Civil Engineering Reference
In-Depth Information
Turbulence
Turbulence
Wind
Wind
Gaussian
distribution of
concentration
Gaussian
distribution of
concentration
Fig. 20.4
Schematic representation of a Gaussian puff model
RandomWalk
Random Walk
Wind
Wind
Wind
Fig. 20.5
Schematic representation of a particle model
differential equations are time-dependent distributions of concentration in the
atmospheric boundary layer. They are computed by codes in time steps on a
space grid; the resolution corresponds to the size of the grid cells. Figure
20.6
illustrates the outcome of a computation of this type. Eulerian grid models are
frequently used for long distance dispersion calculations (scale 100-2000 km).
20.4.2 Modeling Radionuclide Deposition onto Surfaces
20.4.2.1 Dry Deposition
Dry deposition of airborne aerosols and gases on surfaces is characterized by a
deposition velocity v
d
. The deposition velocity is the increase of surface contam-
ination over a given time interval divided by the concentration in the air above the
surface element. The value of v
d
depends on the turbulent transport capacity of the
air layers above the surface and on the adhesion properties of the involved mate-
rials. For instance, v
d
is low for stable atmospheric stratification, smooth surfaces,
and aerosols with low binding capacity. On the other hand, v
d
is high for pro-
nounced vertical mixing, vegetation surfaces with high roughness lengths, and
