Gas-particle partitioning of semivolatile organic compounds (SOCs) on mixtures of aerosols in a smog chamber

The partitioning behavior of a set of diverse SOCs on two and three component mixtures of aerosols from different sources was studied using smog chamber experimental data. A set of SOCs of different compound types was introduced into a system containing a mixture of aerosols from two or more sources. Gas and particle samples were taken using a filter-filter-denuder sampling system, and a partitioning coefficient K-p was estimated using K-p = C-p/(CgTSP). Particle size distributions were measured using a differential mobility analyzer and a light scattering detector. Gas and particle samples were analyzed using GCMS. The aerosol composition in the chamber was tracked chemically using a combination of signature compounds and the organic matter mass fraction (f(om)) of the individual aerosol sources. The physical nature of the aerosol mixture in the chamber was determined using particle size distributions, and an aggregate K-p was estimated from theoretically calculated K-p on the individual sources. Model fits for K-p showed that when the mixture involved primary sources of aerosol, the aggregate K-p of the mixture could be successfully modeled as an external mixture of the K-p on the individual aerosols. There were significant differences observed for some SOCs between modeling the system as an external and as an internal mixture. However, when one of the aerosol sources was secondary, the aggregate model K-p required incorporation of the secondary aerosol products on the preexisting aerosol for adequate model fits. Modeling such a system as an external mixture grossly overpredicted the K-p of alkanes in the mixture. Indirect evidence of heterogeneous, acid-catalyzed reactions in the particle phase was also seen, leading to a significant increase in the polarity of the resulting aerosol mix and a resulting decrease in the observed K-p of alkanes in the chamber. The model was partly consistent with this decrease but could not completely explain the reduction in K-p because of insufficient knowledge of the secondary organic aerosol composition.