Cavity ring-down spectroscopic study of the reactions of Br atoms and BrO radicals with dimethyl sulfide

Article abstract of DOI:10.1021/jp012326f

The title reactions were studied using cavity ring-down spectroscopy in 100 Torr of N₂ diluent at 278-333 K. The equilibrium constant and rates of the forward and reverse reactions of the Br + DMS ? Br-DMS equilibrium were determined to be K =

Full text of DOI:10.1021/jp012326f

J. Phys. Chem. A 2001, 105, 11045-11050
11045
Cavity Ring-Down Spectroscopic Study of the Reactions of Br Atoms and BrO Radicals
with Dimethyl sulfide
Yukio Nakano, Masashi Goto, Satoshi Hashimoto, and Masahiro Kawasaki*
Department of Molecular Engineering, Kyoto UniVersity, Kyoto 606-8501, Japan
Timothy J. Wallington
Ford Motor Company, SRL-3083, P.O. Box 2053, Dearborn, Michigan 48121-2053
ReceiVed: June 19, 2001; In Final Form: September 26, 2001
The title reactions were studied using cavity ring-down spectroscopy in 100 Torr of N2 diluent at 278-333
K. The equilibrium constant and rates of the forward and reverse reactions of the Br + DMS a Br-DMS
equilibrium were determined to be K ) (4.1 ( 0.3) × 10-15 cm3 molecule-1, kforward ) (5.0 ( 0.2) × 10-11
cm3 molecule-1 s-1, and kbackward ) (1.2 ( 0.2) × 104 s-1 at 300 K in 100 Torr of N2. The absorption cross
section of the Br-DMS adduct at 338.3 nm was σ ) (1.2 ( 0.2) × 10-17 cm2 molecule-1. An upper limit
of k < 1 × 10-18 cm3 molecule-1 s-1 was established for the reaction of O2 with the Br-DMS adduct. The
kinetics of the reaction of BrO with DMS were well described by the Arrhenius expression k ) (1.3 ( 0.1)
× 10-14 exp[(1033 ( 265)/T] cm3 molecule-1 s-1 (k ) 4.2 × 10-13 cm3 molecule-1 s-1 at 298 K). The
uncertainties are 2 standard deviations from regression analyses.
1. Introduction
Br + CH3SCH3 + M a Br-DMS + M
Br + CH3SCH3 f HBr + CH3SCH2
(1a,-1a)
Dimethyl sulfide (DMS) is produced by phytoplankton in the
oceans and is released into the atmosphere. DMS is the largest
natural source of atmospheric sulfur.1-5 Atmospheric oxidation
of DMS is initiated by reaction with OH radicals during the
daytime6,7 and NO3 radicals at night,8-10 to give methanesul-
fonic acid (CH3SO3H), dimethyl sulfoxide (DMSO), dimethyl
sulfone (DMSO2), and sulfuric acid (H2SO4). The production
of H2SO4 leads to the formation of cloud condensation nuclei.
It has been proposed that DMS oxidation has a substantial
impact on cloud formation over the oceans and hence influences
the Earth’s radiation balance and climate system.11
Interest in global climate change and the possible biological
regulation of global climate has led to numerous studies of the
atmospheric chemistry of DMS. However, despite a significant
research effort, substantial uncertainties in our understanding
of the atmospheric chemistry of DMS remain. Recent studies
have shown that reactions of DMS with atomic halogens and
halogen monoxide radicals can play roles in global halogen and
sulfur cycles.12 Unfortunately, uncertainties in the kinetic
database for such reactions and in our understanding of the
concentrations of atomic halogens and halogen monoxide
radicals in marine air hinder our ability to quantify the
importance of such reactions in the atmospheric chemistry of
DMS.
