Acid Rain

Scientists have discovered that air pollution from the burning of fossil fuels is the major cause of

acid rain. Acidic deposition, or acid rain as it is commonly known, occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds. This mixture forms a mild solution of sulfuric acid and nitric acid. Sunlight increases the rate of most of these reactions.

These compounds then fall to the earth in either wet form (such as rain, snow, and fog or dry form (such

the atmosphere falls back to earth through dry deposition as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. In some instances, these 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 that happens, the runoff water adds those acids to 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. Prevailing winds transport the compounds, sometimes hundreds of miles, across state and national borders.

Rainwater once was the purest form of water available but now is often contaminated by pollutants in the air. Acid rain is caused when industrial emissions mix with atmospheric moisture. Pollutants may be carried in clouds for long distances before falling, which means that forests and lakes far away from factories may be damaged by acid rain. In the near vicinity of the factories, additional damage is caused by deposition, a kind of dry rain in which the larger pollutant particles fall to the ground. Air pollution has been increasing since the Industrial Revolution but only recently have side effects such as acid rain become severe and widespread enough to evoke international concern.