3.3. Regional and Global-Scale Impact of Aviation on Aerosols

3.3.1. Global Aircraft Emissions and Aerosol Sources

Aircraft emissions may cause changes in the background distribution of soot and sulfuric acid aerosols at regional and global scales. In this section, observations and model results are used to evaluate these potential aircraft-induced changes.

Soot and sulfur mass emissions from aircraft are small compared with other global emissions from anthropogenic and natural sources (Table 3-2). However, aircraft emissions occur in the upper troposphere and lower stratosphere, where background values are lower and removal processes are much less effective than near the Earth's surface. Moreover, aircraft aerosol particles tend to be smaller than background particles, so small emission masses may still cause large changes in aerosol number and surface area densities. In addition, aerosol particles from aircraft can participate in the formation of contrails and clouds in the upper troposphere, hence potentially alter the radiative balance of the atmosphere (Section 3.6).

About 93% of all aviation fuel is consumed in the Northern Hemisphere and 7% in the Southern Hemisphere (Baughcum et al., 1996; see Chapter 9). Within the Northern Hemisphere, 76% of aviation fuel is consumed at mid- and polar latitudes (> 30°N). The geographical and altitude distribution of current aviation fuel consumption implies that the largest changes in aerosol and gas composition from aviation will be at northern mid-latitudes at altitudes of 10 to 12 km.

3.3.2. Sulfate Aerosol

3.3.2.1. Stratosphere

The background stratospheric sulfate layer is believed to be formed largely via the transport of carbonyl sulfide (OCS) into the stratosphere, its subsequent conversion to H2SO4 (Crutzen, 1976), and condensation of H2SO4 onto small particles nucleated primarily near the equatorial tropopause (Brock et al., 1995; Hamill et al., 1997). Current global photochemical models estimate that the natural source from OCS contributes 0.03 to 0.06 Tg S yr^-1 into the stratosphere (Chin and Davis, 1995; Weisenstein et al., 1997). Additional sources of stratospheric sulfur may be required to balance the background sulfur budget (Chin and Davis, 1995), such as a strong convective transport of SO2 precursors (Weisenstein et al., 1997). Large increases in H2SO4 mass in the stratosphere often occur in periods following volcanic eruptions (Trepte et al., 1993). Increased H2SO4 increases the number and size of stratospheric aerosol particles (Wilson et al., 1993). The relaxation to background values requires several years, as Figure 3-6 illustrates with aerosol extinction measurements derived from satellite observations. The relative effect of aircraft emissions will be reduced in periods of strong volcanic activity, particularly in the stratosphere, because the aircraft source of aerosol becomes small compared with the volcanic source (see Table 3-1).

The current subsonic fleet injects ~0.02 Tg S yr^-1 into the stratosphere under the assumption that one-third of aviation fuel is consumed in the stratosphere (Hoinka et al., 1993; Berger et al., 1994) (see Table 3-2). This amount is 1.5 to 3 times less than natural sources of stratospheric sulfur in nonvolcanic periods. The enhanced sulfate aerosol surface area in the stratosphere affects ozone photochemistry through surface reactions that reduce nitrogen oxides and release active chlorine species (Weisenstein et al., 1991, 1996; Bekki and Pyle, 1993; Fahey et al., 1993; Borrmann et al., 1996; Solomon et al., 1997). The chemical impact of sulfate aerosol in the stratosphere is discussed in Chapters 2 and 4.

3.3.2.2. Troposphere

Anthropogenic and natural sources of sulfur are much larger in the troposphere than in the stratosphere (Table 3-2). Anthropogenic emissions of sulfur exceed natural sources by factors of 2 to 3 on a global scale, and emission in the Northern Hemisphere exceeds that in the Southern Hemisphere by a factor of 10 (Langner and Rodhe, 1991). Accordingly, aerosol abundance is larger in the upper troposphere than in the stratosphere and larger in the Northern Hemisphere troposphere than in the Southern Hemisphere counterpart (Hofmann, 1993; Benkovitz et al., 1996; Rosen et al., 1997; Thomason et al., 1997a,b) (Figure 3-6). Tropospheric aerosol concentrations are much larger than lower stratospheric concentrations under nonvolcanic conditions. Condensation nucleus number densities exceeding 1,000 cm^-3 are not uncommon in the troposphere (Schröder and Ström, 1997; Hofmann et al., 1998), whereas values in the lower stratosphere are less than 50 cm^-3 in nonvolcanic periods (Wilson et al., 1993).

