Locatio r USE AND LIMITATIONS OF THE MIDGET X AND H,B. CHARMBURY The Pennsylvania S t a t e College, S t a t e College, P a e
W. L. CHEN
T
H h general publicity The midget impinger method for the collection and b b oi nig. per 100 square given atmospheric conanalysis of air-borne dust has been used to make a dust inches, are presented in Figtaminationandthepassageoi ure 2 and shorn a range oi survey in a small industrial town in Pennsylvania. AIthough only relative and dependent on a number of facI 12 t o 470. The average ordinances by sevelal large cities Cor the prevention of tors, the results indicate the method can be used with value for the 2-week period smoke have caused an inreasonable satisfaction for the locatian of dust-producing was 226 mg., which is eyuivcreased awareness of air pollualent to a deposition oi. 22 areas. The major advantages offered by the method are: tion problems in many of the tons per month per square an extensive survey can be made in a few hours' time; the smallel. cities and towns ,nile or a rate of depositior throughout the country. equipment is rugged and easily portable; and i t is not of 70 tons per mollth lor +,jle Smoke, dust, sulfur dioxide, necessary to employ highly skilled help in the COhctiorP whole town. Values in the of samples. Limitations of the method are discussed. and other types of air polluliterature (10) reported on tion from industrial areas Jimilar basis range frorn 24 tcb 156 tons per month per square have become knorvn to the mile. Although the preliminarh tests indicated in a general way public as a general nuisance, and the demand is iising in iriaiiy coirtthe nature of the dust problem in this particular cornmunit5 R munities for action to correct these nuisances. Such action was rpinore comprehensive and quanti study appeared desiiahl? ceritly called for by the citizens of a small industrial community to determine whether any correl existed betiveen dust conlocated in central Penasylvania, and resulted in the survey herein rent] ations a t various locations and the probable sources of dual reported on the extent and nature of the local dirt problem. from the industrial sections of the toMii. Many such studies have A preliminary study, intended primarily to develop information been made using a variety of techniques, but in general the test% on the nature of the dirt, mas made by collecting the material deme both time-consuming and expensive. In an endeavor to find a posited during a two-week period in eight boxes located at simpler and more rapid method, resort was made t o the use of t h e strategic points in the tonn. li microscopic examination of the midget impinger widely accepted in this country as a staiidard dirt from these boxes revealed that the major portion consisted of (18) for dust determinations and extensively employcd in studies partially burned coal particles, appearing a s cenospheres ranging of dust conditions in coal mines (3, 6, i4? 15) Despite certaiii in size up to 1000 microns; most of the remainder had thc general limitations, the method seems to have ciome merit for atmospk ' appearance of slag or flyash. Coal particles will tend to burn in pollution itridies. spherical shapes in order to expose the minimum amount of surface area during combustion. The spheres are full of h o l l o ~cone>, believed to be caused by escaping gas, which give the pa1 tides a THE IMPINGER AND I T S LIMITATIONS honeycomblike appealance. Figure 1 is a photomiri ograph of L'he Greenburg-Smith impingel method (6) for the collect itm the cenospheres showing particles ranging in siLe up to 150 arid analysis of dust wah deveIoped in 1922 and has been modified microns, but unfortunately the internal str11ctllves of the renoand developed over the subspheres are not clearly sequent years. One of thc evident major improvements ha,An ash determination oi been the development 0 1 the dirt showed the presthe so-called midget irnence of 43% ash and 57% pinger ( 2 j I d ) . This i~ncombustible which rompinger apparatus and oppares reasonably well with erating procedure have beer! the 35% ash and 65% coni-' described elsewhere (,%, id. bustible reported (10)as the Z?). and details nerd not bc average for similar samples repeated. The only difierfrom fourteen American encea between the midge1 cities. arid its predecessor are sim. Although useful only as po\%eelrequirements, nozl;lta qualitative indication as velocity, collection rate, and to the amount of diit dethe amount of eollcctirtg posited 011 this community, liquid required. the weight of dust colAlthough used extensivelj lected in these boxe.: of tor evaluating dust condt104.6 square inches area tioris in almost all dusty inwas determined. The' resiilts, calculated 0 1 1 th(1 Figure 1. Photomicrograph of Deposited Dirt Sample i1iiAtriw and particularly iri ~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
