Air concentrations of PCBs and PAHs were measured in the urban/industrial complex of Chicago, IL, over southern Lake Michigan, and in a non-urban area as part of the AEOLOS (Atmospheric Exchange Over Lakes and Oceans) Project. Air samples were collected simultaneously in intensive experiments during May and July 1994 and January 1995 in order to determine the influence of contaminated air masses from the Chicago urban/industrial complex on the southern Lake Michigan atmosphere. Gas phase ∑PAH concentrations over the lake ∼10-20 km offshore ranged from 0.8 to 70 ng/m 3 while urban air concentrations were 27-430 ng/m 3 . Photooxidation of gas phase PAHs during the day resulted in daytime over-lake concentrations that were ∼75% less than corresponding nighttime concentrations. Gas phase ∑PCB concentrations ranged from 0.14 to 1.1 ng/m 3 over the lake and from 0.27 to 14 ng/m 3 in the urban area. ∑PCB concentrations varied seasonally as a result of higher volatilization during the summer. The highest concentrations occurred when the air flow was from the urban/industrial area encompassing Evanston, IL, to Gary, IN, toward the lake, and concentrations were near regional background when the wind was from any other direction. The urban air emissions increased the average coastal atmospheric concentrations above the continental background signal by factors of 12 and 4 for ∑PAHs and ∑PCBs, respectively. Because of photooxidation of gas phase PAHs during the day, the average daytime concentration was increased by only a factor of 5 while the average nighttime concentration was increased by a factor of 18.

Snow and rain events were collected with concurrent air samples during the winter of 1991−1992 at a suburban site in Minnesota to investigate atmospheric scavenging of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). Particle scavenging was the dominant contributor to the total concentrations in snow and was less important in rain. Gas scavenging was only important for low molecular weight PCB congeners and PAHs. Enrichment of dissolved-phase concentrations over levels predicted from temperature-corrected Henry's Law constants was observed for low molecular weight compounds in both rain and snow. The particulate fraction (φ) of atmospheric PCBs and PAHs was the best indicator of total scavenging (W T) by snow according to the equations log W T = 0.71 (±0.08) log φ + 5.34 (±0.08) for PCB congeners (r = 0.62) and log W T = 0.89 (±0.12) log φ + 6.07 (±0.13) for PAHs (r = 0.73).

Simultaneous air samples were taken in Chicago and over southern Lake Michigan as part of the AEOLOS Project (Atmospheric Exchange Over Lakes and Oceans). Gas and particle phase concentrations of polychlorinated 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 and particle phases was well correlated with the subcooled liquid vapor pressure (p°L) for individual samples, but the relationship differed among samples. For all but a few of the samples the slopes of the log K p vs log p°L lines were statistically greater than -1. Other investigators who have found similar results have concluded that the PCBs/PAHs were not at equilibrium; however, other factors indicate that the PCBs and PAHs in the Chicago/Lake Michigan atmosphere are at equilibrium. Slopes of the regressions of log K p vs log p°L from samples of continental background origin, and therefore assumed to have had sufficient atmospheric residence times to reach equilibrium, are among 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 believed to have been sampled twice, once in the urban area and again ∼3.4 h downwind, shows no difference in partitioning. PCBs and PAHs measured in Chicago and over Lake Michigan were apparently at equilibrium between the gas and particle phases. A slope of -1 in the regression of log K p vs log p°L is not necessary to describe equilibrium partitioning. Differences in particulate matter may be responsible for the shallow slopes observed.

No abstract

Dry deposition was collected from November 1993 to October 1995 at multiple sites within the Lake Michigan basin to estimate fluxes of particulate polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) as part of the Lake Michigan Mass Balance Study (LMMBS). Samples were also collected during the Atmospheric Exchange over Lakes and Oceans (AEOLOS) project to estimate fluxes to coastal waters adjacent to the Chicago urban area. Fluxes of particulate PCBs and PAHs were higher in Chicago than <15 km offshore and at rural sites. Geometric mean dry deposition fluxes across the Lake Michigan basin ranged from 3.6 to 65 mg/m2-day for particle mass, 0.06 to 0.21 μg/m2-day for Σ-PCBs, and 0.25 to 18 μg/m2-day for Σ-PAHs. Similarities in both the distribution pattern of PCB congeners and PAHs and the magnitude of their fluxes between dry deposition and surficial sediment suggest that dry deposition may dominate loadings to the lake. Conservative estimates of loadings to Lake Michigan through particle dry deposition are estimated to be 1100 kg/yr for PCBs and 5000 kg/yr for PAHs. These loadings are more than 3× greater than loadings by wet deposition and, for PCBs, are similar to inputs by air/water exchange.