Rgg

FIGURE 12.46 Model-calculated cumulative loss of ozone from August 1 to the day of maximum ozone depletion as a function of stratospheric bromine ([CI,.] = 12.5 ppbv, [NO,.] = 2 ppb, 70°S at an altitude corresponding to 50 mbar total pressure in these calculations) (adapted from Danilin et al., 1996).

FIGURE 12.46 Model-calculated cumulative loss of ozone from August 1 to the day of maximum ozone depletion as a function of stratospheric bromine ([CI,.] = 12.5 ppbv, [NO,.] = 2 ppb, 70°S at an altitude corresponding to 50 mbar total pressure in these calculations) (adapted from Danilin et al., 1996).

Indeed, these reactions play an important role in the Antarctic ozone hole and they have important implications for control strategies, particularly of the brominated compounds. For example, Danilin et al. (1996) examined the effects of C10x-Br0x coupling on the cumulative loss of 03 in the Antarctic ozone hole from August f 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 CI, 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).

However, although as much as 50% of the loss of 03 could be attributed to bromine chemistry at a Br^ concentration of 25 ppt, a reduction in bromine did not give a proportional change in the total destruction of 03. Figure 12.46 shows the predicted cumulative 03 loss as a function of the Br^ concentration; as bromine decreases, the contribution of the BrO-CIO cycle decreases as well. However, the net effect on total ozone loss is quite small because near-total ozone destruction occurs even without a significant contribution from BrO.

It should be noted that in the Arctic, the integrated concentrations of CIO are generally smaller than in the Antarctic; thus, the BrO + CIO cycle is relatively more important than the CIO + CIO cycle (Salawitch et al., 1993; WMO, 1995). In addition, only -30-50% of the

03 in a given air mass is destroyed over the Arctic, compared to near-total ozone destruction over Antarctica. As a result, bromine does make a significant contribution to the total 03 destruction in the Arctic, and as a result, control of brominated organics is expected to have a greater effect on minimizing ozone destruction in this region.

The BrO + BrO self-reaction occurs in a manner analogous to the CIO + CIO self-reaction:

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