How does CO2 convert infrared radiation to heat

[FrankJ777]

I'm trying to understand the issue of CO2 and global warming better. Can anyone explain to me the mechanism by which CO2 in particular converts infrared radiation to heat. I've had a semester of college chem and three semesters of physics, but I'm not sure what theory I should be able to use to predict that, for instance, when a certain frequency of light applied to x amount of CO2 will raise the temperature to y degrees. I've thought that maybe it has to do with the absorption spectra or emission spectra of CO2, or the specific heat of CO2, but I'm really not sure.

Thanks for the help.

 

[klimatos]

Atmospheric CO2, like other so-called "greenhouse gases", absorbs long-wave radiation emitted primarily by other surrounding emitters such as water vapor, clouds, particulates, ozone and minor emitters such as methane and nitrous oxide. When within a few hundred meters of the Earth's surface, it also absorbs long-wave radiation from that surface. It is a poor absorber of short-wave solar radiation. Absorption of this radiation increases the kinetic energy of translation of the CO2 molecules, and hence their temperature.

In the free atmosphere, it is difficult to measure this absorption accurately because many of the other greenhouse gases (especially water vapor) plus clouds and particulates absorb on some of the same wavelengths. Thus, the global absorption by CO2 is only estimated at somewhere around 53 watts per square meter. There are literally thousands of web sites that discuss these phenomena from both the scientific and the political perspectives.

Unfortunately, the topic of global warming is a no-no on these forums.

 

[FrankJ777]

Ok. So does it have something to do with the emission spectra or absorption spectra or something like that? How would one predict the frequency of radiation that CO2 absorbs and emits? Does it have to do with the energy level of the electrons in CO2. My understanding is that electron occupy discrete specific energy levels and can only absorb and emit photons of the same level, using E = hv. So does infrared light match the energy level of some of the electrons in CO2, more so than say nitrogen or oxygen. From what i remember about the spectra, it was dependent on the elements that make up a compound, so that the spectra of CO2 would just be the spectra of O and C superimposed on each other. Does that fact that the atoms are bonded to another type of atom change there energy level and there for cause the compound molecule to absorb and emit photons of a different energy than its constituent atoms?

 

[Borek]

So does it have something to do with the emission spectra or absorption spectra or something like that?

Definitely. Unfortunately, what you wrote later is seriously wrong - spectrum of CO₂ is not a combination of the C and O spectra (although parts of them can be identified, especially in the x-ray range). The important part of the spectra that describes processes you are interested in is in the IR range, these are mostly vibrational and rotational frequencies, electron transitions are not involved.

 

[FrankJ777]

Is there a theory I can use to calculate something like " ΔT = x moles of CO2 absorb y watts". I was hoping there was something straight forward, like when you calculate the ΔT of using the heat capacity of a chemical and the amount of heat added to it.

 

[Borek]

Nope, it is not that easy. Sure, gases have some heat capacity (described by mcΔT), but it is not the way they change the heat balance. CO₂ doesn't just absorb energy, it reemits it quite fast - but part of the energy is sent back down, instead of going away, so the energy is kept on the planet. So what you have to deal with is an equilibrium system, made of many layers of the atmosphere. (Don't treat this explanation as a correct and a complete one - it is not; I known enough to be able to confusing, but not to explain things).

I suggest you go and look at some textbook devoted to the subject. Several years ago I have downloaded "Principles of Planetary Climate" by R. T. Pierrehumbert from the web, at the time it was freely available from the author's page. Now it is in print and there is no official free version, but perhaps you will be able to locate the old version.