Unpolluted rainfall is slightly acidic (~pH 5.6), due to the dissolution of atmospheric carbon dioxide (CO2) forming the weak acid carbonic acid (H2CO3). However, the term 'acid rain' refers to rainfall containing strong acids. These can originate from both natural sources (especially volcanic emissions) and anthropogenic sources (especially fossil fuel combustion). Volcanic acid rain (pH 2.5-5.0) is typically dominated by hydrochloric acid (HCl) and sulfuric acid (H2SO4) arising from the plume gases HCl and sulfur dioxide (SO2).
The process of acid rain formation is shown in the diagram presented in Gases & Aerosols.
Some water treatment systems may be based on rainfall acidified by volcanic emissions. For further information, refer to the table with pH information in Water Supply.