Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Abstract

Chlorofluorocarbons (CFCs) are ozone-depleting substances as well as strong greenhouse gases, and the control of their production and use under the Montreal Protocol has had demonstrable benefits to both mitigation of increasing surface UV radiation and climate forcing. A global ban on consumption came into force in 2010, but there is evidence of continuing emissions of certain CFCs from a range of sources. One compound has received little attention in the literature, namely CFC-114 (C2Cl2F4). Of particular interest here is the differentiation between CFC-114 (CClF2CClF2) and its asymmetric isomeric form CFC-114a (CF3CCl2F) as atmospheric long-term measurements in the peer-reviewed literature to date have been assumed to represent the sum of both isomers with a time-invariant isomeric speciation. Here we report the first long-term measurements of the two isomeric forms separately, and find that they have different origins and trends in the atmosphere. Air samples collected at Cape Grim (41 degrees S), Australia, during atmospheric background conditions since 1978, combined with samples collected from deep polar snow (firn) enable us to obtain a near-complete record of both gases since their initial production and release in the 1940s. Both isomers were present in the unpolluted atmosphere in comparably small amounts before 1960. The mixing ratio of CFC-114 doubled from 7.9 to 14.8 parts per trillion (ppt) between the start of the Cape Grim record in 1978 and the end of our record in 2014, while over the same time CFC-114a trebled from 0.35 to 1.03 ppt. Mixing ratios of both isomers are slowly decreasing by the end of this period. This is consistent with measurements of recent aircraft-based samples showing no significant interhemispheric mixing ratio gradient. We also find that the fraction of CFC-114a mixing ratio relative to that of CFC-114 increased from 4.2 to 6.9% over the 37-year period. This contradicts the current tacit assumption used in international climate change and ozone depletion assessments that both isomers have been largely co-emitted and that their atmospheric concentration ratio has remained approximately constant in time. Complementary observations of air collected in Taiwan indicate a persisting source of CFC-114a in South East Asia which may have been contributing to the changing balance between the two isomers. In addition we present top-down global annual emission estimates of CFC-114 and CFC-114a derived from these measurements using a two-dimensional atmospheric chemistry-transport model. In general, the emissions for both compounds grew steadily during the 1980s, followed by a substantial reduction from the late 1980s onwards, which is consistent with the reduction of emission in response to the Montreal Protocol, and broadly consistent with bottom-up estimates derived by industry. However, we find that small but significant emissions of both isomers remain in 2014. Moreover the inferred changes to the ratio of emissions of the two isomers since the 1990s also indicate that the sources...

Atmospheric histories and emissions of chlorofluorocarbons CFC-13 (CClF<sub>3</sub>), ΣCFC-114 (C<sub>2</sub>Cl<sub>2</sub>F<sub>4</sub>), and CFC-115 (C<sub>2</sub>ClF<sub>5</sub>)

“…A comparison of the SIO-05 primary calibration scale for CFC-114 with the UEA-2014 calibration scales (University of East Anglia, Laube et al, 2016) is of limited value and not straightforward because of the isomer issues addressed earlier. Nevertheless, for near-modern mole fractions (starting about mid-1990s, ∼ 16 ppt), numerically summed CFC-114 and CFC-114a mole fractions reported on the UEA-2014 calibration scales can be converted to SIO-05-reported CFC-114 by multiplication of 1.025 (see Supplement).…”

“…4. The in situ data are binned into monthly means after applying a pol- Laube et al (2016), and the present study (three separate analysis sets are averaged; see Supplement). Light grey bands denote the uncertainty on our SH model results including calibration uncertainty.…”

“…However, our closer inspection of the literature from laboratory studies, used to derive UV absorption spectra and O( 1 D) reaction rates (Sander et al, 2011), suggests that no consideration was given in these older studies with regard to potential impurities of CFC-114a in CFC-114. Such impurities are likely present (Supplement, Laube et al, 2016), in which case they could have biased the CFC-114 UV absorption spectra, leading to an underestimate of its lifetime, as its absorption is significantly weaker compared to that of CFC-114a in the critical photolysis wavelength region (Davis et al, 2016;J. B. Burkholder, personal communication, November 2017).…”

“…Martinerie et al (2009) modeled atmospheric CFC-114 records based on several firn air profiles and found a much earlier atmospheric appearance and larger abundances until approximately 1980 compared to Sturrock et al (2002). In a recent study, Laube et al (2016) reconstructed atmospheric CFC-114 and CFC-114a histories of abundances and emissions based on CGAA and firn air measurements. The study confirmed the temporally variable ratio of the two isomer abundances, and revealed a CFC-114a / CFC-114 emission ratio that increased sharply in the early 1990s but gradually declined thereafter.…”

“…n Ranges in parentheses are due to the 2σ uncertainty in the UV absorption spectra and O( 1 D) rate coefficients included in the model calculations. o Laube et al (2016) adopted an uncertainty range of 83-133 years analogous with the range for CFC-114 from SPARC (2013).…”