United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/026 May 1985
Project Summary
Development of a Chemical
Kinetic Mechanism for the
U.S. EPA Regional Oxidant
Model
Gary Z. Whitten, Robert G. Johnson, and James P. Killus
The U.S. Environmental Protection
Agency (EPA) is presently developing
an air quality simulation model to sim-
ulate the transport, dispersion, and
transformation of photochemical oxi-
dants on a regional scale. Systems
Applications, Inc., under contract with
the U.S. EPA, provided a chemical
kinetic mechanism for use in this Re-
gional Oxidant Model. The development
of this chemical kinetic mechanism and
its evaluation with smog chamber data
are described in this report.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search 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
For several years, the U.S. Environ-
mental Protection Agency has directed its
attention towards the development of an
air quality simulation model to describe
the formation of photochemical smog on
a regional scale. The objective of the
study described in this Project Report was
to develop a chemical kinetics mechanism
for use in EPA's Regional Oxidant Model
(ROM). Before this study was undertaken,
kinetics mechanisms had been developed
for use in urban oxidant models only.
These mechanisms do not treat a number
of phenomena that are of importance on a
regional scale. The urban oxidant models,
for example, do not treat nighttime chem-
istry and, therefore, they cannot be used
for simulations that extend beyond one
day. In addition, the urban oxidant mech-
anisms do not consider the role of bio-
genie hydrocarbons in the smog-forming
processes. In the rural environment,
these natural hydrocarbons may contrib-
ute to ozone formation. The objective of
this study was to develop a mechanism
for use in the ROM that could treat the
buildup of oxidants over a multi-day
period and consider the role of biogenic
as well as anthropogenic hydrocarbons on
oxidant formation in the rural environ-
ment.
Description of the Mechanism
The ROM mechanism developed in this
effort was designed in accordance with
the carbon-bond reaction concept. In the
carbon-bond mechanisms, organics are
grouped according to the type of carbon
bonding that is found in the various
classes of organics. Breaking the organic
molecule into groups of carbon bonds
offers several advantages over mecha-
nisms that use one organic species to
represent an entire class of organic
compounds. A carbon-bond mechanism
allows the conservation of carbon mass.
It also leads to a more accurate repre-
sentation of kinetic data for an entire
class of organic compounds. For example,
the rates of the reactions of olefins with
OH radicals cover a wide range of values
that depends on the olefin. The use of a
carbon-bond representation allows one
to simulate this spread of reactivity to
some extent, whereas a lumped repre-
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sentation approximates all olefin-OH
reaction rates with a single value. The
principal disadvantage of the carbon-bond
representation is that intramolecular
processes such as decomposition and
side-chain activation may require special
treatment.
The expanded carbon-bond mechanism
that was developed in this effort can be
viewed as consisting of three different
components. The first component, the
central "core" of the mechanisms con-
sists of a set of inorganic reactions and a
set of reactions for those organic species
that are central to most photooxidation
systems. These organic species include
formaldehyde and acetaldehyde. The
"core" is termed the Standard Mechan-
ism and it serves as the basis for all
mechanism development studies at Sys-
tems Applications. All species in the
Standard Mechanism are treated explicit-
ly, that is, there is no lumping or con-
densation of any of the species or reac-
tions.
The second component of the ROM
mechanism consists of those hydrocar-
bons that, because of their importance to
the smog-forming process, are also treat-
ed explicitly. The hydrocarbons for which
detailed, explicit reaction mechanisms
are included are ethene, toluene, m-
xylene, and isoprene.
The third component of the mechanism
consists of those reactions and species
that are treated according to the carbon-
bond, lumped structure approach. The
paraffins and all olef ins except for ethene
are treated in this fashion. One lumped
species, referred to as PAR, is used to
represent the single-bonded carbon
atoms in these species. The lumped
species OLE is used to represent the
carbon-carbon double bonds of olefins.
