Question

Among the following compounds, which one is not responsible for the depletion of ozone layer?

• A. CH\(_4\)
• B. CFC\(l_3\)
• C. NO
• D. Cl\(_2\)

Hint:

Major ozone-depleting substances (ODS): - Chlorofluorocarbons (CFCs), e.g., CFCl\(_3\), CF\(_2\)Cl\(_2\). They release Cl radicals. - Halons (contain bromine), e.g., CBrF\(_3\). They release Br radicals (even more effective than Cl). - Carbon tetrachloride (CCl\(_4\)). - Methyl chloroform (CH\(_3\)CCl\(_3\)). - Nitrous oxide (N\(_2\)O) from natural and anthropogenic sources can lead to NO\(_x\) in the stratosphere. - NO and NO\(_2\) (NO\(_x\)) from aircraft emissions or natural sources. Methane (CH\(_4\)) is a greenhouse gas; its role in ozone chemistry is complex but it's not typically listed as a primary ODS in the same category as CFCs.

Answer 1

The Correct Option is A

Ozone layer depletion is primarily caused by substances that release halogen radicals (like chlorine and bromine) or certain nitrogen oxides in the stratosphere.
- CH\(_4\) (Methane): Methane is a greenhouse gas and contributes to global warming.
In the stratosphere, it can react with chlorine radicals to form HCl ( \( \text{CH}_4 + \text{Cl} \rightarrow \text{HCl} + \text{CH}_3 \) ), which acts as a reservoir for chlorine, temporarily deactivating it from ozone depletion cycles.
Methane itself is not directly considered a primary ozone-depleting substance in the same way as CFCs or halons, though its atmospheric chemistry is complex and can indirectly affect ozone concentrations.
However, compared to the others, it's the least direct cause of depletion.
- CFCl\(_3\) (Trichlorofluoromethane, a CFC - specifically CFC-11): Chlorofluorocarbons (CFCs) are highly stable in the lower atmosphere but break down in the stratosphere under UV radiation to release chlorine radicals (Cl\(\cdot\)).
These chlorine radicals catalytically destroy ozone molecules.
\( \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \); \( \text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2 \).
CFCs are major ozone-depleting substances.
- NO (Nitric Oxide): Nitric oxide and nitrogen dioxide (NO\(_x\)) can participate in catalytic cycles that destroy ozone in the stratosphere.
For example, \( \text{NO} + \text{O}_3 \rightarrow \text{NO}_2 + \text{O}_2 \); \( \text{NO}_2 + \text{O} \rightarrow \text{NO} + \text{O}_2 \).
Sources of stratospheric NO include natural processes and emissions from high-flying aircraft.
- Cl\(_2\) (Chlorine gas): While molecular chlorine itself might not be as stable as CFCs to reach the stratosphere in large quantities from ground-level emissions, if it does reach the stratosphere, UV radiation can dissociate it into chlorine radicals (\( \text{Cl}_2 + h\nu \rightarrow 2\text{Cl} \)), which then deplete ozone.
The primary concern is substances that transport chlorine to the stratosphere.
Comparing the options, CH\(_4\) is the one that is not directly responsible for ozone layer depletion in the same way as CFCs and NO are.
While it interacts with ozone chemistry, its primary role isn't catalytic destruction.
Therefore, CH\(_4\) is the compound not primarily responsible for ozone depletion among the choices.
This matches option (1).