Dust Deposition - Mineral Dust: A Key Player in the Earth System - page 187

errors in the estimation of the dry deposition fluxes. Goossens ( 2005 ) tested many
experimental systems (water surface, glass surface, metal surface, vertical array of
metal plates, and an inverted frisbee filled with glass marbles) to collect dry
deposited dust particles and showed that none of these surrogate systems are really
efficient to collect quantitatively the dry deposition. This is especially critical for
particles in the size range 0.1-10 m, for which the dominating removal process is
associated to the turbulent flux and which are most sensitive to surface properties
(see Sect. 8.2.1 ). On the opposite, the dry deposition of larger particles is controlled
by sedimentation and is therefore less sensitive to surface properties. Due to the
lack of instruments able to perform measurements of dust concentration at high
frequency, experimental determination of the dry deposition velocity of dust can
only be performed using the classical “gradient method” (Gillette and Dobrowolski
1993 ), which defines the turbulent flux of a scalar in the vertical direction as

F D K dC=d z (8.16)

where K is the eddy diffusivity or turbulent transfer coefficient (m 2 s 1 )andd C/ d z
is the scalar concentration gradient (expressed, e.g., for dust in gm 3 m 1 ).
Integration of ( 8.18 ) between two heights ( z 1 and z 2 ) assuming a constant flux
with height and neutral stability results in

u z k
ln . z 2

C
z

F D (8.17)

z 1 /

where C is the scalar concentration difference between heights.
In practice, this method requires measurements of wind velocity and temperature
profiles (since frequently corrections for non-neutral stability conditions are needed
to precisely compute the wind friction velocity) and is often limited by the
precision of the measurements of the scalar concentration difference between
heights.
Wet (or total, when wet deposition strongly dominates the deposition) deposition
measurements of dust are easier to perform. For example, long-time series of total
deposition measurements were conducted in an area close to Barcelona in Spain
(Avila 1996 ), which allow monitoring the intensity and frequency of red rains (i.e.,
precipitation associated with Saharan dust events). These records suggest that the
deposition of Saharan dust is very discontinuous over the western Mediterranean
region (about 10-20 events a year) and that a large part of the annual deposition
flux of dust (up to 30 %) can occur in only two or three days. The deposition
flux of Saharan dust in Miami (Florida, USA) was estimated from measurements
of aluminum in precipitation to be around 1.26 g m 2 year 1 (Prospero et al.
1987 ). More recently, measurements performed during three years at nine stations
in Florida confirmed this value for Saharan dust deposition (deposition fluxes on
theorderof2gm 2 year 1 ;Prosperoetal. 2010 ). Dust deposition measurements
are very scarce in and around the Sahara and Sahel regions, which are probably the
most important dust sources of the world (see Chap. 3 ) . Figure 8.3 shows most of