Dust Observations and Climatology - Mineral Dust: A Key Player in the Earth System - page 153

networks providing continuous PM10 and/or PM2.5 measurements, from which
dust loads can be derived, only a few sites have been dedicated to dust mass
concentration recording over long periods. Many of the available dust datasets cover
periods of 10 years or less, especially in the marine regions surrounding North
Africa, as in the Cape Verde Islands (Chiapello et al. 1995 , 1997 )andinafew
Mediterranean stations. Long-term dust mass concentration records are especially
rare over arid and semi-arid regions located close to sources. To fill this gap, a
transect of ground-based dust concentration measurements at three sites in the Sahel,
one of the dustiest places on Earth, has been started in 2006 in the framework
of AMMA (Marticorena et al. 2010 ). As highlighted by many studies, due to the
sporadic nature of dust events, the regional and daily variability of surface dust
loads provided by this kind of measurements is huge. The mass concentrations in
desert areas where (and when) dust is raised have been reported in the range 100-
100,000 g/m 3 (Goudie and Middleton 2006 ), varying between 0.1 and 100 g/m 3
in other oceanic and continental regions.
The AERONET with hundreds of identical sun/sky photometer sites around the
world, the first starting in the mid-1990s, is a key reference observational system
for aerosols. The photometer measurements provide vertically integrated aerosol
records during daily clear sky conditions, given as AODs at different wavelengths.
Angström exponents are generally used to distinguish the large particles during dust
events (i.e. low Angström values) from other aerosol types. These high-quality data
have been used to investigate dust variability and properties over different regions
of the world (Kim et al. 2011 ). Figure 7.1 shows two illustrations of multiyear
dust observations performed at the site of Banizoumbou (Niger) in West Africa,
with (a) AOD (and Angström exponent) from an AERONET photometer and (b)
surface mass concentrations from a TEOM. These measurements illustrate the large
variability of dust at daily and seasonal timescales.
There are currently more than 20 separate satellite sensors available for aerosol
studies (Lenoble et al. 2013 ), allowing to spatially extend the point observations
from the ground sites. These sensors, because they allow repetitive large-scale
observations and monitoring of dust events, have been widely used in recent years.
Tab le 7.2 gives selected examples of the main satellite sensors suitable and applied
to dust studies, especially for dust load monitoring. As for sun photometers, the
main retrieved aerosol parameter is AOD (i.e. an information of the total amount
of aerosol weight by their extinction coefficient, at one or several wavelengths),
which includes the contribution of all aerosol species. Some algorithms allow
specific retrieval of the dust contribution to AOD (i.e. dust optical depth, DOD).
Other algorithms, especially in the UV (with TOMS and OMI) and in the IR
(with Meteosat and MSG), only provide semi-quantitative indices that have been
used as proxy of dust events or loads, especially over arid regions. Thus, the
different spectral ranges that have been used, that is, besides the visible, the UV
and IR, allow to extent the monitoring to arid and semi-arid surfaces, which is of
primary importance for a better understanding of desert dust emissions (Legrand
et al. 2001 ;Prosperoetal. 2002 , and Sect. 7.3.1 ). Observations from geostationary
sensors, that is, Meteosat and MSG, have been applied for studies of North African