Of all the greenhouse gases regulated under the Kyoto Protocol (United Nations, 1998), SF6 is the most potent, with a global warming potential (GWP) of 23500 over a 100-year time horizon (Myhre et al., 2013). In practical terms, this high GWP means that 1 t of SF6 released to the atmosphere is equivalent to the release of 23500 t of carbon dioxide (CO2). However, the low atmospheric mixing ratio of SF6 relative to CO2 limits its current contribution to total anthropogenic radiative forcing to about 0.2% (Engel et al., 2019). Nevertheless, with a long atmospheric residence time of 3200 years, almost all the SF6 released so far will have accumulated in the atmosphere and will continue to do so (Ravishankara et al., 1993).
Vertical profiles of SF6 mixing ratios, collected from balloon flights up to an altitude of about 37 km, indicated that there is very little loss of SF6 due to photochemistry in the troposphere and lower stratosphere (Harnish et al., 1996; Patra et al., 1997). Using an improved atmospheric chemical transport model, Patra et al. (2018) reported significantly older age of air (AoA) in the stratosphere, and Krol et al. (2018), based on a comparison of six global transport models, showed that upper stratospheric AoA varied from 4 to 7 years among the models. It has been suggested that SF6 may have a shorter atmospheric lifetime of 1937±432 years (Patra et al., 1997), 580–1400 years (Ray et al., 2017), or 1120–1475 years (Kovács et al., 2017). However, these shorter, but still very long, SF6 lifetimes would not significantly affect SF6 emissions estimated from atmospheric trends (Engel et al., 2019). Given the very long lifetime of SF6, compared to the period of our study, uncertainties in this term had a small influence on the outcome. For example, changing the lifetime from 3000 to 1000 years changed the derived emissions by around 1 %, which is smaller than the derived uncertainties.
Since the 1970s, SF6 has been used mainly in high-voltage electrical equipment as a dielectric and insulator in gas-insulated switchgear, gas circuit breakers, high-voltage lines, and transformers. Sales compiled from 1996–2003 by producers in Europe, Japan, USA, and South Africa (not including China and Russia) showed that, on an annual average basis, 80 % of the SF6 produced during this period was consumed by electric utilities and equipment manufacturers for electric power systems (EPA, 2018). Percentage sales, averaged from 1996 to 2003, for other end-use applications included the magnesium industry (4 %), electronics industry (8 %), and uses relating to the adiabatic properties of SF6 (3 %), e.g. incorporating SF6 into tyres, tennis balls, and the soles of trainers as a gas cushioning filler (Palmer, 1996). For example, in 1997 Nike used 277 t (0.25 Gg) of SF6 as a filler in its shoes (Harnish and Schwarz, 2003). Other uses in particle accelerators, optical fibre production, lighting, biotechnology, medical, refining, pharmaceutical, laboratory, university research, and sound-proof windows accounted for around 5 % of sales (Smythe, 2004).
Emissions from electrical equipment can occur during production, routine maintenance, refill, leakage, and disposal (Niemeyer and Chu, 1992; Ko et al., 1993). Random failure or deliberate or accidental venting of equipment may also cause unexpected and rapid high levels of emissions. For example, a ruptured seal caused the release of 113 kg of SF6 in a single event in 2013 (Scottish Hydro Electric, 2013). We assume that such random events are generally not recorded when tabulating bottom-up emission estimates, which would lead to an underestimate in the reported inventories.
Historically, significant emissions of SF6 occurred in magnesium smelting, where it was used as a blanketing gas to prevent oxidation of molten magnesium; in the aluminium industry, also as a blanketing gas; and in semiconductor manufacturing (Maiss and Brenninkmeijer, 1998). These industries and the electrical power industry accounted for the majority of SF6 usage in the USA (Ottinger et al., 2015). A report on limiting SF6 emissions in the European Union also provided estimates of emissions from sound-proof windows (60 Mg) and car tyres (125 Mg) in 1998, although these applications appear to have been largely discontinued due to environmental concern (Schwarz, 2000).
