Document G5xaozoQV7Q21DNvLYEdXEO0V

168 |PMlJTICLESJ SCALE OF ATMOSPHERIC IMPURITIES CHAPTER 10 1948 Guide RATE OF [NUMBER SURFACE StMETFT.UPN.MG.[rj OnFaPtsAwR-(ASRQEUAARINE LAV/5 OF SETTLING FOR (mean INCHES spneresI AIR CONTAINING OF I 0006 0RAINS OF IN RELATION. TO : PARTICLE SIZE .IMPURITIES PER (lines of DEMARCATION appro*. ) !cu.n.(DEN*m-0 ' Compiled by W. G. Frank and Copyrighted. Fig. 1. Sizes and Characteristics of Airborne Particulate Matter 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 continual motion 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 conducted from 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 to Air Contaminants 169! reveal' particles in the 0.1 micron -vicinity! may: have influenced-the determination of average particle size.. "' 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 pf;the light, and'the nature of the background'or the opportunity for contrast. Under ideal conditions a particle of 10-micron size might be recognized, while under less favorable conditions it may be impossible to distinguish a.particle smaller than 50 microns; The lower limit,of visibility probably ranges from 10'to 50 microns. - ' ! .7;7 -. . . 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. - - 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 Table 1. Relation of Screen Mesh to Particle Size U. S. Standard Sieve Mesh,,'-__ Nominal Sieve. Opening in 400 ` 37 325 44 200 . 140 100 ` 60 35 18 7 ... ` ' i 74 1Q5 ; 149/! 250 ; 500 iobo ; shapes deviate considerably- from the spherical form 2. ; The smallest dimension of many such particles will correspond with the maximum permissible distance between the wires of commercial screens made to ASTM Standard1 specifications. Screening does not give sharp'sepkratiqn into size groups, and accordingly such a classification is statistical rather than absolute. - - . .'\~ AIK 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) (month). To the air conditioning engineer thisinformation may indicate the effectiveness of smoke abatement or fuelcornbustioncontrol methods in his locality, but-it does not provide a suitable index'of the .Suspended dust that air cleaners in a ventilating system are expected to Capture * * K Gravimetric or weight data of the type given in Tabled are preferable. In some cases airborne particle counts may be necessary, as for pollen, bacteria, spores, and insoluble dusts causing illness or lung disease. ? - Dust concentrations by weight cannot be converted readily to con centrations by particle, count because of the variability of particle size', shape and specific gravity, arid-the inherent characteristics of. dust Coiniting ' and weighing procedures. - One rnilligram ;of dust: per cubic