Antarctic ozone

Ozone has received increased attention for its occurrence and function in the Earth s atmosphere.For example the decreasing ozone concentration in the stratospheric ozone layer, becoming most obvious with the Antarctic ozone hole. 

Figure 5. Measurements of chlorine monoxide by Anderson et al (19) and of ozone by Proffitt et al (21) carried out in 1987 during the Airborne Antarctic Ozone Experiment.

The Antarctic ozone hole is the result of anthropogenic release of trace gases into the atmosphere (CFCs in particular), causing a decrease in stratospheric ozone and a subsequent increase in solar ultraviolet radiation reaching the earth s surface. 

Fig. 19-2 Depletion of Antarctic ozone during October between 1956 and 1985. (Reprinted with permission from R. S. Stolarski (1988). Changes in ozone over the Antarctic. In F. S. Rowland and I. S. A. Isaksen, "The Changing Atmosphere/ p. 112. John Wiley, Chichester.)...

It now appears that both the extreme magnitude and geographic limitations of the Antarctic ozone depletion are due to meteorologic patterns peculiar to the South Polar regions. The large decrease beyond the small reduction in the rest of the stratosphere apparently involves the circulation of the polar vortex, a complex interaction of Cl with oxides of nitrogen, their physical trapping in extremely cold (T < — 80°C) clouds and preferential removal of some species by precipitation. 

As discussed in Chapter 12, the CIO dimer is a central species in the chemistry of the Antarctic ozone hole. Table 4.32 gives the recommended absorption cross sections (DeMore et al., 1997). The photodissociation can, in principle, proceed by two paths ... 

There are also important differences in the gas-phase chemistry of the Antarctic ozone hole compared to the chemistry at midlatitudes. One is the formation and photolysis of the CIO dimer. In the Antarctic spring, recycling of CIO back to chlorine atoms via reaction (27) with oxygen atoms does not play a major role because of the relatively small oxygen atom concentrations at the low UV levels at that time. Molina and Molina (1987) proposed that the formation of a dimer of CIO could, however, lead to regeneration of atomic chlorine through the following reactions ... 

In short, the heterogeneous chemistry that drives the Antarctic ozone hole can occur not only on solid surfaces but also in and on liquid solutions containing combinations of HN03, H2S04, and HzO. As discussed in the following section, it is believed that this is why volcanic eruptions have such marked effects on stratospheric ozone on a global basis. 

Antarctic ozone hole formation. Outflow to lower latitudes then provides a source of air that has been processed by the polar vortex and PSCs (e.g., see Proffitt et al., 1990, 1993 Randel and Wu, 1995). 

Indeed, these reactions play an important role in the Antarctic ozone hole and they have important implications for control strategies, particularly of the bromi-nated compounds. For example, Danilin et al. (1996) examined the effects of ClO -BrO coupling on the cumulative loss of O-, in the Antarctic ozone hole from August 1 until the time of maximum ozone depletion. Increased bromine increased the rate of ozone loss under the denitrified conditions assumed in the calculations by converting CIO to Cl, primarily via reactions (31b) and (31c) (followed by photolysis of BrCl). Danilin et al. (1996) estimate that the efficiency of ozone destruction per bromine atom (a) is 33-55 times that per chlorine atom (the bromine enhancement factor ) under these conditions in the center of the Antarctic polar vortex, a 60 calculated as a global average over all latitudes, seasons, and altitudes (WMO, 1999). 

Anderson, J. G., D. W. Toohey, and W. H. Brune, Free Radicals within the Antarctic Vortex The Role of CFC s in Antarctic Ozone Loss, Science, 251, 39-46 (1991). 

Angel I, J. K., Reexamination of the Relation between Depth of the Antarctic Ozone Hole, and Equatorial QBO and SST, 1962-1992, Geophys. Res. Lett., 20, 1559-1562 (1993b). 

Brasseur, G. P., X. Tie, P. J. Rasch, and F. Lefevre, A Three-Dimensional Simulation of the Antarctic Ozone Hole Impact of Anthropogenic Chlorine on the Lower Stratosphere and Upper Troposphere, J. Geophys. Res., 102, 8909-8930 (1997). 

Callis, L. B., and M. Natarajan, The Antarctic Ozone Minimum Relationship to Odd Nitrogen, Odd Chlorine, the Final Warming, and the 11-Year Solar Cycle, J. Geophys. Res., 91, 10771-10796 (1986). 

Garcia, R. R and S. Solomon, A Possible Relationship between Interannual Variability in Antarctic Ozone and the Quasi-biennial Oscillation, Geophys. Res. Lett., 14, 848-851 (1987). 

Hofmann, D. J and T. Deshler, Stratospheric Cloud Observations during Formation of the Antarctic Ozone Hole in 1989, J. Geophys. Res., 96, 2897-2912 (1991). 

Hofmann, D. J., and S. J. Oltmans, Anomalous Antarctic Ozone during 1992 Evidence for Pinatubo Volcanic Aerosol Effects, J. Geophys. Res., 98, 18555-18561 (1993). 

Leu, M.-T., Heterogeneous Reactions of N20 with H20 and HCI on Ice Surfaces Implications for Antarctic Ozone Depletion, Geophys. Res. Lett, 15, 853-854 (3988b). 

Tolbert, M. A., M. J. Rossi, and D. M. Golden, Antarctic Ozone Depletion Chemistry Reactions of N205 with H20 and HCI on Ice Surfaces, Science, 240, 1018-1021 (1988b). 

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