Compared to other prime movers, jet engines (gas turbines) are considered to produce very low levels of combustion pollution. The gas turbine emissions of major concern are unburned hydrocarbons, carbon monoxide, oxides of nitrogen (NOx), and smoke. Although the contribution of jet aircraft to atmospheric pollution is less than 1% (Koff, 1994), jet aircraft emissions injected directly into the upper troposphere have doubled between the latitudes of 40 to 60 degrees north, increasing ozone by about 20%. In the stratosphere, where supersonic aircraft fly, NOx will deplete ozone. Both effects are harmful, so NOx reduction in gas turbine operation is a challenge for the 21st century (Schumann, 1993).
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