Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The effect of sulfur emission on SAD depends on how much SO 2 is converted to small particles in the aircraft plume. Conversion of SO 2 to aerosol particles within the
aircraft plume was found to perturb stratospheric SAD to a much greater extent than equivalent SO 2 emissions (Weisenstein et al., 1996, 1998). Atmospheric
measurements in the plume of the Concorde (Fahey et al., 1995) and other aircraft, along with modeling studies (Danilin et al., 1997; Kärcher and Fahey, 1997; Yu and
Turco, 1998), suggest a minimum 10% conversion, with much larger conversion rates possible. However, near-field particle observations (as discussed in Chapters 3
and 7) suggest a typical conversion of sulfur to sulfuric acid (H 2 SO 4 ) of less than 10% with a large, yet not fully understood variability under the assumption that the
aerosol composition is H 2 SO 4 /H 2 O. Model sensitivity studies were designed to examine the full range of this uncertainty with conversion fractions of 0, 10, 50, and
100%.
In most scenarios, the reference atmosphere is consistent with a SAD using SA0. If the HSCT scenario assumes no sulfur emissions, SA0 is used for the HSCT case
as well. In some HSCT calculations, the effect of increased SAD from SO 2 emissions by aircraft is also considered, using a range of different sulfate conversion
fractions (Tables 4-9 through 4-12). The additional SAD fields were constructed by calculating sulfate surface area for background and HSCT conditions using the AER
microphysical model coupled to their 2-D CTM (Weisenstein et al., 1997). The SAD perturbation (HSCT-background) was derived in absolute units and was added to
the SA0 background SAD distribution. The SA5 case assumes a 10% conversion rate for 500 HSCTs with an EI(SO 2 ) of 0.4. This case also roughly approximates a
5% conversion rate, for an EI(SO 2 )=0.8, with the sulfur content of the fuel maintaining its current value. Figure 3-25 shows the annually zonally averaged perturbation
of SAD (in µm 2 cm -3 ) used in the SA5 scenario. SA6 is similar to SA5 but is based on 1,000 HSCTs. A 0% conversion with EI(SO 2 )=0.4 is considered in SA7 for an
aircraft fleet of 500 planes. The assumption of 50% conversion with EI(SO 2 )=0.4 is considered here in SA1 and SA2 for aircraft fleets of 500 and 1,000 planes,
respectively. The assumption of 100% conversion with EI(SO 2 )=0.4 is considered in SA3 and SA4 as an upper limit. SA3 assumes 500 aircraft, whereas SA4 assumes
1,000 aircraft. SAD was also computed for fleets flying 2 km lower. A summary of all SAD distributions used in these scenarios is given in Table 4-9.
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