The accumulation of semivolatile organic compounds (SOCs) in plants is important because plants are the major vector of these compounds into terrestrial food chains and because plants play an important role in scavenging SOCs from the atmosphere and transferring them to the soil. Agricultural plants are of particular interest because they are a key link in the atmosphere-foddermilk/beef food chain that accounts for much of background human exposure to persistent lipophilic organic pollutants such as PCBs and PCDD/Fs. In this study the accumulation of PCBs, PCDD/Fs, PAHs, and some chlorobenzenes was determined in eight grassland species as well as maize and sunflower leaves collected simultaneously at a semirural site in Central Europe. Air samples were collected at the same site during the growth of these plants, and the particlebound and gaseous concentrations were determined. A newly developed interpretive framework was employed to analyze the data, and it was established whether the accumulation of a given compound was due primarily to equilibrium partitioning, kinetically limited gaseous deposition, or particle-bound deposition. The interspecies variability in uptake was then examined, and it was found that for those compounds which had accumulated primarily via kinetically limited gaseous deposition and particle-bound deposition the variation among the 10 species was generally a factor of <4. For most of the plants this variation could largely be explained by differences in the surface area and horizontally projected surface area per unit plant volume. However, for the more volatile compounds for which the plant levels were determined by equilibrium partitioning, the interspecies variability exceeded a factor of 30. This variability was not related to the extractable lipid content or the cuticle volume fraction in the vegetation. Good agreement was found between the partition coefficients measured in the field and published values measured in the laboratory. The results indicate that the interspecies variability in the vegetation/gas-phase partition coefficient is larger than the variability in the net gaseous and particle-bound deposition velocities, yielding a greater interspecies variability in plant levels for more volatile SOCs.

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The concentrations of a variety of semivolatile organic compounds (SOC) including polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls measured in rye grass under field conditions are compared with the concentrations predicted by a mathematical model based on laboratory studies with a fugacity meter. The agreement is excellent, with a maximum difference of 30% between the predicted and the measured concentrations for those compounds where dry gaseous deposition is the main uptake pathway. It was found that compounds with a log octanol/air partition coefficient > 8 did not approach equilibrium in the field study. The uptake of these compounds was Independent of the physical-chemical properties of the substance and was postulated to be governed by the rate at which air is exchanged between the grass canopy and the free atmosphere. For more volatile compounds that approached equilibrium, the bioconcentration factor is determined primarily by the octanol/air partition coefficient of the substance and the temperature. This study represents the first validation of a model of plant uptake of gaseous SOC that we are aware of and demonstrates that the results of fugacity meter studies can be extrapolated to environmental conditions.

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