Abstract Description: AERMOD is the U.S. EPA’s recommended model for evaluating near-field impacts, defined as occurring within 50 km of the sources, caused by pollutant emission sources. Since promulgation of the 1-hour NO2 NAAQS in 2010, U.S. EPA has released guidance and made refinements to AERMOD to make dispersion modeling analyses for NAAQS compliance demonstration purposes more “realistic”, that is, more like an ambient concentration that is actually measured at a monitor. In the latest version of AERMOD and in finalized revisions to Appendix W, released in late 2024, U.S. EPA updated the Generic Reaction Set Method (GRSM) option and made it a regulatory non-default option.
This case study reviews 1-hour NO2 concentrations predicted by AERMOD for the GRSM option and for other regulatory non-default Tier 3 options, the Ozone Limiting Method (OLM) and the Plume Volume Molar Ratio Method (PVMRM), for several hypothetical sources at locations throughout the United States. An analysis of the sensitivity of the model-predicted concentrations to the NO2 conversion methodology used is presented on a design concentration basis. The analysis includes single sources as well as multiple sources to evaluate the effects of more than one plume on the Tier 3 options. Also presented are results showing the sensitivity of AERMOD outputs to those inputs needed for the current regulatory Tier 3 options.
