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

Federal RegisterJul 29, 2015

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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.

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 (PM

2.5

) 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:

A-and-R-Docket@epa.gov

. Include docket ID No. EPA-HQ-OAR-2015-0310 in the subject line of the message.

•

Fax:

(202) 566-9744.

•

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 (202) 566-1742.

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: (919) 541-5563; fax: (919) 541-0044; email:

Bridgers.George@epa.gov

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 NO

5. Status of CALINE3 Models

6. Addressing Single-Source Impacts on Ozone and Secondary PM

2.5

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:

Category

NAICS

Code

Federal/state/territorial/local/tribal government

924110

North American Industry Classification System.

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

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.

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 PM

2.5

National Ambient Air Quality Standards (NAAQS), modeling for compliance demonstration of the 1-hour nitrogen dioxide (NO

) and sulfur dioxide (SO

) NAAQS, and new and emerging models/techniques for future consideration under the

Guideline

to address single-source modeling for ozone and secondary PM

2.5,

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 NO

in January 2010, and a new 1-hour NAAQS for SO

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.

U.S. EPA, 2010. Applicability of Appendix W Modeling Guidance for the 1-hour NO

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

U.S. EPA, 2010. Applicability of Appendix W Modeling Guidance for the 1-hour SO

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

U.S. EPA, 2010. Guidance Concerning the Implementation of the 1-hour SO

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

U.S. EPA, 2010. Guidance Concerning the Implementation of the 1-hour NO

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

Several presentations at the Tenth Modeling Conference addressed issues and challenges associated with demonstrating compliance with these new 1-hour NAAQS for NO

and SO

. 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

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.

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

Prior to the promulgation of the 1-hour NO

NAAQS, compliance with the previous annual NO

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 (NO

) to NO

, referred to as the Ambient Ratio Method (ARM). However, compliance with the new 1-hour NAAQS has typically required a more refined treatment of NO

conversion to NO

. Therefore, several presentations at the Tenth Modeling Conference focused on issues associated with demonstrating compliance with the new 1-hour NO

NAAQS.

These presentations included an overview of an API funded study to develop a Tier 2 ambient ratio method for the 1-hour NO

NAAQS, referred to as ARM2. The ARM2 approach was developed based on an extensive analysis of ambient ratios of NO

/NO

that were analyzed by land use (urban vs. rural) and geographical areas. Based on these analyses of the ambient NO

/NO

ratios, an empirical relationship between ambient concentrations of NO

and NO

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 NO

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 NO

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 NO

In addition, modeling of single-source impacts for ozone and secondarily formed PM

2.5

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

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 PM

2.5.

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 PM

2.5

Permit Modeling

that addressed the need for consideration of secondary PM

2.5

in demonstrating compliance with the PM

2.5

NAAQS.

Subsequently, written comments pertaining to such modeling were submitted to the EPA.

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

U.S. EPA, 2014. Guidance for PM

2.5

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

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 PM

2.5

) 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

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.

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.

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: (919) 541-5563; fax: (919) 541-0044; email:

Bridgers.George@epa.gov

. 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 PM

2.5

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,

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.

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

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 NO

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 NO

Section 5.2.4 of the 2005

Guideline

details a 3-tiered approach for assessing NO

sources, which was recommended to obtain annual average estimates of NO

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 NO

and the subsequent conversion of NO to NO

in the atmosphere. The tiered levels include: (1) Assuming that all NO is converted to NO

(full conversion), (2) using the Ambient Ratio Method (ARM), which applies an assumed equilibrium ratio of NO

to NO

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 NO

to NO

In January 2010, a new 1-hour NO

standard was promulgated. Prior to the adoption of the 1-hour NO

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 NO

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 NO

standard, the EPA is proposing several modifications to the Tier 2 and 3 NO

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.

A recently published study

presented a new analysis of national levels of ambient ratios of NO

to NO

based on

hourly data from the EPA's Air Quality System (AQS). Based on this analysis, a new second tier NO

screening technique, ARM2, has been developed and incorporated into AERMOD. Because ARM2 is based on hourly measurements of the NO

to NO

ratios and provides more detailed estimates of this ratio based on the total NO

present, the EPA is proposing to incorporate a modified version of ARM2 as the new preferred second tier NO

modeling approach.

Chu, S.H. and E.L. Meyer, 1991. Use of Ambient Ratios to Estimate Impact of NO

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).

Podrez, M. 2015. An Update to the Ambient Ratio Method for 1-h NO2 Air Quality Standards Dispersion Modeling.

Atmospheric Environment,

103: 163-170.

