Dust Deposition - Mineral Dust: A Key Player in the Earth System - page 178

dust deposition schemes against in situ data. Moreover, direct dust deposition
measurements are difficult to perform, especially dry, and suffer from a lack of
standard and qualified procedure despite the fact that the existence of such reference
methods should strongly help in combining different measurements in order to
evaluate dust deposition simulations. In fact, networks monitoring dust deposition
remain rare and most of the existing measurements are performed relatively far from
source regions, although a large part of the dust mass is deposited during the first
1,000 km of transport (Schütz 1980 ). Finally, dust deposition, especially after long-
range transport, is significantly affected by wet deposition, meaning that models
dealing with dust (and more generally aerosol) simulations need more accurate
precipitation fields (at least in terms of localization and frequency).
In conclusion, there is an urgent need for further research and measurements
of dust deposition. Dust models are mainly validated against proxies for the atmo-
spheric dust load, e.g., AOD, concentrations, dust vertical profiles, or combinations
of these. However, because of various deficiencies in these datasets, they do not
serve as a sufficient constraint to close the dust mass budget.
In this chapter, we will address the theoretical concepts allowing to describe
the removal processes of dust (i.e., the dry and wet deposition pathways) and will
discuss the importance of the representation of the size distribution into models
to correctly simulate the dust deposition (Sect. 8.2 ). In Sect. 8.3 , we will report
and discuss the few measurements of dust deposition that have been performed in
the past. Finally, we will illustrate how the lack of constraints on the deposition
term limits the ability of dust models to precisely assess the mass budget of dust
(Sect. 8.4 ) and we conclude with some final remarks (Sect. 8.5 ).

8.2 Deposition Processes

In the atmosphere, the vertical mass or number flux, F z (respectively, in g m 2 s 1
and m 2 s 1 ), of a given species at a given height z is linked to the mass or number
concentration (respectively, in g m 3 or m 3 ) of this species at z , C z ,byaparameter
V having the dimension of a velocity (m s 1 ):

F z D V:C z (8.1)

By convention, F z is negative when the flux is downward (i.e., from the
atmosphere to the surface) and in this case, V is called the deposition velocity,
V d .When V d is only controlled by dry deposition processes, V d is named the dry
deposition velocity.
Figure 8.1 shows the number and mass size distributions of dust deduced from
wind-tunnel measurements (Alfaro et al. 1998 ). These distributions are assumed
to be representative of dust size distributions in the vicinity of source regions. It
demonstrates the efficiency of the removal processes for dust particles (i.e., the dry
deposition velocity and the scavenging coefficient of dust by precipitation) versus
the dust particle diameter. It can be noted that similarities exist between the size