This article is from the Ozone Depletion: The Antarctic Ozone Hole FAQ, by Robert Parson firstname.lastname@example.org with numerous contributions by others.
This was a mystery when the hole was first observed, but
it is now well understood. I shall limit myself to a
brief survey of the present theory, and refer the reader to two
excellent nontechnical articles [Toon and Turco] [Hamill and Toon]
for a more comprehensive discussion. Briefly, the unusual
physics and chemistry of the Antarctic stratosphere allows the
inactive chlorine "reservoir" compounds to be converted into ozone-
destroying chlorine radicals. While there is no more chlorine over
antarctica than anywhere else, in the antarctic spring most of
the chlorine is in a form that can destroy ozone.
The story takes place in six acts, some of them occurring
simultaneously on parallel stages:
a.) The Polar Vortex
As the air in the antarctic stratosphere cools and descends during
the winter, the Coriolis effect sets up a strong westerly
circulation around the pole. When the sun returns in the spring the
winds weaken, but the vortex remains stable until November. The air
over antarctica is largely isolated from the rest of the atmosphere,
forming a gigantic reaction vessel. The vortex is not circular, it
has an oblong shape with the long axis extending out over Patagonia.
For further information about the dynamics of the polar vortex see
[Schoeberl and Hartmann], [Tuck 1989], [AASE], [Randel], [Plumb],
and [Waugh]. For a rather short movie (mpeg format) illustrating it,
go to http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/UARS_project.html.
There is some controversy about just how isolated
the air in the vortex is. Some believe that the vortex is better
thought of as a flow reactor than as a containment vessel; ozone-rich
air enters the vortex from above while ozone-poor and ClO-rich air is
stripped off the sides. Recent tracer measurements lend some support
to this view, but the issue is unresolved. See [Randel] and [Plumb].
b.) Polar Stratospheric Clouds ("PSC")
The Polar vortex is extremely cold; temperatures in the lower
stratosphere drop below -80 C. Under these conditions large numbers
of clouds appear in the stratosphere. These clouds are composed
largely of nitric acid and water, probably in the form of crystals
of nitric acid trihydrate ("NAT"), HNO3.3(H2O). Stratospheric
clouds also form from ordinary water ice (so-called "Type II PSC"),
but these are much less common; the stratosphere is very dry and
water-ice clouds only form at the lowest temperatures.
c.) Reactions On Stratospheric Clouds
Most of the chlorine in the stratosphere ends up in one of the
reservoir compounds, Chlorine Nitrate (ClONO2) or Hydrogen Chloride
(HCl). Laboratory experiments have shown, however, that these
compounds, ordinarily inert in the stratosphere, do react on the
surfaces of polar stratospheric cloud particles. HCl dissolves into
the particles as they grow, and when a ClONO2 molecule becomes
adsorbed the following reactions take place:
ClONO2 + HCl -> Cl2 + HNO3
ClONO2 + H2O -> HOCl + HNO3
The Nitric acid, HNO3, stays in the cloud particle.
In addition, stratospheric clouds catalyze the removal of Nitrogen
Oxides ("NOx"), through the reactions:
N2O5 + H2O -> 2 HNO3
N2O5 + HCl -> ClNO2 + HNO3
Since N2O5 is in (gas-phase) equilibrium with NO2:
2 N2O5 <-> 4 NO2 + O2
this has the effect of removing NO2 from the gas phase and
sequestering it in the clouds in the form of nitric acid, a process
called "denoxification" (removal of "NOx"). [Crutzen and Arnold]
[Hamill and Toon]
d.) Sedimentation and Denitrification
The clouds may eventually grow big enough so that they settle out
of the stratosphere, carrying the nitric acid with them
("denitrification"). Denitrification enhances denoxification.
If, on the other hand, the cloud decomposes while in the
stratosphere, nitrogen oxides are returned to the gas phase.
Presumably this should be called "renoxification", but
I have not heard anyone use this language :-).
e.) Photolysis of active chlorine compounds
The Cl2 and HOCl produced by the heterogeneous reactions are
easily photolyzed, even in the antarctic winter when there is
little UV present. The sun is always very low in the polar winter,
so the light takes a long path through the atmosphere and the
short-wave UV is selectively absorbed. Molecular chlorine,
however, absorbs _visible_ and near-UV light:
Cl2 + hv -> 2 Cl
Cl + O3 -> ClO + O2
The effect is to produce large amounts of ClO. This ClO would
ordinarily be captured by NO2 and returned to the ClONO2 reservoir,
but "denoxification" and "denitrification" prevent this by removing NO2.
f.) Catalytic destruction of ozone by active chlorine
As discussed in Part I, Cl and ClO can form a catalytic cycle that
efficiently destroys ozone. That cycle used free oxygen atoms,
however, which are only abundant in the upper stratosphere; it
cannot explain the ozone hole which forms in the lower stratosphere.
Instead, the principal mechanism involves chlorine peroxide, ClOOCl
(often referred to as the "ClO dimer") [Molina and Molina]:
ClO + ClO -> ClOOCl ClOOCl + hv -> Cl + ClOO ClOO -> Cl + O2 2 Cl + 2 O3 -> 2 ClO + 2 O2 ------------------------------- Net: 2 O3 -> 3 O2
ClO + BrO -> Br + Cl + O2 Br + O3 -> BrO + O2 Cl + O3 -> ClO + O2 ----------------------- Net: 2 O3 -> 3 O2