For the Tier 3 technique, the EPA proposes that the existing detailed screening options of the Ozone Limiting Method (OLM)

and PVMRM

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 NO

impacts and NO and NO

speciation with reasonable accuracy when applied appropriately, both OLM and PVMRM are being proposed as preferred Tier 3 screening methods for NO

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 NO

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.

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.

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.

We invite comments on whether we have reasonably addressed technical concerns regarding the 3-tiered demonstration approach and specific NO

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

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.

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

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 PM

2.5,

, and CO hot-spot analyses.

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).

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.

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).

U.S. EPA, 2013, Transportation Conformity Guidance for Quantitative Hot-Spot Analyses in PM

2.5

and PM

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

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 PM

2.5

On January 4, 2012, the EPA granted a petition submitted on behalf of the Sierra Club on July 28, 2010,

that requested the EPA initiate rulemaking to establish air quality models for ozone and PM

2.5

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 PM

2.5

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 PM

2.5

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

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 PM

2.5.

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 PM

2.5

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).

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.

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 PM

2.5

, 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.

Baker, K.R., Kelly, J.T., 2014. Single source impacts estimated with photochemical model source sensitivity and apportionment approaches.

Atmospheric Environment,

96: 266-274.

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

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 PM

2.5

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)

and the Community Multiscale Air Quality (CMAQ)

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 PM

2.5

25 26 27 28

These models have been used extensively to support ozone and PM

2.5

SIPs and to explore relationships between inputs and air quality impacts in the United States and elsewhere.

26 29 30

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.

ENVIRON, 2014. User's Guide Comprehensive Air Quality Model with Extensions version 6,

http://www.camx.com

. ENVIRON International Corporation, Novato.

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.

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.

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.

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.

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.

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.

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.

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 PM

2.5

. 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.

The appropriate tier for a given application should be selected in consultation with the appropriate reviewing authority and be consistent with EPA guidance.

U.S. EPA, 2015. Guidance on the use of models for assessing the impacts from single sources on secondarily formed pollutants ozone and PM

2.5

. Publication No. EPA 454/P-15-001. Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711.

To fully implement these proposed changes to the

Guideline

related to addressing ozone and secondary PM

2.5

impacts, the EPA intends to pursue a separate rulemaking to establish a technical basis and new values for PM

2.5

Significant Impact Levels (SILs) and to introduce a new demonstration tool for ozone and PM

2.5

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 PM

2.5

Permit Modeling.

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 PM

2.5

. Our present understanding of the atmospheric science of ozone and secondary PM

2.5

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 PM

2.5

formation. As part of the separate rulemaking, the EPA intends to demonstrate that a source with precursor emissions (

e.g.,

and SO

for PM

2.5

) 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 PM

2.5

NAAQS or PSD increments. The EPA's

Guidance for PM

2.5

Permit Modeling

provides for a three-tiered approach to address secondary PM

2.5

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 PM

2.5

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 PM

2.5

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 NO

precursors) for ozone using approaches similar to those for PM

2.5

32 33

U.S. EPA, 2015. “Proposed Approach for Demonstrating PM

2.5

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.

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.

While the development of MERPs for ozone and secondary PM

2.5

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.

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 PM

2.5

concentrations.

20 34

The direct decoupled method (DDM) has also been used to estimate ozone and PM

2.5

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

and approaches to differentiate single-source impacts

20 36

show generally similar downwind spatial gradients and impacts.

Baker, K.R., Foley, K.M., 2011. A nonlinear regression model estimating single source concentrations of primary and secondarily formed PM

2.5

Atmospheric Environment,

45: 3758-3767.

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.

Baker, K.R., Kelly, J.T., Fox, T., 2013. Estimating second pollutant impacts from single sources (control #27).

http://aqmodels.awma.org/conference-proceedings

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.

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 PM

2.5

. 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 PM

2.5

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 SO

). 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

), 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.

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.

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

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.

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 PM

2.5.

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.

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.

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.

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 PM

2.5

, the final report

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 PM

2.5

, 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 PM

2.5

, 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.

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

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.

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).

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.

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.

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.

See

70 FR 39104, 39122-23 (July 6, 2005).

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.

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 SO

and NO

, 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.

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.

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

U.S. EPA, 2014. Clarification on the Use of AERMOD Dispersion Modeling for Demonstrating Compliance with the NO

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

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

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, PM

2.5

, 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, SO

, NO

, and primary PM

2.5

and PM

. 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.

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.

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.

