- When the rain water contains large quantities of acids like nitric acid and sulphuric acid formed by dissolution of oxides of nitrogen and sulphur in water it is called as acid rain.
- Term acid rain was coined by Robert August (1872).
- Wet deposition: it is formed of acidic water received on soil through rain, fog and snow.
- Dry deposition: It is formed of wind-blown acidic gases and particles in the atmosphere which fall on ground with rain water.
Two forms of Acid Deposition
About half of acidity in the atmosphere comes to earth through dry deposition. On moist surface of soil, these gases dissolve in water and form acids as in the acid rain.
Composition of Acid Rain
- Acid rain consists 60-70% of oxides of sulphur (e.g., SO, and SO3) and 30-40% of oxides of nitrogen (e.g., NO2 and NO3). Oxides of nitrogen are produced due to combustion process of fossil fuels at high temperatures in industries, automobiles, nitrogen fertilizer plants; while oxides of sulphur are mainly produced due to burning of coal, ore-smelters and oil refineries.
- A study has shown that main sources of oxides of sulphur are volcanoes (67%), industries (22%), vehicles and forest fires. Presence of acids in the rain water lowers its pH value. Rainfall pH value has been estimated to be in between 5.6 to 6.5 in Bangalore, Delhi, Nagpur and Pune. Average pH in rainfall over eastern United States during 1979-1980 was less than 5.0.
Harmful Effects of Acid Rain
- Acid rain damages a number of heritage monuments like Statue of Liberty of New York and Taj Mahal of Agra. Due to acid rain, marble of Tai is changing to magnesium sulphate.
- Acid rain below 5 pH causes death of planktons, molluscs and fish in the aquatic systems so disturbs the food chains. It also causes reproductive failure in the fish.
- Acid rain also kills the useful soil microbial community so disturbing the terrestrial ecosystems.
- Acid rain also damages the foliage so decreasing the growth and yield of the plants.
- Trees become more susceptible to diseases, low temperature, drought, pathogens, etc.
Control of Acid Rain
- Cleaning systems can be fitted into the industrial chimneys to remove harmful gases.
- Use of sulphur-free or low sulphur petroleum in the automobiles.
- Polymer coating of the pipes supplying the drinking water.