Hydrocarbons photochemical oxidation

A few illustrative examples are the following. Photohydrogenation of acetylene and ethylene occurs on irradiation of Ti02 exposed to the gases, but only if TiOH surface groups are present as a source of hydrogen [319]. The pho-toinduced conversion of CO2 to CH4 in the presence of Ru and Os colloids has been reported [320]. Platinized Ti02 powder shows, in the presence of water, photochemical oxidation of hydrocarbons [321,322]. Some of the postulated reactions are ... 

Nitrogen Oxides. From the combustion of fuels containing only C, H, and O, the usual ak pollutants or emissions of interest are carbon monoxide, unbumed hydrocarbons, and oxides of nitrogen (NO ). The interaction of the last two in the atmosphere produces photochemical smog. NO, the sum of NO and NO2, is formed almost entkely as NO in the products of flames typically 5 or 10% of it is subsequently converted to NO2 at low temperatures. Occasionally, conditions in a combustion system may lead to a much larger fraction of NO2 and the undeskable visibiUty thereof, ie, a very large exhaust plume. 

ERA promulgated the basic set of current ambient air-quality standards in April 1971. The specific regulated pollutants were particulates, sulfur dioxide, photochemical oxidants, hydrocarbons, carbon monoxide, and nitrogen oxides. In 1978, lead was added. Table 25-1 enumerates the present standards. 

Describe the role of hydrocarbons in photochemical oxidant formation. 

In the USA, the Clean Air Act of 1970 established air-quality standards for six major pollutants particulate matter, sulfur oxides, carbon monoxide, nitrogen oxides, hydrocarbons, and photochemical oxidants. It also set standards for automobile emissions - the major source of carbon monoxide, hydrocarbons, and nitrogen oxides. An overview of the major standards is given in Tab. 10.2. The levels of, for example, the European Union (1996) are easily achieved with the present catalysts. The more challenging standards, up to those for the ultralow emission vehicle, are within reach, but zero-emission will probably only be attainable for a hydrogen-powered vehicle. 

Some pollutants fall in both categories. Nitrogen dioxide, which is emitted directly from auto exhaust, is also formed in the atmosphere photochemically from NO. Aldehydes, which are released in auto exhausts, are also formed in the photochemical oxidation of hydrocarbons. Carbon monoxide, which arises primarily from autos and stationary sources, is likewise a product of atmospheric hydrocarbon oxidation. 

In the early 1950 s, it was reported by Haagen-Smit that many of the characteristics of photochemical smog could be explained by the presence of ozone and other photochemical oxidants. These substances, he believed, were formed in the atmosphere as a result of chemical reactions involving nitrogen oxides and hydrocarbons present in automobile exhaust. Significant quantities of nitrogen oxides were also emitted by power plants. 

Several approaches have been used to reduce the problem to manageable proportions. The chemistry of photochemical-oxidant formation can best be understood by considering laboratory experiments with one hydrocarbon (two at most) and typical amounts of the nitrogen oxides, carbon monoxide, and water vapor. A model is developed on the basis of all the chemical reactions that are thought to be relevant, with their measured... 

This brief description of oxidant formation in polluted air is based on our current understanding of the chemistry involved. It is evident fix>m an examination of the detailed mechanism that many of the important reactions have not been well studied. For example, the sequences of degradation reactions for the hydrocarbons are only poorly understood. As a result of these uncertainties, it is not possible to make accurate predictions of photochemical oxidant concentrations. However, with another 5 yr of progress similar to the last 5, it should be possible to construct chemical models that will permit ozone predictions accurate to within... 

Johnston, H. S., and K. Dev Jain. Sulfur dioxide sensitized photochemical oxidation of hydrocarbons. Science 131 1523-1524, 1960. 

The main purpose of this chapter is to survi atmospheric concentrations of photochemical oxidants, with emphasis on surface concentrations and the distribution patterns associated with them. The reason for that em> phasis is that the photochemical oxidants that affect public health and welfare are largely concentrated in this region. The whole subject of stratospheric ozone (and its filtering of ultraviolet light and interactions with supersonic-transport exhaust products), nuclear weapon reaction products, and halogenated hydrocarbon decomposition pr ucts is not treated here. 

An international expert panel has issued an air quality criteria document for photochemical oxidants and related hydrocarbons that builds on the U.S. Department of Health, Education, and Welfare (dhew) air quality criteria document for photochemical oxidants. It discusses oxidant concentration patterns in the context of the same tabular material presented earlier. New information is added for the city of Delft the monthly means of daily maximums of hourly ozone concentrations are shown in Table 4-3, and the monthly average ozone concentrations are shown in Table 4-4. As in other cities, the worst month seems to be August, with a mean daily maximum (of hourly concentrations) of 0.071 ppm (140 Mg/m ). Table 4-5 compares the number of days in May through July 1971 when the ozone concentration at one or more sites reached or exceeded the hourly average of 200 Mg/m or of 100 Mg/m (from NATO data °). A comparison is made between Delft and five other monitoring sites in the Netherlands. Amsterdam had a peak value of 0.18 ppm in March 1971. 