Sulfur hexafluoride has also been used as a tracer in atmospheric transport and dispersion studies (Collins et al., 1965; Saltzman et al., 1966; Turk et al., 1968; Simmonds et al., 1972; Drivas et al., 1972; Drivas and Shair, 1974). The combined SF6 emissions from reported tracer studies (Martin et al., 2011) were approximately 0.002 Gg. Unfortunately, the amounts of tracer released are often not reported, and we conservatively assume that these also amounted to 0.002 Gg, providing a total estimate of about 0.004 Gg (4 t) released from historical SF6 tracer studies. Emissions from natural sources are very small (Busenberg and Plummer, 2000; Vollmer and Weiss, 2002; Deeds et al., 2008).
The earliest measurements of SF6 in the 1970s reported a mole fraction of <1 pmol mol1 (or ppt, parts per trillion) (Lovelock, 1971; Krey et al., 1977; Singh et al., 1977, 1979). Intermittent campaign-based measurements during the 1970s and 1980s reported an increasing trend. However, it was not until the 1990s that a near-linear increase in the atmospheric burden, throughout the 1980s, was reported (Maiss and Levin, 1994; Maiss et al., 1996, Geller et al., 1997). Fraser et al. (2004) described gas chromatography with electron capture detection (GC-ECD) measurements of SF6 at Cape Grim, Tasmania, and noted a long-term trend of 0.1 pmol mol1 yr1 in the late 1970s increasing to 0.24 pmol mol1 yr1 in the mid-1990s. However, after 1995 the annual average growth rate from 1996 to 2000 declined by 12.5 % to 0.21 pmol mol1 yr1, coincident with a 32 % decrease in annual sales and prompt releases of SF6 over this same time period (as noted in Table S2 of the RAND report).
Subsequent reports noted a continuing growth in global mole fractions, with an average growth rate of 0.29±0.02 pmol mol1 yr 1 after 2000 (Rigby et al., 2010), reaching 6.7 pmol mol1 at the end of 2008 (Levin et al., 2010). This increase in the atmospheric burden of SF6 was also reported by Elkins and Dutton (2009). Measurement of SF6 in the lower stratosphere and upper troposphere was reported to be 3.2±0.5 pmol mol1 at 200 mbar in 1992 (Rinsland et al., 1993). These atmospheric observations have been used to infer global emission rates (top-down estimates). Geller at al. (1997) derived a global emission rate of 5.9±0.2 Gg yr1 in 1996, which by 2008 had increased to 7.2±0.4 Gg yr1 (Levin et al., 2010) or 7.3±0.6 Gg yr1 (Rigby et al., 2010) and to 8.7±0.4 Gg yr1 by 2016 (Engel et al., 2019).
Regional inverse modelling studies indicated that emissions have increased substantially from non-Annex-1 parties to the UNFCCC, particularly in eastern Asia, and that these increases have offset the reduction in emissions from Annex-1 countries (Rigby et al., 2011, 2014; Fang et al., 2014). Rigby et al. (2010) showed an increasing trend in emissions from Asian countries growing from 2.7±0.3 Gg yr1 in 2004–2005 to 4.1±0.3 Gg yr1 in 2008. This rise was large enough to account for all the global emissions growth between these two periods. Similarly, Fang et al. (2014) found that eastern Asian emissions accounted for between 38±5 % and 49±7 % of the global total between 2006 and 2012, with China the major contributor of emissions from this region. Consistent regional estimates, within the uncertainties, were also reported for China: 0.8 (0.53–1.1) Gg yr1 from October 2006 to March 2008 (Vollmer et al., 2009); 1.3 (0.23–1.7) Gg yr1 in 2008 (Kim et al., 2010); and 1.2 (0.9–1.7) Gg yr1 from November 2007 to December 2008 (Li et al., 2011). Emissions from other Asian countries were found to be substantially smaller by Li et al. (2011) with South Korea emitting 0.38 (0.33–0.44) Gg yr1 in 2008 and Japan emitting 0.4 (0.3–0.5) Gg yr1. For North America, SF6 emission estimates of 2.4±0.5 Gg yr1 were inferred in 1995 (Bakwin et al., 1997), whereas Hurst et al. (2006) reported emissions of 0.6±0.2 Gg yr1 in 2003, consistent with an expectation of declining Annex-1 emissions during this period. Top-down SF6 emissions for western Europe have been reported by Ganesan et al. (2015), indicating larger modelled emission estimates than those reported to the UNFCCC.
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