ACID RAIN Formation of acid rainAcid deposition, more commonly known as acid rain, occurs when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere with water, oxygen, and oxidants to form acidic compounds. This mixture forms a mild solution of sulphuric and nitric acid which then falls to the earth in either wet (rain, snow, sleet or fog) or dry (gas and particles) form. Approximately one-half of the atmosphere's acidity falls back to earth through dry deposition in the form of particles and gases, and are then spread hundreds of miles by winds where they settle on surfaces of buildings, cars, homes, and trees. When acid rain falls, the dry deposited gases and particles are sometimes washed from buildings, trees and other surfaces making the runoff water combine with the acid rain more acidic than the falling acid rain alone. This new combination is referred to as acid deposition. The runoff water is then transported by strong prevailing winds and public sewer systems into lakes and streams. Although some natural sources such as volcanic eruptions, fire and lightening contribute to the emissions of sulfur dioxide and nitrogen oxides in the atmosphere, more than 90% is the result of human activities such as coal burning, smelting of metals such as zinc, nickel and copper, and the burning of oil, coal and gas in power plants and automobiles. When does rain become acidic? Scientists determine whether rain or lake water is acidic by measuring its pH (the measure of acidity or alkalinity of a solution on a scale of 0 to 14). A value of 7 is considered neutral, whereas values less than 7 are acidic and values over 7 are alkaline or basic. A change of one unit on the pH scale represents a factor of ten in acidity; for example, a solution with a
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pH of five is ten times as acid as one with a pH of six. Normal or clean rainfall--without pollutants--is slightly acidic due to carbon dioxide, a natural gas in the air that dissolves in water to form weak carbonic acid. But rain, snow, or other moisture is not called "acid rain" until it has a pH value below 5.6. Rainfall in eastern North America is often acidic with a pH of 4 to 5. Why is North America greatly at risk? Acid rain is more common in the Eastern U.S. and Canada than in the Western U.S. because emissions ...

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