Atmospheric chemistry of 1,2-dichloroethane: UV spectra of CH2ClCHCl and CH2ClCHClO2 radicals, kinetics of the reactions of CH2ClCHCl radicals with O2 and CH2ClCHClO2 radicals with NO and NO2, and fate of the alkoxy radical CH2ClCHClO

Article abstract of DOI:10.1021/jp952149g

A pulse radiolysis technique was used to measure the UV absorption spectra of CH₂ClCHCl and CH₂ClCHClO₂ radicals over the range 230-300 nm. At 250 nm, σ(CH₂ClCHCl) = (1.7 ± 0.2) × 10^-18 and σ(CH₂ClCHClO₂) = (3.5 ± 0.5) × 10^-18 cm² molecule^-1. The observed self-reaction rate constant for CH₂ClCHCl radicals, defined as -d[CH₂ClCHCl]/dt = 2k₄[CH₂ClCHCl]² was k₄ = (2.0 ± 0.3) × 10^-11 cm³ molecule^-1 s^-1. The rate constant for reaction of CH₂ClCHCl radicals with O₂ in one bar of SF₆ diluent was (2.4 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1. The rate constants for the reactions of CH₂ClCHClO₂ radicals with NO and NO₂ were k₅ ≥ 9 × 10^-12 and k₆ = (9.8 ± 0.6) × 10^-12 cm³ molecule^-1 s^-1, respectively. The rate constant for the reaction of F atoms with CH₂ClCH₂Cl was k₃ = (2.6 ± 0.2) × 10^-11 cm³ molecule^-1 s^-1. The reaction of CH₂ClCHClO₂ radicals with NO produced NO₂ and, by implication, CH₂ClCHClO radicals. A FTIR spectroscopic technique showed that CH₂ClCHClO radicals undergo both reaction with O₂ and decomposition via intramolecular 3-center HCl elimination. In 700 Torr of air at 296 K the rate constant ratio K_O2/k_decomp = (2.3 ± 0.2) × 10^-20 cm³ molecule^-1. A lower limit of 4 × 10⁵ s^-1 was deduced for the rate of intramolecular 3-center HCl elimination from CH₂ClCHClO radicals at 296 K in 1 bar of SF₆ diluent. As part of this work a relative rate technique was used to measure rate constants of (1.3 ± 0.2) × 10^-12 and (6.4 ± 1.4) × 10^-14 cm³ molecule^-1 s^-1 for the reactions of Cl atoms with CH₂ClCH₂Cl and CH₂ClC(O)Cl, respectively. All experiments were performed at 296 K. Results are discussed in the context of the atmospheric chemistry of 1,2-dichloroethane.

Full text of DOI:10.1021/jp952149g

J. Phys. Chem. 1996, 100, 5751-5760
5751
Atmospheric Chemistry of 1,2-Dichloroethane: UV Spectra of CH2ClCHCl and
CH2ClCHClO2 Radicals, Kinetics of the Reactions of CH2ClCHCl Radicals with O2 and
CH2ClCHClO2 Radicals with NO and NO2, and Fate of the Alkoxy Radical CH2ClCHClO
Timothy J. Wallington*
Ford Research Laboratory, SRL-3083, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053
Merete Bilde, Trine E. Møgelberg, and Jens Sehested
Section for Chemical ReactiVity, EnVironmental Science and Technology Department,
Risø National Laboratory, DK-4000 Roskilde, Denmark
Ole J. Nielsen*
Ford Motor Company, Ford Forschungszentrum Aachen, Dennewartstrasse 25, D-52068 Aachen, Germany
X
ReceiVed: July 27, 1995; In Final Form: January 24, 1996
A pulse radiolysis technique was used to measure the UV absorption spectra of CH
2
ClCHCl and CH
2
ClCHClO
2
-18
radicals over the range 230-300 nm. At 250 nm, σ(CH
2
ClCHCl) ) (1.7 ( 0.2) × 10 and σ(CH
2
ClCHClO
2
)
-
18
2
-1
)
(3.5 ( 0.5) × 10 cm molecule . The observed self-reaction rate constant for CH
2
ClCHCl radicals,
2
-11
3
-1 -1
defined as -d[CH
2
ClCHCl]/dt ) 2k
4
[CH
2
ClCHCl] was k
4
) (2.0 ( 0.3) × 10 cm molecule s . The
-12
6
rate constant for reaction of CH
2
ClCHCl radicals with O
2
in one bar of SF
diluent was (2.4 ( 0.3) × 10
radicals with NO and NO were
cm molecule s , respectively. The rate constant for the
3
-1 -1
cm molecule s . The rate constants for the reactions of CH
2
ClCHClO
2
2
-
12
-12
3
-1 -1
k
5
g 9 × 10
reaction of F atoms with CH
CH ClCHClO radicals with NO produced NO
spectroscopic technique showed that CH ClCHClO radicals undergo both reaction with O
via intramolecular 3-center HCl elimination. In 700 Torr of air at 296 K the rate constant ratio kO2/kdecomp
and k
6
) (9.8 ( 0.6) × 10
-11
3
-1 -1
2
ClCH Cl was k
2
3
) (2.6 ( 0.2) × 10 cm molecule s . The reaction of
and, by implication, CH ClCHClO radicals. A FTIR
and decomposition
2
2
2
2
2
2
)
-
20
3
-1
5 -1
(
2.3 ( 0.2) × 10 cm molecule . A lower limit of 4 × 10 s was deduced for the rate of intramolecular
3-center HCl elimination from CH ClCHClO radicals at 296 K in 1 bar of SF diluent. As part of this work
2 6
-12
-14
a relative rate technique was used to measure rate constants of (1.3 ( 0.2) × 10 and (6.4 ( 1.4) × 10
3
-1 -1
cm molecule
s
for the reactions of Cl atoms with CH
2
2
ClCH Cl and CH
2
ClC(O)Cl, respectively. All
experiments were performed at 296 K. Results are discussed in the context of the atmospheric chemistry of
,2-dichloroethane.
1
1
. Introduction
Assessment of the environmental impact of organic com-
dichloroethane (CH2ClCH2Cl) will react with OH radicals to
produce alkyl radicals which will, in turn, react with O2 to give
peroxy radicals:
pounds released into the atmosphere requires a detailed under-
standing of their atmospheric degradation. Alkoxy radicals are
key intermediates in the atmospheric oxidation of all organic
compounds. Until 1993 it was believed that the atmospheric
fate of alkoxy radicals was either reaction with O2, isomeriza-
tion, or decomposition (via C-C bond scission or halogen atom
elimination). Recent experimental studies at Ford Motor
Company have demonstrated the existence of another atmo-
CH ClCH Cl + OH f CH ClCHCl + H O
(1)
2
2
2
2
CH ClCHCl + O + M f CH ClCHClO + M (2)
2
2
2
2
In the present work we have used pulse radiolysis to determine
the UV absorption spectra of CH2ClCHCl and CH2ClCHClO2
radicals, and the kinetics of reactions 2-6.
spheric fate for R-monochloroalkoxy radicals; namely intramo-
lecular elimination of HCl.1
-4
At the present time there are
F + CH ClCH Cl f CH ClCHCl + HF
(3)
(4)
2
2
2
significant uncertainties in our understanding of the atmospheric
chemistry of alkoxy radicals.
CH ClCHCl + CH ClCHCl f products
To provide insight into the atmospheric chemistry of halo-
genated alkoxy radicals and hence improve the technical basis
for predictions of the environmental impact of halogenated
organics we have conducted a study of the atmospheric
chemistry of 1,2-dichloroethane. This compound was selected
for study because of its structural similarity to HFC-152
2
2
CH ClCHClO + NO f products
(5)
2
2
CH ClCHClO + NO + M f CH ClCHClO NO + M
2
2
2
2
2
2
(6)
(CH2FCH2F) and HFC-134 (CHF2CHF2) which are under
consideration as CFC replacements. In the troposphere 1,2-
FTIR product studies were used to study the atmospheric fate
of the alkoxy radical CH2ClCHClO. The results presented
herein improve our understanding of the atmospheric chemistry
*
Authors to whom correspondence may be addressed.
