Temperature-dependent rate coefficients for the reactions of Br(2P3/2), Cl(2P3/2), and O(3PJ) with BrONO2

Article abstract of DOI:10.1021/jp001947q

A laser flash photolysis-resonance fluorescence technique has been employed to investigate the kinetics of reactions of the important stratospheric species bromine nitrate (BrONO₂) with ground-state atomic bromine (k₁), chlorine (k₂), and oxygen (k₃) as a function of temperature (224-352 K) and pressure (16-250 Torr of N₂). The rate coefficients for all three reactions are found to be independent of pressure and to increase with decreasing temperature. The following Arrhenius expressions adequately describe the observed temperature dependencies (units are 10^-11 cm³molecule^-1s^-1): k₁ = 1.78 exp(365/T), k₂ = 6.28 exp(215/T), and k₃ = 1.91 exp(215/T). The accuracy of reported rate coefficients is estimated to be 15-25% depending on the magnitude of the rate coefficient and on the temperature. Reaction with atomic oxygen is an important stratospheric loss process for bromine nitrate at altitudes above approximately 25 km; this reaction should be included in models of stratospheric chemistry if bromine partitioning is to be correctly simulated in the 25-35 km altitude regime.

Full text of DOI:10.1021/jp001947q

1416
J. Phys. Chem. A 2001, 105, 1416-1422
Temperature-Dependent Rate Coefficients for the Reactions of Br(2P3/2), Cl(2P3/2), and
†
O(3PJ) with BrONO2
R. Soller,‡,| J. M. Nicovich,§ and P. H. Wine*,‡,§
School of Earth and Atmospheric Sciences and School of Chemistry and Biochemistry,
Georgia Institute of Technology, Atlanta, Georgia 30332
ReceiVed: May 26, 2000; In Final Form: August 10, 2000
A laser flash photolysis-resonance fluorescence technique has been employed to investigate the kinetics of
reactions of the important stratospheric species bromine nitrate (BrONO2) with ground-state atomic bromine
(k1), chlorine (k2), and oxygen (k3) as a function of temperature (224-352 K) and pressure (16-250 Torr of
N2). The rate coefficients for all three reactions are found to be independent of pressure and to increase with
decreasing temperature. The following Arrhenius expressions adequately describe the observed temperature
dependencies (units are 10-11 cm3molecule-1s-1): k1 ) 1.78 exp(365/T), k2 ) 6.28 exp(215/T), and k3 )
1.91 exp(215/T). The accuracy of reported rate coefficients is estimated to be 15-25% depending on the
magnitude of the rate coefficient and on the temperature. Reaction with atomic oxygen is an important
stratospheric loss process for bromine nitrate at altitudes above ∼25 km; this reaction should be included in
models of stratospheric chemistry if bromine partitioning is to be correctly simulated in the 25-35 km altitude
regime.
Introduction
In this paper we report an experimental study of the kinetics
of reactions 1-3 as functions of temperature (224-352 K) and
pressure (16-250 Torr of N2).
Bromine nitrate (BrONO2) is a key stratospheric chemical
species, serving both as a reservoir for BrOx radicals and as an
intermediate in a catalytic cycle that contributes to odd-oxygen
destruction.1-3 Laboratory investigations of bromine nitrate
chemistry reported to date have focused on the kinetics of
BrONO2 formation via the BrO + NO2 association reaction,4-6
quantitative measurement of absorption cross sections7-9 and
photodissociation quantum yields,2,10 and heterogeneous reac-
tions of BrONO2 that represent important nighttime sinks.11,12
Because gas-phase reactions involving BrONO2 have been
thought to be unimportant as stratospheric BrONO2 destruction
pathways, they have received relatively little attention. A single
measurement of the room-temperature rate coefficients for the
reactions of ground-state bromine and chlorine atoms with
BrONO2 represents the entire literature database.2
O(3PJ) + BrONO2 f products
(3)
Kinetic information is obtained by monitoring the temporal
profile of the reactant atom under pseudo-first-order conditions
with BrONO2 in large excess. The values for k1(298 K) and
k2(298 K) obtained in this study agree well with their respective
literature values, and we find that all three reactions are
characterized by negative activation energies, i.e., the rate
coefficients for all three reactions increase with decreasing
temperature. Furthermore, we find that reaction 3 is fast enough
to compete with photodissociation as a stratospheric BrONO2
removal mechanism at altitudes above approximately 25 km.
Experimental Technique
Br(2P3/2) + BrONO2 f products
Cl(2P3/2) + BrONO2 f products
(1)
(2)
The laser flash photolysis (LFP)-resonance fluorescence (RF)
apparatus used in this study was similar to one we have
employed in numerous previous studies of atom-molecule
reactions involving Br,13 Cl,14 and O15 atoms. A schematic
diagram of the current version of the apparatus is published
elsewhere.10,16 Important features of the apparatus and experi-
mental approach that are specific to this study are described
below.
