Environmental Engineering Reference
In-Depth Information
1
J
w
PEF
=
Q
/
C
·
(11.62)
in which
the air dispersion factor or dilution velocity [kg m
−
2
s
−
1
per kg m
−
3
]
Q/C
=
the PM
10
emission flux, [kg m
−
2
s
−
1
].
J
w
=
The expression is based on the approach of Cowherd et al. (
1985
) to calculat-
ing wind erosion and includes two factors. The first factor (
Q/C
) describes the ratio
between particle emission flux and the concentration of dust in air at receptor height.
The same factor is also used for calculating the dilution of vapours released to ambi-
ent air, and has been treated in Section
11.5.2
. Generic values are suggested for
the air dispersion factor by both US EPA and CLEA, with values depending on
geographic location (and thereby on meteorological conditions), on the size of the
contaminated site and on the height of the receptor. The generic values are based on
the results of dispersion models.
The second term, 1/
J
w
, describes the inverse of the PM10 emission flux from the
soil surface.
J
w
is based on a semi-empirical approach described by Cowherd et al.
(
1985
), as follows:
1
3600
3
J
w
=
0.036
·
(
1
−
V
)
·
(
U
m
/
U
t
)
·
F
(
x
)
·
(11.63)
in which
V
the fraction of outdoor surface cover [-]
U
m
=
=
s
−
1
]
the mean annual wind speed at height 10 m [m
·
s
−
1
]
U
t
=
the threshold value of wind speed at height 10 m [m
·
F(x)
=
an empirical function of
x
[dimensionless].
An explanation of how to obtain site-specific estimates of
V
,
U
m
,
U
t
and
F(x)
can
be found in Cowherd et al. (
1985
) and is also given in the technical background of
the CLEA model (Environmental Agency
2008
).
Other models are available for determining the dilution velocity used in calculat-
ing the particulate emission factors. These models are discussed in Section
11.5
of
this chapter on inhalation of vapours outdoors.
11.6.2.3 Calculation of Exposure
The calculation of exposure follows the same methodology as for calculating the
exposure due to inhalation of vapours, see Section
11.5.2.2
.
For contaminants where Toxicological Reference Values are available in the form
of reference concentrations in air, RfC-values (in mg/m
3
), the derived contami-
nant concentration in air is compared with the RfC-value. For contaminants where
Toxicological Reference Values are available only as reference doses, RfD-values
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