Atmospheric chemistry of C4F9O(CH2)3OC4F9 and CF3CFHCF2O(CH2)3OCF3CFHCF2: Lifetimes, degradation products, and environmental impact

Chemical Physics Letters 427 (2006) 41–46 www.elsevier.com/locate/cplett

Atmospheric chemistry of C4F9O(CH2)3OC4F9 and CF3CFHCF2O(CH2)3OCF3CFHCF2: Lifetimes, degradation products, and environmental impact A.M. Toft a, M.D. Hurley b, T.J. Wallington b, M.P. Sulbaek Andersen c, O.J. Nielsen

c,*

a

University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark Ford Motor Company, P.O. Box 2053, Dearborn, MI 48121-2053, USA University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark b

c

Received 15 May 2006; in final form 13 June 2006 Available online 21 June 2006

Abstract FTIR smog chamber techniques were used to measure k(Cl + CF3CFHCF2O(CH2)3OCF2CFHCF3) = (2.97 ± 0.17) · 1012, k(OH + CF3CFHCF2O(CH2)3OCF2CFHCF3) = (2.45 ± 0.14) · 1013, k(Cl + C4F9O(CH2)3OC4F9) = (1.45 ± 0.16) · 1012, and k(OH + C4F9O(CH2)3OC4F9) = (1.44 ± 0.10) · 1013 cm3 molecule1 s1 in 700 Torr of air at 296 K. The atmospheric lifetimes of CF3CFHCF2O(CH2)3OCF2CFHCF3 and C4F9O(CH2)3OC4F9 with respect to reaction with OH radicals are estimated to be 46 and 83 days, respectively. In 700 Torr of N2/O2 diluent at 296 K, decomposition via CAC bond scission and reaction with O2 are competing loss mechanisms for the alkoxy radicals ROCH(O)CH2CH2OR and ROCH2CH(O)CH2OR (R = C4F9, CF3CFHCF2).  2006 Elsevier B.V. All rights reserved.

1. Introduction Recognition of the adverse environmental impact of chlorofluorocarbon (CFC) release into the atmosphere [1,2] has led to an international effort to replace CFCs with environmentally acceptable alternatives. Hydrofluoroethers (HFEs) are a class of fluid compounds which have been developed to replace CFCs in applications such as the cleaning of electronic equipment, heat transfer agents, and carrier fluids for lubricant deposition [3]. C4F9OCH3 (HFE-7100), C4F9OC2H5 (HFE-7200), n-C3F7OCH3 (HFE-7000), and C3F7CF(OC2H5)CF(CF3)2 (HFE-7500) have attracted commercial interest and several assessments of the atmospheric chemistry and environmental impact of HFEs have been reported [4–9]. CF3CFHCF2O(CH2)3OCF2CFHCF3 and C4F9O(CH2)3OC4F9 are being considered for commercialization. They *

Corresponding author. Fax: +45 35320322. E-mail address: [email protected] (O.J. Nielsen).

0009-2614/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.06.047

are volatile liquids, b.p. 180 and 183 C; vapor pressure 0.12 and 0.09 Torr at 20 C, respectively [3], and are likely to be released into the atmosphere. While many of the previous HFEs that have been studied were intended as replacements for CFCs, these two compounds are out of the boiling point range that would normally be considered for CFC alternatives. Rather, these materials are being evaluated as potential alternatives for high GWP compounds such as perfluorocarbons (PFCs), perfluoropolyethers (PFPEs) and hydrofluoropolyethers (HFPEs). These higher boiling materials are primarily used in heat transfer applications in, for example, the semiconductor industry. Prior to large-scale industrial use an assessment of the atmospheric chemistry, and hence environmental impact, of these compounds is needed. Relative rate methods were used to measure the kinetics of their reaction with Cl atoms and OH radicals. FTIR smog chamber techniques were employed to investigate their atmospheric oxidation products. The results are reported herein and discussed with respect to the environmental impact of these compounds.

