[Federal Register Volume 80, Number 145 (Wednesday, July 29, 2015)]
[Proposed Rules]
[Pages 45339-45387]
[FR Doc No: 2015-18075]

Vol. 80

Wednesday,

No. 145

July 29, 2015

Part III

Environmental Protection Agency

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40 CFR Part 51

Revision to the Guideline on Air Quality Models: Enhancements to the
AERMOD Dispersion Modeling System and Incorporation of Approaches To
Address Ozone and Fine Particulate Matter; Proposed Rule

Federal Register / Vol. 80 , No. 145 / Wednesday, July 29, 2015 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 51

[EPA-HQ-OAR-2015-0310; FRL-9930-11-OAR]
RIN 2060-AS54

Revision to the Guideline on Air Quality Models: Enhancements to
the AERMOD Dispersion Modeling System and Incorporation of Approaches
To Address Ozone and Fine Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule; notice of conference.

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SUMMARY:  In this action, the Environmental Protection Agency (EPA)
proposes to revise the Guideline on Air Quality Models (``Guideline'').
The Guideline has been incorporated into EPA's regulations, satisfying
a requirement under the Clean Air Act (CAA) section 165(e)(3) for the
EPA to specify, with reasonable particularity models to be used in the
Prevention of Significant Deterioration (PSD) program. It provides EPA-
preferred models and other recommended techniques, as well as guidance
for their use in predicting ambient concentrations of air pollutants.
The proposed revisions to the Guideline include enhancements to the
formulation and application of the EPA's AERMOD near-field dispersion
modeling system and the incorporation of a tiered demonstration
approach to address the secondary chemical formation of ozone and fine
particulate matter (PM2.5) associated with precursor
emissions from single sources. Additionally, the EPA proposes various
editorial changes to update and reorganize information throughout the
Guideline to streamline the compliance assessment process.
    Within this action, the EPA is also announcing the Eleventh
Conference on Air Quality Modeling and invites the public to
participate in the conference. The conference will focus on the
proposed revisions to the Guideline and part of the conference will
also serve as the public hearing for these revisions.

DATES:  Comments must be received on or before October 27, 2015.
    Public hearing and conference: The public hearing for this action
and the Eleventh Conference on Air Quality Modeling will be held August
12-13, 2015, from 8:30 a.m. to 5:00 p.m.

ADDRESSES:  Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2015-0310, by one of the following methods:
     Federal eRulemaking Portal: http://www.regulations.gov.
Follow the online instructions for submitting comments.
     Email<EMAIL_ADDRESS>Include docket ID No. EPA-
HQ-OAR-2015-0310 in the subject line of the message.
     Fax:<PHONE_NUMBER>.
     Mail: Environmental Protection Agency, Mail code 28221T,
Attention Docket No. EPA-HQ-OAR-2015-0310, 1200 Pennsylvania Ave. NW.,
Washington, DC 20460. Please include a total of two copies.
     Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA
WJC West Building, 1301 Constitution Ave. NW., Washington, DC. Such
deliveries are only accepted during the Docket's normal hours of
operation, and special arrangements should be made for deliveries of
boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2015-0310. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at http://www.regulations.gov, including any personal
information provided, unless the comment includes information claimed
to be Confidential Business Information (CBI) or other information
whose disclosure is restricted by statute. Do not submit information
that you consider to be CBI or otherwise protected through http://www.regulations.gov or email. The www.regulations.gov Web site is an
``anonymous access'' system, which means the EPA will not know your
identity or contact information unless you provide it in the body of
your comment. If you send an email comment directly to the EPA without
going through http://www.regulations.gov, your email address will be
automatically captured and included as part of the comment that is
placed in the public docket and made available on the Internet. If you
submit an electronic comment, the EPA recommends that you include your
name and other contact information in the body of your comment and with
any disk or CD ROM you submit. If the EPA cannot read your comment due
to technical difficulties and cannot contact you for clarification, the
EPA may not be able to consider your comment. Electronic files should
avoid the use of special characters, any form of encryption, and be
free of any defects or viruses. For additional information about the
EPA's public docket, visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm.
    Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although listed in the index, some
information is not publicly available, e.g., CBI or other information
whose disclosure is restricted by statute. Certain other material, such
as copyrighted material, will be publicly available only in hard copy.
Publicly available docket materials are available either electronically
in www.regulations.gov or in hard copy at the Air and Radiation Docket
and Information Center, EPA/DC, Room 3334, WJC West Building, 1301
Constitution Ave. NW., Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is (202)
566-1744 and the telephone number for the Air and Radiation Docket and
Information Center is<PHONE_NUMBER>.
    Public hearing and conference: The public hearing for this action
and the Eleventh Conference on Air Quality Modeling will be held in the
EPA Auditorium, Room C111, 109 T.W. Alexander Drive, Research Triangle
Park, NC 27711.

FOR FURTHER INFORMATION CONTACT:  Mr. George M. Bridgers, Air Quality
Assessment Division, Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Mail code C439-01, Research Triangle
Park, NC 27711; telephone:<PHONE_NUMBER>; fax:<PHONE_NUMBER>; email:
<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

Table of Contents

    The following topics are discussed in this preamble:

I. General Information
    A. Does this action apply to me?
    B. What should I consider as I prepare my comments for the EPA?
    C. Where can I get a copy of this document?
II. Background
    A. The Guideline on Air Quality Models and EPA Modeling
Conferences
    B. The Tenth Conference on Air Quality Modeling
III. Public Participation Regarding Revisions to the Guideline and
Notice of Eleventh Conference on Air Quality Modeling
IV. Proposed Changes to the Guideline
    A. Proposed Actions
    1. Clarifications To Distinguish Requirements From
Recommendations
    2. Updates to EPA's AERMOD Modeling System
    3. Status of AERSCREEN
    4. Updates to 3-Tiered Demonstration Approach for NO2
    5. Status of CALINE3 Models
    6. Addressing Single-Source Impacts on Ozone and Secondary
PM2.5

    7. Status of CALPUFF and Assessing Long-Range Transport for PSD
Increment and Regional Haze
    8. Role of EPA's Model Clearinghouse
    9. Updates to Modeling Procedures for Cumulative Impact Analysis
    10. Updates on Use of Meteorological Input Data for Regulatory
Dispersion Modeling
    11. Transition Period for Applicability of Revisions to the
Guideline
    B. Proposed Editorial Changes
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations

I. General Information

A. Does this action apply to me?

    This action applies to federal, state, territorial, and local air
quality management programs that conduct air quality modeling as part
of State Implementation Plan (SIP) submittals and revisions, New Source
Review (NSR), including new or modifying industrial sources under
Prevention of Significant Deterioration (PSD), Conformity, and other
air quality assessments required under EPA regulation. Categories and
entities potentially regulated by this action include:

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                                                              NAICS \a\
                          Category                               Code
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Federal/state/territorial/local/tribal government..........       924110
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\a\ North American Industry Classification System.

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

    1. Submitting CBI. Do not submit this information to the EPA
through http://www.regulations.gov or email. Clearly mark any of the
information that you claim to be CBI. For CBI information in a disk or
CD ROM that you mail to the EPA, mark the outside of the disk or CD ROM
as CBI and then identify electronically within the disk or CD ROM the
specific information that is claimed as CBI. In addition to one
complete version of the comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.
    2. Tips for preparing your comments. When submitting comments,
remember to:
     Follow directions--The agency may ask you to respond to
specific questions or organize comments by referencing a CFR part or
section number.
     Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
     Describe any assumptions and provide any technical
information and/or data that you used.
     If you estimate potential costs or burdens, explain how
you arrived at your estimate in sufficient detail to allow for it to be
reproduced.
     Provide specific examples to illustrate your concerns and
suggest alternatives.
     Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
     Make sure to submit your comments by the comment period
deadline identified.

C. Where can I get a copy of this document?

    In addition to being available in the docket, an electronic copy of
this proposed rule will also be available on the Worldwide Web (WWW)
through the EPA's Technology Transfer Network (TTN). Following
signature, a copy of this proposed rule will be posted on the TTN's
Support Center for Regulatory Atmospheric Modeling (SCRAM) Web site at
the following address: http://www.epa.gov/ttn/scram. The TTN provides
information and technology exchange in various areas of air pollution
control.

II. Background

A. The Guideline on Air Quality Models and EPA Modeling Conferences

    The Guideline is used by the EPA, other federal, state,
territorial, and local air quality agencies, and industry to prepare
and review new source permits, source permit modifications, SIP
submittals and revisions, conformity, and other air quality assessments
required under EPA regulation. The Guideline serves as a means by which
national consistency is maintained in air quality analyses for
regulatory activities under 40 CFR 51.112, 51.117, 51.150, 51.160,
51.165, 51.166, 52.21, 93.116, 93.123, and 93.150.
    The EPA originally published the Guideline in April 1978 (EPA-450/
2-78-027), and it was incorporated by reference in the regulations for
the PSD program in June 1978. The EPA revised the Guideline in 1986 (51
FR 32176), and updated it with supplement A in 1987 (53 FR 32081),
supplement B in July 1993 (58 FR 38816), and supplement C in August
1995 (60 FR 40465). The EPA published the Guideline as appendix W to 40
CFR part 51 when the EPA issued supplement B. The EPA republished the
Guideline in August 1996 (61 FR 41838) to adopt the CFR system for
labeling paragraphs. The publication and incorporation of the Guideline
by reference into the EPA's PSD regulations satisfies the requirement
under the CAA section 165(e)(3) for the EPA to promulgate regulations
that specify with reasonable particularity models to be used under
specified sets of conditions for purposes of the PSD program.
    To support the process of developing and revising the Guideline
during the period of 1977-1988, we held the First, Second, and Third
Conferences on Air Quality Modeling as required by CAA section 320 to
help standardize modeling procedures. These modeling conferences
provided a forum for comments on the Guideline and associated
revisions, thereby helping us introduce improved modeling techniques
into the regulatory process.
    In October 1988, we held the Fourth Conference on Air Quality
Modeling to advise the public on new modeling techniques and to solicit
comments to guide our consideration of any rulemaking needed to further
revise the Guideline. We held the Fifth Conference in March 1991, which
also served as a public hearing for the proposed revisions to the
Guideline. In August 1995, we held the Sixth Conference as a forum to
update our available modeling tools with state-of-the-science
techniques and for the public to offer new ideas.
    The Seventh Conference was held in June 2000, and also served as a
public hearing for another round of proposed changes to the recommended
air quality models in the Guideline. These changes included the CALPUFF
modeling system, AERMOD modeling system, and ISC-PRIME model.
    Subsequently, the EPA revised the Guideline on April 15, 2003 (68
FR 18440), to adopt CALPUFF as the preferred model for long-range
transport

of emissions from 50 to several hundred kilometers and to make various
editorial changes to update and reorganize information and remove
obsolete models.
    We held the Eighth Conference on Air Quality Modeling in September
2005. This conference provided details on changes to the preferred air
quality models, including available methods for model performance
evaluation and the notice of data availability that the EPA published
in September 2003, related to the incorporation of the PRIME downwash
algorithm in the AERMOD dispersion model (in response to comments
received from the Seventh Conference). Additionally, at the Eighth
Conference, a panel of experts discussed the use of state-of-the-
science prognostic meteorological data for informing the dispersion
models.
    The EPA further revised the Guideline on November 9, 2005 (70 FR
68218), to adopt AERMOD as the preferred model for near-field
dispersion of emissions for distances up to 50 kilometers.
    The Ninth Conference on Air Quality Modeling was held in October
2008, and emphasized the following topics: Reinstituting the Model
Clearinghouse, review of non-guideline applications of dispersion
models, regulatory status updates of AERMOD and CALPUFF, continued
discussions on the use of prognostic meteorological data for informing
dispersion models, and presentations reviewing the available model
evaluation methods.

