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
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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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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.
-------
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
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U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/10605
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