United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/067 Dec. 1985
&EPA Project Summary
EPA Regional Oxidant Model:
Description and Evaluation Plan
K. L. Schere and A. J. Fabrick
The USEPA Regional Oxidant Model
and NEROS data base are described.
The model incorporates a comprehen-
sive description of the physical and
chemical processes thought to be im-
portant to tropospheric O3 production
on 1000 km scales. The data base
employed for the first application of the
ROM was collected during the summers
of 1979 and 1980 in the Northeast U.S.
It contains meteorological and air qual-
ity data from regular monitoring net-
works and from enhanced networks or
special field project measurements
made during that period.
The evaluation procedure that will be
used to determine the ROM perform-
ance on this data base is outlined. A
number of episodes will be simulated
from the period July 23 through August
16,1980, for which performance sta-
tistics will be developed. The evaluation
of any given day within an episode will
proceed in two distinct stages. The first
state will focus on model performance
for an individual model realization,
irrespective of all other realizations.
Model realisations for a given day are
functions of the possible flow fields that
existed for the day. The second state
will attempt to evaluate model per-
formance using the full probabilistic
abilities of the ROM that consider all
realizations concurrently. The focus of
the evaluation will be on O3. The exact
pathway through the evaluation study
will be determined by the resources
available at the time.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search 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
The perception of air quality problems
has increased in recent years from an
urban scale to a larger regional scale as
the effects of multi-day and long-range
transport of air pollutants have been
studied and understood. During the past
five years the U.S. Environmental Protec-
tion Agency (EPA) has undertaken a
model development effort to produce an
air quality simulation model capable of
treating all of the chemical and physical
processes that are thought to affect the
concentrations of air pollutants over
several-day/1000 km scale domains. The
EPA Regional Oxidant Model (ROM) is
now operational. Among the processes it
treats are horizontal transport, atmos-
pheric chemistry, nighttime wind shear
and turbulence episodes associated with
the nocturnal jet, cumulus cloud effects
on vertical mass transport, mesoscale
vertical motions induced by terrain and
the large scale flow, diffusion and deposi-
tion, subgrid scale chemistry processes,
emissions of natural and anthropogenic
precursors, and wet and dry removal.
These processes are simulated in a three-
dimensional (3-D) Eulerian framework
with 3 1/2 vertical layers extending
through the boundary layer and the
capping inversion or cloud layer. In the
present configuration of the ROM domain,
horizontal resolution is approximately 20
km.
The data base that will be used in the
evaluation study is that of the Northeast
Corridor Regional Modeling Project
(NECRMP). The NECRMP ambient data
base consists of measurements made
during four EPA field measurement pro-
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grams in 1979 and 1980 in the north-
eastern U.S. These include the 1979
Northeast Regional Oxidant Study
(NEROS I), the 1980 NEROS II, the
Persistent Elevated Pollution Episodes
Study, and the 1980 Urban Field Studies.
Together, these studies have provided a
variety of air quality and meteorological
measurements on regional, urban, and
site-specific scales. Also, a parallel effort
within the NECRMP program assembled
a complete source emissions inventory
specifically addressing the ROM require-
ments. This emissions inventory will be
supplemented by updated emissions for
the U.S. and southern Canada contained
in the National Acid Precipitation Assess-
ment Program data base, and the revised
inventory will be used in the ROM evalu-
ation study.
The individual grid cells of the model
domain for the NECRMP application are
15 minutes of longitude wide in the E-W
direction and 10 minutes of latitude in the
N-S direction, or about 18.5 km2. There
are 60 cells E-W and42 cells N-S, giving a
total horizontal extent of approximately
1100 km by 780 km. The vertical structure
of the ROM consists of 3 1 /2 layers. The
bottom (1 /2) layer is actually a diagnostic
surface layer less than 100 m deep where
surface deposition and subgrid scale
chemical effects are modeled in a diag-
nostic manner. Layers 1 and 2 are
prognostic model layers and extend
through the depth of the well mixed layer
during the day and the surface inversion
and old mixed layer at night. The top
prognostic model layer extends up to 1 km
above the top of the mixed layer and
includes any convective cloud elements.
