Atmospheric Oxidation of CH3Br: Chemistry of the CH2BrO Radical

Article abstract of DOI:10.1021/jp951813q

The Cl-atom-initiated photooxidation mechanism of CH3Br has been investigated in chamber experiments conducted under conditions applicable to the lower atmosphere.Major carbon-containig products obtained in the presence of O2 and NO, as determined by FITR 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 -> HC(O)Br + HO2, and the three-centered elimination of HBr, CH2BrO -> HCO + HBr, are found to be minor pathways under these conditions (less than 5percent each).Assumption of a rate constant of 6E-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 3E7 s^-1 at 228 K, 1 atm of pressure.

Full text of DOI:10.1021/jp951813q

7
026
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
X
ReceiVed: June 30, 1995; In Final Form: NoVember 30, 1995
The Cl-atom-initiated photooxidation mechanism of CH
conducted under conditions applicable to the lower atmosphere. Major carbon-containing products obtained
in the presence of O and NO, as determined by FTIR absorption spectroscopy, are CH O and CO. In addition,
HC(O)Br is observed in the absence of NO. At low temperature, in the presence of NO, CH BrO NO was
also identified as a reaction product. The results are used to determine the fate of the alkoxy radical, CH
BrO. It is found that unimolecular decomposition of CH BrO to CH O + Br is the dominant process in 1
atm of O at all temperatures studied (between 228 and 298 K), while reaction with O , CH BrO + O
HC(O)Br + HO , and the three-centered elimination of HBr, CH BrO f HCO + HBr, are found to be minor
pathways under these conditions (less than 5% each). Assumption of a rate constant of 6 × 10
3
Br has been investigated in chamber experiments
2
2
2
2
2
2
-
2
2
2
2
2
2
f
2
2
-
14
3
cm
2 2
for the reaction of CH BrO with O (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 × 10 s at 228 K, 1 atm of pressure.
-
1 -1
molecule
s
7
-1
Introduction
Destruction of ozone in the earth’s lower stratosphere is
(298 K, 1 atm of air) is reaction with O2. However, if one
considers the semiempirical rules developed by Atkinson and
1
7
Carter 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,
1
-4
believed to be partly due to the presence of bromine.
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
1
8-22
it has been shown
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.
5
-12
troposphere.
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
1
3
atmosphere and results in the production of the CH2BrO
radical:
Experimental Section
OH + CH Br f CH Br + H O
(1)
(2)
(3)
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
3
2
2
CH Br + O + M f CH BrO + M
2
2
2
2
2
3
previously. Radicals were generated by the UV irradiation
of mixtures of 3 Torr of CH3Br, 98-115 mTorr of Cl2, and
CH BrO + NO f CH BrO + NO
2
2
2
2
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
Under atmospheric conditions, the CH2BrO radical might be
expected to react with O2,
-1
cm . 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,
CH BrO + O f HC(O)Br + HO
2
(4)
2
2
or to undergo unimolecular decomposition Via either of two
possible routes:
-1
1
700-1800, 2050-2250, and 1750-1820 cm , respectively.
CH BrO + M f CH O + Br + M
(5)
(6)
With the exception of HC(O)Br, reference spectra were
2
2
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. Calibra-
tion 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
CH BrO + M f HCO + HBr + M
2
Previously published reports14-16 indicate that the predomi-
nant 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 © 1996 American Chemical Society

Atmospheric Oxidation of CH3Br
J. Phys. Chem., Vol. 100, No. 17, 1996 7027
conditioning of the chamber) with first-order rate constants in
-
1
the range 0.02-0.04 min . The HC(O)Br loss rates observed
in the present study are approximately a factor of 4 slower than
1
4,24
those reported previously
min
and provide an upper limit of 0.02
-
1
to the homogeneous gas phase decomposition of
HC(O)Br in 700 Torr of air at 295 K. CO was the only carbon-
containing product observed from the decomposition of
HC(O)Br. Yarwood et al.24 have shown that HC(O)Br decom-
poses stoichiometrically to give CO and HBr. The increase in
CO concentration provides a means to calibrate the HC(O)Br
-1
spectrum. By using this approach, a value of σ(1805 cm ) )
-
18
2
-1
(
1.7 ( 0.4) × 10 cm molecule 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 ) ) 1.4 × 10 cm molecule estimated from
the calibrated spectrum of HC(O)Br presented in Figure 1c of
the paper by Yarwood et al.24 The 1805 cm 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- broad
-1
-18
2
-1
-1
1
and unresolved at the resolution used. The IR absorption at
805 cm- then will be insensitive to spectral resolution.