Abstract published in AdVance ACS Abstracts, March 15, 1996.
X
0022-3654/96/20100-5751$12.00/0 © 1996 American Chemical Society

5
752 J. Phys. Chem., Vol. 100, No. 14, 1996
Wallington et al.
of alkoxy radicals and hence improve our ability to predict the
environmental impact of the release of organic compounds.
2
. Experimental Section
.1. Pulse Radiolysis System. The pulse radiolysis transient
2
UV absorption setup used for the present experiments has been
described in detail previously and will only be dealt with briefly
5
here. CH2ClCHCl radicals were generated by the radiolysis
of SF6/CH2ClCH2Cl gas mixtures in a 1 L stainless steel reactor
with a 30 ns pulse of 2 MeV electrons from a Febetron 705B
field emission accelerator. SF6 was always in great excess and
was used to generate fluorine atoms:
2
MeV e-
SF6
8 F + products
(7)
Six sets of experiments were performed. First, the kinetics
of the formation of CH2ClCHCl was monitored using its
absorption at 250 nm to provide kinetic data for the reaction of
F atoms with CH2ClCH2Cl. Second, the rate of reaction of
CH2ClCHCl radicals with O2 was established by following the
formation of CH2ClCHClO2 radicals in reaction mixtures
containing O2. Third, the ultraviolet absorption spectra of
CH2ClCHCl and CH2ClCHClO2 radicals were determined by
observing the maximum of the transient UV absorption at short
times (0-2 µs) following the pulse radiolysis of SF6/CH2ClCH2-
Cl and SF6/CH2ClCH2Cl/O2 mixtures, respectively. Fourth,
using a longer time scale (5-400 µs), the decay of the
absorption ascribed to CH2ClCHCl radicals was monitored to
determine the kinetics of the self-reaction (eq 4):
Figure 1. Transient absorbance at 250 nm following the pulsed
radiolysis of a mixture of (A) 10 mbar CH
SF (radiolysis dose was 53% of maximum dose) and (B) 5 mbar
CH ClCH Cl, 30 mbar O , and 965 mbar SF (radiolysis dose was 42%
2 2
ClCH Cl and 990 mbar
6
2
2
2
6
of maximum dose). UV pathlength was 80 cm.
spectra were acquired and calibrated by expanding known
volumes of reference materials into the reactor. The reference
spectrum of HC(O)Cl obtained from the reference library at Ford
-1
CH ClCHCl + CH ClCHCl f products
(4)
Motor Company was calibrated using σ(1793 cm ) ) 1.63 ×
2
2
-18
2
-1 6
1
0
cm molecule .
Fifth, the rate of NO2 formation following the pulse radiolysis
of SF6/CH2ClCH2Cl/O2/NO mixtures was used to measure k5.
Sixth, the rate of decay of NO2 following the pulse radiolysis
of SF6/CH2ClCH2Cl/O2/NO2 mixtures was used to measure k6.
To monitor the transient UV absorption, the output of a pulsed
3
. Results and Discussion
.1. Kinetics of F + CH2ClCH2Cl. Following the pulse
3
radiolysis of CH2ClCH2Cl/SF6 mixtures, a rapid increase in
absorption in the UV at 250 nm was observed, followed by a
slower decay. Figure 1A shows the transient absorption at 250
nm following the radiolysis of a mixture of 10 mbar CH2ClCH2-
Cl and 990 mbar SF6. No absorption was observed when either
1
50 W xenon arc lamp was multi-passed through the reaction
cell using internal White type optics (80 or 120 cm path length).
Reagent concentrations used were as follows: SF6, 960-970
mbar; O2, 0-33 mbar; CH2ClCH2Cl, 0-10.0 mbar; NO, 0-1.57
mbar; and NO2 0-1.16 mbar (1013 mbar ) 760 Torr). All
experiments were performed at 296 K. Ultra-high purity O2
was supplied by L’Air Liquide, SF6 (99.9%) was supplied by
Gerling and Holz, CH2ClCH2Cl (>99.8%) was obtained from
Aldrich, NO (>99.8%) was supplied by Messer Griesheim, and
NO2 (>98%) was supplied by Linde Technische Gase. The
CH2ClCH2Cl sample was degassed before use by repeated
freeze-pump-thaw cycling. All other reagents were used as
received. No evidence for absorption by CH2ClCH2Cl over the
1
0 mbar of CH2ClCH2Cl or 990 mbar of SF6 was radiolyzed
separately. We assume that reaction of F atoms with CH2ClCH2-
Cl proceeds exclusively via H atom abstraction. To the best of
our knowledge, the heat of formation of CH2ClCH2 radicals
has not been reported and we are unable to calculate the
thermochemistry of the chlorine abstraction channel:
F + CH ClCH Cl f FCl + CH ClCH
2
2
2
2
However, relevant data are available to evaluate the thermo-
dynamics of analogous reactions involving CH3Cl, CH2Cl2, and
C2H5Cl. Cl atom abstraction from CH3Cl, CH2Cl2, and C2H5Cl
2
30-300 nm wavelength range was discernable in the present
-
21
2
-1
7
experiments, σ(230 nm) < 10
cm molecule .
-1
2.2. FTIR-Smog Chamber System. The FTIR system was
is endothermic by 23-24 kcal mol . It seems reasonable to
conclude that the reacton of F atoms with CH2ClCH2Cl proceeds
exclusively to give CH2ClCHCl radicals. We ascribe the
absorption shown in Figure 1A to the formation of CH2ClCHCl
radicals and their subsequent loss by self-reaction. The increase
in absorption at short times (0-10 µs) followed pseudo-first-
order kinetics. Figure 2 shows a plot of the pseudo-first-order
rate constants for the appearance of absorption (filled symbols)
as a function of the CH2ClCH2Cl concentration. The solid line
in Figure 2 through the filled symbols is a linear least-squares
interfaced to a 140 L Pyrex reactor. Radicals were generated
by the UV irradiation of mixtures of 110-130 mTorr of
CH2ClCH2Cl, 34-300 mTorr of Cl2, and 15-700 Torr of O2
in 700 Torr total pressure with N2 diluent at 296 K using 22
blacklamps (760 Torr ) 1013 mbar). The loss of reactants and
the formation of products were monitored by FTIR spectroscopy,
using an analyzing path length of 27 m and a resolution of 0.25
-1
cm . Infrared spectra were derived from 32 coadded spectra.
CH2ClCH2Cl, CH2ClC(O)Cl, HC(O)Cl, and CO2 were moni-
tored using their characteristic features over the wavenumber
ranges 1200-1350, 1800-1900, 1720-1820, and 2280-2400
-11
3
5
fit which gives a slope of k3 ) (2.6 ( 0.2) × 10
cm
-1
-1
molecule s . The positive intercept is (3.9 ( 1.8) × 10
-1
-1
cm , respectively. With the exception of HC(O)Cl, reference
s
and is statistically significant. To investigate the cause of

Atmospheric Chemistry of 1,2-Dichloroethane
J. Phys. Chem., Vol. 100, No. 14, 1996 5753
Figure 2. Pseudo-first-order rate constants for the increase in absorp-
tion at 250 nm following radiolysis of mixtures of 1-6 mbar of
Figure 3. Pseudo-first-order rate constants for the increase in absorp-
tion at 250 nm following radiolysis of mixtures of 20 mbar of
2 2 6 2 2
CH ClCH Cl and 1000 mbar of SF vs [CH ClCH Cl]. Filled symbols
CH
2
ClCH
2
2 6 2
Cl, 10-33.5 mbar of O , and 1000 mbar of SF vs [O ].
are observed data; open symbols are the results of simulations described
in the text. The insert shows the experimental transient using a mixture
Filled symbols are observed data; open symbols are the results of
simulations described in the text. The insert shows the experimental
of 1 mbar CH
0 cm and the radiolysis dose was 32% of maximum. The smooth curve
in the insert is the first order fit.
