Photochemical smog, nitrogen oxide role

Although sulfur oxides were recognized as a problem in combustion processes well before the concern for photochemical smog and the role of the nitrogen oxides in creating this smog, much less is understood about the mechanisms of sulfur oxidation. Indeed, the amount of recent work on sulfur oxidation has been minimal. The status of the field has been reviewed by Levy el al. [38] and Cullis and Mulcahy [39] and much of the material from the following subsections has been drawn from Cullis and Mulcahy s article. 

N02 plays a major role in the chemical reactions which generate photochemical smog and ground-level ozone, as well as contributes to the acid rain effect. Nitrogen dioxide is a strong oxidizing agent, which reacts in the air to form corrosive nitric acid, as well as... 

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... 

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. 

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. 

Since the hydrocarbons enter into the atmosphere (e.g. as products of the above combustion processes), they react with further components of the polluted atmosphere, or they participate in photooxidation or photolysis. The reactions resulting in photooxidation smog formation are the most important ones. The mechanism of this smog formation has been investigated by many specialists the sequence of reactions described in Section S.2.4.2 may be considered as the most reasonable one. The production of the photochemical smog is determined by conditions, in which the reactivity of hydrocarbons also plays an important role. It is characterized by the ability of hydrocarbons to participate in photochemical reactions in the presence of nitrogen oxides. According to this, hydrocarbons are divided into four classes of reactivity [34] ... 

Heterogeneous catalysis plays a major role in the fight against urban air pollution. Two components of automobile exhausts that help form photochemical smog are nitrogen oxides and unburned hydrocarbons. 

Heterogeneous catalysis has a role in the atmospheric chemistry of ozone in the troposphere as well. Catalytic converters in automobiles are filled with a porous ceramic material, which provides a surface that catalyzes the removal of CO and NO (nitrogen oxides) ftom the exhaust. (Nitrogen oxides initiate the formation of ozone and other lung irritants in photochemical smog. We will examine this process in detail in Section 11.8.) The process by which catalytic converters operate is shown in Figure 11.14. The types of steps shown there are found in most examples of heterogeneous catalysis. 

Nitrogen oxides play a number of villainous roles in atmospheric chemistry, including the formation of photochemical smog and the removal of ozone in the stratosphere. N2O3 has attracted particular attention... 

This reaction is the most important primary photochemical process involved in smog formation. The roles played by nitrogen oxides in smog formation and other forms of air pollution are discussed in Chapter 7. 

Because nitrogen dioxide dissociates into nitric oxide and atomic oxygen under the action of sunlight as shown in Equation (16.39), it plays a central role in the formation of photochemical smog, as we will discuss in the selected topic in depth. It also serves as a useful nonaqueous solvent. 

---