United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park, NC 27711^
Research and Development
EPA/600/S3-88/008 Apr. 1988
Project Summary
Rocky Mountain Acid Deposition
Model Assessment:
Evaluation of Mesoscale Acid
Deposition Models for Use in
Complex Terrain
R. E. Morris, R. C. Kessler, S. G. Douglas, and K. R. Styles
The hybrid acid deposition/air
quality modeling system for the
Rocky Mountains makes use of a
mesoscale meteorological model,
which includes a new diagnostic
wind model as a driver for a
Lagrangian puff model that treats
transport, dispersion, chemical
transformation, and dry and wet
deposition. Transport will be defined
from the diagnostic wind model
based on the wind at the puff center.
The treatment of dispersion will be
based on the parameterization in the
PNL/MELSAR-POLUT, while retaining
the MESOPUFF-II dispersion
algorithms as an option. Based on
the evaluation of the chemical
mechanisms, the RIVAD chemistry
appears to be the most scientifically
sound as well as consistent with the
Lagrangian puff model formulation.
Treatment of dry deposition will use
the CCADM dry deposition module
with some minor adjustments. Wet
deposition will be based on the
scavenging coefficient approach, as
used in the ERT/MESOPUFF-II.
This modeling approach was
guided by the comments of members
of the Western Acid Deposition Task
Force (WADTF) given in a question-
naire mailed in August 1986 and at a
meeting in May 1987 in Denver. The
modeling approach recommended by
members of the WADTF was use of a
Lagrangian acid deposition model
with a complex-terrain wind model
to calculate long-term source-
specific depos-ition of nitrogen and
sulfur. This modeling approach had
to be cost effective, simple enough
for use by the regulatory agencies,
and similar to models approved by
the EPA for impact assessment. If
possible, it was desirable that the
model have the ability to calculate
PSD increment consumption of SO2
and TSP sources. The hybrid
modeling system meets these
requirements in the most technically
rigorous manner possible, subject to
the cost and complexity constraints.
The modeling approach is not as
comprehensive as the Eulerlan model
development effort (RADM) currently
being carried out by the National
Center for Atmospheric Research
and State University of New Vork at
Albany. However, this approach is
more technically rigorous than those
currently used by regulatory
agencies, and will generate more
defensible estimates of incremental
impacts of acid deposition and
concentrations in regions of complex
terrain in the Rocky Mountains.
This Project Summary was
developed by EPA's Atmospheric
Sciences 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
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Project Report ordering information at
back).
Introduction
Acid deposition has recently become
an increasing concern in the western
United States. Although this problem
may not be as acute in the western U.S.
as it is in the eastern U.S., it is currently
a concern of the public and regulatory
agencies because of the high sensitivity
of western lakes at high altitudes and the
rapid industrial growth expected to occur
in certain areas of the West. An example
of such an area is the region known as
the Overthrust Belt in southwestern
Wyoming. Several planned energy-
related projects, including natural gas
sweetening plants and coal-fired power
plants, may considerably increase
emissions of acid precursors in
northeastern Utah and northwestern
Colorado and significantly affect
ecosystems in the sensitive Rocky
Mountain areas.
Under the 1977 Clean Air Act, the
U.S. Environmental Protection Agency
(EPA), along with other federal and state
agencies, is mandated to preserve and
protect air quality throughout the country.
As part of the Prevention of Significant
Deterioration (PSD) permitting pro-
cesses, federal and state agencies are
required to evaluate potential impacts of
new emission sources. In particular,
Section 165 of the Clean Air Act
stipulates that, except in specially
regulated instances, PSD increments
shall not be exceeded and air quality-
related values (AQRV's) shall not be
adversely affected. Air-quality-related
concerns range from near-source
plume blight to regional-scale acid
deposition problems. By law, the Federal
Land Manager of Class I areas has a
responsibility to protect air-quality-
ralated values within those areas. New
source permits cannot be issued by the
EPA or the states when the Federal
Manager concludes that adverse impacts
on air quality or air-quality-related
values will occur. EPA Region VIII
contains some 40 Class I areas in the
West, including two Indian reservations.
Similar designation is being considered
for several of the remaining 26 Indian
reservations in the region. State and
federal agencies, industries, and
environmental groups in the West need
accurate data concerning western
source-receptor relationships.
