Acid rain is a generic term that can include acidic fog and acidic snow.
The concept of acid rain was rediscovered in the early '70s. Other emissions that affect acidity are hydrochloric acid, ammonia, volatile organic compounds and alkaline dust. There are natural sources of sulfur oxides and nitrogen oxides. Volcanic eruptions and the decomposition of organic matter are the main natural sources of sulfur oxides. The natural sources of nitrogen oxides are bacterial action in soils and in chemical reactions in the upper atmosphere.