i

Table of Contents provided as an aid to reviewers. It will not 

be included in the formal package. Each heading is hyperlinked 

to the text of this preamble; hold down the Control key and 

right click any heading to go to that point in the text.

I. General Information .......................................5

A. Does This Action Apply to Me? ..........................5

B. Where Can I Get a Copy of this Document and Other 

Related Information? ...................................6

II. Purpose ...................................................6

III. Overview of Final PM2.5 PSD Regulations ....................7

A. Increments .............................................7

B. SILs ...................................................8

C. SMC ....................................................9

IV. Background ...............................................10

A. PSD Program ...........................................10

B. History of Particulate Matter (PM) NAAQS ..............14

1. Total Suspended Particulate (TSP) and PM10 NAAQS.... 14

2. PM2.5 NAAQS ......................................... 15

3. Revised PM2.5 and PM10 NAAQS......................... 15

C. Implementation of NSR for PM2.5 ........................16

D. Increments Under the PSD Program ......................18

E. Historical Approaches for Developing Increments .......27

1. Congressional Enactment of Increments for PM and SO2 27

2. EPA’s Promulgation of Increments for NO2 and PM10 ... 28

a. Increments for NO2 Using the “Contingent Safe 

Harbor” Approach Under Section 166(a) of the Act 29

b. Increments for PM10 Using “Equivalent Substitution” 

Approach Under Section 166(f) of the Act ........ 31

V. Final Action on PM2.5 Increments ..........................33

A. Decision to Establish PM2.5 Increments Using “Contingent 

Safe Harbor Approach” Under Section 166(a) ............33

B. Rationale for the Applicability of Section 166(a) .....35

C. EPA’s Interpretation of the Requirements under Sections 

166(a)-(d) of the Act .................................44

1. Regulations as a Whole Should Fulfill Statutory 

Requirements ....................................... 45

2. Contingent Safe Harbor Approach .................... 46

3. The Statutory Factors Applicable under Section 166(c)

48

4. Balancing the Factors Applicable under Section 166(c)

51

5. Authority for States to Adopt Alternatives to 

Increments ......................................... 52

D. Framework for Pollutant-Specific PSD Regulations for 

PM2.5 ..................................................56

1. Increment System ................................... 57
ii

2. Area Classifications ............................... 60

3. Permitting Procedures .............................. 64

4. AQRV Review by Federal Land Manager (FLM) and 

Reviewing Authority ................................ 65

5. Additional Impacts Analysis ........................ 73

6. Installation of BACT ............................... 75

E. Final PM2.5 Increments .................................76

1. Identification of Safe Harbor Increments ........... 78

2. Data Used by EPA for the Evaluation of the Safe Harbor 

Increments for PM2.5. ............................... 81

3. Scope of Effects Considered ........................ 84

4. Evaluation of the Health and Welfare Effects of PM2.5 85

a. Health Effects .................................. 86

b. Welfare Effects ................................. 87

5. Fundamental Elements of Increments ................. 89

6. Evaluation of the Safe Harbor Increments ........... 90

7. Compliance Determinations for the PM2.5 Increments . 112

a. Modeling Compliance with PM2.5 Increments ....... 112

b. Condensable PM ................................. 114

c. PM2.5 Precursors ................................ 115

F. Final Action on Trigger and Baseline Dates for PM2.5 

Increments ...........................................117

G. Definition of “Baseline Area” for PM2.5 ...............124

H. No Final Action with Respect to the Proposed Revocation 

of PM10 Annual Increments .............................126

I. Other Comments on Increments .........................133

VI. Final Action on PM2.5 SILs ...............................137

A. EPA’s Determination on SILs for PM2.5 .................137

B. Response to Comments Concerning the SILs .............139

1. Legal Basis for SILs .............................. 141

2. Levels of the SILs ................................ 142

a. Class I SILs ................................... 145

b. Class II and III SILs .......................... 151

3. Relationship Between SILs and AQRVs ............... 156

4. Form of the SILs .................................. 158

5. SILs for Other Pollutants ......................... 159

VII. Final Action on the PM2.5 SMC ............................162

A. EPA’s Determination on the PM2.5 SMC ..................162

B. Response to Comments Concerning the SMC ..............164

1. Legal Issues ...................................... 164

2. Level of the SMC .................................. 167

C. Correction of Cross Reference in PSD Ambient Monitoring 

Requirements .........................................171

VIII. Dates Associated With Implementation of the Final Rule ..172

A. Effective Date of the Final Rule .....................173

1. State PSD Programs ................................ 177

2. Federal PSD Program ............................... 179
iii

B. Transition Period ....................................182

C. SILs and SMC for PM2.5 ................................185

IX. Other Regulatory Changes ................................189

X. Statutory and Executive Order Reviews ...................190

A. Executive Order 12866 - Regulatory Planning and Review190

B. Paperwork Reduction Act ..............................190

C. Regulatory Flexibility Act ...........................192

D. Unfunded Mandates Reform Act .........................193

E. Executive Order 13132 - Federalism ...................194

F. Executive Order 13175 - Consultation and Coordination 

with Indian Tribal Governments .......................195

G. Executive Order 13045 - Protection of Children from 

Environmental Health and Safety Risks ................195

H. Executive Order 13211 - Actions That Significantly 

Affect Energy Supply, Distribution, or Use ...........196

I. National Technology Transfer and Advancement Act .....196

J. Executive Order 12898 – Federal Actions to Address 

Environmental Justice in Minority Populations and LowIncome Populations ...................................197

K. Congressional Review Act .............................198

XI. Judicial Review .........................................199

XII. Statutory Authority .....................................199
1

6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 51 and 52

[EPA-HQ-OAR-2006-0605; FRL- ]

RIN 2060-AO24

Prevention of Significant Deterioration (PSD) for Particulate 

Matter Less Than 2.5 Micrometers (PM2.5) – Increments, 

Significant Impact Levels (SILs) and Significant Monitoring 

Concentration (SMC)

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

SUMMARY: The EPA is amending the requirements for particulate 

matter less than 2.5 micrometers (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. The SILs for PM2.5 are also being added to two 

other New Source Review (NSR) rules that regulate the 

construction and modification of any major stationary source 

locating in an attainment or unclassifiable area, where the 

source’s emissions may cause or contribute to a violation of the 

national ambient air quality standards (NAAQS).

DATES: This final rule is effective on [INSERT DATE 60 DAYS 

AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER]. 
2

ADDRESSES: The EPA has established a docket for this action 

under Docket ID No. EPA-HQ-OAR-2006-0605. All documents in the 

docket are listed on the www.regulations.gov Web Site. Although 

listed in the index, some information may not be publicly 

available, e.g., Confidential Business Information (CBI) or 

other information whose disclosure is restricted by statute. 

Certain other material, such as copyrighted material, is not 

placed on the Internet and will be publicly available only in 

hard copy form. Publicly available docket materials are 

available either electronically through www.regulations.gov or 

in hard copy at the Air Docket, EPA/DC, EPA West, Room 3334, 

1301 Constitution Avenue, Northwest, Washington, DC. The Public 

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

Friday, excluding legal holidays. The telephone number for the 

Public Reading Room is (202) 566-1744, and the telephone number 

for the Air Docket is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Mr. Dan deRoeck, Air Quality 

Policy Division, Office of Air Quality Planning and Standards 

(C504-03), U.S. Environmental Protection Agency, Research 

Triangle Park, North Carolina 27711, telephone number: (919) 

541-5593, facsimile number: (919) 541-5509, e-mail address: 

deroeck.dan@epa.gov.

SUPPLEMENTARY INFORMATION:

The information in this Supplementary Information section 
3

of this preamble is organized as follows:

I. General Information

A. Does This Action Apply to Me?

B. Where Can I Get a Copy of this Document and Other 

Related Information?

II. Purpose

III. Overview of Final PM2.5 PSD Regulations

A. Increments

B. SILs

C. SMC

IV. Background

A. PSD Program

B. History of Particulate Matter (PM) NAAQS

1. Total Suspended Particulate (TSP) and PM10 NAAQS

2. PM2.5 NAAQS

3. Revised PM2.5 and PM10 NAAQS

C. Implementation of NSR for PM2.5

D. Increments Under the PSD Program

E. Historical Approaches for Developing Increments

1. Congressional Enactment of Increments for PM and SO2

2. EPA’s Promulgation of Increments for NO2 and PM10

a. Increments for NO2 Using the “Contingent Safe 

Harbor” Approach Under Section 166(a) of the Act

b. Increments for PM10 Using “Equivalent Substitution” 

Approach Under Section 166(f) of the Act

V. Final Action on PM2.5 Increments

A. Decision to Establish PM2.5 Increments Using “Contingent 

Safe Harbor Approach” Under Section 166(a)

B. Rationale for the Applicability of Section 166(a)

C. EPA’s Interpretation of the Requirements under Sections 

166(a)-(d) of the Act

1. Regulations as a Whole Should Fulfill Statutory 

Requirements

2. Contingent Safe Harbor Approach

3. The Statutory Factors Applicable under Section 166(c)

4. Balancing the Factors Applicable under Section 166(c)

5. Authority for States to Adopt Alternatives to 

Increments

D. Framework for Pollutant-Specific PSD Regulations for 

PM2.5

1. Increment System

2. Area Classifications

3. Permitting Procedures

4. AQRV Review by Federal Land Manager (FLM) and 

Reviewing Authority

5. Additional Impacts Analysis
4

6. Installation of BACT

E. Final PM2.5 Increments

1. Identification of Safe Harbor Increments

2. Data Used by EPA for the Evaluation of the Safe Harbor 

Increments for PM2.5

3. Scope of Effects Considered

4. Evaluation of the Health and Welfare Effects of PM2.5

a. Health Effects

b. Welfare Effects

5. Fundamental Elements of Increments

6. Evaluation of the Safe Harbor Increments

7. Compliance Determinations for the PM2.5 Increments

a. Modeling Compliance with PM2.5 Increments

b. Condensable PM

c. PM2.5 Precursors

F. Final Action on Trigger and Baseline Dates for PM2.5 

Increments

G. Definition of “Baseline Area” for PM2.5

H. No Final Action with Respect to the Proposed Revocation 

of PM10 Annual Increments

I. Other Comments on Increments

VI. Final Action on PM2.5 SILs

A. EPA’s Determination on SILs for PM2.5

B. Response to Comments Concerning the SILs

1. Legal Basis for SILs

2. Levels of the SILs

a. Class I SILs

b. Class II and III SILs

3. Relationship Between SILs and AQRVs

4. Form of the SILs

5. SILs for Other Pollutants

VII. Final Action on the PM2.5 SMC

A. EPA’s Determination on the PM2.5 SMC

B. Response to Comments Concerning the SMC

1. Legal Issues

2. Level of the SMC

C. Correction of Cross Reference in PSD Ambient Monitoring 

Requirements

VIII. Dates Associated With Implementation of the Final Rule

A. Effective Date of the Final Rule

1. State PSD Programs

2. Federal PSD Program

B. Transition Period

C. SILs and SMC for PM2.5

IX. Other Regulatory Changes

X. Statutory and Executive Order Reviews

A. Executive Order 12866 - Regulatory Planning and Review
5

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132 - Federalism

F. Executive Order 13175 - Consultation and Coordination 

with Indian Tribal Governments

G. Executive Order 13045 - Protection of Children from 

Environmental Health and Safety Risks

H. Executive Order 13211 - Actions That Significantly 

Affect Energy Supply, Distribution, or Use

I. National Technology Transfer and Advancement Act

J. Executive Order 12898 – Federal Actions to Address 

Environmental Justice in Minority Populations and LowIncome Populations

K. Congressional Review Act

XI. Judicial Review

XII. Statutory Authority

I. General Information

A. Does This Action Apply to Me?

Entities affected by this rule include sources in all 

industry groups. The majority of sources potentially affected 

are expected to be in the following groups: 

Industry group NAICSa

Electric services................... 221111, 221112, 221113, 

221119, 221121, 221122

Petroleum refining.................. 32411

Industrial inorganic chemicals...... 325181, 32512, 325131, 

325182, 211112, 325998, 

331311, 325188

Industrial organic chemicals........ 32511, 325132, 325192, 

325188, 325193, 32512, 

325199

Miscellaneous chemical products..... 32552, 32592, 32591, 

325182, 32551

Natural gas liquids................. 211112

Natural gas transport............... 48621, 22121
6

Pulp and paper mills................ 32211, 322121, 322122, 

32213

Paper mills......................... 322121, 322122

Automobile manufacturing............ 336111, 336112, 336712, 

336211, 336992, 336322, 

336312, 33633, 33634, 

33635, 336399, 336212, 

336213

Pharmaceuticals..................... 325411, 325412, 325413, 

325414

a North American Industry Classification System.

Entities affected by this rule also include states, local 

reviewing authorities, and Indian country where affected new and 

modified major stationary sources will locate.

B. Where Can I Get a Copy of this Document and Other Related 

Information?

In addition to being available in the docket, an electronic 

copy of this final rule will also be available on the World Wide 

Web. Following signature by the EPA Administrator, a copy of 

this final rule will be posted in the regulations and standards 

section of our NSR home page located at http://www.epa.gov/nsr.

II. Purpose 

The purpose of this rulemaking is to finalize certain 

program provisions under the regulations to prevent significant 

deterioration of air quality due to emissions of PM2.5 (i.e., 

under the PM2.5 PSD regulations). This final rule supplements 

the final implementation rule for PM2.5, known as the Clean Air 

Fine Particle Implementation Rule (CAFPIR) that we promulgated 
7

on April 25, 2007 (72 FR 20586), and the PM2.5 NSR Implementation 

Rule that we promulgated on May 16, 2008 (73 FR 28321). 

Together, these three rules encompass the elements necessary for

implementation of a PM2.5 program in any area. This final rule

is important because it establishes new increments, SILs, and an 

SMC for PM2.5 to facilitate ambient air quality monitoring and 

modeling under the PSD regulations for areas designated 

attainment or unclassifiable for PM2.5. 

III. Overview of Final PM2.5 PSD Regulations

A. Increments

This rulemaking establishes new increments for PM2.5

pursuant to the legal authority contained in section 166(a) of 

the Clean Air Act (CAA or Act) for pollutants for which NAAQS 

are promulgated after 1977. The final PM2.5 increments were

identified as Option 1 in the 2007 Notice of Proposed Rulemaking 

(NPRM) for this action, and are as follows: 

Averaging 

Period

NAAQS

(μg/m3)

Increments (μg/m3)

Class I Class II Class III

Annual 15 1 4 8

24-hour 35 2 9 18

As discussed in more detail in sections V.F and VIII, the 

new increments for PM2.5 will become effective 1 year from the 
8

date of promulgation of this final rule,1 that is, on [INSERT 

DATE 1 YEAR AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER].

This final rule does not revoke the annual increments for 

particulate matter less than 10 micrometers (PM10) as proposed 

under Option 1 in the 2007 NPRM, but instead retains, in 

addition to the PM2.5 increments, both the annual and 24-hour 

increments for PM10. This outcome is discussed in greater detail 

in section V.H of this preamble.

B. SILs

This rule establishes new SILs for PM2.5 for evaluating the 

impact a proposed new source or modification may have on the 

NAAQS and PSD increments for PM2.5. The SILs for PM2.5 were 

developed by scaling the existing PM10 SILs using a PM2.5-to-PM10

NAAQS ratio. The final SILs were identified as Option 3 in the 

2007 NPRM, and are as follows:

Averaging 

Period

SILs (μg/m3)

Class I Class II Class III

Annual 0.06 0.3 0.3

24-hour 0.07 1.2 1.2

These values will be added to the state implementation plan 

(SIP) requirements for PSD at 40 CFR 51.166 (as an optional 

 

1 We view “promulgation” to occur on the date that a final 

rule is published in the Federal Register. Hence, references in 

this preamble to the date of promulgation of this final rule 

alternatively refer to the date of publication of this final 

rule.
9

screening tool) and the federal PSD program at 40 CFR 52.21, as 

well as under the preconstruction review permit requirements at 

40 CFR 51.165(b) and part 51, Appendix S. See a more detailed 

discussion of the SILs, as well as the relevant comments and our 

responses to them, in section VI of this preamble. The SILs for 

PM2.5 are incorporated into the federal PSD program as well as 

into the regulations for state-implemented PSD programs, 

although they are regarded as optional for state programs. The 

effective date for implementing the new SILs under the federal 

PSD program is the effective date of this final rule. See

section VIII of this preamble for further discussion of the 

effective date.

C. SMC

This final rule establishes the SMC for PM2.5 as 4 μg/m3 PM2.5

(24-hour average). This value has been developed pursuant to 

proposed Option 1; however, it should be noted that the value 

being established in this final rule is lower than the proposed 

value of 10 μg/m3 that was originally developed under Option 1.

A more detailed discussion of the proposed SMC is presented in 

section VII of this preamble, describing the rationale for 

altering the proposed SMC, and the relevant comments on the 

proposed SMC and our responses to them. The SMC for PM2.5 is 

incorporated into the federal PSD program as well as into the 

regulations for state-implemented PSD programs, although they 
10

are regarded as optional for state programs. As with the SILs 

for PM2.5, the effective date for implementing the new SMC under 

the federal PSD program is the effective date of this final 

rule. See section VIII of this preamble for further discussion 

of the effective date.

IV. Background

A. PSD Program

The NSR provisions of the Act are a combination of air 

quality planning and air pollution control technology program 

requirements for new and modified stationary sources of air 

pollution. In brief, section 109 of the Act requires us to 

promulgate primary NAAQS to protect public health and secondary 

NAAQS to protect public welfare. Once we have set these 

standards, states must develop, adopt, and submit to us for 

approval SIPs that contain emission limitations and other 

control measures to attain and maintain the NAAQS and to meet 

the other requirements of section 110(a) of the Act. Part C of 

title I of the Act contains the requirements for a component of 

the major NSR program known as the PSD program. This program 

sets forth procedures for the preconstruction review and 

permitting of new and modified major stationary sources of air 

pollution locating in areas meeting the NAAQS (“attainment” 

areas) and areas for which there is insufficient information to 

classify an area as either attainment or nonattainment 
11

(“unclassifiable” areas). Most states have SIP-approved 

preconstruction permit (major NSR) programs. The federal PSD 

program at 40 CFR 52.21 applies in some states that lack a SIPapproved permit program, and in Indian country.2 The 

applicability of the PSD program to a major stationary source 

must be determined in advance of construction and is a pollutant 

specific determination. Once a major source is determined to be 

subject to the PSD program (PSD source), among other 

requirements, it must undertake a series of analyses to 

demonstrate that it will use the best available control 

technology (BACT) and will not cause or contribute to a 

violation of any NAAQS or increment. For the latter 

demonstration, the PSD regulations generally require sources to 

submit for review and approval a source impact analysis and an 

air quality analysis. 

The source impact analysis is primarily a modeling analysis 

designed to show that the allowable emissions increase from the 

proposed project, in conjunction with other emissions increases 

from existing sources, will not result in a violation of either 

the NAAQS or increments. In cases where the source’s emissions 

 

2 We have delegated our authority to some states to 

implement the federal PSD program. The EPA remains the 

reviewing authority in non-delegated states lacking SIP-approved 

programs and in Indian country.
12

may adversely affect an area classified as a Class I area, 

additional review is conducted to protect the increments and 

special attributes of such an area defined as “air quality

related values” (AQRVs).

The air quality analysis must assess the ambient air 

quality in the area that the proposed project would affect. For 

this analysis, the owner or operator of the proposed project 

must submit as part of a complete permit application air quality 

monitoring data that represent the air quality in the area 

affected by the proposed source for the 1-year period preceding 

receipt of the application. Where data may already exist to 

represent existing air quality, it may be used by the applicant; 

otherwise, the source owner or operator must be responsible for 

the installation and operation of monitors to collect the 

necessary data. 

Historically, EPA has allowed the use of several types of 

screening tools to help facilitate implementation of the 

preconstruction review process to reduce the permit applicant’s 

burden and streamline the permitting process for de minimis

circumstances. These tools include a significant emissions rate 

(SER), SILs, and a SMC. The SER, defined in tons per year (tpy) 

for each regulated pollutant, is used to determine whether the 

emissions increase from any proposed source or modification can 

be excluded from review on the grounds that the increase of any 
13

particular pollutant is de minimis. 

The SIL, expressed as an ambient pollutant concentration 

(micrograms per cubic meter (μg/m

3)), is used to determine 

whether the ambient impact of a particular pollutant (once it is 

determined to be emitted in significant amounts) is significant 

so as to warrant a complete source impact analysis involving 

modeling the collective impacts of the proposed project and 

emissions from other existing sources. 

The PSD regulations generally require each PSD applicant to 

collect 1 year of continuous air quality monitoring data for any 

pollutant determined to be subject to preconstruction review as 

part of complete PSD permit application. Using the SMC as a 

screening tool, expressed as an ambient pollutant concentration 

(μg/m3), sources may be able to demonstrate that the modeled air 

quality impact of emissions from the new source or modification, 

or the existing air quality level in the area where the source 

would construct, are less than the SMC, i.e., de minimis, and 

may be allowed to forego the preconstruction monitoring 

requirement for a particular pollutant at the discretion of the 

reviewing authority.3 See 40 CFR 51.166(i)(5) and 52.21(i)(5).

When the reviewing authority reaches a preliminary decision 

to authorize construction of a proposed major new source or 

 

3 The basic monitoring exemption provision is part of the 

original monitoring requirements adopted in the 1980 PSD 

rulemaking. 45 FR 52710, August 7, 1980.
14

major modification, it must provide notice of the preliminary 

decision and an opportunity for comment by the general public, 

industry, and other persons that may be affected by the 

emissions of the proposed major source or major modification. 

After considering these comments, the reviewing authority may 

issue a final determination on the construction permit in 

accordance with the PSD regulations. 

B. History of Particulate Matter (PM) NAAQS

1. Total Suspended Particulate (TSP) and PM10 NAAQS

The EPA initially established NAAQS for PM in 1971, 

measured by the TSP indicator. Based on the size of the 

particles collected by the “high-volume sampler,” which at that 

time was the reference method for determining ambient 

concentrations, TSP included all PM up to a nominal size of 25 

to 45 micrometers. We established both annual and 24-hour NAAQS 

for TSP.

On July 1, 1987, we revised the NAAQS for PM and changed 

the indicator from TSP to PM10; the latter indicator includes

particles with a mean aerodynamic diameter less than or equal to 

10 micrometers. The PM10 particles are the subset of inhalable 

particles small enough to penetrate to the thoracic region 

(including the tracheobronchial and alveolar regions) of the 

respiratory tract (referred to as thoracic particles). We 

established annual and 24-hour NAAQS for PM10, and revoked the 
15

NAAQS for TSP. (52 FR 24634). 

2. PM2.5 NAAQS

On July 18, 1997, we again revised the NAAQS for PM in 

several respects. While we determined that the NAAQS should 

continue to focus on particles less than or equal to 10 

micrometers in diameter, we also determined that the fine and 

coarse fractions of PM10 should be considered separately. We 

established new annual and 24-hour NAAQS using PM2.5 (referring 

to particles with a nominal mean aerodynamic diameter less than 

or equal to 2.5 micrometers) as the indicator for fine 

particles. The 1997 NAAQS rule also modified the PM10 NAAQS for 

the purpose of regulating the coarse fraction of PM10 (referred 

to as thoracic coarse particles or coarse-fraction particles; 

generally including particles with a nominal mean aerodynamic 

diameter greater than 2.5 micrometers and less than or equal to 

10 micrometers, or PM10-2.5); however, this part of the rulemaking

was vacated during subsequent litigation, leaving the preexisting 1987 PM10 NAAQS in place (62 FR 38652). 

3. Revised PM2.5 and PM10 NAAQS 

On October 17, 2006, we promulgated revisions to the NAAQS 

for PM2.5 and PM10 with an effective date of December 18, 2006 (71 

FR 61144). We lowered the 24-hour NAAQS for PM2.5 from 65 μg/m3

to 35 μg/m3, and retained the existing annual PM2.5 NAAQS of 15 

μg/m3. In addition, we retained the existing PM10 24-hour NAAQS 
16

of 150 μg/m3, and revoked the annual PM10 NAAQS (set at 50 μg/m3). 

C. Implementation of NSR for PM2.5

After we established new annual and 24-hour NAAQS based on 

PM2.5 as the indicator for fine particles in July 1997, we issued 

a guidance document “Interim Implementation for the New Source 

Review Requirements for PM2.5,” John S. Seitz, Director, Office 

of Air Quality Planning and Standards, EPA, October 23, 1997. 

As noted in that guidance, section 165 of the Act implies that 

certain PSD requirements become effective for a new NAAQS upon 

the effective date of the NAAQS. Section 165(a)(1) of the Act 

provides that no new or modified major source may be constructed 

without a PSD permit that meets all of the section 165(a) 

requirements with respect to the regulated pollutant. Moreover, 

section 165(a)(3) provides that the emissions from any such 

source may not cause or contribute to a violation of any 

increment or NAAQS. Also, section 165(a)(4) requires BACT for 

each pollutant subject to PSD regulation. The 1997 guidance 

stated that sources would be allowed to use implementation of a 

PM10 program as a surrogate for meeting PM2.5 NSR requirements 

until certain difficulties were resolved. These difficulties 

included the lack of necessary tools to calculate the emissions 

of PM2.5 and related precursors, the lack of adequate modeling 

techniques to project ambient impacts, and the lack of PM2.5

monitoring sites. 
17

On April 5, 2005, we issued a guidance document entitled 

“Implementation of New Source Review Requirements in PM-2.5 

Nonattainment Areas,” Stephen D. Page, Director, Office of Air 

Quality Planning and Standards, EPA. This memorandum provided

guidance on the implementation of the nonattainment major NSR 

provisions in PM2.5 nonattainment areas in the interim period 

between the effective date of the PM2.5 NAAQS designations 

(April 5, 2005) and when we promulgate regulations to implement 

nonattainment major NSR for the PM2.5 NAAQS. In addition to 

affirming the continued use of the John S. Seitz guidance memo 

in PM2.5 attainment areas, this memo recommended that, until we 

promulgate the PM2.5 major NSR regulations, states should use a 

PM10 nonattainment major NSR program as a surrogate to address 

the requirements of nonattainment major NSR for the PM2.5 NAAQS.

