Regional spatial and temporal interpolation of atmospheric PCBs: Interpretation of Lake Michigan Mass Balance data

During the Lake Michigan Mass Balance (LMMB) Project, over 600 atmospheric samples were collected at eight shoreline sites and during seven cruises. These samples were analyzed for persistent organic pollutants, including PCB congeners, atrazine, and trans-nonachlor. We have developed a method for interpreting the gas-phase data that includes fractionating the observed PCB concentration into land- and water-based sources. This approach accounts for differences in gas-phase atmospheric PCB concentrations over water and over land. Using this fractionation approach, we have interpolated the measured data over time and space to predict PCB air concentrations over the lake during the LMMB field period. The results predict gas-phase PCB(sum of approx. 98 congener groups) concentrations for each of 2318 grid cells over the lake, on a monthly basis. We estimate that lake-wide monthly average PCB gas-phase concentrations range from 0.136 to 1.158 ng/m3, with an annual average PCB concentration of 0.457 ng/m3. As expected, the highest concentrations of PCBs over the lake when the winds are from the southwest (out of the Chicago-Gary region) and when land surface temperatures are elevated. The predicted influence of Chicago is described on a monthly basis as a zone of elevated PCB concentrations for approximately 40 km into Lake Michigan.