Atmospheric Chemistry of n-C3F7OCH3: Reaction with OH Radicals

Jun 17, 2000 - T. J. WALLINGTON*. Ford Research Laboratory, SRL-3083, Ford Motor Company,. Dearborn, P.O. Box 2053, Michigan 48121-2053. Relative ...
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Environ. Sci. Technol. 2000, 34, 2973-2978

Atmospheric Chemistry of n-C3F7OCH3: Reaction with OH Radicals and Cl Atoms and Atmospheric Fate of n-C3F7OCH2O(•) Radicals Y. NINOMIYA AND M. KAWASAKI Department of Molecular Engineering, Kyoto University, Kyoto 606-8501, Japan A. GUSCHIN, L. T. MOLINA, AND M. J. MOLINA Department of Earth, Atmospheric, and Planetary Science, 54-1820, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 T. J. WALLINGTON* Ford Research Laboratory, SRL-3083, Ford Motor Company, Dearborn, P.O. Box 2053, Michigan 48121-2053

Relative rate techniques were used to measure k(OH+nC3F7OCH3) ) (1.2 ( 0.3) × 10-14, k(Cl+n-C3F7OCH3) ) (9.1 ( 1.3) × 10-14, and k(Cl+n-C3F7OC(O)H) ) (8.2 ( 2.2) × 10-15 cm3 molecule-1 s-1 at 295 K. From the value of k(OH+nC3F7OCH3) an estimate of 4.7 years for the atmospheric lifetime of n-C3F7OCH3 is obtained. It was determined that the sole atmospheric fate of n-C3F7OCH2O(•) radicals is reaction with O2 to give n-C3F7OC(O)H (perfluoro-propyl formate). The results are discussed with respect to the atmospheric chemistry and environmental impact of n-C3F7OCH3.

1. Introduction Recognition of the adverse effect 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 for semiconductor and electronics manufacture, and carrier fluids for lubricant deposition (3). Two HFEs (C4F9OCH3, C4F9OC2H5) are in commercial use, and assessments of their atmospheric chemistry have been reported previously (4, 5). n-C3F7OCH3 is the third member of this class being investigated for commercial use. n-C3F7OCH3 is a volatile liquid (bp 34 °C) with a vapor pressure of 523 Torr at 25 °C (3) and will probably be released into the atmosphere during its use. Prior to its largescale industrial use an assessment of the atmospheric chemistry, and hence environmental impact, of n-C3F7OCH3 is needed. The atmospheric oxidation of n-C3F7OCH3 will be initiated by reaction with OH radicals. The alkyl radical produced in reaction 1 will add O2 rapidly to give a peroxy radical. * Corresponding author phone: (313)390-5574; fax: (313)594-2923; e-mail: [email protected]. 10.1021/es991449z CCC: $19.00 Published on Web 06/17/2000

 2000 American Chemical Society

n-C3F7OCH3 + OH f n-C3F7OCH2 + H2O

(1)

n-C3F7OCH2 + O2 + M f n-C3F7OCH2O2 + M

(2)

By analogy to other peroxy radicals (6), n-C3F7OCH2O2 radicals will react with NO, NO2, HO2, and other peroxy radicals in the atmosphere.

n-C3F7OCH2O2 + NO f n-C3F7OCH2O + NO2 (3a) n-C3F7OCH2O2 + NO + M f n-C3F7OCH2ONO2 + M (3b) n-C3F7OCH2O2 + NO2 + M f n-C3F7OCH2O2NO2 + M (4) n-C3F7OCH2O2 + HO2 f products

(5)

n-C3F7OCH2O2 + R′O2 f products

(6)

Experiments have been performed in our laboratories to elucidate the atmospheric chemistry of n-C3F7OCH3. A relative rate approach was used at MIT to measure the kinetics of the reaction of OH radicals with n-C3F7OCH3 and hence to provide an assessment of its atmospheric lifetime. The fate of the alkoxy radical n-C3F7OCH2O produced in reaction 3a was determined using a FTIR spectrometer coupled to a smog chamber at Ford Motor Company. The results are reported herein and discussed with respect to the environmental impact of n-C3F7OCH3.

