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Does emission depend linearly on concentration?

  1. Apr 16, 2009 #1
    Is it possible to find out the relative concentration of c02 by looking at the emission or absorption from a hot gas, T = 1000K, containing among other C02? What I want to find out is if the emission depends linearly on the concentration. (in the IR region)

    What theoretically equations may I used to find this out? I have been looking at literature of Radiative transfer but I do see how this can be possible. I have data from the hitran database (giving wavelength, intensity (or equivalent width), Einstein coeff, statistical weights, FWHM, lower level energy and such)

    Does anyone know?
  2. jcsd
  3. Apr 16, 2009 #2


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    I am not totally sure, but I don't think it will have a strong dependence on concentration beyond a certain point. Once the gas is optically thick, you'll have a kind of radiative equilibrium within the volume, and thermal emissions will only escape from outer edges of your volume. The total power of emission will then be a function of temperature and surface area of the volume. As concentration drops, the optical depth changes to give, I guess, a smaller effective radiating surface. As you become optically thin, I guess it thermal emissions will become linear with concentration of the gases.

    I'm thinking out loud here. If I get better info I'll correct myself. Don't take this as gospel, but as an aspect of the problem that might be worth thinking about.

    Different gases have different emissivity spectra, of course, and that's going to alter the relationship between optical depth and concentration. Some gases will shed energy more efficiently at 1000K than others, but I don't know where CO2 stands on that.

    Cheers -- Sylas
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