v>EPA
United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/042 July 1985
Project Summary
Evaluation of Chemical Reac-
tion Mechanisms for Photo-
chemical Smog: Part III. Sensi
tivity of EKMA to Chemical
Mechanism and Input
Parameters
Toby B. Shafer and John H. Seinfeld
Six chemical reaction mechanisms
for photochemical smog described in
Parts I and II are used to study the effect
of input parameters on volatile organic
compound (VOC) control requirements
needed to meet the National Ambient
Air Quality Standard for ozone. The pa-
rameters studied are initial VOC com-
position, dilution rate, post-8-A.M.
emissions, base-case (present-day) 03
levels, entrainment from aloft of VOC
and ozone, initial MONO and initial
VOC/NOX ratio. The Empirical Kinetic
Modeling Approach (EKMA) was used
to generate ozone isopleths for each
chemical mechanism. The VOC control
needed to reduce the maximum ozone
concentration from, some present-day
value to 0.12 ppm, assuming no IMOX
control and a specified initial VOC/NOX
ratio, was calculated using the six
chemical reaction mechanisms. The ini-
tial VOC/NOX ratio is found to have the
largest effect of all the parameters stud-
ied on VOC control requirements.
Choice of chemical mechanism, ozone
entrainment from aloft and the compo-
sition of the initial VOC mixture also
have a large effect on predicted control
requirements. To reduce the degree of
uncertainty in control predictions using
EKMA it is necessary to establish as ac-
curately as possible the composition of
urban air in early morning. Also, be-
cause of the substantial effect the
choice of chemical mechanism has on
the predicted control requirements
using EKMA, it is important that future
work continue to be directed toward
evaluating candidate chemical mecha-
nisms with respect to their ability to
simulate atmospheric smog chemistry.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research 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
Currently there are several chemical
reaction mechanisms available for use
in predicting hydrocarbon and NOX re-
ductions needed to meet the National
Ambient Air Quality Standard (NAAQS)
for ozone for a particular region. Be-
cause of the numerous organic species
in the atmosphere, it is computationally
infeasible to use a chemical reaction
mechanism that contains every reaction
of every species in carrying out a con-
trol calculation. Each chemical mecha-
nism reduces the number of organic
species in a somewhat different way.
This reduction, or "lumping," would not
be a concern if each of the chemical
mechanisms gave similar predictions
under typical atmospheric conditions.
As discussed in Part II, however, several
investigators have shown that different
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Dchemical smog reaction mecha-
"nisHs predict significantly different
control requirements under the same
conditions. In addition, these investiga-
tors have found that the sensitivity of
the mechanism's predictions to
changes in input parameters varies
among mechanisms. Because of the im-
portance of making intelligent hydro-
carbon control decisions, it is necessary
to understand in as fundamental way as
possible how the choice of chemical
mechanism, as well as the choice of in-
put parameters, affects control predic-
tions. The object of this work, therefore,
was to present a comprehensive, sys-
tematic study of the sensitivity of pre-
dicted hydrocarbon control require-
ments for photochemical smog to the
choice of chemical mechanism and
choice of all input parameters in the
control calculation.
To study the sensitivity of hydrocar-
bon control predictions to choice of
chemical mechanism and input param-
eters, we used the U.S. Environmental
Protection Agency's Empirical Kinetic
Modeling Approach (EKMA) in which
ozone isopleths are generated as a func-
tion of initial volatile organic compound
(VOC) and NOX concentrations using a
chemical reaction mechanism. In the
EKMA methodology, a chemical reac-
tion mechanism for photochemical
smog is used to predict the maximum
ozone concentration achieved over a
fixed time or irradiation, say 10 hours,
as a function of initial precursor concen-
trations (VOC and NOX). The maximum
ozone concentrations corresponding to
each set of initial concentrations are
then represented as isopleths, from
which control requirements are ob-
tained graphically by assuming that the
reductions in initial concentrations
needed to lower the maximum ozone
concentration from one isopleth (the
base-case value) to another (usually the
NAAQSof 0.12 ppmlare linearly related
to the emissions of each precursor. The
chemical reaction mechanism is inte-
grated from early morning, e.g. 0800
hours, to calculate the evolution of
species concentrations in a hypothetical
air parcel that starts from center city and
is advected by the wind. The air parcel
may increase or decrease in size due to
changes in the mixing height and ac-
quire additional pollutants from both
fresh source emissions and entrain-
ment of aged pollutants from aloft. The
simulations on which the isopleths are
based can be performed at two different
levels of detail: Level II, which requires
2
a specific meteorological description of
the air parcel trajectory and emissions
along the trajectory, and Level III, which
makes standard assumptions about the
meteorology and which uses a region-
wide average for emission rates. It has
been shown that the sensitivity of ozone
predictions to choice of chemical mech-
anism does not change appreciably
when one imbeds the mechanisms in
models that are more sophisticated
than the simple Lagrangian box model
on which EKMA is based. Therefore, we
expect that the results of the current
study, which we obtained by use of
EKMA, are a valid indication of the ef-
fects of chemical mechanism and input
parameters on predicted organic con-
trol requirements regardless of the level
of sophistication of the air quality model
in which the mechanism is imbedded.
