Reactions in the atmosphere

Ferguson E E 1979 ion-moiecuie reactions in the atmosphere K/nef/cs of Ion-Molecule Reactions ed P Ausioos (New York Pienum)... 

Deposition. The products of the various chemical and physical reactions in the atmosphere are eventually returned to the earth s surface. Usually, a useful distinction is made here between wet and dry deposition. Wet deposition, ie, rainout and washout, includes the flux of all those components that are carried to the earth s surface by rain or snow, that is, those dissolved and particulate substances contained in rain or snow. Dry deposition is the flux of particles and gases, especially SO2, FINO, and NFl, to the receptor surface during the absence of rain or snow. Deposition can also occur through fog, aerosols and droplets which can be deposited on trees, plants, or the ground. With forests, approximately half of the deposition of SO(, NH+,andH+ occurs as dry deposition. 

The time required for atmospheric chemical processes to occur is dependent on chemical kinetics. Many of the air quality problems of major metropolitan areas can develop in just a few days. Most gas-phase chemical reactions in the atmosphere involve the collision of two or three molecules, with subsequent rearrangement of their chemical bonds to form molecules by combination of their atoms. Consider the simple case of a bimolecular reaction of the following type-. 

Combustion processes are the most important source of air pollutants. Normal products of complete combustion of fossil fuel, e.g. coal, oil or natural gas, are carbon dioxide, water vapour and nitrogen. However, traces of sulphur and incomplete combustion result in emissions of carbon monoxide, sulphur oxides, oxides of nitrogen, unburned hydrocarbons and particulates. These are primary pollutants . Some may take part in reactions in the atmosphere producing secondary pollutants , e.g. photochemical smogs and acid mists. Escaping gas, or vapour, may... 

Scheme 7. Hydrolysis as the first step reaction in the atmospheric moisture condensation cure system.

A reaction mechanism is a series of simple molecular processes, such as the Zeldovich mechanism, that lead to the formation of the product. As with the empirical rate law, the reaction mechanism must be determined experimentally. The process of assembling individual molecular steps to describe complex reactions has probably enjoyed its greatest success for gas phase reactions in the atmosphere. In the condensed phase, molecules spend a substantial fraction of the time in association with other molecules and it has proved difficult to characterize these associations. Once the mecharrism is known, however, the rate law can be determined directly from the chemical equations for the individual molecular steps. Several examples are given below. 

Besides these features, the formation of a layer due to an interaction of a stratified fluid with light is itself noteworthy. Analogs to this phenomenon can be found in other media. Examples include photochemical reactions in the atmosphere near the Earth s surface, photochemical reactions in the surface water of the ocean and biological activity near the ocean surface. 

The oxides are gaseous and do not undergo reactions in the atmosphere that produce aerosol particles. Carbon monoxide is a relatively inert material with its main sinks in the atmosphere via reactions with free radicals, e.g.,... 

Figure 7-11 and its caption (Crutzen, 1983) depict the most important of the gas phase and photochemical reactions in the atmosphere. Perhaps the single most important interaction involves the hydroxyl free radical, OH-. This extremely reactive radical is produced principally from the reactions of electronically excited atomic oxygen, 0( D), with water vapor. Photo-... 

Interestingly, many of these free radicals are produced from photochemical reactions in the atmosphere of O2 and O3, for example... 

Although hydrogen sulfide does not react photochemically, it may be transformed to sulfur dioxide and sulfate by nonphotochemical oxidation reactions in the atmosphere. Its atmospheric residence time is typically less than 1 day (Hill 1973), but may be as high as 42 days in winter (Bottenheim and Strausz 1980). 

For systems with a chemical reaction, an important consideration is reactions that occur at constant pressure. This could represent a reaction in the atmosphere, such as a fire, in which the system is allowed to expand or contract according to the pressure of the surrounding atmosphere. Figure 2.1 illustrates this process for a system contained in a cylinder and also bounded by a frictionless and massless piston allowed to move so that the pressure is always constant on each side. 

In the late 1960s, direct observations of substantial amounts (3ppb) of nitric acid vapor in the stratosphere were reported. Crutzen [118] reasoned that if HN03 vapor is present in the stratosphere, it could be broken down to a degree to the active oxides of nitrogen NO (NO and N02) and that these oxides could form a catalytic cycle (or the destruction of the ozone). Johnston and Whitten [119] first realized that if this were so, then supersonic aircraft flying in the stratosphere could wreak harm to the ozone balance in the stratosphere. Much of what appears in this section is drawn from an excellent review by Johnston and Whitten [119]. The most pertinent of the possible NO reactions in the atmosphere are... 