The effect of aircraft sulfur emissions on aerosol in the upper troposphere and lower stratosphere is far larger than comparison of their amount with global sulfur sources suggests. The major surface sources of tropospheric sulfate aerosol include SO2 and dimethyl sulfide (DMS), both of which have tropospheric lifetimes of less than 1 week (Langner and Rodhe, 1991; Weisenstein et al., 1997). There is large variability in upper tropospheric aerosol particle number and size (Hofmann, 1993; Thomason et al., 1997b) because of variability in tropospheric meteorology and the short lifetime of sulfur source gases. Surface emissions are known to reach the upper troposphere under certain conditions, such as during deep mid-latitude and tropical convection (Arnold et al., 1997; Prather and Jacob, 1997; Dibb et al., 1998; Talbot et al., 1998). Only a small fraction of the surface sulfur emissions reaches the upper troposphere, however, because of the large removal rates of sulfur species near the surface.

3.3.2.3. Differences between the Upper Troposphere and Lower Stratosphere

The effects of aircraft emissions on aerosol particles and aerosol precursors depend on the amounts emitted into the troposphere and stratosphere. In addition to aerosol abundance and sources, stratospheric and tropospheric aerosols also differ in composition and residence time. Typical parameters for sulfate at 12 and 20 km at northern mid-latitudes are summarized in Table 3-1.

Sulfate is considered to be the dominant component of stratospheric aerosol; soot and metals are considered to be minor components (Pueschel, 1996). The composition of tropospheric aerosol, particularly near the surface, also includes ammonium, minerals, dust, sea salt, and organic particles (Warneck, 1988). The role of minor components of aerosol composition in affecting heterogeneous reaction rates is not fully understood. In general, the lower stratosphere contains highly concentrated H2SO4/H2O particles (65-80% H2SO4 mass fraction) as a result of low relative humidity in the stratosphere and low temperatures (Steele and Hamill, 1981; Carslaw et al., 1997). Higher H2O abundances (by a factor of 10 or more) and similar temperatures cause particles in the upper troposphere to be more dilute (40-60% H2SO4). Surface reactions that activate chlorine are particularly effective on dilute H2SO4 particles and cirrus cloud particles at low temperatures in the tropopause region (Chapter 2).

Aircraft emissions injected into the stratosphere have greater potential to perturb the aerosol layer than those emitted into the troposphere, because in the stratosphere the background concentrations are lower and the residence times are longer. The initial residence time (1/e-folding time) of most of the stratospheric sulfate aerosol mass from volcanic eruptions is about 1 year as a result of aerosol sedimentation rates (Hofmann and Solomon, 1989; Thomason et al., 1997b; Barnes and Hofmann, 1997) (see also Figure 3-6). The residence time of the remaining aerosol mass contained in smaller particles is several years. The residence time of upper tropospheric aerosol particles is much smaller, ranging from several days (Charlson et al., 1992) to between 10 and 15 days (Balkanski et al., 1993; Schwartz, 1996). Tropospheric particles are larger than those in the stratosphere (Hofmann, 1990), therefore sediment faster. They are also removed by cloud scavenging and rainout.

3.3.3. Observations of Aircraft-Produced Aerosol and Sulfate Aerosol Changes

Observations of aircraft-induced aerosols have increased substantially in recent years (see Section 3.2). Concentrations of aerosol particles and aerosol precursor gases well above background values have been observed in the exhaust plumes of aircraft operating in the upper troposphere and lower stratosphere. Although aircraft emissions are quickly diluted by mixing with ambient air to near background values, the accumulation of emissions in flight corridors used in the routing of commercial air traffic has the potential to cause notable atmospheric changes.

