Photochemical smog from NOx and hydrocarbons

Steps:

• Identify primary pollutants in vehicle exhaust: nitrogen oxides (NOx) and unburnt hydrocarbons.
• Recall that sunlight triggers photochemical reactions between NOx and hydrocarbons to form smog.
• Eliminate options without both key pollutants.
• Confirm the mixture that reacts under UV light to produce ozone and other oxidants.

Why C is correct:

• Nitrogen oxides (NOx) and unburnt hydrocarbons undergo photochemical oxidation in sunlight, forming peroxyacyl nitrates (PAN) and ozone, as per the reaction: NOx + VOCs + hν → photochemical smog components.

Why the others are wrong:

• A: Carbon monoxide and water vapor do not participate in the key photochemical reactions for smog.
• B: Carbon monoxide alone with hydrocarbons lacks NOx, which initiates the chain reaction.
• D: Sulfur dioxide and water vapor contribute to acid rain, not photochemical smog.

Final answer: C