B. The Tenth Conference on Air Quality Modeling

    The most recent EPA modeling conference was the Tenth Conference on
Air Quality Modeling held in March 2012. This conference covered
multiple topics which have been vital in the development of the
proposed revisions to the Guideline. The conference addressed updates
on the regulatory status and future development of AERMOD and CALPUFF,
review of the Mesoscale Model Interface (MMIF) prognostic
meteorological data processing tool for dispersion models, draft
modeling guidance for compliance demonstrations of the PM2.5
National Ambient Air Quality Standards (NAAQS), modeling for compliance
demonstration of the 1-hour nitrogen dioxide (NO2) and
sulfur dioxide (SO2) NAAQS, and new and emerging models/
techniques for future consideration under the Guideline to address
single-source modeling for ozone and secondary PM2.5, as
well as long-range transport and chemistry. A transcript of the
conference proceedings and a document that summarizes the public
comments received are available at EPA's SCRAM Web site at http://www.epa.gov/ttn/scram/10thmodconf.htm.
    The EPA promulgated a new 1-hour NAAQS for NO2 in
January 2010, and a new 1-hour NAAQS for SO2 in June 2010.
Although AERMOD evaluations that formed the basis of its promulgation
as the EPA's preferred dispersion model demonstrated that AERMOD
provides generally unbiased estimates of ambient concentrations, the
increased stringency of these new standards resulted in increased
scrutiny by the modeling community of AERMOD model performance. In
response, the EPA issued several guidance memoranda to clarify the
applicability of the Guideline and address initial issues with use of
current models and procedures under PSD permitting.1 2 3 4
However, the situation also necessitated the EPA and the modeling
community to more closely evaluate the science and model formulation of
AERMOD to better understand the issues being experienced by
stakeholders and to address performance issues in its use for PSD
permitting under these new standards. As part of this effort, the EPA
reconvened the AERMOD Implementation Workgroup (AIWG) with state and
local agency modelers to evaluate AERMOD across a variety of
hypothetical sources and results from this assessment were also
presented at this conference to inform the modeling community of
potential implications and areas for improvement in the model and
guidance on their use.
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    \1\ U.S. EPA, 2010. Applicability of Appendix W Modeling
Guidance for the 1-hour NO2 NAAQS. Tyler Fox Memorandum
dated June 28, 2010, Office of Air Quality Planning & Standards,
Research Triangle Park, North Carolina 27711. http://www.epa.gov/ttn/scram/guidance/clarification/ClarificationMemo_AppendixW_Hourly-NO2-NAAQS_FINAL_06-28-2010.pdf.
    \2\ U.S. EPA, 2010. Applicability of Appendix W Modeling
Guidance for the 1-hour SO2 NAAQS. Tyler Fox Memorandum
dated August 23, 2010, Office of Air Quality Planning & Standards,
Research Triangle Park, North Carolina 27711. http://www.epa.gov/ttn/scram/guidance/clarification/ClarificationMemo_AppendixW_Hourly-SO2-NAAQS_FINAL_08-23-2010.pdf.
    \3\ U.S. EPA, 2010. Guidance Concerning the Implementation of
the 1-hour SO2 NAAQS for the Prevention of Significant
Deterioration Program. Stephen D. Page, Memorandum dated August 23,
2010, Office of Air Quality Planning & Standards, Research Triangle
Park, North Carolina 27711. http://www.epa.gov/region07/air/nsr/nsrmemos/appwso2.pdf.
    \4\ U.S. EPA, 2010. Guidance Concerning the Implementation of
the 1-hour NO2 NAAQS for the Prevention of Significant
Deterioration Program. Stephen D. Page, Memorandum dated June 29,
2010, Office of Air Quality Planning & Standards, Research Triangle
Park, North Carolina 27711. http://www.epa.gov/ttn/scram/guidance/clarification/ClarificationMemo_AppendixW_Hourly-SO2-NAAQS_FINAL_08-23-2010.pdf.
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    Several presentations at the Tenth Modeling Conference addressed
issues and challenges associated with demonstrating compliance with
these new 1-hour NAAQS for NO2 and SO2. This
included results from a study sponsored by the American Petroleum
Institute (API) that evaluated AERMOD model performance under low wind
speed conditions using additional National Oceanic and Atmospheric
Administration (NOAA) field studies at Oak Ridge, TN, and Idaho Falls,
ID, which were not included in the original 17 databases used to
support AERMOD's promulgation in 2005. The API low wind study \5\
showed significant overprediction of observed concentrations,
especially for the Oak Ridge study where observed wind speeds were
below 0.5 m/s for 10 of the 11 tracer tests, and included wind speeds
as low as 0.15 m/s. The API low wind study also included proposed
modifications to the AERMET meteorological processor and AERMOD model
to address this bias toward overprediction under stable/light wind
conditions.
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    \5\ AECOM, 2010. AERMOD low wind speed evaluation study results,
http://mycommittees.api.org/rasa/amp/Modeling%20Documents/AECOM%202009%20Low%20Wind%20Speed%20Evaluation%20Study%20Report.pdf.
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    Prior to the promulgation of the 1-hour NO2 NAAQS,
compliance with the previous annual NO2 NAAQS was routinely
demonstrated based on the Tier 1 assumption of full conversion or a
Tier 2 option based on an ambient ratio of 75 percent conversion of
nitrogen oxides (NOX) to NO2, referred to as the
Ambient Ratio Method (ARM). However, compliance with the new 1-hour
NAAQS has typically required a more refined treatment of NOX
conversion to NO2. Therefore, several presentations at the
Tenth Modeling Conference focused on issues associated with
demonstrating compliance with the new 1-hour NO2 NAAQS.
    These presentations included an overview of an API funded study to
develop a Tier 2 ambient ratio method for the 1-hour NO2
NAAQS, referred to as ARM2. The ARM2 approach was developed based on an
extensive analysis of ambient ratios of NO2/NOX
that were analyzed by land use (urban vs. rural) and geographical
areas. Based on these analyses of the ambient NO2/
NOX ratios, an empirical relationship between ambient
concentrations of NO2 and NOX was developed. The
EPA subsequently reviewed and evaluated this ARM2 approach and then

incorporated this screening technique as a non-Default/Beta option in
version 13350 of AERMOD in December 2013. Another issue associated with
NO2 NAAQS compliance presented at this conference focused on
the use of relative (instantaneous) dispersion coefficients to define
the plume volume which determines the amount of ozone available to
convert nitrogen (NO) to NO2 using the Plume Volume Molar
Ratio Method (PVMRM) option in AERMOD. The relative dispersion
coefficients originally incorporated in AERMOD for PVMRM are best
representative of daytime convective conditions and may tend to
overestimate plume volumes during stable conditions. Such
overestimation of the plume volume will tend to result in PVMRM to
overestimate concentrations of NO2.
    In addition, modeling of single-source impacts for ozone and
secondarily formed PM2.5 was a topic of discussion at the
Tenth Modeling Conference. On January 4, 2012, the EPA granted a
petition submitted on behalf of the Sierra Club on July 28, 2010 \6\
and committed to engage in rulemaking to evaluate whether updates to
the Guideline are warranted and, as appropriate, incorporate new
analytical techniques or models for ozone and secondarily formed
PM2.5. As a part of satisfying this commitment, there were
presentations of ongoing research at the Tenth Modeling Conference
regarding single-source plume chemistry and photochemical grid modeling
techniques, as well as several public forums. In addition, the EPA
presented an overview along with a panel discussion of its Draft
Guideline for PM2.5 Permit Modeling that addressed the need for
consideration of secondary PM2.5 in demonstrating compliance
with the PM2.5 NAAQS.\7\ Subsequently, written comments
pertaining to such modeling were submitted to the EPA.
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    \6\ U.S. EPA, 2012. Gina McCarthy Letter to Robert Ukeiley dated
January 4, 2012, Washington, DC 20460. http://www.epa.gov/ttn/scram/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-002-1093.pdf.
    \7\ U.S. EPA, 2014. Guidance for PM2.5 Modeling. May
20, 2014, EPA-454/B-14-001. Office of Air Quality Planning &
Standards, Research Triangle Park, NC. http://www.epa.gov/ttn/scram/guidance/guide/Guidance_for_PM25_Permit_Modeling.pdf.
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    As introduced at the Tenth Modeling Conference, the Interagency
Workgroup on Air Quality Modeling (IWAQM) process was formally
reinitiated in June 2013 to inform the EPA's process of updating the
Guideline to address chemically reactive pollutants in near-field and
long-range transport applications. The IWAQM, which consists of
representatives from the EPA, the U.S. Forest Service, the National
Park Service, and the U.S. Fish and Wildlife Service, was initially
formed to support development of technically sound recommendations
regarding assessment of air pollutant source impacts on Federal Class I
parks and wilderness areas. Comments received from stakeholders at the
Tenth Modeling Conference supported reinitiating this interagency
collaborative effort (as ``Phase 3'') to provide additional guidance
for modeling single-source impacts on secondarily formed pollutants
(e.g., ozone and PM2.5) in the near-field and for long-range
transport. Stakeholder comments also support the idea of this
collaborative effort working in parallel with stakeholders to further
model development and evaluation. This renewed \8\ effort included the
establishment of two separate working groups, one focused on long-range
transport of primary and secondary pollutants and the other on near-
field single-source impacts of secondary pollutants. The primary
objectives of this phase of IWAQM include reviewing existing approaches
for estimating single-source secondary pollutant impacts, developing
revisions to the Guideline, and the development of guidance for using
technical methods to estimate downwind secondary pollutant impacts.
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    \8\ Phase 1 of the IWAQM effort focused on review of respective
regional modeling programs, development of an organizational
framework, and formulating reasonable objectives and plans that were
presented to EPA management for support and commitment. Phase 2 of
the IWAQM process continued this work and largely concluded in 1998
with the publication of the Interagency Workgroup on Air Quality
Modeling (IWAQM) Phase 2 Summary Report and Recommendations for
Modeling Long Range Transport Impacts (EPA-454/R-98-019) (USEPA,
1998). This report provided a series of recommendations concerning
the application of the CALPUFF model for use in regulatory long-
range transport (LRT) modeling that supported the revisions in 2003
to the Guideline.
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III. Public Participation Regarding Revisions to the Guideline and
Notice of Eleventh Conference on Air Quality Modeling

    Interested persons may provide the EPA with their views on the
proposed revisions to the Guideline in several ways. This includes
submitting written comments to the EPA, participating in the Eleventh
Conference on Air Quality Modeling, and speaking at the public hearing
that will be conducted as part of the conference. Additional
information on how to submit written comments on the proposed revisions
to the Guideline is provided in the ADDRESSES section above.
    The public hearing for this action and the Eleventh Conference on
Air Quality Modeling will be held August 12-13, 2015, from 8:30 a.m. to
5:00 p.m., in the EPA Auditorium, Room C111, 109 T.W. Alexander Drive,
Research Triangle Park, NC 27711. On August 12, 2015, the first half of
the conference will consist of a structured agenda with presentations.
The second half of the first and all of the second day (August 13,
2015), is reserved for the public hearing on this proposed rule.
Advance requests for reserved time to speak during the public hearing
should be submitted by August 7, 2015, to Mr. George M. Bridgers, Air
Quality Assessment Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail code C439-01,
Research Triangle Park, NC 27711; telephone:<PHONE_NUMBER>; fax: (919)
541-0044; email<EMAIL_ADDRESS>The EPA will also provide an
opportunity for oral presentations by individuals that sign up at the
public hearing. Information submitted to the EPA during the conference
will be placed in the docket for this rule proposing revisions to the
Guideline.
    Background information: Preregistration details, additional
background information, and a more detailed agenda for the Eleventh
Conference on Air Quality Modeling are electronically available at
http://www.epa.gov/ttn/scram/11thmodconf.htm. Preregistration for the
conference, while not required, is strongly recommended due to
heightened security protocols at the EPA-RTP facility.
    REAL ID Act: Because this hearing is being held at a U.S.
government facility, individuals planning to attend the hearing should
be prepared to show valid picture identification to the security staff
in order to gain access to the meeting room. Please note that the REAL
ID Act, passed by Congress in 2005, established new requirements for
entering federal facilities. These requirements took effect July 21,
2014. If your driver's license is issued by Alaska, American Samoa,
Arizona, Kentucky, Louisiana, Maine, Massachusetts, Minnesota, Montana,
New York, Oklahoma, or the state of Washington, you must present an
additional form of identification to enter the federal buildings where
the public hearings will be held. Acceptable alternative forms of
identification include: Federal employee badges, passports, enhanced
driver's licenses and military identification cards. We will list any
additional acceptable forms of identification at: http://www.epa.gov/

ttn/scram/11thmodconf.htm. In addition, you will need to obtain a
property pass for any personal belongings you bring with you. Upon
leaving the building, you will be required to return this property pass
to the security desk. No large signs will be allowed in the building,
cameras may only be used outside of the building and demonstrations
will not be allowed on federal property for security reasons.
    Conference and Public Hearing: The Eleventh Conference on Air
Quality Modeling will be open to the public. No registration fee is
charged. The conference will be conducted informally and chaired by an
EPA official. As required under CAA section 320, a verbatim transcript
of the conference proceedings will be produced and placed in the docket
for this proposed rule.
    The Eleventh Conference on Air Quality Modeling will begin with
introductory remarks by the presiding EPA official. The EPA staff will
then provide an overview of the revisions to the Guideline as proposed
in this document and present on the research that supports those
revisions and supports formulation updates to the preferred models. The
following topics will be presented:

I. Overview of the Eleventh Conference on Air Quality Modeling;
II. Review of the proposed revisions to the Guideline; and
III. Review of the proposed revisions to the preferred air quality
models.

