2023 Appendix W Proposed Rule

Proposed Rule Information:

On October 12, 2023, the U.S. Environmental Protection Agency (EPA) Administrator signed a proposed rule that would revise the Guideline on Air Quality Models (Appendix W to 40 CFR Part 51, "Guideline"). The Guideline has been incorporated into EPA’s regulations, satisfying a requirement under 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.

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, updates to the recommendations for the development of appropriate background concentration for cumulative impact analyses, and various typographical updates to the existing regulation.

Please reference the links below for the specific proposed revisions to the Guideline along with the supporting documentation and information on how to formally submit comments and/or participate in the public hearing for the proposed rule. The public comment period extends through December 22, 2023.

2023 Guideline on Air Quality Models (pdf) (908.42 KB, 10-23-2023) ("Appendix W" to 40 CFR Part 51) - Federal Register Version of Proposed Rule
Rule Docket (ID No. EPA-HQ-OAR-2022-0872)

Model Formulation Updates, Technical Support Material, and Guidance Documents Related to Proposed Rule:

EPA’s AERMOD Modeling System (Version 23132)
Please follow these links to the release version (v23132) of the AERMOD Modeling System. Model code and executables, as well as supporting documentation and test cases, are available. For easy of reference, the technical support and users' guide documentation that is included in the final rule docket is also provided below. Model Code and Executable AERMOD AERMET Supporting Documentation AERMOD Modeling System Transmittal Memorandum (v23132) (PDF) (5 pp, 303 KB, 10-12-2023) AERMOD - Model Change Bulletin #17 - Version Date 23132 (PDF) (6 pp, 216 KB, 10-12-2023) User's Guide for the AMS/EPA Regulatory Model (AERMOD) (PDF) (330 pp, 2.1 MB, 10-12-2023, 454-B-23-008) AERMOD Implementation Guide (PDF) (38 pp, 451 KB, 10-12-2023, 454-B-23-009) AERMOD Model Formulation Document (PDF) (180 pp, 1.7 MB, 10-12-2023, 454-B-23-010) AERMOD Model Evaluation Document (PDF) (59 pp, 2.0 MB, 10-12-2023, 454-B-23-011) AERMET- Model Change Bulletin #13 - Version Date 23132 (PDF) (3 pp, 59 KB, 10-12-2023) User's Guide for the AERMOD Meteorological Preprocessor (AERMET) (PDF) (287 pp, 2.7 MB, 10-12-2023, 454-B-23-005)

EPA’s AERMOD Modeling System (Version 23132)

Please follow these links to the release version (v23132) of the AERMOD Modeling System. Model code and executables, as well as supporting documentation and test cases, are available. For easy of reference, the technical support and users' guide documentation that is included in the final rule docket is also provided below.

Model Code and Executable

Supporting Documentation

AERMOD Modeling System Transmittal Memorandum (v23132) (PDF) (5 pp, 303 KB, 10-12-2023)
AERMOD - Model Change Bulletin #17 - Version Date 23132 (PDF) (6 pp, 216 KB, 10-12-2023)
User's Guide for the AMS/EPA Regulatory Model (AERMOD) (PDF) (330 pp, 2.1 MB, 10-12-2023, 454-B-23-008)
AERMOD Implementation Guide (PDF) (38 pp, 451 KB, 10-12-2023, 454-B-23-009)
AERMOD Model Formulation Document (PDF) (180 pp, 1.7 MB, 10-12-2023, 454-B-23-010)
AERMOD Model Evaluation Document (PDF) (59 pp, 2.0 MB, 10-12-2023, 454-B-23-011)
AERMET- Model Change Bulletin #13 - Version Date 23132 (PDF) (3 pp, 59 KB, 10-12-2023)
User's Guide for the AERMOD Meteorological Preprocessor (AERMET) (PDF) (287 pp, 2.7 MB, 10-12-2023, 454-B-23-005)