To address this problem, EPA Region
VIII needs to designate an air quality
model to estimate mesoscale pollutant
transport and deposition over the
complex terrain of the Rocky Mountain
region for transport distances ranging
from several kilometers to several
hundred kilometers. The EPA recognizes
the uncertainties and limitations of
currently available air quality models and
the need for continued research and
development of air quality models
applicable over regions of complex
terrain.
The primary objective of the Rocky
Mountain Acid Deposition Model
Assessment project is to assemble a
mesoscale air quality model based
primarily on models or model
components currently available for use
by federal and state agencies in the
Rocky Mountain region. To develop
criteria for model selection and
evaluation, the EPA formed an
atmospheric processes subgroup of the
Western Atmospheric Deposition Task
Force, referred to as WADTF/AP. This
group comprises representatives from
the National Park Service, U.S. Forest
Service, EPA Region VIII, the National
Oceanic and Atmospheric Administration,
and other federal, state, and private
organizations. The design of this new
model was based on the comments from
the ADTF, who desired a cost-effective
Lagrangian model capable of calculating
incremental, long-term acid deposition
and short-term concentration impacts
over mesoscale distances in complex
terrain.
A mathematical modeling system for
describing the various physical and
chemical processes associated with acid
deposition and air quality must consist of
several modules. These modules
describe such processes as wind
transport, dispersion, plume rise,
chemical transformation, and wet and dry
deposition. Although the modeling
system must be an integrated, internally
consistent package, it can be
conveniently divided into two distinct
principal parts:
Simulation of meteorological pro-
cesses
Simulation of pollutant transport,
dispersion, chemical transformation,
and deposition.
Procedure
Four mesoscale meteorological and
acid deposition models were selected for
possible use in constructing the new
hybrid acid deposition/air quality
modeling system for the Rocky Mountain
region. The candidate mesoscale
meteorological models were the
find
'1
IE!
California Institute of Technology Wins
Model (CIT/WINDMOD), the Pacif
Northwest Laboratory MELSAR-ME
model (PNL/MELSAR-MET), the Los
Alamos National Laboratory ATMOS1
model (LANL/ATMOS1), and the
Systems Applications, Inc. Complex
Terrain Wind Model (SAI/CTWM). The
candidate acid deposition models were
the Environmental Research and
Technology MESOPUFF-II (ERT/
MESOPUFF-II), the Pacific Northwest
Laboratory MELSAR-POLUT model
(PNL/MELSAR-POLUT), the Systems
Application, Inc. Regional Impact on
Visibility and Deposition model
(SAI/RIVAD), and the Systems
Application, Inc. Comprehensive
Chemistry and Acid Deposition Model
(SAI/CCADM).
The candidate models were evaluated
to determine which models best describe
the complex processes that lead to acid
deposition and air quality impacts in the
complex terrain region of the Rocky
Mountains and yet are consistent with the
modeling approach desired by the
potential users. The potential users
requested a modeling approach that uses
a diagnostic wind model as a driver for a
Lagrangian acid deposition model. The
resultant hybrid modeling system must
be computationally efficient so th^f
annual acid deposition impacts can be
easily obtained and run on smaller
computer systems.
Evaluation of the candidate wind
models consisted of separate simulations
using an idealized terrain obstacle (a
bell-shaped mountain) and terrain from
the Rocky Mountains. Based on these
results a new diagnostic wind model (the
DWM) was developed and further
evaluated using the same tests as the
candidate wind models and then
comparing the results generated by the
DWM with observations from the Rocky
Mountains. The flexibility and adaptability
of the new DWM was further evaluated
by separate simulations in a complex
terrain/coastal environment and within a
large valley.
The evaluation of the candidate acid
deposition models was accomplished by
comparing how each of the candidate
models treats the major processes that
lead to acid deposition; transport,
dispersion, chemical transformation, and
dry and wet deposition. Based on this
evaluation a new hybrid Lagrangian acid
deposition model was constructed using
the most technically rigorous com-
ponents that were internally consistent
with the over all framework of the hybr^j
modeling system.
