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 (Φ); in scenario B, [OH], T, and Φ were averaged in a multibox atmosphere, with a constant t_life; and in scenario C, t_life was varied. In scenario A the strength of the atmospheric sink was 2400-2800 kg/yr; in scenario B it was ∼2100 kg/yr; in scenario C, it was ∼1,800 kg/yr (t_life = 5.4 days) to ∼2,800 kg/yr (t_life = 14 days). The majority of the atmospheric sink was due to the depletion Of Cl₄DFs (1300-1400 kg/yr), followed by Cl₄DDs (360-380 kg/yr) and Cl₅DFs (230-240 kg/yr). On a global scale, major sinks for PCDD/Fs are the deposition to terrestrial soils and the oceans. For Cl_6-8DDs, 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).