the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Oct 2025
Distinct Spatiotemporal Patterns of Atmospheric Total and Soluble Iron from Three Sources Revealed by Shipboard Online Observations in the Northwest Pacific
Abstract. Non-dust emissions have been increasingly recognized as important contributors to atmospheric iron (Fe), influencing marine productivity through enhanced bioavailable Fe inputs. However, accurately quantifying the contributions and spatiotemporal variability of non-dust sources remains challenging due to relatively low time-resolution of traditional filter-based analytical methods. In this study, the contributions of non-dust emissions to atmospheric total and soluble Fe in the Northwest Pacific were quantified based on online measurements from three ship-based observation campaigns in 2021–2022. A Positive Matrix Factorization (PMF) model was applied for source apportionment. Results showed non-dust emissions contributed substantially to atmospheric Fe, accounting for 21 %–48 % of total Fe across different regions and seasons. Importantly, their contributions to soluble Fe were significantly higher, reaching 79 %–98 % and largely dominating the bioavailable Fe supply in the study area. Among non-dust sources, land anthropogenic emissions contributed significant portion of both total and soluble Fe, whereas ship emissions contributed small portion to total Fe but was major source of soluble Fe, particularly in coastal regions. In summer, ship emissions over coastal waters even exceeded land anthropogenic sources, becoming the dominant contributor to soluble Fe. Additionally, Fe from non-dust sources exhibited stronger spatial variability than dust source. The concentrations of land anthropogenic Fe differed by 3–5 times between coastal and open-ocean areas during the same cruises, while ship-derived Fe varied by an order of magnitude or more. This study offers critical observational evidence to advance understanding of how diverse emission sources shape atmospheric composition in Asian continental outflow regions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4699', Anonymous Referee #1, 12 Nov 2025
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RC2: 'Comment on egusphere-2025-4699', Anonymous Referee #2, 23 Dec 2025
This manuscript presents a well-conducted study on atmospheric iron sources in the Northwest Pacific using innovative shipboard online measurements. The research addresses an important knowledge gap in understanding non-dust iron contributions to marine ecosystems. The multi-cruise approach provides a compelling dataset to examine spatiotemporal patterns. The key findings that non-dust sources dominate soluble Fe supply, and that ship emissions can be the primary source of soluble Fe in summer—are clearly demonstrated and have important implications for modeling marine primary productivity. However, the revisions suggested below are aimed at further improving the clarity, precision, and impact of the paper to meet the high standards of ACP:
Major:
(1) The most significant issue in this manuscript is that the seasons of two field campaigns collected aerosol samples with identical particle size are different, while two other campaigns conducted in similar seasons collected aerosol samples with different particle sizes. This compromises the scientific validity of comparative analyses between campaigns, yet the manuscript frequently discusses results from different campaigns together without addressing this critical limitation. For example, Section 3.23 does not differentiate between PM2.5 and PM10 when discussing non-dust source contributions to dissolved Fe, which is scientifically inappropriate given the distinct source characteristics of coarse and fine particles. A comprehensive review and revision of the text is required to prevent the indiscriminate mixing of NWP1 and NWP2 results without explicitly acknowledging the differences in particle size.
(2) Similarly, this issue manifests in Figure 11. It is unclear whether the "marginal seas" referred to in Figures 11b and 11d include both the Yellow and Bohai Seas campaigns and the coastal segments of NWP1/NWP2, and whether the data presented encompass both PM2.5 and PM10 results. If this figure combines PM2.5 and PM10 data, such presentation is inappropriate. Additionally, the criteria used to define "spring" and "summer" seasons in this figure require clarification.
(3) In the abstract, the quantitative claims " Results showed non-dust emissions contributed substantially to atmospheric Fe, accounting for 21%–48% of total Fe across different regions and seasons. Importantly, their contributions to soluble Fe were significantly higher, reaching 79%–98% and largely dominating the bioavailable Fe supply in the study area." are not supported by corresponding data in the Results and Discussion Sections. The same issue occurs in the Conclusions section. The source of these numerical values remains unclear, so does their specific association with either PM2.5 or PM10.
(4) PMF method: In Text S4, the statement "The value of Q relative to Qexpected reached its minimum at seven factors, which suggested that the mathematical diagnostics favored a seven-factor solution." requires clarification. What do Q and Qexpected specifically refer to? Are these equivalent to Qtrue and Qrobust mentioned in the preceding sentence? What’s the methodological basis or reference for determining the optimal factor number using the minimum Q/Qexpected value? Additionally, have the authors evaluated whether PMF solutions with 4 or 5 factors might yield higher physical interpretability?
