Page 353 - Materials Chemistry, Second Edition

L1644_C08.fm Page 317 Tuesday, October 21, 2003 3:03 PM

8.2.4.2 Air Dispersion Models
Pollutants emitted to the air are dispersed depending on the meteorological condi-
tions, e.g., wind speed and solar radiation, and the characteristics of the region, e.g.,
elevations of the terrain and land use. Accordingly, they occur in the atmosphere in
areas even farther from the emission source.
The atmosphere is the starting medium of the environmental fate and transport
of pollutants of the landfill air emissions. The pollutants are dispersed in the air
depending on the meteorological and topographic conditions in the location of the
emission source and can be transported in the atmosphere over large distances.
However, a portion of the pollutants is deposited in the surrounding area of the
emission source and accumulates in other environmental media such as soil, surface
water or vegetation. If air concentrations of pollutants cannot be determined with
measurements, they can be calculated using air dispersion models, which simulate
the atmospheric dispersion using meteorological and topographic information of the
considered region. In the current exercise, the air dispersion of emitted 1,1,1-trichlo-
roethane was modeled for the surroundings of the landfilling of MHSW. In this risk
assessment, the ISCST3 model, described in Chapter 4, was used to estimate air
concentration dispersion of the 1,1,1-trichloroethane emissions.

8.2.4.3 Data for the ISCST3 Model

ISCST3 is based on a Gaussian plume model (see Chapter 4). It is most common
to compute ambient air concentrations and surface deposition fluxes at specific
receptors near a steady-state emission source. The model is capable of simulating
air dispersion of pollutants from point, area, volume, and line sources. A full descrip-
tion of the ISCST3 model and its algorithms can be found in the ISCST3 User’s
Guide (U.S. EPA, 1995).
The results of the air dispersion model rely on four basic data sets: 1) meteorological
conditions; 2) facility characteristics; 3) location of buildings near the emission sources;
and 4) location of receptors (distance to the emission source and elevation on the terrain).
To calculate the air dispersion of contaminants, the ISCST3 model requires hourly
meteorological data. They include values of 1) wind speed and flow vector; 2) ambient
air temperature; 3) atmospheric stability class; and 4) rural and urban mixing height.
In order to calculate the dry and wet deposition fluxes to the ground, additional
information is needed: 1) friction velocity; 2) Monin–Obukhov length; 3) surface
roughness length; and 4) precipitation code.
The meteorological data used in this exercise contained hourly values of wind
speed and wind direction, ambient temperature, precipitation, and solar radiation.
All further parameters can be calculated using this information. Figure 8.4 shows
the wind rose (distribution of the flow vector) corresponding to the meteorological
data used in this case study. It can be observed that wind blowing from the north is
the most frequent and that wind blowing from the east is strongest. The studied
landfill of MSHW is situated in a zone with a high percentage of calm hours. About
27% of all hourly wind speed values did not exceed 0.1 m/s and the average wind
speed was 2.75 m/s.

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