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OCR for page 3396
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3396-3403, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Long-range transport of mineral dust in the global atmosphere:
Impact of African dust on the environment of the
southeastern United States
JOSEPH M. PROSPEROUS
University of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, FL 33149
ABSTRACT Soil dust is a major constituent of airborne
particles in the global atmosphere. Dust plumes frequently
cover huge areas of the earth; they are one of the most
prominent and commonly visible features in satellite imagery.
Dust is believed to play a role in many biogeochemical
processes, but the importance of dust in these processes is not
well understood because of the dearth of information about
the global distribution of dust and its physical, chemical, and
mineralogical properties. This paper describes some features
of the large-scale distribution of dust and identifies some of
the geological characteristics of important source areas. The
transport of dust from North Africa is presented as an
example of possible long-range dust effects, and the impact of
African dust on environmental processes in the western North
Atlantic and the southeastern United States is assessed. Dust
transported over long distances usually has a mass median
diameter < 10 ,um. Small wind-borne soil particles show signs
of extensive weathering; consequently, the physical and chem-
ical properties of the particles will greatly depend on the
weathering history in the source region and on the subsequent
modifications that occur during transit in the atmosphere
(typically a period of a week or more). To fully understand the
role of dust in the environment and in human health, miner-
alogists will have to work closely with scientists in other
disciplines to characterize the properties of mineral particles
as an ensemble and as individual particles especially with
regard to surface characteristics.
There is increased interest in the properties of small airborne
particles (aerosols) because of the role that they play in many
environmental processes. Much of this interest stems from the
possible impact of aerosols on climate-related processes that
involve radiation and clouds. For this reason, aerosol studies
have focused on the chemical and physical properties of
aerosols that relate to radiation and to hydroscopic behavior.
For many years, research efforts have mainly focused on
anthropogenic aerosols, especially sulfate aerosols. Sulfate
receives most attention because humans have massively af-
fected the global cycle of atmospheric sulfur mainly through
emissions from combustion sources (the burning of fossil fuels,
oil and coal) and because the physical and chemical properties
of sulfate particles make them especially efficient for affecting
radiation Propagation in the atmosphere and clo~1 n~cle.~tion
(1~.
~ ~1 1 ' 1 ~
· ~ - - ~ --~ ~ ~ ~ < ^ ^ _ A, _ ~A. . ~ ^ ~ 4 _ ~ it. .~ 1 A
Humans have lived with pollutant aerosols for a relatively
short time in their history, since the beginning of the industrial
age. In contrast, humans have lived with wind-borne mineral
dust over the entire course of their history on earth. This long
history can be clearly read from the record retained in ice and
snow cores and in deep sea sediments. These show that the
PEAS is available online at www.pnas.org.
concentration of dust has varied over an extremely wide range
through time and that dust activity can change very abruptly.
Here, I focus on the present-day transport of mineral dust to
better understand the processes that affect transport and the
possible environmental effects.
Soil dust transport has a number of implications for humans.
At the most fundamental level, the history of human agricul-
ture has been closely tied to loess deposits. Most loess soils are
comprised of mineral particles that are largely derived from till
and outwash at the front of glaciers (24. Winds generate dust
clouds; the larger-sized particles (tens of micrometers in
diameter) because of their high settling velocity are deposited
relatively rapidly (generally within hundreds to a thousand
kilometers of the source) to form deep blankets of soil. Such
soils can be extremely fertile and easily tilled. They lie pre-
dominantly in the mid-latitudes, where weather tends to be
favorable for agricultural pursuits (relatively mild tempera-
tures, a long growing season, adequate rainfall). Massive loess
deposits are found throughout Europe and Asia in regions that
saw the development of many of the early civilizations. In the
new world, the loess deposits of the Midwestern United States
and in southern South America supported a highly productive
agriculture that facilitated colonization and rapid economic
development.
The generation and transport of dust are processes that
continue to this day, playing an important role in geochemical
and geophysical processes, including the addition of nutrients
to soils and to the oceans (1~.
More recently, there is a new focus on the effects of aerosols
on human health. The Environmental Protection Agency
(EPA) is required by the Clean Air Act to set standards for air
quality at levels that protect public health. To this end, the
EPA has established a new standard that focuses on particles
<2.5 ,um in diameter; only these small particles can efficiently
penetrate into the lungs. It has long been known that, in certain
industrial environments (e.g., mines, factories), there were
clearly identifiable health effects associated with specific min-
erals (e.g., silica, asbestos). However, a substantial fraction of
wind-borne soil dust is found in this "respirable" size range.
Thus, we must consider the possible impact of ambient mineral
dust on human health.
Because the most effective particles from the standpoint of
radiation, nucleation, and health are those with diameters of
a few micrometers or less, they will have a relatively long
lifetime in the atmosphere with respect to gravitational set-
tling in the absence of precipitation removal, on the order of
Abbreviations: EPA, Environmental Protection Agency; AOT, aero-
sol optical thickness; AVHRR, Advanced Very High Resolution
Radiometer; TOMS, Total Ozone Mapping Spectrometer; AI, aerosol
index.
