The two principal pollutants that contribute to acid rain are sulfur dioxide (SO2) and nitrogen oxides (NOx). These gases are released into the atmosphere primarily by power plants, factories, and automobiles that burn fossil fuels. Sulfur dioxide and nitrogen oxides can travel hundreds to thousands of miles in the atmosphere before they return to the Earth in acidic precipitation, either wet (i.e., rain or snow) or dry (i.e., particles).