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Infrared active gases

Gases which can absorb and emit longwave radiation are said to be infrared active and act as greenhouse gases.

Most gases whose molecules have two different atoms (such as carbon monoxide, CO), and all gases with three or more atoms (including H O and CO), are infrared active and act as greenhouse gases. (Technically, this is because when these molecules vibrate, those vibrations modify the molecular dipole moment, or asymmetry in the distribution of electrical charge. See Infrared spectroscopy.)

Gases with only one atom (such as argon, Ar) or with two identical atoms (such as nitrogen, N, and oxygen, O) are not infrared active. They are transparent to longwave radiation, and, for practical purposes, do not absorb or emit longwave radiation. (This is because their molecules are symmetrical and so do not have a dipole moment.) Such gases make up more than 99% of the dry atmosphere.