Dissolution of aerosol iron in the surface waters of the North Pacific and North Atlantic oceans as determined by a semicontinuous flow-through reactor method

Authors


Abstract

[1] Aerosol iron dissolution in oceanic waters is determined with a semicontinuous batch-leaching method. In this procedure, aerosol samples that were collected from the marine boundary layer were leached in an all-Teflon flow-through reaction chamber by multiple aliquots of ∼40 mL 0.4 μm filtered oceanic surface water collected using ultraclean techniques. Each aliquot of seawater is permitted to leach the aerosol sample in the reaction chamber for a predetermined time before the seawater solution is separated from the aerosol sample by a 0.4 μm Nucleopore filter membrane. A new aliquot of seawater is then added to the leaching chamber to continue leaching the same aerosol particles retained on the filter. The procedure is repeated with progressive increases in leaching time. The method allows the Fe released from the aerosol particles to be continuously removed from the system before saturating the complexation capacity of the seawater-leaching solution or adsorbing onto the leaching chamber wall. This method results in accurate measurements of seawater-dissolvable labile aerosol Fe that is independent of the total aerosol Fe concentrations in the sample. The aerosol Fe solubility determined using this method ranges from 3.5 ± 1.5% for the North Atlantic during September 2005 to 5.7 ± 2.0% for the North Pacific during April 2005 and April 2006.

1. Introduction

[2] Eolian iron (Fe) deposition is a major source of Fe to the remote regions of the oceans and is considered to have played a critical role in controlling oceanic biological production and the uptake of atmospheric CO2 during geological times [Broecker and Henderson, 1998; Falkowski, 1997; Watson et al., 2000]. Although modeling the oceanic Fe cycle and its effect on natural biogeochemistry of the ocean requires understanding the mechanism and flux of aerosol dust transport and deposition [Mahowald et al., 1999; Fung et al., 2000; Parekh et al., 2004], large uncertainties exist in the estimate of atmospheric available Fe flux to the ocean because of the difficulties in quantifying fluxes of episodic and sporadic eolian Fe input [Gao et al., 2001] and because of the difficulties in measuring accurately the percent dissolution of aerosol Fe in oceanic surface waters [Jickells et al., 2005; Mahowald et al., 2005]. The aerosol Fe solubility reported in the literature ranges across 3–4 orders of magnitude (0.01–80%) [Hodge et al., 1978; Crecelius, 1980; Hardy and Crecelius, 1981; Moore et al., 1984; Zhuang et al., 1990; Chester et al., 1993; Zhu et al., 1992, 1993, 1997; Spokes et al., 1994; Spokes and Jickells, 1996; Jickells, 1999; Siefert et al., 1999; Edwards and Sedwick, 2001; Johansen et al., 2000; Chen and Siefert, 2003; Bonnet and Guieu, 2004; Chen and Siefert, 2003, 2004; Hand et al., 2004; Luo et al., 2005; Baker et al., 2006]. The large variation may result, not only from the inhomogeneous nature of the oceanic and atmospheric system, but also from different procedures used by researchers for aerosol Fe solubility measurements. Aerosol Fe solubility measurements are often influenced by experimental artifacts because of processes such as time-dependent aerosol Fe dissolution, aerosol Fe species modification, Fe adsorption on the leaching chamber walls, and the saturation of seawater Fe-binding natural organic ligands by aerosol Fe.

[3] In this paper, we describe a new method that uses seawater samples collected with ultraclean techniques to leach aerosol samples in a continuous flow reactor. The procedure allows the Fe released from aerosol particles to be carried away from the system before the released Fe saturates the complexation capacity of the seawater sample and/or adsorbs on the wall of leaching chamber, thus minimizing potential artifacts associated with aerosol Fe solubility measurements by conventional batch-leaching methods. Results from this method are shown for samples collected in the North Atlantic and North Pacific oceans during the respective peak dust seasons.

2. Sample Collection and Fe Analysis

[4] Uncontaminated oceanic surface water samples that were used for aerosol dissolution experiments were collected using ultraclean techniques [Vink et al., 2000; Wu and Boyle, 2002] in the tropical North Pacific and subtropical North Atlantic oceans during April and September 2005 R/V Wecoma and Oceanus cruises and an April 2006 R/V Melville cruise. The concentrations of dissolved Fe (0.05–0.8 nM) and Fe-binding organic ligands (1–2 nM) in these samples were determined by isotope dilution Inductively Coupled Plasma Mass Spectrometry (ID-ICPMS) [Wu and Boyle, 1998] and Competitive Ligand Equilibration adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) [Croot and Johansson, 2000]. The 0.4 μm filtered seawater samples were frozen in Teflon bottles without acidification. The frozen samples were thawed to room temperature before they were used for aerosol-leaching experiments.

