Rate coefficients for the gas-phase reaction of OH radicals with dimethyl sulfide: Temperature and O2 partial pressure dependence

Article abstract of DOI:10.1039/b512536g

Rate coefficients for the gas-phase reaction of hydroxyl (OH) radicals with dimethyl sulfide (CH₃SCH₃, DMS) have been determined using a relative rate technique. The experiments were performed under different conditions of temperature (250-299 K) and O₂ partial pressure (～0 Torr O₂-380 Torr O₂), at a total pressure of 760 Torr bath gas (N₂ + O₂), in a 336 l reaction chamber, using long path in situ Fourier transform (FTIR) absorption spectroscopy to monitor the disappearance rates of DMS and the reference compounds (ethene, propene and 2-methylpropene). OH was produced by the photolysis of H₂O ₂. The following Arrhenius expressions adequately describe the rate coefficients as a function of temperature (units are cm³ molecule^-1 s^-1): k = (1.56 ± 0.20) × 10 ^-12 exp[(369 ± 27)/T], for ～0 Torr O₂; (1.31 ± 0.08) × 10^-14 exp[(1910 ± 69)/T], for 155 Torr O₂; (5.18 ± 0.71) × 10^-14 exp[(1587 ± 24)/T], for 380 Torr O₂. The results are compared with previous investigations. the Owner Societies 2006.

Full text of DOI:10.1039/b512536g

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Rate coefficients for the gas-phase reaction of OH radicals with dimethyl
sulfide: temperature and O2 partial pressure dependence
Mihaela Albu,ab Ian Barnes,*a Karl H. Becker,a Iulia Patroescu-Klotz,a
Raluca Mocanub and Thorsten Bentera
Received 5th September 2005, Accepted 1st November 2005
First published as an Advance Article on the web 23rd November 2005
DOI: 10.1039/b512536g
Rate coefficients for the gas-phase reaction of hydroxyl (OH) radicals with dimethyl sulfide
(CH3SCH3, DMS) have been determined using a relative rate technique. The experiments were
performed under different conditions of temperature (250–299 K) and O2 partial pressure (B0
Torr O2ꢀ380 Torr O2), at a total pressure of 760 Torr bath gas (N2 + O2), in a 336 l reaction
chamber, using long path in situ Fourier transform (FTIR) absorption spectroscopy to monitor
the disappearance rates of DMS and the reference compounds (ethene, propene and 2-
methylpropene). OH was produced by the photolysis of H2O2. The following Arrhenius
expressions adequately describe the rate coefficients as a function of temperature (units are cm3
moleculeꢀ1
s
ꢀ1): k = (1.56 ꢁ 0.20) ꢂ 10ꢀ12 exp[(369 ꢁ 27)/T], for B0 Torr O2; (1.31 ꢁ 0.08) ꢂ
10ꢀ14 exp[(1910 ꢁ 69)/T], for 155 Torr O2; (5.18 ꢁ 0.71) ꢂ 10ꢀ14 exp[(1587 ꢁ 24)/T], for 380
Torr O2. The results are compared with previous investigations.
creasing as the partial pressure of oxygen was increased. The
Introduction
‘‘oxygen enhancement’’ was the same for both DMS and its
deuterated analog, showing that there was no kinetic isotope
effect and a significant negative temperature dependence was
found.
The experimental kinetic observations of Hynes et al.11 for
the reaction of OH radicals with DMS and DMS-d6 are
consistent with a two-channel reaction mechanism involving
both hydrogen abstraction (O2-independent channel) and
reversible adduct formation in competition with adduct reac-
tion with O2 (O2-dependent channel):
Dimethyl sulfide (CH3SCH3, DMS) is the dominant natural
sulfur compound emitted from the world’s oceans,1,2 account-
ing for about one quarter of global sulfur gas emissions.
Oceanic DMS, through its oxidation products, is proposed
to play an important role in climate regulation, especially in
the remote marine atmosphere.3
At the low NOx levels that characterizes the remote marine
boundary layer, reaction with OH is the most important loss
process for DMS:
OH + CH3SCH3 - CH3SCH2 + H2O
(1a)
OH + DMS - products
(1)
OH + CH3SCH3 (+M) 2
CH3S(OH)CH3 (+M)
The kinetics of the reaction of OH radicals with DMS have
been extensively studied by a variety of direct and competitive
techniques. The early studies of the reaction4–18 led to conflict-
ing results both with regard to the magnitude of the rate
coefficient and the activation energy where both positive and
negative dependencies were reported. The situation with re-
spect to the rate coefficient and activation energy for the
reaction remained confused until it was demonstrated beyond
doubt by several groups11,12,14 that the rate coefficient was
affected by the presence of O2 in the reaction system and also
that relative kinetic measurements performed in the presence of
NOx resulted in erroneously high values for the rate coefficient.
