Radiative Effects of Dust - Mineral Dust: A Key Player in the Earth System - page 268

Fig. 11.1 Two contrasting
dust volume size
distributions. Data in red are
from AERONET retrievals at
Cape Verde (Dubovik et al.
2002 ) representing
transported dust, whilst data
in black are from aircraft
measurements over the
Sahara (Ryder et al. 2013 ),
representing dust closer to
sources
sources, with a larger coarse mode present, with a peak volume distribution at 20-
30 m diameter. It is possible that the different measurement platforms contribute to
the different size distributions (Müller et al. 2010 ), but equally likely that the coarse
mode is depleted through deposition during transport. Either way, these different
size distributions will be used here to illustrate the impact of the coarse mode on
dust optical properties.
Figure 11.2 shows typical spectral refractive indices in the SW spectrum. The
black line shows data from Otto et al. ( 2007 ), which is a smoothed average of values
from the literature. At 550 nm, this produces an imaginary value of 0.006, which
has been shown to be high compared to recent work (e.g. Kandler et al. ( 2009 );
McConnell et al. ( 2010 ); Balkanski et al. ( 2007 )), although variations in the index
are expected due to variations in the mineral content of the dust source region
(Sokolik and Toon 1999 ). Therefore, a less absorbing adjusted refractive index is
also shown (red line). The higher imaginary values in the ultraviolet are typically
caused by the presence of hematite (see Chap. 2 ) .
The different combinations of refractive index and size distribution can be
applied in a Mie scattering code to generate optical properties for dust, and these
are shown in Fig. 11.3 . The spectral dependence of the mass-specific extinction
coefficient is mainly affected by the size distribution, with the spectral signature
of extinction being fairly flat for both cases. This is because of the presence of
particles across much of the size range. However, extinction decreases substantially
at even longer wavelengths than shown here for the transported case (but not in the
desert case), because fewer coarse particles are present resulting in lower MECs
at longer wavelengths. MECs are higher for the transported case because a larger