Balancing simplicity with accuracy in the use of dispersion modelling in the United States

Abstract

Dispersion modelling has been a requirement of air–quality regulatory agencies in the United States for over 20 years. Over the years, the task of performing this modelling has shifted from the regulatory agency to the applicant. One purpose of the modelling is to show that air–quality 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. Because thousands of modelling studies are performed each year, the United States Environmental Protection Agency guidelines specify a tiered approach: first, screening modelling, and then, refined modelling. Approval for site–specific models can be obtained after extensive validation 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. Since 1979, building downwash effects must be considered and, in the author's experience, these effects are the cause of maximum concentrations in approximately 90% of the studies. Most studies are performed with the use of meteorological data from nearby airports. The 1977 Amendments to the Clean Air Act specified that a violation of an air quality standard could be identified by either modelling or monitoring. As a result, 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 essential that whatever tools the European Union decides to use, they be accessible to ordinary factory environmental engineers and be understandable to the typical staff engineer at a regulatory agency. Thus, practicability will be a hallmark if modelling is to come into widespread use.