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8
THE EARTH' S FRAGILE OZONE SHIELD
Susan Solomon
HI STORY OF THE OZONE DEPLETION PROBLEM
Ozone is an essential part of the earth's ecological balance because
it absorbs certain wavelengths of biologically damaging ultraviolet light
that are not effectively absorbed by any other component of the earth's
atmosphere. The degree of protection provided by the ozone layer is
related to the total amount of ozone between the sun and the planet
surface, and hence to the total integrated column abundance (the total
ozone). It is believed that the evolution of biological life on the
planet surface was closely tied to the evolution of the protective ozone
layer. Most the world's ozone is found in the stratosphere, at altitudes
from about 10 to 35 km.
The study of atmospheric ozone and concern about its possible deple-
tion dates back only to about the 1970s. During the middle and late
1970s, it was recognized that continued use of man-made chlorofluoro-
carbons could significantly perturb the natural ozone abundance.]
Chlorofluorocarbons are used in a wide variety of industrial appli-
cations, including refrigeration, air conditioning, foam blowing, and
cleaning of electronics components. Theoretical studies of the chem-
istry of ozone carried out in the year prior to the discovery of the
antarctic ozone hole suggested that chlorofluorocarbon production would
be expected to decrease ozone by perhaps 5 to 10 percent sometime in the
next century.
In 1985, scientists from the British Antarctic Survey reported ob-
servations of a 50 percent decrease in total ozone during the antarctic
spring.3 Figure 8.1 illustrates some of the observational data that
revealed the ozone hole. This unexpected seasonal decrease in con-
temporary antarctic ozone was quickly dubbed the "antarctic ozone hole,
and it rapidly captured worldwide attention. Laboratory, field, and
theoretical studies over the past 4 years since the discovery of the
antarctic ozone hole have led to a progressively clearer picture of how
it takes place, why it takes place largely in Antarctica, and its likely
implications for other latitudes. These investigations have changed the
understanding of atmospheric ozone chemistry and have led to a heightened
awareness of the importance of global change.
~!
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74
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o
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.
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~ October Mean
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1960 1 970 1 980 1 99C
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o
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Oct.7, 1987
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, 1 , 1 , 1 1
/ ~~O 1~60 1970 1980 1990
Year
Year
FIGURE 8.1 Total ozone. Observational data that first indicated the
existence of the antarctic ozone hole. DU, Dobson units; TOMS, total
ozone mapping spectrometer.
CURRENT THEORETICAL UNDERSTANDING OF ANTARCTIC OZONE DEPLETION
The key to antarctic ozone depletion is the extreme cold temperatures
that occur in the antarctic stratosphere. The stratosphere is extremely
dry, generally precluding significant cloud formation except under the
coldest conditions. The occurrence of clouds changes the chemistry in a
very fundamental way: it allows reactions to occur on surfaces rather
than between gas molecules.4 Chemical reactions take place on these
surfaces, converting chlorine from forms that do not react with ozone to
other, less stable forms that readily break up in the presence of
sunlight and go on to destroy ozone. Both cold temperatures and sunlight
are critical to the ozone depletion process. Therefore, antarctic ozone
depletion does not take place during the winter, when temperatures are
coldest but when the polar regions are largely in darkness, but rather in
the spring, after sunlight returns and temperatures remain cold.
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75
OBSERVATIONAL EVIDENCE
When the antarctic ozone hole was first discovered, little was known
about the antarctic stratosphere beyond the ozone measurements them-
selves. There were virtually no available data on other chemical com-
pounds present in the stratosphere, and there was also a pressing need
for more detailed meteorological information. These needs were rapidly
addressed by ground-based and aircraft expeditions to the Antarctic,
during which state-of-the-art instrumentation was used to measure
chemical compounds, to probe the nature of the polar clouds, and to
further understand the meteorology.
Observations of a broad range of atmospheric compounds, including
chlorine monoxide, chlorine dioxide, hydrochloric and nitric acid,
nitrogen oxide and dioxide, and nitrous oxide, were rapidly obtained.
The observations all display a highly unusual chemistry, greatly per-
turbed by the presence of clouds. The observed abundances of chlorine
and bromine monoxide will result in rapid and substantial ozone loss
similar to that observed in the antarctic spring. The chlorine monoxide
levels found in Antarctica are of particular importance, since this
species participates in catalytic cycles that rapidly destroy ozone.
The abundances of chlorine monoxide have been shown to be about 100 times
greater than expected in the absence of cloud chemistry. The broad range
of experimental techniques used and the consistency of the observed per-
turbations in many different chemical compounds have provided firm evi-
dence that these perturbations account for much if not all of the ant-
arctic ozone loss.5
METEOROLOGICAL PROCESSES: ANTARCTIC AND ARCTIC
The study of atmospheric chemistry is highly interdisciplinary, with
strong links to meteorology and radiative transfer. Meteorology plays an
important role in setting the stage for polar chemistry and modulating
the extent of ozone depletion. For example, some antarctic winters are
warmer than others and are likely to exhibit fewer polar stratospheric
clouds and less ozone depletion. Warmer winters are also likely to
modulate the ozone abundances through direct meteorological effects.