coal mines, there are certain limitations t o the method which have a direct bearing on the present investigation. Particle Size. The midget-impinger method was developed primarily as a means of evaluating hazardous dust conditions of a physiological nature, especially with reference to silicosis and anthracosis. Consequently, it was necessary that the method indicate only the number of air-borne particles in the size range of 1 to 10 microns believed to be harmful (7, 16) to the respiratory system, although more recent studies (8,11,20) have shown that factors other than size must be considered. Nevertheless, the present midget impinger results indicate only the number of particles within this size range and not the total number of particles actually present. Thus, the method yields only a relative result as, undoubtedly, a good portion of the air-borne dust is less than 1 micron in size. Method of Counting. The counting procedure specifies that the sample stand in the Sedgwick-Rafter cell for 25 minutes before the particles are counted. This time limit was established on the premise that a 1-micron silica particle in a n-propyl alcohol suspension would settle through the 1-mm. depth of the cell in the specified 25 minutes. Because the resolving power of the specified microscope ( I ) is approximately 1 micron, longer settling periods appeared unnecessary. However, when used for coal particles (specific gravity about 1.35) or cenospheres of variable but lower specific gravity, the standard settling time is inadequate. Cells of less than 1-mm. thickness could have been used, but it was thought desirable to use the most common size ( 2 ) so that comparisons could be made with other values in the literature. I n addition, shallower cells have not proved as satisfactory ( 2 ) due t o the reduced volume of liquid. Actual tests with sized aoal particles agreed well with theoretical calculations and indicated that a t least 125 minutes are required for all the particles of 1 micron size to settle 1 mm. in %-propyl alcohol into the field of vision. Since longer settling times cause difficulty due to evaporation of the alcohol in the cell, the specified time of settling was used with due recognition to the fact that relative and not absolute results were being secured.
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Sampling and Collection Efficiency. The midget impinger method is based on the determination of dust from a relatively small volume, usually 0.5 t o 2 cubic feet, of dust-laden air collected over a period of 5 to 20 minutes. Although not spot samples, in the generally accepted sense, the data do not represent average conditions over an extended period as do some of the methods employed for dust survey work. According to results reported by several investigators (9, 13, 19) the midget impinger apparatus will retain well over 90% of the dust particles, in the size range of 1 to 10 microns, which enter with the dust-laden air. Stability of Dust Particles. The procedure recommends that the dust counts be made within 24 hours of the time of sampling, but with the collection of a large number of samples this is not always either convenient or possible. Tests made during these studies have shown, however, that with bituminous coal the number of particles in a given sample appears t o decrease progressively for the first few days after collection and then to increase. These results suggest that in the analysis of dusts known to contain bituminous coal, the counts should be made as soon as possible after collection, for best reproducibility of results. Conclusions Regarding Use of Impinger Method. I n view of the foregoing considerations, it is apparent that the midget impinger method is purely empirical and gives only relative results which are subject t o errors in interpretation if consideration is not given the various factors that affect its use and application. Nevertheless, the method is relatively simple, permits a rather extensive survey t o be made in a relatively short time, gives results which are probably fairly indicative of conditions existing a t the time of sampling, and employs equipment that is rugged, easily transported, and operable by semiskilled technicians.
