Monuments and statues are made of either stone or metal M both of which have a tendency to react with acid and therefore, get affected by acid rain which contains $\mathrm{H}_2\mathrm{SO}_4$. $\underset{\text{(metal)}}{M} + \mathrm{H}_2\mathrm{SO}_4$ → $\mathrm{M}_2\mathrm{SO}_4 + \mathrm{H}_2$ $\underset{\text{(from stone)}}{\mathrm{CaCO}_3} + \mathrm{H}_2\mathrm{SO}_4$ → $\mathrm{CaSO}_4 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2$ As a result, the monuments are being slowly disfigured.