United States
                   Environmental Protection
                   Agency
 Environmental Sciences
 Research Laboratory
 Research Triangle Park NC 27711
                   Research and Development
EPA-600/S3-84-063 June 1984
&EPA         Project Summary
                   Evaluation  of Chemical
                   Reaction  Mechanisms  for
                   Photochemical  Smog
                   Part  II:   Quantitative  Evaluation
                   of  the  Mechanisms
                   Joseph A. Leone and John H. Seinfeld
                    Six chemical reaction mechanisms
                   for photochemical smog are analyzed to
                   determine  why, under  identical
                   conditions,  they  predict   different
                   maximum ozone concentrations. To
                   perform the analysis, a counter species
                   analysis technique is used to determine
                   the contributions of individual reactions
                   or sets of reactions  to the overall
                   behavior of a chemical reaction mecha-
                   nism. Using this technique, we can
                   obtain answers to previously inacces-
                   sible questions  such as the relative
                   contributions  of individual organic
                   species to photochemical ozone forma-
                   tion.  Based on the results  of the
                   analysis, we have identified specific
                   aspects of each mechanism that are
                   responsible for the discrepancies with
                   other mechanisms and with a master
                   mechanism  based  on the latest
                   understanding of atmospheric
                   chemistry.  For  each  mechanism,
                   critical areas are identified that, when
                   altered, bring the predictions of the
                   various mechanisms into much closer
                   agreement. Thus, we have been able to
                   identify why the predictions of the
                   mechanisms  are different,  and have
                   recommended research efforts that are
                   needed to eliminate most of the dis-
                   crepancies.
                    This Project Summary was developed
                   by  EPA's  Environmental  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 order-
ing information at back.)

Introduction
  In determining emission control levels,
one must be able to predict how changes
in emission levels will affect ambient air
quality. An important component of such
an approach is a description of atmos-
pheric organic  chemistry. Unfortunately,
the development of a chemical reaction
mechanism  that accurately  describes
atmospheric chemistry and, at the same
time, is computationally tractable, is a
difficult undertaking. Since typical urban
atmospheres  contain  hundreds of
different organic species, it is not feasible
to write a mechanism that includes each
individual species. Thus, these reaction
mechanisms must maintain a balance
between the level of chemical detail and,
for numerical  reasons, the number of
species and reaction pathways.
  Currently,  several chemical  reaction
mechanisms exist that  describe the
organic chemistry of the urban  atmos-
phere and that attempt to maintain a
balance  between chemical detail  and
mechanism length. These mechanisms
are  all  based  on the  same body of
experimental rate constant data,  and
each  mechanism has  been evaluated
against data from various smog chamber
facilities.  In  each mechanism,  the
detailed atmospheric chemistry has been
greatly simplified by a process referred to

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as  lumping.  However,  because  this
simplification, or lumping process, has
been carried out in different ways, no two
mechanisms   are  the  same.  The
differences among  mechanisms would
not be of any  concern if  each of the
mechanisms  gave  similar  predictions
over a range of atmospheric conditions.
However,  several recent investigations
have  shown that different mechanisms
predict substantially different degrees of
emission controls to achieve the same
desired   air  quality  under   identical
conditions. Since tremendous  expenses
are  involved  in   implementing
hydrocarbon and oxides of nitrogen (NOx)
emission  controls  predicted  by  such
mechanisms, there is an urgent need to
understand the fundamental reasons for
these discrepancies.
  This report represents the second part
of a three-part study of lumped reaction
mechanisms  for photochemical smog.
Part  I  (EPA-600/3-83-086)   contains
information   concerning  the  various
approaches  to  lumping, and  how the
particular mechanisms chosen for this
study were selected. Also included in Part
I is a detailed description of each lumped
mechanism.  Part  II  presents  a
quantitative  analysis   aimed  at
determining why  these  mechanisms,
under identical  conditions  predict the
formation  of  substantially   different
amounts of ozone (03). In Part III we will
analyze the emission control require-
ments predicted by the various mech-
anisms under conditions approximating
those occurring in the real atmosphere.

Method of Analysis
  To determine  why  the  O3  yields
predicted  by these  lumped mechanisms
are  so  different,  we would  like to
determine how much  of  the total  03
production can be  attributed to each of
the  initially present organic species in
each mechanism. With this information,
we could determine the relative contribu-
tions of  individual organic species (or
 reactions, reaction pathways, etc.) to the
overall behavior of each mechanism.
   Unfortunately, analyzing the behavior
 of  these  atmospheric reaction mecha-
 nisms is a demanding task because of the
 large number of species and  reactions
 that each contains, and because of the
 interwoven   nature  of  the  free
 radical chain reactions characterizing
 each mechanism. Thus, there is no direct
 way of calculating the relative amounts of
 03 that each organic species  is respon-
 sible for producing in a mechanism. We
 can, however, using a method described
below, keep track of the number of NO to
N02 conversions that arise as a result of
the presence of each organic species. The
amount  of 03 attributable  to  a  given
organic species is well established to be
directly proportional to the number of NO
oxidations affected by the species. With
this information,  corresponding species
or reactions in the various mechanisms
can be compared  directly to one another.
  The technique we use to determine the
number  of  NO  to  NO2 conversions
attributable to individual species is called
counter species analysis. To illustrate the
usefulness of such an analysis, consider
the following simple  mechanism that
describes the photooxidation of formalde-
hyde and acetaldehyde in the presence of
NOx.

