1) Cloud droplets which attract an aerosol into snow, rain or some other water precipitate by coagulation and accretion.
source: [Environmental Particles; v1 - Environmental Analytical and Physical Chemistry Series; Buffle and van Leeuwen, ed.; pages 7, 30-33, 54, 154; 1992; Lewis; Boca Raton, FL.] [Character and Origins of Smog Aerosols; v9 - Advances in Environmental Science and Technology Series; Hidy, ed.; pages 1, 37, 148, 407, 425; 1980; Wiley; New York.]