United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/058 Sept. 1985
Project Summary
Experimental Protocol for
Determining Hydroxyl Radical
Reaction Rate Constants for
Organic Compounds:
Estimation of Atmospheric
Reactivity
James N. Pitts, Jr., Arthur M. Winer, Sara M. Aschmann,
William P. L. Carter, and Roger Atkinson
An experimental protocol is de-
scribed to determine the gas-phase rate
constants for the reactions of hydroxyl
radical with organic compounds at
room temperature. This protocol pro-
vides a basis for estimating the relative
reactivities in terms of the ozone-
forming potential of organic com-
pounds that are emitted into the at-
mosphere and that are consumed
primarily by reaction with hydroxyl rad-
icals.
The experimental technique is based
on monitoring the relative rates of dis-
appearance of the test compound and a
reference compound in an air mixture
containing methyl nitrite and nitric
oxide. The irradiation of methyl nitrite
in air produces hydroxyl radicals. The
reference compound is an organic, the
hydroxyl radical reaction rate constant
of which is accurately known. Irradia-
tions, employing blackllghts emitting in
the 300-400-nm region, are conducted
in -75-1 Teflon bags. The test com-
pound and reference organic are moni-
tored by gas chromatography, and ni-
tric oxide, nitrogen oxides, and ozone
are monitored by chemiluminescence
instruments. Using this technique, OH
radical reaction rate constants
>3 x 10~13 cm3 molecule'1 s"1 can be
measured.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented In a separate report of the same
title (see Project Report ordering Infor-
mation at back).
Introduction
The ozone-forming potential (i.e., re-
activity) of an organic compound can be
defined and measured in a number of
ways. In the past, the ozone-forming po-
tential of an organic compound has
generally been defined in terms of the
amount of ozone formed when the com-
pound is irradiated in the presence of
nitrogen oxides (NOX) in smog cham-
bers. However, the use of smog cham-
bers for the determination of ozone for-
mation has a number of problems; e.g.,
"dirty chamber" effects and wall ad-
sorption/desorption problems. This
makes the results of such experiments
difficult to interpret. Furthermore, the
use of smog chambers to determine re-
activity rankings has been shown to be
particularly unsatisfactory for slowly re-
acting compounds. In addition, smog
chamber experiments are also difficult
and expensive to conduct. Thus an al-
ternative technique for measuring the
ozone-forming potentials of organic
compounds that may be emitted into
the atmosphere would be useful.
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A potentially useful and experimen-
tally straightforward approach for mea-
suring reactivity is to measure the rate
constant for reaction of the organic with
the hydroxyl (OH) radical. This is a
meaningful approach since a large
number of organics are consumed in
the atmosphere primarily by reaction
with OH radicals, and for most of those
compounds the subsequent reactions
of the species formed account for its
ozone-forming potential. Since reaction
with OH radicals is the rate-determining
step, it is reasonable to expect that the
ozone-forming potential of these com-
pounds will be correlated with the rate
constant for this reaction. This is the
basis of the expermental protocol de-
scribed in this report.
For some classes of organic com-
pounds, the assumption that the ozone-
forming potential can be correlated with
the OH radical rate constants may be
either incorrect or an oversimplification.
Thus a number of organic compounds
react in the atmosphere to a significant
extent by other processes, such as by
direct photolysis, reaction with ozone,
reaction with nitrate (NO3) radicals, etc.
In addition, some compounds tend to
act as radical inhibitors and others act
as radical initiators, and this can have a
dramatic effect on the ozone-forming
potential of the compound. Clearly,
these possibilities must be considered
when assessing the reactivities of com-
pounds for which the atmospheric reac-
tion mechanisms are unknown or
highly uncertain. However, for a large
number of classes of organic com-
pounds, the OH radical rate constant
can serve as a useful indicator of atmos-
pheric reactivity.
The experimental procedure de-
scribed in this protocol is designed to
enable rate constants for reactions of
organics with OH radicals to be mea-
sured at room temperature, for the pur-
pose of assessing their relative reactiv-
ity. The experimental approach is based
on measuring the relative disappear-
ance rates of the test compound and of
a reference organic in the presence of
OH radicals. The OH radicals are gener-
ated by the photolysis of varying con-
centrations of methyl nitrite (CH3ONO)
in air.
CH3ONO + hv -> CH30 + NO
CH3O + 02 -> HCHO + H02
HO2 + NO -» OH + NO2
In the presence of added organics, the
OH radicals react as shown below.
OH + test compound -» products (1)
OH + reference organic —» products (2)
where KT and K2 are the rate constants
for reactions (1) and (2), respectively.
In addition to the OH reaction, the test
compound may, in some cases, pho-
tolyze, react with 03, and/or react with
NO3 radicals:
test compound + hi> -» products (3)
test compound + 03 —> products (4)
test compound + N03 -> products (5)
Reactions (4) and (5) are not important
in this protocol since excess NO is
present and NO reacts very quickly with
03 and N03.
