Handbook of Meteorology/The Dust Content of the Air
THE DUST CONTENT OF THE AIR
Dust is usually classed as “foreign matter of the air.” Such a view of the shell of wind-blown dust is permissible. It might also be considered logical as regards the finer dust particles which do not reach the ground except by means other than their own gravity. It is hardly logical to consider the dust of the stratosphere as foreign matter of the air; for, as a matter of fact, it is permanently and not temporarily there. Moreover, cosmic dust seems to pervade every part of the universe which the solar system traverses, and the earth is constantly gathering dust from space.
“Invisible” Dust; Characteristics.—Practically nothing is known of the dust of the stratosphere, except that its presence is revealed in various ways. The particles themselves are too small to be discerned by any mechanical method at present known. En masse they reflect enough sunlight to reveal their existence, but not their form nor their constitution. In part, and probably to a great extent, they constitute the overhead effect noted by observers for more than six thousand years—the sky. An estimate of the size of such dust particles cannot be made with any degree of accuracy. It is safe to say that they are much smaller than the smoke particles that escape from the burning end of a cigar. It safe also to say that they are smaller than the particles which constitute the “blue haze.” As a matter of fact, rapid changes in sky polarization indicate about the only thing that can be authoritatively asserted—namely, that the invisible particles behave much like the molecular constituents of the air.
Measurements.—Under the stratosphere, the dust particles of the air are of every possible size, from those of the blue haze to the coarse rock waste of the simoon. The micromillimeter, practically the twenty-five thousandth part of an inch, is a convenient unit of measurement. It is convenient because of the fact that dust particles of this dimension are very near to the size of the permanently floating dust motes of the air.
The research work of Dr. John Aitkin, the highest authority on the subject, has shown that clean air contains from 3000 to 5000 visible dust particles per cubic inch. The air of schoolrooms and public buildings with undressed wood floors carries from 60,000 to 80,000 particles which are visible under the high power of a microscope, and an unknown number which can be counted only when amplified in size by the condensation of moisture upon their surface.[1] Dr. Aitkin found the cleanest air at snow-clad heights in the Alps, and not over the sea, as one might expect.
Electrification.—To the best of knowledge, the invisible dust, both in the stratosphere and the sphere of convection, does not depend on winds for its distribution. The particles themselves behave as do other ionized bodies, and it is not impossible that their suspension in the air is due to electrification.[2] There seems to be no reason why the ionization of minute dust particles should not occur in the same manner as the ionization of the molecular constituents of the air.
Dust and Condensation.—The experimental work of Dr. Aitkin showed conclusively that the moisture of the air condensed with difficulty in dustless air even when the temperature was several degrees below saturation; in normal, or dust-laden air, condensation took place readily. The repetition of Dr. Aitkin’s experiments under widely diverse conditions has left no doubt that the dust particles of the air, including sulphur gases set free by the combustion of fuel, are the most important factors in condensation. The research of C. T. R. Wilson brought to light additional knowledge; Wilson found that the passage of a beam of ultra-violet light through air caused condensation, even when its temperature was slightly below that of saturation. Saturation temperature, however, is not wholly necessary to condensation; a certain but small amount of condensation goes on below the temperature of saturation. Condensation goes on more freely when the humidity—both absolute and relative—is high.
Dust particles differ greatly as nuclei of condensation; they may be “good,” “indifferent,” or “poor.” The reason for the difference is not known with certainty. It may be that quickly cooling particles are better condensers than slowly cooling particles; it may be that a high degree of ionization favors rapid condensation; or that the more hygroscopic a particle the more freely it condenses. Each is a reasonable hypothesis that remains to be substantiated.
Barus and Pierce have shown that the dust particles over Providence, a manufacturing center, are far more favorable to condensation than those observed contemporaneously at Block Island. The reason therefor may be a difference in the chemical character of the dust particles; it may be due to a difference in the degree of ionization; it may be due to other and unknown causes.
One thing, however, is certain: The dust particles belched from the stacks of manufacturing districts are such excellent nuclei of condensation that the prevalence of fogs over such districts has given rise to the term “city fogs,”[3] as distinguished from the ordinary advection fogs. The distinction is a practical one. It is pertinent to add also that a city fog forms usually under a lid. But while the city fog condenses on particles that are hygroscopic, the fogs of swamp lands, rivers, and ponds condense on particles that are materially different. Condensation does not take place so readily—in other words, the dust particles are indifferent nuclei. A thick fog condensed on nuclei of an indifferent sort may be “eaten up” by a slight rise in temperature; it may rain itself to pieces by a drop in temperature.
Sources of Atmospheric Dust.—Aside from the dust picked up and carried by the winds, there are well-defined sources of floating dust that must be considered. Cosmic, or meteoric dust, is not born of the earth; it is gathered by the earth from space. Large particles fall to the earth; but those materially less than a micromillimeter constitute the floating dust of the air. The character of this dust can be recognized only when the particles fall to the earth or are trapped while floating near to its surface.
