Reaction of OH with HO2NO2 (Peroxynitric Acid): Rate Coefficients between 218 and 335 K and Product Yields at 298 K

Article abstract of DOI:10.1021/jp0363489

HO₂NO₂ (peroxynitric acid, PNA) has an important role in determining the ozone abundance and its changes over time in the lower stratosphere. Rate coefficients (k₃(T)) for the reaction of OH with PNA in the gas phase were

Full text of DOI:10.1021/jp0363489

J. Phys. Chem. A 2004, 108, 1139-1149
1139
Reaction of OH with HO2NO2 (Peroxynitric Acid): Rate Coefficients between 218 and
335 K and Product Yields at 298 K
Elena Jime´nez,†,‡,§ Tomasz Gierczak,†,‡, Harald Stark,†,‡ James B. Burkholder,† and
A. R. Ravishankara*,†,#
Aeronomy Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway,
Boulder, Colorado 80305-3328, and CooperatiVe Institute for Research in EnVironmental Sciences,
UniVersity of Colorado, Boulder, Colorado 80309
ReceiVed: August 7, 2003; In Final Form: NoVember 19, 2003
Rate coefficients (k3(T)) for the reaction of OH with HO2NO2 (peroxynitric acid, PNA) in the gas phase were
measured in the temperature range of 218-335 K by producing OH via pulsed laser photolysis and detecting
it via laser-induced fluorescence. The PNA concentration was measured in situ by UV and IR absorption.
The H2O2, HNO3, and NO2 impurities present in the PNA sample were quantified by mass spectrometry
and/or UV/IR absorption. The measured value of k3(298 K) is (3.4 ( 1.0) × 10-12 cm3 molecule-1 s-1. The
temperature dependence of k3 is best described by the relation k3(T) ) (8.8 ( 2.6) × 10-19T2 exp[(1130 (
20)/T] cm3 molecule-1 s-1. The quoted errors for k3 are at the 2σ level and include estimated systematic
errors, which contribute the most to this uncertainty. The measured values of k3(T) were independent of
pressure between 10 and 100 Torr of helium. The branching ratios of the reaction OH + HO2NO2 f products,
for the production of HO2 and HNO3 and of NO3 and H2O2, respectively, were determined to be <10% and
<5%, respectively, at 298 K. Thus, it was deduced that the main pathway for reaction 3 produces H2O, O2,
and NO2 at 298 K. Our measurements reduce the uncertainties but do not significantly alter the currently
calculated impacts of HO2NO2 in the upper troposphere and lower stratosphere. In the course of this study,
the rate coefficient for the reaction of OH with H2O2 was measured to be k4(T) ) (2.9 ( 1.8) × 10-12
exp[-(110 ( 150)/T] cm3 molecule-1 s-1 in the temperature range of 273-356 K.
1. Introduction
where M represents a third body. The formation reaction has
been extensively studied over the past 20 years, and the rate
Peroxynitric acid (HO2NO2, or PNA) has many roles in the
Earth’s upper troposphere and lower stratosphere (UTLS). First,
it is a significant reservoir in the UTLS for both the HOx (OH
and HO2) and NOx (NO and NO2) species.1-3 Second, its
formation couples the HOx and NOx families. Third, the reaction
of OH with PNA is a significant sink for HOx in the lower
stratosphere (LS), unless HO2 is a significant product of this
reaction. For these reasons, HO2NO2 has an important role in
determining the ozone abundance and its changes over time in
the LS. Sequestering NOx in HO2NO2 provides a mechanism
for its redistribution in the upper troposphere (UT). Therefore,
PNA also has a role in the formation of ozone in the UT.
Understanding the formation and loss processes of PNA under
atmospheric conditions is critical in an evaluation of its role in
ozone changes in the UTLS. PNA is formed in the atmosphere
by the gas-phase association of HO2 with NO2:
coefficient for this reaction is reasonably well-established.4 PNA
is removed from the atmosphere by thermal decomposition, the
reverse of reaction 1 (reaction (-1)),5-7 photodissociation,8-11
and its reaction with OH.12-14
HO2NO2 + hν f products
OH + HO2NO2 f products
(2)
(3)
Heterogeneous removal of PNA on or in atmospheric aerosols
may also have a role. Each of these loss processes may
contribute significantly to the atmospheric loss of PNA but may
have differing impacts in the UTLS, in terms of the partitioning
and removal of HOx and NOx. Thermal decomposition and near-
infrared (NIR) photodissociation via overtone absorption11
contribute to HOx production at low light levels and will affect
the partitioning of HOx and NOx. However, these processes do
not remove OH, HOx, and NOx from the atmosphere. Reactive
HOx and NOx species may be lost from the atmosphere via
heterogeneous loss as well as via the reaction of OH with
HO2NO2 and via UV photolysis, depending on the products of
these two processes.
The rate coefficient for reaction 3 (k3) has been previously
measured in the laboratory by both relative14,15 and absolute
kinetic methods.12,13 Fourier transform infrared (FTIR) spec-
troscopy,12,15 FTIR and gas chromatography,14 and modulated
molecular beam mass spectrometry13 were used in those
experiments to measure the concentrations of PNA and the
reactive impurities present in the PNA sample. The room-
HO2 + NO2 + M T HO2NO2 + M
(1,-1)
* Author to whom correspondence should be addressed. E-mail: ravi@
al.noaa.gov.
† National Oceanic and Atmospheric Administration.
‡ University of Colorado.
§ Current address: Departamento de Qu´ımica F´ısica. Facultad de Ciencias
Qu´ımicas. Universidad de Castilla-La Mancha, Camilo Jose´ Cela, 10, 13071
Ciudad Real, Spain.
Permanent address: Department of Chemistry, Warsaw University, ul.
Zwirki i Wigury 101, 02-089, Warsaw, Poland.
# Also associated with the Department of Chemistry and Biochemistry,
University of Colorado, Boulder, CO 80309.
10.1021/jp0363489 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/24/2004

1140 J. Phys. Chem. A, Vol. 108, No. 7, 2004
Jime´nez et al.
TABLE 1: Absorption Cross Sections and OH Quantum
Yieldsa
λ (nm)
σλ (10-20 cm2)
φOH
HO2NO2
193
248
900
44
0.26b
0.08b
H2O2
193
248
58.9
9
1.5
2
HNO3
193
248
1160
2
0.33
1
Figure 1. Schematic of the experimental apparatus used to measure
rate coefficients for reaction 3. FTIR denotes the Fourier transform
infrared spectrometer, MS represents the electron-impact ionization
mass spectrometer, and PMT is the photomultiplier tube.
a Values taken from Sander et al.4 and references therein, unless noted
otherwise. b Value taken from Jimenez et al.31
OH + H2O2 f HO2 + H2O
OH + HNO3 f H2O + NO3
OH + NO2 + M f HNO3 + M
(4)
(5)
(6)
temperature values k3 reported in these studies range from
4.0 × 10-12 cm3 molecule-1 s-1 to 5.5 × 10-12 cm3 molecule-1
s-1. Such differences are not surprising, given the difficulties
in determining the PNA concentration and accounting for the
influence of reactive impurities. The temperature dependencies
for k3 reported by Barnes et al.,14 Trevor et al.,13 and Smith et
al.12 differ significantly. Barnes et al.14 and Trevor et al.13
reported k3 to be essentially independent of temperature over
the temperature ranges of 268-295 and 246-324 K, respec-
tively. Smith et al.12 reported a negative temperature dependence
(k3(T) ) 5.9 × 10-13 exp(650/T) cm3 molecule-1 s-1) over the
temperature range of 240-330 K. Given the aforementioned
discrepancies in the reported k3 values, further kinetic studies
under temperatures similar to those found in the UTLS are
warranted.
In this work, absolute rate coefficients for the title reaction
were measured at various temperatures by producing OH via
pulsed laser photolysis (PLP) and detecting OH via laser-induced
fluorescence (LIF). Emphasis was placed on quantifying the
concentration of PNA and the impurities in PNA (H2O2, HNO3,
and NO2) that react with OH. We also report the first laboratory
determination of the product branching ratios for reaction 3:
Whenever possible, multiple methods were used to determine
the concentrations of PNA, H2O2, HNO3, and NO2 in the gas
mixture. The PNA concentration was determined using two
independent optical methods: (i) UV absorption using a diode
array spectrometer and (ii) IR absorption using an FTIR
spectrometer. H2O2, which is the reactive impurity in the PNA
samples that contributed most to OH removal (other than PNA),
was quantitatively measured using mass spectrometry (MS) and/
or FTIR. HNO3 was monitored via FTIR and by MS. NO2 was
monitored via UV absorption. Some details of the experimental
hardware and methods are described below.
