Tham khảo tài liệu 'passive sampling techniques episode 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | R. Gioia . Jones and T. Harner by primary emissions rather than secondary emissions from environmental reservoirs. With three sampling intervals it has also been possible to have a first estimate of the clearance rates of PCBs in the atmosphere. Clearance rates ranged from to years for all congeners and locations 15 . This is a further support to the hypothesis that primary emissions still dominate in European background air. Latitudinal distribution and fractionation for PBDEs have been estimated for the sampling campaign 2000-2002 and 2002-2004 14 15 . Results showed that the lighter PBDEs decreased with increasing latitude. This is the opposite of what was observed for PCBs where the sequestered amounts of heavier congeners decrease with increasing latitude which is usually related to distance from sources. In a modelling study Wania and Dugani 41 suggested that lower brominated congeners have an LRAT potential comparable to that of PCBs whereas the highly brominated congeners were predicted to have a low potential to reach remote areas. PBDE congeners are heavier than their PCB analogues. However the C-Br bond in PBDEs is weaker than the C-Cl bond in PCBs. For example tri-BDEs and hepta-CBs have a molar mass of approximately 400 g mol-1 yet the tri-BDEs are predicted to be more volatile and more reactive by over an order of magnitude in air than the hepta-CBs. SPMDs have also been applied in studies at regional local scale. Lohmann et al. 29 deployed SPMDs at 19 sites in northwest England during November December 1999 to test their efficiency as passive samplers for PCDD Fs and PAHs. The study suggested that SPMD efficiently sample vapour-phase species in the atmosphere while species partially or completely associated with particles were found to be sampled by the SPMD with poorer reproducibility. Sequestered PAH and PCDD Fs amounts were compared with active monitoring data where available. Good agreement was found between the active and the passive
