Document zzaQBR318XVZpYv1M81gXd6pB

ELSEVIER Atmospheric Environment 43 (2009) 3717-3724 Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv ATMOSPHERIC ENVIRONMENT Atmospheric chemistry of perfluorobutenes (CF3CF=CFCF3 and CF3CF2CF=CF2): Kinetics and mechanisms of reactions with OH radicals and chlorine atoms, IR spectra, global warming potentials, and oxidation to perfluorocarboxylic acids Cora J. Young a, Michael D. Hurley b, Timothy J. Wallington b' *, Scott A. Maburya'* 'Department of Chemistry, University of Toronto, 80 St George Street, Toronto, ON, Canada MSS 3H6 b Ford Motor Company, Mail Drop SRL-3083, Dearborn, MI 48121, USA ARTICLE INFO Article history: Received 10 February 2009 Received in revised form 6 April 2009 Accepted 7 April 2009 Keywords: Fluorinated alkene Atmospheric oxidation Radiative efficiency Perfluorocarboxylic acid ABSTRACT Relative rate techniques were used to determine k(Cl + CF3CF=CFCF3) = (7.27 0.88) x 10-12, k(Cl + CF3CF2CF=CF2) = (1.79 0.41) x 10-11, k(OH + CF3CF=CFCF3) = (4.82 1.15) x 10-13, and k(OH + CF3CF2CF=CF2) = (1.94 0.27) x 10-12 cm3 molecule-1 s-1 in 700 Torr of air or N2 diluent at 296 K. The chlorine atom- and OH radical-initiated oxidation of CF3CF=CFCF3 in 700 Torr of air gives CF3C(0)F in molar yields of 196 11 and 218 20%, respectively. Chlorine atom-initiated oxidation of CF3CF2CF=CF2 gives molar yields of 97 9% CF3CF2C(0)F and 97 9% COF2. OH radical-initiated oxidation of CF3CF2CF=CF2 gives molar yields of 110 15% CF3CF2C(0)F and 99 8% COF2. The atmospheric fate of CF3CF2C(0)F and CF3C(0)F is hydrolysis to give CF3CF2C(0)0H and CF3C(0)0H. The atmospheric lifetimes of CF3CF=CFCF3 and CF3CF2CF=CF2 are determined by reaction with OH radicals and are approximately 24 and 6 days, respectively. The contribution of CF3CF=CFCF3 and CF3CF2CF=CF2 to radiative forcing of climate change will be negligible. 2009 Elsevier Ltd. All rights reserved. 1. Introduction Following ratification of the Montreal Protocol and related agreements, there has been an international effort to replace ozone-depleting chemicals with more environmentally friendly alternatives. Hydrofluorocarbons are attractive alternatives because the strength of the C--F and H--F bonds precludes catalytic destruction of stratospheric ozone (Wallington et al., 1995). However, C--F bonds absorb in the optically thin spectral region of the atmosphere known as the atmospheric window, leading to a high radiative efficiency of fluorinated compounds. Saturated perfluorocarbons and some hydrofluorocarbons (HFCs) have high radiative efficiencies and are long-lived in the atmosphere, causing them to be classified as long-lived greenhouse gases. Atmospheric lifetimes of HFCs are typically determined by their reaction with hydroxyl radicals. The presence of )C=C( double bond in an HFC leads to an increase in rate of reaction with hydroxyl radicals and reaction with ozone. Fluorinated alkenes have been proposed as potential replacements, because of their greatly reduced atmospheric lifetimes relative to saturated HFCs. The atmospheric chemistry of several fluorinated alkenes has already * Corresponding authors. E-mail addresses: (S.A. Mabury). ford.com (T.J. Wallington), chem.utoronto.ca 1352-2310/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.04.025 been studied. They have been shown to have lifetimes (determined mainly by reaction with OH and to a lesser extent by reaction with ozone) on the order of days or weeks (Acerboni et al., 2001; Hurley et al., 2007; Mashino et al., 2000; Nielsen et al., 2007; Orkin et al., 1997; Papadimitriou et al., 2008; Sondergaard et al., 2007) and consequently negligible global warming potentials (Acerboni et al., 2001; Hurley et al., 2007; Nielsen et al., 2007; Papadimitriou et al., 2008; Sondergaard et