Atmospheric Chemistry of 1, 1, 1, 2-Tetrachloroethane (CCl3CH2Cl

Trine E. Møgelberg, Merete Bilde, and Jens Sehested* ... The observed rate constant for the self-reaction of CCl3CClHO2 radicals was (5.0 ± 1.2) × ...
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J. Phys. Chem. 1996, 100, 18399-18407

18399

Atmospheric Chemistry of 1,1,1,2-Tetrachloroethane (CCl3CH2Cl): Spectrokinetic Investigation of the CCl3CClHO2 Radical, Its Reactions with NO and NO2, and Atmospheric Fate of the CCl3CClHO Radical Trine E. Møgelberg, Merete Bilde, and Jens Sehested* Section for Chemical ReactiVity, EnVironmental Science and Technology Department, Risø National Laboratory, DK-4000 Roskilde, Denmark

Timothy J. Wallington* Ford Research Laboratory, Mail Drop SRL-3083, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053

Ole J. Nielsen* Ford Forschungszentrum Aachen, Dennewartstrasse 25, D-52068 Aachen, Germany ReceiVed: May 22, 1996; In Final Form: September 25, 1996X

A pulse radiolysis technique was used to study the ultraviolet absorption spectra of CCl3CClH and CCl3CClHO2 radicals, the kinetics of the self-reaction of CCl3CClHO2 radicals, and the kinetics of the reactions of CCl3CClHO2 with NO and NO2 in the gas phase at 296 K. At 240 nm, σ(CCl3CClH) ) (303 ( 35) × 10-20, and at 250 nm, σ(CCl3CClHO2) ) (288 ( 48) × 10-20 cm2 molecule-1. The observed rate constant for the self-reaction of CCl3CClHO2 radicals was (5.0 ( 1.2) × 10-12 cm3 molecule-1 s-1. The rate constants for reactions of CCl3CClHO2 radicals with NO and NO2 were k3 > 9.0 × 10-12 and k4 ) (8.9 ( 2.6) × 10-12 cm3 molecule-1 s-1, respectively. A long path length Fourier transform infrared technique was used to show that at 295 K in 700 Torr total pressure of air 76 ( 3% of CCl3CClHO radicals decompose via C-C bond scission and 24 ( 3% undergo three-center intramolecular HCl elimination. As part of this work rate constants for the reaction of F and Cl atoms with CCl3CH2Cl were determined to be (6.4 ( 1.2) × 10-12 and (5.7 ( 1.0) × 10-14 cm3 molecule-1 s-1, respectively. The results are discussed with respect to the atmospheric chemistry of tetrachloroethane. CCl3CClH + O2 + M f CCl3CClHO2 + M

1. Introduction The adverse impact of chlorine released from chlorofluorocarbons (CFCs) on stratospheric ozone has led to a ban on their production beginning in 1996. At this time a variety of compounds are being considered as CFC replacements. Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are two important classes of CFC replacements. The choice of HFCs and HCFCs is motivated by a number of considerations, not least of which is that, unlike CFCs, the HFCs and HCFCs contain at least one C-H bond which makes them susceptible to attack by OH radicals and, hence, to degradation in the lower atmosphere. The probable future large-scale use of HFCs and HCFCs has generated considerable interest in their environmental acceptability and thus their atmospheric chemistry. As part of a collaborative study of the atmospheric chemistry of HFCs and HCFCs we have undertaken a study of CCl3CH2Cl. While CCl3CH2Cl is not expected to assume any major industrial importance in the near future, it possesses structural similarity to CF3CH2F (HFC-134a), which is the most important CFC substitute. Hence, information concerning CCl3CH2Cl provides insight into the atmospheric chemistry of other more important compounds. Following release into the atmosphere CCl3CH2Cl will react with OH radicals:

CCl3CH2Cl + OH f CCl3CClH + H2O X

(2)

By analogy to other peroxy radicals,1 CCl3CClHO2 radicals will react with NO, NO2, HO2, and other peroxy radicals in the atmosphere:

CCl3CClHO2 + NO f CCl3CClHO + NO2

(3a)

CCl3CClHO2 + NO + M f CCl3CClHONO2 + M (3b) CCl3CClHO2 + NO2 + M f CCl3CClHO2NO2 + M (4) CCl3CClHO2 + HO2 f products

