Rainout, atmospheric pollution removal

The tropospheric sulfur chemistry is different. Unlike the nitrogen and carbon chemistry, where combustion is an insignificant source, the combustion source of SO2 appears to be very important. While OH reactions can be shown to convert sulfides to SO2, it is not clear that normal atmospheric chemistry is important in the next step—the conversion of S02 to H2SO, which is then removed from the atmosphere by rainout. It has also been suggested that a large amount of SO2 is removed directly by rainout. Unfortunately we have the fewest data, both kinetic and atmospheric, on sulfur compounds. Most of the kinetic data we do have are at high temperatures, and most of the atmospheric data are for polluted environments. 

In the washout process, pollutants are removed from the atmosphere by falling precipitation but below cloud level, therefore processes which characterise rainout also occur in the washout process. For example, the washout of particulate material is seen to be achieved by impaction, interception by falling raindrops and Brownian diffusion effects, though the latter process is considered to be of little importance to the washout of sulphate and nitrate aerosols. 

Scavenging by precipitation is an efficient means of removing particulate pollutants from the atmosphere. There are two main processes involved rainout, which is the scavenging by water droplets within the cloud layer, and washout, the scrubbing of air by falling raindrops (or snow). Precipitation scavenging is most effective for pollutants having a substantial vertical dispersal in the atmosphere, and the influence of local sources upon wet deposition is unlikely to be... 

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