Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
(Crutzen, 1976), and condensation of H 2 SO 4 onto small particles nucleated primarily near
the equatorial tropopause (Brock et al., 1995; Hamill et al., 1997). Current global
photochemical models estimate that the natural source from OCS contributes 0.03 to 0.06
Tg S yr -1 into the stratosphere (Chin and Davis, 1995; Weisenstein et al., 1997). Additional
sources of stratospheric sulfur may be required to balance the background sulfur budget
(Chin and Davis, 1995), such as a strong convective transport of SO 2 precursors
(Weisenstein et al., 1997). Large increases in H 2 SO 4 mass in the stratosphere often occur
in periods following volcanic eruptions (Trepte et al., 1993). Increased H 2 SO 4 increases the
number and size of stratospheric aerosol particles (Wilson et al., 1993). The relaxation to
background values requires several years, as Figure 3-6 illustrates with aerosol extinction
measurements derived from satellite observations. The relative effect of aircraft emissions
will be reduced in periods of strong volcanic activity, particularly in the stratosphere,
because the aircraft source of aerosol becomes small compared with the volcanic source
(see Table 3-1).
The current subsonic fleet injects ~0.02 Tg S yr -1 into the stratosphere under the
assumption that one-third of aviation fuel is consumed in the stratosphere (Hoinka et al.,
1993; Berger et al., 1994) (see Table 3-2). This amount is 1.5 to 3 times less than natural
sources of stratospheric sulfur in nonvolcanic periods. The enhanced sulfate aerosol surface
area in the stratosphere affects ozone photochemistry through surface reactions that reduce
nitrogen oxides and release active chlorine species (Weisenstein et al., 1991, 1996; Bekki
and Pyle, 1993; Fahey et al., 1993; Borrmann et al., 1996; Solomon et al., 1997). The
chemical impact of sulfate aerosol in the stratosphere is discussed in Chapters 2 and 4.
3.3.2.2. Troposphere
Figure 3-6: Aerosol extinction at 1.02 µm from SAGE II satellite
observations at altitudes of 6.5 to 24.5 km.
Anthropogenic and natural sources of sulfur are much larger in the troposphere than in the stratosphere (Table 3-2). Anthropogenic emissions of sulfur exceed natural
sources by factors of 2 to 3 on a global scale, and emission in the Northern Hemisphere exceeds that in the Southern Hemisphere by a factor of 10 (Langner and
Rodhe, 1991). Accordingly, aerosol abundance is larger in the upper troposphere than in the stratosphere and larger in the Northern Hemisphere troposphere than in
the Southern Hemisphere counterpart (Hofmann, 1993; Benkovitz et al., 1996; Rosen et al., 1997; Thomason et al., 1997a,b) (Figure 3-6). Tropospheric aerosol
concentrations are much larger than lower stratospheric concentrations under nonvolcanic conditions. Condensation nucleus number densities exceeding 1,000 cm-3 are not uncommon in the troposphere (Schröder and Ström, 1997; Hofmann et al., 1998), whereas values in the lower stratosphere are less than 50 cm-3 in
nonvolcanic periods (Wilson et al., 1993).
The effect of aircraft sulfur emissions on aerosol in the upper troposphere and lower stratosphere is far larger than comparison of their amount with global sulfur
sources suggests. The major surface sources of tropospheric sulfate aerosol include SO 2 and dimethyl sulfide (DMS), both of which have tropospheric lifetimes of less
than 1 week (Langner and Rodhe, 1991; Weisenstein et al., 1997). There is large variability in upper tropospheric aerosol particle number and size (Hofmann, 1993;
Thomason et al., 1997b) because of variability in tropospheric meteorology and the short lifetime of sulfur source gases. Surface emissions are known to reach the
upper troposphere under certain conditions, such as during deep mid-latitude and tropical convection (Arnold et al., 1997; Prather and Jacob, 1997; Dibb et al., 1998;
http://www.ipcc.ch/ipccreports/sres/aviation/036.htm (2 von 9)08.05.2008 02:42:01