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

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 Correspondence to: U. Pöschl (u.poschl@mpic.de) (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 intera...

Gas–particle partitioning of atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology

“…However, a value of t hg of this order can impact results inferred from aerosol instruments with residence times of seconds, such as the hygroscopicity tandem differential mobility analyzer (HTDMA) 15,32 and cloud condensation nuclei counter (CCNC). 42 D H 2 O is estimated to decrease to values as low as 10 À20 cm 2 s À1 at temperatures characteristic of the middle to upper troposphere, resulting in kinetic limitations of water mass transport with t hg E hours to days.…”

“…[14][15][16][17] Aqueous solutions of a number of organic substances tend to form semi-solid or amorphous solid (glassy), rather than crystalline, phases as humidity decreases. 15 Ambient particles in boreal forests as well as laboratory-generated particles, consisting predominantly of SOA, have been observed to bounce off the smooth hard surface of an inertial impactor, implying a non-liquid state. 18,19 Upon dilution or heating, SOA particles can evaporate unexpectedly slowly, as a result of retarded mass transfer from the particle bulk to the surface, characteristic of semi-solid behavior.…”

“…7,9 The interfacial tensions, in turn, depend on the compositions of the phases; therefore, the particle morphology changes in response to substantial variations in RH. 9,10 In the case of core-shell structure with a semi-solid or glassy organic shell phase, mass transfer of water and organic compounds can be kinetically limited; 15,25 implications of this situation are discussed below. Fig.…”

Measuring Mass-Based Hygroscopicity of Atmospheric Particles through in Situ Imaging

“…Mikhailov et al suggested that this scenario could lead to higher observed g m values at 300 high RH since the observed initial particle mass will be lower than the theoretical one due to 301 water adsorption being surface limited. 42 Tables S5-S7. 319 The size-based growth factors of the atmospheric particles ( Figure S2 and Table S9, 320 Supporting Information) yield very different growth curves.…”

“…42 Higher 349 values of κ correspond to higher hygroscopicity and for atmospherically relevant particles κ 350 typically ranges from 0 to values exceeding 1. However, instead of a ratio of volumes mass-351 based growth factors can also be used to calculate κ by assuming particle density.…”