A Thermodynamic Approach Using Group Contribution Methods to Model the Partitioning of Semivolatile Organic Compounds on Atmospheric Particulate Matter

Atmospheric particulate matter is a complex mixture consisting of organic and inorganic chemicals. Their sources include various combustion processes, aerosolized dusts and soils, and chemical reactions which produce secondary aerosols. The partitioning of semivolatile toxic organic compounds (SOCs) between particulate matter and the gas phase is strongly influenced by temperature, water concentration, chemical composition of the particulate matter, and the organic fraction of the particulate matter. Many investigations have recently suggested that a considerable portion of the gas−particle (G/P) partitioning in the ambient atmosphere takes place between the liquid phase of organic aerosols and the surrounding gas phase. It has been shown that the equilibrium G/P partitioning constant, K p, of an SOC partitioning to a given particle's liquid medium is inversely related to both the activity coefficient i γ om and its saturated subcooled liquid vapor pressure, . Hence, in principal, the K p of any SOC can be estimated from its vapor pressure and activity coefficient in a given liquid mixture. To calculate activity coefficients of SOCs in the liquid phase of different types of particles, semiempirical thermodynamic models based on additive chemical functional group methods were used. Outdoor chambers were used to generate G/P partitioning data sets for a range of SOCs in the presence of particles from wood and diesel combustion and secondary aerosols from the reaction of α-pinene with ozone. The partitioning SOCs ranged from nonpolar alkanes to polar organic acids. Plots of log ( i γ om K p ) vs log showed a vast improvement over typical log K p vs log plots. These results suggest that equilibrium partitioning of many different types of SOCs can be estimated in almost any organic layer of an atmospheric aerosol.