Octanol-air partition coefficient for describing particle/gas partitioning of aromatic compounds in urban air

Atmospheric concentrations and particle/gas partition coefficients were measured for PAHs, PCBs, and PCNs in urban Chicago during February/March 1995. Average (n = 15) concentrations (pg m(-3)) were Sigma PAH = 58 000, Sigma PCB = 350, and Sigma PCN = 68. The partitioning of these compounds between the particle phase and the gas phase was investigated and compared to the adsorption (Junge-Pankow) model, which utilizes the subcooled liquid vapor pressure (p(L)(o)) as a fitting parameter, and the K(OA) absorption model. The K(OA) model was able to resolve differences between ortho-chlorine substituted groups of PCBs which correlations against p(L)(o) were not able to explain. Partitioning of PCNs was also well described by the K(OA) model. For PAHs, sorption onto particles agreed with both models when the wind was predominantly from the SW sector. When air was sampled from the NE sector, PAHs showed enrichments in the particle phase relative to the chlorinated aromatics in log-log plots of the particle/gas partition coefficient, K(P) versus p(L)(o). Enrichment factors calculated as K(P (PAH))/K(P (multi-ortho PCB)) were 27-100 from the NE sector compared to 2-4 for the SW sector. This suggests source region effects on K(P) for PAHs. It is hypothesized that the enrichment may be due to nonexchangeable PAHs trapped inside combustion aerosols or a result of a slow re-equilibration of these aerosols as they are diluted in ambient air.