can 

only be interpreted to apply to pollutants other than PM and sulfur dioxide (SO2) since 

increments for these pollutants were enacted by Congress in section 163 of the Act. The 

commenter adds that it can be argued that Congress intended to have section 166(a) apply to the 

four other pollutants specifically listed there. The commenter disagrees with EPA’s argument 

that it can treat PM2.5 as a new pollutant under section 166(a) of the Act since it has been 

demonstrated that sub-PM2.5 particles have distinctly different health and welfare effects than the 

other forms of PM (i.e., coarse or PM10). The commenter indicates that just as EPA replaced the 

TSP standards by PM10 as a better indicator of health effects, ongoing research has lead to 

establishment of the PM2.5 standards as a better indicator of certain health effects, and it is the 

natural outcome of such research that has enabled EPA to separate the effect of total particulates 

into two fractions with distinct effects. The commenter adds that given that the definition of 

particulates includes a vast conglomeration of solids and liquids, the finding of differing effects 

should not come as a surprise. The commenter explains that as is the case of different pollutants 

having similar effects and are, nonetheless, treated as separate pollutants, the same concept 

should apply to a range or fraction of particulates found to have different effects in establishing it 

as another indicator and not a different pollutant.

One industry association commenter (0034) also believes that section 166(a) of the Act 

does not provide EPA with authority to promulgate its proposed PM2.5 increments. The 

commenter believes that the legal and Congressional history regarding the establishment of PM 

increments show that Congress added section 166(f) to the Act based on the conviction that 

without it, EPA had no authority to revise the PM increments for PM10 (citing and quoting from 

S. Rep. No. 228, 101st Cong., 2nd Sess. 75 (1990), reprinted in 1990 U.S.C.C.A.N. 3385, 3461). 

The commenter concludes that EPA did not have authority in 1987 under section 166(a) to adopt 

PM10 increments, and does not have authority today under section 166(a) to adopt PM2.5

increments.

Response:

We read section 166(a) of the Act to authorize EPA to promulgate pollutant-specific PSD 

regulations meeting the requirements of sections 166(c) and 166(d) of the Act for any pollutant 

for which EPA promulgates a NAAQS after 1977. Most of the pollutants identified in 

section 166(a) (nitrogen oxides (NOx), photochemical oxidants, carbon monoxide) are pollutants 

for which EPA had established NAAQS in 1977 when Congress adopted section 166 of the Act. 

There was no need for Congress to list the other criteria pollutants, SO2 and PM, in 

section 166(a) because Congress had already established increments for these pollutants in 

section 163 of the Act. In addition to requiring regulations for the enumerated pollutants, 

Congress clearly intended to authorize EPA to establish additional pollutant-specific PSD 

regulations, potentially containing increments, for any additional pollutants for which EPA 

promulgated a NAAQS under section 109 of the Act. Furthermore, because section 166(a) refers 

to pollutants for which EPA promulgates NAAQS after 1977, and does not use the phrase 

“additional pollutants,” or “new pollutants” or some other phrasing that limits the scope of the 

provision, EPA concludes that section 166(a) provides authority for EPA to promulgate new 

increments after revising an existing NAAQS (including NAAQS first promulgated before 1977), 

when we find that such action is appropriate. 
12

Moreover, any new increments developed pursuant to section 166(a) have no effect on 

existing increments, as there is no indication therein that an existing increment should be 

revoked or replaced when additional increments are promulgated. This was the situation 

following the promulgation of new NAAQS for PM in 1987 when EPA replaced the old NAAQS 

based on TSP with new ones based on PM10. Had Congress not added new section 166(f) in 

1990, increments for PM10 could have been developed pursuant to section 166(a) of the Act, but 

such increments would have had no effect on the original statutory increments for PM (based on 

TSP). Consequently, seeing no basis for retaining the original increments, Congress added 

section 166(f) which explicitly authorizes EPA to “substitute” PM10 increments for the statutory 

TSP increments defined by Congress.

Accordingly, we believe that PM2.5 should be considered a “new” pollutant for 

regulatory purposes. This position is supported by the fact that we have retained the PM10

indicator for the 24-hour NAAQS and the PM2.5 and PM10 24-hour NAAQS are regulated 

separately – they co-exist, they are classified independently for attainment purposes, they are 

enforced separately, etc. We acknowledge that we considered section 166(f) as a possible option 

for adopting the annual PM2.5 increment as a justification for eliminating the existing annual 

PM10 increments. We recognized that the annual PM10 increments could not simply be 

eliminated in response to revocation of the annual PM10 NAAQS; they are replacement 

(equivalent substitute) increments for the statutory TSP increments. 

Again, when EPA established the PM10 increments in 1993, we were replacing the 

existing statutory increments for TSP in light of the fact that we had eliminated the TSP NAAQS 

(24-hour and annual). The “replacement” approach was necessitated by the fact that the TSP 

increments were set by statute (and remain in the statute) and, therefore, could not simply be 

removed. Instead, using the explicit authority under section 166(f) of the Act, we could adopt 

substitute increments of equivalent stringency that would exist in lieu of the statutory TSP 

increments. Without this statutory authorization, EPA would have been required to retain the 

existing (statutory) TSP increments, while at the same time considering the need for new 

increments based on the PM10 indicator. Moreover, reliance on section 166(f) has implications 

for the baseline dates and baseline areas that we believe would be problematic for 

implementation of the new PM2.5 increments. We have concluded that section 166(a) provides the 

clearest reading of the Act, as well as the most straightforward approach for developing increments for a 

pollutant or pollutant indicator for which there are no statutory increments. 

Our position also relies, to some extent, on the fact that are a number of practical issues 

related to the implementation of the new PM2.5 increments, including issues related to the 

“major source baseline date” and “trigger date” for the PM2.5 increment system, and highlight 

the complexities involved with having to establish and maintain two sets of emissions inventories 

for the 24-hour and annual PM2.5 increments. The section 166(a) approach clearly provides the 

basis for creating new baseline dates for the PM2.5 increments. The relevant comments and our 

responses to these issues are presented in later sections of this document.
13

2.1.1.2 Comments on Alternatives to Increments

Comment:

Two state/local agency commenters (0019, 0025) and one state/local agency association 

commenter (0018) indicate their preference for the section 166(f) approach because it would 

require numerical increments for PM2.5, whereas the section 166(a) approach would allow states 

to develop alternatives to PM2.5 increments. For one of the state/local agency commenters

(0019), the primary concern with the section 166(a) approach is that the proposal allows states to 

substitute other measures in the place of uniform national increments for PM2.5. The state/local 

agency association commenter (0018) similarly notes that not following the section 166(f) 

provisions would potentially open the door to alternatives to numerical increments that at this 

time are undefined and have the potential to decrease national consistency and rigor in the PSD 

program.

Response:

Unlike the section 166(f) approach, the section 166(a) approach, specifically at 

section 166(d) of the Act, allows for the possibility of states to adopt alternatives to increments to 

prevent the significant deterioration of air quality. We acknowledge the potential problems (e.g., 

lack of uniformity) that could be created if some states with approved State Implementation Plan 

(SIP) provisions for PSD (referred to as “SIP-approved states”) elect to develop alternatives to 

PM2.5 increments and receive approval from EPA for those alternative measures. 

Notwithstanding that concern, consistent with our proposal and our decision to finalize PM2.5

increments under section 166(a) authority (for the reasons discussed in the Federal Register 

notice of this final action, this response to comments, and elsewhere in the record for this 

rulemaking), we have also decided to provide states with the authority to adopt alternatives to 

increments for PM2.5 (where an adequate demonstration can be provided), just as they now can 

for nitrogen dioxide (NO2). See Section 2.1.2 of this document for more on the topic of 

alternatives to increments.

2.1.1.3 Comments on Considerations Related to Surface Mining 

Comment:

One industry commenter’s (0031) primary concern is with how EPA addresses issues 

related to coarse, crustal fugitive dust emissions from western surface mining operations in 

developing measures to prevent significant deterioration from PM emissions. The commenter 

provides extensive comments on the history of the PM NAAQS and increments and associated 

litigation. The commenter asserts that fugitive coarse PM (―fugitive dust‖) at ambient levels is 

of little public health or welfare concern, quoting experts in the field. The commenter contends

that the 1987 24-hour PM10 NAAQS was actually set based on the health effects of fine PM, and

that the increments for PM10 set in 1993 using the ―equivalent substitution‖ approach under 

section 166(f) of the Act did not take into account the difference in effects that result from 

sources of fine PM and course PM (such as surface mines). The commenter points out that the 

mining industry has had an ongoing problem with meeting the resulting PM10 increments, and 
14

the commenter regards PM10 as a confounded indicator for coarse PM because it consists of an 

indeterminate mixture of fine and coarse PM.

Based on this background, the commenter (0031) believes that section 166(f) provides 

the most logical avenue for EPA’s revision of the PM increments precisely because it would 

result in the replacement of the annual and 24-hour PM10 increments. The commenter contends 

that this would resolve the issue of how to address multiple forms of PM NAAQS in the PSD 

program, the confounded PM10 indicator used in the existing PM10 increments, and the mining 

industry’s continued inability to meet the PM10 increments (which, the commenter asserts, are 

essentially premised on concerns with fine PM). In addition, the commenter argues that PM2.5

increments alone would fulfill the requirements of section 166(c) of the Act, and that the Act 

does not require that each NAAQS have a matching increment.

While the commenter (0031) believes that section 166(f) of the Act is the preferred 

avenue for adoption of PM2.5 24-hour increments, the commenter also stated that if EPA decides 

to adopt PM2.5 increments pursuant to section 166(a) of the Act, section 166(a) is also the 

required avenue for adoption of new PM coarse increments. The commenter contends that 

because the 2006 24-hour PM coarse NAAQS is, indeed, a new standard (for the first time, EPA 

is directly regulating coarse PM with a NAAQS), EPA must consider whether the existing PM10

increments should be revised in light of the new PM10 NAAQS. Given the emphasis in the new 

NAAQS on concerns in urban areas and the focus on those areas for enforcement, and the 

confounding of PM10 with PM2.5 as found by the court in American Trucking Ass‟n. v. EPA, 175 

F3d 1027 (D.C Cir. 1999), the commenter believes that the relationship of the new PM coarse 

standard to the statutory TSP increments may well have a different outcome than the 1993 PM10

increments that were adopted in reference to the 1987 PM10 NAAQS.

Response:

For the reasons already discussed above, we have decided to finalize PM2.5 increments 

under section 166(a) authority. We note that the alleged difficulties experienced by the surface 

mining industry with the existing PM10 increments will not be affected (relieved or exacerbated) 

by this action, and we do not believe that the considerations related to surface mining should 

drive this decision in light of the factors discussed previously. 

In the final rule, EPA is not setting or amending any increments for PM10 or otherwise 

taking action with respect to PM10 increments. The pre-existing annual and 24-hour increments 

for PM10 are being retained. (This is discussed further in Section 3 of this document.) Similarly, 

EPA is not taking any action with respect to coarse PM in this rule. For these reasons, the 

commenter‟s arguments on what authority must be used to set increments for PM10 and/or 

coarse PM, and that EPA has some obligation to take action with respect to course PM, are not 

on point for this rule. Thus, no substantive response to this comment is needed. Nevertheless, as 

mentioned earlier, it is clear that Congress provided explicit authority under section 166(f) of 

the Act to address increments for PM10, because it intended for such increments to be substitute 

increments for the original statutory increments for PM, which were measured by the TSP

indicator for PM. Thus, the PM10 increments legally supersede the original statutory increments 

for TSP. Had the PM10 increments been developed under section 166(a), which prior to the 1990 
15

Act Amendments was the only authority available for developing new increments, then the 

original statutory PM increments would have remained in effect in addition to the PM10

increments.

2.1.2 Comments on EPA’s Interpretation of the Requirements under the 

Section 166(a) Contingent Safe Harbor Approach – Sections 166(a)-

(d) of the Act

Comments on the Goals and Purposes of Sections 101 and 160 of the Act:

One environmental group (0040) claims that in describing the contingent safe harbor 

approach, EPA incorrectly asserted that PSD rules satisfying the goals and purposes of 

section 160 of the Act will necessarily satisfy all of those in section 101 (citing 72 FR 54122).

The commenter notes that although there is some overlap between the two sections, they are not 

identical. As an example, the commenter points out that section 101 expressly states that a 

primary goal of the Act is to promote pollution prevention, but that goal is not stated in 

section 160. 

Response:

We disagree with the commenter and continue to believe that measures that satisfy the 

specific goals and purposes of section 160 also satisfy the more general purposes and goals 

identified in section 101 of the Act. As we stated in the 2005 NO2 increment rulemaking, the 

overall goals and purposes of the Act listed in sections 101(b) and 101(c) are general goals 

regarding protecting and enhancing the nation‟s air resources and controlling and preventing 

pollution. Because these broad goals are given more specific meaning in section 160, EPA does 

not believe it is necessary to consider them in detail when evaluating whether PSD regulations 

satisfy the criteria in section 166(c). 70 FR 59587 FN 3, October 12, 2005.

Regarding pollution prevention specifically, we believe that this general goal is 

encompassed in, and given more specific meaning by, sections 160(1), 160(2), and 160(4) of the 

Act. These sections spell out the specific purposes under the PSD program for the general 

section 101 goals of controlling and preventing pollution. We believe that any requirement to 

limit or reduce emissions serves to promote pollution prevention, which is often the most cost 

effective means of lowering pollutant emissions.

Comments on Balancing of Factors Applicable under Section 166(c):

One environmental group (0040) states that EPA ―incorrectly and repeatedly asserts‖ that 

sections 101(b)(1) and 160(3) of the Act pronounce goals of promoting economic growth or 

maximizing opportunities for economic growth (citing 72 FR 54122). The commenter notes that 

these phrases appear nowhere in either section. Instead, the commenter believes that both 

sections state only that any growth that does occur must be consistent with protection of air 

quality.
16

The commenter (0040) believes that EPA further errs by asserting that its PSD rules 

should allow some level of economic growth in Class I areas (citing 72 FR 54123). The 

commenter states that this assertion is not only contrary to the statutory purposes, but to the plain 

language of the Act as well. As examples, the commenter offers that where statutory increments 

are consumed or where emissions will cause or contribute to a violation of the NAAQS, the Act 

plainly does not allow for some level of additional growth – at least not via raising or exceeding 

the increment. The commenter concludes that EPA’s notion that the need to satisfy the other 

requirements of section 166 and the other goals and purposes in sections 101 and 160 can never

preclude additional emissions from economic growth unlawfully elevates such growth over all 

other statutory factors.

Response:

The language in section 160(3) of the Act provides that one of the purposes of the PSD 

program is “to insure that economic growth will occur in a manner consistent with the 

preservation of existing clean air resources.” The commenter suggests that this language can 

only be read as if the statutory phrase “economic growth” actually said “any economic growth 

that does occur” such that section 160(3) says “to insure that any economic growth that does 

occur will occur in a manner consistent with the preservation of existing clean air resources.” 

We disagree. The phrasing used by Congress is “to insure that economic growth will occur....” 

Thus, we believe the plain language of the statute supports EPA‟s reading that section 160(3) 

requires a balancing of the goals of (1) economic growth and (2) preservation of existing clean 

air resources. At a minimum, if the language were to be considered ambiguous enough to allow 

the commenter‟s reading, then the Agency‟s interpretation is also a reasonable reading of the 

statutory language.

We do not agree with the commenter‟s interpretation of our position concerning 

economic growth. It is not our intention to say that the other goals and purposes in sections 101 

and 160 can never preclude additional emissions from economic growth. In recognizing the 

need to balance the different goals set forth under sections 101 and 160, economic growth needs 

to be given fair treatment as a specific goal of the PSD program under the Act. That is not to 

say, however, that it must preclude consideration of other specific goals set forth by the Act. We 

acknowledge that under certain circumstances, such as when a source‟s emissions exceed the 

maximum allowable increment, or would have an adverse effect on an air quality related value 

(AQRV) in a Class I area, then economic growth cannot be allowed to occur unless some 

remedial action can be taken to insure that such other goals of the Act are met. 

Comments on States’ Authority to Adopt Alternatives to Increments:

One state/local agency (0015) and one federal agency (0044) expresses support for our 

proposal that states may develop alternatives to increments, provided that the alternative 

approach otherwise meets the requirements of sections 166(c) and (d) of the Act. The federal

agency (0044) suggests that EPA add specific language to that effect to 40 CFR 51.166 to make 

that intention clear in the regulations. An industry commenter (0031) also supports the use of 

alternatives to increments, arguing that given the differences in the states and the issues

confronting those states, such flexibility is key to making the PSD program work. This 
17

commenter further argues that the flexibility to adopt alternative measures recognizes Congress’ 

grant of significant discretion to states in implementing the PSD program, and continues the 

federal-state partnership that is a key underpinning of the Clean Air Act.

Two state/local agency commenters (0019, 0033), two state/local agency association 

commenters (0018, 0021), and one environmental group (0040) oppose revising 40 CFR 

51.166(c)(2) to allow alternatives to PM2.5 increments. These commenters generally warn 

against sacrificing the national consistency of uniform PM2.5 increments.

For two of the commenters (0018 and 0019), the potential for states not to use the 

uniform national increments for PM2.5 was a primary reason for opposing section 166(a) as the 

basis for PM2.5 increments. One of these commenters (0019) adds that alternative increments

would undermine desired national consistency, and EPA has failed to identify any CAA 

programs which would benefit from this approach.

One of the state/local agency association commenters (0021) disagrees with the legal 

interpretation that would remove nation uniformity from PM2.5 increments. The commenter 

(0021) asserts that a nationally inconsistent regulatory approach to PM2.5 in attainment areas 

could result in a patchwork of state PSD regulations – and the exact kinds of economic 

repercussions that Congress wished to avoid. The commenter explains that some states might 

sacrifice clean air for industrial development, and more industrialized states could see the erosion 

of their industrial base as sources move to attainment areas. The commenter adds that varying 

increment-equivalent measures could also result in an uneven playing field for industry and 

could exacerbate difficulties between states experiencing transport problems.

Similarly, one of the state/local agency commenters (0033) objects to this approach

because it would likely lead to a ―mish-mash‖ of state approaches. The commenter believes that 

this would defeat the intention of Congress that there be uniformity in PSD rules to avoid 

economic dissimilarities from state to state which could allow interstate competition for industry 

based upon which state offers the best (least expensive) environmental compliance regulations.‖ 

The environmental group (0040) agrees that the Act allows for other approaches, but 

believes that such approaches must be in addition to the national increments. Specifically, this 

commenter states that although EPA can provide for states to adopt approaches in addition to 

increments in order to fulfill the statutory purposes, the Agency must make clear that states 

cannot adopt approaches that are less protective than the national increments. The commenter 

believes that the rules will not meet this requirement if they allow states to adopt alternative 

approaches that are less protective of air quality values, parks, and natural areas than EPA itself 

has determined are warranted by the statute. Thus, the commenter argues that to the extent that 

EPA is suggesting that it can allow states to adopt PSD programs that do not include the 

minimum federal increments, that position is contrary to the statute.

Response:

As in the 2005 NO2 increment rulemaking, we have decided to codify the basic principle 

that states can seek to use alternative measures in their plans other than increments, without 

defining any specific type of alternative program that would be approved or otherwise creating 
18

standards beyond the requirements of sections 166(c) and 166(d). Instead, in the event that a 

state proposes a specific alternative approach for EPA to approve as part of a SIP, we plan to 

make determinations on a case-by-case basis. In making those determinations, we will address 

the specific alternative measures as states propose them to the Agency in light of the 

requirements of sections 166(c) and 166(d), including whether the alternative program is “at 

least as effective as the increments established in section 163,” as required in section 166(d).

While we acknowledge the potential problems identified by the commenters associated 

with allowing states to adopt alternative approaches to the numerical increments that we are 

establishing, we also note that section 166(d) of the Act provides that the measures to fulfill the 

goals of section 166 “may contain” increments, emission density requirements, “or other 

measures,” which we conclude is properly read to give states latitude to submit a plan that does 

not include maximum allowable increases (increments) for pollutants for which new or revised 

NAAQS are promulgated. Thus, EPA is authorized to provide that states may consider 

alternatives to the increments established in the final rule for PM2.5 increments, SILs, and SMC. 

That said, the statutory authority is not a blank check for states to do as they please, but enables 

states to consider options that may provide a meaningful way for them to manage their air 

resources within the framework allowed by the statutory PSD requirements.

In EPA‟s proposed rule notice, we discussed this issue and proposed to revise the 

existing 40 CFR 51.166(c)(2) to allow states to adopt alternatives to any increments that we 

would codify pursuant to section 166(a) of the Act (72 FR 54123). Consistent with our 

discussion of this issue, we are confirming our interpretation that the authority to adopt 

alternative measures covers any pollutant subject to section 166(a), and revising 40 CFR 

51.166(c)(2) to make it inclusive to all applicable pollutants rather than just NO2.

2.1.3 Comments on EPA’s Interpretation of the Requirements under the 

Section 166(f) Equivalent Substitution Approach

Comment:

One environmental group (0040), who believes that EPA does not have the authority to 

adopt PM2.5 increments pursuant to section 166(f), states that, even if EPA could adopt PM2.5

increments pursuant to section 166(f), the proposal to use the 1993 ―equivalent to statutory 

increments‖ approach is not consistent with the statute. The commenter explains that under that 

approach, EPA would reject the percentage-of-NAAQS approach used under section 166(a) 

based on the argument that Congress could have easily revised the increments in section 163 of 

the Act instead of providing EPA discretion to establish increments following the standard 

provided in section 166(f) (citing 72 FR 54123). The commenter argues that the fact that 

Congress did not revise the increments does not show that Congress meant for the PM2.5

increments to be any less protective (in terms of percentage of the NAAQS) than increments 

adopted under section 166(a). Instead, the commenter believes that by specifying an equalstringency-in-effect test in section 166(f), rather than just stating minimum increments, Congress 

gave EPA authority to establish increments or other measures that would ensure greater 

protection than a strict percentage-of-the-NAAQS approach, where the latter would not in fact

provide the same level of environmental protection as the TSP increments. The commenter 
19

states that EPA offers no rational explanation, and none exists, why Congress would want to 

provide a less protective benchmark for PM10 increments than for increments for other pollutants 

such as nitrogen dioxide.

The same commenter claims that EPA is also wrong in asserting that the ―equal 

stringency in effect‖ language allows the agency to completely ignore the welfare and visibility 

effects of any new PM2.5 increments. The commenter states that such an approach impermissibly 

ignores the statutory purposes set out in section 160 of the Act. According to the commenter, 

whatever discretion EPA may have in construing the ―equal stringency in effect‖ language, it 

cannot adopt a construction that wholly ignores the statutory purposes. The commenter states

that contrary to the Agency’s assertions, the purpose of increments is not merely to allow the 

same the level of degradation in each area of the same classification, but to protect public health 

and welfare, preserve and enhance air quality in natural areas, and achieve the other purposes in 

section 160 of the Act.

Response:

Under the final rule we did not rely upon the authority of section 166(f) of the Act to 

promulgate the new increments for PM2.5. Irrespective of this, we wish to make one point 

concerning the comment that our proposed increments based on the equal stringency approach 

called for under section 166(f) were flawed by the fact that the resulting increments could not be 

considered fully protective of public health, welfare and visibility effects: Neither the original 

statutory increments for PM nor the substitute PM10 increments are intended to be by themselves

fully protective of public health and welfare, as well as visibility effects. A specific PSD 

provision at section 165(a)(3)(B) in the Act requires sources to demonstrate compliance with the 

health and welfare based NAAQS, even when a portion of the increment remains available. In 

addition, another specific provision of the statutory PSD program provides for the protection of 

AQRVs even when the entire Class I increment is not consumed (section 165(a)(5)(d)). 

Comment:

An industry association commenter (0034), who believes that section 166(f) provides the 

exclusive authority for EPA to adopt PM2.5 increments, asserts that section 166(f) contains 

principles for updating PM increments, which are summarized as follows:

 The EPA is authorized but not required to promulgate a new PSD increment for PM 

when it changes the indicator through the NAAQS process.

 The Administrator may substitute a new PM increment for the pre-existing increment, but 

cannot maintain the pre-existing increment and the new increment at the same time.

 Replacement of PM increments must have the same stringency in effect as the one they 

replace, which differs from the percentage-of-the-NAAQS approach used by Congress to 

define the original PM increments.

 The PSD program must have one effective PM increment at all times. The existing 

increments must remain in place until the replacement increments are in place.
20

One state/local agency commenter (0019), who believes that section 166(f) is the only 

legitimate basis for PM2.5 increments, nevertheless does not disagree with the specific numerical 

increments proposed by EPA under Option 1 (which were developed under section 166(a) 

authority) since the method relies upon the same percentage-of-the-NAAQS approach used in the 

Act to establish the SO2 and TSP increments. The commenter believes that EPA is free to define 

the PM2.5 increments under section 166(f) using either the percentage-of-the-NAAQS approach 

or the ―equivalent substitution‖ approach it used when it set the PM10 increments in 1993, 

asserting that the percentage-of-the-NAAQS approach meets the equal-stringency requirement of 

section 166(f) of the Act. The commenter notes that EPA’s final rulemaking for PM10

increments in 1993 stated that both methods were permissible. The commenter adds that the two 

methods yield very similar values for the PM2.5 increments with only slight differences.

Response:

Where EPA determines that a new PM NAAQS is covered by section 166(a) of the Act, 

we interpret the Act to require EPA to promulgate regulations to prevent significant 

deterioration of air quality as a result of emissions of the new PM indicator, which may or may 

not include increments.

As discussed previously, we agree with the commenter that section 166(f) authorizes EPA 

to substitute a PM10 increment for the previously existing TSP increment, but does not allow us 

to maintain both the new and previously existing increments. We also agree that the PSD 

program must have at least one effective PM increment at all times (as long as a PM NAAQS is 

in existence), but we do not believe that the PSD program is limited to increments for only one 

PM indicator at a time. As explained in Section 2.2.1 of this document, we have developed PM2.5

increments under section 166(a) authority in part because we believe that the PSD program 

should include increments for both PM10 and PM2.5. We do not believe that having only one set 

of PM increments for the 24-hour averaging period would necessarily provide adequate 

protection for both the 24-hour PM10 NAAQS and 24-hour PM2.5 NAAQS. For example, we 

continue to believe that the combination of the PM2.5 increments and the retention of the 24-hour 

PM10 increments will help address non-visibility welfare impacts attributable to deposition of 

particles of any size. 

We agree conceptually with the commenter who indicated that the percentage-of-theNAAQS approach may be used to develop PM increments under section 166(f) of the Act, and we

disagree with the commenter who asserted that such an approach is unacceptable. Although we 

elected to use the “equivalent substitution” to finalize the PM10 increments under section 166(f) 

in 1993 and proposed to use that approach for PM2.5 increments if we finalized them under 

section 166(f), we did not in 1993, and we do not now, interpret section 166(f) to preclude the 

use of the percentage-of-the-NAAQS approach.

Any use of the percentage-of-NAAQS approach, however, would need to ensure that the 

resulting increments are of equal stringency in effect to the PM10 (and TSP) increments that they 

would replace under the procedures set forth in section 166(f) of the Act. This “equivalency” 

condition is an important consideration as part of the unique purpose of section 166(f) to 
21

develop “equivalent” increments that effectively replace the original TSP increments defined by 

Congress under the 1977 Clean Air Act. “Equivalence” under section 166(f) of the Act would 

also seem to necessitate retaining the baseline areas and baseline dates that exist for the PM10

increments (and previous TSP increments), a concept that was opposed by most commenters, 

including this one. 

2.2 Comments on Option 1 for PM2.5 Increments – Contingent Safe 

Harbor Approach for Annual and Short-Term Increments

2.2.1 Comments Supporting and Opposing Finalizing Option 1 at the 

Proposed Levels

Proposed Option 1 is termed the ―contingent safe harbor approach‖ and is based on the 

authority of section 166(a) of the Act. See Environmental Defense Fund v EPA, 898 F.2d 183, 

188-189 (D.C. Cir. 1990); Environmental Defense v. EPA, 489 F.3d 1320, 1325-33 (D.C. Cir. 

2007). This approach involves first deriving the increment values based on the percentage of the 

NAAQS used by Congress to derive the original PM increments in terms of TSP, and then 

evaluating whether alternative increments or additional measures are necessary to meet the 

criteria of section 166(c) of the Act. Under Option 1, we proposed to adopt the safe harbor 

increments for PM2.5.

Comments Favoring Option 1:

Seven commenters (0015, 0020, 0023, 0026, 0027, 0029, 0046) support Option 1 for 

PM2.5 increments, either explicitly or implicitly supporting our method of calculating the safe 

harbor increments and their use as the final PM2.5 increments. These commenters do not take a 

position on the use of section 166(a) of the Act as the legal authority for the increments. Two 

additional tribal commenters (0041, 0049) express support for section 166(a) as the basis for the 

increments without explicitly mentioning Option 1.

In support, a state/local agency (0015) points out that the increments under proposed 

Option 1 are consistent with the approach followed to develop the annual NO2 increments. 

Similarly, an industry commenter (0026) notes that the same Option 1 approach was upheld for 

NO2 by the U.S. Court of Appeals in June 2007. 

In addition, one industry commenter (0016) offers conditional support for Option 1, 

indicating that issues regarding EPA’s legal authority to create increments must be addressed for 

this approach to have value. (See this comment and our response in Section 2.1 of this 

document.) If it is established that EPA has sufficiently stated and, in fact, has legal authority for 

the proposed regulation of PM2.5 in the general manners proposed, the commenter would support 

Option 1 for the establishment of PSD increments (and SILs and SMCs).

One environmental group commenter (0040) agrees with the safe harbor increment 

approach under Option 1, but disagrees with our analysis of the adequacy of the safe harbor 

increments, as discussed in Section 2.2.2 of this document.
22

Comments Opposing Option 1:

An industry association commenter (0036) states that the Option 1 approach for

determining the PM2.5 increments under Option 1 is not scientifically supported. The commenter 

believes that basing the PM2.5 increments on the same percentages of the NAAQS that were used 

to set PM10 increments based on the TSP NAAQS ignores the relationship between PM10 and 

PM2.5 emissions, which may be much different than the relationship between TSP and PM10

emissions. Because the ratio of PM2.5 to PM10 emissions is 0.8, it appears that using the 

percentages proposed by EPA would indirectly restrict PM10/TSP emissions and air quality 

impacts to proportionally lower levels than the PM10 increments in order to avoid exceeding the 

PM2.5 increments. The commenter concedes that using the 0.8 factor to set PM2.5 increments 

may seem too high, but asserted that using the Safe Harbor approach would set increments for 

PM2.5 that are too low.

Another industry association commenter (0014) contends that the PM2.5 increments 

should be twice the proposed levels because scientific studies do not support the need for such 

low levels for protection of health and welfare. The commenter believes that increments at the 

proposed levels would jeopardize the goal of providing opportunities for economic growth. The 

commenter expresses concern over EPA’s use of epidemiologic studies and questioned the 

ability of such studies to provide a reliable evaluation of health risks. The commenter claims

that epidemiologic studies are capable of finding association between a substance or exposure 

and a health effect but rarely capable of determining if there is causation, while toxicological 

studies using randomized trials are specifically designed to determine causation. The commenter 

adds that other factors providing evidence for causation include dose-response relationships, 

consistency, and repeatability of studies, which the commenter says are not present in the studies 

cited by EPA. The commenter specifically refers to two studies, acknowledged by EPA to show 

no evidence of a dose-response relationship gradient between PM2.5 and specific health related 

effects. Because the PM2.5 NAAQS is low relative to the PM10 NAAQS, the commenter asserts 

that it is not appropriate to use the same percentages of the NAAQS for establishing increments 

as were used for TSP and PM10. The commenter indicates that higher levels would still provide 

adequate protection of public health with an adequate margin of safety.

Response:

We agree with the commenters who support Option 1. After determining that 

section 166(a) of the Act is the appropriate basis for PM2.5 increment, as previously discussed in

Section 2.1.1 of this document, we evaluated the Option 1 safe harbor increments based on a 

percentage of the NAAQS approach to determine whether they are sufficient to meet the 

requirements of section 166(c) of the Act. Based on our evaluation of the effects of PM2.5 and a 

balancing of the criteria in section 166(c) of the Act (and the incorporated goals and purposes of 

the Act contained in section 101 and the statutory PSD program in section 160 of the Act), we

concluded that the safe harbor increments for PM2.5 (which satisfy section 166(d) of the Act) are 

sufficient to fulfill the criteria in section 166(c) when combined with the other measures in the 

PSD program that we apply to PM2.5.
23

We agree with these commenters about the appropriateness of using Option 1 to develop 

the new increments for PM2.5. As discussed in Section 2.1, we believe that in the proposal we 

explained our preliminary interpretation of the legal authority available for developing the PM2.5

increments under either Option 1, Option 2A or 2B. While we solicited comments on the 

possibility of using the substitution approach for the 24-hour PM2.5 increment, we have

concluded, among other things, that there is no strong justification for using the substitution 

approach over the approach provided via section 166(a) to replace the existing 24-hour PM10

increments since we are not revoking the 24-hour PM10 NAAQS. However, we believe that the 

section 166(a) approach is the most direct and supportable approach for developing all the 

PM2.5 increments. We have not reached any final conclusion as to whether the Option 2 

equivalent substitution approach under section 166(f) of the Act is permissible under the Act.

The opposing commenter is incorrect in claiming that we based the PM2.5 increments on 

the same percentage of the NAAQS that were used to set the PM10 increments. We adopted the 

PM10 increments using the “equivalent substitution” approach set forth under section 166(f) of 

the Act. Under that approach, rather than calculating the PM10 increments as specific 

percentages of the PM10 NAAQS (using the same percentages that Congress used for setting the 

statutory increments for PM and SO2), EPA determined the levels of the PM10 increments that 

could represent an equivalent amount of increment consumed, as if the TSP increments were still 

in effect. See 58 FR 31622, June 3, 1993, at 31626 - 31627. Nevertheless, the commenter is 

correct that, in cases where the ratio of PM2.5 to PM10 emissions is 0.8 for an individual source, 

the source may have to reduce its PM10 emissions more than would otherwise be necessary to 

meet the PM10 increments in order to control its PM2.5 emissions sufficiently to meet the safe 

harbor PM2.5 increments.3 This is because the safe harbor PM2.5 increments are less than 

80 percent of the PM10 increments. For example, the Class II 24-hour PM2.5 safe harbor 

increment (9 μg/m3) is only 30 percent of the corresponding PM10 increment (30 μg/m3).

The underlying reason that the safe harbor PM2.5 increments are so much lower than the 

PM10 increments is that the PM2.5 NAAQS are considerably lower than the PM10 NAAQS.4 This 

is the result of the evolution in our knowledge about the health and welfare effects of PM, in 

particular the effects of the fine PM represented by PM2.5. We believe that it is fitting for PM2.5

increments to reflect our greater knowledge about PM2.5 effects (as embodied in the NAAQS), 

rather than to simply maintain the control level required by the PM10 increments as suggested by 

the commenter. If this results in PM2.5 increments that are more limiting than PM10 increments, 

we believe that this outcome is appropriate in light of our statutory requirement to prevent 

significant deterioration of air quality as it relates to PM2.5.

We disagree with the second opposing commenter‟s recommendation that the increments 

should be twice the proposed (and final) levels. The scientific studies to which the commenter 

referred pertain to studies that EPA used to determine the health-based NAAQS for PM2.5, and 

we do not believe it is relevant to this rule to respond to comments related to the setting of the 

NAAQS. The NAAQS are designed to protect public health and welfare; increments then are 

 

3 Note that the PM10 increment may still be more limiting in areas where much of that increment has 

already been consumed.

4

The 24-hour PM2.5 NAAQS (35 μg/m3) is about 23 percent of the 24-hour PM10 NAAQS (150 μg/m3).
24

intended to insure that air quality in clean areas is not allowed to deteriorate significantly, and 

the PSD regulations insure that any such deterioration does not lead to air pollution levels that 

exceed the levels defined by the NAAQS. 

As discussed previously, we have decided to finalize this rulemaking using the contingent 

safe harbor approach under section 166(a) of the Act. Using this approach, we calculated the 

“contingent safe harbor” increments as percentages of the NAAQS equal to the percentages that 

Congress used to establish the original statutory increments for PM and SO2. These values 

represent the level of effectiveness necessary to satisfy section 166(d) of the Act, and could be 

tightened if necessary based on further analysis to determine if additional measures are 

necessary to fulfill the requirements of section 166(c) of the Act. Thus, under this approach and 

on this record, we do not conclude that it is appropriate to finalize increments at levels any less 

stringent than the contingent safe harbor increments, as the commenter recommends.

2.2.2 Comments on the Proposed Framework for PSD Regulations for PM2.5

and on EPA’s Evaluation of the Statutory Factors for Increments

One environmental group (0040), while believing that the contingent safe harbor 

approach (Option 1) based on section 166(a) authority is the only option that is consistent with 

the Clean Air Act (see Section 2.1.1), nevertheless asserted that EPA’s proposed application of 

that approach to PM2.5 is flawed. This commenter’s extensive comments are summarized in the 

subsections below, along with the comments of a few other commenters who addressed these 

issues.

