Continued increase of CFC-113a (CCl<sub>3</sub>CF<sub>3</sub>) mixing ratios in the global atmosphere: emissions, occurrence and potential sources

Abstract

Abstract. Atmospheric measurements of the ozone-depleting substance CFC-113a (CCl3CF3) are reported from ground-based stations in Australia, Taiwan, Malaysia and the United Kingdom, together with aircraft-based data for the upper troposphere and lower stratosphere. Building on previous work, we find that, since the gas first appeared in the atmosphere in the 1960s, global CFC-113a mixing ratios have been increasing monotonically to the present day. Mixing ratios of CFC-113a have increased by 40 % from 0.50 to 0.70 ppt in the Southern Hemisphere between the end of the previously published record in December 2012 and February 2017. We derive updated global emissions of 1.7 Gg yr−1 on average between 2012 and 2016 using a two-dimensional model. We compare the long-term trends and emissions of CFC-113a to those of its structural isomer, CFC-113 (CClF2CCl2F), which still has much higher mixing ratios than CFC-113a, despite its mixing ratios and emissions decreasing since the 1990s. The continued presence of northern hemispheric emissions of CFC-113a is confirmed by our measurements of a persistent interhemispheric gradient in its mixing ratios, with higher mixing ratios in the Northern Hemisphere. The sources of CFC-113a are still unclear, but we present evidence that indicates large emissions in East Asia, most likely due to its use as a chemical involved in the production of hydrofluorocarbons. Our aircraft data confirm the interhemispheric gradient as well as showing mixing ratios consistent with ground-based observations and the relatively long atmospheric lifetime of CFC-113a. CFC-113a is the only known CFC for which abundances are still increasing substantially in the atmosphere.

Trends and emissions of six perfluorocarbons in the Northern Hemisphere and Southern Hemisphere

“…As many samples as possible are analysed on both columns to provide a large suite (50+ compounds) of trace gas measurements (e.g. Adcock et al, 2018;Laube et al, 2016;Leedham Elvidge et al, 2018) (Table 2). For the current work, the CP-PLOT column is the most relevant, as it has the ability to separate the higher molecular weight PFCs and their isomers based on their boiling point and symmetry.…”

“…Ion m/z 169 is used for trend analysis, unless a baseline distortion occurred in the chromatogram, in which case the 219 ion is used. Other studies that used the same set-up and method include Laube et al (2016) and Adcock et al (2018).…”

“…The air sampled at Cape Grim under baseline conditions has typically travelled over the open ocean many days prior and is known to be very wellmixed (Fraser et al, 1999). The Cape Grim time series is therefore an important data set used to study Southern Hemisphere baseline trends of various trace gases (Oram et al, 1995;Langenfelds et al, 1996;Fraser et al, 1999;Reeves et al, 2005;Laube et al, 2012;Adcock et al, 2018). The global trend between 1978 and 2010 for c-C 4 F 8 has been discussed in Oram et al (2012) and most recently in Mühle et al (2019), while those for n-C 4 F 10 , n-C 5 F 12 , n-C 6 F 14 , and n-C 7 F 16 have previously been investigated in Laube et al (2012) and Ivy et al (2012a).…”

“…Samples for UEA have been collected since 1994 in electro-polished stainless steel canisters (Rasmussen) until 2000 and in Silcosteel-treated stainless steel canisters (Restek Corp.) since 2001 (Laube et al, 2012;Oram et al, 2012). 4790 E. S. Droste et al: Trends and emissions of six perfluorocarbons (Cape Fuguei) and in the south of Taiwan (Hengchun), starting in the south in 2013 (Laube et al, 2016;Oram et al, 2017;Adcock et al, 2018). Sample collection was aimed at the winter monsoon flow from the north.…”

“…For the lowest molecular weight PFCs, such as CF 4 and hexafluoroethane (C 2 F 6 ), emissions from aluminium production are one of the largest sources. Other sources of PFCs include the use in chemical vapour deposition (CVD) chamber cleaning, plasma etching, refrigerants, fire protection, fluoropolymer production, and -especially for the higher molecular weight PFCs, such as n-C 6 F 14 and n-C 7 F 16 -use as heat transfer fluids and solvents (Robin and Iikubo, 1992;Mazurin et al, 1994;Beu, 2005;EPA, 2008;Hartmann et al, 2013;EDGAR, 2014;Mühle et al, 2019). Atmospheric trends vary, but mixing ratios are increasing for all known PFCs.…”