Uptake and reaction of HOBr on frozen and dry NaCl/NaBr surfaces between 253 and 233 K
[摘要] The uptake and reaction ofHOBr with frozen salt surfaces of variable NaCl / NaBr composition and temperature were investigated with a coated wall flow tube reactor coupledto a mass spectrometer for gas-phase analysis.HOBr is efficiently taken up onto the frozensurfaces at temperatures between 253 and 233 K where it reacts to form thedi-halogens BrCl and Br
2, which are subsequently released into the gas-phase. The uptake coefficient for HOBr reacting with a frozen, mixed salt surface of similar composition to sea-spray was
10-2. The relative concentration of BrCl and Br2 released to the gas-phase was found to be stronglydependent on the ratio of Cl- to Br - in the solution prior to freezing / drying. For a mixed saltsurface of similar composition to sea-spray the major product at low conversion of surfacereactants (i.e. Br - and Cl-) was Br2.
Variation of the pH of the NaCl /NaBr solution used to prepare the frozen surfaces was found to have no significant influence on the results. The observations are explained in terms ofinitial formation of BrCl in a surface reaction of HOBr with Cl-, and conversion ofBrCl to Br2 via reaction of surface Br -. Experiments on the uptake and reaction ofBrCl with frozen NaCl /NaBr solutions served to confirm this hypothesis. The kinetics and products of theinteractions of BrCl, Br2 and Cl2 with frozen salt surfaces were also investigated, and lowerlimits to the uptake coefficients of > 0.034, >0.025 and >0.028 respectively, were obtained.The uptake and reaction ofHOBr on dry salt surfaces was also investigated and the resultsclosely resemble those obtained for frozen surfaces. During the course of this study the gasdiffusion coefficients ofHOBr in He and H2O were also measured as(273 ± 1) Torr cm2 s-1 and (51 ± 1) Torr cm2s-1, respectively, at 255 K. The implications of these results formodelling the chemistry of the Arctic boundary layer in springtime are discussed.
