Temperature Dependence of the Cl Atom Reaction ... - ACS Publications

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Temperature Dependence of the Cl Atom Reaction with Deuterated Methanes Frank Sauer,†,‡,§ Robert W. Portmann,† A. R. Ravishankara,*,†,∥ and James B. Burkholder*,† †

Chemical Sciences Division, Earth System Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305, United States ‡ Cooperative Institutes for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: Kinetic isotope effect (KIE) and reaction rate coefficients, k1−k4, for the gasphase reaction of Cl atoms with 12CH3D (k1), 12CH2D2 (k2), 12CHD3 (k3), and 12CD4 (k4) over the temperature range 223−343 K in 630 Torr of synthetic air are reported. Rate coefficients were measured using a relative rate technique with 12CH4 as the primary reference compound. Fourier transform infrared spectroscopy was used to monitor the methane isotopologue loss. The obtained KIE values were 12CH3D: KIE1(T) = (1.227 ± 0.004) exp((43 ± 5)/T); 12CH2D2: KIE2(T) = (1.14 ± 0.20) exp((191 ± 60)/T); 12CHD3: KIE3(T) = (1.73 ± 0.34) exp((229 ± 60)/T); and 12CD4: KIE4(T) = (1.01 ± 0.3) exp((724 ± 19)/T), where KIEx(T) = kCl+12CH4(T)/kx(T). The quoted uncertainties are at the 2σ (95% confidence) level and represent the precision of our data. The following Arrhenius expressions and 295 K rate coefficient values (in units of cm3 molecule−1 s−1) were derived from the above KIE using a rate coefficient of 7.3 × 10−12 exp(−1280/T) cm3 molecule−1 s−1 for the reaction of Cl with 12 CH4: k1(T) = (5.95 ± 0.70) × 10−12 exp(−(1323 ± 50)/T), k1(295 K) = (6.7 ± 0.8) × 10−14; k2(T) = (6.4 ± 1.3) × 10−12 exp(−(1471 ± 60)/T), k2(295 K) = (4.4 ± 0.9) × 10−14; k3(T) = (4.2 ± 1.0) × 10−12 exp(−(1509 ± 60)/T), k3(295 K) = (2.53 ± 0.6) × 10−14; and k4(T) = (7.13 ± 2.3) × 10−12 exp(−(2000 ± 120)/T), k4(295 K) = (0.81 ± 0.26) × 10−14. The reported uncertainties in the pre-exponential factors are 2σ and include estimated systematic errors in our measurements and the uncertainty in the reference reaction rate coefficient. The results from this study are compared with previously reported room-temperature rate coefficients for each of the deuterated methanes as well as the available temperature dependent data for the Cl atom reactions with CH3D and CD4. A two-dimensional atmospheric chemistry model was used to examine the implications of the present results to the atmospheric lifetime and vertical variation in the loss of the deuterated methane isotopologues. The relative contributions of the reactions of OH, Cl, and O(1D) to the loss of the isotopologues in the stratosphere were also examined. The results of the calculations are described and discussed.

I. INTRODUCTION Methane (CH4) is an important trace gas in the Earth’s atmosphere because it plays many different roles. Methane is a greenhouse gas, it amplifies OH radical production, its degradation can lead to the production of ozone in the troposphere, and its degradation in the stratosphere is a significant source of water vapor. Further, methane is a major component of natural gas, and leakage from this source can be an important contributor to anthropogenic climate change. The atmospheric concentration of methane has increased by roughly 1.15 ppm, though not at a constant rate, since industrialization.1 However, the many known sources of these increases are not well quantified.1 Methane is second only to CO2 in terms of its contributions to radiative forcing of the atmosphere because of its strong absorption in the infrared window region and relatively high atmospheric mixing ratio, ∼1.8 ppm in year 2011.1 In the present-day atmosphere, the contribution of anthropogenic methane to direct radiative forcing is ∼25% of that by CO2.1 CH4 is a greenhouse gas of particular interest because of its relatively short atmospheric lifetime of ∼10 years. Therefore, © XXXX American Chemical Society

decreases in methane emissions would result in a faster atmospheric response with regard to radiative forcing than for the longer-lived greenhouse gases CO2, N2O, and chlorofluorocarbons (CFCs). Atmospheric methane comes from a combination of natural, human influenced, and anthropogenic sources. The total flux of CH4 into the atmosphere is estimated to be ∼500 Tg yr−1. Natural sources include wetlands, termites, oceans, and freshwater. Human influenced and anthropogenic sources include rice production, biomass burning, natural gas leaks, and fossil fuel exploitation activities.2,3 It is estimated that 50 to 65% of atmospheric CH4 originates from anthropogenic sources.1 The main atmospheric loss processes for CH4 are its reactions with OH, Cl, and O(1D), with minor contributions from soil uptake. Special Issue: Mario Molina Festschrift Received: August 28, 2014 Revised: November 4, 2014

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dx.doi.org/10.1021/jp508721h | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. A schematic diagram of experimental apparatus used for measuring k1−k4.

of physical transport of water vapor from the troposphere to the stratosphere. For these reasons, rate coefficients for reactions of Cl with isotopologues of methane are needed.22 Unlike the reactions with OH, data for the reactions of chlorine atoms with methane isotopologues

In addition to being an important climate forcing agent, CH4 also plays key roles in the chemistry of the atmosphere. The primary atmospheric loss process for CH4 is its reaction with the OH radical. The reaction of OH with CH4 can lead to either a net loss or production of odd hydrogen species, HOx, (HOx = OH + HO2) depending on the nitrogen oxide, NOx (NOx = NO + NO2), mixing ratio. Tropospheric oxidation of CH4, in the presence of NOx also leads to the formation of ozone, and this process contributes significantly to background tropospheric ozone.4 CH4 also influences the chemistry of the stratosphere by sequestering catalytic chlorine species in the inactive reservoir form of HCl and by being a major (roughly 30%) source of water vapor in the stratosphere.5 Water vapor in the lower stratosphere is an important gas for controlling outgoing infrared radiation. Water vapor in the stratosphere is also a source of various radicals and provides a medium for heterogeneous and multiphase reactions.6 Although the total methane flux into the atmosphere is reasonably well established, the quantification of the individual sources is less certain. Quantification of individual source strengths is complicated by the diffuse nature of the sources and large temporal and spatial variability in the emission rates. One approach often used to constrain the source strengths is the measurement of 13C/12C and D/H isotope ratios in atmospheric methane since its different sources have different isotope compositions.2,3,7−10 To carry out such inversions of observed isotope compositions to emissions, knowledge of the isotopologue fractionation (i.e., the differences in the rates of removal of the different isotopologues) is essential. The deutero-isotopologues of CH4 (in particular CH3D) are valuable in this analysis due to their relatively large atmospheric fractionation.11−14 Quay et al.15 has described the utilization and utility of measuring isotopologue abundances to constrain emission source strengths. While the main loss process for CH4 in the troposphere is its reaction with OH, its reaction with Cl atom may be important in the marine boundary layer and coastal areas where Cl atom concentrations are expected to be high.16−21 Stratospheric degradation of CH4 is also initiated by its reaction with Cl atom along with OH and O(1D). The Cl and O(1D) reactions contribute most in the stratosphere. This fractionation in the stratosphere is also important since the isotopic signatures of water vapor are used to deduce the extent