2 2 6
ClCH Cl and 999 mbar of SF , the UV pathlength was
transient using a mixture of 20 mbar CH
2 2 2
ClCH Cl, 25.8 mbar of O ,
8
and 960 mbar of SF ; the UV pathlength was 80 cm and the radiolysis
6
dose was 53% of maximum. The smooth curve in the insert is the first
order fit.
the positive intercept, the CHEMSIMUL8 chemical kinetic
modeling package was used to simulate the rise in absorption
by CH2ClCHCl radicals using a chemical mechanism consisting
after the radiolysis pulse to allow for conversion of F atoms
into CH ClCHCl radicals. The subsequent increase in absorp-
2
-
11
3
-1 -1
of reactions 3 and 4 with k3 ) 2.6 × 10 cm molecule
s
tion followed first-order kinetics. Figure 3 shows the observed
pseudo-first-order rate constants (filled circles) as a function of
-11
3
-1 -1
and k4 ) 2.3 × 10
cm molecule
s
(see subsequent
section). Two data points were simulated. The simulated rise
in absorption followed pseudo-first-order kinetics and a first-
order kinetic expression was fit to the simulated data to give
the results shown as the open circles in Figure 2. As seen from
Figure 2, within the experimental uncertainties, the simulated
data and experimental data are indistinguishable. We conclude
that the cause of the positive intercept is the CH2ClCHCl radical
self-reaction which decreases the time taken for the absorption
O concentration. A linear regression analysis of the data in
2
-
12
3
Figure 3 (filled circles) gives k ) (2.4 ( 0.3) × 10
2
cm
molecule- s , the y axis intercept is (3.5 ( 1.2) × 10 s .
1
-1
4
-1
8
To investigate the cause of the positive intercept the CHEMISUL
chemical kinetic modeling package was used to simulate the
rise in absorption by CH ClCHClO radicals using a chemical
2
2
mechanism consisting of reactions 2, 3, and 4 with k ) 2.4 ×
2
-
12
-11
-11
3
-1
10 , k ) 2.6 × 10 , and k ) 2.3 × 10 cm molecule
3
4
1
st
-1
to react a maximum and hence increases the values of k
2 2
s . The CH ClCHClO radical self-reaction was included in
-
11
cm3
-12
3
-1 -1
obtained. No correction to the value of k3 ) 2.6 × 10
the model with kRO2+RO2 ) 3.0 × 10
cm molecule
s
molecule-
1
s
-1
is needed. Quoted errors throughout this paper
which describes the observed decay of absorption at 250 nm
seen in Figure 1B. Two data points were simulated. The
simulated rise in absorption followed pseudo-first-order kinetics.
A first-order kinetic expression was fit to the simulated data to
give the results shown as the open circles in Figure 3. As seen
from Figure 3, within the experimental uncertainties, the
simulated data and experimental data are indistinguishable. We
conclude that the cause of the positive intercept are the self-
reactions of the CH2ClCHCl and CH2ClCHClO2 radicals which
decrease the time taken for the absorption to react a maximum
are two standard deviations. Where appropriate, errors have
been propagated using standard error analysis techniques.
3
.2. Kinetics of CH2ClCHCl + O2. The kinetics of the
reaction of CH2ClCHCl radicals with O2 were investigated by
radiolysis of CH2ClCH2Cl/O2/SF6 mixtures with the resulting
transient absorption monitored at 250 nm. As seen from Figure
1
B, the transient absorption at 250 nm following radiolysis of
mixtures containing O2 was substantially larger than that
observed in the absence of O2. This observation suggests that
a given concentration of peroxy radicals derived from 1,2
dichloroethane (CH2ClCHClO2) absorbs more strongly than the
same concentration of alkyl radicals (CH2ClCHCl). Such a
finding is reasonable upon the basis of the substantial database
1
st
and hence increase the values of k obtained. The modeling
excercise shows that the slope of the data plot in Figure 3 is
not influenced by secondary reactions. Hence, k2 ) (2.4 ( 0.3)
-
12
3
-1 -1
× 10
cm molecule s .
concerning the UV spectra of peroxy radicals which shows that
3.3. Spectrum of CH2ClCHCl Radicals. Measurement of
the absolute absorption spectrum of CH2ClCHCl radicals
requires calibration of the initial F atom concentration. Ad-
ditionally, experimental conditions are needed to ensure that
there is 100% conversion of F atoms to CH2ClCHCl radicals.
The yield of F atoms was established by monitoring the transient
absorbance at 260 nm due to methylperoxy radicals produced
these species absorb strongly at 250 nm.9
,10
Kinetic data for
the reaction of CH2ClCHCl radicals with O2 were obtained by
observing the kinetics of the increase in absorption at 250 nm
following the radiolysis of mixtures of 20 mbar of CH2ClCH2-
Cl, 6-33 mbar of O2, and 960 mbar of SF6 mixtures. The
lifetime of F atoms in the presence of 10 mbar of CH2ClCH2Cl
is 0.08 µs. Hence, the absorption rise was fitted from 1 µs
11
by radiolysis of SF6/CH4/O2 mixtures as described previously.

5
754 J. Phys. Chem., Vol. 100, No. 14, 1996
Wallington et al.
Figure 4. Maximum transient absorbance at 250 nm following the
pulsed radiolysis of mixtures of 10 mbar CH ClCH Cl and 990 mbar
SF (circles) or 5 mbar CH ClCH Cl, 30 bar of O , and 965 mbar SF
squares) as functions of the radiolysis dose. The UV pathlength was
0 cm. The solid lines are linear regressions of the low-dose data (filled
2
Figure 5. UV absorption spectra of radical species: CH ClCHCl (b),
this work; CH ClCHClO (9), this work; CH ClO (2).
2 2 2 2
13
2
2
6
2
2
2
6
(
8
TABLE 1
wavelength
σ(CH
2
ClCHCl)
2 2
σ(CH ClCHClO )
symbols). The dotted lines are second-order regressions to the entire
data set to aid visual inspection.
20
2
(10 cm molecule-1)
-20
2
(
nm)
30
240
(10- cm molecule-1)
2
240
160
170
130
80
53
-
-
390
420
350
290
190
120
57
-
18
2
-1
Using a value of 3.18 × 10
cm molecule for σ(CH3O2)
2
2
2
50
60
70
9
at 260 nm, the yield of F atoms at 1000 mbar of SF6 was
15
-3
determined to be (3.0 ( 0.3) × 10 molecules cm at full
irradiation dose. The quoted error on the F atom calibration
includes both statistical ((2 standard deviations) and potential
systematic errors associated with the 10% uncertainty in
σ(CH3O2). In the following, errors are propagated using
conventional error analysis methods and are shown as (2
standard deviations.
280
290
3
00
31
at 250 nm and converted into absolute absorption cross sections.
Values obtained are given in Table 1 and plotted in Figure 5.
3.4. Spectrum of CH2ClCHClO2 Radicals. Following the
To work under conditions where the F atoms are converted
stoichiometrically into CH2ClCHCl radicals, it is necessary to
consider potential interfering secondary chemistry. A complica-
tion could be reaction 8.
pulsed radiolysis of mixtures of 5 mbar of CH2ClCH2Cl, 30
mbar of O2, and 965 mbar of SF6 a rapid increase, complete
within 5 µs, in UV absorbance in the region 230-300 nm was
observed, followed by a slower decay. An example is shown
in Figure 1B. We ascribe the UV absorbance resulting from
radiolysis of SF6/CH2ClCH2Cl/O2 mixtures to CH2ClCHClO2
radicals.