    At the conclusion of the presentations, the EPA will convene the
public hearing on the proposed revisions to the Guideline. The public
hearing will span the second half of the first day and throughout the
second day of the conference.
    Those wishing to reserve time to speak at the public hearing,
whether to volunteer a presentation on a special topic or to offer
general comment on any of the modeling techniques scheduled for
presentation, should contact us at the address given in the FOR FURTHER
INFORMATION CONTACT section (note the cutoff date). Such persons should
identify the organization (if any) on whose behalf they are speaking
and the length of the presentation. If a scheduled presentation is
projected to be longer than 10 minutes, the presenter should also state
why a longer period is needed. Scheduled speakers should bring extra
copies of their presentation for inclusion in the docket and for the
convenience of the recorder. Scheduled speakers will also be permitted
to enter additional written comments into the record.
    Any person in attendance wishing to speak at the public hearing who
has not reserved time prior to the conference may provide oral comments
on the proposed revisions to the Guideline during time allotted on the
last day of the conference. These parties will need to sign up to speak
on the second day of the hearing and the EPA may need to limit the
duration of presentations to allow all participants to be heard.
    Additional written statements or comments on the proposed revisions
should be sent to the OAR Regulatory Docket (see ADDRESSES section). A
transcript of the conference proceedings and a copy of all written
comments will be maintained in Docket ID No. EPA-HQ-OAR-2015-0310,
which will remain open until October 27, 2015, for the purpose of
receiving additional comments after the conference and the public
hearing on the proposed revisions to the Guideline.

IV. Proposed Changes to the Guideline

    In this action, the EPA is proposing two type of revisions to the
Guideline. The first involve substantive changes to address various
topics, including those presented and discussed at the Tenth Modeling
Conference. These proposed revisions to the Guideline include
enhancements to the formulation and application of the EPA's preferred
dispersion modeling system, AERMOD, and the incorporation of a tiered
demonstration approach to address the secondary chemical formation of
ozone and PM2.5 associated with precursor emissions from
single sources. The second type of revision involves editorial changes
to update and reorganize information throughout the Guideline. These
revisions are not intended to meaningfully change the substance of the
Guideline, but rather to make the Guideline easier to use and to
streamline the compliance assessment process.

A. Proposed Actions

    This section provides a detailed overview of the substantive
proposed changes to the Guideline that are intended to improve the
science of the models and approaches used in regulatory assessments.
1. Clarifications To Distinguish Requirements From Recommendations
    The EPA's PSD permitting regulations specify that ``[a]ll
applications of air quality modeling involved in this subpart shall be
based on the applicable models, data bases, and other requirements
specified in appendix W of this part (Guideline on Air Quality
Models).'' 40 CFR 51.166(l); see also 40 CFR 52.21(l). The applicable
models are the preferred models listed in appendix A to appendix W to
40 CFR part 51. However, there has been some ambiguity in the past with
respect to the ``other requirements'' specified in the Guideline that
must be used in PSD permitting analysis and other regulatory modeling
assessments.
    Ambiguity can result because the Guideline generally contains
``recommendations'' and these recommendations are expressed in non-
mandatory language. For instance, the Guideline frequently uses
``should'' and ``may'' rather than ``shall'' and ``must.'' This
approach is generally preferred throughout the Guideline because of the
need to exercise expert judgment in air quality analysis and the
reasons discussed in the Guideline that ``dictate against a strict
modeling `cookbook'.'' (40 CFR part 51, appendix W, section 1.0(c))
    Considering the non-mandatory language used throughout the
Guideline, the EPA's Environmental Appeals Board has correctly observed
the following:

    ``Although appendix W has been promulgated as codified
regulatory text, appendix W provides permit issuers broad latitude
and considerable flexibility in application of air quality modeling.
Appendix W is replete with references to ``recommendations,''
``guidelines,'' and reviewing authority discretion.''

In Re Prairie State Generating Company, 13 E.A.D. 1, 99 (EAB 2005)
(internal citations omitted).
    Although this approach is typical throughout the Guideline, there
are instances where the EPA does not believe permit issues should have
broad latitude. Some principles of air quality modeling described in
the Guideline must always be applied to produce an acceptable analysis.
Thus, to promote clarity in the use and interpretation of the revised
Guideline, we have, in these cases used mandatory language, and made
specific reference to ``requirements'' throughout the proposed text
where appropriate to distinguish requirements from recommendations in
the application of models for regulatory purposes. We solicit comment
regarding the appropriateness of these revisions in providing the
necessary clarity on the requirements under the proposed revisions to
the Guideline as distinct from the recommendations in the revised text
while noting the continued flexibilities provided for within the
Guideline including but not limited to use and approval of alternative
models.

2. Updates to EPA's AERMOD Modeling System

    Based on studies presented and discussed at the Tenth Modeling
Conference, and additional relevant research since 2010, the EPA and
other researchers have conducted additional model evaluations and
developed changes to the model formulation of the AERMOD modeling
system to improve model performance in its regulatory applications. We
propose the following updates to the AERMOD modeling system to address
a number of technical concerns expressed by stakeholders:
    1. A proposed option incorporated in AERMET to adjust the surface
friction velocity (u*) to address issues with AERMOD model
overprediction under stable, low wind speed conditions. This proposed
option is selected by the user with the METHOD STABLEBL ADJ_U* record
in the AERMET Stage 3 input file.
    2. A proposed low wind option in AERMOD to address issues with
model overprediction under low wind speed conditions. The low wind
option will increase the minimum value of the lateral turbulence
intensity (sigma-v) from 0.2 to 0.3 and adjusts the dispersion
coefficient to account for the effects of horizontal plume meander on
the plume centerline concentration. It also eliminates upwind
dispersion which is incongruous with a straight-line, steady-state
plume dispersion model such as AERMOD. The proposed option is selected
by specifying ``LOWWIND3'' on the CO MODELOPT keyword in the AERMOD
input file.
    3. Modifications to AERMOD formulation to address issues with
overprediction for applications involving relatively tall stacks
located near relatively small urban areas (no user input is required).
    4. Proposed regulatory default options in AERMOD to address plume
rise for horizontal and capped stacks based on the July 9, 1993, Model
Clearinghouse memorandum,\9\ with adjustments to account for the PRIME
algorithm for sources subject to building downwash. These options are
selected by the model user specifying ``POINTCAP'' or ``POINTHOR'' for
source type on the SO LOCATION keyword in the AERMOD input file.
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    \9\ U.S. EPA, 1993. ``Proposal for Calculating Plume Rise for
Stacks with Horizontal Releases or Rain Caps for Cookson Pigment,
Newark, New Jersey'', Memorandum from Joseph A. Tikvart, U.S. EPA/
OAQPS, Research Triangle Park, NC. July 9, 1993. http://www.epa.gov/ttn/scram/guidance/mch/new_mch/R1076_TIKVART_9_JUL_93.pdf.
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    5. A proposed buoyant line source option, based on the BLP model,
has been incorporated in AERMOD. This proposed option is selected by
the model user with the SOURCE type ``BOUYLINE'' to specify the
individual buoyant line source locations and emissions and the new
``BLAVGVAL'' keyword to specify average parameters for a composite
buoyant line.
    6. Proposed updates to the NO2 Tier 2 and Tier 3
screening techniques coded within AERMOD as described more fully later
in this preamble section.
    Model performance evaluation and peer scientific review references
for the updated AERMOD modeling system are cited, as appropriate. An
updated user's guide and model formulation documents for version 15181
have been placed in the docket. We have updated the summary description
of the AERMOD modeling system to appendix A of the Guideline to reflect
these proposed updates. The essential codes, preprocessors, and test
cases have been updated and posted to the EPA's SCRAM Web site, http://www.epa.gov/ttn/scram.
    We invite comments on whether we have reasonably addressed the
technical concerns expressed by the stakeholder community and are on
sound footing to recommend these updates to the regulatory default
version of the AERMOD modeling system which includes its replacement of
BLP as an appendix A model for the intended regulatory applications.
3. Status of AERSCREEN
    In the preamble of the 2005 Guideline, we stated that a screening
version of AERMOD called AERSCREEN was being developed and, in the
meantime, SCREEN3 may be used until AERSCREEN was available. In 2011,
the EPA released AERSCREEN, a program that creates inputs and runs
AERMOD in screening mode. AERSCREEN also interfaces with AERMOD's
terrain processor, AERMAP, the building processor for AERMOD,
BPIPPRIME, and can use AERSURFACE surface characteristics in the
generation of meteorological data for AERMOD via the MAKEMET utility.
In an April 2011 memorandum, the EPA stated that AERSCREEN was the
recommended screening model for simple and complex terrain and replaced
SCREEN3. Since AERSCREEN invokes AERMOD, AERSCREEN represents the state
of the science in screening dispersion models. As part of this proposed
update to AERSCREEN, AERSCREEN now includes inversion break-up and
coastal fumigation, features that were part of SCREEN3. These
fumigation algorithms also take advantage of AERMOD's boundary layer
parameterizations for calculating variables needed by the algorithms.
    We invite comment on incorporation of AERSCREEN into the Guideline
as the screening model for AERMOD that may be applicable in
applications in all types of terrain and for applications involving
building downwash.
4. Updates to 3-Tiered Demonstration Approach for NO2
    Section 5.2.4 of the 2005 Guideline details a 3-tiered approach for
assessing NOX sources, which was recommended to obtain
annual average estimates of NO2 from point sources for
purposes of NSR analysis, including the PSD program and SIP planning
purposes. This 3-tiered approach addresses the co-emissions of NO and
NO2 and the subsequent conversion of NO to NO2 in
the atmosphere. The tiered levels include: (1) Assuming that all NO is
converted to NO2 (full conversion), (2) using the Ambient
Ratio Method (ARM), which applies an assumed equilibrium ratio of
NO2 to NOX, based on observed ambient conditions,
to the annual results from the Tier 1 full conversion, and (3) detailed
screening options focused on determining site-specific ratios of
NO2 to NOX.
    In January 2010, a new 1-hour NO2 standard was
promulgated. Prior to the adoption of the 1-hour NO2
standard, few PSD permit applications required the use of Tier 3
options and guidance available at the time did not fully address the
modeling needs for a 1-hour standard, i.e., tiered approaches for
NO2 in the 2005 Guideline specifically targeted an annual
standard. As a result, several guidance memoranda have been issued by
the EPA to further inform modeling procedures for sources demonstrating
compliance with the new 1-hour standard.1 2 3 4. In response
to the 1-hour NO2 standard, the EPA is proposing several
modifications to the Tier 2 and 3 NO2 screening techniques
incorporated into AERMOD.
    For the Tier 2 technique, the EPA is proposing to replace the
existing ARM with a revised Ambient Ratio Method 2 (ARM2). The existing
Tier 2 technique, ARM, was based on a study that focused exclusively on
long-term averages.\10\ A recently published study \11\ presented a new
analysis of national levels of ambient ratios of NO2 to
NOX based on