Cl + 12CH 3D → Product

k1

Cl + 12CH 2D2 → Product

Cl + 12CHD3 → Product

Cl + 12CD4 → Product

k2

k3

k4

(1) (2) (3) (4)

are more limited (the 12-carbon isotope will be written simply as C from here on). Recent kinetic studies have focused on CH3D, which is the most abundant deuterated methane isotopologue in the atmosphere. Saueressig et al.12 and Tyler et al.23 have reported relative rate data for reaction 1 at several temperatures in the ranges 223−296 K and 273−349 K, respectively. The kinetic data from these two studies are in reasonable agreement over the temperature range common to both studies. Feilberg et al.,24 Boone et al.,25 and Wallington and Hurley,26 also using relative rate methods, have reported room-temperature rate coefficients for reactions 1−4. In addition, Matsumi et al.27 have reported room temperature rate coefficients for reactions 2 and 4, which were measured using a direct technique where the temporal profile of photolytically produced Cl atoms were measured in an excess of methane isotopologues. To the best of our knowledge, there are no experimental data on the temperature dependence of reactions 2 and 3. A complete self-consistent set of temperature dependence rate coefficient data for reactions 1−4 is therefore desired. In addition to its atmospheric importance, accurate temperature-dependent rate coefficient data for the Cl atom reaction with CH4 (and its deutero-isotopologues) is of theoretical interest.28−33 We report the rate coefficients for reactions 1−4 between 223 and 343 K measured using a relative rate technique with CH4 as the primary reference compound in 630 Torr (syn. air) total pressure. Methane was measured using Fourier transform infrared (FTIR) spectroscopy, and Cl atoms were produced by broadband UV/visible photolysis of Cl2. The present results are compared with rate coefficient data from previous studies and B



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experiment, the measured peak heights were good measures of the concentrations. Further, since our rate coefficient measurements relied on the ratio of the concentrations, our approach was well suited to obtain the extent of reaction. First, a mixture of Cl2 and methanes was introduced into the apparatus, mixed well, and multiple infrared spectra were recorded sequentially. Then, the mixture was subjected to photolysis for a known time, mixed by circulating the mixture through the system, and multiple infrared spectra were recorded. This process of photolysis, mixing, and infrared spectral measurements was repeated until significant amounts of the methane isotopologues were depleted. Each of the multiple spectra measured following photolysis for a given time was analyzed to obtain the peak heights of the absorption features noted above. Then the obtained peak heights were averaged to obtain the concentrations in that mixture. Averaged absorption values were then used in the kinetic analysis. It should be noted that eq 7 requires only the ratio of the methane concentrations at various reaction times. Therefore, the rate coefficient determination is independent of the absolute methane infrared cross sections, but dependent on the linearity of the absorption with concentration. The infrared absorption of each methane isotopologue was found in separate measurements to be linear over the range of concentrations used in the relative rate experiments. Chlorine atoms were generated by broadband UV/visible photolysis of Cl2

the discrepancies/agreement between the various studies are discussed. The lifetimes of the deuterated methanes and comparisons of the contribution of the different loss processes to the atmospheric lifetime are computed with a twodimensional atmospheric model.

2. EXPERIMENTAL DETAILS A relative rate technique was used to measure rate coefficients for reactions 1−4 over the temperature range 223−343 K in 630 Torr of synthetic air. In the relative rate method, the loss of a compound due to its reaction with the Cl atom is measured relative to the loss of a reference compound due to its reaction with the same pool of Cl atoms. CH4, CH2D2, and/or CHD3 were used as reference compounds in this study for measuring k1−k4. Cl + Compound → Products

(5)

Cl + Reference → Products

(6)

Provided the loss of the compound and reference are due only to reaction with the Cl atom, the rate coefficients for their reactions are related by ⎛ [Compound]0 ⎞ ⎛ k Compound ⎞ ⎛ [Reference]0 ⎞ ln⎜ ⎟ × ln⎜ ⎟ ⎟=⎜ ⎝ [Compound]t ⎠ ⎝ kReference ⎠ ⎝ [Reference]t ⎠ (7)

where [Compound]t and [Reference]t are the concentrations measured during the course of the reaction and [Compound]0 and [Reference]0 are the initial concentrations. A linear regression analysis of the ln([Compound]0/[Compound]t) versus ln([Reference]0/[Reference]t) data yields (kCompound/ kReference), the ratio of the rate coefficients for the two species. The absolute accuracy of kCompound determined this way, therefore, depends on the accuracy of the reference rate coefficient. The accuracy of the measurement is usually better if the values of kReference and kCompound are of similar magnitude. A diagram of the apparatus used to measure k1−k4 is shown in Figure 1. The concentrations of various methane isotopologues were measured by monitoring their infrared absorption using a Fourier transform infrared spectrometer (FTIR) coupled to a multipass absorption cell. The multipass absorption cell had a base path of 1.6 m, and the total path length was set to 35.2 m for all measurements. The FTIR was equipped with an incandescent carbon rod light source, a CaF2 beam splitter and a liquid nitrogen cooled InSb detector. Absorption spectra were recorded in 50 coadded scans at a spectral resolution of 0.1 cm−1 (approximately the pressure broadened line width). We opted to use the measured peak heights (rather than integrated band strength) of isolated infrared absorption lines to obtain the concentrations of the isotopologues. This is in contrast to how some other investigators have determined the concentrations by either fitting the spectrum over multiple lines and/or using integrated band strengths. Here, for each methane isotopologue, several combinations of ro-vibrational transitions free of spectra interference from other isotopologues were used in the data analysis. The specific spectral lines used in the analyses are presented graphically in the Supporting Information and clearly show that there were no spectral interferences from any of the isotopologues. Further, as the reaction proceeded, spectral features of the products of the reactions (and their subsequent reactions) did not interfere. Since the infrared lines were pressure broadened and the pressure was constant during an

