United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
O,/',
/ i
Research and Development
EPA/600/S3-90/083 Jan. 1991
EPA Project Summary
Adaptation of the Advanced
Statistical Trajectory Regional Air
Pollution (ASTRAP) Model to the
EPA VAX Computer--
Modifications and Testing
Terry L. Clark and Dale H. Coventry
We modified the original 1985 IBM-
3033 version of the Advanced Statistical
Trajectory Regional Air Pollution
(ASTRAP) model to create the ASTRAP-
EPA version for applications on the EPA
VAX-8600 computer using existing EPA
preprocessed meteorological and emis-
sions data files. The cumulative effect of
the model modifications was assessed
by comparing the quarterly 1980 calcu-
lations of sulfur wet deposition of both
versions with screened measurements.
The seasonal correlation coefficients and
standard errors of each model version
are insignificantly different at the 0.05
level, demonstrating that the two model
versions indeed produce similar results.
In general, the Improvements in model
design only slightly enhance model per-
formance.
Sensitivity of ASTRAP-EPA calcula-
tions of sulfur wet deposition was also
assessed for several model assump-
tions and values of model parameters.
ASTRAP-EPA model predictions are
most sensitive to three parameters - the
model time step, the truncation of trajec-
tories near the border of wind-data-void
regions, and the temporal aggregation
of ensemble trajectory statistics. The
maximum quarterly predictions of sulfur
wet deposition, across southwestern
Pennsylvania and northern West Virginia,
decrease by as much as 30% when
either the model time step changes from
3 hours to 6 hours, or when trajectories
are not truncated, or when trajectory
statistics are not temporally aggregated.
This Project Summary was developed
byEPA's AtmosphericResearch andEx-
posure Assessment Laboratory , Re-
search 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 Advanced Statistical Regional Air
Pollution (ASTRAP) model developed by
the Argonne National Laboratory calculates
long-term mean air concentrations and
deposition amounts of oxides of sulfur and
nitrogen and quantifies the linear relation-
ships between emissions from virtual
sources and ambient concentrations at re-
ceptors across North America. Since the
model uses trajectory statistics to param-
eterize atmospheric transport and diffusion,
model applications are restricted to time
periods of a month or more. These long-
term source-receptor relationships have
been used by the Agency in identifying
prime emission control regions and assess-
ing emission reduction strategies for the
acidic precipitation program.
The 1985 version of the ASTRAP model
code was obtained and adapted to the EPA
VAX-8600 computer to improve our under-
standing of the model capabilities, to as-
sess the effects of modeling approaches
and assumptions on the results, and for
possible applications as a screening model
for regional sulfate and fine particle con-
centrations and regional visibility. During
this process, the code was modified to
create a model version named ASTRAP-
EPA, which uses existing EPA meteoro-
logical data files and provides the user the
Printed on Recycled Paper
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flexibility of redefining default model pa-
rameters.
In the final report, we: (1) summarize the
attributes and assumptions of the ASTRAP
model, (2) document the modifications to
adapt the model to the EPA VAX-8600
computer, (3) assess the effects of the
modifications by intercomparing 1980 quar-
terly predictions of both versions with
measurements, and (4) assess the model
sensitivity to input parameters and model
assumptions.
Attributes of the ASTRAP Model
The ASTRAP model simulates the trans-
port, dispersion, and deposition of oxides
of sulfur and nitrogen across North America
with a resolution of one-third that of the
National Meteorological Center (NMC) grid,
or 127 km at 60* N latitude. Since dry and
wet deposition rates and linear chemical
transformation rates are specified in the
model code, only aggregated emission
rates and gridded wind velocities and pre-
cipitation amounts are required for model
applications. Model output consists of long-
term mean air concentrations and deposi-
tion amounts at receptor points; linear
source-receptor relationships are quantified
via a postprocessor. Because of the statis-
tical nature of the model, applications of this
model are not recommended for periods
less than one month.
