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 orga nic and inorganic chemicals. Their sources include various combustion processes, aerosolized dusts and soils, and chemical reactions which p roduce secondary aerosols. The partitioning of semivolatile toxic orga nic compounds (SOCs) between particulate matter and the gas phase is s trongly influenced by temperature. water concentration, chemical compo sition of the particulate matter, and the organic fraction of the part iculate matter. Many investigations have recently suggested that a con siderable portion of the gas-particle (G/P) partitioning in the ambien t 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 part icle's liquid medium is inversely related to both the activity coeffic ient (i) gamma(om) and its saturated subcooled liquid vapor pressure, p(L)(o). 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 di fferent types of particles, semiempirical thermodynamic models based o n additive chemical functional group methods were used, Outdoor chambe rs were used to generate G/P partitioning data sets for a range of SOC s in the presence of particles from wood and diesel combustion and sec ondary aerosols from the reaction of alpha-pinene with ozone. The part itioning SOCs ranged from nonpolar alkanes to polar organic acids. Plo ts of log ((i) gamma(om)K(p)) vs log p(L)(o) showed a vast improvement over typical log K-p vs log p(L)(o) plots. These results suggest that equilibrium partitioning of many different types of SOCs can be estim ated in almost any organic layer of an atmospheric aerosol.