Scavenging Prediction Using Ratios of Concentrations in Air and Precipitation

The ratios of the concentrations of SO2 and of water vapor in precipitation to those in air on a mass basis are in the range 19 to 500. For particulates, the ratio usually falls in the range 290 to 2700. The ratio varies inversely with mixing ratio and precipitation rate. Stronger updrafts and rapid condensation produce lower nuclei concentrations because coalescence of cloud particles contributes less water to the final precipitation particle. The relative contribution to precipitation water of coalescence as compared to condensation is expected to be greater at the beginning and end of storms and at the edges of convective cells, and this partially explains the time variation of pollutants in precipitation. Ratios should be determined by k/χ = ρn/qE2+(1−n)ρα/q+HΛ/R, where k and χ are the concentrations in rain and air, respectively, ρ the density of water, q the absolute humidity, E2 the efficiency of the cloud at removing water vapor as precipitation, n the fraction of the pollutant which nucleates and is subsequently scavenged, α a dimensionless reactivity factor for gases with water, H the height of the cloud base, and Λ the washout coefficient for the precipitation rate R. Measurements of washout ratios will, therefore, provide information on cloud processes.