On November 1, 2005, we proposed a rule to implement the 

PM2.5 NAAQS, including proposed revisions to the NSR program. For 

those states with EPA-approved PSD programs, we proposed to 

continue the 1997 NSR guidance to use PM10 as a surrogate for 

PM2.5, but only during the SIP development period. We also 

indicated in that proposal that we would be developing

increments, SILs, and an SMC in a separate rulemaking, i.e.,

this final rule. Since there was an interim surrogate NSR 

program in place, i.e., the PM10 Surrogate Policy, EPA decided to 

first promulgate the non-NSR part of the implementation rule 
18

(including attainment demonstrations, designations, control 

measures, etc.) – which was promulgated as the CAFPIR on 

April 25, 2007 (72 FR 20586). 

The NSR part of the implementation rule was issued 

separately as a final rule on May 16, 2008 (73 FR 28321), and 

included sets of NSR regulations for both attainment (PSD) and 

nonattainment areas (nonattainment NSR) for PM2.5. In the May 

16, 2008 rule we added one of the important screening tools —

the SER — for PM2.5. The SER for PM2.5 is defined as an emissions 

rate of 10 tpy for direct PM2.5 emissions. We also listed sulfur 

dioxide (SO2) and nitrogen oxides (NOx) as precursors of ambient 

PM2.5 and defined “significant” as 40 tpy or more of either 

precursor pollutant. States were allowed up to 3 years from the 

date of publication in the Federal Register to revise their SIPs 

and submit their revised NSR programs to EPA for approval.

D. Increments Under the PSD Program

Under section 165(a)(3) of the Act, a PSD permit applicant 

must demonstrate that emissions from the proposed construction 

and operation of a facility “will not cause, or contribute to, 

air pollution in excess of any (A) maximum allowable increase or 

maximum allowable concentration for any pollutant ....” 42 U.S.C. 

7475(a)(3). The “maximum allowable increase” of an air 

pollutant that is allowed to occur above the applicable baseline 

concentration for that pollutant is known as the PSD increment. 
19

By establishing the maximum allowable level of ambient pollutant 

concentration increase in a particular area, an increment 

defines “significant deterioration” of air quality in that area.

For PSD baseline purposes, a baseline area for a particular 

pollutant emitted from a source includes the attainment or 

unclassifiable area in which the source is located, as well as 

any other attainment or unclassifiable area in which the 

source’s emissions of that pollutant are projected (by air 

quality modeling) to result in an ambient pollutant increase of 

at least 1 μg/m3 (annual average). See, e.g., 40 CFR 

52.21(b)(15)(i). Once the baseline area is established, 

subsequent PSD sources locating in that area need to consider 

that a portion of the available increment may have already been 

consumed by previous emissions increases. 

In general, the submittal date of the first complete PSD 

permit application in a particular area is the operative

“baseline date.”4 On or before the date of the first complete 

PSD application, emissions generally are considered to be part 

of the baseline concentration, except for certain emissions from 

 

4 Baseline dates are pollutant specific. That is, a 

complete PSD application establishes the baseline date only for 

those regulated NSR pollutants that are projected to be emitted 

in significant amounts (as defined in the regulations) by the 

applicant’s new source or modification. Thus, an area may have 

different baseline dates for different pollutants. 
20

major stationary sources, as explained in the following 

discussion of baseline dates. Most emissions increases that 

occur after the baseline date will be counted toward the amount 

of increment consumed. Similarly, emissions decreases after the 

baseline date restore or expand the amount of increment that is 

available. 

In practice, three dates related to the PSD baseline 

concept are important in understanding how to calculate the 

amount of increment consumed — (1) trigger date; (2) major 

source baseline date; and (3) minor source baseline date. The 

first relevant date is the trigger date. The trigger date, as 

the name implies, triggers the overall increment consumption 

process nationwide. Specifically, this is a fixed date, which 

must occur before the minor source baseline date can be 

established for the pollutant-specific increment in a particular 

attainment area. See, e.g., 40 CFR 52.21(b)(14)(ii). For PM 

(regulated as TSP) and SO2, Congress defined the applicable 

trigger date as August 7, 1977 – the date of the 1977 amendments 

to the Act when the original statutory increments were 

established by Congress. For nitrogen dioxide (NO2), we selected 

the trigger date as February 8, 1988 – the date on which we 

proposed increments for NO2. See 53 FR 40656, 40658; October 17, 

1988. In this final rule, as described later, we are 

establishing a separate trigger date for purposes of 
21

implementing the PM2.5 increments. See section V.F of this 

preamble for additional discussion of the trigger date for PM2.5.

The two remaining dates – “minor source baseline date” and 

“major source baseline date” – as described later, are necessary 

to properly account for the emissions that are to be counted 

toward the amount of increment consumed following the national 

trigger date, in accordance with the statutory definition of 

“baseline concentration” in section 169(4) of the Act. The 

statutory definition provides that the baseline concentration of 

a pollutant for a particular baseline area is generally the air 

quality at the time of the first application for a PSD permit in 

the area. Consequently, any increases in actual emissions 

occurring after that date (with some possible exceptions that we 

will discuss later) would be considered to consume the 

applicable PSD increment. However, the statutory definition in 

section 169(4) also provides that “[e]missions of sulfur oxides 

and particulate matter from any major emitting facility on which 

construction commenced after January 6, 1975, shall not be 

included in the baseline and shall be counted in pollutant 

concentrations established under this part.” 

To make this distinction between the date when emissions 

resulting from the construction at a major stationary source 

consume the increment and the date when emissions changes in 

general (i.e., from both major and minor sources) begin to 
22

consume the increment, we established the terms “major source 

baseline date” and “minor source baseline date,” respectively. 

See 40 CFR 51.166(b)(14) and 52.21(b)(14). Accordingly, the 

“major source baseline date,” which precedes the trigger date,

is the date after which actual emissions increases associated 

with construction at any major stationary source consume the PSD 

increment. In accordance with the statutory definition of 

“baseline concentration,” the PSD regulations define a fixed 

date to represent the major source baseline date for each 

pollutant for which an increment exists. Congress defined the 

major source baseline date for the statutory increments for PM 

and SO2 as January 6, 1975. For the NO2 increments, which we 

promulgated in 1988 under our authority to establish an 

increment system under section 166(a) of the Act, the major 

source baseline date we selected was February 8, 1988 – the date 

on which we proposed increments for NO2. 53 FR 40656. In both 

instances, the major source baseline date for the individual 

increments was set as a date which preceded the date on which 

the regulations pertaining to those increments were issued. In 

this final rule, as described later, we are establishing a 

separate major source baseline date for implementing the PM2.5 

increments. See section V.F of this preamble for further 

discussion of the major source baseline date for PM2.5. 

The “minor source baseline date” is the earliest date after 
23

the trigger date on which a source or modification submits the 

first complete application for a PSD permit in a particular 

area. After the minor source baseline date, any increase in 

actual emissions (from both major and minor sources) consumes

the PSD increment for that area.

The PSD regulations set out the third date – the “trigger 

date” - that is relevant to the PSD baseline concept. These 

regulations provide that the earliest date on which the minor 

source baseline date can be established is the date immediately 

following the “trigger date” for the pollutant-specific 

increment. See, e.g., 40 CFR 52.21(b)(14)(ii). For PM and SO2, 

Congress defined the applicable trigger date as August 7, 1977 –

the date of the 1977 amendments to the Act when the original 

statutory increments were established by Congress. For NO2, we 

selected the trigger date as February 8, 1988 – the date on 

which we proposed increments for NO2. See 53 FR 40656, 40658; 

October 17, 1988.

Once the minor source baseline is established, the new 

emissions increase from that major source consumes a portion of 

the increment in that area, as do any subsequent actual 

emissions increases that occur from any new or existing source 

in the area. When the maximum pollutant concentration increase 

defined by the increment has been reached, additional PSD 

permits cannot be issued until sufficient amounts of the 
24

increment are “freed up” via emissions reductions that may occur 

voluntarily, e.g., via either source shutdowns or control 

requirements imposed by the reviewing authority. Moreover, the 

air quality in a region cannot deteriorate to a level in excess 

of the applicable NAAQS, even if all the increment in the area 

has not been consumed. Therefore, new or modified sources 

located in areas where the air pollutant concentrations are near 

the level allowed by the NAAQS may not have full use of the 

amount of pollutant concentration increase allowed by the 

increment.

Under EPA guidance, the actual increment analysis that a 

proposed new or modified source undergoing PSD review must 

complete depends on the area impacted by the source’s new 

emissions. We have also provided approved air quality models 

and guidelines for sources to use to project the air quality 

impact of each pollutant (over each averaging period) for which 

an increment analysis must be done.5 In addition, we established 

SILs for each pollutant under the permit requirements applicable 

to new and modified major stationary sources locating in 

attainment areas that would cause or contribute to a violation 

of any NAAQS. These SILs have also been used for implementing

the PSD program to identify levels below which the source’s 

 

5 See EPA’s “Guideline on Air Quality Models” at 40 CFR part 

51, Appendix W.
25

modeled impact of a particular pollutant is regarded as de 

minimis. See 40 CFR 51.165(b) and part 51, Appendix S, section 

III.A. In this final rule, we are establishing SILs (24-hour 

and annual) for PM2.5 that are being added to the aforementioned 

regulations containing SILs for other pollutants, as well as to 

the PSD regulations in 40 CFR 51.166 and 52.21. See further 

discussion of the SILs for PM2.5 in section VI of this preamble. 

In the event that a source’s modeled impacts of a 

particular pollutant are below the applicable SIL at all ambient 

air locations modeled, i.e., de minimis everywhere, EPA’s policy 

for PSD provides that no further modeling analysis is required 

for that pollutant. Our longstanding policy under the PSD 

program is that when a preliminary screening analysis based on 

the SIL is sufficient to demonstrate that the source’s emissions 

throughout the area modeled will not cause or contribute to a 

violation of the increment, there is no need for a comprehensive

source impact analysis involving a cumulative evaluation of the 

emissions from the proposed source and other sources affecting 

the area. 

Within the impact area of a source subject to PSD, that is,

the area within which the proposed project’s emissions increase

does have a significant impact, increment consumption is 

calculated using the source’s proposed emissions increase, along 

with other actual emissions increases or decreases of the 
26

particular pollutant from any sources in the area, which have 

occurred since the minor source baseline date established for 

that area. In addition, the emissions increases or decreases

from any major source that has commenced construction since the 

major source baseline date (which precedes the minor source 

baseline date) will consume or expand increment. Thus, an 

emissions inventory of sources whose emissions, in whole or in 

part, of a particular pollutant consume or expand the available 

increment in the area must be compiled. The inventory of 

increment-consuming emissions includes not only sources located 

directly in the impact area, but sources outside the impact area 

that affect the air quality for the particular pollutant within 

the impact area. 

The inventory of increment-consuming emissions includes 

emissions from increment-affecting sources at two separate time 

periods – the baseline date and the current period of time. For 

each source that was in existence on the relevant baseline date 

(major source or minor source), the inventory includes the 

source’s actual emissions on the baseline date and its current 

actual emissions. The change in emissions over these time 

periods represents the emissions that consume increment (or, if 

emissions have gone down, expand the available increment). For 

sources constructed since the relevant baseline date, all their 

current actual emissions consume increment and are included in 
27

the inventory. 

When the inventory of increment-consuming emissions has 

been compiled, computer modeling is used to determine the change 

in ambient concentration that will result from these emissions 

when combined with the proposed emissions increase from the new 

major source or major modification that is undergoing PSD 

review. The modeling has generally been guided by the 

“Guideline on Air Quality Models” (40 CFR part 51, Appendix W), 

which includes provisions on air quality models and the 

meteorological data input into these models. The model output 

(expressed as a change in concentration) for each relevant 

averaging period is then compared to the corresponding allowable 

PSD increment. 

E. Historical Approaches for Developing Increments

1. Congressional Enactment of Increments for PM and SO2

Congress established the first increments defining 

significant deterioration of air quality in the 1977 Amendments 

to the Act. These amendments, among other things, added part C 

to title I, setting out the requirements for PSD. In section 

163, Congress included numerical increments for PM and SO2 for 

Class I, II, and III areas. 

The three area classes are part of the increment system 

originally established by Congress. Congress designated Class I 

areas (including certain national parks and wilderness areas) as 
28

areas of special national concern, where the need to prevent 

deterioration of air quality is the greatest. Consequently, the 

allowable level of incremental change is the smallest relative 

to the other area classes, i.e., most stringent, in Class I 

areas. The increments of Class II areas are larger than those 

of Class I areas and allow for a moderate degree of emissions 

growth. For future redesignation purposes, Congress defined a 

“Class III” classification to allow the redesignation of any 

existing Class II area for which a state may desire to promote a 

higher level of industrial development (and emissions growth). 

Thus, Class III areas are allowed to have the greatest amount of 

pollutant increase of the three area classes while still 

achieving the NAAQS. To date, there have been no redesignations 

made to establish a Class III area.

In establishing these PSD increments, Congress used the 

then-existing NAAQS for those pollutants as the benchmark for 

determining what constitutes “significant deterioration.” 

Congress established the increments for PM as a percentage of 

the then-existing PM NAAQS. At the time the Act was amended in 

1977, the NAAQS for PM were expressed in terms of ambient 

concentrations of TSP. Thus, EPA interpreted the statutory 

increments for PM using the same ambient TSP “indicator.” 

2. EPA’s Promulgation of Increments for NO2 and PM10

Congress also provided authority for EPA to promulgate 
29

additional increments and to update the original PM increments 

created by statute. The EPA has promulgated two regulations 

pursuant to this authority. 

a. Increments for NO2 Using the “Contingent Safe Harbor”

Approach Under Section 166(a) of the Act

Based on section 166(a) of the Act, on October 17, 1988, 

EPA promulgated increments for NO2 to prevent significant 

deterioration of air quality due to emissions of NOx (53 FR 

40656). The EPA based these increments on percentages of the 

NAAQS in the same way that Congress derived the statutory 

increments for PM and SO2. Those NO2 increments were challenged 

in 1988 by the Environmental Defense Fund (EDF) when EDF filed 

suit in the U.S. Court of Appeals for the District of Columbia 

Circuit against the Administrator (Environmental Defense Fund, 

Inc. v. Reilly, No. 88–1882). The EDF successfully argued that 

we failed to sufficiently consider certain provisions in section 

166 of the Act. The court remanded the case to EPA “to develop 

an interpretation of section 166 that considers both subsections 

(c) and (d), and if necessary to take new evidence and modify 

the regulations.” See Environmental Defense Fund v. EPA, 898 

F.2d 183, 190 (D.C. Cir. 1990). Section 166(c) of the Act 

requires the PSD regulations to, among other things, meet the 

goals and purposes set forth in sections 101 and 160 of the Act. 

Section 166(d) requires these regulations be at least as 
30

effective as the increments established for PM (in the form of 

TSP) and SO2 in section 163 of the Act. The court considered the 

NO2 increment values determined using the percentage-of-NAAQS 

approach as “safe harbor” increments which met the requirements 

of section 166(d) of the Act. However, the court also 

determined that EPA’s reliance on such increment levels was 

contingent upon our completing the analyses required under 

section 166(c), which provided that the final increment values 

must address the goals of sections 101 and 160 of the Act to 

protect public health and welfare, parks, and AQRVs6 and to 

insure economic growth.

In response to the court’s decision, we proposed rulemaking 

on increments for NO2 on February 23, 2005 (70 FR 8880) and 

finalized the rule on October 12, 2005 (70 FR 59582). In the 

final rule, we established our policy on how to interpret and 

apply the requirements of sections 166(c) and (d) of the Act. 

In accordance with the court ruling, we conducted further 

 

6 The term “air quality related values” is not defined in 

the Act, but the legislative history provides language saying 

that “The term ‘air quality related values’ of federal lands 

designated as Class I includes the fundamental purposes for 

which such lands have been established and preserved by the 

Congress and the responsible federal agency. For example, under 

the 1916 Organic Act to establish the National Park Service (16 

U.S.C. 1), the purpose of such national park lands ‘is to 

conserve the scenery and the natural and historic objects and 

the wildlife therein and to provide for the enjoyment of the 

same in such manner and by such means as will leave them 

unimpaired for the enjoyment of future generations.’” S. Rep. 

No. 95-127 at 36 (1977).
31

analyses (considering the health and welfare effects of NOx) and 

concluded that the existing NO2 increments were adequate to 

fulfill the requirements of section 166(c). See 70 FR 59586 for 

our detailed analysis of how pollutant regulations satisfy the 

requirements of section 166 of the Act. Hence, we retained the 

existing NO2 increments along with other parts of the existing 

framework of pollutant-specific NO2 increment regulations. We 

also amended the PSD regulations under 40 CFR 51.166 to make it 

clear that states may seek EPA approval of SIPs that utilize a 

different approach than EPA used to establish these NO2

increments. To receive our approval of an alternative program, 

a state must demonstrate that its program satisfies the 

requirements of sections 166(c) and 166(d) of the Act and 

prevents significant deterioration of air quality from emissions 

of NOx.

7

b. Increments for PM10 Using “Equivalent Substitution” Approach 

Under Section 166(f) of the Act

On October 5, 1989, we proposed PM10 increments. See 54 FR 

41218. Although section 163 did not expressly define the 

existing statutory increments for PM in terms of a specific 

indicator, EPA reasoned that Congress’ knowledge that TSP was 

 

7 Under the 2005 NOx regulation, states can adopt measures 

other than increments as long as they can demonstrate that the 

measures selected comply with the same criteria and goals of 

sections 166 (c) and (d) of the Act that must be met for 

increments.
32

the indicator for the PM NAAQS, and that the TSP standards were 

the starting point for the increments levels when the increments 

were established in 1977, meant that TSP was also the 

appropriate measure for the PM increments in section 163. As a 

consequence, EPA believed that the statutory PM increments could 

not simply be administratively redefined as PM10 increments, 

retaining the same numerical values, following the revision of 

the PM NAAQS. Rather, we stated our belief that with the 

promulgation of the PM10 NAAQS, EPA had both the responsibility 

and the authority under sections 166 and 301 of the Act to 

promulgate new increments for PM to be measured in terms of PM10. 

We further concluded that promulgating PM10 increments to 

replace, rather than supplement, the statutory TSP increments 

under section 163 represented the most sensible approach for 

preventing significant deterioration with respect to PM. See 54 

FR 41220 - 41221.

We promulgated PM10 increments to replace the then-existing 

TSP increments on June 3, 1993 (58 FR 31622). In the interim 

between proposal and promulgation, Congress enacted the 1990 Act 

Amendments. As part of these Act Amendments, Congress amended 

section 166 to add a new section 166(f). This section 

specifically authorized EPA to substitute PM10 increments for the 

existing section 163 PM increments based on TSP, provided that 

the substituted increments are “of equal stringency in effect” 
33

as the section 163 increments.

Thus, we were able to replace the TSP increments under 

section 163 of the Act using PM10 increments based directly on 

the newly enacted authority under section 166(f) of the Act. In 

the PM10 rule, we maintained the existing baseline dates and 

baseline areas for PM that had been previously established using 

the TSP indicator. Also, as proposed, we promulgated PM10

increments based on an approach we called the “equivalent to 

statutory increments” approach. Under this approach, we used 

the original TSP increments as a benchmark for calculating the 

PM10 increments, thereby retaining roughly the same limitations 

on future deterioration of air quality as was allowed under the 

TSP increments. 

In using this approach, we considered the historical 

consumption of TSP increment by a sample population of permitted 

PSD sources, and then determined the PM10 increments for each 

area classification and averaging time that would provide 

approximately the same percentage of PM10 increment consumption, 

on average, by the same population of sources. Then, all future 

calculations of increment consumption after the PM10

implementation date would be based on PM10 emissions. See 58 FR 

31622 and 31625.

V. Final Action on PM2.5 Increments

A. Decision to Establish PM2.5 Increments Using “Contingent 
34

Safe Harbor Approach” Under Section 166(a)

The EPA’s 2007 NPRM contained three options for developing 

numerical PM2.5 increments. Option 1 used the authority of 

section 166(a) of the Act to establish increments for PM2.5 as a 

new pollutant for which NAAQS were established after August 7, 

1977, and established 24-hour and annual PM2.5 increments (Class 

I, II, and III) based on the “contingent safe harbor” approach. 

Options 2 and 3 used the contingent safe harbor approach under 

section 166(a) to develop 24-hour PM2.5 increments (Class I, II, 

and III), while using the “equivalent substitution” approach 

under section 166(f) of the Act to develop annual PM2.5

increments. Each of these options is discussed in detail in the 

2007 NPRM. 72 FR 54123 - 54138. In addition, significant 

comments on each of the three options, and our responses to 

them, are provided in this section.

In this final rule, after considering the available 

information and comments from interested parties, EPA has 

decided to select Option 1 and establish increments for PM2.5

using the “contingent safe harbor” approach in accordance with 

the authority provided in section 166(a) of the Act. In this 

section of the preamble, we will summarize the considerations 

that went into our proposed action and describe the final action 

being taken regarding new regulations for preventing significant 

deterioration of PM2.5 air quality — including PM2.5 increments, 
35

baseline dates and other permit requirements for PM2.5 (see

preamble section V.F), and PM10 increments (section V.H). 

This final rule establishes increments for PM2.5 at the 

following levels:

Averaging 

Period

NAAQS

(μg/m3)

Increments (μg/m3)

Class I Class II Class III

Annual 15 1 4 8

24-hour 35 2 9 18

B. Rationale for the Applicability of Section 166(a) 

In the 2007 NPRM, we expressed our belief that it is 

permissible to interpret section 166(a) to apply to PM2.5. 

Section 166(a) requires EPA to develop regulations to prevent 

the significant deterioration of air quality due to emissions of 

certain named pollutants, and to develop such regulations for 

any pollutants for which NAAQS are subsequently promulgated. 

Although EPA has generally characterized the NAAQS for PM2.5 as a 

NAAQS for a new indicator of PM, EPA did not replace the PM10

NAAQS with the NAAQS for PM2.5 when the latter NAAQS were 

promulgated in 1997. Rather, EPA retained the annual and 24-

hour PM10 NAAQS (retaining PM10 as an indicator of coarse 

particulate matter), and established new annual and 24-hour 

NAAQS for PM2.5 as if PM2.5 was a new pollutant, even though EPA 

had already developed air quality criteria for PM generally. 

Thus, for purposes of section 166(a), the promulgation of a 

NAAQS for PM2.5 established a NAAQS for an additional pollutant 
36

after 1977.

Nine commenters supported our proposed Option 1, although 

only one of these explicitly expressed support for the use of 

section 166(a) authority to promulgate PM2.5 increments. Ten 

other commenters specifically opposed the use of section 166(a) 

authority and/or supported the use of section 166(f) authority 

(on which the annual increments under Options 2A and 2B were 

based). 

The commenter (0040) who explicitly agreed with our 

proposed use of section 166(a) authority stated that it is the 

only option that is legally available. This commenter asserted 

that section 166(a) plainly applies to PM2.5 because PM2.5 is a 

pollutant for which NAAQS were promulgated after August 7, 1977. 

This commenter held that EPA’s rulemaking duty under section 

166(a) is not confined to “new pollutants,” but is triggered by 

post-1977 NAAQS promulgations, regardless of whether for new or 

previously regulated pollutants. On the other hand, this 

commenter noted that by its terms, section 166(f) is limited to 

authorizing the adoption of PM10 increments as a substitute for 

the statutory TSP increments and does not provide for 

substitution of PM2.5 increments for TSP or PM10 increments. 

Another commenter (0029) who favors Option 1 stated simply that 

EPA's analysis backing Option 1 was correct, without 

specifically mentioning section 166(a).
37

The opposing commenters did not believe that section 166(a) 

provides a legal basis for EPA to promulgate PM2.5 increments.

One of these commenters (0033) stated that section 166(a) can 

only be used for a new pollutant, and PM2.5 is not a new 

pollutant.

Another commenter (0019) who opposed the use of section 

166(a) authority argued that nothing in section 166(a) of the 

Act can be interpreted to allow it to be used as the basis of 

increments when EPA revises an existing NAAQS. The commenter 

explained that, on its face, section 166(a) can only be 

interpreted to apply to pollutants other than PM and SO2 since 

increments for these pollutants were enacted by Congress in 

section 163 of the Act. The commenter added that it can be 

argued that Congress intended to have section 166(a) apply to 

the four other pollutants specifically listed there.

This commenter (0019) found unpersuasive our argument that 

we are 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 

(72 FR 54121). The commenter asserted that if EPA had defined a 

coarse fraction to the particulate matter 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.
38

The commenter (0019) also disagreed 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 

(0019) indicated 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 particulate matter into two fractions with 

distinct effects. The commenter (0019) added that given that 

the definition of particulate matter includes a vast 

conglomeration of solids and liquids, the finding of differing 

effects should not come as a surprise. The commenter (0019) 

explained 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 particulate matter found to have different effects in 

establishing it as another indicator and not a different 

pollutant.