(5)

CCl3CClHO2 + R′O2 f products

(6)

A pulse radiolysis technique combined with time-resolved UVvisible spectroscopy was used to determine the UV absorption spectra of CCl3CClH and CCl3CClHO2 radicals and to study the kinetics of reactions 3, 4, and 6. In the case of reaction 6 we studied the self-reaction of the peroxy radical (R′ ) CCl3CClHO2). The fate of the CCl3CClHO radical produced in reaction 3a was determined using a FTIR spectrometer coupled to a smog chamber. The results are reported herein. 2. Experimental Section

(1)

Abstract published in AdVance ACS Abstracts, November 1, 1996.

S0022-3654(96)01489-X CCC: $12.00

Two different experimental systems were used. Both have been described in detail in previous publications2,3 and will only be discussed briefly here. © 1996 American Chemical Society

18400 J. Phys. Chem., Vol. 100, No. 47, 1996

Møgelberg et al.

2.1. Pulse Radiolysis System. CCl3CClHO2 radicals were generated by radiolysis of SF6/CCl3CH2Cl/O2 gas mixtures in a 1 L stainless steel reactor with a 30 ns pulse of 2 MeV electrons from a Febetron 705B field emission accelerator. SF6 was always in great excess and was used to generate fluorine atoms:

SF6 + 2 MeV e- f F + products

(7)

F + CCl3CH2Cl f CCl3CClH + HF

(8)

CCl3CClH + O2 + M f CCl3CClHO2 + M

(2)

The absolute yield of F atoms was determined by measuring the absorbance at 260 nm attributable to CH3O2 radicals following the radiolysis of SF6/CH4/O2 gas mixtures. As discussed elsewhere,4 experimental conditions were chosen to ensure complete conversion of F atoms into CH3O2 radicals. Using σ260 nm ) 3.18 × 10-18 cm2 molecule-1,5 the F atom yield was determined to be (3.2 ( 0.3) × 1015 cm-3 at full dose and 1000 mbar of SF6. To monitor the transient UV absorbance, the output of a pulsed 150 W Xenon arc lamp was multipassed through the reaction cell (10 cm physical length) using internal White cell optics (80-120 cm optical path length). A McPherson grating spectrometer, Hamamatsu R 955 photomultiplier, and Le Croy 9450A digital oscilloscope were used to detect and record the light intensity at the desired wavelength. The spectral resolution was 0.8 nm. To obtain spectra of CCl3CClH and CCl3CClHO2 radicals, a Princeton Applied Research OMA-II diode array spectrophotometer was used in place of the photomultiplier. The system consisted of the diode array, image amplifier (type 14201024HQ), controller (type 1421), and personal computer for handling and storage of the data. Spectral calibration was obtained using a Hg pen ray lamp. Reagent concentrations used were as follows: SF6, 897995 mbar; O2, 0-5 mbar; NO, 0-0.72 mbar; NO2, 0-0.5 mbar; CCl3CH2Cl, 0-15 mbar. All experiments were performed at 296 K. Ultra-high-purity O2 was supplied by L’Air Liquide. SF6 (99.9%) was supplied by Gerling and Holz. NO (99.8%) was obtained from Messer Griesheim. NO2 (>98%) was provided by Linde Technische Gase. CCl3CH2Cl (99%) was provided by Aldrich Chemical Products. The CCl3CH2Cl sample was degassed using several freeze-pump-thaw cycles. All reagents were used as received. Six sets of experiments were performed using the pulse radiolysis system. First, to determine the rate of the reaction of F atoms with CCl3CH2Cl radicals, the rate of the formation of the transient absorbance at 240 nm was observed following the radiolysis of SF6/CCl3CH2Cl mixtures. The CCl3CH2Cl concentration was varied, and a rate constant for reaction 8 was derived from first-order fits to the data. Second, an absorption spectrum for CCl3CClH radicals was found by observing the transient absorbance as a function of wavelength using a diode array camera following radiolysis of SF6/CCl3CH2Cl mixtures. Third, the absorption spectrum for CCl3CClHO2 was measured following radiolysis of SF6/CCl3CH2Cl/O2 mixtures. Fourth, the rate constant for reaction 6 was determined by observing the rate of the decay of absorption attributable to CCl3CClHO2 radicals over long time scales (0-1000 µs) following radiolysis of SF6/CCl3CH2Cl/O2 mixtures. Fifth, by monitoring NO2 formation at 400 nm following radiolysis of SF6/CCl3CH2Cl/ O2/NO mixtures, the rate constant for the reaction of CCl3CClHO2 radicals with NO was determined. Finally, the rate constant for the reaction of CCl3CClHO2 radicals with NO2 was measured by monitoring the rate of NO2 decay at 400 nm following radiolysis of SF6/CCl3CH2Cl/O2/NO2 mixtures.

