ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 1935–1944
www.elsevier.com/locate/atmosenv
Study of reaction processes of furan and some furan derivatives
initiated by Cl atoms
B. Cabañas, F. Villanueva, P. Martı́n, M.T. Baeza, S. Salgado, E. Jiménez
Departamento de Quı´mica-Fı´sica. Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha. Avda. Camilo José Cela,
10, 13071, Ciudad-Real, Spain
Received 5 August 2004; received in revised form 22 November 2004; accepted 9 December 2004
Abstract
The reactions of chlorine (Cl) atoms with volatile organic compounds (VOCs) emitted into the atmosphere are of
interest to understand the role of Cl in the marine and coastal chemistry. The rate coefficients for Cl-atom reactions
with organic compounds are typically about one or two order of magnitude larger than the ones corresponding to OH
reaction. We report here the first kinetic measurements of the reactions of atomic Cl with some VOCs: furan, 2methylfuran, 3-methylfuran, 2-ethylfuran, and 2,5-dimethylfuran.
The reactions of atomic Cl with furan and its derivatives were studied at 29872 K and 1 atm pressure using a relative
rate technique with GC-FID/MS detection of the organic compounds. Thionyl chloride and trichloroacetyl chloride
were used as Cl-atom precursors since molecular Cl reacted in the dark with the studied compounds.
The ratios of rate coefficients for Cl-atom reactions with furan and its derivatives relative to n-nonane were as
follows: furan (0.4170.05); 2-methylfuran (0.8570.03); 3-methylfuran (0.8870.04); 2-ethylfuran (0.9570.05); and 2,5dimethylfuran (1.1770.06). Taking k(Cl+n-nonane) ¼ (4.8270.14) 1010 cm3 molecule1 s1 the absolute rate
coefficients obtained (in units of 1010 cm3 molecule1 s1) were: furan (2.070.2); 2-methylfuran (4.170.2); 3methylfuran (4.270.3); 2-ethylfuran (4.670.3); and 2,5-dimethylfuran (5.770.3). All errors are 72s.
The influence of the structure on the reactivity of these compounds and the atmospheric implications are discussed.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Furan; Chlorine-atoms; Rate coefficient; Relative method; Tropospheric chemistry
1. Introduction
There has been long-standing interest in the possible
role of halogen atoms as tropospheric oxidants (Singh
and Kasting, 1988; Chatfield and Crutzen, 1990). The
best evidence so far comes from measurements of
alkanes and acetylene in Arctic surface air (Jobson et
al., 1994) which indicate a sink in April (polar sunrise)
consistent with oxidation by chlorine (Cl) atoms present
Corresponding author. Fax: +34 926 295 318.
E-mail address: Beatriz.Cabanas@uclm.es (B. Cabañas).
at a concentration 1 104 atoms cm3. The source of
the halogen oxidants is not well established but likely
involves chemical production from sea salt accumulated
on the ice over the polar night (Impey et al., 1999).
Generation of halogen oxidants from sea salt would be
of little interest for global tropospheric chemistry if they
were confined to Arctic sunrise. However, measurements
of hydrocarbons and non-radical Cl species in the
marine boundary layer (MBL) at midlatitudes and in the
tropics suggest that Cl atoms may be present at least
occasionally at concentrations in the range of
104–105 atoms cm3 (Keene et al., 1990; Pszenny et al.,
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2004.12.013
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1936
B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
1993; Singh et al., 1996; Wingenter et al., 1996; Spicer et
al., 1998; Wingenter et al., 1999). Therefore, the sources
of Cl atoms are still subject of investigation. Although
the concentration of atomic Cl can be lower than the
OH concentration, Cl reacts with some hydrocarbons
faster than OH does, implying that Cl can play an
important role in the atmospheric chemistry. Extensive
kinetic and mechanistic studies on the oxidation
reactions involving Cl atom are therefore required to
properly describe the fate of VOCs in coastal regions as
well as in the Arctic.