The modeling approach of ASTRAP is
unique. Rather than directly simulating
processes for the pollutants emitted from all
point and area sources in the modeling
domain, the model simply:
(1) Computes for each of 123 virtual
sources (at the center of groups of 9
cells) the coordinates of the en-
semble mean positions, standard
deviations, and spatial correlations of
6-h trajectory endpoints for puffs re-
leased at 6-h intervals and tracked for
periods of 7 days, and
(2) Constructs bivariant normal density
functions that describe the spatial dis-
tribution of mass for the ensemble of
equal-age trajectory endpoints.
Separate sets of statistics are compiled
for airborne pollutant plumes and wet depo-
sition events. With the transport and diffu-
sion statistics, the prescribed dry deposition
velocities, an emissions field, and the
scavenging algorithm, dry and wet deposi-
tion fields can also be generated. Using
these density functions greatly reduces
computational time, thereby making multi-
year applications of this model feasible,
since subprogram VERT must be applied
only once per season and subprogram
HORZ only once per season of each mod-
eled year. Further, for ascertaining the ef-
fects of emission control strategies, the
reapplicatbn of these two subprograms is
not necessary.
A potential limitation of the ASTRAP
model relates to its calculation of transport
near the model domain border and regions
where wind data are non-existent (e.g., the
Atlantic Ocean and Gulf of Mexico). Statis-
tics for ensemble trajectories can be biased
when some trajectory segments extend
beyond the region where wind data are
available. That is, trajectory segments with
endpoints east of the Atlantic Coast are not
considered in the statistical computations,
thereby placing more emphasis on those
remaining trajectory segments, which nor-
mally pertain to lighter winds. As a result,
the simulated transport of an ensemble of
puffs (especially for sources along the East
Coast) would be based on fewer meteoro-
logical cases; thus, a sudden change in the
ensemble trajectory direction and/or speed
would result. The model assumes that any
bias in the ensemble trajectory statistics
would not significantly effect the simula-
tions of air concentrations and wet and dry
deposition amounts.
Differences Between ASTRAP
and ASTRAP-EPA
In 1988, a copy of the 1985 IBM-3033
code of the ASTRAP model was modified
by EPA and renamed ASTRAP-EPA to ac-
complish the following four tasks:
(1) Adapt the code for applications on the
VAX-8600 computer.
(2) Adapt the model to accept EPA-pre-
processed meteorological and emis-
sions data files as input.
(3) Adjust some of the modeling ap-
proaches without substantially in-
creasing the computational require-
ments.
(4) Create a versatile model, one that
affords the less-experienced user
options for the values of such model
parameters as size and location of
modeling domain, model time steps
without necessitating code recom-
pilations.
The most obvious difference between
the two versions of the model is the model
grid configurations. ASTRAP uses two
fixed NMC grids (with a polar stereographic
projection aligned along 80° W longitude
and true at 60° N latitude): A 17-by-19 grid
with a spacing of approximately 380 km for
wind data and a 51 -by-45 grid with a spac-
ing one-third that of the first grid for pre-
cipitation and emissions data and model
calculations of concentrations and deposi-
tions. In contrast, ASTRAP-EPA uses a
single latitude-longitude grid for both input
data and model calculations and affords the
user the opportunity to easily redefine the
grid resolution and geographical domain in
subroutine PARAMETER statements. The
reason for converting to the latitude-longi-
tude grid, which has a resolution similar to
that of the 51-by-45 ASTRAP grid (i.e.,
approximately 120 km), was to accommo-
date the EPA emissions and meteorologi-
cal preprocessors.
ASTRAP was designed for use by the
original model developer with most param-
eterization rates and options internal to the
code. Therefore, an ASTRAP user is re-
quired to modify and recompile the model
code when a change in a single model
parameter is desired, rather than merely
redefining the value of a model parameter
read from an input file. ASTRAP-EPA, on
the other hand, provides the user the follow-
ing options:
(1) Read a preprocessed sulfate emis-
sions grid or default to the original
ASTRAP fixed percentages of SO2
emission rates.
