Article pubs.acs.org/JPCA
Photochemical Properties of trans-1-Chloro-3,3,3-trifluoropropene (trans-CHClCHCF3): OH Reaction Rate Constant, UV and IR Absorption Spectra, Global Warming Potential, and Ozone Depletion Potential Vladimir L. Orkin,* Larissa E. Martynova, and Michael J. Kurylo† National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States S Supporting Information *
ABSTRACT: Measurements of the rate constant for the gas-phase reactions of OH radicals with trans-1-chloro-3,3,3-trifluoropropene (trans-CHClCHCF3) were performed using a flash photolysis resonance−fluorescence technique over the temperature range 220−370 K. The reaction rate constant exhibits a noticeable curvature of the temperature dependence in the Arrhenius plot, which can be represented by the following expression: kt‑CFP (220−370 K) = 1.025 × 10−13 × (T/298)2.29 exp(+384/T) cm3 molecule−1 s−1. The room-temperature rate constant was determined to be kt‑CFP (298 K) = (3.29 ± 0.10) × 10−13 cm3 molecule−1 s−1, where the uncertainty includes both two standard errors (statistical) and the estimated systematic error. For atmospheric modeling purposes, the rate constant below room temperature can be represented by the following expression: kt‑CFP (220−298 K) = (7.20 ± 0.46) × 10−13 exp[−(237 ± 16)/T] cm3 molecule−1 s−1. There was no difference observed between the rate constants determined at 4 kPa (30 Torr) and 40 kPa (300 Torr) at both 298 and 370 K. The UV and IR absorption cross sections of this compound were measured at room temperature. The atmospheric lifetime, global warming potential, and ozone depletion potential of trans-CHClCHCF3 were estimated.
1. INTRODUCTION Despite the broad applicability of simple fully halogenated hydrocarbons in various industries, the production and use of bromo(chloro)fluorocarbons (halons) and chlorofluorocarbons (CFCs) has been phased out because of the danger they pose to the Earth’s stratospheric ozone layer.1 In addition, all halogen-containing hydrocarbons are infrared active gases because of their strong absorption bands in the region of the atmospheric transparency window between ca. 8 and 12 μm. Hence, there is growing concern about the impacts that such compounds have on the Earth’s radiation balance. Chlorofluorocarbons are quite stable with respect to most atmospheric photochemical processes, and their major destruction process is photolysis by solar ultraviolet radiation in the stratosphere with essentially no removal process in the troposphere. Thus, the entire amount of such compounds released into the atmosphere is transported to the upper troposphere and stratosphere where they act as greenhouse gases and/or dissociate under UV radiation to yield Cl atoms, which initiate catalytic ozone destruction. Many CFC substitutes, on the other hand, have been chosen because of their reactivity within the troposphere, thereby limiting their global warming potential (GWP) or ozone depletion potential (ODP). The main removal process for CFC substitutes in the troposphere is the reaction with hydroxyl radicals, which reduces their delivery to the stratosphere. The products of atmospheric oxidation following this initial OH attack are © XXXX American Chemical Society
usually not very-long-lived in the troposphere. The majority of products undergo fast chemical reactions or hydrolysis and dissolution in atmospheric water followed by their washing out from the atmosphere. Thus, the reactivity toward OH radicals often dictates the residence time of a compound in the atmosphere and to large extent controls both global environmental impacts caused by the emission of halogenated hydrocarbons: destruction of stratospheric ozone and greenhouse warming. One class of chemicals under consideration as a CFC substitute is a family of halogenated alkenes. The presence of a carbon−carbon double bond renders these substances highly reactive toward atmospheric hydroxyl radicals (OH). trans-1-Chloro-3,3,3-trifluoropropene (trans-CHCl CHCF3) is one of the new Cl-containing unsaturated candidate replacement compounds that has been proposed for use in applications such as electronics cleaning and foam blowing. We report the results of studies of photochemical properties of this compound. The rate constant for the reaction between transCHClCHCF3 and OH kt ‐CFP
OH + trans‐CHClCHCF3 ⎯⎯⎯⎯→ products
(1)
Received: February 23, 2014 Revised: June 19, 2014
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when the low repetition rate was still required. Test experiments revealed no effect of the repetition rate on the measured decay rate at higher reactant concentrations. The procedure for deriving the reaction rate constant from such data has been described by Orkin et al.4 and in subsequent papers.6,8 At each temperature the rate constant was determined from a fit to all of the decay rates obtained at that temperature at different reactant concentrations. The temperatures for the measurements were chosen to be approximately equally distant along the Arrhenius 1/T scale in order to have them equally weighted in the fitting procedure for determining the temperature dependence. Experiments were always performed at two temperatures that are widely used in other studies, T = 298 K and T = 272 K. The first is standard room temperature, used in evaluations and presentations of the rate constants, whereas the second is the temperature used in estimations of atmospheric lifetimes.9 2.2. Spectral Measurements. 2.2.1. IR Absorption Cross Section Measurements. The IR absorption spectrum of transCHClCHCF3 was measured using a Fourier transform infrared (FTIR) spectrophotometer Nicolet 6700 with a spectral resolution of 0.5 cm−1 and recorded with a step of 0.25 cm−1, and with a spectral resolution of 0.125 cm−1 and a step of 0.067 cm−1. A (10.2 ± 0.03) cm glass absorption cell fitted with KBr windows was used to obtain the absorption spectra at T = (298 ± 1) K. The temperature was measured with a thermocouple attached to the cell body and was used in calculating the compound concentration. Absorption spectra of the evacuated cell and of the cell filled with a gas sample were alternately recorded several times, and the absorption cross sections at the wavenumber ν (reciprocal centimeters) were calculated as
has been studied between 220 and 370 K. UV and IR absorption spectra of the compound were obtained at 298 K.
2. EXPERIMENTAL SECTION2 2.1. OH Reaction Rate Constant Measurements. General descriptions of the apparatus and the experimental method used to measure the OH reaction rate constants are given in previous papers.3−6 Modifications to the apparatus and the measurement procedure, which resulted in significant improvements in the accuracy and precision of the obtained kinetic data, have also been extensively detailed.6,7 The principal component of the flash photolysis−resonance fluorescence apparatus (FP-RF) is a double-walled Pyrex reactor (of approximately 180 cm3 internal volume) equipped with quartz windows. The reactor is temperature-controlled by circulating methanol or water between the outer walls and is located in an evacuated metal housing to prevent ambient water condensation during low-temperature measurements. This also prevents extraneous absorption of the UV radiation from the flash lamp used to produce OH radicals. Reactions were studied in argon carrier gas at a total pressure of 4 kPa (30.0 Torr). Flows of dry argon, argon bubbled through water thermostated at 276 K, and dilute mixtures of each reactant were passed through the reactor at a total flow rate between 0.21 and 2.4 cm3 s−1, STP. The reactant mixtures diluted with Ar were prepared in a 10 L glass bulb equipped with Teflon/glass valves. The concentrations of the gases in the reactor were determined by measuring the gas flow rates and the total pressure with calibrated MKS Baratron manometers. Flow rates of argon, the H2O/argon mixture, and the reactant/argon mixture were measured and maintained using MKS mass flow controllers directly calibrated for every mixture. The calibration procedures for the mass flow controllers and manometers as well as the uncertainties associated with gas handling have been described previously.6 The total uncertainty of the kinetic results was estimated to be ∼2% to 2.5% in the absence of chemical complications for measurements performed after the recent modifications of the apparatus and in the measurement procedures.6 Hydroxyl radicals were produced by the pulsed photolysis of H2O, injected via the 276 K argon/water bubbler. The OH radicals were monitored by their resonance fluorescence near 308 nm, excited by a microwave-discharge resonance lamp (0.8 kPa or 6 Torr of a mixture of H2O in UHP helium) focused into the reactor center. Resonantly scattered radiation from the center of the reaction cell was collimated by the reactor window−lens assembly and detected by a cooled photomultiplier operating in the photon counting mode. The resonance fluorescence signal was recorded on a computerbased multichannel scaler (using a channel width of 100 μs) as a summation of 2000−20000 consecutive flashes. The entire temporal OH profile was recorded and coadded following each flash, thereby minimizing any possible effects of small flash-toflash variations of the initial OH concentration and drift of the resonance lamp intensity. In the absence of any reactant in the reactor, the temporal decay of [OH] is associated with its net diffusion out of the irradiated (photolysis) zone. This relatively long “background” decay was always recorded with a 2.5 Hz flash repetition rate to ensure complete disappearance of the OH between consecutive flashes. [OH] decays were then recorded at various reactant concentrations with a flash repetition rate of 2.5, 5, or 10 Hz for faster data collection except at small reactant concentrations
σ (ν ) =
ln{I0(ν)/I[CHClCHCF3](ν)} [CHClCHCF3] × L
(2)
where [CHClCHCF3] is the concentration of the compound in the absorption cell with the optical path length L. I0(ν) and I[CHClCHCF3](ν) are the radiation intensities measured after the absorption cell when the compound concentration was zero and [CHClCHCF3], respectively. Measurements were performed at various pressures of CHClCHCF3 between 0.27 kPa (2 Torr) and 17 kPa (128 Torr) to verify adherence to the Beer−Lambert absorption law and obtain strong and weak absorption features of the spectrum. The overall instrumental error associated with the optical path length, pressure measurements, temperature stability, and measured absorbance was estimated to be less than 2% for the strong absorption bands. 2.2.2. UV Absorption Cross Section Measurements. The absorption spectrum of trans-1-chloro-3,3,3-trifluoropropene was measured over the wavelength range of 165−300 nm using a single-beam apparatus consisting of a 1 m vacuum monochromator equipped with a 600 lines/mm grating. The radiation source was a Hamamatsu L1385 deuterium lamp, and the detector was a Hamamatsu R166 photomultiplier. Spectra were recorded at increments of 0.5 nm at a spectral slit width of 0.5 nm. The pressure inside the (16.9 ± 0.05) cm absorption cell was measured by a MKS Baratron manometer at T = (298 ± 1) K. Absorption spectra of the evacuated cell and of the cell filled with a gas sample were alternately recorded several times to obtain the absorption cross sections of the compound. The B
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sample pressure between 2.7 Pa (0.02 Torr) and 45.3 kPa (340 Torr) was used to obtain the entire absorption spectrum. 2.3. Materials. A sample of trans-1-chloro-3,3,3-trifluoropropene was provided by Arkema Inc. with a stated purity of 99.2%. The impurity levels of trans-1,3,3,3-trifluoropropene (trans-CHFCHCF 3 ), 3,3,3-trifluoropropene (CH 2 CHCF3), 1,1,3,3,3-pentafluoropropane, and cis-1,3,3,3-tetrafluoropropene (cis-CHFCHCF3) were reported to be 0.51%, 0.10%, 0.10%, and 0.09%, respectively. Our own quantitative IR analysis indicated a smaller amount of the most abundant impurity, trans-CHFCHCF3 (see section 3.2 and Figure 4 below). We found 0.22% to 0.25% of transCHFCHCF3 in the liquid phase and ∼1.5% when sampling from the vapor phase. Therefore, the liquid-phase sampling from the “valve down” storage cylinder was used to prepare 1.5% mixtures of the compound (trans-CHClCHCF3) in a 5 or 10 L glass bulb equipped with a Teflon valve. Thus, the gas sample in the mixture contains the same amount of impurities as the bulk liquid sample that was analyzed. The reactivity of all these compounds (except cis-CHFCHCF3) toward OH have been studied and evaluated.10 The contributions of these reactions to the rate constant measured in this study are small, and the corresponding corrections can be made. Therefore, the available sample of trans-1-chloro-3,3,3-trifluoropropene (transCHClCHCF3) was used without any further purification other than degassing by multiple freeze−pump−thaw−boil cycles. Argon (99.9995% purity; Spectra Gases Inc.) was used as a carrier gas in all kinetic experiments and in the preparation of reactant mixtures.
Figure 1. Available results for the rate constant of the reaction between OH and trans-CHClCHCF3, kt‑CFP(T): (∇ and Δ) relative rate measurements by Sulbaek Andersen et al.14 using C2H4 and C2H2 as reference compounds; (●) this study. The solid line shows a threeparameter fit to our data (eq 3). Lower panel shows ratios of rate constants measured in this study to those calculated from the threeparameter temperature dependence (eq 3; ●) and from the twoparameter Arrhenius dependence (eq 5; ○).
3. RESULTS 3.1. OH Reaction Rate Constant. The rate constants of the title reaction determined between 220 and 370 K are presented in Table 1 and shown in Figure 1. These are results from fits to all measurements performed at the indicated
temperature, 4.00 kPa (30.0 Torr) total pressure, ∼6 × 1014 molecule/cm3 of H2O in the reactor, and ∼8 mJ flash energy. To check for any complication, additional experiments were performed with variation of flash energy, H2O concentration, gas flow through the reactor, flash repetition rate, and the total gas pressure in the reactor. These test experiments allowed us to examine for the possible effect of OH reactions with photolysis products or secondary radicals on the results of our measurements. No statistically significant changes of the measured reaction rate constant were observed in these experiments except for a weak dependence of the measured rate constant on the flash energy. Note that variations of H2O concentration, total gas flow, flash energy, and flash repetition rate result in variations of the OH concentration in the mixture and product accumulation, whereas the variation of flash energy also changes the amount of possible reactant photolysis products. Thus, these test experiments examine for the presence of the OH reaction with photolysis products. Such a possibility is not very surprising because of very strong UV absorption of the reactant below 205 nm (see Figure 3) and reactions between OH and products of photolysis could result in an overestimation of the derived rate constant. Similar complications were described in our earlier papers5,11 and have recently been carefully investigated.12 Hence, sets of experiments at various flash energies between 8 and 50 mJ were performed at various temperatures to quantify this dependence. The values measured at lowest flash energy were then corrected as discussed below.
Table 1. Rate Constants Measured in the Present Work for the Reaction of OH with trans-1-Chloro-3,3,3trifluoropropene (trans-CHClCHCF3)a T (K) 220 230 240 250 260 272 285 298 313 330 350 370 RRSDb
kt‑CFP(T) (× 1013 cm3 molecule−1 s−1)
[trans-CHClCHCF3] (× 1014 molecule/cm3)
± ± ± ± ± ± ± ± ± ± ± ±
0.96−5.6 0.92−6.2 0.88−6.0 0.85−6.5 0.81−5.5 0.78−6.0 0.74−4.3 0.71−6.2 0.65−4.6 0.33−6.2 0.60−4.1 0.29−4.1
2.48 2.54 2.67 2.79 2.90 2.98 3.13 3.28 3.49 3.63 4.04 4.34
0.05 0.03 0.06 0.05 0.06 0.03 0.06 0.03 0.08 0.07 0.08 0.05
1.1% (three-parameter Arrhenius dependence)
a The uncertainties are two standard errors from the least-squares fit of a straight line to the measured OH decay rates versus the reactant concentrations and do not include the estimated systematic uncertainty. We estimate the total uncertainty of our measurements to be less than 3% at T = 298 K. bThe relative residual standard deviation (RRSD)7 is shown to uniformly represent the data deviation from the best fit temperature dependence, see text.
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range of flash energy variations in these experiments was wider than the extrapolation range by a factor of 4−6 and that the rate constants measured at lowest flash energies exceed the “correct” reaction rate constants, kt‑CFP(T), obtained from a short extrapolation and presented in Table 1, by only 1.5% to 4.5%. The reaction represented in eq 1 occurs via addition of OH radical to an unsaturated C atom and, despite the multiatomic nature of trans-CHClCHCF3, there is a possibility that the measured rate constant is pressure-dependent. Therefore, test experiments were also performed at a higher pressure of 40 kPa (300 Torr) to check for a possible pressure dependence of the reaction rate constant. The difference between rate constants measured at 4 kPa (30 Torr) and 40 kPa (300 Torr) was less than 1% at T = 298 K and less than ∼2% at T = 370 K, which is within the uncertainty of the test experiments. This suggests that the reaction has already achieved the high-pressure limit at 4 kPa (30 Torr) of Argon. As mentioned earlier, the presence of reactive impurities in the sample of trans-1-chloro-3,3,3-trifluoropropene also results in a small overestimation of the measured rate constant. Thus, 0.25% of trans-CHFCHCF3 contributes less than 0.5% to the measured rate constant at T = 298 K; 0.10% of CH2CHCF3 contributes ∼0.3%; and 0.10% of CHF2CH2CF3 contributes ∼0.002%. There is no available information on the reactivity of OH with cis-CHFCHCF3. However, we can estimate its reactivity to be the same as that of similar molecules, transCHFCHCF3 or cis-CHFCFCF3, which results in ∼0.2% to 0.3% overestimation in our measurements of kt‑CFP at T = 298 K. Therefore, the presence of the detected impurities makes only a small contribution to the measured values of kt‑CFP and we made the corresponding small corrections to the measured rate constants using the recommended OH reaction rate constants6,10 for these impurities. Thus, we are confident that the reaction rate constant, kt‑CFP(T), reported in this paper has no errors associated with either secondary chemistry or the presence of reactive impurities. The Arrhenius plot shown in Figure 1 exhibits a noticeable curvature that is statistically proved in our study. A threeparameter fit to the data presented in Table 1 yields the following modified Arrhenius expression:
Figure 2 shows the rate constants measured at T = 298 K and T = 230 K at various flash lamp energies (solid circles). The
Figure 2. Results of measurements of the rate constant for the reaction between OH and trans-CHClCHCF3 at various flash lamp energies and T = 230 K and T = 298 K. (●) The rate constants versus the flash energy shown with the bottom axis. The best fits to the data at each temperature (solid lines) are shown with their 95% confidence intervals (dashed lines). The intersects with k(T) axis are the rate constants kt‑CFP at these temperatures with their 95% confidence intervals. (○) The rate constants versus the initial OH concentration shown with the top axis. The best fit to the data are shown with solid lines.