*To whom reprint requests should be addressed. e-mail: jprospero@
. . .
rsmas.m~am~.eau.
3396
OCR for page 3397
Colloquium Paper: Prospero
several weeks. During this time, they can be transported
thousands of kilometers by winds. To assess the impact of such
particles on climate, biogeochemical process, and health, we
must have a good understanding of the sources of the particles,
the processes that affect dust mobilization and transport, and
the consequent global distribution.
I review various aspects of dust transport and show that
mineral particles can have a clearly discernable impact on
atmospheric properties at great distances from the source. I
discuss the global scale distribution of dust and the factors that
affect its mobilization and long-range transport, including
geological and geomorphological factors. Then, I focus on dust
transport over the North Atlantic, showing how dust transport
varies on a temporal and spatial scale. As an example, I present
data from South Florida that show that African dust comprises
an important part of the ambient aerosol, that it plays an
important role in air quality, and that it has implications
regarding geochemical processes and human health. Finally, I
summarize those aspects of long-range dust transport that are
poorly characterized and warrant further study.
The Global Scale Distribution of Mineral Dust
Studies performed over ' the past few decades have clearly
established that large amounts of soil dust are mobilized by
winds, mostly in arid regions, and that substantial quantities
can be carried great distances (reviewed in refs. 2-7~. Many
aspects of global scale mineral dust mobilization and transport
are summarized in an excellent review by Duce (8~. I (9) review
dust transport to the global ocean. Guerzoni and Chester (10)
present papers focusing on African dust transport with em-
phasis on the Mediterranean. The volume by Leinen and
Sarnthein (11) contains papers on dust studies relevant to
paleoclimatic interpretations.
Satellites can give an excellent picture of the transport of
dust and other aerosols on a global scale: e.g., Fig. 1 shows the
global distribution of aerosol optical thickness (AOT) as
estimated from the National Oceanic and Atmospheric Ad
Proc. Natl. Acad. Sci. USA 96 (1999J 3397
ministration Advanced Very High Resolution Radiometer
(AVHRR) (124. AOT is estimated from backscatter radiation
measurements made at an effective wavelength of 0.63 ,um;
high values of AOT usually indicate high concentrations of
suspended particles. Because the AOT algorithm requires that
the underlying surface has a low and constant albedo, AOT
measurements can only be made over oceans. Fig. 1 shows the
mean AOT distributions for July. The most prominent features
in the figure are the very large "plumes" of high values of AOT
that extend westward from the coast of Africa and eastward
from the Middle East. The plume emerging from the west
coast of North Africa is unambiguously attributable to dust
whereas that off the west coast of South Africa is attributed to
biomass burning. The high values of AOT over the Arabian Sea
are due to dust transported from the Middle East.
There are examples of other types of plumes that show
somewhat elevated values of AOT but are not nearly so
prominent, coherent, or persistent as the plumes attributed to
dust and smoke. In particular, in July, pollutant aerosols over
the North Atlantic appear as a plume that emerges from the
east coast of North America and the west coast of Europe and
also along the east coast of Asia. Nonetheless, the effects of
pollutant aerosols (as interpreted from AVHRR) are modest
in comparison to the dust and biomass burning aerosols.
Unfortunately, AVHRR (and other satellites operating in
the visible spectrum) cannot be readily used to obtain infor-
mation about specific source areas because of the restriction of
the algorithm to ocean surface retrievals. Recently, the Total
Ozone Mapping Spectrometer (TOMS) instrument has been
used for detecting absorbing aerosols based on the spectral
contrast at 340 and 380 nm in the upwelling ultraviolet
spectrum (13~. TOMS is sensitive to UV-absorbing aerosols
such as mineral dust, volcanic ash, and soot aerosol from fossil
fuel combustion sources and biomass burning. Because the UV
surface reflectivity is typically low and nearly constant over
both land and water (14), TOMS can detect aerosols over
continents as well as oceans (154. The UV spectral contrast can
be used in a nonquantitative way as an aerosol index (AI). The
Tauel000 |
8330
3
240
~210
51'.50 1
- 1 120 ~
_
a90 1
B~ 1
~_
FIG. 1. AVHRR aerosol optical depth, mean for July, 1988 through July, 1989. (R. Husar, personal communication; see also ref. 12.)
OCR for page 3398
3398 Colloquium Paper: Prospero
temporal and spatial variability of TOMS AI has been matched
to specific types of absorbing aerosols and with known sources:
for example, specific volcanic eruptions, forest fires, and large
dust events (15-17~. Fig. 2 a and b shows global distributions
of the frequency of occurrence of relatively high TOMS AI
values for the months of January and July, 1984 (15~.
A comparison of Fig. 2 with Fig. 1 shows that there is a good
conformity between the plumes located over the oceans. An
exact match is not expected because the data in Fig. 1 are mean
values of AOT, whereas Fig. 2 shows the frequency of occur-
rence of elevated values of TOMS AI. A1SO7 the AI is not a
quantitative measure of aerosol concentration. TOMS is sen-
sitive to a number of aerosol physical properties and, of most
importance, to the altitude of the aerosols layer (13~.
