Quantifying the importance of the atmospheric sink for polychlorinated dioxins and furans relative to other global loss processes

Previous attempts to establish global mass balances for polychlorinated dioxins and furans (PCDD/Fs) have focused on the terrestrial sink, thereby neglecting deposition to the oceans and atmospheric losses. In this study, the atmospheric sink of polychlorinated dioxins and furans (PCDD/Fs) was calculated on the basis of their presence in soils. OH radical ([OH]) depletion reactions compete with atmospheric deposition fluxes for the fate of atmospheric PCDD/Fs. Three different steady state scenarios were considered: scenario A was a one-box atmosphere with globally averaged [OH], temperature (T), atmospheric lifetime (t(life)), and a constant gas-particle partitioning (Phi); in scenario B, [OH], T, and Phi were averaged in a multibox atmosphere, with a constant tlife; and in scenario C, tlife was varied. In scenario A the strength of the atmospheric sink was 2400-2800 kg/yr; in scenario B it was similar to 2100 kg/yr; in scenario C, it was similar to 1,800 kg/yr (t(life) = 5.4 days) to similar to 2,800 kg/yr (t(life) = 14 days). The majority of the atmospheric sink was due to the depletion of Cl(4)DFs (1300-1400 kg/yr), followed by Cl(4)DDs (360-380 kg/yr) and Cl(5)DFs (230-240 kg/yr). On a global scale, major sinks for PCDD/Fs are the deposition to terrestrial soils and the oceans. For Cl(6-8)DDs, deposition to soils outweighs depletion reactions in the atmosphere and ocean uptake. The more volatile Cl4-5DD/Fs, however, are true multimedia'' compounds, with their estimated atmospheric sink being roughly as important as the terrestrial sink (in the case of Cl5DD/Fs) or outweighing it (e.g., Cl4DD/Fs).