Acid rain means the presence of excessive acids in rain water. The pH of acid rain generally varies from 3 to 6. Acid rain mainly contains H2So4 and HNO3 along with weak H3CO3. The oxides of sulphur, nitrogen and carbon (NOx, Sox and Cox) interact with other components of the atmosphere, and form corresponding acids.
Sources of oxides:
- Burning of fossil fuels
- Thermal power plants
- Textile industry
- Steel plants
- Oil refining
- Automobile exhaust
- Explosive industry
- Coal and gas fired furnace
- Fertilizer industry
Adverse Effect of Acid Rain:
(i) Effects on aquatic biota
(a) Many aquatic species disappear due to the acidification of lakes.
(b) Acid rain reduces the production of phytoplankton in the lake.
(c) Many bacteria and blue-green algae are killed due to acidification.
(d) Activity of bacteria and many other microscopic animals are reduced in acidic condition. Thus the dead materials and many other accumulated materials lying at the bottom of the lakes and not decomposed rapidly. Therefore essential nutrients like nitrogen and phosphorous and locked up and less available for use.
(ii) Effects on terrestrial eco-system
(a) Leaves of plants and trees are damaged and become yellow and brown, retarding photo synthesis.
(b) Acid rain weakens the structure of trees, thereby making in vulnerable to pathogens.
(c) The reduction in photosynthesis results in reduced agricultural productivity.
(d) Many essential nutrients like Ca, Mg, S, Fe, Mo, Co are leached out due to acid rain, hampering productivity.
(iii) Effects on building:
(a) Acid rain causes extensive damage to building and sculpture materials of marbles, lime stone, slate, mortar etc. These materials become weakened mechanically as the soluble sulphates are leached out by rain water.
(b) Acid rain attacks monuments, statues, bridges etc. This is one of the reasons why the Taj Mahal is in danger.
(iv) Effects on human beings
(a) Solubilization of heavy metals like Cd, Hg and Cr due to acid water may reach human body via the plants and animals in the food chain or through drinking water supplies.
(b) Acid rain causes respiratory and skin diseases and may attack nervous system in the extreme case.
Control measures for acid rain:
- Short term control of acid deposition problem can be achieved by using lime (CaO).
- The major step can be taken to reduce acid forming gas like NOx and SO2 and if possible, to stop it.
- General public awareness should be created regarding the ill effects of environmental pollution and consequences of acid rain.