Formation of Molecular Bromine from the Reaction of Ozone with Deliquesced NaBr Aerosol:  Evidence for Interface Chemistry

Abstract

The reaction of ozone with aqueous sodium bromide particles is investigated with a combination of aerosol chamber experiments, kinetics modeling, and molecular dynamics simulations. The molecular bromine production in the chamber experiments is approximately an order of magnitude greater than that predicted by known chemistry in the gas and bulk aqueous phases with use of a comprehensive computer kinetics model. Molecular dynamics simulations indicate that ozone has significant residence time at the air−solution interface, while making frequent contacts with bromide ions for as long as 50 ps in the surface layer of a 6.1 M NaBr solution. The formation of a complex between ozone and bromide ion, [O3···Br-], which can lead to production of Br2 by reaction at the air−water interface, is therefore feasible. Experimentally observed Br2 is well predicted by including an interface process with a reaction probability of [1.9 ± 0.8] × 10-6 (1 s) as the first step in a surface mechanism to produce additional gas-phase Br2. An estimate of the impact of this interface reaction on bromine formation in the marine boundary layer shows that several ppt of bromine could potentially be produced during the night from this proposed surface chemistry.

Partitioning of atmospherically relevant ions between bulk water and the water/vapor interface

“…For the formation of molecular chlorine and bromine from aqueous halide aerosols, FinlaysonPitts and coworkers (4,30) have shown that the kinetics model predictions cannot be reconciled with the experimental results unless interfacial reactions are included. Specifically, the amount of molecular bromine formed upon reaction of ozone with the bromide ion in deliquesced sodium bromide aerosols is Ϸ10-fold larger than that predicted by known gas phase and bulk aqueous phase chemistry (30). When an interface reaction was included in the kinetics modeling, the amount of Br 2 formed was reasonably reproduced.…”

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…113,114 In contrast, the interface oxidation of Br À by O 3 does make a significant contribution to the formation of Br 2 in the dark. [115][116][117] Despite the progress on understanding the structure of salt solutions and the impacts on reactivity, much remains speculative. For example, the (OHÁ Á ÁCl) À complex proposed at the interface has not been observed, and the mechanism by which such a complex forms gaseous Cl 2 is not confirmed.…”

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

“…1 However, a combination of computational [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and experimental studies [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] has shown that ions can reside and even be enhanced at the interface.…”

“…15,16 There is also evidence that the photolysis of species at the interface has higher quantum yields than those in the bulk. [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] In short, interfacial kinetics and mechanisms may be significantly different compared to those in the bulk phase.…”