On Composition, Morphology, and Size Distribution of Airborne Mineral Dust - Mineral Dust: A Key Player in the Earth System - page 34

ice cores) to specific source areas and is therefore helpful for a better understanding
of paleoclimatic conditions. Meanwhile, several studies have shown that some
“source tracers” perform much better than others (see compilation by Scheuvens
et al. 2013 ). For mineral dust samples that have originated in northern Africa and
eastern Asia, a combination of different analytical techniques is probably the most
promising approach to assign even far-travelled mineral dusts to a specific source
area. Especially the different isotopes of Nd and Sr, the abundance of carbonates
(calcite Mg)/Fe

are valuable tools for mineral dust source apportionment. However, it has to be
claimed that at present some partial data sets are at best rudimentary and should
be improved in the future. A compilation of the compositional data also reveals that
mineral dust is compositionally heterogeneous and that a “typical” dust composition
does not exist. However, the observed regional compositional trends in northern
Africa and eastern Asia show that average mineral dusts from the different regional
source areas are characterized by specific compositional features (Formenti et al.
2011b ; Scheuvens et al. 2013 ) that in turn will determine their impacts on Earth's
climate, ecosystems, and human health.

C dolomite), the illite/kaolinite ratio, and the elemental ratio (Ca C

2.5.1 Direct Radiative Forcing

Besides the absolute dust concentration and the height and structure of a dust
layer in the atmosphere, the radiative characteristics (direct radiative effect) of
“pure” mineral dust are mainly determined by its particle size distribution, the
composition, and the shape of the particles (see Chap. 11 , Sokolik and Toon 1999 ;
Nousiainen 2009 ). Concerning composition, the abundance and mixing state of the
main light-absorbing minerals in dust - iron-bearing ones like hematite (Fe 2 O 3 )and
goethite (FeOOH) - is of prime importance for estimation of the optical properties
(e.g., Sokolik and Toon 1999 ). However, as the elemental Fe content is only a rough
proxy for the amount of Fe (hydr)oxides in a given sample and the abundance
of hematite/goethite often falls below the detection limit of XRD analysis, other
methods have to be applied to analyze the amount and speciation of iron-bearing
phases (e.g., Lafon et al. 2004 ). Furthermore, individual-particle analysis revealed
that pure Fe (hydr)oxide particles are rare in mineral dust and that these minerals
are more often internally mixed with different (alumino)silicates (e.g., Kandler
et al. 2009 ; Lieke et al. 2011 ; Scheuvens et al. 2011 ). The detailed structure of
these internally mixed particles and hence their radiative characteristics are far from
being understood. Nevertheless, for northern Africa, an indistinct trend with higher
abundances of iron (hydr)oxides towards southern latitudes was shown by Formenti
et al. ( 2008 ), resulting in an elevated value for the imaginary part of the refractive
index for sub-Saharan/Sahelian dust (Kandler et al. 2009 , Table 6, Kandler et al.
2011a , Fig. 12, Müller et al. 2011 ). A more detailed closure is currently limited