Al AND Fe IN PM 2.5 AND PM 10 SUSPENDED PARTICLES IN
SOUTH-CENTRAL FLORIDA: THE IMPACT OF THE LONG RANGE
TRANSPORT OF AFRICAN MINERAL DUST
JOSEPH M. PROSPERO1∗ , ILHAN OLMEZ2 and MICHAEL AMES3
1 University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, U.S.A.;
2 Fatih University, Beylikduzu, Buyuk Cekmece, Istanbul, Turkey
3 Environmental Research and Radiochemistry, Nuclear Reactor Laboratory, Massachusetts
Institute of Technology, Cambridge, U.S.A.
(∗ author for correspondence, e-mail: [email protected]; fax: +1 (305) 3614891)
(Received 6 May 1999; accepted 3 February 2000)
Abstract. Aluminum and iron were measured in daily samples collected at urban and rural sites
near Ft. Myers, Florida, in 1995–1996 using a dichotomous sampler. Al and Fe concentrations were
low during most of the year but they increased dramatically during summer when African dust was
advected into Florida. The ratio of fine (less than 2.5 µm diameter) to coarse (2.5–10 µm) Al and
Fe is relatively constant in African dust events with the fine accounting for a third to a half of the
total. Also the mass ratio of Al-to-Fe is relatively constant at 1.8, a value similar to average crustal
material. In contrast, in non-African dust the fine-to-coarse and Al-to-Fe ratios are extremely variable
and generally much lower than those during African events when dust concentrations ranged up to
86 µg m−3 . The timing and magnitude of the Ft. Myers dust peaks closely matched those measured
concurrently in Miami, 200 km to the southeast. Large areas of the eastern United States are frequently impacted by African dust every summer. Although dust concentrations can reach very high
values it seems unlikely that African dust events alone will cause a violation of the Environmental
Protection Agency’s standards for PM 2.5 or PM 10. However, African dust in conjunction with
emissions from local and regional sources could conceivably present a problem with compliance.
The probability of such an occurrence is heightened by the fact that dust concentrations are highest
in the summer when pollution levels are often at a maximum in the eastern states.
Keywords: aerosols, Africa, air quality, aluminum, iron, mineral dust, PM 2.5, PM 10, Sahara, soil
1. Introduction
Mineral dust has come to be recognized as an important aerosol constituent (Andreae, 1996). On a global scale, dust can affect the radiative properties of the
atmosphere (Tegen et al., 1997; Sokolik and Toon, 1996), it can serve as a reactive
surface for atmospheric gases (Dentener et al., 1996), and it is a major source of
minerals (Duce, 1995; Prospero, 1981, 1996) and associated nutrients (Prospero
et al., 1996) to the global ocean. There is considerable evidence that mineral dust
transported from sources in North Africa could also play an important role in air
quality in the eastern United States. Studies carried out on Barbados, West Indies
(13.17◦ N, 59.43◦ W), continuously since 1965 show that large quantities of mineral
Water, Air, and Soil Pollution 125: 291–317, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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dust are carried from sources in North African, including the Sahara, across the
North Atlantic every year by the trade winds (Prospero and Nees, 1986; Li et al.,
1996; Li-Jones and Prospero, 1998). Recently Prospero (1999) reported the results
of a continuous aerosol sampling program carried out at a coastal site in Miami,
Florida, for the period 1974–1996. These data show that large amounts of African
mineral dust are periodically carried into Florida every summer yielding daily
concentrations that are typically in the range of about 10–100 µg m−3 . Dust events
usually last at least several days and can extend over several weeks. Maximum dust
concentrations usually occur in July (monthly mean, 16.3 µg m−3 ) but relatively
high concentrations are also observed in June (8.4 µg m−3 ) and August (9.8 µg
m−3 ). Over the entire 23 yr period, dust concentrations varied considerably from
year-to-year in response to various meteorological and climatological factors. The
greatest dust amounts were obtained in 1983–1984 at the time of severe drought
in North Africa (Prospero and Nees, 1986). Despite the year-to-year variations in
dust transport, African dust was always the dominant aerosol constituent during
the summer months over the 23 yr of record.
The meteorological conditions associated with African dust advection into the
Caribbean have been well documented (Carlson and Prospero, 1972; Prospero and
Carlson, 1972; Karyampudi and Carlson, 1988; Westphal et al., 1987). Dust is
commonly carried in a layer that extends from the surface to roughly 3 km over
the western Atlantic (Karyampudi et al., 1999) and the southeastern United States
(Prospero et al., 1987; Gatz and Prospero, 1996). Isentropic air mass trajectories
(Merrill, 1994) show that during most of the month of August, trajectories consistently arrive from the southerly sectors; over the tropical Atlantic they hook to
the east, toward Africa. The pattern of these dust-bearing trajectories is consistent
with the general large-scale dust transport patterns observed on Barbados (Savoie
et al., 1989, 1992) and on Bermuda (32.27◦ N, 64.87◦ W) (Arimoto et al., 1992,
1995). These same patterns are observed in Miami every year in conjunction with
dust events. The temporal variability and the spatial scale of African dust events is
consistent with satellite depictions of aerosol distributions over the western North
Atlantic and the Caribbean. The AVHRR aerosol optical depth product (Husar et
al., 1997) and the TOMS absorbing aerosol product (Herman et al., 1997) show
huge ‘plumes’ of aerosol covering this region during much of the summer every
year. These plumes can also be cleary seen in the conventional meteorological
satellites such as GOES and METEOSAT (Karyampudi et al., 1999) and in the
ocean color satellite, SeaWiFS.