The reaction vessel has been described in detail previously.16
It consisted of a 15-cm-long Pyrex jacketed cell with an internal
volume of ∼200 cm3. The reactor was heated or cooled by
circulating a fluid from a thermostated bath through its jacket.
Orthogonal ports on the reactor were used to introduce the laser
beams (collinear photolysis and probe beams), mount the
photomultiplier detector, and allow sample gas flow through
the reactor (see Figure 1).
OH + HO2NO2 f H2O + O2 + NO2
f HO2 + HONO2
(3a)
(3b)
(3c)
Hydroxyl (OH) radicals were produced by excimer laser
photolysis of a mixture of PNA and its impurities in the bath
gas at 193 nm (ArF) or 248 nm (KrF). Photolysis of PNA, H2O2,
and HNO3 at these wavelengths produced OH. Therefore, the
addition of an OH radical precursor was unnecessary. Table 1
summarizes the quantum yields for OH production in the
photolysis of PNA, H2O2, and HNO3 at 193 and 248 nm, as
well as the cross sections of these molecules at these wave-
lengths. HNO3 was the dominant OH radical precursor while
using 193-nm photolysis. In the 248-nm photolysis experiments,
the specific contributions of PNA, H2O2, and HNO3 to the OH
radical production were dependent on the PNA source and the
experimental conditions. The photolysis laser fluence, which
was measured with a power meter at the exit of the LIF cell,
was varied between 0.08 and 2.5 mJ cm-2 pulse-1. The initial
concentration of OH radicals, [OH]0, as estimated from the
measured photolyte concentration, laser fluence, absorption cross
f H2O2 + NO3
2. Experimental Section
Measurements of the kinetics and products of reaction 3
involved recording OH temporal profiles using LIF, determining
the concentrations of PNA (along with those of the reactive
impurities HNO3, H2O2, and NO2), and monitoring the NO3
concentration produced in reaction 3 via cavity ring down
spectroscopy (CRDS). For ease of presentation, they are
described separately in the three sections below.
2.1. Measurement of OH Temporal Profiles. Kinetics of
OH removal was measured under pseudo-first-order conditions
in the OH concentration between 218 and 335 K in 10-100
Torr of helium. The apparatus used for these measurements is
shown schematically in Figure 1 and includes several modifica-
tions to that used recently in our laboratory.16 The modifications
enabled us to measure the concentration of the relatively unstable
PNA in situ and quantify the concentrations of H2O2, HNO3,
and NO2 that are unavoidably present in the PNA samples.
These three molecules react with OH and, thus, contribute to
the measured OH temporal profiles.
sections, and the quantum yields, was kept below 5 × 1011 cm-3
,
to minimize unwanted complications from secondary radical-
radical reactions.
OH radicals were excited at ∼282 nm (A2Σ+,ν′ ) 1 r
X2Π,ν′′ ) 0) from the doubled output of a pulsed Nd:YAG-
pumped dye-laser. The fluorescence signal was detected by a
photomultiplier tube (PMT) oriented perpendicular to the

Reaction of OH with HO2NO2
J. Phys. Chem. A, Vol. 108, No. 7, 2004 1141
excitation beam. A band-pass filter (309-nm peak transmission;
band pass of 20 nm fwhm (full width at half maximum))
mounted in front of the PMT was used to isolate the OH
fluorescence from two transitions (A2Σ+,ν′ ) 1 f X2Π,ν′′ ) 1
and A2Σ+,ν′ ) 0 f X2Π,ν′′ ) 0). The OH temporal profiles
were measured by varying the delay time between the photolysis
and the probe laser pulses.
Rate coefficients for reaction 3 (k3(T)) were measured under
pseudo-first-order conditions in OH radical concentration
([reactant] > 1000 × [OH]0). The OH temporal profile followed
a simple exponential rate law:
H2O2 and HNO3 flowing through the cell were independently
measured via MS and IR absorption measurements, as described
below. The concentration of NO2 was determined by the
measured absorptions between 320 and 450 nm where PNA,
HNO3, and H2O2 do not absorb. The contributions of H2O2,
HNO3, and NO2 to the measured UV absorption signal were
subtracted to determine the UV absorption due to PNA.
Absorption cross sections reported in the literature for PNA,8
H2O2,20 and HNO3 were used in the spectral analysis. The
21
reference spectrum for NO2 in the UV region was recorded
under the same experimental conditions (i.e., pressure and
temperature) and approximately the same NO2 concentrations
observed in the PNA samples in the reactor. Under these
conditions, there were no significant contributions to the
measured absorption by N2O4. As shown in the Results and
Discussion section, the typical contribution of H2O2 and HNO3
to the total absorption signal at 250 nm was typically <10%
and <4%, respectively. This value was 1%-50% for H2O2 and
1%-10% for HNO3.
[OH]t ) [OH]0 exp(-k′t)
(I)
where k′ ) k3[PNA] + k4[H2O2] + k5[HNO3] + k6[NO2] + k7
and k7 is the first-order rate coefficient for the loss of OH in
the absence of PNA, H2O2, HNO3, and NO2.
OH f loss
(7)
The UV absorption cross sections of PNA8 and H2O2,20 which
are the two major contributors to the measured absorption, are
relatively insensitive to temperature in the wavelength range
of 240-300 nm. Therefore, the two 50-cm-long absorption cells
attached to either end of the reactor were at room temperature
while the temperature of the reactor was varied during the kinetic
measurements. The differences in number density in the reactor
and absorption cells due to differences in the temperature were
taken into consideration in calculating concentrations of PNA
and the other species.
The loss of OH in the absence of reactants is attributed to
diffusion out of the detection zone and to reaction with
impurities in the bath gas. The value of k′ was measured at
various concentrations of PNA at each temperature and pressure.
Values of k3 were obtained from the slopes of plots of
k′ - (k4[H2O2] + k5[HNO3] + k6[NO2] + k7) vs [PNA].
Whenever possible, k7 was derived by measuring the first-order
rate coefficient for OH loss as a function of H2O2 under the
same experimental conditions (temperature, pressure, and flow
rates) just prior to the measurement of k3; k7 was the intercept
in plots of the measured first-order rate coefficients for OH loss
vs [H2O2]. Otherwise, k′ - (k4[H2O2] + k5[HNO3] + k6[NO2])
was plotted against [PNA] to obtain k3 as the slope and k7 as
the intercept. The concentrations of PNA, H2O2, HNO3, and
NO2 needed in the aforementioned analysis were measured using
the methods described below, and the values of the k5 and k6
were taken from Brown et al.17 and Wine et al.,18 respectively.
Because reaction 4 contributes the most to the measured value
of k′, k4 was measured during the course of this study under
the same conditions of pressure, temperature, and flow rate.
2.2. Measurement of PNA, HNO3, H2O2, and NO2 Con-
centrations. The concentration of PNA was measured via UV
absorption using two 50-cm-long Pyrex cells coupled to two
ends of the reaction cell, as shown in Figure 1. The windows
on the Pyrex cells were flushed with a small amount of carrier
gas, to limit contact with PNA, HNO3, and H2O2. UV absorption
was measured using a 30-W D2 lamp light source and a 1024-
element diode array detector.19 The spectrograph was set to
cover 200-450 nm, with a resolution of ∼1.5 nm fwhm. The
effective UV absorption path length under gas flow conditions
of the kinetics studies was determined to be 114 ( 1 cm by
flowing a known concentration of O3 and measuring its
absorption; this was essentially the same as the geometrical path
length.
The NO2 concentration in the gas mixture flowing through
the reactor was derived from the UV absorption measurements.
The UV absorption cross sections of NO2 are resolution
dependent. Therefore, UV cross sections of NO2 were measured
under the conditions used for kinetic study by determining [NO2]
via IR absorption measurements and recording the UV spectra.
The obtained value of the absorption cross section at 414 nm
was (6.2 ( 0.3) × 10-19 cm2 molecule-1. Considering the
differences in spectral resolution, it agrees with our previously
measured value of 7.8 × 10-19 cm2 molecule-1 22
.
