Guideline on Air Quality Models; Enhancements to the AERMOD Dispersion Modeling System

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ENVIRONMENTAL PROTECTION AGENCY 40 CFR Part 51 [EPA-HQ-OAR-2022-0872; FRL-10391-01-OAR] RIN 2060-AV92 Guideline on Air Quality Models; Enhancements to the AERMOD Dispersion Modeling System AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule; notification of public hearing and 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)(D) 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. In this action, the EPA is proposing revisions to the Guideline, including enhancements to the formulation and application of the EPA's near-field dispersion modeling system, AERMOD, and updates to the recommendations for the development of appropriate background concentration for cumulative impact analyses. Within this action, the EPA is also announcing the Thirteenth 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 December 22, 2023.

Public hearing and conference: The public hearing for this action and the Thirteenth Conference on Air Quality Modeling will be held November 14-15, 2023, from 8:30 a.m. to 5:00 p.m. Eastern Standard Time (EST).

ADDRESSES:

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2022-0872, by one of the following methods:

• Federal eRulemaking Portal: https://www.regulations.gov. Follow the online instructions for submitting comments

for this action and the Thirteenth Conference on Air Quality Modeling will be held November 14-15, 2023, from 8:30 a.m. to 5:00 p.m. Eastern Standard Time (EST).

ADDRESSES:

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2022-0872, by one of the following methods:

• Federal eRulemaking Portal: https://www.regulations.gov. Follow the online instructions for submitting comments.

• Email: a-and-r-docket@epa.gov. Include Docket ID No. EPA-HQ-OAR-2022-0872 in the subject line of the message.

• Fax: (202) 566-9744.

• Mail: Environmental Protection Agency, EPA Docket Center, Office of Air and Radiation Docket, Mail code 28221T, Attention Docket No. EPA-HQ-OAR-2022-0872, 1200 Pennsylvania Ave. NW, Washington, DC 20460.

• Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA WJC West Building, 1301 Constitution Ave. NW, Washington, DC. The Docket Center's hours of operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal Holidays).

Instructions: All submissions received must include the Docket ID No. for this rulemaking. Comments received may be posted without change to https://www.regulations.gov, including any personal information provided. For detailed instructions on sending comments and additional information on the rulemaking process, see the “Public Participation” heading of the SUPPLEMENTARY INFORMATION section of this document.

The public hearing will be held at 109 T.W. Alexander Drive, Research Triangle Park, North Carolina 27711. The hearing will convene at 8:30 a.m. (local time) and will conclude at 5:00 p.m. (local time). Refer to the SUPPLEMENTARY INFORMATION section below for additional information.

FOR FURTHER INFORMATION CONTACT:

Mr. George M. Bridgers, Office of Air Quality Planning and Standards, Air Quality Assessment Division, Air Quality Modeling Group, U.S. Environmental Protection Agency, Mail code C439-01, Research Triangle Park, NC 27711; telephone: (919) 541-5563; email: Bridgers.George@epa.gov

). Refer to the SUPPLEMENTARY INFORMATION section below for additional information.

FOR FURTHER INFORMATION CONTACT:

SUPPLEMENTARY INFORMATION:

The information in this preamble is organized as follows:

Table of Contents

I. General Information A. Does this action apply to me? B. Where can I get a copy of this document? II. Background A. The Guideline on Air Quality Models and EPA Modeling Conferences B. The Twelfth Conference on Air Quality Modeling C. Alpha and Beta Categorization of Non-Regulatory Options III. Public Participation A. Written Comments B. Notice of Public Hearing and the Thirteenth Conference on Air Quality Models IV. Proposed Revisions to the Guideline A. Proposed Revisions V. Ongoing Model Development VI. Statutory and Executive Order Reviews A. Executive Order 12866: Regulatory Planning and Review and Executive Order 14094: Modernizing Regulatory Review B. Paperwork Reduction Act (PRA) C. Regulatory Flexibility Act (RFA) D. Unfunded Mandates Reform Act (UMRA) 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 Risks 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

045: Protection of Children From Environmental Health Risks 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 a code Federal/State/territorial/local/Tribal government 924110 a North American Industry Classification System. B. Where can I get a copy of this document?

In addition to being available in the docket, an electronic copy of this proposed rule and relative supporting documentation will also be available on EPA's Support Center for Regulatory Atmospheric Modeling (SCRAM) website. Following signature, these materials will be posted on SCRAM at the following address: https://www.epa.gov/scram/13th-conference-air-quality-modeling.

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 Guideline serves as a means by which national consistency is maintained in air quality analyses for regulatory activities under CAA regulations, including 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

rves as a means by which national consistency is maintained in air quality analyses for regulatory activities under CAA regulations, including 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)(D) 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 to 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. Between 1988 and 1995, we conducted the Fourth, Fifth, and Sixth Conferences on Air Quality Modeling to solicit comments from the stakeholder community to guide our consideration of further revisions to the Guideline, update the available modeling tools based on the current state-of-the-science, and advise the public on new modeling techniques

roved modeling techniques into the regulatory process. Between 1988 and 1995, we conducted the Fourth, Fifth, and Sixth Conferences on Air Quality Modeling to solicit comments from the stakeholder community to guide our consideration of further revisions to the Guideline, update the available modeling tools based on the current state-of-the-science, and advise the public on new modeling techniques.

The Seventh Conference was held in June 2000 and also served as a public hearing for the proposed revisions to the recommended air quality models in the Guideline (65 FR 21506). 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

omments 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. To further inform the development of additional revisions to the Guideline, we held the Tenth Conference on Air Quality Modeling in March 2012. 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 fine particulate matter (PM 2.5 ) National Ambient Air Quality Standards (NAAQS), modeling for compliance demonstration of the 1-hour nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ) 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.

The Eleventh Conference on Air Quality Modeling was held August 12-13, 2015, and included the public hearing for the most recently proposed version of the Guideline

) and sulfur dioxide (SO 2 ) 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.

The Eleventh Conference on Air Quality Modeling was held August 12-13, 2015, and included the public hearing for the most recently proposed version of the Guideline. The conference included presentations summarizing the proposed updates to the AERMOD Modeling System, replacement of CALINE3 with AERMOD for modeling of mobile sources, incorporation of prognostic meteorological data for use in dispersion modeling, the proposed screening approach for long-range transport for NAAQS and PSD increments assessments with use of CALPUFF as a screening technique rather than an EPA-preferred model, the proposed 2-tiered screening approach to address ozone and PM 2.5 in PSD compliance demonstrations, the status and role of the Model Clearinghouse, and updates to procedures for single-source and cumulative modeling analyses ( e.g., modeling domain, source input data, background data, and compliance demonstration procedures).

Additionally, the 2015 proposed action included a reorganization of the Guideline to make it easier to use and to streamline the compliance assessment process (80 FR 45340), and also included additional clarity in distinguishing requirements from recommendations while noting the continued flexibilities provided within the Guideline, including but not limited to use and approval of alternative models (82 FR at 45344). These proposed revisions were adopted and reflected in the latest version of the Guideline, promulgated on January 17, 2017 (82 FR 5182).

B. The Twelfth Conference on Air Quality Modeling

The most recent EPA modeling conference was the Twelfth Conference on Air Quality Modeling, which was held in August 2019 in continuing compliance with CAA section 320

of alternative models (82 FR at 45344). These proposed revisions were adopted and reflected in the latest version of the Guideline, promulgated on January 17, 2017 (82 FR 5182).

B. The Twelfth Conference on Air Quality Modeling

The most recent EPA modeling conference was the Twelfth Conference on Air Quality Modeling, which was held in August 2019 in continuing compliance with CAA section 320. While not associated with a regulatory action, the Twelfth Conference was held with the intent to inform the ongoing development of EPA's preferred air quality models and potential revisions to the Guideline. The conference included expert panel discussions and invited presentations covering the following model/technique enhancements: treatment of low wind conditions, overwater modeling, mobile source modeling, building downwash, prognostic meteorological data, near-field and long-range model evaluation criteria, NO 2 modeling techniques, plume rise, deposition, and single source ozone and PM 2.5 modeling techniques. At the conclusion of the expert panels and invited presentations, there were several presentations given by the public, including industrial trade groups, on recommended areas for additional model development and Guideline. The proposed regulatory updates to the AERMOD Modeling System in this action address topics on which there was focused discussion and engagement with the stakeholder community through these expert panels and invited and public presentations during the Twelfth Conference.

All the presentations, along with the transcript of the conference proceedings, are available in the docket for the Twelfth Conference on Air Quality Models (Docket ID No. EPA-HQ-OAR-2019-0454). Additionally, all the materials associated with the Twelfth Conference and the public hearing are available on the EPA's SCRAM website at https://www.epa.gov/scram/12th-conference-air-quality-modeling.

C. Alpha and Beta Categorization of Non-Regulatory Options

With the release of AERMOD version 18181 in 2018, the EPA adopted a new paradigm for engagement with the scientific community to facilitate the continued development of the AERMOD Modeling System. Previously, updates to the scientific formulation of the model were not made available to the public for review, testing, evaluation, and comment prior to the proposal stage of the formal rulemaking process when an update was made to the Guideline. This limited the public's engagement and feedback to a short, predefined comment period, typically only one to two months. The new approach enables the EPA to release potential formulation updates as non-regulatory “alpha” and “beta” options as they are being developed. As non-regulatory options, they can be made available during any release cycle, thereby enabling feedback as they are being developed. This approach allows for more robust testing and evaluation during development, benefitting from the experience of a broad expert community. In addition, the EPA developed a protocol to enable the external community to submit model updates to the EPA for review and consideration for inclusion as new alpha or beta options. A pathway such as this that facilitates more frequent and active engagement with the external community allows for a more informed and timely regulatory update process when the EPA has determined an update has met the criteria required for consideration as a science formulation update to the regulatory version of the model

PA for review and consideration for inclusion as new alpha or beta options. A pathway such as this that facilitates more frequent and active engagement with the external community allows for a more informed and timely regulatory update process when the EPA has determined an update has met the criteria required for consideration as a science formulation update to the regulatory version of the model.

