Deposition, not rain Greg points out that rainwater is just one way in which acidic pollutants reach the ground. Atmospheric scientists believe that just as much acid-causing material is directly deposited from the air. They prefer the term 'acid deposition' to 'acid rain'. One-quarter of the sulfur in the atmosphere is natural, the rest is caused by human activity. Nature releases sulfur through decomposing marine algae and erupting volcanoes.
Industry releases sulfur dioxide when fossil fuels are burnt and sulfide ores smelted.
Sulfur dioxide readily forms sulfuric acid in the air or when it reaches the ground. In the year 1900, global sulfur dioxide emissions were approximately 15 million tonnes. Annual emissions are now close to six times that amount. Nitrogen oxides are generated by lightning and microbes, and by burning of fossil fuel and biomass. In the atmosphere the oxides are often transformed into nitric acid.