Appendix C: Model Information and History

Industrial Source Complex 3 (ISC3)

ISC3 (  HYPERLINK "http://www.epa.gov/scram001/dispersion_alt.htm" 
http://www.epa.gov/scram001/dispersion_alt.htm ) was developed by the
U.S. Environmental Protection Agency (EPA) as a replacement for ISC2. 
ISC3 is a steady-state Gaussian plume model which can be used to assess
pollutant concentrations from a wide variety of sources including point
and area sources.  ISC3 operates in both long-term and short-term modes.
 OPP has operated the model in short-term mode in its fumigant
assessments and used the designation ISCST3.  ISCST3 allows for three
different types of outputs: (1) summaries of high values (highest,
second highest, etc.) by receptor for each averaging period and source
group combination, (2) summaries of overall maximum values (e.g., the
maximum 50) for each averaging period and source group combination, and
(3) tables of concurrent values summarized by receptor for each
averaging period and source group combination for each day of data
processed. The third output option was used when OPP ran the ISCST3
model.  These outputs can be produced all the way down to an hourly
basis.

Up until the end of 2005, ISC3 was the Agency’s recommended air
dispersion model for steady state sources.  It should be noted that ISC3
can still be used as an alternative to the recommended models in
Appendix W in regulatory applications with case-by-case justification
(see Appendix W to 40 CFR Part 51, Section 3.2).

The ISCST3 model allows for the conservative assessment of
concentrations of fumigants coming off of treated fields under specific
meteorological and application conditions.  However, one of the main
weaknesses of ISCST3 is in its treatment of calm periods.  A calm period
in ISCST3 is when the wind speed is less than 1.0 m/s.  When this
occurs, ISCST3 assumes that there is no wind blowing and assigns a wind
speed of 0.0 m/s and this can result in a misrepresentation of the
fumigant plume.  For the Agency’s fumigant assessments, ISCST3 was run
using the “regulatory option” for addressing calm periods.  

American Meteorological Society/Environmental Protection Agency
Regulatory Model (AERMOD)

AERMOD (  HYPERLINK
"http://www.epa.gov/scram001/dispersion_prefrec.htm#aermod" 
http://www.epa.gov/scram001/dispersion_prefrec.htm#aermod ) was
developed by American Meteorological Society (AMS) and the U.S.
Environmental Protection Agency (EPA).  ISC was replaced by AERMOD as
the preferred air dispersion model for near-field, steady state sources
in the Agency’s Guidelines on Air Quality Models as of December 9,
2005.  AERMOD is a Gaussian plume model which can be used to assess
pollutant concentrations from a wide variety of sources including point
and area sources.  AERMOD incorporates air dispersion based on planetary
boundary layer turbulence structure and scaling concepts, including
treatment of both surface and elevated sources, and both simple and
complex terrain.  The AERMOD modeling system consists of two
pre-processors and the dispersion model.  The meteorological
preprocessor AERMET, uses meteorological data and surface
characteristics to calculate boundary layer parameters (e.g. mixing
height, friction velocity, etc.) needed to run AERMOD.  The terrain
pre-processor AERMAP both characterizes the terrain and generates
receptor grids for AERMOD.  AERMOD allows for three different types of
outputs: (1) summaries of high values (highest, second highest, etc.) by
receptor for each averaging period and source group combination, (2)
summaries of overall maximum values (e.g., the maximum 50) for each
averaging period and source group combination, and (3) tables of
concurrent values summarized by receptor for each averaging period and
source group combination for each day of data processed.  These outputs
can be produced all the way down to an hourly basis.

As the replacement to ISC3, AERMOD currently contains new or improved
algorithms for: 1) dispersion in both the convective and stable boundary
layers; 2) plume rise and buoyancy; 3) plume penetration into elevated
inversions; 4) computation of vertical profiles of wind, turbulence, and
temperature; 5) the urban nighttime boundary layer; 6) the treatment of
receptors on all types of terrain from the surface up to and above the
plume height; 7) the treatment of building wake effects; 8) an improved
approach for characterizing the fundamental boundary layer parameters;
and 9) the treatment of plume meander.  Many of these improvements have
little to no effect on OPP’s approach to modeling fumigant
applications as area sources.

AERMOD allows for the conservative assessment of concentrations of
fumigants coming off of treated fields under specific meteorological and
application conditions.  However, AERMOD has a similar weakness to ISC3
in its treatment of calm periods.  A calm period in AERMOD is when the
wind speed is less than 1.0 m/s.  When this occurs, AERMOD assumes that
there is no wind blowing and assigns a wind speed of 0.0 m/s and this
can result in a misrepresentation of the fumigant plume.  Also, AERMOD
does not allow for the probabilistic treatment of variables such as the
meteorological conditions.