N02 + hv-NO + 03               (1)

NO + 03 - N02 + 02               (2)

H02-+ NO - NO2 + OH  + C3      (3)

HCHO + hv -2HO2-+ CO + C4      (4)
          20 2
                                      CH3O(O)02- + NO ~NO2 +
                                      CH3O2-+CO+ C9 °2               (9)

                                      CH3C(0)02- + NO2 - CH3C(0)02N02
                                      (PAN)                            (10)

                                      PAN - CH3C(O)O2' + N02         (11)

                                      CH3O2 • + NO - CH30- + N02 +
                                      C12
                                                                        (12)
                                      CH30 • + O2 - HCHO + HO2-
                                      + C13                           (13)
                                      N02 + OH - HNO3 + C14
                                                                        (14)
HCHO +hv - H2 + CO

HCHO+OH^HO
           U2

CH3CHO + hvo7s
+ C7       2°2
                                (5)

                             C6 (6)
                      HO2.+ CO
                                (7)
CH3CHO
+ C8
   0.080
   0.060
•§  0.040
s
I
o
   0.020
          OH - CH3C(O)02-
              °2
                            H20
                                (8)
In the atmosphere these aldehydes react
to produce  HO2,  CH3O2, and CH3C(O)O2
radicals that can convert NO to NO2, and
thus cause [NO2]/[NO], and consequently
O3,  to increase.  Ozone  formation  will
continue  as long as aldehydes and  NOx
are  both  present. NOx is  consumed via
reactions 10 and 14, so ultimate 03 yields
are  limited by NOx availability as well as
by how fast the aldehydes lead  to O3
formation through the conversion  of NO
to  N02-  The  species Ci  are  fictitious
products  that  are used  to count  the
number  of times  that reaction  i  has
occurred.  By  counting  these  fictitious
species, we can determine the number of
NO to N02 conversions that each reaction
is responsible for.
                              I          I          I

                           NO - /V02 by CH3CHO + OH •
                                             NO- /VO2 by HCHO + OH

                                    NO - /VO2 by CH3CHO +hv

                                         I           I           I
                                                             + Otf^*
                  60
                              120        180        240

                                    Time (Minutes)
                                                              300
                                                                         360
 Figure 1 .
           Counter species results for the formaldehyde/acetaldehyde/NO* simulation.
           Number of NO to NOi conversions due to the photolysis and OH reactions of I
           formaldehyde and acetaldehyde.

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   The results of analyzing this simple
 mechanism with  the  counter  species
 technique  are shown in Figure 1. The
 amount of  NO to N02 conversions due to
 the four key reactions  is shown as a
 function of time. The OH reaction of acet-
 aldehyde is the most important reaction
 from an NO to N02 conversion (and O3
 production) point of  view. With simple
 mechanisms,  such  as  the  aldehyde
 mechanism shown above, there are other
 ways of obtaining the desired informa-
 tion. One method is to make use of the
 pseudo-steady state approximation  to
 eliminate  the   fast-reacting  species.
 Unfortunately, all of the useful chemical
 reaction  mechanisms describing
 photochemical  smog  are much  more
 complicated than  the simple aldehyde
 mechanism presented above. With these
 mechanisms,  it   becomes   extremely
 difficult to  eliminate  the fast-reacting
 species using the pseudo-steady  state
 approximation. Nevertheless, the counter
 species analysis  provides an effective
 means to examine the properties of these
 complex mechanisms

 The Master Mechanism
   The  counter  species  technique
 described above is used to compare the
 structure and behavior of each of the six
 lumped mechanisms. Since each of these
 mechanisms represents  an attempt  to
 simulate  atmospheric  organic/NOx
 chemistry,  it is also desirable to compare
 the structure and behavior of  each
 mechanism to a  fully explicit, detailed
 mechanism containing  as many of the
 important organic species as possible.
 We have constructed such a mechanism
 which contains the detailed chemistry  of
 12 of the  most  important atmospheric
 organic species. We call this mechanism
 the "master mechanism."
Results
  The  results  of applying the counter
species analysis to the six lumped mech-
anisms and the master mechanism are
shown in Figure 2. Shown  for  each
mechanism is the amount of NO to N02
conversions attributable  to each of the
initially present organic species. One can
see that the amount of NO to N02 conver-
sions due to  a given species can vary
substantially  between  the various
mechanisms. Based on results like these,
critical areas of each mechanism are
identified which are most responsible for
the observed discrepancies. When these
critical  areas  are  modified,  the
predictions of the various  mechanisms
       1000
        800
        600
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        400
        200
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m-xylene

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HCHO

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toluene

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other
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mpropene_
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                                                   1
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                                     Mechanism
Figure 2.   Counter species results showing the amount of NO to NOz conversions attributable
           to each of the initially present organics.
are in much  better agreement. This is
shown in Figure 3.