The commenter (0019) did not disagree with the specific 

numerical increments proposed by EPA under Option 1, but did 

have concerns with the potential consequences of the section 
39

166(a) approach. The commenter’s primary concern was the 

proposal to allow states to substitute other measures in the 

place of uniform national increments for PM2.5. (This is 

discussed further in section V.C.5 of this preamble.) Another 

commenter (0018) also expressed this concern.

Another commenter (0034) who opposed the section 166(a) 

approach believes that the legal and Congressional history 

regarding the establishment of PM increments shows 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 (0034) concluded that EPA did not have authority 

in 1987 under section 166(a) to adopt PM10 increments, and does 

not have authority now under section 166(a) to adopt PM2.5

increments.

We read section 166(a) to authorize EPA to promulgate 

pollutant-specific PSD regulations meeting the requirements of 

sections 166(c) and 166(d) for any pollutant for which EPA 

promulgates a NAAQS after 1977. Most of the pollutants 

identified in section 166(a) (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 other criteria pollutants, SO2 and PM, 
40

in section 166(a) because Congress had already established 

increments for these pollutants in section 163 of the Act. In 

addition to requiring regulations for the enumerated pollutants, 

Congress clearly intended to authorize EPA to establish 

additional pollutant-specific PSD regulations, potentially 

containing increments, for any additional pollutants for which 

EPA promulgated a NAAQS under section 109 of the Act. 

Furthermore, because the Act refers to pollutants for which EPA 

promulgates NAAQS after 1977, and does not use the phrase 

“additional pollutants,” section 166(a) provides authority for 

EPA to promulgate new increments after revising an existing 

NAAQS (including one first promulgated before 1977), when we 

find that such action is appropriate. 

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 
41

increments, Congress added section 166(f) which explicitly 

provides for the replacement of the existing increments with PM10

increments.

One commenter (0031) asserted that if EPA establishes 

increments for PM2.5 under the authority of section 166(a) on the 

basis that PM2.5 is a new pollutant, then it must also establish 

PM10 increments under section 166(a) because (according to the 

commenter’s analysis) PM10 is also a new pollutant. In the same 

analysis, the commenter concluded that EPA must adopt new 

measures to prevent significant deterioration from coarse PM 

based on section 166(a). 

In this final rule, EPA is not setting or amending any 

increments for PM10 or otherwise taking action with respect to 

PM10 increments. The preexisting annual and 24-hour increments 

for PM10 are being retained. See section V.H. 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 

coarse 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 
42

increments for the original statutory increments for PM measured 

as TSP. Thus, the PM10 increments legally supersede the original 

statutory increments for PM. Had the PM10 increments been

developed under section 166(a), which prior to the 1990 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.

One commenter (0016) expressed general objections to EPA's 

legal rationale for the PM2.5 increments proposal, asserting that 

we failed to expressly state and support our 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 our legal 

position with clarity. The commenter agreed with EPA’s 

assessment that the PM2.5 increments should and must fulfill the 

legal requirements of the Act (72 FR 54121), and added that it 

is the government’s burden of proof to establish its legal 

authority for action. The commenter stated that it would be 

arbitrary and capricious to promulgate these regulations for 

which EPA has not stated legal authority.

We do not disagree that the 2007 NPRM described two 

different 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 the 
43

Agency was considering for taking action in this rule. In 

particular, we clearly described our legal authority for 

developing the 24-hour increments under section 166(a) of the 

Act, which is the basis on which we are taking final action in 

this rule.

8 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 

54119-120 and 54123 - 54136).

We now believe 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 no increments have yet been established. Our position

also relies, to some extent, on the comments we received which 

supported the delay in implementation of the new increments, 

opposed the potential for two sets of definitions for “major 

source baseline date” and “trigger date” for the PM2.5 increment 

system, and highlighted the complexities involved with having to 

 

8 We also believe that we clearly described how section 

166(f) might provide alternative authority for establishing 

increments for PM2.5 (see, e.g., 72 FR 54120 - 54121), but will 

not address that in detail here because the increments in this 

rule are not based on section 166(f) authority. 
44

establish and maintain two sets of emissions inventories for the 

24-hour and annual PM2.5 increments. (See further description of 

relevant comments in section VIII of this section.)

C. EPA’s Interpretation of the Requirements under Sections

166(a)-(d) of the Act

In section 166(a) of the Act, Congress directed EPA to 

develop pollutant-specific regulations to prevent significant 

deterioration of air quality. Congress further specified that 

such regulations meet specific requirements set forth in 

sections 166(c) and 166(d) of the Act. We stated in the 2007 

NPRM that because we believed that section 166(a) could be 

applied to the development of increments for PM2.5, we would 

follow the interpretation of sections 166(a)-(d) that the Agency 

adopted in its most recent NO2 increments rule. 70 FR 59582,

October 12, 2005. That particular interpretation and 

application was upheld in Environmental Defense v. EPA, 489 F.3d 

1320 (D.C. Cir. 2007). 

The EPA’s interpretation of these provisions is grounded on 

five principles and conclusions. First, we read section 166 of 

the Act to direct EPA to conduct a holistic analysis that 

considers how a complete system of regulations will collectively 

satisfy the applicable criteria, rather than evaluating one 

individual part of a regulatory scheme in isolation. Second, we 

use a “contingent safe harbor” approach which calls for EPA to 
45

first determine an increment with the minimum level of 

effectiveness necessary to satisfy section 166(d) and then to 

conduct further analysis to determine if additional measures are 

necessary to fulfill the requirements of section 166(c). Third, 

we interpret section 166(c) of the Act to identify eight 

statutory factors that EPA must apply when promulgating 

pollutant-specific regulations to prevent significant 

deterioration of air quality. Fourth, where these factors are 

at odds with each other, we interpret the statute to require EPA 

to use its judgment to balance the conflicting factors. Fifth,

we recognize that the requirements of section 166 may be 

satisfied by adopting other measures besides an increment and 

that EPA may allow states to demonstrate that alternatives to 

increments contained in a SIP meet the requirements of sections 

166(c) and 166(d). Below is a brief discussion of each of these 

five principles and conclusions. A more detailed description of 

each of these is contained in the 2007 NPRM at pages 54121 -

54123.

1. Regulations as a Whole Should Fulfill Statutory 

Requirements

Section 166(a) directs EPA to develop pollutant-specific 

regulations to prevent the significant deterioration of air 

quality. Sections 166(c) and 166(d) provide detail on the 

contents of those regulations, but do not necessarily require 
46

the same type of increment system Congress created in section 

163 of the Act. The EPA interprets section 166 to require that 

the entire system of PSD regulations (the framework and details, 

as described in section V.D of this preamble) for a particular 

pollutant must, as a whole, satisfy the criteria in sections 

166(c) and 166(d) of the Act.

2. Contingent Safe Harbor Approach

Section 166(c) of the Act describes the kinds of measures 

to be contained in the regulations to prevent significant 

deterioration of air quality called for in section 166(a) and 

specifies that these regulations are to “fulfill the goals and 

purposes” set forth in sections 160 and 101 of the Act. 

Subsection (d) directs EPA to “fulfill such goals and purposes” 

by providing “specific measures at least as effective as the 

increments established in section 7473 of this title [section 

163 of the Act].” 42 U.S.C. 7476. Thus, EPA reads subsection 

(d) to require that the Agency identify a minimum level of 

effectiveness, or “safe harbor,” for the body of pollutantspecific PSD regulations adopted under section 166. The EPA 

reads subsection (c) to require that the Agency conduct further 

review to determine whether, based on the criteria in subsection 

(c), EPA’s pollutant-specific PSD regulations under section 166 

should contain measures that are different from the minimum 

“safe harbor” identified under subsection (d). The EPA 
47

construes subsection (d) to require that the measures be “at 

least as effective” as the statutory increments set forth in 

section 163.

To apply the “contingent safe harbor” approach for PM2.5, we 

first identified “safe harbor” increments for each area 

classification (Class I, II, or III), using: (1) equivalent 

percentages of the NAAQS as the percentages used for developing 

the statutory increments; (2) the same pollutant as the NAAQS,

i.e., PM2.5, and (3) the same time (averaging) periods as were 

used for the PM2.5 NAAQS. We interpreted that this approach 

would ensure that the increments would be “at least as effective 

as the increments established in [section 163],” as required by 

section 166(d). Second, EPA conducted further review to 

determine whether the “safe harbor” increments, in conjunction 

with existing elements of the PSD program or additional measures 

proposed under section 166 to augment the increments, 

sufficiently fulfill the criteria in subsection (c) of section 

166. In this review, we weighed and balanced the criteria set 

forth in subsection (c) (and the incorporated goals and purposes 

of the Act in section 101 and the PSD program in section 160) to 

determine whether additional measures might be needed to satisfy 

the criteria in subsection (c). See section V.E.6 of this 

preamble for further discussion of our evaluation, comments on 

the evaluation, and our response to them.
48

3. The Statutory Factors Applicable under Section 166(c)

The EPA interprets section 166(c) of the Act to establish 

eight factors to be considered in the development of PSD 

regulations for the pollutants covered by this provision. These 

eight factors included the three criteria stated in section 

166(c) and the five goals and purposes identified in section 160 

of the Act (which, as noted below, also cover the goals and 

purposes set forth in section 101). The three stated criteria 

in section 166(c) indicate that PSD regulations for specific 

pollutants should provide: (1) specific numerical measures for 

evaluating permit applications; (2) a framework for stimulating 

improved control technology, and (3) protection of air quality 

values. 42 U.S.C. 7476(c). The five goals and purposes in 

section 160 are incorporated into the analysis by virtue of the 

fourth criterion in section 166(c), which directs that EPA’s 

pollutant-specific PSD regulations “fulfill the goals and 

purposes” set forth in sections 160 and 101 of the Act. We 

construed the term “fulfill the goals and purposes,” as used in 

section 166(c), to mean that EPA should apply the goals and 

purposes listed in section 160 as factors applicable to 

pollutant-specific PSD regulations established under section 

166. The Agency’s view is that PSD measures that satisfy the 

specific goals and purposes of section 160 also satisfy the more 

general purposes and goals identified in section 101 of the Act. 
49

See 72 FR 54122.

One commenter (0040) disagreed with our interpretation that 

the goals and purposes of section 160 also satisfy all of those 

in section 101. This commenter asserted that although there is 

some overlap between the two sections, they are not identical. 

As an example, the commenter noted that section 101 expressly 

states that a primary goal of the Act is to promote pollution 

prevention — a goal not stated in section 160. The commenter 

asserted that, although the proposed increments would limit some 

pollution increases, there was no provision in the proposal that 

would require or promote pollution prevention.

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 
50

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. As discussed subsequently in 

sections V.D.1 and V.D.6 of this preamble, 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 previously, 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 
51

environmental impacts that result from the use of add-on air 

pollution control devices.

4. Balancing the Factors Applicable under Section 166(c)

While the eight factors in section 166(c) are generally 

complementary, there are circumstances where some of the 

objectives may be in conflict with each other. In these 

situations, some degree of balance or accommodation is inherent 

in the requirement to establish regulations that satisfy all of 

these factors. As first discussed in our 2005 NO2 increments 

rulemaking (70 FR 59582, at 59587), we believe this balancing 

test derives primarily from the third goal and purpose set forth 

in section 160: to insure economic growth consistent with the 

preservation of existing clean air resources. 42 U.S.C.

7406(3). A more detailed discussion of how the balancing of 

factors should be interpreted is contained in the 2007 NPRM at 

72 FR 54122 – 54123. 

One commenter (0040) claimed that EPA “incorrectly and 

repeatedly asserts” that a goal of section 160 of the Act is to 

insure economic growth. The commenter claimed that neither 

section 160 nor section 101 of the Act uses language to support 

a goal of promoting or maximizing opportunities for economic 

growth. Instead, the commenter asserted that both sections 

state only that any growth that does occur must be consistent 

with protection of air quality. The commenter concluded that 
52

“EPA’s notion that the need to satisfy the other requirements of 

Section 166 and 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.”

The language in section 160(c) 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 words were “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. 

5. Authority for States to Adopt Alternatives to Increments

While section 166 of the Act authorizes EPA to promulgate 

increments for pollutants listed under section 166(a), we have 
53

also interpreted the section to allow states to employ 

approaches other than increments to prevent significant 

deterioration of air quality, so long as such an approach 

otherwise meets the requirements of sections 166(c) and 166(d). 

This interpretation was explained in the 2005 NO2 increment 

rulemaking (70 FR 59582, October 12, 2005), in which we amended 

the PSD regulations at 40 CFR 51.166 by adding new paragraph 

(c)(2) to codify this statutory authority. Under the existing 

provision in 40 CFR 51.166(c)(2), states may seek EPA approval 

of SIPs that use an alternative approach to increments if the 

state can demonstrate that the alternative program satisfies the 

requirements of sections 166(c) and 166(d). However, the 

current language at paragraph (c)(2) stated the authority for 

states to adopt alternative measures only with respect to 

increments for NO2. To clarify our interpretation that the 

authority to adopt alternative measures covers any pollutant 

listed in section 166(a), we are revising 40 CFR 51.166(c)(2) to 

make it inclusive to all applicable pollutants rather than just 

NO2. 

Two commenters supported our proposal to revise paragraph 

(c)(2) to include PM2.5, while four state/local agency commenters 

expressed opposition. An environmental commenter (0040) agreed 

that the Act allows for other approaches, but believes that such 

approaches must be in addition to the national increments. 
54

Specifically, this commenter stated 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 that the national increments.” This commenter 

further stated 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.” 

As in the 2005 NO2 increment rulemaking, we are codifying 

the basic principle that states can seek to use alternative 

measures without defining any specific type of alternative 

program that would be approved or otherwise creating standards 

beyond the requirements of sections 166(c) and 166(d). Instead, 

we plan to make determinations on a case-by-case basis when a 

state submits a specific alternative approach for EPA to approve 

as part of a SIP. 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).

The four state/local agency commenters opposing the 

revision to 40 CFR 51.166(c)(2) expressed the importance of 
55

using uniform national increments for PM2.5. One commenter 

(0021) argued that a nationally inconsistent 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 same commenter argued that 

varying increment-equivalent measures could also result in an 

uneven playing field for industry and could exacerbate 

difficulties between states experiencing transport problems. 

Another opposing commenter (0033) was concerned that 

allowing states to adopt alternatives to increments will likely 

lead to a “mish-mash” of state approaches which defeats the 

intention of Congress that there be uniformity in PSD rules to 

avoid economic dissimilarities from state to state that could 

allow interstate competition for industry based upon which state 

offers the best (least expensive) environmental compliance 

regulations. Another commenter (0019) objected to allowing the 

use of alternatives to increments by stating that such 

alternative allowances undermine the desired national 

consistency, and EPA has failed to even identify any Act

programs which would benefit from this approach.

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) expressly gives 
56

EPA some latitude in promulgating regulations that will be at 

least as effective as the increments in section 163, by stating 

that such regulations “may contain air quality increments, 

emission density requirements, or other measures.” Thus, EPA is 

authorized to provide that states may consider alternatives to 

the increments established in this rule. 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.

D. Framework for Pollutant-Specific PSD Regulations for PM2.5

In the 2007 NPRM, we proposed to apply the same basic 

framework for pollutant-specific PSD regulations for PM2.5 that 

we used in our 2005 NO2 increments regulations. Specifically, we 

proposed adopting an increment and area classification system 

for PM2.5 and applying the statutory AQRV review process to PM2.5

as well. We also indicated that while some of the factors 

applicable under section 166(c) are fulfilled by using this type 

of framework for pollutant-specific PSD regulations under 

section 166(a) of the Act, this framework of regulations may not 

fully satisfy the other applicable factors. In these instances, 

the details of our regulations (such as the characteristics of 

the increments themselves) are also important, and we evaluated 

the effectiveness of the framework in conjunction with more 
57

detailed elements of our regulations. As discussed in the 

following subsections, we believe our obligations under section 

166(c) of the Act are satisfied when the PSD regulations 

collectively satisfy the factors applicable under 166(c) of the 

Act.

1. Increment System

An increment-based program satisfies the requirements under 

166(c) to provide “specific numerical measures against which 

permit applications may be evaluated.” An increment is the 

maximum allowable level of ambient pollutant concentration 

increase that is allowed to occur above the applicable baseline 

concentration in a particular area. As such, an increment 

defines “significant deterioration.” Establishing an increment 

system for PM2.5 will fulfill two of the factors applicable under 

section 166(c): (1) providing specific numerical measures to 

evaluate permit applications, and (2) stimulating improved 

control technology. 

First, under section 165(a)(3) of the Act, a permit 

applicant must demonstrate that emissions from the proposed 

construction and operation of a facility “will not cause, or 

contribute to, air pollution in excess of any (A) maximum 

allowable increase or maximum allowable concentration for any

pollutant ....” 42 U.S.C. 7475(a)(3). Once the baseline date 

associated with the application for the first new major 
58

stationary source or major modification in an area is 

established, the new emissions from that source consume a 

portion of the increment in that area, as do any subsequent 

emissions increases that occur from any source in the area. 

When the maximum pollutant concentration increase defined by the 

increment has been reached, additional PSD permits cannot be 

issued until sufficient amounts of the increment are “freed up” 

via emissions reductions that may be required by the reviewing 

authority. Thus, an increment is a quantitative value that 

establishes a “maximum allowable increase” for a particular 

pollutant. It functions, therefore, as a specific numerical 

measure that can be used to evaluate whether an applicant’s 

proposed project will cause or contribute to air pollution in 

excess of allowable levels.

Increments also satisfy the second factor in section 166(c) 

by providing “a framework for stimulating improved control 

technology.” Increments establish an incentive to apply 

improved control technologies in order to avoid violating the 

increment and to “free-up” available increment to promote 

continued economic growth. These control technologies may 

become the basis of BACT determinations elsewhere, as the 

technologies become more commonplace and the costs tend to 

decline. 

One commenter (0040) stated that, although increments may 
59

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 

asserted 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.

We continue to believe that the total program, encompassing

increments and BACT, 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 

application of improved pollution control technology as the 

amount of deterioration increases, and should not be regarded as 

an incidental consequence. As stated in the 2007 NPRM, 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 
60

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. 

Admittedly, the increasing stringency of control technologies 

over time, as observed in EPA’s BACT/Lowest Achievable Emission 

Rate (LAER) Clearinghouse, cannot in itself serve as the 

indicator of how the PSD program has already stimulated 

development of improved control technology; 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 questions 

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. Area Classifications

In this final rule, EPA is establishing the same threetiered area classification system for PM2.5 that is applicable to

the increments for NO2 and other pollutants under the PSD program 

and the Act. Accordingly, areas that are currently Class I for 
61

other pollutants will also be Class I for PM2.5 and all other 

areas will be Class II for PM2.5 unless we redesignate the area 

based on a request by a state or tribe pursuant to the process 

in section 164 of the Act and EPA’s regulations at 40 CFR 

51.166(g) and 52.21(g). 

As explained earlier in section IV.E.1, Class I areas are 

areas where very clean air is most desirable. In contrast, 

Class III areas are designed as those areas in which a state 

wishes to permit the highest relative level of industrial 

development, and thus allow the largest incremental increase in 

pollution. Areas that are not especially sensitive and where 

states have not provided for a higher level of industrial growth 

are classified as Class II. When Congress established this 

three-tiered scheme for SO2 and PM, it intended that Class II 

areas be subject to an increment that allows “moderately large 

increases over existing pollution.” H.R. Rep. 95-294, 4 LH at 

2609. 

Establishing increments at different levels for each of the 

three area classifications helps to fulfill two of the factors 

applicable under section 166(c) of the Act. First, establishing 

the smallest increments in Class I areas helps fulfill EPA’s 

obligation to establish regulations that “preserve, protect, and 

enhance the air quality” in parks and special areas. Class I 

areas are primarily the kinds of parks and special areas covered 
62

by section 160(2) of the Act. Second, by providing for two 

additional area classifications with increment levels that are 

higher but still protective, the area classification system 

helps satisfy the goal in section 160(3) of the Act that EPA 

“insure that economic growth will occur in a manner consistent 

with preservation of clean air resources.” In those areas where 

clean air resources may not require as much protection, more 

growth is allowed. By employing an intermediate level (Class II 

areas) and higher level (Class III areas), this classification 

scheme helps ensure that growth can occur where it is needed 

(Class III areas) without putting as much pressure on existing 

clean air resources in other areas where some growth is still 

desired (Class II areas).

By requesting that EPA redesignate an existing Class II 

area to Class III, states may accommodate economic growth and 

air quality in areas where the Class II increment is too small 

to allow the siting of new or modified sources. The procedures 

specified by the Act for such a redesignation require a 

commitment by the state government to create such an area, 

extensive public review, local government participation in the 

SIP area redesignation process, and a finding that the 

redesignation will not result in the applicable increment being 

exceeded in a nearby Class I or Class II area. See 42 U.S.C. 

7474(a) and (b) (sections 164(a) and (b) of the Act). (No state 
63

has yet requested a Class III redesignation.) The EPA believes 

that the three-tiered classification system has allowed for 

economic growth, consistent with the preservation of clean air 

resources.

However, an area classification system alone may not 

completely satisfy the factors applicable under section 166(c) 

of the Act. The increment that is employed for each class of 

area is also relevant to an evaluation of whether the area 

classification system achieves the goals of the PSD program. We 

briefly discuss the characteristics of increments in section 

V.E.5.

One commenter (0040) took issue with our assessment of the 

two factors that we believe a classification system helps to 

fulfill. As discussed previously in section V.C.4, the

commenter (0040) asserted that EPA has unlawfully interpreted 

section 160(3) of the Act to elevate economic growth over all 

other statutory factors. As explained in greater detail in 

section V.C.4, we disagree that our interpretation elevates 

economic growth over other factors, and believe that 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.

The commenter (0040) also stated that EPA has failed to 

demonstrate that the classification system and safe harbor 
64

increments, in combination with the other elements of the 

regulatory framework, will “preserve, protect, and enhance the 

air quality” in parks and special areas as required under 

section 160(2) of the Act. These comments and our response to 

them are found in section V.E.6 of this preamble where we 

discuss our evaluation of the safe harbor increments.

3. Permitting Procedures

Two of the factors applicable under section 166(c) are 

fulfilled by the case-by-case permit review procedures that are 

built into our existing PSD regulations. The framework of our 

existing PSD regulations employs the preconstruction permitting 

system and procedures required under section 165 of the Act. 42 

U.S.C. 7475. These requirements are generally reflected in 40 

CFR 51.166 and 52.21 of EPA’s PSD regulations. These permitting 

and review procedures, which apply to construction of new major 

sources and to major modifications, fulfill the goals set forth 

in sections 160(4) and 160(5) of the Act. These goals require 

that PSD programs in one state not interfere with the PSD 

programs in other states and that PSD programs assure that any 

decision to permit increased air pollution is made after careful 

evaluation and public participation in the decision-making

process. For the same reasons discussed in our proposal for the 

pollutant-specific NO2 increments regulations (70 FR 8896,

February 23, 2005), we believe these factors are also fulfilled 
65

for PM2.5 by employing the permit review procedures.

4. AQRV Review by Federal Land Manager (FLM) and Reviewing 

Authority

In this final rule, we apply the existing requirements to 

evaluate impacts on AQRVs in Class I areas (see existing 40 CFR 

51.166(p) and 52.21(p)) to PM2.5. These existing requirements 

for an AQRV review provide FLMs with the responsibility to 

review source impacts on site-specific AQRVs in Class I areas 

and to bring any alleged adverse impacts to the attention of the 

reviewing authority. Under an increment approach, we consider 

this review to be an additional measure that helps satisfy the 

factors in sections 166(c) and 160(2) which require EPA’s 

pollutant-specific PSD regulations to protect (1) air quality 

values, and (2) parks and other special areas, respectively.

Two state/local agency commenters supported our proposal to 

apply the requirements to evaluate impacts on AQRV in Class I 

areas to PM2.5 review. However, one commenter (0040) indicated 

that FLM review 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 pointed 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 
66

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 added 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 

addition, the commenter noted 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 commenter (0040) also asserted 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 
67

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.

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 new 40 CFR 51.166(p)(4) 

and 52.21(p)(5).

Section 165(d) creates a scheme in which the FLM and 

reviewing authority must review the impacts of a proposed new or 

modified source’s emissions on AQRVs. The Act assigns to the 

FLM an “affirmative responsibility” to protect the AQRVs in 

Class I areas. 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 
68

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.9 Thus, the showing of compliance 

with the increment determines whether the FLM or the permit 

applicant has the burden of satisfactorily demonstrating whether 

 

9 Even if such a waiver of the Class I increment is allowed 

upon a finding of no adverse impact, the source must comply with 

such emissions limitations as may be necessary to ensure that 

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). 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).
69

or not the proposed source’s emissions would have an adverse 

impact on AQRVs.10 In any event, the FLM plays an important and 

material role by raising these issues for consideration by the 

reviewing authority, which in the majority of cases will be the 

state.

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. As discussed later, we believe 

the factors applicable under section 166(c) of the Act can be 

fulfilled when the review of AQRVs is applied in conjunction 

with increments and other aspects of our PSD regulations. In 

those cases where the increment is not violated and the 

reviewing authority agrees that a proposed project will 

adversely affect AQRVs, the parks and other special areas will 

be protected by denying issuance of the permit or by requiring 

the applicant to modify the project to alleviate the adverse 

impact. 

