U.S. EPA Finalizes Revisions to Appendix W Modeling Guidelines

On November 20, 2024, the United States Environmental Protection Agency (U.S. EPA) published the final rule revising 40 CFR Part 51 Appendix W: Guideline on Air Quality Models (Appendix W). Appendix W outlines the U.S. EPA preferred models for air quality modeling as well as the recommended approaches for using the models for regulatory permit applications. The revisions proposed in Appendix W were around four main areas:

• Changes to modeling procedures for mobile sources of emissions
• Addition of another Tier 3 screening technique for Nitrogen Dioxide (NO₂)
• Incorporation of the Coupled Ocean-Atmosphere Response Experiment (COARE) algorithm into AERMET for use in overwater air quality modeling
• Updates to the guidance for developing background concentrations

The revisions to Appendix W include adding a new source type in AERMOD called RLINE. The RLINE model was originally developed as a standard-alone model and was incorporated into AERMOD in 2019 as a “BETA” option. The change to Appendix W is to remove the BETA tag, thus making the RLINE source type available for regulatory modeling of mobile sources. As part of the revisions U.S. EPA will still allow mobile sources to be modeled using the AREA, LINE or VOLUME source types, giving projects flexibility in selecting the source type for regulatory modeling.

U.S. EPA has also revised Appendix W to include an additional Tier 3 screening technique for NO₂. The three-tiered approach for NO₂ modeling addresses the conversion of emissions oxides of nitrogen (NO_x) to NO₂, and includes the following:

Tier 1 – assuming that all emitted NO_x from a source is converted to NO_2.

Tier 2 – uses the Ambient Ratio Method 2 (ARM2), which applies a ratio converting a portion of NO_x to NO₂

Tier 3 – apply the Ozone Limiting Method (OLM) or Plume Volume Molar Ratio Method (PVMRM), which incorporate ozone data and source-specific NO₂ to NO_x in-stack ratios.

As part of the revision, U.S. EPA is including the Generic Reaction Set Method (GRSM) as a third regulatory non-default Tier 3 screening option. GRSM differs from OLM and PVMRM as it considers the travel time and chemical reaction time of the plume. GRSM requires ozone and NO_x concentrations from a nearby representative monitor as a required input and also allows NO₂ concentrations as an optional input. The revision effectively installs GRSM as another available option when modeling NO₂ in regulatory permit applications. As a reminder, use of any Tier 3 option requires consultation with the appropriate U.S. EPA Regional Office and the reviewing authority.

The third revision to Appendix W incorporates the COARE algorithm into AERMET as a non-default option for use in overwater environments. The current preferred model for overwater air modeling analyses is the Offshore Coastal Dispersion (OCD) model. OCD was developed in the 1980s and has not been replaced by AERMOD because AERMOD is not able to handle shoreline fumigation or platform downwash. However, OCD has many limitations that make it difficult to use for overwater modeling analyses (e.g., construction and operation of offshore wind farms) and as a result many permit applicants were seeking to use AERMOD instead of OCD. In order to use AERMOD in an overwater environment, applicants used the COARE algorithms to help develop the meteorological inputs reflective of overwater meteorology. The COARE algorithms were implemented via a stand-alone program that had not been updated since 2012.

The revisions to Appendix W incorporate the COARE algorithms into AERMET, the meteorological preprocessor to AERMOD. This change will help maintain the algorithms so they stay compatible with the most recent version of AERMOD and will eliminate the alternative model demonstration requirements for using AERMOD in overwater situations when shoreline fumigation and platform downwash effects are adequately accounted for in the modeling demonstration.

The final revision to Appendix W focused on refining the recommendations around background concentrations. Appendix W previously recommended that when background concentrations do not adequately include the impacts of nearby sources, that sources that cause a “significant concentration gradient” in the vicinity of the source under consideration should be included in the air quality modeling analysis. However, there was no guidance or recommended approach for determining what a significant concentration gradient was. The revisions to Appendix W help to clarify this; in addition, U.S. EPA released a separate guidance document entitled “Guidance on Developing Background Concentrations for Use in Modeling Demonstrations”. This guidance document details the framework that should be used to help sources select a credible and representative background concentration for regulatory air modeling analyses. As it is guidance and not part of the Appendix W rule itself it could possibly be revised without going through a formal rulemaking, making it easier to update.

Lastly, U.S. EPA updated AERMOD along with several of the other programs used to develop inputs for AERMOD. U.S. EPA updated:

• AERMOD (v24142),
• AERMET (v24142),
• AERMAP (v24142) (the terrain preprocessor for AERMOD),
• AERSURFACE (v24142) (a processer that determines surface characteristics for AERMET)
• AERPLOT (v24142) (tool for plotting results in Google Earth Format)
• MMIF (v4.1.1) (a tool for converting prognostic meteorological data for AERMOD)

Overall, the revisions to Appendix W are relatively minor, focusing on more specialized regulatory modeling applications, and remain largely unchanged from the draft revisions proposed roughly a year ago. U.S. EPA published the revisions in the Federal Register on November 27, 2024. The effective date for this action is January 28, 2025, with a 1-year transition period. Because the rule falls into the Congressional Review Act lookback period, the rule could potentially be delayed, revised, or even withdrawn completely should the new congress decide to take up such a review.

ALL4 has a team of experienced air dispersion modelers with experience across many source types, from simple sites to offshore wind farms to complex manufacturing facilities. We are well versed in federal and state air dispersion modeling requirements for criteria pollutants and air toxics. Should you have any questions on how these revisions may impact your facility or have an upcoming project that involves air quality modeling, please contact Joe Sabato (jsabato@all4inc.com) or (774)293-8009 to discuss how we can help.