Figure 4.8 Contours of zonally averaged HC1 abundance (ppbv). Plot made from both sunset and sunrise measurements (version 18) by the Halogen Occultation Experiment (HALOE) on the UARS between February 4 and March 16, 1994.

The abundance of CIO is plotted in Figure 4.10. Because [Clf] = [CIO], this quantity can also be considered the abundance of CI,. The figure shows CIO abundance increasing with altitude throughout the stratosphere. In the lower stratosphere this increase is due to increasing C!v abundances. In the mid- and upper stratosphere, it is due to increasing loss rates of ClONO, and HC1 compared to the loss rate of CIO. During winter, the polar region experiences 24 h of darkness every day—this is known as polar night. In this situation, all photolysis reactions cease, and loss of CIO through reaction with NO, to reform ClONO,, or through reaction with other CIO radicals to form ClOOCl, continues unabated. As a result, the abundance of CIO during polar night is approximately zero. The chemistry of the polar night region will be discussed at length in Chapter 7.

Based on our discussions up to this point, we present the following general picture of the partitioning between (he major components of CI,: CI,, ClONO,, and HC1. During the day, the members of CI,. CI and CIO cycle rapidly with each other. On time-scales that are long compared to the lifetimes of CI, and ClONO,, but short compared to the lifetime of HC1, CI, is consumed to produce ClONO,, and

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