We read the legislative history to show that Congress 

 

10 In response to concerns that Class I increment would 

hinder growth in areas surrounding the Class I area, Congress 

established Class I increments as a means of determining where 

the burden of proof should lie for a demonstration of adverse 

effects on AQRVs. See Senate Debate, June 8, 1977 (3 LH at 

725).
70

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.11

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

 

11 See S. Rep. 95-127, at 12, reprinted at 3 LH at 1386, 

1410 (describing the goal of protecting “air quality values” in 

“Federal lands – such as national parks and wilderness areas and 

international parks,” and in the next paragraph and subsequent 

text using the term “air quality related values” to describe the 

same goal); id. at 35, 36 (“The bill charges the Federal land 

manager and the supervisor with a positive role to protect air 

quality values associated with the land areas under the 

jurisdiction of the [FLM]” and then describing the statutory 

term as “air quality related values”). H.R. Report 95-564 at 

532 (describing duty of Administrator to consider “air quality 

values” of the tribal and State lands in resolving an appeal of 

a tribal or State redesignation, which is described in the final 

bill as “air quality related values”).
71

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. As 

discussed further in the next section, one such element 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. 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 may include the impacts 
72

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 
73

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 an increment approach, FLM review for AQRV impacts is an 

additional measure that helps satisfy the factors in sections 

166(c) and 160(2) of the Act (which require EPA’s pollutantspecific 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.

5. Additional Impacts Analysis

The “additional impacts analysis” requirements set forth in 

our part 51 and 52 PSD regulations also help fulfill the 

criteria and goals and purposes in sections 166(c) and 160. The 
74

additional impacts analysis involves a case-by-case review of 

potential harm to visibility, soils, and vegetation in Class II 

and III areas that could occur from the construction or 

modification of a PSD source.

Sections 51.166(o)(1) and 52.21(o)(1) of the PSD 

regulations require that a permit provide the following 

analysis:

an analysis of the impairment to visibility, soils and 

vegetation that would occur as a result of the source 

or modification, and general commercial, residential, 

industrial and other growth associated with the source 

or modification. The owner or operator need not 

provide an analysis of the impact on vegetation having 

no significant commercial or recreational value.

This requirement was based on section 165(e)(3)(B) of the Act, 

which provides that EPA establish regulations that require “an 

analysis of the ambient air quality, climate and meteorology, 

terrain, soils and vegetation, and visibility at the site of the 

proposed major emitting facility and in the area potentially 

affected by emissions from such facility ....” 42 U.S.C. 

7475(e)(3)(B).

As mentioned in the previous section, one commenter (0040) 

argued that the provisions for protection of Class I areas are 

of no help in fulfilling the goal set forth in section 160(2) of 

the Act to preserve and protect air quality in the countless 

“other areas of special national or regional natural, 

recreational, scenic, or historic value” such as state and local 
75

parks, wildlife refuges, recreation areas, lakes and historic 

areas, none of which were originally defined by Congress as 

Class I areas.

We acknowledge that the special provisions for protecting 

Class I areas are not applicable for protecting areas that are 

not designated as “Class I.” However, we believe that the 

“additional impacts analysis” provisions are 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, that are not Class I areas but are worthy of 

special protection beyond what might be provided by the NAAQS 

and increments. See, e.g., 40 CFR 52.21(o). These areas are 

not subject to the special AQRV review that applies only in 

Class I areas. While the additional impacts analysis is not as 

intensive a review as the AQRV analysis required in Class I 

areas, the requirement to consider impairments to visibility, 

soils, and vegetation through the additional impacts analysis 

contributes to satisfying the factors applicable under section 

166(c) of the Act in all areas, including Class II and Class III 

areas.

6. Installation of BACT

The requirement that new sources and modified sources 

subject to PSD apply BACT is an additional measure that helps to 

satisfy the factors in sections 166(c), 160(1), and 160(2) of 
76

the Act. This requirement, based on section 165(a)(4) of the 

Act, is already included in EPA’s PSD regulations for all 

pollutants generally and thus, in the 2007 NPRM we considered it 

to be a part of the regulatory framework for the Agency’s 

pollutant-specific regulations for PM2.5. 40 CFR 52.21(j); 40 

CFR 51.166(j). Our existing regulations define “best available 

control technology” as “an emission limitation ... based on the 

maximum degree of reduction for each pollutant subject to 

regulation under the Act ... which the Administrator, on a caseby-case basis, taking into account energy, environmental, and 

economic impacts and other costs, determines is achievable for 

such source through application of production processes or 

available methods, systems, and techniques ....” 40 CFR 

52.21(b)(12); 40 CFR 52.166(b)(12). This pollutant control 

technology requirement, in practice, has required significant 

reductions in the pollutant emissions increases from new and 

modified sources while also stimulating the on-going improvement 

of control technology. The control of PM2.5 emissions through 

the application of BACT helps to protect air quality values, 

public health and welfare, and parks and other special areas.

E. Final PM2.5 Increments

Based on our evaluation of the effects of PM2.5 and a 

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

incorporated goals and purposes of the Act contained in section 
77

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

EPA has 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 described earlier that we apply to PM2.5. Since 

several of the eight factors applicable under section 166(c) are 

satisfied by adopting the framework and other measures described 

earlier, our development of these increments for PM2.5 was guided 

by the four remaining factors that may not be fully satisfied by 

the framework and other measures: (1) protecting AQRVs; (2) 

protecting the public health and welfare from reasonablyanticipated adverse effects; (3) protecting the air quality in 

parks and special areas, and (4) insuring economic growth.

12 In 

accordance with the “contingent safe harbor” approach, to 

determine the specific characteristics of the proposed 

increments, we first established safe harbor increments 

representing the level of effectiveness necessary to satisfy the 

“at least as effective as” requirement in section 166(d) of the 

Act and then conducted further analysis to determine if 

additional measures are necessary to fulfill the requirements of 

section 166(c). 

 

12 We have paraphrased these factors here and in other 

sections to facilitate the explanation of our reasoning. 

However, we recognize, as we did in our regulation for NOx, that 

the statutory language is broader than the shorthand we use here 

for convenience.
78

1. Identification of Safe Harbor Increments 

Using the percentage-of-NAAQS approach under proposed 

Option 1, as explained in section V.C.2 of this preamble, we 

derived the following safe harbor increments for PM2.5:

Averaging 

Period

NAAQS

(μg/m3)

Increments (μg/m3)

Class I Class II Class III

Annual 15 1 4 8

24-hour 35 2 9 18

The table shows PM2.5 NAAQS levels (primary and secondary 

NAAQS) at 15 μg/m3 for the annual averaging time and 35 μg/m3 for 

the 24-hour averaging time. See 40 CFR 50.7. From these NAAQS 

levels, we calculated the safe harbor increments based on the 

same percentages that were used by Congress to establish the 

original PM increments (measured as TSP) in section 163 of the 

Act, i.e., 6.6 percent of the NAAQS for Class I areas, 25 

percent of the NAAQS for Class II areas, and 50 percent of the 

NAAQS for Class III areas. We have concluded that increments 

with these characteristics are sufficient to satisfy the 

requirement in section 166(d) that we adopt increments (or other 

PSD regulations) that are “at least as effective as” the 

increments established in section 163 of the Act. 42 U.S.C. 

7476(d); See EDF v. EPA, 898 F.2d at 188, 190. 

Nine commenters supported proposed Option 1, either

explicitly or implicitly supporting our method of calculating 

the safe harbor increments used to develop increments for PM2.5. 
79

One of these commenters (0040), while agreeing with the safe 

harbor increment approach under Option 1, disagreed with our 

analysis of the adequacy of the safe harbor increments, as 

discussed in other sections of this preamble. One commenter 

(0019) who opposed Option 1 (based on the belief that section 

166(a) of the Act is not the appropriate basis for PM2.5

increments) nevertheless supported the percentage-of-NAAQS 

approach for developing PM2.5 increments under the statutory 

authority at section 166(f). 

A commenter who opposed our proposal to calculate 

increments using percentages of the NAAQS argued that this 

approach for setting the PM2.5 increments is not scientifically 

supported. This commenter indicated that basing the PM2.5

increments on the same percentage 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. 

The commenter argued that, 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 conceded that using the 0.8 factor to set PM2.5

increments may seem too high, but asserted that using the safe 
80

harbor approach would set increments for PM2.5 that are too low.

We conclude that the commenter is mistaken in saying that 

the PM2.5 increments use the same percentage of the NAAQS that 

were used to set the PM10 NAAQS. 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 pages 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.13 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).

 

13 Note that the PM10 increment may still be more limiting in 

areas where much of that increment has already been consumed.
81

The underlying reason that the safe harbor PM2.5 increments 

are so much less than the PM10 increments is that the PM2.5 NAAQS 

are much less than the PM10 NAAQS.14 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.

2. Data Used by EPA for the Evaluation of the Safe Harbor 

Increments for PM2.5. 

We evaluated whether measures other than the safe harbor

increments are necessary by analyzing primarily the scientific 

and technical information on the health and welfare effects of 

PM2.5 contained in the June 2005 OAQPS Staff Paper which 

accompanied the last full review of the PM NAAQS completed in

 

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

the 24-hour PM10 NAAQS (150 μg/m3).
82

2006.

15

Section 166 of the Act provides that EPA establish 

pollutant-specific PSD regulations, such as increments, after 

the establishment of a NAAQS for the applicable pollutants. 42 

U.S.C. 7476(a). The Act provides that EPA will promulgate new 

PSD regulations under section 166, including new increments if 

appropriate, within 2 years from the promulgation of any NAAQS 

after 1977. 42 U.S.C. 7476(a). Within that time frame, the 

health and welfare information used for the setting of the NAAQS 

would also be “current” for purposes of establishing pollutantspecific PSD regulations. We believe this timing reflects 

Congressional intent that EPA consider the same body of 

information concerning a pollutant’s health and welfare effects 

when it promulgates the NAAQS and subsequent PSD increments (or 

other measures) defining significant air quality deterioration 

for the same pollutant. However, when we used that same 

information as the basis for our proposed pollutant-specific PSD 

regulations, we evaluated that information under the legal 

criteria in section 166 of the Act rather than the criteria in 

section 109 applicable to the promulgation of NAAQS. See EDF v. 

EPA, 898 F.2d at 190. 

At the time of our proposal of PM2.5 increments, we had just 

 

15 The review completed in 2006 updated the previous review, 

which began in 1994 and resulted in revised standards for PM in 

1997.
83

completed a review of the PM2.5 NAAQS. Thus, the information 

used in the NAAQS review was current and timely for purposes of

establishing pollutant-specific PSD regulations for PM2.5. On 

October 17, 2006, based primarily on considerable new data on 

the air quality and human health effects for PM2.5 directly, EPA 

revised the primary and secondary NAAQS to provide increased 

protection of public health and welfare by retaining the level 

of the annual standard and tightening the level of the 24-hour 

standard from 65 to 35 μg/m3 while retaining the 24-hour PM10

NAAQS and revoking the annual PM10 NAAQS. The information 

contained in both the 2004 Criteria Document and 2005 Staff 

Paper that was used for the latest review of the PM NAAQS was 

also considered for the purpose of evaluating the PM2.5

increments that we have established in this final rule. 

The 2004 Criteria Document and 2005 Staff Paper are the 

products of a rigorous process that is followed to validate and 

interpret the available scientific and technical information, 

and provided the basis for recommending the PM2.5 NAAQS. In 

accordance with the Act, the NAAQS process begins with the 

development of “air quality criteria” under section 108 for air 

pollutants that “may reasonably be anticipated to endanger 

public health or welfare” and that come from “numerous or 

diverse” sources. 42 U.S.C. 7408(a)(1). For each NAAQS review, 

the Administrator must appoint “an independent scientific review 
84

committee composed of seven members of the National Academy of 

Sciences, one physician, and one person representing state air 

pollution control agencies,” known as the Clean Air Scientific 

Advisory Committee (CASAC). 42 U.S.C. 7409(d)(2)(A). The CASAC 

is charged with recommending revisions to the criteria document 

and NAAQS, and advising the Administrator on several issues, 

including areas in which additional knowledge is required to 

appraise the adequacy and basis of existing, new, or revised 

NAAQS. 42 U.S.C. 7409(d)(2)(B),(C). 

“Air quality criteria” must reflect the latest scientific 

knowledge on “all identifiable effects on public health or 

welfare” that may result from a pollutant=s presence in the 

ambient air. 42 U.S.C. 7408(a)(2). The scientific assessments 

constituting air quality criteria generally take the form of a 

“criteria document,” a rigorous review of all pertinent 

scientific studies and related information. The EPA also 

develops a “staff paper” to “bridge the gap” between the 

scientific review and the judgments the Administrator must make 

to set standards. See Natural Resources Defense Council v. EPA

(“NRDC”), 902 F.2d 962, 967 (D.C. Cir. 1990). Both documents 

undergo extensive scientific peer review as well as public 

notice and comment. See, e.g., 62 FR 386542. 

3. Scope of Effects Considered 

The effects of ambient PM2.5 concentrations may include
85

effects from secondarily-formed PM2.5. Thus, when we analyzed 

the data in this rulemaking, we evaluated the health and welfare 

effects of both direct PM2.5 and secondarily-formed PM2.5 that may 

result from the transformation of other pollutants such as SO2

and NOx. This was consistent with the approach we described for 

addressing these effects in the review of our pollutant-specific 

NO2 increments regulations. 70 FR 59590. 

4. Evaluation of the Health and Welfare Effects of PM2.5

Airborne PM is not a specific chemical entity, but rather 

is a mixture of liquid and solid particles from different 

sources and of different sizes, compositions, and properties. 

Particle size distributions show that atmospheric particles 

exist in two classes: fine particles and coarse particles. The 

indicator for fine particles is PM2.5, which represents that 

population of particles that is mostly less than 2.5 micrometers 

in size. The indicator for thoracic coarse particles is 

“PM10-2.5,” which represents particles sized between 2.5 and 10 

micrometers. In the last two reviews of the PM NAAQS, EPA 

concluded that these two indicators, because of their different 

sources, composition, and formation processes, should be treated 

as separate subclasses of PM pollution for purposes of setting 

ambient air quality standards.

Fine PM is derived directly from combustion material that 

has volatilized and then condensed to form primary PM or from 
86

precursor gases, such as SO2 and NOx, reacting in the atmosphere 

to form secondary PM. Major components of fine particles are 

sulfates, strong acid, ammonium nitrate, organic compounds, 

trace elements (including metals), elemental carbon, and water. 

Primary and secondary fine particles have long lifetimes in the 

atmosphere (days to weeks) and travel long distances (hundreds 

to thousands of kilometers). They tend to be uniformly 

distributed over urban areas and larger regions, especially in 

the eastern United States. As a result, they are not easily 

traced back to their individual sources. 

a. Health Effects 

The EPA reported important progress since the last PM NAAQS 

review in advancing our understanding of potential mechanisms by 

which ambient PM2.5, alone and in combination with other 

pollutants, is causally linked to a number of key health 

effects. The more extensive and stronger body of evidence used 

by EPA to study the health effects of PM2.5 in our latest review 

identified a broader range of effects than those previously 

documented, involving premature mortality and indices of 

morbidity (including respiratory hospital admissions and 

emergency room visits, school absences, work loss days, 

restricted activity days, effects on lung function and symptoms, 

morphological changes, and altered host defense mechanisms) 

associated with both long-term and short-term exposure to PM2.5. 
87

A more detailed discussion of the health effects associated with 

PM2.5 is contained in the 2007 NPRM. 72 FR 54127 – 54128. In 

addition, an overview of the scientific and technical evidence 

considered in the 2004 Criteria Document and 2005 Staff Paper

can be found in our proposed rule for revising the NAAQS for PM 

(71 FR 2619, January 17, 2006). 

b. Welfare Effects

Ambient PM alone, and in combination with other pollutants, 

can have a variety of effects on public welfare. While 

visibility impairment is the most noticeable effect of fine 

particles present in the atmosphere, both fine and coarse 

particles can have other significant welfare-related effects, 

including effects on vegetation and ecosystems, materials (e.g., 

soiling and corrosion), and climate change processes. 

In reaching our decision in 2006 to revise the suite of PM 

secondary standards, EPA factored in several key conclusions 

from the scientific and technical information contained in the 

2004 Criteria Document and 2005 Staff Paper. These conclusions 

included the following: (1) PM-related visibility impairment is 

principally related to fine particle levels, and most directly 

related to instantaneous levels of visual air quality associated 

with short-term averaging periods; (2) PM2.5 concentrations can 

be used as a general surrogate for visibility impairment in 

urban areas; (3) any secondary NAAQS for visibility protection 
88

should be considered in conjunction with the regional haze 

program as a means of achieving appropriate levels of protection 

against PM-related visibility impairment in urban, non-urban, 

and Class I areas nationwide; (4) the available evidence is not 

sufficient to support distinct secondary standards for fine or 

coarse particles for any non-visibility related welfare effects; 

and (5) the secondary standards should be considered in 

conjunction with protection afforded by other programs intended 

to address various aspects of air pollution effects on 

ecosystems and vegetation, such as the acid deposition program 

and other regional approaches to reducing pollutants linked to 

nitrate or acidic deposition. 

In this rulemaking, EPA has reviewed the scientific and 

technical information concerning welfare related effects 

considered in the 2004 Criteria Document and 2005 Staff Paper to 

determine whether there is any basis for modifying the safe 

harbor increments developed for PM2.5 to satisfy the criteria 

under sections 166(c) and 160 of the Act. Our review included 

information on visibility impairment, and effects on vegetation 

and other ecosystem components, materials and soiling, and 

climate changes. A detailed discussion of the various welfare 

effects we considered for evaluating the safe harbor increments 

for PM2.5 is contained in the 2007 NPRM. 72 FR 54128 – 54133.
89

5. Fundamental Elements of Increments

As we have previously noted, under the model established in

the Act and prior EPA regulations, the function of an increment 

is not like that of the NAAQS in that an increment is not 

intended to set a uniform ambient pollutant concentration 

“ceiling” across the United States. See 70 FR 59600. Instead, 

while both increments and NAAQS generally serve to limit ambient 

air pollution levels, increments are designed to allow a uniform 

amount of pollutant concentration increase for each area in the 

United States having a particular classification, i.e., Class I, 

II, or III. The amount of the allowable increase is measured 

against a baseline air quality level that is typically different 

for each particular area.16 Because the baseline air quality 

level varies from one location to another, and is not 

established for a particular area until a source proposing to 

construct in that area submits a complete PSD permit 

application, it is not possible to determine what the maximum 

ambient pollutant concentration attainable will be for a given 

area (to be used to determine the protection afforded by an 

increment against potential adverse environmental effects) until 

the specific baseline air quality level is known.

 

16 It should be noted, however, that an increment does not 

allow air pollution levels in an area to increase beyond the 

ambient concentration of a pollutant that would exceed the level 

allowed by the NAAQS.
90

For the reasons described in our NO2 increments rule, our 

objective is to establish uniform increments, consistent with 

the increments for SO2 and PM originally established by Congress,

that allow the same level of deterioration for each area of the 

country having the same classification. 70 FR 59601, October 

12, 2005. It is important to understand that increments are not 

intended to reduce ambient concentrations of an air pollutant 

below existing baseline levels in each area, but rather to 

define a level of allowable increase in pollutant concentrations 

above baseline levels, and to identify the level at which

“significant” deterioration occurs for each area, in accordance 

with its specific classification. 70 FR 59600.

6. Evaluation of the Safe Harbor Increments

As indicated earlier (in section V.E.2 of this preamble), 

mindful of the considerations made about the fundamental 

characteristics of the increments, we reviewed the scientific 

and technical evidence available for the 2005 review of the 

NAAQS for PM in order to determine whether, and to what extent, 

the “safe harbor” increments might need to be modified in order 

to protect air quality values, health and welfare, and parks 

while insuring economic growth consistent with the preservation 

of clean air resources in accordance with sections 166(c) and 

160 of the Act. As we did in our evaluation of the safe harbor 

NO2 increments (70 FR 59603 – 59606, October 12, 2005), we relied 
91

on an approach that evaluates how protective the safe harbor 

PM2.5 increments are by comparing the marginal pollutant 

concentration increases allowed by the safe harbor increment 

levels against the pollutant concentrations at which various 

environmental responses occur. 

We analyzed the available evidence from both a quantitative 

and qualitative perspective to reach a decision about whether we 

should modify the contingent safe harbor PM2.5 increments and 

whether we have sufficient information to select a specific 

alternative level, averaging time, or pollutant indicator for 

the increments. As a result of our analysis, we proposed to 

conclude that it was not necessary to modify the safe harbor 

increments to protect human health, address non-visibility 

welfare effects, or further protect visibility. This analysis 

is described in detail in the 2007 NPRM.

After considering the comments on our evaluation of the 

safe harbor increments and the conclusions we reached in the 

NPRM (summarized in the following paragraphs), we continue to 

believe 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) of the Act (and the incorporated 

goals and purposes of the Act in section 101 and the PSD program 

in section 160) when combined with the other measures described 

earlier that we apply to PM2.5. Consequently, this final rule 
92

establishes the PM2.5 increments at the level of the proposed 

safe harbor increments.

An environmental group submitted extensive comments arguing 

that the PM2.5 safe harbor increments are not sufficient to meet 

the Act’s requirements for PSD and that our analysis was 

inadequate, and two other commenters submitted more narrowly 

targeted comments in this area. A summary of the major 

comments, along with our responses, follows. A more detailed 

treatment of the comments can be found in the Response to 

Comments document for this rulemaking, which is available in the 

rulemaking docket.17

The environmental group commenter 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 asserted that EPA 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 show fulfillment of the 

statutory purposes by asserting that it cannot develop a 

 

17 Docket No. EPA-HQ-OAR-2006-0605 can be accessed on line 

at www.regulations.gov. 
93

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).

We conclude that the 2007 NPRM demonstrated that the safe 

harbor increments, in combination with the other aspects of the 

regulatory framework, fulfill the statutory requirements despite 

the scientific uncertainties. We reiterate that finding today. 

The fact that we did not, in the NPRM, explicitly state this as 

a finding does not diminish the demonstration made there and 

reiterated in this preamble. 

The environmental group commenter 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 areaspecific critical loads or equally protective limits, where 
94

necessary to protect and enhance air quality in specific parks 

and natural areas.

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 SIPapproved PSD programs, but sufficient information was not yet 

available for EPA 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
95

increment rule, EPA has addressed the concept of critical loads 

in the last review of the PM NAAQS and currently in the 

secondary NO2/SO2 NAAQS review.18 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 

 

18 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.
96

PM in the ambient air; and (3) the vast majority of 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.” 2005 

Staff Paper at page 7-19. 

The secondary NO2/SO2 NAAQS review, which is currently 

underway, is evaluating ecological effects due to the 

atmospheric deposition of NOx and SOx. The two main targeted 

effects are acidification and nutrient enrichment in both 

aquatic and terrestrial ecosystems. This review has used 

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. 

The environmental group commenter also asserted that the 

safe harbor increments would allow PM2.5 air quality to 

deteriorate to the level of the NAAQS in many locations. 

According to the commenter’s analysis, at 55 percent of the 
97

locations with PM2.5 monitors that were not already exceeding the 

PM2.5 NAAQS, 24-hour PM2.5 concentrations would be allowed to 

increase up to the level of the NAAQS. In addition, the 

analysis 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/m3 or more. The 

commenter believes that allowing such levels would not be 

protective of public health, given that we stated in the NPRM 

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 environmental group commenter’s analysis showed similar 

results for the proposed annual PM2.5 increments. 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/m3

or 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 in the NPRM 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).

We do not believe that increments must be set at levels 
98

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 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 utilize 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 utilize 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 
99

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, as noted previously, 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 NPRM 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.

The environmental group commenter believes that there is a
100

quantifiable relationship between visibility impairment and PM2.5

levels, citing the NPRM discussion (72 FR 54135) as well as the 

most recent Criteria Document and Staff Paper for PM2.5. The 

commenter pointed out that in the 2007 NPRM (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/m3 as an increment to

protect visibility, the commenter believes that this shows that 

the safe harbor increment is inadequate because consumption of 

an increment of 9 μg/m3 combined with background levels alone 

would cause an area to reach within 1 μg/m3 of the staffrecommended value of 20 μg/m3. The commenter added 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 went on to assert that 

the safe harbor PM2.5 increments will not be sufficient to 

protect visibility in parks and other natural areas. In the 

NPRM, we stated that a 24-hour average PM2.5 concentration of 20 

μg/m3 correlates to a visual range of approximately 25 to 35 
101

kilometers. 72 FR 54129. The commenter asserted 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.19 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.

The visibility impairment issue is more complex than 

suggested by the environmental group commenter. 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 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

 

19 The commenter cited 

http://www.nps.gov/shen/naturescience/visibility_and_haze.htm

for historic visibility in national parks.
102

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 PM2.5 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 environmental group 

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.

Another commenter stated that the proposed 24-hour Class I 

increment (2 μg/m3) would not be protective of AQRVs, 

particularly visibility. This commenter noted 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 indicated that 

depending on the constituents of the ambient PM2.5 and the 

humidity, a concentration of 2 μg/m3 in 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 and, therefore, would likely 

constitute "adverse impacts" to Class I visibility. While 

acknowledging that the FLM may still determine that the 
103

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 in the 2007 NPRM 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.

This 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 
104

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 eliminate 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. In the NPRM, 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. 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 
105

national increments.

However, the environmental group commenter asserted that

these other programs will not fulfill the statutory purposes. 

As discussed previously in sections V.D.4 and 5, the commenter 

does not believe that FLM review in the AQRV process and the air 

quality impacts analysis required by section 165(a) of the Act 

are adequate. We disagree; see sections V.D.4 and 5 for more 

detail on the comments and our responses.

The environmental group commenter also noted that we cited

the regional haze program as a justification for adopting less 

protective PSD rules (referring to 72 FR 54135), but the 

commenter pointed 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 noted that we 

stated in the NPRM that “some State and local governments have 

also developed programs to improve visual air quality in 

specific urban areas” (citing 72 FR 54135), and pointed 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 

asked that we identify the specific state and local programs, 

and that we specify how much visibility protection such programs 

are providing.

