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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