(2) Select a subset of the emissions grid
for any model application, or, as be-
fore, use the emissions data for all
virtual sources.
(3) Redefine many preprocessor and
model parameters, such as the di-
mensions of the model domain, grid
resolution, and model time step, with-
out necessitating source code
recompilations.
(4) Apply the model for multiple months
or seasons or, as before, apply the
model for only one month or season
per model execution.
(5) Specify any set of weights for use in
computing a puff transport vector
from boundary-layer-wind-profile data.
(6) Construct source-receptor relation-
ships or just grids of mean air concen-
trations and deposition amounts.
Intercomparisons of
Predictions and Model
Performances
For each quarter of 1980, ASTRAP and
ASTRAP-EPA predictions of sulfur wet
deposition were intercompared at 32 to 37
sites across eastern North America. Based
on the locations and magnitudes of the
maximum quarterly predictions, the two
model versions produce similar results.
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Both model versions anchor the maximum
quarterly predictions of sulfur wet deposi-
tion across the Upper Ohio River Valley
(i.e., northern West Virginia, eastern Ohio,
western Pennsylvania, and southwestern
New York). Further, the magnitudes of the
greatest quarterly predictions are very simi-
lar:
4 to 5 kg S/ha in the Winter quarter,
5 to 6 kg S/ha in the Spring quarter,
5 to 9 kg S/ha in the Summer quarter,
and
2 to 4 kg S/ha in the Autumn quarter.
However, the quarterly mean predic-
tions for ASTRAP-EPA generally are lower
than those for ASTRAP; 22% to 36% less
for all quarters except the Winter quarter
when the ASTRAP-EPA mean is 13%
greater. Because of the relatively few sites
across the large modeling domain, we are
not able to determine whether these dis-
similarities are significant; that is, given the
spacing between sites, a slight shift in the
quarterly ASTRAP patterns of the predictions
could account for much of the differences.
Therefore, based on this aspect of the com-
parison, one cannot state that the two model
versions vastly differ.
The purpose of the intercomparison of
model performances is to determine
whether the two model versions are sub-
stantially different from each other. The
correlations of both sets of predictions with
measurements are similar in that the cor-
relation coefficients do not significantly dif-
fer from each other at the 0.05 level of
statistical significance (Table 1). Further,
the correlations for both sets show the best
agreement with the measurements in the
Spring and Summer quarters, when 25% to
45% of the variance is explained, and the
worst for the other two quarters when only
5% to 15% of the variance is explained.
The S.E.'s are virtually identical for both
the Spring and the Summer quarters; a
slight improvement is noted in the ASTRAP-
EPA performance for these two quarters. In
contrast, the S.E.'s for the other two quar-
ters differ by 40% from those for ASTRAP-
EPA, exceeding those for ASTRAP in the
Winter quarter and vice versa for the Au-
tumn quarter. However, S.E.'s for each
quarter do not significantly differ from each
other at the 0.01 level.
ASTRAP-EPA Sensitivity
Analyses
The sensitivity of ASTRAP-EPA calcula-
tions of sulfur wet deposition to some of the
model parameters and assumptions of both
ASTRAP and ASTRAP-EPA (Table 2) are
assessed in the final report. Rather than
assessing model sensitivity at each of the
nearly 2,300 receptor cells, calculations are
only intercompared at receptor cells abng a
2,000-km band stretching from central Ala-
bama to southern Quebec.
Temporally aggregating the ensemble
trajectory statistics tends to decrease the
sulfur wet deposition predictions across the
two high emission regions - by -40% to
-48% near the Alabama-Georgia border
and -14% to -35% near the Pennsylvania-
West Virginia border, depending on the
season. However, elsewhere predictions
are virtually identical. Thus, the aggregation
appears to smooth the gradients of sulfur
wet deposition near regions of significant
emissions.
