29 April 1994
ELSEVIER
CHEMICAL
PHYSICS
LETTERS
Chemical Physics Letters 221 (1994) 353-358
Kinetic data for the reaction of hydroxyl radicals
with 1, 1,l-trichloroacetaldehyde at 298 + 2 K
John Barry a, Donncha J. Scollard a, Jack J. Treaty a, Howard W. Sidebottom a,
Georges Le Bras b, Giles Poulet b, Sophie T&on b, Alexei Chichinin b,
Carlos E. Canosa-Mas ‘, David J. Kinnison ‘, Richard P. Wayne ‘, Ole John Nielsen d
nDepartment of Chemistry, University College Dublin, Dublin, Ireland
b Laboratoire de Combustion et Systemes Reactifs, CNRS, 45071 OrMans Cedex 2, France
‘Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OXI 3QZ. UK
d Chemical Reactivity Section, ES&T Department, Riw National Laboratory, DK-4000 Roskilde, Denmark
Received 14 February 1994
Abstract
The rate constant for the reaction of the hydroxyl radical with l,l, 1-trichloroacetaldehyde
has been determined at 298 f. 2 K.
Rate data were obtained at atmospheric pressure by a relative rate method. The rate constant was also measured at lower pressures (l-3.4 Torr) using the discharge flow technique with OH radical detection both by resonance fluorescence and electron
paramagnetic resonance. The results provide a value of k(OH+CCI,CHO)=
(1.1*0.2)x
IO-‘* cm3 molecule-’ s-’ at room
temperature giving an atmospheric lifetime for CC13CH0 with respect to reaction with OH radicals of 290 h.
1. Introduction
1, 1,l -trichloroethane
has found widespread application as an industrial solvent and degreasing agent;
however, its possible involvement
in stratospheric
ozone depletion has led to concern regarding its future use [ 1,2]. Reaction with hydroxyl radicals in the
troposphere is relatively slow and it has been estimated that lo- 15Ohof the released Ccl&H,
will enter the stratosphere
[ 1,2]. The available data indicate that the OH radical initiated
oxidation
of
CC13CH3 in the troposphere leads initially to formation of 1,1,1 -trichloroacetaldehyde
[ 1-3 ] which may
undergo further reaction either by photolysis or via
reaction with OH radicals. It is important to determine the tropospheric lifetime of Ccl&HO
and the
eventual oxidation products in order to assess the
0009-2614/94/$07.00
SSDIOOO9-2614(94)
possibility of chlorine-containing
products diffusing
into the stratosphere.
A number of kinetic studies on the reaction of OH
radicals with CCl&HO have been reported [ 3-6 ] :
OH + CC& CHO-+products
.
(1)
The two rate constants values determined in our laboratories [ 3,6] using a relative rate technique, in
which OH radicals were generated from the photolysis of CH30N0 in air, are in good agreement with
that obtained from the discharge flow-resonance
fluorescence method of DobC et al. [4]. However, a
measurement of the rate constant by Balestra-Garcia
et al. [ 5 ] using the pulsed laser photolysis-resonance
fluorescence technique is almost a factor of 2 lower
than these determinations.
Although agreement between the results from our relative rate studies [ 3,6]
and those of DobC et al. [ 41 appear to be satisfactory,
0 1994 Elsevier Science B.V. All rights reserved
00274-T
J. Barry et al. /Chemical Physics Letters 221 (1994) 353-358
354
both sets of experiments may be subject to error. The
relative rate experiments were carried out in the presence of O2 and under these conditions reactions of
OH radicals with chlorinated aldehydes lead to generation of Cl atoms [ 3 1. Although NO or C&H6were
added to these systems in order to scavenge Cl atoms,
it is possible that complications arising from the reaction of Cl atoms with Ccl&HO and the reference
compound may occur. Further, the relative rate studies were carried out under conditions where direct
photolytic removal of CCl&HO may have made
some contribution to the overall loss processes. As
part of their discharge flow-resonance fluorescence
investigation of the reaction of OH with CCl,CHO,
DobC et al. [ 41 also studied the reaction of OH radicals with various aliphatic aldehydes. Although data
for acetaldehyde is in line with previous studies, the
results for the less volatile compounds, (CH,)&HCHO and ( CH3 ) &CHO are considerably higher than
other measurements on these species [ 71.
