the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Aug 2025
Residential burning is a significant source of soluble iron to the ocean
Abstract. Understanding the physicochemical processes that supply atmospheric aerosol iron (Fe) to the ocean is crucial in our understanding of global biogeochemical cycles. Anthropogenic emissions contribute significant fluxes of aerosol Fe to the atmosphere, the soluble fraction of which can modulate marine primary productivity upon its deposition to the ocean surface. However, aerosol Fe solubility remains poorly constrained, due in part to a lack of direct measurements spanning a multitude of anthropogenic sources. We measured solubility of aerosol Fe from several distinct anthropogenic combustion processes and fuel types. The median Fe solubility varied widely by source, ranging from 0.03 % for power plant coal fly ash to 55.87 % for biofuel burning; furthermore, residential coal burning aerosol possessed much higher Fe solubility than industrial coal fly ash. Using new Fe solubilities reported herein, we updated parameters for anthropogenic aerosol Fe within the Mechanism of Intermediate complexity for Modeling Iron, an aerosol Fe module of the Community Earth System Model v2. Such updates led to significant improvement in model performance over ocean regions heavily influenced by anthropogenic emissions, and we identified residential burning as a significant source of soluble aerosol Fe to the ocean. Our work underscores the need to further refine understanding of physicochemical properties of aerosol Fe from a wide variety of anthropogenic sources. In turn, this understanding will aid in characterizing the influences of anthropogenic activities on past, present, and future atmospheric nutrient inputs to marine ecosystems.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(1434 KB) - Metadata XML
-
Supplement
(1579 KB) - BibTeX
- EndNote
Status: open (until 20 Nov 2025)
- RC1: 'Comment on egusphere-2025-4058', Akinori Ito, 16 Oct 2025 reply
-
RC2: 'Comment on egusphere-2025-4058', Anonymous Referee #2, 01 Nov 2025
reply
This manuscript presents new measurements of iron (Fe) content and solubility across a range of anthropogenic fuel sources, with a particular focus on residential combustion (coal and biofuel). These observations are subsequently integrated into the MIMIâCESM2 model framework to assess impacts on soluble Fe deposition to the ocean. The study addresses a critical knowledge gap in quantifying anthropogenic aerosol Fe emissions and their influence on ocean biogeochemistry, especially in high-nutrient, low-chlorophyll (HNLC) regions. The experimental dataset is extensive and covers key combustion sources in China, while the modeling analysis provides valuable constraints on the climatological and future patterns of soluble Fe deposition. The finding that residential combustion disproportionately contributes to soluble Fe fluxes to the ocean is novel and has significant implications for anthropogenic Fe cycling and marine productivity. The manuscript is generally well-written, well-structured, and scientifically solid. However, several points require clarification and enhanced discussion before the manuscript can be recommended for publication.
Major comments
- Line 136â137: Please provide the detection limit for soluble Fe. In addition, briefly describe the QA/QC procedures for the measurement method to ensure data reliability.
- Line 448â490: While the manuscript highlights the importance of acid processing in promoting Fe dissolution, the evidence supporting atmospheric processing-induced release of soluble Fe from power-plant coal fly ash is not clearly demonstrated. Please provide more explicit evidence or discussion.
- Line 556â559: The statement âThis is because Fe in emitted particles is mainly highly soluble Fe sulfates for low combustionâ requires clarification. What is meant by âlow combustionâ? Could the authors elaborate on the formation mechanism and emission source of highly soluble Fe sulfates in this context?
- Line 590â594: Figure 3b shows that the largest relative increases in soluble Fe deposition occur primarily over equatorial/tropical regions such as the Congo Basin and Amazon rainforest. Does this imply that enhanced soluble Fe deposition over the South Atlantic originates largely from long-range transport of soluble Fe associated with South American residential biofuel combustion? Please clarify this interpretation.
- Line 638â645: Figure 4 indicates a systematic model underestimation of total Fe (A1) and a systematic overestimation of soluble Fe (B1âB4), leading to an overestimate of aerosol Fe solubility. Please explain the potential causes of these biases and implications for the modeled Fe dissolution scheme.
- The comparison with ship-based observations is useful; however, as noted by the authors, some ship measurements are outside biofuel-influenced regions. Such data should not be included for model validation in this specific context, as they do not reflect the emission regime of interest.
Minor comments
- Please report the effective number of valid filter samples for each biofuel type in the main text.
- Line 74â75: grammatical issue with âto and from?â â please revise.
- Line 97: clarify whether âaerosol solubilityâ refers specifically to Fe solubility.
- Line 114: revise âin orderâŚ...â to âin order toâŚâŚâ.
- Several minor grammatical issues exist; a careful proofreading is recommended.
Citation: https://doi.org/10.5194/egusphere-2025-4058-RC2 -
RC3: 'Comment on egusphere-2025-4058', Anonymous Referee #3, 02 Nov 2025
reply
Â
This manuscript presents an important and timely topic regarding the role of anthropogenic combustion sources, particularly residential burning, in supplying soluble iron (Fe) to the ocean. The study combines direct laboratory measurements of Fe solubility across a wide range of anthropogenic fuels with the MIMI model. It provides a thorough investigation into the relatively contributions from anthropogenic activities for the present-day and pre-industrial era. The manuscript is generally well written, well referenced, and of interest to the atmospheric chemistry, ocean biogeochemistry, and Earth system modeling communities. Therefore, I would recommend it to be accepted with minor revisions, if the following comments can be properly addressed.
