Chlorofluorocarbons (CFCs) are potent ozone-depleting and greenhouse gases that were phased out of production under the Montreal Protocol agreement (1987) and its subsequent amendments and adjustments. Laube et al. (2014) recently reported the first observation of tetrachloro-1,2-difluoroethane (CCl2FCCl2F, CFC-112), tetrachloro-1,1-difluoroethane (CCl3CClF2, CFC-112a), and 1,1,1-trichloro-2,2,2-trifluoroethane (CCl3CF3, CFC-113a) in the atmosphere with emission sources dating back to the 1960s. The atmospheric mixing ratios in the year 2000 were found to be ∼ 0.5 ppt (parts per trillion) (CFC-112), ∼ 0.08 ppt (CFC-112a), and ∼ 0.3 ppt (CFC-113a), which are minor compared to a total chlorine mixing ratio of 3.3 ppb (parts per billion) (year 2012), where CCl3F (CFC-11), CCl2F2 (CFC-12), and CCl2FCClF2 (CFC-113) account for ∼ 60 % of the total (WMO, 2014). The atmospheric mixing ratio of CFC-112 and CFC-112a was found to have leveled off in the late 1990s, while the mixing ratio of CFC-113a was found to be increasing through to the present day, which is contrary to the objectives of the Montreal Protocol. Laube et al. (2014) estimated the stratospheric lifetimes for these substances, using a tracer–tracer analysis, to be 51 (37–82), 44 (28–98), and 51 (27–264) years for CFC-112, CFC-112a, and CFC-113a, respectively, where the values in parentheses are the range of the lifetimes determined in their analysis. The inferred ozone depletion potentials (ODPs) were 0.88 (0.62–1.44), 0.88 (0.5–2.19), and 0.68 (0.34–3.79) for CFC-112, CFC-112a, and CFC-113a, respectively, where the range in parentheses was derived from the range in the CFC lifetime given above. Atmospheric measurements of CFC-114 are estimated to include a ∼ 10 % fraction due to CFC-114a (WMO, 2014). The atmospheric lifetime of CFC-114a is estimated to be similar to that of CFC-12, i.e., ∼ 100 years (WMO, 2014). It is clear that the CFCs are long-lived compounds and potent ozone-depleting substances and greenhouse gases. It is expected that these compounds would be predominantly removed from the atmosphere via short-wavelength UV photolysis, primarily in the stratosphere. However, to date, there are no UV absorption spectra for these compounds available, which are needed to better evaluate their atmospheric impact.

In this study, UV absorption spectra were measured for CFC-112, CFC-112a, CFC-113a, and 1,1-dichlorotetrafluoroethane (CCl2FCF3, CFC-114a) between 192.5 and 235 nm over the temperature range 207–323 K. The Goddard Space Flight Center (GSFC) 2-D atmospheric model was used to evaluate the reactive and photolytic loss processes and calculate globally averaged lifetimes and ozone depletion potentials. In addition, infrared absorption spectra were measured at 296 K for these compounds and used to estimate their global warming potentials (GWPs). The present results are compared with results from the previous infrared studies of Olliff and Fischer (1992, 1994) (CFCs 112, 112a, 113a, and 114a) and Etminan et al. (2014) (CFC-113a).

The absorption spectrum, σ(λ,T), measurements obeyed Beer's law with fit precisions of ∼ 1 % or less for all wavelengths and temperatures included in this study. Replicate measurements using different sample mixing ratios, bath gas, range of absorption, and optical filtering agreed to within the measurement precision and were combined in a global linear least-squares fit in the final data analysis.

The UV absorption spectra of the CFC-112 and CFC-112a samples were measured at 10 discrete wavelengths between 192.5 and 235 nm at 5 discrete temperatures between 230 and 323 K. The results, not corrected for the isomeric impurity present in the samples, are summarized in Tables S1 and S2 and shown in Figs. S1 and S2 of the Supplement. To account for the isomeric impurity, σ(λ,T) for CFC-112 and CFC-112a were parameterized using the following empirical formula: lnσλ,T=∑iAiλii+T-296∑iBiλii.

The parameterizations reproduced the experimental data to better than ∼ 2 % over the wavelength range most critical to atmospheric photolysis, i.e., between 195 and 215 nm. The results from the 19F NMR sample analysis were then used to obtain the final spectrum parameterizations.

The UV absorption spectra for CFC-113a and CFC-114a were measured at 10 discrete wavelengths between 192.5 and 235 nm at 6 discrete temperatures between 207 and 323 K. The cross section results are given in Tables 1 and 2 and shown in Figs. 1 and 2. The CFC UV absorption spectra were parameterized using Eq. (2). The parameterizations reproduced the experimental data to within ∼ 4 % or better, as shown in Figs. 1 and 2.

The fit parameters are given in Table 3, and a comparison of the parameterized 296 K spectra is shown in Fig. 3. The UV absorption spectra of the CFCs are continuous over the wavelength range included in this study with a precipitous decrease in cross section with increasing wavelength. A decrease in σ(λ,T) with decreasing temperature was observed at nearly all wavelengths included in this study with the temperature dependence being greatest at the longer wavelengths (see Figs. 1, 2, and S1 and S2). The σ(λ, 323 K) measurements, although not entirely atmospherically relevant, were included in the study to better define the absorption spectrum temperature dependence and its parameterization. As shown in Fig. 3, the UV absorption spectra for the CFCs show distinct differences in their absolute cross sections and wavelength dependence over the region most critical for determining their atmospheric photolysis rates, i.e., lifetimes. The spectra demonstrate that CFCs with increased chlorine content are stronger absorbers in this wavelength region, although the molecular structure of the molecule also plays an important role. For example, the C2Cl4F2 isomer with more chlorine atoms on a carbon atom, CFC-112a (CCl3CClF2), absorbs more strongly than CFC-112 (CCl2FCCl2F).

The spectrum parameterizations given in Table 3 reproduce the experimental data very well. The overall 2σ uncertainty in σ(λ,T) for CFC-112, CFC-112a, CFC-113, and CFC-114a, including estimated systematic errors, is estimated to be ∼ 4 % over the range of wavelengths and temperatures included in this study.

The measured infrared spectra for each of the CFCs obeyed Beer's law with a fit precision of ∼ 0.3 % and were independent of total pressure over the pressure range 20–250 Torr (He bath gas). The infrared spectra are shown in Fig. 4, and digitized spectra are available in the Supplement. Table S3 in the Supplement provides a detailed comparison of our results with those of Olliff and Fischer (1992, 1994) for all the CFCs and Etminan et al. (2014) for CFC-113a. Overall the agreement between the studies is better than 10 %.

The atmospheric loss processes, lifetimes, ODPs, and associated uncertainties for the CFCs included in this study were quantified using the Goddard Space Flight Center (GSFC) 2-D atmospheric model (Fleming et al., 2011). The calculations used the UV spectrum parameterizations obtained in this work with an assumed unit photolysis quantum yield at all wavelengths. As discussed in Sect. 3, an overall 2σ uncertainty of 4 % was used at all wavelengths and temperatures for the UV cross sections of the four CFCs. For Lyman-α (121.567 nm), absorption cross sections are not available for these CFCs; values (in units of 10-17 cm2 molecule-1) of 13, 15, 9.8, and 2 were estimated for CFC-112, CFC-112a, CFC-113a, and CFC-114a, respectively, based on values available for similar molecules (see Ko et al., 2013, chap. 3). An estimated Lyman-α cross section uncertainty factor of 2 (2σ) was used. Rate coefficients for the O(1D) reaction with CFC-113a and CFC-114a were taken from Baasandorj et al. (2011) with 2σ uncertainty factors of 1.25 and 1.2, respectively (Burkholder et al., 2015b). Rate coefficients for the O(1D) reaction with CFC-112 and CFC-112a were estimated to be 3 × 10-10 cm3 molecule-1 s-1 with a 0.9 reactive branching ratio and an uncertainty factor of 1.5 (2σ). All other kinetic and photochemical parameters were taken from Sander et al. (2011). All model results presented in this study are for year 2010 steady-state conditions. Surface mixing ratio boundary conditions for 2010 are based on the Laube et al. (2014) results for CFC-112, CFC-112a, and CFC-113a; for CFC-114a, a 2010 value of 1.6 ppt is used, based on measurements of CFC-114, which are a combination of CFC-114 and CFC-114a, with an assumed relative contribution of 10 % for CFC-114a (WMO, 2014).

