the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jul 2025
Factors controlling the concentration of water-soluble pyrogenic carbon in aerosols in Hokkaido, Japan
Abstract. Pyrogenic carbon (PyC) is a continuum of compounds generated during the incomplete combustion of biomass or fossil fuels. Water-soluble PyC (WSPyC), a PyC smaller than 0.7 µm, is transported to the ocean via rivers or atmospheric deposition and is considered a key component in the long-term regulation of the global carbon cycle. Compared to the riverine input flux, the atmospheric deposition flux of WSPyC may have large uncertainties due to limited observations. This study examines the factors influencing WSPyC concentration in atmospheric aerosols in Sapporo, Hokkaido, based on a year-round observation. The WSPyC concentration in the aerosols, determined with the benzene polycarboxylic acid method, ranged from 1.41 to 46.5 ngC m−3, with an average value of 13.7±10.6 ngC m−3. The average concentration was lower than previously observed near combustion sources. Positive Matrix Factorization (PMF) analysis revealed that 60 % of the WSPyC concentration was attributed to a K+-dominated combustion, which included the burning of biomass and biofuels. Meanwhile, 40 % of WSPyC was associated with factors related to the atmospheric aging of aerosols derived from fossil fuel combustion. The global WSPyC deposition flux to the ocean has previously been estimated based on the concentration ratio of WSPyC to elemental carbon (EC) or water-soluble organic carbon (WSOC). The results of this study suggested that these ratios vary due to the atmospheric aging processes, including photochemical production of WSPyC from soot, indicating the need for a reassessment of the global WSPyC deposition flux.
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RC1: 'Comment on egusphere-2025-2525', Anonymous Referee #1, 29 Jul 2025
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General Comments
This manuscript presents a comprehensive year-long observational dataset of water-soluble pyrogenic carbon (WSPyC) in atmospheric aerosols over Sapporo, Japan. The authors combine detailed chemical analyses with source apportionment (via PMF), seasonal air mass trajectory analyses, and statistical relationships with other aerosol properties to investigate the variability and sources of WSPyC. Given the scarcity of observational data on atmospheric WSPyC, particularly outside regions near primary combustion sources, this work contributes valuable information to an underrepresented area of the global carbon cycle.
The authors also engage with broader questions related to global carbon flux estimates, such as the reliability of using fixed WSPyC:EC or WSPyC:WSOC ratios across locations. This is a useful contribution, but certain key aspects of context, clarity, and presentation need refinement to fully support the manuscript’s conclusions.
Some of the methods and reasoning steps are underexplained—for example, the motivation for selecting Sapporo as a study location, how local sources were accounted for, or why specific factor names were used in the PMF. Similarly, while the figures and analyses are generally strong, a few inconsistencies and assumptions (e.g., in how seasonal backtrajectories were presented or in the interpretation of ratios) reduce the interpretability of the results. Addressing these points would enhance both transparency and the broader relevance of the findings.
I recommend publication after minor revisions, which are detailed below. These revisions primarily involve clarifications in text, improved figure presentation, and deeper discussion of certain assumptions or methodological choices.
Specific Comments
- The introduction would benefit from a clearer articulation of the broader significance of atmospheric WSPyC measurements. While the manuscript highlights the oceanic budget imbalance, it is not entirely clear why Sapporo was chosen as the observational site. A short paragraph explaining the strategic relevance of this location—especially in terms of long-range transport and seasonal variability—would help contextualize the study.
- Line 46: the authors should format the “−1” as superscript in “0.83 Tg yr⁻¹”.
- Line 69: The sentence would be clearer if reworded to avoid repeating “previous” and “previously.”
- Lines 71-75: This paragraph does not have any mention to the PMF technique being applied to interpret the results. It would also be good to mention the use of backtrajectories to interpret the sources of the measured particles. The way it is currently written “concentrations were measured … to investigate…” is a bit incomplete, because the measurements alone do not provide the desired insights.
- Line 82: The term “Asian dust” appears here without any explanation. Many readers may not be familiar with this concept. Please include a brief definition with an appropriate reference
- Lines 83-86: It is unclear whether this sample-handling procedure applied to just the Asian dust event or to all filters. Please clarify.
- Figure 1: Both sampling locations appear to be within a densely urbanized area. While the paper focuses on regional transport, local sources—such as road traffic or emissions from nearby buildings—likely influence the measurements. Please indicate how high the rooftop sampler was located and describe any steps taken to minimize or account for local contamination.
- Line 131: The sentence could be restructured for clarity. The current phrasing is difficult to follow.
- Line 169: does this sentence mean that Abs365 was used to determine light-absorbing organic aerosols (brown carbon)?
- Line 200: The paragraph begins straight with a description of what the figures are showing, before presenting any results. Readability and clarity would be improved if the authors would first of all highlight the main results, and only after that indicating where they are presented.
- Lines 202-204: The statement that the highest WSPyC concentration was observed in autumn seems inconsistent with the seasonal means, which are higher in spring and winter. If this refers to the absolute maximum (peak), please specify this explicitly to avoid confusion.
- Line 208: The phrase “relatively but significantly” could be simplified for clarity. Since the statistical significance is shown, you might remove “significantly” altogether.
- Line 220: As in comment 10, consider reorganizing this paragraph to present the key results before describing the figures.
- Line 221-222: If both ratios increase under the influence of biomass burning, how do they indicate the WSPyC sources, as stated in the previous sentence? I recommend the authors to clarify it to the reader less familiar with this concept.
- Lines 225-227: “ratio in winter” is repeated, rephrasing these sentences will improve readability.
- Figures 2 and 3: Please indicate in the legends which season the dust sample was removed from. This improves transparency and interpretability.
- Figure 6: according to Table S1, the filter sampling occurred continuously. However, Figure 6 seems to indicate only one day of sample collection per season, and the multiple colored lines seem to be all in reference to one day only per season. I recommend the authors to clarify this choice, and ideally work with average backtrajectories based on multiple runs of Hysplit, covering ideally all period for each season. One representative day per season can lead to misleading conclusions.
- Figures 5 and 6: If the authors successfully present mean seasonal backtrajectories, this is a much more important scientific information, rather than the local wind direction presented in Figure 5, if authors want to discuss regional transport of particles rather than local emission sources. In this case, Figure 5 is not needed in the main text, since it does not help to explain sources of particles, if they are not influenced by local sources. This type of difference is clearly seen during the spring, when backtrajectories show multiple lines coming from southwest, which is not observed in the local wind direction. During the autumn it is also possible to see how some backtrajectories arriving at the site from southeast actually come from the continent, rather from far away in the ocean.
- Lines 294-302: This paragraph appears to use different line spacing than the rest of the manuscript. Please standardize formatting.
- Lines 335-337: is the conclusion from these sentences that the WSPyC/EC ratio cannot be used to determine if the source of WSPyC is either fossil fuel or biomass burning?
- Line 345: maybe “analyses” instead of “analysis”?
- Lines 353-354: Biomass burning as a source has been discussed earlier. Consider rephrasing to clarify that here it is being identified as a major contributor in a specific season.
- Line 365: The phrase “in spring” is repeated. Rewriting for flow would help.
- Lines 371-373: the authors should rephrase these two sentences. The first one is simply saying what another study aimed to do, without presenting any result or discussing it into the current study context. The second sentence starts with “they also”, but the word “also” would only make sense coming after some result presented before, which was not the case here. If “also” is related to a comparison to the results presented in this paper (meaning that it agrees with the results presented by the authors), it should be clearer.
- Line 376: the authors could indicate what should be the most influential air masses, rather than just indicating which is not.
- Lines 387-388: the authors should refer to my comment 17. I suggest to either discuss this topic in depth here, or remove wind direction from the paper.
- Line 399: the authors should revise this sentence. It does not make much sense that “wind data and backtrajectories” would influence air masses.
- Lines 422-424: the authors should indicate why combustion-derived factors are split into 3 factors. Why do combustion-derived factors should be separated by a dominance of potassium, nitrate and sulfate? What does it tell about the different combustion sources/processing? It should be clearly and briefly stated here, leaving the more detailed explanation for the next paragraph.
- Lines 427-440: to improve clarity, the authors could rephrase and shorten these sentences.
- Line 449: maybe by “greater contribution of fossil fuel-derived”, the authors meant something like “greater contribution of processed fossil fuel emissions”? The way it is currently written can lead to some misunderstanding.
- Figure 10: the names of the factors are interesting, and would probably be good to be defined in the paragraph between lines 412-419. Otherwise, the only place in which they appear is this figure. Regarding factors 4, 5 and 6 – maybe a name explicitly citing what they represent (combustion from biomass, fossil fuel, primary, processed…) rather than their dominant chemical signature would be clearer and improve the understanding of the results.
- Line 461: the authors could revise the title of this section. The way it is, it seems like a second part of an interrupted sentence, or an objective.
- Line 471: the word “spring” is repeated, the authors could rephrase this sentence in order for it to flow better.
- Lines 480-481: maybe this is because of the addition of some portion of ethanol in many gasoline fuels?
- Lines 492-500/527-529: the authors could highlight here how their study contributed to decreasing the uncertainty about the WSPyC deposition fluxes to the ocean, and make it clear which specific contributions to the knowledge of this field this study brings.
- Figure S3: the author could double-check the legend, as the description of the subplots (a, b, c,…) does not match what is written in each subplot.
Citation: https://doi.org/10.5194/egusphere-2025-2525-RC1
Data sets
Data for Factors controlling the concentration of water-soluble pyrogenic carbon in aerosols in Hokkaido, Japan Riku Miyase https://doi.org/10.5281/zenodo.15510865
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