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).
FIGURE 9.29 Structure of elemental carbon.
range from 2 to 20 m2 g 1 (e.g., see Horvath, 1993; Liousse et al., 1993; Hitzenberger et al., 1996; Petzold et al., 1997; Kuhlbusch et al., 1998; and Moosmiiller et al., 1998). Not surprisingly, the absorption depends on the wavelength of the light. Moosmiiller et al. (1998) report that bdp varies with A 2J, whereas Horvath et al. (1997) report that the absorption coefficient for aerosol in Santiago, Chile, assumed to be due to elemental carbon, varied with A °-92. Because of the nature of its sources, its contribution to light extinction varies geographically and temporally. For example, wood-burning fireplaces and diesels are major sources of elemental carbon, and areas with large numbers of these two sources generally have more graphitic carbon in the atmospheric aerosol, and hence more light absorption. Where wood burning is significant, more particulate graphite carbon would be expected in winter than in summer.
Absorption of light by carbon is expected to lead to heating of the atmosphere since the light energy is converted into thermal energy (see Chapter f4.C.fb). This is the opposite effect from scattering of light by particles back into the upper atmosphere. This heating effect would be expected to be most important in polluted urban areas (e.g., see Liu and Smith 1995; and Horvath, 1995).
Table 9.8 summarizes some measurements of the contribution of light absorption by particles, bdp, for different types of locations from urban residential to remote. In urban areas, b.dp varies from ~13 to 42% of bcxl, whereas in very remote areas, the contribution of absorption is much less. This is not surprising since the combustion sources producing graphitic carbon tend to be in urban-industrial areas. However, it is noteworthy that soot has been found associated with many sulfate particles even over the remote oceans (Buseck and Posfai, 1999).
There are other visible light absorbers than elemental carbon present in particles. These include carbonyl compounds from the oxidation of larger organics in the atmosphere (see Section C.2) and some soil components such as hematite (Fe203) (see Chapter 14.C.lb). Their contribution to total light extinction in various regions from urban to remote is not well understood, but it appears that it can be significant. For example, Malm et al. (1996) assessed the contributions of various particle components to scattering and absorption of light at the Lake Mead, Arizona, recreation area and at 18 sites in national parks in the western United States. The contributions of organic carbon, elemental carbon, and soil components to the total absorption of light by particles at Lake Mead were estimated to be approximately equal, at 18, 21, and 19% of the total absorption, respectively (the remainder was due to an unknown absorber or set of absorbers). At the national park sites, the contributions of organic and elemental carbon were again about equal, at 45 and 41% of the total absorption.
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