DUST. Dust is earth or other solid matter in a fine state of subdivision, so that the particles are light enough to be easily raised and carried as a cloud by the wind. The presence of dust in the atmosphere is obvious on the most casual inspection. Far tidies of dust, varying from the motes in the sunbeam to big flakes of soot, are readily visible to the naked eye. The origin of the dust is varied in character. Smoke from domestic and factory chimneys contains particles of carbon (soot), as well as small particles of ashes and drops of liquid tar. These particles are carried by the wind, and even spread upward through considerable heights by the turbulent or eddy motion of air (see METEOROLOGY : Turbulence) .
Over the deserts coarse dust is raised from the ground by wind storms, and can be carried for thousands of miles by the wind. The "red rain" which has been observed in Europe from time to time is due to the washing down of desert dust which has origi nated in tropical deserts. A notable example of "red rain" occur ring on Feb. 21 and 22, 1903, is described in the Quarterly Jour nal Roy. Met. Soc., xxx., 1904 p. 57, where it is shown that the dust must have travelled from north-west Africa round the west ern edge of an anticyclone over southern Europe. The amount of dust which fell in England during the two days in question was estimated at about ten million tons. The harmattan, a dry east erly wind which blows off the west coast of Africa between Cape Verde and Cape Lopez, carries with it quantities of dust, caus ing thick haze for a distance up to 15 m. from the shore. Enor mous quantities of volcanic dust are poured into the atmosphere by the eruption of volcanoes. The eruption of Krakatoa near Java in 1883 produced, among many effects, a vast cloud of small particles which is said to have taken two years to settle down completely. The dust from Krakatoa produced the most extra ordinary colours in the sky. Even in the British Isles sunsets of unusually gorgeous colours were observed. Meteoric matter disintegrating in the air is another source of atmospheric dust. A phenomenal dust storm visited the United States in May Rising from the parched soil of the Western plains, where drifts as deep as six inches often covered the highways, a vast cloud of dust moved slowly eastward, hung for a time in yellow haze over the cities along the coast and finally was precipitated in the At lantic Ocean.
The particles of dust or soot, from whatever source they origi nate, are distributed over a wide area by the wind. Granted a sufficient wind velocity, the smoke from the largest city will be distributed over such a wide area, and through so great a volume of air, that it cannot acquire a sufficient density to be troublesome. But when the wind falls below a certain limit, it ceases to be an effective distributor. Another factor of importance in connec tion with the concentration of smoke is the turbulence in the atmosphere, which scatters the smoke through a greater range of height than it would otherwise attain. When there is an increase of temperature with height (i.e., an inversion) instead of the normal decrease, turbulence is suppressed; since inversions of tem perature at the ground are usually associated with light winds, they generally give smoke fogs in large towns.
The rate of settlement of spherical particles of dust through a still atmosphere is given approximately by the formula, where V is the velocity in centimetres per second and R is the radius of the particle in centimetres. Thus a particle of unit density and of diameter i micron will fall through still air at a rate of about 0.003 centimetres per second, which is equivalent to a fall of 3o metres (ioo ft.) in 278 hours, during which time a wind of 1 o m.p.h. would carry it 2,780 miles. It is thus readily seen that smoke particles can travel over immense distances under favourable conditions. A drop of water of diameter io microns would fall with a velocity of .3 cm/sec. or roughly i o metres in one hour.
The first investigations of the amount of dust in the atmosphere were those of John Aitken in 1880. Aitken succeeded in show ing that water drops will not form, even in supersaturated air, in the complete absence of solid nuclei upon which the molecules of water can collect. It was but a step further, to utilize this idea to count the number of condensation nuclei in a given volume of air. Aitken's method consisted in placing the air under examination in an airtight receiver and saturating it with water vapour. It was then caused to expand adiabatically until con densation was produced, the drops being collected on a reticule and counted by the aid of a short focussed lens. Certain pre cautions were necessary in carrying out this procedure. It was found that if more than 500 nuclei were present in each cubic centimetre of air, they would not all form water drops. In such cases it was necessary first to dilute the air with air which had been carefully filtered, subsequently allowing for the dilution in computing the number of particles per second. Aitken's method was applied in thousands of tests in different parts of the world, and in no case was the air found to be completely free of nuclei. The numbers of particles per c.c. in cities such as London and Paris were often found to exceed 1 oo,000. Fridlander used Aitken's dust counter on a voyage across the Atlantic and never found values below 2,000 per c.c. but in crossing the Pacific and Indian oceans he found values as low as about 250 per c.c. But we can safely conclude that air, no matter where it is sampled, contains an ample supply of particles capable of acting as con densation nuclei. The instrument which Aitken devised for count ing the particles suspended in air was called a "dust-counter," but subsequent research has shown that the particles which Aitken counted were not dust in the ordinary sense of the word, but what are known as hygroscopic nuclei.
