UV-VISIBLE ABSORPTION CROSS-SECTIONS OF GASEOUS BR2O AND HOBR

The absorption cross-section of gaseous HOBr was determined over the w avelength range 235 to 430 nm with a spectral resolution of 0.6 nm ful l width at half maximum (FWHM) using a diode array spectrometer. The s pectrum of HOBr shows two main absorption bands with maxima near 282 n m (sigma=(3.1 +/- 0.4)x10(-19) cm(2) molecule(-1) and 350 nm (sigma=12 .5 +/- 1.6)x10(-20) cm(2) molecule(-1)) extending out to 430 nm. The a bsorption cross-sections in the first absorption band are in good agre ement with a recent determination; the cross-sections in the second ba nd however, are approximately a factor of 2.5 larger than previously d etermined, In addition we provide evidence in support of a weak band i n HOBr around 440 nm (sigma approximate to 7.5x10(-21) cm(2) molecule( -1)) as observed by Barnes et al. [1996]. The absorption cm cross-sect ion of Br2O, which was used to prepare HOBr, was determined over the w avelength range 230 to 750 nm. The spectrum shows four absorption band s with maxima at 314 nm (sigma=(2.1 +/- 0.3)x10(-18) cm(2) molecule(-1 )), 350 nm (sigma=(1.9 +/- 0.2)x10(-18) cm(2) molecule(-1)), 520 nm (s igma=(4.4 +/- 0.5)x10(-20) cm(2) molecule(-1)), and 665 nm (sigma=(6.2 +/- 0.9)x10(-20) cm(2) molecule(-1)). The visible bands at 520 nm cm and 660 nm have not been observed previously The equilibrium constant, for the reaction Br2O + H2O double left right arrow 2HOBr was determi ned to be 0.037 +/- 0.004 at 298 K. Measurement of the equilibrium con stant as a function of temperature enabled values for Delta H-298 K = (13.0 +/- 0.5) kJ mol(-1) and Delta S-298 K = (16 +/- 2) J mol(-1) K-1 to be determined. The absorption cross-section data for HOBr have bee n used in a photochemical box model to investigate the significance of these results in the lower stratosphere. The model results are compar ed with observations during a recent Stratospheric Photochemistry, Aer osols and Dynamics Expedition (SPADE) and show that the revised HOBr c ross-section, coupled to the rapid heterogeneous conversion of BrONO2 to HOBr, can account quantitatively for the abrupt morning rise in HOx .