Upward diffusion

Upward diffusion of water vapor through the cold temperatures of the tropopause is very inefficient in fact, the upper limit of cloud formation often occurs at the tropopause. Thus the stratosphere is so dry as to prevent rain formation, and particles and gases have very much longer residence times there than in the troposphere. Stratospheric removal requires diffusion back through the tropopause, which then may be followed by precipitation scavenging. 

Considering natural stratospheric ozone pro-duction/destruction as a balanced cycle, the NO.v reaction sequence is responsible for approximately half of the loss in the upper stratosphere, but much less in the lower stratosphere (Wennberg et al, 1994). Since this is a natural steady-state process, this is not the same as a long term O3 loss. The principal source of NO to the stratosphere is the slow upward diffusion of tropospheric N2O, and its subsequent reaction with O atoms, or photolysis (McElroy et ai, 1976). 

Murray et al. (1978) found that this value is a factor of 4 larger than what is actually measured and suggest that methane upward diffusion accounts for the missing carbon. <5=... 

As illustrated in Fig. 9.13 the Fe(II), forming complexes with these hydroxy and carboxyl ligands, encounter in their upward diffusion the settling of Fe(III)(hydroxides and interact with these according to the catalytic mechanism, thereby dissolving rapidly the Fe(III)(hydr)oxides. The sequence of diffusional transport of Fe(II), oxidation to insoluble Fe(OH)3 and subsequent settling and reduction to dissolved Fe(II) typically occurs within a relatively narrow redox-cline. 

Suboxic Diagenesis Metals remobilized from reducing sediments. Upward diffusive transport through pore waters supplies metals to nodule bottoms. Accretion is episodic, occurring only when the depth of the redox boundary rises close to the sediment-water interface. 100-200 Todorokite/Birnessite (low Cu and Ni content) 48% 20-70 60-200... 

At the prevailing pH in the Namibian groundwaters, the predicted solubility of carnotite is low and close to saturation. From one hole in the Tubas deposit, carnotite saturation is close to 0 and predicted to be over saturated around the water-table zone and in the near-surface upper 2m of the gypcrete. Where Eh is positive carnotite is predicted to be nearsaturation. This indicates that carnotite accumulation at or above the regional water-table can occur by upward diffusion of uranyl carbonate species with possible precipitation due to nucleation on clay minerals or gypsum, as evidenced in the Tubas River. 

Another oxidant near the sediment-water interface capable of oxidizing Mn(II) is iodate (I03 ), which occurs in the dissolved form as well as adsorbed onto particles (Ullman and Aller, 1985). Iodine is known to be released in porewaters during the remineralization of organic matter (Ullman and Aller, 1983, 1985 Kennedy and Elderfield, 1987). Once produced, iodine is then believed to be oxidized to 103 through microbial processes, where it adsorbs onto metal oxides (Ullman and Aller, 1985). Recent work has shown that an iodide peak can be maintained in sediments through the reduction of 103 by Mn(II), with reoxidation of iodide to IO3 above the iodide peak thus, iodide production is adequate to account for the oxidation of all the upward diffusing Mn(II) via 1 (>3 (Ansch u tz et al., 2000). 

In tropical waters, vertical stratification persists throughout the year and production is permanently limited by nutrient supply rates, which are controlled by internal recycling and slow upward diffusion from deep water. Under these conditions, productivity is low throughout the year. 

The apparently most consistent set of data was obtained from soil site 19. These data suggest that essentially all solution of Ca is occurring at the soil— rock interface. Ca content of samples taken from a depth of 0.64 m ranged from 12 to 22 mg/1 while samples taken on the same days from a lysometer at the soU—rock interface 1.20 m below the surface ranged from 76 to 89 mg/1. No calcite could be detected in soil samples from various depths, and it is believed that aU of the Ca in the shallower samples was derived from upward diffusion. Values of Sq calculated for the six samples obtained from the lysometer installed at the soil-T ock interface were 0.66, 0.44, 0.66,... 

The principal source of NO, in the stratosphere is the slow upward diffusion of N2O, and ifs subsequent reaction with O atoms, or photolysis (McElroy etal., 1976) ... 

Bender, M. L. (1971). Does upward diffusion supply the excess manganese in pelagic sediments J. Geophys. Rev. 76, 4212-4215. 

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