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 (τmix) 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 τmix, 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 τmix 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 τmix 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 β-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.