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United States
Environmental Protection
Agency
Environmental Sciences Research -""?,,
Laboratory "W\
Research Triangle Park NC 27711 ~/lj \
Research and Development
EPA-600/S3-82-043 Sept. 1982
Project Summary
Evaluation of Four
Urban-Scale Photochemical
Air Quality Simulation Models
Jack H. Shreffler and Kenneth L. Schere
This research was initiated with the
intent of determining the accuracy of
four photochemical air quality
simulation models using data from the
Regional Air Pollution Study in St.
Louis. The models evaluated in this
report are: The Photochemical Box
Model (PBM), The Lagrangian
Photochemical Model (LPM), The
Urban Airshed Model (UAM), and The
Livermore Regional Air Quality Model
(LIRAQ). Emphasis is directed at the
ability of the models to reproduce the
maximum 1-hour ozone concentra-
tions observed on 10 days selected
from nearly two years of data. The
PBM, LPM. and UAM have been
tested successfully and show
potential as air quality management
tools. LIRAQ does not show potential
as a model for general use,
irrespective of its accuracy, which
was impossible to judge at this time.
For the three other models, the
standard deviations of the differences
between observed ozone maxima and
predicted concentrations at the same
place and time tend to be large,
ranging 0.04 to 0.1 ppm for maxima
of 0.19 to 0.26 ppm. Possible
resolution of this high variability might
improve performance of the latter
three models.
This Project Summary was devel-
oped by EPA's Environmental Sci-
ences 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 Regional Air Pollution Study
(RAPS) was conducted in the St. Louis
region over the period 1974 to 1977.
RAPS was designed to provide a
comprehensive data set to test and
evaluate numerical air quality
simulation models for an urban area.
While the RAPS field measurements
were in progress, EPA surveyed the
available, state-of-the-art, photo-
chemical air quality simulations
models, and selected three for
evaluation. Since no existing model
embodied a simple box-model
approach, a box model was constructed
by EPA. The four models evaluated by
EPA and described herein are as
follows:
Photochemical Box Model (PBM) - a
single-cell Eulerian model constructed
by EPA.
Lagrangian Photochemical Model (LPM)
- a multi-level parcel model developed
by Environmental Research and
Technology, Inc.
Livermore Regional Air Quality Model -
(LIRAQ) a single-level Eulerian grid
model developed by Lawrence
Livermore Laboratory.
Urban Airshed Model (UAM) - a multi-
level, Eulerian grid model developed by
Systems Applications, Inc.
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The final report describes the results
of simulations for 10 days when
maximum measured 03 concentrations
were 0.19 ppm to 0.26 ppm in St. Louis.
Generally, these days exhibit stagnation
conditions with little cloud cover and
represent situations conducive to the
production of photochemical oxidant
from local emissions.
The models were tested extensively,
and obvious errors or deficiencies were
corrected. However, no effort was made
to adjust or tune the model predictions
to observed concentration values. The
approach to the evaluation was to use
off-the-shelf models, much as they
would be used in a regulatory situation.
Even so, great care was taken in
preparation of data sets and model
executions. Although model
assumptions varied, the authors were
careful to use data in similar manners in
all models. Data preparation and actual
execution of the models was
accomplished solely at EPA. The goal of
the evaluation was to provide a fair and
objective determination of the accuracy
of a set of photochemical models when
tested in an operational mode against a
comprehensive urban data base.
An integral part of RAPS, was
establishment of a network of 25
surface stations in and around St. Louis,
known as the Regional Air Monitoring
System (RAMS). The RAMS stations
continually monitores various meteoro-
logical variables as well as ambient
concentrations of pollutant gases,
providing observational data for com-
parison with model predictions.
Results and Discussion
The Photochemical Box Model (PBM),
a single-cell Eulerian air quality
simulation model, simulates the
transport and chemical transformation
of air pollutants in smog-prone urban
atmospheres. The model's domain is set
in a variable volume, well-mixed
reacting cell where the physical and
chemical processes responsible for the
generation of ozone (03) by its
hydrocarbon (HC) and oxides of nitrogen
(NOx) precursors are mathematically
created. To apply the model to the St.
