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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-789
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Sep 2018

Research article | 12 Sep 2018

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

Modeling CCN activity of chemically unresolved model HULIS, including surface tension, non-ideality, and surface partitioning

Nonne L. Prisle1,2 and Bjarke Molgaard2 Nonne L. Prisle and Bjarke Molgaard
  • 1University of Oulu, Nano and Molecular Systems Research Unit, P.O. Box 3000, 90014, University of Oulu, Finland
  • 2University of Helsinki, Department of Physics, P.O. Box 48, 00014, University of Helsinki, Finland

Abstract. Cloud condensation nuclei (CCN) activity of aerosol particles comprising surface active Nordic Aquatic Fulvic Acid (NAFA) and NaCl was modeled with four different approaches to account for NAFA bulk-to-surface partitioning and the combined influence of NAFA and NaCl on surface tension and water activity of activating droplets. Calculations were made for particles with dry diameters of 30–230nm and compositions covering the full range of relative NAFA and NaCl mixing ratios. Continuous ternary parametrizations of aqueous surface tension and water activity with respect to independently varying NAFA and NaCl mass concentrations were developed from previous measurements on macroscopic bulk solutions and implemented to a Köhler model framework. This enabled comprehensive thermodynamic predictions of cloud droplet activation, including equilibrium surface partitioning, for particles comprising chemically unresolved organic NAFA mixtures. NAFA here serves as a model for surface active atmospheric humic-like substances (HULIS) and for chemically complex organic aerosol in general. Surfactant effects are gauged via predictions of a suite of properties for activating droplets, including critical supersaturation and droplet size, bulk phase composition, surface tension, Kelvin effect, and water activity. Assuming macroscopic solution properties for activating droplets leads to gross overestimations of reported experimental CCN activation, mainly by overestimating surface tension reduction from NAFA solute in droplets. Failing to account for bulk-to-surface partitioning of NAFA introduces severe biases in evaluated droplet bulk and surface composition and critical size, which here specifically affect cloud activation thermodynamics, but more generally could also impact heterogeneous chemistry on droplet surfaces. Model frameworks based on either including surface partitioning and/or neglecting surface tension reduction give similar results for both critical supersaturation and droplet properties and reproduce reported experimental CCN activity well. These perhaps counterintuitive results reflect how the bulk phase is nearly depleted in surface active organic from surface partitioning in submicron droplets with large surface area for a given bulk volume. As a result, NAFA has very little impact on surface tension and water activity at the point of droplet activation. In other words, the predicted surfactant strength of NAFA is significantly lower in sub-micron activating droplets than in macroscopic aqueous solutions of the same overall composition. These results show similar effects of chemically complex surfactants as have previously been seen only for simple surfactants with well-defined molecular properties and add to the growing appreciation of the complex role of surface activity in cloud droplet activation.

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Nonne L. Prisle and Bjarke Molgaard
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Nonne L. Prisle and Bjarke Molgaard
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Short summary
Continuous surface tension and water activity functions were developed for aqueous salt mixtures with Nordic Aquatic Fulvic Acid, an organic reference mixture with properties resembling atmospheric Humic-like Substances, and used for modeling of cloud forming potential. These mixtures show detailed properties closer resembling simple surfactants, than recent results for other complex organic aerosol. The same mechanisms have implications for other aerosol processes, e.g. heterogeneous chemistry.
Continuous surface tension and water activity functions were developed for aqueous salt mixtures...
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