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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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2525', Anonymous Referee #1, 29 Jul 2025
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AC1: 'Reply on RC1', Riku Miyase, 28 Nov 2025
Line numbers shown in the responses are those in the revised manuscript.
Referee#1:
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.
First of all, thank you very much for reviewing our manuscript and providing constructive comments. We have revised the manuscript according to the referee’s comments. We believe that the manuscript has been improved significantly after the revisions.
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.
Sapporo was chosen as a sampling location to observe aerosols in long-range transported air masses before their deposition onto the ocean, because local anthropogenic emissions are limited. In particular, aerosols observed in Sapporo are frequently influenced by the Eurasian continent in winter and spring. According to the comment, we have added a description on these as follows: The tertiary industry is dominant in Sapporo, and the number of manufacturers is lower than the national average, suggesting that industrial emissions are relatively limited (Pavuluri et al., 2013). Because of the influence of the East Asian monsoon, the origin of aerosols in this region varies greatly with the season, making it an ideal location for observing air masses transported over long distances from China, Siberia, and the surrounding seas (Yamamoto et al., 2021; Pavuluri et al., 2013; 2015). (Lines 73-77).
- Line 46: the authors should format the “−1” as superscript in “0.83 Tg yr⁻¹”.
We have revised it (Line 47).
- Line 69: The sentence would be clearer if reworded to avoid repeating “previous” and “previously.”
The first appearance of “previously” was deleted to avoid redundancy. (Line 70).
- 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.
According to the comment, the end of the Introduction (Lines 78-79) has been revised as follows: “Positive matrix factorization (PMF) and backward trajectory analyses were conducted to investigate the sources of WSPyC observed in Sapporo.”
- 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
Asian dust is a meteorological phenomenon that includes soil particles originating from the deserts of the Eurasian continent. The dust is frequently transported over the East China Sea, the Yellow Sea, and the Japan Sea during spring. We have added more detailed descriptions about Asian dust as follows: The samples include an Asian dust sample. Asian dust, consisting of soil particles originating from the deserts of Asia, is frequently transported across the East China Sea and the Yellow Sea to Japan during spring (Iwasaka et al., 1983). (Lines 83-85).
- Lines 83-86: It is unclear whether this sample-handling procedure applied to just the Asian dust event or to all filters. Please clarify.
The sampling procedure was identical for both the regular samples and the Asian dust sample, whereas the sampling duration was different between them. We have reorganized the sentences to clarify it as follows: The aerosol sample was collected on the filter at a flow rate of 1000 L min−1 with a sampling duration of approximately 7 days for standard samples and 31 hours (from 9:00 LT on April 12, 2023, to 16:00 LT on April 13, 2023) for the Asian dust sample (Table S1). (Lines 87-89).
- 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.
The aerosol sampler was located on a rooftop of a building (30 m above ground level) in the extensive university campus, and the height of the building is relatively higher than other buildings in the campus. Therefore, the effect of local sources is likely minimal. We have added a description on these “30 m above ground level” in the revised manuscript (Line 83).
- Line 131: The sentence could be restructured for clarity. The current phrasing is difficult to follow.
We have modified the sentence as follows: “The HPLC analytical settings followed those of Dittmar (2008) and Nakane et al. (2017)” (Line 135).
- Line 169: does this sentence mean that Abs365was used to determine light-absorbing organic aerosols (brown carbon)?
Yes, it does. We have modified the sentence to clarify it. The revised sentence is “In this study, a wavelength of 365 nm, which has minimal absorption by inorganic substances in the water extract, was used as a quantitative parameter of light-absorbing organic aerosols (Hecobian et al., 2010).” (Lines 173-174).
- 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.
According to the comment, we have removed the first sentence to make the paragraph more readable and clearer. The first sentence presents the result with the corresponding figure numbers.
- 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.
The term “highest WSPyC concentration” refers to the absolute maximum value. We have replaced “highest” with “peak” according to the comment (Line 209).
- Line 208: The phrase “relatively but significantly” could be simplified for clarity. Since the statistical significance is shown, you might remove “significantly” altogether.
We have deleted “significance” from the description (Line 215).
- Line 220: As in comment 10, consider reorganizing this paragraph to present the key results before describing the figures.
As with comment 10, we have removed the first sentence to make the paragraph more readable and clearer.
- 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.
WSPyC tends to be more readily produced at relatively low temperatures during biomass burning, which resulted in the higher ratios for both parameters. We have modified the sentence to include the reasons why both ratios increase under the influence of biomass burning as follows: WSPyC/WSOC and WSPyC/EC ratios have been reported to increase under the influence of biomass burning, because biomass burning occurs at relatively low temperatures, it tends to promote the formation of less-condensed aromatic structures, such as WSPyC, compared to EC (Geng et al., 2021). (Lines 227-229).
