A CHEMICALLY REACTIVE PLUME MODEL FOR THE NO-NO2-O3 SYSTEM

A chemically reactive plume model (CRPM) is developed incorporating macro- and micro-mixing. Macromixing processes account for the variation of the mean concentrations while micromixing processes affect the deviations from these mean values. Four chemical reactions of the NOxO3 cycle are used in the CRPM to determine the conversion of nitric oxide (NO) to nitrogen dioxide (NO2). The inert chemical species NOx = NO + NO2 is modelled by a Gaussain plume model. Macromixing of the chemically reactive species is taken into account by dividing the Gaussian plume into N concentric elliptical rings. Exchange between the rings and entrainment of ambient air through the outermost ring are allowed. A combined approach of empirical expressions and the formulation of concentration fluctuations for inert pollutants is used in modelling micromixing effects. The micromixing term NO′NO′ increases the conversion of NO to NO2 while NO′O′3 decreases it, both relative to the rates calculated on the basis of mean concentrations. However, NO′O′3 is expected to have more influence in the long term than NO′NO t ́. Flight measurements were used to evaluate the model performance for different emissions and meteorological conditions. For neutral and stable conditions the results of the CRPM are in close agreement with these measurements. The computations show the importance of the micromixing terms and the necessity of their incorporation into this type of model. © 1990.