7026
J. Phys. Chem. 1996, 100, 7026-7033
Atmospheric Oxidation of CH3Br: Chemistry of the CH2BrO Radical John J. Orlando* and Geoffrey S. Tyndall Atmospheric Chemistry DiVision, National Center for Atmospheric Research, Boulder, Colorado 80303
Timothy J. Wallington Ford Research Laboratory, SRL-3083, Ford Motor Company, Dearborn, Michigan 48121-2053 ReceiVed: June 30, 1995; In Final Form: NoVember 30, 1995X
The Cl-atom-initiated photooxidation mechanism of CH3Br has been investigated in chamber experiments conducted under conditions applicable to the lower atmosphere. Major carbon-containing products obtained in the presence of O2 and NO, as determined by FTIR absorption spectroscopy, are CH2O and CO. In addition, HC(O)Br is observed in the absence of NO. At low temperature, in the presence of NO, CH2BrO2NO2 was also identified as a reaction product. The results are used to determine the fate of the alkoxy radical, CH2BrO. It is found that unimolecular decomposition of CH2BrO to CH2O + Br is the dominant process in 1 atm of O2 at all temperatures studied (between 228 and 298 K), while reaction with O2, CH2BrO + O2 f HC(O)Br + HO2, and the three-centered elimination of HBr, CH2BrO f HCO + HBr, are found to be minor pathways under these conditions (less than 5% each). Assumption of a rate constant of 6 × 10-14 cm3 molecule-1 s-1 for the reaction of CH2BrO with O2 (DeMore, W. B., et al. NASA JPL Publ. 1994, No. 94-26) provides a lower limit to the rate of Br-atom elimination of 3 × 107 s-1 at 228 K, 1 atm of pressure.
Introduction Destruction of ozone in the earth’s lower stratosphere is believed to be partly due to the presence of bromine.1-4 Photochemical destruction of bromine source gases (mainly methyl bromide and the halons) in the lower stratosphere leads to the production of free bromine atoms, which participate in catalytic cycles that result in the net loss of O3. Methyl bromide represents a major source of bromine to the stratosphere; mixing ratios in the range 5-30 pptv have been reported in the troposphere.5-12 Determination of the effect of CH3Br on the chemistry of the atmosphere requires a detailed understanding of the chemistry involved in its photooxidation. Reaction of OH with CH3Br is the dominant loss for CH3Br in the atmosphere13 and results in the production of the CH2BrO radical:
OH + CH3Br f CH2Br + H2O
(1)
CH2Br + O2 + M f CH2BrO2 + M
(2)
CH2BrO2 + NO f CH2BrO + NO2
(3)
Under atmospheric conditions, the CH2BrO radical might be expected to react with O2,
CH2BrO + O2 f HC(O)Br + HO2
(4)
or to undergo unimolecular decomposition Via either of two possible routes:
CH2BrO + M f CH2O + Br + M
(5)
CH2BrO + M f HCO + HBr + M
(6)
Previously published reports14-16 indicate that the predominant fate of the CH2BrO radical under atmospheric conditions X
Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-7026$12.00/0
(298 K, 1 atm of air) is reaction with O2. However, if one considers the semiempirical rules developed by Atkinson and Carter17 to describe the behavior of alkoxy radicals under atmospheric conditions, elimination of Br would be predicted to occur much more rapidly than reaction with O2. In addition, it has been shown18-22 recently that three-center elimination of HCl from both CH2ClO and CH3CHClO is a facile process, and the possibility that the analogous reaction may be an important fate for bromoalkoxy radicals must also be considered. In this paper, the results of chamber experiments are presented that determine the chemistry of the CH2BrO radical under conditions relevant to the lower atmosphere.