- Lines 225-227: “ratio in winter” is repeated, rephrasing these sentences will improve readability.
We have removed “ratio in winter” from the sentence (Lines 233).
- Figures 2 and 3: Please indicate in the legends which season the dust sample was removed from. This improves transparency and interpretability.
The Asian dust sample was collected in the spring. We have added “from the spring data” in the revised captions of Figures 2 and 3 (Lines 243 and 246).
- 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 back trajectories based on multiple runs of Hysplit, covering ideally all period for each season. One representative day per season can lead to misleading conclusions.
We agree with the comment. We have made clustered trajectories for each season in the original Figure 6 (revised Figure 5). This revised figure shows more objective results and supports our conclusions. Following this revision, we have also revised the method regarding the back trajectory as follows: The meteorological data were obtained from the GDAS 1° archive provided by the Air Resources Laboratory (ARL) (www.ready.noaa.gov/archives.php). All the results are shown in Figure S2. To simplify the results of the backward trajectory analysis, the trajectories generated for each season were grouped using the clustering function in the HYSPLIT model and represented by a limited number of mean trajectories (clustered trajectories) calculated by the model. The number of mean trajectories was determined at the point just before the standard change in total spatial variance (TSV) increased sharply. (Lines 201-206).
- 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.
We believe that the figure of local wind direction is necessary to examine whether the local wind direction was consistent with the trajectories on the continental and regional scales. The figures of both local wind directions and trajectories are important to assess the contribution of aerosols of local origin compared to that affected by long-distance transport. To emphasize the figures of trajectories, the original Figure 5 has been moved to the Supplementary Information (revised Figure S6).
- Lines 294-302: This paragraph appears to use different line spacing than the rest of the manuscript. Please standardize formatting.
We have revised it (Lines 293-301).
- 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?
Although the observation has been limited, the WSPyC/EC ratio was successfully applied to identify the combustion sources, namely fossil fuel and biomass burning by Geng et al (2021). We compared the WSPyC/EC ratio observed in Sapporo with those reported by Geng et al (2021). As mentioned in the response to comment#34, fossil fuel combustion-derived aerosols observed by Geng et al (2021) may be influenced by biomass burning. Thus, the WSPyC/EC ratio cannot be used to make a rigid distinction of the origin. We have added this description in the revised manuscript as follows: Geng et al. (2021) reported that some of the observed aerosols in Bachok were derived from fossil fuel combustion, which contained high levels of anhydrosugars, markers of biomass burning. They suggested that fossil fuel combustion-derived aerosols in Bachok may be influenced by biomass burning with a higher WSPyC/EC ratio. (Lines 457-469)
- Line 345: maybe “analyses” instead of “analysis”?
We have revised it (Line 338).
- 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.
We have rephrased the description to indicate that it is one of the major sources of WSPyC in spring as follows: biomass burning is likely one of the major sources of WSPyC in spring in Sapporo (Lines 344-346).
- Line 365: The phrase “in spring” is repeated. Rewriting for flow would help.
We have removed “in Sapporo in spring” from the sentence (Lines 357-359).
- 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.
We intended to compare the results of this study with those of the previous study. Taking into account the comment, we have rephrased the sentence to use “also” appropriately. The revised sentences are “Bao et al. (2017) measured WSPyC concentrations in aerosol samples collected in the open ocean of the western North Pacific and reported that the concentrations were higher in aerosols affected by Asian dust storms, the result of which is similar to that in the current study.” (Lines 363-365).
- Line 376: the authors could indicate what should be the most influential air masses, rather than just indicating which is not.
In summer, back trajectories suggested that the observed air masses were mostly influenced by marine sources in the western North Pacific. We have revised the sentence to read “In summer, backward trajectories indicated the majority of air masses derived from the ocean but not the continent (Fig. 5c).” according to the comment (Line 368).
- 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.
As a response to comment #18, the information on local wind direction is necessary for a better assessment of the source of the aerosols. In Autumn, the primary wind directions were somewhat different between continental and local scales. The dominant local wind direction was southeast, but backward trajectories showed the influence of continental air masses from the northwest. These results imply that aerosols collected in autumn were likely derived from both local and continental origins. Therefore, we have added a description “Such local wind data and air mass origin implied that both local and continental sources likely contributed to autumn aerosols.” in the revised manuscript (Lines 380-381).
- Line 399: the authors should revise this sentence. It does not make much sense that “wind data and backtrajectories” would influence air masses.
We have revised the sentence as follows: In winter, both local wind data and backward trajectories suggested that the air masses were strongly influenced by the continental source (Figs. 5e and S6e) (Lines 392-393).
