Page 374 - Fundamentals of Air Pollution 3E

III. Modeling Nonreactive Pollutants 329

where qa is the area emission rate, u is the mean wind speed over the
simulation period, and the constant C is dependent on the stability, the
effective height of emission of the sources, and the characteristics of the
pollutant. For estimation of annual concentrations with this method,
C = 50, 200, and 600 for SO 2 , particulate matter, and CO, respectively (24).

D. Pollutants That Deposit

The Fugitive Dust Model, FDM (25), was formulated to estimate air
concentrations as well as deposition from releases of airborne dust. It has
a greatly improved deposition mechanism compared with that in previous
models. It considers the mass removed from the plume through deposition
as the plume is moved downwind. Up to 20 particle size fractions are
available. The particle emissions caused as material is raised from the sur-
face by stronger winds is built internally into this model. It has the capacity
of making calculations for point, line, and area sources. The area source
algorithm has two options, simulation of the area source by five finite
line sources perpendicular to wind flow, or a converging algorithm which
provides greater accuracy. Currently, the model should be used for releases
at or below 20 m above ground level.

E. Dispersion from Sources over Water
The Offshore and Coastal Dispersion (OCD) model (26) was developed
to simulate plume dispersion and transport from offshore point sources to
receptors on land or water. The model estimates the overwater dispersion
by use of wind fluctuation statistics in the horizontal and the vertical mea-
sured at the overwater point of release. Lacking these measurements the
model can make overwater estimates of dispersion using the temperature
difference between water and air. Changes taking place in the dispersion
are considered at the shoreline and at any points where elevated terrain is
encountered.

F. Dispersion over Complex Terrain
Development efforts in complex terrain of the EPA using physical model-
ing, both wind tunnel and towing tank, and field studies at three locations
have resulted in the CTDMPLUS, Complex Terrain Dispersion Model plus
the calculation of concentrations for unstable conditions (27). Complex
terrain is the situation in which there are receptor locations above stack
top. Using the meteorological conditions and the description of the nearby
.errain feature, the model calculates the height of a dividing streamline.
Releases that take place below the height of this streamline tend to seek a
path around the terrain feature; releases above the streamline tend to rise