Air dispersion modeling of particulate matter from ground-level area sources
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abstract
The new National Ambient Air Quality Standards (NAAQS) for particulate matter less than 2.5 m in diameter (PM2.5) and ozone, which has recently been under review, will increase the number of nonattainment areas in the United States. Since a facility is responsible for all PM emissions originating from that property, an increased emphasis will be placed on the regulation of fugitive PM sources, as well. Dispersion modeling is often used by State Air Pollution Regulatory Agencies (SAPRA's) in determining whether the contribution of particulate matter from a facility meets the NAAQS. As such, a facility may be granted or denied an operating permit based on the results obtained from a dispersion model. However, the model currently approved by EPA over-predicts downwind concentrations of PM by as much as ten fold. This results in the possibility that a facility is denied a permit when, in fact, its emissions are well within the NAAQS. Dispersion models that provide accurate estimations of downwind concentrations of pollutant from these fugitive sources are needed to ensure reliable and fair regulation of pollutant sources. The current EPA-approved ground-level dispersion model uses a normal distribution in the vertical plane that mathematically reflects the area of the distribution that is below ground back onto the area above ground. This produces a distribution in which the point of maximum concentration is at ground level at any distance downwind of the source, which is intuitively incorrect. With the goal of developing a model that produces a more accurate result, we have replaced the reflected normal distribution in the vertical plane with a triangular distribution. In a triangular distribution the point of maximum concentration is no longer confine to ground level. Additionally, the three indices of the triangular distribution may be adjusted, based on what type of pollutant is being modeled, which provides an additional degree of versatility.