results obtained at each temperature exhibit a linear dependence on flash energy indicated by the solid lines with their confidence intervals corresponding to two standard errors indicated by the dashed lines. An extrapolation of these linear dependences to “zero” flash energy yields the rate constant for the reaction between OH and trans-CHClCHCF3, kt‑CFP(T), not complicated by any reactions with photolysis products. The intersection of the dashed lines illustrates the statistical two standard errors for kt‑CFP(T). Figure 2 also presents complementary supporting dependences of the measured rate constant on the initial OH concentration (i.e., the initial RF signal in the absence of the reactant) at the same various flash energies (open circles corresponding to the top x-axis). The OH concentration itself does not appreciably affect the measured rate constant as it was checked in the experiments with variation of H2O (OH precursor) under the same flash energy. However, the initial OH concentration is a measure of the intensity of the shortestwavelength UV radiation in the flash. Therefore, the open circles in Figure 2 illustrate the dependence of the measured kt‑CFP(T) on the intensity of vacuum UV radiation. They obey a linear dependence also shown with solid lines. Both these complementary dependences on the total flash energy and vacuum UV radiation intensity yield the same kt‑CFP(T) when extrapolated to “zero” values, thus providing additional confidence in our kinetic analysis procedure. Note that the
kt ‐CFP (220−370 K) = 1.025 × 10−13 × (T /298)2.29 exp{+ 348/T } cm 3 molecule−1 s−1
(3)
This temperature dependence (eq 3) is shown in Figure 1 with the solid line. The lower panel in Figure 1 shows the measured reaction rate constants normalized to eq 3 where error bars are the relative errors of measurements reported in Table 1 (filled circles). The data deviation from the fitted line can be numerically represented by the relative residual standard deviation (RRSD) introduced in recent papers;7,13 RRSD = 1.1%. Assuming confidence in the derived uncertainties of the data points, we can apply a χ2-test to check the statistical “goodness” of the three-parameter fit. This yields a probability of P = 0.06 for eq 3, thus suggesting the absence of statistically significant deviation. The recommended room-temperature rate constant derived in this study is kt ‐CFP (298 K) = (3.29 ± 0.10) × 10−13 cm 3 molecule−1 s−1 (4)
where the indicated total uncertainty includes the statistical two standard error listed in Table 1 and the estimated instrumental uncertainty. The quantification of such uncertainties has been D
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discussed in detail elsewhere.6 The data scattering (quantified as a “standard error”) is statistically consistent among the consecutive steps in the rate constant determination: pseudofirst-order kinetic dependences, dependences of the measured rate constant on flash energy (illustrated in Figure 2), and the temperature dependence of the rate constant (shown in Figure 1, lower panel). This consistency indicates the absence of uncontrolled fluctuations in experimental parameters not included in our uncertainty analysis. For atmospheric modeling purposes, kt‑CFP(T) below room temperature can be represented by the two-parameter Arrhenius expression derived from the fit to the data obtained between 220 and 298 K
CHClCHCF3 using the same reference compounds differ randomly with those from our laboratory by −25% to +25% when the same rate constants of the reference reactions are used. Thus, uncertainties in the selected values for the reference reaction rate constants are not solely responsible for differences between the two laboratories. 3.2. UV and IR Absorption Spectra. The ultraviolet absorption spectrum of trans-CHClCHCF3 obtained in this work is presented in Figure 3 and Table 2. It is also available in
kt ‐CFP (220−298 K) = (7.20 ± 0.46) × 10−13 exp{− (237 ± 16)/T } cm 3 molecule−1 s−1
(5)
The experimental data normalized by eq 5 are also shown in the lower panel of Figure 1 by open circles. Their relative errors bars are omitted for clarity. The χ2-test yields a probability of P = 0.07 for eq 5, thus suggesting the absence of statistically significant deviation from the Arrhenius dependence between 220 and 298 K. Therefore, the Arrhenius expression (eq 5) is quite adequate for atmospheric modeling and the calculated relative deviation from this dependence, RRSD = 0.9%. However, eq 5 substantially underestimates the rate constant above room temperature, as shown in the lower panel of Figure 1 with open circles. The only published study of this reaction was performed by Sulbaek Andersen et al.14 who reported a rate constant determined at 295 ± 2 K, kt‑CFP(295 K) = (4.40 ± 0.38) × 10−13 cm3 molecule−1 s−1. The rate constant was determined by a relative rate technique using FTIR detection of the reactants. C2H4 and C2H2 were used as reference compounds resulting in kt‑CFP = (4.52 ± 0.26) × 10−13 cm3 molecule−1 s−1 and kt‑CFP = (4.28 ± 0.26) × 10−13 cm3 molecule−1 s−1, respectively. Both reported values significantly (∼40% and ∼30%, respectively) exceed the result of our measurements at this temperature. A principal source of uncertainty associated with the determination of a reaction rate constant using the relative rate technique is associated with the chosen value of the reference reaction rate constant and its uncertainty. The later should be added to any uncertainties associated with the relative rate measurements themselves. The rate constants of both reference reactions, OH + C2H4 (kC2H4) and OH + C2H2 (kC2H2), used in the Sulbaek Andersen et al.14 study are pressure-dependent, and significant differences exist among various recommendations. Use of the latest recommendation for kC2H4 from Golden15 results in a kt‑CFP(295 K) value that is ∼25% (rather than 40%) larger than that determined in the present work. Using values for kC2H2 recommended by IUPAC16 and JPL10 results in kt‑CFP(295 K) values that are 16−20% (rather than 30%) larger than that determined in the present work. These smaller differences are within the combined uncertainties of both studies when uncertainties in the reference reaction rate constants are included. However, uncertainties in the reference reaction rate constants do not totally explain the systematic differences. For example, the results reported by the same research group for CH2 CHCF 3 , 17 CH 2 CFCF 3 , 18 CH 2 CBrC 2 F 5 , 19 CH 2 CBrCF3,19 CF2CFCF3,20 trans-CHFCHCF3,21 and trans-
Figure 3. UV absorption cross sections of trans-CHClCHCF3. Absorption spectra of trans-CHFCHCF3, CH2CHCF3, and CH3Cl are shown for comparison.
