It is unusual to find clouds in the
stratosphere. It
gets very cold above the S. Pole in the winter and polar stratospheric
clouds do sometimes form (they are made from water and other
materials). This together with an
unusual wind pattern above the S. Pole in the winter are thought to
create the ozone hole when the sun returns in the spring.
The ozone destruction reactions are shown in purple above.
Cl
reacts with O3 to make ClO. This reacts with O to
produce Cl and
O2. The Cl is now available to react again with other
ozone
molecules.
In green are "interference" reactions. ClO reacts with NO2
to
make ClNO3. The Cl in this "reservoir" molecule can't
react with
any more ozone.
Now what happens above the S. Pole in the winter is that the
reservoir
molecules react on the surfaces of the polar stratospheric cloud
particles to make some kind of new compound. This reaction is
shown in orange above. The new compound HOCl accumulates in the
air during the winter. When the sun reappears in the spring, the
UV light splits off all the Cl molecules which react with ozone.
A lot of chlorine suddenly becomes available and the ozone
concentration takes a nosedive.