Stable Isotope Ratios of Combustion Iron Produced by Evaporation in

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Stable isotope ratios of combustion iron produced by evaporation in a steel plant Minako Kurisu, Kouji Adachi, Kohei Sakata, and Yoshio Takahashi ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00171 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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ACS Earth and Space Chemistry

Stable isotope ratios of combustion iron produced by evaporation in a steel plant Minako Kurisu1*, Kouji Adachi2, Kohei Sakata3, and Yoshio Takahashi1* 1

Department of Earth and Planetary Science, Graduate School of Science, The University of

Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan 2

Atmospheric Environment and Applied Meteorology Research Department, Meteorological

Research Institute, 1-1, Nagamine, Tsukuba, Ibaraki, 305-0052, Japan 3

Center for Global Environmental Research, National Institute for Environmental Studies, 16-2,

Onogawa, Tsukuba, Ibaraki, 305-8506, Japan KEYWORDS: size-fractionated aerosol, Fe limitation in the surface ocean, source apportionment of Fe, isotope fractionation, XAFS

Abstract Combustion iron (Fe) in aerosols is one of the sources of dissolved Fe in the surface ocean. The iron isotope ratio (δ56Fe) is an important tool for source apportionment of Fe, because combustion Fe emitted by evaporation possibly yields lower δ56Fe values than natural materials. However, there are insufficient data of δ56Fe for combustion Fe. Hence, δ56Fe values of Fe emitted from a

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steel plant were investigated, and the representative δ56Fe value of combustion Fe was discussed. The presence of a large number of submicron spherical Fe oxide particles suggested that the particles were emitted by high-temperature evaporation. Fine particles yielded much lower δ56Fe (as low as −3.53‰) than original materials, indicating that Fe isotope fractionation occurred during evaporation. Based on this study and our previous data, we suggest −3.9‰ to −4.7‰ as the representative δ56Fe range of combustion Fe. Mass balance calculations using this range suggest that the contribution of combustion Fe is approximately 57-83% of the total soluble Fe in aerosols in the northwest Pacific, implying a large contribution of combustion Fe. The remarkably low δ56Fe value of combustion Fe emitted by evaporation enables us to evaluate its contribution to marine aerosols and to understand Fe cycles in the surface ocean.

Introduction Deficiency of dissolved iron (Fe) is a limiting factor of primary production in some oceanic regions, and related to global climate change.1–4 Natural aerosol (mainly from desert areas) is one of the main Fe sources.5 Combustion aerosol, which is emitted by high-temperature combustion processes, is also considered an important source of dissolved Fe owing to its high solubility in seawater, although its emission is lower than that of natural Fe.6,7 In particular, combustion Fe is emitted by high-temperature processes, such as coal and oil combustion, steel manufacturing, and vehicle emission.8–10 However, the relative contribution of combustion Fe to the surface ocean remains unknown.


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Stable isotope ratios are important tools to identify emission sources of target elements and to understand the subsequent chemical processes.11,12 Iron stable isotope ratios are expressed as follows:

56

( 56Fe/ 54Fe)sample

δ Fe (‰) = � 56 − 1� × 1000 ( Fe/ 54Fe)𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

(1)

Recently, Fe isotopes have been used in marine geochemistry, especially in the evaluation of different Fe sources, including aerosols, sediments, and hydrothermal fluids.13–15 However, only a few studies distinguished combustion Fe from natural Fe, such as mineral dust, because of limited data about the combustion Fe isotope ratios.16,17 In our previous study, we collected size-fractionated aerosol particles in Hiroshima to distinguish combustion aerosols from different sources,18 considering that natural aerosols are usually coarser than combustion ones.19 Fine aerosol particles contained combustion Fe with much lower δ56Fe values (as low as −2.0‰) than crustal Fe (0.0‰).18 We also found that (i) fine aerosol particles collected in a tunnel have δ56Fe values of −1.4–−3.2‰, which are lower than those of possible original materials, such as gasoline, and (ii) the soluble fraction of fly ash collected in an incinerator has a δ56Fe value that is 1.9‰ lower than that of residual bottom ash.20 The low δ56Fe values of combustion Fe may be attributed to isotope fractionation during evaporation and are distinguishable from those of other natural materials.13,21,22 However, the representative δ56Fe value of combustion Fe must be determined for the evaluation of its contribution to the ocean. Thus, sampling aerosols that are obviously emitted by combustion sources, clarifying the δ56Fe values of aerosols from various combustion emission sources, and characterizing Fe isotope fractionation are all necessary steps in our evaluation.