With few exceptions, the doubling of the
time step decreases the sulfur wet deposi-
tion predictions. The most significant
changes occur near the two high emissions
regions; as great as -26% along the Ala-
bama-Georgia segment and -31 % near the
Pennsylvania-West Virginia segment, de-
pending on the season. Across the New
York segment, the summer predictions ac-
tually increase slightly (by less than 4%),
while for the other quarters, the predictions
decrease by as much as 18%.
Table 1. Comparison of Performance Measures of Both Model Versions for the Prediction of 1980
Quarterly Sulfur Wet Deposition at the ISDME Sites
Quarter
Winter
Spring
Summer
Autumn
Number
of sites
32
37
37
35
Correlation
ASTRAP
0.38
0.52
0.67
0.38
Coefficient
ASTRAP
-EPA
0.24
0.61
0.61
0.38
Standard Error
ASTRAP
1.24
1.16
1.73
1.26
ASTRAP
-EPA
1.77
1.11
1.63
0.76
%
Difference
+42
-4
-6
-40
Table 2. Parameter Values and Assumptions
Parameter/Assumptions
That Were Modified for the Sensitivity Analyses
Parameter values
Temporal aggregation for
ensemble trajectory statistics
Model time step
Weighting scheme for
mass transport vector*
Trajectory truncations at
borders of data-void regions
Initial vertical distribution
of emissions
Number of virtual sources
Primary sulfate emissions
Original"
None
3 hours *
6 hours
(1L+1U)/2b
(2L+1U)/3
(1L + 2UJ/3
1L
1U
Truncation"
Extrapolation
Low Layer Only
Upper Layer Only
Mid and Upper Layersb
All 3 Layers
443b
117
5% of SO.
Actual"
• For this investigation, the transport vector is either a weighted average of the surface (the low-level,
L) and the 850- mb (the upper-level, U) wind velocities or is identical to one or the other.
• ASTRAP-EPA base-case conditions.
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ASTRAP computes its transport vectors
using equally weighted means of the lowest
2 (Autumn and Winter), or 3 (Spring and
Summer), 500-meter-layer wind velocities.
Since it preprocesses wind data differently,
the ASTRAP-EPA transport vectors were
computed via a different method-weighted
means of the wind velocities at the surface
and 850-mb level (the latter approximating
1500 meters). This analysis explored the
effects on the winter calculations from the
five sets of weights in Table 2, ranging from
one extreme (the surface wind vector, 1L) to
the other (the 850-mb wind vector, 1U). For
this series of applications, the area and
point source emissions were injected into
layers 3 and 5, respectively (200-300 m and
400-600 m, respectively).
Compared to the base case predictions,
ASTRAP-EPA predictions south of New
York increase with increasing weights of the
surface wind velocities and decrease with
increasing weights of the 850-mb wind ve-
locities. Forthe extreme cases, increases of
80% across Alabama and decreases of
40% across Georgia occur for the 1L and
1U cases, respectively. This relationship
reflects the lower transport speeds at the
surface, compared to those at the 850-mb
level, and the resultant extension of the
residence time of the pollutant mass over
each receptor point. In contrast, the predic-
tions north of Pennsylvania are less than
those of the base case for all four alterna-
tive sets of weights, by as much as 27% for
the low-level wind vector (1 L). Also across
this region, predictions decreased less than
10% for the other sets of weights. It appears
that for the 1L and 2L1U cases, the addi-
tional sulfur removal across West Virginia
and Pennsylvania makes less mass avail-
able for wet deposition along the northeast-
ern segment of the band. Consequently, for
these two cases, the spatial gradient is
markedly enhanced along the West Vir-
ginia-New York segment.