Cl 2 + hv → 2Cl

(8)

using either fluorescent black light or quartz halogen lamps. The UVB fluorescent black lights produced a spectral output over the wavelength range 300 to 400 nm with peak intensity at 350 nm. The Cl atoms from Cl2 photolysis are produced primarily in the ground (2P3/2) spin-state.34 Under the conditions of our experiments, the 1/2 and 3/2 Cl atom spin−orbit states are expected to be in equilibrium35 throughout the course of the reaction, and the kinetic results reported here are for an equilibrated distribution of spin states. The fluorescent lamps (three sets of four lamps) surrounded the Pyrex FTIR multipass absorption cell. Photolysis of the gas mixture in the multipass absorption cell, using the fluorescent lamps, was used only for room temperature measurements. Quartz halogen lamps, 75 W, were used as the photolysis light source for relative rate measurements made in the temperature jacketed reaction cell, as shown in Figure 1. The lamp output was collimated and passed through the length of the reaction cell. Rate coefficients ratios were measured using one or two lamps to vary the photolysis flux in the reaction cell. The reaction cell consisted of a 200 cm long, 5 cm i.d., cylindrical jacketed Pyrex tube. Temperature controlled fluid from a temperature-regulated bath was circulated through the reactor to control its temperature. The temperature of the cell was varied between 223 and 343 K and was measured to be uniform over its length to ±2 K. Relative rate coefficients were measured by first mixing known amounts of the reactant, reference compound, Cl2, and synthetic air buffer gas in the cells at a total pressure of 630 Torr. Several test measurements performed using N2 as the buffer gas yielded the same rate coefficients as in synthetic air. The initial pressures of the reactant and CH4 were varied between 5 and 60 mTorr. The initial Cl2 pressure was in the range 240 to 1700 mTorr, much larger than those of the methane isotopologues. The measured rate coefficients were C



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independent of the initial concentrations. Slightly different procedures were used when Cl2 was photolyzed in the multipass cell and in the reaction cell. Experiments performed using the multipass cell, with the fluorescent lamp photolysis source, followed the commonly used methods in relative rate measurements. First, an infrared absorption spectrum of the initial gas mixture was recorded. The mixture was photolyzed for 10−30 s, depending on the initial Cl2 concentration and number of photolysis lamps used, and then another infrared spectrum was recorded. This cycle was repeated until approximately 80% of the initial methane concentration was consumed. The rate coefficient measurements using the multipass cell were all performed at room temperature. Measurements at other temperatures used the reaction cell and the quartz-halogen photolysis lamps. In these experiments, only a fraction of the total gas mixture, approximately 20%, was exposed to the photolysis lamps at any given time. Following photolysis in the reaction cell, a circulating pump with a Tefloncoated diaphragm thoroughly mixed the contents of the reaction and multipass cells. The concentrations of the methanes were then measured in the multipass cell, and the photolysis and mixing cycles were repeated. Due to the lower photon flux available for Cl2 photolysis in this configuration, the photolysis times in the reaction cell were 10 to 60 min, depending on the Cl2 concentration. 2.1. Materials. Synthetic air (UHP; 80% N2/ 20% O2), N2 (UHP; >99.99%), and the methane samples (>99%) were used as supplied. Pressures were measured using 10, 100, and 1000 Torr capacitance manometers. The presence of impurities in methane or Cl2 do not affect the measured relative rate coefficients as long as no other radical that can react with methane was produced.

Table 1. Summary of Rate Coefficient Ratios Measured in This Work for the Cl + CH3D Reaction, k1(T), Relative to That for the Cl + 12CH4 Reaction, k0(T), over the Temperature Range 248−343 K temperature (K) 343 343 343 323 323 295 295 295 295b 295 272 272 248 248

k1(T)/k0(T) 0.732 ± 0.044a 0.747 ± 0.073 0.702 ± 0.037 average: 0.698 ± 0.060 0.725 ± 0.059 average: 0.731 ± 0.042 0.700 ± 0.030 0.742 ± 0.022 0.722 ± 0.010 0.686 ± 0.020 average: 0.696 ± 0.007 0.697 ± 0.011 average: 0.704 ± 0.046 0.663 ± 0.025 average:

0.720 ± 0.024

0.712 ± 0.036

0.702 ± 0.014

0.696 ± 0.005

0.685 ± 0.026

a

The quoted uncertainties are the 2σ (95% confidence level) values from the unweighted linear least-squares fit of the measured relative methane isotopologue loss using eq 7. The average values and uncertainties are obtained by fitting all the data for a given temperature to eq 7. bRoom temperature measurement performed with photolysis in a reaction cell.

Table 2. Summary of Rate Coefficient Ratios Measured in This Work for the Cl + CH2D2 Reaction, k2(T), Relative to That for the Cl + 12CH4 Reaction, k0(T), over the Temperature Range 248−343 K

3. RESULTS AND DISCUSSION The results from our relative rate coefficient measurements for CH3D, CH2D2, CHD3, and CD4 are summarized in Tables 1−4, respectively. CH4 was used as the reference compound for determining the rate coefficients for Cl atom reactions with CH3D, CH2D2 and CHD3. As indicated in Table 3, CH2D2 was used as the reference compound in several measurements for k3. In these measurements, the rate coefficient for reaction 2 obtained in this work relative to CH4 reaction was used in the data analysis. Rate coefficient data obtained using the two reference compounds were found to be in good agreement and were combined to calculate the Arrhenius parameters for reaction 3. Due to the relatively large difference between the rate coefficients for CH4 and CD4, CHD3 was used as a “transfer” reference compound for the CD4 experiments. The rate coefficients derived in this work for reaction 3 were used in obtaining k4, and ultimately referenced to the reaction of Cl with CH4. A summary of the room temperature (295 K) rate data for reaction 1, measured relative to the rate coefficient for the reaction of Cl with CH4, is given in Table 1 and shown in Figure 2. This data set is representative of the precision of the measurements obtained at other temperatures and for the other deuterated methanes, Tables 2−4. The majority of the room temperature experiments were performed in the multipass cell, however, several room temperature experiments were also performed using the reaction cell and the methods described in the Experimental Details section. The rate coefficients obtained using these two methods are in good agreement, as shown in Figure 2. The ratios of the reaction rate coefficients given in

temperature (K) 343 343 323 323 295 295 295 268 248

k2(T)/k0(T) 0.499 ± 0.025a 0.524 ± 0.019 average: 0.462 ± 0.032 0.472 ± 0.014 average: 0.454 ± 0.016 0.461 ± 0.015 0.464 ± 0.006 average: 0.436 ± 0.006 0.406 ± 0.009

0.518 ± 0.014

0.469 ± 0.012

0.460 ± 0.006

a The quoted uncertainties are the 2σ (95% confidence level) values from the unweighted linear least-squares fit of the measured relative methane isotopologue loss using eq 7. The average values and uncertainties are obtained by fitting all the data for a given temperature to eq 7.

