Temperature and pressure dependence of the rate constant for the HO2 + NO reaction

Article abstract of DOI:10.1021/jp952553f

The rate constant for the reaction HO₂ + NO → OH + NO₂ has been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. At room temperature, th

Full text of DOI:10.1021/jp952553f

4
026
J. Phys. Chem. 1996, 100, 4026-4031
Temperature and Pressure Dependence of the Rate Constant for the HO2 + NO Reaction
†
John V. Seeley, Roger F. Meads, Matthew J. Elrod, and Mario J. Molina*
Department of Earth, Atmospheric and Planetary Sciences, and Department of Chemistry,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
X
ReceiVed: August 30, 1995; In Final Form: December 4, 1995
The rate constant for the reaction HO
2
2
+ NO f OH + NO has been measured using the turbulent flow
technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products.
At room temperature, the rate constant was found to be independent of pressure between 70 and 190 Torr of
-
12
3
-1 -1
N
2
and given by (8.0 ( 0.5) × 10 cm molecule s . The temperature dependence of the rate constant
was investigated between 206 and 295 K; the Arrhenius expression over this temperature range is (3.0 ( 0.4)
-
12
3
-1 -1
×
10 exp[(290 ( 30)/T] cm molecule s . The uncertainties of the room temperature rate constant and
the Arrhenius parameters represent the precision of the data and are reported at the 1 standard deviation
level. We estimate that possible systematic errors limit the accuracy of the determined rate constants to
(
15%. Comparison of our data with the results of previous studies indicates that the rate constant for the
HO
2
+ NO f OH + NO
2
reaction does not depend on the inert gas pressure for 295 K g T g 206 K.
Introduction
to temperatures roughly above 240 K. As a result, the pressure
dependence of the rate constant for reaction 1 remained
uncertain.
Catalytic cycles determine to a large extent the concentration
of ozone in the stratosphere. The cycle involving HOx radicals
is the dominant removal mechanism of ozone in the mid-latitude
In this article we describe our investigations of the kinetics
1
,2
of reaction 1 over a broad range of temperatures and pressures
using a turbulent flow tube coupled to a high-pressure chemical
ionization mass spectrometer. We have previously shown that
the turbulent flow tube technique can be used to accurately
determine the rate constants of reactions at pressures ranging
stratosphere. The effectiveness of this cycle is moderated by
the reaction:
HO + NO f OH + NO
(1)
2
2
6
,7
The importance of this reaction for stratospheric chemistry
became apparent with the pioneering studies of Howard and
from 50 to 760 Torr and at temperatures as low as 180 K. In
order to study reaction 1 we have developed a new detection
system involving high-pressure chemical ionization mass spec-
trometry, which can be used to monitor HO , OH, and NO
3
Evenson, who carried out the first direct measurements of its
rate constant obtaining a value more than an order of magnitude
larger than the value previously derived from indirect studies.
Reaction 1 also plays a significant role in the troposphere, where
it leads to ozone production when combined with the reactions:
2
2
with high sensitivity.
Experimental Section
NO + hν f NO + O
(2)
(3)
2
General Overview of the Experimental Apparatus. A
schematic of the experimental apparatus is shown in Figure 1.
The flow tube was constructed with 2.2 cm id Pyrex tubing
and was 120 cm in total length. A large flow of nitrogen carrier
O + O + M f O + M
2
3
Previous experimental studies have shown that reaction 1 has
a rate constant that increases with decreasing temperature. This
type of behavior in bimolecular reactions often indicates a
complex mode reaction: a reaction involving multiple transition
states and the formation of bound intermediates. Complex mode
reactions can have rate constants which depend upon total
pressure if the lifetime of the intermediate complex is suf-
ficiently long to suffer collisions with the bath gas before going
on to form the final products. Examples of complex-mode
reactions with pressure dependencies include OH + CO, HO2
-1
gas (approximately 50 L(STP) min ) was injected at the rear
of the flow tube. The gases necessary to generate HO2 were
also introduced at the rear of the flow tube. Nitric oxide was
injected through a movable inlet constructed from 5 mm od
stainless steel tubing. The portion of the inlet outside of the
flow tube was encased in collapsible Teflon tubing. This
provision was taken so the inlet could be moved to various axial
positions without breaking any vacuum seals. A fan-shaped
Teflon piece (a “turbulizer”), designed to enhance turbulent
mixing, was fixed to the end of the movable inlet. The ion
source was positioned 15 cm from the end of the flow tube. A
quadrupole mass spectrometer (Balzers QMG 400) was located
at the end of the flow tube. The flowing gases were removed
with a rotary vane vacuum pump (Edwards E2M 80, 1600 L
+
HO2, and OH + HNO3. The intermediate complex of
reaction 1 would most likely be peroxynitrous acid, HOONO.
