Photochemical smog nitrogen oxides

Nitrogen oxides are pollutants. They cause acid rain and photochemical smog. Nitrogen oxides also catalyse the oxidation of sulfur dioxide gas, SO2, in the atmosphere during the formation of acid rain. The sulfur trioxide, SO3, gas that is produced by this oxidation reacts with rainwater, forming sulfuric acid. The reactions below show the catalytic activity of the nitrogen oxides ... 

In the following sections we shall look at the major ingredients of photochemical smog—the oxides of nitrogen and hydrocarbons, and the secondary pollutant ozone. 

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. 

Combustion processes are the most important source of air pollutants. Normal products of complete combustion of fossil fuel, e.g. coal, oil or natural gas, are carbon dioxide, water vapour and nitrogen. However, traces of sulphur and incomplete combustion result in emissions of carbon monoxide, sulphur oxides, oxides of nitrogen, unburned hydrocarbons and particulates. These are primary pollutants . Some may take part in reactions in the atmosphere producing secondary pollutants , e.g. photochemical smogs and acid mists. Escaping gas, or vapour, may... 

Smog A mixture of smoke and tog, that arises from nitrogen oxides and hydrocarbons and the photochemical action of sunlight. 

A second important class of pollutant compounds resulting from combustion processes is the general class of the oxides of nitrogen (NO, NO2, NO3, N2O4, and so forth), typically denoted as NO . These NO molecules are key intermediates in the atmospheric conversion of VOCs into photochemical smog and ozone. There are three identified sources of NO molecules in combustion systems. 

N02 plays a major role in the chemical reactions which generate photochemical smog and ground-level ozone, as well as contributes to the acid rain effect. Nitrogen dioxide is a strong oxidizing agent, which reacts in the air to form corrosive nitric acid, as well as... 

Nitric acid synthesis, platinum-group metal catalysts in, 19 621 Nitric acid wet spinning process, 11 189 Nitric oxide (NO), 13 791-792. See also Nitrogen oxides (NOJ affinity for ruthenium, 19 638—639 air pollutant, 1 789, 796 cardioprotection role, 5 188 catalyst poison, 5 257t chemistry of, 13 443—444 control of, 26 691—692 effect on ozone depletion, 17 785 mechanism of action in muscle cells, 5 109, 112-113 oxidation of, 17 181 in photochemical smog, 1 789, 790 reduction with catalytic aerogels, l 763t, 764... 

Basic rate information permits one to examine these phenomena in detail. Leighton [2], in his excellent book Photochemistry of Air Pollution, gives numerous tables of rates and products of photochemical nitrogen oxide-hydrocarbon reactions in air this early work is followed here to give fundamental insight into the photochemical smog problem. The data in these tables show low rates of photochemical consumption of the saturated hydrocarbons, as compared to the unsaturates, and the absence of aldehydes in the products of the saturated hydrocarbon reactions. These data conform to the relatively low rate of reaction of the saturated hydrocarbons with oxygen atoms and their inertness with respect to ozone. 

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. 

Although sulfur oxides were recognized as a problem in combustion processes well before the concern for photochemical smog and the role of the nitrogen oxides in creating this smog, much less is understood about the mechanisms of sulfur oxidation. Indeed, the amount of recent work on sulfur oxidation has been minimal. The status of the field has been reviewed by Levy el al. [38] and Cullis and Mulcahy [39] and much of the material from the following subsections has been drawn from Cullis and Mulcahy s article. 

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. 

Table 2-6 is only a sampling of the compounds that might be found in photochemical smog in the future. The possible combinations among the many free radicals and the oxides of sulto and nitrogen are almost limitless. Many undiscovered exotic compounds are present in photochemical smog, but their concentration and importance remain to 1 established. 

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

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