Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Case
Optical Depth
t at 0.55 µm
Net Forcing
(W m -2 )
Reference (models FL, M, and N) 0.52 37.1-37.2
Different aspherical particles (models FL, M, and N) 0.4 22-36
Parameters Range
Solar zenith angle 60°-21° 0.52 37-49
Ice water content (IWC) 7.2-42 mg m -3 0.2-1.0 19-51
Particle diameter 10-40 µm 0.85-0.21 41-20
Surface temperature 289-299 K 0.52 35-39
Cloud cover/optical depth of underlying clouds 0-1 / 0-23 0.52 37-40
Surface albedo 0.05-0.3 0.52 31-40
Relative humidity reference - 80% 0.52 37-31
Contrail vertical depth (for fixed ice water path) 200 m - 1 km 0.52 37.1-36.7
Lower contrail top (for fixed IWC) 11-10 km 0.52 37-31
Lower contrail top (for temperature-dependent IWC) 11-10 km 0.52-1.32 37-45
a) Details in Meerkötter et al. (1999). Reference case for mid-latitude summer (McClatchey et al., 1972) as in Table 3-6; spherical particles; solar zenith angle
of 60°; diurnal sunshine fraction of 50%; IWC of 21 mg m-3 ; volume mean particlediameter of 32 µm; surface temperature of 294 K; no low-level clouds;
relative humidity of reference profile (McClatcheyet al., 1972) varying from 77% at the surface to 11% at 12-km altitude; contrail depth 200 m; and contrail top
at 11-km altitude.
Radiative forcing is defined as the net radiative flux change at some level in the atmosphere calculated in response to a perturbation, such as a change in cloud cover.
The definition of radiative forcing used here is the "instantaneous" or "static" flux change at the top of the atmosphere (TOA) (IPCC, 1996) (see Chapter 6). A positive
net flux change represents an energy gain, hence a net heating of the Earth system.
3.6.1. Direct Radiative Impact of Aerosols
The direct radiative impact of aircraft-induced aerosol is much smaller than that of volcanic stratospheric or regional tropospheric aerosol and smaller than other
aircraft-induced radiative forcing values (see Chapter 6). Radiative forcing by aerosol depends on many parameters (Haywood and Ramaswamy, 1998). For aerosol in
the size range 0.1 to 1 µm, the increase in solar reflectance (albedo effect) exceeds the trapping of terrestrial radiation (greenhouse effect), causing a surface cooling
(Lacis et al., 1992, Minnis et al., 1993; Minnis et al., 1998c). Aircraft-induced aerosols could have an importance similar to that of natural stratospheric aerosol
variations if their optical depth in the solar range were of comparable magnitude to that of natural stratospheric aerosol. The optical depth of aerosol is proportional to
its column load. The maximum zonally averaged column load from soot emissions and aircraft-induced sulfuric acid is estimated (see Section 3.3) to be less than 1
and 6 ng cm-2 , respectively. Small sulfuric acid particles are nonabsorbent but scatter strongly in the shortwave spectrum, with mass scattering efficiencies of 7 (4-10)
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