Field and laboratory investigations of particle/gas distribution for polychlorinated biphenyls and other semivolatile organic compounds

Atmospheric deposition of polychlorinated biphenyls (PCBs) takes place mainly by precipitation scavenging and dry deposition of particulate species. Thus the distribution of PCBs between the particle and gas phases in ambient air is a key factor which controls the atmospheric input to lakes and coastal waters. Certain PCB congeners which have no ortho-substituted chlorines (coplanar PCBs) or only one ortho-chlorine produce dioxin-like toxicity and have been assigned 2,3,7,S-tetrachlorodibenzo-p-dioxin toxic equivalency factors. These PCBs have lower vapor pressures than multi-ortho PCBs within the same homolog group, and as a consequence may preferentially sorb to aerosols and be deposited. Two experimental techniques were used to test this hypothesis. The particle/gas distribution of PC3 congeners in Chicago air was operationally defined by sampling with a glass fiber filter followed by a polyurethane foam trap. 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 degrees) of mono-and non-ortho PCBs, although the non-ortho congeners 77 and 126 showed 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 (Kp) were inversely related to the vapor pressures of the congeners, but distinct differences were seen among the ortho-chlorine classes. For a given value of log P-L degrees log K-p increased in the order: multi-ortho < mono-ortho < non-ortho. A multiple linear regression model using log P-L degrees and the dihedral angle between the biphenyl rings explained 98% of the variance in sorption. Experimental estimates of the particle/gas distribution of polycyclic aromatic hydrocarbons (PAHs), PCBs, organochlorine pesticides, and polychlorinated dioxins and dibenzofurans (PCDDs, PCDFs) are often made with high-volume filter-sorbent samplers (hi-vol) such as the one used in this study. Particulate percentages reported in several field investigations with the hi-vol were compared to those predicted by the Junge-Pankow adsorption model. The agreement was quite good for PAHs in urban air. Particle-bound fractions measured in rural air were generally higher than model predictions, especially for compounds in the higher vapor pressure range. The hi-vol sampler underestimated (or the model overestimated) the aerosol fraction of organochlorine compounds. Differences in the behavior of PAHs and organochlorine compounds may be related to greater sampling artifacts for the latter (e.g., blow-off losses from the filter) or differences in sorption strengths between these classes of compounds.