DUST AND SMOKE IN THE ATMOSPHERE
When the moralist reminds us that we are children of the dust and
predestined to a dusty end, there is a grain of comfort in the
discovery that modern science regards dust as one of the most important
things in the whole economy of nature. No longer does dust seem an
appropriate symbol of insignificance and humility when one surveys the
bulk of serious literature that has been written about it, considers
the caliber of the men who have devoted the better part of their lives
to the study of it, or inspects the great array of ingenious apparatus
that has been devised for its investigation.
The dust of which we have to speak in the present chapter embraces all
small particles of solid matter found anywhere, or at any time, in the
earth’s atmosphere. Particular kinds of dust have, of course, their
special names. Soot, the visible part of smoke, is a form of dust that
has played a very conspicuous part in human affairs; hence the separate
mention of smoke in the heading of this chapter.
While there are many agencies that help to charge the atmosphere with
dust, the most important of them all is the wind. Let us see what
happens when the wind blows over the surface of a dusty road, for
example. If the air flowed in a smooth horizontal stream over such
a surface, its friction would drag the dust along on the ground,
but would not lift it. Such surface drifting, due to the horizontal
component of the wind’s motion, does, of course, occur, and its effects
are strikingly visible in the shifting dunes that often form over a
broad surface of sand or snow. All winds near the earth’s surface are,
however, full of waves and eddies, and in many cases, as over a stretch
of strongly heated soil, there are strong updrafts, sometimes extending
to a great height in the atmosphere. All kinds of dust are heavier
than air, and, contrary to popular belief, never truly “float” in the
atmosphere. Dust may enter the atmosphere at high levels, through the
disintegration of meteors, or it may be spouted up by volcanoes, but
dust blown up from the earth’s surface rises only because the air is
rising with it; and, in still air, all dust sinks more or less rapidly
toward the ground. The rate of its fall depends upon its specific
gravity, and upon the size and shape of the dust particles. Other
things being equal, the finest particles fall most slowly. Exceedingly
fine dust, even without upward air movements to support it, requires
months or even years to fall to the ground from the higher levels of
the atmosphere.
Upward movements in the air suffice to carry millions of tons of dust
aloft every year, and horizontal air currents carry the same dust
far and wide over the earth. The transportation of soil by the wind
leads to some results of remarkable interest, practical as well as
scientific. In the first place, far-reaching changes in topography are
brought about by this process. Thus in China vast areas are covered to
a depth of hundreds or even thousands of feet with a fine yellowish
earth, called “loess,” which is believed to have been blown thither by
the winds from the deserts of Central Asia. Less extensive deposits of
this wind-borne material are found in many other parts of the world,
including the Mississippi Valley. Another effect of wind transportation
is the mixing of soils. There is a constant interchange of soil
material between different regions, so that the composition of the
soil on a particular farm, for instance, is not the same now that it
was a few years ago or that it will be a few years hence. Lastly, the
presence of dust in the atmosphere, whether derived from the soil or
otherwise, has various interesting and important effects upon the heat
and light we receive from the sun and modifies, in numerous ways, the
conditions of human life upon our planet.
Several cases in which enormous quantities of solid matter have been
carried to great distances by the wind have formed the subject of
elaborate investigations on the part of meteorologists. Thus, during
the three days, March 8-10, 1901, heavy dust storms occurred in the
deserts of southern Algeria, and the sequel of these storms was
carefully studied by Hellmann and Meinardus. A widespread cyclonic
storm, central over Tunis at the time, sucked up the dust, which was
carried northward by the winds at high altitudes. Deposits from this
dust cloud occurred over an area extending as far as 2,500 miles from
the place of origin. Reports collected from hundreds of observers
indicated that 1,800,000 tons of dust fell over the continent of
Europe, and one-third of this fell north of the Alps. As much more is
believed to have fallen over the Mediterranean, while on the African
coast itself the deposit is supposed to have amounted to 150,000,000
tons. In March, 1918, a shower of dust discolored falling snow at
various places in the United States over an area of at least 100,000
square miles, extending in an east-west direction from Dubuque, Iowa,
to Chelsea, Vt. Reports of this shower were collected by Messrs. E. R.
Miller and A. N. Winchell, who estimate that the amount of dust could
not have been less than a million tons, and may have been several
hundred million. The dust is believed to have been blown up from the
arid regions of the far southwestern United States and to have been
transported a thousand miles or more.
Off the west coast of Africa, between the Canaries and the Cape Verde
Islands, haze due to dust blown up from the Sahara Desert is frequently
encountered by vessels, especially during the first four months of the
year. This haze probably gave rise to the ancient legend of a Sea of
Darkness--the _Mare Tenebrosum_--one of the mysterious terrors of the
ocean reported by the navigators who first sailed toward the New World.