(1b)
Depending on the equilibrium constant for the reaction 1a,-1a
and the reactivity of the Br-DMS adduct, it is conceivable that
the Br-DMS adduct plays a significant role in the chemistry
of marine air. There have been two experimental studies of the
1a,-1a equilibrium. Jefferson et al.13 and Ingham et al.15
employed laser flash photolysis with detection of Br atoms via
resonance fluorescence and concluded that the Br-DMS adduct
is sufficiently short-lived with respect to decomposition via
reaction -1a and that this species is not important in atmo-
spheric chemistry.
BrO radicals react with DMS to form Br atoms and dimethyl
sulfoxide (DMSO):16,17
BrO + CH3SCH3 f Br + CH3S(O)CH3
(2)
DMSO undergoes further oxidation to give aerosols and
condensation nuclei. There have been three studies of the
kinetics of reaction 2. Barnes et al.16 and Bedjanian et al.17 used
discharge flow mass spectrocopy to study the reaction at low
pressure in He diluent. Barnes et al.16 reported a value of k2 )
(2.7 ( 0.5) × 10-13 cm3 molecule-1 independent of total
pressure of He diluent over the range 0.5-3.6 mbar at 298 K.
Bedjanian et al.17 reported a value of k2 ) (2.7 ( 0.2) × 10-13
cm3 molecule-1 in approximately 1 Torr of He diluent at 297
K. Ingham et al.15 used laser flash photolysis combined with
UV absorption to monitor BrO radicals and reported k2 ) (4.40
( 0.66) × 10-13 cm3 molecule-1 independent of total pressure
of N2 diluent over the range 60-200 Torr at 295 K. Barnes et
al.,16 Bedjanian et al.,17 and Ingham et al.15 observed that within
the experimental uncertainties DMSO accounted for 100% of
the loss of BrO radicals in the system. The kinetic data reported
In the present work, we concentrate on the reactions of DMS
with bromine atoms and bromine monoxide radicals. At tem-
peratures typical of the troposphere (<300 K), the reaction of
Br atom with DMS proceeds via an association mechanism to
form the Br-DMS adduct,12-14 whereas at high temperatures
(> 400 K), hydrogen atom abstraction is the dominant reaction
pathway:13
* To whom correspondence should be addressed. E-mail: mkawasa7@
ip.media.kyoto-u.ac.jp. Fax: +81-75-753-5526.
10.1021/jp012326f CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/09/2001

11046 J. Phys. Chem. A, Vol. 105, No. 49, 2001
Nakano et al.
by Barnes et al.16 and Bedjanian et al.17 are in excellent
agreement but are substantially lower than those reported by
Ingham et al.15
and was measured upstream of the reaction tube by monitoring
the absorption at 253.7 nm (σ ) 6.86 × 10-18 cm2 molecule-1 32
)
using a low-pressure Hg lamp as the light source. The
concentration of DMS was measured upstream of the reaction
tube by monitoring absorption of the 213.9 nm (σ ) 1.7 × 10-18
cm2 molecule-1) output of a zinc hollow cathode lamp.33
In the study of the Br + DMS reaction, Br atoms were
generated by the 355 nm photolysis of Br2 ((8-20) × 1013
Cavity ring-down spectroscopy (CRDS) is a new absorption
spectroscopy technique based on measurement of the decay of
a laser pulse confined within a cavity between two highly
reflecting mirrors.18-22 Spectroscopic and kinetic data are
derived by measuring the increased rate of decay of the laser
pulse caused by absorption in the cavity. The CRDS technique
has been used in several kinetic studies23-28 including our studies
of BrO,29 HCO,30 and FCO31 radicals. The sensitivity of CRDS
enables low radical concentrations to be used, thus reducing
potential complications caused by radical-radical reactions. In
this study, we applied CRDS to study the kinetics of the
reactions of Br and BrO with DMS and the UV absorption of
the Br-DMS adduct. The goal of the present work is to provide
further characterization of reactions 1 and 2 to facilitate their
inclusion into global atmospheric chemistry models.
