Sources, Solubility, and Impact of Aerosol Iron on Marine Biogeochemistry

Abstract

Iron (Fe) is an essential micronutrient that constrains primary productivity across approximately 50% of the global ocean, thereby regulating ocean–atmosphere carbon exchange and climate. Atmospheric deposition dominates the external supply of Fe to the open ocean, directly impacting marine biogeochemical cycles. This review systematically synthesizes current knowledge on the sources of total and soluble aerosol Fe and on the key factors and mechanisms governing Fe solubility, including proton- and ligand-promoted dissolution, photoreduction, cloud processing, and their spatiotemporal variability. We critically evaluate the methodologies used to measure Fe solubility across studies, highlighting persistent uncertainties that arise from inconsistent extraction solutions, filter pore sizes, and leaching protocols. By identifying these challenges and integrating field observations, laboratory experiments, and model results, we aim to clarify the controls on atmospheric Fe solubility and provide a more robust assessment of its contribution to marine primary productivity and biogeochemistry.

1. Biogeochemistry of Fe

1.1. Impact of Fe on Marine Primary Productivity

Iron (Fe) is an essential trace nutrient for marine plankton, driving photosynthesis and nitrogen fixation [1,2,3] and thereby regulating energy flow and carbon fixation in marine ecosystems [4,5,6]. The deposition of long-range-transported aerosols supplies Fe that sustains phytoplankton nitrogen assimilation, chlorophyll synthesis, and diazotrophic nitrogen fixation [5,7,8,9]. This process promotes the growth of marine flora and fauna, thereby enhancing primary productivity in the ocean and the absorption of CO₂. Therefore, studying the Fe content in the surface ocean is crucial for understanding marine biogeochemical cycles and their climate-related effects.

Approximately 50% of the world’s ocean areas experience limited primary productivity due to low concentrations of dissolved Fe (dFe), particularly in high-nitrate, low-chlorophyll regions (HNLC) and oligotrophic areas where nitrogen fixation is crucial [8,10]. “Iron fertilization” experiments have demonstrated that enriching surface waters in these regions with bioavailable Fe significantly enhances primary productivity [7]. In the oligotrophic waters of the North Pacific, aerosols deposited via atmospheric transport deliver nitrogen-rich nutrients that directly promote phytoplankton growth [11,12]. Additionally, aerosol Fe stimulates the proliferation of marine diazotrophic bacteria, converting atmospheric nitrogen (N₂) into inorganic nitrogen salts, thereby increasing primary productivity and carbon dioxide (CO₂) uptake in these waters.

Dust aerosols originating from East Asia and Australia can transport substantial amounts of Fe over long distances, alleviating Fe limitation and stimulating marine phytoplankton growth in downwind regions. A schematic diagram illustrating the major emission sources and transport processes of aerosol Fe to the open ocean is presented in Figure 1. A synthesis of global observational and modeling data on Fe-limited regions [13] and soluble Fe sources, shows that Fe concentrations in the Southern Ocean, equatorial Pacific, North Pacific, and Indian Ocean are low enough to restrict phytoplankton growth [14,15,16]. Ice core records further reveal a significant negative correlation between dust-derived Fe flux and atmospheric CO₂ during glacial–interglacial cycles [17], indicating that higher aerosol Fe input during glacial periods intensified the biological pump and drove down CO₂. Consequently, in these Fe-limited regions, the atmospheric deposition flux of soluble Fe exerts an indirect but critical control on marine primary productivity and the biogeochemical cycling of carbon.

1.2. Bioavailability of Fe

Before examining the factors that control Fe solubility and its biogeochemical impacts, it is essential to first consider how Fe is quantified in both seawater and aerosol samples. The reliability and comparability of such data directly influence our ability to assess deposition fluxes, evaluate bioavailability, and constrain biogeochemical models. This section provides an overview of the commonly used operational definitions, extraction protocols, and detection techniques, highlighting how methodological variability creates challenges when comparing solubility and flux estimates across different studies.