The EPA is proposing to add a subsection specifically addressing the modeling recommendations for SO

where, previously, section 4 of the

Guideline

was generally understood to be applicable for SO

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 NO

, 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 PM

2.5

Both ozone and secondary PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

, 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

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

Authority:

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

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

4.2.1.1 AERSCREEN

4.2.1.2 CTSCREEN

4.2.1.3 Screening in Complex Terrain

4.2.2 Refined Models

4.2.2.1 AERMOD

4.2.2.2 CTDMPLUS

4.2.2.3 OCD

4.2.3 Pollutant Specific Modeling Requirements

4.2.3.1 Models for Carbon Monoxide

4.2.3.2 Models for Lead

4.2.3.3 Models for Sulfur Dioxide

4.2.3.4 Models for Nitrogen Dioxide

4.2.3.5 Models for PM

2.5

4.2.3.6 Models for PM

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 PM

2.5

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

6.2.1.1 Models for Estimating Near-Field Visibility Impairment

6.2.1.2 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

7.2.1.1 Dispersion Coefficients

7.2.1.2 Complex Winds

7.2.1.3 Gravitational Settling and Deposition

7.2.2 Stationary Sources

7.2.2.1 Good Engineering Practice Stack Height

7.2.2.2 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

8.4.3.1 Discussion

8.4.3.2 Recommendations

8.4.4 Site-specific data

8.4.4.1 Discussion

8.4.4.2 Recommendations

8.4.5 Prognostic meteorological data

8.4.5.1 Discussion

8.4.5.2 Recommendations

8.4.6 Treatment of Near-Calms and Calms

8.4.6.1 Discussion

8.4.6.2 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

9.2.3.1 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,

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

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 of emissions limits for an existing source due to limitations in spatial and temporal coverage of ambient monitoring networks. The impacts of new sources that do not yet exist and modifications to existing sources that have yet to be implemented can only be determined through modeling. Thus, models have become a primary analytical tool in most air quality assessments. Air quality measurements can be used in a complementary manner to air quality models, with due regard for the strengths and weaknesses of both analysis techniques, and are particularly useful in assessing the accuracy of model estimates.

c. It would be advantageous to categorize the various regulatory programs and to apply a designated model to each proposed source needing analysis under a given program. However, the diversity of the nation's topography and climate, and variations in source configurations and operating characteristics dictate against a strict modeling “cookbook.” There is no one model capable of properly addressing all conceivable situations even within a broad category such as point sources. Meteorological phenomena associated with threats to air quality standards are rarely amenable to a single mathematical treatment; thus, case-by-case analysis and judgment are frequently required. As modeling efforts become more complex, it is increasingly important that they be directed by highly competent individuals with a broad range of experience and knowledge in air quality meteorology. Further, they should be coordinated closely with specialists in emissions characteristics, air monitoring and data processing. The judgment of experienced meteorologists, atmospheric scientists, and analysts is essential.

d. The model that most accurately estimates concentrations in the area of interest is always sought. However, it is clear from the needs expressed by the EPA Regional Offices, by state, local, and tribal agencies, by many industries and trade associations, and also by the deliberations of Congress that consistency in the selection and application of models and databases should also be sought, even in case-by-case analyses. Consistency ensures that air quality control agencies and the general public have a common basis for estimating pollutant concentrations, assessing control strategies, and specifying emissions limits. Such consistency is not, however, promoted at the expense of model and database accuracy. The

Guideline

provides a consistent basis for selection of the most accurate models and databases for use in air quality assessments.

e. Recommendations are made in the

Guideline

concerning air quality models and techniques, model evaluation procedures, and model input databases and related requirements. The guidance provided here should be followed in air quality analyses relative to SIPs, NSR, and in supporting analyses required by the EPA and by state, local, and tribal permitting authorities. Specific models are identified for particular applications. The EPA may approve the use of an alternative model or technique that can be demonstrated to be more appropriate than those recommended in the

Guideline.

In all cases, the model or technique applied to a given situation should be the one that provides the most accurate representation of atmospheric transport, dispersion, and chemical transformations in the area of interest. However, to ensure consistency, deviations from the

Guideline

should be carefully documented as part of the public record and fully supported by the appropriate reviewing authority, as discussed later.

f. From time to time, situations arise requiring clarification of the intent of the guidance on a specific topic. Periodic workshops are held with EPA headquarters, EPA Regional Office, and state, local, and tribal agency modeling representatives to ensure consistency in modeling guidance and to promote the use of more accurate air quality models, techniques, and databases. The workshops serve to provide further explanations of

Guideline

requirements to the EPA Regional Offices and workshop materials are issued with this clarifying information. In addition, findings from ongoing research programs, new model development, or results from model evaluations and applications are continuously evaluated. Based on this information, changes in the applicable guidance may be indicated and appropriate revisions to the

Guideline

may be considered.

g. All changes to the

Guideline

must follow rulemaking requirements since the

Guideline

is codified in appendix W to 40 Code of Federal Regulations (CFR) part 51. The EPA will promulgate proposed and final rules in the

Federal Register

to amend this appendix. The EPA utilizes the existing procedures under CAA section 320 that requires EPA to conduct a Conference on Air Quality Modeling at least every 3 years. These modeling conferences are intended to develop standardized air quality modeling procedures and form the basis for associated revisions to this

Guideline

in support of the EPA's continuing effort to prescribe with “reasonable particularity” air quality models and meteorological and emission databases suitable for modeling National Ambient Air Quality Standards (NAAQS)

and PSD increments (CAA 320, 42 U.S.C. 7620). Ample opportunity for public comment will be provided for each proposed change and public hearings scheduled.

h. A wide range of topics on modeling and databases are discussed in the

Guideline.

