deposition of atmospheric mineral particles in the north pacific ocean

Journal of Atmospheric Chemistry 3 (1985), 123-138. 0167-7764/85.15. 123 © 1985 by D. ReidelPublishing Company.

DEPOSITION OF ATMOSPHERIC MINERAL PARTICLES IN THE NORTH PACIFIC OCEAN

Mitsuo Uematsu, Robert A. Duce Center for Atmospheric Chemistry Studies, Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 U.S.A.

and

Joseph M. Prospero Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 U.S.A.

ABSTRACT. Total deposition of atmospheric mineral particles (wet plus dry) has been measured during consecutive two-week sampling intervals from January, 1981 to March, 1982 at four island stations (Midway, Oahu, Enewetak, and Fanning) of the SEAREX Asian Dust Study Network in the North Pacific. The total deposition of mineral aerosol during the period from February to June is higher than that during the period from July to January at most of the stations. A systematic geographical trend is apparent in the dust flux, with greater fluxes at higher latitudes. The deposition values are correlated with the atmospheric mineral particle concentrations at these stations. The mineral particles are transported from arid regions in Asia to the North Pacific, and the annual dust deposition to the ocean appears to be dominated by sporadic dust events of short duration. Wet deposition dominates the removal of dust particles from the atmosphere over the North Pacific. The total deposition of atmospheric mineral material to the central North Pacific is estimated to be ~20 x 1012 g yr - l .

Key words: Annual dust flux, mineral aerosol, North Pacific, spatial distribution, long-range transport, Asian dust, marine sediments.

1. INTRODUCTION

Recent studies have shown that large quantities of soil material are transported by winds out of Asia and over the North Pacific (e.g., Duce et al. , 1980; Rahn et al., 1981; Darzi and Winchester, 1982; Parrington et a l . , 1983; Uematsu et al . , 1983). A synoptic investiga- tion of this transport has been carried out as part of the S_a/Air

L 24 MITSUO UEMATSU ET AL

represent the maximum values for the dust input to this region of the North Pacific in 1981. Finally, a new estimated dust input to this region was calculated as the sum of the dry deposition calculated from (3) and the wet deposition based on a scavenging ratio of 1000 and the mean atmospheric dust concentrations in 1981 combined with the mean precipitation amount during the 30 - 65 years record at each station (Taylor, 1973). The measured dust flux in the zone from 40 to 50°N could not be obtained for the entire year of 1981 due to the di f f icul- ties of total deposition collection under the severe weather conditions during the winter at Shemya. The new estimated dust flux for the zone from 40 ° to 50°N was computed from the atmospheric concentration data at Shemya (52°44'N, 174°06'E) in the Aleutian Islands (Uematsu et al . , 1983). The dust fluxes obtained by direct measurement agree well with the new estimated dust input values using the scavenging ratio model. The total deposition of atmospheric mineral particles to this area of the North Pacific is ~20 x 1012 g yr-lA which is approximately twice the previous estimate of 6 - 12 x 10 I ( g yr -1 (Uematsu et al . , 1983). Thus atmospherically transported Asian dust appears to be a major source of the clay minerals in the deep-sea sediments of the North Pacific, especially in the higher latitudes, and i ts sporadic input can have a significant impact on the concentration of mineral particles in the underlying ocean waters.

4. CONCLUSIONS

Our continuous one year measurements of the total dust deposition, as measured from wet and dry deposition to collectors at the SADS Network island stations, lead to the following conclusions:

i . A systematic geographical trend is apparent in the dust fluxes, with higher fluxes being observed at stations located at higher latitudes.

2. Most of the periods of high dust flux were observed during periods when both rainfall and atmospheric dust loading were high. These high flux periods occurred during outbreaks of Asian dust in the spring. The annual dust flux to the North Pacific waters appears to be dominated by sporadic dust events of short duration.

3. Large dust particles (>20 ~m diameter) appeared to be transported to much of the centraT North Pacific. These particles may account for from 15 to 35 percent of the total annual dust flux (<20 ~m plus >20 ~m) to the ocean surface.

4. ~ased on calculations of dry deposition, the wet deposition of dust accounts for 75 to ~5 percent of the total deposition measured at each station. The total annual in?~2 ~f du~t to the central North Pacific is estimated to be 20 x yr- . The impact of large sporadic inputs of mineral particles to the surface waters of the North Pacific on chemical and biological processes in the water column remains to be determined.