[5] Aerosol samples were collected from marine boundary layer on board R/V Wecoma in the tropical to subtropical North Pacific in April 2005, R/V Oceanus in the western North Atlantic in September 2005, and R/V Melville in the eastern North Pacific in April 2006. The aerosol-sampling device consists of 0.4 μm Nucleopore polycarbonate membranes mounted on two identical 47 mm diameter open-face Savillex Teflon filter assemblies that were attached to a high-volume air pump (GAST 0623-101Q 0688DX, 0.25 HP, 8 m3/h free flow at atmosphere pressure) through 3/8″ polyethylene tubing and a Y connector. The filter assemblies were mounted vertically with the open end facing downward in an uncapped 5-L wide mouth polyethylene jar. The jar was tied to a pole at the bow. A flow meter was attached to the exhaust port of the pump to monitor the flow rate, although the flow rate is not necessarily known precisely for our purpose of collecting the aerosol samples for solubility work. The pump was turned on to start sample collection when the ship sailed against prevailing wind at 5–10 knots. This procedure prevents air masses passing the ship from circling back because of atmospheric circulation, ensuring that the aerosol samples collected were uncontaminated by ship exhaust. Each aerosol sample was collected during a 12–24 h period at a pumping rate of ∼2000–2500 Lph. Lower pumping rates were encountered in poor weather conditions when the filter pores were clogged by a dry sea salt particle coating due to sea spray. In this situation, because the aerosol samples collected on the Nucleopore filter membranes were suspicious of modification by sea spray, we discuss here only those samples collected in good weather conditions when the pumping rate remained at 2000–2500 Lph for 12–24 h.

[6] Before sample collection the Nucleopore filter membranes were mounted on individual filter holders, cleaned by passing dilute (∼0.05 nM) HCl and deionized (DI) H2O through each filter, and dried in class 100 clean benches. The Fe concentration of the last 30 mL dilute HCl passing the filter was measured, and the cleaning was repeated until the Fe concentrations in the HCl solution were below 0.1 nM.

[7] After sample collection the aerosol sample filters were removed from the holder, placed in acid washed Petri dishes, and stored in the dark at −30°C until they were used for leaching experiments. Although Fe speciation may change during sample storage, this artifact may be minimized by leaching freshly collected samples in a class-100 environment on board the research ship using seawater samples collected from the sea surface. The ability for seawater organic ligands to complex labile Fe species from aerosol particles during the leaching procedure may minimize the influences of the Fe species modification caused by sample storage on the overall Fe solubility if the Fe speciation change due to sample storage does not lead to the formation of stable Fe minerals.

[8] Dissolved Fe in the seawater samples was determined by ID-ICPMS [Wu and Boyle, 1998] after the samples were acidified to pH 2 by adding 2 mL 6N quartz distilled HCl per liter of the sample. Total Fe in the aerosol sample filters was determined by digesting the filter in a mixture of 1 mL 12 N ultrapure HCl (Optima grade purchased from Fisher Scientific) and 200 μl 16 N quartz-distilled HF in a 2-mL Teflon vial placed on a hot plate and warmed at 80°C overnight followed by ID-ICPMS measurements [Wu and Boyle, 1998]. The total aerosol Fe adsorbing on the wall of the leaching chamber was determined by leaching the chamber wall with a mixture of 40 mL 6 N quartz-distilled HCl and 500 μl 16 N quartz-distilled HF at room temperature overnight before Fe analysis by the ID-ICPMS method [Wu and Boyle, 1998].

3. Results and Discussion

[9] In order to develop a technique for accurate measurement of aerosol Fe solubility in seawater, we studied the major processes that influence aerosol Fe solubility measurements. Three factors were found to be the most critical: the role of Fe-binding natural organic ligands in seawater samples, the aerosol Fe speciation, and Fe precipitation and adsorption on the container wall. In the text below we will first discuss major influences of these factors on the accuracy of aerosol Fe measurements and then describe a new method that we have developed to minimize these interferences, and finally we discuss the aerosol solubility results obtained using the new method.

