Abstract Description: In November 2024, the U.S. Environmental Protection Agency (EPA) updated Appendix W to 40 CFR Part 51 (“The Guideline on Air Quality Models” or “Guideline”) and its preferred short-range dispersion model, AERMOD, to include a new regulatory non-default Tier 3 NOx chemistry option called the Generic Reaction Set Method (GRSM). GRSM is the first and only Tier 3 option that addresses photolytic conversion of NO2 to NO and time-of-travel conversion of NO to NOx through titration and entrainment of ambient ozone. The AERMOD modeling system also continues to include the Travel Time Reaction Method (TTRM2) as an ALPHA option for evaluation. TTRM2 is designed to be paired with a Tier 2 or Tier 3 option (i.e., ARM2, OLM, or PVMRM) that doesn’t account for the travel time conversion. GRSM and TTRM2 have been evaluated on a limited number of NO2 datasets, all in rural environments with only a few emission sources of NOx. However, these methods have not been widely tested in practical applications such as 1-hour and annual NO2 cumulative modeling with large source inventories for a Prevention of Significant Deterioration (PSD) application as the Plume Volume Molar Ratio Method (PVMRM) and the Ozone Limiting Method (OLM) have.
This presentation will provide an overview of a 1-hour and annual cumulative NO2 analysis for a PSD application, the ambient air monitoring data available in the area to use for each method, and challenges with incorporating the data into AERMOD for a large industrial facility. For this case study, modeling results utilizing all available NOx to NOx options (Tier 1 through 3) will be evaluated.
