Acid Rain

Acid Rain

We are aware that normally rain water has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rain water with carbon dioxide present in the atmosphere.

H₂O (I)   +   CO₂ (g)    ⇌    H₂CO₃ (aq)

H₂CO₃ (aq)    ⇌    H⁺(aq)   +   HCO₃^-(aq)

When the pH of the rain water drops below 5.6, it is called acid rain.

Acid rain refers to the ways in which acid from the atmosphere is deposited on the earth’s surface. Oxides of nitrogen and sulphur which are acidic in a nature can be blown by wind along with solid particles in the atmosphere and finally settle down either on the ground as dry deposition or in war, fog and snow as wet deposition.

Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere. As mentioned earlier, burning of fossil fuels (which contain sulphur and nitrogenous matter) such s coal and oil in power stations and furnaces or erol and diesel in motor engines produce sulphur dioxide and nitrogen oxides. SO₂ and NO₂ after oxidation and reaction with water are major contributors to acid rain. Because polluted air usually contains particulate matter that catalyse the oxidation.

2SO₂ (g)   +   O₂(g)   +   2H₂O (I)    →   2H₂SO₄ (aq)

4NO₂ (g)   +   O₂ (g)   +   2H₂O (I)    →    4HNO₃ (aq)

Ammonium salts are also formed and can be seen as an atmospheric haze (aerosol of fine particles). Aerosol particles of oxides or ammonium salts in rain drops result in wet deposition. SO₂ is also absorbed directly on both solid and liquid ground surfaces and is thus deposited as dry-deposition

Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth. It causes respiratory ailments in human beings and animals. When acid rain falls and flows as ground water to reach rivers, lakes etc. it affects plants and animal life in aquatic ecosystem. It corrodes water pies resulting in the leaching of heavy metals such as iron, lead and copper into the drinking water. Acid rain damages buildings and other structures made of stone or metal. The Taj Mahal in India has been affected by acid rain.

Taj Mahal and Acid Rain

The air around the city of Agra, where the TAj Mahal is located, contains fairly high levels of sulphur and nitrogen oxides. It is mainly due to a large number of industries and power plants around the area. Use of poor quality of coal, kerosene and firewood as fuel for domestic purposes add up to this problem. The resulting acid rain reacts with marble, CaCO3 of Taj Mahal.

CaCO₃   +   H₂SO₄    →   CaSO₄   +   H₂O   +   CO₂

Causing damage to this wonderful monument that has attracted people from around the world. As a result, the monument is being slowly disfigured and the marble is getting discolored and lusterless. The Government of India announced an action plan in early 1995 to prevent the disfiguring of this historical monument. Mathura refinery has already taken suitable measure to check the emission of toxic gases.

This plan aims at clearing the air in the ‘Taj Mahal’ an area that includes the towns of Agra, Firozabad, Mathura and Bharatpur. Under this plan more than 2000 polluting industries lying inside the trapezium would switch over to the use of natural gas or liquefied petroleum gas instead of coal or oil. A new natural gas pipeline would bring more than half a million cubic meters of natural gas a day to this area. People living in the city will also be encouraged to use liquefied petroleum gas in place of coal, kerosene or firewood, Vehicles plying on highways in the vicinity of Taj would be encouraged to use low sulphur content diesel.