# Proposed Rule To Implement the Fine Particle National Ambient Air Quality Standards

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URL: https://www.frixlaw.com/law-library/documents/fr%3A05-20455

## Record

- **Collection:** Federal Register
- **Document type:** Proposed Rule
- **Published:** November 1, 2005
- **Citation:** 70 FR 65984

## Text

ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 51 and 52
[FRL-7969-1]
RIN 2060-AK74
Proposed Rule To Implement the Fine Particle National Ambient Air Quality Standards

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rulemaking.

SUMMARY:

This proposed rule and preamble describe the requirements that States and Tribes must meet in their implementation plans for attainment of the fine particle (PM
2.5
) national ambient air quality standards (NAAQS). The health effects associated with exposure to PM
2.5
are serious, including premature death, aggravation of heart and lung disease, and asthma attacks. Those particularly sensitive to PM
2.5
exposure include older adults, people with heart and lung disease, and children.

The EPA designated areas not attaining the PM
2.5
standards on December 17, 2004. The PM designations notice was published in the
Federal Register
on January 5, 2005 (70 FR 944) and became effective on April 5, 2005. On this same date, the Administrator signed a supplemental notice making certain changes to the designations based on 2002-2004 air quality data. The supplemental notice was published in the
Federal Register
on April 14, 2005 (70 FR 19844). A total of 39 areas with a population of 90 million were designated as nonattainment.

Within 3 years, each State having a nonattainment area must submit to EPA an attainment demonstration (and associated air quality modeling), adopted State regulations to reduce emissions of PM
2.5
and its precursors, and other supporting information demonstrating that the area will attain the standards as expeditiously as practicable. In order to address PM
2.5
problems, EPA believes that States should implement a balanced program to reduce emissions from regional sources [such as power plants emitting sulfur dioxide (SO
2
) and nitrogen oxides (NO
X
)] and local sources (such as cars, trucks, industrial sources, and various other combustion or burning-related activities). States should take into account national, State, and local emission reduction programs that are already in place and projected to provide future air quality benefits.

DATES:

The comment period on this proposal ends on January 3, 2006. Comments must be postmarked by the last day of the comment period and sent directly to the Docket Office listed in
ADDRESSES
(in duplicate form if possible).

One public hearing will be held prior to the end of the comment period. The dates, times and locations will be announced separately. Please refer to
SUPPLEMENTARY INFORMATION
for additional information on the comment period and public hearings.

ADDRESSES:

Comments may be submitted by mail to: Air Docket, Environmental Protection Agency, Mail code: 6102T, 1200 Pennsylvania Ave., NW., Washington, DC 20460, Attention Docket ID No. OAR-2003-0062.

Comments may also be submitted electronically, by facsimile, or through hand delivery/courier. Follow the detailed instructions provided under
SUPPLEMENTARY INFORMATION
.

Documents relevant to this action are available for public inspection at the EPA Docket Center, located at 1301 Constitution Avenue, NW., Room B102, Washington, DC between 8:30 a.m. and 4:30 p.m., Monday through Friday, excluding legal holidays. A reasonable fee may be charged for copying.

FOR FURTHER INFORMATION CONTACT:

Regarding PM
2.5
implementation issues, contact Mr. Richard Damberg, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Mail Code C504-02, Research Triangle Park, NC 27711, phone number (919) 541-5592 or by e-mail at:
damberg.rich@epa.gov.
Regarding NSR issues, contact Mr. Raj Rao, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Mail Code C339-03, Research Triangle Park, NC 27711, phone number (919) 541-5344 or by e-mail at:
rao.raj@epa.gov.

SUPPLEMENTARY INFORMATION:

Section I of the preamble provides an overview of the PM
2.5
standards, health effects associated with PM
2.5
, legal history, and EPA's overall strategy for reducing PM
2.5
pollution. Section II provides an overview of the pollutants and complex atmospheric chemistry that lead to PM
2.5
formation, the sources of emissions, and a discussion of policy options for addressing PM precursors in the PM
2.5
implemention program and the new source review (NSR) program.

Section III of the preamble describes the various core elements of the PM
2.5
implementation program, based primarily on the subpart 1 requirements of section 172 of the Clean Air Act (CAA). Important topics discussed in section III include attainment dates, attainment demonstrations and modeling, local emission reduction measures [reasonably available control technology (RACT) and reasonably available control measures (RACM)], and reasonable further progress (RFP). Section III also includes a subsection describing options for revising the NSR program to specifically address PM
2.5
. A number of other topics are presented for informational purposes in section III, including innovative program guidance, emission inventory requirements, addressing PM
2.5
under the transportation conformity program, stationary source test methods for PM
2.5
, and approaches for reducing emissions through improved monitoring techniques.

Section IV addresses the various statutory requirements and executive orders applicable to this rule. The final section contains proposed regulatory text for implementation of the PM
2.5
NAAQS, in the form of a proposed subpart Y amending 40 CFR part 51.

Public Hearing

The EPA will hold one public hearing on today's proposal during the comment period. The details of the public hearing, including the time, date, and location will be provided in a future
Federal Register
notice and announced on EPA's PM
2.5
implementation Web site at
http://www.epa.gov/ttn/naaqs/pm/pm25_index.html.

The public hearing will provide interested parties the opportunity to present data, views, or arguments concerning the proposed rule. The EPA may ask clarifying questions during the oral presentations, but will not respond to the presentations or comments at that time. Written statements and supporting information submitted during the comment period will be considered with the same weight as any oral comments and supporting information presented at a public hearing.

How Can I Get Copies of This Document and Other Related Information?

Docket.
The EPA has established an official public docket for this action under Docket ID No. OAR-2003-0062. The official public docket consists of the documents specifically referenced in this action, any public comments received, and other information related to this action. Although a part of the official docket, the public docket does not include Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. The official public docket is the collection of materials that is available

for public viewing at the Air Docket in the EPA Docket Center, (EPA/DC) EPA West, Room B102, 1301 Constitution Ave., NW., Washington, DC. The EPA Docket Center 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. A reasonable fee may be charged for copying.

Electronic Access.
You may access this
Federal Register
document electronically through the EPA Internet under the “Federal Register” listings at
http://www.epa.gov/fedrgstr/.

An electronic version of the public docket is available through EPA's electronic public docket and comment system, EPA Dockets. You may use EPA Dockets at
http://www.epa.gov/edocket/
to submit or view public comments, access the index listing of the contents of the official public docket, and to access those documents in the public docket that are available electronically. Once in the system, select “search,” then key in the appropriate docket identification number.

Certain types of information will not be placed in the EPA Dockets. Information claimed as CBI and other information whose disclosure is restricted by statute, which is not included in the official public docket, will not be available for public viewing in EPA's electronic public docket. The EPA's policy is that copyrighted material will not be placed in EPA's electronic public docket but will be available only in printed, paper form in the official public docket. To the extent feasible, publicly available docket materials will be made available in EPA's electronic public docket. When a document is selected from the index list in EPA Dockets, the system will identify whether the document is available for viewing in EPA's electronic public docket. Although not all docket materials may be available electronically, you may still access any of the publicly available docket materials through the docket facility identified above. The EPA intends to work towards providing electronic access to all of the publicly available docket materials through EPA's electronic public docket.

For public commenters, it is important to note that EPA's policy is that public comments, whether submitted electronically or in paper, will be made available for public viewing in EPA's electronic public docket as EPA receives them and without change, unless the comment contains copyrighted material, CBI, or other information whose disclosure is restricted by statute. When EPA identifies a comment containing copyrighted material, EPA will provide a reference to that material in the version of the comment that is placed in EPA's electronic public docket. The entire printed comment, including the copyrighted material, will be available in the public docket.

Public comments submitted on computer disks that are mailed or delivered to the docket will be transferred to EPA's electronic public docket. Public comments that are mailed or delivered to the Docket will be scanned and placed in EPA's electronic public docket. Where practical, physical objects will be photographed, and the photograph will be placed in EPA's electronic public docket along with a brief description written by the docket staff.

For additional information about EPA's electronic public docket, visit EPA Dockets online or see 67 FR 38102; May 31, 2002.

How and To Whom Do I Submit Comments?

You may submit comments electronically, by mail, by facsimile, or through hand delivery/courier. To ensure proper receipt by EPA, identify the appropriate docket identification number, OAR-2003-0062, in the subject line on the first page of your comment. Please ensure that your comments are submitted within the specified comment period. Comments received after the close of the comment period will be marked “late.” The EPA is not required to consider these late comments. If you wish to submit CBI or information that is otherwise protected by statute, please follow the instructions below under, “How Should I submit CBI to the Agency?” Do not use EPA Dockets or e-mail to submit CBI or information protected by statute.

Electronically.
If you submit an electronic comment as prescribed below, EPA recommends that you include your name, mailing address, and an e-mail address or other contact information in the body of your comment. Also include this contact information on the outside of any disk or CD ROM you submit, and in any cover letter accompanying the disk or CD ROM. This ensures that you can be identified as the submitter of the comment and allows EPA to contact you in case EPA cannot read your comment due to technical difficulties or needs further information on the substance of your comment. The EPA's policy is that EPA will not edit your comment, and any identifying or contact information provided in the body of a comment will be included as part of the comment that is placed in the official public docket, and made available in EPA's electronic public docket. If EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, EPA may not be able to consider your comment.

EPA Dockets.
Your use of EPA's electronic public docket to submit comments to EPA electronically is EPA's preferred method for receiving comments. Go directly to EPA Dockets at
http://www.epa.gov/edocket,
and follow the online instructions for submitting comments. To access EPA's electronic public docket from the EPA Internet Home Page, select “Information Sources,” “Dockets,” and “EPA Dockets.” Once in the system, select “search,” and then key in Docket ID No. OAR-2003-0062. The system is an “anonymous access” system, which means EPA will not know your identity, e-mail address, or other contact information unless you provide it in the body of your comment.

Electronic mail.
Comments may be sent by e-mail to
A-and-R-Docket@epa.gov,
Attention Docket ID No. OAR-2003-0062. In contrast to EPA's electronic public docket, EPA's e-mail system is not an “anonymous access” system. If you send an e-mail comment directly to the Docket without going through EPA's electronic public docket, EPA's e-mail system automatically captures your e-mail address. The e-mail addresses that are automatically captured by EPA's e-mail system are included as part of the comment that is placed in the official public docket, and made available in EPA's electronic public docket.

Disk or CD ROM.
You may submit comments on a disk or CD ROM that you mail to the mailing address identified under Docket above. These electronic submissions will be accepted in WordPerfect or ASCII file format. Avoid the use of special characters and any form of encryption.

By Mail.
Send your comments to Air Docket (in duplicate if possible), Environmental Protection Agency, Mail code: 6102T, 1200 Pennsylvania Ave., NW., Washington, DC, 20460, Attention Docket ID No. OAR-2003-0062.

By Hand Delivery or Courier.
Deliver your comments to: Air Docket, Environmental Protection Agency, 1301 Constitution Avenue, NW., Room B108, Mail code: 6102T, Washington, DC 20004, Attention Docket ID No. OAR-2003-0062. Such deliveries are only accepted during the Docket's normal hours of operation as identified above under Docket.

By Facsimile.
Fax your comments to (202) 566-1741, Attention Docket ID. No. OAR-2003-0062.

How Should I Submit CBI to the Agency?

Do not submit information that you consider to be CBI electronically through EPA's electronic public docket or by e-mail. Send or deliver information identified as CBI only to the following address: Roberto Morales, U.S. EPA, Office of Air Quality Planning and Standards, Mail Code C404-02, Research Triangle Park, NC 27711, telephone (919) 541-0880, e-mail at
morales.roberto@epa.gov,
Attention Docket ID No. OAR-2003-0062. You may claim information that you submit to EPA as CBI by marking any part or all of that information as CBI (if you submit CBI on disk or CD ROM, mark the outside of the disk or CD ROM as CBI and then identify electronically within the disk or CD ROM the specific information that is CBI). Information so marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2.