DEPOSITION OF ATMOSPHERIC MINERAL PARTICLES 125

O*

C ~i:: i • i ~ ~ " , ~ . - "-: . ,~.# "~

~ . : ' . - . . . . : : " . -

V~ENE.WETAK ::P".,/ j / /r-//A /

120°E 150 ° IBO ° 150" 120" 9 0 * W

Figure 1. The total deposition sampling stations in the SEAREX Asian Dust Study (SADS) Network. Shaded area was used for the estimation of dust input to the North Pacific. See text.

the ocean as possible. The deposition samples from Fanning, Enewetak, Oahu and Midway were chosen (see Figure 1) because the remainder of the network stations (Guam, Belau and Shemya) were not favorable sites for this deposition study due to the possibility of local contamination or because of severe weather conditions. Details concerning the collection sites were given by Uematsu et al. (1983).

Total deposition samples were collected using conical polyethylene funnels (10 cm or 25 cm diameter) connected to a 4 l i t e r acid-washed collapsible polyethylene container (CubitainerR). This collection system was constructed to dimensions comparable to those of standard rain gages. Because of the consecutive and extended sampling by untrained local personnel at the remote island stations, the sampling protocol was made as simple as possible. Washing the dry deposition particles on the funnel into the polyethylene container depended upon rain. Dry deposited material was not removed by any ar t i f ic ia l means. I t is recognized that dry deposition to a funnel will be different from that to the ocean surface, especially for submicrometer particles (Sievering, 1984). Most of the mass of the dust over the North Pacific is concentrated on particles from 2 to 10 pm diameter (Duce et al. , 1983). Dry deposition of particles in this size range to a surrogate surface can be measured with some degree of confi- dence.

On arrival at our laboratory, aqueous samples were immediately passed through a-20 pm mesh nylon screen to eliminate the largest particles and local debris (e.g., insects and plant tissues). The samples were then adjusted to a nitr ic acid concentration of 0.15 N

126 MITSUO UEMATSU ET AL.

using quartz-distilled nitr ic acid. Acidification of the total deposition samples minimized the adsorption of trace metals on the wall of the container. I f the amount of liquid in a sample were less than 300 g, the sample was combined with the sample collected during the following period. As blank controls during storage, handling, and chemical analysis, several 300 g aliquots of distilled-deionized water were treated in the same manner as samples.

2.2. Chemical Analysis

The total deposition samples were analysed for AI, which was present in both dissolved and particulate forms, by instrumental neutron activation analysis (INAA). Al is an excellent reference element for mineral dust particles (Uematsu et al. , 1983). As a preliminary experiment, rain samples collected at the Graduate School of Oceanog- raphy, University of Rhode Island, were acidified. After several months, acidified samples were fi l tered with 0.4 ~m poresize Nuclepore f i l te rs . Significant quantities of particulate Al (more than 60 percent of the total Al in the samples) were found in the form of alumi- nosilicates, which did not dissolve in 0.15 N ni t r ic acid even after several months. Strong ni t r ic acid and hydrofluoric acid digestion of the sample to solubilize the aluminosilicate matrix is not required for the determination of Al by INAA. However, large quantities of sea salt present in some samples occasionally caused interference problems. For this reason, coprecipitation of the ionic and particulate Al with ferric hydroxide (Weisel et al . , 1984) was used to preconcentrate the Al and to eliminate the sea salt interference.

Two 50 g aliquots of sample solution were transferred to duplicate 100 ml teflon beakers. Twenty pl of -2 M ferric nitrate solution were added to each sample and stirred well. The pH was then adjusted to 6 with ammonium hydroxide. After one hour, during which time the precipitate was completely formed, the sample solution was f i l tered through a Nuclepore (0.4 pm poresize, 47 mm diameter) f i l t e r . The precipitate was washed with distilled-deionized water to remove any sea salt adhering to the precipitate on the f i l t e r . After drying, the precipitate and f i l t e r were pressed into a pellet (3 mm diameter, -2 mm thick) of uniform geometry. The large particles separated from the samples with a 20 pm mesh screen were washed off with distilled-deionized water and collected on acid-washed Whatman 41 f i l ters (22 mm diameter). These f i l te rs were also pelletized and were analysed for Al by INAA.

