United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-88/042 Feb. 1989
x>EPA Project Summary
Rocky Mountain Acid
Deposition Model Assessment:
Acid Rain Mountain Mesoscale
Model (ARM3)
Ralph E. Morris, Robert C. Kessler, Sharon G. Douglas, Kenneth R. Styles,
and Gary E. Moore
The Acid Rain Mountain
Mesoscale Model (ARMS) is a
mesoscale acid deposition/air quality
model that was developed for
calculating incremental acid depo-
sition (sulfur and nitrogen species)
and pollutant concentration impacts
in complex terrain. The development
of the ARMS was based on
comments and recommendations
from western regulatory agencies
who required an acid deposition/air
quality model to estimate long-term
sulfur and nitrogen deposition and
short-term PSD pollutant concen-
tration impacts at mesoscale
distances (5 to 200 km). The ARMS is
designed to simulate long-term acid
deposition and pollutant concentra-
tions for periods up to a year by
stepping through the year at
approximately hourly time steps.
Although the model was designed
primarily to simulate impacts in
regions within the Rocky Mountains,
it can be applied anywhere, provided
the proper inputs are prepared.
However, since the model uses
pseudo first-order chemistry, it is
not suitable for applications in
regions dominated by nonlinear
chemistry.
The ARMS consists of six
components: a terrain preprocessor,
a land-use preprocessor, a precip-
itation preprocessor, a mesoscale
meteorological model, a Lagrangian
acid deposition/air quality model, and
a postprocessor. The mesoscale
meteorological model contains a new
diagnostic wind model that accounts
for the kinematic, deflection, and
thermal effects that alter the flow
fields due to complex terrain. The
Lagrangian acid deposition/air quality
model has the following attributes:
two options for calculating plume
height above ground; three options
for determining dispersion rates,
including one that accounts for
terrain roughness; a dry deposition
algorithm based on the resistance
approach; a wet deposition algorithm
based on the scavenging approach;
and two options for calculating
chemical transformation.
The primary objective in the
development of the ARMS was to
construct an acid deposition/air
quality model based on existing
models for use by western regulatory
agencies to calculate the incremental
contribution of specific sources to
acid deposition and pollutant
concentrations in the complex terrain
of the Rocky Mountains. The model
was designed to be easy to use,
cost-effective, useful in predicting
both sulfur and nitrogen species
deposition, and adequate to account
for the major processes that lead to
acid deposition and pollutant
transport in complex terrain.
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
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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
Although acid deposition may not be
as acute in the western United States as
it is in the eastern United States, its
presence in the western United States
has become an increasing concern to
public and regulatory agencies. Their
concern stems from the strong sensitivity
exhibited by high western lakes to acid
deposition and from the rapid industrial
growth expected to occur in certain areas
of the West. For example, several
planned energy-related projects in the
Overthrust Belt in southwestern
Wyoming, including natural gas sweet-
ening plants and coal-fired power
plants, may considerably increase
emissions of acid precursors in north-
eastern 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
processes, 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, accept in specially
regulated instances, PSD increments
shall not be exceeded and air-quality-
related values (AQRVs) 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-
related values within those areas. New
source permits cannot be issued by the
EPA or the states when the Federal Land
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 designations are being con-
sidered 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 VIM, the National
Oceanic and Atmospheric Administration,
and other federal, state, and private
organizations. The design of this new
model was based on comments from the
WADTF expressing a desire for 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 describing processes
such as wind transport, dispersion,
plume rise, chemical transformation, and
wet and dry deposition. Although the
modeling system must be an integrated
and internally consistent package, it can
be conveniently divided into two distinct
principal parts:
• Simulation of meteorological
processes.
• Simulation of pollutant transport,
dispersion, chemical trans-
formation, and deposition.
Procedure
The ARM3 was assembled from
existing operational mesoscale
meteorological and acid deposition
models. Four candidate mesoscale
meteorological models were selected for
possible use in constructing the ARM3:
the California Institute of Technology
Wind Model (CIT/WINDMOD), the Pacifij
Northwest Laboratory MELSAR MET
model (PNL/MELSAR-MET), the LOJ
Alamos National Laboratory ATMOS
model (LANL/ATMOS1), and th<
Systems Applications, Inc. Comple:
Terrain Wind Model (SAI/CTWM). Th<
candidate acid deposition models were
the Environmental Research an<
Technology MESOPUFF-II model (ERT
MESOPUFF-II), the Pacific Northwes
Laboratory MELSAR-POLUT mode
(PNL/MELSAR-POLUT), the System
Applications, Inc., Regional Impact oi
Visibility and Deposition model (SAI
RIVAD), and the Systems Applicatior
Inc., Comprehensive Chemistry and Aci<
Deposition Model (SAI/CCADM).
