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.