United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S3-88/012 Apr. 1988
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v°/EPA Project Summary
Development and Testing of the
CBM-IV for Urban and
Regional Modeling
M. W. Gery, G. Z. Whitten, and J. P. Killus
A new chemical kinetics
mechanism for simulating urban and
regional photochemistry based on
the carbon-bond method of
hydrocarbon condensation was
developed and evaluated in this
project. The Carbon-Bond
Mechanism-IV (CBM-IV) is a
condensed version of an expanded
mechanism (CBM-X) that was ini-
tially developed using currently
available laboratory and smog
chamber data. In addition to a
general updating of the mechanism
to include the most recent kinetic,
mechanistic, and photolytic
information, the CBM-IV comprises
extensive improvements to the
chemical representations of aromatic
species, blogenic hydrocarbons and
peroxyacetyl nitrate (PAN). CBM-IV
performance in predicting ozone,
formaldehyde, and PAN con-
centrations was evaluated against
the results of approximately 160
experiments from four different smog
chambers. Both the maximum
predicted concentrations and the
time to the maximum were
compared. Other parameters such
as hydrocarbon and NOX decay rates
were also addressed. The results of
these evaluations indicate
substantial improvement in the ability
of the CBM-IV to simulate aroma-
tic and isoprene systems. For-
maldehyde predictions for the
isoprene experiments were also very
good. The CBM-IV overpredicted
maximum ozone concentrations by
2% and underpredicted for-
maldehyde by 9% for 68 different
hydrocarbon/NOx mixture experi-
ments from three different smog
chambers.
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
The CBM-IV was developed for use
in EPA's EKMA procedure. It can also be
used in large air quality simulation
models (AQSMs) such as the Urban
Airshed Model (UAM) and the EPA
Regional Oxidant Model (ROM). In the
following discussion, the authors present
an overview of the development of the
CBM-IV and summarize the evaluation
of the mechanism.
Development of the CBM-IV
The core of the CBM-IV consists of a
set of explicitly represented inorganic
reactions in addition to the reactions of
common carbonyl species and their
products (e.g., formaldehyde,
acetaldehyde, and PAN). Because the
large number of organics involved in
tropospheric photochemistry precludes a
fully explicit chemical treatment ot
individual organic compounds
condensation techniques must be
employed to represent larger organic
species. In the CBM-IV, reactive
organics are usually treated within <.
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surrogate approximation; however, some
specific compounds that (1) cannot
easily be included in surrogate schemes
and (2) are common constituents of
polluted atmospheres (i.e., ethene,
isoprene, and to a lesser degree,
toluene, xylene, and acetaldehyde) are
treated individually.
Several major improvements that were
made to the CBM to develop the CBM-
IV have led to significant improvement in
the performance of the mechanism in
smog chamber evaluation exercises. The
most important improvements are as
follows:
(1)New chemical kinetics and product
data have been included to update
both the inorganic and organic
components.
(2)The temperature-dependence of
acetylperoxy radical reactions with
NO and NOg has been improved,
and the kinetics of peroxy- peroxy
radical reactions has been updated
to more closely represent
experimental evidence available
from both chemical kinetics and
smog chamber studies
(3) The chemistry of ethene was altered
to account for glycolaldehyde
"formation.
(4)The reaction of acetaldehyde with
HC>2 was eliminated because there is
no substantial proof of its oc-
currence.
(5)The chemical equilibrium between
HC>2, NOa and peroxynitric acid
(PNA) was updated.
(6)The use of formaldehyde (FORM) as
a surrogate for glyoxal was
eliminated, making the species
FORM an explicit representation of
formaldehyde and allowing the
CBM-IV to be used for for-
maldehyde-specific simulations.
(7) The chemistry of aromatic species
(toluene and xylene) has been
significantly altered to account for
the sensitivity of ozone formation to
NMHC/NOX.
(8)A new condensed isoprene (ISOP)
mechanism was developed and
evaluated.
(9)New carbon-bond fractions for
pinene were tested and incorporated
in accordance with the desire to
utilize that species as a biogenic
hydrocarbon surrogate.
It is beyond the scope of this
summary report to directly discuss all of
the enhancements to the CBM-IV.
Therefore, a focus is made on some
significant improvements in the aromatic
and isoprene reaction schemes.
Improvements to the CBM-IV
Aromatic Reaction Scheme
In the CBM-IV, two surrogate
species are utilized to represent the
chemistry of all aromatics. These
surrogate species are TOL for toluene
(mono-alkylbenzenes) and XYL for
xylene (di- and tri-alkylbenzenes).
These surrogate species were selected
because the differentiation between
surrogate chemistries must represent the
widest possible range of aromatic
reactivities if they are to provide the most
appropriate representation of specific
compounds. The most important
chemical features to differentiate in the
surrogate chemistries are the OH
reaction rate and the secondary reaction
scheme. Of these two, appropriate
representation of the secondary reaction
scheme is the most difficult to achieve
because this chemistry is complex and
varies among aromatic species.
