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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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74 400 Cal a' 300 o o ~ Ink ~ ~ . South Pole ~& ~ October Mean 100 , 1 ~ ~ , 1960 1 970 1 980 1 99C Year 400, / 300 a' / ~ / o o 200 / ~ / ~ :: 00 ~ 1 150~1} ~' _ / ~.< 400 ~ \~, 300 o o \20C 0 \ Syowa \ October Mean 100 1 1 1 1 , 1960 1 970 1 980 1 990 TOMS Oct.7, 1987 %:~. ~~. ~ _ Halley Bay October Mean , 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.