Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 287

present as it is determined by a delicate balance between water vapor availability
from cooling (through, e.g., expansion or radiative cooling; Pruppacher and Klett
1997 ) and condensation of water vapor upon existing droplets. Treatments of this
process of droplet formation in models vary significantly, but invariably consider the
strong coupling between water vapor availability and condensation onto CCN that
have activated into droplets (Ghan et al. 2011 and references therein). Particle size
and chemical affinity for water (termed “hygroscopicity” if the affinity is related to
bulk absorption of water and “hydrophilicity” if it involves the adsorption of water
molecules upon the particle surface) eventually determine the particle equilibrium
vapor pressure and its ability to act as a CCN.
Mineral dust close to source region comprises a broad and variable mixture
of minerals, which include insoluble aluminosilicate clays (e.g., kaolinite, mont-
morillonite, illite), tectosilicates (feldspars, quartz), metal oxides (e.g., hematite,
goethite), carbonates (e.g., calcite, dolomite), and sparingly soluble gypsum, as
well as soluble halite salts (Sullivan et al. 2009a ; Murray et al. 2012 ; Chap. 2 ) .
Many field observations have shown that almost 50 % of mineral particles contain
sulfate or nitrate which are formed during their transport in the troposphere by
heterogeneous reactions involving sulfur dioxide and nitrogen oxides (Trochkine
et al. 2003 ; Levin et al. 2005 ;LiandShao 2009a , b ; Matsuki et al. 2010 ). Formation
of sulfate and nitrate is closely related to the mineral composition, as laboratory
experiments and in situ measurements both have revealed that carbonate minerals
are preferentially associated with nitrates, whereas aluminum silicates (i.e., clay
minerals and feldspars) are preferentially associated with sulfates (Gibson et al.
2007 ;Lietal. 2006 ). Calcium sulfate (the reaction product of sulfur on carbonates)
is sparingly soluble in water and, once formed upon dust, can act as a barrier
preventing further uptake of water and other gaseous species. Consequently, the
transformation of sulfur dioxide to sulfate is suppressed in these particles. In
contrast, the major product of nitrate formation on carbonate particles is highly
hydrophilic and hygroscopic (Gibson et al. 2006 ; Vlasenko et al. 2006 ; Sullivan
et al. 2009b ).
Cloud processing of aerosol particles and the coating of mineral dust particles
with sulfate may influence cloud microphysics and cloud development (Yin et al.
2002 ). Insoluble mineral dust particles become increasingly effective CCN after
processing through cloud, owing to layers of soluble material (e.g., sulfate) that
form upon their surface. The introduction of such aerosol into warm or cold clouds
may have two competing effects on precipitation: firstly, a suppression by providing
large concentrations of CCN that compete for the available water, which leads to the
formation of clouds dominated by small droplets with low coalescence efficiencies
(Rosenfeld et al. 2001 ). Secondly, the large mineral particles may act as GCCN,
producing large collector drops that collect small drops in their swath downward,
thus accelerating and enhancing precipitation (Kelly et al. 2007 ). Such changes in
the number concentration of droplets in clouds and their size distribution are thought
to alter the brightness, lifetime, and extent of a cloud.
The ability of particles to act as CCN and form droplets in clouds involves the
effects of surface curvature and depression of water activity from soluble solutes