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6.4. Radiative Forcing from Aircraft-Induced Changes in Aerosols and Cloudiness
There are two mechanisms by which aerosols may exert radiative forcing: the
direct effect, whereby aerosol particles scatter and absorb solar and longwave
radiation; and the indirect effect, whereby aerosol particles act as cloud condensation
nuclei and modify the physical and radiative properties of clouds. Additionally
for aircraft, merely flying through certain meteorological environments can
result in formation of contrails (Section 3.4), which
affect both solar and longwave radiation budgets. The present-day direct radiative
forcing from aircraft emissions of sulfur compounds and black carbon aerosols
is investigated in Sections 6.4.1 and 6.4.2;
radiative forcing from the formation of contrails and the indirect effect of
aerosol emissions is investigated in Section 6.4.3.
Section 6.4.4 derives future RF considering our range
of scenarios for fuel use. The RF models have been described previously (Section
6.3.1). A summary of radiative forcing calculations and related uncertainties
is given in Section 6.4.5.
6.4.1. Direct Radiative Forcing from Sulfate Aerosols
Sulfate aerosol scatters a fraction of incident solar radiation back to space,
thereby leading to negative direct radiative forcing. The direct radiative forcing
of pure sulfate in the longwave spectrum is likely to be negligible as a result
of the size of aerosol particles and the corresponding wavelength dependence
of the specific extinction coefficient (e.g., Haywood and Shine, 1997; Haywood
et al., 1997a). Myhre et al. (1998) summarize 10 detailed studies of the sensitivity
of direct radiative forcing from all anthropogenic sources of sulfate. With
the exception of one study, sensitivities per unit column mass of anthropogenic
sulfate range from -125 to -214 W g-1 SO4.
We reexamined these results by inserting a pure ammonium sulfate in a layer
between 8 and 13 km in the GFDL R30 GCM and assuming an ambient relative humidity
of 45% using the method of Haywood and Ramaswamy (1998). A log-normal distribution
with a dry geometric mean radius of 0.05 µm and a standard deviation of 2.0
was adopted. The resulting global mean sensitivity was found to be approximately
-215 W g-1 SO4, which is adopted
throughout this report. Because the modeled pure sulfate particles scatter incident
radiation with no absorption, the RF is not sensitive to their location relative
to the tropopause. Thus, the RF is well approximated by the instantaneous radiative
forcing at the top of the atmosphere even for aircraft sulfate in the lower
stratosphere.
A study of the distribution of aircraft fuel burned and transported as a passive
tracer from scenario NASA-1992 involved a range of global models and is presented
in Chapter 3 (see also Danilin et al., 1998). The median
global mean column burden of sulfate aerosol is derived in this study by adopting
an emission index for sulfur EI(S) of 0.4 g kg-1 and
a 50% effective conversion factor from fuel-sulfur to optically active sulfate
aerosols; it is approximately 13.5 µg SO4 m-2
(Table 3-4). Thus, global mean radiative forcing
from aircraft emissions of sulfate aerosol in 1992 is estimated to be -0.003
W m-2, with a likely range of -0.001 to -0.009 W m-2
(see also Table 6-1). This value is much smaller
in absolute magnitude than the RF from CO2, O3,
CH4, or contrails. We assume that EI(S) remains constant
through 2050 and scale the sulfate RF with fuel use.
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