In addition to one complete version of the comment that includes any information claimed as CBI, a copy of the comment that does not contain the information claimed as CBI must be submitted for inclusion in the public docket and EPA's electronic public docket. If you submit the copy that does not contain CBI on disk or CD ROM, mark the outside of the disk or CD ROM clearly that it does not contain CBI. Information not marked as CBI will be included in the public docket and EPA's electronic public docket without prior notice. If you have any questions about CBI or the procedures for claiming CBI, please consult the person identified in the
FOR FURTHER INFORMATION CONTACT
section.

What Should I consider as I Prepare My Comments for EPA?

You may find the following suggestions helpful for preparing your comments:

1. Explain your views as clearly as possible.

2. Describe any assumptions that you used.

3. Provide any technical information and/or data you used that support your views.

4. If you estimate potential burden or costs, explain how you arrived at your estimate.

5. Provide specific examples to illustrate your concerns.

6. Offer alternatives.

7. Make sure to submit your comments by the comment period deadline identified.

8. To ensure proper receipt by EPA, identify the appropriate docket identification number in the subject line on the first page of your response. It would also be helpful if you provided the name, date, and
Federal Register
citation related to your comments.

Timing

In a number of places, this document refers to time periods (e.g., x number of years) after designation or after the designation date. By this, we mean the number of years after the effective date of PM
2.5
designations (April 5, 2005).

Table of Contents

I. What Is the PM
2.5
Problem and EPA's Strategy for Addressing It?

A. What are the fine particle standards and the health effects they address?

B. What is the legal history of the PM
2.5
standards?

C. What was the process for designating PM
2.5
attainment and nonattainment areas?

D. What is the geographic extent of the PM
2.5
problem?

E. What is EPA's overall strategy for reducing PM
2.5
pollution?

1. The State implementation plan (SIP) system

2. National rules

II. Fine Particles: Overview of Atmospheric Chemistry, Sources of Emissions, and Ambient Monitoring Data

A. Introduction

B. Concentration, composition and sources of fine PM

C. The role of ammonia in sulfate, nitrate & secondary organic aerosol formation

D. Regional patterns of carbon, sulfate and nitrate, and indications of transport

E. Policy for addressing PM
2.5
precursors

1. Legal Authority to Regulate Precursors

2. Proposed policy options for addressing PM
2.5
precursors in nonattainment plan programs.

III. What Are the Specific Elements of EPA's PM
2.5
Implementation Program?

A. What classification options are under consideration for PM
2.5
nonattainment areas?

1. Background

2. Proposed options for PM
2.5
classifications

a. No classification system based on design values

b. Two-tiered classification system

c. Rural transport classification

B. When are PM
2.5
attainment demonstrations and SIPs due, and what requirements must they address?

C. What are the attainment dates for PM
2.5
nonattainment areas?

1. Background

2. Consideration of existing measures in proposing an attainment date

3. Areas may qualify for two 1-year attainment date extensions

4. Areas may submit a SIP demonstrating that it is impracticable to attain by the 5-year attainment date

5. Areas that fail to attain or do not qualify for an attainment date extension

6. Determining attainment for the PM
2.5
standards

7. How do attainment dates apply to Indian country?

D. What are the incentives for achieving early reductions of PM
2.5
and its precursors?

E. How should the States and EPA balance the need to address long-range transport of fine particle pollution with the need for local emissions reductions when implementing the PM
2.5
standards?

1. Clean Air Act provisions for achieving local and regional emissions reductions

2. Regional emission reduction strategies

3. The role of local and State emission reduction efforts in reducing health risks and achieving the PM
2.5
standards

4. Addressing regionally transported emissions in local area attainment demonstrations

F. How will EPA address requirements for modeling and attainment demonstration SIPs when implementing the 24-hour and annual average PM
2.5
standards?

1. Introduction

2. Areas that need to conduct modeling

3. Modeling guidance

4. Modeled attainment test

5. Multi-pollutant assessments and one-atmosphere modeling

6. Which future year(s) should be modeled?

7. Mid-course review

G. What requirements for RFP apply under the PM
2.5
implementation program?

1. Background

2. What is the baseline year from which States will track emission reductions for meeting RFP requirements?

3. How does EPA propose to address the pollutants associated with PM
2.5
in these RFP requirements?

4. What areas must submit an RFP plan?

a. Areas projected to attain within 5 years of designation

b. Areas projected to attain more than 5 years from the date of designation must submit a 2008 RFP plan

i. For purposes of the 2008 RFP plan, how should a nonattainment area define its emission reduction milestones?

ii. For what pollutants must States reduce emissions?

iii. How should States assess the equivalence of alternative combinations of pollutant emissions reductions?

iv. How would RFP be evaluated for a sample 2008 RFP plan?

v. What potential RFP requirements could apply for “serious” areas under the two-tiered classification option?

5. Other RFP issues

a. How should States account for regional control strategies in evaluating RFP?

b. What geographic area should States address in RFP plans?

c. How should RFP be addressed in multi-state nonattainment areas?

d. How should States compile emission inventories for RFP plans?

e. What RFP requirements apply in Tribal areas?

f. What must States submit to show whether they have met RFP milestones?

H. What requirements for contingency measures should apply under the PM
2.5
implementation program?

I. What requirements should apply for RACM and RACT for PM
2.5
nonattainment areas?

1. General background

2. Background for RACT

3. Emissions inventory analysis supporting RACT options

4. Which PM
2.5
precursors must be addressed by States in establishing RACT requirements?

5. What are the proposed options for implementing the RACT requirement?

6. What factors should States consider in determining whether an available control technology is technically feasible?

7. What factors should States consider in determining whether an available control technology is economically feasible?

8. How should condensable emissions be treated in RACT determinations?

9. What are the required dates for submission and implementation of RACT measures?

10. Under the PM
2.5
implementation program, does a State need to conduct a RACT determination for an applicable source that already has a RACT determination in effect?

11. What policies affect compliance with RACT for electric generating units?

12. Is EPA developing PM
2.5
controlled technique guidelines?

13. Background for RACM

14. What is the proposed approach for implementing RACM?

15. What factors should States consider in determining whether control measures are reasonably available?

16. What specific source categories and control measures should a State evaluate when determining RACM for a nonattainment area?

17. What criteria should be met to ensure effective regulations or permits to implement RACT and RACM?

J. What guidance is available to States and Tribes for implementing innovative programs to address the PM
2.5
problem?

K. What aspects of transportation conformity and the PM
2.5
standard are addressed in this proposal?

1. What is transportation conformity?

2. Why does transportation conformity apply to PM
2.5
?

3. Why is EPA discussing transportation conformity in this proposal?

4. What revisions have been made to the transportation conformity rule to address the PM
2.5
standard?

5. Does EPA plan to revoke the PM
10
standard?

6. Will some areas be demonstrating conformity for both PM
10
and PM
2.5
at the same time?

7. When does transportation conformity apply to PM
2.5
nonattainment areas?

8. How does the 1-year grace period apply in metropolitan areas?

9. How does the 1-year grace period apply in “donut” areas?

10. How does the 1-year grace period apply in isolated rural areas?

L. What requirements for general conformity should apply to the PM
2.5
standards?

1. What is the purpose of the general conformity regulations?

2. How is the general conformity program currently structured?

3. Who runs the general conformity program?

4. How does an agency demonstrate conformity?

5. General conformity regulation revisions for the PM
2.5
standards

a. What
de minimis
emission levels will be set for pollutants that contribute to PM
2.5
concentrations?

b. What impact will the implementation of the PM
2.5
standards have on a State's general conformity SIP?

c. Are there any other impacts on the SIPs related to general conformity based on implementation of the PM
2.5
standards?

6. Is there a 1-year grace period which applies to general conformity determinations for the purposes of the PM
2.5
standards?

M. How will the NSR program address PM
2.5
and its precursors?

1. Background

2. What are the principal elements of the proposed major NSR program for PM
2.5
?

3. Should precursors to the formation of ambient concentrations of PM
2.5
be subject to regulation under NSR?

a. Background

b. Should NSR cover precursor emissions in addition to direct emissions of PM
2.5
?

4. What is a major stationary source (major source) under the major NSR program for PM
2.5
?

a. Background

b. Proposed option

c. What is the effect of this proposed option?

5. What should the significant emissions rate be for direct emissions of PM
2.5
?

a. Background

b. Proposed options

6. What should be the significant emissions rates for PM
2.5
precursors?

a. Background

b. Proposed options

7. What is the role of condensible emissions in determining major NSR applicability?

8. What are the requirements of the Prevention of Significant Deterioration (PSD) program for attainment areas?

9. How should BACT be implemented?

10. What is EPA's plan for preventing significant deterioration of air quality for PM
2.5
?

11. How will the air quality analysis required under section 165(a)(3) be implemented?

12. How should the PSD pre-construction monitoring requirement be implemented for PM
2.5
?

a. Background

b. Options for PSD preconstruction monitoring

13. Nonattainment New Source Review (NA NSR) requirements

14. What are the offset requirements for NA NSR?

a. What is the required offset ratio for PM
2.5
direct emissions?

b. Which precursors shall be subject to the offset requirement?

c. What is the required offset ratio for PM
2.5
precursors?

d. Should EPA allow interprecursor trading to comply with the offset requirement?

15. What are the implementation and transition issues associated with this rule?

16. Implementation of PSD provisions during the SIP Development period

a. Background

b. Proposed options

c. Rationale

17. Implementation of the nonattainment NSR provisions during the SIP development period

a. Background

b. Implementation of NSR under the Emissions Offset Interpretative Ruling (40 CFR part 51, Appendix S) with revisions.

c. Legal basis for requiring States to issue nonattainment NSR permits during the SIP-development period

18. NSR applicability to precursors during the interim period

19. Are there any Tribal concerns?

20. What must a State or local agency do about minor sources of PM
2.5
?

21. Supplemental program option: rural transport areas

a. What flexible implementation options should be available for Transport areas?

b. Which nonattainment areas would be eligible for the transport program?

c. What would be the basic requirements of a transport nonattainment NSR program?

N. How will EPA ensure that the 8-hour ozone standard will be implemented in a way which allows an optimal mix of controls for PM
2.5
, ozone, and regional haze?

1. Could an area's PM
2.5
strategy affect its 8-hour ozone and/or regional haze strategy?

2. What guidance has EPA provided regarding ozone, PM
2.5
and regional haze interaction?

3. What is EPA proposing?

O. What emission inventory requirements should apply under the PM
2.5
NAAQS?

P. What stationary source test methods should States use under the PM
2.5
implementation program?

1. Will the existing stationary source test methods for particulate matter (PM) be acceptable for use in PM
2.5
SIPs?

2. Why are the existing stationary source test methods for PM deficient?

3. If the stationary source test methods are changed, will the existing emission limitations incorporated in SIPs need to be changed?

4. The existing PM test methods and the emission limits based upon these methods have been acceptable since 1971, why do they need to be changed for PM
2.5
?

5. What methods are available for measuring PM size and condensable PM from stationary sources?

6. Why is a new dilution-based test method being developed by EPA?

7. What types of sources should use the new dilution-based test method?

8. What are the main features of the new test method?

9. What is the schedule for finalization of the new test method?

10. How will use of this new method affect an areas emissions inventory and the emissions inventory for individual sources?

11. How will use of this new method affect a State's implementation program more broadly?

Q. How can potentially inadequate source monitoring in certain SIP rules be improved?

1. How does improved PM
2.5
monitoring relate to title V monitoring?

2. Are instrumental techniques more appropriate than visual emissions (VE) techniques for monitoring compliance with PM emissions limits, for some situations and applications?

3. What constitutes improved monitoring?

R. What guidance should be provided that is specific to Tribes?

S. Are there any additional requirements related to enforcement and compliance?

T. What requirements should apply to emergency episodes?

U. What ambient monitoring requirements will apply under the PM
2.5
NAAQS?

IV. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

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

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

H. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution, or Use

I. National Technology Transfer Advancement Act

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations

I. What Is the PM
2.5
Problem and EPA's Strategy for Addressing It?

A. What Are the Fine Particle Standards and the Health Effects They Address?

Fine particles in the atmosphere are made up of a complex mixture of components. Common constituents include: Sulfate (SO
4
); nitrate (NO
3
); ammonium; elemental carbon; a great variety of organic compounds; and inorganic material (including metals, dust, sea salt, and other trace elements) generally referred to as “crustal” material, although it may contain material from other sources. Airborne particulate matter (PM) with a nominal aerodynamic diameter of 2.5 micrometers or less (a micrometer is one-millionth of a meter, and 2.5 micrometers is less than one-seventh the average width of a human hair) are considered to be “fine particles,” and are also known as PM
2.5
. “Primary” particles are emitted directly into the air as a solid or liquid particle (
e.g.
, elemental carbon from diesel engines or fire activities, or condensable organic particles from gasoline engines). “Secondary” particles (
e.g.
, sulfate and nitrate) form in the atmosphere as a result of various chemical reactions. (See section II for a more detailed technical discussion on PM
2.5
, its precursors, formation processes, and emissions sources.)

