Equilibration timescales of anthropogenic and biogenic secondary organic aerosols under dry and humid conditions -

Abstract

The partitioning of secondary organic aerosols (SOA) between the particle and gas phases depends on both their thermodynamic and kinetic properties. Reports of a highly viscous phase state of ambient SOA and the consequent retardation in particle mixing has led to the suggestion that evaporation and condensation may be so kinetically limited that SOA in the atmosphere could exist out of thermodynamic equilibrium. However, while previous work has investigated the effect of phase state on the evaporation kinetics, studies of the equilibration potential of SOA under ambient conditions is still lacking. In this work, SOA generated in an oxidation flow reactor (OFR) from α-pinene and gasoline engine exhaust are diluted in a smog chamber to reach ambient concentrations. The evolution of the particle size distribution is monitored and fit to an evaporation model, allowing the estimation of the volatility and the effective evaporation coefficient. In order to investigate the effect of viscosity on evaporation, the experiments are conducted under low and high relative humidity. The volatility of the SOA exiting the OFR was found to range between 0.2 and 1.2 µgm-3, which is similar to the volatility of OA in both urban and rural environments. After dilution in the chamber, the SOA reestablished equilibrium with the vapor phase on timescales on the order of tens of minutes, exhibiting an evaporation coefficient between 0.06 and 0.24. The SOA generated under higher humidity did not exhibit faster evaporation rates, suggesting that, under ambient concentrations, lower diffusion coefficients do not significantly inhibit equilibration. In either case, the equilibrium was restored on timescales shorter than those of other atmospheric processes, suggesting that the assumption that SOA exist under thermodynamic equilibrium in the atmosphere holds.