OS13F-01 INVITED
Coal Ash Aerosol in East Asian Outflow as a Source for Oceanic Deposition of Iron and Other Metals
While ocean deposition of East Asian dust is given significant emphasis as a source of biologically-active trace elements, iron in particular, dust events are episodic and highly seasonal. There is, however, a constant source of aerosol that is chemically similar to dust (albeit amorphous in structure rather than crystalline) in the ash particles emitted from many hundreds of coal-fired power plants that are sited along the entire coastal region of China and Korea. The emission controls on these facilities vary widely and, in even cases of state-of-the-art emission controls, the secondary release of ash can be significant. There are of course even more small industrial and household sources of coal combustion emissions, in most cases with little or no emissions controls. Ash from a modern coal-fired power facility in Korea has been examined chemically and morphologically with electron microscopic techniques. As is characteristic of all such facilities, two principal types of ash are present: (1) flyash, silicate glass spheres that are emitted with the smoke and removed by electrostatic precipitators; and (2) bottom ash, "clinkers" and noncombustible material sticking to the furnace walls that are mixed with water and ground after cooling, then removed as a slurry to a dumping area. In addition, iron sulfide (pyrite) is a common constituent of coal and provides both a source of sulfur dioxide gas and also molten iron spherical particles in the ash. The iron spheres then are rapidly oxidized upon cooling. Bottom ash is a more complex material than flyash in that it contains more iron and other trace metals, plus it contains varying amounts of uncombusted carbon. The post-combustion handling of bottom ash can lead to significant emissions despite the fact that little or none goes out the stack. The iron oxide spheres can also be emitted by this secondary method. The concentrations of ash can be very high in close proximity to power plants (PM10 of several hundred micrograms per cubic meter of air) and traces of these aerosols have been found in the ACE-Asia and PACDEX experiments above the Sea of Japan, the Yellow Sea and across the width of the North Pacific.
OS13F-02
Iron Solubility Depending on the Mineralogical Composition of Dust Particles
Dust deposition in open ocean is recognised as an important supply of iron for phytoplankton community. Various previous studies have shown an extremely variable solubility (0,01-80%) and numerous factors influencing this solubility, as suspended particules concentration, chemical and photochemical atmospheric process, aerosol sources (Maholwald et al., 2005). Despite these numerous studies, any factor of influence seems to be dominant enough to enable a comprehensive parameterization of iron solubility. Recently, dissolution experiment have been conducted on pure mineral that composed dust, like illite, feldpars, smectite and iron (hydr-)oxide. This study has shown that iron solubility is extremely dependent on the mineral that is considered. Iron coming from aluminosilicates is much more soluble that iron derived from iron (hyd-)oxides (Journet et al., 2008). According to these results, dissolution experiments have been led on dust particles collected in different source areas, in West Africa, and after transport, in tropical Atlantic Ocean. These experiments show that iron solubility is very low, always under 0,6%, in agreement with others observations in these regions (e.g. Baker et al., 2006). Furthermore, from bulk mineralogical analysis of the dust samples, iron solubility in source areas seems exclusively dependent on the mineralogical composition of dust particle. The greater iron solubilities (0,3%) corresponds to dust originated from central Sahara (Algeria, Lybia, Tunisia) where smectite are abundant in comparison to the others studied area (Sahel and Western Sahara) where iron mainly comes from iron (hydr-)oxide and illite. In this case, iron solubility does not exceed 0,13%. From comparison between these results and the lab data issued from Journet et al. (2008), a parameterization to estimate iron solubility from mineralogical composition of dust has been established and validated. Far from the source, iron solubility is usually greater than dust collected in source area, in the range of 0,15 and 0,6%. In these cases, mineralogical factor does not seem to control totally iron solubility. Others factors, probably due to atmospheric transport, knowing to play on iron dissolution process, have to be taken into account. In this study, none of those "external" factors seems to prevail. Finally, besides showing that for non-transported dust, iron solubility can be easily parameterized according to dust mineralogical composition, this work open up a new way of examination for iron dust solubility studies.