To work under conditions where the F atoms are converted
stoichiometrically into CH2ClCHClO2 radicals, it is necessary
to consider potential interfering secondary chemistry. Potential
complications include: (i) Loss of F atoms by reaction with
molecular oxygen
F + CH ClCHCl f products
(8)
2
To check for complications caused by reaction 8, experiments
were performed using [CH2ClCH2Cl] ) 10 mbar and [SF6] )
9
90 mbar in which the maximum transient absorption at 250
nm was measured as a function of the radiolysis dose. Figure
shows the observed maximum transient absorption (optical
4
path length ) 80 cm) ascribed to CH2ClCHCl radicals as a
function of the dose. As shown in Figure 4, the maximum
absorption observed in experiments using maximum dose was
slightly less than that expected on the basis of a linear
extrapolation of the data obtained at lower doses. This
observation suggests that reaction 8 is important in experiments
using the maximum initial F atom concentration.
F + O + M f FO + M
(9)
2
2
and (ii) unwanted radical-radical reactions such as reactions
4, 8, 10, and 11.
F + CH ClCHClO f products
(10)
2
2
The solid line drawn through the CH2ClCHCl data in Figure
is a linear least-squares fit to the low-dose data and has a
CH ClCHCl + CH ClCHClO f
4
2
2
2
slope of 0.178 ( 0.011. Combining this data with the calibrated
CH ClCHClO + CH ClCHClO (11)
2
2
15
-3
yield of F atoms of (3.0 ( 0.3) × 10 molecules cm (full
dose and [SF6] ) 1000 mbar) gives σ(CH2ClCHCl) at 250 nm
Using rate constants for reactions 3 and 9, k3 ) (2.4 ( 0.2) ×
-1
-
18
2
-1
-11
-13
3
)
(1.71 ( 0.20) × 10 cm molecule . Quoted errors reflect
10 (this work) and k9 ) (1.9 ( 0.3) × 10 cm molecule
-1 12
uncertainties in both the F atom calibration and the slope of
the line drawn in Figure 4. To map out the absorption spectrum
of CH2ClCHCl radicals, experiments were performed to measure
the initial absorption between 230 and 280 nm following the
pulsed irradiation of mixtures of 10 mbar of CH2ClCH2Cl and
s , we calculate that 4.5% of the F atoms are converted into
FO2 radicals and 95.5% into CH2ClCHClO2 radicals. Decom-
position of FO2 radicals back to F atoms and O2 is of no
importance over the <5 µs taken for the absorption to reach its
maximum. Corrections for the presence of 4.5% of FO2 radicals
were calculated using the expression σ(CH2ClCHClO2) )
9
90 mbar of SF6. Initial absorptions were then scaled to that

Atmospheric Chemistry of 1,2-Dichloroethane
J. Phys. Chem., Vol. 100, No. 14, 1996 5755
[
σ(observed) - 0.045 × σ(FO2)]/0.955. Values of σ(FO2) are
12
taken from the literature. Corrections to account for FO2
absorption were minor (1-5%).
There are no literature data concerning the kinetics of
reactions 8, 10, and 11, hence we cannot calculate their
importance. To check for these unwanted radical-radical
reactions the transient absorbance at 250 nm was measured in
experiments using [CH2ClCH2Cl] ) 5 mbar, [O2] ) 30 mbar,
and [SF6] ) 965 mbar with the radiolysis dose varied over an
order of magnitude. The UV pathlength was 80 cm. Figure 4
shows the observed maximum of the transient absorbance of
CH2ClCHClO2 radicals at 250 nm as a function of dose. As
seen from Figure 4, the maximum absorption observed in
experiments using maximum dose was less than that expected
on the basis of a linear extrapolation of the data obtained at
lower doses. We ascribe this to incomplete conversion of F
atoms into CH2ClCHClO2 radicals caused by secondary radi-
cal-radical reactions 8, 10, and 11 at high initial F atoms
concentrations.
Figure 6. Plot of the reciprocal half-life for the self-reaction of
The solid line drawn through the data in Figure 4 is a linear
least-squares fit of the low-dose data. The slope is 0.347 (
2
CH ClCHCl radicals as a function of the maximum transient absorption
at 250 nm.
0
.027. From this value and three additional pieces of informa-
15
3
-1
tion, (i) the yield of F atoms of (3.0 ( 0.3) × 10 molecule
linear regression of the data is -(4.8 ( 5.0) × 10 s and is
-3
-18
cm (full dose and [SF6] ) 1000 mbar), (ii) the conversion of
F atoms into CH2ClCHClO2 radicals (95.5%) and FO2 (4.5%),
and (iii) the absorption cross section for FO2 at 250 nm (σ )
not significant. Using σ(CH2ClCHCl) ) (1.7 ( 0.2) × 10
2
-1
5 -1
cm molecule at 250 nm and the slope (6.9 ( 0.5) × 10 s ,
-11
3
-1 -1
gives k4 ) (2.0 ( 0.3) × 10 cm molecule s . This result
-18
2
-1 12
1
.3 × 10
cm molecule ), we derive σ(CH2ClCHClO2)
can be compared to other chloroalkyl radical self-reaction rate
-
18
2
-1
-11
3
-1 -1
at 250 nm ) (3.53 ( 0.45) × 10
cm molecule . The
constants which range from 2.8 × 10 cm molecule
s
in
1
6
-12
17
quoted error includes both statistical and potential systematic
errors and so reflects the accuracy of the measurement.
To map out the spectrum of the CH2ClCHClO2 radical,
experiments were performed to measure the initial absorbance
between 220-300 nm following the pulsed irradiation of SF6/
CH2ClCH2Cl/O2 mixtures. The initial absorbances were scaled
to that at 250 nm and corrected for FO2 to obtain absolute
absorption cross sections. Absorption cross sections are given
in Table 1 and compared to the spectrum of CH2ClO213 in Figure
the case of CH2Cl to 2.4 × 10
for CF3CCl2 and follow
-11
-11
the sequence CH2Cl (2.8 × 10 ) > CH2ClCHCl (2.0 × 10
)
-12 16
-12 18,19
> CHCl2 (9.3 × 10 ) > CCl3 (4.6 × 10
)
> CF3CCl2
-12
(2.4 × 10 ). This order of reactivity can be rationalized in
simple terms on the basis of steric arguments with bulky
substituents slowing down the association reaction.
3.6. Kinetic Data for the Reaction CH2ClCHClO2 + NO
f Products. The kinetics of reaction 5 were studied by
monitoring the increase in the absorbance at 400 nm, attributed
to the formation of NO2, following the pulsed radiolysis (53%
of maximum dose) of SF6/CH2ClCH2Cl/O2/NO mixtures. The
mixtures were combinations of [NO] ) 0.23-1.57 mbar,
[CH2ClCH2Cl] ) 5 mbar, [O2] ) 30 mbar, and [SF6] ) 970
mbar. This method of determining rate constants of reactions
5
. As expected, the CH2ClCHClO2 spectrum is very similar to
that of CH2ClO2.
.5. Kinetic Study of the Self-Reaction of CH2ClCHCl
3
Radicals. Figure 1A shows a typical absorption trace obtained
for the self-reaction of CH2ClCHCl radicals, together with a
nonlinear least-squares second-order decay fit. As discussed
in previous publications,1 the kinetic analysis of second-order
decays can be complicated by the formation of products which
absorb at the monitoring wavelength. This leads to a positive
absorbance after all reactions have ceased. Accordingly, the
experimental data were fit with a three parameter expression
of peroxy radicals with NO has been used extensively in our
4,15
20,21
laboratory and is discussed in detail elsewhere.
absorption transients were fitted using the expression:
The
1st
A(t) ) (A - A )[1 - exp(-k t)] + A
0
(II)
∞
0
where A(t) is the absorbance as a function of time, A∞ is the
1
st
1
/B - 1/B ) ln(10)(2kt)/σ L
(I)
absorbance at infinite time, k is the pseudo-first-order ap-
pearance rate of NO2, and A0 is the extrapolated absorbance at
t ) 0. In the presence of 5 mbar of CH2ClCH2Cl, F atoms
have a lifetime of 0.3 µs with respect to reaction to give
CH2ClCHCl radicals. In the presence of 30 mbar of O2,
CH2ClCHCl radicals have a lifetime of 0.5 µs with respect to
reaction to give CH2ClCHClO2 radicals. To allow sufficient
time for conversion of F atoms into CH2ClCHClO2 radicals the
fits were started 2 µs after the radiolysis pulse. In all cases,
the rise in absorbance was well fit by first-order kinetics.
