In addition to dispersion, wet and dry removal processes as well as atmospheric reactions affect the concentrations of air pollutants in the atmosphere. Atmospheric reactions include ozone or acid rain formation. In dry removal, particles are removed by gravity or impaction, and gases diffuse to surfaces where they are absorbed or adsorbed. Wet removal is the major removal process for most particles and can be a factor in the removal of gaseous contaminants as well. Wet removal can involve the in-cloud capture of gases or particles (rainout) or the below-cloud capture (washout). In washout, raindrops or snowflakes strike particles and carry them to the surface; gases are removed by absorption (Godish 1991).
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