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Steel plants are one of the major sources of combustion Fe emitted by high-temperature processes in Japan.23,24 Steel plants emit a considerably large amount of Fe from a point source relative to other combustion Fe sources, such as oil or coal combustion. Hence, the Fe emission from steel plants can be readily identified. In steel manufacturing, Fe is treated at high temperatures (1000–2000 °C),25 and thus it is expected that particles from steel plants contain a large number of Fe particles which experienced evaporation. Hence, we regarded steel plants as a possible source of combustion Fe produced by evaporation at high temperature. Although a previous study on δ56Fe values of total suspended particles (TSP) from a steel plant did not find large isotope fractionation,26 it is expected that size-fractionated aerosol sampling in this study can specifically detect δ56Fe values of fine Fe particles emitted by high-temperature processes. This study aims to clarify the δ56Fe value in aerosol particles emitted during steel manufacturing and to estimate the representative δ56Fe value of combustion Fe. We discuss formation processes of Fe in coarse and fine particles based on the results of Fe concentration, Fe species, and morphological characteristics. Subsequently, δ56Fe in coarse and fine particles, and δ56Fe in the soluble Fe fraction, are determined to estimate the δ56Fe value of Fe produced by evaporation. Finally, the representative δ56Fe value of Fe in combustion aerosol emitted by various sources and their contribution to marine aerosols in the northwest Pacific are discussed.

Samples and Methods Sample collection. Aerosol sampling was conducted near a steel plant from September 6 to 27, 2016 at the rooftop of Chiba City Office, Chiba, Japan (CCO site, 35.6074°N, 140.1070°E, approximately 30 m above the ground), which is approximately 4 km northeast of the steel plant


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(Figure S1a). Seven sets of samples were collected for 2–4 days for each sampling period (Table 1, samples A to G). For comparison, similar sampling was conducted at the Kemigawa Seminar House, The University of Tokyo, Chiba, Japan (KSH site, 35.6588°N, 140.0731°E, approximately 10 m above the ground), which is approximately 10 km to the north-northwest of the steel plant (Table S1 and Figure S1a). Aerosol particles were collected on filters by a highvolume air sampler (Kimoto, MODEL-123, Osaka, Japan) with a cascade impactor (TE-236, Tisch Environmental Inc., USA) to separate particles into 7 size fractions (stage 1, > 10.2 μm; stage 2, 4.2–10.2 μm; stage 3, 2.1–4.2 μm; stage 4, 1.3–2.1 μm; stage 5, 0.69–1.3 μm; stage 6, 0.39–0.69 μm; backup filter, BF, < 0.39 μm). An acid-washed polytetrafluoroethylene (PTFE) sheet (Naflon tape; thickness: 0.2 mm; Nichias Co., Ltd., Japan) was used for stages 1–6 in order to minimize contamination from the sampling filter. Details of the preparation methods of the PTFE filter are described in Sakata et al.27 Another type of PTFE filter (PF050, Φ90, Advantec, Japan) was used for BF. For washing, both types of filters were soaked into 3 mol/L HNO3, 3 mol/L HCl, and ultra-pure water for 1 day each.

Table 1. Sampling periods of aerosols collected at Chiba City Office (CCO site). sample No.

start (local time)

end

sampling time

CCO-A

September 6, 2:30 p.m.

September 9, 9:30 a.m.

67 h

CCO-B



66 h

CCO-C



44 h

CCO-D



46 h


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CCO-E



96 h

CCO-F



69 h

CCO-G



93 h

Measurement of concentrations of Fe, aluminum, and soluble Fe. The concentrations of total Fe and aluminum (Al) were measured by quadrupole ICP-MS (Agilent 7700, Agilent, Japan). Approximately 1/40–1/20 (3–6 cm2) of a filter was decomposed by 2 mL of 15.2 mol/L HNO3, 2 mL of 9.3 mol/L HCl, and 1 mL of 22 mol/L HF (Tamapure AA-100, Tama Chemical, Japan) in a closed perfluoroalkoxy alkanes (PFA) vial (7 mL) for 1 day at 150 °C. The solution was evaporated nearly to dryness at 120 °C. After that, an appropriate amount of 0.3 mol/L HNO3 was added to each vial for the ICP-MS measurement. All equipment was washed with acid before use and all procedures were conducted under HEPA-filtered environment (SS-MAC 15, Air Tech, Japan). Filter blank concentrations, detection limits, and recoveries of Al and Fe checked by simulated Asian mineral dust material (CJ-2)28 are described in Table S2. The filter blank of Fe was less than 2% of total Fe collected on the PTFE sheet, while 8–50% for PF050. Fractional solubilities of Fe ([Fesoluble]/[Fetotal] ×100 (%)) of stage 1–6 were determined with an extraction experiment. Approximately 1/40–1/20 of a filter was soaked in 15 mL of ultrapure water for an hour with ultrasonic treatment for 30 min. Subsequently, the solution was filtered using 0.20 μm PTFE filter (DISMIC-25HP, ADVANTEC, Japan) which was rinsed with ultrapure water before use. HNO3 was added so that the concentration reached 0.3 mol/L for ICP-MS measurement. This extraction method is based on other studies.29,30 Therefore, we can


