1. Sulphur dioxide accounts for most of the acid rain problems.
   (a) When sulphur dioxide dissolves in water, sulphurous acid is formed.
   SO2(g) + H2O(l) → H2SO3(aq)
   (b) Sulphur dioxide can react with oxygen and water to form sulphuric acid.
   2SO2(g) + O2(g) + 2H2O(I) → 2H2SO4(aq)
2. Acid rain occurs when the pH of rain water falls between 2.0 to 5.5. This is due to the presence of sulphurous acid, sulphuric acid and nitric acid in rainwater.

Go through the detailed information on Acid Rain in the given link.

Further Readings – 

1. Air Pollutants – Classification, Sources and Impacts
2. Rainfall Distribution in India
3. Montreal Protocol
4. Greenhouse Gas