al., 2007). The perfluorobutenes (CF3CF= CFCF3 and CF3CF2CF=CF2) are being considered for industrial applications. Prior to the use of these compounds information on their atmospheric chemistry and environmental impact is needed. Such information is not currently available. The objective of this study was to examine the atmospheric chemistry of CF3CF=CFCF3 and CF3CF2CF=CF2. Experiments were performed to provide the following information: (i) chlorine atom reaction rates; (ii) hydroxyl radical reaction rates; (iii) products of chlorine atom and hydroxyl radical-initiated oxidation; and (iv) infrared spectra. 2. Methods 2.1. Chemicals A commercial sample of CF3CF=CFCF3 (97%) was obtained from ABCR Chemicals (Karlsruhe, Germany) and consisted of 3718 C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 approximately 70% trans and 30% cis isomers. A sample of CF3CF2CF]CF2 was custom synthesized by ABCR Chemicals (Karlsruhe, Germany). CH3ONO was synthesized by the drop-wise addition of concentrated sulfuric acid to a saturated solution of NaNO2 in methanol. All other reagents were obtained from commercial sources. Chemicals were subjected to repeated freeze- pump-thaw cycling to remove volatile impurities before use. 2.2. Kinetics Experiments were performed in a 140 L Pyrex reactor interfaced to a Mattson Sirus 100 FTIR spectrometer. The reactor was surrounded by 22 fluorescent blacklamps (GE F15T8-BL) which were used to photochemically initiate the experiments. Chlorine atoms were produced by photolysis of molecular chlorine: Cl2Dhv/ClDCl Hydroxyl radicals were produced by photolysis of CH3ONO in air: CH3ONODhv/CH3ODNO CH3ODO2/HO2DHCHO HO2DNO/OHDNO2 In relative rate experiments the following reactions take place: OH=ClDReactant/products spectra (which we estimate add an additional 5% uncertainty to the molar product yields) or uncertainties associated with the reference rate constants (which we estimate add an additional 10% uncertainty to the rate constants reported herein). 2.3. Products Products of the chlorine atom- and OH radical-initiated oxidation of CF3CF2CF]CF2 and CF3CF]CFCF3 were monitored using FTIR spectroscopy. Mixtures to study the reaction of chlorine with CF3CF]CFCF3 consisted of 4.9 mTorr CF3CF]CFCF3 and 101 mTorr Cl2. To study the reaction of chlorine with CF3CF2CF]CF2, mixtures of 3.5-8.4 mTorr CF3CF2CF]CF2 and 100 mTorr Cl2 were employed. The reaction of CF3CF2CF]CF2 with hydroxyl radicals was studied using mixtures of 9.7-9.9 mTorr CF3CF2CF]CF2, 100 mTorr CH3ONO, and 50 mTorr NO. Reaction mixtures to study the reaction of CF3CF]CFCF3 with hydroxyl radicals were composed of 4.7 mTorr CF3CF]CFCF3 and 100 mTorr CH3ONO. All product experiments were performed in 700 Torr total pressure of air. 3. Results and discussion 3.1. Kinetics of reactions with Cl atoms The kinetics of reaction (1) were measured relative to reactions (2) and (3) ClDCF3CF]CFCF3/products (1) OH=ClDReference/products It can be shown that: ! ln Xt Xt0 ! kkrXef ln ! ref t ref t0 where Xt0 , [X]t, ref t0 , and [ref]t are the concentrations of the compound of interest and reference at times t0 and t, and kX and kref are the rate constants for the reactant and the reference. Mixtures used to determine k(Cl CF3CF]CFCF3) were composed of 4.1- 11.5 mTorr CF3CF]CFCF3, 100 mTorr Cl2, and either 4.8 mTorr C2H2 or 25.2 mTorr CH3CH2Cl. Reaction mixtures used to determine k(Cl CF3CF2CF]CF2) consisted of 2.8-13.2 mTorr CF3CF2CF]CF2, 100 mTorr Cl2, and either 2.2-2.9 mTorr C2H2 or 26.4 mTorr C2H5Cl. To test for potential complications caused by the formation of CF3O radicals, rate constant ratios for CF3CF2CF]CF2 were measured in the absence and in the presence (1.4-1.6 mTorr) of NO. NO provides an effective scavenging mechanism for CF3O radicals (Wallington et al., 1994). There was no discernable impact of the presence of NO on the rate constant ratios measured suggesting that complications caused by the presence of CF3O radicals are not significant. Mixtures used to determine k(OH CF3CF]CFCF3) were made up of 3.7-26.0 mTorr CF3CF]CFCF3, 100 mTorr CH3ONO and either 4.3-4.7 mTorr C2H2 or 6.9 mTorr C2H4. Reaction mixtures used to determine k(OH CF3CF2CF]CF2) consisted of 7.9-14.8 mTorr CF3CF2CF]CF2, 100 mTorr CH3ONO and either 2.5-4.7 mTorr C2H2 or 3.4-4.1 mTorr C2H4. All experiments were performed at 296 1 K in 700 Torr total pressure of air or N2 diluent. Concentrations of reactants and products were monitored by FTIR spec- troscopy. Infrared spectra were derived from 32 coadded interferograms with a spectral resolution of 0.25 cm1 and an analytical path length of 27.7 m. Quoted uncertainties include two standard deviations from the linear least-squares regression anal- ysis. Quoted error limits do not include uncertainties associated with absolute calibration of the COF2 and CF3C(O)F reference ClDC2H2/products (2) ClDC2H5Cl/products (3) Fig. 1a shows the loss of CF3CF]CFCF3 versus C2H2 and C2H5Cl following UV irradiation. The lines through the data in Fig. 1a are linear least-squares fits, which give k1/k2 0.136 0.010 and k1/k3 0.952 0.058. Using the effective second-order rate constant measured in air at 700 Torr k2 5.07 1011 cm3 molecule1 s1 (Wallington et al., 1990) and k3 8.04 1012 cm3 molecule1 s1 (Wine and Semmes, 1983), we derive values of k1 of (6.89 0.50) 1012 and (7.65 0.47) 1012 cm3 molecule1 s1. Our final value of k1 is the average of the two determinations, with errors that encompass the extremes of the individual measurements: k1 (7.27 0.88) 1012 cm3 molecule1 s1. The sample of CF3CF]CFCF3 used in our experiments was a mixture of 70% trans and 30% cis isomer. There was no discernable difference in the rates of decay of IR features of the CF3CF]CFCF3 sample and we conclude that there is no substantial difference in the reactivity of the two isomers towards chlorine atoms. This conclusion is similar to that reached for reaction of chlorine atoms with the cis and trans isomers of 2-butene (Kaiser et al., 2007) and the E and Z isomers of CF3CF]CHF (Hurley et al., 2007). The kinetics of reaction (4) were measured relative to reactions (2) and (3): ClDCF3CF2CF]CF2/products (4) Fig. 1b shows the loss of CF3CF2CF]CF2 versus C2H2 and C2H5Cl following UV irradiation. The lines through the data in Fig. 1b are linear least-squares fits, which give k4/k2 0.322 0.048 and k4/k3 2.42 0.31. Using k2 5.07 1011 cm3 molecule1 s1 (Wallington et al., 1990) and k3 8.04 1012 cm3 molecule1 s1 (Wine and Semmes, 1983), we derive values of k4 of a 2.0 C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 3719 fluorine substituents, perfluoropropene is more reactive than the perfluorobutenes. Ln ([CF3CF=CFCF3 ]t0/[CF3CF=CFCF3 ]t) 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Ln ([Reference]t0/[Reference]t) b 2.5 Ln ([CF3CF2CF=CF2 ]t0/[CF3CF2CF=CF2 ]t) 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Ln ([Reference]t0/[Reference]t) Fig. 1. Loss of (a) CF3CF]CFCF3 and (b) CF3CF2CF]CF2 versus C2H2 (:) and C2H5Cl (C) following exposure to Cl atoms. (1.63 0.24) 1011 and (1.95 0.25) 1011 cm3 molecule1 s1. Our final value of k4 is the average of the two determinations, with errors that encompass the extremes of the individual measurements: k4 (1.79 0.41) 1011 cm3 molecule1 s1. Chlorine atoms react approximately 2.5 times faster with CF3CF2CF]CF2 than with CF3CF]CFCF3. The reaction of chlorine atoms with perfluorobutenes proceeds via electrophilic addition to the pC]Co double bond. The lower reactivity of CF3CF]CFCF3 presumably reflects the closer proximity of the electron with- drawing fluorine atom