1
1
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 previ-
2
5,26
ously,
consists of a 2 m long, 47 L, stainless steel reaction
vessel interfaced to a BOMEM DA3.01 Fourier transform
Figure 1. IR spectra taken before and after 45 s UV irradiation of a
25
spectrometer. Hanst-type optics 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 700-
mixture of 2.93 Torr of CH
diluent (experiment 1, see Table 1). Reference spectra of methyl
formate (CH OCHO) and CH O are displayed. The result of subtracting
features attributable to CH OCHO and CH O from the “after UV”
3 2
Br and 105 mTorr of Cl in 700 Torr of
O
2
3
2
3
2
spectrum is labeled “residual”. Comparison with the reference spectrum
for HC(O)Br clearly shows the formation of this species.
1
100 Torr, with typical initial concentrations as follows: CH3-
CH2BrO radicals. The Cl-atom-initiated oxidation of CH3Br
was used as the source of CH2BrO radicals:
Br, 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 25-
Cl + hν f Cl + Cl
(7)
(8)
(2)
2
Cl + CH Br f CH Br + HCl
3
2
CH Br + O + M f CH BrO + M
2
2
2
2
In the absence of NO, the main fate of the peroxy radical would
1
4,30
be either self-reaction
or reaction with HO2:
-
1
5
0 interferograms recorded over the range 800-3900 cm at
CH BrO + CH BrO f CH BrO + CH BrO + O
(9a)
-
1
2
2
2
2
2
2
2
a spectral resolution of 1 cm . Components of the IR spectra
CH2O at 1745 cm , NO2 near 1620 cm , NO near 1900 cm ,
and CO near 2150 cm ) were quantified by comparison with
standard spectra, also recorded at 1 cm resolution, with the
exception of HC(O)Br (near 1800 cm ), which was quantified
by using the absorption cross section given earlier. Decomposi-
tion of HC(O)Br was much more rapid in the NCAR stainless
steel chamber than in the Pyrex Ford chamber, with observed
-1
-1
-1
(
f CH BrOH + HC(O)Br + O (9b)
-
1
2
2
-
1
-
1
CH
2
BrO
2
+ HO
2
f CH
2
BrOOH + O
2
(10a)
(10b)
2
f HC(O)Br + H O + O2
-
1
In the presence of NO, reaction 3 will dominate the peroxy
first-order loss rates of about 0.15 min .
-11
3
-1 -1 31
radical chemistry (k3 ) 1.1 × 10 cm molecule s ). CH2-
BrO 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.
Simulations of the chemical system were conducted using
2
7
the ACUCHEM software package.
Rate constants were
when available. The
obtained from recent evaluations,2
8,29
sources of other important rate constant data used in the model
are detailed in the text.
Experiments Conducted in the Absence of NO. Experi-
ments 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
Results
The goal of these experiments was to determine the relative
importance of reactions 4-6 in the atmospheric chemistry of

7
028 J. Phys. Chem., Vol. 100, No. 17, 1996
Orlando et al.
a
TABLE 1: Experimental Results Obtained at Ford
[
CH
3
Br]
0
[Cl
2
]
0
[O
2
]
∆[CH
3
Br]calc
(mTorr)
∆[CH
2
O]
(mTorr)
∆[CO]
(mTorr)
∆[HC(O)Br]
(mTorr)
b
expt
1
(mTorr)
(mTorr) (Torr) t(UV) (s)
comments
2930
105
98
700
5
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
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
<0.13
<0.13
<0.13
<0.13
<0.13
<0.13
<0.13
<0.26
<0.26
<0.13
2
3
4
0
0
5
3.0
2
3
4
5
6
2946
2946
2895
2930
2980
10
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
2
3
4
0
0
0
104
115
108
102
147
147
147
542
10
2
3
4
0
0
0
20
CH
[CH
3
OCH
3
added to initial mix
] added ) 7.0 mTorr
40
60
80
3
OCH
3
10
NO added to initial mix
[NO] ) 18.5 mTorr
20
30
40
0
10
NO added to initial mix
[NO] ) 25.5 mTorr
2
3
4
5
0
0
0
0
0
7
2980
105
594
10
NO added to initial mix
NO] ) 4.7 mTorr
[
0
a
Observed concentrations with no corrections of any kind applied to the data. All experiments conducted at 295 K at 700 Torr of total pressure
diluent). Nominal irradiation times; analysis performed immediately after irradiation.
b
(N
2
(
CH3OCHO, formed from the oxidation of dimethyl ether,32
Experiments were also conducted to demonstrate the lack of
any complications due to the presence of dimethyl ether
impurity. One experiment (experiment 4, Table 1), performed
in the presence of added dimethyl ether, demonstrated that the
yields of CH2O, CO, and HC(O)Br were unaffected by its
presence. In addition, a separate experiment was performed in
which only Cl2, dimethyl ether, and air were irradiated. Methyl
formate and CH3OCH2OOH were the only carbon-containing
products observed, which is consistent with previous results from
which was present as a 0.27% impurity in the initial CH3Br
sample), CH2O, HC(O)Br, and CO. CH3Cl was also observed
as an impurity in the CH3Br samples (0.05%), but was of no
consequence in the analysis because of its limited reactivity with
Cl atoms.