Dust Sources and Regional Characteristics
Fig. 2 also provides information about the distribution of dust
(and smoke) over land, and, of most importance, it yields clues
about the location of dust sources. The most striking feature
is that the greatest number of sources and the most active ones
are distributed across a band of arid regions that extends from
the west coast of North Africa, across the Middle East, and into
central Asia. "Because most dust sources have a strong sea-
sonal activity pattern, they will not necessarily be visible in the
a Stotistics of Smoke or Oust Covered Areas for Jon, 1984
... ..............................................................................................................................
..
., ., , . ~ ~
b Statistics of Smoke or Oust Covered Areas for Jul. 1984
................. ........................................................................................... ................
: : : : : : : : : : :
I=_ Z':~
, - or . Q, ~- , ... , ~. ~; ; it;
.. } _ ..........
>-
O. do.
: : :
_ ~
~0 On 0
0 0 0 0
}1 Or Days
FIG. 2. Statistics on the occurrence of high values of TOMS aerosol
index. The shaded area shows the number of days (lightest shading,
5-10 days; heaviest shading, 25-30 days) when moderate-to-high
concentrations of absorbing aerosol were detected by the TOMS/
NIMBUS-7 satellite. Ia) January, 1984. (b) July, 1984. (Figures
adapted from ref. 15, figure 4.)
Proc. Natl. Acad. Sci. USA 96 (1999J
two examples shown in Fig. 2. Many of the patterns discussed
here are visible in the figures presented in Herman et al. (~15),
where they show maps identical to those in Fig. 2 for each
month of 1984 and 1988.] In contrast, there are many arid
regions, including deserts, that do not show any significant dust
activity. It is notable that there are no prominent dust plumes
associated with any of the southern hemisphere arid regions,
including the deserts in Australia, the Kalahari and Namib
deserts in southern Africa, and the Atacama and Patagonian
deserts in South America. (Note also the absence of any
evidence of persistent absorbing aerosol production and trans-
port over the heavily industrialized and developed regions of
North America and Europe.) This is not to say that there is no
dust mobilization in these regions. Indeed, the daily TOMS AI
product shows relatively large-scale dust episodes in regions
where one might expect to see them for example, in the
southwestern United States (15) but the frequency is rela-
tively low and the AI values tend to be small.
The other striking feature in Fig. 2 is that some regions stand
out as persistent dust sources. Although many source regions
are obscured by dense dust plumes (for example, in West
Africa, where high concentrations of dust are present almost
continuously), many areas are clearly visible as well defined
geometric patterns that persist from year to year. These
features can often be linked to specific geographical locations.
For example, in Fig. 2 a and b, a large circular pattern is located
at the eastern end of the North African dust plume. This area
coincides with the Chad basin. Between 5,000 and 10,000 yr
ago, when the climate was more humid, Lake Chad was much
larger (Mega-Chad); its surface was 320 m above sea level
compared with 200 m today (18~. The floor of the former lake
now forms a monotonous desert plain with dunes, shallow
wadis, and salt flats (18~. The soils in this region are readily
deflated and dust storms are quite frequent throughout the
region (19~. In addition to anecdotal evidence, visibility re-
ports (20) confirm the extremely high frequency of dust storms
and low visibility in this region, and they show the same
seasonal pattern that is seen in TOMS. The dense Harmattan
dust clouds that affect the coastal regions of the Gulf of Guinea
during the winter have long been attributed to dust transported
largely from the Chad basin, especially in the northern region,
in the Bodele Depression (194. The TOMS AI product does
indeed show that this region is persistently active and is the
source of the consistently high AI values; furthermore, an
inspection of daily TOMS AI images shows that, within the
Chad basin, the highest AI values are obtained in the region
of the Bodele depression.
Other active areas are visible in Fig. 2 a and b. In northwest
North Africa, there is an active spot in Tunisia and northern
Algeria, on the south side of the Sahara Atlas mountains. Two
large chotts (salt lakes) are located in this region, which is part
of a large depression that was formerly an arm of the sea and
that extends 400 km westward from the Gulf of Gabes. The
chotts, which receive runoff from the mountains, are covered
with water only in the lowest areas, except after periods of
heavy rains. Thus, large areas of chott sediments are frequently
exposed and subject to deflation. The dust sources observed by
TOMS are most likely associated with these features. There is
a persistently active region in northeastern Libya and western
Egypt.
In West Africa, persistent dust features appear on the
western flanks of the Ahaggar mountains. In Fig. 2b, there is
a prominent area in northeastern Sudan between 30°E and the
Red Sea and flanked to the southwest by the Ethiopian
Highlands. Other well defined source patterns are visible on
the Arabian Peninsula extending up the Tigris-Euphrates
region. A persistent feature is seen in the Aral Sea region.
