United States
Environmental Protection
Agency
Atmospheric Sciences _^i
Research Laboratory ~
Research Triangle Park, NC 27711''
Research and Development
EPA/600/S3-86/013 May 1986
&EPA Project Summary
Validation of OH Radical
Reaction
Rate Constant Test Protocol
E. O. Edney and E. W. Corse
A study was conducted to evaluate the
OH rate constant measurement protocol
developed by researchers at the Universi-
ty of California at Riverside. The protocol,
which is a relative rate method, was used
to measure room temperature OH rate
constants for a series of low molecular
weight, high volatility alkanes, alkenes,
aromatics, oxygen-containing compounds,
and chlorinated compounds. The full
report of this research project provides OH
rate constants for 21 compounds and a
comparison of each value with those
reported in the literature.
The OH rate constants obtained were in
good agreement with literature values for
all classes of compounds except for
chlorinated compounds, for which rate
constants had not been previously deter-
mined for most of the compounds studied.
The protocol can be used to determine OH
rate constants as low as 0.5 x 10~12
cm3/molecule-sec. The results obtained
for chlorinated compounds suggest that
Cl reactions can interfere with the OH rate
constant determination; however, if the
chlorinated compound is irradiated under
conditions of high reference to test com-
pound concentration ratios, the effect can
be reduced. Further research is required
to establish the validity of the protocol for
determining OH rate constants of
chlorinated compounds.
This Project Summary was developed
by EPA's Atmospheric Sciences Research
Laboratory, Research Triangle Park, NC, to
announce key findings of the research pro-
ject that is fully documented in a separate
report of the same title (see Project Report
ordering information at back).
Introduction
Recently, increased attention has been
given to the possible adverse health
effects associated with exposure to
industrial chemicals emitted into the at-
mosphere. To address this issue, the
Environmental Protection Agency (EPA)
Office of Pesticides and Toxic Substances
was established in 1977 to gather the in-
formation necessary to determine the
health risks associated with exposure to
present or future airborne chemicals. To
evaluate the risks, the distribution, toxici-
ty, and environmental fate and lifetime of
industrial chemicals must be established.
The first step in establishing atmos-
pheric lifetimes is to identify the major
removal processes. Gas phase homo-
geneous removal processes have been in-
vestigated extensively in the laboratory
and the results suggest that there are four
major removal pathways. The suggested
pathways are: (1) reaction with hydroxyl
radicals (OH); (2) reaction with ozone
(03); (3) photodissociation; and (4) reac-
tion with nitrate radicals (NO3). To in-
vestigate these processes, a series of
protocols that determine the atmospheric
lifetimes associated with OH and O3
reactions, as well as photolysis have been
developed for EPA by researchers at the
Statewide Air Pollution Research Center
at the University of California at Riverside
(UCR).
To validate the protocols, EPA con-
tracted with Northrop Services, Inc.-
Environmental Sciences to conduct a
series of OH and 03 rate constant deter-
minations, using the UCR methods. The
results of the OH rate constant validation
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study, where room temperature rate con-
stants for 21 compounds were deter-
mined, are presented in the full project
report. Rate constants were determined
for five classes of hydrocarbons: alkanes,
alkenes, aromatics, oxygen-containing
compounds, and chlorinated compounds.
The values obtained were compared,
where possible, with literature values. A
lower limit for which the rate constants
can be evaluated with the protocol was
also estimated. Special emphasis was
placed on the determination of OH rate
constants for chlorinated compounds
where the possibility existed that Cl reac-
tions could occur.
Experimental Approach
The experimental approach employed
was that developed by the UCR. Rate con-
stants for OH were determined by using
a relative rate method, in which the test
compound was irradiated in air in the
presence of methyl nitrite (CH3ONO),
nitric oxide (NO), and a reference com-
pound whose OH rate constant was well
known. The theoretical chemical reaction
sequence is described in the full report.
The rates of reaction with OH are
assumed to satisfy the following
equations:
— [Test] = -k10[Test] [OH] (I)
dt
— [Ref] = -MRefHOH] (II)
dt
where [Test] and [Ref] are the concentra-
tions of the test and reference com-
pounds, respectively, and the kj values are
the associated OH rate constants. Equa-
tions I and II operate under the assump-
tion that the compounds react only with
OH and are invalid if other reactive species
such as chlorine radicals (Cl) are present
during the irradiation.
