Acid rain is linked to both natural and man-made sources. 

1. When a thunderstorm produces lightning,  nitrogen oxides are formed through the extreme heating of air.
2. Sulphurous gases are discharged from erupted volcanoes and rotting vegetation.
3. Man-made sources of acid rain include the burning of any fuel that contains sulphur and nitrogen compounds, including public utilities, industrial broilers, motor vehicles, and chemical plants.

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