Sinks, pollution

These models consist of a number of constantly sized volume elements in a fixed spatial grid which covers the entire region of interest. Pollutants move from box to box as a result of advection, diffusion, release points, and sinks. Pollutants are assumed to be mixed and eractions are considered. Time dependence is introduced by hourly updating of source and meterological measurements... 

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

Early models used a value for that remained constant throughout the day. However, measurements show that the deposition velocity increases during the day as surface heating increases atmospheric turbulence and hence diffusion, and plant stomatal activity increases (50—52). More recent models take this variation of into account. In one approach, the first step is to estimate the upper limit for in terms of the transport processes alone. This value is then modified to account for surface interaction, because the earth s surface is not a perfect sink for all pollutants. This method has led to what is referred to as the resistance model (52,53) that represents as the analogue of an electrical conductance... 

The places from which pollutants emanate are called sources. There are natural as well as anthropogenic sources of the permanent gases considered to be pollutants. These include plant and animal respiration and the decay of what was once living matter. Volcanoes and naturally caused forest fires are other natural sources. The places to which pollutants disappear from the air are called sinks. Sinks include the soil, vegetation, structures, and water bodies, particularly the oceans. The mechanisms whereby pollutants... 

Under low-dose conditions, forest ecosystems act as sinks for atmospheric pollutants and in some instances as sources. As indicated in Chapter 7, the atmosphere, lithosphere, and oceans are involved in cycling carbon, nitrogen, oxygen, sulfur, and other elements through each subsystem with different time scales. Under low-dose conditions, forest and other biomass systems have been utilizing chemical compounds present in the atmosphere and releasing others to the atmosphere for thousands of years. Industrialization has increased the concentrations of NO2, SO2, and CO2 in the "clean background" atmosphere, and certain types of interactions with forest systems can be defined. 

The interactions of air pollutants with forests at low-dose concentrations result in imperceptible effects on the natural biological cycles of these species. In some instances, these interactions may be beneficial to the forest ecosystem. Forests, as well as other natural systems, act as sinks for the removal of trace gases from the atmosphere. 

Vertical air motions affect both weather and the mixing processes of importance to air pollution. Upward vertical motions can be caused by lifting over terrain, lifting over weather fronts, and convergence toward low-pressure centers. Downward vertical motions can be caused by sinking to make up for divergence near high-pressure centers. One must know whether the atmosphere enhances or suppresses these vertical motions to... 

Fig. 23-1. Representation of home with various sources and sinks for indoor air pollutants (A), exchange (B), indoor concentration (C), outgassing of building and furniture materials (D), irvfiltration from soils (E), removal on interior surfaces.

Figure 1.4 Schematic represeotation of mass-integration strategies for pollution prevention segregation. mixing, interception, recycle and sink/gen tor manipulation (El-Halwagi and Spriggs, 1996).

Recycle refers to the utilization of a pollutant-laden stream (a source) in a process unit (a sink). Each sink has a number of constraints on the characteristics (e g, flowrate and composition) of feed that it can process. If a source satisfies these constraints it may be directly recycled to or reused in the sink. However, if the source violates these constraints segregation, mixing, and/or interception may be used to prepare the stream for recycle. 

As mentioned in Chapter One, recycle refers to the utilization of a pollutant-laden stream (a source) in a process unit (a sink). A source may be recycled to a sink... 

On the source-sink mapping diagram, sources are represented by shaded circles and sinks are represented by hollow circles. Typically, process constraints limit the range of pollutant composition and load that each sink can accept. ITie intersection of these two bands provides a zone of acceptable conqKisition and load for recycle. If a source (e.g., source a) lies within this zone, it can be directly recycled to tiie sink (e.g., sink S). Moreover, sources b and c can be mixed using the lever-arm principle to create a mixed stream that can be recycled to sink S. 

Water may be recycled to two sinks the seal pot and the water-jet compression station. The following constreiints on flowrate and composition of the pollutant (heavy organic) should be satisfied ... 

The pollutant sources IG.i and y G 2 niay be without any buoyancy forces or they may be sinks, in other words negative sources or filters. 

R02./R02 Recombinations. Another area of uncertainty is the peroxyl radical recombination reactions described above, which become especially significant when the NO concentration is low. This can occur late in the photooxidation of polluted air undergoing transport, as in some rural environments (60,85) and in clean air. Although reactions of H02 with itself (R33) are reasonably well understood (their rate depends upon total pressure and upon water vapor concentration), reactions of H02 with R02 species and the R02 self reaction are much less well quantified. Since these serve as important radical sink processes under low NO. conditions, their accurate portrayal is important for accurate prediction of HO, concentrations. 

Ocean prevents eutrophication. Much more water flows into the Mediterranean Sea than is required to replace evaporation from it. The excess, high salinity water exits Gibraltar below the water flowing in af fhe surface. Nufrients that enter the Mediterranean Sea from pollution sources are utilized by marine phytoplankton that sinks and exits with the outflow. Another example is that estuaries often have lower salinity or even freshwater at the surface with a denser saline layer at the bottom. An estuarine circulation occurs with nutrients being trapped in the saline bottom water. 

Johnson DW, Lindberg SE. 1995. Sources, sinks, and cycling of mercury in forested ecosystems. Water Air Soil Pollut 80 1069-1077. 

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