molecule cm-3). Br(2P1/2) atoms are rapidly relaxed to Br(2P3/2
)
by collision with N2.34 The initial Br atom concentration was
determined by titration with excess O3 ((4-8) × 1018 molecule
cm-3) and measurement of the resulting BrO concentration.29
Br(2P3/2) atoms react with DMS to give the Br-DMS adduct
which has a broad absorption band in the region from 300 to
450 nm.15 In the present work, the Br-DMS adduct was
monitored using its absorption at 338.3 nm. The CRD decay
curves were measured at two different wavelengths, one
corresponds to the peak position of the BrO absorption (Ia) and
the other at the valley position (Ib). By the subtraction of Ib
from Ia, the contribution of BrO is obtained. The Br-DMS
adduct undergoes rapid decomposition, and an equilibrium is
established given by reaction 1a,-1a. Temporal changes in the
concentration of the Br-DMS adduct were used to determine
the rate constant k1a, equilibrium constant K1a ) k1a/k-1a, and
absorption cross section of the Br-DMS adduct.
2. Experimental Section
The CRDS apparatus used in the present study is described
in detail elsewhere.29 The system employs two pulsed lasers.
The first laser (either 355 or 266 nm output of Nd3+:YAG laser,
Spectra Physics GCR-250) was used to photolyze a suitable
precursor to generate either Br atoms or BrO radicals. The
second laser (Spectra Physics, PDL-3; LDS 698 Dye, pumped
by the 532 nm output of a Nd:YAG laser Quantel Brilliant â)
was used to probe the concentrations of reactive species in the
system. The probe laser beam was frequency doubled by a KDP
crystal and injected through one of two high-reflectivity mirrors
which made up the ring-down cavity. The mirrors (Research
Electro Optics) had a specified maximum reflectivity of 0.9995
at 350 nm, a diameter of 7.8 mm, and a radius of curvature of
1 m and were mounted 1.04 m apart. Light leaking from one
of the mirrors of the ring-down cavity was detected by a
photomultiplier tube (Hamamatsu, R212UH) through a broad
band-pass filter (Toshiba, UVD33S). The decay of the light
intensity was recorded using a digital oscilloscope (Tektronix,
TDS430A) and transferred to a personal computer. The decay
of the light intensity is given by eq I:
In the study of the BrO + DMS reaction, BrO radicals were
produced by the 266 nm photolysis of O3 (typically 4 × 1012
molecule cm-3) in the presence of Br2 ((4-40) × 1013 molecule
cm-3) in N2 diluent at 100 Torr. O(1D) and O(3P) atoms are
generated by the photolysis of O3. O(1D) atoms are rapidly
relaxed to O(3P) by collision with N2. O(3P) atoms react with
Br2 to produce BrO radicals which then react with DMS in
reaction 2. BrO radicals were monitored using their characteristic
absorption at 338.5 nm (A2Π3/2-X2Π3/2 (7,0) band29). The decay
of BrO radicals in the presence of excess DMS was used to
provide kinetic data for the BrO + DMS reaction. Experiments
were performed in 100-200 Torr of N2, or 100 Torr of SF6,
diluent. Uncertainties reported herein are 2 standard deviations
unless otherwise stated.
All reagents were obtained from commercial sources. Br2
(99.0%) and DMS (98.0%) were subjected to repeated freeze-
pump-thaw cycling before use. N2 (>99.999%), O2 (>99.995%),
and SF6 (>99.99%) were used without further purification.
I(t) ) I0 exp(-t/τ) ) I0 exp(-t/τ 0 - σnc(LR/L)t) (I)
where I0 and I(t) are the intensities of light at time 0 and t,
respectively. τ0 is the empty cavity ring-down time (5 µs at
338.3 nm). LR is the length of the reaction region (0.55 m). L
is the cavity length (1.04 m). τ is the measured cavity ring-
down time. n and σ are the concentration and absorption cross
section of the species of interest, and c is the velocity of light.
