Surface tension prevails over solute effect in organic-influenced cloud droplet activation

Abstract

The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory1,2. 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 surfactants1. 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 interface3,4. 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.

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Figure 1: Physical number concentration–size distributions, total mass–size distributions and sulphate and organic-matter mass–size distributions during the NUM event.
Figure 2: CCN, chemical composition and size distribution time series for the NUM event and the CCN closure.
Figure 3: Thermodynamic equilibrium model predictions for an internally mixed organic matter plus sulphuric acid aerosol in the relative humidity range from 94% to about 101%.

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Acknowledgements

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 information

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.

Correspondence to Colin O’ Dowd.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks M. Gysel, K. Wilson and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Ovadnevaite, J., Zuend, A., Laaksonen, A. et al. Surface tension prevails over solute effect in organic-influenced cloud droplet activation. Nature 546, 637–641 (2017). https://doi.org/10.1038/nature22806

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