It was first suggested by Wigand (A. Wigand, Meteor. Zeit schrift. 3o p. 10 Jan. 1913) that the particles counted by Aitken's dust counter were not dust in the ordinary sense of the word. Wigand compared the figures obtained by Aitken's method in air which was artificially made dusty and found that the number of condensation nuclei per c.c. was not dependent on the amount of dust introduced into the atmosphere. This is confirmed by Owen's observations. A recent investigation by Boylan (Proc. Roy. Irish Acad. vol. 37A. N.O. 6, 1926) shows very definitely that the dust and nuclei are different, and that ordinary dust particles will not act as nuclei for condensation.
The horizontal visibility of objects on the earth's surface de pends upon the degree of pollution of the atmosphere. Poor visi bility is due to the obscuration of the atmosphere by water-drops or by solid particles. The condition known as haze is due to the presence in the atmosphere of small solid particles, whose diameter is about .5 micron (0005 mm.), together with varying quantities of water-drops. In a series of observations of haze by Owens it was found that in some cases no water-drops were present, while in other cases large numbers of water-drops were found. The dis tinction is largely a question of relative humidity, since haze is probably always a mixture of insoluble particles and hygroscopic nuclei. If the relative humidity is low, the haze effect is almost entirely due to small solid particles of dust whose diameter is of the order of micron; but if the humidity increases sufficiently, condensation takes place on the hygroscopic nuclei, and the haze changes into a true mist. Similarly fog may be divided into two classes—smoke fogs and water fogs. The diameter of the solid particles in a smoke fog vary from i micron up to several micron, while the diameter of water drops in a water fog vary from 5 to 20 microns. The fine smoke particles which produce the dust horizon frequently seen in the British isles have diameters of about .8 micron, while the Indian dust horizon consists of blown sand of similar size.
The most troublesome aspect of atmospheric dust is provided by the smoke produced in great cities. The problem of pollution produced by smoke has been the study of a special Advisory Committee on Atmospheric Pollution. The work of this committee has been summarized by Sir Napier Shaw and Dr. J. S. Owens in The Smoke Problem of Great Cities. Smoke as it issues from the chimney consists of particles of soot of varying sizes as well as acid products of combustion, small particles of ash, and small drops of tar. The larger aggregations of soot fall in the near neighbourhood of their source, while the smaller particles of diam eter rather less than one micron are carried to considerable dis tances. Shaw and Owens describe a variety of methods for the detection and measurement of the quantities of different forms of pollution in the atmosphere, and the reader is referred to their treatise for fuller details of these methods. Of particular interest, however, is the jet dust counter devised by Dr. J. S. Owens, de pending upon the principle that when air containing dust and a sufficient quantity of water vapour has its pressure suddenly re duced, there is a fall of temperature and a condensation of moisture into water drops.
It is not clear how the ordinary dust particles become attached to their share of water, but it appears probable that the particles which are hygroscopic become nuclei of condensation and that the condensed water captures the dust. When the dust-bearing water drops are brought into contact with a glass surface they adhere to it. In the Owens dust-counter the dusty air is drawn through a slit as a fine ribbon-shapecl jet, and impinges on a coverglass placed at a millimetre from the slit. The air is first passed through a damping chamber in which it acquires sufficient moisture to produce condensation. The lowering of pressure by which the air is drawn through the jet is sufficient to produce condensation in the air striking the coverglass. As the velocity falls off, the pressure and temperature rise, and the waterdrops evaporate from the coverglass, leaving the dust deposited. By a careful adjust ment of the amount of air drawn through the jet, a record is obtained on which the dust particles can be examined and counted by means of a microscope with inch oil-immersion objective.
