Nitrogen, photochemical oxidants

National Ambient Air Quality Standards. Under the Clean Air Act, six criterion pollutants, ie, pollutants of special concern, have been estabhshed by the EPA sulfur oxides (SO ), particulates, carbon monoxide (CO), nitrogen oxides (NO ), o2one (photochemical oxidants), and lead. National Ambient Air QuaUty Standards (NAAQS) were developed by EPA based on threshold levels of air pollution below which no adverse effects could be experienced on human health or the environment. 

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. 

Implications to Humans. Acidic precipitation has not been observed with concentrations that pose a concern to human exposure. The acidic airborne pollutants in the particulate or gaseous forms such as the oxides of sulphur and of nitrogen, and the associated photochemical oxidant ozone, are inhaled. This may lead to the irritation of the respiratory tract, and subsequently to impaired lung function, aggravated asthma and bronchitis. 

Nitrogen Dioxide (NO2) Is a major pollutant originating from natural and man-made sources. It has been estimated that a total of about 150 million tons of NOx are emitted to the atmosphere each year, of which about 50% results from man-made sources (21). In urban areas, man-made emissions dominate, producing elevated ambient levels. Worldwide, fossil-fuel combustion accounts for about 75% of man-made NOx emissions, which Is divided equally between stationary sources, such as power plants, and mobile sources. These high temperature combustion processes emit the primary pollutant nitric oxide (NO), which Is subsequently transformed to the secondary pollutant NO2 through photochemical oxidation. 

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... 

The photochemical oxidants that are observed in the atmosphere are ozone, Oj, nitrogen dioxide, NOj and peroxyacetylnitrate (PAN). Several other substances, such as hydrogen peroxide, HjO, may be classified as photochemical oxidants, but their common presence in smog is not well established. The oxidants are secondary pollutants i.e., they are formed as a result of chemical reactions in the atmosphere. Primary pollutants are those emitted directly by pollution sources. 

FIGURE 2-1 Diurnal variation of nitric oxide, nitrogen dioxide, and ozone concentrations in Los Angeles, July 19, 1965. Reprinted from Air Quality Criteria for Photochemical Oxidants. ... 

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. ... 

The technol( for the routine measurement of the nitrogen oxides (nitrogen dioxide and nitric oxide) is fairly well advanced. The epa is on the verge of officially proposing that chemiluminescence produced by the reaction of nitric oxide with ozone be the reference method for nitrogen dioxide.This method is even more suitable for nitric oxide. Because no national air quality standard has been promulgated for nitric oxide, no reference method will be specified. However, its measurement in the atmosphere is crucial for establishing the relation of its emission to the formation of atmospheric ozone and other photochemical oxidants. 

The diurnal patterns of ozone, nitric oxide, and nitrogen dioxide concentrations observed during photochemical oxidant episodes in California have been confirmed by smog-chamber studies. There may be, however, a decrease in reliability with decreasing concentration of values less than 0.1 ppm that were measured by the colorimetric method. The magnitude of these uncertainties among the various monitoring networks in the United States has still to be assessed. 

The chemical composition of the photochemical-oxidant complex is discussed in Qiapter 2. The major phytotoxic components are ozone, nitrogen dioxide, and the peroxyaiylidtrates. The latter homologous... 

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

Closing volume measurement, 29S-96 Colorimetric analysis for air monhoriqg, 262-64,27S to measure nitrogen oxides, 269 Community Health Effects Surveillance Studies (chess), 9,432 Concentration. See Ozone otmcentration Feroi aoetyliiitrate concentration Photochemical-oxidant concentration... 

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)... 

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