In situ northern mid-latitude observations of ClO, O₃, and BrO in the wintertime lower stratosphere

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.