Nitrogen oxides emissions

Nitrogen is a constituent of both the natural atmosphere and of the biosphere. When industrial metabolism releases nitrogen to the environment it is considered a "pollutant" because of its chemical form: NO, NO₂, and N₂O. These oxides of nitrogen can be toxic to humans and to biota, and they also perturb the chemistry of the global atmosphere.

In the 1980s, United States NO_X emissions from the major sectors were as follows: transportation sector 45 per cent, power plants 35 per cent, and industrial sources 25 per cent (USEPA, 1986). In the transportation sector, the NO_X emissions result from internal combustion engines. In power plants and industrial sources, NO_X is produced in boilers. The overwhelming fraction of nitrogen oxide emissions arises from the high-temperature combustion of fossil fuels, while emissions from metal-processing plants and open-air burning of biomass are relatively low.

In internal combustion engines the main parameter that determines NO production is the combustion temperature, which in turn depends on the air/fuel ratio. In industrial boilers the combustion temperature is also the main factor. In addition, fuel-bound nitrogen in coal and residual oil also contributes about 20 per cent to the NO_X emissions (Darmstadter et al., 1987). For this reason, combustion technology plays a significant role in the quantity of NO_X emissions. In this sense, NO_X emission estimates, as well as the suitable control strategies, are significantly different from those of sulphur.

Estimating historical emission trends of nitrogen oxides is difficult because most of the nitrogen oxide is formed by the fixation of atmospheric nitrogen at high temperatures of combustion rather than by oxidation of the nitrogen contained in the fuel. Thus, nitrogen oxide emissions depend primarily on the combustion temperature and, to a lesser degree, on the fuel properties. Since combustion processes in internal combustion engines and boilers have undoubtedly changed since the turn of the century, it is likely that nitrogen oxide emission factors have also changed. Because combustion parameters can vary over a wide range, and because information on historical combustion processes is generally lacking, assumptions concerning changes in emission factors over time constitute the major source of uncertainty in developing trends in nitrogen oxide emissions.

NO_x emissions can be obtained from fuel consumption data weighted by an appropriate emission factor. For a given source of combustion, this factor is the quantity of nitrogen oxide emitted per unit of fuel consumed. The emission factors used here were derived from extensive inventories that list nitrogen oxide emission factors according to source type of combustion (USEPA, 1977, 1978). The numerous emission factors listed in these compilations were aggregated into four weighted-average emission factors by fuel type: coal, gasoline, natural gas, and other petroleum products. The emission factors before 1970 were estimated to reflect the fact that the average combustion temperature, and hence the production of nitrogen oxides per unit of fuel consumed, was lower, especially for coal combustion, over the past 100 years (fig. 12). A simple linear trend was assumed for all emission factors. For coal combustion, the emission factor was assumed to have increased fivefold from 1880 to 1970. For combustion of gasoline and natural gas, the emission factors were assumed to have increased by 50 and 100 per cent, respectively. The emission factor for other petroleum products was assumed to be constant over time.

Fig. 12 Trends in emission factors of nitrogen oxide by fuel type (Sources: Emission factors for the period 1970-1980 were derived from data presented by the US Environmental Protection Agency (1977, 1978). For the period 1880-1970, trends of historical emission factors were assumed to be linear, with slopes varying by fuel type)

Fig. 13 Trends in emission of nitrogen oxides in the eastern United States

On the basis of these estimates of emission factors and data on fuel consumption, national emission trends were calculated as shown in figure 13. It is evident that there was a monotonic increase in NO_X emissions in the United States from the turn of the century to about 1970; since then the emissions have remained roughly constant.