Figure 7A shows a plot of the observed pseudo-first-order
0
eff
where B ) log(I∞/I), B0 ) log(I∞/I ′), where I ′ is the minimum
transmitted light intensity following the radiolysis pulse, I∞ is
the final light intensity, and I is the transmitted light at time t.
k is the second-order rate constant for the self-reaction of the
radicals, σeff is the absorption cross section of the radical minus
that of any absorbing product formed at the monitoring
wavelength, and L is the monitoring UV path length (80 cm).
The decay of the transient absorption following radiolysis of
CH2ClCH2Cl/SF6 mixtures was monitored at a wavelength of
1
st
2
50 nm. The decay was well represented by this second-order
rate constants, k , as a function of [NO]. The initial F atom
1
5
least-squares fit. Figure 6 shows the reciprocal half-life for the
decay of the absorption at 250 nm as a function of the initial
absorbance due to CH2ClCHCl radicals. A linear least-squares
concentration employed in the present experiments (1.5 × 10
-3
molecule cm ) is a significant fraction (4-27%) of that of the
initial NO concentration and deviations from pseudo-first-order
kinetics may be expected. However, no such deviations were
discernable within the experimental data scatter. Assuming that
5
fit of the data in Figure 6 gives a slope of (6.9 ( 0.5) × 10
-1
s
) (k4 × 2 ln 10)/[σ(CH2ClCHCl)L]. The intercept of the

5
756 J. Phys. Chem., Vol. 100, No. 14, 1996
Wallington et al.
In section 3.9 we show that for the experimental conditions
of the above experiments (30 mbar of O2) 98% of the
CH2ClCHClO radicals will undergo decomposition while 2%
will react with O2. The fact that the NO2 yield is substantially
greater than 100% is therefore due to the rapid decomposition
of CH2ClCHClO radicals. In all experiments the rise in the
transient absorption at 400 nm followed first-order kinetics. It
follows that the rate of decomposition of CH2ClCHClO radicals
must be faster than the fastest pseudo-first-order rate measured,
5
-1
i.e., > 4 × 10 s .
.7. Kinetic Data of the Reaction CH2ClCHClO2 + NO2
M f CH2ClCHClO2NO2 + M. The kinetics of reaction 6
3
+
were studied by monitoring the decrease in absorbance at 400
nm following the pulsed radiolysis (53% of maximum dose) of
mixtures of 5 mbar of CH2ClCH2Cl, 30 mbar of O2, 0.28-
1
.16 mbar NO2, and SF6 to 1000 mbar total pressure. This
method of measuring rate constants of reactions of peroxy
radicals with NO2 has been used extensively in our laboratory
2
5
and is discussed elsewhere.
The rate of decay of the
absorbance at 400 nm followed first-order kinetics and increased
with increasing NO2 concentration. It seems reasonable to
explain the loss in absorbance by a loss of NO2. Three reactions
could be responsible:
F + NO + M f FNO /FONO + M
(13)
(14)
2
2
CH ClCHCl + NO f products
2
2
Figure 7. Plot of the pseudo-first-order rate constant for (A) the
formation of NO as a function of the NO concentration and for (B)
the decay of NO as a function of the NO concentration. Filled symbols
are the observed data; open symbols have been corrected by subtracting
F] /2. The lines are fits to the corrected data.
2
CH ClCHClO + NO + M f CH ClCHClO NO + M
2
2
2
2
2
2
2
2
(6)
[
o
Under the present experimental conditions with [CH2ClCH2-
Cl] ) 5 mbar and [O2] ) 30 mbar, reactions 13 and 14 will be
of negligible importance.
Values of the observed pseudo-first-order rate constants of
NO2 loss derived by fitting a first-order decay to the observed
transients are plotted as a function of [NO2] in Figure 7B. The
initial F atom concentration employed in the present experiments
all F atoms either react directly with NO, or produce species
that react with NO, then the average NO concentration in a given
experiment is less that the initial concentration by an amount
equal to [F]o/2. The open symbols in Figure 7A show the effect
of correcting the [NO] values by this amount. Linear least-
squares analysis of the corrected data gives k5 ) (1.0 ( 0.1) ×
-11
3
-1 -1
1
0
cm molecule s . F atoms also react with NO
15
-3
(
1.5 × 10 molecule cm ) is a significant fraction (5-22%)
-12
3
-1 -1 22
producing FNO, k12 ) 5.5 × 10 cm molecule s . FNO
does not absorb at 400 nm23 and reaction 12 has no influence
on the kinetic analysis used to derive k5.
of that of the initial NO2 concentration and deviations from
pseudo-first-order kinetics may be expected. However, no such
deviations were discernable within the experimental data scatter.
Assuming that all F atoms either react directly with NO2 or
produce species that react with NO2 then the average NO2
concentration in a given experiment is less that the initial
concentration by an amount equal to [F]0/2. The open symbols
in Figure 7B show the effect of this correction. Corrections
applied were in the range 3-11%. Linear least-squares analysis
F + NO + M f FNO + M
(12)
The reaction of CH2ClCHClO2 radicals with NO produces
NO2 and, by inference, CH2ClCHClO radicals. The alkoxy
radical CH2ClCHClO can react with O2, or decompose. If
reaction with O2 or decomposition occurs within the time scale
of the present experiments (0-10 µs) new peroxy radicals will
be formed which can undergo subsequent reaction with NO to
give more NO2. In the present experiments the yield of NO2,
-12
3
of the corrected data gives k6 ) (9.8 ( 0.6) × 10
cm
molecule- s .
1
-1
The reactions of peroxy radicals with NO2 are pressure
dependent and are generally at, or near, the high pressure limit
-
19
2
-1 24
10
calculated using σNO (400 nm) ) 6 × 10 cm molecule ,
at 1 atmosphere total pressure of SF6 diluent. The value of
2
was 162 ( 17%. Corrections were made for the consumption
of F atoms by reaction with O2 and NO. The fact that the NO2
yield is greater than 100% shows that CH2ClCHClO radicals
either react with O2, or decompose, within the experimental
timescale of 0-10 µs. Because of the formation of NO2 from
reactions occurring subsequent to reaction 5 the rate constant
k6 measured here is entirely consistent with the high-pressure
limiting rate constants previously determined for such reactions
-12
3
-1
which typically lie in the range (5-10) × 10 cm molecule
-
1 10
s .
3.8. Kinetics of the Reactions of Cl Atoms with CH2Cl-
CH2Cl and CH2ClC(O)Cl. Prior to investigating the atmo-
spheric fate of CH2ClCHClO radicals, a series of relative rate
experiments was performed using the FTIR system to investigate
the kinetics of the reactions of Cl atoms with CH2ClCH2Cl and
CH2ClC(O)Cl. The techniques used are described in detail
-
11
3
-1 -1
k5 ) (1.0 ( 0.1) × 10
cm molecule
s
determined in
the present work should be regarded as a lower limit for the
-12
3
rate constant for reaction 5. Hence, k5 > 9 × 10
cm
molecule- s . This value is consistent with rate constants
determined previously for reactions of other peroxy radicals with
NO.2
1
-1
2
6
elsewhere. Photolysis of molecular chlorine was used as a
source of Cl atoms.