substituents to the pC]Co double bond. To our knowledge, there are no literature data for k1 and k4 to compare with our measurements. Kinetic data are available for the reaction of chlorine atoms with perfluoropropene; k(Cl CF3CF] CF2) (2.7 0.3) 1011 cm3 molecule1 s1 (Mashino et al., 2000). Consistent with its smaller number of electron withdrawing 3.2. Kinetics of reactions with OH radicals The kinetics of reaction (5) were measured relative to reactions (6) and (7): OHDCF3CF]CFCF3/products (5) OHDC2H2/products (6) OHDC2H4/products (7) Fig. 2a shows the loss of CF3CF]CFCF3 versus C2H2 and C2H4 following UV irradiation. The lines through the data in Fig. 2a are linear least-squares fits that give values of k5/k6 0.525 0.080 and k5/k7 0.061 0.009. Using k6 8.45 1013 (Srensen et al., 2003) and k7 8.52 1012 cm3 molecule1 s1 (Calvert et al., 2000) we derive k5 (4.44 0.68) 1013 and (5.20 0.77) 1013 cm3 molecule1 s1, respectively. We cite a final value of k5 that is the average with error limits that include the extremes of the individual determinations: k5 (4.82 1.15) 1013 cm3 molecule1 s1. The sample of CF3CF]CFCF3 used was a mixture of 70% trans and 30% cis isomer. As a result of spectral congestion caused by the presence of CH3ONO and its oxidation products, we were only able to monitor the loss of CF3CF]CFCF3 via its absorption band at 687 cm1. We are unable to provide information on the relative reactivity of the two isomers. The kinetics of reaction (8) were measured relative to reactions (6) and (7): OHDCF3CF2CF]CF2/products (8) OHDC2H2/products (6) OHDC2H4/products (7) Fig. 2b shows the loss of CF3CF2CF]CF2 versus C2H2 and C2H4 following UV irradiation. The lines through the data in Fig. 2b are linear least-squares fits that give values of k8/k6 2.18 0.18 and k8/k7 0.24 0.02. Using the effective second-order rate constants in air at 700 Torr, k6 8.45 1013 (Srensen et al., 2003) and k7 8.52 1012 cm3 molecule1 s1 (Calvert et al., 2000) we derive k8 (1.84 0.15) 1012 and (2.04 0.17) 1012 cm3 molecule1 s1, respectively. We choose to cite a final value of k5 that is the average of the two values together with error limits that encompass the extremes of the individual determinations: k8 (1.94 0.27) 1012 cm3 molecule1 s1. Hydroxyl radicals react approximately four times faster with CF3CF2CF]CF2 than CF3CF]CFCF3. As with the chlorine atoms, OH radicals react with perfluorobutenes via electrophilic addition to the pC]Co double bond. The closer proximity of the electron withdrawing fluorine atoms to the double bond in CF3CF]CFCF3 and steric hinderance associated with the eCF3 groups presumably contributes to its lower reactivity. While the kinetics of reactions (5) and (8) have not been studied previously, we can compare our results to the measurement of k(OH CF3CF]CF2) (2.4 0.3) 1012 cm3 molecule1 s1 (Mashino et al., 2000). Consistent with its smaller number of electron withdrawing fluorine substituents, perfluoropropene is more reactive than the perfluorobutenes. Ln ([CF3CF=CFCF3 ]t0/[CF3CF=CFCF3 ]t) 3720 a 0.25 0.20 0.15 0.10 0.05 0.00 0.0 b 1.0 0.8 0.6 0.4 C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 0.5 1.0 1.5 2.0 Ln ([Reference]t0/[Reference]t) 0.6 0.3 A: Before Irradiation 0.0 0.6 0.3 B: After Irradiation 0.0 0.6 Absorbance 0.3 C: B - 0.43*A 0.0 0.6 0.4 2.5 0.2 D: CF3C(O)F 0.0 800 1000 1200 1400 1600 1800 2000 Wave number (cm-1) Fig. 3. IR spectra acquired before (A) and after (B) UV irradiation of a mixture of 4.85 mTorr CF3CF]CFCF3 and 100 mTorr Cl2 in 700 Torr of air diluent. Panel (C) shows the product spectrum obtained after subtracting features attributable to CF3CF]CFCF3 from panel (B). Panel (D) is a reference spectrum of CF3C(O)F. CF3C(O)F following chlorine atom or OH radical-initiated oxidation of CF3CF]CFCF3 is indistinguishable from 200%. The line through the data in Fig. 4 has a slope of 2 and is provided to illustrate the data trend. We conclude that the atmospheric oxidation of CF3CF]CFCF3 proceeds via quantitative conversion into CF3C(O)F. Ln ([CF3CF2CF=CF2 ]t0/[CF3CF2CF=CF2 ]t) 0.2 6 0.0 0.0 0.5 1.0 1.5 2.0 Ln ([Reference]t0/[Reference]t) Fig. 2. Loss of (a) CF3CF]CFCF3 and (b) CF3CF2CF]CF2 versus C2H2 (:) and C2H4 (C) following exposure to hydroxyl radicals. 