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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