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 BrONO2 (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.