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United States
Environmental Protection
Agency
Atmospheric Sciences Research - •, " ;
Laboratory ~£, ,-'
Research Triangle Park NC 27711 , ,|,
Research and Development
EPA/600/S3-85/025 May 1985
Project Summary
Modeling of Auto Exhaust
Smog Chamber Data for
EKMA Development
G. Z. Whitten, J. P. Killus, and R. G. Johnson
A new generalized mechanism for
photochemical smog has been devel-
oped. The mechanism is suitable for use
in the Empirical Kinetics Modeling Ap-
proach (EKMA) to estimate the amount
of control of volatile organic compounds
that is needed to achieve the National
Ambient Air Quality Standard for ozone.
The mechanism developed in this study
is called the CBM-X, and it is the fourth
lumped-parameter mechanism to be
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. Carbon atoms with similar
bonding are treated similarly, regardless
of the molecules in which they occur.
The principal features that distinguish
the CBM-X from previous carbon bond
mechanisms include separating formal-
dehyde from the other oxygenates,
treating toluene separately from the
other aromatics and including a more
detailed, up-to-date representation of
aromatic hydrocarbon chemistry.
The CBM-X was tested by comparing
the predictions obtained with the mech-
anism against smog chamber data of
dilute auto exhaust/oxides of nitrogen
mixtures obtained in the outdoor smog
chamber facility operated by the Univer-
sity of North Carolina.
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 in-
formation at back).
Introduction
For several years, the U.S. Environ-
mental Protection Agency has sponsored
a coordinated research program consis-
ting of chemical kinetics studies, smog
chamber experiments, and kinetic model
development, with the ultimate goal of
developing models capable of simulating
the photochemical reactions that take
place in the lower troposphere. As a
result of this program, considerable effort
has been devoted to the development of
reaction mechanisms for photochemical
air pollution, either through the numerous
direct contributions of the EPA program
or through the stimulus the program has
provided others. Part of this effort has
involved developing chemical kinetics
mechanisms for use in the Empirical
Kinetics Modeling Approach (EKMA).
EKMA is a technique for relating ozone
concentrations downwind of an urban
area to the early-morning concentrations
of Oa precursors in that urban area. In the
EKMA approach, a very simple moving
box model is used to generate a series of
ozone isopleths that depict maximum
afternoon ozone concentrations as a
function of 6-9 AM ambient levels of VOC
and NO,. Ozone isopleths can be gener-
ated for any city by using emission and
meteorological inputs that are appropriate
for that city. The isopleths are used to
calculate the amount of VOC control that
is needed to reduce Oa from some
present-day value to the 0.12 ppm air
quality standard.
The mechanism originally developed in
1976 for use in EKMA was a surrogate
species mechanism in which the reactiv-
ity of urban organic emissions was repre-
sented in terms of a simple propylene/
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n-butane mixture. Since that time our
knowledge of the chemical processes
responsible for photochemical oxidant
formation has increased considerably.
The objective of this present study was to
make use of that knowledge to develop an
improved, state-of-the-science mecha-
nism for use in EKMA.
Formulation of the Mechanism
The CBM-X can be viewed as consisting
of three different components. The first
component, the central "core" of the
mechanism, consists of a set of inorganic
reactions and a set of reactions for those
carbonyl species that are central to most
photooxidation systems. These carbonyl
species include formaldehyde, and acetal-
dehyde, glyoxal, and methyl glyoxal. The
core mechanism serves as the basis for
all mechanism development studies at
Systems Applications. All species in the
core mechanism are treated explicitly,
that is, there is no lumping or condensa-
tion of any of the species or reactions.
The second component of the CBM-X
consists of those hydrocarbons that,
because of their importance to the smog-
forming process, are also treated explic-
itly. The hydrocarbons for which detailed,
explicit reaction mechanisms are included
are ethene, toluene, and m-xylene.
The third component of the CBM-X
consists of those reactions and species
that are treated according to the carbon-
bond, lumped structure approach. The
paraffins and all olefins 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 also 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 approximation 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 CBM-
X uses surrogate approximations to de-
scribe a number of compounds. 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 approximation is used for mono-
alkylated aromatics (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 approach, the excess alkyl
carbon in these molecules is treated as
the lumped species PAR. Another surro-
gate approximation is used for chlorinated
ethenes, which are assumed to be similar
to ethene itself. And lastly, aldehydes
with three or more carbon atoms are
treated as acetaldehyde.
The CBM-X contains 146 reactions and
65 species. A complete description of this
mechanism, including a listing of the
individual reactions and species that
comprise the mechanism, is given in the
Project Report.
Testing of the Mechanism
Individual aspects of the CBM-X mecha-
nism were tested against smog chamber
data obtained by the University of North
Carolina (UNC) during the irradiation of
one-component organic and NO, systems.
The components of the CBM-X mecha-
nism that were tested in this fashion
included formaldehyde, acetaldehyde,
methylglyoxal, ethene, toluene, and m-
xylene.
The complete CBM-X mechanism was
tested against data obtained by UNC
during their outdoor chamber study of
auto exhaust/NOx mixtures. In this study,
both sides of the UNC chamber were
charged with dilute auto exhaust and NOX
mixtures from catalyst-equipped auto-
mobiles operated under varying test
cycles and with different fuels. The
experiments covered a wide range of
hydrocarbon-to-NOx ratios. Twenty exper-
iments from the UNC chamber were
simulated in this phase of the study.
Results
The CBM-X mechanism was found to
provide good agreement to the chamber
data obtained for the one-component
organic/NOx systems. The CBM-X also
was able to simulate reasonably well the
auto exhaust runs that were performed in
the UNC chamber. In some cases, how
ever, there were discrepancies betwee
the experimental and simulated results
There were two general areas in whicl
the predictions differed from the experi
mental data. In runs where ozone reachei
a peak and then declined, (generally a
high HC/NOx), the simulations oftei
overpredicted the peak ozone and under
predicted subsequent ozone decay. Thii
overprediction of ozone often occurrei
coincidentally with an underprediction o
PAN. It is quite possible, therefore, tha
some feature of the PAN formation am
decay was responsible for the overpre
diction of peak ozone. There was also <
tendency for the mechanism to under
predict formaldehyde in nearly all cases
In general, however, the predictions foi
most species agreed well with the exper
imental data.
Conclusions
The mechanism developed in this study
is a hybrid of explicit chemistry, surrogate
approximations and lumped/generalizec
chemistry. It was designed to handle the
broad features of urban smog chemistry
and to be applicable for use in EKMA. The
experimental data base used to test the
CBM-X was well suited for developing
mechanisms to describe photochemical
smog formation within urban areas.
Although additional efforts are needed
before all the details of smog photochem-
istry are elucidated, the mechanism for-
mulated in this study has been sufficiently
tested to render it useful for EKMA control
strategy applications.
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G. Z. Wbitten. J. P. Killus, andR. G. Johnson 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 "Modeling of A uto Exhaust Smog Chamber Data for
EKMA Development," (Order No. PB 85-186 492/AS; Cost: $28.00, 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
* U.S. GOVERNMENT PRINTING OFFICE: 1985-559-016/27049
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