CALPUFF

CALPUFF (  HYPERLINK
"http://www.epa.gov/scram001/dispersion_prefrec.htm#calpuff" 
http://www.epa.gov/scram001/dispersion_prefrec.htm#calpuff ) is a
non-steady-state meteorological and air quality modeling system
developed by the Atmospheric Studies Group at TRC Solutions.  It is
maintained by the model developers and distributed by TRC (  HYPERLINK
"http://www.src.com/html/calpuff/calpuff1.htm" 
http://www.src.com/html/calpuff/calpuff1.htm ).  CALPUFF v.5 has been
adopted by the Agency in its Guideline on Air Quality Models as the
preferred model for assessing long range transport of pollutants and on
a case-by-case basis for certain near-field applications involving
complex meteorological conditions (i.e., non-steady state).  The
modeling system consists of three main components and a set of
preprocessing and postprocessing programs.   The main components of the
modeling system are CALMET (a diagnostic 3-dimensional meteorological
model), CALPUFF (an air quality dispersion model), and CALPOST (a
postprocessing package).

The output files that CALPUFF creates for each run include unformatted
data files containing grids of time-averaged concentrations,
time-averaged dry deposition fluxes, and time-averaged wet deposition
fluxes.  These outputs in CALPUFF v.5 can be produced all the way down
to an hourly basis.  The post-processing program CALPOST is designed to
produce ranked tabulations of averages of selected concentration data
from these data files. CALPOST writes a text file containing the input
data summary and output tables.

Although CALPUFF v.5 is on the Agency’s guideline for air models,
there is also currently a CALPUFF v.6 that has not yet been reviewed by
the Agency.  CALPUFF v.6 includes a number of technical enhancements
over v.5 but the major one that could have effects on OPP’s modeling
of fumigant emissions is the option to use subhourly (i.e., 1 minute, 5
minute, etc.) meteorological data.

Probabilistic Exposure and Risk model for FUMigants (PERFUM)

PERFUM (  HYPERLINK
"http://www.exponent.com/practices/health/PERFUM.html" 
http://www.exponent.com/practices/health/PERFUM.html ) was developed to
address the issue of bystander exposures following agricultural
applications of fumigants.  The core of the PERFUM modeling system is
the US EPA dispersion model ISCST3 which at the time PERFUM was
developed was the Agency’s recommended air dispersion model for steady
state sources.  ISCST3 as described above calculates concentrations but
is not designed to determine a buffer zone.  PERFUM was designed to
specifically take the ISCST3 outputs and use them to produce buffer zone
outputs in a distributional format.

PERFUM allows users to develop an understanding of the distributions of
potential bystander exposures and thus more fully characterize the range
of risks resulting to bystanders around treated fields.  ISCST3 is an
integral part of the PERFUM model and the basic physics and code of
ISCST3 remain unchanged.  PERFUM essentially provides ISCST3 with daily
meteorological data over 5 years as well as flux estimates within the
uncertainty of those data.  PERFUM then uses this information to create
distributional outputs for pre-defined receptor locations.

Fumigant Emissions Modeling System (FEMS)

FEMS (http://www.sullivan-environmental.com) was developed to address
the issue of bystander exposures following agricultural applications of
fumigants.  FEMS allows the user to define a number of options prior to
running the model including: the fumigant to be applied, the frequency
of fumigation, the sealing method employed, field size and shape,
consecutive day/contiguous field applications, application season, the
averaging time for the concentrations, and the dispersion model used
(ISCST3, CALPUFF v.5, or CALPUFF v.6).  FEMS also allows the user to
include Monte Carlo treatments of all the key model inputs like
meteorological conditions, emissions data, day the application starts,
etc.  

Once the core dispersion model is selected, FEMS simulates the
application of a fumigant and it’s off-gassing over a 4 day simulation
using 4 hour time steps.  The model estimates fumigant concentrations at
various receptors beyond the perimeter of the applied field that are
matched to the averaging time of interest for the user.  Aside from
estimating the fumigant concentrations, FEMS keeps track of the number
of times that concentrations exceed the concentration of concern at each
receptor.

Once FEMS completes the modeling simulation, the distribution of
concentrations is computed for each receptor.  FEMS produces two main
outputs.  The first is a frequency distribution that looks at the number
of times that concentrations exceed the concentration of concern at each
receptor.  The second involves establishing the distributions of
concentrations for each receptor and then taking the maximum number of
periods per averaging time of interest above the concentration of
concern and computing them as a function of distance from the field. 
Buffer zones are then established based on the most conservative
concentrations that were modeled as a function of distance.

Soil Fumigant Exposure Assessment System (SOFEA)

SOFEA (http://www.epa.gov/oscpmont/sap/meetings/2004/index.htm) was
developed to evaluate and manage human inhalation exposure potential
associated with agricultural applications of fumigants.  SOFEA
calculates fumigant concentrations in air arising from volatility losses
from treated fields for entire agricultural regions using multiple
sources (treated fields), GIS information, agronomic specific variables,
user specified buffer zones and field re-entry intervals.  SOFEA uses a
modified version of ISCST3 as its dispersion model.   SOFEA also uses
Monte Carlo techniques to vary the following parameters: weather
information, field size, application date, application rate, application
method, pesticide degradation rates in air, sealing method, field
re-treatment, and buffer setbacks.

Multi-year, multiple field simulations can be conducted with SOFEA using
random field placement in all agricultural areas or by selectively
placing fields in historical or prospective use areas.  Regional land
use information can be used to refine the placement of treated fields,
dispersion calculations, and exposure assessments.  SOFEA has been
previously used for regulatory decision making in California.

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