The interaction of Cl atoms with VOCs can occur as
sea breezes carry marine air masses inland. Recently,
environmental studies in continental areas show that F,
Cl, and S are the origin of pollutants. In this sense, an
anthropogenic source of precursors of tropospheric Cl
atoms results from atmospheric emissions by brick
factories, local concentrations of HCl and chlorides as
high as 40–200 ppm in the emission plume have been
reported (Galán et al., 2002).
Among volatile organic compounds that can react
with Cl atoms, furan and its derivatives have to be
considered. Furans are a kind of heterocyclic aromatics
emitted into the atmosphere as primary anthropogenic
pollutants from the combustion of fossil fuels, refuse,
plants and, in particular, from biomass burning
(Isidorov et al., 1985; Graedel et al., 1986; Knudsen
et al., 1993; Andreae and Merlet, 2001). These compounds are also known to be the products of
the photooxidation of hydrocarbons with the structure
CH2QCH–CR1QCHR2, such as 1,3-butadiene,
isoprene and 1,3-pentadiene (Ohta, 1984; Gu et al.,
1985; Tuazon and Atkinson, 1990; Ruppert et al., 1992,
Rasmussen and Khalil, 1988; Ruppert and Becker, 2000)
and aromatic compounds such as toluene and o-xylene
(Shepson et al., 1984). Most of these compounds are
mainly biogenic in origin emitted by the ocean such as
isoprene (Bonsang et al., 1992; Ratte et al., 1998).
Heretofore, rate coefficients of furan and some
derivatives have been reported in the literature for the
reaction of OH radical (Atkinson et al., 1983; Bierbach
et al., 1992; Grosjean and Williams, 1992), NO3
(Atkinson and Aschmann, 1985; Atkinson, 1989; Kind
et al., 1996), Br and O3 (Bierbach et al., 1996, 1999), but
there is no data on the reactivity of furan and
alkylfurans with Cl atoms. In this context, the study of
Cl-atoms reaction with a series of VOCs like furans,
whose reactivity is not established, is proposed. So,
in this work we report the first kinetic study of the
reaction of Cl atoms with a series of furans under
atmospheric conditions. The rate coefficients were used
to establish the structure–reactivity relationship for
Cl–furans reactions. In addition, the reactivity of furans
with Cl atoms is compared with other important
atmospheric oxidants and tropospheric fates of these
compounds are discussed.
2. Experimental
A relative kinetic technique was used to determine the
rate coefficients for Cl-atom reaction with furan, 2methylfuran, 3-methylfuran, 2-ethylfuran, and 2,5-dimethylfuran. The experiments were carried out at
29872 K in a 200 L FEP Teflon reaction bag at 1 atm
of total pressure of N2 or synthetic air. That bag was
placed inside a rectangular cage with 4 fluorescent lamps
(Philips TUV G13 36W) mounted on the walls. The Cl
atom precursor was added together with the organic
compound to test for a possible interference from dark
reactions. In preliminary experiments, it was found that
Cl2 could not be used as a Cl atom source because it
reacts in the dark with the furans (furan and furan
derivatives). As a result, photolysis at 254 nm of thionyl
chloride (SOCl2) or trichloroacetyl chloride (CCl3COCl)
was used to generate atomic Cl (Finlayson-Pitts et al.,
1999). Photolysis was typically carried out in steps of 11 s
using SOCl2 and 2 min using CCl3COCl, followed by
turning off the lamps and sampling the reaction mixture.
Total photolysis times were ranged from 1 to 22 min. The
decay of the reactant and reference compound concentrations was followed using gas chromatography
(GC) with mass spectrometry, MS, (Shimadzu GC-17A,
MS-QP5000) or flame ionization detection, FID
(Shimadzu GC-14A). The GC column (size:
30 m 0.32 mm 0.1 mm Teknokroma TRB-1701) was
run either at 45 1C or temperature programmed from
45 1C (9 min) to 100 1C (3 min) at a rate of 10 1C per min.
