When emitted SO₂ and NO_X react with water, oxygen, and other chemicals in the atmosphere and form sulfuric and nitric acids. SO₂ and NO_X can be carried by wind over long distances, so acid rain can occur in places far away from where emissions are occurring.

As the sulfuric and nitric acids in the atmosphere mix with water and other materials, they then fall as precipitation in wet (rain, snow, hail, fog) or dry (dust) forms. Normal rain has a pH level of about 5.6, where acid rain has a lower pH of about 4.2 to 4.4.

Emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_X) predominantly come from the burning of fossil fuels, including sources like electric power generators, vehicles and heavy equipment, oil refineries, and other industries. Small amounts of SO₂ and NO_X are generated from natural sources such as volcanoes.

When acid falls to the earth’s surface as wet or dry deposition, it can cause harmful effects on lakes, streams, soils, and forests. Even dry deposition of sulfur dioxide and nitrogen oxide can harm vegetation and wildlife, as they change into acid when they contact water and flow onto the ground.