Polar vortex

The discovery of ozone holes over Antarctica in the mid-1980s was strong observational evidence to support the Rowland and Molina hypothesis. The atmosphere over the south pole is complex because of the long periods of total darkness and sunlight and the presence of a polar vortex and polar stratospheric clouds. However, researchers have found evidence to support the role of CIO in the rapid depletion of stratospheric ozone over the south pole. Figure 11-3 shows the profile of ozone and CIO measured at an altitude of 18 km on an aircraft flight from southern Chile toward the south pole on September 21, 1987. One month earlier the ozone levels were fairly uniform around 2 ppm (vol). 

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 ... 

After the first reports of this phenomenon, major field campaigns were launched, which clearly established a relationship between ozone destruction and chlorine chemistry. For example, Fig. 1.8 shows simultaneous aircraft measurements of ozone and the free radical CIO as the plane flew toward the South Pole. As it entered the polar vortex, a relatively well-contained air mass over Antarctica, 03 dropped dramati-... 

FIGURE 1.8 Measured concentrations of the chlorine monoxide free radical (CIO) as well as 03 outside and inside the polar vortex on August 23, 1987 (adapted from Anderson, 1989). 

There are several reasons for the dramatic ozone destruction (see Fig. 2.17) low temperatures may have prolonged the presence of polar stratospheric clouds, which play a key role in the ozone destruction, the polar vortex was very stable, there were increased sulfate aerosols from the 1991 Mount Pinatubo volcanic eruption, which also contribute to heterogeneous chemistry, and chlorine levels had continued to increase. These issues are treated in more detail shortly. 

Through a variety of studies, it is now generally accepted that the observed losses are associated with chlorine derived from CFCs and that heterogeneous chemistry on polar stratospheric clouds plays a major role. The chemistry in this region is the result of the unique meteorology. As described in detail by Schoeberl and Hartmann (1991) and Schoeberl et al. (1992), a polar vortex develops in the stratosphere during the winter over Antarctica. The air in this vortex remains relatively isolated from the rest of the stratosphere, allowing photochemically active products to build up... 

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. 

For example, while the vortex-averaged 03 concentration at one altitude in the Arctic in the spring of 1994 was measured to decrease by 10%, the net chemical loss was estimated at 20% but this was partially compensated by an increase due to transport of air containing higher ozone concentrations from higher altitudes (Manney et al., 1995). Similar amounts of chemical ozone loss in the Arctic polar vortex have been calculated based on measurements of CIO, BrO, and 03 (e.g., Brune et al., 1991 Salawitch et al., 1993). 

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... 

FIGURE 12.39 Estimated partitioning of the Cl, reservoir outside and inside the Arctic polar vortex at a potential temperature of 420 K (adapted from Kawa el al., 1992b). 

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). 

When the Arctic polar vortex is not denitrified, more gas-phase HNO, is available as the sunlight intensity increases, and this photolyzes, regenerating N02 ... 

FIGURE 12.40 Loss of O, in Arctic polar vortex as a function of hours of exposure of the air mass to sunlight (adapted from von der Gathen et al., 1995). 

Of course, it is not just the chemistry but also the dynamics that determine the net effect on total column ozone in midlatitudes. Transport of air from the tropics to midlatitudes was discussed earlier in Section A.l. There is also evidence for the influence of high-latitude air on ozone at midlatitudes. It has been proposed, for example, that the Arctic polar vortex acts more like a flowing processor than an isolated air mass. In this scenario, air flows through the polar vortex and as it does, undergoes the chemistry described earlier for the... 

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). 

Kourtidis, K. A., R. Borchers, P. Fabian, and J. Harnisch, Carbonyl Sulfide (COS) Measurements in the Arctic Polar Vortex, Geophys. Res. Lett, 22, 393-396 (1995). 

Manney, G. L and R. W. Zurek, Interhemispheric Comparison of the Development of the Stratospheric Polar Vortex during Fall A 3-Dimensional Perspective for 1991-1992, Geophys. Res. Lett., 20, 1275-1278(1993). 

McIntyre, M. E., The Stratospheric Polar Vortex and Sub-Vortex Fluid Dynamics and Midlatitude Ozone Loss, Philos. Trans. R. Soc. London, A, 352, 227-240 (1995). 

Proffitt, M. H J. J. Margitan, K. K. Kelly, M. Loewenstein, J. R. Podolske, and K. R. Chan, Ozone Loss in the Arctic Polar Vortex Inferred from High-Altitude Aircraft Measurements, Nature, 347, 31-36 (1990). 

Schoeberl, M. R., and D. L. Hartmann, The Dynamics of the Stratospheric Polar Vortex and Its Relation to Springtime Ozone Depletions, Science, 251, 46-52 (1991). 

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