Rate Constants - Modeling Atmospheric Chemistry

The relationship among what goes in, what comes out, and what fluctuates within a system is called process control. Chemical engineering students are required to study this subject in some detail, usually as a course and laboratory in the latter half of the undergraduate curriculum. Prerequisite is a thorough understanding of chemical processes, as well as second-year college calculus and differential equations. 

An important chemical reaction is the decomposition of nitrous oxide into oxygen and nitrogen  

Nitrous oxide contributes to ozone depletion through photochemical reactions in the stratosphere. It reacts with an oxygen atom to form NO N2O is the chief source of NO in the stratosphere through the reaction 

NO catalyzes the removal of ozone in the upper stratosphere through the cycle in reactions (6.66) and (6.67), the net result of which is reaction (6.68)  

Nitrous oxides are formed naturally by decomposition of nitrogen compounds in the soil. N2O is also a man-made chemical used as a propellant for food spray cans and as an anesthetic. It can also be a by-product of pollution control devices installed on smokestacks at power plants. In any case, man-made nitrous oxides formed in the lower part of the atmosphere diffuse to the upper atmosphere where they contribute to ozone depletion. Knowledge of the rate of reaction (6.64) would help us model how long nitrous oxide survives in the atmosphere. 

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Rate constants for some reactions in atmospheric chemistry (Data adapted from NBS technical Note 866, Chemical Kinetic and Photochemical Data Ibr Modeling Atmospheric Chemistry, U.S. Department of Commerce, National Bureau of Standards, 1975.)... 

Second-order rate constants used in modeling atmospheric chemistry are commonly reported in units of cm molecule s. Show that these units are appropriate for a second-order rate law. 

In principle, it is now possible to construct a complete network of interconnecting chemical reactions for a planetary atmosphere, a hot molecular core or the tail of a comet. Once the important reactions have been identified the rate constants can be looked up on the database and a kinetic model of the atmosphere or ISM molecular cloud can be constructed. Or can it Most of the time the important reactions are hard to identify and if you are sure you have the right mechanisms then the rate constants will certainly not be known and sensible approximations will have to be made. However, estimates of ISM chemistry have been made with some success, as we shall see below. 

This section covers some of the more important chemical reactions that occur in the polluted atmosphere and attempts to show how these reactions result in photochemical-oxidant formation. For a more thorough understanding of the chemistry involved, the reader should consult recent reviewsand computer modeling studies by Demeijian, Kerr, and Calvert and by Calvert and MoQuigg. Unless otherwise noted, the mechanisms and rate constants of these modeling studies are used in this discussion. 

As noted above, development of reliable atmospheric models requires the elucidation of detailed rate constants for specific deactivation pathways. Presented below is a discussion of the collisional dynamics of O ( D2) deactivation by molecules of atmospheric interest. Additional species which are not of primary importance to environmental chemistry will also be mentioned in order to illustrate the general behavior of O ( D2) in gas phase encounters with quenching and reactive substrates. 

The rate constant data for the various channels of the H + HO2 reaction are shown in Figs 3.6 to 3.8. The branching ratios have been extensively studied at ambient temperatures because of the importance of the reaction in atmospheric chemistry and are believed to be well known (the results of Keyser [20], which agree with those of Sridharan et al. [21], are usually taken as definitive). However, there are very few studies at higher temperatures and no reliable values above 1000 K. This is not unusual. In most cases there is no information at all for combustion conditions. Current ignorance of reaction pathways in multichannel reactions is possibly the major uncertainty in modelling high-temperature processes. 

Because of its importance in both combustion and atmospheric chemistry, the OH radical has received most attention. Atkinson [65] has produced an extremely comprehensive collection and evaluation of data on OH reactions aimed mainly at atmospheric modelling but evaluating data at higher temperatures as well. Other evaluations of the reactions of OH with alkanes are those of Cohen and Westberg [29], Baulch et al. [66], and, more recently, Cohen s revision of his earlier evaluations [44]. As indicated in Section 3.3 a number of semi-empirical formulae have been derived to predict the rate constants of such reactions. 

A chemical mechanism is the set of chemical reactions and associated rate constants that describes the conversion of emitted species into products. From the point of view of tropospheric chemistry, the starting compounds are generally the oxides of nitrogen and sulfur and organic compounds, and ozone is a product species of major interest. Chemical mechanisms are a component of atmospheric models that simulate emissions, transport, dispersion, chemical reactions, and removal processes (Seinfeld, 1986, 1988). 

The role of reaction (21) in atmospheric chemistry is now well recognized, and general agreement upon the value of its rate constant, of critical importance for modelling calculations of the stratospheric ozone balance, now appears to be... 

The present compilation of kinetic data represents the 12th evaluation prepared by the NASA Panel for Data Evaluation. The Panel was established in 1977 by the NASA Upper Atmosphere Research Program Office for the purpose of providing a critical tabulation of the latest kinetic and photochemical data for use by modelers in computer simulations of stratospheric chemistry. The recommended rate data and cross sections are based on laboratory measurements. The major use of theoretical extrapolation of data is in connection with three-body reactions, in which the required pressure or temperature dependence is sometimes unavailable from laboratory measurements, and can be estimated by use of appropriate theoretical treatment. In the case of important rate constants for which no experimental data are available, the panel may provide estimates of rate constant parameters based on analogy to similar reactions for which data are available. 

The reactions are very fast, with rate constants close to 10 cm molecule s This is even true for the secondary radical C-C6H11O2, in spite of the much lower rate constant observed for the self-reaction (4 x 10 " cm molecule s ). This may be important since the acetylperoxy radical is also an abundant radical in the atmosphere and, if all cross-reactions of this type are as fast as those reported above, they should be taken into account in modelling the chemistry of remote atmospheres. 

Our batch reactor may be used to generate rate constants for a variety of temperatures and pressures. These laboratory-derived rate constants can subsequently be used in quantitative models for atmospheric chemistry. In other words, if we choose the upper atmosphere for the system boundary, then we may write a conservation of mass statement for nitrous oxide that looks like ... 

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