Environ. Sci. Technol. 2007, 41, 7389-7395
Atmospheric Chemistry of 2-ethoxy-3,3,4,4,5-pentafluorotetrahydro-2,5-bis[1,2,2,2-tetrafluoro1-(trifluoromethyl)ethyl]-furan: Kinetics, Mechanisms, and Products of Cl Atom and OH Radical Initiated Oxidation M. S. JAVADI AND O. J. NIELSEN Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark T. J. WALLINGTON* AND M. D. HURLEY System Analytics and Environmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053 J. G. OWENS 3M Company, 3M Center 236-3A-03, St. Paul, Minnesota 55144-1000
Smog chamber/FTIR techniques were used to study the atmospheric chemistry of the title compound which we refer to as RfOC2H5. Rate constants of k(Cl + RfOC2H5) ) (2.70 ( 0.36) × 10-12, k(OH + RfOC2H5) ) (5.93 ( 0.85) × 10-14, and k(Cl + RfOCHO) ) (1.34 ( 0.20) × 10-14 cm3 molecule-1 s-1 were measured in 700 Torr of N2, or air, diluent at 294 ( 1 K. From the value of k(OH + RfOC2H5) the atmospheric lifetime of RfOC2H5 was estimated to be 1 year. Two competing loss mechanisms for RfOCH(O•)CH3 radicals were identified in 700 Torr of N2/O2 diluent at 294 ( 1 K; decomposition via C-C bond scission giving a formate (RfOCHO), or reaction with O2 giving an acetate (RfOC(O)CH3). In 700 Torr of N2/O2 diluent at 294 ( 1 K the rate constant ratio kO2/kdiss ) (1.26 ( 0.74) × 10-19 cm3 molecule-1. The OH radical initiated atmospheric oxidation of RfOC2H5 gives RfOCHO and RfOC(O)CH3 as major products. RfOC2H5 has a global warming potential of approximately 55 for a 100 year horizon. The results are discussed with respect to the atmospheric chemistry and environmental impact of RfOC2H5.
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 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). More recently, this effort has expanded to include the development of alternatives to compounds with high global warming potentials. 2-ethoxy* Corresponding author e-mail:
[email protected]. 10.1021/es071175c CCC: $37.00 Published on Web 10/04/2007
2007 American Chemical Society
3,3,4,4,5-pentafluorotetrahydro-2,5-bis[1,2,2,2-tetrafluoro-1(trifluoromethyl)ethyl]-furan, is a volatile liquid (bp 175 °C) with a vapor pressure of 0.5 Torr at 25 °C (4).
This compound has been developed for use in heat transfer applications as an alternative to perfluorocarbons (PFCs), perfluoropolyethers (PFPEs), and hydrofluoropolyethers (HFPEs) which have been shown to have high global warming potentials (5-7). The molecule is fully fluorinated with the exception of an ethoxy group. For convenience for the rest of this paper we will refer to this compound as RfOC2H5. The atmospheric oxidation of RfOC2H5 will be initiated by reaction with OH radicals. RfOC2H5 + OH f RfOCH(•)CH3 + H2O
(1a)
RfOC2H5 + OH f RfOCH2CH2(•) + H2O
(1b)
The alkyl radicals produced in reaction (1) add O2 rapidly to give peroxy radicals, RfOCH(•)CH3 + O2 + M f RfOCH(OO•)CH3 + M
(2a)
RfOCH2CH2(•) + O2 + M f RfOCH2CH2(OO•) + M
(2b)
The peroxy radicals derived from RfOC2H5 will react with NO, NO2, HO2, and other peroxy radicals in the atmosphere (8, 9), for RfOCH(OO•)CH3 radicals: RfOCH(OO•)CH3 + NO f RfOCH(O•)CH3 + NO2
(3a)
RfOCH(OO•)CH3 +NO + M f RfOCH(ONO2)CH3 + M (3b) RfOCH(OO•)CH3 + NO2 + M f RfOCH(OONO2)CH3 + M (4) RfOCH(OO•)CH3 + HO2 f products
(5)
RfOCH(OO•)CH3 + R′O2 f products
(6)
Prior to its large-scale industrial use an assessment of the atmospheric chemistry, and hence environmental impact, of RfOC2H5 is needed. To address this need the atmospheric chemistry of RfOC2H5 was investigated. Smog chamber/FTIR techniques were used to determine the following: (i) the kinetics of the reaction of chlorine atoms with RfOC2H5, (ii) the products of the chlorine atom initiated oxidation of RfOC2H5, (iii) the kinetics of the reaction of hydroxyl radicals with RfOC2H5, and (iv) the IR spectrum of RfOC2H5. The results are reported herein and discussed with respect to the environmental impact of RfOC2H5.