The advection of African dust into the United States must be considered in the
context of the National Air Quality Standard (NAAQS) for suspended particulate
matter (PM). The Environmental Protection Agency (EPA) recently established a
new standard (Federal Register, 1997) for particles 2.5 µm diameter and smaller
(henceforth, the PM 2.5 standard) that specifies an annual mean of 15 µg m−3
and a 24 hr mean of 65 µg m−3 (based on the 98th percentile of the frequency
distribution averaged over 3 yr). Previously the standard for suspended particulate
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
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Figure 1. Map of South Florida showing the locations of the sampling sites in Ft. Myers and Miami.
matter focused on particles having a diameter of 10 µm and smaller (PM 10). The
EPA, which is required by the Clean Air Act to set standards for air quality at
levels that protect public health with an adequate margin of safety, selected a size
threshold of 2.5 µm diameter based on studies that show that particles less than
this diameter can efficiently penetrate into the lungs (Wilson and Spengler, 1996)
and on epidemiological studies. During African dust events, the concentration of
PM 2.5 dust coupled with particles from local emissions could conceivably yield
aerosol concentrations that challenge the EPA’s PM 2.5 standard (Prospero, 1999).
Because African dust incursions are synoptic-scale events, African dust could affect a much larger region than south Florida. Recently Perry et al. (1997) used data
from the IMPROVE (Interagency Monitoring of Protected Visual Environments)
network to show that African dust has a significant impact on PM 2.5 air quality
over a large area of the United States. The effects are greatest in the southern states
but they are readily discernable in the central and northeastern states as well. Thus
African dust events could affect compliance with EPA standards across a broad
region of the country.
In this article we present the results of daily aerosol measurements of the PM
2.5 and PM 10 concentrations of Al and Fe made at two sites in vicinity of Ft.
Myers, Florida, and we compare them with concurrent measurements of mineral
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dust in Miami, 200 km to the southeast (Figure 1). Both Al land Fe are major soil
components; average crustal material contains 6–8% Al and 5% Fe (Wedepohl,
1995; Taylor and McLennan, 1985). The Ft. Myers aerosol data reveal several
periods in the summer of 1995 and 1996 when Al and Fe concentrations increased
dramatically. Aerosol data collected at a coastal site in Miami (Prospero, 1999)
show that mineral dust concentrations were very high during those same time periods. By comparing the differences and similarities in the concurrently-collected
samples from these sites, we are able to characterize the properties of aerosols
derived from local and regional sources in contrast to those advected across the
Atlantic from Africa.
2. Sampling Sites
2.1. F T. M YERS
Measurements were carried out at two sites in the Ft. Myers-Cape Coral area during 1995–1996. One site was at Terry Park, located adjacent to route 80, midway
between Route 25 and Route 41 on the south shore of the Calloosahatchee River
and on the northern side of the Ft. Myers urban center (Figure 1). We henceforth
refer to the Terry Park location as the ‘Urban’ site because of its proximity to the
cities of Ft. Myers and Cape Coral, which together have a population of about
150 000. A second sampling system was installed 16 km northeast of Terry Park
at Franklin Locks; here the population density is low and consequently we refer to
it as the ‘Rural’ site. At the Urban site sampling began on 2 June 1995 and ended
on 30 October 1996; at the Rural site sampling began later, 26 October 1995, and
ended at approximately the same time as the Urban site, 26 October 1996.
2.2. M IAMI
The Miami data are fully discussed in Prospero (1999). Because the sampling
techniques and analytical protocol are different from that in Ft. Myers, we briefly
discuss them here. Aerosol sampling is carried out on Virginia Key at the campus of
the Rosenstiel School of Marine and Atmospheric Science (RSMAS), University
of Miami, located at the water’s edge on the southern end of Virginia Key, about
4 km east of mainland Miami. During 1995–1996 the filter was mounted at the
top of a 16 m fold-over tower on top of a 12-m high building located 2 m above
mean sea level and 10 m from the water’s edge. The aerosol sampling system was
electronically controlled so that the sampler was activated only when the winds
blew from over the ocean and when the wind speed was greater than 1 m s−1 as
measured by a wind sensor mounted on the tower. The open-ocean sector extended
from approximately NE through E to S. The low-speed cut-off criterion is intended
to eliminate conditions when winds are light and variable. The only land within
the ocean sector is the island of Key Biscayne which lies to the SE and S of the
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
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RSMAS campus. Most of the island is set aside as park areas and, thus, the island is
largely covered with trees and shrubbery; the coast lines of much of the island are
densely covered with mangrove. A small residential community is located towards
the southern end of the island. There are no industrial activities on the island. The
only substantial types of pollution would be emissions from automobiles.
3. Experimental Protocols
3.1. F T. M YERS
At both the Urban and Rural sites, aerosol samples were collected by means of
automatic dichotomous samplers (Graseby/Anderson Instruments, Smyrna, Georgia). The inlet has a 50% cut-off size at 10 µm aerodynamic diameter; the second
stage is a virtual impactor having a cut point at 2.5 µm diameter. The two size fractions are collected on 37 mm diameter TeflonTM membrane filters (pore size 0.2 µm
diameter). Henceforth we refer to the 2.5–10 µm size sample as the ‘coarse’ fraction and the less-than 2.5 µm sample, the ‘fine’ fraction. The flow rate through the
sampler is 1 m3 hr−1 ; flow through the fine and coarse filters was 0.9 and 0.1 m3
hr−1 , respectively.
The sampling program began on 2 June 1995 at the Urban site. Initially each
sample was collected over a nominal 24 hr time period beginning at noon. On
26 October 1995 (the date when sampling was also begun at the Rural site) the
protocol was changed so that the sample duration was 12 hr with sample changes
at noon and midnight. Samples were changed automatically over the course of a
one week period. At weekly intervals, the filters were removed from the sampler,
the flow rate data for each channel was noted, and a new set of filters was installed
in the system.
Exposed filters were packaged in individual sealed petri dishes and shipped to
MIT for instrumental neutron activation (INA) analysis. A subset of samples was
selected for analysis and of these only a limited number of samples were analyzed
for both the fine and coarse fractions; most effort concentrated on the fine fraction.
For a detailed description of the neutron activation procedures, see Olmez (1989).