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'esults and Discussion
Evaluation of the Mesosca/e
Meteorological Models
As an initial test, the four candidate
mesoscale meteorological models were
exercised using a three-dimensional
bell-shaped mountain at a scale
typically found in the Rocky Mountains
and a complex terrain region in the
Rocky Mountains using an initial uniform
flow field. The results indicated that
although the CIT wind model can treat
the kinematic effects of terrain, it lacks
Froude number flow adjustments
(dividing streamline concept) and thus
cannot simulate blocking effects if they
are not defined by the input data. The
CIT wind fields were minimally perturbed
by the terrain.
The MELSAR-MET model was
specifically designed to simulate
blocking and deflection of air flows
typically found in the Rocky Mountains
under weak synoptic conditions.
However, due to the model's unique
interpolation scheme used to define
gridded wind fields, spurious results are
produced near the boundaries of the
modeling domain. The MELSAR-MET
wind fields also were not greatly
perturbed from the initial uniform flow but
did exhibit more terrain effects than the
CIT model. The ATMOS1 model lacks a
Froude number adjustment term to treat
blocking and deflection but can provide a
gross simulation of blocking through a
region-wide stability dependent input
parameter. The ATMOS1 model
exhibited a large deflection of its air flows
due to the terrain. The CTWM alone of
the candidate meteorological models is
designed to generate wind fields using
only a domain mean wind as input. It is
also the only model that can simulate
upslope and downslope thermally
generated flows. However, the CTWM is
also the only candidate model that is
formulated in a Cartesian coordinate
system.
Use of a Cartesian coordinate system
to simulate air flows in complex terrain is
undesirable because air flows tend to
follow terrain and increased vertical
resolution is needed near the surface to
resolve the terrain features. The
problems with converting the CTWM to a
terrain-following coordinate system
were sufficient to eliminate the model
from further consideration as a
candidate. A comparison of the
computation time required for the
idealized test showed that the
MELSAR-MET required the least
computer time of the candidate models.
The CIT wind model, the CTWM model,
and the ATMOS1 model took ap-
proximately 4, 6, and 7 times the com-
puter time that MELSAR-MET required.
Design of a Mesoscale
Meteorological Model for the
Rocky Mountains
The evaluation of the candidate
mesoscale meteorological models
indicated that no one of the candidate
models was significantly superior over
the others. Thus it was decided to
construct a new diagnostic wind model
(the DWM) using the best components
from the candidate meteorological
models. This wind model would utilize all
existing wind observations while
simulating the effects of complex terrain
in regions with sparse observational data.
The generation of the wind field by the
DWM is accomplished in two steps. Step
1 is largely based on the approach used
by the SAI/CTWM but formulated in a
terrain-following coordinate system. The
domain-mean wind for the modeling
region is adjusted for the kinematic
effects of terrain, thermdodynamically
generated slope flows, and blocking
effects. Step I produces a spatially
varying gridded field of u and v wind
components at several vertical levels.
Step 2 involves the incorporation of
wind observations into the wind fields
generated by step 1. An objective anal-
ysis scheme is used to produce a new
gridded wind field. The scheme is
designed so that the observations are
weighted heavily in subregions where
they are deemed representative of the
mesoscale air flow, whereas in
subregions where observations are
deemed unrepresentative, the wind
values produced by step 1 are weighted
heavily. Once the new gridded wind field
is generated, the vertical velocity out of
the top of the modeling domain can be
minimized.
In addition to wind fields, an acid
deposition/air quality model requires
other meteorological inputs, including
boundary layer heights, temperatures,
relative humidities, stability, precipitation,
and other micrometeorological variables
such as friction velocity and Monin-
Obukhov length. The only candidate
meteorological model that also generates
fields of some of these meteorological
variables is the MELSAR-MET model.
The MELSAR-MET was designed
specifically for the western Rocky
Mountains and was written in a highly
modular fashion, which allows for easy
addition, replacement, or modification of
any module. Thus the mesoscale
meteorological model for the hybrid acid
deposition model for the Rocky
Mountains makes use of the MELSAR-
MET framework, with the new DWM as
its wind field generator.
Evaluation of the New
Diagnostic Wind Model (DWM)
As for the candidate wind models, the
DWM was exercised for the idealized
bell-shaped mountain and the terrain
from the Rocky Mountains using an initial
uniform flow field. The DWM was
exercised with its upslope and downslope
parameterizations. These results
produced wind fields consistent with the
expectations for upslope and downslope
flow regimes.
