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FIGURE 10.5 UV absorption spectrum of naphthalene ( = 8 X 10 "6 M in cyclo-hexane) (spectrum taken by Alisa Ezell, 1998).

et al. (1993), Turpin et al. (1993), Gundel et al. (1995a), Kamens and Coe (1997), and Feilberg et al. (1999a).

b. PACs

Compared to PAHs, much less has been published about the gas-particle distributions of PACs in ambient air. Table 10.11 gives average gas- and particle-phase concentrations, and their percentage in the particle phase, of several O-PAC and a widely distributed S-PAC, dibenzothiophene, determined at Portland, Oregon (Ligocki and Pankow, 1989), and in the Los Angeles area (Fraser et al., 1998). Again, an increase in the percentage found in the particle phase is seen with the larger compounds.

In summary, the smallest PAHs and PACs are found primarily in the gas phase and the largest compounds in the particle phase. Those of intermediate sizes partition between the gas and particle phases, with the distribution determined by a number of factors such as the temperature and mass and size of available particles to absorb/adsorb the PAH or PAC (see Chapter 9.D).

5. Absorption and Emission Spectra of Selected PAHs and PACs

As seen in Figs. 10.5-10.11, polycyclic aromatic hydrocarbons absorb in the actinic UV, A > 290 nm (Karcher et al., 1985). Their it —> 77* transitions are strong (much more intense than the corresponding n -> 77* transitions in aromatic carbonyl compounds), so PAHs have relatively large molar extinction coefficients, s (the exception is naphthalene, where the 77 —> 77* transition is "forbidden").

PAHs also generally have well-structured emission spectra (see Figs. 10.6-10.10) and relatively large fluorescence quantum yields. For example, in degassed n-heptane at room temperature, the fluorescence quantum yields are as follows: fluoranthene, 0.35; benz[a]anthracene, 0.23; chrysene, 0.18; BaP, 0.60; BeP, 0.11; and benzo[g/z/]perylene, 0.29 (Heinrich and Güsten,1980). Cyclopenta[c<i]pyrene, however, does not fluoresce.

These large fluorescence quantum yields provide a sensitive method of analysis for PAHs. During the 1950s and 1960s, strongly emitting trace impurities were often a major source of experimental artifacts that could negate the advantage of much greater sensitivity (by factors of ~102-10:!) of fluorescence over UV-visible absorption spectroscopy for PAH analysis. Since then, major advances have been made in the separation procedures and in the spectroscopic detection, identification, and quantification of small amounts of individual PAHs. For example, Mahanama and coworkers (1994) used a combination of absorption and fluorescence spectroscopy to identify and quantify concentrations of key PAHs in simulated and real environmental tobacco smoke (ETS) as well as the NIST Standard Reference Material SRM f 649 "Air Particles" (see Box 10.3). Table 10.12 summarizes the programmed wavelengths selected for the excitation and fluorescence of the individual PAHs and the results of these studies for SRM 1649 (Mahanama et al., 1994;

FIGURE 10.6 UV absorption and fluorescence spectra of phenanthrene in cyclohexane (adapted from Karcher et al, 1985).

FIGURE 10.6 UV absorption and fluorescence spectra of phenanthrene in cyclohexane (adapted from Karcher et al, 1985).

Fluorescence Phenanthrene

FIGURE 10.7 UV absorption and fluorescence spectra of anthracene in cyclohexane (adapted from Karcher et al., 1985).

FIGURE 10.7 UV absorption and fluorescence spectra of anthracene in cyclohexane (adapted from Karcher et al., 1985).

Gundel et al., 1995b). Advantages of using this approach include high sensitivity (50 ng per gram of sample in these particular studies) and the ability to discriminate between compounds such as benzo[6]flu-oranthene and benzo[/c]fluoranthene, which is difficult by other techniques such as gas chromatography.

Solvents also affect the measured absorption spectra. Thus, the ir,ir* bands shift to longer wavelengths (a "red" bathochromic shift) in polar solvents. For example, the long-wavelength band of anthracene shifts from ~375 nm in n-hexane to 381 nm in acetonitrile (Wehry, 1983).

FIGURE 10.8 UV absorption and fluorescence spectra of pyrene in cyclohexane (adapted from Karcher et al, 1985).

FIGURE 10.8 UV absorption and fluorescence spectra of pyrene in cyclohexane (adapted from Karcher et al, 1985).

FIGURE 10.9 uv absorption and fluorescence spectra of fluoranthene in cyclohexane (adapted from Karcher et al., 1985).

Adding N-, O-, or S-atom functionalities to a PAH can cause major changes in its UV-visible absorption spectrum. For example, as seen in Fig. 10.12, addition of N02 groups to BaP to form the 1-, 3-, and 6-nitro isomers results in pronounced red shifts in their absorption spectra (Pitts et al., 1978). This enhanced ability to absorb solar radiation has significant implica tions with respect to the atmospheric reactions, lifetimes, and fates of PACs. Thus, as discussed in Section F, photolysis significantly exceeds OH radical attack as an "efficient" loss process for some gas-phase ni-troarenes such as 1-nitronaphthalene (e.g., see Atkinson et al., 1989; and Feilberg et al, 1999a).

The physical state of a PAH also can have a dra-

Isomers Pyrene

FIGURE 10.10 uv absorption and fluorescence spectra of benzo[a]pyrene in cyclohexane (adapted from Karcher et al., 1985).

FIGURE 10.10 uv absorption and fluorescence spectra of benzo[a]pyrene in cyclohexane (adapted from Karcher et al., 1985).

FIGURE 10.11 UV-visible absorption spectrum of cyclopenta[o/]pyrene in cyclo-hexane (adapted from Karcher et al., 1985). This PAH does not fluoresce.

FIGURE 10.11 UV-visible absorption spectrum of cyclopenta[o/]pyrene in cyclo-hexane (adapted from Karcher et al., 1985). This PAH does not fluoresce.

TABLE 10.12 Concentrations of 10 PAHs in National Institute of Standards and Technology (NIST) Standard Reference Material SRM 1649 Air Particles Determined by Dual Programmable Fluorescence Detector Method

Compared to Their NIST Reference Concentrations"

TABLE 10.12 Concentrations of 10 PAHs in National Institute of Standards and Technology (NIST) Standard Reference Material SRM 1649 Air Particles Determined by Dual Programmable Fluorescence Detector Method

Compared to Their NIST Reference Concentrations"

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