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This figure shows simultaneous measurements of ClO and nitric acid (HNO₃) by MLS during the 1992-93 northern hemisphere winter (top panels) and during the 1992 southern hemisphere winter (bottom panels). It illustrates one aspect of the coupling between chlorine and nitrogen chemistry in the lower stratosphere, and the differences in denitrification between the southern and northern winter vortices. The data shown here have been interpolated to the 465 K isentropic surface (approximately 18 km height). Color bars range from 0 to 2.25 ppbv for ClO and 0 to 13.5 ppbv for HNO₃. White contours are potential vorticity values (2.5 and 3.0 x 10^-5 K m² kg^-1 s^-1 in the north, -2.5 and -3.0 x 10^-5 K m² kg^-1 s^-1 in the south) which identify the approximate edge of the polar vortex. Green contours on the ClO maps, and black contours on the HNO₃ maps, show the 195 K (outer contour) and 190 K (inner contour) temperature contours. The thick black contour concentric with the pole on the ClO maps identifies the daylight edge of MLS measurements: poleward of that contour measurements are in darkness and the decrease in ClO is expected due to formation of ClOOCl. HNO₃is depleted in the southern vortex where PSC particles (containing HNO₃) more readily grow to the larger sizes which can sediment out of the stratosphere. Loss of HNO₃ from the stratosphere reduces the amount of NO₂ (produced by solar photolysis of HNO₃ from evaporating PSCs) available to quench reactive chlorine when temperatures rise and sunlight returns. Denitrification, if any, in the northern hemisphere is much less pronounced. The reduced abundances of HNO₃ seen in the northern hemisphere on 22 February 1993 where temperatures are below 195\,K is expected due to formation of nitric acid trihydrate PSCs at these temperatures, and the increased HNO₃ to the east (downwind) is to be expected from evaporation of the PSCs. See Santee, et al. [1995] for further discussion of these data and results.

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