The health effects associated with exposure to PM
2.5
are significant. Epidemiological studies have shown a significant correlation between elevated PM
2.5
levels and premature mortality. Other important effects associated with PM
2.5
exposure include aggravation of respiratory and cardiovascular disease (as indicated by increased hospital admissions, emergency room visits, absences from school or work, and restricted activity days), lung disease, decreased lung function, asthma attacks, and certain cardiovascular problems. Individuals particularly sensitive to PM
2.5
exposure include older adults, people with heart and lung disease, and children. On July 18, 1997, we revised the NAAQS for particulate matter to add new standards for fine particles, using PM
2.5
as the indicator. We established health-based (primary) annual and 24-hour standards for PM
2.5
(62 FR 38652).
1

The annual standard is a level of 15 micrograms per cubic meter, based on the 3-year average of annual mean PM
2.5
concentrations. The 24-hour standard is a level of 65 micrograms per cubic meter, based on the 3-year average of the 98th percentile of 24-hour concentrations. The EPA established the standards based on significant evidence and numerous health studies demonstrating that serious health effects are associated with exposures to elevated levels of PM
2.5
. Estimates show that attainment of the PM
2.5
standards would be likely to result in tens of thousands fewer premature deaths each year, would be likely to prevent tens of thousands of hospital admissions each year, and would be likely to prevent hundreds of thousands of doctor visits, absences from work and school, and respiratory illnesses in children annually. The research on which EPA based the 1997 standards did not identify a specific threshold concentration below which individuals have no PM-related health effects, meaning that emissions reductions resulting in reduced concentrations below the level of the standards may continue to provide additional health benefits to the local population.
2

At the time we established the primary standards in 1997, we also established welfare-based (secondary) standards identical to the primary standards. The secondary standards are designed to protect against major environmental effects of PM
2.5
such as visibility impairment, soiling, and materials damage. The EPA also established the regional haze regulations in 1999 for the improvement of visual air quality in national parks and wilderness areas across the country. Because regional haze is caused primarily by light scattering and light absorption by fine particles in the atmosphere, EPA is encouraging the States to integrate their efforts to attain the PM
2.5
standards with those efforts to establish reasonable progress goals and associated emission reduction strategies for the purposes of improving air quality in our treasured natural areas under the regional haze program.

1
In the 1997 PM NAAQS revision, EPA also revised the standard for particles with a nominal aerodynamic diameter of 10 micrometers or less (also known as PM
10
). The original PM
10
standard was established in 1987. The revised PM
10
standard was later vacated by the court, and thus the 1987 PM
10
standard remains in effect. Today's proposed implementation rule and guidance does not address PM
10
.

2
Environmental Protection Agency. (1996) Air Quality Criteria for Particulate Matter. Research Triangle Park, NC: National Center for Environmental Assessment-RTP Office; report no. EPA/600/P-95/001aF-cF. 3v.

The scientific assessment that resulted in the establishment of the PM
2.5
standards included a scientific peer review and public comment process. We developed scientific background documents based on the review of hundreds of peer-reviewed scientific studies. The Clean Air Scientific Advisory Committee, a congressionally mandated group of independent scientific and technical experts, provided extensive review of these assessments, and found that EPA's review of the science provided an adequate basis for the EPA Administrator to make a decision. More detailed information on health effects of PM
2.5
can be found on EPA's Web site at:
http://www.epa.gov/air/urbanair/pm/index.html.
Additional information on EPA's scientific assessment documents supporting the 1997 standards is available at:
http://www.epa.gov/ttn/oarpg
(see headings for “Staff Papers” and “Criteria Documents”).

B. What Is the Legal History of the PM
2.5
Standards?

After EPA promulgated the PM
2.5
and 8-hour ozone standards in July 1997, several industry organizations and State governments challenged EPA's action in the U.S. Court of Appeals for the District of Columbia Circuit (the DC Circuit). This action initiated a long legal process, ending with a March 2002 decision by the DC Circuit upholding the standards and the authority on which they were established.

On May 14, 1999, the three-judge panel of the DC Circuit held in a split decision that the CAA, as applied by EPA in setting the 1997 standards for PM and ozone, was unconstitutional as an improper delegation of legislative authority to EPA. The ruling did not question the science or decision-making process used to establish the standards. The Court remanded the PM
2.5
standards to EPA but did not vacate them. In June 1999, the Department of Justice (DOJ) and EPA petitioned the Court for a rehearing en banc with the entire DC Circuit Court. On October 29, 1999, the Court denied the petition for rehearing.

The DOJ and EPA then filed a petition for certiorari with the United States Supreme Court in December 1999 to appeal the decision of the DC Circuit, and the Supreme Court issued its decision to hear the appeal in November 2000. The Supreme Court issued its decision on the merits of the appeal on February 27, 2001.
3

In that decision, the Supreme Court held that EPA's approach to setting the NAAQS in accordance with the CAA did not constitute an unconstitutional delegation of authority. The Supreme Court unanimously affirmed the constitutionality of the CAA provision that authorizes the Agency to set national air quality standards, stating that this provision “fits comfortably within the scope of discretion permitted by our precedent.” The Supreme Court also affirmed that the CAA requires EPA to set standards at levels necessary to protect the public health and welfare, without considering the economic costs of implementing the standards. The Supreme Court remanded several other issues back to the DC Circuit, including the issue of whether EPA acted arbitrarily and capriciously in establishing the specific levels of the standards.

3

Whitman
v.
American Trucking Assoc.,
121 S.Ct. 903, 911-914 (2001) (Whitman).

The DC Circuit heard arguments in this remanded case in December 2001, and issued its decision on March 26, 2002. The DC Circuit found that the Agency had “engaged in reasoned decision making,” rejecting the claim that the Agency had acted arbitrarily and capriciously in setting the levels of the standards. This last decision by the DC Circuit gave EPA a clear path to move forward with implementation of the PM
2.5
standards.

The implementation rule we are proposing today provides specific requirements for State, local, and Tribal
4

air pollution control agencies to address as they prepare implementation plans required by the CAA to attain and maintain the PM
2.5
standards.
5

Each State with an area that is not attaining the PM
2.5
NAAQS will have to develop, as part of its State implementation plan (SIP), emission limits for appropriate sources and other requirements to attain the NAAQS within the timeframes set forth in the CAA.
6

Tribes with jurisdiction over Indian country that is not attaining the PM
2.5
NAAQS could voluntarily submit a Tribal implementation plan (TIP) but are not required to do so. However, in cases where Tribes elect not to submit a TIP, EPA, working with the Tribes, has the responsibility for developing an implementation plan in those areas.

4
The 1998 Tribal Authority Rule (TAR) (40 CFR part 49), which implements section 301(d) of the CAA, provides for Tribes to be treated in the same manner as a State in implementing sections of the CAA. It gives Tribes the option of developing tribal implementation plans (TIPs), but unlike States, Tribes are not required to develop implementation plans. See section III.Q. for further discussion of Tribal issues.

5
When the term “State” is used hereafter, it will refer to States, local air agencies, and Tribal governments electing to be treated as States for the purposes of implementing the CAA.

6
The CAA requires EPA to set ambient air quality standards and requires States to submit plans designed to attain those standards.

C. What Was the Process for Designating PM
2.5
Attainment and Nonattainment Areas?

We issued guidance in April 2003
7

and February 2004
8

on the process for designating attainment and nonattainment areas for PM
2.5
and on factors for States and Tribes to consider in defining boundaries for nonattainment areas. The guidance states that EPA believes the presumptive boundaries for nonattainment areas should be equal to the 1999 Office of Management and Budget (OMB) definitions of the combined metropolitan statistical area, where applicable, or the metropolitan statistical area. We also recognized the fact that in June 2003, OMB released updated definitions of combined statistical areas and core-based statistical areas. We communicated to the States and Tribes that in evaluating potential nonattainment area boundaries, they should include any additional counties that were added in 2003 to the 1999 metro area definitions, plus adjacent counties, in their review of data associated with the nine technical factors discussed in EPA guidance.

7
See “Designations for the Fine Particle National Ambient Air Quality Standard,” memorandum from Jeffrey R. Holmstead, Assistant Administrator, to EPA Regional Administrators, April 1, 2003. Available at:
http://www.epa.gov/pmdesignations/guidance.htm.

8
See “Additional Guidance on Defining Area Boundaries for PM
2.5
Designations,” memorandum from Lydia N. Wegman, Director of Air Quality Strategies and Standards Division, EPA Office of Air Quality Planning and Standards, to EPA Air Division Directors, February 12, 2004. Available at:
http://www.epa.gov/pmdesignations/guidance.htm.

States were required to submit their recommendations to EPA by February 15, 2004.
9

Tribes were encouraged, but not required, to submit designation recommendations to EPA for their reservations or other areas under their jurisdiction. In general, the recommendations were based on the most recent 3 years of air quality data available (e.g. 2001-2003). On June 29, we sent letters to the Governors and Tribal leaders notifying them of any modifications we intended to make to their recommendations. After considering additional comments and information from States and Tribes, EPA issues final PM
2.5
designations on December 17, 2004. They were published in the
Federal Register
on January 5, 2005 (70 FR 944).

9
The Consolidated Appropriations Bill for FY2004 (Pub. L. 108-199), signed by President Bush on January 23, 2004, codifies the required State submittal date (February 15, 2004) and the date for EPA to finalize PM
2.5
designations (December 31, 2004) that were originally included in EPA's April 2003 guidance on PM
2.5
designations.

The nonattainment designation for an area starts the process whereby a State or Tribe must develop an implementation plan that includes, among other things, a demonstration showing how it will attain the ambient standards by the attainment dates required in the CAA. Under section 172(b), States have up to 3 years after EPA's final designations to submit their SIPs to EPA. These SIPs will be due in April 2008, three years from the effective date of the designations.

D. What Is the Geographic Extent of the PM
2.5
Problem?

The PM
2.5
ambient air quality monitoring data for the 2001-2003 period suggest that areas violating the standards are located across much of the eastern half of the United States and in much of central and southern California.

A total of 47 areas comprised of 224 counties and the District of Columbia were designated as nonattainment in December 2004. In April 2005, EPA issued a supplemental notice which changed the designation status of eight areas (with 17 counties) from nonattainment to attainment based on newly updated 2002-2004 air quality data. In addition, four areas previously designated as unclassifiable were changed to attainment in this notice.

The population of the 39 PM
2.5
nonattainment areas is significant—about 90 million, or more than 30% of the U.S. population. Most areas violate only the annual standard, but a few violate both the annual and 24-hour standards. The 2001-2003 data show that no area violates just the 24-hour standard.
10

10
A listing of counties and associated PM
2.5
3-year annual average concentrations, or “design values,” is available on EPA's Web site at:
http://www.epa.gov/airtrends/values.html.

The distribution of the 2001-2003 design values
11

for the 39 nonattainment areas is shown in the table below:

11
The PM
2.5
design value for a nonattainment area is the highest of the 3-year average concentrations calculated for the monitors in the area, in accordance with 40 CFR part 50, appendix N.

Design value range for PM
2.5
nonattainment areas
(in μg/m3)

Number of areas

Percent of all areas
(percent)

15.1-16.0
10
26

16.1-17.0
12
31

17.1-18.0
12
31

18.1-19.0
1
3

19.1 +
4
10

Total
39
100

More than 40% of the nonattainment areas, including many major metropolitan areas, have design values that are 2 μg/m
3
or more above the annual standard.

The EPA believes the PM
2.5
problem has a substantial regional component because the formation and transport of secondarily formed particles, such as sulfates and nitrates, extends over hundreds of miles. The regional nature of PM
2.5
is in contrast to the more localized nature of PM
10
.

In addition, data suggests that ambient PM
2.5
concentrations tend to rise and fall in a consistent manner across very large geographic areas. The transport phenomena associated with PM
2.5
and its precursors has been well-documented for many years. For example, one significant source of information on long-range transport is the National Acid Precipitation Assessment Program (NAPAP) research from the 1980's and its associated reports published in 1991.
12

Additional studies and air quality modeling analyses since that time have added to the body of information documenting the regional nature of PM
2.5
.
13

Since the emissions from one State may contribute significantly to PM
2.5
violations in several other States, we believe that plans to attain the PM
2.5
standards will need to include a combination of national, regional, and local emission reduction strategies.

12
National Acid Precipitation Assessment Program. Acid Deposition: State of the Science and Technology. Washington, DC. 1991. See also: Environmental Protection Agency. (2004) Air Quality Criteria for Particulate Matter. Research Triangle Park, NC: Office of Research and Development; report no. EPA/600/P-99/002a,bF. October. The 2004 PM criteria document is available at:
http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html.

13
NARSTO (2004) Particulate Matter Assessment for Policy Makers: A NARSTO Assessment. P. McMurry, M. Shepherd, and J. Vickery, eds. Cambridge University Press, Cambridge, England. ISBN 0 52 184287 5. For more information, see
http://www.cgenv.com/NARSTO.
See also supporting technical information for the Clear Skies Act,
http://www.epa.gov/clearskies/,
and for the Clean Air Interstate Rule,
http://www.epa.gov/cleanairinterstaterule.

E. What Is EPA's Overall Strategy for Reducing PM
2.5
Pollution?

Our overall strategy for achieving the PM
2.5
standards is based on the structure outlined in the CAA. The CAA outlines important roles for State and Tribal governments and for EPA in implementing national ambient air quality standards.

States have primary responsibility for developing and implementing SIPs that contain local and in-State measures needed to achieve the air quality standards in each area. We assist States and Tribes by providing technical tools, assistance and guidance, including information on control measures. In addition, we set national emissions limits for some sources such as new motor vehicles, certain categories of major new sources, and existing stationary sources of toxic air pollutants. Where upwind sources (such as coal-fired power plants) contribute to downwind problems in other States or Tribes, we can also ensure that the upwind States address these contributing emissions, or we can put in place Federal regulations in situations where the upwind States fail to address these sources. We intend to work closely with States and Tribes to use an appropriate combination of national, regional, and local pollution reduction measures to meet the standards as expeditiously as practicable, as required by the CAA.

1. The State Implementation Plan (SIP) System

A SIP is the compilation of regulations and programs that a State uses to carry out its responsibilities under the CAA, including the attainment, maintenance, and enforcement of NAAQS. (Only certain air quality programs and regulations implemented by States are required to be part of the SIP, however.) States use the SIP process to identify the emissions sources that contribute to the nonattainment problem in a particular area, and to select the emissions reduction measures most appropriate for that area, considering technical and economic feasibility, and a variety of local factors such as population exposure, enforceability, and economic impact. Under the CAA, SIPs must ensure that areas reach attainment as expeditiously as practicable. These plans need to take into consideration emission reductions resulting from national programs (such as mobile source regulations, the acid rain program, or maximum achievable control technology (MACT) standards for air toxics) as well as from State or local programs not directly mandated under the CAA.

The SIP system for nonattainment areas is an important component of the CAA's overall strategy for meeting the PM
2.5
standards, but it is not the only component. As noted below, the CAA also includes requirements for national rules or programs that will reduce emissions and help achieve cleaner air.

2. National Rules

For the States to be successful in developing local plans showing attainment of standards, we must do our part to develop standards and programs to reduce emissions from sources that are more effectively and efficiently addressed at the national level. We also have the responsibility to ensure that interstate transport is addressed through SIPs or other means. As outlined below, we have issued final regulations that will achieve important emissions reductions from power plants, onroad and nonroad engine sources, and other sources that may enable some areas to meet the PM
2.5
standards in the near term and make it easier for others to attain.

The acid rain program, authorized under title IV of the 1990 CAA amendments, was projected to reduce annual SO
2
emissions by 10 million tons from 1980 levels by 2010, and to reduce annual NO
X
emissions by 2 million tons from 1980 levels by 2010. The EPA has implemented the acid rain

program in two phases: Phase I for SO
2
began in 1995 and targeted the largest and highest-emitting coal-fired power plants. Phase I for NO
X
began in 1996. Phase II for both pollutants began in 2000 and sets restrictions on Phase I plants as well as many additional smaller coal-, gas-, and oil-fired plants. Over 2,000 sources (mostly electricity generating facilities) are now affected by the Acid Rain Program. The acid rain emissions trading system had a cap of 8.95 million tons on the total amount of SO
2
that may be emitted by power plants nationwide, about half the amount emitted in 1980. Sulfate particles formed from SO
2
emissions and nitrate particles formed from NO
X
emissions contribute significantly to total PM
2.5
mass in the eastern U.S. (ranging from 30-50 percent), so the reductions already achieved under the Acid Rain Program have led to improvements in PM
2.5
concentrations across the region.

Additional reductions in NO
X
emissions from power plants and large industrial sources were required by May 2004 under our rules to reduce interstate transport of ozone pollution in the eastern U.S. These rules are known as the NO
X
SIP Call, published October 27, 1998 (63 FR 57356), and the Section 126 Rule, published May 25, 1999 (64 FR 28250). We estimate that when fully implemented, this program will result in the reduction of more than one million tons of summertime NO
X
. While this program was established primarily to address the ground-level ozone problem in the East, it will also result in reduced ambient levels of nitrate, one of the main components of PM
2.5
.

The Administration has proposed nationwide legislation—the Clear Skies Act
14

—to address health and environmental concerns associated with power plant emissions of sulfur dioxide, nitrogen oxides, and mercury. However, because passage of the CSA legislation is not assured, EPA has established the Clean Air Interstate Rule (CAIR),
15

a regulatory approach to address interstate transport of pollution under section 110 of the CAA. Section 110 gives EPA the authority to require SIPs to “prohibit * * * any source or other type of emission activity within the State from emitting any air pollutant in amounts which will contribute significantly to nonattainment in, or interfere with maintenance by, any other State with respect to” any NAAQS, and to prohibit sources or emission activities from emitting pollutants in amounts which will interfere with measures required to be included in State plans to prevent significant deterioration of air quality or to protect visibility (such as the protection of 156 mandatory Federal class I areas under the regional haze rule
16

).

14
For more information on the proposed Clear Skies Act, see EPA's website:
http://www.epa.gov/clearskies/.

15
See
http://www.epa.gov/cair.

16
See 64 FR 35714, July 1, 1999.

CAIR, issued by EPA on March 10, 2005, employs the same emissions trading approach used to achieve cost-effective emission reductions under the acid rain program. It outlines a two-phase program with declining power plant emissions caps for 28 eastern states and the District of Columbia: SO
2
caps of 3.6 million tons in 2010, and 2.5 million in 2015; NO
X
caps of 1.5 in 2009 and 1.3 in 2015; and NO
X
ozone season caps of 580,000 tons in 2009 and 480,000 tons in 2015. Emission caps are divided into State SO
2
and NO
X
budgets. By the year 2015, the Clean Air Interstate Rule will result in:

—$85 to $100 billion in annual health benefits, annually preventing 17,000 premature deaths, millions of lost work and school days, and tens of thousands of non-fatal heart attacks and hospital admissions.

—Nearly $2 billion in annual visibility benefits in southeastern national parks, such as Great Smoky and Shenandoah.

—Significant regional reductions in sulfur and nitrogen deposition, reducing the number of acidic lakes and streams in the eastern U.S.

Current emissions standards for new cars, trucks and buses are reducing motor vehicle emissions of volatile organic compounds (VOCs, also referred to as hydrocarbons), NO
X
, and direct PM emissions (such as elemental carbon) as older vehicles are retired and replaced. Other existing rules are reducing emissions from several categories of nonroad engines. The Tier 2 motor vehicle emission standards, together with the associated requirements to reduce sulfur in gasoline, will provide additional benefits nationally beginning in 2004.
17

When the new tailpipe and sulfur standards are fully implemented, Americans will benefit from the clean-air equivalent of removing 164 million cars from the road.

17
See Tier II emission standards at 65 FR 6698, February 10, 2000.

These new standards require passenger vehicles to have emissions 77 to 95 percent cleaner than those on the road today and reduce the sulfur content of gasoline by up to 90 percent. In addition, the 2001 heavy-duty diesel engine regulations
18

will lead to continued emissions reductions as older vehicles in that engine class are retired and fleets turn over. New emission standards will begin to take effect in model year 2007 and will apply to heavy-duty highway engines and vehicles. These standards are based on the use of high-efficiency catalytic exhaust emission control devices or comparably effective advanced technologies. Because these devices are damaged by sulfur, the level of sulfur in highway diesel fuel will be reduced by 97 percent by mid-2006. We project a 2.6 million ton reduction of NO
X
emissions in 2030 when the current heavy-duty vehicle fleet is completely replaced with newer heavy-duty vehicles that comply with these emission standards. By 2030, we estimate that this program will reduce annual emissions of hydrocarbons by 115,000 tons and PM by 109,000 tons. These emissions reductions are on par with those that we anticipate from new passenger vehicles and low sulfur gasoline under the Tier 2 program.

18
See heavy-duty diesel engine regulations at 66 FR 5002, January 18, 2001.

EPA also finalized national rules in May 2004 to significantly reduce PM
2.5
and NO
X
emissions from nonroad diesel-powered equipment.
19

These nonroad sources include construction, agricultural, and industrial equipment, and their emissions constitute an important fraction of the inventory for direct PM
2.5
emissions (such as elemental carbon and organic carbon), and NO
X
. The EPA estimates that affected nonroad diesel engines currently account for about 44 percent of total diesel PM emissions and about 12 percent of total NO
X
emissions from mobile sources nationwide. These proportions are even higher in some urban areas. The diesel emission standards will reduce emissions from this category by more than 90 percent, and are similar to the onroad engine requirements implemented for highway trucks and buses. Because the emission control devices can be damaged by sulfur, EPA also established requirements to reduce the allowable level of sulfur in nonroad diesel fuel by more than 99 percent by 2010. In 2030, when the full inventory of older nonroad engines has been replaced, the nonroad diesel program will annually prevent up to 12,000 premature deaths, one million lost work days, 15,000 heart attacks and 6,000 children's asthma-related emergency room visits.

19
For more information on the proposed nonroad diesel engine standards, see EPA's website:
http://www.epa.gov/nonroad/.

II. Fine Particles: Overview of Atmospheric Chemistry, Sources of Emissions, and Ambient Monitoring Data

A. Introduction

Particulate matter is a chemically and physically diverse mixture of discrete solid particles and liquid droplets. It exists in the air in a range of particle sizes, from submicrometer to more than 30 micrometers in size. The composition of particles varies throughout this range of sizes, depending on the age of the particle, the nature of the source of pollutant emissions, and the source's operating characteristics.

This regulation focuses on reducing ambient concentrations of the PM
2.5
size fraction of PM. The term PM
2.5
is used to describe the fraction of particles whose nominal aerodynamic diameter is less than or equal to 2.5 micrometers. PM
2.5
in the ambient air is defined operationally as the set of particles measured (and associated concentration) by the Federal Reference Method sampling device. Since the cut point of this sampling device is not perfectly sharp, some particles smaller than 2.5 micrometers are not retained and some particles larger than 2.5 micrometers are captured by sampling devices. This is important because there are two relevant modes to the PM size distribution, fine PM (nominally PM
2.5
) and coarse PM (nominally from 2.5 to 10 micrometers aerodynamic diameter). These modes overlap slightly, but they are generally associated with distinctly different source types and formation processes.

Fine particles emitted directly into the air in a stable solid or liquid chemical form are referred to as “primary” particles. Particles formed near their source by condensation processes in the atmosphere are also considered to be primary particles. PM
2.5
that is formed by chemical reactions of gases in the atmosphere is considered to be “secondarily” formed particulate matter.

