Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
atmosphere and in 2050 (see Section 3.3). However, aircraft-induced particles will increase with growing emission rates of condensable sulfur compounds and soot
particle mass. A reduction in fuel sulfur content is not to be expected for the near future (see Chapter 7). The fraction of condensable sulfur compounds formed from
fuel-sulfur depends on the details of the chemistry between the combustor and the engine exit (Brown et al., 1996a; Lukachko et al., 1998). The dependence of this
fraction on expected changes in engine technology is not known (see Chapter 7).
Engines burning liquid hydrogen (liquid methane) instead of kerosene (Wulff and Hourmouziadis, 1997) emit 2.6 (1.5) times more water vapor for the same amount of
combustion heat. Therefore, such engines trigger contrails at about 1 to 2 km lower altitude (4 to 10 K higher ambient temperature) than comparable kerosene
engines. Therefore, an increase in contrail coverage is expected with such fuels. Because of larger water emissions, such contrails will grow to larger diameters before
evaporating in ice-subsaturated ambient air. On the other hand, aircraft using hydrogen (methane) fuels will emit no (little) soot and sulfur compounds, hence may
cause contrails that have fewer and larger ice particles, smaller optical thickness, and a lesser impact on radiative fluxes (Schumann, 1996a) (compare Table 3-7).
3.7.3. Expected Changes for Supersonic Aircraft
The expected emissions of future high speed civil transports (HSCTs) flying above 16-km altitude would substantially add to aerosol amounts in the stratosphere. A
fleet of 500 HSCTs is expected to consume about 72 Tg fuel yr -1 in 2015 (Baughcum and Henderson, 1998). This level of consumption will cause emissions of sulfur
and soot of 14.4 and 2.9 Gg yr -1 , for emission indices of 0.2 g S kg -1 and 0.04 g soot kg -1 , respectively. Microphysical calculations by the AER 2-D model (Weisenstein
et al., 1997) show that 28 Gg of sulfate will accumulate in the global atmosphere, assuming that 10% of sulfur emissions are converted in the plume to new particles
with a radius of 10 nm. The globally averaged aircraft-produced sulfate column is equal to 5.4 ng SO4 cm -2 , with a maximum of 13.6 ng SO4 cm -2 near 50°N. This
value is about twice that computed for present subsonic aviation (Table 3-4). The annually and zonally averaged perturbation of sulfate aerosol SAD as shown in
Figure 3-25 is used for scenario SA5 in Chapter 4 and in calculations in Chapters 5 and 6. The chemical consequences of these SAD changes are discussed in detail
in Section 4.3.3. Though supersonic aircraft may have better engine efficiency than subsonic aircraft (0.38 for the Concorde), supersonic aircraft are expected to form
few persistent contrails because the probability of ice-supersaturated air at cruise altitudes is small, except in the polar regions and near the tropical tropopause (Miake-
Lye et al., 1993). However, the accumulation of supersonic aircraft emissions in the polar atmospheres and local H 2 O, HNO 3 , and aerosol concentration increases in
aircraft plumes may enhance the occurrence of polar stratospheric clouds. The impact on tropospheric cloud formation of supersonic aircraft cruising in the
stratosphere is very likely much smaller than the impact of major volcanic events.
3.7.4. Mitigation Options
In the following discussion, options related to aircraft and aircraft operations are briefly considered for the reduction of volatile and nonvolatile particle emission and
formation and for the reduction of contrail formation and contrail impact.
Volatile particle growth is controlled mainly by oxidized sulfur, chemi-ions, and water vapor present in aircraft exhaust. With current engines and fuels, no practical
options exist to reduce water vapor emission indices. The oxidation of sulfur depends on the emission of SO 3 or the formation of H 2 SO 4 in the engine and plume. The
emission of SO 3 depends on the details of the reactive flow in and beyond the engine combustion chambers (Chapter 7). The processes controlling condensable sulfur
oxides and chemi-ion production are not yet sufficiently understood for a meaningful assessment of mitigation options. A reduction of sulfur content in fuel reduces
plume levels of SO 3 and H 2 SO 4 , but not necessarily by the same factor (Brown et al., 1996a). In addition, for low fuel sulfur content, volatile particles may remain that
result from the emissions of other condensable material (Yu et al., 1998; see Section 3.2) and thus require separate mitigation strategies.
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