1

Atmospheric Chemistry of 1,2-Dichloroethane
J. Phys. Chem., Vol. 100, No. 14, 1996 5757
radicals will be formed in the chamber by the self-reaction of
CH2ClCHClO2 radicals.
Cl + hν f 2Cl
(15)
(16)
(17)
2
Cl + CH ClCH Cl f CH ClCHCl + HCl
CH ClCHClO + CH ClCHClO f
2
2
2
2
2
2
2
CH ClCHClO + CH ClCHClO + O (23a)
2
2
2
Cl + CH ClC(O)Cl f CHClC(O)Cl + HCl
2
CH ClCHClO + CH ClCHClO f
2
2
2
2
The kinetics of reaction 16 were measured relative to reactions
CH ClC(O)Cl + CH ClCHClHOH + O (23b)
2
2
2
18 and 19, reaction 17 was studied relative to reactions 20-
22.
The alkoxy radical will then either react with O2, decompose
via intramolecular HCl elimination, or undergo C-C bond
scission:
Cl + CH Cl f CH Cl + HCl
(18)
(19)
(20)
(21)
(22)
3
2
Cl + CH F f CH F + HCl
CH
2
ClCHClO + O
2
f CH
2
ClC(O)Cl + HO
2
(24)
3
2
Cl + CD f CD + DCl
CH ClCHClO + M f CH ClC(O) + HCl + M (25)
4
3
2
2
Cl + CD H f products
CH ClCHClO + M f CH Cl + HC(O)Cl + M (26)
2 2
2
2
The subsequent reactions of the radical species formed in
reactions 25 and 26 will include:
Cl + CH f CH + HCl
4
3
The observed losses of CH2ClCH2Cl vs CH3Cl and CH3F, and
CH2ClC(O)Cl vs CD4, CD2H2, and CH4 following the UV
irradiation of CH2ClCH2Cl/CH3Cl/Cl2, CH2ClCH2Cl/CH3F/Cl2,
and CH2ClC(O)Cl/methane/Cl2 mixtures, in 700 Torr total
pressure of N2, or air, diluent are shown in Figures 8 and 9.
There was no discernable difference between data obtained in
N2 or air. Linear least-squares analysis gives k16/k18 ) 2.65 (
CH ClC(O) + O + M f CH ClC(O)O + M (27)
2
2
2
2
CH ClC(O)O + RO f CH ClC(O)O + RO + O (28)
2
2
2
2
2
CH ClC(O)O + M f CH Cl + CO + M
(29)
(30)
(31)
(32)
(33)
0
1
1
1
1
.18, k16/k19 ) 3.90 ( 0.12, k17/k20 ) 11.1 ( 1.5, k17/k21 )
2
2
2
.40 ( 0.07, and k17/k22 ) 0.61 ( 0.04. Using k18 ) 4.9 ×
-
-
-
13 24
-13 27
-15 26
0
0
0
,
k19 ) 3.2 × 10
,
k20 ) 6.1 × 10
,
k21 ) 4.5 ×
CH Cl + O + M f CH ClO + M
14 26
-13 24
2
2
2
2
,
and k22 ) 1.0 × 10
gives k16 ) (1.30 ( 0.09) ×
-12 -14
12
, k16 ) (1.25 ( 0.10) × 10 , k17 ) (6.8 ( 0.9) × 10
,
-
14
-14
cm3
k17 ) (6.3 ( 0.3) × 10 , and k17 ) (6.1 ( 0.4) × 10
CH ClO + RO f CH ClO + RO + O
2
2
2
2
2
molecule- s , respectively. Consistent results were obtained
using the different reference compounds. We choose to quote
values for k16 and k17 which are averages of the results above
with error limits which encompass the extremes of the ranges,
1 -1
CH ClO + O f HC(O)Cl + HO
2
2
2
-
12
hence k16 ) (1.28 ( 0.13) × 10
and k17 ) (6.4 ( 1.3) ×
-14
3
-1 -1
10
cm molecule s . We estimate that potential systematic
CH ClO + M f HCO + HCl + M
2
errors associated with uncertainties in the reference rate
constants could add an additional 10% to the uncertainty range.
Propagating this additional 10% uncertainty gives k16 ) (1.3 (
The goal of the FTIR experiments was to determine the relative
importance of reactions 24-26 in the atmospheric chemistry
of CH2ClCHClO radicals. CH2ClC(O)Cl product serves as a
marker for reaction 24. Reaction 25 will lead to an observed
HCl yield (defined as moles of HCl formed per mole of
CH2ClCH2Cl lost) which is in excess of 100% and to substantial
CO2 and HC(O)Cl product formation. If reaction 26 is
important then high yields of HC(O)Cl are expected with no
CO2 and no excess HCl.
In the Cl atom-initiated oxidation of CH2ClCH2Cl, three
carbon-containing products were readily identified by virtue of
their characteristic IR features: CO2, HC(O)Cl, and CH2ClC(O)Cl.
The yields of these species following the UV irradiation of a
mixture of 275 mTorr of CH2ClCH2Cl and 147 mTorr of Cl2
in 700 Torr total pressure of air are shown as a function of the
CH2ClCH2Cl loss in Figure 10. As seen from Figure 10, the
formation of HCl, CO2, HC(O)Cl, and CH2ClC(O)Cl increased
linearly with the loss of CH2ClCH2Cl. Such linear behavior
suggests the absence of significant complications caused by
secondary reactions in the system. At this point it is germane
-12
-14
3
-1
0
s .
.2) × 10
and k17 ) (6.4 ( 1.4) × 10
cm molecule
-
1
The kinetics of reaction 16 have been studied previously by
2
8
29
Wine and Semmes and Tschuikow-Roux et al. Using an
28
absolute rate technique, Wine et al. measured k16 ) 1.5 ×
-12
29
12
1
0
at room temperature while Tschuikow-Roux et al. used
-
3
a relative rate method to derive k12 ) 1.3 × 10
cm
molecule- s . The value of k16 measured in the present work
is consistent with both previous determinations. There are no
literature data for k17 to compare with our result.
1
-1
3
.9. Study of the Atmospheric Fate of CH2ClCHClO
Radicals. To determine the atmospheric fate of the alkoxy
radical CH2ClCHClO, experiments were performed in which
Cl2/CH2ClCH2Cl/O2 mixtures at a total pressure of 700 Torr
(N2 diluent) were irradiated in the FTIR-smog chamber system.
The loss of CH2ClCH2Cl and the formation of products were
monitored by FTIR spectroscopy. By analogy to the behavior
of other peroxy radicals9 it is expected that CH2ClCHClO
,10

5
758 J. Phys. Chem., Vol. 100, No. 14, 1996
Wallington et al.
Figure 10. Yields of HCl (9), CO
b) vs the CH ClCH Cl loss observed following the UV irradiation of
Cl/Cl mixtures in 700 Torr of air diluent at 296 K. The
Cl loss was observed directly for ∆[CH ClCH Cl] > 10
ClCH Cl < 10 mTorr were calculated from the
OCH tracer, see text.
2 2
(O), HC(O)Cl (3), and CH ClC(O)Cl
Figure 8. Observed loss of CH
2
ClCH
2 3 3
Cl vs CH F (circles) and CH Cl
(
2
2
(triangles) in the presence of Cl atoms in 700 Torr of either air (filled
CH
CH
2
ClCH
ClCH
2
2
2
symbols) or N (open symbols) diluent.
