Kinetics of the CH3O2 + NO Reaction: Temperature Dependence of the Overall Rate Constant and an Improved Upper Limit for the CH3ONO2 Branching Channel

Article abstract of DOI:10.1021/jp990469k

The overall rate constant and an upper limit for the CH3ONO2 product channel for the CH3O2 + NO reaction have been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. At room temperature and 100 Torr pressure, the rate constant (and the two standard deviation error limit) was determined to be (7.8 +/- 2.2)E-12 cm³ molecule^-1 s^-1. The temperature dependence of the rate constant was investigated between 295 and 203 K at pressures of either 100 or 200 Torr, and the data was fit to the following Arrhenius expression: (9.2^+6.0_-3.9E-13) exp<(600 +/- 140)/T> cm³ molecule^-1 s^-1. Although the room-temperature rate constant value agrees well with the current recommendation for atmospheric modeling, our values for the rate constant at the lowest temperatures accessed in this study (203 K) are about 50 percent higher than the same recommendation. No CH3-ONO2 product was detected from the CH3O2 + NO reaction (using direct CH3ONO2 detection methods for the first time), but an improved upper limit of 0.03 (at 295 K and 100 torr) for this branching channel was determined.

Full text of DOI:10.1021/jp990469k

4378
J. Phys. Chem. A 1999, 103, 4378-4384
Kinetics of the CH3O2 + NO Reaction: Temperature Dependence of the Overall Rate
Constant and an Improved Upper Limit for the CH3ONO2 Branching Channel
Kurtis W. Scholtens, Benjamin M. Messer, Christopher D. Cappa, and Matthew J. Elrod*
Department of Chemistry, Hope College, Holland, MI, 49423
ReceiVed: February 8, 1999; In Final Form: April 12, 1999
The overall rate constant and an upper limit for the CH3ONO2 product channel for the CH3O2 + NO reaction
have been measured using the turbulent flow technique with high-pressure chemical ionization mass
spectrometry for the detection of reactants and products. At room temperature and 100 Torr pressure, the rate
constant (and the two standard deviation error limit) was determined to be (7.8 ( 2.2) × 10-12 cm3 molecule-1
s-1. The temperature dependence of the rate constant was investigated between 295 and 203 K at pressures
+6.0
of either 100 or 200 Torr, and the data was fit to the following Arrhenius expression: (9.2
× 10-13
)
-3.9
exp[(600 ( 140)/T] cm3 molecule-1 s-1. Although the room-temperature rate constant value agrees well with
the current recommendation for atmospheric modeling, our values for the rate constant at the lowest
temperatures accessed in this study (203 K) are about 50% higher than the same recommendation. No CH3-
ONO2 product was detected from the CH3O2 + NO reaction (using direct CH3ONO2 detection methods for
the first time), but an improved upper limit of 0.03 (at 295 K and 100 torr) for this branching channel was
determined.
Introduction
a tracer of the photochemical age of air masses,1 but the
chemical processes leading to its presence in the atmosphere
The methylperoxy radical (CH3O2) is an important intermedi-
ate species formed in the oxidation of methane in the atmo-
sphere.
remain unclear.2 Recently, Wennberg et al.3 reported the
measurement of higher HOx levels than predicted in the upper
troposphere of the northern hemisphere. Because this result
suggests that this region of the atmosphere is not ordinarily
dominated by NOx chemistry, it is therefore more susceptible
to anthropogenic NOx emissions than previously thought. This
is an important finding since it indicates that increased air traffic
in the upper troposphere may lead to a substantial increase in
ozone levels. Therefore, it is critical to obtain the relative rates
of reactions 4, 7, and 8 and to determine the product distributions
of these reactions in order to address the impact of CH3O2
chemistry on this issue.
CH4 + OH f CH3 + H2O
(1)
CH4 + O(1D) f CH3 + OH
(2)
(3)
CH3 + O2 + M f CH3O2 + M
Ozone levels in the atmosphere are directly affected by CH3O2
reactions, which themselves are dependent on the levels of the
nitrogen oxides (NOx). Under high NOx conditions (generally,
lower tropospheric urban conditions), CH3O2 reactions lead to
the production of ozone, the most deleterious constituent of
photochemical smog.
The CH3O2 + NO reaction has received considerable
study4-10 because of its important role in the formation of
tropospheric ozone. In the most recent study, Villalta et al.11
used low-pressure chemical ionization mass spectrometry
(CIMS) to measure the negative temperature dependence of the
rate constant to temperatures as low as 200 K. Although this
study included temperatures of the upper troposphere, the
experiments were carried out at low pressure (∼ 3 Torr).
Because of this limitation, Villalta et al. suggested that under
the actual high-pressure and low-temperature conditions of the
upper troposphere that the association reaction (channel 4b)
could not be ruled out as an important source of methyl nitrate.