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compare these results with those of other reports. In addition, in order to compare these results with our previous study18, this study also conducted an extraction experiment with simulated rainwater (RW, 0.02 M oxalic acid/ammonium oxalate at pH 4.7) as a ligand-containing solution.18,31 After filtration, the solution was evaporated to dryness at 120 °C. In order to decompose residues, 15.2 mol/L HNO3 was added and heated at 120 °C for 1 day. After that, they were evaporated to dryness and redissolved into 0.3 mol/L HNO3. Filter blank concentration and detection limit of Fe with each extraction solution are described in Table S2. The filter blanks of Fe extracted by ultrapure water and RW were less than 7% and 2% of the extracted sample Fe concentrations, respectively.

Bulk X-ray absorption fine structure (XAFS) and μ-XRF-XAFS analyses. The Fe Kedge X-ray absorption near-edge structure (XANES) spectra of the bulk aerosol samples and references were obtained at the beamline BL-12C at Photon Factory (PF), KEK (Tsukuba, Japan). The Fe species in each sample were estimated by linear combination fitting of the sample spectrum with the reference spectra using the software REX2000 (Rigaku, Japan). The 1st derivative of the XANES spectrum and k3-weighted χ(k) of the extended X-ray absorption fine structure (EXAFS) spectrum were also obtained to confirm the fitting result by XANES. MicroXRF-XAFS analysis was also conducted for individual particle analysis at the beamlines BL-4A of PF and BL05SS and BL37XU of SPring-8 (Sayo, Japan). Sizes of the incident beam at BL4A, BL05SS, and BL37XU were approximately 5 × 5 μm2, 2 × 1 μm2, and 0.6 × 0.8 μm2, respectively. The methods are detailed in the Supporting Information.


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Morphological and elemental composition analyses of individual particles. The morphological and elemental composition analyses of coarse particles were conducted by scanning electron microscopy (SEM, S-4500, Hitachi, Japan). A part of the filter at stage 2 (4.2– 10.2 μm) of sample CCO-A was mounted on a carbon tape without coating. Fine particles were analyzed by transmission electron microscopy (TEM, JEM-1400, JEOL, Japan). Particles were collected directly onto a copper grid (EM Japan Co., Tokyo, Japan) at the impactor stage at 100–700 nm with 50% cutoff aerodynamic diameters by using a TEM sampler (Arios Inc., Tokyo, Japan).32 Five-minute sampling was conducted every 3 or 6 hours during the sampling campaign.

Iron isotope analysis. Iron isotope analysis was conducted by multicollector-inductively coupled plasma mass spectrometry (MC-ICP-MS, Neptune Plus, Thermo Fisher Scientific, USA). The Fe isotope ratio is shown as per mil (‰) relative to a standard material obtained from the Institute for Reference Material and Measurements (IRMM-014, Belgium) as described in Equation (1). The values for the stage BF (< 0.39 μm) were excluded from the discussion because of their high blank/sample ratios. Detailed information for the analysis is described in the Supporting Information.

Results and discussion Characterization of samples. Atmospheric Fe concentrations of samples A, E, and G were higher than those of the other samples in all size fractions (Figure 1a), and those observed


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in other areas in Japan.18,33–35 These high atmospheric Fe concentrations were related to southwest winds from the steel plant (Figure S1b). During the sampling campaign, Ohata et al.36 also quantified the abundance of Fe oxide aerosols (especially magnetite) by a modified singleparticle soot photometer (SP2) with high time resolution, where a large amount of Fe emission was also detected when the wind came from the steel plant. The enrichment factor of Fe (EFFe = (Fe/Al)aerosol/(Fe/Al)crust) was calculated for the estimation of particle sources. The EF was used to indicate the extent of the enrichment of Fe compared with average crust37. The EFFe values of all size fractions in the samples were higher than 1, especially in samples A, E, and G when the wind was mainly from the steel plant (Figure 1b). These high EFFe values suggest that the samples contained Fe-rich materials relative to the crust in all size fractions, partly because of the emission from the steel plant. The EFFe values of stage 6 (0.39–0.69 μm) were higher than those of the other stages, especially in samples A and G.

Figure 1. (a) Size distributions of Fe concentration. (b) Enrichment factors (EFs) calculated as EFFe = (Fe/Al)aerosol / (Fe/Al)crust. Average Fe species of each size fraction. Based on the atmospheric Fe concentration and the wind direction, we selected A and G as the samples mainly derived from the steel plant.