2.2.2.1 Comments on Stimulating Improved Control Technology

One of the requirements of section 166(c) of the Act, which apply to increments 

developed under section 166(a), is to provide ―a framework for stimulating improved control 

technology.‖ 

Comment:

The environmental group commenter (0040) states that, although increments may 

encourage the use of existing control technologies, EPA has not cited any evidence that 

increments actually stimulate the development of improved technologies. Moreover, the 

commenter asserts that even if increments provide the incentive asserted by EPA, any 

encouragement of improved control technology is wholly incidental and hardly amounts to a 

―framework‖ whose purpose is to stimulate such technology.

Response:

We have concluded that the total program, encompassing increments and the 

requirement to install best available control technology (BACT) controls, does provide an 

appropriate framework to stimulate BACT in such a way that it is not simply “wholly 

incidental,” as the commenter claims. The fact that economic growth in an area must occur 

within a defined amount of allowable air quality deterioration should logically lead to the 
25

application of improved pollution control technology as the amount of deterioration increases, 

and should not be regarded as an incidental consequence. As we stated at proposal, Congress 

envisioned that the increments they originally established would serve as an incentive: “the 

incremental ceiling should serve as an incentive to technology, as a potential source may wish to 

push the frontiers of technology in a particular case to obtain greater productive capacity with 

the limits of the increments.” S. Rep. 95-127 at 18, 30 (3 LH at 1392, 1404). We, too, believe 

that as the available increment in an area becomes smaller, and as states try to preserve some of 

the remaining increments for future growth, it will be necessary to require sources to install 

more stringent controls in that area. Such levels of control ultimately must be considered in 

subsequent BACT evaluations in other PSD areas throughout the country. The increasing 

stringency of control technologies over time, as observed in EPA‟s BACT/Lowest Achievable 

Emission Rate (LAER) Clearinghouse, supports that the PSD program has already stimulated 

development of improved control technology, though this does not conclusively demonstrate that 

because there are undoubtedly a number of factors that could cause such trends. Nevertheless, 

even the need to require a more stringent BACT determination in only a few PSD areas (due to 

dwindling increment availability) necessitates consideration of that level of control for all other 

PSD sources wherever they may decide to locate. In any event, while the commenter generally 

questioned the effectiveness of the increments as an incentive for tightening BACT, they provided 

no evidence that more stringent BACT is not related to the increment system established as an 

integral part of the PSD program. 

2.2.2.2 Comments on Protecting National Parks and Other Special Areas

Another requirement of section 166(c) of the Act is to provide ―protection of air quality 

values, and fulfill the goals and purposes‖ in sections 101 and 160 of the Act. Section 160(2) 

sets forth the purpose ―to preserve, protect, and enhance the air quality in national parks, national 

wilderness areas, national monuments, national seashores, and other areas of special national or 

regional natural, recreation, scenic, or historic value.‖

Comment:

The environmental group commenter (0040) stated that EPA has not complied with 

section 166(c) of the Act because the Agency has not made a finding or demonstrated that the 

PM2.5 PSD rules will (as required by section 160(2) of the Act) preserve, protect, and enhance 

the air quality in parks and special areas. The commenter states that EPA concedes that parks 

and natural areas throughout the nation are already suffering from air quality degradation, 

visibility impairment, and significant ecological damage due to fine particle pollution (citing 72 

FR 54129 (PM2.5 contribution to visibility impairment in Class I areas); EPA, Air Quality 

Criteria for Particulate Matter, Vol. 1, Ch. 4 (2004)). The commenter contends that the proposed 

Option 1 increments and related baselines and trigger dates will allow fine particle-related 

pollution to worsen in parks and natural areas whose air quality is already degraded, and 

questions how the proposal can be found to ―preserve, protect, and enhance‖ air quality in parks 

and natural areas. The commenter believes that EPA has not made such a finding or 

demonstration, but instead offered only vague assertions that the proposed increments would 

―satisfy‖ the statutory factors and that they, along with other programs, would ―help‖ to fulfill 

the statutory purposes. The commenter went on to argue that EPA sought to excuse its failure to 
26

show fulfillment of the statutory purposes by asserting that it cannot develop a uniform, 

quantitative, dose-response relationship between fine particle levels and certain ecosystem 

impacts (citing 72 FR 54134), but that, even if true, such a claim does not excuse the agency 

from satisfying its statutory duty under section 166(c).

Response:

We conclude that the proposal demonstrated that the contingent safe harbor increments, 

in combination with the other aspects of the regulatory framework, fulfill the statutory 

requirements despite the scientific uncertainties. We reiterated that finding in the final

rulemaking. The fact that we did not, at proposal, explicitly state this as a finding does not 

diminish the demonstration made at that time and reiterated in the final rulemaking. The 

analysis presented at proposal, as well as the comments and responses in the balance of this 

section, illustrate that the proposed and final Option 1 increments fulfill the requirement to 

“preserve, protect, and enhance” air quality in parks and other special areas. We explained in 

a previous response that provisions of the statutory PSD requirements separate from the 

requirements for increments are intended to provide protections not necessarily provided by the 

increments.

Comments on “Critical Loads”

Comment:

The environmental group commenter (0040) believes that the relationship between PM2.5

and adverse effects can be quantified to a greater extent than stated by EPA. Regarding acid rain 

and other adverse ecological impacts, the commenter asserted that critical loads can be 

established as a way of quantifying and limiting the PM2.5 contribution to degradation, and noted 

that critical loads are now used by authorities in Europe, have been endorsed by leading North 

American scientists, and have been used by federal land management agencies. To comply with 

section 166(c), the commenter believes that EPA must establish a mechanism to supplement the 

nationally uniform increments with additional measures, including a requirement to establish 

area-specific critical loads or equally protective limits, where necessary to protect and enhance 

air quality in specific parks and natural areas.

Response:

With regard to the critical load concept, we agree conceptually with the commenter that 

critical loads could be used to supplement the existing increments, especially as a means of 

protecting the known sensitive ecosystems within Class I areas. While we disagree that the 

critical loads concept can be used as an effective replacement to increments for limiting air 

quality degradation, we believe that the concept offers considerable promise in helping to 

protect sensitive receptors in specific Class I areas. However, we do not believe that it would be 

appropriate at this time to establish a requirement for area-specific critical loads under the PSD 

program. In our 2005 PSD rule for NO2 increments, we indicated that states could propose 

using information on critical loads as part of their approach for managing air quality in their 

individual SIP-approved PSD programs, but sufficient information was not yet available for EPA 
27

to incorporate the use of critical loads into the national PSD program. See 70 FR 59582, 

October 12, 2005 at page 59613.

The concept of critical loads is useful for estimating how much pollution a particular 

ecosystem can experience on a prolonged basis without showing adverse effects. In addition to 

addressing the opportunity for using critical loads under its NO2 increment rule, EPA has 

addressed the concept of critical loads in the last review of the PM NAAQS and currently in the 

secondary NOx/sulfur oxides (SOx) NAAQS review.5 To date in the United States, critical loads 

have had their primary application in the area of atmospheric deposition of sulfur (S) and 

nitrogen (N). In the last review of the PM NAAQS, EPA found that ambient PM was 

contributing to the total load of pollutants entering the U.S. ecosystem annually. However, the 

review also concluded that there was “insufficient data for the vast majority of U.S. ecosystems 

that differentiate the PM contribution to total N [nitrate] or S [sulfate] deposition to allow for 

practical application of this approach as a basis for developing national standards to protect 

sensitive U.S. ecosystems from adverse effects related to PM deposition.” The 2005 Staff Paper 

for the PM NAAQS, in reaching this conclusion, addressed various important factors, including 

(1) the lack of a long-term, historic database of annual speciated PM deposition rates to 

establish relationships between PM deposition and ecosystem responses; (2) uncertainty in 

predicting the amount of PM deposited to sensitive receptors from measured concentrations of 

PM in the ambient air; and (3) the unique nature of each ecosystems and the current inability to 

extrapolate with confidence any effect from one ecosystem to another. The 2005 Staff Paper 

recommended that EPA give serious attention to the critical load concept and recommended the 

collection of data from a “greater variety of ecosystems over longer time scales to determine 

how ecosystems respond to different loading rates over time.” Staff Paper at page 7-19. 

The review of the secondary NAAQS for NOx and SOx, which is currently underway, is 

evaluating ecological effects due to the deposition of NOx and SOx. The two main targeted 

effects are acidification and nutrient enrichment in both aquatic and terrestrial ecosystems. This 

review is attempting to use critical loads to evaluate the impact of current depositional loads and 

alternative loads in several case study areas. However, as mentioned earlier, the estimation of 

ecosystem critical loads expressed in terms of PM requires long-term ecosystem-level data on 

speciated PM deposition rates for which an adequate database is currently lacking for most sites 

in the United States.

Comments on Deterioration of Air Quality

Comment:

The environmental group commenter (0040) states that EPA provided no review of the 

PM2.5 increments added to baseline concentrations to assess how well the proposed increments 

will prevent significant deterioration of air quality relative to the NAAQS. The commenter 

obtained 2006 PM2.5 monitoring data for all U.S. monitor locations reported on EPA’s AirData 

 

5

In the 2005 OAQPS Staff Paper reviewing the NAAQS for PM, EPA cited the following accepted 

definition of “critical load”: “quantitative estimate of an exposure to one or more pollutants below which 

significant harmful effects on specified sensitive elements of the environment do not occur according to present 

knowledge.” See page 6-45.
28

website and evaluated the 98th percentile value of the 24-hour average PM2.5 concentrations and 

the annual average PM2.5 concentrations and added the EPA’s proposed PM2.5 increments to 

those values. The commenter evaluated the most stringent of EPA’s options: 2 μg/m3(24-hour 

average) and 1 μg/m3(annual average) for Class I areas, and 9 μg/m3(24-hour average) and 4 

μg/m3(annual average) for Class II areas (i.e., Options 1 and 2A).‖

The commenter’s evaluation results to the 24-hour increments and NAAQS are as follows: 

• EPA’s AirData website identified 1,174 PM2.5 ambient air monitoring sites operating in 

2006.

• Fourteen percent of 98th percentile values already exceeded the 24-hour PM2.5 NAAQS 

of 35 μg/m3.

• For the remaining monitors, adding the proposed Class II PM2.5 increment of 9 μg/m3to 

the 2006 98th percentile 24-hr average resulted in exceedances of the 24-hour NAAQS at 

55 percent of those locations. 

Thus, the commenter concluded that for 55 percent of the locations with PM2.5 monitors that 

were not already exceeding the PM2.5 NAAQS, 24-hour average PM2.5 concentrations would be 

allowed under EPA’s proposed PM2.5 increments to degrade up to the level of the NAAQS, 

assuming that the PM2.5 baseline date was triggered in 2006 for all of these areas.

Using the same assumptions, the commenter’s analysis also showed that for 84 percent of 

locations not already exceeding the NAAQS, the 24-hour PM2.5 concentrations would be allowed 

to increase to a level of 30 μg/m3or more. The commenter believes that allowing such levels 

would not be protective of public health, given that we stated at proposal that we had previously 

found that PM2.5 concentrations less than a range of 30 - 35 μg/m3(24-hour average) were 

protective of public health (citing 72 FR 54128). The commenter claims that this analysis shows 

that for the majority of areas, the proposed 24-hour average PM2.5 increments would not prevent 

PM2.5 concentrations from increasing up to the level of the NAAQS and also would not be 

protective of public health. 

The environmental group commenter’s analysis showed similar results for the proposed 

annual PM2.5 increments. Based on the same methodology, the commenter asserted that PM2.5

concentrations would be allowed to increase up to the level of the annual NAAQS in 55 percent 

of the locations that are currently in attainment, and that 87 percent of these sites would be 

allowed PM2.5 concentrations of 12 μg/m3or higher. Again, the commenter believes that 

allowing annual concentrations at or above 12 μg/m3 would not be protective of public health, 

based on our statement at proposal that we had previously found that PM2.5 concentrations less 

than a range of 12 - 15 μg/m3(annual average) were protective of public health (citing 72 FR 

54128).

Response:

We do not believe that increments must be set at levels that ensure that the full amount of 

increment will be available in all locations. The statutory provisions in the PSD program have 

always been clear that a source must demonstrate that it will comply with both the NAAQS and 
29

increments for any pollutant. Consistent with Congressional intent, the PSD program does not 

allow a source to violate the NAAQS just because its emissions will not cause the increments to 

be exceeded. If the increments were to be developed in such a way that all areas, taking into 

account current ambient air quality status, would be able to use the full amount of increment, 

then the increment levels would have to be unnecessarily stringent in areas that are substantially 

cleaner than levels allowed by the NAAQS. 

Congress recognized that all areas of the country might not be able to use the full amount 

of increment when they provided provisions within the Act requiring that both the NAAQS and 

increments must continue to be met at all times. In areas where the full amount of increment is 

not available due to levels of pollution approaching the NAAQS, emissions reductions may be 

required to allow for economic growth to occur within the limits allowed by the increments. 

Hence, we do not believe it is reasonable to unduly restrict economic growth in cleaner areas by 

setting more restrictive increments to help maintain air quality levels below the NAAQS in areas 

which are currently only marginally attainment.

 

In addition, we disagree with the commenter‟s assertion that the increments will not 

protect public health. In setting the PM2.5 NAAQS at 35 μg/m3(24-hour) and 15 μg/m3 (annual), 

EPA concluded that these levels protect public health with an adequate margin of safety. 

Regardless of the level at which the increments are set, no source is permitted to cause the 

NAAQS to be exceeded. That is, the upper bound on the permissible concentration of PM2.5 is 

determined by the increment or the NAAQS, whichever is more restrictive in each particular 

case. Thus, the entire framework of the PM2.5 regulations, including the safe harbor increments, 

is protective of public health. In asserting otherwise, the commenter has misconstrued our 

statements in this regard. 

In the proposal preamble‟s section on the health effects of PM2.5 (72 FR 54127 – 54128), 

we discussed the fact that we considered setting the 24-hour NAAQS in the range of 30 to 35 

μg/m3 and the annual NAAQS in the range of 12 to 15 μg/m3. However, we concluded in setting 

the NAAQS that 35 μg/m3(24-hour) and 15 μg/m3(annual) are protective of public health with 

an adequate margin of safety. We did not say, nor do we believe, that PM2.5 concentrations must 

be below 30 μg/m3(24-hour average) or 12 μg/m3(annual average) to protect public health.

Comments on Visibility

Comment:

The environmental group commenter (0040) believes that there is a quantifiable 

relationship between visibility impairment and PM2.5 levels, citing the proposal discussion (72 

FR 54135), as well as the most recent Criteria Document and Staff Paper for PM2.5. The 

commenter points out that at proposal (citing 72 FR 54135), EPA observed that the proposed 

Class II short-term safe harbor increment of 9 μg/m3, if combined with the estimated daily 

background levels in most areas (i.e., 10 μg/m3), would be below the minimum values 

recommended in the 2005 Staff Paper for the secondary short-term standard for PM2.5 (which 

was 20 μg/m3). Rather than supporting the adequacy of 9 μg/m3as an increment level to protect 

visibility, the commenter believes that this shows that the safe harbor increment is inadequate 
30

because consumption of an increment of 9 μg/m3combined with background levels alone would 

cause an area to reach within 1 μg/m3of the staff-recommended value of 20 μg/m3. The 

commenter adds that most areas would have PM2.5 pollution from motor vehicles and stationary 

sources in concentrations substantially greater than background levels, easily placing these areas 

above 20 μg/m3(citing the 2005 Staff Paper at 2-77).

The environmental group commenter (0040) also asserts that the safe harbor PM2.5

increments will not be sufficient to protect visibility in parks and other natural areas. According 

to the commenter, we stated at proposal that a 24-hour average PM2.5 concentration of 20 μg/m3

correlates to a visual range of approximately 25 to 35 kilometers (citing 72 FR 54129). The 

commenter contends that this visual range distance falls far short of what the National Park 

Service considers to be good visibility for national parks, adding that the National Park Service 

has stated that visibility used to be 90 miles (145 km) on average in eastern parks, and 140 miles 

(225 km) on average in western parks.6 The commenter stated that the safe harbor increments 

would allow parks and other natural areas to experience PM2.5 pollution that is correlated with a 

25–35 km visual range.

A federal agency commenter (0044) states that the proposed 24-hour Class I increment 

(2 μg/m3) would not be protective of AQRVs, particularly visibility. This commenter notes that 

the National Park Service uses a 5 percent change in light extinction from estimated natural 

conditions as the threshold for ―adverse impacts‖ to Class I visibility. The commenter indicates

that depending on the constituents of the ambient PM2.5 and the humidity, a concentration of 

2 μg/m3in a typical Class I area would result in a change in light extinction ranging from 13 to 

80 percent in the Western United States and from 8 to 50 percent in the Eastern United States

(depending on the makeup of the PM and the humidity) and, therefore, would likely constitute 

―adverse impacts‖ to Class I visibility. While acknowledging that the federal land manager 

(FLM) may still determine that the visibility in the Class I area is adversely affected by an 

increase in concentration that is less than the increment, this commenter pointed out that we 

stated at proposal that ―generally speaking an increment should not be so large that it routinely 

results in substantially more pollution in Class I areas than is generally acceptable under the 

AQRV approach‖ (citing 72 FR 54135). The commenter concluded that the proposed 24-hour 

PM2.5 increment does not meet this test and recommended that EPA set a lower PM2.5 24-hour 

increment.

Response:

 The visibility impairment issue is more complex than suggested by the commenters. In 

addition to predicting what the maximum ambient change in air quality is for a particular area, 

a visibility impairment assessment considers such things as the frequency, magnitude, and 

duration of visibility impacts in order to conclude that an adverse impact will occur. 

In addition, the environmental group commenter misconstrued the illustration we 

included in the NPRM. We noted that the lowest level we considered as a secondary PM2.5

 

6 The commenter cited http://www.nps.gov/shen/naturescience/visibility_and_haze.htm for historic 

visibility in national parks.
31

NAAQS was 20 μg/m3, which was considered to address visibility issues in urban areas. We also 

noted that in most areas, the estimated 98th percentile of daily background concentrations is less 

than 10 μg/m3. In adding the Class II safe harbor increment (9 μg/m3) to the 98th percentile of 

background levels, we were simply showing that even in the worst case, the combination of the 

safe harbor increment and background PM2.5 would not exceed the most stringent level we 

considered for the secondary NAAQS. The commenter presented this rough, worst-case 

calculation as if it represented the typical situation that would result from the safe harbor 

increments. In addition, the commenter‟s statements do not apply to parks and special areas 

that are classified as Class I areas because the safe harbor increments for such areas are much 

lower.

The federal agency commenter appears to have identified a worst-case scenario in terms 

of increment concentrations, and although we agree with the visibility impacts related to those 

concentrations discussed in the comment, we do not believe the proposed increment level 

compromises the protection of visibility or other AQRVs. Although the “AQRV test” uses 

5 percent light extinction as a screening threshold, the determination of adverse impact is made 

on a case-by-case basis taking into account the geographic extent, intensity, duration, frequency, 

and time of visibility impairment and how these factors correlate with visitation to the Class I 

area. The suggestion that the 5 percent threshold is routinely exceeded by PSD sources or that 

an absolute worst-case scenario is occurring to the geographic extent, intensity, duration, and 

frequency that would warrant an adverse impact determination is unsupported, especially 

considering the relatively few adverse impact determinations that have been made in the past. It 

is, however, important to note that the AQRV analysis is independent of the PSD increment 

analysis; whether or not the increment is projected to be exceeded does not determine the need 

for an AQRV analysis. The determination that a facility does or does not cause an adverse 

impact on a Class I area is not solely contingent upon the PSD increment, so we do not believe 

that lowering the proposed increment is necessarily more protective of the AQRV.

With respect to these two commenters‟ concerns about visibility protection, we continue 

to believe that the increments cannot be expected to be the sole means of protecting various 

welfare concerns. At proposal, we stated that “visibility protection in Class I areas is more 

adequately provided by the AQRV process.” Congress defined AQRVs to specifically include 

visibility and left it for the FLMs to define other special attributes of Class I areas that 

warranted special protection. (See additional discussion of AQRV review below.) We also noted 

that Congress has established several visibility programs that target emissions reductions to 

achieve desired visibility benefits. See 72 FR 54135. Collectively, these protective programs, 

along with the totality of the PSD program, offer an effective means of addressing unique local 

problems that cannot be addressed solely by uniform national increments.

Comment:

The environmental group commenter (0040) states that we cited the regional haze 

program as a justification for adopting less protective PSD rules (referring to 72 FR 54135), but 

the commenter points out that the haze program applies only to Class I areas and does not apply 

at all to the majority of the nation, which is Class II. The commenter further notes that we stated 

at proposal that ―some State and local governments have also developed programs to improve 
32

visual air quality in specific urban areas‖ (citing 72 FR 54135), and points out that we gave no 

specific information on such programs, nor any information about the visibility protection that 

they provide beyond that provided by the proposed increments. The commenter requests that we 

identify the specific state and local programs, and that we specify how much visibility protection 

such programs are providing.

Response:

The commenter is correct that the regional haze program directly addresses only Class I 

areas. These are the areas that Congress defined as deserving of the most protection under 

PSD, including the visibility protection provisions in subpart 2 of title I, part C of the Act, which 

is the statutory basis for the regional haze program. While Class I areas are the target for the 

regional haze program, we believe that many areas of the nation will receive collateral visibility 

benefits from this program. As emissions of the pollutants that cause regional haze are reduced, 

many areas in the paths of transport will benefit. In addition, as discussed previously, PSD 

applicants must prepare an analysis of “other impacts,” including visibility impacts, in areas 

other than Class I areas. 

Regarding state and local visibility programs, the 2005 Staff Paper EPA described 

several existing programs designed to improve visual air quality in urban areas. These 

programs were located in Denver, CO; Phoenix, AZ; and Lake Tahoe, CA. Also, the states of 

California and Vermont have each established standards to protect visibility. See the 2005 Staff 

Paper, pages 6-17 through 6-23. “Review of the National Ambient Air Quality Standards for 

Particulate Matter: Policy Assessment of Scientific and Technical Information. OAQPS Staff 

Paper” EPA-452/R-05-005a, December 2005.

Comment:

The environmental group commenter (0040) cited the proposal (72 FR 54135) where we 

said that the use of ―distinct PM increments for visibility protection is not the most effective 

means of addressing the visibility problem.‖ The commenter believes that this claim is based on 

false premises, including the idea that other programs effectively protect visibility nationwide, 

and the idea that the only option is a ―distinct‖ PM increment for visibility protection. As to the 

latter, the commenter stated that EPA can strengthen the safe harbor increment to ensure 

visibility protection and need not adopt a separate ―visibility‖ increment. In addition, the 

commenter asserted that EPA has ignored the statutory mandate that the PSD rules fulfill the 

statutory goals and purposes, and that we cannot shirk that statutory duty merely because we 

claim some other type of action would be ―more effective.‖ 

Response:

We continue to believe that Class I area visibility protection under the PSD program is 

appropriately addressed via the AQRV process, which is discussed in greater detail below. 

Briefly stated, Congress explicitly included “visibility” as an AQRV for which FLMs would have 

an affirmative responsibility to protect in Class I areas under their jurisdictions. Where the 

FLM successfully demonstrates that there would be an adverse impact on the AQRV (e.g., 
33

visibility), a state cannot issue a PSD permit, even when the source‟s emissions do not violate the 

PM2.5 increments. In addition, we continue to believe that the “other impacts analysis” 

(including analysis of visibility) in non-Class I areas is the appropriate means of addressing 

visibility protection in these areas, as envisioned by Congress when it enacted the PSD 

provisions of the Act.

As a result, we do not believe it is necessary to create a distinct increment (e.g., with a 

different averaging period) or to lower the contingent safe harbor increments to protect visibility 

in urban, non-urban, or Class I areas across the United States. We reach this conclusion in 

proper consideration of the other, more direct approaches being used to address visibility 

problems in the United States. The primary such approach, the regional haze program, is within 

the PSD framework for PM2.5. Note that part C of title I of the Act, “Prevention of Significant 

Deterioration of Air Quality,” includes subpart 2, which is the statutory basis for the regional 

haze program. Regarding our consideration of other state and local visibility protection 

measures that are outside the PSD framework, we do not believe it is reasonable to disregard 

these area-specific measures that focus on the preferences of individual communities where a 

uniform national increment for visibility protection generally cannot.

In addition, as explained earlier, it should be recognized that increments cannot by 

themselves be relied on to protect against visibility impairment problems because increments 

define a maximum allowable increase of pollution above a baseline concentration which, in 

itself, could be at or near concentrations that represent poor visibility conditions. Congress 

explicitly included special provisions to protect AQRVs in the Act; by utilizing the AQRV 

process, FLMs are provided with a mechanism for directly relating to air quality levels 

associated with any existing or potential visibility problem. The AQRVs afford a level of 

protection that can be used even when a source‟s emissions would fall within an acceptable 

range for increment consumption (that is, the source‟s emissions increase would not consume the 

remaining increment).

Comments on Extending AQRV Review to PM2.5

Comment:

Two state/local agency commenters (0015, 0022) support our proposals to develop PM2.5

increments for Class I areas and to apply the requirement to evaluate impacts on AQRVs in 

Class I areas to PM2.5 review. However, the environmental group commenter (0040) indicates

that this review by FLMs does not and cannot assure the prevention of all significant PM2.5-

related deterioration because it applies only to the construction or modification of very large 

stationary sources (e.g., factories and power plants) affecting Class I areas. This commenter 

points out that Class I areas do not include Bureau of Land Management wilderness and 

wilderness study areas (encompassing more than 15 million acres), 341 of the nation’s 390 

national park units (only 49 national parks are Class I), and many U.S. Forest Service lands 

(including a number of wilderness areas). The commenter adds that FLM review does not help 

to fulfill section 160(2)’s goal of preserving and protecting air quality in ―other areas of special 

national or regional natural, recreational, scenic, or historic value,‖ such as state and local parks, 

wildlife refuges, recreation areas, lakes, and historic areas, none of which are Class I areas. In 
34

addition, the commenter notes that FLM review does not apply to emissions increases from 

sources of PM2.5 and precursor pollution other than major stationary sources, such as motor 

vehicles and non-major industrial sources (which are sources that emit substantial amounts of 

PM2.5 and precursors). Alabama Power v. Costle, 636 F.2d 323, 362 (D.C. Cir. 1979) (expressly 

recognizing that ―[s]ignificant deterioration may occur due to increased emissions from 

unregulated minor sources.‖).

The environmental group commenter (0040) also asserts that FLM review is of limited 

reach even where it does apply. Under the current PSD regulations, a state must consider an 

FLM’s objections and must justify its decision in writing when it disagrees with those objections, 

but the state can still issue a PSD permit over those objections unless emissions are predicted to 

cause an exceedance of the applicable increment. The commenter believes that, given these 

limitations, EPA cannot plausibly claim that the existing provision for FLM review ensures the 

preservation, protection, and enhancement of air quality for parks and natural areas throughout 

the nation as required by section 160(2) of the Act.

Response:

In our rulemakings addressing PSD for NOx, EPA extended the AQRV review procedures 

set forth in 40 CFR 51.166(p) and 52.21(p) to cover NO2. These AQRV review procedures were 

established based on section 165(d) of the Act, and they were originally applied only in the 

context of the statutory increments for PM and SO2. However, because they also address many 

of the factors applicable under section 166(c) of the Act, EPA also applied them to NOx through 

regulation. In this final rule, we are amending the existing PSD regulations to extend, as 

proposed, the AQRV review procedures to include PM2.5 by explicitly including PM2.5 in the 

regulatory text that now simply references “particulate matter.” See revised 40 CFR 

51.166(p)(4) and 52.21(p)(5).

Section 165(d) creates a regulatory structure in which the FLM and reviewing authority 

must review the impacts of a proposed new or modified source‟s emissions on AQRVs. The Act 

assigns to the FLM an “affirmative responsibility” to protect the AQRVs in Class I areas. This 

is in notable contrast to the reviewing authority‟s responsibility for protecting the increments –

including Class I increments. The FLM may object to or concur in the issuance of a PSD permit 

based on the impact, or lack thereof, that new emissions may have on any affected AQRV that the 

FLM has identified and for which information is available to the general public. If the proposed 

source‟s emissions are shown not to cause or contribute to a violation of a Class I increment, the 

FLM may still prevent issuance of the permit by demonstrating to the satisfaction of the 

reviewing authority that the source or modification will have an adverse impact on AQRVs. 

Section 165(d)(2)(C). On the other hand, if the proposed source is shown to cause or contribute 

to a violation of a Class I increment, the reviewing authority (state or EPA) shall not issue the 

permit unless the owner or operator demonstrates to the satisfaction of the FLM that there will 

be no adverse impact on AQRVs.7 Thus, the showing of compliance with the increment 

 

7

Even if such a waiver of the Class I increment is allowed upon a finding of no adverse impact, the source 

must comply with such emissions limitations as may be necessary to ensure that alternative increments specified in 

the rules for SO2 or PM are not exceeded. The alternative increments are generally at the level of the Class II 

increments, with the lone exception being a more restrictive 3-hour increment for SO2. Section 165(d)(2)(C)(iv). 
35

determines whether the FLM or the permit applicant has the burden of satisfactorily 

demonstrating whether or not the proposed source‟s emissions would have an adverse impact on 

AQRVs.8 In any event, the FLM plays an important and material role by raising these issues for 

consideration by the reviewing authority, which in the majority of cases will be the state.

Extending the AQRV review procedures of the PSD regulations to PM2.5 helps to provide 

protection with respect to potential adverse effects from PM2.5 for parks and special areas 

(which are generally the Class I areas subject to this review) not afforded by the increment 

system alone. We believe the factors applicable under section 166(c) of the Act can be fulfilled 

when the review of AQRVs is applied in conjunction with increments and other aspects of our 

PSD regulations. In those cases where the increment is not violated and the reviewing authority 

agrees that a proposed project will adversely affect AQRVs, the parks and other special areas 

will be protected by denying issuance of the permit or by requiring the applicant to modify the 

project to alleviate the adverse impact. 

We read the legislative history to show that Congress intended the AQRV review 

provisions of section 165(d) to provide a special layer of protection, beyond that provided by 

increments. The Senate committee report stated the following: “A second test of protection is 

provided in specified federal land areas (Class I areas), such as national parks and wilderness 

areas; these areas are also subjected to a review process based on the effect of pollution on the 

area‟s air quality related values.” S. Rep. 95-127, at 17, 4 LH at 1401. As we stated in the NO2

increment rule, we believe the term “air quality values” should be given the same meaning as 

“air quality related values.” Legislative history indicates that the term “air quality value” was 

used interchangeably with the term “air quality related value” (AQRV) regarding Class I lands.9

The commenter is correct that the FLM (or AQRV) review applies only to Class I areas, 

and not to other “special” areas such as the numerous state and local parks and some other 

areas that could be seen as being covered by the protective purposes of section 160(2) of the Act. 

This level of coverage by FLM review to protect AQRVs was established by Congress when it 

enacted the PSD program, including the purposes set out in section 160(2). Thus, we conclude 

that Congress believed that the special areas not designated as Class I areas were properly 

addressed by the other elements of the PSD program. One such element that we believe is 

 

The EPA made this provision applicable to the PSD provisions for NOx at the level of the NO2 Class II increment 

(53 FR 3704; 53 FR 40656) and substituted the PM10 Class II increments for the statutory alternative PM 

increments, which were based on TSP (58 FR 31622). This final rule expands this provision to include the PM2.5

Class II increments as well. See 40 CFR 51.166(p)(4) and 52.21(p)(5).

8

In response to concerns that Class I increment would hinder growth in areas surrounding the Class I 

area, Congress established Class I increments as a means of determining where the burden of proof should lie for a 

demonstration of adverse effects on AQRVs. See Senate Debate, June 8, 1977 (3 LH at 725).

9

See S. Rep. 95-127, at 12, reprinted at 3 LH at 1386, 1410 (describing the goal of protecting “air quality 

values” in “Federal lands – such as national parks and wilderness areas and international parks,” and in the next 

paragraph and subsequent text using the term “air quality related values” to describe the same goal); id. at 35, 36 

(“The bill charges the Federal land manager and the supervisor with a positive role to protect air quality values

associated with the land areas under the jurisdiction of the [FLM]” and then describing the statutory term as “air 

quality related values”). H.R. Report 95-564 at 532 (describing duty of Administrator to consider “air quality 

values” of the tribal and state lands in resolving an appeal of a tribal or state redesignation, which is described in 

the final bill as “air quality related values”).
36

especially helpful for satisfying the requirements of section 166(c) in Class II and Class III 

areas, including the types of areas described by the commenter, is the requirement for sources to 

conduct an “additional impacts analysis,” which includes an analysis of the impacts on 

visibility, soils, and vegetation of the proposed source and associated growth, regardless of the 

classification of the area impacted by the source. [See 40 CFR 51.166(o) and 52.21(o).] While 

the additional impacts analysis typically is not as intensive a review as the AQRV analysis 

required in Class I areas, it contributes to satisfying the factors applicable under section 166(c) 

of the Act in all areas, including Class II and Class III areas. Note also that states have the

option under the Act of designating additional areas as Class I areas and providing for AQRV 

review for these state Class I areas if they believe that there are areas within their borders that 

merit such protection.

The commenter is not correct in saying that the review to protect AQRVs does not apply 

to emissions increases from sources other than major stationary sources. While it is generally 

true that a major stationary source may trigger the analysis as part of the required PSD review 

for new major stationary sources and major modifications where such source‟s emissions 

increase may affect a Class I area, the review itself generally includes the impacts on an AQRV 

of other emissions in the area, including emissions from non-major sources. In addition, states 

may adopt requirements in their state implementation plans to require certain minor sources 

seeking a permit to undergo an AQRV analysis if they choose to do so. 

We agree with the commenter that the AQRV review has certain limitations in that a state 

can, under some circumstances, issue a PSD permit over the objection of the FLM. Here again, 

Congress enabled this outcome when it provided that a permit would not be issued when the 

FLM demonstrates “to the satisfaction of the State” that the source will have an adverse impact 

on AQRVs in a Class I area. Section 165(d)(2)(C)(ii). We read this provision to reflect 

Congress‟s judgment on the appropriate balance between state and FLM discretion in the reach 

of AQRV review. That said, when a reviewing authority declines to follow a determination of 

adverse impact by the FLM, the reviewing authority is expected to provide a rational basis for 

doing so, and a reviewing authority‟s rejection of an FLM‟s finding may not be arbitrary and 

capricious. As stated by EPA‟s Environmental Appeals Board in In the Matter of: Hadson 

Power 14 – Buena Vista, 4 E.A.D. 258, 1992 WL 345661 (October 5, 1992) (in Section II.A): 

States do not have unfettered discretion to reject an FLM's adverse impact 

determination. If a state determines that an FLM has not satisfactorily 

demonstrated an adverse impact on AQRVs from the proposed facility, the state 

must provide a “rational basis” for such a conclusion, “given the FLMS' 

affirmative responsibility and expertise regarding the Class I areas within their 

jurisdiction.” 50 FR 28549, July 12, 1985. Arbitrary and capricious rejections 

of adverse impact determinations are not sustainable. (Citations omitted.)

 

In sum, the commenter correctly enumerated some of the limitations of the AQRV review 

under the Act. However, such review is only one element of the full PSD program, which must 

be evaluated against the statutory requirements in their entirety. We continue to believe, as 

previously stated, that under the regulatory structure that includes increments, FLM review for 

AQRV impacts is an additional measure that helps satisfy the factors in sections 166(c) and 
37

160(2) of the Act [which require EPA‟s pollutant-specific PSD regulations to protect (1) air 

quality values, and (2) parks and other special areas, respectively] in balance with the other 

statutory factors. We add that the AQRV review requirements of the existing regulations mirror 

these requirements in the Act, which reflect Congress‟ judgment of how AQRV review should 

properly be used to promote the purposes of the program as set out in section 160 of the Act.

Comments on the Enhancement Goal

Comment:

The environmental group commenter (0040) stated that the proposed PSD rules fail to 

ensure fulfillment of the ―enhancement goal‖ set out in the Act. The commenter noted that 

section 101(a) states as the Act’s first purpose: ―to protect and enhance the quality of the 

Nation’s air resources,‖ while section 160(2) states that the purpose of the PSD program is to 

―preserve, protect, and enhance‖ air quality in parks and other special areas. The commenter 

asserted that the proposed rule did not address these enhancement requirements or explain how 

the proposed increments would fulfill those requirements.

Response:

This same issue was raised in the 2005 PSD rule affirming the NOx increments. At that 

time we expressed our belief that the goal to enhance air quality in national parks and 

wilderness areas is implemented through the regional haze program while the PSD program 

focuses on preserving and protecting air quality in these areas. 70 FR 59582, October 12, 2005, 

at page 59589. However, when a PSD increment violation is identified, we agree that EPA may 

require a state to revise its SIP to correct a violation. See 40 CFR 51.166(a)(3). Otherwise, we 

do not interpret these PSD provisions to authorize us to direct states in their SIPs to achieve 

reductions in emissions from existing sources for PSD purposes. 

