Residence time sinks

The StabUity or persistence of a poUutant in the atmosphere depends on the poUutant s atmospheric residence time. Mean residence times and principal atmospheric sinks for a variety of species are given in Table 2. Species like SO2, (NO and NO2), and coarse particles have lifetimes less than... 

Table 2. Mean Atmospheric Residence Times (t) and Dominant Sinks of Air Pollutants...

The residence time is the time spent in a reservoir by an individual atom or molecule. It is also the age of a molecule when it leaves the reservoir. If the pathway of a tracer from the source to the sink is characterized by a physical transport, the word transit time can also be used. Even for a single chemical substance, different atoms and molecules will have different residence times in a given reservoir. Let the probability density... 

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

Balistrieri, L., Brewer, P. G. and Murray, J. W. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean. Deep-Sea Res. 28A, 101-121. 

Buesseler et al. (1992b) proposed a method to circumvent these difficulties in comparing residence times. They argued that the deficiency in total " Th with respect to indicates a flux of " Th in association with particles sinking out of the euphotic zone. If the POC (or particulate organic nitrogen, PON)/ Th ratio of these sinking particles is known, a POC (or PON) flux can be calculated as ... 

POC/ Th (mol C/dpm is the ratio on sinking particles and is the decay constant of " Th (0.029 d ). This approach makes no assumptions about residence times, although it implicitly assumes that sinking biogenic particles are the principal carriers of " Th atoms, that the POC/ Th ratio on sinking particles can be measured, that steady state applies and that horizontal and vertical transport of " Th via advection of water are negligible. 

One way of circumventing the difficulties encountered for systems with widely different time constants is to split the reservoirs into two categories. The first category will comprise the reservoirs with short residence times which will be explicitly required to satisfy the constraints of mass conservation. Reservoirs with long residence times will make up the second category which we will treat as source and sinks. Equation (7.3.8) will be transformed into... 

With hydraulic residence times ranging from months to years, lakes are efficient settling basins for particles. Lacustrine sediments are sinks for nutrients and for pollutants such as heavy metals and synthetic organic compounds that associate with settling particles. Natural aggregation (coagulation) increases particle sizes and thus particle settling velocities (Eq. 7.1) and accelerates particle removal to the bottom sediments and decreases particle concentrations in the water column. 

The biomass is fed overbed through multiple feed chutes using air jets to help distribute the fuel over the surface of the bed. Variable-speed screw conveyors are usually used to meter the fuel feed rate and control steam output. Feedstocks such as bark and waste wood are chipped to a top size of 25 mm (1 in) to ensure complete combustion. The bed usually consists of sand around 1 m (3 ft) deep. This serves to retain the fuel in the furnace, extending its in-furnace residence time and increasing combustion efficiency. It also provides a heat sink to help maintain bed temperature during periods of fluctuating fuel moisture content. 

The goo is then pumped to the top of a vertical, jacketed tower with internal temperature-regulating coils. The vessel is kept full of the styrene/PS mixture. A temperature gradient (change) of 280°F at the top and 400°F at the bottom is maintained. The temperatures are controlled to prevent runaway, but to permit 95% conversion of styrene to PS. As the polystyrene molecules grow, they sink to the bottom of the vessel and can be drawn off The residence time in this vessel is three to four hours. The molten PS is extruded to strands, chopped into pellets, and bagged. 

In Chapter 11, we will discuss a fourth category of elements, one with vertical profiles nearly opposite to the biolimiting elements. These elements have surfece-water enrichments and bottom-water depletions. Most are trace metals that adsorb onto sinking particles enabling their transport to the sediments. These elements tend to have shorter residence times than the biolimiting elements because they lack the remineralization step. Still other elements have a foot in both camps in some locations, they exhibit biolimiting behavior and have profiles with surface-water depletion and bottom-water enrichments, and in other locations, the profiles appear to be controlled by particle adsorption. Iron is an example of such an element. 

Th, Co, and, in some locations, Fe. Surfece-water enrichments are usually caused by rapid rates of supply to the mixed layer via atmospheric deposition or river runoff. Removal usually occurs through relatively rapid precipitation into or adsorption onto sinking particles. Trace elements controlled by scavenging tend to have short (100 to lOOOy) residence times. Since these residence times are less than the mixing time of the ocean, significant geographic gradients are common. 

For elements that have multiple sources or sinks, a fractional residence time (t, ), or turnover time, can be calculated for each supply or removal process. The residence time of the element (t) is then given by... 

Lithium is a conservative element in the ocean with a residence time of abont one million year. Its isotope composition is maintained by inputs of dissolved Li from rivers (average 5 Li + 23%c, Huh et al. 1998) and high-temperature hydrothermal fluids at ocean ridges at one hand and low temperature removal of Li into oceanic basalts and marine sediments at the other. Any variance in these sources and sinks thus should cause secular variations in the isotope composition of oceanic Li. And indeed in a first attempt Hoefs and Sywall (1997) interpreted Li isotope variations in well preserved carbonate shells as indicating secular variations of the oceanic Li-cycle. 

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