Air side resistance

The air-side resistivity of the cobalt chloride modified polyimide film was Increased to the value observed with a nonmodified BTDA-ODA polyimide film while, the cobalt chloride modified BDSDA-ODA polyimide film had an increase in surface resistivity of only about three orders of magnitude after soa)cing this film in water. The variable temperature air-side surface resistivity profiles for the cobalt chloride modified BDSDA-ODA polyimide film before and after a water soa)c are shown in Figure 4. 

Note i lWw measures the size of the air-side resistance relative to the water-side resistance (Eq. 20-30). 

Of the two resistances that limited uptake, the model indicates that the air-side resistance was dominant.37 Expressed in simple terms, for compounds with partition coefficients in the order of 107 or more, the plant does not see enough air in its lifetime to allow it to achieve equilibrium. Given this and the fact that the PCDD/Fs were orders of magnitude away from equilibrium, equation (1) can be simplified to... 

This states that the concentration of the lower chlorinated PCDD/Fs in vegetation is the time integral of the product of the air-side mass transfer coefficient (the inverse of the air-side resistance), the specific surface area of the vegetation and the gaseous air concentration. 

For gases passing between the atmosphere and ocean, two resistances are important (Fig. 2). These resistances are the boimdary layers on the air and water sides of the interface. At some distance from the interface on both sides, the mediums are well mixed. As the interface is approached, turbulent mixing is damped by viscous forces and, very near the interface, diffusion becomes important for mass transfer. The resistance to transfer of any gas is the sum of the air side and the water side resistance. For gases that are highly soluble or react with water, e.g. SO2, NH3 and H2O vapor, the water side resistance becomes less important and the air side resistance dominates. In this case, it is passage through... 

Air-side resistance is calculated using an empirical equation with air diffusivities (Da, cm sec" ), Henry s constant (H, Pam- mol" ), wind speed at 10mheight (t/ o, m/sec)and... 

Air side resistance The partial air side resistance defined as the air side diffusivity divided by the depth of the stagnant air boundary over water (sec cm ). 

The qualitative nature of the air-side resistivity-temperature profiles for the ion-modifi d polyimides categorized em as either semiconductors or insulator. The magnitude (10 -10 ohm) of the... 

Table 19.1 contains a list of chemicals and their properties that are likely to be found in both urban air and urban surface films. At this point we must consider the situation of a gas-phase chemical in relation to a bulk surface film containing a fraction of organic material, /oo, into which the gas-phase chemical will partition. Afa as discussed in Section 19.2.2, can be approximated by/oc oA- In this case, foe becomes a proportionality constant translating between the sorptive capacity of octanol and the film s sorptive capacity. We retain the use/oc as a proportionality constant, which is consistent with its use to describe organic carbon in other matrices such as soil and vegetation. As seen in Table 19.1, A fa is a highly variable parameter and its numerical value will impact the value of Ka- From Equation 19.2, we see that this resistance-in-series expression combines the individual MTCs. (See Chapter 4, Section 4.4.3 for details on its development.) Thibodeaux and Diamond (in preparation) present an analysis and options for the transport of organic molecules within the film by assuming it is composed of various material compositions. Details on the composition and physical structures of surface films are still uncertain at this time. Three film types were assumed air-filled porous material, water-filled porous material, and an organic matter, gel-like material. Calculations were performed using Equation 19.2 and in all three cases, they indicate that the air-side resistances are greater than those for the film-side except for the volatile chemicals such as benzene.

Table 19.2 summarizes air-side MTCs obtained from various literature sources. The MTCs increase as a function of wind speed windy-daytime represented by 5-10 mile h (8-16 km h ) winds and calm nighttime represented by 0.5-3.0 mile h (0.8-5 kmh ) winds with average MTCs of 500 and 100 cmh", respectively. This is equivalent to air-side resistances (i.e., I/ a) of 0.002-0.01 hern", respectively. Therefore, as with other interfacial exchanges, the air-side mass transfer controls the kinetics of chemical sorption to the surface film for lower vapor pressure SOCs whereas the reverse is true for higher vapor pressure volatile organic compounds... 

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