The NO2
concentration, as measured by UV or IR absorption in the
reaction cell during the rate coefficient measurements, was in
the range of 2 × 1013-5 × 1014 molecules cm-3. The NO2
concentration was nominally ∼5% of the PNA concentration.
2.2.2. Mass Spectrometric Measurements: A low-resolution
mass spectrometer was used to monitor the H2O2 and HNO3
concentrations at the output of the UV absorption cell; this
instrument is described in our paper on UV absorption cross
sections of PNA.8 Parent ion signals were used to quantify the
concentrations of both H2O2 (m/z ) 34) and HNO3 (m/z ) 63).
The mass spectrometer sampled the gas flow through a 40-µm
pinhole (total reactor pressure of <20 Torr) or through a 15-
µm pinhole (total reactor pressure of >20 Torr). Absolute
calibrations of the MS signals for H2O2 and HNO3 concentra-
tions were made, relative to UV absorption measurements, just
prior to the kinetic measurements. Calibrations were therefore
performed under the identical flow and pressure conditions used
in the kinetic measurements. The MS signals varied linearly
with H2O2 and HNO3 concentrations over the range observed
in the kinetic measurements (8 × 1013-1 × 1015 molecule cm-3
for H2O2 and 7 × 1013-8 × 1014 molecule cm-3 for HNO3;
the HNO3 concentrations were 5-10 times higher when PNA
was generated using the H2O2/HNO3 source).
2.2.1. UV Absorption Measurements: The absorbance by a
PNA sample at various wavelengths (AT(λ)), as recorded by
the diode array spectrometer, is the sum of the absorptions from
PNA, H2O2, HNO3, and NO2:
AT(λ) ) APNA(λ) + AH O (λ) + AHNO (λ) + ANO (λ) (II)
2
2
3
2
The UV absorption spectra of PNA, H2O2, and HNO3 are
unstructured and overlap with each other. Therefore, the PNA
concentration was not accurately determined from the UV
absorption signal alone. Consequently, the concentrations of
2.2.3. Infrared Absorption Measurements: IR absorption
spectra were recorded at room temperature using a Fourier

1142 J. Phys. Chem. A, Vol. 108, No. 7, 2004
Jime´nez et al.
transform spectrometer between 500 and 4000 cm-1 at a spectral
resolution of 1 cm-1 by co-adding 100 scans. A 15-cm-long,
1.5-cm-diameter, Pyrex absorption cell equipped with germa-
nium windows was coupled to either the entrance or the exit of
the UV absorption section (Figure 1). A capacitance manometer
that was attached to a side port measured the pressure in the
absorption cell. Reference IR spectra of H2O2, HNO3, and NO2
were measured in separate experiments. IR band intensities used
to quantify the concentrations were taken from the HITRAN
database.23 The IR band intensities for PNA were taken from
Smith and co-workers.12,24 The PNA band intensities will be
discussed further in the Results and Discussion section of this
paper.
2.3. Measurement of NO3 Product in Reaction 3 Using
Cavity Ring Down Spectroscopy (CRDS). 2.3.1. Apparatus
and Methodology. The CRDS apparatus used for the NO3 yield
measurements is similar to one described previously.25 A Nd:
YAG laser pumped dye laser tuned to either 623 or 662 nm
with a pulse duration of 6-8 ns was used to measure ring down
signals. The intensity of light escaping from the rear mirror was
collected by a PMT, digitized in an oscilloscope, and processed
in a computer. The high-reflectivity mirrors used in this work
resulted in empty cavity ring down time constants (τ0) between
50 µs (at 623 nm) and 70 µs (at 662 nm). These τ0 values
correspond to mirror reflectivities of 99.994% and 99.996%,
respectively.
Figure 2. Typical OH temporal profiles recorded at 242 K in the
presence of various concentrations of PNA: (O) 1.4 × 1015 molecule
cm-3, (0) 6.9 ×1014 molecule cm-3, and (4) 2.1 ×1014 molecule cm-3
.
Solid lines are weighted linear least-squares fits of the data to eq I.
by nitration of 8 cm3 of concentrated H2O2 (>95%) at 273 K
with 3 g of NO2BF4 in a glovebag purged with dry N2. In the
second method, PNA was prepared via a dropwise addition of
10 cm3 of concentrated H2O2 to a stirred HNO3 solution
maintained at 0 °C. This second method had a tendency to leave
behind large concentrations of H2O2 and HNO3 and, therefore,
was less suitable. PNA was introduced into the gas flow by
passing a small flow of helium over the PNA solution while
maintaining the reservoir at a temperature of 273 K. In the
kinetic measurements, the sample gas flow was then passed
through a cold trap immersed in a salt/ice bath to reduce the
gas-phase H2O2 and HNO3 concentrations.
Gas-flow rates were measured using calibrated mass flow
transducers. The linear gas-flow velocity in the reaction cell
used for OH kinetics was in the range of 10-76 cm s-1; this
flow rate was adequate to avoid an accumulation of photoprod-
ucts between photolysis laser pulses, which were 100 ms apart.
Pressures were measured using 100 and 1000 Torr capacitance
manometers. The temperature of the gas in the reaction zone
was measured with a calibrated thermocouple inserted directly
in the gas flow. The thermocouple was retracted during kinetic
measurements. The flow rate of gases through the CRDS system
was also sufficiently fast, so that the same gas mixture would
not be exposed to more than one photolysis laser pulse; the
pulse rate of the photolysis laser was 2-5 Hz.
The photolysis laser (KrF excimer laser, at 248 nm) beam
was perpendicular to the ring down beam and illuminated
5 cm in the middle of the 100-cm-long cell. The ring down
beam passed through the middle of this photolysis volume. The
purge volumes efficiently suppressed contamination of the
mirrors from exposure to corrosive gases such as PNA. The
total pressure was varied between 25 and 40 Torr. An FTIR
spectrometer was used to determine the concentrations of PNA,
HNO3, H2O2, and NO2. The NO3 radical was monitored at either
623 or 662 nm, where its absorption cross sections are 1.5 ×
10-17 and 2.2 × 10-17 cm2 molecule-1 26
,
respectively. All
experiments were carried out at room temperature (296 ( 2
K). We calibrated the photon laser fluence in the reactor by
detecting NO3 that resulted from the photolysis of chlorine
nitrate (ClONO2) at 248 nm.
The technique of simultaneous kinetics and ring down (SKaR)
was used in this study. In this technique, the ring down time of
the optical cavity is comparable to the time constant for the
reaction that changes the concentration of the monitored species.
The ring down profiles in the absence and presence (with
varying concentration of the absorber) are measured. The ratio
of these two profiles contains the information on the time
dependence of the absorber concentration. The analytical
expression for the time dependence of the absorber concentra-
tion, together with the measured ratio of the profiles, is used to
derive the rate coefficients and yields as described elsewhere.25
3. Results and Discussion
Examples of OH temporal profiles measured in the rate
coefficient studies of reaction 3 are shown in Figure 2. The
OH loss followed first-order kinetics (i.e., obeyed eq I) over at
least three of its lifetimes. Least-squares fits to the decay profiles
yielded the k′ values. PNA, H2O2, HNO3, and NO2 concentra-
tions that corresponded to the measured k′ value were deter-
mined from the ancillary measurements, as described in the
Experimental Section. An example of the measured UV
spectrum of the reaction mixture flowing through the reactor is
shown in Figure 3. Based on the independently measured
concentrations of the HNO3 and H2O2 in this mixture and the
reference spectra of these two species, their contribution to the
measured spectrum are shown in Figure 3. As shown in this
example, H2O2 and HNO3 make relatively small (<10%)
contributions to the UV absorption in the range of 240-290
nm. Thus, the PNA concentration is accurately determined in
the UV absorption method. Therefore, even if the corrections
2.3.2. Materials. Helium (99.999%), N2 (>99.99%), and
NO2BF4 (> 95%) were used as supplied. Concentrated hydrogen
peroxide (>95% H2O2, as determined by titration with a
standard solution of KMnO4) was prepared by bubbling N2
through a H2O2 sample initially at a concentration of 60% for
several days prior to use. Nitric acid (anhydrous) was synthe-
sized under vacuum by slowly adding concentrated sulfuric acid
to 40 g of solid NaNO3 maintained at 330 K. HNO3 vapor was
collected in a glass bubbler immersed in a liquid N2 cooled
trap. ClONO2 was prepared and used as described previously.27
Peroxynitric acid (PNA) was synthesized using two meth-
ods.28 In the preferred method, PNA was prepared in solution

Reaction of OH with HO2NO2
J. Phys. Chem. A, Vol. 108, No. 7, 2004 1143
Figure 3. UV absorption spectrum of a gas-phase PNA sample flowing
through the pulsed laser photolysis-laser-induced fluorescence (PLP-
LIF) reactor. Spectrum was recorded using a diode array spectrometer
with a path length of 114 cm. Also shown are the contributions of
H2O2, HNO3, and NO2 to this absorption spectrum. The contributions
due to H2O2 and HNO3 were determined from auxiliary measurements
of their concentrations via mass spectrometry and FTIR absorption.
