Which one of the following is an essential requirement for the formation of photochemical smog?

AWater vapour

BSunlight ✓ Correct

CHigh temperature

DSnow

Correct answer: (B) Sunlight

Explanation

Sunlight is the essential requirement for photochemical smog to form.

Water vapour (A) relates to classical or industrial smog, not the photochemical type.

High temperature (C) may accompany it but is not the essential trigger.

Snow (D) has no role in smog formation.

Ultraviolet light drives reactions among nitrogen oxides and hydrocarbons to make ozone and PAN.

Photochemical smog is typical of sunny, vehicle-dense cities like Los Angeles.

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