Atmospheric chemistry of CF3CF2OCH3

Chemical Physics Letters 653 (2016) 149–154

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Research paper

Atmospheric chemistry of CF3CF2OCH3 Freja F. Østerstrøm a,⇑, Ole John Nielsen a, Timothy J. Wallington b a b

Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark Research and Advanced Engineering, Ford Motor Company, Dearborn, MI 48121-2053, USA

a r t i c l e

i n f o

Article history: Received 5 February 2016 In final form 27 April 2016 Available online 27 April 2016 Keywords: Gas phase reactions Kinetics OH radicals Cl atoms Oxidation products Atmospheric fate Global warming potential

a b s t r a c t Smog chamber Fourier transform infrared techniques were used to investigate the kinetics of the reaction of CF3CF2OCH3 with Cl atoms and OH radicals: k(Cl + CF3CF2OCH3) = (1.09 ± 0.16)
1013 and k(OH + CF3CF2OCH3) = (1.28 ± 0.19)
1014 cm3 molecule1 s1 in 700 Torr total pressure of N2/O2 at 296 ± 2 K. The Cl-initiated oxidation of CF3CF2OCH3 gives CF3CF2OCHO in a yield indistinguishable from 100%. An estimate of k(Cl + CF3CF2OCHO) = (1.18 ± 0.34)
1014 cm3 molecule1 s1 is provided. Based on the OH reaction rate, the atmospheric lifetime of CF3CF2OCH3 is estimated to be 5.0 years. The 100year time horizon global warming potential of CF3CF2OCH3 is estimated to be 585. The atmospheric impact of CF3CF2OCH3 is discussed. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental methods

The chlorofluorocarbons (CFCs) found use as refrigerants, foam blowing, and cleaning agents [1]. CFCs are unreactive in the troposphere but undergo photolysis in the stratosphere leading to chlorine-based catalytic ozone destruction [2,3]. Springtime depletion of stratospheric ozone over Antarctica known as the ‘‘Ozone Hole” reflects the adverse environmental impact of CFCs [4], which led to an international effort to eliminate their use. Hydrochlorofluorocarbons (HCFCs) were developed as CFC replacements and are themselves being phased out. Hydrofluoroethers (HFEs) are a class of CFC and HCFC replacements, HFEs do not contain Cl atoms and hence do not contribute to ozone depletion. Furthermore, HFEs have shorter lifetimes than the CFCs and do not contribute to global warming to the same extent as the CFCs. It is preferable to investigate the atmospheric chemistry of compounds, before their large scale use. Here the atmospheric chemistry of the HFE CF3CF2OCH3 (1,1,1,2,2-pentafluoro-2-methoxyethane) was studied at Ford Motor Company. We investigated the kinetics of the reaction of CF3CF2OCH3 with Cl atoms and OH radicals as well as the products of Cl atom initiated oxidation. The kinetics of the reaction of CF3CF2OCHO (1,1,2,2,2-pentafluoroethyl formate) with Cl atoms was also investigated. The IR absorption spectrum of CF3CF2OCH3 is presented. The atmospheric lifetime and global warming potential of the HFE was estimated.

Smog chamber studies were performed at Ford Motor Company using Fourier transform infrared (FTIR) techniques to investigate the atmospheric fate of CF3CF2OCH3. The smog chamber is a 140 l Pyrex reactor interfaced to a Mattson Sirius 100 FTIR spectrometer [5]. The smog chamber is surrounded by 22 black light GE F40BLB lamps (UVA, peak wavelength = 368 nm). The spectral resolution used in all experiments was 0.25 cm1 and the analytical path length 27.6 m. All experiments were performed in 700 Torr total pressure of N2/O2 diluent at 296 ± 2 K. Experiments were performed with Cl atoms or OH radicals as reaction initiators. The Cl atoms were produced from the photolysis of Cl2:

⇑ Corresponding author. E-mail address: [email protected] (F.F. Østerstrøm). http://dx.doi.org/10.1016/j.cplett.2016.04.086 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

