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Volume 7, Number 4 13 April 2006 Q04M07, doi:10.1029/2005GC000977
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525-2027
Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise Clifton S. Buck and William M. Landing Department of Oceanography, Florida State University, 325 OSB, Tallahassee, Florida 32306-4320, USA (
[email protected])
Joseph A. Resing and Geoffrey T. Lebon Joint Institute for the Study of the Atmosphere and the Ocean, University of Washington and NOAA-Pacific Marine Environmental Laboratory, NOAA/PMEL Building #3, Seattle, Washington 98115, USA
[1] Dust aerosol samples were collected across the western North Pacific Ocean during May–June 2002. Samples were analyzed for soluble aerosol Fe(II), Fe(II) + Fe(III), and Al as well as major cations and anions. The aerosol samples were leached using a 10 second exposure to either filtered surface seawater or ultrapure deionized water yielding a measure of the ‘‘instantaneous’’ soluble fraction. A variety of analytical methods were employed, including 57Fe isotope dilution high-resolution ICP-MS, energy dispersive X-ray fluorescence, graphite furnace AAS, ion chromatography, and the FeLume chemiluminescent technique. Fe was found to be more soluble in ultrapure deionized water leaches, especially during periods of higher dust concentrations. Fe solubility averaged 9 ± 8% in ultrapure water leaches and 6 ± 5% in seawater leaches. Significant correlations were found between both soluble aerosol FeT and soluble Fe(II) concentrations and aerosol acidity; however, the percentages of soluble aerosol FeT and Fe(II) did not correlate with aerosol acidity We also did not observe significant correlations between total and soluble aerosol Fe concentrations and the concentrations of either particulate Fe or dissolved Fe in surface waters. Components: 8972 words, 14 figures, 1 table. Keywords: dissolved iron; aerosol iron; aerosol aluminum; atmospheric deposition. Index Terms: 4801 Oceanography: Biological and Chemical: Aerosols (0305, 4906); 4875 Oceanography: Biological and Chemical: Trace elements (0489) Received 23 March 2005; Revised 27 September 2005; Accepted 15 December 2005; Published 13 April 2006. Buck, C. S., W. M. Landing, J. A. Resing, and G. T. Lebon (2006), Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise, Geochem. Geophys. Geosyst., 7, Q04M07, doi:10.1029/2005GC000977. ————————————
Theme: Biogeotracers in the Northwest Pacific Ocean Guest Editor: Michiel Rutgers van der Loeff, Williams M. Landing, Catherine Jeandel, and Rodney T. Powel
1. Introduction [2] It is well known that mineral dust serves as an important source for a variety of key trace elements to the surface ocean. The transport of biologically important elements such as Fe is especially critical to primary production in large areas of the world’s Copyright 2006 by the American Geophysical Union
oceans as it is often thought to be the limiting nutrient in High Nutrient Low Chlorophyll areas [Boyd et al., 1999; Coale et al., 1996, 2004; Martin et al., 1994; Martin and Fitzwater, 1988; Martin and Gordon, 1988]. Despite making up 3.1–3.5% of upper continental crustal material [Wedepohl, 1995; Taylor and McLennan, 1995] Fe is often 1 of 21
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available only in trace amounts because of its low solubility and high biological and particle reactivity and therefore is often the limiting nutrient in areas that are replete in other nutrients. [3] While the importance of Fe is well understood, the magnitude of aerosol Fe deposition and its subsequent dissolution in the surface ocean have not been well quantified. Duce et al. [1991] estimated global dust deposition to range around 910 Tg yr 1 with approximately half of this amount being deposited in the North Pacific Ocean (450 Tg yr 1). The source of this mineral dust is primarily from Asian deserts with a large plume generally moving out from the coast in the spring. Islandbased measurements of mineral aerosol concentrations in this area of the Pacific Ocean were used to calculate contours of aerosol Fe flux, including wet and dry deposition, and were found to range from 1000 mg m 2 yr 1 to 10 mg m 2 yr 1 between 140 and 160E. More recent dust transport/deposition models show a wide range of dust flux estimates to the North Pacific. Using the DEAD model, Zender et al. [2003] calculate a dust flux of 31 Tg yr 1 to the North Pacific. The DEAD model uses size dependent deposition velocities and subcloud