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We are all told at school water vapour and carbon dioxide are the top two greenhouse gases, and that they absorb thermal infra red radiation, trap heat and warm up the Earth. My question is how do they do that? Why can't Oxygen or Nitrogen or any other gas not absorb infra red radiation as well as water vapour or CO2?

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up vote 6 down vote accepted

As you can see on these absorption spectra for $\textrm{H}_2\textrm{O}$ and $\textrm{C}\textrm{O}_2$, both molecules have moderate to strong absorbtion in the mid-IR wavelengths, with the absorption of $\textrm{C}\textrm{O}_2$ extending out into the longer wavelengths.

Other molecules common in the atmosphere don't have such strong absorption at the wavelengths given off by thermal radiation. If you are asking why that is, I'm afraid I can't give you a very detailed answer, except to say that the absorption spectra of molecules (and atoms) is governed by quantum mechanics. Maybe somebody else can explain how they would be calculated from principles, but that is beyond my education.

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OP states in the body of the question that water and carbon dioxide absorb IR... not sure what this answer adds – pentane Oct 2 '15 at 14:08
Wow. This is over 4 years old. Anyway, what it adds is essentially a statement that these molecules absorb the wavelengths they absorb because it is an intrinsic property, dictated by the laws of the universe in which we live. – Colin K Oct 3 '15 at 0:12

The absorption of IR radiation is due to vibrations of molecules. When a vibration causes change in charge distribution (or dipole moment to be more specific) the IR radiation is absorbed.

Generally, hetero-polar molecules, like $H_2O \; and \; CO_2$, have permanent dipole moment. The external oscillating electric field in this case perturbs the Hamiltonian and causes IR absorption. Hence, they contribute to "Green House effect" by absorbtion of heat.

$H_2O$, which is non-linear molecule, has three fundamental modes of vibrations. Symmetrical Stretching, asymmetrical Stretching and scissoring (bending). $CO_2$, which is linear molecule, has four fundamental modes of vibrations. Symmetrical stretching, asymmetrical stretching and two degenerate scissoring modes, in planes perpendicular. The symmetric stretching mode in $CO_2$ does not produce or absorb any IR, as it does not cause change in dipole moment, but other modes do change charge distribution causing absorption of IR.

Well, the homo-polar molecules, like $N_2 \; and \; O_2$, does not have any permanent dipole moment. The external oscillating electric field does not perturb the Hamiltonian for nuclear motion, and does not absorb IR, though it perturbs Hamiltonian for electronic motion. Hence, do not contribute for "Green House effect".

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Chutsu was seen here last time 1st of may. And homo-polar is a nonsenseword. This words are used in chemistry for different types of bonds, but for polarity of molecules this words are nonsense. – Georg Oct 27 '11 at 16:50
Actually, CO2 does not have a permanent dipole moment. What is important is the transition dipole moment, i.e. how the dipole moment changes due to the vibrational mode, as you've explained. – Rob Oct 2 '15 at 7:55

Since CO2 re-radiates IR energy, it can't be said that it "absorbs" the energy. It just converts it to another frequency in the electromagnetic spectrum. The end result is transparency.

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In some quantity, everything in the air including nitrogen and oxygen absorbs and emits black body radiation at frequencies which overlap the frequencies absorbed by CO2. In fact, the only reason why there is IR in the air is because the surface of the earth emits black body radiation in proportion to its temperature. The air then does the same thing at some level. The question then is, in what quantity is the atmosphere absorbing and emitting black body radiation. The emissivity of nitrogen and oxygen gasses should be closed to 100%, since they do not reflect IR significantly. But the larger question is how does the quantity of black body absorption compare to the fingerprint absorption of CO2. Actual measurements and numbers do not seem to exist. So promoters use computer models to divide up the heat of the atmosphere between pollutants such as CO2 and everything else. They then pull such numbers out of the hat which say increases in CO2 levels will create a global temperature increase of about 6 deg. This is about 20% of the 33 deg. which the atmosphere is said to contribute to the temperature of the globe.There are about 30 times as many water vapor molecules in the air as CO2 molecules, and water vapor has a more effective fingerprint spectrum. It is also much more variable. This means water vapor will swamp whatever CO2 does. It is obviously not being honest to say CO2 does twenty percent of the heating, when there is a hundred times as much water vapor doing the same thing.

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The real fact is that all of the atmosphere is principally heated by thermal conduction and convection - heating the atmosphere by absorbing infrared radiation is negligible.

Water vapour is accepted to be about 1% of a normal atmosphere. At the Earth's surface the mass of a cubic metre of air is about 1.205 kg. As we go up in the atmosphere pressure decreases and thus so does the mass of air per cubic metre.

At 1% water vapour concentration by volume or about 1.5% by weight there are about 19 grams per cubic metre at sea level - significantly less at higher altitudes. Kiehl & Trenberth claim greenhouse gases produce "backradiation" of 324 Watts / square metre.

A watt is equivalent to a Newton metre per second. A newton is a mass of 1 kilogram accelerated by gravity - 9.8 metres per second.

Hence 324 Watts per square metre is equivalent to 324 Newton metres per second.

324 Newtons is 33 kg subjected to gravity's acceleration.

If anyone can explain to me how a mere 12 grams of water vapour in every cubic metre of normal air at sea level can produce that sort of energy I would feel much enlightened - I simply say it is impossible without furnace like temperatures.

As for Carbon Dioxide at 0.04% by volume which is 0.06% by weight it constitutes approximately 0.732 grams per cubic metre - again significantly less at high altitudes as it is heavier than air.

By similar logic if any one can explain how such an insignificant mass of Carbon Dioxide can produce any detectable quantity of radiation I would feel much enlightened !!

As data from the Moon orbiting satellites a heated surface radiating to space cools at a very slow rate.

True the Moon can become very cold on the night side but that lunar night lasts 354 Earth hours and the surface cools about 290 degrees during that 354 hours - less than a degree per hour.

At that rate the dark side of the Earth would "cool" less than 12 degrees before the Sun rose again.

It is the period of rotation of the Earth that is significant into determining how much the Earth cools.

It is the power of the Solar radiation that determines the maximum daytime temperature.

The atmosphere and oceans reduce the temperature the Solar radiation is capable of causing and there is simple proof in that the Moon's surfaces are heated to over the boiling point of water whilst the Earth never approaches those extremes except when conduction/convection is suppressed or prevented entirely.

Finally do you really believe that although Nitrogen and Oxygen do not significantly absorb Infrared radiation they do not radiate Infrared radiation ?

A well known fact of physics is that everything that has a temperature above absolute zero radiates electromagnetic radiation !

At ambient air temperatures well established physics, Wien's law, predicts that the intensity of the emitted electromagnetic radiation is at a peak well within the Infrared radiation wavelength band.

Almost all of the electromagnetic radiation in the atmosphere is coming from Nitrogen and Oxygen which constitute about 99% of the atmosphere.

The only way Nitrogen and Oxygen would not constitute the vast bulk of this atmospheric radiation is if they were at absolute zero and all the warmth you feel came from 19 grams per cubic metre water vapour and 0.732 grams per cubic metre CO2.

I think that proposition is absurd !

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