United States
Environmental Protection
Agency
Environmental Sciences Research*^.
Laboratory / Tf
Research Triangle Park NC 27711
v-xEPA
Research and Development
EPA-600/S3-82-038 Oct. 1982
Project Summary
Experimental Protocol for
Determining Hydroxyl Radical
Reaction Rate Constants
James N. Pitts, Jr., Arthur M. Winer, Sara M. Aschmann, William P. L Carter,
and Roger Atkinson
An experimental protocol has been
developed to determine the gas-phase
rate constants for the reactions of the
hydroxyl (OH) radical with chemicals
at room temperature. This protocol
provides a basis for evaluating the
relative importance of one atmospheric
reaction pathway (i.e., attack by the
OH radical) for organic substances
that may be emitted into the environ-
ment.
The experimental technique is based
on monitoring the disappearance rates
of the test compound and a reference
organic in irradiated methyl nitrite-
NO-organic-air mixtures. (The refer-
ence is an organic species whose OH
radical reaction rate constant is
accurately known.) Irradiations, em-
ploying blacklamps emitting in the
actinic region, are carried out in
~ 75-liter cylindrical Teflon bags. The
concentrations of the reactants are:
methyl nitrite (CH3ONO), O to ~ 15
ppm; NO, ~ 5 ppm; test compound,
~1 ppm; and reference organic, ~ 1
ppm. The test compound and reference
organic are monitored by gas chro-
matography, NO, NO2, NOX by chem-
iluminescence, and ozone (O3) by
chemiluminescence. Using this tech-
nique, OH radical rate constants ^3 x
10~13 cm3 molecule"1 sec~1 (the range
of primary interest from an atmo-
spheric point of view) can be measured.
This Project Summary was developed
by EPA's Environmental Science
Research Laboratory. Research Triangle
Park. NC. to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Under the sponsorship of the U.S.
Environmental Protection Agency (EPA),
the Statewide Air Pollution Research
Center (SAPRC) at the University of
California, Riverside, has developed and
validated experimental protocols to
assess the atmospheric fates and
lifetimes of organic compounds.
Chemical compounds emitted into the
atmosphere are re moved or degraded by
pathways involving gas-phase reactions
or wet or dry deposition. Laboratory arid
environmental chamber studies have
shown that for the ambient atmosphere,
the following homogeneous gas-phase
removal routes are likely to be important:
• Photolysis, which involves absorp-
tion of lightfollowed by decomposi-
tion or isomerization:
• Reaction with Os;
• Reaction with OH radical; and
• For aromatic compounds containing
an -OH substituent group, reaction
with the nitrate (NOs) radical.
To assess (a) the atmospheric lifetime
of compounds with respect to these gas-
phase removal processes, and (b) the
relative importance of each of these
reaction pathways, rate constants for
photolysis and/or chemical reaction
must be experimentally determined for
individual compounds.
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The experimental procedures detailed
in this protocol are designed to determine
rate constants for reactions of the OH
radical with organics and certain
inorganics at room temperature. Knowl-
edge of these rate constants may allow
estimation of the atmospheric lifetimes
of these compounds with respect to
attack by OH radicals as shown below.
In the atmosphere, where the reaction,
OH + chemical — products occurs, the
decay of the chemical via this reaction is
given by,
-d[chemical]/dt = k°H [OHJchemical] (I)
wheref ] denotes concentration and k°H
is the rate constant for reaction of the
OH radical with the particular chemical.
Equation (I) may be rearranged to yield
-d]n[chemical]/dt = k°H[OH] (II)
and, providing the OH radical concen-
tration remains constant,
/[chemical]t) =
k°H[OH] (t-to) (HI)
where [chemical]t and [chemical]t are
the concentrations of the chemical at
times to and t, and In is the logarithm to
the base e.
In the ambient troposphere, OH
radical concentrations vary as a function
of time of day from a negligibly low level
at night to a peak at around solar noon.
However, for approximate lifetime
calculations, an average OH radica]
concentration of ~8 x 105 molecule cm"
3 may be assumed for the northern
hemisphere. The assumptions inherent
in these calculations must be borne in
mind, especially for organics whose
lifetime is one day or less. The 1/e
lifetime, r°H, of any chemical with
respect to reaction with the OH radical
'(i.e., the time for the concentration of
the chemical to decrease by a factor of
e = 2.7 due to reaction with the OH
radical) is given by,
(IV)
TOH = (koH[OH]-
where [OH] is the ambient atmospheric
OH radical concentrations.
