Remoteness from Emission Sources Explains the Fractionation Pattern of Polychlorinated Biphenyls in the Northern Hemisphere

The global distillation hypothesis states that fractionation patterns of persistent semivolatile chemicals in the environment are determined by the effect of spatially varying environmental temperature on the temperature-dependent phase partitioning coefficients of chemicals. Here, we use a model experiment and an analysis of monitoring data for polychlorinated biphenyls (PCBs) to explore an alternative hypothesis, the differential removal hypothesis, which proposes that fractionation results from different loss rates from the atmosphere, acting along a gradient of remoteness from emission sources Model calculations for a range of PCB congeners demonstrate that fractionation occurs with distance from sources, regardless of the temperature gradient. We have assembled two independent data sets of PCB concentrations in European air that show fractionation, and quantified the remoteness of monitoring sites from PCB sources using the remoteness index, RI Regression analysis of these empirical data against RI and temperature demonstrates that RI determines fractionation patterns Based on this result, we calculate empirical effective residence times in air for a set of PCB congeners from the relationship between measured concentrations and RI. These empirical effective residence times agree well with values calculated by a multimedia mass balance model. Our conclusion from the model experiment and analysis of monitoring data is that temperature is not a driver of the fractionation of PCBs currently observed in European air, but rather that fractionation reflects differential removal from the atmosphere