Seasonal and species differences in the air--pasture transfer of PAHs.

A field plot was established at a semirural site in the U.K. to investigate the atmospheric transfer of PAHs to different pasture species over the whole growing season. The PAHs displayed a range of partitioning behaviors in the atmosphere from exclusively gas phase to exclusively particle bound, resulting in different modes of deposition to the plant surface. The different pasture species had different plant and sward characteristics, e.g., leaf morphologies, yields, etc. For the majority of PAHs, the plant species displayed a seasonality in concentrations, with concentrations being higher in the winter than in the summer. For the lighter PAHs, this seasonality was absent with soil outgassing and/or summer sources of PAHs being implicated. Air-plant transfer factors (scavenging coefficients, with units m3/g dw) typically ranged between 4 and 52 during the summer, increasing to 8-88 during winter. Despite different plant and sward characteristics, the mixtures and concentrations of PAHs were similar for all the plant species. This indicates that there was little difference in the interception and retention behavior of the gas- and particle-phase PAHs. The implications of this for food chain transfer and air-vegetation modeling are discussed.