On the treatment of point source emissions in urban air quality modeling

Abstract

This paper presents the development and evaluation of an urban air quality model, the Plume-Airshed Reactive-Interacting System (PARIS), that is capable of providing a detailed treatment of large point source emissions. The PARIS model treats these large point sources by embedding one or more reactive plume models into the Systems Applications urban airshed model, which is a three-dimensional gridded model governed by the atmospheric diffusion equation. These embedded reactive plume models are used to decribe the chemistry and dynamics of large point source plumes and their interaction with the ambient urban environment. When the plume size becomes comparable to the airshed model grid cell size, a subgrid scale description is no longer necessary and the plume material is mixed into the airshed model grid cells. For purposes of evaluation and comparison, the Systems Applications urban airshed model and the PARIS model were applied to the St. Louis urban area for a one-day simulation and to the South Coast Air Basin (Los Angeles area) for a two-day simulation. Overall absolute errors between predictions and observations for the urban airshed and PARIS model simulations are on the order of 40–50% for O 3 and NO 2 concentrations. The overall differences in absolute errors between airshed and PARIS model predictions are between 1 and 5 %; the difference in overall model performance resulting from the PARIS model subgrid scale treatment of point sources is well within measurement uncertainties for both O 3 and NO 2 concentrations. These results indicate that for these two applications, detailed treatment of large point sources with the PARIS model has little effect on overall model performance. However, the model can provide information necessary for the study of the impact of individual point sources located in an urban environment.