GAS-PARTICLE PARTITIONING OF PCBS AND PAHS IN THE CHICAGO URBAN AND ADJACENT COASTAL ATMOSPHERE - STATES OF EQUILIBRIUM

Simultaneous air samples were taken in Chicago and over southern Lake Michigan as part of the AEOLOS Project (Atmospheric Exchange Over Lake s and Oceans). Gas and particle phase concentrations of polychlorinate d biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and total suspended particles (TSP) were measured over 12 h periods during July , 1994, and January, 1995. Partitioning of PCBs and PAHs between gas a nd particle phases was well correlated with the subcooled liquid vapor pressure p(L)(o) for individual samples, but the relationship differe d among samples. For all but a few of the samples the slopes of the lo g K-p vs log p(L)(o) lines were statistically greater than -1. Other i nvestigators who have found similar results have concluded that the PC Bs/PAHs were not at equilibrium; however, other factors indicate that the PCBs and PAHs in the Chicago/Lake Michigan atmosphere are at equil ibrium. Slopes of the regressions of log K-p vs log p(L)(o) from sampl es of continental background origin, and therefore assumed to have had sufficient atmospheric residence times to reach equilibrium, are amon g the shallowest measured (-0.70 to -0.53 and -0.16 to -0.56 for PAHs and PCBs, respectively). One pair of samples where the air mass is bel ieved to have been sampled twice, once in the urban area and again sim ilar to 3.4 h downwind, shows no difference in partitioning. PCBs and PAHs measured in Chicago and over Lake Michigan were apparently at equ ilibrium between the gas and particle phases. A slope of -1 in the reg ression of log K-p vs log p(L)(o) is not necessary to describe equilib rium partitioning. Differences in particulate matter may be responsibl e for the shallow slopes observed.