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
There is around 0.5 grams/cubic meter of liquid water droplets in the cloud, That is what you see and it can block sunlight even if the cloud is not very thick. Fog is clouds at ground level so you know how much can be blocked with very little water. So you do not dense solid material to block light.
Gases have colors in the sense the let some frequency of light through more than others. Some will block almost all of a specific frequency very efficiently but not block any other.
Take a look at [https://i.sstatic.net/4ubuX.jpg](https://i.sstatic.net/4ubuX.jpg) which shows the absorption spectra of O2 + O3, CO2 and H2O You will notice O2+ O3 block UV light very efficiently but very little in the visible and IR spectrum except for a few narrow peaks. Compared that to H2O (water as a gas) it has a lot of peaks in the IR and block almost all microwaves.
CO2 has peaks in the IR spectra where come do not overlap with H2O, it is those peaks that make is so efficient greenhouse gas, it absorbs wavelengths Earth emits to space that no other gas do to a high degree.
If you look at O2 and O3 separately you get [https://www.researchgate.net/profile/Michael-David-10/publication/288835123/figure/fig1/AS:314356349849600@1451959802764/Absorption-wavelength-of-ozone-135-Advantages-of-optical-absorption-spectroscopy-are.png](https://www.researchgate.net/profile/Michael-David-10/publication/288835123/figure/fig1/AS:314356349849600@1451959802764/Absorption-wavelength-of-ozone-135-Advantages-of-optical-absorption-spectroscopy-are.png) and you can see there are the part where O2 do not block a lot but O3 does,
Nitrogen simply do not block a lot of light on the spectra at all. O3 happen to be the only one in the atmosphere that blocks a part of the UV spectra efficiently.
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’.
A 9 inch line of medium point (around 1mm) sharpie weighs about 0.00000033 pounds. It blocks incoming light from reaching the surface it’s drawn on.
The air above that line weighs about 5.2 pounds.
So the sharpie, despite being about 63 ppb of the material sunlight must pass through to reach the ground, brings it to a halt. Parts per *billion*.
Put the right thing in the way and it doesn’t take very much. It doesn’t take a dense, solid material to stop light. Just a bit of sharpie.
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