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FIGURE 9.9 Particle number size distributions measured at Cheeka Peak, Washington, in 1991 (adapted from Quinn et al., 1993).

FIGURE 9.9 Particle number size distributions measured at Cheeka Peak, Washington, in 1991 (adapted from Quinn et al., 1993).

Based on such observations and proposals, Ondov and Wexler (1998) have offered a modification of the Whitby accumulation mode, summarized in Fig. 9.10. High-temperature combustion sources produce particles in the Aitken nuclei and the accumulation mode. For example, the mass median diameters of particles emitted from a variety of combustion sources, including incinerators, coal- and oil-fired boilers, and automobiles and trucks, typically fall in the range of 0.05-0.35 ¡xm (Hildemann et al., 1991a, 1991 b; Ondov and Wexler, 1998). The relative numbers of particles produced in the Aitken nuclei range compared to the accumulation range depend on the nature of the combustion process (e.g., fuel and operating conditions) as well as the conditions of dilution. Figure 9.11, for example, shows the surface distribution of particles produced by the combustion of several organic compounds as well as by automobiles and by a burning candle. The "dirtier" flames (e.g., the candle and the acetone flame) produced significant numbers of particles in the accumulation mode, whereas the cleaner flames produce more Aitken nuclei (National Research Council, 1979).

Because the particles generated in high-temperature combustion processes contain hygroscopic compounds, water vapor can be taken up or evaporate, depending on the atmospheric conditions. In addition, the particles can be taken up into clouds and fogs. The condensed-phase water provides a medium for atmospheric reactions that generate low-volatility species; the best known example is the oxidation of S02 to sulfate (see Chapter 8.C.3). When the water evaporates, the remaining particle contains this additional material and hence has grown to a larger size (Fig. 9.10; see also Section B-3).

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