Geoscience Reference
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comprises the atomization of a suspension of dust powder in water, drying of the
aerosol, subsequent size selection of generated aerosol by a differential mobility
analyzer (DMA), and a hygroscopic growth, CCN, or IN measurement. Dry
generation varies between studies but mainly involves the resuspension of dust
powder using a jet of gas, soft saltation, or fluidized bed followed by a similar
instrumentation as for the wet generation. The generation technique has been shown
to influence both cases of CCN activity as well as ice nucleation experiments.
The corresponding studies along with their major findings are listed in Table
12.2
.
These experimental studies demonstrate the fact that the ability of dust particles
to act as CCN is closely dependent on both the mineralogical composition of the
dust particles and their mixing state with other atmospheric components. Therefore,
different activities are accordingly expected for different parts of the globe.
A number of studies (e.g., Sullivan et al.
2009a
,
b
; Kumar et al.
2011a
,
b
;
Garimella et al.
2013
) point to the importance of carefully characterizing the effects
of multiple particle charging and the shape factor of particles; both are particularly
strong effects given the large size and highly irregular geometry of the particles.
The same studies (and references therein) point out that wet generation of dust
leads to considerable biases in hygroscopicity and should be avoided. Kumar et al.
(
2011b
) showed that wet-generated particles tend to be bimodal with the smaller size
mode displaying considerable hygroscopicity and following a KT-type dependence
of
S
c
vs.
D
dry
; the larger-mode particles display a FHH-type CCN activity with
characteristics that resembled dry-generated dust.
12.7
Laboratory Experiments on Mineral Dust IN
There have been two recent reviews of ice nucleation that extensively cover the ice-
nucleating properties of mineral dust (Murray et al.
2012
; Hoose and Möhler
2012
).
However, since these reviews were published, there have been some advances in
the field which help us to interpret the information summarized in those reviews.
For many years it was thought that the clay minerals were the most important ice-
nucleating component of mineral dust, but this view was recently challenged for
ice nucleation in the immersion mode (Atkinson et al.
2013
) and deposition mode
(Yakobi-Hancock et al.
2013
). Atkinson et al. (
2013
) performed droplet freezing
experiments with minerals common in mineral dust from the world's arid regions.
The purity of the mineral samples was quantified using X-ray diffraction. The
results are shown in Fig.
12.5
and show that the feldspars, in particular the K-
feldspar (microcline), are the most efficient ice-nucleating minerals in mineral dusts.
This result conflicted with the prevailing view that the clay minerals are the most
active component of mineral dust. Atkinson et al. (
2013
) examined the mineral
composition of several clay samples which had been used in previous studies and
found that many of them contain a significant amount of feldspar. They suggested
that the ice-nucleating ability of these clay samples was in fact determined by
the feldspar content, highlighting the importance of quantifying the composition
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