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y for reactions on NAT and Ice from Table 64 of DeMore et al. [5]; y for reactions on STS are assumed to occur at 195 K; the values are obtained from Figure 6.3. Reactive uptake coefficients are for the first-order loss of CIONO,, HOC1, or HOBr.

y for reactions on NAT and Ice from Table 64 of DeMore et al. [5]; y for reactions on STS are assumed to occur at 195 K; the values are obtained from Figure 6.3. Reactive uptake coefficients are for the first-order loss of CIONO,, HOC1, or HOBr.

All of these PSC reactions convert relatively long-lived reservoir chlorine species (HC1 and CIONO,,) into Cl2 and HOC1, labile forms of chlorine that are easily pho-tolyzed. Upon exposure to sunlight, CI2 and HOC1 are photolyzed within several hours to CI atoms. CI, is in photochemical steady state, so these CI atoms are rapidly converted to CIO or ClOOCl molecules in order to maintain steady-state partitioning of CI,.

Figure 7.8 shows the effect of PSC reactions. Above ~196 K about 50% of the Clv is in the form of HC1, with only a few percent present as CI, (we expect most of the rest is CIONO,). Below ~196 K, at temperatures where PSCs are expected to occur, we see chlorine activation: reduced abundances of HQ and enhanced abundances of CI,.

It is presently unknown whether NAT, NAD, or STS is the dominant form of type I PSC. There is a body of evidence emerging, however, that STS might be the most important [237,248]. It should be noted, however, that while some details of the microphysics are incompletely understood, the net effects of PSCs on CI, are well defined. There is abundant empirical evidence that any time an air parcel is exposed to temperatures in the mid-190s Kelvin and below, chlorine reservoirs in the air mass are converted to CI, [249,250].

It should be pointed out that temperatures below ~196 K are common throughout the Antarctic lower stratosphere in the winter and early spring (see Figure 7.4). As a result, a significant fraction of CI, in this region in the winter and spring is in the form of CI,.

The CIO dimer ClOOCl (also called "the CIO dimer" or simply "the dimer") plays a crucial role in the formation of the ozone hole. ClOOCl is formed in the three-body reaction of CIO with itself:

CIO + CIO ClOOCl

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