Finally, there is a well defined feature in western China, north
of the Indian subcontinent; this is located in the Tarim Basin
between the Tibetan Plateau and the Tian Shan Mountains.
OCR for page 3399
Colloquium Paper: Prospero
Many of the dust sources visible in TOMS have a sharply
defined geometry that appears to be determined by charac-
teristics of the topography. There has been considerable
discussion and debate for many years about the Eves of
_ _ ~ 1
terrains and environments that can serve as a source of fine-
grained dust. Pye (ref. 2, pp. 65-73) identifies (among others)
wadi sediments, lake and playa sediments, alluvial fans, and
alluvial floodplain sediments. Although field investigations
have validated these source types at specific sites, it has been
difficult to extrapolate to larger scales. For example, Herr-
mann et al. (19) summarize the reported locations of dust
activity in North Africa; these would appear to suggest that, in
effect, almost all of North Africa is a source of dust. This
observation may be literally true at ground level. But TOMS
suggests that, from the standpoint of large-scale dust events
and long-range transport, some sources are much more active
and effective than others.
The source terrains identified by the TOMS satellite appear
to be generally consistent with those listed in Pye (2~. These
implied source areas have a number of features in common.
They are located in arid regions. Many prominent sources are
found in well known ancient sedimentary basins (e.g., Mega-
Chad, Tigris-Euphrates) (18~. A common characteristic of the
TOMS sources is that they generally lie in topographical lows
that currently receive runoff from surrounding highlands. The
association of the TOMS-identified dust sources with such
topographical features suggests a mechanism that could ex-
plain the activity of these sources. Precipitation in the high-
lands weathers rocks and soils. Fine particles are carried
downstream to the basin and are deposited in river channels
and wadis; in the dry season, the deposits become exposed, dry
out, crack, and flake. When the wind velocity increases, the
disrupted soil surface is easily deflated, and clouds of fine-
grained dust are carried away (214.
Although many sources can be identified in TOMS, there
are large regions where the sources are obscured by persistent
dust clouds during much of the year in West Africa and the
Arabian Peninsula, for example. In some cases, TOMS shows
active sources that cannot be readily associated with topo-
graphical lows nor with readily identifiable sources of runoff.
Nonetheless, it is reasonable to assume that the dominant dust
sources in such regions will most likely have the same envi-
ronmental attributes as the visible sources cited above and
those listed in Pye (2~.
On the other hand, other types of hypothesized sources are
not validated. For example, Pye (2) specifically mentions stony
deserts and uses the Gobi as an example. TOMS does not show
the Gobi as a major dust source (see below). Furthermore, Fig.
2 shows that many regions that would seem to provide favor-
able conditions for dust activity are, in fact, free of significant
sources. This suggests that there are other factors involved. In
Australia, for example, the absence of significant dust activity
might be attributable to character of the soil particles. Kiefert
et al. (22) compare Saharan and Australian dusts. Australian
suspended dusts display particle size modes between 8 and 12
,um that are surprisingly uniform in time and space. In contrast
(and in agreement with the discussion above) dusts collected
in Mali (West Africa) were much finer, with a mode at 2-3 ,u m.
Australian soils and sediments tend to be highly aggregated,
with large quantities of clay pellets. On the basis of the TOMS
product, we conclude that aridity is a necessary, but not
sufficient, characteristic of dust sources.
It is also informative to compare the distribution of dust
sources with the distribution of loess deposits (e.g., ref. 2~. In
general, the geographical distribution of dust sources is dis-
tinctly different from that of loess deposits. This leads to the
conclusion that loessal soils do not seem to consistently supply
substantial amounts of dust to the atmosphere under present
day conditions. There are certainly some loess deposits, espe-
cially those in the People's Republic of China, that appear to
Proc. Natl. Acad. Sci. USA 96 (1999) 3399
be the source of major dust events (23~. Although these may
have a widespread and significant impact on air quality and
geochemical processes, they are sporadic events and they are
limited to the spring months. Also, dust activity may be
aggravated to a large extent by human activity, especially
agriculture. As pointed out above, in China, the Tarim Basin
is the most persistent feature in the TOMS AI. In contrast, the
Gobi Desert, which lies to the northeast of the Tarim Basin,
does not appear to be a major source of long-range dust,
contrary to common belief. This should not be too surprising
because the Gobi is predominantly a stony desert, with only 5%
covered with sand dunes; indeed, the name "Gob)" in Chinese
means "gravelly, pebbly plain."
=~~~~~~--- ~~~~r ~1- - ~
Processes in Dust Source Areas
In order for mineral dusts to be carried great distances, it is
necessary that a substantial fraction of the deflated dust has a
size under ~10 ,um in diameter. The limitation of the source
dust to this size range has implications regarding the weath-
ering processes by which particles of this size are produced and
also about the consequent physical, chemical, and mineralog-
ical properties of "long-range" dust. The association of dust
sources with topographical lows is consistent with the abun-
dance of fine-grained soil material. We would expect that the
soil particles will be highly weathered; the particle surface can
be physically abused, and it may have acquired surface coatings
(e.g., iron oxide deposits, adhering flecks of very fine clay
particles). In the case of clay minerals, the lattice structure may
be distorted through removal or substitution of elements. In
short. we should not expect to see fresh mineral entities.
