Acid rain is formed when nitrogen oxides or sulfur oxides in the atmosphere react with suspended water droplets and produce acids. The chemical reaction equation for the production of acid rain resulting from sulfur oxides is: SO2 + HOH -> H2SO3. The reaction equation for acid rain produced from nitrogen oxides is: 2NO2 + HOH -> HNO2 + HNO3.
Acid rain refers to any kind of precipitation that transports nitrogen and sulfur compounds to the Earth's surface. The nitrogen oxides and sulfur oxides that form acid rain come from both man-made and natural sources.
The extreme atmospheric temperatures that are produced by lightning during thunderstorms are a natural source of nitrogen oxides. Sulfur oxides in the atmosphere result from decaying vegetation at the Earth's surface and from volcanic eruptions.
Man-made sources for the sulfur and nitrogen compounds that form acid rain are the by-products resulting from the burning of fuels for industry, electricity production and transportation. The United States Environmental Protection Agency estimates that the majority of the sulfur dioxide that forms acid rain comes from power plants generating electricity.