In this work, two separate discharge flow techniques and a relative rate method have been employed to determine the rate constant for reaction of
OH radicals with CCl&HO at 298 &2 K in the pressure range l-760 Tom It was hoped the results would
resolve the discrepancies in the previously reported
rate data and provide a reliable rate constant for this
atmospherically important reaction.
atmospheric pressure in an approximately 50 litre
FEP teflon cylindrical reaction chamber surrounded
by 20 Phillips TU 15W germicidal lamps with an intensity maximum around 250 nm. Measured pressures (MKS Baratron) of the reagents were flushed
from calibrated Pyrex bulbs into the reaction chamber by a stream of zero grade nitrogen (Air Products), which was then filled with ultra-pure air (Air
Products). Quantitative analyses were carried out
using gas chromatography (Shimadzu model 14A,
equipped with a flame ionization detector and temperature programmer). Samples for GC analysis were
obtained with a Valco gas sampling valve.
2.2. Dischargejlow-resonancefluorescence
Hydroxyl radicals were generated by the photolysis
of ozone in the’presence of water vapour,
The flow tube was constructed of stainless steel lined with teflon and had an internal diameter of approximately 34 mm. The flow rate of the helium carrier gas (BOC, commercial grade, purified by passage
through molecular sieves at 77 K) was measured by
a calibrated ball flowmeter (Jencon RS3) and was
1070-1310 cm s-l at 3.35 and 2.6 Torr total pressure, respectively (MKS Baratron). Hydrogen atoms
were generated by flowing Hz (BOC, commercial
grade, purified by passage through molecular sieves
at 77 K) through a microwave discharge. The rapid
reaction between H atoms and NO2 was used to generate OH radicals which were admitted at the upstream section of the flow tube. Hydroxyl radical
concentrations were determined by resonance fluorescence in the A 2E+ +X ‘Il transition. The emitted
light from the resonance lamp was focused into the
centre of the detection cell and fluorescence detected
orthogonally by an EM1 9893 QA350 photomultiplier fitted with a honeycomb filter and a 308 nm
narrow band filter. The signal from the photomultiplier was passed into an EM1 amplifier/discriminator and then into a rate counter which had an analogue output for a chart recorder. Trichloroacetaldehyde entered the flow tube via fmed injection ports
and was measured by a calibrated mass flow controller (Tylan FC260).
09+hv(lx250nm)+O(‘D)+02,
(2)
2.3. Discharge flow-electron paramagnetic resonance
O(‘D) +H,O+OH+OH
(3)
2. Experimental
The relative rate [ 81, discharge flow-resonance
fluorescence [ 9 ] and discharge flow-electron paramagnetic resonance methods [ lo] used in this study
have been described in detail previously and only a
brief outline of the experiments is given.
2.1. Relative rate
.
Kinetic experiments were carried out at 298 + 2 K and
The flow tube was constructed of quartz with an
internal diameter of 22 mm and the internal wall
J. Bany et al. /Chemical PhysicsLetters221(1994) 353-358
coated with halocarbon wax to reduce wall reactions.
A side arm tube and a double axial movable tube were
used to flow the reactants into the reactor. Hydroxyl
radicals were generated by the reaction of hydrogen
atoms with NOz and were detected by electron paramagnetic resonance [ lo]. Hydrogen atoms were produced in the side arm tube by dissociation of H,, diluted in helium, in a microwave discharge. Nitrogen
dioxide, in slight excess over H atoms, was introduced through the external tube of the axial injector.
This excess ensured adequate conversion of H to OH
before the addition of CCl$JHO.
3. Materials
2-methylpropane and nitrogen dioxide (Matheson
research grade) were used as received. Trichloroacetaldehyde (Fluka 99.5%) was vacuum distilled prior
to use. Gas chromatography and infrared analysis
showed no impurities in the samples of CCl&HO.
Ozone was produced by passing zero grade oxygen
(Air Products) through an ozone generator (Monitor labs) directly into the reaction chamber.
4. Results and discussion
The rate constant for reaction of OH radicals with
CCl$HO, Eq. ( 1 ), was determined relative to that
for reaction with a reference compound, in this case
2-methylpropane,
OH + reference+ products .
remove CCLCHO in a chain reaction [ 3 1. Addition
of 100 ppm C2Hs reduced the photolytic removal of
CC13CH0 to less than 1% over the time periods used
for the OH radical kinetic experiments.
Mixtures of 03/H20/CC13CHO/ ( CH3) 3CH in air
were photolyzed for about 7 min at 298+_2 K:
[CC13CH0 ] and [ (CH,),CH] = l-10 ppm; [03 ] =
lo- 1000 ppm; HZ0 = 1OO- 10000 ppm. Under the reaction conditions employed, the substrate and reference compound were found to decay by approximately 70% during the experiments. Concentrationtime data for the runs are plotted in the form of Eq.