Since the consumption of CH3Br was kept low (0.1-0.3%)
to minimize secondary reactions involving Cl atoms, it was not
possible to measure losses of CH3Br directly. However, by
monitoring the consumption of the dimethyl ether, the loss of
CH3Br can be measured indirectly. Chlorine atoms react a
3
2
our laboratory. No CH2O or CO was observed.
The data obtained in experiments 1-3 are plotted in Figure
2. There was no discernible effect of O2 partial pressure over
the range 5-700 Torr on the yields of any of the products. The
yield of CH2O decreased while that of CO increased with
increasing consumption of CH3Br, suggesting the occurrence
of secondary reactions involving these species. The yield of
HC(O)Br was 10 ( 2% (precision only), independent of the
degree of CH3Br loss, suggesting the absence of secondary
consumption of HC(O)Br. The sum of the product yields for
CO, CH2O, and HC(O)Br is shown in Figure 2. Linear least-
squares analysis gives a combined CO + CH2O + HC(O)Br
yield ) 112 ( 12% (precision only). Throughout this paper
errors are 2 standard deviations unless otherwise specified.
Inclusion of possible systematic errors in the relative rate
coefficients for reaction of Cl with CH3Br and dimethyl ether
increases the uncertainty to (30%. Within the experimental
uncertainties, the observed CH2O, CO, and HC(O)Br products
account for all of the CH3Br loss.
-
10
-13 33,28
factor of 475 (1.9 × 10 /4.4 × 10
)
times faster with
dimethyl ether than with CH3Br. During the 45 s irradiation
used to acquire the data shown in Figure 1, the dimethyl ether
decayed to 42% of its initial concentration. If we assume that
dimethyl ether and CH3Br are lost solely by reaction with Cl
atoms, then expression I can be used to calculate that 0.18% of
the CH3Br is consumed:
CH Br loss (%) )
3
100(1 - exp(ln([CH OCH ] /[CH OCH ] )/475)) (I)
3 3 t 3 3 t
0
Similar calculations were performed to derive the values of ∆-
CH3Br]calc listed in Tables 1 and 2. CH3Br loss was also
[
determined in some test experiments at NCAR from a knowl-
edge of the Cl2 photolysis rate. The Cl2 photolysis rate was
determined prior to a CH3Br experiment from the rate of
conversion of CH3OH to CH2O during the irradiation of Cl2/
CH3OH/O2 mixtures. CH3Br losses calculated from the mea-
sured photolysis rates (following correction for loss of Cl atoms
to reaction with dimethyl ether and CH2O) were typically
Results obtained at NCAR in the absence of NO were similar
to those obtained at Ford. If we ignore the production of methyl
formate from dimethyl ether oxidation (which was present as a
250 ppm impurity in the CH Br samples used at NCAR), the
3
5
-10% higher than those obtained from expression I; although
major products observed were CH O and CO, while HC(O)Br
2
the two methods agree within experimental uncertainty, the
apparent discrepancy may be due in part to reaction of Cl atoms
with minor species in the system, which are ignored in the
determination of CH3Br loss from the Cl2 photolysis rate.
was again a minor product (yield ≈ 12% of the total CH O +
2
CO + HC(O)Br yield, see Table 2). The CH2O yield again
decreased with irradiation time, while that of CO increased.
Because CH3Br concentrations were somewhat smaller in the

Atmospheric Oxidation of CH3Br
J. Phys. Chem., Vol. 100, No. 17, 1996 7029
TABLE 2: Results Obtained at NCARa
[
CH
3
Br]
0
[Cl
(mTorr)
2
]
0
[O
(Torr)
2
]
∆[CH
3
Br]calc
(mTorr)
∆[CH
2
O]
(mTorr)
∆[CO]
(mTorr)
∆[HC(O)Br]
(mTorr)
expt
C
(mTorr)
t(UV) (s)
comments
2212
228
20
60
3.84
6.74
3.35
6.54
2.51
4.17
1.21
2.66
1.50
2.43
3.54
5.05
0.68
1.10
1.81
1.73
1.82
1.67
2.01
2.96
4.63
2.16
3.70
0.86
1.27
1.42
1.54
0.83
1.51
1.94
0.74
1.30
1.67
1.70
0.83
1.30
1.48
1.67
0.23
0.52
0.78
0.81
0.86
1.82
4.32
1.67
3.83
0.74
2.25
0.25
0.74
0.49
1.30
2.25
3.10
0.09
0.25
0.50
0.19
0.38
0.62
0.90
0.19
0.38
0.52
0.62
0.06
0.11
0.16
0.20
0.27
0.28
0.42
0.28
700 Torr of total pressure
120
60
120
60
120
60
120
30
D
E
F
2424
2208
2256
2190
229
237
219
226
300
258
700 Torr of total pressure
0.42
<0.15
<0.15
<0.15
<0.15
0.19
0.26
0.30
34.6 mTorr of NO added
710 Torr of total pressure
22 mTorr of NO added
1100
1100
G
6
9
0
0
120
20
0.33
H
J
1993
2190
219
223
1100
710
<0.15
<0.15
<0.15
<0.15
<0.15
<0.22
<0.22
<0.15
<0.15
<0.22
<0.22
<0.15
<0.15
<0.15
<0.15
<0.15
11 mTorr of NO added
4
6
0
0
30
temperature ) 263 K
7.7 mTorr of NO added
6
9
0
0
120
30
K
L
2190
2200
223
220
720
700
temperature ) 263 K
11 mTorr of NO added
45
60
75
15
temperature ) 228 K
8.9 mTorr of NO added
30
45
60
75
a
T ) 295 K except as noted. All product concentrations are reported as observed. Total pressure ) O
). ∆[CH Br]calc not given at low T due to the formation of CH BrO NO
2
pressure, unless otherwise stated
(
balance N
2
3
2
2
2
.