Kinetically, it can be shown that:
d ln[test compound]/dt =
k,[OH] + k3 (I)
and
d ln[reference organic]/dt = k2(OH) (II)
Eliminating the OH radical concentra-
tion and integrating leads to the follow-
ing expression:
1 [test compound],
In
t - t0 [test compound],
In
[reference organic],0
3 k2(t-t0) [reference organic], * '
where [test compound], and [reference
organic], are the concentrations of the
test compound and the reference or-
ganic at time t, respectively, and [test
compound],0 and [reference organic],0
are the corresponding concentrations at
time t0. This equation is independent of
the OH concentration.
A plot of the above equation would
yield a straight line with a slope of k^k2
and an intercept of k3. Since k2 is the
known rate constant of the reference
compound, ki can then be derived. If the
test compound does not photolyze (i.e.,
k3 = 0), then equation (III) when plotted
would go through the origin.
The precision of the derivation of the
rate constant k-i is determined by the
precision of the gas chromatographic
analyses since the rate constant is de-
pendent upon the differences measured
in both the test compound and refer-
ence organic over a period of time. For
the best conditions of reproducibility, it
is expected that rate constants
^3x 10 13 cm3 molecule 1 sec 1 can
be measured with this technique.
Experimental
Irradiations are performed in a -75-1
FEP Teflon bag, constructed from
Teflon sheets heat sealed around the
edges and containing Teflon injection
and sampling ports. Actinic radiation is
provided by a fluorescent lamp assem-
bly consisting of a circular array of 24
15-watt blacklights (GE 15T8-BL 15)
mounted on a cylindrical aluminum
frame. The lamps are arranged on three
electrical circuits, eight lamps per cir-
cuit, thus allowing for three different
light intensities. The bottom of the
chamber contains a fan that circulates a
large volume of air to minimize heating,
and a cylindrical wire mesh screen in-
side the lamp assembly in which the
Teflon bag is placed prevents the bag
from contacting the lamps or the fan.
Methyl nitrite is prepared by the drop-
wise addition of 50% sulfuric acid
(H2SO4) to methanol saturated with
sodium nitrite. The methyl nitrite pro-
duced is swept out of the reaction flask
by a stream of ultra-high purity nitro-
gen, passed through traps containing
saturated sodium hydroxide solution,
and anhydrous calcium chloride (CaCI2)
to remove any H2SO4 and water vapor,
and then collected in a trap at 195 K. The
CH3ONO is then degassed and vacuum
distilled on a greaseless high-vacuum
system and stored under vacuum at
77 K in the dark. Known amounts of
CH3ONO (0 to 15 ppm), NO (~5 ppm),
test compound (~1 ppm), and reference
organic (~1 ppm) are then flushed from
Pyrex bulbs by a stream of ultra-zero air
into the Telfon bag, which is then filled
with additional ultra-zero air.
Discussion
The organic reactants (i.e., the test
and reference organics) are monitored
by gas chromatography prior to and
during the irradiations. With this proto-
col, the irradiation should be terminated
and the last chromatographic samples
taken after ~30 min for full light inten-
sity, ~45 min for two-thirds maximum
light intensity, or ~60 to 90 min for one-
third maximum light intensity. Since it
is preferable to have two to four gas
chromatographic analyses during the ir-
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0.44
a
o
I
o
O
I
O
0.40 -
036 -
032
0.28 -
0.24 U
020 -
0.16 -
0.12 -
0.08 -
0.04
molecule"1 s~1. The controlling factor is
the precision of the gas chromato-
graphic analyses in determining the
slope of equation (III). This protocol has
been validated by comparing the rate
constants obtained by this technique
with a large number of corresponding
literature values. However, it has not
been validated with compounds con-
taining halogen atoms, which may give
rise to halogen atom reactions giving
erroneously large apparent reaction
rate constants.
0.010 0.020
ft-ta ) ~1 In ( [Ethane] >0 / [Ethane] t
0.030
Figure 1 . Plot of equation (V) lor several hydrocarbons using ethane as the reference organic.
radiation, the optimum light intensity is
determined by the retention times ob-
tained for the compounds on the gas
chromatograph.
Care must be taken in making certain
that the test organic does not react with
03 and N03. This is usually the case with
short irradiation times with excess NO
present.
The rate constant for the reaction of
OH radicals with the test compound, rel-
ative to that for the reaction of OH radi-
cals with the reference organic, is ob-
tained from the experimental data by
using equation (III). An example of a
plot of equation (III) is shown in Figure 1
for three hydrocarbons using ethane as
the reference organic.
Conclusions
The experimental protocol described
is applicable to organic compounds
having a lower limit rate constant with
OH radicals of ~3 x 10~13 cm3
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James N. Pitts, Jr.. A. M. Winer. S. M. Aschmann, W. P. L. Carter, andR. Atkinson
are with Statewide Air Pollution Research Center, University of California,
Riverside, CA 92521.
Joseph J. Bufalini is the EPA Project Officer (see below).
The complete report, entitled "Experimental Protocol for Determining Hydroxyl
Radical Reaction Rate Constants for Organic Compounds: Estimation of
Atmospheric Reactivity," (Order No. PB 85-238 558/AS; Cost: $10.00, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-85/058
0000329 PS
IL
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