Prediction of activity coefficients in liquid aerosol particles containing organic compounds, dissolved inorganic salts, and water—Part 2: Consideration of phase separation effects by an X-UNIFAC model

A thermodynamic model is presented for predicting the formation of particulate matter (PM) within an aerosol that contains organic compounds, inorganic salts, and water. Neutral components are allowed to partition from the gas phase to the PM, with the latter potentially composed of both a primarily aqueous (α) liquid phase and a primarily organic (β) liquid phase. Partitioning is allowed to occur without any artificial restraints: when both α and β PM phases are present, ionic constituents are allowed to partition to both. X-UNIFAC.2, an extended UNIFAC method based on Yan et al. (1999. Prediction of vapor–liquid equilibria in mixed-solvent electrolyte systems using the group contribution concept. Fluid Phase Equilibria 162, 97–113), was developed for activity coefficient estimation. X-UNIFAC.2 utilizes the standard UNIFAC terms, a Debye–Hückel term, and a virial equation term that represents the middle-range (MR) contribution to activity coefficient effects. A large number (234) of MR parameters are already available from Yan et al. (1999). Six additional MR parameters were optimized here to enable X-UNIFAC.2 to account for interactions between the carboxylic acid group and Na+, Cl−, and Ca2+. Predictions of PM formation were made for a hypothetical sabinene/O3 system with varying amounts of NaCl in the PM. Predictions were also made for the chamber experiments with α-pinene/O3 (and CaCl2 seed) carried out by Cocker et al. (2001. The effect of water on gas-particle partitioning of secondary organic aerosol. Part I. α-pinene/ozone system. Atmospheric Environment 35, 6049–6072); good agreement between the predicted and chamber-measured PM mass concentrations was achieved.