Physical Transport of Chemicals

Rate of chemical inflow = velocity (m/sec) area (m2) concentration (g/m3). 

The final units of the answer would be (g/sec) with dimensions of [M/T], entirely appropriate to express the rate of chemical inflow to the lake. Alternatively, if (ft/sec) had been used for river velocity, the units of the answer would have been (g ft/m sec), a very good sign that a consistent set of units had not been used in the original expression. If the units for velocity had been omitted, the answer would have had the units of (g/m), which are clearly incorrect, in part because there is no time unit. 

FIGURE 1-4 An example of pollutant advectlon and diffusion in the atmosphere. Smoke from multiple burning oil wells in Kuwait is carried downwind by advection. At the same time, the plumes of smoke widen because of diffusive transport, one of the major Fickian transport processes. Imagery courtesy of Space Imaging, Thornton, Colorado, USA. 

In the second type of transport process, a chemical moves from one location in the air or water where its concentration is relatively high to another location where its concentration is lower, due to random motion of the chemical molecules (molecular diffusion), random motion of the air or water that carries the chemical (turbulent diffusion), or a combination of the two. Transport by such random motions, also called diffusive transport, is often 

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The Reactions and Physical Transport tlie chemical and biological transfornuition, and water movement, that result in different levels of water quality at different locations in time in an aquatic ecosystem. 

Models of atmospheric phenomena are similar to those of combustion and involve the coupling of exceedingly complex chemistry and physics with three-dimensional hydrodynamics. The distribution and transport of chemicals introduced into groundwater also involve a coupling of chemical reactions and transports through porous solid media. The development of groundwater models is critical to understanding the effects of land disposal of toxic waste (see Chapter 7). 

The physical transport of dissolved organic compounds through the subsurface occurs by three processes advection, hydrodynamic dispersion, and molecular diffusion. Together, these three cause the spread of dissolved chemicals into the familiar plume distribution. Advection is the most important dissolved chemical migration process active in the subsurface, and reflects the migration of dissolved chemicals... 

The physical transport of oil droplets into the water column, called dispersion, is often a result of water surface turbulence but may also result from the application of chemical agents (dispersants). These droplets may remain in the water column or coalesce with other droplets and gain enough buoyancy to resurface. Dispersed oil tends to biodegrade and dissolve more rapidly than floating slicks because of high surface area relative to volume. Most of this process occurs from about half an hour to half a day after the spill. 

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

Primary minerals with low surface area (e.g., sihca minerals) and low reactivity mainly affect the physical transport of water, dissolved chemicals, colloids, immiscible (in water) liqnids, and vapors. Secondary minerals generally have high surface area (e.g., clay minerals) and high reactivity that affect the transport of chemicals, their retention and release onto and from the solid phase, and their surface-induced transformations. The sohd phase also can indirectly induce the degradation of chemical compounds, through its effects on the water-air ratio in the system and, thus, on microbiological activity. 

The series Volume 1 The Natural Environment and the Biogeochemical Cycles describes the natural environment and gives an account of the global cycles for elements and classes of natural compounds. The series Volume 2 Reactions and Processes is an account of physical transport, and chemical and biological transformations of chemicals in the environment. 

The fate of pesticides and organic pollutants in natural waters and in soils is strongly dependent on their sorptive behavior (Karickhoff, 1980). Sorption affects not only physical transport of these materials but also their degradation. It is also important to note that the chemical reactivity of pollutants in a sorbed state may be different from their behavior in aqueous solution. Karickhoff (1980) notes that sorbents such as inorganic and organic soil constituents may affect solution-phase processes by changing the solution-phase pollutant concentration or by affecting the release of pollutants into the solution phase. 

Catalytic reaction engineering is a scientific discipline which bridges the gap between the fundamentals of catalysis and its industrial application. Starting from insight into reaction mechanisms provided by catalytic chemists and surface scientists, the rate equations are developed which allow a quantitative description of the effects of the reaction conditions on reaction rates and on selectivities for desired products. The study of intrinsic reaction kinetics, i.e. those determined solely by chemical events, belongs to the core of catalytic reaction engineering. Very close to it lies the study of the interaction between physical transport and chemical reaction. Such interactions can have pronounced effects on the rates and selectivities obtained in industrial reactors. They have to be accounted for explicitly when scaling up from laboratory to industrial dimensions. 

Within the completely online Enviro-HIRLAM (Baklanov et al. 2008a Korsholm et al. 2008b) the transport of chemical species is achieved in the same way like for other variables in HIRLAM (actually it was performed via the Tracer subroutine inside HIRLAM like for other scalars) on the same time steps and with the same grid. There is no need for any interface in Enviro-HIRLAM, because ACT is inside the HIRLAM model, so all the HIRLAM parameters are available for ACT. The model is designed to be used for operational as well as research purposes and comprises aerosol and gas transport, dispersion and deposition, aerosol physics and chemistry, as well as gas-phase chemistry. A Climate version Enviro-HIRHAM is also planned and will be developed in the near future. 

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