Kinetics and product yields in the heterogeneous reactions of HOBr with ice surfaces containing NaBr and NaCl

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.