The pellet and an Al flux monitor were irradiated simultaneously at the Rhode Island Nuclear Science Center swimming pool reactor in a thermal neutron flux of 4 x 1012 n cm -2 s -1 for 60 s. After a 120 s cooling period, the samples were counted for 28AI ( t l /2 = 2.24 min) for 500 s on a 25 percent Ge(Li) detector (resolution of 2.5 keV for the 1332 keV gamma ray of 60Co) coupled to a multi-channel analyzer. Concentrations were calcuTated by correcting for dead time using the computer programs GAMNAL (Ioyuye et al . , 1969) and PIDAQ (Maney et al . , 1977). Total concentrations were corrected for the concentrations of Al in the blank control samples.

DEPOSITIONOFATMOSPHERICMINERALPARTICLES [27

The detection l imit for AI in the samples was 0.5 ~g kg-l. This enabled accurate determination of Al over the concentration range of several ,g kg -1 to several hundred ~g kg -1. The coefficient of variation for the Al concentrations for duplicate samples was less than 10 percent.

The mineral dust concentrations in the samples were calculated by multiplying the Al concentrations by 12.5, since Al is -8 percent of mineral dust (Taylor, 1964; Uematsu et al., 1983).

~,~o. / z~ / ~ / ~ / - ~ / MD OA EN FA

(~ug cm-2period -1)

/ ~ / ~ ~ ~ /

.,OO~T / ~ / ~ / @ / @ S~ASON / 22 / 13 / 2, / 132 j

MD OA EN FA (cm period -1)

Figure 2. Comparison of atmospheric dust f lux for par t ic les smaller than 20 ~m diameter ( a ) , and prec ip i ta t ion amount (b) at Midway (MD), Oahu (OA), Enewetak (EN), and Fanning (FA) from the SADS Network during the high dust and clean season and for the entire year in 1981.

128 MITSUO UEMATSU ET AI

3. RESULTS AND DISCUSSION

3.1. Seasonal and Areal Variation of the Dust Flux Over the North Pacific

Measurements of total deposition from the four SADS stations in 1981 and early 1982 should be representative of North Pacific conditions. The dust concentrations in 1981 appeared to be similar to the average atmospheric dust concentrations observed at Mauna Loa Observatory (elevation 3400 m) from 1979 to 1982 (Parrington et al . , 1983) and at the SADS stations from 1981 to 1984 (Uematsu et al . , in preparation). The precipitation amounts measured by the total deposition collectors at the four stations also were within one standard deviation of the average precipitation for a 30 - 65 year period up to 1972 at each station (Taylor, 1973). Exceptions were the period from July to January at Midway and Fanning, when rainfall amounts at both these stations were -60 percent lower than average.

The precipitation quantities, atmospheric dust concentrations and dust fluxes to the ocean (particle sizes <20 ~m diameter) are summa- rized in Table 1 for the high dust season (February - June), the low dust (clean) season (July - January), and the entire year, 1981.

The average daily dust deposition amounts during the five months of the high dust season were greater than or equal to those during the seven months of the clean season at all stations. For example, the average daily deposition amount during the high dust season at Midway (0.30 ~g cm-2day -±) was 3.6 times higher than that during the clean season (0.084 ~g cm-2day-1).

The measured dust fluxes to the ocean at the four stations were approximately twice the high end of the range of dust fluxes estimated by Uematsu et al. (1983) using simple deposition models and measured atmospheric dust concentrations to make the estimates. The measured annual dust fluxes at Midway and Oahu (64 and 43 ~g cm -2 yr -1, respectively, Figure 2a) were comparable to the flux of mineral particles determined from sediment traps near the Aleutian Islands (100 - 200 ~g cm -2 yr -1 at 48°N, 176*E; Tsunogai et. a l . , 1982) and over

• 0 0

the East Hawaii Abyssal Plaln (50 ~g cm -Z yr -± at 15 N, 151 E; Honjo et al . , 1982). At Enewetak, the 1981 measured dust flux of 30 pg cm -~ yr -1 agreed well with the dust flux obtained from th R results of the major SEAREX experiment there in 1979 (30 ~g cm -L yr-1; Arimoto et al . , 1985).

There appears to be a systematic geographical trend in the dust fluxes, with higher dust fluxes observed at stations located at higher latitudes• This trend was most pronounced in the high dust season. The dust flux at Fanning, near the equator, was 19 percent of the dust flux at Midway for the entire year. In contrast, the annual mean atmospheric dust concentration at Fanning was 7 percent of that at Midway. Although atmospheric dust concentrations at Fanning showed only small variations throughout the year (Uematsu et al . , 1983), seasonal differences in the dust flux were pronounced, with almost 3 times as much dust entering the ocean during the high dust season. This is probably due to the much greater amount of precipitation dur-


Table 1. Precipitation, Atmospheric Dust Concentration, and Dust Flux at the SADS Stations

Station Season Measured (1981) Precip.

amount

(cm day -I )

Mean Atmos. Measured Estimated Dust Conc.* Dust Flux Dust Flux*

(<20 ~m)

(~g m -3) (~g cm-2day -1) (~g cm-2day -1)

Midway

28°13'N High-Dust 0.14 177°21'W Clean 0.12

Annual 0.13

1,6 0.30 0.28 0.084

0.84 0.17 0.052 - 0.12

Oahu

21°20'N High-Dust 0.085 157°42'W Clean 0.22

Annual 0.16

1.3 0.14 0.21 0.099

0.66 0.12 0.038 - 0.063

Enewetak

11°20'N High-Dust 0.19 162°20'E Clean 0.44

Annual 0.34

0,68 0,082 0.05 0.086

0.31 0.084 0.019 - 0.041

Fanning

3°55'N High-Dust 0.87 0.08 0.060 159°20'W Clean 0.15 0.04 0.014

Annual 0.45 0.06 0.031 0.003 - 0.019

High-Dust Season: February - June, Clean Season: J u l y - January. * Uematsu e t a l . (1983)


~ . the high dust season than during the clean season (see Figure I t should be noted that Fanning, although located at 4°N, gener-

ally lies within, or the south of, the intertropical convergence zone (ITCZ). Therefore, Asian dust should have a relatively small impact at Fanning.

3.2. Relationships Among Precipitation Amounts, Atmospheric Dust Concentrations, and Dust Fluxes to the Ocean

Figure 3 presents the normalized temporal variations of the dust flux, atmospheric dust concentration, and rainfall amount for the four SADS stations. The weekly atmospheric dust concentrations were combined to produce the mean atmospheric dust concentration during each period in which total deposition samples were collected. The data were normalized to the annual means (see Table I) for each of these parameters. During the high dust season in 1981, two distinct peaks in the dust flux were found synoptically at three stations (Midway, Oahu and Enewetak) during late February and late April early May. The latter period of high dust deposition was also observed at Fanning. As seen in Figure 3, most of the periods of large dust flux were observed during periods of high atmospheric dust loading and rainfall. The two periods of large dust flux discussed above were related to dust storm

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ENEWETAK

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OAHU

I I i i ii

/ \ ',I i '\ ~ . ',, - ~

' J ' F ' M ' A ' M ' J ' J ' A ' S ' O ' N ' D ' J ' F '

1981

;I FANNING

' d ' F ' M ' A ' M' J ' d ' A ' S ' O ' N.' D ' d ' F '

1981

Figure 3. The normalized temporal variations of dust flux for particles smaller than 20 ~m diameter (solid l ine), atmospheric dust concentration (circles), and precipitation amount (dashed line) in 1981 and part of 1982.


outbreaks in Asia (Uematsu et al . , 1983). During the high dust season, westerly winds at high elevations over Asia carry dust raised from the surface rapidly out over the North Pacific. As a result of general atmospheric subsidence and particle settling, the dust is carried to lower altitudes and latitudes and is then advected from the east to the west by the trade wind regime (Duce et al . , 1980; Merrill et al. , 1985). Wind records at the island sites indicated that the northeast trades were blowing steadily from the ocean to the coastal sampling sites during these periods in the high dust season. Conse- quently, contamination from local sources is believed to be unlikely.

Occasionally low dust fluxes were observed during periods of relatively high atmospheric dust concentrations, particularly when the precipitation amounts were low (e.g., much of March and April at Midway and mid-May to mid-June at Oahu). Th is suggests that wet removal dominates the atmospheric deposition of dust particles. As one might expect, large dust fluxes were never found during periods when both the atmospheric dust concentration and the precipitation amount were low. This reinforces our belief that contamination by the dry depositon of material derived from local sources is not a problem.

The dust flux at Midway varied greatly during the high dust season. Midway is the northernmost of the four sites and is often directly beneath prevailing westerly winds from Asia. The sum of the dust deposition at Midway during the two periods of February 12 - 26 and April 30 - May 7 was 29 ~g cm -2. This was almost half of the annual dust deposition at Midway, and i t occurred during only 6 percent of the year. In fact, a few extremely high atmospheric dust concentrations, 7 - 9 times higher than the annual means, were observed from the one-week air sampling intervals at several SADS stations during the high dust season in 1981 (Uematsu et al. , 1983). Uematsu et al. (1985) found that the high concentrations of mineral particles in the surface waters of the northwestern North Pacific in 1978 resulted from an Asian dust event of short duration. These particles were removed to deeper waters within one month. Thus i t is l ikely that most of the annual deposition of dust occurs during very short time intervals, perhaps just a few days, at these sites. The effect of these short duration but high intensity bursts of mineral matter on chemical and biological processes in the North Pacific should be investigated.

During the clean season at Midway and Oahu the dust fluxes were quite low, while the atmospheric dust concentrations were always below the mean values. No relationship was apparent between dust fluxes and rainfall during this season at these sites. In contrast, the dust flux appeared to be correlated with the bi-weekly precipitation amount during the clean season at Enewetak and during the entire year at Fanning, where the atmospheric dust concentrations were low, generally less than 0.1 pg m -3. As an example, a strong correlation between the dust flux and the precipitation amount (correlation coefficient, r = 0.94) was found at Fanning. This again suggests strongly that wet deposition dominates the removal of the dust particles.


3.3. Evaluation of Scavenging Ratios for Dust

Uematsu et al. (1983) estimated the atmospheric input of dust to the North Pacific in 1981 by using three simple models based on the mean atmospheric dust concentrations at the SADS stations (see Table 1). Of the three models, the highest estimated dust fluxes were obtained from the one based on an estimated scavenging ratio (or washout factor) of 500 * 300. The scavenging ratio, W, is the ratio of the concentration in rain of any substance to i ts concentration in the atmosphere, and is given by Equation 1:

W = Kp C -1 ( I )

where K = the dust concentration in rain (~g cm -3 or ~g g-l) , p the density of air (0.0012 g cm-J), aod C the dust concentration in air (~g cm-J).

I t is convenient to estimate wet fluxes by using the scavenging ratio, when the concentration in rain is not available. Thus we can compute the deposition fluxes i f we know the concentration in air, the scavenging ratio, and the precipitation amount at any place. The annual estimates of dust deposition made by Uematsu et al. (1983) using scavenging ratios were approximately half of the directly measured dust fluxes reported here. This suggests that the scavenging ratio used by Uematsu et al. (1983) was too low. We can determine actual scavenging ratios for the SADS Network total deposition samples by using the directly measured total dust fluxes of particles with diameters <20 ~m, the atmospheric dust concentrations, and calculations of the dry deposition of dust at each station.

The total deposition flux, F t , (~g cm -2 period -1) includes both the wet (F w) and dry (F d) deposition, i .e . ,

F t = F d + F w. (2)

F d is given by

F d = CVTf (3)

where V = the dry deposition velocity for dust (cm day-l), T = the sampling period (days), and f = the proportion of the time when no precipitation was fall ing

during the period T.

From studies of trace elements at Enewetak in 1979 (Arimoto et al. , 1985), the dry deposition ,velocity for Al to a circular plastic plate was found to be 2.8 x 10 ~ cm day -~ . This value was used for V in (3) for all stations. The value for f for each station was determined from the annual mean percentage of weather observations that reported precipitation (U.S. Navy, 1977). F d calculated from equation (3) was 15 25 percent of the measured F t during the high dust season at Midway, Oahu, and Enewetak. F d was less than 10 percent of F t


during the clean season at these stations and during the entire year at Fanning. This is consistent with the results of the direct measurement of dry and wet deposition at Enewetak in 1979 (Duce et al . , 1980; Arimoto et al . , 1985).

F w is given by

F w = KP'T(1-f) = KP (4)

where P' = the precipitation amount (cm day-l), and P = the precipitation amount during the period T (cm period-i).

From ( i ) and (4), F w is given by

F w = WPC p-l. (5)

Figure 4 presents a plot of the values of log Fw, as determined from the measured values for F t and the calculated values for F d, vs log (PC p-l] during the high dust season, the clean season, and the entire year at the four SADS stations. These plots enabled values for the scavenging ratio, W, to be determined. Lines of constant W of 500, 1000, and 2000 are also shown on the figure. Scavenging ratios for the high dust season and for the entire year ranged between 500

3.~ [] ANNUAL /~b~//~" o CLEAN */~.~

• u.K. / 2 * / / , "

,/~/111 ""

1 . ,/"D//~/

0~/ / I . . . . . . . . . I . . . . . . . . . I . . . . . . . . . I

- 3 - 2 -1 0

log (PC/p)

Figure 4. Scavenging ratios at the SADS Network stations and those observed by investigators in the United Kingdom (U.K.). See text.

134 MITSUO UEMATSU ET AL

and 2000, while the scavenging ratios for the clean season were slightly higher, over 2000. Also plotted on Figure 4 are annual values for F w at six non-urban sites in the United Kingdom from January to December 1976 (Cawse, 1977). These values were obtained from measurements of F t, Fd, P, and C at the sites. The scavenging ratios for mineral aerosol particles at the United Kingdom sites were in the same range as those obtained for the North Pacific network sites, even though there are large differences in rainfall amounts, atmospheric dust concentrations and dust fluxes. The mean scavenging ratio at all the stations in Figure 4 is -1000. A scavenging ratio of 580 for dust at Enewetak in 1979 was computed from measurements of K and C (Arimoto et al . , 1985). The mean scavenging ratio observed for eleven rain events at Miami during the Sahara dust events was 730 (Prospero et al . , 1984). Therefore a scavenging ratio of -1000 for mineral aerosol can be used to estimate the annual dust wet deposition, with a l ikely uncertainly of a factor of -2 over the North Pacific, even though there are large seasonal and geographical variations of both the precipitation amount and the atmospheric dust concentration in this region.

3.4. Consideration of Particles Larger Than 20 Micrometers in Total Deposition Samples

Dust particles larger than 20 ~m had been considered as an unavoidable local soil contamination in the total deposition collectors at these stations. At Enewetak, the annual dust flux for particles larger than 20 ~m (large-particle) was found to be -20 percent of the annual dust flux for particles smaller than 20 ~m (small-particle). However, the large-particle dust flux showed a pronounced seasonal variation; e.g., the larQe-particle flux during the high dust season (0.028 ~g cm-2day -~) was 6 times greater than that during the clean season (0.005 ~g cm-2day-1). The large-particle dust flux was approximately 33 percent of the small-particle dust flux during the high dust season and less than 6 percent of the small-particle flux during the clean season. Similar results were also observed at Oahu and Midway. The large-particle dust flux during the high dust season was 5 and 3 times higher than that during the clean season at Oahu and Midway, respectively.

The station at Enewetak was particularly important relative to these large particles, because this small atoll is essentially all coral ( i .e . , calcium carbonate) with l i t t l e aluminosilicate soil on the island i tsel f . The concentration of sea-salt aerosol particles and the record of wind direction at Enewetak showed steady winds from the ocean throughout the sampling periods, with the exception of part of August and September, 1981. I t is possible, of course, that small aluminosilicate Asian dust particles previously deposited on the island could become attached to coral particles and could subsequently be remobilized; these could conceivably contribute to the apparent large-particle dust flux. Another possibility is that small numbers of large dust particles actually reached the atol l , following trajec- tories some 8,000 to 10,000 km from their Asian source. This is cer-

DEPOSITION OF ATMOSPHERICMINERALPART1CLES 135

Table 2. The Measured and Estimated Total Atmospheric Dust Input to the North Pacific Ocean*

Measured Measured New Latitude Dust Flux Total Dust Flux Estimated Dust Flux

(°N) (<20 ~m) (<20 ~m + >20 ~m) (<20 ~m)

(1012 g y r -1) (1012 g yr - I ) (1012 g yr - I )

40 - 50 - - 4.2 * 1.1

25 - 40 7.8 12 7.5 * 1.8

15 - 25 4.0 5.6 3.8 * 1.3

5 - 15 3.1 3.7 4.4 * 0.8

0 - 5 0.6 0.7 0.5 * 0.1

Total 20 ± 3

*0 - 50*N, 150°E - 130°W: 43 x 106 km 2

ta in ly not without precedent. Rahn et al. (1981) found -16 um diameter mineral part ic les in arct ic haze originating from an Asian dust storm. Transport paths for these part icles were 12,000 to 15,000 km.

Windom's (1975) simple calculation of potential transport dis- tances for dust part ic les suggested that i f dust part icles of 20 pm in diameter are injected to an al t i tude of 7 - 8 km at the location of dust storms generated in Asia (Iwasaka et a l . , 1983), these dust par- t ic les could be transported to the central North Pacif ic, up to 10,000 km from the source. Dauphin (1984) found that eolian quartz part icles larger than 20 pm in diameter were common in North Pacific deep-sea sediments. Inclusion of the dust f lux of part icles larger than 20 um would raise the estimate of the total annual dust f lux at each site by 20 - 50 percent.

3.5. Total Annual Atmospheric Dust Input to the North Pacific

The tota l annual atmospheric dust input was determined for the area of 43 x 106 km 2 between the equator and 50°N and between 150°E and 130*W in the North Paci f ic (see Figure 1 and Table 2). This area was divided into 5 l a t i t ud ina l zones, 4 of which were represented by Network stat ions. The measured dust f lux at each stat ion was used for the annual dust input to the ocean in the four zones. The measured to ta l dust f luxes (small plus large par t ic les) are also presented, and

136 MITSUO UEMATSU ET AL

Exchange Program (SEAREX; Duce, 1982); in this study, the atmospheric mineral aerosol (dust) concentrations are being measured at seven island stations of the SEAREX Asian Dust Study (SADS) Network in the North Pacific (Uematsu et al . , 1983). These stations are located from 4°N to 53°N. A strong seasonal variabil i ty has been observed in dust concentrations at these stations, with high dust concentrations occur- ring from February to dune and low concentrations from July to January each year. A latitudinal gradient in the mean annual atmospheric dust concentrations was also observed, with the highest concentrations oc-

• • °

curring in the mdlatltudes, 30 to 50°N. Wind transported dust appears to be a significant source of marine sedimentary material in the North Pacific. Several investigators (Rex and Goldberg, 1958; Griffin and Goldberg, 1968; Dauphin, 1983) have found the highest concentrations of eolian mineral components such as quartz and i l l i t e in sediments in the midlatitude regions of the North Pacific. In the western North Pacific, the mineralogy of the mineral aerosol is identical to that of the underlying sediments (Blank et al . , 1985).

There has been only one direct measurement of the present-day deposition of mineral particles from the atmosphere to the North Pacific; Arimoto et al. (1985) measured the input of mineral aerosol at Enewetak in 1979. Tsunogai et al. (1982) and Honjo et al. (1982) measured fluxes of mineral particles in the water column of the North Pacific.

Atmospheric dust concentrations varied from less than 0.01 ~g m -3 to 5 pg m -3 at the SADS Network sites over an annual cycle (Uematsu et al . , 1983). Precipitation amounts and frequencies also varied significantly from season to season and from station to station over the North Pacific. Because of these great variabil it ies, an accurate evaluation of the atmospheric dust flux over and into the North Pacific requires intensive sampling over an extended time and space scale.

We report here on the total atmospheric deposition of mineral particles collected at four SADS Network sites using a device constructed to dimensions comparable to those of standard rain gages. The collection period for these deposition samples was synoptic with the aerosol sampling program from January, 1981 to March, 1982, as presented by Uematsu et al. (1983). The interrelationships between the dust deposition rates and the atmospheric dust concentrations are discussed in this paper.

2. COLLECTION AND ANALYTICAL METHODS

2.1. Sample Collection and Handling

Total (wet plus dry) deposition samples of mineral particles were collected at seven stations of the SADS Network. The collectors were open continuously over the two-week sampling intervals. Each two-week deposition sample coincided with two one-week air f i l t e r samples. The deposition collectors were placed in close proximity to the air sampling systems on the windward coasts of the islands and as close to

DEPOSITION OF ATMOSPHERIC MIN ERAL PARTICLES 137

ACKNOWLEDGEMENTS

We wish to thank the following for their continuous efforts to collect samples during this extended period: the staff at the Naval Oceanog- raphy Command Detachment at Midway; Dr. R. McDonald at the University of Hawaii; Dr. P. Colin and the staff at the Mid-Pacific Research Laboratory at Enewetak; Dr. M. Vitousek from the University of Hawaii and his co-workers at Fanning; and the numerous personnel at the SADS Network stations. We also thank Mr. V. Chisholm, the SEAREX coordina- tor, for his excellent management of the network. We gratefully acknowledge the staff of the Rhode Island Nuclear Science Center for providing irradiation and counting fac i l i t ies. Supported by NSF grants OCE 77-13072, OCE 79-17877, OCE 81-11895, OCE 81-12106, and OCE 84-05605 as part of the SEAREX program.

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deposition of atmospheric mineral particles in the north pacific ocean

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