Â
Â
Major Comments
Â
A major innovative contribution of this study is quantifying the contributions of Fe from multiple main sources with newly measured emission factors and modeling simulations. The authors mention ranges of solubility and provide multiple simulations. While there are several processes that may retain large uncertainties with the modeling effort (e.g., Fe-to-BC ratio variability, emission inventory biases, solubility parameter assumptions), the propagation of uncertainties is not fully quantified. A sensitivity analysis is recommended to improve the reliability of the conclusions. For example, how much do uncertainties in residential coal solubility affect global estimates? A sensitivity analysis may help provide more robust policy-relevant implications for the conclusions.
Â
Minor Comments:
(1) In the abstract it would be better to briefly describe the relative importance of residential burning compared to other anthropogenic sources (e.g., percentage contribution to total soluble Fe flux).
Â
(2) line#83: âA study ⌠is neededâ or âstudies ⌠are neededâ.
Â
(3) line#473 âwas found to rangeâ should be ârangedâ
Â
Citation: https://doi.org/10.5194/egusphere-2025-4058-RC3
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 1,738 | 59 | 17 | 1,814 | 36 | 25 | 22 |
- HTML: 1,738
- PDF: 59
- XML: 17
- Total: 1,814
- Supplement: 36
- BibTeX: 25
- EndNote: 22
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
General comments
There are still large uncertainties in source attribution of soluble iron (Fe) in atmospheric aerosols from different anthropogenic sources, despite substantial advancement in the source attribution between natural and anthropogenic sources. Due to low iron content in residential burning aerosols, residential burning is expected to be negligible source of Fe to the ocean. This study argues that residential burning is a significant source of soluble Fe to the ocean based on the measurements of high Fe solubility at emission for residential coal burning aerosol and residential biofuel burning than industrial coal fly ash. This hypothesis is based on the assumption on a representation of iron mineralogy for anthropogenic combustion sources, which suggests that the emissions of soluble iron from residential coal and wood combustion could be an important contributor to soluble iron production (Rathod et al. 2020). The comprehensive measurements and their application to the model simulations may help us to advance modeling anthropogenic soluble iron. However, my major concern is the inconsistency between Fe content used in the model and Fe content measured from this work with the high Fe solubility at emission measured in this work. I have some comments and questions to improve this paper.
Â
Major comments
Â
Specific comments
l.116: Please describe the information about the size of ash samples.
Â
l.121 and Table S5: Please specify the sources of oil fly ashes, because Fe content is substantially different between the two samples. Why is this?
Â
l.129: You described that the acetate buffer was used to simulate cloud water (Li et al., 2022b). Here, you applied Fe solubility from the acetate buffer to that at emission before the cloud processing. Why did you choose the sodium acetate buffer over the deionized water? Please show the comparison of Fe solubility near the source regions, for example, at Guangzhou.
Â
l.198: You updated Fe solubility at emission but did not Fe content. I suggest additional simulations with both updated Fe content and solubility at emission (see below more details).
Â
l.214: The anthropogenic coal Fe sources from residential sector can be separately estimated from industrial sector, as is described on l.383. Do you mean that you segregated 1) industrial fossil fuel (coal) in the dataset provided by Rathod et al. (2020) into industrial and residential coal burning sources? Please rephrase the sentence.
Â
l.216: How did you assume that the Fe-to-BC ratios were matched between sources? Iron emissions from residential wood and coal combustion is only about 2% (Table S6 in Rathod et al. 2020) of the global fine Fe emissions due to their low-Fe emission factors. Thus, the industrial-to-residential BC is higher than industrial-to-residential Fe (see Table 3 in Ito et al. 2021), as you described for carbon content on l.386 and iron content in Figure 2. If you used the same Fe-to-BC ratios between the two sources, you assigned larger Fe emissions (in other words, higher Fe content than Rathod et al., 2020) into residential sector than industrial sector by more than a factor of 10. It is straightforward to calculate the Fe-to-BC ratios for residential sector and industrial sector because the anthropogenic coal Fe sources from residential sector is separately estimated from industrial sector (Rathod et al., 2020). Please show the global emissions for Fe and BC from coal combustion for each sector and final Fe content at emission.
Â
l.320: Please indicate the references for the additional measurements.
Â
l.383: Table S2 shows negative relationship between Fe content and Fe solubility, possibly because Fe species inside the particles might not be transformed to labile form in combustion process. If you used higher Fe content (> 0.5 mg/g) for aerosols, you would apply larger Fe emissions (> 13 times) to residential coal combustion aerosols which consist of the median Fe content (0.038 mg/g) measured at higher Fe solubility. Why donât you update Fe content, too?
Â
l.347, Table 2: Please indicate the size information and elucidate the differences between ash and aerosol.
Â
l.430: If you used higher Fe content (0.58 mg/g) for aerosols, you would apply larger Fe emissions (45 times) to residential biofuel combustion aerosols which consist of the median Fe content (0.013 mg/g) measured at higher Fe solubility. Why donât you update Fe content, too?
Â
l.568 and Table 3: Why did you use higher Fe solubility of oil bottom ash (25%) than the measurements of Fe solubility in Table 2 for oil fly ash (12.56%)?
Â
l.658: Please report the statistics for soluble Fe over Southeastern Asia, the Bay of Bengal, the North Pacific, and the North Atlantic to support the improvement.
Â
l.678: Please report the statistics for Fe to support the improvement of dust Fe.
Â
l.685: Please report the statistics for Fe solubility.
Â
l.768: Since you increased soluble Fe from anthropogenic sources, you would estimate lighter Fe isotopes than your previous estimates. How can this be reconciled with the aerosol Fe isotope measurements? If higher Fe solubility is compensated by lower Fe content in the additional simulations, your Fe isotope might be consistent. Please show the comparison of Fe isotopes.
Â
Technical comments
l.57: What is Fe availability? Do you mean bioavailability?
Â
Fig. 5: Please rename PD-OIL.