Airborne.
Rain and snow are considered by some individuals to be relatively "pure," yet gases and particles introduced into the atmosphere by human activities and natural phenomena, such as volcanoes, can contaminate precipitation that falls back to Earth. The combustion of fossil fuels adds carbon dioxide to the atmosphere as well as nitrogen and sulfur compounds that tend to make precipitation more acidic. In areas of the world where the geology does not contain minerals that buffer the effects of the acids, the acidity of lakes and rivers may be increased as a result of "acid rain," derived, for example, from industrial areas. In addition, precipitation may also contain nutrients, heavy metals (e.g., mercury) and organic compounds that have entered the atmosphere and become entrained into precipitation.