In a comprehensive flash photolysis-resonance fluorescence
(FP-RF) investigation Hynes et al.11 found that the effective
rate coefficient for OH + DMS and its deuterated analog
(DMS-d6) was dependent on the oxygen concentration, in-
(1b, 1ꢀb)
(1c)
CH3S(OH)CH3 + O2 - products
OH + CD3SCD3 - CD3SCD2 + HDO
(2a)
OH + CD3SCD3 (+M) 2
CD3S(OH)CD3 (+M)
(2b, 2-b)
(2c)
CD3S(OH)CD3 + O2 - products
In the work of Hynes et al.11 the temperature dependence of
the rate coefficient for DMS + OH in 730 ꢁ 30 Torr air,
together with temperature dependence of the branching ratio
were determined. Using the data the authors derived the
following expression for kobs for one atmosphere of air:
kobs = {T exp(ꢀ234/T) + 8.46 ꢂ 10ꢀ10 exp(7230/T) + 2.68
ꢂ 10ꢀ10 exp(7810/T)}/{1.04 ꢂ 1011 T + 88.1 exp(7460/T)}
a Bergische Universitat Wuppertal, Physikalische Chemie/FB C, Gauss
¨
where kobs is the total overall rate coefficient for the abstrac-
tion and addition reaction channels.
Strasse 20, D-42097 Wuppertal, Germany. E-mail: barnes@uni-
wuppertal.de
b ‘‘Al.I. Cuza’’ University of Iasi, Faculty of Chemistry, Department
of Analytical Chemistry, Carol I Boulevard 11, 700 506 Iasi,
Romania
The branching ratio for the abstraction channel is given by:
k1a/kobs = 9.6 ꢂ 10ꢀ12 exp(ꢀ234/T) /kobs
ꢃc
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For many years, the two-channel mechanism has been widely
accepted as the operative mechanism for the OH-initiated
oxidation of DMS, and expressions for the rate coefficient
and inferred branching ratio recommended as the best cur-
rently available for mechanistic interpretation and atmo-
spheric modeling purposes. The original expressions of
Hynes et al. were also modified to take into account the O2
dependence of the reaction rate.19
FTIR spectrometer (Nicolet Magna 550, KBr beam splitter,
MCT detector) for in situ detection of reactants and products.
A detailed description of the experimental setup can be found
elsewhere.22
The kinetic experiments to determine rate coefficients for the
reaction of OH with DMS were performed at a total pressure
of 760 Torr diluent gas (N2, synthetic air, or N2/O2 mixture) at
three different O2 partial pressures (B0, 155 and 380 Torr) and
six different temperatures (250, 260, 270, 280, 290 and 299 K).
The photolysis of hydrogen peroxide (H2O2) was used as the
OH radical source:
A re-evaluation of the rate coefficient and the branching
ratio has been made by Williams et al.20 using the pulsed laser
photolysis-pulsed laser induced fluorescence (PLP-PLIF) tech-
nique. The effective rate coefficient for the reaction of OH +
DMS and OH + DMS-d6 was determined as a function of O2
partial pressure at 600 Torr total pressure in N2/O2 mixtures;
measurements were made at 240 K for DMS and at 240, 261,
and 298 K for DMS-d6. This new work showed that at low
temperatures the recommended expression at that time11,19
underestimated both the effective rate coefficient for the reac-
tion and also the branching ratio between addition and
abstraction. For example, at 261 K a branching ratio of 3.6
was obtained as opposed to the value of 2.8 obtained in the
work of Hynes et al.11 (the branching ratio is defined here as
(kobs ꢀ k1a)/k1a); at 240 K the discrepancy was found to
increase between a measured value of 7.8 by Williams et al.
and a value of 3.9 obtained from an extrapolation using the
empirical expression of Hynes et al.11 In addition, at 240 K the
expression for kobs in 760 Torr of air based on the work of
Hynes et al.11 predicts a value which is a factor of 2 lower then
the measured value at 600 Torr of Williams et al.
H2O2 + hn (l o 370 nm) - 2OH
Ethene, propene and 2-methylpropene were used as the re-
ference compounds in the investigations. The rate coefficients
for the reactions of these compounds with OH are well
established23 and are listed in Table 1 as a function of
temperature.
The starting concentrations (calculated at the working
temperature) of DMS and of the reference organic compounds
were typically in the range 1 to 3 ppm and that of H2O2 was
approximately 20 ppm. After introducing the reactants into
the chamber, the reaction chamber was pressurized to 760
Torr with nitrogen, synthetic air, or nitrogen/oxygen depend-
ing on the desired partial pressure of O2.
The concentration–time behavior of DMS and the reference
organic compounds was followed over 40–50 min time periods
by FTIR spectroscopy. Spectra were obtained by co-adding 64
scans which yielded a time resolution of 1 min.