At MIT, sample processing was carried out in a class-100 laminar flow clean
hood. The filters were placed in HNO3-washed polyethylene vials which were
subsequently irradiated in the MIT Research Reactor (MITR-11) in a neutron flux
of 8×1012 n cm−2 s−1 . The samples were irradiated for 10 min and the emitted
radiation measured shortly thereafter to observe the decay of radioisotopes with
short half lives, ranging from 2.2 min (28 Al) to 15 hr (24 Na). After a ‘coolingoff’ period of 2–3 days the samples were irradiated again for a period of 12 hr to
enable the measurement of radioisotopes with long half lives. Blanks were included
with each sample batch along with samples of several standard reference materials from the National Institute of Standards and Technology (NIST): coal fly ash
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(SRM1633), mercury in sediment (RM8408), and orchard leaves (SRM1571). The
concentration of individual elements in the sample was determined on the basis of
the intensity of specific gamma rays that are characteristic for the element (Olmez,
1989). Gamma spectra from the irradiated samples and standards were measured
using four High Purity Germanium (HPGe) detector systems (Canberra Industries,
Meriden, CT) connected to a VAX 3100 workstation which runs interactive neutron
activation analysis software (Canberra Industries). Because the focus of this article
is on the role of mineral dust, at this time we report only on the results of the
analyses of Fe and Al.
3.2. M IAMI
The Miami protocol is presented in detail by Prospero (1999). Samples are collected daily except over weekends when samples are three-days long and over
holidays when samples may be longer. Bulk aerosol samples were collected by
drawing air through 20 × 25 cm Whatman 41 (W41) filters at a flow rate of about
1.1 m3 min−1 . The filter cassette is covered by a protective hat. The hat does not
significantly affect the collection efficiency of the filters with regard to particle
size. Comparisons were made of an upward-facing uncovered filter and a hatted
filter at a site in the Canary Islands, off the coast of Africa, where the particle
size distribution had a considerably larger fraction of large particles; there was no
significant difference in the amounts of mineral dust collected by the two systems
(Maring et al., 2000). The water-soluble ions are extracted with Milli-Q de-ionized
water. Dust is determined by ashing the extracted filter at 500 ◦ C for about 14 hr
(i.e., overnight) to destroy all organic matter. The weight of the ash residue less
that of the average blank is referred to as the ‘mineral ash’. Dust concentrations are
calculated using the ash weight multiplied by factor of 1.3 to correct for the loss
of soluble minerals during the extraction procedure and, during ashing, for the loss
of volatiles and combustibles (e.g., organics) and the breakdown of some mineral
phases (e.g., carbonates) (Prospero, 1999).
4. Results Ft. Myers
4.1. Al AND Fe CONCENTRATION
TRENDS
The time series of Al-fine (Alf ) and Fe-fine (Fef ) are shown in Figure 2 for both
the Urban and Rural sites. In general, concentrations are low except for several
periods during which concentrations increase sharply; as will be shown in a latter
section, these periods were concurrent with the appearance of high concentrations
of African dust at the Miami site.
Because operations at the Rural site began later than at the Urban site and
because sampling was more intermittent, the Rural site only captured a portion of
one major African dust event, the one that peaked on 7 July 1996. Figure 3 shows
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
297
Figure 2. The time series of PM 2.5 Al (Al-fine or Alf ) and PM 2.5 (Fe-fine or Fef ) for the Urban
sites (left side of figure) and Rural sites (right side of figure) in Ft. Myers.
a scatter plot of Urban-fine versus Rural-fine for both Al and Fe during periods
of concurrent operations. The four points along the 1:1 line are those obtained
during the single dust event in July 1996. There are a number of moderately high
concentration values that do not match up well. Especially notable is the Urban Alf
value of 991 ng m−3 which matches to a Rural value close to zero; this corresponds
to the isolated Urban Alf peak on 31 May 1996 (Figure 2). A value at 503 ng m−3
corresponds to the single-value peak on 14 February 1996; the matching value in
the Rural data is 70 ng m−3 . A third high value at 275 ng m−3 is matched in the
Rural data by one at 15 ng m−3 . Similarly in the Rural Alf data there are values at
344 ng m−3 (20 November 1995) and 324 ng m−3 (3 June 1996) that are associated
with single-point peaks; the corresponding samples from the Urban site are quite
low (28 ng m−3 and 35 ng m−3 , respectively) and within the limits of ‘background’
values. Because these dust events are short (i.e., they occur only during one 12–
24 hr sampling period) and because the events are not concurrent at both the Urban
and Rural sites, we conclude that these values are associated with localized dust
sources. The good agreement between sites during the large African dust event in
July 1996 and the poor agreement for other dust ‘events’ suggests that a good test
for the occurrence of an African dust event is the uniformity of dust concentrations
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Figure 3. Top: Scatter plot of Urban Alf against Rural Alf . Bottom: Scatter plot of Urban Fef
against Rural Fef .
across a network of two or more widely-separated samplers. This point is further
discussed in a later section.
4.2. T HE
RELATIONSHIP BETWEEN FINE AND COARSE AEROSOL
CONCENTRATIONS
In Figure 4 the concentrations of the fine fraction of Al (Alf ) and Fe (Fef ) are
plotted against the respective coarse fractions (Alc and Fec ) for both the Urban and
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
299
Figure 4. Scatter plots of the concentrations of the fine fraction of Al and Fe against the coarse
fraction for the Rural and Urban sites.
Rural sites (note the differences in the concentration scales). At the Urban site when
Alf and Fef concentrations are high there is an excellent correlation between the
two size fractions in both the Al and the Fe data; in contrast at the Rural site where
no dust events occurred during the time when both fine and coarse factions were
collected, the fine and coarse fractions are poorly correlated. In the Urban Al data
set, linear regression through zero yields a line with slope 0.455 (r 2 = 0.973). The
regression for Fef /Fec is essentially identical, 0.442 (r 2 = 0.951). Both the slope
of the lines and the high correlation are driven by 7 high concentration values. The
6 highest were obtained during the period 24–29 June 1995; the remaining value
was obtained on 9 July 1995.
