Air-water PCB fluxes from southwestern Lake Michigan revisited.

From simultaneous air and water polychlorinated biphenyl (PCB) measurements collected in September 2010, we re-evaluated the direction and magnitude of net air-water exchange of PCBs in southwest Lake Michigan and compared them with estimations made using similar approaches 15 years prior. Air and water samples were collected during a research expedition on Lake Michigan at 5 km off the coast of Chicago, with prevailing winds from the southwest of our location. Gas-phase ΣPCB concentrations ranged from 190 to 1100 pg m-3 with a median of 770 pg m-3, which is similar to the concentrations measured in the City of Chicago at the same time and similar to concentrations measured in this part of the lake over the last 20 years. Water dissolved-phase ΣPCB concentrations ranged from 150 to 170 pg L-1 with a median of 160 pg L-1, which is one-tenth of that measured in the 1990s. ƩPCB net fluxes showed a slightly absorptive behavior, with a median of (-) 21 ng m-2 day-1 and an interquartile range of (-) 47 to (+) 5 ng m-2 day-1, where (-) and (+) fluxes indicate absorption and volatilization, respectively. Airborne PCB concentrations were higher when the winds were coming from Chicago and drive the deposition. Our fluxes are not significantly different from estimations from 1994 and 1995 and suggest that absorption of PCBs into the waters is slightly more prevalent than 15 years ago. It was confirmed that Chicago remains an important atmospheric source of PCBs to Lake Michigan.