A global re-analysis of regionally resolved emissions and atmospheric mole fractions of SF₆ for the period 2005–2021

Abstract

Abstract. We determine the global emission distribution of the potent greenhouse gas sulfur hexafluoride (SF6) for the period 2005–2021 using inverse modeling. The inversion is based on 50-day backward simulations with the Lagrangian Particle Dispersion Model (LPDM) FLEXPART and on a comprehensive observation data set of SF6 mole fractions, in which we combine continuous with flask measurements sampled at fixed surface locations, and observations from aircraft and ship campaigns. We use a global distribution-based (GDB) approach to determine baseline mole fractions directly from global SF6 mole fraction fields at the termination points of the backward trajectories. We compute these fields by performing an atmospheric SF6 re-analysis, assimilating global SF6 observations into modeled global three-dimensional mole fraction fields. Our inversion results are in excellent agreement with several regional inversion studies in the USA, Europe, and China. We find that (1) annual U.S. SF6 emissions strongly decreased from 1.25 Gg in 2005 to 0.48 Gg in 2021, however, they were on average twice as high as the reported emissions to the United Nations. (2) SF6 emissions from EU countries show an average decreasing trend of -0.006 Gg/yr during the period 2005 to 2021, including a substantial drop in 2018. This drop is likely a direct result of the EU’s F-gas regulation 517/2014, which bans the use of SF6 for recycling magnesium die-casting alloys from 2018 and requires leak detection systems for electrical switch gear. (3) Chinese annual emissions grew from 1.28 Gg in 2005 to 5.16 Gg in 2021, with a trend of 0.21 Gg/yr, which is even higher than the average global total emission trend of 0.20 Gg/yr. (4) National reports for the USA, Europe, and China all underestimated their SF6 emissions. (5) The global total SF6 emissions are captured well by the inversion, however, results are sensitive to the a priori emission estimates, given that substantial biases of these estimates in regions poorly covered by the measurement network (e.g. Africa, South America) can be improved but not entirely corrected. (6) Monthly inversions indicate that SF6 emissions in the Northern Hemisphere were on average higher in summer than in winter throughout the study period.