2. Experimental Procedures 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. The loss of RfOC2H5 and formation of VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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products were monitored by FTIR spectroscopy. IR spectra were derived from 32 co-added interferograms with a spectral resolution of 0.25 cm-1 and an analytical path length of 27.4 m. Two sets of experiments were performed. First, relative rate techniques were used to determine the rate constant for reaction of Cl atoms and OH radicals with RfOC2H5. Second, the products of the atmospheric oxidation of RfOC2H5 were investigated by irradiating RfOC2H5/Cl2/O2/N2 mixtures. All samples of RfOC2H5 used in this work were supplied by the 3M Company at a purity >99.9% and were used without further purification. The oxidation of RfOC2H5 was initiated by reaction with Cl atoms, which were generated by the photolysis of molecular chlorine in N2 or air diluent at 700 Torr total pressure at 294 ( 1 K, Cl2 + hν f 2Cl
(7)
Cl + RfOC2H5 f RfOCH(•)CH3 + HCl
(8a)
Cl + RfOC2H5 f RfOCH2CH2(•) + HCl
(8b)
OH radicals were produced by photolysis of CH3ONO in the presence of NO in air, CH3ONO was synthesized by the CH3ONO + hv f CH3O + NO
(9)
CH3O + O2 f HO2 + HCHO
(10)
HO2 + NO f OH + NO2
(11)
dropwise addition of concentrated sulfuric acid to a saturated solution of NaNO2 in methanol. All experiments were performed at 294 ( 1 K. The relative rate method is a well-established technique for measuring the reactivity of Cl atoms and OH radicals with organic compounds. Kinetic data were derived by monitoring the loss of RfOC2H5 relative to one, or more, reference compounds. Providing that RfOC2H5 and the reference are lost only by reaction with the radical of interest (OH or Cl in this work) and neither RfOC2H5 nor reference are reformed in any process, it can be shown that
(
Ln
)
[RfOC2H5]to [RfOC2H5]t
)
(
)
[reference]to kreactant Ln kreference [reference]t
(12)
Where [RfOC2H5]t0, [RfOC2H5]t, [reference]t0, and [reference]t are the concentrations of RfOC2H5 and reference at times t0 and t, and kreactant and kreference are the rate constants for reactions of RfOC2H5 and the reference with OH (or Cl). Plots of Ln([RfOC2H5]t0/[RfOC2H5]t) versus Ln([reference]t0/[reference]t) should be linear, pass through the origin and have a slope of kreactant/kreference. The uncertainties reported in this paper are two standard deviations unless stated otherwise.
3. Results and Discussion 3.1. Relative Rate Study of k(Cl + RfOC2H5). Relative rate experiments were performed using the FTIR system to investigate the kinetics of reaction 8. The techniques used are described in detail elsewhere (11). The rate of reaction 8 was measured relative to reactions 13 and 14: Cl + RfOC2H5 f products
(8)
Cl + C2H5Cl f products
(13)
Cl + CH3OCHO f products
(14)
Reaction mixtures consisted of 2.1-7.8 mTorr of RfOC2H5, 98.5-105.9 mTorr of Cl2, and either 16.0-42.7 mTorr of C2H57390
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FIGURE 1. Top panel, decay of RfOC2H5 versus C2H5Cl and CH3OCHO in the presence of Cl atoms in 700 Torr of either N2 (open symbols) or air (filled symbols). Bottom panel, decay of RfOCHO versus CHD3 and CD4 in the presence of Cl atoms in 700 Torr of air. Cl or 3.1-5.6 mTorr of CH3OCHO, in 700 Torr of air or N2 diluent. The observed loss of RfOC2H5 versus those of reference compounds is plotted in Figure 1. There was no discernible difference between the results obtained in N2 or air diluent. Linear least-squares analysis gives k8/k13 ) 0.36 ( 0.03 and k8/k14 ) 1.86 ( 0.15, errors include two standard deviations and our estimate of uncertainties associated with the IR analysis. Using k13 ) 7.82 × 10-12 (12, 13) and k14 ) 1.38 × 10-12 (14-16) gives k8 ) (2.82 ( 0.24) × 10-12 and k8 ) (2.57 ( 0.20) × 10-12 cm3 molecule-1 s-1, respectively. We choose