The commenter is correct that the regional haze program 
106

directly addresses only Class I areas. As we have discussed 

before, 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 in section V.D.5 of this preamble, 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, in the 2005 

Staff Paper EPA described several existing programs 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.

The environmental group commenter cited the 2007 NPRM (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 
107

this claim is based on false premises, including the idea 

(discussed previously) 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.” 

We continue to believe that Class I area visibility 

protection under the PSD program is appropriately addressed via 

the AQRV process. As mentioned previously, 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., 

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 analysis of other 

impacts, including 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 
108

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 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. In regards 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.

The environmental group commenter also stated that the 

proposed PSD rules fail to ensure fulfillment of “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 
109

“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.

This same issue was raised in the 2005 PSD rule affirming 

the NO2 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. However, when a PSD increment violation is 

identified, we agree that EPA may require a state to revise its 

SIP to correct the 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 
110

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.

Another commenter stated that the PM2.5 increments should be 

twice the recommended 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 expressed 

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 claimed 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 added that other factors providing 

evidence for causation include dose-response relationships, 

consistency, and repeatability of studies, which the commenter 

said are not present in the studies cited by EPA. The commenter 

specifically referred to two studies, acknowledged by EPA to 

show no evidence of a dose-response relationship gradient 
111

between PM2.5 and specific health related effects.

We disagree with the 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 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 are finalizing this 

rulemaking using the safe harbor approach under section 166(a) 

of the Act. Using this approach, we calculated the “safe 

harbor” increments as percentages of the NAAQS comparable to the 

percentages that Congress used to establish the original 

statutory increments for PM and SO2. These values represent the 

minimum 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 
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any less stringent than the safe harbor increments, as the 

commenter recommends.

7. Compliance Determinations for the PM2.5 Increments

a. Modeling Compliance with PM2.5 Increments

Section 163(a) of the Act provides that “In the case of any

maximum allowable increase ... for a pollutant based on 

concentrations permitted under the national ambient air quality 

standards for any period other than an annual period, such 

regulations shall permit such maximum allowable increase to be 

exceeded during one such period per year [emphasis added].”

Accordingly, the existing PSD rules allow one exceedance per 

year of each short-term increment defined by the rules. See,

e.g., 40 CFR 51.166(c). With the addition of the PM2.5 increments 

to the list of maximum allowable concentrations in the PSD 

rules, the existing provision allowing one exceedance per year 

applies equally to the PM2.5 increments as well. Thus, when 

modeling increment compliance, the highest value of the secondhighest modeled increase in estimated PM2.5 concentrations at 

each model receptor for the 24-hour averaging time should be 

less than or equal to the maximum allowable increase for PM2.5. 

For the annual increments, the modeled annual averages should 

not exceed the annual maximum allowable increase for PM2.5. 

We did not expressly state in the 2007 NPRM the 

implications of adding PM2.5 increments to the existing list of 
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increments in paragraphs 51.166(c) and 52.21(c) of the PSD 

regulations. Nevertheless, it should have been clear at the 

time that, in the absence of alternative language for PM2.5, the 

existing provision allowing one exceedance for the short-term 

increments would apply to the increments for PM2.5 along with the 

increments already listed. We did not receive any comments 

either supporting or opposing these methods for determining 

compliance with the PM2.5 increments.

We recognize that the above approach for determining 

compliance with the 24-hour PM2.5 increments differs from the 

approach used for the 24-hour PM2.5 NAAQS, which is based on 

compliance with the 98th percentile value monitored concentration 

(or the highest eighth-highest modeled concentration at a 

location). A similar situation exists for the 24-hour PM10

increments where compliance with the 24-hour PM10 NAAQS is based 

on an expected exceedance form of the standard, resulting in 

compliance based on impacts less than a modeled Highest (n+1) 

Highest concentration at a receptor location, where n is equal 

to the number of years of meteorological data that is used for 

modeling. Often 5 years of meteorological data are used, so 

that the Highest 6th Highest modeled concentration is used to 

determine whether a violation is predicted. This inconsistency, 

however, should not cause complications in carrying out the 

required compliance determinations for the NAAQS and increments 
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for the same pollutant because these determinations are 

typically different anyway, based on separate, independent

analyses resulting from the inherently different emissions data 

bases from which ambient air quality concentrations are 

predicted under the NAAQS and increments analyses.

b. Condensable PM 

Initially, the EPA will not require PSD applicants under 

the federal PSD program to consider condensable PM in emissions

calculations to determine whether a proposed project is subject 

to the PSD requirements. In addition, we will not require the 

condensable portion to be considered in the required PM2.5 air 

quality analyses either. In our May 2008 PM2.5 NSR 

Implementation Rule, we announced that we would not require that 

states address condensable PM in establishing enforceable 

emissions limits for either PM10 or PM2.5 in NSR permits until the 

completion of a transition period. Further, we indicated that 

the transition period would end January 1, 2011 unless EPA 

advanced the date through the rulemaking process. We also 

indicated that such rulemaking would involve the assessment and 

possible revision of test methods for measuring condensable 

emissions and taking comment on an earlier closing date for the 

transition period in the NSR program if we are on track to meet 

our expectations to complete the test methods rule much earlier 

than January 1, 2011. In addition, states that have developed 
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the necessary tools are not precluded from acting to include 

condensable PM emissions in NSR permit actions prior to the end

of the transition period, especially if it is required in an 

applicable SIP. See 73 FR 28321 at 28334 – 28336.

c. PM2.5 Precursors 

In the 2007 NPRM, we proposed to add SILs for PM2.5 to the

PSD regulations at 40 CFR 51.166 and 52.21. (The SILs are 

described more fully in section VI of this preamble.) 

Accompanying these SILs, we proposed to add 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 
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impacts to the prescribed SILs for PM2.5.

The impacts of PM2.5 precursors on ambient concentrations of 

PM2.5 cannot be determined from the dispersion models that EPA 

has currently approved for modeling individual PSD sources.

Such models are not designed to consider chemical 

transformations that occur in the atmosphere after the precursor 

emissions have been released from the source. Consideration of 

these transformations is necessary to be able to add precursor 

impacts to the total impact that ambient PM2.5 concentrations 

have in order to compare the results to the 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. Estimating techniques are 

being developed that 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. 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 
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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 this final 

rule now enables the test to be based on “the emissions 

increase...that would cause...air quality impacts less than [the 

SILs].” See, e.g., new 40 CFR 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. 

F. Final Action on Trigger and Baseline Dates for PM2.5

Increments

In the 2007 NPRM, we proposed as part of Option 1 to 

require the implementation of the PM2.5 increment system (annual 

and 24-hour increments) with new baseline areas, baseline dates,

and trigger date. Specifically, we proposed that the major 

source baseline date and trigger date, both fixed dates, would 

be defined as the effective date of the final rule and would 

reflect a date 1 year from the date of publication, in 

accordance with section 166(b) of the Act. In contrast, under 

Option 2 (both 2A and 2B), we proposed to establish new baseline 

dates for the 24-hour PM2.5 increments, but to retain the 

existing baseline areas and dates for the annual PM2.5 increments 

because the annual increments would be equivalent substitutes 
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for the existing annual PM10 increments.

In light of the then-current and expected trends in PM2.5 

concentrations, our judgment was that starting with new baseline 

dates on or after the effective date of this rule would make the 

new PSD increments more protective. We proposed that any 

emissions reductions occurring prior to the effective date of 

this rule would be counted toward the baseline concentration 

rather than be used for expanding the PM2.5 increment. If a 

retroactive baseline date were to apply, emissions reductions 

occurring prior to the effective date of this rule would serve 

to expand the available increments, enabling more new pollution 

than would otherwise be allowed to occur.

We also expressed our belief that starting with new 

baseline dates to implement increments for PM2.5 would be 

appropriate because Option 1 treats PM2.5 essentially as a “new”

pollutant for purposes of PSD and section 166 of the Act. We 

continue to believe that establishing a new baseline also 

overcomes significant implementation concerns that would 

otherwise exist if the existing PM baseline were maintained. If 

we were to require sources and reviewing authorities to conduct 

PM2.5 increment analyses based on the minor source baseline dates 

previously established under the TSP or PM10 program, they would 

have to attempt to recreate the PM2.5 emissions inventory as of 

the minor source baseline date in order to determine the 
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baseline PM2.5 concentration for the area. For early minor 

source baseline dates in particular (e.g., 1976 in some areas of 

the United States), establishing the emissions inventory for 

PM2.5 would be extremely difficult, cumbersome, and potentially 

inaccurate because historic emissions inventories did not 

include PM2.5 emissions. For all of these reasons, we proposed 

Option 1 as our preferred option and requested comment on this 

contingent safe harbor approach for annual and 24-hour PM2.5

increments under Option 1.

Under Option 1, we proposed that the PM2.5 increments would 

be subjected to a 1-year delay consistent with the procedures 

under section 166(b) of the Act, which provides in general that 

these rules “shall become effective 1 year after the date of 

promulgation.” Alternatively, we sought comment on a 60-day 

delay as part of our proposal under Option 1. In the proposal 

we requested comment on the argument that, while the Act 

includes a 1-year implementation delay for new increments, the 

same provision calls for EPA to promulgate new increments within 

2 years of the promulgation of the NAAQS, and given that these 

new increments are being promulgated after the 2-year interval, 

we expressed our belief that the overall Congressional intent 

reflected in section 166 of the Act could possibly be met by 

advancing the effective date of the PM2.5 increments. 

Twelve commenters supported our proposal under Option 1 to 
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establish new trigger and baseline dates for PM2.5, regardless of 

the particular increment option that they otherwise supported. 

These commenters generally saw new dates as being the best 

approach because of various problems that would result from 

retaining existing trigger and baseline dates. Some commenters 

claimed that it would be technically difficult to try to 

reconstruct old inventories to determine the amount of PM2.5

emitted by sources in the past.

One commenter stated that establishing new PM2.5 increment 

inventories using existing PM10 baseline dates would be 

“extremely difficult, cumbersome, and necessarily inaccurate and 

unreliable as historic emissions did not speciate PM2.5

emissions.” A state/local agency commenter said that 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.” 

Yet another commenter claimed that “resurrecting PM2.5

inventories based on the PM10 baseline dates would be 

insurmountable.” Similar comments were echoed by several 

commenters who supported the use of legal authority set forth in 

section 166(f) (“equivalent substitution” approach) for 

developing the numerical values for the PM2.5 increments. One of 

these commenters stated that he did not “believe the 
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establishment of new baseline dates for PM2.5 would abandon past 

cases of increment consumption for PM10, because the 24-hour PM10

increments would still be in effect ....”

One commenter suggested 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 explained 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. 

The commenter 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 claimed, “this will minimize any inconsistent 

applications of the regulations for PM2.5.” 

One commenter noted that our proposed PM2.5 increments were 

very low and “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.”

One commenter indicated that the historic TSP/PM10 baseline 

dates should be retained. This commenter favored the equivalent 

substitution approach and, consistent with that approach, 

retention of the existing baseline dates.

Having considered all the comments, we believe that the 
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most reasonable approach for addressing the relevant dates 

associated with the PM2.5 increments is to start anew with the 

baseline date concept. As mentioned earlier, the commenters 

tended to support our proposed approach under Option 1 to 

establish new dates for implementing the PM2.5 increments even 

when not supporting the proposed Option 1 approach for defining 

increment values, generally citing the difficulties that would 

occur if the PM2.5 emissions inventory for increment analyses had 

to be created for an earlier period of time. Also, these new 

dates will not undo increment already consumed under the 

existing increments for PM because we are not revoking the 24-

hour or annual PM10 increments under this new rule. Accordingly, 

this 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 used for the annual and 24-

hour PM2.5 increments and will be established when a source 

applies for a PSD permit any time on or after the new trigger 

date for PM2.5. (See also the discussion about changes to the 

definition of “baseline area” in section V.G. of this preamble) 

The “major source baseline date” for PM2.5 is being set as 

[INSERT DATE OF PUBLICATION IN THE FEDERAL REGISTER] — the date 

of publication of this final rule. The setting of this date 
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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 at [INSERT DATE 1 YEAR 

AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER], which is 1 

year after the date of publication of this final rule. We are 

using this approach to define the date on which the PM2.5

increments become effective as 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 new increments from the 

effective date of this final rule in general, 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 this 

final rule. The implementation of these dates as part of the 

new PM2.5 increment system is discussed in greater detail in 

section VIII of this preamble. 

We recognize that some may still have a concern about our 

decision to set the major source baseline date as the date of 
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publication of this 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 due to the 

lack of adequate technical tools and emissions data for PM2.5. 

Hence, 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. 

G. Definition of “Baseline Area” for PM2.5

No changes were proposed with respect to the definition of 

“baseline area” for PM2.5 increments. One commenter (0040), 

however, noted that fact in claiming that we did not adequately 

account for significant impacts of PM2.5 for purposes of defining 

the “baseline area” for the PM2.5 increments. Under the existing 

regulations, the establishment of a baseline area for any PSD 

increment results from the submittal of the first complete PSD 

application, and is based on both the location of the proposed 

source and the impact of the source’s emissions on the area. In 

accordance with the definition, the attainment or unclassifiable 

area in which the proposed source would construct is always part 

of the baseline area in which the minor source baseline date is 
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established and the increment analysis is conducted. In 

addition, the definition provides that any surrounding 

attainment or unclassifiable area in which the proposed source’s 

impact is greater than 1 μg/m3, annual average, would also become 

part of the baseline area, assuming the area had not already 

been established as a baseline area by a previous application 

for a PSD permit. See, e.g., 40 CFR 52.21(b)(15). 

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 level of significance for both SO2 and PM when the definition 

was originally established.20 There was no mandate at that time 

that a 1 μg/m3 impact be used to determine the baseline area for 

increments for other pollutants; however, the use of a 1 μg/m3

impact 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. The EPA has determined, however, that “significant” 

for PM2.5 ambient impacts should be considered to occur at a 

lower concentration than 1 μg/m3. Elsewhere in this preamble, we 

have indicated that the SIL for PM2.5 in this final rule is 0.3

 

20 “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. 
126

μg/m3, annual average. 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 e.g., revised 40 CFR 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 action 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 this 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.

H. No Final Action with Respect to the Proposed Revocation of 

PM10 Annual Increments

In the 2007 NPRM, we proposed to either revoke or replace 

the annual increments (Class I, II, and III) for PM10 to conform 

to the earlier revocation of the annual PM10 NAAQS. We proposed 

to revoke the annual increments, based on the same technical 

evidence that led us to revoke the annual PM10 NAAQS, if we 
127

decided to use Option 1 for adopting PM2.5 increments, and 

discussed our authority and rationale for doing so. 72 FR 

54136. In comparison, we proposed to replace the existing 

annual PM10 increments with equivalent substitute PM2.5 increments 

using the authority under section 166(f) of the Act. After 

further analysis and consideration of the comments on this 

issue, we have decided not to take final action on our proposal 

to revoke those increments for PM10 as part of this rulemaking. 

The effect of not taking final action with respect to the PM10

annual increments is to leave those increments in place and 

unchanged.

Three commenters agreed with EPA’s proposal to “adopt the 

24-hour and annual PM2.5 increments and to revoke the annual PM10

increments.” One commenter stated, “counting and tracking 

increment is confusing enough without adding the confusion of 

potentially overlapping PM standards.” The commenter noted that 

the “cleanest approach is to establish a single new PM2.5 

increment and work from there.” The commenter suggested that 

EPA first “develop a coarse fraction increment, once EPA 

establishes coarse PM NAAQS.” The commenter added that the 

removal of the PM10 annual increment is supported by the removal 

of the “health based standard for annual PM10.”

One of the commenters agreed, “it makes no sense for EPA’s 

regulations to contain an annual increment for PM10 even though 
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an annual PM10 NAAQS no longer exists.” The commenter added, 

“EPA is without authority under Section 166(f) to retain the PM10

annual increment if it adopts a PM2.5 annual increment. This 

commenter explained, “EPA is compelled by law to eliminate the 

PM10 annual increment.”

We agree with this commenter that section 166(f) is a 

“substitution” approach; 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, some 

commenters challenged our authority under section 166(f) to 

replace the PM10 increments. In our response to the following 

comments, we address the legal issues that we believe prevent us 

from simply revoking the PM10 increments. 

 One environmental commenter (0040) claimed, “the agency has 

no authority to repeal an existing PM10 increment without at the 

same time restoring the corresponding TSP increment.” The 

commenter noted, “Congress established the TSP increments by 

statute and gave EPA no authority to revoke them,” and “instead, 

Congress gave EPA only limited authority to substitute PM10 

increments for TSP increments under the conditions specified in 

Section 166(f).” The commenter explained, “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 increment.” The commenter noted, “retention of 
129

the PM10 annual increment is also entirely compatible with the 

statutory purposes, notwithstanding EPA’s revocation of the 

annual PM10 NAAQS.” The commenter further noted 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.” 

72 FR 54136. 

 “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 capacity of the ecosystem and/or affects the 

nutrient status of the ecosystem.” 72 FR 54131. 

Five state/local agency commenters opposed the revocation 

of PM10 annual increments “until EPA makes a determination on a 

PM-coarse NAAQS” and/or “establishes equivalent increments for 

PM-coarse.” One of these commenters added, “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.” The commenter claimed that, “if EPA retains the annual PM10

increments” “then the determination of PM2.5 increments can 
130

complement the continuation of PM10 increment determinations 

without any discontinuities or unwanted degradation concerns.” 

Another one of these commenters stated, “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 noted, “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.” The commenter added, “it 

is prudent for EPA to retain the PM10 increment until such time 

that it makes a final determination on the coarse component of 

particles.” 

One state/local agency commenter (0019) noted, “EPA also 

proposes to replace the PM10 annual increment with the 

corresponding PM2.5 increment under the Section 166(f) options 2A 

and 2B as well, but does not provide a substantive basis for 

such an action.” The commenter (0019) 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 we believe it can.” The commenter (0019) 

explained, “under this approach, if EPA determines that coarse 
131

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 asserted 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.

One commenter (0028) requested that EPA “keep the PM10 PSD 

program (especially the increments) in place until the full PM2.5 

program is adopted and in place.” 

One commenter (0033) “does not support revoking the annual 

PM10 increments,” because the commenter feels that “there are too 

many uncertainties regarding PM2.5.” The commenter (0033) 

provided the following example: “the 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 (0033) explained 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 

suggested, “PM10 increments and NAAQS should remain in effect 

until these issues have been resolved to the satisfaction of the 
132

Administrator.” This commenter believed 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 can be

supported under this section because the two sets of increments 

complement each other. The commenter indicated 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.

One state/local agency association commenter (0021) 

recommended that “EPA can and should continue both the 24-hour 

and annual average PM10 PSD increment program until PM10-2.5 

standards are promulgated.” The commenter (0021) explained that 

“EPA has the discretion to accomplish this under CAA §166(f)” 

and “at a minimum, the agency should continue the 24-hour PM10 

increments in conjunction with the continuation of the 24-hour 

PM10 NAAQS.”

As stated previously, in this rule we are taking no final 

action on our proposal to revoke the annual PM10 increments even 

though the annual PM10 NAAQS has been revoked. Based on 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. 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
133

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 NAAQS, because annual increments are not contingent upon the 

existence of annual NAAQS. This is clear from the court’s 

decision 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.”)

I. Other Comments on Increments

Ten commenters (four state/local agencies (0019, 0025, 

0028, 0033), two state/local agency associations (0018, 0021), 

two industry commenters (0031, 0042), and two industry 
134

associations (0034, 0039)) supported section 166(f) of the Act 

as the basis for PM2.5 increments. These commenters typically 

voiced the belief that when Congress enacted section 166(f), it 

authorized EPA to update PM increments when another indicator 

was defined, and that section 166(f) allows EPA to continue do 

so as long as these increments are of equal stringency to the 

prior increments. Some of these commenters believe that section 

166(f) is the only legitimate approach under the Act, while 

others indicated simply that it is preferable to section 166(a). 

Some of the commenters believe that section 166(f) authority can 

be used to add PM2.5 increments to the existing PM10 increments. 

Others believe that PM2.5 increments finalized under section 

166(f) must fully replace the existing PM10 increments, and 

recommended doing so.

For the reasons discussed previously in this preamble, EPA 

has decided to finalize the PM2.5 increments under the authority 

of section 166(a) of the Act. With respect to the potential 

creation of PM2.5 increments under section 166(f) (as discussed 

in the 2007 NPRM at 72 FR 54120 – 54121), we have not reached 

any final conclusion as to whether that approach is authorized 

by the statute, but believe that such an approach raises 

significant legal issues. Because the Agency is not relying on 

section 166(f) in this rulemaking, we do not address these 

issues in this preamble, though some additional discussion is 
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included in the Response to Comments document for this rule.

One industry association (0029) that supported the Option 1 

approach based on section 166(a) authority also acknowledged 

that EPA is authorized to use the Option 2 approach based on 

section 166(f) authority. An industry commenter (0043) 

indicated that 2007 NPRM’s arguments regarding the alternative 

legal authorities under section 166(a) and (f) were not 

compelling; the commenter recommended setting the PM2.5

increments at the levels proposed as Option 2B because they 

would have the lowest economic impact.

As noted previously, we have decided to finalize Option 1 

based on section 166(a) authority because we believe that 

provision provides the clearest statutory authority for purposes 

of developing the new increments based on PM2.5. We would point 

out, however, that any conclusion as to which option would yield 

increments that “have the lowest economic impact” must include a 

consideration of not only the levels of the increments but also 

the associated baseline dates that define when emissions changes 

must be considered to affect the amount of increment consumed. 

Under options 2 and 3, the PM2.5 increments would be regarded as 

replacement increments for the PM10 increments and, as such, 

would include amounts of increment (based upon the PM2.5

component) already consumed under the existing PM10 increment

system. Thus, portions of the substitute PM2.5 increments could 
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have already been consumed by previous PSD sources that emit PM. 

If, in fact, a portion of the PM2.5 increments have already been 

consumed by the prior PM10 increment consumption process, than 

there would be a basis to conclude that less additional economic 

growth would be allowed under a set of replacement PM2.5

increments as compared to PM2.5 increments based on separate, 

independent baseline dates. 

One industry commenter (0016.1) suggested 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 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 PSD increments (or SILs or SMCs) where the 

conditions of concern are not capable of control.

As previously discussed, this final rule establishes an 
137

area classification system with prescribed, uniform PM2.5

increments for each class. We do not believe that it is 

necessary to develop different increments (or SILs or SMC) for 

different areas of the country. Emissions from natural 

conditions such as those described by the commenter would not 

consume increment due to 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)).

VI. Final Action on PM2.5 SILs

A. EPA’s Determination on SILs for PM2.5

It is EPA’s longstanding policy to allow the use of the 

SILs as de minimis thresholds under the NSR programs at 40 CFR

51.165(b) and part 51, Appendix S, to determine whether the 

predicted ambient impact resulting from the emissions increase 

at a proposed major new stationary source or modification is 

considered to cause or contribute to a violation of the NAAQS. 

As described in section IV.D of this preamble, we have also 

allowed the SILs under the PSD program to determine: (1) when a 

proposed source’s ambient impacts warrant a comprehensive 

(cumulative) source impact analysis; (2) the size of the impact 

area within which the air quality analysis is completed, and (3) 

whether the emissions increase from a proposed new major 
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stationary source or major modification is considered to cause 

or contribute to a violation of any NAAQS. 

We proposed three separate options for setting SILs for 

PM2.5. The first option relied upon the same approach we proposed 

for PM10 in the 1996 NSR Reform proposal. This set included 

Class I SILs set at 4 percent of the Class I PM2.5 increments. 

For class II and III areas, we proposed to codify the SIL values 

that already existed for PM10, i.e., 1.0 μg/m3 (annual) and 5.0 

μg/m3 (24-hour). Options 2 and 3 relied on scaling the PM10

SILs, as codified in 40 CFR 51.165(b), by a particular ratio. 

Specifically, for Option 2, the multiplier was the emissions 

ratio of PM2.5 to PM10 for point sources in the 1999 NEI; for 

Option 3 the multiplier was the ratio of the PM2.5 NAAQS to the 

PM10 NAAQS. The resulting SILs were proposed as follows: 

Option

Proposed SILs (μg/m3)

Class I Class II Class III

Annual 24-hr Annual 24-hr Annual 24-hr

1 0.04 0.08 1.0 5.0 1.0 5.0

2 0.16 0.24 0.8 4.0 0.8 4.0

3 0.06 0.07 0.3 1.2 0.3 1.2

We have decided to finalize the PM2.5 SILs proposed under 

Option 3. As explained earlier, these values will be used in 

the federal PSD preconstruction review process consistent with 

our proposal. See 72 FR 54122 at pages 54138-41 and 54143. 

States are not required to adopt SILs in their NSR or PSD 
139

programs; the analyses for PM2.5 required by each applicable 

regulation can be carried out without using a SIL.

21 Therefore, 

we do not intend for any specific deadlines to apply under the 

regulations at 40 CFR 51.165(b), 51.166, or part 51, Appendix S

for states to submit SILs for PM2.5, should they choose to do so, 

as part of their revisions to incorporate the final rules for 

PM2.5 into SIPs. Nonetheless, we believe that the availability 

of SILs as a screening tool greatly improves PSD program 

implementation by streamlining the permit process and reducing 

labor hours necessary to submit and review a complete permit 

application where the projected impact of the proposed source is 

de minimis in the relevant area. For these reasons, we are 

including the PM2.5 SILs in the federal PSD regulations at 40 CFR 

52.21 to screen proposed projects concerning the need for a 

cumulative source impact analysis for PM2.5.

