Step 1: Sort products by smog type:
Classical smog holds sulphur dioxide, sulphur trioxide and sulphuric acid. Photochemical smog holds ozone, PAN and aldehydes.
Step 2: Place each option:
Ozone goes with photochemical smog. Aldehydes go with photochemical smog. PAN goes with photochemical smog.
Step 3: Place the last option:
Sulphuric acid goes with classical smog, since it needs sulphur from burnt coal or oil.
Step 4: Test with the cause:
Vehicle exhaust has nitrogen oxides and hydrocarbons, not much sulphur. So no acid forms in the sunlit reactions.
Step 5: Conclude:
The odd one out is sulphuric acid.
Extra check:
A simple memory aid helps here. Sulphur gases lead to acid mist and belong to coal smoke. Nitrogen oxides and hydrocarbons lead to ozone, PAN and aldehydes and belong to traffic smoke in strong light. The acid mist item is therefore the one that does not appear in the sunlit traffic-type smog described in the passage.
Final Answer:
The answer is sulphuric acid.
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