Day of 1994

Figure 7.10 Time series of HNO, abundance at several equivalent latitudes in the high southern latitudes. MLS HNO, measurements (version 4) at 465 K in a ±1° equivalent latitude (derived from UKMO PV) band are averaged to produce an average UNO, abundance for each day. The lime series has been smoothed to reduce the day-to-day variability.

increases throughout the winter; i.e. there is no hint of denitrification at this latitude. At 65°S, HNO, decreases by about 20% between May and September, suggesting slight denitrification. At higher latitudes there is a pronounced drop in HNO, starting in the first few days of June. At 70°S, about two-thirds of the HNO, has been removed by the end of June. At 80°S, virtually all of the HNO, has removed from the gas phase by the end of June. There is little change in August and September. Particle extinction measurements as well as in situ data [252] indicate that the NOv is not simply sequestered in particles but has been irreversibly removed from the lower stratosphere. Also, note that air in the polar vortex is descending during the late fall and early winter (see Figure 7.7). Because the abundance of HNO, (and NO,.) increases with altitude in the polar lower stratosphere, such descent brings down air with higher values of HNO,. This explains, for example, the increase in HNO, seen at 60°S. Descent cannot therefore explain the rapid decrease in HNO, shown in Figure 7.10.

Why is denitrification important? It turns out that NO, can reverse chlorine activation. If HNO, is present, then reactions (4.46) and (4.47),

produce NO,, which can then react with CIO (reaction (4.10)),

CIO + NO, ClONO, leading to conversion of CI, into C10N02. However, the removal of NOv from the air mass through sedimentation of NOv-rich PSCs shuts off this deactivation pathway and allows CI, abundances to remain high for a longer period than without denitrification.

The net result of PSC processing and denitrification is enhanced CI, abundances inside the vortex (Figure 7.11) for a sustained length of time. At 465 K (~20 km), CIO in the chemically perturbed regions of the vortex averages about 1700 pptv,

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