Supporting Information with smaller wavelength steps of 0.2−1 nm. We are unaware of any other UV absorption data for this compound. trans-CHClCHCF3 exhibits very strong absorption over the stratospheric transparency window near 200 nm, the narrow spectral region where the solar radiation penetrates to the lower stratosphere. This absorption is due to Cl atom adjacent to a double bound. Figure 3 also shows the UV absorption spectra of CH2CHCF3, trans-CHFCHCF3, and CH3Cl for comparison. The UV absorption band of transCHClCHCF3 is similar to that for the double bond of CH2CHCF3 and trans-CHFCHCF3 with approximately the same maximum intensity. However, the presence of Cl atom adjacent to a double bond results in red shift (∼26 nm and ∼22 nm, respectively) and thus drastically increases the double-bond absorption near 200 nm. Note that saturated Clcontaining compounds (CH3Cl is shown as an example) have much weaker absorption over this spectral interval. The IR absorption spectrum of trans-CHClCHCF 3 between 500 and 1900 cm−1 is shown in Figure 4. The spectrum was combined from the results of measurements at various pressures of trans-CHClCHCF3 between 0.27 to 17 kPa (2−128 Torr). The top panel shows the main IR absorption bands while the lower panel shows the spectrum in log scale to illustrate weaker absorption features. There is a good agreement between spectra measured using a deuterated triglycine sulfate detector (DTGS) for spectral resolutions of 0.125 and 0.5 cm−1, with less than 0.3% difference between the total absorption cross sections integrated between 500 and E
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Table 2. UV Absorption Cross Sections of trans-CHClCHCF3 at 298 K wavelength (nm)
σ (× 1020 cm2 molecule−1)
wavelength (nm)
σ (× 1020 cm2 molecule−1)
166 168 170 172 174 176 178 180 182 184 186 188 190 192 194 196
596.4 859.5 1275.1 1745.4 2285.8 2781.0 3197.4 3498.6 3703.6 3896.3 4028.2 4005.1 3781.6 3318.8 2636.1 1928.4
198 200 202 204 206 208 210 212 214 216 218 220 222 224 226
1275.1 729.3 373.1 177.0 82.3 38.3 16.96 7.91 3.56 1.66 0.748 0.342 0.151 0.067 0.031
1.805 × 10−16 cm molecule−1 when integrated over the same spectral interval. The four individual integrated band intensities reported by Sulbaek Andersen et al.14 are consistently slightly smaller than the corresponding values derived from our measurements. These differences are within the reported uncertainty: 3.2% (1600−1790 cm−1), 3.2% (1030−1350 cm−1), 3.7% (750−1030 cm−1), and 6% (600−690 cm−1). There are also three very weak absorption bands beyond this interval (see lower panel of Figure 4): 2.3 × 10−19 cm molecule−1 (between 520 and 620 cm−1), 1.5 × 10−19 cm molecule−1 (between 1790 and 1840 cm−1), and 0.96 × 10−19 cm molecule−1 (between 1840 and 1900 cm−1). The stated level of the major impurity in the sample, 0.5% of trans-CHFCHCF3, can affect our measured IR absorption cross sections, which span more than 3 orders of magnitude (see Figure 4). Using the IR absorption measurements we were able to check and quantify the presence of trans-CHFCHCF3 in the sample. This is illustrated in the circled area of the lower panel in Figure 4 and the corresponding inset in which a small section of the spectrum around 696.2 cm−1 is magnified. For illustrative purpose only, the circled area of the lower panel shows both the final reported spectrum and the spectrum of the vapor phase of the sample, which contains a higher concentration of the trans-CHFCHCF3 (as manifested by the resolved structure with a peak at 696.2 cm−1). Although less pronounced, the same spectral feature is still visible when the liquid-phase sampling was used. The upper part of the inset shows the absorption spectrum of 0.5% of trans-CHF CHCF36 (the expected concentration based on the stated sample impurity). The lower part of the inset shows (from the top to the bottom) the absorption spectra measured when vapor- and liquid-phase sampling of trans-CHClCHCF3 was used. These measured spectra were combined with the spectrum of trans-CHFCHCF3 to find the ratio when the structured absorption feature of the latter one disappears from the measured spectrum of trans-CHClCHCF3 similar to the bottom spectrum in the inset, which is reported here. Thus, we found that the vapor phase of the original trans-CHCl CHCF3 sample contains ∼1.5% of trans-CHFCHCF3, whereas the bulk liquid sample has only ∼0.25% of this impurity. The main absorption band of trans-CHFCHCF3 also clearly manifested itself near 1103 cm−1 and was used to calculate the same impurity level. Although 0.25% impurity in
Figure 4. IR absorption cross sections of trans-CHClCHCF3 shown on both linear and logarithmic scales to visualize small absorption features. Lower panel shows both the final absorption spectrum of trans-CHClCHCF3 and the measured spectrum of the vapor phase of the sample near 696 cm−1 (circled). Inset shows the spectrum of the main impurity, trans-CHFCHCF3 (upper panel), and measured spectra of samples taken from vapor and liquid phase of trans-CHCl CHCF3 along with the final reported absorption spectrum (lower panel).