Previously, there had been two studies of the product branching
channels for reaction 4 at 298 K, and in neither case was
evidence found for branching into channel 4b. Ravishankara et
al. established an upper limit branching ratio [k4b/(k4a + k4b)]
of 0.24 for channel 4b by following the production of the
channel 4a product, NO2, and applying mass balance arguments
to determine the maximum branching into channel 4b that was
consistent with the data.6 Zellner et al. determined an upper
limit of 0.20 for channel 4b by following the production of the
other channel 4a product, CH3O, and again applying mass
CH3O2 + NO f CH3O + NO2
NO2 + hν (λ < 380 nm) f NO + O
O + O2 + M f O3 + M
(4a)
(5)
(6)
CH3O2 can also be temporarily removed from the ozone
production cycles by the formation of reservoir species.
CH3O2 + NO + M f CH3ONO2 + M
CH3O2 + NO2 + M f CH3O2NO2 + M
CH3O2 + HO2 f CH3OOH + O2
(4b)
(7)
(8)
A potential product of reaction 4, methyl nitrate (CH3ONO2),
has been directly measured in the atmosphere and suggested as
* Corresponding author. E-mail: elrod@hope.edu. Phone: (616) 395-
7629. Fax: (616) 395-7118.
10.1021/jp990469k CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/14/1999

Kinetics of the CH3O2 + NO Reaction
J. Phys. Chem. A, Vol. 103, No. 22, 1999 4379
device was placed at the end of the injector in order to enhance
turbulent mixing. The polonium-210 R-emitting ionization
source was placed between the temperature regulated flow tube
and the inlet to the quadrupole mass spectrometer. All gas flows
were monitored with calibrated mass flow meters. The flow tube
pressure was measured upstream of the ionization source using
a 0-1000 Torr capacitance manometer. The temperature was
determined at both the entrance and exit points of the temper-
ature regulated region of the flow tube using Cu-constantan
thermocouples.
Reactant Preparation. CH3O2 was generated using the
following reactions:
Figure 1. Experimental apparatus.
CH4 + F f CH3 + HF
(9)
CH3 + O2 + M f CH3O2 + M
(10)
balance arguments to determine the maximum branching into
channel 4b that was consistent with their data.9 In addition,
Atkinson and co-workers have developed an empirical relation-
ship for the yield of organic nitrates from environmental
chamber photolysis studies of C3-C9 hydrocarbon/NO mixtures
at 298 K.12 Although this relationship probably provides only
a qualitative estimate of potential methyl nitrate formation from
reaction 4, it is important to note that the branching ratio is
predicted to rise from 0.005 for the methyl case to 0.16 for the
n-heptyl case at 298 K and 760 torr, thus indicating the
importance of the association channel for the larger alkyl peroxy
radicals.
In this article we describe our investigation of the kinetics
of the CH3O2 + NO reaction conducted at pressures at 100 and
200 Torr and at a range of temperatures extending to those found
in the lower stratosphere using a turbulent flow (TF) tube
coupled to a high-pressure chemical ionization mass spectrom-
eter (CIMS). It has been previously shown that TF technique
can be used to accurately determine the rate constants of
reactions at pressures ranging from 50 to 760 Torr and at
temperatures as low as 180 K.13 As in previous kinetics studies
of HO2 + NO (ref 14), HO2 + BrO (ref 15), and OH + ClO
(ref 16) using the coupled TF-CIMS approach, we are able to
directly access atmospheric pressure and temperature conditions
and sensitively monitor many of the relevant reactants and
products for the CH3O2 + NO reaction. In the OH + ClO work
mentioned above, the CIMS detection methodology allowed for
the first identification of a minor HCl-producing channel for
the OH + ClO reaction, thus resolving a long-standing
inconsistency between predicted and measured stratospheric HCl
concentrations.16 In this work, we describe our development of
a new chemical ionization detection scheme for CH3ONO2,
which allows for the first direct product study of reaction
channel 4b.
(k9 ) 6.7 × 10-11 cm3 molecule-1 s-1 and k10 ) 4.9 × 10-13
cm3 molecule-1 s-1 at 100 Torr).17 Fluorine atoms were
produced by passing a dilute F2/He mixture through a micro-
wave discharge produced by a Beenakker cavity operating at
50 W. The dilute F2/He mixture was obtained by combining a
2.0 STP L min-1 flow of helium (99.999%), which had passed
through a silica gel trap immersed in liquid nitrogen, with a
0.5-5.0 STP mL min-1 flow of a 1% F2/He mixture (Excimer
grade). To generate CH3, the fluorine atoms were then injected
into a sidearm and mixed with an excess of CH4 (CP grade,
∼1015 molecule cm-3) in order to ensure that no fluorine atoms
were introduced into the main flow. CH3O2 was then produced
by the addition of a large excess of O2 (99.995%; ∼5 × 1017
molecule cm-3) just downstream of the production of CH3.
