Acid rain is a generic term that can include acidic fog and acidic snow. Acid rain was essentially "discovered" in Great Britain in the 1800's when scientists found that precipitation was more acidic than "natural" rainwater. The concept of acid rain was rediscovered in the early 1970's.

"Natural" rain water is already slightly acidic, with a pH of 5.6. This is because CO2 (carbon dioxide) dissolves into the rainwater and produces carbonic acid.  Any precipitation that has a pH less than 5, is considered "acid rain." The two principal acids in acid rain are H2SO4 (sulfuric acid) and HNO3 (nitric acid). The source of sulfuric and nitric acids are SO2 (sulfur dioxide) and NOx (nitrogen oxide). Under the term "acid deposition" defined later in this document, "dry deposition" consists of sulfur dioxide gas and nitrogen salts. Both can cause the same damage as "wet deposition." Other emissions that affect acidity are hydrochloric acid, ammonia, volatile organic compounds and alkaline dust.

Natural sources of sulfur oxides are volcanic eruptions and the decomposition of organic matter.  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. Humans are the cause of 90% of sulfur emissions and 95% of nitrogen emissions in eastern North America. Sulfur dioxide is released mainly by the combustion of coal for electrical generating plants, the refinement of oil and sometimes from natural gas wells.

Nitrous oxides are released mainly by the burning of fuels at high temperatures. About 40% of these anthropogenic nitrous oxides are from cars, trucks, buses and trains, while 25% results from thermoelectric generating plants and 35% result from industrial combustion processes.  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 1970's and 1980's, Canada and the United States did not have a joint agreement in emission controls. In 1984, Canada designed a program to reduce emissions by 50%. 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 be precipitated somewhere else. Thus, it was essential for the US to curb emissions so that acid rain would not fall on southern and eastern Canada. In fact, 50% of the sulfates deposited on Canada are originally from the U.S. It was ironic for industrial plants to build extremely tall smokestacks because it made local pollution problems into regional problems. In the Ohio Valley, coal-burning power plants with 700 -1000 foot stacks allowed pollution to travel 1000 miles to form acid rain in Canada, the Adirondack Mountains and New England.

Acid rain is still a very large environmental problem that the world is trying to solve. Until the atmospheric release of pollutants is stopped, acid rain will continue to fall and become one of the greatest examples of the destructive behavior of mankind.

Source: U.S. EPA Airmarkets and Acid Rain Project websites

What are the factors that contribute to the formation of acid rain?

Scientists have discovered that air pollution from the burning of fossil fuels is the major cause of acid rain.  The main chemicals in air pollution that create acid rain are sulfur dioxide (S02) and nitrogen oxides (NOx). Acid rain usually forms high in the clouds where sulfur dioxide and nitrogen oxides react with water, oxygen and oxidants. This mixture forms a mild solution of sulfuric acid and nitric acid.  Sunlight increases the rate of most of these reactions. Rainwater, snow, fog and other forms of precipitation containing those mild solutions of sulfuric and nitric acids fall to earth as acid rain.

Rain does not account for all of the acidity that falls back to earth from pollutants. About one-half of the acidity in the atmosphere falls back to earth due to dry deposition as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes and trees. In some instances, the gases and particles can eat away the things on which they settle. Dry deposited gases and particles are sometimes washed from trees and other surfaces by rainstorms. When this happens, the runoff water combines those acids with the acid rain, making the combination more acidic than the falling rain alone. The combination of acid rain plus dry deposited acid is called acid deposition.

Nitrogen Oxides

The highly reactive Nitrous Oxide formed above reacts with the oxygen diatomic molecule to produce Nitrous Dioxide:
2NO(g) + 0₂ ===> 2NO₂(g)

Sunlight and hydroxide radical combine to form nitric acid:
NO₂(g) + OH(g) ===> HN0₃(g)

Sulfur Dioxides

Carbon in coal burns with oxygen to from carbon dioxide:
C(in coal) + 0₂(g) ===> C0₂(g)

Sulfur in coal also burns in oxygen to form sulfur dioxide:
S(in coal) + 0₂(g) ===> S0₂(g)

Sulfur dioxide reacts with oxygen in the atmosphere to form Sulfur trioxide:
2 S0₂(g) + 0₂(g) ===> 2 S0₃(g)

Sun-light and hydroxide radical combine to from sulfuric acid:
SO₂(g) + 2 OH(g) ===> H₂SO₃(g)

Source: U.S. EPA Airmarkets and Acid Rain Project websites

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