Acid Rain

All rainfall, even that in the most pristine (unpolluted) areas, is acidic. This acidity results from the presence of carbon dioxide in the atmosphere, which reacts with water to produce carbonic acid. (The same reaction also occurs in a carbonated beverage.) Acid rain is rain with an acidity greater than that which normally occurs.

The cause of acid rain is sulfur oxide and nitrogen oxide air pollution. After being discharged into the atmosphere, these pollutants are chemically converted into sulfuric acid and nitric acid.

The most observable effect of acid rain is the slow, but definite, corrosion of carbonate-based building materials; this phenomenon is sometimes called "stone leprosy." Objects, including statues, made of limestone and marble, which are forms of calcium carbonate (CaCO3), are the most severly affected materials.

[source: Chemical Principles by H. Stephen Stoker]