Extensive deposits of atmospheric dust have attracted attention from
the earliest times. Ehrenberg, in 1849, collected records of 349 such
cases, and published a map showing their distribution, which embraces
the greater part of the world. Atmospheric dust is always brought down
in greater or less quantities by rain. When it consists of fine powdery
sand, the rain sometimes acquires a brownish or reddish tinge, staining
objects on which it falls and constituting the “showers of blood” that
have been regarded as prodigies from remote antiquity. Homer describes
such a shower, and many similar occurrences are recorded by the Roman
historians. Italy, owing to its proximity to the African coast, is
often visited by these showers, which still strike superstitious terror
into the hearts of the peasantry.
The millions of meteors that enter the earth’s atmosphere every day
contribute their quota of dust, though the total amount is small
compared with that of the material lifted from the earth. Fine
ferruginous particles are often seen on the snowy summits of high
mountains and the polar ice fields, and both their appearance and their
composition indicate that they are derived from meteors.
Forest fires, burning peat beds, and other conflagrations on a large
scale discharge quantities of dust into the atmosphere. Cinders from
the great Chicago fire spread over a large part of the globe. They are
said to have reached the Azores some forty days after the beginning
of the catastrophe. In Europe, the once common practice of burning
the moors to prepare them for cultivation gave rise to huge volumes
of smoke, which was carried by the wind hundreds and even thousands
of miles. The stronghold of this old custom--which still survives to
some extent--was East Friesland, in northwestern Germany, and the
characteristic haze to which it gave rise, known as “moor smoke”
(German, _Moorrauch_), was sometimes observed as far away as Spain,
Italy, and Greece.
The famous “dark days” that figure in both ancient and modern history,
though in a few cases probably due to eclipses of the sun, have
generally been the result of an abnormal accumulation of smoke or
dust in the air; sometimes arising from volcanic eruptions, but more
often from burning forests, moors, or prairies. Forest fires are the
principal cause of dark days in the United States. Probably the most
celebrated of such days was May 19, 1780, when, in consequence of
great forest fires along Lake Champlain and down to the vicinity of
Ticonderoga, darkness like that of night prevailed in New England.
All but the most necessary business was suspended, the schools were
dismissed, and the greater part of the population flocked to church to
prepare for the end of the world, which was believed to be at hand.
The great Idaho fire of August, 1910, was responsible for dark days
over a larger area than in any other case on record in this country.
Artificial light was required in the daytime over a broad belt,
extending from Idaho to northern Vermont, but smoke was observed far
beyond this area. The British ship _Dunfermline_ reported that on the
Pacific Ocean, 500 miles west of San Francisco, the smell of smoke was
noticed and haze prevailed for ten days. When smoke in the air forms
a rather thin layer, through which the sunlight penetrates feebly,
we sometimes get an effect similar to the golden glow of sunset, a
yellow or coppery tinge being cast over the landscape. Such was the
cause of the “yellow day” still remembered in New England--September
6, 1881--attributed to the burning of the immense peat bogs of the
Labrador barrens.
Another occasional cause of atmospheric dustiness is the eruption of
volcanoes, especially those of an explosive character, which carry fine
dust to heights at which it cannot be washed out of the atmosphere by
rain. The remarkable dry fog of 1783--the most famous in history--which
covered the greater part of Europe and North America for three or
four months--was undoubtedly due to the violent eruptions of that
year in Iceland and Japan. Its connection with the Iceland eruption
was suggested even by contemporary writers. The outbreak of Krakatoa,
in the East Indies, in 1883, spread a veil of dust over the greater
part of the globe. For two or three years its presence in the air was
the cause of striking optical phenomena, including gorgeous sunset
glows. The story is told of an American fire brigade which, deceived
by one of these brilliant sunsets, set out to extinguish what was
mistaken for a great fire in a neighboring village. A large species of
corona around the sun, known as “Bishop’s ring,” because it was first
observed by the Rev. Sereno Bishop of Honolulu, appeared shortly after
the eruption and reached its maximum intensity the following year.
This was due to the diffraction of light by the exceedingly fine dust
from the volcano, and the same phenomenon has been seen after other
great explosive eruptions; e. g., that of Mont Pelée, in 1902. Some
authorities believe that the finest particles of dust from the Krakatoa
eruption were carried to an altitude of over fifty miles above the
earth, and remained suspended at very high levels for several years,
constituting the strange “noctilucent clouds,” seen on summer nights
from 1885 onward. These clouds glowed with a silvery luster, attributed
to reflected sunlight.
A persistent veil of volcanic dust in the upper air is thought to
exercise marked effects upon terrestrial temperatures, and prolonged
periods of intense vulcanism have been regarded as the cause, or one
of the causes, of the recurrent ice ages of which geology furnishes
the record. This explanation of ice ages was advanced by P. and F.
