Which of the following is a greenhouse gas?

Concept: A greenhouse gas is an atmospheric gas that absorbs infrared (heat) radiation and re-radiates it back toward the Earth's surface, warming the lower atmosphere -- the greenhouse effect. The gases recognised for this heat-trapping role are carbon dioxide, methane, nitrous oxide, ozone, water vapour, and the fluorinated gases; each has a molecular structure able to vibrate in a way that absorbs infrared radiation. A gas whose structure cannot vibrate this way -- a symmetric diatomic element or a lone noble-gas atom -- plays no part in the greenhouse effect.

Application: Checking each option against this rule:

• Methane (CH4): a tetrahedral molecule with four polar C-H bonds; its bending and stretching vibrations continually shift the dipole moment, letting it absorb infrared radiation and re-emit it as heat -- exactly the heat-trapping behaviour of a greenhouse gas, and methane’s global warming potential runs many times that of carbon dioxide over a 100-year horizon.

• Oxygen (O2, O=O): a symmetric, homonuclear diatomic molecule; its vibration does not shift the dipole moment, so it plays no part in absorbing infrared radiation or in the greenhouse effect.

• Argon (Ar): a monatomic noble gas with no chemical bonds and so no vibrational modes; like oxygen, it takes no part in the greenhouse effect.

• Sulphur dioxide (SO2): a real atmospheric pollutant, but its recognised environmental role is different from a greenhouse gas’s: after oxidising in the air, it reacts with water vapour to form sulphuric acid -- acid rain -- and forms light-scattering sulphate aerosols that cool rather than warm the atmosphere. It is classed as an air pollutant and acid-rain precursor, not as a greenhouse gas.

Cross-check: The IPCC's recognised list of greenhouse gases -- carbon dioxide, methane, nitrous oxide, ozone, water vapour, and the fluorinated gases -- is built around exactly this heat-trapping vibrational behaviour; sulphur dioxide is absent from that list because its principal effect (acid rain and cooling aerosols) is not heat-trapping, and oxygen and argon are absent because their molecules cannot vibrate in the way infrared absorption requires.

Methane is therefore the greenhouse gas among the given options.