PM
2.5
in the atmosphere is composed of a complex mixture of constituents: Sulfate; nitrate; ammonium; particle-bound water; black carbon (also known as elemental carbon); a great variety of organic compounds; and miscellaneous inorganic material (sometimes called “crustal material,” which includes geogenic dust and metals). Atmospheric PM
2.5
also contains a large number of elements in various compounds and concentrations. Some organic materials such as pollen, spores, and plant detritus are also found in both the fine and coarse particle modes but from different sources or mechanisms. Crustal materials such as calcium, aluminum, silicon, magnesium, and iron are found predominately in coarse mode particles. Nitrate is generally found in the fine particle mode, but it is also found in the coarse mode particles, coming primarily from the reaction of gas-phase nitric acid with preexisting coarse particles.

Primary coarse particles are usually formed by mechanical processes. This includes material emitted from such sources as wind-blown dust, road dust, and particles formed by abrasion, crushing, and grinding. Some combustion-generated particles such as fly ash and soot also are found in the coarse mode. Primary PM
2.5
includes soot from diesel engines, a wide variety of organic compounds condensed from incomplete combustion or cooking operations, and compounds such as arsenic, selenium, and zinc that condense from vapor formed during combustion or smelting. The concentration of primary PM
2.5
in the air depends on source emission rates, transport and dispersion, and removal rate from the atmosphere.

Secondary PM is formed by chemical reactions of gas-phase precursors in the atmosphere. These reactions form condensable vapors that either form new particles or condense onto other particles in the air. Most of the sulfate and nitrate and a portion of the organic compounds in the atmosphere are formed by such chemical reactions. Secondary PM formation depends on numerous factors including the concentrations of precursors; the concentrations of other gaseous reactive species such as ozone, hydroxyl radicals, peroxy radicals, or hydrogen peroxide; atmospheric conditions including solar radiation, temperature, and relative humidity (RH); and the interactions of precursors and pre-existing particles with cloud or fog droplets or in the liquid film on solid particles. Several atmospheric aerosol species, such as ammonium nitrate and certain organic compounds, are semivolatile and are found in both gas and particle phases. Given the complexity of PM formation processes, new information from the scientific community continues to emerge to improve our understanding of the relationship between sources of PM precursors and secondary particle formation.

Certain particles, such as sulfates, nitrates, and certain organics, readily take up water and are considered to be hygroscopic. As a result of the equilibrium of water vapor with liquid water in hygroscopic particles, many ambient particles contain some amount of liquid water. When filter samples are weighed at lower relative humidity levels according to the PM
2.5
Federal reference method specifications, the filters are desiccated and much of this water is removed, but some particle-bound water will be measured as a component of the particle mass. Particle-bound water in the ambient air increases with higher relative humidities. This phenomenon is important because it affects the size of certain particles, and in turn, their properties of light scattering and aerodynamics. Differences in relative humidity can result in different measured particle size distributions, mass concentrations, and resulting visibility impairment levels. Regional emission reduction strategies to reduce PM
2.5
, particularly hygroscopic particles such as sulfates and nitrates, should also provide significant visibility improvements, both in urban areas and in federal class I areas (national parks and wilderness areas).

The following discussion elaborates on the relationship between source types and the composition of PM
2.5
. More information and references on the composition of PM may be found in the EPA 2004 PM Air Quality Criteria Document.
20

20
Environmental Protection Agency. (2004) Air Quality Criteria for Particulate Matter. Research Triangle Park, NC: Office of Research and Development; report no. EPA/600/P-99/002a,bF. October. The 2004 PM criteria document is available at:
http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html.

B. Concentration, Composition and Sources of Fine PM

The relative contribution of PM
2.5
components varies significantly by region of the country. Data on PM
2.5
composition primarily in urban areas is available from the EPA Speciation Trends Network beginning in 2001. PM
2.5
composition data for primarily rural areas (
e.g.
national parks and wilderness areas) is available from the IMPROVE visibility monitoring network beginning in 1988. Speciation data from September 2001 to August 2002 are summarized for urban and rural areas in nine regions in table 2.

BILLING CODE 6560-50-U

EP01NO05.000

BILLING CODE 6560-50-C

This discussion focuses on the eastern U.S. and California since most nonattainment areas will be located in those regions. In general, urban areas have higher annual average PM
2.5
concentrations than nearby rural areas. In the eastern U.S. urban areas, ammonium sulfate and total carbon (comprised of black carbon and organic carbon) are the dominant species, each accounting for 30-40 percent of total reconstructed mass in most locations. (Reconstructed mass is the PM mass calculated by adding together the mass from each of the main components of PM as obtained from chemical composition monitoring.) Nitrate plus associated ammonium ion is a more significant component of PM mass in northern regions, such as the midwest and east coast, but is a less significant fraction in the southeast. In California, the main species contributing to urban PM
2.5
mass are ammonium nitrate (35-40 percent) and total carbon (43 percent), while sulfate and associated ammonium accounts for approximately 10-15 percent.

Table 3 compares chemical composition data for 13 pairs of urban and nearby non-urban sites in order to identify the primary components that make up the “urban increment.” To conduct this analysis, for each species the PM
2.5
mass in the rural location is subtracted from the species mass for the urban location. The amount by which the urban site exceeds the nearby rural site is the “urban increment.”
21

21
V. Rao, N. Frank, A. Rush, F. Dimmick, “Chemical Speciation of PM
2.5
in Urban and Rural Areas,” In the Proceedings of the Air & Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, November 13-15, 2002.

Table 3.—Urban Increment Analysis for 13 Urban/Rural Pairs
[All values in micrograms per cubic meter]

Chemical species

West
(3 site pairs)

Min.
Max.
Avg.

East
(10 site pairs)

Min.
Max.
Avg.

Sulfate
0.2
0.7
0.5
−0.5
1.1
0.3

Est. Ammonium
0.2
2.2
1.2
0.1
0.8
0.4

Nitrate
0.6
6.9
3.7
0.4
1.4
0.8

Total Carbon
4.8
9.8
6.6
2.1
5.3
3.1

Crustal
0.1
0.6
0.4
−0.1
0.8
0.3

Total Excess
5.8
20.1
12.4
2.0
9.4
4.8

Carbonaceous mass is the largest contributor to urban increments in all regions of the country. In east coast and midwestern urban areas, carbon can account for as much as 70-90 percent of the total urban increment. The highest local increment of carbon as calculated from available data appears to be about 10 μg/m
3
in Fresno, CA. Nonroad diesel, onroad diesel, gasoline highway vehicles, and fire related activities are regarded to be important major contributors to this urban excess of carbon. The relative amounts of primary versus secondary organic compounds in the ambient air vary with location and time of year. While it is difficult to generalize, it is clear that both primary and secondary organic compounds are significant contributors to ambient PM
2.5
mass in many parts of the country.

The urban increment for sulfate, on the other hand, appears to be fairly low in most locations. Rural and urban sulfate levels are often very similar, indicating that sulfate is a regional pollutant that can be transported long distances. This is consistent with the fact that power plants are the principal sources of SO
2
, the precursor to sulfate, and in general, these plants are located outside urban core areas. In some eastern cities, the small estimated urban excess (up to 0.5 μg/m
3
) may be attributed to a range of source types, including power plants located within the metro area, the combustion of sulfur-laden fuel oil used for commercial or institutional heating, and fuel combustion by diesel and gasoline motor vehicles.

Excess nitrate concentrations are observed predominantly in northern, midwestern, and western locations, comprising a larger local contribution than sulfate or crustal material. Nitrate is particularly high in the winter time partly because it is less volatile at colder temperatures and partly because SO
2
is less prone to react preferentially with ammonium in the winter as opposed to the summer. Local sources of NO
X
leading to excess urban nitrate likely include mobile sources and other types of fuel combustion.

Some locations also show a small urban excess of crustal material (
e.g.
inorganic material including metals, dust, sea salt, and other trace elements). The estimation procedure used in the IMPROVE protocol includes the measurement of iron and other trace elements. Therefore, this difference also reflects oxidized particulate metals, some of which may be attributed to road dust or industrial sources in urban areas.

We have developed a National Emissions Inventory (NEI) inventory for use in analyzing trends in emissions, conducting various regulatory analyses for PM, and for use in regional scale modeling.
22

The NEI covers all 50 States plus some of the U.S. territories, and includes point, area, onroad and nonroad mobile sources, biogenic, and geogenic emissions. Large stationary sources are located individually in the inventory while county tallies are used for smaller stationary sources, and area and mobile source category groups. Spatial, temporal and compositional profiles are used to allocate these emissions to time-resolved grids for chemical transport modeling. The inventory includes emissions of SO
2
, NO
X
, VOC, NH
3
, PM
10
, and PM
2.5
. A brief discussion of each particle type, their principal sources (based on the NEI), formation mechanisms, and spatial and temporal patterns follows.

22
USEPA,
National Air Quality and Emissions Trends Report: 2003 Special Studies Edition,
Report Number EPA-454/R-03-005, Research Triangle Park, NC, September 2003. USEPA,
National Air Pollutant Emissions Trends,
Report Number EPA-454/R-00-002, Research Triangle Park, NC, March 2000. See also:
http://www.epa.gov/ttn/chief/trends/.

Primary PM (Crustal and Carbonaceous).
This section addresses inorganic and organic forms of primary PM. The main anthropogenic sources of inorganic (or crustal) particles are: Entrainment by vehicular traffic on unpaved or paved roads; mechanical disturbance of soil by highway, commercial, and residential construction; and agricultural field operations (tilling, planting and harvesting). However, much of these emissions are coarse PM rather than fine PM.

Industrial processes such as quarries, minerals processing, and agricultural crop processing can also emit crustal materials, but their influence is most important close to the source and they are not generally significant contributors to regional scale PM problems. Even so, during certain high wind events, fine crustal PM has been shown to be transported over very long distances. Satellite data and other studies have shown that dust has been transported into the U.S. as a result of Asian or African dust storms.

Emission estimates of mechanically suspended crustal PM from sources within the U.S. are often quite high. However, this PM is often released very close to the ground, and with the exception of windblown dust events, thermal or turbulent forces sufficient to lift and transport them very far from their source are not usually present. Thus, as shown in table 1, crustal material is only a minor part of PM
2.5
annual average concentrations.

Primary carbonaceous particles are largely the result of incomplete combustion of fossil or biomass fuels. This incomplete combustion usually results in emissions of both black carbon and organic carbon particles. High molecular weight organic molecules (
i.e.
, molecules with 25 or more carbon atoms) are either emitted as solid or liquid particles, or as gases that rapidly condense into particle form. These heavy organic molecules sometimes are referred to as volatile organic compounds, but because their characteristics are most like direct PM emissions, they will be considered to be primary emissions for the purposes of this regulation. Primary organic carbon

also can be formed by condensation of semi-volatile compounds on the surface of other particles.

The main combustion sources emitting carbonaceous PM
2.5
are mobile sources (both onroad and nonroad), managed burning, wildland fires, open burning of waste, residential wood combustion, certain industrial processes, and coal and oil-burning boilers (utility, commercial and industrial). Certain organic particles also come from natural sources such as decomposition or crushing of plant detritus. Most combustion processes emit more organic particles than black carbon particles. A notable exception to this are diesel engines, which typically emit more black carbon particles than organic carbon. Because photochemistry is typically reduced in the cooler winter months for much of the country, studies indicate that the carbon fraction of PM mass in the winter months is likely dominated by direct PM emissions as opposed to secondarily formed organic aerosol.

Particles from the earth's crust may contain a combination of metallic oxides and biogenic derived organic matter. The combustion of surface debris will likely entrain some soil. Additionally, emissions from many processes and from the combustion of fossil fuels contain elements that are chemically similar to soil. Thus, a portion of the emissions from combustion activities may be classified as crustal in a compositional analysis of ambient PM
2.5
.

