Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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3.3.5. Soot
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The primary atmospheric source of soot or black carbon particles is combustion of fossil fuels and biomass burning at the Earth's surface, with total emission values
near 12 Tg C yr -1 (Liousse et al., 1996). This value exceeds reasonable estimates of the aircraft source of black carbon by several orders of magnitude (Bekki, 1997).
For example, aircraft are estimated to have emitted 0.0015 to 0.015 Tg C as soot into the atmosphere in 1992 [with EI(soot) of 0.01 to 0.1 g C/kg fuel] (Friedl, 1997;
Rahmes et al., 1998). As in the case of sulfate aerosol, deposition and scavenging of black carbon near surface sources creates large vertical gradients in the lower
atmosphere, with soot concentrations falling by 1 to 2 orders of magnitude between the surface and the lower stratosphere (Penner et al., 1992; Cooke and Wilson,
1996; Liousse et al., 1996). A possible meteoritic source of soot in the lower stratosphere has been considered but is not well quantified at present (Chuan and Woods,
1984).
Few direct measurements of soot abundance are available in the upper troposphere and
lower stratosphere. The most extensive measurements in these regions are from aircraft
impactor measurements (Pueschel et al., 1992, 1997; Blake and Kato, 1995). The accuracy
of such measurements depends on knowledge of the impactor for small soot particles. The
results (Figure 3-10) are considered to represent a lower limit for soot number and mass
because of size-selective sampling and because of scavenging of soot by background
aerosol particles. Features of the measurements include a large gradient between the
Northern and Southern Hemispheres and large variability with altitude at northern midlatitudes.
The large vertical variability of soot at northern mid-latitudes cannot be explained
by the accumulation of aircraft soot emissions. With typical soot mass densities observed to
be approximately 1 ng m -3 , soot is estimated to represent approximately 0.01% of the
stratospheric aerosol mass (Pueschel et al., 1992). In other sampling flights over southern
Germany, measurements of absorbing mass (probably soot) at 8-12 km altitude and partly
within cirrus clouds showed concentrations above 10 ng m -3 , with higher values correlated
with local aviation fuel consumption (Ström and Ohlsson, 1998).
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