Gas/particle partitioning of neutral and ionizing compounds to single and multi-phase aerosol particles. 1. Unified modeling framework

A single framework is developed for describing the equilibrium gas/particle partitioning of organic and inorganic constituents wherein all partitioning molecules are handled in the same manner, and multiple phases may be present in the particulate matter (PM). The model assumes an equilibrium distribution of all constituents between the gas phase and all of the condensed PM phases. The partitioning species may include neutral non-ionizing compounds (e.g., alkanes) as well as acids, bases, and metal ions. Acid-related species are assumed to be volatile only when present in the neutral “free-acid” form. Base-related species are assumed to be volatile only when present in the neutral “free-base” form. Metal ions are assumed to be non-volatile. The behavior of systems involving multiple liquid and solid phases are discussed using analogs of equations that have been developed, tested, and verified in prior efforts. Schemes are discussed for seeking the overall equilibrium composition of a gas and multi-phasic PM system. The framework developed thus provides a basis for solving a wide variety of complex partitioning problems. Because effective algorithms exist for predicting liquid-phase activity coefficients for neutral molecules as a function of composition and temperature, the framework can currently be used to solve a wide variety of gas/particle partitioning problems that involve non-ionizing constituents in a gas and multi-phasic PM system. Application of the framework to more general problems involving both ionic constituents and neutral molecules in liquid PM will become possible when reliable algorithms are developed for predicting activity coefficients of ions and neutral molecules in a general liquid mixture of such species.