to cite a final value of k8 which is the average of the individual determinations together with error limits which encompass the extremes of the determinations, hence, k8 ) (2.70 ( 0.36) × 10-12 cm3 molecule-1 s-1. This result can be compared to k(Cl + C2H5OC2H5) ) (3.6 ( 0.3) × 10-10 (17), k(Cl + n-C4F9OC2H5) ) k(Cl + i-C4F9OC2H5) ) (2.7 ( 0.6) × 10-12 (18), and k(Cl + n-C3F7CF(OC2H5)CF(CF3)2) ) (2.3 ( 0.7) × 10-12 cm3 molecule-1 s-1 (19). The molecules n-C4F9OC2H5, i-C4F9OC2H5, n-C3F7CF(OC2H5)CF(CF3)2, and RfOC2H5 are ethers consisting of one fully fluorinated alkyl group and an ethyl group. The fluorinated alkyl group will not be reactive toward Cl atoms. Given their molecular similarity it might be expected that these molecules would react with Cl atoms at similar rates. Consistent with expectations, the reactivities of n-C4F9OC2H5, i-C4F9OC2H5, n-C3F7CF(OC2H5)CF(CF3)2, and RfOC2H5 are indistinguishable within the experimental uncertainties. The reactivity of the ethyl group in these fluorinated ethers is
FIGURE 2. Infrared spectra acquired before (A and C) and after (B and D) UV irradiation of mixtures of 7.4 mTorr RfOC2H5, 104 mTorr Cl2, and 100 Torr O2 (A and B) or 6.7 mTorr RfOC2H5, 98 mTorr Cl2, and 700 Torr O2 (C and D). The experiments were performed in a total pressure of 700 Torr made up with N2. substantially (approximately 70 times) less than in diethyl ether. 3.2. RfOC2H5 Oxidation Mechanism. The atmospheric degradation mechanism of RfOC2H5 was studied using the UV irradiation of RfOC2H5/Cl2/O2/N2 mixtures. Reaction mixtures consisted of 6.7-8.2 mTorr RfOC2H5, 100-400 mTorr Cl2 and 10-700 Torr O2 at a total pressure of 700 Torr in N2 diluent. Comparison of the IR features formed in low and high [O2] experiments revealed that two distinct products, or sets of products, were formed in the chamber. We will label these two products “X” and “Y”. The difference between the products was most pronounced in the carbonyl stretching region. Figure 2 shows IR spectra at 1700-1900 cm-1 obtained before A and after B subjecting a mixture containing 7.4 mTorr RfOC2H5, 104 mTorr Cl2, and 100 Torr of O2 to 50 s of UV irradiation. The doublet features at 1802 and 1810 cm-1 scaled linearly to the loss of RfOC2H5 over the range of RfOC2H5 consumptions of 5-90% in all experiments. This linearity suggests they are attributable to one product which we will call “X”. Similarly the feature at 1834 cm-1 also scaled linearly with RfOC2H5 consumption and we will attribute this feature to product “Y”. The linearity of the formation of X and Y shown in Figure 3 suggests the absence of significant loss of these compounds via secondary reactions. The doublet at 1802 and 1810 cm-1 is characteristic of a fluorinated formate (20-22), whereas the feature at 1834 cm-1 is characteristic of a fluorinated acetate (18). The relative yields of X and Y varied with [O2]. In experiments conducted with high [O2] the yield of X decreased while that of Y increased. Panels C and D in Figure 2 show spectra obtained before C and after D subjecting a mixture containing 6.7 mTorr RfOC2H5, 98 mTorr Cl2, and 700 Torr of O2 to 24 s of UV irradiation. Comparing panel B with D in Figure 2 it can be seen that increased [O2] favors product Y at the expense of X. We have observed similar behavior in
FIGURE 3. Top panel, formation of X and Y versus loss of RfOC2H5 observed following the UV irradiation of a mixture of 6.7 mTorr RfOC2H5, 98 mTorr Cl2 and 700 Torr O2. Bottom panel, observed molar yields of RfOC(O)CH3 and RfOCHO versus the O2 partial pressure following the UV irradiation of RfOC2H5/Cl2/N2/O2 mixtures at 700 Torr total pressure and 294 ( 1 K. studies of n-C4F9OC2H5 (18), i-C4F9OC2H5 (18), and n-C3F7CF(OC2H5)CF(CF3)2 (19). Based upon the previous studies, the observed dependence of X and Y on oxygen partial pressure, and the frequencies of the carbonyl features discussed above, we assign X to the formate RfOCHO and Y to the acetate RfOC(O)CH3. The reaction of Cl atoms with RfOC2H5 in the presence of O2 gives rise to two different peroxy radicals, which undergo self- and cross-reaction to give the corresponding alkoxy radicals: RfOCH(OO•)CH3 + RO2 f RfOCH(O•)CH3 + RO + O2
