Proposed USEPA Revisions to the Guideline on Air Quality Models

On October 23, 2023, the United States Environmental Protection Agency (USEPA) proposed revisions to the Guideline on Air Quality Models,[1] (“Guideline”). The Guideline has been used in USEPA models to comply with the Clean Air Act and to predict ambient concentrations of air pollutants. Additionally, it is used across federal, state, and local programs to comply with USEPA regulations and at all regulatory levels to review air quality-related permits, including preconstruction permits, industrial modification permits, and new source permits.

The USEPA proposed the following:

  1. Enhancements to the air quality dispersion model system (AERMOD), which include:
    • Additional capabilities to process marine-based meteorology data to assist in overwater applications
    • The addition of a third tier for dispersion modeling regarding the conversion of nitrogen oxide emissions to nitrogen dioxide
    • A new source type available in AERMOD that can be used for transportation modeling applications, such as roadway segments.
  2. A more robust framework for characterizing background concentrations for cumulative modeling to:
    • More clearly define the methodology and support greater consistency in its application
    • Allow the same level of flexibility as previous versions of the Guideline but shift the emphasis away from the use of significant concentration gradients

The proposed revisions are supported by the Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations,[2] which includes a description of what constitutes nearby sources and other sources for consideration in National Ambient Air Quality Standards implementation modeling demonstrations.

These new additions to the model are optional and add flexibility, clarity, and consistency for future modeling needs.

The USEPA held the 13th Conference of Air Quality Models on November 14-15, 2023, to update the public on the proposed revisions to the Guideline and allow time for public comment. The comment period is open until December 22, 2023.

If you have questions as to what this might mean for your organization please reach out to Liz Hoerning, Katie Milk, or Andy Sheppard.

[1] Guideline on Air Quality Models; Enhancements to the AERMOD Dispersion Modeling System from Federal Register:

[2] USEPA. 2023. Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations. Retrieved on November 30, 2023.



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