United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S9-87'024 Jan. 1988
Project Summary
Workshop on Evaluation/
Documentation of Chemical
Mechanisms
Roger Atkinson, Harvey Jeffries, Gary Whitten and Fred Lurmann
Atmospheric photochemists and
developers and users of air quality
models need to discuss the
evaluation and documentation of
chemical mechanisms used in air
quality simulation models. A
workshop, therefore, was organized
and conducted on December 1-3,
1986 by EPA to discuss the latest
evidence and viewpoints on the
subject and to solicit from experts
recommendations on optimum
approaches to mechanistic model
evaluation, documentation, and
further development. Previous
practices and underlying issues in
the subject areas were reviewed and
discussed in background documents
prepared and distributed in advance
of the workshop. Participants agreed
that smog chamber data provide the
most unambiguous test of urban
atmospheric photochemistry
mechanisms. They also agreed,
however, that there are uncertainties
associated with the representation of
chamber radical sources and of
photolytic rates in outdoor
chambers, with smog chamber
measurement errors, and with the
representation of as yet unknown
reaction pathways. The participants
recommended that task forces and/or
review groups be established to
discuss and resolve existing smog
chamber methodology issues, to
assemble a required smog chamber
data base for mechanism testing,
and to review/evaluate relevant
kinetic and smog chamber data and
mechanism testing results.
Recommendations were also
developed on future research needs
and on mechanism documentation
procedures.
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 EPA conducted this workshop to
discuss the evaluation and
documentation of chemical mechanisms
used in air quality simulation models. (A
chemical mechanism is the set of
chemical reactions and associated rate
constants which describes the
transformation of emitted chemicals into
intermediate and final products. In the
context of this workshop, the initially
emitted chemicals are hydrocarbons and
oxides of nitrogen, and ozone is the
product species of major interest.) Goals
of this workshop were:
to assess present practice in
photochemical reaction mechanism
development and testing for those
mechanisms intended for use in urban
air quality control calculations;
to determine if there might be a
commonly agreed upon mechanism
evaluation procedure; and
to determine if there might be a
standard data base that would be
useful in distinguishing among
different mechanisms.
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A review of previous practice and a
discussion of the underlying issues was
presented in background documents
"The Science of Photochemical Reaction
Mechanism Development and
Evaluation" by Jeffries and Arnold and
"Need for Chemical Mechanism
Documentation" by Sexton and Jeffries,
which were distributed to workshop
participants prior to the meeting.
Four scientists and an EPA user of
models were asked to respond to the
background documents and to offer their
viewpoints on evaluation procedures and
testing data bases. These were Kenneth
Demerjian, Roger Atkinson, Michael
Gery, Allan Dunker, and Joseph Tikvart.
These four scientists were in agreement
that there is, and has been, a generally
accepted procedure for testing the extent
of "reasonable agreement" between
model predictions and experimental
measurements. This procedure involves
the use of laboratory kinetic,
mechanistic, and product data, the use of
smog chamber data, and the use of other
test data, such as captive air irradiation
measurements and ambient air
measurements. Many participants
believed, however, that the latter type of
comparisons (e.g. model predictions
compared to ambient measurements
and, for some, even captive air studies)
require so many approximations and
suffer from such instrumental limitations
that the extent of agreement expected
would be quite limited and thus such
efforts would not be clear tests of our
understanding of the chemical
transformation processes. All four
scientists agreed that, although there
certainly were problems with their data,
environmental or smog chambers still
provided the most unambiguous data for
the testing of urban chemical
transformation mechanisms. Subsequent
discussion confirmed that this approach
or method was generally the accepted
approach used by the workshop
attendees.
The EPA model user (J. Tikvart)
strongly supported the draft proposal for
mechanism documentation. Other
workshop attendees, including William
Carter, Fred Lurmann, Gary Whitten, and
Gregory McRae described their current
practice and recent model testing
strategies and results.
While various mechanisms have been
developed and tested against limited
numbers of smog chamber experiments,
only two mechanisms, the SAPRC/ERT
mechanism and the latest Carbon Bond
Mechanism, have been tested against a
large number of chamber experiments
(ca. 500) from different chambers.
The Steering Committee concluded
that, without further work, it was not
possible to choose one of these
mechanisms over the other on the basis
of scientific evidence, and that the EPA
should be encouraged to use both
mechanisms as a method to estimate the
present uncertainties in control
requirement predictions. The Steering
Committee also concluded that several
review groups should be assembled to
review the kinetic and chamber data
bases and to review the extent of
agreement among the models and these
data bases. These recommendations will
be described below.
It is clear from the data presented at
this workshop, together with work
published over the past five years, that a
vast amount of progress has been made
during the past decade, both with
respect to urban-area chemical
mechanism development and the data
base upon which these mechanisms are
based and tested. Nevertheless,
additional development is clearly
needed. Recommendations for future
chemical mechanism development,
documentation, and testing intended to
extend the present urban oxidant-only
mechanisms to the new areas that will
confront EPA in the coming years are
also given below.
