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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
02 Mar 2018
Review status
This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP).
Establishing the Impact of Model Surfactants on Cloud Condensation Nuclei Activity of Sea Spray Aerosols
Sara D. Forestieri1,a, Sean M. Staudt2, Thomas M. Kuborn2, Katharine Faber3, Christopher R. Ruehl4, Timothy H. Bertram2, and Christopher D. Cappa1 1Dept. of Civil and Environmental Engineering, University of California, Davis, CA, USA
2Dept. of Chemistry, University of Wisconsin, Madison, WI, USA
3Dept. of Chemistry, Carleton College, Northfield, MN, USA
4California Air Resources Board, Sacramento, CA, USA
anow at: California Air Resources Board, Sacramento, CA, USA
Abstract. Surface active compounds present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency by reducing the surface tension (σ) in the growing droplets. However, the importance of this effect is poorly constrained by measurements. Here we present estimates of droplet surface tension near the point of activation derived from direct measurement of droplet diameters using a continuous flow stream-wise thermal gradient chamber (CFSTGC). The experiments used sea spray aerosol mimics composed of NaCl coated by varying amounts of (i) oleic acid, palmitic acid or myristic acid, (ii) mixtures of palmitic acid and oleic acid, and (iii) oxidized oleic acid. Significant reductions in σ relative to that for pure water were observed for these mimics at relative humidity (RH) near activation (~ 99.9 %) when the coating was sufficiently thick. The calculated surface pressure (π = σH2O − σobserved) values for a given organic compound or mixture collapse onto one curve when plotted as a function of molecular area for different NaCl seed sizes and measured RH. The observed critical molecular area (A0) for oleic acid determined from droplet growth was similar to that from bulk experiments conducted in a Langmuir trough. However, the observations presented here suggest that oleic acid in microscopic droplets may exhibit larger π values during monolayer compression. For myristic acid, the observed A0 compared well to bulk experiments on a fresh subphase, for which dissolution has an important impact. A significant kinetic limitation to water uptake was observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid being able to form a solid film. However, for binary palmitic acid-oleic acid mixtures there was no evidence of a kinetic limitation to water uptake. Oxidation of oleic acid had a minor impact on the magnitude of the surface tension reductions observed, potentially leading to a slight reduction in the effect compared to pure oleic acid. A cloud condensation nuclei (CCN) counter was also used to assess the impact on critical supersaturations of the substantial σ reductions observed at very high RH. For the fatty acid-coated NaCl particles, when the organic fraction (εorg) was > 0.90 small depressions in critical supersaturation were observed. However, when εorg < 0.90 the impact on critical supersaturation was negligible. Thus, for the fatty acids considered here, the substantial σ reductions observed at high RH values just below activation have limited impact on the ultimate critical supersaturation. A surface film model is used to establish the properties that surface active organic molecules must have if they are to ultimately have a substantial impact on the activation efficiency of SSA. To influence activation, the average properties of surface-active marine-derived organic molecules must differ substantially from the long chain fatty acids examined, having either smaller molecular volumes or larger molecular areas. The model results also indicate that organic-driven surface tension depression can serve to buffer the critical supersaturation against changes to the organic-to-salt ratio in particles in which the organic fraction is sufficiently large.
Citation: Forestieri, S. D., Staudt, S. M., Kuborn, T. M., Faber, K., Ruehl, C. R., Bertram, T. H., and Cappa, C. D.: Establishing the Impact of Model Surfactants on Cloud Condensation Nuclei Activity of Sea Spray Aerosols, Atmos. Chem. Phys. Discuss.,, in review, 2018.
Sara D. Forestieri et al.
Sara D. Forestieri et al.

Data sets

Water uptake, cloud condensation nuclei and surface tension: results from the MadFACTS campaign S. Forestieri, S. Staudt, T. Kuborn, C. Ruehl, T. Bertram, and C. Cappa
Sara D. Forestieri et al.


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Short summary
Our work establishes how surface tension reduction influences droplet growth and activation of simple sea spray mimics, i.e. NaCl coated with fatty acids. Fatty acids can substantially reduce droplet surface tension near activation, but have limited impact on activation. Coating of NaCl by stearic acid (a wax) impedes water uptake, but this impedance is removed if oleic acid (a liquid) is mixed in. The properties that surface active compounds need to impact activation are theoretically examined.
Our work establishes how surface tension reduction influences droplet growth and activation of...