hourly data from the EPA's Air Quality System (AQS). Based on this
analysis, a new second tier NO2 screening technique, ARM2,
has been developed and incorporated into AERMOD. Because ARM2 is based
on hourly measurements of the NO2 to NOX ratios
and provides more detailed estimates of this ratio based on the total
NOX present, the EPA is proposing to incorporate a modified
version of ARM2 as the new preferred second tier NOX
modeling approach.
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    \10\ Chu, S.H. and E.L. Meyer, 1991. Use of Ambient Ratios to
Estimate Impact of NOX Sources on Annual NO2
Concentrations. Proceedings, 84th Annual Meeting & Exhibition of the
Air & Waste Management Association, Vancouver, B.C.; 16-21 June
1991. (16pp.) (Docket No. A-92-65, II-A-9).
    \11\ Podrez, M. 2015. An Update to the Ambient Ratio Method for
1-h NO2 Air Quality Standards Dispersion Modeling. Atmospheric
Environment, 103: 163-170.
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    For the Tier 3 technique, the EPA proposes that the existing
detailed screening options of the Ozone Limiting Method (OLM) \12\ and
PVMRM \13\ be formally incorporated into the regulatory version of
AERMOD. Both OLM and PVMRM have been available as non-regulatory, non-
default options in AERMOD for many years, but their usage in a NAAQS
compliance demonstration required approval by the reviewing authority.
Based on the EPA's evaluation and external studies available on their
performance, which show that OLM and PVMRM are capable of modeling 1-
hour NO2 impacts and NO and NO2 speciation with
reasonable accuracy when applied appropriately, both OLM and PVMRM are
being proposed as preferred Tier 3 screening methods for NO2
modeling. In addition, the EPA is proposing to incorporate a revised
version of the PVMRM option, referred to as PVMRM2, that utilizes
relative dispersion coefficients to estimate plume volume during
convective conditions and total dispersion coefficients during stable
conditions. These adjustments to the calculation of plume volume are
intended to mitigate potential overprediction of NO2
conversion in multisource applications, especially during stable
meteorological conditions. The EPA is proposing to replace the existing
PVMRM with the new PVMRM2 with both versions being made available in
the proposed version of AERMOD to facilitate testing and evaluation of
the EPA's proposed replacement of PVMRM option with new PVMRMR2 option.
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    \12\ Cole, H.S. and J.E. Summerhays, 1979. A Review of
Techniques Available for Estimation of Short-Term NO2
Concentrations. Journal of the Air Pollution Control Association,
29(8): 812-817.
    \13\ Hanrahan, P.L., 1999. The Polar Volume Polar Ratio Method
for Determining NO2/NOX Ratios in Modeling--Part I: Methodology.
Journal of the Air & Waste Management Association, 49: 1324-1331.
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    We invite comments on whether we have reasonably addressed
technical concerns regarding the 3-tiered demonstration approach and
specific NO2 screening techniques within AERMOD and whether
we are on sound foundation to recommend the updates described above.
5. Status of CALINE3 Models
    The 2005 Guideline identified CALINE3 \14\ and its variants
(CAL3QHC and CAL3QHCR) as the preferred model for mobile source
modeling for carbon monoxide (CO), particulate matter (PM), and lead.
CALINE3 was developed in the late 1970's using P-G stability classes as
the basis for the dispersion algorithms. AERMOD, on the other hand,
uses a planetary boundary layer scaling parameter to characterize
stability and determine dispersion rates, which has been found to be
superior to dispersion parameterizations based on P-G stability
classes.\15\ In addition, the LINE and AREA source options in AERMOD
implement a full numerical integration of emissions across the LINE or
AREA sources, whereas the CALINE3 family of models incorporate a much
less refined approach. Thus, AERMOD provides a more scientifically
credible and accurate characterization of plume dispersion than
CALINE3. Recent model performance studies \16\ have shown that the
CALINE models performed poorly when compared to AERMOD and other modern
dispersion models which also employ state-of-the-science dispersion
parameters. AERMOD is also able to model multiple years in a single
model run, while the CALINE3 variants are limited to either a single
meteorological condition (CALINE3 and CAL3QHC) or a single year of
meteorological data (CAL3QHCR). Additionally, AERMOD is able to utilize
more recent, and more representative, meteorological observations than
are readily available for modeling with CAL3QHCR. Based on the more
scientifically sound basis for AERMOD, improved model performance over
CALINE3, and the availability of more representative meteorological
data, the EPA proposes replacing CALINE3 with AERMOD as the preferred
appendix A model for determining near-field impacts for primary
emissions from mobile sources, including PM2.5,
PM10, and CO hot-spot analyses.\17\
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    \14\ Benson, Paul E., 1979. CALINE3--A Versatile Dispersion
Model for Predicting Air Pollutant Levels Near Highways and Arterial
Streets. Interim Report, Report Number FHWA/CA/TL-79/23. Federal
Highway Administration, Washington, DC (NTIS No. PB 80-220841).
    \15\ Cimorelli, A. et al., 2005. AERMOD: A Dispersion Model for
Industrial Source Applications. Part I: General Model Formulation
and Boundary Layer Characterization. Journal of Applied Meteorology,
44(5): 682-693.
    \16\ Heist, D., V. Isakov, S. Perry, M. Snyder, A. Venkatram, C.
Hood, J. Stocker, D. Carruthers, S. Arunachalam, AND C. Owen.
Estimating near-road pollutant dispersion: a model inter-comparison.
Transportation Research Part D: Transport and Environment. Elsevier
BV, AMSTERDAM, Netherlands, 25:93-105, (2013).
    \17\ U.S. EPA, 2013, Transportation Conformity Guidance for
Quantitative Hot-Spot Analyses in PM2.5 and
PM10 Nonattainment and Maintenance Areas. Publication No.
EPA-420-B-13-053, Office of Transportation and Air Quality, Ann
Arbor, MI. http://www.epa.gov/otaq/stateresources/transconf/policy/420b13053-sec.pdf.
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    We solicit comments on our proposal to identify AERMOD as a
replacement for CALINE3 as an appendix A model for its intended
regulatory applications.
6. Addressing Single-Source Impacts on Ozone and Secondary
PM2.5
    On January 4, 2012, the EPA granted a petition submitted on behalf
of the Sierra Club on July 28, 2010,\18\ that requested the EPA
initiate rulemaking to establish air quality models for ozone and
PM2.5 for use by all major sources applying for a PSD
permit. In granting that petition, the EPA explained that the ``complex
chemistry of ozone and secondary formation of PM2.5 are
well-documented and have historically presented significant challenges
to the designation of particular models for assessing the impacts of
individual stationary sources on the formation of these air
pollutants'' and further explained that ``[b]ecause of these
considerations, the EPA's judgment in the past has been that it was not
technically sound to designate with particularity specific models that
must be used to assess the impacts of a single source on ozone
concentrations,'' but rather the EPA had established a process for
determining on a case-by-case basis the analytical techniques that
should be used for ozone, as well as for secondary formation of
PM2.5.
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    \18\ U.S. EPA, 2012. Gina McCarthy Letter to Robert Ukeiley
dated January 4, 2012, Washington, DC 20460. http://www.epa.gov/ttn/scram/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-002-1093.pdf.
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    In the petition grant, the EPA committed to engage in rulemaking to
evaluate whether updates to the Guideline are warranted and, as
appropriate, incorporate new analytical techniques or models for ozone
and secondarily formed PM2.5. This rulemaking satisfies the
EPA's commitment in the petition grant. As a part of this commitment
and in compliance with CAA section 320, the EPA conducted the Tenth
Modeling Conference in March 2012, where there were presentations of
ongoing research of single-source plume chemistry and photochemical
grid modeling techniques, as well as several public forums, and the EPA
subsequently received written comments pertaining to such modeling.

    The EPA initiated Phase 3 of the IWAQM process in June 2013 to
inform this process to update the Guideline to address chemically
reactive pollutants for near-field and long-range transport
applications. Comments received from stakeholders at the Tenth Modeling
Conference supported this collaborative effort to provide additional
guidance for modeling single-source impacts of secondarily formed
pollutants in the near-field and for long-range transport. Stakeholder
comments also supported the idea of this collaborative effort occurring
in parallel with stakeholders' efforts to further model development and
evaluation. The EPA's recommended revisions to the Guideline are
largely based on detailed review and assessment of this input.
    For this proposed revision to the Guideline, the EPA has determined
that advances in photochemical modeling science indicate it is now
reasonable to provide more specific, generally-applicable guidance that
identifies particular models or analytical techniques that may be used
under specific circumstances for assessing the impacts of an individual
source on ozone and secondary PM2.5.
    Quantifying secondary pollutant formation requires simulating
chemical reactions and thermodynamic partitioning in a realistic
chemical and physical environment. Chemical transport models treat
atmospheric chemical and physical processes such as deposition and
transport. There are two types of chemical transport models, which are
differentiated based on a fixed frame of reference (i.e., Eulerian
models, specifically photochemical grid models) or a frame of reference
that moves with parcels of air between the source and receptor point
(i.e., Lagrangian models).\19\
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    \19\ McMurry, P.H., Shepherd, M.F., Vickery, J.S., 2004.
Particulate matter science for policy makers: A NARSTO assessment.
Cambridge University Press.
---------------------------------------------------------------------------

    Comparing these two types of chemical transport models,
photochemical grid models are integrated, three-dimensional grid-based
models that treat chemical and physical processes in each grid cell and
use Eulerian diffusion and transport processes to move chemical species
to other grid cells.\19\ While some Lagrangian models also treat in-
plume gas and particulate chemistry, to do so these models require time
and space varying oxidant concentrations, and in the case of
PM2.5, neutralizing agents such as ammonia, because
important secondary impacts happen when plume edges start to interact
with the surrounding chemical environment.20 21 These
oxidant and neutralizing agents are not routinely measured, but can be
generated with a three-dimensional photochemical transport model and
subsequently input to a Lagrangian modeling system.
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    \20\ Baker, K.R., Kelly, J.T., 2014. Single source impacts
estimated with photochemical model source sensitivity and
apportionment approaches. Atmospheric Environment, 96: 266-274.
    \21\ ENVIRON, 2012. Evaluation of chemical dispersion models
using atmospheric plume measurements from field experiments, EPA
Contract No: EP-D-07-102. September 2012. 06-20443M6. http://www.epa.gov/ttn/scram/reports/Plume_Eval_Final_Sep_2012v5.pdf.
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    In light of these differences between photochemical grid models and
Lagrangian models that address chemistry, the EPA believes
photochemical grid models are generally most appropriate for addressing
ozone and secondary PM2.5 because they provide a spatially
and temporally dynamic realistic chemical and physical environment for
plume growth and chemical transformation.20 22 Publically
available and documented Eulerian photochemical grid models such as the
Comprehensive Air Quality Model with Extensions (CAMx) \23\ and the
Community Multiscale Air Quality (CMAQ) \24\ model treat emissions,
chemical transformation, transport, and deposition using time and space
variant meteorology. These modeling systems include primarily emitted
species and secondarily formed pollutants such as ozone and
PM2.5.25 26 27 28 These models have been used
extensively to support ozone and PM2.5 SIPs and to explore
relationships between inputs and air quality impacts in the United
States and elsewhere.26 29 30
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    \22\ Zhou, W., Cohan, D.S., Pinder, R.W., Neuman, J.A.,
Holloway, J.S., Peischl, J., Ryerson, T.B., Nowak, J.B., Flocke, F.,
Zheng, W.G., 2012. Observation and modeling of the evolution of
Texas power plant plumes. Atmospheric Chemistry and Physics, 12:
455-468.
    \23\ ENVIRON, 2014. User's Guide Comprehensive Air Quality Model
with Extensions version 6, http://www.camx.com. ENVIRON
International Corporation, Novato.
    \24\ Byun, D., Schere, K.L., 2006. Review of the governing
equations, computational algorithms, and other components of the
models-3 Community Multiscale Air Quality (CMAQ) modeling system.
Applied Mechanics Reviews, 59: 51-77.
    \25\ Chen, J., Lu, J., Avise, J.C., DaMassa, J.A., Kleeman,
M.J., Kaduwela, A.P., 2014. Seasonal modeling of PM 2.5 in
California's San Joaquin Valley. Atmospheric Environment, 92: 182-
190.
    \26\ Civerolo, K., Hogrefe, C., Zalewsky, E., Hao, W., Sistla,
G., Lynn, B., Rosenzweig, C., Kinney, P.L., 2010. Evaluation of an
18-year CMAQ simulation: Seasonal variations and long-term temporal
changes in sulfate and nitrate. Atmospheric Environment, 44: 3745-
3752.
    \27\ Russell, A.G., 2008. EPA Supersites program-related
emissions-based particulate matter modeling: initial applications
and advances. Journal of the Air & Waste Management Association, 58:
289-302.
    \28\ Tesche, T., Morris, R., Tonnesen, G., McNally, D., Boylan,
J., Brewer, P., 2006. CMAQ/CAMx annual 2002 performance evaluation
over the eastern US. Atmospheric Environment, 40: 4906-4919.
    \29\ Cai, C., Kelly, J.T., Avise, J.C., Kaduwela, A.P.,
Stockwell, W.R., 2011. Photochemical modeling in California with two
chemical mechanisms: model intercomparison and response to emission
reductions. Journal of the Air & Waste Management Association, 61:
559-572.
    \30\ Hogrefe, C., Hao, W., Zalewsky, E., Ku, J.-Y., Lynn, B.,
Rosenzweig, C., Schultz, M., Rast, S., Newchurch, M., Wang, L.,
2011. An analysis of long-term regional-scale ozone simulations over
the Northeastern United States: variability and trends. Atmospheric
Chemistry and Physics, 11: 567-582.
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    For assessing secondary pollutant impacts from single sources, the
degree of complexity required to assess potential impacts varies
depending on the nature of the source, its emissions, and the
background environment. In order to provide the user community
flexibility in estimating single-source secondary pollutant impacts and
given the emphasis on the use of photochemical grid models for these
purposes, the EPA is proposing a two-tiered demonstration approach for
addressing single-source impacts on ozone and secondary
PM2.5. The first tier involves use of technically credible
relationships between precursor emissions and a source's impacts that
may be published in the peer-reviewed literature; developed from
modeling that was previously conducted for an area by a source, a
governmental agency, or some other entity and that is deemed
sufficient; or generated by a peer-reviewed reduced form model. The
second tier involves application of more sophisticated case-specific
chemical transport models (e.g., photochemical grid models) to be
determined in consultation with the EPA Regional Office and conducted
consistent with new EPA single-source modeling guidance.\31\ The
appropriate tier for a given application should be selected in
consultation with the appropriate reviewing authority and be consistent
with EPA guidance.
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    \31\ U.S. EPA, 2015. Guidance on the use of models for assessing
the impacts from single sources on secondarily formed pollutants
ozone and PM2.5. Publication No. EPA 454/P-15-001. Office
of Air Quality Planning & Standards, Research Triangle Park, North
Carolina 27711.
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    To fully implement these proposed changes to the Guideline related
to addressing ozone and secondary PM2.5 impacts, the EPA
intends to pursue a separate rulemaking to establish a technical basis
and new values for PM2.5 Significant Impact Levels (SILs)
and to introduce a new demonstration tool for ozone and
PM2.5 precursors referred to as Model Emissions Rates for
Precursors (MERP). When completed, this rule

