Re: why doesn't the ozone layer sink?

There are several parts to the answer:

(1) Gases that are mixed together stay mixed. A heavier gas does not settle 
out to the bottom of a gas mixture. The rates at which a molecule like ozone 
or carbon dioxide will settle out from a mixture with molecules like oxygen 
or nitrogen is absolutely tiny. It takes only the slightest bit of wind or 
turbulence to cancel out the settling effect and keep the gases mixed.

You may have seen experiments where a heavy gas like carbon dioxide sinks 
and "pours" in air to extinguish a candle flame, or where a light gas like 
hydrogen rises in air. These experiments are a little misleading. Those 
things only happen while the gases remain separate. As soon as gases mix, 
they stay mixed. They do not settle out from one another.

(If gases did separate out, we would smother if we went to bed at night 
downstairs or out camping, because all of the argon and carbon dioxide would 
have collected at the bottom of the atmosphere. And we would not have any 
oxygen at mountain tops, because oxygen is heavier than nitrogen!)

(2) The "ozone layer" is not much like you think of a layer. It stretches 
roughly over 35 kilometres, from 15 to 50 kilometres altitude. But even in 
the middle of the ozone layer the gas mixture is nearly all nitrogen, 
oxygen, and argon, just as it is at ground level. Only 2 or 3 parts per 
million of the air in the middle of the ozone layer is ozone!

(3) Ozone is a very reactive molecule. It cannot and does not just sit in 
the atmosphere and move around. It is continually being formed from ordinary 
oxygen in chemical reactions, and removed by other chemical reactions, which 
turn it back into ordinary oxygen again. Most of these reactions 
can only happen when there is a lot of ultraviolet light around, and that 
means above 15 kilometres altitude, because the ozone itself blocks out the 
ultraviolet light from getting any lower.