Contrary to general belief, sand dunes are not usually good
sources of fine particles. This point is emphasized by the
observation made above that the TOMS sources are associated
with topographical lows that suggest that wet processes are
important. This is not to say that substantial amounts of dust
are not (or could not be) generated from sand dunes but,
rather, that other types of sources are clearly important.
The size distribution of the deflated dust is a strong function
of many factors, including the physical properties of the soil
matrix (e.g., the size distribution of soil particles, soil moisture,
cohesiveness), the condition of the surface (e.g., whether it has
been disturbed, the degree of protection afforded by vegeta-
tion), and the characteristics of the wind field above the
surface (21~. Studies of dust mobilization in soils from arid
regions show that particles < 10 ,um in diameter can be released
in large numbers. For example, measurements in Mali, West
Africa. show a bimodal distribution with one mode at 44 ,um
in diameter and another at 5 ~m; the large particle mode is
attributed to relatively localized sources, and the 5-~m mode
is attributed to long-distance transport (24~. Measurements in
dust storms in Tadzhjikistan show a substantial mass mode in
the size range <10 ,um; at a distance of 100 km downwind of
a dust source, the dust mass peak was in the range 2-5 ,um in
diameter (7~. In a study of windblown dust from agricultural
fields in the Pacific Northwest, Clairborn et al. (25) compared
size distributions during windy conditions with those during
nonwindy conditions. They found that concentrations in the
size range of 1-10 ,um in diameter increased by as much as a
factor of 5.
A substantial fraction of the wind-borne dust particles are in
the size range <1.0 ,um in diameter. These are released from
the soil by a "sandblasting" process (26~. SEM studies show
that the larger (1-10 ,um) airborne particles are often coated
with clay-like platelets (see also ref. 1~. The impaction of
wind-driven large particles dislodges the fine particles. Studies
in wind tunnels using different types of soils (27) show that the
concentration of submicrometer particles increases sharply
when the wind velocity attains a threshold value that imparts
OCR for page 3400
3400 Colloquium Paper: Prospero
enough momentum to impacting particles to effect the dis
lodgment of the fine particles.
Given the fact that the suspended dust has such a small mass
median diameter, we would expect that the composition will be
largely determined by the composition of the small particles in
the soil and that clay minerals will be strongly represented.
Schutz (28-29) reviews the literature on the mineralogical
composition of African dusts. He shows that the dominant clay
minerals are illite and kaolinite with additional amounts of
smectite, montmorillonite and chlorite. Quartz and calcite are
also important. As one might expect, there are regional
differences in the composition that reflect the regional differ
ences in geology and weathering processes. For example,
kaolinite is more prominent in dust from the low latitudes of
West Africa, and illite is more prominent in dust from the
northern regions, toward the Mediterranean coast; this lati
tudinal distribution is consistent with the idea that kaolinite is
a favored weathering product in warm, humid environments
(e.g., the tropics), whereas illite is a product of midlatitude
weathering conditions. This latitudinal distribution of illite is
observed in dust collected over the world oceans (3~. Schutz
also shows differences in the composition of eastern North
African dusts versus western dusts. Nonetheless, the dusts
derived from sources within relatively large regions have a
relatively uniform mineralogical composition. Caquineau et al.
(30) show that, in wind-borne dusts over the eastern tropical
Atlantic, the illite/kaolinite ratios fall into consistent patterns;
in contrast, the amounts of quartz can vary widely and show no
regional pattern. Although some regional mineralogical dif
ferences are noted, in general, these differences are not great.
As a consequence, it is difficult to identify unique character
istics that would allow for a straightforward and unambiguous
attribution to specific source regions (31-32~.
The elemental composition of North African dust is also
rather homogeneous. If one normalizes composition data to Al
or Fe (29), one finds that, for the vast majority of elements, the
ratios are quite close to those of average crustal abundances
(i.e., generally within a factor of about 0.5 to 2-5~. Much higher
ratios are found for some elements that are known to have
substantial anthropogenic sources (e.g., Zn, As, Sb, etch; also,
the size distribution of these elements is shifted strongly to the
submicrometer fraction, as one might expect for pollution
sources (33) in contrast to the soil-derived elements, which
peak in the supramicrometer size range.
The uniformity in dust composition reflects the fact that the
mobilized fraction of the soil is a highly weathered product that
has been derived from number of sources in the region.
Dust-sized particles in soils have gone through repeated cycles
of mobilization and deposition, mixing with materials from
other regional sources. Thus, dust that travels across the
Atlantic is the homogenized product of a long chain of
geological, weathering, and meteorological processes. so
African Dust Transport to Miami: An Example of Possible
Air Quality Issues
General Characteristics of Miami Aerosols. Thus far, I have
discussed the general character of dust sources and some of the
characteristics of the aerosols generated in those regions.
