On the effect of emissions from aircraft engines on the state of the atmosphere

Emissions from aircraft engines include carbon dioxide, water vapor, nitrogen oxides, sulphur components, and various other gases and particles. Such emissions from high-flying global civil subsonic aircraft contribute to anthropogenic climate changes by increase of ozone and cloudiness in the upper troposphere, and by enhanced greenhouse effect. The absolute emissions by air traffic are small (a few percent of total) in comparison to surface emissions. However, the greenhouse effect of emitted water and of nitrogen oxides at cruise altitude is large in comparison to that of the same emissions near the earth's surface because of relatively large residence times at flight altitudes, low background concentrations, low temperature, and large radiative efficiency. At present, it appears that the emissions of nitrogen oxides have changed the background concentration in the upper troposphere in between 40 deg N and 60 deg N by 100 percent, causing an increase of ozone by about 20 percent. Regionally the observed annual mean change in cloudiness is of order 0.4 percent. The resultant greenhouse effect of changes in ozone and thin cirrus cloud cover causes a climatic surface temperature change of the order 0.01 to 0.1 K. These temperature changes are small in comparison to the natural variability. Recent research indicates that the emissions at cruise altitude may increase the amount of stratospheric aerosols and polar stratospheric clouds and thereby may have an impact on the atmosphere environment, to a yet unknown degree. Air traffic is increasing by about five to six percent per year; fuel consumption grows by about three percent per year. Moreover, the climatic changes due to air traffic enhance other environmental problems originating, e.g., from anthropogenic carbon dioxide or methane emissions. Hence, air traffic induced emissions are of growing importance. This calls for the development of efficient and low-emission propulsion systems and other means to reduce the emissions. This paper surveys the state of knowledge and describes several items of results from recent and ongoing research.