system
Given the following post, respond as if you're commenting on Reddit:
human
Why is there a layer of O3 (ozone layer), but all of the other gases ae mixed together?

Also, where does O3 come from, and what is causing the hole in the ozone layer? I've heard greenhouse gases, but what do they actually do to ozone that causes it to deteriorate?
gpt
O3 is highly reactive, it doesn't tend to last very long in the lower atmosphere where there are more things to react with. However, in the upper atmosphere O3 is continuously produced through UV bombardment of O2. The steady state of these reactions is a layer of significantly elevated ozone abundance within the upper stratosphere. However, even there the concentration of ozone is only a few parts per million (if the ozone layer were actually a proper layer of 100% ozone it would only be a few millimeters thick). Nevertheless, ozone absorbs UV-b rays very strongly so more than 95% of all the UV-b radiation hitting the Earth's atmosphere doesn't make it to the surface.

One of the advantages to using CFCs as refrigerants is that they don't break down easily, which is a good safety measure as they are more or less inert in terms of reacting with things. However, that means they will stick around in the atmosphere indefinitely and they will float up to the stratosphere and get hit by hard UV rays causing them to crack apart and form free Chlorine radicals. And those free Chlorine radicals can *catalytically* break down ozone (meaning that the free radical is not consumed during the reaction). A single Chlorine free radical can break down on average 100,000 ozone molecules before it gets consumed by some reaction. With a high enough CFC production this could have led to a global depletion of stratospheric ozone levels and a consequent dramatic increase in UV-b radiation levels at the surface of the Earth (even if the ozone layer was still filtering out 90% of all UV-b radiation that would be a factor of 5x or more increase in UV-b exposure).

The phenomena responsible for causing seasonal holes in the ozone layer around the poles is more complex but is due to similar mechanisms.