Figure 5 presents the same data as in Figure 4 (urban site only) but with the
scales expanded at the lower end. In each panel the data are split into two groups,
one for the time period 2 June through 7 September 1995 (i.e., a ‘summer’ group
that encompasses the dust peaks in Figure 2) and the second group for the period
13 September to 29 November 1995 (i.e., a ‘fall’ group, when dust concentrations
were consistently low). In Figure 5, the ‘summer’ Al values yield a regression line
through zero with a slope of 0.459 (r 2 = 0.983); the regression is largely driven by
the high concentration Al-dust values seen in Figure 4 but which lie off the graph in
Figure 5. Above the principal dust regression line lies a series of 7 values obtained
during the period 7 June to 1 September; a line through these 7 values has a slope
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Figure 5. Expanded scale scatter plots of Urban concentrations of the fine fraction of Al and Fe
against the coarse fraction for the ‘summer’ season samples (filled square) and the ‘fall’ samples
(X’s). Top: Alf vs. Alc . The solid line is the linear regression through all summer samples, many
of which are off-scale (see Figure 5). The heavy dashed line shows the trend of the intermediate
‘summer’ dust concentrations; the fine dashed line shows the trend of the intermediate ‘fall’ dust
concentrations. Bottom: Same as top but for Fef and Fec . The solid line is the regression through all
‘summer’ samples.
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
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of 0.75. Thus, these two lines with slopes of (roughly) 0.5 to 0.8 encompass all the
high and intermediate summer dust Al values.
In contrast, the ‘fall’ Al values in Figure 5 are more scattered and a regression line through zero is not warranted. An unforced linear regression yields an
equation: y = –0.021 + 42.2 (r 2 = 0.0022). The dust in the ‘fall’ population is characterized by much higher concentrations of Alc particles relative to Alf particles,
a feature consistent with impacts from proximate sources. The topmost 7 data
points on this line were obtained from samples collected between 26 October
and 28 November 1995, 5 between 16–28 November. A line through these 7 values yields a Alf /Alc slope of 0.12. Thus, even at intermediate dust-Al concentrations, there appears to be a clear distinction between African dust and local
dust. The well-defined slope obtained from the November 1995 dust ‘events’ suggests that there may be local sources that could be responsible for moderately
enhanced dust concentrations and that these sources have a fairly well defined
ratio of Alf -to-Alc which, in conjunction with other factors, could serve as an
identifying characteristic.
Figure 5 also shows an expanded scatter plot of Fef against Fec with the values separated into ‘summer’ and ‘fall’ groups. The summer group yields a linear
regression that is essentially identical to that of the entire ensemble shown in Figure 4. In contrast to the Al data in Figure 5, the fall group is very widely scattered
and shows no coherent pattern. Thus the data in Figure 5 suggest that during the
‘fall’ months Al and Fe have different sources that are relatively independent of
one another.
The relationship between the fine and coarse fractions of Al and Fe is depicted
in a different way in Figure 6 which shows scatter plots of the Urban fine/coarse
ratios against the combined fine and coarse fractions of Al and of Fe. At concentrations less than several hundred ng m−3 the Urban fine/coarse ratios for both Al
and Fe are widely scattered; most values fall in the range from about 0.2 to 1. In
contrast, at high concentrations of Al and Fe (that is, during dust events) the ratio
fine/coarse sharply peaks at about 0.5. The Urban data suggest that dust events have
a characteristic ratio of fine/coarse for both Al and Fe and that the fine fraction (PM
2.5) dust mass is about 1/3 of the total dust mass. However, it should be noted that
fine and coarse fractions were collected for only one major African dust event,
the one occurring in late June 1995, and only at the Urban site. Also, as pointed
out in the discussion of Figure 5, low-concentration African dust events yielded
ratios between about 0.5 and 0.8. On the basis of this limited data set we can not
definitively conclude that the ratio of about 0.5 is applicable to all large African
dust events. Nonetheless, it is perhaps significant that the ratio is so stable over the
week-long period during which the large dust event occurred.
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Figure 6. Scatter plots of the fine/coarse ratio against the combined fine and coarse fractions for Al
and for Fe from the Urban site. The data sets are truncated on the ordinate to show more detail of the
ratio distributions in the dust peak.
4.3. Al- BASED
DUST TIME SERIES
Previous studies (Li et al., 1996; Li-Jones et al., 1998; Prospero, 1999) have shown
that African dust collected in the western North Atlantic yields an Al/dust mass
ratio of about 0.08, a value consistent with that reported for average crustal material
(Taylor and McLennan, 1985; Wedepohl, 1995). We can estimate dust concentrations based on the assumptions that Alf constitutes about 1/3 of the total Al (Alf
plus Alc ) concentration (as shown in Section 4.2, above) and that Al comprises 8%
of the total dust:
dustAl−f = (Alf ) × 3/0.08 = (Alf ) × 37.5
(1)
Using Equation (1), we construct the African dust time series for both the Rural and
Urban sites (Figure 7). In late June 1995 there was a large dust event that lasted
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
303
about one week; for four days, dust concentrations were in excess of 50 µg m−3 . In
the summer of 1996 there was a long period of enhanced dust concentrations. The
first event began on 4 July and lasted for four days. The second event began on 25
July and continued through 15 August. Thus for a period of about 25 days during
the summer, dust concentrations exceeded 10 µg m−3 ; because there are long gaps
in the data during this period, the number of dust days was actually greater as we
will show in a later section.
4.4. Al/Fe RATIO
TRENDS RELATED TO AEROSOL SOURCE
Previous studies of African dust events (Glaccum, 1978; Glaccum and Prospero,
1980) suggest that the mineralogical and elemental composition of dust in individual dust episodes are relatively constant. To investigate this aspect with the
Ft. Myers data, we use the full data set to generate a scatter plot of Urban Alf
against Fef (Figure 8). This shows a very good relationship at the higher aerosol concentrations, yielding a Alf /Fef ratio of 1.81 (r 2 = 0.932). At low aerosol
concentrations (Figure 8, bottom) the relationship is poor. The low-concentration
samples reflect the impact of local sources; these clearly have a much more variable
Alf /Fef composition which might reflect the variability of the soils or the impact
of other types of sources for these two elements. In particular, it appears that at
low Al and Fe concentrations, Fe becomes increasingly more important and more
variable relative to Al in the fine fraction. The constancy of the ratio Alf /Fef in
African dust is displayed more clearly in Figure 9 which is a scatter plot of the
Alf /Fef against the estimated dust concentration based on Alf using Equation (1).
