Ion Partitioning at the Liquid/Vapor Interface of a Multicomponent Alkali Halide Solution: A Model for Aqueous Sea Salt Aerosols

Abstract

The chemistry of Br species associated with sea salt ice and aerosols has been implicated in the episodes of ozone depletion reported at Arctic sunrise. However, Br -is only a minor component in sea salt, which has a Br -/Cl -molar ratio of ~0.0015. Sea salt is a complex mixture of many different species, with NaCl as the primary component. In recent years experimental and theoretical studies have reported enhancement of the large, more polarizable halide ion at the liquid/vapor interface of corresponding aqueous alkali halide solutions. The proposed enhancement is likely to influence the availability of sea salt Br -for heterogeneous reactions such as those involved in the ozone depletion episodes. We report here ambient pressure x-ray photoelectron spectroscopy studies and molecular dynamics simulations showing direct evidence of Br -enhancement at the interface of an aqueous NaCl solution doped with bromide. The experiments were carried out on samples with Br -/Cl -ratios in the range 0.1% to 10%, the latter being also the ratio for which simulations were carried out. This is the first direct measurement of interfacial enhancement of Br -in a multi-component solution with particular relevance to sea salt chemistry.

Surface Chemical Properties of Eutectic and Frozen NaCl Solutions Probed by XPS and NEXAFS

“…The peak position in the solution spectrum at 256 K agrees well with literature, [4,25] while in the frozen solution, the position of the Na 2s peak is shifted to higher BE, Figure 2. Photos of the sample holder with a) the solution droplet and b) the frozen solution.…”

“…Both chloride and bromide are strongly implicated in the depletion of the stratospheric ozone layer [1] and in the global budget of reactive halogens emitted from Arctic snowpack and sea ice in the troposphere. [2,3] The microstructure of frozen sea-salt solution pockets, with chloride and bromide as major constituents (Br À /Cl À molar ratio in sea salt 0.0015 [4] ) is key in photochemical cycles leading to ozone depletion in the regional boundary layer and export of reactive halogens to the global troposphere. [3] Chloride ions in glacier ice are used to reconstruct past atmospheric composition and climate.…”

The effect of low solubility organic acids on the hygroscopicity of sodium halide aerosols

“…The most important is the drop of surfacelevel ozone concentrations in the Arctic at polar sunrise. This is due to the tendency of Br − to segregate to the salt surface in the presence of water, substantially increasing Br / Cl surface molar ratios, and to the fact that bromide ions exhibit a higher surface reactivity than chloride (Baker, 2005;Ghosal et al, 2008;Zangmeister et al, 2001). Sea salt particles have been shown to be the source of BrO, which is involved in catalytic cycles that destroy ozone (FinlaysonPitts, 2009;Frinak and Abbatt, 2006;Hunt et al, 2004;Read et al, 2008;Von Glasow, 2008).…”

Ion Spatial Distributions at the Air– and Vacuum–Aqueous K2CO3 Interfaces

“…In this regard, three prominent analytical techniques have recently proved noteworthy complements to the well established macroscopic techniques of surface tension [8][9] and surface potential [10][11] for the study of air (vacuum)-water interfaces: second harmonic generation (SHG), [12][13][14][15][16][17][18][19][20][21][22] sum-frequency generation (SFG) spectroscopy [23][24][25][26][27][28][29][30][31] and liquid based X-ray photoelectron spectroscopy (XPS). [32][33][34][35][36][37][38][39][40][41][42][43][44][45] All three of these methods are capable of interrogating the microscopic structure of the air (vacuum)-water interface, and often provide complementary information due to the different properties probed. For instance, SFG is a second order non-linear vibrational spectroscopy typically used to investigate the fundamental OH stretching region (between 3100−3500 cm −1 ), which provides detailed information on the structure and orientation of water within the non-centrosymmetric region at the interface.…”