Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Talbot et al., 1998). Only a small fraction of the surface sulfur emissions reaches the upper troposphere, however, because of the large removal rates of sulfur species
near the surface.
Table 3-3: Non-volcanic upper tropospheric annual mean optical depth and % change per year, along with standard deviation, from SAGE satellite
observations during 1979-97. Values in parentheses are for the Southern Hemisphere (adapted from Kent et al., 1998).
Latitude Band Annual Mean Optical Depth (10 -4 ) Change per Year (%)
80-60° N(S) 25.1 ± 4.7 (2.9 ± 0.9) -0.4 ± 0.2 (-0.7 ± 0.6)
60-40° N(S) 18.1 ± 4.7 (7.0 ± 1.0) 0.4 ± 0.2 (1.4 ± 0.3)
40-20° N(S) 19.3 ± 2.9 (15.2 ± 2.5) 0.2 ± 0.1 (1.2 ± 0.2)
20-0° N(S) 19.6 ± 0.9 (18.2 ± 1.6) 0.2 ± 0.1 (0.8 ± 0.1)
Hemisphere N(S) 0.1 ± 0.1 (0.9 ± 0.3)
Globe 0.5 ± 0.2
3.3.2.3. Differences between the Upper Troposphere and Lower Stratosphere
The effects of aircraft emissions on aerosol particles and aerosol precursors depend on the amounts emitted into the troposphere and stratosphere. In addition to
aerosol abundance and sources, stratospheric and tropospheric aerosols also differ in composition and residence time. Typical parameters for sulfate at 12 and 20 km
at northern mid-latitudes are summarized in Table 3-1.
Sulfate is considered to be the dominant component of stratospheric aerosol; soot and metals are considered to be minor components (Pueschel, 1996). The
composition of tropospheric aerosol, particularly near the surface, also includes ammonium, minerals, dust, sea salt, and organic particles (Warneck, 1988). The role of
minor components of aerosol composition in affecting heterogeneous reaction rates is not fully understood. In general, the lower stratosphere contains highly
concentrated H 2 SO 4 /H 2 O particles (65-80% H 2 SO 4 mass fraction) as a result of low relative humidity in the stratosphere and low temperatures (Steele and Hamill,
1981; Carslaw et al., 1997). Higher H 2 O abundances (by a factor of 10 or more) and similar temperatures cause particles in the upper troposphere to be more dilute
(40-60% H 2 SO 4 ). Surface reactions that activate chlorine are particularly effective on dilute H 2 SO 4 particles and cirrus cloud particles at low temperatures in the
tropopause region (Chapter 2).
Aircraft emissions injected into the stratosphere have greater potential to perturb the aerosol layer than those emitted into the troposphere, because in the stratosphere
the background concentrations are lower and the residence times are longer. The initial residence time (1/e-folding time) of most of the stratospheric sulfate aerosol
mass from volcanic eruptions is about 1 year as a result of aerosol sedimentation rates (Hofmann and Solomon, 1989; Thomason et al., 1997b; Barnes and Hofmann,
1997) (see also Figure 3-6). The residence time of the remaining aerosol mass contained in smaller particles is several years. The residence time of upper
tropospheric aerosol particles is much smaller, ranging from several days (Charlson et al., 1992) to between 10 and 15 days (Balkanski et al., 1993; Schwartz, 1996).
Tropospheric particles are larger than those in the stratosphere (Hofmann, 1990), therefore sediment faster. They are also removed by cloud scavenging and rainout.
http://www.ipcc.ch/ipccreports/sres/aviation/036.htm (3 von 9)08.05.2008 02:42:01