Preferential Sorption of Non- and Mono-ortho-polychlorinated Biphenyls to Urban Aerosols

Abstract

Two experimental techniques were used to test the hypothesis that mono- and non-ortho-substituted polychlorinated biphenyls (PCBs) are sorbed to urban aerosols to a greater extent than multi-ortho congeners of the same homolog. Particulate and gaseous PCBs in Chicago air were operationally defined by sampling with a glass fiber filter followed by a polyurethane foam trap. After separation from organochlorine pesticides, PCBs were fractionated into non-ortho, mono-ortho, and multi-ortho groups on an activated carbon-silicic acid column and determined by capillary gas chromatography (GC) with electron capture detection or GC/mass spectrometry. Within a homolog group (e.g., pentachlorobiphenyls), the percentage of particulate PCBs increased in the order : multi-ortho < mono-ortho < non-ortho. This order was explained for the most part by the lower liquid-phase vapor pressures (p° L ) of mono- and non-ortho-PCBs, although the non-ortho congeners 77 and 126 showed slightly enhanced sorption relative to expectations based on vapor pressure. To gauge the relative extent of sorption for PCBs having different numbers of ortho-chlorines, filters loaded with Chicago aerosols were exposed to gaseous PCBs at a constant temperature. Particle/gas partition coefficients (K p ) were inversely related to p° L , but distinct differences were seen among the ortho-chlorine classes. For a given value of log p° L , values of log K p increased in the order : multi-ortho < mono-ortho < non-ortho. A multiple linear regression model using log p° L and the dihedral angle between the biphenyl rings explained 98% of the variance in sorption.