Kinetics atmospheric reactions

Hampson RF. 1980. Chemical kinetic and photochemical data sheets for atmospheric reactions. Washington, DC U.S. Department of Transportation. 

Cadle, R. D. Experimental studies of Los Angeles air and kinetics of atmospheric reaction, pp. 27-59. In L. H. Rogers, Ed. Proceedings of the Conference on Chemical Reactions in Urban Atmospheres. Technical Report 15. Los Angeles Air Pollution Foundation, 1956. 

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

The above methods of investigating the order of the reaction with respect to each species independently, although simple and practical for many reactions (such as atmospheric reactions and aqueous reactions) studied by chemists and geochemists, is often difficult to apply to homogeneous reactions in a silicate melt or mineral because the concentration of each species may not be varied freely and independently. This will become clear later when the kinetics for the Fe-Mg order-disorder reaction in orthopyroxene and the interconversion reaction between molecular H2O and OH groups in silicate melt are discussed. 

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

We therefore first briefly discuss the analysis of systems that involve diffusion in the gas and liquid phases, uptake, and reaction in the bulk liquid or at the interface. Following that, we give a brief description of some of the most common methods used to measure mass accommodation coefficients and reaction kinetics for heterogeneous atmospheric reactions. Included are some new approaches that appear to be especially promising. For a review of this area, see Kolb et al. (1995, 1997). 

Fortunately, there are several compendia of kinetic data applicable for atmospheric reactions that are... 

Atkinson, R., Atmospheric Reactions of Alkoxy and /3-Hydroxy-alkoxy Radicals, Ini. J. Chem. Kinet., 29, 99-111 (1997b). 

Chemical component of models As we have seen from the examination of kinetics and mechanisms of atmospheric reactions thus far, the chcmistiy of even relatively simple organics can be quite complex. This chemistry has been described in terms of explicit chemical mechanisms, that is, a listing of the individual chemical reactions. The oxidation of even one organic in air includes hundreds of reactions. 

James N. Pitts, Jr., is a Research Chemist at the University of California, Irvine, and Professor Emeritus from the University of California, Riverside. He was Professor of Chemistry (1954-1988) and cofounder (1961) and Director of the Statewide Air Pollution Research Center (1970-1988) at the University of California, Riverside. His research has focused on the spectroscopy, kinetics, mechanisms, and photochemistry of species involved in a variety of homogeneous and heterogeneous atmospheric reactions, including those associated with the formation and fate of mutagenic and carcinogenic polycyclic aromatic compounds. He is the author or coauthor of more than 300 research publications and three books Atmospheric Chemistry Fundamentals and Experimental Techniques, Graduate School in the Sciences—Entrance, Survival and Careers, and Photochemistry. He has been coeditor of two series, Advances in Environmental Science and Technology and Advances in Photochemistry. He served on a number of panels in California, the United States, and internationally. These included several National Academy of Science panels and service as Chair of the State of California s Scientific Review Panel for Toxic Air Contaminants and as a member of the Scientific Advisory Committee on Acid Deposition. 

Environmental Fate. Sensitized photolysis studies in water and oxidation/reduction studies in both air and water are lacking, as are biodegradation studies in surface and groundwaters. These kinds of studies are important, since they represent the fundamental removal mechanisms available to isophorone in the environment. In addition, the kinetic studies for the atmospheric reactions are important for understanding the significance of a removal mechanism and predicting the reactions that may control the fate of a chemical in the environment. 

Peroxy radicals are intermediates in the atmospheric oxidation of air pollutants and in oxidation reactions at moderate temperatures. They are rapidly formed from free radicals by addition of 02. Free radicals in the atmosphere are quantitatively converted to R02 with a half-time of about 1 fis. The peroxy radicals are then removed by reaction with other trace species. The dominant pathways are reactions with NO and NOz. Only a few peroxy radicals have been detected with a mass spectrometer, and extensive research on these reactions has been done by UV absorption spectroscopy with the well-known and conveniently accessed band in the 200- to 300-nm region. Nevertheless, FPTRMS has been used for some peroxy radical kinetics investigations. These have usually made use of the mass spectrometer to observe more than one species, and have given information on product channels. The FPTRMS work has been exclusively on atmospheric reactions of chlorofluoromethanes and replacements for the chlorofluoromethanes. 

Due to the time-resolution limitation of the method, FPTRMS can be used to determine the kinetics of only those unimolecular reactions that occur on millisecond time scales or longer. However, even if a unimolecular reaction occurs too rapidly for time resolution of the kinetics, the occurrence of a reaction can be shown by mass spectrometric detection of the products. If the unimolecular reaction is rate limited by a preceding slow step so that the product formation rates are time resolved, then a lower limit to the unimolecular rate coefficient can be estimated. In the case of atmospheric reactions this will frequently be enough information to permit reaction mechanisms to be sorted out. 

