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. Aquatic plants grow best between pH 7.0 and 9.2 (Bourodemos). As acidity increases (pH numbers become lower), submerged aquatic plants decrease and deprive waterfowl of their basic food source. At pH 6, freshwater shrimp cannot survive. At pH 5.5, bottom-dwelling bacterial decomposers begin to die and leave undecomposed leaf litter and other organic debris to collect on the bottom. This deprives plankton–tiny creatures that form the base of the aquatic food chain–of food, so that they too disappear. Below a pH of about 4.5, all fish die.
Acid rain harms more than aquatic life. It also harms vegetation. The forests of the Federal Republic of Germany and elsewhere in Western Europe, for example, are believed to be dying because of acid rain. Scientists believe that acid rain damages the protective waxy coating of leaves and allows acids to diffuse into them, which interrupts the evaporation of water and gas exchange so that the plant no longer can breathe. This stops the plant’s conversion of nutrients and water into a form useful for plant growth and affects crop yields.
Perhaps the most important effects of acid rain on forests result from nutrient leaching, accumulation of toxic metals and the release of toxic aluminum. Nutrient leaching occurs when acid rain adds hydrogen ions to the soil which interact chemically with existing minerals. This displaces calcium, magnesium and potassium from soil particles and deprives trees of nutrition.