Acid rain is the general term used to describe the removal, by rainfall, of acidic pollutants from the atmosphere. Acids also can be removed by other forms of precipitation, such as snow or fog. Acid pollutants may also fall as dry particles or gases that form acids when later combined with moisture. The term acid deposi-
4 sources of acidity 895
tion is used to include all the possible forms of acid pollutant removal from the atmosphere, but acid rain remains the popular term.
The majority of the deposited acids are nitric acid and sulfuric acid. In some of the more rural regions of the world, organic acids also are important. In very remote areas where the level of acid is low, the "natural background" acid is carbonic acid, which is associated with carbon dioxide in the air. The overall acidity of precipitation also depends on the basic (or alkaline) constituents of the precipitation. Major bases include ammonia and geologic materials, such as dust and fly ash.
Acidity is measured in terms of a pH scale, which is a measure of the log of the hydrogen ion concentration in the precipitation. The scale runs from 0 to 14 with 0 being very acidic and 14 being very alkaline. A midscale value of 7 is considered neutral. A change in 1 pH unit indicates 10-fold increase or decrease in acidity. Unpolluted rain water is considered to have a pH of about 5.6. This acidity is assumed to contain only carbonic acid. In the highly polluted eastern states of the United States., the average acidity of water has a pH between 4 and 5. Even in some remote areas of the world, pH values of 5.2 have been found. These acidity levels suggest that there is a long-range transport of nitrogen and sulfur chemicals.
Was this article helpful?