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I am looking for a molecular-level understanding of the greenhouse effect.

What is it about the carbon-dioxide molecule (and methane, and water, etc) that is different from other gasses (particularly, N2 and O2) such that it works in the atmosphere to trap heat?

Is it, say, the distance between nuclei in the molecules relative to the wavelengths of infrared light? Dipolarity of the molecule? A combination of various factors?

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2 Answers 2

up vote 5 down vote accepted

To absorb infrared light, a stretching or bending vibration of the molecule must change the molecule's dipole moment. In $N_2$ and $O_2$ there is no dipole moment regardless of how you stretch the bond. On the other hand, O=C=O can change dipole moment by the C moving toward one O and away from the other O, or by bending with the C becoming a vertex of an obtuse angle. Water and methane molecules can also change dipole moment.

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Excellent (+1). So is there a metric for this "bendability" -- the ability of a molecule to change dipole moment that I could look up the comparative values for and quote in discussions on the topic? –  Faust Feb 26 '14 at 20:42
Why must a stretching or bending occur? Consequence of the low energy? –  BMS Feb 26 '14 at 20:53
Finally, I think I've found my reference, which supports this answer: acs.org/content/acs/en/climatescience/greenhousegases/… –  Faust Feb 26 '14 at 22:17
All bonds of molecules can stretch, and any molecule with 3 or more atoms can bend. For diatomic molecules, if the two atoms are the same element, the dipole moment cannot change by stretching, so it is not InfraRed active. Diatomic molecules with two different atoms do change dipole moments by stretching and are IR active. Monoatomic gas don't have any bonds, so they are not IR active. Molecules with 3 or more atoms are generally IR active, I can't think of any counter examples. The bends and stretches correspond to quantized osillations, the IR energy matching energy level differnces. –  DavePhD Feb 26 '14 at 22:58

IR emission and absorption for major Greenhouse Effect gases. Note that adding more absorbance to as saturated line (A greater than ~2, less than 1% tranmission) increases absorbance by log[concentration] not Beer's law [concentration]. ALWAYS look at the wavelength scale and the tranasmission or absorbance scale. The open transmisison windows are marvelously filled by HCFCs that were supposed to mitigate the Greenhouse Efect. Oop$ie. enter image description here

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