Ozone

With spring coming to the southern hemisphere, the blossom and the daffodils are out and the weather is definitely warmer (OK some of the time). But there is a also something bad that also happens, as the sun rises over the polar horizon, and the polar vortex (winds that rotate about the pole) begins to shut down for the summer there is a large patch of stratosphere which has been enclosed in the polar vortex that has an appreciably lower Ozone content. This Ozone "hole" breaks up with the shut down of the polar vortex and regions of lower Ozone content spread out over parts of the southern hemisphere as the two regions diffuse into one an other.

What this means for us that live closer to the South Pole than the equator is that around the time of the southern vernal equinox we tend to get a period of very low ozone over head. So in honour of this and the dangers it can present, I have a series of posts on Ozone.

Firstly, some basics. The Ozone hole is not a region of no Ozone (which is the reason for the scare quotes in the above paragraph) but merely a region of lesser density of Ozone. The hole is defined as a region of Ozone less than 220 Dobson units (DU). This level was chosen as the reference since ozone levels had never been seen lower than this before 1979.

Ozone itself is a relatively unstable allotrope of oxygen (O₃). It forms when UV radiation from the Sun breaks the bond between the atoms in the O₂ molecule, some of the liberated oxygen atoms then bond with other O₂ molecules making O₃. This same interaction with UV radiation also breaks down the O₃ and it is these two together that are responsible for the UV protection we get from the Ozone layer. Ozone is unstable and quite reactive, especially with molecules containing nitrogen, hydrogen, chlorine, or bromine.

That is enough for today, next in this series we will look at how the Ozone hole has formed and what we have done about it.