2. Experimental Section The two experimental systems used are described in detail elsewhere (7, 8). All samples of n-C3F7OCH3 used in this work were supplied by the 3M Company at a purity > 99% and were used without further purification. The uncertainties reported in this paper are 2 SD unless otherwise stated. Standard error propagation methods were used to combine uncertainties where appropriate. 2.1. FTIR-Photolysis System at MIT. The rate constant for the OH + n-C3F7OCH3 reaction was obtained by monitoring the disappearance rate of n-C3F7OCH3 relative to that of a reference compound (CH4 or CH3Cl) in the presence of OH radicals at 295 K. The decay of the sample was measured using infrared spectroscopy (7). The concentration of n-C3F7OCH3 was monitored by spectral subtraction. OH radicals were generated by photolysis of ozone at 254 nm in the presence of water vapor:

O3 + hν f O(1D) + O2

(7)

O(1D) + H2O f 2OH

(8)

The long-path absorption cell, made of Pyrex glass, had a volume of 7.6 L and a base length of 60 cm, which was adjusted to give a total of 24 passes and an optical path of 14.4 m. The concentrations of the reactants and products were monitored with an FTIR spectrometer (Nicolet 20SX). The mercury photolysis lamp (Ace Hanovia 450-Watt medium-pressure mercury lamp) was placed inside the absorption cell, enveloped in a Vycor tube which transmits 254-nm radiation but absorbs the 185-nm Hg line; no decay of the n-C3F7OCH3 sample was observed in the absence of ozone. The organic reactants were mixed with helium in a 3-L glass reservoir to yield mole fractions of ∼1%. Ozone was VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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prepared by first trapping the effluent from an ozonizer in cold silica gel and then desorbing the sample into a 12-L glass reservoir and subsequently mixing it with helium. The experiments were performed at room temperature in ∼200 Torr helium as a buffer gas in the presence of 3-5 Torr ozone and 2-3 Torr of water vapor. 2.2. FTIR-Smog Chamber System at Ford Motor Company. Experiments were performed in a 140-L Pyrex reactor interfaced to a Mattson Sirus 100 FTIR spectrometer (8). The reactor was surrounded by 22 fluorescent blacklamps (GE F15T8-BL) which were used to photochemically initiate the experiments. The oxidation of n-C3F7OCH3 was initiated by reaction with Cl atoms which were generated by the photolysis of molecular chlorine in N2 diluent at 700 Torr total pressure at 295 ( 2 K

Cl2 + hν f 2Cl

(9)

Cl + n-C3F7OCH3 f n-C3F7OCH2 + HCl

(10)

n-C3F7OCH2 + O2 + M f n-C3F7OCH2O2 + M

(2)

The loss of n-C3F7OCH3 and the formation of products were monitored by Fourier transform infrared spectroscopy using an infrared path length of 27 m and a resolution of 0.25 cm-1. Infrared spectra were derived from 32 co-added interferograms. Two sets of experiments were performed. First, relative rate techniques were used to determine the rate constant for the reaction of Cl atoms with n-C3F7OCH3. Second, the products of the atmospheric oxidation of n-C3F7OCH3 were investigated by irradiating n-C3F7OCH3/Cl2/O2/N2 mixtures with, and without, added NO. Initial concentrations of the gas mixtures for the relative rate experiments were 4.7 mTorr of n-C3F7OCH3, 15-38 mTorr of the reference organic compounds (CH4 or CH3Cl), and 0.15 Torr of Cl2 in 700 Torr of N2 diluent. In the study of the oxidation of n-C3F7OCH3, reaction mixtures consisted of 4.6 mTorr of n-C3F7OCH3, 0.15 Torr of Cl2, 0-18 mTorr of NO, and 20-700 Torr of O2 at a total pressure of 700 Torr in N2 diluent. All experiments were performed at 295 K.

