The influence of uptake by leaves on atmospheric deposition of vapor-phase organics

A model is described which can be used to study the processes involved in vapor phase uptake of organic compounds from the atmosphere by plants. In the model the transport pathways for the compounds and the uptake capacity of “model” leaves are described by resistances and partition coefficients estimated on the basis of a description of the leaves and the physical and chemical properties of the compounds. These leaves are incorporated into an existing multilayer atmospheric model. Example calculations were performed with an oak forest exposed to the three test compounds (2,4-dichlorophenoxy)acetic acid, HCB and phenol. Those calculations demonstrated that lipophilic compounds can diffuse efficiently through the cuticles into the leaves. Such compounds, however, cannot be transported to other parts of the plant and are accumulated in the leaves. After an initial rise of the leaf concentration, it will approach equilibrium with that in the atmosphere, and net transport from the atmosphere to the leaves will strongly be reduced. Hydrophilic compounds, however, can be readily transported to other parts of the plant and are not accumulated in the leaves. The transport to the vegetation will therefore maintain its initial rate. The model can thus provide a framework to estimate the importance of atmosphere-vegetation transport pathways for individual compounds.