3.7. Parameters of Future Changes in Aircraft-Produced Aerosol and Cloudiness

The future effects of aircraft depend on trends in climate and air traffic amount and changes in the technical properties of aircraft. Our current understanding of the formation of aviation-induced aerosol and cloudiness can be used to estimate how future changes may affect the impacts of aviation and to identify mitigation options that would be effective in reducing these impacts.

3.7.1 Changes in Climate Parameters

If climate change occurs in the future, atmospheric parameters related to aerosols and contrails will also have changed. Of particular importance to aviation-induced aerosol and cloudiness are changes in temperature and humidity in the upper troposphere and lower stratosphere; changes in the height, temperature, and humidity of the tropopause region; changes in the abundance of particles; and changes in cloudiness. Table 3-10 summarizes how changes in these parameters may be reflected in aviation-related impacts. General circulation models of the atmosphere predict that the climate of 2050 will reflect global warming from the accumulation of greenhouse gases. In this new climate, models predict increases in the amounts of cirrus clouds, the height of the tropopause, and upper tropospheric temperature (IPCC, 1996; Timbal et al., 1997). A higher tropopause would cause more contrails, at least at high latitudes. Observed temperature changes (e.g., Parker et al., 1997) do not reveal the expected temperature increase in the upper troposphere. Some models predict a higher tropopause if the surface temperature increases (about 200-m altitude increase for 1 K surface temperature increase) (Thuburn and Craig, 1997). Increases on the order of 100 m were analyzed in polar regions and at mid-latitudes (Hoinka, 1998; Steinbrecht et al., 1998). Such changes may be forced by cooling of the lower stratosphere as a result of changes in ozone concentration (Hansen et al., 1997) and increases in moisture as a result of increasing methane concentrations. Stratospheric temperatures between 50 and 100 hPa have decreased by about 1 to 2 K since 1980 (Ramaswamy et al., 1996; Halpert and Bell, 1997). An increase in water vapor concentration has been observed in the lower stratosphere, with the largest trend (0.8%/yr) in the 18- to 20-km region (Oltmans and Hofmann, 1995). Because few contrails currently form in the lower stratosphere, small changes in stratospheric conditions will not create significant changes in contrail abundance. Aerosol loading in the troposphere and lower stratosphere may increase because of changed climate conditions and increased surface emissions. Surface emissions from fossil fuel burning were projected to grow by a factor of 1.5 to 2.1 from 1990 to 2040 (Wolf and Hidy, 1997).

3.7.2. Changes in Subsonic Aircraft

By the year 2050, the number of aircraft flying in the upper troposphere is expected to have increased significantly (see Chapter 9). In scenario Fa1, global annual aviation fuel consumption in 2050 will have increased by a factor of 3.2 compared with 1992, with a larger increase (factor of 4.3) above 500 hPa. Scenarios Fc1, Fe1, and Eah (see Chapter 9) assume increases by factors of 1.8, 5, and 14, respectively, in total fuel consumption compared to 1992. The frequency of contrail formation is expected to increase with traffic because large regions of the atmosphere are humid and cold enough to allow persistent contrails to form and because such regions are not at present fully covered with optically thick cirrus or contrail clouds (see Sections 3.4.1 and 3.4.3). The number of aircraft may grow slightly less rapidly than fuel consumption when smaller aircraft are replaced by larger ones. This factor is important because the amount of persistent contrail cover may depend mainly on the number of aircraft triggering contrails and less on fuel consumption.

As aircraft engines become more fuel efficient, contrails will form more frequently at lower flight levels because exhaust plumes of more efficient engines are cooler for the same water content (see Section 3.2.4.1). The overall efficiency h (Cumpsty, 1997) with which engines convert fuel combustion heat into propulsion of cruising subsonic aircraft was close to 0.22 in the 1950s, near 0.37 for modern engines in the early 1990s, and may reach 0.5 for new engines to be built by 2010 (see Figure 3-22). An increase of h from 0.3 to 0.5 in a standard atmosphere increases the threshold formation temperature of contrails by about 2.8 K (equivalent to 700-m lower altitude) (Schumann, 1996a).

The change in persistent contrail coverage because of changed traffic has been determined using thermodynamic analysis of meteorological data from 1983 to 1992 and fuel consumption data (Sausen et al., 1998) (see Section 3.4). This method has been used to estimate future changes in contrail cover resulting from changes in air traffic, assuming a fixed climate, fuel consumption scenarios, and expected engine performance specifications (Gierens et al., 1998). The computed contrail cover (Figure 3-23) for the 2050 Fa1 fuel scenario using present analysis data and h of 0.5 shows a global and annual mean contrail cover of 0.47%, with values of 0.26% and 0.75% for scenarios Fc1 and Fe1. Values may be as high as 1 to 2% for scenario Eah, which does not specify the spatial distribution of future traffic and in which contrail cover may become limited by the amount of cloud-free ice-supersaturated air masses. In comparison, values are 0.087% for the 1992 DLR fuel inventory with h of 0.3 and 0.38% for the 2050 scenario with h of 0.3. Hence, contrail cover is expected to increase by a factor of about 5 over present cover for a 3.2-fold increase in annual aviation fuel consumption from 1992 to 2050, even under constant climate conditions. Increased efficiency of propulsion by future engines causes about 20% of the computed increase in contrail cover. In the year 2050, the maximum contrail coverage is expected to occur over Europe (4.6%, 4 times more than 1992), the United States of America (3.7%, 2.6 times more), and southeast Asia (1.2%, 10 times more). Contrail-induced increases in cirrus cloud cover may depend also on wind shear, vertical motions, and existing cirrus cover, which this thermodynamic analysis does not take into account. In addition, changes in climate conditions may influence future contrail formation conditions.

