The acid that is believed to be mainly responsible for the damage...

The air around the city of Agra, where the Taj Mahal is located, contains fairly high levels of sulphur and nitrogen oxides. It is mainly due to a large number of industries and power plants around the area. Use of poor quality of coal, kerosene and firewood as fuel for domestic purposes add up to this problem. The resulting acid rain reacts with marble, ${CaCO}_{3}$ of Taj Mahal \left({CaCO}_{3} + {H}_{2}{SO}_{4} \to {CaSO}_{4} + {H}_{2}O + {CO}_{2}\right)$ causing damage to this wonderful monument.