Abstract

Abstract. We have investigated CCN properties of internally mixed particles composed of one organic acid (oxalic acid dihydrate, succinic acid, adipic acid, citric acid, cis-pinonic acid, or Nordic reference fulvic acid) and one inorganic salt (sodium chloride or ammonium sulphate). Surface tension and water activity of aqueous model solutions with concentrations relevant for CCN activation were measured using a tensiometer and osmometry, respectively. The measurements were used to calculate Köhler curves and critical supersaturations, which were compared to measured critical supersaturations of particles with the same chemical compositions, determined with a cloud condensation nucleus counter. Surfactant surface partitioning was not accounted for. For the aqueous solutions containing cis-pinonic acid and fulvic acid, a depression of surface tension was observed, but for the remaining solutions the effect on surface tension was negligible at concentrations relevant for cloud droplet activation. The surface tension depression of aqueous solutions containing both organic acid and inorganic salt was approximately the same as or smaller than that of aqueous solutions containing the same mass of the corresponding pure organic acids. Water activity was found to be highly dependent on the type and amount of inorganic salt. Sodium chloride was able to decrease water activity more than ammonium sulphate and both inorganic salts are predicted to have a smaller Raoult term than the studied organic acids. Increasing the mass ratio of the inorganic salt led to a decrease in water activity. Water activity measurements were compared to results from the E-AIM model and values estimated from both constant and variable van't Hoff factors. The correspondence between measurements and estimates was overall good, except for highly concentrated solutions. Critical supersaturations calculated with Köhler theory based on measured water activity and surface tension, but not accounting for surface partitioning, compared well with measurements, except for the solutions containing sodium chloride and oxalic acid or one of the more surface active organic compounds. In such cases, significantly lower values were obtained from Köhler theory than the measured critical supersaturations with deviations above 50% for a 60 nm particle containing 50% (dry mass) of Nordic reference fulvic acid, suggesting that surfactant partitioning and/or an effect of sodium chloride on solubility of the organic component is important.

Highlights

  • Atmospheric aerosols contain numerous organic and inorganic compounds

  • No fit is shown for the mixture of oxalic acid dihydrate and ammonium sulphate, because the surface tension depression of these mixtures is negligible

  • The cloud condensation nuclei (CCN) ability of particles composed of different organic acid-inorganic salt mixtures was investigated by measuring the surface tension and water activity of droplet solutions and comparing Kohler theory to measured critical supersaturations

Read more

Summary

Introduction

Atmospheric aerosols contain numerous organic and inorganic compounds. The organic fraction has been estimated to account for 20% to 90% of the total fine aerosol mass (Kanakidou et al, 2005). Prisle et al, 2008) can be used to model cloud droplet formation and determine the critical supersaturaion of aerosol particles if parameters such as surface tension and water activity of the aqueous solution droplets are available. The presence of inorganic salts in the solution can enhance the surface tension depression of organic surfactants by forcing the organic compound to the solution-air interface Such a phenomenon was observed when cis-pinonic acid, fulvic acids or humic acids were dissolved in a 2 M ammonium sulphate aqueous solution (Shulman et al, 1996; Kiss et al, 2005). Water activity and surface tension have been determined experimentally for a series of aqueous solutions of mixtures of organic acids and inorganic salts and parameterized as a function of solute concentration. The parameterizations are used in Kohler theory to calculate critical supersaturation as a function of the dry particle diameter, and the predictions are compared to experimentally determined critical supersaturations

Theory
Experimental
Surface tension
Water activity
Critical supersaturations
Conclusions
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call