The NO2 contribution was determined from its absorption at wave-
lengths longer than 340 nm and auxiliary FTIR measurements. In this
example, the PNA, H2O2, HNO3, and NO2 concentrations are 1.2 ×1015,
2.6 ×1014, 6.1 ×1014, and 2 ×1014 molecule cm-3, respectively. The
residual upon subtraction of all the contributions is shown at the bottom.
Clearly, the residual is small (<1%) and shows no wavelength-
dependent features.
due to H2O2 and HNO3 are uncertain by 20%-30%, their
contribution to the uncertainty in the PNA concentration is <5%.
Figure 4a shows examples of the measured first-order rate
coefficient (k′) vs [PNA] at 296 and 221 K. Figure 4b shows
plots of k′ - (k4[H2O2] + k5[HNO3] + k6[NO2] + k7) values vs
[PNA] at 298 K. The corrections applied to k′(T), to account
for the loss of OH due to reaction with H2O2, HNO3, and NO2,
were typically 5%-25% of the measured value (see Figure 4a).
The corrections for OH loss due to reaction with H2O2 and
HNO3 were smaller at lower temperatures. The major correction
was due to reaction 4 (OH + H2O2) and ranged from 4% to
∼50%; the correction due to reactions 5 and 6 were typically
<10% and 2%, respectively. Thus, the total correction was
<25% and the uncertainty in the value of k′ - (k4[H2O2] +
k5[HNO3] + k6[NO2]) due to the correction was <10%. Note
that the derived value of k3 was essentially independent of the
magnitude of the correction for reactions 4-6, showing that
the corrections were accurate.
The measured rate coefficient and conditions used in the OH
experiments are summarized in Tables 2 and 3. A plot of k3
(logarithmic scale) vs 1/T is shown in Figure 5. The rate
coefficient data shows a slight detectable systematic deviation
from Arrhenius behavior. However, note that the magnitude of
the deviation lies within the absolute accuracy of the individual
rate coefficient measurements. The kinetic data are well-
reproduced over the entire temperature range using the rate
coefficient expression
Figure 4. Plots of measured pseudo-first-order rate coefficients for
the loss of OH in the presence of PNA (and its associated impurities),
as a function of [PNA]. Panel a shows the data at 298 K (circles) and
221 K (squares). Solid lines are the weighted least-squares fits that
take into consideration the contribution of the impurities, whereas the
dashed lines represent the fits to the uncorrected data. Panel b shows
a plot of the measured values of kcorr′ ) [k′ - (k4[H2O2] + k5[HNO3]
+ k6[NO2] + k7)] at 298 K (see Table 2), as a function of [PNA]. A
weighted least-squares fit yields k3(298 K) ) (3.44 ( 0.12) × 10-12
cm3 molecule-1 s-1
.
of the nonlinear least-squares fit. Inclusion of the estimated
systematic errors at the 2σ level in determining the concentration
of PNA to the first term in the aforementioned expression will
lead to k3(T) ) (8.8 ( 2.6) × 10-19T2 exp[(1130 ( 20)/T] cm3
molecule-1 s-1. An Arrhenius fit, i.e., a fit of ln k3 vs 1/T to a
straight line using a linear least-squares analysis, to k3 values
measured at or below 298 K yields
k3(T) ) (3.18 ( 0.30) × 10-13
×
690 ( 40
exp
cm3 molecule-1 s-1 (IV)
(
)
T
Ea
k (T) ) AT2 exp -
(
)
3
where the quoted errors are 2σ and σA ) AσlnA. This expression
reproduces the data well over this restricted temperature range
(Figure 5). Again, inclusion of the estimated systematic errors
at the 2σ level in determining the concentration of PNA into
the first term in the aforementioned expression will lead to
k3(T) ) (3.2 ( 1.0) × 10-13 exp[(690 ( 40)/T] cm3 molecule-1
RT
) (8.79 ( 0.67) × 10-19T2 ×
1130 ( 20
exp
cm3 molecule-1 s-1 (III)
(
)
T
The quoted uncertainties are the 2σ values from the precision
s-1
.

1144 J. Phys. Chem. A, Vol. 108, No. 7, 2004
Jime´nez et al.
TABLE 2: Summary of the Rate Coefficients for the OH + PNA Reaction at 298 K and Experimental Conditions Used To
Measure Them
gas flow
photolysis
laser fluence
pressure velocitya
(Torr)
[OH]0
[PNA]b
k3(298 K)c
(cm s-1
)
(mJ cm-2 pulse-1
)
(1011 molecule cm-3
)
k7 (s-1
)
(1015 molecule cm-3
)
k′ (s-1
)
(10-12 cm3 molecule-1 s-1
)
10
14
14
14
14
14
14
14
14
14
14
14
15
16
16
16
17
91
100
100
22
29
30
30
30
30
30
30
72
26
72
26
50
51
65
62
53
16
14
36
0.9-2.5
0.4-0.7
0.6-2.1
0.9
0.7-23.0
0.4-1.8
0.1-1.1
0.3-1.8
0.3-1.2
0.5-1.8
0.6-2.0
0.05-1.0
0.03-0.2
0.05-0.5
0.03-0.2
0.05-0.5
0.3-5.4
0.09-1.0
0.5-4.0
2.2-22
0.3-3.9 (8)
0.2-1.8 (8)
0.03-1.9 (11)
0.2-2.6 (8)
0.1-3.2 (9)
0.3-2.6 (9)
0.3-2.6 (9)
0.3-3.9 (9)
0.2-1.5 (7)
0.1-2.1 (7)
0.2-1.5 (7)
0.1-2.1 (7)
0.04-1.7 (10)
0.1-1.4 (9)
0.06-1.7 (7)
0.2-1.5 (9)
0.2-1.3 (5)
0.1-3.4 (7)
0.4-2.5 (8)
0.3-1.6 (5)
2200-14000
600-5700
340-5600
1000-8800
500-9400
1700-7600
1700-7300
2000-11500
1200-5600
700-7000
1200-5600
700-7100
500-6500
700-5000
1100-7400
1400-8000
600-4600
200-13500
1000-7700
500-5000
3.41 ( 0.68
3.16 ( 0.24
3.19 ( 0.30
3.59 ( 0.08
3.17 ( 0.10
3.03 ( 0.08
3.17 ( 0.08
3.05 ( 0.12
3.38 ( 0.12
3.37 ( 0.08
3.29 ( 0.08
3.40 ( 0.06
3.77 ( 0.04
3.70 ( 0.08d
3.82 ( 0.04 e
3.27 ( 0.14f
3.18 ( 0.10
3.65 ( 0.04d
3.24 ( 0.08
3.44 ( 0.16
470 ( 150
150 ( 25
430 ( 55
680 ( 40
630 ( 40
690 ( 50
400 ( 40
280 ( 20
500 ( 20
235 ( 30
200 ( 25
290 ( 20
350 ( 30
330 ( 20
290 ( 10
310 ( 10
310 ( 10
310 ( 10
0.4-0.7
0.4-1.0
0.4-1.0
0.1-0.3
0.1-0.2
0.1-0.2
0.1-0.2
0.1-0.2
2.7-3.1
0.2-0.7
1.7-2.2
0.3
0.7-1.4
0.4-0.5
0.4-0.5
0.4-0.5
0.4-3.6
0.3-2.0
0.3-2.0
0.3-0.9
a Gas flow velocity was calculated for the center of the reaction cell, where the OH temporal profiles were measured. b Number of PNA
concentrations used in k3 determination given in parentheses. c Uncertainties are the 2σ precision of the measurement values. d PNA source is
HNO3/H2O2 instead of NO2BF4/H2O2. e Photolysis wavelength, λ ) 193 nm. f In the presence of 7 × 1015 molecules cm-3 of NO.
the quoted uncertainty is the 2σ precision of the slope of the
k′ vs [PNA] plot. If we include the estimated uncertainty at the
2σ level (see Section 3.1, “Error Analysis”) in the determination
of PNA concentration, we obtain k3(298 K) ) (3.4 ( 1.0) ×
10-12 cm3 molecule-1 s-1
.