Estimated changes from aircraft emissions (Schumann, 1994; WMO, 1995) are small compared with natural variability, hence are not always apparent in observational data sets. However, regional enhancements in concentrations of aircraft-produced aerosol have been observed near air traffic corridors. During measurement flights across the North Atlantic flight corridor over the eastern Atlantic, signatures of NOx, SO2, and condensation nuclei (CN) were clearly evident in the exhaust plumes of 22 aircraft that passed the corridor at this altitude in the preceding 3 h, with values exceeding background ambient levels by 30, 5, and 3 times, respectively (Schlager et al., 1997). A mean CN/NOx abundance ratio of 300 cm^-3 ppbv-1 was measured. This ratio corresponds to a mean particle emission index of about 1016 kg^-1 and implies CN increases of 30 cm^-3 in corridor regions where aircraft increase NOx by 0.1 ppbv (cf. Chapter 2). The regional perturbation was found to be detectable at scales of more than 1,000 km under special meteorological conditions within a long-lasting stagnant anticyclone (Schlager et al., 1996). In an analysis of 25 years of balloon measurements in Wyoming in the western United States of America, subsonic aircraft are estimated to contribute about 5-13% of the CN concentration at 8-13 km, depending on the season (Hofmann et al., 1998). This estimate provides only a lower bound of the aircraft contribution because smaller aircraft-produced particles (radius < 10 nm) are not detected. Additionally, regular lidar measurements have been made of aerosol optical depth at aircraft altitudes (10-13 km) in an area of heavy air traffic in southern Germany (Jäger et al., 1998). Large optical depths on the order of 0.1 that could be attributed to the accumulation of aircraft aerosol were observed very rarely at this location.

Global changes in sulfate aerosol properties at subsonic air traffic altitudes were small over the last few decades. The examination of long-term changes in aerosol parameters suggests that aircraft operations up to the present time have not substantially changed the background aerosol mass. Multiyear observations for the upper troposphere and lower stratosphere are available from satellite and balloon platforms and ground-based lidar systems. Long-term variations of the optical depth in the upper troposphere from the SAGE satellite (Figure 3-6 and Table 3-3) have been analyzed with periods of volcanic influence excluded (Kent et al., 1998). Data indicate that changes are less than about 1% yr^-1 between 1979 and 1998, when observations are averaged over either hemisphere. A significant change in aerosol amounts is also not found in the 15- to 30-km region examined with lidar over Mauna Loa (20°N) for the period 1979 to 1996 (Barnes and Hofmann, 1997). Similarly, aerosol mass data above the tropopause derived from lidar soundings show no trend in a region of heavy air traffic in Germany (48°N) over the last 22 years (Figure 3-7) (Jäger and Hofmann, 1991; Jäger et al., 1998).

Long-term changes in aerosol parameters measured in situ are also small. In situ measurements are important because the number of particles in the upper troposphere and lowermost stratosphere is dominated by sizes that are too small (< 0.15-mm radius) to be remotely detected. Long-term changes in CN are small in the 10- to 12-km region of the mid-latitude troposphere, where most of the current aircraft fleet operates (Hofmann, 1993). The 5% yr^-1 increase in larger particle (radius > 0.15 mm) abundances found in lower stratospheric balloon measurements made between 1979 and 1990 was considered consistent with the accumulation of aircraft sulfur emissions (Hofmann, 1990, 1991). However, the absence of a change in observed stratospheric CN number suggests that the trend in the larger particles is the result of the growth of existing particles rather than nucleation of new particles. Model results show that the contribution of the current subsonic fleet to aerosol mass amounts between 15 and 20 km is about 100 times smaller than the observed aerosol amounts (Section 3.3.4; Bekki and Pyle, 1992). Previous balloon-borne CN counters did not measure particles below 10 nm in radius, which are now detected with more modern CN counters and dominate aerosol number in aircraft plumes. Moreover, the attribution of aerosol changes to aircraft is complicated by changes in surface sources of sulfur and episodic strong injections of sulfur from volcanic eruptions (Hitchman et al., 1994; Barnes and Hofmann, 1997; Thomason et al., 1997a,b). Hence, the contribution of aircraft emissions to changes or possible trends in these regions is difficult to determine at present.

