UPSC Prelims 2003 Q118: Environment & Ecology – Pollution & Waste Management
Which one of following is produced during the formation of photochemical smog?
- (a) Hydrocarbons
- (b) Nitrogen Oxides
- (c) Ozone
- (d) Methane
Answer: (c) Ozone
Explanation
Products of photochemical smog
| Substance | Role in photochemical smog |
|---|---|
| Ozone | Produced by reaction of nitrogen oxides with hydrocarbons in sunlight |
| Nitrogen oxides | Precursors, emitted by vehicles |
| Hydrocarbons | Precursors, emitted by vehicles |
| Methane | Not significantly involved |
Ozone is a main product formed during photochemical smog, together with peroxyacetyl nitrate (PAN) and aldehydes; nitrogen oxides and hydrocarbons are the precursors.
Answer: (c) Ozone.
Why each option is right or wrong
- (a) Incorrect. Hydrocarbons are precursors, not products.
- (b) Incorrect. Nitrogen oxides are precursors, not products.
- (c) CORRECT. Ozone is produced in photochemical smog.
- (d) Incorrect. Methane does not take part significantly.
Key point
Photochemical smog: NOx + hydrocarbons + sunlight gives O3, PAN, aldehydes.
Remember
Ozone and PAN are products of photochemical smog.
Common mistakes
- Taking the precursors as the products.
- Mixing photochemical smog with the classical smog of SO2.
- Forgetting peroxyacetyl nitrate (PAN) as a product.
Exam tip
Los Angeles smog: oxidising; London smog: reducing.
Sources
- NCERT Class XI Chemistry — Environmental Chemistry
- Encyclopaedia Britannica — smog