The gaseous oxides (CO2, NO2 and SO2) presentin the air as chemical pollutants with water in air to form acid such as carbon acid, nitric acid, and sulphuric acid. these acid comes down as rain and cause corrosion of marble statues. calcium carbonate present in marble is transformed into other compounds by reaction with acids as follows:
CO2+H2O+CaCO3 sign of rection Ca(HCO3)2
CaCO3+H2SO4 sign of reaction CaSO4+CO2+H2O
the componds formed are water soluble and easily washed away.
blackening of marble surface is due to the presence of carbonate, sulphate, and nitrate salts