School of Chemistry, University of Bristol, BS8 1TS Bristol, United Kingdom;
Nano and Molecular Systems Research Unit, University of Oulu, 90014 Oulu, Finland.
Proc Natl Acad Sci U S A. 2020 Apr 14;117(15):8335-8343. doi: 10.1073/pnas.1915660117. Epub 2020 Apr 1.
Surface tension influences the fraction of atmospheric particles that become cloud droplets. Although surfactants are an important component of aerosol mass, the surface tension of activating aerosol particles is still unresolved, with most climate models assuming activating particles have a surface tension equal to that of water. By studying picoliter droplet coalescence, we demonstrate that surfactants can significantly reduce the surface tension of finite-sized droplets below the value for water, consistent with recent field measurements. Significantly, this surface tension reduction is droplet size-dependent and does not correspond exactly to the macroscopic solution value. A fully independent monolayer partitioning model confirms the observed finite-size-dependent surface tension arises from the high surface-to-volume ratio in finite-sized droplets and enables predictions of aerosol hygroscopic growth. This model, constrained by the laboratory measurements, is consistent with a reduction in critical supersaturation for activation, potentially substantially increasing cloud droplet number concentration and modifying radiative cooling relative to current estimates assuming a water surface tension. The results highlight the need for improved constraints on the identities, properties, and concentrations of atmospheric aerosol surfactants in multiple environments and are broadly applicable to any discipline where finite volume effects are operative, such as studies of the competition between reaction rates within the bulk and at the surface of confined volumes and explorations of the influence of surfactants on dried particle morphology from spray driers.
表面张力会影响大气粒子中成为云滴的那一部分。尽管表面活性剂是气溶胶质量的重要组成部分，但激活气溶胶粒子的表面张力仍未得到解决，大多数气候模型假设激活粒子的表面张力与水相等。通过研究皮升级液滴聚并，我们证明了表面活性剂可以显著降低有限大小液滴的表面张力，使其低于水的值，这与最近的现场测量结果一致。重要的是，这种表面张力降低与液滴尺寸有关，并不完全对应于宏观溶液值。一个完全独立的单层分配模型证实了观察到的有限尺寸依赖性表面张力来自于有限大小液滴的高表面积与体积比，并能够预测气溶胶的吸湿性增长。这个模型受到实验室测量的限制，与目前假设水表面张力下的激活临界过饱和度降低的情况一致，这可能会显著增加云滴数浓度，并改变相对于当前估计的辐射冷却效果。这些结果强调了需要在多种环境中对大气气溶胶表面活性剂的身份、性质和浓度进行更好的约束，并且广泛适用于任何存在有限体积效应的领域，例如研究在受限体积内部和表面的反应速率之间的竞争，以及探索表面活性剂对喷雾干燥器中干燥颗粒形态的影响。
