The kicker is the "upper atmosphere" part. Ozone is naturally created by ultraviolet light. When short-wave UV light hits regular oxygen it breaks the molecule apart into atomic oxygen. The two newly-freed oxygen atoms are then able to find other oxygen molecules and combine with them to form ozone. Medium-wave UV light is absorbed by ozone - it breaks the ozone apart, which becomes regular molecular oxygen and a free oxygen atom. The atom can then find another regular molecule to form ozone, or hook up with an atom and become molecular oxygen again.
Originally Posted by RocketMike
Chlorine and Bromine enter the picture by catalytically converting ozone back to oxygen - they do so without reacting (so they're available to do it many times), and they destroy the ozone much quicker than it can be created.
The types of UV are important here - UVA, UVB, and UVC. You hear about UVA and UVB all the time in sunscreen commercials. UVA is long-wave UV, which isn't blocked by the ozone layer. UVC is the short-wave UV, which is the type absorbed by oxygen, so depletion of the ozone layer doesn't let more of this in. UVB is the medium-wave stuff that's blocked by ozone, and coincidentally a major factor in sunburn and skin cancer.
CFCs enter the picture because they belong to a group of molecules called "Halocarbons." Halocarbons are carbon-based molecules that have one or more halogen atoms bound to them. Fluorine, chlorine, and bromine are halogens. (Iodine is, too, but to my knowledge nobody has used it for refrigerants). Fluorine bonds extremely tightly to carbon, so it won't come off in the upper atmosphere, and can't participate in ozone depletion. This is why modern refrigerants (R-134a and many others) only use fluorine. Chlorine and bromine come off of a halocarbon molecule relatively easily, so when they reach the upper atmosphere they can effectively 'detach' from their host molecule and go play with the ozone. CFC-based refrigerants (Freon, also known as R-12) had chlorine. Bromine was usually used in fire suppression - Halon being the big usage.
SO back to the original question: why ozone created at ground level doesn't help. Other people have answered this already, but I'll repeat: it's just too darned reactive to stick around long enough to do any good. Plus, since it's destroyed in the act of absorbing UVB radiation, and it can only be regenerated (in any significant quantity) by UVC radiation, it's not going to be regenerated at ground level.
It's also really nasty stuff - strong oxidizing agents really aren't good for you. It's actually beneficial in some ways, though - white blood cells produce it as a means to rapidly destroy bacteria or other things they don't like.
And just in case you thought the new refrigerants were great... they're all STRONG greenhouse gases. Way more powerful than CO2. So you just can't win.
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