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 pHlower than the 5.6 average of rainwater. Acid rain kills aquatic life, trees,crops and other vegetation, damages buildings and monuments, corrodes copper andlead piping, damages such man-made things as automobiles, reduces soil fertilityand can cause toxic metals to leach into underground drinking water sources.
Rain is naturally acidic because carbon dioxide, found normally in the earth’satmosphere, 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 placedepending upon the type and amount of other gases present in the air, such assulphur 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 thoseabove it are basic or alkaline. Every point on the pH scale represents a tenfoldincrease over the previous number. Thus, pH 4 is 10 times more acidic than pH 5and 100 times more so than pH 6. Similarly, pH 9 is 1O times more basic than pH8 and 100 times more basic than pH 7. The acid in acid rain comes from two kindsof air pollutants– sulphur dioxide (SO2) and nitrogen oxides (NOx). These areemitted primarily from utility and smelter “smokestacks” andautomobile, truck and bus exhausts, but they also come from burning wood. Whenthese pollutants reach the atmosphere they combine with gaseous water in cloudsand change to acids–sulphuric acid and nitric acid.
Then, rain and snow washthese acids from the air. Acid rain affects lakes, streams, rivers, bays, pondsand other bodies of water by increasing their acidity until fish and otheraquatic creatures can no longer live. Aquatic plants grow best between pH 7.
0and 9.2 (Bourodemos). As acidity increases (pH numbers become lower), submergedaquatic plants decrease and deprive waterfowl of their basic food source. At pH6, freshwater shrimp cannot survive. At pH 5.
5, bottom-dwelling bacterialdecomposers begin to die and leave undecomposed leaf litter and other organicdebris to collect on the bottom. This deprives plankton–tiny creatures thatform 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 andelsewhere in Western Europe, for example, are believed to be dying because ofacid rain. Scientists believe that acid rain damages the protective waxy coatingof leaves and allows acids to diffuse into them, which interrupts theevaporation 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 forplant growth and affects crop yields.
Perhaps the most important effects of acidrain on forests result from nutrient leaching, accumulation of toxic metals andthe release of toxic aluminum. Nutrient leaching occurs when acid rain addshydrogen ions to the soil which interact chemically with existing minerals. Thisdisplaces calcium, magnesium and potassium from soil particles and deprivestrees of nutrition.