Atmospheric pollution has become a worldwide concern. With the prospect of supersonic transports flying in the stratosphere came initial questions as to how the water vapor ejected by the power plants of these planes would affect the stratosphere. This concern led to the consideration of the effects of injecting large amounts of any species on the ozone balance in the atmosphere. It then became evident that the major species that would affect the ozone balance were the oxides of nitrogen. The principal nitrogen oxides found to be present in the atmosphere are nitric oxide (NO) and nitrogen dioxide (NO₂)—the combination of which is referred to as NO_x—and nitrous oxide (N₂O). As Bowman [1] reported, the global emissions of NO_x and N₂O into the atmosphere have been increasing steadily since the middle of the nineteenth century. Although important natural sources of the oxides of nitrogen exist, a significant amount of this increase is attributed to human activities, particularly those involving combustion of fossil and biomass fuels. For details as to the sources of combustion-generated nitrogen oxide emissions, one should refer to Bowman’s review [1].

Improvement of the atmosphere continues to be of great concern. The continual search for fossil fuel resources can lead to the exploitation of coal, shale, and secondary and tertiary oil recovery schemes. For instance, the industrialization of China, with its substantial resource of sulfur coals, requires consideration of the effect of sulfur oxide emissions. Indeed, the sulfur problem may be the key in the more rapid development of coal usage worldwide. Furthermore, the fraction of aromatic compounds in liquid fuels derived from such natural sources or synthetically developed is found to be large, so that in general, such fuels have serious sooting characteristics.

This chapter seeks not only to provide better understanding of the oxidation processes of nitrogen and sulfur and the processes leading to particulate (soot) formation, but also to consider appropriate combustion chemistry techniques for regulating the emissions related to these compounds. The combustion—or, more precisely, the oxidation—of CO and aromatic compounds has been discussed in earlier chapters. This information and that to be developed will be used to examine the emission of other combustion-generated compounds thought to have detrimental effects on the environment and on human health. How emissions affect the atmosphere is treated first.