Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 284

CCN activity are significantly enhanced. The theoretical frameworks of adsorption
activation and Köhler theory are presented to explain dust CCN activity, together
with a summary on the potential contributions of dust to cloud droplet number
concentration (CDNC), and its role in regulating supersaturation. Mineral dust
aerosol is an effective IN and, combined with their concentration, can dominate
ice production in cirrus and mixed-phase clouds even at great distances from source
regions. The pathways to nucleation of ice are different for different cloud types
and have distinct effects in those clouds. Our fundamental understanding of ice
nucleation lags behind that for CCN activation, and a key challenge is that we
cannot predict a priori which aerosol materials will make effective IN. Nevertheless,
numerous field and laboratory studies have shown that mineral dust from deserts is
one of the most important ice-nucleating aerosol types around the globe.

Keywords Cloud condensation nuclei ￿ Ice nuclei ￿ Water ￿ Clouds ￿
Hydrological cycle ￿ Particles ￿ Droplets ￿ Cloud formation ￿ Hygroscopicity ￿
Warm clouds ￿ Ice clouds ￿ Laboratory experiments ￿ Observations

12.1 CCN, IN, and Their Impacts on Clouds,

the Hydrological Cycle, and Climate

Atmospheric aerosol particles originate from different sources with a wide composi-
tional range, including soil and desert dust (see Chaps. 2 and 3 ) , sea salt, volcanoes,
biogenic activity, biomass burning, resource extraction activities, and burning of
fossil fuels. Aerosol composition and size distribution are continuously modified
in the atmosphere by the mixing of aerosol particles and their aging through
interactions with clouds (see Chap. 4 ) . The interactions of aerosols with clouds,
radiation, and the hydrological cycle, referred to as “aerosol-cloud interactions,”
constitute the largest source of uncertainty in assessments of climate sensitivity and
anthropogenic climate change (Boucher et al. 2013 ; Carslaw et al. 2013 ). Cloud
feedbacks and the resulting buffered response of clouds and cloud systems pose
a grand challenge in understanding the sign and magnitude of aerosol impacts
on clouds (Stevens and Feingold 2009 ). Nevertheless, considerable progress in
understanding the links between aerosol and clouds has been established and
constitutes the physical basis upon which aerosol-cloud-climate interaction studies
are carried out with.
Increasing concentrations of cloud condensation nuclei (CCN) in low-level
stratus results in higher droplet concentrations and smaller droplet sizes relative to
pristine clouds (Twomey 1977 ). These microphysical changes increase shortwave
albedo and longwave emissivity (Penner et al. 2006 ; Lohmann et al. 2007 ;Kirkevag
et al. 2008 ; Takemura et al. 2005 ;Ghanetal. 2001 , 2012 ; Menon et al. 2002 ;
Topping et al. 2013 ) and are less likely to form precipitation-size drops (Albrecht
1989 ), delay the onset of freezing, and decrease riming efficiency (Lohmann and
Feichter 2005 ; Lance et al. 2011 ). Ice latent heating will increase the buoyancy