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For reference, we used B as the sample mainly derived from non-steel plant sources since (i) wind directions were not from the steel plant during this sampling period (Figure S1) and (ii) sample B showed the lowest EFFe values among all the samples (Figure 1b). The bulk XAFS spectra showed similar characteristics among the samples A, B, and G (Figure 2a). Fine particles had sharper peaks at around 7.129 keV, whereas peak tops of coarse particles were broader and located at around 7.128 keV. Species with a higher valence have higher peak energy due to different binding energies,38 as is also seen from the reference spectra. Therefore, the higherenergy peak position indicates that fine particles contain a larger fraction of ferric species than coarse particles. Iron species in all the samples are composed of two or three of the following Fe (hydr)oxides (Figure 2): wüstite (FeO), magnetite (Fe3O4), ferrihydrite, maghemite (γ-Fe2O3), and hematite (α-Fe2O3). The similar fitting results of the EXAFS and the1st derivative of XANES support the results of the XANES analysis (Figures S2 and S3). It should be noted that the fitting results are semi-quantitative and we show only dominant species in each sample. Unlike aerosols in suburban areas in Japan,7,18 natural Fe sources, including aluminosilicates (e.g., biotite and illite), were not the main Fe species, even when the influence of Fe particles from the steel plant was relatively small (sample B). Wüstite was detected only in the coarse particles, whereas maghemite was detected only in the fine particles. Fractions of wüstite and maghemite were larger in samples A and G than in B, indicating that wüstite and maghemite were mainly derived from the steel plant.


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Figure 2. (a) Iron K-edge XANES spectra with reference spectra and (b) fitting results of the samples A, B, and G. Individual particle analysis by μ-XRF-XAFS and SEM/TEM. The spot analysis by μXRF-XAFS identified Fe (hydr)oxides, including ferrihydrite and hematite, in coarse particles of sample A at stage 2 (4.2–10.2 μm), which was consistent with the results of the bulk XAFS analysis (Figure S4). Iron coexisted with various elements such as calcium (Ca), titanium (Ti), manganese (Mn), vanadium (V), and zinc (Zn). SEM observations also revealed that several types of particles were contained in coarse particles. The most abundant particle type was a debris-like type (spots 1 and 2 in Figure S5a), which mainly consisted of Fe and Ca, as well as Al and silicon. Other particles included were (i) large spherical types (approximately 10 μm)


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consisting mainly of Fe (spot 3 in Figure S5a) and (ii) aluminosilicate types (spot 4 in Figure S5a). TEM observation revealed that fine particles of sample A at stage 6 (0.39–0.69 μm) contained Fe-rich spherical particles, which consisted of Fe oxide with 10-100 nm and were commonly aggregated with each other (Figure 3). Spot analysis by the μ-XAFS showed that individual Fe species were ferrihydrite, magnetite, maghemite, and hematite (Figure S6e), which is consistent with the bulk XAFS analysis. Unlike coarse particles, Fe in fine particles did not coexist with Ca, Mn, and Zn from the μ-XRF elemental maps (Figure S6), indicating that the formation process of fine particles was different from that of coarse particles, even if similar Fe species were found in the particles. Calcium, which is more refractory than Fe, was not found frequently in fine particles, whereas Mn and Zn, which are more volatile than Fe, were abundant in fine particles.39


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Figure 3. TEM and STEM-EDS images of aerosol particles collected at 10:30, September 26, 2016, during sampling period G. (a) TEM images of aerosol particles after the STEMEDS analysis. (b) Magnified TEM images of Fe oxide particles circled in (a). Tens of Fe oxide particles with sizes smaller than 100 nm occur as aggregates. (c) Distribution of Fe and O by STEM-EDS analysis. White particles in the STEM image were mostly Fe oxide.

Formation processes of coarse and fine particles in the steel plant. During steel manufacturing, various kinds of particles, such as raw materials (mainly magnetite and hematite), slags, and particles emitted via high-temperature processes, are emitted.40,41 Slags are formed in a blast furnace, where Fe oxides are reduced by reacting with coke, and in a converter, where oxygen is blown into molten Fe. Impurities, including sulfur, silicon, aluminum, calcium, and magnesium, are removed as slags by separating them from molten Fe in terms of different densities. Slags are stored in storage areas after cooling them. High amounts of Fe remain in the slags, where ferrous and ferric Fe oxides are found.41–43 Particles emitted via high-temperature processes are mainly from a sinter, a blast furnace, and a converter.40 On the basis of Fe concentration, Fe species, and individual particle analyses, we discuss different formation processes of coarse and fine particles emitted from the steel plant. In coarse particles, Fe-rich debris-like particles were dominant. The elemental compositions and morphological characteristics indicated that these particles originated from slags or Fe ore.41–44 Debris-like particles were possibly emitted by the suspension of particles from storage areas.40 Wüstite (FeO) was found in the coarse particles, which is frequently found in slags.41–43 In addition, Fe-rich spherical particles detected in coarse particles were considered to be emitted by melting and cooling processes in a blast furnace or a converter.41


