3.5 Major air pollutants

(ii) Source of the species in atmosphere such as hydroxyl $\left( {H\mathop O\limits^ \bullet } \right)$ & perhydroxyl $\left( {\mathop {HO\mathop O\limits^ \bullet }\limits^{} } \right)$ radicals, atomic oxygen & ozone help in the oxidation of $CO$ & $C{O_2}$ & consequently remove it from atmosphere.

$C{O_2}$ 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.$C{O_2}$ is also produced by biological decay of plants.

Ocean is a main sink for $C{O_2}$. In a balanced ecological system, the $C{O_2}$ is released into the atmosphere & is rexampleularly removed by green plants for photo synthesis.

$C{O_2}$ causes narcotic effect, stimulation of respiratory centre & leads to asphyxiation. The increasing concentration of $C{O_2}$ also changes climatic conditions especially by raising the general temperature.

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 ${\bf{S}}{{\bf{O}}_{\bf{2}}}$ undergoes photolytic & catalytic oxidation to form ${\bf{S}}{{\bf{O}}_{\bf{2}}}$ The $S{O_3}.$ so formed gets converted to ${H_2}S{O_4}$ in the presence of moisture. This acid comes down from the atmosphere in the form of sulphuric acid rain.

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.

$S{O_2}$ irritates the respiratory system of animals & human. $S{O_2}$ produces leaf injuries (called necrotic bloating) to board leaved plants & gases. It also causes deterioration of fabric (cotton, rayon) paper & leather.

Nitrogen & oxygen forms five oxides of nitrogen. Out of various oxides of ${N_2}$ only three namely nitric oxide $NO$, nitrogen dioxide $N{O_2}$ and nitrous oxide ${N_2}O$ are present in the atmosphere in significant amounts. Of these three, only two oxides $\left( {NO{\text{ }}\& {\text{ }}N{O_2}} \right)$ are atmospheric pollutants sources. $N{O_2}$ is produced in small amounts by microbiological processes in soil. However significant amount of ${NO{\text{ }}\& {\text{ }}N{O_2}}$ are emitted in to the atmosphere by natural activity which causes considerable pollution.

The main harmful effects of oxides of nitrogen are:

(i) NO is biologically less active & less toxic than $N{O_2}$ 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 ${{\bf{N}}{{\bf{O}}_{\bf{x}}}}$ causes leaf spotting & break down of plant tissues.

(iv) The sunlight reacts with $N{O_2}$ to produce highly active oxygen atoms.

Many natural processes acts as sink for oxides of nitrogen. These oxides are inherently unstable & decompose to ${N_2}\& {\text{ }}{O_2}$ after some time. Therefore, the concentration of nitrogen oxides in the atmosphere tends to remains low.

The main natural sources of ${H_2}S$ 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 ${H_2}S$

${H_2}S$ gas is extremely toxic to humans. In higher atmospheric concentration, it can be fatal. The reaction of ${H_2}S$ 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.

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 $N{O_2}$$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.