$\mathrm{SO_2}$ present in atmosphere combines with water to give sulphuricacid. $2\mathrm{SO_2} + \mathrm{O_2} + 2\mathrm{H_2O} \rightarrow 2\mathrm{H_2SO_4}$ This is called acid rain. Acid rain causes extensive damage to building andstatues made of marble, limestone, etc. due to its reaction to give $\mathrm{CaSO_4}$. This way $\mathrm{SO_2}$ acts as pollutant.