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Air contains about 78% nitrogen and 21% oxygen independent of altitude (up to 100 km). Why is this? Shouldn't the concentration of nitrogen increase with higher altitudes since nitrogen has a lower density than oxygen?

composition of air with altitude

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  • $\begingroup$ why do you think, that it doesn't? The picture is not sooo precise, is it? $\endgroup$ – Ilja Apr 5 '16 at 19:34
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    $\begingroup$ Because there are large mixing terms from, e.g. updrafts, weather systems, etc. $\endgroup$ – Jon Custer Apr 5 '16 at 19:34
  • $\begingroup$ Like Ilja, I doubt the accuracy of those plots; keep in mind that O$_2$ is only 10% heavier than N$_2$ (and this gradient is fighting against entropic forces) so you wouldn't expect a large difference in any case. $\endgroup$ – lemon Apr 5 '16 at 19:39
  • $\begingroup$ the air molecules only widen there gap between each other as the altitude heightens. $\endgroup$ – user5434678 Apr 5 '16 at 19:48
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    $\begingroup$ @SeanLake -- Apparently it's wordpress.mrreid.org/2014/08/01/… Note that the ordinate in this graph is percent composition and that the total falls below 100% at 100 km or so. That means our atmosphere is something else than molecular nitrogen, molecular oxygen, argon, and other trace species above 100 km. The author of that blog post used the NASA MSIS E-90 model (there are others) as the basis for his plot. $\endgroup$ – David Hammen Sep 20 '16 at 10:28
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Shouldn't the concentration of nitrogen increase with higher altitudes since nitrogen has a lower density than oxygen?

No, it shouldn't, at least not up to 100 km or so. Look at your graph, which shows that even argon is well-mixed throughout the lower atmosphere (the troposphere, stratosphere, and mesosphere). Argon atoms are considerably more massive than are carbon dioxide molecules, which in turn are considerably more massive than oxygen and nitrogen molecules, and yet all of these (along with all of the long-lived gases in the atmosphere) are well-mixed throughout the lower atmosphere.

The reason is that the lower atmosphere is dense enough to support turbulence while the upper atmosphere is not. The turbopause marks the somewhat fuzzy boundary below which turbulent mixing dominates over diffusion and above which it's diffusion that dominates.

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  • $\begingroup$ Bonus points for also explaining why the gasses in question drop off so steeply at 100km altitude. $\endgroup$ – Asher Apr 6 '16 at 20:43

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