CDS (I) 2024 General Knowledge · Question 85 of 120

Correct — C, N₂O. Nitrous oxide is the longest lived of the four. Using the lifetimes in the IPCC's Sixth Assessment Report, nitrous oxide stays in the atmosphere for about 109 years, CFC-12 for about 100 years and CFC-11 for about 52 years, methane for about 12 years, and water vapour for only about nine or ten days before it rains out. So the ranking runs N₂O, then the common CFCs, then methane, then water vapour — and nitrous oxide comes out on top. This long residence is also why nitrous oxide keeps warming the planet for a century after it is released, even though far less of it is emitted than carbon dioxide.

• (a)CFC — The two CFCs that dominate the textbook picture are shorter lived than nitrous oxide — CFC-12 about 100 years and CFC-11 about 52 years, against nitrous oxide's 109 years.
• (b)CH₄ — Methane lasts only about 12 years. It is a powerful greenhouse gas per molecule but a comparatively short-lived one.
• (d)Water vapour — Water vapour is the shortest lived of all — a molecule spends about nine to ten days in the troposphere before it condenses and falls as precipitation.

Atmospheric residence time, or lifetime, is roughly how long a molecule of a gas stays in the air before it is removed by chemical breakdown, by photolysis in the stratosphere, or by rainfall. It matters because a long-lived gas keeps warming the planet long after the emission stops, while a short-lived one stops contributing quickly. Water vapour is removed by condensation in days, methane is oxidised in about a decade, and nitrous oxide and the CFCs survive for a century or so because they are broken down only slowly, high in the stratosphere.

One honest qualification worth carrying. CFC is not a single substance but a family, and a few rare members really do outlive nitrous oxide — CFC-115 and CFC-13, for instance, run to several hundred years. The option in this question is written as the general class, and the members that carry almost all the atmospheric burden and appear in every textbook are CFC-11 and CFC-12, at about 52 and 100 years. On that reading nitrous oxide wins, which is what the official key records. As of the 2024 exam, and still today, CFC concentrations are falling under the Montreal Protocol while nitrous oxide keeps rising, mainly from nitrogen fertiliser use — so the gap in importance between the two options has widened since the paper was set.

• IPCC Sixth Assessment values give the atmospheric lifetime of nitrous oxide as about 109 years and of methane as about 12 years.
• CFC-11 lasts about 52 years and CFC-12 about 100 years, both shorter than nitrous oxide.
• A water molecule spends about nine to ten days in the troposphere before it is rained out.
• Nitrous oxide is not covered by the Montreal Protocol, and its main human source is nitrogen fertiliser applied to soils.

• Picking CFC because CFCs are famous for persistence — the commonly cited CFC-11 and CFC-12 are in fact shorter lived than nitrous oxide.
• Confusing residence time with abundance or with warming power; carbon dioxide is the most abundant of the long-lived gases, methane the most potent per molecule over twenty years, nitrous oxide among the most persistent.
• Picking water vapour because it is the largest greenhouse contributor by effect, forgetting that individual molecules are removed within days.

Asked as a single-best-answer ranking item — four greenhouse gases, one property, and the candidate must know the order of magnitude of each lifetime.

• practice — not a real PYQ

  Which of the following greenhouse gases has the shortest average residence time in the atmosphere?

  • (a)Nitrous oxide
  • (b)Methane
  • (c)Water vapour
  • (d)CFC-12

  Answer(c) Water vapour — a molecule stays in the troposphere for about nine to ten days before it condenses and falls as precipitation.

• practice — not a real PYQ

  The principal human source of nitrous oxide emissions is

  • (a)refrigerant leakage from air conditioners
  • (b)nitrogen fertiliser applied to agricultural soils
  • (c)paddy cultivation and enteric fermentation in cattle
  • (d)aluminium smelting

  Answer(b) nitrogen fertiliser applied to agricultural soils — paddy and cattle are methane sources, and refrigerants are fluorinated gases.