The new work of Williams et al.20 has been incorporated
into the latest IUPAC Gas Kinetic Data Evaluation.21 The
preferred value for the rate coefficient of reaction (1a) from the
review is now k1a = 1.13 ꢂ 10ꢀ11 exp(ꢀ253/T) cm3 moleculeꢀ1
Rate coefficients for the reactions of DMS with OH radicals
were determined using a relative rate method in which the
relative disappearance rates of DMS and a reference com-
pound are monitored parallelly in the presence of OH radicals:
s
ꢀ1 over the temperature range 240–400 K (k1a = 4.8 ꢂ 10ꢀ12
cm3 moleculeꢀ1 sꢀ1 at 298 K). This value is based on the
studies on Wine et al.7, Hynes et al.,11 Hsu et al.,13 Abbatt et
al.,16 and Barone et al.18 The preferred value for the rate
coefficient of reaction (1b) from the review is now k1b = 1.0 ꢂ
10ꢀ39 [O2] exp(5820/T)/{1 + 5.0 ꢂ 10ꢀ30 [O2] exp(6280/T)}
cm3 moleculeꢀ1 sꢀ1 over the temperature range 240–360 K
(k1b = 1.7 ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1 at 298 K, 760 Torr of
air). This value is based on the studies of Hynes et al.11
and Williams et al.20
OH + DMS - products kDMS
OH + reference - products kref
Providing that DMS and the reference are removed solely by a
reaction with the OH radicals, then
ꢀ
ꢁ
ꢀ
ꢁ
½DMSꢄt
½DMSꢄt0
½refꢄt
½refꢄt0
kDMS
kref
ln
¼
ln
ðIÞ
The aim of the present study is to obtain an independent
confirmation, using a relative kinetic technique, of the large
increase in the rate coefficients for OH with DMS measured at
low temperatures by Williams et al.20 as opposed to the
extrapolated values obtained with the empirical fit given by
Hynes et al.11
where [DMS]t0 and [ref]t0 are the concentrations of DMS and
the reference compound, respectively, at time t0; [DMS]t and
[ref]t are the corresponding concentrations at time t; kDMS and
kref are the rate coefficients for the reaction with OH radicals
of DMS and the reference compound, respectively.
Experimental
Table 1 Rate coefficients for reaction of the reference compounds
with OH radicals (k = A eꢀE /RT, T B 250–425 K)23
a
The experiments were carried out in a 336 l glass reaction
chamber equipped with a system for temperature regulation to
within ꢁ1 K from 230 to 300 K and 2 types of lamps: 6 Philips
TLA 40 W (300 r l r 450 nm, lmax = 360 nm) and 6 Philips
TUV 40 W (lmax = 254 nm). The reaction chamber is also
equipped with an internally mounted multi-reflection White
mirror system (47.7 m total optical path length) coupled to a
Reference compound
1012 k (298 K)a
1012 Aa
ꢀEa/R (K)
Ethene
Propene
2-Methylpropene
8.52
26.3
51.4
1.96
4.85
9.47
438
504
504
a
cm3 moleculeꢀ1 sꢀ1
ꢃc
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Under the experimental conditions, DMS and the reference
compounds may additionally undergo photodissociation and/
or be lost to the chamber surface.
2 lists the reference compounds used for each set of experi-
mental conditions, the measured rate coefficient ratios and the
rate coefficients derived for the reaction of OH radicals with
DMS. The errors given for the rate coefficient ratios in Table 2
are two least-squares standard deviations (ꢁ2s); the quoted
errors for the rate coefficients in Table 2 are a combination of
the 2s statistical errors from the linear regression analysis plus
an additional 20% to cover uncertainties associated with the
values of the reference rate coefficients. For every temperature
The photostability of DMS and of each reference com-
pound was established by irradiation of DMS–reference com-
pound–diluent gas mixtures in the absence of OH radical
precursors for a time period as long as that employed in the
kinetic experiments, after switching on the lamps. The refer-
ence compounds employed were not photolysed and their wall
losses were also very low, often below the detection limit. For
DMS, wall losses accounted for approximately 5% of its
measured decay at room temperature and increased gradually
to around 30% at the lowest temperature employed in the
experiments:
and pO = 155 Torr, the rate coefficients were measured
2
relative to two or three different references, thus, the final
values given in Table 2 for these conditions of the rate
coefficients (kDMS(average)) are averages of those determined
using the two or three different reference compounds together
with error limits which encompass the extremes of the indivi-
dual determinations. Given in Table 2 is also the tropospheric
lifetime of DMS at each temperature calculated using the
annually averaged global tropospheric hydroxyl radical concen-
tration of B1 ꢂ 106 molecule cmꢀ3 (24 h average) (Prinn et al.24).
Several trends can be seen in the kinetic data listed in
Table 2:
DMS (+wall) - products kloss
The wall loss of DMS was measured at the beginning of every
experiment by recording 10–15 spectra prior to irradiation. To
check that the wall loss of DMS was constant over the time
period of the experiments the loss was measured on a regular
basis after termination of the irradiation. No significant
change in the DMS wall loss rate was ever observed between
the pre- and post-irradiation periods. Incorporating the first-
order loss process of DMS into eqn (I) leads to eqn (II):
(i) at a fixed temperature the overall rate coefficient for
DMS + OH increases with increasing partial pressure of
oxygen;
ꢀ
ꢁ
ꢃ
ꢀ
ꢁꢂ
ꢄ
(ii) at a constant O2 partial pressure the rate coefficient is
observed to increase quite appreciably with decreasing tem-
½DMSꢄt
½DMSꢄt0
½refꢄt
½refꢄt0
ln
ꢀ kloss ꢂ t ¼ kDMS ln
kref
ðIIÞ
perature, i.e. by factors of 3.6 and 3.0 for pO = 155 and 380
2
Torr between 299 and 250 K, respectively;
where t is the reaction time and kloss is the loss rate of DMS.
