beren
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beren asked in EnvironmentGlobal Warming · 1 decade ago

Do gases (at terrestrial temperatures) act like black bodies?

If they can, what are the quantrum transitions that occur that allow for a near continuum of emitted radiation.

If not, can you use an approximation that the gas acts like a black body over a very narrow wavelength range? Does this range have to have a vibrational band near the peak frequency of the black body radiation? Does changing the temperature of the gas change which rotational levels are emitting within the vibrational band and does this follow Planck's law? What are the limits of such an approximation?

Update:

Actually BZ this was not directed at you but more to First Grade Rocks. If you do not know that electrons (and also vibrational modes) can work different in solids than in gases, you should really take a course in solid state physics. You will learn about band theory:

http://en.wikipedia.org/wiki/Electronic_band_struc...

Update 2:

"Are you actually suggesting that gases can't emit IR without greenhouse gases?"

No, I am suggesting that gases do not emit IR unless a transition is available.

Update 3:

BZ, if gases are a black body and according to Kirchhoff's law for a black body emissivity = absorptivity. This would mean that the IR spectrum of any gas would follow Planck's Law and the spectrum of any gas would only depend on the temperature. Is this your position?

Update 4:

BZ, are you suggesting that N2 spontaneously emits IR? What is the mechanism for this process? The only thing I can think of is Raman scattering. What are the thermodynamic implications of this? Why would a molecule loss energy to the surroundings when it is in approximate thermal equilibrium with its surroundings? Please explain the quantum mechanical processes that would cause N2 to emit IR.

Update 5:

Actually I made an error on Kirchhoff's law, that applies to grey bodies, however the point is the same since there is no such thing as a perfect black body.

Update 6:

Another minor error. Individual molecules can loss energy to their surroundings regardless of the environment. The concept of thermal equilibrium does not exist for individual molecules. What I should have asked is: Why would an ensemble of molecules lose energy to their surroundings when they are in approximate thermal equilibrium with their surroundings.

Update 7:

BMR, excellent point about thermal equilibrium. I make the assumption about thermal equilibrium because it makes the discussion easier. Obvioulsy when get down to the real nitty gritty of what is going on local non-equilibrium conditions are important. A quick google search found this:http://ams.confex.com/ams/17Fluid15Middle/techprog...

Anytime a gas absorb photons and excites more vibrational states than dictated by the partition function, then a non-equilibrium condition exists until that energy can be thermalized and the temperature of the gas increases.

Temperature itself can be a tricky thing to define. In gases, when we measure temperature we are really measuring the average kinetic energy of the molecules. In a non-equilibrium condition, over short time periods, you can define things like rotational temperature, vibrational temperature..etc, which can be different from kinetic energy temperature.

Update 8:

BMR, I don't know the answer to what impact non-equilibrium has to this overall discussion. Thanks for the input.

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  • 1 decade ago
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    This question already is a hairball with side issues and I don't intend to address all of them. Paul already has touched on most. The short answer is no, gasses at normal temperatures do not behave like a black body. There is not a continum of quantum states and where states exist, there are issues with the density of states. A rotational band has (in most cases) a PQR structure in which the increasing degeneracy j(j+1) with quantum number j dominates for intermediate values of j. The partition function gives the proper weights, which are different from the Planck distribution alone. Yes, you can measure the temperature (although perhaps defined a bit differently) by measuring the strength of rotational bands and correcting for degeneracy. The temperature depends on the frame of reference. In a molecular beam, all of the molecules might be travelling at 1000 m/s in the z direction, yet the temperature in that direction may be < 10 K because all the molecules are moving at nearly the same velocity. The temperature in the x and y directions is higher than in the z direction in this example.

    I think that part of the confusion about the fate of IR photons comes from the mistaken notion that once thermalized, the energy is disconnected from the radiative process. At thermal equilibrium, a molecule is just as likely to convert kinetic energy from a collision into a photon as to absorb a photon and convert the energy into motion. Paul's Alias notes that conservation of momentum holds for absorption. It holds for emission also. See for example the Compton effect.

    In response to BMR's question about the non-equilibrium case, the result is that the probabilities for thermalizing a photon and emitting a photon are no longer equal.

  • 1 decade ago

    bravozuluL<<CO2 absorbs IR energy by bending or stretching the bond. That is how it is commonly explained and it is an analogy like black body radiation was meant to be in the point that was apparently lost. That energy is transferred to other molecules as heat. The energy required to bend the bond is the same energy that is contained in IR frequencies that are absorbed by CO2. If it acted as a reflector in that sort of absorb/emission sequence, there would be no warming of the air. >>

    You are saying contradictory things. You claim that the emitted frequency is the same as the absorbed frequency, and you are claiminhg that the molecules heat up. It is not possible for both to be the case--the photon fequencies are proportional to the energies, therefore if the emitted photon has the same frequency as the absorbed photon, no net energy has been added to the molecule, and thus it could not heat up.

