To explain why the hole in the ozone layer is mostly over the Antarctic (from where it somtimes spreads over Australia), I’ll start with a bit about ozone itself. Ozone is a molecule of three oxygen atoms. Mostly, oxygen atoms form pairs, which is what we breathe, but high up in the atmosphere short wave length radiation from the sun splits up these oxygen pairs leaving two free oxygen atoms. Oxygen atoms by themselves are not very stable, so they react with a nearby oxygen pair to form ozone.
One of the quirks of ozone is that it is very good at absorbing ultraviolet radiation (240-320nm). When ultraviolet rays are hit by an ultraviolet photon, they split up into a pair and a single atom again, absorbing the energy, and then the single atom reacts with a pair again to form a new ozone molecule, with the UV photon totally used up in the process. This is called the “ozone cycle”. When the free oxygen atoms end up together as a pair, then the cycle is broken. In the ozone layer, the recombining of single oxygen atoms into pairs is balanced by the splitting of pairs, and there is an abundance of ozone left to form the layer.
Things all begin to change when something else interrupts the ozone cycle. Some of the main culprits are chlorine and bromine atoms. For example, a chlorine atom reacts with an ozone molecule and produces ClO and an oxygen pair. The ClO then reacts with a free oxygen atom and you get another oxygen pair and the free chlorine atom back with no UV radiation absorbed by the process. And because the chlorine reactions happen faster than the ozone cycle, the whole balance has been shifted back towards oxygen pairs and there is much less ozone around to absorb the UV radiation.
So how does the chlorine get all the way up to the ozone layer? This is where CFCs (chlorofluorocarbons) come in to the picture. CFCs used to be used a lot in refrigerators, air conditioner and aerosol sprays in the 1980's, although they have now been banned. There are no natural sources of CFCs that we know about, so just about all the CFCs in the atmosphere are manmade. Further, because CFCs are generally stable and the chlorine isn’t used up in the process described above, it will take a long time before the hole goes away again.
Because the CFCs don’t react to anything in the lower atmosphere, they get caught up in the general air movement and get mixed throughout the whole lower atmosphere and are carried up to the stratosphere, where the ozone layer lives. Once there, the short wave radiation (the same radiation that breaks up the oxygen pairs) is the first thing the CFCs encounter that can break them up, releasing the chlorine atoms right in the ozone layer.
Finally I’m back to our original question; why is the hole over the Antarctic? Firstly, the cooling air in the winter and the rotation of the earth combine to form a large vortex over the South Pole. Because the vortex is so cold, clouds form (mostly from nitric acid; there’s very little water that high in the atmosphere – it all falls back to earth as rain much lower down). These clouds act as catalysts for the creation of the free chlorine atoms (i.e. they take part in the reaction, but are not used up by it, and in general make the reaction happen faster), so you have even more ozone destroying chlorine atoms then usual. This means you have much less ozone over Antarctica than elsewhere and this is what we call the hole.
There is also a small hole, sometime called a dimple, over the arctic. The vortex over the arctic isn’t as cold or as powerful as the one over the Antarctic due to the distribution of land masses. Correspondingly, there are less clouds formed and the whole (hole) problem isn’t as bad.
Image courtesy of NASA (note this image is in the public domain)This is obviously a complex phenomenon, so if you are after more detail on the ozone layer and its hole, here are some good websites:
http://www.faqs.org/faqs/ozone-depletion/
http://en.wikipedia.org/wiki/Ozone_depletion
http://www.theozonehole.com/
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