An Improvement to Virtual Point Source Model

The virtual point source model (VPSM), which is based on the combination of the Gaussian dispersion modeling and the point source approximation, is widely applied to estimate the ambient air concentrations of pollutants dispersed and transported downwind of an area emission source. However, as is well known, this modeling becomes less accurate as the receptor comes closer to the source, and becomes inapplicable for the estimation of on-site ambient concentration. This paper proposes an improvement for VPSM to overcome its above-mentioned shortcomings. The improvement is based on the virtual point approximation and the assumption that only part of the upwind plume of emissions within a narrow sector contributes to the concentration at a certain point. As a result, when it comes to the calculation of concentration near the source, we can consider only part of the source strength instead of the whole one. And if the calculated point is in the concerned area source, we consider not only a smaller source strength, but also an additional assumed backward distance. In our practice, this approach has been proved to be effective to avoid obtaining much too high concentrations and approach the actual concentrations near the sources.