So the question returns to ... How does an intact, heavy, stable, almost completly inert, gas molecule get into the presence of O3 in the upper atmosphere?
Need I picture a bowling ball somehow making its way to 80,000 feet thanks to a tornado?
There's your physics question of the day.
Ask yourself this question-
How does a light, inert gas such as helium ever get down to ground level?
By your rationale, it should all be somewhere at the top of the atmosphere. Carbon dioxide also weighs more than air and is quite stable (average lifetime around 200 years). Yet it easily diffuses through the atmosphere. If the atmosphere stratified by molecular weight, we'd all die.
How does the composition of the atmosphere change with altitude? (Or, how can CFC's get up to the stratosphere when they are heavier than air?)In the earth's troposphere and stratosphere, most _stable_ chemical species are "well-mixed" - their mixing ratios are independent of altitude. If a species' mixing ratio changes with altitude, some kind of physical or chemical transformation is taking place. That last statement may seem surprising - one might expect the heavier molecules to dominate at lower altitudes. The mixing ratio of Krypton (mass 84), then, would decrease with altitude, while that of Helium (mass 4) would increase. In reality, however, molecules do not segregate by weight in the troposphere or stratosphere. The relative proportions of Helium, Nitrogen, and Krypton are unchanged up to about 100 km.
Why is this? Vertical transport in the troposphere takes place by convection and turbulent mixing. In the stratosphere and in the mesosphere, it takes place by "eddy diffusion" - the gradual mechanical mixing of gas by motions on small scales. These mechanisms do not distinguish molecular masses. Only at much higher altitudes do mean free paths become so large that _molecular_ diffusion dominates and gravity is able to separate the different species, bringing hydrogen and helium atoms to the top. The lower and middle atmosphere are thus said to be "well mixed." [Chamberlain and Hunten] [Wayne] [Wallace and Hobbs]
Experimental measurements of the fluorocarbon CF4 demonstrate this homogeneous mixing. CF4 has an extremely long lifetime in the stratosphere - probably many thousands of years. The mixing ratio of CF4 in the stratosphere was found to be 0.056-0.060 ppbv from 10-50 km, with no overall trend. [Zander et al. 1992]
An important trace gas that is *not* well-mixed is water vapor. The lower troposphere contains a great deal of water - as much as 30,000 ppmv in humid tropical latitudes. High in the troposphere, however, the water condenses and falls to the earth as rain or snow, so that the stratosphere is extremely dry, typical mixing ratios being about 5 ppmv. Indeed, the transport of water vapor from troposphere to stratosphere is even less efficient than this would suggest, since much of the small amount of water in the stratosphere is actually produced _in situ_ by the oxidation of stratospheric methane. [SAGE II]