The
Antarctic Ozone hole
What
is it?
The
antarctic ozone hole is an area of the antarctic stratosphere in which
the recent (since about 1975) ozone levels have dropped to as low
as 33% of their pre-1975 values. The ozone hole occurs during the
antarctic spring, from September to early December, as strong westerly
wind start to circulate around the continent and create an atmospheric
container. In this container over 50% of the lower stratospheric ozone
is destroyed.
Why
is it important?
While
the effective of the antarctic hole in decreasing the global ozone
is relatively small, estimated at about 4% per decade, the hole has
generated a great deal of interest because:
- The
decrease in the ozone layer was predicted in the early 1980's
to be roughly 7% over a sixty-year period. The sudden and, at
that time unexplained, disappearance of over 50% of the ozone
layer in a localized area of antarctica created quite a stir.
Many were worried that ozone holes might start to appear over
other areas of the globe but to date the only other significant,
localized depletion is a much smaller ozone ''dimple''(R.Parson
FAQ) ; observed during the arctic
spring over the north pole.
- The
antarctic hole is a warning that if conditions become more antarctic:
cooler stratospheric temperatures, more stratospheric clouds,
more active chlorine; then global ozone will decrease at a much
greater pace. Some of the more popular senarios of global warming
predict that these changes could occur in larger portions of the
stratosphere.
- When
the antarctic ozone hole does break-up, the ozone-depleted air
drifts out into nearby areas. Decreases in the ozone level of
up to 10% have been reported in New Zealand in the month following
the break-up of the antarctic ozone hole [WMO 1991] [Atkinson
et al.1989] [Roy et al. 1990]
What
is so special about Antarctic conditions?
Polar
regions get a much larger variation in sunlight than anywhere else,
and during the 3 months of winter spend most of time in the dark without
solar radiation. Temperatures hover around or below -80'C for much
of the winter and the extremely low antarctic temperatures cause cloud
formation in the relatively ''dry''stratosphere. These Polar
Stratospheric Clouds (PSC's) are composed of ice crystals that
provide the surface for a multitude of reactions, many of which speed
the degredation of ozone molecules.
Summary
of what happens in the Anarctic hole
For
details of the chemistry of ozone depletion see the Ozone
chemistry tutorial . Most of the following infornation on the
Anarctic hole comes from Robert Parson's FAQ on the subject.
1. As mid-May brings on the onset of winter, the antarctic stratosphere
cools and descends closer to the surface. The Coriolis effect (caused
by the earths rotation) sets up a strong westerly circulation around
the south pole, forming an oblong vortex which varies in size from
year to year. Current theory [Tuck 1989] holds that the vortex is
like a semi-sealed reaction vessel with most of the antarctic air
staying trapped inside the vortex. As temperatures in the lower
stratosphere cools below -80'C, Polar Stratospheric Clouds (PSC's)
start to form.
2. Most of the anarctic stratospheric chlorine ends up in resevoir
compounds such as ClONO2 or HCl. Resevoir compounds are so named
because they hold the atmospheric chlorine in an inactive form but
can react later, usually after a hit by ultraviolet radiation, and
release reactive chlorine molecules. On the surface of the PSC crystals,
nitrogen compounds are readily absorbed and chlorine resevoir compounds
are converted to far more reactive compounds such as Cl2 and HOCl.
3. The small amounts of visible light during the antarctic winter
are sufficient to convert much of the atmospheric Cl2 to ClO:
-
Cl2 + light ---> 2 Cl
-
Cl + O3 ---> ClO + O2
Ordinarily
much of the ClO would be captured by atmospheric NO2 and returned
to the ClONO2 resevoir, but the polar clouds have absorbed most of
the Nitrogen compounds such as NO2.
4. Spring brings an increase of ultraviolet light to the lower antarctic
stratosphere, providing the energy needed needed for the rapid catalytic
break-down of ozone by ClO and its dimer ClOOCl. Another mechanism
involving Bromine adds another 33% to the depletion total. Over
50% of the stratospheric ozone is destroyed by these two mechanims,
most of the damage occuring in the lower stratosphere.
5. Towards the end of spring (mid-December) the warming temperatures
cause the vortex to break up; ozone-rich air from the surrounding
area comes flooding in and masses of ozone-depleated air go wandering
off, temporarly lowering the ozone in areas of South America and
New Zealand by up to 10%.
Factors
influencing the magnitude of the hole are essentially
the same as those factors
affecting global ozone levels but an area of great uncertainty
are the surface reactions that happen in the polar stratospheric
clouds. Reactions have been proposed and tested in labs in which
chlorine, bromine and nitogen-oxides can cooperate to break down
ozone many times faster than they could alone, and the presence
of the polar clouds is important in this cooperation.
Author: Brien Sparling
Return
to the ozone homepage
|