OS13F-03
Effect of Atmospheric Organic Complexation on Iron Dust Solubility
Atmospheric dust deposition is a major external iron source for the remote surface ocean. Organic complexation is known to play a role on the dissolution of iron containing minerals. Here we study the oxalate contribution on the dust iron solubility in simulated rainwater for various African dust source. Our results show that a strong positive linear correlation between the solubility of iron carried by analogues of Sahelian dust and oxalate concentrations. Soluble iron (SFe) increases from (0.037 +/- 0.009)% to (0.27 +/- 0.02)% of total iron with an oxalate concentration ranging from 0 to 8 μM, typical values of this ion in atmopsheric waters. The effect of oxalate on iron solubility is also fonction of source of dust: Higher impact for Saharan dust is found in southern areas, and lower impact in the north. This is probably caused by the mineralogy of iron species in dust. Considering mineralogical composition of those particles, experiments with pure minerals (hematite, goethite and illite) have been performed to study the dissolution process. Our results show that oxalate promotes the solubility of iron contained in clay and hence confirm that 90% of SFe from soil dust is provided by illite. Organic complexation appears consequently to be an important process which leads to a better solubility, and thus probably bioavailability, of dust iron. Field measurements showed a positive relation between oxalate and soluble Fe concentrations in air masses over North Atlantic region, suggesting a positive effect of organic complexation on dust iron dissolution solubility. However, considering anthropogenic source of oxalate, the authors concluded that it is difficult to exclude the potential effect of anthropogenic iron on the measured soluble Fe concentrations. These results indicate that the observed effect of oxalate on iron solubility from aerosols collected over the Atlantic is at least partly due to an increase in dust iron solubility rather than the presence of more soluble anthropogenic iron.
OS13F-04
Evidence of DMS and other biogenic gases affecting iron bioavailability in remote marine aerosols
Iron availability limits open-ocean phytoplankton growth, and because phytoplankton account for half of the Earth's photosynthesis they are key players in modulating global climate. Atmospherically transported dust particles provide an important source of iron to remote regions, yet, the mechanisms that control iron speciation and thus its bioavailability remain ill-defined. The present study pertains to elucidating processes that occur on atmospheric dust particles before deposition into the ocean. Laboratory experiments have identified a chemical link between iron reductive dissolution of synthesized iron(oxy)hydroxides and methanesulfinic acid (MSIA), an oxidation product of dimethyl sulfide (DMS) emitted by iron-starved phytoplankton. We present evidence for the existence of this mechanism in aerosol particles collected over the equatorial Pacific Ocean. Furthermore, results suggest that biogenically emitted isoprene oxidation products also affect iron speciation. These findings support the hypothesis that phytoplankton can actively affect iron availability through a direct biogeochemical feedback cycle.
OS13F-05 INVITED
Aerosol Chemistry from the Pacific and North Atlantic Oceans: Results from the CLIVAR/Repeat Hydrography Program
Daily-integrated aerosol samples were collected on CLIVAR/Repeat Hydrography cruises from 2003-2006 using a bow-mounted sector-controlled aerosol sampling system. In addition, a Micro-Orifice Deposit Impactor (MODI) was deployed to provide size-fractionated aerosol samples. The goals of this research were to measure the concentration and solubility of aerosol Fe and other trace elements, to estimate their deposition to the oceans, and to study the factors responsible for creating soluble aerosol trace elements. Aerosol filters were leached with 100 mL of either 0.2 um filtered surface seawater (pH 8.2) or unacidified ultrapure water (pH 5.6). The seawater filtrates were analyzed for soluble Fe(II) by the FeLume chemiluminescent method and for total dissolved Fe using ion-exchange column chromatography and isotope dilution ICP-MS. Ultrapure water filtrates were analyzed for soluble major anions and soluble oxalate by ion chromatography, as well as soluble Fe and other elements by ICP-MS. Replicate aerosol filters were analyzed at NOAA/PMEL for total Fe (and other elements) by energy dispersive X-ray fluorescence, and using synchrotron radiation (XANES) at the Brookhaven synchrotron (NSLS). Air-mass back trajectory analysis was conducted using the NOAA/HYSPLIT model. Rainfall samples were also collected to quantify the fraction of soluble aerosol Fe and other trace elements using ICP-MS. These data are used to discuss the sources, transport, and deposition of soluble and total aerosol trace elements and major ions. Samples from the North Atlantic include those impacted by anthropogenic emissions and the Saharan dust plume. Samples from the Pacific Ocean (including samples from the South Pacific and the Southern Ocean) illustrate the influences of anthropogenic emissions and mineral dust from Asia and Australia, and the extremely low aerosol loads found south of the equator.
OS13F-06
The Fractional Solubility of Aluminium From Mineral Aerosols Collected in Hawaii and Implications for Atmospheric Deposition of Biogeochemically Important Trace Elements
Understanding the systematics of the atmospheric deposition and dissolution of mineral dust in the surface ocean is of key importance to geochemistry and biogeochemistry. The atmospheric deposition route represents an important but poorly quantified part of the geochemical cycle as well as a vector for the delivery of biogeochemically important trace elements such as Fe to the surface ocean. The magnitude of this delivery process plays a vital role in biological processes as well as having consequences for the utilization of macro-nutrients and moderating atmospheric carbon dioxide levels. In order to further our understanding of this process we collected mineral aerosols from a 20m tower on the windward side of Oahu, Hawaii between February and May 2002, the high dust season. The fractional solubility of Al in the mineral aerosols ranged from 0.09 to 14.3% with a mean of 4.6%. Most of the dissolution took place within the first 24 hours of the suspension of these dusts in filtered surface seawater, but a further approx. 20% dissolved over an additional 3 day period. The fractional solubility of Al decreased with increased dust loading on the filter. However, the absolute amount of Al that dissolved from each filter correlated positively with the amount of non sea salt sulphate and nitrate on replicate filters. If confirmed, this result implies that the contribution of trace elements to surface waters is more likely regulated by the amount of acidity in the atmosphere rather than the amount of mineral aerosol. However, since much of the mineral dust that arrives in Hawaii has passed through air masses affected by intense industrialisation in Asia, the results we find may not be applicable to regions or periods that are not impacted by anthropogenic effects. Determining the underlying processes controlling, the solubility of mineral aerosols will require comparison of results from many different regions of the oceans. There are however no currently accepted methodologies for estimating this solubility which limits our ability to compare results from different research groups and different regions.