Figure 1. Pseudo-first-order rate constants for the appearance of absorption as a function of CCl3CH2Cl concentration. The insert shows a typical transient absorbance at 240 nm observed following the pulsed radiolysis of a mixture of 5.2 mbar of CCl3CH2Cl and 990 mbar of SF6 (radiolysis dose ) 32%, optical path length ) 120 cm).

2.2. FTIR-Smog Chamber System. The FTIR system was interfaced to a 140 L Pyrex reactor. Radicals were generated by UV irradiation of mixtures of 30-650 mTorr of CCl3CH2Cl, 40-200 mTorr of Cl2, 0-15 mTorr of NO, and 10-700 Torr of O2 in 700 Torr total pressure of N2 diluent at 296 K using 22 blacklamps (760 Torr ) 1013 mbar). The loss of reactants and the formation of products were monitored by FTIR spectroscopy, using an analyzing path length of 27 m and a resolution of 0.25 cm-1. Infrared spectra were derived from 32 coadded spectra. CCl3CH2Cl, CCl3C(O)Cl, HC(O)Cl, CO, and CO2 were monitored using their characteristic features over the wavenumber range 800-2000 cm-1. Reference spectra were acquired by expanding known volumes of reference materials into the reactor. 3. Results and Discussions 3.1. The Reaction of F Atoms with CCl3CH2Cl. We ascribe the increase in absorption at 240 nm following the radiolysis of SF6/CCl3CH2Cl mixtures to formation of CCl3CClH radicals via reaction 8, see insert in Figure 1. The increase in absorption was complete within 5 µs and was followed by a slower decay. We ascribe the decay to loss via reactions such as

CCl3CClH + CCl3CClH f products

(9)

The radiolysis dose employed in the experiments was 32% of full dose, and the SF6 concentration was 990 mbar. For the experimental conditions used to determine k8 the initial F atom concentration was 1 × 1015 cm-3. With 1-13 mbar of CCl3CH2Cl, the formation of CCl3CClH radicals via reaction 8 followed pseudo-first-order kinetics. A first-order rise expression was fit to the rise of the transient absorption, and pseudofirst-order rate constants were derived. In Figure 1 the pseudofirst-order rate constants are plotted as a function of the CCl3CH2Cl concentration. A linear least-squares analysis gives k8 ) (6.4 ( 1.2) × 10-12 cm3 molecule-1 s-1, where the quoted error is two standard deviations. There are no literature data to compare with our result. 3.2. Absorption Spectrum of CCl3CClH Radicals. To determine the absorption spectrum of CCl3CClH radicals a series

Atmospheric Chemistry of CCl3CH2Cl

J. Phys. Chem., Vol. 100, No. 47, 1996 18401

Figure 2. Maximum transient absorbance versus radiolysis dose following radiolysis of two different reaction mixtures. Triangles are data obtained using 10 mbar of CCl3CH2Cl and 990 mbar of SF6 (monitoring wavelength ) 240 nm). Circles are data obtained using mixtures of 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6 (monitoring wavelength ) 250 nm). In both cases the UV path length was 120 cm. The lines are linear regressions of the low-dose data (filled symbols).

of experiments was performed using mixtures of 10 mbar of CCl3CH2Cl and 990 mbar of SF6. The radiolysis dose was varied by 1 order of magnitude, and the maximum transient absorbance was monitored at 240 nm. Figure 2 shows the measured maximum absorbance as a function of radiolysis dose (triangles). For low doses, the measured maximum absorbance is proportional to the dose. At full dose the observed absorbance is lower than expected from a linear extrapolation of the lowdose data. We ascribe this curvature to unwanted radicalradical reactions such as reaction 9. A linear least-squares regression through the low-dose data gives a slope of 0.500 ( 0.028. The optical path length used was 120 cm. The F atom yield at full dose and 1000 mbar SF6 was (3.2 ( 0.3) × 1015 molecules cm-3. Using this information the absorption cross section at 240 nm can be derived:

Figure 3. Absorption spectra of CCl3CClH (A) and CCl3CClHO2 (B) radicals. For comparison the spectrum of CH2ClCClH6 is shown by the filled circles in panel A. The line through the CCl3CClHO2 data is a third-order fit to aid visual inspection. Literature cross section data for CF3CFHO28 (filled circles) and CClH2CClHO26 (hollow circles) are given for comparison.