In this alpha/beta construct, alpha options are updates to the scientific formulation that are thought to have merit but are considered experimental, still in the research and development stage. Alpha options have not yet been fully tested, evaluated, or vetted through peer review and should not be considered for use as an alternative model for regulatory applications of the model.

Beta options, on the other hand, have been demonstrated to be applicable on a theoretical basis, have undergone scientific peer review, and are supported with performance evaluations using available and adequate databases that demonstrate unbiased, improved model performance. In general, beta options have met the necessary criteria to be formally proposed and adopted as updates to the regulatory version of the model but have not yet been proposed through the required rulemaking process, which includes a public hearing and formal comment period. Beta options are mature enough in the development process to be considered for use as an alternative model, provided an appropriate site-specific modeling demonstration is completed to show the alternative model is appropriate for the site and conditions where it will be applied and the requirements of the Guideline, section 3.2, are fully satisfied, including formal concurrence by the EPA's Model Clearinghouse.

III. Public Participation

Interested persons may provide the EPA with their views on the proposed revisions to the Guideline in several ways

stration is completed to show the alternative model is appropriate for the site and conditions where it will be applied and the requirements of the Guideline, section 3.2, are fully satisfied, including formal concurrence by the EPA's Model Clearinghouse.

III. Public Participation

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 Thirteenth Conference on Air Quality Modeling, and speaking at the public hearing that will be conducted as part of the conference. Additional information on where to submit written comments on the proposed revisions to the Guideline is provided in the ADDRESSES section above.

A. Written Comments

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2022-0872, at https://www.regulations.gov (our preferred method), or the other methods identified in the ADDRESSES section. Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received to its public docket. Do not submit to EPA's docket at https://www.regulations.gov any information you consider to be Confidential Business Information (CBI), Proprietary Business Information (PBI), or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission ( i.e., on the web, cloud, or other file sharing system). Please visit https://www.epa.gov/dockets/commenting-epa-dockets for additional submission methods; the full EPA public comment policy; information about CBI, PBI, or multimedia submissions; and general guidance on making effective comments.

ill generally not consider comments or comment contents located outside of the primary submission ( i.e., on the web, cloud, or other file sharing system). Please visit https://www.epa.gov/dockets/commenting-epa-dockets for additional submission methods; the full EPA public comment policy; information about CBI, PBI, or multimedia submissions; and general guidance on making effective comments.

B. Notice of Public Hearing and the Thirteenth Conference on Air Quality Models

The public hearing for this action and the Thirteenth Conference on Air Quality Modeling will be held on November 14-15, 2023, in the EPA Nantahala Auditorium, Room C111, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711. The hearing and conference will convene each day at 8:30 a.m. EST and will conclude at 5:00 p.m. EST.

The Thirteenth Conference on Air Quality Modeling will be open to the public. No registration fee is charged. The conference will be formally conducted 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 action. The conference will begin with introductory remarks by the presiding EPA official. The EPA staff and EPA invited speakers will then provide a structured 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 Thirteenth Conference on Air Quality Modeling;

II. Review of the proposed revisions to the preferred air quality models; and

III. Review of the proposed revisions to the Guideline.

ity Modeling;

II. Review of the proposed revisions to the preferred air quality models; and

III. Review of the proposed revisions to the Guideline.

At the conclusion of these presentations, the EPA will convene the public hearing on the proposed revisions to the Guideline. The public hearing will span a portion of the afternoon of the first day and throughout the second day of the conference. The EPA will make every effort to follow the schedule as closely as possible on the days of the conference; however, please plan for the public hearing to run either ahead of schedule or behind schedule. The EPA may close the hearing 15 minutes after

Those wishing to reserve time to speak at the public hearing, whether to offer specific comments on the proposed rule, volunteer a presentation on a special topic, or to offer recommendations on any regulatory modeling techniques, should contact us at the address given in the FOR FURTHER INFORMATION CONTACT section by no later than November 10, 2023. 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 in advance may provide oral comments on the proposed revisions to the Guideline during time allotted on the last day. 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.

The EPA may ask clarifying questions during the oral presentations but will not respond to the presentations at that time

vide oral comments on the proposed revisions to the Guideline during time allotted on the last day. 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.

The EPA may ask clarifying questions during the oral presentations but will not respond to the presentations at that time. Information submitted to the EPA during the public hearing will be placed in the docket for this proposed action. Written statements and supporting information submitted during the comment period will be considered with the same weight as oral testimony and supporting information presented at the public hearing.

Conference background information. Preregistration details, additional background information, and a more detailed agenda for the Thirteenth Conference on Air Quality Modeling are electronically available at https://www.epa.gov/scram/13th-conference-air-quality-modeling. Preregistration for the conference, while not required, is strongly recommended due to heightened security protocols at the EPA-RTP facility.

Access to U.S. government facility. Because this hearing is being held at a U.S. government facility, individuals planning to attend the conference and/or public 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. For purposes of the REAL ID Act, EPA will accept government-issued IDs, including drivers' licenses, from the District of Columbia and all States and territories except from American Samoa. If your identification is issued by American Samoa, you must present an additional form of identification to enter the Federal building where the public hearing will be held

nts for entering Federal facilities. For purposes of the REAL ID Act, EPA will accept government-issued IDs, including drivers' licenses, from the District of Columbia and all States and territories except from American Samoa. If your identification is issued by American Samoa, you must present an additional form of identification to enter the Federal building where the public hearing will be held. Acceptable alternative forms of identification include Federal employee badges, passports, enhanced driver's licenses, and military identification cards. For additional information for the status of your State regarding REAL ID, go to: https://www.dhs.gov/real-id-enforcement-brieffrequently-asked-questions. Any objects brought into the building need to fit through the security screening system, such as a purse, laptop bag, or small backpack. Demonstrations will not be allowed on Federal property for security reasons. Attendees are encouraged to arrive at least 15 minutes prior to the start of the meeting to allow enough time for security screening.

IV. Proposed Revisions to the Guideline

In this action, the EPA is proposing updates to the Guideline corresponding to updates to the scientific formulation of the AERMOD Modeling System and updates to the recommendations for the development of appropriate background concentration for cumulative impact analyses. When and where appropriate, the EPA has engaged with our Federal partners, including the Bureau of Ocean Energy Management (BOEM) and the Federal Highway Administration (FHWA), to collaborate on these proposed updates to the Guideline. There are additional editorial changes proposed to the Guideline to correct minor typographical errors found in the 2017 Guideline and update website links.

A. Proposed Revisions

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.

o the Guideline. There are additional editorial changes proposed to the Guideline to correct minor typographical errors found in the 2017 Guideline and update website links.

A. Proposed Revisions

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. Proposed Updates to EPA's AERMOD Modeling System

Based on studies presented and discussed at the Twelfth Conference on Air Quality Models held on October 2-3, 2019, 1 and additional relevant research since 2017, 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. One update is to the AERMET meteorological preprocessor for AERMOD. This update provides the capability to process measured and prognostic marine-based meteorology for offshore applications. Separate updates are related to the AERMOD dispersion model and include (1) a new Tier 3 screening method for the conversion of nitrogen oxides (NO X ) emissions to NO 2 and (2) a new source type for modeling vehicle roadway emissions.

1 https://www.epa.gov/scram/12th-conference-air-quality-modeling.

Each of the proposed formulation updates to the AERMOD Modeling System is provided as a non-regulatory beta option in the release of the relevant modeling system components that is occurring concurrent with this proposed rule. If EPA adopts these formulation updates in a subsequent final rule, the beta categorization would be removed and the respective model option(s) could be considered regulatory model options.

The EPA proposes the following updates to the AERMOD Modeling System to address several technical concerns expressed by stakeholders:

ystem components that is occurring concurrent with this proposed rule. If EPA adopts these formulation updates in a subsequent final rule, the beta categorization would be removed and the respective model option(s) could be considered regulatory model options.

The EPA proposes the following updates to the AERMOD Modeling System to address several technical concerns expressed by stakeholders:

a. Incorporation of COARE Algorithms Into AERMET for Use in Overwater Marine Boundary Layer Environments

As the number of overwater applications has increased in recent years, the EPA is proposing to add the Coupled Ocean-Atmosphere Response Experiment (COARE) 2 3 algorithms to AERMET for meteorological data processing in applications using either observed or prognostic meteorological data in overwater marine boundary layer environments. One of the first notable uses of AERMOD for an overwater application was an alternative model application—AERMOD-COARE was used in 2011 in an ice-free arctic environment of Alaska. 4 5 In this 6 processor that can be used with either measured or prognostic data for overwater applications in lieu of AERMET. AERCOARE has been approved as an alternative model for several overwater applications since 2011. 7

2 Fairall, C.W., E.F. Bradley, J.E. Hare, A.A. Grachev, and J.B. Edson, 2003: “Bulk Parameterization of Air-Sea Fluxes: Updates and Verification for the COARE Algorithm.” Journal of Climate, 16, 571-591.

3 Evaluation of the Implementation of the Coupled Ocean-Atmosphere Response Experiment (COARE) algorithms into AERMET for Boundary Layer Environments. EPA-2023/R-23-008, Office of Air Quality Planning and Standards, RTP, NC.