B. Response to Comments Concerning the SILs 

The primary purpose of the SILs is to identify a level of 

ambient impact that is sufficiently low relative to the NAAQS or

increments that such impact can be considered trivial or de 

minimis. Hence, the EPA considers a source whose individual 

 

21 We note that, under the 2007 NPRM, we proposed that the 

SILs for PM2.5 would not be treated as a minimum program element 

for state PSD programs; however, the proposed regulatory 

language incorrectly stated the “the plan shall provide”, which 

would indicate that the use of the SILs for PM2.5 was required in 

the state plan. This final rule corrects this error.
140

impact falls below a SIL to have a de minimis impact on air 

quality concentrations that already exist. Accordingly, a 

source that demonstrates that the projected ambient impact of 

its proposed emissions increase does not exceed the SIL for that 

pollutant at a location where a NAAQS or increment violation 

occurs is not considered to cause or contribute to that 

violation. In the same way, a source with a proposed emissions 

increase of a particular pollutant that will have a significant 

impact at some locations is not required to model at distances 

beyond the point where the impact of its proposed emissions is 

below the SILs for that pollutant. 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 evaluation of 

the proposed source or modification. 

While some commenters opposed all of the proposed options

for PM2.5 SILs, most commenters generally supported the use of a 

SIL as a screening tool for PM2.5 air quality analyses. 

Commenters who supported one of the proposed options for the 

SILs were divided as to their support of a particular approach 

for selecting the SIL value, with each option receiving some 

support. Commenters also tended to agree that the SILs should 
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not be used for determining significant impacts on AQRVs in 

Class I areas. 

Those commenters supporting the concept of the SILs, yet 

opposing all proposed options, believed that all options yielded 

SILs that were too low. Another commenter, an environmental 

group, presented extensive legal and policy arguments against 

the SILs concept in general. Some of the significant comments 

and our responses to them are addressed herein, while others are 

covered in the Response to Comments document which we have 

placed in the docket for this rulemaking. 

1. Legal Basis for SILs

One commenter opposed all three proposed options on both 

legal and policy grounds claiming that EPA has no legal 

authority to promulgate SILs and that the de minimis doctrine 

endorsed by the court does not apply to increment analyses, 

where Congress has expressly directed that the letter of the law 

applies in all circumstances, as it has in this case. (The 

commenter’s policy concerns about SILs are discussed later in 

this section of this preamble.) The commenter stated that 

“Congress codified increments in section 163 of the Act, 

directing that SIPs contain measures assuring that the 

increments shall not be exceeded.” According to the commenter, 

“The Act plainly provides that no major source may be 

constructed unless it meets this requirement, and may not 
142

contribute to an exceedance ‘for any pollutant in any area.’” 

The commenter further stated that “the de minimis doctrine is 

inapplicable because it applies only where the regulations will 

yield a gain that is demonstrably trivial or zero.” 

We disagree with this commenter’s claim that there is no 

legal basis for SILs. As stated in the 2007 NPRM, the concept 

of a SIL 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). In this case reviewing EPA’s 1978 PSD 

regulations, the court recognized that “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. See the 2007 NPRM preamble for more 

on how we have applied the de minimis principle in the past.

2. Levels of the SILs

Several commenters opposed all three proposed options on 

the grounds that all yielded levels of SILs that are too low. 

One of these commenters argued that the proposed SILs “imply a 

level of monitoring and modeling sophistication that is 

currently absent in our regulatory scheme.” This commenter 

recommended 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.” 

We disagree with these commenters’ concerns about all the
143

proposed SILs being too low. While we did not select the Option 

1 levels, the Class II and III SILs for PM2.5 are the same 

ambient concentration levels that are used for the SILs for the 

other criteria pollutants under 40 CFR 51.165(b), and those 

existing SILs values are associated with NAAQS that are 

considerably higher than the NAAQS for PM2.5. Clearly, it would 

have been inappropriate to select Class II and III SILs for PM2.5

that represent relatively higher values than the existing SIL 

values for other pollutants in light of the more stringent NAAQS 

levels that exist for PM2.5. We also disagree that the SILs 

should be consistent with current monitoring capabilities for 

PM2.5. The SILs are a screening tool used in comparison with 

modeled predictions — not monitored concentrations — of PM2.5. 

Monitoring accuracy is not a relevant concern in predicting with 

air quality dispersion models the concentrations of a pollutant 

that a source will cause if its construction and operation are 

allowed to occur.

Two commenters expressed concern about national de minimis

values. One stated that “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.” The other commenter recommended that 

EPA “develop geographic area-specific ... levels that take local 

conditions into account.” This commenter reasoned that some PSD 
144

areas “should not be ‘penalized’ by a single, national PSD 

increment, significant impact levels and significant monitoring 

level, where the conditions of concern are not capable of 

control.” 

With regard to the first of these commenters, our 

longstanding policy has been that when a source has a de minimis

impact on an existing air quality problem, that source should 

not necessarily be required to bear the burden of addressing its 

small contribution to a problem caused primarily by other 

sources. However, notwithstanding the existence of a SIL, 

permitting authorities should determine when it may be 

appropriate to conclude that even a de minimis impact will 

“cause or contribute” to an air quality problem and to seek 

remedial action from the proposed new source or modification. 

For example, where a proposed modification to an existing major 

stationary source would have what is normally considered to be a 

de minimis impact on air quality but would nevertheless result 

in air quality concentrations exceeding the NAAQS or increments, 

it might be appropriate to require the source to take remedial 

action despite its “de minimis” contribution. This could be the 

case if the review authority finds that the source’s existing

(that is, pre-modification) emissions are not being sufficiently 

controlled and the source is a primary contributor to the 

overall air quality problem.
145

We do not agree with the second of these comments 

concerning the development of regional SILs based on a concern 

that some amounts of PM2.5 in a particular area are “not capable 

of control.” The PM2.5 SILs define a threshold level for 

determining whether a predicted ambient impact by a proposed 

major stationary source or major modification of PM2.5 needs to 

undergo a more thorough analysis of the 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 causes the 

existing PM2.5 concentrations, rather by the impact of a single 

source relative to the levels of the NAAQS and increments that 

must be protected. Therefore, we do not see why the SILs should 

be influenced by the geographic area of concern, or how 

different levels of SILs for the same pollutant and averaging 

period would be necessary.

With regard to the commenters that supported at least one 

of the proposed SILs options, they generally did not prefer the 

entire suite of SILs (Class I, II, and III SILs) from a single 

option, but instead supported parts of different options, 

primarily divided by drawing a distinction between the Class I 

SILs and the SILs for Class II and III areas. Consistent with 

the way that commenters addressed the Class I, II, and III SILs, 

we will address the comments separately herein as well.

a. Class I SILs
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Support and opposition for the proposed PM2.5 SILs for Class 

I areas was fairly evenly divided. The PM2.5 SILs for Class I 

areas proposed under Option 2 received the support of some 

commenters, but also received an equal amount of opposition. 

Option 1, which yielded the lowest (most restrictive) values for 

the Class I area SILs for PM2.5 (annual and 24-hour averages), 

was supported by some commenters, including a federal agency 

that serves as a FLM for federal Class I areas under the PSD 

program, but was equally opposed. Finally, comments supporting

the Class I SILs proposed under Option 3 (from which we derived 

the values included in the final rules) were matched by comments 

that opposed the Class I SILs under Option 3.

One commenter (0043) opposing the Option 3 SILs for Class I 

areas said that the values “appear to be unrealistically low 

and, if selected, would point to the need for EPA to conduct an 

economic impact analysis.” We disagree that adopting the Option 

3 SILs for Class I areas (and Class II and III areas) will 

result in economic impacts significant enough to warrant an 

economic impact analysis. Under the Paperwork Reduction Act, 

EPA is required to analyze, and receive approval from the Office 

of Management and Budget for, the recordkeeping and reporting 

burden imposed by its regulations (referred to as the 

“Information Collection Request” for the regulation). For the 

PSD program, this includes the burden associated with the entire 
147

permitting process, including any required modeling analyses. 

In our analysis for this rulemaking, we have concluded that the 

number of PSD permits issued annually will be unchanged (at an 

estimated 274 per year), while the total burden across all PSD 

permit applicants of adding PM2.5 analyses will increase by a 

total of approximately 29,000 hours per year at a cost of 

approximately $2.8 million per year. This total annual impact 

on industry is a small fraction of the threshold ($100 million 

per year) that is considered “significant” under Executive Order 

12866 (Regulatory Planning and Review) and the Unfunded Mandates 

Reform Act. See sections X.B and X.D of this preamble for more 

on the Paperwork Reduction Act and the Unfunded Mandates Reform 

Act, respectively. Our analysis of the recordkeeping and 

reporting burden of this rulemaking can be found in the docket 

for this Information Collection Request.22

Another commenter (0037) stated that the use of a NAAQSbased ratio under Option 3 for the proposed SILs does not 

“translate back to the emissions point level when comparing PM10

and PM2.5.” This commenter continued, “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 

 

22 See “Information Collection Request (ICR) for the 

Prevention of Significant Deterioration for PM2.5 - Increments, 

Significant Impact Levels and Significant Monitoring 

Concentration,” Docket No. EPA-HQ-OAR-2007-0628.
148

accounting for precursors and EPA does not have a quantifiable 

sense of the portion of PM2.5 that is condensable for various 

industries.”

We disagree with the commenter’s concern that the use of 

NAAQS-based ratios is an invalid method for developing the PM2.5

SILs. The purpose of using the NAAQS ratio with the PM10 SILs to 

develop PM2.5 SILs is to establish values that have a comparable 

relationship between ambient concentrations of PM10 and PM2.5 and 

their respective NAAQS levels. Whether a particular ambient 

concentration of PM2.5 results from direct PM2.5 emissions or from 

precursor emissions is not relevant to this particular approach. 

The PM2.5 SILs in this final rule are intended to be compared to 

the ambient concentrations of PM2.5 that are predicted by

modeling the emissions of a proposed new project. Ambient 

concentrations of PM2.5 can be the result of direct PM2.5

emissions, which may include condensable particulate matter, as 

well as precursor emissions, e.g., SO2 and NOx. 

We note that the 2007 NPRM included proposed regulatory 

language providing that demonstrations of whether the air 

quality impact of a major new source or modification would be 

less than the PM2.5 SILs be based on direct PM2.5 emissions from 

the proposed project. The intent of this was to recognize the 

technical limitations associated with modeling precursor 

emissions to predict ambient PM2.5 impacts. However, in this 
149

final rule we have removed that limitation by removing the 

reference to “direct” PM2.5 emissions.

One commenter (0029), who did not support any of the 

proposed SILs options, was especially critical of the Class I

SILs for PM2.5 under Option 1, stating that multiplying the 

proposed PM2.5 increment by 4 percent is without legal or 

practical merit. The commenter stated that 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.” The commenter added that if the PM2.5 SILs 

for Class I areas under Option 1 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.

In contrast, the federal agency commenter supporting the 

PM2.5 SILs for Class I areas under Option 1 explained that they 

analyzed the effectiveness of the three sets of proposed SILs by 

modeling four different coal-fired power plant scenarios using 

an EPA-approved long-range transport model. The modeled plants 

included a large 1,500 MW facility, a moderate-sized 500 MW 

facility, and two medium 800 MW facilities. Based on this 

modeling analysis, the commenter concluded that the proposed 

levels of the Class I 24-hour SILs based on Option 1 and Option 
150

3 are “more appropriately protective of the proposed Class I 

PM2.5 increment and impacts to visibility than the level obtained 

under Option 2.” This commenter supported the consistency of 

using 4 percent of the Class I increments that was used by EPA 

in proposing Class I SILs for SO2, NOx, and PM10 in 1996.

We chose the Class I SILs under Option 3 because we believe 

that this option yields the most appropriate combination of SILs 

for all area classifications. Whether a particular source will 

have a significant impact on an area is determined to some 

extent by the amount of its emissions, but also by other factors 

such as the height of release, pollutant transport distance, 

terrain features, and meteorological factors. Thus, we did not 

select SILs values to address a certain size source or the 

degree of control of that source, but the ambient impact of that 

source relative to the NAAQS and increments that will result 

from the source’s emissions. While the annual Class I SIL under 

Option 3 represents a level that is somewhat greater than 4

percent of the PM2.5 annual increment for Class I areas, it is

sufficiently close (as derived from a ratio of the PM2.5 NAAQS to 

the PM10 NAAQS) so as to provide a reasonable threshold 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 NPR for a PSD rulemaking; however, that particular 
151

component of the proposal was never finalized. See 61 FR 38250 

beginning at page 38291. We will further discuss our rationale 

for selecting the SILs under Option 3 in the discussion which 

follows for the Class II and III SILs.

b. Class II and III SILs 

While many commenters tended to favor Option 2 with regard 

to the proposed Class I increments, they tended clearly to

support Option 1 for defining Class II and III SILs for PM2.5. 

These particular SILs for PM2.5 were proposed so as to be equal 

to the existing Class II and III SILs for the existing 

pollutants (e.g., SO2, PM10, NOx). In all, six commenters 

supported Option 1. One of these commenters stated that Option 

1 SILs for Class II and III areas are “sufficiently stringent 

and fully consistent with the de minimis justification for 

SILs.” The commenter added that “when conducting an air quality 

impact analysis ... most applicants assume all coarse PM10 to be 

PM2.5.” The commenter claimed that this assumption is 

conservative and “overestimates the amount of fine particles 

being emitted and renders the effective SIL thresholds for PM2.5

lower than those written into the regulations.”

We strongly disagree that the SILs proposed under Option 1 

as applied to PM2.5 are sufficiently stringent. The application 

of such values as SILs for PM2.5 would result in ambient 

concentrations of PM2.5 that consume a much larger portion of 
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both the PM2.5 NAAQS and increments than either of the other two 

options proposed for PM2.5 in light of the correspondingly more 

stringent levels of the PM2.5 NAAQS and increments than those for 

the other pollutants. We believe that of the 3 options 

proposed, the PM2.5 SILs based on Option 3 represent values that 

are more closely aligned percentage-wise with the SILs that have 

been or are being used for other forms of PM when compared to 

their respective NAAQS and increments.

We also disagree with the commenter’s suggestion that the 

development of the SILs for PM2.5, or any other pollutant, should 

in any way be influenced by the possibility that some sources

may use conservative techniques for estimating a source’s 

emissions rate. Such conservative techniques may be needed on 

an interim basis while technical issues associated with the 

calculation of PM2.5 emissions exist, and can certainly be used 

at any time as a simplified methodology for estimating PM2.5

emissions. But when such an overly conservative approach fails 

to yield de miminis results, the source may find it necessary to 

rely upon more accurate techniques for determining the amount of 

PM2.5 that the source will emit. 

Finally, one commenter, objecting to all of the proposed 

SILs, stated that EPA must assure that SILs are truly de minimis

and must also include limitations on the use of SILs as 

necessary to prevent air quality from significantly 
153

deteriorating. We acknowledge that we did not conduct any new 

modeling or other types of analyses of the proposed SILs in 

order to explicitly show that the final PM2.5 SILs values in this 

final rule are de minimis. Instead, we have relied on past 

actions regarding the setting of de minimis levels to illustrate 

that the PM2.5 values selected via Option 3 represent values that 

are as stringent as the previous levels that have been 

established to define de minimis for PM10 and TSP. See 45 FR 

52676, 52706-708 (August 7, 1980)(using modeling and 

representative data). 

Using the 24—hour and annual NAAQS ratios of PM2.5 to PM10, 

and multiplying them by the corresponding existing PM10 SILs, we 

are assured that the PM2.5 SILs define de mimimis for the PM2.5

NAAQS in the same way as the PM10 SILs do for PM10 NAAQS. With 

regard to the increments, the annual and 24-hour PM2.5 SILs

represent about 7.5 and 13 percent of the annual and 24-hour 

PM2.5 increments, respectively. By comparison, the annual and

24-hour PM10 SILs represent about 5 and 17 percent of the annual 

and 24-hour PM10 increments, respectively. We believe the PM2.5

SILs fall into a comparable relative range with the PM10 SILs and 

can be considered de minimis.

 In EPA’s 1980 final rule for PSD, EPA adopted significant 

emissions rates for the pollutants then subject to regulation 

under the PSD requirements. The significant emissions rate 
154

adopted for PM (then measured as TSP) was 25 tpy, which 

represented an emissions rate for which EPA modeled impacts that

represented about 4 percent of the TSP 24-hour NAAQS and about 

28 percent of the 24-hour TSP increment. Thus, EPA considered 

it acceptable under the de minimis assessment for PM that a 

source of particulate matter capable of consuming around 28 

percent of the applicable 24-hour TSP increment could be 

exempted from the requirements to complete a comprehensive

source impact analysis for the PM NAAQS and increments. 45 FR 

at 52708.

In looking at the amount of increment that could be 

consumed by a source that is ultimately exempted from having to 

complete a comprehensive modeling analysis, it should be pointed 

out that the maximum modeled concentration typically occurs in a

relatively limited area, as compared to the entire modeling 

domain. In particular, for the short-term averaging periods, 

such as the 24-hour averaging period, modeled concentrations 

across the modeled area generally drop off from the peak value 

fairly rapidly, so that the peak modeled concentrations 

represent the source’s impact at only a relatively few receptors 

within the modeled area. In addition, it is important to note 

that the temporal and spatial conditions which lead to a maximum 

impact by one source are seldom the same for other sources, such 
155

that maximum impacts of individual sources do not typically 

occur at the same location.

Thus, in an area where several sources can demonstrate that 

their modeled impacts are de minimis, it generally should not be 

assumed that their individual maximum (albeit de minimis) 

impacts on the increment are additive. For example, four

sources with de minimis PM2.5 impacts, each consuming 12 percent 

of the 24-hour PM2.5 increment, would not necessarily consume 

almost half of the 24-hour increment. Increment consumption is 

determined by the cumulative impact of source emissions on each 

individual receptor or modeling point in the area of impact 

within the baseline area defined for the affected PSD sources.

The preamble for the August 7, 1980 final rule for PSD

included a description of a modeling analysis that EPA conducted

to illustrate that a number of major sources each making only a

de minimis emissions increase for SO2 could locate in an area (in 

that case, the Dayton area) and not cause a violation of either 

the applicable SO2 increment or NAAQS. In that particular case, 

the modeling indicated that the maximum aggregate increment 

consumption for 37 sources emitting 40 tpy of SO2 (the de minimis

emissions rate for SO2) would have a cumulative impact at any 

location of less than 1.5 μg/m3 on a 24-hour basis—well below the 

NAAQS and increments for SO2. 45 FR 52708.
156

With regard to the commenter’s recommendation that we place 

limitations on the use of SILs, we earlier provided an example 

of when it might be appropriate to require a modified source to 

mitigate its contribution to a violation of a NAAQS or increment 

even when the predicted ambient impact of the proposed emissions 

increase would result in what is normally considered to be de 

minimis. In addition, we have historically cautioned states 

that the use of a SIL may not be appropriate when a substantial 

portion of any NAAQS or increment is known to be consumed. We 

have indicated elsewhere in this preamble that States are not 

required to adopt the SILs for PM2.5 in this final rule. At 

their discretion they may choose not to rely on SILs to screen 

applicants or they may establish more stringent values 

contingent upon a demonstration that such values represent de 

minimis values for their permitting purposes.

Finally, it should be noted that while a source having only

de miminis impacts may not be required to complete a 

comprehensive source impact analysis, the emissions from such 

sources are still considered to consume increment and would be 

counted as part of the next increment analysis required to be 

completed by a PSD applicant in that same area, or by the state 

under a periodic increment review.

3. Relationship Between SILs and AQRVs 

While commenters generally supported EPA’s position that 
157

the SILs should not be used in any way to determine effects of 

emissions increases on the AQRVs in a Class I area, two 

commenters urged that the de minimis concentration be used for 

analyzing Class I area impacts under certain circumstances. 

That is, they believed that the SILs should be used to determine 

the need for a Class I area air quality analysis when an FLM has 

not identified a specific AQRV related to the pollutant under 

evaluation or obtained ambient monitoring data to confirm that 

predicted concentrations from air dispersion models are 

representative of actual AQRV impacts in the Class I area. The 

commenters claimed that without this flexibility, applicants 

would be required to conduct complex and extensive Class I air 

dispersion modeling without any 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 if potentially costly controls should be 

required. 

These commenters appear to be suggesting that an FLM may 

needlessly call for an analysis of a particular Class I area, 

involving “complex and extensive Class I area dispersion 

modeling” despite the fact that no AQRV has been identified for 

that Class I area. We agree that a Class I analysis in the 

absence of any known AQRVs would be unnecessary because any 

demonstration of an adverse impact must be made with respect to 
158

a pollutant adversely affecting an AQRV. We believe, however, 

that such analyses would be avoided under the procedures set 

forth in section 165(d)(2)(C) of the Act which require that a 

notice be filed alleging that a proposed source may cause or 

contribute to adverse effects, and identifying the adverse 

impact. Insofar as the FLM must also demonstrate “to the 

satisfaction of the State that emissions from such facility will 

have an adverse impact on the air quality related values,” it 

would be difficult to require the source to undertake any kind 

of detailed analysis in the absence of an AQRV on which such 

adverse impacts must be demonstrated. Thus, we have concluded 

that it is not necessary to use the SILs as a safeguard against 

unnecessary Class I area analyses. Instead, we believe that the 

need for a Class I analysis, other than the required analysis of 

the NAAQS and Class I increments (both for which the SILs were 

intended to be used), should be based on the potential for 

adverse effects on an AQRV that the FLM has identified and 

believes could be affected by a pollutant that would be emitted 

by the proposed project. 

4. Form of the SILs

One commenter (0038) stated that “the Proposal does not 

indicate how the proposed PM2.5 SILs are to be interpreted.” 

This commenter believed that “the form of the SILs should be 

consistent with the form of the PM2.5 NAAQS” adding that “the 
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current PM2.5 NAAQS requires that compliance with the 24-hour and 

annual standards be determined using 3-year averaging.” 

Specifically, “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 highest-8

th high value) of 24-hour 

concentrations.”

We agree that the form of the standard should be given 

consideration and we will address this in forthcoming guidance 

on how the PM2.5 SILs are to be interpreted. 

5. SILs for Other Pollutants

In proposing Option 1, we noted that many who commented on 

the 1996 NSR Reform proposal supported this approach and 

believed that the proposed PM10 SIL values would serve as 

appropriate de minimis values. In fact, we are aware that many 

states have been using these proposed SILs for PM10 as screening 

tools since 1996 or earlier. 

Regarding the proposed Class I SILs under Option 1, we 

expressed our belief that where a proposed source consumes less 

than 4 percent of the Class I increment, the source’s impact is 

sufficiently low so as not to warrant requiring the source to 

carry out a detailed analysis of the combined effects of the 

proposed source and all other increment-consuming emissions in 

the area. 72 FR 54112, September 21, 2007, at 54140. We 
160

previously used a similar rationale to establish the significant 

emissions rates for PSD applicability purposes, concluding in 

part that emissions rates that resulted in ambient impacts less 

than 4 percent of the 24-hour standards for PM and SO2 were 

sufficiently small so as to be considered de minimis. 45 FR 

52676, August 7, 1980, at 52707-8. 

The original SIL values of 1.0 and 5.0 μg/m3 for TSP and 

PM10 were interpreted by EPA as representing the minimum amount 

of ambient impact that is significant. This formed the basis of 

the proposed PM2.5 SIL values of 1.0 and 5.0 μg/m3 for the annual 

and 24-hour averaging periods for Class II and III areas. 

The SILs currently appear in EPA’s regulations at 40 CFR 

51.165(b). That particular NSR regulation provides that states 

must include a preconstruction review permit program for any new 

major stationary source or major modification that proposes to 

locate in an attainment or unclassifiable area and would cause 

or contribute to a violation of the NAAQS. These values, added 

to 40 CFR 51.165(b) on July 1, 1987, have previously been 

referred to as “significant ambient impact concentrations” and 

are used to enable a source to determine whether its emissions 

would cause or contribute to a NAAQS violation at “any locality 

that does not or would not meet the applicable national 

standard.” 52 FR 24672, April 2, 1985, at 24688.

In 1985, when EPA proposed to add “significant ambient 
161

impact levels” for PM10, we also indicated that for PSD purposes 

the requirements under section 51.165(b)23 “would be applied to 

all applicable PSD requirements.” The EPA has since applied

these values in other analogous circumstances under the PSD 

program. Based on EPA interpretations and guidance, SILs have

also been widely used in the PSD program as a screening tool for 

determining when a new major source or major modification that 

wishes to locate in an attainment or unclassifiable area must 

conduct a more extensive air quality analysis to demonstrate 

that it will not cause or contribute to a violation of the NAAQS 

or PSD increment in the attainment or unclassifiable area. The 

SILs are also used to define the extent of the Significant 

Impact Area where, using air dispersion models and ambient 

monitoring data, a cumulative source impact analysis accounting 

for emissions changes from affected sources is performed.24 See

the 2007 NPRM preamble for additional information on the history 

of EPA’s guidance related to SILs (72 FR 54138-39).

In the 1996 NSR Reform proposal, we proposed to add the 

SILs for PM10 and other pollutants already contained in 40 CFR 

 

23 In 1985, the requirements now contained in 40 CFR 

51.165(b) were contained in 40 CFR 51.18(k), which was later 

part of a major restructuring of the part 51 SIP requirements. 

24 In the case of a NAAQS compliance analysis, all sources 

in the area are considered to contribute to the air quality 

levels; for increments, however, “all” refers only to those

sources whose emissions, in whole or in part, consume PSD 

increment for a particular pollutant.
162

51.165(b)(2) directly into the PSD regulations at 40 CFR 51.166 

and 52.21. Because the SILs in 40 CFR 51.165(b) did not include 

thresholds for Class I areas, we proposed to set Class I SILs at 

the level of 4 percent of the respective Class I increments. 

