Oxides of sulfur and nitrogen combine with water in the atmosphere to form acid rain. Non-metal oxides are acid anhydrides, which form acids in combination with water. Sulfur oxides produces sulfuric acid and sulfurous acid, H2SO4 and H2SO3. Nitrogen oxides produce nitric acid and nitrous acid, HNO3 and HNO2. Acid rain has a pH lower than 7 (7 is neutral) and is a source of hydrogen ions that cause corrosion of metals, rocks and marble and limestone building materials. It also affects the pH of lakes and streams which has an adverse affect on aquatic life.
Sulfur and nitrogen oxides show up in the atmosphere as a result of human activity. Burning of coal to produce electricity releases sulfur oxides. Automobile exhaust is another source of chemicals that cause acid rain. The production of nitrogen oxides from petroleum exhaust is catalyzed by ultraviolet light, hence the name photochemical smog.