The vast majority of the data points yield Alf /Fef ratios below about 1.5; these
are associated with low estimated dust concentrations. In contrast, the higher dust
concentrations (values above about 5–10 µg m−3 ) have ratios that are mostly in
the range 1.5–2.2 with the peak centered on 1.8. This ratio is consistent with that
measured for average upper continental crustal material: 1.84 (Wedepohl, 1995);
2.30 (Taylor and McLennan, 1985).
Note that during low dust conditions (below about 5–10 µg m−3 ), the use of
Alf as the estimator for mineral dust becomes less precise because of two factors:
the ratios of fine-to-coarse particles and of Al-to-Fe become much more variable
at low concentrations.
5. Comparison of Ft. Myers and Miami Aerosol Data
In this section, we compare on a day-to-day basis the dust concentrations at Miami
with those at Ft. Myers so as to characterize the spatial coherence of dust concentrations. Because the protocols were different at the two sites, it was necessary to
make adjustments in the data format. This is because most Ft. Myers samples were
collected over a 12 hr period. In contrast, at Miami the samples were collected over
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Figure 7. Ft. Myers dust concentrations calculated on the basis of Alf concentrations assuming that
the Alf /Alc ratio is 1:2 and that Al constitutes 8% of the mineral dust mass. Top: Urban site; Bottom:
Rural site.
a 24 hr period but there were many multi-day samples which ran over the weekends
or holidays. Also, the Miami samples were wind-sectored. Nonetheless, under most
conditions, the multi-day samples in Miami can be regarded as the average concentration over the sampling period; this is especially true during the summer when the
winds are consistently in the sampling sector because of the dominant trade-wind
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305
Figure 8. Scatter plot of the Ft. Myers Urban fine aerosol concentrations of Al against the fine aerosol
Fe concentrations. The top plot shows the full data set; the bottom plot shows a data subset of the
low concentration samples.
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Figure 9. Scatter plot of the ratio Al/Fe against the estimated dust concentration based on fine-particle
Al. The top panel shows the full data set; the bottom panel presents the same data with the ordinate
restricted to show more clearly the ratios at lower values.
conditions. To meld the data sets, blank lines were inserted for days in which there
were no Miami or Ft. Myers samples. For multi-day Miami samples, the average
for the period was inserted for the appropriate days. The data from both sites are
plotted as time series in Figure 10. In most cases, the major peaks are closely
matched in terms of the timing of the dust peaks at the two sites and the absolute
concentrations. The latter is especially impressive in light of the two very different
approaches used to estimate dust concentrations and the different protocols that
were used (that is, one based on the concentration of Al in the fine fraction and
one based on the ash weight of a total aerosol filter). The agreement between the
two data sets substantiates our assumptions about the relative constancy of the Al
fine/course ratio and that the Al concentration in African mineral dust is about 8%.
The data during the summer of 1996 are particularly impressive. Although the
dust concentrations were not very high during the summer of 1996 relative to the
long-term record (Prospero, 1999), they were substantial and they persisted over a
relatively long time period (almost two months). During this period, the concentrations varied considerably, producing many peaks. Both the Miami and Ft Myers
sites captured this variability rather well, most notably the events around 2–12 July
and 23–31 July. Indeed, where concurrent data are available at both sites, the four
major dust peaks (26–27 June 1995; 7–8 July, 25–26 July, 12–13 August, 1996)
are exactly coincident. This suggests that the dust-laden air masses traversed the
200 km between the two sites quite rapidly.
The very large seasonal variability in dust concentrations is clearly evident in
Figure 11 which shows the monthly mean concentrations at both sites. The same
TABLE I
Ft. Myers urban and Miami monthly mineral aerosol statistics
Month
Year
Ft. Myers – Urban
Miami
Estimated PM 2.5 dusta
1995
1995
1995
1995
1995
1995
1995
1996
1996
1996
1996
1996
1996
1996
1996
1996
1996
Estimated total dustb
Count
Median
Max
Mean
Count
Median
Max
Mean
Mean
28
5
5
4
5
28
8
2
11
6
1
9
4
28
19
19
19
0.75
0.59
1.48
0.57
0.38
0.44
0.41
0.38
0.45
0.48
0.39
0.73
0.31
2.09
1.79
0.26
0.16
28.75
6.06
2.72
3.09
0.47
3.47
0.96
0.48
6.29
0.94
0.39
12.39
1.10
19.68
10.17
4.85
4.85
4.51
1.65
1.28
1.06
0.34
0.54
0.46
0.38
1.02
0.55
0.39
1.94
0.44
4.95
2.87
0.66
0.62
28
5
5
4
5
28
8
2
11
6
1
9
4
28
19
19
20
2.25
1.78
4.45
1.70
1.14
1.23
1.23
1.14
1.34
1.44
1.16
2.18
0.94
6.26
5.38
0.77
0.42
86.25
18.17
8.16
9.27
1.40
2.69
2.88
1.45
18.87
2.83
1.16
37.18
3.29
59.04
30.51
14.54
14.54
13.62
4.95
3.85
3.19
1.02
1.30
1.37
1.14
3.05
1.64
1.16
5.82
1.31
14.86
8.62
1.98
1.77
15.81
8.97
8.61
4.78
0.6
1.39
0.98
0.81
2.07
1.64
1.56
1.01
1.45
14.73
7.48
0.88
0.91
1
2
3
2
4
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
June
July
Augustus
September
October
November
December
January
February
March
April
May
June
July
Augustus
September
October
Notes
a Estimated dust in the PM 2.5 fraction based on Al and assuming 8% Al content in dust.
b Estimated total (fine + coarse) dust based on PM 2.5 Al assuming a third of total in fine fraction.
