Dust Observations and Climatology - Mineral Dust: A Key Player in the Earth System - page 160

this transport, especially to the northern tropical Atlantic, the dominant direction
of export, have been the subject of many studies. Multiyear time series of dust
concentrations recorded at different sites, including Barbados (Prospero and Lamb
2003 ), Miami (Prospero 1999 ), the Canary Islands and the Cape Verde Islands,
have allowed assessing and understanding the large seasonal variability of the
dust transport (Chiapello et al. 1995 ). Since the 1990s, the seasonal and monthly
distributions of dust provided by satellite observations have allowed new insights
into the spatial distribution associated to this seasonal variability of dust transported
from North Africa and from other desert regions of emissions of the world (Swap
et al. 1996 ;Hermanetal. 1997 ; Husar et al. 1997 ;Cakmuretal. 2001 ). Table 4.7
of Goudie and Middleton ( 2006 ) summarizes current knowledge of seasons when
maximum dust storm activity has been observed over the main affected areas of the
globe.

AOD, Dust Discrimination and Dust Above Clouds

As indicated in Table 7.2 , most of the algorithms applied to satellite sensors allow
retrievals of AOD, defined as the vertically integrated aerosol extinction. AOD can
be considered as an optical measure of the whole amount of aerosols (all types) with
some wavelength dependence. The uncertainties on the retrieved AOD have been
significantly reduced from the early sensors in operation in the 1980s and 1990s
(AVHRR, GOES, Meteosat) to the current satellite missions dedicated to aerosol
research, especially those of the A-Train (MODIS, MISR, POLDER). The accuracy
varies between sensors and also depends on the underlying surface (land or ocean),
but has been estimated to be on order of 20 % or

0.05 (Yu et al. 2013 ). Some of the
most recent sensors allow partitions of AOD into fine and coarse modes, the latter
being dominated by mineral dust and sea salt. POLDER measurements of spectral
and angular polarization provide additional partition into the spherical and non-
spherical components within the coarse mode (Herman et al. 2005 ). This product,
available only over oceans under optimal geometric conditions, is expected to repre-
sent the desert dust component (Tanré et al. 2011 ; Peyridieu et al. 2010 ). Figure 7.4
illustrates seasonal distributions of the nonspherical coarse component of AOD
derived from POLDER-3 over global oceans for the year 2009. The most widespread
and intense transported dust plumes occur in summer (from Arabia and the Sahara)
and spring (from Asia) with lowest dust loads generally occurring in autumn.
Other studies have attempted to separate the dust component from other aerosol
types (i.e. marine, fine mode particles from pollution and smoke) by combining
parameters derived from the satellite measurements (Angström exponent, size
fraction, absorption), especially with MODIS (Kaufman et al. 2005 ; Ginoux
et al. 2012 ). Because these approaches require assumptions, the unambiguous
identification of dust is still challenging. Nevertheless, most of the current satellite
sensors attempt to discern dust from the total aerosol signal, with an improved
accuracy compared to early sensors. Figure 7.5 shows an illustration of dust AOD
retrieved from MODIS, MISR and PARASOL over the North Atlantic. Although

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