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Pratsinis et al., 1984

" From Japar et al. (1986). h At altitudes from 0.2 to 3 km.

" From Japar et al. (1986). h At altitudes from 0.2 to 3 km.

Graphitic carbon also scatters light; in one study, its mass scattering coefficient was found to be approximately the same as that for sulfate (Appel et al., 1985). Because of its contribution to both scattering and absorption of light, graphitic carbon tends to play a proportionally much greater role in light extinction than its contribution to the particulate mass would suggest. For example, Horvath et al. (1997) report that 13% of the total light extinction in Santiago, Chile, was due to absorption by elemental carbon, but an additional 14% was due to scattering; as a result, more than a quarter of the total light extinction by aerosol particles could be attributed to elemental carbon.

This disproportionate effect on light extinction of elemental carbon compared to its mass fraction has been reported in a number of areas. For example, in Denver, graphitic carbon was found to represent 15% of the fine particle mass but contribute ~35% to the total light extinction (Groblicki et al., 1981); at Zilna Mesa, Arizona, during one sampling period, graphitic carbon comprised 1.4% of the total aerosol mass and 3.5% of the fine particle mass but was responsible for 15% of the light extinction (Ouimette and Flagan, 1982). Similarly, in one Los Angeles area study, graphitic carbon was found to represent ~ 8.5-10% of the fine particle mass but to account for ~ 14-21% of the total light extinction (Pratsinis et al., 1984).

The absorption of light by carbon is sufficiently strong that it has been used as a marker of long-range transport in unusual circumstances. Thus, the plumes from the burning of oil wells in Kuwait were detected at the Mauna Loa Observatory in Hawaii in the form of increased 6ap values. Before the oil well burning, 6ap was in the (0.1-1) X 10 7 m 1 range, whereas during the burning, values as high as 8.6 X 10 7 m 1 were measured (Bodhaine et al., 1992).

Table 9.9 summarizes the results of some recent studies of the contribution of various particle components to light scattering. The contribution of absorp tion, when measured, is also shown. Such data suggest that sulfate and organics are major contributors to light scattering, with the contribution of nitrate being more variable. Light absorption, even in these non-urban areas, appears to be significant as well. (It should be noted, however, that apportionment of light scattering to various components of the particles is sensitive to whether it is assumed that the particles are internally or externally mixed, which is not well established in most cases; e.g., see Malm and Kreidenweis, 1997.)

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