This species has been observed in matrix isolation studies;
however, it has never been successfully detected in the gas
4
5
phase.
-1
min ). All gas flows were monitored with calibrated mass
All previous direct determinations of the rate constant of
reaction 1 have been carried out in conventional discharge flow
tubes which are limited to pressures below 15 Torr and usually
flow meters (Tylan). The pressure was determined immediately
upstream of the ion source using a 0-1000 Torr capacitance
manometer (MKS Baratron). The temperature was determined
using copper-constantan thermocouples located at the rear of
the flow tube, at the end of the movable inlet, and immediately
upstream of the ion source.
†
Current address: Ionospheric Effects Division, PL/GPID, Phillips
Laboratory, 29 Randolph Rd., Hanscom AFB, MA 01731-3010.
X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4026$12.00/0 © 1996 American Chemical Society

Rate Constant for the HO2 + NO Reaction
J. Phys. Chem., Vol. 100, No. 10, 1996 4027
with NO2 to determine their concentration after injection into
the flow tube, as follows: the O2 was replaced with a known
1
1
quantity of NO2 (Matheson, 99.5%), normally near 3 × 10
-3
molecules cm . The mass spectrometer signal from NO2 was
then monitored with the microwave discharge turned off and
then on. The decrease in the NO2 signal that was observed
was attributed to
H + NO f NO + OH
(5)
2
Our calculations show that reaction 5 should proceed to greater
than 98% completion before reaching the ion source. Using
this method we found that on average 15% of the H2 was
dissociated into H atoms. Under normal operating conditions
the initial concentration of hydrogen atoms in the flow tube
1
1
-3
was approximately 1 × 10 molecules cm .
Purified NO/N2 mixtures were prepared by the following
process: First, NO (Matheson CP grade) was condensed in a
cold finger at 77 K. The resulting crystals were evacuated to
remove volatile impurities (i.e., O2, N2, etc.). After 5 min of
pumping, the cold finger was isolated from the pump and
warmed to 143 K (pentane slush) which vaporized the NO while
leaving other oxides of nitrogen still in the condensed phase.
The desired amount of NO was then mixed with N2 (Airco
Grade 5) and stored in a stainless steel cylinder. Before being
injected into the flow tube, the NO mixture was further diluted
with a 200 mL(STP) min- flow of N2 and then passed through
a silica gel trap at 195 K (ethanol slush). The concentration of
NO in the flow tube was determined from calibrated mass flow
1
1
2
12
meter readings and ranged from 1 × 10 to 15 × 10 molecules
-3
cm . In all cases [NO] . [HO2], thus ensuring pseudo-first-
order conditions.
Detection of Reactants and Products. HO2, OH, and NO2
were chemically ionized with selected reagent ions and then
detected with the mass spectrometer. The reagent ions were
produced in the ion source by passing approximately 10 ppm
of ion precursor (NF3 or SF6) over the corona discharge. The
ion precursor was carried by a 10 L(STP) min- flow of N2.
The corona discharge took place at the end of a steel needle
that was maintained at an electrical potential of -4000 V using
a Bertan high-voltage power supply. The discharge current was
approximately 30 µA. An electrically grounded 6 mm od
stainless steel tube served as the counter electrode. The needle
was separated from the stainless steel tube with a 3 mm od
glass tube. After passage over the needle, the ion source gas
was directed through a 5 cm long Pyrex tube and then into the
main flow tube. A turbulizer was placed at the end of the small
Pyrex tube to help the reagent ions mix with the main flow
tube gas. The end of the ion injector tube was 7 cm from the
front aperture of the mass spectrometer.