OS13F-07
Chemical Composition and Size Distributions of Coastal Aerosols Observed on the U.S. East Coast
Aerosol input is an important source of certain limiting nutrients, such as iron, for phytoplankton growth in several large oceanic regions. As the efficiency of biological uptake of nutrients may depend on the aerosol properties, a better knowledge of aerosol properties is critically important. Characterizing aerosols over the coastal ocean needs special attention, because the properties of aerosols could be altered by many anthropogenic processes in this land-ocean transition zone before they are transported over the remote ocean. The goal of this experiment was to examine aerosol properties, in particular chemical composition, particle-size distributions and iron solubility, over the US Eastern Seaboard, an important boundary for the transport of continental substances from North America to the North Atlantic Ocean. Our field sampling site was located at Tuckerton (39°N, 74°W) on the southern New Jersey coast. Fourteen sets of High-Volume aerosol samples and three sets of size segregated aerosol samples by a 10-stage MOUDI impactor were collected during 2007 and 2008. The ICP-MS methodology was used to analyze aerosol samples for the concentrations of thirteen trace elements: Al, Fe, Mn, Sc, Cd, Pb, Sb, Ni, Co, Cr, Cu, Zn and V. The IC procedures were applied to determine five cations (sodium, ammonium, potassium, magnesium and calcium) and eleven anions (fluoride, acetate, propionate, formate, MSA, chloride, nitrate, succinate, malonate, sulfate and oxalate). The UV spectrometry was employed for the determination of iron solubility. Preliminary results suggest three major sources of aerosols: anthropogenic, crustal and marine. At this location, the concentrations of iron (II) ranged from 2.8 to 29ng m-3, accounting for ~20% of the total iron. The iron concentrations at this coastal site were substantially lower than those observed in Newark, an urban site in northern NJ. High concentrations of iron (II) were associated with both fine and coarse aerosol particles, suggesting the impacts of different sources on the particle-size distributions of atmospheric iron. More results will be discussed during the presentation at the meeting.
OS13F-08
Measurements of Atmospheric Gaseous Mercury, Aerosol Trace Metals and Stable Lead Isotopes Over the South-Western Indian Ocean
During November 2007, continuous measurements were made of total gaseous mercury (TGM) over the
Indian Ocean during a two week cruise aboard the R/V Revelle from the Seychelles to Mauritius. Hg
concentrations were consistently low during the cruise (1.0-1.4 ng m
-3) similar to concentrations observed between 1994 and 2006 at an observatory in South Africa (Slemr et
al., 2008). There was no significant diurnal signal observed during the cruise and the low variability in Hg is
consistent with well mixed air masses and a long lifetime of Hg in the atmosphere.
During this cruise size segregated and bulk aerosol samples were also collected using a high volume aerosol
sampler. The aerosols were analysed for major ions, trace metals (Al, Fe, Ba, Mn, Co, V, Cr, Mo, Sr, Pb, Cd,
As, Zn, Cu and Ni) and stable lead isotope ratios. The concentrations of most of the metals were similar to
those observed in previous aerosol studies over similar regions of the Indian Ocean in 1986 (Chester et al.,
1991) and 2002 (Witt et al., 2006). Aerosols were enriched above crustal and oceanic sources in many trace
metals such as Pb, Cd, Ni and Zn although air mass back trajectories suggest air encountered had been
over the ocean for at least 5 days prior to collection. Metal concentrations over the remote Indian Ocean
appear to be intermediate between values reported for the remote Pacific and Atlantic oceans. Lead isotope
ratios (206Pb, 207Pb and 208 Pb) in the aerosols fell into a group with a relatively radiogenic
signature different to the Pb characteristic of Australian ores, where trajectories suggest air originated. The
isotope ratios also differ to those observed in South African cities and are closer to the lead composition
more typical of coals.
Chester et al., (1991) Mar. Chem., 34; 261-290
Slemr et al., (2008) GRL, 35 (11) doi:10.1029/2008GL033741
Witt et al., (2006) Atmos. Env., 40; 5435-5451