Vii

Dibenz[a,//]acridine

The physical and chemical complexity of primary combustion-generated POM is illustrated in Fig. 10.1 (Johnson et al., 1994), a schematic diagram of a diesel exhaust particle and associated copollutants. The gasphase regime contains volatile (2-ring) PAHs and a fraction of the semivolatile (3- and 4-ring) PAHs. The particle-phase contains the remainder of the semivolatile PAHs ("particle-associated") along with the 5- and 6-ring heavy PAHs adsorbed/absorbed to the surface of the elemental carbon spheres that constitute the "backbone" of the overall diesel soot particle. Also present is sulfate formed from oxidation of sulfur present in the diesel fuel and gas- and particle-phase PACs.

The elemental carbon core particles ("black carbon") range from ca. 0.01- to 0.08-/um aerodynamic diameter. They not only add significantly to the total mass of the aerosol particle but, as seen in Chapter 9, are also a

Solid elemental carbon spheres

;0J> Surface sulfate

FIGURE 10.1 Schematic of a diesel soot particle consisting of an agglomeration of elemental carbon spheres (0.01- to 0.08-/im diameter). Its surface is covered with absorbed/adsorbed particle-phase organics, including 5-ring (e.g., BaP) and 6-ring PAHs. Gas-phase organics include all of the highly volatile 2-ring PAHs (e.g., naphthalene and methylnapthalenes). Semivolatile 3-ring (e.g., phenanthrene and anthracene) and 4-ring PAHs (e.g., pyrene (II) and fluoranthene (V)) are distributed between both phases. Sulfate is also associated with diesel particles. (Adapted with permission from Johnson et al., 1994, SAE Paper 940233 © 940233 Society of Automotive Engineers, Inc.; see also Schauer et al., 1999.)

Solid elemental carbon spheres

;0J> Surface sulfate

Gas phase ■ organics

Surface-absorbed / adsorbed organics

FIGURE 10.1 Schematic of a diesel soot particle consisting of an agglomeration of elemental carbon spheres (0.01- to 0.08-/im diameter). Its surface is covered with absorbed/adsorbed particle-phase organics, including 5-ring (e.g., BaP) and 6-ring PAHs. Gas-phase organics include all of the highly volatile 2-ring PAHs (e.g., naphthalene and methylnapthalenes). Semivolatile 3-ring (e.g., phenanthrene and anthracene) and 4-ring PAHs (e.g., pyrene (II) and fluoranthene (V)) are distributed between both phases. Sulfate is also associated with diesel particles. (Adapted with permission from Johnson et al., 1994, SAE Paper 940233 © 940233 Society of Automotive Engineers, Inc.; see also Schauer et al., 1999.)

significant cause of visibility degradation in polluted atmospheres (e.g., see Larson et al., 1989).

Polycyclic aromatic hydrocarbons and their associated more polar (hence more water soluble) heteroatom derivatives (polycyclic aromatic compounds, PACs; see later) not only are present in air as gases and particles but also are present, for example, in urban "street dust" samples (Takada et al., 1990) and other soil and water environments throughout the world due to wet and dry deposition (see, for example, Schwarzenbach et al. (1993), Wild and Jones (1995), and Neilson (1998)). Furthermore, through long-range transport of polluted air masses, PAHs can be found in ambient air at receptor sites far from their original sources. For example, PAHs from the European continent and Great Britain have been observed at a "background site" at Birkenes, Norway, and in Copenhagen, Denmark (Bjorseth et al., 1979; Bjorseth and Olufsen, 1983; Nielsen et al., 1999a, 1999b). PAHs from the former Soviet Union have been reported to reach the Norwegian Arctic (Pacgna and Oehme, f988) and sources in Africa and in northern Europe (e.g., Germany, Belgium, and France) have been reported to impact Corsica, France (Masclet et al., 1988).

Of particular interest in terms of atmospheric chemistry are reactions of certain PAHs in VOC-NOx-air atmospheres to form biologically active polycyclic aromatic compounds, PACs. Thus, not only is the fundamental chemistry of the formation and fates of these secondary air pollutants of interest, but it can also have major toxicological implications. For example, in some airsheds certain PACs that are reaction products (e.g., nitro-PAH and nitro-PAH lactones) contribute signifi cantly more to the overall, direct-acting bacterial mutagenicities of the gaseous and particle phases of ambient air than do the PACs in the primary emissions directly emitted by sources. Furthermore, certain of them, e.g., 2-nitrofluoranthene (XXVII), are human cell mutagens:

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