United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/024 May 1986
4>EPA Project Summary
Preliminary Evaluation Studies
With the Regional Acid
Deposition Model (RADM)
This summary focuses on the find-
ings of preliminary evaluation studies
of the National Center for Atmospheric
Research (NCAR) Regional Acid Deposi-
tion Model (RADM). Current efforts in
evaluating the gas-phase chemistry
submodel have met with significant
success, and an understanding of the
smog chamber databases has been de-
veloped. Due to lack of adequate obser-
vation data, evaluation of aqueous-
phase chemistry and dry deposition
submodels has not been possible. The
cloud process submodel is consistent
with the available but small data set.
Preliminary evaluation of the full RADM
system using OSCAR (Oxidation and
Scavenging Characteristics of April
Rains) meteorology and wet chemical
deposition data has achieved initial
success, though it is far from conclu-
sive. Much larger and extensive data-
bases are required to test the system
thoroughly. The meteorology driver for
the RADM has shown considerable skill
in forecasting the OSCAR IV meteorol-
ogy. Simulations of sulfate and nitrate
wet deposition for the first day of
OSCAR IV are quite good. RADM-
simulated three-dimensional chemical
species distributions are consistent
with preliminary data measured by
NCAR under a separate program. The
first test of RADM's capability for ana-
lyzing "what if studies with hypotheti-
cal reductions in sulfur emissions is
demonstrated. The findings confirm the
complexity of directly observing poten-
tial benefit from emissions reductions.
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
A comprehensive Regional Acid Dep-
osition Model (RADM) is being devel-
oped at the National Center for Atmos-
pheric Research. The RADM is an
evolving advanced Eulerian computer
model simulating the processes and
pathways related to acid deposition in
eastern North America. The RADM sys-
tem consists of a mesoscale meteoro-
logical model which drives a transport/
deposition model containing modular
descriptions of the gas-phase chem-
istry, cloud processes and aqueous-
phase chemistry, and dry deposition.
The objectives for the RADM are
threefold: (1) to result in a state-of-the-
art modeling system suitable for con-
ducting source-receptor assessment
studies, (2) to be sufficiently flexible to
integrate current and developing repre-
sentations of the relevant physical and
chemical processes, and (3) to describe
the spatial and temporal distributions of
pollutants resulting from known source
emissions. Progress toward these ob-
jectives has been very favorable. A first
version of the RADM was completed in
January 1985 and since that time has
undergone numerous sensitivity stud-
ies and preliminary evaluation against
existing field and laboratory data. A
summary of the preliminary evaluation
of the RADM and of sensitivity and ap-
plication studies is presented here. The
evaluation of the RADM submodels are
discussed first. Next, 72-h RADM simu-
lations of two OSCAR (Oxidation and
Scavenging Characteristics of April
Rains field study, conducted in the
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northeastern United States in April
1981) cases are compared to measured
precipitation chemistry data. For one of
these cases, a preliminary evaluation of
the meterological model is presented.
Emissions sensitivity studies and emis-
sions reduction experiments are also
presented for one of the OSCAR cases.
Finally, databases available for evalua-
tion of the RADM and their limitations
are discussed.
RADM Submodel Evaluation
The RADM version I gas-phase chem-
istry mechanism has been compared
with both smog chamber data and more
complex chemical mechanisms. In gen-
eral, these tests show that the RADM
mechanism gives a good representa-
tion of those aspects of tropospheric
chemistry necessary for the modeling
of acid deposition. Initial tests were
made with data obtained from the
Satewide Air Pollution Research Center
(SAPRC), and agreement between
RADM simulations and experimental
values is generally very good for NO2,
NO, and 03. The agreement between
the predicted and experimental values
for all the reactive species is excellent.
Since reactions with HO radicals is the
dominant loss process for reactive or-
ganics during photooxidation, the
agreement between the simulation and
measured concentrations of reactive or-
ganic species shows that the RADM
mechanism is correctly predicting HO
concentrations within the uncertainty
imposed by wall sources.
Comparison of the RADM gas-phase
chemistry predictions with the Univer-
sity of North Carolina smog chamber
data is only in the beginning stages.
Procedures used to establish the error
bounds of these data, along with com-
parisons with RADM predictions for a
single case, are reported. In general,
RADM predictions for O3, NO, and PAM
are in good agreement with the
chamber data, considering the uncer-
tainties of the J-values. The simulation
also suggests that for definitive tests of
the chemical mechanisms for the pro-
duction of H202 in the atmosphere, the
photolytic rate constant must be accu-
rately known.
The RADM gas-phase chemical mech-
anism was compared to the explicit
mechanism of Leone and Seinfield and
the carbon bond mechanism, version
CBM-X, for conditions ranging from
very clean to highly polluted. Both of
these latter mechanisms were modified
to include sulfur chemistry. Oxidation
rates for S02 and NOX are in reasonable
agreement by the three mechanisms,
while large differences exist for the total
H202 produced. A sensitivity analysis
was performed to determine reasons
for the differences in H2O2.
The RADM cloud processes and
aqueous-phase chemistry module was
subjectively evaluated against limited
field data and more complex models.
Comparisons with OSCAR aircraft ob-
servations of cloudwater composition
suggest that the RADM submodel is ca-
pable of inferring cloudwater pH over
the range of initial conditions encoun-
tered during the observation period.