Secondary PM. Although some sulfate and nitrate salts (
i.e.
calcium sulfate, calcium nitrate) and acids (
i.e.
sulfuric acid, nitric acid) are directly emitted by sources under certain circumstances, sulfates and nitrates are predominately formed as a result of chemical reactions with ammonia and other compounds in the atmosphere. (See next sections for more detail.) During combustion, very small combustion nucleation particles (ultrafine particles, less than 0.1μm) are produced. These small particles act as nucleation sites where gases, water vapor, and other nucleation particles can condense or coagulate and therefore cause particle growth in both particle size and particle mass. Ammonium sulfate, ammonium nitrate, and secondarily formed organic aerosols, as well as agglomerating fine particles, all may use these ultrafine particles in their formation and growth in the atmosphere. The secondary organic aerosol (SOA) component of PM
2.5
is a complex mixture of perhaps thousands of organic compounds. A brief discussion of the sources of SO
2
, NO
X
, NH
3
, and organic gases (including VOC and semi-volatile compounds), and the formation of sulfate, nitrate and secondary organic aerosol follows. More detailed discussions of the formation and characteristics of secondary particles can be found in the U.S. EPA Criteria Document,
23

and in the NARSTO Fine Particle Assessment,
24

on which much of the following discussion is based.

23
USEPA, 2003. Air Quality Criteria for Particulate Matter (Fourth External Review Draft). EPA/600/P-99/002aD and bD. U.S. Environmental Protection Agency, Office of Research and Development, National Center For Environmental Assessment, Research Triangle Park Office, Research Triangle Park, NC. June 2003. Available electronically at
http://cfpub.epa.gov/ncea/cfm/partmatt.cfm.

24
North American Research Strategy for Tropospheric Ozone and Particulate Matter (NARSTO) (2004) Particulate Matter Assessment for Policy Makers: A NARSTO Assessment. P. McMurry, M. Shepherd, and J. Vickery, eds. Cambridge University Press, Cambridge, England. ISBN 0 52 184287 5. For more information, see
http://www.cgenv.com/NARSTO.
See also supporting technical information for the Clear Skies Act,
http://www.epa.gov/clearskies/,
and for the Clean Air Interstate Rule,
http://www.epa.gov/cleanairinterstaterule.

Sulfate. SO
2
is emitted mostly from the combustion of fossil fuels in boilers operated by electric utilities and other industry. Less than 20 percent of SO
2
emissions nationwide are from other sources, mainly from other industrial processes including oil refining and pulp and paper production.

The formation of sulfuric acid from the oxidation of SO
2
is an important process for most areas in North America. There are three different pathways for this transformation. First, gaseous SO
2
can be oxidized by the hydroxyl radical (OH) to create sulfuric acid. This gaseous SO
2
oxidation reaction occurs slowly and only in the daytime. The hydroxl radical is an important product of the atmospheric chemistry process that forms ozone through the oxidation of NO
X
to form nitric acid. It is also involved in the formation of secondary organics.

Second, SO
2
can dissolve in cloud water (or fog or rain water), and there it can be oxidized to sulfuric acid by a variety of oxidants, or through catalysis by transition metals such as manganese or iron. If ammonia is present and taken up by the water droplet, then ammonium sulfate will form as a precipitant in the water droplet. After the cloud changes and the droplet evaporates, the sulfuric acid or ammonium sulfate remains in the atmosphere as a particle. This aqueous-phase production process involving oxidants can be very fast; in some cases all the available SO
2
can be oxidized in less than an hour.

Third, SO
2
can be oxidized in reactions in the particle-bound water in the aerosol particles themselves. This process takes place continuously, but only produces appreciable sulfate in alkaline (dust, sea-salt) coarse particles.
25

Oxidation of SO
2
has been also observed on the surfaces of black carbon and metal oxide particles. During the last twenty years, much progress has been made in understanding the first two major pathways, but some important questions still remain about the smaller third pathway. Models indicate that more than half of the sulfuric acid in the eastern United States and in the overall atmosphere is produced in clouds.
26

25
Sievering, H., Boatman, J., Gorman, E., Kim, Y., Anderson, L., Ennis, G., Luria, M., Pandis, S.N., 1992. Removal of sulfur from the marine boundary layer by ozone oxidation in sea-salt. Nature 360, 571-573.

26
McHenry, J.N., Dennis, R.L., 1994. The relative importance of oxidation pathways and clouds to atmospheric ambient sulfate production as predicted by the Regional Acid Deposition Model. Journal of Applied Meteorology 33, 890-905. Also: Langner, J., Rodhe, H., 1991. A global three dimensional model for the tropospheric sulfur cycle. Journal of Atmospheric Chemistry 13, 225-263.

The sulfuric acid formed from the above pathways reacts readily with ammonia to form ammonium sulfate, (NH4)2SO
4
. If there is not enough ammonia present to fully neutralize the produced sulfuric acid (one molecule of sulfuric acid requires two molecules of ammonia), part of it exists as ammonium bisulfate, NH4HSO
4
(one molecule of sulfuric acid and one molecule of ammonia) and the particles are more acidic than ammonium sulfate. In extreme cases (in the absence of sufficient ammonia for neutralization), sulfate can exist in particles as sulfuric acid, H2SO
4
. Sulfuric acid often exists in the plumes of stacks where SO
2
, SO
3
, and water vapor are in much higher concentrations than in the ambient atmosphere, but these concentrations become quite small as the plume is cooled and diluted by mixing.

Nitrate. The main sources of NO
X
are combustion of fossil fuel in boilers and onroad mobile sources. Together they account for more than 60 percent of NO
X
emissions in PM
2.5
nonattainment areas (based on 2001 emission inventory information), with stationary and mobile source fuel combustion each accounting for about half of these emissions. Nitrates are formed from the oxidation of oxides of nitrogen into nitric acid either during the daytime

(reaction with OH) or during the night (reactions with ozone and water).
27

27
Wayne, R.P.,
et al.
, 1991. The nitrate radical: physics, chemistry and the atmosphere. Atmospheric Environment 25A, 1-203.

27

Nitric acid continuously transfers between the gas and the condensed phases through condensation and evaporation processes in the atmosphere. However, unless it reacts with other species (such as ammonia, sea salt, or dust) to form a neutralized salt, it will volatize and not be measured using standard PM
2.5
measurement techniques.
28

The formation of aerosol ammonium nitrate is favored by the availability of ammonia, low temperatures, and high relative humidity. Because ammonium nitrate is not stable in higher temperatures, nitrate levels are typically lower in the summer months and higher in the winter months. The resulting ammonium nitrate is usually in the sub-micrometer particle size range. Reactions with sea-salt and dust lead to the formation of nitrates in coarse particles. Nitric acid may be dissolved in ambient aerosol particles.

28
Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. J. Wiley, New York.

Secondary Organic Aerosol (SOA). The organic component of ambient particles is a complex mixture of hundreds or even thousands of organic compounds. These organic compounds are either emitted directly from sources (
i.e.
primary organic aerosol) or can be formed by reactions in the ambient air (
i.e.
secondary organic aerosol, or SOA).

Volatile organic compounds
29

are key precursors in both the SOA and ozone formation processes. The lightest organic molecules (
i.e.
, molecules with six or fewer carbon atoms) occur in the atmosphere mainly as vapors and typically do not directly form organic particles at ambient temperatures due to the high vapor pressure of their products. However, they participate in atmospheric chemistry processes resulting in the formation of ozone and certain free radical compounds (such as the hydroxyl radical [OH]) which in turn participate in the oxidation of semivolatile organic compounds to form secondary organic aerosols, sulfates and nitrates. These VOCs include all alkanes with up to six carbon atoms (from methane to hexane isomers), all alkenes with up to six carbon atoms (from ethene to hexene isomers), benzene and many low-molecular weight carbonyls, chlorinated compounds, and oxygenated solvents. The relative importance of organic compounds in the formation of organic particles varies from area to area, depending upon local emissions sources, atmospheric chemistry, and season of the year. Intermediate weight organic molecules (
i.e.
, compounds with 7 to 24 carbon atoms) often exhibit a range of volatilities and can exist in both the gas and aerosol phase. For this reason they are also referred to as semivolatile compounds. Semivolatile compounds react in the atmosphere to form secondary organic aerosols. These chemical reactions are accelerated in warmer temperatures, and studies show that SOA typically comprises a higher percentage of carbonaceous PM in the summer as opposed to the winter.

29
29 As discussed earlier, high molecular weight organic molecules (
i.e.
, molecules with 25 or more carbon atoms) are either emitted directly as particles or as liquids that rapidly condense onto existing particles. Because these condensable emissions act primarily as direct PM emissions, they are to be regulated as direct PM
2.5
emissions, not as VOC precursors, for the purposes of this regulation.

The production of SOA from the atmospheric oxidation of a specific VOC depends on four factors: Its atmospheric abundance, its chemical reactivity, the availability of oxidants (O
3
, OH, HNO
3
), and the volatility of its products. In addition, recent work by Jang and others suggests that the presence of acidic aerosols may lead to an increased rate of SOA formation.
30

30
Jang, M.; Czoschke, N.; Lee, S.; Kamens, R. Heterogenous Atmospheric Aerosol Production by Acid-Catalyzed Particle-Phase Reactions,
Science
, vol. 298, p. 814-817, October 25, 2002.

Aromatic compounds such as toluene, xylene, and trimethyl benzene are considered to be the most significant anthropogenic SOA precursors and have been estimated to be responsible for 50 to 70 percent of total SOA in some airsheds.
31

As organic gases such as aromatics are oxidized in the gas phase by species such as the hydroxyl radical (OH), ozone (O
3
), and the nitrate radical (NO
3
) their oxidation products accumulate. Some of these products have low volatility and condense on available particles in an effort to establish equilibrium between the gas and condensed phases. Man-made sources of aromatics gases are mobile sources, petrochemical manufacturing and solvents. The experimental work of Odum and others
32

showed that the secondary organic aerosol formation potential of gasoline could be accounted for solely in terms of its aromatic fraction.

31
Grosjean, D., Seinfeld, J.H., 1989. Parameterization of the formation potential of secondary organic aerosols. Atmospheric Environment 23, 1733-1747.

32
Odum, J.R., Jungkamp, T.P.W., Griffin, R.J., Flagan, R.C., Seinfeld, J.H., 1997. The atmospheric aerosol-forming potential of whole gasoline vapor. Science 276, 97-99.

Some of the biogenic hydrocarbons emitted by trees are also considered to be important precursors of secondary organic particulate matter. Terpenes (α- and β-pinene, limonene, carene, etc.) and the sesquiterpenes are expected to be major contributors to SOA in areas with significant vegetation cover, but isoprene is not. Terpenes are very prevalent in forested areas, especially in the southeastern U.S. The rest of the anthropogenic hydrocarbons (higher alkanes, paraffins, etc.) have been estimated to contribute 5-20 percent to the SOA concentration depending on the area.

Table 4.—Role of Organic Gases in Secondary Organic Aerosol Formation

SOA-forming organic gases
Non SOA-forming organic gases

Anthropogenic

—Aromatics (esp. toluene, xylenes, trimethyl-benzenes)
—Higher alkanes (>6 C atoms)

—Lower alkanes <6 C atoms, (ethane to hexane isomers).
—Benzene.
—Lower MW carbonyls, chlorinated compounds & oxygenated solvents.

Biogenic

—Terpenes (esp. α- and β-pinene, limonene, carene)
—Sesquiterpenes

—Isoprene.

The contribution of the primary and secondary components of organic aerosol to the measured organic aerosol concentrations remains a controversial issue. Most of the research performed to date has been done in southern California, and more recently in central California, while fewer studies have been completed on other parts of North

America. Early studies suggested that the majority of the observed organic particulate matter was secondary in nature. Later investigators focusing on the emissions of primary organic material proposed that 80 percent or so of the organic aerosol in Southern California on a monthly basis resulted from direct organic particle emissions.
33

More recent studies suggest that the primary and secondary contributions are highly variable even during the same day. Studies of pollution episodes indicated that the contribution of SOA to the organic particulate matter varied from 20 percent to 80 percent during the same day.
34

33
Hildemann, L.M., Cass, G.R., Mazurek, M.A., Simoneit, B.R.T., 1993. Mathematical modeling of urban organic aerosol properties measured by high resolution gas-chromatography. Environmental Science and Technology 27, 2045-2055.

33

34
Turpin, B.J., Lim, H.J., 2000. Species contributions to PM mass concentrations: Revisiting common assumptions for estimating organic mass, Aerosol Science and Technology, vol. 35, no. 1, p. 602-610.

