Ciooci o710

net: CIO + CIO-> CI + CI + O, provides an efficient mechanism to convert CIO to CI. The result is a high [QJ/JCIJ ratio, and consequently a conversion of CI, to HC1 at a rate of several tenths of a part per billion by volume per day [267,2681. Other research has pointed out that the reaction between OH and CIO to produce HQ (reaction (4.13)) might also be important 1269]. Combined with the slow formation of ClONO, due to the low abundance of NO, (and therefore the slow formation rate of NO,), the formation of HQ dominates the formation of ClONO,.

It should be noted that deactivation is going on continuously throughout the winter and spring. However, during the Antarctic winter and early spring the temperatures are sufficiently cold that PSC processing recurs frequently enough to reverse any deactivation and prevent a significant build-up of the reservoirs of HQ and ClONO,. As a result, a high [Q,]/[C1V1 ratio is maintained in the polar lower stratosphere. At some point during the spring, however, the Antarctic polar vortex has warmed sufficiently that PSC formation ceases. Thereafter, CI, decreases as it is converted back to the Cl,; reservoir species. As the polar vortex breaks up and HNO, mixes into the region, its photolysis produces NOr, and the equilibrium between HQ and ClONO, is re-established on a time-scale of weeks. The decline in C1T and the reformation of HC1 and ClONO, signals the end of O, loss due to polar chemistry.

The timing of the deactivation of CI, is a crucial factor in determining how much polar O, is destroyed by chlorine chemistry. As discussed previously, the amount of O, destroyed during a day is a function of both the CI, abundance and the amount of sunlight the vortex is exposed to. Deactivation of CI, in the late winter, after the vortex has been exposed to relatively little sunlight, would result in comparatively little Ov loss there. In the Antarctic, however, Clv remains high for several weeks into the spring, so the high-ClA air experiences many hours of sunlight every day. This allows the destruction of virtually all of the O, in the polar lower stratosphere and the formation of an ozone hole.

Meteorological variability means that the Antarctic vortex is warmer in some years than others (e.g. see WMO [13), Figure 3.3), and exactly how far into the spring PSCs occur shows interannual variability. For example. Figures 7.1 and 7.2 show that ozone hole during the relatively warm year 1988 was both smaller and had higher minimum ozone than the ozone holes of the colder years 1987 and 1989.

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