In addition to using explicit chemistry
and the lumped structure approach to
simplify the reaction scheme, a surrogate
mechanism approach is used to represent
some of the organics in the atmospheric
mix. The surrogate species approach
consists of using chemical reactions for
one species to represent the chemistry of
another, similar species. The surrogate
approximation is used when a compound
is sufficiently similar in its photooxidation
behavior to an already existing class that
it can be included in that class without
modifications to the chemical parameters
of the mechanism. The surrogate approx-
imation is also used for compounds
whose behavior is not known in detail;
the behavior of such compounds must be
estimated by analogy with other known
compounds. The ROM mechanism uses
surrogate approximations to describe
several species. The behavior of olefins
with two or more alkyl groups (e.g.,
isobutene, internal olefins) is simulated
as a mixture of the aldehyde and ketone
products. In this case the surrogate
approximation is justified by the fact that
such very reactive olefins oxidize to their
products so rapidly that product behavior
dominates. A second surrogate approx-
imation is used for mono-alkylated arom-
atics (e.g., ethylbenzene). These species
are assumed to be similar to toluene for
which a condensed explicit mechanism is
used. In keeping with the carbon balance
considerations of the carbon bond ap-
proach, the excess alkyl carbon in these
molecules is treated as the lumped spec-
ies PAR. Another surrogate approxima-
tion is used for chlorinated ethenes,
which are assumed to be similar to ethene
itself. I n addition, aldehydes with three or
more carbon atoms are treated as acetal-
dehyde. Surrogate approximations are
also used for some of the products formed
during the photooxidation of isoprene. In
particular, such approximations are used
to treat methyl vinyl ketone and meth-
acrolein.
Testing of the Mechanisms
The ROM mechanism was tested
against smog chamber data obtained by
the University of California at Riverside
(UCR) and the University of North Carolina
(UNC) at Chapel Hill. The UCR study was
designed to provide a data base for
simulating multi-day effects. A 50,000-1
dual-mode outdoor Teflon chamber was
used in that study. Experiments consisted
of irradiating NO, and an eight-compon-
ent surrogate mix for two to four days. In
some experiments NO, was injected on
subsequent days to simulate NO, emis-
sions into an aged air mass.
The UNC study was designed to provide
a data base for simulating the photooxida-
tion of isoprene. In the UNC experiments,
mixtures of isoprene and NO, were irrad-
iated in their outdoor smog chamber
facility. A range of isoprene-to-NO, ratios
were used in these experiments.
In general, very good agreement was
obtained between the two sets of cham-
ber data and the ROM mechanism. Details
of the results obtained during the testing
of the mechanism are contained in the
Project Report.
Condensation of the
Mechanism
The ROM mechanism that was tested
against the UCR and UNC chamber data
consisted of 170 reactions and 78 species.
Subsequent to the testing of this mechan-
ism, the mechanism was condensed, and
the predictions obtained with this con-
densed mechanism were compared to
those obtained with the original, more
expanded mechanism. Excellent agree-
ment was found between the two sets of
simulations. The condensed ROM mech-
anism contains 115 reactions and 47
species. A complete description of these
mechanisms is contained in the Project
Report.
Summary
The ROM mechanism is based on the
carbon-bond representation and provides
an expanded treatment of hydrocarbon
chemistry in comparison to previous
carbon-bond mechanisms. It treats, for
example, aromatics in two major classes
(toluene and xylene), and carbonyl com-
pounds in four classes (formaldehyde,
two aldehyde compounds and ketones).
The mechanism also includes a reaction
scheme for biogenic hydrocarbons.
The condensed ROM mechanism that
was developed in this effort is suitable for
use in EPA's sophisticated Regional Ox-
idant Model. Included in the Project
Report are instructions on how to exercise
the ROM mechanism in this model. In
particular, detailed guidance is given on
how emissions data should be partitioned
into the various organic groupings that
are used in the mechanism.
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G. Z. Whitten, R. G. Johnson, andJ. P. Killus are with Systems Applications, Inc.,
San Rafael, CA 94903; the EPA author Marcia C. Dodge (also the EPA Project
Officer, see below) is with the Atmospheric Sciences Research Laboratory,
Research Triangle Park, NC 27711.
The complete report, entitled "Development of a Chemical Kinetic Mechanism for
the U.S. EPA RegionalOxidant Model,"(Order No. PB 85-185 858/AS; Cost:
$22.00, 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
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