3. Results and Discussion 3.1. Reaction of OH with n-C3F7OCH3 Investigated at MIT. The kinetics of reaction 1 were measured relative to reactions 11 and 12.

OH + n-C3F7OCH3 f n-C3F7OCH2 + H2O

(1)

OH + CH4 f products

(11)

OH + CH3Cl f products

(12)

The loss of n-C3F7OCH3 versus those of the references is shown in Figure 1. Linear least-squares analysis of the data in Figure 1 gives k1/k11 ) 1.68 ( 0.20 and k1/k12 ) 0.35 ( 0.03. Using k11 ) 6.3 × 10-15 and k12 ) 3.6 × 10-14 cm3 molecule-1 s-1 (9), we derive k1 ) (1.06 ( 0.13) × 10-14 and (1.26 ( 0.11) × 10-14 cm3 molecule-1 s-1. We estimate that potential systematic errors associate with uncertainties in the reference rate constants add 10% uncertainty range for k1. Propagating this additional uncertainty gives k1 ) (1.06 ( 0.16) × 10-14 and (1.26 ( 0.17) × 10-14 cm3 molecule-1 s-1. We choose to cite a final value for k1 which is the average of those determined using the two different reference compounds together with error limits which encompass the extremes of the individual determinations. Hence, k1 ) (1.2 ( 0.3) × 10-14 cm3 molecule-1 s-1, the quoted uncertainty reflects the accuracy of the measurements. Our result is in excellent 2974

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FIGURE 1. Decay of n-C3F7OCH3 versus CH4 (triangles) and CH3Cl (circles) in the presence of OH radicals in 200 Torr of helium at 295 K. agreement with the recent determination of k1 ) (1.18 ( 0.05) × 10-14 cm3 molecule-1 s-1 at 298 K by Tokuhashi et al. (10). This result can be compared to k(OH+CF3OCH3) ) 1.2 × 10-14 (average from refs 11 and 12), k(OH+n-C4F9OCH3) ≈ 1.2 × 10-14 (4), k(OH+n-C4F9OC2H5) ) (6.4 ( 0.7) × 10-14 (5), and k(OH+i-C4F9OC2H5) ) (7.7 ( 0.8) × 10-14 cm3 molecule-1 s-1 (5). As might be expected for such structurally similar molecules, the reactivity of OH radicals toward CF3OCH3, n-C3F7OCH3, and n-C4F9OCH3 is indistinguishable. Replacement of the methyl group by an ethyl group in this class of compound leads to a substantial (factor of 5) increase in reactivity of the molecule toward OH radicals. Assuming an atmospheric lifetime for methane of 8.9 years (13) and a rate constant for the CH4 + OH reaction of 6.3 × 10-15 cm3 molecule-1 s-1 leads to an n-C3F7OCH3 atmospheric lifetime against reaction with OH of 4.7 years. 3.2. Relative Rate Studies of the Reactions of Cl with n-C3F7OCH3. Prior to investigating the atmospheric fate of n-C3F7OCH2O(•) radicals, relative rate experiments were performed using the FTIR system at Ford Motor Company to investigate the kinetics of reaction 14. The techniques used are described in detail elsewhere (14). Cl atoms were generated by photolysis of Cl2 molecules and initiated the following reactions.

Cl2 + hν f 2Cl

(13)

Cl + n-C3F7OCH3 f n-C3F7OCH2 + HCl

(14)

The kinetics of reaction 14 were measured relative to reactions 15 and 16.

