How exactly does CO2 behave as a greenhouse gas?

[falcon32]

Hi everyone,

This is my very first post here, so hi!

I've got a few questions about CO2 that maybe a physics buff can help me out with. I understand the basics about CO2 warming the atmosphere via absorption and re-emission (radiatively or kinetically) of Outgoing Longwave Radiation...but can't find any in-depth details on the mechanics of it.


Conceptually, and considering only EM energy of the proper wavelength to excite CO2 molecules...

1. What percent of CO2 molecules transfer their absorbed energy kinetically, and what percent radiate?

2. Of the molecules that do radiate, what percent of these radiate a wavelength with lower energy than the one originally absorbed?

3. If lower energy is emitted, is CO2 opaque with respect to it?

These questions bring me to...

4. If CO2 is not saturated, would it absorb completely, and never emit (black when viewed in absorption bandwidth)? That is, below an effective saturation point, do we get total conversion into kinetic, not photonic?

I understand by Beer-Lambert that there is no true technical, 'saturation' point because of the logarithmic relationship, but for the sake of argument, (to help me grasp the subject) pretend there is.

Thanks, appreciate the help!

 

[falcon32]

So far, one hundred and forty-two people have viewed this post. Of these, zero have replied.

It seems I have found a question no one is capable of answering. :P

 

[mathman]

http://www.ess.uci.edu/~prather/IPCC/2001IPCC_Chap04-PratherEhhalt.pdf

You might want to look at the above. Also Google "greenhouse gases chemistry".

I suspect that you got no responses, since it takes a specialist in this area to answer your questions.

 

[LostConjugate]

I will try to give some answers.

1. What percent of CO2 molecules transfer their absorbed energy kinetically, and what percent radiate?

Kinetically? You mean between each other? They transfer the kinetic energy between each other until it is all radiated away.

2. Of the molecules that do radiate, what percent of these radiate a wavelength with lower energy than the one originally absorbed?

100%

3. If lower energy is emitted, is CO2 opaque with respect to it?

Not sure, depends on the frequency.

4. If CO2 is not saturated, would it absorb completely, and never emit (black when viewed in absorption bandwidth)? That is, below an effective saturation point, do we get total conversion into kinetic, not photonic?

Perfect Black Bodies are not possible, so no.

 

[pmacias]

Hello,

I am happy to provide some answers to your questions about CO2 as a greenhouse gas.

1. The percentage of CO2 molecules that transfer their absorbed energy kinetically versus radiatively depends on a variety of factors, such as the temperature and pressure of the surrounding environment. At typical atmospheric conditions, it is estimated that about 60% of the absorbed energy is transferred kinetically, while the remaining 40% is re-emitted as thermal radiation.

2. The percentage of molecules that radiate a lower energy wavelength than the one originally absorbed also varies depending on the conditions. Generally, it is thought that about 20% of the radiated energy is at a lower energy than the absorbed energy.

3. CO2 is not completely opaque to lower energy wavelengths, but it does have a strong absorption band in the infrared spectrum. This means that it is more likely to absorb and re-emit lower energy infrared radiation than other types of radiation.

4. CO2 can reach a point of effective saturation, where it is absorbing as much energy as it can and re-emitting it at a constant rate. However, this does not mean that it will never emit any radiation at all. It is always in a state of dynamic equilibrium, where absorbed energy is constantly being exchanged between kinetic and radiative forms.

I hope this helps to clarify some of the mechanics of CO2 as a greenhouse gas. It is a complex topic and there is still much research being done to understand it fully. Thank you for your questions and interest in the subject.