Sarasin, in 1901, and was first put upon a scientific basis by Dr. W.
J. Humphreys in 1913; but the idea that volcanic dust might be the
cause of cold seasons was suggested by Benjamin Franklin as early as
1784. Franklin’s speculations on this subject were prompted by the cold
winter of 1783-1784, which followed the extraordinary fog of 1783,
already mentioned. Humphreys has published a list of all the great
volcanic outbreaks recorded since 1750, and has shown that each of them
registered itself in the temperatures of the earth and also, since
accurate measurements began to be made of solar radiation, in these
instrumental records. Thus, the intensely cold winters of 1783-1785
followed the tremendous eruptions of Asama, Japan, and Skaptar Jökull,
Iceland, in 1783; the famous “year without a summer” (1816) was the
sequel of the gigantic outbreak of Tomboro, in the Sunda Islands, in
1815, which is said to have hurled thirty-six cubic miles of solid
matter into the atmosphere; and definite periods of low temperatures
and reduced sunshine were observed after the eruptions of Mont Pelée,
in 1902, and Mount Katmai, Alaska, in 1912.
The effect of a volcanic dust veil in lowering temperatures on earth
is attributed chiefly to the fact that, while the fine grains of dust
are able to reflect back into space the short waves of radiation coming
from the sun, they do not bar the passage of the long heat waves
radiated outward from the earth. According to Humphreys’s calculations,
such a veil is about thirtyfold more effective in shutting solar
radiation out than in keeping terrestrial radiation in. This process is
just the reverse of the familiar effect of the greenhouse; where the
glass lets in the short waves of solar radiation but does not readily
let out the long waves of earth radiation.
A small contingent of atmospheric dust consists of common salt (sodium
chloride) due to the evaporation of spray from the ocean. This
substance is frequently found in rain, as well as in samples of air,
not only near the seashore, but even in the interior of continents and
on high mountains. According to Du Bois the amount of sodium chloride
annually deposited on the dunes of Holland is at least 6,000,000
kilograms (more than 6,600 tons).
One of the striking phenomena of arid regions is the dust whirlwind;
exemplified in the “devils” of India and South Africa, the “twisters”
of Texas, etc. E. E. Free, in his treatise on “The Movement of Soil
Material by the Wind” (U. S. Bureau of Soils, Bulletin 68), says of
these whirls:
“They may be seen nearly every hot day, sometimes running rapidly over
the surface; sometimes remaining nearly, if not quite, stationary,
but never losing their rapid rotation. They usually last only a few
minutes, but occasionally persist much longer. One observed by Pictet
lasted for over five hours. They are largest and last longest on the
flat, bare plains of the desert, and are usually seen in a calm or
when only a light breeze is blowing, although their occurrence in
windy weather is not unknown. These whirls have been noticed by many
travelers in desert and steppe regions and have been carefully observed
by Baddeley in India, and by Pictet in Egypt. They are frequent in
China and on the pampas of South America, and occasionally occur during
the dry season even in the humid regions. One of the most interesting
phenomena in connection with the dust whirls is the occurrence of
systems of several whirls, each revolving rapidly about its own center
and also moving about a common center in a more or less perfect circle
a few rods in diameter.”
The little whirls often seen on dusty roads are a miniature variety of
the same phenomenon.
One very important class of dust particles in the atmosphere consists
of organic matter, living or dead, including the pollen of plants and
the countless myriads of microorganisms, as well as a variety of other
products of the animal and vegetable kingdoms. An abundance of pollen
in the air accounts for the occasional fall of yellow rain, described
as “sulphur rain,” “golden showers,” etc. The promptness with which
a piece of stale bread becomes moldy in a damp atmosphere is one of
many proofs of the omnipresence in the atmosphere of the microscopic
spores of fungi, ready to propagate their species with amazing rapidity
as soon as they light upon a suitable nutrient medium. Last, but not
least, bacteria, the most minute of all known organisms--so small that
thousands or millions of them clustered together would make a mass not
larger than the head of a pin--swarm in the air, as they do in water,
the soil, and the bodies of animals. Fortunately, while certain species
of bacteria carry disease and death with them, the great majority are
harmless to mankind.
[Illustration: AITKEN’S DUST COUNTER]
A great many different methods are in use for determining the total
amount of solid matter present in a given volume of air, counting the
number of particles, or gathering samples for microscopic examination.
