When I think about blocking radiation, I imagine using dense solid materials, in meaningful quantities. Not a gas, let alone tiny constituents of that gas (20 ppm is 0.002%), primarily concentrated in a region of the atmosphere that is itself incredible thin. With nitrogen being 78% of the atmosphere, that means there’s at least 39,000x as much nitrogen up there, right? Eli5, how can ozone be the protective shield we think it is, and how do we know?
In: Planetary Science
Tiny amounts of chemicals can have disproportionately large effects.
Consider [this list](https://en.wikipedia.org/wiki/List_of_highly_toxic_gases), and how at the top of it there’s chemicals that are [immediately dangerous](https://en.wikipedia.org/wiki/IDLH) at 5 ppm. Less even than ozone in its layer, it would still be dangerous to a human.
Now, consider that UV is basically light. Light is absorbed by different chemicals at different rates. Some light (10-100 nm wavelength) is absorbed really well by nitrogen, for example. Meanwhile, 200-300 nm wavelength is absorbed really well by ozone. So well, that measurements from above the ozone layer has 350 000 000 times as much light from those wavelengths.
So the how is just ‘weird chemicals do weird things’, and the how do we know is ‘we measured it with weather balloons and near space testing’.
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