Which one of the following is NOT a greenhouse gas found naturally in the atmosphere?
- (a)Nitrogen oxide
- (b)Carbon dioxide
- (c)Methane
- (d)Ozone
Correct — A, Nitrogen oxide. The greenhouse gases that occur naturally in Earth's atmosphere are water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3). 'Nitrogen oxide' — that is, nitric oxide (NO) and nitrogen dioxide (NO2), together called NOx — is an air pollutant and a precursor to ozone and smog, but it is NOT itself counted among the greenhouse gases. Watch the trap: nitrous oxide (N2O) IS a greenhouse gas, but the nitrogen oxides (NO/NO2) named here are not — so nitrogen oxide is the odd one out.
- (b)Carbon dioxide — CO2 is the principal long-lived greenhouse gas and occurs naturally in the atmosphere — so it is not the exception.
- (c)Methane — Methane (CH4) is a potent naturally occurring greenhouse gas (from wetlands, rice paddies, livestock, etc.) — not the exception.
- (d)Ozone — Ozone (O3) is a naturally occurring greenhouse gas (tropospheric ozone traps heat); it also forms the protective stratospheric layer — so it is a GHG, not the exception.
Greenhouse gases absorb and re-emit infrared radiation, warming the lower atmosphere (the greenhouse effect). The naturally occurring ones are water vapour, carbon dioxide, methane, nitrous oxide and ozone; industrial gases (CFCs, HFCs, PFCs, SF6) are additional, human-made GHGs. Nitrogen oxides (NO, NO2) are a separate family — reactive air pollutants that drive smog and tropospheric-ozone formation but are not classed as greenhouse gases.
The exam bait is the near-identical wording 'nitrogen oxide' versus 'nitrous oxide'. Nitrous oxide (N2O, 'laughing gas') is a recognised GHG; nitrogen oxide (NO/NO2) is not. Since CO2, methane and ozone are all clearly GHGs, elimination leaves nitrogen oxide as the answer.
- Naturally occurring GHGs: water vapour, CO2, CH4, nitrous oxide (N2O), ozone (O3).
- Nitrogen oxides (NO, NO2 / NOx) are air pollutants and ozone precursors, NOT greenhouse gases.
- Nitrous oxide (N2O) is a GHG — do not confuse it with nitrogen oxide.
- CFCs/HFCs/PFCs/SF6 are human-made GHGs, additional to the natural ones.
- Confusing nitrogen oxide (NO/NO2 — pollutant) with nitrous oxide (N2O — a GHG).
- Thinking ozone is only protective; tropospheric ozone is also a greenhouse gas.
UPSC/UPPSC test which gases are GHGs, their sources, and the pollutant-vs-GHG distinction; watch for name look-alikes like nitrogen oxide vs nitrous oxide.
Consider the following: 1. Rice fields 2. Coal mining 3. Domestic animals 4. Wetlands. Which of the above are sources of methane, a major greenhouse gas?
- (a) 1 and 4 only
- (b) 2 and 3 only
- (c) 1, 2 and 3 only
- (d) 1, 2, 3 and 4
Answer(d) 1, 2, 3 and 4 — rice fields, coal mining, domestic animals and wetlands are all sources of methane.
Same concept — greenhouse gases. This UPPSC item identifies which gas is NOT a natural GHG (nitrogen oxide); the UPSC item treats methane as a 'major greenhouse gas' and tests its sources.
- practice — not a real PYQ
Which of the following is a greenhouse gas but is often confused with an air pollutant of a similar name?
- (a)Nitric oxide (NO)
- (b)Nitrogen dioxide (NO2)
- (c)Nitrous oxide (N2O)
- (d)Sulphur dioxide (SO2)
Answer(c) Nitrous oxide (N2O) — a recognised greenhouse gas, unlike the nitrogen oxides NO/NO2, which are pollutants.
- practice — not a real PYQ
Tropospheric ozone is best described as:
- (a)A greenhouse gas and a pollutant
- (b)Only a protective layer
- (c)Chemically inert
- (d)Not present near the surface
Answer(a) A greenhouse gas and a pollutant — ground-level ozone traps heat and harms health, while stratospheric ozone is protective.