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MLS has observed enhanced ClO in the northern hemisphere polar vortex which is comparable, in both abundances and spatial extent (but not in temporal duration), to that observed in the southern vortex during formation of the Antarctic ozone hole. This map shows lower stratospheric ClO measured by MLS on 11 January 1992. The color bar indicates ClO abundances in parts per billion. The data have been interpolated to an isentropic surface having potential temperature of 465 K, corresponding to about 20 km height (air parcels move on isentropic surfaces when they don't significantly exchange energy with their surroundings - generally a good approximation for periods up to a week or so). The black contour concentric with the pole gives the daylight edge of MLS measurements; poleward of that contour measurements are in darkness where ClO abundances are expected to be low due to formation of ClOOCl (the area of enhanced ClO in darkness north of eastern Siberia can be explained by thermal dissociation of ClO at the higher temperatures occurring in that region). The white contour gives the approximate edge of the polar vortex (as identified by the 2.75x 10-5 K m2 kg-1 s-1 contour of potential vorticity); it can be seen that the enhanced ClO is mostly within the vortex. The green contour in the region of Scandinavia indicates where temperatures are below 195 K, the approximate threshold below which polar stratospheric clouds (PSCs) can form. Air circulation within the vortex is from west to east, and larger abundances of ClO are seen in, and downwind from, the region where PSCs are expected. This picture of enhanced ClO is generally consistent with current understanding of the activation of stratospheric chlorine by PSCs, and shows that this process can essentially `fill' the lower stratospheric vortex with chemically-reactive chlorine. See Waters, et al. Nature,  for more discussion of these data. The patches of smaller abundances of ClO seen well outside the vortex appear generally consistent with that expected from the instrument noise level [Schoeberl, et al., Geophys. Res. Lett. 20, 2861-2864, 1993]. See Waters, et al. Nature 362, 597-602  for more discussion of these data.
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