In the experiments with GC-MS the kinetics were
followed using the single ion-monitoring (SIM) mode.
However previously, a run was performed in the scanning
mode to determine the retention time of each reactant.
The m/z values to be monitored by the mass spectrometer
were selected taking into account the abundance in the
mass spectra of the reference or reactant compounds.
According to the column temperature, the measured
retention time varied from 1.8 min for propane to
13.8 min for n-nonane.
A separate set of runs were also carried out with the
individual reference compound or the furans to ensure that
the reactions did not produce products with retention
times which could interfere with the reactant peaks used in
the kinetic analysis. Additionally, all organics were
subjected to the 254 nm radiation alone to study the
possible photolysis and to ensure that there were no
unrecognized reactions occurring in the absence of the Cl
atom source. Finally, for each mixture of organics, a
number of injections (typically 12 or more) of the
unreacted mixture were carried out to obtain an estimate
of the precision associated with the measurements in order
to use in the error analysis (Brauers and Finlayson-Pitts,
1997). The standard deviations (2s) of these replicate
injections were typically in the range 1–4% for the
furans and 3–6% for the reference compounds. These
ARTICLE IN PRESS
B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
measurements also included the losses with the walls of the
reaction bag. The reproducibility of the results show that
the wall losses of the reactants were not significant.
The furans and the reference compound were
introduced into the reaction bag by injecting a measured
amount of these compounds into a stream of the carrier
gas (either air or N2). Concentrations of furans and
reference organic ranged from 3 to 13 ppm and from 5 to
18 ppm, respectively. Concentration ranges of 5–16 and
32–80 ppm were used for the internal standard and for
Cl atom sources, respectively. Sampling after the
measurement of photolysis times was carried out using
a VICI VALCO gas-sampling valve. The reaction
mixture was slowly pumped through the sampling loop
(100 mL) to be sure that it was thoroughly flushed and
then allowed to come to equilibrium with the reaction
chamber pressure of 1 atm prior to injection.
The reactions of interest are the simultaneous reaction
of the furans (substrate) and the reference compound
with Cl atoms,
Cl þ substrate ðSÞ ! products
(1)
ðkS Þ
Cl þ reference compound ðRÞ ! products
ðkR Þ
(2)
As described in detail elsewhere (Atkinson and
Aschmann, 1985; Finlayson-Pitts and Pitts, 2000), the
decay of the substrate from [S]0 at time t ¼ 0 to [S]t at
time t, and the simultaneous loss of the reference
compound from an initial concentration of [R]0 to [R]t
at time t is given by
½S 0
½R 0
kS
ln
¼
(I)
ln
½S t
½R t
kR
Thus, a plot of {ln [S]0/[S]t} versus {ln [R]0/[R]t}
should be a straight line passing through the origin and
whose slope yields the ratio of rate coefficients kS/kR.
Benzene was used as an internal standard, as its reaction
rate with Cl is neglected due to its low rate coefficient
(k298 ¼ (1.370.3) 1015 cm3 molecule1 s1) (Shi and
Bernhard, 1997). At each sampling, the ratio of
concentration of the reference or substrate against the
concentration of internal standard was used in rate
coefficients calculations. This method allowed us to
minimize the sampling errors.
Previously to the determination of the rate kS, the
possible photolysis of these compounds was studied. The
results showed a significant decrease in the concentration
of 2-methylfuran, 3-methylfuran, 2-ethylfuran, and 2,5dimethylfuran after 10 min of UV irradiation at 254 nm.
Therefore, when CCl3COCl is used (total time of
photolysis of 14–22 min), it is necessary to correct the
observed second-order rate coefficient in the kinetic
experiments in order to obtain the real second-order rate
coefficient (kS).