Cl2 þ hm ! 2Cl

ð1Þ

OH radicals were produced by photolysis of CH3ONO in air in the presence of NO:

CH3 ONO þ hm ! CH3 O þ NO

ð2Þ

CH3 O þ O2 ! HCHO þ HO2

ð3Þ

HO2 þ NO ! OH þ NO2

ð4Þ

All reagents except CH3ONO were obtained from commercial sources. CF3CF2OCH3 was purchased from SynQuest Laboratories at a purity of 98% and was further purified by repeated freeze– pump–thaw cycling. Ultrapure N2 (<99.999%), O2 (>99.994%) and

F.F. Østerstrøm et al. / Chemical Physics Letters 653 (2016) 149–154

synthetic air were used as received. CH3ONO was synthesized from the drop-wise addition of 50% H2SO4 to a saturated solution of NaNO2 in methanol in an ice bath, dried by passing through a column of CaCl2, purified by fractional distillation, and stored in the dark at 296 K. The CH3ONO sample was devoid of detectable impurities as analyzed by FTIR. The relative rate method was used to determine the kinetics of the reactions of OH radicals and Cl atoms with CF3CF2OCH3. Kinetic data were derived by monitoring the loss of CF3CF2OCH3 relative to one or more reference compounds. The loss of CF3CF2OCH3 and the reference compounds were then plotted using Eq. (I) [6]:



 ½CF3 CF2 OCH3 t0 kCF3 CF2 OCH3 ½Referencet0 ¼ ln ln ½CF3 CF2 OCH3 t kReference ½Referencet

ðIÞ

[CF3CF2OCH3]t0, [CF3CF2OCH3]t, [Reference]t0, and [Reference]t are the concentrations of CF3CF2OCH3 and the reference compound at times t0 and t. kCF3CF2OCH3 and kReference are the rate coefficients for reactions of OH radicals or Cl atoms with CF3CF2OCH3 and the reference. For the Cl atom kinetic experiments CH4 and CH3Cl were used as reference compounds. For the OH radical kinetic experiments C2H2 was used as the reference compound. To test for unwanted heterogeneous reactions and dark chemistry, typical reaction mixtures were left to stand in the dark for 30 min. No discernable loss of reactants or products was observed suggesting the absence of complicating reactions. The quoted uncertainties include two standard deviations from the linear least squares analyses and the inherent uncertainty from the analysis, usually ±1% of the original reactant concentration. 3. Results and discussion 3.1. Relative rate study of CF3CF2OCH3 + Cl The rate of reaction (5) was measured relative to reactions (6) and (7) using initial reaction mixtures of 6.46–10.58 mTorr CF3CF2OCH3, 147 mTorr Cl2, and either 11.75 mTorr CH4 or 7.35 mTorr CH3Cl in a total pressure of 700 Torr of N2. Mixtures were subjected to UV irradiation for a total of 7–14 s.

CF3 CF2 OCH3 þ Cl ! CF3 CF2 OCH2 þ HCl

ð5Þ

CH4 þ Cl ! Products

ð6Þ

CH3 Cl þ Cl ! Products

ð7Þ

Linear least-squares analyses of the data in Fig. 1 give rate coefficient ratios of k5/k6 = 1.13 ± 0.12 and k5/k7 = 0.22 ± 0.02. Using k6 = 1.0
1013 [7] and k7 = 4.8
1013 [8] gives k5 = (1.13 ± 0.12)
1013 and (1.06 ± 0.11)
1013 cm3 molecule1 s1. We choose to report a final rate coefficient as the average of the two determinations, with an uncertainty which encompasses the uncertainties of the individual determinations. Hence, k5 = (1.09 ± 0.16)
1013 cm3 molecule1 s1. This result is in good agreement with determination by Nohara et al. [9] of k5 = (1.10 ± 0.14)
1013 cm3 molecule1 s1. The rate coefficient is similar to that of other fluorinated ethers with the general molecular formula CxF2x+1OCH3: k(CF3OCH3 + Cl) = (1.4 ± 0.2)
1013 [10], k(n-C3F7OCH3 + Cl) = (9.1 ± 1.3)
1014 [11], and k(n-C4F9OCH3 + Cl) = (9.7 ± 1.4)
1014 cm3 molecule1 s1 [12]. All CxF2x+1OCH3 ethers seem to have rate coefficients that are around 1
1013 cm3 molecule1 s1. 3.2. Product study of CF3CF2OCH3 + Cl Smog chamber studies were performed to investigate the products of the Cl atom initiated degradation of CF3CF2OCH3. The initial