    The underlying origin of your error is actually subtle and non-trivial. An incoming photon does not simply knock the electron to a higher energy state. Textbooks treat it as if it does, but that is not quite correct. The molecule also picks up a center-of-masss momentum--if it did not, momemtum would not be conserved being that the incoming photon's momentum disappears. This conservation of momentum does not need to be added extraneously, it actually will naturally come out of Schrodinger's Equation if one does the time-evolution very carefully. We are guarenteed that, because conservation of momentum turns out to be guaranteed automatically by quantUm physics--for example at vertices in Feynman Diagrams.

  • 1 decade ago

    This post is a bit "chewey" for my limited knowledge of Physics but I am bookmarking the post for reference. I will try and digest it in further depth later.

    Thank you to both Bravozulu and Paul for some actual science to think about rather than rhetoric and ad homonyms.

    EDIT:

    A lot of discussion on "thermal equilibrium" as a basic assumption, when it would appear that thermal equilibrium, except very locally, cannot and does not exist. What is the impact? Also apparent widespread agreement that much of the energy is thermalized through kinetic interactions between molecules of various types. What percentage of the energy is thermalized and at what rate?

    beren - Thank you for the response and thank you for posting the question. When I first started coming here this is more of what I thought that I might read. I truly appreciate it. My objections to AGW theory are on the media and the process, not on the science. I hate hyperbole mixed in with science and I really think that it is causing a problem with this whole mess. I have my strengths and physics and thermodynamics are not them. Unfortunately, if you could see the Stratigraphic Charts and reconstructed relative and absolute sea level curves (proxy for warming) related to them that I use daily, you would quickly see where my objections stem from. I have tried to get it across in here sometimes to a chorus of "thumbs down" but never any feedback or refutations based on anything resembling scientific discourse!

    Thanks again!

  • 1 decade ago

    Oh dear!

    "Many alarmists seem to have this silly notion that CO2 transfers photons from one CO2 molecule to the other or it simply reflects the same frequency. That is apparently a distortion of how photons are absorbed as electrons jump to higher energy levels and emit the same frequency of photon. "

    Infrared radiation depends, not on electronic, but on vibrational excitation. There is no way in which the gas resembles a black body, which is defined as a body at perfect equilibrium with the temperature-dependent radiation field. A gas at thermal equilibrium with its environment will be continually absorbing photons from the radiation field, and re-emitting them. However, since a gas is nothing like a black body, such processes are less important than vibrational excitation and de-excitation by molecular collisions, excitation by absorption, and de-excitation by emission.

    The upshot is that net emission at a greenhouse gas infrared absorption frequency takes place on average from a depth beneath the top of the troposphere, inversely proportional both to the absorption coefficient at that frequency (itself, BTW, a function of rotational excitation and hence of temperature), and to the concentration of the gas.

    More gas, smaller depth from top of troposphere, greater height, colder gas, fewer molecules excited and less emission.

    "Are you actually suggesting that gases can't emit IR without greenhouse gases?"

    Precisely. Any gas that can emit IR is also a gas that can absorb IR, and therefore a greenhouse gas, through the mechanism described above.

    All this is established physics, the well-known theory of "grey body emission", beautifully explained by John Houghton in his book on global warming. John Houghton, FRS, sometime Prof of atmospheric sciences at the University of Oxford, is therefore in clear disagreement with bravozulu.

    I leave it to the reader to decide which one is probably correct.

    BTW, the sun, being a plasma, is a pretty good black body. However, its upper atmosphere is not. Elements in this atmosphere absorb light (the so-called Fraunhofer lines) from which we can infer their presence. This light, obviously, is not escaping as efficiently as it would from a black body, and so the lines are illustrating the principle of the greenhouse effect.

    Response to BZ: It is simply not true that gases absorb and emit as you say. If the atmosphere were anything like a black body, we wouldn't be able to see the sun. Very hot bodies do approximate to black bodies, but only because they are plasmas and the electron gas approaches a continuum of states.

    "gases ... do emit thermal radiation in ways that have nothing to do with greenhouse gases". There is no mechanism by which a gas can emit infrared radiation, other than transition from a higher to a lower vibrational/rotational state. But if the emission is symmetry-allowed, so is the absorption. In other words, it's a greenhouse gas.

    "radiation from the sky matches the bandwidth of IR absorption of the various greenhouse gases". If the atmosphere were a black body, it would emit continuously. We know that the radiation leaving the earth corresponds roughly to blackbody radiation from the surface (this is possible because the surface has a near continuum of possible energy states), MINUS energy absorbed by greenhouse gases nearer the surface (warm) but only re-emitted near the top of the stratosphere (cold). This slowing down of the escape of energy IS the greenhouse effect.

    I trust that BZ does at least accept that the greenhouse effect exists. I wonder how he imagines it works.