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In fine particles, aggregates of Fe-rich spherical nanoparticles were found, and their species were mainly classified as Fe (hydr)oxides, such as maghemite, magnetite, hematite, and ferrihydrite. EFFe values at stage 6 were higher than those at the other stages possibly because of the dominance of these Fe-rich particles. This finding was also supported by Ohata et al.36 showing that particles in the fine-sized fractions contained a large number of Fe oxides. Similar Fe oxide nanoparticles, including maghemite, magnetite, and hematite, are often observed in urban areas.45–47 Such nanoparticles are usually aggregates of 20–40 nm spherical particles, which are formed at high temperatures soon after emission.47,48 Spherical morphologies and sizes suggested that they were formed by either evaporation or melting at high temperature. However, the particle sizes of spherical particles formed by melting and cooling are basically larger than those formed by evaporation.49 According to a previous study that investigated particles emitted from a liquid steel bath, the number of fine particles emitted by evaporation is larger than that emitted by melting and cooling.50 Model calculations also suggest that Fe particles formed from Fe gas show similar particle sizes of up to 100 nm.48 Therefore, a large number of Fe in fine particles was formed via evaporation. The presence of various Fe species is possibly due to different oxidation conditions of each particle; small Fe particles tend to be readily oxidized,51 and Fe species can differ in terms of Fe and oxygen fugacities at high temperatures. Maghemite, which was observed only in fine particles, is a metastable species possibly formed within a short oxidation period before it is emitted to the atmosphere, although it remains unclear whether this species is emitted via evaporation or melting.

Iron isotope ratios. Coarse particles (stages 1–4; >1.3 μm) of samples A, B, and G showed δ56Fe values from −0.42±0.08‰ to 0.33±0.08‰ (on average 0.04‰, Figure 4a, Table


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S3). The main Fe source in coarse particles was probably steel slags or raw materials, and the δ56Fe values of steel slags are possibly consistent with those of raw materials.52 Steel plants use several types of ores from different localities with δ56Fe values ranging from −1‰ to 1‰.53 The values in Fe ore can vary within the range, and actually a δ56Fe value for one of the Fe ores used in the plant was ‒0.27±0.13‰. Although the δ56Fe value of the ore in this study and the coarse particles cannot be simply compared, the similarity of these values suggests that the Fe in coarse particles was not subject to large isotope fractionation during emission. The δ56Fe values of fine particles (stages 5 and 6; 0.39–1.3 μm) ranged from −0.37‰ to −3.53‰, which were significantly lower than those of coarse particles and the measured Fe ore (Figure 4a). Fine particles were also originated from Fe ores from the steel plant and the original materials became liquid before emission. Therefore, fine particles possibly had similar δ56Fe values with coarse particles and ores before emission. The presence of a large number of evaporated Fe particles suggests that large isotope fractionation during evaporation occurred in fine particles under high-temperature processes in the steel plant. The values at stage 6 (0.39– 0.69 μm) of sample G showed much lower δ56Fe values (−3.53±0.30‰) than the other samples, indicating that sample G contained a larger amount of evaporated Fe from the steel plant, which was also suggested from its high EFFe value at stage 6. To determine temporal and spatial variations of δ56Fe values, the stage 6 of the other sample sets (CCO-C, D, E, and F) and the samples collected at the other site (KSH site) were also analyzed (Figure S7). The δ56Fe values were basically lower than −1.5‰ and similar between the two sites. Further details on the comparison of the samples at the two sites (CCO and KSH) are described in the Supporting Information.


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Figure 4. (a) Size distributions of Fe isotope ratios of samples A, B, and G. (b) Size distributions of Fe isotope ratios of extracted Fe with simulated RW and ultrapure water (UPW). (c) Relationship between Fe isotope ratios and inverse of Fe atmospheric concentrations.

Fractional solubility of Fe and isotope ratios of soluble Fe. The fractional solubility of Fe depends on various possible factors, such as sources, species, and atmospheric processing, including interaction with acids during transportation.7,31,54–59 In the present study, the influence


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of atmospheric processing should be low, because the sampling site is close to the source. Therefore, fractional solubility probably reflects the solubilities of the source materials. The fractional solubility of combustion Fe (up to 80%) is higher than that of Fe in natural soil ( 2.1 μm) of all the samples were low (0.1–2%), which is attributable to the dominance of Fe (hydr)oxides that originated from slags or raw materials (Figure 5a). The fractional solubilities of Fe extracted by ultrapure water at stages 4-6 (0.39–2.1 μm) were higher (up to 19%) than those in the coarse particles, especially in sample G, although Fe (hydr)oxides were the main Fe species at these stages. These fractional solubilities were lower than those in oil fly ash57, and this result was consistent with previous findings.58,59 However, a weak correlation between fractional solubilities and δ56Fe values was observed in all of the samples (Figure 5b). Considering that the low δ56Fe values indicate isotope fractionation during evaporation, we assume that this correlation indicates the partial contribution of the presence of Fe emitted by evaporation to the enhancement of Fe solubility, which is partly due to the large surface areas of spherical Fe nanoparticles emitted by evaporation. Although the fractional solubility of Fe in these particles can be enhanced due to atmospheric processing during transportation, the evaporation process is possibly one of the factors controlling fractional solubility of Fe in this study.