Tables 1−4 were obtained from unweighted linear least-squares fits of the measured relative losses to eq 7. The precision of the fits was good in all cases with deviations of a few percent. The fits also yielded zero intercepts to within the uncertainty of the fit. The final rate coefficients were calculated with intercept set to zero. At each temperature with multiple measurements, we report an average value for the rate coefficient ratio and use this D



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Table 3. Summary of Rate Coefficient Ratios Measured in This Work for the Cl + CHD3 Reaction, k3(T), Relative to That for the Cl + 12CH4, k0(T), and Cl + CH2D2, k2(T), Reactions over the Temperature Range 225−343 K k3(T)/k0(T)

temperature (K) 343 343 343 323 323 295 295b 295b 272 272 248 225 296 308 333

0.333 ± 0.011a 0.296 ± 0.007 0.295 ± 0.003 average: 0.264 ± 0.007 0.280 ± 0.008 average: 0.249 ± 0.008 0.241 ± 0.005 0.272 ± 0.010 average: 0.242 ± 0.001 0.248 ± 0.005 average: 0.229 ± 0.005 0.214 ± 0.002

Table 4. Summary of Rate Coefficient Ratios Measured in This Work for the Cl + CD4 Reaction, k2(T), Relative to That for the Cl + 12CHD3 Reaction, k3(T), over the Temperature Range 248−343 K

(k3(T)/k2(T))

temperature (K)

k4(T)/k3(T)

343 343

0.393 ± 0.007a 0.394 ± 0.014 average: 0.399 ± 0.020 0.379 ± 0.004 average: 0.314 ± 0.008 0.329 ± 0.013 0.296 ± 0.005 0.299 ± 0.008 0.346 ± 0.008 0.297 ± 0.014 0.311 ± 0.005 0.304 ± 0.011 0.286 ± 0.004 0.320 ± 0.018 0.321 ± 0.008 average: 0.296 ± 0.006 0.226 ± 0.003

0.300 ± 0.004

323 323

0.272 ± 0.006

296 296b 296 296 296 296 296 296 296 296b 296

0.251 ± 0.005

0.245 ± 0.002

0.575 ± 0.004 0.587 ± 0.004 0.626 ± 0.004

272 248

a The quoted uncertainties are the 2σ (95% confidence level) values from the unweighted linear least-squares fit of the measured relative methane isotopologue loss using eq 7. The average values and uncertainties are obtained by fitting all the data for a given temperature to eq 7. bRoom temperature measurement performed with photolysis in a reaction cell.

0.393 ± 0.006

0.383 ± 0.007

0.310 ± 0.004

a

The quoted uncertainties are the 2σ (95% confidence level) values from the unweighted linear least-squares fit of the measured relative methane isotopologue loss using eq 7. The average values and uncertainties are obtained by fitting all the data for a given temperature to eq 7. bRoom temperature measurement performed in a reaction cell.

average value in the determination of the Arrhenius parameters and kinetic isotope effects for reactions 1-4. A parameter commonly used in the discussion of atmospheric isotope fractionation is the kinetic isotope effect (KIE). KIE is simply defined as the rate coefficient for the reaction of Cl with CH4 divided by the rate coefficient for the reaction of Cl with the isotopologue of interest. The KIE values for the deuterated methanes obtained from our temperaturedependent data are given in Table 5 in the form KIE = BeC/T. The temperature dependence of the KIE values, the C parameter, increases with increasing deuterium substitution, while the absolute rate coefficient decreases. The absolute rate coefficients for the deuterated methanes were calculated using the recommended rate coefficient for Cl atom reaction with CH4 at that specific temperature, 7.3 × 10−12 exp(−1280/T) cm3 molecule−1 s−1.22 Arrhenius parameters, A and E/R, were derived by linear least-squares analyses of the equation E (9) RT for each of the deuterated methane reactions. They are given in Table 5 and included in Figure 3. 3.1. Uncertainty Analysis. The relative rate coefficients obtained in this study were found to be independent of various experimental parameters such as the initial concentrations of the methane isotopologues and Cl2, Cl2 photolysis rate, extent of reaction, and use of N2 or synthetic air. The relative rate data, e.g., as shown in Figure 2, obeyed eq 7 over the entire range of methane isotopologue loss and yielded a zero intercept to within the uncertainty of the fit. Room temperature relative rate coefficients measured using the multipass absorption cell ln(k(T )) = ln(A) −

Figure 2. A plot of the measured loss of CH3D relative to that of CH4 at 295 K. The different symbols represent separate experiments performed under different experimental conditions. The filled circles represent measurements made using the reaction cell at room temperature, 295 K.

agreed with those measured using the reaction cell and thus provided additional assurance that the methods used in the temperature-dependent rate coefficient measurements are valid. Several experiments were also performed to verify that secondary losses of the methanes were not significant. In the absence of the photolysis light, mixtures of reactant, reference compound, and Cl2 in the reaction cell showed that “dark” E



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Table 5. Summary of the Arrhenius Parameters and KIE Values Obtained in This Work and Previous Studies for the Cl Atom Reaction with CH3D, CH2D2, CHD3 and CD4a k(T)b

KIE(T) = BeC/T temperature range (K)

B

C (K)

KIE(295 K)

Ac

E/R (K)

k(295 K)d

reference this work Feilberg et al.24 Boone et al.25 Wallington and Hurley26 Saueressig et al.12 Tyler et al.23 Combined Fitf NASA/JPL Rec.22 This Work Feilberg et al.24 Boone et al.25 Matsumi et al.27 Wallington and Hurley26 This Work Feilberg et al.24 Boone et al.25 Wallington and Hurley26 This Work Feilberg et al.24 Boone et al.25 Matsumi et al.27 Chiltz et al.36 Clyne and Walker37 Wallington and Hurley26

