Insights into the role of dicarboxylic acid on CCN activity: implications for surface tension and phase state effects

Abstract. Dicarboxylic acids are ubiquitous in atmospheric aerosol particles, but their roles as surfactants in cloud condensation nuclei (CCN) activity remain unclear. In this study, we investigated CCN activity of inorganic salt (sodium chloride and ammonium sulfate) and dicarboxylic acid (including malonic acid (MA), phenylmalonic acid (PhMA), succinic acid (SA), phenylsuccinic acid (PhSA), adipic acid (AA), pimelic acid (PA) and octanedioic acid (OA)) mixed particles with varied organic volume fraction (OVF), and then directly determined their surface tension and phase state at high relative humidity (over 99.5 %) by atomic force microscopy (AFM). Our results showed that CCN derived κCCN of studied dicarboxylic acids ranged in 0.003–0.240. A linearly positive relation between κCCN and solubility was obtained for slightly dissolved species, while negative relation was found between κCCN and molecular volume for highly soluble species. For most inorganic salt/dicarboxylic acid (MA, PhMA, SA, PhSA and PA), a good closure within 30 % relative bias between κCCN and chemistry derived κChem were obtained. However, κCCN values of inorganic salt/AA and inorganic salt/OA systems were surprisingly 0.3–3.0 times higher than κChem, which was attributed to surface tension reduction as AFM results showed that their surface tensions were 20 %–42 % lower than that of water (72 mN m-1). Meanwhile, semisolid phase states were obtained for inorganic salt/AA and inorganic salt/OA and may also affect hygroscopicity closure results. Our study highlights that surface tension reduction should be considered to investigate aerosol-cloud interactions.