GAS-PHASE SPECTRA OF HOBR AND BR2O AND THEIR ATMOSPHERIC SIGNIFICANCE

The HOBr molecule is a potential reservoir of Br compounds in the atmo sphere. In this work, the UV-visible spectrum of HOBr was measured ove r the range 242-400 nm. Its absorption consists of two maxima at 280 n m (sigma(max) = 2.7 +/- 0.4 x 10(-19) cm(2) molecules(-1)) and 355 nm (sigma(max) = 7.0 +/- 1.1 x 10(-20) cm(2) molecules(-1)), respectively , where the error is +/-1 sigma. Atmospheric photolysis lifetime calcu lations for HOBr in the lower stratosphere have been made using the PH OTOGT model. The results show a strong dependence on the solar zenith angle (SZA) implying a longer lifetime at high latitudes and a relativ ely short lifetime at low latitudes for example 714 s (albedo of 25%, SZA of 20 degrees and an altitude of 17 km), and 3226 s (albedo of 25% , SZA of 88 degrees and an altitude of 17 km). The UV-visible absorpti on 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 (s igma(max) = 2.3 +/- 0.3 x 10(-18) cm(2) 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 lif etime and is not likely to have a large daytime concentration, for exa mple, 20 s (albedo of 25%, SZA of 20 degrees and an altitude of 17 km) , and 83 s (albedo and 25%, SZA of 88 degrees and an altitude of 17 km ).