248−343 298 ± 2 298 ± 5 295 ± 2

1.227 ± 0.004

43 ± 5

1.420 ± 0.038 1.47 ± 0.03 1.47 ± 0.1 1.36 ± 0.04e

5.95 ± 0.70

1323 ± 50

6.7 ± 0.8 6.5 ± 0.1 6.5 ± 0.5 6.98 ± 0.2

223−296 273−350

1.278 ± 0.095 0.894 ± 0.13

51.3 ± 19 145 ± 42

1.521 ± 0.041 1.464 ± 0.02

CH2D2

248−343 298 ± 2 298 ± 5 RTg 295 ± 2

1.14 ± 0.20

191 ± 60

2.17 2.45 2.27 1.36 2.19

5.63 ± 0.42 8.16 ± 1.2 6.22 7.0 6.4 ± 1.3

1328 ± 34 1425 ± 42 1345 1380 1471 ± 60

6.24 ± 0.5 6.51 ± 0.9 6.5 6.5 4.4 ± 0.9 3.89 ± 0.02 4.2 ± 0.5 7.0 ± 0.8 4.35 ± 0.2

CHD3

225−343 298 ± 2 298 ± 5 295 ± 2

1.73 ± 0.34

229 ± 60

3.76 ± 0.7 4.7 ± 0.1 5.0 ± 0.8 4.31 ± 0.10

4.22 ± 1.0

1509 ± 60

2.53 ± 0.60 2.03 ± 0.04 1.9 ± 0.3 2.21 ± 0.05

CD4

248−343 298 ± 2 298 ± 5 RTg 300−475 298−1018 295 ± 2

1.01 ± 0.30

724 ± 19

2000 ± 120

952 ± 69 884 ± 28

± ± ± ± ± ± ±

7.13 ± 2.3

0.48 ± 0.12 0.70 ± 0.11

11.8 14.7 17.6 11.6 12.1 14.0 16.4

15.2 ± 3.8 10.4 ± 1.6

2232 ± 69 2164 ± 28

0.81 0.65 0.54 0.82 0.79 0.68 0.58

CH3D

d

± ± ± ± ±

0.2 0.05 0.27 0.16 0.10

3.5 0.3 1.3 1.4 2.9 2.0 1.60

± ± ± ±

0.26 0.01 0.04 0.1

± 0.06

The quoted error limits for KIE in this work are 2σ from the precision of the fit to the average KIE values obtained at each temperature. The error limits for the A factor include estimated systematic measurement errors (5%) and the NASA/JPL recommended 2σ uncertainty in k(Cl + 12CH4) at 298 K of 10%. The quoted uncertainty in E/R was taken to be the greater of the uncertainty in C coefficient in the KIE equation or 50 K. The error limits for the previous studies are 2σ values reported by the authors. bk(295 K) values and Arrhenius parameters were calculated with k(Cl + 12CH4) = 7.3 × 10−12 exp(−1280/T) cm3 molecule−1 s−1. cUnits: 10−12 cm3 molecule−1 s−1. dUnits: 10−14 cm3 molecule−1 s−1. eA revised KIE(295 K) value of 1.47 ± 0.09 was reported in Saueressig et al.12 as a private communication with Wallington and Hurley. fA fit to the results from this work, Saueressig et al.,12 and Tyler et al.;23 the latter two values have been adjusted for k(295 K). gReported as room temperature. a

production of another reactive radical in the system could influence the measured rate coefficients. If O2 were present, it can scavenge CH3 radicals and form CH3O2, which can subsequently lead to species such as OH if NOx is also present. We used UHP N2 in our experiments and the levels of O2 and NOx (= NO + NO2) were, respectively, 5 ppm and 7 ppt (NO < 2 ppt and NO2 < 5 ppt). Therefore, in N2, much of the chemistry that can lead to reactive radicals such as OH is greatly suppressed. Further, we made measurements in air and obtained the same results as in N2, as noted in the Results and Discussion section. To evaluate contributions of secondary reactions, we explicitly analyzed the potential sequence of reactions that could occur. Based on the numerical analysis and the observation that the obtained results are the same in air and N2, we contend that the contribution of secondary reactions to the loss of the methanes, for example, due to reactions with OH, was negligibly small. As noted above, CH3Cl (and its isotopologues) were formed in the reaction mixture. However, these products undergo further reactions with Cl atoms in our system often faster than the reaction of Cl with the isotopologues of methane. Therefore, we did not measure the changes in the

methane loss was not measurable over the duration of a typical relative rate experiment. Photolysis of mixtures of the reactant and reference compound with no Cl2 present showed no measurable methane loss. An advantage of using a relative rate technique, where the relative loss of two stable reactants is followed, is that secondary chemistry of the free radical with impurities or reaction products do not contribute to the measured rate coefficients. In a system where Cl2 is photolyzed in the presence of methane, to a first order approximation, the concentration of chlorine atoms does not change much during the course of the reaction because it is regenerated by the following sequence of reactions Cl 2 + hv → 2Cl

(8)

Cl + CH4 → HCl + CH3

CH3 + Cl 2 → CH3Cl + Cl

When the concentration of Cl2 gets low enough for the CH3 radical to be lost by other processes, for example, reaction with itself or with an impurity in the system, the Cl atom concentration would decrease. However, the relative rate of the two reactants will not be affected. On the other hand, the F