The reaction cell consisted of a Pyrex glass tube (21 mm
i.d.), which was evacuated by a combination of an oil rotary
pump and a mechanical booster pump. The temperature of the
gas flow region was controlled by circulation of a thermostated
mixture of ethylene glycol and water and was controllable over
the range of 275-325 K. The difference between the temper-
ature of sample gas at the entrance and exit of the flow region
was <1 K. The pressure in the cell was monitored by an absolute
pressure gauge (Baratron). Gas flows were measured and
regulated by mass flow controllers (KOFLOC, 3650). A slow
flow of nitrogen diluent gas was introduced at both ends of the
ring-down cavity close to the mirrors to minimize deterioration
caused by exposure to reactants and products. The total flow
rate was kept constant at 1.0 × 103 cm3 min-1 (STP). Ozone
was produced by a silent electrical discharge in oxygen gas flow
3. Results
3.1. Kinetics of the Reaction of Br with DMS. By
measuring the concentration of BrO radicals produced following
the laser flash photolysis of Br2/O3/N2 mixtures,29 the initial Br
atom concentration in the present set of experiments, [Br]0, was
determined to be (0.4-2) × 1012 molecule cm-3
:
Br + O3 f BrO + O2
k3 ) 1.7 × 10-11 exp(800/T) cm3 molecule-1 s-1 (3)
Following the generation of Br atoms in gas mixtures containing
(0.3-3.2) × 1014 molecule cm-3 of DMS in 100 Torr of N2
diluent, a rapid increase in absorption at 338.3 nm was observed
(see Figure 1). This absorption is attributed to the formation of
the Br-DMS adduct which absorbs strongly in the region of
300-450 nm.15
The rate of increase in this absorption contains information
concerning the kinetics of reaction 1a. Diffusion of Br-DMS
adduct from the detection region can be ignored over the short
experimental time scale (10-200 ms). The DMS concentration

Cavity Ring-Down Spectroscopic Study
J. Phys. Chem. A, Vol. 105, No. 49, 2001 11047
Figure 3. Absorbance of the Br-DMS adduct at 338.3 nm as a
function of [DMS]. The solid line is a fit of eq IV to the data.
Figure 1. Pseudo-first-order rise of the Br-DMS adduct in 100 Torr
of N2 diluent at 300 K. The curve line is a fit of eq II to the data.
[CH3SCH3] ) 3.2 × 1014 molecule cm-3; [Br]0 ) 8.3 × 1011 molecule
Following addition of Br atoms to DMS to produce the Br-
DMS adduct, the reaction system reaches an equilibrium given
by reaction 1a,-1a. Figure 3 shows the absorbance at 338.3
nm of the Br-DMS adduct at equilibrium versus the concentra-
tion of DMS used in the experiment ((1-20) × 1014 molecule
cm-3) for experiments conducted at 300 K. As expected, the
equilibrium concentration of the Br-DMS adduct increased with
increasing DMS concentration and tended toward a limiting
value reflecting the condition where essentially all of the Br
atoms are associated with DMS. The dependence of the absorp-
tion at equilibrium on [DMS] can be expressed by eq IV:
cm-3
.
A ) σ (λ)LRK1a[Br]0[DMS]eq/(1 + K1a[DMS]eq) (IV)
where A and σ(λ) are absorbance and absorption cross section
of Br-DMS adduct at wavelength λ ) 338.3 nm, respectively.
K1a is an equilibrium constant of the reaction 1a,-1a. LR is the
effective path length, and [Br]0 is the initial Br atom concentra-
tion (determined via titration with ozone in a separate experi-
ment). Dimethyl sulfide is in large excess, and its concentration
at equilibrium is essentially the same as that measured prior to
generation of Br atoms in the reaction vessel.
Figure 3 shows a fit of eq IV to the data with σ(λ) and K1a
varied simultaneously to provide the best fit. As seen from
Figure 3, eq IV provides a good description of the data trend.
