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161
depth, pollutants are diluted by mixing with air over a
thicker layer resulting in lower ground concentrations.
the pollutant is approximately uniformly mixed or
changes slowly with height up to the top of the mixing
taken over land in the early morning during clear skies
and winds less than about 10 mph, a ground based inversion is normally found. As this inversion begins to
dissipate shortly after sunrise, the vertical temperature
fective height of the mixing layer from the readings of
When one examines vertical temperature soundings
gradient approaches or exceeds the adiabatic lapse rate
over an increasingly thicker layer. As a result, the lower-
most layer is unstable but is capped by an inversion.
Pollutants in this unstable layer from the ground upward are uniformly mixed. Equation (1) describes the
concentration of pollutants within such a layerresulting
from a continuouspoint source located below the inversion lid
Xx
___@
Tu exp { _¥|
5g,3f
(1)
= concentration gr/m?
@ = emission rate of pollutants gr/sec
u% = wind speed downwind of source meters/sec
x
H = mixing depth in meters
y = crosswind distance in meters
a, = the perpendicular distance in meters from
centerline of the plume in the horizontal diree-
tion to the point where the concentrationfalls
layer, but then falls sharply, we can determine theef-
pyrheliometers at twoor three levels. Several techniques
for doing this are described below.
THE CASE OF UNIFORM MIXING OR CONSTANT DENSITY
OF POLLUTANTS
Technique for Obtaining Mixing Depths with Two
Pyrheliometers and Two Solar Zenith Distances
In this method, one pyrheliometer is located at the
ground and the other on the rooftop of a building a feet
tall. (See Figure 129.) Readings are taken about one
hour apart; it is assumed that the mixing depth has
changed little during this period. This assumption, of
course, is not valid during transitional periods; it may
be tolerable during the middle of the day.
Theintensity of the solar beam passing from the top
of the atmosphere to the top of the building on which
the upper pyrheliometer is mounted may be given by
f7(i) = [o(h1) exp {—pL(6)},
where
?,(@) = intensity of all band solar radiation at top
to 0.61 times the centerline value
As the ground based inversion is dissipated and the
lower unstable layer increases to the level at which a
particularly strong source is located (the chimney top)
6: =
the material is brought to the ground in high concentrations and a condition known as fumigation results.
Equation (1) may be used under fumigation conditions.
Thus, the mixing depth must undergo diurnal variations. Information on the behavior of the mixing depth
is useful in forecasting air pollution levels or devising
strategies for incident control. Continuous measure-
Zo(#:) =
areas or during pronounced convection would assist in
gaining a fundamentalinsight into those meteorological
processes of importance to the air pollution problems.
The normal constituents of the atmosphere, oxygen,
nitrogen, water vapor, ozone, carbon dioxide, and aerosols attenuate solar radiation. Attenuation is caused to
some extent by absorption but to a larger extent by
scattering.
Measurements of the attenuation of solar radiation
within a given layer may be determined by comparing
L{#) =
ments of the mixing depth over rural as well as urban
the radiation readings at the top and bottom of the
layer. Three or four pyrheliometers at different. levels
would provide information on the variation of attenuation with height, which in turn would be related to the
vertical distribution of aerosol.
If we assume that in the lower layers above ground
(2)
T =
uw =
of building with the sun’s zenith distance
Ay
solar zenith distance in radians at initial
time
intensity of normal incidence solar radiation at top of atmosphere with the sun’s
zenith distance 6,
subscript representing instrument on rooftop of building serving as a platform
attenuation coefficient in meters~!, This eoefficient is the sum of the absorption and
scattering coefficients.
length of solar beam from top of mixing
layer to uppermost pyrheliometer with solar
zenith distance, 6, .
Vor a solar zenith angle of 62, the equation becomes
I7(62) = Iq(@2) exp { —pLh(62).
(3)
Similar equations for pyrheliometers at groundlevel are
where
Fy((1) = Lo(@1) exp {—p[L(a) + L{61)]}
(4)
T4(02) = Lo{82) exp { —u[L (62) + L,(62)]}
(3)
#8 = subscript which refers to the bottom of the
building
Lq(fi) and L_(62) = length of solar beam from top to
bottom of building
k