Mid‐latitude observations of the seasonal variation of BrO: 2. Interpretation and modelling study

A 1‐D photochemical model, initialised with output from a chemical transport model and coupled to a radiative transfer model, is used to calculate BrO slant columns. Calculated BrO slant columns are shown to be in good agreement with measured values, which supports our current understanding of stratospheric bromine chemistry at middle latitudes, and argues against the need to invoke new chemistry or a new photolabile night‐time bromine reservoir. The model calculations show that the seasonal variation of BrO slant columns results from seasonal variations in NO2 abundances. The occurrence of BrONO2 hydrolysis on sulphate aerosols is found to affect twilight BrO abundances, but no other heterogeneous reactions involving bromine species are important under the range of stratospheric conditions sampled. The zenith‐sky measurements from Aberdeen in the winter of 1994–5 can be explained with present photochemical schemes and current estimates of approximately 20 pptv for the bromine content of the stratosphere.