Dust Deposition - Mineral Dust: A Key Player in the Earth System - page 182

In this approach, the deposition surface is considered as perfectly “sticky,” since
no bounce-off or remobilization of particles on the surface is considered. When
turbulence increases, the thickness of the quasi-laminar layer decreases, while the
wind friction velocity increases; as a consequence, V d increases when u * increases.
It is worth noting that theoretical developments have been relatively rare during
the past twenty years. Some efforts have been made to better reproduce dry
deposition of aerosols over vegetated surface (e.g., Zhang et al. 2001 ; Petroff et al.
2008 , 2009 ) or to add additional effects (such as phoretic effects) to the actual
formulation of the dry deposition (Petroff and Zhang 2010 ). The same authors also
propose a new formulation of the Brownian diffusion more efficient to estimate dry
deposition over smooth and water surfaces.

8.2.2 Wet Deposition

The term wet deposition includes all depositional processes by which aerosols are
removed from the atmosphere due to the presence of water, i.e., mainly cloud, snow,
and fog.
If, as already mentioned, dry deposition is by far the dominant removal process
of atmospheric mineral particles in the vicinity of dust source areas, the relative
importance of wet scavenging processes increases with the distance from the source
regions (Tegen and Fung 1994 ;Zhaoetal. 2003 ).
For atmospheric particles, there are two distinct ways of wet removal:

￿ Rainout or in-cloud scavenging corresponds to the scavenging of aerosol particles

acting as condensation or freezing nuclei or to collision with preexisting cloud
droplets or ice crystals inside the cloud.

￿ Washout or below-cloud scavenging corresponds to the impaction of aerosol

particles by the falling droplets.

The below-cloud scavenging process is much more efficient for the removal
of particles in the coarse mode (Fig. 8.1 ), while the in-cloud scavenging process
is important for the removal of submicron particles. Moreover, in their initial
composition (see Chap. 2 ) , dust particles are generally assumed to be hydrophobic
(Fan et al. 2004 ), reducing the in-cloud scavenging efficiency and suggesting that
below-cloud scavenging is the dominant wet deposition process for pure dust
particles. Nevertheless, recent work of Twohy et al. ( 2009 ) suggests that pure
dust particles could act as condensation nuclei. Dust particles are also known to
be efficient ice nuclei and Atkinson et al. ( 2013 ) demonstrate that it is strongly
dependent on the aerosol mineralogy. Once more, the processes involved are, at the
moment, too poorly known to be included in dust models.
During transport, dust particles may undergo transformations, aggregating more
hydroscopic compounds like sulfates on their surface (Levin et al. 1996 ; Würzler
et al. 2000 ; Perry et al. 2004 ; see also Chaps. 4 and 12 ) . This coating could allow