Abstract. We reconstruct atmospheric abundances of the potent
greenhouse gas c-C4F8 (perfluorocyclobutane, perfluorocarbon
PFC-318) from measurements of in situ, archived, firn, and aircraft air
samples with precisions of ∼1 %–2 % reported on the SIO-14
gravimetric calibration scale. Combined with inverse methods, we found near-zero atmospheric abundances from the early 1900s to the early 1960s, after
which they rose sharply, reaching 1.66 ppt (parts per trillion dry-air mole
fraction) in 2017. Global c-C4F8 emissions rose from near zero in
the 1960s to 1.2±0.1 (1σ) Gg yr−1 in the late 1970s to
late 1980s, then declined to 0.77±0.03 Gg yr−1 in the mid-1990s
to early 2000s, followed by a rise since the early 2000s to 2.20±0.05 Gg yr−1 in 2017. These emissions are significantly larger than
inventory-based emission estimates. Estimated emissions from eastern Asia
rose from 0.36 Gg yr−1 in 2010 to 0.73 Gg yr−1 in 2016 and 2017,
31 % of global emissions, mostly from eastern China. We estimate
emissions of 0.14 Gg yr−1 from northern and central India in 2016 and
find evidence for significant emissions from Russia. In contrast, recent
emissions from northwestern Europe and Australia are estimated to be small
(≤1 % each). We suggest that emissions from China, India, and Russia
are likely related to production of polytetrafluoroethylene (PTFE,
“Teflon”) and other fluoropolymers and fluorochemicals that are based on
the pyrolysis of hydrochlorofluorocarbon HCFC-22 (CHClF2) in which
c-C4F8 is a known by-product. The semiconductor sector, where
c-C4F8 is used, is estimated to be a small source, at least in
South Korea, Japan, Taiwan, and Europe. Without an obvious correlation with
population density, incineration of waste-containing fluoropolymers is
probably a minor source, and we find no evidence of emissions from
electrolytic production of aluminum in Australia. While many possible
emissive uses of c-C4F8 are known and though we cannot
categorically exclude unknown sources, the start of significant emissions
may well be related to the advent of commercial PTFE production in 1947.
Process controls or abatement to reduce the c-C4F8 by-product were
probably not in place in the early decades, explaining the increase in
emissions in the 1960s and 1970s. With the advent of by-product reporting
requirements to the United Nations Framework Convention on Climate Change
(UNFCCC) in the 1990s, concern about climate change and product stewardship,
abatement, and perhaps the collection of c-C4F8 by-product for use
in the semiconductor industry where it can be easily abated, it is
conceivable that emissions in developed countries were stabilized and then
reduced, explaining the observed emission reduction in the 1980s and 1990s.
Concurrently, production of PTFE in China began to increase rapidly. Without
emission reduction requirements, it is plausible that global emissions today
are dominated by China and other developing countries. We predict that
c-C4F8 emissions will continue to rise and that c-C4F8
will become the second most important emitted PFC in terms of
CO2-equivalent emissions within a year or two. The 2017 radiative
forcing of c-C4F8 (0.52 mW m−2) is small but emissions of
c-C4F8 and other PFCs, due to their very long atmospheric
lifetimes, essentially permanently alter Earth's radiative budget and should
be reduced. Significant emissions inferred outside of the investigated
regions clearly show that observational capabilities and reporting
requirements need to be improved to understand global and country-scale
emissions of PFCs and other synthetic greenhouse gases and ozone-depleting
substances.
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