Question

Why is the concentration of ozone less over the north and south poles?
(A) CFCs accumulate only in areas where the air is cold
(B) CFC use is highest in these areas
(C) CFCs stick to frozen water vapour and are able to act as catalysts
(D) UV rays are stronger in the atmosphere

Answer 1

Hint: Ozone destruction takes place at low temperatures, and chemical reaction by CFCs takes place in the clouds formed by water vapours (due to low temperatures) so the destruction of ozone occurs in the lower stratosphere.
Step-by-step solution:
There are two types of ozone known one is good ozone and the other one is bad ozone. The ozone layer found in the stratosphere is known as good ozone and the ozone layer found in the troposphere is known as bad ozone. The thickness of ozone is measured in terms of “Dobson units” (DU).
Ozone gas is continuously formed by UV rays, it is continuously formed and degraded into molecular oxygen in the stratosphere. There has to be a balance between the production and degradation of ozone in the stratospheric layer. Lately, this balance has been disturbed due to an increase in ozone-depleting substances one such example is chlorofluorocarbons (CFCs). CFCs are used as refrigerants. CFCs which are discharged in the lower part of the atmosphere move upward and reach the stratosphere. Their UV rays act on them and chlorine is released in atomic form. Chlorine degrades ozone which further releases molecular oxygen.
The locations which have low temperatures cause the destruction of ozone present in the atmosphere. At such low temperature, clouds are formed in the lower stratosphere from water vapour, and CFCs allows a chemical reaction to take place inside them. This process leads to the fast destruction of ozone when sunlight returns over the pole. Therefore the concentration of ozone is less over both the poles.
so, the correct answer is option (C)
Note: The chlorine atom only acts as catalysts in the complete degradation reaction, chlorine degrades ozone releasing molecular oxygen; so chlorine atoms are not consumed in the reaction. Hence, once CFCs are added to the stratosphere they have a permanent and continuous effect on ozone.