Effects of volatility, viscosity, and non-ideality on particle-particle mixing timescales of secondary organic aerosols

Different populations of aerosol are constantly mixed throughout the atmosphere. Large-scale models often assume no particle-particle mixing or fast mixing among aerosol populations, so that they stay externally mixed or instantaneously form internal mixtures. We apply the kinetic multilayer model of gas-particle interactions (KM-GAP) to simulate the evaporation of semi-volatile species from one particle population and partitioning into another population with various phase states and nonideal mixing conditions. We find that the particle-particle mixing timescale (& tau;mix) is prolonged when the semi-volatile species transport to a population in which it is miscible, as more mass must be transported. Extremes of volatility prolong the & tau;mix, as low-volatility species evaporate slowly, while high-volatility species condense slowly. When the bulk diffusivities of the two populations are greater than 10-15 cm2 s-1, semi-volatile species mix rapidly; otherwise, the & tau;mix can be prolonged beyond 1 h. We apply KM-GAP to particle-particle mixing experiments of H-toluene SOA into D-toluene SOA and limonene SOA, showing that & tau;mix is prolonged when toluene SOA is highly viscous, while initial partitioning of gas phase semi-volatile species from toluene SOA into limonene SOA is rapid because of the low viscosity of limonene SOA. Simulations of mixing toluene SOA and & beta;-caryophyllene SOA indicate that the apparent discrepancy of limited mixing under conditions where both are predicted to have low viscosity are explained by limited miscibility of the semi-volatile components. Our study demonstrates that particle-particle mixing timescales are affected by a complex interplay among volatility, diffusion limitations, and non-ideal miscibility.