In seawater, dFe is operationally defined as the fraction passing through 0.2 μm or 0.45 μm filters. Based on reactivity, Fe species can be categorized into three pools: Fe′ (labile inorganic complexes), FeL (organic ligand complexes exchangeable on timescales of <1 day), and Fe_inert (Fe bound in essentially non-labile matrices); Fe′ and FeL together are considered bioavailable Fe [18]. In terms of particle size, Fe is further divided into particulate (>0.45 or 0.2 μm), colloidal (passing through 0.45/0.2 μm but retained by 0.02 μm filters), and soluble (<0.02 μm) fractions.

Because bioavailable Fe cannot be measured directly in the ocean, many studies use dFe as a practical proxy, yet comparisons are complicated by methodological differences. In aerosol research, dFe (or soluble Fe) is typically defined as the fraction that passes through a 0.22 or 0.45 μm filter after extraction, and the ratio of soluble Fe to total Fe is referred to as Fe solubility [19]. The measured solubility depends strongly on the choice of extraction solution, leaching protocol, digestion method, and detection technique.

Common extraction solutions include ultrapure water, ammonium acetate, formate buffer, and seawater. Ultrapure water can approximate conditions in non-acidified cloud water, but its very low buffering capacity causes pH to vary with the dust sample, hampering inter-study comparison. To control pH, buffered solutions such as ammonium acetate (pH ≈ 4.7) or formate buffer (pH ≈ 4.5) have been used [20,21], enabling more consistent comparisons. However, some buffers may themselves modify Fe solubility. Seawater, with its high buffering capacity at pH ≈ 8.2, provides the most representative medium for simulating the release of dFe from dust entering the ocean. Notably, Fe solubility in seawater is generally low, and several studies have found that seawater extracts consistently less Fe than ultrapure water [22].

Experimental approaches include continuous flow leaching, static extraction, ultrasonic extraction, and oscillatory shaking extraction [23]. Continuous flow leaching reduces saturation effects but cannot account for the equilibrium between adsorption and dissolution during particle settling [24]. Static extraction in ultrapure water may require long equilibration times (e.g., >1000 h at low pH), leading to strong time dependence in measured solubility [25]. Ultrasonic and oscillatory shaking extraction substantially shorten processing times, although quantitative differences in extraction efficiency between these two techniques remain to be fully resolved.

Therefore, when discussing Fe solubility across studies, it is essential to explicitly state the extraction solution, method, and filter pore size employed. Aerosol Fe solubility is a key indicator for evaluating Fe bioavailability and currently constitutes the largest source of uncertainty in model-based estimates of soluble Fe deposition flux.

2. Sources and Deposition Fluxes of Fe in Marine Aerosols

2.1. Concentration and Sources of Marine Aerosol Fe

As shown in

Table 1. Global total iron fluxes to the ocean (Data source: [26]).

Source	Flux (Fe, Tg/yr)
Fluvial particulate total iron	625–961
Fluvial dissolved iron	1.5
Glacial sediments	34–211
Atmospheric	16
Coastal erosion	8
Hydrothermal	14
Authigenic	5

, the main sources of Fe to surface seawater include aerosol deposition, riverine input, coastal erosion, and hydrothermal activity [5,26]. Riverine and glacial particulate Fe are largely retained in coastal and nearshore environments, where they serve as a major source of Fe to regional surface waters. Although riverine inputs dominate the total global Fe flux, they are mainly confined to coastal zones; in the open ocean, away from coastal and benthic sources, atmospheric aerosol deposition is the primary supply of Fe to surface waters. In contrast, hydrothermal and sedimentary Fe inputs are released predominantly in the deep ocean, although a fraction may eventually reach the surface through upwelling and mixing. However, only a portion of the deposited Fe is directly usable by marine organisms; the bioavailable fraction is often assumed to correspond largely to dFe [27,28].

Atmospheric deposition dominates the external supply of bioavailable Fe to the open ocean. Fung et al. [29] estimated a total phytoplankton Fe requirement of ~0.67 Tg yr⁻¹, whereas the total Fe input from atmospheric deposition is ~5.37 Tg yr⁻¹, with the bioavailable fraction ranging from 0.05 to 0.53 Tg yr⁻¹ (equivalent to roughly 1–10% dFe in total deposition). By comparison, the upward flux of bioavailable Fe from the base of the mixed layer supplies only ~0.04 Tg yr⁻¹, highlighting the critical role of aeolian input.

The solubility and bioavailability of aerosol Fe are controlled by both source characteristics and atmospheric processing. Anthropogenic SO₂ in Yellow Sea aerosols can acidify dust particles during transport, increasing Fe solubility and thereby alleviating Fe limitation in HNLC regions of the Pacific [30]. Combustion aerosols can dominate the supply of dFe, contributing up to 70% and 85% at Bermuda and Ireland, respectively [31]. At the process level, total Fe content has been observed to decline with transport time, while Fe solubility tends to increase [32]. Seasonal and regional variability are also large: in the tropical and subtropical North Atlantic, higher wintertime solubility and greater solubility in the subtropics than in the tropics have been reported [33]. Observations from the Southern Ocean and Antarctica report total Fe concentrations ranging from 0.1 to 150 ng m⁻³ and solubilities spanning 0.01–90% [34], underscoring the complexity of bioavailable Fe formation.

Stable Fe isotopes (δ⁵⁶Fe) have recently been used to fingerprint anthropogenic vs. natural sources and to trace their impacts on marine Fe cycles. Wei et al. [35] compiled a global dataset of aerosol δ⁵⁶Fe and, using an MixSIAR model, identified coal combustion as the dominant anthropogenic Fe source on a hemispheric scale. Chen et al. [36] applied δ⁵⁶Fe in deep-sea sediments to assess how human-derived Fe influences the deep Fe cycle in the North and South Pacific. Together, these studies demonstrate that atmospheric Fe solubility varies significantly with particle size, season, and region, and that future model-based assessments of dFe deposition flux will require both expanded field observations and a deeper mechanistic understanding of aerosol Fe solubility.

2.2. Deposition Flux and Internal Circulation Process

Fe is an essential component of electron-transport proteins such as cytochromes and ferredoxin, which are central to phytoplankton photosynthesis. Extremely low dFe concentrations (typically <0.2 nmol L⁻¹) can constrain chlorophyll synthesis and reduce carbon fixation efficiency [18,37,38]. Iron fertilization experiments have repeatedly shown that Fe addition enhances primary productivity in the surface ocean. For instance, during the IronEx II experiment in the equatorial Pacific, the addition of Fe led to a 27-fold increase in phytoplankton chlorophyll, a more than four-fold rise in primary productivity, and a notable decrease of ~90 μatm in surface-water CO₂ partial pressure [39]. Fe is also required for the nitrogenase of nitrogen-fixing cyanobacteria (e.g., Trichodesmium) and thereby modulates community structure through its effects on N₂ fixation. In tropical regions such as the North Atlantic, Fe and P co-limit nitrogen fixation; atmospheric dust deposition, especially from the Sahara, can enhance fixation rates by 2-to 8-fold, increasing the supply of bioavailable nitrogen and stimulating primary productivity [40]. Moreover, Fe enrichment directly alters phytoplankton community composition, for example, by triggering diatom blooms in the subarctic Pacific [41] and by shifting Southern Ocean communities from small prokaryote-dominated assemblages toward larger diatoms [42]. Thus, as an essential micronutrient for marine phytoplankton, Fe exerts a critical control on surface-ocean primary productivity, particularly in HNLC regions such as the Southern Ocean and the equatorial Pacific.

As shown in Figure 2, the bioavailability of Fe to marine primary producers varies greatly despite its complex chemical speciation. Fe(II) exhibits the highest bioavailability but is rapidly oxidized by O₂ and H₂O₂ in oxygenated seawater to Fe(III), which forms highly insoluble hydroxides. Most dFe in seawater is complexed by organic ligands as FeL, accounting for >99% of total dFe [18]. These soluble Fe and some colloidal Fe pools can dissociate to release Fe′ (labile inorganic Fe) on timescales of <1 day and are therefore considered bioavailable [43]. In contrast, the remaining colloidal Fe, together with particulate Fe (>0.2 μm), contributes little to the bioavailable pool because it is not directly accessible to phytoplankton.

Globally, the concentration of Fe in atmospheric aerosols varies significantly by region, with higher mass concentrations observed near North Africa and East Asia, influenced by mineral dust and anthropogenic pollution. In contrast, remote areas such as the mid-latitude Atlantic, Southern Ocean, and eastern Pacific exhibit lower concentrations. The Southern Ocean, identified as an HNLC region, shows total Fe deposition fluxes ranging from 1.31 × 10⁻⁶ to −3 × 10⁻⁴ g·m⁻²·yr⁻¹, with coastal zones near Antarctica experiencing markedly higher fluxes than open waters. This region is sensitive to atmospheric deposition, exemplified by the significant increase in soluble Fe following the 2019 Australian wildfires, which contributed to rapid phytoplankton growth and algal blooms [44]. In the North Pacific, total Fe fluxes are typically below 10 ng·m⁻²·d⁻¹ but can rise to 50–200 ng·m⁻²·d⁻¹ during spring dust storms [34]. In East Asia’s coastal waters, soluble Fe fluxes are typically higher than in the open ocean, with some impacted areas exhibiting 2 to 3 times greater fluxes than natural backgrounds, averaging 0.5 to 2.5 ng·m⁻²·d⁻¹ [45].

In summary, the spatiotemporal distribution of aerosol Fe deposition is a key control on oceanic primary productivity, the carbon cycle, and ecosystem structure. Even small changes in deposition fluxes to HNLC regions can trigger significant biological responses. Accurate quantification of the regional variability in atmospheric Fe deposition and its solubility is therefore essential for assessing potential impacts on marine ecosystems and biogeochemical cycles.

3. Aerosol Fe Solubility During Transportation

3.1. Dissolution Mechanisms of Aerosol Fe

The dissolution of aerosol Fe is primarily controlled by three abiotic mechanisms: proton-promoted, ligand-promoted, and photoreductive dissolution [46,47]. Once deposited into seawater, bioreductive processes may also become significant. Laboratory and field studies have collectively elucidated how these mechanisms operate during atmospheric transport and at the ocean surface.

Proton-promoted dissolution occurs when high H⁺ concentrations weaken Fe–O bonds in acidic media. Mineral dust from arid regions is typically alkaline, and its surface pH remains buffered as long as the particles contain neutralizing components such as CaCO₃. However, when acidic gases (e.g., SO₂, NO_x) accumulate beyond the buffering capacity, the surface pH drops sharply, accelerating Fe release [48,49,50,51]. Aerosol acidity is therefore considered a key factor in transforming insoluble Fe into labile and dissolved forms.

Ligand-promoted dissolution is particularly effective for Fe bound in aluminosilicates and oxides, with the latter dissolving markedly faster [52]. Small organic acids, such as oxalic, humic, and malonic acids, adsorb onto particle surfaces and form Fe–organic complexes, lowering the activation energy for dissolution and enhancing solubility [53]. Field studies have reported significant positive correlations between oxalic acid concentrations and soluble Fe [54,55], and laboratory experiments [56] confirm that Fe solubility increases with organic acid concentration, with the specific acid type also influencing the valence state of the dFe.

Photoreductive dissolution further amplifies Fe solubility. Organic ligands facilitate light-induced electron transfer that reduces Fe(III) to the more soluble Fe(II). Meskhidze et al. [30] observed that the dissolution rate of hematite increased 5- to 10-fold under illumination, and Fu et al. [57] confirmed that photoreduction measurably enhances Fe solubility in mineral dust. Field observations consistently detect organic acids in aerosols and at the ocean surface [58,59], where they associate with particulate matter and promote the formation of soluble Fe–organic complexes, thereby increasing the bioavailable Fe pool.

3.2. Emission Source on the Fe Solubility

Atmospheric Fe solubility is strongly source dependent. Mineral dust, the most abundant aerosol type globally [60,61], typically exhibits low initial solubility, generally below 0.5% [62,63,64,65]. Measurements near source regions confirm this: Fe solubility in total suspended particles from Dunhuang was <1% [66], and Qingdao dust averaged 0.27% [51]. Even after long-range transport, solubility often remains modest; for example, mineral aerosols reaching Hokkaido exhibit a solubility of just 0.52% [67] and Saharan dust arriving over the Atlantic shows a median solubility of only 1.7% [68]. Therefore, despite its overwhelming mass emission, dust likely supplies only a limited fraction of bioavailable Fe.

Anthropogenic aerosols from combustion and industrial processes exhibit considerably higher solubility, sometimes >80% [51,62,69]. For example, biomass-burning aerosols reach 56 ± 31% and residential coal-combustion particles 33 ± 28%, whereas industrial fly ash remains near zero [70]. Urban fine particles across Chinese cities show 2.7–5.0% solubility [71], and in the Sargasso Sea, Saharan dust solubility rose from 0.44% to 19% after mixing with North American industrial emissions [72]. Such observations clearly demonstrate that anthropogenic sources can disproportionately increase the bioavailable Fe input, despite their smaller total Fe emission fluxes.

However, the contribution of anthropogenic versus natural sources to surface-ocean bioavailable Fe remains highly uncertain, and this uncertainty is not only due to variable emission strengths. A critical but often overlooked issue is the lack of measurement standardization: studies employ different leaching solutions (ultrapure water, acetate buffer, seawater), filter pore sizes, and extraction times, rendering direct inter-comparisons of reported solubility values questionable. Furthermore, the concept of a fixed “initial solubility” linked to source type is an oversimplification because dynamic processes, such as acidification, organic acid coating, and photoreduction during transport, can profoundly alter Fe solubility downstream. For instance, Mahowald et al. [13] modeled high solubility over the Atlantic and central Africa, contrasting with high total Fe concentrations near East Asia, illustrating the spatial decoupling of Fe loading and bioavailability driven by transit chemistry. Southern Hemisphere wildfires add a further layer of complexity: their Fe contribution can rival that of dust, yet solubilities vary erratically from 0.5% to 46%, and the large interannual variability in fire activity makes it difficult to define a representative background for global models.

Therefore, reducing these uncertainties will require not only more field data but also a concerted effort toward harmonized solubility protocols and process-level parameterizations that capture the evolving bioavailability of aerosol Fe during transport. Such advances are essential for accurately assessing the impact of atmospheric deposition on marine productivity and carbon cycling.

3.3. Atmospheric Processes Affecting Fe Solubility

Gravitational settling causes coarser particles to be removed more rapidly than fine ones during atmospheric transport, producing a size differentiation that increases with distance [73]. Numerous field studies have reported a general negative correlation between Fe solubility and total Fe or total Al [31,33,49,74], which Baker et al. [49] attributed to the larger specific surface area of finer particles that favors chemical reactions. However, this inverse relationship is not universal: Buck et al. [75] found no significant trend between Fe solubility and particle size across nine size-segregated aerosol samples from the North Atlantic, indicating that size alone cannot reliably predict solubility.

Fe solubility is strongly pH-dependent [76,77,78]. During transport, acidic gases such as SO₂ and NO_x are oxidized to sulfuric and nitric acids through heterogeneous reactions, altering aerosol acidity and the chemical forms of Fe [79,80]. Consequently, Fe solubility typically increases along transport pathways: Buck et al. [81] measured solubilities of 9.0% and 8.1% in TSP over the North Atlantic and Pacific, Hsu et al. [82] reported 7.7% in the East China Sea, and Chuang et al. [66] documented a range of 0.1–10% in Asian coastal aerosols that correlated with black carbon, linking solubility enhancement to anthropogenic emissions. The most striking demonstration is the contrast between fresh Saharan dust (~0.5% solubility) and the same dust after passing over North America, where solubility reached 19% [72]. Acidification by anthropogenic secondary species is central to this enhancement, as indicated by positive correlations between Fe solubility and sulfate or nitrate content [51,83,84] and by TEM observations of Fe-bearing particles from coal combustion and steel production encapsulated in secondary sulfates [85]. Relative humidity (RH) facilitates these acid-driven reactions [86,87]; accordingly, Fe solubility is highest on foggy or hazy days [51,88], and in Xi’an the median solubility rose from <1% at RH <90% to 1.68% when RH exceeded 90% [89] (Figure 3). In contrast, during dust events with limited chemical processing in Qingdao [51,88], Jeju Island [66], and Hokkaido [67], Fe solubility generally remains below 0.5%, confirming the importance of acidification and humidity. Nevertheless, it should be noted that the chemical environment experienced by individual particles is highly variable, and the statistical relationships with bulk aerosol sulfate or nitrate do not fully capture the micro-scale conditions that control dissolution.