A Pyrex, jacketed reaction cell with an internal volume of
∼150 cm3 was used in all experiments. The cell was maintained
at a constant temperature by circulating ethylene glycol (T >
298 K) or methanol (T < 298 K) from a thermostated bath
through the outer jacket. A copper-constantan thermocouple
with a stainless steel jacket was periodically injected into the
reaction zone through a vacuum seal to measure the gas
temperature under the precise pressure and flow rate conditions
of the experiment.
Harwood et al.2 monitored the temporal profile of NO3 appear-
ance following 352.5 nm laser flash photolysis of X2/BrONO2/
N2/O2 mixtures (X ) Br, Cl), and used these data to obtain the
298 K rate coefficients k1 ) (6.7 ( 0.7) × 10-11 cm3 molecule-1
s-1 and k2 ) (1.27 ( 0.16) × 10-10 cm3 molecule-1 s-1
.
† Part of the special issue “Harold Johnston Festschrift”.
* To whom correspondence should be addressed: School of Chemistry
and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-
0400. E-mail: pw7@prism.gatech.edu.
‡ School of Earth and Atmospheric Sciences.
§ School of Chemistry and Biochemistry.
| Present address: Department of Chemistry, Emory University, Atlanta,
GA 30322.
10.1021/jp001947q CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/20/2000

Reactions of Halides with BrONO2
J. Phys. Chem. A, Vol. 105, No. 9, 2001 1417
Oxygen and chlorine atoms were produced by 355 nm laser
flash photolysis of NO2 and Cl2, respectively, using third
harmonic radiation from a Nd:YAG laser (Quanta Ray model
DCR-2A) as the photolytic light source. In some experiments,
oxygen atoms were generated by 248 nm laser flash photolysis
of the BrONO2 reactant using a KrF excimer laser (Lambda
Physik model Compex 102) as the photolytic light source; it
has been demonstrated that 248 nm photolysis of BrONO2
produces O atoms with a relatively high yield of ∼66%.10,17
Bromine atoms were generated by laser flash photolysis of
BrONO2 at a variety of Nd:YAG and excimer laser wavelengths
(248, 266, 308, and 355 nm); the sources of 248, 266, and 355
nm radiation were the lasers mentioned above, while the source
of 308 nm radiation was a Lambda Physik model Lextra 200
excimer laser.
N2 (and with CO2 when appropriate) before entering the reaction
cell. To prevent dark reactions of BrONO2 with the photolytic
precursors Cl2 and NO2, these species were injected into the
reaction cell just upstream from the reaction zone. To minimize
the rate of BrONO2 hydrolysis on the walls of the flow system,
N2O5 was flowed through the system for about 15 min before
each series of experiments; this treatment converted nearly all
adsorbed H2O to gaseous HNO3.
Species concentrations in the reaction mixtures were evaluated
using a combination of photometric, mass-flow-rate, and total
pressure measurements. The concentration of bromine nitrate
was measured both upstream and downstream of the reaction
cell by UV photometry. Both absorption cells were 200 cm in
length, and cadmium penray lamps were used as the light source
in both cells. Quantitative measurements of BrONO2 were made
in every experiment using 228.8 nm as the monitoring wave-
In experiments where Br kinetics were studied, approximately
0.5 Torr of CO2 was added to the reaction mixture in order to
rapidly deactivate any photolytically generated spin-orbit
excited bromine atoms, Br(2P1/2); the rate coefficient for
electronic-to-vibrational energy transfer from Br(2P1/2) to CO2
length: the BrONO2 absorption cross section at 228.8 nm was
7-9
taken to be 2.17 × 10-18 cm2.
Periodically, absorption
measurements were also made at 326.1 nm (also a cadmium
lamp emission line) in order to assess the level of Br2O impurity
in the BrONO2 sample and the degree to which Br2O was
generated by hydrolysis of BrONO2 on the walls of the slow
flow system:
is known to be 1.5 × 10-11 cm3molecule-1s-1 18
.
Photodisso-
ciation of Cl2 at 355 nm is known to produce >99% of the
2
chlorine atoms in the P3/2 ground state.19,20 The three fine
structure levels of O(3PJ) are sufficiently closely spaced in
energy that it is safe to assume rapid equilibration via collisions
with the N2 buffer gas. The above considerations along with
the magnitude of the observed rate coefficients (i.e., very fast)
suggests that the reactant atoms in our studies of reactions 1-3
can be taken to be Br(2P3/2), Cl(2P3/2), and a thermalized mixture
of O(3PJ) (J ) 0, 1, 2), respectively.