4 U.S. EPA, 2011: COARE Bulk Flux Algorithm to Generate Hourly Meteorological Data for Use with

5 U.S. EPA, 2011: Model Clearinghouse Review AERMOD-COARE as an Alternative Model in an Arctic Ice Free Environment. George Bridgers Memorandum dated May 6, 2011, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina 27711.

6 U.S

lity Planning and Standards, RTP, NC.

4 U.S. EPA, 2011: COARE Bulk Flux Algorithm to Generate Hourly Meteorological Data for Use with

6 U.S. EPA, 2012: User's Manual AERCOARE Version 1.0. EPA-910-R-12-008. U.S. EPA, Region 10, Seattle, WA.

7 Please reference the EPA Model Clearinghouse Information Storage and Retrieval System (MCHISRS) database for more information regarding AERCOARE alternative model approvals ( https://cfpub.epa.gov/oarweb/MCHISRS, text Search term “AERCOARE”).

For overwater applications, the algorithms in COARE are better suited for overwater boundary layer calculations than the existing algorithms in AERMET that are better suited for land-based data. These calculations include calculation of surface roughness, stability classification, effects of moisture on Monin-Obukhov length, and the use of Bowen ratio by AERMET for heat flux calculations. 5 The EPA proposes to add COARE to AERMET in order to ensure that the COARE algorithms are updated regularly as part of routine AERMET updates, to provide consistent data handling among land based and overwater based meteorological data ( e.g., treatment of missing data and treatment of calms), and to have all meteorological processing for AERMOD applications in one program.

The addition of the COARE algorithms to AERMET would replace the standalone AERCOARE program and the AERCOARE output option in MMIF for prognostic data overwater. This proposed option is selected by the user with the METHOD COARE RUN-COARE record in the AERMET Stage 2 input file. For prognostic applications processed through the MMIF, the user can run MMIF for AERMET input for overwater applications

e addition of the COARE algorithms to AERMET would replace the standalone AERCOARE program and the AERCOARE output option in MMIF for prognostic data overwater. This proposed option is selected by the user with the METHOD COARE RUN-COARE record in the AERMET Stage 2 input file. For prognostic applications processed through the MMIF, the user can run MMIF for AERMET input for overwater applications.

The addition of COARE to AERMET would eliminate the previous alternative model demonstration requirements for use of AERMOD in marine environments, and this elimination is contingent upon consultation with the EPA Regional Office and appropriate reviewing authority. This consultation will ensure that platform downwash and shoreline fumigation are adequately considered in the modeling demonstration.

b. Proposed Addition of a New Tier 3 Detailed Screening Technique for NO 2

Section 4.2.3.4 of the 2017 Guideline details a 3-tiered approach for evaluating the modeled impacts of NO X sources, which was recommended to assess hourly and annual average NO 2 impacts from point, volume, and area sources for the purposes of the PSD program, SIP planning, and transportation general conformity. This 3-tiered approach addresses the co-emissions of NO and NO 2 and the subsequent conversion of NO to NO 2 in the atmosphere. The tiered levels include:

Tier 1—assuming that all emitted NO is converted to NO 2 (full conversion).

Tier 2—using the Ambient Ratio Method 2 (ARM2), which applies an assumed equilibrium ratio of NO 2 to NO X , based on analysis of and correlation with nationwide hourly observed ambient conditions.

Tier 3—applying the Ozone Limiting Method (OLM) and Plume Volume Molar Ratio (PVMRM) screening options based on site-specific hourly ozone data and source-specific NO 2 to NO X in-stack ratios. 8 9 10 11

8 Podrez, M. 2015. An Update to the Ambient Ratio Method for 1-h NO 2 Air Quality Standards Dispersion Modeling. Atmospheric Environment, 103: 163-170.

9 Cole, H.S. and J.E. Summerhays, 1979

8 Podrez, M. 2015. An Update to the Ambient Ratio Method for 1-h NO 2 Air Quality Standards Dispersion Modeling. Atmospheric Environment, 103: 163-170.

9 Cole, H.S. and J.E. Summerhays, 1979. A Review of Techniques Available for Estimation of Short-Term NO 2 Concentrations. Journal of the Air Pollution Control Association, 29(8): 812-817.

10 Hanrahan, P.L., 1999. The Polar Volume Polar Ratio Method for Determining NO 2 /NO X Ratios in Modeling—Part I: Methodology. Journal of the Air & Waste Management Association, 49: 1324-1331.

11 Chu, S.H. and E.L. Meyer, 1991. Use of Ambient Ratios to Estimate Impact of NO X Sources on Annual NO 2 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).

As further discussed in section 4.2.3.4(e) of the Guideline, regulatory application of Tier 3 screening options shall occur in consultation with the EPA Regional Office and appropriate reviewing authority.

The EPA proposes to include the Generic Reaction Set Method (GRSM) as a regulatory non-default Tier 3 NO 2 screening option. Following a peer-reviewed publication in 2017, GRSM was added to AERMOD as an alpha option in version 21112 and updated as a beta option in version 22112. 12 The primary motivation behind the formulation and development of the GRSM NO 2 screening option was to address photolytic conversion of NO 2 to NO and to address the time-of-travel necessary for NO X plumes to convert the NO portion of the plume to NO 2 via titration and entrainment of ambient ozone

ded to AERMOD as an alpha option in version 21112 and updated as a beta option in version 22112. 12 The primary motivation behind the formulation and development of the GRSM NO 2 screening option was to address photolytic conversion of NO 2 to NO and to address the time-of-travel necessary for NO X plumes to convert the NO portion of the plume to NO 2 via titration and entrainment of ambient ozone. The existing regulatory non-default Tier 3 NO 2 screening options, PVMRM and OLM, do not address or provide for treatment of these mechanisms, and have been shown to over-predict for some source characterizations and model configurations at project source ambient air boundaries and within the first 1-3 km. 13

12 David J. Carruthers, Jenny R. Stocker, Andrew Ellis, Martin D. Seaton & Stephen E. Smith (2017) Evaluation of an explicit NO X chemistry method in AERMOD, Journal of the Air & Waste Management Association, 67:6, 702-712, DOI: 10.1080/10962247.2017.1280096.

13 Jenny Stocker, Martin Seaton, Stephen Smith, James O'Neill, Kate Johnson, Rose Jackson, David Carruthers (CERC). Evaluation of the Generic Reaction Set Method for NO 2 conversion in AERMOD. The modification of AERMOD to include ADMS chemistry. August 8, 2023. Cambridge Environmental Research Consultants (CERC) Technical Report.

The functionality of the GRSM implementation in AERMOD is similar to that of the PVMRM and OLM schemes, with exception to some additional input requirements necessary for treatment of the reverse NO 2 photolysis reaction during daytime hours. Modeled source inputs for GRSM require NO 2 /NO X in-stack ratios, with similar assumptions as applied to PVMRM and OLM according to section 4.2.3.4 of the Guideline. Ambient inputs for GRSM require hourly ozone concentrations taken from an appropriately representative monitoring station or selection of monitoring stations for varying upwind sector concentrations

sis reaction during daytime hours. Modeled source inputs for GRSM require NO 2 /NO X in-stack ratios, with similar assumptions as applied to PVMRM and OLM according to section 4.2.3.4 of the Guideline. Ambient inputs for GRSM require hourly ozone concentrations taken from an appropriately representative monitoring station or selection of monitoring stations for varying upwind sector concentrations. GRSM also requires hourly NO X concentration inputs to resolve the daytime photolysis of NO 2 reaction in equilibrium with ozone titration conversion of the NO portion of the NO X plume. GRSM hourly NO X concentration inputs can also vary by upwind sector concentration, as appropriate. Background NO 2 concentrations are accounted for in the GRSM daytime equilibrium NO 2 concentration estimates based on the chemical reaction balance between ozone entrainment and NO titration, photolysis of NO 2 to NO, and ambient background NO 2 participation in titration and photolysis reactions. Nighttime GRSM NO 2 estimates are based on ozone entrainment and titration of available NO in the NO X plume. Note that all hourly ozone and NO X ambient inputs to GRSM must coincide with the hourly meteorological i.e., minimum of 1 year for on-site data, 3 years of prognostic data, and 5 years of airport data).

Updates to the GRSM formulation in AERMOD version 22112 were developed in late 2022 to address more realistic building effects on instantaneous plume spread, accounting of multiple plume effects on entrainment of ozone, and the tendency of GRSM to over-predict in the far-field ( e.g., beyond approximately 3 km for typical point source releases). Sensitivity testing and model performance evaluations of these updates to GRSM in AERMOD version 23132 have shown consistent or improved model behavior and performance. 14

14 Environmental Protection Agency, 2023. Technical Support Document (TSD) for Adoption of the Generic Reaction Set Method (GRSM) as a Regulatory Non-Default Tier-3 NO2 Screening Option, Publication No. EPA-454/R-23-009

ases). Sensitivity testing and model performance evaluations of these updates to GRSM in AERMOD version 23132 have shown consistent or improved model behavior and performance. 14

c. Proposed Addition of RLINE as Mobile Source Type

As a culmination of an Interagency Agreement between EPA and FHWA, the EPA proposes to add the RLINE source type as a new source type applicable for regulatory modeling of mobile sources. This is in addition to the AREA, LINE, and VOLUME source types already available for mobile source modeling. The proposed addition of RLINE as a mobile source type is an extension of the 2017 update to the Guideline in which AERMOD replaced CALINE3 as the addendum A 15 model for mobile source modeling. At that time, AERMOD's AREA, LINE, and VOLUME sources were available for mobile source modeling. The basis of the RLINE source type is the EPA's Office of Research and Development (ORD) Research LINE (RLINE) model 16 released in 2013. The RLINE model was designed for near-surface releases to simulate mobile source dispersion with an emphasis on the near-road environment. The RLINE model was first incorporated into AERMOD as a beta source type in AERMOD version 19191 in 2019.

