Ultraviolet−Visible Absorption Cross Sections of Gaseous HOBr

Abstract

The UV−visible absorption cross sections for HOBr between 260 and 545 nm have been measured at 295 K in either 80 or 300 Torr of He bath gas. HOBr was generated in situ by laser photolytic production of OH radicals in the presence of Br2, and its absorption of light was detected using both a gated diode array camera (220 nm windows) and a photomultiplier. Calibration was performed relative to Br2 loss. The UV−visible spectrum of HOBr consists of three broad absorption bands between about 240 and 550 nm, with λmax at 284 nm (σ = (25.0 ± 1.2) × 10-20 cm2), 351 nm (σ = (12.4 ± 0.6) × 10-20 cm2), and 457 nm (σ = (2.3 ± 0.2) × 10-20 cm2). The existence of the third, long-wavelength absorption band has been confirmed unambiguously in this work. J-value calculations show that absorption into the third band is responsible for up to 50% of the total photolysis rate at high zenith angles close to the surface and at least 25% at all other altitudes and zenith angles down to 40°. At this zenith angle the total J value ranges from 3.2 × 10-3 s-1 at 15 km to 2.2 × 10-3 s-1 at the surface, corresponding to lifetimes of 5.2 and 7.6 min, respectively.