The range of temperatures over which we could accurately
measure k3 was limited at high temperature by the thermal
decomposition of PNA (and the consequent uncertainties in the
PNA concentration along the UV absorption cell) and at the
low temperatures by condensation of PNA. We have established
that our measured values of k3 in the temperature range of 218-
335 K were free of problems associated with uncertainties in
PNA concentration and large contributions to the measured k3
value by the presence of H2O2, HNO3, and NO2.
In a few experiments, under some specific conditions, the
OH temporal profile was nonexponential and indicated a
regeneration of OH at longer reaction times. Such OH regenera-
tion was only observed when a freshly prepared PNA sample
was first introduced into the reactor. We suspect that the
regeneration was due to the presence of very small concentra-
tions of NO that reacted with HO2 radicals formed from PNA
photolysis, reaction 4, or reaction 3b. An NO concentration of
∼1 × 1013 molecule cm-3 is needed to explain the observed
OH temporal profiles. Unfortunately, we were unable to detect
such a small [NO] in our system. The addition of large
concentrations of NO did make the OH temporal profiles
exponential after 100 µs. Addition of very small amounts of
NO made the profiles nonexponential. In any case, after flushing
the bath gas through the PNA source for a few minutes, the
OH regeneration was absent and all kinetic data were taken
under such conditions.
3.1. Error Analysis. A major uncertainty in the determination
of k3 is associated with knowing the concentration of PNA in
the reactor. We have used UV and IR absorption measurements
to quantify the PNA concentration in our kinetic measurements.
The UV measurements were performed within the reactor,
whereas the IR measurements were external to the reactor. The
UV and IR measurements yielded PNA concentrations that were
within 10% of each other. Molina and Molina10 and Singer et
Figure 5. Plot of the k3 value (on a logarithmic scale) from this work
versus 1/T. Dotted line is a weighted least-squares fit to the equation
k3(T) ) AT2 exp[-Ea/(RT)] and yields k3(T) ) (8.79 ( 0.67) × 10-19T2
[exp((1130 ( 20)/T)] cm3 molecule-1 s-1. Solid line is a fit to the data
at T e 298 K (shown in the figure over that range) to an Arrhenius
expression, which yields k3(T) ) (3.2 ( 0.3) × 10-13 exp[(690 ( 40)/
T] cm3 molecule-1 s-1. All the quoted errors are 2σ precision obtained
via least-squares analyses.
The experimental conditions were varied during the course
of the kinetic experiments in an effort to identify potential
systematic errors in the rate coefficient determinations. The
majority of the experimental variations were performed at room
temperature (see Table 2). The rate coefficient data was
independent of the following: (i) the linear gas flow velocity
(V ) 10-76 cm s-1) and the location of PNA addition to the
apparatus; (ii) the PNA source, either H2O2 + NO2BF4 or
H2O2 + HNO3; (iii) the photolysis wavelength (193 or 248 nm);
(iv) the initial OH radical concentration; and (v) total pressure
(10-100 Torr of helium). This independence is demonstrated
in the plot of all k′ values measured at room temperature, as a
function of [PNA] in Figure 4b. A linear least-squares fit of
these data (a total of 168 first-order rate coefficients) yields
k3(298 K) ) (3.44 ( 0.12) × 10-12 cm3 molecule-1 s-1, where

Reaction of OH with HO2NO2
J. Phys. Chem. A, Vol. 108, No. 7, 2004 1145
TABLE 3: Summary of the Rate Coefficient for the OH + PNA Reaction, Obtained as a Function of Temperature (T)
gas-flow
photolysis
laser fluence
pressure velocitya
[OH]0
[PNA]b
k3
c
T (K) (Torr) (cm s-1
)
(mJ cm-2 pulse-1
)
(1011 molecule cm-3
)
k7 (s-1
)
(1015 molecule cm-3
)
k′ (s-1
)
(10-12 cm3 molecule-1 s-1
)
218 19
221 20
224 20
224 95
230 20
232 19
232 20
232 96
242 15
249 16
253 15
255 14
266 15
273 17
59
60
64
10
69
55
67
11
38
48
47
27
33
48
0.4-1.4
0.9-0.9
0.08-0.5
0.3-0.3
0.9-1.1
0.2-0.3
0.5-0.7
0.3-0.4
0.1-0.2
1.3-1.8
1.7-1.8
0.4-0.4
0.4-0.4
0.5-0.5
0.02-0.3
0.05-0.5
0.02-0.1
0.1-0.3
0.1-0.7
0.05-0.4
0.05-0.6
0.2-0.5
0.02-0.4
0.8-2.5
0.3-1.5
0.04-0.8
0.04-0.8
0.1-1.0
0.05-0.29 (5)
0.06-0.62 (6)
0.09-0.84 (6)
0.06-0.80 (5)
0.15-0.91 (5)
0.09-1.10 (8)
0.07-0.91 (7)
0.13-1.18 (8)
0.07-1.70 (8)
0.50-1.73 (8)
0.05-0.45 (8)
0.09-1.90 (11)
0.40-2.18 (8)
0.04-0.60 (8)
1300-3400
1100-5600
1000-4000
1000-6000
1200-5600
700-6300
1200-6100
1400-8700
800-9400
2700-8600
500-2400
1000-8400
1600-9500
600-2700
7.82 ( 0.64
6.67 ( 0.21
6.82 ( 0.40
7.59 ( 0.14
6.05 ( 0.21
6.17 ( 0.10
6.49 ( 0.19
6.25 ( 0.11
5.55 ( 0.09
5.16 ( 0.04 d
4.66 ( 0.06e
4.57 ( 0.10
4.35 ( 0.06
4.02 ( 0.06e
3.44 ( 0.12 f
3.26 ( 0.05
2.96 ( 0.06
2.85 ( 0.06e
320 ( 10
440 ( 20
420 ( 20
390 ( 20
298 10-100 14-72
315 13
335 13
335 16
33
44
68
0.5-0.5
0.5-0.6
0.6-0.6
0.06-0.4
0.2-1.5
0.1-1.3
140 ( 20
850 ( 25
490 ( 30
0.07-2.68 (8)
0.23-2.56 (9)
0.31-0.85 (7)
200-10000
1300-8500
1300-3000
a Gas-flow velocity was calculated for the center of the reaction cell, where the OH temporal profiles were measured. b Number of PNA
concentrations used in k3 determination given in parentheses. c Uncertainties are the 2σ precision of the measurement values. d In the presence of
NO added. e PNA source is HNO3/H2O2 instead of NO2BF4/H2O2. f Obtained from the slope of the plot of all k′ values at 298 K versus [PNA] (see
Figure 4b).
TABLE 4: Reaction Mechanism Used in Simulationsa
al.9 estimated the uncertainty in the UV absorption cross section
of PNA at 250 nm to be approximately (10%. (Molina and
rate coefficient
(cm3 molecule-1 s-1
)
Molina10 quoted this to be at the 1σ level; we assume this to be
at the 2σ level for the study by Singer et al.,9 who did not specify
their quoted uncertainty.) Uncertainties in the absorption path
length, total pressure, temperature, and flows are all relatively
small. The estimated uncertainty (approximately (5%) in the
UV spectral analysis, including uncertainties in the H2O2 and
HNO3 UV absorption cross sections, leads to an additional
uncertainty in [PNA] of ∼10%. The calculated most-probable
error in [PNA] that is due to uncertainty in the absorption cross
sections, and accounting for contributions of H2O2 and HNO3
to the measured absorbance, is estimated to be <20% at the 2σ
level.
reaction
Thermal Decomposition
HO2NO2 + M f HO2 + NO2 + M
0.013d
k(298 K)b
k(532 K)c
4498d
Hydroxyl Radical Reactions
280d,e,f
OH f loss
OH + PNA f H2O + O2 + NO2
3.6 × 10-12 e 2.1 × 10-12e
f HO2 + HNO3
f H2O2 + NO3
OH + H2O2 f HO2 + H2O
OH + NO2 + M f HNO3 + M
OH + HNO3 f H2O + NO3
OH + NO + M f HONO + M
OH + HONO f H2O + NO2
2.0 × 10-12 e 2.15 × 10-12 e
2.0 × 10-12
1.3 × 10-13
4.9 × 10-14
5.8 × 10-14
8.6 × 10-12
8.7 × 10-13 e 4.1 × 10-14
4.5 × 10-12
Several experiments were conducted to check the accuracy
of the measured PNA concentration. In several experiments,
the PNA concentration at the entrance and exit of the UV
absorption cell were measured using IR absorption while
measuring the concentration in the cell via UV absorption: they
agreed to within (10%, implying a lack of a gradient in the
PNA concentration along the length of the UV absorption cell.
