Framework for the interpretation of measurements of SOCs in plants

Plant uptake of semivolatile organic compounds (SOCs) occurs primarily from the atmosphere via one of three processes: equilibrium partitioning between the vegetation and the gas phase, kinetically limited gaseous deposition, or wet and dry particle-bound deposition. Each of these processes depends on different atmospheric concentrations, plant properties, and environmental variables. Hence, in interpreting measurements of SOCs in plants, it is imperative that the major process responsible for the accumulation of a given compound be known. Beginning with basic equations describing gaseous and particle-bound deposition to vegetation, a framework for identifying the major uptake process and further interpreting measurements of plant concentrations is developed. This framework makes use of the relative differences in accumulation behavior as a function of the octanol-air partition coefficient (K-OA) of the chemical. The mathematical analysis leads to two interpretive tools, both log-log plots, one of the quotient of the vegetation and gas-phase concentrations vs K-OA and one of the quotient of the vegetation and particle-bound concentrations vs the quotient of the particle-bound and gaseous concentrations. Each of these plots contains three distinct and easily recognizable segments, and each segment corresponds to one of the three deposition processes. When the experimental data are plotted and the three segments are identified, it is possible to determine the dominant uptake process for a given compound, and this in turn opens the door to further interpretation of the plant uptake behavior.