Hess M, Krieger U K, Marcolli C, Huthwelker T, Ammann M, Lanford W A, Peter Th
Institute for Atmospheric and Climate Science, ETH Zurich, 8093 Zurich, Switzerland.
J Phys Chem A. 2007 May 24;111(20):4312-21. doi: 10.1021/jp0674120. Epub 2007 Apr 27.
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.
海盐气溶胶和海冰中释放的溴以其高化学反应活性而闻名。先前的研究表明，与主体相比，通过使溴在气溶胶表面的浓度增加的分离过程，可以提高其在气相中的可用性。然而，对于近表面区域（即最外层约100个单分子层）内的组成了解甚少。我们使用卢瑟福背散射光谱法（RBS）测量了掺溴的氯化钠单晶中约750纳米深度的溴浓度分布，以表征在潮湿空气中溴向近表面区域分离的热力学和动力学。这些实验是在熔体生长和退火溶液生长晶体的解理面上于室温及相对湿度（RH）过低以至于无法形成稳定液相的条件下进行的。在溴/氯 = 0.01的熔体生长晶体上，溴的分离低于检测限。在溴/氯 = 0.002的退火溶液生长晶体的情况下，在50%和65%的相对湿度下，分别观察到平均分离量为(0.24±0.11)×10¹⁵和(0.42±0.12)×10¹⁵个溴原子/平方厘米。在20%的相对湿度下未发现分离现象。观察到的溴分离可以通过形成吸附液层（取决于晶体表面性质和相对湿度）以及根据溴在水相和固体氯化钠之间的分配系数，溴优先、扩散受限地溶解到该层中来解释。吸附液层的厚度取决于晶体表面几何形状和相对湿度，根据测量的溴浓度和分配系数估计其范围为4至最多59纳米。将这种分配概念应用于天然海盐表明，与海水中约0.0015的溴/氯摩尔比相比，结晶天然气溶胶颗粒吸附的表面水中溴/氯摩尔比高达0.2。这将对干燥条件下（如冻干的北极边界层）海盐颗粒上的非均相反应产生重大影响。
