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.
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