Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?
Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?
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DOI：
10.5194/acp-15-13599-2015
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发表时间：
2015-01-01
影响因子：
6.3
通讯作者：
Peter, T.
中科院分区：
文献类型：
--
作者：
Lienhard, D. M.;Huisman, A. J.;Peter, T.
本文提出了水在由α -蒎烯氧化产生的二次有机气溶胶（SOA）材料和许多有机/无机模型混合物（3-甲基丁烷-1,2,3-三羧酸（3-MBTCA）、左旋葡聚糖、左旋葡聚糖/NH4HSO4、棉子糖）中的扩散的新测量方法。这表明，水的扩散系数是由气溶胶物质的几个性质决定的，不能从玻璃化转变温度或弹跳性质来推断。我们的研究结果表明，水在SOA颗粒中的扩散速度比通常假设的要快，并且在220 K以上的温度下，对水的吸收和释放没有明显的动力学限制。水的快速扩散表明，在这些系统中，玻璃核上的非均相冰不太可能成核。在低于220 K的温度下，SOA粒子的模型模拟表明，当经历快速上升气流时，非均质冰核可能在浸入模式下发生在玻璃核上，而玻璃核仍然嵌入在液体壳中。在这些上升气流中，颗粒吸收了大量的水，这些水使它们的外层塑化，使得这些层在达到均匀冰核阈值之前很容易与气相湿度平衡。因此，玻璃的形成不太可能限制均匀的冰核。只有在非常高的热带对流层顶附近的最极端条件下，均匀冰成核速率系数才会由于缓慢的凝结水扩散而降低。由于即使在非常高的热带对流层顶，受扩散限制和不受扩散限制的行为之间的差异也很小，因此在对流层条件下，凝结水的扩散率不太可能对SOA颗粒的直接气候效应产生重大影响。
New measurements of water diffusion in secondary organic aerosol (SOA) material produced by oxidation of alpha-pinene and in a number of organic/inorganic model mixtures (3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA), levoglucosan, levoglucosan/NH4HSO4, raffinose) are presented. These indicate that water diffusion coefficients are determined by several properties of the aerosol substance and cannot be inferred from the glass transition temperature or bouncing properties. Our results suggest that water diffusion in SOA particles is faster than often assumed and imposes no significant kinetic limitation on water uptake and release at temperatures above 220 K. The fast diffusion of water suggests that heterogeneous ice nucleation on a glassy core is very unlikely in these systems. At temperatures below 220 K, model simulations of SOA particles suggest that heterogeneous ice nucleation may occur in the immersion mode on glassy cores which remain embedded in a liquid shell when experiencing fast updraft velocities. The particles absorb significant quantities of water during these updrafts which plasticize their outer layers such that these layers equilibrate readily with the gas phase humidity before the homogeneous ice nucleation threshold is reached. Glass formation is thus unlikely to restrict homogeneous ice nucleation. Only under most extreme conditions near the very high tropical tropopause may the homogeneous ice nucleation rate coefficient be reduced as a consequence of slow condensed-phase water diffusion. Since the differences between the behavior limited or non limited by diffusion are small even at the very high tropical tropopause, condensed-phase water diffusivity is unlikely to have significant consequences on the direct climatic effects of SOA particles under tropospheric conditions.