DUST SURVEY A survey of the air-borne dust concentrations in the aforementioned community has been made using the midget impinger method. Additional data regarding weather conditions a t the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
time of sampling were obtained as an aid in interpreting the results. The methods employed were as follows: Forty stations as indicated in Figure 2 were spotted on the map of the area to be surveyed, and a group of stations was assigned to each of three operators with midget impinger sets. At a prescribed time each operator started the collection of a 5-minute sample a t one of his assigned stations. On securing his sample each operator passed on to the next station as rapidly as convenient and continued in this manner until samples had been secured at each of the stations. T h e total period over which the 40 samples were collected was approximately 3 hours. At the start and end of the sampling, weather conditions were recorded from the roof of the tallest building in the town. A11 samples were returned to the laboratory for examination and analysis. At the start of the survey the temperature was 86" F., the relative humidity 36%, and the wind velocity 3 miles per hour from the north-northwest. At the completion of the tejt, the temperature had dropped to 68" F., the relative humidity had increased SO%, and the wind velocity and direction had changed to 2 miles per hour from the north-northeast. The results calculated into terms of dust concentrations in millions of particles per cubic foot were plotted on the map of the area (Figure 2), and dust concentration contour lines were drawn. DISCUSSION
The data shown in Figure 2 indicate that the dust concentrations range from 0.2 to 9.5 million particles per cubic foot of air, and that the dust concentration contours assume a definite pattern around the major dust-producing areas, Area A is the center of the town's railroad tracks where considerable dust might be expected from the stacks of coal-burning locomotives and where dust already present would be disturbed and resuspended in the atmosphere by the local and through railroad traffic. Area B is one of many railroad sidings where coal-burning locomotives are employed in frequent shunting operations. Area C is primarily one of industrial plants where various drying and combustion operations are performed. There are twelve major stacks in this area most of which are frequently emitting smoke or dust. The concentration contour lines definitely show a decrease in dust concentration as the distance from these areas increases. The one in the south section of the town shows a particularly sharp decrease in concentration within a relatively short distance. This may be explained by the fact that the wind was coming, in general, from the north and that the stacks from which the primary dust was emitted are comparatively low. The rather peculiar pattern shown by the contour lines in the northeastern section can only be explained by the double dust source and the change in wind direction during the test period. Undoubtedly, the dust concentrations and contour
Vol. 41, No. 11
pattern would be altered by major changes in wind velocity and direction, by thermal inversions, and by other atmospheric conditions (4,y ) , but it appears that the essential features of the pattern would remain substantially unchanged. I n fact, evidence to support this was obtained by securing samples a t a few of the stations on other dates. I n each case the dust concentrations decreased with an increase in the distance from the dust source. This same relation may also be observed with the deposited dust in the preliminary box test. ACKNOWLEDGMENT
The authors want to express their appreciation to T. R. Kemmerer, Jr., for his assistance in collecting and analyzing the impinger samples. LITERATURE CITED (1) Brown, C. E., arid Feicht, F. L., U.S. Bur.:Mines, Rept. Invest. 3821 (1945). (2) Brown, C . E., and Schrenk, H, H., Ibid., Inform. Circ. 7026
(1938). and Gardner, C:. R., Trans. Am. Inst. M ~ ? LMet. . (3) Davis, D.H., Engrs., Coal Technol., 149, 193 (1942). (4) Dept. Sci. Ind. Research (Brit.), Atm. Pollution Research, Tech. Paper 1 (1945). ( 5 ) Flinn, R. H., Seifert, 11. E., Brinton, H. P., Jones, J. L., and ' .S . Pub. Health Service, Bull. 270 (1941). Franks, It. W., 6 (6) Greenburg, L., and Smith, G. W., U . 8.Bur.'Mines, Rept. Invest. 2392 (1922). (7) Harrington, D., and Davenport, S. J., U . S.IBur. Mines, Bull. 400 (1937). (8) Hatch, T.,and Hemeon, W. C . L., J . I n d . Hug. ToXi~ol.,30, 172 (1945). (9) Hatch, T., Warren, H., and Drinker, P., Ibid.,14,301 (1932). (10) Ives, J. E., Britten, It. H., Armstrong, D. W., Gill, W. A,, and Goldman, F. H., U.S . Pub. Health Service, Bull. 224 (1936). (11) Landahl, €1. D., and IIerrniann, R. G., J . Ind. Hug. Toxicol., 30,181(1948). (12) Littlefield, J. B., Feicht, F. b.,and Schrenk, H. H., U . S. BUT. Mines, Rept. Invesf. 3360 (1937), (13) Ibid., 3401 (1938). (14) Owings, C. W., Mechanization, 10, No. 11,
Carbon Black Deposited from Smoke of Burning Benzene
51 (1946). (15) Owings, C. W.,U . S . Bur. of Mines, Rept. Invest. 3681 (1942). (16) Sayers, R. It., and Jones, R. R., Pub. Health Repts., 55, No. 33,
1453 (1938). (17) Schrenk, H. II., arid Feicht, F. L., U . 8 . Bur. M i n e s , Inform,. Circ. 7076 (1939). (18) Silverman, L., Ind. Hug. Am., Foundation Chem. and Toxicol. Ser., Bull. 1 (1947). (19) U . S. Pub. Health Seroice, Bull. 144 (1928). (20) Wilson, I. B., and La Mer, V. K., J . I n d , Hug.'Toxicol., 30,!265
(1948). RECEIVED March 21, 1949.