(5) in Fig. 1 and show the expected linear relationship. The rate constant ratio k,/k4=0.45f0.01
(error is _+2s and represents precision only), was independent of reaction time, relative reactant
concentration and light intensity in agreement with
the proposed mechanism. A possible source of error
in the experiments is the reaction of 0( ID) with
CC13CH0 and (CH3),CH, however, the rate constant for reaction ofO(‘D) with HzO, k3=2.2 x lo-”
cm3 molecule-’ s-r [ 111 is sufficiently high that under the experimental conditions employed with
[H,O] r 100 [CC13CHO] or [ (CH3)3CH], O(‘D)
will be effectively scavenged in reaction ( 3 ) . Variations in the concentration of water vapour from 1OO10000 ppm had no measurable effect on the value of
k,/k, providing support for this assumption. Reaction of OH radicals with CC13CH0 in air has been
shown to yield Cl atoms [ 3 1, however, these species
will be rapidly removed by reaction with 03. In order
to ensure Cl atom reactions were unimportant when
(4)
Assuming that reaction with OH is the only signiflcant loss process for both trichloroacetaldehyde and
2-methylpropane then
]f
0.5
I
2
g
0.4
-
0.3
-
0.2
-
0.1
-
”
-”
::
c
2
[reference ] o
[reference
355
’
(5)
4
2
”
where the subscripts 0 and t indicate concentrations
at the beginning of the experiment and at time t, respectively. Under the conditions employed for the kinetic experiments photolytic loss of 10 ppm
CC13CH0 was rapid with a half-life of around 2 min.
Photolysis of CC13CH0 in the presence of O2 has been
shown to lead to the generation of Cl atoms which
y
0.0,
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fig. 1. Plot of ln( [CC13CHO],/[CC13CHO],)
against
ln( [referencelo/ [reference],) from the photolysis of OJH,O/
CCl,CHO/ (CH,),CH mixtures in air.
J. Barry et al. /Chemical Physics Letters 221(1994)
356
the O3 concentrations
were depleted towards the end
of the photolyses, all experiments were carried out
with 100 ppm of added ethane. Combination
of the
rate constant ratio k, /k4 with the evaluated value of
the rate constant for the reference reaction, k4 (OH +
(CH,)3CH)=2.34x10-12cm3molecule-1
s-l [7],
gives a value of k, = (1.05 20.18) x lo-‘* cm3 molecule-’ s-l assuming a 15% error in the rate constant
for the reference reaction.
The loss of OH radicals in the discharge flow-resonance fluorescence system can be attributed to either
wall removal,
OH + wall+products
,
(6)
80
7
n
[CC13CHO] +ka [OH] .
(7)
For fixed positions of observation and generation of
OH, as in the present system, it can be shown that Eq.
(7) reduces to [ 12,131
ln( [OH],/
[OH],)
=k, [CC13CHO]t=k’t,
60.
;
40
-
[Cclpo].
160
120
‘-
80
It
40
01
00
(8)
where [OH],, and [OH] t are proportional
to the
measured fluorescence signal intensity in the absence
and presence of CC13CH0 for the same injector position. For the above analysis to hold, the heterogeneous loss of OH must be first order and independent
of the presence of CC13CH0. The procedure involved measurement of the normalized ratio [OH ] o/
[OH] Las a function of contact time to obtain k’ and,
as implied by Eq. ( 8) the rate coefftcient for wall
loss, k5, is not obtained from this analysis. An estimate of k6 was obtained by measuring the difference
in the observed resonance fluorescence intensity for
addition of NO2 at several of the fixed injectors in
the flow tube. Typical values of k6 were in the range
of 5- 15 s- ’ for the reaction conditions used in this
work. A plot of the pseudo-first-order
rate constant,
k’, against [ CCl,CHO],
Fig. 2, gives a value of the
bimolecular rate constant for reaction of OH radicals
with CC13CH0, k, = ( 1.28 ? 0.25) x IO-‘* cm3 molecule-’ s-i.
In the discharge flow-EPR experiments,
the OH
10’Jmolec”le
cm-’
Fig. 2. Plot of k’ against [CCl,CHO] from the discharge flowresonance fluorescence technique.
or via reaction with CC13CH0, reaction ( 1). Reactions with NO, NO2 and H2 also present in the system, are negligible on the time-scale of the experiments. The rate of loss of OH radicals is thus
-d[OH]/dt=k,[OH]
353-358
I
I
0.4
[CCI,CHO].
0.8
10”
12
molecule
1.6
cm-’
Fig. 3. Plot of k’ against [CCl,CHO] from the discharge flowelectron paramagnetic resonance method.
decays were also observed under pseudo-first-order
conditions
at 298 K. Initial concentrations
of the
reactants were [OH],= (0.4-2.8) x IO’* molecule
cme3 and [CC13CHO] = (0.03-1.79) X lOi molecule cmm3. The total pressure was about 1 Torr and
the flow velocity ranged from 1000 to 1700 cm s-i.