3.3. Products of Cl atom- and OH radical-initiated oxidation of CF3CF]CFCF3 and CF3CF2CF]CF2 Fig. 3 shows IR spectra acquired before (A) and after (B) a 10 s UV irradiation of a mixture of 4.85 mTorr CF3CF]CFCF3 and 100 mTorr Cl2 in 700 Torr of air diluent. Subtraction of IR features attributable to CF3CF]CFCF3 gives the residual spectrum shown in panel (C). Comparison of panel (C) with a reference spectrum of CF3C(O)F in panel (D) shows the formation of CF3C(O)F as a major product. Fig. 4 shows a plot of the observed formation of CF3C(O)F versus the loss of CF3CF]CFCF3 following irradiation of CF3CF]CFCF3/Cl2 and CF3CF]CFCF3/CH3ONO/NO mixtures in 700 Torr of air diluent. Linear least-squares fits to the data give slopes of 196 11% for the CF3CF]CFCF3/Cl2 experiments and 218 20% for the CF3CF]CFCF3/CH3ONO/NO experiments. The molar yield of 5 CF3C(O)F Production (mTorr) 4 3 2 1 0 0 1 2 3 CF3CF=CFCF3 (mTorr) Fig. 4. Yield of CF3C(O)F following chlorine atom- (C) and hydroxyl radical- (:) initiated oxidation of CF3CF]CFCF3. The line has a slope of two. C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 3721 Fig. 5 shows IR spectra acquired before (A) and after (B) a 10 s UV irradiation of a mixture of 7.64 mTorr CF3CF2CF]CF2 and 100 mTorr Cl2 in 700 Torr of air diluent. Subtraction of IR features attributable to CF3CF2CF]CF2 from panel (B) gives the spectrum shown in panel (C). Comparison of panel (C) with the reference spectrum in panel (D) shows the formation of COF2 as a major product. Subtraction of IR features attributable to COF2 from panel (C) gives the residual spectrum shown in panel (E). As illustrated by comparison with the reference spectrum for CF3COF in panel (F), the feature centered at approximately 1890 cm1 in panel (E) is consistent with the carbonyl absorption band expected for the formation of an acyl fluoride product. The product responsible for the carbonyl absorption band shown in panel (E) increased linearly with CF3CF2CF]CF2 consumption. Fig. 6a shows a plot of the formation of COF2 versus the loss of CF3CF2CF]CF2 observed following the UV irradiation of CF3CF2CF]CF2/Cl2 and CF3CF2CF]CF2/CH3ONO/NO mixtures in 700 Torr of air diluent. Linear least-squares fits give molar COF2 yields of 97 9% and 99 8% for the chlorine atom and hydroxyl radical-initiated oxidation, respectively. Thus, COF2 is formed with a molar yield which is indistinguishable from 100%. It is expected that CF3CF2C(O)F is formed as a co-product of COF2. We do not have a reference spectrum for CF3CF2C(O)F to compare with the residual spectrum in panel E of Fig. 5. However, as discussed above, the frequency of the absorption band of the unknown product is consistent with that expected for CF3CF2C(O)F. If we assume that the unknown product responsible for the absorption feature at 1890 cm1 shown in Fig. 5e is formed in a molar yield of unity then from the integrated absorption in Fig. 5e over the range 1850- 1930 cm1 and the loss of CF3CF2CF]CF2 (6.69 mTorr) we can calculate an integrated absorption cross-section of 2.62 1017 cm molecule1 for this absorption band. This result is indistinguishable from the integrated absorption cross-section of 2.56 1017 cm molecule1 for the analogous band (1850-1930 cm1) in our CF3COF reference spectrum shown in Fig. 5f. The frequency and magnitude of the absorption in