It was thought38 that the mechanism was intermediate between SE1 and SE2. Other workers39 have suggested that the kinetic results of Reutov et al 1 suffer from irregularities arising from atmospheric oxidation of dibenzylmercury. When conducted under a nitrogen atmosphere, reaction (26) followed second-order kinetics in solvent aqueous acetonitrile39. 

A collaborative effort between the groups of Mabury and Wallington investigated the atmospheric kinetics and reaction dynamics of the FTOHs. 

In such models the OH concentration field is computed using measured or estimated concentration fields of the precursor molecules and photon flux data. The resulting OH field is then tuned such that it correctly predicts the lifetime of methyl chloroform (CH3CCI3) with respect to OH radical attack. From measurements of the atmospheric turnover time of CH3CCI3 (4.8 years) [20], its lifetime with respect to loss in the stratosphere (45 years), and its lifetime with respect to loss in the oceans (85 years) the tropospheric lifetime of CH3CCI3 with respect to OH radical attack has been inferred to be 5.7 years [17,21], Methyl chloroform is the calibration molecule of choice because it has a long history of precise atmospheric measurements, it has no natural sources, its industrial production is well documented, and because the kinetics of reaction Eq. 20 are well established, feo = 1.8 x 10-12 exp(- 1500/T) cm3 molecule-1 s-1 [22]. 

This chapter discusses the fluid-solid and solid-solid reactions used to produce ceramic powders. The first aspect of this discussion is the spontaneity of a particular reaction for a given temperature and atmosphere. Thermodynamics is used to determine whether a reaction is spontaneous. The thermod3mamics of the thermal decomposition of minerals and metal salts, oxidation reactions, reduction reactions, and nitridation reactions is discussed because these are often used for ceramic powder synthesis. After a discussion of thermodynamics, the kinetics of reaction is given to determine the time necessary to complete the reaction. Reaction kinetics are discussed in terms of the various rate determining steps of mass and heat transfer, as well as surface reaction. After this discussion of reaction kinetics, a brief discussion of the types of equipment used for the synthesis of ceramic powders is presented. Finally, the kinetics of solid—solid interdiffusion is discussed. 

Chemical reaction processes account for the production of a variety of contaminant species in the atmosphere. Each of the basic airshed models above includes reaction phenomena in the conservative equations. The reaction term, denoted by R accounts for the rate of production of species i by chemical reaction and depends generally on the concentrations of each N species. The conservation equations are thus coupled through the Ri terms, the functional form of each term being determined through the specification of a particular kinetic mechanism for the atmospheric reactions. 

General Considerations. The nature and characteristics of atmospheric contaminants suggest certain diflBculties in the formulation of a kinetic mechanism of general validity. First, there is a multiplicity of stable chemical species in the atmosphere. Most species are present at low concentrations, thereby creating major problems in detection and analysis. A number of atmospheric constituents probably remain unidentified. Also, there are a large number of short-lived intermediate species and free radicals which participate in many individual chemical reactions. However, while we must admit to only a partial understanding of atmospheric reaction processes, it remains essential that we attempt to formulate quantitative descriptions of these processes which are suitable for inclusion in an overall simulation model. 

Ion-molecule reactions in interstellar clouds Radiation chemistry in interstellar grain mantles Condensation in stellar outflows Equilibrium reactions in the solar nebula Surface catalysis (Fischer-Tropsch) in the solar nebula Kinetically controlled reactions in the solar nebula Radiation chemistry (Miller-Urey) in the nebula Photochemistry in nebular surface regions Liquid-phase reactions on parent asteroid Surface catalysis (Fischer-Tropsch) on asteroid Radiation chemistry (Miller-Urey) in asteroid atmosphere... 

The reverse processes, CF30 + HX, are considerably more important for the kinetic modeling of atmospheric reactions, especially the reactions CF30 + HC1 and CF30 + HBr, which are exothermic and proceed with small energy barriers. The temperature dependence of the rate constants kHx calculated by Brudnik et al.134 is given by the expressions... 

Oxidation of aqueous phenol solutions was studied over various catalysts in a semibatch slurry and continuous-flow fixed-bed reactors at temperatures up to 463 K and pressures slightly above atmospheric. The results show that due to a complex consecutive-parallel reaction pathway and a heterogeneous-homogeneous free-radical mechanism both kinetics and reaction selectivity are strongly dependent on the type of reactor used. Although the catalysts employed were found to be active in converting aqueous phenol solutions to nontoxic compounds, neither metal oxides nor zeolites were stable at the reaction conditions. 

DOT. 1980. Chemical kinetic and photochemical data sheets for atmosphere reactions. Washington, DC U S. Department of Transportation, High Altitude Pollution Program National Aeronautics and Space Administration, Upper Atmosphere Research Office and National Bureau of Standards, Office of Standard Reference Data. FAA-EE-80-17. 

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