Ozone is above, nitrogen is below

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

• Weight does not matter at all, otherwise all atmosphere would separate and we would drown in $\ce{Ar}$ and $\ce{CO2}$. Ozone is formed where UV is the strongest; being not very stable, it does not reach us down below. – Ivan Neretin Nov 25 '15 at 15:43
• Because, in an ideal gas mixture, the gases behave as if they are independent of one another. There will be a slight density stratification of all the gases. But, if you had ozone and air in a room and calculated the ratio of the mole fractions at the ceiling compared to the ratio at the floor, you would find that the difference would only be a tiny fraction of a percent. As Ivan points out, more important than this, ozone in the atmosphere is controlled by photochemistry, while air is not. physicsforums.com/threads/… – Chet Miller Nov 25 '15 at 16:05
• LOL - molecular weight does matter. The earth's gravity isn't enough to hold helium. – MaxW Nov 25 '15 at 17:02
• @MaxW Good point! Well, it does matter in a way, but keep in mind the time scale. – Ivan Neretin Nov 25 '15 at 18:01
• I have to point out that the Physics Forums thread linked by Chester has an amazing reference inside. Definitely worth a look. – Nicolau Saker Neto Nov 27 '15 at 8:38

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.

• What you say is correct but it doesn't answer the actual question. – matt_black May 19 '17 at 17:07

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.