Bromine Enrichment in the Near-Surface Region of Br-Doped NaCl Single Crystals Diagnosed by Rutherford Backscattering Spectrometry

Abstract

Bromine released from sea-salt aerosols and seawater ice is known for its high chemical reactivity. Previous studies have suggested that its availability to the gas-phase could be enhanced by segregation processes increasing Br concentration on the aerosol surface as compared to the bulk. However, little is known about the composition within the near-surface region, that is, the outermost approximately 100 monolayers. We used Rutherford backscattering spectrometry (RBS) to measure Br concentration profiles to a depth of about 750 nm of Br-doped NaCl single crystals to characterize the thermodynamics and kinetics of Br segregation to the near-surface region in moist air. These experiments were carried out on cleavage planes of melt-grown and of annealed solution-grown crystals at room temperature and relative humidities (RH) too low for formation of a stable liquid phase. Segregation of Br was below the detection limit on melt-grown crystals with Br/Cl = 0.01. In the case of annealed solution-grown crystals with Br/Cl = 0.002, average segregations of (0.24 +/- 0.11) x 10(15) and (0.42 +/- 0.12) x 10(15) Br atoms cm-2 were observed at 50% and 65% RH, respectively. No segregation was found at 20% RH. The observed Br segregation can be explained by the formation of an adsorbed liquid layer (depending on crystal surface properties and relative humidity) and preferential, diffusion-limited dissolution of Br into this layer according to the partition coefficient of Br between aqueous and solid NaCl. The thickness of the adsorbed liquid layer, which depends on crystal surface geometry and on relative humidity, can be estimated to range from 4 to at most 59 nm on the basis of measured Br concentrations and partition coefficients. Applying this concept of partitioning to natural sea salt suggests a Br/Cl molar ratio of up to 0.2 in adsorbed surface water of crystallized natural aerosol particles compared to about 0.0015 in seawater. This would have a major impact on heterogeneous reactions on sea-salt particles under dry conditions such as in the freeze-dried Arctic boundary layer.

Release of gas-phase halogens from sodium halide substrates: heterogeneous oxidation of frozen solutions and desiccated salts by hydroxyl radicals

“…However, it is now recognized that Br − and I − ions concentrate at the surface of humidified solid salts and deliquesced solutions. This has been shown by ambient pressure electron spectroscopy and Rutherford back scattering studies (Zangmeister et al 2001, Ghosal et al 2005, Hess et al 2007. Molecular dynamics simulations indicate that polarizable species such as Br − and I − associate with the surface more than do less polarizable species such as Cl − and F − (Jungwirth and Tobias 2006).…”

Diffusion constants of Br in NaCl measured by Rutherford backscattering spectroscopy

“…However, it was also shown that the diffusion constant of Br through the NaCl crystal lattice plays a crucial role in describing the kinetics of the Br surface enrichment. 6 This is basically the motivation for the study presented here.…”

“…1 Together with atmospheric radiation, meteorology, and cloud physics, these aerosols strongly affect the chemistry of the atmosphere, e.g., 2-4 the release of chlorides and bromides from sea salt aerosols into gas phase strongly influences the halogen budget and therefore can have a substantial impact on chemistry of hydrocarbons or ozone in the troposphere. 5 In a recent paper, 6 it was shown that partial liquification of the surface of a Br doped NaCl crystal at high relative humidity can lead to an enrichment of Br on the surface compared to bulk. Since chemical reactions between aerosols and the surrounding gas phase occur mainly on the aerosol surface, such discrepancies between the surface and the bulk composition may have significant effect on the chemical composition of the atmosphere.…”

Uptake of nitric acid on NaCl single crystals measured by backscattering spectrometry

“…We used Rutherford backscattering spectrometry [5,6] (RBS) to measure the Cl depth profile and the resonant cross section of O at approximately 3.05 MeV [5,6] to measure the O depth profile. A beam of 4 He + ions was used with energies between 3.00 and 3.26 MeV and typical beam currents below 5 nA.…”