    "increasing greenhouse gases would cause them to emit more heat": Increasing greenhouse gases causes them to absorb more heat, as well as emit more. His analogy of an overcast night sky is a very good one. Of course, if there are more clouds, there is more radiation to space from clouds, but that is less than the additional radiation they reflect back to the surface, rather than allowing the surface to radiate straight out into space.

    BMR: equilibrium is always an approximation, but one that is good enough to answer this question.

    Note, BMR, that either BZ or I must be talking nonsense. To help you decide if I am credible, you might look at my answers in other sections esp. chemistry and science-other to evaluate my scientific expertise.

    d/dx is spot on about the nature of the subtle fallacy that BZ is (with some additional errors of his own) repeating.

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  • 1 decade ago

    I was making a different point when I said they act as black bodies. The point was that warm gases emit IR radiation. That was in response to the idea that gases without greenhouse gases don't emit IR. I am perfectly aware that it doesn't act the same as solid. It is three dimensional and the interactions of electrons apparently happen when the atoms are in proximity. In a solid they would continuously be in proximity and the direction of the IR would be limited by physical constraints that don't exist in gases. Many alarmists seem to have this silly notion that CO2 transfers photons from one CO2 molecule to the other or it simply reflects the same frequency. That is apparently a distortion of how photons are absorbed as electrons jump to higher energy levels and emit the same frequency of photon.

    CO2 absorbs IR energy by bending or stretching the bond. That is how it is commonly explained and it is an analogy like black body radiation was meant to be in the point that was apparently lost. That energy is transferred to other molecules as heat. The energy required to bend the bond is the same energy that is contained in IR frequencies that are absorbed by CO2. If it acted as a reflector in that sort of absorb/emission sequence, there would be no warming of the air.

    The fact that air warms proves that heat is transferred from the CO2. When the heat is transferred, it doesn't only transmit heat back in the same bandwidth that CO2 absorbs. That is what would happen if only CO2 and other greenhouse gases emitted heat. It is clearly not the same as solid objects which would emit them on the surface in a single direction. It makes me wonder what is going on their minds. Do they think that warmed air is waiting for greenhouse gas molecules to emit IR so they can cool down to maintain heat balance. If that were true, then the greenhouse gas would be acting as a cooling influence where it wasn't exposed to IR directly from the ground. It would have to absorb the energy from the warmed gas to emit and it would emit in the same frequency it absorbs. I am not saying that it doesn't. I am saying that isn't what greenhouse warming is. Greenhouse warming is the warming of the atmosphere by absorbing IR radiation. Everything I have read on the subject besides obviously unscientific notions from alarmists have stated directly that gases emit longwave radiation.

    Perhaps you could explain to me why electrons from different molecules in gases don't interact in the same way that electrons interact in solids besides the obvious extra bonds. Gases in space emit radio waves. That is common knowledge in astrophysics and it is based on the temperature of the gas. The sun is a gas or more precisely a plasma. It OBVIOUSLY emits electromagnetic radiation. Do they need greenhouse gases on the sun to emit radiation. I know that is nonsense. It is emitting as a black body with a spectrum spread like a bell curve based on the temperature with some bands being absorbed by hydrogen and helium. It isn't a solid so I don't get your point.

    Are you actually suggesting that gases can't emit IR without greenhouse gases?

    <edit in response to paul>

    "Infrared radiation depends, not on electronic, but on vibrational excitation."

    That was precisely my point.

    "There is no way in which the gas resembles a black body, which is defined as a body at perfect equilibrium with the temperature-dependent radiation field."

    Oh really. Why do gases at 4K emit radio and gases at 10,000,000 emit x-rays and I assume the radiation of the electromagnetic spectrum is continuous throughout and beyond that range. They emit in the same bell curve as black body radiation which emits at essentially all real world temperatures though the amount is obviously much less as temperature is reduced. I have read the literature about how it is theorized that vibrational interactions are what cause the radiation in solids. I have read that it isn't clear that gases behave in the same way but clearly they do emit thermal radiation in ways that have nothing to do with greenhouse gases.

    "A gas at thermal equilibrium with its environment will be continually absorbing photons from the radiation field, and re-emitting them. " No kidding. It absorbs thermal radiation by greenhouse gases absorbing the energy and translating the vibrational energy to momentum of other molecules that contact it. The gas warms and emits the energy in wide bands like a black body. The CO2 doesn't emit the heat. That isn't to say that it never emits heat by the bond returning to ground state or however you want to define that.

    "However, since a gas is nothing like a black body, such processes are less important than vibrational excitation and de-excitation by molecular collisions, excitation by absorption, and de-excitation by emission."

    The real world would contradict that unless you can demonstrate that radiation from the sky matches the bandwidth of IR absorption of the various greenhouse gases. The mechanism you propose has a logical flaw. It means that increasing greenhouse gases would cause them to emit more heat so the nighttime would cool faster with increased greenhouse gases rather than slower as is known by cloudy or humid nights holding in the heat. You will have to work out the logic of that yourself.

    That is enough said to make the point. You can lead a horse to water but you can't make it drink.

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