The process of the removal of atmospheric constituents by precipitation. One item necessary for a rain drop to form is a condensation nucleus. Pollutants in the air such as nitric acid and sulfuric acid form nucleation sites (particles) upon which water vapour often condenses and forms a droplet. As these droplets grow they collect more moisture and may ultimately fall out as precipitation. This accounts for some of the acid rain deposition on the planet's surface and a means by which atmospheric aerosols, gases, and particles are removed.