The * candidate models were eva
uated and the most appropriat
components were implemented in th
ARM3. Model components were selecte
based on the rigor of their technics
approach and consistency with th
overall modeling approach. The modelin
approach was based on the recorr
mendations of the western regulator
agencies.
Results and Discussion
Overview of the Modeling
System
The ARMS consists of seven
interrelated programs and input data file;
The ARM3 modeling package include
input data to simulate any region with
the master modeling domain, whic
contains Colorado, Wyoming, and mo
of Utah. To simulate other regions, tr
user must supply the input dat
Accompanying the ARMS programs a
sufficient meteorological, terrain, ar
land-use data to simulate an impa
assessment for the calendar year 198
The ARMS was designed to be a se
contained modeling system for which tt
user defines only the mesosca
modeling domain, the grid spacing, tl
meteorological update interval, and tl
sources and receptors of interest.
Model Structure and Program
Interaction
The ARMS modeling packa<
consists of six Fortran 77 programs a'
several related data files. The six Fortr
programs that make up the ARM3 are
follows:
PRELND. the land-use pr
processor. This program uses data frc
the Geographic Information Syster
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>(GIS) land cover data base, supplied for
the entire United States, and creates
gridded fields of surface roughness and
fractional land cover for the user-
selected mesoscale modeling domain.
PRETER, the terrain preprocessor.
This program calculates the average
terrain heights and terrain roughness
values for the user-selected mesoscale
modeling domain. Terrain data is
supplied for the master modeling grid
(Colorado, Wyoming, and most of Utah);
other regions can be simulated if the
user provides the 1-km terrain data.
PRECIP, the precipitation preproc-
essor. This program uses 24-hour and
hourly precipitation measurements to
create gridded fields of precipitation
amounts on the user-selected meso-
scale modeling domain at the user-
selected meteorological update
frequency.
METDWM. the mesoscale meteoro-
logical model. The meteorological model
uses the data supplied by the
preprocessors and surface and upper air
meteorological observations to create
several vertical levels of gridded wind
fields and gridded fields of mixing
heights, temperatures and dew points,
temperature and dew point lapse rates,
stability, and other meteorological
I variables.
CONDEP, the Lagrangian concen-
tration and deposition simulation model.
This program calculates hourly
concentration and deposition amounts
from user-specified sources at user-
specified receptors. The CONDEP can
be run for several different sources or
source configurations using the same
meteorological data from the preproc-
essors and meteorological model
PSTPRC, the postprocessor. This
program creates tables of maximum
concentrations at several averaging times
and cumulative deposition amounts from
the concentration and deposition data
created by CONDEP.
METDWM, the Mesoscale
Meteorological Model
The heart of the mesoscale
meteorological model is a new diagnostic
wind model (the DWM) that was
constructed from the most advanced
components of the four candidate
meteorological models. The new DWM
uses all existing wind observations while
simulating the effects of complex terrain
on wind flows in regions with sparse
observational data. The generation of the
Pwind field by the DWM is accomplished
in two steps. Step 1 is largely based on
the approach used in 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,
thermodynamically generated slope
flows, and blocking effects. Step 1
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
analysis 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; in subregions where
observations are deemed unrepresenta-
tive, 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.
The procedures used to generate
gridded fields of other meteorological
data required by an acid deposition/air
quality model (including boundary-layer
heights, temperatures, relative humidi-
ties, stability, precipitation, and other
micro-meteorological variables, such as
friction velocity and Monin-Obukhov
length) were based on the PNL/
MELSAR-MET model. Gridded fields of
nonwind meteorological data are
interpolated to the mesoscale modeling
domain using orographic adjustments
based on an analysis of climatological
data from the western Rocky Mountains.
CONDEP, the Lagrangian Acid
Deposition/Air Quality Model
The formulation of the Lagrangian
acid deposition/air quality component of
the ARM3 was based on the evaluation
of the four candidate acid deposition/air
quality models. As no one of these
models is the best choice for calculating
source-specific acid deposition impacts
in the Rocky Mountain region, a new
Lagrangian Gaussian puff model was
designed, making use of the best
components from the candidate models.