307
1. Ft. Myers missed the high dust conditions in early July. 2. Ft. Myers had no samples for dust events in early and late August. 3. Ft. Myers
missed an event in late September. 4. Ft. Myers had only a few samples and one of these was a big ‘spike’ on 31 May.
308
J. M. PROSPERO, I. OLMEZ AND M. AMES
Figure 10. Dust concentration data from Ft. Myers and Miami plotted as a time series. Top:
June–December, 1995; Bottom: January-October, 1996. (Note the difference in the ordinate scales).
The Ft. Myers dust concentrations are estimated on the basis of the fine-particle Al concentrations;
Miami dust data are based on filter ash residue weights. Note that in the case of the dust event in late
July 1996, the data points for the Ft. Myers and Miami samples for 25–26 July lie directly on top of
one another.
data are presented in Table I along with the monthly maximum and median values.
Note that at Ft. Myers the sample counts for many months are rather low which
makes it difficult to make a valid comparison with Miami values. In particular the
Ft. Myers means are low relative to Miami in July, August and September 1995
because no samples were taken during some of the large dust events that occurred
during that period as can be seen in Figure 10. The agreement is excellent in 1996
when the sample continuity from the Ft. Myers Urban site is quite good.
Finally, it should be noted (in Figures 10, 11 and in Table I) that winter-spring
dust concentrations are low at both sites, typically around 1 µg m−3 . The monthly
mean dust concentration for the period October 1995 to April 1996 was 1.47 µg
m−3 for Ft. Myers and 1.30 µg m−3 for Miami. An exception is the period during
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
309
Figure 11. Monthly mean mineral dust concentrations at Ft. Myers and Miami. Ft. Myers dust
concentrations are estimated on the basis of the fine-Aluminum fraction as described above.
February 1996 when concentrations were relatively high at both sites; if the February values are excluded the mean for the Fall-Winter period is about 1.2 µg m−3
for both sites. Because the Miami sampling system is gated so as to sample only
when winds blow from the ocean, we would not expect to see substantial impacts
from local dust sources and, thus, low dust values are expected. But the low values
at Ft. Myers are surprising. Winter is the dry season in Florida (Henry et al., 1994).
It is also the agricultural season, when fields are plowed and cultivated across much
of the State. Also, wind speeds are often quite high in winter, especially with the
passage of vigorous cold fronts which typically begin in early winter and extend
into the spring. Despite this, the Ft. Myers Alf and Fef concentrations remain
low and relatively steady. The Ft. Myers data suggest that the impact of local dust
sources is small all year long, even during fall and winter.
6. Trajectory Studies
Air mass trajectories were computed for both the Miami and Ft. Myers sites using
the National Oceanic and Atmospheric Administration (NOAA) Air Resources
Laboratory (ARL) HYSPLIT (HYbrid Single-Particle Lagrangian Integrated
Trajectory) Model which was accessed through the NOAA ARL web site
310
J. M. PROSPERO, I. OLMEZ AND M. AMES
(http://www.arl.noaa.gov/ ready.html). Trajectories (96 hr) were computed for three
altitudes: 500, 1000 and 2000 m for 1800Z using the model version that incorporates vertical velocities. Several conclusions can be made from this brief study. On
most occasions the trajectories at Ft. Myers and Miami track one another fairly
closely; this is not surprising since they are separated by only 200 km. At both sites
high dust concentrations are associated with easterly and southerly trajectories. On
some occasions high dust concentrations are measured at Ft. Myers with westerly
winds, but the back trajectories eventually hook to the south and east.
Trajectories from the north are associated with low dust concentrations. This
suggests that the state of Florida itself and the continental United States as a whole
are poor sources of mineral dust that can be transported over great distances, a
factor emphasized by Perry et al. (1997). This is consistent with our earlier observation that at Ft. Myers dust concentrations were quite low during the winter when
soils were dry and when fronts passed through the regions. This conclusion can not
be stated so emphatically for the Miami data because of the fact that the sampler is
sectored to the open ocean; but the results from Miami are none-the-less consistent
in this regard with respect to northeasterly trajectories.
The trajectory studies are consistent with satellite imagery showing the time
progression of plumes across the Atlantic (Husar et al., 1997; Herman et al., 1997;
Chiapello et al., 1999). Over the western Atlantic the African dust plumes follow
two types of paths: one moves over the windward islands directly to the southeastern states; another travels deep into the Caribbean and subsequently moves
north over the Gulf of Mexico and either into the Central United States or to the
east over Florida and the southeastern states. The association between African dust
and satellite products is clearly shown in Chiapello et al. (1999) who compared
the TOMS satellite aerosol index (Herman et al., 1997) with concurrent measurements of mineral dust made at four sites in the North Atlantic: the Canary
Islands and Cape Verde Islands, located off the west coast of Africa; Barbados and
Miami. They found a very close correlation between major dust events as indicated
by high concentrations of dust and the TOMS index. Indeed, individual events
could be followed across the Atlantic in both the satellite product and the ground
measurements.
As an example of a typical dust event scenario, we briefly discuss the meteorological conditions associated with the large dust event that occurred on 22–23
June 1995 (Figure 10). The sequence described here is representative of summer dust events. For the few days prior to the event, conditions were relatively
stagnant; trajectories were slow and confused, and concentrated closely over the
very southern end of Florida. At both Miami and Ft. Myers, the lowest trajectory
(950 hpa, 500 m) trends more clearly to the NE. On 21 June, the trajectories at both
sites change dramatically, switching to the S at all three levels; at this time, dust
concentrations remain quite low. On the 23rd, the trajectories continue from the S
and dust concentrations start to rise at both Miami and Ft. Myers. The southerly
trajectories persist through the 25th and 26th when the dust concentration increased
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
311
dramatically. On the 27th, the dust concentrations reached their maximum at both
sites, over 80 µg m−3 ; at both sites, the trajectories at all levels arrive from the SW
but they hook strongly to the SE over Cuba. On the 28th, dust concentrations start
to fall; the trajectories arrive from the SW but, as on the 27th, they hook strongly
to the SE a short distance to the S of Florida. On the 29th and 30th, trajectories at
both sites are strongly from the N but, once again, there is a very pronounced turn
in the trajectories so that several days upstream the trajectories come from the S.
