Profile vertical concentration

Fig. 21. Deuterium concentration profiles, obtained by SIMS, for n-type doped a-Si H (10 4[PH3]/[SiH4]) with columnar microstructure. The bottom curve is the profile for the as grown sample, while the middle and top profiles (vertical scale offset) are obtained after annealing at 240°C for 35 min. and 24 hours, respectively (Street and Tsai. 1988).

As mentioned, the type of concentration-depth profiles observed in oceans should also be observed in lakes. However, the vertical concentration differences in lakes are often not as pronounced as in the ocean. The reason for this is, that the water column in lakes is much shorter mixing and stagnation in lakes is much more dynamic than in the oceans. Due to the presence of high concentrations of different particles in lakes, the release of trace elements from biogenic particles may not be clearly observed, due to readsorption to other particles. This would mean that low concentrations are observed throughout the water column, but that concentration differences are small. Atmospheric inputs to the upper water layers may also make it more difficult to observe a depletion of certain elements in the epilimnion. 

At any level in the transition region, there will be a balance between the mixing effects attributable to (a) axial dispersion and to (b) the segregating effect which will depend on the difference between the interstitial velocity of the liquid and that interstitial velocity which would be required to produce a bed of the same voidage for particles of that size on their own. On this basis a model may be set up to give the vertical concentration profile of each component in terms of the axial mixing coefficients for the large and the small particles. 

Elements that are not biolimiting have quite different vertical concentration profiles. Thus, the shapes of vertical concentration profiles can be used to infer the most important bio-geochemical processes acting on the chemical of interest. In this chapter and the next, we will explore several sets of vertical profiles for nitrogen, phosphorus, and silicon, obtained from different parts of the world s ocean. In Chapter 11, we will investigate the vertical profiles of the micronutrients, such as iron and zinc. 

Vertical concentration and temperature profiles are commonly used to assess the nature and relative rates of biogeochemical processes. An example was provided in Chapter 4... 

The dissolved oxygen data follow depth trends that are nearly a mirror image of the nutrients. The OMZ lies at depths slightly above the core of the AAIW. Why is the OMZ located at these depths To answer this question, oceanographers use the vertical concentration profiles of O2, nutrients, and TDIC to assess the relative rates of aerobic respiration and photosynthesis as a function of depth. (The TDIC concentration is used as a measure of how much CO2 has been taken up from or released into the water.)... 

The results of concentration measurements are presented as vertical profiles similar to those for the water column, with the vertical axis representing increasing depth below the sediment-water interfece. Depth profiles of concentrations can be used to illustrate downcore variations in the chemical composition of pore waters or in the solid particles. Dissolved concentrations are typically reported in units of moles of solute per liter of pore water. Solid concentrations are reported in mass/mass units, such as grams of carbon per 100 grams of dry sediment (%C) or mg of manganese per kg of dry sediment (ppm Mn). 

As we saw with the steady-state water-column application of the one-dimensional advection-diffusion-reaction equation (Eq. 4.14), the basic shapes of the vertical concentration profiles can be predicted from the relative rates of the chemical and physical processes. Figure 4.21 provided examples of profiles that exhibit curvatures whose shapes reflected differences in the direction and relative rates of these processes. Some generalized scenarios for sedimentary pore water profiles are presented in Figure 12.7 for the most commonly observed shapes. 

Vertical concentration profiles of (a) temperature, (b) potential density, (c) salinity, (d) O2, (e) % saturation of O2, (f) bicarbonate and TDIC, (g) carbonate alkalinity and total alkalinity, (h) pH, (i) carbonate, ( ) carbon dioxide and carbonic acid concentrations, and (k) carbonate-to-bicarbonate ion concentration ratio. Curves labeled f,p have been corrected for the effects of in-situ temperature and pressure on equilibrium speciation. Curves labeled t, 1 atm have been corrected for the in-situ temperature effect, but not for that caused by pressure. Data from 50°27.5 N, 176°13.8 W in the North Pacific Ocean on June 1966. Source From Culberson, C., and R. M. Pytkowicz (1968). Limnology and Oceanography, 13, 403-417. 