- 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.
We used PMF analysis to determine the source of the aerosols. The PMF showed WSPyC is related to three combustion-derived sources. Three factors were strongly affected by K+, nitrate, and sulfate, most likely due to the contribution of biomass/biofuels burning, NOx-related combustion sources, and SO2-related combustion sources, respectively. Interestingly, WSPyC was only associated with the three factors, confirming its combustion origin. Using this evidence, we could quantitatively separate the source of WSPyC. This part is a novel point of the present study. We had emphasized the point in the original manuscript as “It should be noted that the observed WSPyC concentrations were controlled by only three factors (i.e., F4, F5, and F6), all attributed to combustion-related factors, confirming the combustion origin of the WSPyC” (Lines 418-419). Furthermore, according to comments #29 and #31 of Referee#1 and comment #5 of Referee#2, we have reorganized the first three paragraphs in the original 4.2 subsection to shorten the paragraphs and make our message more straightforward. Please also see our response to comment #29.
- Lines 427-440: to improve clarity, the authors could rephrase and shorten these sentences.
According to the comment, comment #31 of Referee #1 and comment #5 of Referee #2, we have reorganized the first three paragraphs in the original 4.2 subsection to make it more concise (Line 405-433). The relevant sentences of this comment are as follows: F4 was attributable to WSOC, Abs365, nss-K+, and EC (21–44%). Because nss-K+ is known to originate from biomass and biofuel burning (Chow et al., 2004; Pant and Harrison, 2012), F4 is defined as “potassium-dominated combustion” sources primarily influenced by biomass and biofuel burning. F5 showed the largest contribution of NO3- (62%), followed by NH4+ (46%), and defined as “nitrate-dominated combustion” source. F6 was dominated by nss-SO42-and NH4+ (>40%) with minor contributions of WSOC and Abs365 (15–21%), which is defined as “sulfate-dominated combustion” source. NOx is precursor gas species of NO3-, which is emitted from various anthropogenic combustion activities, such as mobile sources (e.g., traffic and ships) being particularly prominent (Geng et al., 2024; Ni et al., 2024). SO2 is precursor gas species of SO42-, which primarily originates from coal combustion in factories and power plants (Lin et al., 2022). Therefore, F5 and F6 indicate the aerosols associated with combustion sources that had undergone atmospheric ageing processes. (Line 408-417).
- 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.
The sentence has been revised “WSPyC derived from aged fossil fuel combustion aerosols” (Line 429).
- 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.
According to comment #29 and this comment, the sentences have been reorganized. Please see our response to comment #29. In the revised manuscript, the names of each factor have been defined in Lines 405-413. Regarding the names of the factors, we think that it is difficult to assign specific names to combustion sources based solely on the parameters used in this study, given the inherent uncertainty in those parameters. For example, to assign the biomass burning source, levoglucosan is needed to specify the combustion sources. Similarly, nitrate- and sulfate-dominated sources include several types of combustion sources rather than a specific one. Therefore, we decided to keep the names of the factors in the revised manuscript.
- 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.
According to the comment, we have changed the title of this section as follows: Possible factors controlling WSPyC in relation to WSPC and EC (Line 440).
- Line 471: the word “spring” is repeated, the authors could rephrase this sentence in order for it to flow better.
We have revised it (Line 451).
- Lines 480-481: maybe this is because of the addition of some portion of ethanol in many gasoline fuels?
We consider that high levels of anhydrosugars and elevated WSPyC/EC ratios observed simultaneously suggest an influence from biomass burning. The sentence has been restructured to make the point clearer. The revised sentence is “Geng et al. (2021) reported that some of the observed aerosols in Bachok were derived from fossil fuel combustion, which contained high levels of anhydrosugars, markers of biomass burning. They suggested that fossil fuel combustion-derived aerosols in Bachok may be influenced by biomass burning with a higher WSPyC/EC ratio.” (Lines 457-460).
- 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.
Line 492–500 in the original manuscript: We have revised the sentences as follows: The slopes between WSPyC and WSOC, as well as between WSPyC and EC, observed near the combustion sources have been used to estimate the global flux of atmospheric deposition of WSPyC to the ocean (Bao et al. 2017; Geng et al., 2021). The slopes were reported to differ between combustion sources (Zhang, Qiao, et al., 2023). This study also showed that the slopes changed with the addition of biogenic WSOC as well as atmospheric aging processes such as photooxidation of soot and subsequent production of WSPyC. Therefore, the global flux of atmospheric deposition of WSPyC to the ocean is likely to be highly uncertain, as it is based on a limited number of observations near combustion sources. To reduce uncertainty in the global flux of atmospheric WSPyC deposition to the ocean, WSPyC, WSOC, EC, and other aerosol parameters in the open ocean, far from combustion sources, are needed. Furthermore, it is necessary to determine changes in WSPyC, WSOC, and EC concentrations during atmospheric aging. (Lines 470-478).
