Abstract
A finite-difference model incorporating the mechanisms of thermal coagulation and gravitational sedimentation in the equation of diffusion is used to predict the particulate size distribution downstream from an urban area source. The particulate size range is 0.001 to 100 μm dia and the domain is 1 km vertically and 100 × 80 km horizontally in the mean and cross-wind directions, respectively. The predicted particulate concentrations, coagulation rate and deposition rate are presented. Comparison between the cases without particles larger than 10 μm and with all the particles included shows that the larger particles serve as an efficient collection agent for the particles smaller than 0.05 μm. The predicted spectral shape simulates fairly well the observations by Whitby et al. and Junge in the range smaller than 0.1 μm, and appears closer to Deirmendjian's observations in the range of 0.1–10μm. The effect of gravitational sedimentation and coagulation is found to be negligible for particles in the range of 0.1–1 μm. Consequently, the number spectrum should peak at this range in an equilibrium state.
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