Section 2 gives an overview of models and their suitability for use in regulatory applications. Section 3 provides specific guidance on the determination of preferred air quality models and on the selection of alternative models or techniques. Sections 4 through 6 provide recommendations on modeling techniques for assessing criteria pollutant impacts from single and multiple sources with specific modeling requirements for selected regulatory applications. Section 7 discusses general considerations common to many modeling analyses for stationary and mobile sources. Section 8 makes recommendations for data inputs to models including source, background air quality, and meteorological data. Section 9 summarizes how estimates and measurements of air quality are used in assessing source impact and in evaluating control strategies.

i. Appendix W to 40 CFR part 51 contains an appendix: Appendix A. Thus, when reference is made to “appendix A” in this document, it refers to appendix A to appendix W to 40 CFR part 51. Appendix A contains summaries of refined air quality models that are “preferred” for particular applications; both EPA models and models developed by others are included.

2.0 Overview of Model Use

a. Increasing reliance has been placed on concentration estimates from air quality models as the primary basis for regulatory decisions concerning source permits and emission control requirements. In many situations, such as review of a proposed new source, no practical alternative exists. Before attempting to implement the guidance contained in this document, the reader should be aware of certain general information concerning air quality models and their evaluation and use. Such information is provided in this section.

2.1 Suitability of Models

a. The extent to which a specific air quality model is suitable for the assessment of source impacts depends upon several factors. These include: (1) The topographic and meteorological complexities of the area; (2) the detail and accuracy of the input databases,

i.e.,

emissions inventory, meteorological data, and air quality data; (3) the manner in which complexities of atmospheric processes are handled in the model; (4) the technical competence of those undertaking such simulation modeling; and (5) the resources available to apply the model. Any of these factors can have a significant influence on the overall model performance, which must be thoroughly evaluated to determine the suitability of an air quality model to a particular application or range of applications.

b. Air quality models are most accurate and reliable in areas that have gradual transitions of land use and topography. Meteorological conditions in these areas are spatially uniform such that observations are broadly representative and air quality model projections are not further complicated by a heterogeneous environment. Areas subject to major topographic influences experience meteorological complexities that are often difficult to measure and simulate. Models with adequate performance are available for increasingly complex environments. However, they are resource intensive and frequently require site-specific observations and formulations. Such complexities and the related challenges for the air quality simulation should be considered when selecting the most appropriate air quality model for an application.

c. Appropriate model input data should be available before an attempt is made to evaluate or apply an air quality model. Assuming the data are adequate, the greater the detail with which a model considers the spatial and temporal variations in meteorological conditions and permit-enforceable emissions, the greater the ability to evaluate the source impact and to distinguish the effects of various control strategies.

d. There are three types of models that have historically been used in the regulatory demonstrations applicable in the

Guideline,

each having strengths and weaknesses that lend themselves to particular regulatory applications.

i. Gaussian plume models use a “steady-state” approximation, which assumes that over the model time step, the emissions, meteorology and other model inputs, are constant throughout the model domain, resulting in a resolved plume with the emissions distributed throughout the plume according to a Gaussian distribution. This formulation allows Gaussian models to estimate near-field impacts of a limited number of sources at a relatively high resolution, with temporal scales of an hour and spatial scales of meters. However, this formulation allows for only relatively inert pollutants, with very limited considerations of transformation and removal (

e.g.,

deposition), and further limits the domain for which the model may be used. Thus, Gaussian models may not be appropriate if model inputs are changing sharply over the model time step or within the desired model domain or if more advanced considerations of chemistry are needed.

ii. Lagrangian puff models, on the other hand, are non-steady-state, and assume that model input conditions are changing over the model domain and model time step. Lagrangian models can also be used to determine near and far-field impacts from a limited number of sources at a high resolution. Traditionally, Lagrangian models have been used for relatively inert pollutants, with slightly more complex considerations of removal than Gaussian models. Some Lagrangian models treat in-plume gas and particulate chemistry. However, these models require time and space varying concentration fields of oxidants and, in the case of fine particulate matter (PM

2.5

), neutralizing agents, such as ammonia. Reliable background fields are critical for applications involving secondary pollutant formation because secondary impacts generally occur when in-plume precursors mix and react with species in the background atmosphere.

