Forests as Filters of Airborne Organic Pollutants:  A Model

Abstract

The role of forests in filtering airborne organic pollutants from the atmosphere and transferring them to soil was examined. A mathematical model was assembled to calculate the filter factor, defined as the quotient of the net deposition of a given compound to a forest and its net deposition to a bare soil. A simple equation was obtained that expressed the filter factor as a function of just two physical-chemical properties of the chemical: the octanol/air and air/water partition coefficients (K OA and K AW ). The model was then applied to a spruce and to a beech/oak canopy close to Bayreuth for which measured deposition velocities were available. The model simulations indicated that there was little filter effect for volatile compounds with log K OA < 7 and for hydrophilic substances with log K AW < -6. Similarly, although forest canopies are thought to be effective filters of airborne particles, the filter factor for compounds that are primarily particle bound was also close to 1, since the dry particle bound deposition to the canopies was overshadowed by the wet deposition of particle bound chemical at these sites. However, for semivolatile compounds with 7 < log K OA < 11 and log K AW > -6, a pronounced filter effect was predicted with filter factors as high as 10 at the sites studied. The results are in good agreement with deposition measurements conducted at the same sites. Thus, forests influence the atmospheric deposition of only a small subset of organic chemicals. However, this subset includes many contaminants of particular concern such as chlorinated dioxins, furans, biphenyls, and pesticides. Forests can be expected to play a key role in the environmental fate of these compounds, decreasing their atmospheric half-lives (and hence long-range transport, input into agricultural ecosystems and human exposure) and transferring the chemical to forest soils.

Dechlorane Plus (DP) in air and plants at an electronic waste (e-waste) site in South China

“…At the e-waste site, however, DP may penetrate into the interior through leaf cuticles or stomata due to the long-term exposure of leaves to high air DP concentrations. The observation also suggests that uptake can efficiently reduce air DP concentration, in agreement with previous model calculations, which indicate that the forest filter effect can notably decrease SOC concentrations in the air, particularly for those with higher octanolair partition coefficients (log K OA ) McLachlan and Horstmann, 1998;Wania and McLachlan, 2001).…”

Occurrence of fluorotelomer alcohols at two Alpine summits: sources, transport and temporal trends

“…The forest filter effect of the Alps was observed for compounds such as polychlorinated biphenyls (PCBs), [42] while it was probably not applicable to FTOHs. There was little forest filter effect for compounds with log K OA , 7, [43] and the log K OA of FTOH fell in this range with values of 5.26-5.72. [35,36] This is also supported by the investigation that FTOHs could not be detected in the spruce needle samples implying a low tendency for FTOH to partition to needle surfaces.…”

Environmental fate and behavior of persistent organic pollutants in Shergyla Mountain, southeast of the Tibetan Plateau of China

“…Forests can act as efficient filters for many airborne POPs (McLachlan and Horstmann, 1998). Their canopies can capture POPs on leaves and barks from air and redirect POPs to the soil by different processes: rain washout, wax erosion, and transport due to litter fall.…”