Reactive Uptake of ClNO₂on Aqueous Bromide Solutions

Abstract

The reactive uptake of ClNO2 on pure water, NaBr, and mixed NaBr/NaCl solutions was studied using a wetted wall flow tube equipped with a quadrupole mass spectrometer. The measured uptake coefficient, γexpt, varied between approximately 1 × 10-5 and 1 × 10-4, depending on the aqueous salt concentration. Parameters governing the reactive uptake on NaBr solutions were determined interdependently as H2D1 = (0.101 ± 0.015) M cm2 atm-2 s-2. The accommodation coefficient, α, for ClNO2 on aqueous solutions at 275 K was found to be (9 ± 4) × 10-3. The major gas-phase products of the uptake and reaction of ClNO2 and Br- were found to be Br2 and BrNO2. BrNO2 is formed initially and is converted to Br2 in a secondary reaction with Br- in the aqueous phase. BrCl is a minor product. The presence of 1M Cl- had no effect on either the kinetics of uptake or the products formed. By exchanging the halide solutions for 0.1 M OH-, the uptake rate could be increased so that the diffusion limit was reached in the reactor. Variation of the pressure yielded the diffusion coefficients of ClNO2 in He and N2 as 275 ± 26 and 75 ± 6 Torr cm2 s-1, respectively. The implications of these results for the halogen chemistry of the marine boundary layer are discussed.