1900 cm−1. These spectra are reported here after a small correction described below. The spectrum measured using a cold mercury cadmium telluride detector (MCT) integrated between 600 and 1900 cm−1 is ∼0.5% larger. The coincidence of the spectra measured with different spectral resolution is reasonable because the IR absorption spectrum of transCHClCHCF3 does not exhibit narrow spectral features. Sulbaek Andersen et al.14 reported the total IR absorption cross-section of trans-CHClCHCF3 integrated between 600 and 1800 cm−1 to be (1.74 ± 0.20) × 10−16 cm molecule−1, which agrees within the reported uncertainty with our value of F
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atmospheric lifetime due to photolysis in the stratosphere to be τph t‑CFP ≈ 24 years, based on the UV absorption spectrum measured here and a semiempirical approach presented in a recent paper.23 It should be noted that τph t‑CFP was also estimated under the assumption of the compound being well-mixed in the troposphere, which is not correct for very short-lived substances such as trans-CHClCHCF3, thus resulting in an overestimation of its stratospheric removal rate. Hence, the estimation shows that stratospheric photolysis makes a negligible contribution to the atmospheric lifetime derived OH from eq 6 because τph t‑CFP ≫ τt‑CFP. 4.2. Estimated GWP. The global warming potential of a compound under the assumption of complete mixing throughout the atmosphere can be estimated using a semiempirical approach.24 The global warming potential of transCHClCHCF3 was first calculated relative to CFC-11, CFCl3 (halocarbon global warming potential, HGWP), using its IR absorption spectrum, the measured spectrum of Earth’s outgoing radiation, and the estimation of τOH t‑CFP ≅ 46 days:
our samples made only a small difference, we corrected both IR absorption cross sections and OH reaction rate constant for its presence. These slightly corrected IR absorption cross sections are available in Supporting Information.
4. ATMOSPHERIC IMPLICATIONS We report results of the first measurement of rate constant for the reaction of trans-CHClCHCF3 with OH over the temperature range of atmospheric interest and its spectral data in regions of atmospheric importance. These data allow simple estimations of atmospheric properties of the compound. 4.1. Estimated Atmospheric Lifetime. The lifetime of an atmospherically well-mixed compound due to its reaction with tropospheric hydroxyl radicals can be estimated using a simple scaling procedure that is based on the results of field measurements22 and detailed atmospheric modeling.9 Under this well-mixed assumption τtOH ‐CFP =
kMCF(272) OH ·τMCF ≈ 0.126 years = 46 days kt ‐CFP(272)
(6)
HGWPt ‐CFP(t ) =
where kt‑CFP(272 K) ≅ 3.0 × 10−13 cm3 molecule−1 s−1 and kMFC(272 K) = 6.14 × 10−15 cm3 molecule−1 s−1 are the rate constants for the reactions of OH with trans-1-chloro-3,3,3trifluoropropene and methyl chloroform,10 respectively, at T = OH 272 K, and τOH t‑CFP and τMFC are their lifetimes due to reactions with hydroxyl radicals in the troposphere. The value of τOH MFC = 6.13 years was calculated from the measured lifetime of MCF of 5.0 years (reported in the latest WMO/UNEP ozone assesement1) after taking into account an ocean loss of 89 years and a stratospheric loss of 39 years. The actual local atmospheric lifetime for trans-CHClCHCF3 depends on the local chemical environment in which its emissions occur (including such aspects as latitude, season, etc.) because it is significantly shorter than the characteristic time of mixing processes in the troposphere. The variations of the local lifetime of a compound in the atmospheric boundary layer due to the different emission scenarios can be illustrated using the OH concentration field calculated by Spivakovsky et al.9 using a 3-D model. Using Table 5 of Spivakovsky et al.,9 we have estimated the local lifetime of trans-1-chloro-3,3,3-trifluoropropene in the tropics (20 oS to 20 oN) to be between ∼30 and 40 days year round. Outside of the tropics the local lifetime in July slowly increases with latitude from ∼25 days at 28 oN to ∼55 days at 60 oN. In contrast, the local lifetime in January dramatically increases with latitude from ∼83 days at 28 oN, ∼210 days at 36 oN, and up to ∼1800 days at 60 oN. Thus, trans-1-chloro-3,3,3-trifluoropropene emitted into the atmosphere at higher latitudes during winter would behave essentially as a well-mixed compound that could survive into spring and summer or be transported to lower latitudes to be destroyed by the higher local OH concentrations. The globally averaged single-value lifetime derived using eq 6 can be used to characterize a compound’s fate in the atmosphere in the absence of a specific emission scenario and can be useful for comparing similar compounds. Emission-scenario-specific 3-D modeling is required to obtain a more detailed prognosis. As mentioned above, this unsaturated double bond adjacent Cl-containing compound exhibits very strong absorption in the stratospheric transparency window near 200 nm, which could affect its atmospheric lifetime because of photodissociation by solar UV radiation. We can estimate this compound’s
