EPA Guideline on Air Quality Modeling Revisions

The United States Environmental Protection Agency (EPA) is required under the Clean Air Act to conduct a conference to reevaluate Air Quality Modeling every 3 years. The EPA conducted the Thirteenth Conference on Air Quality Modeling on November 14th and 15th of 2023, focusing on proposed revisions to the Guideline on Air Quality Modeling (GAQM). As part of the federal rulemaking process, the proposed revisions are currently available for public comments until December 22, 2023.

Who do these modeling revisions apply to?

The GAQM proposed revisions will impact facilities that conduct air quality modeling as part of State Implementation Plan (SIP) submittals, New Source Review (NSR), sources under Prevention of Significant Deterioration (PSD), Conformity, and other air quality assessments required under EPA regulation.

What is the current Modeling Guideline structure?

The current GAQM has three classifications of modeling software options: alpha, beta, and regulatory. Modeling systems under alpha are considered experimental and are ultimately in the trial phase. Modeling conducted with an alpha system is not considered to be cleared for regulatory permitting. Modeling systems under beta are further along in the development side than alpha systems but are not quite approved by the EPA to be used in regulatory permitting. Finally, modeling systems under regulatory have been vetted and tested by the EPA and are approved for use for regulatory permitting. The majority of air permit applications only use regulatory modeling systems, which is typically conducted via AERMOD, an air dispersion modeling software provided by the EPA.

What are the proposed revisions to the Modeling Guideline?

The EPA has proposed several revisions to the GAQM. The biggest update would be the addition of 3 new model formulations to AERMOD. A smaller update includes refining the method to determine background concentrations for modeling. Also included in the proposal are typographical and formatting updates.

The first new formulation would incorporate COARE into AERMET as a regulatory system, allowing for modeling to be conducted more accurately for offshore/overwater facilities.

The second new formulation would incorporate a new Tier 3 screening option as a regulatory system when addressing co-emissions of NO and NO2 and the subsequent conversion of NO to NO­2 in the atmosphere. This new Tier 3 option, GRSM, has proven to be more accurate than the current Tier 3 screening options, which have been known to over-predict for NO conversion, leading to higher NO conversion rates.

The third new formulation would incorporate the RLINE source type to AERMOD as a regulatory system, allowing for improved accuracy for mobile source modeling.

The revision to determining background concentration is only used in NAAQS implementation modeling demonstrations (such as PSD compliance demonstrations). Currently, facilities must evaluate all nearby sources and include them in the cumulative NAAQS evaluation based on a significant concentration gradient. The proposed rule replaces the recommendation to model nearby sources that cause a significant concentration gradient, removing ambiguity for both the facility and the state agency, ensuring consistency across all states, and avoiding using an overly conservative model.

What does this mean for your facility?

The best thing you can do for your facility is to stay informed about the upcoming changes proposed by the EPA. If your facility is close to offshore, currently is regulated as a PSD major source, is a mobile source, and/or is required to perform a NAAQS demonstration, these proposed changes will have an impact on future Air Quality Modeling. These updates will likely improve your facility’s air dispersion modeling, as the improved modeling systems and improved specificity will help remove overly conservative models.

If you have any comments regarding the proposed changes to the Guideline on Air Quality Modeling, please submit all comments to the EPA by December 22, 2023. Comments can be submitted electronically at https://www.regulations.gov using Docket ID No. EPA-HQ-OAR-2022-0872.

If you believe your facility needs to conduct any air dispersion modeling under the current or future GAQM, Conversion Technology, Inc. (CTI) can provide consulting assistance to ensure that your facility is in compliance under the Clean Air Act. CTI has had over 30 years of experience working with industrial facilities to assist with environmental compliance. This includes calculating air emissions estimates, submitting air quality permit applications, and developing site-specific air quality modeling. We will be happy to assist you with whatever consulting services you are in need of.

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