Results
We have examined the sensitivity of
predicted VOC control requirements to
the choice of chemical mechanism and
to the variation of parameters associ-
ated with the chemical and meteorolog-
ical nature of the atmosphere. Each
chemical reaction mechanism was
found to respond to a variation of the
parameters in a somewhat different
way; in many cases the relative sensitiv-
ity of the mechanism can be explained
from an analysis of the intrinsic chemi-
cal features of the mechanism. The sen-
sitivity calculations can be summarized
concisely as follows:
Initial VOC Composition
The choice of chemical mechanism
has a large effect on VOC control predic-
tions and on the sensitivity of the pre-
dictions to a change in VOC composi-
tion. The ERT mechanism is most
sensitive to a change in composition;
the CBM least sensitive.
Aldehyde Content
Predicted VOC control is quite sensi-
tive to the aldehyde content of the initial
VOC mixture. Generally, the ERT mech-
anism exhibits the greatest sensitivity
to changes in aldehyde content.
VOC/NOX Ratio
The VOC/NOX ratio was generally the
parameter having the greatest influence
on VOC control predictions. Generally,
the ERT mechanism showed the great-
est sensitivity to a change in the
VOC/NOX ratio, and the CBM exhibited
the least sensitivity. In addition, the sen-
sitivity of predictions to a change in the
VOC composition is greatest at low
VOC/NOX ratios.
Base Case Ozone
Effects of changes in VOC composi-
tion are more pronounced the lower the
base case ozone. The DEM mechanism
is most sensitive to the choice of base
case ozone.
Dilution
Differences among the predictions of
the mechanisms are somewhat smaller
at higher dilution rates than they are at
low dilution rates.
Emissions
When emissions are assumed to be
present, a given percent VOC reduction
represents a larger reduction in total
VOC in the atmosphere than in the ab-
sence of emissions.
VOC Entrainment from Aloft
Predicted VOC control requirements
change by between 4 and 21% when
VOC entrainment from aloft is included,
regardless of the reactivity of the mix-
ture assuming a base case ozone con-
centration of 0.24 ppm, and between 6
and 40% when a base case ozone con-
centration of 0.18 is assumed.
Ozone Entrainment from Aloft
All mechanisms exhibit a strong sen-
sitivity to ozone entrainment from aloft.
Ozone aloft is therefore a key variable in
control studies. The sensitivity of VOC
control to the VOC/NOX ratio is influ-
enced more by the entrainment of VOC
than by the entrainment of ozone.
Initial MONO
Predictions obtained with the ERT
mechanism are most sensitive to initial
MONO; predictions from the PW mecha-
nism are least sensitive.
Summary and Recommenda-
tions
The results obtained in the series of
reports, of which this is the third, paint
a consistent picture of the behavior of
chemical reaction mechanisms for pho-
tochemical smog. Several key issues
emerged that are suggestive of future
work:
(1) In much of the existing laboratory
smog chamber data base, chamber
wall effects are important enough to
make accurate discrimination
among rival chemical reaction
mechanisms difficult. Continuing
extensive analysis of data from out-1
door chamber facilities is recom- "
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mended as these systems seem to
be characterized by smaller wall ef-
fects than indoor facilities.
(2) We have found that predicted VOC
control requirements are highly
sensitive to the assumed VOC/NOX
ratio and the VOC composition of
the initial mixture and emissions.
Therefore, it is important that these
variables be as accurately character-
ized as possible for areas for which
control strategies are to be de-
signed.
(3) Because predicted VOC control can
be quite sensitive to entrainment
from aloft, the organic and espe-
cially the ozone concentration of en-
trained air aloft must be better char-
acterized in areas where control
strategies are to be designed.
(4) Continued effort should be placed
on the evaluation of the perfor-
mance of chemical reaction mecha-
nisms in simulating both laboratory
and outdoor smog chamber experi-
ments. These experiments should
include a wide range of conditions
in order to gain a better understand-
ing of the effects of input parame-
ters (for example, initial VOC/NOX,
emissions and initial VOC composi-
tion) on control predictions.
(5) When determining control strate-
gies, regulatory agencies should
use several chemical mechanisms
because at this time it is impossible
to determine which chemical mech-
anism best simulates atmospheric
chemistry.
(6) It is most important to know the in-
put parameters as accurately as
possible since the sensitivity of one
input parameter may depend on the
values assumed for other parame-
ters.
T. B. Shafer and J. H. Seinfeld are with California Institute of Technology,
Pasadena. CA91125.
Marcia C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Chemical Reaction Mechanisms for
Photochemical Smog: Part III. Sensitivity of EKMA to Chemical Mechanism and
Input Parameters," (Order No. PB 85-210 888; Cost: $11.50, 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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