UPSC Prelims 2013 Q49: Environment & Ecology – Pollution & Waste Management
Photochemical smog is a resultant of the reaction among
- (a) NO₂, O₃ and peroxyacetyl nitrate in the presence of sunlight
- (b) CO, O₂ and peroxyacetyl nitrate in the presence of sunlight
- (c) CO, CO₂ and NO₂ at low temperature
- (d) high concentration of NO₂, O₃ and CO in the evening
Answer: (a) NO₂, O₃ and peroxyacetyl nitrate in the presence of sunlight
Explanation
Photochemical smog
Photochemical smog forms when nitrogen oxides and hydrocarbons from vehicle exhaust react in sunlight.
The products are ozone and peroxyacetyl nitrate (PAN).
| Option | Verdict | Reason |
|---|---|---|
| (a) NO2, O3 and peroxyacetyl nitrate in the presence of sunlight | Correct | These are its main components |
| (b) CO, O2 and peroxyacetyl nitrate in sunlight | Incorrect | Carbon monoxide and oxygen are not the key reactants |
| (c) CO, CO2 and NO2 at low temperature | Incorrect | It needs warmth and sunlight |
| (d) NO2, O3 and CO in the evening | Incorrect | It peaks around midday, not evening |
Two kinds of smog:
| Feature | Classical (London) smog | Photochemical (Los Angeles) smog |
|---|---|---|
| Made of | Smoke, fog, sulphur dioxide | Ozone, PAN, nitrogen oxides |
| Climate | Cool, humid | Warm, dry, sunny |
| Nature | Reducing | Oxidising |
Answer: (a) NO2, O3 and peroxyacetyl nitrate in the presence of sunlight.
Why each option is right or wrong
- (a) CORRECT. It results from NO2, O3 and peroxyacetyl nitrate in sunlight.
- (b) Incorrect. CO and O2 are not the defining components.
- (c) Incorrect. It does not form at low temperature; it needs sunlight.
- (d) Incorrect. It is strongest at midday, not in the evening.
Key point
Photochemical smog: nitrogen oxides + hydrocarbons + sunlight give ozone and PAN; oxidising; warm sunny cities. London smog: smoke + SO2; reducing.
Remember
Photochemical smog = NOx + sunlight = ozone + PAN.
Common mistakes
- Confusing photochemical smog with classical London smog.
- Forgetting that 'photo' means light is needed.
- Thinking ozone at ground level is good; it is a pollutant there.
Exam tip
Photo = sunlight. Any option without sunlight is wrong.
Sources
- NCERT Class XI — Chemistry (Environmental Chemistry: smog)
- Central Pollution Control Board — Ground-level ozone and air quality