FIGURE 12.18 Energetics of the CIO + CIO reaction (adapted from Nicko-laisen et al., 1994).
However, the formation of the dimer in the ter-moleeular reaction is sufficiently fast under stratospheric conditions that the bimolecular reactions are not important. For example, using the recommended termolecular values (DeMore et al., 1997) for the low-pressure-limiting rate constant of = 2.2 X f0~32
and the high-pressure-limiting rate constant of k:T = 3.5 X 10
-12 1 cm molecule"
with temperature-dependent coefficients n = 3.1 and m = f.O (see Chapter 5), the effective rate constant at 25 Torr pressure and 300 K is 1.6 X f0~14 cm3 molecule s-1, equal to the sum of the bimolecular channels (Nickolaisen et al., 1994). At a more typical stratospheric temperature of 220 K and only 1 Torr pressure, the effective second-order rate constant for the termolecular reaction already exceeds that for the sum of the bimolecular channels, 2.4 X f0~15 versus f.9 X 10"15 cm3 molecules-1.
In short, under stratospheric conditions the self-reaction of CIO to form the dimer (C10)2 is the most important channel and has been shown to be consistent with observations of ozone destruction in the Antarctic (Molina and Molina, 1987; Sander et al., 1989; Trolier et al., 1990; Nickolaisen et al., 1994).
This cycle is believed to be responsible for approximately 75% of the ozone destruction in the f3- to 19-km region in the Antarctic ozone hole (Anderson et al., 1991). The cycles involving CIO + O, reaction (27), and CIO + H02, reactions (28) and (29), each account for approximately 5% of the ozone loss and the remainder is due to the CIO + BrO interaction, reactions (31) and (32) (Anderson et al., 1991).
It should be noted that around the edges of the vortex where exchange with the surrounding air occurs, there is less extensive denitrification in this "collar"
region (e.g., Toon et al., 1989; Ricaud et al., 1998). As a result, CIO was trapped as C10N02 and there was less ozone destruction.
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 (31 a). 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 If). 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 Br0N02 on aerosol particles (e.g., Erie et al., 1998).
ft is interesting that enhanced OCIO levels were observed at McMurdo as early as late June, which is not expected since light is not available at that time to generate CI and Br and hence the CIO and BrO precursors (Sanders et al., 1993). It appears that portions of the polar vortex can be exposed to sunlight even during the polar winter due to the size of the vortex and some displacement of the vortex edge into sunlit regions. This leads to the generation of enhanced CIO, BrO, and their product OCIO as well as reduced N03 and increased N02 (e.g., see Tuck, 1989; Solomon et al., 1993; Jiang et al., 1996). This effect has also been proposed to have increased the wintertime loss of 03 so that the threshold for development of the ozone hole is lowered (Jiang et al., 1996). [It should be noted that another mechanism for producing OCIO is reaction (43d) of CIO + CIO; for example, this has been invoked to explain the observed levels of CIO in the Arctic vortex under warmer (225 K) conditions (Pierson et al., 1999).]
Evidence for the role of chlorine is seen in Fig. 12.19. This shows CIO and 03 measured on August 23, 1987, prior to development of the ozone hole, and on September 16, after the hole had formed, as the sampling aircraft flew south into the polar vortex (Anderson et al., 1991). The rapid drop in 03 and accompanying increase in CIO due to the chemistry discussed above is clearly seen on September 16. On August 23, CIO is observed because some of the air in the polar vortex has been exposed previously to sunlight around the edges of the vortex, forming CIO. This leads to small amounts of ozone destruction via reactions (44), (45), and (26), with 03 losses of ~f0-20% prior to development of the hole. However, the total available light at this time is not sufficient to drive substantial ozone depletion, which is in effect determined by the total integrated solar exposure available to cause the chemistry. As the solar exposure increases through September, the chain destruction of 03 above is greatly enhanced, as seen in Fig. 12.19. Typical ozone loss
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