ATMOSPHERIC POLLUTION 
sity and duration. From the foregoing brief discussion 
it will be obvious that the processes of mutual inter- 
action between pollution and fog need further study. 
LIMITATION AND CONTROL OF ATMOSPHERIC 
POLLUTION 
There are in existence various laws whose purpose it 
is to limit pollution at the source [82]. Thus, in the 
United States, California has an air-pollution-control 
law and several counties and many municipalities have 
ordinances. This regulation of pollution is being under- 
taken by the following methods: by the sampling and 
inspection of discharges, in the case of smoke mainly 
through the use of the Ringelmann chart; by permits 
to operate existing equipment; by permits to build or 
install new equipment from which atmospheric con- 
taminants may be emitted; and by control of the 
quality and kind of fuel used. Limitation of sulfur 
dioxide emission has in a few cases been specified. Thus 
the regulations of the Los Angeles County Air Pollution 
Control District permit a maximum emission of 2000 
parts per million. In Great Britain the limitation tends 
to be more restrictive: the London County Council 
permits a maximum concentration of 35 ppm. Some of 
the possible means of limitation are mentioned below. 
Limitation of Contaminants Emitted. There are two 
possibilities: to change plant processes so as to reduce 
contaminants in effluents to acceptable levels; or to 
remove or treat contaminants before they reach the 
atmosphere. Since the latter procedure usually involves 
less dislocation of plant facilities and less expense, it has 
been much more widely adopted. Methods of removing 
sulfur compounds and dust and fumes from waste 
gases have been described by Johnstone [43] and else- 
where [1]. 
Effect of Stack Height. It is clear from the discussion 
of the theory that high stacks are useful in alleviating 
surface pollution. However, it has been pointed out by 
Bosanquet and Pearson [12] that any increase above 
a reasonable height [68] has no practical effect. In 
other words, the use of high stacks leads to ameliora- 
tion up to a certain point; to obtain further improve- 
ment, other methods must be used. 
Effect of Stack Temperature. When effluent gases are 
warmer than the ambient air they have a buoyancy 
which causes them to rise. Their ascent ceases when, or 
shortly after, their temperature drops to that of the 
surrounding air. In this connection adiabatic cooling is 
of secondary importance in comparison with turbulent 
transfer of heat from the effluent gases to the air through 
which they rise. For given temperatures of effluent and 
atmosphere, the height to which the gases rise depends 
on the volume emitted per unit time, on the turbulence 
of the surrounding atmosphere, and on the wind speed. 
With low turbulence and low winds the eddy transfer 
of heat is relatively small and the gases rise nearly 
vertically to considerable heights, the height attained 
being greater with greater volume emitted per unit 
time. With large volumes emitted the gases may rise 
400 to 600 ft; with even greater volumes they may 
1149 
rise 1000 to 1200 ft. With a highly turbulent atmos- 
phere and higher winds the initial rise may be only one 
or two hundred feet or less. The height to which gases 
rise before leveling off has been called the “effective 
stack height” by Beers [8]. 
The problem has been discussed theoretically by 
Schmidt [71] and by Sutton [85]. Schmidt used ap- 
proximate methods of solution involving infinite series, 
whereas Sutton postulated that if temperature differ- 
ences between the vertical jet and the surrounding air 
are not too large, the spread of a vertical stream of hot 
gas must bear a close resemblance to the diffusion in 
the atmosphere of a cloud of cold smoke from a con- 
tiuous point source. No wind and a dry adiabatic 
lapse rate are assumed, and the turbulence of the 
general environment is considered negligible in com- 
parison with that induced by the jet itself. For the 
upward velocity w of the warm effluent Schmidt obtains 
an expression of the general form 
—1/3 
WM = Cou e -, (10) 
the constant being evaluated in terms of infinite series, 
whereas Sutton’s corresponding equation is 
a = ( 79Q nee 
37C, pCT ab 
where g is the acceleration of gravity, Q is the heat 
supplied at the source per unit time, c, is the specific 
heat of dry air at constant pressure, p is the air density, 
C is a diffusion coefficient with an approximate value 
of 0.3 em’, and T is the mean absolute temperature of 
the undisturbed air. Strictly speaking, since the actual 
source will not be a point, but an area, the height z is 
not measured from the mouth of the stack but from 
some lower level whose position depends on the size 
of the orifice. Both theories give results of the correct 
order of magnitude, but detailed checks with actual 
observations are required. Furthermore, the important 
case of calm conditions associated with an inversion is 
not covered by the theories. Sutton also obtains an 
approximate solution of the problem when there is a 
wind but, as he points out, the uncertainties then are 
much greater. 
Meteorological Control]. The possibility of alleviating 
pollution by varying the emission of contaminants as 
the diffusing power of the atmosphere varies has as 
yet received relatively little attention. Such a pro- 
cedure has been used successfully by the smelter in the 
Columbia River valley at Trail, British Columbia. 
There, in order to prevent damage to vegetation down- 
valley (to the south) in the state of Washington, an 
upper limit to emission of sulfur dioxide has been set: 
with marked turbulence and hence rapid diffusion of 
the sulfur dioxide the maximum permissible rate of 
emission is much greater than when turbulence and 
diffusion are small; the upper limit is also a function of 
wind direction and speed [19, 37, 38]. The upper limits 
as specified by the Trail Smelter Arbitral Tribunal are 
given in Table I. 