We recognized at that time, and continue to believe, that the growth management goals of 

PSD may also be fulfilled when the states adopt controls on existing sources that would reduce 

emissions and allow growth from new sources and major modifications to existing sources 

without causing significant deterioration. Under the increment approach, we have interpreted 

the PSD rules to allow states to require reductions from existing sources in order to expand the 

allowable increments and, thereby, allow for more growth under the PSD program. However, 

we have never required states to do so because, in the absence of an increment violation, we do 

not believe section 166 and other provisions in part C of title I of the Act give us the legal 

authority to mandate such reductions for PSD purposes.

2.2.2.3 Comments on Promoting Pollution Prevention

Comment:

The environmental group commenter (0040) states that the proposal contains no 

discussion of how the rules will fulfill the pollution prevention purpose set out in section 101 of 

the Act, as required under section 166(c) of the Act. The commenter contends that although the 
38

proposed increments will limit some pollution increases, there is no provision in the proposal 

that requires or promotes pollution prevention.

Response:

As discussed in Section 2.1.2 of this document, we believe that measures that satisfy the 

specific goals and purposes of section 160 of the Act also satisfy the more general purposes and 

goals identified in section 101 of the Act. As we stated in the 2005 NO2 increment rulemaking, 

the overall goals and purposes of the Act listed in sections 101(b) and 101(c) are general goals 

regarding protecting and enhancing the nation‟s air resources and controlling and preventing 

pollution. Because these broad goals are given more specific meaning in section 160, EPA does 

not believe it is necessary to consider them in detail when evaluating whether PSD regulations 

satisfy the criteria in section 166(c). 70 FR 59587 FN 3, October 12, 2005.

Regarding pollution prevention specifically, we believe that this general goal is 

encompassed in, and given more specific meaning by, sections 160(1), 160(2), and 160(4) of the 

Act. These sections spell out the specific purposes under the PSD program for the general 

section 101 goals of controlling and preventing pollution. We believe that any requirement to 

limit or reduce emissions serves to promote pollution prevention, which is often the most cost 

effective means of lowering pollutant emissions.

In addition to citing the purposes set out in section 160, section 166(c) includes the 

criterion that pollutant-specific PSD regulations should provide a framework for stimulating 

improved control technology. We believe that this criterion is fulfilled by the system of 

increments for PM2.5 and by the requirement for PSD permittees to apply BACT to minimize 

PM2.5 emissions. In stimulating improved control technology generally, these elements of the 

PSD program also promote pollution prevention. As noted above, pollution prevention is often 

the most cost effective means of control, particularly for new sources and new process lines at 

existing sources. In addition, because BACT is a case-by-case determination that considers cost 

and collateral environmental impacts, pollution prevention, where technically feasible, often 

fairs well in BACT analyses because it is typically free from the negative environmental impacts 

that result from the use of add-on air pollution control devices.

2.3 Comments on Option 2 for PM2.5 Increments – Equivalent 

Substitution Approach for Annual Increments

Proposed Option 2 is termed the ―equivalent substitution approach‖ and is based on the 

authority of section 166(f) of the Act. Under section 166(f), increments for a new PM indicator 

completely replace the increments for the previous PM indicator, and the new increments are to 

be ―of equal stringency in effect‖ as the old PM increments that they replace. We proposed two 

different methods for determining annual PM2.5 increments to replace the existing annual PM10

increments, Option 2A and 2B. We did not propose to use this approach to replace the 24-hour 

PM10 increments because the 24-hour NAAQS for PM10 remains in place, although the annual 

NAAQS for PM10 has been revoked.
39

We received some comments that addressed Option 2 in general, which are addressed 

below in Section 2.3.1. Comments specific to Options 2A and 2B are addressed in 

Sections 2.3.2 and 2.3.3, respectively.

2.3.1 General Comments on Option 2

Comment:

One industry association commenter (0032) states that the ―equivalence approach‖ is 

rational and should be adopted, without stating a preference for Option 2A or 2B. Another 

industry association commenter (0034) and two industry commenters (0031, 0042) also support 

the Option 2 equivalent substitution approach under section 166(f) of the Act, but have issues 

with the methodology that EPA used to derive the increments proposed under Options 2A and 

2B (as discussed further below). While preferring the Option 1 approach under section 166(a) of 

the Act, another industry association commenter (0029) believes that EPA is authorized to use 

the Option 2 equivalent substitution approach but also has issues with EPA’s methodology (as 

discussed further below).

One state/local agency commenter (0051) does not believe that using the Option 2 

equivalent substitution approach to determine the increment would be appropriate since it has not 

yet been determined if any precursors will be regulated in regards to PM2.5. The commenter adds 

that since measurements of PM10 have not considered precursors or the formation of secondary 

PM10, it seems comparing PM2.5 to PM10 inventories would likely be inaccurate.

Response:

For the reasons discussed in the previous sections of this document, we have decided to 

finalize Option 1 for the entire suite of PM2.5 increments, based on authority under section 

166(a) of the Act. Thus, while EPA acknowledges that there are a number of issues (as 

described by some of the comments provided in this section) with aspects of the methodology 

used to develop Option 2, those issues do not need to be resolved in this action, and EPA does 

not need to respond to these comments in detail at this time." 

Comment:

Six commenters do not agree with the methodology used to develop the proposed 

increments under Option 2.

Although favoring Option 2’s equivalent substitution approach under section 166(f) of 

the Act, one industry association commenter (0034) and one industry commenter (0042) believe 

that the equivalent substitution methodology EPA employed under Option 2A and 2B of the 

proposed rule is inconsistent with prior EPA methodology used to derive the PM10 increments 

under section 166(f). The commenters argue that the ratios used under both Option 2A and 2B 

are not well supported and do not provide source-specific scrutiny to demonstrate that the new 

increments are of equal stringency to the PM10 increments they are designed to replace. The 

commenters note that for the PM10 increments, EPA compared the TSP impact to the PM10
40

impact for each 249 separate sources in the NSR data base and determined each such source’s 

PM10 impact by multiplying its TSP impact times the PM10/TSP emissions ratio for that source. 

The commenters point out that for Option 2A, EPA determined a single PM2.5/PM10 emission 

ratio for all utilities and industrial point sources combined and multiplied that number time the 

value of the PM10 increment. The commenters contend that this gross approximation approach 

fails to satisfy the requirements of section 166(f). 

The commenters (0034, 0042) also claim that the methodology used under Option 2B 

was deficient. One commenter (0034) asks whether multiplying the PM10 increment times the 

ratio of the PM2.5 NAAQS to the PM10 NAAQS can accurately represent the ―equivalent effect‖ 

in terms of PM10 and PM2.5 increment consumption for the population of PSD sources. The 

other commenter (0042) recommends that EPA revise the proposed rule to comply with 

requirement to provide for equal stringency in compliance with the requirements of 

section 166(f). 

One industry association commenter (0037) states that EPA should not apply NAAQSbased ratios because these do not translate back to the emissions point level when comparing 

PM10 and PM2.5. The commenter believes that this is an invalid method because EPA has not 

shown that there is a correlation between the NAAQS and direct PM2.5 since there is no accounting for precursors, and EPA does not have a quantifiable sense of the portion of PM2.5 that is 

condensable for various industries. The commenter expresses concern that if EPA were to 

proceed with a NAAQS ratio approach, it would clearly result in a very small PM2.5 increment 

and this would not take into account the expected condensable fraction.

The same commenter indicates that based on a review of the regulatory history of the 

establishment of increments for Class I - III areas, the record is very clear that the original 

increments were established at levels which were designed to accommodate moderate, wellcontrolled development. 39 FR 42510 (December 5, 1974) (―The Administrator continues to 

feel that a Class II increment should be compatible with moderate, well-controlled development 

in a nationwide context...‖); see also 45 FR 52676 (August 7, 1980). The commenter questions

whether EPA’s proposal to use the NAAQS ratios to set the increments (and SILs) will 

accommodate moderate, well-controlled development, or if this approach will actually inhibit 

additional growth. The commenter suggests that the EPA should provide a demonstration 

assuring that this proposal will not conflict with the prior regulatory history and intent of 

Congress and will allow for sufficient growth consistent with air quality needs.

Another industry association commenter (0036) states that EPA inappropriately rejected 

the 80 percent ratio of PM2.5 to PM10 emissions in setting the PM2.5 increments. The commenter 

argues that ignoring this basic scientific fact and using lower percentages to set the PM2.5

increments results in effectively lowering the PM10 increments, because the PM2.5 increments 

will, in effect, be much more stringent than the PM10 increments and, consequently, will require 

sources to reduce total PM emissions and PM10 emissions further to comply with PM2.5 

increments. 

Another industry commenter (0031) who favors Option 2’s equivalent substitution 

approach under section 166(f) of the Act contends that EPA’s methodology in adopting the 1993 
41

PM10 increments was flawed, and so is the analogous methodology used to derive the PM2.5

increments. The commenter believes that the methods used by EPA all rely on a fundamental 

assumption that there is a fixed relationship between TSP and PM10 and between PM10 and 

PM2.5, both in terms of mass percent and in terms of health and welfare impact.

One industry association commenter (0029) questions EPA’s approach to determining the

―stringency ratio‖ of 0.3 used under Option 2 to convert the PM10 increments into PM2.5

increments, which was derived for the annual averaging period by computing the ratio of the 

annual PM2.5 NAAQS to the annual PM10 NAAQS. The commenter believes this is an 

inappropriate ―apples to oranges‖ comparison, and that a more appropriate approach would be to

use a multiplier based on the ratio of the PM10 increment to the PM10 NAAQS for each averaging 

period. The commenter provides an example for the PM2.5 Class II 24-hour increment, which he

would derive by (a) dividing the PM10 Class II 24-hour increment (30 μg/m3) by the PM10

24-hour ambient standard (150 μg/m3); (b) multiplying that by the PM2.5 24-hour ambient 

standard (35 μg/m3); and then (c) rounding that product to the nearest whole number: 7 μg/m3. 

Response:

For the reasons discussed in other sections of this document, we have decided to finalize 

Option 1, based on authority under section 166(a) of the Act. We have not reached any final 

conclusion as to whether the Option 2 equivalent substitution approach under section 166(f) of 

the Act is permissible under the Act, but we acknowledge problems associated with the specific 

methodologies we relied on to propose equivalent increments. For example, we agree with 

commenters that the emissions ratio approach does adequately account for precursor emissions. 

The percentage of NAAQS approach used in Option 1 did not suffer from this problem 

associated with the precursor source contribution because the calculated values do not 

distinguish between direct and secondarily formed PM2.5; that is, the calculated values represent 

total ambient concentrations of PM2.5 regardless of their origin. This further supports our 

decision to select Option 1.

Regarding the last comment on our use under Option 2 of the ratio of the PM2.5 NAAQS 

to the PM10 NAAQS, we note that our methodology under Option 2B and the commenter‟s 

example methodology are computationally identical. While we did not propose under Option 2 

to use section 166(f) authority to substitute PM2.5 increments for PM10 increments for the 

24-hour averaging period, had we done so, our input values and results under Option 2B would 

have been identical to the commenter‟s example. Specifically, the commenter‟s methodology is 

as follows:

Our Option 2B methodology is as follows:
42

When looked at closely, one can see that these two expressions are identical except for the order 

of the multiplicands in the numerator and, therefore, yield identical results.

2.3.2 Comments on Option 2A 

Comment:

One commenter explicitly expresses support for Option 2A, and another explicitly 

expresses opposition to that option. As discussed above, other commenters support and oppose 

Option 2 generally without specifying Option 2A or 2B, and some of these raise issues with the 

methodology that we used to develop both the Option 2A and 2B increments.

One state/local agency (0033) supports EPA’s proposed Option 2A. The commenter 

believes that Option 2A meshes well with the intent of achieving equal stringency as stated in 

section 166(f), but only if the annual and 24-hour increments are determined in the same manner. 

One industry association commenter (0039) opposes Option 2A. The commenter asserts

that deriving the PM2.5 increments using a multiplier determined from the ratio of PM2.5

emissions to PM10 emissions (which yields PM2.5 increments that are up to 87 percent of the 

PM2.5 NAAQS) results in increments that are too high. Accordingly, the commenter believes 

that EPA should not perpetuate this calculation method as part of Option 2A. The commenter 

notes that while EPA corrected this overstated PM2.5 increment in Option 2A by multiplying the 

PM2.5/PM10 emissions ratio by the ratio of the PM2.5 NAAQS to the PM10 NAAQS, this method 

in Option 2A is not well founded.

Response:

For the reasons discussed in other sections of this document, we have decided to finalize 

Option 1, based on authority under section 166(a) of the Act. While we solicited comments on 

the possibility of using the substitution approach for the 24-hour PM2.5 increment, we did so 

because we found no strong justification for using the substitution approach to replace the 

existing 24-hour PM10 increments since we were not revoking the 24-hour PM10 NAAQS. In the 

final rule, we have concluded that the section 166(a) approach is the most direct and 

supportable approach for developing all the PM2.5 increments. 

W e agree that deriving the PM2.5 increments based solely on the 0.8 multiplier derived 

from the ratio of PM2.5 emissions to PM10 emissions would have resulted in increments that were 

inappropriately high. As one commenter noted, it was for this reason that we proposed in 

Option 2A also to use a multiplier based on the ratio of the PM2.5 and PM10 NAAQS. 

Nevertheless, we agree that the use of the emissions ratio component does not adequately 

account for the precursor emissions that significantly affect PM2.5 air quality. 
43

2.3.3 Comments on Option 2B

Comment:

One state/local agency association commenter (0021), one industry commenter (0043), 

and one industry association commenter (0039) explicitly express support for Option 2B. One 

industry association commenter (0037) opposes the methodology we used to develop the 

Option 2B increments. As discussed above, other commenters support and oppose Option 2 

generally without specifying Option 2A or 2B, and some of these raise issues with the 

methodology that we used to develop both the Option 2A and 2B increments.

The supportive industry commenter (0043) opines that the arguments for one option 

versus another are not compelling, and notes that the various proposed increment options are 

similar in magnitude. The commenter recommends Option 2B because this option appears to 

have the lowest economic impact.

The supportive industry association commenter (0039) strongly recommends that EPA 

adopt Option 2B. The commenter believes that the new PM2.5 increments should remain as 

consistent with Congress’ original PM increments as feasible. The commenter contends that 

Option 2B is the appropriate method of adopting PM2.5 increments consistent with Congress’ 

intent for the following reasons:

 When Congress enacted the CAA section 163 PM increments, it established each 

increment as a percentage of the PM NAAQS.

 Option 2B does not use the ―flawed‖ ratio of PM2.5/ PM10 emissions used in Option 2A. 

(See this commenter’s comments in Section 2.3.2 above.)

 The use of the NAAQS ratio is generally consistent with Congress’ original statutory 

determination of PM increments as a percentage of the PM NAAQS. 

Response:

For the reasons discussed in other sections of this document, we have decided to 

promulgate PM2.5 increments using Option 1, based on authority under section 166(a) of the Act. 

Although we have not reached any final conclusion as to whether the Option 2 equivalent 

substitution approach under section 166(f) is permissible under the Act, we believe that the 

NAAQS ratio approach could part of a reasonable way to develop equivalent increments..

The industry association commenter did not provide an explanation of why Option 2B 

would have the lowest economic impact. That alleged outcome is questionable in light of the fact 

that, as proposed, Option 2B would involve annual and 24-hour PM2.5 increments that were

based on two separate statutory authorities and would require two separate implementation 

approaches. For example, the 24-hour increments (based on section 166(a) authority) would 

use new baseline dates, while the annual increments (based on section 166(f) authority) would 

use existing baseline dates and areas that had been established previously for PM10 or TSP. 

This would likely involve the maintenance of two separate source/emissions inventories, and the 
44

inventory for the annual increments would have to be converted from PM10 emissions to PM2.5

emissions. 

2.4 Other Comments on Increments

2.4.1 Comments on Baseline Areas

Comment:

A state/local agency association commenter (0021) notes that with regard to identifying 

air quality control regions, states currently have the authority to permit major sources and track 

increments in accord with 40 CFR 52.21(b)(15)(i), which defines ―baseline areas‖ as areas 

classified as attainment or unclassifiable in which a proposed major stationary source or 

modification has a 1 μg/m3impact. The commenter indicates that states have considerable 

latitude, within this regulatory boundary, to identify geographic areas for the purpose of 

managing and implementing their PSD programs. The commenter adds that some states have 

made adjustments in their original identification of air quality control regions, redefining section 

107 areas into reasonably-sized, geographically related areas that enhance their ability to manage 

the complex increment program. The commenter believes that such redefinition is within the 

intent of the Act and regulations and can rightfully be undertaken by states with regard to PM2.5 

increments.

One state agency commenter (0019) believes that states should be required to use the 

baseline areas previously established for their PSD program, unless the process for redefining 

these areas strictly follow procedures in the PSD regulations and EPA policy. The commenter 

indicates that this will minimize any inconsistent applications of the regulations for PM2.5. The 

commenter notes that in establishing the PSD increments and baseline dates for the NO2

increment, EPA did not allow the establishment of new baseline areas.

Response:

We agree with the first commenter‟s interpretation that states have flexibility to redefine 

section 107 area designations in order to redefine the boundaries of baseline areas once they 

have been established for a pollutant. The commenter‟s position seems to be taken in 

conjunction with their support for section 166(f) of the Act as the appropriate legal path for 

implementing new PM2.5 increments. While we did not use section 166(f) as the legal path for 

developing and implementing of the new increments, we believe that the same flexibility is 

available to states under the section 166(a) approach which we believe is the most legally 

defensible approach for promulgating the PM2.5 increments. While a PSD source establishes the 

baseline area in all intrastate attainment and unclassifiable areas in which it would have a 

significant impact – not just the area in which it would locate – longstanding EPA guidance 

provides that states may request the redefinition of a baseline area by seeking the redefinition of 

the attainment or unclassifiable area boundaries affecting the baseline area. Principles set out 

in 45 FR 52676, August 8, 1980 at 52716 are available to assist states in the proper manner for 

redefining existing baseline areas. Generally, as long as the redefined area (that is, a redefined 

boundary for an attainment or unclassifiable area) continues to include all locations where the 
45

source‟s impact is significant for PM2.5 on an annual basis, we believe that states may request 

the desired adjustments.

 

We are unclear as to what the second commenter means when saying that states should 

be “required to use the baseline areas previously established for their PSD program....” 

Baseline areas are established independently for each pollutant for which increments have been 

established in accordance with the definition of “baseline area” under 40 CFR 51.166(b)(15) 

and 52.21(b)(15), and they are established consistent with the area designation boundaries that 

the state established for the particular pollutant. We have not made any changes to the 

definition of “baseline area” in these provisions to alter the way in which the baseline area 

concept is applied for purposes of PM2.5. The only change we have made to the definition of 

“baseline area” at 40 CFR 51.166(b)(15) and 52.21(b)(15) adds the significant impact level 

(annual average) for PM2.5 to determine what attainment or unclassifiable areas need to be 

included in the baseline area established by a PSD source (see below). Accordingly, baseline 

areas for PM2.5 would be defined consistent with area designations for PM2.5, and such areas 

may be altered if the state properly redefines the applicable attainment and unclassifiable areas 

as described in the response provided above. 

Comment:

One industry association commenter (0032) indicates that EPA did not include proposed 

regulatory language to establish new baseline areas to accompany the new baseline dates for the 

24-hour PM2.5 increments. The commenter states that baseline areas are addressed in the 

definitions at 40 CFR 52.21(b)(15) but EPA proposes no amendment to this provision, making it 

unclear how a minor source baseline date could be established when the area for PM2.5 is not 

established. The commenter contends that EPA needs to establish the baseline areas so that 

sources can clearly understand the geographic boundaries of the baseline area when the baseline 

date for that area is triggered.

Response:

We disagree with this comment claiming that new regulatory language is needed as part 

of the definition of “baseline area” (40 CFR 52.21(b)(15) and 51.166(b)(15)) to allow the 

establishment of new baseline areas for the PM2.5 increments. We believe the existing definition 

of “baseline area” is applicable to any pollutant for which area designations of “attainment” or 

“unclassifiable” have been made. We published the area designations for PM2.5 in the Federal 

Register at 74 FR 58688, November 13, 2009. The existing definition of “baseline area” is 

adequate to enable the establishment of new baseline areas for the PM2.5 increments for any 

PSD source emitting PM2.5 emissions that submits a complete PSD application and thereby 

establishes the “minor source baseline date” for PM2.5. The definition of “baseline area” refers 

to air quality impacts “for the pollutant for which the baseline date is established.” Under 40 

CFR 52.21(b)(14)(iii) and 51.166(b)(14)(iii), a baseline date “is established for each pollutant 

for which increments...have been established.”
46

Comment:

One environmental group commenter (0400) states that EPA must revise its existing 

definition of ―baseline area‖ with respect to the PM2.5 increment. The commenter notes that if a 

proposed PSD source’s impact would be less than 1 μg/m3(annual average) in a PM2.5

attainment area adjacent or near to the area in which the source proposes to locate, the minor 

source baseline date for PM2.5 would not be triggered in that area. The commenter indicates that 

a comparison of the 1 μg/m3(annual average) significant level to the NAAQS for PM10, SO2, 

NO2, and PM2.5 shows that this significant level could allow greater deterioration of air quality 

for PM2.5 pollution than is currently allowed for PM10, SO2, and NO2 pollution. The commenter 

points out that a 1 μg/m3(annual average) significant level reflects 1-2 percent of the annual 

NAAQS for PM10, SO2, and NO2. In contrast, the commenter notes that this significant level 

represents 6.7 percent of the annual PM2.5 NAAQS of 15 μg/m3, as well as the entire amount of 

what EPA proposed for the annual Class I PM2.5 increment and 25 percent of what EPA 

proposed for the annual Class II PM2.5 increment. 

The commenter (0040) argues that allowing for such a lax significant level to define 

where the PM2.5 minor source baseline date is triggered will allow for PM2.5 pollution growth to 

be considered part of baseline concentration rather than consuming the available PM2.5 increment 

in many locations. The commenter also points out that, in a departure from its past practice with 

respect to PM10, SO2, and NO2 (for which EPA often designated one attainment area 

encompassing an entire state, except the areas designated nonattainment), EPA has designated 

PM2.5 attainment areas by counties. 70 FR 944, January 5, 2005. The commenter adds that these 

much smaller attainment area designations greatly increase the likelihood that the PM2.5 minor 

source baseline date will not be triggered for many areas, especially if EPA adopts new baseline 

areas and baseline dates for PM2.5. The commenter further adds that this is very likely for Class I 

areas, because PSD sources do not usually propose to locate in Class I areas, and thus the PM2.5

minor source baseline date will only be trigged for most Class I areas based on a proposed PSD 

source having a ―significant‖ impact on PM2.5 concentrations in that area. The commenter 

asserts that the use of this significant level for PM2.5 will not ensure the prevention of significant 

deterioration of air quality or fulfill the statutory purposes as mandated by section 166 of the Act. 

The commenter argues that such a high significance level will illegally and arbitrarily allow 

worsening of air quality in parks and natural areas in many parts of the nation, and recommended 

that the PM2.5 significant level should be no higher than an annual average of 0.15 – 0.30 μg/m3

(i.e., no more than 1-2 percent of the annual PM2.5 NAAQS) to be consistent with the approach 

used for the definition of ―significant‖ for PM10, SO2, and NO2.

Response:

We agree with the commenter‟s concern about the use of a 1 μg/m3impact level to 

determine whether a particular attainment or unclassifiable area should be included in a 

baseline area established by a PSD application for PM2.5, and, as explained below, we have 

revised the definition of “baseline area” to address this concern. As explained in the preamble 

for the 1980 PSD regulations, EPA selected an impact of 1 μg/m3, annual average, for the 

definition of “baseline area” because that value was considered the significant impact level for 
47

both SO2 and PM when the definition was originally established.10 There was no mandate at 

that time that a 1 μg/m3impact be used to determine the baseline area for increments for other 

pollutants; however, the use of a 1 μg/m3impact in the definition of “baseline area” was not 

changed when EPA developed increments for NO2 in 1988 because EPA also defined 

“significant” for NO2 using the same annual average concentration of 1 μg/m3. We have

determined, however, that “significant” for PM2.5 ambient impacts should be considered to 

occur at a lower concentration than 1 μg/m3. In finalizing the rule, we decided that the 

significant impact level (SIL) for PM2.5 should be 0.3 μg/m3, annual average. (See Section 5 of 

this document for more on SILs.) Consequently, although no change to the definition of 

“baseline area” was proposed in this rule, we believe it is necessary and appropriate to define 

in this final rule a level of significance of 0.3 μg/m3, annual average, for establishing a new 

baseline area for purposes of PM2.5 increments. See revised 40 CFR 51.166 (b)(15)(i) and 

52.21(b)(15)(i). 

Had we established the SIL at 1 μg/m3, annual average, as proposed under Option 1 for 

SILs, then the definition of “baseline area” would not need to be revised. However, the revised 

definition in this final rule is consistent with our decision to establish a SIL of 0.3 μg/m3, annual 

average, for PM2.5. We consider this revision to the definition of “baseline area” to be a logical 

outgrowth of our decision to establish a SIL for PM2.5 and the comment concerning the effect of 

that action on the definition of “baseline area.” Thus, we believe that our failure to initially

propose such a change to the definition of “baseline area,” based on the possibility of selecting 

Option 3 for defining the SIL for PM2.5, does not warrant a reproposal.

2.4.2 Comments on Baseline Dates for the PSD Increments for PM2.5

Three dates comprise the ―baseline date‖ concept for the PSD increments – ―trigger date‖ 

and ―major source baseline date‖ (both of which are fixed dates), and ―minor source baseline 

date‖ (which is set based on the submittal date of the first PSD application submitted for a source 

located within an area or with emissions that have a significant impact in an area). In the 

proposal, EPA proposed two options for defining the trigger date and major source baseline date 

for the new PM2.5 increments. Under Option 1, based on the provisions of section 166(a) of the 

Act, we proposed that these fixed baseline dates be defined as the effective date of the final rule, 

thereby creating new baseline dates for PM2.5 for both the annual and 24-hour averaging periods.. 

Under Options 2A and 2B, we proposed to retain the existing baseline dates being used for 

TSP/PM10 for the annual PM2.5 NAAQS based on the provisions of section 166(f), while 

following Option 1 for defining new fixed baseline dates for the 24-hour PM2.5 increments. We 

also requested comment under Options 2A and 2B on the alternative of new baseline dates for 

both the annual and 24-hour PM2.5 increments. In addition, we requested comment on the issue 

of the multiple emissions inventories that would be required if we finalized different baseline 

dates for the annual and 24-hour PM2.5 increments as proposed under Options 2A and 2B. 

 

10 “A source will be considered to impact an area if it has an impact of 1 μg/m3

or more of SO2 or PM on 

an annual basis. This figure has been selected because it corresponds to levels of significance used in previous 

Agency determinations for SO2 and PM.” 45 FR 52676, August 7, 1980 at 52716. 
48

Comment:

Eleven commenters, including four state/local agency commenters (0015, 0019, 0022, 

0033), two state/local agency association commenters (0018, 0021), two industry commenters 

(0026, 0031), and three industry association commenters (0023, 0029, 0036), support the 

establishment of new baseline dates for PM2.5 for both the 24-hour and annual averaging periods 

rather than retaining the existing baseline dates that were previously established for TSP/PM10, 

even in cases where they support section 166(f) of the Act as the basis for the PM2.5 increments. 

One industry association commenter (0032) supports retaining the TSP baseline dates for both 

the 24-hour and annual PM2.5 increments.

The commenters that support new baseline dates for PM2.5 generally contend that

retaining existing baseline dates would create problems. For example, one state/local agency 

(0015) indicates that for early minor source baseline dates (circa the mid-late 1970’s), 

establishing the PM2.5 emissions inventory would be extremely difficult, cumbersome, and 

necessarily inaccurate and unreliable because historic emissions inventories did not speciate 

PM2.5 emissions. Similarly, a state/local agency association commenter (0018) and a state/local 

agency commenter (0019) contend that any attempt to calculate the PM2.5 component of 

previously established PM10 increment consumption would be an insurmountable task due to 

lack of appropriate data, especially in cumulative impact analysis cases, and another state/local 

agency association commenter (0022) indicates that resurrecting PM2.5 inventories based on the 

PM10 baseline dates would be insurmountable.

Another state/local agency association commenter (0021) supports new baseline dates for 

PM2.5 because the commenter believes this will assist in insuring the accuracy of increment 

consumption calculations for the fine and coarse particle fractions. The commenter indicates 

that, as a practical matter, it would be virtually impossible for states to calculate the PM2.5

component of previously consumed PM10 increments because data on the fine and coarse 

fractions of source emissions are largely unavailable. 

One industry association commenter (0023) believes that making the PM2.5 increments 

effective retroactively would cause unreasonable burdens on both permittees and permitting 

agencies, and would not provide any real benefits. One industry commenter (0026) and one 

industry association commenter (0029) opine that prospective application of PM2.5 increments is 

sensible and more consistent with the PSD program, because it will give more time for 

development of PM2.5 emissions inventories rather than forcing affected permit applicants to try 

to recreate PM2.5 inventories for some earlier baseline date. Another industry commenter (0031) 

believes that EPA made a significant change in 1997 when it identified coarse and fine PM as 

separate pollutants, and that given the significant strides in the understanding of PM since the 

original adoption of the PM NAAQS in 1977, it no longer makes sense to draft new PM 

increments onto an outdated baseline date/baseline concentration system.

As further support for new baseline dates, two commenters (0018, 0019) point out that 

the establishment of new dates for PM2.5 would not abandon past cases of increment 

consumption for PM10 because the 24-hour PM10 increments will still be in effect. Another of 

the commenters (0036) supporting new baseline dates adds that because EPA’s proposed 
49

increments are very low, facilities may find themselves immediately out of compliance with the 

PM2.5 increments upon promulgation of the rule, based on a January 1975 or 1977 baseline date.

Four of the commenters (0015, 0026, 0029, 0036) that support new baseline dates for 

PM2.5 explicitly raise issues with the Option 2 proposal to have new baseline dates for the 

24-hour averaging period while retaining the existing baseline dates for the annual averaging 

period. One of these commenters (0015) believes this would unnecessarily complicate the 

review process without providing a significant environmental benefit. Another of the 

commenters (0029) contends that a system with different baseline dates would create an 

unrealistic burden on both permit applicants and permit reviewers, and suggests that EPA should 

modify the Option 2 approach to avoid relying on the retroactive application of the annual PM10

major source baseline date. Without expressing a preference for the use of new or existing 

baseline dates, another industry commenter (0043) states that EPA should establish a way to 

select just one baseline date in order to avoid the need for facilities to maintain emission 

inventories for both the 24-hour and annual increment reviews.

Three of the commenters (0015, 0036, 0038) that support new baseline dates for PM2.5

suggest that EPA should define the major source baseline and trigger date as the effective date of 

the rule as proposed under Option 1. (Note that under proposed Option 1, the effective date of 

the rule was to have been the date 1 year after promulgation of the final rule.) However, another 

of the supportive commenters (0019) recommends that EPA establish the trigger date as of the 

date when it officially established the nonattainment and attainment areas for PM2.5, that is, 

April 5, 2005. The commenter explains that this approach is consistent with the PSD regulations 

from their inception and partially mitigates EPA’s delays in implementing the PSD program for 

PM2.5.

One industry association commenter (0032) supports the ―equivalence approach‖ 

(presumably referring to Option 2) for developing the PM2.5 increments, and recommends that 

EPA not use new baseline dates for the 24-hour increments as proposed. The commenter urges 

EPA to retain the TSP baseline dates for both the 24-hour and annual PM2.5 increments, stating 

that recent reductions in PM2.5 will not oversubscribe the increment because states will retain 

authority in their SIPs to assure attainment and maintenance of the NAAQS. 

Response:

Having considered all the comments, we believe that the most reasonable approach for 

addressing the relevant dates associated with the PM2.5 increments is to start anew with the 

baseline date concept. As already mentioned, the commenters have identified difficulties that 

would occur if the PM2.5 emissions inventory for increment analyses had to be created for an 

earlier period of time, and the existence of these difficulties supports the approach under Option 

1 to establish new dates for implementing the PM2.5 increments. Also, these new baseline dates

for PM2.5 increments will not undo the current protection provided by the existing increments for 

PM because we are not revoking the 24-hour or annual PM10 increments under the new rule. 

Accordingly, the final rule establishes independent PM2.5 increments using a “trigger date” and 

“major source baseline date” that are separate from the dates defined for the PM10 increments. 

Consequently, new minor source baseline dates and the corresponding baseline areas will be 
50

used for the annual and 24-hour PM2.5 increments, and will be established when a source 

submits a complete PSD permit application any time on or after the new trigger date for PM2.5. 

(See also the discussion about changes to the definition of “baseline area” above in Section 

2.4.1 of this document.) 

The “major source baseline date” for PM2.5 is being set as the date of publication of the 

final rule in the Federal Register. The setting of this date differs from previous major source 

baseline dates which were set as the date of publication of the proposed rule, but is similar to the 

major source baseline date set for the other increments in that the date precedes the effective 

date for implementing the increments, and thereby requires that certain major source emissions 

increases that occur before the trigger date retroactively count toward the amount of increment 

consumed. 

The “trigger date” is being set as the date 1 year after publication of the final rule in the 

Federal Register. We are using this approach to define the date on which the PM2.5 increments 

become effective as the date 1 year from the date of publication, consistent with the 1-year delay 

required under section 166(b) of the Act. This date for the trigger date separates the effective 

date of the PM2.5 increments from the effective date of the final rule in general (which is 60 days 

after publication in the Federal Register), but also ensures that the “minor source baseline 

date” for PM2.5 for any particular PM2.5 attainment or unclassifiable area cannot be established 

until after the increments become effective in the final rule. The implementation of these dates as 

part of the new PM2.5 increment system is discussed in greater detail in Section 6 of this 

document. 

We recognize that some commenters may still have a concern about our decision to set 

the major source baseline date as the date of publication of the final rule in light of the fact that 

the PM2.5 NAAQS have been in place since 1997; however, we believe that the selection of 

possible earlier dates would require states to retroactively establish PM2.5 emissions inventories 

for increment analyses during a period when sources were generally not required to conduct 

PM2.5 air quality analyses. Hence, given the lack of information, and considering the technical 

difficulties in doing so, we do not believe that it would be appropriate to require states and 

sources to retroactively account for PM2.5 increment consumption by setting the major source 

baseline date at an earlier date than the date we have selected. 

Comment:

Four of the commenters (0018, 0019, 0021, 0033) that support new baseline dates for 

PM2.5 do so despite explicitly supporting section 166(f) authority and/or Option 2 as the basis for 

PM2.5 increments. These commenters believe that EPA has ample authority under section 166(f) 

to define new baseline dates for PM2.5.

Response:

As discussed above, we are finalizing the PM2.5 increments under the authority of 

section 166(a) of the Act and establishing new baseline dates for PM2.5 for both the 24-hour and 

annual averaging periods. Thus, the issue of the permissibility of establishing new baseline 

dates under section 166(f) authority does not need to be addressed here. Nevertheless, as we 
51

pointed out in the 2007 NPRM, section 166(f) does not explicitly address how EPA should 

handle baseline dates for a substituted increment. We proposed to retain the existing baseline 

dates for TSP/PM10 for the annual averaging period under Option 2, consistent with the 

approach we took when we replaced the statutory PM increments with PM10 increments 

pursuant to section 166(f). However, we also pointed out at proposal that given PM2.5 emissions 

trends, our judgment is that establishing new baseline dates for PM2.5 may be more effective at 

preventing significant deterioration because the baseline concentrations will reflect emissions 

reductions, and we took comment on whether this would provide sufficient justification for 

establishing new baseline dates under the section 166(f) substitution approach. See 72 FR 

54138. 

Because we have not used section 166(f) authority in the current rulemaking, we need not 

make a final determination at this time on whether new baseline dates are permissible under that 

section of the statute. Accordingly, we do not adopt or rule out any interpretation of 

section 166(f) in this regard at this time.

2.4.3 Miscellaneous Comments on Increments for PM

Comment:

One industry commenter (0016) suggests that EPA develop geographic area-specific 

increments (and SILs and SMCs) that take local conditions into account. The commenter 

pointed out that PM2.5 levels in PSD areas proximate to international borders may be elevated by 

sources outside the legal and practical control of the United States and state authorities. The 

commenter also notes that PM2.5 levels may be elevated by natural conditions, such as drought, 

fires, geologic formations (sandy or fine-grained surface features), high winds, etc., leading to 

excessively dusty ambient conditions over which the local area has no control. The commenter 

indicates that local area baselines must reflect these PM emissions, though they are not reflected 

in the local area’s emissions inventory. The commenter urges EPA not to penalize such PSD 

areas by imposing uniform national PSD increments (or SILs or SMCs) where the conditions of 

concern are not capable of control.

