A

Figure 7.16 O profiles measured in the Antarctic vortex during September 1998 by the POAM HI instrument. (J. D. Lurnpe, personal communication, 1999.)

has essentially stopped by the late winter/early spring period (see Figure 7.7), so vertical transport is negligible. And other analyses suggest that horizontal transport into and out of the vortex is strongly suppressed by the polar night jet [187,222,266]. As a result, it is generally agreed that changes in the abundance of (), in the Antarctic polar vortex during the formation of the ozone hole arise almost entirely from chemical loss.

Finally, for completeness we note that production of O, is extremely slow in this region. While the photolysis of O, can occur by the absorption of photons with wavelengths as long as 242 nm, most of the photolysis occurs from absorption of photons with wavelengths of 200 nm or less. This flux of these photons falls off rapidly as the Sun moves to higher SZAs, and as a result is negligible in the polar lower stratosphere during the period of rapid O, loss.

7.15 Deactivation

At the same time that (), is being destroyed by CI,- and Br,-mediated catalytic cycles. CI, is also being converted back into Cly reservoir species in a process known as deactivation. Deactivation occurs primarily through two processes. As NO, is produced from destruction of HNO, (reactions (4.46) and (4.47)), it reacts to reform CIONO-, (reaction (4.10)):

Second, the reaction between CI and CH4 forms HC1 (reaction (4.11)):

Under nonpolar conditions, formation of NO. from HNO, destruction and the subsequent formation of ClONO, would rapidly deplete CI, on a time-scale of a week or so—much faster than formation of HQ would. Equilibration between ClONO, and HQ would then be re-established on time-scales of several weeks.

In the low-NOv, low-O, environment of the Antarctic lower stratosphere, however, production of HQ from CI, is much faster than under nonpolar conditions for two reasons. First, the conversion of CI to CIO through reaction with O, (reaction (4.8)) is slowed due to low O, abundances. Second, the reaction scheme

CIO + CIO ClOOCl

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