BrONO2 + H2O(adsorbed) f HOBr + HNO3
2 HOBr T Br2O + H2O
(4)
(5)
The absorption cross section for BrONO2 at 326.1 nm was taken
to be 9.1 × 10-20 cm2,7-9, while absorption cross sections for
Br2O at 228.8 and 326.1 nm were taken to be 1.51 × 10-18
cm2 and 2.04 × 10-18 cm2, respectively.21 Solution of two
equations (one for total absorbance at 228.8 nm and one for
total absorbance at 326.1 nm) in two unknowns yielded
estimates of Br2O impurity levels. To assess the possibility that
some of the absorbance observed at 228.8 nm and/or at 326.1
nm resulted from material adsorbed to absorption cell windows,
some additional experiments were carried out where a third
(short) absorption cell with a path length of 10 cm was
incorporated into the slow flow system. When the BrONO2/N2
mixture was flowing through the system, the 228.8 nm absor-
bances in the long and short absorption cells scaled as they
should if all absorption was attributable to gas-phase species.
However, the (much smaller) absorbances at 326.1 nm did not
scale appropriately, i.e., the absorbance in the 10 cm cell was
typically more than 5% of the absorbance in the 200 cm cell.
Therefore, gas-phase absorbances at 326.1 nm were taken to
be the difference between those observed in the long and short
cells, and the path length used to convert absorbance to Br2O
concentration was taken to be 190 cm. To check impurity levels
of Br2 and NO2 in BrONO2 samples, some experiments were
carried out where the flowing sample was passed through a
multipass absorption cell that employed 457.9 nm radiation from
an Ar+ laser as the probe radiation and had a total absorption
path length (88 passes) of ∼3300 cm. Both Br2 and NO2 have
absorption cross sections of 4.5 × 10-19 cm2 at 457.9 nm.22,23
Bromine nitrate was synthesized from the reaction of BrCl
with ClONO2, as first described by Spencer and Rowland.7
Chlorine nitrate (ClONO2) was synthesized from the reaction
of Cl2O with N2O5 as first described by Schmeisser,24 while
Cl2O was prepared by passing Cl2(g) over solid yellow mercuric
oxide as described by Cady25 and N2O5 was prepared from the
gas-phase reaction of NO2 with O3 as described by Ravishankara
et al.26 BrCl was prepared by mixing Br2 with an excess of Cl2
at 195 K as described by Burkholder et al.8 Before being used
An atomic resonance lamp, situated perpendicular to the
photolysis laser, excited resonance fluorescence in the pho-
tolytically produced atoms. The resonance lamp consisted of
an electrodeless microwave discharge through about one Torr
of a flowing mixture containing a trace of Br2, Cl2, or O2 in
helium. Radiation passed from the lamp into the reaction cell
through MgF2 optics. The region between the lamp and the
reaction cell was purged with a flowing gas filter mixture that
effectively prevented simultaneous detection of multiple
species.13-15 For detection of Br, Cl, and O atoms, the filter
gases were 50 Torr cm CH4, 3 Torr cm N2O, and 60 Torr cm
O2, respectively. Resonance fluorescence was collected by an
MgF2 lens on an axis orthogonal to both the photolysis laser
beam and the resonance lamp beam, and imaged onto the
photocathode of a solar blind photomultiplier. The region
between the reaction cell and the photomultiplier was purged
with N2 to prevent absorption of resonance fluorescence by
atmospheric gases such as O2 and H2O. Signals were processed
using photon-counting techniques in conjunction with multi-
channel scaling.
To avoid accumulation of photolysis or reaction products,
all experiments were carried out under “slow-flow” conditions.
The linear flow rate through the reactor was typically 3.5 cm
s-1 (range was 1.7-5.3 cm s-1), and the laser repetition rate
was typically 5 Hz (range was 2-10 Hz). Since photolysis
occurred on an axis perpendicular to the direction of flow, no
volume element of the reaction mixture was subjected to more
than a few laser shots. The photolytic precursors NO2 and Cl2
were flowed into the reaction cell from 12 L bulbs containing
dilute mixtures in nitrogen buffer gas, while CO2 and additional
N2 were flowed directly from their high-pressure storage tanks.
Bromine nitrate was introduced into the reaction cell by passing
a flow of N2 over the BrONO2 sample that was kept in a trap
at 220-230 K. The BrONO2 flow was premixed with additional

1418 J. Phys. Chem. A, Vol. 105, No. 9, 2001
Soller et al.
to prepare BrONO2, the BrCl sample was purified by trap-to-
trap distillation using a series of traps held at 195, 158, and 77
K. We found that it was easier to separate Br2 from BrCl than
it was to separate Br2 from BrONO2. Additional details
concerning the syntheses of Cl2O, N2O5, ClONO2, BrCl, and
BrONO2 can be found elsewhere.10
The gases used in this study had the following stated
minimum purities: N2, 99.999%; CO2, 99.99%; Cl2, 99.99%;
O2, 99.99%; NO2, 99.5%. The stated purities of Cl2 and NO2
refer to the liquid phase in the high-pressure storage cylinder.
