Abstract
Abstract. Interactions with water are crucial for the properties, transformation and climate effects of atmospheric aerosols. Here we present a conceptual framework for the interaction of amorphous aerosol particles with water vapor, outlining characteristic features and differences in comparison to crystalline particles. We used a hygroscopicity tandem differential mobility analyzer (H-TDMA) to characterize the hydration and dehydration of crystalline ammonium sulfate, amorphous oxalic acid and amorphous levoglucosan particles (diameter ~100 nm, relative humidity 5–95% at 298 K). The experimental data and accompanying Köhler model calculations provide new insights into particle microstructure, surface adsorption, bulk absorption, phase transitions and hygroscopic growth. The results of these and related investigations lead to the following conclusions: (1) Many organic substances, including carboxylic acids, carbohydrates and proteins, tend to form amorphous rather than crystalline phases upon drying of aqueous solution droplets. Depending on viscosity and microstructure, the amorphous phases can be classified as glasses, rubbers, gels or viscous liquids. (2) Amorphous organic substances tend to absorb water vapor and undergo gradual deliquescence and hygroscopic growth at lower relative humidity than their crystalline counterparts. (3) In the course of hydration and dehydration, certain organic substances can form rubber- or gel-like structures (supramolecular networks) and undergo transitions between swollen and collapsed network structures. (4) Organic gels or (semi-)solid amorphous shells (glassy, rubbery, ultra-viscous) with low molecular diffusivity can kinetically limit the uptake and release of water and may influence the hygroscopic growth and activation of aerosol particles as cloud condensation nuclei (CCN) and ice nuclei (IN). Moreover, (semi-)solid amorphous phases may influence the uptake of gaseous photo-oxidants and the chemical transformation and aging of atmospheric aerosols. (5) The shape and porosity of amorphous and crystalline particles formed upon dehydration of aqueous solution droplets depend on chemical composition and drying conditions. The apparent volume void fractions of particles with highly porous structures can range up to ~50% or more (xerogels, aerogels). (6) For efficient description of water uptake and phase transitions of aerosol particles, we propose not to limit the terms deliquescence and efflorescence to equilibrium phase transitions of crystalline substances. Instead we propose generalized definitions according to which amorphous and crystalline components can undergo gradual or prompt, partial or full deliquescence or efflorescence. We suggest that (semi-)solid amorphous phases may be important not only in the upper atmosphere as suggested in recent studies of glass formation at low temperatures. Depending on relative humidity, (semi-)solid phases and moisture-induced glass transitions may also play a role in gas-particle interactions at ambient temperatures in the lower atmosphere.
Highlights
For efficient description of water uptake and phase transitions of amorphous and crystalline organic and inorganic aerosol particles and particle components, we propose not to limit the terms deliquescence and efflorescence to equilibrium phase transitions of crystalline substances interacting with water vapor
The efflorescence transition resulting in the formation of salt crystals and the evaporation of liquid water was observed at ∼30% relative humidity (RH) in good agreement with literature data of RHe for submicron ammonium sulfate particles (Table 2)
In this study we have presented a conceptual framework and investigated the interaction of amorphous and crystalline aerosol particles with water vapor by hygroscopicity tandem differential mobility analyzer (H-TDMA) experiments and Kohler model calculations
Summary
The interaction of aerosol particles with water vapor is a central issue of research in atmospheric and climate science (Martin, 2000; Kanakidou et al, 2005; Lohmann and Feichter, 2005; Poschl, 2005; McFiggans et al, 2006; Fuzzi et al, 2006; Andreae and Rosenfeld, 2008; Rosenfeld et al, 2008; Wang et al, 2008a, b; Heintzenberg and Charlson, 2009; and references therein). Aerosol particles composed of certain watersoluble inorganic salts can reversibly take up water vapor without the prompt deliquescence and efflorescence phase transitions and hysteresis effects typically observed for the corresponding crystalline bulk material: CaCl2 and MnSO4 (Cohen at al., 1987); NH4NO3 (Tang, 1980; Dougle et al, 1998; Cziczo and Abbatt, 2000; Lee and Hsu, 2000; Mikhailov et al, 2004; Schlenker et al, 2004; Schlenker and Martin, 2005; Svenningson et al, 2006), NH4HSO4 (Tang and Munkelwitz, 1994; Cziczo et al, 1997; Cziczo and Abbatt, 2000; Lee and Hsu, 2000; Martin et al, 2003; Colberg et al, 2003, 2004; Schlenker et al, 2004; Schlenker and Martin, 2005), NaNO3 (McInnes et al, 1996; Lee and Hsu, 2000, Gysel et al, 2002; Hoffmann et al, 2004), Ca(NO3) (Liu et al, 2008), and sea salt (Tang et al, 1997).
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