Forests as filters of airborne organic pollutants: A model

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.