(6)
RfOCH2CH2(OO•) + RO2 f RfOCH2CH2O• + RO + O2
(15)
Decomposition via C-C bond scission or reaction with O2 are the likely fates of these alkoxy radicals. For RfOCH(O•)CH3 radicals: RfOCH(O•)CH3 + M f RfOCHO + CH3 + M
(16)
RfOCH(O•)CH3 + O2 f RfOC(O)CH3 + HO2
(17)
whereas for RfOCH2CH2O• radicals RfOCH2CH2O• + M f RfOCH2• + HCHO + M VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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RfOCH2CH2O• + O2 f RfOCH2CHO + HO2
(19)
The RfOCH2• radical formed in reaction 18 will add O2 to give a peroxy radical which will undergo subsequent reactions leading to the formation of the alkoxy radical, RfOCH2O•, which reacts with O2 to give the formate RfOCHO (22). Reaction 19 gives an aldehyde, which is expected to be more reactive toward Cl atoms than RfOC2H5 (23) and will lead to the formation of RfOCH2C(O)(O•) radicals which will eliminate CO2 to give RfOCH2• radicals which then are converted to the formate RfOCHO. Increased O2 partial pressure led to an increase in the yield of the acetate and a corresponding decrease in the yield of the formate. The simplest explanation of this observation is that reactions 16 and 17 compete for the available RfOCH(O•)CH3 radicals. In principle the RfOCH(O•)CH3 radicals could decompose via RfO elimination. However, this is unlikely as (i) it would involve C-O bond scission which would be less thermodynamically favorable than C-C bond scission, and (ii) it would lead to a substantial yield of COF2 which was not observed. We can place the yield of RfOC(O)CH3 on an absolute basis by equating the increase in its yield to the decrease in the yield of RfOCHO and by assuming that these two species account for all of the observed loss of RfOC2H5. The yields in Figure 3 were calibrated on the basis of this assumption. The dependence of the product yields in Figure 3 on [O2] can be used to extract a value for k17/k16. Assuming that the fate of RfOCH(O•)CH3 radicals is either unimolecular decomposition via reaction 16 or bimolecular reaction with O2 via reaction 17 and that all RfOCH2CH2(O•) radicals are converted into RfOCHO, then the yield of RfOCHO can be related to the molar yields of the two alkoxy radicals and k17/k16: Y(RfOCHO) ) Y(RfOCH(O•)CH3)
[
]
1 + k17 [O2] + 1 k16 Y(RfOCH2CH2O•) (20)
[ ]
whereas the yield of RfOC(O)CH3 is given by
Y(RfOC(O)CH3) ) Y(RfOCH(O•)CH3)
k17 [O ] k16 2
k17 [O ] + 1 k16 2
+C (21)
The C term in eq 21 accounts for the nonzero intercept in the RfOC(O)CH3 yield versus [O2] plot, which is apparent from Figure 3. There are several possible sources of RfOC(O)CH3 which may explain the intercept: (i) H atom elimination from RfOCH(O•)CH3 radicals, (ii) the molecular channel of the peroxy radical self- and cross-reactions, or (iii) the RfOCH(OO•)CH3 + HO2 reaction. We are not able to distinguish between these possibilities at the present time. It should be noted that the important conclusion from this study, namely that reaction with O2 and decomposition via C-C bond scission are competing atmospheric fates of RfOCH(O•)CH3 radicals, is insensitive to the cause of the intercept. The curves in Figure 3 are fits of the above expressions to the experimental data. The fit to the RfOCHO data gives Y(RfOCH(O•)CH3) ) 0.77 ( 0.08, Y(RfOCH2CH2O•) ) 0.06 ( 0.06, and k17/k16 ) (4.6 ( 1.9) × 10-3 Torr-1. The fit to the RfOC(O)CH3 data gives Y(RfOCH(O•)CH3) ) 0.88 ( 0.09, C ) 0.11 ( 0.04, and k17/k16 ) (3.6 ( 1.3) × 10-3 Torr-1. Indistinguishable values of k17/k16 were obtained from the analyses of the RfOCHO and RfOC(O)CH3 data. We choose to cite a final value of k17/k16 as the average of the two determinations with error limits that encompass the extremes 7392
