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Is this because of some equilibrium between diffusion and gravity ? What exactly is responsible?

Martin - マーチン
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  • Not sure why this is considered a dupe. It asks about ozone where some of the issues are specific to the gas and not adequately answered in the other questions. – matt_black Apr 02 '20 at 12:51
  • The absolute concentration of ozone in the ozone layer is very, very small. – Karl Nov 26 '20 at 22:57

2 Answers2

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While gravity does cause some separation of atmospheric constituents at high altitudes, the reason why ozone doesn't migrate to the ground has more to do with its chemistry

Ozone is pretty unstable and is formed from some chemical reactions and from the interaction of oxygen with UV light. But it is unstable and decays fairly quickly. This means that is only appears anywhere when the reaction forming it keeps happening. This is true in the upper atmosphere which is why we have an ozone layer. Some hydrocarbons (not just from pollution from cars but also from volatile stuff released by trees) can react to generate ozone at ground-level. But we only see a significant concentration when the reaction forming it is happening.

The effect of gravity on the separation of the components of the atmosphere is small, especially in the lower atmosphere where turbulent circulation mix things up thoroughly. In the upper atmosphere the dominant process is diffusion and gravity can have more of an effect. This paper describes the process and has some details of the calculations about the size of the effect. It summarises the situation like this:

Turbulent mixing keeps the relative concentrations of gases nearly constant in the lowest 100 km. At higher altitudes, molecular diffusion controls the concentrations, with the lighter gases becoming relatively more abundant with increasing altitude.

It is a worthwhile read if you want the detail.

But the core issue is that, in the lower atmosphere, all gasses are well mixed so there is not separation by density. Ozone only appears there if there are some very specific reaction conditions that create it. The bulk of ozone in the ozone layer occurs because there are constant reactions there driven by UV light from the sun. Separation by density isn't relevant there either.

matt_black
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  • Hi Matt: I reckon only the second part of your post is the correct answer. Up to the turbopause/homopause turbulence causes a well mixed atmosphere. Gravity can't compete with eddy diffusion. How heavy a gas is (see question), does hence not play a role. However, while ozone does have a complicated photo chemistry, the answer does not have to do with that. The atmosphere is also well mixed for O2, N2, H2O, He, Ar, Ne, ... due to turbulence, and chemistry plays no role. – tipavi Jul 16 '16 at 15:46
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    @tipavi The reason why ozone is found where it is is entirely to do with photochemistry and nothing to do with its relative mass. So this is relevant to the question. And I don't disagree with you on atmospheric mixing: it is certainly irrelevant here, though it does happen for some parts of the upper atmosphere. – matt_black Jul 16 '16 at 15:52
  • I agree. But while its correct, that's not what he asked for. Many other correct things could be said, but the more concise and precise an answer is, the more helpful it will be. :) – tipavi Jul 16 '16 at 16:03
  • @tipavi "... that's not what he asked for" That the OP accepted this is proof enough (in addition to the upvotes, one of which I cast myself) that the answer is better off without the omission you suggested. Thank you for sharing your opinion though :-) – paracetamol Dec 04 '17 at 14:54
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    @paracetamol I respectfully disagree. The question whether or not an answer is correct does not depend on a single opinion (e.g., the OP's) or a popular vote (e.g., 5 upvotes). Neither is "proof enough", see discussions about "alternative facts". While the photochemistry of ozone is interesting, it's not relevant in a question about mass ("being heavy than air"), gravity, and diffusion. If you turned off all the (photo)chemistry, ozone's altitude distribution would look different, but ozone would still not settle. Molecular diffusion and turbulence would keep it well mixed up to the turbopause – tipavi Dec 05 '17 at 15:53
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    "If you turned off all the photochemistry, ozone's altitude distribution would look different, but ozone would still not settle" True, but in the real world there is photolytic decomposition going on, and to me it seems prudent to mention it. As I understand it, Matt didn't form his answer solely upon the photochemistry bit, rather, the photochemistry part was an insightful addendum, because it aims to justify why ozone is localized to the upper atmosphere. Matt then builds on this "In the upper atmosphere the dominant process is diffusion and gravity can have more of an effect." O:) – paracetamol Dec 05 '17 at 16:10
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    In short: This answer isn't fixated over the photochemistry of ozone. ;-) – paracetamol Dec 05 '17 at 16:12
  • "But while its correct, that's not what he asked for" @tipavi though Matt's already posted an answer, that mustn't stop you from posting yours. If you're willing to write up a sufficiently well-tailored answer, by all means go ahead and post it :-) If the OP finds it helpful enough, he might accept it. Since you also understand this topic well-enough (although you might want to frame the answer different), in all likelihood, your answer will get my upvote as well O:) – paracetamol Dec 05 '17 at 16:17
  • @tipavi If you are going to complain that ozone photochemistry isn't what the OP asked for, you might want to also read the second sentence which is "What exactly is responsible?" – matt_black Dec 05 '17 at 16:22
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If ozone wasn't so reactive that it only exists in regions where it is produced, it would still never settle down, because no gases can unmix due to gravitational fields. Not even in the complete absence of winds and turbulence.

That is exept if the flight path of molecules is significantly influenced by an accelerating field on the lenght scale of the mean free path lenght. That's why ultracentrifuges can do it, and helium and hydrogen slowy escape out of our atmosphere.

Bulk gases of course have buoyancy in the surrounding atmosphere. That's a different issue. The concentration of ozone in the stratosphere is however below 10 ppm.

Karl
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