Accumulation parameters and seasonal trends for PCBs in temperate and boreal forest plant species.

The concentration of polychlorinated biphenyls (PCBs) in the air and vegetation was measured periodically in two alpine forests, during the growing season. Foliage samples from nine plant species typical of the temperate and boreal environment were collected and analyzed. Leaf concentrations of tri- and tetra-CBs showed fast response times with changing temperature and gas-phase concentrations, suggesting that a partitioning equilibrium is approached relatively rapidly (few days) in the field. Heavier compounds showed kinetically limited accumulation trends, not reaching equilibrium during the growing season. Results were used to estimate the bioconcentration factors or equilibrium plant/air partition coefficient (KPA) for each species. Values of log KPA (calculated on a mass/volume basis) ranged between 0.78 and 1.96 and were correlated to the log KOA. Uptake trends of the higher chlorinated compounds showed intraspecific differences which were partially explained by the specific leaf area (SLA).

Seasonality of the air-forest canopy exchange of persistent organic pollutants.

Forest canopies represent an extensive organic surface available for partitioning of semivolatile organic pollutants with the atmosphere. To date, the ability of forests to sequester such compounds (the so-called "forest filter effect") has been investigated using indirect methods that yield time integrated deposition fluxes and scenario-dependent deposition velocities. In the present study, experimental data collected at three different alpine forest sites were used to assess the dynamics of PCB deposition fluxes (F, ng m(-2) d(-1)) during the growing season. Estimated values of Fwere consistent with previously reported data. Furthermore, this study showed that maximum levels of F in late spring can be a factor of 1.4-3.4 higher than their seasonal mean value. These data, in conjunction with a simple modelframeworkthatincludesthe main forcing parameters of air concentration, temperature, foliage structure, and biomass dynamics, are used to estimate the plant-air mass transfer coefficient (ku, m d(-1)) and its variation with time in one of the forests. ku did not appear to significantly vary during the season, and its mean seasonal value ranged between 43 and 95 m d(-1) for selected compounds. The proposed framework was successfully applied to predict the variation in canopy concentration with time in the other two forests.