Despite significant progress that has been made in understanding the origins and properties of SOA, it remains the least understood component of PM
2.5
. The reactions forming secondary organics are complex and the number of intermediate and final compounds formed is voluminous. Some of the best efforts to unravel the chemical composition of ambient organic aerosol matter have been able to quantify the concentrations of hundreds of organic compounds representing only 10-20 percent of the total organic aerosol mass. For this reason, SOA continues to be a significant topic of research and investigation.

C. The Role of Ammonia in Sulfate, Nitrate & Secondary Organic Aerosol Formation

Ammonia (NH
3
) is a gaseous pollutant that is emitted by natural and anthropogenic sources. Emissions inventories for ammonia are considered to be among the most uncertain of any species related to PM. One recent estimate shows, however, that livestock (73 percent) and fertilizer application (17 percent) are the two primary sources of emissions.
35

(Note that these estimates do not include natural emissions from soil, which can be significant.)

35
Anderson, N., R. Strader, and C. Davidson (2003) Airborne reduced nitrogen: Ammonia emissions from agriculture and other sources, Environment International, 29: 277-286.

Ammonia serves an important role in neutralizing acids in clouds, precipitation and particles. In particular, ammonia neutralizes sulfuric acid and nitric acid, the two key contributors to acid deposition (acid rain). Deposited ammonia also can be an important nutrient, contributing to problems of eutrophication in water bodies.
36

Ammonia would not exist in particles, if not for the presence of acidic species with which it can combine to form a particle. In the eastern U.S., sulfate, nitrate, and the ammonium associated with them can together account for between roughly 30 percent and 75 percent of the PM
2.5
mass. The ammonium itself roughly accounts for between 5 percent and 20 percent of the PM
2.5
.
37

36
Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. J. Wiley, New York.

37
NARSTO, 2003. Particulate Matter Science for Policy Makers—A NARSTO Assessment. Parts 1 and 2. NARSTO Management Office (Envair), Pasco, Washington.
http://www.cgenv.com/NARSTO
.

The NARSTO Fine Particle Assessment indicates that sulfates form preferentially over nitrates and that particle nitrate formation is affected by a number of factors, including the availability of sulfates, NO
X
, ammonia, nitric acid and VOCs. The report also notes that implementing decreasing ammonia emissions where sulfate concentrations are high can reduce PM
2.5
mass concentrations, but may also increase particle and precipitation acidity.
38

As noted above, this acidification of particles may result in an increase in the formation of secondary organic compounds. Moreover, the relationship between ammonia and sulfate-nitrate equilibrium may also impact SOA formation, although this link is not well understood. Recent studies of ammonia sources and possible emission reduction measures indicate that ammonia controls are a maturing science, but that ongoing research will greatly improve our understanding of such control measures.

38
Ibid, at S-31 (table S.4).

The same can be said of our understanding of the role of ammonia in aerosol formation. Based on the above information and further insights gained from the NARSTO Fine Particle Assessment, it is apparent that the formation of sulfate, nitrate and SOA compounds is a complex, nonlinear process. The control techniques for ammonia and the analytical tools to quantify the impact of reducing ammonia emissions on atmospheric aerosol formation are both evolving sciences. Also, there are indications that there may be considerable ambiguity concerning the results of reducing ammonia emissions and in some cases, there may be undesired consequences of ammonia reductions. Therefore, based on our current understanding of ammonia's role in these complex precursor interactions and emission reduction processes, it seems prudent to continue research on ammonia control technologies and the ammonia—sulfate—nitrate—SOA equilibrium before one undertakes broad national programs to reduce ammonia emissions. However, as States and EPA develop a greater understanding over the coming years about the potential air quality effects of reducing ammonia emissions in specific nonattainment areas, it may be appropriate for ammonia reduction strategies to be included in future SIPs. At this time, however, we believe that reducing SO
2
and NO
X
will allow us to move with greater certainty toward achieving our nation's air quality goals. We encourage you to provide comments on the resolution of this issue.

D. Regional Patterns of Carbon, Sulfate and Nitrate, and Indications of Transport

Table 2 above shows that much of the eastern U.S., both urban and non-urban areas alike, is subject to high PM
2.5
concentrations, with the highest concentrations occurring in urban areas. Table 3 above compares the urban and rural concentrations of sulfate, nitrate, and carbon particles. The data show that there are high concentrations of sulfate across the region and that sulfate at urban monitoring sites is only slightly higher than at nearby non-urban sites. In contrast, the carbon mass at urban sites is significantly higher than at the nearby non-urban sites. This seems to indicate that sulfate is present on a much more regional scale and likely is associated with significant pollutant transport. On the other hand, a sizeable fraction of the carbonaceous mass seems to be more associated with urban sources. Mobile sources are much more concentrated in urban areas and may explain much of the elevated urban carbon concentrations. However, black carbon and organic aerosols still make up a large percentage of the non-urban air quality composition, indicating that there is a regional background level of carbon that is enhanced in urban areas by local sources.

The atmospheric lifetimes of particles and thus the distances they can be transported vary with particle size. The regional nature of PM
2.5
reflects the fact that fine particles can be transported over long distances. Ultra-fine and fine particles rapidly grow in size into a relatively stable size range, generally less than 2 μm. These fine particles are kept suspended by normal air motions and have very low deposition rates to surfaces. They can be transported

thousands of kilometers and remain in the atmosphere for a number of days. Thus, they are important when considering regional PM transport. Coarse particles can settle rapidly from the atmosphere within hours and normally travel only short distances. However, when mixed high into the atmosphere, as in some dust storms, the smaller-sized coarse-mode particles may have longer lives and travel greater distances.

Meteorology also plays a role in the size and characteristics of particles. High temperatures increase reaction rates, which may explain why sulfate concentrations are generally greatest in the summer. Conversely, lower temperatures result in a greater fraction of nitrates being in the particle phase. Fine particles, especially particles with a hygroscopic component, grow as the relative humidity increases, serve as cloud condensation nuclei, and grow into cloud droplets. If the cloud droplets grow large enough to form rain, the particles are removed in the rain. Falling rain drops impact coarse particles and remove them. Very fine particles are small enough to diffuse to the falling drop, be captured, and be removed in rain. However, falling rain drops are not nearly as effective in removing PM
2.5
as the cloud processes mentioned above. Sulfuric acid, ammonium nitrate, ammonium sulfates, and organic particles also are deposited on surfaces by dry deposition. Therefore, reductions in SO
2
and NO
X
emissions will decrease both acidic deposition and PM concentrations.

E. Policy for Addressing PM
2.5
Precursors

1. Legal Authority To Regulate Precursors

The Clean Air Act authorizes the Agency to regulate criteria pollutant precursors. The term ‘air pollutant” is defined in section 302(g) to include “any precursors to the formation of any air pollutant, to the extent the Administrator has identified such precursor or precursors for the particular purpose for which the term 'air pollutant' is used.”' The first clause of this second sentence in section 302(g) explicitly authorizes the Administrator to identify and regulate precursors as air pollutants under other parts of the Act. In addition, the second clause of the sentence indicates that the Administrator has discretion to identify which pollutants should be classified as precursors for particular regulatory purposes. Thus, we do not necessarily construe the Act to require that EPA identify a particular precursor as an air pollutant for all regulatory purposes where it can be demonstrated that various Clean Air Act programs address different aspects of the air pollutant problem. Likewise, we do not interpret the Act to require that EPA treat all precursors of a particular pollutant the same under any one program when there is a basis to distinguish between such precursors. For example, in a recent rule addressing PM
2.5
precursors for purposes of transportation conformity, we chose to adopt different approaches for some precursors based on the degree to which the various precursors emitted by transportation-related sources contributed to the PM
2.5
air quality problem. 70 FR 24280 (May 6, 2005).

Other provisions of the Act reinforce our reading of section 302(g) that Congress intended precursors to NAAQS pollutants to be subject to the air quality planning and control requirements of the Act, but also recognized that there may be circumstances where it is not appropriate to subject precursors to certain requirements of the Act. Section 182 of the Act provides for the regulation of NO
X
and VOCs as precursors to ozone in ozone nonattainment areas, but also provides in Section 182(f) that major stationary sources of NO
X
(an ozone precursor) are not subject to emission reductions controls for ozone where the State shows through modeling that NO
X
reductions do not decrease ozone. Section 189(e) provides for the regulation of PM
10
precursors in PM
10
nonattainment areas, but also recognizes that there may be certain circumstances where it is not appropriate to apply control requirements to PM
10
precursors. In providing that the Agency was to issue guidelines for the control of PM
10
precursors, the legislative history of Section 189(e) recognized the complexity behind the science of precursor transformation into PM
10
ambient concentrations and the need to harmonize the regulation of PM
10
precursors with other provisions of the Act:

The Committee notes that some of these precursors may well be controlled under other provisions of the Act. The Committee intends that * * * the Administrator will develop models, mechanisms, and other methodology to assess the significance of the PM
10
precursors in improving air quality and reducing PM
10
. Additionally, the Administrator should consider the impact on ozone levels of PM
10
precursor controls. The Committee expects the Administrator to harmonize the PM
10
reduction objective of this section with other applicable regulations of this Act regarding PM
10
precursors, such as NO
X
.

See H. Rpt. 101-490, Pt. 1, at 268 (May 17, 1990), reprinted in S. Prt. 103-38, Vol. II, at 3292.

In summary, section 302(g) of the Act clearly calls for the regulation of precursor pollutants, but the Act also identifies circumstances when it may not be appropriate to regulate precursors and gives the Administrator discretion to determine how to address particular precursors under various programs required by the Clean Air Act. Due to the complexities associated with precursor emissions and their variability from location to location, we believe that in certain situations it may not be effective or appropriate to control a certain precursor under a particular regulatory program or for EPA to require similar control of a particular precursor in all areas of the country.

In the following section II.E.2, we discuss our proposal for how States should address PM
2.5
precursors for the majority of the nonattainment program issues in PM
2.5
implementation plans, such as RACT, RACM, reasonable further progress and most of the other issues discussed in section III. This discussion is linked to related discussions of precursor issues in the NSR section of this package (see section III.M.), the transportation conformity program (see section III.K. of this package, and the conformity regulations
39

), and the general conformity program (see section III.L. of this package. All of these programs take effect prior to approval of SIPs for attaining the PM
2.5
NAAQS. In the case of NSR, the program applies on the effective date of the nonattainment area designation. In the case of transportation conformity and general conformity, the program takes effect one year from the effective date of designation of the nonattainment area (i.e., April 5, 2006). Thus, for each of these programs there is an interim period between the date the program becomes applicable to a given nonattainment area and the date the State receives EPA approval of its overall PM
2.5
implementation plan. Options for addressing PM
2.5
precursors in the NSR program are discussed in section III.M. below. For the transportation conformity program, precursor policies are addressed in the final rule on PM
2.5
precursors.
40

39
See the final transportation conformity rule (69 FR 40004; July 1, 2004); the conformity rule amendments addressing PM
2.5
precursors (70 FR 24280; May 6, 2005); and transportation conformity regulations at 40 CFR Parts 51 and 93.

40
The final transportation conformity rule on PM
2.5
precursors was published in the
Federal Register
on May 6, 2005 at 70 FR 24280.

2. Proposed Policy Options for Addressing PM
2.5
Precursors in Nonattainment Plan Programs

This section discusses potential options for addressing the PM
2.5
precursors SO
2
, ammonia, NO
X
and volatile organic compounds in PM
2.5
nonattainment plan programs other than NSR and transporation conformity. Several other preamble sections in today's notice, including those on RFP, RACT, RACM, and modeling and attainment demonstrations refer the reader to this overall section. Our approach to precursors of PM
2.5
in these areas will be decided after consideration of comments through this rulemaking process and our policy for PM
2.5
precursors will be stated in the final rule.

