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 lifet ime against photolysis is likely to be very short (around a few s) whi ch prevents any appreciable concentration during sunlit hours. This ra pid photolysis also prevents the gas-phase production of significant O BrO in the model during twilight, as possible precursors (e.g. BrO) ar e converted to their nighttime reservoirs. Using a range of possible g as-phase production reactions, the maximum (nighttime) OBrO volume mix ing ratio produced in the model in the lower stratosphere is around 0. 01 x 10(-12) (0.01 pptv). These model results contradict recent tentat ive 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 p roduce appreciable amounts of OBrO in the stratosphere. These constrai nts show that if OBrO is indeed present in the nighttime stratosphere at the pptv level, our current understanding of atmospheric bromine ch emistry is severely flawed.