Thus, for PM10, the proposed Class I SILs were 0.2 μg/m3 (annual) 

and 0.3 μg/m3 (24-hour), and the proposed Class II and III SILs 

were 1.0 μg/m3 (annual) and 5.0 μg/m3 (24-hour). The EPA has not 

yet taken final action on the 1996 proposal on SILs for 

pollutants other than PM2.5; therefore, we rely upon our 

longstanding policy to use those values, as codified in

40 CFR 51.165(b)(2), for PSD permitting.

VII. Final Action on the PM2.5 SMC

A. EPA’s Determination on the PM2.5 SMC

As with the increments and SILs for PM2.5, we proposed three 

different options for establishing an SMC for PM2.5. The first 

option, referred to as the “lowest detectable concentration” 

approach, relied on the method we used in 1980 to develop the 

SMCs for all the criteria pollutants then subject to PSD. This 

particular method focused on development of the SMC value based 

on the current capability of providing a meaningful measure of 

the pollutants. See relevant discussion later in this section 

and at 45 FR 52676, August 7, 1980 at 52710. Options 2 and 3, 

called the “PM2.5 to PM10 emissions ratio” and the PM2.5 to PM10

NAAQS ratio,” respectively, used the SMC for PM10 as the base for 
163

multiplying the emissions and NAAQS ratios to derive an SMC for 

PM2.5. See 72 FR at 54141. The three proposed options yielded 

the following numerical levels for the SMC:

 Option 1: 10 μg/m3, (24-hour average);

 Option 2: 8.0 μg/m3 (24-hour average); and 

 Option 3: 2.3 μg/m3 (24-hour average).

We are taking final action on the SMC for PM2.5 using the 

“lowest detectable concentration” approach (Option 1). However, 

we have determined that it is necessary to modify the 

methodology to reflect “current capability” with respect to the 

measurement of ambient PM2.5 concentrations. The result of such 

modification is that the SMC value in this final rule is 

different from (more stringent than) the proposed level. The 

revised value is 4 μg/m3 (24-hour average). Our basis for the

modified approach and the resulting lower value is described in 

greater detail later in this section.

The EPA and its delegated reviewing authorities will use 

the PM2.5 SMC to determine when it may be appropriate to exempt a 

proposed new major stationary source or major modification from 

the ambient monitoring data requirements under the PSD rules. 

Similarly, states with EPA-approved PSD programs that adopt the 

SMC for PM2.5 may use the SMC, once it is part of an approved 

SIP, to determine when it may be appropriate to exempt a 

particular major stationary source or major modification from 
164

the monitoring requirements under their state PSD programs (see,

e.g., 40 CFR 51.166(m)(1)). 

B. Response to Comments Concerning the SMC

1. Legal Issues

Under the Act and EPA regulations, an applicant for a PSD 

permit is required to gather preconstruction monitoring data in 

certain circumstances. Section 165(a)(7) of the Act calls for 

“such monitoring as may be necessary to determine the effect 

which emissions from any such facility may have, or is having, 

on air quality in any areas which may be affected by emissions 

from such source.” 42 U.S.C. 7475(a)(7). In addition, section 

165(e) of the Act requires an analysis of the air quality in 

areas affected by a proposed major facility or major 

modification and calls for gathering 1 year of monitoring data 

unless the reviewing authority determines that a complete and 

adequate analysis may be accomplished in a shorter period. 42 

U.S.C. 7575(e)(3). These requirements are codified in EPA’s PSD 

regulations at 40 CFR 51.166(m) and 52.21(m). 

In 1980, EPA adopted regulations that included pollutantspecific SMCs as a screening tool for sources to determine 

whether they should conduct site-specific preconstruction 
165

ambient monitoring.25 We explained our position that it was 

appropriate to exempt sources from preconstruction monitoring 

requirements for a pollutant if the source could demonstrate 

that its ambient air impact was less than a value known as the 

Significant Monitoring Concentration or SMC. At the time the

SMCs were adopted, EPA described them as “air quality 

concentration de minimis level[s] for each pollutant ... for the 

purpose of providing a possible exemption from monitoring 

requirements.” The EPA explained that it believed there was 

“little to be gained from preconstruction monitoring” where a 

source could show that its projected impact of a pollutant 

within the affected area was below the de minimis concentration

for that pollutant. 45 FR 52676, August 7, 1980, at 52710). 

One commenter (0040) opposed our proposed establishment of 

any SMC for PM2.5, claiming that SMCs in general are contrary to 

the Act. The commenter stated that “in Section 165(e) Congress 

mandated a full year of continuous air quality monitoring for 

each major source subject to the PSD program.” With this in 

mind, the commenter indicated that there are no exceptions, 

other than the limited statutory provisions, discussed above, 

which allow for less than a year’s worth of monitoring based on 

 

25 The provision for the monitoring exemption was originally 

promulgated at 40 CFR 51.24(i)(8) and 52.21(i)(8); it should be 

noted, however, that this provision is now found at 40 CFR 

51.166(i)(5) and 52.21(i)(5). 
166

a determination that a complete and adequate analysis of such 

purposes may be accomplished in a shorter period. The commenter 

then argued that “the allowance for a ‘shorter period’ hardly 

amounts to authority to waive monitoring entirely, which is what 

EPA’s SMC proposal would do.”

As with the SMCs adopted by EPA in 1980, the SMCs that we 

proposed for PM2.5 are supported by the de minimis doctrine set 

forth in the Alabama Power v. Costle opinion. Like the other 

pollutants for which EPA has promulgated SMCs, EPA believes 

there is little to be gained from preconstruction monitoring of 

PM2.5 concentrations when the increased emissions of PM2.5 from a 

proposed source or modification have a de minimis impact on 

ambient concentrations of PM2.5. If a source can show through 

modeling of its emissions alone that its impacts are less than 

the corresponding SMC, there is little to be gained by requiring 

that source to collect additional monitoring data on PM2.5

emissions to establish background concentrations for further 

analysis. 

Therefore, in developing the three proposed options for an

SMC, EPA sought to use methods that would identify levels 

representing a de minimis or insignificant impact on PM2.5

ambient air quality that makes the collection of additional 

monitoring data extraneous. 
167

2. Level of the SMC

As indicated earlier, the SMC for PM2.5 in this final rule 

is 4 μg/m3, 24-hour average. This value may be used by 

permitting authorities to determine when they may exempt a 

proposed major stationary source or major modification for PM2.5

from the air quality monitoring requirements for PM2.5 under 40 

CFR 51.166. The EPA and its delegated state/local programs will 

also use this new value under the federal PSD program at 40 CFR 

52.21. 

We proposed three options for developing the SMC for PM2.5; 

each option yielded a notably different concentration value. In 

choosing between the three options, EPA proposed to select the

option that reflected the degree of ambient impact on PM2.5

concentrations that could be considered truly de minimis and 

used to justify exempting a source from the requirement to 

gather 1 year of ambient monitoring data for PM2.5. Ultimately, 

we have selected the “lowest detectable concentration” approach 

(Option 1) that relies directly upon ambient monitoring 

measurement sensitivity and precision. That is, if the 

predicted source impact or estimated existing air quality in an 

area is estimated to be below a concentration that can be 

accurately measured, then it would not be reasonable to require 

a source to attempt to collect such ambient data. 

In 1980, EPA determined the SMCs based on the then current 
168

capability of providing a meaningful measure of ambient

pollutant concentrations. The EPA promulgated values that 

represented five times the lowest detectable concentration in 

ambient air that could be measured by the instruments available 

for monitoring the pollutants. 45 FR 52710. The factor of 

“five” took into account the measurement errors associated with 

the monitoring of these low pollutant levels or small 

incremental changes in concentration. These measurement errors 

were said to arise from various sources, such as sample 

collection, analytical measurement, calibration, and 

interferences. See the internal EPA memorandum from Rehme, K. 

A., to Warren Peters dated May 20, 1980, contained in the docket 

for this rulemaking. Accordingly, in the 2007 proposal for 

PM2.5, we voiced our belief that this was a reasonable approach, 

since it was also used for PM10 and TSP. Id.

Eight commenters expressed support for the SMC based on 

Option 1, albeit at the higher level as originally proposed. In 

some cases, it is not clear whether these commenters supported 

the particular approach (i.e., link to lowest detectable level) 

or the fact that the calculated value was simply the highest 

value of the values proposed under three options. Clearly, some 

of the commenters indicated their support for the approach 

because it is consistent with the approach used for setting the 

original SMCs in 1980. Two commenters opposed the use of Option 
169

1 because it resulted in a value that was too high. These 

latter commenters noted that the SMC derived via Option 1 (10 

μg/m3, 24-hour average) was greater than the proposed 24-hour 

PM2.5 increment for Class II areas and argued that such an 

outcome is inappropriate. We believe that this important 

concern is adequately addressed by the new level of the SMC.

Several commenters supported the levels derived from either 

Option 2 or Option 3, but were concerned that the justification 

for choosing either of these values would need to be further 

explained. Some of these commenters were specifically concerned 

about the use of a 0.8 PM2.5-to-PM10 emissions ratio which, they 

argued, relied on inventory data that did not adequately address 

all sources that would likely affect ambient concentrations of 

PM2.5 in an area.

We conclude that Option 1 is the most supportable option 

for defining the SMC for PM2.5. The ability to accurately

measure ambient 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. Our original 

concern was that, while Option 1 linked the SMC directly to the 

concept of a minimum detectable concentration (in order to 

identify de minimis monitoring circumstances), the value 

originally derived from that approach in the 2007 NPRM was high 
170

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 for PM2.5. Our re-examination utilized 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 micro-gravimetric 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 that ‘2’ at 
171

the concentration equal to the MDL of 2 μg/m3.”26 Accordingly, 

the lowering of the uncertainty factor from “five” to “two” 

under Option 1 yields an SMC of 4 μg/m3 PM2.5, 24-hour average, 

rather than the proposed concentration of 10 μg/m3. 

We conclude that the modified level of 4 μg/m3 PM2.5, 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 capability 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.

C. Correction of Cross Reference in PSD Ambient Monitoring 

Requirements

In the 2007 NPRM, we proposed to take final action to 

correct a cross reference contained in paragraph (i) of the part 

51 and 52 PSD regulations. Specifically, at the time of the 

proposal, paragraphs (ii) and (iii) in 40 CFR 51.166(i)(5), and 

paragraph (ii) in 40 CFR 52.21(i)(5), each referred to 

 

26 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.
172

concentrations listed in paragraph (i)(8)(i) of both 

regulations. However, there is no paragraph (i)(8)(i) in

existing 40 CFR 51.166, and no concentration values are 

contained in existing section (i)(8)(i) of 40 CFR 52.21. The 

cross reference in these provisions was intended to reference 

the SMCs in paragraph (i)(5)(i) of the two PSD regulations, but 

EPA failed to make this change when the paragraphs were 

renumbered in an earlier rulemaking. We did not receive any 

comments concerning this proposed corrective action. We made 

the necessary correction as part of the May 16, 2008 final rule 

for PM2.5 NSR implementation (see 73 FR 28321 at 28348 and 

28349); therefore it is not necessary to take any further action 

in this final rule with regard to the proposed correction. 

VIII. Dates Associated With Implementation of the Final Rule

This section describes the key dates that we have 

established for implementing the final rule. In the 2007 NPRM, 

we indicated that different dates appeared to be appropriate for 

implementing the PM2.5 increments, each date depending on the 

legal authority that we relied upon to promulgate them. We 

described and took comment on some alternative effective dates 

for increments, as well. In addition, we discussed and took 

comment on potential implementation dates for the SILs and SMC 

components of the proposed rule, which we indicated were not 

subject to the same statutory considerations as the increments. 
173

We received a number of comments on the different proposed 

dates. We carefully considered these comments in selecting the 

dates described below for the final rule. Some of the 

significant comments and our responses to those comments are 

provided below. The remaining comments and our responses are 

contained in the Response to Comments document included in the 

docket for this rulemaking. 

A. Effective Date of the Final Rule

In the 2007 NPRM, we took comments on the effective date of 

the final rule by presenting the different options available for 

implementing the PM2.5 increments. Under Option 1 for developing 

the increments, we stated that section 166(b) of the Act 

specifies that new increments promulgated pursuant to section 

166(a) are to become effective 1 year following their 

promulgation. In contrast, there is no such 1-year delay or any 

other date prescribed for increments promulgated in accordance 

with section 166(f) of the Act, upon which we based Options 2 

and 3 for the annual PM2.5 increments. Thus, increments 

promulgated under Option 1, which relies on the procedural 

provisions of section 166(b) of the Act, would normally be 

subject to a 1-year delay in implementation, while increments 

promulgated under either Option 2 or 3, relying on section 

166(f) of the Act, could follow a 30- or 60-day effective date, 

typical of the effective date for most new rules in general. In 
174

either case, our consideration of the effective date for the new 

increments assumed that the selected date would also be the 

effective date of the final rule. 

In the 2007 NPRM, we took comment on some alternative 

approaches to establishing the effective date for PM2.5

increments. Specifically, while proposing a 1-year effective 

date under Option 1, we requested comment on whether we could 

promulgate these increments under section 166(a) of the Act with 

an effective date of only 60 days. See 72 FR 54142.

Nine commenters supported our proposal to establish the 

effective date of the part 51 and 52 PSD regulations for PM2.5 as

1 year from the date of publication. Alternately, two 

commenters encouraged us to apply the 60-day effective date, 

while three other commenters supported other effective dates, as 

described in this section.

Seven industry and industry association commenters 

supported our proposal to make the final rule for PM2.5 increments 

effective 1 year after promulgation. Most of these commenters 

cited the additional time necessary to develop the needed PM2.5 

inventories needed for implementation of the PM2.5 PSD program.

Two of the commenters urged EPA to allow state programs 

sufficient time to adopt increments, particularly if condensable 

particulate matter is included in the increment and its 

analysis. These commenters stated that the federal rule should 
175

not be effective for 1 year. (They also stated that states 

should have 3 years for the associated SIP revisions.) These 

same commenters added that this delay 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 specifically 

voiced 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). This commenter also explained that this 

additional time may give the Agency time to promulgate better 

measurement methods for sources of condensable particulate

matter.

Another of these commenters noted that, at the time of the 

proposal, the NSR portion of the CAFPIR had not yet been 

promulgated, and that states would need time to incorporate that 

rule as well as the requirements of the proposal into their 

SIPs. This commenter added 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. 

 In contrast, the two commenters supporting the shorter 

effective date encouraged us to apply the 60-day period for the 

effective date under whatever option is finalized. One of these 
176

commenters urged us to take measures to expedite the 

implementation of the PM2.5 final rule and suggested that we 

choose the shortest of the proposed effective dates which are 

allowed under any of the applicable regulations. This commenter 

indicated 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 be applied under 

EPA’s preferred option. 

In light of our decision to promulgate PM2.5 increments 

under the authority of section 166(a) of the Act (proposed 

Option 1), we are faced with the decision as to how to most 

effectively implement the long-awaited PM2.5 increments, 

recognizing that the Act provides for a 1-year implementation 

delay. We have concluded that it is most appropriate to follow 

the plain language 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 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 at 1 year from the date of 
177

promulgation of this final rule. We are doing this by setting 

the “trigger date” for PM2.5 as [INSERT DATE 1 YEAR AFTER DATE OF 

PUBLICATION IN THE FEDERAL REGISTER]. 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). At the same time, we are 

establishing an effective date for the other provisions, i.e., 

the SILs and SMC for PM2.5, in this final rule as [INSERT DATE 60 

DAYS FROM THE DATE OF PROMULGATION IN THE FEDERAL REGISTER], 

which is 60 days from the date of promulgation. This will

enable the implementation of these key elements of this rule

under the federal PSD program as soon as possible. 

1. State PSD Programs

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 [INSERT DATE 21 MONTHS AFTER DATE OF PUBLICATION IN THE 

FEDERAL REGISTER]. Section 166(b) also specifies that we must 

approve or disapprove these revisions within 25 months of 
178

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.

It is very unlikely that states will be able to 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 [INSERT DATE 1 YEAR AFTER DATE OF PUBLICATION IN THE FEDERAL 

REGISTER]. Therefore, there is likely to be a period of time 

after [INSERT DATE OF PUBLICATION 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. 
179

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 

[INSERT DATE OF PUBLICATION OF THIS 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. 

2. Federal PSD Program

The federal PSD regulations under 40 CFR 52.21 apply where 

states do not have approved PSD programs and in Indian lands. 

In such cases, either EPA implements the PSD program or the 

state will implement it under authority granted by EPA through a 

delegation agreement. 

We proposed to begin implementing the federal PSD program

for PM2.5 on the effective date of the final rule, i.e., either 1

year from the date of publication in the Federal Register or 60 

days from date of publication, if we developed the new 

increments pursuant to proposed Option 1. Alternatively, we 
180

requested comment on whether we should delay implementation of 

the federal PSD program until 25 months after promulgation, 

which is the latest date by which EPA is required to approve 

state SIP revisions. This is the same approach we took in 1988 

to implement the then new NO2 increments. See 53 FR 40658, 

October 17, 1988. We did not propose the 24-month delay for the 

PM2.5 increments because of the significant delay that had

already occurred between the time we promulgated the PM2.5 NAAQS 

and the time the PM2.5 increment rulemaking would be finalized. 

However, we sought comment on this alternative approach because 

we recognized that it might not be equitable to begin 

implementation of the new program requirements in those few 

areas where the federal program applies before the majority of 

states are required to implement the program. 

Two commenters (0026, 0043) urged EPA to hold off 

implementation of state programs administered under the federal 

PSD program in order to provide a uniform and consistent 

national approach. One state agency (0015) supported 

implementing the federal PSD program with a delayed effective 

date of 1 year after the effective date of the final rule 

instead of 60 days.

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. That is, the revised 
181

regulations at 40 CFR 52.21 will become effective in 60 days, on 

[INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION 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 this preamble. 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. However, PSD sources subject to 

PM2.5 that receive their PSD permit after the date of publication 

of this final rule will be considered to consume PM2.5 increments 

by virtue of the fact that they will commence construction after 

the major source baseline date for PM2.5, which is the date of 

publication of this final rule. 

Thus, sources in an area subject to the federal PSD program 

for PM2.5 will be able to use the SILs and SMC as screening tools 

for the required PM2.5 NAAQS compliance demonstration, but in 

most cases will not be required to submit a PM2.5 increment 

analysis as part of a complete PSD permit application for a 

federal PSD permit unless the application is submitted on or 

after [INSERT DATE 1 YEAR AFTER DATE OF PUBLICATION IN THE 

FEDERAL REGISTER]. On or after that date, when an applicant 

submits a complete PSD permit application that is required to 
182

address PM2.5 under the federal PSD program, that first 

application will establish the minor source baseline date for 

PM2.5 in the applicable attainment or unclassifiable area. 

As with the state PSD program requirements, prior to the 

establishment of the minor source baseline date in an area, 

emissions increases from minor sources in the area will be 

counted toward the baseline concentration, rather than to the

PM2.5 increment. As described earlier, the emissions from major 

stationary sources that commence construction after the major 

source baseline date, regardless of the date on which their PSD

application is submitted, must be counted toward consumption of

the PM2.5 increments. While these sources will not be required 

to submit an increment analysis for PM2.5 as part of their 

complete application as long as they receive their PSD permit 

before the trigger date for PM2.5 (see discussion that follows in 

section VIII.B), the emissions increases resulting from the

permitting of these sources ultimately must be counted toward 

the PM2.5 increments when the first PSD permit application 

submitted after the trigger date establishes the minor source 

baseline date for the area of concern, and in all subsequent 

PM2.5 increment analyses for that area.

B. Transition Period 

In the 2007 NPRM, we proposed a transition period to 

clarify when PSD permit applications must contain an increment 
183

analysis demonstrating compliance with the PM2.5 increments

following the date the PM2.5 increments become effective in any 

state or federal PSD program. Specifically, we proposed to 

establish a grandfathering provision to allow complete 

applications submitted before the increment effective date, but 

for which the permit had not yet been issued by the effective 

date, to continue being processed using the PM10 Surrogate Policy 

to satisfy the requirement to demonstrate compliance with the 

new PM2.5 requirements. The grandfathering provision for PM2.5

was originally proposed in the 2007 NPRM at 40 CFR 51.166(i)(10) 

and 40 CFR 52.21(i)(11) for state and federal PSD programs, 

respectively. See 72 FR 54112, September 21, 2007, at 54149 and 

54154.

Three commenters (0015, 0022, 0026) supported the proposed 

grandfathering provision for sources that submitted a complete 

application before the effective date of the applicable PSD 

rules. Another commenter (0043) felt that it was reasonable to 

allow states a choice between using PM10 or PM2.5 increments 

during a transition period including SIP approval, where 

applicable.

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 
184

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 [INSERT DATE 1 YEAR AFTER 

DATE OF PUBLICATION 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 incrementconsuming 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.

Thus, we are requiring each source that receives its PSD 

permit after the effective date of the PM2.5 increments, 

regardless of when the application was submitted, to provide a 

demonstration that the source’s proposed emissions increase, 

along with other increment-consuming emissions, will not cause 

or contribute to a violation of the PM2.5 increments. 

Under this final rule, sources applying for a PSD permit 
185

under the federal PSD program after the major source baseline 

date for PM2.5 (i.e., after the date of publication of this final 

rule), but before the PM2.5 increments become effective (i.e., 

the date 1 year after publication of this final rule), will be 

considered to consume PM2.5 increment. While EPA will not 

require any such source to include a PM2.5 increment analysis as 

part of its initial PSD application, an increment analysis 

ultimately will be required before the permit may be issued if 

the date of issuance will occur after the trigger date, when the 

PM2.5 increments become effective under the federal PSD program. 

Finally, for the same reasons that we are not adopting the 

proposed transition period that would have exempted PSD 

applicants with pending permit applications from demonstrating 

compliance with the PM2.5 increment requirements under the 

federal PSD program, we have decided not to provide an option 

for states to apply a transition period under 40 CFR 51.166. We 

believe it is appropriate for all increment-consuming sources 

subject to PM2.5 to demonstrate compliance with the PM2.5

increments when the required permit is issued after the PM2.5

increments become effective in the state’s PSD regulations. 

C. SILs and SMC for PM2.5

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 
186

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. 

One state/local commenter (0033) agreed that the SILs and 

SMCs should not be a required element of the PSD SIP. Another 

state/local commenter (0015) agreed with our proposal, but 

stated that EPA has the authority to include SILs and SMCs as 

minimum program requirements per the opinion set forth in 

Alabama Power v. Costle. This commenter added 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 the source’s emissions would 

“cause or contribute” to a NAAQS or increments violation under 

section 165(a)(3) of the Act. 

Two commenters (0035, 0037) disagreed with our proposed 

position and argued that SILs and SMCs should be mandatory 
187

elements of a state PSD program. One of these commenters argued 

that the requirement to model without the use of 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 other opposing commenter 

stated that the increments, SILs, and SMCs need to be adopted as 

a single regulatory approach because the SILs and SMCs define 

when additional work is needed to ensure that PSD requirements, 

such as maintaining adequate increment, are met. This commenter 

added that there is 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.

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 SIPrelated deadline for revising their PSD programs to add these 
188

screening tools.

Using the SILs for PM2.5, when a proposed major new source 

or major modification of PM2.5 predicts (via air quality 

modeling) an impact less than the PM2.5 de minimis value, the 

proposed source or modification is not considered to have a 

significant air quality impact and would not need to complete a 

cumulative impact analysis involving an analysis of other 

sources in the area. Also, a source with a de minimis ambient 

impact would not be considered to cause or contribute to a 

violation of either the PM2.5 NAAQS or increments. 

The PM2.5 SILs will become effective under the federal PSD 

program on the effective date of this final rule, that is, on

[INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION IN FEDERAL 

REGISTER], when either EPA, or a state acting under a delegation 

of EPA’s authority, implements the revised PSD permitting 

requirements for PM2.5 pursuant to 40 CFR 52.21. The SILs will 

be for use initially with the compliance demonstration for the 

PM2.5 NAAQS, and later for the PM2.5 increment analysis, under the 

federal PSD program. We emphasize, however, that the PM2.5 SILs 

are not intended to be used as part of the determination of 

adverse impacts on AQRVs for PM2.5 in Class I areas. 

Similarly, we intend to use the PM2.5 SMC (4 μg/m3, 24-hour 

average) as a screening tool in the federal PSD permit program

beginning on [INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION IN 
189

THE FEDERAL REGISTER]. Accordingly, when either the modeled 

PM2.5 impact of, or the existing ambient air quality within the 

area of, the proposed new major source or major modification is 

less than the PM2.5 SMC, the reviewing authority may exempt the 

source or modification from the monitoring data requirements for 

PM2.5 under 40 CFR 52.21(m).

IX. Other Regulatory Changes

The Act provides that the PSD regulations apply to areas 

designated as “attainment” or “unclassifiable” as defined by the 

Act. When the original regulations were written, the Act

provisions for designating areas as either “attainment” or 

“unclassifiable” were contained in sections 107(d)(1)(D) and 

(E), respectively. In 1990, Congress revised section 107 and 

changed the relevant paragraphs defining “attainment” and 

“unclassifiable” areas to sections 107(d)(1)(A)(ii) and (iii), 

respectively. In accordance with these statutory changes, we 

are correcting the references to the statutory classifications 

contained in the existing PSD rules to match the revised

paragraphs in the Act. See revised 40 CFR 51.166(b)(14)(iii)(a) 

and (15)(i) and (ii), and 40 CFR 52.21(b)(14)(iii)(a) and 

(15)(i) and (ii).

