Acid Rain

Acid rain fall-out of acids caused by sulphur dioxide and nitrogen oxides with rainfall and thus increases in the amount of acidity of rainwater. As pointed   out above rainwater is not pure because atmospheric carbon dioxide is dissolved in the rainwater which thus because moderately acidic, the pH being generally 5. The water with Ph value of 7.0 is called neutral water whereas the pH value below it makes the water alkaline. The water becomes more injurious when the pH fall below 4.

Sulphur dioxide emitted from man made sources in large quantities into the atmosphere combines with water to form sulphur and sulphuric acids (H2SO4). These acids fall on the earth's surface with rainfall and thus the fall of highly acidic water are called acid rain. The pH of rainwater in some localities of the USA mainly in West Virginia has gone down to 1.5 pH of rainwater in Europe is as low as 2.4 whereas the normal range of pH of neutral water must be 7.0.

The main sources of acid rains are oxides of sulphur and nitrogen which are emitted from industrial establishments and different types of vehicles. These pollutions are spread in atmosphere by wind and from acids after reacting with water in the atmosphere. It may be pointed out that acid rains are not confined to the source areas of the emissions of oxides of sulphur and nitrogen rather they cover much larger areas far away from the source pollutants being in gaseous phase are carried away and spread over larger areas through winds and clouds. For example, oxides of sulphur and nitrogen spewed from several mills in Germany and U.K have caused widespread acid rains in Scandinavia countries with the result most of the lakes in Scandinavia countries have lost their biological communities and are now biologically termed as dead lakes.

Dead lake means destruction and death of all aquatic lives including plants and animals in lakes, ponds, and rivers. Acid rains also affect human community adversely, though the mode of human disease and death due to acid rains has not been properly understood as yet. The productivity of soils is also significantly lowered because of acid rains as increased acidity destroys mineral elements and other nutrients of the soils.

Acid rain is not a local problem or a problem of a particular country, rather it is an international problem because air pollutants emitted from a particular country are carried in the atmosphere by the winds and are spread over other countries. Thus the problem of acid rains should be tackled at international level.