3.3.4. Modeling Sulfate Aerosol Perturbations Caused by Aircraft

3.3.4.1. Subsonic Aircraft

Global models are required to evaluate the atmospheric impact of aerosol generated by subsonic aircraft (Friedl, 1997; Brasseur et al., 1998). The global distribution of tropospheric sulfur species has been investigated using various three-dimensional (3-D) models (Langner and Rodhe, 1991; Penner et al., 1994; Chin et al., 1996; Feichter et al., 1996; Pham et al., 1996; Schwartz, 1996). Because most models have been developed to investigate regional or global effects of surface emissions in the lower or middle troposphere, few have addressed the potential impact of aviation sources on aerosol parameters in the upper troposphere and lower stratosphere.

A systematic model study has been carried out with a suite of two-dimensional (2-D) and 3-D atmospheric models to determine upper bounds for the accumulation of aviation aerosol in the atmosphere (Danilin et al., 1998). Each model computed the steady-state global distribution of a passive tracer emitted into the model atmosphere with the same rate and distribution as aviation fuel use, based on the NASA 1992 database (see Chapter 9). The only sink for the passive tracer is below 400 hPa (approximately 7 km), where it is removed with a 1/e-folding time of 5 days. The resultant global tracer distribution can be used to provide estimates of steady-state concentration change from a specific emission by multiplying the tracer value by the associated aircraft engine emission index (EI). Figure 3-8 shows steady-state tracer distributions in tracer-to-air mass mixing ratio units and annually and zonally averaged fuel source used in the simulation. Table 3-4 summarizes the main results of these simulations.

All models predict the largest perturbation at mid-latitudes in the Northern Hemisphere in the altitude range of 10-12 km. However, the magnitude of the perturbation varies by a factor of 10, ranging from 12.6 (ECHAm³) to 122 ng g^-1 (GSFC-2D), reflecting differences in model resolution and current uncertainties in modeling of global atmospheric dynamics and turbulent diffusion. To mitigate the effects of model resolution, the tracer amount was summed in the 8- to 16-km altitude region between 30 and 90°N (shown by the thick dashed line in Figure 3-8). This region contains 34 (AER, ECHAm³) to 61% (GSFC-2D) of the total accumulated tracer. The absolute amount of tracer mass in this volume ranges from 2.9 (ECHAm³) to 14.5 Tg (GSFC-2D). The amount of the tracer above 12 km, which serves to diagnose the fraction of aircraft emissions transported toward the stratospheric ozone maximum, ranges from 14 (UCI/GISS, GSFC-2D) to 45% (UMICH, AER) of each model's global tracer amount. The global residence time of the fuel tracer, defined as the ratio of the steady-state tracer mass to the tracer source, varies from 21 days (Tm³, ECHAm³) to 65 days (GSFC-2D, LLNL). The lower values are similar to the global residence times (approximately 18 days) found for air parcels uniformly released at 11 km between 20 and 60°N and followed with a trajectory model using assimilated wind fields (Schoeberl et al., 1998). The 1/e-folding aircraft emissions lifetime of 50 days computed by Gettelman (1998) is consistent with the results of the fuel tracer experiment described here (Danilin et al., 1998).

The model simulation results indicate that aircraft contribute little to the sulfate mass near the tropopause. For example, the sulfate aerosol mass density from the GSFC-2D model (see Figure 3-8) is 0.055 mg m^-3 at 10 km at 55°N, in contrast to background concentrations of 1 to 2 mg m^-3 (Yue et al., 1994). Other recent model studies (Danilin et al., 1997; Kjellström et al., 1998) show similar results for aerosol mass; these studies further conclude that aircraft emissions may noticeably enhance the background number and surface area densities (SAD) of sulfate aerosol (see Tables 3-1 and 3-4) because of the smaller radii of aircraft-produced particles.

