MEASUREMENT OF PARTICLE SIZE DISTRIBUTION IN AEROSOLS 83 SPECIAL INSTRUMENTAL METHODS The difficulties and time-consuming character of the direct determina- tion of particle size distribution in aerosols have led to the development of a number of instrumental methods(5, 6, 7). The objectives are to avoid some of the sampling problems by examining the particles while in sus- pension, or to decrease the time for a determination through the use of indirect measurements such as light scattering or automatic particle count- ing. Figure 3.--Aerosoloscope. An instrument named "aerosoloscope" for the automatic counting and sizing of aerosol particles in suspension has been recently described by Fisher, Katz, Lieberman and Alexander(8). A photograph of the aerosolo- scope is shown in Fig. 3. The instrument operates in the following manner: (1) The aerosol to be studied is pumped into the instrument at a rate of 1.8 1./min. (2) The aerosol is mixed and diluted as required. Liquid as well as solid particles or mixtures of the two can be handled without excessive par- ticle loss or evaporation. (3) The aerosol particles pass through an illuminated region, where each particle produces a pulse of scattered light.

84 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (4) The light scattered by each individual particle is received on a photo- multiplier, giving rise to an electrical pulse whose amplitude is related to the size of the particle. (5) The electrical pulses are amplified and then sorted into one of 12 chan- nels corresponding to 12 size ranges of aerosol particles. The counts are registered separately on a set of 12 electro-mechanical counters. (6) The instrument has built in provisions for monitoring air flow rates, electrical calibration and standardization, time of sampling and other ' necessary controls. The general performance of the aerosoloscope has been evaluated using a number of aerosol materials such as glass beads, iron particles, air cleaner test dust, pollens, water droplets and dibutyl phthalate. Good agree- ment with other methods has been obtained using these materials. A 8O 60 50 40 30 20 % IO _• 8 • 5 4 ß Aeroso[oscope o M•croscope I 5 I0 20 50 70 90 99 Figure 4.--Comparison of counting methods for glass beads. 9 - 4 - 3 - 2 - I comparison of aerosoloscope data with direct microscopic measurement for glass beads is shown in Fig. 4. Since the aerosoloscope actually meas- ures the light scattering ability of the particles, and not the size directly, it is necessary to calibrate the instrument for particular aerosols. Thus, the data shown in Fig. 4 serve as a calibration for glass beads. Once this has been accomplished for a specific material, a particle size measurement can be conducted automatically in less than a minute. Aerosol particles ranging from 1 to 64: microns in diameter can be measured at a maximum rate of 2000 per minute. REFERENCES (l) Fulton, R. A., Proc. Chem. Specialties Mfg. ,,tssoc., 40th Ann. Meeting, 157 (1953).