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of the individual determinations; k17/k16 ) (4.1 ( 2.4) × 10-3 Torr-1. Using k17/k16 ) 4.1 × 10-3 Torr-1 we calculate that reaction with O2 accounts for approximately 40% of the fate of RfOCH(O•)CH3 radicals in 760 Torr of air at 294 K. Temperature and oxygen concentration decrease with increasing altitude and the rates of reactions 16 and 17 will both decrease with increasing altitude within the troposphere. Reaction 16 is a unimolecular decomposition reaction and its rate will decrease more rapidly than reaction 17 with increasing altitude, hence, reaction with O2 will be become a more important loss mechanism for RfOCH(O•)CH3 with increasing altitude. The value of k17/k16 ) (4.1 ( 2.4) × 10-3 Torr-1 can be compared to rate constant ratios of kO2/kdiss ) 0.026 ( 0.010 and 0.013 ( 0.006 Torr-1 reported for the C4F9OCHO(•)CH3 (18) and n-C3F7CF(OCH(O•)CH3)CF(CF3)2 radicals (19), respectively. The decomposition channel is relatively more important for RfOCH(O•)CH3 radicals than for the other two structurally similar fluorinated alkoxy radicals. Finally, it should be noted that for the experimental data presented thus far, the RfOCH(O•)CH3 alkoxy radicals were prepared by the self- and cross-reaction of peroxy radicals. Such reactions are close to thermoneutral and do not lead to the production of chemically activated alkoxy radicals. In the real atmosphere the RfOCH(O•)CH3 alkoxy radicals will be produced by the reaction of RfOCH(OO•)CH3 peroxy radicals with NO. It has been shown that such peroxy + NO reactions can produce chemically activated alkoxy radicals which decompose promptly (24). To test, chemical activation experiments were performed using the UV irradiation of RfOC2H5/Cl2/NO/O2 mixtures in 700 Torr of N2. If chemical activation were important we would expect to observe an increase in the yield of formate and a decrease in the yield of acetate. However, in the experiments conducted in the presence of NO, no such effect was observed. The yields of both the formate and acetate were approximately 20% lower than those observed in the absence of NO (probably reflecting loss of the alkoxy radical via reaction with NO or NO2 giving a nitrite or nitrate). However, the ratio of formate/acetate products in the experiment conducted in the presence of NO was indistinguishable (within 10%) from that observed in the absence of NO. We conclude that chemical activation of RfOCH(O•)CH3 radicals formed in the RfOCH(OO•)CH3 + NO reaction is not significant. 3.3. Relative Rate Study of k(Cl + RfOCHO). As discussed in section 3.2, RfOCHO was readily generated in the chamber by UV irradiation of gas mixtures of 7.8-8.1 mTorr RfOC2H5, 176-328 mTorr Cl2, and 10-700 Torr O2 in 700 Torr of N2 diluent. To provide information on the reactivity of the formate RfOCHO, after all (>97%) of RfOC2H5 was consumed, a reference compound (CHD3 or CD4) was added and the UV irradiation was resumed. RfOCHO + Cl f products
(22)
CHD3 + Cl f products
(23)
CD4 + Cl f products
(24)
The rate constant k22 was derived by observing the relative loss rate of RfOCHO and the reference compounds. Figure 1 shows loss of RfOCHO versus those of the reference compounds. Linear least-squares analysis of the data in Figure 1 gives k22/k23 ) 0.61 ( 0.05 and k22/k24 ) 2.07 ( 0.15. Using, k23 ) 2.32 × 10-14 (11) and k24 ) 6.1 × 10-15 (11), we derive k22 ) (1.42 ( 0.12) × 10-14 and (1.26 ( 0.09) × 10-14 cm3 molecule-1 s-1, respectively. We choose to cite a final value for k22, which is the average of those determined using the two different reference compounds together with error limits that encompass the extremes of the individual determina-
