The natural sources of nitrogen oxides are bacterial action in soils and in chemical reactions in the upper atmosphere. However, natural sources only account for a small percentage of the pollutants. Sulfur dioxide is released mainly by the combustion of coal for electrical generating plants, the refinement of oil and sometimes from natural gas wells. The nitrous oxides are released mainly by the burning of fuels at high temperatures. Thus, whatever pollutants are released into the atmosphere, must come down, and acid rain is the mechanism.
Acid rain is a pollution problem because it transcends political boundaries.
After much research and understanding of the acid rain problem in the 70s and 80s, Canada and the United States did not have a joint agreement in emission controls. However, the US held off making any policies until late 1990 after amendments to the Clean Air Act . Atmospheric pollution can be transported large distances by air masses and then precipitated somewhere else. It was ironic for industrial plants to build extremely tall smokestacks because it made local pollution problems become a regional one. Acid rain is still a very large environmental problem that the world is trying to solve.