deposition rate of particulate and dissolved aluminum...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. D12, PAGES 14,723-14,731, DECEMBER 20, 1987

DEPOSITION RATE OF PARTICULATE AND DISSOLVED ALUMINUI! DERIVED FROM SAHARAN DUST IN PRECIPITATION AT MIAMI, FLORIDA

Joseph M. Prospero and Ruby T. Nees

Division of Marine and Atmospheric Chemistry Rosenstiel School of •rine and Atmospheric Science

University of Miami, Miami, Florida

Mitsuo Uematsu

Center for Atmospheric Chemistry Studies, Graduate School of Oceanography University of Rhode Island, Narragansett, Rhode Island

Abstract. Precipitation was collected for a significant transport of mineral aerosol from 1-year period in Miami, Florida. The deposition Asian sources over a large portion of the North rate of A1 in samples containing Saharan dust was Pacific [Duce et al., 1980; Uematsu et al., 1983; 10.1 Bg/cm2; this flux is equivalent to a mineral Parrington et al., 1983]. In these ocean waters deposition rate of 126 Bg/cm 2 per year, a value the vertical distributions of dissolved A1 comparable to the mineral accumulation rate in [Hydes, 1983; Measures et al., 1984; Orians and sediments of the tropical North Atlantic. Bruland• 1985, 1986] and particulate A1 [Carder Mineral deposition rates in rain were highly et al., 1986; Uematsu et al., 1985a] suggest that variable, with 22% of the total occurring in ! atmospheric inputs of soil materials might be a day and 68% occurring in 4 days in two separate dominant factor in the A1 cycle in seawater. dust episodes. The volume-weighted average Saharan dust transport is of special interest concentration of dissolved A1 in dust-related because of the massive amounts of material that rain events was 9.8 •g/L; if normalized to the are transported over such a large area. total volume of rain that fell that year, the Knowledge of deposition rates is important in average would have a minimum value of 3.0 •g/L. light of evidence that the rate of deflation of The dissolved A1 fraction (defined as all A1 that soil material in North Africa is highly sensitive passes through a 0.45-•m filter) ranged from 0.5 to climatic factors [Prospero and Nees, 1986]; to 48%, with a volume-weighted mean of 5%; hence an understanding of the factors controlling solubility tended to increase with decreasing dust transport and deposition to the ocean would rain pH and decreasing mineral concentration. If assist in the paleoclimatic interpretation of the the 5% solubility applies to all Saharan dust sedimentary record [Prospero, 1985]. Nonetheless, deposited in the tropical Atlantic, then the no measurements have been made of deposition annual deposition rate of soluble A1 in this rates to the North Atlantic, despite the fact region would be 2-8 x 1011 g, a rate commensurate that there is a relatively large body of data on with that carried by the Amazon; on a global the atmospheric concentration of mineral aerosol. basis the annual input to the oceans of soluble All current literature estimates of dust A1 in dust would be 20-40 x 1011 g, a rate comparable to that carried by rivers. These atmospheric input rates of dissolved A1 are sufficient to account for many of the distribution features of A1 in the oceans.

Introduction

deposition rates are based on models which are of questionable validity when applied to atmospheric dust and which yield a wide range of values [Prospero, 1981; Chester, 1982; Uematsu et al., 1983].

A second source of difficulty in assessing the oceanic cycle of A1 derives from the uncertainty about the solubility of aerosol A1 in

Mineral dust is a major atmospheric precipitation (the dominant deposition pathway for most of the aerosol mass) and in seawater constituent in many oceanic regions [Prospero, [Moore, 1981]. To date there has been no 1981]. Over the North Atlantic, large adequate measurement of dissolved A1 in properly concentrations of Saharan dust are frequently

measured as far west as the Caribbean [Carlson collected precipitation samples. and Prospero, 1972• Prospero and Nees, 1986; In this report we present the results of a Prospero et al., 1981; Talbot et al., 1986] and study of mineral aerosol deposition in Miami [Savoie and Prospero, 1977; Glaccum and precipitation collected at Miami, Florida, over a Prospero, 1980; Prospero et al., 1979] and as far period of 1 year (mid-June 1982 until mid-June

1983). Saharan dust is the dominant mineral north as Bermuda [Chen and Duce, 1983]. African dust constitutes a major fraction of the component in the onshore winds at Miami during nonbiogenic component of the sediments in this summer months [Prospero et al., 1979; Prospero region [Prospero, 1981]. Also, there is a and Carlson, 1981; Savoie and Prospero, 1977; Carder et al., 1986]. When Saharan dust is

present in the atmosphere, aerosol filters Copyright 1987 by the American Geophysical Union. acquire a very distinct beige color after a few

hours of sampling; in the absence of Saharan Paper number 7D0754. dust, the filters are grey or black because of 0148-0227/87/007D-0754505.00 pollutants derived from local sources or advected

14,723

14,724 Prospero et al.' Deposition of Aluminum in Dust

over the ocean from the higher latitudes. X ray selected. These 18 events yielded a rain volume diffraction studies of beige-colored samples have of 29.6 L which constituted 31% of the total rain shown that the mineralogy is identical to that of deposited during the course of the yearlong study samples collected at BarBados and on islands and (151 cm). We then selected the aerosol filter ships off the west coast of North Africa [Glaccum samples for the periods corresponding to the and Prospero, 1980]; the mineralogy of Miami soil precipitation events. A quarter section of the aerosols is different from that of Saharan dust aerosol filters was extracted into 20 mL of in a number of respects, most obviously in the Milli-Q water in three washes [Savoie and much higher concentration of calcite. Prospero, 1982] and the extracted filter reserved

In this study we collected precipitation for A1 analysis; aerosol filtrates were not samples concurrently with aerosol samples. We analyzed for A1. A second quarter section of analyzed these samples for A1 and a number of each aerosol filter was extracted as described other chemical species that could affect the and then ashed at 500øC.; the residue is ascribed solubility of A1. At this time we present the to mineral matter. results of measurements of the deposition rates The A1 content of the precipitation filters of total A1 and of dissolved A1. and filtrate and of the extracted aerosol filters

was determined by neutron activation at the Rhode Procedures Island Nuclear Reactor Center [Uematsu et al.,

1983, 1985b]. Aliquots of filtrate were adjusted Aerosol and precipitation samples were to a nitric acid concentration of 0.15N to

collected continuously for a 1-year period on the minimize the adsorption of A1 on the container roof of a three-story building on the campus of walls during storage. Dissolved A1 was the Rosenstiel School of Marine and Atmospheric preconcentrated by coprecipitation with ferric Science, University of Miami. The site is hydroxide [Weisel et al., 1984a], a procedure located on the east coast of Virginia Key, which which also eliminated sea-salt interference. The lies about 6 km east of the mainland. detection limit for the dissolved A1 was 0.5 Precipitation was sampled on an event basis Bg/kg. The coefficient of variation for (except over weekends) using an automatic device duplicate samples (aerosol or precipitation) was that exposed the collection vessel (high-density less than 10%. polyethylene, 28.6-cm diameter) only when rain The pH of an aliquot of the precipitation was falling; this device is similar to that used samples was measured. The filter extracts and in the National Acid Deposition Program [Gibson, the filtered precipitation solutions were also 1984]. A total of 98 events were sampled. Of analyzed for C1-, NOB- and S04-, using ion these, we filtered 34 samples that were visibly chromatography, and for Na +, using atomic turbid or that were collected during conditions absorption; however, we do not present these that suggested that Saharan dust clouds might be results at this time. Precipitation data are present in the area; such evidence included the presented in Table 1. The term "dissolved Ai" presence of a beige color on the aerosol filters, refers to all A1 that passes through a 0.45-Bm the occurrence of local haze conditions, and Millipore filter. This operational definition of meteorological evidence including satellite "dissolved" is the same as that commonly used in images [Prospero and Car]son, 1981]. Samples the study of the concentration of dissolved and were filtered through prewashed 0.45-Urn Millipore particulate species in seawater. filters as soon as possible after the rain event; the elapsed time could be as long as 16 hours for Results rain events that occurred during the n•ght to 2 days for weekend events. Filtrates were stored _Deposition Rate of A1 and Mineral Dust at 4øC until the time of analysis.

Aerosol samples were collected [Savoie and The deposition rate of total A1 on an event Prospero, 1982] by drawing air through 20 x 25 cm basis was highly variable, ranging from 0.02 Whatman-41 (W-41) filters at a rate of about •g/cm 2 to 1.92 ug/cm 2. On the basis of the 18 1 m 3/min. W-41 filters have a relatively high analyzed events, the total deposition for the collection efficiency for soil particles; in the year was 10.1 Bg A1/cm 2. In the larger central North Pacific• tests with two in-line deposition events a layer of beige mud was filters show that 99.4% of the A1 in ambient clearly visible in the bottom of the collection aerosols is collected on the front filter (M. vessel and a well-defined cake was obtained when Uematsu, unpublished data, 1987). The A1 blank the sample was filtered. The major fraction of is 2.7 Bg per filter, which for a typical the annual deposition occurred in a small sampling volume is equivalent to an atmospheric fraction of the days' 22% on ! day (June 8, A1 concentration of about 1 ng/m 3. The sampling 1983); 68% in 4 rain event days that occurred apparatus was controlled by wind sensors, which during two Saharan dust episodes (June 23 and 24, activated the system only when the wind blew from 1982; June 8-10, 1983). Although we feel that we the ocean at a speed greater than 5 km/h. Aerosol sampled and analyzed all major dust deposition sampling duration was dictated by other experi- events, some significant events may have been mental protocols and ranged from 1 day to ! week. missed. Consequently, our value should be

We visually examined the precipitation sample regarded as a lower limit. filters from the 34 events. Filter colors ranged l]•e atmospheric concentration of A1 in from dark grey to the beige color that is a aerosols ranged from 0.08 to 2.62 Bg/m 3 and characteristic of Saharan dust. We selected for averaged 0.82 •g/m 3 on a volume weighted basis A1 analysis all filters that showed evidence of (Table 2). The ratio of the aerosol A1 Saharan dust, plus a few that yielded totally concentration to the aerosol filter ash weight grey or black samples. A total of 18 events were was relatively constant and averaged 0.080

Prospero et al.: Deposition of Aluminum in Dust 14,725

TABLE 1. Particulate and Dissolved A1 Concentration in Rain and A1 Deposition Rates

Rain Event Date

Total A1 Total A1 Ratio Sample Particulate Dissolved Deposition Deposition Dissolved/ Volume, A1, a A1, b Rate, Rate, Total Rain

mL •g •g •g/cm 2 % of Total % pH

1 June 23, 1982 2 June 24; 1982 3 Aug. 3, 1982 4 Aug. 6, 1982 5 Feb. 21, 1983 6 Mar. 8, 1983 7 Mar. 18, 1983 8 Apr. 26, 1983 9 May 31, 1983

10 June 1, 1983 11 June 6, 1983

1000 956.0 --- 1.49 14.2 --- 6.4 630 968.0 24.60 1.55 14.7 2.5 750 76.8 --- 0.12 1.1 6.7

1600 122.0 112.00 0.36 3.5 47.9 4.9 1600 61.8 25.30 0.14 1.3 29.0 3.9 3435 177.0 15.20 0.30 2.8 7.9 5.4 1680 112.0 6.77 0.19 1.8 5.7 5.2

30 44.4 --- 0.07 0.7 --- 1520 70.4 11.60 0.13 1.2 14.1 4.7 4660 232.0 20.50 0.39 3.7 8.1 5.0

155 9.3 1.70 0.02 0.2 15.5 4.6 235.0 3.53 0.37 3.5 1.5 4.6

1210.0 16.10 1.92 18.2 1.3 4.8 227.0 3.01 0.36 3.4 1.3 5.0 608.0 3.51 0.95 9.1 0.6 5.3 562.0 2.75 0.88 8.4 0.5 5.1 486.0 12.30 0.78 7.4 2.5 4.9

44.9 12.70 0.09 0.9 22.0 4.8

12 June 8 1983(AM) 495 13 June 8, 1983(PM) 1315 14 June 9, 1983 790 15 June 10, 1983(AM) 1545 16 June 10, 1983(PM) 655 17 June 13, 1983 2550 18 June 14, 1983 5180

aTotal particulate A1 in sample. bTotal dissolved A1 in sample.

(Figure 1), a value identical to that of average dust concentrations, shown in Table 2, were soil material [Taylor, 1964]. The highest A1 and computed on this basis. ash weights were associated with those filters The highest aerosol A1 concentrations and A1 that had the most pronounced beige coloration; deposition rates occurred during the summer the lowest weights were obtained from months when Saharan dust events generally occur predominantly grey or black filters. These in the Miami area. The high summer deposition results are consistent with our assumption that rate is also partially attributable to the fact the A1 in the aerosol (and presumably in the that in southeast Florida a third to a half of rain) is derived from mineral matter. On the the annual rainfall (140-150 cm) falls in the basis of the average A1 concentration of 8% in summer [Henry, 1983]. aerosol ash, the A1 concentration in rain can be The fact that the highest dust concentrations converted to an equivalent mineral weight by and deposition rates occurred predominantly in multiplying by a factor of 12.5; the equivalent the summer when Miami is under the influence of

TABLE 2. Aluminum and Mineral Dust Concentration in Aerosol Samples Collected Concurrently With Rain Events

Volume, A1, Dust ,a Date On Date Off m 3 Bg/m 3 Bg/m B

1 June 22, 1982 June 23, 1982 1423 1.260 11.24 2 June 23, 1982 June 24, 1982 1394 2. 620 27.37 3 Aug. 2, 2963 Aug. 3, 1982 1497 0.925 10.69 4 Aug. 5, 1982 Aug. 9, 1982 5423 1.950 21.12 5 Feb. 18, 1983 Feb. 21, 1983 3940 0.079 1.36 6 Mar. 4, 1983 Mar. 11, 1983 2544 0.15.2 3.51 7 Mar. 11, 1983 Mar. 18, 1983 2609 0.103 1.49 8 Apr. 21, 1983 Apr. 29, 1983 2464 0.519 8.39 9 May 27, 1983 Jun. 3, 1983 2524 0.164 2.20

10 Jun. 3, 1983 Jun. 10, 1983 2567 0.907 9.33 11 Jun. 10, 1983 Jun. 17, 1983 2416 0.293 3.34

a Based on filter ash weight.

14,726 Prospero et al.: Deposition of Aluminum in Dust

3.0

2.5

E 1.5 ._c 1.o E •: 0.5

0.0. 0

deleted, the dissolved fraction for the set is reduced to 0.026. Although we have no reason to believe that such adjustments to the data set are warranted, they do suggest that a value of 3 to 5% for the dissolved fraction is valid.

The concentration of dissolved A1 in precipitation appears to increase with decreasing pH (Figure 3). The pH of the rain is determined by a number of factors, principally by_ the concentration of the acidifying species NO B and nss S04 = and by the neutralizing effect of mineral species such as CaCO B in the dust [Loye-Pilot et al., 1986]. The average CaCO B

5 10 15 20 25 30 concentration of Saharan dust at Miami is 7% Mineral Aerosol (•g/m3) [Glaccum and Prospero, 1980]. Because the

concentration of aerosol NO B and nss S04 is Fig. 1. The A1 content of aerosols (as measured about twice as high in the winter as it is in the by neutron activation) versus the total ash weight summer, low pHs tend to be associated with low of the filter. dust concentrations.

Precipitation in relatively remote ocean regions generally has a pH of 5 or higher [Galloway et al., 1982, 1983]. If we consider

persistent easterly winds is taken as evidence only those Miami samples that had a pH above 5, that local sources do not contribute then the mean volume-weighted dissolved aluminum significantly to the total A1 deposition. In fraction is about 3%. contrast, we would expect local dust sources to Our dissolved A1 results are consistent with have the greatest impact during the winter, which those of Maring and Duce [1987] who measured the is the dry season and the time when agricultural seawater solubility of A1 in wind-borne Asian activity is at its peak in south Florida. Also, dust collected by filter on Enewetak. They found in the winter, cold fronts pass through the that about 5% of the A1 was leached after region at frequent intervals; the vigorous immersion in seawater (pH 8) for periods of up to westerly winds that are associated with these 6 hours. About the same amount was leached into fronts would be expected to enhance the impact of dionized water at pH 5.5 in 6 hours. Thus our local dust sources. Nonetheless, dust concentra- results and those of Maring and Duce suggest that tions and deposition rates are at a minimum in a substantial fraction of the A1 in soil aerosols the winter; in contrast, the concentrations of can be readily mobilized during relatively short --

NO B and non-sea-salt (nss) SO 4- in precipitation immersion times in a variety of aqueous at our site are at a maximum in the winter, a solutions. fact that we attribute to anthropogenic impacts [Savoie et al., 1987]. Discussion Dissolved A1 Total A1 Deposition Rates

The primary objective of this research was to The total A1 deposition rate in rain during characterize the deposition rate of total A1 in this study was 10.1Bg/cm2/yr (0.028 •g/cm2/day) rain. To this end we measured both particulate and the equivalent mineral deposition rate was and dissolved A1. The dissolved A1 results 126.2 Bg/cm2/yr (0.35 Bg/cm2/day). This rate is revealed some trends that warrant comment despite comparable to that of the nonbiogenic sediment the fact that the experimental conditions were accumulation rate in the central tropical North not ideally suited for a study of A1 solubility.

The ratio of dissolved A1 to total A1 in the precipitation samples is highly variable, ranging from 0.005 to 0.48. The average ratio for individual events is 0.11. However, the higher ratios are generally associated with the samples having the smaller total A1 concentrations (Figure 2). On the basis of the sum of the dissolved and particulate A1 weights in all rain samples, the average dissolved A1 fraction is 0.05.

The dissolved-to-total A1 ratio is relatively insensitive to extreme values in the sample set. A number of samples yielded total dissolved A1 values of only a few micrograms. We could assume that there was a systematic bias in our blank corrections; if we set the dissolved A1 values of these samples (samples 7,11,12,14,15, and 16) to zero, the dissolved-to-total A1 fraction for the

Atlantic: 4 mm/Kyr [Ku et al., 1968] or 0.55

•- 5o

• 40

•- 30 , I

o 25 ! 2o

•,15 • 0 10 oo •n 5 e • O. : :• : : ; e,• : • , , .,e .

0.0 0.2 0.4 0.6 0.8 1.0 1.2 TOTAL ALUMINUM (rag/sample)

set is only reduced to 0.047. Furthermore, there Fig. 2. The dissolved A1 concentration in is one sample (sample 9) that yielded a very high precipitation expressed as a percentage of the dissolved A1 concentration. If this value is total A1 concentration in the sample.


20

18 70 '•16 ß

ß --• 12 ß

•10

• 8 ee 0 6 -- 4 ß

:2 ß ß

3.5 3.7 ,:3.9 4.1 4,3 4.5 4.7 4.9 5.1 5.3 5.5

pH

rain drops on the surface was relatively dense (with a mean spacing of several millimeters) but relatively few touched. When the drops dried, they left behind perfectly formed extremely fine- textured beige mud spots that were recognizable as Saharan dust. (We had known that high concentrations of Saharan dust were in the area

from satellite photographs and also from the dense beige coloration of the aerosol filters collected at that time. The mineral aerosol

concentration during the course of the dust event was unusually high, ranging from 31 to 71 Hg/m3.) Dust was recovered from the surface of the car by making repeated sweeps with a moistened Whatman- 41 filter wrapped over the edge of a moistened

Fig. 3. The dissolved A1 concentration in cellulose sponge. The filter, which developed a precipitation versus the pH. deep beige color as the wiping progressed, was

subsequently extracted with water and ashed; it yielded a residue of 68.2 mg from an area of 1 m2 The dust deposition rate in this one event

Hg/cm2/day (assuming a typical in situ bulk was 6.8 Hg/cm 2, roughly 20 times the average sediment density of 0.5 g/cm3). Such high measured daily wet removal rate. No measurable deposition rates suggest that Saharan dust could rain occurred during the dust event. We have also be a significant contributor to soil subsequently noted that cars are frequently formation on some islands in this region, many of spotted with reddish mud because of events such which are developed on exposed coral platforms. as these. Brief showers could be especially The rate of dust deposition is probably greater important for the deposition of Saharan dust in the lower latitudes and closer to Africa because the meteorological conditions associated because of the increased dust concentrations in with dust outbreaks tend to suppress convection these areas [Savoie and Prospero, 1977; Schlitz, and hence reduce the frequency and intensity of 1980]. rain events [Prospero and Carlson, 1981]. Under

It should be noted that the deposition rates such conditions the occurrence of brief rain reported here could be higher than average. events such as that described here would be North Africa has experienced drought in varying favored. degrees since 1968. Aerosol measurements made in It is conceivable that some fraction of the Barbados since 1965 show that mineral aerosol dissolved and particulate A1 that we collect concentrations have increased markedly since the could be derived from spray ejected from the sea onset of drought [Prospero and Nees, 1986]. The surface by ruptured bubbles. A number of trace drought was especially severe in 1983 and 1984, metals are known to be enriched in spray droplets when dust concentrations at Barbados were at relative to bulk seawater [Weisel et al., 1984b]; unprecedented levels, about 4 times the average for A1 the mean enrichment factor (EF) is about concentrations measured prior to the drought. 5000. (The EF is defined as the ratio of the Unfortunately, we do not have as extensive an aerosol concentration of A1 to Na divided by the aerosol record of Saharan dust at Miami, and we concentration ratio of the same elements in cannot compare the measurements reported here seawater.) In contrast, the EFs computed for our with any predrought values. However, comparison soluble A1 fraction alone in rain were generally with measurements made sporadically in Miami about 100 times greater than those reported by since the early 1970s suggests that the 1982-1983 Weisel et al. (Table 3). We can estimate the values are indeed higher than average. worst-case impact of spray-derived A1 by assuming

We have made no effort to collect that the EF for A1 in Miami sea-salt aerosol is

dry-deposited mineral aerosol: because there are the same as that measured by Weisel eta!. and no satisfactory techniques for making such that all the A1 derived from the rupture of collections [Hicks et al., 1980; Dolske and Gatz, bubbles is dissolved. The equivalent bubble- 1984]. The dry-deposition velocity is estimated spray-derived dissolved A1 computed on the basis [Sehmel, 1980] to be about 0.2 cm/s, based on the of the Na content of the precipitation, is shown size distribution of Saharan dust in the Miami in Table 3. Except for three samples, all of area (mass median diameter of 2 •m, Savoie and which yielded low deposition rates, the Prospero, [1977]). The mean annual mineral contribution of A1 from bubble spray is at most aerosol concentration in Miami during onshore several percent; of the three exceptions, only winds for the late 1970s (the last period for one had an unusually high ratio of dissolved-to- which we have extensive data) was about 6 Hg/mB; total A1 (June 14). On the basis of these therefore the computed dry-deposition flux should calculations, we conclude that in the tropical be 0.1 Bg/cm2/day, about a quarter of the wet- North Atlantic, A1 derived from sea-salt spray deposition rate. could not have a significant impact on the total

It is possible that significant amounts of A1 flux or on the dissolved component of A1 in dust are being deposited in brief showers that do aerosols or precipitation. not activate the automatic collectors. Such

showers are quite common in Miami and other Dissolved A1 Deposition Rates tropical regions. On one occasion (July 31, 1982), a brief shower occurred shortly after one The volume-weighted concentration of dissolved of us (JMP) had waxed his car. The coverage of A1 for all analyzed samples was 9.8 Bg/L. If we


TABLE 3. Enrichment Factors for Dissolved A1 in comparison, the annual discharge of dissolved A1 Miami Rain (Relative to Na) and the Possible in streams to the oceans is estimated to be in

Impact of A1 Derived From Salt Spray the range 4-29 x 1011 g [Stoffyn and Mackenzie, 1982]; moreover, much of the river-borne A1 would not reach the open ocean because a substantial

Spray A1 Spray A1 fraction of the dissolved A1 could be for b /Dissolved precipitated in estuaries and coastal regions c

Rain

Event EF(A1/Na) EF = 5000, A1, Date /1000, a Bg/L %

June 23, 1982 --- June 24, 1982 1222 0.2 0.4 Aug. 3, 1982 ...... Aug. 6, 1982 1328 0.3 0.4 Feb. 21, 1983 124 0.6 4.0 Mar. 8, 1983 141 0.2 3.5 Mar. 18, 1983 29 0.7 17.2 Apr. 26, 1983 0 4.9 --- May 31, 1983 327 0.1 1.5 June 1, 1983 209 0.1 2.4 June 6, 1983 209 0.3 2.4 June 8, 1983 194 0.2 2.6 June 8, 1983 362 0.2 1.4 June 9, 1983 239 0.1 2.1 June 10, 1983 50 0.2 10.1 June 10, 1983 212 0.1 2.4 June 13, 1983 359 0.1 1.4 June 14, 1983 78 0.2 6.5

[Sholkovitz, 1976]. The A1 solubility value of 3-5% reported here

is considerably larger than the value of 1% that has commonly been used in assessing the impact of eolian material on the A1 cycle in seawater. The latter value is based on the one known study of dissolved A1 in precipitation [Hodge et al., 1978] and is suspect for a number of reasons. Hodge used continuously open buckets, a collection technique that is now known to yield anomalously high and unrepresentative deposition samples [Hicks et al., 1980; Dolske and Gatz, 1984]. Also, because sampling periods ranged from about 2 weeks to 3 months, extensive reactions could have occurred in the samples.

There is reason to believe that the bucket-collected material in the Hodge study was predominantly dry-deposited local dust. First of all, precipitation fell only during 6 of the 13 collection periods; nonetheless, the A1 deposition rates for samples during which no rain fell were not appreciably different from the rates for samples where rain did fall. Normally, wet-deposition fluxes are at least several times

aEnrichment factor (EF) for dissolved A1 in rain greater than those for dry deposition [Slinn et computed relative to Na and assuming an A1 al., 1978]. The deposition velocities that we concentration of 0.35 Bg/L in seawater. compute from the Hodge data are 2.7 cm/s and 3.9

bConcentration of total A1 in rain that would be cm/s. These values are extremely high (7-10 derived from seawater bubble spray, assuming an A1 times the total deposition velocity measured at enrichment factor of 5000 [Weisel et al., 1984b]. Miami, 0.40 cm/s) and equivalent to the Stokes

CRatio (in percent) of computed spray-derived A1 settling velocity of a spherical mineral particle (assuming an EF = 5000) to the measured dissolved of about 20-Bin diameter. These high velocities A1 in rain sample. suggest that the samples consisted predominantly

of very large particles derived from proximate sources. Such large particles would have short atmospheric residence times and, consequently,

assume that the remainder of the rain that fell minimal exposure to atmospheric chemical at Miami (but was not analyzed) contained no processes that could affect solubility. Also, dissolved A1, then the mean dissolved A1 because of the low surface-to-volume ratio, only concentration would be 3 Bg/L. In comparison the a small fraction of the A1 would be readily dissolved A1 for the Amazon is 20-60 Bg/L; •or leachable from such large soil particles. some California streams, 1-10 Bg/L; and springs, There is little information available on the 10-80 Bg/L [Stoffyn and Mackenzie, 1982]. We can solubility of trace metals in carefully collected use the Miami data to compute a lower limit precipitation samples. The most extensive effort

deposition flux of so lU•ole A1 to the North was that of Gatz et al. [1984], who Atlantic between 0 ø and 3 N; for concentrations simultaneously collected wet-only, dry-only, and of 3 and 9.8 Bg/L and an annual rainfall of 100 bulk deposition samples to study the solubility cm [Rao et al., 1976{, the rates would be 6 x of trace metals as a function of a number of 1010 g and 2 x 10 1 g A1/yr respectively. parameters, including precipitation pH and the Alternatively, we can assume t•at the 3-5% A1 clay mineral concentration. The solubility solubility measured in Miami applies to all trends observed by Gatz et al. are similar to Saharan dust deposited in this region (variously those obtained by us for A1 and further help to estimated to be in the range of 0.6-2 x 10 1• explain the discrepancy between our A1 results g/yr, Prospero [1981]); on this basis the annual and those reported by Hodge et al. All of the deposition rate of soluble A1 is 1.4-4.8 x 1011 g metals were relatively soluble in the wet-only to 2.4-8 x 1011 g. By way of comparison, the samples; in contrast, the metal solubilities in Amazon• with an annual flow of 5.5 x 1018 g H20 the dry and•rbelk samples were lower, and they and a mean dissolved A1 concentration of 40 Bg/L were much m variable than in the wet-only [Stoffyn and MacKenzie, 1982] carries 2.2 x 1011 samples. In wet-only samples, solubilities g of A1 to the Atlantic. If the 3-5% solubility tended to increase with decreasing pH and applies for all mineral dust deposited to the decreasing concentration of insoluble materials oceans (5-10 x 101• g/yr, Prospero [1981]), then (assumed to be primarily clays). It is also the global deposition rate of soluble A1 would be noteworthy that the solubilities for Pb, Cu, Cd, 12-24 x 1011 g to 20-40 x 1011 g/yr. In and Zn, as reported by Gatz et al., are


substantially larger than those reported by Hodge Conclusions et al.

The dissolved A1 results reported here resolve The deposition rate of Saharan dust in rain is some questions regarding the importance of •olian sufficiently great to have a major impact on the inputs with regard to the A1 distribution in some sediment accumulation rates in a large area of ocean regions. On the basis of the previously the North Atlantic. If 3-5% of the A1 in mineral reported solubility of 1%, Moore [1981] concluded dust is soluble, as we found in our rain samples, that the input of soil aerosol was insufficient then atmospheric inputs of dust will play a to explain the vertical distribution of dissolved dominant role in the oceanic A1 cycle in this A1; using as an example the Sargasso Sea, he region and. most likely, in other regions as computed a residence time of 500 years for A1 in well. surface waters, a value that is well in excess of The fact that the dissolved fraction of A1 the generally accepted value of 80 years. appears to increase with decreasing dust However, with an A1 solubility of 5%• the concentration and decreasing ptl has some residence time is reduced to 100 years, a value important implications for the high latitudes, that is clearly acceptable. where dust concentrations are relatively small

Factors Affecting A1 Solubility in Rain

The chemistry of A1 in precipitation is undoubtedly quite complex because of the involvement of mineral phases. It is noteworthy that the concentrations of dissolved A1 in our rain samples fall in the general range observed for A1 in pore waters in marine sediments (roughly 2-70 Bg A1/L; Caschetto and Wollast [1979]), although pH values in the sediments were considerably higher (typically in the range 7.4-8.1). Furthermore, the dissolved A1

[Prospero, 1979] but where pollutant concentrations are high [Galloway et al. , 1983]. The solubility of A1 and other trace metals in precipitation in these latitudes [Church eta]., 1982, 1984] could be considerably greater than in the low latitudes.

We found that 68% of the annual mineral dust deposition occurred in 4 days; in other words, during 1% of the sampling cycle. This means that the distribution of dissolved and particulate A1 (and other dust-related species) in seawater could be extremely variable and that the large deposition events could be easily missed in any short-term sampling program. The impact of these

concentrations in most of our precipitation large input pulses of particles on biological samples were comparable to those measured in productivity also warrants consideration [Duce, laboratory studies of clay mineral solubility in 1986]. seawater [Hydes, 1977]. However, in the case of sediment pore waters, there is no consistent relationship between concentration data and the values computed from equilibrium constants for mineral dissolution-precipitation reactions [Caschetto and Wollast, 1979].

In our samples the pH and the concentrations _

of A1 and SO 4- are more similar to those found in acidified surface waters, although the range of

Acknowledgments. We thank T. Snowdon for his technical assistance in this program and H. Maring for helpful discussions. Supported by NSF grants OCE 81-12106, OCE 84-05609, and OCE 81-11895 as part of the SEAREX program, and by NSF ATM 8016127.

References

values in the latter is much greater [Nordstrom Andreae, M. 0., R. J. Charlson, F. Bruynseels, H. and Ball, 1986; Hooper and Shoemaker, 1985]. The Storms, R. Van Grieken, and W. Maenhaut, dissolve• A1 in surface waters shows a sharp Internal mixture of sea salt, silicates and transition in the pH range of 4.6-4.9. This excess sulfate in marine aerosols, Science, transition corresponds to the pK for the first 232, 1620-1623, 1986. hydrolysis constant of the aqueous A1 ion. Above Carder, K. L., R. G. Steward, P. R. Betzer, D. L. a pH of about 5, the A1 concentration appears to Johnson, and J. M. Prospero, Dynamics and be related to the equilibrium solubility of composition of particles from an aeolian input microcrystalline gibbsite or amorphous aluminum event to the Sargasso Sea, J. Geophys. Res., hydroxide. 91, 1055-1066, 1986.

The chemistry of A1 in precipitation will Carlson• T. N., and J. M. Prospero, The large- differ from that in sediments and ground waters scale movement of Saharan air outbreaks over in that the mineral particles have been subjected the northern equatorial •Atlantic, J. Appl.

to a complex chemical history in the atmosphere. Meteor?l•, 11, 283-297, 19•2. Aerosols in the marine boundary layer often Caschet•o• ., •nd R. Wollast Dissolved aluminum consist of internal mixtures of various salts and in interstitial waters of recent marine insoluble particles, including minerals [Andreae sediments, Geochim. Cosmochim. Acta, 43, et al. , 1986]. The composition of these 425-428, 1979. particles suggests that they have been formed by Chen• L., and R. A. Duce, The sources of sulfate, cloud processes and that they have passed through vanadium and mineral matter, in aerosol repeated cycles of condensation and evaporation. particles over Bermuda, Atmos. Environ., 17, Such processes would subject soil particles to a 2055-2064, !983. wide range of chemical environments and to Chester, R., Particulate aluminum fluxes in the prolonged leaching, conceivably at pHs eastern Atlantic, Mar. Chem., 11, 1-16, 1982. considerably lower than that observed in Church, T. M., J. N. Galloway, T. D. Jickells, precipitation. Thus in addition to other and A. H. Knap, The chemistry of western factors, the A1 chemistry in precipitation could Atlantic precipitation at the mid-Atlantic be dependent on the past history of the aerosol coast and on Bermuda, J. Geophys. Res., 87, particles. 11,013-11,018, 1982.


Church, T. M., J. M. Tramontano, J. R. Scudlark, Loye-Pilot, M.D., J. M. Martin, and J. Morelli, T. D. Jickells, J. J. Tokos, Jr., A. H. Knap, Influence of Saharan dust on the rain acidity and J. N. Galloway, The wet deposition of and atmospheric input to the Mediterranean, trace metals to the western Atlantic Ocean, at Nature, 321, 427-428, 1986. the Mid-Atlantic coast and on Bermuda, Atmos. Maring, H. B., and R. A. Duce, The impact of Environ., 18, 2657-2664, 1984. atmospheric aerosols on the trace metal

Dolske, D. A., and D. F. Gatz, Field inter- chemistry of open ocean surface seawater, 1, comparison of sulfate dry deposition Aluminum, Earth Planet ß Sci. Lett., 84, monitoring and measurement methods' 381-392, 1987. Preliminary results, in Deposition, Both Wet Measures, C. I., B. Grant, M. Khadem, D. S. Lee and Dry, edited by B. B. Hicks, pp. 121-131, and J. M. Edmond, Distribution of Be, A1, Se Butterworth, Stoneham, Mass., 1984. and Bi in the surface waters of the western

Duce, R. A., The impact of atmospheric nitrogen, North Atlantic and Caribbean, Earth Planet. phosphorus, and iron species on marine Sci. Lett., 71, 1-12 1984. biological productivity, in The Role of the Moore, R. M., Oceanographic distribution of Ocean in Geochemical Cycling, NATO ASI Ser., dissolved zinc, cadmium, copper and aluminum vol. 185, edited by P. Buat-Menard, pp. in waters of the Central Arctic, Geochim. 497-529, D. Reidel, Hingham, Mass., 1986. Cosmochim. Acta, 45, 2475-2482, 1981.

Duce, R. A., C. K. Unni, B. J. Ray, J. M. Nordstrom, D. K., and J. W. Ball, The geochemical Prospero, and J. T. Merrill, Long-range behavior of aluminum in acidified surface atmospheric transport of soil dust from Asia waters, Science, 232, 54-56, 1986. to the tropical North Pacific: Temporal Orians, K. J. , and K. W. Bruland• Dissolved variability, Science 209, 1522-1524, 1980. aluminum in the central North Pacific, Nature,

Galloway, J. N., G. E. Likens, W. C. Keene, and 316, 427-429, 1985. J. M. Miller, The composition of precipitation Orians, K. J., and K. W. Bruland, The biogeo- in remote areas of the world, J. Geophys Res., chemistry of aluminum in the Pacific Ocean, 87, 8771-8786, 1982. Earth Planet. Sci. Lett., 78, 397-410, 1986.

Galloway, J. N., A. H. Knap, and T. M. Church, Parrington, J. R., W. H. Zoller and N. K. Aras, The composition of western Atlantic Asian dust: Seasonal transport to the Hawaiian precipitation using shipboard collectors, J. Islands, Science, 220, 195-198, 1983. Geophys. Res., 88, 10,859-10,864, 1983. Prospero, J. M., Mineral and sea-salt aerosol

Gatz, D. F., B. K. Warner, and L-C. Chu, concentrations in various ocean regions, J. Solubility of metal ions in rainwater, in Geophys. Res., 84, 725-731, 1979. Deposition, Both Wet and Dry, edited by B.B. Prospero, J. M., Aeolian transport to the world Hicks, pp. 131-151, Butterworth, Stoneham, ocean, in The Oceanic Lithosphere, The Sea, Mass., 1984. vol. 7, edited by C. Emiliani, pp. 801-874,

Gibson, J. H., Evaluation of wet chemical Wiley-Interscience, New York, 1981. deposition in North America, in Deposition, Prospero, J. M., Records of past continental Both Wet and Dry, edited by B. Hicks, pp. climates in deep-sea sediments, Nature, 315, 1-14, Butterworth, Stoneham, Mass., 1984. 279-280, 1985.

Glaccum, R. A., and J. M. Prospero, Saharan Prospero, J. M., and T. N. Carlson, Saharan air aerosols over the tropical North Atlantic - outbreaks over the tropical North Atlantic, Mineralogy, Mar. Geol., 37, 295-321, 1980. Pure Appl. Geophys , 119, 677-691, 1981

Henry, J. A., Precipitation and cloud Prospero, J. M., and R. T. Nees, Impact of the climatologies of Florida, in Acid Deposition North African drought and E1 Nino on mineral Causes and Effects: A Statement Assessment dust in the Barbados trade winds, Nature, 320, Model. edited by A. E. S. Greenand and W. H. 735-738, 1986. Smith, pp. 201-208, Government Institutes, Prospero, J. M., D. L. Savoie, T. N. Car]sonl and Inc., Rockville, Md., 1983. R. T. Nees, Monitoring Saharan aerosol

Hicks, B. B., M. L. Wesely, and J. L. Durham, Critique of Methods to Measure Dry Deposition, EPA-600/9-80-050, Environ. Sci. Res. Lab., U.S. Environ. Prot. Agency, Research Triangle Park, N.C., 1980.

Hodge, V., S. R. Johnson, and E. D. Goldberg,

transport by means of atmospheric turbidity measurements, in Saharan Dust: Mobilization, Transport, Deposition, edited by C. Morales, pp. 171-186, John Wiley, New York, 1979.

Prospero, J. M., R. A. Glaccum, and R. T. Nees, Atmospheric transport of soil dust from Africa

Influence of atmospherically transported to South America, Nature, 289, 270-572, 1981. aerosols on surface ocean water composition, Rao, M. S. V., M. V. Abbott III, and J. $. Theon, Geochem. J., 12, 7-20, 1978. Satellite-derived global oceanic rainfall

Hooper, R. P., and C. A. Shoemaker, Aluminum atlas (1973 and 1974), NASA SP-410, 1976. mobilization in an acidic headwater stream: Saveie, D. L., and J. M. Prospero, Aerosol Temporal variation and mineral dissolution concentration statistics for the northern disequilibria, Science, 229, 463-465, 1985.

Hydes, D. J., Dissolved aluminum concentration in sea water, Nature, 268, 136-137, 1977.

Hydes, D. J., Distribution of aluminum in waters of the North East Atlantic 25øN to 35øN, Geochim. Cosmochim. Acta, 47, 967-973, 1983.

tropical Atlantic, J. Geophys. Res., 82, 5954-5964, 1977.

Savoie, D. L., and J. M. Prospero, Particle size distribution of nitrate and sulfate in the

marine atmosphere, Geophys. Res. Lett., 9, 1207-1210, 1982.

Ku, T.-L., W. S. Broecker, and N. Opdyke, Savoie, D. L., J. M. Prospero, and R. T. Nees, Comparison of sedimentation rates measured by Washout ratios of nitrate, non-sea-salt paleomagnetic and ionium methods of age sulfate, and sea salt on Virginia Key, determination, Earth Planet. Sci. Lett., 4, Florida, and on American Samoa, Atmos.

___

1-16, 1968. Environ., 21, 103-112, 1987.


SchHtz, L., Long-range transport of desert dust with speõial emphasis on the Sahara, in Ann. N.Y. Acad. Sci., 338, 515-532, 1980.

Sehmel, G. A., Particle and gas dry deposition: A review, Atmos. Environ., 14, 983-1011, 1980.

Sholkovitz, E. R., Flocculation of dissolved and inorganic matter during the mixing of river water and seawater, Geochim. Cosmochim. Acta, 40, 831-845, 1976.

Slinn, W. G. N., L. Hasse, B. B. Hicks, A. W. Hogan, D. Lal, P. S. Liss, K. O. Munnich, G. A. Sehmel, and O. Vittori, Wet-dry removal process es, Atmos. Environ., 12, 2055-2087, 1978.

Stoffyn, M., and F. T. Mackenzie, Fate of dissolved aluminum in the oceans, Mar. Chem., 11, 105-127, 1982.

Talbot, R. W., R. C. Harriss, E. V. Browell, G. L. Gregory, E. I. Sebacher, and S. M. Beck, Distribution and geochemistry of aerosols in

Tsunogai, Short-term temporal variability of eolian particles in surface waters of the northwestern North Pacific, J. Geophys. Res., 90, 1167-1172, 1985a.

Uematsu, M., R. A. Duce, and J. M. Prospero, Deposition of atmospheric mineral particles in the North Pacific Ocean, J. Atmos. Chem., 3, 123-138, 1985b.

Weisel, C. P., R. A. Duce, and J. L. Fasching, Determination of aluminum, lead, and vanadium in North Atlantic seawater after coprecipitation with ferrichydroxide, Anal. Chem., 56, 1050-1052, 1984a.

Weisel, C. P., R. A. Duce, J. L. Fasching, and R. W. Heaton, Estimates of the transport of trace metals from the ocean to the atmosphere, J. Geophys. Res., 89, 11,607-11,618, 1984b.

R. T. Nees and J. M. Prospero, Division of the tropical North Atlantic troposphere: Marine and Atmospheric Chemistry, Rosenstiel Relationship to Saharan dust, J. Geophys. School of Marine and Atmospheric Science, Res., 91 5173-5182, 1986. University of Miami, 4600 Rickenbacker Causeway,

Taylor, S. R., Abundance of chemical elements in Miami, FL 33149. the continental crust: A new table, Geochim. M. Uematsu, Center for Atmospheric Chemistry Cosmochim. Acta, 28, 1273-1285, 1964. Studies, Graduate School of Oceanography,

Uematsu, M., R. A. Duce, J. M. Prospero, L. Chen• University of Rhode Island, Narragansett, RI J. T. Merrill, and R. L. McDonald, The 02882. transport of mineral aerosol from Asia over the North Pacific Ocean, J. Geophys. Res., 88, (Received December 2, 1986; 5343-4352, 1983. revised September 16, 1987;

Uematsu, M., R. A. Duce, S. Nakaya and S. accepted September 22, 1987.)

deposition rate of particulate and dissolved aluminum...

Documents