For a typical troposphere OH radical
concentration of 8 x 105 molecule cm"3,
the rate constants which yield 1/e
lifetimes of one hour, one day, one
week, one month, and one year are
given in Table 1 for the three sets of
most commonly used units. For lifetimes
longer than about one day, the variation
of temperature with altitude must be
taken into account for rigorous calcula-
tions.
Table 1. Rate Constants for Reaction with the OH Radical which Yield Selected 1/e
Lifetimes in the Presence of 8 x 10s molecule cm~3 of OH Radicals
Rate Constant
Lifetime
One hour
One day
One week
One month
One year
ppm'^ min 1
5.2 x 10s
2. 1 x 10"
3.0 x W3
7.0 x W2
5.8 x 10'
liter mole 1 sec 1
2.7 x 10"
8.8 x JO9
1.3x JO9
2.9 x 10s
2.4 x 107
cm+3
molecule'^ sec"1
3.5 x 70"10
1.5 x 70~11
2.7 x 70"12
4.8 x 70"13
4.0 x 70~15
The experimental approach described
below is based on measuring the
relative disappearance rates of the test
compound and of a reference organic in
the presence of OH radicals.
CH3ONO + hi/ - CH30 + NO
CH3O + O2 - HCHO + H02
HO2 + NO-OH + N02
OH radicals are generated from the
photolysis of varying concentrations of
CH3ONO in air. In the presence of added
organics, the OH radicals react as
shown below. The reactions are identified
with Arabic numerals so as to facilitate
interpretation with the corresponding
rate constants, k-i, k2, etc.
OH + test compound
OH + reference organic
• products (1)
products (2)
In addition, thetest compound may, in
some cases, also photolyze, react with
03, and/or react with the N03 radical:
test compound + h v — products (3)
test compound + O3 — products (4)
test compound + NO3 — products (5)
Reactions (4) and (5) will be unimportant
provided that excess NO is present si nee
O3 and NO3 both react rapidly with NO:
NO + 03- NO2 + O2
NO + NO3 - 2 NO2
If the organics are lost only by reaction
with OH radical, and, for the test
compound, by photolysis, then,
-d[test compounds]/dt =
ki[OH][test compound]
+ ksttest compound] (V)
-d[reference organic]/dt =
k2 [OHJreference organic] (VI)
where ki and k2 are the OH radical rate
constants for reaction (1) and (2),
respectively, and k3 is the photolysis rate
constant. Hence,
dln[testcompound]/dt=k,[OH] + k3,(VII)
and
dereference organic]/dt = k2 [OH]. (VIII)
Elimination of the OH radical concentra-
tion and integration leads to the following
expression:
.!_,„(
t-to) |^
[test compound]to
[test compound]!
k3+ kl
k2 (t-to)
[reference organic]to
[reference organic]t
.(IX)
where [test compound]t and [reference
organicjt are the concentrations of the
test compound and the reference
organic, respectively, at time t0; [test
compound]t and [reference organic]t
are the corresponding concentrations at
time t. Note that Equation (IX) is valid
even if the OH radical concentration
baries with time.
Hence, a plot of (t-t0) 1 Inftest com-
pound]! /[test compound]! against (t-to) 1
ln([reference organic]t /[reference
organicji) should yield a straight line of
slope k!/k2 and intercept k3. Knowing k2,
the rate constant, ki, may then be
derived. If the test compound does not
photolyze (i.e., k3 = 0), then Equation (IX)
can be simplified to yield the following
relationship:
[test compound]!
[test compound]!
[reference organic]t>
[reference organic]t
(X)
A plot of ln([test compound]io/[test
compound]t) versus ln([reference organic]to
/[reference organic]t) should yield a
straight line of slope ^/k2 with a zero
intercept.
The concentrations of the test com-
pound and the reference organic are
monitored before and during irradiation
of the CH3ONO/NO/test compound/
reference organic/air mixtures by gas
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chromatography. Hence, the lower limit
to ki that can be determined is set by the
precision of the gas chromatographic
analyses, but is expected to be of the
order of < 3 x 10~13cm3molecule"1 sec"1
which corresponds (Table 1) to an
atmospheric lifetime of greater than one
month.
Experimental
Irradiations are carried out in a —75-
liter cylindrical Teflon bag which is
made from FEP Teflon sheet heat-
sealed around the edges and fitted with
a Teflon injection and sampling port, as
shown in Figure 1. Actinic radiation is
provided by a fluorescent lamp assembly
as shown in Figure 2. It consists of a
circular array of twenty-four 15-Watt
blacklights (GE F15T8-BL15) mounted
in a cylindrical aluminum frame. The
lamps are arranged on three electrical
circuits, eight lamps per circuit, with
every third lamp being on a given circuit,
an arrangement which allows for three
different light intensities. In the bottom
of the chamber is a fan that circulates a
large volume of air to minimize heating.