15 Under the codification requirements of the Administrative Committee of the Federal Register (ACFR), only subparts, parts, subchapters, and chapters may have appendices. Therefore, we have changed the naming convention from “appendix A” to “addendum A”.

16 Snyder, M.G., Venkatram, A., Heist, D.K., Perry, S.G., Petersen, W.B. and Isakov, V., 2013. RLINE: A line source dispersion model for near-surface releases. Atmospheric environment, 77, pp.748-756

ive Committee of the Federal Register (ACFR), only subparts, parts, subchapters, and chapters may have appendices. Therefore, we have changed the naming convention from “appendix A” to “addendum A”.

16 Snyder, M.G., Venkatram, A., Heist, D.K., Perry, S.G., Petersen, W.B. and Isakov, V., 2013. RLINE: A line source dispersion model for near-surface releases. Atmospheric environment, 77, pp.748-756.

The RLINE source type for this proposed action has undergone significant evaluation by the EPA and the FHWA as part of the Interagency Agreement and has shown improved performance. 17 18 This proposed option is selected by the model user with the SOURCE type “RLINE”. In addition to proposing RLINE as a new source type, the EPA is also proposing the use of the AERMOD urban option (accounting for urban heat island effect in stable conditions) and terrain with the RLINE source type. However, the inclusion of terrain with RLINE does not supersede the EPA's PM Hot-spot guidance where FLAT terrain is recommended for modeling applications. 19 The EPA also emphasizes that the inclusion of RLINE as a source type for mobile source modeling does not preclude the use of the existing AREA, LINE, and VOLUME source types thereby extending the flexibility of users in best characterizing mobile source for regulatory modeling.

17 Incorporation and Evaluation of the RLINE source type in AERMOD for Mobile Source Applications. EPA-2023/R-23-011, Office of Air Quality Planning and Standards, RTP, NC.

18 Heist, D., et al., 2023. Integration of RLINE dispersion model into EPA's AERMOD: updated formulation and evaluations. Journal of the Air & Waste Management Association, Manuscript submitted for publication.

19 U.S. EPA, 2021: PM Hot-spot Guidance; Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM 2.5 and PM 10 Nonattainment and Maintenance Areas. EPA-42-B-21-037. U.S. EPA, Office of Transportation and Air Quality, Ann Arbor, MI.

to EPA's AERMOD: updated formulation and evaluations. Journal of the Air & Waste Management Association, Manuscript submitted for publication.

d. Support Information, Documentation, and Model Code

Model performance evaluation and peer-reviewed scientific references for each of these three proposed updates to the AERMOD Modeling System are cited and placed in the docket, as appropriate. An updated user's guide and model formulation documents for version 23132 have also been placed in the docket. We have updated the summary description of the AERMOD Modeling System to addendum 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 website, https://www.epa.gov/scram.

2. Proposed Updates to Recommendations on the Development of Background Concentration

Based on permit modeling experiences since the 2017 revisions to the Guideline, the EPA proposes revisions to section 8 of the Guideline to refine the recommendations regarding the determination of appropriate model input data, specifically background concentration, for use in NAAQS implementation modeling demonstrations ( e.g., PSD compliance demonstrations, SIP demonstrations for inert pollutants, and SO 2 designations). The Guideline recommends that a representative background concentration should include contributions from all sources, including both nearby and other sources

rmination of appropriate model input data, specifically background concentration, for use in NAAQS implementation modeling demonstrations ( e.g., PSD compliance demonstrations, SIP demonstrations for inert pollutants, and SO 2 designations). The Guideline recommends that a representative background concentration should include contributions from all sources, including both nearby and other sources. When identifying nearby sources that may not be adequately represented by ambient monitoring data, the Guideline recommends selecting sources “that cause a significant concentration gradient in the vicinity of the source(s) under consideration.” The EPA recognizes that the recommended method for identifying nearby sources lacks specificity, is used and referenced inconsistently, and may lead to overly conservative modeling exercises. The proposed revisions to section 8 are intended to provide a more robust framework for characterizing background concentrations for cumulative modeling with particular attention to identifying and modeling nearby sources in multi-source areas.

The EPA proposes to revise recommendations for the determination of background concentrations in constructing the design concentration, or total air quality concentration in multi-source areas ( see section 8.3), as part of a cumulative impact analysis for NAAQS implementation modeling demonstrations. The EPA's proposed framework includes a stepwise set of considerations to replace the narrow recommendation of modeling nearby sources that cause a significant concentration gradient. This framework focuses the inherent discretion in defining representative background concentrations through qualitative and semi-quantitative considerations within a transparent process using the variety of emissions and air quality data available to the permit applicant

considerations to replace the narrow recommendation of modeling nearby sources that cause a significant concentration gradient. This framework focuses the inherent discretion in defining representative background concentrations through qualitative and semi-quantitative considerations within a transparent process using the variety of emissions and air quality data available to the permit applicant. To construct a background concentration for model input under the framework, permit applicants should consider the representativeness of relevant emissions, air quality monitoring, and pre-exiting air quality modeling to appropriately represent background concentrations for the cumulative impact analysis.

In conjunction with the proposed revisions to section 8 of the Guideline, the EPA developed the Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations. 20 This draft guidance

20 U.S. Environmental Protection Agency, 2023. Draft Guidance on Developing Background

3. 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 existing 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

her 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 existing 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.

Such a transition period is consistent with previous revisions to the Guideline and 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 proposed revisions to the Guideline are intended as incremental improvements to the Guideline, and such improvements do not necessarily invalidate past practices under the previous editions of the Guideline. The requirements and recommendations in the existing (2017) 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, if finalized.

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.

4. Proposed Revisions by Section

a. Section 1.0—Introduction

The EPA proposes to correct paragraph (i) by combining the inadvertently created paragraph (A), which is actually part of the phrase “addendum A” in the first sentence.

b. Section 3.0—Preferred and Alternative Air Quality Models

The EPA proposes to revise an outdated website link in section 3.0(b)

plications during this period.

4. Proposed Revisions by Section

a. Section 1.0—Introduction

The EPA proposes to correct paragraph (i) by combining the inadvertently created paragraph (A), which is actually part of the phrase “addendum A” in the first sentence.

b. Section 3.0—Preferred and Alternative Air Quality Models

The EPA proposes to revise an outdated website link in section 3.0(b).

In sections 3.1.1(c) and 3.1.2(a), the EPA proposes to correct the sections by combining the inadvertently created paragraph (A), which is actually part of the phrase “addendum A” in the first sentence.

c. Section 4.0—Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen Dioxide and Primary Particulate Matter

The EPA proposes to update reference numbers where necessary due to added references.

In sections 4.1(b) and 4.2.2(a), the EPA proposes to correct the sections by combining the inadvertently created paragraph (A), which is actually part of the phrase “addendum A” in the first sentence.

In section 4.2.2.1, the EPA proposes to add a new paragraph (f) regarding the use of AERMOD in certain overwater situations. A typographical correction is proposed in section 4.2.2.1(b).

The EPA proposes amendments to section 4.2.2.3 to account for circumstances where OCD is available to evaluate situations where shoreline fumigation and/or platform downwash are important.

In section 4.2.3.4, the EPA proposes to revise paragraph (e) to adopt the Generic Reaction Set Method (GRSM) as a regulatory Tier 3 detailed screening technique for NO 2 modeling demonstrations. Sentences in this section would be updated to incorporate GRSM with the existing regulatory Tier 3 screening techniques OLM and PVMRM. An additional statement is proposed indicating GRSM model performance may be better than OLM and PVMRM under certain source characterization situations

Reaction Set Method (GRSM) as a regulatory Tier 3 detailed screening technique for NO 2 modeling demonstrations. Sentences in this section would be updated to incorporate GRSM with the existing regulatory Tier 3 screening techniques OLM and PVMRM. An additional statement is proposed indicating GRSM model performance may be better than OLM and PVMRM under certain source characterization situations. The EPA also proposes to add two references to the section including one for the peer-reviewed paper on development and evaluation of GRSM, and a second reference to the EPA Technical Support Document (TSD) on GRSM.

The EPA proposes to revise Table 4-1 in section 4.2.3.4(f) to include GRSM as a Tier 3 detailed screening option.

d. Section 5.0—Models for Ozone and Secondarily Formed Particulate Matter

The EPA proposes to update reference numbers where necessary due to added references. In section 5.2, the EPA proposes to revise paragraph (c) to include a reference for guidance on the use of models to assess the impacts of emissions from single sources on secondarily formed ozone and PM 2.5 .

e. Section 6.0—Modeling for Air Quality Related Values and Other Governmental Programs

The EPA proposes to update reference numbers where necessary due to added references and revise an outdated website link in section 6.3(a).

f. Section 7.0—General Modeling Considerations

The EPA proposes to update reference numbers where necessary due to added references.

In section 7.2.3, the EPA proposes to revise paragraph (b) to include the addition of RLINE as a source type for use in regulatory applications of AERMOD and remove references to specific distances that receptors can be placed from the roadway.

