Air-Pasture Transfer of PCBs GARETH THOMAS, † ANDREW J. SWEETMAN, † WENDY A. OCKENDEN, † DONALD MACKAY, ‡ AND KEVIN C. JONES* ,† Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and Environmental and Resource Studies, Trent University, Peterborough, Ontario, Canada, K9J 7B8 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 (m 3 of air g -1 of plant dry weight sdefined here as the scavenging coefficient) and log octanol -air (K oa ) partition coefficients. Pasture typically retained amounts of PCB per g dry weight equivalent to that in ∼7m 3 of air for congener 18 and ranging up to ∼64 m 3 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. Introduction There is growing interest in and awareness of the important role vegetation plays in the global cycling and food chain transfer of persistent, semivolatile organic contaminants (SOCs) (1). Because ofthe propensityfor vegetation to retain gas-phase SOCs and to scavenge particle-phase SOCs, it can playa role in ‘cleansing’the atmosphere (2, 3)and introducing SOCs, many of which can bioaccumulate, to natural and agriculturalterrestrialfood chains (4-8). Vegetation should be viewed as an important, dynamic, and active environ- mental compartment that influences the atmospheric trans- port,globalturnover,and cyclingofmanySOCsorpersistent organic pollutants (POPs). However, at the present time manyuncertaintiesremain in our knowledge ofthe processes of air -vegetation exchanges of SOCs and in our ability to model these transfer processes (4-7). This largely arises from a lack of detailed, mechanistic, field-based studies on the kinetics and fugacity relationships of air -vegetation transfer. To date, the focus has been on acute exposure studies in the laboratory and studies in controlled growth chambers that may not adequately mimic the processes of deposition in the field (8-10). The few field studies that have been conducted are typically limited to simple obser- vations of the relationship between SOC concentrations in air and vegetation (e.g., refs 11 and 12); such investigations ignore potentially important environmental and vegetation factors that will influence the transfer processes. These shortcomings place uncertainties, at least in the short term, on the extent to which vegetation can be used as a biomonitor of spatial and temporal variations in atmospheric SOC concentrations (1, 13, 14). This paper reports the results from a detailed field study conducted to investigate and quantify the transfer of one well-characterized class of SOCs, the polychlorinated bi- phenyls (PCBs),from the atmosphere to an established sward ofupland grazingpasture at a site in northwest England.Our interest was both in the propensity of the vegetation to interact with atmospheric PCBs,which have been wellstudied at this field site (15-17), and the supply of atmospherically derived PCBs to grazing livestock (4, 6, 7). Experimental Section Field Site and Experiment. Afield plot was isolated on an established sward of unimproved upland pasture at a semirural field station site owned by Lancaster University, in northwest England. The sward comprised Lolium perenne (∼30%), Holcus lanatus (∼30%), Agrostis capillaris (∼10%), Poa pratensis (∼10%), Cynosurus cristatus (∼10%), and a range of other grasses and forbs. Ameteorological site and ambient PCBmonitoringequipment have been based at the site for a number of years, and PCB air data have been reported previously (15-17). The field plot was established in early April 1996 and divided into 1 m 2 subplots. Three management regimes were established on randomlyselected subplots;‘simulated grazing’in which triplicate subplots were harvested every2weeks through the growingseason between April and October 1996; ‘simulated silage production’ in which triplicate subplots were harvested at 6-week intervals (i.e., late May, late July and mid September); ‘long-term growth’ in which triplicate subplots were harvested every 3 months (i.e. in July, September, and January). Pasture yields (g m -2 ) were always recorded. Air samples were also taken over 1 or 2 week periods (∼600 m 3 sampled, with a filter and polyurethane foam plug on a high volume air sampler; 15) and coincided with pasture harvesting. The glass fiber filters were spiked with PCB recovery standards before sampling commenced. Great care was taken to avoid contamination of the vegetation duringsamplehandling,byavoidingan air-drying step and ensuring minimal air contact after sampling (18, 19). Samples were placed into bags in the field,immediately sealed, and frozen until required. The sample for analysis (30 g wet weight) was frozen with liquid nitrogen, ground with powdered sodium sulfate (50 g), spiked with labeled recoverystandards([ 13 C] PCB-28, 52, 101, 153, 138, 180, 209), and Soxhlet extracted for 8 h with 250 mL of 4:1 hexane: acetone. The extract was first cleaned up on a silica/acidified silica double-layered column, eluted with hexane, and secondly on a Biobeads S-X3 GPC column, eluted with 1:1 DCM:hexane. Internal standards and keeper solvent (dode- cane)were then added,and the finalsample was taken down to 50 μL. Air filter and PUF samples were spiked with labeled *Corresponding author e-mail: k.c.jones@lancaster.ac.uk. † Lancaster University. ‡ Trent University. Environ. Sci. Technol. 1998, 32, 936-942 936 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 7, 1998 S0013-936X(97)00761-X CCC: $15.00  1998 American Chemical Society Published on Web 02/24/1998