3.1. Role of Fe-Binding Organic Ligands in Seawater

[10] In organic-free seawater at room temperature, Fe (III) solubility is ∼0.08 nM for freshly precipitated amorphous Fe oxyhydroxide [Wu et al., 2001] and ∼0.01 nM for aged and more crystalline Fe oxides [Liu and Millero, 1999, 2002]. In natural seawater containing Fe-binding dissolved organic ligands, Fe solubility can be >1.0 nM [Kuma et al., 1996, 1998]. The Fe complexation/stabilization by dissolved organic ligands results in surface and deep oceanic water dissolved (<0.4 μm) Fe concentrations (∼0.2–0.7 nM [Wu et al., 2001]) much higher than the Fe (III) inorganic hydrolysis limit (∼0.08 nM [Wu et al., 2001]). Because surface water dissolved Fe concentrations have exceeded Fe (III) inorganic solubility, if free forms of Fe-binding organic ligands are not present, the oceanic surface water will not be able to further leach Fe from the aerosol particles, unless the aerosol Fe dissolution is caused by the release of aerosol Fe(II). In order to simulate conditions of aerosol Fe dissolution in oceanic surface waters where free Fe-binding organic ligands are present (in the surface waters of the subtropical North Atlantic and North Pacific, concentrations of free Fe-binding organic ligands are 1–2 nM [Rue and Bruland, 1995; Wu and Luther, 1995; van den Berg, 1995]), saturation of seawater organic ligands by the Fe released from aerosol dust particles must be minimized. The seawater samples used for leaching the aerosol samples should be collected using clean techniques to prevent Fe contamination. The contamination may introduce Fe to titrate the excess Fe-binding dissolved organic ligands in the water sample.

3.2. Chemical and Physical Speciation of Fe in Aerosol

[11] Fe species in aerosols collected from the marine boundary layer can be categorized into three groups. The first group is Fe (II) species that is produced by photochemical reduction of Fe-bearing minerals during long-range transport of aerosol particles in the atmosphere and at the relatively low pH encountered at ambient humidity or in a cloud solution [Zhu et al., 1993; Spokes et al., 1994; Spokes and Jickells, 1996; Siefert et al., 1999]. When aerosol particles contact seawater, Fe (II) can be released from aerosol particles into the solution almost instantaneously (within seconds). The released Fe is then rapidly (within minutes) oxidized to Fe (III) and strongly hydrolyzes to Fe (III) oxyhydroxide colloidal particles at seawater pH of 8.1. These particles are small enough to pass through the 0.4 μm filter membranes commonly used to distinguish between dissolved and particulate Fe and can be mistakenly identified as dissolved Fe even when free forms of Fe-binding organic ligands are not present in seawater. As shown in Figure 1, within a few hours after the Fe addition all the 24 nM Fe (II) added to a seawater sample can still pass through 0.4 μm Nucleopore membrane. Because of the fast oxidation kinetics of Fe (II) in pH 8.1 oxygenated seawater, by the time these samples were filtered through 0.4 μm Nucleopore membrane, much of the 24 nM Fe (II) added into the samples should be in the form of Fe (III) oxyhydroxide colloids which apparently are sufficiently small in size to pass through the filter pores. The concentrations of the dissolved organic ligands in our seawater samples are less than 2 nM. These concentrations are too low to keep 24 nM Fe in solution by forming dissolved Fe-organic complexes.

Figure 1.

Dissolved Fe(II) (24 nM) was added to an aliquot of unacidified 0.4 μm filtered Sargasso Sea surface water (background Fe ∼0.5 nM). The mixture was incubated at room temperature followed by filtration through 0.4 μm Nucleopore filter. The filtrate was acidified and measured for dissolved Fe by ID-ICPMS [Wu and Boyle, 1998]. The unfiltered sample was measured as total Fe.

[12] The second group of aerosol Fe species is composed of amorphous Fe oxyhydroxides and Fe oxides. These Fe species are naturally present on the dust source mineral particles or result from Fe (II) oxidation and the subsequent Fe (III) hydrolyzation. Fe (II) oxidation can occur during aerosol sample collection or when aerosol samples are stored frozen in the dark (which is the common method for storing aerosol samples). The Fe (II) oxidation also can occur in natural conditions such as cloud formation and evaporation (aerosol particles typically experience ∼10 wetting and drying cycles before rainout [Junge, 1964; Spokes et al., 1994]). This second group of Fe species has a “loose” structure [Zhu et al., 1993] and thus can be slowly released from aerosol particles into seawater solutions by complexation with natural dissolved organic ligands in seawater [Rue and Bruland, 1995; van den Berg, 1995; Wu and Luther, 1995], leading to a time-dependent Fe dissolution. If the organic complexation capacity of the water is saturated at a high dust to seawater ratio or when the seawater solution is contaminated by Fe prior to use in the leaching experiment, this group of Fe species will not be released into the seawater solution because of low-Fe (III) inorganic hydrolysis solubility in seawater at pH 8.1 (∼0.01–0.08 nM, [Liu and Millero, 1999; 2002; Wu et al., 2001]). However, when a low dust to seawater ratio is used to determine the dissolution of this Fe group from aerosol particles, the low-aerosol Fe dissolution kinetics and the substantial Fe adsorption on the container walls may result in an undetectable signal for this Fe group. These processes can be illustrated by our batch-leaching experiments where aerosol samples collected using ultraclean procedures [Wu et al., 2001; Wu and Boyle, 2002] from the marine boundary layer in the tropical North Atlantic near Barbados were leached in acid-washed 250 mL polyethylene bottles in the dark at room temperature (Figure 2). As total aerosol Fe concentration decreases, the relative percent Fe dissolution from aerosol samples increases to reach a maximum of ∼0.8% at 100 nM aerosol Fe and then decreases to a negative value at 23 nM aerosol Fe. While the saturation of Fe solubility/organic complexation capacity of the seawater by the released Fe at high-aerosol Fe to seawater ratios may result in a decrease in measured aerosol Fe solubility with increasing total aerosol Fe, the Fe adsorption on bottle wall at low total aerosol Fe content may lead to the negative Fe dissolution with time.

Figure 2.

Aerosol samples (F0–F8) were collected from marine boundary layer using 0.4 μm Nucleopore membranes in a sector field aerosol sampler on board R/V Knorr during the MP3 cruise in the tropical North Atlantic near Barbados (0–10°N, 45–49°W) in July 2001. Aerosol samples were frozen and stored for 2 months before use for experiments. Thawed aerosol samples were leached with ∼200 mL 0.4 μm filtered surface seawater in 250 mL acid washed polyethylene bottles at room temperature for 24 h before filtration and measurement of dissolved Fe in the solution using ID-ICPMS method [Wu and Boyle, 1998]. The percent Fe dissolution is based on Fe concentration difference in seawater before and after the leaching of Fe from aerosol samples.

[13] The third group of Fe resides inside aerosol Al-silicate lattice or is present as crystalline Fe minerals such as goethite and hematite. This group of Fe species is kinetically more resistant to weak acid leaching dissolution than the above two Fe groups. Although this group of Fe may be slowly leached from aerosols by strong acids [Kim et al., 1999], the rate of dissolution for this group of Fe by strong Fe-binding natural organic ligands are too slow to be detected in laboratory experiments [Barbeau and Moffett, 2000; Kraemer, 2004] although the dissolution of this inert Fe phase at prolonged leaches (months) by seawater at natural oceanographic environments may potentially contribute to the overall eolian flux of dissolved Fe to the ocean.

3.3. Fe Precipitation and Adsorption on Container Wall

[14] Fe adsorption on the bottle wall can result in an underestimation of aerosol Fe solubility. As shown in Figures 1 and 2, when the amount of Fe released from aerosol particles in the form of Fe (II) exceeds the Fe-binding capacity of the water (as may occur at a high aerosol to seawater ratio), the Fe (II) in excess of free organic ligands in the water would be oxidized and converted to Fe (III) oxyhydroxide colloids. Some of these colloids can be converted to filterable-size particles, whereas others can be absorbed on the container wall and be removed from the solution, leading to an underestimation of the dissolvable Fe in the aerosol. As shown in Figure 1, when 24 nM Fe (II) were added to the seawater sample in 250 mL polyethylene bottles and incubated at room temperature in the dark, the amount of Fe remaining in the solution decreases with increasing sitting time, suggesting that the added Fe (II) were removed from the solution by precipitation and adsorption onto the bottle wall. These results suggest that time-dependent aerosol Fe dissolution cannot be accurately measured in a batch-leaching mode using a high aerosol to seawater ratio because the Fe released from aerosol particles can be lost from the solution by Fe precipitation and adsorption at prolonged leaching time.

[15] Although this problem can be overcome by using a low aerosol to seawater ratio, the small amount of Fe released from the aerosol particles at low aerosol to seawater ratio tends to adsorb on the container wall, resulting in an underestimation of the true aerosol Fe solubility. When aerosol to seawater ratios are low, the organic complexation capacity of the seawater samples are not saturated and all the released Fe should be complexed by excess dissolved organic ligands. However, these newly formed Fe-organic complexes tend to adsorb onto the wall surfaces of the acid-washed polyethylene bottles. As shown in Figure 3, when 1 nM Fe(III) is added to a filtered seawater sample containing 0.5 nM dissolved Fe in a acid-washed polyethylene bottle, the amount of dissolved Fe lost from the solution by adsorption onto bottle wall increases from 0.2 nM at 2 h to ∼0.5 nM at >30 h of incubation. Although this problem can be minimized by using Teflon bottles or by conditioning the bottle with seawater samples, the time-dependent increases in dissolved Fe concentrations in the bottle as aerosol Fe is released into solution with time in batch-leaching mode make it difficult to accurately determine the influence of Fe adsorption.

Figure 3.

Sargasso Sea surface water samples were collected using ultraclean sampling methods, filtered through a 0.4 μm Nucleopore membranes, and stored in polyethylene bottles at room temperature for 1 month before use for the experiment. The aged seawater sample was spiked with 1 nM dissolved Fe(III), incubated in 250 mL polyethylene bottles at room temperature, and filtered again. Dissolved Fe was determined by ID-ICPMS [Wu and Boyle, 1998].

3.4. A Semicontinuous Batch-Leaching Procedure for Determining Aerosol Fe Solubility in Seawater

[16] We have developed a semicontinuous batch-leaching method to minimize the above potential artifacts. The aerosol-leaching experiments were conducted in an all-Teflon flow-through reaction chamber (Figure 4) inside a class-100 clean bench at room temperature. The reactor is composed of an acid-washed 0.4 μm Nucleopore filter membrane mounted on an open-face Savillex filter holder. The draining port of the filter holder is attached to a Teflon valve to control the rate of the solution flowing through the filter membrane. During the leaching experiments, aerosol sample filters were placed on the clean 0.4 μm Nucleopore filter membrane mounted on the filter holder, and seawater was added to the holder and allowed to sit at room temperature. At the end of the leaching time the valve below the holder was opened, and the samples were collected in 250 mL polyethylene bottles. The valve was then closed, and a new batch of seawater was poured onto the filter holder to leach the aerosol samples retained by the Nucleopore filter membrane. This procedure was repeated with gradual increases in leaching time. Before the experiments the device was cleaned, and a procedural blank was measured as the difference in Fe concentrations between seawater samples before and after passing through the device. The frozen aerosol sample filters were first thawed at room temperature before being used for leaching experiments.

Figure 4.

A flow-through reactor for aerosol Fe solubility measurement.

[17] To minimize the saturation of seawater organic complexation capacity by the Fe released from aerosol particles, each aerosol sample was sequentially leached with multiple replicates of ∼40 mL 0.4 μm filtered seawater sample. Each replicate of seawater sample leaches aerosol samples inside the reaction chamber over predetermined time intervals (ranging from ∼8 min to ∼4 h). At the end of each time interval the seawater solution is filtered through the 0.4 μm Nucleopore filter membrane mounted in the flow-through leaching chamber. A new aliquot of seawater is then poured into the leaching chamber to continue leaching the same aerosol particles retained on the Nucleopore filter. In this procedure the Fe released from aerosol particles is continuously drained from the system before the released Fe adsorbs onto the leaching chamber wall. The sequential leaching of one aerosol sample using multiple batches of seawater ensures that the aerosol Fe is repeatedly leached by fresh seawater samples containing free forms of Fe-binding organic ligands without saturating the complexation capacity of the seawater. Thus the procedure can be used to accurately measure the amount of labile Fe released from aerosol particles in seawater that is independent of the total aerosol Fe in the experiments (Figure 5).

Figure 5.

Aerosol Fe solubility determined using the flow-through reactor is plotted as a function of total aerosol Fe in the sample used in the experiments. Aerosol samples were collected at peak dust season in the North Pacific (22°N, 158°W in April 2005 and 22–32°N, 123–158°W in April 2006) and in the Sargasso Sea (29°N, 69°W, in September 2005).

[18] To accurately determine time-dependent dissolution of Fe from atmospheric aerosol particles, we allow the time that the leaching solution stays in the leaching chamber to increase from 8 to 15, 40, 70, 130, and 250 min. The Fe adsorption on the Teflon-leaching chamber wall was found to be ∼0.03 nM during 8–70 min incubation and ∼0.1 nM during 250 min incubation. These values for the Teflon chamber wall are lower than those for the walls of the polyethylene bottle (Figure 3). Such low Fe adsorption on the Teflon-leaching chamber wall has a negligible effect on aerosol Fe solubility determined using this flow-through reactor method because over 80% of labile Fe is released into the leaching solution during the 8–70 min leaching steps when the Fe adsorption is near the detection limit of the method.

[19] During each leaching interval, 6–8 batches of fresh seawater were used to sequentially leach the same aerosol sample until the Fe dissolution rate had decreased to the point where increases in seawater-dissolved Fe concentrations were sufficiently low allowing for using longer leaching times. By monitoring dissolved Fe concentrations in each batch of seawater samples flowing out of the reactor, we can determine how many batches of repeated leaching at each leaching time interval are needed to allow the total Fe concentrations in the last two batches of seawater samples flowing out of the leaching chamber to be lower than the total complexation capacity of the water. Because the aerosol sample is repeatedly leached by many batches of seawater sample at each leaching time, even if organic ligands are saturated by aerosol Fe in the first few batches of leaching solution when a large aerosol sample is determined, the subsequent batches of seawater sample will have sufficient amount of organic ligands to continuously leach aerosol Fe.

[20] For aerosol samples containing less than 4000 ng Fe (at ∼3% solubility) it was found that six batches (∼40 mL each) of seawater at each leaching time were sufficient. All seawater samples at each leaching time were subsequently combined into one 250 mL sample for total dissolved Fe measurement during routine aerosol Fe solubility measurements. The total Fe concentrations in each 250 mL sample are often below 2 nM and are lower than the organic complexation capacity of oceanic surface water, especially for the samples that experienced longer leaching times (Figure 6). For small aerosol samples collected in low-dust seasons, smaller batches of seawater (20 mL instead of 40 mL per batch) can be used, and Fe concentrations in each batch of seawater can be measured, to determine aerosol Fe solubility in as low as ∼8 ng total Fe aerosol samples. For large aerosol samples such as those collected in the Atlantic near Barbados a portion of the aerosol samples collected on each Nucleopore filter, instead of the whole filter, can be used to determine aerosol Fe solubility in samples that exceed 40,000 ng total aerosol Fe. Thus this procedure is accurate for aerosol concentrations that differ by ∼4 orders of magnitude range of aerosol concentrations which is sufficient for aerosol samples collected at global ocean marine air-water boundary layer without a prerequisite of knowing the exact (within a factor of 10) amount of aerosol Fe on the sample filter. In addition, the detection limit of this method is not influenced by the background dissolved Fe concentration in the original ocean surface samples. Our ID-ICPMS method [Wu and Boyle, 1998] has a minimum resolution of ∼0.02 nM Fe or 3% (depending on whichever is larger) at a Fe concentration of 0.05–1.0 nM Fe level. When the background total Fe concentration in the original fresh seawater leaching solution is low (∼0.05 nM) (such as in samples taken in the equatorial Pacific, the subarctic Pacific, and the Southern Ocean), a high-sensitivity method [Wu, 2007] can be used to determine picomolar Fe in the samples. This new method can resolve as low as ∼0.002 nM at the 0.05 nM Fe level.

Figure 6.

Iron concentrations in 250 mL solution flowing out of aerosol leaching chamber (Figure 4). Aerosol samples were collected at peak dust season in the North Pacific (22°N, 158°W in April 2005), equatorial Pacific (0–4°N, 158°W in April 2005), and in the Sargasso Sea (29°N, 69°W in September 2005). Seawater samples used for leaching the aerosol were collected from the equatorial Pacific Ocean and have background dissolved Fe concentrations of ∼0.06 nM.

3.5. Aerosol Fe Solubility Determined by the Flow-Through Reactor Method in the North Atlantic and North Pacific Oceans

[21] The aerosol Fe solubility determined with the above flow-through reactor method is shown in Figure 7. In all cases the accumulated percent dissolution of aerosol Fe in seawater increases with increasing leaching time, suggesting that various Fe species were continuously leached from aerosol particle surfaces with time. The Fe fraction that was released at the shortest time interval (∼30 min) is 45 ± 20% of the total Fe released. This Fe fraction may include labile Fe species such as Fe (II) species, Fe (III)-containing fine grains particles smaller than the pore size of 0.4 μm Nucleopore filter membrane, and Fe (III) species that are complexed by dissolved organic ligands on the aerosol particles or in the leaching seawater solution. There is a continuous release of Fe from aerosol samples during 30 min to 60 h (Figure 7), but the rates of dissolution decreases with increasing leaching time (Figure 8), suggesting that labile Fe species such as amorphous Fe (II) oxyhydroxide and Fe (III) oxide and ferrihydrite phases on aerosol particles can be slowly released into solution by free forms of Fe-binding dissolved organic ligands in seawater, with lower amount or lability of labile Fe on the aerosol particle surfaces at the longer leaching time. The labile Fe phases released during 30 min to 60 h leaching time interval can either be originally present in the dust source or result from Fe (II) oxidation and the subsequent Fe (III) hydrolyzation during aerosol sample collection or when aerosol samples are stored frozen in the dark. The time-dependent dissolution of aerosol Fe in seawater is in sharp contrast to the dissolution kinetics of other aerosol heavy metals (such as Pb, Cu, and Cd) in seawater that tend to be released into solution within 30 min of leaching [Chester et al., 1993]. The difference may be due to the fact that these heavy metals are predominantly anthropogenic and present on the surfaces of aerosol particles in labile forms that can be released into leaching solution more quickly than the aerosol Fe. At the 60 h leaching time the rate of dissolution (Figure 8) approaches the dissolution rate of stable Fe minerals such as goethite in the laboratory experiments [Kraemer, 2004] suggesting that most of the labile Fe species may have been released by the leaching solution before this time. The Fe dissolution rate after 60 h leaching time can be limited by the slow release of Fe bound inside the matrix of aluminosilicate minerals. In all cases the Fe dissolution from aerosol samples after 60 h leaching time was 3 orders of magnitude lower than the initial rates (Figure 8). The dissolution rate after 60 h leaching time cannot be measured reliably using our new procedure (Figure 8) and was neglected in our calculations of total percent dissolution of aerosol Fe in seawater, although slow aerosol Fe dissolution may potentially occur in natural oceanographic environments and influence ocean Fe cycling in longer timescales (such as months to years).

Figure 7.

Aerosol Fe solubility in seawater was determined using the flow-through reactor (Figure 4). Aerosol samples were collected at peak dust season in the North Pacific (22°N, 158°W in April 2005 and 22–32°N, 123–158°W in April 2006) and in the Sargasso Sea (29°N, 69°W, in September 2005). Samples 4/20A and 4/20B, C5 and B5, C6 and B6, C7 and B7, C3 and B3 are duplicates collected simultaneously by our aerosol-sampling device.

Figure 8.

Rate of Fe released from aerosol samples at different leaching time for aerosol samples collected in the North Pacific, North Atlantic, and equatorial Pacific oceans. Procedures for sample collection, aerosol leaching, and Fe analysis are the same as in Figures 67.

[22] Figures 5 and 7 show that the total aerosol Fe dissolution ranges from ∼2% to ∼9% with mean values of 5.7 ± 2.0% for the North Pacific and 3.5 ± 1.5% for the North Atlantic Ocean. These values are summarized in Table 1 together with our tropical North Atlantic and Pacific results. The aerosol Fe solubility values that we obtained using the new flow-through reactor method are higher than the reported Fe solubility of ∼0.5% for soil by Fung et al. [2000] and Hand et al. [2004], the 0.001–0.02% for Saharan aerosol reported by Guieu and Thomas [1996], the <0.013% for Saharan aerosol collected near the source by Spokes and Jickells [1996], and the ≤1% for marine aerosol measured by Jickells and Spokes [2001]. The higher-Fe solubility determined by our flow-through reactor method may be due to the fact that the flow-through reactor method includes Fe dissolution at >30 min leaching time which is as high as 55% of total Fe dissolved (Figure 7) but has not been considered by the above researchers. The difference between our measured aerosol Fe solubility values and the above literature values can also result from the different aerosol samples that may be affected differently by cloud processing, or from the influence by modification during transport and mixing with anthropogenic aerosol [Prospero, 1996; Baker et al., 2006]. Baker et al. [2006] reported Fe solubility of 1.7% for Saharan dust and 5% for aerosols from other sources.

Table 1. Dissolution of Aerosol Fe in Seawater During Peak Dust Seasons in the Atlantic and Pacific Oceans as Determined by Batch Leaching and Semicontinuous Leaching Methodsa
Sampling LocationSampling DatePercent Fe DissolutionAerosol-Leaching Method
  • a

    Aerosol samples were frozen and stored for 1–6 months before use for experiments.

Tropical Atlantic (0–10°N, 45–49°W)July 20010.05–0.9batch leaching, in 250 mL Polyethylene bottles
Tropical Pacific (0–4°N, 158°W)April 20052.4semicontinuous leaching in Teflon flow-through reactor
North Atlantic (29°N, 69°W)July 20051.9–5.9semicontinuous leaching in Teflon flow-through reactor
North Pacific (22°N, 158°W)April 20052.4–5.6semicontinuous leaching in Teflon flow-through reactor
North Pacific (22–32°N, 123–158°W)April 20063.0–9.5semicontinuous leaching in Teflon flow-through reactor

[23] The 3.5–5.7% mean Fe solubility that we observed in this study falls in the range (0.7–6%) of several recent reports of aerosol Fe solubility in seawater [Chen et al., 2006; Buck et al., 2006; Baker et al., 2006]. Chen et al. [2006] used a batch-leaching method to determine a Fe solubility of ∼0.7%. Buck et al. [2006] used a flow-through leaching method similar to our 30 min leaching procedure to measure a Fe solubility of 6 ± 5% in seawater. Baker et al. [2006] used 1 M ammonia acetate (pH ∼4.7) to extract Fe from aerosol samples and reported a Fe solubility of 5%. The ammonia acetate leaching solution of pH 4.7 may be weaker than pH 1 HCl but stronger than ultrapure H2O in extracting labile Fe from the aerosol particles. The 5% solubility reported by Baker et al. [2006] for the Atlantic aerosol is slightly higher than 3.5 ± 1.5% that we observed for the Atlantic (Figure 7), although a time-dependent Fe dissolution was not considered by Baker et al. [2006]. If the difference of aerosol Fe speciation between the two studies is not considered, the above difference in measured Fe solubility may suggest that the ammonia acetate leaching releases more Fe from aerosol particles than the seawater leaching. In fact, higher-aerosol Fe solubility in ultrapure H2O than in seawater has been reported [Chen et al., 2006; Buck et al., 2006]. Although the lower-Fe (III) hydrolysis solubility in seawater is thought to be the cause of the difference, more studies are needed to understand the exact cause of these differences in Fe solubility.

[24] Figure 9 shows the aerosol Fe solubility determined with weak acid (0.1 N HCl) extraction in our flow-through reactor. The aerosol samples used in the 0.1 N HCl extractions were replicates of the same sample in which Fe solubility had been determined by the seawater-leaching method described in section 3.4. The amount of Fe released from the aerosol samples increases with leaching time (Figure 9), indicating that the labile Fe is continuously leached out of the aerosol particles by the 0.1 N HCl solution. However, at each leaching time interval the amount of Fe released by the 0.1 N HCl solution is higher than that released by seawater, especially at leaching times >1 h, suggesting that 0.1 N HCl is more effective at releasing Fe from aerosol particles than are the organic ligands in natural seawater. These results indicate that dilute acid solution such as 0.1 N HCl can leach not only amorphous Fe oxyhydroxide and Fe oxide phases from aerosol particles in a way similar to natural Fe-binding dissolved organic ligands in seawater, but also a portion of less labile crystalline Fe oxides and Fe resides inside aerosol Al-silicate lattice that cannot be released by seawater organic ligands at prolonged leaching time, leading to an overestimate of aerosol Fe solubility in seawater by the dilute acid leaching method, although the use of acids more dilute than 0.1 N may avoid such overestimation.

Figure 9.

Aerosol samples were collected in the subtropical North Pacific during April 2006 on board R/V Melville. Sample 1 was collected at 32°N 123°W. Sample 2 was collected at 31°N 128°W. Samples were leached in the flow-through reactor (Figure 4) using high-purity 0.1 N HCl and equatorial Pacific surface water.

[25] The percent aerosol Fe soluble in seawater that we observed in both oceans is higher at lower-atmospheric dust loading (Figure 10). This result is consistent with the findings by Siefert et al. [1999], Chuang et al. [2005], Baker and Jickells [2006], and Sedwick et al. [2007] which show that small size aerosol particles that are characteristics of anthropogenic origin tend to have a higher-Fe solubility. The aerosol Fe solubility that we determined in the North Pacific in April (5.7 ± 2.0%) is a factor of two of those in the Sargasso Sea in September (3.5 ± 1.5%, Figures 7 and 8) with large variations relative to the mean, suggesting that the two oceans do not have very different aerosol Fe solubility during their peak dust season. These results are surprising because anthropogenic activities in Asia are thought to result in higher-aerosol Fe solubility in the Pacific than in the Atlantic [Johnson et al., 2003; Boyle et al., 2005]. The low-aerosol Fe solubility that we observed in the Pacific may be due to the fact that our aerosol samples were collected only at the peak dust input season (in April [Merrill, 1989; Prospero, 1996; Gao et al., 2003]) when anthropogenic influences are relatively small. Higher-aerosol Fe solubility in the low-dust seasons than in high-dust seasons has been reported [Baker et al., 2006; Sedwick et al., 2005]. Because the majority of eolian Fe deposition to the ocean occurs during peak dust seasons [Gao et al., 2003; Jickells et al., 2005; Mahowald et al., 2005], the dissolution of aerosol Fe in seawater at the peak dust season with relatively low anthropogenic influences is useful for understanding natural oceanic Fe cycle, especially during preindustrial times.

Figure 10.

Aerosol Fe solubility determined using the flow-through reactor is plotted as a function of total aerosol Fe per unit air. Aerosol samples were collected at peak dust season in the North Pacific (22–32°N, 123–158°W in April 2006), and in the North Atlantic (29°N, 69°W in September 2005).

4. Conclusion

[26] Results from this study indicate that aerosol Fe dissolution in seawater is a time-dependent process that can be underestimated if (1) a short leaching time is used, (2) the complexation capacity of Fe-binding organic ligands in leaching seawater is saturated by the released Fe at high aerosol to seawater ratios, or (3) the released Fe is absorbed on the leaching chamber wall at prolonged leaching time. To minimize these artifacts, we developed a new method that uses seawater samples collected from ocean surface mixed layer using ultraclean techniques to leach aerosol samples collected from marine boundary layer in a semicontinuous flow-through reaction chamber. In this procedure, aerosol samples are continuously leached by seawater samples containing free forms of Fe-binding organic ligands, and the released Fe from the aerosol samples are carried away from the system before the Fe adsorbs to the wall of the leaching chamber. Aerosol Fe solubility measured by this procedure ranges from 3.5 ± 1.5% for the North Atlantic Ocean to 5.7 ± 2.0% for the North Pacific Ocean, suggesting that the two oceans do not have very different aerosol Fe solubility at their respective peak dust seasons.

Acknowledgments

[27] This work is supported by funding from NSF (OCE-0325031, OCE 0220978, OCE-0321402, and ARC-0612538 to J.W) and CNSF (40528007 and 90411016 to J. Wu and M. Chen) and SCSIO. We are grateful to Brian Cohn, Nathan Buck, and the technician and the crew of R/V Wecoma, R/V Endeavor, and R/V Melville for their help in sample collection during the WO503C and Tracer2 cruises. This paper benefited from fruitful discussions with Stephan Kraemer.

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