Also in section 7.2.3, the EPA proposes to revise paragraph (c) to include RLINE as a source type that can be used to model mobile sources and clarify that an area source can be categorized in AERMOD using the AREA, LINE, or RLINE source type.

a source type for use in regulatory applications of AERMOD and remove references to specific distances that receptors can be placed from the roadway.

g. Section 8.0—Model Input Data

The EPA proposes to update reference numbers where necessary due to added references.

The EPA proposes to revise Table 8-1 and Table 8-2 to correct typographical errors and update the footnotes in each of the tables.

The EPA proposes to revise section 8.3.1 to address current EPA practices and recommendations for determining the appropriate background concentration as model input data for a new or modifying source(s) or sources under consideration for a revised permit limit. This revision would provide a stepwise framework for modeling isolated single sources and multi-source areas as part of a cumulative impact analysis. The EPA also proposes to remove the term “significant concentration gradient” and its related content in section 8.3.1(a)(i) due to the ambiguity and lack of definition of this term in the context of modeling multi-source areas.

The EPA proposes to remove paragraph (d) in section 8.3.2 and renumber paragraphs (e) and (f) to (d) and (e), respectively. The content of

In section 8.3.3, the EPA proposes revisions to the content in section 8.3.3(b) on the recommendations for determining nearby sources to explicitly model as part of a cumulative impact analysis. The EPA proposes to remove the content related to the term “significant concentration gradient” in section 8.3.3(b)(i), section 8.3.3(b)(ii), and section 8.3.3(b)(iii) due to the lack of definition of this term in the context of modeling multi-source areas

content in section 8.3.3(b) on the recommendations for determining nearby sources to explicitly model as part of a cumulative impact analysis. The EPA proposes to remove the content related to the term “significant concentration gradient” in section 8.3.3(b)(i), section 8.3.3(b)(ii), and section 8.3.3(b)(iii) due to the lack of definition of this term in the context of modeling multi-source areas. The EPA also proposes to revise the example given in section 8.3.3(d) to be consistent with the discussion of other sources in section 8.3.1(a)(ii) and the proposed revisions to Tables 8-1 and 8-2.

In section 8.4.1, the EPA proposes to include buoy data as an example of site-specific data as a result of the inclusion of the Coupled-Ocean Atmosphere Response Experiment (COARE) algorithms to AERMET for marine boundary layer processing. The EPA proposes to revise paragraph (a) of section 8.4.2 to note that MMIF should be used to process prognostic meteorological data for both land-based and overwater applications, and to revise paragraph (b) to clarify that AERSURFACE should be used to calculate surface characteristics for land-based data and AERMET calculates surface characteristics for overwater applications. Also, the EPA proposes to revise paragraph (e) of this section to clarify that at least 1-year of site-specific data applies to both land-based and overwater-based data.

The EPA proposes to revise paragraph (a) of section 8.4.3.2 to remove references to specific weblinks and to state that users should refer to the latest guidance documents for weblinks.

The EPA proposes to add a new section 8.4.6 to discuss the implementation of COARE for marine boundary layer processing and to renumber the existing section 8.4.6 (in the 2017 Guideline) to a new section 8.4.7. References to specific wind speed thresholds are proposed to be replaced with guidance to consult the appropriate guidance documents for the latest thresholds.

documents for weblinks.

h. Section 9.0—Regulatory Application of Models

The EPA proposes to update reference numbers where necessary due to added references.

In section 9.2.3, the EPA proposes to revise the example given in section 9.2.3(a)(ii) to be consistent with the discussion of other sources in section 8.3.1(a)(ii) and the proposed revisions to Tables 8-1 and 8-2.

i. Section 10.0—References

The EPA proposes updates to references in section 10.0 to remove outdated website links and reflect current versions of guidance documents, user's guides, and other supporting documentation where applicable. The EPA also proposes to add references to support proposed updates to the AERMOD Modeling System described in this proposed update to the Guideline.

5. Proposed Revisions to Addendum A 21 to Appendix W to Part 51

21 Formerly designated as appendix A.

a. Section A.0

The EPA proposes to revise section A.0 to remove references that indicate there are “many” preferred models while the number is currently only three.

b. Section A.1

The EPA proposes to revise the References section to include additional references that support our proposed updates to the AERMOD Modeling System.

In the Abstract section, the EPA proposes to add line type sources as one of the source types AERMOD can simulate.

add line type sources as one of the source types AERMOD can simulate.

The EPA proposes to revise section A.1(a) to include overwater applications for regulatory modeling where shoreline fumigation and/or platform downwash are not important to facilitate the use of AERMOD with COARE processing. This revision would remove the need to request an alternative model demonstration for such applications. The EPA also proposes to clarify elevation data that can be used in AERMOD, specifically the change in the name of the U.S. Geological Survey (USGS) National Dataset (NED) to 3D Elevation Program (3DEP). For consistency, references to NED would be updated to 3DEP throughout section A.1.

The EPA proposes to revise section A.1(b) to include prognostic data as meteorological input to the AERMOD Modeling System, as applicable.

The EPA proposes to revise section A.1(l) to include the proposed Generic Reaction Set Method in the discussion on chemical transformation in AERMOD. We also propose to clarify the status of the different deposition options in A.1(l).

The EPA proposes to revise section A.1(n) to include references to additional evaluation studies to support our proposed updates to the AERMOD Modeling System.

c. Section A.3

In section A.3, the EPA proposes to remove the reference to the Bureau of Ocean Energy Management's (BOEM) outdated guidance.

V. Ongoing Model Development

In addition to the proposed beta options above, AERMOD version 23132 also includes alpha options that are thought to have scientific merit and that are still being developed or evaluated and peer reviewed. These alpha options are not being proposed as updates to the regulatory formulation of the AERMOD Modeling System, and the EPA is not taking comment on the alpha options during this rulemaking. A list of alpha options on which the EPA has placed a high priority for continued research and development for model improvement is included below. Refer to the AERMOD User's Guide for details and usage of each option

options are not being proposed as updates to the regulatory formulation of the AERMOD Modeling System, and the EPA is not taking comment on the alpha options during this rulemaking. A list of alpha options on which the EPA has placed a high priority for continued research and development for model improvement is included below. Refer to the AERMOD User's Guide for details and usage of each option.

The AERMOD Modeling System, version 23131, includes but is not limited to the following alpha options:

• Low Wind Default Overrides (LOW_WIND).

LOW_WIND was first implemented as a collection of non-regulatory beta test options in AERMOD version 12345 (LOWWIND1 and LOWWIND2) and expanded in version 15481(LOWWIND3). Each of these options altered the default model values for minimum sigma-v, minimum wind speed, and the minimum meander factor with different combinations of hardcoded values. Though the original LOW_WIND beta test options are no longer implemented in AERMOD, the LOW_WIND option was recategorized as an alpha option in AERMOD version 18181. The LOW_WIND option in version 23132 enables the user to override AERMOD default values with user-defined values for one or more of the following parameters:

○ Minimum standard deviation of the lateral velocity to the average wind direction;

○ Minimum mean wind speed;

○ Minimum and maximum meander factor;

○ Minimum standard deviation of the vertical wind speed; and

○ Time scale for random dispersion.

• Modifications to PRIME Building Downwash (AWMADWNW and ORD_DWNW).

Beginning with AERMOD version 19191, two distinct sets of alpha options were added that modify the building downwash algorithm, PRIME. The two sets of options were developed

• Downwash from Offshore Drilling Platforms (PLATFORM).

t modify the building downwash algorithm, PRIME. The two sets of options were developed

• Downwash from Offshore Drilling Platforms (PLATFORM).

To enhance AERMOD's offshore modeling capabilities, the platform downwash algorithm, adapted from the Offshore Coastal Dispersion (OCD) dispersion model, was incorporated in AERMOD version 22112 and requires further development, testing, and evaluation. The PLATFORM option simulates the building downwash effect from platforms commonly used for offshore drilling, made up of both porous and solid structures and which are elevated with airflow beneath them.

• Extended RLINE Source Type Including Barriers and Depressed Roadways (RLINEXT).

The RLINEXT source type was implemented in AERMOD version 18181 and is an extended version of the RLINE source type that allows for a more refined characterization of a road segment. It accepts separate inputs for the elevations of each end of the road segment and extended options for modeling with roadway barriers (RBARRIER) and depressed roadways (RDEPRESS).

• TTRM and TTRM2 for Conversion of NO X to NO 2 .

The Travel Time Reaction Method (TTRM) was implemented in AERMOD version 21112 as a stand-alone NO X -to-NO 2 conversion option that accounts for plume travel time, applicable only in the near field. TTRM was further integrated in AERMOD version 22112 as TTRM2 which can be paired with any one of the Ambient Ration Method (ARM2), OLM, or PVMRM. When paired with one of these, TTRM is applied in the near field and the other specified option is applied in the far field where travel time is not as relevant.

• Highly Buoyant Plume (HBP).

aired with one of these, TTRM is applied in the near field and the other specified option is applied in the far field where travel time is not as relevant.

• Highly Buoyant Plume (HBP).

A Highly Buoyant Plume (HBP) option was implemented as an alpha option that can be applied to POINT source types beginning with AERMOD version 23132 to further explore AERMOD's treatment of the penetrated plume. A penetrated plume occurs when a plume released into the mixed layer, and a portion of the plume eventually penetrates the top of the mixed layer during convective hours as it continues to rise due to either buoyancy or momentum.

• Aircraft Plume Rise (AREA/VOLUME Source Types).

Beginning with AERMOD version 23132, the characterization of AREA and VOLUME sources was extended to account for the buoyancy and horizontal momentum of aircraft emissions. The aircraft plume rise formulation and code for AREA and VOLUME sources was independently developed and provided by the Federal Aviation Administration (FAA). EPA continues to collaborate with FAA on model evaluation and peer review of the aircraft plume rise formulations.