Incorporation of COARE algorithms into AERMET for use in overwater marine boundary layer environments
The EPA is proposing to add the Coupled Ocean-Atmosphere Response Experiment (COARE) algorithms to AERMET for meteorological data processing in applications using either observed or prognostic meteorological data in overwater marine boundary layer environments. 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. If finalized, the addition of COARE to AERMET would replace the standalone AERCOARE program and the AERCOARE output option in MMIF for prognostic data overwater and would eliminate the previous alternative model demonstration requirements for use of AERMOD in marine environments. Future use of AERMOD in marine environments would be 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. Model Code and Executable AERMET (v23132) MMIF (v4.1) Supporting Documentation Evaluation of the Implementation of the Coupled Ocean Atmosphere Response Experiment (COARE) Algorithms into AERMET for Marine Boundary Layer Environments (PDF) (51 pp, 1.2 MB, 10-12-2023, 454-R-23-008) Evaluation of Prognostic Meteorological Data in AERMOD Overwater Applications (PDF) (49 pp, 1.3 MB, 10-12-2023, 454-R-23-010) User's Guide for the AERMOD Meteorological Preprocessor (AERMET) (PDF) (287 pp, 2.7 MB, 10-12-2023, 454-B-23-005) User's Guide for the Mesoscale Model Interface (MMIF) Program (PDF) (70 pp, 1.4 MB, 10-30-2023) Guidance on the Use of the Mesoscale Model Interface Program (MMIF) for AERMOD Applications (PDF) (22 pp, 230 KB, 10-12-2023, 454-B-23-006) User’s Manual AERCOARE Version 1.0 (PDF) (32 pp, 633 KB, 10-01-2012, 910-R-12-008) Evaluation of the Combined AERCOARE/AERMOD Modeling Approach for Offshore Sources (PDF) (89 pp, 26 MB, 10-01-2012, 910-R-12-007) Combined WRF/MMIF/AERCOARE/AERMOD Overwater Modeling Approach for Offshore Emission Sources (PDF) (299 pp, 24MB, 10-01-2015, 910-R-15-001c)

Incorporation of COARE algorithms into AERMET for use in overwater marine boundary layer environments

The EPA is proposing to add the Coupled Ocean-Atmosphere Response Experiment (COARE) algorithms to AERMET for meteorological data processing in applications using either observed or prognostic meteorological data in overwater marine boundary layer environments. 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.

If finalized, the addition of COARE to AERMET would replace the standalone AERCOARE program and the AERCOARE output option in MMIF for prognostic data overwater and would eliminate the previous alternative model demonstration requirements for use of AERMOD in marine environments. Future use of AERMOD in marine environments would be 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.

Model Code and Executable

Supporting Documentation

Evaluation of the Implementation of the Coupled Ocean Atmosphere Response Experiment (COARE) Algorithms into AERMET for Marine Boundary Layer Environments (PDF) (51 pp, 1.2 MB, 10-12-2023, 454-R-23-008)
Evaluation of Prognostic Meteorological Data in AERMOD Overwater Applications (PDF) (49 pp, 1.3 MB, 10-12-2023, 454-R-23-010)
User's Guide for the AERMOD Meteorological Preprocessor (AERMET) (PDF) (287 pp, 2.7 MB, 10-12-2023, 454-B-23-005)
User's Guide for the Mesoscale Model Interface (MMIF) Program (PDF) (70 pp, 1.4 MB, 10-30-2023)
Guidance on the Use of the Mesoscale Model Interface Program (MMIF) for AERMOD Applications (PDF) (22 pp, 230 KB, 10-12-2023, 454-B-23-006)
User’s Manual AERCOARE Version 1.0 (PDF) (32 pp, 633 KB, 10-01-2012, 910-R-12-008)
Evaluation of the Combined AERCOARE/AERMOD Modeling Approach for Offshore Sources (PDF) (89 pp, 26 MB, 10-01-2012, 910-R-12-007)
Combined WRF/MMIF/AERCOARE/AERMOD Overwater Modeling Approach for Offshore Emission Sources (PDF) (299 pp, 24MB, 10-01-2015, 910-R-15-001c)

Proposed Addition of a new Tier 3 detailed screening technique for NO₂
The EPA proposes to include the Generic Reaction Set Method (GRSM) as a regulatory non-default Tier 3 NO₂ screening option. The primary motivation behind the formulation and development of the GRSM NO₂ screening option was to address photolytic conversion of NO₂ to NO and to address the time-of-travel necessary for NO_X plumes to convert the NO portion of the plume to NO₂ via titration and entrainment of ambient ozone. The existing regulatory non-default Tier 3 NO₂ 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. Supporting Documentation Technical Support Document (TSD) for Adoption of the Generic Reaction Set Method (GRSM) as a Regulatory Non-Default Tier-3 NO₂ Screening Option (PDF) (25 pp, 1.2 MB, 10-12-2023, 454-R-23-009) Jenny Stocker, Martin Seaton, Stephen Smith, James O’Neill, Kate Johnson, Rose Jackson, David Carruthers (CERC). Evaluation of the Generic Reaction Set Method for NO₂ conversion in AERMOD. The modification of AERMOD to include ADMS chemistry. Cambridge Environmental Research Consultants (CERC) Technical Report (PDF) (38 pp, 1.5 MB, 08-08-2023)

Proposed Addition of a new Tier 3 detailed screening technique for NO₂

The EPA proposes to include the Generic Reaction Set Method (GRSM) as a regulatory non-default Tier 3 NO₂ screening option. The primary motivation behind the formulation and development of the GRSM NO₂ screening option was to address photolytic conversion of NO₂ to NO and to address the time-of-travel necessary for NO_X plumes to convert the NO portion of the plume to NO₂ via titration and entrainment of ambient ozone. The existing regulatory non-default Tier 3 NO₂ 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.