Aldehydes may also be thought of as photochemical oxidants. The definition here becomes a bit hazy, because aldehydes in themselves are photooxidative reactants, as well as secondary pollutants that have adverse health effects. Referring to Figure 4-4, we note that aldehyde concentration throughout the day in Rome, Italy, seems to decay at roughly the same rate as the nitric oxide concentration. It would be expected to track the reactive fraction of the hydrocarbons, and this is also borne out approximately by the Rome data. A maximal formaldehyde concen-... 

Neligan, R. E., and R. M. Angus. The Validity of the Strategy of Linear Rollback of Hydrocarbons to Achieve Oxidant Air Quality Standards. Paper Presented at the UC-ARB (University of California-Air Resources Board) Conference on Technical Bases for Contrd Strategies of Photochemical Oxidant Current Status and Priorities in Research, Dec. 16-17, 1974, Riverside, California. 

North Atlantic Treaty Organization. Committee on the Challenges of Modem Society. Atmospheric concentrations, pp. 2-1-2-52. In Air Quality Criteria for Photochemical Oxidants and Related Hydrocarbons. N.29. 1974. 

Photochemical oxidants are atmospheric pollutants produced by a series of reactions between hydrocarbons and oxides of nitrogen in the presence of sunlight. The recognized photochemical oxidants that have been measured in ambient air are ozone, the peroxyacylnitrates (mostly as PAN), and hydrogen peroxide. ... 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

For the measurement of the hydrocarbon precursors of photochemical oxidants, the naturally occurring methane must be separated from the other so-called nonmethane hydrocarbons. Only one procedure, gas chromatography coupled with flame ionization detection, is available for this separation and measurement. Although instrumentation for routinely accomplishing this process is commercially available, its maintenance (continued operation) requires a degree of operational know-how that may be too costly for most public agencies in the United States to support. Consequently, the data currently are insufficient to relate the occurrence of photochemical oxidants and ozone accurately to some of their most important precursors, the nonmethane hydrocarbons. 

Before ozone - and PAN were identified as specific phytotoxic components of the photochemical complex, researchers used a number of artificial chemical reaction systems to simulate the ambient photochemical-oxidant situation. These efforts involved a number of irradiated and nonirradiated reaction systems unsaturated hydrocarbon-ozone mixtures, unsaturated hydrocarbon-NOx mixtures, and dilute auto exhaust). Most research before 1960 involved one or more of these reaction systems. This research has been well reviewed " - 451.459.488.505 extenslvely covered here. Although the... 

Even if hydrocarbons are completely removed from the air aldehydes and NOx can generate high concentrations of photochemical oxidants. 

Nonmethane hydrocarbons and both oxides of nitrogen should be monitored concurrently whenever photochemical oxidant or ozone is monitored. 

Ambient air. See also Photochemical-oxidant concentration hydrocarbons in, %, 97 monitoring procedure for, 6, 259-68 photochemical oxidants in, 128-29, 400 plant damage from oxidants in, 461-62 Ammonium... 

Altshuller, A.P., Kopczynski, S.L., Lonneman, W.A., Sutterfield, F.D., and Wilson, D.L. Photochemical reactivities of aromatic hydrocarbon-nitrogen oxide and related systems. Environ. ScL TechnoL, 4(l) 44-49, 1970. 

In many cases, a combination of physical, chemical, physico-chemical, and biotechnological treatments may be more efficient than one type of treatment (Table 3). Efficient pretreatment schemes, used prior to biotechnological treatment, include homogenization of sohd wastes in water, chemical oxidation of hydrocarbons by H2O2, ozone, or Fenton s reagent, photochemical oxidation, and preliminary washing of wastes by surfactants. 

Phlogiston a material once thought to be an element responsible for combustion Phospholipid lipid containing phosphorus derived from phosphoric acid Photochemical Oxidants air pollutants produced when hydrocarbons, nitrogen oxides, and other chemicals react under the influence of sunlight, for example, ozone, peroxyacylnitrates (PAN)... 

Hisham, M. W. M., and D. Grosjean, Sulfur Dioxide, Hydrogen Sulfide, Total Reduced Sulfur, Chlorinated Hydrocarbons, and Photochemical Oxidants in Southern California Museums, Atmos. Environ., 25A, 1497-1505 (1991b). 

Farley FF. 1977. Photochemical reactivity classification of hydrocarbons and other organic compounds. In EPA-600/3-77-001B. Inter Conf Photochemical Oxidant Pollut Control. Dimitriades B, ed. Research Triangle Park, NC, 713-726. 

In later experiments, Izumi et al. (47, 48) examined aerosol formation during photooxidation of a variety of hydrocarbons in an evacuable smog chamber. No seed particles were used in these experiments, but good estimates of the yield of aerosol from photochemical oxidation of the hydrocarbon precursors were obtained by using EAA data. In some cases, the volumetric yield was found to decrease with decreasing precursor concentration (Figure 4), so the finite vapor pressure of the reaction products limited nucleation, particle growth, or both. 

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