Transfers of airborne PCDD/Fs to bulk deposition collectors and herbage

PCDD/F air concentrations, deposition fluxes, and grass concentrations/offtakes (pg m(-2) day(-1)) were measured concurrently in Bolsover, Derbyshire, England over 1 year during 1992-1993. The data are used here to discuss spatial and temporal differences in atmospheric PCDD/F concentrations and how the airborne PCDD/Fs transferred to the bulk deposition collectors and grass. Three sample sites are discussed, namely, a regional background site (RES), an urban site (US), and a site close to an industrial complex (IS) that has been a suspected source of selected PCDD/F homologs. Generally, the mixture of PCDD/F 2,3,7,8-substituted congeners and homologs was the same in the air, in the bulk deposition, and in the grass over a given sampling period and site. This suggests that PCDD/Fs of different levels of chlorination were transferring with similar efficiencies from the air to the collectors and grass, despite different gas-particle partitioning in the air. Deposition or transfer velocities for individual congeners/homologs to the bulk collectors throughout the whole study ranged between 0.034 and 0.82 cm s(-1) at the sites, averaging 0.27 cm s(-1) for PCDD/Fs at the RES. An experiment conducted to assess potential volatilization of soilborne PCDD/Fs from contaminated areas near the IS showed that all the compounds had a strong affinity for the soil and that volatilization is not likely to significantly contaminate herbage growing in the contaminated soil or the overlying atmosphere near this site. Herbage PCDD/Fs were therefore derived almost exclusively from the atmosphere in this study. The similarities in the PCDD/F mixture in air and grass suggest that the different PCDD/Fs are scavenged by the grass sward from the atmosphere with similar efficiencies; the implications of this are discussed.