Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 311

growth which eventually quenches the supersaturation. Homogeneous nucleation
of haze particles occurs around 155-160 % RH i depending on temperature, but
independently of solute composition (Koop et al. 2000 ). Since the nucleation rate
for homogeneous nucleation increases rapidly with increasing saturation, many of
these droplets freeze before the supersaturation is quenched by the growth of the
nucleated ice crystals. In the presence of heterogeneous IN capable of nucleating
ice at lower supersaturations, far fewer ice crystals typically form (Gettelman et al.
2012 ; Lohmann and Feichter 2005 ; Barahona and Nenes 2007 , 2009 ; Barahona
et al. 2010 ; Murray et al. 2010 ; Karcher and Lohmann 2003 ; Ren and Mackenzie
2005 ;Jensenetal. 2010 ). Several studies have shown that nucleation by dusts, and
other materials, below water saturation results in a much more gradual nucleation
of ice crystals on increasing saturation than in the case of homogeneous nucleation
(Koehler et al. 2010 ; Möhler et al. 2006 ). Wheeler and Bertram ( 2012 ) reach
a similar conclusion through showing that nucleation by mineral dust cannot
be represented with classical nucleation theory assuming a single contact angle.
Models show that the growth of heterogeneously nucleated ice crystals can result
in a peak saturation below that required for homogeneous nucleation with the result
that a small number of ice crystals nucleate, but these crystals grow to larger sizes
(Murray et al. 2010 ; Barahona and Nenes 2007 , 2009 ; Barahona et al. 2010 ).

12.9 Conclusion

In this chapter, we discuss the role of mineral dust as CCN and IN as well as
the subsequent influence on cloud microphysics and climate. Field observations
have shown that on numerous occasions, secondary sulfate and nitrate are het-
erogeneously formed on many dust particles and aerosol particles appear to be
internally mixed, especially during long-range transport. It has been found that
carbonate minerals are preferentially associated with nitrates, whereas aluminum
silicates are preferentially associated with sulfates. Apart from internal mixtures,
nearly hygrophobic mineral particles can also be found as external mixtures with
hygroscopic particles. In all cases of either internal or external mixtures, the
presence of hygroscopic components can substantially alter the hygroscopic growth
and CCN activity, as well as the IN ability and nucleation pathway, of mineral dust
particles.
For the majority of studies, the CCN activity of mineral dust particles is solely
described by Köhler theory, which is based only on the contribution of the solute and
curvature effects upon water equilibrium vapor pressure using the hygroscopicity
parameter introduced by Petters and Kreidenweis ( 2007 ). Kumar et al. ( 2011b )
proposed a new framework of CCN activation that takes into account not only the
effects of solute but also water vapor adsorption, based on a core-and-shell model
with the core representing insoluble dust and shell consisting of a layer of soluble
salt. These attempts to parameterize better the activity of mineral dust aerosols
are crucial in order to better comprehend the aerosol-cloud-climate interactions.