the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jul 2026
Size-resolved characterizations of Fe in aerosols in East Asian outflow in winter and spring: Source apportionment and bioaccessibility
Abstract. Trace metals in aerosol particles affect the Earth's radiative budget, human health, and ocean biogeochemistry. Semi-continuous measurements of the elemental composition of fine-mode (PM2.5) and total (PM10) aerosols and high-volume air sampling (5-d)/offline chemical analyses were conducted on a remote island of Japan in winter–spring 2023–2024 to characterize the source apportionment and water-soluble (i.e., bioaccessible) concentrations/fractions of trace elements such as Fe in the East Asian outflow region. Temporal variations in PM2.5 and PM10 Fe concentrations were classified into dust and non-dust contributions based on multilinear regression using concentrations of tracer species. Size-segregated water-soluble Fe fraction (fFe,sol) was investigated using dust Fe contributions and aging processes (e.g., transport time). Source apportionment (dust vs. non-dust) was essential to account for fFe,sol variations for PM2.5 aerosols, whereas those for coarse-mode aerosols were largely affected by aging processes. Temporal variations in total water-soluble Fe (Fesol) concentrations were strongly correlated with black carbon (BC) concentrations, indicating the role of continental combustion sources in enhancing Fesol concentrations in the outflow regions. The enhancement ratios of Fesol to BC concentrations were 53.8 (±11) ng µg−1 on average during the observation period and increased from ~40 ng µg−1 in winter to ~90 ng µg−1 in spring. This may reflect increased dust contributions to Fesol and reduced BC emissions from residential sector, as indicated by model analyses. These offer useful constraints for developing, validating, and refining numerical models of aerosol Fe behavior in the East Asian outflow.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Status: open (until 24 Aug 2026)
- RC1: 'Comment on egusphere-2026-3778', Anonymous Referee #1, 08 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3778', Anonymous Referee #2, 16 Aug 2026
reply
This study conducted observations at Fukue Island, located in the East Asian outflow region, from December 2023 to May 2024, covering both winter and spring seasons. The authors collected aerosol samples with different size fractions and analyzed a wide range of chemical components. Overall, this work provides a valuable observational dataset for improving our understanding of aerosol chemistry in the East Asian outflow region and its potential implications for ocean biogeochemistry. However, the current manuscript could be further improved in terms of data analysis and the interpretation of the results. Below, I provide several comments and suggestions that may help strengthen the manuscript.
Major Comments
(1) Line 262: The authors reported an uncertainty of 60% for Si. It would be helpful if the authors could provide further details on how this uncertainty was estimated. In addition, since Si was subsequently used as an important tracer for estimating dust contributions, the authors may consider discussing whether such a relatively large uncertainty could introduce substantial impacts on the derived dust contributions.
Although Al was not measured in this study, the authors may consider evaluating the applicability of alternative dust tracers, such as Ti or non-sea-salt Ca (nss-Ca), for source identification. Furthermore, given that the observations were conducted at an island site, if the relevant data are available, the use of nss-SO42- instead of total SO42- as a tracer for secondary aerosol sources may also be considered.
(2) Section 3.3.1: For the source apportionment of PM2.5 and PM10, the authors only considered BC, sulfate, and Si as source tracers, which may not fully represent the diverse sources contributing to atmospheric particulate matter. The authors have acknowledged this limitation and indicated that the intercept term in the multiple linear regression model represents contributions from other sources, such as sea salt. However, the use of the term “background levels (line 328)” in the current statement may require further clarification, as it could imply that contributions from these additional sources remain relatively constant. In fact, even sea salt emissions can exhibit temporal variability, for example, due to variations in wind speed and meteorological conditions across different seasons.
In addition, the statement that BC represents both anthropogenic sources and a part of secondary sources (line 313) may require further justification. The authors may consider providing a clearer explanation of the rationale for using BC as a tracer for secondary sources. Although BC can undergo atmospheric aging and some source apportionment studies have identified aged BC as a source factor, the carbonaceous core of aged BC is still associated with primary emissions rather than secondary formation.
Therefore, the authors may consider incorporating additional source tracers to achieve a more comprehensive source apportionment of atmospheric particulate matter, if such measurements are available. Alternatively, the authors may consider performing separate multiple linear regression analyses for winter and spring to account for potential seasonal variations in the contributions represented by the intercept term.
(3) Section 3.3.2: Similar to the above comment, the authors may consider conducting separate multiple linear regression analyses for winter and spring to evaluate the source contributions of Fe. This consideration is particularly relevant because previous studies have demonstrated that the regression coefficients between BC and anthropogenic Fe can differ substantially between winter and spring (Zhang et al., Source Apportionment of Atmospheric Iron in Northern China Based on 3-Year Continuous Measurements). Therefore, accounting for seasonal variations in the regression relationships may help improve the accuracy and reliability of the estimated Fe source contributions.
(4) Figure 7: The use of “transport time from the continent” as an indicator of aerosol aging degree should be interpreted with caution. For different air masses, variations in meteorological conditions (e.g., temperature and relative humidity) and chemical compositions can substantially influence atmospheric processing, making transport time alone difficult to serve as a metric for aerosol aging.
