ACID RAIN-AN INVISIBLE THREAT

Abstract

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.