The role of surfactants in Köhler theory reconsidered

Abstract

Abstract. Atmospheric aerosol particles typically consist of inorganic salts and organic material. The inorganic compounds as well as their hygroscopic properties are well defined, but the effect of organic compounds on cloud droplet activation is still poorly characterized. The focus of the present study is in the organic compounds that are surface active i.e. they concentrate on droplet surface and decrease droplet surface tension. Gibbsian surface thermodynamics were used to find out how partitioning in binary and ternary aqueous solutions affects the droplet surface tension and the droplet bulk concentration in droplets large enough to act as cloud condensation nuclei. Sodium dodecyl sulfate was used as a model compound together with sodium chloride to find out the effect the correct evaluation of surfactant partitioning has on the solute effect (Raoult effect). While the partitioning is known to lead to higher surface tension compared to a case in which partitioning is neglected, the present results show that the partitioning also alters the solute effect, and that the change is large enough to further increase the critical supersaturation and hence decrease the droplet activation. The fraction of surfactant partitioned to droplet surface increases with decreasing droplet size, which suggests that surfactants might enhance the activation of larger particles relatively more thus leading to less dense clouds. Cis-pinonic acid-ammonium sulfate aqueous solution was studied in order to relate the partitioning to more realistic atmospheric situation and to find out the combined effects of dissolution and partitioning behaviour. The results show that correct partitioning consideration alters the shape of the Köhler curve when compared to a situation in which the partitioning is neglected either completely or in the Raoult effect.

Ternary solution of sodium chloride, succinic acid and water; surface tension and its influence on cloud droplet activation

“…We made calculations of critical supersaturations for 40 nm and 170 nm particles. Partitioning was calculated using the Gibbs adsorption equation (Sorjamaa et al, 2004). By including the partitioning in the calculations we obtained almost the same critical supersaturations as using the surface tension of water without the partitioning, with an absolute error of less than 0.05%.…”

“…Because succinic acid is a surface active compound, also the partitioning of the acid between the bulk solution and the surface should be taken into account (Sorjamaa et al, 2004). We made calculations of critical supersaturations for 40 nm and 170 nm particles.…”

Accurate Determination of Aerosol Activity Coefficients at Relative Humidities up to 99% Using the Hygroscopicity Tandem Differential Mobility Analyzer Technique

“…The presence of surfactants may bias the inferred water activity via Equation (3). In this case, a more sophisticated theory that accounts for the radial surfactant partitioning within the droplet (Sorjamaa et al 2004;Li et al 1998;Petters and Kreidenweis 2012) may be needed to accurately infer water activity from growth factor data. We also assumed that the same scaling factor (i.e., dynamic shape factor) applies to the organic compounds and the ammonium sulfate.…”

Cloud droplet activation and surface tension of mixtures of slightly soluble organics and inorganic salt

“…Therefore surface tension measurements of the mixtures at relevant concentrations were conducted, parameterized as a function of the carbon content of the individual acids and incorporated in a Köhler model taking into account limited solubility. It has recently been suggested (Li et al, 1998;Sorjamaa et al, 2004) that partitioning of surface active organic compounds between bulk and surface in activating droplets r curves accounting for limited solubility for initially dry particles with a diameter rnary mixtures of adipic and succinic acid and sodium chloride. For comparison the e adipic and succinic acid are also given.…”