In adding the SILs for PM2.5 in this final rule, we 

restructured paragraph (k) (“Source impact analysis”) in the 

existing PSD regulations at 40 CFR 51.166 and 52.21. Under the 
190

restructuring of paragraph (k), old paragraph (k)(2) is now 

paragraph (k)(1)(b). To accommodate this restructuring change, 

we are also revising grandfathering provisions that are 

contained in existing paragraphs (i)(8) and (i)(9) at 40 CFR

51.166, and paragraphs (i)(9) and (i)(10) at 40 CFR 52.21, which 

contained references to requirements contained in paragraph 

(k)(2). As revised, the grandfathering provisions now reference 

new paragraph (k)(1)(b).

X. Statutory and Executive Order Reviews

A. Executive Order 12866 - Regulatory Planning and Review

Under Executive Order 12866 (58 FR 51735, October 4, 1993), 

this action is a “significant regulatory action” because it 

raises novel legal or policy issues arising out of legal 

mandates, the President’s priorities, or the principle set forth 

in the Executive Order. Accordingly, EPA submitted this action 

to the Office of Management and Budget (OMB) for review under 

Executive Order 12866 and any changes made in response to OMB 

recommendations have been documented in the docket for this 

action.

B. Paperwork Reduction Act 

The information collection requirements in this rule have 

been submitted for approval to the OMB under the Paperwork 

Reduction Act, 44 U.S.C. 3501 et seq. The information 

collection requirements are not enforceable until OMB approves 
191

them.

Pursuant to title I, part C, of the Act, the PSD program 

requires the owner or operator to obtain a permit prior to

either constructing a new major stationary source of air 

pollutants or making a major modification to an existing major 

stationary source. The information collection for sources under

PSD results from the requirement for owners or operators to 

submit applications for NSR permits. In some cases, sources 

must conduct preconstruction monitoring to determine the 

existing ambient air quality. For reviewing authorities, the

information collection results from the requirement to process 

permit applications and issue permits, and to transmit 

associated information to EPA. The EPA oversees the PSD

program, and the information collected by sources and reviewing 

authorities is used to ensure that the program is properly

implemented.

The final rule will increase the PSD permitting burden for 

owners and operators of major stationary sources of PM2.5

emissions by adding PM2.5 increments to the list of existing 

increments for which air quality impact analyses must be carried 

out to track the amount of increment consumed by the proposed 

source and other sources in the area. Over the 3-year period 

covered by the Information Collection Request (ICR), we estimate 

an average annual burden totaling about 29,000 hours and $2.8 
192

million for all industry entities that will be affected by the 

final rule. For the same reasons, we also expect the final rule 

(when fully implemented) to increase burden for the state and 

local authorities reviewing PSD permit applications. In 

addition, there will be additional burden for state and local 

agencies to revise their SIPs to incorporate the proposed 

changes. Over the 3-year period covered by the ICR, we estimate 

that the average annual burden for all state and local reviewing 

authorities will total about 7,500 hours and $581,000. Burden 

is defined at 5 CFR 1320.3(b).

An agency may not conduct or sponsor, and a person is not 

required to respond to, a collection of information unless it 

displays a currently valid OMB control number. The OMB control 

numbers for EPA’s regulations in 40 CFR are listed in 40 CFR 

part 9. When this ICR is approved by OMB, the Agency will 

publish a technical amendment to 40 CFR part 9 in the Federal 

Register to display the OMB control number for the approved 

information collection requirements contained in this final 

rule.

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) generally requires an 

agency to prepare a regulatory flexibility analysis of any rule 

subject to notice and comment rulemaking requirements under the 

Administrative Procedure Act or any other statute unless the 
193

Agency certifies that the rule will not have a significant 

economic impact on a substantial number of small entities. 

Small entities include small businesses, small organizations, 

and small governmental jurisdictions. 

For purposes of assessing the impacts of this rule on small 

entities, “small entity” is defined as: (1) a small business as 

defined by the Small Business Administration’s regulations at 13 

CFR 121.201; (2) a small governmental jurisdiction that is a 

government of a city, county, town, school district or special 

district with a population of less than 50,000; and (3) a small 

organization that is any not-for-profit enterprise which is 

independently owned and operated and is not dominant in its 

field.

After considering the economic impacts of this final rule 

on small entities, I certify that this action will not have a 

significant economic impact on a substantial number of small 

entities. This final rule will not impose any requirements on 

small entities because small entities are not subject to the 

requirements of this rule.

D. Unfunded Mandates Reform Act 

This action contains no federal mandates under the 

provisions of Title II of the Unfunded Mandates Reform Act of 

1995 (UMRA), 2 U.S.C. 1531-1538 for state, local, or tribal 

governments or the private sector. The action imposes no 
194

enforceable duty on any state, local or tribal governments or 

the private sector. The final rules adds only a relatively 

small number of new requirements to the existing permit 

requirements already in place under the PSD program, since 

states are currently implementing a PM10 surrogate program 

pursuant to EPA guidance. Thus, this action is not subject to 

the requirements of sections 202 or 205 of UMRA.

This rule is also not subject to the requirements of 

section 203 of UMRA because it contains no regulatory 

requirements that might significantly or uniquely affect small 

governments. The final rule applies only to new major 

stationary sources and to major modifications at existing major 

stationary sources.

E. Executive Order 13132 - Federalism 

This final rule does not have federalism implications. It 

will not have substantial direct effects on the states, on the 

relationship between the national government and the states, or 

on the distribution of power and responsibilities among the 

various levels of government, as specified in Executive Order 

13132. The final rule makes relatively minor changes to the 

established PSD program, simply making it possible for states to 

implement PSD for PM2.5 instead of relying on PM10 as a surrogate.

Thus, Executive Order 13132 does not apply to this rule. In the 

spirit of Executive Order 13132, and consistent with EPA policy 
195

to promote communications between EPA and state and local 

governments, EPA specifically solicited comment on the proposed 

rule from state and local officials.

F. Executive Order 13175 - Consultation and Coordination with 

Indian Tribal Governments

This action does not have tribal implications, as specified 

in Executive Order 13175 (65 FR 67249, November 9, 2000). The 

final rule provides the elements to implement a PM2.5 PSD program 

in attainment areas. The Act provides for states to develop 

plans to regulate emissions of air pollutants within their 

jurisdictions. The Tribal Air Rule (TAR) under the Act gives 

tribes the opportunity to develop and implement Act programs to 

attain and maintain the PM2.5 NAAQS, but leaves to the discretion 

of the tribes the decision of whether to develop these programs 

and which programs, or appropriate elements of a program they 

will adopt. Thus, Executive Order 13175 does not apply to this 

action.

The EPA did reach out to national tribal organizations in 

2006 to provide a forum for tribal professionals to provide 

input to the rulemaking. However, not much participation or 

input was received.

G. Executive Order 13045 - Protection of Children from 

Environmental Health and Safety Risks 

This action is not subject to Executive Order 13045 (62 FR 
196

19885, April 23, 1997) because it is not economically 

significant as defined in Executive Order 12866, and because the 

Agency does not believe the environmental health or safety risks 

addressed by this action present a disproportionate risk to 

children. One of the basic requirements of the PSD program is 

that new and modified major sources must demonstrate that any 

new emissions do not cause or contribute to air quality in 

violation of the NAAQS.

H. Executive Order 13211 - Actions That Significantly Affect 

Energy Supply, Distribution, or Use 

This action is not a “significant energy action” as defined 

in Executive Order 13211 (66 FR 28355, May 22, 2001) because it 

is not likely to have a significant adverse effect on the 

supply, distribution, or use of energy. Further, we have 

concluded that this rule is not likely to have any adverse 

energy effects.

I. National Technology Transfer and Advancement Act 

Section 12(d) of the National Technology Transfer and 

Advancement Act of 1995 (NTTAA), Public Law No. 104-113, 12(d) 

(15 U.S.C. 272 note) directs EPA to use voluntary consensus 

standards in its regulatory activities unless to do so would be 

inconsistent with applicable law or otherwise impractical.

Voluntary consensus standards are technical standards (e.g., 

materials specifications, test methods, sampling procedures, and 
197

business practices) that are developed or adopted by voluntary 

consensus standards bodies. The NTTAA directs EPA to provide 

Congress, through OMB, explanations when the Agency decides not 

to use available and applicable voluntary consensus standards. 

This action does not involve technical standards.

Therefore, EPA did not consider the use of any voluntary 

consensus standards.

J. Executive Order 12898 – Federal Actions to Address 

Environmental Justice in Minority Populations and Low-Income 

Populations

Executive Order 12898 (59 FR 7629, Feb. 16, 1994)

establishes federal executive policy on environmental justice. 

Its main provision directs federal agencies, to the greatest 

extent practicable and permitted by law, to make environmental 

justice part of their mission by identifying and addressing, as 

appropriate, disproportionately high and adverse human health or 

environmental effects of their programs, policies, and 

activities on minority populations and low-income populations in 

the United States. 

The EPA has determined that this final rule will not have 

disproportionately high and adverse human health or 

environmental effects on minority or low-income populations 

because it does not affect the level of protection provided to 

human health or the environment. This final rule will provide 
198

regulatory certainty for implementing the preconstruction NSR 

permitting program for PM2.5. However, the requirements are 

similar to the existing requirements of the PM10 program and 

hence do not impact the human health or environmental effects.

K. Congressional Review Act

The Congressional Review Act, 5 U.S.C. 801 et seq., as 

added by the Small Business Regulatory Enforcement Fairness Act 

of 1996, generally provides that before a rule may take effect, 

the agency promulgating the rule must submit a rule report, 

which includes a copy of the rule, to each House of the Congress 

and to the Comptroller General of the United States. The EPA 

will submit a report containing this rule and other required 

information to the U.S. Senate, the U.S. House of 

Representatives, and the Comptroller General of the United 

States prior to publication of the rule in the Federal Register. 

A major rule cannot take effect until 60 days after it is 

published in the Federal Register. This action is not a “major 

rule” as defined by 5 U.S.C. 804(2). Nevertheless, this rule 

needs to be reviewed for the new increments being promulgated 

herein so that they can be scrutinized by Congress as intended 

under section 166(b) of the Act. Even though the PM2.5

increments will not become effective for 1 year, the final rule 

will become effective 60 days from the date of publication, that 

is, on [INSERT DATE 60 DAYS AFTER THE DATE OF PUBLICATION IN THE 
199

FEDERAL REGISTER], for the screening tools (SILs and SMC) being 

established in this rule.

XI. Judicial Review

Under section 307(b)(1) of the Act, petitions for judicial 

review of this action must be filed in the United States Court 

of Appeals for the District of Columbia Circuit by [INSERT DATE 

60 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER]. Any 

such judicial review is limited to only those objections that 

are raised with reasonable specificity in timely comments. 

Filing a petition for reconsideration by the Administrator of 

this final rule does not affect the finality of this rule for 

the purposes of judicial review nor does it extend the time 

within which a petition for judicial review may be filed, and 

shall not postpone the effectiveness of such rule or action. 

Under section 307(b)(2) of the Act, the requirements of this 

final action may not be challenged later in civil or criminal 

Page 200 of 216 – 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)

proceedings brought by us to enforce these requirements. 

XII. Statutory Authority

The statutory authority for this final action is provided 

by sections 101, 160, 163, 165, 166, 301, and 307(d) of the Act 

as amended (42 U.S.C. 7401, 7470, 7473, 7475, 7476, 7601, and 
200

7607(d)).

List of Subjects

40 CFR Part 51

Administrative practices and procedures, Air pollution 

control, Environmental protection, Intergovernmental relations.

40 CFR Part 52

Administrative practices and procedures, Air pollution 

control, Environmental protection, Intergovernmental relations.

___________________________ 

Dated:

__________________________

Lisa P. Jackson,

Administrator.
201

For the reasons set out in the preamble, title 40, chapter I of 

the Code of Federal Regulations is amended as follows:

PART 51 - [Amended]

1. The authority citation for part 51 continues to read as 

follows:

Authority: 23 U.S.C. 101; 42 U.S.C. 7401 - 7671q.

Subpart I - [Amended]

2. Section 51.165 is amended by revising the table in 

paragraph (b)(2) to read as follows:

§51.165 Permit requirements.

* * * * *

(b) * * *

(2) * * *

Pollutant Annual

Averaging time (hours)

24 8 3 1

SO2 .......

PM10 ......

PM2.5......

NO2 .......

CO........

1.0 μg/m3

1.0 μg/m3

0.3 μg/m3

1.0 μg/m3

........

5 μg/m3 ..

5 μg/m3

1.2 μg/m3

........

........

0.5 mg/m3

25 μg/m3

........ 2 mg/m3

* * * * *
202

3. Section 51.166 is amended as follows:

a. By revising paragraph (a)(6)(i);

b. By revising paragraph (b)(14)(i)(a);

c. By removing the period at the end of paragraph 

(b)(14)(i)(b) and adding “; and” in its place;

d. By adding paragraph (b)(14)(i)(c);

e. By revising paragraph (b)(14)(ii)(a);

f. By removing the period at the end of paragraph

(b)(14)(ii)(b) and adding “; and” in its place;

g. By adding paragraph (b)(14)(ii)(c);

h. By revising paragraph (b)(14)(iii)(a);

i. By revising paragraph (b)(15)(i) and paragraph 

(b)(15)(ii) introductory text;

j. By revising the table in paragraph (c)(1);

k. By revising paragraph (c)(2);

l. By revising paragraph (i)(5)(i)(c);

m. By redesignating existing paragraphs (i)(5)(i)(d) 

through (j) as paragraphs (i)(5)(i)(e) through (k);

n. By adding new paragraph (i)(5)(i)(d);

o. By removing “(k)(2)” from paragraph (i)(8) and adding 

“(k)(1)(b)” in its place;

p. By removing in two places “(k)(2)” from paragraph

(i)(9) and adding “(k)(1)(b)” in those places;

q. By revising paragraph (k);
203

r. By removing the words “particulate matter” in the last 

sentence of paragraph (p)(4) and adding in their place “PM2.5, 

PM10,”; and

s. By revising the table in paragraph (p)(4).

§51.166 Prevention of significant deterioration of air quality.

(a) * * *

(6) * * * 

(i) Any State required to revise its implementation plan by 

reason of an amendment to this section, with the exception of 

amendments to add new maximum allowable increases or other 

measures pursuant to section 166(a) of the Act, shall adopt and 

submit such plan revision to the Administrator for approval no 

later than 3 years after such amendment is published in the

Federal Register. With regard to a revision to an 

implementation plan by reason of an amendment to paragraph (c) 

of this section to add maximum allowable increases or other 

measures, the state shall submit such plan revision to the 

Administrator for approval within 21 months after such amendment

is published in the Federal Register.

* * * * * 

(b) * * *

(14)(i) * * *

(a) In the case of PM10 and sulfur dioxide, January 6, 

1975;
204

* * * * *

(c) In the case of PM2.5, [INSERT DATE OF PUBLICATION IN 

THE FEDERAL REGISTER].

(ii) * * *

(a) In the case of PM10 and sulfur dioxide, August 7, 1977;

* * * * *

(c) In the case of PM2.5, [INSERT DATE 1 YEAR AFTER DATE OF 

PUBLICATION IN THE FEDERAL REGISTER].

(iii) * * *

(a) The area in which the proposed source or modification 

would construct is designated as attainment or unclassifiable 

under section 107(d)(1)(A)(ii) or (iii) of the Act for the 

pollutant on the date of its complete application under 40 CFR 

52.21 or under regulations approved pursuant to 40 CFR 51.166; 

and

* * * * *

(15)(i) Baseline area means any intrastate area (and every 

part thereof) designated as attainment or unclassifiable under 

section 107(d)(1)(A)(ii) or (iii) of the Act in which the major 

source or major modification establishing the minor source 

baseline date would construct or would have an air quality 

impact for the pollutant for which the baseline date is 

established, as follows: equal to or greater than 1 μg/m3 (annual 

average) for SO2, NO2, or PM10; or an air quality impact equal or 
205

greater than 0.3 μg/m3 (annual average) for PM2.5.

(ii) Area redesignations under section 107(d)(1)(A)(ii) or 

(iii) of the Act cannot intersect or be smaller than the area of 

impact of any major stationary source or major modification 

which:

* * * * *

(c) * * *

(1) * * *

Pollutant

Maximum 

allowable 

increase

(micrograms 

per cubic 

meter)

Class I Area

PM2.5:

Annual arithmetic mean.........................

24-hr maximum..................................

PM10: 

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean..............................

1

2

4

8

2

5

25

2.5

Class II Area

PM2.5:

Annual arithmetic mean.........................

24-hr maximum..................................

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

4

9

17

30

20

91
206

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean.........................

512

25

Class III Area

PM2.5:

Annual arithmetic mean.........................

24-hr maximum..................................

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean.........................

8

18

34

60

40

182

700

50

* * * * *

(2) Where the State can demonstrate that it has 

alternative measures in its plan other than an area 

classification plan, including maximum allowable increases as 

defined under paragraph (c)(1) of this section, that satisfy the 

requirements in sections 166(c) and 166(d) of the Act for a 

regulated NSR pollutant for which the Administrator has 

established maximum allowable increases pursuant to section 

166(a) of the Act, the requirements for an area classification 

plan and maximum allowable increases for that pollutant under 

paragraph (c)(1) of this section shall not apply upon approval 

of the plan by the Administrator. The following regulated NSR 

pollutants are eligible for such treatment:

(i) Nitrogen dioxide.
207

(ii) PM2.5.

* * * * *

(i) * * *

(5) * * *

(i) * * *

(c) PM2.5—4 μg/m3, 24-hour average;

(d) PM10—10 μg/m3, 24-hour average;

* * * * *

(k) Source impact analysis. 

(1) Required demonstration. The plan shall provide that 

the owner or operator of the proposed source or modification 

shall demonstrate that allowable emission increases from the 

proposed source or modification, in conjunction with all other 

applicable emissions increases or reduction (including secondary 

emissions), would not cause or contribute to air pollution in 

violation of:

(i) Any national ambient air quality standard in any air 

quality control region; or

(ii) Any applicable maximum allowable increase over the 

baseline concentration in any area.

(2) Significant impact levels. The plan may provide that, 

for purposes of PM2.5, the demonstration required in paragraph 

(k)(1) of this section is deemed to have been made if the 

emissions increase from the new stationary source alone or from 
208

the modification alone would cause, in all areas, air quality 

impacts less than the following amounts:

Pollutant

Averaging 

time

Class I 

Area

Class II

Area

Class III

Area

PM2.5 Annual ........ 006 μg/m3 ....0.3 μg/m3 .....0.3 μg/m3

24-hour ....... 007 μg/m3 ....1.2 μg/m3 .....1.2 μg/m3

* * * * *

(p) * * *

(4) * * *

Pollutant

Maximum 

allowable 

increase 

(micrograms 

per cubic 

meter)

PM2.5:

Annual arithmetic mean..............................

24-hr maximum.......................................

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean.........................

4

9

17

30

20

91

325

25

* * * * *
209

4. Appendix S to part 51 is amended by revising the table 

in Section III.A to read as follows:

Appendix S to Part 51—Emission Offset Interpretative Ruling

* * * * *

III. * * *

A. * * *

Pollutant Annual

Averaging time (hours)

24 8 3 1

SO2 .......

PM10 ......

PM2.5......

NO2 .......

CO........

1.0 μg/m3

1.0 μg/m3

0.3 μg/m3

1.0 μg/m3

........

5 μg/m3 ..

5 μg/m3

1.2 μg/m3

........

........

0.5 mg/m3

25 μg/m3

........ 2 mg/m3

* * * * *
210

PART 52 - [Amended]

5. The authority citation for part 52 continues to read as 

follows:

Authority: 42 U.S.C. 7401, et seq.

Subpart A - [Amended]

6. Section 52.21 is amended as follows:

a. By revising paragraph (b)(14)(i)(a);

b. By removing the period at the end of paragraph 

(b)(14)(i)(b) and adding “; and” in its place; 

c. By adding paragraph (b)(14)(i)(c);

d. By revising paragraph (b)(14)(ii)(a);

e. By removing the period at the end of paragraph 

(b)(14)(ii)(b) and adding “; and” in its place;

f. By adding paragraph (b)(14)(ii)(c);

g. By revising paragraph (b)(14)(iii)(a);

h. By revising paragraph (b)(15)(i) and paragraph 

(b)(15)(ii) introductory text;

i. By revising the table in paragraph (c); 

j. By revising paragraph (i)(5)(i);

k. By removing “(k)(2)” from paragraph (i)(9) and adding 

“(k)(1)(b)” in its place;

l. By removing in two places “(k)(2)” from paragraph 

(i)(10) and adding “(k)(1)(b) in those places;

m. By revising paragraph (k);
211

n. By removing the words “particulate matter” in the last 

sentence of paragraph (p)(5) and adding in their place “PM2.5, 

PM10,”; and

o. By revising the table in paragraph (p)(5).

§52.21 Prevention of significant deterioration of air quality.

* * * * *

(b) * * *

(14)(i) * * *

(a) In the case of PM10 and sulfur dioxide, January 6, 

1975;

* * * * *

(c) In the case of PM2.5, [INSERT DATE OF PUBLICATION IN 

THE FEDERAL REGISTER].

(ii) * * *

(a) In the case of PM10 and sulfur dioxide, August 7, 1977;

* * * * *

(c) In the case of PM2.5, [INSERT DATE 1 YEAR AFTER DATE OF 

PUBLICATION IN THE FEDERAL REGISTER].

(iii) * * *

(a) The area in which the proposed source or modification 

would construct is designated as attainment or unclassifiable 

under section 107(d)(1)(A)(ii) or (iii) of the Act for the 

pollutant on the date of its complete application under 40 CFR 
212

52.21 or under regulations approved pursuant to 40 CFR 51.166; 

and

* * * * *

(15)(i) Baseline area means any intrastate area (and every 

part thereof) designated as attainment or unclassifiable under 

section 107(d)(1)(A)(ii) or (iii) of the Act in which the major 

source or major modification establishing the minor source 

baseline date would construct or would have an air quality 

impact for the pollutant for which the baseline date is 

established, as follows: equal to or greater than 1 μg/m3 (annual 

average) for SO2, NO2, or PM10; or an air quality impact equal or 

greater than 0.3 μg/m3 (annual average) for PM2.5.

(ii) Area redesignations under section 107(d)(1)(A)(ii) or 

(iii) of the Act cannot intersect or be smaller than the area of 

impact of any major stationary source or major modification 

which:

* * * * *

(c) * * *

Pollutant

Maximum 

allowable 

increase 

(micrograms 

per cubic 

meter)

Class I Area

PM2.5:

Annual arithmetic mean.........................

24-hr maximum..................................

1

2
213

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean..............................

4

8

2

5

25

2.5

Class II Area

PM2.5:

Annual arithmetic mean.........................

24-hr maximum..................................

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean.........................

4

9

17

30

20

91

512

25

Class III Area

PM2.5:

Annual arithmetic mean.........................

24-hr maximum..................................

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean.........................

8

18

34

60

40

182

700

50

* * * * *

(i) * * *

(5) * * *

(i) The emissions increase of the pollutant from the new 
214

source or the net emissions increase of the pollutant from the 

modification would cause, in any area, air quality impacts less 

than the following amounts:

(a) Carbon monoxide—575 μg/m3, 8-hour average;

(b) Nitrogen dioxide—14 μg/m3, annual average;

(c) PM2.5—4 μg/m3, 24-hour average;

(d) PM10—10 μg/m3, 24-hour average;

(e) Sulfur dioxide—13 μg/m3, 24-hour average;

(f) Ozone;1

(g) Lead—0.1 μg/m3, 3-month average;

(h) Fluorides—0.25 μg/m3, 24-hour average;

(i) Total reduced sulfur—10 μg/m3, 1-hour average;

(j) Hydrogen sulfide—0.2 μg/m3, 1-hour average;

(k) Reduced sulfur compounds—10 μg/m3, 1-hour average; or

1 No de minimis air quality level is provided for ozone.

However, any net emissions increase of 100 tons per year or more 

of volatile organic compounds or nitrogen oxides subject to PSD 

would be required to perform an ambient impact analysis, 

including the gathering of ambient air quality data.

* * * * *

(k) Source impact analysis. 

(1) Required demonstration. The owner or operator of the 

proposed source or modification shall demonstrate that allowable 
215

emission increases from the proposed source or modification, in 

conjunction with all other applicable emissions increases or 

reductions (including secondary emissions), would not cause or 

contribute to air pollution in violation of:

(i) Any national ambient air quality standard in any air 

quality control region; or

(ii) Any applicable maximum allowable increase over the 

baseline concentration in any area.

(2) Significant impact levels. For purposes of PM2.5, the 

demonstration required in paragraph (k)(1) of this section is 

deemed to have been made if the emissions increase from the new 

stationary source alone or from the modification alone would 

cause, in all areas, air quality impacts less than the following 

amounts:

Pollutant

Averaging 

time

Class I

Area

Class II 

Area

Class III

Area

PM2.5 Annual ........ 0.06 μg/m3 ....0.3 μg/m3 .....0.3 μg/m3

24-hour ....... 0.07 μg/m3 ....1.2 μg/m3 .....1.2 μg/m3

* * * * *

(p) * * *

(5) * * *

Pollutant

Maximum 

allowable 

increase 

(micrograms 

per cubic 
216

meter)

PM2.5:

Annual arithmetic mean..............................

24-hr maximum.......................................

PM10:

Annual arithmetic mean.........................

24-hr maximum..................................

Sulfur dioxide:

Annual arithmetic mean..............................

24-hr maximum.......................................

3-hr maximum........................................

Nitrogen dioxide: 

Annual arithmetic mean.........................

4

9

17

30

20

91

325

25

* * * * *