Glass transition and phase state of organic compounds: dependency on molecular properties and implications for secondary organic aerosols in the atmosphere

Abstract

Recently, it has been proposed that organic aerosol particles in the atmosphere can exist in an amorphous semi-solid or solid (i.e. glassy) state. In this perspective, we analyse and discuss the formation and properties of amorphous semi-solids and glasses from organic liquids. Based on a systematic survey of a wide range of organic compounds, we present estimates for the glass forming properties of atmospheric secondary organic aerosol (SOA). In particular we investigate the dependence of the glass transition temperature T(g) upon various molecular properties such as the compounds' melting temperature, their molar mass, and their atomic oxygen-to-carbon ratios (O:C ratios). Also the effects of mixing different compounds and the effects of hygroscopic water uptake depending on ambient relative humidity are investigated. In addition to the effects of temperature, we suggest that molar mass and water content are much more important than the O:C ratio for characterizing whether an organic aerosol particle is in a liquid, semi-solid, or glassy state. Moreover, we show how the viscosity in liquid, semi-solid and glassy states affect the diffusivity of those molecules constituting the organic matrix as well as that of guest molecules such as water or oxidants, and we discuss the implications for atmospheric multi-phase processes. Finally, we assess the current state of knowledge and the level of scientific understanding, and we propose avenues for future studies to resolve existing uncertainties.

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

“…37 Viscosity of materials can be converted to bulk diffusivities using the Stokes-Einstein equation, which is a suitable approach for organic molecules in a liquid or semi-solid phase. 16,17 The bulk diffusivities are used as input parameters for the KM-GAP simulations. The particle mass concentration attains a new equilibrium more slowly at lower RH (Fig.…”

“…The bulk diffusivity of water in the electrolyte-rich ''inorganic'' phase is taken to be 10 À5 cm 2 s À1 , a value typical for water in low-viscosity liquids. 16,17 For the (a-b) structure the surface accommodation coefficient of water vapor on the electrolyte-rich phase is set to be unity, and for the (b-a) structure the surface accommodation coefficient on the organic-rich phase is assumed to be 0.1. 41 When the electrolyte-rich phase is the shell (a-b), t hg E 10 À5 s (Fig.…”

Evaluation of a Conceptual Model for Gas-Particle Partitioning of Polycyclic Aromatic Hydrocarbons Using Polyparameter Linear Free Energy Relationships

“…53 The minimum and maximum of these surface areas were also applied in order to explore the sensitivity of the model predictions. We acknowledge that the phases considered in the present study may not be fully available for PAH partitioning due to internal mixing and kinetic limitations related to OM phase-state 54 (slow diffusion in semisolid and solid phases) and that under high relative humidity (NH 4 ) 2 SO 4 and NH 4 Cl may be present in dissolved aqueous phase and subject to liquid−liquid phase separation with OM. 55 Moreover, as PM composition varies significantly with particle size, SOC sorption behavior is expected to be particle-size dependent.…”

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

“…81,82 The size-dependent data support the hypothesis that SOA particles from ozonolysis of trans-3-hexene are initially formed by nucleation of higher molecular weight oligomers (n Z 4) and then grow via condensation of shorter oligomers and other less oxygenated, more volatile species. 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.…”

“…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''.…”