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UPSC Prelims 2003 · Environment · Pollution Files
Which one of the following is produced during the formation of photochemical smog?
- (a) Hydrocarbons
- (b) Nitrogen Oxides
- (c) Ozone
- (d) Methane
Show the answer and explanation
Answer: (c) Ozone
In sunlight, NO₂ splits and its oxygen atom forms ozone — the main product (with PAN).
The idea
A primary pollutant comes straight out of a source, such as an exhaust pipe. A secondary pollutant forms later in the air from primary ones. The question asks what is produced during smog formation, so the answer must be a secondary pollutant.
Inputs (primary): nitrogen oxides + hydrocarbons → Sunlight → Outputs (secondary): ozone + PAN
Why the others are wrong
(a) Hydrocarbons are raw ingredients (emitted by vehicles), not products.
(b) Nitrogen oxides are raw ingredients from vehicle exhaust — they go into the smog.
(d) Methane is not a product of smog formation.
The trap
NOₓ is in every smog answer, so it feels right. Ask: put in, or produced?
Also know
Ozone is “good up high, bad nearby”: in the stratosphere it shields us from ultraviolet rays; near the ground it harms lungs and reduces crop yields. Ground-level ozone is one of the eight pollutants in India’s Air Quality Index.
🔒 Smog inputs: NOₓ + VOCs. Products: O₃ + PAN.