Figure 1. Schematic of the experimental apparatus.
All of the experiments were performed in turbulent flow
conditions. Turbulent flow is established when the Reynolds
number, Re, is greater than 2000. This number is given by
1
2
a uj F
Re )
µ
where a is the internal radius of the flow tube, uj is the average
velocity of the gas, F is the density of the gas, and µ is the
viscosity coefficient of the gas.
Generation of Reactants. HO2 was produced in the rear
portion of the flow tube via
H + O + M f HO + M
(4)
2
2
Hydrogen atoms were generated in the following manner: A 2
L(STP) min- flow of He (Airco Grade 5) was combined with
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 -130 V. The ions were focused by three lenses
constructed from 3.8 cm id, 4.8 cm od copper gaskets. The
front vacuum chamber was pumped by a 6 in. diffusion pump
(Varian VHS-6, 2400 L s- ) backed by a mechanical pump
1
-1
a 0.2 mL(STP) min flow of a 1% H2/He mixture (Air Products
UHP). The resulting mixture was passed through a silica gel
trap immersed in liquid nitrogen and then through a microwave
discharge produced by a Beenakker cavity operating at 70 W.
The microwave discharge dissociated some of the H2 molecules
into hydrogen atoms. The H atoms were then injected into the
flow tube through a side-arm inlet located near the rear of the
flow tube. Oxygen (Matheson UHP) was injected into the flow
tube 3 cm downstream of the H-atom injection point. The
1
-
1
(Varian SD-700, 765 L min ). When the flow tube was at a
pressure of 100 Torr, the front chamber pressure was roughly
-6
5 × 10 Torr. The sampled gases entered the rear vacuum
chamber through a skimmer cone with a 1.2 mm orifice (Beam
Dynamics Model 2). The distance between the front aperture
and the skimmer cone was approximately 5 cm. The skimmer
cone was held at a potential of -20 V. Once the ions passed
through the skimmer cone, they were mass filtered and detected
with a Balzers QMG 400 quadrupole mass spectrometer. The
15
density of O2 in the flow tube was approximately 10 molecules
-3
cm . The end of the movable inlet was always further than
0 cm downstream of the O2 injection point. At the pressures
4
and velocities used in this study, we calculate that the H + O2
reaction went to greater than 99% completion before reaching
the tip of the movable inlet. H atoms were periodically titrated

4
028 J. Phys. Chem., Vol. 100, No. 10, 1996
Seeley et al.
rear vacuum chamber was pumped by a turbomolecular pump
-
1
(
(
Edwards EXT 501, 500 L s ) backed by a mechanical pump
-
1
Edwards E2M 12, 289 L min ). When the flow tube was at
a pressure of 100 Torr, the rear chamber pressure was roughly
-7
5
× 10 Torr.
Two reagent ions were used in this study: F and SF6-.
Fluorine anions were produce by passing trace amounts of NF3
Air Products, electronic grade) over the corona. A typical den-
-
(
1
3
sity of NF3 injected into the flow tube was 1 × 10 molecules
-3
-
cm . The signal at 19 amu (F ) was approximately 50 000
-
1
-
counts s . Smaller quantities of fluorine hydrates (F ‚H2O,
-
-
and F ‚2H2O) were also produced. SF6 ions were generated
by passing trace amounts of SF6 (Matheson, CP Grade) over
the corona discharge. A typical density of SF6 injected into
1
2
-3
the flow tube was 5 × 10 molecule cm . The signal at 146
-
-1
amu (SF6 ) was approximately 300 000 counts s .
