Acid rain is caused when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground. Source: https://www.thehindu.com/sci-tech/energy-and-environment/significant-drop-in-so2-levels-in-india-in-last-decade-iit-kgp-study/article65913462.ece