2
2
2
2
mTorr. Losses of CH
observed loss of CH
2
2
3
3
ments in which a more reactive species such as CH3OCH3 is
present as a tracer. Experiments were performed using a mixture
of 979 mTorr of CH2ClCH2Cl, 11.4 mTorr of CH3OCH3, and
1
47 mTorr of Cl2 in 700 Torr of air diluent. CH3OCH3 reacts
-10
-12
with Cl atoms a factor of 146 (1.9 × 10 /1.3 × 10 ) times
faster than does CH2ClCH2Cl. The consumption of CH3OCH3
was monitored by virtue of its IR features at 1300-1400 cm-
and used in the expression, ln([CH3OCH3]to/[CH3OCH3]t) ) 146
ln([CH2ClCH2Cl]to/[CH2ClCH2Cl]t), to compute the consump-
tion of CH2ClCH2Cl. The calculated consumption of CH2ClCH2-
Cl was 1.9-9.5 mTorr (0.2-1.0% of the initial concentration).
The Cl atom-initiated oxidation of CH3OCH3 gives CH3OCHO
1
3
2
and CH3OCH2OOH and is not expected to interfere with the
oxidation of CH2ClCH2Cl. The observed yields of CO2,
HC(O)Cl, and CH2ClC(O)Cl are plotted vs the calculated
CH2ClCH2Cl loss in Figure 10. For experiments in which
CH3OCH3 was used as a tracer for the CH2ClCH2Cl loss the
HCl yield does not provide any useful information and so it is
not plotted in Figure 10. As seen from Figure 10, there is good
agreement between the data obtained using CH3OCH3 as a tracer
for CH2ClCH2Cl loss and those experiments in which the
CH2ClCH2Cl loss was observed directly. In the combined data
set shown in Figure 10 the CH2ClCH2Cl consumption ranged
from 0.2-11%. The consistency of the results over this
consumption range precludes the possibility that the CO2 product
is formed indirectly by secondary reactions involving Cl atom
attack of some unknown reactive intermediate.
Figure 9. Observed loss of CH
diamonds), and CH (circles) in the presence of Cl atoms in 700 Torr
of either air (filled symbols) or N (open symbols) diluent.
2 4 2 2
ClC(O)Cl vs CD (triangles), CD H
(
4
2
to consider possible losses of these products in the chamber.
None of the products are expected to be photolyzed by the UV
blacklamps used in this work. To check for possible hetero-
geneous losses, reaction mixtures were allowed to stand in the
dark for 20 min; there were no observable losses (<2%) of any
of these compounds. There is no reaction of Cl atoms with
-
12
CO2 or HCl. CH2ClCH2Cl reacts 1.8 (1.3 × 10 /7.3 ×
-
13 30,31
-12
-14
10
) and 20 (1.3 × 10 /6.4 × 10 ) times more rapidly
with Cl atoms than HC(O)Cl and CH2ClC(O)Cl, respectively.
Hence, for the consumptions of CH2ClCH2Cl employed in the
present work (<11%) loss of HC(O)Cl and CH2ClC(O)Cl via
Cl atom attack is of minor importance.
The CO2, HC(O)Cl, and CH2ClC(O)Cl products shown in
Figure 10 together account for 74 ( 15% of the loss of
CH2ClCH2Cl. In addition to these identified products IR
features due to unidentified products were observed at 1052,
-1
The observation of CO2 as a primary product following Cl
atom attack of CH2ClCH2Cl in air is interesting as such
formation of CO2 requires a rather dramatic change in molecular
configuration with the breakage of a C-C bond, C-Cl bond,
and two C-H bonds and the formation of two CdO bonds. To
check for a contribution of secondary reactions involving Cl
atoms to the observed CO2 yield requires experiments to be
performed in which the consumption of CH2ClCH2Cl is less
than 1%. Such a small consumption cannot be measured
directly. However, it can be measured indirectly using experi-
1117, 1187, 1362, and 1450 cm . As described above there
are a variety of different peroxy radicals formed in the present
experimental system; CH2ClCHClO2, CH2ClC(O)O2, CH2ClO2,
and HO2. These species may undergo cross reactions to give a
variety of products. The fact that the combined yields of CO2,
HC(O)Cl, and CH2ClC(O)Cl products only account for 75% of
the loss of CH2ClCH2Cl is not surprising in light of the
complexity of the peroxy radical chemistry.
Linear least-squares analysis of the data in Figure 10 gives
yields of 154 ( 8%, 59 ( 4%, 56 ( 4%, and 16 ( 2% for

Atmospheric Chemistry of 1,2-Dichloroethane
J. Phys. Chem., Vol. 100, No. 14, 1996 5759
expressed as a function of the rate constant ratio k24/k25 and the
3
3
CH2ClC(O)Cl yield from reaction 23b, Yo:
CH ClC(O)Cl yield )
2
Y + (1 - 2Y )({(k /k )[O ]}/{1 + (k /k )[O ]})
o
o
24 25
2
24 25
2
The solid line in Figure 11 is a fit of the above expression to
-
20
3
the data which gives k24/k25 ) (2.3 ( 0.2) × 10
cm
molecule- and Yo ) 0.08 ( 0.02. From this rate constant ratio
it can be calculated that in one atmosphere of air at 296 K,
1
89% of the CH2ClCHClO radicals decompose via HCl elimina-
tion while 11% react with O2. Intramolecular elimination of
HCl dominates the fate of CH2ClCHClO radicals at sea level.
With increasing altitude in the atmosphere the temperature, total
pressure, and partial pressure of O2 all decrease. Decreasing
temperature and total pressure favor reaction 24 over reaction
2
2
5 while decreasing partial pressure of O2 will favor reaction
5 over reaction 24. Reaction 25 is a unimolecular decomposi-
Figure 11. Yields of HC(O)Cl (triangles) and CH
vs the O
Cl/Cl /O
2
ClC(O)Cl (circles)
tion reaction and is expected to be very sensitive to changes in
temperature. Considering the conditions pertinent to the United
States standard atmosphere over the altitude range 0-15 km
and neglecting the effect of total pressure, reaction 25 will slow
2
partial pressure following the UV irradiation of CH
2 2
ClCH -
2
2
mixtures in 700 Torr total pressure made up with N
2
diluent
34
at 296 K. Open symbols have been corrected for the effect of reaction
3ssee text for details. The curve through the CH ClC(O)Cl data is a
fit of the expression discussed in the text.
3
2
down faster than reaction 24 if the activation energy for reaction
1
2
5 is > 4 kcal mol- more than that for reaction 24. By analogy
6
to the behavior of CH2ClO radicals it seems likely that the
barrier to decomposition of CH2ClCHClO radicals will be more
than 4 kcal mol- larger than that for reaction with O2. Hence,
it is expected that reaction with O2 will increase in importance
with increasing altitude. In the absence of data concerning the
temperature dependence of the rate constant ratio k24/k25 we are
unable to provide an overall assessment of the relative impor-
tance of reactions 24 and 25 in the atmospheric chemistry of
CH2ClCHClO radicals.
HCl, CO2, HC(O)Cl, and CH2ClC(O)Cl, respectively. The
observation of a yield of HCl which is greater than 100%
together with a substantial CO2 yield shows that intramolecular
elimination of HCl via reaction 25 is an important loss
mechanism for CH2ClCHClO radicals in the chamber. This
1
behavior is similar to that displayed by CH3CHClO radicals
which undergo rapid HCl elimination.1
,2,4
It is interesting to
note that the HCl yield in Figure 10 is in excess of the expected
00% yield attributable to reaction 16 by an amount (54 ( 8%)
1
Finally, it is of interest to compare the behavior of
CH2ClCHClO radicals observed here with that reported for
which is indistinguishable from the observed CO2 and HC(O)Cl
yields. In 700 Torr of air diluent each CO2 molecule formed
in reaction 29 will be accompanied by the formation of one
molecule of HC(O)Cl via reactions 30-32. The fact that the
CO2 and HC(O)Cl yields shown in Figure 10 are indistinguish-
able shows that reaction 26 does not play a significant (<10%)
role in the chemistry of CH2ClCHClO radicals. The CH2ClC(O)-
Cl product shown in Figure 10 can be formed via either the
molecular channel of the self-reaction of CH2ClCHClO2 radicals,
reaction 23b, or via reaction of O2 with the alkoxy radical
CH2ClCHClO, reaction 24. If reaction 24 is a substantial source
of CH2ClC(O)Cl then its yield should be sensitive to the partial
pressure of O2.