FIGURE 4. Decay of RfOC2H5 versus C2H2 in the presence of OH radicals in 700 Torr of air. tions. Hence, k22 ) (1.34 ( 0.20) × 10-14 cm3 molecule-1 s-1. We can compare this result with k(Cl + CF3OCHO) ) (9.8 ( 1.2) × 10-15 (22), k(Cl + C3F7OCHO) ) (8.2 ( 2.2) × 10-15 (21), and k(Cl + n-C3F7CF(OCHO)CF(CF3)2) ) (9.7 ( 1.4) × 10-15 cm3 molecule-1 s-1 (19). Given the similar structure of the formates CF3OCHO, C3F7OCHO, n-C3F7CF(OCHO)CF(CF3)2, and RfOCHO is not surprising that their reactivities toward Cl atoms are comparable. 3.4. Relative Rate Study of k(OH + RfOC2H5). The kinetics of reaction 1 were measured relative to reaction 25. RfOC2H5 + OH f products
(1)
C2H2 + OH f products
(25)
Reaction mixtures consisted of 40-57 mTorr RfOC2H5, 7.47.5 mTorr C2H2, and 203-210 mTorr CH3ONO in 700 Torr air diluent. The loss of RfOC2H5 was barely discernible (a few %) and difficult to measure directly. To improve the precision of the measurements the loss of RfOC2H5 was monitored indirectly by observing the formation of the formate (RfOCHO) and acetate (RfOC(O)CH3). We assume that for every RfOCHO and RfOC(O)CH3 molecule that is formed a molecule of RfOC2H5 is lost. The results, presented in section 3.2, support this assumption. Figure 4 shows the loss of RfOC2H5 (calculated from the formation of RfOCHO and RfOC(O)CH3) versus the reference compound C2H2. Linear least-squares analysis of the data in Figure 4 gives k1/k25 ) (6.32 ( 0.64) × 10-2. Using k25 ) 8.45 × 10-13 (25), we derive k1) (5.34 ( 0.54) × 10-14 cm3 molecule-1 s-1. Finally, we need to consider the possibility that some alkoxy radicals are lost via reaction with NOx in the system and, hence, that the sum of RfOCHO and RfOC(O)CH3 underestimates the RfOC2H5 loss. The amount of NOx formed in the OH rate experiments was approximately a factor of 2 less that present in the RfOC2H5/ Cl2/NO/O2 mixtures described in section 3.2 where it was observed that the RfOCHO and RfOC(O)CH3 yields were reduced 20%. To be conservative we opt to assume that we might be underestimating the RfOC2H5 loss by up to 20% (i.e., 10 ( 10%); hence, we arrive at a final value of k1) (5.93 ( 0.85) × 10-14 cm3 molecule-1 s-1 This result can be compared to k(OH + C2H5OC2H5) ) 1.3 × 10-11 (26), k(OH + n-C4F9OC2H5) ) (6.4 ( 0.7) × 10-14 (18), k(OH + (CF3)2CFCF2OC2H5) ) (7.7 ( 0.8) × 10-14 (18), and k(OH + n-C3F7CF(OC2H5)CF(CF3)2) ) (2.6 ( 0.6) × 10-14 cm3 molecule-1 s-1 (19). The molecules n-C4F9OC2H5, (CF3)2CFCF2-
FIGURE 5. Infrared spectrum of RfOC2H5. OC2H5, n-C3F7CF(OC2H5)CF(CF3)2, and RfOC2H5 are ethers consisting of a fully fluorinated alkyl group which will not react with OH and an ethyl group which will react with OH. Given their molecular similarity it might be expected that these molecules would react with OH radicals at similar rates. Consistent with expectations, the reactivities of n-C4F9OC2H5, (CF3)2CFCF2OC2H5, and RfOC2H5 are very similar. For reasons which are unclear, n-C3F7CF(OC2H5)CF(CF3)2 is approximately 2-3 times less reactive than the other fluorinated ethers. The reactivity of the -OC2H5 group in these fluorinated ethers is substantially (approximately 100 times) less than in diethyl ether. This is probably explained by the fact that the introduction of fluorine substituents into organic compounds typically leads to an increase in the strength of the remaining C-H bonds (27). The value of k(OH + RfOC2H5) measured in the present work can be used to provide an estimate of the atmospheric lifetime of RfOC2H5. Scaling k(OH + RfOC2H5) to k(OH + CH3CCl3) ) 1.0 × 10-14 cm3 molecule-1 s-1 (28) and assuming a lifetime of 6.1 years for CH3CCl3 (29) provides an estimate of approximately 1.0 years for the lifetime of RfOC2H5 with respect to reaction with OH radicals in the troposphere. It should be noted that such scaling should ideally be conducted using OH rate constants at 272 K. Such data are not available for RfOC2H5, and it is not expected that the temperature dependencies of k(OH + RfOC2H5) and k(OH + CH3CCl3) would be sufficiently different to have a material impact on the lifetime assessment. The atmospheric lifetime of RfOC2H5 is expected to be dominated by loss via OH radical attack. 