-
HO2 was detected as O2 by the proton transfer reaction
-
-
F + HO f HF + O
(6)
2
2
Reaction 6 has not yet been directly studied; however, the
-
1 8
reaction is exothermic by approximately 20 kcal mol . NO2
-
was detected as NO2 using the charge transfer reaction with
-
SF6 :
SF6- + NO f SF + NO
2
-
(7)
2
6
9
The rate constant of reaction 7 has been measured by Streit to
-
10
3
-1 -1
be 1.3 × 10 cm molecule s . OH was detected through
a two-step chemical ionization scheme. The first step is charge
transfer with SF6 :
-
SF6- + OH f SF + OH-
(8)
6
The rate constant of reaction 8 has been estimated by Lovejoy
1
0
-9
3
-1 -1
et al. to be 2 × 10 cm molecule s . The second step
is:
-
-
OH + CO + M f HCO + M
(9)
2
3
1
1
Reaction 9 has been reported by Fehsenfeld and Ferguson to
-
28
6
-2 -1
have a rate constant of 8.6 × 10
cm molecule s . We
-
found that the HCO3 signal was proportional to [OH]. A
maximum signal to noise ratio was obtained when approximately
1
4
-3
2
× 10 molecules cm of CO2 (Matheson, 99.8%) was
injected into the flow tube at a position immediately upstream
of the ion source. We found that using the first step alone yields
-
an OH signal which is nonlinear with [OH], most likely due to
secondary reactions.
Temperature Control. The temperature of the reaction
region (the area between the end of the movable inlet and the
ion source) was controlled to within 1 K. The walls of the
flow tube were cooled to the desired temperature by flowing a
temperature-controlled liquid (either ethanol or HCFC-123)
through the jacket on the exterior of the flow tube. The nitrogen
carrier gas was precooled to the desired temperature by passage
through a copper coil immersed in the temperature-controlled
liquid before injection into the flow tube.
Figure 2. (a) Signal intensity at 46 amu (NO -
concentration. Data points were collected under the following flow tube
2
) as a function of NO
2
conditions: P ) 100 Torr; T ) 292 K; average velocity ) 1300 cm
-
1
-
2
s
; Re ) 3000. (b) Signal intensity at 32 amu (O
) as a function of
Results and Discussion
HO concentration. Data points were collected under the following flow
2
tube conditions: P ) 96 Torr; T ) 292 K; average velocity ) 1430
Assessment of Detector Sensitivity. Trace quantities of
NO2, HO2, and OH were individually injected into the flow tube
in order to calibrate the mass spectrometer signals. Dilute
mixtures of NO2 were injected at the rear of the flow tube
through the port normally used for O2 injection; a plot of the
-1
-
3
cm s ; Re ) 2600. (c) Signal intensity at 61 amu (HCO ) as a function
of OH concentration. Data points were collected under the following
flow tube conditions: P ) 96 Torr; T ) 292 K; average velocity )
1430 cm s- ; Re ) 2600.
1
-
NO2 signal vs [NO2] is shown in Figure 2a. At a flow tube
of NO2 yield a signal to noise ratio equal to 1. This corresponds
to 0.1 ppb detection limit (at 100 Torr).
8
-3
pressure of 100 Torr, we estimate that 5 × 10 molecules cm

Rate Constant for the HO2 + NO Reaction
J. Phys. Chem., Vol. 100, No. 10, 1996 4029
The concentration of HO2 was estimated as described in the
experimental section; it was assumed that the microwave
discharge dissociated 15% of the H2 into H atoms, which were
then converted into HO2 via reaction 4. A plot of the O2- signal
vs [HO2] is shown in Figure 2b. We found that such plots were
11
-3
linear when [HO2] was less than 3 × 10 molecules cm . The
positive intercept is due to HO2 produced by residual H2 in the
silica gel trap. The magnitude of this intercept was found to
slowly decrease after the H2 was shut off. At a flow tube
9
pressure of 100 Torr, we estimate that [HO2] ) 1 × 10
molecules cm- produces a signal to noise ratio equal to 1. As
3
the flow tube pressure was raised, the sensitivity decreased due
-
to increased clustering of F with water. At pressures above
2
00 Torr we estimate that the sensitivity was reduced by
approximately a factor of 3.
OH was produced by reaction 5. Hydrogen atoms were
injected at the rear of the flow tube, and NO2 was injected
through the O2 port. The NO2 concentration in the flow tube
Figure 3. Typical set of O -
position. This data set was obtained under the following conditions: P
signal values as a function of injector
2
11
-3
was approximately 5 × 10 molecules cm . The concentration
of OH was estimated by assuming that the microwave discharge
dissociated 15% of the H2 into H atoms and that all of the H
atoms were converted into OH by reaction 5. A plot of the
-1
)
149 Torr; T ) 292 K; average velocity ) 1235 cm s ; Re ) 3550.
-
HCO3 signal vs [OH] is shown in Figure 2c. The positive
intercept is due to OH produced by residual H2 in the silica gel
trap. At a flow tube pressure of 100 Torr, we estimate that
9
-3
[
OH] ) 5 × 10 molecules cm produces a signal to noise
ratio equal to 1.
