University of Oulu

Lin, J. J., Kristensen, T. B., Calderón, S. M., Malila, J., & Prisle, N. L. (2020). Effects of surface tension time-evolution for CCN activation of a complex organic surfactant. Environmental Science: Processes & Impacts, 22(2), 271–284.

Effects of surface tension time-evolution for CCN activation of a complex organic surfactant

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Author: Lin, Jack J.1; Kristensen, Thomas B.2; Calderón, Silvia M.1;
Organizations: 1Nano and Molecular Systems Research Unit, University of Oulu, P. O. Box 3000, Oulu,FI-90014, Finland
2Lund University, Division of Nuclear Physics, S-22100 Lund, Sweden
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.7 MB)
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Language: English
Published: Royal Society of Chemistry, 2020
Publish Date: 2020-01-15


The physical processes and time scales underlying the evolution of surface tension in atmospheric solution droplets are largely unaccounted for in present models describing cloud droplet formation. Adsorption of surface-active molecules at the surface of a solution droplet depresses the droplet surface tension but also depletes solute from the droplet bulk, which have opposing and sometimes canceling effects in cloud droplet formation. In this work, we study the effect of time-evolving surface tension for cloud droplet activation of particles composed of Nordic Aquatic Fulvic Acid (NAFA) mixed with sodium chloride (NaCl). We model the formation of cloud droplets using Köhler theory with surface tension depression and bulk/surface partitioning evaluated from two different thermodynamic surface models. Continuous ternary parameterizations were constructed from surface tension measurements of macroscopic droplets at different time steps after the formation of a droplet surface. The predicted results are compared to previous measurements of mixed NAFA–NaCl cloud condensation nuclei (CCN) activity and a bulk solution model that does not take the NAFA bulk/surface partitioning equilibrium into account. Whereas the bulk model shows a trend in cloud droplet formation following that of macroscopic surface tension depression with time, the variation with time essentially disappears when bulk/surface partitioning is taken explicitly into account during droplet activation. For all equilibrium time steps considered, the effect of surface tension depression in the NAFA–NaCl system is counteracted by the depletion of solute from the finite-sized droplet bulk phase. Our study highlights that a comprehensive data set is necessary to obtain continuous parameterizations of surface tension and other solution properties required to fully account for the bulk/surface partitioning in growing droplets. To our knowledge, no similar data set currently exists for other aqueous organic systems of atmospheric interest. Additional work is necessary to deconvolve the effects of bulk/surface partitioning in the context of time-evolution on cloud droplet activation and to determine whether the results presented here can be further generalized.

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Environmental significance

The time-evolution of aqueous surface tension is a well-known result of partitioning of surface-active compounds from the bulk to the surface. Surfactants are ubiquitous components of atmospheric organic aerosol. Equilibration time scales potentially impact interpretation of all measurements of cloud condensation nuclei (CCN) activity and modeled effects taking surface tension into account, including closure studies between instruments with different residence times and analysis using κ-Köhler theory. Using continuous surface tension parameterizations at different times, we provide a thermodynamically consistent analysis of the impact of time-dependency for predictions of CCN activity. We show that although time-evolution is clearly seen in surface partitioning, its signature decreases in droplet surface tension and we find no trends in CCN activity with time.

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Series: Environmental science. Processes & impacts
ISSN: 2050-7887
ISSN-E: 2050-7895
ISSN-L: 2050-7887
Volume: 22
Issue: 2
Pages: 271 - 284
DOI: 10.1039/C9EM00426B
Type of Publication: A1 Journal article – refereed
Field of Science: 114 Physical sciences
1171 Geosciences
1172 Environmental sciences
Funding: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme, Project SURFACE (Grant Agreement No. 717022). NLP, JJL, and JM also gratefully acknowledge the financial contribution from the Academy of Finland (Grant No. 308238, 314175, and 290145). NLP is furthermore grateful to the Carlsberg Foundation for funding (2010_01_0391 and 2009_01_0366). TBK also acknowledges funding of Prof. Merete Bilde from the Carlsberg Foundation (2009_01_0515) and from the Nordic Center of Excellence CRAICC.
EU Grant Number: (717022) SURFACE - The unexplored world of aerosol surfaces and their impacts.
Academy of Finland Grant Number: 308238
Detailed Information: 308238 (Academy of Finland Funding decision)
314175 (Academy of Finland Funding decision)
290145 (Academy of Finland Funding decision)
Copyright information: © The Authors. This Open Access Article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence