Evidence for HBr production due to minor channel branching at mid-latitudes

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Abstract

Recently, Nolt et al. [1997] have presented HBr measurements over the height range 20–36 km. We have compared model calculations with these measurements and find definitive evidence for non-zero branching ratios for HBr production from either HO2+BrO, k1b, or OH+BrO, k2b. For k1b, best agreement between modelled and observed HBr concentrations is obtained if the branching ratio increases from ∼0.4% at 20 km, to about 2% at 34 km with no additional contribution from the OH+BrO branch. However, a height-independent value of 1% also produces reasonable results. This branching ratio contradicts the study of Mellouki et al. [1994]. For OH+BrO, the best value of k2b appears to be 2–3%, if k1a = 0 which is comparable to the recently determined branching ratio for HCl production from the analogous chlorine reaction, OH+ClO.

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