Line 527–529 in the original manuscript: We have revised the sentences as follows: The former two effects, first noted by this study, are likely to become more pronounced as aerosols are transported over long distances from land to the open ocean. This study indicates that it is essential to experimentally elucidate the solubilization processes of WSPyC from soot in both the atmosphere and the ocean surface, in addition to aerosol observations in open ocean regions, to estimate the global WSPyC deposition flux to the ocean precisely. (Lines 504-508).
- 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.
We have corrected it.
Citation: https://doi.org/10.5194/egusphere-2025-2525-AC1 -
AC3: 'Revised Reply on RC1', Riku Miyase, 28 Nov 2025
In the ‘Reply on RC1’, the comment numbers were not displayed correctly, so I have corrected them.
Line numbers shown in the responses are those in the revised manuscript.
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.
First of all, thank you very much for reviewing our manuscript and providing constructive comments. We have revised the manuscript according to the referee’s comments. We believe that the manuscript has been improved significantly after the revisions.
Specific Comments
1. 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.
Sapporo was chosen as a sampling location to observe aerosols in long-range transported air masses before their deposition onto the ocean, because local anthropogenic emissions are limited. In particular, aerosols observed in Sapporo are frequently influenced by the Eurasian continent in winter and spring. According to the comment, we have added a description on these as follows: The tertiary industry is dominant in Sapporo, and the number of manufacturers is lower than the national average, suggesting that industrial emissions are relatively limited (Pavuluri et al., 2013). Because of the influence of the East Asian monsoon, the origin of aerosols in this region varies greatly with the season, making it an ideal location for observing air masses transported over long distances from China, Siberia, and the surrounding seas (Yamamoto et al., 2021; Pavuluri et al., 2013; 2015). (Lines 73-77).
2. Line 46: the authors should format the “−1” as superscript in “0.83 Tg yr⁻¹”.
We have revised it (Line 47).
3. Line 69: The sentence would be clearer if reworded to avoid repeating “previous” and “previously.”
The first appearance of “previously” was deleted to avoid redundancy. (Line 70).
4. 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.
According to the comment, the end of the Introduction (Lines 78-79) has been revised as follows: “Positive matrix factorization (PMF) and backward trajectory analyses were conducted to investigate the sources of WSPyC observed in Sapporo.”
5. 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
Asian dust is a meteorological phenomenon that includes soil particles originating from the deserts of the Eurasian continent. The dust is frequently transported over the East China Sea, the Yellow Sea, and the Japan Sea during spring. We have added more detailed descriptions about Asian dust as follows: The samples include an Asian dust sample. Asian dust, consisting of soil particles originating from the deserts of Asia, is frequently transported across the East China Sea and the Yellow Sea to Japan during spring (Iwasaka et al., 1983). (Lines 83-85).
6. Lines 83-86: It is unclear whether this sample-handling procedure applied to just the Asian dust event or to all filters. Please clarify.
The sampling procedure was identical for both the regular samples and the Asian dust sample, whereas the sampling duration was different between them. We have reorganized the sentences to clarify it as follows: The aerosol sample was collected on the filter at a flow rate of 1000 L min−1 with a sampling duration of approximately 7 days for standard samples and 31 hours (from 9:00 LT on April 12, 2023, to 16:00 LT on April 13, 2023) for the Asian dust sample (Table S1). (Lines 87-89).
7. 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.
The aerosol sampler was located on a rooftop of a building (30 m above ground level) in the extensive university campus, and the height of the building is relatively higher than other buildings in the campus. Therefore, the effect of local sources is likely minimal. We have added a description on these “30 m above ground level” in the revised manuscript (Line 83).
8. Line 131: The sentence could be restructured for clarity. The current phrasing is difficult to follow.
We have modified the sentence as follows: “The HPLC analytical settings followed those of Dittmar (2008) and Nakane et al. (2017)” (Line 135).
9. Line 169: does this sentence mean that Abs365was used to determine light-absorbing organic aerosols (brown carbon)?
Yes, it does. We have modified the sentence to clarify it. The revised sentence is “In this study, a wavelength of 365 nm, which has minimal absorption by inorganic substances in the water extract, was used as a quantitative parameter of light-absorbing organic aerosols (Hecobian et al., 2010).” (Lines 173-174).
10. 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.
According to the comment, we have removed the first sentence to make the paragraph more readable and clearer. The first sentence presents the result with the corresponding figure numbers.
11. 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.