N2, CO2, and O2 were used as supplied, while Cl2 and NO2
were degassed repeatedly at 77 K before being used to prepare
dilute mixtures in N2 buffer gas. Ozone was prepared by passing
UHP O2 through a commercial ozonator. It was collected and
stored on silica gel at 195 K. The liquid Br2 sample had a stated
minimum purity of 99.94%; it was transferred into a vial fitted
with a high vacuum stopcock and degassed repeatedly at 77 K
before use.
Figure 1. Typical resonance fluorescence temporal profiles observed
in studies of reactions 1-3. Reaction: O(3PJ) + BrONO2. Experimental
conditions: T ) 227 K; P ) 50 Torr; [BrONO2] in units of 1014
molecules cm-3 ) (A) 0.80, (B) 2.04, (C) 5.16; number of laser shots
averaged ) (A) 2500, (B) 2600, (C) 3500. Solid lines are obtained
from least squares analyses and give the following first-order decay
rates in units of s-1: (A) 3650, (B) 10500, (C) 25800.
Results and Discussion
1. Experimental Results. All experiments were carried out
under pseudo-first-order conditions with BrONO2 in large excess
over the atomic reactant. Hence in the absence of secondary
reactions that enhance or deplete the reactant atom concentration,
the atom temporal profile is dominated by the reactions
Ai + BrONO2 f products
Ai + Ij f products
i ) 1-3
(i)
(6j)
Ai f loss by diffusion from the detector field of
view and/or reaction with background impurities (7i)
In the above reaction scheme, Ai represents the reactant atom
in reaction i, Ij is the jth impurity in the BrONO2 sample, and
“background” impurities are those which are present even when
the BrONO2 flow is shut off (including the photolytes Cl2 and
NO2). Integration of the rate equations for the above scheme
yields the relationship
Figure 2. Typical plots of k′ versus [BrONO2] for data obtained in
studies of reactions 1-3. Reaction: O(3PJ) + BrONO2. Solid lines are
obtained from least squares analyses; their slopes give the following
bimolecular rate coefficients in units of 10-11 cm3molecule-1s-1: 4.99
at 227 K and 3.59 at 339 K.
ln([A]0/[A]t) ) ln(S0/St) )
{(k + (R k ))[BrONO ] + k }t ) k′t (I)
∑
i
j
6j
2
7i
where S0 and St represent the resonance fluorescence signal
levels at times 0 and t, respectively, and Rj represents the ratio
[Ij]/[BrONO2].
play an important role in all three reactions. The following
Arrhenius expressions are derived from the data:
For all three reactions studied, atom temporal profiles were
found to be exponential, and observed pseudo-first-order decay
rates were found to increase linearly as a function of BrONO2
concentration as predicted by eq I. Furthermore, observed
temporal profiles were independent of laser fluence and varied
as predicted by eq I when concentrations of the photolytic
precursors NO2 and Cl2 were varied (if the atom reacts with its
photolytic precursor at a significant rate, then k7i will increase
with increasing Cl2 (i ) 2) or NO2 (i ) 3) concentration even
if the atom concentration is held constant as the photolyte
concentration is varied). Typical atom temporal profiles are
shown in Figure 1 and typical plots of k′ versus [BrONO2] are
shown in Figure 2. The kinetic data obtained for reactions 1-3
are summarized in Tables 1 - 3, and Arrhenius plots for
reactions 1-3 are shown in Figure 3. For all three reactions,
the Arrhenius plots are linear over the temperature range
investigated, and all three reactions display small negative
activation energies, suggesting that long-range attractive forces
k1 ) (1.78 ( 0.33) ×
10-11 exp{(365 ( 52)/T} cm3 molecule-1 s-1
k2 ) (6.28 ( 1.28) ×
10-11 exp{(215 ( 59)/T} cm3 molecule-1 s-1
k3 ) (1.91 ( 0.34) ×
10-11 exp{(215 ( 50)/T} cm3 molecule-1 s-1
Uncertainties in the above expressions are 2σ and represent
precision only. The uncertainties in the above expressions refer
to the Arrhenius parameters only. Realistic error estimates for
individual rate coefficients are derived below. As described in
eq I, the above Arrhenius expressions only represent the
reactions of interest accurately if the rate of atom removal from

Reactions of Halides with BrONO2
J. Phys. Chem. A, Vol. 105, No. 9, 2001 1419
BrONO2 sample or is produced by heterogeneous removal of
BrONO2 in the slow flow system. The impurities of this type
that are most likely to contribute to Br(2P3/2), Cl(2P3/2), and/or
O(3PJ) removal are Br2, Br2O, and NO2; literature values and
estimates (where data are unavailable) of the rate coefficients
for these potential impurity reactions are summarized in Table
4.
