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 polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) sources and sinks to and from the atmosphere. Much of this effort has focused on estimating emissions from combustion sources and comparing those estimates to atmospheric deposition measurements. Despite significant growth in the available data for emissions and for deposition, it still appears that total PCDD/F deposition exceeds emissions by well over a factor of 2. We have further investigated this phenomenon by first developing a method to estimate PCDD/F emissions for countries where these data are lacking. Second, we have investigated the global mass balance of PCDD/F on a homologue-specific basis, taking into account degradation by reactions with the OH radical. We have found that most of the mass balance discrepancy is due to the octachlorinated dibenzo-p-dioxin (OCDD) congener. We suggest that the photochemical synthesis of OCDD from pentachlorophenol (PCP) in atmospheric condensed water is the most significant source of OCDD to the environment. Further research directions in this area are suggested.