Hygroscopic behavior of inorganic–organic aerosol systems including ammonium sulfate, dicarboxylic acids, and oligomer

Abstract The hygroscopic behavior of complex mixed organic–organic and organic–inorganic particles consisting of various dry mass ratios of ammonium sulfate (AS), oxalic acid (OA), malonic acid (MA) and polyethylene glycol-300 (PEG) has been studied using a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) setup. The measured growth factors (GFs) are compared to values predicted by the AIOMFAC-based thermodynamic equilibrium model and the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. The measured GFs for PEG–OA and PEG–MA systems are found to be significantly lower than those predicted by the two models, which assume the complete dissolution of the organic compounds. The observed behavior suggests that the presence of PEG substantially affects the solubility of organic acids and associated water uptake. For quinary systems containing PEG, organic acids, and AS, a complete deliquescence of the mixed particles is observed during hydration conditions at RH ∼78–80% close to the deliquescence point of pure AS particles. A clear disagreement was observed between measurements and predictions from AIOMFAC and ZSR for hydration conditions prior to the full particle deliquescence. We provide indirect evidence for the presence of an organic solid alongside solid AS. Hypothetically, the observed disagreement could also be due to a preferential interaction between –COOH and –CH2OCH2- groups, which may prevent a fraction of the organic acid amount to interact with water. For fully deliquesced particles, good agreement between model predictions and measurements are found for the mixed PEG–organic acids–AS systems. Upon dehydration, when the mass fraction of PEG