The DWM was then exercised for the
Rocky Mountain terrain region using
actual surface and upper-air
meteorological observations. The DWM
generated six vertical levels of gridded
horizontal wind fields for each hour
between 1600 on 17 September 1984 to
1500 on 18 September 1984. This period
was selected because of the availability
of three supplementary upper-air
observations, in addition to the routine
National Weather Service (NWS) surface
and upper-air measurements, collected
as part of the Atmospheric Studies in
Complex Terrain (ASCOT) Brush Creek
experiments. The Brush Creek
experiments were designed to study
drainage winds in the Brush Creek
canyon. The formation of drainage winds
generally requires clear, stagnant nights.
If there is significant synoptic flow it will
overpower the drainage winds.
The DWM was exercised twice for
each hour of the 24-hour period, once
using the routine NWS data only and
once with the additional supplementary
data. The DWM was thus evaluated
qualitatively by comparing the wind fields
generated with and without the
supplemental data, and quantitatively by
comparing the wind speeds and wind
direction calculated in the simulation
without the supplemental data and the
supplemental observations themselves. A
comparison of the wind speeds
calculated by the DWM with the
supplemental observations showed that
the DWM underpredicted the wind
speeds by 0.6 m/s out of an average
observed wind speed of 2.1 m/s. The
stagnant nature of the simulation period
is confirmed by the fact that over 50
percent of the predicted and observed
wind speeds at the supplemental data
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sites were calm. A comparison of the
wind directions calculated by the DWM
with the supplemental observations
showed that the positive and negative
deviations from the observed wind
direction exactly cancel each other out,
resulting in a net zero bias. Removal of
the calm wind periods from the wind
deviation distribution results in a much
better match between the predicted and
observed wind directions.
The new DWM was further evaluated
by simulating two regions in California: a
complex terrain/coastal region centered
around Santa Barbara, and a region
containing the southern California Central
Valley and the Sierra Nevada mountains.
For the complex terrain/coastal region,
the DWM was exercised with up to 80
surface and 20 upper-air wind
observation sites to produce hourly wind
fields for 15 days. The DWM replicated
the slope flows and sea breezes quite
well. The flexibility of the formulation of
the DWM was illustrated in the
simulations within the Central Valley by
using results from a two-dimensional
simulation of a primitive equation model
as input into the DWM. Again the DWM
produced complicated nighttime
downslope and daytime upslope flows.
Evaluation of the Candidate
Acid Deposition Models
The candidate acid deposition/air
quality simulation models were evaluated
by comparing how each model treats the
processes of transport, dispersion,
chemical transformation, dry deposition,
and wet deposition.
Transport. All of the candidate acid
deposition models, except the CCADM,
define transport by using the wind at the
center of the Lagrangian plume or puff.
The CCADM relies on user input for its
trajectory definition. The sensitivity of
trajectory definition to height above
ground was examined by calculating air
parcel trajectories at heights of 10, 300,
and 1,000 m above ground, and four
different release times using the DWM-
generated wind fields from the Rocky
Mountains. Results from the trajectory
analysis can be summarized as follows.
The different transport characteristics
between surface and elevated
releases confirms the need for
multilevel wind fields and the correct
prescription of plume rise. Obtaining
an upper-level wind by use of the
power law relationship on the surface
wind speed cannot accurately
characterize transport in complex
terrain,
When an emission release becomes
well mixed, the advection of air
parcels near the surface and parcels
aloft should ideally be handled
differently.
Dispersion. The candidate plume
segment model, the RIVAD, and the two
puff models, the MESOPUFF-II and
MELSAR-POLUT, all use different
parameterizations for defining the
horizontal and vertical plume dispersion
parameters, oy and oz. The CCADM
requires user input for its diffusion and
thus requires too much user interaction.
The dispersion algorithms of the three
models were evaluated by examining
curves of the
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For S02 the MESOPUFF-II and
OCADM dry deposition algorithms
predicted similar dry deposition velocity
that agreed with measured values
reported in the literature over all surfaces
and almost all meteorological conditions.
The exception to this was under F
stability at approximately 2.5 m/s, where
the MESOPUFF-II produced an
anomalously high SO2 dry deposition
velocity peak.