In addition to carbon, DOM also contains large amounts of nitrogen and phosphorus. As shown in Table 23.1, concentrations of DOC are on the order of tens of micromolar, a few micromolar fitr DON, and tenths of micromolar for DOP. The vertical concentration distributions typically exhibit stratification as exemplified by the profiles in Figure 23.4. 

Vertical concentration profiles of (a) nitrate, (b) nitrite and phosphate, and (c) O2 and temperature in the eastern tropical North Pacific in August 1962 (15°N 100°W). The calculated nitrate concentrations were estimated by multiplying the observed phosphate concentrations by the average nitrate to phosphate ratio in the three deepest samples (11.9 1.6 xmolN/L). Source From Thomas, W. H. (1966). Deep-Sea Research 13, 1109-1114. 

Mylne, K. R. The vertical profile of concentration fluctuations in near-surface plumes. Bound.-Lay. Meteorol. 65, 111-136 (1993). 

In Part V of this book we will discuss models of different environmental systems. Usually, such models will be extended in time as well as in space. To describe the variation in space we can either adopt the simpler scheme of box models (see Chapter 21) or introduce one or more continuously varying space coordinates (Chapter 22). Boundaries will be an essential part of both kinds of models. In the former, the boxes are separated by (interface or non-interface) boundaries their appropriate choice can turn the construction of a model into a piece of art. In the latter kind of models, the continuous functions (such as vertical concentration profiles of chemicals in the ocean) are framed by so-called boundary conditions which can either be... 

Figure 21.2 The interplay of transport and reaction, exemplified by the hypothetical vertical concentration profile of phenanthrene in a lake, (a) The rate of photolysis decreases with depth due to the diminishing light intensity with water depth, (b) Two possible vertical profiles of phenanthrene concentration if vertical mixing in the water column is strong, the profile is constant (profile 1). If vertical mixing is slow, a distinct vertical concentration gradient develops with small values at the water surface where photolysis is strongest (profile 2).

In the epilimnion/hypolimnion two-box model the vertical concentration profile of a chemical adopts the shape of two zones with constant values separated by a thin zone with an abrupt concentration gradient. Often vertical profiles in lakes and oceans exhibit a smoother and more complex structure (see, e.g., Figs. 19.1a and 19.2). Obviously, the two-box model can be refined by separating the water body into three or more horizontal layers which are connected by vertical exchange rates. 

In Section 21.1 we discussed the simultaneous influence of transport and transformation processes on the spatial distribution of a chemical in an environmental system. As an example we used the case of phenanthrene in the surface water of a lake. In Fig. 212b two situations were distinguished which differed by the relative importance of the rate of vertical mixing versus the rate of photolysis. Yet, neither was a quantitative method given to calculate the resulting vertical concentration profile (profiles 1 and 2 in Fig. 21.26), nor did we explain how the rates of such diverse mechanisms as diffusion, advection, and photolysis should be compared in order to calculate their relative importance. In this section we will develop the mathematical tools which are needed for dealing with such situations. 

The goal of the lDV-model is to calculate the time-dependent continuous vertical concentration profile of a compound, Cw(z,t), where the depth coordinate z is the height above the deepest point of the lake (thus the vertical coordinate z is chosen as positive upward). Let us consider a horizontal layer of thickness Az confined by the cross sections at depthz and z + Az,A(z) and A(z + Az), respectively (Fig. 23.6). The volume of the layer, AF, can be approximated by A(z)Az, and the sediment contact area AA by [A(z + Az) - A(z)]. Note that bottom slopes of lakes are commonly so small that AA, the horizontal projection of the inclined sediment surface, is usually a good approximation for the real contact area between water and sediment surface. In... 