Fig. 5e is consistent with the formation of CF3CF2C(O)F. Assuming that the absorption in Fig. 5e is a6 5 COF2 Production (mTorr) 0.6 0.3 A: Before Irradiation 0.0 0.6 0.3 B: After Irradiation 0.0 0.6 0.3 C: B - 0.47*A Absorbance 0.0 0.6 0.3 D: COF2 0.0 0.4 E: C - 0.64*D 0.2 (CF3CF2C(O)F) 0.0 0.6 F: CF3C(O)F 0.3 0.0 800 1000 1200 1400 1600 Wave number (cm-1) 1800 2000 Fig. 5. IR spectra acquired before (A) and after (B) UV irradiation of a mixture of 7.64 mTorr CF3CF2CF]CF2 and 100 mTorr Cl2 in 700 Torr of air diluent. Panel (C) shows the product spectrum obtained after subtracting features attributable to CF3CF2CF]CF2 from panel (B). Panels (D) and (F) are reference spectra of COF2 and CF3C(O)F. Panel E is the residual spectrum obtained after subtracting features attributable to COF2 from the product spectrum (C). 4 3 2 1 0 0 1 2 3 4 5 6 CF3CF2CF=CF2 (mTorr) b6 5 CF3CF2C(O)F Production (mTorr) 4 3 2 1 00 1 2 3 4 5 6 CF3CF2CF=CF2 (mTorr) Fig. 6. Yields of COF2 (a) and CF3CF2C(O)F (b) following chlorine atom-initiated (C) and hydroxyl radical-initiated (:) oxidation of CF3CF2CF]CF2. The lines have slopes of unity. 3722 C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 attributable to CF3CF2C(O)F, and the carbonyl absorption band centered at 1890 cm1 has the same integrated absorption crosssection as the analogous feature in CF3COF, then we can calibrate the yields of CF3CF2C(O)F. This approach was used to derive the CF3CF2C(O)F data shown in Fig. 6. Product yields for CF3CF2C(O)F of 97 9% and 110 15% were observed from chlorine atom and hydroxyl radical addition, respectively. The atmospheric oxidation of CF3CF2CF]CF2 leads to the formation of both COF2 and CF3CF2C(O)F in molar yields indistinguishable from 100%. 3.4. Proposed oxidation mechanisms The reaction of chlorine atoms and hydroxyl radicals with CF3CF]CFCF3 and CF3CF2CF]CF2 is expected to proceed via addition to the pC]Co double bond leading to a radical which will add O2 to give a peroxy radical. The peroxy radical will react with NO or with another peroxy radical to give an alkoxy radical. The symmetry of CF3CF]CFCF3 leads to a single alkoxy radical (see Fig. 7). In principle this alkoxy radical could decompose via scission of one of two different CeC bonds giving two sets of products. In practice we observed only one product, CF3C(O)F, showing that decomposition occurs essentially exclusively via scission of the central CeC bond. This preference presumably reflects thermochemical factors. The observed formation of COF2 and CF3CF2C(O)F from CF3CF2CF]CF2 in molar yields indistinguishable from 100% shows that irrespective of the attacking radical (Cl or OH) or the carbon atom to which it adds (terminal or interior) the fate of the resulting alkoxy radical is decomposition via scission of the terminal CeC bond. This mechanism is illustrated in Fig. 7b for the radicals generated following addition of OH to the terminal carbon atom. 3.5. Infrared spectra and radiative efficiency of CF3CF]CFCF3 and CF3CF2CF]CF2 IR spectra were recorded at 296 K using 2.2-7.5 mTorr of CF3CF]CFCF3 and 1.79-9.05 mTorr of CF3CF2CF]CF2 (separately) in 700 Torr of air diluent. The IR features scaled linearly with the perfluorobutene concentration. The absolute absorption spectra are shown in Fig. 8. The integrated cross-sections (650-1500 cm1) of CF3CF]CFCF3 and CF3CF2CF]CF2 are 2.88 and 2.14 1016 cm molecule1, respectively. Uncertainties in the cross-section measurement arise from following sources: sample concentration (2%), sample purity (2%), path length (1.5%), spectrum noise (1020 cm2 molecule1), and residual baseline offset after subtraction of background (1.5%). From these individual uncertainties, the total (random) uncertainty in the integrated absorption cross-section is 4%. We prefer to