Fuel tracer simulation also provides estimates of soot and sulfate column amounts that can be used to calculate the direct radiative forcing of current subsonic fleet emissions (see Chapter 6). Maximum tracer column values are located near 50 to 60°N and range from 4.1 (ECHAm³) to 22.9 mg cm^-2 (GSFC-2D). To calculate instantaneous direct radiative forcing at the top of the atmosphere from aircraft soot emissions, globally averaged tracer column values (which are smaller than their maximum values by a factor of 3 to 4) are first multiplied by EI(soot) to obtain soot column values. For aircraft sulfur emissions, the tracer column is scaled by EI(S), the ratio of molar mass of SO4 and S, and a 100% conversion fraction of sulfur to sulfate (see Table 3-4). For the suite of models in Table 3-4, the upper bound for the average soot column is 0.1 ng cm^-2, with a range from 0.07 to 0.20 ng cm^-2; for the average sulfate column the upper bound is 2.9 ng cm^-2, with a range from 2 to 6 ng cm^-2. If photochemical oxidation lifetime and tropospheric washout rate are taken into account, a 50% conversion fraction of sulfur to sulfate is a more suitable value than 100%. In this case, the average sulfate column is 1.4 ng cm^-2, with a range of 1 to 3 ng cm^-2.

In addition to the passive tracer simulation, the AER 2-D model also calculated the evolution of aerosol using a sulfur photochemistry and aerosol microphysics model designed for stratospheric conditions (Weisenstein et al., 1997). This model calculates about 3.4 ng cm^-2 for the perturbation in sulfate aerosol at 55°N, consistent with the AER model value in Table 3-4 but almost 30 times smaller than the background sulfate column amounts (~100 ng cm^-2). Figure 3-9 depicts the annually averaged increase of sulfate aerosol SAD calculated with the AER 2-D model, assuming 5% conversion of sulfur emissions into new particles (as recommended in Section 3.2) with a radius of 5 nm and fuel with 0.4 g S/kg. The maximum SAD perturbation, located at about 10-12 km in northern mid-latitudes, is about 0.3 mm² cm^-3, which is comparable to ambient values in nonvolcanic periods (Hofmann and Solomon, 1989; Thomason et al., 1997b).

Table 3-1 presents upper-bound estimates of soot and sulfate aerosol number and surface area densities from 1992 fuel simulations. Present aircraft emissions noticeably increase the number and surface area densities of aerosol particles in the tropopause region despite the large CN background concentration in the upper troposphere. The estimates use the range of values of computed tracer concentrations from all models and the effective EIs of soot and sulfate mass and assume a mean particle size of 10(20) nm for sulfate (soot) particles and a 5% conversion of sulfur to sulfate aerosol. The results represent order-of-magnitude estimates of zonal mean maximum values at 12 km and can be compared with background aerosol properties in the lowermost stratosphere as given in Table 3-1. For these estimates, 5% of the emitted sulfur dioxide is assumed to be converted to sulfuric acid before dispersal out of the zonal region of maximum air traffic.

Tracer simulations strongly suggest that aircraft emissions are not the source of observed decadal H2O changes at 40°N. The simulation results can be scaled by EI(H2O) to provide an upper bound (neglecting precipitation from the upper troposphere) for the accumulation of water vapor above the tropopause as a result of aircraft emissions. The LLNL model shows the largest tracer accumulation at 40°N, with equivalent H2O values smoothly decreasing from 55 ppbv at 10 km to 12 ppbv at 24 km. These values are small in comparison to current ambient values of 59 ppmv at 10-12 km and 4.2 ppmv at 22-24 km (Oltmans and Hofmann, 1995). Assuming 5% yr^-1 growth in fuel consumption and EI(H2O) of 1.23 kg/kg, the change in aircraft-produced H2O ranges from 3.4 ppbv yr^-1 at 10 km to 0.8 ppbv yr^-1 at 24 km. These values represent a change of +0.006% yr^-1 at 10 km and +0.018% yr^-1 at 24 km and are more than of 20 times smaller than those found in long-term balloon observations (Oltmans and Hofmann, 1995).

3.3.4.2. Supersonic Aircraft

The impact of a future fleet of supersonic aircraft on sulfate aerosol abundance in the stratosphere has been discussed using measurements and model results (Bekki and Pyle, 1993; Fahey et al., 1995a; Stolarski et al., 1995; Weisenstein et al., 1996, 1998). The results suggest that aerosol surface area density will be substantially greater than nonvolcanic background values in proposed fleet scenarios (Section 3.7 and Chapter 4). The consequences for stratospheric ozone changes depend on the simultaneous emissions of nitrogen oxides and chlorine and aerosol loadings of the atmosphere (Solomon et al., 1997; Weisenstein et al., 1998; see also Chapter 2).

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