Atmospheric Chemical Reactions

In addition to being removed from the atmosphere by physical processes, atmospheric chemicals can be removed by chemical transformations. Chemical transformations also can be sources of atmospheric pollutants a notorious example is the production of urban smog by reactions involving hydrocarbons, nitrogen oxides, and oxygen. 

In addition to chemical reactions brought about directly by light energy, there are other atmospheric chemical reactions that can occur in the dark, but that typically involve reactive chemical species previously produced photo-chemically (cf. indirect photolysis in surface waters). The rates of such thermal, or dark, reactions depend on the temperature and on the concentrations of the reactive chemicals involved. For example, consider the rate of the dark reaction between the photochemically produced compounds ozone (03) and nitric oxide (NO). The rate is proportional to the concentration of each reactant 

Water droplets and particulate matter often influence the rates of chemical transformations in the atmosphere. Whereas homogeneous reactions involve only gaseous chemical species in the atmosphere, reactions involving a liquid phase or a solid surface in conjunction with the gas phase are called heterogeneous reactions. Reactions that occur much more rapidly in water than in air may occur primarily in droplets, even though the droplets constitute only a small fraction of the total atmospheric volume. Solid surfaces also can catalyze reactions that would otherwise occur at negligible rates specific examples are discussed in the following sections on acid deposition and stratospheric ozone chemistry. 

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Fig. 12-1. Precursor-product relationship of atmospheric chemical reactions.

Atmospheric chemical reactions are classified as either photochemical or thermal. Photochemical reactions are the interactions of photons with species which result in the formation of products. These products may undergo further chemical reaction. These subsequent chemical reactions are called thermal or dark reactions. 

Atmospheric chemical reaction after release of emissions. 

A kinetic mechanism describing the rates of atmospheric chemical reactions as a function of the concentrations of the various species present. 

Although atmospheric chemical reactions are a subject of great interest and importance, they are not discussed in this article. 

Expressions of volume per volume units (ppm, pphm, or ppb) simplify measurements, because their value is independent of atmospheric temperature and barometric pressure. The volume units are equivalent to the ratio of the number of molecules of ozone to the number of molecules of air. This facilitates quantification of the atmospheric chemical reactions that lead to the formation of ozone. These units are also preferable when the molecular weight of a substance is uncertain, as in the reporting of total nitrogen oxides or total aldehydes. 

Thorium may change from one chemical species to another in the atmosphere (such as Th02to Th(S04)2) as a result of chemical reactions, but nothing definitive is known about the atmospheric chemical reactions of thorium. The chemical forms in which thorium may reside in the atmosphere are also not known, but it is likely to be present mostly as Th02. 

Cadle, R. D., and R. C. Robbins, Kinetics of Atmospheric Chemical Reactions Involving Aerosols, Disc. Faraday Soc., 30, 155-161 (1960). 

Aerosol A moisture-coated, microscopic, airborne particle up to 0.01 millimeter in diameter that is a site for many atmospheric chemical reactions. 

Generally the indoor environment allows different chemical transformation reactions to occur than usually predominate in the outside atmosphere. So called night-time chemistry (atmospheric chemical reactions not driven by photochemistry) is usually a good starting point to consider the in-museum chemistry that goes on. 

Liquid-water clouds (5) represent a potentially important medium for atmospheric chemical reactions in view of their high liquid water content [104 to 105 times that associated with clear-air aerosol (6)] and high state of dispersion (typical drop radius 10 pm). Clouds are quite prevalent in the atmosphere (fractional global coverage 50%) and persistent (lifetimes of a few tenths of an hour to several hours). The presence of liquid water also contributes to thermochemical driving force for production of the highly soluble sulfuric and nitric acids. 

The principal processes that govern the concentration and properties of atmospheric aerosols are emission of aerosol particles and precursor gases, gas-to-particle conversion and other pertinent atmospheric chemical reactions, transport, and processes by which particles are removed from the atmosphere. There is a substantial hterature on the characterization of these processes from laboratory studies and field measurements (cf. Section 4.04.1), so only a brief overview is provided here. 

Gas-to-particle conversion processes in the atmosphere consist of formation of low-vapor-pressure gases formed by atmospheric chemical reactions followed by new particle formation (nucleation), condensation of the low-vapor-pressure material on existing particles, or both. Chemical reaction producing the low-vapor-pressure product also occurs in cloud droplets, with the product remaining in the condensed phase of clear-air aerosol particles following cloud evaporation. Substances of intermediate vapor pressure may reversibly... 

The secondary component results from atmospheric chemical reactions that produce inorganic ionic species of which the most important are NH. and NOJ. Organic vapors also react in the atmosphere to form condensable products. For example, cyclic olefins react with ozone to form less volatile dicarboxyiic acids. The secondary chemical species nomially reported in studies ofaliiiospheric aerosol composition are relatively stable reaction products they have usually survived in the atmosphere and on filter or impactor... 

Cadle, R. D. and Powers, J. W., 1966 Some aspects of atmospheric chemical reactions of atomic oxygen. Tellus 18, 176-185. 

To assure the reliability of the analytical information obtained by air analysis it is necessary to look at the photochemistry of the atmosphere. It is very important to know the catalytic influence of inorganic compounds on atmospheric processes. For studying atmospheric chemical reactions and aerosols,... 

Chemistry is basically a practical science, and most of our knowledge about atmospheric chemical reactions has been established by laboratory experiments. Despite considerable advances in laboratory techniques it is often difficult to find experimental conditions whereby the reaction of interest can be sufficiently isolated from other concurrent ones so that its rate behavior can be unambiguously determined. Rate data thus are considered reliable only if two independent experimental techniques lead to the same results. As a consequence, the accumulation of dependable information on reactions deemed important to atmospheric chemistry has been a slow process. Nevertheless, a fairly large body of data now exists, at least for homogeneous gas-phase reactions. Tables A-4 and A-5 provide a compilation of reaction rate data as a reference for all subsequent discussions. The numbering of the reactions is used throughout this book. 

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