To complete an air quality impact assessment (AQIA) and health risk
assessment (HRA) for the CEQA process, modeling is usually required.
The SDAPCD requires that AERMOD, the EPA approved regulatory air
dispersion model, be used to perform the air dispersion modeling for
AQIAs and HRAs. The AERMOD executable is available for free from EPA at:
https://gaftp.epa.gov/Air/aqmg/SCRAM/models/preferred/aermod/aermod_exe.zip.
The AERMOD user’s guide is at:
https://gaftp.epa.gov/Air/aqmg/SCRAM/models/preferred/aermod/aermod_userguide.pdf
There are also fee-based software options available that incorporate
a user-friendly interface. The SDAPCD does not endorse any specific
interface, but one such product is Lakes Environmental AERMOD View
software, which is used by SDAPCD modeling staff for all modeling
projects using AERMOD. Lakes AERMOD View software can be obtained at
the Lakes website at:
https://www.weblakes.com/
The use of Lakes AERMOD view by the Lead Agency may help with the
ease of review and sharing of files. AERMOD contains several
regulatory options, include default options, and non-regulatory
options. For most modeling projects, the regulatory default options
should be used in the modeling.
Emission Sources
AERMOD requires the input of emission source information. There are
four different types of sources - point, area, volume, and line.
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Point Sources – A point source is the most common type of
release and is characterized by a stack or vent. Examples of point
sources are exhausts from emergency engines, stacks coming off
combustion equipment, and roof vents. There are different types of
point sources that can be modeled in AERMOD including non-capped
vertical stacks, a stack with a flapper valve, and stacks with a
fixed rain cap. Point sources can also be modeled as having a
horizontal orientation. Point sources have stack parameters
associated with them including exhaust temperature, stack diameter,
flow rate, and stack height.
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Area Sources – Area sources are used to model releases that
occur over an area. Examples of area sources include landfills and
open tanks and stockpiles. Different types of area sources are
rectangular, circular, and polygonal (to represent an area that is
irregularly shaped and has up to 20 sides). For an area source, you
must determine the release height above ground. For example, a tank
open to the atmosphere would have a release height equal to the tank
height. For a landfill surface, the release height would be
zero.
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Volume Sources – Volume sources are used to model releases
that occur over a three-dimensional volume. Examples of volume
sources include fugitive leaks, multiple vents, gas stations, wipe
cleaning and solvent use. Volume sources require a release height
which is the height above ground at the center of the volume. An
irregularly shaped volume can be represented by dividing the volume
source into multiple smaller volume sources.
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Line Sources – Line sources are used to model releases from a
variety of sources including roads, rail lines, and conveyor belts.
AERMOD also allows line volume sources, which are volume sources
arranged in a line. The SDAPCD typically uses this source type to
represent heavy duty truck travel on unpaved haul roads as they are
good at simulating the kicking up of dust by the trucks’
wheels.
Meteorological Data
SDAPCD-processed meteorological data should be used. The SDAPCD has
processed meteorological data with the latest version of the EPA
AERMET preprocessor that converts the raw data into an AERMOD-ready
meteorological data input file. For more information on AERMET the
user’s guide is at:
https://gaftp.epa.gov/Air/aqmg/SCRAM/models/met/aermet/aermet_userguide.pdf
Please Contact the Monitoring and Technical Services Division at APCDWX.LUEG@sdapcd.org or
858-586-2769 to get the meteorological data for the site most
appropriate for a modeling project.
Receptors
The receptor network must include adequate coverage to capture the
maximum ground level concentration. The receptor network should
include a regularly spaced grid and include property boundary
receptors. To limit the total number of receptors in a modeling,
there is the option to have a denser grid of receptors closer to the
source, and a coarser grid further away from the source.
