Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 299

then isolated using a counterflow virtual impactor and then probed with an aerosol
mass spectrometer. They found that 33 % of the ice crystal residues contained either
mineral dust or fly ash (these materials could not be distinguished with the mass
spectra), whereas this category only made up 1 % of the background aerosol. On
the basis of electron microscopy analysis, they suggest that 80 % of these particles
were mineral dust. Later, Baustian et al. ( 2012 ) sampled from the same site using a
comparable technique and showed that more than 50 % of the fine mode IN were
mineral dust under conditions relevant for cirrus clouds.
Measurements have also been performed where ice crystals in natural cirrus
clouds were isolated, the ice sublimed away, and analyzed by mass spectrometry
or offline with electron microscopy. Cziczo et al. ( 2013 ) summarize the results from
several campaigns where the composition of the ice crystal residues was determined.
They show that mineral dust/metallic particles were enhanced in most cirrus clouds
sampled. Of notable exception is cirrus which forms in the tropical tropopause
where nucleation may occur on glassy aerosol or crystalline solid salts (Cziczo et al.
2013 ; Murray et al. 2010 ; Wise et al. 2012 ) and mineral dusts are not enhanced in
ice crystal residues (Froyd et al. 2010 ). Nevertheless, these studies in combination
reveal that mineral dust is one of the most important ice nuclei for cirrus-type clouds.
In addition to cirrus, ice crystal residues have been examined in mixed-phase
cloud types. Pratt et al. ( 2009 ) used a counterflow virtual impactor to separate
ice crystals from supercooled cloud droplets in a wave cloud during ICE-L (Ice
in Clouds Experiment - Layer clouds). They found that 50 % of the ice crystals
contained mineral dust particles. Mineral dust was also found to be the dominant ice
crystal residue in mixed-phase clouds at the high alpine observatory at Jungfraujoch
in the Swiss Alps (Kamphus et al. 2010 ). Ebert et al. ( 2011 ) used electron
microscopy to characterize ice crystal residues sampled at the same site and found
that aerosols with silicates or metal oxides as a major component were enhanced by
a factor of 11, but also commented that mixing state may play a very important role
in determining ice nucleation behavior.
These studies of ice crystal residues from both natural ice crystals and ice crystals
nucleated from ambient aerosol in a CFDC collectively show that mineral dust is one
of the most important IN species in both mixed-phase and cirrus clouds. Despite it
being dominantly emitted in the African and Asian dust belts, it is transported in
sufficient concentrations that it contributes to IN concentrations on a global basis.
However, it is unclear if mineral dust is also the dominant IN aerosol type in regions
very remote from dust sources, such as the Southern Ocean or the Arctic (Burrows
et al. 2013 ); hence, more measurements are needed.

12.6 Laboratory Experiments on Mineral Dust CCN

The hygroscopic growth and CCN activity of various, mostly calcium, mineral
dust particles have been extensively studied in several laboratory experiments.
Dust aerosols are generated from either aqueous solution or dry; wet generation