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Day of year

Figure 7.12 Time series of average CI, abundance at 465 K potential temperature for 1992, 1993, and 1994 in the Antarctic polar vortex. CI, is derived from daytime UARS MLS CIO data (version 4) assuming photochemical steady state. (After Wu and Dessler [262-1.)

time. Averaging over the entire vortex, CI, is -2.4 ppbv, with little variation throughout this time.

It should be noted that even in June and early July, CIO measurements show that PSC processing has activated a significant faction of Clt. While CI, is elevated at this time, little of the vortex is exposed to sunlight at this time of the year, so chemical loss of O, is negligible.

There are few measurements of bromine species in the Antarctic polar vortex. Taking measurements from the Arctic [263] as a guide, it is likely that BrO is -10 pptv throughout the sunlit Antarctic polar vortex.

An aside: can Mount Erebus be supplying chlorine to the Antarctic stratosphere ?

Mount Erebus is an active volcano on Ross Island in Antarctica. In 1991 the volcano pumped 13.3 Gg of HC1 into the Antarctic troposphere [264], It has been speculated that chlorine from Mount Erebus might be a significant contributor to stratospheric chlorine. Could chlorine from Mount Erebus be making it into the Antarctic stratosphere, where it could be contributing to the formation of the ozone hole?

Almost certainly not. First, the budget of stratospheric chlorine is well balanced: the formation of inorganic chlorine closely matches the destruction of CFCs and methyl chloride. Any additional source of stratospheric CI, would have to be small (less than a few percent of CI,,). Second, the mean overturning circulation features air rising over the tropics and descending at high latitudes. Air descends from the stratosphere to the troposphere over Antarctica; air in the Antarctic troposphere is not ascending into the stratosphere.

It should be noted that Mount Erebus' effluents remain in the troposphere because the eruption is not energetic. Energetic eruptions, such as that of Mount Pinatubo in 1991, can inject their effluents well into the mid-stratosphere. Such eruptions can significantly perturb the chemistry of the stratosphere (e.g. see Chapter 6).

7.1.4 Odd Oxygen loss

Under the high-Cl, conditions of the Antarctic polar vortex, Ox is destroyed through two catalytic cycles. The most important one is based on the ClO-dimer molecule [265 J:

The rate-limiting step of this cycle is the photolysis of the CIO dimer. Thus, the rate of O, loss from this cycle is 27nnorl(C!OOCl|. As we mentioned earlier, thermal decomposition of ClOOCl is negligible at the cold temperatures found in the Antarctic polar vortex. As a result, the formation rate of the dimer (the self-reaction of CIO, reaction (7.6)) is equal to the photolysis rate of the dimer (reaction (7.7)). One can therefore approximate the rate of O, loss due to this cycle as twice the rate of formation of the dimer (2/c*no+( IO[C10]2). In the warmer Arctic, thermal decomposition can be important, and therefore this approximation is less valid.

Hie second important catalytic cycle is the ClO-BrO cycle discussed in Chapter 3 (cycles (3.23) and (3.24)). Finally, the ClO-O cycle (cycle (3.16)) also contributes.

It is crucial to recognize that all three of these catalytic cycles require sunlight to destroy O,. For the ClO-dimer cycle (7.9), it is obvious why it does not run in darkness: one of the steps in the cycle is the photolysis of ClOOCl, which ceases without sunlight. The ClO-BrO cycles (cycles (3.23) and (3.24)) cease because CIO is greatly reduced at night because most of CI, is tied up in ClOOCl, and BrO is depleted through reaction with any remaining CIO to form BrCl, which is stable at night. Finally, the CIO O cycle (cycle (3.16)) does not run because, at night, the abundance of CIO is low and the abundance of O is zero.

Figure 7.13 shows the O, loss per day in the lower stratosphere during the last month of winter. The O, loss per day is relatively small in the middle of August,

Day of year

Figure 7.13 Antarctic vortex-averaged daily O, loss per day versus day of the year. Calculated as the sum of the rates of the CIO-CIO and ClO-BrO catalytic cycles, using UARS MLS CIO measurements (version 4) and assuming IBrO] = 12 pptv. This sum is adjusted upward by 5% to account for other cycles, such as CIO + O. (Adapted from Wu and Dessler 1262].)

Day of year

Figure 7.13 Antarctic vortex-averaged daily O, loss per day versus day of the year. Calculated as the sum of the rates of the CIO-CIO and ClO-BrO catalytic cycles, using UARS MLS CIO measurements (version 4) and assuming IBrO] = 12 pptv. This sum is adjusted upward by 5% to account for other cycles, such as CIO + O. (Adapted from Wu and Dessler 1262].)

increasing rapidly through the end of August and the first half of September. From Figure 7.12 we see that daytime average C1A is approximately constant during this time, so changes in Clr abundance cannot explain the increase in 0A loss per day during this time. Instead, it is the rapidly rising number of hours of sunlight per day during this time period, from about 5 h of sunlight per day in mid-August to nearly 12 h in mid-September (Figure 7.14) that causes the increase. As previously mentioned, the catalytic cycles that destroy polar Ot require sunlight to operate, so we expect the Ot loss per day to be proportional to the number of hours of sunlight per day.

At the same time that the days are lengthening the average SZA is also decreasing (Figure 7.14), leading to increased photolysis frequencies. The increased photolysis frequencies drive the photochemistry faster, again leading to enhanced O, loss. This effect is, however, less important than the increasing length of day.

Because of the rapid increase in the O, loss per day during the last month of winter and first month of spring, most of the Ot destroyed in the formation of the ozone hole is destroyed between mid-September and early October. Therefore, in order to destroy enough Ot to form an ozone hole, CI, must remain enhanced well into the first few weeks of spring. This occurs in the Antarctic, but as we will discuss later in this chapter, CI, in the Arctic is generally not activated past the end of winter and, as a result, total O, loss is much smaller.

The ClO-dimer cycle destroys about two-thirds of the O, lost in the Antarctic. The ClO-BrO cycle destroys most of the remainder, with the C10-0 cycle contributing a few percent of the loss.

9/10/93

9/10/93

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