scavenging efficiencies. By comparison, the GOCART model [Ginoux et al., 2001] predicted a dust flux of 92 Tg yr 1. This figure was derived using a deposition velocity that was assumed to be equivalent to the exchange velocity for heat and moisture at the surface as derived from GEOS DAS data [Takacs et al., 1994]. Wet deposition rates were estimated using the same scavenging rate as was used for sulfate aerosols as described by Chin et al. [2000]. Using the assumption that mineral dust is 3.5% Fe, the calculated total aerosol Fe fluxes ranged from 1–17 Tg Fe yr 1. In the DEAD and GOCART models, the majority of this dust flux (90%) is deposited by wet deposition. Duce et al. [1991], using a scavenging ratio of 1000, estimated that 70% of the aerosol Fe deposition to the North Pacific was associated with wet deposition. In contrast, Gao et al. [2001] emphasized the seasonal nature of the dust flux to the North Pacific, showing maximum fluxes in Spring and Fall, a total annual flux of 3 Tg Fe yr 1, and roughly 50% being deposited by wet deposition on an annual basis. This figure was derived assuming that the wet removal of dust in the atmosphere was proportional to a precipitation rate of 1 cm d 1. The uncertainties in estimates given by these models stem not only from the episodic nature of dust events but also from the significant uncertainty in the factors used to estimate model param-
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eters such as deposition velocities and scavenging ratios. In addition, estimates of mineral dust concentrations have not been compared with actual aerosol sampling data for the vast majority of the open ocean. [4] While studying Fe deposition, it is also advantageous to measure Al deposition and solubility. Al is a useful tracer of mineral dust input to the oceans because it exists in continental materials at a relatively constant concentration of about 8.0% [Wedepohl, 1995]. In the ocean, Al is found in very low concentrations (95% of the soluble Fe is recovered in the first 100 mL of leachate. The filtration rig was rinsed 3 times with ultrapure deionized water between samples to prevent cross contamination of samples. At Florida State University, the samples were analyzed for soluble Fe using the 57Fe isotope dilution method of Wu and Boyle [1998] on a Finnigan Element I high-resolution magnetic sector ICP-MS.
2.4. DI Water Solubility Measurements [10] A replicate 47 mm polypropylene filter was leached with unacidified DI water (pH 5.6) using the same method as described above. In this case, the samples were immediately frozen before analyses at FSU. These samples were then analyzed for major anions, including NO3 , SO24 , and oxalate using a Dionex 4500i ion chromatograph before being acidified to 0.024M HCl. A sequential leaching experiment was performed confirming that 99% of soluble anions were leached in the first 100 mL of leachate. The samples were subsequently analyzed by graphite furnace atomic absorption spectrometry for soluble Fe and Al on a Varian Spectra AA-640Z GFAAS, as well as soluble Na using flame AAS.
2.5. Aerosol FeT and AlT [11] The 47 mm Nuclepore polycarbonate filters were analyzed at the NOAA/PMEL laboratory using energy-dispersive X-ray fluorescence (EDXRF) for aerosol FeT and AlT, as well as for other elements. Sample analysis was conducted using a variation of methods discussed by Feely et al. [1991] on a Spectro X-Lab 2000 X-ray Fluorescence Spectrometer. Standardization was accomplished by suspending various certified particulate matter standards in water and depositing them onto filters as evenly distributed thin films. The standards used were BHVO, GSD-5, GSD-6, GSD-7, GXR-1, GXR2, GXR-4, GXR-6, SS-1, MAG-1 MESS-1, NIST-1206, NIST-1633, and PACS-2. Govindaraju [1994] provides a description of these standards (except SS-1) and their elemental content. Elements that were detectable included Fe, Al, Na, Mg, Si, S, Cl, K, Ca, Br, Sr, Cu, Zn, and Ba. This method also analyzes for P, Ti, V, Cr, Mn, Co, Ni, Ga, As, Se, Rb, W, Pb, and 3 of 21
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Bi but these elements were all below their respective detection limits. It is important to note that the presence of Br in these samples can create an interference with the measurement of Al by EDXRF in samples containing high sea salt. The problem is caused by a secondary Br peak that lies directly beneath the primary Al peak resulting in an overestimation of Al. The AlT values we report have been corrected for this interference.
2.6. Back-Trajectory Analyses [12] The NOAA Air Resources Laboratory Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT, FNL data set) was used to calculate 5-day back-trajectories for the start of each sampling period [Draxler and Rolph, 2003; Rolph, 2003]. Though our aerosol samples were collected at a height of 10 meters above the sea surface, there is much uncertainty as to the behavior of air masses with the marine boundary layer (MBL) of the atmosphere. Analysis of sea level pressure during the cruise period and more recent measurements of MBL thickness showed that the MBL is typically between 400 and 1200 meters thick. We ran simulations for each time period with ending heights of 600, 1000, and 1400 meters and found similar results for each. The ensemble form of the model was also used (ending height of 1000 meters) to provide offsets of 191 km in the X-Y plane to allow for better evaluation of the effects of small-scale meteorological conditions on the trajectories. We recognize that the accuracy of these trajectories decreases as they move back in time and thus did not model the trajectories for longer than 5 days.
3. Results and Discussion 3.1. Aerosol AlT and FeT [13] Figures 1a and 1b and Table 1 show the concentrations of aerosol AlT and FeT. What is clear from these figures is that there were three periods of distinctly elevated dust during the course of this cruise in addition to a fourth, smaller dust event. Other researchers conducting aerosol measurements during this cruise agree with both the timing and intensity of these dust events (Uematsu, personal communication, 2004). The first event occurred between May 3 and 5 (29N 141E to 34N 147E) to the southwest of Japan and continued as the ship moved to the northeast. This event resulted in the highest AlT concentrations, on the order of 49,000 pmol m 3 of filtered
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air. Assuming a dry deposition velocity of 1 cm s 1, this would result in a dry deposition of AlT of 43,000 nmol m 2 d 1 to the surface ocean. FeT concentrations were also high, at nearly 14,000 pmol m 3, yielding an estimated dry flux on the order of 12,000 nmol m 2 d 1. The 5-day back trajectory presented in Figure 2a shows that the air mass associated with this dusty period moved over northern China and Mongolia and was subsequently cycled over the northern portion of Japan. Recent published estimates of the relative importance of wet versus dry deposition to the open ocean vary tremendously, with dry deposition ranging from as low as 10% [Zender et al., 2003) to as high as 50% [Gao et al., 2003] of the FeT deposition. Thus our calculated dry deposition fluxes may be 2–10 times lower than the total (wet plus dry) FeT deposition in these waters. [14] The second dust event occurred as the ship moved northeast around 45N. The highest concentration of FeT was measured on May 13 (44N 155E) at 4,200 pmol m 3 and a flux of 3,500 nmol m 2 d 1. AlT concentrations during this same period were measured at nearly 15,000 pmol m 3 and totaled a flux of 13,000 nmol m 2 d 1. Five day back-trajectories for this event were examined and show that the air masses responsible for this event passed over Russia before moving east past the islands of Japan (Figure 2b). [15] Our cruise track followed 170E from May 16–23 (50N to 24N) and crossed through a third dust event between the latitudes of 41 and 34N. The peak AlT concentration occurred on May 18 at 3,800 pmol m 3 with an FeT concentration of 1,450 pmol m 3. These concentrations are much lower than observed during the first two events. The estimated dry fluxes were 3,300 and 1,200 nmol m 2 d 1 for AlT and FeT, respectively. Figure 2c shows that this air mass appears to have been over the ocean for most of the 5-day period prior to sampling and much of the original load of continental dust may have already been removed by wet and/or dry deposition. [16] The final dust signal was detected on May 31 northwest of the Hawaiian Islands at 24N 158W. It was the smallest of the four events but was still quite significant, with AlT and FeT concentrations 3,800 pmol m 3 and 1,100 pmol m 3, respectively. These concentrations yield estimated fluxes of 3,200 and 1,000 nmol m 2 d 1 for AlT and FeT, respectively. This dust event is important because neither the 5-day back-trajectory shown in Figure 2d, nor a 10-day back-trajectory simulation 4 of 21
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Figure 1. (a) Concentrations of aerosol AlT in pmol m . Each symbol represents one 10-hour sampling period. All samples that have anomalously high AlT concentrations relative to SiT (noted in Table 1) have been plotted as