Louis RAPS data base, the horizontal
scale of the single cell was 20 km and
the vertical scale was time-varying,
proportional to the depth of the mixed
layer. The model domain was centered
on downtown St. Louis in such a
manner that the 20 x 20 km area
encompassed most of the major
emissions sources on either side of the
Mississippi River . Uniform distribution
of source emissions was assumed
across the surface of the cell. Twelve of
the RAMS surface monitoring stations
were located within the cell's
boundaries.
The following statistics summarize
the differences between the observed
hourly maximum concentration of 03 for
each day and the PBM. prediction at the
same time. The concentrations are
averages over the PBM domain.
AC = Obs. - Pred (ppm)
AC = -0.033
s.d.(AC) = 0.041
|AC| = 0.039
The evaluation of the PBM is
continuing in EPA's Meteorology
Division. This analysis is the first major
step toward a thorough understanding
of model performance. Initial evidence
shows that the model is a useful tool in
assessing the urban air quality for
photochemically reactive pollutants,
especially in stagnation conditions. The
PBM is relatively simple to use and its
data requirements are far less stringent
than most other numerical air quality
simulation models. The areas of further
study that should be pursued include: (a)
the hysteresis problem during advection
conditions, (b) the relationship between
the average O3 concentration observed
within the model domain and the
maximum O3 level observed at a single
station, (c) the continued testing and
refinement of the chemical kinetic
mechanism within the PBM, and (d) the
sensitivity of the model to variations in
selected parameters such as initial and
uoundary concentrations, initial cell
depth, emissions, wind speed, and solar
radiation.
The Lagrangian Photochemical
Model (LPM) was developed by
Environmental Research and
Technology, Inc. and adapted for use
with the RAPS data base. (The LPM is
essentially identical to the general-use
model named ELSTAR). The LPM
considers a portion of the atmosphere
as an identifiable parcel which can be
tracked from early morning to late
afternoon. As the parcel moves over the
various emission sources, pollutants
are assimilated, vertically mixed, and
subjected to photochemical reactions in
the presence of solar radiation.
The following statistics summarize the
differences between the observed
hourly maximum 03 concentration and
the LPM prediction at the same time and
place.
AC = Obs - Pred (ppm)
A~C = -0.004
s.d.(AC) = 0.11
|AC| = 0.080
The LPM has shown promise as an
effective tool to understand 03
production in an urban region. The
model is relatively easy to use,
inexpensive to execute, and seems
immune to various execution errors
which tend to arise unexpectedly in
complex computations of this sort.
Areas of further research should incude
the following: (a) the importance of the
initial conditions versus the emissions
accumulated in the parcel, (b)
reasonable methods to include
horizontal diffusion, (c) the vertical
oiffusivity as it relates to unrealistic
build-up of pollutants at ground level.
The Livermore Regional Air Quality
Model (LIRAQ) was developed at
Lawrence Livermore Laboratory (LLL)
under funding of the National Science
Foundation. The model is an Eulerian
grid type and was specifically
constructed for use in the San Francisco
Bay area, where the Bay Area Quality
Management District (BAAQMD)
retains an interest in LIRAQ
applications. Reflecting the originally
intended region of study, the model has
provision for generating mass-
consistent wind fields in complex
terrain but allows only one grid cell
vertically. In 1975, EPA entered into an
Interagency Agreement (IAG) with LLL
to have LIRAQ adapted for use with the
RAPS data base in St. Louis. Because
LIRAQ uses special features of the CDC
7600 computing system, adapting it to
run on the EPA's UNIVAC 1110 was
deemed impractical. Therefore, LIRAQ
was transferred to the Lawrence
Berkely Laboratory (LBL) computing
facility where EPA could have direct
access to the model.
The effort to evaluate LIRAQ
performa nee was beset with difficulties,
and EPA is considering eliminating the
model from its evaluation program.
With limited testing, LIRAQ did not
produce significant concentrations of
ozone, suggesting errors in the
emission and/or chemistry module.