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previously reported values. Indeed, the agreement between the past five studies and ours is excellent, with an average k(298 K) of 6.57 ± 0.50 (2σ of the average). Our measured value of k(298 K) for CH2D2 agrees, to within the combined 2σ uncertainties, with the values reported by Feilberg et al.,24 Boone et al.,25 and Wallington and Hurley,26,38 with an average of 4.21 × 10−14 cm3 molecule−1 s−1. The value reported by Matsumi et al.,27 is roughly 65% higher than this average value. Matsumi et al. measured the loss of Cl atoms in an excess of CH2D2, while all the other studies were relative rate coefficient measurements. It is not clear as to why there is such a large difference. The larger value reported by Matsumi et al. could suggest the presence of a reactive impurity in their CH2D2 sample, as noted previously by Boone et al. Matsumi et al. did not report the chemical purity of their methane samples, but they did note that the isotopic purity of their CH2D2 sample was 98% or better. Therefore, the reason for their reported higher value cannot be due to the presence of the nondeuterated methane. Furthermore, they would need an impurity level of almost 0.1% (assuming that their sample contained an impurity that reacts with Cl with a rate coefficient of 1 × 10−11 cm3 molecule−1 s−1), which seems unlikely. Therefore, we are at a loss to explain this difference for their reported value for k2 with those of many other studies. The rate coefficient data for CHD3 and CD4 show slightly larger differences among the various studies with the values reported by Wallington and Hurley26 and Clyne and Walker37 for CD4 falling within the combined range of the quoted uncertainties. The rate coefficients reported by Chiltz et al.36 and Matsumi et al.27 for CD4 agree best with our results. Literature values for the temperature dependence of the rate coefficients are available only for reactions 1 and 4. For reaction 1, Saueressig et al.12 measured the relative rate coefficients at 223 and 296 K, while Tyler et al.23 made measurements at 273, 299, 323, and 349 K. The Arrhenius parameters and the temperature dependence of the KIE values from these two studies are given in Table 5. The temperature dependence of the KIE measured between 248 and 343 K in this work agrees with those reported by Saueressig et al. and Tyler et al. The largest deviation is ∼7% at the lowest temperature of our measurements. Table 5 and Figure 3 also contain a fit to the combined data from this work and that of Saueressig et al. and Tyler et al. For reaction 4, Chiltz et al.36 and Clyne and Walker37 reported relative rate coefficients at room temperature and above, 300−475 K and 298−1018 K, respectively. Both of these studies report E/R values that are somewhat larger, by about 180 K, than obtained in our study. This is not surprising in that the rate coefficient for the reaction of Cl with isotopologues of methane shows non-Arrhenius behavior (curvature, with a steeper temperature dependence at higher temperatures). However, over the temperature range common to our study, the rate coefficient data from Chiltz et al.36 are in agreement with our values while the Clyne and Walker37 values are systematically lower by ∼15%. The observed trend in the values of E/R going from reactions 1−4 is consistent with the increasing barrier height for the reaction.39 Further, the role of tunneling in these reactions will be greater for the abstraction of an H atom than a D atom. Thus, one would expect the measured activation energies for these reactions to increase going from reaction 1 to reaction 4. Both these factors can influence the increasing E/R and the measured trend in E/R is to be anticipated.

Figure 3. Arrhenius plot of the Cl + deuterated methane (CH3D, CH2D2, CHD3, and CD4) rate coefficients obtained in this work (solid circles and solid lines). The rate coefficient data is given in Tables 1−4, and the Arrhenius parameters are summarized in Table 5. The Cl + CH4 rate coefficient used in our data analysis is shown for comparison. Results from previous studies are for CH3D: Saueressig et al.12 results at 223, 243, 263, and 296 K (bowtie), and Tyler et al.23 results at 273, 299, 323, and 349 K (diamond); the dashed line is the combined fit (see Table 5). For CD4: Chiltz et al.36 results over the temperature range 300 to 475 K (dotted line), and Clyne and Walker37 results over the temperature range 298 to 1018 K (dashed line). The room temperature results for CH3D, CH2D2, CHD3, and CD4 from Wallington and Hurley26 (open circles), Boone et al.25 (open squares), and Feilberg et al.24 (open triangles) and results for CH2D2 and CD4 from Matsumi et al.27 (open inverted triangles) are included for comparison. [Note: Accidentally, the value of k2 measured by Matsumi et al., falls close to the value of k1 shown in the figure.]

concentrations of CH3Cl (and its isotopologues) to extract information about relative rates of reactions 1−4. The rate coefficient for the reaction of Cl with CH4, 7.3 × 10−12 exp(−1280/T) cm3 molecule−1 s−1, used in this work was taken from kinetic data evaluation of Sander et al.22 This reaction is an excellent reference reaction for our relative rate studies because it has been extensively studied (see Sander et al.22 for discussion of measurements) and its rate coefficient over the range of temperatures used in the present experiments is well established. Sander et al. give the estimated uncertainty in this rate coefficient as f(T) = f(298 K) exp|g[(1/T) − (1/ 298)]| where f(T) is the uncertainty factor at temperature T and f(298 K) = 1.05 and g = 50 K. These uncertainties have been propagated in the values quoted in Table 5. Although the uncertainty in the rate coefficient for the reaction of Cl with CH4 is relatively small, it still adds to the uncertainty in the determination of the absolute rate coefficients for reactions 1−4, i.e., the deuterium KIEs determined from the relative rate data are more accurately determined than are the individual absolute rate coefficients. 3.2. Comparison with Previous Work. A summary of the rate coefficient data available for the deuterated methanes, reactions 1−4, is given in Table 5. It compares both the KIE (relative to CH4), which is the measured quantity, and the value of k for the isotopologue obtained using k(Cl + 12CH4) = 7.3 × 10−12 exp(−1280/T) cm3 molecule−1 s−1. The results from previous studies are also included in Figure 3. Overall, the room temperature rate coefficient data for CH3D agrees well with all G



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Table 6. Summary of KIEs for Methane Isotopologue Reactions with OH, O(1D), and Cl Atoms, Where KIEx(T) = kCl+12CH4(T)/kCl+x KIE(T) = BeC/T molecule 13

CH4

reactant OH

O(1D) Cl

CH3D

CH2D2 CHD3 CD4 a

OH O(1D) Cl OH Cl OH Cl OH Cl

KIE(298 K)a

B

C (K)

method

reference

1.010 1.0054 1.0039 1.001 1.013 1.026 1.066 1.066 1.234 1.294 1.06 1.42 1.80 2.16 3.25 3.73 7.32 11.5

1.0054 1.013 1.0137 1.043 0.600 1.06 1.227 0.817 1.138 1.29 1.73 1.88 1.01

0 0 3.56 6.455 215 0 43 235 191 275 229 405 724

relative rate relative rate relative rate relative rate relative rate ab initio relative rate relative rate PP-LIF relative rate relative rate relative rate PP-LIF relative rate PP-LIF relative rate PP-LIF relative rate

Davidson et al.42 Cantrell et al.44 Saueressig et al.43 Davidson et al.42 Saueressig et al.43 Tanaka et al.45 Saueressig et al.11 Crowley et al.46 Gierczak et al.13 Saueressig et al.43 Saueressig et al.43 this worka Gierczak et al.13 this worka Gierczak et al.13 this worka Gierczak et al.13 this worka

See Table 5 for comparison of Cl atom KIE values from this work with previous measurements.

of CF3CH2OCHF2 and CF3CHClOCHF2 kinetics along with their previously measured k4(295 K) value. The present k4(295 K) results are approximately 20% larger than used in those studies. Using our values, the previously reported rate coefficients for the reactions of Cl with halogenated ethers would be ∼20% larger.

