This presentation reviews our current understanding of the source and behaviour of bromine in the troposphere and in particular of bromine explosion events. These events occur in the Arctic and Antarctic springtime, when heterogeneous processes on the snow pack and sea ice lead to an autocatalytic release of reactive bromine (e.g. Br2) from sea-salt. Br2 is then photolysed, yielding free bromine atoms that react with ozone to form BrO and, in turn, BrO reacts with itself, releasing bromine atoms again and thereby completing the ozone destroying cycle while preserving the bromine radicals. With the return of sun light in spring, the rate of Br2 photolysis sharply increases and this in turn results in very high concentrations of BrO.

These bromine explosion events can lead to substantial or even complete boundary layer ozone depletion which has been observed at several Arctic and Antarctic sites. And since in the planetary boundary layer the turbulence mixing timescale is very fast and an exchange of gases with the surface occurs quickly (within hours), this explains the sudden onset of these events. This sudden onset can be observed as strong enhancements in the BrO concentration. Moreover, some evidence of BrO in the free troposphere has also been presented in the literature. Speculations about the sources of tropospheric BrO include the possibility that BrO is transported from the boundary layer into the free troposphere through convection occurring over large ice leads (i.e. openings in the pack ice).

We have been measuring BrO using zenith sky UV/vis absorption spectroscopy at Arrival Heights, Antarctica since 1995. Examples of BrO explosion events in this data set will be presented. To further resolve the mechanisms discussed here, a new retrieval technique developed to better distinguish between tropospheric and stratospheric partial column of BrO is presented in the associated paper by R. Schofield et al.