Response:

The final rule establishes an area classification system with prescribed, uniform PM2.5

increments for Class I, II and III areas. We have not defined different increments for different 

areas of the country, although states can propose to use more stringent increments or propose 

alternative measures within their individual jurisdictions where they deem it appropriate to do 

so. With regard to the commenter‟s concern about emissions from natural conditions (such as 

those described by the commenter) we do not believe that such emissions would consume 

increment because of their natural and temporary nature. In addition, if a state wishes to 

disregard new emissions from sources outside the United States, the state‟s PSD program may 

provide that such emissions do not consume increment (see 40 CFR 51.166(f)(1)(iv)).
52

Comment:

One federal agency (0044) points out a typographical error in the proposed regulatory 

language at 72 FR 54147 for Option 1 for the 24-hour Class I PM2.5 increment in 40 CFR 

51.166(c). The commenter (0044) notes that this number should have been 2 μg/m3to be 

consistent with the remainder of the proposal.

Response:

The commenter is correct in pointing out that the proposed Option 1 value for the 

24-hour Class I increment was intended to be 2 μg/m

3

as discussed in the preamble and reflected 

in the regulatory language proposed in 40 CFR 52.21(c). The correct value appears in the final 

regulatory language.

Comment:

One industry commenter (0031) believes that PM2.5 increments must take into account 

the evidence that significant levels of coarse PM can intrude into PM2.5 measurements. In this 

argument, the commenter discusses the bimodal nature of PM, indicating that ―fine particles‖ as 

evaluated for regulation are formed primarily by combustion or chemical reaction of gases and 

typically have a diameter smaller than 1 μm, while ―coarse particles‖ are formed by the 

mechanical breakdown of mineral, crustal material and organic debris and typically have a 

diameter greater than 3 μm. These types of particles have distinct health effects, with the effects 

of fine particles being of concern. The commenter notes that the PM2.5 indicator was adopted as 

a means to regulate fine PM, and argues that with a 2.5 μm threshold, the PM2.5 indicator also 

inappropriately includes some coarse particles of crustal origin. The commenter contends that 

coarse particle intrusion into PM2.5 is a significant issue, particularly in arid western areas where 

a high proportion of PM2.5 can be coarse PM. The commenter believes that it is a basic 

distortion to adopt a PM2.5 NAAQS based on fine PM and then to measure compliance based on 

coarse PM, and that increments derived from the PM2.5 NAAQS are similarly flawed. Based on 

this argument, the commenter asserts that PM2.5 increments or other measures to prevent 

significant deterioration must address and account for coarse intrusion into PM2.5 measurements.

Response:

This commenter is critical of the Agency‟s selection of an indicator for particulate matter 

that includes particulate matter of a size that the commenter feels should not be included. The 

EPA made the decision concerning the PM2.5 indicator in defining the NAAQS under a separate 

rulemaking. The final rule to adopt PM2.5 increments, SILs, and the SMC for PM2.5 is not the 

appropriate rule to address the commenter‟s concerns about the appropriate indicator for PM. 

Thus, we provide no formal response to the comment herein. 
53

3 Comments on the PM10 Increments

Under our preferred option (Option 1) for developing PM2.5 increments, we proposed to 

rely on the authority under section 166(a) of the Act to establish annual and 24-hour increments. 

In addition, we proposed to revoke the annual PM10 increments based on the same technical 

evidence that led us to revoke the annual PM10 NAAQS, and retain the existing 24-hour PM10

increments. Alternatively, under Options 2A and 2B, we proposed to again use the 24-hour 

increment values derived under Option 1 to promulgate 24-hour PM2.5 increments, but to develop 

annual PM2.5 increments to replace the existing annual PM10 increments using the equivalent 

substitution approach under the authority of section 166(f) of the Act. In addition, under 

Options 2A and 2B we sought comment on whether we should rely on section 166(f) of the Act 

to develop 24-hour PM2.5 increments to replace the existing 24-hour PM10 increments, even 

though the primary and secondary 24-hour PM10 NAAQS had not been revoked. 

Comments Supporting Revocation of PM10 Increments:

Six commenters, including two state/local agency commenters (0015, 0022), three 

industry commenters (0031, 0034, 0043), and one industry association commenter (0042), 

support EPA’s proposal to revoke the annual PM10 increments. In addition, five of these 

commenters (0015, 0031, 0034, 0042, 0043) recommend that EPA revoke the 24-hour PM10

increments as well.

One state/local agency commenter (0022) agrees with the proposal to adopt the 24-hour 

and annual PM2.5 increments and revoke the annual PM10 increments. The commenter indicates 

that counting and tracking increment is confusing enough without adding the confusion of 

potentially overlapping PM standards. According to the commenter, the cleanest approach is to 

establish a single new PM2.5 increment and work from there – EPA can develop a coarse fraction 

increment, once EPA establishes coarse PM NAAQS. The commenter adds that the removal of 

the PM10 annual increment is supported by the removal of the health-based standard for annual 

PM10.

Another state/local agency commenter (0015) also supports proposed Option 1 for 

developing the annual and 24-hour PM2.5 increments and revoking the annual PM10 increments. 

However, the commenter does not agree that the EPA should retain the 24-hour PM10 increments 

since the PM10 emissions include PM2.5 emissions. The commenter believes that the 

combination of the current 24-hour PM10 increments and the proposed 24-hour PM2.5 increments 

will be unnecessarily conservative. The commenter contends that the proposal does not 

adequately discuss the potential for double-counting of 24-hour PM2.5 emissions within a 

24-hour PM10 increment analysis.

An industry association commenter (0034) and an industry commenter (0042) agree that 

EPA’s regulations should not contain an annual increment for PM10 since an annual PM10

NAAQS no longer exists. In addition, the commenters believe that section 166(f) provides the 

only authority under the Act to establish increments for PM, and that under section 166(f) any 

update to the PM increments must be achieved only by a substitution of the new increment for 
54

the old increment. Thus, the commenters believe that EPA is without authority to retain the 

PM10 annual increment if it adopts a PM2.5 annual increment. 

These commenters (0034, 0042) also argue that EPA’s rationale for retaining the 24-hour 

PM10 increments is flawed. As noted above, these commenters believe that EPA must address 

PM increments under section 166(f) of the Act, and they believe that section 166(f) allows 

increments for only one indicator for PM at a time. Thus, the commenters assert that EPA does 

not have the authority to retain the 24-hour PM10 increments if it adopts the 24-hour PM2.5

increments. In addition, the commenters point out that in previous actions, EPA has stated that a 

dual-increment system would be unnecessarily burdensome and cumbersome (variously citing 

52 FR 24685 and 24701, July 1, 1987; 54 FR 41220, October 5, 1989; and 58 FR 31,624, 

June 3, 1993).

The industry association commenter (0034) also states that EPA seems to believe that 

because it has retained the 24-hour PM NAAQS for PM10, it must retain the 24-hour PM 

increment for PM10. The commenter argues that there is no statutory provision that requires such 

a one-to-one correlation of NAAQS and increment when the NAAQS is expressed by more than 

one indicator. The commenter refers to two examples as support for that comment: (1) EPA’s 

1984 proposal to replace the primary PM NAAQS expressed as TSP with a primary PM10

NAAQS while retaining the secondary PM NAAQS and PM increments expressed as TSP; and 

(2) the fact that the PM2.5 NAAQS have been in place for over 10 years with no PM2.5

increments, and PM10 increments have been used as a surrogate for PM2.5.

In addition, the industry association commenter (0034) contends that section 166(f) 

ensures that a single increment system protects public health and welfare by requiring that 

―substituted maximum allowable increases [of the new increments] shall be of equal stringency 

in effect as those specified in the [increment] provisions for which they are substituted.‖ The 

commenter claims that the elimination of the 24-hour PM10 increment would not result in a 

diminution of public health or welfare because the new 24-hour PM2.5 increment must provide 

the same level of protection. 

Another industry commenter (0031) argues against retention of either the annual or 

24-hour PM10 increment, and supports replacing both with annual and 24-hour PM2.5 increments. 

The commenter argues that the existing PM10 increments were developed based on the 1987 

PM10 NAAQS which have been found by the courts to ―confounded‖ because the coarse PM 

regulated under that standard depends ‖quite arbitrarily‖ on the existing levels of PM fine in an 

area. This commenter recommends that EPA use section 166(f) of the Act to replace both the 

annual and 24-hour PM10 NAAQS with annual and 24-hour PM2.5 NAAQS or with alternative 

measures. Alternatively, the commenter indicates that, should EPA decide to establish new 

PM2.5 increments based on section 166(a) of the Act, then EPA must also establish PM10

increments under section 166(a) because (according to the commenter’s analysis) PM10 is also a 

new pollutant. In this case, the commenter believes that EPA has the freedom under 

section 166(a) to evaluate alternatives to increments for PM10, which could include a finding that 

the PM2.5 increments adopted pursuant to section 166(a) adequately address coarse PM emissions 

for PSD purposes. Regardless of the statutory basis, the commenter believes that EPA could 

more appropriately address the goals of the PSD program for coarse PM by separating PSD 
55

protection from the level of the NAAQS, which the commenter believes is excessively 

conservative as applied to coarse, crustal PM of a rural type. Thus, the commenter recommends 

that EPA consider alternative measures designed to better address the goals of the PSD program, 

which include protection of economic development, such as western surface mines.

The other industry commenter (0043) recommends that EPA revoke the 24-hour PM10

standard as we did the annual PM10 standard. From the context of the comment, we presume that 

by recommending that both PM10 standards be revoked, the commenter also recommends that 

both the annual and 24-hour increments should be revoked as well.

Comments Opposing Revocation of Annual PM10 Increments:

Five state/local agency commenters (0018, 0019, 0025, 0028, 0033), one state/local 

agency association commenter (0021), and one environmental group commenter (0040) oppose 

the revocation the annual PM10 increments. The commenters generally believe it is premature to 

revoke the annual PM10 increments and/or that these increments continue to play protective role.

Three of the state/local agency commenters (0018, 0019, 0025) oppose revoking the 

annual PM10 increments until EPA makes a determination on a NAAQS for PM-coarse and 

establishes equivalent increments for PM-coarse. Two of the state/local agency commenters

(0018, 0019) believe that it is prudent to maintain the PM10 increments until EPA makes a 

determination on the health and environmental effects of the coarse fraction of PM. These commenter indicate that PM2.5 and PM10 increments can play complementary roles to avoid any 

discontinuities or unwanted degradation concerns. 

One of the state/local agency commenters (0019) elaborates that the basis for dismissing 

the annual PM10 NAAQS by the substitution of fine particle NAAQS to address certain health 

and welfare effects does not provide a basis for dismissing a PSD increment which is meant to

stop significant degradation of air quality. The commenter notes that as refinements are made to 

estimation of fine particle emissions or in instances where these are deemed not to be a major 

component of particulate emissions, the PM10 annual increment could prevent long term 

deterioration of air quality associated with the coarse component. 

This state/local agency commenter (0019) also states that EPA proposed to replace the 

PM10 annual increment with the corresponding PM2.5 increment under the section 166(f) options 

2A and 2B, but did not provide a substantive basis for this action. The commenter does not see 

the tension noted by EPA between sections 166(a) and (f) with respect to reaching a holistic 

solution if EPA views PM2.5 as a new indicator of PM, as the commenter believes it can. The 

commenter (0019) explains that under this approach, if EPA determines that coarse particle 

levels are necessary to protect the public from certain exposures not addressed by PM2.5, then it 

will be appropriate for EPA to define complementary increments for coarse particulates as 

another indicator of PM. The commenter also asserts that the 24-hour increments for PM2.5 must 

be based on section 166(f) authority, but believes that the PM2.5 increment need not replace the 

PM10 increment for this averaging period. 
56

Similarly, a state/local agency association commenter (0021) believes that EPA can and 

should continue both the 24-hour and annual average PM10 PSD increment program until 

NAAQS for PM with aerodynamic diameter less than or equal to a nominal 10 μm but greater 

than a nominal 2.5 μm (PM10-2.5) are promulgated, and that EPA has the discretion to accomplish 

this under section 166(f). The commenter asserts that, at a minimum, EPA should continue the 

24-hour PM10 increments in conjunction with the continuation of the 24-hour PM10 NAAQS.

Two of the state/local agency commenters (0028, 0033) request that EPA keep the PM10

PSD increments in place until the full PM2.5 program is adopted and in place. One of these 

commenters (0033) believes that there are too many uncertainties regarding PM2.5 to revoke the 

annual PM10 increments. The commenter notes that the PM2.5 program has been dragging for 

years, analytical methods are not formulated, the NSR part of the implementation rule has not 

issued, condensables are not yet included, and the impact of precursors has not been definitively 

explored. The commenter contends that under these conditions, nothing concerning PM10 should 

be revoked until the reasons for doing so are clearly understood and the overall impact on 

ensuring clean air and the public health and welfare have been fully explored. The commenter 

suggests that PM10 increments and NAAQS should remain in effect until these issues have been 

resolved to the satisfaction of the Administrator. This commenter believes that Options 2A and 

2B must be based entirely on section 166(f) of the Act, but that the presence of increments for 

both PM10 and PM2.5 is supportable under this section because they complement each other. The 

commenter indicates that the problem will be resolved when sufficient data are available to 

revoke the PM10 NAAQS and increments and/or PM10 is replaced by PM10-2.5.

The environmental group commenter (0040) claims that EPA has no authority to repeal 

an existing PM10 increment without at the same time restoring the corresponding TSP increment. 

The commenter notes that Congress established the TSP increments by statute and gave EPA no 

authority to revoke them, but instead gave EPA only limited authority to substitute PM10

increments for TSP increments under the conditions specified in section 166(f). The commenter 

asserts that EPA cannot revoke the annual PM10 increments, either by ―replacing‖ them with 

PM2.5 increments or otherwise, unless EPA at the same time restores the annual TSP increments. 

The commenter also contends that retention of the annual PM10 increments is entirely compatible 

with the statutory purposes, notwithstanding EPA’s revocation of the annual PM10 NAAQS. The 

commenter notes that the courts have observed that the unique congressional objectives of the 

PSD program may necessitate the imposition of increment metrics that differ from the related 

NAAQS (citing Environmental Defense Fund v. EPA, 898 F.2d 183, 190 (D.C. Cir. 1990).

The environmental group commenter (0040) gives the following examples/evidence that 

retention of the annual PM10 increments is important to achieving the goals of the Act’s PSD 

provisions: 

 While EPA attributes the visibility impairing impacts of PM pollution primarily to 

elevated short-term fine particle concentrations, EPA recognizes that PM10 plays a 

significant role in the other welfare related impacts of PM pollution (citing 72 FR 54136).

 The EPA also states that the most significant PM-related ecosystem-level effects result 

from long term cumulative deposition . . . that exceeds the natural buffering or storage 
57

capacity of the ecosystem and/or affects the nutrient status of the ecosystem (citing 72 FR 

54131). 

Response:

As explained elsewhere in this document and the preamble to the final rule, we are not 

using a section 166(f) legal path to promulgate PM2.5 increments to replace the existing PM10

increments, and we are not taking final action on our proposal to otherwise revoke the PM10

increments. We agree that section 166(f) of the Act is a “substitution” approach, as mentioned 

by some commenters supporting the use of that approach as the legal path to substitute the new 

PM2.5 increments for the existing PM10 increments. However, as we stated in our 2007 NPRM, 

we expressed some concern about using section 166(f) to substitute PM2.5 increments for PM10

increments. In fact, as described above, some commenters challenged our authority under 

section 166(f) to replace the PM10 increments with new PM2.5 increments. We have not reached 

any final conclusion as to whether the Option 2 equivalent substitution approach under 

section 166(f) is permissible under the Act. 

In the final rule, EPA is promulgating new PM2.5 increments under the authority of 

section 166(a) of the Act, but is not amending in any way the existing increments for PM10. We 

continue to believe, as we indicated in the 2007 NPRM, that the combination of the new PM2.5

increments and the existing 24-hour PM10 increment will address non-visibility welfare impacts

attributable to deposition (primarily sulfate and nitrate particles larger than PM2.5). 

The obvious effect of not taking final action with respect to the PM10 annual increments is 

to leave those increments in place and unchanged. See section V.H of the preamble to the final 

rule. Similarly, EPA is not taking any action with respect to coarse PM in this rule as 

recommended by a commenter. For these reasons, the commenter‟s arguments on what 

authority must be used to set increments for PM10 and/or coarse PM, and that EPA has some 

obligation to take action with respect to coarse PM, are not on point for this rule. Thus, no 

substantive response to this comment is needed. 

We disagree with the comment that the removal of the PM10 annual increment is 

supported by the removal of the health-based standard for annual PM10. Based on legal 

arguments contained in some of the comments and our own legal analysis of the PM10

increments, we have concluded that there is a strong legal basis for not revoking the annual 

increments at this time, even though we have revoked the annual PM10 NAAQS. The PM10

increments were promulgated on June 3, 1993 (58 FR 31622) as replacement increments for the 

then existing statutory increments for PM measured as TSP. The fact that EPA promulgated the 

PM10 increments as “equivalent” replacements for the TSP increments under the authority of 

section 166(f) of the Act is important in that EPA does not have authority to simply remove the 

TSP increments that were explicitly defined within the PSD program requirements in the Act. 

Accordingly, we believe that the annual TSP increments would be restored by default should we 

decide to revoke the annual PM10 increments as proposed. However, even if the original annual 

TSP increments were not restored, there is no basis for automatically revoking the annual PM10

increments simply because we have revoked the annual PM10 NAAQ, because annual increments 

are not contingent upon the existence of annual NAAQS. This is clear from the court‟s decision 
58

in the earlier NO2 increment litigation stating that increments for a particular pollutant do not 

necessarily need to match the averaging periods that have been established for NAAQS for the 

same pollutant. Environmental Defense Fund, Inc. v. EPA, 898 F.2d 183, 189-190 (D.C. Cir. 

1990) (“... the „goals and purposes‟ of the PSD program, set forth in § 160, are not identical to 

the criteria on which the ambient standards are based.”)

The commenter‟s concerns about the alleged excessive conservativeness of the PM10

NAAQS is not relevant to this final rule establishing increments, SILs, and SMC for PM2.5 under 

the PSD program. Therefore, we are not responding to that particular concern in this document.
59

4 Comments on Significant Impact Levels (SILs) for PM2.5

4.1 Comments on the Legal Basis for SILs

Comment:

One environmental group (0040) presented extensive legal and policy arguments against 

SILs. The legal arguments are summarized in this section, while the policy arguments are 

discussed below in Section 4.2 of this document. In brief, the main points of the commenter’s 

legal concerns are as follows:

 The EPA has no authority to promulgate SILs. Congress codified increments in 

section 163 of the Act, directing that SIPs contain measures assuring that the 

increments shall not be exceeded (emphasis added in the comments). A primary 

method for assuring that increments are not exceeded is the requirement that PSD 

sources must demonstrate that their emissions ―will not cause or contribute‖ to a 

violation. The Act plainly provides that no major source may be constructed unless it 

meets this requirement, and may not contribute to an exceedance ―for any pollutant in 

any area.‖ Congress did provide exemptions where they were intended, but did not 

provide for exemption based on SILs or anything similar. Thus, SILs are plainly 

incompatible with the statutory language. The EPA cannot justify SILs as being just 

a shortcut to determining compliance with increments and NAAQS because there is 

no data or analysis in EPA’s proposal showing any necessary or rational connection –

the SILs are arbitrary numbers based on ratios that have no demonstrated or rational 

relationship to compliance with increments and NAAQS.

 The de minimis doctrine does not apply to increment analyses and preconstruction 

monitoring requirements. This doctrine does not apply where Congress has expressly 

directed that the letter of the law applies in all circumstances [Shays v. FEC, 414 F.3d 

76 (D.C.Cir. 2005)], as it has in this case. There is no room for de minimis exceptions 

in the statute. In any case, the de minimis doctrine is inapplicable because it applies 

only where the regulation will yield a gain that is demonstrably trivial or zero. The 

EPA has offered no evidence that sources with impacts below the proposed SILs will 

never cause or contribute to violations of increments or NAAQS, or that the gain 

from regulating such sources will in fact be trivial., The proposed SIL values are 

hardly trivial, amounting to a quarter to a half of proposed increments in some cases. 

Moreover, the idea that a single national number can define ―trivial‖ is flawed, given 

that even very small impact can be of great significance in an area that is close to an 

increment or NAAQS.

Response:

As EPA explained in the preamble to the proposed rule, the legal basis for SILs is 

grounded on the de minimis principles described by the court in Alabama Power Co. v. Costle, 

636 F.2d, 323, 360 (D.C. Cir. 1980) (“Categorical exemptions may also be permissible as an 

exercise of agency power, inherent in most statutory schemes, to overlook circumstances that in 
60

context may fairly be considered de minimis.”). See 72 FR 54112, 54139 (September 21, 2007). 

Further, as the Alabama Power court explained, the proper use of a de minimis exemption is not 

a departure from the statute, but is instead “a tool to be used in implementing the legislative 

directive.” 636 F.2d at 360. The Alabama Power court concluded that “most regulatory 

statutes, including the Clean Air Act, permit [de minimis] showings in appropriate cases. 636 

F.2d at 360. The court went on to say that, “unless Congress has been extraordinarily rigid, 

there is likely a basis for an implication of de minimis authority to provide exemption when the 

burdens of regulation yield a gain of trivial or no value.” 636 F.2d at 360-361. Although this 

comment identifies several statutory provisions that the commenter contends are inconsistent 

with a de minimis exemption based on SILs, the commenter does not provide any basis for 

concluding that these provisions are the kind of “extraordinary rigid” provision that forecloses 

the use of a de minimis exemption. Thus, where the SIL values set by a rule identify a level of 

impact such that “the burdens of regulation” (in this case certain requirements under CAA 

section 165) will “yield a gain of trivial or no value,” the establishment of SILs is a permissible 

de minimis exemption.

Further, the comment cites Shays v. FEC, 414 F.3d 76 (D.C. Cir. 2005) as support for 

the conclusion that the de minimis doctrine does not apply here. The Shays case, however, does 

not support the contention that Congress in the CAA foreclosed the use of the de minimis 

exemption. The Shays case simply confirms the limits of the de minimis doctrine from the 

Alabama Power decision: that the de minimis doctrine does not apply where the statute is 

“extraordinarily rigid” or where “the regulatory function does provide benefits, in the sense of 

furthering the regulatory objectives.” 414 F.3d at 114.

As explained above, the SIL is expressly used as a screening tool to determine whether a 

source‟s impact is considered to be significant enough to cause or contribute to a violation of an 

increment or NAAQS. Accordingly, a relatively small contribution (an air quality impact below 

the SIL) to an existing modeled violation will generally not be considered to contribute to the 

violation, recognizing that the violation results from existing facilities causing more substantive 

impacts that result in the air quality levels exceeding the NAAQS or increments.

4.2 General Comments on SILs for PM2.5

EPA received overwhelming support for the development of SILs to serve as a screening 

tool for implementing the air quality impact requirements under PSD. Nevertheless, commenters 

varied in their support of a specific option for defining the PM2.5 SILs. This section summarizes 

both the supporting and opposing comments that we received.

Comments Supporting SILs:

Two state/local agency commenters (0015, 0025) and four industry and industry 

association commenters (0013, 0035, 0038, 0043) express general support for EPA’s proposal to 

codify PM2.5 SILs. One of the state/local agency commenters (0025) recommends that EPA 

finalize SILs as expeditiously as possible to facilitate their use for both determining the 

significance of a source on its surroundings (in attainment and nonattainment areas) and 

determining when and if a cumulative impacts review is necessary. 
61

One industry commenter (0013) supports EPA’s efforts to address the final elements that 

aid implementation of the PSD program for PM2.5, which includes SILs (and increments and 

SMC). An industry association commenter (0038) strongly supports the establishment of PM2.5 

SILs (and increments and SMCs) that properly balance the economic and environmental goals of 

the PSD program.

Another industry association commenter (0035) supports the use of SILs (and SMCs) in

screening models to estimate the impact of proposed projects in order to avoid costly air quality 

impact analyses for projects that are unlikely to have consequential impacts on air quality. 

Similarly, an industry commenter (0043) agrees that SILs can serve as an effective screening tool 

to establish de minimis impact levels so that more time-consuming, extensive analyses are not 

required for situations with only minimal potential impact.

Comments Opposing SILs:

One environmental group commenter (0040) opposes SILs on legal and policy grounds. 

The policy grounds are summarized in this section, while the legal grounds are discussed above 

in Section 4.1 of this document. In brief, the main points of the commenter’s policy concerns are 

as follows:

 A review of the implementation of the proposed SILs for other PSD pollutants shows 

that impacts that are not de minimis are often exempted. The Class I SIL for NO2 is 

virtually never exceeded, with the result that almost all PSD sources have been 

permitted with no review of whether the facility will cause or contribute to an NO2

increment violation in a Class I area. Compliance with the NO2 increments has 

become a significant issue in recent years, in large part because of a boom in NOx

emissions associated with oil and gas development, much of which is from minor 

sources. Under the SILs for NO2, virtually no new PSD source locating in an area 

affected by such minor sources would be required to evaluate whether its additional 

NOx emissions would actually be contributing to NO2 increment violations. The 

commenter presented examples of cases where these concerns are alleged to have 

been born out. The commenter added that experience shows similar issues with PM10

and SO2 SILs. Finally, the commenter gave examples of some facilities impacting 

areas known to be in violation of increments that have been permitted based on 

modeling showing that their contributions will be at levels less than the proposed 

SILs.

 The EPA must assure that SILs are truly de minimis and must also include limitations 

on the use of SILs as necessary to prevent significant deterioration of air quality. The 

specific numbers proposed for PM2.5 SILs cannot rationally be characterized as trivial 

or insignificant. Moreover, the proposed rule would exempt a source meeting SILs 

from conducting an increment analysis even if there is an existing or projected 

increment violation in the area – no situation where an a NAAQS or increment is 

being violated can lawfully or rationally be considered de minimis relative to the 

requirement in section 165(a)(3)(A) of the Act that PSD sources cannot be allowed to 

cause or contribute to a violation of any increment or NAAQS
62

Response:

We believe that the PM2.5 SILs will provide a useful screening tool that provides an 

effective means of implementing the requirements for an air quality impact analysis for PSD. 

The SILs in general help to streamline the PSD process for determining which PSD sources must 

conduct a comprehensive cumulative air quality impact for PM2.5 under the PSD rules. 

Alternatively, sources whose impacts are considered de minimis are allowed to avoid the 

expenditure of completing a comprehensive cumulative analysis that will have only minimal 

effects on the environment. (Our legal rationale for the SILs is provided in the next section.) 

When a proposed source‟s impact by itself is not considered to be “significant,” EPA has long 

maintained that any further effort on the part of the applicant to complete a cumulative source 

impact analysis involving other source impacts would only yield information of trivial or no 

value with respect to the required air quality impact assessment for the proposed source or 

modification.

We believe that we have the discretion to use the de minimis concept to help determine 

whether an emissions increase will “cause or contribute to” a violation of a NAAQS or 

increment. Consequently, we believe that it is reasonable to allow new or modified sources that 

contribute only minimally (SILs represent relatively small percentages of the corresponding 

NAAQS and increments) to a modeled violation of a NAAQS or increment to receive their 

construction permits in light of the fact that the modeled violation can be attributed to emissions 

from other existing sources whose operation generally plays a more dominant role in the air 

quality exceeding the NAAQS or increment. Accordingly, states that consider de minimis 

impacts not to cause or contribute to a violation must still substantiate and then address any 

violations of a NAAQS or an increment, if appropriate. For example, a state might choose to use

their SIP planning process to address the violations by requiring the a reduction of emissions 

from existing sources, where more meaningful remedies, producing more substantial air quality 

improvements, are available from such existing sources . Further reducing the emissions from 

an already well-controlled source (such as the proposed PSD source) with relatively small air 

quality impacts would naturally yield only minimal air quality improvements (as illustrated by 

the level associated with the SIL) and would not produce the levels of air quality improvement 

that could come from the control of older, existing sources. 

In short, it is not our intent that the SILs be used as a means of allowing known adverse 

air quality situations to be ignored just because they are small. If, for example, the cumulative 

effect of a number of de minimis impacts in an area can be shown to cause air quality problems 

that are not being addressed simply because no source‟s impact is “significant” then the 

permitting authority must address the air quality problems, whether by using lower de minimis 

levels or temporarily cease using the de minimis concept altogether where it is known to allow 

the adverse conditions to worsen without remedy, or taking some other step to address the 

problems. 
63

Comment:

The same environmental group commenter (0040) provides that EPA’s adoption of any 

SILs must include detailed requirements as to how a source must evaluate its impact to compare 

to the SILs. The EPA must adopt rules setting criteria for this analysis under section 165(e)(3). 

These criteria must ensure a conservative analysis, such as modeling direct and precursor 

emissions, modeling worst case potential emissions for each averaging time, comparing 

maximum modeled impact against the SILs, following the Guideline on Air Quality Models, and 

following EPA requirements for acceptable receptor spacing, among other things.

Response:

We agree with the commenter‟s recommendation that we provide information for 

comparing a source‟s impact against the SILs. We do not, however, believe that this information 

needs to be in the form of “detailed requirements,” which presumably would be contained in the 

PSD rules. Instead, we believe that procedures in the form of guidance are appropriate for 

assisting permit applicants and permitting authorities. To this end, we have already provided 

guidance for comparing the impacts of a proposed project‟s PM2.5 emissions against the SIL for 

purposes of complying with the NAAQS. This guidance specifically takes into account the form 

of both the annual and 24-hour PM2.5 standards and recognizes that the application of the SILs 

should follow a conservative approach for estimating a source‟s modeled ambient impacts. See 

EPA memorandum titled “Modeling Procedures for Demonstrating Compliance with PM2 5

NAAQS.” (March 23, 2010.) We also plan to address our recommended approach for using the 

PM2.5 SILs for purposes of meeting the new PM2.5 increments in forthcoming guidance. 

Comment:

One industry association commenter (0029) states that SILs must be consistent with the 

twin goals and purposes of the PSD program: preventing significant deterioration while allowing 

reasonable economic growth. The commenter explains that EPA must consider the 

administrative burdens associated with setting SILs at very low levels, which would require PSD 

permit applicants to undertake onerous and time-consuming cumulative impact assessments for 

proposed sources that are well controlled, have trivial impacts, and are located up to 300 km

from any Class I areas. The commenter argues that imposing such an administrative burden 

upon the reviewing authority and the permit applicant is neither reasonable nor legally justified 

when it is triggered by a trivial predicted impact on the PSD increments for a Class I area, 

especially when other PSD program elements – e.g., the AQRV-evaluation provisions of section 

165(d) of the Act – will fully protect Class I areas. 

Response:

This comment contains a number of misconceptions about how SILs are set. First, SILs 

are not set based on the need to balance the goal of preventing significant deterioration in air 

quality SILs and the goal of allowing reasonable economic growth. Second, SILs are not set at a 

level to ensure that permit applicants will avoid the “administrative burdens” of meeting the 

statutory requirements for getting a PSD permit. Third, the fact that a proposed source is well-
64

controlled or located a far distance from where it causes impacts does not enter into the 

calculus. Instead, as EPA explained in the preamble to the proposed rule, the legal basis for 

SILs is grounded on the de minimis principles described by the court in Alabama Power Co. v. 

Costle, 636 F.2d, 323, 360 (D.C. Cir. 1980). See 72 FR 54139. Thus, the proper value to select 

for a SIL is a value that “implements the legislative design” by avoiding “trifling matters,” 

“pointless expenditures of effort,” and “trivial items.” 636 F.2d at 360 (“The ability ... to 

exempt de minimis situations from a statutory command is not an ability to depart from a 

statute, but rather a tool to be used in implementing the legislative design.”). 

4.3 Comments on the Relationship of SILs to AQRVs

Comment:

One industry association commenter (0038) and one federal agency commenter (0044) 

support position set out by EPA in the 2007 NPRM that PM2.5 SILs for Class I areas are not 

intended to serve as thresholds for determining the need for an AQRV analysis or whether an 

adverse impact on an AQRV will occur. The federal agency commenter (0044) indicates that the 

2007 NPRM makes clear that the fact that a source’s predicted impact is less than the Class I SIL 

would neither relieve the source from having to complete an AQRV impact analysis, nor 

automatically allow the reviewing authority to reject the FLM’s demonstration of adverse AQRV 

impacts (citing 72 FR 54139), and adds that this an important clarification given the separate 

Class I increment and AQRV impact tests of the PSD regulations.

The generally supportive industry association commenter (0038) and one state/local 

agency commenter (0015) believe that SILs should be used as an appropriate de minimis

threshold when an FLM has not identified a specific AQRV related to the pollutant under 

evaluation. The state/local agency commenter (0015) believes that SILs also should be used as a 

de minimis threshold when an FLM has not obtained ambient air monitoring to confirm that 

predicted concentrations from air dispersion models are representative of actual AQRV impacts 

in a Class I area. This commenter added that without this flexibility, applicants would be 

required to conduct complex and extensive Class I air dispersion modeling with no clear 

objective, and regulatory agencies would have to review the modeling with limited information 

to determine if the emissions could cause an ―adverse‖ impact or to determine if potentially 

costly controls should be required.

Response:

We agree with the commenters who support the position on the relationship between SILs 

and AQRVs that EPA set out in the 2007 NPRM. 72 FR at 54139 (September 21, 2007). We 

reiterate that this remains our position 

With regard to the comments recommending the use of SILs for AQRVs, we believe that 

by following these commenters‟ recommendation, a source would be required to complete an air 

quality analysis for a Class I area when the source‟s emissions would exceed the Class I SIL –

even though the FLM has not identified a specific AQRV that may experience adverse effects. It 

is difficult to see what kind of analysis would be conducted in the absence of any known AQRV 
65

that could be adversely affected by a pollutant emitted by the proposed source. We believe that 

the need for a Class I analysis, other than the required analysis of the NAAQS and Class I 

increments (for both of which the SILs were intended to be used), should be based solely on the 

potential for adverse effects on an AQRV that the FLM identifies as being sensitive to the 

pollutant under consideration. When the FLM does not identify an AQRV for which an adverse 

effect could occur, sources should not be required to perform a Class I analysis, other than the 

analysis of the NAAQS and Class I increments, regardless of the relation between the source 

emissions and the Class I SIL.

4.4 Comments on the Proposed Levels for SILs

4.4.1 General Comments on Proposed SIL Levels

Comment:

One industry commenter (0016) suggests that EPA develop geographic area-specific SILs

(and increments and SMC) that take local conditions into account. The commenter pointed out 

that PM2.5 levels in PSD areas proximate to international borders may be elevated by sources 

outside the legal and practical control of the United States and state authorities. The commenter 

also noted that PM2.5 levels may be elevated by natural conditions, such as drought, fires, 

geologic formations (sandy or fine-grained surface features) high winds, etc., leading to 

excessively dusty ambient conditions over which the local area has no control. The commenter 

indicated that local area baselines must reflect these PM emissions, though they are not reflected 

in the local area’s emissions inventory. The commenter urged EPA not to penalize such PSD 

areas by imposing uniform national SILs (and increments and SMC) where the conditions of 

concern are not capable of control.

Response:

We do not believe it is necessary or appropriate to establish geographic area-specific 

SILs to identify a significant impact. Regardless of the existing air quality and what sources of a 

pollutant may contribute to such levels, we believe that it is reasonable to identify a source‟s 

“significant” impact consistently from one area to another. The SILs define a projected ambient 

impact that a stationary source of PM2.5 would cause relative to the applicable PM2.5 NAAQS or 

increments. This value is not directly affected by the total amounts of PM2.5 that may exist in an 

area or by what the source of such PM2.5 concentration may be. We believe that the use of areaspecific SILs is best left up to the discretion of the individual state/local air pollution control 

agencies that are charged with managing the air quality in a particular area. Thus, individual 

states may adopt significance levels that differ from the EPA-defined levels as long as the level 

selected is at least as stringent as the EPA definition. Moreover, states are not required to 

consider any impact as being insignificant, and may alternatively choose to require proposed 

sources with minimal impacts to complete a comprehensive analysis under the PSD 

requirements.
66

Comment:

One industry commenter (0046) generally opposes all the proposed SILs, contending that 

all the proposed SILs are extremely low. The commenter also notes that EPA has proposed the 

same concentrations for Class II and III SILs for all three options. The commenter claims that

the proposed SILs imply a level of monitoring and modeling sophistication that is currently 

absent in our regulatory scheme and suggests that EPA rethink the level of the proposed SILs 

and select concentrations less likely to be within the level of error inherent in current monitoring 

and modeling methods.

Response:

We disagree with this commenter‟s view that all the proposed SILs are too low. For all 

of the reasons that we have provided on the record of this rulemaking, the SILs must be set 

sufficiently low so as to be de minimis. 

