Clean Air Act Permit Modeling Guidance

This section provides recommendations on modeling techniques and guidance for estimating pollutant concentrations in order to assess control strategies and determine emission limits. These recommendations were originally published in April 1978 as the "Guideline on Air Quality Models" and were incorporated by reference in the regulations for the Prevention of Significant Deterioration of Air Quality, Title 40, Code of Federal Regulations (CFR) sections 51.166 and 52.21 in June 1978 [Federal Register, 43 (118), 26 382-26 388]. The purpose of these guidelines is to promote consistency in the use of modeling within the air quality management process. These guidelines are periodically revised to ensure that new model developments or expanded regulatory requirements are incorporated. This section provides guidance associated with permit modeling and is divided into three components: Appendix W Guidance, Screening Guidance, and Other Permit Modeling Guidance.

Guideline on Air Quality Models ("Appendix W" to 40 CFR Part 51)
Appendix W (PDF)(191 pp, 1.1 MB, 11-20-2024) - Guideline on Air Quality Models ("Appendix W" to 40 CFR Part 51), November 2024, Prepublication Version 2024 Appendix W Final Rule Supporting Information and Docket Website Previous 2017 Appendix W (PDF)(54 pp, 761 KB, 01-17-2017) - January 2017 version provided for reference. 2017 Appendix W Final Rule Supporting Information and Docket Website Previous 2005 Appendix W (PDF)(45 pp, 822 KB, 11-09-2005) - November 2005 version provided for reference. Preferred/Recommended Models listed in Appendix W - A list of referred and recommended models as provided in Appendix W. Use of Alternative Models - A list of dispersion models for use on a case-by-case basis with approval by reviewing authority. Air Quality Analysis Checklist (PDF)(13 pp, 77 KB, 12-20-2016) - The Air Quality Analysis Checklist assists the regulatory reviewer in assuring that important aspects of the NSR/PSD compliance demonstrations for NAAQS and PSD increments are accounted for and appropriately examined. Additionally, this checklist is useful for those conducting these regulatory demonstrations by identifying the necessary pieces of information required to assure an appropriate air quality assessment. A previous version of the Checklist (PDF)(14 pp, 114 KB, 04-18-2003) formerly appeared as Appendix C of the "Guideline on Air Quality Models" as explained in the Explanatory Note(1 KB, 04-18-2013). Clarification Memos - Provides access to memoranda issued by EPA as clarifications of Appendix W and technical aspects of dispersion modeling in general.

Guideline on Air Quality Models ("Appendix W" to 40 CFR Part 51)

Appendix W (PDF)(191 pp, 1.1 MB, 11-20-2024) - Guideline on Air Quality Models ("Appendix W" to 40 CFR Part 51), November 2024, Prepublication Version

2024 Appendix W Final Rule Supporting Information and Docket Website

Previous 2017 Appendix W (PDF)(54 pp, 761 KB, 01-17-2017) - January 2017 version provided for reference.
2017 Appendix W Final Rule Supporting Information and Docket Website

Previous 2005 Appendix W (PDF)(45 pp, 822 KB, 11-09-2005) - November 2005 version provided for reference.

Preferred/Recommended Models listed in Appendix W - A list of referred and recommended models as provided in Appendix W.
Use of Alternative Models - A list of dispersion models for use on a case-by-case basis with approval by reviewing authority.

Air Quality Analysis Checklist (PDF)(13 pp, 77 KB, 12-20-2016) - The Air Quality Analysis Checklist assists the regulatory reviewer in assuring that important aspects of the NSR/PSD compliance demonstrations for NAAQS and PSD increments are accounted for and appropriately examined. Additionally, this checklist is useful for those conducting these regulatory demonstrations by identifying the necessary pieces of information required to assure an appropriate air quality assessment. A previous version of the Checklist (PDF)(14 pp, 114 KB, 04-18-2003) formerly appeared as Appendix C of the "Guideline on Air Quality Models" as explained in the Explanatory Note(1 KB, 04-18-2013).

Clarification Memos - Provides access to memoranda issued by EPA as clarifications of Appendix W and technical aspects of dispersion modeling in general.

Screening Guidance
Screening Procedures for Estimating the Air Quality Impact of Stationary Sources Revised (PDF)(93 pp, 3 MB, 10-01-1992, 454-R-92-019) Screening Models - SCRAM webpage providing information on screening models, including model code and documentation

