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log Kqa

FIGURE 9.64 Plot of log Kp for 10 PAHs in ambient air samples collected at seven sites worldwide as a function of calculated values of log KnA. FLE fluorene, PH phenanthrene, AN anthracene, PY pyrene, FL fluoranthene, BaA benz a anthracene, CHR chrysene, BaP benzo a pyrene, BeP benzo e pyrene, BkF benzo k fluoranthene (adapted from Finizio et al„ 1997).

urban Chicago area in February and March (Harner and Bidleman, 1998). The solid line shows the predicted percentages calculated using the Junge-Pankow adsorption model, Eq. (TT). Figure 9.65b treats the same data set in terms of absorption into a liquid using the octanol-water partitioning coefficient assuming the fraction of organic matter, fom, in the particles is either 10 or 20%. Both models are in reasonable agreement with the PAH data for winds from the southwest. With winds from the northeast, enrichment of the PAH in particles is observed compared to model predictions using the absorption model (Fig. 9.65b), which the authors suggest could be due to nonattainment of equilibrium or to the trapping of nonexchangeable PAHs in the particles. The adsorption model overpre-dicts the percentages of PCBs expected to be found in the particles, but the KOA absorption model is effective in explaining the field partitioning data for PCBs and PCNs.

In short, data such as those in Figs. 9.64 and 9.65 support the use of the octanol-air partitioning coefficient as a useful parameter for characterizing gas-particle partitioning of SOC into liquid particles or liquid layers on particles in air.

As discussed elsewhere in this book, there is increasing evidence for reactions at the air-water interface in the atmosphere. Pankow (f 997) has treated partitioning of gases to the interface as well and predicts that as for adsorption on a solid and absorption into a liquid, there should be a linear relationship between log Kp and In pL with a slope of approximately 1.

This discussion of gas-particle partitioning has focused on the idea that equilibrium between the two phases is attained in the atmosphere. However, it should be noted that equilibrium cannot always be assumed in the atmosphere. For example, Wania et al. (1998) ex amined the exchange of SOC between the air and the surface under two assumptions, one being equilibrium and one that treats the kinetics of the exchange of the SOC with the soil and with incoming and outgoing air. They show that some atmospheric observations can be explained by the kinetic effects. For example, the concentration of y-hexachlorocyclohexane (HCH) in the gas phase has a significant temperature dependence while a-HCH does not. This is unexpected for two such similar molecules if equilibrium gas-particle partitioning controls the gaseous concentrations. Wania et al. (1998) suggest that a-HCH concentrations are determined largely by the kinetics of transport of air containing this compound to the measurement sites, since it is distributed globally and hence there are large reservoirs such as the oceans. y-HCH, on the other hand, has lower global "background" levels but has been used extensively in industrialized countries, leading to higher concentrations downwind. Hence they attribute the -y-HCH levels, measured at sampling sites located downwind, to a shift in the equilibrium toward the gas phase as soil temperatures increased, leading to increased evaporation.

Because of such "real-world" nonequilibrium situations, some efforts, for example, by Turco and coworkers (Jacobson et al., 1996), Kamens and co-workers (Odum et al., 1994), and Seinfeld and co-workers (Bowman et al., 1997), have focused on developing dynamic models to describe gas-particle distributions.

In a similar vein, the time scales to achieve equilibrium for inorganics have been examined by Meng and Seinfeld (1996), who show that small (submicron) particles can come to equilibrium with the gas phase in less than a few hours typically but that larger particles may not. The major factors determining the time needed to reach equilibrium are the aerosol size distribution,

FIGURE 9.65 Observed percentages in the particle phase of PAHs (( ) N-E air; (*) S-W air), PCBs (( ) nonortho, ( ) monoortho, and multiortho PCBs, respectively), and (•) PCNs in Chicago air compared to model predicted values, (a) Solid line is calculated with Junge-Pankow (J-P) adsorption model (Eq. (TT)). (b) Solid and dotted lines are calculated with absorption model for aerosols assumed to contain 10 and 20% organic matter (om), respectively (adapted from Harner and Bidleman, f998).

FIGURE 9.65 Observed percentages in the particle phase of PAHs (( ) N-E air; (*) S-W air), PCBs (( ) nonortho, ( ) monoortho, and multiortho PCBs, respectively), and (•) PCNs in Chicago air compared to model predicted values, (a) Solid line is calculated with Junge-Pankow (J-P) adsorption model (Eq. (TT)). (b) Solid and dotted lines are calculated with absorption model for aerosols assumed to contain 10 and 20% organic matter (om), respectively (adapted from Harner and Bidleman, f998).

temperature, and accommodation coefficient for uptake of the gas into the particle (see Chapter 5).

In short, traditional equilibrium theories of gas adsorption provide at least a qualitative framework for describing the partitioning of SOC between the gas and particle phases in the atmosphere, and more recent theories are providing further insight into these partitioning processes in the atmosphere.

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