On Composition, Morphology, and Size Distribution of Airborne Mineral Dust - Mineral Dust: A Key Player in the Earth System - page 26

C-dominated (soot, “tar balls”) and K-S rich particles) is mostly restricted to
the fine (submicron) size fractions and is in most case studies mainly externally
mixed with mineral dust (Chou et al. 2008 ; Formenti et al. 2008 ; Deboudt et al.
2010 , but see also Hand et al. 2010 ; Matsuki et al. 2010a ; Kandler et al. 2011a ;
Lieke et al. 2011 ).
Individual-particle analysis on Asian dust mainly focused on the interaction
between (long-range) transported dust and urban (polluted) aerosols of mega-
cities as Xi'an and Beijing and/or mixing and processing during transport towards
eastern China and Korea and further to Japan and the North Pacific. On the other
hand, a number of studies exist that investigate near-ground or balloon-borne
dust samples collected in the most important potential source areas of eastern
Asia (e.g., Taklamakan) although the number of analyzed particles is often low
(<500). In general, individual-particle analysis shows that Asian dust is composed
of external and internal mixtures of quartz, aluminosilicates (mainly different clay
minerals), and carbonates to varying amounts (Okada and Kai 1995 ; Gao and
Anderson 2001 ; Trochkine et al. 2003 ; Matsuki et al. 2005a ;Yuanetal. 2006 ; Jeong
2008 ) comparable to northern African dust. Jeong ( 2008 ) describes the occurring
clay minerals in more detail and shows their association with nanosized calcite, iron
oxides, and other minerals. In dusts from the Taklamakan Desert, elevated amounts
of halite particles were detected (Okada and Kai 2004 ). The older idea that freshly
emitted Asian dust is mostly devoid of sulfates (Iwasaka et al. 2003 ) has recently
been challenged by a study of Wu et al. ( 2012 ) who could show that the sulfur
content in samples from the Taklamakan Desert is of primary soil-derived origin
(see also Shi et al. 2005 ;Yuanetal. 2006 ;Wangetal. 2012 ). Yuan et al. ( 2006 )also
proposed that elevated phosphorus and chlorine contents in their dust samples are
related to saline soils in the source area (here: Inner Mongolia).
In particular, the carbonate calcite is subjected to heterogeneous reactions at the
particle surface and readily reacts during transport to form calcium sulfate (Gao
and Anderson 2001 ; Trochkine et al. 2003 ; Matsuki et al. 2005a ), calcium nitrate,
and/or calcium chloride (Tobo et al. 2009 ) depending mainly on the abundance of
the different acidic gases H 2 SO 4 , HCl, and HNO 3 in the atmosphere. Whereas the
reaction from calcite to calcium nitrate under participation of nitric acid is a self-
reinforcing process resulting in complete transformation and genesis of spherical
(aqueous) particles, the conversion from calcium carbonate to calcium sulfate occurs
more slowly and is often incomplete (Matsuki et al. 2005a , b ; Formenti et al. 2011a ).
During long-range transport (alumino)silicate particles are also strongly affected by
internal mixing with non-crustal components (e.g., sea salt, Niimura et al. 1998 ;
Zhang et al. 2003a ; Zhang and Iwasaka 2004 ; Ma and Choi 2007 ) and/or uptake of
sulfur, nitrogen, chlorine or organic compounds on the particle surface (Ma et al.
2001 , 2012 ; Zhang et al. 2003a ; Matsuki et al. 2005a ; Sullivan et al. 2007 ). The
internal mixing of Asian dust with sea salt on its way to Japan is mainly restricted
to near-ground samples and is not found in the cloud-free troposphere (Trochkine
et al. 2003 ).