So in the case of the compounds that are subjected to
photolysis under the conditions of our experiments
1937
(alkylfurans ¼ 2-methylfuran, 3-methylfuran, 2-ethylfuran, and 2,5-dimethylfuran), the decay of these
compounds and the reference compound are governed
by the following rate laws (Bierbach et al., 1992, Olariu
et al., 2000):
d½alkylfuran
¼ kS ½Cl ½alkylfuran þ kph ½alkylfuran
dt
(II)
d½reference
(III)
¼ kR ½Cl ½reference
dt
Integration and rearrangement of Eqs. (II) and (III)
leads to the following expression:
ln
½alkylfuran 0
kS ½reference 0
kph t ¼
,
ln
½alkylfuran t
kR ½reference t
(IV)
where kS and kR are the rate coefficients for reaction of
the alkylfuran and reference compound (n-nonane) with
Cl atoms, respectively; kph is the photolysis rate constant
of alkylfurans and t is the reaction time. Plotting
ln([alkylfuran]0/[alkylfuran]t)-kpht versus ln([n-nonane]0/[n-nonane]t) gives kS/kR as the slope of the straight
lines according to Eq. (6). When the Cl atom source used
is SOCl2, photolysis is carried out in steps of 10 s, the
total time of photolysis being 70–110 s. During this time
the photolysis undergone by the furans is totally
negligible; thus, it was not necessary to make any
correction.
The chemicals were as follows: propane (Praxair,
99%), 1-butene (Aldrich, 99%), butane (Aldrich, 99%),
n-nonane (Aldrich, 99.5%), benzene (Panreac, 99%),
thionyl chloride (Aldrich, 99%) and trichloroacetyl
chloride (Aldrich, 99%) used as received. a-pinene
(Aldrich, 98%), furan (Aldrich, 99%), 2-methylfuran
(Aldrich, 99%), 3-methylfuran (TCI America 98%), 2,5dimethylfuran (Aldrich, 99%) and 2-ethylfuran (Aldrich, 97%) were purified by several trap-to-trap
distillations. N2 and air (Praxair, ultrahigh purity,
99.999%) were purified by means of an Oxisorb trap
and a molecular-sieve trap.
3. Results and discussion
3.1. Control experiments
Two Cl reactions were studied in order to validate the
GC-MS as a detection technique in the kinetic studies.
First, the rate coefficients for Cl+a-pinene and nnonane obtained in our system by using GC-MS
detection were compared with the ones obtained by
Finlayson-Pitts et al. (1999) and Atkinson et al. (1994)
using GC-FID (see Table 1). Secondly, the reaction
between Cl atoms and n-nonane was studied using
CCl3COCl as an alternate to Cl2, comparing with
ARTICLE IN PRESS
1938
B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
secondary reactions or heterogeneous processes are insignificant. The second-order rate coefficient for n-nonane,
kR, is accurately known allowing kS to be determined.
Fig. 2 shows a plot of Eq. (6) for the reaction of Cl with 2methyl, 3-methyl, 2-ethyl and 2,5-dimethylfuran taking
into account kph previously determined. These plots
present a very good linearity and have intercepts close to
zero, within the experimental error. Table 2 summarizes
the rate coefficients for the reactions of Cl atom and furans
using SOCl2 and CCl3COCl sources in either N2 or air.
The error limit for the ratio of rate coefficients kS/kR and
kS includes only the precision of the fit to our experimental
data ( 2s). The data show that within the error bars there
is no significant difference between runs in N2 and in air,
indicating that interference from OH reaction is negligible.
As it can be seen in Table 2, the large values of the rate
coefficients (order of 1010 cm3 molecule1 s1) show a
great reactivity of Cl towards furan and alkylfurans. The
larger inductive effect of methyl group increases the
reactivity of the cycle in Cl reaction, although the rate
coefficient is hardly affected by the length or the position
of the alkyl group (compare rate constants of 2/3methylfuran with 2-ethylfuran).
