Q.18 Explain four major
air pollutants and their consequences.
(AKTU. - 2006-07)
Related Questions -
Q. Discuss the
natural and man made (synthetic) pollutant that cause air pollution. (AKTU. - 2009 - 10)
Ans. Major Air
Pollutants: - (AKTU. -2010 - 11)
(i) Carbon Dioxide (CO2): -
It is a colourless, tasteless, odourless
and non-combustible gas that provides the carbonation in soft drinks and
beverages. At present the carbon dioxide concentration in the atmosphere is
approximately 350 ppm. At this concentration, it has no known harmful effects
to humans. It is totally necessary for photosynthesis (i.e., production of
organic matter), as represented by the following chemical equation:
Photosynthesis
equation
It is produced when organic
matter (or carbon) is burnt. Animal (including human) bodies produce carbon
dioxide as they utilize foods; and it leaves their bodies in exhaled breath.
Sources and Sinks: -
The major reservoirs of carbon dioxide are
atmosphere, soil, vegetation, oceans and fossil fuel reserves which contain
735, 1500, 560, 36000 and 5000-10000 billion metric tons of CO2 respectively.
On a geological scale, the
process for removing CO2 from the atmosphere is
which
occurs in oceans, depositing solid CaCO3 (limestone or
some other variants, such as, calcite, etc.) on the ocean floor. Oceans are,
thus, the sinks of atmospheric carbon dioxide.
Effect: -
Carbon dioxide, at natural level of concentration, has
no harmful effects. Being a radiatively active gas (RAG) or green house gas
(GHG), it helps in raising the equilibrium temperature of the earth; thereby,
making life on earth possible. Increased emmissions of CO2 are the largest
single cause of global warming.
Control: -
In order to slow or stop the build up of CO2 in the
atmosphere, the use of fossil fuel should be reduced and we should switch to
solar, wind, hydroelectric, geothermal and nuclear energy which do not release
CO2.
(ii) Carbon
Monoxide (CO): -
It is colourless, tasteless and odourless gas. It is
slightly lighter than air (0.965 times as heavy as air ) and is insoluble in
water. It is chemically inert under normal conditions and has an estimated
atmospheric life of about two and a half months. It is a poisonous gas and is
generally classified as an asphyxiant. The atmospheric background of CO is 0.1
ppm. It is produced by
(i) incomplete burning of the carbon in fossil fuels
(ii) reaction between carbon dioxide and carbon containing
materials at very high temperatures in industrial processes, such as in
electric and blast furnaces
(iii) and by dissociation of carbon dioxide at higher
temperatures
Sources and Sinks:
-
Carbon monoxide sources are both natural and
anthropogenic. The natural sources are volcanic eruptions, natural gas
emissions, forest fires, oxidation of methane gas from decaying vegetation,
electrical discharge during storms, etc. The anthropogenic sources are motor
vehicles, aircrafts, railways, industries (such as iron and steel, petroleum
and paper industries, electrical and blast furnaces, etc.), fuel combustion in
stationary sources for power and heating, agricultural burning, solid waste
disposal, etc.
The major carbon monoxide sink
is some soil micro-organisms. These soil sinks can take care of atmospheric
carbon monoxide, but neither CO nor the sinks are distributed uniformly. In
fact, the highly populated urban areas having the highest ambient CO
concentration often have the least amount of available soil sinks.
Effect: -
At present ambient levels, carbon monoxide has little,
if any, effect on property, vegetation or materials. But it can seriously
affect human aerobic metabolism, due to its high affinity for hemoglobin (Hb).
It reacts with the hemoglobin of blood and displaces oxygen to form
carboxyhemoglobin (COHb), thus, reducing the capabitity of the blood to carry
oxygen.
Control: -
The four basic technical control methods used for CO
are adsorption, absorption, condensation and combustion.
(iii) Hydrogen
Sulphide (H2S):
-
Hydrogen
Sulphide is well known for its rotten egg smell. It is produced by the
reduction of sulphur. It is a very strong smelling substance and is gaseous at
room temperature having a boiling point at - 600C. It is slightly soluble in water at atmospheric
conditions.