42

A.M. Toft et al. / Chemical Physics Letters 427 (2006) 41–46

2. Experimental method Experiments were performed in a 140 liter Pyrex reactor interfaced to a Mattson Sirus 100 FTIR spectrometer [10]. 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 Cl2 + hv ! 2Cl ð1Þ OH radicals were produced by photolysis of CH3ONO  ð2Þ CH3 ONO þ hv ! CH3 Oð Þ þ NO  CH3 Oð Þ þ O2 ! HO2 þ HCHO ð3Þ HO2 þ NO ! OH þ NO2

ð4Þ

In the relative rate experiments the following reactions take place: Cl þ reactant ! products ð5Þ Cl þ reference ! products OH þ reactant ! products

ð6Þ ð7Þ

OH þ reference ! products

ð8Þ

Reaction mixtures consisted of 2.9–13.2 mTorr of reactant, 7.2–7.6 mTorr of reference, and 88.5–94.0 mTorr of Cl2 in 700 Torr total pressure of air diluent. The observed loss of CF3CFHCF2O(CH2)3OCF2CFHCF3 and C4F9O(CH2)3OC4F9 versus those of the reference compounds is shown in Fig. 1. Linear least squares analysis of the data in Fig. 1 gives k10/k12 = 6.20 ± 0.32, k10/k13 = 0.37 ± 0.02, k11/k12 = 2.90 ± 0.21, and k11/k13 = 0.187 ± 0.014. Using k12 = 4.8 · 1013 cm3 molecule1 s1 [11] and k13 = 8.0 · 1012 cm3 molecule1 s1 [12] gives k10 = (2.98 ± 0.15) · 1012, k10 = (2.96 ± 0.16) · 1012, k11 = (1.39 ± 0.11) · 1012 and k11 = (1.50 ± 0.11) · 1012 cm3 molecule1 s1. Indistinguishable values of k10 and k11 are obtained using the two different references. We choose to cite final values which are the average together with error limits which encompass the extremes of the individual determinations; k10 = (2.97 ± 0.17) · 1012 and k11 = (1.45 ± 0.16) · 1012 cm3 molecule1 s1. 3.2. Relative rate study of k(OH + CF3CFHCF2O(CH2)3OCF2CFHCF3) and k(OH + C4F9O(CH2)3OC4F9) The kinetics of reactions (14) and (15) were measured relative to those of (16)

It can be shown that     ½reactantt0 ½referencet0 k reactant ln ln ¼ ½reactantt k reference ½referencet

ð9Þ

where ½reactantt0 , [reactant]t, ½referencet0 and [reference]t are the concentrations of reactant and reference compound at times t0 and t, and kreactant and kreference are the rate constants for the reactions of reactant and of the reference. Plots of lnð½reactantt0 =½reactantt ) versus lnð½referencet0 =½referencet ) should be linear, pass through the origin and have a slope of kreactant/kreference. Concentrations of the reactants and the products were monitored by FTIR spectroscopy. IR spectra were derived from 32 co-added interferograms with a spectral resolution of 0.25 cm1 and an analytical path length of 27 m. Unless stated otherwise, the quoted uncertainties are two standard deviations from least squares regression analyses. Samples of the HFEs were supplied by the 3M Company at purities >99% (by GC). The C4F9O(CH2)3OC4F9 sample contained mixed C4F9 isomers. 3. Results 3.1. Relative rate study of k(Cl + CF3 CFHCF2O(CH2)3OCF2CFHCF3) and k(Cl + C4F9O(CH2)3OC4F9)

OH þ CF3 CFHCF2 OðCH2 Þ3 OCF2 CFHCF3 ! products ð14Þ OH þ C4 F9 OðCH2 Þ3 OC4 F9 ! products

ð15Þ

OH þ C2 H2 ! products

ð16Þ

Initial reaction mixtures consisted of 3.78–9.55 mTorr reactants, 0.74–9.97 mTorr C2H2, and 97–200 mTorr CH3ONO in 700 Torr total pressure of air diluent. Fig. 2 shows the loss of RO(CH2)3OR plotted versus the loss of C2H2. Linear least squares analysis gives k14/k16 = 0.290 ± 0.017 and k15/k16 = 0.170 ± 0.012. Using k16 = 8.45 · 1013 [13] gives k14 = (2.45 ± 0.14) · 1013 and k15 = (1.44 ± 0.10) · 1013 cm3 molecule1 s1. 3.3. Product study of C4F9O(CH2)3OC4F9 and CF3CFHCF2O(CH2)3OCF2CFHCF3 The products of the Cl atom initiated oxidation were studied using mixtures of 5.8–8.16 mTorr HFE, 85.2– 93.0 mTorr Cl2 and 10–700 Torr O2 in 700 Torr total pressure of N2 diluent at 296 ± 2 K. The reaction of Cl atoms with RO(CH2)3OR is expected to proceed via two channels:

The kinetics of reactions (10) and (11) were measured relative to reactions (12) and (13):

Cl þ ROðCH2 Þ3 OR ! ROCHð ÞCH2 CH2 OR þ HCl

Cl þ CF3 CFHCF2 OðCH2 Þ3 OCF2 CFHCF3 ! products

Cl þ ROðCH2 Þ3 OR ! ROCH2 CHð ÞCH2 OR þ HCl

Cl þ C4 F9 OðCH2 Þ3 OC4 F9 ! products

ð10Þ ð11Þ

Cl þ CH3 Cl ! products

ð12Þ

Cl þ C2 H5 Cl ! products

ð13Þ

ð11aÞ ð11bÞ The alkyl radicals produced in reactions (11a) and (11b) will add O2 to give the corresponding peroxy radicals. Selfand cross-reactions of the peroxy radicals will generate the

A.M. Toft et al. / Chemical Physics Letters 427 (2006) 41–46

43

2.5

A: CF3CHFCF2O(CH2)3OCF2CHFCF3 2.0

0.12

Ln ([HFE]t0/[HFE]t)

Ln ([HFE]t0/[HFE]t)

A: CF3CHFCF2O(CH2)3OCF2CHFCF3

0.14

CH3Cl 1.5

1.0

0.10 0.08 0.06 0.04

C2H5Cl 0.5

0.02

0.0 0.0 1.2

0.5

1.0

1.5

2.0

2.5

0.00 0.0

3.0

Ln ([HFE]t0/[HFE]t)

Ln ([HFE] t0/[HFE]t)

0.3

0.4

0.5

0.4

0.5

0.06

CH3Cl

0.6

C2H5Cl

0.04

0.02

0.3

0.0 0.0

0.2

B: C4F9O(CH2)3OC4F9

B: C4F9O(CH2)3OC4F9 0.9

0.1

0.00

0.5

1.0

1.5

2.0

2.5

Ln ([Reference]t0/[Reference]t) Fig. 1. Loss of the HFEs versus those of CH3Cl (squares) and C2H5Cl (triangles) in the presence of Cl atoms in 700 Torr of air at 296 ± 2 K.

alkoxy radicals ROCH(O)CH2CH2OR and ROCH2CH(O)CH2OR, which will either react with O2, or decompose via CAC bond scission [14]. Hence, depending upon the relative importance of channels (11a) and (11b) and the fate of the alkoxy radicals formed, there are four expected products: ROC(O)H, ROCH2C(O)H, ROC(O)(CH2)2OR, and ROCH2C(O)CH2OR. The top two panels in Fig. 3 show spectra recorded following 15 s UV irradiations of mixtures of 7 mTorr C4F9O(CH2)3OC4F9 and 90 mTorr Cl2 in 700 Torr of N2/ O2 diluent with [O2] = 10 (panel A) or 700 Torr (panel B). The 15 s irradiation led to a 17% consumption of C4F9O(CH2)3OC4F9. As seen from comparison of panels A and B there was a significant change in the relative importance of the products with a change in [O2]. Furthermore, it was observed that for a given reaction mixture the relative importance of the reaction products changed with the degree of C4F9O(CH2)3OC4F9 consumption. By comparing product spectra obtained using different [O2] and reactant consumptions, it was possible to separate the three product spectra shown in panels C, D, and E. The product spectra in all experiments could be described by a linear combination of the spectra shown in panels C, D, and E.

0.0

0.1

0.2

0.3

Ln ([C2H2]t0/[C2H2]t) Fig. 2. Loss of the HFEs versus that of C2H2 in the presence of OH radicals in 700 Torr of air at 296 ± 2 K.