Equations I and II can be combined and
integrated. The result is
[Test], k10 [Ref],
In - - = — In - ^ (III)
[Test],
[Ref],
where [Test],0 and [Ref],0 are the concen-
trations of the test compound and the
reference compound at time t0, respec-
tively, and [Test], and [Ref], are the
respective concentrations at time t.
If ln([Test],0/[Test]t) is plotted against
ln[Ref],0/[Ref]t), a straight line should be
obtained with slope equal to k^/k^ and
intercept equal to zero. The value for the
test compound OH rate constant is im-
mediately obtained from the slope since
the reference compound OH rate constant
is known.
Experimental Method
All irradiations were conducted in pillow-
shaped 2-mil Teflon bags. Bags were con-
structed by heat-sealing three sides of a
4- x 8-ft sheet of Teflon. A Teflon
Swagelok O-Seal straight-thread connec-
tor was mounted on the bag and was used
for filling and evacuating the bag, as well
as sampling its contents. The volumes of
the bags employed ranged from 50 to 60
L. The irradiation chamber consisted of a
wood cylindrical frame, split lengthwise
with two light banks mounted on the inner
side of the frame. Each light bank con-
tained five black lamps (General Electric
F40 BLB). A 180 cfm blower was mounted
at the top of the frame and was used to
remove heat from the chamber. To further
minimize temperature increases during the
irradiation, the frame was left open ap-
proximately 15 cm during the experiments.
A thermometer was mounted inside the
frame and the maximum temperature in-
crease found during all the experiments
was 2 °C. The initial bag temperature was
controlled by the laboratory temperature
and the temperatures measured during all
the experiments ranged from 21 ° to 26 °C.
A typical OH rate constant determina-
tion consisted of first filling the bag with
approximately 30 L of clean air (Zero 1.0
Grade from MG Scientific), mixing its con-
tents, and then evacuating it. The bag was
then filled with clean air to a final volume
of approximately 55 L. The flow rate was
10L/min and was measured with a cali-
brated rotometer. During the filling pro-
cess, NO, CH3ONO, and the test and
reference hydrocarbons were introduced
into the bag by injecting the compounds
into a glass tee that was mounted in the
clean air fill line. All glass syringes were
used for the liquid and gas injections. The
purity of all hydrocarbons employed was
greater than 99%, and they were not fur-
ther purified. Laboratory lights were turned
off and the reaction chamber was covered
during the time the bag was being filled,
and these conditions were maintained
until the irradiation began. After the bag
was filled and required amounts of gases
added, it was kneaded to ensure good
mixing.
The inital concentrations of CH3ONO
and NO were approximately 25 ppm and
20 ppm, respectively. The initial test
compound concentration ranged from 0.5
ppm to 4.0 ppm, whereas the reference
compound concentration ranged from 0.5
ppm to 64 ppm. The initial test and
reference compound concentrations were
determined by gas chromatography (GC).
The chromatographic conditions employed
to detect the compounds are listed in the
full report. The initial CH3ONO and NO
concentrations were based on the amount
of compound injected.
A series of GC samples were taken
before the irradiation began in order to
determine the stability of the test and
reference compounds in the dark. In
general, the sampling continued until suc-
cessive peak heights of the compounds
differed by less than 2%. Once the irradia-
tion began, the NO was monitored con-
tinuously and GC samples were taken
every 5 or 10 min, depending on the reten-
tion times of the compounds. Typical ir-
radiation times ranged from 30 min to 60
min.
The hydrocarbon analyses were con-
ducted using two gas chromatographs
(Perkin Elmer Model 900 and a GOW-MAC
Model 750). Both gas chromatographs
contained flame ionization detectors.
Nitric oxide concentrations were deter-
mined with a Bendix Model 8101-B
NO/N02/NOX analyzer.
The CH3ONO was prepared by drop-
wise addition of 50% sulfuric acid
(H2S04) to a stirred saturated solution of
sodium nitrite (NaNO2) in methanol
(CH3OH). A 50 ml/min N2 stream was
used to transfer the CH3ONO from the
reaction system to a liquid N2 cold trap,
where it was collected. The transfer line
consisted of two bubblers in series in front
of a collection trap used to remove H2SO4
and H2O. The first bubbler contained a
saturated solution of sodium hydroxide
(NaOH) and the second contained Drierite
dessicant.
