U.S. EPA Releases New Dispersion Modeling Guidance

U.S. EPA’s Air Quality Modeling Group (AQMG) released guidance on the U.S. EPA Support Center for Regulatory Atmospheric Modeling (SCRAM) website. The first memorandum entitled “Applicability of Appendix W Modeling Guidance for the 1-hour SO₂ NAAQS” was released in conjunction with the second memo entitled “Guidance Concerning the Implementation of the 1-hour SO₂ NAAQS for the Prevention of Significant Deterioration Program.” The first memo outlines the applicability of 40 CFR Part 51, Appendix W (Appendix W) as it relates to the new 1-hour SO₂ National Ambient Air Quality Standards (NAAQS). Of importance from this memo is that the current guidance in Appendix W for demonstrating compliance with the previous 24-hour and annual NAAQS and current 3-hour NAAQS is generally applicable for the new 1-hour NAAQS. In addition, the memo points out that while the new 1-hour SO₂ NAAQS is defined in terms of a 3-year average for monitor design values to determine attainment of the NAAQS, this definition does not preempt or alter the Appendix W requirement for use of five (5) years of National Weather Service (NWS) meteorological data or one (1) year of site specific meteorological data. The second memo outlines guidance on procedures that should be followed when modeling the new 1-hour SO₂NAAQS as part of Prevention of Significant Deterioration (PSD) permit applications. The memo also provides an interim 1-hour SO₂ Significant Impact Level (SIL) of 3 parts per billion (ppb) or 7.9 µg/m³. The SIL can be used to determine if a full NAAQS and PSD Increment air quality modeling analysis is required. 

U.S. EPA’s AQMG also released a beta version of AERSCREEN on the SCRAM website. AERSCREEN is a screening model based on AERMOD. AERSCREEN will produce estimates of “worst-case” 1-hour concentrations for a single source, without the need for location-specific hourly meteorological data. The program also includes conversion factors to estimate “worst-case” 3-hour, 8-hour, 24-hour, and annual ambient concentrations. AERSCREEN was developed to produce concentration estimates that are equal to or greater than the estimates produced by AERMOD with a fully developed set of meteorological and terrain data. In certain cases, AERSCREEN may be used to demonstrate compliance with ambient standards without the time and expense associated with a full AERMOD evaluation. A final version of AERSCREEN is expected to be released no later than December 31, 2010.