Numerical Dust Models - Mineral Dust: A Key Player in the Earth System - page 208

@c. z /
@t D

ƒc. z /

where c ( z ) is the particle concentration at height level z and ƒ is the removal rate
for either 'rainout' or 'washout' (see Chap. 8 , this topic).
Since dust deposition velocities strongly depend on particle sizes, the dust
aerosol size distribution shifts to smaller sizes during transport from the source
areas to remote regions due to the fast removal of large particles. Close to dust
source regions, gravitational sedimentation is responsible for most of the dry dust
deposition, while wet deposition is responsible for a large part of the removal of far-
travelled dust over remote ocean areas (Jickells et al. 2005 ). Atmospheric lifetimes
of dust particles depend on their size and may range from few hours for particles
larger than 20 m, which are quickly removed by gravitational settling, to some
weeks for submicron particles that are mostly removed by wet deposition (Seinfeld
and Pandis 1998 ).

9.6 Optical Properties of Dust Used in Dust Models

In large-scale models that compute the effect of radiative heating by mineral dust
aerosol, the absorption and scattering by dust particles is parameterized as part
of the radiative transfer schemes (see Chap. 11 ) . Dust optical properties depend
on particle sizes, which are modified during transport, since larger particles are
removed more quickly (see Sect. 9.5 ). Atmospheric dust size distributions (Chap. 2 )
are represented either by assigning dust aerosol particles to the coarse mode in
modal schemes (e.g. Stier et al. 2005 ) or as 5-10 bins in sectional schemes (e.g.
Ginoux et al. 2001 ), considering particle size ranges from 0.1 to 10 morin
some cases up to 50 m. The size distributions of dust transported away from the
sources are mostly well reproduced by such models (e.g. Haustein et al. 2012 ).
However, for dust near the source areas, information on size distribution is sparse. In
emission models the treatment of dust size distribution can vary considerably; large
dust particles may be strongly underestimated near the source areas, which causes
uncertainties in simulations of their optical properties and radiative forcing. Field
measurements during the Fennec 2011 field campaign show large contributions of
coarse mode dust particles downwind of Saharan dust source regions (Ryder et al.
2013 ). Models that neglect such large particles underestimate absorption of solar
radiation by airborne dust. In addition to size, dust optical properties depend on
the particles' complex refractive indices, which are prescribed as constant in most
models. Possible differences in mineral composition of dust particles (Chap. 2 ) are
not taken into account in current dust models when computing their radiative effects
(e.g. Balkanski et al. 2007 ). Particle properties like single scattering albedo that may
relate to mineral composition remain unrelated to the mineral composition at source
areas in the models.