Acid Deposition

Unpolluted rainwater is naturally slightly acidic, with a pH of about 5.6, because atmospheric carbon dioxide dissolves in it to form the weak carbonic acid: $CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)$. The term acid deposition, commonly called acid rain, is reserved for precipitation and airborne particles with a pH below about 5.6, caused by additional pollutant gases, chiefly the oxides of sulfur and nitrogen. Acid deposition includes both wet deposition (acidic rain, snow, fog, and mist) and dry deposition (acidic gases and particles that settle out and later dissolve on contact with moisture).

The main culprits are sulfur dioxide ($SO_2$) and the nitrogen oxides collectively written $NO_x$. Sulfur dioxide is produced mainly by burning fossil fuels that contain sulfur impurities