From Microsoft Encarta '98 Deluxe CD-ROM
Acid rain is the term given to all forms of acid precipitation (rain, hail, snow, fog etc). It is caused when pollutants (mainly sulfur and nitrogen oxides) are converted to acids in the atmosphere when mixed with water vapour making the atmosphere acidic. Rain is naturally weakly acidic due to the carbon dioxide in the atmosphere being converted to carbonic acid. However nitric and sulfuric acid are stronger acids than carbonic acid thus causing a problem. The equations below show the reactions that occur:-
Sulfur and nitrogen oxides are found naturally in the atmosphere but due to man's activities, especially the combustion of fossil fuels, the increased concentration results in acid rain. Acid rain is a global problem as the acidic clouds can be blown 1000's of kilometre's before precipitation occurs. The effects of acid rain are damage to forests, fresh water lakes, buildings, wildlife and soils. Acidification of lakes results in a decrease in species diversity as fewer and fewer species can survive as the pH decreases. Trees are not killed directly but are weakened by leaf damage or poisoning by toxic metals. Acid rain tends to result in toxic metal ions being released from the soil into lakes or soil water thus killing fish and trees. The diagram below illustrates the process:-
From Microsoft Encarta '98 Deluxe CD-ROM
For information on the international agreements to try and stop acid rain go to Environmental Regulations