1.5

Ln([CF3CF2OCH3]t0/[CF3CF2OCH3]t)

150

CH4

1.0

CH3Cl 0.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ln([Reference]t0/[Reference]t) Fig. 1. Loss of CF3CF2OCH3 versus the loss of the reference compounds: CH4 (triangles) and CH3Cl (circles) in the presence of Cl atoms.

reaction mixtures were 4.55–8.82 mTorr CF3CF2OCH3, 109.9– 132 mTorr Cl2, and 0–14.7 mTorr NO in 700 Torr of air diluent. Mixtures were subjected to UV irradiation for a total of 145 s–60 min. Fig. 2 shows IR spectra of a mixture of 4.55 mTorr CF3CF2OCH3 and 110 mTorr Cl2 in 700 Torr of air before (panel A) and after (panel B) 70 s UV irradiation. Panel C is a reference spectrum of C(O)F2. Panel D show the residual IR spectrum obtained by subtraction of features attributable to CF3CF2OCH3 (0.57
panel A) and C(O)F2 (panel C) from panel B. Previous studies of the atmospheric oxidation of HFEs of the general formula CxF2x+1OCH3 have shown they are converted into the corresponding formates [13]. Similar behavior is expected for CF3CF2OCH3. The degradation of CF3CF2OCH3 is initiated by Cl atom abstraction of a hydrogen atom from the ACH3 group:

CF3 CF2 OCH3 þ Cl ! CF3 CF2 OCH2 þ HCl

ð5Þ

The alkyl radical that is formed can then react with O2 to form a peroxy radical, this will then react with RO2 (or NO) forming an alkoxy radical:

CF3 CF2 OCH2 þ O2 ! CF3 CF2 OCH2 O2

ð8Þ

CF3 CF2 OCH2 O2 þ RO2 ! CF3 CF2 OCH2 O þ RO þ O2

ð9Þ

The alkoxy radical can react with O2 to give the formate CF3CF2OCHO:

CF3 CF2 OCH2 O þ O2 ! CF3 CF2 OCHO þ HO2

ð10Þ

The residual spectrum shown in panel D in Fig. 2 is assigned to the formate CF3CF2OCHO based on the carbonyl (C@O) stretching features at 1805 and 1822 cm1. For CF3CF2OCH3 consumptions less than 20% these and other features in the IR spectrum shown in Fig. 2D scaled linearly with the loss of CF3CF2OCH3 indicating that CF3CF2OCHO is the sole primary degradation product. As described by Nohara et al. [9] and Wallington et al. [12] the syn and anti conformers of perfluoroalkyl formates can be discerned in their IR spectra. The two features in the carbonyl stretching region Fig. 2D are attributed to the syn (1805 cm1) and anti (1822 cm1) C@O stretching modes in CF3CF2OCHO [9,12]. As seen in Fig. 3, for conversions of CF3CF2OCH3 up to approximately 50% the formate increased linearly with CF3CF2OCH3 loss, after which the slope deviated from linearity and significant amounts of C(O)F2 product are observed. We attribute this behavior to loss of the primary oxidation product CF3CF2OCHO via

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reaction with Cl atoms leading to the formation of C(O)F2 as a secondary product.

