Photochemical bromine production implicated in Arctic boundary-layer ozone depletion

Abstract

RECENT measurements^1–7 in the Arctic have revealed episodic destruction of boundary-layer ozone from 30–40 parts per 10⁹ by volume (p.p.b.v.) to undetectable levels on a timescale of less than a day, during periods when the boundary layer is very stable. The ozone destruction begins at polar sunrise, continues for the months of March and April, and is strongly associated with levels of filterable bromine which are much greater than during the rest of the year. Here we suggest that sea-salt Br⁻ reaches high concentrations in the snow pack during the long polar night, and is evolved into the atmosphere as Br₂at polar sunrise. Ordinarily, gas-phase photochemistry would convert Br₂ to HBr or brominated organic compounds with consequently little destruction of boundary-layer ozone. In view of several laboratory experiments^8–11, and by analogy with the marine boundary layer¹², we propose that the HBr and brominated organic compounds will be scavenged by the ambient aerosols and ice crystals, and that these heterogeneous reactions release Br₂ back to the atmosphere. We argue that this cycling of bromine between the aerosol and the gas phase should maintain sufficiently high levels of Br atoms and BrO radicals to destroy ozone, in agreement with observations^1–7.