(a)
B
-140cm-
T
45cm
1
E
A cylindrical wire mesh screen inside
the lamp assembly in which the Teflon
bag is placed (Figure 2) prevents the
reaction bag from contacting the lamps
or the fan.
Methyl nitrite is prepared by the
dropwise addition of 50% H2SC>4 to
methanol saturated with sodium nitrite.
The CH3ONO produced is swept out of
the reaction flask by a stream of ultra-
high purity nitrogen, passed through a
trap containing saturated NaOH solution
to remove any HzSO.4, dried by passage
through an anhydrous CaCI2 trap and
collected in a trap at 196 K. The
CHaONO 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 the
CH3ONO, NO, and the reference and
reactant organics are flushed from
Pyrex bulbs by a stream of ultra-zero air
into the Teflon reaction bag, which is
then filled with additional ultra-zero air
Discussion
Initial concentrations of the reactants
are typically: CH3ONO, zero to~15 ppm;
NO, ~5 ppm; and test and reference
organics, ~1 ppm. The NO is present to
minimize Oa formation and any reaction
with the organics. The organic reactants
Wire Mesh
Screen
GE F15T8-BL
Blacklights
Fan
are monitored by gas chromatography
prior to and during the irradiations. With
this protocol, the irradiation should be
terminated and the last gas chromato-
grahic samples taken after ~30 min for
full light intensity, —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 irradiation, the optimum light
intensity is then determined by the gas
chromatographic retention times. As an
example, Figure 3 shows a typical set of
gas chromatograms for an n-butane +
propene system, for which sampling
periods of 15 min were employed.
For organics that react with O3 (i.e.,
the alkenes), care should be taken not to
obtain data when the reaction with 03
becomes important (i.e., reaction with Oa
should contribute <10% of the organic
reaction rate with the OH radical).
The rate constant for the reaction of
OH radicals with the test compound,
relative to that for the reaction of OH
radicals with the reference organic, is
then obtained from the experimental
data by using Equations (IX) or (X). An
example of a plot of Equation (IX) is
shown in Figure 4 for a series of
carbonyls, with cyclohexane as the
reference organic.
Conclusions
The experimental technique presented
has been validated by demonstrating
excellent agreement between rate
constants obtained by this method and
corresponding literature values. A list of
recommended reference organics is
provided in the detailed protocol along
with their room temperature OH radical
rate constants.
(b)
(c)
Heat Seal BC to DE; A to BD. F to CE
Figure 1. Construction of the Teflon
reaction bag.
,50cm
— 50cm-
Wire Mesh
Screen
Fan
Lamps
\
Aluminum
Casing
Figure 2. Fluorescent lamp assembly.
U.S. GOVERNMENT PRINTING OFFICE. 19M-559-017/0835
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Hi
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Q.
to
^
0>
o
I
0
<
-c
Cj
Vi
u
Before Irradiation After Irradiation
15 min. 30 min. 45 min.
nC*
C~ „ C*
I
[
v_
X2
\^
nC*
X2
CT~
I
I L
n(Jt,
X2
cr
III
v_
1 min.
0 10r
2,6-Dimethyl-3-/
Heptanone
2,4-Dimethyl-
3-Pentanone
Time
Figure 3. Example of gas chromatographic analysis of propene (C^j and n-butane
(nCn ) during a CH3ONO/NO/propene/n-butane/air irradiation.
:
V0.06
Q> 0.04
^~,
^
^•0.02
0 I 0.010 \ 0.020 \ 0.030
0.005 0.015 0.025 0.03i
(t-toj-^ In ffcyclohexanej
to /[cyclohexane]t ) mm '
Figure 4. Plot of equation (IX) for a
series of ketones, using
cyclohexane as the
reference organic.
James N. Pitts, Jr., Arthur M. Winer, Sara M. Aschmann, William P. L. Carter,
and Roger Atkinson are with the Statewide Air Pollution Research Center,
University of California. Riverside, CA 92521.
Bruce W. Gay, Jr.. is the EPA Project Officer (see below).
The complete report, entitled "Experimental Protocol for Determining Hydroxyl
Radical Reaction Rate Constants," (Order No. PB 82-256 066; Cost: $7.50.
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:
Environmental 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
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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