1.2 1.0 0.8 0.6

A: Before UV

0.4 0.2 0.8

CF3 CF2 OCHO þ Cl ! COF2 þ other products

ð12Þ

½CF3 CF2 CHOt a ¼ ð1  xÞfð1  xÞðk12 =k11 1Þ  1g ½CF3 CF2 OCH3 t0 1  k12 k11

0.6

B: After UV

0.4

Absorbance

ð11Þ

The concentration of CF3CF2OCHO is related to its yield, a, in reaction (11) and the rate coefficient ratio k12/k11 of reactions (11) and (12) by the expression [14]:

0.0

ðIIÞ

0.2

where x is defined as the fractional conversion of CF3CF2OCHO:

0.0

x1

1.0 0.8 0.6

C: C(O)F2

0.4 0.2 0.0 0.6 0.5 0.4

D: Residual

0.3 0.2 0.1 0.0 800

1000

1200

1400

1600

1800

2000

-1

Wavenumber (cm ) Fig. 2. IR spectra before (panel A) and after (panel B) 70 s of UV irradiation of a mixture of 4.55 mTorr CF3CF2OCH3 and 110 mTorr Cl2 in 700 Torr of air diluent. Panel C shows a reference spectrum of C(O)F2. Panel D is the residual spectrum obtained after subtracting features attributable to CF3CF2OCH3 (0.57
panel A) and C(O)F2 from panel B.

1.0

[CF3CF2OCHO]t/[CF3CF2OCH3 ] t0

CF3 CF2 OCH3 þ Cl ! aCF3 CF2 OCHO þ other products

0.8

0.6

0.4

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Δ[CF3CF2OCH3]/[CF3CF2OCH3 ]t0 Fig. 3. Observed formation of CF3CF2OCHO (black symbols) and C(O)F2 (gray symbols) normalized to the initial concentration of CF3CF2OCH3 versus the fractional loss of CF3CF2OCH3 in the presence of Cl atoms. Experiments were performed in the absence (circles) and presence (triangles) of NO in air. The lines through the black symbols are fits to the data, see text for details. The lines through the gray symbols connect the data to aid visual inspection of the data trends.

½CF3 CF2 CHOt ½CF3 CF2 CHOt0

ðIIIÞ

The curve through the CF3CF2OCHO data in Fig. 3 obtained in the absence of NO (circles) is a fit of expression II to the data with the formate yield, a, assumed to be 1.0 which gives k12/ k11 = 0.09 ± 0.02. Using the value for k11 = (1.09 ± 0.16)
1013 determined above, gives k12 = (1.01 ± 0.24)
1014 cm3 molecule1 s1. The fit of expression II to the data obtained in the presence of NO (triangles) in Fig. 3 gives a = 0.89 ± 0.09 and k12/ k11 = 0.12 ± 0.02. Using the k11 value gives k12 = (1.35 ± 0.23)
1014 cm3 molecule1 s1. The rate coefficient ratios k12/k11 obtained in the absence and presence of NO are indistinguishable within the experimental uncertainties. The average of the two determinations gives k12 = (1.18 ± 0.34)
1014 cm3 molecule1 s1. This result is in agreement with that from Nohara et al. [9] of k12 = (1.2 ± 0.5)
1014 cm3 molecule1 s1. For similar formates the rate coefficients of the reaction with Cl atoms have been found to be k(CF3OCHO + Cl) = (9.8 ± 1.2)
1015 [10], k(n-C3F7OCHO + Cl) = (8.2 ± 2.2)
1015 [11], k(i-C3F7OCHO + Cl) = (1.47 ± 0.56)
1014 [15], and k(n-C4F9OCHO + Cl) = (1.6 ± 0.7)
1014 cm3 molecule1 s1 [12]. These values are indistinguishable within the uncertainties, suggesting that the reactivity of perfluorinated formates with the general formula CxF2x+1OCHO toward Cl atoms is independent of the size of the CxF2x+1 group and is approximately an order of magnitude lower than for the corresponding ethers CxF2x+1OCH3. Previous studies of the degradation of methyl perfluorinated alkyl HFEs of the general formula CxF2x+1OCH3 have found that they are converted into their corresponding formates, CxF2x+1OCHO [9– 12,16]. The results from the present study are consistent with, and extend, this finding to C2F5OCH3. We present results of product studies of the chlorine-atom initiated oxidation CF3CF2OCH3 both in the presence and in the absence of NO. As seen from Fig. 3, for small conversions of the parent ether there is no discernable difference in the formate yield between the experiments performed in the absence and presence of NO, with an initial yield which is indistinguishable from 100% in the absence of NO, and a yield of 89 ± 9% in the presence of NO. The small change of 10% in the formate yield in the presence of NO is close to the uncertainty of the yield. There is no evidence for the so-called ‘‘hot alkoxy effect” [17,18] associated with formation of an alkoxy radical from the peroxy radical reaction with NO that has enough internal energy to decompose immediately. If the hot alkoxy effect was important formation of C(O)F2 would be observed from the beginning of the experiment as a result of CAC scission eventually giving a CF3 radical that would react to form C(O)F2. A maximum of 3% yield of C (O)F2 was observed in the experiment with NO present with a 52% consumption of CF3CF2OCH3 indicating that neither the hot alkoxy effect nor thermal decomposition of the alkoxy radical are important. A possible explanation of the slight decrease in yield of formate in the presence of NO is formation of organic nitrates