Model calculations of the CFC fractional atmospheric loss processes are given in Table 4, and the altitude profiles for CFC-112 are shown in Fig. 5. The calculated atmospheric profiles for CFC-112a, CFC-113a, and CFC-114a are provided in the Supplement. UV photolysis is the predominant atmospheric loss process for each of the CFCs. Lyman-α photolysis is important only in the mesosphere above 65 km; it has a negligible contribution to the overall global loss (< 0.001). The O(1D) reaction is a minor stratospheric loss process, ∼ 2 %, for CFC-112, CFC-112, and CFC-113a, but more significant for CFC-114a, ∼ 7 %. The UV photolysis and O(1D) reactive loss of the CFCs leads to the direct release of reactive chlorine and the formation of chlorine containing radicals (Burkholder et al., 2015a).

The CFC lifetimes were computed as the ratio of the annually averaged global atmospheric burden to the vertically integrated annually averaged total global loss rate (Ko et al., 2013). The total global lifetime (τTot) was also separated by the troposphere (τTrop, surface to the tropopause, seasonally and latitude-dependent), stratosphere (τStrat), and mesosphere (τMeso, < 1 hPa) using the total global atmospheric burden and the loss rate integrated over the different atmospheric regions such that 1τTot=1τTrop+1τStrat+1τMeso. The 2-D model total global annually averaged lifetimes and the range in lifetimes are given in Table 5. The 2σ range in the lifetime was calculated using the absolute 2σ maximum and minimum in the UV absorption spectra and estimated Lyman-α cross sections reported in the present work, along with the 2σ uncertainties in the O(1D) rate coefficients taken from Sander et al. (2011). The CFCs are long-lived and primarily removed in the stratosphere by UV photolysis. The uncertainty in the calculated lifetime due to the uncertainty in the UV absorption spectra measured in this work is small, < 3 %. The absolute lifetime uncertainty due to the kinetic and photochemical input parameters is expected to be small compared to that calculated using different atmospheric models due to the individual model treatment of dynamics, chemistry, radiation, numerics, and other processes (Chipperfield et al., 2014; Ko et al., 2013). In general, the lifetime uncertainty due to the kinetic and photochemical input parameters is also expected to be small compared to the uncertainty due to transport processes and actinic fluxes (due to, for example, uncertainty in the J[O2] cross sections; e.g., see Ko et al., 2013). However, evaluation of all processes that contribute to uncertainty in the total CFC lifetime is beyond the scope of the present paper.

The model-calculated stratospheric lifetimes for CFC-112, CFC-112a, and CFC-113a are in reasonable agreement with the values of 51 (37–82), 44 (28–98), and 51 (27–264) years reported by Laube et al. (2014) (uncertainty ranges in parentheses). The lifetimes reported by Laube et al. were based on a tracer–tracer analysis (see Plumb and Ko, 1992; Volk et al., 1997, for method details) using a reference CFC-11 lifetime of 45 years. Scaling to the 52-year CFC-11 lifetime given in WMO (2014) brings the results into better agreement with the present work. The range of lifetimes obtained in the model results, which was determined solely based on the uncertainty in the kinetic and photochemical input parameters, is, however, significantly less than that obtained in the tracer–tracer analysis. It is worth noting that while the total global lifetimes of the isomers CFC-112 and CFC-112a are similar, the lifetimes of CFC-113a (55.4 years) and CFC-114a (105.3 years) are substantially shorter (by ∼ 60 %) than those of the isomers CFC-113 (93 years) and CFC-114 (189 years) (WMO, 2014).

Short-wavelength UV absorption spectra for CCl2FCCl2F (CFC-112), CCl3CClF2 (CFC-112a), CCl3CF3 (CFC-113a), and CCl2FCF3 (CFC-114a) measured in this work between 192.5 and 235 nm and at temperatures in the range 207 to 323 K were combined with 2-D atmospheric model calculations to assess their atmospheric loss processes, lifetimes, and ozone depletion potentials (ODPs). Short-wavelength UV photolysis was shown to be the predominant loss process for the CFCs with global annually averaged lifetimes of 63.6, 51.5, 55.4, and 105.3 years, for CFC-112, CFC-112a, CFC-113a, and CFC-114a, respectively. The uncertainty in the model-calculated lifetimes due to the 2σ uncertainty in the UV absorption spectra reported in this work was found to be small, < 3 %. These CFCs are potent ozone-depleting substances with 2-D model-calculated ODPs of 0.98, 0.86, 0.73, and 0.72 for CFC-112, CFC-112a, CFC-113a, and CFC-114a, respectively. The uncertainty in the model calculated ODPs due to the uncertainty in the UV spectra and O(1D) reactive loss was small, < ± 0.015. These CFCs are also potent greenhouse gases with GWPs comparable to those of the most abundant CFCs present in the atmosphere.

Additional data and figures can be found in the Supplement.