United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 2771 1
/ \ \"
Research and Development
EPA/600/S3-86/012 Mar. 1986
SERA Project Summary
Development of CBM-X
Mechanisms for Urban and
Regional AQSMs
G. Z. Whitten and M. W. Gery
A series of chemical kinetic mechan-
isms, each of which describes the
formation of photochemical smog from
nitrogen oxide and multiple organic
precursors, has been developed. The
most condensed version of the Carbon-
Bond Mechanism series now available
is known as the CBM-IV. The formula-
tion and testing of the CBM-IV is the
subject of this project summary. This
mechanism has been carefully tested
against results obtained from an ex-
panded version of the CBM and several
intermediate versions that represent
steps of increasing condensation. This
approach was used to (1) ensure that
the CBM-IV did not contain compensat-
ing errors, (2) verify mechanism per-
formance under a range of conditions,
and (3) verify the operating boundaries
of the mechanism at each condensation
step. The condensation of the expanded
CBM mechanism to CBM-IV was tested
in an incremental manner by simulating
seven days of outdoor smog chamber
data from two different experimental
facilities. These data provided a wide
range of time, temperature, light inten-
sity, and concentration conditions for
comparing simulation results. The de-
velopment effort yielded a mechanism
containing 28 species and 70 reactions.
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
Since 1976, Systems Applications has
been developing Carbon Bond Mechan-
isms (CBM) for use in urban and regional
air quality simulation models. The series
of mechanisms developed to date is given
in Table 1. The main objective of the
mechanism development effort reported
here was to prepare a highly condensed
version (CBM-IV) of the expanded Carbon
Bond Mechanism (CBM-X) for use in
complex atmospheric models such as the
EPA Regional Oxidant Model (ROM). A
further objective was to develop new
methods for condensing and testing
chemical mechanisms. Thus, in addition
to documenting the development and
testing of the CBM-IV mechanism, the
Project Report also outlines a potential
protocol for mechanism condensation and
discusses a set of techniques (some of
which are newly conceived) and tests that
can be used to condense and test other
chemical kinetic mechanisms.
Condensation Technique
As shown in Table 1, the most expand-
ed Carbon Bond Mechanism developed to
date is called the CBM-XR with 170
reactions and 78 species. The initial steps
to condense this mechanism led to the
CBM-RR version, which contains only
113 reactions and 47 species. However,
these two versions of essentially the
same chemistry did not require extensive
condensation testing since most of the
condensation techniques were simple
algebraic transformations. Such trans-
formations do not significantly affect the
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Table 1.
The Carbon-Bond Mechanism Series
Mechanism
CBM-I
CBM-II
CBM-III
CBM-X
CBM-XR
CBM-RR
CBM-IV
Description
Original CBM
Update/ Expansion of
CBM-I
Update of CBM-II with
improved aromatics
chemistry
Expanded version of
CBM-III suitable for
use in EKMA
CBM-X with isoprene
chemistry for regional
scale modeling
Condensed version of
CBM-XR
Highly condensed version
of CBM-RR
Number of
Reactions/
Species
35/20
65/27
75/36
146/67
170/78
113/47
70/28
Reference
Whitten and Hogo (1977)
Whittenet al. (1980a)
Whitten et al. (1980b)
Killus and Whitten (1984)
Whitten et al. (1985a)
Whitten et al (1985b)
Whittenet al. (1985b)
This work
Killus, J. P.. and G.Z. Whitten. 1984. Technical Discussion Relating to the Use of the Carbon-Bond
Mechanism in OZIPM/EKMA. SYSAPP-84/117, Systems Applications, Inc., San Rafael, CA
Whitten, G. 2., and H. Hogo. 1977. Mathematical Modeling of Simulated Photochemical Smog.
EPA/600/3-77/011, U.S. Environmental Protection Agency, Research Triangle Park, NC
Whitten, G. 2., H. Hogo, and J. P. Killus. 1980a. The Carbon-Bond Mechanism A Condensed
Kinetic Mechanism for Photochemical Smog Environ. Sci. Techno/., 14:699.
Whitten, G. Z, J. P. Killus, andH. Hogo. 1980b. Modeling of Simulated Photochemical Smog with
Kinetic Mechanisms. EPA/600/3-80/028a, U.S. Environmental Protection Agency, Research
Triangle Park, NC.
Whitten, G. 2., J. P. Killus, and R. G. Johnson. 1985a Modeling of Auto Exhaust Smog Chamber
Data for EKMA Development. EPA/600/3-85/025, U.S. Environmental Protection Agency,
Research Triangle Park, NC.
