74
J. M. Prospero
deposition of Fe in dust in this latitude band (which includes the Sargasso Sea where enhancedfixation is most prominent) is 2.8 x 1010 mol Fe yr- 1 • This amount of Fe could support an annual excess (i.e. "new") nitrate production of as much as 28 x 10 12 mol N yr- 1 if
all the Fe were available to the Trichodesmium. Even if only a relatively small fraction
of the Fe was available (about 10-25%), it could support a nitrate production comparable to the excess production observed in this region. This suggests that the excess nitrate in the North Atlantic may be largely controlled by dust transport and deposition.
If dust is playing a large role in nitrate nutrient production, then dust could cause
large interannual changes in nitrate production (and primary productivity in general)
in the North Atlantic. As shown in Fig. 2.9, there is a very large year-to-year variability
in dust transport; Prospero and Nees (1986) showed that this variability was highly
correlated to drought in sub-Saharan Africa (see also the inset in Fig. 2.9). Over the
past three decades, dust concentrations in the Trade Winds has increased by a factor
of three to four compared to dust transport during the mid-60s when rainfall rates
were higher. Over longer time scales, the variability could be much larger. Palaeoclimate
studies in North Africa show that this region has gone through extreme changes in
climate. About 6-8 kyr B.P., the region experienced a pluvial phase which must have
greatly reduced dust emissions. For example, the largest and most intense dust source
on Earth today is found in northern Chad, in the Bodele Depression (Prospero et al.
2002). During the pluvial phase, the Bodele Depression was filled with water forming
the palaeo Lake Chad, which covered a large area of the present -day country of Chad.
Today Lake Chad covers a very small area far to the south of the depression. Further
back in time, during glacial periods, dust activity must have been much greater than
that today. Ice core studies in Greenland show that during the last glacial maximum,
dust deposition rates were orders of magnitude greater than at present (Mahowald et al.
1999). Similarly, large changes in dust deposition have been observed in ice cores from
the Antarctic; dust most likely was transported from sources in Australia and southern South America (Prospero et al. 2002).
2.6
Other Aerosol Species and the Impact of Continental Sources
This paper has focused on a limited number of species found in marine aerosols. There
are many other species that could play an important role in the marine environment.
Of particular interest are organic aerosols. Compared to the aerosol species discussed
in this review, we know very little about the organic fraction. There are three major
classes of primary organic aerosols: biological particles, carbonaceous material emitted by biomass burning, and particles emitted from anthropogenic sources. Secondary organic particles can be formed from the oxidation of volatile organic carbon
(VOC) compounds that are emitted from biogenic and anthropogenic sources (Andreae
and Crutzen 1997; Scholes and Andreae 2000). The chemical characterization or organic aerosols is extremely difficult because of the huge range of compounds present.
Usually, only a small percentage of the total organic aerosol mass can be accounted
for (Peltzer and Gagosian 1989). An alternative approach has been to broadly characterize the total organic carbon (OC) and that of an important subset of organics - BG.
Both OC and BC can be measured relatively easily by a variety of techniques, although
the data are subject to considerable uncertainty (Heintzenberg et al. 1997; Jacobson
J. M. Prospero
deposition of Fe in dust in this latitude band (which includes the Sargasso Sea where enhancedfixation is most prominent) is 2.8 x 1010 mol Fe yr- 1 • This amount of Fe could support an annual excess (i.e. "new") nitrate production of as much as 28 x 10 12 mol N yr- 1 if
all the Fe were available to the Trichodesmium. Even if only a relatively small fraction
of the Fe was available (about 10-25%), it could support a nitrate production comparable to the excess production observed in this region. This suggests that the excess nitrate in the North Atlantic may be largely controlled by dust transport and deposition.
If dust is playing a large role in nitrate nutrient production, then dust could cause
large interannual changes in nitrate production (and primary productivity in general)
in the North Atlantic. As shown in Fig. 2.9, there is a very large year-to-year variability
in dust transport; Prospero and Nees (1986) showed that this variability was highly
correlated to drought in sub-Saharan Africa (see also the inset in Fig. 2.9). Over the
past three decades, dust concentrations in the Trade Winds has increased by a factor
of three to four compared to dust transport during the mid-60s when rainfall rates
were higher. Over longer time scales, the variability could be much larger. Palaeoclimate
studies in North Africa show that this region has gone through extreme changes in
climate. About 6-8 kyr B.P., the region experienced a pluvial phase which must have
greatly reduced dust emissions. For example, the largest and most intense dust source
on Earth today is found in northern Chad, in the Bodele Depression (Prospero et al.
2002). During the pluvial phase, the Bodele Depression was filled with water forming
the palaeo Lake Chad, which covered a large area of the present -day country of Chad.
Today Lake Chad covers a very small area far to the south of the depression. Further
back in time, during glacial periods, dust activity must have been much greater than
that today. Ice core studies in Greenland show that during the last glacial maximum,
dust deposition rates were orders of magnitude greater than at present (Mahowald et al.
1999). Similarly, large changes in dust deposition have been observed in ice cores from
the Antarctic; dust most likely was transported from sources in Australia and southern South America (Prospero et al. 2002).
2.6
Other Aerosol Species and the Impact of Continental Sources
This paper has focused on a limited number of species found in marine aerosols. There
are many other species that could play an important role in the marine environment.
Of particular interest are organic aerosols. Compared to the aerosol species discussed
in this review, we know very little about the organic fraction. There are three major
classes of primary organic aerosols: biological particles, carbonaceous material emitted by biomass burning, and particles emitted from anthropogenic sources. Secondary organic particles can be formed from the oxidation of volatile organic carbon
(VOC) compounds that are emitted from biogenic and anthropogenic sources (Andreae
and Crutzen 1997; Scholes and Andreae 2000). The chemical characterization or organic aerosols is extremely difficult because of the huge range of compounds present.
Usually, only a small percentage of the total organic aerosol mass can be accounted
for (Peltzer and Gagosian 1989). An alternative approach has been to broadly characterize the total organic carbon (OC) and that of an important subset of organics - BG.
Both OC and BC can be measured relatively easily by a variety of techniques, although
the data are subject to considerable uncertainty (Heintzenberg et al. 1997; Jacobson