Model calculations of stratospheric OBrO indicating very small abundances

We have used a one‐dimensional photochemical model to investigate the potential role of OBrO in stratospheric photochemistry. The OBrO lifetime against photolysis is likely to be very short (around a few s) which prevents any appreciable concentration during sunlit hours. This rapid photolysis also prevents the gas‐phase production of significant OBrO in the model during twilight, as possible precursors (e.g. BrO) are converted to their nighttime reservoirs. Using a range of possible gas‐phase production reactions, the maximum (nighttime) OBrO volume mixing ratio produced in the model in the lower stratosphere is around 0.01 × 10−12 (0.01 pptv). These model results contradict recent tentative nighttime balloon observations of large OBrO [Renard et al., 1997, 1998]. We have used our model results to put constraints on the rates of gas‐phase and heterogeneous reactions that would be necessary to produce appreciable amounts of OBrO in the stratosphere. These constraints show that if OBrO is indeed present in the nighttime stratosphere at the pptv level, our current understanding of atmospheric bromine chemistry is severely flawed.