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" Adapted from Kamens et al. (1995).

h Subcooled liquid vapor pressures in Torr at 25°C calculated by Kamens et al. (1995) from Hawthorne et al. (1989).

' Concentration of aerosol, ¿ig particle density 1.25.

" Adapted from Kamens et al. (1995).

h Subcooled liquid vapor pressures in Torr at 25°C calculated by Kamens et al. (1995) from Hawthorne et al. (1989).

' Concentration of aerosol, ¿ig particle density 1.25.

and PACs being sampled. However, Arey and coworkers (1989a) found that even with four PUF plugs, fluorene (and certainly not naphthalene and the meth-ylnaphthalenes) was not collected quantitatively and some breakthrough of phenanthrene also occurred, especially during daytime sampling periods. Thus seven of the volatile 2- and 3-ring PAHs were collected on Tenax-GC high-flow cartridges, and naphthalene, because of its very high concentrations (up to 6f00 ng m-3), on a low-flow cartridge. Tenax-TA solid adsorbent also effectively traps these volatile PAHs (Zielinska et al., 1996).

Because of these sampling difficulties, the reported gas-phase concentrations of the semivolatile 3- and 4-ring PAHs are "operationally defined"; i.e., they are the quantity of a given PAH extracted from the solid adsorbent PUF plugs (or other solid sorbents such as Tenax cartridges). Similarly, the 5- and 6-ring PAHs along with that fraction of the semivolatile PAHs collected on the "upstream" filter are "operationally" defined as being in the particle phase.

We emphasize the term "operational" when describing the gas- and particle-phase concentrations of PAHs sampled using this popular filter/sorbent technique, because it has been recognized for some time that this and other methods are subject to both positive and negative artifacts (e.g., see Van Vaeck and Van Cauwenberghe, f984; Fitz et al., 1984; Coutant et al., 1988; Baek et al., 1991b; Kaupp and Umlauf, 1992; Hart and Pankow, 1994; Kamens and Coe, 1997; and Feilberg et al., 1999a). These artifacts may lead to significant deviations of the measured gas and particle concentrations of semivolatile organics (SOCs) from their "true" equilibrium concentrations used in theoretical treatments of the gas-particle partitioning of PAHs (see Chapter 9.D and references cited above).

As examples, the term "blow off" refers to a phenomenon in which the pressure drops across a filter can cause particle-associated SOCs (e.g., 3- and 4-ring

PAHs) to be stripped from the collected particulate matter and trapped downstream on the PUF sorbent, leading to an underestimate of their true particle-phase concentrations and an overestimate of their gas-phase levels. However, several studies (e.g., McDow and Huntziker, 1990; Turpin et al., 1994) suggest that this is less of a problem than adsorption of organics; see following discussion).

Conversely, gaseous semivolatile organics can adsorb to filter surfaces, resulting in artificially overestimating their particle-phase concentrations and underestimating their gas-phase levels. For example, Hart and Pankow (1994) conducted a study using two filter/PUF sorbent samplers similar to the apparatus in Fig. 10.4 but each incorporating a "backup" filter immediately downstream from the regular filters used to collect particle-phase PAHs. They found that for a typical sampling event in ambient air, gas adsorption to a single quartz fiber filter in a conventional Hi-Vol sampler could shift the apparent gas-particle partitioning parameter in favor of the particle phase, leading to overestimations by factors ranging from ~f.2 to 1.6. However, a second sampler operating in parallel and using a single Teflon membrane filter gave gas-particle distributions approximately the same as the corrected quartz fiber filter values.

Gas-particle partitioning is also impacted by changes in ambient temperature and aerosol concentrations during sampling, resulting in artifacts in the relative particle- and gas-phase concentrations (see Hart and Pankow, 1994).

A sampling system designed to reduce artifact problems was employed by Liang and co-workers (1997) in an experimental chamber study of the gas-particle partitioning of several PAHs (and n-alkanes) on three types of model aerosols and ambient urban particulate matter. The system consisted of a dual glass fiber filter system as described by Hart and Pankow (1994), followed by two parallel sampling trains for trapping gas-phase species. One had two sequential PUF plugs and the other had two Tenax-GC cartridges in sequence to trap the more volatile species.

An alternative approach is that using denuder-based samplers for determining gas-particle partitioning of semivolatile PAHs and other organics (see Feilberg et al., 1999a; Chapter f f ,A.3b has a description of denud-ers.) In principle, gas-phase PAHs in ambient air are first trapped on a denuder surface coated with a sor-bent material (e.g., a resin); particles pass through and are collected on a follow-up filter. In practice, experimental problems exist, for example, in extracting quantitatively species from the sorbent coating on the denuder walls. Gundel and co-workers (1995a) addressed

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