ATMOSPHERIC POLLUTION 
tion. It must be emphasized, however, that although 
the above analyses are the best available, they still 
await experimental verification. Only Lowry’s treat- 
ment has any direct experimental backing, and further 
verification of it is needed. Many more measurements 
of concentrations and of the associated meteorological 
variables are required in order to check the forms of the 
expressions and, more particularly, to evaluate the 
various parameters under a wide range of conditions. 
Furthermore, under certain atmospheric conditions de- 
seribed in the next two sections, the expressions given 
above break down completely. 
Concentrations Near the Source. Whether or not 
significant concentrations of suspended impurities occur 
at the surface near a stack depends largely on the 
stability of the atmosphere. Etkes and Brooks [21] 
have described the behavior of smoke from a stack 
during various conditions of stability and wind speed. 
Their analysis indicates that smoke comes to the sur- 
face near a stack only with light winds and a super- 
adiabatic lapse rate. Such conditions occur often on 
clear summer afternoons or occasionally after the pas- 
sage of a cold front, especially during the early autumn 
over continental areas. This behavior of the smoke 
near the source is confirmed in a study by Church [16], 
who classifies it as “looping,’”’ a descriptive term for 
the oscillating motion of the narrow plume of smoke. 
He finds that the large convective eddies which produce 
looping occur only with winds of 20 mph or less. On 
the average, the plume first reaches the ground 80 ft 
away from a 200-ft stack with a wind speed of 1 mph; 
the distance is proportionately greater for higher speeds 
up to 20 mph. For a 200+t stack, Church gives the 
following tentative average values of the dilution at 
the surface during unstable conditions: 2000 at 450 ft; 
4000 at 900 ft; and more than 10,000 beyond 1700 ft. 
The dilution is defined as the number of volumes of 
air with which a unit volume of stack effluent has been 
mixed; a dilution figure of 500, for example, signifies a 
concentration 1499 of that in the stack. Lowry [52] 
gives three stack heights as the approximate distance 
at which maximum concentrations occur in unstable 
air and with light winds. Bosanquet and Pearson [12] 
point out that under such conditions mean surface 
concentrations for periods from a few minutes to an 
hour are given by equation (1) if a suitable value of a 
is used. 
Large particles, such as fly ash, will be deposited 
near the source irrespective of weather conditions; the 
lighter the wind the nearer to the source will the deposit 
occur. 
Some general conclusions concerning the occurrence 
of high concentrations at the surface near a stack may 
be reached by inference. The diurnal variation of such 
concentrations will show a maximum frequency of oc- 
currence during the early afternoon and a minimum 
during the night and early morning. The annual varia- 
tion will consist of a maximum during the summer and a 
minimum during the winter and early spring. 
A considerable amount of atmospheric pollution 
comes not from stacks but from sources at or near the 
1143 
earth’s surface as a result of various operations near 
industrial plants. The most reliable data available are 
measured concentrations downwind from point and line 
sources taken during average meteorological conditions 
on downland on Salisbury Plain, England [83]. Sutton’s 
theoretical expressions [83] for such concentrations are 
in satisfactory agreement with these data, but measured 
values neither of concentrations nor of Sutton’s param- 
eters are available for extreme conditions, such as large 
temperature lapses or inversions, or for various types of 
terrain. The solution of a number of pollution problems 
thus awaits further measurements of concentrations 
from known surface sources and of the various mete- 
orological parameters required for an evaluation of 
theoretical approaches. Such measurements would be 
most valuable if made at various locations and heights 
among and around typical industrial plants under both 
average and extreme meteorological conditions. 
Concentrations Distant from the Source. The general 
picture of concentrations distant from a stack is given 
by the theoretical and semitheoretical analyses of 
Bosanquet and Pearson, Sutton, Lowry, and Barad. 
Church’s empirical analysis [16] also provides informa- 
tion of interest, especially concerning the physical be- 
havior of the smoke under various conditions and the 
degree of its dilution as related to wind speed and 
stability. The latter for a 200-ft stack is portrayed in 
Fig. 2; the degree of stability is expressed by 1/@ X 
+18 = +] = 
T.+9 = = — 02 
He fo) == SS =| SS RSRSEESESESS ie 
© ut 
'o ro) 
2-8 Sty ice 
2 oS VS 
=IG IO |of Je -3 © 
= ~ = 
0-25 L -4 
= / i ae 
n-34 Xe) 9, g Loye} 7 DRY mont) 
ODS SINGS |! ADIABATIC_|_ 
ae Ai ee | RATE Ee 
-5! ine eee La 
a 
2 4 6 8 10 l2 14 I6 IS 20 22 24 26 28 2 
WIND SPEED M.P.H. F 
Fic. 2.—Least dilutions (maximum concentrations) at the 
ground as related to stability and wind speed, according to 
Church. A dilution of 800 denotes a concentration which is 
Zoo of that in the stack. Stack height: 200 ft. (Reproduced by 
permission of the American Chemical Society.) 
dé/dz. It will be noted from the figure that the smallest 
dilutions (greatest concentrations) at the ground occur 
with light winds and unstable air, as indicated in the 
previous section. The least dilutions increase with the 
wind speed for a specified degree of stability. For a 
given wind speed, the least dilutions diminish with 
decreasing stability until a minimum is reached and 
then increase; the minima occur at progressively greater 
values of the stability as the wind speed increases. 
When considering the range of usefulness of these 
approaches, the assumptions on which they are based 
should be kept in mind. These approaches provide a 
picture of the diffusion of smoke or gas from a stack 
under any one of a given set of conditions, with the 
