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Day Number of 1993

Figure 7.14 Hours of daylight per day (defined as hours per day with SZA < 90°) and average daytime SZA (daytime is defined as SZA < 90°) versus day number. Calculated for a parcel located at 70°S, which is typical of the Antarctic polar vortex.

In Figure 7.15, we plot vortex-average 0, at several levels in the lower and mid-stratosphere. The vast majority of O. loss occurs between mid-August and mid-September, the time period featuring both high CI, (see Figure 7.12) and long periods of sunlight (see Figure 7.14). At 465 K, about two-thirds of the O, in the vortex is lost during the last month of winter. At higher altitudes the loss becomes progressively smaller, consistent with lower values of CI, (see Figure 7.11). The increase in O, between the end of September and the beginning of November is likely due to the weakening of the polar vortex by rising polar temperatures, followed by horizontal transport of ozone-rich air from mid-latitudes to the high latitudes.

Another dramatic view of the decrease of Antarctic O, is shown in Figure 7.16. During the month of September virtually all of the 0¡ between 14 and 22 km is destroyed. O, above -23 km. where the CIO abundance is much lower, is unaffected. It has been verified that models using realistic values of chlorine and bromine can accurately reproduce such rapid loss [262].

It is important to point out that the observed changes in lower stratospheric polar O, cannot be caused by the transport into that region of low-O, air. First, as we mentioned before, calculations [224[ suggest that vertical motion of air in the vortex



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