Smog formation, reactions

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. 

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. 

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],... 

Under the conditions of photochemical smog formation, nitric oxide is converted to nitrogen dioxide by the following overall reaction ... 

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. ... 

This reaction sequence increases the rate of NO oxidation, thereby increasing the rate of photochemical smog formation and the ambient level of O3 since less O3 is consumed by reacting with NO. Westberg and Cohen (2) incorporated this reaction sequence in a computer program to estimate the effect of 100 ppm of CO on isobutene and NO2. Their calculations predicted that the ozone concentration in this system is 50% greater than that in a similar system without CO. 

During the decade prior to 2003 there was a major expansion in the range of ambient measurements of species related to smog formation, especially in polluted rural environments. These measurements included primary NO and VOCs, a range of long-lived secondary reaction products and, in latter years, direct measurement of the OH... 

In Section 7.2.2 we saw the role that nitric oxide plays in smog formation and the incentive we would have for reducing its concentration in the atmosphere. It is proposed to reduce the concentration of NO in an effluent stream from a plant by passing it through a packed bed of spherical porous carbonaceous solid pellets. A 2% NO-98% air mixture flows at a rate of 1 X 10" mVs (0.001 dm /s) through a 2-in,-ID tube packed with porous solid at a temperature of 1173 K and a pressure of 101.3 kPa. The reaction... 

Pending this information and consideration of secondary reactions, the foregoing analysis supports the conclusion already reached by other investigators that, although other primary processes undoubtedly contribute, the photodissociation of nitrogen dioxide into nitric oxide and oxygen atoms is of major importance in smog formation. 

Incinerator exhaust gases do, however, contain a variety of pollutants -not only dioxin derivatives but also other compounds such as odours, volatile organic compounds (VOCs) and the reaction intermediates of dioxins [516], VOCs are a major contributor to air pollution because of their toxic and malodorous nature and their contribution to ozone and smog formation [131], They are emitted from a wide range of industrial processes and transportation activities [131,162,519-521], Some representative VOCs are methanol, ethanol, 2-propanol, acetone and toluene. 

Olefins. Olefins are the most reactive class of hydrocarbons in photochemical smog and have been studied extensively (I, 17, 18, 19). In general, as was perhaps first noted by Schuck and Doyle (20), the mechanism for olefin decomposition apparently involves electrophilic attack (by atomic oxygen, ozone, and other species) on the double bond. Thus, for most of the chemical reactions related to smog formation, olefin reactivity generally increases with additional alkyl (or other electron-donating) groups attached to the two carbon atoms joined by the double bond. 

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