Atmospheric Chemical Reactions Involving Particles

FIGURE 6.12 Particles provide sites for many important atmospheric chemical processes. 

Access to and use of the Internet is assumed in answering all questions, including general information, statistics, constants, and mathematical formulas required to solve problems. These questions are designed to promote inquiry and thought rather than just finding material in the chapter. So in 

Give two examples each of Earth s natural capital in terms of (1) protective function and (2) raw materials. 

In what respect is the composition of gases in the troposphere not uniform (which atmospheric constituent varies widely in time and location)  

Other than avoiding turbulence due to lower altitude weather, suggest an advantage for commercial aircraft to cruise at a relatively high altitude of around 10 km. 

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Chemical reactions involving aerosol particles in the atmosphere derive from the interaction of gaseous species with the liquid water associated with aerosol particles and with dissolved electrolytes. For example, the generation of HONO from nitrogen oxides... 

Particulate carbon in the atmosphere exists predominantly in three forms elemental carbon (soot) with attached hydrocarbons organic compounds and carbonates. Carbonaceous urban fine particles are composed mainly of elemental and organic carbon. These particles can be emitted into the air directly in the particulate state or condense rapidly after Introduction into the atmosphere from an emission source (primary aerosol). Alternatively, they can be formed in the atmosphere by chemical reactions involving gaseous pollutant precursors (secondary aerosol). The rates of formation of secondary carbonaceous aerosol and the details of the formation mechanisms are not well understood. However, an even more fundamental controversy exists regarding... 

Airborne solid particles such as ash, soot, metal oxides, and even sea salts play a major role in air pollution. Particles up to 0.01 millimeter in diameter (too small to be seen with the naked eye) attract water droplets and thereby form aerosols that may be visible as fog or smoke. Aerosol particles remain suspended in the atmosphere for extended periods of time and, as Figure 17.8 shows, serve as sites for many chemical reactions involving pollutants. 

In recent years, there has been an increasing recognition of the importance in atmospheric chemistry of chemical processes that occm on particle surfaces and in solution in liquid particles (Figure 6.12). Challenging as it is, gas-phase atmospheric chemistry is relatively straightforward compared to the heterogeneous chemistry that involves particles. Particles may serve as somces and sinks of atmospheric chemical reaction participant species and are very much involved in atmospheric... 

Gas-phase reactions play a fundamental role in nature, for example atmospheric chemistry [1, 2, 3, 4 and 5] and interstellar chemistry [6], as well as in many teclmical processes, for example combustion and exliaust fiime cleansing [7, 8 and 9], Apart from such practical aspects the study of gas-phase reactions has provided the basis for our understanding of chemical reaction mechanisms on a microscopic level. The typically small particle densities in the gas phase mean that reactions occur in well defined elementary steps, usually not involving more than three particles. 

Surface Chemical Analysis. Electron spectroscopy of chemical analysis (ESCA) has been the most useful technique for the identification of chemical compounds present on the surface of a composite sample of atmospheric particles. The most prominent examples Include the determination of the surface chemical states of S and N in aerosols, and the investigation of the catalytic role of soot in heterogeneous reactions involving gaseous SO2, NO, or NH3 (15, 39-41). It is apparent from these and other studies that most aerosol sulfur is in the form of sulfate, while most nitrogen is present as the ammonium ion. A substantial quantity of amine nitrogen also has been observed using ESCA (15, 39, 41). 

Three examples of chemistry occurring on or in atmospheric particles are provided below. These examples demonstrate how particulate-based chemical reactions can alter the composition both of particles and of the gas phase. However, the examples are by no means comprehensive of all chemical processes that occur in the troposphere. The particles involved in these examples are generally accumulation or coarse mode, though... 

A number of important chemical reactions in the atmosphere involve a gas molecule striking the surface of an airborne particle. For gas molecules A in three-dimensional random motion, the number of molecules of A striking a unit area per unit time is... 

In large part, the chemistry we meet in practice takes place in a solution of some kind, but a quantitative description of the chemical kinetics involved is much more complex than for gaseous reactions. The key difference lies in the interparticle distances. In a gas at atmospheric pressure, the particles occupy less then 1 % of the total volume and, effectively, move independently of each other. In a solution the solute and solvent molecules, with the latter being in the majority, take up more than 50% of the available space, the distances between the various species are relatively small, and each particle is in continuous contact with its neighbours. It is these interactions which greatly complicate the formulation of a satisfactory theory of chemical kinetics in solution. Indeed, the rate of an elementary reaction and for that matter a composite reaction, can be significantly influenced by the choice of solvent. 

Probably all of the recently suggested modifications of the systems of combustions using conventional fuels are based on techniques usually used by chemical engineers. One example is the use of fiuidized beds— either atmospheric or pressurized—as developed by chemical engineers. Usually such beds have assisted chemical reactions which are more involved than simple combustion. Such fluidized combustion of coal particles under boilers has significant advantages in some instances. 

UFP can be emitted directly from anthropogenic sources such as vehicles, furnaces or other technologies involving the combustion of fossil fuels. They can also form through a series of secondary chemical reactions in the atmosphere. There are five main processes that govern the formation, transformation and removal of UFP nucleation, condensation or evaporation of semi-volatile species, coagulation, deposition, and dilution. The effect of each of these processes on the total number of particles in the atmosphere and their size is summarized in Table 1. Each process affects the number and size of UFP and operates over vastly different characteristic times. Further, these processes have varying importance for different sized particles. 

FIGURE 3.9 Most important environmental chemical processes in the hydrosphere involve interactions between water and another phase. Aquatic biochemical processes take place inside the cells of organisms suspended in water, materials are exchanged between water and sediments, gases are emitted to and absorbed from the atmosphere, very small colloidal particles are snspended in water and aggregate to form solids that settle into sediments, and water-immiscible liquids such as hydrocarbons may be present as films on the water surface. The inset shows a phenomenon observed in some sediments in which layers of white CaCOj precipitated as the result of photosynthesis during the summer alternate with black layers of FeS produced by the reaction of Fe and HjS formed by anoxic bacterial processes during the winter. 

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