Since Cl atom could compete with OH radical in the
oxidation of some organics in the marine boundary
layer, coastal regions (Finlayson-Pitts et al., 1999) and
continental areas, it seems reasonable to compare the
reactivity of these oxidants with the same group of
compounds. So Bierbach et al. (1992, 1995) proposed as
the main reaction channel for OH-furans reactions the
electrophilic addition to the p system; the mechanism
also proposed here for Cl-furans due to the electrophilic
nature of Cl atom. In the case of furan, the attack of the
radical for both Cl and OH reactions could happen at C2 (or C-5) and C-3 (or C-4). Both positions undergo
radicals stabilized by resonance. Nevertheless, the attack
at ortho position (C-2 or C-5) leads to a radical
intermediate with an additional resonant structure,
Atkinson’s data (Atkinson, 1994) where Cl2 is used as a
precursor of Cl atoms.
In Table 1, the obtained results for the studied
reactions are shown. In light of the agreement between
our rate coefficients and those from bibliography, it
could be concluded that CCl3COCl is a good alternate
chlorinate source for these kind of studies. The
photolysis rate constants (kph) were measured in
separate experiments under typical experimental kinetic
conditions obtaining a range of kph of (4–6) 105 s1
for 2-methylfuran, 3-methylfuran and 2-ethylfuran and
of (1.970.8) 104 s1 for 2,5-dimethylfuran. Photolysis represents a contribution less than 20% of the total
decay of the alkylfuran concentrations. For furan and
the reference compounds photolysis were not observed
under our experimental conditions.
3.2. Rate coefficients of Cl+furan and alkylfurans
reactions
In this work, all reactants were studied relative to nnonane. In addition for furan, the rate coefficient was
also measured relative to propane, 1-butene, trans-2butene in order to ensure that the rate coefficient was
not dependent on the reference compound used. This
fact is showed by the data obtained for kS that are equal
within the experimental errors (see Table 2). So, the rate
coefficient for furan was obtained from the weighted
average of the several rate coefficients. Also, separate
sets of experiments were carried out for all organic pairs
using air or N2 as a carrier gas to test for potential
systematic errors due to OH reaction with the organics
(Kaiser and Wallington, 1996).
Fig. 1 shows some typical data plotted in the form of
Eq. (I) for the Cl+furan reaction. In agreement with
this equation, the data yield straight lines that passes
through the origin with a slope of kS/kR indicating that
Table 1
Rate coefficients for the reactions of n-nonane and a-pinene with Cl atoms at atmospheric pressure and room temperature.
Comparison between our results and the bibliographic ones
Reaction
Chlorine source
k298 K/1010 (cm3 molecule1 s1)
k/kreference
a
c
Technique
n-Nonane+Cl
Cl2
CCl3COCl
2.2170.06
2.3770.29a
4.870.1
5.270.8d
GC-FID
GC-MS
a-Pinene+Cl
CCl3COCl
CCl3COCl
CCl3COCl
0.9570.24b
1.1070.02b
0.9970.06b
4.671.2e
5.370.1f
4.870.3d
GC-FID
GC-FID
GC-MS
a
With a reference rate coefficient of butane of (2.1870.22) 1010 cm3 molecule1 s1 (Atkinson et al., 1995).
With a reference rate coefficient of n-nonane of (4.8270.14) 1010 cm3 molecule1 s1 (Atkinson, 1997).
c
Atkinson et al. (1995).
d
This work.
e
Finlayson-Pitts et al. (1999).
f
Timerghazin and Parisa, (2001).