Therefore, we conclude that the PNA concentration measured
via UV absorption in the reactor is an accurate measure of the
PNA concentration in the region where OH temporal profiles
were measured. The PNA IR band intensities were determined,
relative to the UV absorption measurements. The relative and
absolute band intensities agreed well with those reported by
Smith24 and by Molina and Molina.10 This agreement demon-
strates a self-consistency between the UV and IR measurements
made in these studies.
The PNA concentration was also indirectly measured by
converting PNA to NO2 in the presence of an excess of NO
([NO] ) (1.3-4.0) × 1016 molecule cm-3) at ∼532 K. A
mixture of PNA, whose concentration was measured using UV
absorption, and NO was passed through a heated (∼532 K)
Pyrex tube. The PNA rapidly decomposed via reaction -1 to
produce HO2 and NO2. The HO2 radical formed in the PNA
decomposition was converted to OH and NO2:
HO2 Radical Reactions
HO2 + NO2 + M f HO2NO2 + M
2.5 × 10-13
7.5 × 10-15
5.6 × 10-12
7.1 × 10-13
HO2 + NO f OH + NO2
8.1 × 10-12
HO2 + HO2 + M f H2O2 + O2 + M 1.7 × 10-12
NO3 Radical Reactions
NO3 + NO f 2 NO2
2.6 × 10-11
3.5 × 10-13
3.5 × 10-12
2.1 × 10-11
3.8 × 10-14
3.5 × 10-12
NO3 + NO2 + Mf N2O5 + M
NO3 + HO2 f OH + NO2 + O2
a Unless noted, the rate coefficients are taken from Sander et al.4
b Pressure-dependent rate coefficients calculated for 45 Torr of helium.
c Pressure-dependent rate coefficients calculated for 15 Torr of helium.
d Units for this value are s-1 e Measured in this study. f First-order loss
.
rate coefficients measured in reaction cell via laser-induced fluorescence
(LIF) detection of OH (see text).
which leads to the formation of another NO2 molecule. The
loss of OH via reaction 3a will produce yet another NO2
molecule. Therefore, the loss of one PNA molecule leads to
the formation of three NO2 molecules. However, the efficiency
of conversion of PNA to NO2 will be slightly less than three,
because of secondary chemistry of the OH radical (see the
reaction mechanism listed in Table 4). The measured conversion
efficiency of 2.5 is in good agreement with that expected from
the reaction mechanism and the measured H2O2, HNO3, PNA,
NO, and NO2 concentrations. This agreement adds to our
confidence in the accuracy of the concentration of PNA
measured via UV absorption.
HO2 + NO f OH + NO2
(8)

1146 J. Phys. Chem. A, Vol. 108, No. 7, 2004
Jime´nez et al.
TABLE 5: Summary of the Rate Coefficients for the OH + H2O2 Reactiona
temperature,
number of
experiments
[H2O2]
[OH]0
k4 ( 2σ
b
T (K)
(1014 molecule cm-3
)
(1011 molecule cm-3
)
k4′ (s-1
)
(10-12 cm3 molecule-1 s-1
)
254
266
273
298
315
335
356
10
7
1.2-13.3
0.7-8.3
0.7-5.75
0.8-16.5
2.4-13.3
0.9-25.4
0.5-22.2
0.3-4.7
0.3-3.4
0.3-2.5
0.1-25.0
1.3-6.5
1.5-9.3
0.1-2.2
600-3200
500-1825
550-1550
600-3400
680-3200
550-5800
635-4900
2.16 ( 0.48
1.86 ( 0.10
1.99 ( 0.10
2.00 ( 0.08
2.51 ( 0.10
2.10 ( 0.26
2.21 ( 0.38
6
67c,d
6
11
20
a Pressure range was 13-16 Torr and H2O2 was photolyzed at 248 nm. b The quoted errors are 2σ precision of the linear least-squares fits of
k4′ vs [H2O2] data relative to the expression k4′ ) k4[H2O2] + constant. c In 42 experiments, H2O2 was photolyzed at 193 nm. d One experiment was
performed at 100 Torr.
TABLE 6: Comparison of OH + HO2NO2 Reaction Rate Coefficient Studies
temperature, pressure
k3(298 K)
A
concentration
a
T (K)
(Torr)
(10-12 cm3 molecule-1 s-1
)
(10-12 cm3 molecule-1 s-1
)
Ea/R (K)
see text
techniqueb measurementc
reference
218-335
298
268-295
246-324
240-330
10-100
1
1-300
3-15
760
3.4 ( 1.0
5.0d
5.5 ( 1.4
4.0 ( 1.6
5.2 ( 1.1
4.6
see text
PLP/LIF
UV/MS/FTIR this work
relative rate FTIR
relative rate FTIR/GC
Barnes et al.15
Barnes et al.14
Trevor et al.13
Smith et al.12
Sander et al.4
independent of T
8.05
(0.59 ( 0.04)
(193 ( 194) PLP/RF
MS
FTIR
-(650 ( 30) PLP/RF
1.3
-380
evaluation
a Uncertainties reported by the authors. b PLP/LIF, pulsed laser photolysis/laser-induced fluorescence; PLP/RF, pulsed laser photolysis/resonance
fluorescence. c UV, ultraviolet absorption spectroscopy; MS, mass spectrometry; FTIR, Fourier transform infrared spectroscopy; and GC,
gas chromatography. d Revised in Barnes et al.14 from the originally reported value of (4.1 ( 1.0) × 10-12 cm3 molecule-1 s-1, using
(OH + propene) ) 1.86 × 10-12 cm3 molecule-1 s-1
.
A check of our methods was also accomplished through
measurements of the rate coefficient for the reaction of OH with
H2O2 (reaction 4). The summary of the measured rate coef-
ficients for reaction 4 (k4) and the conditions under which they
were measured are given in Table 5. These rate coefficients
were measured by monitoring the H2O2 concentration after the
mass spectrometer was calibrated prior to most determinations
of k3. We obtained the value of k4 in the temperature range of
254-356 K by fitting the ln k4 vs 1/T data to a line using a
weighted linear-least-squares methodology:
k4(T) ) (2.9 ( 1.8) × 10-12
×
110 ( 150
exp -
cm3 molecule-1 s-1 (V)
(
)
T
Our data yields k4(298 K) ) (2.00 ( 0.15) × 10-12 cm3
molecule-1 s-1. The quoted errors in A and k4(298 K) are at
the 2σ level and include estimated systematic errors. These
values of k4 are in excellent agreement with the value reported
by Vaghjiani et al.29 These measurements were performed in
the same system as that used for the determination of k3 and
using the same methods and the same conditions as those used
to quantify the concentration of H2O2, i.e., MS determination.
Therefore, this agreement further validates our determined
concentrations of H2O2 and, more importantly, establishes the
accuracy of the corrections made to k′ for the contributions due
to reaction 4, which was the largest contributor to the correc-
tions.
As illustrated in Table 2, the precision of the individual k3(298
K) determinations (approximately (5%) is significantly better
than the reproducibility of k3 on different sets of determinations
((15%). This uncertainty, combined with our estimated uncer-
tainty in [PNA] of ∼20%, leads us to estimate the overall
maximum uncertainty in k3(T) to be (30%. We believe that
the precision of our measurements, to be conservative, is best-
represented by the reproducibility of the measured values of
k3. Furthermore, this assignment of the precision accounts for
the variations in the precisions of the corrections and the
determinations of the PNA concentrations.
