Heterogeneous Hydrolysis and Reaction of BrONO2 and Br2O on Pure Ice and Ice Doped with HBr

Abstract

The rate of uptake of bromine nitrate (BrONO2) and dibromine monoxide (Br2O) on different types of ice, such as condensed (C), bulk (B), and single-crystal ice (SC) have been investigated in a Teflon-coated Knudsen flow reactor in the temperature range 180−210 K using mass spectrometric detection. For the whole temperature range the Br2O uptake kinetics is first order in [Br2O] with a mean initial uptake coefficient of γ0 = 0.24 ± 0.10, which leads to the exclusive formation of HOBr. The BrONO2 hydrolysis has been measured on B-, C-, and SC-type ice and leads to HOBr and Br2O on all types of ice. At a fixed temperature the rate law is first order in [BrONO2] with γ ≈ 0.3 at 180 K. The observed negative temperature dependence for the heterogeneous hydrolysis of BrONO2 on pure ice leads to Ea of −2.0 ± 0.2, −2.1 ± 0.2, and −6.6 ± 0.3 kcal/mol on C-, B- and SC-type ice, respectively. Despite the high reactivity of BrONO2 on ice substrates, the kinetics of interaction of BrONO2 on ice nevertheless depends on the type of ice used. No saturation of the uptake coefficient has been observed during the BrONO2 hydrolysis on ice in contrast to the ClONO2/ice system. On ice samples doped with approximately 5 × 1016 molecules HBr per cm3 the kinetics of the interaction of BrONO2 with HBr leads to an uptake coefficient similar to that for BrONO2 hydrolysis. The interaction of BrONO2 with HBr occurs via the hydrolysis of BrONO2 to HNO3 and HOBr where the latter reacts with HBr in a fast secondary reaction to produce Br2 with Ea = −1.2 ± 0.2 kcal/mol.