Figure 2. Second-order plot for the reaction of Br with DMS in 100
Torr of N2 at 300K. The line is a linear least-squares fit.
was in great excess over that of Br atoms, and the rise of
absorption attributable to the Br-DMS adduct followed pseudo-
first-order kinetics. According to reaction 1a, the formation of
the Br-DMS adduct can be analyzed using the following
equations:
The least squares regression analysis provides values of K1a
)
(4.1 ( 0.3) × 10-15 cm3 molecule-1 and σ(Br-DMS, 338.3
[Br-DMS]t ) [Br]0{K1a[DMS]/(K1a[DMS] + 1)}{1 -
nm) ) (1.2 ( 0.2) × 10-17 cm2 at 300 K in 100 Torr of N2
exp(-k11sat - (k1st/K1a[DMS])t)} (II)
+1.6
diluent, which can be compared to the cross-section (2.74
)
-1.1
1a
× 10-17 cm2, reported by Ingham et al.15 The rate constant for
the back reaction, Br + Br-DMS, was reported to be 4.2 ×
10-10 cm3 molecule s-1 by Ingham et al.15 With a kinetics
simulation program, we found that the equilibrium for the adduct
formation is achieved first, and then, the adduct is lost slowly
by attack of Br. Thus, the absolute concentrations of the BrDMS
complex were estimated with eq IV.
k11sat ) k1a[DMS]
(III)
where [Br-DMS]t is the concentration of Br-DMS at time t
1st
1a
and [Br]0 is the concentration of Br at time 0. k1a and k are
the second order and pseudo-first-order rate constants for
reaction 1a, respectively. The equilibrium constants K1a ) k1a/
k-1a at various temperatures and the absorption cross section
Equilibrium constants, K1a, were measured at various tem-
peratures over the range of 275-325 K and are given in Table
1. The van’t Hoff plot of ln(K1a[M]) versus the inverse of
temperature is shown in Figure 4. The enthalpy change, ∆H,
and entropy change, ∆S, in the equilibrium reaction 1a,-1a were
determined using eq V:
of the Br-DMS adduct at 338.3 nm are described in the
1st
1a
following section. Figure 2 shows a plot of k versus [DMS]
for experiments conducted at room temperature in a total
pressure of 100 Torr of N2 diluent. The second-order rate
constant k1a is obtained from a linear least-squares analysis of
the data in Figure 2: k1a ) (5.0 ( 0.2) × 10-11 cm3 molecule-1
s-1 at 300 K.
ln(K1a[M]) ) -∆H/RT + ∆S/R
(V)

11048 J. Phys. Chem. A, Vol. 105, No. 49, 2001
Nakano et al.
for the lifetime of the Br-DMS adduct with respect to thermal
decomposition in one atmosphere of air at ambient temperature
of 70 ms. The only atmospheric species capable of scavenging
the Br-DMS adduct on a time scale competitive with reaction
-1a is O2:
Br-DMS + O2 f DMSO + BrO
(4)
To search for BrO production from reaction 4, experiments using
the 355 nm photolysis of Br2 in DMS/O2 mixtures were
performed. However, generation of BrO radicals was not
observed with our CRDS study (maximum detection limit is 5
× 1010 cm-3). These results indicate that the Br-DMS adduct
is not reactive toward O2. Using the detection limit for BrO (5
× 1010 cm-3), the concentration of Br-DMS adduct (2.7 ×
1012 cm-3) and O2 (3.2 × 1018 cm-3), and the first-order decay
rate constant of BrO (200 s-1) via diffusion and reaction with
DMS, an upper limit value of k4 < 1 × 10-18 cm3 molecule-1
s-1 was determined. In one atmosphere of air, the pseudo first-
order loss of Br-DMS adduct with respect to reaction with O2
is <5 s-1. The atmospheric fate of the Br-DMS adduct is
decomposition via reaction -1a.
3.2. Kinetics of the Reaction of BrO with DMS. BrO
radicals were produced by the 266 nm photolysis of O3 in the
presence of Br2 at a total pressure of 100 Torr with N2 diluent
at temperatures of 278-333 K in the presence of DMS
((4-40) × 1013 molecule cm-3). Under these conditions, BrO
radicals are lost via reaction with DMS and via self-reaction:27
Figure 4. van’t Hoff plot for the equilibrium 1a,-1a; Br + DMS a
Br-DMS adduct.