Figure 2. Yields of CO (O), CH
following irradiation of mixtures of 2.9 Torr of CH
Cl , and 5-700 Torr of O at 700 Torr of total pressure made up with
where appropriate (experiments 1-3, see Table 1). The solid lines
through the CO and CH O data are second-order fits to aid in visual
inspection of the data trends. The straight line through the HC(O)Br
data is a first-order fit. The sum of CO, CH O, and HC(O)Br is given
2
O (2), and HC(O)Br (b) observed
Figure 3. Yields of CO (O), and CH
irradiation of mixtures of 3 Torr of CH Br, 0.1 Torr of Cl
Torr of O , and 4.7-25.5 mTorr of NO in 700 Torr of total pressure
made up with N (experiments 5-7, see Table 1). The solid lines
through the CO and CH O data are second-order fits to aid in visual
inspection of the data trends. The sum of CO and CH O yields is given
2
O (2) observed following
3
Br, 0.1 Torr of
2
, 147-594
3
2
2
2
N
2
2
2
2
2
2
by the solid diamonds; the line through the data is a first-order fit. The
open squares were obtained by correcting the observed CO yields for
by the solid diamonds; the line through the data is a first-order fit.
2
the impact of the Cl + CH O reaction.
NCAR experiments than in those at Ford (2 compared to 3 Torr),
secondary consumption of CH2O was more severe. No
systematic trends in the product yields were observed over a
large range of O2 partial pressures (20-1100 Torr).
Experiments Conducted in the Presence of NO. To shed
further light on the chemistry of CH2BrO radicals, experiments
were performed in which NO was added to the initial reaction
mixture. Addition of NO ensures the complete conversion of
CH2BrO2 to CH2BrO Via reaction 3 and eliminates complica-
tions due to the occurrence of reactions 9 and 10. Also, as will
be discussed in more detail later, the NO and NO2 present in
these experiments act as sinks for Cl and Br atoms and for OH
radicals, thus limiting the secondary consumption of CH2O.
Following the irradiation of CH3Br/NO/Cl2/O2/N2 mixtures,
two carbon-containing products were observed, CH2O and CO
(methyl formate from CH3OCH3 oxidation was also detected).
No HC(O)Br was observed in any of these experiments. Results
from Ford are shown in Table 1 and Figure 3. Values of ∆-
[CH3Br]calc were again determined from the observed loss of
CH3OCH3 by using expression I. This calculation assumes that
CH3OCH3 is lost solely by Cl-atom attack. In the presence of

7
030 J. Phys. Chem., Vol. 100, No. 17, 1996
Orlando et al.
BrO2 with NO2:
CH BrO + NO + M f CH BrO NO + M (11)
2
2
2
2
2
2
The analogous CH3OONO2, observed in the Cl-atom-initiated
3
5
oxidation of CH4, has absorption features at 1730 and 1300
-1
-1
cm (the 1300 cm feature was not observed here for BMPN
due to strong absorption in this region by CH3Br). If it is
assumed that the thermal stability of BMPN is similar to that
of ClCH2O2NO2, thermal decomposition rates (the reverse of
-
6
-1
reaction 11) at 700 Torr of about 2 × 10 and 0.3 s can be
3
6
estimated for 230 and 300 K, respectively. With this rapid
thermal dissociation at room temperature, followed by reaction
of CH2BrO2 with NO, BMPN does not build up as a reaction
product. However, with the anticipated enhancement of its
thermal stability at low temperature, the formation of BMPN
becomes essentially irreversible on the time scale of an
experiment, and it becomes a major product of the CH3Br
oxidation under the conditions of this study.