The term “highest WSPyC concentration” refers to the absolute maximum value. We have replaced “highest” with “peak” according to the comment (Line 209).
12. Line 208: The phrase “relatively but significantly” could be simplified for clarity. Since the statistical significance is shown, you might remove “significantly” altogether.
We have deleted “significance” from the description (Line 215).
13. Line 220: As in comment 10, consider reorganizing this paragraph to present the key results before describing the figures.
As with comment 10, we have removed the first sentence to make the paragraph more readable and clearer.
14. 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.
WSPyC tends to be more readily produced at relatively low temperatures during biomass burning, which resulted in the higher ratios for both parameters. We have modified the sentence to include the reasons why both ratios increase under the influence of biomass burning as follows: WSPyC/WSOC and WSPyC/EC ratios have been reported to increase under the influence of biomass burning, because biomass burning occurs at relatively low temperatures, it tends to promote the formation of less-condensed aromatic structures, such as WSPyC, compared to EC (Geng et al., 2021). (Lines 227-229).
15. Lines 225-227: “ratio in winter” is repeated, rephrasing these sentences will improve readability.
We have removed “ratio in winter” from the sentence (Lines 233).
16. Figures 2 and 3: Please indicate in the legends which season the dust sample was removed from. This improves transparency and interpretability.
The Asian dust sample was collected in the spring. We have added “from the spring data” in the revised captions of Figures 2 and 3 (Lines 243 and 246).
17. 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 back trajectories based on multiple runs of Hysplit, covering ideally all period for each season. One representative day per season can lead to misleading conclusions.
We agree with the comment. We have made clustered trajectories for each season in the original Figure 6 (revised Figure 5). This revised figure shows more objective results and supports our conclusions. Following this revision, we have also revised the method regarding the back trajectory as follows: The meteorological data were obtained from the GDAS 1° archive provided by the Air Resources Laboratory (ARL) (www.ready.noaa.gov/archives.php). All the results are shown in Figure S2. To simplify the results of the backward trajectory analysis, the trajectories generated for each season were grouped using the clustering function in the HYSPLIT model and represented by a limited number of mean trajectories (clustered trajectories) calculated by the model. The number of mean trajectories was determined at the point just before the standard change in total spatial variance (TSV) increased sharply. (Lines 201-206).
18. 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.
We believe that the figure of local wind direction is necessary to examine whether the local wind direction was consistent with the trajectories on the continental and regional scales. The figures of both local wind directions and trajectories are important to assess the contribution of aerosols of local origin compared to that affected by long-distance transport. To emphasize the figures of trajectories, the original Figure 5 has been moved to the Supplementary Information (revised Figure S6).
19. Lines 294-302: This paragraph appears to use different line spacing than the rest of the manuscript. Please standardize formatting.
We have revised it (Lines 293-301).
20. 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?
Although the observation has been limited, the WSPyC/EC ratio was successfully applied to identify the combustion sources, namely fossil fuel and biomass burning by Geng et al (2021). We compared the WSPyC/EC ratio observed in Sapporo with those reported by Geng et al (2021). As mentioned in the response to comment#34, fossil fuel combustion-derived aerosols observed by Geng et al (2021) may be influenced by biomass burning. Thus, the WSPyC/EC ratio cannot be used to make a rigid distinction of the origin. We have added this description in the revised manuscript as follows: Geng et al. (2021) reported that some of the observed aerosols in Bachok were derived from fossil fuel combustion, which contained high levels of anhydrosugars, markers of biomass burning. They suggested that fossil fuel combustion-derived aerosols in Bachok may be influenced by biomass burning with a higher WSPyC/EC ratio. (Lines 457-469)
21. Line 345: maybe “analyses” instead of “analysis”?
We have revised it (Line 338).
22. 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.
We have rephrased the description to indicate that it is one of the major sources of WSPyC in spring as follows: biomass burning is likely one of the major sources of WSPyC in spring in Sapporo (Lines 344-346).
23. Line 365: The phrase “in spring” is repeated. Rewriting for flow would help.
We have removed “in Sapporo in spring” from the sentence (Lines 357-359).
24. 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.
We intended to compare the results of this study with those of the previous study. Taking into account the comment, we have rephrased the sentence to use “also” appropriately. The revised sentences are “Bao et al. (2017) measured WSPyC concentrations in aerosol samples collected in the open ocean of the western North Pacific and reported that the concentrations were higher in aerosols affected by Asian dust storms, the result of which is similar to that in the current study.” (Lines 363-365).
25. Line 376: the authors could indicate what should be the most influential air masses, rather than just indicating which is not.
