Question

Assertion:
An equilibrium is established between generation and destruction of ozone, leading to a steady state concentration of ozone layer in the stratosphere at an altitude of 20-30 km above sea level.

Reason:
The thickness of the ozone layer is generally larger above the equator and smaller above the poles.
a. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
b. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
c. Assertion is correct but Reason is incorrect
d. Both Assertion and Reason are incorrect

Answer 1

The ozone layer is a region of the Earth's stratosphere, roughly lying at an altitude of 15-30 km above Earth's surface. The thickness of the ozone layer changes seasonally and geographically. The rise in temperature in the stratosphere is caused by the ozone layer.

Complete answer:
The ozone layer or ozone shield is found in the upper part of the Earth's atmosphere called the stratosphere (ozonosphere). Ozone layer acts as an umbrella for the UV radiation. Ozone gas is formed continuously in the stratosphere by the action of UV rays on molecular oxygen. The cycle of formation and destruction of ozone involves the following steps:

a. In the first step, UV rays split molecular oxygen into free oxygen atoms.
b. In the second step, free oxygen atoms combine with the molecular oxygen to form ozone.
c. In the third step, ozone absorbs UV rays and splits into molecular oxygen and free oxygen atoms.

Ozone is thermodynamically unstable and decomposes to molecular oxygen. There is dynamic equilibrium in between the generation and destruction of ozone. Equilibrium is established between formation and destruction of ozone that leads to a steady state concentration of ozone layer in the stratosphere at an altitude of 20-30 km above sea level.

The ozone hole is not a hole in reality. The ozone hole is actually a region of extremely depleted ozone in the stratosphere over Earth's polar region. Ozone hole or depletion of ozone is primarily caused by human activities. Chlorofluorocarbons (CFCs), solvents, propellants and nitrogenous compounds are mainly responsible for ozone layer depletion. Ozone depletion is a major environmental issue because it increases the amount of UV rays that reach the Earth's surface and increases the risk of skin cancer, cataracts and genetic and immune system damages.

Hence, the correct answer is option (B).

Note: The ozone layer is less over the poles and more around the equator. In the stratosphere at an altitude of 20-30 km above sea level, equilibrium is maintained in between generation and destruction of ozone causes a steady state concentration of ozone layer.