In order to test photochemical theories linking chlorofluorocarbon derivatives to ozone (O3) depletion at high latitudes in the springtime, several related atmospheric species, including O3, chlorine monoxide (ClO), and bromine monoxide (BrO) were measured in the lower stratosphere with instruments mounted on the NASA ER-2 aircraft on 13 February 1988. The flight path from Moffett Field, California (37°N, 121°W), to Great Slave Lake, Canada (61°N, 115°W), extended to the center of the polar jet associated with but outside of the Arctic vortex, in which the abundance of O3 was twice its mid-latitude value, whereas BrO levels were 5 parts per trillion by volume (pptv) between 18 and 21 kilometers, and 2.4 pptv below that altitude. The ClO mixing ratio was as much as 65 pptv at 60°N latitude at an altitude of 20 kilometers, and was enhanced over mid-latitude values by a factor of 3 to 5 at altitudes above 18 kilometers and by as much as a factor of 40 at altitudes below 17 kilometers. Levels of ClO and O3 were highly correlated on all measured distance scales, and both showed an abrupt change in character at 54°N latitude. The enhancement of ClO abundance north of 54°N was most likely caused by low nitrogen dioxide levels in the flight path.
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