As discussed in the Experimental Section, the sum of the
impurity levels of Br2 and NO2 was evaluated using long path
absorption at 457.9 nm as the probe technique. These observa-
tions showed that ([Br2] + [NO2])/[BrONO2] was less than 0.03
in all bromine nitrate samples examined. Using the measure-
ments of upper limit Br2 and NO2 levels in conjunction with
the information in Table 4, we conclude that reactions of these
impurities with the reactant atoms contributed e0.2% to the
measured value of k1, e3% to the measured value of k2, and
e1.5% to the measured value of k3.
Despite our best efforts, the dual wavelength photometric
measurements at 228.8 and 326.1 nm (see Experimental Section)
could only constrain the Br2O impurity level to be e10% of
the BrONO2 level. The relatively weak absorbance of Br2O at
326.1 nm and some problems with weak absorption by material
collected on absorption cell windows (see above) precluded
more sensitive Br2O measurements. Fortunately, additional
information about the Br2O-to-BrONO2 concentration ratio in
the reactor was attainable from studies of BrONO2 photochem-
istry at 355 nm. The photochemistry results are (being) published
elsewhere,10,17 but the key findings relevant to establishing the
Br2O impurity level are summarized below: (1) The “apparent”
bromine atom quantum yield Φ(Br) from BrONO2 photodis-
sociation at 355 nm is 0.77 ( 0.19 (error is 2σ); (2) The
“apparent” ratio Φ(Br)/Φ(BrO) from BrONO2 photodissociation
at 355 nm is 3.2 ( 1.0 (error is 2σ); (3) At 355 nm the Br2O
absorption cross section is a factor of 29 larger than the BrONO2
absorption cross section;7-9 and (4). On the basis of thermo-
chemical considerations, the quantum yields for Br and BrO
production from Br2O photodissociation at 355 nm are equal,
i.e., the channel producing 2Br+O is energetically forbidden.
In the above discussion, an “apparent” quantum yield is one
that includes contributions to the measured photoproduct from
BrONO2 photolysis and possibly from photolysis of impurities
in the BrONO2 sample as well. It is a virtual certainty that Br2O
is the only bromine-containing impurity which contributes
significantly to generation of the observed photoproducts at λ
) 355 nm (the contribution from Br2 is very small). The
maximum possible Br2O level that could be present is the
amount needed to generate all of the observed BrO; under the
(almost certainly correct) assumption that the Br2O photodis-
sociation quantum yield is unity, this constraint places a very
conservative upper limit value for [Br2O]/[BrONO2] at 0.03.
Assuming gas kinetic rate coefficients for all Br2O impurity
reactions (see Table 4) and a Br2O impurity level of 3%, reaction
with Br2O would contribute 6-12% to the measured value for
k1, 4-5% to the measured value for k2, and 12-17% to the
measured value for k3. In all cases, the maximum impurity
contribution is at the highest temperature studied and the
minimum contribution is at the lowest temperature studied. It
is worth noting that a very low Br2O impurity level could only
be established in experiments involving 355 nm photolysis of
reaction mixtures containing only the BrONO2 sample and N2
(i.e., no added reactant atom photolytic precursor). There is no
reason to suspect higher Br2O levels in other experiments and,
in fact, values for k1(298 K) measured using 355 nm photolysis
of BrONO2 as the Br(2P3/2) source are in good agreement with
Figure 3. Arrhenius plots for reactions 1-3. The solid lines are
obtained from least squares analyses (of data from this study only)
and represent the Arrhenius expressions given in the text. Solid data
points are from this study while open data points are from ref 2.
reactions with impurities that enter the reactor with the BrONO2
flow (or are produced via dark reactions involving BrONO2) is
negligible compared to the rate of atom removal by BrONO2
itself. Potential contributions from impurity reactions are
assessed below.
2. Estimated Accuracy of Reported Rate Coefficients.
Examination of Tables 1-3 shows that the precision of the rate
coefficient measurements is quite good. However, systematic
errors resulting from impurities in the BrONO2 sample and from
heterogeneous reactions in the slow flow system are potentially
important and need to be addressed. In addition, the accuracy
of BrONO2 concentration measurements makes a significant
contribution to the overall uncertainty in reported rate coef-
ficients.
