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Overview of the REMSAD Modeling
System
Prepared by
Systems
Applications International, Inc. (SAI)
15 November 2005
The
Regional Modeling System for Aerosols
and Deposition (REMSAD) is designed to
support a better understanding of the
distributions, sources, and removal
processes relevant to fine particles
and other airborne pollutants,
including soluble acidic components
and toxics. Consideration of the
different processes that affect
primary and secondary (i.e., formed by
atmospheric processes) particulate
matter at the regional scale is
fundamental to advancing this
understanding and to assessing the
effects of proposed pollution control
measures. These same control measures
will, in most cases, affect ozone,
particulate matter and deposition of
pollutants to the surface.
The
REMSAD modeling system was developed
and is maintained and distributed by
Systems Applications International,
Inc. (SAI), an affiliate of ICF
Consulting. The U.S. Environmental
Protection Agency (EPA) funded
portions of the development of REMSAD.
The
REMSAD modeling system was originally
intended as a screening tool – a
model that could be run (quickly) for
a continental-scale modeling domain
(specifically the continental U.S.)
and for a full-year simulation period
– to provide information (although
not very detailed) on the distribution
and composition of particulate matter,
the deposition of pollutant (including
toxic) species onto the surfaces of
inland and coastal bodies of water,
and the expected change in air quality
and deposition that results from
changes in emissions. All of these
parameters were intended to be
primarily represented in terms of
seasonal or annual averages or
deposition totals. What began as a
simple screening tool has evolved into
a more complex “one atmosphere”
modeling system that simulates the
chemistry, transport, and deposition
of airborne pollutants (with emphasis
on particulate matter (PM), ozone, and
mercury) using algorithms that reflect
the state-of-the science and current
knowledge of the important physical
and chemical processes.
MODELING
SYSTEM FEATURES
The
REMSAD system is built on the
foundation of the variable-grid Urban
Airshed Model (UAM-V)
– a regional-scale photochemical
modeling system (SAI, 1999). Thus many
of features of the UAM-V are also
available in REMSAD. The REMSAD model
is capable of "nesting" one
or more finer-scale subgrids within a
coarser overall grid. This two-way
fully interactive nesting feature
allows the user to apply higher
resolution over selected source and/or
receptor regions. The modeling system
may be applied at scales ranging from
a single metropolitan area to a
continent containing multiple urban
areas. To date, most applications have
focused on the continental-scale.
REMSAD
is a three-dimensional grid model
designed to calculate the
concentrations of both inert and
chemically reactive pollutants by
simulating the physical and chemical
processes in the atmosphere that
affect pollutant concentrations. The
basis for the model is the atmospheric
diffusion or species continuity
equation. This equation represents a
mass balance in which all of the
relevant emissions, transport,
diffusion, chemical reactions, and
removal processes are expressed in
mathematical terms. The REMSAD system
consists of a series of preprocessor
programs, the core model, and several
postprocessing programs.
Fine
particles (or aerosols) are currently
thought to pose one of the greatest
problems for human health impacts from
air pollution. The major factors that
affect the concentration and
distribution of aerosols include:
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spatial
and temporal distribution of
toxic and particulate emissions
including sulfur dioxide (SO2),
oxides of nitrogen (NOx),
volatile organic compounds
(VOC), and ammonia (NH3)
(both anthropogenic and
non-anthropogenic),
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size
composition of the emitted PM,
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spatial
and temporal variations in the
wind fields,
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dynamics
of the boundary layer, including
stability and the level of
mixing,
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chemical
reactions involving PM, SO2,
NOx and other
important precursor species,
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diurnal
variations of solar insolation
and temperature,
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Loss
of primary and secondary
aerosols and toxics by dry and
wet deposition, and
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ambient
air quality immediately upwind
and above the region of study.
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The
REMSAD model simulates these processes
when it is used to simulate aerosol
distribution and deposition. The model
solves the species continuity equation
using the method of fractional steps,
in which the individual terms in the
equation are solved separately in the
following order: emissions are
injected; horizontal
advection/diffusion is solved;
vertical advection/diffusion and
deposition is solved; and chemical
transformations are performed for
reactive pollutants. The model
performs this four-step solution
procedure during one half of each
advective (driving) time step, and
then reverses the order for the
following half time step. The maximum
advective time step for stability is a
function of the grid size and the
maximum wind velocity or horizontal
diffusion coefficient. Vertical
diffusion is solved on fractions of
the advective time step to keep their
individual numerical schemes stable. A
typical advective time step for coarse
(50–80 km) grid spacing is 10–15
minutes, whereas time steps for fine
grid spacing (10–30 km) are on the
order of a few minutes.
Gridded
model inputs are prepared to represent
meteorological conditions and
emissions for each simulation day.
Once the model has been evaluated and
determined to perform within
prescribed levels, a projected emission
inventory can be used to simulate
possible future policy-driven emission
scenarios.
REMSAD
provides gridded, averaged surface and
multi-layer instantaneous
concentrations, and surface deposition
output for all species and grids
simulated. The averaged surface
concentrations and depositions are
intended for comparison with
measurements and ambient standards.
The instantaneous concentration output
is primarily used to restart the
model, and to examine model results in
the upper levels. Concentrations of
particulates are passed as input to a
module that estimates atmospheric
visibility. Wet and dry deposition
fluxes are calculated hourly and may
be accumulated for any desired
interval.
