BALANCING SIMPLICITY WITH ACCURACY IN THE USE OF DISPERSION MODELING IN THE UNITED-STATES

Dispersion modelling has been a requirement of air-quality regulatory agencies in the United States for over 20 years. Over the years, the t ask of performing this modelling has shifted from the regulatory agenc y to the applicant. One purpose of the modelling is to show that air-q uality standards or air toxic guidelines will not be exceeded when the proposed modification or new facility is constructed. Thus, the role of modelling is to estimate reliably the maximum concentrations likely to occur sometime, somewhere beyond the boundary of the facility. Bec ause thousands of modelling studies are performed each year, the Unite d States Environmental Protection Agency guidelines specify a tiered a pproach: first, screening modelling, and then, refined modelling. Appr oval for site-specific models can be obtained after extensive validati on studies. The vast majority of studies are performed for modest-size factories and installations. The guideline also specifies a 'level of significance' below which concentrations are considered trivial. Sinc e 1979, building downwash effects must be considered and, in the autho r's experience, these effects are the cause of maximum concentrations in approximately 90% of the studies. Most studies are performed with t he use of meteorological data from nearby airports. The 1977 Amendment s to the Clean Air Act specified that a violation of an air-quality st andard could be identified by either modelling or monitoring. As a res ult, the modelling performed by applicants must include data from all 'nearby sources', which in turn hinges on the availability of accurate emission inventories to anyone from regulatory agencies. It is essent ial that whatever tools the European Union decides to use, they be acc essible to ordinary factory environmental engineers and be understanda ble to the typical staff engineer at a regulatory agency. Thus, practi cability will be a hallmark if modelling is to come into widespread us e.