CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
65
tensively studied during the past two decades. For reviews of various areas of research
relevant to the ocean, see Andreae 1995; Prospero 1981, 1996a,b; Pye 1987; Middleton
et al.1986; Duce et al.1991; Duce 1995; Goudie 1983; Leinen and Sarnthein 1989; Golitsyn
and Gillette 1993; Guerzoni and Chester 1996.
Satellite images provide the most graphic evidence of the widespread occurrence
of dust. In Figs. 2.3 and 2.4, huge plumes of dust are seen to emerge from arid continental regions and extend over large ocean areas. Indeed, excluding clouds, dust is the
most prominent aerosol feature over the oceans. Dust plumes are much more prominent than pollution plumes; they cover larger areas and are more persistent. Because
of the prominence of dust over such large regions, dust has become a major focus of
climate studies. Dust is a strong absorber and scatterer of solar and terrestrial radiation (Sokolik and Toon 1999). Thus, dust can be an agent of climate change. Conversely,
the generation of dust is strongly dependent on climatic factors such as aridity and
wind conditions. Thus, there is a strong possibility that dust can provide strong feedback in the climate forcing system.
Satellite images show that the distribution of dust over the oceans is highly variable. The most prominent dust plumes are found in the Northern Hemisphere. Satellite images show a dust belt that extends from the western coast of North Africa,
through the Middle East, into Central Asia and reaching into China almost to the Pacific coast. This huge dust belt dominates the global dust budget. In this belt, the largest
sources by far are found in North Africa. In contrast, there are almost no major dust sources
in the Southern Hemisphere. This is surprising in light of the widespread arid and desert
regions in southern Africa, South America, and Australia. The absence of major dust
sources in Australia, a continent where about 80% of the area is arid, is especially notable.
2.5.2
Sources of Dust
Mineral dust is a primary aerosol product; it is lifted directly from soils in the source
regions by winds. Soil particles are produced by the chemical weathering of rocks and
by mechanical processes (e.g. grinding, fracturing, impaction, etc.). These processes
mostly produce particles that are relatively large, tens to hundreds of micrometres in
diameter, but there is a substantial yield of smaller particles as well. When winds lift
soil particles into the air, the suspended mass initially consists largely of particles with
sizes greater than 10 11m (e.g. silt and sand particles) (Duce 1995). Such large particles
have a very short residence time in the atmosphere because of their high settling velocity. For example, a sand particle (density 2.6 gm/cm 3 ) with a diameter of 100 11m
diameter has a Stokes settling velocity of 78.5 cm S-1 (67.9 km d- 1 ). Because of the rapid
fallout of large particles during transport, the peak in the suspended dust size distribution rapidly shifts to smaller particles; at distances of about 1 000 km or more from
the source, the dust attains a relatively stable size distribution with a mass median
diameter of several 11m (Duce 1995), consistent with the size distributions shown schematically in Fig. 2.1. Dust particles in this size range can be carried great distances by
winds; for example; a dust particle 2 11m diameter has a settling velocity of 0.034 cm S-1
(0.029 km d- 1 ).
There is considerable uncertainty as to the specific sources of dust. It is known that
certain types of soil environments readily yield large amounts of dust. Prospero et al.
65
tensively studied during the past two decades. For reviews of various areas of research
relevant to the ocean, see Andreae 1995; Prospero 1981, 1996a,b; Pye 1987; Middleton
et al.1986; Duce et al.1991; Duce 1995; Goudie 1983; Leinen and Sarnthein 1989; Golitsyn
and Gillette 1993; Guerzoni and Chester 1996.
Satellite images provide the most graphic evidence of the widespread occurrence
of dust. In Figs. 2.3 and 2.4, huge plumes of dust are seen to emerge from arid continental regions and extend over large ocean areas. Indeed, excluding clouds, dust is the
most prominent aerosol feature over the oceans. Dust plumes are much more prominent than pollution plumes; they cover larger areas and are more persistent. Because
of the prominence of dust over such large regions, dust has become a major focus of
climate studies. Dust is a strong absorber and scatterer of solar and terrestrial radiation (Sokolik and Toon 1999). Thus, dust can be an agent of climate change. Conversely,
the generation of dust is strongly dependent on climatic factors such as aridity and
wind conditions. Thus, there is a strong possibility that dust can provide strong feedback in the climate forcing system.
Satellite images show that the distribution of dust over the oceans is highly variable. The most prominent dust plumes are found in the Northern Hemisphere. Satellite images show a dust belt that extends from the western coast of North Africa,
through the Middle East, into Central Asia and reaching into China almost to the Pacific coast. This huge dust belt dominates the global dust budget. In this belt, the largest
sources by far are found in North Africa. In contrast, there are almost no major dust sources
in the Southern Hemisphere. This is surprising in light of the widespread arid and desert
regions in southern Africa, South America, and Australia. The absence of major dust
sources in Australia, a continent where about 80% of the area is arid, is especially notable.
2.5.2
Sources of Dust
Mineral dust is a primary aerosol product; it is lifted directly from soils in the source
regions by winds. Soil particles are produced by the chemical weathering of rocks and
by mechanical processes (e.g. grinding, fracturing, impaction, etc.). These processes
mostly produce particles that are relatively large, tens to hundreds of micrometres in
diameter, but there is a substantial yield of smaller particles as well. When winds lift
soil particles into the air, the suspended mass initially consists largely of particles with
sizes greater than 10 11m (e.g. silt and sand particles) (Duce 1995). Such large particles
have a very short residence time in the atmosphere because of their high settling velocity. For example, a sand particle (density 2.6 gm/cm 3 ) with a diameter of 100 11m
diameter has a Stokes settling velocity of 78.5 cm S-1 (67.9 km d- 1 ). Because of the rapid
fallout of large particles during transport, the peak in the suspended dust size distribution rapidly shifts to smaller particles; at distances of about 1 000 km or more from
the source, the dust attains a relatively stable size distribution with a mass median
diameter of several 11m (Duce 1995), consistent with the size distributions shown schematically in Fig. 2.1. Dust particles in this size range can be carried great distances by
winds; for example; a dust particle 2 11m diameter has a settling velocity of 0.034 cm S-1
(0.029 km d- 1 ).
There is considerable uncertainty as to the specific sources of dust. It is known that
certain types of soil environments readily yield large amounts of dust. Prospero et al.