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 pla nts. In the model the transport pathways for the compounds and the upt ake capacity of ''model'' leaves are described by resistances and part ition coefficients estimated on the basis of a description of the leav es and the physical and chemical properties of the compounds. These le aves are incorporated into an existing multilayer atmospheric model. E xample calculations were performed with an oak forest exposed to the t hree 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, howe ver, cannot be transported to other parts of the plant and are accumul ated in the leaves. After an initial rise of the leaf concentration, i t will approach equilibrium with that in the atmosphere, and net trans port from the atmosphere to the leaves will strongly be reduced. Hydro philic compounds, however, can be readily transported to other parts o f the plant and are not accumulated in the leaves. The transport to th e vegetation will therefore maintain its initial rate. The model can t hus provide a framework to estimate the importance of atmosphere-veget ation transport pathways for individual compounds.