Background Concentrations of Polychlorinated Dibenzo-p-Dioxins, Dibenzofurans, and Biphenyls in the Global Oceanic Atmosphere

The remote oceans are among the most pristine environments in the world, away from sources of anthropogenic persistent organic pollutants (POP), but nevertheless recipients of atmospheric deposition of POPs that have undergone long-range atmospheric transport (LRAT). In this work, the background occurrence of gas and aerosol phase polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and dioxin like polychlorinated biphenyls (dl-PCB) is evaluated for the first time in the atmosphere of the tropical and subtropical Atlantic, Pacific, and Indian oceans. Thirty-nine air samples were collected during the eight-month Malaspina circumnavigation cruise onboard the R/V Hespérides. The background levels of dioxins and dl-PCBs remained very low and in many cases very close to or below the limit of detection. Expectedly, the levels of PCBs were higher than dioxins, PCB#118 being the most abundant compound. In the particular case of dioxins, octachlorodibenzo-p-dioxin (OCDD) was the most abundant PCDD/F congener. Distribution of dl-PCB is dominated by the gas phase, while for PCDD/F the aerosol phase concentrations were higher, particularly for the more hydrophobic congeners. The Atlantic Ocean presented on average the highest PCDD/F and dl-PCB concentrations, being lower in the southern hemisphere. The assessment of air mass back trajectories show a clear influence of continental source regions, and lower concentrations when the air mass has an oceanic origin. In addition, the samples affected by an oceanic air mass are characterized by a lower contribution of the less chlorinated dioxins in comparison with the furans, consistent with the reported higher reaction rate constants of dibenzo-p-dioxins with OH radicals than those of dibenzofurans. The total dry atmospheric deposition of aerosol-bound ∑PCDD/F and ∑dl-PCB to the global oceans was estimated to be 354 and 896 kg/year, respectively.