We have used a constrained one-dimensional photochemical model to investigate the effect of a potential minor channel of the fast reaction between BrO + OH to produce HBr. There is no direct evidence for this reaction but the analogous yield of HCl from ClO + OH is thought to be about 5%. With only a 1–2% yield of HBr the modelled HBr mixing ratio between 20–30 km increases from around 0.5 parts per 1012 by volume (pptv) to 1–2 pptv. This brings the model into agreement with recent balloon-borne observations of stratospheric HBr. Should BrO + OH produce HBr with around 1–2% yield then this reaction will dominate HBr production between 20–35 km. As the main loss of HBr is reaction with OH this will lead to steady state HBr:BrO partitioning which is independent of other species, and temperature.
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