On Composition, Morphology, and Size Distribution of Airborne Mineral Dust - Mineral Dust: A Key Player in the Earth System - page 24

are shown in Fig. 2.1 . In principle, individual-particle analysis confirms the XRD
results of bulk samples. It demonstrates that the major fraction of the particle
number abundance consists of Si-dominated particles (mainly quartz, Fig. 2.1 a;
occasionally diatoms, Fig. 2.1 i), different aluminosilicates (Fig. 2.1 b-d), and car-
bonates (Fig. 2.1 e), whereas sulfates (Fig. 2.1 e), Fe-dominated particles (Fig. 2.1 f),
Cl-bearing, and carbonaceous particles only occur in minor amounts. Minor com-
ponents such as Ca sulfates (gypsum), sodium chloride (halite), and C-dominated
components may or may not be of desert origin. Size-segregated analysis of Kandler
et al. ( 2009 ) clearly shows that sulfates and carbonaceous particles are significantly
enriched in the smallest size fraction (<500 nm) and at least in their samples cannot
be classified as mineral dust particles in a strict sense. Furthermore, it is important
to note that the majority of particles are represented by internal mixtures between
different phases and that “pure” end-member particles are rather rare (Falkovich
et al. 2001 ; Deboudt et al. 2010 ). For example, a detailed study by Deboudt et al.
( 2012 ) shows that Fe oxides and Fe hydroxides in dust samples stemming from
northwestern Africa are mostly internally mixed with aluminosilicates (see also
Lieke et al. 2011 ; Scheuvens et al. 2011 ). Iron-bearing particles often seem to be
positioned at the surface of silicate particles probably due to rubification processes
in the parent soil. Other studies emphasize the role of coatings of sulfur-, nitrogen-,
and phosphorus-bearing or carbonaceous phases on northern African mineral dust
particles (Falkovich et al. 2001 ; Kandler et al. 2007 , 2011a ; Crumeyrolle et al.
2008 ; Dall'Osto et al. 2010 ; Scheuvens et al. 2011 ;Pósfaietal. 2013 ) due to post-
entrainment processing in the atmosphere. It is somehow surprising that even freshly
emitted near-source mineral dust collected in Morocco contains significant amounts
of internally mixed silicate-sulfate particles (Kandler et al. 2011a ; Scheuvens et al.
2011 ). An extraordinary case study was presented by Matsuki et al. ( 2010b ) from
airborne dust samples collected in southwestern Niger where calcium carbonate
particles completely reacted to form spherical (aqueous) calcium nitrate particles.
Concerning the characterization of potential source areas, different SEM/EDX
studies indicate that samples originating in the northwestern regions of northern
Africa (Morocco, northern Algeria) exhibit elevated abundances of Ca-rich particles
(calcite, dolomite) and palygorskite and are characterized by illite/kaolinite ratios
>1 in full agreement with the mineralogical data obtained by XRD (Blanco et al.
2003 ;Morenoetal. 2006 ; Kandler et al. 2007 , 2009 ; Coz et al. 2009 ; Scheuvens
et al. 2011 ; Deboudt et al. 2012 ). On the other hand, SEM-EDX analysis was used
to determine a shift in source area with time (2002-2005) for sediment trap material
in the north Atlantic (Brust and Waniek 2010 ) based on the estimated abundances
of different phyllosilicates (illite, palygorskite, smectite). Manual inspection of
individual particles have also yielded some hints to specific source regions (e.g.,
findings of the diatoms Aulacoseira and Stephanodiscus as source markers for the
Bodélé depression in samples from Niger, Chou et al. 2008 ).
An important topic for northern African mineral dust (especially from more
southern latitudes) is its mixing state with aerosols stemming from biomass burning
(and/or urban emissions). Several large field campaigns were dedicated to this
problem. In summary, it could be shown that biomass burning aerosol (mainly