Air-boreal forest transfer and processing of polychlorinated biphenyls

The exchange of persistent organic pollutants (POPs) between different compartments of a typical mature boreal forest was investigated. The study focused on fluxes of polychlorinated biphenyls (PCBs) between the atmosphere, vegetation and soil, and within the soil to assess whether this type of forest acts as a final sink or temporary repository for POPs. The study, at a Swedish site, suggested total PCB air-to-forest floor fluxes of 1.4 microg m(-2) year(-1). Much of this could be attributed to compounds bound to particles that may originate from needle surfaces. Degradation half-lives in soil between 6.4 and 30 years for tetra- to hepta-PCBs were obtained using a mass balance approach. This field data-based method derived degradation rates of POPs in background soils, although it may have underestimated the persistence of the heavy PCB congeners. Compounds reaching the forest soil appear to be stored efficiently and degraded slowly. As a first approximation, applying the findings from this study site to boreal forests on a global scale suggests that 2-21% (depending on the congener) of the estimated global atmospheric emission deposits to these ecosystems.