Multiple irradiations were conducted for
each compound in order to generate a data
base. Different reference compounds and
initial reference to test compound concen-
tration ratios were employed. A special
protocol was developed to investigate the
OH rate constants of chlorinated com-
pounds. In this case, the initial concentra-
tion of test compound was held fixed at
4.0 ppm and the initial reference com-
pound concentration was varied from 1.0
ppm to 64 ppm. The inital CH3ONO con-
centration was also held constant during
the set of experiments. In addition to using
this protocol to investigate chlorinated
compounds, it was also employed to
determine rate constants for propane and
isobutane.
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Results and Discussion
Data for each OH rate constant deter-
mination are provided in the appendix of
the full report. The data include the iden-
tity of the reference compound, the initial
concentrations of the test and reference
compounds, the average temperature and
percent deviation, the slope, intercept, and
correlation coefficient obtained from Equa-
tion III, and the experimentally determined
value for ROH- The results are further
summarized in the report; the average OH
rate constants are listed, as well as the
average temperatures and percent devia-
tions associated with the rate constants.
The report also provides, for comparison,
rate constants reported by other
researchers.
The OH rate constants obtained for the
alkane class consisting of ethane, propane,
isobutane and cyclohexane were in agree-
ment with those in the literature. The
ethane results suggested that the method
can be used to measure rate constants as
small as 3 x 10~13 cm3/molecule-sec.
However, this may not be the appropriate
lower limit for compounds that are more
difficult to detect than ethane. Results of
this study support the fact that the rate
constants obtained for propane and
isobutane were independent of the refer-
ence to test compound ratio and were
consistent with the model in that the only
reactive species in the irradiated mixture
was OH.
The results for the two alkenes studied
(rra/?s-2-butene and isoprene), the five
aromatic compounds (benzene, toluene,
ortho-xylene, meta-xylene, and para-
xylene) and the oxygen-containing com-
pounds (acrolein, methacrolein, and
methylethyl ketone) were also in good
agreement with the literature. The rate
constant found for propylene oxide (1.11
x 10~12 cm3/molecule-sec) agreed with a
value obtained by another researcher using
the relative rate method. However, it was
a factor of two larger than a value obtained
by another researcher who used a flash
photolysis resonance fluorescence
method. The percent deviation for propy-
lene oxide results reported in the full report
was 68%. Much of the deviation was
probably due to the difficulty in measur-
ing propylene oxide.
A large number of experiments were
conducted in this study to evaluate the OH
rate constants of chlorinated compounds.
Many potentially hazardous compounds
contain chlorine, and the potential for reac-
tions of the test and reference compound
with Cl (if it is produced) is great. A value
of 1.86 x 10~12 cm3/molecule-sec was
found for the OH rate constant for
trichloroethylene. The relatively large value
for the percent deviation (± 36%) from the
recommended literature value was surpris-
ing since the magnitude of the rate con-
stant found was well above the lower limit
for the protocol, and there were no
inherent difficulties in measuring
trichloroethylene. Results for this study
suggested the possibility of reactions
other than OH attack since the measured
effective rate constant was shown to be
a function of the initial n-butane to
trichloroethylene concentration. Recent
evidence shows that reactive atomic
chlorine is produced in irradiated
trichloroethylene/nitrous acid (HOMO)/
NO/air mixtures. Once produced, Cl readily
reacts with trichloroethylene to produce
trichloroacetyl chloride, which releases Cl
and thereby propagates a Cl chain
reaction.
It is reasonable to expect that.CI is pro-
duced in this system, and in addition it is
likely that the production rate of Cl is pro-
portional to the trichloroethylene concen-
tration. At low [Ref]/[Test] ratios there ex-
ists the possibility that Cl reactions will be
important, and therefore the relative
decreases in the test and reference con-
centrations as a function of time will
reflect the tendency of the two com-
pounds to react with both OH and Cl. The
results, therefore, cannot be used to ob-
tain the OH rate constant. However, at
high [Ref]/[Test] ratios the system will tend
to be dominated by OH reactions since the
source of Cl compared to OH is reduced,
and a major sink for Cl, hydrogen abstrac-
tion from n-butane, will have been en-
hanced. Results of this study indicated
that for [Ref]/[Test]>10, the effective rate
constant was essentially independent of
the concentration ratio. The OH rate con-
stant obtained in this region was 2.85 x
10 ~12 cm3/molecule-sec.
