The heterogeneous reactions of HOBr with ice surfaces formed by freezing sodium halide solutions were studied using a coated-wall, low-pressure flow tube coupled to a quadrupole mass spectrometer. Experiments were conducted at 233 and 248 K with films containing bromide, chloride, or a mixture of the two, and with HOBr gas-phase concentrations of about 1012 molecules/cm3. Gas-surface uptake coefficients and product yields are presented for a range of halide and hydrogen ion concentrations. Compared to analogous reactions with HOCl, HOBr reactions were slower than expected with a maximum uptake coefficient of approximately 0.01. At both 233 and 248 K, gas-phase Br2 was formed exclusively from bromide-only films and only BrCl was formed from chloride films. When both ions were present, BrCl was the sole gas-phase product observed at 233 K, while both gas-phase Br2 and BrCl were formed at 248 K. The relative yields of the gas-phase products depend on the temperature, composition, and acidity of halide-ice surfaces. A mechanism consistent with the observations involves initial formation of a HOBr·X- complex, where X- is either chloride or bromide, followed by reaction of the complex with a proton. For this reason, reaction probabilities for the formation of BrCl or Br2 were higher on acidified films than on films formed pH-neutral solutions. The atmospheric implications of these results are discussed.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry