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Emissions from fuel combustion and agricultural activities contribute significantly to the continuous atmospheric entrainment of pollutants into protected and vulnerable ecosystems where long-term monitoring is often a challenge. Here, results are presented from a 26-year study (1989–2015) of wet precipitation collected at Paradise Station (1654 m above sea level), Mount Rainier National Park, Washington, USA. Weekly samples were analyzed for pH, conductivity and major anions and cations. While precipitation concentrations for sulfate, nitrate, protons and conductivity peaked in the early 2000s, overall trends decreased by 54%, 46%, 41%, and 37%, respectively. Associated pH values increased from 5.2 to 5.6, and were largely controlled by non-sea-salt contributions of sulfate and neutralizing calcium, potassium and magnesium. Between 1999 and 2015, nitrogen (N) deposition rates from ammonium increased by a factor of 3.6, from 0.27 to 0.96 kg N ha−1 yr−1 (p = 0.02), while nitrate deposition did not change statistically (0.91–0.74 kg N ha−1 yr−1, p = 0.30). Combined, these N sources are reaching reported critical loads of 2.0 kg N ha−1 yr−1. Results indicate that emission regulations focused on stationary sources have effectively decreased apparent acid precipitation, however, increased nitrogen deposition from ammonium may lead to further fertilization and acidification of delicate soils and waters. Continued long-term monitoring is thus imperative to track continued anthropogenic inputs to vulnerable ecosystems.

Air quality models are widely used to estimate pollutant deposition rates and thereby calculate critical loads and critical load exceedances (model deposition > critical load). However, model operational performance is not always quantified specifically to inform these applications. We developed a performance assessment approach designed to inform critical load and exceedance calculations, and applied it to the Pacific Northwest region of the U.S. We quantified wet inorganic N deposition performance of several widely-used air quality models, including five different Community Multiscale Air Quality Model (CMAQ) simulations, the Tdep model, and ‘PRISM x NTN’ model. Modeled wet inorganic N deposition estimates were compared to wet inorganic N deposition measurements at 16 National Trends Network (NTN) monitoring sites, and to annual bulk inorganic N deposition measurements at Mount Rainier National Park. Model bias (model – observed) and error (|model – observed|) were expressed as a percentage of regional critical load values for diatoms and lichens. This novel approach demonstrated that wet inorganic N deposition bias in the Pacific Northwest approached or exceeded 100% of regional diatom and lichen critical load values at several individual monitoring sites, and approached or exceeded 50% of critical loads when averaged regionally. Even models that adjusted deposition estimates based on deposition measurements to reduce bias or that spatially-interpolated measurement data, had bias that approached or exceeded critical loads at some locations. While wet inorganic N deposition model bias is only one source of uncertainty that can affect critical load and exceedance calculations, results demonstrate expressing bias as a percentage of critical loads at a spatial scale consistent with calculations may be a useful exercise for those performing calculations. It may help decide if model performance is adequate for a particular calculation, help assess confidence in calculation results, and highlight cases where a non-deterministic approach may be needed.

It is well recognized that the atmospheric deposition of iron (Fe) affects ocean productivity, atmospheric CO2 uptake, ecosystem diversity, and overall climate. Despite significant advances in measurement techniques and modeling efforts, discrepancies persist between observations and models that hinder accurate predictions of processes and their global effects. Here, we provide an assessment report on where the current state of knowledge is and where future research emphasis would have the highest impact in furthering the field of Fe atmosphere-ocean biogeochemical cycle. These results were determined through consensus reached by diverse researchers from the oceanographic and atmospheric science communities with backgrounds in laboratory and in situ measurements, modeling, and remote sensing. We discuss i) novel measurement methodologies and instrumentation that allow detection and speciation of different forms and oxidation states of Fe in deliquesced mineral aerosol, cloud/rainwater, and seawater; ii) oceanic models that treat Fe cycling with several external sources and sinks, dissolved, colloidal, particulate, inorganic, and organic ligand-complexed forms of Fe, as well as Fe in detritus and phytoplankton; and iii) atmospheric models that consider natural and anthropogenic sources of Fe, mobilization of Fe in mineral aerosols due to the dissolution of Fe-oxides and Fe-substituted aluminosilicates through proton-promoted, organic ligand-promoted, and photo-reductive mechanisms. In addition, the study identifies existing challenges and disconnects (both fundamental and methodological) such as i) inconsistencies in Fe nomenclature and the definition of bioavailable Fe between oceanic and atmospheric disciplines, and ii) the lack of characterization of the processes controlling Fe speciation and residence time at the atmosphere-ocean interface. Such challenges are undoubtedly caused by extremely low concentrations, short lifetime, and the myriad of physical, (photo)chemical, and biological processes affecting global biogeochemical cycling of Fe. However, we also argue that the historical division (separate treatment of Fe biogeochemistry in oceanic and atmospheric disciplines) and the classical funding structures (that often create obstacles for transdisciplinary collaboration) are also hampering the advancement of knowledge in the field. Finally, the study provides some specific ideas and guidelines for laboratory studies, field measurements, and modeling research required for improved characterization of global biogeochemical cycling of Fe in relationship with other trace elements and essential nutrients. The report is intended to aid scientists in their work related to Fe biogeochemistry as well as program managers at the relevant funding agencies.

This work presents iron isotope data in the western equatorial Pacific. Marine aerosols and top core margin sediments display a slightly heavy Fe isotopic composition (δ 56 Fe) of 0.33 ± 0.11‰ (2SD) and 0.14 ± 0.07‰, respectively. Samples reflecting the influence of Papua New Guinea runoff (Sepik River and Rabaul volcano water) are characterized by crustal values. In seawater, Fe is mainly supplied in the particulate form and is found with a δ 56 Fe between A0.49 and 0.34 ± 0.07‰. The particulate Fe seems to be brought mainly by runoff and transported across continental shelves and slopes. Aerosols are suspected to enrich the surface Vitiaz Strait waters, while hydrothermal activity likely enriched New Ireland waters. Dissolved Fe isotopic ratios are found between A0.03 and 0.53 ± 0.07‰. They are almost systematically heavier than the corresponding particulate Fe, and the difference between the signature of both phases is similar for most samples with Δ 56 Fe DFe – PFe = +0.27 ± 0.25‰ (2SD). This is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases. The dissolved phase being heavier than the particles suggests that the exchanges result in a net nonreductive release of dissolved Fe. This process seems to be locally significantly more intense than Fe reductive dissolution documented along reducing margins. It may therefore constitute a very significant iron source to the ocean, thereby influencing the actual estimation of the iron residence time and sinks. The underlying processes could also apply to other elements.

The oxidation state of four transition metals (Fe, Mn, Cu, and Cr) in cloudwater has been investigated during several cloud events at Whiteface Mountain (NY), one cloud event at San Pedro Hill (CA), and one fog event at Bakersfield (CA). Samples were collected and immediately analyzed for the oxidation states of four transition metals: Fe(II) [44 measurements], Cu(I) [30 measurements], Mn(IV) [27 measurements], and Cr(III) [3 measurements]. Extreme care was taken to minimize contamination and interferences when measuring these oxidation states. Particulate and dissolved concentrations of these metals were also determined. Other measurements performed—relevant to the redox chemistry of these metals—included pH, total elemental concentrations (Fe, Cu, Mn, Cr, Al, K, Ca, Na, and Mg), organic anions (formate, acetate, glycolate, oxalate), inorganic anions (chloride, sulfate, nitrate, sulfite), cations (sodium, calcium, magnesium, potassium), peroxides, and formaldehyde. These measurements were then used in thermodynamic-speciation models to understand the speciation of ambient fog and cloudwater. From this analysis, two different cases were found for Fe(III)soluble speciation. Fe(III) was found to exist either as Fe(OH)_2 ^+ or Fe(Oxalate)_2. However, an unidentified strong chelating ligand with Fe(III) was also suggested by the data. Cu(I) and Cu(II) were calculated to be predominantly Cu^+ and Cu^(2+) (with less than 10% as Cu(II)-oxalate complexes). A chemical kinetic model was also used to investigate the transition-metal chemistry. The model results indicate that Fe(II) should be the predominant chemical form of Fe during daylight conditions. This prediction is in agreement with the field measurements in which the highest ratios of Fe(II)/Fe total were found in samples collected during the day. The model results also indicated that Fe(III) should be the predominant form of Fe during nighttime conditions, which is also in agreement with the field measurements. In the model, Cu(II) and Mn(II) were the predominant oxidation states during daylight and nighttime conditions with Cu(I) and Mn(III) increasing during daylight conditions. Mn(III) concentrations were never high enough to influence the redox chemistry of Cr. Overall, Cr(VI) in cloudwater is predicted to be reduced to Cr(III) if free S(IV) is present.

Atmospheric deposition of trace elements and isotopes (TEI) is an important source of trace metals to the open ocean, impacting TEI budgets and distributions, stimulating oceanic primary productivity, and influenc ing biological community structure and function. Thus, accurate sampling of aerosol TEIs is a vital component of ongoing GEOTRACES cruises, and standardized aerosol TEI sampling and analysis procedures allow the com parison of data from different sites and investigators. Here, we report the results of an aerosol analysis intercal ibration study by seventeen laboratories for select GEOTRACES-relevant aerosol species (Al, Fe, Ti, V, Zn, Pb, Hg, NO 3 ‐ , and SO 4 2‐ ) for samples collected in September 2008. The collection equipment and filter substrates are appropriate for the GEOTRACES program, as evidenced by low blanks and detection limits relative to analyte concentrations. Analysis of bulk aerosol sample replicates were in better agreement when the processing proto col was constrained (± 9% RSD or better on replicate analyses by a single lab, n = 7) than when it was not (gen erally 20% RSD or worse among laboratories using different methodologies), suggesting that the observed vari ability was mainly due to methodological differences rather than sample heterogeneity. Much greater variabil ity was observed for fractional solubility of aerosol trace elements and major anions, due to differing extraction methods. Accuracy is difficult to establish without an SRM representative of aerosols, and we are developing an SRM for this purpose. Based on these findings, we provide recommendations for the GEOTRACES program to

The ultrafine size fraction of ambient particles (ultrafine particles [UFP], diameter < 100 nm) has been identified as being particularly potent in their adverse health effects, yet, the detailed mechanisms for why UFP display such distinctive toxicity are not well understood. In the present study, mitochondria were exposed to ambient UFP while monitoring mitochondrial electron transport chain (ETC) activity as a model system for biochemical toxicity. UFP samples were collected in rural and urban environments, and chemically characterized for trace metals, ferrous (Fe(II)) and easily reducible ferric (Fe(III)) iron, polycyclic aromatic hydrocarbons (PAHs), and surface constituents with X-ray photoelectron spectroscopy (XPS). Fixed doses of UFP (8 μg mL−1) inhibited mitochondrial ETC function compared to controls in 94% of the samples after the 20 min of exposure. Significant moderate to weak correlations exist between initial %ETC inhibition (0 – 10 min) and Fe(II) (R = 0.55, P = 0.03, N = 15), anthracene...

Atmospheric deposition of iron (Fe) to certain regions of the oceans is an important nutrient source of Fe to the biota, and the ability of the biota to uptake Fe is dependent on the speciation of the Fe. Therefore understanding the speciation of Fe in the atmosphere is critical to understanding the role of Fe as a nutrient source in surface ocean waters. Labile ferrous iron (Fe(II)) concentrations as well as total concentrations for Fe and other important trace metals, cations, and anions were determined over the Arabian Sea for two nonconsecutive months during 1995. Ambient aerosol samples were collected during the Indian Ocean intermonsoon and southwest monsoon seasons over the Arabian Sea. Sampling took place aboard the German research vessel Meteor in the months of May (leg M32/3; intermonsoon) and July/August (leg M32/5; southwest monsoon). Both cruise tracks followed the 65th east meridian, traveling for 30 days each (from north to south during leg M32/3 and from south to north during leg M32/5). A high-volume dichotomous virtual impactor with an aerodynamic cutoff size of 3 μm was used to collect the fine and coarse aerosol fractions for metal analysis. A low volume collector was used to collect aerosol samples for anion and cation analysis. The analysis for labile-Fe(II) was done immediately after sample collection to minimize any possible Fe redox reactions which might occur during sample storage. The analytical procedure involved filter extraction in a formate/formic acid buffered solution at pH 4.2 followed by colorimetric quantification of soluble Fe(II). Metals, anions, and cations were analyzed after the cruise. Total atmospheric aqueous-labile-Fe(II) concentrations during the intermonsoon were between 4.75 and 80%) was present in the fine fraction (

LVe encountered an extensive surface bloom of the N, fixing cyanobactenum Trichodesrniurn erythraeum in the central basin of the Arabian Sea during the spring ~nter-n~onsoon of 1995. The bloom, which occurred dunng a penod of calm winds and relatively high atmospher~c iron content, was metabollcally active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by h:: flxation could account for a major fraction of the estimated 'new' N demand of pnmary production. Isotopic measurements of the N in surface suspended material confirmed a direct contribution of N, fixation to the organic nltrogen pools of the upper water column. Retrospective analysis of NOAA-12 AVHRR imagery indicated that blooms covered up to 2 X 106 km2, or 20% of the Arabian Sea surface, during the period from 22 to 27 May 1995. In addition to their biogeochemical impact, surface blooms of this extent may have secondary effects on sea surface albedo and light penetration as well as heat and gas exchange across the air-sea interface. A preliminary extrapolation based on our observed, non-bloom rates of N, fixation from our limited sampling in the spring intermonsoon, including a conservative estimate of the input by blooms, suggest N2 fixation may account for an input of about 1 Tg N yr-I This is substantial, but relatively minor compared to current estimates of the removal of N through denitrification in the basin. However, N2 fixation may also occur in the central basin through the mild winter monsoon, be considerably greater during the fall intermonsoon than we observed during the spring intermonsoon, and may also occur at higher levels in the chronically oligotrophic southern basin. Ongoing satellite observations will help to determine more accurately the distribution and density of Trichodesmium in this and other tropical oceanic basins, as well as resolving the actual frequency and duration of bloom occurrence.

Mineral aerosol deposition is recognized as the dominant source of iron to the open ocean and the solubility of iron in the dust aerosol is highly variable, with measurements ranging from 0.01–80%. Global models have difficulty capturing the observed variations in solubility, and have ignored the solubility dependence on aerosol size. We introduce two idealized physical models to estimate the size dependence of mineral aerosol solubility: a diffusion-controlled model and a surface-area-controlled model. These models produce differing time- and space-varying solubility maps for aerosol Fe and Al given the dust age at deposition, size-resolved dust entrainment fields, and the aerosol acidity. The resulting soluble iron deposition fluxes are substantially different, and more realistic, than a globally uniform solubility approximation. The surface-area-controlled solubility varies more than the diffusion-controlled solubility and better captures the spatial pattern of observed solubility in the Atlantic. However, neither of these two models explains the large solubility variation observed in the Pacific. We then examine the impacts of spatially variable, size-dependent solubility on marine biogeochemistry with the Biogeochemical Elemental Cycling (BEC) ocean model by comparing the modeled surface ocean dissolved Fe and Al with observations. The diffusion-based variable solubility does not significantly improve the simulation of dissolved Fe relative to a 5% globally uniform solubility, while the surface-area-based variable solubility improves the simulation in the North Atlantic but worsens it in the Pacific and Indian Oceans.

The passive behaviour of commercially pure Be (99.0 and 99.8%) has been studied by means of electrochemical measurements, solution analyses and electron microscopy. Aqueous test solutions (25°C, pH 1–15) of orthophosphate, orthoborate, carbonate, formiate, acetate, oxalate or NaOH were employed. In some solutions, Be exhibited a potential independent passive current density up to the oxygen evolution potential, whereas pitting corrosion was initiated in other solutions. The passive current depended on the pH and the concentration of the weak acids in the solution. The passive film was identified (ex situ) as microcrystalline BeO.

International audience; Atmospheric inputs of iron to the open ocean are hypothesized to modulate ocean biogeochemistry. This review presents an integration of available observations of atmospheric iron and iron deposition, and also covers bioavailable iron distributions. Methods for estimating temporal variability in ocean deposition over the recent past are reviewed. Desert dust iron is estimated to represent 95% of the global atmospheric iron cycle, and combustion sources of iron are responsible for the remaining 5%. Humans may be significantly perturbing desert dust (up to 50%). The sources of bioavailable iron are less well understood than those of iron, partly because we do not know what speciation of the iron is bioavailable. Bioavailable iron can derive from atmospheric processing of relatively insoluble desert dust iron or from direct emissions of soluble iron from combustion sources. These results imply that humans could be substantially impacting iron and bioavailable iron deposition to ocean regions, but there are large uncertainties in our understanding.

Iron contained in dust-derived aerosol particles deposited into remote oceans is essential for phytoplankton productivity, which controls photosynthesis rate and the uptake and release of climate forcing gases. Understanding chemical mechanisms that control iron bioavailability, that is, its speciation, is therefore crucial for global climate predictions. In the present study, the photoredox chemistry of iron in marine atmospheric aerosol particles was investigated by using ferrihydrite as a surrogate iron phase in the presence of dimethyl sulfide (DMS) derived oxidation products: dimethyl sulfoxide (DMSO), dimethyl sulfone (DMS02), methane sulfinic acid (MSIA), and methane sulfonic acid (MSA). Reactants and products were analyzed with UV-vis absorption spectroscopy, ion chromatography, and a hydrogen peroxide sensitive electrode. Results show that MSIA enhances the photoreductive dissolution of iron in a ligand-to-metal charge transfer reaction producing Fe(II), MSA, and H2O2. The rate law for Fe(II) is close to first order (0.79) with regard to adsorbed MSIA and has an empirical rate constant of 1.4 x 10(-4) s(-1). This mechanism may represent a significant pathway through which iron becomes more bioavailable, and it contributes to models of iron and sulfur chemistries in the marine atmosphere.

[1] Within open-ocean regions where excess macronutrients are present, phytoplankton growth is limited by the bioavailability of iron supplied to these areas primarily within atmospheric aerosols of crustal origin. However, processes that control the abundance of biologically accessible iron in these aerosols are largely unknown. Here we show that dissolution of ferrihydrite, a surrogate iron(oxy)hydroxide phase found in atmospheric waters, is enhanced in the presence of methanesulfinic acid (MSIA, CH3SO2H, a dimethylsulfide (DMS) oxidation intermediate) in laboratory irradiation experiments with aqueous suspensions that simulate marine aerosol particles. The increased release of soluble Fe(II) is attributed to a species specific and direct photochemical reduction rather than a proton-promoted effect, and suggests an efficient mechanism by which iron-starved phytoplankton can actively increase aerosol iron-bioavailability by increasing DMS emissions.

[1] Mineral aerosol deposition is the dominant source of iron to the open ocean. Soil iron is typically insoluble and understanding the atmospheric processes that convert insoluble iron to the more soluble forms observed over the oceans is crucial. In this paper, we model several proposed processes for the conversion of Fe(III) to Fe(II), and compare with cruise observations. The comparisons show that the model results in similar averaged magnitudes of iron solubility as measured during 8 cruises in 2001–2003. Comparisons show that results of cases including cloud, SO2 and hematite processing are better than the other approaches used using the reaction rates we assume in this paper; unfortunately the reaction rates are not well known, and this hampers our ability to conclusive show one process is more likely than another. The total soluble iron deposited to the global ocean is estimated by the model to range from 0.36 to 1.6 Tg y−1, with 0.88 Tg y−1 being the mean estimate; however there are large uncertainties in these estimates. Comparison shows that the regions with largest differences between the model simulations and observations of iron solubility are in the Southern Atlantic near South America coast and North Atlantic near Spain coast. More observations in these areas or in the South Pacific will help us identify the most important processes. Additionally, laboratory experiments that constrain the reaction rates of different compounds that will result in a net solubilization of iron in aerosols are required to better constrain iron processing in the atmosphere. Additionally, knowing what forms of iron are most bioavailable will assist atmospheric scientists in providing better budgets of iron deposited to the ocean surfaces.

Ambient aerosol samples were collected over the Arabian Sea during the month of March 1997, aboard the German R/V Sonne, as part of the German JGOFS project (Joint Global Ocean Flux Study). This is the third study in a series of analogous measurements taken over the Arabian Sea during different seasons of the monsoon. Dichotomous high volume collector samples were analyzed for anions and cations upon return to the laboratory. Anthropogenic pollutant concentrations were larger during the first part of the cruise, when air masses originated over the Indian subcontinent. Total NSS-SO_4^(2−) concentrations amounted to 2.94 ± 1.06 μg m^(−3) of which 92.1 ± 4.5% was present in the fine fraction. NSS-SO_4^(2−) source apportionment analysis with multivariate linear regression models revealed that in the coarse fraction half is biogenically and half anthropogenically derived, while in the fine fraction only 6% seemed of biogenic origin and 84% anthropogenic and 10% crustal in nature. Chloride deficits up to 99.1% in the fine fraction were observed. The average Cl^− deficit in the fine fraction was 89.0 ± 9.4%, potentially related to NSS-SO_4^(2−) acid displacement and Cl reactive species formation, while in the coarse fraction it was 25.6 ± 21.3%, with NO_3^− being the preferred species for acid displacement.

[1] Ambient aerosol samples were collected over the Arabian Sea during the month of March of 1997, aboard the German R/V Sonne, as part of the German Joint Global Ocean Flux Study (JGOFS) project. This is the third study in a series of analogous measurements taken over the Arabian Sea during different seasons of the monsoon. Dichotomous high-volume collector samples were analyzed for ferrous iron immediately after collection, while trace metals, anions, and cations were determined upon return to the laboratory. The main crustal component was geochemically well represented by the average crustal composition and amounted to 5.94 ± 3.08 m gm 3 . An additional crustal constituent of clay-like character, rich in water-soluble Ca and Mg, was seen in the fine fraction in air masses of Arabian origin. Total ferrous iron concentrations varied from 3.9 to 17.2 ng m 3 and averaged 9.8 ± 3.4 ng m 3 , with 87.2% of Fe(II) present in the fine aerosol fraction. Fe(II) concentrations accounted for on average 1.3 ± 0.5% of the total Fe. While ferrous iron in the coarse fraction appeared to be correlated with the main crustal component, the fine Fe(II) fraction exhibited a more complex behavior. The anthropogenic contribution to the aerosol, as traced by Pb, Zn, and some anions and cations, was found to be considerably larger, especially during the first 10 days of this cruise, than in previously collected samples from the inter-monsoon and southwest monsoon of 1995. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0365 Atmospheric Composition and Structure: Troposphere— composition and chemistry; 4801 Oceanography: Biological and Chemical: Aerosols (0305); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4875 Oceanography: Biological and Chemical: Trace elements; KEYWORDS: ferrous iron, trace metals, aerosol particles, Indian Ocean Citation: Johansen, A. M., and M. R. Hoffmann, Chemical characterization of ambient aerosol collected during the northeast monsoon season over the Arabian Sea: Labile-Fe(II) and other trace metals, J. Geophys. Res., 108(D14), 4408, doi:10.1029/2002JD003280, 2003.

Ambient aerosol samples were collected over the tropical northern Atlantic Ocean during the month of April 1996 onboard the R/V Seward Johnson. Dichotomous high-volume collector samples were analyzed for ferrous iron immediately after collection, while trace metals, anions, and cations were determined upon return to the laboratory. Data are analyzed with the aid of enrichment factor, principal component, and weighted multiple linear regression analyses. Average mineral aerosol concentrations amounted to 19.3±16.4 μg m^(−3) whereby the chemical characteristics and air mass back trajectories indicated the dust to be of a typical shale composition and Saharan origin. Calcite accounted for 3.0 and 7.9% of the mineral aerosol during the first and second halves of the cruise, respectively. Total iron concentrations (averaging 0.84±0.61 μg m^(−3)) are crustally derived, of which 0.51±0.56% is readily released as Fe(II). Eighty-six percent of this Fe(II) is present in the fine (

Despite its importance for the global oceanic nitrogen (N) cycle, considerable uncertainties exist about the N fluxes of the Arabian Sea. On the basis of our recent measurements during the German Arabian Sea Process Study as part of the Joint Global Ocean Flux Study (JGOFS) in 1995 and 1997, we present estimates of various N sources and sinks such as atmospheric dry and wet depositions of N aerosols, pelagic denitrification, nitrous oxide (N_2O) emissions, and advective N input from the south. Additionally, we estimated the N burial in the deep sea and the sedimentary shelf denitrification. On the basis of our measurements and literature data, the N budget for the Arabian Sea was reassessed. It is dominated by the N loss due to denitrification, which is balanced by the advective input of N from the south. The role of N fixation in the Arabian Sea is still difficult to assess owing to the small database available; however, there are hints that it might be more important than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; however, they play only a minor role for the overall N cycling. Emissions of N_2O and ammonia, deep-sea N burial, and N inputs by rivers and marginal seas (i.e., Persian Gulf and Red Sea) are of minor importance. We found that the magnitude of the sedimentary denitrification at the shelf might be ∼17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycling in the Arabian Sea than previously thought. Sedimentary and pelagic denitrification together demand ∼6% of the estimated particulate organic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating that the N cycle of the Arabian Sea might be sensitive to variations of the intermediate water circulation of the Indian Ocean.

Ambient aerosol samples were collected over the northern Indian Ocean during two 1 month-long research cruises (German R/V Meteor) that took place during the intermonsoon (May) and SW monsoon (July/August) of 1995. A high volume and two small volume collectors were used to collect samples, which were subsequently analyzed for ferrous iron, 32 elements, and anions and cations. The present paper focuses on the bulk aerosol material, the ions, while utilizing some of the trace metal data that were presented in more detail in our previous paper [Siefert et al., 1999]. Data are analyzed and interpreted with the aid of principal component and multiple linear regression analyses. Intermonsoon samples were strongly influenced by continental material, both of crustal and anthropogenic origin. The crustal component (24.5±13% of the total suspended particulate mass (TSP), 6.0±4.4 μg m^(−3)) contained 3.2% gypsum (CaSO_4). While more than half of the TSP (21.2±9.6 μg m^(−3)) during the SW monsoon was sea-salt-derived due to the strong winds prevailing during this season, only 1.7±1.1% (0.7±0.4 μg m^(−3)) was found to be of crustal origin. Sulfate (SO_4^(2−)) sources were determined and quantified with linear regression analyses utilizing specific tracers for the independent variables. Lead (Pb) was found to be a more reliable surrogate for anthropogenic SO_4^(2−) compared to nitrate (NO_3^−) during the relatively polluted intermonsoon. Soluble calcium (Ca^(2+)) served as the tracer for gypsum, and methane sulfonate (MSA) served as the tracer for biogenically derived SO_4^(2−) during both seasons. On the basis of this analysis, 75% of the non-sea-salt sulfate (NSS-SO_4^(2−)) (0.8±0.2 μg m^(−3), representing ∼2.4% of TSP) was found to be of biogenic origin during the SW monsoon with the remaining 25% of anthropogenic origin. During the intermonsoon, NSS-SO_4^(2−) accounted for 2.1±1.2 μg m^(−3) (∼9.2% of TSP) and had a composition that was 65% anthropogenic, 21% biogenic, and 14% gypsum-derived. Linear regression analyses revealed that the bio-SO_4^(2−)/MSA weight ratios appear to be consistent with the temperature dependence proposed by Hynes et al. [1986]. In this case the yield of SO_4^(2−) increased relative to MSA with an increase in temperature. Three samples during the SW monsoon, near the coast of Oman, showed lower temperatures, due to coastal upwelling, than the rest of the samples; at 24°C the bio-SO_4^(2−)/MSA weight ratio was 6.8±0.5. The remainder of the SW monsoon samples were collected at an average temperature of 27.2°C, for which the bio-SO_4^(2−)/MSA weight ratio was 13.5±4.4. At an average temperature of 28.9°C during the intermonsoon, sampling gave a ratio of 17.7±4.8. These observations indicate a temperature dependence factor between 24° and 29°C of 2.2 (i.e., a 2.2 increase in the ratio of bio-SO_4^(2−)/MSA with every degree temperature increase). Cl− deficits determined during both seasons appear to indicate that different mechanisms may govern the observed depletion of Cl− in each season.