The role of Br₂ and Brcl in surface ozone destruction at polar sunrise

Bromine atoms are believed to play a central role in the depletion of surface-level ozone in the Arctic at polar sunrise. Br₂, BrCl, and HOBr have been hypothesized as bromine atom precursors, and there is evidence for chlorine atom precursors as well, but these species have not been measured directly. We report here measurements of Br₂, BrCl, and Cl₂ made using atmospheric pressure chemical ionization - mass spectrometry at Alert, Nunavut, Canada. In addition to Br₂ at mixing ratios up to ∼25 parts per trillion, BrCl was found at levels as high as ∼35 parts per trillion. Molecular chlorine was not observed, implying that BrCl is the dominant source of chlorine atoms during polar sunrise, consistent with recent modeling studies. Similar formation of bromine compounds and tropospheric ozone destruction may also occur at mid-latitudes but may not be as apparent owing to more efficient mixing in the boundary layer.