Analyses similar to that employed for
trichloroethylene were used to evaluate
the rate constants for vinylidene chloride,
allyl chloride, benzyl chloride, and
chlorobenzene. The results for vinylidene
chloride, allyl chloride, and benzyl chloride
also suggested similar evidence for Cl
reactions. The OH rate constants derived
for vinylidene chloride, allyl chloride, and
benzyl chloride using high [Ref]/[Test]
ratios were 14.5 x 10'12, 17.2 x 10'12,
and 2.82 x 10~12 cm3/ molecule-sec,
respectively. Rate constants for these
compounds have not been reported
previously.
The average value found for the OH rate
constant for chlorobenzene was 0.55 x
10 12 cm3/molecule-sec with a percent
deviation of 37%. The data were too scat-
tered to determine whether there were any
Cl reactions occurring in the system. The
value obtained for the rate constant was
close to that cited as the lower limit for
the UCR protocol (0.3 x 10~12 cm3/
molecule-sec) and therefore the scatter
was not surprising. The value for the rate
constant was in reasonable agreement
with two recent measurements.
The average OH rate constant found for
epichlorohydrin was 0.55 x 10 ~12
cm3/molecule-sec and the percent devia-
tion was 22%. This result agrees with a
literature value obtained by the flash
photolysis resonance fluorescence tech-
nique. However, data were not taken at
enough reference points during this study
to test concentration ratios in order to
detect the presence of Cl reactions. It is
unlikely that a difference would be
detected with such a low value for the OH
rate constant.
Experiments were also conducted to
determine the OH rate constants with
three slowly reacting compounds. The ex-
periments were unsuccessful because the
decreases in the test compound concen-
trations during the irradiations were within
the experimental uncertainty of the
measurements. These results further
substantiate the validity of the lower limit
of 3 x 10~13 cm3/molecule-sec for OH
rate constants obtained with the UCR
protocol.
Conclusions and
Recommendations
The OH rate constants determined with
the UCR protocol for compounds in the
alkane, alkene, and aromatic classes were
in good agreement with literature values,
as were those for oxygen-containing com-
pounds. It was found that there was a
potential for Cl reactions to interfere with
the determination of OH rate constants of
chlorinated compounds. However, it was
found that the interference could be
decreased by measuring the effective rate
constant as a function of the initial
reference to test hydrocarbon ratio and by
obtaining the OH rate constant from the
asymptotic region (large values for the
reference to test compound ratio), where
the effective rate constant is independent
of the concentration ratio. Verification of
this method is required and could be ob-
tained by comparing the results of this
study with those obtained with the flash
photolysis resonance fluorescence
method. In the flash photolysis method,
the OH rate constant is determined from
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the decay of the OH concentration and
therefore should not be influenced by Cl
reactions.
Rate constants as small as 3 x 10~13
cm3/molecule-sec (ethane) were obtained
using the UCR method. For compounds
that are more difficult to measure, it is like-
ly that the lower limit is 5 x 10 ~13 cm3/
molecule-sec.
All compounds investigated in this study
were .low molecular weight species
(molecular weight <200) with substantial
vapor pressures at room temperature. If
the UCR method is to be used for the
determination of high molecular weight,
low volatility compounds, for which wall
loss may be substantial and quantitative
detection difficult, additional studies will
have to be conducted to validate the
protocol.
Edward 0, Edney and E. W. Corse are with Northrop Services, Inc., Research
Triangle Park, NC 27709.
Bruce W. Gay. Jr.. is the EPA Project Officer (see below).
The complete report, entitled "Validation of OH Radical Reaction Rate Constant
Test Protocol," (Order No. PB 86-166 758/A S; Cost: $9.95. subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
Center for Environmental Research
21ALMAIL
Environmental Protection
Agency
Official Business
Penalty for Private Use $300
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