Deposition of air pollutants is dominated by precipitation, whereby rain can scavenge particles and chemically react with gases. The rain deposits the pollutants (usually in a modified form) onto the surface of the Earth. Dry deposition also occurs, especially with "sticky" compounds like ammonia. However, dry deposition is not nearly as efficient as wet deposition. Though, if particles are large enough, they will settle out of the atmosphere relatively quickly and deposit on the surface.

I suggest you read up a little on the topic here to get you more familiar with the different factors that affect air pollutants.