We will denote the compounds that give rise to these spectra as product(s) #1, #2, and #3. Experiments were conducted using [O2] = 10, 140, and 700 Torr. The observed formation of products #1, #2, and #3, normalized to the initial C4F9O(CH2)3OC4F9 concentration are plotted as a function of the C4F9O(CH2)3OC4F9 consumption in Fig. 4. We do not have calibrated reference spectra for the products and hence their yields are plotted in arbitrary units. There was no observable formation of COF2 (<2% molar yield for experiments employing <30% HFE consumption) indicating that the ether linkage is retained in the primary products; breaking of the CAO ether linkage would lead to ‘unzipping’ [4] in which COF2 units are eliminated. Comparison of panel E in Fig. 3 with a reference spectrum of n-C4F9OC(O)H given in panel F [4] suggests that product #3 is C4F9OC(O)H. The C4F9-groups in the HFE sample are a mixture of isomers and product #3 is therefore a mixture of C4F9OC(O)H isomers. The small difference in the relative intensity of the absorption features in panels E and F presumably reflects the different isomeric composition of the C4F9OC(O)H samples. Further support for the identification of product #3 as C4F9OC(O)H comes

44

A.M. Toft et al. / Chemical Physics Letters 427 (2006) 41–46

[Product #1]/[HFE]t0

0.08 A: product spectrum [O2] = 10 Torr

0.04 0.00 0.04 0.02

B: product spectrum [O2] = 700 Torr

0.00

0.15

A: C4F9OCH2C(O)H

0.10

0.05

0.00

[Product #2]/[HFE]0

C: C4F9OCH2C(O)H

Absorbance

0.01 0.00 0.10 D: C4F9OC(O)CH2CH2OC4F9 C4F9OCH2C(O)CH2OC4F9

0.05 0.00

0.15

B: C 4F9OCH2C(O)CH2OC4F9 C4F9OC(O)(CH2)2OC4F9

0.10 0.05 0.00

0.2

[Product #3]/[HFE]t0

E: C4F9OC(O)H

0.1 0.0

0.1

0.20

F: n-C4F9OC(O)H

0.0 1600

1700

1800

1900

2000

-1

Wavenumber (cm ) Fig. 3. Product spectra obtained following UV irradiation of C4F9O(CH2)3OC4F9/Cl2/O2/N2 mixtures at 700 Torr total pressure with [O2] = 10 Torr (A) or 700 Torr (B). Panel C shows the spectrum of ‘product #1’ (C4F9OCH2C(O)H). Panel D shows the spectrum of ‘product #2’ (C4F9OC(O)(CH2)2OC4F9 and/or C4F9OCH2C(O)CH2OC4F9). Panel E shows the spectrum of ‘product #3’ (C4F9OC(O)H). Panel F shows the reference spectrum of n-C4F9OC(O)H.

from the observation (see Fig. 4C) that the yield of product #3 decreases with increasing [O2], and increases with increasing C4F9O(CH2)3OC4F9 consumption. C4F9OC(O)H can be formed both as a primary (via decomposition of ROCH(O)CH2CH2OR radicals) and a secondary product (via Cl initiated oxidation of ROCH2C(O)H) in the system. Reaction with O2 will compete with decomposition for the available ROCH(O)CH2CH2OR radicals, hence the yield of C4F9OC(O)H will decrease with increasing [O2]. The importance of Cl initiated oxidation of ROCH2C(O)H, and hence the yield of C4F9OC(O)H, will increase with increasing C4F9O(CH2)3OC4F9 consumption. The product profiles shown in Fig. 4, provide three pieces of information regarding product #1. First, it undergoes secondary oxidation presumably to give C4F9OC(O)H. Second, its yield increases with decreasing [O2], i.e., it is formed via a process that is in competition with reaction with O2. Third, it is reactive towards Cl atoms. It is logical to conclude that product #1 is C4F9OCH2C(O)H. Further support for this identification comes from the frequency of the carbonyl stretching mode centered at 1760 cm1 (see Fig. 3C) which is typically for an aldehyde (HCHO, CH3CHO, and C3H7CHO have carbonyl bands

C: C 4F9OC(O)H 0.3

0.2

0.1

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Δ[HFE]/[HFE]0 Fig. 4. The yields (in arbitrary units) of C4F9OCH2C(O)H (product #1), C4F9OC(O)CH2CH2OC4F9 and/or C4F9OCH2C(O)CH2OC4F9 (product #2), and C4F9OC(O)H (product #3) observed following the Cl atom initiated oxidation of C4F9O(CH2)3OC4F9 in 700 Torr total pressure of N2/O2 diluent at 296 K with [O2] = 10 Torr (squares), 140 Torr (circles), or 700 Torr (triangles).