b
ARTICLE IN PRESS
1939
B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
Table 2
Summary of relative rate coefficients and the absolute values derived from them for the reactions of furan and furan derivatives with Cl
atoms at atmospheric pressure and 29872 K. The absolute rate coefficients are in units of 1010 cm3 molecule1 s1
Organic
Reference
Chlorine
atom source
Carrier
gas
Detection
system
ks/kr ( 2s)
Average ks/
kr ( 2s)e
k298 k ( 2s) Average
k̄298 k
Propanea
CCl3COCl
N2
GC-MS
1.470.1
1.970.4
1-Buteneb
n-Onanec
CCl3COCl
CCl3COCl
N2
N2
GC-MS
GC-MS
0.6870.02
0.4170.05
2.070.3
2.070.2
E-2-butened
CCl3COCl
Air
N2
Air
N2
GC-MS
GC-MS
GC-MS
GC-MS
0.6370.05
0.5870.04
0.670.1
0.5770.1
CCl3COCl
Air
GC-MS
0.9470.06
SOCl2
N2
Air
N2
GC-MS
GC-MS
GC-MS
0.8070.03
0.9970.08
0.9170.05
CCl3COCl
Air
GC-FID
1.070.2
SOCl2
N2
Air
N2
GC-FID
GC-MS
GC-MS
0.8370.05
0.9970.10
0.9770.10
CCl3COCl
Air
GC-FID
1.170.2
SOCl2
N2
Air
N2
GC-FID
GC-MS
GC-MS
0.9370.05
1.170.2
0.970.3
CCl3COCl
Air
GC-MS
1.370.2
SOCl2
N2
Air
N2
GC-MS
GC-MS
GC-MS
1.1670.06
1.270.2
1.1670.1
2.070.2e
Furan
O
SOCl2
2-Methylfuran
O
n-Nonane
0.5970.03
2.470.3
0.8570.03
4.170.2
CH3
3-Methylfuran
n-Nonane
4.270.3
CH3
O
2-Ethylfuran
O
n-Nonane
CH2CH3
2,5-Dimethylfuran
CH3
0.8870.04
O
n-Nonane
0.9570.05
4.670.3
1.1770.06
5.770.3
CH3
a
Rate constant of the reaction Cl+propane ¼ (1.470.27) 1010 cm3 molecule1 s1 (Atkinson, 1997).
Rate constant of the reaction Cl+1-butene ¼ (3.070.4) 1010 cm3 molecule1 s1 (Orlando et al., 2003).
c
Rate constant of the reaction Cl+trans-2-butene ¼ (4.070.5) 1010 cm3 molecule1 s1 (Orlando et al., 2003).
d
Rate constant of the reaction Cl+n-nonane ¼ (4.8270.14) 1010 cm3 molecule1 s1 (Atkinson and Aschmann, 1995) which has
been adjusted to the recommended value of k (Cl+n-butane) ¼ 2.18 1010 cm3 molecule1 s1 (Atkinson, 1997).
e
Weighted average according to the precision of the measurement (w ¼ 1=s2 ).
b
reason why this position would be more favoured than
another one (Bierbach et al., 1992). In reference to
alkylfurans, there are several sites of attack for Cl or OH
that leads to different radical intermediates (see
Scheme 1). For 2-methylfuran only the attack at C-5
leads to an intermediate stabilized by resonance with
three resonant structures versus 3-methylfuran that
presents two sites of attack (C-2 and C-5) that leads to
ARTICLE IN PRESS
1940
B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
3
2.5
ln ([Furan]o/[Furan] t)
Propane
2
1-Butene
1.5
1
Trans-2-butene
0.5
n-nonane
0
0
1
2
ln ([R]o/[R]t)
3
4
Fig. 1. ln([furan]0[furan]t) versus ln([R]0/[R]t) for the reactions
of Cl atoms and furan using different reference compounds.
3
ln [Alkylfuran]0/[Alkylfuran]t-kpht
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
ln ([n-nonane]0/[n-nonane]t)
3
Fig. 2. Relative rate plots according to Eq. (6) for alkylfurans
with Cl atoms using CCl3COCl in N2, (’) 2,5-dimethylfuran,
(K) 2-ethylfuran, (J) 3-methylfuran, and (m) 2-methylfuran.
For all, the reference compound is n-nonane. There are no error
bars since they are negligible (estimated at 1%).
an intermediate stabilized by resonance with three
resonant structures in each position.
By comparison between the rate coefficients measured
in this work (kS) and those found in the literature for the
corresponding OH reaction, it can be seen that there is a
clear positive reactivity dependence on the number of
–CH3 groups for OH and Cl as it is expected due to the
electrodonating nature of the methyl substituent. In
contrast, the effect of the position of such a group on the
rate coefficient is different for Cl and OH reactions.
