Sulfur hexafluoride (SF6) is the most potent greenhouse gas and its atmospheric abundance, albeit small, has been increasing rapidly. Although SF6 is used to assess atmospheric transport modeling and its emissions influence the climate for millennia, SF6 emission magnitudes and distributions have substantial uncertainties. In this study, we used NOAA’s ground-based and airborne measurements of SF6 to estimate SF6 emissions from the U.S. between 2007 and 2018. Our results suggest a substantial decline of U.S. SF6 emissions, a trend also reported in the U.S. Environmental Protection Agency (EPA)’s national inventory submitted under the United Nations Framework on Climate Change, implying that U.S. mitigation efforts have had some success. However, the magnitudes of annual emissions derived from atmospheric observations are 40 – 250% higher than the EPA national inventory and substantially lower than the Emissions Database for Global Atmospheric Research inventory. The regional discrepancies between atmosphere-based estimate and EPA’s inventory suggest that emissions from electric power transmission and distribution (ETD) facilities and an SF6 production plant that did not or does not report to EPA may be underestimated in the national inventory. Furthermore, the atmosphere-based estimates show higher winter than summer emissions of SF6. These enhanced wintertime emissions may result from increased maintenance of ETD equipment in southern states and increased leakage through aging brittle seals in ETD in northern states in winter. The results of this study demonstrate the success of past U.S. SF6 emission mitigation and suggest substantial additional emission reductions might be achieved through efforts to minimize emissions during servicing or through improving sealing materials in ETD.