Experimental Section Experiments were conducted in the environmental chambers at both Ford and NCAR. The FTIR system at Ford Motor Company was interfaced to a 140 L Pyrex reactor as described previously.23 Radicals were generated by the UV irradiation of mixtures of 3 Torr of CH3Br, 98-115 mTorr of Cl2, and 5-700 Torr of O2 in 700 Torr of total pressure with N2 diluent at 296 K using 22 black lamps. 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. CH3Br, CH3OCH3 (an impurity in the CH3Br samples), CH2O, CO, and HC(O)Br were monitored by using their characteristic features over the wavenumber ranges 800-1600, 1080-1200, 1700-1800, 2050-2250, and 1750-1820 cm-1, respectively. With the exception of HC(O)Br, reference spectra were acquired by expanding known volumes of reference materials into the reactor. The reference spectrum of HC(O)Br was taken from the IR reference library at Ford Motor Company. Calibration of the HC(O)Br spectrum was achieved by allowing reaction mixtures containing HC(O)Br to stand in the chamber in the dark. The rate of HC(O)Br loss progressively decreased throughout the course of the experiments (presumably reflecting © 1996 American Chemical Society
Atmospheric Oxidation of CH3Br conditioning of the chamber) with first-order rate constants in the range 0.02-0.04 min-1. The HC(O)Br loss rates observed in the present study are approximately a factor of 4 slower than those reported previously14,24 and provide an upper limit of 0.02 min-1 to the homogeneous gas phase decomposition of HC(O)Br in 700 Torr of air at 295 K. CO was the only carboncontaining product observed from the decomposition of HC(O)Br. Yarwood et al.24 have shown that HC(O)Br decomposes stoichiometrically to give CO and HBr. The increase in CO concentration provides a means to calibrate the HC(O)Br spectrum. By using this approach, a value of σ(1805 cm-1) ) (1.7 ( 0.4) × 10-18 cm2 molecule-1 was obtained. The quoted uncertainty reflects both statistical uncertainties and potential systematic errors associated with uncertainties in the CO calibration spectrum. This result is consistent with the value of σ(1805 cm-1) ) 1.4 × 10-18 cm2 molecule-1 estimated from the calibrated spectrum of HC(O)Br presented in Figure 1c of the paper by Yarwood et al.24 The 1805 cm-1 IR feature of HC(O)Br selected as a calibration point is the peak of an R branch rotational envelope that is approximately 15 cm-1 broad and unresolved at the resolution used. The IR absorption at 1805 cm-1 then will be insensitive to spectral resolution. Ultrapure oxygen, nitrogen, synthetic air, chlorine, and methyl bromide (>99.5%) were supplied by Matheson Gas Products and used without further purification. The NCAR chamber, which has been described previously,25,26 consists of a 2 m long, 47 L, stainless steel reaction vessel interfaced to a BOMEM DA3.01 Fourier transform spectrometer. Hanst-type optics25 housed inside the reaction vessel provided a total absorption path of 32.6 m. Experiments were conducted at temperatures of 298, 263, and 228 K. Temperature regulation was accomplished by flowing cooled ethanol from a NESLAB ULT-80DD circulating bath around the cell. Experiments were carried out at total pressures of 7001100 Torr, with typical initial concentrations as follows: CH3Br, Linde, ≈2 Torr; O2, UHP, U.S. Welding, 100-1100 Torr; N2, boil-off from a liquid N2 dewar, 0-600 Torr; Cl2, Linde, High Purity, ≈0.2 Torr; and NO, Linde, 0-20 mTorr. Minor components of the mixture were added to the reaction chamber Via expansion from smaller calibrated volumes, while N2 and O2 were added directly to the chamber. Photolysis was conducted by using a filtered Xe arc lamp (320-410 nm) for periods ranging from 10 to 60 s, and products were detected by Fourier transform infrared spectroscopy following each irradiation. Spectra were obtained from the coaddition of 2550 interferograms recorded over the range 800-3900 cm-1 at a spectral resolution of 1 cm-1. Components of the IR spectra (CH2O at 1745 cm-1, NO2 near 1620 cm-1, NO near 1900 cm-1, and CO near 2150 cm-1) were quantified by comparison with standard spectra, also recorded at 1 cm-1 resolution, with the exception of HC(O)Br (near 1800 cm-1), which was quantified by using the absorption cross section given earlier. Decomposition of HC(O)Br was much more rapid in the NCAR stainless steel chamber than in the Pyrex Ford chamber, with observed first-order loss rates of about 0.15 min-1. Simulations of the chemical system were conducted using the ACUCHEM software package.27 Rate constants were obtained from recent evaluations,28,29 when available. The sources of other important rate constant data used in the model are detailed in the text.