The reaction of Cl with methane is expected to proceed via a direct abstraction. Therefore, the reaction of Cl with mixed isotopes (CH3D, CH2D2, and CHD3) can be viewed as two different reactions, one that involves the abstraction of an H atom, and one that involves the abstraction of a D atom, as noted by Feilberg et al.24 for comparing rate coefficient at 298 K. Here we further assume that the temperature dependence and the A factor for abstracting D and H are independence of the presence of the other isotopes. With this assumption, it is possible to represent the overall rate coefficient and its temperature dependence for the isotopologues containing both H and D as k(CHxDy ) = (AH × x)e

⎛E ⎞ −⎜ H ⎟ ⎝ RT ⎠

+ (AD × y)e

4. ATMOSPHERIC IMPLICATIONS The atmospheric abundance of methane isotopologues containing carbon-13 and deuterium are enhanced relative to 12 CH4 due to fractionation, i.e., slightly slower removal reactions for the heavier isotopologue, and hence their longer atmospheric lifetimes, i.e., KIE values >1. The most important gas-phase atmospheric sinks for methane include its reaction with OH, Cl, and O(1D). A summary of KIE values determined experimentally using either relative rate or absolute methods is presented in Table 6. The relative contributions of the OH, Cl, and O(1D) reactions for the removal of methanes are dependent on its location in the atmosphere. The KIE value for the O(1D) reaction, which is only relevant in the stratosphere, with 13CH4 was determined by Davidson et al.42 to be relatively small, KIE(298 K) = 1.001. In a more recent study, Saueressig et al.43 report a larger value, 1.013, independent of temperature between 223 and 296 K. Saueressig et al. presented a comparison of model calculations with the methane fractionation measured in the stratosphere to support their measurements. In the same study, Saueressig et al. also reported a KIE value of 1.06 for the reaction of O(1D) with CH3D. The atmospheric lifetime of all the isotopologues of methane is sufficiently long (>10 years) that its removal via the Cl atom reaction in the stratosphere could be important to the fractionation of the isotopologues. Although the atmospheric abundance of Cl atoms is significantly less than that of OH, its reaction rate coefficients and KIE are larger than those for the corresponding OH reactions. Therefore, the impact of Cl

⎛E ⎞ −⎜ D ⎟ ⎝ RT ⎠

where AH and AD are one-fourth the A factor for the reactions of Cl atom with CH4 and CD4, and EH and ED are the activation energies for the abstraction of H and D. Similar analyses has been made for the reactions of OH with isotopologues of methane by Gierczak et al.13 Indeed, a fit of our measured values of k1−k4 to this expression, while allowing AD and ED to vary, reproduced all measured rate coefficients to within 20%. The values of AH and EH were fixed at the literature value of 1.82 × 10−12 cm3 molecule−1 s−1 (A factor divided by 4) and 2534 cal mol−1 K−1. A plot of the measured rate coefficients and the fits are shown in the Supporting Information. Reliable rate coefficient data for the methane isotopologues can improve the accuracy of laboratory kinetic measurements for other compounds measured using the relative rate method. For example, Hickson and Smith40 have reported rate coefficients for the reaction of Cl atoms with the halogenated ethers CF3CH2OCHF2, CF3CHClOCHF2, and CF3CH2OCClF2 as a function of temperature using reaction 4 as the reference and the KIE results from Clyne and Walker37 (see Table 5), k4(298 K) = 7.4 × 10−15 cm3 molecule−1 s−1. Wallington et al.41 used reaction 4 as a reference in their study H



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Table 7. Lifetimes and Breakdown of the Loss Processes for the Deuterated Methane Species Computed with the “NOCAR” Two-Dimensional Model Using Year 2000 Boundary Conditions molecule 12

CH4 CH3D CH2D2 CHD3 CD4 a

lifetime (years)

fractional loss in the stratosphere (%)

9.4 11.8 16.9 29.3 60.5

6.0 6.6 8.1 12.1 21.2

OH lossa (%) 96.4 95.7 94.2 90.2 80.6

(44.1) (38.5) (32.1) (21.4) (10.5)

Cl lossa (%)

O(1D) lossa (%)

1.4 (19.9) 1.3 (16.8) 1.2 (12.3) 1.2 (8.7) 0.6 (2.5)

2.1 (34.9) 2.9 (43.1) 4.4 (53.4) 8.2 (66.8) 17.9 (82.8)

photolysis lossa (%) 0.1 0.1 0.2 0.4 0.9

(1.1) (1.6) (2.2) (3.1) (4.2)

Overall atmospheric loss percentage is given first, and the percentage relative to the total stratospheric loss is in parentheses.

reaction fractionation needs to be accounted for quantitatively in atmospheric model calculations.20,21,43 We explore the effect of the different reaction rate coefficients for the deuterated methane species using the “NOCAR” two-dimensional model.47 This model includes dynamics (including parametrized planetary and gravity waves), detailed photolysis and radiative transfer, and comprehensive chemistry in the stratosphere. The model was designed to be a stratospheric/mesospheric model and, hence, has a simplified representation of the troposphere transport and chemistry. Because of this limitation, the lifetimes of gases dominated by tropospheric loss processes (e.g., reaction with OH) are not computed accurately. Therefore, we scaled the tropospheric OH values to reproduce the known lifetime of methyl chloroform via its reaction with OH of 6.1 years.48 The scaled OH does not affect the chemistry in the rest of the model. Using this value, we computed a total atmospheric lifetime of 5.4 years and a stratospheric lifetime of 43.2 years for methyl chloroform. These values compare well with 5.0 and 39 years, respectively, given in WMO-2010. It is important to note that our modeling study was focused on what happens in the stratosphere. Indeed, our model is not designed to examine tropospheric processes in detail because our model cannot include the highly variable tropospheric emissions (since they are highly zonally asymmetric). A more comprehensive atmospheric model with a comprehensive three-dimensional tropospheric chemistry representation would be useful. The lifetimes of the deuterated methane species are computed with the model. In the absence of data on the UV absorption cross sections for all isotopologues and the rate coefficients for their reactions with O(1D), we assumed them to be the same as those for CH4. Table 7 shows the fractional contribution of the various loss mechanisms for these compounds with the total chlorine abundances set at that in the year 2000, i.e., there is no temporal trend in Cl atom concentration in the stratosphere. Due to their progressively smaller rate coefficients for reaction with OH, the lifetimes of the deuterated methanes are progressively longer as the number of deuterium increases. For example, CD4 has a 60 yr lifetime, which is 6 times longer than that for CH4. Figure 4 shows the vertical distribution of the deuterated methanes compared to that for CH4. Due to the large differences in the tropospheric abundances of these molecules, the profiles are normalized to their surface values. In the stratosphere, there is an increasing amount of deuterated species relative to their surface values. Clearly, while CH4 at the stratopause is roughly 25% of the surface value, roughly 50% CD4 still remains at the stratopause. The falloff in the abundances for other partially deuterated methanes are bounded by the curves for CH4 and CD4, as expected. This vertical distribution also suggests that the mesosphere will be highly selectively enriched in deuterated isotopologues of methane relative to CH4.