Whitten, G. 2., J. P. Killus, and R. G. Johnson. 1985b Development of a Chemical Kinetic
Mechanism for the U.S. EPA Regional Oxidant Model. EPA/600/3-85/026, U.S. Environmental
Protection Agency, Research Triangle Park, NC.
numerical output of the integrated dif-
ferential equations based on the chemical
reaction set. However, much of the
descriptive nature of the original expand-
ed reaction set (CBM-XR) becomes lost in
the algebraic transformation. The more
extensive condensation employed during
this project involved assumptions con-
cerning the bounding conditions on var-
ious reactions and species. This type of
procelure is not trivial and requires
stepwise verification over a range of
possible conditions.
The techniques used to condense the
CBM-XR to the CBM-RR involved elim-
ination of (1) stable products, (2) constant
concentration species, (3) mass balance
species, (4) unit stoichiometric factors, (5)
species that only rapidly decay unimolec-
ularly, and (6) the addition of universal
peroxy radicals which convert NO to NC"2
or nitrates. With the exception of the last
technique, no testing of the results is
required except to verify the algebra and
arithmetics involved. For the last tech-
nique, the use of universal peroxy rad-
icals, rather obvious tests were i mplicated
since the maximum errors would be
expected for conditions of near zero NO
concentrations because the main reac-
tions of peroxy radicals are with NO.
Smog chamber data were therefore used
from experiments in which NO approach-
ed zero several times during runs up to
four days.
For this project the new techniques
used for condensation were (1) elimina-
tion of unimportant species, (2)the use of
implicit steady-state approximations, and
(3) combinations or extensions of previous
techniques. When species and reactions
were eliminated, much care was taken to
first search for and simulate the condi-
tions where such species were most
important to ensure that, even under
such conditions, these species were still
unimportant to the overall process of
oxidant formation. For some species a
boundary of importance was noted. For
example, peroxynitric acid (PNA) and
methylperoxynitric acid (MPNA) were
found to be important species below 290
K under conditions of high oxidant forma-
tion such as in a pure propene oxidation
system. Hence, it might be wise to include
PNA and MPNA if winter conditions are to
be simulated where significant ozone
formation occurs.
Tests of Mechanism
Condensation
Since the expanded Carbon-Bond
Mechanisms (CBM-X and CBM-XR) had
already been validated against smog
chamber data, the goal in this part of the
project was to ensure that the condensa-
tion steps produced a condensed version
(CBM-IV) that gives model results similar
to the expanded versions. To ensure that
one condensation step did not produce
compensating effects that might cancel
some previous condensation step, tests
were performed at each step in the
condensation process rather than only
with the completely condensed CBM-IV
mechanism.
Three multi-component outdoor smog
chamber experiments encompassing a
wide range of chemical, temporal, and
meteorological conditions were used to
demonstrate most of the condensation
steps. Two of these experiments were
multi-day runs performed at the Univer-
sity of California at Riverside and the third
experiment was performed at the Univer-
sity of North Carolina (UNC). All these
experiments involved multicomponent,
urban-like mixtures of hydrocarbons.
Temperature ranges were tested using
two propene outdoor experiments per-
formed at the UNC facility in December
and October. An isoprene experiment at
UNC was used to test the isoprene
condensation steps.
Summary
The project report provides the details
of the condensed versions (CBM-IV) of
the extended Carbon-Bond Mechanisms
(CBM-X and CBM-XR). The report des-
cribes the various methods of mechanism
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condensation employed and how each of
these methods were tested. The conden-
sation of the expanded CBM to CBM-IV
was tested in an incremental manner by
simulating seven days of outdoor smog
chamber data from two different experi-
mental facilities. These data provided a
wide range of time, temperature, light
intensity, and concentration conditions
for comparing simulation results from
increasingly condensed mechanisms
with the expanded CBM results. This
approach was enacted to minimize com-
pensating errors and to verify the operat-
ing boundaries of the mechanism at each
condensation step. This development
yielded a mechanism containing 28 spec-
ies and 70 reactions (including isoprene
chemistry) compared with 78 species and
170 reactions contained in the CBM-XR.
Because the CBM-IV includes isoprene
chemistry, the mechanism is suitable for
use in gridded regional oxidant models.
With the further addition of sulfur chem-
istry, along with PNA, MPNA, and organic
acid chemistry, it may also be suitable for
modeling homogeneous gas phase chem-
istry in acid rain models. Moreover,
additional product species that account
for organic nitrates, organic peroxides, or
peroxy acids can easily be included in the
CBM-IV. However, the CBM-IV must be
used with caution under conditions for
which it has not yet been tested.
Although the CBM-IV may provide the
performance of a highly detailed mechan-
ism in an efficient and compact form, all
the chemistry is not visible and mechan-
ism updates may require both recalcula-
tion of parameters and retesting.
G. Z. WhinenandM. W. Geryare with Systems Applications, Inc.. SanRafael, CA
94903.
Marcia C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Development ofCBM-X Mechanisms for Urban and
Regional AQSMs," (Order No. PB 86-155 033/AS; Cost: $16.95, 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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