Antarctic stratosphere

The discovery of the Ozone Hole in the Antarctic stratosphere has led to the realization that previously unsuspected heterogeneous chemical reactions occuring on the surface of ice and other stratospheric cloud particles play a critical role in atmospheric ozone depletion — not only in the Antarctic stratosphere,... 

This is probably the key heterogeneous reaction for Antarctic stratospheric ozone depletion, and serves as a useful focus for the discussion of the theoretical challenges that must be addressed in dealing with fairly complex chemistry in a complex environment, challenges enlivened — as will be seen below — by the evident chemical involvement of the ice surface environment. 

Crutzen, P. J., and F. Arnold, Nitric Acid Cloud Formation in the Cold Antarctic Stratosphere A Major Cause for the Springtime Ozone Hole, Nature, 324, 651-655 (1986). 

Leu, M.-T., Laboratory Studies of Sticking Coefficients and Heterogeneous Reactions Important in the Antarctic Stratosphere, Geophys. Res. Lett., 15, 17-20 (1988a). 

Tolbert, M. A., M. J. Rossi, R. Malhotra, and D. M. Golden, Reaction of Chlorine Nitrate with Hydrogen Chloride and Water at Antarctic Stratospheric Temperatures, Science, 238, 1258-1260 (1987). 

F. S. Rowland, Chlorofluorocarbons and the depletion of stratospheric ozone Am. Sci. 77, 36-45 (1989) T.-L. Tso, L. T. Molina, and F. C.-Y. Wang, Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride and ice release of active chlorine. Science 238, 1253-1260 (1987) J. G- Anderson, D. W. Toohey, and W. H. Brune, Free radicals within the Antarctic vortex the role of CFCs in Antarctic ozone loss. Science 251, 39-46 (1991) P. S. Zurer, Complexities of ozone loss continue to challenge scientists. Chem. Eng. News June 12, 20-23 (1995). 

Mechanisms and rates of transport of nuclear test debris in the upper and lower atmosphere are considered. For the lower thermosphere vertical eddy diffusion coefficients of 3-6 X 106 cm.2 sec. 1 are estimated from twilight lithium enhancement observations. Radiochemical evidence for samples from 23 to 37 km. altitude at 31° N indicate pole-ward mean motion in this layer. Large increases in stratospheric debris in the southern hemisphere in 1963 and 1964 are attributed to debris from Soviet tests, transported via the mesosphere and the Antarctic stratosphere. Most of the carbon-14 remains behind in the Arctic stratosphere. 210Bi/ 210Pb ratios indicate aerosol residence times of only a few days at tropospheric levels and only several weeks in the lower stratosphere. Implications for the inventory and distribution of radioactive fallout are discussed. 

In 1985, the British Antarctic Survey reported that the total ozone in the Antarctic stratosphere had decreased by 50% in early spring, relative to levels observed in the preceding 20 years. Ground, airborne, and satellite observations have since shown that this ozone hole occurs only in early spring (Figure 1-1) and continued to deepen until the year 2000. 

An important piece of evidence that some of this chlorine comes from the breakdown or CFCs was the unusually high levels of fluorine compounds detected in the Antarctic stratosphere. Whereas chlorine compounds come from a number of natural sources, fluorine compounds in nature are relatively rare. The source of this stratospheric fluorine, therefore, is most likely chlorofluoro-carbons. In addition to elevated fluorine levels, evidence of ozone depletion... 

During what time of the year is Antarctic stratospheric ozone depletion the greatest ... 

Crutzen, P.J., and Arnold, F. (1986) Nitric acid cloud formation in the cold Antarctic stratosphere a major cause for the springtime ozone hole. Nature 324,651-655. 

If there were no new inputs of chlorine into the stratosphere, eventually all of the chlorine would be inactivated (i.e., it would eventually not be in the form of CI2, Cl, or CIO). The inactivation reactions produce HC1 and C10N02. In the relatively warm months in the Antarctic stratosphere, these two compounds are in the gas phase as opposed to condensed on solid phases (such as ice particles). Note that we will use abbreviations in the reactions to indicate the phases (s) and (g) mean in the solid and gas phases, respectively. 

When it gets cold, small ice crystals form in the Antarctic stratosphere these are called polar stratospheric clouds. HC1 condenses onto these surfaces. Unfortunately for the ozone, there is a reaction of HCl(s) with C10N02(g), which is catalyzed by the surface. 

This happens in the early spring in the Antarctic or in the months of September and October. The Cl atoms then catalytically destroy ozone, resulting in the rapid loss of ozone that we call the ozone hole. As time goes by, the Antarctic stratosphere warms up, the ice crystals melt, Cl starts to be inactivated, and the ozone hole heals. 

Figure 29 shows a recent picture of the ozone hole taken from space. Strictly speaking the use of the word hole to describe what happens to ozone in the Antarctic is an exaggeration. There is undoubtedly a massive depletion of ozone, particularly between 12 and 20 km in the Antarctic stratosphere (up to 100%) but the total column of ozone is depleted rather than removed altogether (see Figure 28). The exact location and size of the hole varies with meteorological conditions, but the area covered has increased over the past 10 years or so (see Figure 30). Currently, in the austral spring the hole extends over the entire Antarctic continent, occasionally including the tip of South America, covering an area equivalent to the North American continent (ca. 22 million km ) (see Figure 31).

Lowest values of ozone measured from satellite each year in the ozone hole. Global average ozone is about 300 Dobson Units. Before 1980 ozone less than 200 Dobson Units was rarely seen. In recent years ozone near 100 Dobson Units has become normal in the ozone hole. Ozone in the year 2002 ozone hole was higher than we have come to expect because of unusually high temperatures in the Antarctic stratosphere (from NASA data)... 

The reason for the dehydration and denitrification of the Antarctic stratosphere is the formation of the PSCs, whose chemistry perturbs the composition in the Antarctic stratosphere. Polar stratospheric clouds can be composed of small (< 1 pm diameter) particles rich in HNO3 or at lower temperatures (<190 K) larger (10 pm) mainly ice particles. These are often split into two categories, the so-called Type 1PSC, which contains the nitric acid either in the form of liquid ternary solutions with water and sulfuric acid or as solid hydrates of nitric acid, or Type II PSCs made of ice particles. The ice crystals on these clouds provide a surface for reactions such as... 

In the winter of 1984, massive losses of stratospheric ozone were detected in Antarctica over the South Pole (Halley Bay). This ozone depletion is known as the ozone hole. We know now that it also forms over the Arctic, although not as dramatically as in the Antarctic. Stratospheric ozone protects life on the surface of the Earth by screening harmful UV radiation coming from the sun through a photodissociation mechanism (see Chapter 4). 

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