MadSci Network: Earth Sciences |
Daphne, Basically, the conditions above the South Pole are unique in how they permit dramatic ozone depletion. During the polar winter, when sunlight is absent, the air over the pole gets cold enough that chemicals that could remove chlorine freeze and form clouds. This is "de-nitrification" and it has two consequences: first, the clouds formed by the freezing chemicals speed up the reactions that turn chemically inert CFCs into reactive free chlorine. Second, the clouds of frozen chemicals become heavy and sink, leaving the chlorine. Once the sunlight returns, the built-up chlorine begins reacting very quickly to destroy ozone, and this is why there is an "ozone hole" over Antarctica from August (the end of Antarctic winter) until November. You can find a more complete description of this process here (it's pretty complicated): http://www.atm.ch.cam.ac.uk/tour/part3.html
Try the links in the MadSci Library for more information on Earth Sciences.