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0.1 1 10 20 Particle diameter (urn)

FIGURE 11.61 Theoretical response of a typical single-particle counter (adapted from Whitby and Willeke, 1979; data from Cooke and Kerker, 1975).

importance of instrument calibration using standardized aerosols with an index of refraction as close as possible to the sample being measured; since the refractive index of atmospheric particles varies from 1.33 for water to 1.7 for minerals, they recommend using a calibration aerosol with m = 1.5. Because light scattering is very dependent on the particle shape, when measuring irregularly shaped particles such as coal dust, one should calibrate the instrument with aerosols generated from the same material. Figure 11.62, for example, shows the instrument response as a function of particle diameter for an ideal calibration aerosol of dioctyl phthalate and for coal dust particles.

Potential problems with using single-particle counters in ambient measurements and ways to minimize these are discussed in detail by Whitby and Willeke (1979).

Optical counters allow relatively rapid measurements of the size distribution and, unlike some of the other methods of size fractionation, include volatile particles in the measurement. However, some care must be taken in interpreting the detailed shape of the size distribution spectrum because of some anomalies that have been observed; for example, around the 1-pm region, interference from light that is reflected or refracted from the front and back of the particle gives a "knee" in many calibration curves of number of particles versus their diameter (LBL, 1979).

Electron and optical microscopies. Counting the particles and measuring their sizes can be done by optical or electron microscopy, the former for particles with diameters from ~0.4 pm to several hundred microns,

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