About 90% of U.S. energy comes from fossil fuel combustion. Fossil Fuels include petroleum, natural gas, and coal. Some fossil fuels, especially coal, contain significant amounts of sulfur impurities. During combustion, these impurities react with oxygen to from SO2, In addition, during combustion of any fossil fuel, nitrogen from the air reacts with oxygen to from NO2. The SO2 and NO2 emitted from fossil fuel combustion reacts with water in the atmosphere to form sulfuric acid and nitric acid.
These acids combine with rain to form acid rain. In the United States, the problem is greatest in the northeastern portion of the country because many midwestern power plants burn coal. The sulfur and nitrogen oxides produced from coal combustion in the Midwest are carried toward the Northeast by natural air currents, making rain in that portion of the country significantly acidic.
Rain is naturally somewhat acidic because of atmospheric carbon dioxide. Carbon dioxide combines with rainwater to form carbonic acid.
However, carbonic acid is a relatively weak acid. Even rain that is saturated with Co2 has a pH of only about 5.6, which is mildly acidic. However, when nitric acid and sulfuric acid mix with rain, the pH of the rain can fall as low as 4.4 (Figure 20). Remember that, because of the logarithmic nature of the pH scale, rain with a pH of 4.4 has an [H3O+] about 16 times greater than that of rain with a pH of 5.6. Rain that is this acidic has negative consequences for the environment.
(Figure 20) Acid rain in the United States Average pH of precipitation in the United States the for 2008