For sulfate, dry deposition velocities
calculated by the MESOPUFF-II and
CCADM again respond in a similar
fashion to changes in environmental
conditions. However, the MESOPUFF-II
predicts dry deposition velocities for
sulfate that are always less than 0.1
cm/s, while the CCADM numbers tend to
peak at around 0.3 to 0.8 depending on
the surface type.
The predicted dry deposition
velocities for NOX agree very closely with
the ones for SO2 except that the
anomalous peak at F stability and 2.5
m/s calculated by MESOPUFF-II is
absent. The NOX dry deposition
velocities calculated by MESOPUFF-II
and CCADM generally agree over all
types of surfaces except for water.
Nitric acid has a very high deposition
rate compared to the other gases studied
because of its high solubility. The
MESOPUFF-II and CCADM agree
remarkably well on their predictions of
nitric acid dry deposition velocities.
There are very few measurements of the
dry deposition velocity for nitric acid, but
the few there are agree with the models'
predictions.
Wet Deposition. Only the MESO-
PUFF-II and RIVAD wet deposition al-
gorithms were consistent with the
desired modeling approach and thus
were evaluated by comparing their
predicted wet scavenging rates for
several species at different precipitation
rates. For SOa the response of the wet
scavenging rates in the two models to
changes in precipitation were similar,
although the MESOPUFF-II rates were
approximately twice those of the RIVAD.
Despite the differences in their
formulations, the MESOPUFF-II and
RIVAD produce remarkably similar
scavenging rates for sulfates for a liquid
hydrometer. The MESOPUFF-II
predicts lower scavenging rates for a
frozen hydrometer, reflecting the fact that
it is difficult for the particles to become
embedded into ice crystals except
through the process of riming. The
RIVAD model predicts a wet scavenging
rate of 100 %/h for nitric acid and all
precipitation rates studied. The
MESOPUFF-II also predicts high wet
scavenging rates for nitric acid, but
requires a precipitation rate of 1 in/h to
obtain a scavenging rate of 100 %/h.
Design of the Acid
Deposition/Air Quality Model for
the Rocky Mountains
The evaluation of the four candidate
acid deposition/air quality models
indicated that no one of these models is
the best choice for calculating source-
specific acid deposition impacts in the
Rocky Mountain region. Thus a new
Lagrangian Gaussian puff model was
designed, making use of the best
components from the candidate models.
Transport within this new puff model
would be defined by the wind at the
plume center from the DWM. The
dispersion algorithm from the MELSAR-
POLUT model has been implemented in
the new model, although the MESO-
PUFF-II dispersion algorithms have also
been retained as an option. The RIVAD
parameterization of chemical trans-
formation appears to be superior to the
mechanism in the MESOPUFF-II and is
the recommended mechanism in the new
model. However, the MESOPUFF-II
chemical mechanism has also been
implemented as an option. The CCADM
and MESOPUFF-II dry deposition
algorithms produced very similar results;
the CCADM algorithm has been
implemented because it is more similar
to the algorithms currently used in the
state-of-the-art scientific acid de-
position models, the RADM and ADOM.
Finally, because of its ability to
parameterize wet scavenging rates for
both liquid and frozen precipitation, the
MESOPUFF-II wet deposition al-
gorithms have been implemented.
Conclusions and
Recommendations
A model for calculating incremental
impacts of acid deposition and pollutant
concentrations in the Rocky Mountains
has been designed using the
components from existing models that
are scientifically sound and also
internally consistent with the overall
modeling approach. Before each
component was inserted into the
modeling system, it was thoroughly
evaluated to assure its scientific
accuracy. The hybrid modeling system
was designed in a highly modular fashion
so that when new modules describing
atmospheric processes become available
they can be easily integrated into the
modeling system. The authors recognize
the inherent uncertainties and limitations
in all air quality simulation models.
•frll < GnvnMiUtMTPMMTIMGncnrE. iaaa/c/.e_ica/c-Mn/.
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R. E. Morris, R. C. Kessler, S. G. Douglas, and K. R. Styles are with Systems
Applications, Inc., San Rafael, CA 94903.
Alan H. Huber is the EPA Project Officer (see below).
The complete report, entitled "Rocky Mountain Acid Deposition Model
Assessment: Evaluation of Mesoscale Acid Deposition Models for Use in
Complex Terrain," (Order No. PB 88-167 481/AS; Cost: $25.95, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-88/008
0000329 f*S
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*61!ICY
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