A comparison of the sulphur and nitrogen emissions reveals significant differences:
1. National sulphur emissions have fluctuated between 8 and 16 million tons a year since the turn of the century; nitrogen oxides, on the other hand, monotonically increased until the 1970s.
2. Since the 1970s, the main source of sulphur oxides has been coal combustion in power plants, while nitrogen oxides are contributed primarily by internal combustion engines.
3. Sulphur emissions result from oxidation of the sulphur impurity contained in fossil fuels and metal ores; nitrogen oxides are formed primarily by fixation of atmospheric nitrogen at high temperatures, and to a lesser degree by oxidation of fuel-bound nitrogen.
4. The control of sulphur oxides will have to rely on the removal of sulphur from the fuel or flue gases. Nitrogen oxides controls can be accomplished by technological changes in the combustion itself.
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