Formation photochemical smog

Benefits depend upon location. There is reason to beheve that the ratio of hydrocarbon emissions to NO has an influence on the degree of benefit from methanol substitution in reducing the formation of photochemical smog (69). Additionally, continued testing on methanol vehicles, particularly on vehicles which have accumulated a considerable number of miles, may show that some of the assumptions made in the Carnegie Mellon assessment are not vahd. Air quaUty benefits of methanol also depend on good catalyst performance, especially in controlling formaldehyde, over the entire useful life of the vehicle. 

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). 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

VOCs - A VOC is any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metal carbides or carbonates and ammonium carbonate, which participate in atmospheric photochemical reactions1. VOCs are precursors to ground-level ozone production and various photochemical pollutants and are major components in the formation of smog through photochemical reactions2,3. There are many sources of VOCs, as will be discussed later. 

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

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 following reaction is involved in the formation of photochemical smog. 

Despite uncertainties concerning the causative agents and their effects, we must proceed with the regulation of emissions that lead to the formation of photochemical smog. At the same time, research should continue on identifying the individual harmful agents in photochemical smog and... 

The available information on aerosol formation in photochemical smog... 

Three properties of photochemical smog were evident first in Los Angeles eye irritation haze (aerosol) formation and the d adation of rubber products. All three are associated with oxidants, although aerosols can also be formed by other pollutants, particularly sulfur dioxide. 

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. 

Wilson, W. E., Jr., A. Levy, and D. B. Wimmer. A study of sulfur dioxide in photochemical smog. II. Effect of sulfur dioxide on oxidant formation in photochemical smog. J. Air Pollut. Control Assoc. 22 27-32, 1972. 

Air quality simulation models for photochemical pollutants were reviewed by Johnson et al. for a new edition of Air Pollution. Some of the models developed for simulating photochemical smog were reviewed from the viewpoints of module logic and evaluation. The Los Angeles-based developments were outlined, including the format and preprocessing of emission inventory data and meteorologic data. Lumped-param-... 

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

Sanderson, H. P. The effects of photochemical smog on materials, pp. 71-87. In Photochemical Air Pollution Formation, Transport and Effects. NRC Associate Committee on Scientific Criteria for Environmental Quality. Report no. 12. NRCC no. 14096. Ottawa National Research Council of Canada, 1975. 

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. 

The interplay of HO, peroxy radicals, VOCs, and NO , species has substantial implications for tropospheric air quality. For instance, VOCs, NO , , and sunlight result in poor visibility from ozone and aerosol formation, together denoted as photochemical smog, which can lead to adverse health effects in sensitive individuals. Normally, we think of minimizing either class of compounds as beneficial to the atmosphere. However, minimizing VOC emissions only impacts ozone concentration in high-NO , areas. Moreover, in VOC-sensitive areas, reductions in NO , may lead to the overproduction of ozone. We can examine a simplified scheme for ozone production ... 

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. 

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