Chemical pollutants (gaseous oxides such as CO2​,NO2​  and SO2​) present in the atmosphere react with water to form carbonic acid, nitric acid and sulphuric acid. This causes acid rain which corrodes marble statues and monuments. The reaction between the calcium carbonate present in the monoumnets and the acid of acid rain is given below.CO2​+H2​O→ CaCO3​ or Ca(HCO3​)2​ CaCO3​+H2​SO4​→CaSO4​+CO2​+H2​OThe products are water soluble and washed away. The presence of carbonate, sulphate and nitrate salts causes blackening of marble surfaces.