Unpolluted rainfall is slightly acidic (~ph 5.6), due to the dissolution of carbon dioxide (CO2) forming weak carbonic acid (H2CO3). However, the term 'acid rain' refers to rainfall containing elevated levels of strong acids, which lead to pH values less than 5.6 and as low as pH 4. These strong acids can originate from both natural sources (especially volcanic emissions) and anthropogenic sources (especially fossil fuel combustion). Rain falling through volcanic plumes containing strong mineral acids such as HCl, HF and H2SO4.
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.