Global Emissions of Perfluorocyclobutane (PFC-318, c-C4F8) Resulting from the Use of Hydrochlorofluorocarbon-22 (HCFC-22) Feedstock to Produce Polytetrafluoroethylene (PTFE) and related Fluorochemicals

Emissions of the potent greenhouse gas perfluorocyclobutane (c-C4F8, PFC-318, octafluorocyclobutane) into the global atmosphere inferred from atmospheric measurements have been increasing sharply since the early 2000s. We find that these 20 inferred emissions are highly correlated with the production of hydrochlorofluorocarbon-22 (HCFC-22, CHClF2) for feedstock (FS) uses, because almost all HCFC-22 FS is pyrolyzed to produce (poly)tetrafluoroethylene ((P)TFE, Teflon) and hexafluoropropylene (HFP), a process in which c-C4F8 is a known by-product, causing a significant fraction of global c-C4F8 emissions. We find a global emission factor of ~0.003 kg c-C4F8 per kg of HCFC-22 FS pyrolyzed. Mitigation of these c-C4F8 emissions, e.g., through process optimization, abatement, or different manufacturing processes, such as 25 electrochemical fluorination, could reduce the climate impact of this industry. While it has been shown that c-C4F8 emissions from developing countries dominate global emissions, more atmospheric measurements and/or detailed process statistics are needed to quantify country to facility level c-C4F8 emissions.

al., 2021). Mühle et al. (2019) reported that global atmospheric emissions of c-C 4 F 8 began in the late-1960s, reaching a plateau of ~1.2 Gg yr -1 during late-1970s to the late-1980s, followed by a decline to a plateau of ~0.8 Gg yr -1 during the early-1990s to early-2000s, and then increased sharply reaching ~2.2 Gg yr -1 in 2017. Emissions of c-C 4 F 8 from developed countries are regulated and reported under the Kyoto Protocol of the United Nations Framework Convention on Climate 35 Change (UNFCCC). However, these reports from developed countries to UNFCCC only account for a small fraction of global emissions of c-C 4 F 8 inferred from atmospheric measurements (Mühle et al., 2019), similar to the emissions gaps observed for other synthetic GHGs (e.g., Montzka et al., 2018;Mühle et al., 2010;Stanley et al., 2020). This emissions gap results partly from emissions in developing countries, which do not have to be reported to UNFCCC and are therefore missing, and/or from uncertainties in emissions reported by developed countries. To understand the sources of recent global 40 c-C 4 F 8 emissions, Mühle et al. (2019) used Bayesian inversions of atmospheric c-C 4 F 8 measurements made at sites of the Advanced Global Atmospheric Gases Experiment (AGAGE, Prinn et al., 2018) in East Asia and Europe and from an aircraft campaign over India. For 2016, these limited regional measurements allowed Mühle et al. (2019) to allocate ~56% of global c-C 4 F 8 emissions to specific regions with significant emissions from Eastern China (~32%), Russia (~12%), and India (~7%). Spatial patterns of these regional c-C 4 F 8 emissions were roughly consistent with facilities that produce 45 polytetrafluoroethylene (PTFE, Teflon) and related fluoropolymers and the necessary precursor monomers tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), which are produced via the pyrolysis of hydrochlorofluorocarbon-22 (HCFC-22, CHClF 2 ). c-C 4 F 8 , essentially the dimer of TFE, is one of several byproducts/intermediates of this process (Chinoy and Sunavala, 1987;Broyer et al., 1988;Gangal and Brothers, 2015;Harnisch, 1999;Ebnesajjad, 2015). Process control and optimization to reduce the formation of c-C 4 F 8 and other by-products 50 are complex, and under unsuitable conditions c-C 4 F 8 by-production could be as high as 14% (Ebnesajjad, 2015). On the other hand, Murphy et al. (1997) demonstrated that co-feeding several percent of c-C 4 F 8 to the HCFC-22 feed could reduce additional c-C 4 F 8 formation to less than 0.5% of the combined TFE and HFP yield, thus increasing combined TFE and HFP yield to more than 96%. But they also stated that perfect process control may be impractical. In 2018, one of China's largest TFE producer confirmed c-C 4 F 8 by-product formation (Mühle et al., 2019). Unless c-C 4 F 8 is recovered or recycled, excess 55 c-C 4 F 8 may therefore be emitted to the atmosphere, consistent with the observations. Historically, similar c-C 4 F 8 by-product venting occurred in the US and Europe (Mühle et al., 2019), unnecessarily increasing the carbon footprint of this industry.
Note that Ebnesajjad (2015) and e.g. Mierdel et al. (2019) discuss research into the use of electrochemical fluorination (ECF) which may offer significantly reduced by-product formation rates in addition to energy savings and overall waste reduction.
Closely related to c-C 4 F 8 (as a by-product of HCFC-22 pyrolysis) is hydrofluorocarbon-23 (HFC-23, CHF 3 ), also a strong 60 GHG, which has long been known to be a by-product of the actual production of HCFC-22 from chloroform (CHCl 3 ), that is often vented to the atmosphere, unnecessarily increasing the carbon footprint of this industry, despite technical solutions, regulations, and financial incentives (e.g., Stanley et al., 2020).
Here we show that global emissions of c-C 4 F 8 since 2002 are highly correlated with the amount of HCFC-22 produced for feedstock (FS) uses, because almost all this FS HCFC-22 is pyrolyzed to produce TFE/HFP, a process with c-C 4 F 8 as a known by-product. This supports the hypothesis that recent global c-C 4 F 8 emissions are dominated by c-C 4 F 8 by-product emissions from the production of TFE/HFP, PTFE and related fluoropolymers and fluorochemicals.

