Back-trajectory analysis of atmospheric polychlorinated biphenyl concentrations over Lake Superior

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Vapor-phase concentrations of three polychlorinated biphenyl (PCB) congeners measured at a site on the Lake Superior shoreline are analyzed in conjunction with back-trajectories to assess the contribution of long-range transport to the measured concentrations. Most previous attempts to use back-trajectories to identify a potential signature of long-range transport of semivolatile organic compounds have been confounded by correlation between air mass source region and temperature, through temperature-dependent air/surface exchange processes in the sampling region. This effect is minimized in the present study through the use of a model of equilibrium air/surface exchange to remove temperature dependence from the data set prior to trajectory analysis. Back-trajectories corresponding to periods when observed concentrations were significantly greater than those expected based on the observed temperature dependence indicate rapid transport from urban and industrial regions well south of the sampling site; transport from south of 40° north latitude results in concentrations 1.6-1.9 times those observed during periods without such transport. The method presented here should be applicable to other semivolatile organic compounds for which long-range transport signals are obscured by highly temperature-dependent regional air/surface exchange processes.

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Environmental Science and Technology