Since the authors have quantified secondary source contributions in PM2.5 and PM10, they may consider using these contributions as an alternative indicator of aerosol aging. Besides, the molar ratio of secondary inorganic anions [NO3-+2×nss-SO₄²⁻] to total Fe may also serve as a potential indicator of atmospheric aging.
(5) Lines 658-660, The authors suggested that the increase in ΔFesol/ΔBC from ~40 ng µg-1 in winter to ~90 ng µg-1 in spring was primarily associated with an increased contribution of dust to Fesol, and secondarily with the larger contribution of the residential sector to BC compared with Fesol. It would be helpful if the authors could provide further evidence or quantitative analysis to support the conclusion that the former factor plays a more important role.
In addition, the authors may consider clarifying whether this interpretation represents a general seasonal pattern or is mainly influenced by the strong dust event observed in April 2024. Since dust events may substantially alter soluble Fe concentrations but have a relatively limited impact on BC concentrations, the contribution of this episodic event to the observed increase in ΔFesol/ΔBC should be carefully evaluated.
Minor Comments
(1) The manuscript contains several statements that could benefit from more precise and rigorous wording. For example, in line 65, the authors state that “anthropogenic aerosols often contain less Fe but show substantially higher solubility due to atmospheric processing”. The phrase “due to atmospheric processing” may require further clarification, as it appears to attribute the higher Fe solubility of anthropogenic aerosols primarily to atmospheric processing, while potentially overlooking the fact that Fe from some anthropogenic sources may already exhibit relatively high solubility upon emission.
(2) Line 487: The term “significantly correlated” is used. However, the statistical significance tests are not provided in Table S4. The authors may consider adding the relevant statistical results or revising the wording accordingly.
(3) Lines 495-498: “This can be partly derived from the uncertainties in the dry deposition fluxes of coarse particles in the model (Kok et al., 2021). As the high temporal resolution variations in the source-resolved Fe concentrations were reasonably well simulated, we concluded that the removal processes as well as emissions were also reasonably simulated”. The authors first attribute the discrepancy partly to uncertainties in dry deposition fluxes, but subsequently conclude that the removal processes were reasonably simulated. The authors may consider clarifying how these two statements can be reconciled.
(4) Lines 508-509 “Therefore, the possible reason for the discrepancies between the observations and model simulations (NMB of 21 %) in this study was the type of leaching method used to derive the Fesol concentrations. ” The statement may be too definitive. The authors may consider revising it to indicate that this is one of the possible reasons contributing to the discrepancy.
(5) Lines 555-558 “The model simulations implied that the long-range transport of dust Fe from the continent in East Asia and its deposition in the outflow oceanic regions over the NWPO can control the atmospheric supply of bioaccessible Fe to the ocean surface in the HNLC region and contribute to the significant amount of bioavailable Fe required for sustaining the springtime ocean biological activities in the NWPO.” This statement represents a broad conclusion. However, the current analysis may not provide sufficient evidence to fully support this statement.
(6) Line 655: The authors state that “indicating the significance of continental combustion sources in enhancing Fesol concentrations in the outflow regions.” The authors may consider whether the term “continental” is sufficiently precise in this context, as BC in the outflow regions may also include contributions from ship emissions.
(7) The authors may consider revising several expressions throughout the manuscript to ensure more accurate terminology. For example, the term “total (PM10) aerosols” may need clarification, as PM10 is generally not considered equivalent to the total aerosol fraction. In addition, terms such as “PM2.5 aerosols” and “Si aerosols” may need to be revised for greater terminological accuracy. The authors may consider using “PM2.5 particles” or “the PM2.5 fraction” and “Si-containing particles”.
(8) When describing the agreement between two datasets, the authors frequently use expressions such as “within ~30%”, “< ±30%”, and “within ±30%”. The authors may consider using a consistent expression throughout the manuscript and providing a clear definition of these terms. For example, it is unclear whether “within ±30%” refers to a relative bias (e.g., NMB/NME less than 30%), or whether it indicates that the slope of the linear regression between the two datasets falls within the range of 0.7–1.3.
(9) If lidar depolarization ratio measurements are available, the authors may consider comparing them with the dust contributions derived from PM source apportionment. Such a comparison could provide additional independent evidence for evaluating the reliability of the estimated dust contributions. However, this analysis is not essential and could be considered as a potential improvement if the relevant data are available.
(10) I am interested in the use of particle-laden spots on the filter roll tape from the PX-375 for subsequent IC and ICP-MS analyses. Since the filter is collected on a roll tape, the authors may consider clarifying how potential cross-contamination between particle-loaded spots from different positions on the roll (e.g., inner and outer layers) was avoided or evaluated during sample handling and analysis.
Citation: https://doi.org/10.5194/egusphere-2026-3778-RC2
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This manuscript presents field observations of total and water-soluble Fe in PM2.5 and PM10 aerosols and attempts to distinguish their sources using a multilinear regression approach. The topic is relevant to atmospheric iron cycling and aerosol source apportionment. However, the manuscript currently overstates several conclusions, and its novelty is not clearly demonstrated. In addition, several interpretations require stronger evidence, and comparisons with previous studies are insufficient. Therefore, I recommend revision before the manuscript can be considered for publication.