Many of the particles thus caught are tiny meteorites. These, in many instances, are metallic globules or floating metal bubbles. They are essentially different from the metallic particles of smeltery dust, emery-wheel dust, and brake-shoe dust, which also are metallic. The cosmic dust of non-metal character cannot be recognized with any degree of certainty; indeed, recognition of any sort of dust whose particles are less than a micromillimeter is difficult. The gathering of cosmic dust seems to be constant rather than sporadic.
Additions of volcanic dust to the floating dust of the air are made irregularly, but they come in enormous quantities. Much of the ash[4] falls to the ground, but a very large part consists of particles fine enough to constitute floating matter. The eruption of Krakatoa,[5] in 1883, projected so much floating dust into the air that the trail, which girdled the earth several times, was visible at sunset for nearly two years. The blood-red sky[6] at times rivaled the northern lights. Less marked red sunsets followed the eruptions of La Soufrière and Mont Pelếe in 1902. The explosion of a great quantity of munitions in New York Harbor was followed for several days by red sunsets observable as far west as the Weather Bureau station at Ithaca, N. Y. The dust mantle of the Greenland glacier is apparently of volcanic origin. Indeed, volcanic dust is always an important constituent of the floating dust of the air; at times it is the chief constituent.
The floating dust of the air has a marked effect upon its temperature. Benjamin Franklin noted this fact. During several months in 1783, the air was filled with floating volcanic dust. “The sun’s rays were indeed rendered so faint in passing through it that, when collected in the focus of a burning glass, they would scarcely kindle brown paper.” The heating power of the sun was so feeble that freezing temperatures began nearly a month before their normal occurrence. “Delaware River was closed in November and remained ice-bound until late in March.”[7]
The years 1812-1816 were years of great volcanic activity, and the air was loaded with floating dust. As a result, the year 18 16 has gone into history as the “year without any summer”—the year of “eighteen hundred and froze-to-death.” In Vermont snow fell and frosts occurred every month of that year. On the 8th of June, snow on the uplands was 5 or 6 inches deep.[8]
Humphreys has shown that, with a blanket of volcanic dust in the air, while the earth is receiving a lessened amount of heat from the sun it is radiating into space about thirty times as much.[9]
The products of combustion must also be taken into consideration as having a similar effect on absorption and radiation of heat. The world’s fuel consumption each year is the equivalent of about 1,500,000,000 tons of coal. Forest fires and grass fires add to the total of combustion whose products in part escape into the air. By their means an enormous number of dust particles are projected into the air and distributed through it. As a rule, the dust particles of fuel combustion are nuclei favorable to condensation. One cannot estimate even broadly the extent of air pollution from this source; it can be measured chiefly in terms of city fogs.
The suspended matter of combustion products has been measured at times. Systematic measurements both of suspended matter and of matter which is brought to the ground by rainfall have been made in various parts of England, at regular stations. The insoluble matter caught in gauges consisted chiefly of smoke carbon, a mixture of free carbon and heavy hydrocarbons, minute globules of liquid tar and insoluble ash. The soluble matter consisted of various sulphates, chlorine, ammonia, and soluble ash. The amount varied from a few hundred tons per year on each square mile to nearly 6000 tons per square mile. Measurements in several manufacturing districts of Pennsylvania showed an average of about 1900 tons per square mile per year falling to the ground.[10]
In regions where smokeless fuel is used there practically is no smoke problem, and the pollution of the air is confined almost wholly to wind-blown dust and to local sources of pollution. In localities swept by sea winds, salt derived from wind-whipped spray is usually a factor in the floating dust. In most of the large seaports of the United States the chlorine content from this source is made a matter of systematic measurement. The tendency of tools and polished steel articles to become rusty in the vicinity of the sea is probably due as much to the chlorine content of the air as to the presence of excessive moisture.
Wind-blown Dust.—In regions of sparse vegetation, where the ground is bare, enormous amounts of loose rock waste are moved hither and thither by the wind. The increase of the carrying power of the wind with increment of velocity is almost beyond belief. When the velocity of the wind is doubled its carrying power is increased sixty-four fold. In regions of loose rock waste the wind becomes a wonderful physiographic factor. The broad, intermontane valleys of the plateau region have been filled with rock waste, much of which is wind-blown; and the floors of the deeply filled valleys have been made level by wind-blown dust. The plains to the eastward of the Rocky Mountains are deep with wind-blown dust. More dust and rock waste is deposited in the rivers of this region than they are able to carry. Platte River, popularly described as “a mile wide, an inch deep, and bottom on top,” is an instance of a river drowned by the rock waste which it cannot carry.
Winds blowing steadily for centuries have carried fine rock waste from the Gobi far into eastern China, choking the gorge of the Hoang in places with wind-blown dust more than ioo feet deep. The loess deposits in the lower course of the Hoang are also of wind-blown dust, which has been dumped into the river in quantities greater than the river could carry. In 1851 the channel had become clogged to the extent that the river broke its banks near the city of Kaifeng, abandoned the old channel to the delta of the Yangste, and made a new channel to the Pechili. The sediment with which the river had clogged its channel was wind-blown dust. In general, the action of the wind in unswarded regions is one of leveling. It wears away the high spots and fills the low spots.