(iii) in N2 with pO =B0 Torr where the abstraction path-
According to eqn (II), a plot of ln{[DMS]t0/[DMS]t} ꢀ kloss
ꢂ
2
way for the reaction will dominate the rate coefficient for DMS
+ OH is observed to increase very slightly with decreasing
temperature. However, the observed increase does not exceed
the combined error limits of the measurements.
t against ln{[ref]t0/[ref]t}/kref gives straight lines directly yield-
ing kDMS as the slope. Eqn (II) allows the data for all reference
hydrocarbons employed for a set of experimental conditions to
be plotted and evaluated simultaneously and gives a direct
visual view of the quality of the agreement of the experimental
data obtained from the different reference compounds.
For each temperature and pO = B0 Torr and pO = 380
The data given in Table 2 are plotted in Arrhenius form in
Fig. 2 for the three O2 partial pressures investigated. The solid
lines are the least-squares fits to the data. The Arrhenius
expressions derived from these fits are given in Table 3 for
the individual O2 partial pressures.
2
2
Torr, one reference compound (ethene) was employed, and for
each temperature and pO = 155 Torr, two or three different
2
reference compounds (ethene, propene, 2-methylpropene)
were employed. For each set of experimental conditions, at
least three experimental runs were performed.
Possible source of errors in the system
Although we report here results from experiments performed in
All the reactants were used without further purification.
Dimethyl sulfide was supplied by Sigma Aldrich with a stated
purity of 99%. H2O2 (85% wt) was supplied by Peroxide
Chemie GmbH. The reference hydrocarbons: ethene, propene,
and 2-methylpropene, were supplied by Messer-Griesheim
with stated purities of 99.95, 99.95, and Z 99%, respectively.
The following gases were supplied by Messer-Griesheim:
nitrogen (99.999%), oxygen (99.999%), and hydrocarbon free
synthetic air (99.999%) with a N2 : O2 ratio of 20.5 : 79.5%.
760 Torr N2 with pO = 0 Torr in reality the system will contain
2
very small concentrations of O2. Apart from O2 entering the
system through leaks in the apparatus the use of hydrogen
peroxide, as the OH radical source, can lead to production of
O2 via (i) heterogeneous decomposition on the reactor surface
(possibly the largest source) and (ii) chemical reactions in the
system. HO2 radicals are produced in the reaction of OH with
DMS and they are produced also by the reaction:
H2O2 + OH - H2O + HO2
(3)
Results and discussion
The self-reaction of HO2 and reaction with OH result in the
formation of O2:
The kinetic data obtained from the investigations on the
reaction of OH with DMS at 6 temperatures for different
partial pressures of O2 are shown plotted according to eqn (II)
in Fig. 1. Reasonably linear plots were obtained in all cases.
Significant scatter in the measurement points was only ob-
served at the lowest temperature investigated, i.e. 250 K. Table
HO2 + HO2 - H2O2 + O2
HO2 + OH - H2O + O2
(4)
(5)
The above reactions in combination with heterogeneous decay
of H2O2 will always provide small quantities of oxygen in the
ꢃc
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Fig. 1 Plot of the kinetic data according to eqn (II) for the reaction of DMS with OH radicals at different temperatures and oxygen partial
pressures: (a) =B0 Torr O2, (b) = 155 Torr O2, (c) = 380 Torr O2; references: J-ethene, &-propene, and n-2-methylpropene.
ꢃc
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Table 2 Rate coefficient ratios kDMS/kref and rate coefficients kDMS for the gas phase reaction of OH radicals with DMS at different temperatures
and O2 partial pressures
O2 partial
pressure/Torr
tDMS = 1/kDMS
[OH] (h)
Temperature/K
299
Reference
kDMS/kref
kDMS ꢂ 1011a
0.50 ꢁ 0.10
0.80 ꢁ 0.16
0.75 ꢁ 0.15
0.95 ꢁ 0.19
kDMS (average) ꢂ 1011a
0.78 ꢁ 0.18
B0
155
Ethene
Ethene
Propene
Ethene
0.59 ꢁ 0.01
0.94 ꢁ 0.02
0.28 ꢁ 0.01
1.13 ꢁ 0.02
35.6
28.3
23.1
18.4
13.9
380
290
280
270