would differ from the current process recommended in the EPA's Guidance
for PM2.5 Permit Modeling.\7\ A MERP would neither replace the existing
Significant Emissions Rates (SERs) for these pollutants nor serve as
the basis for the applicability of PSD requirements to sources with
emissions above the SER. However, a MERP would represent a level of
emissions of precursors that is not expected to contribute
significantly to concentrations of ozone or secondarily-formed
PM2.5. Our present understanding of the atmospheric science
of ozone and secondary PM2.5 formation indicates that MERP
values will likely be higher than the SERs and more appropriate for
evaluating the impacts of these criteria pollutants as precursors to
ozone and PM2.5 formation. As part of the separate
rulemaking, the EPA intends to demonstrate that a source with precursor
emissions (e.g., NOX and SO2 for
PM2.5) below the MERP level will have ambient impacts that
will be less than the SIL and, thereby, provide a sufficient
demonstration that the source will not cause or contribute to a
violation of the PM2.5 NAAQS or PSD increments. The EPA's
Guidance for PM2.5 Permit Modeling \7\ provides for a three-
tiered approach to address secondary PM2.5 with (1) a
qualitative assessment; (2) a hybrid qualitative/quantitative
assessment utilizing existing technical work; and (3) a full
quantitative modeling exercise. The EPA expects that MERPs as a
demonstration tool will replace the first tier of a qualitative
assessment as sources that currently would provide a qualitative
assessment are expected to have precursor emissions levels below the
MERP. The second and third tier of assessment will then be consistent
with the EPA's proposed two-tiered demonstration approach for
PM2.5 reflected in this proposed revisions to the Guideline.
To specifically assist the public in commenting on this rule within the
overall context of the NSR program, including PSD, the EPA has added
two separate memoranda to the docket of this proposed rule. These
memoranda provide more details on how this future approach to PSD
compliance demonstrations will work for secondary PM2.5 and
also describe our expectations for how such an approach might work for
ozone based on a future, separate action to similarly establish a SIL
and MERPs (for VOC and NOX precursors) for ozone using
approaches similar to those for PM2.5.32 33
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    \32\ U.S. EPA, 2015. ``Proposed Approach for Demonstrating
PM2.5 PSD Compliance'', Memorandum to Docket No. EPA-HQ-
OAR-2015-0310 by Tyler J Fox, U.S. EPA/OAQPS, Research Triangle
Park, NC. June 30, 2015.
    \33\ U.S. EPA, 2015. ``Proposed Approach for Demonstrating Ozone
PSD Compliance'', Memorandum to Docket No. EPA-HQ-OAR-2015-0310 by
Tyler J Fox, U.S. EPA/OAQPS, Research Triangle Park, NC. June 30,
2015.
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    While the development of MERPs for ozone and secondary
PM2.5 precursors is expected to address a number of PSD
permitting situations, the EPA believes that most of the remaining
situations in which a source must demonstrate compliance under the
proposed Guideline will be addressed sufficiently under the proposed
first tier where existing technical information could be used in
combination with other supportive information and analysis for the
purposes of estimating secondary impacts from a particular source. The
existing technical information should provide a credible and
representative estimate of the secondary impacts from the project
source. In these situations, a more refined approach for estimating
secondary pollutant impacts from project sources may not be necessary.
The EPA has been compiling and reviewing screening approaches that are
based on technically credible tools (e.g., photochemical grid models)
that relate source precursor emissions to secondary impacts. In review
of existing approaches detailed in peer reviewed journal articles and
non-peer reviewed forms (e.g., technical reports, conference
presentations), it is not clear that a single approach has been clearly
proposed to and evaluated by the modeling community for estimating
screening level secondary impacts from single sources. Other screening
level alternatives to photochemical grid model application may include
the use of existing credible photochemical model impacts for sources
deemed to be similar in terms of emission rates, release parameters,
and background environment. The EPA will continue to engage with the
modeling community to identify credible alternative approaches for
estimating single-source secondary pollutant impacts which provide
flexibility and are less resource intensive for permit demonstration
purposes.
    For those situations for which existing modeling or screening
estimates are not available or appropriate, the second tier proposed by
the EPA would apply and involve use of more sophisticated case-specific
chemical transport models (e.g., photochemical grid models) to be
determined in consultation with the appropriate EPA Regional Office
based upon new EPA single-source modeling guidance.\31\ Based on
several scientific studies, the EPA proposes to determine that
photochemical grid models are appropriate for assessment of near-field
and regional scale reactive pollutant impacts from specific sources
20 22 34 35 or a group of multiple sources impacting an
area.25 27 28 Even though single-source emissions are
injected into a grid volume, photochemical transport models have been
shown to adequately capture single-source impacts when compared with
downwind in-plume measurements.20 22 Where set up
appropriately for the purposes of assessing the contribution of single
sources to primary and secondarily formed pollutants, photochemical
grid models can be used with a variety of approaches to estimate these
impacts. These approaches generally fall into the category of source
sensitivity (how air quality changes due to changes in emissions) and
source apportionment (what air quality impacts are related to certain
emissions). Source apportionment has been used to differentiate the
contribution from single sources on model predicted ozone and
PM2.5 concentrations.20 34 The direct decoupled
method (DDM) has also been used to estimate ozone and PM2.5
impacts from specific sources 20 35 as well as the simpler
brute-force sensitivity approach.20 22 35 Limited comparison
of single-source impacts between models \36\ and approaches to
differentiate single-source impacts 20 36 show generally
similar downwind spatial gradients and impacts.
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    \34\ Baker, K.R., Foley, K.M., 2011. A nonlinear regression
model estimating single source concentrations of primary and
secondarily formed PM2.5. Atmospheric Environment, 45:
3758-3767.
    \35\ Bergin, M.S., Russell, A.G., Odman, M.T., Cohan, D.S.,
Chameldes, W.L., 2008. Single-Source Impact Analysis Using Three-
Dimensional Air Quality Models. Journal of the Air & Waste
Management Association, 58: 1351-1359.
    \36\ Baker, K.R., Kelly, J.T., Fox, T., 2013. Estimating second
pollutant impacts from single sources (control #27). http://aqmodels.awma.org/conference-proceedings.
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    Near-source in-plume aircraft based measurement field studies
provide an opportunity for evaluating model estimates of (near-source)
downwind transport and chemical impacts from single stationary point
sources.\21\ Photochemical grid model source apportionment and source
sensitivity simulation of a single source downwind impacts compare well
against field study primary and secondary ambient measurements made in
Tennessee and Texas.20 21 This work indicates photochemical
grid models and source

apportionment and source sensitivity approaches provide meaningful
estimates of single-source impacts on ozone and secondarily-formed
PM2.5. Additional evaluations for longer time periods and
more diverse environments, both physical and chemical, would be
valuable to generate broader confidence in these approaches for this
purpose.
    We invite comments on whether the proposed two-tiered demonstration
approach and related EPA guidance is appropriately based on sound
science and practical application of available models and tools to
address single-source impacts on ozone and secondary PM2.5.
7. Status of CALPUFF and Assessing Long-Range Transport for PSD
Increment and Regional Haze
    The 2003 Guideline recommended CALPUFF as the preferred model for
long-range transport (i.e., source-receptor distances of 50 to several
hundred kilometers) of emissions from point, volume, area, and line
sources for primary criteria pollutants (e.g., PM and SO2).
Since that time, as discussed previously in this preamble, the EPA has
received input from stakeholders and has worked through the IWAQM
process on analytical techniques to address chemically reactive
pollutants for near-field and long-range transport applications. As a
result, in order to provide the user community flexibility in
estimating single-source secondary pollutant impacts and given the
availability of more appropriate modeling techniques, such as
photochemical transport models (which address limitations of models
like CALPUFF \37\), the EPA is proposing that the Guideline no longer
contain language that requires the use of CALPUFF or another Lagrangian
puff model for long-range transport assessments. Additionally, the EPA
is proposing to remove the CALPUFF modeling system as an EPA-preferred
model for long-range transport due to concerns about the management and
maintenance of the model code given the frequent change in ownership of
the model code since promulgation in the previous version of the
Guideline.\38\ The EPA recognizes that long-range transport assessments
may be necessary in certain limited situations for PSD increment. For
these situations, the EPA is proposing a screening approach where
CALPUFF along with other appropriate screening tools and methods may be
used to support long-range transport PSD increment assessments.
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    \37\ U.S. EPA, 2009. Reassessment of the Interagency Workgroup
on Air Quality Modeling (IWAQM) Phase 2 Summary Report; Revisions to
Phase 2 Recommendations. Draft. Office of Air Quality Planning &
Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/ttn/scram/guidance/reports/Draft_IWAQM_Reassessment_052709.pdf.
    \38\ U.S. EPA, 2015. ``Summary of CALPUFF Ownership Since 2003
Promulgation'', Memorandum to Docket No. EPA-HQ-OAR-2015-0310 by
Tyler J Fox, U.S. EPA/OAQPS, Research Triangle Park, NC. June 30,
2015.
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    To determine if a Class I PSD increment analyses may be necessary
beyond 50 km (i.e., long-range transport assessment), the EPA is
recommending a screening approach to determine if a significant impact
will occur with particular focus on Class I areas that may be
threatened at such distances. The first step relies upon the near-field
application of the appropriate screening and/or preferred model to
determine the significance of ambient impact at or about 50 km from the
new of modifying source. If this initial analysis indicates there may
be significant ambient impacts at that distance, then further analysis
is necessary. For assessment of Class I ambient impacts, under the
proposed Guideline, there will not a preferred model for distances
beyond 50 km. Typically, a Lagrangian model is the type of model
appropriate to use for these screening assessments; however, applicants
should establish approaches (models and modeling parameters) on a case-
by-case basis in consultation with the appropriate reviewing authority,
Regional Office, and the affected Federal Land Manager(s) (FLM(s)). If
a cumulative increment analysis is necessary, for these limited
situations, the selection and use of an alternative model shall occur
in agreement with the appropriate reviewing authority (paragraph
3.0(b)) and approval by the EPA Regional Office based on the
requirements of section 3.2.2(e).
    As previously noted, Phase 3 of the IWAQM process was reinitiated
in June 2013 to inform the EPA's commitment to update the Guideline to
address chemically reactive pollutants in near-field and long-range
transport applications. This Phase 3 effort included the establishment
of a working group composed of EPA and FLM technical staff focused on
long-range transport of primary and secondary pollutants with an
emphasis on use of consistent approaches to those being developed and
applied to meet near-field assessment needs for ozone and secondarily-
formed PM2.5. The EPA expects that such approaches will be
focused on state of the science chemical transport models (CTMs) as
detailed in IWAQM reports 39 40 and published literature.
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    \39\ U.S. EPA, 2015. Interagency Workgroup on Air Quality
Modeling Phase 3 Summary Report: Near-Field Single Source Secondary
Impacts. Publication No. EPA 454/P-15-002. Office of Air Quality
Planning & Standards, Research Triangle Park, North Carolina 27711.
    \40\ U.S. EPA, 2015. Interagency Workgroup on Air Quality
Modeling Phase 3 Summary Report: Long Range Transport and Air
Quality Related Values. Publication No. EPA 454/P-15-003. Office of
Air Quality Planning & Standards, Research Triangle Park, North
Carolina 27711.
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    To inform future consideration of visibility modeling in regulatory
applications consistent with proposed changes for addressing chemistry
for single-source impact on ozone and secondary PM2.5, the
final report \40\ of the IWAQM long-range transport subgroup identified
that modern CTMs have evolved sufficiently and provide a credible
platform for estimating potential visibility impacts from a single or
small group of emission sources. Chemical transport models are well
suited for the purpose of estimating long-range impacts of secondary
pollutants, such as PM2.5, that contribute to regional haze
and other secondary pollutants, such as ozone, that contribute to
negative impacts on vegetation through deposition processes. These
multiple needs require a full chemistry photochemical model capable of
representing both gas, particle, and aqueous phase chemistry for
PM2.5, haze, and ozone.
    Photochemical transport models are suitable for estimating
visibility and deposition since important physical and chemical
processes related to the formation and transport of PM are
realistically treated. Source sensitivity and apportionment techniques
implemented in photochemical grid models have evolved sufficiently and
provide the opportunity for estimating potential visibility and
deposition impacts from one or a small group of emission sources using
a full science photochemical grid model. Photochemical grid models
using meteorology output from prognostic meteorological models have
demonstrated skill in estimating source-receptor relationships in the
near-field 20 21 and over long distances.\41\
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    \41\ ENVIRON, 2012. Documentation of the Evaluation of CALPUFF
and Other Long Range Transport Models using Tracer Field Experiment
Data, EPA Contract No: EP-D-07-102. February 2012. 06-20443M4.
http://www.epa.gov/ttn/scram/reports/EPA-454_R-12-003.pdf.
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    It is important that modeling tools used for single-source long-
range transport impacts assessments demonstrate skill in adequately
replicating source-receptor relationships that are not in close
proximity. For

