Reconsideration of surface tension and phase state effects on cloud condensation nuclei activity based on the atomic force microscopy measurement

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 fractions
(OVFs), and then directly determined their surface tension and phase state at
high relative humidity (over 99.5 %) via atomic force microscopy (AFM). Our
results show that CCN-derived κCCN of studied dicarboxylic
acids ranged from 0.003 to 0.240. A linearly positive correlation between
κCCN and solubility was obtained for slightly dissolved
species, while negative correlation was found between κCCN and
molecular volume for highly soluble species. For most inorganic
salts and dicarboxylic acids (MA, PhMA, SA, PhSA and PA), a good closure within
30 % relative bias between κCCN and chemistry-derived
κChem was 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 also affected hygroscopicity closure results. Our study
highlights that surface tension reduction should be considered when
investigating aerosol–cloud interactions.