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and peroxynitrates from reactions of either the peroxy radical or the alkoxy radical with NO2.

-12.0

(CF3)2CHOCH3

-12.5

3.3. Relative rate study of CF3CF2OCH3 + OH

CF3 CF2 OCH3 þ OH !! CF3 CF2 OCHO

ð13Þ

C2 H2 þ OH ! Products

ð14Þ

An indirect approach was used in the analysis, because OH radicals are much less reactive toward CF3CF2OCH3 than toward C2H2. The formation of the product CF3CF2OCHO was analyzed and used to estimate the loss of CF3CF2OCH3. The calibrations of CF3CF2OCH3 and CF3CF2OCHO were used to quantify the amount of CF3CF2OCH3 remaining in the spectra. Linear least squares analysis of the data in Fig. 4 gives a rate coefficient ratio k13/k14 = 0.015 ± 0.002. Using k14 = 8.45
1013 [19] gives k13 = (1.28 ± 0.14)
1014 cm3 molecule1 s1. Our result agrees with the previous determination by Tokuhashi et al. of k13 = (1.21 ± 0.09)
1014 cm3 molecule1 s1 within the experimental uncertainty [20]. The rate coefficient is indistinguishable from those of similar fluorinated ethers with OH radicals; k(n-C3F7OCH3 + OH) = (1.2 ± 0.3)
1014 [11] and k(n-C4F9OCH3 + OH) = 1.2
1014 cm3 molecule1 s1 [12]. As with the reactivity of Cl atoms toward CxF2x+1OCH3 and CxF2x+1OCHO, the effect of the deactivating effect of the electron withdrawing CxF2x+1 group on the reactivity of OH radicals toward CxF2x+1OCH3 is independent of the size of the CxF2x+1 group. Fig. 5 shows a plot of the OH reactivity versus the Cl atom reactivity for HCFCs and HFEs adapted from Sulbaek Andersen et al. [21]. The data presented as open circles are from Sulbaek Andersen et al. and references in [21]. The data presented as filled circles are collected in Calvert et al. [13] and from other studies [10,15,22–27]. Data for CF3CF2OCH3 from the present work is indicated by the gray star and is consistent with the overall trend for HFEs. The relationship between log(k(OH)) and log(k(Cl)) is described by Eq. (IV) from Sulbaek Andersen et al. [21] and is indicated by the solid line in Fig. 5.

Ln([CF3CF2OCH3]t0/[CF3CF2OCH3]t)

0.006

0.004

0.002

0.000 0.2

0.4

Ln([C2H2]t0/[C2H2]t) Fig. 4. Loss of CF3CF2OCH3 versus the loss of C2H2 in the presence of OH radicals. The line is a linear regression fit to the data.

Log(k(OH))

-13.0

The rate of reaction (13) was measured relative to reaction (14). The initial reaction mixture contained 446.7 mTorr CF3CF2OCH3, 147.1 mTorr CH3ONO, and 8.37 mTorr C2H2 in a total pressure of 700 Torr air. The mixture was subjected to UV irradiation for a total 45 min.

