Major air pollutants
Major air pollutants can be divided into 2 categories : Inorganic gases & particular gases.
(A) Inorganic gases
1. Carbon monoxide (CO)
CO is a colourless, poisonous, lethal gas, which is one of the most serious air pollutants.
Source: Some of the sources of CO are
(i) Incomplete combustion of carbonaceous matter, automobile engines & also in defective furnaces.
(ii) Incomplete combustion of fossil fuels, agricultural & slash matter.
(iii) Reaction of carbon dioxide & carbon at high temperatures in industrial processes.
(iv) Dissociation of carbon dioxide at high temperatures.
Toxic effects of CO
Its toxic effect arises from its capacity to bind with haemoglobin of blood more strongly than oxygen. As a result of this amount of haemoglobin available in the blood binds, so the transport of oxygen to the blood cells decreases. The normal metabolism is thus impaired due to less O2 level. The presence of CO in blood can cause mental retardation, muscular weakness, dizziness & even death depending on its concentration in blood.
Sinks of CO
Human activities are releasing CO in the atmosphere & it is expected to double its concentration in surrounding atmosphere every 5 years. However the actual increase in CO in atmosphere is less because of some natural sinks.
(i) Soil is major sink for CO.
(ii) Source of the species in atmosphere such as hydroxyl & per hydroxyl
radicals, atomic oxygen & ozone help in the oxidation of CO & CO2 & consequently remove it from atmosphere.
2. Carbon dioxide (CO2)
CO2 is a natural constituent of atmosphere (0.03% by volume) & is vital to all forms of plant life.
It is released mainly into the atmosphere by the combustion of fossil fuels (coal, oil etc) in factories & also at homes. CO2 is also produced by biological decay of plants.
Ocean is a main sink for CO2. In a balanced ecological system, the CO2 is released into the atmosphere & is regularly removed by green plants for photo synthesis.
CO2 causes narcotic effect, stimulation of respiratory centre & leads to asphyxiation. The increasing concentration of CO2 also changes climatic conditions especially by raising the general temperature.
Major air pollutants-II
3. Oxides of sulphur (SOx)
These are probably the most harmful of the gaseous pollutants.
The common oxide of sulphur emitted by pollution sources is sulphur dioxide. This pollutant is released into atmosphere through volcanic eruptions (natural activity) & also through combustion of sulphur bearing fuels such as coal & oil (human activity). This pollutant is also produced during roasting & smelting of sulphide ores (human activity)
A part of SO2 undergoes photolytic & catalytic oxidation to form SO3. The SO3, so formed gets converted to H2SO4 in the presence of moisture. This acid comes down from the atmosphere in the form of sulphuric acid rain.
Toxic effects of SO2
It is wide spread & serious air pollutant. It has been reported that even at lower concentration, it causes cough, shortness of breath & spasm of larynx. It causes acute irritation to the membrane of gas resulting tears & reduces hearing ability.
SO2 irritates the respiratory system of animals & human. SO2 produces leaf injuries (called necrotic bloating) to board leaved plants & gases. It also causes deterioration of fabric (cotton, rayon) paper & leather.
4. Oxides of Nitrogen (NOx)
Nitrogen & oxygen forms five oxides of nitrogen. Out of various oxides of N2 only three namely nitric oxide NO, nitrogen dioxide NO2 and nitrous oxide N2O are present in the atmosphere in significant amounts. Of these three, only two oxides (NO & NO2) are atmospheric pollutants sources. NO2 is produced in small amounts by microbiological processes in soil. However significant amount of NO & NO2 are emitted in to the atmosphere by natural activity which causes considerable pollution.
Toxic effect of oxides of Nitrogen
The main harmful effects of oxides of nitrogen are:
(i) NO is biologically less active & less toxic than NO2. Like CO it binds haemoglobin & decreases oxygen transport efficiency of blood.
(ii) Inhaling of nitrogen oxides by human results in pulmonary odema & haemorrhage.
(iii) The oxides of nitrogen cause damage to plants. Exposure of plants to NOx causes leaf spotting & break down of plant tissues.
(iv) The sunlight reacts with NO2 to produce highly active oxygen atoms.
The active oxygen attacks traces of hydrocarbons in the air & produces irritants, which constitute photochemical smog. This is a health hazard.
Sinks of NOx
Many natural processes acts as sink for oxides of nitrogen. These oxides are inherently unstable & decompose to N2 & O2 after some time. Therefore, the concentration of nitrogen oxides in the atmosphere tends to remains low.
5. Hydrogen sulphide
The main natural sources of H2S are volcanic activity & biological decay of protein matter in stagnant water & swamps. A large number of industries such as petroleum refining, manufacture of paper, manufacture of sulphur dyes, tanning etc also produce considerable amount of H2S.
Toxic effect of H2S
H2S gas is extremely toxic to humans. In higher atmospheric concentration, it can be fatal. The reaction of H2S with essential protein is primarily responsibly for its toxic effects.
These are compounds of carbon & hydrogen only. The hydrocarbon involved in air pollution are those which use volatile or gases under ordinary conditions. The hydrocarbons in air by themselves alone cause no harmful effects. However they undergo chemical reaction in the presence of sunlight & nitrogen oxides forming photochemical oxidants.
The natural processes such as bacterial decomposition of organic matter, forest fires & vexampleetation are important sources of hydrocarbons in the environment. Gasoline fuelled vehicles industrial emissions accounts for 1/6th of all hydrocarbons in atmosphere.
7. Other pollutants
In addition to the pollutants discussed, some aldehydes such as acrolin, formaldehyde, aromatic aldehydes etc in high concentration are toxic. They are produced due to incomplete combustion of hydrocarbons or by interaction of hydrocarbon with NO2 under the influence of sunlight.
Chlorofluoro carbons (CFC) which are widely used as propellants & refrigerants escape into atmosphere. When they reach the upper sphere of the atmosphere they deplete the ozone layer.
Illustration 1. How are NO and NO2 formed in the atmosphere?
Solution: NO is formed due to reaction between N2 and O2 during lightning or combustion of fossil fuels. It is further oxidized to NO2.
Illustration 2. How are flue gases from industries freed from oxides of nitrogen and sulphur?
Solution: By scrubbing them with conc. H2SO4 or with alkaline solution like Ca(OH)2 and Mg(OH)2
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