Declining, seasonal-varying emissions of sulfur hexafluoride from the United States point to a new mitigation opportunity

Abstract. 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 influences 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. The substantial decline in U.S. SF₆ emissions derived from atmospheric observations agrees with the reported trend in the U.S. Environmental Protection Agency (EPA)’s national inventory submitted under the United Nations Framework on Climate Change, suggesting mitigation efforts have had some success. However, the magnitudes of derived annual emissions 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 do 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. These results demonstrate the success of past U.S. SF₆ emission mitigations, and suggest substantial additional emission reductions might be achieved through efforts to minimize emissions during servicing or through improving sealing materials in ETD.