Cl + CH4 f products

(15)

Cl + CH3Cl f products

(16)

The observed losses of n-C3F7OCH3 versus those of reference compounds in the presence of Cl atoms are shown in Figure 2. Linear least-squares analysis gives k14/k15 ) 0.93 ( 0.05 and k14/k16 ) 0.18 ( 0.01. Using k15 ) 1.0 × 10-13(9) and k16 ) 4.9 × 10-13 (9) gives k14 ) (9.3 ( 0.5) × 10-14 and k14 ) (8.8 ( 0.3) × 10-14 cm3 molecule-1 s-1, respectively. We estimate that potential systematic errors associated with uncertainties in the reference rate constants could add an additional 10%

FIGURE 2. Decay of n-C3F7OCH3 versus CH4 (open symbols) and CH3Cl (filled symbols) in the presence of Cl atoms in 700 Torr of N2 at 295 K. uncertainty range for k14. Propagating this additional uncertainty gives k14 ) (9.3 ( 1.1) × 10-14 and k14 ) (8.8 ( 1.0) × 10-14 cm3 molecule-1 s-1. We choose to cite a final value of k14 which is an average of those determined using the two different reference compounds together with error limits which encompass the extremes of the individual determinations. Hence, k14 ) (9.1 ( 1.3) × 10-14 cm3 molecule-1 s-1. This result can be compared to k(Cl+CF3OCH3) ) (1.4 ( 0.2) × 10-13 (15), k(Cl+n- and i-C4F9OCH3) ) (9.7 ( 1.4) × 10-14 (4), and k(Cl+n-C4F9OC2H5) ) (2.7 ( 0.6) × 10-12 (5). The reactivity of Cl atoms toward CF3OCH3, C4F9OCH3 (nand i-isomers), and n-C3F7OCH3 is very similar. Replacement of the methyl group by an ethyl group in this class of compound leads to a substantial (approximately a factor of 30) increase in reactivity of the molecule toward Cl atoms. 3.3. Determination of the Atmospheric Fate of nC3F7OCH2O(•) Radicals. The atmospheric fate of n-C3F7OCH2O radicals was studied using the FTIR-smog chamber system at Ford Motor Company (8). Experiments were performed using the UV irradiation of n-C3F7OCH3/Cl2/O2/ N2 mixtures with, and without, added NO. In these experiments n-C3F7OCH2O radicals were formed either by reaction 17a or 3a.

n-C3F7OCH2O2 + n-C3F7OCH2O2 f n-C3F7OCH2O + n-C3F7OCH2O + O2 (17a) n-C3F7OCH2O2 + n-C3F7OCH2O2 f n-C3F7OCHO + n-C3F7OCH2OH + O2 (17b) n-C3F7OCH2O2 + NO f n-C3F7OCH2O + NO2 (3a) n-C3F7OCH2O2 + NO + M f n-C3F7OCH2ONO2 + M (3b) Once formed, the n-C3F7OCH2O radicals are expected to undergo either reaction 18 or 19.

n-C3F7OCH2O + O2 f n-C3F7OC(O)H + HO2 (18) n-C3F7OCH2O + M f n-C3F7O + HCHO + M (19)

FIGURE 3. IR spectra of (A) 1.84 mTorr of n-C3F7OC(O)H and (B) 1.88 mTorr of n-C4F9OC(O)H; the IR path length was 27 m. Reaction 18 produces perfluoropropyl formate. The unimolecular decomposition reaction 19 gives a perfluoropropyl alkoxy radical which will decompose and “unzip” via the reaction sequence 20-22.

CnF2n+1O + M f Cn-1F2n-1 + COF2 + M

(20)

Cn-1F2n-1 + O2 f Cn-1F2n-1O2 + M

(21)

Cn-1F2n-1O2 + NO f Cn-1F2n-1O + NO2

(22)

