ATMOSPHERIC POLLUTION 
By E. WENDELL HEWSON 
Massachusetts Institute of Technology 
INTRODUCTION 
Contamination of the atmosphere by human activ- 
ities has occurred in many countries and from early 
times [86]; with increasing industrialization such pollu- 
tion is becoming more widespread and severe. Atmos- 
pheric pollution presents a problem the solution of 
which requires contributions from many fields, such 
as chemistry [60], city planning [87], legislation [93], 
public health [46, 93, 99], mechanical engineering [77, 
93], meteorology [9, 19, 24, 39, 104], and plant phys- 
iology [45, 93, 97, 106]. Thus the analysis, measure- 
ment of concentrations, and methods of removal at 
the source of certain offending contaminants are chemi- 
cal problems; city planning, by locating industrial plants 
most advantageously relative to population centers, 
makes its contribution; legal authorities draw up en- 
forceable codes; industrial hygiene and public health 
authorities may specify acceptable upper limits to con- 
taminants; mechanical engineers play their part through 
development of more efficient combustion processes and 
design of dust collectors; meteorology describes the 
manner in which contaminants are transported and 
diffused by the atmosphere and offers methods of al- 
leviation; and plant physiology examines the effects of 
pollution on vegetation. General discussions of atmos- 
pherie pollution are available, either brief [1, 8, 34, 36, 
51, 63], or comprehensive [56, 93]. For a very complete 
bibliography of meteorological literature on atmos- 
pheric pollution, see [48]. 
Before launching into the discussion of the strictly 
meteorological aspects of atmospheric pollution, it is 
desirable to survey briefly several closely related topics, 
namely types of atmospheric contaminants and methods 
of measuring them. 
TYPES OF CONTAMINANTS 
Man-made contaminants in the atmosphere may be 
divided broadly into two categories, particulate matter 
and gases. 
Particulate Matter. The largest of the atmospheric 
impurities are certain of the dusts, which tend to settle 
out in a relatively short time. Grinding for size reduc- 
tion is a prolific source. Dusts are also frequently 
formed by industrial combustion processes and ejected 
into the atmosphere by flue gases of sufficiently high 
velocity to carry heavier particles up the stacks; such 
high stack velocities are characteristic of industrial 
furnaces. These impurities consist of coal and coke 
dusts and fly ash. The smaller particles tend to remain 
suspended and are frequently termed “aerosols.” 
Fumes, formed by volatilization and condensation of 
solids, and smokes are typical; many of these smaller 
particles are composed of tar and other combustible 
matter; some of them are hygroscopic and act as con- 
densation nuclei [50]. Finally, there are the mists, con- 
sisting of liquid droplets which are often of a per- 
manent nature. For detailed discussions, see [24, 36, 
56, 98, 102]. 
Gases and Vapors. The presence of such contami- 
nants, although invisible, may be revealed by their 
smell, or by a stinging or burning, as of the eyes, or by 
their corrosive effect on materials or damage to vegeta- 
tion, or, in special cases, by the radiation emitted by 
them. Sulfur dioxide is one of the most prevalent of 
such gases. Acid vapors have a corrosive action and 
certain organic compounds in extremely small con- 
centrations are malodorous [36, 80, 81, 98]. Radio- 
active waste gases from nuclear reactors present special 
features [8, 76]. Radiation hazards from such gases 
decrease with time because of their decay, argon-41, 
for example, having a half-life of 110 minutes. 
MEASUREMENT OF ATMOSPHERIC POLLUTION 
Various methods of measuring pollution have been 
devised: some of these methods, along with an ex- 
tensive bibliography of 120 references, are given in [13]; 
the basic considerations which should be kept in mind 
when designing instruments are described in [85]. The 
more widely used methods of measuring pollution are 
described briefly below; fuller treatments are available 
elsewhere [24, 93], and suggested improvements in in- 
strumentation have also been outlined [24, pp. 132-136]. 
The Deposit Gauge. This instrument, designed for 
the purpose of collecting the total amount of material 
deposited on a given area during a given time, is one 
of the first widely used for the measurement of atmos- 
pheric pollution [66]. One form is similar to a rain 
gauge, and consists essentially of a funnel with a col- 
lecting bottle below; the impurities and raiwater col- 
lected are analyzed by chemical methods. Its limita- 
tions are also similar to those of the rain gauge [61], 
since its “catch” is very sensitive to exposure, and 
depends not only on pollution present but also on rain- 
fall, wind direction, and turbulence [4; 25, pp. 18-21; 
59]. A detailed description of the significance of deposit- 
gauge measurements has been given [25, pp. 8-11]. Other 
more refined deposit gauges have been designed, but 
they all suffer from the above-mentioned limitation. 
The Smoke Filter. Samples of the solid matter sus- 
pended in the atmosphere may be obtained by draw- 
ing a measured volume of air through a filter paper by 
means of a pump. The solids are filtered out, leay- 
ing a gray stain on the paper; their amount is deter- 
mined by visual comparison with a standard scale of 
shades or by photometric methods. An automatic filter, 
which has been widely used, was designed by Owens 
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