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In order to compare these results with our previous studies7,18,31, we conducted an extraction experiment involving simulated RW. The fractional solubilities of Fe extracted by simulated RW were higher than those extracted by ultrapure water in all of the size fractions possibly because the amount of Fe that could be extracted by oxalic acid in RW is larger than that obtained by ultrapure water or other acids, such as acetic acid and sulfuric acid (Figures 5a and S8).54 The fractional solubility was higher in fine particles than in coarse particles (Figure S8), and this result was similar to our previous findings on aerosols collected in a suburban area in Japan, suggesting that combustion Fe is highly soluble.18 However, the trend was unclear. The fractional solubilities of Fe in the coarse particles in our study are relatively higher than those in the previous study.18 The simulated RW can preferentially dissolve Fe (hydr)oxides, especially ferrihydrite, compared with aluminosilicates.31 The coarse particles in this study were mainly composed of Fe (hydr)oxides, but were not dominated by aluminosilicates. Thus, the fractional solubility of Fe in coarse particles is higher in this study than that in the previous study.18 We do not discuss these coarse particles from the steel plant because they are not an important source of soluble Fe since they were not dominant in other sites.7,18,31 δ56Fe values of Fe in sample G extracted by ultrapure water for all stages except for stage 1 (due to low solubility) were analyzed, considering that evaporated Fe could be preferentially extracted.18 In addition, δ56Fe values of soluble Fe extracted by simulated RW in all of the samples were analyzed. Although RW extracted a larger amount of Fe than ultrapure water, evaporated Fe could be readily extracted in comparison with other components. Most of the δ56Fe values of fine particles extracted by simulated RW and ultrapure water were lower than those of acid-digested Fe. The δ56Fe values of fine particles of sample G extracted by simulated RW were consistent with those extracted by ultrapure water despite that the fractional solubilities


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were different. This result suggests that the particles with low δ56Fe values (i.e. evaporated Fe) were preferentially extracted, while fractional solubility probably differs among particles with low δ56Fe values. Sample G at stage 6 extracted by simulated RW (−4.08±0.22‰), which contained the largest fraction of evaporated Fe among all the samples, possibly yielded a δ56Fe value closer to that of evaporated Fe. δ56Fe values of coarse particles extracted by simulated RW were similar to those of aciddigested Fe, presumably because almost all coarse particles consist of Fe (hydr)oxides in slags or raw materials, which were partially dissolved (Figure 4b).

Figure 5. (a) Fractional solubility of Fe extracted by ultrapure water defined as [Fesoluble] / [Fetotal] × 100 (%). (b) Relationship between fractional solubilities and Fe isotope ratios. Estimation of the δ56Fe value for evaporated Fe. Each sample set has a correlation between δ56Fe values and inverses of Fe concentration (Figure 4c). These correlations mean that the δ56Fe values can be expressed by a linear combination of two endmembers with different δ56Fe values. Similar correlations found for samples A and G with a large contribution of Fe particles from the steel plant suggest that each size fraction has similar endmembers in terms of


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δ56Fe. For samples A and G, we assume that the two components are (i) particles originated from slags or raw materials ([Fe]original, δ56Feoriginal) and (ii) isotope-fractionated particles by evaporation ([Fe]evaporated, δ56Feevaporated). Concentrations of Fe in aerosol samples ([Fe]aerosol) are described as follows: [Fe]aerosol = [Fe]evaporated × f + [Fe]original × (1-f),

(2)

where f means proportion of evaporated Fe in aerosol samples. In addition, the following equation is obtained from mass balance: δ56Feaerosol × [Fe]aerosol = δ56Feevaporated × [Fe]evaporated × f + δ56Feoriginal × [Fe]original × (1-f) (3) From these two equations, the following is obtained: δ56Feaerosol = a/[Fe]aerosol + b,

(4)

where a and b are expressed by combinations of δ56Feevaporated, [Fe]evaporated, δ56Feoriginal, and [Fe]original. The a and b become constant when δ56Feaerosol and 1/[Fe]aerosol values are correlated. In the case of sample A, a and b are -155±20 and 0.21±0.05, respectively, whereas they are 152±6 and 0.72±0.06, respectively, for sample G (Figure 4c). To calculate the δ56Feevaporated values, [Fe]evaporated was estimated from the Fe/Al ratio. We employed the Fe/Al ratios at stages 6 (0.39–0.69 μm) and 1 (> 10.2 μm), assuming that stage 6 contained the largest fraction of isotope-fractionated Fe particles, whereas stage 1 mostly consists of slags or raw materials. Considering that all Al in stage 6 was derived from slags or raw materials, the following equation is obtained: [Fe]evaporated = [Fe]stage 6 − [Al]stage 6 × [Fe]/[Al]stage 1

(5)


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Subsequently, δ56Feevaporated calculated from the correlation lines were −4.0±0.5‰ and −4.9±0.25‰ for samples A and G, respectively. The average for δ56Feevaporated calculated in this study was −4.7±0.7‰.

