Ozone is above, nitrogen is below

Question

Ozone is heavier than nitrogen. Then why is ozone in stratosphere and nitrogen in troposphere?

Answer 1

The entirety of the Earth's atmosphere can be divided into two regions: the homosphere and the heterosphere. The former extends from the earth's surface up to about $\pu{100km}$. Within this region, turbulence and mixing, even within the relatively calm, persistent temperature inversion known as the stratosphere, prevents any segregation of gases based on molecular/atomic weight. With the exception of water vapor, the composition of the major constituent gases only varies by a few hundred ppm from any given region of the homosphere to another.

The heterosphere extends from about $\pu{100km}$ above the earth's surface upward until merging with the interplanetary medium at around $\pu{100,000km}$. Within this region, the mean free path of any gas is great enough that gravitational forces have a chance to segregate the different species based on their mass, rather than by bulk processes like turbulence.

The Encyclopaedia Britannica describes the heterosphere this way:

In the heterosphere, the concentrations of the heavier constituents, such as nitrogen or oxygen, decrease more rapidly with increasing altitude than do the concentrations of lighter gases, such as hydrogen or helium; and eventually the atmosphere is dominated by the lighter gases. Under mean daytime conditions, helium and its ion become dominant around $\pu{1,000 km}$ ($\pu{620 miles}$) and hydrogen and protons above $\pu{2,500 km}$ ($\pu{1,555 miles}$).

At sufficient altitude, $\ce{H+}$ becomes the dominant species and hydrogen is the only element of significance that escapes the atmosphere based on its low mass.

So, to reiterate the answer to the question; the troposphere and stratosphere both lie well within the homosphere, and thus segregation based on mass plays no roll in their distribution throughout this region of the atmosphere.

Answer 2

The primary reason is because ozone is unstable and the major reactions that form it only occur in the upper atmosphere.

The lower part of the atmosphere (the troposphere) is well-mixed and there is no separation of gases by density. This is less true in some of the higher layers of the atmosphere, but this is irrelevant to why we have an ozone layer.

Ozone is unstable and reactive. The reactions that create it in the upper atmosphere are photochemical. The concentration is a balance between the rate of formation and the rate of decay and it is this balance that leads to an ozone layer. Lower in the atmosphere there is less UV and the ozone would decay faster so the concentration is usually very low. That is why we find it up top: nothing to do with density.

Having said that, we sometimes do get ozone in the lower atmosphere. This happens when certain other chemicals promote its formation (as many pollutants like nitrogen oxides do and also hydrocarbons from trees). But, being reactive, it doesn't hang around long.

It is the reactivity that matters not the density.