0.0

CF3CH2OCH3 CH3OCF2CHF2

-13.5

CHF2OCH2CF3 CF3OCH3

(CF3)2CFOCH3

-14.0 CHF2OCF2OCF2CF2OCHF2

-14.5

CF3CF2OCH3

CHF2OCF2OCHF2

CHF2OCHF2 CF3OCF2CHF2

-15.0

CF3CHFCF2OCHF2 (CF3)2CHOCF3

-15.5

CF3CHFCF2OCF3 CF3OCHF2

-16.0 -20

-18

-16

-14

-12

-10

Log(k(Cl)) Fig. 5. Log(k(OH)) versus log(k(Cl)) for HCFCs and HFCs (open circles), HFEs (filled circles), and CF3CF2OCH3 (gray star), see text for details.

logðkðOHÞÞ ¼ ð0:412  0:049Þ
logðkðClÞÞ  ð8:16  0:72Þ

ðIVÞ

Using Eq. (IV) and k12(CF3CF2OCHO + Cl) = (1.18 ± 0.34)
1014 cm3 molecule1 s1 obtained here gives an estimate of k(CF3CF2OCHO + OH) of approximately 1
1014 cm3 molecule1 s1 with an uncertainty of approximately a factor of two. Chen et al. found k(CF3CF2OCHO + OH) = (1.50 ± 0.11)
1014 cm3 molecule1 s1 [28] consistent with our estimate. 3.4. Atmospheric lifetime, IR spectrum, and global warming potential Organic compounds are removed from the atmosphere via a number of processes: photolysis, wet and dry deposition, and reaction with NO3 radicals, O3, Cl atoms or OH radicals. CF3CF2OCH3 and CF3CF2OCHO do not absorb light with wavelengths >200 nm, so loss due to photolysis is not important in the troposphere [29]. Reactions of NO3 and O3 with saturated compounds such as CF3CF2OCH3 and CF3CF2OCHO are too slow to be of importance in the atmosphere [30]. Kutsuna et al. have estimated an atmospheric lifetime of the order of 4–40 years for CF3CF2OCHO dissolution into the oceans [31]. Reaction of CF3CF2OCH3 with Cl atoms is one order of magnitude faster than the reaction with OH radicals, but the global average concentration of Cl atoms is low; on the order of [Cl] = 1.0
103 molecules cm3 [32]. The global average OH radical concentration is approximately three orders of magnitude larger than the Cl atom concentration, [OH] = 1.0
106 molecules cm3 [33]. Species with lifetimes >1 year are mixed throughout the troposphere and lifetimes can be estimated using the OH rate coefficient at 272 K, with an assumed tropospheric average [OH]  106 molecule cm3 [13]. For the reaction of OH radicals with closely related compound, CF3CF2CF2OCH3, Bravo et al. [34] determined the temperature dependence Ea/R factor to be 2130 which corresponds to a decrease in rate coefficient between 296 and 272 K of approximately 50%. Assuming similar behavior and rate data at 296 K from the present work and from Chen et al. [28] we estimate atmospheric lifetimes for CF3CF2OCH3 and CF3CF2OCHO of 5.0 and 4.2 years, respectively. We conclude that the major loss process of CF3CF2OCH3 and CF3CF2OCHO is via reaction with OH radicals [6,29,35,36]. This in accordance with the WMO estimate of the atmospheric lifetime for CF3CF2OCH3 of 4.9 years [37]. Chen et al. report a lifetime for CF3CF2OCHO of 3.6 years, in accordance with the present estimate [28]. Fig. 6 shows IR spectra of CF3CF2OCH3 and CF3CF2OCHO recorded in 700 Torr of air diluent. The IR spectrum of CF3CF2OCH3

CF3CF2OCH3

6

A

σ (10-18 cm-2 molecule-1)

4

Absorbance (arb. units)

F.F. Østerstrøm et al. / Chemical Physics Letters 653 (2016) 149–154

the radiative efficiency of compound x, and sx is its atmospheric lifetime. The denominator in Eq. (VI) is the absolute global warming potential (AGWP) for CO2 which for a 100-year time horizon is 9.17
1014 W m2 yr kg1 (= 0.722 W m2 yr ppm1) [41]. The atmospheric lifetime and radiative efficiency of CF3CF2OCH3 is used to calculate the GWP of a 100-year time horizon to be GWP100 = 585.