The absorption cross section for BrONO2 at 228.8 nm is only
known to about 10% accuracy,8 and this source of uncertainty
contributes directly to the uncertainties in our reported values
for k1-k3. An additional source of uncertainty in our measured
BrONO2 concentrations results from the fact that, despite our
best efforts to “condition” the flow system, some BrONO2 was
lost upon traversal of the system. In experiments with the
reaction cell at 298 K and above, the BrONO2 concentration
measured downstream from the reaction cell was typically less
than 5% lower than the concentration measured upstream from
the reaction cell. However, at the lowest temperatures where
kinetic data were obtained (∼225 K), the downstream BrONO2
concentration was typically ∼30% lower than the upstream
concentration. Apparently, more efficient binding of BrONO2
to the reactor walls at lower temperatures facilitates heteroge-
neous reactions that destroy BrONO2. All concentrations
reported in Tables 1-3 are the averages of concentrations
measured in the upstream and downstream absorption cells
(corrected for small pressure gradients and, of course, for
temperature differences between the absorption cells and the
reaction cell). On the basis of the above considerations, we
estimate the accuracy of measured BrONO2 concentrations to
be (12% for kinetics experiments at T g 298 K, increasing to
(20% for kinetics experiments at T ∼ 225 K.
As mentioned above, impurities that react rapidly with the
atom reactants are sources of potential systematic error in
measured rate coefficients if the impurity is present in the

1420 J. Phys. Chem. A, Vol. 105, No. 9, 2001
Soller et al.
TABLE 1: Summary of Kinetic Data for the Reaction Br(2P3/2) + BrONO2 f Products
no. of
Ta
Pa
λa,b
[BrONO2]a
[Br]0
expts.d
range of k′ a
ka,e
a,c
224
228
245
247
265
298
298
298
298
298
298
298
298
298
298
343
352
52
51
52
50
50
20
24
26
48
51
51
140
201
206
251
51
266
266
266
266
266
266
248
308
266
266
355
308
355
266
266
266
266
1000-4090
1050-3550
1510-4480
1440-3240
1660-3930
1420-3890
1180-6470
1480-7070
1120-8520
1580-4050
1090-8670
2170-5210
880-7260
8.0-24
3.2-11
4.9-20
2.3-12
1.9-12
5.0-12
0.5-3.4
1.3-4.8
2.1-11
4.6-9.9
3.0-23
1.7-4.7
2.5-21
5.8-12
2.2-9.2
3.6-18
5.4-13
4
5
10
6
5
6
5
4
19
3
17
4
6300-47800
7310-30900
12400-32500
10700-23900
11100-26800
9780-26200
4490-37400
8520-40700
6940-52300
9370-24000
5660-49900
14000-32000
4150-42100
9420-19700
4850-27500
8510-30800
6530-19100
9.89 ( 0.79
8.89 ( 0.83
7.77 ( 0.75
7.30 ( 0.34
6.87 ( 0.27
6.77 ( 0.22
5.83 ( 0.21
5.84 ( 0.21
6.23 ( 0.23
5.96 ( 0.22
5.82 ( 0.18
6.01 ( 0.39
5.89 ( 0.15
6.19 ( 0.45
6.27 ( 0.19
5.29 ( 0.20
5.13 ( 0.20
5
4
6
5
1570-3220
1020-4400
1670-5770
1270-3800
51
5
a Units are T (K), P (Torr), λ (nm); concentrations (1011 per cm3), k′ (s-1), k (10-11 cm3 molecule-1 s-1). b λ ≡ photolysis laser wavelength.
c Calculated based on the measured laser fluence and BrONO2 concentration. d expt ≡ determination of a single pseudo-first-order decay rate.
e Uncertainties are 2σ and represent precision only.
TABLE 2: Summary of Kinetic Data for the Reaction Cl(2P3/2) + BrONO2 f Productsa
no. of
Tb
Pb
[BrONO2]b
[Cl2]b
320
270
170-280
280
[Cl]0
expts.d
range of k′ b
kb,e
b,c
238
298
298
298
334
20
21
21
100
21
0-4530
0-4130
0-3520
0-3730
0-3500
12
12
6.8-16
11
8.9
5
5
11
5
23-68800
28-53900
32-43900
18-50400
51-40600
15.5 ( 0.8
13.1 ( 0.6
12.4 ( 0.6
13.1 ( 1.0
12.0 ( 0.8
210
5
a The photolysis laser wavelength was 355 nm in all experiments. b Units are T (K), P (Torr), concentrations (1011 per cm3), k′ (s-1), k (10-11 cm3
molecule-1 s-1). c Calculated based on the measured laser fluence and Cl2 concentration. d expt ≡ determination of a single pseudo-first-order
decay rate. e Uncertainties are 2σ and represent precision only.
