CBSE Class 12 Chemistry Notes: Enviromental Chemistry – Stratospherical Pollution

STRATOSPHERICAL POLLUTION: (Ozone Layer & its depletion)

Ozone layer in atmosphere is very significant because it protect us from the harmful effect of U.V. radiations of sun.

Role of Ozone Layer

The stratosphere or ozonosphere plays a significant role in protecting earth from the UV radiation coming from the sun. Ozone absorbs UV radiation & prevent most of them in reaching the earth’s surface.

Ozone is simultaneously being formed & destroyed by naturally occupying chemical reactions.

Depletion of Ozone Layer

The equilibrium between formation & destruction of ozone has been upset by influx of several substances into the atmosphere which react with ozone to destroy it. The rate at which ozone is being destroyed is much faster than the rate at which it is being formed. The factors which accelerate the ozone depletion are as follows:

1. Effect of chlorofluoro carbons (Freon’s)

A very serious threat to the existence of ozone in the stratosphere comes from the use of chlorofluorocarbons (CFC). CFC’s find a wide & varied applications.

Once they enter the atmosphere, they do not react with any substance due to their chemical inertness & thus can not be eliminated from the atmosphere.

In stratosphere they absorb the UV radiations and break up liberating free chloride atoms. The chlorine atoms react with O₃ to form a species, chlorine monoxide (CIO) which further combines with O to give O₂.

2. Use of Halons

Halons are bromo fluorocarbons which are also used as fire extinguishers & pesticides. Halons produce bromine atoms by the similar process which further results in consumption of very large number of ozone molecules.

3. Oxides of Nitrogen

The sources of oxides of nitrogen are mainly explosion of thermonuclear weapons in the atmosphere, industrial emission & agricultural fertilizers. Like Cl & Br atoms NO molecules also catalytically converts O_­3 to O₂.

Effect of Depletion of Ozone layer

Depletion of ozone layer poses severe threat to mankind. As a result of decreased concentration of ozone in stratosphere, the influx of UV radiation reaching the surface of earth would increase which would increase in risk to skin cancer due to exposure to UV radiation, UV radiations also tend to damage the immune system.

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