In summer, back trajectories suggested that the observed air masses were mostly influenced by marine sources in the western North Pacific. We have revised the sentence to read “In summer, backward trajectories indicated the majority of air masses derived from the ocean but not the continent (Fig. 5c).” according to the comment (Line 368).
26. 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.
As a response to comment #18, the information on local wind direction is necessary for a better assessment of the source of the aerosols. In Autumn, the primary wind directions were somewhat different between continental and local scales. The dominant local wind direction was southeast, but backward trajectories showed the influence of continental air masses from the northwest. These results imply that aerosols collected in autumn were likely derived from both local and continental origins. Therefore, we have added a description “Such local wind data and air mass origin implied that both local and continental sources likely contributed to autumn aerosols.” in the revised manuscript (Lines 380-381).
27. Line 399: the authors should revise this sentence. It does not make much sense that “wind data and backtrajectories” would influence air masses.
We have revised the sentence as follows: In winter, both local wind data and backward trajectories suggested that the air masses were strongly influenced by the continental source (Figs. 5e and S6e) (Lines 392-393).
28. 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.
We used PMF analysis to determine the source of the aerosols. The PMF showed WSPyC is related to three combustion-derived sources. Three factors were strongly affected by K+, nitrate, and sulfate, most likely due to the contribution of biomass/biofuels burning, NOx-related combustion sources, and SO2-related combustion sources, respectively. Interestingly, WSPyC was only associated with the three factors, confirming its combustion origin. Using this evidence, we could quantitatively separate the source of WSPyC. This part is a novel point of the present study. We had emphasized the point in the original manuscript as “It should be noted that the observed WSPyC concentrations were controlled by only three factors (i.e., F4, F5, and F6), all attributed to combustion-related factors, confirming the combustion origin of the WSPyC” (Lines 418-419). Furthermore, according to comments #29 and #31 of Referee#1 and comment #5 of Referee#2, we have reorganized the first three paragraphs in the original 4.2 subsection to shorten the paragraphs and make our message more straightforward. Please also see our response to comment #29.
29. Lines 427-440: to improve clarity, the authors could rephrase and shorten these sentences.
According to the comment, comment #31 of Referee #1 and comment #5 of Referee #2, we have reorganized the first three paragraphs in the original 4.2 subsection to make it more concise (Line 405-433). The relevant sentences of this comment are as follows: F4 was attributable to WSOC, Abs365, nss-K+, and EC (21–44%). Because nss-K+ is known to originate from biomass and biofuel burning (Chow et al., 2004; Pant and Harrison, 2012), F4 is defined as “potassium-dominated combustion” sources primarily influenced by biomass and biofuel burning. F5 showed the largest contribution of NO3- (62%), followed by NH4+ (46%), and defined as “nitrate-dominated combustion” source. F6 was dominated by nss-SO42-and NH4+ (>40%) with minor contributions of WSOC and Abs365 (15–21%), which is defined as “sulfate-dominated combustion” source. NOx is precursor gas species of NO3-, which is emitted from various anthropogenic combustion activities, such as mobile sources (e.g., traffic and ships) being particularly prominent (Geng et al., 2024; Ni et al., 2024). SO2 is precursor gas species of SO42-, which primarily originates from coal combustion in factories and power plants (Lin et al., 2022). Therefore, F5 and F6 indicate the aerosols associated with combustion sources that had undergone atmospheric ageing processes. (Line 408-417).
30. 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.
The sentence has been revised “WSPyC derived from aged fossil fuel combustion aerosols” (Line 429).
31. 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.
According to comment #29 and this comment, the sentences have been reorganized. Please see our response to comment #29. In the revised manuscript, the names of each factor have been defined in Lines 405-413. Regarding the names of the factors, we think that it is difficult to assign specific names to combustion sources based solely on the parameters used in this study, given the inherent uncertainty in those parameters. For example, to assign the biomass burning source, levoglucosan is needed to specify the combustion sources. Similarly, nitrate- and sulfate-dominated sources include several types of combustion sources rather than a specific one. Therefore, we decided to keep the names of the factors in the revised manuscript.
32. 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.
According to the comment, we have changed the title of this section as follows: Possible factors controlling WSPyC in relation to WSPC and EC (Line 440).
33. Line 471: the word “spring” is repeated, the authors could rephrase this sentence in order for it to flow better.
We have revised it (Line 451).
34. Lines 480-481: maybe this is because of the addition of some portion of ethanol in many gasoline fuels?
We consider that high levels of anhydrosugars and elevated WSPyC/EC ratios observed simultaneously suggest an influence from biomass burning. The sentence has been restructured to make the point clearer. The revised sentence is “Geng et al. (2021) reported that some of the observed aerosols in Bachok were derived from fossil fuel combustion, which contained high levels of anhydrosugars, markers of biomass burning. They suggested that fossil fuel combustion-derived aerosols in Bachok may be influenced by biomass burning with a higher WSPyC/EC ratio.” (Lines 457-460).