Absolute CH3O2 concentrations (needed to ensure pseudo-first-
order kinetics conditions and for branching ratio modeling) were
determined by the titration reactions
CH3O2 + NO f CH3O + NO2
CH3O + O2 f CH2O + HO2
(11)
(12)
(k11 ) 7.8 × 10-12 cm3 molecule-1 s-1 and k12 ) 1.9 × 10-15
cm3 molecule-1 s-1 17
) and subsequent calibration of the CH2O
mass spectrometer signal. Computer modeling of these titration
reactions indicates that not all of the CH3O2 initially present is
converted to the stable species, CH2O. This is mainly due to
reactions of CH3O with other species in the chemical system
(which will be addressed in the discussion section). Therefore,
the CH3O2 concentrations were calculated from the CH2O
concentrations by explicitly determining the conversion factor
(usually ∼0.7) from computer modeling for the specific
experimental conditions. The NO (CP grade) used in reaction
11 was purified by several freeze/pump/thaw cycles to remove
NO2 impurities. The NO2/NO ratio of the purified sample was
typically less than 1% as determined from CIMS NO2 detection
and calibration methods. The standard CH2O samples required
for calibration of the mass spectrometer signal were prepared
by heating paraformaldehyde and trapping the gaseous product.
For this study, CH3O2 concentrations ranged from 0.5 to 15.0
Experimental Section
Turbulent Fast Flow Kinetics. A schematic of the experi-
mental apparatus is presented in Figure 1 and is similar to that
used in a previous study of HO2 + NO (ref 14). The flow tube
was constructed with 2.2 cm i.d. Pyrex tubing and was 100 cm
in total length. A large flow of nitrogen carrier gas (ap-
proximately 30 STP L min-1) was injected at the rear of the
flow tube. The gases necessary to generate CH3O2 were
introduced through a 10 cm long 12.5 mm diameter sidearm
located at the rear of the flow tube. NO was added via an
encased movable injector. The encasement (made from cor-
rugated Teflon tubing) was used so that the injector could be
moved to various injector positions without breaking any
vacuum seals, as well as to prevent ambient gases from
condensing on cold portions of the injector. A fan-shaped Teflon
× 1011 molecule cm-3
.
For the actual kinetics experiments, NO was purified before
use as described above and was added to the flow reactor as a
10% mixture in N2 through the movable injector. NO concentra-
tions used in this study ranged from 0.5 to 20.0 × 1012 molecule
cm-3. To ensure pseudo-first-order kinetics conditions, [NO]
was kept more than 10 times larger than [CH3O2].
For the low-temperature studies, liquid nitrogen cooled
silicone oil was used as the coolant for the jacketed flow tube.

4380 J. Phys. Chem. A, Vol. 103, No. 22, 1999
Scholtens et al.
Nitrogen carrier gas was precooled by passing it through a
copper coil immersed in a liquid N2 reservoir followed by
resistive heating. The temperature was controlled in the reaction
region to within 1 K.
pump. The gases entered the rear chamber through a skimmer
cone with a 1.0 mm orifice (held at (130 V) which was placed
approximately 5 cm from the front aperture. The rear chamber
was pumped by a 250 L s-1 turbomolecular pump. Once the
ions passed through the skimmer cone, they were mass filtered
and detected with a quadrupole mass spectrometer.
Chemical Ionization Schemes. To perform pseudo-first-order
kinetics studies of the overall rate constant of reaction 4, a
chemical ionization scheme for the CH3O2 reactant is required.
Villalta et al.11 developed the following chemical ionization
scheme for CH3O2:
CH3ONO2 Branching Channel Measurements. In these
studies, the potential production of CH3ONO2 from reaction 4b
was monitored directly over a reaction time of ∼25 ms. Because
no CH3ONO2 product was actually observed, computer model-
ing was used to determine an upper limit for the rate constant
of reaction 4b. The method involved using the initial measured
concentrations of CH3O2, NO, and all precursors and then
determining the largest rate constant (k4b) for which CH3ONO2
production would remain below the detection level of the mass
spectrometric analytical method (defined as a signal-to-noise
ratio of 2:1). For these experiments, higher CH3O2 levels were
used than in the determination of the bimolecular rate constant
(to increase the concentration of the potential product). A side
reaction resulting from the reaction of the products of the main
channel (4a) of reaction 4,
CH3O2 + O2+ f CH3O2+ + O2
(14a)
(14b)
CH3O2 + O2+ f CH3+ + O2 + O2
They found that the fragmentation reaction 14b was the
predominant (75%) channel for ionization by O2+. Although it
is somewhat unusual to observe fragmentation in chemical
ionization processes, the fragmentation observed in this process
is easily explained by the very large difference in ionization
potentials for O2 [12.07 eV (ref 18)] and CH3O2 [7.36 eV (ref
19)]. The relatively large exothermicity of this charge-transfer
process (454 kJ mol-1) drives the dissociation of CH3O2+ cation.
