Abstract
Abstract. Organic compounds with surfactant properties are commonly found in atmospheric aerosol particles. Surface activity can significantly influence the cloud droplet forming ability of these particles. We have studied the cloud droplet formation by two-component particles comprising one of the organic surfactants sodium octanoate, sodium decanoate, sodium dodecanoate, and sodium dodecyl sulfate, mixed with sodium chloride. Critical supersaturations were measured with a static diffusion cloud condensation nucleus counter (Wyoming CCNC-100B). Results were modeled from Köhler theory applying three different representations of surfactant properties in terms of surfactant surface partitioning and reduced droplet surface tension. We here confirm previous results for single-component organic surfactant particles, that experimental critical supersaturations are greatly underpredicted, if reduced surface tension is used while ignoring the effects of surface partitioning in droplets. Furthermore, disregarding surfactant properties by ignoring surface partitioning and assuming the constant surface tension of pure water can also lead to significant underpredictions of experimental critical supersaturations. For the mixed particles comprising less than 50% by mass of surfactant, this approach however still provides a good description of the observed droplet activation. A comprehensive account for surfactant properties, including both surface tension reduction and effects of surface partitioning in activating droplets, generally predicts experimental critical supersaturations well.
Highlights
The influence of atmospheric aerosol particles on cloud formation and properties constitutes the single largest uncertainty in assessing anthropogenic climate forcing (IPCC, 2007)
In panels (a) and (b), it is seen that with Wp,SFT≤50%, differences in SScexp for particles of a given dry particle size (Dp) comprising different surfactants are comparable to experimental uncertainties
Any potential differences in droplet activation behavior due to individual molecular properties of the surfactants are dominated by the presence of the inorganic salt
Summary
The influence of atmospheric aerosol particles on cloud formation and properties constitutes the single largest uncertainty in assessing anthropogenic climate forcing (IPCC, 2007). Cloud droplets form when water vapor condenses onto particle surfaces. Particle constituents may dissolve into the aqueous phase and form solution droplets. Surface active molecules (surfactants) concentrate in the surface and can reduce the surface tension of an aqueous solution. Reduced surface tension, compared to that of pure water, has been demonstrated in bulk samples of atmospheric cloud and fog water (Facchini et al, 1999, 2000) and in aqueous extracts of collected atmospheric aerosol samples from a wide variety of sources and environments, including biomass (Asa-Awuku and Sullivan, 2008) and coal burning (Oros and Simoneit, 2000), and marine (Mochida et al, 2002), rural (Kiss et al, 2005), and polluted environments (Dinar and Taraniuk, 2006). The goal of this work is to advance the fundamental understanding of the role of surfactants in cloud microphysics, which is essential for consistent representations of aerosol effects in atmospheric models
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