Which of the following is NOT a green house gas ?

Question

UnlockIAS · Accepted Answer

Correct — C, Argon. A gas can act as a greenhouse gas only if it absorbs the long-wave infrared radiation that the warmed Earth sends back upward, and that requires a molecule whose vibrations change its electrical dipole. Argon is a noble gas and its molecules are single atoms — one atom has nothing to vibrate against, so it has no infrared-active vibration at all and is transparent to terrestrial radiation. This is the same reason nitrogen and oxygen, which together make up about 99 per cent of dry air, are not greenhouse gases either: they are symmetrical two-atom molecules whose stretching produces no change in dipole. Argon is the third most abundant constituent of dry air at roughly 0.93 per cent by volume, and it still contributes nothing to the greenhouse effect. The other three options — CFCs, carbon dioxide and methane — all have the required asymmetric vibrations and are all recognised greenhouse gases.

Why the other options are wrong — (a) CFCs are greenhouse gases, and unusually powerful ones: each molecule traps far more heat than a molecule of carbon dioxide, which is why even their tiny atmospheric concentrations matter. They are better known for a different offence — destroying stratospheric ozone, which is why they are being phased out under the Montreal Protocol — but that does not stop them being greenhouse gases as well.  (b) Carbon dioxide is the reference greenhouse gas; global warming potentials of all other gases are quoted relative to it, and it is the largest single driver of human-caused warming because of the sheer quantity emitted by burning fossil fuels.  (d) Methane is a greenhouse gas and, molecule for molecule over the short term, a considerably stronger one than carbon dioxide, though it is far less abundant and breaks down in the atmosphere within a couple of decades. Its major sources are wetlands, flooded paddy fields, livestock, landfills and fossil-fuel extraction.

Concept: The greenhouse effect works on a division of labour in the atmosphere. Short-wave sunlight passes through and warms the surface; the surface re-radiates that energy as long-wave infrared; and a handful of trace gases absorb the infrared and re-emit part of it downward, keeping the surface far warmer than it would otherwise be. Whether a gas can do this is decided by molecular structure, not by abundance. Monatomic gases such as argon, helium and neon have no vibrational modes; symmetrical diatomic gases such as nitrogen and oxygen have vibrations that do not change the dipole moment. Only molecules with three or more atoms, or two different atoms, absorb in the infrared — hence water vapour, carbon dioxide, methane, nitrous oxide, ozone and the halocarbons.

State PCS 2022 Q25 — Environment & Ecology PYQ with Answer

State PCS 2022 Q25 — Environment & Ecology PYQ with Answer