Environmental Engineering Reference
In-Depth Information
Measurements of CFCs at the South Pole
indicating a continuous increase of 5 per cent
per year tended to support this, although the
original fluctuation had been present before CFCs
were released into the atmosphere in any quantity
(Schoeberl and Krueger 1986). Other researchers
suggested that gases such as NO
x
(Callis and
Natarajan 1986) and the oxides of chlorine
(ClO
x
) and bromine (BrO
x
) (Crutzen and Arnold
1986) were the culprits. As the investigation of
the Antarctic ozone layer intensified, it became
clear that the chemistry of the polar stratosphere
is particularly complex. Although the
stratosphere is very dry, it becomes saturated at
the very low temperatures reached during the
winter months, and clouds form. Nitric and
hydrochloric acid particles in these polar
stratospheric clouds enter into a complex series
of reactions—involving, for example,
denitrification and dehydration—which
ultimately lead to the release of chlorine. In the
form of ClO it then attacks the ozone (Shine
1988). The evaporation of the polar stratospheric
clouds in the spring, as temperatures rise, brings
an end to the reactions, and allows the recovery
of the ozone layer.
The chemical reactions which lead to the
destruction of the ozone following the formation
of polar stratospheric clouds, take place initially
on the surface of ice particles in the clouds.
Similar heterogeneous chemical reactions on the
surface of sulphate aerosol particles have also
been identified as contributing to major thinning
of the Antarctic ozone layer (Brasseur and
Granier 1992; Deshler
et al.
1992; Keys
et al.
1993). The injection of large volumes of sulphate
into the lower stratosphere during the eruptions
of Mount Pinatubo and Mount Hudson in 1991
was followed by rapid destruction of the ozone
layer at altitudes of 9-13 km. During September
1991 alone, almost 50 per cent of the ozone in
the lower stratosphere was destroyed (Deshler
et al.
1992). The role of the sulphate aerosol was
confirmed by modelling chemical, radiative and
dynamical processes in the stratosphere (Braseur
and Granier 1992), by direct measurement of the
levels of volcanic aerosol and ozone depletion
(Deshler
et al.
1992), and by the presence of
chemicals normally associated with
heterogeneous chemical reactions at a time when
stratospheric temperatures remained too high for
polar stratospheric clouds to form. The
background sulphate aerosols in the stratosphere
provided an alternative surface upon which the
reactions took place (Keys
et al.
1993).
Following the initial identification of the
sulphate/ozone relationship in the Antarctic in
late 1991, additional evidence was obtained from
Thule in Greenland. Measurements taken in the
first three months of 1992 showed a negative
correlation between aerosol counts and ozone
levels in the middle stratosphere, with
fluctuations of as much as 50 per cent in ozone
content (di Sarra
et al.
1992). One year later over
Canada, record low ozone values were measured
at altitudes at which aerosols from the Mount
Pinatubo eruption had been observed (
Kerr et
al.
1993).
When the aerosols from Pinatubo and Hudson
initially spread into the Antarctic in 1991, they
were unable to penetrate beyond an altitude of
14-15 km because of the presence of the
circumpolar vortex. As a result, the thinning of
the ozone layer in the upper stratosphere
remained close to normal levels. By late 1992,
however, the aerosols had spread evenly over the
polar region. Record low levels of ozone were
reported over the pole and over southern
Argentina and Chile, while the thinning of the
ozone layer also reached record levels over the
northern hemisphere (Gribbin 1992), and global
ozone levels were more than 4 per cent below
normal (Kiernan 1993).
In contrast to these approaches which
emphasize the chemistry of the stratosphere, there
are the so-called dynamic hypotheses, which seek
to explain the variations in ozone levels in terms
of circulation patterns in the atmosphere. Certain
observations do support this. For example, at the
time the ozone level in the hole is at its lowest, a
ring of higher concentration develops around the
hole at between 40 and 50°S. The hole begins to
fill again in November, seemingly at the expense
of the zone of higher concentration (Stolarski
et