Guidelines for Mechanism
Development and Testing
Demerjian described what most
workshop participants accepted as a
general approach to mechanism
development and testing. The major
components of this approach are shown
in Figure 1. Within this generally
accepted approach, however, there are
differences in the practice among
different modeling groups. With respect
to mechanism evaluation, there was
general agreement that:
mechanism testing should be
performed according to a "hierarchy
of species,"
data from at least two, and preferably
more, chambers should be used in the
testing,
testing should include at least two
phases:
-testing and refining the
representations of chamber-
dependent phenomena, and
-testing and refining for comple,.
organic species and mixtures of
species,
and
as much data as is available should
used m the testing, and at a minimum.
15 experiments should be used in the
first phase and 50 experiments in the
second phase for each chamber.
Detailed
Chemical
Mechanism
I
\
i.
r
Condensed
Chemical
Mechanism
_-- '""^ i
i
k
r
Testing against
Amb/ent Air Data
for Consistency
Figure 1. Schematic Method for
Development and Eval-
uation of Chemical
Mechanisms. After Ken
neth Demerjian.
Workshop discussions indicated
further that assessing goodness-to-fit
between the experimental data and the
model predictions is a topic that requires
further work and one for which the
Steering Committee recommends the
creation of a Task Group. One approach
to showing the"goodness-to-fit," the
developers of the SAPRC/ERT
mechanism used simple statistical
measures to describe the goodness-
to-fit for their model As an example of
another approach, the developers of the
Carbon Bond Four mechanism used
several hundred plots of model
predictions-vs-smog chamber
observations to illustrate the adequacy of
their fit. Their reports also generally
contain one or two tables giving the
maximum 03 and time to maximum for
the model predictions and the observed
values, but no scatter plots or error
distribution plots are given. So-
members of the Steering Committee .
that both of these approaches to showing
"goodness-to-fit" are incomplete.
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The detailed chemical mechanism,
developed and evaluated as discussed
above, must be condensed (see Figure
1) for use in air quality simulation models
- control strategy assessment
purposes. Procedures for condensing
mechanisms and testing the condensed
mechanism against simulations of the
expanded or explicit mechanisms were
discussed in three reports: Whitten,
Johnson, and Killus ("Development of a
Chemical Kinetic Mechanism for the U.S.
EPA Regional Oxidant Model," EPA-
600/3-85-026, 1985); Whitten and
Gery ("Development of CBM-X
Mechanisms for Urban and Regional
AQSMs," EPA-600/3-86-012, 1986),
and Lurmann, Carter, and Coyner ("A
Surrogate Species Chemical Mechanism
for Urban-Scale Air Quality Simulation
Models, Adaptation of the Mechanism,"
EPA-600/3-86-031, 1986). Specific
methods for mechanism condensation
include eliminations of species that are
unreactive or do not change in
concentration during the reactions and
thus should not change the numerical
results of simulations using the
intermediate condensed mechanism
«hen compared to the detailed
echanism. Other steps include the use
~'6f fractional stoichiometric coefficients
and intermediate species which have
unimolecular branched pathways of
reaction. As long as the unimolecular
;cay lifetimes are reasonably short, the
**"rTumerical simulation results will not be
significantly affected. Tests are required
for lifetimes longer than a few minutes.
Other techniques include the use of a
"mass balance" or "counter species"
which are given arbitrarily high decay
rates to form"intentional" steady-
state-like species. Condensation steps
beyond those above involve assumptions
about typical atmospheric situations, and
the condensed versions of the
mechanism must be tested to determine
the bounds of such assumptions.
Techniques have been developed to
identify unimportant reactions and
species. Elimination of unimportant
reactions provides little benefit to
simulation costs, because the costs are
most sensitive to the number of species.
Of course, fewer reactions do make a
mechanism easier to understand.
In regards to mechanism
documentation, it was stressed by the
Steering Committee that clear and
omprehensive documentation of
Chemical mechanisms by their
developers is needed to ensure their
proper use. In his presentation, Joseph
Tikvart reviewed and strongly supported
the approach proposed in the workshop
background document "Need for
Chemical Mechanism Documentation,"
by Sexton and Jeffries. In this document
an example outline of a guidance
document for the application of chemical
mechanisms was proposed. Computed
solutions to mechanism test problems
are essential to proper implementation of
chemical mechanisms by nondeveloper
users. The Steering Committee believes
that a minimum of four test problems are
needed. These initial test problems
should not involve dilution, entrainment,
emissions injection, or deposition, but
rather be examples of the pure chemical
kinetics. Ideally, the solutions should be
computed using a high quality algorithm
such as the Gear algorithm with tight
error control. In these test cases, the
algorithm used, the method of computing
the Jocobian, the use of absolute or
relative error tolerances, and the
minimum, initial, and maximum integrator
step sizes should be documented. All
species should be integrated rather than
determined from the steady-state
assumptions. Another documentation
item discussed by the Steering
Committee was the need for a standard
set of conditions for comparing
mechanism prediction of VOC control
requirements when using the OZIPM
program. It was suggested that the
regulatory groups in EPA should be
involved in developing these test cases
to insure that they cover typical cases of
interest for regulatory purposes.