quote a conservative uncertainty of 5%. Hence, the integrated cross-sections of CF3CF]CFCF3 and CF3CF2CF]CF2 are (2.88 0.14) and (2.14 0.11) 1016 cm molecule1, respectively. Pinnock et al. (1995) have presented a simple method that can be used to estimate radiative efficiency from IR absorption spectra. In this method, the region between 0 and 2500 cm1 is divided up into 250 bands 10 cm1 wide. The radiative efficiency for a 0 / 1 ppbv change in atmospheric concentration of the compound can then be calculated using the expression: Radiative efficiency X 250 10 cm1si Fi av s i1 where savi is the average absorption cross-section in band i in units of cm2 molecule1, and Fsi is the radiative forcing per unit crosssection per wavenumber per part-per-billion in band i in units of W m2 (cm1)1 (cm2 molecule 1)1. Using this method, the IR spectra of CF3CF]CFCF3 and CF3CF2CF]CF2 shown in Fig. 8, and the IR spectrum of CFC-11 reported elsewhere (Ninomiya et al., 2000), we calculate radiative efficiencies for CF3CF]CFCF3, CF3CF2CF]CF2, and CFC-11 of 0.32, 0.29, and 0.26 W m2 ppb1, respectively. Although CF3CF]CFCF3 and CF3CF2CF]CF2 are isomers, the radiative efficiency of CF3CF]CFCF3 is about 10% greater reflecting the fact that chemical structure impacts the radiative efficiency of fluorinated compounds (Young et al., 2008). a F F3C CF3 OH F F OH +O2/+NO F O OH CF3 CF3 F3C F /-NO2 F3C F b F F3C FF O HO2 F3C F O2 F OH O CF3 F3 C F F F OH F O OH OH F3C F +O2/+NO F3C F F F /-NO2 F FF FF O O HO2 FF OH F3C O2 F FF FF Fig. 7. Proposed mechanism for the atmospheric oxidation of (a) CF3CF]CFCF3 and (b) CF3CF2CF]CF2 initiated by hydroxyl radicals. Stable products are given in boxes. In part (b), only addition to the terminal carbon is depicted for simplicity. C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 3723 a5 4 (10-18 cm2 molecule-1) 3 2 1 0 600 b5 800 1000 1200 1400 1600 1800 2000 Wave number (cm-1) 4 (10-18cm2 molecule-1) 3 2 1 0 600 800 1000 1200 1400 1600 1800 2000 Wave number (cm-1) Fig. 8. Infrared spectra of (a) CF3CF]CFCF3 and (b) CF3CF2CF]CF2. It should be noted that the model of radiative efficiency utilized here (Pinnock et al., 1995) assumes a uniform distribution of the chemical in question. For short-lived compounds such as CF3CF]CFCF3 and CF3CF2CF]CF2, greater concentrations will be found in the boundary layer, where radiative effects are lower. Radiative efficiencies based on distributions calculated using chemical transport models can be substantially lower for fluoroalkenes (by factors of 23 for C2F4 and 8 for C3F6) than those based on uniform distributions (Acerboni et al., 2001). Thus, our values should be considered upper limits for the radiative efficiencies of CF3CF]CFCF3 and CF3CF2CF]CF2. 4. Atmospheric implications Assuming an average global concentration of hydroxyl radicals of 1 106 molecules cm3 (Prinn et al., 2001) gives lifetimes for CF3CF]CFCF3 and CF3CF2CF]CF2 with respect to reaction with OH of about twenty-four and six days, respectively. The approximate nature of these lifetimes must be stressed. The concentration of hydroxyl radicals, and hence atmospheric lifetime, varies substan- tially with season and latitude. Tropospheric concentrations of chlorine atoms are highly vari- able and uncertain, levels in the marine boundary layer of up to 1 105 molecules cm3 have been reported (Spicer et al., 1998), with a tropospheric average of <5-10 102 molecules cm3 (Singh et al., 1996). Thus, typical lifetimes for CF3CF]CFCF3 and CF3CF2CF]CF2 with respect to reaction with chlorine atoms will be on the order of a few years in the free troposphere, and a few weeks under marine boundary layer conditions. Reaction with chlorine atoms is not expected to be a significant fate of CF3CF]CFCF3 or CF3CF2CF]CF2. Reactions of alkenes