Effects of turbulence on gas-phase atmospheric chemistry: Calculation of the relationship between time scales for diffusion and chemical reaction

Summary Non-uniform mixing of gas-phase trace species may limit the accuracy of the predictions of Eulerian transport/transformation models if the chemical reactions are rapid enough to be diffusion limited. If a reaction is diffusion limited, its average reaction rate might not be accurately represented by those models that assume instantaneous uniform mixing. One possible consequence of this artificial dilution is the overprediction of ozone and hydroxyl radicals. We have determined which reactions in the Regional Acid Deposition Model Gas-Phase Chemical Mechanism (Stockwell et al., 1990) are diffusion limited for a typical atmospheric condition through the calculation of Damköhler numbers. Damköhler numbers are defined to be the ratio of the diffusion mixing time to the chemical reaction time for a given chemical reaction (McRae et al., 1982; Hill, 1976). The reactions of hydroxyl radicals and the reactions of peroxy radicals with NO are diffusion limited under typical atmospheric conditions. Both sets of reactions are especially significant because NO_x and organic species strongly affect ozone and hydroxyl radical concentrations. It is suggested that Damköhler numbers could be used to help determine the placement of Eulerian model boundaries and to determine model grid structure.With 2 Figures