Figure 4. Comparison of measured and modeled CH
2
O, CO, and HC-
O]; (3)
Interpretation of Yield DatasThe Chemistry of CH2BrO.
It is clear that, at low CH3Br conversions, CH2O accounts for
the majority of CH3Br loss in experiments conducted with and
without NO. In experiments conducted without NO, there are
two possible sources of CH2O: reaction 9a followed by reaction
5 or reaction 9b followed by reaction 12.
(O)Br for experiment H of Table 2: (b) measured [CH
2
measured [CO]; (1) measured upper limits of [HCOBr]. The curves
show the product concentrations obtained from the model, assuming
that reaction 6 does not occur (see text for details).
NO, reaction with HO2 radicals generates OH radicals, which
-12
-14 28,34
are 80 (2.3 × 10 /2.9 × 10
)
times more reactive toward
CH3OCH3 than toward CH3Br and are thus less discriminating
in their reactivity toward CH3OCH3 and CH3Br than Cl atoms.
Accordingly, the calculated loss of CH3Br based upon the
observed CH3OCH3 is an underestimate of the true consumption
of CH3Br, and thus the sum of the CH2O and CO yields is
greater than the loss of CH3Br calculated by using expression
I. The linear least-squares fit shown in Figure 3 gives a
combined yield for CH2O and CO of 142 ( 15% (precision
error only); the y-axis intercept is 0.3 ( 0.4 mTorr and is not
statistically significant. It is evident (see Figures 2 and 3) that
the temporal profile of CH2O is more linear in the presence of
NO than in its absence; secondary consumption of CH2O is more
severe when NO is not present.
CH BrOH f CH O + HBr
(12)
2
2
The decomposition of BrCH OH (reaction 12) is likely to be
2
37
quite rapid, as is the case for the analogous ClCH OH species.
2
CH2O formed by reactions 9b and 12 would be accompanied
by an equal amount of HC(O)Br, which is not observed, and it
follows that most, if not all, of the CH2O product is formed by
reaction 9a followed by reaction 5. Since CH2O is the most
prevalent product at low conversions of CH3Br, it can be
concluded that the majority of the loss of CH2BrO2 radicals
proceeds Via reaction 9a and that the fate of CH2BrO radicals
is dominated by reaction 5.
In the presence of NO, there are again two possible routes to
CH2O formation, reaction 5 or reaction 13a:
Results obtained at NCAR were similar to those observed at
Ford (see Table 2 and Figure 4). Experiments were conducted
CH BrO + NO f CH O + BrNO
(13a)
(13b)
(13c)
14
2
2
over a range of initial [NO] [(3.2-10.0) × 10 molecules
-3
cm ), [O2] (250-1100 Torr), and total pressure (710-1100
Torr). At low conversion, CH2O was the major product while
small yields of CO were observed. The CH2O yield decreased
at higher conversion while that of CO increased. The results
are again consistent with secondary consumption of CH2O
leading to CO production, although the secondary consumption
of CH2O was much less severe when NO was present than when
it was not. No HC(O)Br was observed in any of these
experiments, and its yield (relative to the sum of CO and CH2O)
was at most 5%.
CH BrO + NO f HC(O)Br + HNO
2
CH BrO + NO f CH BrONO
2
2
However, some experiments were conducted at very low [NO]/
O2] ratios, where reaction 13 would not be expected to compete
with reaction 4. Reaction 4 is likely to proceed with a rate
[
-
14
3
-1 -1
constant of ≈10
reactions of alkoxy radicals with O2. (A rate constant of 6 ×
cm molecule is recommended on the basis of
the similar reaction of CH2ClO with O2.) In some experiments
e.g., experiment 7 of Table 1 and experiments F, H, and J-L
cm molecule s , by analogy to other
-14
3
-1 -1
28
1
0
s
Some experiments were also conducted at NCAR at reduced
temperature (262 and 228 K) in the presence of NO. Oxygen
partial pressures were kept high (700 Torr) to provide conditions
that would favor the occurrence of reaction 4. Results were
similar to those obtained at room temperature. At low conver-
sion, the CH2O yield was significantly higher than that of CO,
while enhanced yields of CO were observed at higher conversion
(
4
of Table 2), the initial [O2]/[NO] ratio was at least 5 × 10 ;
thus, reaction 13 could not compete with reaction 4. Since the
NO concentration decreases with time, the possibility of a
significant contribution of reaction 13 to the observations is
further limited. Hence, the CH2O product observed in these
high [O2]/[NO] experiments must be produced in reaction 5,
again indicating that this is the dominant fate of the CH2BrO
(again consistent with secondary conversion of CH2O to CO).
As was the case in room temperature experiments containing
NO, no HC(O)Br was observed. An additional absorption
radical, even in the presence of 1100 Torr of O (see experiment
2
-1
feature, centered near 1730 cm , was noted in the FTIR spectra
at low temperatures. This feature is assigned to bromomethyl
peroxynitrate (BMPN), formed from the association of CH2
H of Table 2).
