Antarctic vortex

Atmospheric measurements of CIO, BrO, 03, and NzO confirm the importance of reactions (43a)-(45) in the destruction of 03. For example, Anderson et al. (1989) showed that this cycle is the largest contributor to ozone loss in the Antarctic vortex from 14-18 km. 

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). 

Tuck, A. F., Synoptic and Chemical Evolution of the Antarctic Vortex in Late Winter and Early Spring, 1987, J. Geophys. Res., 94, 11687-11737 (1989). 

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). 

Figure 33 Schematic representations of the changes in the concentration of some stratospheric species on entering the chemically perturbed region of the Antarctic vortex (after ref. 57)...

Anderson, J.G., W.H. Brune, and M.H. Proffitt, Ozone destruction by chlorine radicals within the Antarctic vortex The spatial and temporal evolution of CIO-O3 anticorrelation based on in situ ER-2 Data. J Geophys Res 94, 11,479, 1989. 

Tuck, A.F., Synoptic and chemical evolution of the Antarctic vortex in late winter and early spring, 1987. J Geophys Res 94, 11,687, 1989. 

In addition to catalyzing Reaction (7.40), the ice particles play another role they remove nitrogen from the stratosphere (as HNO3), which limits the forward Reaction of (7.39), thereby providing more CIO than would otherwise be available. Thus, on both counts, during the austral winter the ice particles that comprise PSCs in the Antarctic vortex set the stage for the destruction of ozone by enhancing the concentrations... 

Anderson, J.G. Bmne, W.H. Proffitt, M.H., 1989 Ozone Destmetion by Chlorine Radicals Within the Antarctic vortex The Spatial and Temporal Evolution of CIO-O3 Anticorrelation Based on In Sim ER-2 Data , in Journal of Geophysical Research, 94 11465. 

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. 

During the long Antarctic night, appreciable amounts of molecular chlorine, Cl, and hypochlorous acid, HOCl, accumulate within the polar vortex. When the sun returns during the spring (in September in Antarctica), ultraviolet radiation decomposes the accumulated molecular chlorine and hypochlorous acid to produce atomic chlorine. Cl. Atomic chlorine is a free radical. Free radicals are atoms or molecules that contain an unpaired or free electron. The Lewis structures of free radicals contain an odd number of electrons. The unpaired electron in free radicals makes them very reactive. The free radical Cl produced from the decomposition of CI2 and HOCl catalyzes the destruction of ozone as represented by the reaction ... 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

Second, the northern polar vortex is much less stable and hence less isolated from mixing with external air masses compared to the Antarctic case events in January and February in which there was substantial mixing of air from midlatitudes into the vortex have been reported (e.g., see Browell et al., 1993 Plumb et al., 1994). This makes it particularly important to make both measurements and model predictions with sufficient resolution (Edouard et al., 1996). In addition, the Arctic polar vortex tends to break up earlier than the Southern Hemisphere polar vortex since ozone destruction is determined to a large degree by the extent of exposure to sunlight, the earlier breakup and mixing with air external to the vortex cuts the ozone loss short. 

As is the case for the Antarctic polar vortex, the extent of ozone depletion is governed to a large extent by the number of hours of sunlight available to drive... 

While there is a variety of evidence from these and other measurements (e.g., see Notholt, 1994) that denitrification is more episodic and less widespread in the Arctic compared to the Antarctic, it does not mean that it does not occur. Indeed, there is good evidence for denitrification of the Arctic polar vortex under some conditions (e.g., see Schlager et al., 1990 Kondo et al., 1992 Wilson et al., 1992 Dye et al., 1992 Pueschel et al., 1992b Tuck et al., 1994 and Hopfner et al., 1996). For example, direct measurements of stratospheric water and NO>. at the edge of the Arctic polar vortex in one study showed that in that case, both dehydration and denitrification had occurred (Hintsa et al., 1998a). 

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). 

Shindell, D. T., and R. L. de Zafra, Limits on Heterogeneous Processing in the Antarctic Spring Vortex from a Comparison of Measured and Modeled Chlorine, J. Geophys. Res., 102, 1441-1449(1997). 

A similar observation of record low total column ozone over Lauder, New Zealand, down to 222 DU compared to the 1985-1996 average of 340 DU was reported by Brinksma et al. (1998). They attributed the low ozone in part to a portion of the Antarctic polar vortex passing over this location at altitudes of 25-35 km and in part to injection at lower altitudes ( 22 km) of ozone-poor subtropical air. 

Figure 1. This graph shows the rapid variation of CIO and 03 as the edge of the chemically perturbed region in the Antarctic polar vortex is penetrated by the National Aeronautics and Space Administration (NASA) ER-2 high-altitude aircraft over the Palmer Peninsula of Antarctica on September 16, 1987 (5). It is one of a series of 12 snapshots, or individual flights, during the Airborne Antarctic Ozone Experiment (AAOE) that show the development of an anticorrelation between CIO and 03 that began as a correlation in mid-August. When these two measurements are combined with all the others from the ER-2 aircraft, the total data set provides a provocative picture of how such chemistry occurs and what it is capable of doing to ozone.

A third issue is the estimates of ozone loss associated with the polar vortex. Will the Antarctic ozone hole expand and will the Arctic ozone hole begin Can the loss of ozone be better quantified, and what are the zonal asymmetries in the CIO and BrO fields How much additional ozone is lost when the polar vortex breaks up ... 

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