Effects of the intermixing process between the Lagrangian particles on the estimation of concentration in the Lagrangian particle dispersion model

A 3-D Lagrangian particle dispersion model, including the photochemical reaction developed by Song et al. (Atmos. Environ. 37 (2003) 4607), has been applied to the data obtained from the Southern Oxidants Study field campaign to examine the effect of the intermixing process between the superposed Lagrangian particles on the spatial distribution of the non-reactive pollutant of SO2 and the photochemical compound of O3 with several different Lagrangian particle releasing time intervals. It is found that the intermixing process tends to disperse the centerline maximum concentration of the non-reactive gas toward the edges of the plume and to enhance the entrainment rate into the plume, thereby reducing the maximum concentration at the center of the plume and widening the width of the plume. The Lagrangian particle releasing time interval of up to 5 min does not significantly affect the non-reactive SO2 concentration across the plume within 49 km (T4 traverse) in the downwind direction from the source. However, a much shorter releasing time interval (an order of minute) is required to simulate the reactive pollutant of O3 concentration compared to the non-reactive SO2 concentration due to excessive entrainment of the background reactive pollutant into the plume center and the overweighed mass of each Lagrangian particle. This result suggests that the present model with the Lagrangian particle releasing time interval of one minute and with the intermixing process between the superposed Lagrangian particles can be used more practically to simulate concentration of both the non-reactive SO2 and the photochemical compound of O3.