3
5
CH2FCHFO radicals.
We show here that CH2ClCHClO
radicals undergo 3-center intramolecular HCl elimination. No
such analogous process is observed for CH2FCHFO radicals
which undergo C-C bond scission exclusively under ambient
conditions.
3
.10. The Mechanism of the Reaction of F Atoms with
CH2ClCH2Cl. As discussed in section 3.1, we assume that
reaction of F atoms with CH2ClCH2Cl proceeds exclusively via
H atom abstraction. For completeness, the FTIR system was
used to study the F atom initiated oxidation of CH2ClCH2Cl.
HC(O)Cl and CH2ClC(O)Cl were observed in yields of 39 (
2
% and 38 ( 2% following the irradiation of a mixture of 121
A series of experiments were performed using mixtures of
mTorr of CH2ClCH2Cl and 444 mTorr of F2 in 700 Torr of O2.
These yields are indistinguishable from those observed in Cl
atom initiated experiments (see Figure 11) providing experi-
mental support for the expectation that reaction of F atoms with
CH2ClCH2Cl gives CH2ClCHCl radicals.
1
20 mTorr of CH2ClCH2Cl, 34-176 mTorr of Cl2, 15-700
Torr of O2, in 700 Torr total pressure of N2 diluent. Figure 11
shows the yields of HC(O)Cl and CH2ClC(O)Cl plotted as a
function of the O2 partial pressure. HC(O)Cl is formed by
reaction of CH2ClO radicals with O2, reaction 32. For the two
lowest O2 partial pressures investigated, CH2ClO radicals will
be lost via both reactions 32 and 33. While for [O2] > 100
Torr reaction 32 totally dominates reaction 33. To place the
observed HC(O)Cl yields on an equal footing, the loss of
CH2ClO radicals via reaction 33 was computed using k32/k33 )
4
. Implications for Atmospheric Chemistry
Following release into the atmosphere, CH2ClCH2Cl will react
predominantly with hydroxyl radicals. At 298 K the rate
constant for reaction of OH radicals with CH2ClCH2Cl is 25
-
18
3
-1 3
-13 36
-15 37
5
.0 × 10 cm molecule . The observed yields of HC(O)Cl
(2.4 × 10
/9.6 × 10
) times greater than that of methyl
for the experiments using low [O2] were then corrected upward
to give values expected if all CH2ClO radicals were converted
into HC(O)Cl.
chloroform which is known to be removed from the atmosphere
solely by reaction with OH radicals with an atmospheric lifetime
3
7
of 5.7 years. Hence, the atmospheric lifetime of CH2ClCH2-
As seen from Figure 11, the yield of CH2ClC(O)Cl increased
with increasing O2 concentration while the HC(O)Cl yield
decreased. Assuming that CH2ClC(O)Cl is formed in the
chamber solely by reactions 23b and 24 then its yield can be
Cl is ∼3 months. Reaction with OH gives CH2ClCHCl radicals.
-12
3
-1 -1
Using k2 ) 3.0 × 10 cm molecule s , it can be calculated
that in 760 Torr of air, CH2ClCHCl radicals have a lifetime of
0.06 µs with respect to reaction with O2 to give peroxy radicals,

5
760 J. Phys. Chem., Vol. 100, No. 14, 1996
Wallington et al.
CH2ClCHClO2. CH2ClCHClO2 radicals react rapidly with NO
to produce NO2 and (by inference) CH2ClCHClO radicals. Using
(11) Wallington, T. J.; Ellermann, T.; Nielsen, O. J. J. Phys. Chem. 1993,
7, 8442.
9
(12) Ellermann, T.; Sehested, J.; Nielsen, O. J.; Pagsberg, P.; Wallington,
-12
3
-1 -1
k5 g 9.0 × 10 cm molecule
background tropospheric NO concentration of 2.5 × 10 cm
the lifetime of CH2ClCHClO2 radicals with respect to reaction
is calculated to be less than 7 min. Reaction 5 is likely to be
s
together with an estimated
T. J. Chem. Phys. Lett. 1993, 218, 287.
(13) Dagaut, P.; Wallington, T. J.; Kurylo, M. J. Int. J. Chem. Kinet.
8
-3 38
1
988, 20, 815.
14) Kurylo, M. J.; Ouellette, P. A.; Laufer, A. H. J. Phys. Chem. 1986,
0, 437.
(
5
9
an important atmospheric loss of CH2ClCHClO2 radicals.
Reaction with O2 and intramolecular HCl elimination are
competing atmospheric fates of CH2ClCHClO radicals. In one
atmosphere of air at 296 K, 89% of the CH2ClCHClO radicals
decompose via HCl elimination while 11% react with O2.
Reaction with O2 gives CH2ClC(O)Cl while HCl elimination
leads to the formation of HC(O)Cl. Both compounds have
lifetimes in the lower atmosphere which are much shorter than
that required for transport to the stratosphere and so are
incapable of delivering significant amounts of chlorine to the
stratosphere.
(
15) Sander, S. P.; Watson, R. T. J. Phys. Chem. 1981, 86, 2960.
(16) Roussel, P. B.; Lightfoot, P. D.; Caralp, F.; Catoire, V.; Lesclaux,
R.; Forst, W. J. Chem. Soc., Faraday Trans. 1991, 87, 2367.
17) Wallington, T. J.; Ellermann, T.; Nielsen, O. J. Res. Chem. Intermed.
994, 20, 265.
18) Danis, F.; Caralp, F.; Veyret, B.; Loirat, H.; Lescluax, R. Int. J.
Chem. Kinet. 1989, 21, 715.
(
1
(
(
(
(
19) Ellermann, T. Chem. Phys. Lett. 1992, 189, 175.
20) Wallington, T. J.; Nielsen, O. J. Chem. Phys. Lett. 1991, 187, 33.
21) Sehested, J.; Nielsen, O. J.; Wallington, T. J. Chem. Phys. Lett.
1
993, 213, 457.
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Chem. 1994, 98, 2346.
(23) Sehested, J.; Nielsen, O. J. Chem. Phys. Lett. 1993, 206, 369.
We show here that CH2ClCHClO radicals undergo rapid (k25
(24) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
5
-1
>
4 × 10 s ) intramolecular HCl elimination. This behavior
Kurylo, M. J.; Howard, C. J.; Ravishankara A. R.; Kolb, C. E.; Molina, M.
J. Jet Propulsion Laboratory Publication 94-26; Jet Propulsion Laboratory.
Pasadena, CA, 1994.
has also been reported from the structurally similar CH2ClO
and CH3CHClO radicals.1 It appears that intramolecular HCl
elimination is a general mechanism for R-monochloroalkoxy
radical loss which should be considered in future assessments
of the atmospheric chemistry of chlorinated organic compounds.
-6
(
25) Platz, J.; Nielsen, O. J.; Sehested, J.; Wallington, T. J. J. Phys.
Chem. 1995, 99, 6570.
26) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437.
(
(27) Wallington, T. J.; Ball, J. C.; Nielsen, O. J.; Bartkiewicz, E. J.
Phys. Chem. 1992, 96, 1241.
(
(
28) Wine P. H., Semmes, D. M. J. Phys. Chem. 1983, 87, 3572.
29) Tschuikow-Roux, E.; Faraji, F., Niedzielski, J. Int. J. Chem. Kinet.
Acknowledgment. We thank Steve Japar (Ford Motor
Company) for a critical reading of the manuscript and Marchoe
Dill for technical assistance.
1
986, 18, 513.
(30) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Int. J.
Chem. Kinet. 1980, 12, 915.
31) Libuda, H. G.; Zabel, F.; Fink, E. H.; Becker, K. H. J. Phys. Chem.
(
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2
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