Gas/particle partitioning of neutral and ionizing compounds to single- and multi-phase aerosol particles. 2. Phase separation in liquid particulate matter containing both polar and low-polarity organic compounds

A modeling method is developed for use in predicting the stability of multiple liquid phases in atmospheric particulate matter (PM). The method utilizes a pseudo-diffusion process that simulates the multicomponent inter-phasic movement of constituents between adjacent PM phases. It can be used as a stand-alone application, and can also be incorporated in overall gas/particle (G/P) partitioning models of aerosol PM formation. Previously studied (Environ. Sci. Technol. 35 (2001) 1806) relative-humidity (RH)-dependent PM compositions resulting from the ozone oxidation of five different volatile organic compounds (VOCs) were verified as being stable as single phases. A number of additional cases were considered in which RH-dependent secondary PM compositions were amended with a mix of low-polarity (LP) organic compounds. It was determined that, depending on the mass fraction of added LP-mix, the PM can be more stable as a two-phase system. Phase separation into two phases can occur even in the absence of water in the PM. Assuming a single liquid PM phase when two are actually present will generally lead to an underprediction of TPM. For the cases considered here, the calculated errors in predicted TPM levels were found to range from −3.9% to −21.8%; conditions can be envisioned that would lead to even larger errors.