Environmental Engineering Reference
In-Depth Information
al.
1986). Bowman (1986) has suggested that the
main mechanism involved is the circumpolar
vortex. The Antarctic circumpolar vortex is a
particularly tight, self-contained wind system,
which is most intense during the southern winter,
when it permits little exchange of energy or
matter across its boundaries. Thus it prevents any
inflow of ozone from lower latitudes, where most
of it is produced, allowing the cumulative effects
of the catalytic, ozone-destroying chemicals to
become much more obvious. The breakdown of
the vortex allows the transfer of ozone from
lower latitudes to fill the hole. Observations show
that when the breakdown of the vortex is late,
ozone levels become very low, and when the
breakdown is early, the ozone hole is less well-
marked. The possibility also exists that the
cooling of the atmosphere above the Antarctic,
resulting from ozone depletion, will encourage
the vortex to persist longer, become more intense
and perhaps even spread to lower latitudes, thus
compounding the problem (Gribbin 1993).
Most authorities tend to place the hypotheses,
which attempt to explain the Antarctic ozone
levels, into either chemical or dynamic categories
(Rosenthal and Wilson 1987), but there are
several hypotheses which might be placed in a
third group, combining both chemical and
dynamic elements. The paper which first drew
attention to the increased thinning of the
Antarctic ozone layer, for example, might be
classed in the chemical group since it attributes
the decline to CFCs (Farman
et al.
1985). It does,
however, have a dynamic element in its
consideration of the polar vortex, which appears
to be a necessary prerequisite for the ozone
depletion.
Another hypothesis which combines dynamic
and chemical elements is that proposed by Callis
and Natarajan (1986). They suggest that the
depletion of the ozone in the Antarctic is a natural
phenomenon, caused by elevated levels of NO
x
in the atmosphere during periods of increased
solar activity. Sunspot activity did peak at a
particularly high level in 1979, and continued
into the early 1980s, but measurements by Dr
Susan Solomon, released at a special meeting of
the Royal Meteorological Society in 1988,
indicated that NO
x
levels in the lower
stratosphere in the Antarctic were too low to have
the necessary catalytic effect (Shine 1988).
Comparison of the situation in Antarctica with
that in the Arctic provides indirect support for
the role of the circumpolar vortex in the thinning
of the ozone layer. The circumpolar vortex in the
northern hemisphere is much less intense than
its southern counterpart, which may explain, in
part at least, the absence of a comparable hole
in the ozone over the Arctic (Farman
et al.
1985).
A smaller, more mobile hole was identified in the
ozone above the Arctic in the late 1980s, however,
and further investigation set in motion (Shine
1988).
The European Arctic Stratospheric Ozone
Experiment (EASOE)—involving scientists
from the European Community, the United
States, Canada, Japan, New Zealand and
Russia—was undertaken during the northern
winter of 1991-92, using ground
measurements, balloons, aircraft and a variety
of modelling techniques to establish the nature
and extent of ozone depletion over the Arctic
(Pyle 1991). Preliminary results tended to
confirm the absence of a distinct hole over the
Arctic, but noted the higher than average ozone
loss in middle latitudes. Because the Arctic
stratosphere is generally warmer than its
southern counterpart, polar stratospheric
clouds are less ready to form, and ozone
destruction is less efficient. The less developed
Arctic circumpolar vortex also allows the loss
of ozone at the pole to be offset to some extent
by the influx of ozone from more southerly
latitudes. The relatively free flow of air out of
the Arctic in the winter might also contribute
the peculiar patterns of ozone depletion in the
north. Chemicals incapable of destroying ozone
because of the lack of energy available during
the Arctic winter night become energized when
carried south into the sunlight of mid-latitudes,
and cause greater thinning there than at the pole
(Pyle 1991). By March 1992, the EASOE had
detected decreases of 10-20 per cent in Arctic
ozone (Concar 1992). Although this was less
