A wintertime in situ profile of BrO between 17 and 27 km in the Arctic vortex

The vertical profile of bromine oxide (BrO) was measured in situ from a balloon launched near Kiruna, Sweden (68°N, 21°E) at sunrise on February 3, 1995. BrO mixing ratios of 10±2 parts per trillion by volume (pptv) were observed at high solar zenith angles (90–92°) between 20 and 23 km in a layer that coincided with significantly enhanced abundances of ClO. Above 23 km, where ClO mixing ratios typical of midlatitudes were observed, BrO mixing ratios ranged from 5 to 11 pptv, increasing nearly monotonically with increasing altitude and decreasing solar zenith angle. The measurements indicate that 30 to 60% of the total bromine (assuming 18 pptv total bromine) is in the form of BrO between 23 and 27 km at these latitudes in winter. These results are consistent with lower altitude measurements of BrO from the NASA ER‐2 at similar latitudes and season. Observations of higher fractions of BrO (50 to 95% of total bromine) in the chemically perturbed region at sunrise imply the rapid release of bromine from a photolabile reservoir species, such as BrCl.