Differences Among Well-
Tested Mechanisms
When the guidelines given here for
chemical mechanism development are
followed by multiple research groups, it
is expected that large sections of the
mechanisms developed will be very
similar, if not identical. The portions of
the detailed chemical mechanism,
however, which are either unknown or
only poorly understood will have to be
assembled using estimations or
arguments by analogy, or perhaps
simply be parameterized. Thus, different
methods of representing these unknown
or poorly known sections of the
chemistry will arise in different
mechanism developments. The latter
process will most likely lead to detailed
(and subsequently condensed) overall
mechanisms which, based on past
experience, may differ in their control
strategy predictions, despite the fact that
each chemical mechanism may be
consistent within the bounds of
reasonable agreement with the
elementary reaction kinetics,
mechanisms, and products data bases
and with environmental chamber data.
Other than further efforts to refine the
measure of uncertainty in both the
kinetics and chamber data and new
formulations and testing of the
mechanisms, there may well be no
scientific reason to accept or reject one
of these chemical mechanisms over the
other. Given that EPA must proceed with
applications of the mechanisms, it is
recommended that the EPA not select
only a single mechanism for making
control strategy predictions.
Task or Review Groups Needed
The workshop participants
recommended that several interacting
sets of review processes or review
groups be established, each with a goal
of assessing and documenting the state
of knowledge and degree of reasonable
agreement to be expected in a given
domain. At least four different domains
were identified for such activities:
1) kinetic and mechanistic data needed
in constructing photochemical
transformation mechanisms;
2) environmental chamber data needed
for comparison with mechanism
predictions;
3) mechanism intercomparisons tests;
and
4) user or application tests.
Short-Term Recommendations
The Steering Committee believes that
it may be possible to further refine
existing mechanisms on a short-time
scale and without the need for any
new experimental data. While this
advancement needs no new
experimental data, recommendations
for further well-defined research may
result from this effort. The first priority
is to use the presently available
environmental chamber data base to
refine the testing of present chemical
mechanisms. The Steering Committee
therefore recommends the
establishment of a task force of non-
EPA scientists to evaluate and resolve
chamber effects and light
intensity/spectral distribution issues.
One of the goals of this workshop was
to identify a standard data base that
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would be useful in distinguishing
among different mechanisms. The
Steering Committee concluded that it
was possible to assemble a
"necessary" environmental chamber
data set, that is, one containing
chamber experiments which all
mechanisms for use in urban areas
would have to simulate. The data base
is described as "necessary" because
if a mechanism could not simulate
these experiments, it mostly likely
would be considered unsatisfactory
for EPA applications, but the data
base would not be "sufficient" to
resolve all questions associated with
oxidant prediction. The Steering
Committee recognized that to
assemble such a data base would
require significant input from chamber
operators at both UNC and UCR and
from model developers at SAI and
UCR, as well as other interested
parties who might wish to use the
data. It is envisioned that the data
base would contain about 100
experiments from several chambers.
Each experiment would contain
detailed recommendations and
supporting information on photolytic
rates, chamber wall processes, and
initial and temporal conditions. These
would be the result of review,
discussion, and consensus among
task force members.
Longer-Term
Recommendations
Establishment of Review Group for
Evaluation of Fundamental Kinetic and
Mechanistic Data for Use in Model
Development
Establishment of Review Group for
Evaluation of Environmental Chamber
Data
Establishment of Review Group for
Mechanism Intercomparison
Establishment of Review Group for
Mechanism Predictions in
Applications
Obtain Additional Data on
Atmospheric Chemistry of Organics.
Specifically obtain data (a) on
absorption cross-section and
photodissociation quantum yields and
product data for the carbonyl
compounds formed as intermediate
products in the degradation schemes
or organics, (b) to determine the fates
of HO-aromatic adducts under
atmospheric conditions, (c) to
determine the reactions of the >Ce
alkoxy and alkylperoxy radicals under
atmospheric conditions and the
subsequent reactions of their
products, and (d) to determine the
radicals formed, and their yields, from
ozone-alkene reactions under
atmospheric conditions.
Obtain additional Environmental
Chamber Data of higher quality and
also for the purposes of (a)
discriminating between present
approaches to representing chamber
effects, and (b) testing of acid
deposition and regional and
tropospheric models.
Conduct New Studies of Chamber-
Dependent Effects
Investigate the Applicability of the
present EKiVIA Method
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Roger Atkinson is with the University of California, Riverside, CA 92521; Harvey
J Jeffries is with the University of North Carolina, Chapel Hill, NC 27514; Gary
Whitten is with Systems Applications, Inc., San Rafael, CA 94903; and Fred
Lurmann is with Environmental Research and Technology, Inc., Newbury Park,
CA 91320.
Basil Dimitriades is the EPA Project Officer (see below).
The complete report, entitled "Workshop on Evaluation/Documentation of
Chemical Mechanisms," (Order No. PB 88-134 358/AS; Cost: $32.95, 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:
Atmospheric 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
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S9-87/024
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