Nitrogen transport coefficients

Gas transport coefficients of PVC and PVC modified with pyridine groups were studied. It was found that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. There is a two-fold reduction for PVC of the diffusion coefficients during the first two days,... 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

The model fits adequately the experimental data allowing the extraction of the adsorption parameters and the diffusion coefficients for the transport inside the micropores. Table 3 reports these parameters for both samples. Similar diffusion parameters were found for both samples. Despite the difference in volume of the micropores as calculated by the HK method (Table 1), a big difference between Sbet and Sdr for each sample is found indicating problems of accessibility for nitrogen molecules at the low temperature at which the adsorption process is carried out (77 K). The micropore volume according to the DR method gives similar values for both samples (Table 1) The values of the diffusion parruneters found by modelling of the transient responses for both samples are very close. 

Wall deactivation of vibrationally excited nitrogen should however be considered at number density lower than 10ls cm-3 as pointed out in Ref.7. The influence on the kinetics of hydrogen dissociation of the transport of atoms to the walls and their recombination thereon has been considered in Ref.16. Calculated values of kw are affected by the scattering in the literature values of the recombination coefficients. 

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