The photochemical oxidants that are observed in the atmosphere are ozone, Oj, nitrogen dioxide, NOj and peroxyacetylnitrate (PAN). Several other substances, such as hydrogen peroxide, HjO, may be classified as photochemical oxidants, but their common presence in smog is not well established. The oxidants are secondary pollutants i.e., they are formed as a result of chemical reactions in the atmosphere. Primary pollutants are those emitted directly by pollution sources. 

The surface layer composition may effect catalytic activity. Surface enrichments of trace metals, for example, may enhance the catalytic role of particles in heterogeneous reactions in the atmosphere involving gaseous pollutants such as SO2 (54, 55). 

The chemical characterization of atmospheric pollutants is of great importance for determining their primary sources, elucidating chemical reactions in the atmosphere, determining potential risk to the environment and developing a reasonable control strategy. 

The kinetics of reactions in the atmosphere depends on the concentration of the hydroxyl radical. 

Reaction 1-5, 203(gas) 302(gas), is an overall reaction. Both Reactions l-6f and l-6b, 2CO(gas) + 02(gas) 2C02(gas), are also overall reactions. Both Reactions l-9f and l-9b are elementary reactions. Whether a reaction is an elementary reaction or an overall reaction can only be determined experimentally, and cannot be determined by simply looking at the reaction. Many simple gas-phase reactions in the atmosphere involve intermediate radicals and, hence, are complicated overall reactions. 

The fraction of species i remaining after chemical reaction in the atmosphere was determined in the previous section by comparison of concentrations at the source and in the atmosphere, with the assumption that one species is non-reacting. The chemical reactivity of these species can also be determined from laboratory studies under controlled conditions of radiation, and... 

Acid rain occurs when reactions in the atmosphere lead to the formation of various acids lowering the pH below the natural pH of rain, which is approximately 5.5. The acidic pH of natural rain water is due to the formation of carbonic acid by the reaction ... 

In summary, rate constants for addition reactions in the atmosphere can be estimated as a function of temperature and pressure if values are available for the low- and high-pressure limiting rate constants as a... 

There are many different types of surfaces available for reactions in the atmosphere. In the stratosphere, these include ice crystals, some containing nitric acid, liquid sulfuric acid-water mixtures, and ternary solutions of nitric and sulfuric acids and water. In the troposphere, liquid particles containing sulfate, nitrate, organics, trace metals, and carbon are common. Sea... 

We now turn to a brief description of typical laboratory techniques used to determine kinetic parameters that characterize heterogeneous reactions in the atmosphere. 

In addition, in the nighttime atmosphere, reaction of the R02 radicals with N03 may occur as discussed earlier. As a result, the products of the isoprene-N03 reaction in the atmosphere will depend on the concentrations of NO, N03, H02, and R02. 

In short, while kinetics are very important in determining the importance of reactions in the atmosphere, other aspects such as the formation of reactive intermediates, and the physical and chemical nature of reaction surfaces, should also be taken into account. 

Barbara J. Finlayson-Pitts is Professor of Chemistry at the University of California, Irvine. Her research program focuses on laboratory studies of the kinetics and mechanisms of reactions in the atmosphere, especially those involving gases with liquids or solids of relevance in the troposphere. Reactions of sea salt particles to produce photochemically active halogen compounds and the subsequent fates of halogen atoms in the troposphere are particular areas of interest, as are reactions of oxides of nitrogen at aqueous and solid interfaces. Her research is currently supported by the National Science Foundation, the Department of Energy, the California Air Resources Board, the Dreyfus Foundation, and NATO. She has authored or coauthored more than 80 publications in this area, as well as a previous book, Atmospheric Chemistry Fundamentals and Experimental Techniques. 

The interest in the reaction still continues and has acquired special importance due to deleterious effect on stratospheric ozone layer of haloalkane free radicals generated by photochemical reactions in the atmosphere. 

Since most of the reactions discussed in the following chapters take place in aqueous media, we confine our thermodynamic considerations to reactions occurring in dilute aqueous solutions. For the gas-phase reactions of organic compounds with highly reactive oxidants (i.e., reactions in the atmosphere Chapter 16), we will assume that these reactions are always energetically favorable and, thus, proceed spontaneously. 

Tihe atmosphere contains many radionuclides which result from nuclear weapons testing and from natural processes. The nuclear weapons-produced radionuclides include both fission products and activation products from the construction materials of the device. The natural radionuclides include the decay products of radon and thoron, the natural radionuclides in the airborne dust, and the cosmic-ray-produced radionuclides which result from spallation reactions in the atmosphere. Through the determination of the absolute and relative concentrations of this wide spectrum of radionuclides, it should be possible to define the rates of both the long term stratospheric processes and the shorter term tropospheric processes. At the beginning of 1962 a ground-level... 

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