Which one of the following is mainly responsible for the depletion of the ozone layer?

Chlorofluorocarbons

The correct answer is option 3.  chlorofluorocarbons (CFCs).

Here is a detailed explanation of how chlorofluorocarbons (CFCs) contribute to the depletion of the ozone layer:

The ozone layer is located in the stratosphere, a layer of the Earth's atmosphere. It absorbs most of the sun's harmful ultraviolet (UV) radiation, particularly the UV-B and UV-C wavelengths, which can cause skin cancer and cataracts in humans and are harmful to other forms of life.

Chlorofluorocarbons (CFCs)

CFCs are composed of chlorine, fluorine, and carbon. They were commonly used in refrigeration, air conditioning, foam blowing agents, and as propellants in aerosol sprays due to their stability and non-flammability. CFCs are very stable in the lower atmosphere (troposphere) and do not break down easily. This stability allows them to persist and eventually diffuse up to the stratosphere.

Breakdown in the Stratosphere

UV Radiation Exposure: When CFCs reach the stratosphere, they are exposed to high-energy UV radiation, which is not present in the lower atmosphere.

Photodissociation: The UV radiation breaks down CFC molecules, releasing chlorine atoms in the process:

\(\text{CCl}_2\text{F}_2 + \text{UV} \rightarrow \text{CClF}_2 + \text{Cl}\)

Ozone Depletion Mechanism

Chlorine Atom Reaction: The free chlorine atom released from the CFC molecule reacts with an ozone molecule (\( \text{O}_3 \)):

\(\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2\)

This reaction breaks down one ozone molecule and forms a chlorine monoxide (ClO) molecule and an oxygen molecule (\( \text{O}_2 \)).

Catalytic Cycle: The chlorine monoxide can then react with a free oxygen atom (\( \text{O} \)) in the stratosphere:

\(\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2\)

This reaction regenerates the chlorine atom and produces another oxygen molecule.

Catalytic Destruction

Cycle Continuation: The regenerated chlorine atom is free to react with another ozone molecule, continuing the cycle. This means one chlorine atom can destroy thousands of ozone molecules before it is deactivated or removed from the stratosphere.

Net Effect: The overall effect is a significant reduction in the ozone concentration in the stratosphere, particularly noticeable in the formation of the ozone hole over Antarctica.

Global Impact

Increased UV Radiation: The depletion of the ozone layer results in increased UV-B and UV-C radiation reaching the Earth's surface, leading to higher risks of skin cancer, cataracts, and other health and environmental problems.

Regulatory Actions

Montreal Protocol: In response to the harmful effects of CFCs, the Montreal Protocol was established in 1987. It is an international treaty aimed at phasing out the production and use of ozone-depleting substances, including CFCs.

Summary: Chlorofluorocarbons (CFCs) are responsible for ozone layer depletion due to their release of chlorine atoms when exposed to UV radiation in the stratosphere. These chlorine atoms participate in catalytic cycles that break down ozone molecules, leading to thinning of the ozone layer and increased UV radiation reaching the Earth's surface. This understanding led to international agreements to reduce and eventually eliminate the use of CFCs to protect the ozone layer.