Air-pasture transfer of PCBs.

A field experiment was conducted to study the air to pasture transfer of PCBs at a rural site in northwest England. Strong positive linear correlations were obtained between the log plant−air partition coefficients (m3 of air g-1 of plant dry weightdefined here as the scavenging coefficient) and log octanol−air (Koa) partition coefficients. Pasture typically retained amounts of PCB per g dry weight equivalent to that in 7 m3 of air for congener 18 and ranging up to 64 m3 for congener 170, regardless of whether the pasture growth (exposure) time had been 2, 6, or 12 weeks. This indicates that airborne PCBs partition onto freshly grown pasture and approach plant surface−air gas-phase equilibrium rather rapidly at this site, i.e., within 2 weeks of exposure. In late April−June, when grassland production is at a maximum, sequestering rates could approach 1.2 ng of PCB-18, 0.17 ng of PCB-170, and 12 ng of ∑PCB m-2 day-1. With 7 million ha of managed and rough grassland in the U.K., fresh pasture production in the spring and summer is estimated to remove an average of 0.8 kg of ∑PCB day-1 from the air during these times (80 kg of ∑PCB per growing season). Some buffering influence may be exerted on surface air concentrations during the most active periods of plant biomass production, while the incorporation of PCBs into pasture following air−pasture transfer processes controls the supply of PCBs to grazing animals and the human food chain