Rain water absorbs carbon dioxide from the air it passes through on its way down. The carbon dioxide reacts with the water to form carbonic acid. This mechanism is one of the principal ways in which atmospheric carbon is scrubbed from the air and put back into circulation among living systems.
Carbonic acid is only weakly acidic. The compound is commonly used in sparkling wines and carbonated beverages, and it is a minor component of human blood. In contact with ferrous metals, carbonic acid is an effective oxidizing agent. By dissolving the surface layer of iron or steel, carbonic acid facilitates the transfer of electrons and promotes the growth of rust. Excess acidic rain can also have negative impacts on soil bacteria and the economically important crops that depend on them.
The natural acidity of rain is due to the trace amounts of carbonic acid present in aqueous solution. It is not to be confused with the kind of acid rain that is sometimes the result of excess sulfur dioxide and other industrial pollutants in the atmosphere. Sulfur dioxide reacts with rain to form sulfuric acid, which has a much more extreme pH than carbonic acid and is much more destructive.