Are PCBs in the Canadian Arctic atmosphere declining? Evidence from 5 years of monitoring

A long-term database of weekly air concentrations was examined to establish temporal trends of PCBs in the Arctic atmosphere. Several methods were emp loyed to reduce the intra-annual variability allowing the elucidation of lo ngterm trends for a selection of congeners at Alert located in the Canadian Arctic. These methods included temperature normalization (TN), multiple li near regression (MLR), and digital filtration (DF). Estimation of the slope (m) resulting from the linear regression between the natural logarithm of the partial pressure in air versus reciprocal temperature (In P = m/T + b), required for TN and MLR, proved difficult due to the poor correlation with temperature experienced for the majority of congeners. Values of mwere con siderably lower than those obtained from temperate studies, implying that r egional air-surface exchange plays a minor role in supporting the observed air concentrations in the Arctic. The lighter congeners generally showed ve ry low slopes, and some even showed positive correlation with 1/T. This mig ht be a result of their relatively fast reaction rates with OH radicals fol lowing the onset of 24-h sunlight in spring. Use of DF (in combination with TN and MLR) revealed declining trends for several of the lower chlorinated congeners in the high Arctic atmosphere, with estimated first-order half-l ives, t(1/2), ranging from similar to3 to 20 yr. Declining trends of the lo wer congeners probably reflect falling levels in source regions, as a resul t of long-range transport to this Arctic site. There were no apparent trend s for the higher chlorinated congeners (penta-substituted and above), excep t for PCB 180, in marked contrast to temperate studies, indicating a lag ti me for decline between the Arctic and source regions.