Explain the meaning of the topographic function, a(z). Why does a(z) generally increase toward the lake bottom Imagine a chemical with a constant sediment boundary flux and give a qualitative picture of the vertical concentration profiles in the water column which evolve as a result of such a flux. Assume that the chemical does not react in the water column. 

An aircraft has three obvious advantages over a surface site. First, the aircraft can measure vertical and horizontal profiles of concentrations such measurements are not possible at the surface. However, because the typical aircraft travels horizontally much faster than it ascends or descends, it may be difficult to deconvolute vertical from horizontal variations. Second, the aircraft allows the investigator to choose the general type of air parcels to study rather than simply allowing sampling of whatever parcels are brought to a site (as on the surface). Third, the aircraft can measure concentrations free from the surface influences discussed in the preceding sections. 

This approach is based on the premise that Al can be used as a tracer for bottom sediment material and that the concentration of Al in resus-pendable surface sediment is fairly uniform basinwide. Detailed profiles of size-fractionated particulate aluminum concentrations spaced closely in time over the unstratified period show vertical concentration profiles at nearly uniform levels, indicating that a pseudosteady state had been achieved. The mean areal pool of Al during this period was designated as the net resuspended pool (80-90% settles from the water column by September), and the quantity of surface sediment required to supply this pool was calculated. 

Figure 1. Vertical concentration profiles for DMS, chlorophyll a and DMSP in the western basin of the Cariaco Trench off Venezuela (10° 39 N, 65°30 W). DMS was determined by sparging and gas chromatography with a flame photometric detector. Particulate DMSP was determined bv base treatment of material collected on 0.22 pm filters and analysis of the DMS released free DMSP was determined as DMS released upon base treatment of sparged water samples obtained after initial DMS analysis. Chlorophyll a data from W. Cooper and R. Zika (personal communication).

Tuncel et al. tabulated S/Se ratios for particles from many locations (4). The ratio is about 3000 at rural sites downwind, but outside of coal-burning areas. In the midst of the ORV, it is depressed to about 1700, in agreement with the model. In the midst of cities in which substantial coal is burned, the ratio is depressed to 1000 or less. Except for a few samples at Allegheny Mt. collected downwind from three power plants, Tuncel et al. did not see sudden drops in the S/Se ratio that one would expect to see occasionally in fresh plumes from coal-fired plants. A major flaw in the Gor-don/Olmez model is the assumption of uniform vertical concentration profiles, which is surely a poor assumption just beyond a source. Most power plants have tall stacks, whereas, measurements are at ground level. The S/Se ratio will surely be depressed near the plume centerline, but the effect will usually be washed out before the plume hits ground level. However, around cities, there are probably some ground level sources. 

This paper proposes a system of 10 non-linear, simultaneous differential equations (Table I) tdiich upon further development and validation, may serve as a comprehensive model for predicting steady state, vertical profiles of chemical parameters in the sulfide dominated zones of marine sediments. The major objective of the model is to predict the vertical concentration profiles of H2S, hydrotriolite (FeS) and p3nrite (FeS2). As with any model there are a number of assumptions involved in its construction that may limit its application. In addition to steady state, the major limiting assumptions of this model are the assumptions that the sediment is free of CaC03, that the diffusion coefficients of all dissolved sulfur species are equivalent and that dissolved oxygen does not penetrate into the zone of sulfate reduction. 

Figures 15, 16, 17 and 18 show examples of the paths of air masses estimated in the manner just described. The arrival times in El Monte are used to identify each trajectory in any subsequent references in this report. The date of Sept. 29, 1969, is chosen because of the variety of data that we have available for that day. The earlier morning meandering patterns give way to the dominant onshore flows for all trajectories 0900 to 1000 hours Pacific Standard Time (PST). The meteorological formulation that we have adopted takes the time and location information from these trajectories to establish the initial conditions and the boundary conditions. Initial conditions are specified as vertical profiles of concentration and boundary conditions as time histories of surface-based pollutant emissions. These trajectories are not used in the tests of the slab model they are used to test the moving air parcel model.

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