United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park, NC 2771 1
v '
Research and Development
EPA/600/S3-87/046 Jan. 1988
Project Summary
Review of the UCR Protocol for
Determination of OH Rate
Constants with VOCs and Its
Applicability to Predict
Photochemical Ozone
Production
Joseph J. Bufalini, Robert R. Arnts
The experimental protocol for
determining the rate constants for
reactions of hydroxyl radicals (OH)
with volatile organic chemicals
(VOCs) as developed by the
University of California-Riverside
group Is evaluated. Limits of
detection and precision are
discussed. This protocol is to be
used as a measure of the ozone
forming potential of a VOC; a
compound with a high OH rate
constant also very often produces
high levels of ozone. Adaptations of
the protocol for compounds
containing halogen atoms are
suggested. The protocol may not be
applicable for compounds that do
not produce ROa and HOa radicals
such as carbon disulfide. Also,
compounds that are free radical
scavengers such as phenol,
benzaldehyde and amines may not
give high levels of ozone even
though they may have a high rate of
reaction with OH radicals. The long
chained paraffins also present
problems with the protocol since the
RO2 radicals produced after reacting
with OH radicals in air do not oxidize
NO to NO2 but instead combine with
the NO to form nitrates. When this
occurs, it is recommended that the
protocol be complemented with
smog chamber experiments in order
to establish the reactivities of VOCs.
This Project Summary was
developed by EPA's Atmospheric
Sciences 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
An experimental protocol for
determining the rate constants for the
reactions of hydroxyl radicals (OH) with
volatile organic chemicals (VOCs) has
been developed by a research team at
the University of California-Riverside.
The protocol provides a basis for
determining the photochemical reactivity
of organics in the atmosphere. The
reason for this is that it has been shown
that if an organic compound reacts
quickly with OH radicals, then in the
presence of NOX, ozone could be
produced. This is demonstrated with the
following sequence of reactions:
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(1)RH
(2) R»+ O
(3)RRO*+NO2
(4) NO2 + sunlight =» O + NO
M
(5) O + O2 =» O3
Reactions (2) through (5) are very fast
and the rate at which ozone is produced
(reaction (5)) is primarily dependent upon
reaction (1). However, not all organic
compounds will undergo the sequence of
reactions shown above and these may
not produce ozone. The formation of
ozone is dependent upon the free radical
chain process shown above. If a
compound is a free radical scavenger,
less ozone will be produced.
The protocol is based upon
simultaneously monitoring the
disappearance of two compounds; one a
reference compound R, the other a test
compound (T).
ke
(6) T + OH =» products
k7
(7) R + OH => products'
(8)
kp
T + hv=5 products"
(9) T + walls => loss to walls
It can be shown that the combined
integrated differential equations
expressing the rate of disappearance of
both the test and reference compounds
is:
Equation I
t-t
k,, t-t
7 o
R
where k' = kg + kg
A plot of (t-t0)-1 In (T /T) versus (t-
t0)'1 In (Ro/R) yields a straight line with
a slope of kg/k? and an intercept of k'.
The rate constant of the test compound,
kg, is equal to the slope X ky. Since ky Is
known, then the rate constant for the test
compound can be calculated. If k'= o,
which is the case if a compound does
not photodissociate or adhere to the
walls, equation I can be reduced to.
Equation II
The OH radicals are generated by the
photolysis of methyl nitrite in the
presence of excess NO:
(10) CH3ONO + hv =» CH3O
+ NO
(11)CH3O + O2=>HCHO
+ H02
(12) HO2 + NO => OH + NO2
The NO drives reaction (12) to
completion and also prevents the buildup
of ozone since the NOa produced from
reaction (12) will also photodissociate.
(4) NO2 + hv + => NO + 0
M
(5) O + O2 => O3
(13) O3 + NO=»NO2 + O2
Limit of Detection and
Precision
Error analysis was not addressed in
the original UCR protocol. However,
since this protocol will be considered as
a test for possible VOC emission
controls, it is necessary to address this
issue. The salient characteristic of this
technique is that knowledge of the
absolute value for RQH is not necessary.
If compounds A and B are reacted
together in a smog chamber using the
UCR protocol, then one can establish
which compound is more reactive by
determining the slope as defined by
equations I or II. Regression analysis can
be used to determine the ratio kA-OH/
ke-OH along with a statistical measure
of its random variation, i.e., standard
error. Thus, if the error bar doesn't
include 1.0 then compound A can be
ranked faster or slower than B. The
method is not subject to concentration
calibration errors since absolute
concentration.s are not required; only
relative loss of compounds are
measured.
If the protocol is used to calculate an
absolute rate constant, then the error of
the reference compound rate constant
must be considered. The overall rate of
kg and its relative standard deviation
(rsd) is calculated by
k6=k7X(slope)
Halogenated Organics
When an organic compound contains
a halogen atom, the compound after
reacting with OH radicals may release a
halogen atom. Free halogen can then
attack the test compound and/or the
reference compound. This problem can
be circumvented by the use of different
reference-test compound ratios. At a
high reference to test compound ratio,
equation II becomes:
Equation III
InR /R T k,
o t _ _o 7
InT /T ~QR k~
o t 06
where a is the branching ratio, the
halogen (X) released resulting from OH
attack on the test compound.
(13) OH + T => products + a X
In equation III, the slope is equal to a and
the intercept is equal to ky/kg. This
technique has been used to measure the
OH rate constants for 1,2-
dichloroethane, 1,2-dibromoethane, and
p-dichlorobenzene.
Special Cases: Cases where the
OH protocol may not be an
indicator of VOC reactivity
As outlined earlier, the key assumption
as to the validity of the OH protocol in
establishing the reactivity of a VOC lies
in the fact that free radicals (R02 and
H02> are produced which oxidize NO to
NO2- If, however, no free radicals are
produced, then no oxidation of NO can
occur and no ozone is subsequently
produced Some compounds which may
not produce ozone even though they
have a high OH rate constant are: carbon
disulfide, phenol, benzaldehyde, amines,
and long-chained paraffins.
Carbon Disulfide
The OH rate constant for CS2 was
measured with this protocol and found to
be 10 times greater than ethane. This
would suggest that CS2 would produce
ozone when photooxidized in the
presence of NO. However, there is no
evidence to suggest that X02 (R02 or
HO2) radicals are produced in this
system. Additional smog chamber work
is needed to determine if C$2 does
produce ozone when irradiated in the
presence of NOX.
Phenol, Benaldehyde, and
Amines
These compounds all have rate
constants higher than ethane. However,
little or no ozone is produced when
placed in smog chambers. These
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compounds presumably inhibit free
radical formation as long as they are
present. Diethyl hydroxyl amine (DEHA)
for example has been proposed as a
smog inhibitor and it appears to work
satisfactorily for a one-day irradiation.
However, upon prolonged irradiation,
DEHA produces both acetaldehyde and
peroxyacetyl nitrate. Both of these
compounds produce ozone in the
presence of NOX and sunlight.
Long-Chained Paraffins
Long-chained paraffins such as
those used in printing oils (> CTS) react
quickly with OH radicals. Smog chamber
data indicate that little ozone is produced
when these compounds are irradiated in
the presence of NOX. The probable
reason for this is that these compounds
react with NO to produce nitrates instead
of N02 as suggested by reaction (14)
below
(14)
+ NO=>aCi5H31ON02
+ (l-a)Ci5H310* + (l-a)NO2
The nitrate yield can be as high as
40% which is sufficient to deter
significant ozone buildup.
Concluding Comments
The protocol has been used by
several laboratories on many different
compounds Excellent agreement has
been obtained amongst the laboratories
suggesting that the protocol is simple to
use and requires no special training.
It is also apparent that a significant
number of compounds are not
amendable to this protocol. We
recommend that if there are any doubts
concerning the reactivity of a specific
compound either because of its structure
or because it contains dissimilar atoms,
then smog chamber experiments should
be performed.
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The EPA authors Joseph J. Bufalini and Robert R. Arnts are with the
Atmospheric Sciences Research Laboratory, Research Triangle Park, NC
27711.
The complete report, entitled "Review of the UCR Protocol for Determination
of OH Rate Constants with VOCs and Its Applicability to Predict Photochemical
Ozone Production," (Order No. PB 88-130 059/AS; Cost: $12.95) 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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EPA/600/S3-87/046
6 i i J i s; J I
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