At higher CH Br conversion, yields of CH O are seen to
3
2
decrease in experiments conducted with and without added NO.

Atmospheric Oxidation of CH3Br
J. Phys. Chem., Vol. 100, No. 17, 1996 7031
Br are all possible:
OH + HC(O)Br f BrCO + H O
(17)
2
Cl + HC(O)Br f BrCO + HCl
Br + HC(O)Br f BrCO + HBr
(18)
(19)
although rate constants for these processes are not known. There
are indications, however, that these processes are not significant.
Reactions of OH and Cl with HC(O)Cl are not particularly fast
-
13
3
-1 -1
-13
(
kCl ) 7.8 × 10
cm molecule
s
and kOH < 5 × 10
3
-1 -1 39
cm molecule s ), and hence the analogous HC(O)Br
reactions might also be anticipated to be slow. In addition, the
yield of HC(O)Br in the experiments conducted without NO
was linear, indicating that secondary consumption is slow. For
the purpose of obtaining a conservative upper limit to the yield
of HC(O)Br in these experiments, simulations were conducted
in which rate constants of 10- , 10 , and 10
11
-10
-12
were
Figure 5. Comparison of modeled and measured yields of CH
and HC(O)Br for experiment 3 of Table 1: (b) measured [CH
(
2
O, CO,
O];
3) measured [CO]; (1) measured [HCOBr]. The curves show the
product concentrations obtained from the model, assuming that reaction
does not occur (see text for details).
employed for reactions 17, 18, and 19, respectively. Even with
these losses incorporated into the model, the observed upper
limit for HC(O)Br is consistent with reaction 4 contributing only
5% to the overall loss of CH2BrO at 298 K and 540 Torr of O2
2
6
(
experiment 6 of Table 1). Modeling of experiment H of Table
This the expected result given the high reactivity of CH2O with
Cl atoms, Br atoms, and OH:28
2 also revealed an upper limit of 5% for reaction 4 in the
presence of 1100 Torr of O . Thus, for 1 atm of air, reaction
2
with O2 will contribute at most 1% to the fate of CH2BrO
radicals.
Cl + CH O f HCl + HCO
(14)
(15)
(16)
2
The CO yield can be used to estimate the importance of
reaction 6, provided other sources and sinks of CO are properly
accounted for. In all experiments, yields of CO at low
conversions were substantially less than those of CH2O.
Experiment 6 of Table 1, for example, shows that the maximum
contribution of reaction 6 to the loss of CH2BrO radicals is 11%
of that of reaction 5. However, this ignores the fact that sources
of CO exist other than reaction 6, e.g., destruction of CH2O
(reactions 14-16) and possibly destruction of HC(O)Br (reac-
tions 17-19) followed by thermal decomposition of BrCO.
Because of the complexity of this chemical system, in which
Cl, Br, and OH are important in the production of CO, modeling
was conducted to determine the relative importance of reaction
6. Model results for experiment H (Table 2) are shown as the
solid lines in Figure 4. The model results were obtained by
assuming that reaction 6 does not occur at all, while rate
Br + CH O f HBr + HCO
2
OH + CH O f H O + HCO
2
2
Indeed, Cl atoms are 165 times more reactive with CH2O than
2
8
with CH3Br, while OH (produced from the reaction of HO2
with NO in experiments containing NO) is more reactive with
CH2O than with CH3Br by a factor of 330.28 Br atoms,
produced in reaction 5, are unreactive toward CH3Br and
represent a further loss mechanism for CH2O. As noted earlier,
the roll-off of [CH2O] is less severe when NO is present. This
results from the added sinks for Cl, Br, and OH, mainly addition
to NO and NO2, leading to the production of BrNO, ClNO,
ClNO2, BrNO2, HNO3, and HONO. Further reactions of Br
and Cl with BrNO, ClNO, ClNO2, and BrNO and of OH with
all of the aforementioned species are also possible, leading to
a NOx-catalyzed recombination of the reactive species and thus
allowing [CH2O] to build to a higher value. As will be
discussed in more detail later, models of the chemical system
were capable of matching the observed roll-off in CH2O yield
in experiments conducted with NO (Figure 4) and without NO
-
14
3
constants for reactions 4 and 5 were set at 6 × 10
cm
molecule-
1
s
-1
and 4 × 10 s , respectively. Most of the CO
7 -1
production can be explained from the consumption of CH2O
produced in reaction 5. Best agreement between measured and
modeled CO yields was obtained when a 5% channel to HCO
6
-1
and HBr for CH2BrO was assumed (k6 ) 2 × 10 s ).
However, when the same chemical model was used to examine
Ford experiment 6 (Table 1), no contribution from reaction 6
was required to explain the observed levels of CO. (Model
results actually overestimated the CO yield in this experiment.)
