Models sulfate concentration

Figure 8.23 Sulfate concentrations in pore waters as a function of the depth below the water-sediment interface of the Saanich Inlet Murray et al. (1978). The exponential curve supports the diffusional diagenetic model.

Haywood, J. M., R. J. Stouffer, R. T. Wetherald, S. Manabe, and V. Ramaswamy, Transient Response of a Coupled Model to Estimated Changes in Greenhouse Gas and Sulfate Concentrations, Geophys. Res. Lett., 24, 1335-1338 (1997b). 

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

Several whole-lake ion budgets have shown that internal alkalinity generation (IAG) is important in regulating the alkalinity of groundwater recharge lakes and that sulfate retention processes are the dominant source of IAG (3-5)1 and synoptic studies (6-9) have shown that sulfate reduction occurs in sediments from a wide variety of softwater lakes. Baker et al. (10) showed that net sulfate retention in lakes can be modeled as a first-order process with respect to sulfate concentration and several "whole ecosystem" models of lake acidification recently have been modified to include in-lake processes (11). 

Inputs and outputs to the lake have been measured to calculate net retention for the pre-acidified lake. Precipitation inputs of sulfate were based on data from wet collectors (1980-1983) compiled by the National Atmospheric Deposition Program (NADP). SO2 inputs were calculated from regional ambient air concentrations (22) usinga deposition velocity of 0.5 cm/sec. Aerosol sulfate was estimated from NADP dry bucket measurements and from dry bucket and snow core measurements made in this study (22). Groundwater inputs occur largely at the southeast corner of the lake and were calculated from modeled inseepage (21) and measured sulfate concentrations in a well located in the major inseepage area. Sulfate output was estimated from mean lakewater sulfate concentration and modeled outflows. 

Tracer Hybrid Receptor Model (Lewis). Lewis and Stevens (3) have derived a hybrid receptor model for describing the secondary sulfate from an SO2 point source. The resulting expression for secondary sulfate concentration M o at the receptor has the form... 

FIGURE 4-28 An example of output from ACID, a regional-scale model that simulates transport of S02 and sulfate, oxidation of S02 to sulfate, and sulfate deposition. Each contour represents the average airborne sulfate concentration in micrograms per cubic meter that would result in the Adirondacks region of New York per 1014 g of sulfur emitted annually anywhere along the contour line. For example, if a 1014 g/year source of S02 were sited in Tennessee, the resultant average addition to the airborne sulfate concentration in the Adirondacks would be 20 /cg/m3. 

However, this interpretation does not fit with the distribution of sulfate concentrations (Table I) and a more elaborate model must be found. Fig. 4 shows that samples may be divided into four groups ... 

On the basis of data compiled, E. Meszaros (1978) proposes model background concentrations for different part of rural and remote areas of Europe, as tabulated in Table 13. One can see that the S02 level may be as high as IS fig m 3, while the sulfate concentration ranges from 0.3 to 10.5 /igm 3 STP. The values given agree reasonably well with data gained in North America, as discussed by the same author. 

The coincidence of maxima in the methane oxidation rate and the sulfate reduction rate in Saanich Inlet strongly suggests that the methane oxidizing agent was sulfate, either via direct reaction, or coupled indirectly through reactions with other substrates (Devol, 1983). A methane-sulfate coupled reaction diffusion model was developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska), the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. In Saanich Inlet sediments, from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%. 

Fig. 15.4 Geochemical model calculation using the program PHREEQC. In an anoxic system (state at the end of the model calculation from Fig. 15.3), the gradual addition of organic matter to the redox reaction is continued, whereby the system is kept open for calcite equilibrium and sealed from the gaseous phase. Initially, the dissolved sulfate will be consumed, in the course of which low amounts (logarithmic scale) of methane will emerge. Only after the sulfate concentration has become sufficiently low, will the generation of methane display its distinct increase.

Fig. 15.12 Model calculation of the sulfate concentration profile in the deposition of a sediment avalanche on the continental slope off the coast of Uruguay (see also. Fig. 15.11). Red circles denote the measured sulfate concentrations. Blue lines indicate the model calculation at various time points after the avalanche Left Immediately after the occurrence of the avalanche. Center approx. 21 years later. Right approx. 500 years later.

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

Table II displays the PLMA MW in the model system as a function of the alkyl chain length of the anionic surfactant, all other concentrations remaining constant. When the prepolymerization solution contains only C12EO5, the solution is opaque and the PLMA MW is large this result implies the existence of large micelles. Addition of sodium 2-ethylhexyl sulfate to the C12EO5-LMA prepolymerization solution at the concentration used for SDS (0.035 M) dramatically increases the MW. This result is presumably due to an electrolyte effect that increases nonionic surfactant aggregation numbers (3i), because the sodium 2-ethylhexyl sulfate concentration is far below the surfactant s CMC. However, doubling the sodium 2-ethylhexyl sulfate con-...

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