Water is important for various uses of humans. This water is mainly received from rainfall.
During present times, sulphur dioxide (SO2) emitted from man-generated sources, reaches the atmosphere and forms sulphate and sulphuric acid (H2SO4) after mixing with water.
When it reaches the ground along with rain, it is called acid rain.
Acidity of water is measured in terms of pH. When pH of water is 7, it is called neutral water, pH of normal rainfall is 5, but when this standard is less than 4, it is harmful for biotic community, pH of rain water would reduce in proportion to increase in concentration of sulphur dioxide and nitrogen oxide in the atmosphere.
Scientists believe that the effect of acid rain is not limited only to areas where sulphur dioxide and nitrogen oxide are emitted by industrial and transportation sources, but it affects areas far and wide because pollutant factors (eg., sulphur dioxide) are in gaseous form which spread with air and clouds in far off areas.
For example, sulphur dioxide and nitrogen oxides emitted by mills located in England and Germany cause acid rain in Norway and Sweden where biotic life has vanished in most of the lakes. They are called ‘Dead Lakes’ from the biotic point of view.
Oxide emitted by industries of United States of America is visible in the form of acid rain in Mexico, while smoke emitted from the fire ignited in natural oil wells by Iraq during the Gulf War, had an effect on Iran, and Jammu and Kashmir in India. CO2 existing in carbon smoke came along with rain water in Jammu and Kashmir, and it remained frozen in the midst of ice layers.
Acid rain has assumed dangerous dimensions in industry dominated countries like United States of America and Western Europe. Acid rain is also sometimes called ‘Lake Killer’ because acid rain is the main factor responsible for the death of many water creatures in different water sources like lakes and other big and small water reservoirs.
Thousands of lakes of Ontario province of Canada are affected by acid rain and 140 lakes have been declared as unsuitable for biotic life and have been declared ‘dead lakes’, pH value of their water has become extremely low. According to Hamilton of United States of America, 7,500 to 12,000 people die and lakhs of people suffer from skin cancer and related diseases in the world on account of acid sulphate generated by burning of fossil fuels like coal and petroleum.
Soil productivity is also affected by acid rain. Minerals and other nutritive elements of the soil are destroyed due to acidity. Acid rain has its impact on forests also. It has destroyed many forests of North America (United States of America, Canada, Mexico, and the Western Island group), and European countries (Norway, Sweden, Germany, and Central European Countries).
The growth of leaves and roots of trees is hindered by the acid. Ancient historical buildings constructed of marble or red stone are also affected by acid rain. Harm is caused to these buildings due to corrosion on account of acid rain. In India, Taj Mahal, Moti Masjid, Red Fort etc., are facing damage on account of acid rain. Effect of sulphur dioxide emitted from Mathura Oil Refinery is visible in the form of yellowness on the marble walls of Taj Mahal.
The problem of acid rain is not so serious in India as compared to that in America or in other western countries. Researchers at Bhabha Atomic Research Centre and World Meteorological Organization have found that acidity of rain water in most of the Indian cities is less than the safe limit. In a few big cities (Delhi, Bombay, Chennai and Kolkata), increase in level of sulphur has been found in atmosphere.