Pasquill`s influence: on the evaporation from various liquids into the atmosphere Page: 4 of 8
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PASQUILL'S INFLUENCE: ON THE EVAPORATION FROM VARIOUS
LIQUIDS INTO THE ATMOSPHERE
C. H. Huang
Westinghouse Hanford Company
P.O. Box 1970
Richland, Washington 99352
Pasquill's development of an evaporation model as
well as his experimental work on the subject are important
in view of the recent emphasis on toxic chemical releases to
the environment. Pasquill's contributions to the field of
"atmospheric diffusion" are enormous and well-known. The
Pasquill stability classification enables us to apply the
Gaussian diffusion model in our daily life. The Gaussian
diffusion model has been widely and routinely applied in
industry for the estimate of the air concentration and to
manage the radioactive and hazardous wastes in recent
years. Equally important but no less than his contribution
to the Gaussian diffusion model is Pasquill's influence on
the subject of evaporation from various liquid surfaces into
An evaporation model taking into account the vertical
variations of the mean wind speed and the eddy exchange
coefficient was first introduced by Sutton (1934). The
Sutton model made it-possible to estimate toxic chemical
release from a smooth liquid surface to the environment.
However, in the Sutton model, the process of vapor transfer
is based on the momentum exchange involving a parameter
of the air viscosity. Subsequently, Pasquill (1943) modified
Sutton's evaporation theory by introducing another new
parameter, the molecular diffusivity. This replaced the
viscosity in the Sutton model, an important missing
parameter. The Sutton-Pasquill evaporation model has
found wide applications in industry for half a century.
However, out of these two parameters, a non-dimensional
parameter can be formed; it can be used to modify the
Pasquill evaporation model. Experimental data in the
laboratories and the fields indicate that the rate of
evaporation from a liquid surface is dependent on a Schmidt
number. Thus, in this study, we will consider the
modification of the Sutton-Pasquill model and provide some
2. SUTTON'S EVAPORATION MODEL
A theoretical treatment of the removal of water vapor
from a free-liquid surface into the atmosphere was first
introduced by Sutton (1934). Sutton in his theoretical
treatment of evaporation considered the analogy between the
vapor transfer and momentum-transfer. The basic equation
Sutton considered is a two-dimensional diffusion equation
which may be written as:
u(z) _a = a (Kj ')
ax az z
The evaporation problem considered by Sutton is a strip
of a free-liquid surface with an infinite extent in cross wind
direction and finite dimension in down wind direction.
Where u(z) is the horizontal mean wind speed, x the
concentration, K, the eddy diffusivity, and x and z the
coordinates in the down wind and the vertical directions,
respectively. The wet surface area is defined by 0 < x <
x for z=0, while for x >0 or x < 0, the ground is assumed
to be dry as shown in Fig. 1.
With the wind profile and the eddy diffusivity specified,
the solution for Eq.(1) for the rate of evaporation is obtained
Er (u, 4) = A u'tI xr") or
Er (u,,) =Aulxa' forn0.25
where A = K x",
Thus, the total rate of evaporation per unit width in the
cross wind direction over a fetch x from a smooth surface
can be. calculated from Eq.(2) or Eq.(3). For practical
applications, the values of K for various values of the
atmospheric intensity index, n, were calculated by Pasquill
(1943). The equation has often been used to estimate the
release of hazardous materials into the atmosphere.
3. PASQUILL'S EXPERIMENTAL WORK
Various researchers considered the evaporation from a
plane, liquid surface. However, insufficient attention has
been paid to study the aerodynamic effects on the
evaporation. Thus, Pasquill (1943) set out to conduct his
experiments and to investigate the effects of the airflow over
a liquid surface on the rate of evaporation. He examined the
experimental results conducted in a wind tunnel in the light
of the evaporation theory in turbulent flow developed by
Sutton (1934). In recognizing the shortcoming of the
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Huang, C.H. Pasquill`s influence: on the evaporation from various liquids into the atmosphere, report, September 1, 1995; Richland, Washington. (digital.library.unt.edu/ark:/67531/metadc665839/m1/4/: accessed January 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.