VI. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 14094: Modernizing Regulatory Review

This action is not a significant regulatory action as defined in Executive Order 12866, as amended by Executive Order 14094, and was therefore not subject to a requirement for Executive Order 12866 review.

B. Paperwork Reduction Act (PRA)

This action does not impose an information collection burden under the PRA. This action does not contain any information collection activities, nor does it add any information collection requirements beyond those imposed by existing New Source Review requirements.

Order 14094, and was therefore not subject to a requirement for Executive Order 12866 review.

B. Paperwork Reduction Act (PRA)

C. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial number of small entities under the RFA. This action will not impose any requirements on small entities. This action proposes revisions to the Guideline, including enhancements to the formulation and application of the EPA's near-field dispersion modeling system, AERMOD, and updates to the recommendations for the development of appropriate background concentration for cumulative impact analyses. Use of the models and/or techniques described in this action is not expected to pose any additional burden on small entities.

D. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect small governments. This action imposes no enforceable duty on any State, local or Tribal governments or the private sector.

E. Executive Order 13132: Federalism

This 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.

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

This action does not have Tribal implications, as specified in Executive Order 13175

ons. 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.

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

This action does not have Tribal implications, as specified in Executive Order 13175. This action provides proposed revisions to the Guideline which is used by the EPA, other Federal, State, territorial, local, and Tribal air quality agencies, and industry to prepare and review preconstruction permit applications, SIP submittals and revisions, determinations of conformity, and other air quality assessments required under EPA regulation. Separate from this action, the Tribal Air Rule implements the provisions of section 301(d) of the CAA authorizing eligible Tribes to implement their own Tribal air program. Thus, Executive Order 13175 does not apply to this action.

The EPA provided information regarding this action to the Tribes during a monthly National Tribal Air Association (NTAA) call and will continue to provide any new or subsequent updates to EPA modeling guidance and other regulatory compliance demonstration related topics upon request of the NTAA. Additionally, the EPA specifically solicits any comments on this proposed action from Tribal officials.

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

EPA interprets Executive Order 13045 as applying only to those regulatory actions that concern environmental health or safety risks that EPA has reason to believe may disproportionately affect children, per the definition of “covered regulatory action” in section 2-202 of the Executive Order.

Therefore, this action is not subject to Executive Order 13045 because it does not concern an environmental health risk or safety risk. Since this action does not concern human health, EPA's Policy on Children's Health also does not apply.

A has reason to believe may disproportionately affect children, per the definition of “covered regulatory action” in section 2-202 of the Executive Order.

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

This action is not subject to Executive Order 13211, because it is not a significant regulatory action under Executive Order 12866.

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 and Executive Order 14096: Revitalizing Our Nation's Commitment to Environmental Justice for All

The EPA believes that this action does not have disproportionate and adverse human health or environmental effects on communities with environmental justice concerns because it does not establish an environmental health or safety standard. This action proposes revisions to the Guideline, including enhancements to the formulations and application of EPA's near-field dispersion modeling system, AERMOD, that would assist and expand assessment options in Environmental Justice determinations. While the EPA does not expect this action to directly impact air quality, the proposed revisions are important because the Guideline is used by air permitting authorities and industry to prepare and review NSR permits and serves as a benchmark of consistency across the nation. This consistency has value to all communities including communities with environmental justice concerns

ce determinations. While the EPA does not expect this action to directly impact air quality, the proposed revisions are important because the Guideline is used by air permitting authorities and industry to prepare and review NSR permits and serves as a benchmark of consistency across the nation. This consistency has value to all communities including communities with environmental justice concerns.

List of Subjects in 40 CFR Part 51

Environmental protection, Administrative practice and procedure, Air pollution control, Carbon monoxide, Criteria pollutants, Intergovernmental relations, Lead, Mobile sources, Nitrogen oxides, Ozone, Particulate matter, Reporting and recordkeeping requirements, Stationary sources, Sulfur oxides.

Michael S. Regan, Administrator.

For the reasons stated in the preamble, the Environmental Protection Agency is amending title 40, chapter I of the Code of Federal Regulations 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.

andardization 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 the EPA's Regional Offices

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 Sources 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 Marine Boundary Layer Environments 8.4.6.1 Discussion 8.4.6.2 Recommendations 8.4.7 Treatment of Near-Calms and Calms 8.4.7.1 Discussion 8.4.7.2 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 Addendum A to Appendix W of Part 51—Summ

ndations 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 Addendum 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 Inputs for SIP Revisions of Inert Pollutants. 8-2 Point Source Model Emission Inputs for NAAQS Compliance in PSD Demonstrations. 1.0 Introduction

a. The Guideline provides air quality modeling techniques that should be applied to State Implementation Plan (SIP) submittals and revisions, to New Source Review (NSR), including new or modifying sources under Prevention of Significant Deterioration (PSD), 1 2 3 conformity analyses, 4 and other air quality assessments required under EPA regulation. Applicable only to criteria air pollutants, the Guideline is intended for use by the EPA Regional Offices in judging the adequacy of modeling analyses performed by the EPA, by State, local, and Tribal permitting authorities, and by industry. It is appropriate for use by other Federal government agencies and by State, local, and Tribal agencies with air quality and land management responsibilities. The Guideline serves to identify, for all interested parties, those modeling techniques and databases that the EPA considers acceptable. The Guideline is not intended to be a compendium of modeling techniques. Rather, it should serve as a common measure of acceptable technical analysis when supported by sound scientific judgment.

agencies with air quality and land management responsibilities. The Guideline serves to identify, for all interested parties, those modeling techniques and databases that the EPA considers acceptable. The Guideline is not intended to be a compendium of modeling techniques. Rather, it should serve as a common measure of acceptable technical analysis when supported by sound scientific judgment.

b. Air quality measurements 5 are routinely used to characterize ambient concentrations of criteria pollutants throughout the nation but are rarely sufficient for characterizing the ambient impacts of individual sources or demonstrating adequacy 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

d 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.

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 Offices, 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.

nts 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 the EPA to conduct a Conference on Air Quality Modeling at least every 3 years (CAA 320, 42 U.S.C. 7620). 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) 6 and PSD increments. 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

r 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 addendum: addendum A. Thus, when reference is made to “addendum A” in this document, it refers to addendum A to appendix W to 40 CFR part 51. Addendum 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

sts. 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

s. 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

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. 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. 9 Eulerian models assume that emissions are spread evenly throughout each model grid cell. At coarse grid resolutions, 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

tween grid cells. 9 Eulerian models assume that emissions are spread evenly throughout each model grid cell. At coarse grid resolutions, 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 grid 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 sophisticated models are used or the area 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

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 16 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. 17 The statement of model accuracy is based on statistical tests or performance measures such as bias, error, 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 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

nsitivity analyses);

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

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

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 data 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

Analyses and Models

a. It is desirable to begin an air quality analysis by using simplified and conservative methods 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 and 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 increase in concentration attributable to 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 unless appropriate controls or operational restrictions are implemented based on the screening modeling.

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

ally 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.

ing a complex, numerical air quality model such as a photochemical grid model.

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) website at https://www.epa.gov/scram . This is a website 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 be available from the National Technical Information Service (NTIS), https://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

s and regulations. Codes and documentation may also be available from the National Technical Information Service (NTIS), https://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 23 (section 3.3); EPA modeling conferences; periodic Regional, State, and Local Modelers' Workshops; and the EPA's SCRAM website (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 permitting under programs approved into SIPs. In some EPA regions, Federal authority to administer NSR 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 Region. 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

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 ( https://www.epa.gov/scram/air-modeling-regional-contacts ), 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

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

ates 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 24 25 and model performance evaluation exercises 26 27 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 28 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. 29

c. When a single model is found to perform better than others, it is recommended for application as a preferred model and listed in addendum A. If no one model is found to clearly perform better through the evaluation exercise, then the preferred model listed in addendum 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 addendum 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.

ough the evaluation exercise, then the preferred model listed in addendum 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 addendum 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 addendum 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.

ance 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 addendum 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. Addendum 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 addendum 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 addendum 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 preferred model code is another example of a modification that does not affect concentrations

rations 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 preferred model code 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 concentrations are not affected.

c. A preferred model must be operated with the options listed in addendum A for its intended regulatory application. If the regulatory options are not applied, 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, refined models or techniques, or alternative models approved under section 3.2.

3.2 Alternative Models

3.2.1 Discussion

dels 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. 29 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 addendum 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)

g. 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 addendum 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 have delegated authority to issue such approvals under section 3.2 of the Guideline, provided that such approval is issued after consultation with the EPA's Model Clearinghouse and formally documented in a concurrence memorandum from the 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 the EPA's Model Clearinghouse as discussed in paragraphs 3.0(b) and 3.2.1(b). Where the Regional Administrator finds that an

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 addendum A; or

3. If there is no preferred model.

a and the results of that evaluation indicate the alternative model performs better for the given application than a comparable model in addendum 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 website (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 appropriate regulatory metric(s) 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 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 that 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

l 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; a and

a 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 not 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 EPA Regional 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 process so that fairness, consistency, and transparency in modeling decisions are fostered among the EPA Regional Offices and the State, local, and Tribal agencies. To satisfy that need, the EPA established the Model Clearinghouse 23 to serve a central role of coordination and collaboration between EPA headquarters and the EPA Regional Offices

is a need for assistance and guidance in the selection process so that fairness, consistency, and transparency in modeling decisions are fostered among the EPA Regional Offices and the State, local, and Tribal agencies. To satisfy that need, the EPA established the Model Clearinghouse 23 to serve a central role of coordination and collaboration between EPA headquarters and the EPA Regional Offices. Additionally, the EPA holds periodic workshops with EPA Headquarters, EPA Regional Offices, and State, local, and Tribal agency modeling representatives.

b. The appropriate EPA Regional Office should always be consulted for information and guidance concerning modeling methods and interpretations of modeling guidance, and to ensure that the air quality model user has available the latest most up-to-date policy and procedures. As appropriate, the EPA Regional Office may also request assistance from the EPA's Model Clearinghouse on other applications of models, analytical techniques, or databases or to clarify interpretation of the Guideline or related modeling guidance.

c. The EPA Regional Office will coordinate with the EPA's Model Clearinghouse after an initial evaluation and decision has been developed concerning the application of an alternative model. The acceptability and formal approval process for an alternative model is described in section 3.2.

