by Michael Riley

Created on: January 24, 2009   Last Updated: February 08, 2009

Acid rain is a very destructive product of chemical pollutants dispersed into the atmosphere from various sources. Industrial pollution, vehicle emmissions, wildfires and even volcanoes contribute to the chemical process that lowers the pH of atmospheric precipitation to harmful levels. As the pH lowers and moisture in the atmoshere becomes more acidic the damage becomes evident where it returns to Earth.

Rain drops are naturally acidic. The typical pH of a "clean" raindrop or snowflake is about 5.2 pH units. This is a result of chemicals naturally present and carbon dioxide reacting with water to form carbonic acid. Carbonic acid is a "weak" acid that, in naturally occurring quantities, is not harmful to the environment. When rain is contaminated with Sulphur (S) and Nitrogen (N) oxides, the pH is lowered further and is now considered "acid rain".

Sulphur Dioxide is produced by the burning of fossil fuels like coal and oil. The biggest contributor is the burning of coal. As the coal is burned, sulphur dioxide is emitted into the atmosphere where it reacts with moisture that is present there. When the sulphur dioxide combines with water, it creates sulfuric acid. Sulfuric acid is a "strong" acid which further reduces the pH of atmospheric precipitation.

Nitrogen Dioxide is generally produced by burning fuel with internal combustion engines. As the use of automobiles has increased throughout the world, the amount of nitrogen dioxide emitted into the atmosphere has increased exponentially. When nitrogen dioxide enters the atmosphere it reacts with water to form nitric acid. Like sulfuric acid, nitric acid is a "strong" acid that also lowers the pH of moisture that is eventually deposited on Earth.

In the early stages of the Industrial Revolution, the effects of acid rain were evident in localities surrounding factories that used coal as an energy source. At that time, the smoke stacks or flues used to disperse the combustion gases were not very tall and the sulfuric acid produced was deposited very near the place it was generated. This was evident with the local degradation of areas in England and New York state during the early and mid-1900s. The pollution was also apparent to the local residents as health problems rose from breathing the contaminated air. This can still be seen in areas like Malaysia, Thailand, parts of India and China.

As the effects of acid rain became known, smoke stacks and flues were built taller and hence, the sulfuric acid produced was dispersed

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