Determination of the evaporation coefficient of lubricating oil aerosols -

Abstract

Unburned engine lubricating oil (LO) accounts for a major fraction of the primary organic aerosol (POA) emissions of internal combustion engines. Because LO spans a wide volatility range, its gas-particle partitioning behavior is of interest both for determining engine emission factors and for estimating gas precursor inventories when predicting secondary aerosol formation in the atmosphere. While some investigators have used chambers to study POA under thermodynamic equilibrium conditions, using thermodenuders to study aerosol volatility under non-equilibrium conditions is of practical importance. In these circumstances, data must be fitted to kinetic models of POA evaporation. Models in turn require knowledge of the evaporation coefficient, for which there are currently no reliable estimates. In this study we sought to determine the effective evaporation coefficient of LO aerosols over a range of particle concentrations (25-250 μg-m3) relevant to atmospheric pollution. The approach involved using isothermal dilution to perturb an aerosol system from its initial equilibrium state, and then tracking particle volume versus time as the aerosol returned to phase equilibrium, as it flowed through an inert flow tube. The resulting normalized volume change versus time curve was fitted for evaporation coefficient using a numerical model. The studied aerosols were generated by condensing vapors produced by bubbling nitrogen through a 150°C column of LO, and total particle volume was simultaneously measured by two electrical mobility spectrometers (TSI SMPS 3936 equivalent) upstream and at various axial distances along the flow tube. The aerosols were also analyzed by GC-MS. We found the observed phase equilibration kinetics were well described by effective evaporation coefficients approaching unity. We also found that the effective vapor pressure of the LO aerosol was well correlated to particle concentration, with higher concentrations yielding higher effective vapor pressures, as predicted by equilibrium