For the cases of NO2 and HO2, the chemical ionization
technique provides greater sensitivity than other flow tube
detection techniques commonly employed in the past. For
example, electron impact mass spectrometry for NO2 is fre-
12
quently reported to have a sensitivity on the order of 100 ppb,
which is roughly 3 orders of magnitude less sensitive than the
chemical ionization scheme employed in this work. The most
successful flow tube detection technique for HO2 is laser
13
magnetic resonance (LMR); Howard reports a detection limit
8
-3
of 4 × 10 molecules cm of HO2 at pressures near 1 Torr of
He with this technique. However, LMR sensitivity is ap-
proximately inversely proportional to the square of the total
pressure due to line broadening.14 Thus, at pressures high
enough for turbulent flow (P > 50 Torr), LMR would be at
least an order of magnitude less sensitive than the chemical
ionization detection technique. For the case of OH, our
chemical ionization detection scheme is not as sensitive as the
commonly used laser induced fluorescence (LIF) detection
I
Figure 4. Typical plot k as a function of [NO]. This data set was
obtained under the following conditions: P ) 149 Torr; T ) 292 K;
-1
average velocity ) 1235 cm s ; Re ) 3550.
shows a portion of a typical data set. Such plots were essentially
linear for all of the experiments, indicating that secondary
reactions occurred to a minimal extent. The k values were then
plotted vs [NO], and the data points were fitted with a linear
least squares routine, the slope of the fit provided the bimo-
lecular rate constant, k. Figure 4 shows a typical plot of k vs
I
15
technique; e.g., Abbatt et al. report LIF detection limits of 2
I
8
-3
×
10 molecules cm for OH in 100 Torr of N2, which is at
[
NO]. We estimate that rate constants can be determined with
least 1 order of magnitude more sensitive than our chemical
ionization detection technique. However, it should be noted
that chemical ionization mass spectrometry has been used to
an accuracy of (15%, considering possible systematic errors
in the measurement of gas flows, temperature, detector signal,
and pressure.
The approach for determining rate constants described above
assumes that deviations from plug flow are negligible. The
1
6
detect OH in the atmosphere at sub-parts per trillion levels;
thus, it is likely that an improved chemical ionization scheme
will enable much greater sensitivity for OH in laboratory flow
tube studies.
validity of this assumption was examined with two-dimensional
numerical flow tube model.
17
For the flow simulation we
Determination of Rate Constants. The rate constant of
reaction 1 was determined as follows: first, the NO mixture
was injected at a predetermined rate, and [NO] was calculated
by assuming complete mixing in the region downstream of the
assumed that the detection technique samples the entire cross
section of the flow tube. This is a reasonable assumption
because the flow is thoroughly mixed by the ion source
turbulizer immediately before sampling. We also assumed that
-
-4
movable inlet. The O2 signal intensity was then recorded as
the reaction probability of HO on the walls, γ, is 10 , on the
2
a function of the movable injector position. The logarithm of
the measured signal intensity was then plotted as a function of
injector-detector distance. The data points were fitted with a
linear least squares routine. The value of the pseudo-first-order
rate constant, k , was determined by multiplying the resulting
slope by the average gas velocity. This process was repeated
for approximately 10 different concentrations of NO; Figure 3
basis of experimental results for the loss of HO on the walls
2
3
,18
of similar flow tubes.
Using these assumptions, the model
predicts that the plug flow approximation yields rate constants
which are 3% lower than the actual rate constant. This deviation
is small when compared with our estimated uncertainty of (15%
due to errors in the measurement of gas flows, temperature,
detector signal, and pressure. The deviation from plug flow
I

4
030 J. Phys. Chem., Vol. 100, No. 10, 1996
Seeley et al.