(5) Given that aerosols in different samples experience different aging processes during atmospheric transport (i.e., variable Rj values in equations 3-5 should be different in different samples), this fixed-parameter approach should theoretically produce less accurate results. However, Figure 8a demonstrates the reliability of the fixed iron solubility parameterization scheme combined with PMF results for estimating aerosol iron solubility. Does this mean that atmospheric processing may have minimal modification effects on the initial iron solubility of different source-derived aerosols? Please provide further detailed explanation of this issue in the article.
(6) Additionally, numerous studies have used PMF-based methods to estimate Fe solubility in aerosols from different sources. While the solubility estimated by these methods shows similar trends to that of source-specific samples collected directly from emission sources, there are significant discrepancies in absolute values. A key reason for this discrepancy is that the sources identified by PMF are not pure but may instead represent mixtures of multiple pollution sources, whose individual components can exhibit drastically different solubility. For example, fly ash typically exhibits very low solubility, whereas some industrial sources may have much higher solubility; yet these distinct sources might be lumped into a single source factor in PMF analysis. Another case in point is ship emissions: in many studies, PMF-estimated solubility for ship emissions is far below 70%. Given these issues, it is recommended that the authors include a discussion on the limitations and uncertainties of this method.
(7) During the Yellow and Bohai Seas campaign, both Xact 625 measurements and filter sample analyses (ICP-MS) were employed for elements detection. How consistent are the results obtained from these two analytical methods?
(8) The authors have not contextualized their findings alongside recent studies on Fe sources in Chinese coastal regions or other marine environments. To strengthen the scientific significance of this research and more clearly articulate its contributions, it is strongly recommended that the authors undertake a comparative analysis engaging with both recent and prior literature.
Minor:
(1) In the abstract, the phrase " whereas ship emissions contributed small portion to total Fe but was major source of soluble Fe" requires the indefinite article "a" before "major" for grammatical correctness. It should read: "...but was a major source of soluble Fe."
(2) Figure 1: The cruise track should be clearly marked with dates, particularly highlighting periods when the instrument was operational versus malfunctioning during the NWP2 leg. If the plotted track represents only the periods when the instrument was functioning normally, this must be explicitly stated either in the figure caption or the main text.
(3) In Figure 3c, the use of V for linear regression is puzzling. Given that dust factor also contributes substantially to V, while Ni is predominantly explained by ship emissions, it would be more appropriate to use Ni for the regression analysis. The authors should clarify their choice of V or consider switching to Ni to better align with the source apportionment results.
(4) Line 33: "Iron (Fe) in marine aerosols have been extensively studied..." should be "has been" (subject "Iron" is singular).
(5) Line 42: "This made non-dust emissions especially important..."→"This makes..." (present tense is more appropriate for a general statement).
(6) Line 75 and Line 526: "Combing" should be "Combining"
(7) Line 164: The phrase "Other than Fe" suggests that Fe is excluded from the analysis, whereas the PMF analysis actually includes Fe. It is recommended to rephrase this to: "In addition to Fe, other elements were primarily used as input variables…" to avoid potential misunderstanding.
(8) Line 179: The term "downwind" appears to be an oversight. To avoid contamination from the vessel’s own emissions, the sampler should be located upwind of the stack. Please verify and correct this wording.
(9) Line 344：“northern Yellow Seas” should be “northern Yellow Sea”
(10) Line 343: The authors note that "Excluding the EP events" indicates Figure 5 results do not include several extreme pollution episodes. This exclusion should be explicitly stated in the figure caption for clarity. Additionally, while the authors cite prior studies to justify the higher dust contribution at low-latitude sites compared to high-latitude ones, this finding remains somewhat counterintuitive. First, dust transport pathways can vary significantly across different years and events. Second, it is generally accepted that northern regions are more susceptible to dust influences. The authors should provide a more thorough discussion to substantiate the rationality of this observation in their study context.
(11) Line 607-621: The difference in Fe concentrations between coastal and open-ocean areas is much more pronounced in NWP2 than in NWP1. Could this be related to the difference in aerosol size fractions sampled between the two campaigns? Please discuss this potential factor.
Citation: https://doi.org/10.5194/egusphere-2025-4699-RC2
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The authors did a very good job of writing this manuscript. It is appropriate in scope for ACP and I enjoyed reading the paper.