Atmospheric chemical

E. Graedel, D. T. Hawkins, and L. D. Cld,si.toQ., Atmospheric Chemical Compounds, Academic Press, Odando, Fla., 1986, p. 263, cited in Hazardous Substances Data Bank, Acetone from Toxicology Data Network (TOXNET), National Library of Medicine, Bethesda, Md., Jan. 1990, NATS section in the review. 

T. E. Graedel, D. T. Hawkias, and L. D. CExton, Atmospheric Chemical Compounds Sources, Occurrence and Bioassay, Academic Press, New York, 1986. Atmospheric O ne 1985, World Meteorological Organization, Geneva, Switzerland (3 vols.) an excellent compendium on tropospheric and stratospheric processes. 

Representation of Atmospheric Chemistry Through Chemical Mechanisms. A complete description of atmospheric chemistry within an air quaUty model would require tracking the kinetics of many hundreds of compounds through thousands of chemical reactions. Fortunately, in modeling the dynamics of reactive compounds such as peroxyacetyl nitrate [2278-22-0] (PAN), C2H2NO, O, and NO2, it is not necessary to foUow every compound. Instead, a compact representation of the atmospheric chemistry is used. Chemical mechanisms represent a compromise between an exhaustive description of the chemistry and computational tractabiUty. The level of chemical detail is balanced against computational time, which increases as the number of species and reactions increases. Instead of the hundreds of species present in the atmosphere, chemical mechanisms include on the order of 50 species and 100 reactions. 

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. 

Finally, atmospheric chemical transformations are classified in terms of whether they occur as a gas (homogeneous), on a surface, or in a liquid droplet (heterogeneous). An example of the last is the oxidation of dissolved sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur in the gas phase, forming secondary products such as NO2 and O3 in the liquid phase, such as SO2 oxidation in liquid droplets or water films and as gas-to-particle conversion, in which the oxidized product condenses to form an aerosol. 

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-. 

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

The atmosphere is a dynamic system, with gases and particulate matter entering, undergoing transformation, and leaving. Atmospheric chemical... 

Graedel, T. E., Hawkins, D. T., and Claxton, L. D., "Atmospheric Chemical Compounds Sources, Occurrence, and Bioassay," Academic Press, Orlando, FL, 1986,... 

Unlike the chemistry of simple mixtures of small numbers of reactants as observed in the laboratory, the chemistry of the atmosphere involves complex interactions of large numbers of species. However, several key aspects of these interactions have been identified that account for major observable properties of the atmospheric chemical system. It is convenient to separate the description into gas phase and condensed phase interactions, not the least because different chemical and physical processes are involved in these two cases. 

Finally, Figs 19-3g-j illustrate the accelerated rate of change that has occurred for some of these atmospheric chemical variables for the three decades from 1957 to the present. The O3 column data (g) are for the month of October at Flalley Bay, Antarctica, while the remainder are global mean values. 

We conclude this review with an example of the application of SPFM to corrosion studies. Atmospheric chemical corrosion constitutes a severe threat to the structural integrity of... 

Hayman GD, RG Derwent (1997) Atmospheric chemical reactivity and ozone-forming potentials of potential CFG replacements. Environ Sci Technol 31 317-336. 

The modeling package, delivered to the EPA, includes nationwide data bases for emissions, dispersion meteorology, and population patterns. These data are used as input for a Gaussian plume model for point sources and a box model for urbanwide area sources. Prototype modeling is used for point sources that are too numerous to define individually. Building wake effects and atmospheric chemical decay are addressed. 

Chemical Reactivity—Some are nonreactive some decay by atmospheric chemical processes and some are created by such processes. 

Atmospheric chemical reaction after release of emissions. 

Baur, M. E. (1978), "Thermodynamics of Heterogeneous Iron-Carbon Systems Implications for the Terrestrial Promotive Reducing Atmosphere", Chemical Geology 22,189-206. 

Whether the prediction scheme is a simple chart, a formula, or a complex numerical procedure, there are three basic elements that must be considered meteorology, source emissions, and atmospheric chemical interactions. Despite the diversity of methodologies available for relating emissions to ambient air quality, there are two basic types of models. Those based on a fundamental description of the physics and chemistry occurring in the atmosphere are classified as a priori approaches. Such methods normally incorporate a mathematical treatment of the meteorological and chemical processes and, in addition, utilize information about the distribution of source emissions. Another class of methods involves the use of a posteriori models in which empirical relationships are deduced from laboratory or atmospheric measurements. These models are usually quite simple and typically bear a close relationship to the actual data upon which they are based. The latter feature is a basic weakness. Because the models do not explicitly quantify the causal phenomena, they cannot be reliably extrapolated beyond the bounds of the data from which they were derived. As a result, a posteriori models are not ideally suited to the task of predicting the impacts of substantial changes in 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. 

A number of interfaces such as thermospray (TSP), ionspray (IS), atmospheric chemical ionization (APCI) and electrospray (ES) can tolerate much higher flow rates without requiring that the flow be split at the end of the LC column. Ions that are produced in atmospheric pressure ionization sources are moved directly into the mass spectrometer through small apertures. 

Davis, D. D., W. A. Payne, and L. J. Stief. The hydroperoxyl radical in atmospheric chemical dynamics Reaction with carbon monoxide. Science 179 280-282. 1973. 

Because of the dominance of distributed sources over local single sources in the production of photochemical oxidants, point-source models are not discussed here. Related research regarding the measurement of diffusion or the development of atmospheric chemical submodels are not emphasized. Giapter 2 is devoted to the chemical processes that govern atmospheric transformation and removal, and this aspect of the models is not repeated here. 

In another review, Hoffert discussed the social motivations for modeling air quality for predictive purposes and elucidated the components of a model. Meteorologic factors were summarized in terms of windfields and atmospheric stability as they are traditionally represented mathematically. The species-balance equation was discussed, and several solutions of the equation for constant-diffusion coefficient and concentrated sources were suggested. Gaussian plume and puff results were related to the problems of developing multiple-source urban-dispersion models. Numerical solutions and box models were then considered. The review concluded with a brief outline of the atmospheric chemical effects that influence the concentration of pollutants by transformation. 

The atmospheric chemical processes undergone by most pollutants are not readily describable by first-order kinetics. Hence, the simple Gaussian plume solution in Equation 5-3 is inapplicable in most cases where physiochemical transformations significantly alter concentrations on a time scale or space scale appropriate to an urban airshed. 

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. 

Laffort, P. and Gortan, C. (1987). Olfactory properties of some gases in hyperbaric atmosphere. Chemical Senses 12,139-142. 

HYDRATION ATMOSPHERE CHEMICAL KINETICS Solvatochromic relationship,... 

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. 

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