COF2 is readily detectable by its absorption features at 774 and 1850-2000 cm-1 and serves as a convenient marker for the importance of reaction 19. The formation of perfluoropropyl formate serves as a marker for reaction 18. Two sets of experiments were performed with a sample of n-C3F7OCH3 in the presence and absence of NO. For small (1-10%) conversions of n-C3F7OCH3 there was no detectabe formation of COF2 in the system showing that reaction 19 is not an important loss of n-C3F7OCH2O radicals. Following UV irradiation of n-C3F7OCH3/Cl2/NO/air mixtures IR product features were observed at 754, 827, 964, 992, 1016, 1053, 1074, 1152, 1180, 1205, 1244, 1338, 1806, and 1821 cm-1 which we assign to the formate n-C3F7OC(O)H formed in reaction 18. As shown in Figure 3 the IR spectrum of n-C3F7OC(O)H is similar to that of n-C4F9OC(O)H (4). A number of vibrational doublets are expected for the syn- and antiisomers, but the most prominent are likely obscured by the large number of intense C-F stretch bands from the perfluoropropyl chains (10). As discussed below we conclude that reaction 18 is the dominant loss of n-C3F7OCH2O radicals in the system. Interestingly, there was no discernible difference between the formate yields in experiments with, and without, added NO and no evidence for the formation of VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Observed concentration of n-C3F7OC(O)H normalized to the initial n-C3F7OCH3 concentration versus the fractional loss of n-C3F7OCH3 following irradiations of mixtures of 4.6 mTorr n-C3F7OCH3, 150 mTorr Cl2 and either 20 Torr (triangles), 160 Torr (circles), or 700 Torr (squares) of O2 in 700 Torr total pressure of N2 diluent at 295 K. The curve is a fit to the data; see text for details. other carbon containing products. The only reasonable interpretation is that n-C3F7OCH3 is converted into formate in both systems in essentially 100% yield. On this basis we are able to calibrate the formate yields shown in Figure 4. From the fact that there was no discernible difference between the formate yields in experiments with, and without, added NO we conclude that chemical activation of n-C3F7OCH2O radicals formed in reaction 3a does not affect their atmospheric fate. For Cl atom initiated oxidation experiments conducted in the absence of NO we usually observe the formation of hydroperoxides via the reaction RO2 + HO2 f ROOH + O2 (R ) C4F9OCH2). The absence of hydroperoxides in the present experiments can be rationalized several ways: (i) the RO2 + HO2 reaction is unusually slow, (ii) the RO2 + HO2 reaction does not give ROOH but instead produces the formate (the analogous CF3OCH2O2 + HO2 reaction gives a substantial formate yield (15)), (iii) ROOH is formed in the system but is consumed by secondary reaction with Cl atoms to regenerate the peroxy radical, or (iv) a combination of the above. Following the irradiation of n-C3F7OCH3/Cl2/O2/N2 mixtures the concentration of the formate increased linearly with loss of n-C3F7OCH3 up until about 50% loss of the parent n-C3F7OCH3. For conversions of n-C3F7OCH3 >85-90% the formate yield reached a plateau and then decreased. We ascribe this behavior to reaction of Cl atoms with the formate. By monitoring the formation and subsequent loss of the formate as a function of the fractional conversion of n-C3F7OCH3 we can establish the rate constant for reaction of Cl atoms with the formate.

Cl + n-C3F7OC(O)H f HCl + n-C3F7OC(O)

(23)

The relevant reactions are (23) and (24)

Cl + n-C3F7OCH3 f R formate + other products (24) where R is the yield of formate from n-C3F7OCH3 (0 < R e 1). The corresponding rate equations can be solved analytically to relate the amount of formate at any time t to the 2976

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FIGURE 5. Product yields following the Cl atom initiated oxidation of n-C3F7OC(O)H; see text for details. corresponding conversion of n-C3F7OCH3 at time t, as a function of R and the rate constant ratio k23/k24, where k23 and k24 are the bimolecular rate constants of reactions 23 and 24, respectively. The expression, as derived in reference (16), is

[formate]t [n-C3F7OCH3] t0

)

