In situ observations of ClO in the Antarctic - ER-2 aircraft results from 54 deg S to 72 deg S latitude

The spatial and temporal distributions of ClO in the latitude belt from 54 to 72 deg S during the later stages of austral winter were obtained from measurements aboard the ER-2 aircraft flying at the altitude of about 18 km into the Antarctic polar vortex. Mixing ratios of about 10 parts per trillion by volume (pptv) were found in the vicinity of 55 deg S, increasing to 50 pptv at 60 deg S. A steep gradient of the ClO mixing ratio occurred in the vicinity of 65 deg S, where, at a fixed potential temperature, the value of the ClO mixing ratio increased by an order of magnitude within a very few degrees of latitude, defining the edge of the 'chemically perturbed region' (CPR). At the southern extension of the flight track, peak mixing ratios increased sharply from 800 pptv on August 23 to 1100 pptv in early September, and then increased more slowly to 1200 pptv on September 22. This corresponds to enhancements 500 times the typical mid-latitude values.