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

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 glycol300 (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 similar to 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 <20%, the signature of effloresced AS in solid-liquid equilibrium with the remaining solution was observed. However, with higher organic volume fraction, the particles release water gradually without a noticeable efflorescence of AS down to 20% RH. For quinary PEG-organic acids-AS systems, the AIOMFAC-based equilibrium model predicts that liquid-liquid phase separation (LLPS) occurs; with a clear distinction between a predominantly electrolyte-rich phase alpha (composed mainly of ammonium and sulfate ions, organic acids and water) and an organic-rich phase beta (composed mainly of PEG). The onset of LLPS is predicted at RH levels of 83-89% depending on the mixed particle's composition. We also show that a residence time of similar to 10 s in the humidified section of the HTDMA instrument is sufficient for establishing gas-particle equilibrium of the 100 nm sized organic-inorganic particles studied in this work; this may differ in other cases when highly viscous particles are involved. The measurements offer valuable data for future work on the development and validation of organic solid-liquid equilibrium in thermodynamic models.