source-receptor distances greater than 50 km, regional scale
photochemical grid models may be applied for the assessment of
visibility impacts due to one or a small group of sources. Skill in
estimating source-receptor relationships on this scale can be
illustrated by evaluating modeling systems against regional scale inert
tracer release experiments. Historically, several regional tracer
release experiments have been used to demonstrate skill in long-range
transport of inert pollutants: 1980 Great Plains Mesoscale Tracer Field
Experiment, the 1983 Cross-Appalachian Tracer Experiment (CAPTEX), the
1987 Across North American Tracer Experiment (ANATEX), and 1994
European Tracer Experiment (ETEX).41 42 Photochemical grid
models have been shown to demonstrate similar skill to Lagrangian
models for pollutant transport when compared to measurements made from
multiple mesoscale field experiments.\41\ Use of CTMs for Air Quality
Related Values (AQRV) analysis requirements, while not subject to
specific EPA model approval requirements outlined in 40 CFR
51.166(l)(2) and 40 CFR 52.21(l)(2), should be justified for each
application following the general recommendations outlined in section
3.2, and concurrence sought with the affected FLM(s).
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    \42\ Hegarty, J., Draxler, R.R., Stein, A.F., Brioude, J.,
Mountain, M., Eluszkiewicz, J., Nehrkorn, T., Ngan, F., Andrews, A.,
2013. Evaluation of Lagrangian particle dispersion models with
measurements from controlled tracer releases. Journal of Applied
Meteorology and Climatology, 52: 2623-2637.
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    In 2005, the EPA issued guidelines for implementation of the best
available retrofit technology (BART) requirements under the Regional
Haze Rule. In these BART Guidelines, the EPA addressed the question of
how states could best predict a single source's contribution to
visibility impairment.\43\ At the time, the EPA recognized that CALPUFF
had not yet been fully evaluated for secondary pollutant formation, but
the EPA still considered CALPUFF to be the best application for
assessing a single source's impact on visibility in a Class I area for
purposes of the regional haze program. The EPA took note of the
limitations of CALPUFF for this purpose but concluded that CALPUFF was
the best modeling application for use in evaluating BART, especially
given how the modeling results would be used. Based on this assessment,
the EPA recommended that the states use CALPUFF. The EPA also made
clear, however, that states could use other alternative approaches,
including photochemical grid models, if done in consultation with the
appropriate EPA Regional Office.
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    \43\ See 70 FR 39104, 39122-23 (July 6, 2005).
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    The current version of the Guideline does not contain any explicit
recommendation regarding the use of CALPUFF in the regional haze
program, but in advising states and in making its own BART
determinations, the EPA has looked to the Guideline to resolve
questions regarding the proper application of the model. In particular,
the EPA has guided states to use the applicable regulatory version of
CALPUFF for such assessments. Following the EPA's recommendations,
states have used the EPA-preferred version of CALPUFF in hundreds of
BART determinations since 2005. Although most assessments of BART are
now complete, a handful of BART determinations remain outstanding. We
expect most of the remaining actions addressing the BART requirements
to be completed within the next two years.
    The proposed changes to the Guideline do not affect the EPA's
recommendation in the 2005 BART Guidelines to use CALPUFF in the BART
determination process. Given that the overwhelming majority of BART
determinations have been made using CALPUFF, we consider it appropriate
for states (or the EPA) to continue to use this application for the
remaining assessments under the current Guideline with approved
protocols. This approach assures consistency across and within states
in the regional haze program. In addition, in many instances, the
modeling of visibility impacts has already been completed even though
the BART determination process is not yet done. Allowing states to
continue to rely on CALPUFF avoids additional time and expense in
developing a new assessment of visibility impacts for a SIP initially
due in 2007. We intend to continue to advise states with respect to the
EPA-preferred version of CALPUFF that should be used in specific BART
cases. Consistent with the BART Guidelines, states may also use
alternative modeling approaches, in consultation with the appropriate
EPA Regional Office.
    The EPA is seeking comment on its proposed screening approach to
address long-range transport for purposes of assessing PSD increments;
its decision to remove CALPUFF as a preferred model in appendix A for
such long-range transport assessments; and its decision to consider
CALPUFF as a screening technique along with other Lagrangian models to
be used in consultation with the appropriate reviewing authority. It is
important to note that the EPA's proposed action to remove CALPUFF as
an appendix A model in this Guideline does not affect its use under the
FLM's guidance regarding AQRV assessments (FLAG 2010) nor previous use
of this model as part of regulatory modeling applications required
under the CAA. Similarly, this proposed action does not affect EPA's
recommendation that States use CALPUFF to determine the applicability
and level of BART in regional haze implementation plans.\43\
8. Role of EPA's Model Clearinghouse
    The EPA's Model Clearinghouse has been a fundamental aspect of
communication between the EPA Region Offices and with the broader
permitting community on technical modeling and compliance demonstration
issues for almost three decades. The Model Clearinghouse serves a
critical role in helping resolve issues that arise from unique
situations that are not specifically addressed in the Guideline or
necessitate the consideration of an alternative model or technique for
a specific application or range of applications. The Model
Clearinghouse ensures that fairness, consistency, and transparency in
modeling decisions are fostered among the Regional Offices and the
state, local, and tribal agencies.
    In this action, we are proposing to codify the long-standing
process of the Regional Offices consulting and coordinating with the
Model Clearinghouse on all approvals of alternative models or
techniques. While the Regional Administrators are the delegated
authority to issue such approvals under section 3.2 of the Guideline,
all alternative model approvals will only be issued after consultation
with the EPA's Model Clearinghouse and formal documentation through a
concurrence memorandum which demonstrates that the requirements within
section 3.2 for use of an alternative model have been met.
    We invite comment on our proposal to codify existing practice of
requiring consultation and coordination between the EPA Regional
Offices and the EPA's Model Clearinghouse on all approvals under
section 3.2 of alternative models or techniques.
9. Updates to Modeling Procedures for Cumulative Impact Analysis
    Based on input from the Tenth Modeling Conference and recent permit
modeling experiences under new short-term NAAQS for SO2 and
NO2, the EPA is proposing to make modifications to section 8
of the Guideline regarding model inputs and background concentrations
to provide much needed

clarity associated with input and database selection for use in PSD and
SIP modeling. Many of these revisions are based on the EPA
clarification memoranda issued since 2010 that were intended to provide
the necessary clarification regarding applicability of the Guideline to
PSD modeling for these new standards.1 2 44 45 The EPA has
specifically cautioned against the literal and uncritical application
of very prescriptive procedures for conducting NAAQS and PSD modeling
compliance demonstrations as described in chapter C of the draft New
Source Review Workshop Manual.\46\ Our main concern is that following
such procedures in a literal and uncritical manner has led to practices
that are overly conservative and unnecessarily complicate the
permitting process. The proposed changes to section 8 are intended to
modify these practices and provide a more appropriate basis for
selection and use of modeling inputs through the Guideline itself and
supporting guidance.
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    \44\ U.S. EPA, 2011. Additional Clarification Regarding
Applicability of Appendix W Modeling Guidance for the 1-hour NO2
NAAQS. Tyler Fox Memorandum dated March 1, 2011, Office of Air
Quality Planning & Standards, Research Triangle Park, North Carolina
27711.http://www.epa.gov/ttn/scram/guidance/clarification/NO2_Clarification_Memo-20140930.pdf.
    \45\ U.S. EPA, 2014. Clarification on the Use of AERMOD
Dispersion Modeling for Demonstrating Compliance with the
NO2 National Ambient Air Quality Standard. R. Chris Owen
and Roger Brode Memorandum dated September 30, 2014, Office of Air
Quality Planning & Standards, Research Triangle Park, North Carolina
27711. http://www.epa.gov/ttn/scram/guidance/clarification/NO2_Clarification_Memo-20140930.pdf.
    \46\ U.S. EPA, 1990. New Source Review Workshop Manual:
Prevention of Significant Deterioration and Nonattainment Area
Permitting (Draft). Office of Air Quality Planning & Standards,
Research Triangle Park, North Carolina 27711. http://www.epa.gov/nsr.
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    We have provided a more definitive definition of the appropriate
modeling domain and how to best characterize the various contributions
to air quality concentrations within that domain. Specifically, we
provide the following recommendations:
     Definition and/or factors to consider in determining
appropriate modeling domain for NAAQS and PSD increment assessments and
for SIP attainment demonstrations (see section 8.1).
     Revised requirements on how to characterize emissions from
nearby sources to be explicitly modeled for purposes of a cumulative
impact assessment under PSD and new language regarding how to
characterize direct and precursor emissions from modeled sources for
SIP attainment demonstrations for ozone, PM2.5, and regional
haze (see section 8.2).
     Revised recommendations on how to determine background
concentrations in constructing the design concentration, or total air
quality concentration, as part of a cumulative impact analysis for
NAAQS and PSD increments. Specific recommendations are proposed for
situations involving isolated single-source(s) and multi-source areas
(see section 8.3) with an emphasis on how to determine which nearby
sources to explicitly model based on the concept of significant
concentration gradients and the use of monitored background to
adequately represent ``other sources'' (i.e., that portion of the
background attributable to natural sources, other unidentified sources
in the vicinity of the project, and regional transport contributions
from more distant sources (domestic and international)). It is
important to note the interconnectedness of these issues as the
question of which nearby sources to include in cumulative modeling is
inextricably linked with the question of what ambient monitoring data
are available and what these data represent for a specific application.

More specific data requirements and the format required for the
individual models are described in detail in the users' guide and/or
associated documentation for each model.
    Given the added complexity of the technical issues that arise in
the context of demonstrating compliance with NAAQS through dispersion
modeling, we strongly encourage adherence to the recommendations in
section 9.2.1 of the proposed Guideline regarding development of a
modeling protocol, i.e., that ``[e]very effort should be made by the
Regional Office to meet with all parties involved in either a SIP
revision or a PSD permit application prior to the start of any work on
such a project. During this meeting, a protocol should be established
between the preparing and reviewing parties to define the procedures to
be followed, the data to be collected, the model to be used, and the
analysis of the source and concentration data.'' We expect by providing
more clarity in the Guideline of the factors to be considered in the
cumulative impact assessment, permit applicants and permitting
authorities will be able to find the proper balance of the competing
factors that contribute to these analyses.
    We invite comments on whether the updates proposed in section 8 of
the Guideline and associated guidance are appropriate and sufficient to
provide the necessary clarification in selecting and establishing the
model inputs for conducting the regulatory modeling for PSD and SIP
applications.
10. Updates on Use of Meteorological Input Data for Regulatory
Dispersion Modeling
    For near-field dispersion modeling applications using National
Weather Service (NWS) Automated Surface Observing Stations (ASOS), the
EPA released a pre-processor to AERMET, called AERMINUTE, in 2011 that
calculates hourly averaged winds from 2-minute winds reported every
minute at NWS ASOS sites. AERMET substitutes these hourly averaged
winds for the standard hourly observations, thus reducing the number of
calms and missing winds for input into AERMOD. The presence of calms
and missing winds were due to the METAR reporting methodology of
surface observations. In March 2013, the EPA released a memorandum
regarding the use of ASOS data in AERMOD as well as the use of
AERMINUTE. When using meteorological data from ASOS sites for input to
AERMOD, hourly averaged winds from AERMINUTE should be used in most
cases.
    For a near-field dispersion modeling application where there is no
representative NWS station, and it is prohibitive or not feasible to
collect adequately representative site-specific data, it may be
necessary to use prognostic meteorological data for the application.
The EPA released the MMIF program that converts the prognostic
meteorological data into a format suitable for dispersion modeling
applications. The most recent 3 years of prognostic data are preferred.
Use of the prognostic data is contingent on the concurrence of the
appropriate reviewing authorities and collaborating agencies that the
data are of acceptable quality and representative of the modeling
application.
    We solicit comments on our proposed updates regarding use of
meteorological input data for regulatory application of dispersion
models.
11. Transition Period for Applicability of Revisions to the Guideline
    In previous rulemakings to revise the Guideline, we have
traditionally communicated that it would be appropriate to provide 1
year to transition to the use of new models, techniques and procedures
in the context of PSD permit applications and other regulatory modeling
applications. We invite comments whether it would be appropriate to
apply a 1-year transition after promulgation of the revised Guideline
(i.e., from its effective

date) such that applications conducted under the current Guideline with
approved protocols would be acceptable during that period, but new
requirements and recommendations should be used for applications
submitted after that period or protocols approved after that period.
    The EPA believes such a transition period is appropriate to avoid
the time and expense of revisiting modeling that is substantially
complete, which would cause undue delays to permit applications that
are pending when the proposed revisions to the Guideline are finalized.
The revisions that the EPA is proposing to the Guideline are intended
as incremental improvements to the Guideline, and such improvements do
not necessarily invalidate past practices under the previous edition of
the Guideline. The requirements and recommendations in the current
(2005) version of the Guideline were previously identified as
acceptable by the EPA, and they will continue to be acceptable for air
quality assessments during the period of transition to the revised
version of the Guideline.
    Where a proposed revision to the Guideline does raise questions
about the acceptability of a requirement or recommendation that it
replaces, model users and applicants are encouraged to consult with the
appropriate reviewing authority as soon as possible to assure the
acceptability of modeling used to support permit applications during
this period.

