The aerosol flow rates for the cylindrical and conical centrifuges may appear to be rather low compared to other instruments used to precipitate airborne particles by centrifugal force. In comparing different instruments used to precipitate airborne particles within a given size range by centrifugal force, a parameter of great interest is the number density of particles precipitated per unit of time. Instruments which precipitate particles in a cumulative distribution have a larger total aerosol flow than spectrometers such as ours which precipitate particles in a discrete size distribution, provided the total gas flow is comparable through both types. In instruments of similar dimensions and comparable flow rates, the number density of a monodisperse aerosol precipitated per unit time is the same for both types of instruments; for the instrument precipitating particles in the discrete size distribution, however, the area of deposition is smaller by the ratio of the aerosol flow rates than the deposit in the instrument precipitating particles in a cumulative distribution. For the instruments described in this paper, a 0.557-pm. polystyrene aerosol is deposited over an area of 63 mm. * in the cylindrical centrifuge and over an area of 69 mm. * in the conical centrifuge at the volume flow rates and rotational speeds given above. In contrast, the instrument described by Goetz, Stevenson, et al. (1960) operated under similar conditions but with a total aerosol flow of 2.5 liters per minute, will precipitate the same sized aerosol over an area of 1500 mm.2. In the case of a polydisperse aerosol, an instrument yielding a cumulative size distribution is a disadvantage if the deposition is to be analyzed for particle size distribution because of the difficulty of analyzing large particles in the presence of a large number of smaller particles. Application to Measurement of Aerodynamic Diameter Any instrument, such as the ones described in this paper, which precipitates particles in a centrifugal force field can be used to measure the aerodynamic diameter of aerosol particles. As can be seen in the photographs (Figures 3 and 5 ) , the aerodynamic diameter of aggregates of various numbers of single particles can be determined. The ratio of the aerodynamic diameter of an aggregate of spheres of the same diameter to the diameter of a single sphere in the aggregate is shown in Table I. These values agree with those of Stober, Berner, et al. (1969) which were compiled from the experimental results of several investigators. Greater significance is placed on the values for the ratio of the aerodynamic diameters of doublets and triplets to single particles because these are the averages of many measurements taken with different particle sizes and rotational speeds. No significant dependence in the aero-
Table I. Ratio of Aerodynamic Diameter of an Aggregate of Spheres of the Same Diameter to the Diameter of a Single Sphere in the Aggregate 1
2 3 4 5 6 7 8 9 10 11
1.185 1.336 1.45 1.53 1.65 1.73 1.80 1.87 1.92 1.97
1.190 1.335 1.44 1.57 1.65 1.71 1.80 1.85 1.93 2.00
dynamic diameter ratios upon the centrifugal force field or particle size was found. Acknowledgment The authors are indebted to H. Paul Geisert for his skilled machining of the spectrometers and the preliminary scanning electron microscope work done by Jack Hutchinson, International Business Machines, Boulder, Colo. Literature Cited Berner, A., Reichelt, H., Staub28, 158 (1968). Goetz, A., Geojis. Pura Appl. 36,49-69 (1957). Goetz, A., Stevenson, H. J. R., Preining, O., J. Air Pollution Control Assoc. 10, 378-414 (1960). Hauck, H., Schedling, J. A., Staub28,18-21 (1968). Kast, W., Staub 21,215-23 (1961). Keith, C. H., Derrick, J. C., J . Coll. Sci. 15, 340-56 (1960). Sawyer, K. F., Walton, W. H., J . Sci. Instr. 27,272-6 (1950). Stober. W.. Berner., A.., Blaschke. R., J . Coll. Interf. Sci. 29. 710-19 (1969). Stober, W., Zessack, V., Zentralblatt fiir biol. Aerosolforschung, 13,263-81 (1966). Walkenhorst, W., Bruckmann, E., Staub 26,221-5 (1966). Wells, W. F., Am. J . Public Health 23,58-9 (1933). Receiced for reciew December 16, 1968. Accepted June 9, 1969. This study was supported, in part, by the National Science Foundation in connection with its contract with the National Center for Atmospheric Research. D. H. was supported by a Public Health Sercice International Postdoctoral Research Fellowdiip (No. I F05-TW-1090-01). Presented in part at the Dicision of Colloid and Surface Chemistry, 156th meeting, A C S , Atlantic City, N . J., September 1968.
END OF SYMPOSIUM
Correction CHEMISTRY OF MANGANESE IN LAKE MENDOTA, WISCONSIN In this article by J. J. Delfino and G. F. Lee [ENVIRON. SCI. TECHNOL. 2, 1094 (1968)], in Table 11, footnote c should read 1.35 x 10-7.
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