Conclusions
  In  this report we have  presented a
quantitative  analysis  of  six  lumped
mechanisms describing  photochemical
smog.   We   determined  why   these
mechanisms, under identical conditions.
                                   gave rise to widely different predictions of
                                   peak-03 levels. Based on the results of
                                   this analysis, we  have identified specific
                                   areas  in each mechanism that are most
                                   responsible for the observed discrepan-
                                   cies in O3 predictions. For each mechan-
                                   ism, several recommendations have been
                                   made that are aimed at eliminating these
                                   discrepancies.  Some of these recom-
                                   mendations amount merely to updating

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      7000
       SOO
  I
  i
3
o
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i
     600
       400
       200
                         .h
                                   II

                                  Mechanism
                                            ll
                                            I*
II
figure 3.
         A comparison of the total amount of NO to NOsConversions predicted by the original
         (unshaded) and modified (shaded) versions of five lumped mechanisms.
  area needing improvement concerns
  the  mechanism of  aromatic  ring
  opening. The products formed during
  aromatic ring opening must be eluci-
  dated before any faith can be placed
  in the predictions of  photochemical
  smog mechanisms. The photolysis
  products of the important a-dicarbo-
  nyls such  as  methyl glyoxal  also
  represents a   critical  area   of
  uncertainty in aromatic chemistry.

• much greater resources be devoted
  to  the  determination  of  the
  fundamental mechanism leading to
  chamber radical sources.  It is an
  unfortunate fact that mechanisms
  cannot be unambiguously evaluated
  using chamber data or compared to
  each other until the  radical source
  issue is resolved. It now appears that
  the  only   way to  resolve  this
  controversy  is   to determine  the
  mechanism which gives rise to this
  radical source.

• the counter species  analysis tech-
  nique be used by each investigator
  when future versions of these mech-
  anisms are developed. We have only
  applied  this analysis to  a  single
  primary  hydrocarbon distribution, at
  3  RHC  to NOx ratios.  Individual
  investigators should  make  use  of
  counter  species analysis to test their
  mechanisms with a variety of initial
  hydrocarbon distributions and RHC
  to NOx ratios.

• future  work  in evaluating  these
  lumped  mechanisms include  an
  analysis of the  methods for  estab-
  lishing  initial   conditions   when
  detailed hydrocarbon composition
  profiles are  not  available. The
  emission  control  requirements
  predicted by these  lumped mecha-
  nisms should  be evaluated  under
  real world conditions of continuous
  pollutant  emissions,  continuous
  dilution, and  in the  presence  of
  background pollutants. Problems  of
  this type will be addressed in Part III
  of this study.
rate  constants,  while others  involve
developing   completely  new  reaction
sequences. When the lumped mechan-
isms are modified to include our sugges-
tions, their predictions are in much closer
agreement. However, changes such as
those that we have suggested should not
be adopted until the performance of the
entire mechanism is reevaluated.
                                        Several recommendations for future
                                      work are apparent upon completing this
                                      study. These would include recommend-
                                      ing that:

                                        • future work be directed at several
                                         important  areas  of  atmospheric
                                         chemistry where our knowledge is
                                         lacking.  Perhaps the  most critical
                           that a significant effort be devoted
                           toward measuring the composition
                           and amounts   of organic species
                           emitted  into  urban  environments.
                           Without   this  information,  the
                           uncertainties involved  in specifying
                           input data could nullify any improve-
                           ments in the reaction  mechanisms
                           themselves.

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    J. A. Leone and J. H. Seinfeld are with the California Institute of Technology,
       Pasadena, CA 91125.
    Marc/a C. Dodge is the EPA Project Officer (see below).
    The complete report, entitled "Evaluation of Chemical Reaction Mechanisms for
       Photochemical Smog.  Part II.  Quantitative Evaluation of the Mechanisms,"
       (Order No. PB 84-196  740; Cost: $19.00, subject to change) will be available
       only from:
            National Technical Information Service
            5285 Port Royal Road
            Springfield, VA22161
            Telephone: 703-487-4650
    The EPA Project Officer can be contacted at:
            Environmental Sciences Research Laboratory
            U.S. Environmental Protection Agency
            Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
                                  &  nt
                                r u, ^-» -J
         o"!:>
                                                                                   U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/10605

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