Understanding the Fe isotope fractionation process. The δ56Feevaporated values can be described by the Rayleigh distillation law as a function of Fe fraction in the solid phase (f) and the fractionation coefficient (αsolid):60 δ56Feevaporated (‰) = (1000 + δ56Feoriginal)×(1−𝑓𝑓 1⁄𝛼𝛼𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 )/(1−f) −1000.

(6)

Adopting δ56Feoriginal as 0‰ (average δ56Fe value in coarse particles) and δ56Feevaporated as calculated above, αsolid was calculated as a function of f (Figure S9). Since it was difficult to estimate f from our results, we roughly estimated the minimum f from the results of Ohata et al.36 They analyzed Fe oxide concentration during the same sampling campaign and showed that the emission of Fe oxide from the steel plant (mass equivalent diameters 170 nm-2,100 nm) was approximately 0.49±1.03 Gg/yr, which was 0.012% of the total crude steel production per year. In this case, even if all the Fe is emitted via evaporation, more than 99.9% (≈100%−0.012%) of Fe remains in the crude steel, indicating that the minimum ratio in the solid phase f is 0.999 (Figure S9). As a result, αsolid values for samples A and G were calculated as 1.0040 and 1.0049, respectively. When f ranges from 0.999 to 1, the αsolid values are almost constant from Figure S9. When only a small proportion of Fe evaporates, lighter Fe preferentially evaporates and the gaseous phase has an extremely low δ56Fe value. Subsequently, Fe condensates soon after evaporation to form spherical nanoparticles as observed in the TEM analysis. It should be noted


21


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that even though only a small fraction of Fe evaporates, its amount can be large enough for detection because the Fe amount in original materials is extremely large. It was somewhat difficult to evaluate the influence of Fe emitted by evaporation in bulk (all size-fraction) aerosols due to the dominance of Fe from other sources in coarse particles. The size-fractionated sampling made it possible to clearly detect the evaporated Fe fraction in this study. The αsolid value mainly depends on the mass weight of evaporated species. Therefore, we focused on the chemical species during evaporation based on the theoretical αsolid values calculated as the square root of the mass ratio.61,62 According to Sylvestre et al.,40 the main sources of Fe-containing fine aerosol particles in steel plants are the converter, sinter plant, and blast furnace. In converters, Fe evaporates as Fe(II)O gas when oxygen gas is blown into the pig Fe pool.63 However, the theoretical αsolid value of Fe(II)O (1.0142) is much higher than our estimated value. Another possible species is Fe chloride (FeCl2 or FeCl3). Thermodynamic calculations suggest that Fe evaporates as Fe chloride gas under the presence of Cl.64 Source materials for sintering contain several hundred g/t of Cl.65 In addition, the boiling temperatures of FeCl2 (1000 °C) and FeCl3 (650 °C)66 are lower than the temperatures in the steel plant (1000– 2000 °C). Hence, some amount of Fe can evaporate as Fe chloride during the steel production. Although Fe chlorides were not detected in the XAFS and morphological analyses, Fe chlorides possibly react with water or oxygen to finally form Fe (hydr)oxides under high-temperature conditions.67 The αsolid values of FeCl2 and FeCl3 are 1.0080 and 1.0062, respectively, which are closer to our estimated values (1.0040 and 1.0049) than Fe(II)O. Physicochemical conditions during evaporation such as partial pressure of the species and coexistent reactive chemical compounds can also affect αsolid and the main chemical process.61 Thus, further experimental studies are necessary to clarify the evaporated Fe species.