Radiative forcing from contrails was calculated for 2050 using the Fa1 fuel scenario and the same method as described in Section 3.6.3 (Minnis et al., 1999). For the contrail cover shown in Figure 3-23, values of SW, LW, and net forcing were found to be about 6 times larger than in 1992 (see Table 3-8). The increase in radiative forcing from 1992 to 2050 is larger than the increase in contrail cover (factor of 5) during the same period because additional contrails in the subtropics and over Asia over relatively warm and cloud-free surfaces are more effective in increasing radiative forcing. The global distribution of radiative forcing calculated with this procedure is shown in Figure 3-24 for an assumed optical depth of 0.3. Radiative forcing grows more strongly globally than in regions of present peak traffic. Global mean forcing is 0.1 W m^-2 in this computation, with maximum values of 3.0 and 1.4 W m^-2 (3.3 and 2.4 times more than 1992) over northeast France and the eastern United States of America, respectively.

For an optical depth of 0.3, the best-estimate value of the global radiative forcing in 2050 (scenario Fa1) is 0.10 W m^-2. The uncertainty range is a little larger than in 1992, and estimated to amount to a factor of 4. Hence, the likely range of forcing extends from 0.03 to 0.4 W m^-2 (see Table 3-9). The forcing for other scenarios has not been computed in detail, but rough estimates scale with the fuel consumption. The climatic consequences of this forcing are discussed in Chapter 6.

An estimate of the range of aviation-induced cirrus cloudiness in 2050, as required for this assessment, is not available in the scientific literature. For 1992, a range for the best estimate of the additional aviation-induced cirrus clouds was derived from decadal trends in high fuel-use regions (0-0.2% global cover; Section 3.5.1.5). For the 2050 time period, a different approach is required. Observed contrail frequencies and trends in cirrus occurrence have been found to correlate with aviation fuel consumption (see Figures 3-14 and 3-18). Therefore, the aviation-induced cirrus cloudiness between 1992 and 2050 is projected to grow in proportion to the total aviation fuel consumption in the upper troposphere. This fuel consumption grows by a factor of 4 between 1992 and 2050 in scenario Fa1. Hence, the best-estimate of additional global cirrus cover in 2050 would range from 0 to 0.8%. For the same radiative sensitivity as in 1992, the associated radiative forcing could be between 0 and 0.16 W m^-2 or up to 1.6 times the value given for line-shaped contrail cirrus in 2050 (see Table 3-9). The forcing could be outside this range if future aviation causes strong changes in the optical properties of the cirrus clouds. Saturation effects (Sausen et al., 1999) will likely limit any increase in cirrus cover in heavy air traffic regions. Because of these uncertainties the status of understanding of radiative forcing from additional aviation-induced cirrus clouds in 2050 is very poor. The assessment of the other indirect effects (Section 3.6.5) is beyond the scope of present understanding.

Modern subsonic aircraft cruise most efficiently at flight altitudes of 9 to 13 km. Trends in aircraft cruise altitudes are discussed in Chapter 7. If mean flight levels of global air traffic were to increase, the frequency of persistent contrails in the troposphere at mid-latitudes would be reduced and the frequency in the upper troposphere in the tropics would be increased. In addition, the formation of polar stratospheric clouds in the lower polar stratosphere (Peter et al., 1991) may be enhanced as a result of increased emissions in the stratosphere. At mid-latitudes, a 1-km flight-level increase causes a moderate reduction of contrail cover because of increased flights in the dry stratosphere (e.g., 12% less contrail cover over the North Atlantic compared with the nominal-altitude cover). Despite these changes, the global change in contrail cover from an altitude increase is small because of compensating changes in the tropics. The stronger increase of contrail cover in the tropics may cause a stronger positive radiative forcing because of the warmer surface in the tropics compared with mid-latitudes (see Table 3-7). A reduction in flight levels generally has the opposite effect (more contrails at high latitudes and fewer contrails in the tropics). Results for Europe and parts of the United States of America are different in that computed contrail coverage decreases slightly for both an increase and a decrease in mean flight levels because air traffic currently occurs in the cold and humid upper troposphere in those regions, and a shift toward the drier stratosphere or the warmer mid-troposphere reduces contrail coverage (Sausen et al., 1998). A change in mean altitude of contrails may change their radiative impact even for constant areal coverage. A contrail at higher altitude in the troposphere will likely contain less ice mass and produce less radiative forcing, therefore, despite lower ambient temperatures (see Table 3-7).

Trends in soot emissions would be important if soot influences ice particle formation (see Section 3.4) or the chemistry of ozone (see Chapter 2). A soot mass emission index of 0.5 to 1 g kg^-1 (and larger) is not uncommon for older aircraft engines. The soot mass emission index of the present aircraft fleet is estimated as 0.04 g kg^-1 (see Chapter 7). The soot emission index decreased with new engine technology until about 1980 but has showed no significant trend thereafter (Döpelheuer, 1997). The mass of soot emitted may decrease despite increases in fuel consumption. No data exist on trends for the number and surface area of soot aerosol emissions.

Atmospheric models and fuel consumption scenarios suggest that aircraft emissions contribute little to the tropospheric mass of sulfate and soot aerosol in today's 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 H2O, HNO3, 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 SO3 or the formation of H2SO4 in the engine and plume. The emission of SO3 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 SO3 and H2SO4, 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.

Options to reduce soot emissions require changes in the combustion process (Chapter 7). Soot may be activated by H2SO4 and possibly other exhaust species. If soot activation by H2SO4 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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