Surface tension prevails over solute effect in organic-influenced cloud droplet activation
Surface tension prevails over solute effect in organic-influenced cloud droplet activation
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DOI：
10.1038/nature22806
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发表时间：
2017-06-29
期刊：
影响因子：
64.8
通讯作者：
O'Dowd, Colin
中科院分区：
文献类型：
--
作者：
Ovadnevaite, Jurgita;Zuend, Andreas;O'Dowd, Colin
用经典的Kohler理论(1，2)描述了过饱和水汽条件下云凝结核(CCN)自发成长为云滴的过程。CCN的这种自发活化取决于Raoult效应和开尔文效应之间的相互作用，Raoult效应的活化势随着水活度的降低或溶质浓度的增加而增加，开尔文效应的活化势随着液滴尺寸的减小而减小，或者随着表面张力的降低而增加(1)。有机表面活性剂引起的表面张力降低会减弱开尔文效应，但由于表面活性剂分子从液滴本体置换到液滴-蒸气界面(3，4)，拉乌尔效应预计会随之减弱。在这里，我们提供了观测和理论证据，说明在环境空气中，表面张力的降低可以超过Raoult效应的减少，从而导致云滴浓度的大幅增加。我们认为，考虑液-液分离，导致吸湿颗粒核心完全或部分被疏水的富有机相吞没，可以解释为什么没有伴随的Raoult效应的减少，同时保持表面张力的大幅降低，即使是部分表面覆盖。除了粒子大小和组成在液滴激活中的重要性外，我们通过观察和模拟表明，将相分离效应纳入激活热力学可以导致CCN数浓度达到气候模型预测的十倍，从而改变云的性质。对于气候模型中的云的预测来说，对CCN激活过程的适当表示是至关重要的，鉴于云对地球能量平衡的影响，改进对气溶胶-云-气候相互作用的预测可能会改进对未来气候变化的评估。
The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Kohler theory(1,2). This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants(1). Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet-vapour interface(3,4). Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increases in cloud droplet concentrations. We suggest that consideration of liquid-liquid phase separation, leading to complete or partial engulfing of a hygroscopic particle core by a hydrophobic organic-rich phase, can explain the lack of concomitant reduction of the Raoult effect, while maintaining substantial lowering of surface tension, even for partial surface coverage. Apart from the importance of particle size and composition in droplet activation, we show by observation and modelling that incorporation of phase-separation effects into activation thermodynamics can lead to a CCN number concentration that is up to ten times what is predicted by climate models, changing the properties of clouds. An adequate representation of the CCN activation process is essential to the prediction of clouds in climate models, and given the effect of clouds on the Earth's energy balance, improved prediction of aerosol-cloud-climate interactions is likely to result in improved assessments of future climate change.