Gas phase spectra of HOBr and Br2O and their atmospheric significance

The HOBr molecule is a potential reservoir ofBr compounds in the atmosphere. In this work, the UV-visible spectrum of HOBrwas measured over the range 242–400 nm. Its absorption consists of two maxima at280 nm (\u3c3_max=2.7±0.4×10^–19 cm²molecules^–1) and 355 nm (\u3c3_max=7.0±1.1×10^–20cm² molecules^–1), respectively, where the error is±1Σ. Atmospheric photolysis lifetime calculations for HOBr in thelower stratosphere have been made using the PHOTOGT model. The results show astrong dependence on the solar zenith angle (SZA) implying a longer lifetime athigh latitudes and a relatively short lifetime at low latitudes for example 714s (albedo of 25%, SZA of 20^° and an altitude of 17 km), and 3226 s (albedoof 25%, SZA of 88^° and an altitude of 17 km). The UV-visibleabsorption spectrum of Br₂O, which is an intermediate in thepreparation, used in this study and is together with H₂O inequilibrium with HOBr, was measured from 205 to 450 nm. The spectrum shows amaximum at 315 nm (\u3c3_max=2.3±0.3×10^–18 cm²molecules^–1) with a shoulder at 355 nm. From the results of theatmospheric lifetime calculations for Br₂O, it is clear that thismolecule has a short stratospheric lifetime and is not likely to have a largedaytime 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^° andan altitude of 17 km)