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The representative δ56Fe value of combustion Fe for the evaluation of its contribution to marine aerosols. Evaporated Fe can yield as low as −4.7‰ as was suggested from the calculations based on the correlation line. In our previous study,20 fine aerosol particles (< 0.7 μm) emitted from vehicles in a tunnel yielded δ56Fe values as low as −3.2‰ (Figure 6). The soluble fraction of fly ash collected in a waste incinerator gave a δ56Fe of −1.9‰. In addition, such low values were also observed in fine aerosols collected in Higashi-Hiroshima, as an example of Fe in the suburban environment in Japan.18 Furthermore, the soluble fraction of Fe in the fine aerosol particles in Higashi-Hiroshima, which possibly contains a larger fraction of evaporated Fe than total Fe does, yielded much lower δ56Fe values down to −3.9‰. We found that such low δ56Fe values were obtained for particles emitted from various combustion sources. However, δ56Fe values of evaporated Fe are possibly lower than these measured values, because these fine particles also contain some amount of natural Fe. Thus, we here tentatively propose −3.9‰ (the lowest δ56Fe value of soluble Fe observed in a suburban environment) to −4.7‰ (the calculated δ56Fe value of evaporated Fe from the steel plant in this study) as the representative δ56Fe range for combustion Fe emitted by evaporation from various sources. The δ56Fe range (−3.9‰ to −4.7‰) for the combustion Fe estimated in this study can be used to determine the contribution of combustion Fe to marine aerosols coupled with the δ56Fe value assumed for natural Fe (= 0.0‰)68. As an example, we estimated the contribution of combustion Fe to marine aerosols collected in the northwest Pacific reported by Kurisu et al.18 (KH-14-3 sample 1, Figure 5). We adopted mass balance equations for coarse (> 2.5 μm) and fine (< 2.5 μm) particles as follows:


23


δ56Fefine×[Fefine] = δ56Fecombustion×[Fecombustion-fine] + δ56Fenatural×[Fenatural-fine]

Page 24 of 37

(7)

δ56Fecoarse×[Fecoarse] = δ56Fecombustion×[Fecombustion-coarse] + δ56Fenatural×[Fenatural-coarse] (8) where [Fe] is atmospheric Fe concentration. The ratio of combustion Fe to the total Fe was calculated as follows: [Fecombustion]/[Fetotal] (%) = 100×([Fecombustion-fine] + [Fecombustion-coarse])/[Fetotal] (9) The [Fecombustion]/[Fetotal] ratio was 10–22%. Furthermore, we took into account the fractional solubility. We assumed fractional solubility of natural Fe as 1%, on the basis of typical solubility observed in marine aerosols originated from dust,29,56,69 while combustion Fe as 10–20% on the basis of the highest fractional solubility of Fe in the fine fraction (extracted by ultrapure water) and observational data of marine aerosols.69,70 The ratio of combustion soluble Fe to the total soluble Fe was 57–83%. These results suggest that combustion Fe is one of the more important sources of soluble Fe supplied to the marine environment. This study clearly shows that the δ56Fe value of combustion Fe emitted by evaporation is much lower than that of Fe from most other sources due to the large isotope fractionation during evaporation. The unique δ56Fe value for combustion Fe gives us important implications on source apportionment of Fe in aerosols and on the Fe cycle in the surface ocean.


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1

> 2.5 µm 0.7 - 2.5 µm < 0.7 µm < 0.7 µm, soluble

0 -1

δ56Fe(‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(< 2.5 µm)

-2 -3 -4 -5

Steel plant Tunnel Higashi- KH-14-3 Fly ash (This study) (incinerator) Hiroshima Sample 1

Figure 6. Iron isotope ratios of various aerosols collected near steel plant (this study), in a tunnel19, at an incinerator19, in Higashi-Hiroshima18, and during R/V Hakuho-maru KH-143 cruise18. The values of incinerator samples are total fly ash (circle) and soluble fly ash (open square). The fly ash was extracted by 1 mol/L HCl to measure soluble fraction possibly containing evaporated fraction.

ASSOCIATED CONTENT Supporting Information. Details of XAFS, and Fe isotope analyses and temporal/spatial variation of Fe isotope ratio; Sample information collected at KSH site (Table S1), analytical data (blanks, detection limits, and recovery of a reference material) (Table S2), numerical data for Fe isotope ratios (Table S3), maps of the sampling sites and wind directions (Figure S1), EXAFS and 1st derivative of XANES spectra and fitting results (Figures S2, S3), results of μXRF-XAFS analysis for coarse particles (Figure S4), SEM images and EDS spectra (Figure S5), results of μ-XRF-XAFS analysis for fine particles (Figure S6), temporal variation of Fe isotope


25


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ratios of stage 6 (Figure S7), fractional solubility of Fe (Figure S8), and the relationship between fractionation factor and fraction in solid phase (Figure S9).

AUTHOR INFORMATION Corresponding Author Minako Kurisu ([email protected]), Yoshio Takahashi ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This study was supported by the Grant-in-Aid for JSPS Research Fellow Grant Number 17J06716, Grant-in-Aid for Scientific Research (A) Grant Number 18H04134, and Grant-in-Aid for Challenging Exploratory Research Grant Number 16K13911.

ACKNOWLEDGMENT We appreciate staffs in Chiba City Office and Kemigawa Seminar House for cooperation for the sampling. This study was supported by JSPS KAKENHI Grant Numbers 18H04134, 17J06716, and 16K13911. XAFS analysis was conducted with approval of KEK-PF (2016G632) and SPring8 (Proposal Nos. 2016B172 and 2015A0118).


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Stable Isotope Ratios of Combustion Iron Produced by Evaporation in

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