HTDMA analysis of multicomponent dicarboxylic acid aerosols with comparison to UNIFAC and ZSR

Abstract

The deliquescence and hygroscopic growth of laboratory‐generated aerosols containing inorganic salts and/or mixtures of 1–10 different multifunctional dicarboxylic acids were observed using an hygroscopic tandem differential mobility analyzer (HTDMA), and the results were compared to predictions from the Zdanovskii‐Stokes‐Robinson (ZSR) mixing rule and Universal Quasi‐Chemical Functional Group Activity Coefficient (UNIFAC) group contribution method. Dicarboxylic acids were chosen because of their atmospheric ubiquity. The results show that mixtures of five or more multifunctional dicarboxylic acids exhibit similar hygroscopic growth, to within experimental uncertainty, despite different chemistries. UNIFAC predictions of water uptake were observed to be in good agreement with the complex organic mixture data, while Modified UNIFAC overpredicted water uptake. Continuous growth curves for these mixtures indicate that particles are likely hydrated even at low RHs. Predictions were inconsistent for fewer‐component mixtures, and ZSR was used to model incomplete dissolution of some species to better approximate the HTDMA data. Individual mixture species do not necessarily contribute equally to overall water uptake and are discussed in detail. Finally, it was observed that the deliquescence point and water uptake were significantly depressed for mixtures of organics and an inorganic salt, relative to the pure inorganic salt. Predictions from the ZSR mixing rule based on experimental measurements were in good agreement with the data. These results suggest that the behavior of complex mixtures of dicarboxylic acids exhibit water uptake consistent with the total number of moles present in solution, but irrespective of individual component hygroscopicity. This simplification will aid in incorporating these compounds into parameterizations of aerosol growth.