B. Proposed Editorial Changes

    The EPA is proposing to make editorial changes to update and
reorganize information throughout the Guideline. These revisions are
not intended to meaningfully change the substance of the Guideline, but
rather to make the Guideline easier to use. One way this is
accomplished is by grouping topics together in a more logical manner to
make related content easier to find. This in turn should streamline the
compliance assessment process.
    Editorial changes are described below for each affected section. We
invite comment on any of the changes proposed below for the Guideline
text.
1. Preface
    Only a few minor text revisions are proposed to this section for
consistency with the remainder of the Guideline.
2. Section 1
    The EPA propose to update the introduction section to reflect the
reorganized nature of the revised Guideline. Minor text revisions are
proposed throughout this section for additional clarity. Additional
information is provided regarding the importance of CAA section 320 to
amendments of the Guideline.
3. Section 2
    The EPA proposes to revise section 2 to more appropriately discuss
the process by which models are evaluated and considered for use in
particular applications. We propose to incorporate information from the
previous section 9 pertaining to model accuracy and uncertainty within
this section to clarify how model performance evaluation is critical in
determining the suitability of models for particular application.
    We also propose to provide a discussion in section 2.1 (Model
Accuracy and Uncertainty) of the three types of models historically
used for regulatory demonstrations. For each type of model, some
strengths and weaknesses are listed to assist readers in the
understanding of the particular regulatory applications to which they
are most appropriate.
    In addition, the EPA proposes revisions to section 2.2 with respect
to the recommended practice of progressing from simplified and
conservative air quality analysis toward more complex and refined
analysis. In this section, the EPA proposes to clarify distinctions
between various types of models that have previously been described as
screening models. In addition, this section clarifies distinctions
between models used for screening purposes and screening techniques and
demonstration tools that may be acceptable in certain applications.
4. Section 3
    The EPA proposes minor modifications to section 3 to more
accurately reflect current EPA practices and by moving the discussion
of the EPA's Model Clearinghouse to a revised section 3.3 for ease of
reference and prominence within the Guideline. A change is proposed to
require Regional Office consultation with the Model Clearinghouse on
all alternative model approvals. Previously, section 3 included various
requirements under recommendation subheading that were not clearly
identified as requirements. Accordingly, the EPA is proposing to modify
section 3 with the incorporation of requirement subsections to
eliminate any ambiguity.
5. Section 4
    The EPA proposes to significantly revise section 4 to incorporate
the modeling approaches recommended for air quality impact analyses for
the criteria pollutants of CO, lead, SO2, NO2,
and primary PM2.5 and PM10. In many respects, the
proposed revisions to section 4 are a combination of the previous
sections 4 and 5, reflecting inert criteria pollutants only. The EPA
also proposes to modify section 4 to incorporate requirement
subsections to provide clarity of the various requirements where
previously sections 4 and 5 included various requirements under
recommendation subheadings.
    As proposed, this section provides an in-depth discussion of
screening and refined models, including the introduction of AERSCREEN
as the recommended screening model for simple and complex terrain for
single sources and options for multi-source screening with AERMOD.\47\
The EPA proposes to include a clear discussion of each appendix A
preferred model in section 4.3 (Refined Models). The EPA also proposes
to modify the discussion for each preferred model (i.e., AERMOD
Modeling System, CTDMPLUS, and OCD) from the previous section 4 with
appropriate edits and some streamlining based on information available
in the respective model formulation documentation and users guides.
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    \47\ U.S. EPA, 2015. Technical Support Document (TSD) for
Replacement of CALINE3 with AERMOD for Transportation Related Air
Quality Analyses. Publication No. EPA-454/B-15-002. Office of Air
Quality Planning & Standards, Research Triangle Park, NC.
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    The EPA is proposing to add a subsection specifically addressing
the modeling recommendations for SO2 where, previously,
section 4 of the Guideline was generally understood to be applicable
for SO2. Minor updates are proposed with respect to the
modeling recommendations for each of the other inert criteria
pollutants that were previously found in section 5. For NO2,
the ARM2 is proposed to be added as a Tier 2 option, and the Tier 3
options of OLM and PVMRM are proposed to become part of the regulatory
version of AERMOD. For any pollutant that had significant emissions
from mobile sources, our previous recommendation to use the CALINE3
models is proposed to be replaced with AERMOD.
6. Section 5
    As already stated, much of the previous section 5 with respect to
the inert criteria pollutants is proposed to be incorporated into the
revised section 4. As proposed, the revised section 5 is now focused
only on the modeling approaches recommended for ozone and secondary
PM2.5.
    Both ozone and secondary PM2.5 are formed through
chemical reactions in the atmosphere and are not

appropriately modeled with traditional steady-state Gaussian plume
models, such as AERMOD. Chemical transport models are necessary to
appropriately assess the single-source air quality impacts of precursor
pollutants on the formation of ozone or secondary PM2.5.
    While the proposed revision to section 5 do not specify a
particular EPA-preferred model or technique for use in air quality
assessments, a two-tiered screening approach is proposed for ozone and
secondary PM2.5 with appropriate references to the EPA's new
single-source modeling guidance. The first tier consists of technically
credible and appropriate relationships between emissions and the
impacts developed from existing modeling simulations. If existing
technical information is not available or appropriate, then a second
tier approach would apply, involving use of sophisticated chemical
transport models (e.g., photochemical grid models) as determined in
consultation with the appropriate EPA Regional Office on a case-by-case
basis based upon the EPA's new single-source modeling guidance.
7. Section 6
    Revisions to section 6 are proposed to more clearly address the
modeling recommendations of other federal agencies, such as the FLM(s),
that have been developed in response to EPA rules or standards. While
no attempt is made to comprehensively discuss each topic, the EPA
proposes to provide appropriate references to the respective federal
agency guidance documents.
    The proposed revision to section 6 focus primarily on AQRVs,
including near-field and long-range transport assessments for
visibility impairment and deposition. The interests of the Bureau of
Ocean Energy and Management for Outer Continental Shelf permitting
situations and of the Federal Aviation Administration for airport and
air base permitting situations are represented in proposed section 6.3
(Modeling Guidance for Other Governmental Programs).
    The discussion of Good Engineering Practices (GEP) for stack height
consideration is proposed to be modified and moved to section 7. The
EPA proposed to remove the discussion of long-range transport for PSD
Class I increment and references to the previously preferred long-range
transport model, CALPUFF, in accordance with the more detailed
discussion in the Proposed Actions section of this Preamble.
8. Section 7
    We propose to revise section 7 to be more streamlined and
appropriate to the variety of general modeling issues and
considerations that are not already been covered in sections 4, 5, and
6 of the Guideline. The EPA proposes to move the information concerning
design concentrations and receptor sites to section 9. The discussion
of stability categories is proposed to be removed from section 7 since
it is specifically addressed in the model formulation documentation and
guidance for the dispersion models that require stability categories to
be defined. As already stated, the GEP discussion from the previous
section 6 is proposed to be incorporated into this section.
    The EPA proposes to expand the recommendations for determining
rural or urban dispersion coefficients to provide more clarity with
respect to appropriate characterization within AERMOD, including a
discussion on the existence of highly industrialized areas where
population density is low that may be best treated with urban rather
than rural dispersion coefficients. References to CALPUFF in the
Complex Winds subsection are proposed to be removed due to technical
issues described in the Proposed Actions section of this preamble. As
proposed, if necessary for special complex wind situations, the setup
and application of an alternative model should now be determined in
consultation with the appropriate reviewing authority. Finally, the EPA
proposes to revise section 7 to include a new discussion of modeling
considerations specific to mobile sources.
9. Section 8
    The EPA propose extensive updates and modifications to section 8 to
reflect current EPA practices, requirements, and recommendations for
determining the appropriate modeling domain and model input data from
new or modifying source(s) or sources under consideration for a revised
permit limit, from background concentrations (including air quality
monitoring data and nearby and others sources), and from meteorology.
As with earlier sections, the EPA proposes to modify section 8 to
incorporate requirement subsections where previously section 8
ambiguously included various requirements under recommendation
subheadings.
    The Background Concentration subsection is proposed to be
significantly modified from the existing Guideline to include a more
clear and comprehensive discussion of nearby and other sources. This is
intended to eliminate confusion of how to identify nearby sources that
should be explicitly modeled and all other sources that should be
generally represented by air quality monitoring data. In addition to
air quality monitoring data, a brief discussion on the use of
photochemical grid modeling to appropriately characterize background
concentrations has been included in this proposed section. Updates to
Tables 8-1 and 8-2 are proposed per changes in the considerations for
nearby sources, as discussed in the Proposed Actions section of this
Preamble.
    The use of prognostic mesoscale meteorological models to provide
meteorological input for regulatory dispersion modeling applications is
proposed to be incorporated throughout the Meteorological Input Data
subsection, including the introduction of the MMIF as a tool to inform
regulatory model applications. Other than additional minor
modifications to the recommendations through this subsection based on
current EPA practices, the most substantive proposed edits relate to
the recommendation to use the AERMINUTE meteorological data processor
to calculate hourly average wind speed and direction when processing
NWS ASOS data for developing AERMET meteorological inputs to the AERMOD
dispersion model.
10. Section 9
    The EPA proposes to move all of the information previously in
section 9 related to model accuracy and evaluation into other sections
in the revised Guideline (primarily to the revised section 2 and some
to the revised section 4). This provides for greater clarity in those
topics as applied to selection of models under the Guideline. However,
the EPA proposes to remove subsection on the ``Use of Uncertainty in
Decision Making.''. After removing this content, the EPA proposes to
totally revise section 9 to focus on the regulatory application of
models, which would include the majority of the information found
previously in section 10.
    The EPA proposes to revise the discussion portion of section 9 to
more clearly summarize the general concepts presented in earlier
sections of the Guideline and to set the stage for the appropriate
regulatory application of models and/or, in rare circumstances, air
quality monitoring data. The importance of developing and vetting a
modeling protocol is more prominently presented in a separate
subsection.

    The information related to design concentrations is proposed to be
updated and unified from previous language found in sections 7 and 10.
An expanded discussion of receptor sites is proposed based on language
from the previous section 7 and new considerations given past practices
of model users tending to define an excessively large and inappropriate
number of receptors based on vague guidance.
    The recommendations for NAAQS and PSD increment compliance
demonstrations are proposed to be overhauled to more clearly and
accurately reflect the long-standing EPA recommendation and practice of
performing a single-source impact analysis as a first stage of the
NAAQS and PSD increment compliance demonstration and, as necessary,
conducting a more comprehensive cumulative impact analysis as the
second stage. The appropriate considerations and applications of
screening and/or refined model are described in each stage.
    Finally, the section on Use of Measured Data in Lieu of Model
Estimates subsection is proposed to be revised to provide more details
on the process for determining the rare circumstances in which air
quality monitoring data may be considered for determining the most
appropriate emissions limit for a modification to an existing source.
As with other portions of the revised section 9, the language
throughout this subsection is proposed to be updated to reflect current
EPA practices, as appropriate.
11. Section 10
    As discussed, the majority of the information found previously in
section 10 is proposed to be incorporated into the revised section 9.
As proposed, section 10 consists of the references that were in the
previous section 12. We also propose to update each reference, as
appropriate, based on the text revisions throughout the Guideline.
12. Section 11
    In a streamlining effort, the EPA proposes to remove this
bibliography section from the Guideline.
13. Section 12
    As stated earlier, this references section is now proposed as
section 10 with appropriate updates.
14. Appendix A to the Guideline
    The EPA proposes to revise appendix A to the Guideline to remove
the Buoyant Line and Point Source Dispersion Model (BLP), CALINE3, and
CALPUFF as refined air quality models preferred for specific regulator
applications. The rational for the removal of these air quality models
from the preferred status can be found in the Proposed Actions section
of this Preamble.