Results The goal of these experiments was to determine the relative importance of reactions 4-6 in the atmospheric chemistry of
J. Phys. Chem., Vol. 100, No. 17, 1996 7027
Figure 1. IR spectra taken before and after 45 s UV irradiation of a mixture of 2.93 Torr of CH3Br and 105 mTorr of Cl2 in 700 Torr of O2 diluent (experiment 1, see Table 1). Reference spectra of methyl formate (CH3OCHO) and CH2O are displayed. The result of subtracting features attributable to CH3OCHO and CH2O from the “after UV” spectrum is labeled “residual”. Comparison with the reference spectrum for HC(O)Br clearly shows the formation of this species.
CH2BrO radicals. The Cl-atom-initiated oxidation of CH3Br was used as the source of CH2BrO radicals:
Cl2 + hν f Cl + Cl
(7)
Cl + CH3Br f CH2Br + HCl
(8)
CH2Br + O2 + M f CH2BrO2 + M
(2)
In the absence of NO, the main fate of the peroxy radical would be either self-reaction14,30 or reaction with HO2:
CH2BrO2 + CH2BrO2 f CH2BrO + CH2BrO + O2
(9a)
f CH2BrOH + HC(O)Br + O2 (9b) CH2BrO2 + HO2 f CH2BrOOH + O2 f HC(O)Br + H2O + O2
(10a) (10b)
In the presence of NO, reaction 3 will dominate the peroxy radical chemistry (k3 ) 1.1 × 10-11 cm3 molecule-1 s-1).31 CH2BrO would then react Via any or all of the pathways (4-6), with each of the three reactions leading to the production of a unique carbon-bearing product, HC(O)Br, CH2O, or CO. Quantification of these species can then be used as a marker for their occurrence, provided other sources and losses of the three species are properly accounted for. Experiments Conducted in the Absence of NO. Experiments were conducted in the absence of NO both at Ford and at NCAR. Figure 1 shows IR spectra taken before and after a 45 s irradiation of the reaction mixture used in experiment 1 at Ford (see Table 1). Products observed included methyl formate
7028 J. Phys. Chem., Vol. 100, No. 17, 1996
Orlando et al.
TABLE 1: Experimental Resultsa Obtained at Ford expt
[CH3Br]0 (mTorr)
[Cl2]0 (mTorr)
[O2] (Torr)
1
2930
105
700
2
2946
98
5
3
2946
104
147
4
2895
115
147
5
2930
108
147
6
2980
102
542
7
2980
105
594
t(UV)b (s)
∆[CH3Br]calc (mTorr)
∆[CH2O] (mTorr)
∆[CO] (mTorr)
∆[HC(O)Br] (mTorr)
10 20 30 45 10 20 30 40 10 20 30 40 20 40 60 80 10 20 30 40 10 20 30 40 50 10
0.93 2.20 3.36 5.35 1.30 2.66 3.68 5.20 1.45 2.65 4.10 6.06 1.92 3.98 6.39 9.79 1.47 2.70 3.98 5.29 1.48 2.51 3.87 4.60 6.40 1.02
1.20 2.1 2.7 3.0 1.35 2.25 2.70 3.00 1.43 2.10 2.55 2.85 1.80 2.55 3.00 3.00 2.25 3.60 4.65 6.45 2.34 3.69 4.98 6.05 6.75 1.67
0.19 0.64 1.30 2.51 0.25 0.86 1.59 2.39 0.29 0.89 1.55 2.54 0.48 1.52 3.37 5.28 0.25 0.54 1.08 1.46 0.25 0.61 0.95 1.43 1.88 0.25
0.13 0.28 0.41 0.58 0.11 0.25 0.35 0.48 0.14 0.27 0.39 0.52 0.20 0.44 0.70 0.97
