Burning of fossil fuels emits sulphur dioxide.
S (in coal) + O2SO2
Sulphur dioxide reaches the atmosphere, it oxidizes to first form a sulphate ion.
2 SO2 + O2 2 SO3
The sulphate ion joins with hydrigen atoms in the air and becomes sulphuric acid. This then falls back down to earth as "acid rain"
SO3 + H2O H2SO4
Hope this at least helped a little c: