Al and Fe in PM 2.5 and PM 10 suspended

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. PROSPERO 1∗ , ILHAN OLMEZ 2 and MICHAEL AMES 3

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: jprospero@rsmas.miami.edu; 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 (An- dreae, 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

J. M. PROSPERO, I. OLMEZ AND M. AMES

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

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

J. M. PROSPERO, I. OLMEZ AND M. AMES

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 dur-

ing 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

295 RSMAS campus. Most of the island is set aside as park areas and, thus, the island is

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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, Geor- gia). 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 Teflon TM 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 m 3 hr −1 ; flow through the fine and coarse filters was 0.9 and 0.1 m 3 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 HNO 3 -washed polyethylene vials which were subsequently irradiated in the MIT Research Reactor (MITR-11) in a neutron flux

of 8×10 12 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 ‘cooling- off’ 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 materi- als from the National Institute of Standards and Technology (NIST): coal fly ash

J. M. PROSPERO, I. OLMEZ AND M. AMES

(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 col-

lected 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 m 3 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 (Al f ) and Fe-fine (Fe f ) 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

Figure 2. The time series of PM 2.5 Al (Al-fine or Al f ) and PM 2.5 (Fe-fine or Fe f ) 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 Al f value of 991 ng m −3 which matches to a Rural value close to zero; this corresponds

to the isolated Urban Al f 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 Al f 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

J. M. PROSPERO, I. OLMEZ AND M. AMES

Figure 3. Top: Scatter plot of Urban Al f against Rural Al f . Bottom: Scatter plot of Urban Fe f against Rural Fe f .

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 (Al f ) and Fe (Fe f ) are plotted against the respective coarse fractions (Al c and Fe c ) for both the Urban and

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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 Al f and Fe f 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 Fe f /Fe c 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

J. M. PROSPERO, I. OLMEZ AND M. AMES

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: Al f vs. Al c . 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 Fe f and Fe c . The solid line is the regression through all ‘summer’ samples.

301 of 0.75. Thus, these two lines with slopes of (roughly) 0.5 to 0.8 encompass all the

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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 char- acterized by much higher concentrations of Al c particles relative to Al f 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 val-

ues yields a Al f /Al c 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 Al f -to-Al c which, in conjunction with other factors, could serve as an identifying characteristic. Figure 5 also shows an expanded scatter plot of Fe f against Fe c 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 Fig- ure 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.

J. M. PROSPERO, I. OLMEZ AND M. AMES

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 concentra-

tions based on the assumptions that Al f constitutes about 1/3 of the total Al (Al f plus Al c ) concentration (as shown in Section 4.2, above) and that Al comprises 8% of the total dust:

(1) Using Equation (1), we construct the African dust time series for both the Rural and

dust Al−f = (Al f ) × 3/0.08 = (Al f ) × 37.5

Urban sites (Figure 7). In late June 1995 there was a large dust event that lasted

303 about one week; for four days, dust concentrations were in excess of 50 µg m −3 . In

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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 Al f

against Fe f (Figure 8). This shows a very good relationship at the higher aerosol concentrations, yielding a Al f /Fe f 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

Al f /Fe f 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 Al f /Fe f in African dust is displayed more clearly in Figure 9 which is a scatter plot of the Al f /Fe f against the estimated dust concentration based on Al f using Equation (1). The vast majority of the data points yield Al f /Fe f 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 Al f 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

J. M. PROSPERO, I. OLMEZ AND M. AMES

Figure 7. Ft. Myers dust concentrations calculated on the basis of Al f concentrations assuming that the Al f /Al c 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

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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.

J. M. PROSPERO, I. OLMEZ AND M. AMES

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 Notes

PM Count

Estimated PM 2.5 dust a Estimated total dust b

Mean 2.5 AND

June 1995 28 0.75 28.75 4.51 28 2.25 86.25 13.62 15.81 July

1995 5 0.59 6.06 1.65 5 1.78 18.17 4.95 8.97 1 PM Augustus

1995 5 1.48 2.72 1.28 5 4.45 8.16 3.85 8.61 2 10 September

1995 4 0.57 3.09 1.06 4 1.70 9.27 3.19 4.78 3 SUSPENDED October

1995 5 0.38 0.47 0.34 5 1.14 1.40 1.02 0.6 2 November

1995 28 0.44 3.47 0.54 28 1.23 2.69 1.30 1.39 December

1995 8 0.41 0.96 0.46 8 1.23 2.88 1.37 0.98 P January

1996 2 0.38 0.48 0.38 2 1.14 1.45 1.14 0.81 AR TICLES February

1996 11 0.45 6.29 1.02 11 1.34 18.87 3.05 2.07 March

1996 6 0.48 0.94 0.55 6 1.44 2.83 1.64 1.64 April IN 1996 1 0.39 0.39 0.39 1 1.16 1.16 1.16 1.56

SOUTH May

1996 9 0.73 12.39 1.94 9 2.18 37.18 5.82 1.01 4 June

1996 4 0.31 1.10 0.44 4 0.94 3.29 1.31 1.45 July FLORID 1996 28 2.09 19.68 4.95 28 6.26 59.04 14.86 14.73

Augustus 1996 19 1.79 10.17 2.87 19 5.38 30.51 8.62 7.48 September

1996 19 0.26 4.85 0.66 19 0.77 14.54 1.98 0.88 A October

1996 19 0.16 4.85 0.62 20 0.42 14.54 1.77 0.91 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. 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.

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

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 Feb- ruary 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 Al f and Fe f 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

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 incorpor- ates 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 obser- vation 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

311 dramatically. On the 27th, the dust concentrations reached their maximum at both

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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); low- concentration 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).

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

313 and Prospero, 1972; Karyampudi and Carlson, 1988; Westphal et al., 1987, 1988;

PM 2.5 AND PM 10 SUSPENDED PARTICLES IN SOUTH FLORIDA

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 af- fected 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 assump- tion 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 McClel- lan, 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

J. M. PROSPERO, I. OLMEZ AND M. AMES

first row transition metals, the concentrations of V, Cr, Mn, Fe, Co and Ni appear to

Al and Fe in PM 2.5 and PM 10 suspended