7 8

These oxidant and neutralizing agents are not routinely measured, but can be generated with a three-dimensional photochemical grid model.

iii. Photochemical grid models are three-dimensional Eulerian grid-based models that treat chemical and physical processes in each grid cell and use diffusion and transport processes to move chemical species between grid cells.

Eulerian models assume that emissions are spread evenly throughout each model grid cell. Typically, Eulerian models have difficulty with fine scale resolution of individual plumes. However, these types of models can be appropriately applied for assessment of near-field and regional scale reactive pollutant impacts from specific sources

7 10 11 12

or all sources.

13 14 15

Photochemical gird models simulate a more realistic environment for chemical transformation,

7 12

but simulations can be more resource intensive than Lagrangian or Gaussian plume models.

e. Competent and experienced meteorologists, atmospheric scientists, and analysts are an essential prerequisite to the successful application of air quality models. The need for such specialists is critical when the more sophisticated models are used or the area being investigated has complicated meteorological or topographic features. It is important to note that a model applied improperly or with inappropriate data can lead to serious misjudgments regarding the source impact or the effectiveness of a control strategy.

f. The resource demands generated by use of air quality models vary widely depending on the specific application. The resources required may be important factors in the selection and use of a model or technique for a specific analysis. These resources depend on the nature of the model and its complexity, the detail of the databases, the difficulty of the application, the amount and level of expertise required, and the costs of manpower and computational facilities.

2.1.1 Model Accuracy and Uncertainty

a. The formulation and application of air quality models are accompanied by several sources of uncertainty. “Irreducible” uncertainty stems from the “unknown” conditions, which may not be explicitly accounted for in the model (

e.g.,

the turbulent velocity field). Thus, there are likely to be deviations from the observed

concentrations in individual events due to variations in the unknown conditions. “Reducible” uncertainties

are caused by: (1) Uncertainties in the “known” input conditions (

e.g.,

emission characteristics and meteorological data); (2) errors in the measured concentrations; and (3) inadequate model physics and formulation.

b. Evaluations of model accuracy should focus on the reducible uncertainty associated with physics and the formulation of the model. The accuracy of the model is normally determined by an evaluation procedure which involves the comparison of model concentration estimates with measured air quality data.

The statement of model accuracy is based on statistical tests or performance measures such as bias, noise, correlation, etc.

18 19

c. Since the 1980's, the EPA has worked with the modeling community to encourage development of standardized model evaluation methods and the development of continually improved methods for the characterization of model performance.

16 18 20 21 22

There is general consensus on what should be considered in the evaluation of air quality models; namely, quality assurance planning, documentation and scrutiny should be consistent with the intended use and should include:

• Scientific peer review;

• Supportive analyses (diagnostic evaluations, code verification, sensitivity analyses);

• Diagnostic and performance evaluations with data obtained in trial locations; and

• Statistical performance evaluations in the circumstances of the intended applications.

Performance evaluations and diagnostic evaluations assess different qualities of how well a model is performing, and both are needed to establish credibility within the client and scientific community.

d. Performance evaluations allow the EPA and model users to determine the relative performance of a model in comparison with alternative modeling systems. Diagnostic evaluations allow determination of a model capability to simulate individual processes that affect the results, and usually employ smaller spatial/temporal scale date sets (

e.g.,

field studies). Diagnostic evaluations enable the EPA and model users to build confidence that model predictions are accurate for the right reasons. However, the objective comparison of modeled concentrations with observed field data provides only a partial means for assessing model performance. Due to the limited supply of evaluation datasets, there are practical limits in assessing model performance. For this reason, the conclusions reached in the science peer reviews and the supportive analyses have particular relevance in deciding whether a model will be useful for its intended purposes.

2.2 Levels of Sophistication of Air Quality Analyses and Models

a. It is desirable to begin an air quality analysis by using simplified or conservative methods (or both) followed, as appropriate, by more complex and refined methods. The purpose of this approach is to streamline the process and sufficiently address regulatory requirements by eliminating the need of more detailed modeling when it is not necessary in a specific regulatory application. For example, in the context of a PSD permit application, a simplified or conservative analysis may be sufficient where it shows the proposed construction clearly will not cause or contribute to ambient concentrations in excess of either the NAAQS or the PSD increments.

