Atmospheric data diffusion

Interpretation of available data is frustrated by lack of knowledge of certain fundamental quantities such as Interfacial area, mass transfer coefficients, solubility data, diffusion coefficients, bubble sizes, etc.. Existing equations for almost all of these variables have been developed on the basis of experiments conducted at atmospheric pressure and around room temperature. Use of such predictive equations at the reacting conditions involves large extrapolation, and the combined errors would make the analysis of kinetic data very suspect. In spite of this, most work reported in the literature does use such correlations. 

The observed noble-gas abundances and isotopic ratios on Venus are summarized in Tables 3 and 4. The helium mixing ratio is a model-dependent extrapolation of the value measured in Venus upper atmosphere, where diffusive separation of gases occurs. The main differences between Venus and Earth are that Venus is apparently richer in He, Ar, and Kr than the Earth, and the low " Ar/ Ar ratio of — 1.1 on Venus, which is —270 times smaller than on Earth. The low " Ar/ Ar ratio may reflect more efficient solar-wind implantation of Ar in solid grains accreted by Venus and/or efficient early outgassing that then stopped due to the lack of plate tectonics. Wieler (2002) discusses the noble-gas data. Volkov and Frenkel (1993) and Kaula (1999) describe implications of the " Ar/ Ar ratio for outgassing of Venus. 

LIQUID DIFFUSIVITIES. The theory of diffusion in liquids is not as advanced or the experimental data as plentiful as for gas diffusion. The diffusivities in liquids are generally four to five orders of magnitude smaller than in gases at atmospheric pressure. Diffusion in liquids occurs by random motion of the molecules, but the average distance traveled between collisions is less than the molecular diameter, in contrast to gases, where the mean free path is orders of magnitude greater than the size of the molecule. 

The (en) compound developed nuclei which advanced rapidly across all surfaces of the reactant crystals and thereafter penetrated the bulk more slowly. Kinetic data fitted the contracting volume equation [eqn. (7), n = 3] and values of E (67—84 kJ mole"1) varied somewhat with the particle size of the reactant and the prevailing atmosphere. Nucleus formation in the (pn) compound was largely confined to the (100) surfaces of reactant crystallites and interface advance proceeded as a contracting area process [eqn. (7), n = 2], It was concluded that layers of packed propene groups within the structure were not penetrated by water molecules and the overall reaction rate was controlled by the diffusion of H20 to (100) surfaces. 

Most of the data used for compiling such maps have been obtained for the flow of water and air at near atmospheric temperature and pressure, and scaling factors have been introduced to extend their applicability to other systems. However, beating in mind the diffuse nature of the boundaries between the regimes and the relatively minor effect of... 

The diffusivity of the vapour of a volatile liquid in air can be conveniently determined by Winkdmann s method in which liquid is contained in a narrow diameter vertical tube, maintained at a constant temperature, and an air stream is passed over the top of the tube sufficiently rapidly to ensure that the partial pressure of the vapour there remains approximately zero. On the assumption that the vapour is transferred from the surface of the liquid to tile air stream by molecular diffusion alone, calculate the diffusivity of carbon tetrachloride vapour in air at 321 K and atmospheric pressure from the experimental data given in Table 10.3. 

An important issue that remains unresolved, because of the lack of adequate quantitative data on reservoirs and fluxes, is the location of the so called "missing" carbon. Missing carbon is the carbon added to the atmosphere from the burning of fossil fuel that cannot be accounted for by the measured increase in atmospheric concentration or by diffusion into the ocean (5). 

The results of this study show (99) the involvement of fragments such as Cr(CO) , (3 < jc < 6) which react with CO molecules which come from any of several sources fragmentation of the original molecules, bulk radiolysis of the compound, application of an external atmosphere, or perhaps from intermolecular exchange. It was concluded from the data that diffusion processes are involved and that the relative rates of reaction and of diffusion away are important in determining the height of the annealing plateaus. 

DBCP. The predictions suggest that DBCP is volatile and diffuses rapidly into the atmosphere and that it is also readily leached into the soil profile. In the model soil, its volatilization half-life was only 1.2 days when it was assumed to be evenly distributed into the top 10 cm of soil. However, DBCP could be leached as much as 50 cm deep by only 25 cm of water, and at this depth diffusion to the surface would be slow. From the literature study of transformation processes, we found no clear evidence for rapid oxidation or hydrolysis. Photolysis would not occur below the soil surface. No useable data for estimating biodegradation rates were found although Castro and Belser (28) showed that biodegradation did occur. The rate was assumed to be slow because all halogenated hydrocarbons degrade slowly. DBCP was therefore assumed to be persistent. 

Biesenberger and Kessidis were able to correlate their experimental data at atmospheric pressure in terms of N in accordance with Eq. (36) [their Eqs. (4)-(9)], but the diffusion coefficient which they computed using Eq. (36) along with the experimental data was found to be roughly D = 10 m sec" at 177°C. Again, if the ethylbenzene-polystyrene system is used as a basis for comparison, Duda et al. (1982) report a value of 3 x 10"" m /sec at 178°C, which is considerably smaller than the value obtained by Biesenberger and Kessidis (1982). In the experiments con-... 

Trends in air pollutant concentrations can be predicted with simple empirical models based on atmospheric and laboratoiy data. Concentrations of nonreactive pollutants from point sources can be predicted vfith accuracy well within a factor of 2 predictions are more likely to be too high than too low, especially predictions of concentration peaks. Concentrations of reactive pollutants, such as ozone and other photochemical oxidants, can be predicted reasonably well with photochemical-diffusion models when detailed emission, air quality, and meteorolc c measurements are available most such predictions of air pollution in Los Angeles, California, have been accurate to within approximately 50% for ozone. Detailed performance analyses are found elsewhere in this chapter. 

---