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Thread: Atmospheric IR absorption mechanisms elucidated

  1. #1 Atmospheric IR absorption mechanisms elucidated 
    Administrator KALSTER's Avatar
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    It seems that the specific mechanisms responsible for the absorption of IR radiation by the atmosphere has been further elucidated by studies mensioned in this article:http://www.aip.org/pnu/2008/split/862-1.html.

    The question is: How will this affect global warming forecasts?


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    Forum Isotope Bunbury's Avatar
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    CO2 molecules one at a time can absorb light. But molecules can also absorb light when they collide with other molecules. This collision-induced absorption, occurring at wavelengths different from those for single molecules and accounting for about 10% of overall IR absorption, is insufficiently understood.
    This is a bit confusing. Are they referring to "light" as the entire electromagnetic spectrum, thus including IR in that category? CO2 is transparent to visible light, AFAIK.


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    Administrator KALSTER's Avatar
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    Quote Originally Posted by Bunbury
    CO2 molecules one at a time can absorb light. But molecules can also absorb light when they collide with other molecules. This collision-induced absorption, occurring at wavelengths different from those for single molecules and accounting for about 10% of overall IR absorption, is insufficiently understood.
    This is a bit confusing. Are they referring to "light" as the entire electromagnetic spectrum, thus including IR in that category? CO2 is transparent to visible light, AFAIK.
    I think they are talking of light in general, which includes IR.
    Here is the EM spectrum which shows 3 categories of IR radiation (I'm sure this is nothing new to you :wink: )

    .

    According to the wiki article, CO2 absorption varies with elevation and the wavelengths at which CO2 absorbs light are close to that of the spectral lines of the molecule. The major absorption bands are 2350 and 667 cm<sup>-1</sup>. I am not sure how to convert these to specific absorption wavelengths, but I am guessing that the two bands represent the two ways IR is absorbed by the CO2 molecule, which is (1) by individual molecules and (2) during collisions (contributing about 10% to total absorbtion).

    The studies mentioned by the article, I think, implies that the absorption during collisions can be understood using classical means (momentum, etc), while individual molecule absorption is gauged using quantum mechanics (the molecule stretches, forming a dipole I think).

    So being a layman, I don't really know how to make sense of all this and how it will affect greenhouse effect forecasts :? . Still interesting though!

    Reason for Edit: Collisions account for about 10% of IR absorbtion, not the greater percentage as I stated originally.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  5. #4  
    Forum Isotope Bunbury's Avatar
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    Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm). In a scientific context, the word light is sometimes used to refer to the entire electromagnetic spectrum.
    Well, all right then. Wiki (above) hedges its bets. I always think of "light" as referring to the visible spectrum, but evidently that's too restrictive.

    I too don't know how or if it will affect climate change forecasts. I expect http://www.realclimate.org/ will write about it before long.
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