any where a factory is producing smoke that contains a gas which produces an acid when combined with water. Carbon-dioxide does, but very weakly. The main "acid rain" producer is Sulphur Dioxide, dissolving in water to form sulphurous acid.
Rain is naturally somewhat acidic (pH 5.5) due to carbon dioxide dissolved in the water. Problem acid rain also has dissolved sulphur dioxide and nitrogen oxides which form sulphurous and nitrous acids.
when gases like SO2, NO2, etc (effluents from the vehicles) are released into the atmosphere the combine with water vapour in the air to form mild acids of sulphuric and sulphurous acid, and nitric acid, etc. These when pour upon the earth fall down as acid rain.
Sulphur dioxide is an intermediate towards producing Sulphurous acid. It is a chemical compound produced by volcanoes, burning coal and petrol, such fumes passing into the air produce acid rain thus are a cause for being an environmental concern.
Sulphurous acid is formed when sulphur dioxide dissolves in water. Sulphuric acid is formed when sulphur trioxide dissolves in water. Over time, sulphurous acid will oxidize to sulphuric. SO2 + H20 -> H2SO3 (sulphurous acid) SO3 + H20 -> H2SO4 (sulphuric acid)