RADM predictions of aqueous oxidation
within clouds are shown to agree rea-
sonably with the difference in the ratio
of S02 to total sulfur at the base and top
of typical cumulus clouds. Scavenging
ratios predicted by the RADM are
shown to lie between the extremes ob-
served in field observations. In addition
to predicting cloud and rainwater com-
position, the RADM cloud processes
model also predicts the rate at which
pollutants are vertically transported by
sub-grid scale cumulus clouds. RADM
parameterization for this process was
compared to a high resolution cloud
model, and both predicted similar sig-
nificant depletion of a passive tracer
from the boundary layer and "venting"
of this material to the cloud layer.
The method used by the RADM to
model dry deposition assumes a series
of three resistances (aerodynamic, sub-
layer, and surface) and is based on a
highly empirical parameterization, rely-
ing heavily on a relatively sparse data-
base of dry deposition measurements.
Therefore, it was not possible to evalu-
ate independently the performance of
the dry deposition model since all avail-
able measurements of dry deposition
have been used to tune the current
model parameterizations. Data needs
for comprehensive dry deposition
model evaluation include heat and mo-
mentum fluxes in the surface layer,
wind speed, isolation, land type (onto
which substances are deposited), sur-
face roughness, and a quantification of
surface moisture.
Preliminary Evaluation of
RADM with OSCAR Database
The overall performance of the mete-
orological model and RADM in predict-
ing the meteorological processes and
wet concentration/deposition during
the OSCAR IV period (April 22-24,1981)
was examined. A number of quantita-
tive measures of the accuracy of the
mesoscale meteorological model are
described. These measures will be usec
to compare the accuracy of different
versions of the mesoscale model and tc
provide quantitative estimates of the
error in the meteorological data sup
plied to the RADM. Interpretation of the
verification scores and methods from
the OSCAR IV case are provided.
Correlations between RADM model
results and OSCAR measurements oi
precipitation chemistry variables are
presented for two OSCAR cases for both
hourly and event totals of concentra-
tions and deposition. Parameters com
pared were sulfate, nitrate, ammonium
and hydrogen ions in rainwater. In al
cases, the correlations were much
higher for the total event comparisor
and for concentrations. Correlations
were also higher for the OSCAR IV case
than OSCAR I.
RADM Sensitivity Studies and
Applications
Ultimately, RADM will be used to
evaluate source-receptor relationships
and the effects of changes in emissions
on downwind receptors. This requires
an understanding of the basis of the pat-
terns of concentration and deposition
forecast by the RADM along with sensi-
tivity of the forecasts to various input
parameters and physical parameteriza-
tions. Toward this end, wet and dry dep-
osition patterns for the OSCAR I and IV
cases were analyzed. Results are alsc
presented for RADM simulations of twc
"what if" scenarios of 50% and 90°/<
source reduction in the Ohio Valley re
gion.
The patterns and magnitudes of dn
deposition of S02 over the northeasterr
United States and southeastern Canada
integrated over the three-day episode:
were similar for both the OSCAR I anc
IV cases. In Canada, peaks showed uf
near Sudbury and Noranda. In the
United States, the peaks showed UF
over south-central Pennsylvania; at th(
conjunction of Ohio, Pennsylvania, anc
West Virginia; and in New Jersey, f
maximum dry deposition of 1.14 kg/hi
after three days occurred over south
central Illinois during OSCAR IV. Fo
HNO3, the accumulated dry depositor
for the two three-day episodes wen
again quite similar, and mass deposi
tion was considerably less than S02. /
maximum deposition of 0.24 kg/ha oc
curred from northern New Jersey t<
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southern New Hampshire. The accumu-
lated wet deposition differed more be-
tween the two experiments than did dry
deposition. This reflects primarily the
difference in rainfall patterns for the two
episodes. The percentage of total sulfur
deposition that was wet was about 50%
and 60% fo the OSCAR I and IV cases,
respectively. The percentage of nitro-
gen deposition that was wet was 45%
and 60% for OSCAR I and IV, respec-
tively.
Several sensitivity tests were run in
which the SOX emissions were reduced
by 50% and 90% in the Ohio Valley
(states of Indiana, Ohio, Kentucky, Ten-
nessee, and West Virginia). In response
to this regional reduction in emissions,
the maximum regional reduction for dry
and wet deposition occurred in the re-
duced emissions area, but only weakly
in more distant receptor states. The de-
crease in dry deposition was greater
than the decrease in wet deposition
only in the four states with decreased
emissions. The decrease in sulfur emis-
sions had very little effect on nitrogen
chemistry or nitrogen deposition.
Databases for RADM Evalua-
tion Studies
The evaluation of RADM performance
requires the assessment of the data-
bases used for model execution and
evaluation. Included in the report are a
description and evaluation of the avail-
able air quality databases for evaluating
regional models, a description of qual-
ity control procedures employed in
preparing the emissions databases
(EPRI and NAPAP) for model applica-
tions, and the precipitation database
used in verification of the precipitation
predictions. Also discussed are the limi-
ations of smog chamber data in evaluat-
ing RADM chemistry.
The National Center for Atmospheric Research is located in Boulder, CO 80307.
John F. Clarke is the EPA Project Officer (see below).
The complete report, entitled "Preliminary Evaluation Studies with the Regional
A cid Deposition Model (RA DM)," (Order No. PB 86-175 692/A S; Cost: $22.95,
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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