68
J. M. Pro spero
20
18
•
if' 16
E
•
C\
3- 14
<:
.!2
~ 12
1;
~ 10
8
tii 8
=s
-c
~ : 11
'"
1
•
<:
~
•
2
0
~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~
Fig. 2.10. Mineral dust concentrations at Midway Island in the central North Pacific. Data points are
concentrations obtained from one-week-long samples collected at a coastal site on the island (Prospero
et al. 1989; unpublished data, R. Arimoto and D. 1. Savoie)
and Chester 1996; Pye 1987; Merrill et al.1994; Leinen and Sarnthein 1989). The elemental composition of the dust is broadly similar to that of average crustal material
(Zhuang et al. 1992; Uematsu et al. 1985; Arimoto et al. 1989, 1990, 1992; Prospero 1981;
Prospero et al. 1987, 1989; Talbot et al. 1986; Graham and Duce 1979; Spokes et al. 1994).
Indeed, there is a remarkable consistency in composition of dust from specific source
regions. As an example, Fig. 2.11 presents data on the elemental composition of dust
samples collected from eight different large African dust events that passed over Barbados, West Indies, over a two year period. The figure shows the mean ratio of the
concentration of a specific element (Fe, Ti, Cd, Sb, etc.) measured in the dust to that of
Al in the dust; these mean ratios are plotted against the same ratio in average crustal
material (Taylor and McLennan 1985). Aluminum is used as the normalizing element
because it is a major component in the Earth's crust and there are no major anthropogenic sources of AI-rich aerosol; but other major elements could have been (and are)
used (e.g. Fe, Ti, and Si). As can be seen in Fig. 2.11, most elements fall very close to the
1:1 line, which suggests that the composition of the dust is very similar to that of average crustal material. Indeed, the composition of the individual samples shows a very
similar pattern, suggesting that there is very little difference in composition from one
dust event to another. When element ratios deviate markedly from the 1:1 line (for
example, As, Sb, and Cd in Fig. 2.11), it suggests that other sources or processes could
be influencing the sample, for example, pollution (Guieu and Thomas 1996; Chester
et al. 1996; Gullu et al. 1996). In comparing aerosol composition to hypothesized
sources, it is convenient to use enrichment factors (EF). The EF is based on the ratio
J. M. Pro spero
20
18
•
if' 16
E
•
C\
3- 14
<:
.!2
~ 12
1;
~ 10
8
tii 8
=s
-c
~ : 11
'"
1
•
<:
~
•
2
0
~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~
Fig. 2.10. Mineral dust concentrations at Midway Island in the central North Pacific. Data points are
concentrations obtained from one-week-long samples collected at a coastal site on the island (Prospero
et al. 1989; unpublished data, R. Arimoto and D. 1. Savoie)
and Chester 1996; Pye 1987; Merrill et al.1994; Leinen and Sarnthein 1989). The elemental composition of the dust is broadly similar to that of average crustal material
(Zhuang et al. 1992; Uematsu et al. 1985; Arimoto et al. 1989, 1990, 1992; Prospero 1981;
Prospero et al. 1987, 1989; Talbot et al. 1986; Graham and Duce 1979; Spokes et al. 1994).
Indeed, there is a remarkable consistency in composition of dust from specific source
regions. As an example, Fig. 2.11 presents data on the elemental composition of dust
samples collected from eight different large African dust events that passed over Barbados, West Indies, over a two year period. The figure shows the mean ratio of the
concentration of a specific element (Fe, Ti, Cd, Sb, etc.) measured in the dust to that of
Al in the dust; these mean ratios are plotted against the same ratio in average crustal
material (Taylor and McLennan 1985). Aluminum is used as the normalizing element
because it is a major component in the Earth's crust and there are no major anthropogenic sources of AI-rich aerosol; but other major elements could have been (and are)
used (e.g. Fe, Ti, and Si). As can be seen in Fig. 2.11, most elements fall very close to the
1:1 line, which suggests that the composition of the dust is very similar to that of average crustal material. Indeed, the composition of the individual samples shows a very
similar pattern, suggesting that there is very little difference in composition from one
dust event to another. When element ratios deviate markedly from the 1:1 line (for
example, As, Sb, and Cd in Fig. 2.11), it suggests that other sources or processes could
be influencing the sample, for example, pollution (Guieu and Thomas 1996; Chester
et al. 1996; Gullu et al. 1996). In comparing aerosol composition to hypothesized
sources, it is convenient to use enrichment factors (EF). The EF is based on the ratio