Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 308

generally increases the threshold nucleation saturation. However, at water saturation
when particles are immersed in very dilute solutions, chemically inert, then soluble
coatings have no impact on ice nucleation (Koehler et al. 2010 ; Tobo et al. 2012 ).
In contrast, sulfuric acid irreversibly reduced the ice-nucleating behavior of the
dust which is consistent with the surface modifications observed by Reitz et al.
( 2011 ) when dust was exposed to sulfuric acid. More recently Wex et al. ( 2013 )
showed that only a kaolinite sample with feldspar impurities was susceptible to
sulfuric acid, whereas a sample with no detectable feldspar was not. Feldspars are
known to be converted to clay minerals and amorphous silica by acids (Zhu et al.
2006 ), and these reactions are therefore consistent with the observations of Wex
et al. ( 2013 ).

12.8 Modeling Studies on the Interaction of Dust with Clouds

A thorough treatment of all studies focused on dust interactions with clouds and
climate is beyond the scope of this section; we however present representative
examples of studies on aspects of dust impacts and their major conclusions.
Mineral dust, sea salt aerosol, and their mixtures have been simulated in an
attempt to quantify their effect on precipitation development and rain amounts.
Based on the chemical and physical properties of particles obtained during MEI-
DEX, Levin et al. ( 2005 ) concluded that allowing the soluble component of
the mixed aerosol to act as efficient GCCN enhances the development of the
warm rain process in continental clouds with higher rain amounts by as much as
37 % compared to the case without GCCN. Introducing similar coarse particles
into maritime-type clouds on the other hand does not have a significant effect
on neither cloud nor amount of rainfall. When dust particles are active as both
GCCN and effective ice nuclei (IN), continental clouds become wider, there are
higher concentrations of droplets, and the cloud lifetime is longer, while once
more the effects on more maritime clouds is very uncertain. Smoydzin et al.
( 2012 ) also arrived at the conclusion that the contribution of dust to the CCN
population is potentially significant only over land. They investigate the impact of
the coarse mode hygroscopicity on aerosol activation and the largest impact was
found for warm phase clouds. Precipitation was not influenced by changing the
hygroscopicity, except when dust entered into an area of orographic ascent, causing
glaciations of the clouds and leading to local enhancement of rainfall.
The effect of GCCN on global climate, warm clouds, and precipitation was also
simulated within the ECHAM5 General Circulation Model (GCM) (Posselt and
Lohmann 2008 ). It was found that measured distributions agree much better with
GCCN concentrations when considering an upper size limit of 10 m particles, than
concentrations for chosen cutoff radii of 5 m. The incorporation of GCCN in the
results does not affect precipitation globally as regions with increasing precipitation
rates alternate with regions with decreasing precipitation rates so that the zonal
average does not change. Nonetheless, depending on the amount of added GCCN,