TABLE 1: Summary of Rate Constants for Reaction 1
Obtained at Temperatures around 295 K
TABLE 2: Summary of Experimental Determinations of the
Rate Constant of Reaction 1
T (K) P (Torr) velocity (cm s-1)
Re
k ( σ (10-12 cm3s-1
)
k ( σ (10-12 cm3 s-1
)
T (K)
pressure range
ref
2
2
2
2
2
2
2
2
2
92
94
94
95
93
92
92
95
91
70
82
94
2397
1717
1404
1605
1375
1164
1235
1035
910
3231
2660
2530
2960
2800
3030
3550
3440
3520
8.92 ( 0.42
7.89 ( 0.27
7.33 ( 0.24
8.39 ( 0.28
8.21 ( 0.39
7.92 ( 0.18
8.40 ( 0.31
7.64 ( 0.28
7.41 ( 0.35
8.1 ( 0.8
8 ( 1
7.9 ( 0.5
8.0 ( 0.5
9.0 ( 1.4
11 ( 2
296
298
298
296
298
297
293
298
297
294
1.1-1.7 Torr of He
1.0 Torr of He
1.0 Torr of He
0.9 Torr of He
2 Torr of He
0.75-1.2 Torr of He
2-12 Torr of He
2 Torr of Ar
3
20
21
13
12
22
23
24
25
97
107
135
149
175
199
7.6 ( 0.9
6.9 ( 0.3
8.5 ( 0.7
0.8-13 Torr of N
70-199 Torr of N
2
8
.0 ( 0.5
2
this work
Figure 5. Plot of the HO + NO rate constant as a function of flow
2
tube pressure. Each point was obtained at a temperature near 293 K.
The solid line represents the average of the previously reported values,
Figure 6. Signal intensity as a function of injector position. The curve
passing through the HO and NO data represents the values calculated
2
2
-
12
3
-1 -1
8
.3 × 10 cm molecule
s
(see Table 2).
for a first-order decay of reactants and a first-order appearance of
products, respectively. The curve passing through the OH data is based
on the combined effect of reaction 1 and reaction 10 on the OH
concentration. The data points were obtained under the following flow
tube conditions: P ) 108 Torr; T ) 294 K; average velocity ) 1420
increases as the walls become more reactive: the rate constants
determined using the plug flow approximation are predicted by
the model to be accurate to within 2% in the absence of wall
reactions (γ ) 0) and to within 8% assuming that HO2 molecules
are removed with every wall collision (γ ) 1).19
The rate constant for reaction 1 was determined at flow tube
pressures ranging from 70 to 190 Torr. Table 1 contains a
summary of the rate constants determined near room temper-
ature; these results are also shown in Figure 5. As can be seen
in the figure, the rate constant is independent of pressure to
within experimental error. The average value of the rate
-1
cm s ; Re ) 2930.
TABLE 3: Summary of Rate Constants for Reaction 1
Obtained in the Temperature Range 295 K > T > 200 K
velocity (cm s-1
k ( σ (10-12 cm3s-1
T (K) P (Torr)
)
Re
)
2
2
2
269
260
253
93
80
80
107
97
92
98
97
96
85
94
93
95
91
95
93
93
99
94
1375
1695
1570
1690
1460
1505
1410
1490
1405
1423
1375
1445
1450
1280
1750
1275
2800
3400
3040
3720
3370
3610
3040
3760
3685
3890
3870
4260
4300
3980
5950
4300
8.21 ( 0.78
9.08 ( 0.80
7.71 ( 0.44
8.84 ( 0.52
9.44 ( 0.36
8.69 ( 0.46
9.86 ( 0.57
9.53 ( 0.21
11.4 ( 0.5
10.2 ( 0.5
10.7 ( 0.4
11.7 ( 0.5
10.5 ( 0.5
12.2 ( 0.8
11.8 ( 0.5
12.0 ( 0.5
-
12
3
-1
constant near 294 K is k ) (8.0 ( 0.5) × 10 cm molecule
-1
s ; the uncertainty represents the scatter in the data at the 1
standard deviation level and is not an estimate of systematic
errors. If a linear fit is made to the data, the following
252
243
235
-12
expression is k ) [(8.7 ( 0.5) - (0.006 ( 0.005)P] × 10
234
224
3
-1 -1
cm molecule s , where P is the flow tube pressure in Torr.
