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| - Deposition of PolybrominatedDiphenyl Ethers, PolychlorinatedBiphenyls, and Polycyclic AromaticHydrocarbons to a Boreal DeciduousForest
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| - The atmospheric deposition of several groups of semi-volatile organic compounds to a deciduous forest inCanada was determined using an indirect technique basedon ratios of measured canopy interception and airconcentrations. Air (gas and particle phase) and bulkdeposition were sampled for 14 months from October 2001to December 2002 at both a forest and a nearby clearing,and extracts were quantified for polybrominated diphenylethers (PBDEs), polychlorinated biphenyls (PCBs), andpolycyclic aromatic hydrocarbons (PAHs). Long-term averagedry deposition velocities for vapors and particle-boundspecies were then derived for the canopy growing period.The mean dry gaseous deposition velocity for PBDEsand PCBs to the Canadian deciduous forest was 2.7 ±0.52 cm·s-1, which is similar to the only other measuredvalue for a deciduous canopy. Particle-bound depositionvelocities to the canopy due to diffusion and impactionwere 0.8 cm·s-1 for the PBDEs and 0.11 cm·s-1 for the PAHs.Differences in the particle-bound deposition velocitiesbetween PBDEs and PAHs and between deciduous canopiesin Canada and Germany are explainable by differencesin particle size distribution. The interception/concentrationratios for several PAHs were too low to be interpretableas dry gaseous deposition velocities. This is likely becausethe measured deposition flux under the canopy was lessthan the deposition flux to the canopy, possibly as a resultof photodegradation in the canopy. From the ratio ofcanopy interception and average gas-phase concentrationof less chlorinated PCBs, a predictive relationshipbetween the canopy/air partition coefficient KPA and theoctanol/air partition coefficient KOA was derived (KPA = 110KOA0.67). Despite differences in local climate and canopycomposition and structure, the deposition velocities and thecanopy uptake capacity measured in Canada wereremarkably similar to those reported in Germany, lendingcredibility to the suggestion that high gaseous depositionvelocities are common throughout boreal and temperatedeciduous forests. These extraordinarily high depositionvelocities of semi-volatile organic compounds to deciduousforest canopies are at the core of the hypothesis of asignificant filter effect of forests on a regional and globalscale.
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