Info

" From Pankow and Bidleman (1992). Original data were for 25°C and published by Yamasaki et al. (1984). They were corrected to 20°C by Pankow and Bidleman (1992). h Temperature dependence given by log px (Torr) e T d.

" From Pankow and Bidleman (1992). Original data were for 25°C and published by Yamasaki et al. (1984). They were corrected to 20°C by Pankow and Bidleman (1992). h Temperature dependence given by log px (Torr) e T d.

faces will not be significant in urban areas (but may be more important in remote areas).

It should be noted that a similar trend in Kp with relative humidity appears to apply to the gas-to-particle distribution of PAH in urban areas (Pankow et al., 1993), although in studies by Cotham and Bidleman (1992), RH did not appear to significantly affect the adsorption of organochlorine pesticides, at least over the range of RH of 30-95%.

While many sets of data appear to follow Eq. (VV) relatively well, with slopes of m, f as predicted, deviations in the values of mT and br are often observed. There are a number of reasons for such deviations (e.g., see Pankow and Bidleman, 1992). For example, changes in temperature, concentrations of SOC, and relative humidity during sampling, nonattainment of equilibrium, and sampling artifacts can all lead to deviations from the predicted, equilibrium relationship. In addition, if (AHd AHvap) in Eq. (UU) is not constant along the series, relationship (VV) will not hold because the value of br is changing.

However, it is also the case that slopes different from mT 1 can occur even under equilibrium conditions, due to the nature of the molecular interactions involved in adsorption on a surface. For example, Goss and Schwarzenbach (1998) propose a modified formulation of Eq. (VV) that more explicitly takes into account the interactions between the SOC and the surface on a molecular level. Thus, based on work by Goss (1997), they express the relationship between a gas-particle partition coefficient if,ads, defined as the adsorbed con centration on the surface (in mg m 2) divided by the gas-phase concentration (in mg m 3), and pL in the following form:

ln*C,ads [ 0.133(yvdw)°'5]ln pL 2.09(yvdw)"'5

The first two terms represent van der Waals interactions between the adsorbed SOC and the surface, which would apply to all SOC. The second two terms represent Lewis acid-base interactions, which can be important for compounds containing O, N, or aromatic rings, for example, the adsorption of alkyl ethers on the polar surface of quartz. The y coefficients (in mJ m 2) describe the surface properties, where yvdw is associated with its van der Waals interactions with adsorbing gases, y describes its electron-acceptor interactions, and y describes the electron-donor interactions of the surface. On the other hand, the properties of the adsorbing species are described by In pL for the van der Waals interactions and by the dimensionless parameters and which relate to the electron-donor and electron-acceptor properties (if any), respectively, of the adsorbing molecule.

This formulation explicitly accounts for both the properties of the surface and those of the adsorbing SOC that determine on a molecular level the amount of adsorption of the gas on the surface, and hence the gas-particle partitioning. For example, Goss and Schwarzenbach (1998) describe the implications of three

PAHs

Was this article helpful?

0 0

Post a comment