I understand that CO2 absorbs IR photons at two specific energies because the molecular bond has more than one allowable vibrational energy. These energy levels are quantised. The energy thus absorbed (in bond vibration) does not increase the kinetic energy of the molecule which means that the energy absorbed is not thermalized. Because of the quantised nature of the bond it can only relax to the ground state by emitting a quantum of energy at the same frequency as it absorbed it. Most atmospheric gases cannot absorb such quanta, so how is the absorbed energy thermalized in the atmosphere?
In a confined volume, the emitted IR will be thermalized at the walls of the containment, hence the measurement of fine line absorption in a spectrometer.
I am no expert in these matters so any correction/elucidation of the above understanding would be welcome!

I think Martin was being ironic, RKS.

Ronaldo

I understand that CO2 absorbs IR photons at two specific energies because the molecular bond has more than one allowable vibrational energy. These energy levels are quantised. The energy thus absorbed (in bond vibration) does not increase the kinetic energy of the molecule which means that the energy absorbed is not thermalized.

You are right that here are two discrete 'vibration modes' when it comes to CO2 absorption (bond stretching and bond bending) which are quantised (i.e. there are only two narrow windows of energy at which they operate) but why do you think that absorbed vibrational energy doesn't increase the kinetic energy of the molecule?

Vibrational energy IS kinetic energy. The molecule IS thermalized, it's hotter after the collision.

It will shortly collide with diatomics (as it was doing every picosecond before the collision anyway) or other GHG molecules and within about 10 collisions (on average, at average temp and pressure) will have transferred that quantum of energy to another molecule - a vibration in a CO2 molecule bond can easily translate into a rotational, translational or vibrational energy in other molecules remember, they are not bound by the same vibrational modes as the CO2 is.

Actually I think it's three frequencies: The O=C=O bending, the asymmetric stretching mode, and the more feeble symmetric stretch.

Someone has kindly put up a video on youtube:
http://www.youtube.com/watch?v=W5gimZlFY6I

Symmetric stretch does not produce a change in dipole moment (in laymans terms, the central node must move) so is unable to absorb or emit IR photons.

Fair enough. Mind you, I've also just learned a bit about Fermi resonance in CO2. I wonder how often that gets incorporporated in the GCMs?

Martin,

I remember reading that mean time to IR emission (relaxation time) is in the order of 10 microseconds, whereas in a fairly dense warm atmosphere, e.g. air at sea-level - kinetic collisions are in the order of 3 nanoseconds or so (not every collision results in a transfer of energy, though).

I would take from this that IR photon exchange at sea-level is a minor component of heat exchange. At the top of the atmosphere where density and temperature are lower and provide fewer collisions, the IR component becomes more likely and becomes the dominant means of heat exchange right at the edge of the atmosphere.

Aug 1, 2014 at 6:55 PM | Martin A

This must be well known from the lab. It must be important for the models to work out where the energy goes??

When I went out this afternoon I thought you were pulling my leg, but I see now I was confused. What I was thinking of as concentration, wasn't, or not in the sense of ppm (the units should have alerted me but didn't). I was thinking in terms of number of CO2 molecules per unit volume.

So I'll restate what I said to be what I meant:

Rhoda, if at height h in the stratosphere the number of CO2 molecules per cubic meter is some nominal X, do you accept that after doubling the % CO2 in the atmosphere h will increase for the same X? It all hangs on this.

On metaphors, have you never used metaphors to describe concepts to those who do not have the necessary domain knowledge to understand a full explanation (for example). You would be unusual I think. Your mice show that you like metaphor. Why would explanations without metaphor be so superior to those with metaphor when explaining a concept for which years of training are necessary to come close to understanding?

On those vibrational modes and discrete energies, how do they relate to the "almost 315,000 individual absorption lines for CO2 recorded in the HITRAN database" mentioned by SoD? Maybe those are all minor compared to the two modes mentioned, but they are all discrete energies. What do they correspond to physicaly?.

TBYJ
Thank you for your reply. However I don't see how an increase in the bond energy within the molecule contributes to the kinetic energy of the molecule. Is not the absorbed photon energy held as elastic energy until the bond relaxes to its ground state by emitting a photon of electromagnetic energy? Is there any experimental evidence for thermalisation in an open system like the atmosphere? I am genuinely puzzled and ask for my further education.

I'm sure someone will tell me there is a world of difference but metaphor/analogy are such similar words that I see no way of saying metaphor bad, analogy good. Neither can I say for sure whether trapping heat is a metaphor or an analogy. When it comes to explaining science or computing etc to my grandmother analogies are essential aids. They do distort the true meaning but that is inevitable. The general population is, educationally, my grandmother. So trapping heat like a blanket is wholly appropriate. That is why people, even sceptics, use it.

I had the same understanding of energy levels etc as you but found the large number surprising. Sure, permutation can grow hugely but that assumes that a molecule can go between any two states. The large number implies maybe it can.

Does my question to Rhoda make more sense now?

"However I don't see how an increase in the bond energy within the molecule contributes to the kinetic energy of the molecule."

Consider what happens when two molecules, either or both of them spinning and bucking and bouncing like wild things, bump into one another. What happens?

Ronaldo, it's a good question

"However I don't see how an increase in the bond energy within the molecule contributes to the kinetic energy of the molecule."

and NiV gave a good answer. Without going into statistical mechanics and thermodynamics you just need to think about large numbers of molecules and the fact that the internal energy of a molecule consists of translational, vibrational, rotational and electronic components. Whilst the quantum levels for these different energy components are very far apart (electronic > vibrational > rotational > translational) conversion between them is possible. You do this experiment every day when you boil the water in a saucepan. The energy required to raise the temperature of the metal of the saucepan is very much less than that to raise the temperature of the water. This is because the specific heat capacity of the water is very much greater than it is for the metal in your saucepan. Yet if you think about it the translational kinetic energy of the atoms in the saucepan is the same as the water (3/2kT). So why all the extra energy to heat the water? It's because it is being converted into internal energy (largely vibrational and rotational) of the water molecule.

Martin A if the vibrational state relaxes to a translational one then the temperature increases, yes. See my answer above because it's easier to think about these things in terms of large ensembles of molecules. Also the translational energy is quantised. The levels are so close together however it looks continuous.

"What governs the time between a CO2 molecule absorbing a photon and then re-radiating a photon of the same wavelength? Since CO2 molecules don't have any time-recording device on board, presumably it has an exponential (memoryless) distribution. But what sets the average time to re-emit?"

It involves calculations with quantum wavefunctions and is possibly a bit complicated for pop-science explanations. But out of interest, you might want to look up Fermi's Golden Rule.

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/fermi.html

Paul Dennis

Thanks, but I don't think that heating a mass of water by thermal conduction in a pan is much of an analogy for shining electromagnetic radiation onto CO2. I accept that the specific heat capacity of water is greater than the metallic pan, but the mass of the water, the heat transfer properties of the water/pan interface the heat losses from the water due to vapour pressure increases and evaporation before the boiling point is reached will have a much greater influence on the rate of heating.
Reverting to CO2, I am still not clear that thermalisation can take place in a non-bounded system and would love to see the experimental evidence for this.