Smog formation

The most important reaction in the initiation of smog formation is the production of ozone, a strong oxidant generated through the N02 photodissociation cycle known as the null cycle.  

In turn, these free radicals may react with atomic or molecular oxygen, nitric oxide, and nitrogen dioxide without necessarily sacrificing the ozone present. 

A series of chain reactions then take place, which depend on radical initiation, propagation, and termination. These are aleatory processes where reactions occur among the species as they come upon each other, depending on the individual reactivities. The oxidation of hydrocarbons in the termination reactions leads to aldehydes, which may be further oxidized to form aldehyde peroxides, peroxides, hydroperoxides, and peroxyacids  

Some termination reactions also include the formation of acids, alcohols, and nitro compounds. They may take place through combination or disproportionation (dismutation) reactions  

Among the very important secondary pollutants related to smog are the peroxy-nitro compounds, PAN (peroxyacetyl nitrate) and PBN (peroxyben-zylnitrate), which are responsible for the phenomenon of eye redness in polluted cities. Ozone— as well as these organic peroxides—is part of the end of the chain mechanisms of photochemical smog. Considering acetaldehyde as the initial organic reactant we have  

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Soot. Emitted smoke from clean (ash-free) fuels consists of unoxidized and aggregated particles of soot, sometimes referred to as carbon though it is actually a hydrocarbon. Typically, the particles are of submicrometer size and are initially formed by pyrolysis or partial oxidation of hydrocarbons in very rich but hot regions of hydrocarbon flames conditions that cause smoke will usually also tend to produce unbumed hydrocarbons with thek potential contribution to smog formation. Both maybe objectionable, though for different reasons, at concentrations equivalent to only 0.01—0.1% of the initial fuel. Although thek effect on combustion efficiency would be negligible at these levels, it is nevertheless important to reduce such emissions. 

In recent years, the use of solvent-borne adhesives has been seriously restricted. Solvents are, in general, volatile, flammable and toxic. Further, solvent may react with other airborne contaminants contributing to smog formation and workplace exposure. These arguments have limited the use of solvent-bome adhesives by different national and European regulations. Although solvent recovery systems and afterburners can be effectively attached to ventilation equipment, many factories are switching to the use of water-borne rubber adhesives, hot melts or 100% solids reactive systems, often at the expense of product performance or labour efficiency. 

The reader can easily estimate whether or not the local conditions in his/her region are suitable for photochemical smog formation. 

Early attention focused on the most reactive of the hydrocarbons, the olefins, because it was expected and was observed by atmospheric sampling that they were preferentially consumed during smog formation. Lab-oratoiy studies confirm that olefln-NO mixtures are very prolific sources of ozone. However, these olefins are not essential to oxidant formation. 

Calvert, J. G., and R. D. McQuigg. The computer simulation of the rates and mechanisms of photochemical smog formation. Int. J. Chem. Kinet. Symp. 1 (Chemical Kinetics Data for the Lower and Upper Atmosphere) 113-154, 1975. 

Dimitriades, B. Effects of hydrocarbon and nitrogen oxides on photochemical smog formation. Environ. Sci. Technol. 6 253-260, 1972. 

Takeuchi, K., Yazawa, T., andibusuki, T. Heterogeneous photocatal34ic effect of zinc oxide on photochemical smog formation reaction of C4H8-N02-Air, Atmos. Environ., 17(ll) 2253-2258,1983. 

In NO, smog formation (NO, is a mixture of NO, N2O, NO2, N2O4, and N2O5) the NO is produced by reaction of N2 and O2 at the high temperatures of combustion in automobiles and fossil fuel power plants, and NO2 and the other NO, species are produced by subsequent low-temperature oxidation of NO in air. NO is colorless, but NO2 absorbs visible radiation and produces brown haze. We write these reactions as a set of two reactions among four species,... 

One such reaction in smog formation is the formation of the acetyl radical such as by sunlight photolysis of acetaldehyde... 

Smog formation in the atmosphere is caused by such reaction. Nitrogen dioxide is rapidly oxidized by ozone to form nitrogen pentoxide ... 

Clearly, environmental chamber studies are very useful tools in examining the chemical relationships between emissions and air quality and for carrying out related (e.g., exposure) studies. Use of these chambers has permitted the systematic variation of individual parameters under controlled conditions, unlike ambient air studies, where the continuous injection of pollutants and the effects of meteorology are often difficult to assess and to quantitatively incorporate into the data analysis. Chamber studies have also provided the basis for the validation of computer kinetic models. Finally, they have provided important kinetic and mechanistic information on some of the individual reactions occurring during photochemical smog formation. 

In short, while the OH reactivity scale has a number of caveats associated with its use, it has proven useful in providing at least an initial assessment of relative contributions of organics to photochemical smog formation. 

Chock, D. P A. M. Dunker, S. Kumar, and C. S. Sloane, Effect of NOa. Emission Rates on Smog Formation in the California South Coast Air Basin, Environ. Sci. TechnoL, 15, 933-939(1981). 

Impact Depends on the exact compound from health effects (cancer) to photochemical smog formation. 

Finally, enrichment of isotopic species has been achieved for a number of atoms and molecules using an appropriate monochromatic light source that preferentially excites an isotopic species of interest in mixtures of other isotopic species. The photochemistry associated with isotopic enrichment is briefly described in Chapter VIII. Great efforts have been made recently to obtain information on the detailed photochemical processes involving smog formation, stratospheric pollution, and atmospheres of other planets, and brief discussions of these subjects are also presented in the chapter. 

The OH (X2n) can be generated from the photolysis of water, hydrogen peroxide, and nitric and nitrous acid. Reactions of OH(A2n) with various hydrocarbons arc important in understanding photochemical smog formation (see Section VIII—2). 

In addition, near infrared absorption bands at 1.255 and 1.425 /tm have recently been found by Hunziker and Wendt (493), who have attributed the bands to a transition 2 A <- 2A". The band at 1.504 /emission bands of H02 have been detected recently by Becker et al. (83, 86). The H02 radical is an important reaction intermediate in combustion, in polluted atmospheres, and in the photolysis of H202. The reaction of H02 with NO is considered as a key reaction in photochemical smog formation, which is discussed in Section VIII 2. 

Likewise, 03 reacts with hydrocarbons to produce unknown numbers of H02 and R02 (or RC002) [see below]. From the computer analysis of simulated smog formation involving the hypothetical illumination of N0-N02-H20-butene-aldehydes-C0-CH4 mixtures in air, Calvert and McQuigg (184) estimate that H02 and R02 radicals, formed mainly by the addition of OH to butene, account for 10% of NO to N02 conversion. The H02 and R02 radicals formed from the photolysis of aldehydes and OH reactions with aldehydes are responsible for 25% of the conversion. Carbon monoxide is only 5% effective for the NO to N02 conversion. The effect of paraffins on the NO to N02 conversion rate is very small. 

Calvert and McQuigg have also suggested that the rate of decay of truns-2-butene in the initial stage is mainly determined by the reaction of OH with the hydrocarbon. In the later stage of smog formation OH and 03 attacks on the hydrocarbon must be equally important. 

Thus, H2CO may play a significant role for photochemical smog formation in polluted atmospheres [Calvert ct al. (182) sec Section (VI11-2), p. 335],... 

Cox (246) estimates the concentration of HN02 to be 109 molec cm"3 in the daytime natural troposphere. The photolysis of HN02 may be an important source of OH in the troposphere, since HN02 absorbs the sun s radiation above 3000 A. The reactions of OH with hydrocarbons (either hydrogen abstraction from paraffins or addition to the double bond in olefins) in the troposphere are known to be the initial steps for photochemical smog formation [see Section VIII-2, p. 333],... 

Mechanism of Smog Formation. A mechanism initially proposed t > explain the time history of air pollutants was the dissociation of N02 l> solar radiation since other primary pollutants NO and hydrocarbons d<<... 

Photochemical oxidation or smog formation potential (PCOP). 

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