We also disagree that the SILs should be consistent with current monitoring capabilities 

for PM2.5. The SIL represents a screening tool that is used in comparison with modeled 

prediction – not monitored concentrations – of PM2.5. Ambient air quality monitoring accuracy 

is not a relevant concern in predicting concentrations of a pollutant that a source will cause if its 

construction and operation are allowed to occur. While the values that we selected are lower 

than the values under Option 1, we believe that they represent values that are in line percentagewise with the other pollutants in comparison to the respective NAAQS. We provide further 

discussion of our reasons for selecting the Option 3 values in section VI.B.2.a and VI.B.2.b of the 

preamble to the final rulemaking.

Comment:

One industry association commenter (0014) argues that the proposed SILs (and 

increments) should be twice the proposed levels. The commenter (0014) claims that scientific 

studies do not support the need for this low a level for protection of public health and welfare,

and that EPA is jeopardizing the goal of providing opportunities for economic growth, especially 

given that EPA has provided no guidance on how modeling will be done and what factors will be 

used. The commenter contends that EPA has relied too much on epidemiological studies (which 

cannot establish causation) and not enough on laboratory-based toxicological studies specifically 

designed to determine causation.

Response:

The commenter‟s inference that the SIL should be set at a level that provides protection 

of health and welfare is unfounded, as is the commenter‟s linking of the SIL and opportunities 

for economic growth. It is the express purpose of the NAAQS to define levels of air quality 

consistent with the protection of health and welfare. The SILs, on the other hand, are intended 

as a screening tool to determine the level of air quality analysis required for any proposed new 

or modified source under the PSD preconstruction review process. As explained elsewhere, EPA 

has discretion to allow the exemption of de minimis impacts from the burdens associated with 
67

completing a comprehensive, cumulative air quality analysis. Such de minimis impacts are not 

based on scientific findings related to the health and welfare effects of a particular pollutant.

The SIL is used largely as a screening tool to determine the level of air quality analysis 

that a proposed source or modification must undertake to satisfy the requirements for an air 

quality impact analysis. We do not believe that being used in this capacity is a factor in 

determining whether source will decide whether it will construct or modify its facility. The SIL‟s 

ability to affect opportunities for economic growth may become a factor if the source, based on 

the use of the SIL, is found to contribute to a modeled violation of a NAAQS or increment. The 

SIL is thereby used to determine whether the source‟s contribution is significant enough to be 

considered to “cause or contribute to” such modeled violation. We believe we have set the 

levels of the SILs for PM2.5 relative to the PM2.5 NAAQS to where they are comparable to the 

SILs for other pollutants relative to their corresponding NAAQS. 

Comment:

One industry association commenter (0029) submitted comments discussing all the 

proposed options for setting SILs without endorsing any option. The commenter believes that 

proposed Option 1 for Class II and III SILs is consistent with the Act, but contends that the 

Class I SILs proposed under Option 1 are without legal or practical support. (The commenter’s

arguments opposing the Option 1 Class I SILs are summarized below in Section 4.2.2 of this 

document.) 

The commenter agrees that EPA is authorized under the Act to use approaches like those 

underlying Options 2 and 3. However, the commenter requests that if EPA chooses to adopt 

Option 2, which uses the 0.8 multiplier (based on the overall ratio of PM2.5 emissions to PM10 

emissions), EPA make clear that this ratio should not be used for other regulatory purposes. 

Despite agreeing that an approach like that underlying Option 3 is acceptable under the Act, the 

commenter contends that EPA’s use the 0.3 ―stringency factor‖ based on the ratio of the PM2.5

NAAQS to the PM10 NAAQS is not appropriate for the same reasons discussed above in 

Section 2.3.1 in relation to increments.

Response:

For the final rule, we decided to finalize Option 3 for the PM2.5 SILs for the reasons 

discussed further in the sections that follow. We agree with the commenter that the SILs 

promulgated in this rule are consistent with the Act. With respect to the commenter‟s support of 

some of the other proposed SILs, we will not address those comments here, as those options were

not selected for the final rule.

Regarding the use of the 0.8 multiplier for other regulatory purposes, see our response to 

this issue in Section 8.3 of this document. Regarding the comment on the “stringency factor” 

derived based on the ratio of the PM2.5 NAAQS to the PM10 NAAQS, see our response to the 

commenter‟s similar assertion related to increments in Section 2.3.1 of this document. The 

commenter did not provide specifics in this comment about SILs as he did in the comment on 

increments, but we believe that the same computational principles apply.
68

Comment:

An environmental group commenter (0040) claims that the proposal illegally and 

arbitrarily ignores PM2.5 precursors. The commenter points out that the proposed text of the rule 

would allow a source to show compliance with a SIL by analyzing only the impact of direct 

PM2.5 emissions. The commenter asserts that this approach is illegal and irrational given that 

EPA itself has found that precursors can and do contribute significantly to ambient PM2.5

concentrations.

Response:

The commenter is correct that in the 2007 NPRM we proposed to add SILs for PM2.5 to 

the PSD regulations at 40 CFR 51.166 and 52.21 by adding a new paragraph to the regulations 

explaining that the requirements for a source impact analysis for PM2.5 would be considered to 

be satisfied, without further air quality modeling, if it were to be shown that the increase in 

direct PM2.5 emissions from the source or modification will cause air quality impacts less than 

the prescribed SILs for PM2.5. The reasoning at the time was that state-of-the-art modeling 

would not be available to adequately account for secondary PM2.5 impacts resulting from 

emissions of precursors of PM2.5, e.g., SO2 and NOx. Nevertheless, the existing PSD rules 

currently define potential precursors of PM2.5. Based on the proposed language, the required 

compliance demonstration for the PM2.5 NAAQS and the PM2.5 increments (when promulgated) 

would be limited by regulation to an analysis of direct PM2.5 emissions, and would not include 

consideration of emissions of PM2.5 precursors for comparing the modeled source impacts to the 

prescribed SILs for PM2.5.

The technical tools needed to complete a comprehensive analysis of all emissions that 

contribute to ambient concentrations of PM2.5 are only in the developmental stage; nevertheless, 

we believe that it would be inappropriate to restrict the regulatory language in such a way that 

future regulatory amendments would be required to enable the inclusion of precursor impacts in 

the PM2.5 analysis as the necessary technical tools become available. When EPA develops the 

necessary techniques for estimating the impacts of precursor emissions on ambient PM2.5

concentrations, we intend to consider those impacts in determining whether a source will have a 

significant impact using the SIL values in the final rule, which we could not do if the regulatory 

language precluded that from being done. We acknowledge the concerns that have been 

expressed by some commenters about the shortcomings of not considering the impacts of PM2.5

precursors under the PM2.5 air quality analyses. Accordingly, we believe that the new provision 

for applying the SILs for PM2.5 to the required analyses for the NAAQS and increments should 

not be self-limiting by specifying the use of only direct PM2.5 emissions. Instead, the new 

provision contained in the final rule provides that the test will be based on whether “the 

emissions increase ... would cause ... air quality impacts less than [the PM2.5 SILs].” See new 

40 CFR 51.166(k)(2) and 52.21(k)(2). We believe that it would be more effective to rely on 

interim policy and guidance as appropriate to help determine the best methods available to make 

the required assessment of source impacts on ambient PM2.5 resulting from any emissions. 
69

4.4.2 Comments on Proposed Levels for Class I SILs

In this section, we present all the comments supporting and opposing the proposed 

options for Class I SILs and advocating other levels for the Class I SILs, and then provide a 

single response to all the comments.

Comments Supporting Option 1 Level for Class I SILs:

One state/local agency commenter (0015) and two tribal commenters (0041, 0049) 

support SIL Option 1 for all areas, including Class I areas. The state/local agency commenter 

(0015) indicates that the use of a percentage of the NAAQS is consistent with Option 1 for 

increments. [We note that the commenter is incorrect; the Option 1 approach for the Class I 

increments relies on the use of 4 percent of the Class I increments – not the NAAQS.] The tribal 

commenters believe that the advantages of Option 1 better suit the tribal and environmental 

perspectives.

In addition, one industry commenter (0016) offers conditional support for Option 1, 

indicating that if it is established that EPA has sufficiently stated and, in fact, has legal authority 

for the proposed regulation of PM2.5 in the general manners proposed, the commenter would 

support Option 1 for the establishment of SILs (and increments and SMCs). 

One federal agency commenter (0044) that serves as the FLM for some Class I areas 

supports proposed Option 1 level for the annual SIL for Class I areas and either the Option 1 or 

Option 3 level for the 24-hour SIL for Class I areas. The commenter reports on a modeling study 

of four different coal-fired power plant scenarios using the EPA approved long-range transport 

CALPUFF model with the associated three dimensional CALMET metrological data for each 

particular scenario to assess the effectiveness of the different proposed PM2.5 Class I SILs. The 

commenter explains that the analysis consisted of four different analyses representing different 

geographic and climatic areas of the country where the National Park Service is seeing largescale electrical energy development from new coal-fired power plants, summarized as follows:

1. A large 1,500 MW coal-fired power plant in the desert southwest Four Corners area.

2. A moderate sized 500 MW coal-fired plant in central Wyoming.

3. A medium 800 MW coal-fired power plant located in the northern Great Lakes area.

4. A medium 800 MW coal-fired power plant located in the central Mid-Atlantic States 

area.

Based on the analysis, the commenter argues that the 24-hr proposed SILs of 0.07 μg/m3

(Option 3) and 0.08 μg/m3(Option 1) are more appropriately protective of the proposed Class I 

PM2.5 increment and impacts to visibility than the Option 2 proposed level of 0.24 μg/m3(see 

this commenter’s objections to the Option 2 level below). 

For consistency with the SO2, NO2, and PM10 SILs that EPA proposed in 1996, the same 

commenter recommends that EPA set the PM2.5 SILs at 4 percent of the respective Class I 

increments. The commenter notes that if EPA lowers the proposed Class I PM2.5 24-hour 
70

increment as the commenter recommends (see Section 2.2.2.2 on visibility), 4 percent of the 

lower level would be less than the values currently proposed by EPA. 

Comments Opposing Option 1 Levels for Class I SILs:

Two industry commenters (0020,0026) and two industry association commenters (0023, 

0029) express opposition to Option 1 levels for Class I SILs. 

One of the industry association commenters (0029) contends that establishing the Class I 

SILs by multiplying the proposed PM2.5 increment by 4 percent is without legal or practical 

support. The commenter adds that if these SILs for Class I areas were codified, emissions from 

even the most well-controlled coal-fired electric generating station, located as far away as 

300 km from a Class I area, could well exceed the threshold levels, triggering an extensive, timeconsuming, and expensive cumulative air quality analyses of the projected PM2.5 emissions from 

the proposed source and from sources located within a 300 km radius, possibly including sources 

located in multiple states, which would be even more burdensome when applied to a new 

pollutant like PM2.5 for which no historic data exist. The commenter stated that such an 

undertaking makes no sense when it is triggered by projected emissions impacts so trivial as to 

be barely measurable, and when other PSD program elements serve to address concerns 

associated with the protection of AQRVs in Class I areas.

One of the industry commenters (0020) similarly argues that there is no legal or 

reasonable justification for promulgating a level based on the historical use (1996 NSR reform 

proposal) of 4 percent of PM2.5 increments in setting PSD applicability levels when it is not 

needed to ensure protection of Class I areas. 

The other industry commenter (0026) questions EPA’s divergence from the method used 

to derive the Class II and III SILs under Option 1, which was based on the SILs for PM10. The 

commenter argues that Option 1 produces Class I SILs that are very stringent and will likely 

require extensive and costly cumulative air quality impact analyses even when predicted impacts 

would be minimal.

Comments Supporting Option 2 Levels for Class I SILs:

One state/local agency commenter (0033), three industry commenters (0026, 0027, 

0043), and one industry association commenter (0039) support the Option 2 levels for Class I 

SILs. The state/local agency commenter (0033) believes it is appropriate to achieve some degree 

of consistency by using Option 2 for both SILs and the SMC. One of the industry commenters

(0027) favors the Option 2 for Class I SILs because they are sufficiently low to protect Class I 

areas. Another of the industry commenters (0043) believes that the Option 2 values established 

by converting the PM10 SIL values to PM2.5 seem to properly balance the protection of Class II 

and Class I areas.

The industry association commenter (0039) supports Option 2 for Class I, II, and III SILs 

and expresses concerns over the already extensive timeline required for new or modified sources 

to obtain PSD permits. The commenter believes that if EPA sets the SILs unnecessarily low, 
71

many more projects than necessary would be required to conduct air quality increment

assessments, which would take at least an additional year to perform. The commenter urges

EPA to adopt SILs that will only require a full increment assessment where the SIL would 

accurately predict an exceedance of a PM2.5 increment for the project. The commenter agrees

with EPA’s statement in the 2007 NPRM that Option 2 SILs would serve as an appropriate de 

minimis SIL value and represent insignificant impact on ambient air quality (72 FR 54140).

Additional detail regarding this commenter’s arguments, and our responses to them, are 

presented below in Section 4.4.3 of this document addressing the Class II and III SILs.

One industry commenter (0026) that is supportive of the Class I SILs proposed under 

Option 2 expressed concern regarding the use of the uniform 0.8 PM2.5/PM10 ratio EPA has 

applied under this approach. The commenter states that if EPA uses this approach, it should 

clarify that the 0.8 ratio should not be used broadly for other regulatory purposes because the 

conversion ratio of PM10 to PM2.5 will differ for individual sources and from one area to another. 

(See our response to this issue in Section 8.3 of this document.)

Comments Opposing Option 2 Levels for Class I SILs:

One federal agency commenter (0044) that serves as the FLM for some Class I areas

opposes Option 2 for Class I areas. The commenter (0044) reports on the results of a modeling 

analysis conducted for four different geographic and climatic areas of the country where the 

National Park Service is seeing large-scale electrical energy development from new coal-fired 

power plants (see description above). Based on the results, the commenter states that the 

proposed Option 2 24-hr PM2.5 SIL of 0.24 μg/m3 would not be protective of the proposed 

Class I PM2.5 increment since it is approximately one-eighth of the proposed increment and is not 

exceeded in some scenarios as close as 20 km from a new coal-fired power plant. The 

commenter (0044) notes that the proposed 0.24 μg/m3 SIL would preclude a cumulative Class I 

increment analysis of the new source with other existing PSD source(s) located farther than 

20 km from the new source, but still close enough to a Class I area to affect its ambient PM2 .5

concentration and visibility impacts. 

One state/local agency commenter (0015), one industry commenter (0020), and one 

industry association commenter (0036) oppose Option 2 for SILs because it uses the 0.8 

multiplier derived from the ratio of PM2.5 to PM10 emissions for point sources in the 2001 

extrapolation of the final 1999 National Emissions Inventory (NEI). The state/local agency

commenter (0015) indicates that EPA has not demonstrated that this ratio reasonably represents 

PM2.5 emissions from all source types, and explains that that an increment analysis should 

include emissions from minor point sources, area sources, and mobile sources, The commenter 

believes that the proposal did not adequately explain how a ratio derived from utilities and 

industrial point source emissions would be representative or appropriate.

The industry association commenter (0036) questions the use of the 1999 NEI for PM2.5

that is based on the ratio of direct PM2.5 to direct PM10 emissions. The commenter (0036) 

believes that the inventory is outdated and incomplete, noting that the state of knowledge 

regarding both direct and indirect PM2.5 emissions has advanced significantly since 1999. The 
72

commenter suggests that an updated inventory that incorporates both direct and indirect PM2.5

emissions should be used for this purpose.

Another state/local agency commenter (0019) disagrees with EPA’s statement in the 

2007 NPRM that the SIL Option 2 approach is very similar to Option 2A for the increments. 

The commenter points out that in increment Option 2A, EPA used the emission ratio approach 

only for the annual PM2.5 increment and used this emission ratio in concert with the ratio of the 

corresponding PM2.5-to-PM10 NAAQS to derive the increments. The commenter indicates that if 

EPA had used both of these factors, Option 2 would have resulted in Class I SILs of 0.05 and 

0.07 μg/m3for the annual and 24-hour cases, respectively, which are very close to the Option 1 

Class I SILs.

Comments Supporting Option 3 Levels for Class I SILs:

Two state/local agency commenters (0019, 0025) and two state/local agency association

commenters (0018, 0021) support Option 3 for Class I SILs (or for all Classes). One of the

state/local agency commenters (0019) states that the Option 3 SILs are the SILs which can 

properly be interpreted to be truly de minimis. The other state/local agency commenter (0025) 

believes that Option 3 is the most health-protective method by which to derive SILs. 

One environmental group (0040) states that all the proposed options for SILs are 

irrational and illegal (see Sections 4.1 and 4.2 above), but Option 3 SILs are the least 

objectionable since they are the lowest, and therefore allow the least circumvention of the Act.

One federal agency commenter (0044) that serves as the FLM for some Class I areas 

supports proposed Option 1 level for the annual SIL for Class I areas and either the Option 1 or 

Option 3 level for the 24-hour SIL for Class I areas. 

Comments Opposing Option 3 Levels for Class I SILs:

One industry commenter (0043) and two industry association commenters (0037, 0038)

believe that the Option 3 Class I SILs are unreasonably low. The industry commenter (0043) 

argues that the values appear to be unrealistically low and, if selected, would point to the need 

for EPA to conduct an economic impact analysis.

One industry association commenter (0037) states that EPA should not simply apply 

NAAQS-based ratios because these do not translate back to the emissions point level when 

comparing PM10 and PM2.5. The commenter believes that this is an invalid method of 

proceeding because EPA has not shown that there is a correlation between the NAAQS and 

direct PM2.5 since there is no accounting for precursors and EPA does not have a quantifiable 

sense of the portion of PM2.5 that is condensable for various industries. 

The commenter questions whether EPA’s proposal of using the NAAQS ratios to set the 

SILs (and increments) will accommodate moderate, well-controlled development, as intended by 

Congress when it set the original increments, or if this approach will inhibit additional growth. 

The commenter suggests that EPA should provide a demonstration assuring that this proposal 
73

will not conflict with the prior regulatory history and intent of Congress and will allow for 

sufficient growth consistent with air quality needs. The commenter gave citations related to the 

congressional intent regarding increments (39 FR 42510, December 5, 1974; 45 FR 52676,

(August 7, 1980). 

Comments Suggesting Other Levels for Class I SILs:

Two industry commenters (0020, 0026) suggest that EPA revise the Option 1 SILs for 

Class I areas by using the PM10 Class I increments (4.0 μg/m3annual and 8.0 μg/m3 24-hours) as 

the starting point, which would result in Class I SILS of 0.16 μg/m3(annual) and 0.32 μg/m3

(24-hour). One industry association commenter (0023) also recommends that SILs for Class I 

areas should be raised to 0.16 μg/m3(annual) and 0.32 μg/m3(24-hour) without specifying the 

methodology for deriving these levels. The commenters believe that Class I SILs at these levels 

would meet EPA goals of preventing significant deterioration while ensuring economic growth.

One industry association commenter (0029) offers the following alternatives for 

determining Class I SILs under Option 1:

 Use the currently accepted Class I SILs for PM10, 0.2 μg/m3(annual) and 0.3 μg/m3

(24-hour). These values were interpreted by EPA as complying with the goals and 

purposes of the PSD program when they were proposed in 1996 (61 FR 38338, July 23, 

1996). This approach also would be consistent with how the Class II and III SILs were 

determined under Option 1.

 Start with the PM10 increments that already exist, 4.0 μg/m3(annual) and 8.0 μg/m3(24-

hour). These values could be used for SILs, or multiplied by a suitable multiplier. If a 

4-percent multiplier were used, the Class I SILs would be 0.16 μg/m3(annual) and 

0.32 μg/m3(24-hour). This level would accomplish the goals and purposes of the PSD

program and would reasonably limit the scope of the complex – and environmentally 

unwarranted – air quality analyses that would be required under the proposed Option 1 

level

 If starting with the proposed PM2.5 increments, use a reasonable multiplier. Just because 

4 percent may have been a reasonable multiplier to use in establishing a significant 

emission rate threshold does not mean that the multiplier should be used for a completely 

different regulatory purpose.

Response:

We chose the Class I SILs based on Option 3 because we believe that this option yields 

the most appropriate combination of SILs for all area classifications. While the Class I SILs 

under Option 3 represent levels that are somewhat different than 4 percent of the respective 

PM2.5 increments for Class I areas (6 percent for the annual SIL and 3.5 percent for the 24-hour 

SIL), they are sufficiently close (as derived from a ratio of the PM2.5 NAAQS to the PM10

NAAQS) so as to provide reasonable thresholds for defining de minimis for purposes of 

conducting a Class I increment analysis. We had proposed the use of 4 percent of the existing 

Class I increments to develop SILs for pollutants in a 1996 NPRM for a PSD rulemaking; 

however, that particular component of the proposal was never finalized. See 61 FR 38250 
74

beginning at page 38291. We will further discuss our rationale for selecting the suite of SILs 

under Option 3 in the discussion which follows for the Class II and III SILs.

With regard to the comment alleging trivial impacts from sources located up to 300 km 

from any Class I areas, we note that it is not supported by any technical documentation provided 

by the commenter. However, while we have not substantiated the reported results, a modeling 

analysis carried out and reported by a federal agency commenter (0044) did predict that certain 

new coal-fired power plants locating within 125 km from a Class I area could trigger the Class I 

24-hour SIL for PM2.5 as defined under either Option 1 or 3. That commenter further noted that 

under some scenarios a moderate to large coal-fired power plant could trigger those SILs values 

at distances of 300 km or greater. Distance is an important factor in determining the impact that 

a source‟s emissions may have on a Class I area, but many other factors play an important role, 

including stack height and other release parameters, topographic influences along the plume 

trajectory, and prevailing wind and other meteorological patterns across the modeling domain. 

Therefore, a source located a relatively great distance away from a particular Class I area 

should not simply be ignored because of its distance. 

Regarding the comment that the Option 3 Class I SILs are unrealistically low and, if 

selected, would point to the need for EPA to conduct an economic impact analysis, we wish to 

point out that EPA is required to complete and submit for Office of Management and Budget 

approval an Information Collection Request (ICR), which accounts for the burdens placed on 

state and local agencies and the permit applicants to meet the PSD program requirements. The 

ICR includes an estimate of the burdens associated with completing a pollutant-specific air 

quality analysis for each permit applicant. In estimating such burden, we do not take into 

account the various screening tools, including SILs. Thus, whether or not a source is actually 

exempted from having to complete a comprehensive cumulative impact analysis for a particular 

pollutant is not considered. Hence, the use of a SIL, regardless of its actual level, only serves to 

lower the burden on an applicant that is estimated as part of the ICR. 

4.4.3 Comments on Proposed Levels for Class II and III SILs 

Comments Supporting Option 1 Levels for Class II and III SILs

Comment:

One industry commenter (0026) supports Option I for Class II and III SILs, while another 

industry commenter (0027) supports Option I for Class II SILs. Two tribal commenters (0041, 

0049) support Option 1 for the SILs for all areas. These commenters believe that the advantages 

of this option better suit the tribal and environmental perspectives.

One industry commenter (0016) provides conditional support for the Option I SILs (and 

increments). The commenter indicates that other issues raised by the commenter (related to legal 

authority, precursors, lack of accurate stack tests, and the superiority of regional SILs (and 

increments) over national levels) must be addressed for this approach to have value.
75

One industry association commenter (0029) indicates that the approach used in Option 1 

for Class II and III SILs is consistent with the requirements of the Act, although the commenter 

does not explicitly express support for Option 1.

Response:

While these commenters support Option 1 for selecting the PM2.5 SILs levels, they do not 

provide any substantive basis for their support. As we indicate in our responses to the following 

commenters, we believe that the SILs defined under Option 1 for Class II and III areas are too 

large to reasonably represent de minimis levels for PM2.5 impacts.

Comment:

One state/local agency commenter (0015) agrees with EPA’s SIL Option 1. The 

commenter (0015) states that the use of a percentage of the NAAQS is consistent with increment 

Option 1. 

Response:

The commenter is incorrect in stating that the SILs under Option 1 are based on a 

percentage of the NAAQS. The Class I SILs were based on a percentage (4 percent) of the 

respective Class I increments, while the Class II and III SILs under Option 1 are the same SILs 

values used for other criteria pollutants, including PM10, currently contained in 40 CFR 

51.165(b). 

Comment:

One industry association commenter (0038) believes that the daily and annual SILs 

proposed under Option 1 would be sufficiently stringent and fully consistent with the de minimis

justification for SILs. The commenter adds that when conducting an air quality impact analysis 

to determine compliance with the SIL, most applicants assume all coarse PM10 to be PM2.5. The 

commenter claims that this is a conservative assumption, which overestimates the amount of fine 

particles being emitted and renders the effective SIL thresholds for PM2.5 lower than those 

written into the regulations. The commenter believes that meeting a modeled 5.0 μg/m3limit in 

the 24-hour air quality analysis would likely equate to an actual impact of 4.0 μg/m3or less. 

The commenter contends that this conservative skewing of the PM2.5 SIL analysis is 

compounded in the case of the daily SIL because the 24-hour SIL analysis typically uses the 

worst case meteorological data, assumes the highest emitting fuel for the source (even if that fuel 

is used only as back up fuel), and assumes maximum operating conditions. The commenter adds 

that the daily SIL modeling includes a conservative emission rate (usually based on the 

manufacturer’s guarantee) and, where applicable, an additional conservative estimate of PM 

contributions after selective catalytic reduction and oxidation catalyst control technologies. The 

commenter encourages EPA to set the 24-hour SIL to 5.0 μg/m3because of these built-in

conservatisms.
76

The same commenter further asserts that setting the 24-hour PM2.5 SIL at the 5.0 μg/m3

level would match the PM10 daily SIL, while establishing a daily PM2.5 SIL that differs from the 

daily PM10 SIL would introduce unnecessary regulatory burden for even the cleanest fossil-fuel 

burning electric generating units. The commenter explains that new, low emitting units are 

constrained by Good Engineering Practice stack height requirements and do not have a longrange, downwind impact; thus, such units generally do not have to undertake the cumulative 

impact analysis for predicted PM10 impact. The commenter notes that these units could be 

forced to conduct that analysis if EPA were to set the PM2.5 daily SIL lower than the PM10 daily 

SIL. 

Finally, the commenter concludes that this new burden would yield little environmental 

benefit because unless the other sources in the cumulative impact analysis align with the wind 

direction and are in the immediate vicinity of the source project, the in-depth, complex analysis 

will ultimately demonstrate that the new source will have virtually no impact. The commenter 

claims that having to perform a 24-hour cumulative analysis would be a needless exercise, and 

broad, national application of a PM2.5 SIL that differs from the PM10 daily SIL could 

unnecessarily delay or even derail the installation of even the cleanest-burning electric 

generating units (EGUs). 

Response:

We are particularly concerned about the class II and III SILs values being too high to use 

for PM2.5 purposes. In our final determination for selecting the SILs, we concluded that the 

option 1 SILs for class II and III areas represented too large a portion of the PM2.5 NAAQS to be 

considered de minimis. Most of the commenters supporting this option failed to provide 

substantive reasons for selecting those particular values. 

We disagree with the comment suggesting that the development of the SILs for PM2.5, or 

any other pollutant, should be influenced by the possibility that some sources currently may use 

“conservative techniques” (such as assuming that all coarse PM is PM2.5) for estimating a 

source‟s emissions rate. Conservative techniques, such as the ones described by the commenter, 

may be needed on an interim basis until technical issues associated with the calculation of PM2.5

emissions for a particular source are resolved, and can always be used as a screening 

methodology when appropriate. However, over time more accurate techniques will be available 

and should be relied upon whenever an alternative approach yields results that overestimate the 

source‟s impact. 

The commenter recommends that EPA set the PM2.5 SIL at same level as the PM10 SIL 

(i.e., based on Option 1), because of the various conservative assumptions factored into the 

modeling analysis. We recognize the conservative nature of the screening analysis and believe 

that a screening technique by its very nature should have a conservative tendency. We 

established the SIL for PM2.5 on the basis of what we believe to be a de minimis source impact on 

air quality relative to the 24-hour PM2.5 NAAQS and increments. Moreover, the 24-hour PM2.5

NAAQS is considerably lower than the 24-hour PM10 NAAQS, but the commenter does not 

account for this fact. Thus, the commenter‟s recommendation that the two 24-hour SILs should 

be the same does not result in an equitable representation of the relative impact that each SIL 
77

has on the corresponding NAAQS. That is, while the 24-hour PM10 SIL represents about 3

percent of the 24-hour PM10 NAAQS, the 24-hour PM2.5 SIL would represent about 14 percent of 

the 24-hour PM2.5 NAAQS. Moreover, that value would represent about 55 percent of the 

24-hour PM2.5 increment (Class II) contained in this final rule. We do not believe that such a 

result would provide an acceptable showing of de minimis consistent with other SIL values for 

criteria pollutants. 

Comments Opposing Option 1 Levels for Class II and III SILs:

One state/local agency commenter (0051) states that Option 1 (and Option 2) result in 

unacceptably high SILs values, allowing impact levels as much as 50 percent of the proposed 

increments for Class II areas. The commenter notes that her state’s air monitoring network 

shows that many areas are within 1 to 3 μg/m

3

of the annual PM2.5 NAAQS of 15 μg/m3and 

within 3 to 5 μg/m3 of the 24-hour PM2.5 NAAQS of 35 μg/m3, while Option 1 would allow for 

impacts of up to 1 μg/m3and 5 μg/m3on an annual and 24-hour basis, respectively. The 

commenter (0051) asserts that to allow sources to construct or modify with the potential to have 

air quality impacts that would threaten the NAAQS, yet not require them to perform an extensive 

air quality analysis, appears to undermine the state’s effort and ability to maintain attainment 

status for PM2.5 through regulation of stationary sources.

Another state/local agency commenter (0019) indicates that EPA provides no rationale 

for continuing the use of the current PM10 SILs under Option 1, except their historical use. In 

addition, the commenter points out that under Option 1 EPA proposed to use 4 percent of the

PM2.5 increments for the Class I SILs, but did not propose to use this approach to define the 

Class II and III SILs. The commenter notes that had EPA done so, the resultant PM2.5 Class II 

SILs would be 0.6 and 1.4 μg/m3for the annual and 24-hour averages, respectively.

Response:

As explained above, we agree that the SILs proposed under Option 1 for the Class II and 

III areas represent ambient concentrations of PM2.5 that are unreasonably high for defining de 

minimis impact levels relative to the existing PM2.5 NAAQS and for the increments selected in the 

final rule.

Comments Supporting Option 2 Levels for Class II and III SILs:

Three commenters (0033, 0039, 0043) support Option 2 for the Class II and III SILs (and 

the Class I SILs). One supportive state/local agency commenter (0033) believes it is appropriate 

to achieve some degree of consistency by using Option 2 for both SILs and the SMC. One 

supportive industry commenter (0043) believes that the Option 2 values established by 

converting the PM10 increment values to PM2.5 seems to properly balance the protection of 

Class II and Class I areas. 

One supportive industry association commenter (0039) points out that the SIL is a 

screening tool that estimates whether a source may cause or contribute to a violation of a PSD 

increment or the NAAQS. The commenter indicates that typically, if the ambient impact from 
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the proposed source would exceed the SIL, states and EPA require the project applicant to 

conduct 1 year’s worth of ambient air quality modeling to assess such future impacts from the

project. The commenter expresses concerns over the already extensive timeline required for new 

or modified source projects to obtain PSD permits. The commenter believes that if EPA sets the 

SILs unnecessarily low, many more projects than necessary would be required to conduct air 

quality increment assessments, which would take at least an additional year to perform. 

Accordingly, the commenter urges EPA to adopt SILs that will only require a full increment 

assessment where the SIL would accurately predict an exceedance of a PM2.5 increment for the 

project. 

The same commenter believes that the simplest, most straightforward approach would be 

for EPA to build on the SILs proposed for PM10 in the 1996 NSR Reform proposal and by 

further adapting those SILs by the emissions ratio of PM2.5 to PM10. The commenter agrees with

EPA’s statement in the 2007 NPRM that Option 2 SILs would serve as an appropriate de minimis 

SIL value and represent insignificant impact on ambient air quality (72 FR 54140).

Response: 

We find no relevance in being consistent in the methods used for selecting the screening 

tools – SILs and SMC – for PM2.5 as recommended by one of the commenters. The SILs 

represent ambient impacts, determined by modeling, that are counted against the respective 

NAAQS and increments. The SMC, as discussed later in this document, represents an ambient 

concentration that can be reliably measured by an ambient monitoring device. Thus, 

“significant” is considered in a completely different context for those two types of screening 

tools.

We disagree with the comments that support Option 2 as the most appropriate option for 

the Class II and III SILs. In particular, the comment suggesting that a lower SIL level will 

adversely affect the time required to obtain a permit is somewhat confusing. The commenter is 

correct in saying that the SILs are used to determine whether a proposed source would cause or 

contribute to a violation of either a NAAQS or PSD increment; however, the remainder of the 

comment then focuses only on impacts associated with the PSD increments. It is important to 

understand that applicants that model ambient concentrations above the SIL must complete a 

comprehensive cumulative air quality analysis for both the NAAQS and the increments, where 

applicable. The commenter then indicates that the applicant must submit 1 year‟s worth of 

ambient air quality modeling to assess such future impacts from the project. Typically, 

applicants must submit a modeling analysis including modeled impacts over a 5-year period 

based on 5 years of National Weather Service meteorological data. The commenter may be 

confusing this with the fact that 1 year of ambient air quality monitoring data may need to be 

collected as part of the analysis. The monitoring data is used primarily for the NAAQS 

analysis – not the increment analysis. The analysis for increment consumption tends to be based 

only on modeling projections. 

The same commenter‟s recommendation that EPA adopt SILs (based on Option 2) that 

will only require a full increment assessment where the SIL would accurately predict an 

exceedance of a PM2.5 increment for the project is also confusing. For one thing, the commenter 
79

only addresses the use of the SIL for purposes of analyzing the PM2.5 increments, and not the 

NAAQS. Secondly, it is not clear what is meant by “the SIL would accurately predict an 

exceedance of a PM2.5 increment for the project.” We explained in the proposal that the SIL 

would be compared against the ambient concentration that would result from a proposed 

source‟s emissions increase to determine whether that ambient impact was “significant.” As 

such, the SIL does not predict violations of increments, unless the source‟s estimated ambient 

impact is in itself greater than the applicable increment. Nor is it clear why a predicted 

increment violation should be the trigger for a comprehensive, cumulative air quality impact 

analysis. In general, the permitting authority should be aware of, and the public should be given 

an opportunity to comment on, the amount of increment that is available in an area because this 

may determine the amount of subsequent development that can occur in the area. The PSD 

regulations provide that the public be apprised of the amount of increment being consumed as 

part of the public notification process for each PSD permit.

Finally, we do not believe the comment recommending that the SILs be based on 

adapting the PM10 SILs using a ratio of PM2.5 to PM10 emissions adequately accounts for the 

levels of the PM2.5 NAAQS upon which de minimis impact is being defined. In addition, as 

pointed out by various commenters, that ratio does not account for precursor impacts on 

ambient PM2.5 concentrations, which are included in the provision containing the PM2.5 SILs in 

the final rule. Thus, we believe that scaling the existing PM10 SILs using a PM2.5-to-PM10

NAAQS ratio represents a more reasonable approach for defining the Class II and III SILs for 

PM2.5.

Comments Opposing Option 2 Levels for Class II and III SILs:

One state/local agency commenter (0051) believes that Option 2 (and Option 1) result in 

unacceptably high SILs values, allowing impact levels as much as 50 percent of the proposed 

increments for Class II areas. As discussed above in quantitative terms for Class I, the 

commenter’s state is quite close to the PM2.5 NAAQS in many area, and adoption of the Option 2 

SILs would allow sources to construct or modify with the potential to have air quality impacts 

that would threaten the NAAQS, yet not require them to perform an extensive air quality 

analysis. The commenter believes that this appears to undermine the state’s effort and ability to 

maintain attainment status for PM2.5 through regulation of stationary sources.

One state/local agency commenter (0015), one industry commenter (0020), and one 

industry association commenter (0036) oppose Option 2 for SILs because it uses the 0.8 

multiplier derived from the ratio of PM2.5 to PM10 emissions for point sources in the 2001 

extrapolation of the final 1999 NEI. The state/local agency commenter (0015) indicates that 

EPA has not demonstrated that this ratio reasonably represents PM2.5 emissions from all source 

types, and explains that that an increment analysis should include emissions from minor point 

sources, area sources, and mobile sources. The commenter believes that the proposal did not 

adequately explain how a ratio derived from utilities and industrial point source emissions would 

be representative or appropriate.

The industry association commenter (0036) questions the use of the 1999 NEI for PM2.5

that is based on the ratio of direct PM2.5 to direct PM10 emissions. The commenter (0036) 
80

believes that the inventory is outdated and incomplete, noting that the state of knowledge 

regarding both direct and indirect PM2.5 emissions has advanced significantly since 1999. The 

commenter suggests that an updated inventory that incorporates both direct and indirect PM2.5

emissions should be used for this purpose.

