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Definition:
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Removal of pollutants from the air by either rain or snow. Rainout (or snowout), which is the in-cloud capture of particulates as condensation nuclei, is one form of scavenging. The other form is washout, the below-cloud capture of particulates and gaseous pollutants by falling raindrops. Large particles are most efficiently removed by washout. Small particles (especially those less than 1 'm in diameter) more easily follow the airstream flowing around raindrops and generally avoid capture by raindrops except in heavy rain events.
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