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(a) Why is the ozone layer required in the stratosphere? How does it get degraded? Explain.

(b) Why is the ozone depletion a threat to mankind?                                                       5mark


(a) (i) Ozone is a form of oxygen. The molecule of ozone contains three oxygen atoms O3. In the stratosphere, ozone is continuously formed by the action of UV rays on molecular oxygen.

(ii) There is a 'good ozone' found in the upper part of the atmosphere and absorbs ultraviolet radiation from the Sun.

(iii) Ozone gas is continuously formed by the action of UV rays on molecular oxygen and also degraded into molecular oxygen in the stratosphere. Of late, the balance has been disrupted due to enhancement of ozone degradation by chlorofluorocarbons (CFCs). These are used as refrigerants. CFCs discharged in the lower part of atmosphere, move upward and reach stratosphere. UV rays act on them releasing Cl atoms. Cl degrades ozone releasing molecular oxygen, with these atoms acting merely as catalysts; Cl atoms are not consumed in the reaction. Hence whatever CFCs are added to the stratosphere, they have permanent and continuing affects on ozone levels.Although ozone depletion is occurring widely, it is particularly marked over Antarctic region. It has resulted in formation of large area of thinned ozone layer, commonly called as ozone hole. 

(b) Threats of ozone depletion to mankind: 

The thin layer of ozone around the atmosphere that prevents entry of harmful ultra-violet rays (UV-rays) is called ozone shield. Methane and chloro-fluorocarbons are the two ozone depleting substances.

Chlorofluorocarbons release active chlorine when UV rays act on them in the stratosphere. Cl-atoms degrade ozone releasing molecular oxygen. Depletion of ozone allows the entry of UV radiation to the earth.

UV-B rays damage DNA and may also cause mutation. UV-radiation causes aging of skin, damages skin cells and causes various types of cancers. It also leads to inflammation of cornea called snow-blindness, cataract etc.                                                                                   5 mark