Socads

If the gas-phase pressure of SOC(g) is P, then k{P(S SQ) k ,S(). (NN)

Rearranging, one obtains

vided by the total concentration of total suspended particles, TSP (ng m 3), where the proportionality constant M contains the conversion constants to convert the TSP concentration into the total surface area and F into the area of the adsorbed molecules:

where bL k[ k_{ (the subscript L is used for Langmuir). That is, the fraction of the total surface sites that are occupied by adsorbed molecules is given by the expression in (OO) and is usually denoted 0L.

In the limit of low gas-phase pressures of the adsorbing compound, which is true for SOC in the atmosphere, 1 » bLP and (OO) reduces to

Similarly, the right-hand side of Eq. (PP) can be converted into a term involving the gas-phase concentration A in Eq. (MM) using the ideal gas law, P (n V)RT. The number of moles per m3, (n V), is converted to ng m 3 using the molecular weight (MW) of the SOC. Combining the MW, R, T, and other conversion factors into one constant N, P NA, and the right side of Eq. (PP) becomes bLP bLNA. Equation (PP) therefore becomes

However, as first developed by Yamasaki et al. (1982) and subsequently by Pankow (1987), the fraction of the total surface sites that hold adsorbed molecules, Al, is just proportional to the concentration of adsorbed species on the particles, F (ng m 3), di-

However, as seen in Eq. (OO), bL k[ k_{, i.e., bL is in effect an equilibrium constant for the adsorption of SOC(g) on the solid and the reevaporation of SOC(ads) from the solid surface. Hence bL k[ k_[ K' and Eq. (RR) becomes

In short, the form of the gas-particle partitioning defined in Eq. (MM) is consistent with Langmuir adsorption of the SOC on the surface of the TSP.

the temperature dependence of pL as log pL (Torr) c T d.

Figure 9.61 shows some typical plots to test Eq. (VV) for some SOC in the form of higher molecular weight alkanes and some PAH (see Chapter 10) at various relative humidities (Storey et al., 1995). In this case, the gas-particle partitioning coefficient Kp has been normalized to take into account different surface areas by using a surface area normalized coefficient, K , defined as

K Kp (Specific surface area of adsorbing substrate). (WW) Figure 9.61 shows that log K s is indeed linear with log pL, and the slopes are typically close to 1 as expected. Fig. 9.61 also illustrates the effects of relative humidity (RH) on the adsorption of these compounds on a quartz fiber filter (QFF); the values of K decreased by about an order of magnitude as the RH increased from ~30 to 70% which was attributed to changes in the properties of the surface as water adsorbed onto it. Furthermore, the values for adsorption of these compounds on urban particulate matter (UPM) that had been observed in other studies (Yamasaki et al., 1982; Foreman and Bidleman, 1990) are clearly much larger than on the QFF. This suggests that if mineral oxide particle surfaces in the atmosphere behave like the QFF, adsorption to such inorganic sur

TABLE 9.24 Subcooled Liquid Vapor Pressures (pL ) at 20°C for a Series of Polycyclic Aromatic Hydrocarbons" and Their Temperature Dependenceb
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