In regions of generous rainfall, the surface is covered with vegetation to the extent that very little rock waste is exposed to the action of the wind. About the only physiographic action consists of the formation of sand dunes to the leeward of ocean and lake shores. In various instances these have gradually traveled a distance of several miles inland, ceasing to advance when growing vegetation has anchored the sand in place.
In cities and much-traveled rural districts, the wind-blown dust is picked up mainly from dirt streets, school playgrounds, and other unswarded areas. The wind-blown dust from these consists chiefly of loose dirt, paving material, garbage, finely pulverized horse dung, and foliage dust. The dust carried by winds blowing over areas of orchard and shrubbery usually contains the spores of fungi, pollen in season, the spores of various moulds, the eggs of insects and the dust scales of moths. Winds blowing over swampy areas are apt to have a generous content of the micro-organisms common to swamps. Dry air contains the spores of micro-organisms; moist air is often rich in the organisms themselves.
Bacterium Content of Dust.—Dr. T. M. Prudden exposed Petri dishes, each varnished with a gelatine culture medium, for five minutes in different parts of New York City. The dishes were set aside for several days. Each micro-organism falling on the plates developed into a “colony.” The colonies were counted with the following result:
| 1. | Ball ground, Central Park, a westerly wind |
499 |
| 2. | Union Square, at fountain |
214 |
| 3. | A private library |
34 |
| 4. | An uptown dry-goods store, near Broadway |
199 |
| 5. | Broadway and 35th Street, small park |
941 |
| 6. | A cross street, after sweeping |
5810 |
Examination of dust collected by the author in school rooms and from the book shelves of a public library yielded results similar to those obtained by Prudden.
The foregoing presents general principles worth noting: micro-organisms may fall to the ground and become a part of the dust of a public street. They also float a long time—some of them permanently—in the air. Exposures 1, 5, and 6 show that, when the air is in motion, the bacterium content is much greater than when the air is still. Measurements made at the direction of the Transvaal Chamber of Mines showed that dust particles 1 micromillimeter in dimension required about five and one-half hours to fall a distance of 7 feet. At the Mount Vernon laboratory, particles of the same dimension required from six to ten hours to fall 9 feet. Equally important is the fact that, when the air is stirred by sweeping, the tramp of footsteps, or the passage of vehicles, its bacterium content—and also its dust content—is much greater than when it is still.
- ↑ The dust-counter used by Dr. John Aitken consisted of a chamber or receiver, into which a measured portion of air was drawn. The receiver contained a small amount of water—enough to keep the air pretty nearly at saturation. A slight reduction of temperature by means of an air pump causes almost instant condensation. By counting the droplets condensed on a ruled silver plate within the receiver, using a magnifying lens therefor, the number of droplets per cubic centimeter, or per cubic inch, may be estimated. Dr. Aitken obtained the best results when the dust content of the air was small. In practice he therefore mixed the air to be examined with a measured quantity of air made dustless by filtration. A modified dust-counter, the “koniscope” is a more practical instrument, though not so accurate.
- ↑ A solid of 1 inch cubic measurement, weighing 1 ounce, has 6 square inches of surface. If it be shaved into slices one one-thousandth of an inch in thickness, each slice loses 999 parts of the original weight but only a little more than 4 parts of the original surface. That is, in subdivision, a substance loses weight much more rapidly than surface. The weight of a dust particle one twenty-five thousandth part of an inch in dimension is less than one fifteen trillionth of an ounce. The surface is almost infinitely great in comparison. Now, the electric charge of a dust particle, condensed on its surface, is of the same kind as that of the earth. Therefore they mutually repel. It is only fair to add that the theory of the electrification of dust is not fully substantiated.
- ↑ It has been pointed out that sulphur dixoide molecules in themselves are not “good” nuclei. Sulphur dioxide is a gas and is not to be included in the dust content of the air. But the intense heat of combustion has separated it from the combination in which it existed. The chemical affinity of the nascent gas is strong, and in the air it is apt to enter into combination again with dust particles of basic character, the resulting combination forming nuclei favorable to rapid condensation.
- ↑ Volcanic “ash” is not a product of combustion. It is the convenient name applied to lava blown into fine dust by the expansive force of steam, or by other forces.
- ↑ The eruption, which threw the ash into the air, had proceeded for several days, during which time the coarser dust fell on the nearby islands and into the sea. This, a normal eruption, was separate and distinct from the explosion which shattered the island.
- ↑ By reflected light, fine dust particles tend to a whitish color, and to a bluish tint if very fine and fewer in number. The purity of the tint depends, to a certain degree, on the size of the particles. By transmitted light, especially when the sun is near the horizon, the blue and the violet rays are absorbed and scattered and the red rays reach the eye of the observer. When the air is full of floating dust, the scattering of blue and violet rays is very great.
- ↑ The Philadelphia Inquirer.
- ↑ Thompson’s History of Vermont.
- ↑ Physics of the Air.
- ↑ H. H. Kimball.