Another state/local agency commenter (0019) disagrees with EPA’s statement in the 

2007 NPRM that the SIL Option 2 approach is very similar to Option 2A for the increments. 

The commenter points out that in increment Option 2A, EPA used the emission ratio approach 

only for the annual PM2.5 increment and used this emission ratio in concert with the ratio of the 

corresponding PM2.5-to-PM10 NAAQS to derive the increments. The commenter indicates that if 

EPA had used both of these factors, the SILs for Option 2 would have resulted in Class II annual 

and 24-hour SILs of 0.24 and 1.4 μg/m3, respectively, which are much lower than the values 

proposed for Option 1 and very similar to those of Option 3.

Response:

We agree with these commenters‟ concern about the Option 1 and 2 SILs for Class I and 

II areas being too high. We acknowledge as well that the use of the 0.8 ratio for PM2.5-to-PM10 

emissions does not adequately represent the impacts on ambient PM2.5 concentrations caused by 

PM2.5 precursor emissions. We also concluded that basing the SILs levels on an emissions ratio 

does not adequately consider the stringency of the PM2.5 NAAQS in comparison to the PM10

NAAQS. Since the SIL is intended to be a measure of the impact of source‟s emissions relative to 

the NAAQS (and increments), we believe the level of the NAAQS must be an essential 

consideration for formulating the SIL level.

Comments Supporting Option 3 Levels for Class II and III SILs:

Two state/local agency commenters (0019, 0025) and two state/local agency association 

commenters (0018, 0021) support Option 3 SILs for Class II and III areas (or all areas).

One of the supportive state/local agency commenters (0019) argues that the Option 3

SILs can properly be interpreted to be truly de minimis and notes that the Class II SILs 

established under Option 3 are at 2 and 3.4 percent of the corresponding PM2.5 NAAQS, which is 

well in line with the 4 percent criterion used previously by EPA and the states to establish other 

de minimis values. The commenter adds that these percentages are in line with Class II SILs for 

all other pollutants (PM10, SO2, and NO2) established by EPA and used by the states, which range 

from 1 percent (annual NO2) to 3.3 percent (24-hour PM10) of the corresponding NAAQS. The 

commenter also notes that Options 1 and 2 would establish SILs at about 25 and 50 percent of 

the proposed PM2.5 Class II increments for the annual and 24-hour cases, respectively. The 

commenter asserts that these can hardly be defined as de minimis values.

The other supportive state/local agency commenter (0025) believes that Option 3 is the 

most health-protective method by which to derive SILs. 
81

One environmental group (0040) states that all the proposed options are irrational and 

illegal (see Sections 4.1 and 4.2 above), but Option 3 SILs are the least objectionable since they 

are the lowest, and therefore allow the least circumvention of the Act.

Response:

We agree with these commenters who support Option 3, without necessarily agreeing 

with all the statements that the commenters make in conjunction with their support for that 

option. First, we did not select the SILs under Option 3 under the premise that such SILs are

more protective of health. The NAAQS are established to protect against adverse health effects 

and are defined to provide a margin of safety. The SILs are a screening tool to determine, based 

on a source‟s ambient impact relative to the NAAQS, the extent to which a source must complete 

an air quality analysis to satisfactorily demonstrate compliance with the NAAQS and increments. 

The use of the SIL should not enable exceedances of the NAAQS to go unaddressed. To the 

extent that a particular SIL value enables a proposed source that completes a comprehensive, 

cumulative air quality impact analysis that shows violations of the NAAQS to be constructed 

without itself being considered to cause or contribute to such modeled violation, the state must 

still substantiate and address, as appropriate, any potential violation and ultimately demonstrate 

attainment with the NAAQS as part of the applicable implementation plan.

Secondly, we clearly do not agree with the comment that the SILs are “irrational and 

illegal.” We address the legality of the SILs in section 4.2 of this document.

Comments Opposing Option 3 Levels for Class II and III SILs

Comment:

One industry commenter (0043) argues that the Option 3 values appear to be 

unrealistically low and, if selected, would point to the need for EPA to conduct an economic 

impact analysis.

As summarized above in relation to Class I increments, one industry association

commenter (0037) states that EPA should not simply apply NAAQS-based ratios because these 

do not translate back to the emissions point level when comparing PM10 and PM2.5. The 

commenter believes that this is an invalid method of proceeding because EPA has not shown that 

there is a correlation between the NAAQS and direct PM2.5 since there is no accounting for 

precursors and EPA does not have a quantifiable sense of the portion of PM2.5 that is condensable 

for various industries. The commenter expresses concern that if EPA were to proceed with a 

NAAQS ratio approach, it would result in very low PM2.5 increments, and this would not take 

into account the expected condensable fraction.

The same commenter questions whether EPA’s proposal of using the NAAQS ratios to 

set the SILs (and increments) will accommodate moderate, well-controlled development, as 

intended by Congress when it set the original increments, or if this approach will inhibit 

additional growth. The commenter suggests that EPA should provide a demonstration assuring 

that this proposal will not conflict with the prior regulatory history and intent of Congress and 
82

will allow for sufficient growth consistent with air quality needs. The commenter gave citations 

related to the congressional intent regarding increments (39 FR 42510, December 5, 1974; 45 FR

52676, (August 7, 1980).

Response:

See our response concerning the alleged need for an economic impact analysis in 

Section 4.4.2 of this document.

By using the NAAQS ratio to set the levels of the SILs, we believe we are establishing 

comparable SILs for both PM10 and PM2.5. That is, the SILs represent similar impacts relative to 

the NAAQS with which they are associated. Because the NAAQS consider all ambient 

concentrations of PM2.5, they include ambient concentrations resulting from both direct PM2.5

(which includes the condensable fraction) and the secondary (precursor) component. Similarly, 

as set forth in the final rule, the PM2.5 SILs values represent ambient concentrations of PM2.5

that include the effects of both direct PM2.5 emissions and indirect (PM2.5 precursor) emissions. 

We disagree with these two commenters‟ conclusion that the SILs levels have an adverse 

effect on economic development, as the SILs are generally not directly indicative of economic 

growth. The SILs are a screening tool for determining whether individual PSD applicants must 

complete a comprehensive cumulative analysis of source impacts to satisfy the air quality impact 

analysis requirement. As such, the SILs do not prohibit source construction but merely establish 

the level of analysis required to satisfy the preconstruction requirements associated with the 

permit review process. Even when the SIL is used to determine that a proposed source‟s 

emissions would cause or contribute to a modeled violation of a NAAQS or increment, the source 

has opportunities to mitigate its impact so as not to have its permit denied. 

Comment:

One industry association commenter (0038) believes that the Option 3 SILs would be 

unreasonably low, especially in the case of the daily SIL as shown by the burden such a 

threshold could have on EGUs burning clean fossil fuels (e.g., natural gas and ultra-low sulfur

distillate). The commenter claims that based on preliminary, internal modeling and analysis, the 

1.2 μg/m3daily SIL proposed under Option 3 would exacerbate the problem of modeling 

demonstrations in urban areas when the source must demonstrate impacts below the PM2.5 SILs 

for both ground level and elevated receptors. The commenter explains that rules for modeling 

demonstrations for stable nonattainment pollutants (e.g., PM10 and presumably PM2.5) require 

that a net benefits analysis be performed at all ambient receptors, and because elevated receptors 

generally experience higher concentrations since these locations are closer to the centerline of 

the plume, the net benefit is more difficult to demonstrate at elevated receptors due to the higher 

impacts predicted at these receptors.

Response:

The commenter seems to recommend that we develop different SILs for use at different 

receptor elevations, or at least consider a higher SIL that accounts for potentially higher 
83

ambient air impacts at elevated receptors. We do not believe that it is necessary or appropriate 

to make such a distinction. The ceiling imposed by any NAAQS makes no distinction based on 

receptor elevation and the SILs are intended to define a threshold for significant impacts on the 

NAAQS (as well as the increments). We do not find a compelling argument here to lead us to 

consider a SIL that accounts for receptor elevation.

Comment: 

One industry commenter (0020) expresses concern over the use of 0.8 PM2.5/PM10

emissions ratio in SIL Options 2 and 3 because the ratio is not technically accurate. Similarly, 

one state/local agency commenter (0015) states that SIL Options 2 and 3 used a ratio of PM2.5 to 

PM10 emissions for point sources in the 2001 extrapolation of the final 1999 NEI.

Response:

These commenters stated that SIL Options 2 and 3 use the PM2.5/PM10 emissions ratio, 

but this is not correct with regard to the approach used for Option 3.

4.5 Other Comments on SILs

4.5.1 Comments on SILs for Other Pollutants

Two state/local agency commenters (0019, 0022), one state/local agency association 

commenter (0018), and one federal agency commenter (0044) request that EPA finalize SILs for 

all pollutants (including PM2.5). The state/local agency association commenter (0018) notes that 

SILs have been established as a PSD program screening tool to determine when cumulative 

reviews are necessary, but permitting authorities have also widely used SILs to determine the 

significance of the impact from a source in an attainment area on its surroundings, both within 

and outside nonattainment areas. One of the state/local agency commenters (0022) encourages

EPA to codify the Class I SILs for NOx, SO2, and PM10 proposed in 1996 because permitting 

agencies rely on the 1996 proposed levels as well as the promulgated Class II SILs in PSD 

permitting actions.

The federal agency (0044) recommends that EPA promulgate the 1996 proposed SILs for 

pollutants other than PM2.5 at this time for the reasons that EPA stated in the preamble of the 

proposed PM2.5 increment (citing 72 FR 54138-54139). The commenter explains that although 

the 1996 SILs are only proposed, those levels are commonly used by states, permit applicants, 

and FLMs to determine the need for a cumulative increment analysis. The commenter notes that

promulgating PM2.5 SILs provides EPA a great opportunity to close the regulatory loop by 

promulgating SILs for the other pollutants as well. The commenter supports the use of SlLs

because otherwise, a cumulative increment analysis would be required of every source, 

regardless of impact.

The other state/local agency commenter (0019) also suggests that the current action offers 

the opportunity for EPA to incorporate the Class I and Class II SILs for other pollutants into the 

PSD regulations. The commenter notes that many states have been using the Class I SILs in the 
84

1996 NSR Reform proposal since that time. The commenter adds that the SILs for sources in 

Class II attainment areas which are used to judge the contribution of these sources to nearby 

nonattainment areas pursuant to 40 CFR 51.165(b) have also traditionally been used by EPA and 

states to determine the significance of the impact from a source in an attainment area on its

surroundings, not just for impacts on nearby nonattainment areas. The commenter points out that

states and their organizations (e.g., NESCAUM and Western States Air Resources Council 

(WESTAR)) have previously asked EPA to finalize these SILs in the NSR programs to facilitate 

regulatory applications.

The same commenter recommends that EPA extend the applicability of the SILs in

40 CFR 51.165(b)(2), which currently apply only for sources in attainment areas that impact 

nonattainment areas, to sources in nonattainment areas as well. The commenter notes that these 

SILs have traditionally been used by states not only for the purposes of addressing sources in 

attainment areas impacting on nonattainment areas, but also for determining the contribution to 

standards violations by sources in nonattainment areas. The commenter notes that this 

requirement is clearly mandated by section 110(a)(2)(D) of the Act which prohibits any source 

within a state from significantly contributing to nonattainment or to projected NAAQS 

violations. Thus, the commenter urges EPA to clarify this requirement in the nonattainment 

major NSR provisions of the federal regulations and in the approval of state SIPs.

 

Response:

We have amended 40 CFR 51.165(b)(2) to define PM2.5 SILs as a screening tool to 

determine whether a major stationary source or major modification locating in an attainment 

area causes or contributes to a NAAQS violation at any location (including nonattainment 

areas). See 40 CFR 51.165(b)(2) in the final rule. It should be noted that a proposed major 

source or modification of a pollutant for which an area is designated nonattainment is not 

excluded from satisfying the relevant nonattainment NSR requirements based on a de minimis air 

quality impact in the nonattainment area; that source must obtain offsets because of the fact that 

it proposes to locate in the nonattainment area.

The EPA is not taking any action in the final rule for PM2.5 increments, SILs, and SMC to 

codify the Class I SILs for various pollutants that we proposed in 1996. The EPA will consider 

taking such action in a subsequent rulemaking.

4.5.2 Miscellaneous Comments on SILs

Comment on Repowering or Replacement Projects:

One industry association commenter (0038) explains that repowering and replacement 

projects can result in the substitution of cleaner, more efficient electric generating units operating 

in the place of older units, yet the repowered or replacement unit may have direct PM emissions 

that exceed the PM SIL, particularly at receptors proximate to the source (especially if the 

Option 3 SILs are adopted). The commenter adds that such units also have lower emissions of 

PM2.5 precursors (SO2 and NOx) compared to the replaced unit, and so would help lower PM2.5

levels in downwind nonattainment areas. The commenter states that EPA should recognize the 
85

net positive PM2.5 impact resulting from the reduction of emissions of PM precursors in the 

modeling demonstration for repowering or replacement projects by crediting them for their 

overall impact, perhaps through the use of a standard conversion rate across the board that would 

provide some credit. The commenter also suggests that another method worth considering would 

be to use the average PM2.5 receptor reduction throughout the nonattainment area attributable to 

the SO2 and NOx reductions.

Response:

In this final rulemaking for PM2.5 increments, SILs, and SMC, we are not taking any 

action to address the commenter‟s recommendations to allow PM2.5 netting credit for reductions 

in precursor (SO2, NOx) emissions. We are currently unable to quantify the “net positive PM2.5

impacts” resulting from reductions in precursor reductions. As we make progress in terms of 

estimating (modeling) precursor impacts on PM2.5, we will give additional consideration to the 

issue of precursor netting credits.

Comment on the Form of the SILs:

One industry association commenter (0038) notes that the proposal did not indicate how 

the proposed PM2.5 SILs are to be interpreted. The commenter contends that the form of the SIL 

(and, therefore, the predicted impact value to be compared with the SIL) should be consistent 

with the form of the PM2.5 NAAQS, which require that compliance with the 24-hour and annual 

standards be determined using 3-year averaging. Specifically, the commenter points out that the 

annual standard is calculated based upon the 3-year average of annual mean PM2.5

concentrations, and the 24-hour standard is based on the 3-year average of the 98th percentile (or 

high-8th-high) of 24-hour concentrations. 

The same commenter claims that in employing a 3-year test, the form of this NAAQS

differs significantly from that of the PM10 NAAQS and other criteria pollutants for which SILs 

have been long-established. The commenter notes that SILs have historically been applied in 

modeling through comparison of the highest predicted impact at any receptor during any 

individual year of the analysis with the appropriate SIL, and usually 5 consecutive years of 

meteorological data are employed in the analysis. The commenter believes that this is a 

reasonable approach because, in the cumulative, multi-source modeling analysis, the contribution 

of the new or modified source to a potential exceedance of the NAAQS could, theoretically, be 

as much as that highest value.

The same commenter explains that when a 3-year averaging is employed to determine 

compliance as in this case, it is not possible for a source to contribute its single year maximum 

predicted 24-hour impact to an exceedance of the 24-hour PM2.5 NAAQS; rather, the most that a 

new or modified source could theoretically contribute would be its highest 3-year average of 

maximum predicted 24-hour average impacts. The commenter adds that the use of a single year 

maximum 24-hour predicted impact for comparison with the SIL is therefore overly conservative 

and unnecessary to assure that sources which cannot significantly contribute are eliminated from 

cumulative analysis. Accordingly, the commenter suggests that the use of a rolling 3-year 

average of the three predicted single year 24-hour highest concentrations at any receptor is more 
86

appropriate and is still conservative in that highest concentrations are used rather than the high8th-high. 

For comparison with the annual PM2.5 SIL, the commenter suggests the use of a typical 

5-year meteorological database and determination of the predicted annual PM2.5 impact 

concentration at each receptor for each year. The commenter explains that once these values 

were determined, the facility would then generate three PM2.5 3-year average impacts at each 

receptor (i.e., for Years 1-3, Years 2-4, and Years 3-5). The commenter indicates that the overall 

maximum 3-year average value (at any receptor) would be selected for comparison with the SIL, 

rather than the single-year maximum. 

The commenter suggests that the methodology for comparison of the 24-hour PM2.5

predicted impacts to the 24-hour SIL would be similar and use the same basic procedure, but 

averaging maximum 24-hour impact predictions at each receptor across 3 years rather than 

annual impact predictions. The commenter expresses the opinion that despite the standard being 

based upon the 3 year average of the 98th-percentile 24-hour impacts for each year, the 98th 

percentile values cannot be used; instead, the 3-year average of maximum concentrations must 

be used because the facility could contribute up to that amount to a potential exceedance. 

Response:

We agree that the use of the SIL for PM2.5 should take into account the form of the 

NAAQS. Accordingly, in a March 23, 2010 EPA memorandum titled “Modeling Procedures for 

Demonstrating Compliance with PM2.5 NAAQS,”11 we provided guidance for using the SILs in 

conjunction with the 24-hour and annual PM2.5 NAAQS, which takes into account the statistical 

form of the NAAQS, consistent with the rationale presented by the commenter that the form of 

the SIL should reflect the potential contribution that the source could have to a potential 

exceedance. We note, however, that the recommended method for using modeling data does not 

rely on 3-year rolling averages but generally relies on a 5-year average calculation based on 

5 years of National Weather Service data. Thus, for the annual NAAQS, we recommend that the 

highest average of the modeled annual averages across 5 years for National Weather Service 

(NWS) meteorological data or the highest modeled annual average of 1 year of site-specific data 

be compared to the annual SIL for PM2.5. For the 24-hour NAAQS, we recommend that the 

highest average of the maximum 24-hour averages across 5 years for NWS data or the highest 

modeled 24-hour average for 1 year of site-specific meteorological data should be compared to 

the 24-hour SIL for PM2.5. In the March 23 guidance memo, we note that by using the average 

of the highest values across the years modeled, we preserve one aspect of the form of the PM2.5

NAAQS, and using the average of the first highest 24-hour averages rather than the 98th

percentile (8th highest) values from the distribution is consistent with the screening-level nature 

of the analysis. We also note that, following promulgation of the PM2.5 increments in the final 

rule, we intend to provide guidance for interpreting the SILs for their use with the 24-hour and 

annual PM2.5 increments as well. 

 

11 This EPA memorandum is available on EPA‟s website containing NSR/PSD guidance: 

http://www.epa.gov/region07/air/nsr/nsrmemos/pm25memo.pdf.
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5 Comments on the Significant Monitoring Concentration 

(SMC) for PM2.5

5.1 General Comments on an SMC for PM2.5

Three commenters (0013, 0035, 0038) express general support for EPA’s proposal to 

codify a PM2.5 SMC without indicating a preferred SMC level. Another commenter (0051) 

appears to support the SMC concept, but cautions that necessary actions should be completed 

prior to promulgating the SMC for PM2.5. In addition, other commenters indirectly indicate their 

support of the SMC concept by providing comments on their preferred option for setting the 

SMC level. These comments on the level of the SMC are presented in Section 5.3 of this 

document.

One commenter (0040) opposes the concept of SMC for PM2.5 under the PSD program 

claiming that the Act does not authorize EPA to exempt sources from the PSD ambient 

monitoring requirements. 

Comments Supporting an SMC:

One industry commenter (0013) supports EPA’s efforts to complete the rulemaking 

process required to address the final elements that aid implementation of the PSD program for

PM2.5, which includes an SMC (and increments and SILs).

One industry association commenter (0035) supports the use of SMCs (and SILs) as 

screening models to estimate the impact of proposed new and major modifications of major 

sources because they will greatly assist companies and regulators in managing resources for air 

quality impact analyses that truly have the potential to result in environmental impacts. The 

commenter indicates that the use of the SMC (and SILs) in screening models will avoid costly analyses 

for projects that are unlikely to have consequential impacts on air quality.

One industry association commenter (0038) strongly supports the establishment of PM2.5 

SMCs (and increments and SILs) that properly balance the economic and environmental goals of 

the PSD program. 

Although appearing to support the concept of an SMC (and increments and SILs), one 

state/local agency commenter (0051) expresses general concerns over the proposed regulations 

aimed at establishing the SMC value (and increment and SIL values) without first promulgating 

regulations or developing tools that (1) define PM2.5 and any precursors, (2) establish approved 

test methods or emission factors for determining PM2.5 from various common sources, and (3) 

provide an approved air dispersion model for determining PM2.5 air quality impacts from a 

source.
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Comments Opposing an SMC:

One environmental group (0040) opposes EPA’s proposed development of an SMC for 

PM2.5, believing that such screening tools are contrary to the requirements of the Act. The 

commenter’s extensive legal arguments are presented in Section 5.2 of this document.

Response:

We agree with the commenters who support promulgation of an SMC for PM2.5. 

Accordingly, the final rule includes an SMC for PM2.5 in revised 40 CFR 51.166(i)(5)(i)(c) and 

52.21(i)(5)(i)(c).

With regard to the comment that the SMC for PM2.5 should be delayed pending action 

concerning definitions for PM2.5 and its precursors, approved test methods or emissions factors, 

and approved air dispersion modeling, we acknowledge the importance of each of those 

components of the overall PM2.5 implementation program, and note that these issues are now 

being addressed. The EPA has in fact taken action on each of the commenter‟s concerns as 

follows:

 In the May 16, 2008 final rule for PM2.5 NSR implementation, EPA identified precursors 

of PM2.5 that must be regulated (73 FR 28321) under the PSD permit program. 

 For a number of years, EPA has been posting guidance on its website for measuring 

emissions of PM2.5, including the condensable fraction. The equipment, supplies, and 

procedures provided by this guidance have been improved over time by stakeholders who 

have submitted constructive comments. We believe this posted guidance has provided a 

reasonable means to quantify emissions that are suitable for use in developing emissions 

inventories; for developing information that is useful in developing appropriate 

achievable emissions levels for sources; and for assessing the performance of a source‟s 

particulate matter controls. Currently, existing published test methods for measuring 

direct PM2.5 emissions, as well as the condensable fraction, are being reviewed following 

formal notice and comment, and we expect them to be promulgated within the same 

timeframe as this final rule. 

 The EPA issued guidance on PM2.5 modeling and the recommended procedures for 

carrying out a PM2.5 source impact analysis (March 23, 2010 EPA memorandum signed 

by Stephen D. Page, Director, Office of Air Quality Planning and Standards, titled 

“Modeling Procedures for Demonstrating Compliance with PM2.5 NAAQS).

We respond to the comments concerning legal opposition to the SMC for PM2.5 in the 

following section.
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5.2 Comments on the Legal Basis for the SMC

Comment:

One environmental commenter (0040) asserts that the proposal to establish SMCs for 

PM2.5 is illegal and arbitrary for the same reasons that the commenter gave for SILs (see 

Section 4.1 of this document). The commenter adds that in section 165(e)(2) of the Act 

Congress mandates a full year of continuous air quality monitoring for each major source subject 

to the PSD program. The commenter indicates that there are no exceptions, other than the 

limited provision allowing for less than a year’s worth of monitoring based on a determination 

―that a complete and adequate analysis for such purposes may be accomplished in a shorter 

period.‖ The commenter argues that the allowance for a ―shorter period‖ hardly amounts to 

authority to waive monitoring entirely, which is what the SMC proposal would do. 

The same commenter claims that there is no data or analysis in the proposal showing any 

necessary or rational connection between compliance with SMCs and compliance with 

increments and NAAQS, or to the need for monitoring. The commenter adds that the SMC 

levels are arbitrary numbers based on application of an arbitrary multiplication factor to 

detection levels. The commenter contends that under the proposed rule, a source showing 

impacts below the SMCs will be totally exempt from the monitoring requirement even if the 

source will cause or contribute to an increment or NAAQS violation, which has occurred under 

prior EPA guidance to allow such an unlawful and arbitrary results.

The same commenter claims that EPA’s attempt to invoke the de minimis theory to 

justify SMCs fails for the same reasons as discussed with respect to SILs in Section 4.1 of this 

document. The commenter adds that the statute is explicit that monitoring is always required, 

leaving no room for de minimis exceptions. In addition, the commenter contends that Congress’

intent to foreclose EPA from inventing de minimis exceptions is shown by the fact that Congress 

did specify a narrow test for allowing less than a full years’ monitoring, and by the fact that 

Congress itself defined the universe of sources too insignificant to justify regulation by applying 

the permit program only to major sources. 

The same commenter adds that the de minimis theory is particularly inapplicable to 

monitoring requirements, which are by their nature meant to provide information to accurately 

determine the nature and degree of a source’s impact on air quality. The commenter claims it is 

illegal and irrational for EPA to assert that monitoring will yield trivial or no benefit due to the 

supposed insignificance of a source’s air quality impact, when Congress required such 

monitoring precisely ―for the purpose of determining whether emissions from such facility will 

exceed the maximum allowable increases or the maximum allowable concentration permitted‖ 

under the Act (citing section 165(e)(2) of the Act). The commenter states that EPA apparently 

believes it can allow substitution of modeling for the Act’s monitoring requirement, but the 

statute forecloses such substitution. Based on a comparison of sections 165(e)(2) and 

165(e)(3)(D), the commenter believes that Congress plainly knew the difference between 

monitoring and modeling, and did not allow EPA to substitute the latter for the former.
90

The same commenter notes that the continuous air quality monitoring data is typically 

used to determine background concentration for the NAAQS analysis conducted by a PSD 

permit applicant (citing EPA’s NSR Workshop Manual at C.27, C.34, C.46; as well as 40 CFR 

part 51, Appendix W, section 8.2.1.b). The commenter argues that without preconstruction 

monitoring data on existing PM2.5 concentrations, the results of the permit applicant’s NAAQS 

analysis will lack credible support. The commenter notes that EPA’s proposed regulations

would exempt a PSD permit applicant from preconstruction monitoring without any 

determination that existing, current, quality-assured monitoring data exist and that such data 

accurately represents background concentrations in the vicinity of the proposed source and 

would prevent an adequate or credible assessment of whether a proposed new or modified source 

would cause or contribute to a NAAQS violation. The commenter claims that for many areas, 

the NAAQS will be a more restrictive limitation on deterioration of air quality than the PM2.5

increments proposed by EPA, and EPA cannot allow any exemption from collecting the air 

quality data necessary to accurately define background concentrations of PM2.5. 

The same commenter opines that even if use of the de minimis doctrine was not 

precluded by the statute, it would be inapplicable in this case. The commenter explains that the 

de minimis doctrine applies only where regulation will yield a gain that is demonstrably trivial or 

zero. The commenter states that EPA has offered no evidence that sources with impacts below 

the proposed SMC levels will never cause or contribute to violations of increments or NAAQS, 

or that the gain from requiring monitoring by such sources will, in fact, always be trivial. In 

addition, the commenter believes that, objectively, the specific SMC numbers proposed by EPA 

are hardly trivial. The commenter points out that proposed Option 1 would set the 24-hour SMC 

at 10 μg/m3, which is higher than the proposed 24-hour Class II increments; thus Option 1 

proposes to waive monitoring for sources that are projected to violate the increments. The 

commenter contends that this is an unlawful and arbitrary result. The commenter also notes that 

the proposed Option 2 and 3 SMCs (24-hour) are both higher than the proposed 24-hour Class I 

increments, and would allow projected consumption of significant portions of the proposed 

Class II increments without any monitoring: the Option 2 SMC is 89 percent of the Class II 

increment level while the Option 3 SMC is 26 percent of that level. The commenter believes that 

it is absurd to claim that a source projected to consume such a large portion of the allowable 

increment is too trivial to warrant monitoring. The commenter adds that the notion that EPA can 

develop a single national number that defines ―trivial‖ impact for all attainment areas is 

fundamentally flawed, given that even a very small impact can be of great significance in an area 

that is very close to exceeding an increment or a NAAQS. 

The same commenter argues that the proposed rule is further flawed because it fails to 

specify how a source is to demonstrate compliance with an SMC. The commenter believes that 

EPA intends that such compliance will be shown through some sort of modeling, but contends 

that the statute does not allow EPA to wholly supplant with modeling the monitoring mandated 

by section 165(e)(2). The commenter adds that even if modeling were allowed to supplant 

monitoring, EPA would need to adopt rationally justified criteria for determining when to allow 

such substitution, and would have to do so by rule (citing section 165(e)(3)(D)). The commenter 

states that such criteria would need to include requirements to ensure a conservative analysis, 

such as modeling PM2.5 primary and precursor emissions, modeling worst-case potential 

emissions for the averaging time in question, comparing maximum modeled impact against the 
91

SMC, following the Guideline on Air Quality Models to ensure sufficient and representative 

meteorological data, use of EPA-approved models (and EPA-approved model settings), and 

following EPA requirements for acceptable receptor placement and spacing. 

The same commenter believes that even assuming that the Act did allow for exempting a 

source from preconstruction monitoring, the proposed SMCs are arbitrary because the specific 

numbers proposed cannot rationally be characterized as trivial or insignificant. The commenter 

adds that the proposal is also arbitrary because it allows a waiver of monitoring even in the 

absence of a demonstration that current, quality assured monitoring data reflective of background 

concentrations in the area of the proposed source is available for use in the NAAQS modeling, 

or, if no such data is available, a requirement that all other sources of PM2.5 or precursors to 

PM2.5 (including area and mobile sources) are included in the air quality modeling done in lieu of 

monitoring.

Finally, the commenter argues that EPA has no authority to allow the use of SMCs (or 

SILs) to obviate the statutorily required protection of AQRVs, or to obviate or impair FLM 

review of and objection to permits under section 165 of the Act.

Response:

As with the SMCs adopted by EPA in 1980, the SMC that we have selected for PM2.5 is

supported by the de minimis doctrine set forth in the Alabama Power opinion, contrary to the 

assertion of the commenter. As previously discussed in Section 4.1 of this document in response 

to this commenter‟s legal objections to SILs, the Alabama Power court concluded that “most 

regulatory statutes, including the Clean Air Act, permit [de minimis] showings in appropriate 

cases. 636 F.2d at 360. The court went on to say that, “unless Congress has been 

extraordinarily rigid, there is likely a basis for an implication of de minimis authority to provide 

exemption when the burdens of regulation yield a gain of trivial or no value.” 636 F.2d at 360-

361. Although the commenter contends that the monitoring requirements of section 166(e)(2) 

are inconsistent with a de minimis exemption based on an SMC, the commenter does not provide 

any basis for concluding that these provisions are the kind of “extraordinary rigid” provision 

that forecloses the use of a de minimis exemption. Thus, where the SMC values set by a rule 

identify a level of impact such that “the burdens of regulation” (in this case certain requirements 

under CAA section 165) will “yield a gain of trivial or no value,” the establishment of an SMC is 

a permissible de minimis exemption.

Like the other pollutants for which EPA has promulgated SMCs, EPA believes there is 

little to be gained from requiring the collection of preconstruction monitoring data for PM2.5

concentrations that are at levels so low that they cannot be accurately measured. As indicated in 

the preamble to the final rule, we selected a final value for the PM2.5 SMC based on the principle 

underlying Option 1 (i.e., derived from the lowest detectable concentration (LDC) as modified

based on the appropriate uncertainty correction factor). However, the promulgated value is 

lower than the value proposed under Option 1, which the commenter considered to be too high,

because we adjusted the correction factor being applied to the LDC. The updated analysis of the 

correction factor is contained in a March 12, 2009 internal EPA memorandum that we have 

placed in the docket for this rulemaking, which describes “compelling data from 9 years of more 
92

advanced ambient monitoring technology and associated quality assurance measurements of 

PM2.5....” See further discussion of this analysis in the response to comments in Section 5.3 of 

this document. 

Because the final SMC is based on the capabilities of the ambient monitors and 

associated quality assurance procedures to measure PM2.5, there is little benefit to be gained 

from preconstruction monitoring of ambient concentrations that are below the SMC. In this 

situation, the source would be trying to measure concentrations that are in the range where 

measurement uncertainties exist, and the results of the monitoring could not be used for any 

quantitative purposes because they would not provide the desired confidence of accuracy and 

precision. Thus, the time and expense of a year of preconstruction monitoring essentially would

be wasted.

A similar situation would exist where a source‟s emissions will result in ambient impacts 

that are less than the final SMC. Any difference in the ambient concentration due to such a 

source would be too small to be measured accurately, so any attempt to quantify the impact of 

the source based on preconstruction and post-construction monitoring in the area would be 

inconclusive. In addition, increment analyses typically are based solely on modeling of the 

emission increases and decreases at sources that affect the increment, without the use of 

monitoring data. Thus, preconstruction monitoring data generally are not useful for increment 

analyses for any sources, including sources with impacts below the SMC.

However, as pointed out by the commenter, preconstruction monitoring can be useful in 

determining the background concentration for purposes of a NAAQS analysis. Accordingly, 

where representative ambient concentration data are not available from other sources and the 

ambient air quality is at levels near the PM2.5 NAAQS, we do not encourage the continued use of 

the SMC to exempt sources from collecting preconstruction monitoring data. We believe that the 

PSD regulations provide discretion to individual reviewing authorities to determine when it is 

appropriate to require the collection of ambient data as part of a complete application 

regardless of whether or not the SMC is being exceeded. Specifically, the PSD regulations 

provide that the reviewing authority may exempt a source from the preconstruction monitoring

data requirements based on the SMC, but neither require such an exemption nor require that the 

exemption be established without exceptions or limitations.

We do not agree with the commenter that the final SMC for PM2.5 is too large to be 

considered trivial based on comparisons with the PM2.5 increments. The SMC is based on the 

measurement capability of ambient monitors and associated quality assurance procedures, 

which are unrelated to the level at which increments are set. In addition, as noted above, 

increment analyses typically are modeling exercises, and ambient monitoring results do not 

enter into these analyses. Thus, the level of the increments is not material to the level of the 

SMC that we are setting in this final rule.

We also do not agree with the commenter that setting a single national SMC is 

“fundamentally flawed.” The final SMC is based on the ambient concentration of PM2.5 that can 

be accurately measured, which is the same everywhere. In locations where other considerations 

enter into the determination of whether preconstruction monitoring data should be required, as 
93

discussed previously the reviewing authority retains discretion under the PSD regulations to

require the collection of monitoring data regardless of whether the source‟s projected ambient 

impact is above or below the SMC, and should do so when warranted.

The commenter also expresses concern about our alleged failure to demonstrate how a 

source is to show compliance with the SMC for PM2.5. The SMCs for regulated NSR pollutants 

have been defined in the PSD regulations since 1980; in order to compare a proposed source‟s 

impact against the applicable SMC we have previously indicated that the source should use the 

highest modeled concentration for the applicable averaging time or the existing air pollution 

concentration in the area of concern. See EPA‟s New Source Review Workshop Manual (Draft, 

October 1990) at page C.16. The SMC values are linked to direct measurements or predictions 

of an air pollutant concentration and are not affected by the form of the individual standards 

upon which compliance is based. Nevertheless, we will continue to review our current guidance 

and provide clarifications as deemed appropriate.

5.3 Comments on the Proposed Level of the SMC for PM2.5

While the SMC level proposed under Option 1, based in part on the sensitivity of existing 

ambient monitors for PM2.5, received the most support, the other two options received some 

support as well. Some commenters that opposed Option 1 did not indicate a strong preference 

between Options 2 and 3.

Comments on Option 1:

A total of 10 commenters express support for the SMC level proposed under Option 1. 

The reasons for support given by these commenters are summarized below.

Four of those commenters prefer Option 1 because it is the only option that considers 

factors associated with actual sampling of ambient PM2.5 concentrations. Two industry 

commenters (0026, 0027) state that the option is reasonable because it reflects pollutant levels at 

which low level concentrations or small incremental changes in pollutant concentrations can be 

reasonably determined, which is the same methodology originally used to set the SMC for TSP. 

Two industry association commenters (0032, 0037) prefer Option 1 because it is the only option 

that relates to monitoring measurement sensitivity.

One state/local agency commenter (0015) prefers Option 1. The commenter notes that 

EPA has retained the 24-hour PM10 NAAQS, and the development of the PM2.5 SMC under this 

option is consistent with past SMC development for PM10.

One industry commenter (0020) expresses a preference for Option 1 by default. The 

commenter explains that the other options cannot be supported for reasons explained below in 

the comments pertaining to Options 2 and 3. Another industry association commenter (0029) 

also supports Option 1 and believes 10 μg/m3is a reasonable value. However, this commenter 

also believes that EPA is authorized under the Act to choose Option 2. One industry association 

commenter (0046) provides general support for Options 1 and 2 because there is little difference 

between them.
94

One supportive industry association commenter (0039) recommends that the SMC values 

be set at a level that would avoid unnecessary collection of pre-application monitoring data, and 

believes that the proposed Option 1 SMC is sufficiently conservative for this purpose. The 

commenter argues that if the SMC were set correctly, modeled exceedances of the SMC would 

better demonstrate an actual need for collection of a year of pre-application monitoring data. 

The commenter adds that collecting such data, if found to not be needed, unduly delays 

successful permitting of much needed power plant projects in the West.

Finally, one industry commenter (0016) indicates support for Option 1 for the SMC, yet 

proposes that EPA develop area-specific SMCs (and increments and SILs) that take local 

conditions into account.

Two commenters oppose the level proposed under Option 1 because of its relation to the 

level of the proposed Class II increment. Specifically, a state/local agency commenter (0019) 

and a state/local agency association commenter (0018) oppose the proposed Option 1 SMC

(10 μg/m3) because it exceeds any of the corresponding proposed Class II 24-hour increments for 

PM2.5 (9 μg/m3).

