Figure 4.9 Contours of zonal!y averaged daytime CIONO, abundance (pptv) for December 1992, derived from UARS CLAF.S data (version 8).

photolysis of CIONO , reforms CI,. On time-scales that are long compared to the lifetimes of HC1, CI, is converted to HC1, and HC1 is destroyed to reform Clr

Because of the different time-scales, we can divide the problem of Cly partitioning into three separate parts: the partitioning between CI and CIO, the partitioning between CI, and C10N02, and the partitioning between HCI and the sum of CI, and CIONO, (Figure 4.11).

The partitioning of CI, was discussed at length earlier in this section, and we will not repeat it here. The partitioning between CI, and CIONO, can be seen in Figure 4.12. The abundance of CI, rises just after sunrise due to the initiation of CIONO, photolysis (reaction (4.15)) and, in the upper stratosphere, CIONO, oxidation. The abundance of CI, increases until the rate of formation of CIONO, (reaction (4.10)) balances the rate of destruction of C10N02. At this point, the C1,-C10N02 system is in photochemical steady state [69], and the relative abundance of C10N02 and CI, can be written as

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