Document nkyZMvKGzqQK1GgkK6p3E4Ro6

186 CHAPTER IQ 1946 Gyide Compiled.by W. G. Frank and Copyrighted. Fig. 1. Sizes and Characteristics, of Airborne Particulate Matter in size: Tremendous numbers are also present in the sub-microscopic range below 0.5 micron, but those below 0.1 micron are not believed'at present to be of any practical importance, possibly due to their exceed ingly small mass in comparison with the balance of airborne matter. In fact, particles this small may become the permanent atmospheric impuri ties that have little if any opportunity of settling because of the continualmotion imparted to them by air currents and the molecular activity of their supporting gases (Brownian Movement). The'survey1 of atmospheric pollution, in 14 American cities conductedfrom 1931 .to 1933 indicated the average size of outdoor dust particle - to be 0.5 micron, as collected by the Owens jet dust counter and measured under:the microscope. The inability of the light field microscope i to /Air Contaminants 187 reveal particles in. the 0.1 micron vicinity may-account in part for this observed statistical average of 0.5 micron. The lower limit of particle size visible to the naked eye cannot be stated definitely. It depends not only upon the individual eye, but,also upon the shape and color of the particle, the intensity and quality of the light, and the nature of the background or the opportunity for contrast. Under ideal conditions a particle of 10-micron size may be recognized, while under less favorable conditions it may be impossible to distinguish a particle smaller than 50 microns. The lower limit of visibility should, therefore, be considered as a physiological range, probably 10 to 50 microns. Dusts, powders and granular materials are frequently classified by reference to the size of screens used for separation. Particles above 40 microns are said to be the screen sizes and those below, the sub-screen or microscopic sizes. The approximate or theoretical sizes of particles corresponding to the mesh scale of the U. S. Standard Sieve Series are given in Table 1. . Table 1. Relation of Screen Mesh to Particle Size U. S. Standard Sieve Mesh. 400 325. 200 140 100 60 35 18 Nominal Sieve - Opening in Microns----------- 37 44 74 105 149 250 500. 1000 Microscopic examination of screened dust indicates, that the average diameter of a sample of irregular particles may be substantially larger than the openings of the screen through which it has passed, if the particle shapes deviate considerably from the spherical form2. The smallest dimension of many such particles will correspond with the maximum, permissible distance between the wires of commercial screens made to ASTM Standard specifications. Screening does not give sharp separation into size groups, and accord ingly such a classification is statistical rather than absolute. Mineral particles, such as grains of sand, bits of rock, volcanic ash, or fly-ash, can be transported long distance's under unusual circumstances. Thus, the dust storms' of 1935 in the Kansas district resulted' in vast amounts of fine top soil being thrown high into the air. Solar illumination as far east as Boston was affected noticeably and particles as large as 40 to 50 microns were actually carried half way across the continent before they settled out. In similar manner volcanic ash has been carried even farther. It is not surprising, therefore, that fly-ash from furnace gases, cement dust and the like, -can be carried for considerable distances and that, occasionally, the engineer is confronted with the problem- of remov ing such material before the air in question is suitable for use in building ventilation.- AIR POLLUTION BY SMOKE, ASH AND CINDERS The total airborne solids settling in urban areas are usually reported as soot fall in tons per (square mile) (month). Such data published'for the cities in this country range-from 20 to 200 tons per (square mile)