Factsheet

Final Revisions to the Guideline on Air Quality Models; Enhancements to the AERMOD

Dispersion Modeling System

Action

•	On November 20, 2024, EPA finalized revisions to "Guideline on Air Quality Models"
(also known as Appendix W or "Guideline,") and to the AERMOD dispersion modeling
system.

•	EPA's Guideline revision will provide states, Tribes and private industry with updated air
quality models and techniques, modeling procedures, and enhanced technical guidance.
These revisions will improve efficiency, accuracy, and ability of regulatory modeling
applications, increasing certainty in the degree and type of air pollution emission
controls necessary to achieve health protective air quality levels.

•	In this final action, EPA is improving AERMOD with changes to the modeling of:

o Near-roadway mobile source emissions;

o Conversion of NOx to NO2; and

o Marine boundary layer environments for offshore sources.

•	These improvements can now be used in a regulatory context without special
permission. Before this action, industry, Tribes, states, and local air pollution agencies
had to receive alternative model approval from the appropriate EPA regional
administrator and concurrence with the EPA's Model Clearinghouse (as required per the
Guideline, sections 3.2 and 3.3) to use these three non-preferred alternative models.

•	EPA is also updating recommendations for the development of appropriate background
concentrations for cumulative impact analyses.

Background

•	The Guideline establishes EPA's preferred air pollution models and other recommended
techniques, as well as guidance for their use in estimating ambient concentrations of air
pollutants. It is incorporated into the EPA's regulations, satisfying a requirement under
the Clean Air Act (CAA) for the EPA to specify, with reasonable particularity, models to
be used in the Prevention of Significant Deterioration (PSD) program.

•	The Guideline is used by EPA, states, Tribes, and private industry in the review and
preparation of permits for new sources for Prevention of Significant Deterioration (PSD),
for state or Tribal Implementation Plan (SIP) revisions, and other regulatory modeling
applications.

•	For more information, read the final rule: https://www.epa.gov/scram.

-------