Here, I report on the properties of dust after it has been
transported across the North Atlantic, a distance of at least
5,000 km from the sources in North Africa. I focus primarily
on measurements made in Miami, Florida, where continuous
daily aerosol measurements have been made at a coastal site
since 1974 (34~. This data set may well be one of the longest
records of daily aerosol composition measurements in the U.S.
Data for the years 1989-1996 (Fig. 3) show a clear seasonal
periodicity, with the maximum dust concentrations in June,
July, and August. This temporal pattern is consistent with that
Proc. Natl. Acad. Sci. USA 96 (1999j
120
20
O
a', at ~ a, ~ ~ ~ it Hi, ~ ~ ~ be, ~ &, &, ~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ a, ~ ~ ~ (~' ~ C
Date
FIG. 3. Daily mineral dust concentrations in Miami, 1989 to 1996.
Measurements are made at a coastal site during on-shore wind
conditions.
seen in the satellite products, AVHRR (ref. 12; Fig. 1) and
TOMS (ref. 15; Fig. 2~.
Dust episodes usually extend over several days or more;
given the persistence of the trade-wind flow, this suggests that
the scale of the dust events is on the order of several hundred
to 1,000 km; this scale is consistent with satellite depictions of
aerosol distributions over the western North Atlantic and the
Caribbean (Figs. 1 and 2) (12, 15~. Thus, measurements of
aerosol concentration and composition in Miami should be
representative of a very large region.
The seasonal pattern of dust concentrations in Miami is
similar to that at Barbados, West Indies (13° 15' N. 59° 30' W),
where the University of Miami aerosol group has carried out
a continuous sampling program since 1965 (35-374. Fig. 4
shows the Barbados and Miami dust record for the period 1989
to 1996 (34~. The major differences between the records from
the two sites are that the dust concentrations are consistently
higher at Barbados and that the dust transport season on
Barbados is longer than that in Miami, where transport starts
later in the year and ends earlier. As a result, the annual mean
concentration at Barbados is ~2.5 times that in Miami (9~. The
mineralogical composition of dust collected in the western
Atlantic is identical to that collected off the coast of Africa
(38~; the dominant constituents are clay minerals and quartz.
Long-Term Record and Relation to Climate. Summer dust
transport has been a persistent feature throughout the 23 years
of measurements in Miami, 1974 to 1996, as shown by the
monthly mean dust concentrations in Fig. 5 (34~. Nonetheless,
50r
-
~i
cr. 30
-
cr
o
~ 20
Q
m
10
O
_ Mi~mi _ R_'rh~nc
W mu' ~ W ~ ~ _
I I I I I I I I I I I i l i l l l i l l I I I I I I I I I I I I I i l l l l l l l l l l I I I I I I I I I I I I i l l l l I I I I I I i l l l l l l l l I I I I I I I I I I I I I I i i l l l v
1 89 1 90 1 91 1 92 1 93 1 94 1 95 1 96
Year
FIG. 4. Monthly mean mineral dust concentrations at Barbados
and Miami for the period 1989 to 1996. (Note the difference in the
scales for Barbados and Miami.)
OCR for page 3401
Colloquium Paper: Prospero
40
35
30
, 25
20
<,, 15
._
10
0
U-) (D 1- 0 Cal) 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ O ~ N
he ~ 1- he ~ al ED Do 0 co 0 OD Gt) CD GO a) al a) a) a) a) a)
Year
FIG. 5. Monthly mean dust concentrations measured in Miami,
Florida, for the period 1974-1996.
there are very substantial variations in dust concentration over
this period. Concentrations were consistently high during the
period 1983-1987. The early 1980s was a time of severe
drought in North Africa. Previous work has shown that sum-
mer dust concentrations measured in the trade winds at
Barbados are anticorrelated with rainfall in the sub-Saharan
(Sahel) region of North Africa (35, 39~. In this regard, note that
the highest monthly mean dust concentrations in Miami were
obtained in 1983, the year after the onset of one of the most
intense El Nino events in recent history; a similar sharp
increase in dust also was observed on Barbados (354. The
longer-term variability of dust transport could be linked in a
complex way to other climate variables, such as the North
Atlantic Oscillation (40~.
Size Distributions. In the discussion of dust properties in
North Africa, it was noted that there was a clearly identifiable
mode in the mass distribution in the size range below 10 ,um.
Measurements over the western North Atlantic show that the
dominant size in the mass distribution is in the range of several
micrometers and that there is very little mass above ~10 ,um;
thus, the larger particles have been deposited during transit.
Studies on Barbados (41) and Miami (42) during African dust
events show that about one-third to one-half of the dust mass
was less than 2.0-2.5 ,um in aerodynamic diameter. In general,
the mass median diameter of mineral dust over the oceans is
typically 2-3 ,um (see ref. 8 for a review of the literature on the
size distribution of mineral dust over the oceans).
Distribution of African Dust in the Eastern United States.