35. 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.
Line 492–500 in the original manuscript: We have revised the sentences as follows: The slopes between WSPyC and WSOC, as well as between WSPyC and EC, observed near the combustion sources have been used to estimate the global flux of atmospheric deposition of WSPyC to the ocean (Bao et al. 2017; Geng et al., 2021). The slopes were reported to differ between combustion sources (Zhang, Qiao, et al., 2023). This study also showed that the slopes changed with the addition of biogenic WSOC as well as atmospheric aging processes such as photooxidation of soot and subsequent production of WSPyC. Therefore, the global flux of atmospheric deposition of WSPyC to the ocean is likely to be highly uncertain, as it is based on a limited number of observations near combustion sources. To reduce uncertainty in the global flux of atmospheric WSPyC deposition to the ocean, WSPyC, WSOC, EC, and other aerosol parameters in the open ocean, far from combustion sources, are needed. Furthermore, it is necessary to determine changes in WSPyC, WSOC, and EC concentrations during atmospheric aging. (Lines 470-478).
Line 527–529 in the original manuscript: We have revised the sentences as follows: The former two effects, first noted by this study, are likely to become more pronounced as aerosols are transported over long distances from land to the open ocean. This study indicates that it is essential to experimentally elucidate the solubilization processes of WSPyC from soot in both the atmosphere and the ocean surface, in addition to aerosol observations in open ocean regions, to estimate the global WSPyC deposition flux to the ocean precisely. (Lines 504-508).
36. 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.
We have corrected it.
Citation: https://doi.org/10.5194/egusphere-2025-2525-AC3
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AC1: 'Reply on RC1', Riku Miyase, 28 Nov 2025
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RC2: 'Comment on egusphere-2025-2525', Anonymous Referee #2, 11 Sep 2025
Miyase et al. explored the seasonal changes in the concentration and source of water-soluble pyrogenic carbon in aerosols, and they found the ratio of WSPyC to EC or WSPyC to WSOC varied seasonally and regionally, and they concluded that we should re-estimate global atmospheric deposition of dissolved black carbon to the ocean. They manuscript is well-written, they combined multiple tools to track the source of the WSPyC. I don't have any major comments, here are only some minor suggestions that might help to improve the manuscript.
1. Introduction, Lines 44-45: a recent study indicate that submarine groundwater discharge could also be another source of DBC (Zhao, Bao et al., 2025, Global Biogeochemical Cycles).
2. Introduction, lines 58-59: for aerosols, there might only the three publications, but there were also some other studies on atmospheric depositions, e.g.,, rainwater (Bao et al., 2022, Science of the total Environment, 153846;; Wagner et al., 2019, Biogeochemistry, 146: 191-207. I suggest you to include those references.
3. Methods: Lines 97, 102: you mentioned the filter poresize twice, I suggest to keep only one.
4. For results: normally, the environment parameters were firstly presented, including the airmass trajectory and ions concentration, then organic carbon and WSPyC. I suggest the authors to re-arrange the results.
5. some paragraphs were too long: for example, lines 305-322, lines 425-453.
Citation: https://doi.org/10.5194/egusphere-2025-2525-RC2 -
AC2: 'Reply on RC2', Riku Miyase, 28 Nov 2025
Referee#2
Miyase et al. explored the seasonal changes in the concentration and source of water-soluble pyrogenic carbon in aerosols, and they found the ratio of WSPyC to EC or WSPyC to WSOC varied seasonally and regionally, and they concluded that we should re-estimate global atmospheric deposition of dissolved black carbon to the ocean. They manuscript is well-written, they combined multiple tools to track the source of the WSPyC. I don't have any major comments, here are only some minor suggestions that might help to improve the manuscript.
First of all, thank you very much for reviewing our manuscript and providing constructive comments. We have revised the manuscript according to the referee’s comments. We believe that the manuscript has been improved significantly after the revisions.
1. Introduction, Lines 44-45: a recent study indicate that submarine groundwater discharge could also be another source of DBC (Zhao, Bao et al., 2025, Global Biogeochemical Cycles).
We have added a description on the submarine groundwater with the reference (Lines 44-45). The revised sentence is “The primary sources of oceanic WSPyC are riverine transport and atmospheric deposition, although the seafloor and submarine groundwater are also considered sources (Wagner et al., 2018, (Zhao et al., 2025)Zhao et al., 2025).”.
