Is combustion the major source of polychlorinated dibenzo-p-dioxins and dibenzofurans to the environment? A mass balance investigation

We have summarized some of the significant work characterizing polychlorina ted dibenzo-p-dioxin and dibenzofuran (PCDD/F) sources and sinks to and fro m the atmosphere. Much of this effort has focused on estimating emissions f rom combustion sources and comparing those estimates to atmospheric deposit ion measurements. Despite significant growth in the available data for emis sions and for deposition, it still appears that total PCDD/F deposition exc eeds emissions by well over a factor of 2. We have further investigated thi s phenomenon by first developing a method to estimate PCDD/F emissions for countries where these data are lacking. Second, we have investigated the gl obal mass balance of PCDD/F on a homologue-specific basis, taking into acco unt degradation by reactions with the OH radical. We have found that most o f the mass balance discrepancy is due to the octachlorinated dibenzo-p-diox in (OCDD) congener. We suggest that the photochemical synthesis of OCDD fro m pentachlorophenol (PCP) in atmospheric condensed water is the most signif icant source of OCDD to the environment. Further research directions in thi s area are suggested.