Photochemical smog consists of excessive amount of X, in addition to aldehydes, ketones, peroxy acetyl nitrile (PAN). X is:

Photochemical smog is a type of air pollution formed when sunlight reacts with pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs) released from vehicles and industries. The chemical reactions under sunlight produce secondary pollutants.

The question states that photochemical smog contains an excessive amount of X, along with aldehydes, ketones, and peroxy acetyl nitrile (PAN). We need to identify X from the options.

Recall the key components of photochemical smog. The primary pollutants are nitrogen oxides (NO and NO2) and VOCs. Sunlight triggers reactions that form secondary pollutants, including:

• Ozone (O3)
• Aldehydes (e.g., formaldehyde)
• Ketones
• Peroxyacyl nitrates (PAN)

Ozone is a major product. It forms when nitrogen dioxide (NO2) absorbs sunlight and breaks down:

Then, the oxygen atom reacts with oxygen molecules:

This ozone accumulates and is a defining component of photochemical smog, causing respiratory issues and environmental harm.

Now, evaluate the options:

• Option A (CH4): Methane is a greenhouse gas but is not reactive enough to form photochemical smog. It does not produce aldehydes, ketones, or PAN in significant amounts under typical smog conditions.
• Option B (CO2): Carbon dioxide is a greenhouse gas from combustion but is not a key reactant or product in photochemical smog reactions. It does not contribute to smog formation.
• Option C (O3): Ozone is a central component of photochemical smog, produced in high concentrations from NOx and VOCs. It is present excessively and aligns with the given pollutants (aldehydes, ketones, PAN).
• Option D (CO): Carbon monoxide is emitted from incomplete combustion but is primarily a primary pollutant. It does not lead to the formation of aldehydes, ketones, or PAN in photochemical smog.

Thus, X must be ozone (O3), as it is excessively produced and characteristic of photochemical smog.

Hence, the correct answer is Option C.