Abstract
Abstract. Volatility and hygroscopicity are two key properties of organic aerosol components, and both are strongly related to chemical identity. While the hygroscopicities of pure salts, di-carboxylic acids (DCA), and DCA salts are known, the hygroscopicity of internal mixtures of these components, as they are typically found in the atmosphere, has not been fully characterized. Here we show that inorganic–organic component interactions typically not considered in atmospheric models can lead to very strongly bound metal–organic complexes and greatly affect aerosol volatility and hygroscopicity; in particular, the bi-dentate binding of DCA to soluble inorganic ions. We have studied the volatility of pure, dry organic salt particles and the hygroscopicity of internal mixtures of oxalic acid (OxA, the dominant DCA in the atmosphere) and a number of salts, both mono- and di-valent. The formation of very low volatility organic salts was confirmed, with minimal evaporation of oxalate salt particles below 75 °C. Dramatic increases in the cloud condensation nuclei (CCN) activation diameter for particles with di-valent salts (e.g., CaCl2) and relatively small particle volume fractions of OxA indicate that standard volume additivity rules for hygroscopicity do not apply. Thus small organic compounds with high O : C ratios are capable of forming low-volatility and very low hygroscopicity particles. Given current knowledge of the formation mechanisms of OxA and M–Ox salts, surface enrichment of insoluble M–Ox salts is expected. The resulting formation of an insoluble coating of metal-oxalate salts can explain low-particle hygroscopicities. The formation of particles with a hard coating could offer an alternative explanation for observations of glass-like particles without the need for a phase transition.
Highlights
Pure salt particles are shown in solid curves while salt particles with Oxalic acid (OxA) deposited are shown with dashed curves
High-oxidation state, soluble organic species can strongly decrease hygroscopicity in particles dominated by di-valent salts
Our results are specific to OxA, but strong binding may occur for other di-carboxylic acids (DCA)
Summary
The ubiquity of di-carboxylic acids (DCA) is well documented both in urban and rural terrestrial aerosol and marine aerosol (Warneck, 2003; Wang et al, 2006; Wang and Kawamura, 2006; Tan et al, 2010; Sempéré and Kawamura, 1996; Pavuluri et al, 2010; Ma et al, 2013; Kundu et al, 2010; Kawamura et al, 1996, 2007; Gierlus et al, 2012; Altieri et al, 2008). A recent study of marine aerosol from around the globe found that DCA contribute on average ∼ 15 % to total marine organic aerosol (OA) mass (Fu et al, 2013; Myriokefalitakis et al, 2011). It was shown to contribute more than 50 % on average of the total mass of marine DCA, and mineral dust is often enriched in OxA (Sullivan et al, 2009; Fu et al, 2013). High oxidation states and the ability to strongly interact with inorganic compounds (especially crustal/marine salts) make the effects of DCA on aerosol composition and physical properties unique and complex. In this study we focus on DCA interactions with inorganic salts and their impacts on hygroscopicity and volatility
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