While the value of the rate coefficients of OH-reaction
with furans drops with the stability of the radicals
formed (for example, kS for 3-methylfuran is rather
larger than that for 2-methylfuran, because more stable
radicals are produced in the first case, as explained
above), the Cl reactions are independent of methyl
position, as it can be seen from the data of Table 3. This
could be explained by the fact that Cl atoms are less
selective although it is more reactive than hydroxyl
radical. Its selectivity can be due to its high reactivity,
with rate coefficients approaching collision control
(collision limits estimated using collision theory,
1.6 1010 for furan to 2.5 1010 cm3 molecule1 s1
for 2,5 dimethylfuran and 2-ethylfuran) and therefore
there is no difference of reactivity in function of the
position or the length of alkyl chain. The effect of a
second methyl group on the reactivity of furan is again
larger in the case of OH radical than Cl atom due to the
same reason (see Table 3).
Although the rate coefficients obtained in this work
indicate that the first step is an electrophilic addition of
Cl to one of the CQC of furans, the determination
of products are necessary to establish the reaction
mechanism. There are no data about product determination in Cl reactions. Only Bierbach et al. (1995) reported
the reaction products of OH with furan, 2-methylfuran
and furanaldehydes. That study demonstrated that
unsaturated dicarbonyls were formed in high yields
in the OH-initiated oxidation of heterocyclic aromatics
rings. It suggested a primary ring opening product of the
OH-initiated degradation of furan and its alkylated
derivatives.
Other reaction partners for furan and its derivatives in
the atmosphere apart from OH and Cl include NO3
and O3. Rate coefficients reported in the literature for
the reactions of O3 and NO3 with the compounds
studied in this work are also shown in Table 3. These
oxidants show a similar trend in the reactivity to that of
OH. As it can be seen in this table, the rate coefficients
for NO3 (1012–1011 cm3 molecule1 s1) and O3
(1017 cm3 molecule1 s1) are significantly lower than
the ones for OH and Cl reactions. According to the rate
coefficients reported in Table 3, we can compare the
reactivity of the different tropospheric oxidants. So, the
rate coefficients, in decreasing reactivity order, can be
written as kCl 4kOH 4kNO3 bkO3 :
ARTICLE IN PRESS
B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
1941
Sterically hindered
C-2
X
O
O
2-methylfuran
X
C-5
O
O
X
X
O
X
O
C-4
O
X
X
C-3
+X
X
O
X
O
X
O
Radical more stable
by resonance
O
Radical more stable
by resonance
C-2
X
O
C-3
+X
X
O
O
X
X
X
Sterically hindered
O
3-methylfuran
O
X
O
X
C-4
C-5
O
O
X
O
X
O
X
O
Radical more stable
by resonance
Scheme 1. Possible attack positions of X (Cl or OH radical) to methylfurans.
4. Conclusion and atmospheric implications
To date, the measurements reported here constitute
the first determination of the rate coefficients for the
reaction of Cl atoms with furan, 2-methylfuran, 3methylfuran, 2-ethylfuran and 2,5-dimethylfuran. Our
measurements confirm that these reactions are very fast,
and therefore there is no difference of reactivity in the
function of the position or the length of alkyl chain.
Although the reactivity of furan and its derivatives
towards the tropospheric oxidants is Cl4OH4NO3
O3 ; the role of these oxidants in the degradation
tropospheric process depends on their atmospheric
concentrations.
Using the kinetic data obtained in this work in
combination with an average tropospheric concentrations or a peak value of the different oxidants, upper
limits for atmospheric lifetime (t) of furan and its alkyl
derivatives have been calculated according to the
relationship t ¼ 1=k½X ; X ¼ OH, Cl, O3 or NO3.
The estimated lifetimes are summarized in Table 3.
