Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Options to reduce soot emissions require changes in the combustion process (Chapter 7). Soot may be activated by H 2 SO 4 and possibly other exhaust species. If soot
activation by H 2 SO 4 is to be avoided, fuel sulfur contents of less than 10 ppm would be required.
Simulations suggest that contrails would form even without any soot and sulfur emissions by activation and freezing of background particles (Jensen et al., 1998b;
Kärcher et al., 1998a). Hence, the formation of contrails cannot be avoided completely by reducing exhaust aerosol emissions. Contrails formed in plumes with fewer
exhaust particles are likely to be composed of fewer and larger particles, have smaller optical depths (Schumann, 1996a), hence cause less radiative forcing. Reduced
soot and sulfate particle emissions may also lead to the formation of cirrus clouds with fewer but larger particles and less radiative forcing.
An increase in engine efficiency may change the global effects of contrails. Improvements in engine efficiency measured as specific fuel consumption (SFC) per unit
thrust or overall efficiency, h, would reduce fuel consumption at cruise altitudes for a given amount of air traffic. Because more efficient engines increase the altitude
range over which persistent contrails form (see Section 3.2.4.1 and 3.7.2), contrail frequency and cover would likely increase for a given air traffic amount. On the other
hand, the number of ice crystals forming per aircraft-km would likely be reduced for lower SFC because aerosol and aerosol precursor emissions would be reduced.
Fewer ice crystals could result in less radiative impact for a given amount of air traffic in altitude regions where contrails form at present. Hence, the balance of
changes in contrail occurrence and the radiative impact that would result from changes in engine efficiency depend on a variety of factors, not all of which are well
known enough at present.
Reducing the frequency of contrails for a given amount of air traffic could otherwise be effected by reducing the number of flights in the humid and cold regions of the
upper and middle troposphere. Numerical weather prediction schemes may be used to predict and circumvent such regions on long-distance flights. Contrail-forming
regions could also be avoided by flying at generally higher altitudes, but the climatic impact of contrails may not be reduced because of counteracting effects. For
example, higher flight altitudes at low latitudes could increase contrails, possibly causing a net increase instead of a decrease in global radiative forcing by contrails. In
addition, more flights in the lower stratosphere could result in enhanced aerosol and chemical impacts not related to contrails.
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