σ(240 nm) ) [0.500 ln(10)]/[(120)(3.2 × 1015)(0.99)] ) (303 ( 35) × 10-20 cm2 molecule-1 The quoted error includes uncertainty in both the slope of the data in Figure 2 and the F atom yield. The UV absorption spectrum of CCl3CClH radicals between 225 and 310 nm is shown in Figure 3A. The spectrum was obtained by recording the absorbance following pulse radiolysis (half dose) of a mixture of 10 mbar of CCl3CH2Cl and 990 mbar of SF6 using a diode array camera (spectral resolution, 1.6 nm; delay, 3 µs; gate, 8 µs). Absorption cross sections were normalized to σ(240 nm) ) 3.03 × 10-18 cm2 molecule-1. Cross sections for selected wavelengths are listed in Table 1. In Figure 3A the spectrum of the CCl3CClH radical is compared to that of the CH2ClCClH radical.6 As expected, the UV spectra are similar. 3.3. Absorption Spectrum of CCl3CClHO2 Radicals. Following the radiolysis of mixtures of 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6, a rapid increase in absorbance at 250 nm was observed followed by a slower decay. A typical absorption transient is shown in Figure 4. Using k8 ) 6.4 × 10-12 cm3 molecule-1 s-1 (see section 3.1) it follows that in the presence of 10 mbar of CCl3CH2Cl the lifetime of F atoms with respect to conversion into CCl3CClH radicals is 0.6 µs. Assuming that CCl3CClH radicals behave like other alkyl

Figure 4. Transient absorbance at 250 nm following the pulsed radiolysis (full dose) of 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6. The UV path length was 120 cm.

radicals and add O2 with a rate constant on the order of 10-12 cm3 molecule-1 s-1, the lifetime of CCl3CClH radicals with respect to conversion to CCl3CClHO2 radicals is on the order of 2 µs. Thus, it seems reasonable to ascribe the absorption shown in Figure 4 to the formation and subsequent decay of CCl3CClHO2 radicals.

18402 J. Phys. Chem., Vol. 100, No. 47, 1996

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TABLE 1: Measured UV Absorption Cross Sections wavelength (nm)

σ(CCl3CClH) × 1020 (cm2 molecule-1)

σ(CCl3CClHO2) × 1020 (cm2 molecule-1)

220 225 230 235 240 245 250 255 260 270 280

386 351 305 303 223 217 193 172 118 55

70 156 219 263 286 293 288 267 238 162 74

Unwanted radical-radical reactions that may play a role in the ≈5 µs taken for the absorbance to reach a maximum include

CCl3CClHO2 + CCl3CClHO2 f products

(6)

F + CCl3CClHO2 f products

(10)

Figure 5. Plot of 1/t1/2 as a function of Amax at 250 nm.

CCl3CClHO2 + CCl3CClH f products

(11)

absorption spectrum of the CCl3CClHO2 radical reported here is consistent with such general trends. 3.4. The Self-Reaction of the CCl3CClHO2 Radical. As shown in Figure 4, the rapid increase in absorption at 250 nm observed on radiolysis of SF6/CCl3CH2Cl/O2 mixtures was followed by a slower decay. To investigate the rate of reaction 6 a series of experiments was performed in which mixtures of

The rates of these unwanted radical-radical reactions increase as the square of the radical concentration. To check for such reactions experiments were performed with the radiolysis dose varied over 1 order of magnitude. The result is shown as circles in Figure 2. For radiolysis doses less than 42% of the maximum, the absorbance increased linearly with radiolysis dose. A linear least-squares fit through the low-dose data gives a slope of 0.441 ( 0.016. The absorbance at high doses falls below that expected from the low-dose data. This behavior is ascribed to reactions 6, 10, and 11. In all subsequent experiments a radiolysis dose of 42% was used unless otherwise stated. Before we can calculate an absorption cross section at 250 nm, we have to recognize that reactions 8 and 12 compete for the available F atoms:

