Evaporation kinetics of secondary organic aerosol formed from gasoline engine emissions -

Abstract

The discrepancy between measured and modeled concentrations of secondary organic aerosols (SOA) in the atmosphere is triggering studies to challenge the adopted theories of partitioning between the gas and the condensed phase of aerosols. Absorptive partitioning theory, a traditionally adopted model for SOA concentration estimation, predicts rapid reversible partitioning of particles in the atmosphere due to evaporation; however reports have been emerging on the limitations of the applicability of this theory, citing low evaporation coefficients due to the amorphous state of SOA. In our approach we studied the evaporation kinetics of SOA produced from the photochemical oxidation of gasoline engine emissions. We evaluated the equilibration time and calculated the evaporation coefficient using thermodynamic data calculated in our experiments rather than using previously published data. SOA particles were produced in a flow reactor and isothermally diluted in a smog chamber; the particles were allowed to evaporate for a period of time starting at atmospherically relevant concentrations. In the evaporation model, we used mass transfer equations to describe diffusion of molecules between the surface and the interface at the transition flow regime using the Fuch-Sutugin factor. We also accounted for losses due to particle deposition on the walls of the smog chamber. To fit the recorded results from the chamber experiment, we represented the size distribution of SOA using the concept of the condensation sink diameter. Based on the experiment results we were able to observe equilibrium nearly after an hour, and calculated an average value of the evaporation coefficient of 0.06. We can report that SOA from anthropogenic sources did not exhibit hindered evaporation rates and reached equilibrium within the timescales of the experiment.