66
J. M. Prospero
(2002) used the TOMS satellite to identify the most active dust sources all over the
Earth. They show that there is a clear geographical pattern to the frequency and intensity of dust activity. This relationship is apparent in Fig. 2.4, which shows the frequency of occurrence of high concentrations of dust (and smoke). Many of the source
regions in Fig. 2-4 have a characteristic shape. These features (most clearly seen in the
January distribution in Fig. 2.4) appear in TOMS year after year over a broad region
of Africa, the Middle East and Asia. The pattern of activity in the most active dust
sources can be closely matched to terrain contours and shows that the most active sites
are located in topographical lows. Most of these regions had been inundated during
pluvial periods in the Pleistocene/Holocene. Thus, these basins contain deep alluvial
deposits, which in modern (arid) times are rapidly being deflated by winds. Finally,
all major dust sources are located in regions where rainfall is less than about
200 mmyr- 1 •
As stated earlier, the most active dust sources are concentrated in a band that extends from the western coast of North Africa almost to the North Pacific coast of China.
In contrast, there are many deserts and arid regions in southern Africa, North America,
South America and Australia that produce so little dust that they are essentially insignificant in the global dust budget. The absence of major dust sources in Australia is
especially notable. About 80% of the area of Australia is arid and there are extensive
deserts and arid regions. Yet there is very little dust activity in Australia. Nonetheless,
it is significant that dust activity in Australia largely takes place over the Lake Eyre
basin, a region that has the characteristics that Prospero et al. (2002) identify as important: it is a topographical low that was inundated in the Pleistocene; it has deep
fluvial deposits; and it is now in an arid region with rainfall in the range of about 100200 mmyr- 1 •
Of all the dust regions identified in Prospero et al. (2002), North Africa is clearly
the largest source of dust that can be transported great distances. Large amounts of
dust are carried to the north across the Mediterranean to Europe (Guerzoni and
Chester 1996) and to the west (Chiapello et al. 1995) across the North Atlantic to the
Caribbean (Prospero and Nees 1986) and the eastern coast of North America (Prospero
1999; Perry et al. 1997). Figure 2.9 shows the monthly mean mineral dust concentration measured in the trade winds at Barbados starting in 1965. There is a clear seasonal cycle, which is linked to the seasonal shift in the large-scale wind patterns. Dust
concentrations are relatively low during winter when dust from Africa is carried in
the lower latitudes to South America (Prospero et al. 1981; Swap et al. 1992) as shown
in the AVHRR (Fig. 2.3) and TOMS (Fig. 2.4) aerosol products.
Satellite imagery of aerosol optical thickness (BAOn such as that in Fig. 2.3 shows
that the highest values of BAOT and the largest areal coverage over the oceans is clearly
related to dust sources (Husar et al.1997). In contrast, the pollution plumes that emerge
from the eastern coast of the United States and the western coast of Europe are relatively small and weak in comparison to the African dust plume. Furthermore, dense
African dust plumes are highly visible all year long while the European and North
American pollution plumes are prominent only during spring and summer.
Large amounts of dust are also transported out of Asia each spring (Prospero et al.
1989; Perry et al. 1999; Arimoto et al. 1996. Zhang et al. 1997). The dust is intermixed
with substantial amounts of pollution aerosol as well (Savoie et al.1989b; Arimoto et al.
1996). The dust and pollution plume is clearly visible in the AVHRR aerosol optical
J. M. Prospero
(2002) used the TOMS satellite to identify the most active dust sources all over the
Earth. They show that there is a clear geographical pattern to the frequency and intensity of dust activity. This relationship is apparent in Fig. 2.4, which shows the frequency of occurrence of high concentrations of dust (and smoke). Many of the source
regions in Fig. 2-4 have a characteristic shape. These features (most clearly seen in the
January distribution in Fig. 2.4) appear in TOMS year after year over a broad region
of Africa, the Middle East and Asia. The pattern of activity in the most active dust
sources can be closely matched to terrain contours and shows that the most active sites
are located in topographical lows. Most of these regions had been inundated during
pluvial periods in the Pleistocene/Holocene. Thus, these basins contain deep alluvial
deposits, which in modern (arid) times are rapidly being deflated by winds. Finally,
all major dust sources are located in regions where rainfall is less than about
200 mmyr- 1 •
As stated earlier, the most active dust sources are concentrated in a band that extends from the western coast of North Africa almost to the North Pacific coast of China.
In contrast, there are many deserts and arid regions in southern Africa, North America,
South America and Australia that produce so little dust that they are essentially insignificant in the global dust budget. The absence of major dust sources in Australia is
especially notable. About 80% of the area of Australia is arid and there are extensive
deserts and arid regions. Yet there is very little dust activity in Australia. Nonetheless,
it is significant that dust activity in Australia largely takes place over the Lake Eyre
basin, a region that has the characteristics that Prospero et al. (2002) identify as important: it is a topographical low that was inundated in the Pleistocene; it has deep
fluvial deposits; and it is now in an arid region with rainfall in the range of about 100200 mmyr- 1 •
Of all the dust regions identified in Prospero et al. (2002), North Africa is clearly
the largest source of dust that can be transported great distances. Large amounts of
dust are carried to the north across the Mediterranean to Europe (Guerzoni and
Chester 1996) and to the west (Chiapello et al. 1995) across the North Atlantic to the
Caribbean (Prospero and Nees 1986) and the eastern coast of North America (Prospero
1999; Perry et al. 1997). Figure 2.9 shows the monthly mean mineral dust concentration measured in the trade winds at Barbados starting in 1965. There is a clear seasonal cycle, which is linked to the seasonal shift in the large-scale wind patterns. Dust
concentrations are relatively low during winter when dust from Africa is carried in
the lower latitudes to South America (Prospero et al. 1981; Swap et al. 1992) as shown
in the AVHRR (Fig. 2.3) and TOMS (Fig. 2.4) aerosol products.
Satellite imagery of aerosol optical thickness (BAOn such as that in Fig. 2.3 shows
that the highest values of BAOT and the largest areal coverage over the oceans is clearly
related to dust sources (Husar et al.1997). In contrast, the pollution plumes that emerge
from the eastern coast of the United States and the western coast of Europe are relatively small and weak in comparison to the African dust plume. Furthermore, dense
African dust plumes are highly visible all year long while the European and North
American pollution plumes are prominent only during spring and summer.
Large amounts of dust are also transported out of Asia each spring (Prospero et al.
1989; Perry et al. 1999; Arimoto et al. 1996. Zhang et al. 1997). The dust is intermixed
with substantial amounts of pollution aerosol as well (Savoie et al.1989b; Arimoto et al.
1996). The dust and pollution plume is clearly visible in the AVHRR aerosol optical