Activation of Br2 and BrCl via uptake of HOBr onto aqueous salt solutions

The reactive uptake of HOBr onto aqueous solutions containing Cl− and Br− has been studied using a wetted‐wall flow tube reactor. The uptake was found to be limited by gas phase transport, enabling the measurement of the diffusion coefficient of HOBr in He (319±48 torr cm2 s−1) and N2 (84±7 torr cm2 s−1) at 274 K (errors are 2 σ). The same experiments allowed a lower limit to the accommodation coefficient of α > 1×10−2 to be calculated. Both Br2 and BrCl products were observed in the gas phase, the relative yield of which was dependent on the ratio of Cl− to Br− in the aqueous phase. At [Cl−] = 1M, and [Br−] = 10−3 M, >90% of HOBr taken up by the film was released into the gas phase as Br2. At low Br− concentrations, BrCl was the dominant product. The role of pH in the efficiency of bromine release from the aqueous phase was examined by carrying out HOBr‐uptake experiments with 1 M Cl−, 10−3 MBr−, and pH between 4 and 10. At a pH of less than 6.5 at least 90% of the HOBr taken up onto the aqueous Cl− / Br− solution was released into the gas phase as Br2. Bromine was not released following HOBr uptake onto non‐acidified solution. These experiments confirm the efficient activation of bromine predicted by recent models of halogen chemistry in the marine boundary layer.