ACID RAIN-AN INVISIBLE THREAT

Acid Rain is defined as precipitation (rain) that has a pH lower than 5.6, which is the pH expected in distilled water exposed to the atmosphere. (pH is a measure ot the acidity or alkalinity of a water sample.) The pH Of precipitation undoubtedly is affected by a variety of natural sources of acidic and alkaline materials (e.g. volcanic gases, gases from decompoSng Organic matter and soil dust). However it has recently become apparent that rain and snow in certain regions of the earth are consistently more acidic than expected. The European Atmosphere Chemistry Net Work first recognized that the pH of precipitation was declining in Scandinavia during the late 1960's. Current data indicates that the mean annual pH in this region was 5.0 - 5.5 in the late 1950's which declined to 4.2 • 4.4 in the mid 1970's. In Eastern North America precipitation is now more acidic than in Scandinavia. The median pH for 1978 • 1979 ranged from 4.0 to 4.4 in North Eastern U.S. and South Eastem Canada. Although there is disagreement over the Source and nature Of acidic precipitation , the most widely accepted view is that the increased acidity is a result of the presence Of increased quantities Of sulphuric and nitric acids. These acids are believed to result from oxidation Of sulphur and nitrogen oxide gases. Oxides of sulphur and nitrogen are produced from combustion of fossil fuels, metal smelting and various industrial processes