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FIGURE 9.28 Size distribution of particle geometric cross section (A) as a function of geometric diameter for a typical rural aerosol (adapted from Hegg et al, 1993).

defined as the ratio of 6sp at 80% RH to that at a low reference RH (e.g., Waggoner et al., 1983a; Hegg et al., 1993). Changes in light scattering due to changes in RH again reflect the effects on particle size and the index of refraction. Hegg and co-workers (1993) treat the effects on particle size in terms of two components, the first a change on the geometric cross section of the individual particles, and the second a change in the particle size distribution. Figure 9.28 shows the size distribution of the geometric cross section of particles under low RH conditions and at an RH of 80%. The initial distribution represents particles typical of a rural continental site having a number mean diameter of Dn 0.076 /¿m, a geometric standard deviation of 2, and a number concentration of 2300 particles cm 3. Increasing the relative humidity to 80% results in a larger fraction (41 vs 16%) of the particles being in the range that is most efficient for light scattering, giving a hygroscopic growth factor in this case of 4.2. Values in the range of 1-3 are typical (e.g., see Hegg et al., 1996; Svenningsson et al., 1992; Ten Brink et al., 1996; and Mclnnes et al., 1998). The increase in RH is thus seen in this case to have major effects primarily through this shift in the particle size distribution into a more effective range for light scattering. Hegg and co-workers (1993) point out that this effect is expected to be the largest for particles in rural areas, where the size distributions often peak below the efficient light scattering range and which can therefore take up water and grow into this range. On the other hand, aged urban particles often already exist with sizes in efficient light scattering ranges so that the uptake of water has relatively less effect. For example, Mclnnes et al. (1998) measured hygroscopic growth factors for particles (using a reference RH of 40%) at Sable Island, Nova Scotia, Canada. Particles in air masses from the northeastern

United States that had been impacted by anthropogenic emissions had a hygroscopic growth factor of 1.7 0.1, compared to 2.7 0.4 for particles in marine air from the open ocean.

In short, the contribution of various components of particles to the scattering of light in the troposphere is complex, depending not only on the particular species of interest but also on its interactions with other constituents, on the initial particle size distribution, and on the relative humidity.

Significant advances have also been made in illustrating the effects of visibility reduction by generating simulated photographs calculated to mimic the scattering and absorption of light by particles typically found in the particular area of interest. Both ground-based photography (see, for example, Williams et al., 1980; Malm et al., 1983; Larson and Cass, 1989; Eldering et al., 1993; and Molenar et al., 1994) and satellite images (Eldering et al., 1996) have been mimicked using this approach, both very effective ways to demonstrate the effects of particles on visibility reduction. Significant progress has also been made on developing grid-based models that incorporate both physical and chemical aerosol processes to predict effects on visibility (e.g., see Eldering and Cass, 1996).

Not only light scattering but also light absorption by particles can occur. It is generally agreed that the major contributor to light absorption in the visible region is black or graphitic carbon, often referred to as elemental carbon. On a practical level, this is the aerosol component that is insoluble in organic solvents and is not oxidized at temperatures below 400°C (Penner and Novakov, 1996). However, as discussed by Chang et al. (1982), carbon particles in the air are made up of a number of crystallites 2-3 nm in diameter, with each crystallite consisting of several carbon layers having the hexagonal structure of graphite, Fig. 9.29. Because of the presence of defects, dislocations, and discontinuities, there are unpaired electrons that constitute active sites in the carbon; during the formation of the carbon particle in combustion processes, these active sites can react with gases to incorporate other elements such as oxygen, nitrogen, and hydrogen into the structure. Thus elemental, black, or graphitic carbon found in atmospheric particles is not chunks of highly structured pure graphite, but rather is a related, but more complex, three-dimensional array of carbon with small amounts of other elements. As discussed later, the presence of polar groups on the surface is believed to play an important role in determining their properties, such as uptake of water.

Being black, this atmospheric carbon is a strong absorber of visible radiation. The specific mass absorption coefficient, b , has been measured to be in the o 100 E

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