The heterogeneous loss rate of OH was found to be
in the range 7-20 s- ‘. Changes in [OH] ,, or flow velocity had no effect on the value of the rate constant.
Seven independent
series of experiments were performed. A plot of the pseudo-first-order
rate constant, k’, against [ CC13CHO], for one series of experiments is given in Fig. 3. The mean value of the
bimolecular rate constant for reaction ( 1), weighted
by the number
of plots for each series, is
k1=(0.89f0.15)x10-‘*cm3molecule-’s-l.
The values of the rate constant for reaction of OH
radicals with CC13CH0 determined by the three dif-
J. Barry et al. /Chemical PhysicsLetters221 (1994) 353-358
ferent experimental techniques employed in this work
are in reasonable agreement and provide an average
value of ki = (1.1 kO.2) X 1O-12 cm3 molecule-’
s-’
at 298 f 2 K independent
of pressure from 1 to 760
Torr. This value is about 30% lower than the rate
constants determined previously in our laboratories
using the methyl nitrite photolytic
relative rate
method [ 3,6] and with the discharge flow-resonance
fluorescence investigation of D6bC et al. [ 41 and provides support for the lower value of the rate constant
reported by Balestra-Garcia
et al. [ 5 ] using the laser
photolysis-resonance
fluorescence method.
The available evidence indicates that reaction of
OH radicals with trichloroacetaldehyde
involves hydrogen abstraction from the aldehyde group [ 61. The
lack of any pressure effect on the rate constant provides support for this conclusion. The rate constant
determined in this work allows an estimate of the atmospheric lifetime of the CC4CHO with respect to
reaction with OH. Using an average tropospheric
concentration
of 8.7 x lo5 molecule
cm-3
[ 21,
the lifetime of CC13CH0 is 290 h (lifetime= l/
bH [ OH] ). Absorption cross-section data suggests
that photolysis may also contribute to atmospheric
removal [2]. Calculated photolysis lifetimes are of
the order of a few hours assuming a quantum yield
for decomposition
of unity at 298 K [ 141, however,
decomposition
at the lower temperatures
of the upper troposphere may be a minor channel for decay of
electronically excited CC13CH0. Thus, the lifetime of
CC13CH0 is relatively short and hence CC13CH0 will
not contribute directly to Cl atom concentrations
in
the stratosphere. It is of some interest to consider the
products of the OH initiated oxidation of CCbCHO.
CCl&O radials, formed by the reaction of OH with
CCl,CHO, may either decompose or react with 02:
cc13c0+cc13
+co
cc13co+o~+ccl3c(o)o~.
)
(9)
(10)
Oxidation of Ccl3 radicals will eventually yield phosgene [ l-3 1, which is unreactive toward OH and is
stable to photolysis in the troposphere. It has been
suggested that the major fate of CClzO in the troposphere is aqueous phase hydrolysis [ 21. The major
loss process for CC13C (0) O2 radicals is via reaction
with NO:
CCl,C(O)O,
+NO-+CC13C(0)O+N02,
cc13c(o)o~cc13+co~.
357
(11)
(12)
It is, however, possible that under conditions of high
NOz to NO concentrations,
formation of the peroxyacyl nitrate may be important
CCl,C(O)O,
+NOz +M+
CC13C(0)OzN0,
+M .
(13)
Studies on the stabilities of peroxyacyl nitrates by
Barnes et al. [ 15 ] indicate that these species are stable with respect to thermal decomposition
under
conditions typical of the upper troposphere (220 K
and 100 Torr) and that their lifetimes are limited by
their photolysis rates, estimated by comparison with
CH3C( 0)02N02
to be around 100 days. However,
these authors showed that under all tropospheric
conditions, thermal decomposition
of CC13C0, reaction (9)) is the dominant reaction charnel for this
radical. At 298 K and 760 Torr it was estimated that
92% of CC13C0 decomposes while at 220 K and 75
Torr decomposition accounts for 89% of CC13C0 loss
and hence CCl,C (0 ) 02N02 can only provide a very
minor flux of Cl atoms into the stratosphere. The results of this study and previous work indicate that the
degradation products arising from CH3CC13 removal
in the troposphere do not contribute to chlorine atom
loading in the stratosphere.
References
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and Monitoring Project-Report
No. 20, Scientific
Assessment tof Stratospheric Ozone, Vol. II, Appendix:
AFEAS Report, ch. 6 (1989).
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