Reason: Acid rain contains acids like nitrous acids, which corrode the upper layers of the buildings and monuments that are made up of marbles and damages them.
The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively. Acid rain can cause damage to buildings and historical monuments because the acid present in the rain reacts with the upper layers of the buildings or monuments and corrodes them.