Nitrogen oxides atmospheric chemistry

Photochemistry plays a significant role in nitrogen s atmospheric chemistry by producing reactive species (such as OH radicals). These radicals are primarily responsible for all atmospheric oxidations. However, since the photochemistry of the atmosphere is quite complex, it will not be dealt with in detail here. For an in-depth review on tropospheric photochemistry, the reader is referred to Logan et al. (1981), Finlayson-Pitts and Pitts (1986), Crutzen and Gidel (1983) or Crutzen (1988). 

Ishihara T,Kagawa M, Mizuhara Y et al (1992) Selective reduction of nitrogen monoxide with propene over Cu-silicoaluminophosphate (SAPO) under oxidizing atmosphere. Chemistry Letters 2119-2122. 

Gas-phase oxidation of thiols has been discussed in some depth (33). This review mainly emphasi2es atmospheric processes, but a section on nitrogen oxides and thiols appears to be broadly appHcable. The atmospheric oxidation chemistry of thiols is quite different from that of alcohols. 

Nitrogen forms several oxides, with oxidation numbers ranging from - -l to +5. All nitrogen oxides are acidic oxides and some are the acid anhydrides of the nitrogen oxoacids (Table 15.2). In atmospheric chemistry, where the oxides play an important two-edged role in both maintaining and polluting the atmosphere, the) are referred to collectively as NO (read nox ). 

NOx An oxide, or mixture of oxides, of nitrogen, typically in atmospheric chemistry, noble gas A member of Group 18/VIlI of the periodic table (the helium family). 

Smog contains nitrogen oxides, ozone, and iarger moiecuies. The chemistry of smog is compiex and not fully understood. Atmospheric scientists are studying how smog forms and how it can be prevented. 

Historically, the sulfur oxides have long been known to have a deleterious effect on the atmosphere, and sulfuric acid mist and other sulfate particulate matter are well established as important sources of atmospheric contamination. However, the atmospheric chemistry is probably not as well understood as the gas-phase photoxidation reactions of the nitrogen oxides-hydrocarbon system. The pollutants form originally from the S02 emitted to the air. Just as mobile and stationary combustion sources emit some small quantities of N02 as well as NO, so do they emit some small quantities of S03 when they bum sulfur-containing fuels. Leighton [2] also discusses the oxidation of S02 in polluted atmospheres and an excellent review by Bulfalini [3] has appeared. This section draws heavily from these sources. 

Grosjean et al. (1996) investigated the atmospheric chemistry of cyclohexene with ozone and an ozone-nitrogen oxide mixture under ambient conditions. The reaction of cyclohexene and ozone in the dark yielded pentanal and cyclohexanone. The sunlight irradiation of cyclohexene with ozone-nitrogen oxide yielded the following carbonyls formaldehyde, acetaldehyde, acetone, propanal, butanal, pentanal, and a C4 carbonyl. 

Oxides of nitrogen play a central role in essentially all facets of atmospheric chemistry. As we have seen, N02 is key to the formation of tropospheric ozone, contributing to acid deposition (some are toxic to humans and plants), and forming other atmospheric oxidants such as the nitrate radical. In addition, in the stratosphere their chemistry and that of halogens interact closely to control the chain length of ozone-destroying reactions. 

E. ATMOSPHERIC CHEMISTRY OF HNO, TABLE 7.3 Oxidized Nitrogen Distribution at Selected Locations" (Percentage of Total) 285... 

Barbara J. Finlayson-Pitts is Professor of Chemistry at the University of California, Irvine. Her research program focuses on laboratory studies of the kinetics and mechanisms of reactions in the atmosphere, especially those involving gases with liquids or solids of relevance in the troposphere. Reactions of sea salt particles to produce photochemically active halogen compounds and the subsequent fates of halogen atoms in the troposphere are particular areas of interest, as are reactions of oxides of nitrogen at aqueous and solid interfaces. Her research is currently supported by the National Science Foundation, the Department of Energy, the California Air Resources Board, the Dreyfus Foundation, and NATO. She has authored or coauthored more than 80 publications in this area, as well as a previous book, Atmospheric Chemistry Fundamentals and Experimental Techniques. 

In this chapter we discuss the detailed chemistry of selected high-temperature processes where gas-phase reactions are important. Most research on gas-phase reactions has been motivated by environmental issues in atmospheric chemistry or in combustion. Significant advances in the detailed understanding of fuel-oxidation chemistry, as well as nitrogen, sulphur, and chlorine chemistry, have allowed development of modeling tools that can be used for design purposes for a number of combustion and industrial processes. 

Society is facing several crucial issues involving atmospheric chemistry, Species containing nitrogen are major players in each. In the troposphere, nitrogen species are catalysts in the photochemical cycles that form ozone, a major urban and rural pollutant, as well as other oxidants (references 1 and 2, and references cited therein), and they are involved in acid precipitation, both as one of the two major acids (nitric acid) and as a base (ammonia) (3, 4). In the stratosphere, where ozone acts as a shield for the... 

Li, and Cal profiles ai altitudes of Xrt to I00 km. The method also has been useful lor studying ihe hydroxyl free radical (OH), This radical is of principal inlerest because or ihe cataly tic role which it exerts in atmospheric chemistry. The OH radical, along with chlorine and nitrogen oxides, is involved in the ozone destruction cycle. 

Studies of nitrogen oxide radicals in various condensed media by means of the EPR technique started about 45 years ago. Initial results were collected in [88, 28]. NxOy radicals are of interest first of all because of their toxicity and a key role in atmospheric chemistry. From this point of view, formation, stability and reactivity of these species adsorbed on the surface of nanosized metal-oxide semiconductor particles, which are photoactive and widely presented in atmosphere, are of essential importance. Principal values of g- and A-tensors for some cases are picked up in the following Table 8.4. 

In this chapter we will examine the atmospheric degradation mechanisms of the following important classes of anthropogenic molecules alkanes, alkenes, aromatics, nitrogen oxides, S()2, CFCs and Halons, and finally HFCs and HCFCs. Our intent is not to give an exhaustive account of the photochemical oxidation of every man-made chemical species but rather to present examples of the degradation mechanisms of a few representative members of each class of pollutant. First, we need to consider the general features of atmospheric chemistry. 

Gas-phase, solution-phase, and heterogeneous reactions all play important roles in atmospheric chemistry. The mean atmospheric composition is given in Table 1. N2, O2, and Ar comprise 99.9% of the atmosphere and, for all practical purposes, the relative proportion of these gases is constant in the lower 100 km of the atmosphere. We are concerned here with the fate of pollutants such as CO, volatile organic compounds, halocarbons, sulfur compounds, and nitrogen oxides, which are present in trace amounts and whose concentrations vary significantly both spatially and temporally. 

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