The results derived by the use of the Owens dust-counter show a wide variation in the number and nature of the particles in the atmosphere. For example, in a dense fog on Jan. 22, 1922, there were 21,75o particles per cubic centimetre. The average diameter of the particles was o.85 microns, but a large proportion had diameters twice as great as the average. Somewhat similar results were obtained from a fog on Oct 26, 1921, but in this case there were numerous small spherical particles of diameters up to 0.85 microns. A slight haze in dry sunny weather yielded 1 oo to 200 particles per cubic centimetre, of sizes from 0.3 microns up to 1.7 microns. Further, it was found that when the damping chamber of the Owens dust-counter was slightly warmed, and a large quan tity of air drawn through the jet, the condensed water flowed out sideways in streams, and on evaporation left the soluble matter crystallized in the dried-up stream beds. The crystals could then be examined microscopically and micro-chemically. The deposit on the coverglass frequently showed needle-shaped and rhomboid crystals, sometimes with a tarry deposit. Most of the records obtained showed a number of transparent spherical particles, occa sionally accounting for as much as 5o% of the total number of particles counted. When very small these particles appear opaque, but when the diameter exceeds about 0.75 micron they show a bright centre. The spherical particles are insoluble in water, xylol, and cedarwood oil.
Perhaps the most remarkable fact brought out by the observa tions mentioned above is the remarkable uniformity as to the size of the particles. Shaw and Owens in the book referred to (p. 185) give a comparison between numbers of particles counted by the jet dust-counter, and the total impurity present in the atmosphere, showing a remarkably close proportionality. The number of particles counted should therefore give a measure of the degree of obscurity of the atmosphere. These numbers bear no relationship to the numbers of particles obtained in the Aitken dust-counter, since the latter may give extremely high values in apparently clear air.
The fact that the particles measured by the Owens dust-counter differ from those observed by Aitken is also confirmed by the observations made by Owens while crossing the Atlantic. No dust was to be recognized in any of the records, though some showed numbers of crystals, some of which were not hygroscopic and were only sparingly soluble in water. Aitken, on the other hand, never found less than about 2,000 particles in one cubic centimetre of air over the Atlantic.
During the first two or three years after the eruption of Krakatoa a reddish brown corona was often observed around the sun. It had an angular radius of to and was i o° to 12° wide. Pernter explained this phenomenon, which was known as Bishop's ring, as a result of diffraction of sunlight by small dust particles, and assuming the particles to be spherical he found their diameter to be 1.85 microns. It is of interest to combine this figure with Stokes's formula for the rate of fall of particles. We find their rate of fall should be 0.02 cm/sec. or io km. in a time between one or two years.
Another aspect of the optical effects of dust in the atmosphere upon sunlight relates to the loss of ultra-violet rays which is directly produced by dust and smoke pollution. In the Times of Dec. 22, 1924, Professor Leonard Hill gave a comparison of the relative amounts of ultra-violet light at different places in the British isles, showing the lowest values in the centre of London. In view of the importance of ultra-violet rays to human health, the results are highly suggestive.
It has been frequently suggested that a smoke fog such as a London fog might be penetrated by the longer infra-red rays. On theoretical grounds we should expect that infra-red rays of wave length large by comparison with the mean diameter of the smoke particles (o.8 micron), should show some degree of pene tration. In Wood's Physical Optics (3rd Edition p. 416), there is a statement that a film of smoke which was absolutely opaque to light was transparent to infra-red rays of wave length 1 oo microns. This result in itself is not suggestive of any practical solution of penetration, since there still remain the problems of finding not only suitable sources for such radiation but also means of rendering visible the rays which have penetrated the fog.
The problem is a long way from solution, and no definite observa tions of the penetration of fogs by infra-red rays of different wave length appear to have been published.
Country fogs, composed of drops of water whose diameter is of the order of 1 o microns, show no appreciable selective effect in the transmission of light, and there is no obvious reason for supposing that infra-red rays would penetrate such fogs.
F. Entwistle (Jour. Ry. Aer. Soc. vol. xxxii., p. 374, repro duces two interesting photographs by C. J. P. Cave, of a landscape—the one photograph taken in the usual way without a screen, and the second with a red screen, which cuts out all the blue light. The first shows a light fog, while in the second only slight traces of the fog are visible.
Humphreys develops in considerable detail the theory that the emission of large quantities of volcanic dust into the atmosphere can produce large variations of climate, of a sufficient magnitude to account for ice-ages. It is clearly established that after the eruption of Krakatoa in 1883 there were marked changes in the pyrheliometric measurements of solar radiation (Arctowski, An nals New York Acad. Sci. 26, 1915, p. 149, and Kimball, Monthly Weather Review, 1918, P. 355)• The theory, known as the vulcanism theory, has by no means met with general acceptance, though there is some evidence, especially in eastern Australia, of the association of glaciation with volcanic activity, which appears to bear out the theory. In any case, volcanic dust may well have been a deciding factor in starting glaciation when other factors were also favourable. For further details the reader is referred to Humphreys' Physics of the Air, and to works on climatology.