260
B0
155
Ethene
Ethene
Propene
Ethene
0.64 ꢁ 0.02
1.13 ꢁ 0.02
0.33 ꢁ 0.01
1.51 ꢁ 0.04
0.57 ꢁ 0.11
1.00 ꢁ 0.20
0.93 ꢁ 0.18
1.34 ꢁ 0.27
0.98 ꢁ 0.23
1.20 ꢁ 0.26
1.51 ꢁ 0.34
1.99 ꢁ 0.47
380
B0
155
Ethene
Ethene
Propene
Ethene
0.67 ꢁ 0.02
1.27 ꢁ 0.03
0.42 ꢁ 0.01
1.64 ꢁ 0.03
0.62 ꢁ 0.13
1.19 ꢁ 0.24
1.22 ꢁ 0.24
1.54 ꢁ 0.31
380
B0
155
Ethene
Ethene
Propene
Ethene
0.64 ꢁ 0.02
1.53 ꢁ 0.06
0.47 ꢁ 0.01
1.87 ꢁ 0.05
0.63 ꢁ 0.14
1.52 ꢁ 0.31
1.47 ꢁ 0.30
1.85 ꢁ 0.37
380
B0
155
Ethene
Ethene
Propene
2-Methylpropene
Ethene
0.60 ꢁ 0.03
1.93 ꢁ 0.04
0.58 ꢁ 0.01
0.30 ꢁ 0.01
2.25 ꢁ 0.12
0.64 ꢁ 0.13
2.04 ꢁ 0.40
1.97 ꢁ 0.39
2.00 ꢁ 0.40
2.38 ꢁ 0.47
380
250
B0
155
Ethene
Ethene
Propene
2-Methylpropene
Ethene
0.58 ꢁ 0.05
2.37 ꢁ 0.30
0.77 ꢁ 0.04
0.41 ꢁ 0.02
2.51 ꢁ 0.23
0.66 ꢁ 0.14
2.68 ꢁ 0.63
2.80 ꢁ 0.57
2.96 ꢁ 0.60
2.83 ꢁ 0.62
2.82 ꢁ 0.77
9.85
380
cm3 moleculeꢀ1 sꢀ1
a
system, leading to the formation of oxygenated compounds
even when ‘‘nitrogen’’ is used as the bath gas. Using the known
leak rate of the reactor under vacuum it is estimated that in the
‘‘nitrogen’’ experiments pO is probably less than 30 ppm.
2
Based on the measured O2 enhancement on the rate coefficient
for DMS + OH these low O2 concentrations in the N2
experiments will make a negligible contribution to the mea-
sured rate coefficient.
Another potential source of error is the reaction of HO2
radicals with DMS:
HO2 + CH3SCH3 - OH + CH3S(O)CH3
(6)
An upper limit of the rate coefficient for reaction (6) of o5 ꢂ
10ꢀ15 cm3 moleculeꢀ1 sꢀ1 at 298 K has been reported.21
However, the good agreement between the rate coefficient
for OH + DMS determined in N2 at 299 K in this study with
those obtained by other workers (as discussed later) leads to
the conclusion that both reaction (6) and the low traces of O2
have probably a negligible influence on the measurements.
Table 3 Arrhenius expressions valid in the temperature range from
250 to 299 K
O2 partial presurre/Torr
kDMS/cm3 moleculeꢀ1 sꢀ1
Fig. 2 Arrhenius plots of the rate coefficients determined for DMS +
OH in the temperature range 250 to 299 K for various O2 partial
pressures.
B0
155
380
(1.56 ꢁ 0.20) ꢂ 10ꢀ12 exp[(369 ꢁ 27)/T]
(1.31 ꢁ 0.08) ꢂ 10ꢀ14 exp[(1910 ꢁ 69)/T]
(5.18 ꢁ 0.71) ꢂ 10ꢀ14 exp[(1587 ꢁ 24)/T]
ꢃc
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Table 4 Room temperature rate coefficient for DMS + OH reaction
in absence of O2
ported room temperature rate coefficient in the absence of O2,
which are substantially lower. Potential production of O
impurities in the systems which can react with DMS to
regenerate OH radicals on the time scale of the experiment
and thus lead to a low effective rate constant has been
postulated as an explanation for the low values. O atoms
can be formed either in high concentrations from microwave
discharges (which could affect flow tube experiments) or from
high concentrations of OH, via the reaction OH + OH -
H2O + O. The experiments of Martin et al.10 and Nielsen
et al.,15 for example, used extremely high OH radical concen-
trations.
kDMS ꢂ 1012/cm3
Pressure
(Torr)/bath gas
moleculeꢀ1 sꢀ1
T/K
Technique
Ref.
4.26 ꢁ 0.56
3.22 ꢁ 1.16
4.09 ꢁ 1.16
4.44 ꢁ 0.23
4.75 ꢁ 0.15
4.80 ꢁ 0.11
3.60 ꢁ 0.20
5.50 ꢁ 1.00
3.50 ꢁ 0.20
4.98 ꢁ 0.46
4.95 ꢁ 0.35
5.30 ꢁ 0.50
4.40 ꢁ 0.40
4.80
298
293
298
298
298
298
297
298
295
297
298
296
298
298
299
50/Ar
0.5/He
40/Ar
30/Ar
500/SF6
40/N2
50–400/Ar
0.8–3/He
760/Ar
10–100/N2
100/He/N2/SF6
740/Ar
760/N2
—
FP-RFa
7
DF-EPRa
FP-RFa
10
11
11
11
11
12
13
15
16
18
12
14
21
tw
FP-RFa
PLP-PLIFa
PLP-PLIFa
FP-RFa
DF-RFa
PR-KSa
HPF-LIFa
PLP-PLIFa
DS-FTIRb
CP-GCb
The new recommended Arrhenius expression for reaction
(1a) of k1a = 1.13 ꢂ 10ꢀ11 exp(ꢀ253/T) cm3 moleculeꢀ1 sꢀ1
over the temperature range 240–400 K from the review of
Atkinson et al.21 gives k1a = 4.8 ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1
at 298 K. The value of (5.00 ꢁ 1.00) ꢂ 10ꢀ12 cm3 moleculeꢀ1
sꢀ1 determined in this study at 299 K is in good agreement
with the recommended value.