Other Permit Modeling and Related Guidance
Guidance on Developing Background Concentrations for Use in Modeling Demonstrations (PDF) (75 pp, 2.2 MB, 11-20-2024, 454-R-24-003). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Guidance for Ozone and Fine Particulate Matter Permit Modeling (PDF) (142 pp, 3.6 MB, 07-29-2022, 454-R-22-005). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Guidance on the Development of Modeled Emission Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for Ozone and PM_2.5 under the PSD Permitting Program (PDF) (87 pp, 3.4 MB, Revised 04-30-2024, 454-R-19-003). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. MERPs VIEW Qlik applications - Provides easy access to EPA’s updated hypothetical single source inventory of modeled O₃ and PM_2.5 impacts to support appropriate PSD applications. Photochemical Model Estimated Relationships Between Offshore Wind Energy Project Precursor Emissions and Downwind Air Quality (O₃ and PM_2.5) Impacts (PDF) (68pp, 11 MB, 12-21-2022, 454-R-22-007). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program (Reference Link to EPA Website) - April 2018. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Guidance on the Use of Models for Assessing the Impacts of Emissions from Single Sources on the Secondarily Formed Pollutants: Ozone and PM_2.5 (PDF) (33 pp, 273 K, 12-02-2016, 454-R-16-005). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Haul Road Workgroup Final Report (PDF) (22 pp, 876 K, 03-02-2012). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Disclaimer: The recommendations presented in the Haul Road Workgroup Final Report are not an endorsement by the USEPA as the definitive methodology for characterizing and addressing fugitive dust emissions from haul roads but should be considered a best practice approach based on the broad involvement of the co-regulating community in the development of this recommendation. Area Source Estimated Concentrations Related Update (PDF) (1 pg, 26 K, 03-28-2002) Guideline for Determination of Good Engineering Practice (GEP) Stack Height - Revised (PDF) (101 pp, 4 MB, 06-01-1985, 450-4-80-023R). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Protocol for Determining the Best Performing Model (PDF) (38 pp, 1 MB, 09-01-1992, 454-R-92-025). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Guidance Concerning the Implementation of the 1-hour SO2 NAAQS for the Prevention of Significant Deterioration Program (PDF) (19 pp, 995 K, 08-23-2010). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Guidance Concerning the Implementation of the 1-hour NO2 NAAQS for the Prevention of Significant Deterioration Program (PDF) (30 pp, 1 MB, 06-29-2010). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. PM Hot-Spot Guidance: Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas (PDF) (191 pp, 4.1 MB, 10-01-2021, 420-B-21-037). U.S. Environmental Protection Agency, Office of Transportation and Air Quality.

Other Permit Modeling and Related Guidance

Guidance on Developing Background Concentrations for Use in Modeling Demonstrations (PDF) (75 pp, 2.2 MB, 11-20-2024, 454-R-24-003). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Guidance for Ozone and Fine Particulate Matter Permit Modeling (PDF) (142 pp, 3.6 MB, 07-29-2022, 454-R-22-005). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Guidance on the Development of Modeled Emission Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for Ozone and PM_2.5 under the PSD Permitting Program (PDF) (87 pp, 3.4 MB, Revised 04-30-2024, 454-R-19-003). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

MERPs VIEW Qlik applications - Provides easy access to EPA’s updated hypothetical single source inventory of modeled O₃ and PM_2.5 impacts to support appropriate PSD applications.

Photochemical Model Estimated Relationships Between Offshore Wind Energy Project Precursor Emissions and Downwind Air Quality (O₃ and PM_2.5) Impacts (PDF) (68pp, 11 MB, 12-21-2022, 454-R-22-007). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program (Reference Link to EPA Website) - April 2018. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Guidance on the Use of Models for Assessing the Impacts of Emissions from Single Sources on the Secondarily Formed Pollutants: Ozone and PM_2.5 (PDF) (33 pp, 273 K, 12-02-2016, 454-R-16-005). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Haul Road Workgroup Final Report (PDF) (22 pp, 876 K, 03-02-2012). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.
Disclaimer: The recommendations presented in the Haul Road Workgroup Final Report are not an endorsement by the USEPA as the definitive methodology for characterizing and addressing fugitive dust emissions from haul roads but should be considered a best practice approach based on the broad involvement of the co-regulating community in the development of this recommendation.

Area Source Estimated Concentrations Related Update (PDF) (1 pg, 26 K, 03-28-2002)

Guideline for Determination of Good Engineering Practice (GEP) Stack Height - Revised (PDF) (101 pp, 4 MB, 06-01-1985, 450-4-80-023R). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Protocol for Determining the Best Performing Model (PDF) (38 pp, 1 MB, 09-01-1992, 454-R-92-025). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Guidance Concerning the Implementation of the 1-hour SO2 NAAQS for the Prevention of Significant Deterioration Program (PDF) (19 pp, 995 K, 08-23-2010). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Guidance Concerning the Implementation of the 1-hour NO2 NAAQS for the Prevention of Significant Deterioration Program (PDF) (30 pp, 1 MB, 06-29-2010). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

PM Hot-Spot Guidance: Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas (PDF) (191 pp, 4.1 MB, 10-01-2021, 420-B-21-037). U.S. Environmental Protection Agency, Office of Transportation and Air Quality.

Clean Air Act Permit Modeling Guidance

Support Center for Regulatory Atmospheric Modeling (SCRAM)