Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 296

While n s has become widely used for mineral dust and other IN species, it is
important to bear in mind its inherent limitations. A key limitation is that if the
measurement timescale is very different to the timescale available for nucleation in
the atmosphere, n s values (or time-independent IN concentrations) must be used
with caution. In convective cloud systems, the singular description most likely
captures the key features of the IN spectrum, whereas in stratus clouds, it has been
suggested that slow stochastic nucleation could account for prolonged production
of ice crystals (Westbrook and Illingworth 2013 ). Recently, Herbert et al. ( 2014 )
proposed a means of representing the time dependence of nucleation using the
singular description through the recognition that the quantity ln J het / T ,where J het
is for each component in an IN population, defines the time-dependent nature of ice
nucleation.

12.4 Field Observations of Dust CCN

Activity/Hygroscopicity

During the last decade, several field measurements have been dedicated to the
physicochemical properties of mineral dust, such as the Saharan Dust Experiment
(SHADE), the Aerosol Characterization Experiment - Asia (ACE-Asia), the African
Monsoon Multidisciplinary Analysis (AMMA), the Mediterranean Israeli Dust
Experiment (MEIDEX), the Dust and Biomass-burning Experiment (DABEX), and
the Saharan Mineral Dust Experiment (SAMUM) (Manktelow et al. 2010 ; Formenti
et al. 2011 and references within). The foci in these campaigns were the study of dust
composition, the impacts of atmospheric aging thereon, and in some the hygroscopic
and CCN properties of the dust samples; a list of studies and major findings is shown
in Table 12.1 .
From these observations, it is often found that dust particles are internally
mixed with sulfate and/or nitrate, as a result of their atmospheric processing. The
production of the different aged dust particles depending on their transport pathways
has important implications on their different effects on cloud formation, radiation
balance, and health (see Chaps. 4 , 11 and 16 ) . Of all the studies cited above,
few studies directly measure the CCN activity of dust in situ. Given the wide
range of soluble fraction, combined with the importance of both adsorption (FHH)
and absorption (Raoult) of water on the equilibrium vapor pressure above dust,
necessitates more field measurements directly addressing the origin of dust aerosol
hygroscopicity.