Presently, three days (195, 226, and |
275 of 1976) have been tested on the
LBL system by EPA. The Day 275
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simulation produced an error in LIRAQ
execution of unknown origin. On Days
195 and 226, little ozone was generated
by the model, and nitrogen oxides
seemed to totally dominate the
photochemical system. Even if current
problems are resolved, there are cost-
benefit considerations centered on the
adequacy of the model's theoretical
framework (single vertical cell), cost,
and inaccessibility for general use. The
LIRAQ conversion was initiated with the
intent of having a complete spectrum of
state-of-art models in existence in 1975
for testing by EPA. Reconsiderations of
its eventual utility have made LIRAQ
less than attractive for further research.
Since LIRAQ is tied to a specific
computer system, its prospects for use
in a general regulatory setting - the
ultimate goal of the model evaluation
program - are extremely dim.
The Urban Airshed Model (UAM) is a
three-dimensional (3-D) grid-type or
Eulerian, photochemical air quality
simulation model developed by
Systems Applications, Inc. (SAI) of San
Rafael, California. The structure of the
model consists of a lattice array of cells,
arranged so that the total volume
represents an urban domain and in
which the physical and chemical
processes responsible for photochem-
ical smog are mathematically
simulated. The horizontal dimensions of
each cell are constant but the heights of
the cells vary throughout a model
simulation according to changes in the
depth of the mixed layer. To apply the
model to the St. Louis RAPS data base,
the area modeled was 60 km wide and
80 km long. Each individual cell was 4
km on a horizontal side. Vertically, there
were four layers of cells in total; the
bottom two layers simulated the mixed
layer and the top two represented the
region immediately above the mixed
layer. The domain of the UAM was
centered just west of downtown St.
Louis and included the entire
metropolitan area.
The following statistics summarize
the differences between the observed
hourly maximum 03 concentration and
the UAM prediction at the same time
and place.
AC = Obs - Pred (ppm)
AC = -0.074
s.d.(AC) = 0.033
| AC) = 0.074
The evaluation of the UAM is
continuing at the Meteorology Division
of EPA, and additional test days may be
chosen to add to those already done.
From the work performed to this point, it
is clear that the potential use of a grid-
type model such as the UAM is great,
although the complexity of the model
often makes solving the problems which
arise more difficult. Areas of further
study that should be pursued include: (a)
the adequacy of the wind-field
methodology, and (b) the sensitivity of
the model to variations in selected
parameters such as initial and boundary
concentrations, emissions, wind fields,
and solar radiation.
Conclusions
The evaluations of urban photo-
chemical air quality simulation models
presented in this report represent the
first comprehensive effort to examine
model performance under controlled
conditions. Great care was taken in
establishing the same data base and
impartial running procedures for all
models. The following points briefly
summarize the general conclusions
emerging from the study.
1. The PBM model performs the best
in near-stagnation conditions.
2. The LPM fixed-box formulation for
the air parcel needs modification.
Unrealistically high ozone predic-
tions result under certain circum-
stances.
3. LIRAQ lacks potential as a model
for general-use for a variety of
reasons and might be phased out
of the evaluation program. No
decision was made on use by
specialized groups for specific
locales.
4. The UAM tends to consistently
underpredict ozone, the effect
possibly results from spurious
numerical diffusion.
5. For all models, the user should
have a strong scientific back-
ground and be extremely careful
in implementing air quality
simulations.
6. For all models, there is substantial
variabilty between specific 1 -hour
predicted and observed concen-
trations at a particular location.
Decision makers in regulatory
agencies should be cognizant of
this variability.
Using the results described in this
report and the possible model problems
they elucidate, a final investigation of
the structure of each model should be
carried out. If any further problems are
identified, with workable solutions, they
should be corrected. Model modifica-
tions should be based on sound
scientific judgements and not solely on
a desire to improve the results. A
decision should be made on whether to
pursue further development and testing
in the special case of LIRAQ. When
model changes are final, another set of
runs should be made and analyzed. Up
to 10 additional days from the RAPS
could be included in the final analyses.
TheEPA authorsJackH. Shreftier (also the EPA Project Officer, see below) and
Kenneth L. S chore are with the Environmental Sciences Research Laboratory,
Research Triangle Park. NC 27711.
The complete report, entitled "Evaluation of Four Urban-Scale Photochemical
Air Quality Simulation Models." (Order No. PB 82-239 278; Cost: $16.50,
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:
Environmental Sciences Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
t, US GOVERNMENT PRINTING OFFICE: 1M2 -559-017/0806
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