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. T hese samples were analyzed for persistent organic pollutants, including PCB congeners, atrazine, and trans-nonachlor. We have developed a method for i nterpreting the gas-phase data that includes fractionating the observed PCB concentration into land- and water-based sources. This approach accounts f or differences in gas-phase atmospheric PCB concentrations over water and o ver land. Using this fractionation approach, we have interpolated the measu red data over time and space to predict PCB air concentrations over the lak e during the LMMB field period. The results predict gasphase Sigma PCB (sum of similar to 98 congener groups) concentrations for each of 2318 grid cel ls over the lake, on a monthly basis. We estimate that lake-wide monthly av erage Sigma PCB gas-phase concentrations range from 0.136 to 1.158 ng/m(3), with an annual average PCB concentration of 0.457 ng/m(3). As expected, th e highest concentrations of PCBs over the lake when the winds are from the southwest (out of the Chicago-Gary region) acid when land surface temperatu res are elevated. The predicted influence of Chicago is described on a mont hly basis as a zone of elevated PCB concentrations for approximately 40 km into Lake Michigan.