By this time, dust concentrations have decreased sharply.
During the winter, trajectories reflect the dominant air flow in Central and South
Florida which is from the NE except during the passage of cold fronts. As stated
earlier, these winds tend to carry very low concentrations of mineral dust. The
results of the trajectory studies for Miami and Ft. Myers are entirely consistent
with past trajectory studies of dust transport across the Atlantic (Savoie et al., 1992;
Merrill et al., 1994; Ellis et al., 1993).
7. Discussion
This study shows that large areas of south and central Florida are frequently impacted by advected air masses that contain high concentrations of African mineral
dust. Perry et al. (1997) show that the influence of African dust events can extend
over a large area of the central and eastern United States. In their study they assess
the temporal and spatial variability of PM 2.5 particle concentrations in a network
of approximately 70 sites located in national parks and wilderness areas during the
period 1992–1995. They note that the highest individual PM 2.5 dust 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 synoptic scale processes that are typical of incursions
of African dust. Perry et al. (1997) focus primarily on PM 2.5 concentrations and
composition characteristics. Although they do not present any data on fine/coarse
soil dust concentrations for the southeastern states, they do present data for a
limited number of African dust events observed in Shenandoah National Park,
Virginia, from March 1993 to March 1994. Five dust events yielded a fine/coarse
soil dust ratio of 0.8 (as estimated from their Figure 6). In comparison at Ft. Myers
we obtained fine/coarse ratios of 0.46 for Al and 0.44 for Fe (our Figure 4); lowconcentration dust events (our Figure 5) yielded values up to 0.8. For non-African
soil dust samples, Perry et al. (1997) obtained a value of about 0.2 (estimated from
their Figure 6). Our spread of values was larger than those of Perry et al. (1997)
(possibly because their measurements were made in national parks and were most
likely less impacted by local soil sources) but many fall-winter samples show a
clear pattern with a slope of about 0.13 (Figure 5).
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J. M. PROSPERO, I. OLMEZ AND M. AMES
The comparison of the Perry et al. (1997) fine/coarse concentration data and
ours is limited by two factors: first, they report data from a site in Virginia for
the period 1993–1994; second, our fine/coarse data only include one large dust
event and it is for the period 1995–1996 and a larger set of low dust concentration
events. Nonetheless the data presented here and that in Perry et al. suggest that in
African dust events a third to a half of the PM 10 soil dust mass falls into the PM
2.5 fraction. Our conclusion is consistent with other aerosol size measurements of
African dust made over the western North Atlantic. Li-Jones and Prospero (1998)
present the results of size distribution (cascade impactor) measurements made on
Barbados in April 1994 during which time four large African dust events occurred
(Li et al., 1996); 43% of the dust mass was less than 2.5 µm aerodynamic diameter
and 18% less than 1.25 µm diameter. Hardy et al. (1976) made size distribution
measurements with a 5 stage cascade impactor at three sites in the Miami area
during 8–18 July 1974. Independently, aerosol measurements had begun at the
University of Miami site in early 1974 (Prospero, 1999); these data show that
African dust concentrations were high during much of the Hardy et al. study, ranging from 2.3–33.0 µg m−3 (average 14.7 µg m−3 ). The African dust event was not
recognized as such by Hardy et al. at the time of their field study nor in the ensuing
publication although they do comment on the surprisingly high concentration of
crustal material which they attribute to local sources such as road dust. The size
distribution of Fe (computed from Table I, Hardy et al. 1976) shows that 32% of
the mineral dust mass was less than 2 µm diameter and 73% was below 4 µm
diameter.
Viewed in the context of the 23 yr of African dust measurements made at the
Miami site (Prospero, 1999), the dust events that occurred in 1995 and 1996 were
somewhat below average. The mean dust concentrations during the months of June
and July were 11.96 and 8.84 µg m−3 in 1995 and 1996, respectively. Over the
period of our entire Miami record, the June–July average was lower in only four
years: 1975, 8.75 µg m−3 ; 1988, 3.35 µg m−3 ; 1990, 6.94 µg m−3 ; and 1991,
8.21 µg m−3 . In contrast the highest mean values for June–July were obtained
in the period 1983–1986 and in 1993 with means in the low 20’s µg m−3 . Thus,
the presence of mineral dust during the summer months over the southeast United
States should be regarded as a normal situation, a point emphasized by Perry et
al. (1997) based on their more limited period of measurements, 1992–1995. The
incursion of African dust into south Florida is readily recognized by the very hazy
appearance of the sky which is normally quite clear in the summer. However,
in other regions of the southeast summer pollution events are common and the
appearance of haze and reduced visibility due to dust could be misinterpreted as
a pollution event. Dust can have a strong impact on visibility; in the Caribbean,
dust has forced the closure of airports (Li et al., 1996) because of poor visibility.
Thus it is important that local air quality personnel learn to recognize the meteorological indicators of African dust events (Prospero and Carlson, 1972; Carlson
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
313
and Prospero, 1972; Karyampudi and Carlson, 1988; Westphal et al., 1987, 1988;
Karyampudi et al., 1999).
Although dust concentrations can reach very high values, it seems unlikely that
an African dust event in itself will cause a violation of the PM 10 or PM 2.5
standards either on the basis of the annual standard or the 24 hr standard. During
the 1990’s (Prospero, 1999) the maximum dust concentration occurred on 29–30
June 1993, 121 µg m−3 ; if we assume that a third to a half of the dust is under
2.5 µm diameter, then the PM 2.5 concentration would be about 40–60 µg m−3 .
