Atmospheric Chemical Mechanisms

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. 

Environmental chambers, Atmospheric chemical mechanisms, Ozone, Volatile organic compounds,... 

SENSI nVITY/UNCERTAINTY ANALYSIS OF ATMOSPHERIC CHEMICAL MECHANISMS 219... 

A chemical mechanism is evaluated by comparing its predictions to laboratory or field data. Both the measurements and the predictions have uncertainties associated with them since the agreement between measurements and predictions is never perfect, the question then arises as to whether the differences exhibited are significant. When dealing with an atmospheric chemical mechanism, it is desirable to place uncertainty bounds on simulations. There have been a number of such studies for stratospheric chemical mechanisms (Stolarski et al., 1978 Falls et al., 1979 Ehhalt et al., 1979 Tilden et al., 1981 Tilden and Seinfeld, 1982 Stolarski and Douglass, 1986). 

There are several sources of uncertainty in an atmospheric chemical mechanism. First, some important reactions may be completely unknown and therefore not included in the mechanism. Second, for those reactions included in the mechanism, rate constants are known to varying degrees of accuracy. Third, the products or the distribution of products for some reactions are in question. In addition, when a chemical mechanism is used for a non-steady-state simulation, initial concentrations must be supplied for all the species in the mechanism. Some of these initial concentrations may not be available for simulation, and those that are measured may have uncertainties associated with them. The first source... 

A second approach is the Fourier method (Cukier et al., 1978 McRae et al., 1982). Here, c-,o, and A , are varied simultaneously in a systematic fashion and the resulting variations in Cj(t kj c,o) are analyzed by Fourier series to evaluate the multidimensional integrals. The method was originally developed for the sensitivity analysis of complex models, and Falls et al. (1979) have applied the method to an atmospheric chemical mechanism. Whereas calculations of or, (0 have not been explicitly reported, this quantity can be obtained by the Fourier method. The Fourier method is more efficient than the Monte Carlo method, but the computing time required can still be large. 

Determining the sensitivity of all concentrations to one parameter essentially doubles the computational burden over that associated with. solving (4.A.3). Nonetheless, one is often interested in the. sensitivity of all n concentrations to all ni parameters. Equation (4.A.7) is a system of nm differential equations, and, if n and m are large, as they frequently are in atmospheric chemical mechanisms, the computing burden can be considerable. 

Appendix 4 Sensitivity/Uncertainty Analysis of Atmospheric Chemical Mechanisms 219 4.A. 1 Sensitivity Coefficients 222 4.A.2 The Direct Decoupled Method 223 4. A.3 Adjoint Methods 224 4.A.4 Green s Function Methods 224 References 226 Problems 230... 

The number of reaction steps in the mechanism is decreased by one, but since lumping of the reaction steps resulted in exactly the same set of ODEs, there is no gain in simulation speed. Nevertheless, the lumping of multicharuiel reactions as above is common in atmospheric chemical mechanisms, because it may clarify the main reaction routes for the user. 

Using the rules of mass action kinetics, (almost) the same equations can be derived for the production rates of all species but D. The presence of a negative stoichiometric coefficient is perhaps surprising at first glance, but there are several lumped atmospheric chemical mechanisms (Gery et al. 1989) that contain negative stoichiometric coefficients on the right-hand side of some chemical equations. 

Chen, S., Brune, W.H., Oluwole, O.O., Kolb, C.E., Bacon, F., Li, G.Y., Rabitz, H. Global sensitivity analysis of the regional atmospheric chemical mechanism an application of random sampling-high dimensional model representation to urban oxidation chemistry. Environ. Sci. Technol. 46, 11162-11170 (2012)... 

In the atmosphere, the cmicentrations of radicals are low thus, the products of two radical concentrations are very small making the rates of radical radical reactions also very small. For this reason, radical—radical reactions are usually not considered in atmospheric chemical mechanisms with the exception of peroxy—peroxy radical reactions. The rate coefficients of molecule—molecule reactions are also usually very small. Therefore, atmospheric chemical mechanisms (unlike, e.g. combustion mechanisms) usually do not contain reaction steps in which identical species react with each other (reaction type 2A B). For this reason, in atmospheric chemistry, the production rates for most species can be calculated using the general equation ... 

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