with ozone can contribute to degradation of these compounds in the atmosphere. The reaction of ozone with perfluoropropene proceeds with a rate constant of 6.2 1022 cm3 molecule1 s1 (Acerboni et al., 2001). As discussed for chlorine atom and OH radical reactions we would expect the reactivity of the perfluorobutenes to be lower than per- fluoropropene. Hence, the lifetime of the perfluorobutenes with respect to reaction with O3 will probably be greater than 8 years and not significant. We conclude that the atmospheric lifetimes of CF3CF]CFCF3 and CF3CF2CF]CF2 are determined by reaction with hydroxyl radicals and are approximately twenty-four and six days, respectively. The halocarbon global warming potential for CF3CF2CF]CF2 (relative to CFC-11) can be estimated using the expression: HGWP REX sXMCFC-11 1 expt=sX X RECFC-11 sCFC-11MX 1 exp t=sCFC-11 where REX, RECFC-11, MX, MCFC-11, sX, and sCFC-11 are the radiative efficiencies, molecular weights, and atmospheric lifetimes of the molecule of interest and CFC-11, and t is the time horizon over which the forcing is integrated. Assuming s(CF3CF]CFCF3) 24 days, s(CF3CF2CF]CF2) 6 days, and s(CFC-11) 45 years (Forster et al., 2007), we estimate the HGWPs of CF3CF]CFCF3 and CF3CF2CF]CF2 (relative to CFC-11) are 1.3 103 and 2.5 104 for a 100 year horizon, respectively. Relative to CO2, the GWP of CFC-11 on a 100 year time horizon is 4750 (Forster et al., 2007). Thus, we estimate that relative to CO2, the GWPs of CF3CF]CFCF3 and CF3CF2CF]CF2 are approximately 6 and 1, respectively, for a 100 year time horizon. It should be noted that these GWP values are upper limits, due to the likelihood that the radiative efficiency values are over-estimated (see Section 3.5). It is clear that neither CF3CF]CFCF3 nor CF3CF2CF]CF2 will contribute to radiative forcing of climate change. CF3CF]CFCF3 and CF3CF2CF]CF2 do not contain chlorine, bromine, or iodine and hence they will not have any significant impact on stratospheric ozone. Atmospheric oxidation of CF3CF]CFCF3 gives CF3C(O)F, while oxidation of CF3CF2CF]CF2 gives COF2 and CF3CF2C(O)F. COF2 is a common product of the atmospheric oxidation of fluorinated compounds and is the dominant product of the degradation of numerous hydrofluorocarbons (Taketani et al., 2005; Tuazon and Atkinson, 1993), hydrochlorofluorocarbons (Tuazon and Atkinson, 1993) and hydrofluoroethers (Sulbaek Andersen et al., 2005; Tua- zon, 1997). The atmospheric fate of COF2 is hydrolysis to yield CO2 and HF. At the levels expected from atmospheric oxidation of per- fluorobutenes (and HFCs in general) the formation of HF is not of any environmental significance. The fate of CF3C(O)F and CF3CF2C(O)F is hydrolysis to form CF3C(O)OH (trifluoroacetic acid, TFA) and CF3CF2C(O)OH (perfluoropropionic acid, PFPrA), respectively. TFA and PFPrA are members of the class of compounds called perfluorocarboxylic acids (PFCAs) which have attracted attention because they are persistent and accumulate in biota. However, 3724 C.J. Young et al. / Atmospheric Environment 43 (2009) 3717-3724 PFCAs with less than eight carbons such as TFA and PFPrA do not appear to be bioaccumulative (Martin et al., 2003a,b). TFA is widespread in the environment (Scott et al., 2005, 2006) and is produced during the atmospheric degradation of several anthropogenic pollutants (World Meteorological Organization, 2007). TFA has been detected in deep ocean water and appears to be a natural trace component of the oceanic environment (Frank et al., 2002). It is generally accepted that any additional environmental burden of TFA resulting from the atmospheric degradation of HCFCs and HFCs will not have any significant environmental impact (World Meteorological Organization, 2007). 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