Figure 4. Annual mean mixing ration profiles for deuterated methanes calculated using the “NOCAR” 2D model calculation for 25°S−25°N. The mixing ratios are normalized relative to the surface value so that all the profiles can be shown together, and the relative losses of the isotopologues as a function of altitude can be compared. The reaction rate coefficients reported here were used along with those given in Table 6 for reactions of the isotopologues with OH. The rate coefficient for the reactions of all the isotopologues with O(1D) and the photolysis rates of the isotopologues were assumed to be the same as those for CH4.

Figure 5 (lower panel) shows the fractional contributions of Cl atom reaction to the removal of various isotopologues of methane. The importance of reaction with Cl is largest for CH4 and decrease as more deuterium is added (ranging from 20% to only 2% of the total stratospheric loss going from CH4 to CD4, respectively, for 2000 chlorine levels). This decrease (in the stratosphere) is somewhat larger than the decrease in loss from reaction with OH due to the larger change in the rate coefficient for the Cl reaction with methanes with more deuterium. The loss rate due to reaction with Cl peaks just above 40 km and falls off approximately exponentially above the peak. The factional loss increases above the peak due to the even larger decrease of the OH induced loss, but these larger fractions occur in a region with very small total loss rates and so are not significant to the overall loss rates. Figure 5 (upper panel) shows the rates of loss of CH3D due to reactions with OH, Cl, and O(1D), and to photolysis. At most altitudes in the stratosphere, its reaction with O(1D) is the dominant loss process for CH3D, with reactions with OH and Cl being the next most important. The O(1D) reaction plays an even larger role in the removal of deuterated methanes as the number of deuterium atoms in methane increases. For CD4, its reaction with O(1D) contributes approximately 18% toward its overall loss and 84% of the stratospheric loss (compared to only 2% and 35% for CH4, respectively) (see Table 7). For all I



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are given in Table 8. Overall lifetime increases by less than 1% due to the decreased chlorine levels. However, in the stratosphere the breakdown of the loss mechanisms is more significantly altered with OH and O(1D) contributing an even larger amount to the loss. Reaction with Cl accounts for approximately 5% to 0.5% of the stratospheric loss for the 1950 conditions, when the contributions of the anthropogenic chlorinated ozone depleting substances were negligibly small. To a first approximation, the 1950 conditions also represent the chlorine levels in the future, say beyond 2100, when most of the anthropogenic ODSs would be cleansed out of the atmosphere. It is interesting to note that in the absence of the Montreal Protocol, the chlorine levels could have been much higher and, then, the Cl atom reaction would have contributed more to the removal of the methane isotopologues. Even though the fractionation of CH3D relative to CH4 via reaction with Cl is larger (e.g., a factor of 1.46 at 250 K) than that due to its reaction with OH (a factor of 1.025 at 250 K), the atmospheric fractionation via Cl atom reaction is less significant than that due to reaction with OH. This is because the Cl atom reaction contributes very little to the overall removal of atmospheric methanes since it contributes about 1.5% to stratospheric loss, which is only about 6% of the overall loss. However, the reaction of O(1D) contributes little to fractionation although it is significant for the removal of methanes. The rate coefficients for the removal of the deuterated methanes by their reaction with O(1D) are, however, not well-known and needs further study.

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ASSOCIATED CONTENT

S Supporting Information *

(I) Infrared absorption spectra and IR lines of the isotopologues of methane that were used in the relative rate kinetic analysis; (II) graphs of relative loss rate data on CH2D2, CHD3, and CD4 obtained at 295 K; (III) a unified Arrhenius analysis of rate coefficients for Cl atom reactions with the four isotopologues of methane; and (IV) graphs of model calculated annual mean loss rates for CH4, CH3D, CH2D2, CHD3, and CD4, isotopologue calculated using a 2-dimensional transportchemistry model. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. Top panel: Annual mean loss rates of CH3D calculated using the “NOCAR” 2D model for 25°S−25°N. The total loss rate is separated into the individual processes that contribute. The reaction rate coefficients reported here were used along with those given in Table 6 for reactions of the isotopologues with OH. The rate coefficient for the reactions of all the isotopologues with O(1D) and the photolysis rates of the isotopologues were assumed to be at the same as those for CH4. Bottom panel: The vertical variation of the fraction of total loss rate due to the loss via reaction with chlorine for the five isotopologues.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.R.R.). *E-mail: [email protected] (J.B.B.).

of the deuterated methane species, the dominant contributor to the stratospheric loss is O(1D), followed by OH. Indeed, the chlorine atom reactions are the smallest contributors. Such plots for each of the isotopologues is given in the Supporting Information. The lifetimes of the deuterated methane species computed using chlorine levels for the year 1950 (i.e., nearly preindustrial)

Present Addresses §

F. Hoffmann-La Roche, Ltd., LPBS - Patent Department, Building 675/Third floor, 4070 Basel, Switzerland. ∥ Departments of Chemistry and Atmospheric Science, Colorado State University, 1872 Campus Delivery, Fort Collins, CO 80523.

Table 8. Same as Table 7 except for 1950 Boundary Conditions for the Halocarbons molecule 12

CH4 CH3D CH2D2 CHD3 CD4 a

lifetime (years)

fractional loss in the stratosphere (%)

9.4 11.9 17.0 29.5 60.7

5.2 5.9 7.5 11.4 20.8

OH lossa (%) 97.1 96.4 94.9 90.9 81.0

(51.2) (43.7) (35.1) (22.8) (10.7)

Cl lossa (%) 0.6 0.5 0.4 0.5 0.2

(5.4) (4.4) (3.1) (2.1) (0.6)

O(1D) lossa (%)

photolysis lossa (%)

2.2 (41.9) 3.0 (49.9) 4.5 (59.2) 8.3 (71.7) 17.8 (84.4)

0.1 0.1 0.2 0.4 0.9

(1.5) (2.0) (2.6) (3.4) (4.3)

Overall atmospheric loss percentage is given first, and the percentage relative to the total stratospheric loss is in parentheses. J



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank E. Dlugokencky for helpful discussions and Dr. Stefan Bauerle for help in the initial setting up the multipass cell used in these experiments. This work was supported in part by the NOAA’s Health of the Atmosphere and Climate Programs and in part by NASA’s Atmospheric Composition Program. Some of the manuscript preparation was funded by Colorado State University. We thank Mario Molina for his contribution to our science and decades of friendship- “You have always been a scholar and a gentleman, Mario.”

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