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

Mixing ratios of approximately 10 parts per trillion by volume (pptv) are found at cruise altitude in the vicinity of 55°S increasing to 50 pptv at 60°S. In the vicinity of 65°S latitude a steep gradient in the ClO mixing ratio is observed. At a fixed potential temperature, the ClO mixing ratio increases by an order of magnitude within a very few degrees of latitude through this sharp transition, thus defining the edge of a "chemically perturbed region'. From the edge of that region to the southern extension of the flights, 72°S, a dome of slowly increasing ClO best describes the distribution. -from Authors