Figure 6. Comparison of the measured k3 values with those from the
previous work; this work (Line B), Barnes et al.15 (open triangle), Barnes
et al.14 (solid triangles), Trevor et al.13 (open squares), and Smith et
al.12 (solid diamonds). Line A represents the values recommended by
Sander et al.4 for atmospheric modeling. Line C represents the Smith
et al. data. Dashed components represent extrapolation beyond the range
of the measurements.
3.2. Comparison of k3(T) with Previous Studies. Graham
et al.6 published the first estimate of k3(298 K), which was
<3 × 10-12 cm3 molecule-1 s-1; it was based on an indirect
analysis of PNA thermal decomposition data. However, the
more-recent “direct” measurements of k3 (see Table 6 and Figure
6) have shown that the reported value of Graham et al.6 is low.
Our measured value of k3(298 K) agrees with those determined
using both relative rate14,15 and absolute methods12,13 within the
quoted uncertainty limits. The independence of k3(298 K) on
pressure, in the pressure range of 10-100 Torr (helium),
observed in this work also agrees with a similar lack of pressure
dependence in the pressure ranges of 1-300 Torr (N2) reported
by Barnes et al.14 and 3 and 15 Torr (helium) noted by Trevor
et al.13 Note that the scatter in the data of Barnes et al.14 and
Trevor et al.13 is quite large and indicates the difficulty in
measuring k3. The precision of these measurements may not

Reaction of OH with HO2NO2
J. Phys. Chem. A, Vol. 108, No. 7, 2004 1147
reflect the real accuracy of their measurements. For example,
the method used by Trevor et al.13 to deduce their [PNA] value
was rather indirect and would have led to significant variations
from experiment to experiment. In the experiments where OH
temporal profiles are measured, the largest contributors to the
uncertainty in k3 are the [PNA] and the contributions to OH
loss by reaction with impurities. We have established our
measured [PNA] value by multiple methods and have paid
careful attention to the possible depletion of PNA along the
length of the reactor. In contrast to the k3 derived from OH
temporal profile measurements, as in the case of Trevor et al.,13
Smith et al.,12 and us, the studies of Barnes et al.14 do not suffer
from the inaccuracies in [PNA] and impurity reactions, because
they monitored the fractional depletion of PNA. Barnes et al.14
had to account for secondary reactions and, thus, were not
immune to other sources of error. Yet, the reasonable agreement
between the reported values of Barnes et al.14 and our study
lends support to the quoted accuracy of k3.
by Smith et al.12 to correct k′ were measured in the same study
in the temperature range of 240-370 K and the pressure range
of 50-762 Torr of helium. The temperature and pressure
dependence of the rate coefficient for reaction 6 has since been
revised.17 However, the revised values are not necessarily the
best values to use for correction, because their measured values
of k4, k5, and k6 should be applicable to their system. The revised
values would increase the magnitude of the corrections by
∼30%, i.e., increase the rate coefficient by ∼10%. Even such
a correction, which may not be appropriate, probably will not
be enough to completely account for the differences in the k3
values. Note that even the uncorrected (raw) values of k′
measured in this work would yield values of k3 that are less
than those reported by Smith et al.12 Therefore, the discrepancies
in our studies may not entirely be due to uncertainties in the
corrections to k′ or the quantification of [PNA].
The currently recommended value of k3(T) for stratospheric
modeling is an average of the values reported by Barnes et
al.,14,15 Trevor et al.,13 and Smith et al.12 The recommended
values are included in Figure 6 for comparison. The currently
recommended4 room-temperature value of k3 is higher than our
value. However, the values, when extrapolated to 200 K, only
differ by ∼15% with our determination. This difference is well
within the quoted, rather large, uncertainty. The implications
of the results obtained in this work, which reduces the
uncertainty in k3 considerably, are discussed in Section 4,
“Atmospheric Implications”.
There are significant differences between the temperature
dependence of k3 reported by Barnes et al.,14 Trevor et al.,13
and Smith et al.12 Barnes et al.14 used an adaptation of the
relative rate technique to determine k3(T) at 268, 278, and 295
K. They did not observe a statistically significant change in
k3(T) over this narrow temperature range. Trevor et al.13 used a
laser photolysis resonance fluorescence technique to study
reaction 3 at low pressure over the temperature range of 246-
324 K. They observed a slightly negative temperature depen-
dence on k3. However, the overall low precision of their kinetic
data limited them from defining the temperature dependence;
they reported an E/R value of -(193 ( 194) K. The authors
attributed the scatter in the data to possible wall reactions of
OH and PNA in the flow tube. Uncertainties in the MS
determination of [PNA] may also have contributed to the scatter.
We expect the uncertainty in E/R to be less than that in A,
because the largest sources of error in k3 are the determination
of [PNA] and corrections for impurities. This is to be expected
because the thermal decomposition of PNA is not very rapid at
and below 298 K and the rate coefficients for the other reactions
that contribute to the measured OH loss (i.e., k4, k5, and k6) are
not very dependent on temperature over the range of temper-
atures studied.
Smith et al.12 reported a systematic increase in k3 with
decreasing temperature in the temperature range of 240-330
K with a derived E/R value of -600 K, i.e., a negative
temperature dependence. Our measured temperature dependence
for k3 is in good agreement with that reported by Smith et al.12
However, the magnitude of the k3 value reported by Smith et
al. are systematically higher by ∼30% from our values. At
present, we do not have an explanation for this discrepancy.
One possible source of such a systematic difference is the
corrections to the measured values of k′, which is the first-order
rate coefficient for a loss of OH, because of the presence of
H2O2, HNO3, and NO2. Smith et al.12 stated that their kinetic
data corrections (in all cases) accounted for <30% of the
measured k′ values. This is not much different from the level
of corrections to k′ applied in our study. Smith et al.12 used IR
absorption to monitor the H2O2 and HNO3 impurity concentra-
tions, as was done in this work. They also used UV absorption
to measure the PNA concentration, which is again similar to
this work. The main correction to k3 in our study is due to
reaction 4, with minor contributions from reactions 5 and 6. In
the study by Smith et al.,12 the corrections due to reactions 5
and 6 were somewhat higher, because of the higher HNO3 and
NO2 concentrations in their experiments. The k6 values used
3.3. Product Measurements of the OH + HO2NO2 Reac-
tion. The most likely reaction products, which are thermody-
namically allowed, for the OH + PNA reaction are
OH + HO2NO2 f H2O + NO2 + O2
(∆H° ) -45.8 kcal mol-1) (3a)
298
f HO2 + HNO3
(∆H° ) -24.9 kcal mol-1) (3b)
298
f H2O2 + NO3
(∆H° ) -11.1 kcal mol-1) (3c)
298
Currently, atmospheric models assume reaction 3a to be the only
significant pathway. Reaction products of reaction 3a are
difficult to detect in the laboratory, because of their high
background concentrations in a PNA sample and the lack of a
good way to measured H2O and O2 in a pulse photolysis system.
In this work, we have measured the branching ratios for
reactions 3b and 3c: reaction 3b through analysis of the OH
temporal profile in the presence of NO and reaction 3c by direct
measurement of the NO3 radical. The branching ratio for reaction
3a is inferred from these values.
The branching ratio for reaction 3b (Φ3b ) k3b/(k3a + k3b
+
k3c)) was determined at T ) 298 K by following the temporal
profiles of OH in experiments that were identical to the kinetic
measurements but with NO added to the sample. The addition
of NO converts HO2 radicals (produced either in the initial
photolysis pulse or through subsequent reactions, e.g., reactions
3b and 4) to detectable OH radicals via reaction 8. Numerical
simulations of the OH temporal profile using the chemical
mechanism outlined in Table 4 and the measured experimental
conditions were then used to estimate Φ3b. An example of the
measured OH temporal profiles in the presence of NO is shown
in Figure 7. The increase in the OH signal at early times in the
presence of NO results from the conversion of HO2 produced
in the photolysis of PNA at 248 nm.30,31 The quantum yield for

1148 J. Phys. Chem. A, Vol. 108, No. 7, 2004
Jime´nez et al.
Figure 7. OH temporal profile observed following the pulsed
photolysis of a sample that contained PNA (1.1 × 1015 molecule cm-3),
NO (2.3 × 1015 molecule cm-3), and H2O2 (1.1 × 1014 molecule cm-3).
