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Discussion papers
https://doi.org/10.5194/acp-2018-1201
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1201
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Technical note 22 Nov 2018

Technical note | 22 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Technical Note: The Role of Evolving Surface Tension in the Formation of Cloud Droplets

James F. Davies1, Andreas Zuend2, and Kevin R. Wilson3 James F. Davies et al.
  • 1Department of Chemistry, University of California Riverside, CA USA
  • 2Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
  • 3Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA USA

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, 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. However, 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.

James F. Davies et al.
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Short summary
The formation of cloud droplets involves the condensation of water onto preexisting particles in the atmosphere. The efficiency of this process depends on the nature of the particles, and recent work has shown that organic-rich particles may exhibit a suppressed surface tension that promotes the formation of cloud droplets. In this technical note, we discuss the mechanism for this and highlight the evolution of surface tension as the key factor in the extent of surface effects.
The formation of cloud droplets involves the condensation of water onto preexisting particles in...
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