4.0 Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen Dioxide and Primary Particulate Matter

4.1 Discussion

a. This section identifies modeling approaches generally used in the air quality impact analysis of sources that emit the criteria pollutants carbon monoxide (CO), lead, sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), and primary particulates (PM 2.5 and PM 10 ).

t the criteria pollutants carbon monoxide (CO), lead, sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), and primary particulates (PM 2.5 and PM 10 ).

b. The guidance in this section is specific to the application of the Gaussian plume models identified in addendum A. Gaussian plume models assume that emissions and meteorology are in a steady-state, which is typically based on an hourly time step. This approach results in a plume that has an hourly-averaged distribution of emission mass according to a Gaussian curve through the plume. Though Gaussian steady-state models conserve the mass of the primary pollutant throughout the plume, they can still take into account a limited consideration of first-order removal processes ( e.g., wet and dry deposition) and limited chemical conversion ( e.g., OH oxidation).

c. Due to the steady-state assumption, Gaussian plume models are generally considered applicable to distances less than 50 km, beyond which, modeled predictions of plume impact are likely conservative. The locations of these impacts are expected to be unreliable due to changes in meteorology that are likely to occur during the travel time.

d. The applicability of Gaussian plume models may vary depending on the topography of the modeling domain, i.e., simple or complex. Simple terrain is considered to be an area where terrain features are all lower in elevation than the top of the stack(s) of the source(s) in question. Complex terrain is defined as terrain exceeding the height of the stack(s) being modeled.

e. Gaussian models determine source impacts at discrete locations (receptors) for each meteorological and emission scenario, and generally attempt to estimate concentrations at specific sites that represent an ensemble average of numerous repetitions of the same “event.” Uncertainties in model estimates are driven by this formulation, and as noted in section 2.1.1, evaluations of model accuracy should focus on the reducible uncertainty associated with physics and the formulation of the model

gical and emission scenario, and generally attempt to estimate concentrations at specific sites that represent an ensemble average of numerous repetitions of the same “event.” Uncertainties in model estimates are driven by this formulation, and as noted in section 2.1.1, evaluations of model accuracy should focus on the reducible uncertainty associated with physics and the formulation of the model. The “irreducible” uncertainty associated with Gaussian plume models may be responsible for variation in concentrations of as much as +/−50 percent. 30 “Reducible” uncertainties 16 can be on a similar scale. For example, Pasquill 31 estimates that, apart from data input errors, maximum ground-level concentrations at a given hour for a point source in flat terrain could be in error by 50 percent due to these uncertainties. Errors of 5 to 10 degrees in the measured wind direction can result in concentration errors of 20 to 70 percent for a particular time and location, depending on stability and station location. Such uncertainties do not indicate that an estimated concentration does not occur, only that the precise time and locations are in doubt. Composite errors in highest estimated concentrations of 10 to 40 percent are found to be typical. 32 33 However, estimates of concentrations paired in time and space with observed concentrations are less certain.

f. Model evaluations and inter-comparisons should take these aspects of uncertainty into account. For a regulatory application of a model, the emphasis of model evaluations is generally placed on the highest modeled impacts. Thus, the Cox-Tikvart model evaluation approach, which compares the highest modeled impacts on several timescales, is recommended for comparisons of models and measurements and model inter-comparisons

inter-comparisons should take these aspects of uncertainty into account. For a regulatory application of a model, the emphasis of model evaluations is generally placed on the highest modeled impacts. Thus, the Cox-Tikvart model evaluation approach, which compares the highest modeled impacts on several timescales, is recommended for comparisons of models and measurements and model inter-comparisons. The approach includes bootstrap techniques to determine the significance of various modeled predictions 34 35 Because of the uncertainty in paired modeled and observed concentrations, any attempts at calibration of models based on these comparisons is of questionable benefit and shall not be done.

4.2 Requirements

a. For NAAQS compliance demonstrations under PSD, use of the screening and preferred models for the pollutants listed in this subsection shall be limited to the near-field at a nominal distance of 50 km or less. Near-field application is consistent with capabilities of Gaussian plume models and, based on the EPA's assessment, is sufficient to address whether a source will cause or contribute to ambient concentrations in excess of a NAAQS. In most cases, maximum source impacts of inert pollutants will occur within the first 10 to 20 km from the source. Therefore, the EPA does not consider a long-range transport assessment beyond 50 km necessary for these pollutants if a near-field NAAQS compliance demonstration is required. 36

b. For assessment of PSD increments within the near-field distance of 50 km or less, use of the screening and preferred models for the pollutants listed in this subsection shall be limited to the same screening and preferred models approved for NAAQS compliance demonstrations.

ssessment beyond 50 km necessary for these pollutants if a near-field NAAQS compliance demonstration is required. 36

c. To determine if a compliance demonstration for NAAQS and/or PSD increments may be necessary beyond 50 km ( i.e., long-range transport assessment), the following screening approach shall be used to determine if a significant ambient impact will occur with particular focus on Class I areas and/or the applicable receptors that may be threatened at such distances.

i. Based on application in the near-field of the appropriate screening and/or preferred model, determine the significance of the ambient impacts at or about 50 km from the new or modifying source. If a near-field assessment is not available or this initial analysis indicates there may be significant ambient impacts at that distance, then further assessment is necessary.

ii. For assessment of the significance of ambient impacts for NAAQS and/or PSD increments, there is not a preferred model or screening approach for distances beyond 50 km. Thus, the appropriate reviewing authority (paragraph 3.0(b)) and the EPA Regional Office shall be consulted in determining the appropriate and agreed upon screening technique to conduct the second level assessment. Typically, a Lagrangian model is most appropriate to use for these second level assessments, but applicants shall reach agreement on the specific model and modeling parameters on a case-by-case basis in consultation with the appropriate reviewing authority (paragraph 3.0(b)) and EPA Regional Office. When Lagrangian models are used in this manner, they shall not include plume-depleting processes, such that model estimates are considered conservative, as is generally appropriate for screening assessments.

l reach agreement on the specific model and modeling parameters on a case-by-case basis in consultation with the appropriate reviewing authority (paragraph 3.0(b)) and EPA Regional Office. When Lagrangian models are used in this manner, they shall not include plume-depleting processes, such that model estimates are considered conservative, as is generally appropriate for screening assessments.

d. In those situations where a cumulative impact analysis for NAAQS and/or PSD increments analysis beyond 50 km is necessary, 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 paragraph 3.2.2(e).

4.2.1 Screening Models and Techniques

a. Where a preliminary or conservative estimate is desired, point source screening techniques are an acceptable approach to air quality analyses.

b. As discussed in paragraph 2.2(a), screening models or techniques are designed to provide a conservative estimate of concentrations. The screening models used in most applications are the screening versions of the preferred models for refined applications. The two screening models, AERSCREEN 37 38 and CTSCREEN, are screening versions of AERMOD (American Meteorological Society (AMS)/EPA Regulatory Model) and CTDMPLUS (Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations), respectively. AERSCREEN is the recommended screening model for most applications in all types of terrain and for applications involving building downwash. For those applications in complex terrain where the application involves a well-defined hill or ridge, CTSCREEN 39 can be used.

g building downwash. For those applications in complex terrain where the application involves a well-defined hill or ridge, CTSCREEN 39 can be used.

c. Although AERSCREEN and CTSCREEN are designed to address a single-source scenario, there are approaches that can be used on a case-by-case basis to address multi-source situations using screening meteorology or other conservative model assumptions. However, the appropriate reviewing authority (paragraph 3.0(b)) shall be consulted, and concurrence obtained, on the protocol for modeling multiple sources with AERSCREEN or CTSCREEN to ensure that the worst case is identified and assessed.

d. As discussed in section 4.2.3.4, there are also screening techniques built into AERMOD that use simplified or limited chemistry assumptions for determining the partitioning of NO and NO 2 for NO 2 modeling. These screening techniques are part of the EPA's preferred modeling approach for NO 2 and do not need to be approved as an alternative model. However, as with other screening models and techniques, their usage shall occur in agreement with the appropriate reviewing authority (paragraph 3.0(b)).

e. As discussed in section 4.2(c)(ii), there are screening techniques needed for long-range transport assessments that will typically involve the use of a Lagrangian model. Based on the long-standing practice and documented capabilities of these models for long-range transport assessments, the use of a Lagrangian model as a screening technique for this purpose does not need to be approved as an alternative model. However, their usage shall occur in consultation with the appropriate reviewing authority (paragraph 3.0(b)) and EPA Regional Office.

reviewing authority (paragraph 3.0(b)) and EPA Regional Office.

f. All screening models and techniques shall be configured to appropriately address the site and problem at hand. Close attention must be paid to whether the area should be classified urban or rural in accordance with section 7.2.1.1. The climatology of the area must be studied to help define the worst-case meteorological conditions. Agreement shall be reached between the model user and the appropriate reviewing authority (paragraph 3.0(b)) on the choice of the screening model or technique for each analysis, on the input data and model settings, and the appropriate metric for satisfying regulatory requirements.

