Reactions mechanisms of smog

NIKI, DABY, AND WEINSTOCK Mechanisms of Smog Reactions... 

Niki, H., E. E. Daby, and B. Weinstock (1972). Mechanisms of smog reactions. In "Photochemical Smog and Ozone Reactions" (R. F. Gould, ed.), pp. 16-57. American Chemical Society, Washington, D.C. 

Ndd, H., Davy, E., Weinstock, B. Mechanism of smog reactions. In Photochemical Smog and Ozone Reactions, Advances in chemistry, vol. 113, pp. 16-57 (1972)... 

Nitric oxide is the primary nitrogen oxide emitted from most combustion sources. The role of nitrogen dioxide in photochemical smog has already been discussed. Stringent emission regulations have made it necessary to examine all possible sources of NO. The presence of N20 under certain circumstances could, as mentioned, lead to the formation of NO. In the following subsections the reaction mechanisms of the three nitrogen oxides of concern are examined. 

Strong support for fundamental gas-phase kinetics is needed. Most of the reaction mechanisms and rate constants that are needed to construct realistic and detailed models of the polluted atmosphere are determined in laboratory studies under very special conditions, not in smog simulations at a pressure of 1 atm. Because there are still very... 

Understanding the kinetics and mechanisms of the production of HONO from N02 reaction is very important in that it appears to be a major source of HONO in smog chambers, inside homes, from automobiles (see Chapter 15), and perhaps in the troposphere at large. Clearly, more studies are warranted, especially those that can elucidate the nature and concentrations of species at the interface itself. 

As discussed in Sections B.3 and C of Chapter 7, this reaction has been shown to be too slow in aqueous solution to be significant in the atmosphere it is faster on surfaces and has been proposed as a source of HONO in smog chambers (e.g., see Sakamaki et al., 1983 Pitts et al., 1984 Leone et al., 1985 and Chapter 7.0. Since HONO is a major OH source in the early stages of irradiation in smog chambers, it is important to understand the mechanism of its formation and to quantify its rate of production under various experimental conditions. Thus, if this reaction only occurs at... 

It is apparent from (VIII- 18a) that a rapid conversion of NO to N02 is needed for the buildup of O, concentration levels. It was recognized that the conversion rate of NO to N02, observed in the real atmosphere, was several hundred times as fast as the rate of the reaction 2NO + 02 — 2NOz in the range of NO concentrations of 0.05 to 0.5 ppm. Furthermore, in simulated smog chamber experiments it was found that the rate of butene consumption by illumination of a NO- N02-H20-butene mixture in air was much faster than that calculated on the basis of reactions of O(3P) atoms and 02 with butene. Apparently a new mechanism based on radical chain reactions was needed to explain these observations. Two groups of scientists, Heicklen and coworkers and Weinstock and coworkers, were the first to propose independently the following sequence of reactions in 1970. 

These data and the resulting mechanism are consistent with earlier observations of smog chamber processes conducted in the presence and absence of added S02, as discussed in references (2 61. In the atmosphere, reaction (4) is expected to be the primary fate of SO3 due to the presence of large quantities of water and the presumed lack of other important reactions for SO3. Under atmospheric conditions, this mechanism suggests that HO radicals are effectively converted to HO2 radicals, if reaction (3) is sufficiently fast and there... 

Table I gives a complete statement of the mechanism reference should be made to this table throughout the following discussion. The major inorganic species that participate in photochemical smog reactions are NO, NO2, O2, CO, O3, and H2O. In Table I Reactions 1-8 and 14 represent most of the important reactions among these species and account for the following experimentally observed phenomena ...

Validation of the Mechanism. The process of matching the predictions of the mechanism to experimental smog chamber data is termed validation of the mechanism. The first step in a validation procedure is to establish values for the two major classes of parameters that appear in the mechanism—the reaction rate constants and the stoichiometric coeflBcients. Base values of the rate constants can be estimated from the chemical literature. However, with the sacrifice of chemical detail present in the new, simplified mechanism is a loss in the ability to associate the rate constant values with particular reactions. Therefore, the rate constants in the simplified mechanism are more a quantitative assessment of the relative rates of competing reactions than a reflection of the exact values for particular reactions. Base values for the parameters that appear in the kinetic mechanism are thus established on the basis of published rate constants. However, we must expect that final validation values will consist of those values which produce the best fit of the mechanism to actual smog chamber data. A recent summary of rate constants for specific hydrocarbon systems was made by Johnston et al. 40) from which rate constants for the Reactions in Table I can be estimated for a number of hydrocarbons. 

Hess Q. D., F. Camovale. M. E., Cope, and Q. M. Johnson The evaluation of some photochemical smog reaction mechanisms - 1 temperature and initial composition effects, Atmos. Environ., 26A, 625-641. (1992)... 

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