On Composition, Morphology, and Size Distribution of Airborne Mineral Dust - Mineral Dust: A Key Player in the Earth System - page 35

by the lack of detailed particle structure information and the resulting ambiguity of
the shape and structure influence on the radiative properties (Merikallio et al. 2011 ;
Nousiainen et al. 2011 ).
Particle size strongly influences the shortwave direct radiative forcing of mineral
dust by changing the effective scattering cross section, in particular in the absence of
clouds (Liao and Seinfeld 1998 ). In contrast to other aerosol types, the contribution
of large mineral dust particles is non-negligible for the radiative forcing in a
size range, where exact measurement is more difficult and variability is high
(Miller et al. 2004 ). As a result, the aerosol size distribution is still a considerable
uncertainty in determining the dust radiative forcing (see Chap. 11 , Claquin et al.
1998 ; Miller et al. 2004 ;Durantetal. 2009 ;Zhaoetal. 2010 ; Köhler et al.
2011 ; Osborne et al. 2011 ). Although a better modeling based on more exact
measurements of the size distribution decreased the uncertainties in the last decade
(e.g., Heinold et al. 2011 ; Johnson et al. 2012 ), there still seems to be some way to
go (e.g., Haustein et al. 2009 ;Kok 2011 ; Müller et al. 2012 ).
The shape and morphology of the particles is another important parameter
influencing the optical properties of mineral dust. The dust particle's nonspherical
shape exhibits quite different radiative characteristics compared to the formerly
assumed spherical particles (Nousiainen 2009 ; see, e.g., Otto et al. 2009 for a forcing
simulation as function of particle shape; Redmond et al. 2010 ). The largest database
is available for the aspect ratio of the particles (see compilation above), but except
for a characteristic distribution function, the variability seems to be low; however, a
shape-preferential particle removal (Li and Osada 2007a ) will change the radiative
properties. More complex shapes and morphologies have been investigated recently
(Nousiainen et al. 2009 ; Gasteiger et al. 2011 ; Kahnert and Rother 2011 ; Lindqvist
et al. 2013 ), but for a broad application of these methods, more knowledge on the
real particle shape and morphology has to be acquired.

2.5.2 Indirect Radiative Forcing

The indirect effects of aerosol particles are associated with their ability to provide
surfaces for the nucleation of cloud droplets (cloud condensation nuclei, CCN) or
ice crystals (ice nuclei, IN; see also Chap. 12 ) . Based on the classical Köhler theory,
dust particles are considered to represent poor CCN (Pruppacher and Klett 1997 ).
However, several experimental and field studies revealed that mineral dust in general
(Twohy et al. 2009 ; Kumar et al. 2011 ) and (aged) calcite in particular may act as
CCN (Gibson et al. 2006 ). Hence, the CCN activity of pristine mineral dust may
largely depend on its carbonate (or Ca) content. On the other hand, individual-
particle studies for northern African and eastern Asian dust reveal that pure mineral
dust hardly exists and that, even shortly after entrainment, dust particles are the
sites of heterogeneous reactions finally resulting in the deposition of highly soluble