EFFECTS OF TURBULENCE ON GAS-PHASE ATMOSPHERIC CHEMISTRY - CALCULATION OF THE RELATIONSHIP BETWEEN TIME SCALES FOR DIFFUSION AND CHEMICAL-REACTION

Non-uniform mixing of gas-phase trace species may limit the accuracy o f the predictions of Eulerian transport/transformation models if the c hemical reactions are rapid enough to be diffusion limited. Ifa reacti on is diffusion limited, its average reaction rate might not be accura tely represented by those models that assume instantaneous uniform mix ing. One possible consequence of this artificial dilution is the overp rediction of ozone and hydroxyl radicals. We have determined which rea ctions in the Regional Acid Deposition Model Gas-Phase Chemical Mechan ism (Stockwell et al., 1990) are diffusion limited for a typical atmos pheric condition through the calculation of Damkohler numbers, Damkohl er 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 react ions of peroxy radicals with NO are diffusion limited under typical at mospheric conditions. Both sets of reactions are especially significan t because NOx and organic species strongly affect ozone and hydroxyl r adical concentrations. It is suggested that Damkohler numbers could be used to help determine the placement of Eulerian model boundaries and to determine model grid structure.