Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 495

D e , w
H

D eff = D e , g + (11.61)

in which

H = the dimensionless Henry's Law constant [-].

11.5.3.2 Meteorological Parameters

The estimation of the dilution of contaminants in the ambient air requires informa-
tion on a number of meteorological parameters, primarily wind speed and mixing
height. In order to calculate the mixing height, data concerning atmospheric stabil-
ity and surface properties are needed. If an external atmospheric dispersion model
is used, a number of site-specific meteorological data will be needed. However,
external models are not likely to be used for specific sites, but rather to be used
to derive typical regional air dispersion factors, as has been done in the US and
the UK. Thus, the type of data needed for these models will not be discussed
here.
Depending on the model used, wind speed may refer to different heights above
the ground surface. The wind speed used in the RBCA model is the ambient air
velocity in the mixing zone, using a default value of 2.25 m s − 1 . The Danish model
(Miljøstyrelsen 2002 ) also uses the air velocity in the mixing zone, but uses a lower
wind velocity (0.1 m s − 1 ) corresponding to calm weather conditions. However, for
contaminants where the Toxicological Reference Values are based on long-term
effects, e.g., carcinogens, a higher wind velocity (1 m s − 1 ) is used.

The Dutch and the Flemish models (CSOIL and Vlier-Humaan) use as input
the wind speed at a height of 10 m, corresponding to the wind speeds given in
meteorological statistics. The default value used in the models is 5 m

·

s − 1 . This data
is used in the model to calculate the wind speed at the height at which human beings
inhale contaminated air, i.e., at the height of the breathing zone.
The RBCA model assumes a fixed mixing height (default value 2 m) related to
the height of the breathing zone. The Danish model assumes that the mixing height
is proportional to the length of the contaminated area and is therefore not included as
input. In the Dutch and Flemish models the mixing height is calculated in the model
based on data on atmospheric stability, surface roughness and the dimensions of the
contaminated area.

·

11.5.3.3 Receptor Height

In both the RBCA and the Danish models a complete mixing is assumed within the
mixing zone so receptor height is not used as a parameter. The Dutch and Flemish
models use a height at which human beings inhale contaminated air, i.e., at the
height of the breathing zone of 1.5 m for adults and 1 m for children. The UK model
(CLEA) uses a height of the breathing zone for adults of 1.6 m and for children of
0.8 m.