Transport within this new puff model
is defined by the wind at the plume
center, as determined by the DWM. The
user has the option of calculating vertical
transport of the Lagrangian puff using
either empirical techniques or the vertical
velocities generated by the DWM. Since
there is considerable uncertainty in the
vertical wind velocities generated by any
wind model, the use of empirical
adjustments is recommended for modi-
fying the plume height above terrain due
to complex terrain effects. The initial
plume rise of the source emissions is
calculated using formulas proposed by
Briggs that have been adapted from the
EPA-recommended CRSTER model.
Three options are available for
defining dispersion in the ARMS: two
from the MELSAR-POLUT model (with
one accounting for the effects of terrain
roughness on the dispersal of pollutants)
and one from the dispersion algorithms of
the MESOPUFF-II, which are a fit to
Turner's dispersion curves. The square
of total horizontal diffusion is defined as
the sum of the squares of three
components: (1) an initial diffusion
resulting from nonatmospheric processes
(e.g., buoyant ptume rise), (2) diffusion
resulting from atmospheric turbulence,
and (3) diffusion resulting from horizontal
wind shear. The square of the total
vertical diffusion is the sum of the
squares of two components: (1) an initial
diffusion resulting from nonatmospheric
processes, and (2) diffusion resulting
from atmospheric turbulence.
Two different treatments of chemical
transformation are available in the ARM3,
based on the algorithms from the
SAI/RIVAD and the ERT/MESOPUFF-II.
The RIVAD and MESOPUFF-II chemical
mechanisms are both called pseudo-
first-order chemical mechanisms be-
cause any nonlinearities in the chemical
transformation rates must be based on
conditions that can be described within
the Lagrangian puff (i.e., independent of
background concentrations). The RIVAD
mechanism is a highly condensed
chemical mechanism that estimates the
hydroxyl radical concentration (the
primary gas-phase oxidizer of SOg and
NOa) based on the puff SOg and NOg
concentrations and the ambient tem-
perature and water vapor concentration.
On the other hand, the MESOPUFF-II
oxidation rates are based on an empirical
fit to chemical box model simulations
over a range of environmental conditions.
Both pseudo-first-order mechanisms
contain a surrogate heterogeneous
(aqueous-phase) oxidation rate that is
added to the homogeneous (gas-phase)
to account for the aqueous-phase
oxidation of SOg to sulfate. These
chemical mechanisms are not appro-
priate for use in regions dominated by
nonlinear chemistry.
The treatment of dry deposition in
the ARM3 is based on the resistance
approach, in which the deposition of
pollutants to the ground is limited by a
series of three resistances: (1) an
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aerodynamic resistance that depends on
meteorological conditions and surface
resistance, (2) a quasi-laminar layer
resistance that depends on meteor-
ological conditions, surface roughness,
and species type, and (3) a surface
resistance that is species- and
surface-type-dependent. In addition,
the deposition of particulate species
contains a settling velocity that acts in
parallel to the other resistances. The
dry-deposition algorithms were adapted
from the SAI/CCADM dry-deposition
module, which in turn was based on dry
deposition paramaterizations from three
advanced Eulerian models: (1) the Acid
Deposition and Oxidant Model (ADOM)
developed by Environmental Research
and Technology, (2) the Regional Acid
Deposition Model (RADM) developed by
the National Center for Atmospheric
Research and the State University of
New York at Albany, and (3) the Regional
Transport Model Version III (RTM-III)
developed by Systems Applications, Inc.
Wet deposition in the ARMS is
treated using the scavenging coefficient
approach that was adapted from the
MESOPUFF-II model. This algorithm
contains provisions for both liquid and
frozen precipitation scavenging.
Conclusions and
Recommendations
A model for calculating incremental
impacts of acid deposition and pollutant
concentrations in the Rocky Mountains,
the ARM3, has been developed using
components from existing models that
are scientifically sound and 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 integratec
easily into the modeling system. The
ARMS has not undergone any rigorous
operational testing, sensitivity analysis, oi
validation. A limited model performance
evaluation has been performed. However
further testing and evaluation of the
ARMS are needed in order to gair
confidence in the model predictions
Model evaluation requires new research
studies to collect field data on regional
scale meteorology and air chemistry o
the Rocky Mountain region. The author;
recognize the inherent uncertainties anc
limitations in all air quality simulatiot
models.
Ralph E. Morris, Robert C. Kessler, Sharon G. Douglas, Kenneth R. Styles, and
Gary E. Moore 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: Acid Rain Mountain Mesoscale Model (ARM3)," (Order No.
PB 89-124 408/AS; Cost: $36.95, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-88/042
OQOG32? PS
U S
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