Question

Ozone layer of upper atmosphere is being destroyed by:
(a)- Chlorofluorocarbons
(b)- $S{{O}_{2}}$
(c)- Photochemical oxidants / ${{O}_{2}}$ and $C{{O}_{2}}$
(d)- Smog

Answer 1

The ozone layer is an important layer in the stratosphere that is used to protect us from harmful ultraviolet radiation. This layer is easily depleted by the compounds that contain chlorine, fluorine, and carbon atoms.

Complete step-by-step answer: The ozone layer is an important layer in the stratosphere that is used to protect us from harmful ultraviolet radiation, that's why the ozone layer is known as the earth's protective umbrella.
But the ozone layer is being destroyed by the compounds that contain chlorine, fluorine, and carbon, and these compounds are commonly known as freons. These compounds are chlorofluorocarbons. The aerosol sprays and refrigerants are the main source from which these chlorofluorocarbons. Their formula is $C{{F}_{2}}C{{l}_{2}}$. They are chemically unreactive, odorless, and non-toxic, but they stay in the atmosphere for a very long time due to which they slowly reach the stratosphere, in which they start to decompose by the UV-light.
When they reach the stratosphere, a photochemical reaction occurs in which chlorine-free radicals are released. The reaction is given below:
$C{{F}_{2}}C{{l}_{2}}\xrightarrow{h\nu }\cdot C{{F}_{2}}Cl+Cl\cdot$
These chlorine-free radicals react with the ozone to form oxygen as the reaction is given below:
$Cl\cdot +{{O}_{3}}\to ClO\cdot +{{O}_{2}}$
$ClO\cdot +O\to Cl\cdot +{{O}_{2}}$
As the molecules of ${{O}_{3}}$ molecules destroy the layer becomes thinner. It has been seen that one thousand ozone molecules can be destroyed by only one chlorofluorocarbon molecule (CFCs).

Therefore, the correct answer is an option (a).

Note: Not only CFCs (chlorofluorocarbons) cause the depletion of the ozone layer, Nitric oxide is also responsible for this. The NO (nitric oxide) reacts with ozone to form oxygen gas. The reactions are given below:
$NO+{{O}_{3}}\to N{{O}_{2}}+{{O}_{2}}$
$N{{O}_{2}}+O\to NO+{{O}_{2}}$