TABLE 1: Values of the Br + DMS a Br-DMS
Equilibrium Constant Measured in the Present Work in 100
Torr of N2 Diluent
temp (K)
K1a (10-15 cm3 molecule-1
)
275
285
295
300
305
315
325
19.2 ( 1.8
10.3 ( 0.7
4.08 ( 0.29
4.16 ( 0.22
2.22 ( 0.22
1.58 ( 0.20
0.54 ( 0.18
2BrO f 2Br + O2
k5a ) 2.4 × 10-12 exp(40/T) cm3 molecule-1 s-1 (5a)
f Br2 + O2
where [M] is a concentration of third body. R is the gas constant.
By fitting eq V to the data in Figure 4, values of ∆H ) -51 (
4 kJ mol-1 and ∆S ) -93 ( 12 J mol-1 K-1 were obtained.
These values are in excellent agreement with the measurements
by Jefferson et al.13 (∆H ) -61 ( 5 kJ mol-1 and ∆S ) -96
( 16 J mol-1 K-1 measured at 334 K for 760 Torr). The Br-
S bond is formed in reaction 1a, and the value of ∆H ) -51
( 4 kJ mol-1 is the Br-S bond strength in the Br-DMS adduct.
The value of K1a ) (4.1 ( 0.3) × 10-15 cm3 molecule-1 at
300 K can be combined with our determination of k1a ) (5.0 (
0.3) × 10-11 cm3 molecule-1 s-1 to give the rate constant for
unimolecular decomposition of the Br-DMS adduct at 300 K;
k-1a ) (1.2 ( 0.2) × 104 s-1. This result is consistent with the
value of k-1a ) (1.02 ( 0.07) × 104 s-1 at 295 K in 100 Torr
of N2 reported recently by Ingham et al.15
The rapid unimolecular decomposition of the Br-DMS
adduct reflects the weakness of the Br-S bond (51 ( 4 kJ
mol-1). It is interesting to contrast the instability of the Br-
DMS adduct to the relative stability of the Cl-DMS adduct
which does not show any evidence of decomposition on the
millisecond time scale.35 The experimental data indicate that
the Cl-DMS bond is considerably stronger than the Br-DMS
bond.35 There have been three computational determinations of
the Cl-DMS bond strength.36-38 McKee36 and Resende and De
Almeida37 calculated Cl-S bond strengths of 51.0 and 51.5 kJ
mol which are indistinguishable from the experimental data for
the Br-S bond strength in the Br-DMS adduct. In contrast,
Wilson and Hirst38 report a value of 80.9 kJ mol for the Cl-S
bond which is significantly greater than that of the Br-S bond.