Villalta et al. were able to overcome the fragmentation problem
CH3O + NO2 + M f CH3ONO2 + M
(13)
(k13 ) 1.2 × 10-11 cm3 molecule-1 s-1 at 100 Torr),17 is a
potential source of CH3ONO2, and experimental conditions must
be designed to minimize this reaction. By keeping O2 concentra-
tions very high, reaction 12 will consume most of the CH3O
produced by reaction 4a and minimize production of CH3ONO2
by reaction 13.
+
in their study by careful calibration of the CH3 signal.
However, we sought to find a less exothermic chemical
ionization process for our work with CH3O2. In another study,
we have undertaken extensive ab initio G2-level electronic
structure and thermodynamics studies of the HOOH and CH3-
OOH neutral and ionic species as part of the development of
chemical ionization mass spectrometric detection methods for
peroxides.20 From this work, we identified the following
chemical ionization scheme for CH3O2:
To determine the detection sensitivity of the mass spectrom-
eter for CH3ONO2, standard samples of CH3ONO2 were
prepared. CH3ONO2 was prepared by dropwise addition of a
solution of 5 mL of 95% H2SO4 and 15 mL of reagent grade
CH3OH to an ice-bath cooled solution containing 30 mL of 95%
H2SO4 and 30 mL of 70% HNO3. The resulting solution was
transferred to a separatory funnel, and the lower layer (acid)
was drained off. The organic layer (containing CH3ONO2) was
then washed several times with a 22% NaCl solution, which
was stored in a refrigerated flask. The high purity of the CH3-
ONO2 sample was confirmed by FTIR spectroscopy, and the
samples did not undergo degradation over the course of several
weeks. Standard samples were prepared by drawing an ap-
propriate amount of CH3ONO2 vapor from the liquid sample
and mixing with N2 to make 1% CH3ONO2 mixtures appropriate
for mass spectrometric calibration.
CH3O2 + H3O+ f CH3OOH+ + H2O
(15)
The calculated exothermicity of this reaction is only 25 kJ mol-1
(because of the very similar proton affinities of CH3O2 and
H2O), thus ensuring the stability of the ionic product CH3OOH+.
This chemical ionization detection scheme was experimentally
tested, and reaction 15 was found to be sufficiently fast to serve
as a sensitive method for the detection of CH3O2. Depending
on the amount of the water in the flow system, CH3O2 was
detected as CH3OOH+(H2O)n, where n was typically 2 or 3.
To perform branching ratio measurements for reaction 4b,
chemical ionization schemes are needed for CH2O (for CH3O2
absolute concentration determinations) and CH3ONO2. In a
previous study, formaldehyde was found to react quickly with
H3O+,
Chemical Ionization Mass Spectrometric Detection. A
positive ion chemical ionization scheme (with H3O+ as the
reagent ion) was used to detect CH3O2 and CH2O, and a negative
chemical ionization scheme (F-) was used to detect CH3ONO2
with the quadrupole mass spectrometer. H3O+ was produced
in the ion source by passing a large O2 flow (10 STP L min-1
)
through the polonium-210 R-emitting ionization source (with
H2O impurities being sufficiently abundant to produce adequate
quantities of reagent ions). F- was produced in the ion source
by passing a large N2 flow (10 STP L min-1) and 0.1 STP mL
min-1 of NF3 through the ionization source. The commercial
ionization source consisted of a hollow cylindrical (69 by 12.7
mm) aluminum body with 10 mCurie (3.7 × 108 disintegrations/
s) of polonium-210 coated on the interior walls.
Ions were detected with a quadrupole mass spectrometer
housed in a two-stage differentially pumped vacuum chamber.
Flow tube gases (neutrals and ions) were drawn into the front
chamber through a 0.1 mm aperture, which was held at a
potential of (210 V. The ions were focused by three lenses
constructed from 3.8 cm i.d., 4.8 cm o.d aluminum gaskets.
The front chamber was pumped by a 6 in. 2400 L s-1 diffusion
CH2O + H3O+ f CH2OH+ + H2O
(16)
(k16 ) 3 × 10-9 cm3 molecule-1 s-1).21 Again, depending on
the amount of the water in the flow system, CH2O was detected
as CH2OH+(H2O)n, where n was typically 2 or 3. The third
species for which a chemical ionization scheme must be
developed is CH3ONO2. Recent experimental22 and theoretical23
studies have definitively shown that CH3ONO2 has a larger
proton affinity than water, suggesting that CH3ONO2 can also
be detected with H3O+.
CH3ONO2 + H3O+ f CH3ONO2‚H+ + H2O (17)
However, we found that our experimental sensitivity using this

Kinetics of the CH3O2 + NO Reaction
J. Phys. Chem. A, Vol. 103, No. 22, 1999 4381
reaction for the detection of CH3ONO2 was not sufficient for
branching ratio studies. Presumably, this result is due to a
relatively slow rate constant for reaction 17. Interestingly, Lee
and Rice have predicted that the proton actually bonds to the
bridging oxygen atom rather than the terminal oxygen atoms.23
Thus, the fraction of reactive orientations achieved by collisions
of H3O+ and CH3ONO2 molecules may be sterically limited
and may help explain why reaction 17 is relatively slow. In
work to be published,24 we investigated several potential
chemical ionization schemes for CH3ONO2 with ab initio G2-
level electronic structure and thermodynamics calculations and
found the following reaction to be exothermic.
