Which of the following is a greenhouse gas?
2018
Which of the following is a greenhouse gas?
- A.
Nitrogen
- B.
Carbon dioxide
- C.
Argon
- D.
Oxygen
Show answer & explanation
Correct answer: B
A greenhouse gas is any atmospheric gas that absorbs and re-emits infrared (heat) radiation, trapping outgoing heat and warming the atmosphere - the greenhouse effect. Among simple atmospheric gases, the dominant mechanism behind this heat-trapping behaviour is a vibration that changes the molecule's dipole moment; symmetric, non-polar molecules such as homonuclear diatomics (N2, O2) and monatomic noble gases (argon) have no dipole-active vibrational mode, so they are not counted among the greenhouse gases.
Among the given gases, carbon dioxide (CO2) is a triatomic, linear molecule whose bending and asymmetric-stretching vibrations do change its dipole moment, so it strongly absorbs and re-radiates infrared energy - this heat-trapping behaviour is exactly what defines a greenhouse gas.
Contrasting the four gases by molecular structure and infrared behaviour:
Gas | Molecular structure | Infrared / greenhouse behaviour |
|---|---|---|
Nitrogen | Homonuclear diatomic (N2) | No dipole-moment change on vibration - no vibrational infrared absorption, despite forming about 78% of the atmosphere |
Argon | Monatomic noble gas | No molecular vibrations at all - not ordinarily classified as a greenhouse gas |
Oxygen | Homonuclear diatomic (O2) | No net dipole-moment change on vibration - no vibrational infrared absorption |
Carbon dioxide | Triatomic, linear (CO2) | Bending and asymmetric-stretch vibrations change the dipole moment - strongly absorbs and re-radiates infrared energy (greenhouse gas) |
Independent confirmation: authoritative bodies such as the US EPA and NASA list carbon dioxide, methane, water vapour, nitrous oxide, and ozone as the principal greenhouse gases - none of these is nitrogen, oxygen, or argon - so carbon dioxide is the correct answer.