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Fig. 11.4
Dust refractive indices in the terrestrial spectrum from WMO (WCP
1986
)(
black
),
Fouquart et al. (
1987
)(
red
), OPAC Hess et al. (
1998
)(
green
)andVolz(
1973
)(
blue
)
number of smaller particles are present, which interact more with the incident solar
wavelengths. Changing the imaginary part of the refractive index does not affect the
extinction significantly.
The SSA is notably lower towards the smaller wavelengths in the UV (indicating
greater absorption), due to the higher imaginary refractive indices here. It is also
sensitive to the size that also affects the amount of absorption. The more absorbing
refractive index results in lower SSA values (solid lines). The size distribution
with a greater number concentration of larger particles also results in lower SSA
values (black lines). Therefore, the absorption properties of dust are sensitive to
both the dust composition, and hence dust source region (via the imaginary part
several factors including dust grain size at the source, uplift mechanism and strength
Values shown in Fig.
11.3
at 550 nm wavelength range from 0.73 to 0.96, but higher
values up to 0.99 have also been measured (Johnson and Osborne
2011
)(seealso
Sect.
11.2.4
).
Due to the large particle sizes present in mineral dust aerosol, dust also interacts
effectively with LW radiation. As with SW radiation, the characteristics of dust
optical properties in the IR are a function of the composition, the size distribution
and the particle shape. Composition can be characterised shown by the complex
refractive index, as in Fig.
11.4
. Here, refractive indices from a number of different
studies are shown, including Volz (
1973
) (Saharan dust transported to Barbados)
and Fouquart et al. (
1987
) (sand in Niger), based on chemical analysis of samples.
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