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Can anyone clarify the CO2 band saturation thing?

post #1 of 8
Thread Starter 

I'm still not fully grasping some aspects of RealClimate's explanation (1 and 2) of how CO2 overcomes the band saturation of IR radiation. It seems they have two general explanations of it (correct me if I got it wrong):

 

1. Adding more CO2 increases the height in the atmosphere at which IR radiation can escape, thus "filling up the sink" and keeping the waves of IR in the atmosphere for a longer time. This also has the effect of increasing CO2 concentrations even higher than the height of this emission point, where it is still less saturated and thus much more effective as a GHG.

 

2. Increasing partial pressure of CO2 widens the wavelength that can be absorbed by CO2 (see the first two graphs in article 2).

 

I think I understand the first argument but I want to make sure...basically it is saying that since CO2 from the surface quickly mixes itself with all levels of the atmosphere, and since CO2 tapers off as you get higher and higher, that increasing the concentration at the ground level essentially increases concentrations at all altitudes equally, including these areas of lighter concentration high up. This in turn has the effect of raising the level of its greenhouse effect to a higher altitude--right? (By the way how high up are we talking about here? They just talk about the 'cold, dry' areas but they don't say how high...) 

 

Where I really have trouble is the second part. You can see in the graphs in the second part of the explanation that adding CO2 increases the IR range where it has an effect as a GHG. But how does this happen?

 

It seems the only explanation they give is this:

 

CO2 colliding with itself in a tube of pure CO2 broadens the lines about 30% more than does CO2 colliding with N2 or O2 in air, which results in an additional slight overestimate of the absorption in the laboratory experiment.


...so adding more CO2 in effect causes itself to broaden the range, because collisions with itself (and I'd imagine other trace gasses) increase the range compared to collisions with nitrogen and oxygen. But...why? Is there a theory behind this or is it just an observation?

They also mention 'pressure broadening' which states that as you decrease pressure by increasing altitude, you increase the range of IR absorbed (by the way is there a theory behind this?) but that shouldn't be affected by increasing concentration right?

post #2 of 8

CO2 increases broadening because it is heavier and there is more momentum exchanged in CO2-CO2 collisions than in CO2-N2 (or CO2-O2) collisions.  The difference in mass is about 30%, hence the 30% increase in broadening.  See here.  Other trace gases will only increase the broadening if they are heavier than N2/O2, but there aren't any of those at significant concentration for this to be a factor. 

 

post #3 of 8

I forgot the first parts, sorry. 

 

The cold parts they are taking about are from about 6km up to 10 km, really the top part of the troposphere.  This is why deep convection is so important in the global heat budget, since it lifts heat above the layer of the atmosphere where most of the IR absorption occurs. 

post #4 of 8
Thread Starter 
Quote:
Originally Posted by gcnp58:

CO2 increases broadening because it is heavier and there is more momentum exchanged in CO2-CO2 collisions than in CO2-N2 (or CO2-O2) collisions.  The difference in mass is about 30%, hence the 30% increase in broadening.  See here.  Other trace gases will only increase the broadening if they are heavier than N2/O2, but there aren't any of those at significant concentration for this to be a factor. 

 

 

Okay, so from that article I'm guessing this is the answer:

 

Impact pressure broadening: The collision of other particles with the emitting particle interrupts the emission process. The duration of the collision is much shorter than the lifetime of the emission process. This effect depends on both the density and the temperature of the gas. The broadening effect is described by a Lorentzian profile and there may be an associated shift.

 

I still don't get exactly how this happens, but I guess if it's too complicated to be covered on RealClimate and Wikipedia then it'll be too complicated for me.

 

Thanks for the answer gcnp.

post #5 of 8

It's a collision thing, where the impact between molecules gets molecules into excited rotational levels.  These rotational levels couple into the vibration spectra, broadening the lines.  Heavier molecules colliding lead to higher rotational levels, and the higher the level (and more rotational modes that are active), the more options the molecule has to relax into once a vibrational transition occurs.  Since rotational level changes take far less energy than the changing vibrational energy levels, and there are so many rotational levels available compared to vibration, you don't see large shifts in the wavelength of the vibration transition.  Instead, what you see is the wavelength of the vibrational transition gets smeared over a larger range.  But what is really going on is that you are seeing the effect of more rotational levels being active in the process.

 

Sort of.  A real spectroscopist would beat me to death with his Jerzy-Turner monochromator for explaining it like that. 

 

Pretty much any good p-chem book will have a discussion of this.  Classic references like Hertzberg's spectra of molecules series and Levine's book Molecular Spectroscopy also go into it in some detail. 

 

post #6 of 8

I think that the effects of temperature and pressure are getting confused here. 

 

Temperature has two effects.  First, there is a Gaussian broadening effect due to the relative motion of emmiting (or absorbing) molecule and the observer.  Secondly, the temperature determines the population of the rotational and vibrational states via the partition function.  The states are described by a vibrational quantum number n and rotational quantum number j. Except at high temperatures only n=0 and low j numbers have significant population.  The symmetry of the molecule determines which delta j's are allowed.  While the initial population distribution is significant for emission, it does not matter much for absorption (except at very high photon densities where there is a photobleaching effect) because all allowed transitions are available. 

 

Pressure:  All quantum states in molecules and atoms have an associated natural lifetime.  A longer lifetime gives a greater certainty in the energy of the state (Heisenberg Uncertainty Principle) and hence a narrower line width.  The effect of collisions is to perturb state and shorten the lifetime and hence increase the uncertianty in energy.  This leads to a Lorentzian broadening.  gncp was describing the effect of mass on the perturbation.   

post #7 of 8
Thread Starter 

Thanks guys--a lot of that went over my non-quantum physics educated head, but it's definitely more clear than it was.

post #8 of 8
Thread Starter 

double post


Edited by dawei - Mon, 19 Jan 2009 01:17:57 GMT
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