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Q & A: air absorbing IR radiation

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Most recent answer: 09/07/2013
Q:
Infrared Radiation: Infrared radiation doesn't require a media to transfer heat energy. So it can travel through a vacuum. But if it is absorbed by all objects, presumably when infrared radiation travels through the air it is absorbed by air molecules (of H20, nitrogen, C02, etc.). If objects absorb infrared radiation, is some of this heat energy lost when infrared radiation travels through the air, being absorbed by various air particles? Many thanks.
- Alex (age 16)
UK
A:
Certainly some of the molecules in air absorb IR energy. H2O, CO2, CH4 are some of the most important. (N2 isn't.) The energy isn't lost, however. It leaves the molecules again, mostly still as IR but in a new random direction. This process slows the escape of IR energy into space and is the cause of the greenhouse effect, which keeps the Earth substantially warmer than it would be otherwise.  Changing the concentration of the molecules in the atmosphere changes the size of the greenhouse effect, so that's why our emissions of CO2 and CH4 are warming the Earth. (The H2O concentration adjusts more quickly, via evaporation and rain, to a level set mainly by the average surface temperature.)

Mike W.

(published on 03/29/2013)

Follow-Up #1: greenhouse gases

Q:
Carbon dioxide absorbs infrared light, as I understand it. Does that mean that carbon dioxide also retains more heat? Because it absorbs more energy? [non visible light which I guess is still energy}. What other gases absorb infrared. Is there a homemade tool that would allow me to see which gases absorb infrared? Where does infrared energy go?
- Brigitte (age 25)
Montreal
A:

Hi Brigitte- I've moved your question to follow-up a more closely related thread.

To answer the new parts:

Those IR-absorbing gases do have some heat capacity associated with the same modes that do the absorbing. At a given temperature, the average thermal energy of a CO2 molecule is a little more than that of an O2 molecule for that reason. However, there's not a consistent simple connection between those properties, since molecules can have modes that pick up thermal energy but don't contribute to the IR absorption.

I guess you can do a simple IR-absorption experiment without any lab equipment. It's not very expensive to buy an IR meter, but it's even cheaper if you don't insist on the name. Those little ear thermometers you get in the drugstore measure temperature via IR radiation. I bet you could rig up some setup with one of them to test gases for IR absorption.

Here's a possibility. Use a covered ceramic pot of water kept barely boiling on an electric stove as an IR source. I say ceramic because it's likely to absorb/emit IR better than metal. The water in the pot is just to keep the temperature steady near the boiling point. The cover is to avoid having too much water vapor come out. A vent fan would help with that. An electric stove is preferable because it won't give off various combustion products  (including CO2) to confuse things, unlike a gas stove. (You could use yourself as an IR source, but the pot will give more radiation and be easier to detect.)

Mount the ear thermometer, with its little plastic sensor cover, some distance away, pointed toward the pot. Adjust the distance until the reading comes out in the thermometer's range.  I bet you can see the reading change if you blow water vapor or CO2 between the stove and the thermometer.

It's possible that you could do the same thing just by feeling the radiant heat with your hand instead of a thermometer, but you'd have to be sure that the blowing gases weren't directly cooling it.

Please let us know if this works!

Mike W.

 


(published on 09/07/2013)

Follow-up on this answer.