The temporal and geographical extent of African dust trans-
port to the United States is nicely depicted by Perry et al. (43),
who studied PM 2.5 soil dust particle concentrations (based on
the measured concentration of Al, Ca, Fe, Si, and Ti in
twice-weekly daily samples) in a network of approximately 70
sites located in national parks and wilderness areas during the
period 1992-1995. They observed that the highest individual
PM 2.5 soil concentrations were associated with sites in the
eastern United States during the summer, not in the arid
southwest, as one might expect. Furthermore, there was a
large-scale coherence in the temporal variability of the high
PM 2.5 values, suggesting that they were associated with a
large-scale forcing process. The elemental composition of the
samples in these large-scale PM 2.5 events was distinctly
different from other types of soil-dominated samples; further-
more, the composition of the samples at sites in the eastern
United States during these events was identical to samples
collected in the Virgin Islands. These observations are con-
sistent with the hypothesis that the high PM 2.5 episodes were
associated with incursions of African dust. Indeed, the
progress of some of these dust incursions could be followed in
Proc. Natl. Acad. Sci. USA 96 (1999) 3401
the data as they moved from the Caribbean and Gulf of Mexico
into the southern states and across the northeast United States.
Independently, in a retrospective study of data from a field
program held in central Illinois in the summer of 1979, Gatz
and Prospero (44) noted the occurrence of unusually high
concentrations of Si, Al, and other crustal elements carried by
winds from the Gulf of Mexico. Concurrent mineral dust
measurements at Miami show that there was a strong influx of
North African dust at the same time. High concentrations of
African dust are also routinely observed on Bermuda during
the summer (45-46~.
Geological and Geochemical Implications of Dust Deposi-
tion. Geologically and geochemically significant amounts of
dust are deposited in precipitation. In Miami, rain collected
during intense summer dust events can have a turbid appear-
ance. When filtered, some rains yield a "cake" of red-brown
mud (47) which, when dried and rubbed with the fingers,
produces an extremely fine, rouge-like powder. In contrast,
during winter rain events, there is very little sediment, and the
sediment is often gray-colored and gritty, characteristics that
are associated with particles derived from pollution sources
and from local soils that often have a high content of calcium
carbonate. Measurements of dust deposition in precipitation in
Miami over a 1-yr period (1982-1983) yielded a total Al
deposition rate of 0.10 g m-2 yr-i (47), which is equivalent to
a mineral dust deposition rate (8% Al) of ~1.25 g m-2 arm.
This deposition rate is comparable to the long-term accumu-
lation rate of aeolian minerals in the deep sea sediments of the
tropical North Atlantic (47~.
When placed in the context of the meteorology and clima-
tology of the region, it is clear that dust events are large-scale
phenomena that can affect a very large region. This conclusion
is supported by other studies. Landing et al. (48) measured the
concentration of soil-related species (Al, Fe) in a network of
sampling stations in Florida from the panhandle to the Florida
Keys. Data show a well defined summer maximum at all sites,
a seasonality that is consistent with the summer dust maximum
observed at the University of Miami site (47~. Concentrations
were remarkably uniform during the summer months through-
out the state. The deposition rate of Al in precipitation
measured by Landing et al. (48) at five sites distributed over the
length of Florida in 1993-1994 ranged from 0.062 to 0.148
g m-2 yr-i (equivalent to dust deposition rates of 0.78 to 1.9
gm-2yr-~. Almost all of the deposition of soil dust takes
place during the summer months, which is also the rainy season
in Florida.
Dust deposition appears to be high throughout the western
North Atlantic (49~. Bermuda has an extensive cover of
fine-grained red clayey paleosols. Herwitz et al. (50) found that
the Zr/Y Zr/La ratios of these soils closely resembled that of
the <2-,um fraction of African dust; contributions from two
other hypothesized sources on North America (Great Plains
loess and Mississippi River Valley loess) could not be detected.
Muhs et al. (51) concluded on the basis of elemental compo-
sition that the soils on Barbados, Jamaica, the Florida Keys,
and the Bahamas that African dust was the most important
contributor to the soils on all of the islands.
Mineral Dust and the EPA PM 2.5 Standard for Suspended
Particles. There is renewed interest in aerosol properties
because of evidence that the exposure of humans to high
concentrations of airborne particulate matter can have a
detrimental effect on health (52~. Much evidence is based on
epidemiological studies of death rates and respiratory-related
hospital admissions (e.g., aggravated asthma, severe respira-
tory symptoms, and chronic bronchitis). Regions that have
relatively high mean concentrations of particles tend to have
higher rates of admissions than those with lower mean levels.
Short-term exposures also appear to have an impact, as
reflected in sharply increased admissions during severe pol-
lution episodes. Until recently, the EPA standard for sus
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3402 Colloquium Paper: Prospero
pended particulate matter focused on particles having a di-
ameter of 10 ,um or smaller. In July, 1997, the EPA established
a new standard for particles 2.5 ,um in diameter or smaller
(henceforth, the PM 2.5 standard) (52~. EPA's new PM 2.5
standard specifies an annual mean of 15 ,ug m-3 and a 24-hour
mean of 65 ,ug m-3.