3.5. Infrared Spectrum of RfOC2H5. IR spectra were recorded at 294 ( 1 K using 1.5-6.6 mTorr of RfOC2H5 in 700 Torr of N2 diluent. Peak absorbances scaled linearly with the RfOC2H5 concentration. The absorption spectrum of RfOC2H5 is shown in Figure 5. The integrated cross section (700-1400 cm-1) of RfOC2H5 is 5.59 × 10-16 cm molecule-1. Uncertainties in the cross-section measurement arise from the following sources (with estimated contribution): sample concentration (2%), sample purity (0.1%), path length (2%), spectrum noise (10-20 cm2 molecule-1), and residual baseline offset after subtraction of the 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 section is (5.59 ( 0.28) × 10-16 cm molecule-1. VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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4. Atmospheric Chemistry and Environmental Impact of RfOC2H5 The atmospheric oxidation of RfOC2H5 gives a fluorinated acetate RfOC(O)CH3 and a fluorinated formate RfOCHO. Combining k(Cl + RfOCHO) ) (1.34 ( 0.20) × 10-14 measured in the present work with the relationship log(k(OH)) ) 0.412 log(k(Cl)) - 8.16 (30) we estimate that k(OH + RfOCHO) is approximately 1 × 10-14 cm3 molecule-1 s-1. Hence, the atmospheric lifetime of RfOCHO with respect to reaction with OH is approximately 3 years. This result is consistent with the rate constants measured for the reaction of OH with C2F5OCHO and n-C3F7OCHO which lead to atmospheric lifetimes of approximately 3-4 years (31). The acetate RfOC(O)CH3 likely displays similar reactivity. Dissolution and hydrolysis in cloud, rain, and seawater are potential loss mechanisms for fluorinated esters (32). However, while hydrolysis is competitive with, or dominates, the OH reactivity for C2F5OCHO, it is likely to be less significant for RfOCHO and RfOC(O)CH3 since these esters are expected to be considerably less soluble in water. Hydrolysis would give formic acid, acetic acid, and a fluorinated alcohol RfOH. Formic acid and acetic acid are ubiquitous naturally occurring compounds in the atmosphere and the additional burden associated with the oxidation of RfOC2H5 is of no consequence. In contrast, RfOH is not a naturally occurring compound. The fate of R-haloalcohols is heterogeneous elimination of hydrogen halide (e.g., CF3OH eliminates HF, CCl3OH eliminates HCl). However, RfOH does not contain a halogen in the R-position, and hence, this decomposition route is not possible. The fate of RfOH is unclear, further work is needed to clarify the atmospheric fate of this compound. RfOC2H5 does not contain any chlorine and will not contribute to stratospheric ozone depletion via the wellestablished chlorine based chemistry. As with all hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs) the ozone depletion potential of RfOC2H5 is, for all practical purposes, zero. Using the method of Pinnock et al. (33) and the IR spectra of RfOC2H5 shown in Figure 5 and CFC-11 reported elsewhere (21), we calculate instantaneous forcings for RfOC2H5 and CFC-11 of 0.60 W m-2 and 0.26 W m-2, respectively. Values of the halocarbon global warming potential, HGWP (34), for RfOC2H5 (relative to CFC-11) can then be estimated using the following expression: HGWPRfOC2H5 )
(
)(
)
IFRfOC2H5 τRfOC2H5 MCFC-11 IFCFC-11 τCFC-11 MRfOC2H5
(
)
1 - exp(-t/τRfOC2H5) 1 - exp(-t/τCFC-11)
where IFRfOC2H5, IFCFC-11, MRfOC2H5, MCFC-11, τRfOC2H5, and τCFC-11 are the instantaneous forcings, molecular weights, and atmospheric lifetimes of the two species, and t is the time horizon over which the forcing is integrated. Using τRfOC2H5 ) 1 year, τCFC-11) 45 years (29), and following the suggestion by Freckleton et al. (35) that for compounds with atmospheric lifetimes