CH3ONO2 + F- f CH3ONO2‚F-
(18)
Upon experimental testing of the above ionization scheme, it
was found that reaction 18 provided sufficient sensitivity for
the branching ratio studies. The electronic structure calculations
indicate that the fluoride atom bonds with one of the methyl
hydrogen atoms. Again, depending on the amount of the water
in the flow system, CH3ONO2 was detected as CH3ONO2‚
F-(H2O)n, where n was typically 2 or 3.
Figure 2. Pseudo-first-order CH3O2 decay curves for the CH3O2 +
NO reaction at 200 Torr, 295 K, and 960 cm s-1 velocity (NO
concentrations in molecule cm-3).
Results and Discussion
Detection Sensitivity. In earlier kinetics work with TF-
CIMS methodology, it was reported that various negative ion
chemical ionization detection schemes resulted in sensitivities
of 100, 200, and 1000 ppt (at 100 Torr) for NO2, HO2, and
OH, respectively.14 As part of the branching ratio measurements,
we carried out formal calibrations of the mass spectrometer for
CH3O2, CH2O, and CH3ONO2. It was apparent that these species
(detected in both negative and positive ion mode) could be
detected with similar sensitivity to that obtained for the species
listed above, thus demonstrating the generality of the CIMS
technique. The mass spectrometer signals for these compounds
were found to be linear over the range of concentrations used
in this work. The stated sensitivity was more than adequate for
the present work; actually, it was necessary to degrade the
sensitivity of the spectrometer (by decreasing the ion-molecule
reaction time) to allow the introduction of greater amounts of
reactants. This was necessary in order to study the relatively
slow CH3O2 + NO reaction without inducing complications
from secondary ion-molecule processes.
Rate Constant Determination. Bimolecular rate constants
were obtained via the usual pseudo-first-order approximation
method, using NO as the excess reagent. Typical CH3O2 decay
curves as a function of injector distance are shown in Figure 2.
The first-order rate constants obtained from fitting the CH3O2
decay curves were plotted against NO in order to determine
the bimolecular rate constant, as shown in Figure 3. This
approach for determining bimolecular rate constants assumes
that deviations from the plug flow approximation (molecular
velocities are equal to the bulk flow velocity) are negligible.
Under the conditions present in our turbulent flow tube
(Reynolds number > 2000), Seeley et al. estimated that these
deviations result in apparent rate constants which are at most
8% below the actual values.25 Hence, the flow corrections were
neglected as they are smaller than the sum of the other likely
systematic errors in the measurements of gas flows, temperature,
detector signal, and pressure. Considering such sources of error,
we estimate that rate constants can be determined with an
accuracy of (30% (2σ).
Figure 3. Determination of bimolecular rate constant for the CH3O2
+ NO reaction from data in Figure 2.
TABLE 1: CH3O2 + NO Temperature and Pressure
Dependence Data
temperature pressure
velocity
Reynolds
number
k4a (10-12 cm3
(K)
(Torr)
(cm s-1
)
molecule-1 s-1
)
295
295
295
295
273
263
248
243
238
233
228
223
218
218
213
213
208
203
100
100
100
200
200
100
100
100
200
100
100
100
100
100
100
200
100
100
1240
1240
1200
960
2200
2200
2300
3700
3700
2300
2800
2200
4100
3400
2400
3200
2800
2400
2780
4100
3255
3444
6.5 ( 1.9
8.5 ( 1.3
8.3 ( 1.6
7.1 ( 1.1
7.3 ( 0.9
8.2 ( 2.6
12.2 ( 3.5
9.2 ( 3.0
8.5 ( 2.5
10.5 ( 3.6
15.4 ( 4.7
11.4 ( 3.8
13.5 ( 6.2
15.8 ( 5.5
18.1 ( 4.0
15.6 ( 4.5
17.6 ( 5.9
18.3 ( 4.0
930
1060
1240
1020
800
1120
1000
1100
900
1000
950
810
925
890
a Errors are 2σ.
of experimental conditions and measured rate constants) and
arrived at the mean value of k ) (7.8 ( 2.2) × 10-12 cm3
molecule-1 s-1; the uncertainty represents the two standard
deviation statistical error in the data and is not an estimate of
systematic errors. Table 2 contains a comparison of all reported
We performed three separate determinations of the rate
constant at 100 Torr and 295 K (see Table 1 for a complete list

4382 J. Phys. Chem. A, Vol. 103, No. 22, 1999
Scholtens et al.
