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Interactions with water are crucial for the properties, transformation andclimate effects of atmospheric aerosols. Here we present aconceptual framework for the interaction of amorphous aerosol particles withwater vapor, outlining characteristic features and differences incomparison to crystalline particles. We used ahygroscopicity tandem differential mobility analyzer (H-TDMA) tocharacterize the hydration and dehydration of crystallineammonium sulfate, amorphous oxalic acid and amorphouslevoglucosan particles (diameter ~100 nm, relativehumidity 5–95% at 298 K). The experimental data andaccompanying Köhler model calculations provide new insights intoparticle microstructure, surface adsorption, bulk absorption, phasetransitions and hygroscopic growth. The results of these and relatedinvestigations lead to the following conclusions:  (1) Many organic substances, including carboxylic acids,carbohydrates and proteins, tend to form amorphous rather thancrystalline phases upon drying of aqueous solution droplets. Depending onviscosity and microstructure, the amorphous phases can be classified asglasses, rubbers, gels or viscous liquids.  (2) Amorphous organic substances tend to absorb water vapor and undergogradual deliquescence and hygroscopic growth at lower relative humiditythan their crystalline counterparts.  (3) In the course of hydration and dehydration, certain organic substancescan form rubber- or gel-like structures (supramolecular networks) andundergo transitions between swollen and collapsed networkstructures.  (4) Organic gels or (semi-)solid amorphous shells (glassy, rubbery,ultra-viscous) with low molecular diffusivity can kinetically limit theuptake and release of water and  may influence the hygroscopic growth and activation ofaerosol particles as cloud condensation nuclei (CCN) and ice nuclei (IN).Moreover, (semi-)solid amorphous phases may influence the uptake ofgaseous photo-oxidants and the chemical transformation and aging ofatmospheric aerosols.  (5) The shape and porosity of amorphous and crystalline particles formed upondehydration of aqueous solution droplets depend on chemical composition anddrying conditions. The apparent volume void fractions of particles withhighly porous structures can range up to ~50% or more (xerogels,aerogels).  (6) For efficient description of water uptake and phase transitionsof aerosol particles, we propose not to limit the termsdeliquescence and efflorescence to equilibrium phase transitions ofcrystalline substances. Instead we propose generalized definitionsaccording to which amorphous and crystalline components can undergo gradualor prompt, partial or full deliquescence or efflorescence.  We suggest that (semi-)solid amorphous phases may be important notonly in the upper atmosphere as suggested in recent studies of glassformation at low temperatures. Depending on relative humidity, (semi-)solidphases and moisture-induced glass transitions may also play a role ingas-particle interactions at ambient temperatures in the lower atmosphere

atmospheric chemistry and physics

Amorphous and crystalline aerosol particles interacting with water vapor: conceptual framework and experimental evidence for restructuring, phase transitions and kinetic limitations