PAHs associated with the leaves of three deciduous tree species. II: uptake during a growing season.

Leaves from three species of deciduous tree (oak, ash and hazel) were sampled at intervals through a growing season in a mature, mixed-deciduous woodland. Polycyclic aromatic hydrocarbon (PAH) concentrations remained within a small range for all species between May and September, deviating significantly only when increases in atmospheric concentrations of PAHs (notably from the 'Bonfire night' festival in early autumn) have been shown. We concluded that the influence of air concentrations was more important than meteorological conditions (temperature, humidity and rainfall) in determining plant concentrations of PAHs over a growing season. Concentrations of 4-, 5- and 6-ring PAHs were positively correlated with time for all species, but there were significant differences in the PAH profile between species sampled from the canopy (oak and ash) compared with the understorey (hazel). Oak and ash had similar PAH profiles, while hazel leaves had proportionally greater concentrations of the heavier molecular weight (4-, 5- and 6-ring) PAHs, and the ratios of these compounds to 3-ring PAHs was positively correlated with time. This affirms earlier work conducted on the same species in the same woodland, where we concluded that the canopy was filtering particles and attendant PAHs from air passing over or through it, and that these particles were transferred to the understorey and the woodland floor.