Balanced chemical equation: S(s) + O₂(g) → SO₂(g).
Sulfur change oxidation number from 0 to +4 (oxidation) and oxygen change oxidation number from 0 to -2 (reduction).
Sulfur dioxide (SO₂) is very common volcanic gas.
Fossil fuel combustion increases the acidity of rain because the sulfur dioxide is produced.
Because of fuel combustion, sulfur dioxide goes up into the atmosphere as the hot gases rise, than it reacts with water and oxygen in the air and form sulfuric acid.