One critical difference between the
product yields of toluene and those of
higher molecular weight aromatic species
is the inability of toluene to form high
yields of reactive products (such as
dicarbonyl species) promptly after OH
reaction. Such products are stable but
thermally and photolytically reactive.
Therefore, their presence perpetuates
the reactive nature of the system even
after the initial hydrocarbon is exhausted.
Available smog chamber data indicate
that toluene systems rapidly consume
NO, but that toluene reaction continues
well after the period during which
reactive product formation would be
severely limited by diminished NO
concentrations. Ozone formation in these
toluene experiments initially appears to
be very fast but then rapidly terminates,
and ozone is often consumed in later
stages of an experiment. This
dichotomous behavior is somewhat
unique for a "reactive" hydrocarbon and
may indicate that, unlike other aromatics
and olefins, the product mixture formed
in the initial portion of toluene
experiments may form less reactive
species as the experiment progresses
and NO is consumed.
Prior to development of the CBM-IV,
most kinetics simulation mechanisms
represented the addition of OH to
aromatics with the following simplified
form:
OH + ARO (+ 02) -» ARO-02.
ARO-02. + NO->NO2 +
reactive products
where the aromatic R02 radical reacted
exclusively with NO to form reactive
organic products and NO2. This reactio
effectively produces highly reactiv
products in a rather prompt fashion. On
the basis of smog chamber and product
yield data for toluene, we believe that this
prompt formation of products, especially
highly reactive and conjugated
dicarbonyls, incorrectly describes the
toluene oxidation reaction when NO
concentrations become limited. Possible
pathways that had not been considered
were alternate reactions of the aromatic
R02 radicals that do not involve oxidation
of NO. The aromatic R02 radicals
probably do not have long half-lives,
and if they cannot react with NO, they
must be lost by an alternate reaction
pathway. Our methodology in developing
the CBM-IV has been to estimate the
mechanism, products, and kinetics of an
alternate reaction of the aromatic R02
radical. Such a reaction becomes
important only at low NO concentrations
and produces less reactive products than
the NO-to-N02 conversion process.
This reaction has been included in the
CBM-IV and has led to significantly
better predictions.
In the development and performance
evaluation of the new CBM-IV aromatics
reaction schemes, an attempt was made
to simulate all usable toluene and xylene
experiments from the UNC and UCR-
EC chambers. Also a number of the
hydrocarbon-reactivity-mixture
experiments performed at UNC were
utilized, in which aromatic species were
substituted into otherwise identical
mixtures. For toluene, the UNC outdoor
smog chamber experiments were the
basis of the development efforts. On the
average, maximum ozone was over
predicted for all UNC days by only 4 ±
8%. Similar improved results were
obtained for the UCR-EC experiments
and the xylene simulations. The average
maximum ozone overprediction for the
combined toluene and xylene simulations
from both chambers was 1 ± 12%. The
standard deviation error is much smaller
than in earlier work.
Isoprene and Pinene
The objective in developing this
portion of the CBM-IV was to provide a
chemical representation of biogenic
hydrocarbons by deriving a condensed
mechanism for isoprene and estimating
valid carbon bond splits for pinene. In the
former task, the product species
considered in the condensed isoprene
mechanism were limited to those already
included in the CBM-IV. In the
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svelopment of the isoprene mechanism
Re following significant phenomena to
be associated with isoprene oxidation
were found:
Highly reactive products, including
methacrolein and methylvinyl
ketone. Some reaction of these
species with ozone was also
indicated as would be expected from
the olefinic character of these two
products;
A high rate of radical production,
presumably from photolysis of
isoprene products;
A high yield of PAN as well as a
PAN-like compound, probably from
methacrolein; and
A high yield of formaldehyde.
The general methodology for the
formulation of the condensed isoprene
mechanism was based on the following
rationale: The olefinic nature of isoprene
products would best be simulated by
ethene, since the alkene bonds of
methacrolein and methylvinyl ketone are
partly deactivated and resemble ethene
in their reactivity to OH and 03. This
representation also gives a high yield of
formaldehyde from the secondary
oxidation of ethene, which resembles the
formaldehyde yield of isoprene. The
" rmation of PAN and PAN-like com-
ounds from isoprene was simulated by
assuming that both acetaldehyde and
acetylperoxy radicals are formed during
isoprene oxidation. Inevitably, this can
result in an overprediction of measured
PAN, since simulated PAN represents
PAN analogues as well. The radical yield
for products was simulated by the
formation (and photolysis) of methyl-
glyoxal. The product yields were
adjusted to give the best fits to the mid-
range of hydrocarbon-to-NOx-ratio
isoprene experiments, in which notable
double ozone peaks become evident.
The double ozone peaks in isoprene/NOx
experiments are caused by ozone
reaction with isoprene and isoprene
products, and continued ozone formation
due to PAN decomposition at elevated
temperatures. Simulation of the double
peak effect gives some indication that a
reasonable balance has been achieved
for the multiple processes occurring in
isoprene oxidation.
In addition to the isoprene simulations,
pinene experiments were used to
develop a new carbon-bond pa-
rameterization for pinene. The new
"epresentation is 0.5 OLE, 1.5 ALD2, and
.0 PAR, which is a slight alteration to
ihe representation used earlier.
