UV‐visible absorption cross sections of gaseous Br₂O and HOBr

The absorption cross‐section of gaseous HOBr was determined over the wavelength range 235 to 430 nm with a spectral resolution of 0.6 nm full width at half maximum (FWHM) using a diode array spectrometer. The spectrum of HOBr shows two main absorption bands with maxima near 282 nm (σ = (3.1 ± 0.4) × 10 −19 cm 2 molecule −1 and 350 nm (σ = 12.5 ± 1.6) × 10 −20 cm 2 molecule −1 ) extending out to 430 nm. The absorption cross‐sections in the first absorption band are in good agreement with a recent determination; the cross‐sections in the second band however, are approximately a factor of 2.5 larger than previously determined. In addition we provide evidence in support of a weak band in HOBr around 440 nm (σ ≈ 7.5 × 10 −21 cm 2 molecule −1 ) as observed by Barnes et al. [1996]. The absorption cross‐section of Br 2 O, which was used to prepare HOBr, was determined over the wavelength range 230 to 750 nm. The spectrum shows four absorption bands with maxima at 314 nm (σ = (2.1 ± 0.3) × 10 −18 cm 2 molecule −1 ), 350 nm (σ = (1.9 ± 0.2) × 10 −18 cm 2 molecule −1 ), 520 nm (σ = (4.4 ± 0.5) × 10 −20 cm 2 molecule −1 ), and 665 nm (σ = (6.2 ± 0.9) × 10 −20 cm 2 molecule −1 ). The visible bands at 520 nm and 660 nm have not been observed previously. The equilibrium constant, for the reaction Br 2 O + H 2 O ⇔ 2HOBr was determined to be 0.037 ± 0.004 at 298 K. Measurement of the equilibrium constant as a function of temperature enabled values for ΔH 298 K = (13.0 ± 0.5) kJ mol −1 and ΔS 298 K = (16 ± 2) J mol −1 K −1 to be determined. The absorption cross‐section data for HOBr have been used in a photochemical box model to investigate the significance of these results in the lower stratosphere. The model results are compared with observations during a recent Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) and show that the revised HOBr cross‐section, coupled to the rapid heterogeneous conversion of BrONO 2 to HOBr, can account quantitatively for the abrupt morning rise in HO x .