Influence of temperature on the plant/air partitioning of semivolatile organic compounds

Dry gaseous deposition is the main pathway of many SOCs to vegetation. It can be understood as a partitioning process between the plant and the gas phase. In this paper, the temperature dependence of the partitioning of polychlorinated biphenyls between air and ryegrass (Lolium multiflorum) was investigated in the laboratory using a solid-phase fugacity meter, and the results were incorporated into a mathematical model of plant uptake of SOCs. The measured plant/air partition coefficients were exponentially proportional to the reciprocal temperature, in agreement with theoretical expectations. The enthalpy of phase change (plant/air) was linearly proportional to the enthalpy of vaporization of the subcooled liquid, but the agreement between the two parameters was poor, the enthalpy of phase change (plant/air) being lower than the enthalpy of vaporization for the lower chlorinated PCBs and much higher for the higher chlorinated PCBs. The model simulations showed that under environmental conditions the temperature dependence of the partitioning coefficient does not influence the plant concentrations of most SOCs. The slow uptake/clearance kinetics prevent the plant/air system from reacting quickly to the new equilibrium state resulting from the temperature-induced change in the partition coefficient. Only for more volatile compounds such as trichlorobiphenyls or phenanthrene can the plant/air concentration ratio be expected to react to changes in temperature.