Geoscience Reference
In-Depth Information
60N
40N
20N
0
20S
40S
60S
180
120W
60W
0
60E
120E
180
0
3
6
9
12
TOMS Aerosol Index
15
18
21
24
27
30
Figure 20.7
Long-term mean annual (1979-1993) TOMS aerosol index.
20.2
Key source areas
in general. The southern hemisphere as a whole and the
Americas are notable for their relatively low AI values.
Thus, for example, the AI values of the Bodele Depression
of the south-central Sahara are around four times greater
than those recorded for either the Great Basin or the Salar
de Uyuni in the Altiplano.
The total dust burden in the atmosphere is estimated
from 15 global models to be 8-37 Tg. Simulated global
dust emission is between about 1000-3000 Tg/yr (En-
gelstaedter, Tegen and Washington, 2006; Textor et al. ,
2006), with the uncertainty range of about a factor of 3.
20.2.1
Bodele Depression, Chad
Many studies have pointed to the Bodele Depression as
one of the key dust sources in the world (Kalu, 1979;
Herrmann, Stahr and Jahn, 1999; Brooks and Legrand,
2000; Goudie and Middleton, 2001; Prospero et al. ,
2002; Koren and Kaufman, 2004; Washington and Todd,
2005; Washington et al. , 2003, 2006a, 2006b; Schepan-
ski et al. , 2007). The TOMS AI data indicate that the
Bodele is the most intense source, not only in the Sa-
hara, but also in the world, with annual mean AI val-
ues that exceed 3.0. Dust plumes that originate in the
Bodele from a region of exposed diatomite sediment (cen-
tred near 17 N, 18 E) deposited under Lake Mega-Chad
extend for hundreds of kilometres and are evident in
the MODIS true colour imagery (e.g. Koren and Kauf-
man, 2004; Washington and Todd, 2005). The dust load-
ing from these plumes is reflected in the AERONET
data from Ilorin in Nigeria, some 1700 km downwind
from
Table 20.2 Maximum mean aerosol index (AI) values for
major global dust sources determined from TOMS (after Wash-
ington et al. , 2003).
Location
Mean AI value
Bodele Depression of south central Sahara
> 3.0
West Sahara in Mali and Mauritania
> 2.4
Arabia (southern Oman/Saudi border)
> 2.1
the
Bodele
(Pinker
et
al. ,
2001;
Todd
et
al. ,
Eastern Sahara (Libya)
> 1.5
Southwest Asia (Makran coast)
> 1.2
2008).
Bodele dust loadings, as estimated from a variety of
data sources, follow a semi-annual cycle with peaks in the
boreal Spring and Autumn (Figure 20.8). There is close
agreement between TOMS AOT data and the annual cycle
of dust plumes over the Bodele from MODIS imagery.
TOMS AI data, on the other hand, peaks later in the year,
Taklimakan/Tarim basin
> 1.1
Etosha Pan (Namibia)
> 1.1
Lake Eyre basin (Australia)
> 1.1
Makgadikgadi basin (Botswana)
> 0.8
Salar de Uyuni (Bolivia)
> 0.7
Great Basin of the United States
> 0.5
 
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