Effects of Volatility, Viscosity, and Non-ideality on Particle-Particle Mixing Timescales of SOA

MEREDITH SCHERVISH, Manabu Shiraiwa, University of California, Irvine

     Abstract Number: 55
     Working Group: Aerosol Chemistry

Abstract
Different populations of aerosol are constantly being mixed throughout the atmosphere, e.g. indoor-to-outdoor mixing, concentrated pollution plumes mixing with ambient aerosol, or simple transport of ambient aerosol from different sources. Large-scale models often represent fast mixing between different aerosol populations relative to other atmospheric processes, due to computational limitations, so that they rapidly form an internal mixture. Previous experiments aimed at probing particle-particle mixing timescales have shown that mixing between different SOA components can be rapid at high RH, but can be hours at low RH. The differences in mixing timescales can be attributed to different volatility distributions, diffusions limitations due to higher particle viscosity at low RH, or non-ideal miscibility. Here we apply the kinetic multilayer model of gas-particle partitioning (KM-GAP) to simulate mixing of two particle populations to probe the complex interplay of mass transfers kinetics and non-ideal thermodynamic partitioning. We simulate that population 1 contains a semi-volatile species that evaporates and re-partitions into population 2 by varying bulk diffusivity and activity coefficient as well as the pure saturation concentration of the semi-volatile species. We found that equilibration timescale is prolonged when the semi-volatile species is favorably miscible in population 2, as more mass needs to move to population 2 at equilibrium. Extremes of volatility prolong the equilibration timescale as low volatility species evaporate slowly from population 1, while high volatility species condense slowly to population 2. When population 1 is liquid, semi-volatile species equilibrate relatively fast for semi-solid population 2 particles, but not for solid particles. We apply the model to experimental mixing experiments to show consistency between measurements of SOA viscosity, SOA miscibility, and observed mixing. We show extremes of volatility, high viscosity, and favorable miscibility lead to long mixing timescales, while semi-volatile species mix rapidly in less viscous particles especially if they have low miscibility.