Atmospheric observations of c-C4F8 and inverse modeling of global emissions
We have extended the 1970-2017 AGAGE in situ c-C 4 F 8 atmospheric measurement record used by Mühle et al. (2019) and 70 produced updated global emissions through 2020. For this we used measurements of c-C 4 F 8 by "Medusa" gas chromatographic systems with quadrupole mass selective detection (GC/MSD) (Arnold et al., 2012;Miller et al., 2008)  In situ data were filtered with the AGAGE statistical method to remove pollution events (Cunnold et al., 2002). Fig. 1 shows the continued increase of pollution free monthly mean c-C 4 F 8 mole fractions in the global atmosphere. The data were then 85 used in conjunction with the AGAGE 12-box two-dimensional model (Rigby et al., 2013) and a Bayesian inverse method to update global emissions (Table 1 and

HCFC-22 feedstock (FS) production data
To investigate if the chemical relationship between HCFC-22 pyrolysis and c-C 4 F 8 by-product (as discussed in the introduction) results in a correlation between HCFC-22 feedstock (FS) production and c-C 4 F 8 emissions, we compiled 100 HCFC-22 FS production statistics (Table 1 and Table 4-1 in the TEAP (2020) report for 2008 to 2018; this report contains data used for the determination of the funding requirement for the Multilateral Fund (MLF) for the implementation of the MP. It also lists totals for A5 countries which show small inconsistencies with the UNEP (2021) data, probably due to recent updates.

