Activation of Br-2 and BrCl via uptake of HOBr onto aqueous salt solutions

The reactive uptake of HOBr onto aqueous solutions containing Cl- and Br- h as 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 diff usion coefficient of HOBr in He (319 +/- 48 torr cm(2) s(-1)) and N-2 (84 /- 7 torr cm(2) s(-1)) at 274 K (errors are 2 sigma). The same experiments allowed a lower limit to the accommodation coefficient of alpha > 1x10(-2) to be calculated. Both Br-2 and BrCl products were observed in the gas phas e, 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 Br-2. At low Br concentr ations, 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-up take experiments with 1 M Cl-, 10(-3) M Br-, 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 Br,. Bromine was not releas ed following HOBr uptake onto non-acidified solution. These experiments con firm the efficient activation of bromine predicted by recent models of halo gen chemistry in the marine boundary layer.