Evidence of heterogeneous bromine chemistry on cold stratospheric sulphate aerosols

Simultaneous detection of BrO and unexpectedly elevated OClO total columns outside the polar vortex are reported. The 10‐day backward trajectories show that the observed halogen activated air masses were not recently extruded from the vortex, or processed by PSCs formed outside the vortex. 1‐D photochemical model calculations indicate that for the probed conditions, detectable amounts of OClO can only be formed when heterogeneous activation of BrONO 2 (and to a much lesser extent HOBr) on moderately cold (200–210 K) sulphuric acid aerosol particles is taken into account. The inclusion of these heterogeneous reactions has only a small effect on the observed concentrations of bromine oxide (BrO), it increases, however, the amount of OClO by a factor of 2.