F + CCl3CH2Cl f CCl3CClH + HF

(8)

F + O2 + M f FO2 + M

(12)

Using k8 ) 6.4 × 10-12 (see section 3.1), k12 ) 1.9 × 10-13 cm3 molecule-1 s-1,7 and partial pressures of 10 mbar of CCl3CH2Cl and 20 mbar of O2, we calculate that 5.6% of the F atoms react with O2 instead of CCl3CH2Cl. By combining this information with (i) σFO2(250 nm) ) 130 × 10-20 cm2 molecule-1,7 (ii) the optical path length of 120 cm, and (iii) the F atom yield at full dose and 1000 mbar of SF6, 3.2 × 1015 cm-3, we can correct the observed absorbance at 250 nm for that attributable to FO2 radicals and calculate an absorption cross section for CCl3CClHO2 at 250 nm. The result is σ(250 nm) ) (288 ( 48) × 10-20 cm2 molecule-1. The absorption spectrum from 230 to 290 nm was measured using a diode array camera with a spectral resolution of 1.6 nm, a gate of 8 µs, and a delay of 8 µs. Absorbances were corrected for absorption by FO2 and placed on an absolute basis using the absorption cross section at 250 nm determined above. The result is shown in Figure 3B. The smooth line through the data is a polynomial fit through the data to aid visual inspection. Absorption cross sections are listed in Table 1. For comparison the UV spectra of CF3CFHO28 and CClH2CClHO26 are also given in Figure 3B. The UV spectra of peroxy radicals are structureless with one or more broad Gaussian-shaped absorption bands.5,9 Fluorine substituents cause a blue-shift of the spectrum, while chlorine substituents tend to broaden the spectrum and decrease the magnitude of the maximum. The

CCl3CClHO2 + CCl3CClHO2 f products

(6)

10 mbar CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6 were subject to pulse radiolysis and the resulting transient absorption at 250 nm was monitored over long time scales (0-1000 µs). The initial CCl3CClHO2 radical concentration was varied by varying the radiolysis dose. The decays of the transient absorptions were fitted using a second-order expression to obtain the half-life of the decay. The half-life, t1/2, is related to the absorbance by

1/t1/2 ) [(2.303)Amax2k6obs]/[(120)σ(CCl3CClHO2)] where Amax is the observed maximum absorbance and 120 cm is the optical path length. Figure 5 shows 1/t1/2 plotted versus the maximum absorbance, from which we derive k6obs ) (5.0 ( 1.2) × 10-12 cm3 molecule-1 s-1. The quoted error includes two standard deviations from the linear least-squares analysis of the data in Figure 5, the uncertainty on σ(250 nm), and an additional 10% experimental uncertainty. As shown in section 3.8, reaction 6 produces a substantial yield of CCl3CClHO radicals which decompose via C-C bond scission to give CCl3 and, hence, CCl3O2 radicals. CCl3O2 radicals absorb appreciably at 250 nm5 and probably react with CCl3CClHO2 at a non-negligible rate. In the absence of information concerning this cross reaction we are unable to correct the measured value of k6obs for complications caused by the formation of CCl3O2 radicals. Our reported value of k6obs is the observed rate constant which describes the decay of absorption at 250 nm in the present chemical system. The observed rate constant for reaction 6 can be compared to the rate constants for other peroxy radicals: kCH3CH2O2 + CH3CH2O2 ) 6 × 10-14 cm3 molecule-1 s-1,9 and kCF3CFHO2 + CF3CFHO2 ) 6 × 10-12 cm3 molecule-1 s-1.10 The observed rate constant increases with halogenation, and the rate constant observed here is very similar to that for CF3CFHO2 radicals. 3.5. Kinetic Data for the Reaction CCl3CClHO2 + NO f Products. To study reaction 3 experiments were conducted

