Photochemical reactions computational modeling

In order to estimate the extent of ozone depletion caused by a given release of CFCs, computer models of the atmosphere are employed. These models incorporate information on atmospheric motions and on the rates of over a hundred chemical and photochemical reactions. The results of measurements of the various trace species in the atmosphere are then used to test the models. Because of the complexity of atmospheric transport, the calculations were carried out initially with one-dimensional models, averaging the motions and the concentrations of chemical species over latitude and longitude, leaving only their dependency on altitude and time. More recently, two-dimensional models have been developed, in which the averaging is over longitude only. 

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. 

The Original Scheme. The photochemical reactions lead to most of the unusual new requirements placed on this model. Before treating atmospheric studies, we must understand the action of the chemistry and describe it simply enough to keep the ultimate computer requirements within reasonable bounds. First, an extremely simple version of a kinetic mechanism 27) will be explained and examined. 

The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. 

A similar computational modelling approach has been shown to be useful, for example, in studying the mechanism of low-temperature oxidation of alkanes (4), pyrolysis of alkanes (5-7), other gas-phase reactions (8), the formation of photochemical smog (9,10), and peroxide decomposition (11), among others. It is not uncommon to begin with all possible species and by permutation and combination derive a complete set of reactions, and then eliminate a subset by chemical... 

An alternative to the simultaneous solution of (4.A.3) and (4.A.7) is to solve (4.A.3) first and store the computed values of c, (t), / = 1, 2,. .., n, for use in subsequent solution of (4.A.7). Because this method, called the direct decoupled method (DDM), solves the model and sensitivity equations separately, only n differential equations need to be solved at one time that is, the n equations (4.A.7) are solved m times for each of the values of j. Applications of the DDM are given by Dunker (1981, 1984). Milford et al. (1992) have used this method to evaluate. sensitivities of urban photochemical reaction mechanisms. The DDM is a computationally attractive alternative to the full, simultaneous. solution of (4.A.3) and (4.A.7). 

The first section deals specifically with the spectroscopic/ microscopic tools that can be used in concert with macroscopic techniques. The second section emphasizes computer models that are used to elucidate surface mediated reaction mechanisms. The remainder of the volume is organized around reaction type. Sections are included on sorption/desorption of inorganic species sorption/desorption of organic species precipitation/dissolution processes heterogeneous electron transfer reactions photochemically driven reactions and microbially mediated reactions. What follows are a few highlights taken from the work presented in this volume. 

Gery, M.W., Whitten, G.Z., Killus, J.P., Dodge, M.C. A photochemical kinetics mechanism for urban and regional scale computer modeling. J. Geophys. Res. D94, 12925-12956 (1989) Gilbert, R.G., Luther, K., Troe, J. Theory of thermal unimolecular reactions in the fall-off range. 

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