The physical processes and time scales underlying the evolution of surface tension in atmospheric solution droplets are largely unaccounted for in present models describing cloud droplet formation. Adsorption of surface-active molecules at the surface of a solution droplet depresses the droplet surface tension but also depletes solute from the droplet bulk, which have opposing and sometimes canceling effects in cloud droplet formation. In this work, we study the effect of time-evolving surface tension for cloud droplet activation of particles composed of Nordic Aquatic Fulvic Acid (NAFA) mixed with sodium chloride (NaCl). We model the formation of cloud droplets using Köhler theory with surface tension depression and bulk/surface partitioning evaluated from two different thermodynamic surface models. Continuous ternary parameterizations were constructed from surface tension measurements of macroscopic droplets at different time steps after the formation of a droplet surface. The predicted results are compared to previous measurements of mixed NAFA-NaCl cloud condensation nuclei (CCN) activity and a bulk solution model that does not take the NAFA bulk/surface partitioning equilibrium into account. Whereas the bulk model shows a trend in cloud droplet formation following that of macroscopic surface tension depression with time, the variation with time essentially disappears when bulk/surface partitioning is taken explicitly into account during droplet activation. For all equilibrium time steps considered, the effect of surface tension depression in the NAFA-NaCl system is counteracted by the depletion of solute from the finite-sized droplet bulk phase. Our study highlights that a comprehensive data set is necessary to obtain continuous parameterizations of surface tension and other solution properties required to fully account for the bulk/surface partitioning in growing droplets. To our knowledge, no similar data set currently exists for other aqueous organic systems of atmospheric interest. Additional work is necessary to deconvolve the effects of bulk/surface partitioning in the context of time-evolution on cloud droplet activation and to determine whether the results presented here can be further generalized.

中文翻译：

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表面张力时间演化对复杂有机表面活性剂CCN活化的影响。

在描述云滴形成的当前模型中，大气溶液小滴中表面张力的演变所基于的物理过程和时间尺度在很大程度上未得到解释。溶液液滴表面的表面活性分子的吸附会降低液滴的表面张力，但也会消耗液滴中的溶质，这在云滴的形成中具有相反的作用，有时甚至是相互抵消的作用。在这项工作中，我们研究了随时间变化的表面张力对由北欧水产黄腐酸（NAFA）与氯化钠（NaCl）混合组成的颗粒的云滴活化的影响。我们使用Köhler理论对云滴的形成进行建模，并从两个不同的热力学表面模型评估了表面张力，并进行了体积/表面分配。从液滴表面形成后不同时间步长的宏观液滴表面张力测量结果构建连续的三元参数化。将预测结果与以前的混合NAFA-NaCl云凝结核（CCN）活性测量结果和不考虑NAFA体积/表面分配平衡的本体溶液模型进行了比较。尽管体模型显示了随着时间的推移宏观表面张力下降之后云滴形成的趋势，但是当在液滴激活过程中明确考虑体/表面分配时，随时间的变化基本上消失了。对于所有考虑的平衡时间步长，NAFA-NaCl系统中表面张力降低的影响被有限大小的液滴本体相中的溶质耗竭所抵消。我们的研究强调，需要全面的数据集来获得表面张力和其他溶液特性的连续参数化，以充分考虑生长液滴的体积/表面分配。据我们所知，目前尚无其他有关大气关注的水性有机体系的类似数据集。需要进行额外的工作来消除卷积/表面分区在时间演化的背景下对云滴活化的影响，并确定是否可以进一步概括此处介绍的结果。我们的研究强调，需要一个综合的数据集来获得表面张力和其他溶液特性的连续参数化，以充分考虑生长液滴的体积/表面分配。据我们所知，目前尚无其他有关大气关注的水性有机体系的类似数据集。需要进行额外的工作来消除卷积/表面分区在时间演化的背景下对云滴活化的影响，并确定是否可以进一步概括此处介绍的结果。我们的研究强调，需要全面的数据集来获得表面张力和其他溶液特性的连续参数化，以充分考虑生长液滴的体积/表面分配。据我们所知，目前尚无其他有关大气关注的水性有机体系的类似数据集。需要进行额外的工作来消除卷积/表面分区在时间演化的背景下对云滴活化的影响，并确定是否可以进一步概括此处介绍的结果。