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

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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 gases 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.

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    $\begingroup$ @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. $\endgroup$
    – matt_black
    Commented Jul 16, 2016 at 15:52
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    $\begingroup$ @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 $\endgroup$
    – tipavi
    Commented Dec 5, 2017 at 15:53
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    $\begingroup$ "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:) $\endgroup$
    – paracetamol
    Commented Dec 5, 2017 at 16:10
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    $\begingroup$ In short: This answer isn't fixated over the photochemistry of ozone. ;-) $\endgroup$
    – paracetamol
    Commented Dec 5, 2017 at 16:12
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    $\begingroup$ @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?" $\endgroup$
    – matt_black
    Commented Dec 5, 2017 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 gases cannot unmix due to gravitational fields. Not even in the complete absence of winds and turbulence, only due to Brownian motion / diffusion.

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

All gases that are not taking part in local chemical reactions or phase transitions (water) are perfectly homogeneously distributed in the stratosphere and below.

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.

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  • $\begingroup$ Gases certainly can unmix due to gravitational or similar fields or forces; you contradict yourself in paragraph 2 and again in 3 and 4 is just a nonsequitor. Just because it doesn't seem to happen on Earth does not make it universal. All mass and Energy obeys the same Laws. $\endgroup$
    – jimchmst
    Commented Sep 24 at 20:18
  • $\begingroup$ @jimchmst your latin is great $\endgroup$
    – Karl
    Commented Sep 27 at 22:16