Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
have been performed by the AER and CSIRO 2-D models, described in detail in Chapter
4. For these calculations, the models have used the approach described in Section
2.2.1.1; namely, they have set up a base case without aircraft NO x emissions and a
perturbed case with the aircraft source added. They also have both used the subsonic
fleet emission inventory described in Chapter 9. The results of the two studies are in close
agreement and yield the following conclusions:
Approximately 20% of subsonic emissions are released directly into the
stratosphere; this result is on the low end of empirically based estimates,
resulting in roughly a 1% increase in summertime NO x levels existing just
above the tropopause at 45°N latitude.
The aircraft-induced increase in lowermost stratospheric NO x causes an
increase in ozone. Peak ozone increases are approximately 0.5% at
northern mid-latitudes and at altitudes immediately above the tropopause.
The total column ozone change resulting from stratospheric aircraft
emissions is approximately 0.1%.
A very small amount of aircraft NO x is transported to altitudes above 22
km, where it acts to decrease ozone concentrations slightly.
2.2.1.3. Effects of Aircraft Aerosol Emissions
A number of previous model studies have examined the effects of HSCT sulfate aerosols
on stratospheric chemistry; several new studies are discussed in Chapter 4. A key
uncertainty in these calculations has been the degree to which emitted sulfur forms
submicron sulfate particles in the aircraft plume (see Section 3.2.3). As part of the Chapter
4 studies, the AER and UNIVAQ stratospheric models have calculated the response of
stratospheric ozone to the subsonic fleet (year 2015) for a range of assumed SO2-to particle conversion efficiencies. Details of those studies appear in Section 4.4.1.2. A rough
estimate of the effect of sulfur emissions from the 1992 fleet can be made by scaling the
2015 results to 1992 emission levels. This calculation results in an estimate for the column
ozone change at 45°N of approximately -0.1%.
Figure 2-4: Estimate of mean trend using all four measurement
systems (i.e., Umkehr, ozonesondes, SBUV, and SAGE I/II) at northern
mid-latitudes (heavy solid line). Combined uncertainties are also shown
as 1s (light solid lines) and 2s (dashed lines). Combined trends and
uncertainties are extended down to 10 km as shown by the light dotted
lines. The results below 15 km are a mixture of tropospheric and
stratospheric trends, and the exact numbers should be viewed with
caution. Combined trends have not been extended lower into the
troposphere because the small sample of sonde stations have an
additional unquantified uncertainty concerning their representativeness
of mean trends (WMO, 1998).
The potential role of subsonic aircraft-generated soot in stratospheric ozone depletion has been investigated by Bekki (1997) using a 2-D model that includes
heterogeneous reactions of HNO 3 , NO 2 , and ozone on soot. Using modeled aircraft soot aerosol distributions that were broadly consistent with observations, the
model yields increased losses of ozone (approximately -0.2% column change at 45°N for the 1992 fleet) relative to the same model without soot chemistry. Moreover,
inclusion of soot chemistry improves model agreement with the observed trend in LS ozone depletion. However, recent laboratory measurements indicating that soot is
a reaction consumable do not support a major assumption of the Bekki model that soot acts as a catalyst.
The degree to which soot consumption affects the Bekki calculations can be estimated as follows: If the
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