Joint effect of organic acids and inorganic salts on cloud droplet activation

摘要

We have investigated CCN properties of internally mixed particles composedof one organic acid (oxalic acid dihydrate, succinic acid, adipic acid,citric acid, cis-pinonic acid, or Nordic reference fulvic acid) and oneinorganic salt (sodium chloride or ammonium sulphate). Surface tension andwater activity of aqueous model solutions with concentrations relevant forCCN activation were measured using a tensiometer and osmometry,respectively. The measurements were used to calculate K?hler curves andcritical supersaturations, which were compared to measured criticalsupersaturations of particles with the same chemical compositions,determined with a cloud condensation nucleus counter. Surfactant surfacepartitioning was not accounted for. For the aqueous solutions containingcis-pinonic acid and fulvic acid, a depression of surface tension wasobserved, but for the remaining solutions the effect on surface tension wasnegligible at concentrations relevant for cloud droplet activation. Thesurface tension depression of aqueous solutions containing both organic acidand inorganic salt was approximately the same as or smaller than that ofaqueous solutions containing the same mass of the corresponding pure organicacids. Water activity was found to be highly dependent on the type andamount of inorganic salt. Sodium chloride was able to decrease wateractivity more than ammonium sulphate and both inorganic salts are predictedto have a smaller Raoult term than the studied organic acids. Increasing themass ratio of the inorganic salt led to a decrease in water activity. Wateractivity measurements were compared to results from the E-AIM model andvalues estimated from both constant and variable van't Hoff factors. Thecorrespondence between measurements and estimates was overall good, exceptfor highly concentrated solutions. Critical supersaturations calculated withK?hler theory based on measured water activity and surface tension, butnot accounting for surface partitioning, compared well with measurements,except for the solutions containing sodium chloride and oxalic acid or oneof the more surface active organic compounds. In such cases, significantlylower values were obtained from K?hler theory than the measured criticalsupersaturations with deviations above 50% for a 60 nm particlecontaining 50% (dry mass) of Nordic reference fulvic acid, suggestingthat surfactant partitioning and/or an effect of sodium chloride onsolubility of the organic component is important.