TABLE 2: Selected Pressure- and Temperature-Dependent
Results from Kinetics Studies of the CH3O2 + NO Reaction
temperature
(K)
pressure
(Torr)
k4 (10-12 cm3
study
molecule-1 s-1
)
Simonaitis and
Heicklen7
296
218
298
298
218
295
218
∼100
∼200
100
2-5
2-5
100
7.7
13.5
7.5
Ravishankara et al.6
Villalta et al.11
7.5
10.0a
7.8
this work
100
14.4a
a Values calculated from fitted Arrhenius expressions.
Figure 5. Arrhenius plot of all temperature-dependent data for the
CH3O2 + NO reaction: (O) this work; (0) ref 11; (4) ref 7; (3) ref 6.
ature dependence (exp[(600 ( 140)/T]) for the rate constant
than in the Villalta et al. analysis (exp[(285 ( 60)/T]). In Figure
5 we present Arrhenius plots for all four temperature-dependent
studies. It is apparent that all four studies are in good agreement
at temperatures above 250 K, but our work and that of
Simonaitis and Heicklen indicate a faster rate constant at
temperatures below 250 K.
Pressure Dependence. Because of the disagreement between
the low-pressure and high-pressure studies of the low-temper-
ature rate constant for the CH3O2 + NO reaction, it is possible
that the rate constant has a greater pressure-dependent compo-
nent at low temperatures. Due to the negative Arrhenius behavior
of the overall rate constant, it is reasonable to assume that the
reaction proceeds through an intermediate in the following
fashion:
Figure 4. Arrhenius plot of the temperature dependence of the rate
constant for the CH3O2 + NO reaction determined in this work: (b)
100 Torr; (O) 200 Torr.
rate constants for the CH3O2 + NO reaction near room
temperature and at 218 K for the studies in which the
temperature dependence was investigated. Our room-temperature
rate constant is in excellent agreement with previous studies
and thus with the JPL recommended value for atmospheric
modeling.17
[CH OONO]*
CH3O2 + NO T
f CH3O + NO2 (19)
3
V[M]
CH3ONO2
(20)
Temperature Dependence of the Rate Constant. We
performed several measurements at temperatures between 295
and 203 K in order to establish the temperature dependence of
the rate constant for conditions relevant to the upper troposphere
(and lower stratosphere). The rate constant approximately
doubled as the temperature was lowered over this range. From
The possibility of a increased pressure-dependent component
to the rate constant is reasonable given that the lower temper-
atures should increase the lifetime of the [CH3OONO]* colli-
sional intermediate complex. It is then possible that an increase
in total pressure from 3 to 100 Torr would increase the rate of
collisional stabilization of the energized intermediate by the bath
gas and thus increase the lifetime of the [CH3OONO]* complex
further. This, in turn, would lead to a higher proportion of [CH3-
OONO]* collisional pairs going on to form products (either
CH3O + NO2 or CH3ONO2) and thus result in a higher
measured rate constant. To explore this potential effect, we
investigated the rate constant at an increased pressure of 200
Torr at temperatures of 295, 273, 238, and 213 K. We did not
observe any significant difference in the temperature dependence
of the rate constant compared to our studies at 100 Torr, so the
200 Torr data was included in the Arrhenius analysis described
above. However, since our work covers a relatively narrow
pressure range, these results are not conclusive concerning the
possibility of a pressure-dependent component to the rate
constant. Therefore, it is important to emphasize that the low-
pressure results of Villalta et al., the high-pressure results of
Simonaitis and Heicklen, and those presented here may be
reconciled by the existence of a pressure-dependent component
to the rate constant which is greater at lower temperatures.
CH3ONO2 Branching Ratio Determination. We performed
several measurements aimed at establishing a value for the CH3-
the data listed in Table 1 and plotted in Figure 4, we obtain the
+6.0
-3.9
Arrhenius expression k(T) ) (9.2
× 10-13) exp[(600 (
140)/T] cm3 molecule-1 s-1. There have been three previous
studies of the temperature dependence of this reaction. Two
relatively high-pressure (∼ 100 Torr) studies by Simonaitis and
Heicklen7 and Ravishankara et al.6 at a few selected temperatures
and a thorough low-pressure (∼3 Torr) study by Villalta et al.11
(which allowed a similar Arrhenius analysis to that described
above for our study at 100 Torr) have been performed for this
reaction. In Table 2, the rate constants determined from the
previous temperature-dependent work and this work are com-
pared at a temperature of 218 K (which is one of the
temperatures included in the Simonaitis and Heicklen work; the
rate constants at 218 K for the Villalta et al. study and this
work are calculated from the appropriate Arrhenius expressions).