2 3

b. There are two general levels of sophistication of air quality models. The first level consists of screening models that provide conservative modeled estimates of the air quality impact of a specific source or source category based on simplified assumptions of the model inputs (

e.g.,

preset, worst-case meteorological conditions). In the case of a PSD assessment, if a screening model indicates that the concentration contributed by the source could cause or contribute to a violation of any NAAQS or PSD increment, then the second level of more sophisticated models should be applied.

c. The second level consists of refined models that provide more detailed treatment of physical and chemical atmospheric processes, require more detailed and precise input data, and provide spatially and temporally resolved concentration estimates. As a result they provide a more sophisticated and, at least theoretically, a more accurate estimate of source impact and the effectiveness of control strategies.

d. There are situations where a screening model or a refined model is not available such that screening and refined modeling are not viable options to determine source-specific air quality impacts. In such situations, a screening technique or reduced-form model may be viable options for estimating source impacts.

i. Screening techniques are differentiated from a screening model in that screening techniques are approaches that make simplified and conservative assumptions about the physical and chemical atmospheric processes important to determining source impacts while screening models make assumptions about conservative inputs to a specific model. The complexity of screening techniques ranges from simplified assumptions of chemistry applied to refined or screening model output to sophisticated approximations of the chemistry applied within a refined model.

ii. Reduced-form models are computationally efficient simulation tools for characterizing the pollutant response to specific types of emission reductions for a particular geographic area or background environmental conditions that reflect underlying atmospheric science of a refined model but reduce the computational resources of running a complex, numerical air quality model such as a photochemical grid model.

In such situations, an attempt should be made to acquire or improve the necessary databases and to develop appropriate analytical techniques, but the screening technique or reduced-form model may be sufficient in conducting regulatory modeling applications when applied in consultation with the EPA Regional Office.

e. Consistent with the general principle described in paragraph 2.2(a), the EPA may establish a demonstration tool or method as a sufficient means for a user or applicant to make a demonstration required by regulation, either by itself or as part of a modeling demonstration. To be used for such regulatory purposes, such a tool or method must be reflected in a codified regulation or have a well-documented technical basis and reasoning that is contained or incorporated in the record of the regulatory decision in which it is applied.

2.3 Availability of Models

a. For most of the screening and refined models discussed in the

Guideline,

codes, associated documentation and other useful information are publicly available for download from the EPA's Support Center for Regulatory Atmospheric Modeling (SCRAM) Web site at

http://www.epa.gov/ttn/scram

. This is a Web site with which air quality modelers should become familiar and regularly visit for important model updates and additional clarifications and revisions to modeling guidance documents that are applicable to EPA programs and regulations. Codes and documentation may also available from the National Technical Information Service (NTIS),

http://www.ntis.gov

, and, when available, is referenced with the appropriate NTIS accession number.

3.0 Preferred and Alternative Air Quality Models

a. This section specifies the approach to be taken in determining preferred models for use in regulatory air quality programs. The status of models developed by the EPA, as well as those submitted to the EPA for review and possible inclusion in this

Guideline,

is discussed in this section. The section also provides the criteria and process for obtaining EPA approval for use of alternative models for individual cases in situations where the preferred models are not applicable or available. Additional sources of relevant modeling information are the EPA's Model Clearinghouse

(section 3.3), EPA modeling conferences, periodic Regional, State, and Local Modelers' Workshops, and the EPA's SCRAM Web site (section 2.3).

b. When approval is required for a specific modeling technique or analytical procedure in this

Guideline,

we refer to the

“appropriate reviewing authority.”

Many states and some local agencies administer NSR and PSD permitting under programs approved into SIPs. In some EPA regions, federal authority to administer NSR and PSD permitting and related activities has been delegated to state or local agencies. In these cases, such agencies

“stand in the shoes”

of the respective EPA regions. Therefore, depending on the circumstances, the appropriate reviewing authority may be an EPA Regional Office, a state, local, or tribal agency, or perhaps the Federal Land Manager (FLM). In some cases, the

Guideline

requires review and approval of the use of an alternative model by the EPA Regional Office (sometimes stated as

“Regional Administrator”

). For all approvals of alternative models or techniques, the EPA Regional Office will coordinate and shall seek concurrence with the EPA's Model Clearinghouse. If there is any question as to

the appropriate reviewing authority, you should contact the EPA Regional Office modeling contact (

http://www.epa.gov/ttn/scram/guidance_cont_regions.htm

), whose jurisdiction generally includes the physical location of the source in question and its expected impacts.

c. In all regulatory analyses, early discussions among the EPA Regional Office staff, state, local, and tribal agency staff, industry representatives, and where appropriate, the FLM, are invaluable and are strongly encouraged. Prior to the actual analyses, agreement on the databases to be used, modeling techniques to be applied, and the overall technical approach helps avoid misunderstandings concerning the final results and may reduce the later need for additional analyses. The preparation of a written modeling protocol that is vetted with the appropriate reviewing authority helps to keep misunderstandings and resource expenditures at a minimum.

d. The identification of preferred models in this

Guideline

should not be construed as a determination that the preferred models identified here are to be permanently used to the exclusion of all others or that they are the only models available for relating emissions to air quality. The model that most accurately estimates concentrations in the area of interest is always sought. However, designation of specific preferred models is needed to promote consistency in model selection and application.

