Burning of fossil fuels emits sulphur dioxide.
The sulphate ion joins with hydrigen atoms in the air and becomes sulphuric acid. This then falls back down to earth as "acid rain"
Acid rain is a result of air pollution. 
some of the polluting gases  (especially nitrogen oxides and sulphur dioxide)          react with the tiny droplets of water in clouds to form sulphuric and nitric acids. The rain from these clouds then falls as very weak acid - which is why it is known as "acid rain".
Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air.