The Virtual Chembook of Elmhurst College explains that acid rain considerably increases the rate of corrosion on marble, sandstone and limestone buildings. Sulfur dioxide and water forms sulfurous acid, and sulfur trioxide and water creates sulfuric acid, which reacts with limestone in a neutralization reaction. Limestone dissolves and crumbles as calcium sulfate is soluble in water.
Acid rain mainly affects stone through dissolution and alteration, according to the U.S. Geological Survey website. Calcite dissolves when sulfurous, sulfuric and nitric acids in polluted air react with the calcite found in limestone and marble. Exposed regions of buildings often show roughened surfaces, loss of carved details and removal of material due to acid rain. Sheltered parts on marble and limestone buildings also reveal dark crusts that have peeled off in some areas, showing crumbling stone beneath them. The black crust consists of gypsum, which is a mineral created from the reaction among sulfuric acid, calcite and water.
Gypsum is soluble in water, and it stays only on protected surfaces that are not directly exposed to the rain, the U.S. Geological Survey elaborates. The crust appears black, as the crystals of gypsum form networks that trap particles of dirt and pollutants. The dark crusts reveal crumbling stone when they peel off in some areas of a building.