Atmospheric Chemistry of CCl3CH2Cl

J. Phys. Chem., Vol. 100, No. 47, 1996 18403 (moles of NO2 formed per mole of CCl3CClHO2 radicals consumed) of 140 ( 23%. Corrections were made for loss of F atoms by reaction with O2 and NO. The fact that the NO2 yield is greater than 100% can be explained by the formation of NO2 via reactions of NO with decomposition products of the alkoxy radical CCl3CClHO (see section 3.8). In all experiments the rise in the transient absorption at 400 nm followed first-order kinetics. It follows that the rate of decomposition of CCl3CClHO radicals must be faster than the fastest pseudo-first-order rate measured, i.e., >1.8 × 105 s-1. The formation of NO2 from reactions which occur after reaction 3 introduces a delay in the overall appearance of NO2. The value of k3 ) (1.04 ( 0.14) × 10-11 determined in the present work should be regarded as a lower limit; hence, k3 > 9.0 × 10-12 cm3 molecule-1 s-1. This value is consistent with rate constants for other peroxy radicals derived from alkanes5 and halogenated ethanes11 with respect to reaction with NO: kCF3CF2O2 > (1.07 ( 0.15) × 10-11,11 kCF3CF2CFHO2 > 8 × 10-12,12 kCF3CFHO2 ) (1.31 ( 0.41) × 10-11,13 and kCF3CH2O2 ) (1.2 ( 0.3) × 10-11 cm3 molecule-1 s-1.14 3.6. Kinetic Data for the Reaction CCl3CClHO2 + NO2 + M f CCl3CClHO2NO2 + M. The reaction between CCl3CClHO2 radicals and NO2 was investigated by monitoring the decay of absorbance at 400 nm following the radiolysis of mixtures of 0.22-0.73 mbar of NO2, 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6. The insert in Figure 6B shows the decay in absorption observed in an experiment using 0.22 mbar of NO2. NO2 absorbs at 400 nm, and we ascribe the decay of absorption following radiolysis to loss of NO2 via reaction 4:

CCl3CClHO2 + NO2 + M f CCl3CClHO2NO2 + M (4)

Figure 6. (A) Plot of k1st versus [NO]. The insert shows the transient absorbance at 400 nm observed following pulsed radiolysis of a mixture of 0.40 mbar of NO, 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6. The UV path length was 120 cm. The solid line is a first-order rise fit which gives k1st ) 9 × 104 s-1. (B) Plot of k1st versus [NO2]. The insert is the transient absorbance at 400 nm observed following pulsed radiolysis of a mixture of 0.22 mbar of NO2, 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6. The UV path length was 120 cm. The solid line is a first-order decay fit which gives k1st ) 6 × 104 s-1.

using pulse radiolysis (42% of full dose) of mixtures of 0.220.71 mbar of NO, 10 mbar of CCl3CH2Cl, 20 mbar of O2, and 970 mbar of SF6.

CCl3CClHO2 + NO f CCl3CClHO + NO2

(3)

The rate of NO2 formation was monitored at 400 nm using a UV path length of 120 cm. The increase in absorption caused by NO2 formation was fitted using the expression A(t) ) (Ainf - A0)[1 - exp(-k1stt)] + A0 with the fit starting 2-3 µs after the electron pulse to allow for the time taken for formation of the peroxy radical. The insert in Figure 6A shows a typical transient absorption, and the smooth line is the first-order fit with k1st ) 9.0 × 104 s-1. Figure 6A shows a plot of k1st versus [NO] with a linear least-squares fit which gives k3 ) (1.04 ( 0.14) × 10-11 cm3 molecule-1 s-1. The increase in absorption can be combined with σ(NO2) at 400 nm ) 6.0 × 10-19 cm2 molecule-1 to derive the NO2 yield

The experimental traces were fitted using a first-order decay expression to derive pseudo-first-order rate constants. In Figure 6B the pseudo-first-order rate constants are plotted as a function of NO2 concentration. The slope in Figure 6B gives k4 ) (8.9 ( 2.6) × 10-12 cm3 molecule-1 s-1. This result is consistent with the available database for reactions of peroxy radicals with NO2 which have rate constants which lie in the range (5-10) × 10-12 cm3 molecule-1 s-1.5 3.7. Kinetics of the Reaction of Cl Atoms with CCl3CH2Cl. Prior to investigating the atmospheric fate of CCl3CClHO radicals, relative rate experiments were performed to study the kinetics of reaction 13. The techniques used have been described

