Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Atmospheric CO2 and diurnal Asymmetry

  1. May 15, 2018 #1
    Something that has bothered me is that CO2 appears to have a much greater role after dark than in the daytime. I was wondering if this is because of a CO2 population inversion during the sunlight hours.
    I am thinking that Sunlight and daytime blue sky, excite atmospheric nitrogen,
    The nitrogen vibration ally passes the energy to CO2, which spontaneously decays back to ground state.
    In a CO2 laser, helium is added to speed up the decay of the .2 eV back to ground.
    In the atmosphere, there is insufficient helium for this task, and so the CO2 energy cycle would slow down.
    The evidence of something like this happening would be 9.6 and 10.6 um emission spectra,
    present during sunlight hours, but not after dark.
    Does anyone have a source for daytime vs nighttime infrared sky spectra?
     
  2. jcsd
  3. May 15, 2018 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Can you be more specific? E.g. in daylight hours the CO2 would be intercepting IR in both directions, whereas at night it is all from ground.
    Not to any great extent, other than in extreme UV, surely?
     
  4. May 16, 2018 #3
    What I am thinking is that during the sunlight hours, the CO2 quickly gets into a population inversion state,
    and a much lower percentage of CO2 molecules are available to intercept IR.
    Since the observed diurnal asymmetry is roughly 3:1 night vs daytime warming,(Karl et al 1993) the effective daytime CO2
    level would be around 320 ppm.
    If this idea is correct, the evidence would be a greater level of 9.6 and 10.6 um atmospheric emissions, during sunlight hours than after sundown.
     
  5. May 16, 2018 #4

    DrClaude

    User Avatar

    Staff: Mentor

    Why would you say that? First, sunlight is not strong enough (it is not laser light), and second, CO2 can't be considered as a two-level system. Even if there was significant excitation of the molecules, they could still absorb more at the same frequencies (absorption in the IR is mostly related to vibrational excitation).
     
  6. May 16, 2018 #5
    Nitrogen does not need laser light to be excited, simply a photon in it's absorption band (Roughly 3.8 to 5 um)
    CO2 does have a meta stable state at the .2 eV level, which can cause a population inversion and limit power in a laser.
    Helium is used in the gas mixture, to allow more molecules to get back to ground state where they can be excited again.
    The same forces are at work in the atmosphere, but without the helium.
    If I am correct, then the excited nitrogen would be vibration ally exciting the CO2 to the .3 eV level, during the sunlight hours.
    The evidence of this would be higher levels of 9.6 and 10.6 um bands in the daylight than at night.
     
  7. May 16, 2018 #6

    mfb

    User Avatar
    2017 Award

    Staff: Mentor

    Please give a reference demonstrating that this can happen in the atmosphere.
     
  8. May 16, 2018 #7
    We know it happens at atmospheric range pressures in the lab, the question is why would it not happen in the atmosphere?
    The real question is if the artifacts of these energy transitions are detectable,
    i.e. are the 9.6 and 10.6 um bands more pronounced during the sunlight hours?
     
  9. May 16, 2018 #8

    mfb

    User Avatar
    2017 Award

    Staff: Mentor

    Because we don't have the specific pumping conditions needed for a CO2 laser in the atmosphere.
    I asked for a reference, which means a peer-reviewed publication, not for "why not" questions.
     
  10. May 17, 2018 at 6:19 AM #9
    Sorry, Most of the discussions of energy states of atmospheric nitrogen revolve around much higher energy states.
    http://iopscience.iop.org/article/10.1088/1367-2630/11/6/065011/meta
    This air fluorescence demonstrates that excited nitrogen exists in the atmosphere.
    They even mention that,
    "2.2. Fluorescence quenching
    Non-radiative molecular de-excitation by collision with other molecules of the medium (collisional quenching) becomes very important even at moderate pressures."
    The reasoning is that if we have excited atmospheric nitrogen, as it cycles back to ground state, it can pass energy
    to any CO2 molecule it encounters, the question is weather this behavior is asymmetrical with day and night?
     
  11. May 17, 2018 at 7:08 AM #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It would clearly be a daytime effect, but is it strong enough to explain the observation you raised in post #1?

    Anyway, as I mentioned in post #2, it is not clear what observation you have in mind. Can you be more precise about CO2 having a "greater rôle" at nght?
     
  12. May 17, 2018 at 7:28 AM #11

    mfb

    User Avatar
    2017 Award

    Staff: Mentor

    @johnbbahm: This has nothing to do with the claim of a population inversion, or CO2 in general. I closed this thread, if you find a reference discussing such a population inversion send me a message then I will open the thread again.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook




Loading...
Similar Threads for Atmospheric diurnal Asymmetry
B Basic rule for Atmospheric Reentry of Glider Class vehicles