Review
CP-FTIRb
5.00 ꢁ 1.00
760/N2
a
Absolute technique: FP = flash photolysis, RF = resonance fluor-
escence, DF = discharge flow, EPR = electron paramagnetic reso-
nance, PLP = pulsed laser photolysis, PLIF = pulsed laser induced
fluorescence, PR = pulse radiolysis, KS = kinetic spectroscopy, HPF
The Arrhenius expression derived in this work from the
b
measurement in N2 with pO = B0 Torr for DMS + OH in
= high pressure flow, LIF = laser induced fluorescence; Relative
2
technique: DS = dark source of OH (O3 + N2H4), FTIR = Fourier
transform infrared spectroscopy, CP = continuous photolysis, GC =
gas chromatography; tw = this work.
the temperature range 250 to 299 K is compared with other
determinations in the absence of O2 and also with the most
recent kinetic review given in Table 5.
The activation energy of reaction (1a) is still not well
characterized. The studies of Wine et al.,7 Hynes et al.,11
Hsu et al.,13 Abbatt et al.16 report a positive Arrhenius
activation energy, while Wallington et al.12 report a negative
Arrhenius activation energy. The latest IUPAC data evalua-
tion recommends a positive Arrhenius activation energy. The
observation of a positive activation energy has been taken as
indicating the dominance of the hydrogen abstraction reaction
(1a) in the systems used to study the reaction, while measure-
ment of a negative Arrhenius activation energy has been taken
as an indication that the association reaction (1b) is also
probably contributing to the rate coefficient measurement
under the conditions employed. In this work a negative
Arrhenius activation energy has been obtained. Although we
will always have very low concentrations of O2 in our system
even in 760 Torr N2, a reasonable agreement with other
determinations of the rate coefficient would suggest that the
O2 is not making a large contribution, at least at room
temperature. However, with decrease in temperature, the
formation of the (CH3)2S-OH adduct will become more im-
portant and it can not be excluded that the system may be
more sensitive to the presence of small quantities of O2 at
Comparison with previous work
For ease of comprehension, rate coefficients determined in the
absence of O2 will first be discussed, followed by the determi-
nations performed in the presence of O2. Earlier determina-
tions which are now thought to be flawed because of reactive
impurities in the DMS samples4,5 or heterogeneous effects8 are
not considered in the comparison. Also excluded from the
comparison are relative studies where NO was present,6,9,12,15
since the studies are believed to be in error due to secondary
reactions in the systems. The exclusions, however, underline
the variety of complications which can arise in studies of the
kinetics of the DMS + OH reaction.
In Table 4 a comparison is made of the data obtained in this
work at 299 K and 760 Torr N2 (pO = B0 Torr) with other
room temperature values reported in the literature.
2
This comparison shows that the value obtained in this study
(kDMS = (5.00 ꢁ 1.00) ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1) is in very
good agreement with most of the other determinations ob-
tained using absolute7,11,13,16,18 and relative methods.12,14
There are, however, three absolute studies10,12,15 which re-
Table 5 Arrhenius expressions for DMS + OH reaction in different ranges of temperature in the absence of O2
kDMS Arrhenius expression/cm3 moleculeꢀ1 sꢀ1
T range/K
Pressure (Torr)/bath gas
Technique
Ref.
(6.8 ꢁ 1.1) ꢂ 10ꢀ12 exp[(ꢀ138 ꢁ 46)/T]
(13.6 ꢁ 4.0) ꢂ 10ꢀ12 exp[(ꢀ332 ꢁ 96)/T]
(2.5 ꢁ 0.9) ꢂ 10ꢀ12 exp[(130 ꢁ 102)/T]
(11.8 ꢁ 2.2) ꢂ 10ꢀ12 exp[(ꢀ236 ꢁ 150)/T]
(13.5 ꢁ 6.2) ꢂ 10ꢀ12 exp[(ꢀ285 ꢁ 135)/T]
1.13 ꢂ 10ꢀ11 exp(ꢀ253/T)
248–363
276–397
297–400
260–393
297–368
240–400
250–299
50–200/Ar
30–40/Ar
50/Ar
0.8–3/He
10ꢀ100/N2
—
FP-RFa
FP-RFa
FP-RFa
DF-RFa
HPF-LIFa
Review
7
11
12
13
16
21
tw
(1.56 ꢁ 0.20) ꢂ 10ꢀ12 exp[(369 ꢁ 27)/T]
a
Absolute technique: FP = flash photolysis, RF = resonance fluorescence, DF = discharge flow, HPF = high pressure flow, LIF = laser
760/N2
CP-FTIRb
b
induced fluorescence. Relative technique: CP = continuous photolysis, FTIR = Fourier transform infrared spectroscopy; tw = this work.
