United States
                   Environmental Protection
                   Agency     	
Atmospheric Sciences
Research Laboratory
Research Triangle Park, NC 27711
                   Research and Development
EPA/600/S3-88/012  Apr. 1988
                                                                                .''•4 >
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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