Air dispersion modeling of particulate matter from ground-level area sources

Abstract

State Air Pollution Regulatory Agencies (SAPRAs) often use dispersion modeling to predict downwind concentrations of particulate matter (PM) from a facility. As such, a facility may be granted or denied an operating permit based on the results obtained from a dispersion model. The model currently approved by EPA, the Industrial Source Complex, Version 3 - Short Term (ISC3-ST), over-predicts downwind concentrations of PM by as much two orders of magnitude. As a result, a facility may be denied a permit when, in fact, its emissions result in off-property concentrations that would not cause a violation of the National Ambient Air Quality Standard (NAAQS) for PM. To ensure fair and reliable regulation of pollutant sources, dispersion models that provide accurate estimations of downwind concentrations of pollutant from these fugitive sources are needed. The dispersion model currently approved by EPA for use with ground-level sources uses a normal distribution in the vertical plane that mathematically reflects that portion of the distribution that is below ground back onto the area above ground. The resulting distribution necessarily predicts that the point of maximum concentration is at ground level at any distance downwind of the source, which is not always true. 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 confined to ground level. Additionally, the three indices of the triangular distribution may be adjusted based on pollutant density which provides an additional degree of versatility.