Investigation of thermodynamic properties of ambient and laboratory-generated multi- component organic aerosols

Ambient aerosol, a significant portion of which is composed of a complex mixture of semi-volatile organic compounds, has substantial impact on human welfare via adverse health effects and global climate change. Prediction of ambient semi-volatile organic aerosol remains highly problematic and air quality models often do not agree with observations. One of the pieces of knowledge needed for better predictions of ambient semi- volatile organic aerosol is understanding of the partitioning of semi-volatile compounds between the gas and the aerosol phases. In this study the equilibrium thermodynamic properties of a number of multi-component aerosols were investigated under controlled laboratory conditions, as well as in the field. In the laboratory experiments, model mixtures of organic compounds as well as modified ambient aerosols were tested. The ambient aerosols were modified in a controlled way by adding known amounts of different organic substances of known thermodynamic properties. The equilibrium gas / aerosol partitioning in a temperature range relevant to ambient conditions was investigated using the Integrated Volume Method (IVM). The field measurements of ambient aerosol equilibrium properties were carried out during June 2007 - January 2008 at the FACTS research facility in Duke Forest (Chapel Hill, NC). The results can be used to derive equilibrium vapor pressures and activity coefficients of test compounds and to verify and improve the parameterizations used in group contribution models, such as UNFAC.