The highest dust concentration obtained over the 23 yr of measurements in Miami
was 149 µg m−3 , obtained on 16–18 July 1983; this would have yielded a PM
2.5 concentration in the range of 49–74 µg m−3 . Although dust concentrations
in themselves are unlikely to trigger a violation of the standard, dust in conjunction with emissions from local and regional sources could conceivably present
a problem. The probablity of such an occurrence is heightened by the fact that
dust concentrations are highest in the summer when pollution levels are often at a
maximum in the eastern states.
The EPA has the authority to discount or de-weight air quality data that are affected by ‘exceptional events’ caused by natural sources (Federal Register, Vol. 62,
No. 138, appendix K, 2.4a; 18 July 1997). In this category the EPA (Nichols, 1996)
specifically identifies volcanic eruptions, wild land fires, and high-wind events; the
last category implicitly addresses the possible impact of soil dust under the assumption that soil dust will be derived from local sources and that a direct association can
be made on the basis of the occurrence of high wind speeds. However African dust
incurrsions are not associated with high winds. Thus, in order to account for the
impact of such events, it will be necessary to develop a set of diagnostic indicators
for African dust.
As previously stated, the ratio of Al/Fe in the Ft. Myers samples collected
during dust events yields a characteristic ratio of 1.8. Perry et al. also report on
the ratio of Al/Fe during a widespread African dust event over the eastern United
States 23 June–7 July 1993; they obtain a ratio of 2.11. These values are close to
those obtained for average crustal material (Wedepohl, 1995; Taylor and McClellan, 1985). While the ratio of Al/Fe alone would not be a reliable indicator of the
presence of African dust, the constancy of the ratio across a network of sampling
sites would serve as strong evidence of subtantial dust impacts. The constancy in
composition has been noted for other elements as well. Perry et al. (1997) and
Gatz and Prospero (1996) showed that during African dust events the ratios of a
wide variety of other elements were quite constant and distinctly different from
elemental ratios observed during periods when African dust was not present.
Numerous studies in North Africa and the Mediterranean region have shown
that the elemental composition of African dust is relatively uniform and that the
concentration of many elements is quite similar to that of average crustal material (Schutz, 1989; Molinaroli, Guerzoni, and Rampazzo, 1993; Gullu, Olmez and
Tuncel, 1996; Guieu and Thomas, 1996; Chester et al., 1996). For example, of the
314
J. M. PROSPERO, I. OLMEZ AND M. AMES
first row transition metals, the concentrations of V, Cr, Mn, Fe, Co and Ni appear to
be close to average crustal values (Guieu and Thomas, 1996; Chester et al., 1996;
Gullu et al., 1996). In PM 2.5 African dust particles Zn can be as high as ten times
greater than in average crustal material (Gullu et al., 1996; Molinaroli et al., 1993)
but in some regions it is close to average crustal abundances (Schutz, 1989; Rahn
et al., 1979; Chester et al., 1996). In contrast, the concentrations of many elements
that are often associated with pollutant sources are often greatly increased in dust.
In particular Mo, Cd, As, Sb, and Se are enriched by factors ranging from about
20 to 500 (Gullu et al., 1996) and Pb, 42 (Molinaroli et al., 1993). Nonetheless,
it is not clear to what extent these enriched elements are due to pollution inputs
and how much is naturally a part of the dust matrix. The origin of the elements is
important because it is well established that the anthropogenic component is much
more soluble than the soil dust component (Chester et al., 1996).
Thus, on the basis of the limited data currently available, it seems that it should
be possible to develop a consistent set of diagnostic elemental ratio indicators and
particle size relationships for African dusts which, in conjunction with meteorological analyses and satellite products, could provide strong evidence of African dust
impacts and the temporal and spatial coverage of such impacts.
Finally, it should be noted that African dust events are often associated with
2−
+
increased concentrations of NO−
3 , nss-SO4 , and NH4 which are attributed to pollutant sources (Savoie et al., 1989, 1992; Prospero et al., 1995; Prospero, 1999).
Pollutants from Europe are advected over North Africa where they can mix and
react with dust raised from the surface (Dentener et al., 1996). Although the concentrations of pollutant species is relatively low they could be significant. For
example, the presence of acid species on dust could conceivably affect the solubility or biological availability of trace elements that might otherwise be rigidly
locked into the mineral particle matrix. However, studies thus far do not show
a strong effect. Consider the behavior of iron. African dust particles are heavily
coated with iron which accounts for the characteristic red-brown color of filters
samples collected during dust events. In cascade impactor samples, a considerable
portion of the nss-SO2−
4 and NO3 is found in the supramicrometer size fraction
(Li-Jones and Prospero, 1998). Even so, only about 6% of the total Fe content of
dust is easily extracted from dust samples under acid conditions (Zhu et al., 1992,
1997).
8. Conclusions
The Ft. Myers and Miami aerosol data show that African dust events can have a
dramatic impact on soil aerosol concentrations in south and central Florida. In the
absence of African dust, the concentration of soil aerosol was negligibly small.
The work of Perry et al. (1997) shows that the effects of African dust on air quality
can be discerned over much of the central and eastern United States. Although it is
PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA
315
unlikely that African dust events in themselves will cause a violation of EPA’s new
PM 2.5 standard, the concentration of dust combined with that of particles emitted
from local and regional sources could challenge the standard. The presence of
high concentrations of African dust will also complicate the assessment of sourcereceptor relationships. To that end, it will be necessary to characterize the elemental
‘signature’ of African dust and its temporal and spatial variability. The work carried
out thus far, although limited, suggests that the composition of African dust and
its size distribution are relatively uniform and could serve as identifying features
of dust events. These conclusions could be easily verified through a coordinated
study of aerosol composition in a network of sites in the Southeastern United States
during the summer months.
Acknowledgements
We thank L. Custals for assistance in processing the Miami dust samples and T.
Snowdon for operating and maintaining the aerosol sampling site in Miami. The
aerosol study performed in the Ft. Myers region was supported by Florida Power
and Light Company, Juno Beach, Florida. The Miami aerosol data were obtained
with partial support from National Science Foundation grant ATM-9414812 as a
part of the Atmosphere-Ocean Chemistry Experiment (AEROCE).
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