The
particulate matter species modeled by
REMSAD include a primary coarse
fraction (corresponding to
particulates in the 2.5 to 10 micron
size range), a primary fine fraction
(corresponding to particulates less
than 2.5 microns in diameter), and
several secondary particulates (e.g.,
sulfates, nitrates, and organics). The
sum of the primary fine fraction and
all of the secondary species is
assumed to be representative of PM2.5.
REMSAD
users have the option of choosing
between two photochemical mechanisms.
The full mechanism included in REMSAD,
the Carbon-Bond Chemical Mechanism
Version 5 (CB-V), is a
state-of-the-science mechanism that
includes updates to chemical reaction
rates based on the most recent
literature. The other option uses a
photochemical mechanism module that is
a reduced-form version of CB-V. This
reduced-form version is termed
“micro-CB” and is based on a
reduction in the number of different
organic compound species that are
included. The inorganic and radical
parts of the mechanism are identical
to CB-V. The organic portion of the
chemistry is based on three primary
organic compound species (VOC,
representing an average anthropogenic
hydrocarbon species, and ISOP and TERP,
representing biogenic hydrocarbon
species) and one carbonyl species (CARB).
Secondary
organic aerosol species (SOA) are
known to result from the reactions of
hydrocarbons in the atmosphere.
REMSAD Version 8 includes a
calculation of the yield of SOA from
both anthropogenic and biogenic
hydrocarbon species.
Of the anthropogenic
hydrocarbon emissions, the aromatic
hydrocarbons are the principal
contributors to SOA. Anthropogenic SOA
is formed from reactions of TOL and
XYL in CB-V. For microCB, a provision
is included in REMSAD to establish the
aromatic fraction of VOC as a function
of space and time.
Biogenic emissions of the
species TERP, representing
monoterpenes, are the principal
biogenic precursors of SOA.
REMSAD
simulates both wet and dry deposition
of gaseous and particulate species.
Wet deposition occurs as a result of
precipitation scavenging. Dry
deposition is calculated for each
species based on land-use
characteristics and meteorological
parameters.
The
chemical transformations of mercury
included in Version 8 of REMSAD are
based on recent reviews of the current
status of atmospheric chemistry of
mercury. Species representing the
oxidation state of mercury and the
phase (gas or particulate) are
tracked. These include HG0 (elemental
mercury vapor), HG2 (divalent mercury
compounds in gas phase), and HGP
(divalent mercury compounds in
particulate phase). A tagging scheme (PPTM,
see below) for the mercury (and other)
species is an optional feature of
REMSAD.
The
Particle and Precursor Tagging
Methodology (PPTM) implemented in
REMSAD allows one to tag and track the
release, transport, chemical
transformation, and deposition of
precursor species and toxics (sulfur,
nitrogen, mercury, cadmium, dioxin,
and lead) from emissions sources,
source categories, or source regions
throughout the REMSAD modeling domain.
A
number of issues are particularly
important to a successful application
of REMSAD for evaluating the
atmospheric transport and deposition
of pollutants. These include the
accuracy and representativeness of the
meteorological and emission inventory
inputs; the resolution, structure and
extent of the modeling grid; and the
treatment of urban areas in both the
source and receptor areas of the
computational grid.
ATTRIBUTES
AND LIMITATIONS
The
REMSAD modeling system provides a
relatively simple and cost effective
means to begin to study and
understand, through modeling, the
factors that contribute to PM,
mercury, and toxics concentrations and
deposition totals, and the relative
effectiveness of emission reductions
measures in reducing the associated
air quality related values. Attributes
of the REMSAD modeling system,
relevant to its use for current air
quality modeling studies, include the
following:
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Since
the REMSAD modeling system is
based on the UAM-V modeling
system framework, many existing
datasets that were developed for
ozone modeling can be readily
adapted for use with REMSAD
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The
options for simplification of
the photochemical and other
processes enables one to use
REMSAD to obtain regional and
annual estimates of PM,
visibility, and deposition using
readily available (standard)
computer technology in a
reasonable amount of time (a
typical continental-scale (for
the U.S.), annual simulation
takes about two weeks of
computer run time on a high-end
(by today’s standards) work
station)
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Many
of the key features and
components of the REMSAD model
are consistent with the
state-of-the-science knowledge
and techniques as related to
particulates, mercury, and other
toxic species
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The
availability of the CB-V
mechanism and detailed treatment
of the formation of SOA allows
detailed treatment of processes
when necessary.
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PPTM
accommodates detailed
source-attribution analyses to
support the development of
emission-control strategies.
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There
is still much that is not known about
the formation, composition, transport,
and deposition of particulate and
toxic species in the atmosphere.
Although the availability, quality,
and spatial and temporal
representativeness of current
measurements and experimental results
do not support a comprehensive
understanding of these processes,
current PM modeling systems, including
REMSAD, attempt to simulate these
processes in a one-atmosphere modeling
system. Further, the ambient air
quality data are largely insufficient
for a thorough evaluation of
performance for REMSAD and other PM
models.
Version
8 of the REMSAD modeling system has a
few limitations that should be
considered prior to its use in a
regulatory or research application.
These include:
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Use
of only the simplified ozone
chemistry may not adequately
represent ozone concentrations
as well as the interactions of
ozone with other pollutant
species (e.g., PM). Use of the
CB-V mechanism may be advisable
in some cases.
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REMSAD
(v8) does not include a
plume-in-grid treatment for the
detailed treatment of plume
chemistry and dynamics for large
point sources
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