As expected, when trajectories are ex-
trapolated, sulfur wet deposition predictions
indeed decrease everywhere along the band
from 8% to 23%, averaging 16%. The larg-
est decreases occur across Quebec and
northern New York. When summer trajecto-
ries are extrapolated rather than truncated,
the predictions tend to decrease by as
much as 20% across the Southeast and
Quebec, but actually increase by as much
as 22% across West Virginia The increase
could be caused by returning trajectories
from the Atlantic Ocean. Therefore, when
trajectories are truncated, as they are in
ASTRAP, the predictions of sulfur wet
deposition generally would be biased on the
high side. The magnitude of this bias is
expected to be highest for those receptor
cells along the Atlantic coast with signifi-
cant sulfur sources (e.g., New York City).
The sensitivity analysis of the initial ver-
tical distribution of emissions shows the
greatest differences from the base case
predictions result when all the emissions
are injected into Layer 5. For this initial
distribution, predictions increase virtually
everywhere along the band by 15% to 25%
in summer and 5% to 10% in winter. The
increases resulting from the even initial
distribution are half that of the Layer 5
predictions. Predictions decrease when the
emissions are injected into Layer 1 (10% to
15% in summer and 5% to 10% in winter).
For all model applications, the predictions
are most sensitive across New York and
Quebec.
The sensitivity of ASTRAP-EPA to the
manner in which sulfate emissions are de-
fined is slight. Along the entire band, the
predictions from the ASTRAP approach are
less than those of ASTRAP-EPA by no
more than 5%.
Conclusions and
Recommendations
During the process of adapting the
ASTRAP modeltothe EPA VAX-8600 com-
puter, the model design was modified to
utilize existing EPA meteorological and
emission files and to provide the user more
flexibility in changing parameterization rates.
As a means of assessing the cumulative
effects of these changes on model predic-
tions, the sulfur wet deposition calculations
from both model versions were compared to
measurements. Although a slight improve-
ment in model performance is observed in
this performance assessment, the differ-
ences in the correlation coefficients and
standard errors are not statistically signifi-
cant at the 0.05 level. That is, based on the
model performance assessments, the
ASTRAP-EPA results are very similar to
those of ASTRAP.
The ASTRAP-EPA sensitivity analyses
demonstrate that the predictions of sulfur
wet deposition along the band of receptors
stretching from Alabama to Quebec are
sensitive to several user-specified param-
eters. When only one parameter is
changed for each model application, some
quarterly predictions of sulfur wet deposi-
tion decrease from base case values by as
much as 30% when either the model time
step is doubled from 3 hours, or when the
trajectory statistics are not temporally ag-
gregated, or when trajectories are not
truncated at the borders of wind-data-void
regions. The most significant decreases
along the band typically occur in the
regions where annual sulfur wet deposition
is relatively high (i.e., across Alabama,
western Pennsylvania and West Virginia).
One should apply this model with caution
until we can recommend values of these
and other model parameters after ASTRAP-
EPA is evaluated in 1992 with the ACID-
MODES data base. The 1988 and 1989
ACID-MODES data will, for the first time,
enable us to assess the performance of
regional air pollution models for more than
one modeled parameter, providing a more
rigorous evaluation and a better under-
standing of model behavior. Therefore, we
recommend that the performance of
ASTRAP-EPA be assessed using all perti-
nent data from this field study. Although the
relationships between sulfur emissions and
regional visibility are not well defined, sta-
tistical comparisons of modeled and ob-
served spatial patterns of sulfate air con-
centrations will provide insight to the pre-
dictive capability of the model.
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Terry L Clark, also the EPA Project Officer (see below), and Dale H. Coventry are
with the National Oceanic and Atmospheric Administration of the U.S. Department of
Commerce on assignment to the Atmospheric Research and Exposure Assessment
Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled 'Adaptation of the Advanced Statistical Trajectory
Regional Air Pollution (ASTRAP) Model to the EPA VAX Computer - Modifications
and Testing," (Order No. PB91-127 720/AS; Cost: $15.00, 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 Research and Exposure Assessment 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/S3-90/083
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