4.2.1.1 AERSCREEN

a. Released in 2011, AERSCREEN is the EPA's recommended screening model for simple and complex terrain for single sources including point sources, area sources, horizontal stacks, capped stacks, and flares. AERSCREEN runs AERMOD in a screening mode and consists of two main components: (1) the MAKEMET program which generates a site-specific matrix of meteorological conditions for input to the AERMOD model; and (2) the AERSCREEN command-prompt interface.

b. The MAKEMET program generates a matrix of meteorological conditions, in the form of AERMOD-ready surface and profile files, based on user-specified surface characteristics, ambient temperatures, minimum wind speed, and anemometer height. The meteorological matrix is generated based on looping through a range of wind speeds, cloud covers, ambient temperatures, solar elevation angles, and convective velocity scales (w*, for convective conditions only) based on user-specified surface characteristics for surface roughness (Z o ), Bowen ratio (B o ), and albedo (r). For unstable cases, the convective mixing height (Z ic ) is calculated based on w*, and the mechanical mixing height (Z im ) is calculated for unstable and stable conditions based on the friction velocity, u*.

c. For applications involving simple or complex terrain, AERSCREEN interfaces with AERMAP

pecified surface characteristics for surface roughness (Z o ), Bowen ratio (B o ), and albedo (r). For unstable cases, the convective mixing height (Z ic ) is calculated based on w*, and the mechanical mixing height (Z im ) is calculated for unstable and stable conditions based on the friction velocity, u*.

c. For applications involving simple or complex terrain, AERSCREEN interfaces with AERMAP. AERSCREEN also interfaces with BPIPPRM to provide the necessary building parameters for applications involving building downwash using the Plume Rise Model Enhancements (PRIME) downwash algorithm. AERSCREEN generates inputs to AERMOD via MAKEMET, AERMAP, and BPIPPRM and invokes AERMOD in a screening mode. The screening mode of AERMOD forces the AERMOD model calculations to represent values for the plume centerline, regardless of the source-receptor-wind direction orientation. The maximum concentration output from AERSCREEN represents a worst-case 1-hour concentration. Averaging-time scaling factors of 1.0 for 3-hour, 0.9 for 8-hour, 0.60 for 24-hour, and 0.10 for annual concentration averages are applied internally by AERSCREEN to the highest 1-hour concentration calculated by the model for non-area type sources. For area type source concentrations for averaging times greater than one hour, the concentrations are equal to the 1-hour estimates. 37 40

4.2.1.2 CTSCREEN

a. CTSCREEN 39 41 can be used to obtain conservative, yet realistic, worst-case estimates for receptors located on terrain above stack height. CTSCREEN accounts for the three-dimensional nature of plume and 42 CTSCREEN is designed to execute a fixed matrix of meteorological values for wind speed (u), standard deviation of horizontal and vertical wind speeds (σv, σw), vertical potential temperature gradient (dθ/dz), friction velocity (u*), Monin-Obukhov length (L), mixing height (z i ) as a function of terrain height, and wind directions for both neutral/stable conditions and unstable convective conditions

SCREEN is designed to execute a fixed matrix of meteorological values for wind speed (u), standard deviation of horizontal and vertical wind speeds (σv, σw), vertical potential temperature gradient (dθ/dz), friction velocity (u*), Monin-Obukhov length (L), mixing height (z i ) as a function of terrain height, and wind directions for both neutral/stable conditions and unstable convective conditions. The maximum concentration output from CTSCREEN represents a worst-case 1-hour concentration. Time-scaling factors of 0.7 for 3-hour, 0.15 for 24-hour and 0.03 for annual concentration averages are applied internally by CTSCREEN to the highest 1-hour concentration calculated by the model.

4.2.1.3 Screening in Complex Terrain

a. For applications utilizing AERSCREEN, AERSCREEN automatically generates a polar-grid receptor network with spacing determined by the maximum distance to model. If the application warrants a different receptor network than that generated by AERSCREEN, it may be necessary to run AERMOD in screening mode with a user-defined network. For CTSCREEN applications or AERMOD in screening mode outside of AERSCREEN, placement of receptors requires very careful attention when modeling in complex terrain. Often the highest concentrations are predicted to occur under very stable conditions, when the plume is near or impinges on the terrain. Under such conditions, the plume may be quite narrow in the vertical, so that even relatively small changes in a receptor's location may substantially affect the predicted concentration. Receptors within about a kilometer of the source may be even more sensitive to location. Thus, a dense array of receptors may be required in some cases.

rs within about a kilometer of the source may be even more sensitive to location. Thus, a dense array of receptors may be required in some cases.

b. For applications involving AERSCREEN, AERSCREEN interfaces with AERMAP to generate the receptor elevations. For applications involving CTSCREEN, digitized contour data must be preprocessed 42 to provide hill shape parameters in suitable input format. The user then supplies receptor locations either through an interactive program that is part of the model or directly, by using a text editor; using both methods to select receptor locations will generally be necessary to assure that the maximum concentrations are estimated by either model. In cases where a terrain feature may “appear to the plume” as smaller, multiple hills, it may be necessary to model the terrain both as a single feature and as multiple hills to determine design concentrations.

c. Other screening techniques may be acceptable for complex terrain cases where established procedures 43 are used. The user is encouraged to confer with the appropriate reviewing authority (paragraph 3.0(b)) if any unforeseen problems are encountered, e.g., applicability, meteorological data, receptor siting, or terrain contour processing issues.

4.2.2 Refined Models

a. A brief description of each preferred model for refined applications is found in addendum A. Also listed in that addendum Are availability, the model input requirements, the standard options that shall be selected when running the program, and output options.

4.2.2.1 AERMOD

standard options that shall be selected when running the program, and output options.

4.2.2.1 AERMOD

a. For a wide range of regulatory applications in all types of terrain, and for aerodynamic building downwash, the required model is AERMOD. 44 45 The AERMOD regulatory modeling system consists of the AERMOD dispersion model, the AERMET meteorological processor, and the AERMAP terrain processor. AERMOD is a steady-state Gaussian plume model applicable to directly emitted air pollutants that employs best state-of-practice parameterizations for characterizing the meteorological influences and dispersion. Differentiation of simple versus complex terrain is unnecessary with AERMOD. In complex terrain, AERMOD employs the well-known dividing-streamline concept in a simplified simulation of the effects of plume-terrain interactions.

b. The AERMOD Modeling System has been extensively evaluated across a wide range of scenarios based on numerous field studies, including tall stacks in flat and complex terrain settings, sources subject to building downwash influences, and low-level non-buoyant sources. 27 These evaluations included several long-term field studies associated with operating plants as well as several intensive tracer studies. Based on these evaluations, AERMOD has shown consistently good performance, with “errors” in predicted versus observed peak concentrations, based on the Robust Highest Concentration (RHC) metric, consistently within the range of 10 to 40 percent (cited in paragraph 4.1(e)).

c. AERMOD incorporates the PRIME algorithm to account for enhanced plume growth and restricted plume rise for plumes affected by building wake effects. 46 The PRIME algorithm accounts for entrainment of plume mass into the cavity recirculation region, including re-entrainment of plume mass into the wake region beyond the cavity.

d. AERMOD incorporates the Buoyant Line and Point Source (BLP) Dispersion model to account for buoyant plume rise from line sources

ed plume growth and restricted plume rise for plumes affected by building wake effects. 46 The PRIME algorithm accounts for entrainment of plume mass into the cavity recirculation region, including re-entrainment of plume mass into the wake region beyond the cavity.

d. AERMOD incorporates the Buoyant Line and Point Source (BLP) Dispersion model to account for buoyant plume rise from line sources. The BLP option utilizes the standard meteorological inputs provided by the AERMET meteorological processor.

e. The state-of-the-science for modeling atmospheric deposition is evolving, new modeling techniques are continually being assessed, and their results are being compared with observations. Consequently, while deposition treatment is available in AERMOD, the approach taken for any purpose shall be coordinated with the appropriate reviewing authority (paragraph 3.0(b)).

f. The AERMET meteorological processor incorporates the COARE algorithms to derive marine boundary layer parameters for overwater applications of AERMOD. 47 48 AERMOD is applicable for some overwater applications when platform downwash and shoreline fumigation are adequately considered in consultation with the Regional Office and appropriate reviewing authority. Where the effects of shoreline fumigation and platform downwash need to be assessed, the Offshore and Coastal Dispersion (OCD) model is the applicable model (paragraph 4.2.2.3).

4.2.2.2 CTDMPLUS

a. If the modeling application involves an elevated point source with a well-defined hill or ridge and a detailed dispersion analysis of the spatial pattern of plume impacts is of interest, CTDMPLUS is available. CTDMPLUS provides greater resolution of concentrations about the contour of the hill feature than does AERMOD through a different plume-terrain interaction algorithm.

4.2.2.3 OCD

a. The OCD (Offshore and Coastal Dispersion) model is a straight-line Gaussian model that incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. OCD can determine the impact of offshore emissions from point, area, or line sources on the air quality of coastal regions. OCD is also applicable for situations that involve platform building downwash.

4.2.3 Pollutant Specific Modeling Requirements

4.2.3.1 Models for Carbon Monoxide

a. Models for assessing the impact of CO emissions are needed to

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