In general, the pH value of rainwater is 5.6. If the pH value of rainwater is below 5.6, it is called acid rain. This acid rain is caused by gases like sulphur dioxide and oxides of nitrogen present in the atmosphere.
Sulphur dioxide and oxygen present in the atmosphere react with each other in presence of sunlight to form sulphur trioxide. This sulphur trioxide dissolves in rainwater and forms sulphuric acid.
Oxides of nitrogen present in the atmosphere dissolve in rainwater and form sulphuric acid.
These acids then pour down on the earth's surface in the form of acid rain.