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Published June 29, 2017 | Supplemental Material
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Surface tension prevails over solute effect in organic-influenced cloud droplet activation


The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory. 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. 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. 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.

Additional Information

© 2017 Macmillan Publishers Limited, part of Springer Nature. Received 20 August 2016; Accepted 04 May 2017; Published online 21 June 2017. The research leading to these results received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) project BACCHUS under grant agreement number 603445, the Irish Environmental Protection Agency, the HEA PRTLI4 project and the Academy of Finland Center of Excellence programme (grant number 272041). The work was further supported by the CNR (Italy) under AirSEaLab: Progetto Laboratori Congiunti, the US Office of Naval Research, and the Natural Sciences and Engineering Research Council of Canada (NSERC, grant RGPIN/04315-2014). N.H. was supported by a National Science Foundation Atmospheric and Geospace Sciences Postdoctoral Research Fellowship (award number 14433246). Author Contributions: J.O. conducted and analysed the AMS measurements and led the scientific development (in conjunction with C.O'D.). D.C. organized and conducted the aerosol measurements. G.R. and K.J.S. conducted the CCN measurements. M.C.F., S.D. and M.R. provided surface tension measurements. A.L. provided the partitioning model. A.Z. did the thermodynamic modelling. C.O'D., J.O., N.H., A.Z. and J.H.S. wrote the paper. The authors declare no competing financial interests.

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