If it is known, however, what the Pu host-particle characteristics are both in
terms of number-size distribution in the total suspended aerosol, and in terms
of total aerosol activity, it turns out in many cases to be better to study
the host-particles rather than the Pu activity directly.
If we assume the
host-particles are dust, originating from the contaminated soil below, we find
that the dust-flux method gives considerable degree of detail and allows a
more intimate study of the processes of transporte and the rate limiting factors
at the ground surface. The purpose of this presentation is to review two
approaches to measurement of dust-flux, to describe experiments in which the
methods have been applied, to show the limitations and assumptions of the
methods, and to summarize some new questions about Pu transport which have
been revealed by our experiences in applying the methods.
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THEORY OF ATMOSPHERIC DUST-FLUX AT THE SURFACE
F=-y' w'
Wind, m/sec
Dust concentration is a scaler quantity which obeys lawa of classical micrometeorology near the ground. These laws are explained in several standard texts
and apply to gases which are exchanged with the surface (Sutton, 1953; Priestly,
1959; Chamberlain, 1975).
Dust particles behave like gases unless they exceed
about 20 pm diameter, above which their gravitational settling-velocity becomes
increasingly important in determining their rate of deposition or suspension;
let us confine our discussion to the respirable range of sizes below 20 um.
Given an "ideal" detector during a high mass-loading observation period in the
atmosphere, we find that dust concentrations (and therefore Pu concentrations)
will have the same apparent random variations as does a sensitive wind speed
detector; see Fig. 1.
The vertical flux of duse (F) is the mass passing
through a given horizontal area per unit time and can be defined by an eddycorrelation:
(1)
where x' is the instantaneous deviation in dust concentration from stationary
mean (e.g., ten minute mean on Fig. 1), w' is the instantaneous vertical wind
velocity component, and the overbar denotes a time average of the covariance
product. The negative sign indicates a loss from the ground surface (either
By analogy to molecular transport processes, the
suspension or resuspension).
flux-gradient equation is:
~xp Xx
Fe-kK
ag
(2)
where K is a diffusion coefficient and dy/d% is the vertical gradient of dust
concentration.
It is generally accepted that we can estimate K from boundary
layer theory, in an analogy to momentum-flux:
K = utk'd
{___
0
1202
31 MAR 74
'
1206
Jttg
1214
1210
Time, hr - min
4___t
_t
.
.
Fig. 1.
Detatled information of wind-speed and dust which can be
observed in the atmosphere when sensors are appropriately
sensitive.
(3)
where u* is the friction velocity proportional to horizontal wind speed, k' is
Karman's constant adjusted for atmospheric stability, and % is the vertical
height ordinate.
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