Gas--particle partitioning of organic compounds and its interpretation using relative solubilities

A review is presented of the current understanding of the partitioning phenomena of relatively low vapor pressure organic chemicals between the gas phase and atmospheric particulates. The phenomena have been interpreted as adsorption, absorption, or a combination of both and corresponding theoretical equations suggested to quantify partitioning. Empirical correlations involving the chemical's vapor pressure and octanol--air partition coefficient are discussed. It is suggested that insights into selecting preferred correlations and into the nature of the partitioning phenomena can be enhanced by treating the partition coefficient as a ratio of a solubility or pseudo-solubility of the chemical in the aerosol particle to that in air. Such particle solubilities when calculated for PCBs, alkanes, and PAHs show remarkable constancy and are generally consistent with near-ideal absorption into organic matter. An exception occurs when the PAH is generated simultaneously with the aerosol and unusually high solubilities are observed, indicative of adsorption to active carbon surfaces. Recommendations are made for interpreting experimental partitioning data and for the use of correlations for predictive purposes.