The ROM System
The ROM described here is the 2nd
generation version of the model. It is
designed to simulate air pollution chem-
istry and dispersion over a 1000-km by
1000-km area for multiple day periods.
The simulation of photochemical air
pollution over such scales is a complex
problem. The ROM reflects this complex-
ity, consisting of some 25 programs
accessing almost 100 files.
The input files contain the "raw" data
accessed by the modeling system. Actu-
ally, these data files are produced by a
combination of manual and computer
manipulation of raw data. However, the
input data files constitute a boundary
between the ROM and the rest of the
world, and therefore define the input data
for the model. The initial preprocessors
take the "raw" input data and transform
them for use by other preprocessors.
These data are transferred between pre-
processor programs through the proces-
sor input files (PIF). The preprocessor
programs are used to develop interme-
diate parameters that are then trans-
formed into final model inputs. Examples
of these intermediate parameters are the
wind fields, emissions, and turbulent
fluxes. The preprocessors transfer the
processed data through the PIF files and
the model input files (MIF).
The data contained in the MIF files are
converted into the form required by the
CORE model through the execution of the
b-matrix compiler (BMC) program. The
BMC translates the parameter fields in
the MIFfiles(layer thicknesses, horizontal
winds, interfacial fluxes, deposition ve-
locities, etc.) into the matrix and vector
elements necessary to operate the CORE
model. The BMC transfers the information
needed by the CORE model via the final
data input files.
The CORE model is the computer
language analogue of the differential
equations that describe the processes
involved in the chemistry and dispersion
of regional photochemical air pollution.
The CORE model is expressed in funda-
mental mathematical form. All inputs to
the model are matrices and vectors whose
elements are composites of meteorologi-
cal parameters, chemical rate constants,
etc.
The output of the CORE model consists
of the layer-averaged pollutant concen-
trations for every grid cell every 30
minutes. However, these data are not the
final model results. The ROM incorporates
two new concepts in air pollution model-
ing. The first is a method for simulating
the physical and chemical processes that
occur within about the first 100 meters of
the ground. The result of this scheme is
that both the ground-level concentration
and the root-mean-square concentration
variation within each selected cell are
produced. These data can be extracted
using the results of the CORE model
output (contained in the model output
files) and parameters contained in the
model input files.
The other concept is the incorporation
of the uncertainty of the wind field
representation. It is known that many
different wind fields can be constructed
"that match the observed wind data and
empirical and theoretical constraints. In
the ROM, a family of possible wind fields
is used in the model and leads to a result
of an ensemble of concentration fields.
Therefore, the model results will consist
of a distribution of concentration values
for each grid cell for each time step. The
implementation of the multiple wind field
concept requires that the model be run
many (~10) times for each simulation.
ROM postprocessor programs will be used
to transform the results of the individual
CORE outputs into ensemble concentra-
tion distributions. Other model postpro-
cessors will transform the layer 1 concen-
tration values into ground level (layer 0)
concentration distributions. These data
constitute the model results. Measures of
model accuracy will be made by compar-
ing the model predictions of concentration
probability distributions with observed
measurements, as well as examining
individual model runs (realizations) on a
single wind field.
Model Evaluation
The comprehensive evaluation of a 3-D
gridded air quality simulation model is a
complex task. At a basic level it is
necessary to test whether the mathe-
matical representations of the individual
physical and chemical processes are
correct. This is done for each component
process in the ROM. The validity of the
chemical kinetic mechanism, for example, i
may be tested with data from controlled '
smog chamber experiments. The vertical
cloud flux algorithm can be validated with
aircraft measurements of material below,
within, and above a convective-type cloud
from the NEROS field program. The wind
field algorithm cannot be precisely tested
in an independent fashion, although
individual trajectories can be evaluated
from tetroon data from the NEROS study,
and the individual wind realizations can
be analyzed for their consistency with
measurements from fixed monitoring
sites. The model evaluation outlined here
takes the model as a whole and attempts
to compare its predictions with ambient
observations.
