Technical note: The role of evolving surface tension in the formation of cloud droplets

James F Davies,Kevin R Wilson,Andreas Zuend

doi:10.5194/acp-19-2933-2019

Abstract

Abstract. The role of surface tension (σ) in cloud droplet activation has long been ambiguous. Recent studies have reported observations attributed to the effects of an evolving surface tension in the activation process. However, the adoption of a surface-mediated activation mechanism has been slow and many studies continue to neglect the composition dependence of aerosol–droplet surface tension, using instead a value equal to the surface tension of pure water (σw). In this technical note, we clearly describe the fundamental role of surface tension in the activation of multicomponent aerosol particles into cloud droplets. It is demonstrated that the effects of surface tension in the activation process depend primarily on the evolution of surface tension with droplet size, typically varying in the range 0.5σw≲σ≤σw due to the partitioning of organic species with a high surface affinity. We go on to report some recent laboratory observations that exhibit behavior that may be associated with surface tension effects and propose a measurement coordinate that will allow surface tension effects to be better identified using standard atmospheric measurement techniques. Unfortunately, interpreting observations using theory based on surface film and liquid–liquid phase separation models remains a challenge. Our findings highlight the need for experimental measurements that better reveal the role of composition-dependent surface tensions, critical for advancing predictive theories and parameterizations of cloud droplet activation.

Highlights

The formation of a cloud involves a complex series of steps as nanometer-sized aerosol particles, termed cloud condensation nuclei (CCN), grow by condensation of water vapor to become supermicron-sized cloud droplets in a process known as CCN activation
We offer a perspective on the role of surface tension in the activation process, drawing on recent studies and interpretations of cloud droplet activation measurements (e.g., Forestieri et al, 2018; Ovadnevaite et al, 2017; Ruehl et al, 2016; Ruehl and Wilson, 2014)
Surface tension effects can lead to significant differences from classic, hygroscopicity mixing rule mechanisms for CCN activation (Hansen et al, 2015; Kristensen et al, 2014)

Summary

Introduction

The formation of a cloud involves a complex series of steps as nanometer-sized aerosol particles, termed cloud condensation nuclei (CCN), grow by condensation of water vapor to become supermicron-sized cloud droplets in a process known as CCN activation. In order to gain a robust and predictive understanding of CCN activation, a molecular-level theory must be developed and adopted by the atmospheric chemistry community In this technical note, we offer a perspective on the role of surface tension in the activation process, drawing on recent studies and interpretations of cloud droplet activation measurements (e.g., Forestieri et al, 2018; Ovadnevaite et al, 2017; Ruehl et al, 2016; Ruehl and Wilson, 2014). Our aim is to provide a platform for discussion and help foster a molecular-based interpretation of the role of organic material in the activation of aerosol to cloud droplets

Clarifying how surface tension alters cloud droplet activation

Surface tension evolution during activation

Identifying surface tension effects from critical supersaturation

Summary and conclusions