Question

Ozone layer is getting depleted because of:
A. Excessive use of automobiles
B. Excessive formation of industrial units
C. Excessive use of man-made compounds containing both fluorine and chlorine
D. Excessive deforestation

Answer 1

Ozone layer is defined as a region of concentration of ozone molecule, ${{\text{O}}_{\text{3}}}\text{ }\\!\\![\\!\\!\text{ }{{\text{O}}_{\text{2}}}\text{+ }\\!\\![\\!\\!\text{ O }\\!\\!]\\!\\!\text{ }\\!\\!]\\!\\!\text{ }$, ​ in the earth's atmosphere.
- Ozone layer depletion can be defined as the gradual thinning of the earth's ozone layer in the upper atmosphere caused mainly due to the release of a chemical compound called chlorofluorocarbon and other ozone depleting substances, mainly containing chlorine, bromine and fluorine.

Complete Solution :
- The Ozone layer lies in the stratosphere at an altitude of 18-50 kms. The ozone layer naturally shields the earth's life from the harmful effect of the Sun's ultraviolet radiations.
- Ozone-layer is getting depleted day by day because of excessive use of man-made compounds containing both fluorine and chlorine. These are in general called ozone depleting substances.
- Ozone depleting substances (ODS) are mainly chlorofluorocarbons, carbon tetrachloride, and halons. The sources of ODS are refrigerants, air-conditioners, solvents, adhesives, and fire extinguishers.
- Chlorofluorocarbon (CFC) molecules are made up of chlorine, fluorine and carbon atoms which are extremely stable. The extreme stability of these molecules allows CFC's to slowly make their way into the stratosphere.

This prolonged life in the atmosphere of earth allows them to reach great altitudes where photons are more energetic than lower levels, where they undergo photolysis:
$\text{CCl}{{\text{F}}_{3}}+hv\to \text{C}{{\text{F}}_{3}}+\text{C}{{\text{l}}^{-}}$
Cl formed above reacts with ozone to form oxygen:

& \text{Cl+}{{\text{O}}_{\text{3}}}\to \text{ClO+}{{\text{O}}_{\text{2}}} \\\ & \text{ClO+O}\to \text{Cl+}{{\text{O}}_{\text{2}}} \\\ \end{aligned}$$​ Thus, many ozone molecules can be destroyed for each chlorine atom produced. **So, the correct answer is “Option C”.** **Note:** Ozone layer depletion causes increased UV radiation levels at the Earth's surface, which is damaging to human health and also other forms of life on earth. Negative effects of this include increases in certain types of skin cancers, eye cataracts and immune deficiency disorders. These harmful UV rays are also known to affect plant growth, reducing agricultural productivity.