Which one of the following is NOT a greenhouse gas found naturally in the atmosphere?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) CO2 is the principal long-lived greenhouse gas and occurs naturally in the atmosphere — so it is not the exception.  (c) Methane (CH4) is a potent naturally occurring greenhouse gas (from wetlands, rice paddies, livestock, etc.) — not the exception.  (d) 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.

Concept: 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.

State PCS 2020 Q139 — Environment & Ecology PYQ with Answer

State PCS 2020 Q139 — Environment & Ecology PYQ with Answer