Cl + CCl3CH2Cl f CCl3CClH + HCl

(13)

elsewhere.15 Photolysis of molecular chlorine was used as a source of halogen atoms. The kinetics of reaction 13 were studied relative to reactions 14 and 15. The observed loss of

Cl + CH3Cl f CH2Cl + HCl

(14)

Cl + CH3D f products

(15)

CCl3CH2Cl versus CH3Cl and CH3D following the UV irradiation of CCl3CH2Cl/CH3Cl/Cl2 and CCl3CH2Cl/CH3D/Cl2 mixtures in 700 Torr of air is shown in Figure 7. Linear leastsquares analysis of the data in Figure 7 gives k13/k14 ) 0.12 ( 0.01 and k13/k15 ) 0.75 ( 0.07. Using k14 ) 4.9 × 10-13 16 and k15 ) 7.35 × 10-14 15 gives k13 ) (5.9 ( 0.5) × 10-14 and (5.5 ( 0.5) × 10-14 cm3 molecule-1 s-1. We estimate that potential systematic errors associated with uncertainties in the reference rate constants add an additional 10% to the uncertainty range. Propagating this additional uncertainty gives k13 ) (5.9 ( 0.8) × 10-14 and (5.5 ( 0.8) × 10-14 cm3 molecule-1 s-1. Consistent results were obtained using the two different refer-

18404 J. Phys. Chem., Vol. 100, No. 47, 1996

Møgelberg et al.

CCl3CClHO + M f CCl3C(O)H + Cl + M

(20)

CCl3CClHO + M f CCl3 + HC(O)Cl + M

(21)

Subsequent reactions of the radical species formed in reactions 18 and 21 will include

CCl3CO + O2 + M f CCl3C(O)O2 + M

(22)

CCl3CO + M f CCl3 + CO + M

(23)

CCl3C(O)O2 + RO2 f CCl3C(O)O + RO + O2 (24)

Figure 7. Loss of CCl3CH2Cl versus CH3Cl (2) and CH3D (b) when mixtures containing these compounds were exposed to Cl atoms in 700 Torr of air.

ence compounds. We choose to quote a final value which is an average of the two individual determinations with error limits which encompass the extremes of both determinations. Hence, k13 ) (5.7 ( 1.0) × 10-14 cm3 molecule-1 s-1. The kinetics of reaction 13 have been studied previously by Cillien et al.17 at 323-423 K. Using a relative rate technique Cillien et al. measured k13/k16 ) 0.2 exp(450/T).

Cl + CHCl3 f CCl3 + HCl

(16)

Taking k16 ) (4.9 × 10-12) exp(-1240/T)16 gives k13 ) (9.8 × 10-13) exp(-790/T), i.e., k13 ) 6.7 × 10-14 cm3 molecule-1 s-1 at 295 K. This result is consistent with the value measured herein. 3.8. Atmospheric Fate of CCl3CClHO Radicals. To determine the atmospheric fate of the alkoxy radical CCl3CClHO, experiments were performed in which Cl2/CCl3CH2Cl/O2/N2 mixtures were irradiated in the FTIR-smog chamber system with, and without, added NO. All experiments were performed at 295 ( 2 K in 700 Torr total pressure of N2 diluent. Loss of CCl3CH2Cl and formation of products were monitored by FTIR spectroscopy. By analogy to the behavior of other peroxy radicals5,9 it is expected that CCl3CClHO radicals will be formed in the chamber by the self-reaction of CCl3CClHO2 radicals.

CCl3CClHO2 + CCl3CClHO2 f CCl3CClHO + CCl3CClHO + O2 (6a) CCl3CClHO2 + CCl3CClHO2 f CCl3C(O)Cl + CCl3CClHOH + O2 (6b) The alkoxy radical will then either react with O2, decompose via intramolecular three-center HCl elimination, undergo H or Cl atom elimination, or undergo C-C bond scission:

CCl3CClHO + O2 f CCl3C(O)Cl + HO2

(17)

CCl3CClHO + M f CCl3CO + HCl + M

(18)

CCl3CClHO + M f CCl3C(O)Cl + H + M

(19)

CCl3C(O)O + M f CCl3 + CO2 + M

(25)

CCl3 + O2 + M f CCl3O2 + M

(26)

CCl3O2 + RO2 f CCl3O + RO + O2

(27)

CCl3O + M f COCl2 + Cl + M

(28)