Supporting Documentation

Technical Support Document (TSD) for Adoption of the Generic Reaction Set Method (GRSM) as a Regulatory Non-Default Tier-3 NO₂ Screening Option (PDF) (25 pp, 1.2 MB, 10-12-2023, 454-R-23-009)
Jenny Stocker, Martin Seaton, Stephen Smith, James O’Neill, Kate Johnson, Rose Jackson, David Carruthers (CERC). Evaluation of the Generic Reaction Set Method for NO₂ conversion in AERMOD. The modification of AERMOD to include ADMS chemistry. Cambridge Environmental Research Consultants (CERC) Technical Report (PDF) (38 pp, 1.5 MB, 08-08-2023)

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 RLINE model was designed for near-surface releases to simulate mobile source dispersion with an emphasis on the near-road environment. 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 Appendix A model for mobile source modeling. 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. 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. Supporting Documentation Incorporation and Evaluation of the RLINE Source Type in AERMOD For Mobile Source Applications (PDF) (55 pp, 5.2 MB, 10-12-2023, 454-R-23-011) Evaluation of Addition of Terrain Treatment to the RLINE Source Type in AERMOD (PDF) (33 pp, 1.7 MB, 10-12-2023, 454-R-23-012) Availability of RLINE Source Type Journal Publication Docket Memorandum (PDF) (2 pp, 269 KB, 10-12-2023)

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 RLINE model was designed for near-surface releases to simulate mobile source dispersion with an emphasis on the near-road environment. 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 Appendix A model for mobile source modeling.

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. 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.

Supporting Documentation

Incorporation and Evaluation of the RLINE Source Type in AERMOD For Mobile Source Applications (PDF) (55 pp, 5.2 MB, 10-12-2023, 454-R-23-011)
Evaluation of Addition of Terrain Treatment to the RLINE Source Type in AERMOD (PDF) (33 pp, 1.7 MB, 10-12-2023, 454-R-23-012)
Availability of RLINE Source Type Journal Publication Docket Memorandum (PDF) (2 pp, 269 KB, 10-12-2023)

Proposed Updates to Recommendations on the Development of Background Concentration
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 SO₂ 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’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. 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-existing 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. This draft guidance document details the EPA-recommended framework with stepwise considerations to assist permit applicants in characterizing a credible and appropriately representative background concentration for cumulative impact analyses including the contributions from nearby sources in multi-source areas. Supporting Documentation Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations (pdf) (596.34 KB, 10-12-2023, 454-P-23-001)

Proposed Updates to Recommendations on the Development of Background Concentration

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 SO₂ 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’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. 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-existing 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. This draft guidance document details the EPA-recommended framework with stepwise considerations to assist permit applicants in characterizing a credible and appropriately representative background concentration for cumulative impact analyses including the contributions from nearby sources in multi-source areas.

Supporting Documentation

Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations (pdf) (596.34 KB, 10-12-2023, 454-P-23-001)

13th Conference on Air Quality Modeling and Public Hearing Information for Proposed Rule
On October 12, 2023, the EPA also announced the 13th Conference on Air Quality Modeling and invited the public to participate in the conference. The conference, mandated by Section 320 of the Clean Air Act, will be held on the EPA RTP Campus from November 14th through 15th, 2023. The conference will be focused on the proposed revisions to the Guideline on Air Quality Models (CFR Title 40, Part 51, Appendix W) and will serve as the public hearing for these proposed revisions. 13th Modeling Conference - Informational Website Please direct any additional questions concerning the 13th Conference on Air Quality Modeling or related to the proposed rulemaking to Mr. George Bridgers, email: Bridgers.George@epa.gov.

13th Conference on Air Quality Modeling and Public Hearing Information for Proposed Rule

On October 12, 2023, the EPA also announced the 13th Conference on Air Quality Modeling and invited the public to participate in the conference. The conference, mandated by Section 320 of the Clean Air Act, will be held on the EPA RTP Campus from November 14th through 15th, 2023. The conference will be focused on the proposed revisions to the Guideline on Air Quality Models (CFR Title 40, Part 51, Appendix W) and will serve as the public hearing for these proposed revisions.

13th Modeling Conference - Informational Website

Please direct any additional questions concerning the 13th Conference on Air Quality Modeling or related to the proposed rulemaking to Mr. George Bridgers, email: Bridgers.George@epa.gov.

2023 Appendix W Proposed Rule

Proposed Rule Information:

Model Formulation Updates, Technical Support Material, and Guidance Documents Related to Proposed Rule:

Support Center for Regulatory Atmospheric Modeling (SCRAM)