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Particle diameter (|rm)

FIGURE 9.23 (a) Aerosol particle size distribution measured at Pomona during the 1972 State of California Air Resources Board ACHEX program, (b) Calculated optical scattering by particles, 6sp, for measured size distribution (adapted from Waggoner and Charl-son, 1976).

Particle diameter (|rm)

FIGURE 9.23 (a) Aerosol particle size distribution measured at Pomona during the 1972 State of California Air Resources Board ACHEX program, (b) Calculated optical scattering by particles, 6sp, for measured size distribution (adapted from Waggoner and Charl-son, 1976).

For example, Fig. 9.23a shows the measured volume distribution of one ambient aerosol sample. When this volume distribution is multiplied by the size distribution of the scattering coefficient per unit volume in Fig. 9.22, one obtains the calculated curve for light scattering in Fig. 9.23b. It is seen that the particles in the 0.1-to f-/j,m-diameter range, that is, in the accumulation mode, are clearly expected to predominate the light scattering.

This is supported by the correlation between the total aerosol volume of particles with diameters in the 0.1- to I-pm range and the experimentally determined values of 6sp obtained using a nephelometer in many studies (e.g., Fig. 9.24). The slopes of lines such as that in Fig. 9.24, however, depend critically on the nature and history of the air mass and can vary by more than a factor of fO from clean, nonurban air to highly polluted air in the vicinity of sources. For example, Sverdrup and Whitby (f980a) have shown that the ratio of submicron aerosol volume to 6sp, which corresponds to the slope of the line in Fig. 9.24, varies from 5 to 80, depending on the nature of the air mass. In addition, the correlation between 6sp and fine particles is usually not as clear-cut as seen in Fig. 9.24.

Consistent with the relationship between the aerosol fine particle volume and the particle scattering coefficient, a number of studies have shown that the fine particle mass and bsp are also related. Figure 9.25 shows the scattering coefficient observed in studies in Denver, Colorado, by Groblicki and co-workers (1981) as a function of the observed mass in the tine and coarse particle ranges, respectively. It is seen that a good linear relationship exists between bsp and the fine particle mass (FPM) but not between 6sp and the coarse particle mass. This has been observed in a number of areas ranging from pristine to industrial, with the ratio of the scattering coefficient to the fine particle mass concentration (bsp FPM) being approximately 3 in many areas (Waggoner et al., 1981; Conner et al., 1991).

Because of the dependence of Mie scattering on the refractive index and hence chemical composition of the particles, one would expect the light scattering coeffi

Refractive Index Mie Scattering

FIGURE 9.24 Plot of measured aerosol fine particle volume (including only those particles of 0.1- to l.O-jiim diameter) versus measured 6sp. Measurements were part of the State of California Air Resources Board ACHEX program (adapted from Waggoner and Charlson, 1976; data supplied by Dr. Clark of North American Rockwell).

FIGURE 9.24 Plot of measured aerosol fine particle volume (including only those particles of 0.1- to l.O-jiim diameter) versus measured 6sp. Measurements were part of the State of California Air Resources Board ACHEX program (adapted from Waggoner and Charlson, 1976; data supplied by Dr. Clark of North American Rockwell).

100 150 200 250 Fine particle mass (p.g m"3)

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