1140 
in 1918 [26]; a modified design more suitable for the 
North American climate has been produced [18]. A 
variety of more elaborate instruments for the study 
of particulate matter have recently been used [98]; 
these include the condensation-nuclei counter [94], the 
midget impinger [15], the cascade impactor and the 
electron microscope [57]. 
The Ringelmann Chart. In the United States the 
emission of solids from stacks has been estimated by 
the Ringelmann chart. The observer compares the shade 
of grayness of the smoke with a series of shade diagrams 
consisting of black horizontal and vertical lines on a 
white ground; the thickness of these lines increases 
progressively through the series. The method is highly 
subjective and open to criticism, the results obtained 
being influenced by such factors as background, il- 
lumination, ete. [33, 55]. 
Sulfur Dioxide Apparatus. Two methods are widely 
used. In one, air is drawn through a bubbler containing 
dilute hydrogen peroxide. Sulfur dioxide in the air 
combines with the hydrogen peroxide to form sulfuric 
acid. The acidity of the solution is determined either by 
titration [24] or by measuring its electrical conductivity 
[89, 90, 91]; the concentration of the sulfur dioxide is 
then computed from the measured acidity of the solu- 
tion. A recording instrument utilizing the titration prin- 
ciple provides a measure of materials, such as sulfur di- 
oxide, that exert a reducing action on an acid solution 
of bromine [96, 98]. 
The other method, developed by Wilsdon and Mc- 
Connell [105], depends on the corrosive action of sulfur 
dioxide. A prepared surface of lead peroxide is exposed 
to the atmosphere for a specified period of time, usually 
a month; sulfur dioxide reacts with the peroxide to 
form lead sulfate. The yield of sulfate is proportional 
to the mean concentration of sulfur dioxide. Of the 
various meteorological elements, only rainfall influences 
the reaction rate [24]. 
The Ultraviolet Daylight Integrator. The amount of 
ultraviolet light from the sun cut off is a measure of 
pollution by particulate matter and is significant for the 
health of human and plant life. A number of methods of 
measuring ultraviolet radiation received at the earth’s 
surface have been devised [56], the best of which con- 
sists of two coaxial translucent fused silica bulbs, one 
inside the other, and with necks downward. With this 
arrangement, a nearly constant intensity of diffuse 
light passes down the neck of the inner bulb for any 
position of the light source over a hemisphere; a silver 
filter transmits only ultraviolet radiation; the intensity 
of the ultraviolet light is varied by an optical wedge; 
and finally, the light falls on suitable photographic 
paper [24]. 
Visibility. Visibility data permit a determination of 
pollution by solid and liquid particles [27]. Objective 
methods of measuring the attenuation of light by par- 
ticulate contaminants im the atmosphere have been 
devised [29, 78, 79, 98], and laboratory studies have 
been made. In one important laboratory study [10] two 
types of suspensoids were used, one consisting of trans- 
parent liquid drops produced by burning phosphorus, 
ATMOSPHERIC POLLUTION 
and the other of black opaque carbon particles produced 
by burning camphor. For the carbon particles, the 
glare and diffusion were negligible, and the logarithm of 
the transmission was proportional to the total amount 
of carbon. For a water cloud there was considerable 
scattering of light and the transmission did not fall off 
with concentration nearly so rapidly as for carbon. 
Mixed clouds showed intermediate properties. In an- 
other laboratory study [98], measurements were made 
of the opacity of pollutants such as sulfur trioxide 
which combine with water to form sulfuric acid mists. 
Studies of the interrelationship of visibility and various 
parameters have been made; in one, it was deduced 
that the product of the number of particles per unit 
volume, the relative humidity, and the visibility is a 
constant [47]; in another, that visibility is mversely 
proportional to smoke concentration [24, pp. 118-119]. 
A start has been made in the application of theory to 
the problem [10, 30, 78], but there are uncertainties 
involved. For example, since some of the particles are 
hygroscopic, their size at higher relative humidities 
may increase by condensation, thus decreasing the 
visibility even though there may have been no in- 
crease in pollution [62]. The contribution of visibility 
studies has to date been strictly limited. 
POLLUTION BY SINGLE SOURCES 
Atmospheric pollution by a single source is frequently 
of importance, as in the case of an industrial plant 
emitting contaminants from its stack. A number of 
theoretical studies of this problem have been made. 
Basic Equations. One of the first theoretical treat- 
ments was that by Roberts [70] in 1923. He developed 
an equation for the concentrations downwind from a 
continuous point source using as a parameter the co- 
efficient of eddy diffusivity. Subsequent studies [69] 
have indicated the inadequacy of an approach based 
on the assumption that the coefficient is constant and 
suggest that other parameters are required. 
In 1932, Sutton [82] developed an equation for a 
point source at the earth’s surface by assigning an 
expression, deduced from dimensional considerations, 
to the correlation coefficient introduced by Taylor in 
his theory of diffusion by continuous movements. 
Using statistical concepts fundamentally very similar 
to those of Taylor and Sutton, Bosanquet and Pearson 
[12] and Bosanquet [11] developed a number of useful 
expressions for the pollution from a point source. One 
such expression gives the concentration which is effec- 
tive in cumulative processes, such as the blackening of a 
neighborhood by soot and the attack of structures by 
acid constituents of the stack effluents. Thus, if the frac- 
tion of the year during which the wind direction falls 
within an arc @ is a6, then the average value*of the con- 
centration for the whole year is given by 
QA —n/pzx 
go 8 pie, (1) 
pux 
where @ is the mass of contaminant emitted per unit 
time, u is the wind speed, x is the distance downwind 
from the source, h is the height of the stack, and pis a 
