To text describing this figure   To MLS home page    To MLS scientific results page maps of ClO at 465K in 1991-1992 NH winter
This figure shows the evolution of lower stratospheric ClO observed by MLS during the 1991-92 northern hemisphere winter. 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 thick black contour concentric with the pole shows the daylight edge of MLS measurements, and the thin white contour concentric with the pole indicates the edge of polar night. The green contour indicates the region of temperatures below 195 K, where polar stratospheric clouds can form. The irregular white contours indicate the approximate edge of the polar vortex (potential vorticity contours of 2.5 and 3.0 x 10-5 K m2 kg-1 s-1. Temperatures `episodically' dropped below the threshold for PSC formation starting in early December 1991, and remained below this threshold during early January 1992. The lower stratosphere warmed above the PSC threshold in late January 1992, and MLS observed the enhanced ClO to decay during February and March. This picture appears generally consistent with current understanding of the activation of chlorine by polar stratospheric clouds, and its deactivation by NO2 released by photolysis of HNO3 from evaporating PSCs. See Waters, et al. Nature [1993] and Douglass, et al. [1993] for more discussion of these data.

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