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FIGURE 9.11 Surface distribution of particles from the combustion of several organics and from automobiles and a candle (adapted from National Research Council, 1979).

Two peaks in the accumulation mode may also result if the particles are externally mixed rather than internally mixed (the latter meaning that all particles have the same composition corresponding to a mixture of the various components). That is, the composition of individual particles may not be the same as the overall bulk particle composition, with some having more hygroscopic components than others. These are referred to as "externally mixed" particles; indeed single-particle analyses suggest that externally mixed particles are common (see Chapter 11.B.4). In this case, a

Aitken Nuclei

Accumulation Mode

Coarse Particles

Fresh

Aged

High temperature combustion ^

Accumulation Mode

Coarse Particles

Fresh

Aged

High temperature combustion ^

Coagulation/condensation Conversion

(near plume)

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