Declining, seasonal-varying emissions of sulfur hexafluoride from the United States

<p>Sulfur hexafluoride (SF<span class="inline-formula">₆</span>) is the most potent greenhouse gas (GHG), and its atmospheric abundance, albeit small, has been increasing rapidly. Although SF<span class="inline-formula">₆</span> is used to assess atmospheric transport modeling and its emissions influence the climate for millennia, SF<span class="inline-formula">₆</span> emission magnitudes and distributions have substantial uncertainties. In this study, we used NOAA's ground-based and airborne measurements of SF<span class="inline-formula">₆</span> to estimate SF<span class="inline-formula">₆</span> emissions from the United States between 2007 and 2018. Our results suggest a substantial decline of US SF<span class="inline-formula">₆</span> emissions, a trend also reported in the US Environmental Protection Agency's (EPA) national inventory submitted under the United Nations Framework Convention on Climate Change (UNFCCC), implying that US mitigation efforts have had some success. However, the magnitudes of annual emissions derived from atmospheric observations are 40 %–250 % higher than the EPA's national inventory and substantially lower than the Emissions Database for Global Atmospheric Research (EDGAR) inventory. The regional discrepancies between the atmosphere-based estimate and EPA's inventory suggest that emissions from electric power transmission and distribution (ETD) facilities and an SF<span class="inline-formula">₆</span> production plant that did not or does not report to the EPA may be underestimated in the national inventory. Furthermore, the atmosphere-based estimates show higher emissions of SF<span class="inline-formula">₆</span> in winter than in summer. 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 during winter. The results of this study demonstrate the success of past US SF<span class="inline-formula">₆</span> emission mitigations and suggest that substantial additional emission reductions might be achieved through efforts to minimize emissions during servicing or through improving sealing materials in ETD.</p>