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lower temperatures. As stated previously our observed in-
crease in the DMS + OH rate coefficient in N2 with decreasing
temperature does not exceed the combined error limits of the
measurements at the temperature extremes. Similarly, for all
of the other investigations the determined activation energy is
associated with quite sizeable errors and the observed increase
in the rate coefficient does not exceed the combined error
limits of the measurements at the temperature extremes.
Further measurements with higher precision are required to
determine more accurately the Arrhenius parameters for the
abstraction channel (1a) in DMS + OH.
Table 6 Rate coefficients for DMS + OH reaction at room tem-
perature in air
kDMS ꢂ 1012/cm3
moleculeꢀ1 sꢀ1
T/K
Pressure/Torr
Technique
Ref.
6.28 ꢁ 0.10
8.50 ꢁ 0.20
8.00 ꢁ 0.50
7.80 ꢁ 1.80
a
298
296
298
299
750
740
760
760
PLP-PLIFa
DS-FTIRb
CP-GCb
11
12
14
tw
CP-FTIRb
Absolute technique: PLP = pulsed laser photolysis, PLIF = pulsed
b
laser induced fluorescence. Relative technique: DS = dark source of
OH (O3 + N2H4), FTIR = Fourier transform infrared spectroscopy,
CP = continuous photolysis, GC = gas chromatography; tw = this
work.
The observed increase in the rate coefficient for the reaction
of OH with DMS with increase in the partial pressure of O2 for
each temperature studied is in agreement with the observations
from the studies at room temperature of Wallington et al.12
and Barnes et al.,14 and from the studies of Hynes et al.11 and
Williams et al.20 made at different temperatures. In the work
of Wallington et al.,12 the experiments were carried out at 296
K and 740 Torr total pressure at different O2 partial pressures
between 0 and 740 Torr; the rate coefficient of 5.3 ꢂ 10ꢀ12 cm3
moleculeꢀ1 sꢀ1 measured with 0 Torr O2 was found to increase
by factors of 1.3, 1.6, and B2.0 in the presence of 50, 160, and
740 Torr O2, respectively. In the work of Barnes et al.14 carried
out at a total pressure of 760 Torr at 298 K, the rate coefficient
was observed to increase from 4.4 ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1
in the absence of O2 by factors of 1.2, 1.5, 1.8, and 2.6 with 50,
100, 155 and 760 Torr partial pressures of O2. In this work at
299 K, the rate coefficient increases from 5.0 ꢂ 10ꢀ12 cm3
moleculeꢀ1 sꢀ1 with B0 Torr O2 by factors of approximately
1.6 and 2.0 with 155 and 380 Torr of O2. In the work of Hynes
et al.,11 at 298 K the measured rate coefficient was found to
increase from 4.8 ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1 in 40 Torr N2 by
a factor of 1.3 when using 750 Torr air, and at 261 K from
4.29 ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1 in 700 Torr N2 by a factor of
approximately 3.0 when measured in 700 Torr air. In the work
of Williams et al.20 at a temperature of 240 K and a total
pressure of 600 Torr, the rate coefficient was found to increase
from 5.4 ꢂ 10ꢀ12 cm3 moleculeꢀ1 sꢀ1 with 5 Torr O2 by factors
of approximately 2.9, 3.9, and 5.2 when using 15, 30, and
120 Torr partial pressures of O2, respectively. In this work at
260 K, the rate coefficient increases from 6.4 ꢂ 10ꢀ12 cm3
moleculeꢀ1 sꢀ1 with B0 Torr O2 by factors of approximately
3.1 and 3.7 with 155 and 380 Torr of O2.
(iii) values calculated from the new recommended fit expres-
sion of Atkinson et al.21 for 760 Torr and pO = 155 Torr;
2
(iv) the values measured in this work at 760 Torr and
pO = 380 Torr; and
2
(v) values calculated from the new recommended fit expres-
sion of Atkinson et al.21 for 760 Torr and pO = 380 Torr.
2
Unfortunately, the rate coefficient data in the paper of
Williams et al.20 was not given in a tabulated form and thus
could not be plotted in Fig. 3. It is, however, incorporated into
the fit expression given by Atkinson et al.21 which is plotted in
Fig. 3.
An inspection of Fig. 3 shows that for 760 Torr of air and in
the temperature range 270 to 299 K, the values determined in
this work are somewhat higher but are generally in fair
agreement with those calculated from the old fit of Hynes et
al.11 and the new recommended fit given in Atkinson et al.21
which incorporates the new Williams et al.20 data. Also shown
in Fig. 3, just for comparison purposes, is a plot of the rate
coefficients measured in this study for pO = 380 Torr and the
2
Table 6 shows a comparison between the overall rate
coefficient obtained in this work for DMS + OH in 1 atm
of synthetic air (pO = 155 Torr) at 299 K and those reported
2
in other studies under similar conditions which are considered
free of potential artifacts.