Comments on Option 2:

One state/local agency commenter (0019) and one state/local agency association 

commenter (0018) express support for either Option 2 or Option 3. However, the commenters

express concern that the justification for choosing either Option 2 or 3 would need to be clearly 

explained.

One state/local agency commenter (0033) believes it is appropriate to achieve some 

degree of consistency by using Option 2 for both SILs and SMCs.

One industry association commenter (0046) provides general support for Options 1 and 2

because there is little difference between them.

One state/local agency commenter (0015) and two industry commenters (0020, 0026) 

who support Option 1 also explicitly oppose Option 2. These commenters express concern with 

the factor of 0.8 (based on the NEI ratio of total PM2.5 emissions to PM10 emissions) that was 

used to derive the Option 2 SMC. The state/local agency commenter (0015) and one of the 

industry commenters (0020) believe that the ratio is technically inaccurate. The two industry 

commenters (0020, 0026) are concerned that use of this factor could set an inappropriate

precedent for other, source-specific other regulatory processes.

Comments on Option 3:

One state/local agency commenter (0025) supports EPA’s proposed Option 3 because

this option is the most stringent.

As noted above, one state/local agency commenter (0019) and one state/local agency 

association commenter (0018) express support for either Option 2 or Option 3. However, the 
95

commenters express concern that the justification for choosing either Option 2 or 3 would need 

to be clearly explained.

One environmental group commenter (0040), although believing that all of the SMC 

options are unlawful and irrational, indicates that the Option 3 SMC is the least objectionable 

because it is the lowest and therefore allows the least amount of circumvention of the Act’s 

requirements.

Five commenters oppose the use of Option 3 to set levels for the SMC for PM2.5, 

expressing their reasons as follows:

 One industry commenter (0046) believes that the Option 3 SMC is too low. 

 Two industry commenters (0020, 0026) and one industry association commenter (0029) 

believe that the Option 3 SMC is too close to the LDC for PM2.5 to be used effectively. 

Two of these commenters (0026, 0029) add that the determination as to whether or not a 

source needs to monitor emissions should be based on a value that can be expected to be 

measured accurately, and an SMC level set so close to the LDC does not provide this 

assurance.

 One industry association commenter (0038) states that the final PM2.5 SMC should 

complement the adopted SILs and notes that the SMC under Option 3 is smaller than the 

proposed 24-hour Class II SILs under Options 1 and 2. The commenter explains that 

under this scenario, a PSD applicant could potentially trigger preconstruction monitoring 

but not be required to conduct a cumulative modeling analysis, which the commenter 

believes would be inappropriate. To avoid requiring preconstruction monitoring where 

no cumulative impact analysis is required, the commenter believes that EPA should not 

adopt Option 3 for the SMC.

Other Comments on the SMC Level:

One industry commenter (0029) (who supports Option 1) indicates that EPA also is 

authorized to use its Option 2 approach. However, this commenter cautions that EPA should 

make it clear that the PM2.5/PM10 emissions ratio used in this option is intended to be used only 

for the setting of the nationwide threshold values being addressed in this rulemaking and is not 

intended to be used for other regulatory purposes.

One environmental group (0040), while believing that all the proposed SMCs are 

unlawful and irrational, indicates that a less objectionable (though still impermissible) option 

would be to set an SMC based on the LDC of 2.0 μg/m3(24-hour average).

Response:

In developing the three proposed options for an SMC, EPA sought to use methods that 

would attempt to identify levels representing a de minimis or insignificant impact on PM2.5

ambient air quality that makes the requirement to collect additional continuous air monitoring 

data extraneous. We agree with the commenters who conclude that Option 1 is the most 

supportable option for defining the SMC for PM2.5. The ability to accurately measure ambient 
96

PM2.5 concentrations is not related to a ratio of PM2.5 to PM10 either directly in terms of 

emissions or as expressed by the respective NAAQS, which were used to define the SMC for 

PM2.5 under Options 2 and 3, respectively. We selected the Option 1 approach because it relies 

on the concept of a minimum detection limit (MDL) to argue that there is little to be gained by 

trying to monitor ambient concentrations lower than the concentration that available monitoring 

techniques and quality assurance procedures can reliably measure. Our original concern with 

Option 1 was that, while it linked the SMC directly to the concept of the MDL (in order to 

identify de minimis monitoring circumstances), the value originally derived from that approach 

in the 2007 NPRM was high in relationship to the concentrations of PM2.5 defined by the existing 

NAAQS and increments for PM2.5. 

In considering the use of Option 1 for developing the SMC in the final rules, however, we 

recognized during the administrative process after publication of the proposed rule that it was 

necessary to re-examine the assumptions that we relied upon in 1980 to develop the numerical 

values for the original SMCs so that we could most accurately reflect current monitoring 

techniques and quality assurance procedures for PM2.5. Our re-examination for the final rule 

used the most current information concerning the physical capabilities of the PM2.5 Federal 

Reference Method Samplers, and addresses uncertainties introduced to the measurement of 

PM2.5 due to variability in the mechanical performance of the PM2.5 samplers and the microgravimetric analytical balances that weigh filter samples. 

The minimum detection limit (MDL) of 2 μg/m3, originally used in 1980 for the SMC for 

PM and promulgated for PM2.5 in 1997 (see 40 CFR part 50, Appendix L, section 3.1), has been 

reaffirmed by 9 years of field blank data collected by EPA through the PM2.5 Performance 

Evaluation Program. However, we found that new data exist to “indicate a conservative 

estimate of the aggregate uncertainty factor is no greater than „2‟ at the concentration equal to 

the MDL of 2 μg/m3.”12 Accordingly, the lowering of the uncertainty factor from “five” to 

“two” under Option 1 yields an SMC of 4 μg/m3PM2.5, 24-hour average, rather than the 

proposed concentration of 10 μg/m3.

 

We conclude that the modified level of 4 μg/m3PM2.5, 24-hour average, for the SMC 

under Option 1, based upon a more current understanding of monitoring precision for PM, 

especially fine PM, addresses commenter support for the use of a method that is consistent with 

the way other SMCs were developed and most directly reflects monitoring technology and 

quality assurance measurement capabilities for the pollutant of concern, while at the same time 

responding to the concern of other commenters that a value in the lower range of proposed SMC 

values is most reasonable considering the levels of the NAAQS and increments for PM2.5. 

 

12 This information is contained in a March 12, 2009 internal EPA memorandum from Dennis Crumpler to 

Raj Rao, titled “PSD Monitoring De Minimis Concentration for PM2.5,” which has been placed in the docket for this 

rulemaking.
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5.4 Other Comments on SMCs

Comment:

One industry commenter (0027) states that if ambient air monitoring is required as part of 

a PSD permit review (i.e., the source exceeds the SMC) EPA should, either in rulemaking or 

associated guidance, enable the permit applicant to use and rely on ambient air measurements 

from established PM2.5 ambient air quality monitors that are operated by EPA, state agencies, or 

established air quality research organizations. The commenter states that the PSD permitting 

program should encourage the use of existing data and data networks, and should only require 

the installation of a new ambient air monitoring system on a limited case-by-case basis. The 

commenter explains that in many cases, existing PM2.5 ambient air monitors should be sufficient 

to establish baseline or pre-project ambient air concentrations.

Response:

The PSD regulations do not require that the ambient monitoring requirements be 

satisfied with data collected specifically by the PSD applicant. Rather, the PSD regulations 

require PSD applications to “contain an analysis of ambient air quality” and that “the 

continuous air monitoring data that is required shall have been gathered over a period of one 

year.” (See, e.g., 40 CFR 51.166(m)(1)(i) and (m)(1)(iv), respectively.) Accordingly, EPA‟s 

longstanding policy is to allow PSD applicants to rely upon existing representative data, where 

available, to satisfy the preconstruction monitoring requirements. We continue to believe that, to 

the extent that existing data is representative of air quality in the area that the proposed sources 

would affect, such existing monitoring data may be used in the PSD application lieu of requiring 

applicants to establish separate ambient monitoring networks to collect new air quality data.

Comment:

One industry commenter (0016) suggests that EPA develop geographic area-specific 

SMCs (and increments and SILs) that take local conditions into account. The commenter points 

out that PM2.5 levels in PSD areas proximate to international borders may be elevated by sources 

outside the legal and practical control of the United States and state authorities. The commenter 

also notes that PM2.5 levels may be elevated by natural conditions, such as drought, fires, 

geologic formations (sandy or fine-grained surface features) high winds, etc., leading to 

excessively dusty ambient conditions over which the local area has no control. The commenter 

indicates that local area baselines must reflect these PM emissions, though they are not reflected 

in the local area’s emissions inventory. The commenter urges EPA not to penalize such PSD 

areas by imposing a uniform national SMC (or increments or SILs) where the conditions of 

concern are not capable of control.

Response:

We do not agree with the commenter‟s recommendation for area-specific SMC levels for 

PM2.5. The air quality analysis for determining compliance with the annual and 24-hour PM2.5

NAAQS must consider the existing air quality in the area of concern, regardless of its origin. We 
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do not allow certain emissions to be excluded from measures of actual air quality, except in 

specific cases of “exceptional events” that may occur during a period when the ambient 

concentrations are showing levels exceeding the NAAQS. We do not find any basis for setting 

different SMCs for PM2.5 based on geographic location “where the conditions of concern are not 

capable of control.”
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6 Comments on Transition Issues

6.1 Comments on the Effective Date for PM2.5 Increments

We proposed that if we promulgated PM2.5 increments under section 166(a) of the Act 

(Option 1), the effective date of the final rules would be 1 year after the date of promulgation 

consistent with the timeline of section 166(b) of the Act. However, we also took comment on 

whether we could make the increment regulations effective 60 days after promulgation under 

Option 1, noting that Congress intended increments and other measures to become effective 

within 3 years after promulgation of a NAAQS and that there has a been an extended delay of 

over 10 years since we established the PM2.5 NAAQS. Further, we pointed out that the 1 year 

delay embodied in section 166(b) of the Act was intended to give Congress time to intervene if it 

did not agree with EPA’s rules. Since section 166(b) was added to the Act, the Congressional 

Review Act (CRA) has been enacted, which provides an expedited mechanism for Congress to 

block any final rule within 60 days of promulgation. Thus, we requested comment on whether, 

given the procedures under the CRA, a 60-day delay in the effective date of the final PM2.5

increment rules could satisfy the Congressional intent of section 166(b). 

In addition, we proposed that if we promulgated PM2.5 increments under section 166(f) of 

the Act (Option 2), the effective date of the final rules would be 60 days after promulgation 

consistent with the CRA timeline. We do not interpret section 166(b) as applying to increments 

promulgated under section 166(f).

Although in the proposal preamble we discussed the effective date of the final rule in the 

context of the section of the Act under which we promulgate the PM2.5 increments, many

commenters expressed preferences on this timing either regardless of which of the two options 

we select or without reference to the options. As a result, the comments on the effective date of 

the final rule for PM2.5 increments are organized around the preferred effective date rather than 

the options discussed in the preamble. However, where commenters included comments on the 

proposed options, these are noted as well.

In addition, it should be noted that at proposal we equated the effective date of the final 

rule with the effective date of the PM2.5 increments (either 60 days or 1 year after promulgation). 

However, in the final rule we have decoupled the effective date of the increments from the 

effective date of the final rule, which will be 60 days after promulgation in the Federal Register. 

Comments Supporting an Effective Date 1 Year after Promulgation and/or Opposing 

60 Days:

A total of nine commenters support a 1-year delay in the effective date for the PM2.5

increments, for the following reasons: 

Two state/local agency commenters (0015, 0022) support delaying the effective date by 

1 year rather than 60 days because states need time to establish a process and revise rules. One 

of these commenters (0015) specifically recommends that we follow section 166(a) of the Act 

under Option 1 and does not concur with the proposal to make increments effective within 
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60 days of promulgation under Option 1 or with Option 2. The same commenter notes that the 

PM2.5 NAAQS still have not been implemented in the NSR program, and indicates that states 

will need time to establish a process and revise rules that include PM2.5 NAAQS, PM2.5 and PM10

increments, SILs, SMCs, and significant related PSD issues, such as coordination with the 

FLMs.

Eight industry and industry association commenters support EPA’s proposal to make the 

final rule for PM2.5 increments effective 1 year after promulgation. Three of these commenters 

(0023, 0026, 0029) indicate that this time is necessary to develop the needed PM2.5 inventories. 

Two of these (0023, 0029) add that accurate inventories of PM2.5 emissions data that are 

necessary for implementation of the PM2.5 PSD program.

Three of the industry association commenters (0032, 0035, 0037) urge EPA to allow state

programs sufficient time to adopt increments, particularly if condensable particulate is included 

in the increment and its analysis. The commenters state that the federal rule should not be 

effective for at least 1 year. (They also state that states should have 3 years for the associated 

SIP revisions.) These commenters add that this time would provide time for sources that have 

permits in the pipeline or are just about to submit an application to be able to complete the 

permitting process without undue delay. One of the commenters (0035) specifically voices 

support for Option 1 for the effective date of the final rule (1 year) and Option 2B for the period 

granted for SIP revisions (3 years). Another of the commenters (0032) supports Option 1 if the 

increments are based on filterable PM2.5 only, and proposes a modified Option 2 if condensables 

are included that would postpone the effective date for the federal program for 3 years and allow 

a 3-year period for SIP submittal in states with approved PSD programs. The latter two 

commenters explain that this additional time may give the Agency time to promulgate changes to 

improve test methods for sources of condensable particulate.

Another of the industry association commenters (0038) notes that, at the time of the 

proposal, the NSR portion of the Clean Air Fine Particle Implementation Rule had not yet been 

promulgated, and that states will need time to incorporate that rule as well as the requirements of 

the proposal into their SIPs. The commenter adds that making the PM2.5 increments effective 

before states and sources have had a reasonable opportunity to begin, let alone complete, the SIP 

process for the two related rulemakings would unnecessarily complicate an already-complex 

regulatory process. 

One of the industry commenters (0043) states simply that the proposed 60-day timeline 

was unreasonably short. 

Comments Supporting an Effective Date 60 Days after Promulgation and/or Opposing 

1 Year:

Two commenters, one state/local agency commenter (0019) and one state/local agency 

association commenter (0018), support EPA’s proposal to implement the PM2.5 PSD program as 

soon as possible, and encouraged EPA to apply the 60-day period for the effective date under 

whatever option is finalized. The state/local agency (0019) urges EPA to take measures to 

expedite the implementation of the PM2.5 final rule and suggested the Agency choose the shortest 
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of the proposed effective dates which are allowed under any of the applicable regulations. The 

commenter (0019) states that one of the detrimental effects of EPA’s preferred approach of 

developing PM2.5 increments under section 166(a) of the Act (Option 1) could be to delay the 

regulatory effective date of applicability by 1 year, versus the 60 days time frame required under 

the section 166(f) approach. The commenter (0019) indicates that in light of the excessive delay 

in the implementation of the PM2.5 PSD program since the NAAQS were promulgated, the 

60-day effective date should also apply under EPA’s preferred option. 

Comments Supporting Other Effective Dates:

Two industry commenters (0016, 0020) and one industry association commenter (0036) 

suggest different approaches to setting the effective date of the final rule for PM2.5 increments. 

One of these industry commenters (0020) states that the rule should take effect only when 

sufficient PM2.5 emissions data become available to derive the 24-hour baseline, and no earlier 

than 1 year after promulgation to allow time for all to address inventory issues.

Another of the commenters (0016) argues that the proposed effective dates relative to 

permitting are unrealistic, arbitrary, and capricious. The commenter notes that the foundation of 

the proposed PM2.5 program is emissions inventory data, and stated that many PSD areas of the 

United States have no data, others have an insufficient amount, and all the data are of 

questionable accuracy due to testing concerns. The commenter recommends that the proposed 

deadlines be rewritten, couched in terms of the dates by which accurate PM2.5 emissions 

inventory data can be realistically be developed using an EPA-approved stack test methodology, 

integrated, and made available for use by the PSD-regulated community in convenient format.

The same commenter adds that the regulations and preamble of the final regulations 

should state that the obligation to develop the necessary baseline PM2.5 data rest, for verification, 

quality assurance, and cost reasons, solely on EPA and state governments throughout the 

country. The commenter believes that to impose such data development on the regulated 

community and, in particular, on the first of the PSD permit applicants in a PSD area is 

unreasonable, and would raise the cost for that applicant to prohibitive levels. The commenter 

believes that finalizing the rules before there are adequate and accurate emissions inventory data 

will grind PSD permitting to a halt until there is an accurate EPA-approved stack test method for 

PM2.5. The commenter states that this is inconsistent with one of the goals of the PSD program 

as set forth in section 160 of the Act – to insure economic growth will occur in a manner that is 

consistent with the preservation of existing clean air resources. The commenter also states that 

the proposed effective dates fail to take into account the lack of emissions data, the time 

necessary to integrate PM2.5 emissions data by and between adjoining PM2.5 areas in the same 

and different states, and the potential for selection of alternative increment approaches by states.

The industry association commenter (0036), while agreeing with EPA’s approach to 

establish the effective date of the rule as expeditiously as possible for Options 1 and 2, also states 

that EPA must recognize that, to address the air quality impacts of PM2.5 emissions, adequate air 

dispersion models must be made available for the modeling community. The commenter notes 

that there are no adequate air dispersion models to address the direct and indirect PM2.5

emissions for near-field applications. The commenter asserts that there needs to be appropriate 
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modeling guidance provided by EPA and sufficient time to evaluate this guidance to ensure 

consistent application of air dispersion modeling to assess PM2.5 impacts, prior to requiring the 

PM2.5 increments be demonstrated through modeling analysis. 

Response:

In light of our decision to promulgate PM2.5 increments under the authority of 

section 166(a) of the Act (proposed Option 1), we faced the issue of to how to most effectively 

implement the long-awaited PM2.5 increments, recognizing that section 166(b) provides for a 

1-year implementation delay. We have concluded that it is most appropriate to follow the plain 

language of section 166(b), which calls for a 1-year effective date for implementing new 

increments developed under section 166(a) of the Act. We agree with the commenters who 

suggested that a shortened implementation delay was desirable because of the substantial delay 

in the promulgation of measures to prevent significant air quality deterioration with respect to 

PM2.5. Nevertheless, we believe it would be inappropriate in this action to disregard the 

statutory language which plainly calls for a 1-year delay. Accordingly, we are setting the 

effective date of the PM2.5 increments as the date 1 year after the date of publication of this final 

rule. See new 40 CFR 51.166(b)(14)(i)(c) and (ii)(c), and new 40 CFR 52.21(b)(14)(i)(c) and 

(ii)(c). It should be noted that the same trigger date applies to both the federal PSD rule at 

40 CFR 52.21 and the state (SIP-approved) programs consistent with the 40 CFR part 51 PSD 

regulations. 

At the same time, we are establishing a different effective date for the other provisions, 

i.e., the SILs and SMC for PM2.5, in this final rule by establishing the effective date of the final 

rule as the date 60 days after publication of the final rule in the Federal Register. This will 

enable the implementation of these key elements of this rule under the federal PSD program as 

soon as possible. 

6.2 Comments on the Implementation Date for PM2.5 Increments

6.2.1 Comments on the Schedule for Submitting SIP Revisions 

We proposed that if we promulgated PM2.5 increments under section 166(a) of the Act 

(Option 1), we would require states to submit revised implementation plans within 21 months of 

promulgation of the final rule according to timeline specified in section 166(b) of the Act, 

regardless of whether we made the effective date of the rule 1 year or 60 days after promulgation 

(see Section 6.1 above). Similarly, we proposed under Option 1 that we would approve or 

disapprove the SIP revisions within 4 additional months, that is, by 25 months after 

promulgation. 

In contrast, we proposed that if we promulgated PM2.5 increments under section 166(f) of 

the Act (Option 2A or 2B), we would follow the existing regulatory provisions of 40 CFR 

51.166(a)(6) and require states to submit revised implementation plans within 3 years of 

promulgation. However, we also requested comment on whether we should require a shorter 

timeline for SIP submission, such as the 21 months required under Option 1.
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Although in the proposal preamble we discussed the timeline for implementation of state 

programs in the context of the section of the Act under which we promulgate the PM2.5

increments, many commenters expressed preferences on this timing without regard to the options 

that they supported. As a result, the comments on the implementation timeline for state 

programs are organized around the preferred schedule rather than the options discussed in the 

preamble. However, where commenters included comments on the proposed options, these are 

noted as well.

Comments Supporting 21 Months:

One industry commenter (0026) indicates that her company was receptive to EPA’s 

suggestion to apply the statutory minimum of 21 months for requiring SIP revision submittals 

and 4 additional months for EPA approval. The commenter (0026) agrees that allowing states up 

to a full 3 years to adopt and submit PM2.5 implementation plan revisions would delay 

implantation of a PM2.5 PSD process. An industry association commenter (0037) indicates that if 

condensable PM is not included in the PM2.5 increment rules, a period shorter than 3 years, such 

as the 21 months proposed in Option 1, makes sense. However, this commenter favors 3 years if 

condensables are included under the rule (see below).

Comments Supporting 3 Years:

A total of four commenters support the 3-year SIP submittal period, consistent with the 

proposed use of section 166(f) of the Act for developing the PM2.5 increments, for the following 

reasons:

 Three industry association commenters (0032, 0035, 0037) urge EPA to allow state

programs sufficient time to adopt increments, particularly if condensable particulate is 

included in the increment and its analysis. The commenters recommend that states be 

allowed 3 years for the associated SIP revisions. (They also state that the federal rule 

should not be effective for at least 1 year.) These commenters add that this amount of 

time would enable sources that have permits in the pipeline or are just about to submit an 

application to be able to complete the permitting process without undue delay. One of 

the commenters (0035) specifically voices support for Option 2B for the period granted 

for SIP revisions (3 years) and Option 1 for the effective date of the final rule (1 year). 

Another of the commenters (0032) supports the Option 1 implementation time line if the 

increments are based on filterable PM2.5 only, and proposes a modified Option 2 if 

condensables are included that would allow a 3-year period for SIP submittal in states 

with approved PSD programs and postpone the effective date for the federal program for 

3 years. The latter two commenters also explain that this recommended amount of time 

may give the Agency time to promulgate changes to improve test methods for sources of 

condensable particulate.

 One state/local agency commenter (0033) believes that Option 2, consistent with the use 

of section 166(f) as the basis for the increment determination, is sensible and provides 

sufficient time for the effort required to implement the rule. The commenter indicates 

that the PM2.5 program is a good and sorely needed program, and it is important to be 
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sure that it is properly structured. Thus, EPA should resist a last minute rush to get it up 

and running.

Comments Supporting Other Dates for Submitting SIP Revisions:

One state/local agency commenter (0019) urges EPA to establish a time shorter than 

21 months for requiring states to submit revised PSD rules for SIP approval. The commenter 

believes that any further delays in the implementation of the PM2.5 increments or the NSR rule 

are contrary to the intent of Act for the PM2.5 NAAQS established in 1997.

Response:

In this final rule, we are establishing the final PM2.5 increments as minimum program 

elements for all state PSD programs. Accordingly, states must submit for EPA‟s approval 

revised SIPs that incorporate the final PM2.5 increments or alternative measures that can be 

demonstrated to EPA‟s satisfaction to provide an equivalent level of protection as the PM2.5

increments. In accordance with section 166(b) of the Act, we are requiring states to submit 

revised implementation plans to EPA for approval within 21 months of promulgation, that is, by 

the date 21 months after the date of publication of the final rule in the Federal Register. 

Section 166(b) also specifies that we must approve or disapprove these revisions within 25 

months of promulgation (4 months from the statutory deadline for SIP submittal). We regard 

these statutory deadlines as maximum allowed timeframes for action. Moreover, we do not 

believe that the Act restricts our ability to approve SIP revisions requested by a state at any time 

before these deadlines. In this final rule, we are amending the regulatory provisions at 40 CFR 

51.166(a)(6)(i) to articulate the deadline set forth by the statute for the SIP submittals involving 

the PM2.5 increments pursuant to section 166(a) of the Act.

As described earlier, the trigger date and major source baseline date are fixed dates 

applicable to the PSD program nationwide. This means that the same trigger date and major 

source baseline date apply to the federal PSD rule under 40 CFR 52.21 as well as to all SIPrelated PSD rules developed consistent with the requirements under 40 CFR 51.166. Only a few 

states continue to rely on the federal PSD permit program (40 CFR 52.21) for issuing PSD 

permits; most states now have their own PSD programs under an EPA-approved SIP. The 

1-year delay in the trigger date will mean that sources applying for PSD permits under the 

federal PSD rules must address the PM2.5 increments beginning on or after the date 1 year after 

the date of publication of the final rule in the Federal Register, depending on the date the first 

complete PSD application for a source of PM2.5 is submitted (the minor source baseline date for 

PM2.5). 

Sources applying for PSD permits under a state PSD program generally will not be 

required to address the PM2.5 increments until the effective date of the applicable state rule in 

which the PM2.5 increments are contained, which will be some time after the trigger date and 

major source baseline date for the PM2.5 increments. In either case, the date of submittal of the 

first complete application for a PSD permit after the effective date of the applicable PSD rule 

will establish the “minor source baseline date” in an attainment or unclassifiable area. As 

described in section IV.D of the preamble, the “minor source baseline date” is not a fixed date, 
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but instead applies independently to each baseline area based on the submittal date of a 

complete application for a PSD permit, and marks the date after which actual emissions changes 

from all sources affect the available increment.

The EPA does not expect that states will revise their SIPs and submit them to EPA for 

approval prior to the effective date of the PM2.5 increments in this final rule, which is the date 1 

year after the date of publication of this final rule in the Federal Register. Therefore, there is 

likely to be a period of time after the date of publication of this final rule in the Federal Register

when state laws will not require PSD applicants otherwise subject to PSD for PM2.5 to complete 

an increment analysis for the PM2.5 increments, even though the PM2.5 increments, major source 

baseline date, and trigger date have been established as a result of this final rule. Similarly, it is 

not clear whether states will have the authority to consider such applicants as having triggered 

the minor source baseline date during this interim period before their revised PSD rules 

containing the new PM2.5 increments and relevant baseline dates become effective. 

The EPA does not intend to prescribe the implementation timeline for state programs; 

rather, each state will need to determine how increment consumption and the setting of the minor 

source baseline date for PM2.5 will occur under its own PSD program. Nevertheless, regardless 

of when a state begins to require PM2.5 increment analyses and how it chooses to set the PM2.5

minor source baseline date, the emissions from sources subject to PSD for PM2.5 on which 

construction commenced after the date of publication of the final rule in the Federal Register 

(the major source baseline date) will consume PM2.5 increment and must be included in 

increment analyses occurring after the minor source baseline date is established for an area 

under the state‟s revised PSD program. 

6.2.2 Comments on Coordinating Implementation of PM2.5 Increments in 

the State and Federal Programs

Under Option 1, we proposed to begin implementing PM2.5 increments in the federal 

part 52 PSD program on the effective date of the final rules, that is, either 1 year or 60 days after 

promulgation. We also requested comment on the option of delaying implementation of the 

federal part 52 PSD program until 25 months after promulgation to coordinate it with the outside 

date by which EPA would be required to approve SIP revisions under the Option 1 timeline. 

Under Option 2, we proposed to begin implementing PM2.5 increments in the federal part 52 PSD 

program on the effective date of the final rules, that is, 60 days after promulgation. We did not 

propose to consider coordinating implementation with the timeline under Option 2, which could 

take up to 4 years (3 years for SIP submissions and 1 year for EPA approval).

Comment:

Two commenters support an effort to coordinate the timing for implementation of new 

PSD requirements.

 One industry commenter (0043) recommends that states with their own PSD programs 

and states implementing the federal program should have approximately the same 

implementation schedule. 
106

 One industry commenter (0026) urges the Agency to adopt a national implementation 

date in order to provide consistency across the states. The commenter (0026) urges the 

Agency to hold off implementation of state programs administrated under Part 52 PSD 

regulations in order to provide a uniform and consistent national approach. 

Response:

As explained in section 6.1 of this document, we have concluded that it is most 

appropriate to follow the plain language of section 166(b) of the Act, which calls for a 1-year 

effective date for implementing new increments developed under section 166(a) of the Act. We 

agree that there is some merit to coordinating the implementation commencement date of both 

the federal PSD program and individual state PSD programs. However, one problem is that it 

would be difficult to predict exactly when individual state programs will be approved. We have 

decided to begin implementing the revised federal PSD program as set out previously in our 

introductory discussion of this issue in section VIII.A of the preamble to the final rule. That is, 

the revised regulations at 40 CFR 52.21 will become effective 60 days after the publication of the

final rule in the Federal Register. This will allow EPA or the delegated state agency to begin 

using the SILs and SMC for PM2.5 on that date, as described in section VIII.C of the preamble to 

the final rule. However, the date established in the regulations for the trigger date will ensure 

that the PM2.5 increments do not become effective for 1 year, consistent with section 166(b) of 

the Act, and that the minor source baseline date cannot be established until the PM2.5 increments 

become effective. With the 1-year delay in implementing the new increments, which primarily 

affects the implementation of the federal PSD program for PM2.5, there will be less of a gap in 

implementing the new requirements in the federal program as compared with the state programs 

than if the increments were to become effective immediately in the federal PSD program. The 

“trigger date” and “major source baseline date” for PM2.5 increments will be the same fixed 

dates for both the federal program and all state PSD programs. 

In accordance with section 166(b) of the Act, we are requiring states to submit revised 

implementation plans to EPA for approval within 21 months of the date of publication of the 

final rule in the Federal Register. Section 166(b) of the Act also specifies that we must approve 

or disapprove these revisions within 25 months of publication (that is, 4 months from the 

statutory deadline for SIP submittal). We regard these statutory deadlines as maximum allowed 

timeframes for action. Moreover, we do not believe that the Act restricts our ability to approve 

SIP revisions requested by a state at any time before these deadlines. In this final rule, we are 

amending the regulatory provisions at 40 CFR 51.166(a)(6)(i) to articulate the deadline set forth 

by the statute for the SIP submittals involving the PM2.5 increments pursuant to section 166(a) of 

the Act.

EPA does not expect that states will revise their SIPs and submit them to EPA for 

approval prior to the effective date of the PM2.5 increments in this final rule, which is the date 

one year after the date of publication of this final rule in the Federal Register. Therefore, there 

is likely to be a period of time when state laws will not require PSD applicants otherwise subject 

to PSD for PM2.5 to complete an increment analysis for the PM2.5 increments, even though the 

PM2.5 increments, major source baseline date, and trigger date have been established as a result 
107

of this final rule. Similarly, it is not clear whether states will have the authority to consider such 

applicants as having triggered the minor source baseline date during the interim period between 

the national trigger date for the PM2.5 increments and the date their revised PSD rules 

containing the new PM2.5 increments and relevant baseline dates become effective. 

The EPA does not intend to prescribe the implementation timeline for state programs; 

rather, each state will need to determine how increment consumption and the setting of the minor 

source baseline date for PM2.5 will occur under its own PSD program. Nevertheless, regardless 

of when a state begins to require PM2.5 increment analyses and how it chooses to set the PM2.5

minor source baseline date, the emissions from sources subject to PSD for PM2.5 on which 

construction commenced after the date this final rule is published in the Federal Register (the 

major source baseline date) will consume PM2.5 increment and must be included in increment 

analyses occurring after the minor source baseline date is established for an area under the 

state‟s revised PSD program. 

6.2.3 Other Comments on Implementation

Comment:

One state/local agency commenter (0019) states that EPA should impose a requirement 

similar to the one it imposed under the PM2.5 nonattainment provisions on April 5, 2005, where 

EPA used applicable EPA regulations (Appendix S of 40 CFR part 51) in the transition period 

between the effective date of the rule and the state SIP submissions. The commenter notes that 

for states with appropriate regulations, EPA urged the use of existing PM10 nonattainment 

provisions to address PM2.5 nonattainment requirements. The commenter suggests that EPA 

should use similar approaches and time frames for the implementation of the PM2.5 PSD 

requirements. The commenter believes that any further delays in the implementation of the 

PM2.5 increments or the NSR rule are contrary to the intent of Act per the 1997 PM2.5 NAAQS.

One state/local agency association commenter (0018) similarly encourages EPA to 

impose mandatory requirements that would provide for EPA PM2.5 PSD regulations to serve as a 

backstop during the transition period between the effective date of EPA regulations and 

completion of the state SIP submissions and adoption process.

Response:

As one commenter notes, EPA currently does not have a rule comparable to the 

Emissions Offset Interpretative Rule (40 CFR part 51, Appendix S) to provide an interim means 

for states to implement the PSD requirements in advance of an approved PSD program for PM2.5

in their SIPs. The EPA‟s “backstop” rule is contained in 40 CFR part 52, where EPA has 

established a federal PSD program to be implemented in states with SIPs that lack an adequate 

state PSD program. However, that program cannot be used as an interim implementation 

measure, by either EPA or a state agency, for SIP-approved programs that are in the process of 

being revised to address new PSD requirements. The EPA first would have to make a finding 

that the state rule is deficient in order to impose those part 52 PSD program requirements on a 

state that currently has a SIP-approved PSD program. The EPA is giving consideration to a 
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part 51 PSD program similar to the existing Interpretative Rule in Appendix S and may take 

some action in the future. In this rule, however, no final decision has been made and no action is 

being taken.

6.3 Comments on Timing for the Revocation of the PM10 Increments

Comment:

One state/local agency (0015) supports the proposal to revoke the PM10 annual increment 

in conjunction with the commencement of the PM2.5 increment system under the part 51 and 

part 52 PSD regulations. A second state/local agency (0022) similarly agrees with the proposal 

to revoke the PM10 annual increment from any SIP on or after the date EPA approves the SIP for 

PM2.5 increments. (The second commenter is silent regarding the federal part 52 PSD program.)

One state/local agency (0033) does not support specification of a fixed PM10 increment 

revocation date at this time. The commenter (0033) explains that this date depends upon too 

many other factors whose time impact is unknown.

One industry commenter (0016) states that if it is necessary to delay withdrawal of the 

PM10 PSD increment (and SILs and SMC) to allow for the orderly development, integration, and 

public availability of PM2.5 emissions inventory data, the commenter would support such a delay. 

[This commenter argues that the proposed effective dates for the final rules for PM2.5 increments

are unrealistic, arbitrary, and capricious, given that the foundation of the PM2.5 PSD program is 

emissions inventory data and that many areas have insufficient and inaccurate data (see 

Section 6.1 of this document).]

Response:

As discussed in Sections 2 and 3 of this document, the PM2.5 increments in the final rule 

are based on proposed Option 1, the “contingent safe harbor” approach under section 166(a) of 

the Act, and we are taking no final action on our proposal to revoke the annual PM10 increments 

at this time. We also explained our position that the PM10 increments cannot be revoked because 

they are statutory increments set forth as numerical limitations in the Act itself. Thus, the 

potential timing of such a revocation is no longer at issue, and no further response to these 

comments is warranted at this time.

6.4 Comments on a Transition Period for Permit Applicants

Comment:

Two state/local agency commenters (0015, 0022) support the proposal to give states the 

discretion to continue the existing PM10 increment program or to begin implementing the new 

PM2.5 increment programs during the transition period. One of these commenters (0015) states 

that it is unclear how the implementation of the new PM2.5 increment program would be 

federally enforceable during the transition period. 
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One industry commenter (0026) urges that we retain the exemption for permit 

applications submitted before the implementation date of the new increment regulations from 

having to transition to the new program that EPA has allowed in past programs. The commenter 

asserts that where a permit application has already been submitted and deemed complete, the 

applicants must be allowed to continue the permit process under existing regulations, and urged

EPA to explicitly clarify this in the final rule.

Another industry commenter (0043) states that it is reasonable to allow states a choice 

between using PM10 or PM2.5 increments during a transition period involving SIP approval, 

where applicable. This commenter also indicates that transition periods with a mixture of both 

PM10 and PM2.5 requirements should be avoided, even if PM2.5 implementation must be delayed 

be several months.

Response:

During the time since the proposal of this rule in 2007, we have reconsidered the need 

for the proposed transition period in the federal PSD program to effectively implement the PM2.5

increments. In light of the importance of preventing significant deterioration of PM2.5 air quality 

and the amount of time that has passed since the initial promulgation of the PM2.5 NAAQS, we do 

not believe that further delay is warranted. We expect that most permits issued after the effective 

date of the increments in the final rule, i.e., 1 year from publication of the final rule in the 

Federal Register, will be from sources that submitted their PSD applications after the major 

source baseline date for PM2.5, which is defined as the date of publication of this final rule, so 

that they will be increment-consuming sources. Therefore, when these sources apply for their 

PSD permits, they will have had significant advance notice of when the new increments will 

become effective, i.e., 1 year from the date of publication of this final rule. The review and 

permitting of permit applications submitted prior to the publication date of this final rule should 

generally be completed prior to the effective date of PM2.5 increments and thus effectively have a 

transition period of 1 year to complete processing.

