Question

Which of the following is not essentially required for photochemical smog formation?

• A. Nitrogen oxides
• B. Sunlight
• C. Water Vapour
• D. Volatile hydrocarbons

Hint:

Los Angeles smog forms in dry sunny air. Which item belongs to foggy smog?

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Photochemical smog forms when sunlight drives reactions between primary pollutants. The passage says it forms from nitrogen oxides and volatile hydrocarbons in presence of sunlight.

Step 2: Key Approach:
List what is essential: the two precursors and sunlight. Then find the option outside this list.

Step 3: Check option (1).
Nitrogen oxides give nitrogen dioxide, which splits in light to start ozone formation. They are essential. So (1) is needed.

Step 4: Check option (2).
Sunlight supplies the energy for the photochemical reactions. Without it no smog forms. So (2) is needed.

Step 5: Check option (3).
Water vapour is the main part of classical smog, where it makes fog. Photochemical smog forms even in dry warm air, so it is not essential. So (3) is the answer.

Step 6: Check option (4).
Volatile hydrocarbons react with radicals to make PAN and aldehydes. They are essential. So (4) is needed.

Final Answer:
The item not essential is water vapour, option 3.
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