Results and Discussion
In agreement with Mühle et al. (2019), our updated global inversion results show that c-C 4 F 8 emissions were relatively stable 110 at ~0.8 Gg yr -1 in the early-1990s to early-2000s. However, in 2002 c-C 4 F 8 emission growth resumed, reaching levels not seen before, with a relatively steady increase to 2.26 Gg yr -1 in 2017 (Table 1 and Fig. 2, black diamonds). Here, we find a stabilization at this emission level from 2017 to 2019, followed by a possible resumed increase in emission growth to 2.32 Gg yr -1 in 2020 (however, differences between the 2017-2020 emissions are not statistically significant). In comparison, global HCFC-22 production for feedstock (FS) uses has increased relatively steadily since the early 1990s, initially driven by 115 FS production in non-A5 (developed) countries (Fig. 2, red circles). This non-A5 growth slowed down in the early-2000s and non-A5 HCFC-22 FS production has been relatively stable since then. The global growth in HCFC-22 FS production since 2002 has been driven by the increase in production in A5 (developing) countries (Fig. 2, blue squares), dominated by China (Fig. 2, open orange squares). This is the time frame of a steady increase of inferred global c-C4F8 emissions. We find a strong correlation between HCFC-22 FS production in A5 (developing) countries and inferred global c-C 4 F 8 emissions (R 2 = 0.97, p < 0.01) (Fig. 3, blue squares and fit, 2002-2019). While HCFC-22 FS production itself does not lead to c-C 4 F 8 by-production and emissions (HFC-23 is by-produced in this process and emitted, Stanley et al. (2020)), the fact that 98-99% of global HCFC-22 FS production is used to produce TFE (~87%) and HFP (~13%), to in turn produce PTFE and related fluoropolymers and fluorochemicals, causes the observed strong correlation with HCFC-22 FS production. This 130 would probably not be the case if a significant fraction of HCFC-22 FS production were used for other processes without c-C 4 F 8 by-production and emissions. Note that the HCFC-22 to TFE route (with c-C 4 F 8 by-product) can also be used to produce HFC-225 isomers and hydrofluoroolefin HFO-1234yf (CF 3 -CF=CH 2 ) (Sherry et al., 2019), with HFO-1234yf being the preferred replacement for HFC-134a (CF 3 -CFH 2 ) in mobile air conditioning (MAC). Note that the EFs of ~0.003 kg/kg or ~0.3% (by weight) of c-C 4 F 8 emitted per HCFC-22 FS used are similar to the optimal production conditions explored by Murphy et al. (1997) of less than 0.5% c-C 4 F 8 by-product of the combined TFE and HFP yield (excluding other by-products).
From 1996 to 2001, before the start of any significant production of HCFC-22 for FS uses in A5 countries, c-C 4 F 8 emissions and non-A5 HCFC-22 FS production were relatively stable (Fig. 2). Assuming that all of the HCFC-22 produced for FS uses 160 in non-A5 countries was pyrolyzed to TFE/HFP with c-C 4 F 8 by-product emissions, an EF of 0.0052 ± 0.0004 kg/kg could be calculated, which is larger than the EF for A5 (developing) countries (or the total global) in recent years. However, it cannot be excluded that other sources, such as the semi-conductor industry, caused emission during this timeframe (but see the small emissions from the semiconductor producing countries Japan and South Korea in Mühle et al., 2019) or that EF reductions have occurred since then. Still, if we multiply this EF with the HCFC-22 FS production in non-A5 countries we 165 could estimate non-A5 country c-C4F8 emissions in recent years and subtract these from total global emissions. From an investigation of the correlation of the remaining c-C 4 F 8 emissions against HCFC-22 FS production in A5 countries, we find the same EF (0.0031 ± 0.0001 kg/kg) as for A5 countries determined earlier, but a negative offset (-0.21 ± 0.05 Gg yr -1 c-C 4 F 8 ). This negative offset indicates that the subtracted estimates of non-A5 c-C 4 F 8 emissions were too high, and thus that an EF of 0.0052 kg/kg (from 20 years ago) may not be applicable to today's non-A5 country  Ultimately, atmospheric measurements covering more facilities that pyrolyze HCFC-22 and/or detailed mass balance statistics would be needed to determine EFs for A5 and non-A5 countries, and how EFs may differ from facility to facility.  (Table 1), results in an EF of 0.0021 ± 0.0003 kg/kg. This is lower than the EF determined for non-A5 countries (or the 175 total global) in recent years, which seems unlikely, as total A5 country HCFC-22 FS production is dominated by China (Fig.   2). Most probably, total Chinese c-C 4 F 8 emissions are larger than those determined for eastern China as several Chinese facilities that likely emit c-C 4 F 8 are outside of the inversion domain used in Mühle et al. (2019). More measurements would be needed to answer this question and similar questions for other parts of the world. and India and that spatial emission patterns were roughly consistent with facilities that produce tetrafluoroethylene (TFE) and/or hexafluoropropylene (HFP) and from these polytetrafluoroethylene (PTFE, Teflon) and related fluoropolymers and fluorochemicals. TFE and HFP are produced via the pyrolysis of hydrochlorofluorocarbon-22 (HCFC-22), a process in which c-C 4 F 8 is a known by-product. In this investigation, we find that this chemical relationship between the HCFC-22 185 pyrolysis and c-C 4 F 8 by-product leads to tight correlations between a) HCFC-22 FS production in A5 (developing) countries and global c-C 4 F 8 emissions and between b) total global HCFC-22 FS production and global c-C 4 F 8 emissions (both from 2002 to 2019). These correlations arise as ~98% of the HCFC-22 FS production is used to produce TFE and HFP via HCFC-22 pyrolysis, with c-C 4 F 8 as by-product. Our results support the hypothesis that current global c-C 4 F 8 emissions are mostly due to avoidable by-product venting during the production of TFE/HFP, PTFE and related fluoropolymer and 190 fluorochemicals. Emission factors are estimated to be ~0.003 kg c-C 4 F 8 emitted per kg of HCFC-22 FS (to produce TFE and HFP) or ~0.3% (by weight). In 2018, one of the largest TFE producer in China confirmed c-C 4 F 8 by-product formation, which, unless recovered or recycled, may lead to c-C 4 F 8 emissions. Historically, similar c-C 4 F 8 by-product venting occurred in the US and Europe, unnecessarily increasing the carbon footprint of this industry. Due to the relatively stable HCFC-22 FS production in non-A5 (developed) countries since 2002, it is not possible to determine whether facilities that pyrolyze 195 HCFC-22 to TFE/HFP in non-A5 (developed) and A5 countries (developing) currently emit c-C 4 F 8 at similar rates.

Summary and Conclusions
Atmospheric measurements covering c-C 4 F 8 emissions from more HCFC-22 pyrolyzing facilities in non-A5 and in A5 countries and/or detailed mass balance statistics would be needed to investigate this further and to determine contributions of other countries to global c-C 4 F 8 emissions. Similarly, more atmospheric measurements and/or data are needed to determine Closely related to emissions of c-C 4 F 8 are emissions of hydrofluorocarbon-23 (HFC-23), also a strong GHG, which has long been a known by-product of the actual production of HCFC-22 from chloroform (CHCl 3 ). Emissions of HFC-23 contribute unnecessarily to the carbon footprint of HCFC-22 industry despite technical solutions, regulations, and financial incentives