Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 306

Fig. 12.7 Electron microscope images of a typical NX-illite particle showing that a micron-scaled
particle is made up of many smaller primary grains (Image from Broadley et al. ( 2012 ))

the atmospheric counterpart. This issue is most acute when the primary grain size is
small in comparison to the overall size of the aggregate particle and is a particular
problem for materials which contain clay minerals (which tend to be made up of
nanometer-scaled primary particles). For feldspars and quartz, the primary grain
sizes tend to be much larger which means the surface area determined from a
spherical approximation and gas adsorption techniques is in reasonable agreement
(Atkinson et al. 2013 ).
Finally, also shown in Fig. 12.6 are n s values for K-feldspar discussed above
(Atkinson et al. 2013 ). It was argued above that K-feldspar is the most efficient
single mineral in mineral dusts; hence, one would expect n s values for K-feldspar
to exceed those of the natural dusts. Below 18 ı C this is the case, but there are

15 ı C for the natural desert dust sampled near Cairo
which has similar n s values to K-feldspar in our study. This might suggest that there
is another component of this dust which becomes more important than K-feldspar
at these warmer temperatures. One candidate might be ice-nucleating biological
residues which have been observed to nucleate in fertile soils (Conen et al. 2011 ;
O'Sullivan et al. 2014 ). Further work at temperatures of

three data points at around

15 ı C and above is clearly

required.
A selection of data for ice nucleation by natural desert dusts and ATD is shown
in Fig. 12.8 for conditions pertinent for both mixed-phase and cirrus clouds (Hoose
and Möhler 2012 ). As was discussed in Sect. 12.3 , mineral dusts tend not to
nucleate below water saturation at above about 30 ı C, and this is consistent

with field observations where ice only forms after the development of a liquid
cloud. Under conditions pertinent for cirrus clouds, mineral dusts can nucleate
ice well below water saturation. Interestingly, ATD tends to nucleate ice at lower
threshold saturations than dusts sampled from arid regions (Fig. 12.8 ). This echoes
the findings in the immersion mode shown in Fig. 12.6 where ATD tended to have
a greater n s than natural desert dusts. One possible explanation for this difference,