Understanding and Adapting to New Dispersion Models
The U.S. Environmental Protection Agency (EPA) has proposed revisions to its air dispersion modeling guidance1 (“Guideline on Air Quality Models”, or simply “Guideline”), found in 40 CFR 51 Appendix W.2 These revisions, once finalized, will create significant changes in the way that air quality analyses are performed in the US, and perhaps the most significant changes since the Guideline was first published in 1978. EPA views the Guideline as the federal standard in reviewing analyses for federal programs (e.g., PSD) and approving state implementation plans (SIPs). Oregon’s air quality rules require that all air quality analyses use only models approved in the Guideline, unless DEQ approves an alternate model.3 Advances in dispersion modeling techniques and desktop computing resources over the last ten years have generated newer dispersion models that offer significant improvements over current Guideline models in their ability to analyze atmospheric transport and dispersion. Shortcomings of the ISCST3 Model The Industrial Source Complex (ISC) dispersion model has been the “mainstay” tool used to predict ambient air concentrations from industrial sources for more than 20 years. The most current version, ISCST3, is approved in the Guideline. ISCST3 is best described as a Gaussian steady-state plume dispersion model with a minimum one-hour timestep. This definition essentially means that for each hour of the modeled period, the model calculates downwind concentrations as if the meteorological conditions had been constant throughout the modeled domain and had been the same for some time, this approach ignores the fact that the atmosphere is continuously varying in both time and space, and that plumes rarely set up into the “steady state” condition. When compared to the current “new generation” of dispersion models, ISCST3 has several shortcomings. These flaws include poor characterization of building downwash and terrain features, a lack of chemical transformation and physical removal processes, and a useful range limit of approximately 40 to 50 km. Because newer dispersion models address these shortcomings, EPA has proposed revising the Guideline to phase out the ISCST3 model and require the use of newer dispersion models. We discuss each of ISCST3’s shortcomings and the new models that EPA has proposed to take its place.