Number, Mass, Surface, and. Volume Distributions

The atmosphere, whether in remote or urban areas, always contains significant concentrations of particles, up to 10x cm These may have diameters anywhere within the entire range from molecular clusters to 100 /xm. Because the size of atmospheric particles plays such an important role in both their chemistry and physics in the atmosphere as well as in their effects, it is important to know the distribution of sizes. We thus consider first how these size distributions are characterized. 

A second problem in expressing the size distribution of aerosols is that the intervals of diameter over which it is experimentally convenient to measure the number of particles are not constant in terms of either D or log 

For example, as discussed in Chapter 11, a multistage impactor is often used to measure the number of particles in certain size ranges. The size intervals are 

FIGURE 9.2 Plot of number of particles (AO against D (D aerodynamic diameter) determined using a four-stage impactor with the cutoff points given in Table 9.1. It has been assumed that the particles are spherical with density 2.6 g cm 3. Data from Wesolowski et al. (1980). 

However, it is not only the number of particles in each size interval that is of interest but also how other properties such as mass, volume, and surface area are distributed among the various size ranges. For example, the U.S. Environmental Protection Agency s air quality 

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