Proposed Revisions to the “Guideline on Air Quality Models”

On October 23, 2023, the United States Environmental Protection Agency (U.S. EPA) published a proposed rule to revise 40 CFR Part 51, Appendix W (the “Guideline on Air Quality Models” or “Guideline”). The proposed revisions include enhancements to the formulation and application of the U.S. EPA’s preferred near-field air dispersion model (AERMOD) and its meteorological pre-processor (AERMET), and updates to the recommendations for development of representative background concentrations for cumulative National Ambient Air Quality Standards (NAAQS) air quality modeling demonstrations. The 13^th Conference on Air Quality Modeling, mandated by Section 320 of the Clean Air Act (CAA), scheduled for November 13-14, 2023, at U.S. EPA’s RTP, NC Campus will serve as the public hearing where public comments will be taken and placed in the docket. Public comments on the proposed revisions to the Guideline will be accepted through December 22, 2023.

The proposed regulatory updates to AERMOD include:

• Addition of a new regulatory non-default Tier 3 nitrogen oxides (NO_X) to nitrogen dioxide (NO₂) conversion option, the Generic Reaction Set Method (GRSM),
• Incorporation of the Coupled Ocean-Atmosphere Response Experiment (COARE) algorithms into AEMRET, and
• Addition of RLINE as a mobile source type.

GRSM NO₂ Screening Option

The GRSM NO₂ screening option was developed to address photolytic conversion of NO₂ to nitrogen oxide (NO) and to address the time-of-travel necessary for NO_X plumes to convert the NO portion of a plume to NO₂ via titration and entrainment of ambient ozone. The existing Tier 3 NO₂ options, Plume Volume Molar Ration Method (PVMRM) and Ozone Limiting Method (OLM), do not address these mechanisms and have been shown to overpredict ambient impacts within the first one to three kilometers (km) from the emissions source. To run the GRSM option, in addition to the NO₂/NO_X in-stack ratios and hourly ozone (O₃) ambient monitoring data required for PVMRM and OLM, GRSM also requires hourly NO_X ambient monitoring data.  The hourly ozone and NO_X ambient monitoring data must coincide with the hourly meteorological dataset utilized in the model. As a regulatory non-default option, the GRSM will be able to be used with prior approval of the regulatory jurisdiction and be another tool for NO₂ modeling demonstrations.

COARE Addition

With the recent surge in offshore wind air permitting projects the proposed Guideline revisions include adding the COARE algorithms to AERMET for meteorological processing of observed or prognostic meteorological data in overwater marine boundary layer environments. The addition of COARE to AERMET will replace the current standalone AERCOARE program.  Lastly, this revision would remove the current lengthy and tedious process of completing an alternative modeling demonstrate to utilize the COARE algorithms.  However, use of the COARE algorithms will still require U.S. EPA Regional approval similar to the current process for utilizing Tier 3 NO₂ non-default screening options.

RLINE Addition

Based on an Interagency Agreement between U.S. EPA and the Federal Highway Administration (FHWA) a new source type, RLINE, is being proposed to be added for regulatory modeling of mobile sources. Like COARE, RLINE was originally developed as a stand-along model known as the Research Line Model (R-LINE). The RLINE model was designed for near-surface releases to simulate mobile source dispersion focusing on the near-road environment.  U.S. EPA has noted that RLINE does not replace the existing AREA, LINE, and VOLUME source types, so all four can be utilized for characterizing mobile source emissions.

General AERMOD Enhancements

In addition to the updates discussed above, a new proposed version of AERMOD (v. 23132) has been released which includes a slew of other bug fixes and general enhancements to improve the model.  The new version of AERMOD can be used immediately for regulatory modeling demonstrations.  However, the updates discussed above are considered regulatory and, therefore, won’t be able to be utilized until the Appendix W revisions are finalized.

Updates to Recommendations on the Development of Background Concentrations

U.S. EPA is proposing updates to Section 8 (Model Input Data) of the Guideline to refine the guidance for identifying ambient monitoring data to be utilized as part of NAAQS modeling demonstrations. The proposed updates are to address the current lack of specificity and inconsistent approaches regulatory agencies have implemented based on the lack of specificity.  The proposed updates outline a framework that focuses on defining representative background concentrations through qualitative and semi-quantitative considerations.  In addition, to the proposed updates to Section 8 U.S. EPA has also developed “Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations.”  The Draft Guidance outlines U.S. EPA recommended stepwise considerations designed to identify representative background concentrations for cumulative impacts analyses and to avoid conservative “double counting” of local sources. What the Draft Guidance does not address, or update, is the conservative approach of combining design value (e.g., Tier 1) or near design value (e.g., Tier 2) monitoring data with modeled design values.

Takeaways

The proposed revisions to the Guidelines provide some additional regulatory non-default options that will be useful in specific modeling applications (i.e., NO₂ modeling, overwater modeling, mobile source modeling) and require less “hoops to jump through” by way of being regulatory approved.  The updates to Section 8 related to identifying background concentrations and the supporting background concentration guidance document will have the broadest impacts and will be especially important with the pending proposal to lower the particulate matter less than 2.5 microns (PM_2.5) annual NAAQS from 12 micrograms per meter cubed (µg/m³) to 9 µg/m³ or 10 µg/m³.  However, the proposed revision doesn’t represent a stepwise improvement in the air quality modeling process and as a result regulatory air quality modeling will continue to be a difficult and critical step in the air permitting process. U.S. EPA has indicated they hope to finalize the revisions to the Guideline by Spring of 2024.

Several ALL4 staff will be in attendance at the 13^th Conference on Air Quality Models on November 14-15, 2023, in RTP, NC.  If you have any questions about how the proposed revisions will impact your facility or your next project, please contact Dan Dix.