Assuming an average global concentration of
104 molecule cm3 for Cl atoms, the tropospheric lifetimes vary from 136 h for furan to 58 h for 2,5dimethylfuran. Nevertheless, the contribution of Cl
atoms may be significant in those areas with higher
concentration. In fact, peak concentrations as high as
1 105 atoms cm3 (Spicer et al., 1998) are expected in
the marine boundary layer at dawn and much earlier
than OH where [OH] is 5 105 radicals cm3 (Brauers et
al., 1996). Under such eventual conditions the tropospheric lifetimes for Cl vary from ca. 13.6 h for furan to
ca. 5.8 h for 2,5-dimethylfuran versus tropospheric OH
lifetimes of 34 h for furan to 10 h for 2,5-dimethylfuran.
ARTICLE IN PRESS
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B. Cabañas et al. / Atmospheric Environment 39 (2005) 1935–1944
Table 3
Summary of the rate coefficients for the reactions of Cl, OH, NO3 and O3 with the compounds studied in this work and typical
tropospheric lifetimes (k in units of cm3 molecule1 s1)
a
VOCs
kCl/1010
kOH/1011
Furan
2-Methylfuran
3-Methylfuran
2.0
4.1
4.2
4.05b
6.19c
9.35d
2-Ethylfuran
2,5-Dimethylfuran
4.6
5.7
10.77c
13.21c
kNO3 =1011
kO3 =1017
tCl/h
0.14e
2.57f
2.86f
1.31g
—
5.78f
0.24h
—
2.05g
—
—
—
136i
73i
70i
—
63i
58i
i
tOH/h
6.8
4.5
3
—
2.5
2.0
j
tNO3 = min
k
23.8
1.3
1.2
1.9
—
0.6
tO3 =h
l
165
—
19
—
—
—
a
This work.
Atkinson (1994).
c
Bierbach et al. (1992).
d
Atkinson, 1989.
e
Atkinson, 1991.
f
Kind et al. (1996).
g
Alvarado et al. (1996).
h
Atkinson et al. (1983).
i
Assuming an average global concentration of [Cl] ¼ 1 104 molecule cm3 (Wingenter et al., 1996).
j
Assuming a 12-h concentration of [OH] ¼ 1 106 radicals cm3 as a 12-h daytime average (Spivakovsky et al., 2000).
k
Assuming a 12-h concentration of [NO3] ¼ 5 108 radicals cm3 (Shu and Atkinson, 1995).
l
Assuming a 24-h concentration of [O3] ¼ 7 1011 molecule cm3 (Logan, 1985).
b
These values show a loss of the furans by reaction with
Cl atoms that is comparable to that by reaction with
OH. Hence, Cl atoms clearly can contribute significantly
to the loss of this kind of compounds in competition
with OH radical playing a significant role in the
degradation of studied furans at dawn in the marine
boundary layer and the coastal areas. That may also be
the case of some urban contaminated areas where high
levels of Cl may be originated from industrial emission
(Galán et al., 2002). The tropospheric removal of these
compounds in the presence of NO3 radical, assuming a
12-h night time average of [NO3] ¼ 5 108 radicals cm3
(Shu and Atkinson, 1995) also constitutes a very
important sink (lifetime about few minutes) that is
going to dominate the nocturnal chemistry of furans.
Regarding to the reaction with ozone this oxidant hardly
contributes to the degradation of these kinds of
compounds since the lifetimes are too large in comparison to the other tropospheric oxidants. All these
numbers should be treated with caution because the
Cl, OH, NO3 and O3 concentrations vary substantially
depending on the environment, location and season. The
identification of reaction products for these reactions is
of great importance because if these compounds are
unsaturated carbonyls they are secondary pollutants
that are involved in processes such as the photochemical
smog (Comittee on Aldehyde, 1981) or peroxyacyl
nitrates (PANs) formation (Wayne, 2000). This fact
justified the need of the characterization of reaction
products for furan and its derivatives with Cl and shows
the atmospheric implications of these reactions.
Acknowledgment
Florentina Villanueva Garcı́a thanks ‘‘Junta de
Comunidades de Castilla La Mancha’’ for a personal
grant.
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