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Figure 7.11 CIO abundance versus equivalent latitude on the 465, 565, and 665 K potential temperature surface on September 15, 1992. Dots are individual daytime measurements of CIO by the UARS MLS (version 4), interpolated to theta surfaces using UKMO temperatures [259]. The equivalent latitude and vortex edge are derived from UKMO PV. The vertical line denotes the edge of the vortex. Much of the scatter in the data, and especially the negative values of CIO, are due to precision uncertainty in the MLS CIO measurements [260,261],

Equivalent Latitude

Figure 7.11 CIO abundance versus equivalent latitude on the 465, 565, and 665 K potential temperature surface on September 15, 1992. Dots are individual daytime measurements of CIO by the UARS MLS (version 4), interpolated to theta surfaces using UKMO temperatures [259]. The equivalent latitude and vortex edge are derived from UKMO PV. The vertical line denotes the edge of the vortex. Much of the scatter in the data, and especially the negative values of CIO, are due to precision uncertainty in the MLS CIO measurements [260,261],

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while CIO near the edge and outside of the vortex is tens of pptv. If the CIO dimer makes up —50—60% of CI,, then the total reactive chlorine CI, abundance in the vortex is -2800-3400 pptv. In other words, -80-100% of the CI has been converted to CI, in the chemically perturbed regions of the vortex. At 565 K (-24 km), vortex CIO abundances are lower. Noting that the CIO dimer should also make a smaller contribution at this level, we conclude that less Clv has been converted to CI,. This is consistent with warmer temperatures (less PSC processing and denitrification) and higher photolysis rates (faster NO, production rates) with increasing altitude. At 665 K (27 km), there is evidence of only a slight enhancement of CI, inside the vortex compared to mid-latitudes. Aircraft measurements indicate that CIO is also enhanced below 465 K [258],

It can be seen in Figure 7.11 that the edge of the high-CIO region is not coincident with the edge of the vortex. This should not be surprising: the vortex is defined by the meteorological field ¡ 219], while the region of high CIO is determined by cold temperatures. While these two geophysical parameters are physically related, there is no reason that the vortex, as defined by PV, will exactly overlap the region containing PSC-processed high-Cl, air. Because of this, researchers will sometimes define a chemically perturbed region to be that region of the vortex containing PSC-processed air.

As will be discussed later, the late winter and early spring is the critical time period for the formation of the ozone hole. Figure 7.12 shows a time series of vortex-averaged lower-stratospheric CI, abundance in the Antarctic vortex during this

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