Anthropogenic nitric oxide

One of the key culprits in the formation of photochemical smog is NO (nitric oxide), a colorless but reactive gas. Its nominal concentration in our atmosphere is quite low, and there are natural sources of nitric oxide about which we can do little. It is the anthropogenic nitric oxide about which we are most concerned, as it is a pollutant over which we may have some control. Where does this nitric oxide come from You are probably aware that an... 

A substantial portion of fhe gas and vapors emitted to the atmosphere in appreciable quantity from anthropogenic sources tends to be relatively simple in chemical structure carbon dioxide, carbon monoxide, sulfur dioxide, and nitric oxide from combustion processes hydrogen sulfide, ammonia, hydrogen chloride, and hydrogen fluoride from industrial processes. The solvents and gasoline fractions that evaporate are alkanes, alkenes, and aromatics with relatively simple structures. In addition, more complex... 

For many years the presence of nitric oxide (NO) and nitrogen dioxide (N02) in the atmosphere has been a cause for concern on account of both the scale of anthropogenic emissions of these compounds and their impacts on health and the environment. There are various atmospheric reactions which cycle NO and N02, and it is therefore convenient to think of the two compounds as a group. By convention the sum total of oxides of nitrogen (i.e. NO + N02) is termed NOx and is expressed as N02 mass equivalents. 

The tropospheric nitric oxide is produced from molecular nitrogen or its compounds during high temperature combustion processes thus it is mostly of anthropogenic origin. Nitrogen dioxide is produced by further oxidation of NO from these compounds other nitrogenous species are formed. 

In effect, the reactions in Eqns 7.3 and 7.4 bring about the reverse ofEqns 7.1 and 7.2. Another anthropogenic gas, nitric oxide (NO), scavenges ozone in a similar way (to form nitrogen dioxide, N02) and is produced by the dissociation of N20, which can be generated from fossil fuel combustion and from the decomposition... 

The practical importance of the process is related, in particnlar, to the rednction of atmospheric emission of N2O, which is a greenhouse gas and also contributes to stratospheric ozone depletion. One of the most important anthropogenic somces of N2O emission is adipic acid production by nitric oxidation of cyclohexanol and/or cyclohexanon Since the concentration of N2O in adipic-acid-production off-gases is high, the most effective way of reducing N2O emission, according to the Rhone-Poulenc approach, is conversion back to NO, ... 

The natural concentrations of H, OH, NO, and Cl (most of which originate at the Earth s surface) in the stratosphere serve to catalyze Reaction (7.29) and to maintain approximately steady-state concentrations of ozone. However, if the concentrations of the catalysts X in Reaction (7.28) are increased significantly by anthropogenic activities, the delicate balance between the sources and sinks of atmospheric ozone will be disturbed, and stratospheric ozone concentrations can be expected to decrease. One of the first concerns in this respect were aircraft flying in the stratosphere (particularly supersonic aircraft). This is because aircraft engines emit nitric oxide (NO) which, as shown by Reactions (7.30), can serve as the X in Reaction (7.28). However, there are not sufficient numbers of aircraft flying in the stratosphere at the present time to perturb stratospheric ozone significantly. 

As for the primary trace chemical species in the remote natural atmosphere where there is no influence of anthropogenic activities, methane (CH4) emitted from lakes and marshes, biogenic volatile organic compounds (BVOCs), nitric oxide (NO) from natural soils and lightning, dimethyl sulfide (DMS) from marine organisms, O3 descended from the stratosphere can be conceived. Among them, the most important chemical species that is subjected to photolysis by the actinic flux (see Sect. 3.5) in the troposphere is O3 and NO2 (see Sects. 4.2.1 and 4.2.2). 

DMS is photochemically oxidized in the atmosphere to methanesulfonic and sulfuric acids. These strong acids contribute, along with nitric and organic acids, to the natural acidity of precipitation. Recent problems with acid rain have aroused interest in the anthropogenic and natural sources of volatile sulfur compounds (2). 

The maj or environmental concern for nitric acid oxidation of either cyclohexanol or the KA Oil is the emission of the greenhouse gas nitrous oxide, N2O. Theemission factor is estimated to be around 300 kg of N2O per ton of AA indeed, the real value is between 260 and 330 (around 0.75-1.2 mol of N2O per mole of AA), depending on the amount of catalyst used and the KA Oil composition. For a global AA production of 3.0 M-tonyr, the total amount of N2O produced is estimated to be about 0.9 M-tonyr. However, nowadays less than 5% of total emission of N2O into the atmosphere is from anthropogenic origin, and of this 5% only a small fraction is due to AA production, because almost all the AA producers have installed N2O decomposition units. 

In contrast to sulfur species, there are no differences in principle between natural and anthropogenic processes in the formation and release of reactive nitrogen species. Industrial nitrogen fixation (in separated steps N2 NH3, N2 NOx, NOx NO3) proceeds via the same oxidation levels as biotic fixation and nitrification, either on purpose in chemical industries (ammonia synthesis, nitric acid production) or unintentionally in all high-temperature processes, namely combustion, as a byproduct due to N2 + O2 2 NO. 

---