[thedoc (OP)]

Why does the ozone layer float, when the molar mass of ozone is greater than the molar mass of oxygen?
Asked by Daniel


                                        Visit the webpage for the podcast in which this question is answered.

[chapter podcast=1000454 track=13.09.05/Naked_Scientists_Show_13.09.05_1001257.mp3]  ...or Listen to the Answer[/chapter] or [download as MP3]

[thedoc (OP)]

We answered this question on the show...

Laura - Obviously, the ozone layer is really important because it protects us from[img float=right]/forum/copies/RTEmagicC_Redskyatnight_02.jpg.jpg[/img] UV rays. But actually, that's one of the reasons why it actually is up there because it’s continually being reformed. It needs to be up there, getting the UV rays, absorbing them, having that chemistry going on in the atmosphere. So, it is actually sort of heavier, but actually, it’s because it’s continually being reformed.
Dominic - I guess, another factor here is that because the ozone layer is absorbing all this ultraviolet radiation from the sun it's actually getting really quite hot. We think of temperatures decreasing with altitude, but when you get up to these quite high altitudes that air is getting hotter. Of course, hot air floats because it’s less dense. That's how a hot air balloon flies. And so, this ozone is naturally quite buoyant because it’s hot.

[alancalverd]

It isn't really a "layer" in the sense of a static shell of pure ozone around the atmosphere. The oxygen content of air actually decreases with altitude, so the stuff up there is still mainly nitrogen with a bit less oxygen than at sea level. Solar ultraviolet radiation converts some oxygen to ozone at every level of the atmosphere, but the ozone is very short-lived at lower altitudes because there is a high probability of an ozone molecule colliding with something else, and very little UV radiation to promote the forward reaction. At high altitudes there is more UV radiation hence a greater proportion of the oxygen is converted to ozone, and a lower probability of an ozone molecule colliding with another molecule once it has formed, so as you go higher, so the instantaneous proportion of ozone increases.       

[CliffordK]

Note that the ozone holes occur over the poles during the winter when the sun doesn't shine.  For the most part, they fill back in once the sun returns.  Of course, CFC's did contribute to making the holes worse

[alancalverd]

Of course, CFC's did contribute to making the holes worse

Interesting hypothesis, entirely contrary to the laws of catalysis!

Catalysts reduce the energy barrier of a process, but don't alter the equilibrium concentrations.

So the entire petrochemical, fertiliser and artificial diamond  industries, to name but a few,  have got it wrong for centuries, and a conference of treehuggers and politicians got it right?

[MGraham]

The amount of ozone should not increase as you go higher, as it is a molecule heavier than what lies above it. And unless ozone is a weak molecule, it shouldn't come apart when colliding with other molecules as it falls. Things that are hot must cool and heavy molecules must sink into lighter molecules. I wonder if any fall to us at the surface, and if trace amounts can be gathered.