Ice Crystallization in Ultrafine Water-Salt Aerosols: Nucleation, Ice-Solution Equilibrium, and Internal Structure
Ice Crystallization in Ultrafine Water-Salt Aerosols: Nucleation, Ice-Solution Equilibrium, and Internal Structure
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DOI：
10.1021/ja503311r
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发表时间：
2014-06-04
影响因子：
15
通讯作者：
Molinero, Valeria
中科院分区：
文献类型：
--
作者：
Hudait, Arpa;Molinero, Valeria
大气气溶胶对地球的气候有很大的影响。阐明大气气溶胶的物理状态和内部结构对于预测气溶胶的气体和水分吸收以及大气重要非均相反应的位置和速率至关重要。在对流层和对流层顶大量检测到尺寸在3至15纳米之间的超细气溶胶。在实验室和现场实验中，已经确定了天然和人工海水气泡破裂产生的纳米气溶胶。然而，这些气溶胶的内部结构和相态还不能在实验中确定。在这里，我们使用分子模拟研究的相行为和内部结构的液体，玻璃化，结晶水盐超细气溶胶的半径从2.5到9.5 nm，高达10%摩尔的离子。我们发现，冰结晶和玻璃化的nanodroplets导致从解决方案的纯水分层：玻璃化的水性nanodroplets产生nanodomains的纯低密度无定形冰与玻璃化的溶质丰富的水性玻璃共存。气溶胶中冰的融化温度随溶质分数的增加和半径的减小而单调降低。模拟结果表明，冰的成核均匀地发生在水-盐纳米颗粒的表面下。随后的冰生长产生相分离，内部混合，气溶胶与两相平衡：浓缩的水盐无定形混合物和球形帽状冰纳米相。结晶气溶胶的表面是不均匀的，冰和溶液暴露在蒸汽中。自由能计算表明，随着盐在颗粒中的浓度、结晶的进展或颗粒尺寸的增加，相对于其中冰核被溶液完全包围的替代结构，球冠结构的稳定性增加。我们预测微米级颗粒和纳米颗粒具有相同的平衡内部结构。液-汽表面张力随溶质浓度的变化是决定气溶胶平衡结构是溶液包埋冰芯还是蒸汽暴露冰帽的关键因素。与实验一致，我们预测，混合相硝酸-水粒子的结构，极地平流层云的代表，包括由冷冻浓缩溶液包围的冰核。研究结果对确定大气相关条件下海雾超细气溶胶和其他混合相粒子的相态和内部结构具有重要意义。
Atmospheric aerosols have a strong influence on Earth's climate. Elucidating the physical state and internal structure of atmospheric aqueous aerosols is essential to predict their gas and water uptake, and the locus and rate of atmospherically important heterogeneous reactions. Ultrafine aerosols with sizes between 3 and 15 nm have been detected in large numbers in the troposphere and tropopause. Nanoscopic aerosols arising from bubble bursting of natural and artificial seawater have been identified in laboratory and field experiments. The internal structure and phase state of these aerosols, however, cannot yet be determined in experiments. Here we use molecular simulations to investigate the phase behavior and internal structure of liquid, vitrified, and crystallized water salt ultrafine aerosols with radii from 2.5 to 9.5 nm and with up to 10% moles of ions. We find that both ice crystallization and vitrification of the nanodroplets lead to demixing of pure water from the solutions: Vitrification of aqueous nanodroplets yields nanodomains of pure low-density amorphous ice in coexistence with vitrified solute rich aqueous glass. The melting temperature of ice in the aerosols decreases monotonically with an increase of solute fraction and decrease of radius. The simulations reveal that nucleation of ice occurs homogeneously at the subsurface of the water-salt nanoparticles. Subsequent ice growth yields phase-segregated, internally mixed, aerosols with two phases in equilibrium: a concentrated water salt amorphous mixture and a spherical cap-like ice nanophase. The surface of the crystallized aerosols is heterogeneous, with ice and solution exposed to the vapor. Free energy calculations indicate that as the concentration of salt in the particles, the advance of the crystallization, or the size of the particles increase, the stability of the spherical cap structure increases with respect to the alternative structure in which a core of ice is fully surrounded by solution. We predict that micrometer-sized particles and nanoparticles have the same equilibrium internal structure. The variation of liquid vapor surface tension with solute concentration is a key factor in determining whether a solution-embedded ice core or vapor-exposed ice cap is the equilibrium structure of the aerosols. In agreement with experiments, we predict that the structure of mixed-phase HNO3-water particles, representative of polar stratospheric clouds, consists of an ice core surrounded by freeze-concentrated solution. The results of this work are important to determine the phase state and internal structure of sea spray ultrafine aerosols and other mixed-phase particles under atmospherically relevant conditions.