Mechanism performance was
evaluated using 12 isoprene/NOx smog
chamber experiments conducted by
UNC. The average of the difference
between predicted and measured
maximum ozone was about 6 ± 23%.
We consider this very good evidence
that the new condensed isoprene
chemistry successfully represents the
photochemical processes in these
systems.
The simulation results indicate that the
characteristic second ozone peak caused
by thermal decomposition of PAN or
PAN-like species under high afternoon
temperatures was successfully simulated
for most experiments. In addition,
because isoprene is often used to
represent the photochemistry of biogenic
hydrocarbons, it was also the authors'
intention to develop a condensed
isoprene representation that would
predict formaldehyde concentrations as
accurately as possible. Maximum
formaldehyde concentrations were
overpredicted by only 5 ± 16% for the
isoprene/NOx systems.
Performance Evaluation Using
Complex Hydrocarbon Mixtures
Many individual hydrocarbon/NOx
experiments from the UNC and UCR-
EC smog chambers were used to
evaluate individual portions of the CBM-
IV. In addition to the aromatic and
biogenic hydrocarbon systems, individual
species experiments for formaldehyde,
acetaldehyde, ethene, and larger olefins
were also simulated. To test CBM-IV
performance in simulating different
reactive hydrocarbon mixtures, some
well characterized smog chamber data
sets from the UNC dual outdoor
chambers and the UCR-EC and indoor
Teflon chamber (ITC) were selected. The
authors simulated 21 synthetic
automobile exhaust and urban surrogate
hydrocarbon-mixture experiments from
UNC, along with a few experiments of
authentic automobile exhaust. These
experiments were complemented by a
set of 28 hydrocarbon reactivity
experiments to focus more closely on the
chemical effects of substituting aromatics
and some other species in surrogate
hydrocarbon mixtures. Eleven EC
seven-component-mixture
experiments and five ITC multiday
experiments from the UCR chambers
were simulated, resulting in a total of 65
mixture experiments from these chamber
groups. Including the less complex tests,
slightly fewer than 200 experiments were
simulated during CBM-IV evaluation.
For the hydrocarbon mixture
experiments, the average overprediction
of maximum 03 concentration (excluding
one point) was 2 ± 22%. Formaldehyde
was underpredicted by 9 ± 34 %. Fi-
nally, for all experiments simulated
(including those for all individual
hydrocarbon/NOx systems), maximum
ozone overprediction was 4 ± 23%.
Conclusions
The development of the CBM-IV
began with the gathering of recent
chemical kinetic and mechanistic data for
tropospheric gas-phase chemistry. The
reaction rates and product yields were
then updated and tested. In particular,
large portions of the inorganic section
were altered to reflect current knowledge.
The authors also improved a few specific
portions of the organic chemistry section
for which there were ample data to test
the assumptions. Mechanism evaluation
and demonstration of the chemical
dynamic characteristics of the CBM-IV
were performed using approximately 170
smog chamber experiments from the
different UNC and UCR smog chambers.
The authors compared maximum
experimental ozone, PAN, and
formaldehyde concentrations with
predicted values and provided plots of
these comparisons for each experiment
so that the dynamic processes could be
discussed and the goodness-of-fit over
the entire experimental period could be
compared.
The improvements to the isoprene
condensation and the representation of
toluene (and other aromatics) oxidation
processes provided much better
predictions of ozone and formaldehyde
than those of previous mechanisms.
For the isoprene tests, the mechanism
overpredicted maximum ozone
concentrations by 6 ± 22%; for the aro-
matic experiments, the overprediction
was 1 ±12%. These results indicate that
the new mechanistic representations
significantly diminish the uncertainty
associated with these calculations.
Less obvious in the overall results
were the improvements to predictive
capabilities provided by the enhanced
organic radical and peroxyacetyl reaction
chemistry. These changes appear to
have increased the accuracy of the
radical concentration predictions during
the midday period, resulting in better
agreement between PAN formation and
decay rates and hydrocarbon decay rates
over a large range of temperatures.
These improvements and others just
described led to a slight overprediction (2
± 22%) of the maximum ozone
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concentration averaged for simulations of
68 different experimental mixtures in
three different smog chambers.
Maximum formaldehyde concentrations
were underpredicted by 9 ± 34% for
these mixture experiments. Given the
larger uncertainties in this calculation,
due to both ambiguous organic oxidation
processes and less precise
measurement methods (than for ozone),
these results can be considered to be
good.
M. W. Gery, G. 2. Whitten, and J. P. Killus are with Systems Applications, Inc., San
Rafael, CA 94903.
Marc/a C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Development and Testing of the CBM-IV for Urban
and Regional Modeling," (Order No. PB 88-180 039/AS; Cost: $38.95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22T61
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
HO .25;
Official Business
Penalty for Private Use $300
EPA/600/S3-88/012
0000529 PS
•ft-U.S. GOVERNMENT PRINTING OFFICE: 1988—548-013/870
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