AirWare User Manual
AERMOD: A Dispersion Model
This documentation is extracted from Perry et al. (1998).
for Industrial Source Applications
The primary meteorological variables used by the
ISC2 model are hourly surface-level observations of wind speed,
wind direction, and temperature. The major derived parameter used by ISC2
is PG stability class. Stability class is used to characterize dispersion,
select the appropriate plume rise equation, and set the temperature gradient.
With the exception of wind speed, the approach taken by ISC2 assumes that
these meteorological parameters do not vary with height.
However, it is well known that significant height variations can occur in
all of these variables.
One of the major improvements which AERMOD brings is
its ability to characterize the PBL through both surface and mixed layer
scaling. AERMOD constructs vertical profiles of needed meteorological
variables based on measurements and extrapolations of those measurements
using similarity (scaling) relationships.
Therefore, vertical profiles of wind speed, wind direction,
s v, temperature, and temperature
gradient can be estimated by the model without requiring significant input
AERMOD is designed to operate as a refined model, without the need to
collect extensive data. Furthermore, it has been AERMIC's goal to have
the model applied, where appropriate, using readily available
(e.g. National Weather Service data) representative meteorological data.
Specific meteorological requirements for regulatory applications of AERMOD
will be established in the future.
Estimation and use of the mixing height is another area
in which AERMOD represents a major change. In ISC2, the mixing height
is used only as an elevated reflecting surface. In addition to a
reflecting surface, AERMOD uses zi as the primary height
scale in the CBL. As such, the vertical profiles of wind direction and
s w explicitly depend on
zi. In the SBL, ISC2 does not account for the
surface-based mechanical mixing layer.
In AERMOD, the mechanical mixed-layer height (h) is calculated and
used both as a reflecting surface and as the height scale for calculating
the vertical profiles of s
v and s w
within the surface layer. To estimate mixing height, ISC2 uses a simple
linear interpolation scheme to describe both the growth and collapse of
the mixing layer. During the transition between day and night, this
technique can produce unrealistic mixing heights.
The approach taken by AERMOD, which is similar to the method used in CTDMPLUS,
is to calculate zi and h from knowledge of the heat
flux and momentum flux properties of the boundary layer.