Geoscience Reference
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phases at their surface (e.g., coatings). Meanwhile, different types of coatings
(sulfate, nitrate, phosphate, organic; see Chap. 4 ) were verified by several studies
(Shi et al. 2008 ;LiandShao 2009 ; Matsuki et al. 2010a , b ), and even a combination
of different coatings has to be taken into account. The deposition of highly soluble
material at the (dust) particle surface will significantly change the hygroscopicity
and hence enhance their ability to act as cloud condensation nuclei (CCN).
In contrast to its possibly restricted CCN activity, dust particles are readily
activated as ice nuclei (DeMott et al. 2003 ; Sassen et al. 2003 ) so that the long-
range transport of mineral dust is associated with elevated IN number concentrations
(Klein et al. 2010 ; Chou et al. 2008 ). However, a soluble coating acquired through
atmospheric processing during transport may suppress the initiation of ice formation
at the same time as enhancing their action as CCN (Hoose et al. 2008 ; Möhler
et al. 2008 ; Niedermeier et al. 2011 ; Reitz et al. 2011 ; Tobo et al. 2012 ). A high
atmospheric burden of dust particles, that is, readily available CCN or IN, may
have a significant impact on cloud processes (Rosenfeld et al. 2001 ; Yin et al.
2002 ; Levin et al. 2005 ), so the absolute number concentration of mineral dust
plays a considerable role. The mineral dust size distribution should, in contrast,
not affect activation, as most of the dust particles are larger than the usual activation
size for cloud droplets (Pruppacher and Klett 1997 ). However, for particle cloud
dynamic and precipitation development, the presence of large CCN and a large
number of IN should have an influence, which most probably depends on the actual
thermodynamic situation and cannot be generalized.
2.5.3
Ecosystem Nutrient Supply and Human Health Effects
The major nutrients that may control the functionality of marine and terrestrial
ecosystems are iron, phosphorus, and nitrogen (Duce and Tindale 1991 ; Mills
et al. 2004 ). Nitrogen concentrations in freshly emitted mineral dust are generally
low. However, several studies emphasized that especially calcite particles may
react to form calcium nitrate under conditions of elevated NO x and/or HNO 3
concentrations in the atmosphere. These nitrate (or internally mixed calcite/nitrate)
particles are highly soluble and may be an important source for nitrogen when
finally deposited (Fan et al. 2004 ; Doney et al. 2007 ). Northern African dust is
generally enriched in phosphorus with respect to the composition of the upper
continental crust (Scheuvens et al. 2013 ). Special attention has recently been drawn
to large phosphorite deposits in northwestern Africa which may be the source of P-
enriched dust deposits (Rodríguez et al. 2011 ). The major carrier of phosphorus in
dust samples is the mineral apatite which is in general only slightly soluble. Studies
on the determination and conditions of phosphorus solubility in dust samples are
comparatively rare (see also Mahowald et al. 2008a , e.g., Nenes et al. 2011 ;Okin
et al. 2011 ) and will be a challenge for future studies due to the comparatively low
P abundance and the high fraction of generally insoluble apatite in dust samples.
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