Animated Solution for Chemistry - Environmental Chemistry: The reaction that is not involved in the ozone layer depletion mechanism in the stratosphere is

To truly understand the chemistry of our atmosphere, we must first visualize its structure. We live in the troposphere, the lowest layer of Earth's atmosphere, which extends up to about $10\text{ km}$. Right above it sits the stratosphere, stretching from $10\text{ km}$ to $50\text{ km}$. This layer is incredibly important because it houses the protective ozone ($O_3$) layer, which acts as Earth's sunscreen, shielding us from harmful ultraviolet (UV) radiation.

CFCs

So, what causes the destruction of this vital ozone layer? The main culprits are chlorofluorocarbons, commonly known as CFCs, such as freon-12 ($C{F}_{2}C{l}_2$). These compounds are highly stable in the troposphere. Because they are so chemically inert, they do not break down near the ground. Instead, they slowly diffuse upwards over many years, eventually crossing the boundary and entering the stratosphere.

Once CFCs reach the stratosphere, the environment changes drastically. They are exposed to powerful, high-energy ultraviolet radiation from the sun. This UV light is strong enough to break the carbon-chlorine bond in the CFC molecule, a process known as photodissociation:

This reaction releases highly reactive chlorine free radicals ($C{l}^{\bullet }$). This perfectly matches the reaction given in option (d), confirming it is a key step in stratospheric ozone depletion.

This newly formed chlorine radical is an absolute menace. It attacks an ozone molecule, stealing an oxygen atom to form chlorine monoxide ($Cl{O}^{\bullet }$) and oxygen gas ($O_2$).

The chlorine monoxide then reacts with a free oxygen atom (which is naturally present in the stratosphere due to the UV breakdown of $O_2$), regenerating the original chlorine radical:

This sets off a devastating catalytic chain reaction! A single chlorine radical can destroy up to $10^5$ ozone molecules before it is finally removed from the stratosphere. This regenerating step matches the reaction in option (b).

Additionally, the chlorine radical can react with trace amounts of water vapor in the stratosphere to form hypochlorous acid ($HOCl$). However, this is only a temporary reservoir. UV light breaks down $HOCl$, releasing the chlorine radical back into the wild to continue its rampage:

This matches the reaction in option (c).

Now, let's look at methane ($C{H}_4$). Methane is generated on the Earth's surface and stays predominantly in the troposphere. It is highly reactive with hydroxyl radicals near the ground, meaning it cannot easily diffuse into the stratosphere in large quantities like CFCs do.

The reaction of methane with ozone:

is actually a part of the complex chemistry that forms photochemical smog near the ground (tropospheric pollution), not stratospheric ozone depletion.

Therefore, the reaction involving methane is the odd one out. It does not play a role in the ozone layer depletion mechanism in the stratosphere. This makes option (a) our correct answer. Always remember to distinguish between the pollutants that cause trouble in our immediate breathing air (troposphere) versus those that destroy our planetary shield (stratosphere)!