It is apparent that the rate constant determined in this study at
218 K is more consistent with the Simonaitis and Heicklen value
than with the Villalta et al. value. It is important to note that
the Villalta et al. 218 K rate constant is within the 2σ error
limit of our calculated 218 K rate constant. However, our
analysis does indicate a statistically significant larger temper-

Kinetics of the CH3O2 + NO Reaction
J. Phys. Chem. A, Vol. 103, No. 22, 1999 4383
TABLE 3: Chemical Reactions Used in Kinetics Modeling
for Branching Ratio Determination
overall rate constant indicate the possibility of a low-temperature
pressure-dependent channel and CH3ONO2 production would
be expected to be maximized under these conditions, we
attempted to investigate the temperature dependence of the
branching ratio. However, reduced CIMS sensitivity for CH3-
ONO2 at the lower temperatures precluded the determination
of a comparably good upper limit as that obtained at 295 K.
k (cm3 molecule-1 s-1
)
reaction
at 295 K, 100 Torra
CH3O2 + NO f CH3O + NO2
CH3O2 + CH3O2 f 2CH3O + O2
CH3O2 + CH3O2 f CH2O + CH3OH + O2
CH3O2 + NO2 f CH3O2NO2
CH3O + NO f CH3ONO
CH3O + NO2 f CH3ONO2
CH3O + O2 f CH2O + HO2
HO2 + NO f OH + NO2
OH + NO f HONO
7.0 × 10-12
1.4 × 10-13
2.8 × 10-13
1.8 × 10-12
1.3 × 10-11
1.2 × 10-11
1.8 × 10-15
8.2 × 10-12
1.8 × 10-12
1.1 × 10-10
1.9 × 10-12
3.5 × 10-12
1.0 × 10-11
Conclusions
The results presented here extend the measurements of the
overall rate constant for CH3O2 + NO reaction to the temper-
ature and pressure conditions representative of the upper
troposphere and lower stratosphere. Although our value for the
room temperature rate constant agrees well with the current
recommendation for stratospheric modeling, our values for the
rate constant at the lowest temperatures attained in this study
(203 K) are about 50% higher than the same recommendation,
which was largely determined from low-pressure measurements
(∼3 Torr). However, our low-temperature results are in agree-
ment with previous studies at similar pressure (∼100 Torr),
suggesting the possibility of an increased pressure-dependent
component to the rate constant at low temperature. No CH3-
ONO2 product was detected (using CH3ONO2 specific detection
methods for the first time) from the CH3O2 + NO reaction, but
an improved upper limit of 0.03 (at 295 K and 100 torr) for
this branching channel [k4b/(k4a + k4b)] was determined.
OH + HO2 f H2O + O2
OH + OH f H2O2
OH + NO2 f HNO3
OH + CH2O f H2O + HCO
a From ref 17.
TABLE 4: Branching Ratio Measurement Conditions and
Analysis
parameter
value
pressure (Torr)
temperature (K)
100
295
20
initial observation time (ms)
final observation time (ms)
initial [CH3O2] (molecule cm-3
47
)
1.43 × 1012
1.75 × 1013
5.0 × 1017
2.8 × 109
initial [NO] (molecule cm-3
)
initial [O2] (molecule cm-3
)
[CH3ONO2] (molecule cm-3) resulting from
CH3O + NO2 reaction
Acknowledgment. This research was funded by grants from
the Camille and Henry Dreyfus Foundation, American Chemical
Society-Petroleum Research Fund, Research Corporation, the
Michigan Space Grant Consortium, and the National Science
Foundation.
minimum detectable [CH3ONO2] (molecule cm-3
)
3.3 × 109
upper limit for k4b (cm3 molecule-1 s-1) determined
2.0 × 10-13
from kinetics conditions and kinetic modeling
TABLE 5: Results from Direct Branching Ratio Studies of
the CH3O2 + NO Reaction
References and Notes
product temperature pressure
k4b/
(1) Bertman, S. B.; Roberts, J. M.; Parrish, D. D.; Buhr, M. P.; Goldan,
P. D.; Kuster, W. C.; Fehsenfeld, F. C.; Montzka, S. A.; Westberg, H. J.
Geophys. Res. 1995, 100, 22805.
study
monitored
(K)
(Torr) (k4a + k4b)
Ravishankara et al.6 NO2
298
298
295
40
8
100
<0.24
<0.20
<0.03
(2) Flocke, F.; Atlas, E.; Madronich, S.; Schauffler, S. M.; Aikin, K.;
Margitan, J. J.; Bui, T. P. Geophys. Res. Lett. 1998, 25, 1891.
(3) Wennberg, P. O.; Hanisco, T. F.; Jaegle´, L.; Jacob, D. J.; Hintsa,
E. J.; Lanzendorf, E. J.; Anderson, J. G.; Gao, R.; Keim, E. R.; Donnelly,
S. G.; Negro, L. A.; Fahey, D. W.; McKeen, S. A.; Salawitch, R. J.; Webster,
C. R.; May, R. D.; Herman, R. L.; Proffitt, M. H.; Margitan, J. J.; Atlas, E.
L.; Schauffler, S. M.; Flocke, F.; McElroy, C. T.; Bui, T. P. Science 1998,
279, 49.
