Photochemical Smog Chemistry

In the 1970s, after it became clear that the alkoxyl-02 reaction was important in photochemical smog chemistry, an effort was made to determine the rate coefficient for the CH3O + O2 reaction. Wiebe et al. (1973) examined the reaction at 25°C in the photooxidation of CH3ONO at 366 nm in the presence of NO2. They monitored the CH3ONO2 produced in the competitive reaction... 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

Table 12-4 are sufficient for an understanding of smog chemistry. The major undesirable components of photochemical smog are NOj, O3, SOj, H2SO4, PAN, and aldehydes. Air quality standards have been established in several countries for SO2, NO2, and O3 H2SO4 contributes to acidic deposition and reduction in visibility and PAN and aldehydes can cause eye irritation and plant damage if their concentrations are sufficiently high. 

The atmospheric chemistry of nitrogen is quite complex and involves literally hundreds or thousands of chemical reactions. Although the fluxes are much smaller than the biological fluxes, these processes are important for a variety of reasons, including impacts on climate, stratospheric ozone, and photochemical smog. In this section we present an overview of the most important processes. 

Nitric acid synthesis, platinum-group metal catalysts in, 19 621 Nitric acid wet spinning process, 11 189 Nitric oxide (NO), 13 791-792. See also Nitrogen oxides (NOJ affinity for ruthenium, 19 638—639 air pollutant, 1 789, 796 cardioprotection role, 5 188 catalyst poison, 5 257t chemistry of, 13 443—444 control of, 26 691—692 effect on ozone depletion, 17 785 mechanism of action in muscle cells, 5 109, 112-113 oxidation of, 17 181 in photochemical smog, 1 789, 790 reduction with catalytic aerogels, l 763t, 764... 

Laity JL, Burstain IG, Appel BR. 1973. Photochemical smog and the atmospheric reactions of solvents. In Tess RW, ed. Solvents theory and practice. Advances in Chemistry Series 124, Washington, DC American Chemical Society, 95-112. 

Three major questions on the overall atmospheric chemistry of photochemical smog, not readily answered in the early studies, are ... 

Photochemistry of course plays a crucial role in the whole of tropospheric atmospheric chemistry, not just in locally confined events such as those involving acid deposition discussed above, and in the weU-documented photochemical smog of Californian cities, which is now a local problem of very widespread incidence throughout the world. 

A model is developed to account for the chemical features of photochemical smog observed in laboratory and atmospheric studies. A detailed mechanism consisting of some 60 reactions is proposed for a prototype smog system, the photooxidation in air of propylene in the presence of oxides of nitrogen at low concentrations. The rate equations for this detailed mechanism have been numerically integrated to calculate the time-concentration behavior of all the constituents of the system. The model has been used to examine the effects of varying relative and absolute concentrations of the reactants. The conclusions of this examination provide a framework for the analysis of the more complicated atmospheric problem. Some of the key questions related to the atmospheric chemistry have been discussed in terms of the detailed model. 

Tn recent years, a number of reaction models have been proposed to account for the chemical features of photochemical smog observed in atmospheric and laboratory studies (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11). Because of the complexity of smog chemistry and a lack of detailed knowledge of many relevant elementary reactions, numerous assumptions and simplifications are made in these mechanistic interpretations. A model for the chemistry of smog is presented here with a critical evaluation of the factors that control the major course of the reactions. The photooxidation of propylene (CsHq) in the presence of nitric oxide and nitrogen dioxide (NO + NO2 = NO, ) is used as a prototype for this study. 

The Role of OH Radicals in Photochemical Smog, American Chemical Society Division of Water Air and Waste Chemistry, Chicago (Sept. 13-18,1970). 

Photochemistry, chemistry caused by light, is responsible for many of the atmospheric reactions that determine the quality of our air. Photochemical smog is the name given to the red-brown haze that can form over cities under certain weather conditions. Nitrogen (as N2) and oxygen... 

The primary and secondary ambient air quality standards for nitrogen oxides are somewhat higher than for sulfur dioxide because the needs for control measures are driven by very similar requirements, namely, health effects and their influence on the pH of precipitation (Table 3.7). The significant role of nitrogen oxides (NOx) in the chemistry of photochemical smog episodes is a further factor used to determine ambient air quality standards. 

See the Chemistry in Use essay Nitrogen Otddes and Photochemical Smog in Chapter 24. 

In the mid-1960 s when Rule 66 was being promulgated, the problem of photochemical smog was technologically still in its infancy. We are now much more aware of the complexities of the atmospheric chemistry that are associated with the reactions of hydrocarbons and nitrogen oxides in the presence of sunlight. As a result, although an extensive amount of... 

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