R (1 - x)[(1 - x){k23/k24 - 1} - 1] k23 1k24

where x is the conversion of n-C3F7OCH3, defined as

x≡1-

[n-C3F7OCH3]t [n-C3F7OCH3]t0

Figure 4 shows the observed concentration of n-C3F7OC(O)H normalized to the initial n-C3F7OCH3 concentration versus the fractional loss of n-C3F7OCH3 following irradiation of mixtures of 4.6 mTorr n-C3F7OCH3, 150 mTorr Cl2, and 20-700 Torr O2 in 700 Torr total pressure of N2 diluent. A fit of the above expression (with R fixed at 1.0) to the data in Figure 4 gives k23/k24 ) 0.09 ( 0.02. Using k(Cl+ n-C3F7OCH3) ) (9.1 ( 1.3) × 10-14, we arrive at k23 ) (8.2 ( 2.2) × 10-15 cm3 molecule-1 s-1. This result can be compared to k(Cl+CF3OC(O)H) ) (9.8 ( 1.2) × 10-15 (15) and k(Cl+C4F9OC(O)H) ) (1.6 ( 0.7) × 10-14 cm3 molecule-1 s-1 (4). The size of the CnF2n+1 group in the CnF2n+1OC(O)H formate appears to have little impact on the reactivity of the molecule toward Cl atoms. For completeness a product study of the Cl atom initiated oxidation of the formate was performed. The experimental method was as follows. Mixtures of 5 mTorr of n-C3F7OCH3, 150 mTorr of Cl2, 18 mTorr of NO, and 40 Torr of O2 in 50 Torr total pressure of N2 diluent were prepared and irradiated for successive 23 min periods until all IR features attributed to the n-C3F7OCH3 had disappeared. The product mixtures were then subjected to further UV irradiation, and the loss of formate and formation of COF2 were monitored. The results are shown in Figure 5. The radical formed in reaction 23 is expected to add O2 and react with NO to give the n-C3F7OC(O)O radical which will decompose to give CO2 and the n-C3F7O radical which will “unzip” via reactions 20-22. As

The atmospheric degradation of HFEs produces essentially the same fluorinated radical species (e.g., CxF2x+1O2 and CxF2x+1O) as formed during the degradation of hydrofluorocarbons, HFCs. HFCs do not impact stratospheric ozone (6), and the same conclusion applies to HFEs. n-C3F7OCH3 has an ozone depletion potential of zero. Finally we need to consider the potential for n-C3F7OCH3 to impact the radiative balance in the atmosphere. Using the method of Pinnock et al. (19) with the IR spectra shown in Figure 6 we calculate instantaneous forcings for n-C3F7OCH3 and CFC-11 of 0.34 W/m2 and 0.26 W/m2, respectively. Values of the GWP (global warming potential) for n-C3F7OCH3 (relative to CFC-11) can then be estimated using the expression 20:

GWPn-C3F7OCH3 )

FIGURE 6. IR spectra of n-C3F7OCH3 and CFC-11. expected, the oxidation of n-C3F7OC(O)H gave a molar COF2 yield, 269 ( 36%, which was indistinguishable from 300%. Quoted errors include 2 SD from linear regressions of the data in Figure 5 plus an additional 15% uncertainty associated with uncertainties in the calibrations of the COF2 and formate reference spectra.

4. Implications for Atmospheric Chemistry We present herein a large body of kinetic and mechanistic data pertaining to the atmospheric chemistry of n-C3F7OCH3. The atmospheric lifetime of n-C3F7OCH3 is determined by its reaction with OH radicals and is estimated to be 4.7 years. Reaction with OH gives the alkyl radical n-C3F7OCH2 which will be rapidly converted into the peroxy radical n-C3F7OCH2O2 which reacts with NO to produce n-C3F7OCH2O radicals. The sole fate of the alkoxy radical, n-C3F7OCH2O, is reaction with O2 to give the formate n-C3F7OC(O)H. This behavior is entirely consistent with the available database concerning the behavior of similar nonfluorinated alkoxy radicals, e.g. CH3OCH2O and (CH3)3COCH2O radicals are known to exclusively react with O2 to give formates (17). It is reported herein that the formate n-C3F7OC(O)H is rather unreactive toward Cl atoms and is likely to be similarly unreactive toward OH radicals. Organic compounds which react with Cl atoms with rate constants in the range 10-1410-13 cm3 molecule-1 s-1 generally react faster with Cl atoms than with OH radicals (9). Hence, we can use the value of k23 ) 8.2 × 10-15 cm3 molecule-1 s-1 as an upper limit to k(OH+nC3F7OC(O)H). Assuming an atmospheric lifetime for methane of 8.9 years (13) and a rate constant for the CH4 + OH reaction of 6.3 × 10-15 cm3 molecule-1 s-1 leads to an n-C3F7OC(O)H atmospheric lifetime against reaction with OH of a lower limit of 7 years. Photolysis of esters is only important below 240 nm (18); it is unlikely that n-C3F7OC(O)H undergoes photolysis in the lower atmosphere. In view of the polar nature of n-C3F7OC(O)H it seems likely that the main atmospheric removal mechanism of this compound will be via wet/dry deposition. There are no available data for the rate of wet/dry deposition, and hence it is not possible to provide an accurate estimate of the atmospheric lifetime of n-C3F7OC(O)H at this time.