3.1 Preferred Models

3.1.1 Discussion

a. The EPA has developed some models suitable for regulatory application, while other models have been submitted by private developers for possible inclusion in the

Guideline.

Refined models that are preferred and required by the EPA for particular applications have undergone the necessary peer scientific reviews

and model performance evaluation exercises

that include statistical measures of model performance in comparison with measured air quality data as described in section 2.1.1.

b. An American Society for Testing and Materials (ASTM) reference

provides a general philosophy for developing and implementing advanced statistical evaluations of atmospheric dispersion models, and provides an example statistical technique to illustrate the application of this philosophy. Consistent with this approach, the EPA has determined and applied a specific evaluation protocol that provides a statistical technique for evaluating model performance for predicting peak concentration values, as might be observed at individual monitoring locations.

c. When a single model is found to perform better than others, it is recommended for application as a preferred model and listed in appendix A. If no one model is found to clearly perform better through the evaluation exercise, then the preferred model listed in appendix A may be selected on the basis of other factors such as past use, public familiarity, resource requirements, and availability. Accordingly, the models listed in appendix A meet these conditions:

i. The model must be written in a common programming language, and the executable(s) must run on a common computer platform.

ii. The model must be documented in a user's guide or model formulation report which identifies the mathematics of the model, data requirements and program operating characteristics at a level of detail comparable to that available for other recommended models in appendix A.

iii. The model must be accompanied by a complete test dataset including input parameters and output results. The test data must be packaged with the model in computer-readable form.

iv. The model must be useful to typical users,

e.g.,

state air agencies, for specific air quality control problems. Such users should be able to operate the computer program(s) from available documentation.

v. The model documentation must include a robust comparison with air quality data (and/or tracer measurements) or with other well- established analytical techniques.

vi. The developer must be willing to make the model and source code available to users at reasonable cost or make them available for public access through the Internet or National Technical Information Service. The model and its code cannot be proprietary.

d. The EPA's process of establishing a preferred model includes a determination of technical merit, in accordance with the above six items including the practicality of the model for use in ongoing regulatory programs. Each model will also be subjected to a performance evaluation for an appropriate database and to a peer scientific review. Models for wide use (not just an isolated case) that are found to perform better will be proposed for inclusion as preferred models in future

Guideline

revisions.

e. No further evaluation of a preferred model is required for a particular application if the EPA requirements for regulatory use specified for the model in the

Guideline

are followed. Alternative models to those listed in appendix A should generally be compared with measured air quality data when they are used for regulatory applications consistent with recommendations in section 3.2.

3.1.2 Requirements

a. Appendix A identifies refined models that are preferred for use in regulatory applications. If a model is required for a particular application, the user must select a model from appendix A or follow procedures in section 3.2.2 for use of an alternative model or technique. Preferred models may be used without a formal demonstration of applicability as long as they are used as indicated in each model summary in appendix A. Further recommendations for the application of preferred models to specific source applications are found in subsequent sections of the

Guideline.

b. If changes are made to a preferred model without affecting the modeled concentrations, the preferred status of the model is unchanged. Examples of modifications that do not affect concentrations are those made to enable use of a different computer platform or those that only affect the format or averaging time of the model results. The integration of a graphical user interface (GUI) to facilitate setting up the model inputs and/or analyzing the model results without otherwise altering the model kernel is another example of a modification that does not affect concentrations. However, when any changes are made, the Regional Administrator must require a test case example to demonstrate that the modeled concentration are not affected.

c. A preferred model must be operated with the options listed in appendix A for its intended regulatory application. If other options are exercised, the model is no longer “preferred.” Any other modification to a preferred model that would result in a change in the concentration estimates likewise alters its status so that it is no longer a preferred model. Use of the modified model must then be justified as an alternative model on a case-by-case basis to the appropriate reviewing authority and approved by the Regional Administrator.

d. Where the EPA has not identified a preferred model for a particular pollutant or situation, the EPA may establish a multi-tiered approach for making a demonstration required under PSD or another CAA program. The initial tier or tiers may involve use of demonstration tools, screening models, screening techniques, or reduced-form models; while the last tier may involve the use of demonstration tools, refinded models or techniques, or alternative models approved under section 3.2.