This view would suggest that the ROM
could then be viewed as a "black box"
model producing results that could be
statistically analyzed against observa-
tions. Although it is tempting to take this
view, it must be avoided. The blind appli-
cation of statistical tests will not provide
sufficient understanding of model per-
formance to draw any meaningful con-
clusions from the study. The goal of a
model evaluation study, such as the one
proposed here, is to gain insight into the
model predictions and the observed data.
One wants to know if the model is
producing the right answer for the right
reason and whether the model prediction
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is good enough for the user's purposes.
The careful analysis of observational data
will provide the direction for a meaningful
path through the course of a model
evaluation. The proper choice of statistical
comparisons between observations and
predictions can be established from an
understanding of the phenomena shown
by the data sets. This is the approach that
we take in the ROM evaluation.
The approach will consist of three levels
of analysis. The first level will explore the
observed data set with diagnostic tests to
bring out the important features, both on
temporal and spatial scales, and to order
the data according to the features that are
found. Because the principal pollutant of
interest included in the ROM simulations
is 03, the analysis of observed data will
focus on it. The second level of analysis
will be an evaluation of the ROM results
for an individual realization of an episode.
This approach is a deterministic analysis
of each simulated realization without
regard to the other members in the family
of realizations. Finally, the third analysis
level will consider concurrently the entire
family of simulated realizations and the
resulting probability distributions of con-
centration. This analysis structure is
actually a hierarchy, with the results
obtained at each lower level guiding the
steps taken at the next higher level.
The ROM model evaluation must ac-
count for the stochastic nature of the
predicted concentration field. This is a
result of the multiple wind field realiza-
tions (interpolations) that the ROM wind
field processor generates from a given set
of wind observations. These multiple wind
fields are each consistent with both the
observations and physical laws governing
atmospheric flow and are assigned prob-
abilities of occurrence based on the
inherent kinetic energy contained within
the field. When all resulting wind fields
are considered, the model generates a
concentration probability distribution for
a given receptor site within the domain
instead of a single concentration value.
When all realizations are considered
concurrently, this aspect of the model
prediction makes the ROM different than
most other air quality simulation models
and presents more of a challenge to the
evaluation effort.
The first and second moments of the
predicted concentration distribution can
be used to determine the expected fre-
quency with which the observed concen-
trations should fall within a given interval.
The utility of the model for regulatory use
is measured partly by the width of this
interval. The second moment of the
distribution (the concentration variance)
is the parameter that defines this width. It
is a measure of the inherent uncertainty
of the model. If the width is very large the
model may provide no more information
than one would gather by guessing the
expected concentration. This is true even
if the model is shown to be accurate in
other respects. Therefore part of the ROM
evaluation should consist of an analysis
of the predicted concentration variances.
For regulatory use, the predicted concen-
tration for maximum ozone at a given
receptor location for a single realization is
also a parameter of interest. This value
may be compared to the corresponding
observed value. Also, if groups of recep-
tors can be identified having similar
predicted concentration distributions, the
observed distribution composed of the
measurements from each receptor with in
the group can be compared to the pre-
dicted distribution from the group. Such
groups might be stratified by receptor
location with respect to downwind dis-
tance from major source emissions areas.
Base level evaluation studies with the
ROM will be conducted for four or five
regional smog episodes from the data
base. The individual episodes range from
2 to 6 days in duration, and are mostly
from the 1980 summer period. A full 2-
week simulation will also be performed.
The EPA author K. L. Schere (also the EPA Project Officer, see below) is with the
Atmospheric Sciences Research Laboratory, Research Triangle Park, NC
27711. and A. J. Fabrick is with MEF Environmental Inc.. Austin, TX 78758.
The complete report, entitled "EPA Regional Oxidant Model: Description and
Evaluation Plan," (Order No. PB 86-103 090/AS; Cost: $11.95, 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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Environmental Protection
Agency
Center for Environmental Research
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