centered at 1745, 1746, and 1753 cm1, respectively). Fluorine substitution leads to an increase in the carbonyl stretching frequency (e.g., 1944 cm1 for COF2, 1895 cm1 for CF3C(O)F). The carbonyl group in C4F9OCH2C(O)H is remote from fluorine substitution and is expected to have a carbonyl stretching frequency similar to those of un-substituted aldehydes. The shape of the product plots in Fig. 4A contain information on the reactivity of C4F9OCH2C(O)H towards Cl atoms [15], the curves through the data are fits of the following expression to the data ðk =k Þ1

½C4 F9 OCH2 CðOÞH að1  xÞfð1  xÞ 17 11 ¼ ½C4 F9 OðCH2 Þ3 OC4 F9 0 f1  ðk 17 =k 11 Þg

 1g

where x is the fractional consumption of HFE, a is the yield of RCH2C(O)H in the Cl atom initiated oxidation of the HFE, and k11 and k17 are the rate constants of reactions (11) and (17) Cl + C4 F9 OCH2 C(O)H ! products

ð17Þ

Values of k17/k11 = 10.3 ± 1.6, 8.3 ± 2.1, and 7.8 ± 1.9 were obtained from fits to the [O2] = 10, 140, and 700 Torr data, respectively. Within the admittedly large uncertain-

A.M. Toft et al. / Chemical Physics Letters 427 (2006) 41–46

ties, consistent results were obtained from experiments with [O2] = 10–700 Torr. Taking the average of the determinations with uncertainties which encompass the extremes of the individual determinations, k17/k11 = 8.8 ± 3.1, and using the value of k11 reported in Section 3.1 gives k(Cl + C4F9OCH2C(O)H) = (1.3 ± 0.5) · 1011 cm3 molecule1 s1. By a process of elimination, and from the observation that its yield increases with increased [O2] (reflecting the competition for the alkoxy radicals discussed above), we conclude that product #2 is either C4F9OC(O)(CH2)2OC4F9, or (C4F9OCH2)2C(O), or both. The linearity of the yield plots for product #2 in Fig. 4B shows that this product(s) are significantly (at least a factor of 10) less reactive towards Cl atoms than the parent HFE. Esters and ketones are substantially less reactive towards Cl atoms than the corresponding ethers [19] and the identification of product #2 as C4F9OC(O)(CH2)2OC4F9 or (C4F9OCH2)2C(O) is hence consistent with the kinetic observations. As might be anticipated based on its similar chemical structure, the same general observations were made in product studies of the Cl atom initiated oxidation of CF3CFHCF2O(CH2)3OCF2CFHCF3. Three products were observed and they displayed the same behaviour as described above in the C4F9OC(CH2)3OC4F9 experiments. The rate of reaction of Cl atoms with CF3CFHCF2OCH2OC(O)H was determined to be (2.3 ± 0.7) · 1011 cm3 molecule1 s1. 4. Discussion 4.1. Kinetic data The reactivity of chlorine atoms with the HFEs studied in the present work can be compared with values of k(Cl + CH3OCH3) = 1.91 · 1010 [16], k(Cl + CF3OCH3) = 1.4 · 1013 [17], k(Cl + C4F9OCH3) = 9.7 · 1014 [4], k(Cl + C4F9OC2H5) = 2.7 · 1012 [5], and k(Cl + CF3CFHCF3) = (4.5 ± 1.2) · 1017 cm3 molecule1 s1 [18]. The reactivity of Cl atoms towards CF3CFHCF3 is extremely low. There is not expected to be any appreciable reaction of Cl atoms with the CF3CFHCF2A groups in CF3CFHCF2O(CH2)3OCF2CFHCF3. For both of the HFEs investigated herein the reactivity of Cl atoms will be confined to the AO(CH2)3OA moiety. Comparing the reactivity of CH3OCH3 with HFEs, it is clear that fluorination leads to a substantial decrease in the reactivity of the molecule and that the deactivating effect of the fluorine substituents extends across the ether linkage. Our observation that RO(CH2)3OR has a lower reactivity when R = C4F9A is consistent with the fact that the C4F9A group has a greater level of fluorine substitution than the CF3CFHCF2A group. Our determination of k(Cl + C4F9OCH2C(O)H) = (1.3 ± 0.5) · 1011 and k(Cl + CF3CFHCF2OCH2C(O)H) = (2.3 ± 0.7) · 1011 can be compared to k(Cl + CH3C(O)H) = (8.0 ± 1.4) · 1011 [19], k(Cl + C4F9OC(O)H) = (1.6 ± 0.7) · 1014 [4], and k(Cl + C4F9CH2C(O)H) =

