Health Assessment Document for Diesel Emissions - NSCEP | US ...

1 silica gel, and alumina. The extrapolation of sometimes contradictory results reported by
2 different laboratories to atmospherically realistic conditions presents major problems.
3 The atmospheric fate of particle-bound P AHs has received much attention since their
4 potential toxicity was first observed. In their recent publication, Behymer and Hites (1988)
5 define two opposite schools of thought on this subject. One says that particle-bound P AHs
6 degrade quickly in the atmosphere with lifetimes as short as a few hours (e.g., Kamens et al.,
7 1988; Nielsen, 1988; Behymer and Hites, 1988). The other says that PAHs degrade slowly, if at
8 all, in the atmosphere and eventually deposit on soil or water. The latter conclusion is supported
9 by the studies of marine and lacustrine sediments (the ultimate environmental sinks ofPAHs)
1 0 that have shown that the relative abundances of P AHs, even at the most remote locations, are
11 similar to those in combustion sources and in air particulate matter (Laflamme and Hites, 1978;
12 Hites et al., 1980; Me Veety and Hites, 1988).
13
14 2.4.2.2.1. Photooxidation of particulate polycyclic aromatic hydrocarbons. Laboratory studies
15 of photolysis of P AHs adsorbed on 18 different fly ashes, carbon black, silica gel, and alumina
16 (Behymer and Hites, 1985, 1988) and several coal stack ashes (Yokley et al., 1986; Dunstan et
17 al., 1989) showed that the extent of photodegradation of P AHs depended very much on the
18 nature of the substrate to which they are adsorbed. The dominant factor in the stabilization of
19 P AHs. adsorbed on fly ash was the color of the fly ash, which is related to the amount of black
20 carbon present. It appeared that P AHs were stabilized if the black carbon content of the fly ash
21 was greater than ::::5%. On black substrates, half-lives ofPAHs studied were on the order of
22 several days (Behymer and Hites, 1988).
23 Similar conclusions were reached from studies of photolysis of P AHs adsorbed on coal
24 stack ashes (Yokley et al., 1986; Dunstan et al., 1989). The relative quantity of carbon in coal
25 ash was the main factor determining the extent of photochemical degradation of pyrene and
26 benzo[a]pyrene adsorbed on the surface. In addition, in coal ashes that contained a relatively
27 large quantity of iron; the magnetic particles played a minor role in stabilizing adsorbed pyrene
28 toward photodegradation (Dunstan et al., 1989).
29 On the other hand, the environmental chamber studies of Kamens and co-workers (1988)
30 on the daytime decay of PAH present on residential wood smoke particles and on gasoline
31 internal combustion emission particles showed PAH half-lives on the order of 1 hat moderate
32 humidities and temperatures. At very low-angle sunlight, very low water-vapor concentration, or
33, very low temperatures, P AH daytime half-lives increased to a period of days.
34 Atmospheric studies by Nielsen (1988), carried out in rural areas during the winter and
35 early spring when ambient temperatures and concentrations ofN0 2 and 0 3 were low, showed
36 · evidence for atmospheric decay of more reactive PAHs, such as benzo[a]pyrene and
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