Surface Tension and Critical Supersaturations for Mixed Aerosol Particles Composed of Inorganic and Organic Compounds of Atmospheric Relevance

The interaction between water vapor and aerosol particles in the atmosphere has implications on important processes. Among these are cloud droplet formation and growth, which impact cloud properties and therefore have an indirect effect on climate. A significant fraction of the dry submicron mass of atmospheric aerosols is composed of water-soluble organic carbon (WSOC). Although the WSOC fraction contains a large amount of compounds, most yet unidentified, it can be partitioned into three main categories in order to use a set of model substances to reproduce its behavior. In this study, we chose levoglucosan, succinic acid and Nordic Reference fulvic acid (NRFA) to represent the WSOC categories of neutral compounds, mono-/di-carboxylic acids, and polycarboxylic acids, respectively. We measured the surface tension of aqueous pure NRFA and of five of its mixtures at 298 K using the Wilhemy plate method. Langmuir adsorption parameters for the organic mixtures were extracted by fitting the surface tension measurements and corresponding solute concentrations to the Szyszkowski-Langmuir equation. The measured surface tension as a function of aqueous NRFA concentration was identical to that of Suwannee River (SR) and Waskish Peat fulvic acids below 0.02 g/L but up to 12% and 15% higher, respectively, at higher concentrations. Similar to previous findings by Aumann et al. (2010) with SRFA, the surface tension of a NRFA/inorganic salt solution was mainly controlled by the organic compound even when the salt comprised 75% of the added solute mass. This effect was observed for mixtures of NRFA with both sodium chloride and ammonium sulfate salts up to 5 g/L of NRFA. From 5 g/L to about 50 g/L of NRFA, the surface tension for both NRFA/salt mixtures stopped decreasing, remained constant at 52-53 mN/m and then started slowly increasing indicating that the salt component might start dominating at higher concentrations. For a solution of 25% NRFA / 75% levoglucosan, the surface tension lowering with increasing concentration was very similar to that of pure aqueous NRFA even to the maximum measured concentration of 50 g/L of NRFA. However, the surface tension of the NRFA/saccharide mixture exhibited a more linear decrease when plotted against ln(NRFA concentration) in the 0.1 to 50 g/L range. We also measured the surface tension of two additional mixtures based on chemical composition data for different aerosol types. The measured surface tension for the solution representing organic aerosols (40% NRFA / 40% succinic acid / 20% levoglucosan) was bounded by that of pure NRFA and the NRFA/levoglucosan mixture up to a concentration of ~28 g/L of NRFA, where it remained constant at around 46.6 mN/m until 80 g/L of NRFA. The solution representing biomass burning aerosols (25% NRFA/ 27% succinic acid / 18% levoglucosan / 30% ammonium sulfate) had a similar surface tension to pure NRFA up to a concentration of ~5 g/L of NRFA, from where the surface tension drop continued between that of pure NRFA and the NRFA/salt mixtures. Critical supersaturations as a function of dry particle diameter were estimated by using measured water activity as a function of concentration and surface tension data to calculate the maximum of each Köhler curve for the mixtures studied.