The main air pollutant(s) in metropolitan cities is/are
- (a)O₃
- (b)CO and SO₂
- (c)CO₂ and NO₂
- (d)None of the above
This question was cancelled by the commission and marks were awarded to all candidates, so no option is treated as correct. It asked for the 'main' air pollutant of metropolitan cities, but the real answer is a set — particulate matter (PM2.5/PM10), nitrogen dioxide (NO₂), carbon monoxide (CO), sulphur dioxide (SO₂) and ground-level ozone (O₃) — while carbon dioxide (CO₂) is a greenhouse gas, not a criteria air pollutant tracked in the Air Quality Index. Because more than one option contains a genuine urban pollutant yet none names the complete or correct set, the item was ambiguous and the commission voided it. Use it only to revise which gases actually pollute city air.
- (a)O₃ — Not accepted as correct — the question was cancelled. Ground-level ozone is a genuine secondary urban pollutant (a component of photochemical smog), but it is only one of several main pollutants, not 'the' main one.
- (b)CO and SO₂ — Not accepted as correct — the item was voided. CO and SO₂ are real urban pollutants, but this pair omits the equally major NO₂ and particulate matter, so it is not a complete answer.
- (c)CO₂ and NO₂ — Not accepted as correct — the item was voided. NO₂ is a major urban pollutant, but CO₂ is a greenhouse gas, not a criteria air pollutant tracked in the AQI, so the pair is mismatched.
- (d)None of the above — Not accepted as correct — several of the listed gases (CO, SO₂, NO₂, O₃) are indeed real urban air pollutants, so 'none' is not right either; the commission cancelled the question and gave marks to all.
In cities the health-relevant air pollutants are the 'criteria pollutants' tracked by India's National Air Quality Index: particulate matter (PM2.5 and PM10), nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), ground-level ozone (O₃), ammonia (NH₃) and lead (Pb). Carbon dioxide (CO₂) is a greenhouse gas driving climate change, but it is NOT one of these AQI criteria pollutants. Ground-level ozone is a 'secondary' pollutant — it is not emitted directly but forms when sunlight acts on NOₓ and volatile organic compounds (photochemical smog).
The trap is that every option holds a partly-true item: CO, SO₂, NO₂ and O₃ are all genuine urban pollutants, so each option looks defensible while none names the correct or complete set — which is exactly why the commission could not settle on a single key and voided the item.
- India's National AQI is based on eight pollutants: PM2.5, PM10, NO₂, SO₂, CO, O₃, NH₃ and Pb.
- CO₂ is a greenhouse gas, not one of the AQI criteria air pollutants.
- Ground-level (tropospheric) ozone is a secondary pollutant formed by sunlight acting on NOₓ and VOCs — the basis of photochemical smog.
- In metros, vehicles are the dominant source of CO and NOₓ, while particulate matter (PM2.5/PM10) is the pollutant of greatest health concern.
- The commission cancelled this 2018 item and awarded marks to all candidates.
- Treating CO₂ as an AQI / criteria air pollutant — it is a greenhouse gas, not tracked in the AQI
- Assuming SO₂ is still the dominant metro pollutant — with cleaner fuels it has fallen; PM and NOₓ now dominate
UPSC asks which gases feed the AQI or cause smog / acid rain; MPPSC asks you to identify the main pollutant or match a pollutant to its effect. Anchor: criteria pollutants (PM, NO₂, SO₂, CO, O₃) vs the greenhouse gas CO₂.
In the cities of our country, which among the following atmospheric gases are normally considered in calculating the value of Air Quality Index? 1. Carbon dioxide 2. Carbon monoxide 3. Nitrogen dioxide 4. Sulfur dioxide 5. Methane
- (a) 1, 2 and 3 only
- (b) 2, 3 and 4 only
- (c) 1, 4 and 5 only
- (d) 1, 2, 3, 4 and 5
Answer(b) 2, 3 and 4 only
Directly the same concept — which gases count as the main urban air pollutants (CO, NO₂, SO₂) in the city AQI, while CO₂ and methane do not.
Consider the following: 1. Carbon monoxide 2. Nitrogen oxide 3. Ozone 4. Sulphur dioxide. Excess of which of the above in the environment is/are cause(s) of acid rain?
- (a) 1, 2 and 3
- (b) 2 and 4 only
- (c) 4 only
- (d) 1, 3 and 4
Answer(b) 2 and 4 only
Same family of urban air pollutants — tests recognising SO₂ and NOₓ among the key gaseous pollutants (here as the agents of acid rain).
- practice — not a real PYQ
Which of the following is NOT among the criteria pollutants used to calculate India's National Air Quality Index (AQI)?
- (a)PM2.5
- (b)Carbon dioxide (CO₂)
- (c)Ozone (O₃)
- (d)Sulphur dioxide (SO₂)
Answer(b) Carbon dioxide (CO₂) — a greenhouse gas, not an AQI criteria pollutant.
- practice — not a real PYQ
Ground-level ozone, a component of photochemical smog over cities, is best described as a:
- (a)primary pollutant emitted directly by vehicle exhaust
- (b)secondary pollutant formed by sunlight acting on NOₓ and VOCs
- (c)greenhouse gas with no local health effect
- (d)naturally abundant, harmless gas of the lower atmosphere
Answer(b) a secondary pollutant formed photochemically from NOₓ and VOCs.