Deposition of PCBs in mountains: the forest filter effect of different forest ecosystem types.

The effect of canopy composition and density on the forest filter effect (the deposition flux under the canopy and deposition to bare soil) for polychlorinated biphenyls (PCBs) was investigated for forests of the Italian Alps. Deposition fluxes were measured in situ using deposition samplers below canopies and in adjacent clearings at three altitudes (1100, 1400, and 1800 m above sea level). Forest sites were selected on the basis of canopy composition and density. Net forest fluxes (NFFs) were calculated by subtracting the deposition flux under canopies by deposition fluxes to clearings and represent the net contribution of forests to PCB deposition. NFF trends are discussed in relation to canopy development. Mean deposition velocities were also calculated and a direct correlation with the/octanol air partition coefficient (K(OA)) was found. A leaf area index (LAI) was used to calculate a specific, canopy density-independent, deposition velocity for each forest type. This parameter can be used to calculate the deposition of PCBs in forests, given their LAI. Results show that forests significantly enhance the deposition of PCBs to soil and that this effect is controlled by temperature, leaf dynamics, and K(OA).