There is, therefore, no conclusive evidence for the occurrence
of reaction 6 in these studies, and we estimate an upper limit
of 5% for its occurrence relative to reaction 5. The inability to
perfectly model the CH2O/CO product ratios in these experi-
ments perhaps is not surprising given the numerous reactions
that consume and interconvert the Cl, Br, and OH species and
given that many of the rate constants for these reactions had to
be estimated.
(
Figure 5).
Yields of HC(O)Br can be used to determine the importance
of reaction 4 in the chemistry of the CH2BrO radical. While
HC(O)Br is observed in experiments conducted without NO,
its yield is independent of O2 (10-12% yield, O2 pressure )
-1000 Torr). This again shows that even in the presence of
100 Torr of O2, reaction 4 does not compete effectively with
reaction 5 and that the source of the observed HC(O)Br is either
reaction 9b or 10b. HO2 radicals are produced in the oxidation
of dimethyl ether and of CH2O, and a reaction channel analogous
to that of reaction 10b is known to be of importance in the
reaction of CH2FO2 with HO2 radicals.38
5
1
No HC(O)Br was observed in experiments containing NO,
again confirming the unimportance of reaction 4 relative to
reaction 5. It is important to note, however, that there is the
potential for secondary loss processes of HC(O)Br that could
occur in the system. Reaction of HC(O)Br with OH, Cl, and
Modeling of experiments conducted without NO is compli-
cated due to the lack of knowledge of the branching ratios of
reactions 9 and 10 and the potential for reaction of CH2BrO2
radicals with CH3OCH2O2 obtained from dimethyl ether (for
which no rate data are available). Simulation of the observed

7
032 J. Phys. Chem., Vol. 100, No. 17, 1996
Orlando et al.
HC(O)Br yield requires that either reaction 9b or 10b or both
occur. For modeling of these experiments, the branching ratio
position for CH2BrO can be obtained if assumptions are made
regarding the rate constant for its reaction with O2. If we use
-14
3
-1
(
k9b/k9) was assumed to be zero (the coproduct of reaction 9b,
the currently recommended value of 6 × 10 cm molecule
1
4
-1 28
BrCH2OH, has not been observed in previous experiments),
s , and if we consider that unimolecular decomposition is at
least 20 times faster than reaction with O2 in the presence of
1100 Torr of O2 (experiment H, Table 2), the rate of unimo-
and k10b/k10 was adjusted until the HC(O)Br yield was properly
modeled. It was found that a significant temporary sink for Br
atoms was required to properly model the shape of the CH2O
profile. A relatively fast reaction of Br atoms with CH2BrO2
is proposed, which could lead to the formation of BrO and/or
7
-1
lecular decomposition must be at least 4 × 10 s . The same
arguments applied to the experiments conducted at 228 K (in
700 Torr of O2) yield a lower limit to decomposition of 3 ×
4
0
7
-1
CH2BrOOBr, both of which are unreactive toward CH3Br:
10 s . Lower limits to the rate of Cl-atom elimination from
4
2,43
other halomethoxy radicals have been reported.
Chlorine
Br + CH BrO f BrO + CH BrO
(20a)
(20b)
(21)
2
2
2
atom elimination from CCl3O (at 233 K and 7 Torr of total
pressure), CFCl2O (at 253 K and 7 Torr of total pressure), and
CF2ClO (298 K, 50-200 Torr of total pressure) was reported
T CH BrOOBr
2
5
42
4 42
5 -1 43
to be >1 × 10 , >3 × 10 , and >7 × 10 s , respectively.
BrO + CH BrO f Br + CH BrO + O
2
2
2
2
Some conclusions can also be drawn regarding the activation
energy for Br-atom elimination from CH2BrO. By using the
Model simulations, conducted by assuming k6 ) 0, again show
that the CO produced in experiments without NO added can be
explained solely from CH2O destruction. Model results for
experiment 3 of Table 1 are compared with observed product
yields in Figure 5.
7
-1
lower limit of 3 × 10 s for CH2BrO decomposition at 228
14 -1 44
K and by assuming an A-factor of 5 × 10 s , an upper
limit to the activation energy of 7.5 kcal/mol can be obtained.
This value is similar to reported upper limits of the activation
energy for Cl-atom elimination from CCl3O (<9.6 kcal/mol)
Comparison with Previous Studies. As mentioned earlier,
42
and CFCl2O (<11.0 kcal/mol).
previously published studies1
4-16
of CH3Br oxidation have
Let us now consider the thermodynamics of the reactions of
the CH2BrO radical. The ∆Hf of CH2BrO is not known.
