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Why ozone layer doesn't fall?

  1. Jan 19, 2010 #1
    The molecular mass of ozone is 48. But the average molecular mass of air on ground is 29.....

    So, doesn't the ozone layer have to fall?
  2. jcsd
  3. Jan 19, 2010 #2


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    Gases don't separate out by mass until well above the stratosphere, I believe. It doesn't take much mixing to prevent this "falling".

    The reason we have an ozone layer is chemistry. Ozone in the stratosphere exists as a dynamic chemical equilibrium. It is continually being formed and destroyed again in reactions with oxygen and sunlight. The ozone layer is a region in which the equilibrium of these reactions maintain a significant amount of ozone -- although it is still only a few parts per million.

    Cheers -- sylas
  4. Jan 19, 2010 #3

    D H

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    A rhetorical question: The molecular mass of CO2 is 44, so why aren't we suffocating in a 3.2 meter high pool of CO2 at the bottom of the atmosphere?

    The answer: While CO2 will temporarily pool when quickly emitted in large volumes such as in volcanic and limnic eruptions, the mixing due to winds and molecular dispersion make CO2 fairly well-mixed throughout the troposphere.

    So a better question is why isn't ozone well-mixed, unlike other gases? Instead, ozone appears only near the surface due to pollution and high in the atmosphere in the ozone layer. The reason ozone is not well-mixed is that ozone is a particularly unstable compound. Due to its high reactivity and instability ozone molecules don't live long enough for those dispersive effects to take hold. The half-life of an ozone molecule is about half an hour. This means ozone is concentrated near the places where it is produced -- the surface (pollution) and the stratosphere (the ozone-oxygen cycle).
  5. Jan 19, 2010 #4
    This is why ozone is such a strong pollutant when produced near the ground. It hangs around at low levels where we can breath it.
  6. Jan 28, 2010 #5
    That was very informative, learned new things about ozone.
  7. Jan 28, 2010 #6


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    Gas molecules are continuously bumping into each other. This produces a seemingly random movement of all gas molecules that results in all constieunts of a gas (regardless of mass) becoming evenly dispersed in a fixed volume. It is a fundamental property of gases that they always expand to fully and evenly occupy any fixed volume.

    Of course, since the atmosphere is not a fixed volume, then gravity will eventually separate out the heavier molecules. However, turbulence and thermal convection continuously mix upper and lower layers of the atmosphere to about 60 miles. Above this altitude the atmosphere begins to separate out according to molecular mass. The vertical distance upwards, over which the pressure of a particular constituent decreases by a factor of e can be calculated as follows:

    H = KT/MG

    K = Boltzmann constant = 1.38 x 10−23 J·K−1
    T = Temperature in kelvins
    M = Molecular mass
    G = Acceleration due to gravity (m/s²)

    So, at the highest altitudes of the atmosphere, hydrogen gas is the predominate molecule.
  8. Jan 30, 2010 #7
    Thx guys.
    It helped a lot :)
  9. Jan 30, 2010 #8


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    Mostly but not always. Ask anyone who has to install propane systems. It has a nasty habit of pooling and creating a highly explosive situation.

    And we're not talking about enclosed systems. Powerboats and sailboats have to be specially designed with storage tanks sealed outside to ensure there is no possible way propane can leak into and pool in the cabin.
  10. Jan 31, 2010 #9


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    It's always, but not necessarily fast enough to suit our needs and wants.

    Us humans are by nature impatient.
  11. Jan 31, 2010 #10


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    Are you saying that dispersion occurs in two stages? First it falls to the lowest point and pools there, and then second it disperses to fill the volume?
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