However, as shown above, African dust is the dominant
aerosol constituent in south Florida during the summer.
African air masses would, in effect, bring into the region very
high PM 2.5 "background" aerosol concentrations. During
dust events, the concentration of dust, coupled with particles
from local emissions, could conceivably yield aerosol concen-
trations that exceed the EPA's recently implemented PM 2.5
standard. Thus, to account for the impact dust on air quality,
it will be necessary to develop a set of diagnostic indicators for
African dust, such as the mineral composition of the dust (38),
its elemental composition (43-46, 50, 54), or its morphological
characteristics (1~. As previously stated, African dust has a
remarkably uniform mineralogical and elemental composi-
tion. In contrast, locally or regionally derived dusts in the
eastern United States appear to have distinctly different
characteristics and a much more variable composition, as
suggested by the studies of elemental composition (9, 43-44,
484.
Mineral Particles and Health. The EPA standards for
airborne particulate materials are largely based on epidemio-
logical studies of specific at-risk communities in regions that
experience relatively high concentrations of anthropogenic
particles and other airborne pollutants. Although these studies
suggest that increased illness and morbidity are associated with
increased concentrations of airborne particles, it has not been
possible to unambiguously identify specific cause-effect rela-
tionships with specific components in aerosols nor to validate
mechanisms by which low concentrations of particles could
cause cardiopulmonary toxicity (55~. Amdur (56) and Utell
and Samet (55) review various hypotheses regarding mecha-
nisms; they present evidence that suggests that aerosols coated
with first row transition metals are especially efficient in
producing an inflammatory response in the lungs. In particu-
lar, they suggest that surface-complexed iron can generate
hydroxyl radicals in lung tissue and that these radicals have
acute lung toxicity. In this regard, it should be noted that
African dust particles collected on Barbados have a total Fe
content of 3.4% (57~. The dust particles are heavily coated with
iron, which accounts for the characteristic red-brown color of
filters collected during dust events. Under acid conditions,
6.2% of the total Fe content of the aerosol was readily soluble
(57~; thus, it might be expected that a substantial fraction of the
Fe on dust could be readily mobilized in the lung once the
particles are deposited on lung tissue.
The ease with which trace species are desorbed or extracted
from mineral particles will strongly depend on the specific
properties of the mineral particles and their weathering his-
tory. Consequently, we might expect that the health effects of
dust particles could vary greatly from region to region. Thus,
any strategy to address the health issues of mineral dust
particles will have to incorporate studies of the properties of
the mineral particles themselves.
Conclusions
The presence of high concentrations of mineral dust over such
large areas of the Earth has implications in many areas of
science: meteorology, climate, and biogeochemical processes.
To properly assess the role of dust in these processes, we must
have a better understanding of the properties of the airborne
soil particles. The mass median diameter of long-range dust is
generally <10 ,um, typically about several micrometers. The
soil particles in this size fraction usually show signs of severe
chemical and physical weathering. The particle surface may be
Proc. Natl. Acad. Sci. USA 96 (1999J
coated with oxides or salts, other particles (e.g., clays) could be
attached to the surface, or the particle itself might be made up
of an agglomeration of smaller particles. Consequently, par-
ticles will have very complex physical and chemical properties
that cannot be elucidated by simply studying the bulk proper-
ties of the dust; individual particle analysis will be required.
Dust generation is a highly nonlinear process that is very
sensitive to climate change. Indeed, the geological record
shows that dust mobilization has varied tremendously through
time. Recent research suggests that mineral dust also plays a
significant role in climate forcing (see, for example, refs.
59-63~. Thus, there could be feedback between the climate-
dust generation processes and the climate-forcing effects of
mobilized dust. To understand the role of dust in climate, it will
be necessary to develop models that can both characterize the
radiative effects of dust and predict the location and output
rates of dust sources as a function of climate. The modeling of
dust sources (58-61) is perhaps the most difficult task facing
climate modelers. To accomplish this goal, it will be necessary
for geologists and mineralogists to work closely together to
study the processes that affect dust mobilization and also to
relate the mineral properties of dust to the source terrains.
Finally, there is a renewed interest in the health effects of
fine particles as reflected in the new EPA regulations regard-
ing suspended particles <2.5 ,um in diameter. In many regions
of the world, mineral dust is the dominant aerosol constituent,
and, consequently, dust could constitute a widespread health
threat. Although some types of mineral particles (e.g., asbes-
tos, silica dust) are unambiguously linked to health issues, the
health effects of ambient soil dust are unknown. A number of
hypotheses focus on the chemical and physical properties of the
particle surface, but these remain unproven. Nonetheless, it is
clear that the investigation of health effects will require the
participation of mineralogists who can characterize the surface
properties of individual particles.
This work was carried out as a part of the Atmosphere/Ocean
Chemistry Experiment supported by the National Science Foundation,
Grants ATM-94-14812, ATM-94-14808, and ATM-94-14846.
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Representative terms from entire chapter:
dust sources