However, if one considers that ∆Hf for CH2ClO is -2.4 kcal/
concluded that reaction with O2 was the dominant fate of CH2-
BrO under atmospheric conditions, leading to HC(O)Br produc-
tion, in disagreement with the results obtained here. In an
22
mol and that Br bonds are typically ≈10 ( 2 kcal/mol weaker
14
earlier, more limited study conducted at Ford Motor Company,
28
than the analogous Cl bond, the ∆Hf for CH2BrO is estimated
a calibrated HC(O)Br spectrum was not available, the partial
O2 pressure was not varied, experiments employing NO to
convert CH2BrO2 into CH2BrO were not performed, and
quantification of CH3Br loss was not possible. Although the
observations made in that study were similar to those reported
here for experiments conducted in the absence of NO (HC(O)-
Br and CH2O observed as products, with CH2O levels reaching
steady state), the observation of HC(O)Br led the authors to
suggest that Br-atom elimination from CH2BrO was not
important.
to be 5.4 kcal/mol. Application of the same logic for ∆Hf(HC-
21
(O)Br) relative to ∆Hf(HCOCl) ) -44.3 kcal/mol yields a
value of -36.5 kcal/mol for this species. Thus, the ∆Hr values
for reactions 4-6 are estimated to be -39, -4.7, and -4.1
17
kcal/mol, respectively. Atkinson and Carter have shown that,
for bond fission processes involving alkoxy radicals, decom-
position will dominate over reaction with O2 for atmospheric
conditions when the difference in ∆Hf for the two processes is
less than about 40 kcal/mol. Clearly, by using this criterion,
elimination of a Br atom is predicted to be favored relative to
reaction with O2, as is observed.
Weller et al.16 also observed HC(O)Br formed in the
photooxidation of CH3Br in the presence of Cl2 and air.
Addition of NO was found to suppress the formation of HC-
Reaction rates for alkoxy radical decomposition have been
estimated by using the Evans-Polanyi relationship, in which
the ∆Hr for thermal decomposition is used to predict the
activation energy for the bond rupture process. For C-C and
C-H bond ruptures in simple alkoxy radical radicals, the
following relationship has been reported (in kilocalories/
(O)Br, as was the case in our experiments, but this observation
was interpreted by Weller et al. to be due to direct reaction of
CH2BrO with NO (reaction 13). While this reaction may occur,
it cannot make a significant contribution in our system since
-
5
the ratio of NO to O2 was kept very low (<2 × 10 ) for most
4
4,45
mole): Ea ) 12.8 + 0.71(∆Hr).
In the present case, use of
experiments, as discussed earlier.
a ∆Hr of about -5 kcal/mol from earlier leads to the prediction
of an activation energy of 9 kcal/mol, which is somewhat higher
than the derived upper limit of 7.5 kcal/mol. It should be
1
5
Locke et al. also found HC(O)Br as a major product of the
Cl-atom-initiated oxidation of CH3Br in air and did not observe
CH2O. It is possible that the CH2O concentration was kept
below the detection limit due to loss Via reactions 14 and 15.
As in our experiments without NO present, HC(O)Br formation
was likely due to reaction 10b rather than reaction 5.
4
5
pointed out that in the earlier literature it was assumed that
the activation energy never decreased below 12.8 kcal/mol for
exothermic dissociation reactions, and hence an Ea of 12.8 kcal/
mol would have been predicted for reaction 5.
During the course of this work, a study of the oxidation of
CH3Br similar to ours was published.41 Experiments were
Atmospheric Implications. The experiments presented here
show that the atmospheric fate of CH2BrO radical is unimo-
lecular decomposition to CH2O and Br, even at the low
temperatures of the upper troposphere and lower stratosphere.
Combination of reactions 1-3 and 5 gives the overall net
reaction for OH-initiated photooxidation of CH3Br in the
atmosphere:
conducted at room temperature at 700 Torr of total pressure,
with varying O2 partial pressure (70-700 Torr). From an
analysis of the yields of CH2O, CO, and HC(O)Br, it was
concluded that Br-atom elimination was the dominant fate of
CH2BrO in 700 Torr of air (g79%), while upper limits for
reaction with O2 and HBr elimination of 10 and 11%,
respectively, were found. These results are in excellent agree-
ment with those reported here. These authors also identified
BMPN in experiments conducted with added NO2, confirming
our identification of this molecule at low temperature.
OH + CH Br + O + NO f CH O + Br + NO + H O
3
2
2
2
2
Contrary to previous reports,1
4-16
the atmospheric oxidation of
Kinetics, Energetics, and Thermodynamics of CH2BrO
Reactions. A lower limit to the rate of unimolecular decom-
CH3Br in the presence of NO does not lead to the formation of
HC(O)Br.

Atmospheric Oxidation of CH3Br
J. Phys. Chem., Vol. 100, No. 17, 1996 7033
Acknowledgment. The authors thank Sue Schauffler and
Fred Eisele of NCAR and Robert Lesclaux (Universit e´ Bor-
deaux) for helpful comments regarding this work. NCAR is
partially supported by the NSF.
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