It appears that Cl-DMS bond strength has been underestimated
in the work of McKee36 and Resende and De Almeida.37
The value of k-1a > (1.4 ( 0.2) × 104 s-1 measured herein
in 100 Torr total pressure can be used to estimate an upper limit
k5b ) 2.8 × 10-14 exp(860/T) cm3 molecule-1 s-1 (5b)
Assuming that BrO radicals are lost via reactions 2, 5a, and
5b and diffusion from the detection region, the decay of BrO
can be analyzed as a sum of first- and second-order kinetics
using eq VI:
2k5
2k5
1
1
)
+
exp(k12stt) -
(VI)
k12st
k2
1st
{(
)
(
)}
[BrO]t
[BrO]0
where [BrO]t and [BrO]0 are the concentrations of BrO at time
t and 0, respectively. k5 is the rate constant of reactions 5a and
5b (k5 ) k5a + k5b). k1st is the sum of the pseudo-first-order
2
rate losses of BrO radicals via reaction 2 and diffusion from
the detection region:
k12st ) k2[DMS] + kd
(VII)
The regeneration reaction of BrO from Br + O3 is slow under
our experimental conditions (typical time scale for BrO regen-
eration was a few hundred ms). Because of the high concentra-
tion of CH3SCH3, the following successive reactions of Br were
considered:
BrO + DMS f Br + DMSO
Br + DMS f Br-DMS
(2)
(1a)
(6)
Br-DMS + Br f Br2 + DMS
Even with k6 ) 4.2 × 10-10 cm3 molecule s-1 reported by
Ingham et al.,15 eq VI is applicable under our experimental

Cavity Ring-Down Spectroscopic Study
J. Phys. Chem. A, Vol. 105, No. 49, 2001 11049
Figure 5. Pseudo-first-order decay profile of BrO. The curve is a fit
of eq VI to the data in 100 Torr of N2 diluent at 300 K. P(N2) ) 100
Torr; [CH3SCH3] ) 3.7 × 1014 molecule cm-3; [O3]0 ) 4 × 1012
Figure 7. Arrhenius plot for the reaction of BrO with DMS. Present
work (closed circle) and Ingham et al. of ref 15 (open circle).
molecule cm-3
.
TABLE 2: Values of the Rate Constant of the BrO + DMS
Reaction Measured in the Present Work in 100 Torr of N2
Diluent
temp (K)
k2 (10-13 cm3 molecule-1 s-1
)
278
283
300
308
313
333
5.13 ( 1.66
4.76 ( 0.77
4.60 ( 0.60
4.07 ( 0.47
2.87 ( 0.56
2.88 ( 0.15
energy (Ea ) - 8.6 ( 2.2 kJ mol-1) determined herein is
consistent with that determined in the low pressure study of
Bedjanian et al.17 (Ea ) - 7.0 ( 1.5 kJ mol-1). However, the
absolute values of k2 measured herein are substantially greater
(by 50% at ambient temperature) than the values reported by
Bedjanian et al.17 The negative activation energy suggests that
the reaction of BrO with DMS proceeds via an associated
complex as shown below:
Figure 6. Second-order plot for the reaction of BrO with DMS in
100 Torr of N2 diluent at 300K. The line is a linear least-squares fit.
BrO + DMS a [BrO-DMS] f Br + DMSO
(2)
conditions, that is, under relatively low radical concentrations.
Figure 5 shows an example of the temporal profile of BrO and
a fit of eq VI to the data. Figure 6 shows k12st versus [DMS] in
100 Torr of N2 diluent at 300 K, linear least-squares analysis
gives k2 ) (4.6 ( 0.6) × 10-13 cm3 molecule-1 s-1. This result
is in excellent agreement with the finding of Ingham et al.15
that k2 ) (4.4 ( 0.7) × 10-13 cm3 molecule-1 s-1 at 295 K.
The aim of the present work was to provide kinetic data for
reaction 2 suitable for inclusion into atmospheric models. The
question arises as to whether a value of k2 obtained in 100 Torr
of N2 diluent provides a good description of the kinetics in 760
Torr of air diluent. To test for an effect of the nature or total
pressure of diluent gas, additional experiments were performed
using 100 Torr of SF6 or 200 Torr of N2 diluent which gave
values of k2 ) (4.5 ( 0.3) × 10-13 and (4.5 ( 0.8) × 10-13
cm3 molecule-1 s-1, respectively. The insensitivity of k2 to the
nature and pressure of diluent gas suggests that values of k2
determined in 100 Torr of N2 are appropriate for use in
atmospheric models. This conclusion is consistent with the
observation of Ingham et al.,15 that there was no effect of total
pressure over the range 60-200 Torr of N2 diluent.
As discussed above, the available experimental data suggest
that the Arrhenius expression derived in this work from
experiments conducted in 100 Torr of N2 diluent is appropriate
for representing the kinetics of this reaction at atmospheric
pressure. Consequently, we recommend use of k2 ) (1.3 ( 0.1)
× 10-14 exp[(1033 ( 265)/T] cm3 molecule-1 s-1 in global
atmospheric models.