(4) Cox, R. A.; Tyndall, G. S. Chem. Phys. Lett. 1979, 65, 357.
(5) Sander, S. P.; Watson, R. T. J. Phys. Chem. 1980, 84, 1664.
(6) Ravishankara, A. R.; Eisele, F. L.; Kreutter, N. M.; Wine, P. H. J.
Chem. Phys. 1981, 74, 2267.
(7) Simonaitis, R.; Heicklen, J. J. Phys. Chem. 1981, 85, 2946.
(8) Plumb, I. C.; Ryan, K. R.; Steven, J. R.; Mulcahy, M. F. R. J.
Phys. Chem. 1981, 85, 3136.
(9) Zellner, R.; Fritz, B.; Lorenz, K. J. Atmos. Chem. 1986, 4, 241.
(10) Sehested, J.; Nielson, O. J.; Wallington, T. J. Chem. Phys. Lett.
1993, 213, 457.
(11) Villalta, P. W.; Huey, L. G.; Howard, C. J. J. Phys. Chem. 1996,
100, 5808.
(12) Carter, W. P. L.; Atkinson, R. J. Atmos. Chem. 1989, 8, 165.
(13) Seeley, J. V.; Jayne, J. T.; Molina, M. J. Int. J. Chem. Kinet. 1993,
25, 571.
(14) Seeley, J. V.; Meads, R. F.; Elrod, M. J.; Molina, M. J. J. Phys.
Chem. 1996, 100, 4026.
(15) Elrod, M. J.; Meads, R. F.; Lipson, J. B.; Seeley, J. V.; Molina, M.
J. J. Phys. Chem. 1996, 100, 5808.
Zellner et al.9
this work
CH3O
CH3ONO2
ONO2 branching ratio [k4b/(k4a + k4b)]. These measurements
were performed with much higher sensitivity than previous
efforts because the CH3ONO2 product was directly monitored
by the CIMS method described above. Table 3 lists the chemical
reactions and relevant rate constants used in the kinetics
modeling to determine the experimental upper limit for the
CH3ONO2 branching ratio. Table 4 lists a typical set of
experimental conditions, maximum predicted [CH3ONO2] pro-
duced from the side reaction CH3O + NO2 (reaction 13), and
the overall CH3ONO2 detection sensitivity for the branching
ratio measurement (defined as a signal/noise ratio of 2:1). As
can be seen from this table for these experimental conditions,
the amount of CH3ONO2 produced from reaction 13 is roughly
equal to the detection level for CH3ONO2. Thus, the ultimate
lower limit on the value for the branching ratio for reaction 4b
is constrained by both the secondary chemistry from reaction
13 and the sensitivity level of the analytical method used here.
None of our branching ratio experiments indicated the produc-
tion of CH3ONO2 from the CH3O2 + NO reaction. However,
we were able to determine a much improved upper limit for
k4b/(k4a + k4b) of 0.03 at 295 K and 100 Torr. Table 5 compares
(16) Lipson, J. B.; Elrod, M. J.; Beiderhase, T.; Molina, L. T.; Molina,
M. J. Faraday Trans. 1997, 93, 2665.
(17) DeMore, W. B.; Sander, S. P.; Howard, C. J.; Ravishankara, A.
R.; Golden, D. M.; Kolb, C. E.; Hampson, R. F.; Kurylo, M. J.; Molina,
M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric
Modeling; JPL Publication 97-4; Jet Propulsion Laboratory: Pasadena,
California, 1997.
(18) Chemistry Webbook (http://webbook.nist.gov/chemistry); National
Institute of Standards and Technology: Gaithersberg, Maryland, 1999.
(19) Cheung, Y.-S.; Li, W.-K. Chem. Phys. Lett. 1994, 223, 383.
the previously indirectly determined upper limits for k4b/(k4a
+
k4b) with the value determined here. It is clear that our value
for k4b/(k4a + k4b) places a much tighter constraint on the amount
of CH3ONO2 that can be formed from the CH3O2 + NO
reaction. Because our temperature-dependent results for the

4384 J. Phys. Chem. A, Vol. 103, No. 22, 1999
Scholtens et al.
(20) Messer, B. M.; Stielstra, D. E.; Cappa, C. D.; Scholtens, K. W.;
Elrod, M. J. In preparation.
(21) Adams, N. G.; Smith, D.; Grief, D. Int. J. Mass Spectrom. Ion Phys.
1978, 26, 405.
(23) Lee, T. J.; Rice, J. E. J. Am. Chem. Soc. 1992, 114, 8247.
(24) Messer, B. M.; Elrod, M. J. In preparation.
(25) Seeley, J. V. Experimental Studies of Gas-Phase Radical Reactions
Using the Turbulent Flow Tube Technique. Ph.D. Thesis, Massachusetts
Institute of Technology, 1994.
(22) Sunderlin, L. S.; Squires, R. R. Chem. Phys. Lett. 1993, 212, 307.