The value obtained in this work for atmospheric conditions
is in reasonable agreement with the other values determined
using relative kinetic techniques12,14 but is approximately 20%
higher than that determined using an absolute technique.11
Fig. 3 shows plots of the overall rate coefficients kDMS
(abstraction + addition channels) for DMS + OH as a
function of temperature for:
(i) values calculated from the fit expression of Hynes et al.11
for 760 Torr total pressure and pO = 155 Torr;
Fig. 3 Variation in the overall rate coefficient kDMS for DMS + OH
as a function of temperature as estimated or measured (see text for
details).
2
(ii) values measured in this work at 760 Torr and pO = 155
2
Torr;
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results will only be of atmospheric significance if DMS can
reach the upper troposphere/lower stratosphere (UT/LS)
where such low temperatures prevail. Updrafts in deep con-
vective clouds can raise the boundary layer air into the upper
troposphere within a matter of minutes. It has been suggested
from observations of near-zero ozone concentrations over the
convective Pacific that rapid convective transport of marine
gaseous emissions such as DMS between the marine boundary
layer and the uppermost troposphere could occur, which can
lead to sulfate particle formation.25
The distribution of the products from the reaction of OH
with DMS is dependent on the ratio of the two reaction
channels, addition and subtraction, which is controlled by
temperature and the oxygen partial pressure. The results of
this study and that of Williams et al.20 have shown that for 1
atm of air and at temperatures below B260 K the contribution
of the addition pathway is considerably more than was pre-
dicted by extrapolation of the expression of Hynes et al.11
Product studies on the OH initiated oxidation of DMS as a
function of temperature and oxygen partial pressure26 have
shown that dimethyl sulfoxide (CH3SOCH3; DMSO) is the
major product from the addition channel with molar yields of
around 75% at 260 K in the presence of low levels of NOx.
Only very low yields of SO2 and DMSO2 are observed. The
products from the oxidation of DMSO at low temperature are
not currently known. At room temperature it has been shown
that the major product from the OH initiated oxidation of
DMSO is methanesulfinic acid (MSIA; CH3S(O)OH).27–29
The SO2 formed will be converted to H2SO4 and eventually
through gas-to-particle conversion to sulfate aerosol.
Fig. 4 Temperature dependence of the branching ratio between the
addition and abstraction channels for DMS + OH as estimated or
measured (see text for details).
corresponding values calculated with the Atkinson et al.
expression. The agreement here is also reasonably good.
At temperatures below 270 K our rate coefficient values
show a strong increase compared to the values predicted by
the empirical fit expression of Hynes et al.11. The values are,
however, in good agreement with the new preferred values
calculated from the recommended expression of Atkinson
et al.21 which took into account the data of Williams et al.20
This work thus confirms the measurements reported by
Williams et al.20
The above discussion has been for conditions of 1 atm of
air, however, in the UT/LS where temperatures between 210
and 260 K can prevail, the increase in the rate coefficient
caused by the decrease in temperature will be compensated, at
least in part, by the decrease in the O2 partial pressure which
will fall, for example, from approximately 110 Torr at 3 km
altitude to approximately 40 Torr around 10 km. In regions of
the troposphere with an O2 partial pressure of B100 Torr
where temperatures of approximately 250 K prevail, the
present results show that the addition channel will account
for B70% of the overall reaction of OH with DMS. Tem-
peratures of 210 K prevail at 10 km altitude where the partial
pressure of O2 is approximately 40 Torr. We do not have
kinetic data for these conditions but an extrapolation of the
available data set suggests that under these conditions the
addition channel could account for B80% of the overall
reaction. Thus, it would appear that under the conditions
prevailing in the UT/LS the contribution of the addition
channel to the reaction of OH with DMS will vary within a
relatively narrow range, i.e. approximately 70–80%, since the
increase in rate with decrease in temperature is being more or
less compensated by the decrease in the O2 partial pressure.
Fig. 4 compares the temperature dependence of the branch-
ing ratio between the addition and abstraction channel for
DMS + OH obtained from:
(i) the empirical expression given in Hynes et al.;11
(ii) the values given in Williams et al.;20
(iii) the values measured in this study for the overall reaction
at pO = 155 Torr and the value for the abstraction channel
2
taken from Atkinson et al.21
The branching ratio is defined as (kobs ꢀ k1a)/k1a. As is to be
expected from the kinetic data, above 270 K there is good
agreement between the branching ratios calculated from the
different data sets, while below 270 K there is a sharp increase
in the branching ratio calculated using the data from this work
which matches very well that obtained in the study of Williams
et al.,20 but deviates strongly from the values obtained from an
extrapolation of the empirical expression of Hynes et al.11
Atmospheric implication
The results reported in this work have confirmed the sharp
increase in the rate coefficient for OH with DMS at tempera-
tures below 260 K observed by Williams et al.20
Acknowledgements
Although these results are of interest from a mechanistic
viewpoint they will have no impact on DMS chemistry in the
lower troposphere since at temperatures below B273 K emis-
sions of DMS from the oceans will be extremely low. The
Financial support of this work by the European Commission
within the 5th Framework Programme Project El CID
(EVK2-CT-1999–00033) is gratefully acknowledged.
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Kerr, M. J. Rossi and J. Troe, J. Phys. Chem. Ref. Data, 1997, 26,
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