2. Introduction, lines 58-59: for aerosols, there might only the three publications, but there were also some other studies on atmospheric depositions, e.g.,, rainwater (Bao et al., 2022, Science of the total Environment, 153846;; Wagner et al., 2019, Biogeochemistry, 146: 191-207. I suggest you to include those references.
We have added a description on rainwater with the references (Lines 58-59). The revised sentence is “To the best of our knowledge, studies on atmospheric WSPyC observations are limited, with only two focusing on rainwater (Wagner et al., 2019; Bao et al., 2022) and three on aerosols (Bao et al., 2017; Geng et al., 2021; Zhang et al., 2023).”.
3. Methods: Lines 97, 102: you mentioned the filter pore size twice, I suggest to keep only one.
We have deleted the first description related to the pore size.
4. For results: normally, the environment parameters were firstly presented, including the airmass trajectory and ions concentration, then organic carbon and WSPyC. I suggest the authors to re-arrange the results.
We presented the results of OC and WSPyC first, because WSPyC as well as OC are the main target of this study and the air mass characterization by the trajectories and relevant parameters should follow the result of WSPyC to interpret it. We believe that this order of the descriptions can enhance the readability and make the impact/importance of the study clearer.
5. Some paragraphs were too long: for example, lines 305-322, lines 425-453.
Lines: 305-322 in original manuscript: We have shortened the paragraph to make the sentences clearer. The revised paragraph is as follows: The WSPyC/WSOC and WSPyC/EC observed in Sapporo fell within the range previously reported for aerosols influenced by fossil fuel combustion which were lower than those influenced by biomass burning (Figs. 7 and 8). WSPyC/WSOC of the Asian dust sample was one-third of that observed in the Asian dust samples collected in the western North Pacific Ocean (Bao et al., 2017). Variations in the WSPyC concentrations, WSPyC/WSOC, and WSPyC/EC have also been reported to be controlled not only by their sources but also by atmospheric aging processes and meteorological conditions (e.g., precipitation frequency and air temperature) (Zhang, Qiao et al., 2023). For example, the susceptibility to wet deposition varies depending on particle size and hydrophobicity, with EC being relatively less prone to removal (Snowani et al., 2019; Petters et al., 2006; Weingartner et al., 1997). The atmospheric aging processes include the production of WSPyC from soot via photodegradation (Li et al., 2019; Roebuck et al., 2017; Li et al., 2022; Decesari et al., 2002). The photodegradation of WSPyC, and the formation of water-soluble secondary organic aerosols (SOA) through the photochemical oxidation of water-insoluble organic carbon (WIOC) and volatile organic compounds (VOCs) (Lim et al., 2019; Grieshop et al., 2009; Zhang, Cheng et al., 2023; Dzepina et al., 2011). Thus, the WSPyC/WSOC and WSPyC/EC ratios observed in Sapporo are also likely influenced by the atmospheric aging processes of continental origin during long-range transport. (Lines 304-316).
Lines: 425-453 in original manuscript: We moved some sentences to the previous paragraph and shortened the corresponding paragraph as follows: It should be noted that the observed WSPyC concentrations were controlled by only three factors (i.e., F4, F5, and F6), all attributed to combustion-related factors, confirming the combustion origin of the WSPyC. The observed WSPyC in Sapporo was 60% from potassium-dominated combustion sources (F4), 25% from nitrate-dominated combustion (F5), and 15% from sulfate-dominated combustion (F6), demonstrating that biomass burning and secondary aerosols from fossil fuel combustion are the primary sources of WSPyC in Sapporo. Figure 10 illustrates how the contribution of each factor to the WSPyC concentration varies over time, as well as showing the seasonal mean. The potassium-dominated combustion sources were the dominant factor controlling the WSPyC concentration in autumn in Sapporo. The highest concentration of WSPyC observed in this study was in autumn. It was associated with the highest concentration of WSPyC with the potassium-dominated combustion sources (Fig. 10). Back trajectory analysis of the sample with the highest WSPyC concentration (11/8, 2022, in Fig. S2) revealed a strong influence of the continental air masses. The combined contribution of the nitrate- and sulfate-dominated combustion sources was about half of the WSPyC concentration in spring and winter, indicating a greater contribution of WSPyC derived from aged fossil fuel combustion aerosols compared to autumn. The larger contribution of sulfate-dominated combustion sources in winter compared to spring may be influenced by coal use for heating in Northeast China because it has been reported that the leading cause of winter smog in Northeast China is the use of coal for residential heating (Zhang et al., 2020). The back trajectories in winter show a strong continental origin (Fig. 5e), suggesting that they may more closely reflect combustion activities on the continent (Line 418-433).
Citation: https://doi.org/10.5194/egusphere-2025-2525-AC2
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AC2: 'Reply on RC2', Riku Miyase, 28 Nov 2025
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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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