Acid rain refers to all types of precipitation--rain, snow, sleet, hail, fog--that is acidic in nature. Acidic means that these forms of water have a pH lower than the 5.6 average of rainwater. Acid rain kills aquatic life, trees, crops and other vegetation, damages buildings and monuments, corrodes copper and lead piping, damages such man-made things as automobiles, reduces soil fertility and can cause toxic metals to leach into underground drinking water sources. .
Rain is naturally acidic because carbon dioxide, found normally in the earth's atmosphere, reacts with water to form carbonic acid. While "pure" rain's acidity is pH 5.6-5.7, actual pH readings vary from place to place depending upon the type and amount of other gases present in the air, such as sulphur oxide and nitrogen oxides. .
The term pH refers to the free hydrogen ions (electrically charged atoms) in water and is measured on a scale from 0 to 14. Seven is considered neutral and measurements below seven are acidic while those above it are basic or alkaline. Every point on the pH scale represents a tenfold increase over the previous number. Thus, pH 4 is 10 times more acidic than pH 5 and 100 times more so than pH 6. Similarly, pH 9 is 1O times more basic than pH 8 and 100 times more basic than pH 7. .
The acid in acid rain comes from two kinds of air pollutants-- sulphur dioxide (SO2) and nitrogen oxides (NOx). These are emitted primarily from utility and smelter "smokestacks" and automobile, truck and bus exhausts, but they also come from burning wood. .
When these pollutants reach the atmosphere they combine with gaseous water in clouds and change to acids--sulphuric acid and nitric acid. Then, rain and snow wash these acids from the air. Acid rain affects lakes, streams, rivers, bays, ponds and other bodies of water by increasing their acidity until fish and other aquatic creatures can no longer live. As undecomposed organic leaf-litter increases, owing to the loss of bottom-dwelling bacteria, toxic metals such as aluminum, mercury and lead within the litter are released.