TABLE 3: Summary of Kinetic Data for the Reaction O(3PJ) + BrONO2 f Products
Ta
Pa
λa,b
[BrONO2]a
[NO2]a
[O]0
no. of exptsd
range of k′ a
ka,e
a,c
227
245
298
298
298
298
298
298
339
50
50
16
21
33
355
355
355
248
248
355
248
355
355
0-5160
0-5580
140
6.8-8.6
6.0-8.3
6.7
11-29
3.8-17
5.7
2.7-6.9
11
5.5-7.7
6
3
5
3
4
5
3
4
5
87-25800
93-24700
156-37900
4510-19800
92-33300
141-24200
2470-8580
122-42300
78-25300
4.99 ( 0.12
4.47 ( 0.31
3.96 ( 0.11
4.13 ( 0.07
3.99 ( 0.21
3.79 ( 0.39
3.92 ( 0.96
3.76 ( 0.13
3.59 ( 0.24
110
95
0
70-190
95
0
0-9450
930-4770
0-8100
0-6010
910-2280
0-11200
0-6880
50
100
157
51
160
90-140
a Units are T (K), P( Torr), λ (nm), concentrations (1011 per cm3), k′(s-1), k(10-11 cm3molecule-1s-1). b λ ≡ photolysis laser wavelength. c Calculated
based on the measured laser fluence and photolytic precursor (NO2 or BrONO2) concentration. d expt ≡ determination of a single pseudo-first-order
decay rate. e Uncertainties are 2σ and represent precision only.
TABLE 4: Literature or Estimated Rate Coefficients for
values obtained using shorter wavelength photolysis to generate
Potentially Important Impurity Reactions
Br(2P3/2).
reaction
k (298 K)a
Ea/Ra
ref
Considering the three most important factors that limit the
accuracy of reported rate coefficients to be precision, determi-
nation of the BrONO2 concentration, and contributions from
impurity reactions, we estimate that the accuracy of reported
rate coefficients falls in the range 15-25%. The room temper-
ature determination of the fastest rate coefficient (k2) is the most
accurate, while the lowest temperature determination of the
slowest rate coefficient (k3) is the least accurate. The following
format is widely used to express uncertainties in rate parameters
that are used in modeling atmospheric chemistry:27
Br(2P3/2) + NO2
0.021 at 15 Torr
0.20 at 200 Torr
20
15
-814
-788
39
39
40
41
42
42
Br(2P3/2) + Br2O
Cl(2P3/2) + Br2
Cl(2P3/2) + NO2
0b
144
-765
-740c
0b
-40
-209
0b
0.078 at 15 Torr
0.92 at 200 Torr
Cl(2P3/2) + Br2O
O(3PJ) + Br2
20d
2.0
41
43
O(3PJ) + NO2
O(3PJ) + Br2O
1.06
20d
a Units of k( 298 K) are 10-11 cm3 molecule-1 s-1 and units of Ea/R
are degrees Kelvin. b In the absence of data, it is assumed that Ea/R )
0. c In the absence of data, it is guesstimated that Ea/R is a little less
negative at 200 Torr than at 15 Torr. d In the absence of data, it is
assumed that k (298 K) is gas kinetic.
f(T) ) f(298) exp{(∆Ea/R) (T-1 - 298-1)}
(II)
Applying eq II to our results leads to the following estimated

Reactions of Halides with BrONO2
J. Phys. Chem. A, Vol. 105, No. 9, 2001 1421
TABLE 5: Thermochemistry of X + BrONO2 Reaction
Channels (X ) Br(2P3/2), Cl(2P3/2), O(2PJ))
reaction
no.
-∆rH°
reactants
Br(2P3/2) + BrONO2 f Br2 + NO3
f Br2 + NO + O2
products
(298 K)a
(1a)
(1b)
(1c)
(2a)
(2b)
(2c)
(3a)
(3b)
(3c)
(3d)
(3e)
(3f)
(3g)
(3h)
49
33
7
75
58
25
93
220
187
151
147
106
76
f Br2O + NO2
Cl(2P3/2) + BrONO2 f BrCl + NO3
f BrCl + NO + O2
f ClNO2 + BrO
f BrO + NO3
O(3PJ) + BrONO2
f BrNO2 + O2
f BrONO + O2
f BrOO + NO2
f Br + NO2 + O2
f OBrO + NO2
f BrO + NO + O2
f BrNO + O3
Figure 4. Altitude dependence of BrONO2 destruction rates via
photolysis (solid line) and reaction with O(3PJ) (dashed line) under
conditions of 40° north latitude, March 15, local noon.
67
a Units of ∆rH° are kJ mol-1
.
parameters (subscripts are reaction numbers): f1(298) ) 1.17,
(∆Ea/R)1 ) 40; f2(298) ) 1.15, (∆Ea/R)2 ) 50; f3(298) ) 1.20,
(∆Ea/R)3 ) 30.
NO3 from reaction 3 is greater than 0.85. Hence, it appears that
reactions of Br(2P3/2), Cl(2P3/2), and O(3PJ) with BrONO2
proceed predominantly via channels 1a, 2a, and 3a, respectively.
