Why is the ozone layer in the upper part of he stratosphere?

[planetmartin]

OK - I was at work the other day conducting air sampling for Ozone which is essentially 3 oxygen atoms bonded together. This got me thinking that if three oxygen atoms forming an ozone mollecule weigh more than an oxygen mollecule, why then is the ozone layer up there and what keeps it there?  Why doesn't it sink to ground level? 

[Peter Pedant]

you  need a green ecologist for this!

The life cycle of ozone up there (where it comes from, where it goes) is different to the life cycle of ozone down here, which is closely concerned with incomplete combusion of car engines.

Similarly for the CFCs, the reactions they indulge in (zapping the ozone layer and getting broken down themselves) are different up there than on the way up.

So it is not one equation fits all, and  i found it all quite taxing and complicated

[Hippy]

The atmosphere is not homogenous like, say, a column of water where lighter bits go up and heavier stuff goes down. It exists in layers that are kept stratified by the balance of radiation and gravity. High in the atmosphere oxygen exists as two atoms combined to form one molecule just as at ground level. However, the mix  and density of gasses at this height is very different than at ground level. The gases are kept aloft by convection and ionisation, that is the excitation or addition of electrons to atoms.

At the outer parts of the atmosphere short wavelength ultraviolet light from the Sun excites the outermost electron of the oxygen atom causing it to increase its energy state and migrate outwards from the nucleus (similar to like poles of magnets repelling each other). This increase of energy is the result of the absorption of ultraviolet light and it weakens the molecular bond causing single atoms to exist. These bond with unaffected double atom molecules to form three-atom oxygen molecules. The new bond although relatively weak, conserves the energy absorbed from the UV.

The newly formed Ozone is not only highly unstable but is also reactive. It readily donates its extra oxygen molecule to free radical species like nitrogen, hydrogen, bromine, and chlorine. These naturally occur in the stratosphere, released from sources such as soil, water vapour, and the oceans. The energy absorbed from the UV is conserved in these new chemical bonds.

Slightly lower in the atmosphere, ozone molecules are broken by longer wavelength UV to form two atom molecules and single atoms that then combine with each other to form more two atom molecules. It is this creation at top level and destruction lower down combined with the catalytic action with other elements that absorbs the dangerous ultraviolet light and prevents our planet from becoming a runaway furnace of hydrochloric fumes like Venus.

[planetmartin]

Thanks Guys - you're very helpful.