Freons and Stratospheric Ozone Depletion

Freons is the trade name for a family of chlorofluorocarbons (CFCs) -- simple haloalkanes bearing only fluorine and chlorine on a short carbon chain, of which Freon-12 ($\text{CCl}_2\text{F}_2$, dichlorodifluoromethane) is the best known. CFCs were adopted on a huge scale through the twentieth century as refrigerants (in refrigerators and air conditioners) and as aerosol-can propellants, precisely because they are chemically inert, non-toxic, non-flammable and easy to liquefy under modest pressure -- an outstanding safety profile at ground level.

That same chemical inertness is the source of the problem. Because CFCs do not react with anything in the lower atmosphere, they survive, essentially unchanged, long enough to slowly diffuse all the way up into the stratosphere. There, however, they are exposed to a far more intense flux of high-energy ultraviolet radiation than reaches the ground (most of it having already been absorbed by the ozone layer itself before reaching lower altitudes), and this UV light is energetic enough to photolyse the relatively weak $\text{C--Cl}$ bond, releasing a highly reactive chlorine radical:

The chlorine radical then destroys ozone catalytically, through a two-step chain that regenerates the radical at the end of each cycle:

with the net effect $\text{O}_3 + \text{O} \to 2\,\text{O}_2$ -- ozone (and a free oxygen atom that would otherwise have gone on to form more ozone) is converted to ordinary oxygen, while the chlorine atom itself is regenerated unchanged at the end of the cycle and is free to repeat the process. Because the chlorine radical is a true catalyst rather than a reactant that is consumed, a single chlorine atom can go on to destroy a very large number of ozone molecules -- of the order of a hundred thousand, by common estimates -- before it is eventually removed from the cycle by reacting with another atmospheric species to form a more stable, unreactive chlorine compound. …