Meteorological processes also play a critical role in determining whether
or not the depletion of polar ozone can spread to lower latitudes through
mixing and large-scale overturning of the atmosphere.
There are a number of important differences between the antarctic and
arctic stratospheres. Satellite and ground-based observations show ozone
losses of about 5 to 10 percent in the arctic winter at high latitudes.6
Clearly, the ozone depletion in the arctic stratosphere is thus far much
smaller than that of the antarctic stratosphere. This is partly due to
the fact that winter arctic temperatures are warmer on average than those
of the Antarctic. Perhaps more importantly, the arctic stratosphere
generally warms up much earlier in the spring season than does the
antarctic stratosphere. This likely leads to a critical difference in
the temporal overlap between the cold temperatures and the sunlight
required for ozone depletion. It is critical to understand how the
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76
temperature history interacts with chemical processes and to evaluate
whether an unusually cold and late arctic spring would result in
substantial ozone losses there.
In Antarctica, the ozone loss of perhaps 50 percent is accompanied by
a much more spectacular increase in chlorine monoxide by a factor of 100.
The latter perturbation is much more readily identified as compared to
natural variability, and implies that measurements of chemical species
such as chlorine monoxide can help to evaluate the present and future
potential for ozone loss in those environments where direct identi-
fication of small ozone losses may be difficult. These considerations
motivated studies of the chemical composition of the arctic stratosphere
during the winters of 1987 and 1988, in which researchers sought to
understand the chemistry of the Arctic during winter and to determine the
extent to which it too may be influenced by polar stratospheric clouds.
IMPLICATIONS
Many scientists view the antarctic ozone hole as a sort of global
early warning system. The unusual chemistry of polar stratospheric
clouds has clearly made the antarctic ozone layer more w lnerable to
anthropogenic chlorine than the rest of the contemporary atmosphere. An
area of increasing concern is the possibility of similar chemical
reactions occurring on the type of particles present at warmer latitudes,
especially following major volcanic eruptions, which can greatly enhance
the particles present in the stratosphere around the world.
It is clearly fortunate that the ozone hole has so far occurred
largely in that part of the globe that contains the least biological
life. Ongoing research is, however, aimed at studying the possible
effects of ozone depletion on phytoplankton and, by extension, other
creatures such as Frill, penguins, and seals. It is of paramount
importance to determine the origin of the smaller ozone changes measured
at other latitudes and to evaluate the future changes that can be
expected worldwide if mankind continues the emission of chlorofluoro-
carbons.
NOTES
1. Molina, M. J., and F. S. Rowland, Nature, 249, 810, 1974; Stolarski,
R. S., and R. J. Cicerone, Can. J. Chem., 52, 1610, 1974; a recent
review has been given in McElroy, M. B., and R. J. Salawitch,
Science, 243, 763, 1989.
2. National Research Council, Causes and Effects of Changes in
Stratospheric Ozone: Update 1983, National Academy Press, Washington,
D.C., 1984.
Farman, J. C., B. G. Gardiner, and J. D. Shanklin, Nature, 315, 207,
1985.
4. Solomon, S., R. R. Garcia, F. S. Rowland, D. J. Wuebbles, Nature,
321, 755, 1986; McElroy, M. B., R. J. Salawitch, S. C. Wofsy, J. A.
Logan, Nature, 321, 759, 1986; Toon, O. B., P. Hamill, R. P. Turco,
3.
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77
J. Pinto, Geophys. Res. Lett., 13, 1308, 1986; McElroy, M. B
Salawitch, S. C. Wofsy, Geophys. Res. Lett., 13, 1296, 1986;
Crutzen, P. J., and F. Arnold, Nature, 324, 651, 1986; Molina, L. T.,
and M. J. Molina, J. Phys. Chem., 91, 433, 1986; Molina, M. J., T. L.
Tso, L. T. Molina, F. C. Y. Wang, Science, 238, 1253, 1987; Tolbert,
M. A., M. J. Rossi, R. Malhotra, D. M. Golden, Science, 238, 1258,
1987.
deZafra, R. L., M. Jaramillo, A. Parrish, P. Solomon, B. Connor,
J. Barrett, Nature, 328, 408, 1987; Brune, W. H., J. G. Anderson, K.
R. Chan, submitted to J. Geophys. Res., 1989; Solomon, S., G. H.
Mount, R. W. Sanders, A. L. Schmeltekopf, J. Geophys. Res., 92, 8329,
1987; Farmer, C. B., G. C. Toon, P. W. Shaper, J. F. Blavier, L. L.
Lowes, Nature, 329, 126, 1987; the status of antarctic ozone research
prior to August 1987 was reviewed in Solomon, S., Rev. Geophys., 26,
13, 1988; important new findings from airborne experiments will
appear shortly in a special issue of J. Geophys. Res., 1989.
NASA reference publication 1208, Present State of Knowledge of the
Upper Atmosphere 1988: An Assessment Report, National Aeronautics
and Space Administration, Washington, D.C., 1988.
., R. J.
Representative terms from entire chapter:
ozone depletion
