Time scales of physical and chemical processes in chemically reactive plumes

Abstract

The rate of non-linear chemical reactions in a dispersing plume, such as the formation of NO2, is discussed on the basis of various models and measurements of the oxidation rate of NO. Results of calculations with four different reactive plume models are compared with a set of plume measuremeent and discussed in terms of the ration of the time scales of physical (Tp) and chemical processes (Tc), the Damköhler number:Tp/Tc. In modelling a fast non-linear chemical reaction an interaction between physical processes of dispersion and mixing and chemical reactions can be expected. Models may differ in that they assume either chemical equilibrium (Tp/Tc⪢1) or no chemical equilibbrium (Tp/Tc⪡1).From our study it is concluded that for the first 5 km from a stack concentrations in the plume change more rapidly by dilution due to dispersion of the plume than by chemical reactions. Farther off it is the other way around and the chemistry is fast enough to achieve chemical equilibrum in the plume parcels.Conversion of NO is then inhibited by the mixing rate of the plume with its surrounding air. To incorporate the mixing rate of the plume correctly into a model, in principle the concentration distribution in a momentary plume has to be used. If a non-linear chemical reaction is modelled it is therefore important to establish the averaging times on which the dispersion parameters in the model are based. The contribution of plume meandering to dispersion, which is especially important in the daytime, can be eliminated by taking shorter averaging times. This leads to a marked improvement of modelling results. Further research is required to investigate how the contribution of fluctuations in the concentrations around the mean has to be incorporated into a model of a chemically reactive plume.