The average value of the rate constant determined in this work
is in excellent agreement with the results of previous low-
pressure flow tube studies. A summary of the previous
measurements of the rate constant near room temperature is
listed in Table 2.
2
2
2
2
2
24
20
15
11
06
effect of reaction 1 and reaction 10 on the OH concentration
In several cases the products of reaction 1 were also
monitored. Figure 6 shows a plot of the HO2, NO2, and OH
signals as a function of injector position. The curve passing
through the HO2 and NO2 data represents the values calculated
for a first-order decay of reactants and a first-order appearance
of products, respectively. Although OH is a product of reaction
-
31
3
-1 -1 26
assuming k10 ) 7.0 × 10
cm molecule s .
We also measured the rate constant for reaction 1 at
temperatures between 206 and 293 K. A summary of these
results is given in Table 3 and in Figure 7. The majority of the
rate constants were determined at a pressure of approximately
9
5 Torr. The rate constant was observed to increase by
1
, the OH signal slowly decays as the injector is pulled back
approximately 50% as the temperature was lowered from 300
to 200 K. From the data listed in Table 3, we obtain the
due to the secondary reaction with NO:
-
12
OH + NO + M f HONO + M
(10)
Arrhenius expression k(T) ) (3.0 ( 0.4) × 10
exp[(290 (
3
-1 -1
3
0)/T] cm molecule s ; the uncertainty is given at the 1
standard deviation level. Howard has performed the only other
13
The curve passing through the OH data is based on the combined

Rate Constant for the HO2 + NO Reaction
J. Phys. Chem., Vol. 100, No. 10, 1996 4031
cessfully coupled to turbulent flow tube systems for chemical
kinetic studies at relatively high pressures and low temperatures,
that is, under conditions outside the operating range of the well-
established conventional fast flow technique.
Acknowledgment. The research described in this article was
supported by a grant from the NASA Upper Atmospheric
Research Program to the Massachusetts Institute of Technology.
References and Notes
(1) Wennberg, P. O.; Cohen, R. C.; Stimpfle, R. M.; Koplow, J. P.;
Anderson, J. G.; Salawitch, R. D.; Fahey, D. W.; Woodbridge, E. L.; Keim,
E. R.; Gao, R. S.; Webster, C. R.; May, R. D.; Toohey, D. W.; Avallone,
L. M.; Proffitt, M. H.; Lowenstein, M.; Podolske, J. R.; Chan, K. R.; Wofsy,
S. C. Science 1994, 266, 398.
(2) Cohen, R. C.; Wennberg, P. O.; Stimpfle, R. M.; Koplow, J. P.;
Anderson, J. G.; Fahey, D. W.; Woodbridge, E. L.; Keim, E. R.; Gao, R.
S.; Proffitt, M. H.; Lowenstein, M.; Chan, K. R. Geophys. Res. Lett. 1994,
2
1, 2539.
(3) Howard, C. J.; Evenson, K. M. Geophys. Res. Lett. 1977, 4, 437.
Figure 7. Arrhenius plot for the reaction HO
2
+ NO. The open circles
(4) Cheng, B. M.; Lee, J. W.; Lee, Y. P. J. Phys. Chem. 1991, 95,
are experimental data from this work, and the line is the resulting
2814.
(5) Burkholder, J. B.; Hammer, P. D.; Howard, C. J. J. Phys. Chem.
987, 91, 2136.
6) Seeley, J. V.; Jayne, J. T.; Molina, M. J. Int. J. Chem. Kinet. 1993,
5, 571.
(7) Seeley, J. V.; Jayne, J. T.; Molina, M. J. Submitted for publication.
(8) Harrison, A. G. Chemical Ionization Mass Spectrometry, 2nd ed.,
CRC Press: Boca Raton, FL, 1992.
9) Streit, G. E. J. Chem. Phys. 1982, 77, 826.
10) Lovejoy, E. R.; Murrells, T. P.; Ravishankara, A. R.; Howard, C.
J. J. Phys. Chem. 1990, 94, 2386.
1
3
Arrhenius fit. The data of Howard are represented by filled circles.
