Infrared energy absorption of H20 molecules as a percentage of energy

[darksat]

I eventually managed to find that a CO2 molecule can absorb about 8% of infrared energy passing through it that's radiated from Earth but its actually been incredibly hard to find a comparable figure for H2O molecules. Has anyone got a percentage figure for H2O in the form of atmospheric water vapour. I want to do a direct comparison of the infrared absorption capabilities of the 2 molecules.

 

[Andrew Mason]

HI Darksat. Welcome to PF!

Interesting question. I have done a bit of digging for you and for what it is worth here is my take on this.

The ability of a gas to absorb radiation in the depends on several factors such as: the concentration of the gas; the ability of the gas molecule to absorb radiation at different energies; the spectrum of radiation that is incident on the gas; and the path length through the gas that is being considered. So I am not sure what your 8% figure actually refers to.

For CO₂ the concentration is fairly uniform throughout the lower atmosphere (troposphere) whereas water vapor concentrations tend to have large local variations.

The ability of gas molecules to absorb IR radiation depends on the ability of the molecule to vibrate or rotate in sympathy with the incident radiation. Water and CO₂ have different geometries. The CO₂ molecule is linear whereas the H₂O molecule is bent. This gives the water molecule more vibrational modes than the CO₂ molecule (two kinds of stretch vibrations where the H atoms move toward/away from the O atom, and a bent mode where the H atoms oscillate toward/away from each other, around the O atom). The H₂O molecule also has a lower moment of inertia because the H atoms have much less mass than O and are closer together than the O atoms in CO₂, so it rotates more easily. So the H₂O molecule is able, therefore, to use vibration and rotation to absorb lower energy (longer wavelength) radiation than CO₂.

As a result, the CO₂ molecule can absorb certain wavelengths of IR (infrared) radiation: at wavelengths mainly around 15 μm and between 2-4.3 μm. The H₂O molecule has a much broader absorption spectrum and absorbs IR very well at wavelengths above 20 μm and well into the microwave range, and from about 4 to 8 μm. It does not absorb very well in that 8-20 μm range, and is especially poor at absorbing in the 8-14 μm range. The reasons for this have to do with quantum physics.

Earth temperature averaging around 15C or 288K radiates at a peak wavelength of 10 μm (Wien's law). That is a part of the spectrum that water does not absorb very well with that "window" through water between 8 and 20 μm (the window is clear between 8 and 14 μm and from 14 μm to about 20 μm it starts closing).

CO₂, on the other hand, absorbs very well around 15 μm so it blocks part of that H₂O window. As Earth temperature increases, there will be a bit more IR at that 15 μm wavelength to absorb but much more in the 4-8 μm range that water vapor is very effective in absorbing. As Earth temperature increases, there will also be a higher concentration of water vapor present in the troposphere. This is the water vapor feedback effect from the radiative forcing of CO₂.

The effectiveness in absorbing IR radiation is shown by the molecule's Absorption Coefficient which is a function of frequency/wavelength and concentration over a certain path-length. So a gas' effectiveness in absorbing the total amount of radiation that is incident on the gas depends on its Absorption Coefficient for a given wavelength, concentration and path length, the spectrum of that incident radiation, the concentration of the molecule in the gas and the path length.

What we can say is that CO₂ tends to block IR in at least 2 wavelengths that water vapor does not block well and that one of those wavelengths, 15 μm, is near the peak (10 μm) of the IR radiation spectrum that the Earth surface emits.

References:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6174548/http://230nsc1.phy-astr.gsu.edu/hbase/wien.html#c3https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_waterAM

 

[Andrew Mason]

Just as a follow-up, it is important to keep in mind that atmospheric absorption of IR radiation is already significant due mainly to water vapor and CO₂. Otherwise, Earth surface temperature would be at its natural black-body temperature of about 250K (-23C) instead of what it is: 288K (15C). It is also important to note that most of this IR absorption is by water vapor, the concentration of which is determined by atmospheric temperature and proximity to bodies of water.

As seen from Fig. 1 in the Wei paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6174548/, the combination of CO₂ and water vapor in the troposphere (at present concentrations) absorbs about 70% of the IR emitted from the Earth surface. Most of the IR that gets through the atmosphere (and, therefore, does not contribute to atmospheric warming) is in the 8 μm to 14 μm window. If it were not for the presence of CO₂ that window would be wider - 8 μm to 20 μm.

The CO₂ concentration, therefore, appears to be an important factor that controls the amount of IR that gets through that window.

An increase in CO₂ concentration decreases the amount of IR that gets through the window, which causes an increase in the atmospheric temperature that, in turn, causes an increase in the ability of the atmosphere to hold water vapor. Since water vapor is effective in absorbing IR across a broad IR spectrum, this slight increase in CO₂ absorption is amplified by causing an increase in water vapor concentration, which then increases absorption of IR across the IR spectrum.

AM