3.2 Alternative Models

3.2.1 Discussion

a. Selection of the best model or techniques for each individual air quality analysis is always encouraged, but the selection should be done in a consistent manner. A simple listing of models in this

Guideline

cannot alone achieve that consistency nor can it necessarily provide the best model for all possible situations. As discussed in section 3.1.1, the EPA has determined and applied a specific evaluation protocol that provides a statistical technique for evaluating model performance for predicting peak concentration values, as might be observed at individual monitoring locations.

This protocol is available to assist in developing a consistent approach when justifying the use of other-than-preferred models recommended in the

Guideline

(

i.e.,

alternative models). The procedures in this protocol provide a general framework for objective decision-making on the acceptability of an alternative model for a given regulatory application. These objective procedures may be used for conducting both the technical evaluation of the model and the field test or performance evaluation.

b. This subsection discusses the use of alternate models and defines three situations when alternative models may be used. This subsection also provides a procedure for implementing 40 CFR 51.166(l)(2) in PSD permitting. This provision requires written approval of the Administrator for any modification or substitution of an applicable model. An applicable model for purposes of 40 CFR 51.166(l) is a preferred model in appendix A to the

Guideline.

Approval to use an alternative model under section 3.2 of the

Guideline

qualifies as approval for the modification or substitution of a model under

40 CFR 51.166(l)(2). The Regional Administrators are delegated authority to issue such approvals under section 3.2 of the

Guideline,

provided that such approval is issued after consultation with EPA's Model Clearinghouse and formally documented in a concurrence memorandum from EPA's Model Clearinghouse which demonstrates that the requirements within section 3.2 for use of an alternative model have been met.

3.2.2 Requirements

a. Determination of acceptability of an alternative model is an EPA Regional Office responsibility in consultation with EPA's Model Clearinghouse as discussed in paragraphs 3.0(b) and 3.2.1(b). Where the Regional Administrator finds that an alternative model is more appropriate than a preferred model, that model may be used subject to the approval of the EPA Regional Office based on the requirements of this subsection. This finding will normally result from a determination that (1) a preferred air quality model is not appropriate for the particular application; or (2) a more appropriate model or technique is available and applicable.

b. An alternative model shall be evaluated from both a theoretical and a performance perspective before it is selected for use. There are three separate conditions under which such a model may be approved for use:

1. If a demonstration can be made that the model produces concentration estimates equivalent to the estimates obtained using a preferred model;

2. If a statistical performance evaluation has been conducted using measured air quality data and the results of that evaluation indicate the alternative model performs better for the given application than a comparable model in appendix A; or

3. If there is no preferred model.

Any one of these three separate conditions may justify use of an alternative model. Some known alternative models that are applicable for selected situations are listed on the EPA's SCRAM Web site (section 2.3). However, inclusion there does not confer any unique status relative to other alternative models that are being or will be developed in the future.

c. Equivalency, condition (1) in paragraph (b) of this subsection, is established by demonstrating that the maximum or highest, second highest concentrations are within+/− 2 percent of the estimates obtained from the preferred model. The option to show equivalency is intended as a simple demonstration of acceptability for an alternative model that is so nearly identical (or contains options that can make it identical) to a preferred model that it can be treated for practical purposes as the preferred model. However, notwithstanding this demonstration, models that are not equivalent may be used when one of the two other conditions described in paragraphs (d) and (e) of this subsection are satisfied.

d. For condition (2) in paragraph (b) of this subsection, established statistical performance evaluation procedures and techniques

28 29

for determining the acceptability of a model for an individual case based on superior performance should be followed, as appropriate. Preparation and implementation of an evaluation protocol which is acceptable to both control agencies and regulated industry is an important element in such an evaluation.

e. Finally, for condition (3) in paragraph (b) of this subsection, an alternative model or technique may be approved for use provided that:

i. The model or technique has received a scientific peer review;

ii. The model or technique can be demonstrated to be applicable to the problem on a theoretical basis;

iii. The databases which are necessary to perform the analysis are available and adequate;

iv. Appropriate performance evaluations of the model or technique have shown that the model or technique is not inappropriately biased for regulatory application;

and

For PSD and other applications that use the model results in an absolute sense, the model should not be biased toward underestimates. Alternatively, for ozone and PM

2.5

SIP attainment demonstrations and other applications that use the model results in a relative sense, the model should note be biased toward overestimates.

v. A protocol on methods and procedures to be followed has been established.

f. To formally document that the requirements of section 3.2 for use of an alternative model are satisfied for a particular application or range of applications, a memorandum will be prepared by the EPA's Model Clearinghouse through a consultative process with the Region Office.

3.3 EPA's Model Clearinghouse

a. The Regional Administrator has the authority to select models that are appropriate for use in a given situation. However, there is a need for assistance and guidance in the selection proces

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