MCFC‐11 τtOH 1 − exp( − t /τtOH ‐CFP ‐CFP) M t ‐CFP τCFC‐11 1 − exp( − t /τCFC‐11) ν
∫ν 2 σt ‐CFP(ν) × Φ(ν) dν 1
ν
∫ν 2 σCFC‐11(ν) × Φ(ν) dν
(7)
1
where Φ(ν) is the intensity of outgoing Earth’s radiation; ν1 and ν2 are the integration limits (500 and 1600 cm−1, respectively, in our calculations); Mt‑CFP and σt‑CFP are the molecular weight and absorption cross sections of transCHClCHCF3, respectively. We then use the presently accepted global warming potential of CFCl3 relative to CO2 (GWPCFC‑11)1 calculated using a radiative transfer model of the atmosphere to obtain GWP of trans-CHClCHCF3 (relative to CO2) GWPt ‐CFP(t ) = HGWPt ‐CFP(t ) × GWPCFCl3(t )
(8)
Thus, calculated values of GWPt‑CFP(t) are presented in Table 3 for time horizons of 20, 100, and 500 years. Note that Table 3. Radiative Forcing Relative to CFC-11 and Global Warming Potentials of trans-CHClCHCF3 (Mass Basis) Relative to CO2 under Assumption of Well-Mixed Atmospherea GWP at time horizon lifetime (days)
RRF (relative CFC-11)
20 years
100 years
500 years
46
0.86
48
14
4
a
The atmospheric lifetime, RRF, and GWP were calculated under assumption of a uniform compound distribution in the atmosphere using the measured spectrum of Earth’s outgoing radiation.24 They can be accepted as upper limits of GWP. The use of a model-derived specific radiative forcing (per unit absorption cross section) as suggested by Pinnock et al.25 would result in ∼9% smaller values of RRF and GWP.
the IR spectral information is needed to calculate the very last term in eq 7, the relative radiative forcing (RRF). Using the satellite-measured intensity of the outgoing Earth’s radiation,24 Φ(ν), we calculate RRF = 0.86 relative to CFC-11. Alternatively, this parameter can be calculated using a modelderived specific radiative forcing (per unit absorption cross G
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section) as suggested by Pinnock et al.25 instead of Φ(ν) in eq 7 to yield RRF = 0.79. Use of this smaller RRF results in the same ∼9% decrease in the GWPs presented in Table 3. Note that this estimation procedure was suggested for greenhouse gases that are well-mixed through the atmosphere. It is not valid for gases with very short atmospheric lifetimes because they do not have a uniform mixing ratio either vertically in the upper troposphere and tropopause region or geographically with latitude as does the CFC-11 reference compound. Absorption of the Earth’s outgoing radiation takes place in the middle and upper parts of the troposphere at colder temperatures. Therefore, the estimation under the well-mixed atmosphere assumption results in an overestimation of GWPs of short-lived compounds. Nevertheless, such estimations do provide a useful scaling of the GWPs of very short-lived compounds in the absence of any other simple single indices. This estimate can be considered as an upper limit of the GWP for trans-1-chloro-3,3,3-trifluoropropene. 4.3. Estimated ODP. The ODP of trans-CHClCHCF3 was recently calculated by Patten and Wuebbles26 who used a 3-D chemical-transport model of the atmosphere and the temperature-independent OH reaction rate constant kt‑CFP = 4.4 × 10−13 cm3 molecule−1 s−1 reported by Sulbaek Andersen et al.14 Thus, they obtained ODPt‑CFP = 0.00034 for emissions from land surfaces at latitudes 30 to 60 °N. We have measured significantly smaller values of kt‑CFP(T), which exhibit a noticeable, although weak, temperature dependence. In the absence of recalculating the ODP using a 3-D model, we can apply a simple ∼47% correction for the difference in the rate constant at T = 272 K to yield ODPt‑CFP = 0.00050. This corrected value is more consistent with the residence time in the atmosphere due to reaction with OH derived in the present work.
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ASSOCIATED CONTENT
The UV absorption cross sections of trans-CHClCHCF3 with wavelength steps of 0.2−1 nm and the IR absorption cross sections of trans-CHClCHCF3 obtained with spectral resolutions of 0.125 and 0.5 cm−1. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* V.L.O.: E-mail:
[email protected]. Phone: 301-9754418. Fax: 301-869-4020. Present Address †
M.J.K.: Goddard Earth Sciences, Technology, and Research (GESTAR), Universities Space Research Association, Greenbelt, MD 20771. Notes
The authors declare no competing financial interest.
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REFERENCES
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S Supporting Information *
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Article
ACKNOWLEDGMENTS
This work was performed under a cooperative research and development agreement CN-5094 with Arkema Inc. (2008) and supported by the Upper Atmosphere Research Program of the National Aeronautics and Space Administration. The authors thank Arkema Inc. for providing the samples. H
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