Mathematical Modeling of Chemically Reactive Plumes in an Urban Environment

Abstract

Mathematical models used to simulate photochemical urban air pollution are generally based upon the solution of the atmospheric diffusion equation, which describes the rate of change of chemical species in each cell of a three-dimensional grid-mesh. In such models, point source emissions are assumed to be immediately dispersed into a grid cell. This approximation generally leads to an overestimation of plume dispersion and thus considerably affects the accurate treatment of photochemical kinetic processes.

A model has been developed that offers a realistic treatment of large point source emissions in urban areas. This Plume-Airshed Reactive-Interacting System (PARIS) is based upon the embedment of one or more reactive plume models in a conventional airshed model. These plume models describe the chemistry and dynamics of large point source plumes, including their interaction with the ambient environment. Plume trajectories are calculated from the gridded wind field used for the airshed model, and plume dispersion is computed according to the local temperature lapse rate. As the plume size becomes comparable with the grid cell size, the plume material is mixed into a grid of the airshed model with the background airshed material.

This paper presents some results of PARIS model simulations performed for the St. Louis urban area; in addition, the performance of the model is analyzed and implications regarding its use in the simulation of air quality are discussed.