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Air dispersion models for release scenarios

Under the hazard assessment requirements of 40 CFR Part 68, Subpart B, an owner or operator is required to analyze a worst-case release scenario and more likely alternative release scenarios. Has EPA developed any air dispersion models for conducting these evaluations? Is EPA's TScreen model an appropriate technique?

EPA has developed the RMP Offsite Consequence Analysis Guidance  document which provides technical guidance and reference tables for worst-case and alternative release scenario assessments. Stationary source owners and operators may, however, use any generally recognized, commercially or publicly available air dispersion modeling techniques, provided the modeling parameters specified in the rule are used (61 FR 31672; June 20, 1996).

EPA's Office of Air Quality Planning and Standards has prepared a publicly available modeling tool called TScreen that can assist owners and operators with consequence assessments (61 FR 31684; June 20, 1996). TScreen can be used to simulate a release and to calculate the dispersion characteristics and pollutant concentrations of the resulting plume. TScreen can be downloaded from EPA's Support Center for Regulatory Atmospheric Modeling (SCRAM). EPA has also developed an emergency response model called Areal Locations of Hazardous Atmospheres (ALOHA). ALOHA can be used to plot the area downwind of a release where concentrations may exceed a user-set threshold level. ALOHA can be downloaded from EPA's Web site (