As an initial matter, it is helpful to clarify the terminology we use throughout this notice to discuss precursors. We recognize NO
X
, SO
2
, VOCs, and ammonia as precursors of PM
2.5
in the scientific sense because these pollutants can contribute to the formation of PM
2.5
in the ambient air. However, the degree to which these individual precursors and pollutants contribute to PM
2.5
formation in a given location is complex and variable. This requires that we further consider in this action how States should address these PM
2.5
precursors in their PM
2.5
nonattainment plan programs. Thus, where we believe that all states should address a given precursor of PM
2.5
under a specific PM
2.5
nonattainment plan requirement, we refer to it more specifically as a “PM
2.5
nonattainment plan precursor, transportation conformity precursor, or NSR precursor. We request comment on all aspects of the proposed options set forth below.

Sulfur dioxide. We believe the previous technical discussion and analysis of speciated air quality data provides an appropriate basis for requiring States to address sulfur dioxide as a PM
2.5
nonattainment plan precursor in all areas.
41

The fact that sulfate is a significant contributor (e.g. ranging from 9 percent to 40 percent) to PM
2.5
nonattainment and other air quality problems in all regions of the country is a critical piece of evidence supporting this approach. The EPA requests comments on the requirement that SO
2
is a PM
2.5
nonattainment plan precursor in all nonattainment areas.

41
As stated in the May 6, 2005 (70 FR 24282) final transportation conformity rule on PM
2.5
precursors, on-road emissions of sulfur dioxide would only be addressed in conformity determinations if the state air agency or EPA Regional Administrator found that the on-road emissions are a significant contributor to the area's PM
2.5
problem or if the area's SIP established a motor vehicle emissions budget for sulfer oxides.

Ammonia. In regard to ammonia, however, we believe there is sufficient uncertainty about emissions inventories and about the potential efficacy of control measures from location to location such that the most appropriate approach for proposal is a case-by-case approach. Ammonia reductions may be appropriate in selected locations, but in others such reductions may lead to increased atmospheric acidity, exacerbating acidic deposition problems. Each State should evaluate whether reducing ammonia emissions would lead to PM
2.5
reductions in their specific PM
2.5
nonattainment areas. Under this proposed policy, however, States are not required to address ammonia as a PM
2.5
nonattainment plan precursor, unless the State or EPA makes a technical demonstration that ammonia emissions from sources in the State significantly contribute to the PM
2.5
problem in a given nonattainment area or to other downwind air quality concerns. As noted above, ammonia reductions may be effective primarily in areas where nitric acid is in abundance and ammonia is the limiting factor to ammonium nitrate formation. Where the State or EPA has determined that ammonia is a significant contributor to PM
2.5
formation in a nonattainment area, the State would address ammonia emissions in its nonattainment SIP due in 2008. From that point in time, the implementation of the PM program and other associated programs (e.g. the NSR program and transportation conformity program) in that area would proceed in accordance with this determination.
42

Ammonia will be addressed under the transportation conformity program if the SIP establishes a budget specifically for on-road ammonia emissions. The EPA requests comments on this approach to addressing ammonia emissions under the PM
2.5
program.

42
As stated in the May 6, 2005 (70 FR 24282) final transportation conformity rule on PM
2.5
precursors, on-road emissions of ammonia would also be addressed in conformity determinations before a SIP is submitted and budgets are found adequate or approved if the state air agency or EPA Regional Administrator found that the on-road emissions of ammonia are a significant contributor to the area's PM
2.5
problem.

Nitrogen oxides. Based on a review of speciated monitoring data analyses, it is apparent that nitrate concentrations vary significantly across the country. For example, in some southeastern locations, annual average nitrate levels are in the range of 6 to 8 percent of total PM
2.5
mass, whereas nitrate comprises 40 percent or more of PM
2.5
mass in certain California locations. Nitrate formation is favored by the availability of ammonia, low temperatures, and high relative humidity. It is also dependent upon the relative degree of nearby SO
2
emissions because ammonia reacts preferentially with SO
2
over NO
X
.

The sources of NO
X
are numerous and widespread, including motor vehicles, power plants, and many other combustion activities. We believe the previous technical discussion and analysis of speciated air quality data provides an appropriate basis for presuming that states must evaluate and implement reasonable controls on sources of NO
X
in all nonattainment areas. Under this policy, States are required to address NO
X
under all aspects of the program, unless the State and EPA makes a finding that NO
X
emissions from sources in the State do not significantly contribute to the PM
2.5
problem in a given area or to other downwind air quality concerns. An additional consideration is that the majority of potential PM
2.5
nonattainment areas are already designated as nonattainment for the 8-hour ozone standard. For PM
2.5
areas that are also violating the 8-hour ozone standard, strategies to reduce NO
X
emissions will help address both air pollution problems. The EPA requests comments on this approach to addressing NO
X
emissions under the PM
2.5
program.

Volatile Organic Compounds (VOC). Section II.B. discusses the main categories of organic compounds with varying degrees of volatility: Highly reactive, volatile compounds with six or fewer carbon atoms which indirectly contribute to PM formation through the formation of oxidizing compounds such as the hydroxyl radical and ozone; and semivolatile compounds with between seven and 24 carbon atoms which can exist in particle form and can readily be oxidized to form other low volatility compounds. High molecular weight organic compounds (with 25 carbon atoms or more and low vapor pressure) are emitted directly as primary organic particles and exist primarily in the condensed phase at ambient temperatures. For this reason, these organic compounds will be regulated as primary PM
2.5
emissions and not VOCs for the purposes of the PM
2.5
implementation program.

Current scientific and technical information clearly shows that carbonaceous material is a significant fraction of total PM
2.5
mass in most areas, and that certain aromatic VOC emissions such as toluene, xylene, and trimethyl-benzene are precursors to the formation of secondary organic aerosol. Further, analyses of ambient data

indicate that a considerable fraction of the total carbonaceous material is likely from local as opposed to regional sources.

However, while significant progress has been made in understanding the role of gaseous organic material in the formation of organic PM, this relationship remains complex. We recognize that further research and technical tools are needed to better characterize emissions inventories for specific VOC compounds, and to determine the extent of the contribution of specific VOC compounds to organic PM mass.

In light of the factors discussed above, EPA proposes that States are not required to address VOC's as PM
2.5
nonattainment plan precursors, unless the state or EPA makes a finding that VOC's significantly contribute to a PM
2.5
nonattainment problem in the State or to other downwind air quality concerns. In proposing this policy, we are mindful of the fact that a majority of areas that have been designated as nonattainment for PM
2.5
are already designated as nonattainment for the 8-hour ozone standard. Thus, these areas will already be required to evaluate VOC control measures for ozone purposes. (The inventory of VOC as defined here, including gaseous organic compounds, is essentially identical to the inventory of VOC for ozone control purposes.) The few PM
2.5
areas not designated as nonattainment for the 8-hour ozone standard will not be required to regulate VOC emissions sources unless the State or EPA makes a relevant technical finding. We request comments accompanied by detailed technical supporting information on this proposed policy approach for addressing VOC's under the PM
2.5
implementation program.

In general. Any State or EPA technical demonstration to modify the presumptive policy approach for ammonia, NO
X
, or VOC should be developed well in advance of the SIP submittal date. In addition, the development of such a technical demonstration should include consultation with appropriate State, local, and EPA technical representatives representing air quality and transportation agencies.

III. What Are the Specific Elements of EPA's PM
2.5
Implementation Program?

A. What classification options are under consideration for PM
2.5
nonattainment areas?

1. Background

Section 172 of subpart 1 contains the general requirements for SIPs for all nonattainment areas. Section 172(a)(1) states that on or after the date of designation, the Administrator may classify the area for the purpose of applying an attainment date or for some other purpose. Thus, a classification system is allowed under section 172, but is not required for the purposes of implementing a national ambient air quality standard.

If we choose to establish a classification system, the Act states that we may consider certain factors in doing so, such as the severity of nonattainment in such areas, and the availability and feasibility of the pollution control measures that may be needed to achieve attainment. We must publish a notice in the
Federal Register
announcing any classifications and provide for at least 30 days for written comment. Classifications are not subject to notice and comment rulemaking requirements, however, nor are they subject to judicial review until we take any action on plan submissions (under sections 110(k) or 110(l)), or sanctions in cases where the State fails to submit a plan (under section 179).

2. Proposed Options for PM
2.5
Classifications

This section describes two implementation approaches for classifying or not classifying PM
2.5
nonattainment areas. The first and preferred option is to not have any classification system. The second option would have a two-tiered classification system, with areas classified as “moderate” or “serious” based on specific criteria. These options are discussed below.

a. No Classification System Based on Design Values

In today's notice, our preferred option is to not have any system for classifying PM
2.5
nonattainment areas or assigning attainment dates and control strategy requirements based on the severity of the nonattainment problem (e.g. the area's design value). We believe that an advantage of this approach is that it will provide a relatively simple implementation structure for state implementation of the PM
2.5
standards. This approach also will allow flexibility to determine attainment dates and control strategies appropriate for each area under Clean Air Act requirements.

We believe that with the variable mix of sources contributing to PM
2.5
concentrations in various regions of the country and the variable set of appropriate control measures, it may not be advantageous to have a classification system which automatically requires a longer list of control strategies, and allows a later attainment date, for areas with higher current levels of PM
2.5
pollution.

Under our proposed approach, the State will be required to submit an attainment demonstration for each nonattainment area proposing an attainment date that is as expeditious as practicable for each area. (Attainment date issues are discussed in more detail in section III.C.) In determining what attainment date is considered “as expeditious as practicable,” the State will need to demonstrate that it is achieving RFP (see section III.G.), and it will have to adopt rules to implement the RACT and RACM requirements within the nonattainment area (see section III.I.) in order to attain the standard as expeditiously as practicable. In determining an expeditious attainment date, the State will need to take into consideration the air quality improvements that are expected due to other emission reduction programs at the national level (e.g. Tier II vehicle standards, heavy-duty diesel program, etc.), regional level reductions (e.g. NO
X
SIP call), any additional regional SO
2
or NO
X
reductions that may be achieved under a legislative or regulatory approach, and State level (e.g. Clean Smokestacks legislation in North Carolina).

b. Two-Tiered Classification System

Another option on which we are seeking comment is a two-tiered classification system. Under this approach, areas with higher PM
2.5
levels (i.e. design values) would qualify for an attainment date extension beyond April 2010 to no later than April 2015. In return, consistent with the approach in subpart 2, part D of Title I for ozone, such areas would be required to include certain mandatory measures in their SIPs.

Definition of serious and moderate areas. This option would establish two nonattainment classification categories: “moderate” and “serious.” These categories could be based on the severity of nonattainment (e.g., serious areas would be those with a design value above a specific threshold), the attainment date for the area (e.g., serious areas would be those with attainment dates after April 2010), or some other measure. We invite comment on appropriate ways to define moderate and serious areas and request that any recommended approach be accompanied by adequate supporting information.

Under a potential two-tiered classification system, all areas not classified as “serious” would be classified as “moderate.” However, any

moderate area that needed an attainment date longer than five years would be reclassified to serious. This would ensure that areas with a more persistent PM
2.5
problem are subject to more stringent requirements, even if they are not one of the areas with the highest current design values. For such areas, the state would be required to request reclassification and ensure that the 2008 attainment SIP submission for the area includes all measures needed to meet serious area requirements.

Serious area requirements. Serious areas would be required to meet RACM and RACT requirements described elsewhere in this notice. The attainment date would be as expeditiously as practicable, but no later than 10 years after designation, depending on the year in which the area would be projected to attain considering existing control requirements and the effect of RACM, RACT and RFP.

Various approaches can be considered for outlining additional requirements for serious areas beyond those required for all areas by subpart 1. More stringent requirements for serious areas could be established for RFP, RACT, and/or RACM.

For RFP, one approach could involve setting a more prescriptive or higher RFP requirement for serious areas from the 2002 base year to the attainment year. For example, the required rate could be a specific annual percentage reduction in direct PM
2.5
and all PM
2.5
precursors, analogous to the 3% per year reduction requirement for the 1-hour ozone program in section 182 of the Act. This approach is described among the options in the RFP section of this proposal (see section III.G.5). Progress would be evaluated in 2008 and every 3 years thereafter. An alternative could be to require a specific weighted average annual reduction in direct PM
2.5
and all precursors, based upon the PM
2.5
speciation profile for the relevant urban area.

An additional requirement for serious areas could be to define a lower emissions threshold for major sources for purposes of determining applicability

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A05-20455. Public record. Not legal advice.