Evidence has accumulated that secondary organic aerosols (SOA) exhibit complex morphologies with multiple phases that can adopt semisolid or glassy phase states. However, experimental analysis and numerical modeling on formation and evolution of SOA still often employ equilibrium partitioning with an ideal mixing assumption in the particle phase. Here we apply the kinetic multilayer model of gas-particle partitioning (KM-GAP) to simulate condensation of semi-volatile species into a core-shell phase-separated particle to evaluate equilibration timescales of SOA partitioning. By varying bulk diffusivity and activity coefficient of the condensing species in the shell, we probe the complex interplay of mass transfer kinetics and thermodynamics of partitioning. The effect of non-ideality on SOA partitioning is slight for liquid particles, but becomes prominent in semi-solid or solid particles. If the condensing species is miscible with the viscous shell phase, the particle can reach equilibrium with the gas phase long before the dissolution of concentration gradients in the particle bulk. For the condensation of immiscible species with he semisolid shell, the mass concentration in the shell may overshoot its equilibrium concentration due to slow bulk diffusion through the viscous shell to the core phase. As the volatility increases, re-evaporation becomes significant as desorption is faster for volatile species than bulk diffusion in semisolid matrix, leading to an increase in equilibration timescale. We also show that equilibration timescale is longer in an open system relative to a closed system especially for partitioning of miscible species; hence, caution should be taken when interpreting and extrapolating closed system chamber experimental results to atmosphere conditions. Our results may reconcile apparent discrepancies between experimental observations of fast particle-particle mixing and predictions of long mixing timescales in viscous particles and provide useful insights into description and treatment of SOA in aerosol models.

Impact of phase state and non-ideal mixing on equilibration timescales of secondary organic aerosol partitioning

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