Thus a known volume of air may be drawn through a filter of cotton
wool or bubbled through distilled water, and the dust detained by the
cotton or deposited in the water may be weighed. In certain types of
apparatus the air is drawn or forced against a plate or tube coated
with glycerin, oil, varnish, gelatin, or other adhesive surface, to
which the dust remains attached. Several devices depend for their
operation upon the fact that when a volume of confined air is cooled
by expansion a point is eventually reached at which the water vapor
present condenses to form a fog, each droplet of which is supposed
to have a single particle of dust as its “nucleus.” This is the
principle involved in the well-known Aitken dust counter, which has
been so extensively used in different parts of the world, and has
furnished most of the impressive statistics of air dustiness found
in textbooks and reference books. Thus, from indications supplied by
this instrument, it is stated that a cubic inch of town air contains
50,000,000 particles of dust; that a room, near the ceiling, was found
to contain 88,000,000 particles per cubic inch; and that a cigarette
smoker sends 4,000,000,000 particles into the air at every puff. Recent
authorities are inclined to look upon these figures as misleading,
for the reason that the nuclei counted with Aitken’s instrument are
probably so infinitesimal in size that they hardly deserve to be called
dust; indeed there is good reason to believe that an indefinitely
large proportion of them may actually be molecules of gases.
The effects of dust, both inorganic and organic, upon the health of
humanity will be considered in another chapter. Certain kinds of
dust are of economic importance on account of their inflammable and
explosive character when mixed with the right proportions of air.
Thus the cereal dusts made in the handling and working up of grain
into food products occasionally give rise to serious accidents. These
occur in cereal, flour, and feed mills, grain elevators, starch and
glucose factories, and on farms in connection with the use of threshing
machines. During a period of ten years, 1906-1916, cereal dust
explosions resulted in the loss of eighty lives and the destruction
of property to a value of $2,000,000 in the United States. A study
of this subject has been made by the United States Department of
Agriculture, and various recommendations have been published with a
view to preventing the occurrence of sparks in the neighborhood of
these dangerous dusts. Coal dust in mines likewise causes numerous
explosions. Preventive measures include wetting the dust, moistening
the air, and powdering the walls, roof and floor of the mine with a
nonexplosive rock dust, which has the effect of stifling an incipient
fire or explosion.
The last species of dust that we have to consider in this chapter is
one that constitutes a literal blot on civilization, since the noblest
cities and monuments of mankind are defaced with it. Neither are
the evils of this kind of dust wholly æsthetic, for it is extremely
injurious to health and enormously expensive. After enduring coal
smoke as a necessary evil for generations, civilised humanity has
now embarked upon a vigorous campaign for its elimination, and very
encouraging results have already been achieved in many parts of the
world. The war against smoke is carried on by numerous societies in
Europe and America; a multitude of laws and ordinances (not all of them
effective) have been enacted on the subject; it has been the occasion
of international conferences and expositions; and its literature has
grown so copious that a partial bibliography of the subject, published
a few years ago by the Mellon Institute, of Pittsburgh, fills 164 pages.
The smoking of chimneys is costly, in the first place, because it is
due to imperfect combustion and the waste of part of the heating value
of the fuel, and, in the second place, on account of the damage wrought
by the deposit of the soot. Thus a smoky atmosphere entails big laundry
and dry-cleaning bills, frequent repainting of houses, injury to metal
work, damage to goods in shops, and excessive artificial lighting in
the daytime. Throughout the United States it is said that smoke causes
an annual waste and damage amounting to five hundred million dollars.
In Pittsburgh alone--before the reform produced by vigorous legislative
and scientific measures, following an exhaustive investigation by the
Mellon Institute of Industrial Research--the cost of the smoke nuisance
was estimated at nearly ten million dollars a year. Means of mitigating
this evil include the introduction of improved appliances for burning
soft coal, and the use of other kinds of fuel. The electrification of
the railway lines entering cities is an important measure of relief.
It is estimated that more than one-third of the smoke found in certain
American cities comes from locomotives.
Systematic measurements of the amount of solid matter contributed
to the atmosphere by smoke have been made at various places in this
country and abroad, and yield startling figures. Measures of the
“sootfall” in Pittsburgh, before the evil there was mitigated, showed
an annual average deposit amounting to 1,031 tons per square mile.
London’s average is 248 tons per square mile for the whole city and
426 tons in the central districts. In the heart of Glasgow the annual
sootfall is 820 tons per square mile.
In Great Britain measurements and analyses of soot and the study of
its effects have been carried out on a large scale for a number of
years by the Advisory Committee on Atmospheric Pollution, attached
to the Meteorological Office. The Committee has installed “pollution
gauges,” of uniform type, at about twenty-five places in England and
Scotland. The soot that falls into these gauges is collected once
a month, weighed and analyzed. This organization also makes direct
measurements of the purity of the air, and has acquired a unique body
of observations that can be used to test the success of efforts made
to abate the smoke nuisance, besides providing interesting comparisons
between the incidence of respiratory diseases and the amount of solid
matter in the air.