Grass-air exchange of polychlorinated biphenyls

Three field experiments were performed to assess the clearance, uptake, and exchange kinetics of polychlorinated biphenyls (PCBs) between grass and th e atmosphere using mixed- and single-species grass (Holcus lanatus). In the clearance experiment, the grass was artificially contaminated by equilibra tion with diluted Aroclor vapor then exposed to field air, and the rates of depletion were monitored by sampling at regular intervals to determine cle arance rate constants. In the uptake experiment, the uptake of PCBs from th e ambient atmosphere was followed in growing grass at ambient concentration s for 3 and 6 weeks by analysis of segmented samples along the length of th e sward. In the third experiment, diurnal temperature-driven changes in gra ss concentrations were measured. The results indicate that the grass is beh aving as a two-compartment system: (1) a fast-exchanging surface adsorption site with a response time of hours and a capacity essentially independent of K-OA, the octanol-air partition coefficient and (2) a slow responding si te with a response time of weeks, the capacity of which is related to K-OA. The kinetic and equilibrium phenomena involved in grass-air exchange are t hus complex and are not adequately described by simple first-order rate con stants and equilibrium partitioning coefficients.