4. Atmospheric Implications
The equilibrium constant for the Br-DMS adduct K1a
determined here can be combined with an estimate of [Br] ) 5
× 105 molecule cm-3 and a typical value of [DMS] ) 5 × 109
molecule cm-3 to derive an estimate of 10 molecule cm-3 for
the concentration of the Br-DMS adduct in the marine
boundary layer.39,40 This extremely small concentration com-
bined with the fact that the adduct does not react rapidly with
O2 renders the Br-DMS adduct of essentially no importance
in atmospheric chemistry.
The present study is the first study of the kinetics of the BrO
+ DMS reaction as a function of temperature at high pressure.
It was determined that k2 ) (4.6 ( 0.6) × 10-13 cm3 molecule-1
s-1 in 100 Torr of N2 at 300 K. This result is approximately
1.8 times greater than the NASA data panel’s recommendation32
of k2 ) 2.6 × 10-13 cm3 molecule-1 s-1 which was based upon
the low-pressure data of Barnes et al.16 and Bedjanian et al.17
However, our result is in an excellent agreement with that
To determine the Arrhenius expression for the reaction of
BrO with DMS, rate constants of reaction 2 were measured at
various temperatures (278-333 K). The results are listed in
Table 2 and plotted in Figure 7. Linear least-squares analysis
of the data in Figure 7 gives k2 ) (1.3 ( 0.1) × 10-14 exp-
[(1033 ( 265)/T] cm3 molecule-1 s-1. The negative activation

11050 J. Phys. Chem. A, Vol. 105, No. 49, 2001
Nakano et al.
reported recently by Ingham et al.,15 k2 ) (4.4 ( 0.6) × 10-13
cm3 molecule-1 s-1 in 60-200 Torr of N2 diluent at 295 K. It
appears that previous measurements carried out at low pressures
(<4 Torr of He) do not provide a reliable measure of the kinetics
of the BrO + DMS reaction under atmospheric conditions.
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The kinetic data for the BrO + DMS reaction measured herein
support and extend (to temperatures other than ambient) the
recent findings of Ingham et al.15 The present work taken
together with that of Ingham et al.15 provides compelling
evidence that the values of k2 used in present global atmospheric
models are too low by approximately a factor of 1.8. Ingham
et al.15 used a chemical box model (MOCCA40,41) to assess the
importance of reaction 2 in the atmospheric chemistry of DMS.
The MOCCA model provides a detailed description of the loss
of DMS via reaction with OH, NO3, Cl, Br, BrO, and IO radicals
in the marine boundary layer. Using their value of k2 ) (4.4 (
0.7) × 10-13 cm3 molecule-1 s-1 at 295 K, Ingham et al.15 found
that incorporation of reaction 2 into the MOCCA model lead
to a decrease in [DMS] by approximately a factor of 2 and an
increase in [DMSO] by approximately a factor of 4. It is clear
that reaction with BrO radicals is an important sink for DMS
and an important source of DMSO in marine environments. As
seen in Figure 7, the results from the present work show that
reaction 2 has a negative temperature dependence given by k2
) (1.3 ( 0.1) × 10-14 exp[(1033 ( 265)/T] cm3 molecule-1
s-1. Use of this expression in the MOCCA model would increase
the importance of reaction 2 slightly (by approximately 10%)
over that reported by Ingham et al.15
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Finally, it should be noted that BrO radicals are regenerated
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BrO + DMS f Br + DMSO
Br + O3 f BrO + O2
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Ozone is present in the marine boundary layer at a concentration
which is typically 1000 times greater than that of DMS.
Although reaction 2 is an important loss mechanism for DMS
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Acknowledgment. The authors thank Mr. Shinji Nakamichi
for his help with the experiments. This work was supported by
a Grant-in-Aid for the priority research field “Radical Reactions”
from the Ministry of Education, Japan, and a NEDO Interna-
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