2024 Appendix W Final Rule

Final Rule Information:

On November 20, 2024, the U.S. Environmental Protection Agency (EPA) Administrator signed a final rule that revises 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.

In this final rule, the EPA is revising 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 concentrations for cumulative impact analyses, and various typographical updates to the existing regulation. Please reference the links below for the specific revisions to the Guideline along with the supporting documentation.

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

EPA’s AERMOD Modeling System (Version 24142)

Please follow these links to the release version (v24142) 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
Incorporation of COARE algorithms into AERMET for use in overwater marine boundary layer environments

The EPA is adding 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, which 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.

The addition of COARE to AERMET replaces 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. The use of AERMOD in marine environments is contingent upon consultation with the EPA Regional Office and appropriate reviewing authority. This consultation ensures that platform downwash and shoreline fumigation are adequately considered in the modeling demonstration.

Model Code and Executable

Supporting Documentation
Addition of a new Tier 3 detailed screening technique for NO2

The EPA is including 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 NOX plumes to convert the NO portion of the plume to NO2 via titration and entrainment of ambient ozone. The other 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
Addition of RLINE as mobile source type

As a culmination of an Interagency Agreement between EPA and FHWA, the EPA is adding 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 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 finalizing RLINE as a new source type, the EPA will also allow 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
Updates to Recommendations on the Development of Background Concentration

The EPA is revising 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 2017 version of the Guideline recommended selecting sources “that cause a significant concentration gradient in the vicinity of the source(s) under consideration.” The EPA recognizes that the previously recommended method for identifying nearby sources lacks specificity, is used and referenced inconsistently, and may lead to overly conservative modeling exercises. The 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 recommended 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 revisions to section 8 of the Guideline, the EPA is releasing the final version of the Guidance on Developing Background Concentrations for Use in Modeling Demonstrations.  This 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
13th Conference on Air Quality Modeling and Public Hearing Information for Proposed Rule

The 13th Conference on Air Quality Modeling, mandated by Section 320 of the Clean Air Act, was held on the EPA RTP Campus from November 14th through 15th, 2023. The conference was focused on the proposed revisions to the Guideline on Air Quality Models (CFR Title 40, Part 51, Appendix W) and served as the public hearing for these proposed revisions.

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

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Last updated on January 28, 2025