Below-cloud scavenging gases

Typical values of scavenging ratio lie within the range 300-2000. Scavenging ratios are rather variable, dependent upon the ehemieal nature of the trace substance (particle or gas, soluble or insoluble, etc) and the type of atmospheric precipitation. Incorporation of gases and particles into rain can occur both by in-cloud scavenging (also termed rainout) and below-cloud scavenging (termed washout). 

GEM-AQ only has a simplified aqueous phase reaction module for oxidation of SO2 to sulphate. Thus, for the gas phase species, wet deposition processes are treated in a simphfied way. Only below-cloud scavenging of gas phase species is considered in the model. The efficiency of the rainout is assumed to be proportional to the precipitation rate and a species-specific scavenging coefficient. The coefficients apphed are the same as those used in the MATCH model (Multiscale Atmospheric Transport and Chemistry Model) used by the Swedish Meteorological and Hydrological Institute (SMHl) (Langner et al. 1998). 

If the species exists only in the gas phase (not in the aerosol), and if the contribution of rainout is negligible compared to washout," the wet deposition flux of the species will be equal to the below-cloud scavenging rate as given by (20.4). 

Below-Cloud Scavenging Coefficient The concentration of gas A is lOpgm 3 below a raining cloud. Assuming a constant scavenging coefficient of 3.3 h "1, calculate the concentration of A in the atmosphere after 30 min of rain, and the overall wet deposition flux. Assume a cloud base at 2 km. 

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