45

(1.84 ± 0.30) · 1011 cm3 molecule1 s1 [21]. The reaction of Cl atoms with C4F9CH2C(O)H proceeds via abstraction of the aldehydic hydrogen atom [20]. The deactivating influences of the C4F9OA and CF3CFHCF2OA groups in ROCH2C(O)H are comparable, modest (factor of 4–5 reduction in reactivity compared to that of CH3CHO), and similar to the effect of the C4F9A group in C4F9CH2C(O)H. Consistent with the expectations based upon the discussion above for the Cl atom reactions, it was observed that the reactivity of OH radicals was somewhat greater towards the less fluorinated HFE (R = CF3CFHCF2, kOH = (2.45 ± 0.14) · 1013; R = C4F9, kOH = (1.44 ± 0.10) · 1013 cm3 molecule1 s1). DeMore et al. [21] have proposed an expression relating the rate of hydrogen abstraction from organic compounds by Cl atoms and OH radicals; log k(OH) = (0.412 · log k(Cl))  8.16 [21]. Using the measured k(Cl) values we can predict values of k(OH) = 9.1 · 1014 (R = C4F9) and 1.2 · 1013 (R = CF3CFHCF2) which are within a factor of 2 of the measured values. The relative reactivity of the HFEs studied in the present work towards Cl atoms and OH radicals are consistent with expectations based upon the existing database. The rate constants for the OH radical reactions with the HFEs can be used to estimate the atmospheric lifetime of the two compounds. Using a global weighted-average OH concentration of 1.0 · 106 molecules cm3 [22] leads to estimated lifetimes with respect to reaction with OH radicals of 46 days for CF3CFHCF2O(CH2)3OCF2CFHCF3 and 83 days for C4F9O(CH2)3OC4F9. The approximate nature of these estimates should be stressed. The average daily concentration of OH radicals in the atmosphere varies significantly with both location and season [23]. The values above are therefore estimates of global average lifetimes and local lifetimes could be longer (e.g., in winter with lower temperatures and [OH]), or shorter (e.g., in summer with higher temperatures and [OH]), than those quoted above. As a result of their short atmospheric lifetimes, CF3CFHCF2O(CH2)3OCF2CFHCF3 and C4F9O(CH2)3OC4F9 will have negligible global warming potentials and will not contribute to radiative forcing of climate change. 4.2. Mechanistic data We show that the atmospheric degradation of RO(CH2)3OR (R = C4F9, CF3CFHCF2) leads to the formation of ROC(O)H, ROCH2C(O)H, and either ROC(O)(CH2)2OR, ROCH2C(O)CH2OR, or both. ROC(O)H, ROC(O)(CH2)2OR, and ROCH2C(O)CH2OR are substantially less reactive than their parent ethers. The atmospheric lifetime of C2F5OC(O)H and n-C3F7OC(O)H with respect to reaction with OH radicals is approximately 3–4 years [24]. Similar lifetimes with respect to reaction with OH radicals are expected for C4F9OC(O)H and CF3CFHCF2OC(O)H. The OH radical initiated oxidation of C4F9OC(O)H and CF3CFHCF2OC(O)H is expected to

