Comparing the mechanism of water condensation and evaporation in glassy aerosol

Abstract

Atmospheric models generally assume that aerosol particles are in equilibrium with the surrounding gas phase. However, recent observations that secondary organic aerosols can exist in a glassy state have highlighted the need to more fully understand the kinetic limitations that may control water partitioning in ambient particles. Here, we explore the influence of slow water diffusion in the condensed aerosol phase on the rates of both condensation and evaporation, demonstrating that significant inhibition in mass transfer occurs for ultraviscous aerosol, not just for glassy aerosol. Using coarse mode (3-4 um radius) ternary sucrose/sodium chloride/aqueous droplets as a proxy for multicomponent ambient aerosol, we demonstrate that the timescale for particle equilibration correlates with bulk viscosity and can be ≫10 3 s. Extrapolation of these timescales to particle sizes in the accumulation mode (e.g., approximately 100 nm) by applying the Stokes-Einstein equation suggests that the kinetic limitations imposed on mass transfer of water by slow bulk phase diffusion must be more fully investigated for atmospheric aerosol. Measurements have been made on particles covering a range in dynamic viscosity from <0.1 to >10 13 Pa s. We also retrieve the radial inhomogeneities apparent in particle composition during condensation and evaporation and contrast the dynamics of slow dissolution of a viscous core into a labile shell during condensation with the slow percolation of water during evaporation through a more homogeneous viscous particle bulk.water uptake | whispering gallery modes | Raman spectroscopy | optical tweezers | viscous aerosol A tmospheric aerosol particles are typically complex mixtures of organic and inorganic species with correspondingly complex equilibria and temporal responses to changes in humidity. Secondary organic aerosols (SOA) continue to receive a great deal of attention due to their impact on radiative forcing, mainly through the indirect effect (1). Ambient aerosol typically contains a significant organic fraction, arising from the oxidation of volatile organic compounds (2). SOA has been largely thought of as existing as a liquid phase, or as a combination of a solid phase within a liquid droplet, but the reality is likely to be far more nuanced. Recently, Virtanen et al. demonstrated that ambient SOA particles can have similar mechanical properties to crystalline ðNH 4 Þ 2 SO 4 particles at 10-20% RH but exist as amorphous glasses to low relative humidity (RH) rather than forming crystalline phases (3). This picture is consistent with the conclusions of Mikhailov et al. (4) and Zobrist et al. (5), who suggested that the existence of glassy states may have profound consequences for the properties of atmospheric aerosol particles, particularly at low temperatures.The term glassy refers to an amorphous, highly viscous state with a dynamic viscosity (η) of greater than 1 × 10 12 Pa s and the mechanical properties of a solid (6). In thermodynamic terms, a glass is in a nonergodic metastable state...

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

“…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. 40,43 As a consequence, glassy organicinorganic aerosol particles (not fully equilibrated with the ambient RH) may act preferentially as heterogeneous ice nuclei, rather than as cloud condensation nuclei forming liquid water droplets. 44,45 …”

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…12,30,31,38,39 Phase state of SOA The phase of atmospheric particles (liquid/solid) plays a crucial role in various physical and chemical processes of particles such as growth, 49,50,52 aging, 47,48,52,53 and water uptake. 48,51 In the present study, particle impaction/ATR-FTIR measurements were performed to offer some insight into the phase/viscosity of trans-3-hexene SOA since the impaction patterns on the Ge crystal have been shown to be an indicator of phase. 54 For example, viscous liquid particles hit and stick to the crystal to form a row of spots, whereas solid particles above a certain size bounce and can be subsequently captured on the crystal, forming a disperse pattern dubbed ''clouds''.…”

“…We report here an expanded investigation of the ozonolysis of trans-3-hexene in which particle composition is probed as a function of size. In addition, the influence of water vapor, OH and SCI scavengers, as well as the phase state of SOA particles (which plays critical roles in various physical and chemical processes of aerosols) [47][48][49][50][51][52][53][54][55][56] are elucidated and possible mechanisms are examined in light of predictions of a box model for this system. Initial studies of the ozonolysis of the much larger and complex biogenic alkene a-cedrene are also reported, and it is shown that the mechanism of particle formation for this sesquiterpene is different from that for trans-3-hexene.…”

Oxidation of a model alkane aerosol by OH radical: the emergent nature of reactive uptake

“…In this context, well-mixed means behavior consistent with complete compositional uniformity. Recent studies [3][4][5][6] suggest that some organic aerosols exist in a glassy or semisolid phase, which can significantly affect the rates and mechanisms of the uptake of water 7,8 and other gas phase species as well as heterogeneous oxidation chemistry. 1 Laboratory studies of model systems are essential to unravel these factors and provide a rigorous foundation for climate model parameters.…”