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FIGURE 9.6 Number, surface, and volume distributions for a typical urban model aerosol (adapted from Whitby and Sverdrup, 1980).

eters between 0.01 and 0.08 /¿m, known as the transient or Aitken nuclei range.

As the technology for measuring small particles has improved (see Chapter 11.B), ultrafine particles have also been increasingly studied. While there is no fixed definition of these particles, they are usually taken to mean those with diameters less than 0.01 /jlm, i.e.,

In short, particles in the atmosphere are now frequently treated in terms of the four modes summarized in Fig. 9.7, which also shows the major sources and removal processes for each one. Although the vertical axis is not shown, it could in theory be any of the distributions discussed, that is, number, mass, surface, or volume.

Particles in the coarse particle range are usually produced by mechanical processes such as grinding, wind, or erosion. As a result, they are relatively large and hence settle out of the atmosphere by sedimentation, except on windy days, where fallout is balanced by reentrainment. Schmidt et al. (1998) also point out that particles generated in blizzards and sandstorms develop an electrostatic charge and that the electrostatic forces between the particles and the surface may be similar in magnitude to gravitational forces. These large particles

Chemical conversion of gases to low volatility vapors

Chemical conversion of gases to low volatility vapors

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