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review

    This proposed action is not a ``significant regulatory action''
under the terms of Executive Order 12866 (58 FR 51735, October 4, 1993)
and is, therefore not subject to OMB review under Executive Orders
12866 and 13563 (76 FR 3821, January 21, 2011).

B. Paperwork Reduction Act

    This proposed action does not impose an information collection
burden subject to OMB review under the provisions of the Paperwork
Reduction Act, 44 U.S.C. 3501 et seq.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of this rule on small
entities, small entity is defined as (1) a small business as defined by
the Small Business Administration's (SBA) regulations at 13 CFR
121.201; (2) a small governmental jurisdiction that is a government of
a city, county, town, school district or special district with a
population of less than 50,000; and (3) a small organization that is
any not-for-profit enterprise which is independently owned and operated
and is not dominant in its field.
    The modeling techniques described in this proposed action are
primarily used by air agencies and by industries owning major sources
subject to NSR permitting requirements. To the extent that any small
entities would have to conduct air quality assessments, using the
models and/or techniques described in this proposed action are not
expect to pose any additional burden (compared to the existing models
and/or techniques) on these entities. The proposal features updates to
the existing EPA-preferred model, AERMOD, that serves to increase
efficiency and accuracy by changing only mathematical formulations and
specific data elements. Also, this proposed action will streamline
resources necessary to conduct necessary modeling with AERMOD by
incorporating model algorithms from the BLP model and replacing CALINE3
for mobile source applications. Although this proposed action calls for
new models and/or techniques for use in addressing ozone and secondary
PM2.5, we expect most small entities will generally be able
to rely on existing modeling simulations; so, we expect minimal burden
associated with these assessments. Therefore, we do not believe that
that this proposal poses a significant or unreasonable burden on any
small entities.
    After considering the economic impacts of this rule on small
entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. We continue
to be interested in the potential impacts of the proposed rule on small
entities and welcome comments on issues related to such impacts.

D. Unfunded Mandates Reform Act

    This proposed action contains no federal mandates under the
provisions of Title II of the Unfunded Mandates Reform Act of 1995
(UMRA), 2 U.S.C. 1531-1538 for state, local, or tribal governments or
the private sector. This action imposes no enforceable duty on any
state, local or tribal governments or the private sector. Therefore,
this action is not subject to the requirements of sections 202 or 205
of the UMRA. This action is also not subject to the requirements of
section 203 of UMRA because it contains no regulatory requirements that
might significantly or uniquely affect small governments.

E. Executive Order 13132: Federalism

    This proposed action does not have federalism implications. It will
not have substantial direct effects on the states, on the relationship
between the national government and the states, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. This rule does not create a
mandate on state, local or tribal governments nor does it impose any
enforceable duties on these entities. This action would add better,
more accurate techniques for conducting air quality assessments and
does not add

any additional requirements for any of the affected parties covered
under Executive Order 13132. Thus, the requirements of section 6 of the
Executive Order do not apply to this proposal. In the spirit of
Executive Order 13132, and consistent with the EPA policy to promote
communications between the EPA and state and local governments, the EPA
specifically solicits comment on this proposed rule from state and
local officials.

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

    This proposed action does not have tribal implications, as
specified in Executive Order 13175 (65 FR 67249, November 9, 2000).
This proposed rule imposes no requirements on tribal governments.
Accordingly, Executive Order 13175 does not apply to this action. In
the spirit of Executive Order 13175, the EPA specifically solicits
additional comment on this proposed action from tribal officials.

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

    The EPA interprets Executive Order 13045 as applying only to those
regulatory actions that concern environmental health or safety risks
that the EPA has reason to believe may disproportionately affect
children, per the definition of ``covered regulatory action'' in
section 2-202 of the Executive Order. This action is not subject to
Executive Order 13045 because it does not concern an environmental
health risk or safety risk.

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

    This action is not a ``significant energy action'' as defined in
Executive Order 13211 (66 FR 28355 (May 22, 2001)), because it is not
likely to have a significant adverse effect on the supply,
distribution, or use of energy.

I. National Technology Transfer and Advancement Act

    This rulemaking does not involve technical standards.

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

    The EPA has determined that this proposed rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it does not
affect the level of protection provided to human health or the
environment.

List of Subjects in 40 CFR Part 51

    Environmental protection, Administrative practice and procedure,
Air pollution control, Carbon monoxide, Intergovernmental relations,
Nitrogen oxides, Ozone, Particulate Matter, Reporting and recordkeeping
requirements, Sulfur oxides.

    Dated: July 14, 2015.
Gina McCarthy,
Administrator.

    For the reasons stated in the preamble, title 40, chapter I of the
Code of Federal Regulations is proposed to be amended as follows:

PART 51--REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF
IMPLEMENTATION PLANS

0
1. The authority citation for part 51 continues to read as follows:

    Authority:  23 U.S.C. 101; 42 U.S.C. 7401-7671q.

0
2. Appendix W to part 51 is revised to read as follows:

APPENDIX W TO PART 51--Guideline on Air Quality Models Preface

    a. Industry and control agencies have long expressed a need for
consistency in the application of air quality models for regulatory
purposes. In the 1977 Clean Air Act (CAA), Congress mandated such
consistency and encouraged the standardization of model
applications. The Guideline on Air Quality Models (hereafter,
Guideline) was first published in April 1978 to satisfy these
requirements by specifying models and providing guidance for their
use. The Guideline provides a common basis for estimating the air
quality concentrations of criteria pollutants used in assessing
control strategies and developing emissions limits.
    b. The continuing development of new air quality models in
response to regulatory requirements and the expanded requirements
for models to cover even more complex problems have emphasized the
need for periodic review and update of guidance on these techniques.
Historically, three primary activities have provided direct input to
revisions of the Guideline. The first is a series of periodic EPA
workshops and modeling conferences conducted for the purpose of
ensuring consistency and providing clarification in the application
of models. The second activity was the solicitation and review of
new models from the technical and user community. In the March 27,
1980, Federal Register, a procedure was outlined for the submittal
to the EPA of privately developed models. After extensive evaluation
and scientific review, these models, as well as those made available
by the EPA, have been considered for recognition in the Guideline.
The third activity is the extensive on-going research efforts by the
EPA and others in air quality and meteorological modeling.
    c. Based primarily on these three activities, new sections and
topics have been included as needed. The EPA does not make changes
to the guidance on a predetermined schedule, but rather on an as-
needed basis. The EPA believes that revisions of the Guideline
should be timely and responsive to user needs and should involve
public participation to the greatest possible extent. All future
changes to the guidance will be proposed and finalized in the
Federal Register. Information on the current status of modeling
guidance can always be obtained from EPA's Regional Offices.

Table of Contents

List of Tables

1.0 Introduction
2.0 Overview of Model Use
2.1 Suitability of Models
    2.1.1 Model Accuracy and Uncertainty
2.2 Levels of Sophistication of Air Quality Analyses and Models
2.3 Availability of Models
3.0 Preferred and Alternative Air Quality Models
3.1 Preferred Models
    3.1.1 Discussion
    3.1.2 Requirements
3.2 Alternative Models
    3.2.1 Discussion
    3.2.2 Requirements
3.3 EPA's Model Clearinghouse
4.0 Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen
Dioxide and Primary Particulate Matter
4.1 Discussion
4.2 Requirements
    4.2.1 Screening Models and Techniques
    <IP_ADDRESS> AERSCREEN
    <IP_ADDRESS> CTSCREEN
    <IP_ADDRESS> Screening in Complex Terrain
    4.2.2 Refined Models
    <IP_ADDRESS> AERMOD
    <IP_ADDRESS> CTDMPLUS
    <IP_ADDRESS> OCD
    4.2.3 Pollutant Specific Modeling Requirements
    <IP_ADDRESS> Models for Carbon Monoxide
    <IP_ADDRESS> Models for Lead
    <IP_ADDRESS> Models for Sulfur Dioxide
    <IP_ADDRESS> Models for Nitrogen Dioxide
    <IP_ADDRESS> Models for PM2.5
    <IP_ADDRESS> Models for PM10
5.0 Models for Ozone and Secondarily Formed Particulate Matter
5.1 Discussion
5.2 Recommendations
5.3 Recommended Models and Approaches for Ozone
    5.3.1 Models for NAAQS Attainment Demonstrations and Multi-
Source Air Quality Assessments
    5.3.2 Models for Single-Source Air Quality Assessments
5.4 Recommended Models and Approaches for Secondarily Formed
PM2.5
    5.4.1 Models for NAAQS Attainment Demonstrations and Multi-
Source Air Quality Assessments
    5.4.2 Models for Single-Source Air Quality Assessments

6.0 Modeling for Air Quality Related Values and Other Governmental
Programs
6.1 Discussion
6.2 Air Quality Related Values
    6.2.1 Visibility
    <IP_ADDRESS> Models for Estimating Near-Field Visibility Impairment
    <IP_ADDRESS> Models for Estimating Visibility Impairment for Long-
Range Transport
    6.2.2 Models for Estimating Deposition Impacts
6.3 Modeling Guidance for Other Governmental Programs
7.0 General Modeling Considerations
7.1 Discussion
7.2 Recommendations
    7.2.1 All sources
    <IP_ADDRESS> Dispersion Coefficients
    <IP_ADDRESS> Complex Winds
    <IP_ADDRESS> Gravitational Settling and Deposition
    7.2.2 Stationary Sources
    <IP_ADDRESS> Good Engineering Practice Stack Height
    <IP_ADDRESS> Plume Rise
    7.2.3 Mobile Sources
8.0 Model Input Data
8.1 Modeling Domain
    8.1.1 Discussion
    8.1.2 Requirements
8.2 Source Data
    8.2.1 Discussion
    8.2.2 Requirements
8.3 Background Concentrations
    8.3.1 Discussion
    8.3.2 Recommendations for Isolated Single Source
    8.3.3 Recommendations for Multi-Source Areas
8.4 Meteorological Input Data
    8.4.1 Discussion
    8.4.2 Recommendations and Requirements
    8.4.3 National Weather Service Data
    <IP_ADDRESS> Discussion
    <IP_ADDRESS> Recommendations
    8.4.4 Site-specific data
    <IP_ADDRESS> Discussion
    <IP_ADDRESS> Recommendations
    8.4.5 Prognostic meteorological data
    <IP_ADDRESS> Discussion
    <IP_ADDRESS> Recommendations
    8.4.6 Treatment of Near-Calms and Calms
    <IP_ADDRESS> Discussion
    <IP_ADDRESS> Recommendations
9.0 Regulatory Application of Models
9.1 Discussion
9.2 Recommendations
    9.2.1 Modeling Protocol
    9.2.2 Design Concentration and Receptor Sites
    9.2.3 NAAQS and PSD Increments Compliance Demonstrations for New
or Modified Sources
    <IP_ADDRESS> Considerations in Developing Emissions Limits
    9.2.4 Use of Measured Data in Lieu of Model Estimates
10.0 References
Appendix A to Appendix W of Part 51--Summaries of Preferred Air
Quality Models

                             List of Tables
------------------------------------------------------------------------
             Table No.                              Title
------------------------------------------------------------------------
8-1...............................  Point Source Model Emission Input
                                     for SIP Revisions of Inert
                                     Pollutants.
8-2...............................  Point Source Model Emission Input
                                     for NAAQS Compliance in PSD
                                     Demonstrations.
------------------------------------------------------------------------

1.0 Introduction

    a. The Guideline recommends air quality modeling techniques that
should be applied to State Implementation Plan (SIP) submittals and
revisions, to New Source Review (NSR), including new or modifying
sources under Prevention of Significant Deterioration
(PSD),1 2 3 conformity analyses,\4\ and other air quality
assessments required under EPA regulation. Applicable only to
criteria air pollutants, the Guideline is intended for use by the
EPA Regional Offices in judging the adequacy of modeling analyses
performed by the EPA, by state, local, and tribal permitting
authorities, and by industry. It is appropriate for use by other
federal government agencies and by state, local, and tribal agencies
with air quality and land management responsibilities. The Guideline
serves to identify, for all interested parties, those modeling
techniques and databases that the EPA considers acceptable. The
Guideline is not intended to be a compendium of modeling techniques.
Rather, it should serve as a common measure of acceptable technical
analysis when supported by sound scientific judgment.
    b. Air quality measurements \5\ are routinely used to
characterize ambient concentrations of criteria pollutants
throughout the nation but are rarely sufficient for characterizing
the ambient impacts of individual sources or demonstrating adequacy