Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 286

soluble salts, glassy aqueous materials, and volcanic ash (Hoose and Möhler 2012 ;
Murray et al. 2012 ). Of these, mineral dusts have been identified as one of the most
important ice-nucleating aerosol types around the globe and in clouds ranging from
mixed-phase wave clouds and stratus to cirrus clouds (Cziczo et al. 2013 ; Pratt et al.
2009 ; Baustian et al. 2012 ; Kamphus et al. 2010 ;DeMottetal. 2003b ; Creamean
et al. 2013 ).
Mineral dust is one of the most abundant components of particulate matter in
the atmosphere, accounting for more than 50 % of the global aerosol mass load
(Yin et al. 2002 ;Cakmuretal. 2006 ). Large amounts are annually emitted into
the atmosphere by wind-driven erosion over arid and semiarid areas (Chap. 5 ) ,
and it is observed even at the most remote regions of the world (Chap. 7 ) . Source
regions are found close to coastal regions, for example, the Atlantic Ocean and
Mediterranean Sea neighboring northern Africa and the Arabian Peninsula, the
Indian Ocean surrounding Australia, the Yellow Sea, and East China Sea located
leeward to Eastern China (Chap. 3 ) . Dust is also emitted close to anthropogenically
influenced areas such as near industrial and/or highly populated cities. During
their atmospheric transport, mineral dust particles form internally mixed particles
through coagulation with other types of aerosol, heterogeneous reactions of gas
species, and processing through clouds (Yamashita et al. 2011 ; Chap. 4 ) . Due to this
internal mixing, particles undergo profound changes in their size, shape, surface,
and physical-chemical and optical properties, which affect atmospheric lifetime,
radiative properties (Chap. 11 ) , as well as their CCN and IN activity (e.g., Dentener
et al. 1996 ; Sokolik et al. 2001 ; Kelly et al. 2007 ; Crumeyrolle et al. 2008 ; Cheng
et al. 2009 ).
In this chapter, we discuss the role mineral dust plays in cloud microphysical
processes. We first concentrate on the fundamentals of mineral dust CCN and
IN and how to describe their activity. For CCN activity we discuss the role of
condensable material versus water uptake on bare uncoated dusts, while for IN we
address the various pathways of nucleation and how these pathways are relevant in
different parts of the atmosphere. We then summarize field and laboratory studies
of mineral dust CCN and IN before briefly discussing modeling studies of mineral
dust interactions with clouds.

12.2 The CCN Activity of Mineral Dust

Cloud droplet formation depends on the condensation of water vapor on ambient
aerosols under conditions that favor the formation of a liquid phase. The main
physical-chemical principles involved in the transformation (“activation”) of CCN
into cloud droplets depend on whether the particle can experience unconstrained
water uptake from the vapor phase. For this to occur, the ambient water vapor
pressure has to persistently exceed the equilibrium water vapor pressure over the
dust particle throughout most of its residence in cloud. The level of water vapor
available in clouds for condensation is however affected also by the amount of CCN