EPA Proposes Revisions to Air Quality Modeling Guidelines

On October 23, 2023, in the Federal Register, the Environmental Protection Agency (EPA) proposed updates to the Guideline for air quality models, aiming to improve their scientific accuracy in regulatory assessments. These updates include changes to the AERMOD Modeling System, as well as a new approach for determining background concentrations in air quality modeling. Comments will be received until December 22, 2023.

EPA proposes updates to the Guideline for air quality models to enhance their scientific precision in regulatory assessments. These updates encompass changes in the AERMOD Modeling System, including COARE algorithms for marine environments, a new Tier 3 NO2 screening technique, and the addition of RLINE for mobile sources. These modifications aim to improve model accuracy, supported by comprehensive resources for user transparency and accessibility.

EPA proposes revisions to section 8 of the Guideline to improve the determination of background concentrations in air quality modeling for NAAQS implementation. These changes offer a more robust framework for characterizing background concentrations, particularly in multi-source areas. The proposed approach replaces the previous recommendation of modeling nearby sources causing significant concentration gradients with a more qualitative and semi-quantitative process that considers emissions and air quality data. Additionally, the EPA provides a Draft Guidance on Developing Background Concentrations to assist permit applicants in creating credible and representative background concentrations for cumulative impact analyses.

Finally, EPA is considering a 1-year transition period after the revised Guideline’s promulgation. During this time, applications adhering to the existing Guideline with approved protocols would remain acceptable. New requirements and recommendations would apply to subsequent applications or protocols approved after the transition. This transition aligns with past practices, aiming to prevent delays for pending permit applications that have substantially completed modeling. The proposed revisions are viewed as incremental improvements that do not necessarily invalidate past practices.