The reactions of Cl(2P3/2) and O(3PJ) with ClONO2 are also
known to produce NO3 with near unit yield.28,31
3. Comparison of Reported Rate Coefficients with Lit-
erature Values. As mentioned in the Introduction, the entire
literature database concerning the kinetics of reactions 1-3
consists of a single 298 K measurement of k1 and k2 by Harwood
et al.2 These authors obtained their kinetic information from
time-resolved observations of the reaction product NO3. The
results of Harwood et al.2 are plotted along with our data in
Figure 3. Clearly, the agreement between the two studies is
excellent.
5. Implications for Atmospheric Chemistry. While kinetic
information on all three of the reactions studied is useful for
interpreting laboratory studies of BrONO2 chemistry, it appears
that only reaction 3 is an important stratospheric process. The
rate coefficient for the O(3PJ) + BrONO2 reaction is high enough
that this reaction can make a significant contribution to BrONO2
removal in the stratosphere. Furthermore, since the dominant
bromine-containing product of reaction 3 appears to be BrO,37
the occurrence of reaction 3 affects stratospheric BrOx levels.
The value of k3 reported in this study has been used in one
modeling study which focused on polar lower stratospheric
conditions and, as is appropriate for such conditions, incorpo-
rated heterogeneous as well as gas-phase chemistry of BrONO2.38
The inclusion of reaction 3, which has been ignored in all
previous simulations of stratospheric bromine chemistry, in-
creased stratospheric BrO levels by approximately 3%.38
Because O atom concentrations increase dramatically with
increasing altitude, the relative importance of reaction 3
(compared to photolysis) as a BrONO2 destruction pathway,
will increase with increasing altitude. First-order BrONO2
removal rates via reaction 3 and via photolysis are plotted as a
function of altitude in Figure 4. These altitude profiles, which
are for 40° north latitude, March 15, local noon conditions, are
constructed using the value for k3 reported in this study in
conjunction with the O atom altitude profile taken from ref 27
and BrONO2 photolysis frequencies provided by R. W. Port-
mann of the National Oceanic and Atmospheric Administration’s
Aeronomy Laboratory. Reaction 3 becomes competitive with
photolysis as a BrONO2 loss process at altitudes above 25 km,
and it becomes more important than photolysis at altitudes above
30 km. Although bromine nitrate chemistry is most important
in the lower stratosphere, BrONO2 is an important stratospheric
Bry species at altitudes up to 35 km1. It is clear that reaction 3
should be included in model simulations of stratospheric
chemistry in order to correctly predict the partitioning of reactive
bromine between its reservoir and catalytically active (i.e., Br
and BrO) forms.
It is interesting to compare the kinetics of reactions 1-3 with
available kinetic information for the analogous reactions with
ClONO2 replacing BrONO2 as the molecular reactant:
Br(2P3/2) + ClONO2 f products
Cl(2P3/2) + ClONO2 f products
O(3PJ) + ClONO2 f products
(8)
(9)
(10)
There are no kinetic data in the literature for reaction 8, but
reactions 9 and 10 have been studied by several groups.28-34
Like reaction 2, the Cl(2P3/2) + ClONO2 reaction has a small
negative activation energy; however, k2 ∼ 13 k9 at 298 K.28-30
Unlike reaction 3, the O(3PJ) + ClONO2 reaction has a
significant positive activation energy of ∼3.5 kJ mol-1 31,33,34
,
and k3 ∼ 200 k10 at 298 K. Theoretical studies that identify the
structures and energetics of the transition states for reactions
1-3 and 8-10 would be very useful for developing a molecular
level understanding of the kinetics and dynamics of these
interesting reactions.
4. Reaction Mechanisms. Reactions 1-3 each have multiple
sets of energetically accessible products as summarized in Table
5 (enthalpies of formation used to compute the enthalpies of
reaction given in Table 5 are taken from DeMore et al.,27
Chase,35 and Orlando and Tyndall36). The experiments reported
in this paper provide no information concerning the identity of
the reaction products. However, through time-resolved studies
of NO3 production following laser flash photolysis of X2/
BrONO2/N2/O2 mixtures, Harwood et al.2 have obtained NO3
yields of 0.88 ( 0.08 and 1.04 ( 0.24 for reactions 1 and 2,
respectively. In a very recent study, Burkholder37 has employed
time-resolved detection of NO3 following 248 nm laser flash
photolysis of O3/BrONO2/N2mixtures to show that the yield of
Acknowledgments. This research was supported by the
National Aeronautics and Space AdministrationsUpper Atmo-
sphere Research Program through Grants NAG5-3634 and

1422 J. Phys. Chem. A, Vol. 105, No. 9, 2001
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