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

5

10 15 20 25 30

[CF3CF2OCH3] (mTorr)

2

4. Atmospheric impact The results from an experimental study of the kinetics and mechanism of the reactions of Cl atoms and OH radicals with CF3CF2OCH3 are presented and discussed. The atmospheric lifetime of CF3CF2OCH3 is determined to be 5.0 years, a radiative efficiency of 0.288 W m2 ppb1 and a GWP100 of 585 are calculated. A radiative efficiency of CF3CF2OCHO was determined to be 0.408 W m2 ppb1. The GWP100 is significantly lower than that of the CFCs which are generally in the range 5,000–15,000 [42]. Atmospheric oxidation of CF3CF2OCH3 initiated by Cl atoms gives CF3CF2OCHO. CF3CF2OCH3 and CF3CF2OCHO do not contain Cl atoms and thus do not contribute to stratospheric ozone depletion. Neither CF3CF2OCH3 nor its oxidation product pose an environmental risk in the expected atmospheric concentrations.

0

CF3CF2OCHO

6

B 4

2

0 800

1200

1600

2000

Wavenumber (cm-1)

Acknowledgements

Fig. 6. IR spectra of CF3CF2OCH3 (A) and CF3CF2OCHO (B) measured in 700 Torr of air diluent at 296 K. The inset shows the linearity of the absorption features.

was obtained by the expansion of known volumes of the ether in the smog chamber and recording IR spectra. The absorbance scales linearly with the concentration of CF3CF2OCH3, as can be seen from the inset in Fig. 6. The IR spectrum of CF3CF2OCHO was obtained from the product study. To calculate the radiative efficiencies of CF3CF2OCH3 and CF3CF2OCHO the method described by Pinnock et al. was used [38]. The integrated absorption cross sections of the spectra in Fig. 6 are found to be (2.25 ± 0.11)
1016 cm molecule1 over the range 600–2000 cm1 for CF3CF2OCH3 and (3.31 ± 0.17)
1016 cm molecule1 over the range 700–2000 cm1 for CF3CF2OCHO. The integrated absorption cross section found here for CF3CF2OCH3 is consistent within the likely combined experimental uncertainties with the study by Imasu et al. of 2.46
1016 cm molecule1 over the range 700–1500 cm1 [39]. The radiative efficiencies are then 0.312 and 0.448 W m2 ppb1 for CF3CF2OCH3 and CF3CF2OCHO, respectively. For compounds that are not well-mixed in the atmosphere a correction to this value is needed. The correction factors, f(s), based on the lifetime of the compounds are calculated using the method by Hodnebrog et al. [40]:

f ðsÞ ¼

153

asb 1 þ c sd

ðVÞ

where a, b, c, and d are constants with values of 2.962, 0.9312, 2.994, and 0.9302, respectively. The correction factors are calculated to be f(s) = 0.92 and 0.91 for CF3CF2OCH3 and CF3CF2OCHO, respectively. The corrected radiative efficiencies are then 0.288 and 0.408 W m2 ppb1 for CF3CF2OCH3 and CF3CF2OCHO, respectively. The radiative efficiency is used in the calculation of the global warming potential (GWP) for a given time horizon, t0 , of the compound using Eq. (VI):

R t0 GWPðxðt 0 ÞÞ ¼

0

F x expðt=sx Þdt R t0 F CO2 RðtÞdt 0

ðVIÞ

where FCO2 is the radiative efficiency of CO2, R(t) is a response function that describes the decay of an instantaneous pulse of CO2, Fx is

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