The goal of the FTIR experiments was to determine the relative importance of reactions 17-21 in the atmospheric chemistry of CCl3CClHO radicals. In the Cl atom initiated oxidation of CCl3CH2Cl, five carbon containing products were readily identified by virtue of their characteristic IR features: COCl2, CO, CO2, HC(O)Cl, and CCl3C(O)Cl. At this point it is germane to consider possible loss of these compounds in the chamber. To check for possible heterogeneous loss, reaction mixtures containing these compounds were allowed to stand in the dark for 20 min; there was no observable loss. None of the products photolyze in our experimental system. There is no reaction of Cl atoms with COCl2, CO2, or CCl3C(O)Cl. Cl atoms do react with CO and HC(O)Cl. Reaction of Cl with CO proceeds with a rate constant which is 0.56 (3.2 × 10-14 16/ 5.7 × 10-14) times that of CCl3CH2Cl at 700 Torr and is of minor importance for small consumptions of CCl3CH2Cl. Cl atoms react 13.7 (7.3 × 10-13 18,19/5.7 × 10-14) times faster with HC(O)Cl than with CCl3CH2Cl and can be an important loss of HC(O)Cl (see below). Of all the products seen, only HC(O)Cl provides a unique identification of one reaction pathway, namely reaction 21. For example, COCl2 is formed via CCl3 radicals which are produced in reaction 21, or reaction 18 followed by 23. Figure 8 shows plots of the observed formation of COCl2, HC(O)Cl, CCl3C(O)Cl, CO, and CO2 versus the loss of CCl3CH2Cl following irradiation of mixtures of 45 mTorr of CCl3CH2Cl, 40 mTorr of Cl2, and either 10 or 700 Torr of O2 at a total pressure of 700 Torr of N2 diluent. Within the experimental uncertainty (approximately 10%) the combined formation of the five products observed accounted for 100% of the observed CCl3CH2Cl loss. As seen from Figure 8, the COCl2 and CCl3C(O)Cl products increased linearly with consumption of CCl3CH2Cl. In contrast the CO2 increased nonlinearly with CCl3CH2Cl loss, showing that it is formed in large degree as a secondary, rather than primary, product. In the low O2 experiments the CO yield plot was distinctly curved, while for the experiment conducted in 700 Torr of O2 the CO yield was, within the experimental uncertainties, a linear function of the CCl3CH2Cl loss (the molar CO yield was 15 ( 2% in 700 Torr of O2). Linear least-squares analysis gives molar yields of COCl2 and CCl3C(O)Cl of 98 ( 4% and 96 ( 4% and 7.5 ( 0.8% and 9.1 ( 0.8% for experiments with 10 and 700 Torr partial pressure of O2, respectively. Variation of the O2 partial pressure by a factor of 70 had little, or no, effect on the measured CCl3C(O)Cl yield, suggesting that reaction 17 does not compete effectively for the available CCl3CClHO radicals. The observation of HC-

Atmospheric Chemistry of CCl3CH2Cl

J. Phys. Chem., Vol. 100, No. 47, 1996 18405

Figure 9. Formation of COCl2 (2), CCl3C(O)Cl (b), HC(O)Cl ([ ) observed, ] ) after correction for Cl atom attack), CO (0), and CO2 (4) versus the loss of CCl3CH2Cl observed following the UV irradiation of mixtures of 7 mTorr of CH3F, 170 mTorr of CCl3CH2Cl, and 100 mTorr of Cl2 in 700 Torr of air. The loss of CCl3CH2Cl was calculated from the observed loss of CH3F tracer, see text.

Figure 8. Formation of COCl2 (2), CCl3C(O)Cl (b), HC(O)Cl ([), CO (0), and CO2 (4) versus the loss of CCl3CH2Cl observed following the UV irradiation of mixtures of 45 mTorr of CCl3CH2Cl, 40 mTorr of Cl2, and either 10 (A) or 700 Torr (B) of O2 in 700 Torr total pressure with N2 diluent.

(O)Cl shows that reaction 21 is an important loss of CCl3CClHO radicals. The curved yield plot for HC(O)Cl reflects its loss via reaction with Cl atoms. To operate under conditions where loss of HC(O)Cl via Cl atom attack is less important it is necessary to employ small consumptions of CCl3CH2Cl (