(

)(

)

IFn-C3F7OCH3 τn-C3F7OCH3MCFC-11 IFCFC-11

τCFC-11Mn-C3F7OCH3

(

)

1-exp(-t/τn-C3F7OCH3) 1-exp(-t/τCFC-11)

where IFn-C3F7OCH3, IFCFC-11, Mn-C3F7OCH3, MCFC-11, τn-C3F7OCH3, 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 τn-C3F7OCH3 ) 4.7 years and τCFC-11 ) 50 years (19) we estimate that the GWP of n-C3F7OCH3 is ≈0.27 for a 20 year horizon and ≈0.10 for a 100 year time horizon. The GWP of n-C3F7OCH3 is a factor of 5-10 less than those of the CFCs that it replaces.

Acknowledgments We thank John Owens (3M Specialty Materials) for supplying the samples of n-C3F7OCH3 used in this work, 3M Specialty Materials for funding the work performed at MIT, and the Japanese Government for a NEDO grant which made this collaborative research project possible.

Literature Cited (1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (2) Farman, J. D.; Gardiner, B. G.; Shanklin, J. D. Nature 1985, 315, 207. (3) Owens, J. 3M Specialty Materials, private communication, 1999. (4) Wallington, T. J.; Schneider, W. F.; Sehested, J.; Bilde, M.; Platz, J.; Nielsen, O. J.; Christensen, L. K.; Molina, M. J.; Molina, L. T.; Wooldridge, P. W. J. Phys. Chem. A 1997, 101, 8264. (5) Christensen, L. K.; Sehested, J.; Nielsen, O. J.; Bilde, M.; Wallington, T. J.; Guschin, A.; Molina, L. T.; Molina, M. J. J. Phys. Chem. 1998, 102, 4839. (6) Wallington, T. J.; Schneider, W. F.; Worsnop, D. R.; Nielsen, O. J.; Sehested J.; Debruyn, W. J.; Shorter, J. A. Environ. Sci. Technol. 1994, 28, 320 (7) Tang, Y. M.S. Thesis, Massachusetts Institute of Technology, 1993. (8) Wallington, T. J.; Japar, S. M. J. Atmos. Chem. 1989, 9, 399. (9) DeMore W. B.; Sander S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. JPL Publication No. 94-26; NASA Jet Propulsion Lab.: Pasadena, CA, 1997. (10) Tokuhashi, K.; Takahashi, A.; Kaise, M.; Kondo, S.; Sekiya, A.; Yamashita, S.; Ito, H. Int. J. Chem. Kinet. 1999, 31, 846. (11) Hsu, K.-J.; DeMore, W. B. J. Phys. Chem. 1995, 99, 11141. (12) Orkin, V. L.; Khamaganov, V. G.; Guschin, A. G.; Huie, R. E.; Kurylo, M. J. The 13th International Symposium on Gas Kinetics; University College Dublin: Ireland, 1994. (13) Prinn, R. G.; Weiss, R. F.; Miller, B. R.; Huang, J.; Alyea, F. N.; Cunnold, D. M.; Fraser, P. J.; Hartley, D. E.; Simmonds, P. G. Science 1995, 269, 187. (14) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437. VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Received for review December 30, 1999. Revised manuscript received April 26, 2000. Accepted May 3, 2000. ES991449Z