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4.8.7 AIR QUALITY MODELS
A special feature of EcoSense is the fact that air quality models are included. Apart
from the local-scale ISCST-2 or -3 model, for which a set of site-specific meteoro-
logical data must be added by the user, a long-range pollutant transport model is
included; both models have also been applied separately in this study for the calcu-
lation of site-dependent impact factors.
Close to the plant, i.e., at distances of some 10–100 km from the plant, chemical
reactions in the atmosphere have little influence on the concentrations of primary
pollutants. For these reasons, the computation of ambient air concentrations of
primary pollutants on a local scale is done with a model that neglects chemical
reactions, but is detailed enough in the description of turbulent diffusion and vertical
mixing. An often-used model that meets these requirements is the Gaussian plume
model. The concentration distribution from a continuous release into the atmosphere
is assumed to have a Gaussian shape (see Expression (4.5)):

z h) 
 2    ( − 2  ( + 2
z h) 
cx y z(, , ) = Q ⋅ exp −  y  ⋅  exp −  + exp −  (4.5)


u2πσ σ 2σ 2 y    2σ 2 z  2σ 2 z 
y z    
where: c(x,y,z) = concentration of pollutant at receptor location (x,y,z)
Q = pollutant emission rate (mass per unit time)
u = mean wind speed at release height
σy = standard deviation of lateral concentration distribution at downwind
distance
σ = standard deviation of vertical concentration distribution at downwind
z
distance x
h =plume height above terrain

The assumptions embodied into this type of model include those of idealized terrain
and meteorological conditions so that the plume travels with the wind in a straight
line, mixing with the surrounding air, both horizontally and vertically, to produce
pollutant concentrations with a normal (Gaussian) spatial distribution (Figure 4.8).
Dynamic features that affect the dispersion, for example vertical wind shear, are
ignored. These assumptions generally restrict the range of validity of the application
of these models to the region within some 100 km of the source. Pollution transport
however, extends over much greater distances. The assumption of a straight line is
justified for a statistical evaluation of a long period, where mutual changes in wind
direction cancel out each other, rather than for an evaluation of short episodes.
In this study the Industrial Source Complex Short Term model, version 2 (ISCST-
2) of the U.S. EPA (1992) and version 3 (ISCST-3) of U.S. EPA (1995a) in the form
of BEEST (Beeline, 1998) have been applied. The model calculates hourly concentra-
tion values of gases and particulate matter for 1 year at the center of each specified
grid. Effects of chemical transformation are neglected. Annual mean values are obtained
by temporal averaging of the hourly model results. Currently U.S. EPA has proposed
the establishment of a new regulatory dispersion model AERMOD (U.S. EPA, 2002).

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