Organic acids, ubiquitous in aerosols and at the ocean surface [58,59], can penetrate or adsorb onto particles and complex Fe, potentially enhancing its solubility. However, the role of oxalic acid remains debated: some studies observe a significant positive correlation between oxalate and soluble Fe [52,90], while others find no clear relationship [51,84]. This discrepancy may arise because oxalic acid can also be produced by atmospheric oxidation, so an observed correlation does not necessarily imply that oxalate directly drives Fe dissolution [91]. Resolving this ambiguity requires process-level studies that discriminate between primary emissions and secondary formation of organic acids and that quantify their true ligand-promoted dissolution rates.

Cloud processing adds further complexity. Insoluble Fe can dissolve during repeated condensation–evaporation cycles (typically 5–10 before reaching the ocean) [92] because evaporating droplets produce wet aerosols with pH values as low as 2 [93,94,95]. Laboratory simulations confirm that Fe solubility increases at low pH [96], and particles from biomass burning, due to their hygroscopicity, preferentially act as cloud condensation nuclei [97]. Single-particle measurements at Nanling showed that combustion-related Fe-rich particles comprised 84% of Fe-containing cloud residuals, and cloud processing increased nitrate, chloride, and oxalate in these residuals [98]. Yet, a counterpoint is provided by Shi et al. [99], who found that while acidic wet aerosols promote dissolution, the higher pH typical of actual cloud droplets can inhibit it. Thus, the net effect of cloud processing on Fe solubility depends on the droplet pH history, which itself is a function of aerosol composition and trace gas uptake, a complexity that current large-scale parameterizations rarely capture. Collectively, these uncertainties highlight the need for field-based, chemically resolved observations and for models that explicitly track aerosol acidity and organic ligand evolution along trajectories rather than relying on fixed solubility fractions assigned to source types.

4. Conclusions

Atmospheric deposition of aerosol Fe is a primary pathway supplying bioactive Fe to the open ocean, with dust and combustion sources dominating total Fe loading. This review has integrated field observations, laboratory experiments, and model evidence to show that Fe solubility is governed by a complex interplay of source characteristics, chemical aging during transport (acidification, ligand-promoted dissolution, and photoreduction), and cloud processing, all modulated by relative humidity and particle size. A consistent finding is that anthropogenic emissions and secondary acidification can elevate solubility by an order of magnitude above that of fresh mineral dust, while the net effect of cloud cycles is highly dependent on droplet pH history. These processes collectively decouple soluble Fe delivery from total Fe emission, challenging the common practice of assigning fixed solubility fractions to source types. However, it is important to acknowledge that the synthesis of existing data is complicated by inconsistent methodologies. Studies employ a wide range of leaching solutions, filter pore sizes, and extraction times, which introduces significant uncertainty into the comparability of reported solubility values and may partly mask or exaggerate the true environmental trends. Moreover, the widespread concept of a static initial solubility linked to a given source type is an oversimplification that ignores rapid dynamic alterations in Fe speciation during transit, and the high interannual variability of sources such as wildfires further renders static parameterizations unreliable.

Despite this progress, significant uncertainties persist. First, the relative contribution of anthropogenic versus natural soluble Fe remains poorly constrained in ambient samples, largely because most studies examine single-source aerosols and lack the chemical fingerprints needed to deconvolve mixed plumes. This mismatch between model parameterizations and observational realities undermines the accuracy of deposition flux estimates. Second, the ecosystem-scale impacts of altered Fe bioavailability, such as its interplay with nitrogen and phosphorus limitation and its long-term effects on community structure and carbon export, are still primarily inferred from short-term enrichment experiments. This leaves a critical gap in understanding ecological dynamics over seasonal to decadal timescales.

Addressing these challenges will require systematic field campaigns that resolve anthropogenic and natural iron fractions with chemical markers, process-level models that explicitly simulate aerosol acidity and ligand evolution along transport pathways, and sustained multidisciplinary studies linking atmospheric inputs to biogeochemical responses across trophic levels. A concerted effort to harmonize solubility measurement protocols is equally essential, as it is a prerequisite for synthesizing a globally representative database. Such integrated efforts will be indispensable for improving projections of marine productivity and carbon sequestration under changing emission and climate regimes.