1
(
thorough temperature dependence study for T < 300 K; a
comparison of his results with the results of this study is also
shown in Figure 7. Given the close agreement between
Howard’s low pressure results and our higher pressure values,
we conclude that the rate constant does not have a significant
pressure dependence for temperatures between 200 and 300 K.
If Howard’s data set is combined with our data set, an Arrhenius
2
(
(
(
(
(
11) Fehsenfeld, F. C.; Ferguson, E. E. J. Chem. Phys. 1974, 61, 3181.
12) Leu, M. T. J. Chem. Phys. 1979, 70, 1662.
13) Howard, C. J. J. Chem. Phys. 1979, 71, 2352.
(14) Jemi-Alade, A. A.; Thrush, B. A. J. Chem. Soc., Faraday Trans.
990, 86, 3355.
15) Abbatt, J. P. D.; Demerjian, K. L.; Anderson, J. G. J. Phys. Chem.
990, 94, 4566.
16) Eisele, F. L.; Tanner, D. J. J. Geophys. Res. 1991, 96, 9295.
(17) (a) Seeley, J. V.; Molina, M. J. Manuscript in preparation for
submission to Int. J. Chem. Kinet. (b) Seeley, J. V. Experimental Studies
of Gas Phase Radical Reactions Using the Turbulent Flow Tube Technique,
Ph.D. Thesis, MIT, 1994.
-
12
3
expression of (2.67 ( 0.31) × 10
exp[(316 ( 30)/T] cm
molecule-
1
s
-1
is obtained.
1
1
The negative temperature dependency for the rate constant
for reaction 1 indicates that the reaction proceeds through the
formation of an intermediate; the absence of a measurable
pressure dependency points out that such an intermediate is too
short-lived to be affected by collisions with the carrier gas. The
magnitude of a possible pressure effect is expected to increase
as the temperature is lowered; however, we see no evidence
for such an effect down to 200 K. It is also possible that species
such as water vapor or oxygen give rise to a pressure effect
which is larger than that due to nitrogen, as is the case, for
example, with the HO2 self-reaction.26 Additional studies of
such pressure effects would be useful.
(
(
(18) Chang, J. S.; Kaufmann, F. J. Phys. Chem. 1978, 82, 1683.
19) Previously, we have reported that in turbulent flow conditions the
(
plug flow approximation yields rate constants which are approximately 12%
6,7,17
less than the actual value.
In these cases the greater predicted deviation
was a result of using a detection technique (resonance fluorescence) which
preferentially samples the faster moving central portion of the flow tube
cross section.
(
20) Margitan, J. J.; Anderson, J. G. Results presented at 13th Informal
Conference on Photochemistry, Clearwater Beach, FL, 1978.
(21) Kaufman, F.; Reimann, B. Results presented at 13th Informal
Conference on Photochemistry, Clearwater Beach, FL, 1978.
Conclusions
Our data indicate that reaction 1 does not have a substantial
pressure dependence at temperatures between 200 and 300 K.
Our results are in excellent agreement with previous results
obtained in low-pressure discharge flow systems. This work
also shows that key radical species, such as HO2 and OH, can
be monitored using chemical ionization mass spectrometry with
greater sensitivity than that provided by conventional electron
impact mass spectrometers; work in progress in our laboratory
also indicates that many stable species can be monitored as well
with superb sensitivity. Furthermore, our results show that high-
pressure chemical ionization mass spectrometry can be suc-
(
22) Glaschick-Schimpf, I.; Leiss, A.; Monkhouse, P. B.; Schurath, U.;
Becker, K. H.; Fink, E. H. Chem. Phys. Lett. 1979, 67, 318.
23) Hack, W.; Preuss, W.; Temps, F.; Wagner, H. G. G. Int. J. Chem.
Kinet. 1980, 12, 850.
24) Thrush, B. A.; Wilkinson, J. P. T. Chem. Phys. Lett. 1981, 81, 1.
(
(
(25) Jemi-Alade, A. A.; Thrush, B. A. J. Chem. Soc., Faraday Trans.
990, 86, 3355.
1
(26) 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 94-26; Jet Propulsion Laboratory: Pasadena,
CA, 1994.
JP952553F