[daveshorts]

I thought the reason for the southern hemisphere ozone hole being bigger was that the ozone destruction is done on the surface of stratospheric ice crystals in sunlight. The crystals form during the winter and the destruction is done in the spring before they evaporate. The South pole has more stratospheric ice crystals (possibly because the ground is colder so there is less infra red emmitted to warm the stratosphere in the long winter) so more ozone depletion.

[anthony]

Dave, I didn't like you ice-crsytal photochemistry reason so I did some digging. and have found this:

The air of the Northern and Southern hemisphere is pretty much isolated into two
separate parcels.
About 90% of the CFCs released were in the Northern hemisphere.
Why then isn't the Ozone hole (the springtime depletion phenomenon) more
prominent in the North, and less prominent in the South?

It is true that the air parcels of the North and the South are largely isolated
from one another. But there is some mixing. The average mixing time for a
particular molecule released in the Northern hemisphere to get anywhere else in
the Northern hemisphere (e.g. Cernobyl fallout) is a matter of 2 or 3 weeks. This
compares with an average crossing time from North to South of between 6 months
and a year. There is always a little casual mixing at the equator, and twice a
year there is relatively major transfer associated with the monsoon seasons in
each hemisphere. The average time it takes for a molecule released at the Earth's
surface to reach the stratosphere is between 20 and 50 years. And most of the
molecules entering the stratosphere from below do so very close to the equator.
So as far as the molecules entering the stratosphere are concerned there is no
North/South difference -- there has been plenty of time for thorough mixing.

Remember that in order to help cause ozone destruction in the polar phenomenon, a
CFC molecule must rise from the surface into the stratosphere to at least 30 km
altitude, break up into smaller free radicals, and then be stored in the
stratosphere as hydrogen chloride or chlorine nitrate until it finds its way to
the polar regions in the stratosphere.

The reason that the ozone hole forms over the Antarctic rather than the Arctic
has to do with climatic differences which ultimately tie in with the different
surface geography in the two polar regions. The Antarctic lower stratosphere is
typically about 5 to 8 deg C colder than the Arctic.

This is from: http://www.madsci.org/posts/archives/mar2000/953164126.Es.r.html
The original source, I think, being the CRC for Southern Hemisphere Meteorology which is partially based at Monash University, Melbourne.