Gas phase spectra of HOBr and Br2O and their atmospheric significance

The HOBr molecule is a potential reservoir of Br compounds in the atmosphere. In this work, the UV-visible spectrum of HOBr was measured over the range 242–400 nm. Its absorption consists of two maxima at 280 nm (σmax=2.7±0.4×10–19 cm2 molecules–1) and 355 nm (σmax=7.0±1.1×10–20 cm2 molecules–1), respectively, where the error is ±1Σ. Atmospheric photolysis lifetime calculations for HOBr in the lower stratosphere have been made using the PHOTOGT model. The results show a strong dependence on the solar zenith angle (SZA) implying a longer lifetime at high latitudes and a relatively short lifetime at low latitudes for example 714 s (albedo of 25%, SZA of 20° and an altitude of 17 km), and 3226 s (albedo of 25%, SZA of 88° and an altitude of 17 km). The UV-visible absorption spectrum of Br2O, which is an intermediate in the preparation, used in this study and is together with H2O in equilibrium with HOBr, was measured from 205 to 450 nm. The spectrum shows a maximum at 315 nm (σmax=2.3±0.3×10–18 cm2 molecules–1) with a shoulder at 355 nm. From the results of the atmospheric lifetime calculations for Br2O, it is clear that this molecule has a short stratospheric lifetime and is not likely to have a large daytime concentration, for example, 20 s (albedo of 25%, SZA of 20° and an altitude of 17 km), and 83 s (albedo and 25%, SZA of 88° and an altitude of 17 km)