Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Modeling studies of soot distribution in the upper troposphere and lower stratosphere differ
on the importance of aircraft sources of soot. Some model simulations suggest that groundlevel
sources of soot (12 Tg C yr-1 ) are as important as aircraft sources in the upper
troposphere and predict maximum soot values there [2 to 5 ng m-3 in Liousse et al. (1996)
and 10 to 50 ng m-3 in Cooke and Wilson (1996)] that exceed observed values near 200
hPa at northern mid-latitudes (Pueschel et al., 1997). Other model results show that current
aircraft could be a noticeable source of the soot near the tropopause at northern midlatitudes
(Bekki, 1997; Danilin et al., 1998; Rahmes et al., 1998). Tracer simulation results
from the AER 2-D model (Section 3.5.1) were multiplied by EI(soot) of 0.04 g/kg fuel
(Döpelheuer, 1997) to estimate the global distribution of soot. The results (Figure 3-11)
show maximum values of 0.6 ng m-3 near 12 km at northern mid-latitudes. These values are
in the middle of the range of other model results and are in the range of observed values
(Figure 3-10). However, because the fleet-mean EI(soot) is uncertain and may range from
0.01 to 0.1 g/kg fuel, the effective range of the maximum in Figure 3-11 is 0.15 to 1.5 ng m- 3 . At 20 km, fuel tracer simulations show aircraft-induced zonal mean soot perturbations to
be approximately 100 times smaller than maximum observed values (Figure 3-10).
Observations of soot in the upper troposphere and lower stratosphere are too limited to
provide an estimate of any long-term changes in soot concentrations in those regions. The
possible consequences of heterogeneous reactions on soot (Bekki, 1997; Lary et al., 1997)
are discussed in Chapter 2.
3.3.6. Polar Stratospheric Clouds and Aircraft Emissions
During winter in the polar regions, low temperatures lead to the formation of polar
stratospheric cloud (PSC) particles, which contain H 2 SO 4 , HNO 3 , and H 2 O (e.g., WMO,
Figure 3-11: Latitude and altitude distribution of annually averaged
increase in soot mass density calculated with the AER 2-D model,
assuming a 1992 aircraft fuel-use scenario and a soot emission index
of 0.04 g C/kg fuel. Contour values are in ng m-3 (adapted from
Weisenstein et al., 1997).
1995; Carslaw et al., 1997; Peter, 1997). PSCs activate chlorine, leading to significant
seasonal ozone losses in the lower stratosphere, particularly in the Southern Hemisphere
(WMO, 1995). PSC formation may be enhanced by the atmospheric accumulation of aircraft
emissions of NO x , H 2 O, and sulfate, as well as through direct formation in aircraft plumes in polar regions (Section 3.2 and Chapter 4). If aircraft emissions change the
frequency, abundance, or composition of PSCs, the associated ozone loss may also be modified (Peter et al., 1991; Arnold et al., 1992; Considine et al., 1994; Tie et
al., 1996; Del Negro et al., 1997). The effects of subsonic aircraft emissions on PSCs and stratospheric ozone are expected to be smaller than those of similar
emissions from supersonic aircraft because subsonic emissions occur in the 10- to 12-km region, whereas supersonic emissions will most likely occur in the 15- to 20km
region. Ambient temperatures in the 10- to 12-km region are usually too high (> 200 K) for PSCs to form with available H 2 O and HNO 3 , and ozone and total
inorganic chlorine concentrations are much lower than near 20 km.
The impact of the subsonic fleet on PSC formation has not been well studied. The results of the fuel tracer simulation discussed in Section 3.3.4 can be used to
estimate the increase of PSC SAD as a result of aircraft emissions of H2O and NOx . Assuming an EI(H2O) of 1,230 g/kg, EI(NOx ) of 15 g/kg, complete conversion of
NOx to HNO3 , and formation of NAT particles at threshold temperatures, AER model results for a 1992 subsonic fleet show an additional condensation of HNO3 on
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