46

A.M. Toft et al. / Chemical Physics Letters 427 (2006) 41–46

give CO2 and COF2 as major products. Dissolution and hydrolysis in cloud and sea water are competing atmospheric loss mechanisms for fluorinated esters [25]. The lifetime of ROC(O)H, ROC(O)(CH2)2OR, and ROCH2C(O)CH2OR with respect to hydrolysis is unclear and further work is needed. Hydrolysis will give perfluorocarboxylic acids. Perfluorocarboxylic acids with a chain length > 6 carbon atoms are bioaccumulative and the subject of some current research interest. The perfluorocarboxlic acids formed in the atmospheric degradation of CF3CFHCF2O(CH2)3OCF2CFHCF3 and C4F9O(CH2)3OC4F9 have carbon chain lengths < 6, will not bioaccumulate, and are not expected to be of any environmental significance. In conclusion, CF3CFHCF2O(CH2)3OCF2CFHCF3 and C4F9O(CH2)3OC4F9 have short atmospheric lifetimes and will make no significant direct contribution to radiative forcing of climate change. Their atmospheric degradation products are, in the concentrations anticipated in the environment, expected to be benign. Acknowledgements We thank John Owens (3M) for providing the HFE samples and for helpful discussions. M.P.S.A. and O.J.N. acknowledge the financial support from the Danish Natural Science Research Council. References [1] M.J. Molina, F.S. Rowland, Nature 249 (1974) 810. [2] J.D. Farman, B.G. Gardiner, J.D. Shanklin, Nature 315 (1985) 207. [3] J. Owens, 3M Specialty Materials, Private communications, 1999 and 2005. [4] T.J. Wallington et al., J. Phys. Chem. A 101 (1997) 8264.

[5] L.K. Christensen et al., J. Phys. Chem. A 102 (1998) 4839. [6] Y. Ninomiya, M. Kawasaki, A. Guschin, L.T. Molina, M. Molina, T.J. Wallington, Environ. Sci. Technol. 34 (2000) 2973. [7] M. Goto et al., Environ. Sci. Technol. 36 (2002) 2395. [8] L. Chen, S. Kutsuna, K. Tokuhashi, A. Sekiya, R. Tamai, Y. Hibino, J. Phys. Chem. A 109 (2005) 4766. [9] L. Chen, S. Kutsuna, K. Tokuhashi, A. Sekiya, Chem. Phys. Lett. 403 (2005) 180. [10] T.J. Wallington, S.M. Japar, J. Atmos. Chem. 9 (1989) 399. [11] S.P. Sander, et al., JPL Publication No. 02-25, NASA Jet Propulsion Lab., Pasadena, Calif, 2003. [12] P.H. Wine, D.H. Semmes, J. Phys. Chem. 87 (1983) 3572. [13] M. Sørensen, E.W. Kaiser, M.D. Hurley, T.J. Wallington, O.J. Nielsen, Int. J. Chem. Kinet. 35 (2003) 191. [14] J.J. Orlando, G.S. Tyndall, T.J. Wallington, Chem. Rev. 103 (2003) 4657. [15] R.J. Meagher, M.E. McIntosh, M.D. Hurley, T.J. Wallington, Int. J. Chem. Kinet. 29 (1997) 619. [16] M.E. Jenkin, G.D. Hayman, T.J. Wallington, M.D. Hurley, J.C. Ball, O.J. Nielsen, T. Ellermann, J. Phys. Chem. 97 (1993) 11712. [17] L.K. Christensen, T.J. Wallington, A. Guschin, M.D. Hurley, J. Phys. Chem. A 103 (1999) 4202. [18] T.E. Møgelberg, J. Sehested, M. Bilde, T.J. Wallington, O.J. Nielsen, J. Phys. Chem. 100 (1996) 8882. [19] R. Atkinson et al., IUPAC Subcommittee on Gas Kinetic Data Evaluation, Data Sheet X_VOC10. , 2006 (accessed on April 2006). [20] M.D. Hurley, J.C. Ball, T.J. Wallington, M.P. Sulbaek Andersen, D.A. Ellis, J.W. Martin, S.A. Mabury, J. Phys. Chem. A 108 (2004) 5635. [21] M.P. Sulbaek Andersen, O.J. Nielsen, T.J. Wallington, M.D. Hurley, W.B. DeMore, J. Phys. Chem. A 109 (2005) 3926. [22] B.J. Finlayson-Pitts, J.N. Pitts, Chemistry of the Upper and Lower Atmosphere, Academic Press, London, 2000. [23] G. Klecka et al., Evaluation of Persistence and Long-range Transport of Organic Chemicals in the Environment, Academic Press, London, 2000. [24] L. Chen, S. Kutsuna, K. Tokuhashi, A. Sekiya, Chem. Phys. Lett. 400 (2004) 563. [25] S. Kutsuna, L. Chen, T. Abe, J. Mizukado, T. Uchimaru, K. Tokuhashi, A. Sekiya, Atmos. Environ. 39 (2005) 5884.