Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
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.
Figure 3-10: Latitude and altitude distribution of measured values of
soot concentrations (BCA = black carbon aerosol) in upper
troposphere and lower stratosphere (in ng m-3).
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
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