Environmental Engineering Reference
In-Depth Information
normal ozone cycles in the Antarctic gave values of ca 300 DU during winter and until
late spring in October. Thereafter a rise to ca 400 DU by the beginning of December
was typical, with a gradual falling off again towards March. Since the 1980s this cycle
of events has been replaced by a superimposed thinning of the ozone with values below
100 DU during the Antarctic spring. This spring-time decrease was first reported by
Farman and co-workers
2
of the British Antarctic Survey team in 1985. They had
observed a mean loss during October 1984 from ca 300 to 180 DU. Subsequent
recordings have shown values of less than 100 DU. A press release by NASA in
September 2000 announced that an ozone depletion area three times larger than the land
mass of the United States, had occurred. This depleted area spanned ca 28 million km
2
,
and was considerably larger than that noted two years previously.
3. Contributing factors for the ozone loss
There is general consensus that the ozone layer is being destroyed mainly as a
consequence of the release of chlorofluorocarbons (CFCs) into the atmosphere by
industry
3
. The chlorine monoxide molecule (ClO) and other important chemical species
such as bromine monoxide (BrO) and nitrogen oxides are involved. Reservoirs of
bromine, nitrogen oxide and chlorine are transported to the upper atmosphere, although
bromine reservoirs are more unstable than chlorine, and occur mainly as Br and BrO.
The chlorine reservoirs may consist of hydrochloric acid and chlorine nitrate, while
dinitrogen pentoxide and nitric acid are the nitrogen oxide reservoirs. Catalytic
breakdown reactions of ozone involve heterogeneous interactions with bromine,
chlorine and nitrogen compounds. Of importance are also sulphate aerosol particles and
sulphur dioxide, the latter of which has been found in high amounts following volcanic
eruptions. During polar spring conditions of ca -80°C, the sulphate aerosols take up
water and nitric acid and form the so-called polar stratospheric clouds (PSC) which
become solidified as temperatures drop further. These clouds serve as catalytic surfaces
where ozone-degrading substances are released and become concentrated, and react
with ozone when the polar regions warm up.
Another important climatic factor is the increasing trend of CO
2
levels, which
also negatively affect the ozone chemistry by preventing re-emission of the radiation
from the surface of the earth. This greenhouse phenomenon cools the upper atmosphere,
which in turn results in favourable conditions for the formation of the polar stratospheric
clouds over the polar regions, especially over the Antarctic.
4. Antarctic versus Arctic climates
The geographical features of the Arctic region apparently result in more irregular
polar vortex winds and higher temperatures compared to the Antarctic, which is
surrounded by ocean rather than by mountainous continents. Consequently, polar
stratospheric clouds are not as frequent in the Arctic. Despite this, recent recordings of
ozone losses exceeding 60% have coincided with increased sightings of PSCs and a
more stable polar vortex. The lower temperatures found in the Arctic in recent years
show a positive correlation with stratospheric ozone loss
4
. These findings suggest that
the recovery of the ozone layer may be delayed longer than predicted.
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