HNO3 (2.1 × 1014 molecule cm-3) and NO2 (2.4 × 1014 molecule cm-3
)
at 298 K. The lines represent simulations of the OH temporal profiles
using the chemical mechanism given in Table 4 with the branching
ratio for reaction 3b (Φ3b) set equal to 0 (curve a), 0.1 (curve b), 0.2
(curve c), 0.4 (curve d), and 1.0 (curve e). This analysis clearly shows
Φ3b < 0.1.
HO2 in the 248-nm photolysis of PNA is reported to be >0.5.
The rate of rise in the OH signal is determined by the NO
concentration. The best fit to the OH temporal profile is obtained
with Φ3b ) 0. Simulations with several different Φ3b are also
shown in Figure 7 for comparison. We conclude from this
analysis that Φ3b is <0.10 and could be zero for the case shown
in the figure. Such profiles at various concentrations of PNA
and NO were measured and analyzed to derive an upper limit
of Φ3b < 0.1.
The yield of NO3 in reaction 3 (i.e., reaction 3c) was
determined using the SKaR technique, as noted earlier. The
measured CRDS profiles in the absence of OH production and
upon OH production via 248-nm photolysis of a mixture of
PNA/HNO3/H2O2 are shown in Figure 8a. The concentrations
of PNA, H2O2, HNO3, and NO2 were determined via IR
absorptions. The reactions that occur in this system include the
following:
Figure 8. Cavity ring down spectroscopy (CRDS) profiles measured
in the determination of the branching ratio for reaction 3c (Φ3c). Panel
a shows profiles measured in the absence of PNA (solid line) and in
the presence of PNA (dotted line). Panel b shows the ratio of the two
profiles shown in panel a (solid line); dashed lines are model simulations
(see text for details) (with Φ3c ) 0% (coincident with solid line),
Φ3c ) 4% (dotted line), and Φ3c ) 20% (dashed line). The time of the
photolysis is marked with an arrow.
with HO2NO2, HNO3, NO2, and H2NO2, whereas reaction 11a
represents the loss of OH that leads to NO3 formation via
reactions with HO2NO2 and HNO3. Reaction 12 represents the
loss of NO3 in this system. The pseudo-first-order rate coefficient
for the production of NO3 is equal to the pseudo-first-order OH
loss rate coefficient and is given by
HO2NO2 + hν (λ ) 248 nm) f OH + NO3
(2a)
k11′ ) k3[HO2NO2] + k4[H2O2] + k5[HNO3] +
k6[NO2] + k7 (VI)
f other products (2b)
HNO3 + hν (λ ) 248 nm) f OH + NO2
H2O2 + hν (λ ) 248 nm) f 2OH
(9)
The rate coefficient for the loss of NO3 was measured by
monitoring the NO3 concentrations at long times by varying
the time delay between the photolysis and probe (CRDS) laser;
k1′2 was always <2000 s-1. Because this loss is slower than the
time scale of the ring down signal, we could not accurately
quantify this rate coefficient in the SKaR profile. The overall
pseudo-first-order rate coefficient for NO3 production was on
the order of 10 000 s-1 (i.e., the same time scale for the ring
down signal); therefore, we use the SKaR method to measure
the NO3 production kinetics. This method has been described
in detail in a previous publication from our laboratory.25 Figure
8b shows the logarithm of the ratio of ring down signals with
and without photolysis. The photolysis event is clearly identified
at ∼30 µs. Integration of the differential equation that represents
reactions 2a, 11, and 12 yields the time dependence of the NO3
concentration in this system, which is given by
(10)
and the reactions listed in Table 4. In this system, NO3 radicals
are produced via reaction 5 and also via reaction 3c. This
mechanism can be further simplified to three processes of (i)
the photolytic production of NO3 that is coincident with the
photolysis laser, (ii) the production of NO3 as the reactions
proceed, and (iii) the loss of NO3 via reactions and physical
removal:
HO2NO2 + hν (λ ) 248 nm) f OH + NO3
OH f NO3
(2a)
(11a)
f other losses
(11b)
(12)
NO3 f products
[NO3]t ) A exp(-k′ t) + B exp(-k′ t)
(VII)
Here, reaction 11 represents the loss of OH via its reactions
11
12

Reaction of OH with HO2NO2
J. Phys. Chem. A, Vol. 108, No. 7, 2004 1149
Here, A and B are constants related to the initial concentrations
of OH and NO3. This temporal profile of NO3 was included
with the measured ratio profiles (such as that shown in Figure
8b) and the yield of NO3 in PNA photolysis31 to derive the
yield of NO3, as described elsewhere.25 The calculated lines
for various yields of NO3 are also shown in Figure 8b. Several
such experiments and analysis were performed to obtain an
upper limit of <0.05 for the NO3 yield in reaction 3.
Acknowledgment. This work was funded in part by NASA’s
Upper Atmospheric Research Program.
References and Notes
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Wiley: New York, 1998.
(2) WMO/UNEP Scientific Assessment of Ozone Depletion: 1998
(Geneva, Switzerland); National Aeronautics and Space Administration:
1998.
We have measured upper limits for Φ3b and Φ3c. Assuming
that there are no channels other than reactions 3a, 3b, and 3c
for reaction 3, the lower limit for Φ3a is quoted to be >0.85.
Note that we really did not obtain a measurable value for
reaction 3b or 3c, and, therefore, the branching ratio for reaction
3a to produce H2O, O2, and NO2, could be 1.0.
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G. K.; Orkin, V. L.; Ravishankara, A. R. Chemical Kinetics and Photo-
chemical Data for Use in Atmospheric Studies, Evaluation Number 14.
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The values of k3(T) at the temperatures of the upper
troposphere and lower stratosphere (UTLS) are needed to
evaluate the atmospheric lifetime of HO2NO2 and to elucidate
the role of peroxynitric acid (HO2NO2, PNA) in affecting both
the HOx and NOx budgets. Furthermore, because reaction 3 is
a major sink for HOx, its rate coefficient is needed to assess the
HOx loss rates and the stratospheric O3 trends that are due to
the anthropogenic emission of nitrogen oxides and halogen-
containing molecules. The results from the present study
significantly reduce the uncertainties in k3. Our k3 results are
different from previously reported values; however, the differ-
ences from the values recommended for stratospheric modeling
are not large. For instance, k3(200 K) extrapolated using the
results of this work is 1 × 10-11 cm3 molecule-1 s-1; this value
is only ∼15% lower than the current recommended value.
Consequently, our results will lead to small changes in the
calculated global lifetime of PNA in the UTLS and the impact
on ozone trends. However, the uncertainty in these values that
is due to uncertainties in the HO2NO2 chemistry is considerably
reduced.
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The product branching ratio measurements presented in this
work are consistent with the current assumptions used in
atmospheric model calculations. The main reaction channel leads
to the formation of NO2, O2, and H2O (reaction 3a). This
reaction channel will lead to the net removal of HOx in the
UTLS:
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Photolysis-Resonance Fluorescence, Ph.D. Thesis, University of California,
Irvine, CA, 1983.
HO2 + NO2 + M f HO2NO2 + M
OH + HO2NO2 f H2O + NO2 + O2
net reaction: OH + HO2 f H2O + O2
(1)
(3a)
(13)
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(31) Jime´nez, E.; Gierczak, T.; Burkholder, J. B.; Stark, H.; Ravishan-
kara, A. R. manuscript in preparation.
However, if reactions 3b and 3c were larger, reaction 3 would
have been a smaller sink for HOx in the lower stratosphere (LS).
Recently, Salawitch et al.32 suggested a few possible explana-
tions for their inability to match the measured [OH]/[HO2] ratio
in the LS with their model calculations. One possibility proposed
was that k3 was at the lower limit of the NASA/JPL recom-
mendation4 at the temperatures of the LS. Another possibility
was that the branching ratio for reaction 3a was smaller than
recommended. On the basis of our results, we excluded these
two possibilities. Measurement of the product yields at tem-
peratures that are characteristic of the LS would be beneficial.
(32) Salawitch, R. J.; Wennberg, P. O.; Toon, G. C.; Sen, B.; Blavier,
J. F. Geophys. Res. Lett. 2002, 29, 10.1029/2002GL015006.