6.5 Comments on Whether SILs and SMCs Should Be Mandatory 

Program Elements

Comment:

One state/local agency (0033) believes SILs and SMCs should not be a required element 

of the SIP.

Another state/local agency (0015) believes that EPA has the authority to include SILs and 

SMCs as minimum program elements per the opinion set forth in Alabama Power v. Costle. The 

commenter adds that the EPA Environmental Appeals Board has affirmed EPA’s interpretation 

of the Act to allow EPA to evaluate the significance of a source’s impact when determining 

whether it would ―cause or contribute‖ to a NAAQS or increment violation under section 

165(a)(3) of the Act.‖ In Re: Prairie State Generating Company, PSD Appeal No. 05-05, slip 

op. at 139-144 (Aug. 24, 2006).
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Three industry association commenters (0032, 0035, 0037) state that SILs and SMCs 

should be mandatory elements of the PSD program. One of these commenters (0032) believes 

that the SILs and SMC provide an important way for sources to show that there is not an adverse 

impact without conducting expensive modeling. Another of the commenters (0035) believes that 

modeling PM2.5 (or any other pollutant) without using screening models with SILs and SMCs is 

so unreasonable that EPA must require that states adopt the SILs and SMCs to meet the Purpose 

clause of the Act, which requires a balancing of environmental and economic considerations. 

The commenter also argues that section 116 of the Act, which allows states to be ―more 

stringent‖ absent state law that provides otherwise, does not authorize states to be irresponsible 

in the use of their air quality resources.

Another of the industry association commenters (0037) states that the increment, SILs,

and SMC need to be adopted as a single regulatory approach because the SILs and SMC define 

when additional work is needed to ensure that PSD requirements, such as maintaining adequate 

increment are met. The commenter stresses the importance of sources having the ability to use 

screening models to show they will not have a significant impact and that modeling is futile 

where the impact is below the measurement sensitivity of the monitor. The commenter claims 

that there is simply no reason for sources to be placed in the position of conducting expensive 

modeling that can delay a project when it is unnecessary from an air quality perspective.

Response:

In the 2007 NPRM, we explained our position that SILs and SMCs are not minimum 

required elements of an approvable SIP. While these de minimis values are widely considered to 

be useful components for implementing the PSD program, they are not absolutely necessary for 

the states to implement their PSD programs. That is, states can satisfy the statutory 

requirements for a PSD program by requiring each PSD applicant to submit air quality 

monitoring data and to conduct a comprehensive air quality impacts analysis for PM2.5 without 

using de minimis thresholds to exempt certain sources from such requirements. Because the de 

minimis values for PM2.5 (and other pollutants) are not mandatory elements, we proposed not to 

establish specific deadlines for submitting revisions to incorporate the specific values for PM2.5

into SIPs. 

We agree that the SILs and SMCs used as de minimis thresholds for the various 

pollutants are useful tools that enable permitting authorities and PSD applicants to screen out 

“insignificant” activities; however, the fact remains that these values are not required by the Act 

as part of an approvable SIP program. We believe that most states are likely to adopt the SILs 

and SMCs because of the useful purpose they serve regardless of our position that the values are 

not mandatory. Alternately, states may develop more stringent values if they desire to do so. In 

any case, states are not under any SIP-related deadline for revising their PSD programs to add 

these screening tools.
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7 Comments on Statutory and Executive Order Reviews

7.1 Comments on Review under the Unfunded Mandates Reform Act

Comment:

Two industry commenters (0024, 0043) and two industry association commenters (0014,

0046) express concern that EPA has dramatically underestimated the impact of these regulations 

on the regulated community. The commenters believe that the cost impact could exceed the 

threshold of $100 million in any 1 year that triggers additional review under the Unfunded 

Mandates Reform Act.

Two of these commenters (0024, 0046) focus on the costs of source testing for PM2.5.

One (0024) states that, although the proposed rule adds a relatively small number of new 

requirements to the existing permit requirements, EPA is simultaneously requiring (through the 

implied need for source knowledge) sources to test for PM2.5 emissions when there exists no 

qualified method for testing stationary sources. The commenter (0024) believes that when such 

a method is finalized that it will be neither simple nor cheap, and that such testing will cost many 

10’s of thousands of dollars with no demonstrable benefit to air quality. The second of these 

commenters (0046) also expresses the opinion that source testing alone could exceed the 

threshold, and encouraged EPA to revisit the cost impacts of the proposed rule and consider 

regulatory alternatives.

The other two commenters (0014, 0043) express more general concerns about the cost 

impacts of the proposed rule. One of these commenters (0043) states that the cost burden on 

each facility will be significant and that the overall impact on industry will be significantly 

greater than $100 million, with the proposed options with lower values requiring incrementally 

more analysis and more permit effort, subjecting more sources to BACT control, and requiring 

more modeling and more ambient air monitoring. The second of these commenters (0014) 

asserts that gathering 1 year of baseline PM2.5 data and doing the modeling for PM2.5 for any 

facility could easily cost $20,000 to $30,000 or even more, and pointed out that if 5,000 facilities 

do PSD permitting the cost would be over the $100 million threshold. This commenter (0014) 

urges EPA to do a cost-benefit analysis.

Response:

We believe that the commenters‟ claims that we have dramatically underestimated the 

impact of this final rule on the regulated community are incorrect. Clearly, there is a cost 

associated with the testing of individual sources to generate needed information on PM2.5

emissions. Similarly, there are costs associated with the modeling and air monitoring that will 

need to be done to meet the requirements for ambient impacts. Nevertheless, the need for such 

data does not stem solely from this rule, but from the series of EPA regulations that have been 

issued to meet the requirements associated with implementing the PM2.5 NAAQS. This includes 

the requirements associated with the 2007 Clean Air Fine Particle Implementation Rule, the 

2008 PM2.5 NSR Implementation Rule, as well as the forthcoming PM Test Methods Rule. Thus, 
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we believe that it is inappropriate to link to this particular rule the burdens described by the 

commenters (source testing for PM2.5 emissions, subjecting sources to BACT control and 

requiring more modeling and ambient air monitoring). Moreover, with the promulgation of the 

new SILs and SMC for PM2.5 in this final rule, sources will have important screening tools that 

can be used to help to reduce the costs associated with implementing the various PSD/NSR 

requirements for PM2.5. Thus, there will be fewer situations where modeling and monitoring 

must be done than would be the case in the absence of these tools. 

7.2 Comments on Review under Executive Order 13211: Actions that 

Significantly Affect Energy Supply, Distribution, and Use

Comment:

One industry association commenter (0014) disagrees with EPA’s argument that this 

action will cause no significant energy impacts, asserting that until modeling factors are 

determined and modeling actually done, EPA has no idea what the impacts will be. The 

commenter states that it is very possible that increment under these very low levels could easily 

already be consumed or be so close to it that no more coal development or power plant 

development could occur, for example in the Powder River Basin. The commenter concluded 

that the impacts could be very significant and that EPA should be required to address this.

Response:

Although the commenter suggests that the new PM2.5 increments could have significant 

energy impacts with regard to coal development or power plant development, no specific 

information was provided to convince us that such significant impacts will actually occur. In 

order for an energy project to be prevented under the PSD program, the project‟s emissions 

must be shown to cause or contribute to a violation of a NAAQS or PSD increment. Even where 

modeling shows that a PM2.5 violation may occur, it must further be shown that the project‟s 

proposed emissions will cause or contribute to the modeled violations. The required analysis 

involves a case-by-case determination that is difficult to make in the abstract because of the

importance of knowing the specific location, source design, and meteorological conditions that 

interact as part of the source impact analysis that must be completed for permit approval.

In addition, the commenter appears to imply that the PM2.5 increment may already have 

been consumed by other sources in the areas where energy projects seek to locate. However, 

under the final rule, new baseline dates will be established for PM2.5, and no PM2.5 increment 

has been consumed in any area.
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8 Other Comments on the Proposed Rulemaking

8.1 Comments on Secondary PM2.5

8.1.1 Comments on Inclusion of Secondary PM2.5 in PSD Modeling 

Analyses

Comment:

Several commenters express concern over the lack of mention of secondary PM2.5

formation in the proposal and indicate that the final rule should address whether secondary PM2.5

is to be included in modeling analyses for PM2.5. The commenters suggest a range of courses for 

the handling of secondary PM2.5 as summarized below.

In identical comments, two tribal commenters (0041, 0049) state that secondary particles, 

derived from the oxidation of primary gases such as sulfur and NOx, should be taken into 

account for all analysis and modeling in Class I and Class II areas. The tribal commenters note

that EPA has acknowledged that the secondary pollutants contribute a large portion of total 

PM2.5. The commenters add that secondary sulfate and nitrate aerosols are strong lightscatterers, which is important for Regional Haze analysis and the analysis of permits that could 

affect Class I areas. The commenters also note that the inclusion of secondary particulates is 

important for Class II areas where sources are clustered, such as in northeast Minnesota, and the 

accumulation of these pollutants could create issues for the health and welfare of the 

environment and the people.

Two state/local agency association commenters (0018, 0021) state that the presumed 

norm in a Class II PM2.5 increment analysis should be the exclusion of secondary PM2.5 because 

most Class II modeling addresses near-source impact with short travel distances and minimal 

time for sulfate and nitrate formation. These commenters add, however, that if a permitting 

agency believes that contributions from secondary particulate formation may be important,

secondary PM2.5 should be included in the Class II air impact analysis.

The same state/local agency association commenters further state that the presumed norm 

in most Class I PM2.5 increment analysis should be the inclusion of secondary PM2.5 because 

most Class I analyses involve long-range transport and there is ample time for secondary sulfate 

and nitrate formation, which can become a significant fraction of a source’s total PM2.5 impact. 

The commenters add that the permitting authority is responsible for determining whether the 

contribution of secondary particles is or is not significant.

One state/local agency commenter (0025) recommends that EPA develop a protocol for 

determining which sources must consider secondary species in their evaluations against PM2.5

SILs and increments in Class II areas. The commenter suggests a protocol whereby a source’s 

secondary species are considered against the PM2.5 SILs, increments, and NAAQS if (1) the 

source’s maximum potential emissions of SO2 emissions exceed 100 tons per year, or (2) the 

source’s SO2 impacts exceed the SO2 SILs of 5 μg/m3(24-hour) and 1 μg/m3 (annual). The 
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commenter also recommends that EPA include secondary species in evaluations against PM2.5

increments, SILs, and NAAQS in Class I areas.

One federal agency commenter (0044) states that the rule should make clear that 

secondary species are to be included in the PM2.5 increment consumption calculation. The 

commenter explains that, given the long-range transport common to most Class I analyses, there 

is ample time for secondary sulfate and nitrate formation, and that such secondary species can 

become a significant fraction of a source’s total PM2.5 impacts. 

Without explicitly expressing a preference for or against inclusion, one state/local agency 

commenter (0028) states that EPA should address the treatment of secondary particulates (and

condensables) in testing, modeling, and all the various aspects of the program. 

One state/local agency commenter (0022) does not believe that secondary particulate 

formation of PM2.5 should be included in any Class I or Class II increment or SIL analyses

because of concerns with the current models. The commenter points out deficiencies with the 

atmospheric chemistry algorithms built into CALPUFF [Note: this is discussed further in the 

next section], which is the model used for far-field Class I modeling. The commenter also notes

that Class II modeling is near-field analysis and generally does not provide adequate residence 

time for secondary sulfate and nitrate formation. The commenter adds that AERMOD, the 

regulatory model for near-field analysis, does not include an atmospheric chemistry algorithm, 

and that the use of CALPUFF for near-field analysis (for which it is not recommended) raises the 

same issues discussed above for Class I analyses.

One industry association commenter (0037) interprets the proposal not to include 

precursor emissions in increments, SILs, or SMCs, and agreed with this presumed approach. 

The commenter argues that including precursor emissions would be inappropriate because the 

models of impacts and atmospheric chemistry related to formation of PM2.5 are not sufficiently 

developed to allow sources to conduct appropriate analyses for permit applications. The 

commenter adds that the increment levels do not take into account precursor contributions so 

additional analysis would be needed to include precursors in any increment issued by EPA.

Another industry association commenter (0032) simply notes that models to predict PM2.5

concentration due to precursor emissions from single source have not been developed and, while 

a regional model is available, it appears unsuitable for single source analyses. The commenter 

adds that this lack of models makes PSD impact analyses difficult if not impossible.

Response:

The EPA recognizes the significant contribution to ambient PM2.5 concentrations that 

precursor emissions can make. The foundation for regulating PM2.5 precursors under PSD were 

established in the 2008 PM2.5 NSR Implementation Rule. 73 FR 28321. Unfortunately, the 

technical tools needed to complete a comprehensive analysis of all emissions (i.e., including 

secondary emissions) that contribute to ambient concentrations of PM2.5 are only in the 

developmental stage. Nevertheless, we believe that the final rule for PM2.5 increments, SILs, and 

SMC adequately considers all emissions that contribute to ambient PM2.5 concentrations (the 
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levels of the increments, SILs, and SMC are all intended to account for secondary emissions 

contributions when it is feasible to do so). (See also the response at Section 4.2 of this document 

for further information about the inclusion of precursor emissions for the PM2.5 SILs.) It would 

be inappropriate to restrict the regulatory language in such a way that future regulatory 

amendments would be required to enable the inclusion of precursor impacts in the PM2.5

analysis as the necessary technical tools become available. Estimating techniques are being 

developed that we anticipate will be able to be applied to the PM2.5 analysis in the near future, 

which could not be required if the regulatory language precluded them until being revised. 

8.1.2 Comments on Models for Secondary PM2.5

Comment:

One state/local agency commenter (0022), one state/local agency association commenter 

(0018), one industry association commenter (0035), and one federal agency (0044) provide 

comments on CALPUFF, the regulatory model for far-field modeling of PM2.5. All these 

commenters urge EPA to improve the atmospheric chemistry algorithms in the CALPUFF model 

to better estimate PM2.5 concentrations.

The state/local agency commenter (0022) states that the chemical transformation 

mechanisms built into the CALPUFF model have been described as dated and inadequate (R. 

Morris, 2005, Review of “Protocol for the Application of the CALPUFF Model for Analysis of 

Best Available Retrofit Technology,” Revised Draft prepared for VISTAS, September 20, 2005). 

The commenter explains that nitrate and sulfate formation is believed to be overestimated in cold 

conditions because the default chemistry scheme (MESOPUFF II) was developed using 

temperature points that were all above 50°F (T. Moore, 2005, Regional Modeling for Stationary 

Source Control Strategy Evaluation, WESTAR Conference on BART Guidelines and Trading), 

while in-cloud formation of sulfate may be greatly underestimated because it is approximated 

using relative humidity to simulate the effect of cloud water. The commenter also expresses

concern that the inclusion of secondary particle formation in PM2.5 increment analyses could lead 

to errors due to imbalances between emitted pollutants and background ammonia levels. As an 

example, the commenter posits a modeling domain that includes 4,000 tons of NOx and SO2

emissions, but only 500 tons of the SO2 are increment-consuming emissions. The commenter 

notes that only 500 tons of SO2 would be input into the model, and excess nitrate formation 

could be predicted because of the relative lack of both sulfate formation and scavenging of the 

background ammonia. As a result of these types of problems, the commenter does not believe 

that precursor emissions should be considered in the Class I or II increment and SIL analyses for 

PM2.5 until improvements are made in CALPUFF’s atmospheric chemistry algorithms and a

protocol is developed for modeling increment-consuming emissions.

The state/local agency association commenter (0018) states that EPA should improve the 

atmospheric chemistry algorithms in CALPUFF with respect to secondary formation of sulfate 

and nitrate species, but should allow the use of the current chemistry algorithm in CALPUFF as 

a screening tool for sulfate and nitrate formation in Class I and Class II increment analyses, SILs, 

and NAAQS analyses for PM2.5. The commenter adds that the same improved atmospheric 

chemistry algorithm should be added to the EPA guideline model AERMOD.
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The industry association commenter (0035) notes that, although the proposal suggested 

that CALPUFF is acceptable for purposes of PM2.5 modeling, the National Academy of Sciences

has stated, ―Because the primary focus of monitoring strategies has been on documenting 

NAAQS attainment, there is little motivation to develop methods to measure ambient 

concentrations that are substantially below the standards. Such measurements are needed to 

provide critical data for understanding atmospheric processes and providing input for air quality 

models.‖ (National Research Council, 2004, Air Quality Management, pp. 236-237.) The 

commenter believes that, ideally, EPA would invest in such models before requiring cumulative 

air impact or increment analyses of PM2.5 for NSR purposes, but realizing that this is not 

possible, the commenter urges EPA to wait to finalize this rule.

Without specifically mentioning CALPUFF, another state/local agency association 

commenter (0021) encourages EPA to undertake scientific study and analysis of secondary 

particulate formation. The commenter urges EPA to develop an understanding of the far-field 

impacts of sulfates and nitrates, and to undertake a rulemaking addressing SILs for them and 

other PSD tools as appropriate.

Response:

We acknowledge the concerns regarding the limitations of the chemical transformation 

mechanisms available in the current EPA-approved version of the CALPUFF model. We also 

note that while CALPUFF is mentioned in section 6.2 of Appendix W for visibility, where 

secondary PM2.5 plays a role, the chemistry options within CALPUFF have no regulatory status 

for NAAQS or increment assessments. We also acknowledge the concerns raised by one of the 

commenters regarding the additional challenges involved with the inclusion of chemical 

transformation within the context of an increment modeling analysis due to the complex 

interactions of emissions from baseline sources, increment-consuming sources, and incrementexpanding sources. Guidance will be developed to address these issues as the technical tools 

needed to conduct such analyses become available. 

8.2 Comments on Inclusion of the Condensable Fraction of PM2.5 in 

PSD Modeling Analyses

Comments Supporting Inclusion of Condensables:

Three state/local agency commenters (0019, 0022, 0025), two state/local agency 

association commenters (0018, 0021), two tribal commenters (0041, 0049), and one federal 

agency (0044) support the immediate inclusion of condensable PM2.5 emissions in PSD modeling 

analyses. However, these commenters vary as to which types of analyses they specifically list 

for inclusion condensable PM emissions, as follows:

 Six commenters (0018, 0019, 0021, 0022, 0025, 0044) specifically state that 

condensables should be included in increment analyses.

 Five commenters (0018, 0019, 0021, 0022, 0025) also specifically state that 

condensables should be included in SIL analyses.
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 Three commenters (0018, 0019, 0025) also specifically state that condensables should 

be included in NAAQS analyses.

 One commenter (0019) also specifically states that condensables should be included 

in SMC analyses.

 Two commenters (0041, 0049) do not specify, but state that condensable should be 

included in any analysis or modeling in Class I and Class II areas for the effects of 

Regional Haze and health related concerns.

One of the state/local agency commenters (0019) notes that the 2007 Clean Air Fine 

Particle Implementation Rule identified certain concerns with the stack measurement techniques 

for condensables which EPA intends to address over the next few years. 72 FR 20586, April 25, 

2007. The commenter adds, however, that these concerns have been misconstrued and 

misrepresented with respect to the development of emissions inventories and NAAQS 

compliance demonstrations. The commenter quotes the Clean Air Fine Particle Implementation 

Rule preamble, which stated, ―For the purpose of developing emissions inventories and 

attainment demonstrations, the final rule reflects a requirement to account for significant 

contributions of direct PM2.5 emissions, both filterable and condensable PM2.5‖ and ―States 

should continue to describe the impacts of baseline emissions and develop future air quality 

strategies using information available on direct PM2.5 emissions including condensable PM.‖ 

72 FR 20652. Thus, the commenter believes that EPA has made a very clear distinction between 

developing emission limits for permits versus the NAAQS demonstration requirements. On this 

basis, the commenter asserted that for the purposes of the proposal on PM2.5 increments, SILs,

and SMCs, which are solely dependent on emissions estimates and model calculations of impacts 

from these estimates, EPA must require the inclusion of the condensable fraction in the 

determination of emissions and for impact calculations for comparison to these numerical limits, 

in addition to NAAQS compliance. The commenter urges EPA to make this clarification in the 

requirements of the final PM2.5 NSR rule and warns that without the consideration of the 

condensable fraction in the emission and impact calculations, EPA will only perpetuate its 

current inadequate policy of addressing PM2.5 using PM10 requirements.

Two of the commenters (0018, 0025) point out that some air agencies already require 

condensable emissions to be included in air quality modeling. Another of the commenters 

(0022) predicates support for inclusion of condensables on the availability of EPA test methods.

In identical comments, the two tribal commenters (0041, 0049) state that excluding 

condensables would disallow aerosols and precursors to other pollutants that greatly contribute to 

the composition of PM2.5 and can be toxic in nature. The commenters (0041, 0049) note that the 

condensable portion of PM2.5 is a significant contributor of total PM2.5 emissions since 

condensables primarily fit into the PM2.5 size range.

Comments Opposing Inclusion of Condensables:

Two industry association commenters (0035, 0037) assert that EPA should either not 

include condensables in the increment and SILs, limiting them to filterable PM2.5, or should defer 

this rulemaking until an adequate, reliable, commercially-viable test method has been 

promulgated to include condensables. A third industry association commenter (0032) simply 
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contends that PM2.5 increments and SILs should be based only filterable PM2.5 emissions and not 

include condensables. These commenters state that in the absence of an accurate test method, 

one cannot characterize air emissions correctly, and, without a correct characterization of 

emissions, one cannot complete the air dispersion modeling needed to determine increment 

consumption for condensable PM2.5.

An industry commenter (0016) similarly points out that condensables cannot be 

accurately measured by any existing EPA-approved stack testing method. Given this fact, the 

commenter believes that EPA must not intend the scope of these regulations to extend to 

condensables. Moreover, the commenter states that if condensables are intended by EPA to fall 

within the scope of these regulations, an EPA-approved accurate stack testing methodology is a 

prerequisite for the establishment of local PSD area baselines and other critical PSD parameters. 

The commenter asserted that it is premature, arbitrary, and capricious for EPA to finalize 

regulations with which the regulated community cannot comply. The commenter indicates that 

the existing inaccurate testing methodology negatively affects state and local efforts to build an 

accurate PM2.5 emissions inventory and precludes accurate modeling of proposed new PM2.5

emissions for purposes of SIL and PSD increment determinations. The commenter adds that the 

use of a ratio of PM2.5 to PM10 approach to determining emission rates for new sources is legally 

unenforceable.

One of the industry association commenters (0035) indicates that finalizing the 

rulemaking based on filterable particulate matter would, in the interim, provide a reasonable 

basis for predicting air quality and increment impacts. The commenter believes this is 

particularly true if EPA uses a ratio approach (the equivalent substitution approach discussed in 

the proposal preamble) and if the ratio were based on the PM10/TSP increments and SILs, since 

these values were based entirely on filterable particulate.

Another of the industry association commenters (0032) expands on the issue of ratios 

used in the rulemaking, indicating that for both increments and SILs the 2007 NPRM relied 

primarily on using ratios to the PM10 increments/SILs, which were themselves based on a ratio to 

the TSP increments/SILs. The commenter interprets the proposed rules to define the PM2.5

increments and SILs such that the impact is determined by measuring ―direct‖ emissions of 

PM2.5, which includes condensable emissions. By using a ratio that reaches back to the TSP 

increment/SILs, the commenter believes EPA is relying on TSP increments/SILs using test 

methods that did not include condensables. Thus, the commenter contends that EPA has

included in the direct PM2.5 emissions a large portion of emissions that were not counted in the 

value from which it conducted its ratio process. The commenter believes that that if the 

condensable fraction had been considered in the TSP increments, the PM10 increments would 

have been set higher, possibly higher than the TSP increments, and this error is propagated (and 

perhaps compounded) by using the PM10 increment as the basis for a ratio to establish a PM2.5

increment. The commenter contends that given the lack of reliable, commercially viable, and 

replicable test methods for PM2.5 condensables, the most prudent and technically and legally

defensible approach is to use a ratio directly to TSP (thereby avoiding propagating errors in the

PM10 values) and to only include the filterable portion of PM2.5. The commenter asserts that the 

approach of using a ratio but not taking into account that the TSP method did not address 

condensables is arbitrary and cannot reasonably be termed an ―equivalent substitution‖ under 
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section 166(f) of the Act, and that establishing an increment or SIL under the section 166(a) safe 

harbor approach based on ―direct‖ emissions of PM2.5 without considering the amount of those 

emissions, or the availability and reasonableness of controls, is also unfounded.

Referring to the association’s comments on the earlier Proposed Rule to Implement Fine 

Particle NAAQS (EPA-HQ-OAR-2003-0062-0121/0121.1), the third of the industry association

commenters (0037) states that to include condensable PM2.5, EPA needs to finalize the test 

method for condensables and then re-propose to receive comments in light of that final method. 

(Commenter 0032 also advocates this approach.) Commenter 0037 encourages the Agency to 

use the final improved Method 202 to conduct analyses on an appropriately sized sample of 

sources to determine the impacts of the current proposal and offer alternatives in light of this 

testing. The commenter explains that this approach is necessary because the current PM10

increment simply does not characterize condensables accurately. 

Two of the industry association commenters (0032, 0037) also express concern that EPA 

has not analyzed how many sources would exceed the increment, whether controls are available 

to reduce emissions to meet the increment, or the cost-effectiveness of such controls, as it did 

when it established the PM10 increments. One of the commenters (0037) warns that there is a 

real question as to whether controls would be available to reduce the emissions, since the 

controls that work well for filterable PM may not be applicable to condensables. This

commenter states that EPA should ensure that any rule it issues does not have the unintended 

consequence of halting modifications and new construction, and that without this analysis, EPA 

and the public are unable to determine the effects of the rule. The second of these commenters 

(0032) notes that no impact analysis has been made available to the public for comment, and that 

the Radian report that derived the PM10 increments likewise has not been made available in the 

docket. The commenter opines that the lack of information in the docket and appropriate 

analyses are important reasons that EPA should re-propose and reconsider its approaches in this 

rulemaking.

Other Comments on Inclusion of Condensables:

Without explicitly expressing a preference for or against inclusion, one state/local agency 

commenter (0028) states that EPA should address the treatment of condensables (and secondary 

particulates) in testing, modeling, and all the various aspects of the program. 

One industry commenter (0013) expresses support for EPA’s establishment of a 

transition period for condensables in the Clean Air Fine Particle Implementation Rule (72 FR 

20652, April 25, 2007), and noted that this transition period was not addressed in this proposal. 

The commenter interprets this transition period, which extends until January 1, 2011, to apply to 

PSD modeling analyses so that condensables would not be used in the determination of PSD 

increments and comparison of a project’s direct PM2.5 concentrations with the SILs and the 

SMC. In addition, the commenter believes that EPA should explicitly continue to provide 

technical support to states in establishing appropriate PM2.5 emission limitations, particularly in 

the transition period where the condensable portion of direct PM2.5 is not required to be included.
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Response:

The above comments that either support or oppose certain actions associated with the 

regulation of condensable PM raise issues that are not relevant to this rulemaking, but that we 

are addressing through other rulemakings or policy decisions. The final rule on PM2.5

increments, SILs, and SMC does not explicitly regulate the inclusion or exemption of 

condensable PM. The final rule anticipates the inclusion of condensable PM as part of the PM2.5

that will be counted against the PM2.5 increments, and that will be used to compare source

impacts of PM2.5 against the ambient PM2.5 screening levels – SILs and SMC. 

The comments expressing concern that EPA has not analyzed how many sources would 

exceed the increment, or whether controls are available to reduce emissions to meet the 

increment, appear to suggest that EPA should consider loosening the increment values if 

necessary to ensure that economic growth will not be stymied. The EPA‟s longstanding 

interpretation is that the “at least as effective” language in CAA section 166(d) should be read 

as requiring increments at least as stringent as the statutory increments in CAA section 163. See 

EDF v. EPA, 898 F.2d 183, 187 (D.C. Cir. 1990) (describing EPA‟s reading of the statutory 

language). The EPA‟s interpretation is supported by the applicable court opinions, which note 

that this reading “is the most workable” and “well supported by the legislative history.” EDF v. 

EPA, 898 F.2d at 188. Although there is language in the opinions suggesting that the agency 

could consider a less stringent increment in light of CAA section 166(c) (see Environmental 

Defense v. EPA, 489 F.3d 1320, 1325 (D.C. Cir. 2007) (EPA must determine that the criteria 

under section 166(c) do not call “for a more, or a less, stringent standard.”) (quoting EDF v. 

EPA, 898 F.2d 183, 189 (D.C. Cir. 1990)), the court's final analysis was clear that it was 

reasonable to conclude that the only deviation from the contingent safe harbor would be towards 

greater stringency. EDF v. EPA, 898 F.2d at 189 (stating that the court‟s reading of the statute 

is that EPA must first establish the contingent safe harbor increment under CAA section 166(d) 

and then “determine that the inquiry under subsection (c) does not require a more stringent 

standard.”)

Under a separate rulemaking (the 2008 PM2.5 NSR Implementation Rule published at 

73 FR 28321 (May 16, 2008), we allowed sources to temporarily exclude condensable PM from 

consideration in permit actions under the PSD program or under nonattainment NSR until 

January 1, 2011, unless the applicable SIP otherwise required that such emissions be considered

in the permit action for applicability and enforcement purposes. Thus, independent of this final 

rule for PM2.5 increments, any permit issued on or after the January 1, 2011 sunset date will be 

required to address condensable PM. 

The temporary exclusion for condensable PM for was designed to allow EPA time to 

evaluate the existing test methods for measuring PM emissions and condensable PM emissions 

from stationary sources. Under a separate rulemaking, EPA has undertaken a re-evaluation of 

the existing test methods for measuring emissions of PM from stationary sources. In a 2009 

notice of proposed rulemaking, EPA proposed amendments to Method 202 to revise the sample 

collection and recovery procedures of the method to reduce the formation of reaction artifacts 

that could lead to inaccurate measurements of condensable PM. 74 FR 12970 (March 25, 
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2009). That rulemaking containing the revised test methods is expected to be finalized within the 

same timeframe as the final rule for PM2.5 increments, SILs, and SMC. 

Comment:

One industry association commenter (0032) indicates that EPA needs to reconsider the 

apparent underlying assumption that direct condensable emissions all convert to PM2.5 within the 

modeled area, which is a conservative assumption that tends to inflate a source’s potential 

impact. The commenter advocates that, similar to the approach taken for modeling NO2

concentrations when nitric oxide (NO) is a portion of the emissions, EPA should allow for a 

refinement that allows a deduction for a portion of the condensable plume that does not convert 

to fine particulate in the modeled area.

Response: 

Condensable PM is considered to be a component of direct PM2.5 emissions along with 

the filterable portion. It is our standard practice to include both the condensable portion and the 

filterable portion as input data for direct PM2.5 emissions in the model, and to treat them 

collectively as a non-reactive pollutant. The commenter provides no explanation as to why all of 

the condensable PM should not be considered to convert to ambient PM2.5 within the modeled 

area, since they are measured as condensable PM when they are released into the atmosphere 

from the stack. We believe that the commenter may be confusing this with the fact that some 

precursor emissions may not undergo chemical transformation to form PM2.5 within the distance 

defined for the modeling area.

8.3 General Comments on the Proposed Rulemaking

Comment:

One industry commenter (0016) alleges that EPA stated in the proposal preamble that 

secondary particulate matter acids primarily formed as a result of chemical reaction in the 

atmosphere are the intended focus of this action (citing 72 FR 54127, column 3), and that EPA is 

concerned exclusively with the adverse environmental impacts of these secondary particulate 

matter acids (citing 72 FR 54130-54132). The commenter goes on to point out, nevertheless, 

that the NPRM includes a list of industry groups by North American Industry Classification 

System (NAICS) code whose direct emissions are presumed to be subject to the proposed PM2.5

standards. The commenter urges EPA to write the regulations in a more specific manner in order 

to clarify that emissions that lead to the formation of secondary acidic PM2.5 are intended to be 

addressed, and others are not.

Response:

The commenter is mistaken in interpreting the 2007 NPRM to say that our intent in the 

this rulemaking for PM2.5 increments, SILs, and SMC is to focus on secondary particulate matter 

acids primarily formed as a result of chemical reactions in the atmosphere. On page 54127 
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(column 3) of the 2007 NPRM, we discuss the components of fine PM as part of our description 

of the health and welfare effects of PM2.5 and simply say the following:

Major components of fine particles are sulfates, strong acid, ammonium nitrate, organic 

compounds, trace elements (including metals), elemental carbon, and water.

Elsewhere in the 2007 NPRM, we indicate that hygroscopic components (particularly

sulfates and nitrates) of fine PM make a disproportionate contribution to visibility impairment 

under high humidity conditions, but we do not state (nor did we intend to state) that such 

particles are “exclusively” responsible for visibility impairment or are the focus of the 

rulemaking (page 54130, at column 1). Similarly, portions of the discussion of welfare effects 

(pages 54130-54132) tend to focus on secondary particles because those particles are most 

relevant in identifying effects on such things as vegetation and ecosystems. Nevertheless, EPA‟s 

concern for the effect of directly emitted particles on visibility impairment is clearly stated on 

page 54129 (column 1). 

Finally, on page 54128 (column 1), we state that “[w]hile there remains uncertainty 

about the role and relative toxicity of various components of fine PM, the current evidence 

continues to support the view that fine particles should be addressed as a group for purposes of 

public health protection.” Hence, the final rule regulates fine particulate matter or PM2.5 in all 

forms as it is emitted directly or formed from precursor gases in the atmosphere. Thus, there is 

no reason to revise the final rule to limit affected sources only to those emitting secondary acid 

PM2.5.

Comment:

One industry commenter (0024) requests that EPA withdraw the proposed rule, noting 

that there are many other programs currently underway that will pave the way for massive 

reductions in PM2.5 on a national basis. The commenter points out that programs such as the 

Clean Air Interstate Rule (CAIR), NOx SIP call, Acid Rain Program, controls for off-road and 

on-road diesel engines, and mobile source controls will all be phased in over the coming years. 

The commenter believes that a more prudent approach to reducing PM2.5 emissions and gaining 

associated health benefits would be to rely on the cost effective application of regulation to these 

true sources of the problem.

Response:

The commenter apparently fails to recognize that that the Act, at section 166(a), requires 

EPA to promulgate regulations to prevent significant deterioration of air quality, including 

increments or other measures, for pollutants for which NAAQS have been defined. Moreover, 

the PSD permit program is a statutorily-based permit program as well, which requires proposed 

new major stationary sources and major modifications to existing major stationary sources to 

undergo preconstruction review as a prerequisite to being approved for construction. The PSD 

program exists in conjunction with other programs for the regulation of PM2.5 emissions to 

provide for the requisite health and welfare protection associated with recognized air pollutants. 

Thus, even if some of the provisions in the final rule were permissive rather than mandatory, 
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EPA believes that they are an important part of the overall program for all of the reasons 

discussed in the preamble to the final rule, this response to comments, and elsewhere in the 

record.

Comment:

Three commenters are concerned that the PM2.5/PM10 ratio of 0.8 we derived from the 

―2001 National Emissions Estimates by Source Categories‖ could be misused in the future. 

They suggest that EPA make clear that this ratio is not to be used for purposes beyond the 

proposed increments, SILS, and SMC. 

One industry commenter (0026) notes that the ratio will differ for individual sources and 

from one area to another based on the composition of different sources in any given area. The 

commenter urged EPA to clarify that the 0.8 ratio should not be used broadly for other regulatory 

purposes, and should not be used subsequently to convert monitored and/or modeled PM10 values 

for individual locations to PM2.5 values. 

One industry association commenter (0029) urges EPA to set limitations on the use of 

that ratio for other purposes since EPA’s AP-42 Compilation of Emissions Factors confirms that 

the percentage of PM10 that is likely to be PM2.5 is not going to be 80 percent for all industries or 

for all sources within one industry. The commenter does not suggest that EPA reconsider use of 

its proposed 0.8 multiplier in this rulemaking, but does request that EPA make it clear that use of 

such a multiplier would be appropriate only for purposes of helping to set nationwide threshold 

values in this rulemaking and not for other regulatory purposes. 

One industry commenter (0020) contends that the ―one-size-fits-all‖ approach based upon

the aggregate ratio calculation is technically inaccurate and could cause problems in individual 

permitting cases where PM10 data are available to be used in modeling demonstrations but PM2.5 

emissions data are not available.

Response:

We agree with the commenters that the fraction of PM10 that is PM2.5 varies from area to 

area, industry to industry, and source to source. We did not see that as a problem in using the 

ratio as part of a methodology for selecting PM2.5 increments that would be affected by all types 

of sources. However, we acknowledge that the ratio does not adequately account for PM2.5

precursors that can contribute a significant portion of the ambient PM2.5 concentrations in an 

area. While we have not reached any final conclusion as to whether the Option 2 equivalent 

substitution approach under section 166(f) is permissible under the Act, we acknowledge that use 

of the emissions ratio approach was problematic as an equivalent substitution approach for the 

PM2.5 increments. Moreover, we do not intend to apply the 0.8 ratio that we derived based on a 

large aggregation of point sources for other regulatory purposes, including the examples given 

by the commenters.