The manuscript presents BC data, based on absorption measurements from 5 Arctic ship-based campaigns (2016-2020). It is a nice data-set, and considering the scarcity of available data from this remote area, very valuable. I would recommend the paper for publication in ACP, however have some comments that are needed to be addressed before.

Both the AE33 and COSMOS are instruments, that measure absorption coefficients. It is already challenging and makes different corrections necessary to retrieve the correct absorption coefficient from filter-based measurements. The absorption coefficients are then used to estimate the value of the BC mass concentrations. Reading the paper, one could think that the instrument would measure directly the BC mass, or at least it would be a trivial or straight-forward thing to get the BC concentrations correctly from filter-based absorption measurements. Only very few details are given according to the measurement uncertainty and possible corrections and assumptions. I would like to read a more thorough discussion/introduction about it, how well these instruments can be used for direct BC mass measurements. The manuscript from Backmann et al., 2017 (https://doi.org/10.5194/amt-10-5039-2017) deals with the uncertainties of Aeth measurements from the Arctic and proposes a region-specific correction factor. I am not even completely sure, how and if the aethalometer data was corrected, but this would be maybe an option to consider.

If I understood correctly, the AE33 was used in 2020, otherwise the COSMOS was deployed. The wavelength dependence of the AE33 measurement is also often used to differentiate between biomass burning and anthropogenic BC. At least for 2020, you could do this differentiation and compare that to the model results. That would be a nice validation for the model, considering the moderate agreement between the measured and modeled total BC concentration.

Specific comments:

1. P2,L9: “can work as CCN” please rephrase, e.g. to can act as
2. P4, L8: Please mention already here, that the specific placement of your inlet was not the only method you have used to get rid of the ship emission influence, and that that the data sorting will be explained later.
3. P4, L10-22: what mass absorption efficiency is used in the COSMOS instrument?
4. P4, L23: you state that the BC mass concentration is derived from the AE33 880 nm measurements. What about the other wavelengths? Why not another one? The COSMOS uses 565nm, why not the closest wavelength of the AE33 was used to be more consistent?
5. P4, L28-29: did you use these “default parameter settings”? And please provide more details about this fact. Please see my general comment about requesting much more details on how BC concentration was derived, and what are your uncertainties!
6. P5, L13-22: If I understood correctly, your method of discriminating the data which might be influenced by the ship emissions was the following: firstly the discrimination by wind direction, and in the years when the mixing ration of O3 was also measured you used that as an extra criterion. How much more data was considered invalid using the O3 criteria in 2017 and 2018? Which was not already discriminated by the wind direction? Can you estimate how much bias could it cause not having O3 data for the other years, and with that still using some data that might have been influenced by the ship?
7. Figure 2, panel a: could you make the y axis logarithmic? Or do something else to see the lower values as well?
8. Figure 3: a logarithmic y axis for the BC concentration would help here a lot too. Caption: “short dashed gray line” it is enough if you write dashed gray line, I was looking for short lines that are dashed and gray :).
9. P6, L30-31: This is what you do not see based on Fig2a, and therefore would be a logarithmic axis nice
10. P7, L25-26: what was this similar value reported by Taketani et al.?
11. P11, L13: “anthropogenic productive activities producing” strange wording, please change
12. P11, L14: “may export” please change to “may be exported”
13. P11, L26-27: if I understood correctly, the AE33 and the COSMOS were not running parallel, and the AE33 measured in 2020 and otherwise the COSMOS. You state that the background periods were defined based on the measurements of the COSMOS. Does this mean that you did not even try to look for background periods in 2020? Why?
14. P11, L29: your definition of a background period included that the BC mass concentration had to be above 1ng/m3, which is the 1 h detection limit of the instrument. My problem is with this, that with such a criterion you define that your background BC mass concentration has to be above 1 ng/m3. I understand that you are not able to reliably measure such low concentrations, but these values should be considered, otherwise the background concentration that you report is false, and at most you can call it as the upper limit, and the real background concentration could still be well below this value.
15. P11, L33- P1, L2: You state that the background value (that you have determined) being smaller that the overall mean BC concentration measured during a full year in the central Arctic suggests that even in the summer months the central Arctic is influenced by imported BC pollutants. Please compare the background to the mean summer values that were measured during MOSAIC. And if the relation is still the same, then your statement will be correct.
16. P12, L6-7: “The result should be representative as the data covered the summer seasons of three years.” Why? Representative for what? I would just delete this statement, or make it more specific, please.
17. Section 4.4: you use the GEOS-Chem simulated biomass burning BC to total BC ratio to evaluate the high BC episodes as well. Table 2 shows in some cases of the episodes, enormous difference between the measured and modelled BC concentration (e.g. E1 20 vs 1 ng/m3) exits. In such cases, if the modelled vs. the measured BC concentration are many factors away from each other, I do not think that you can use the modelled biomass burning to total BC ratio to assume anything about the real measurement. Even for 2 out of the 3 selected episodes, the modelled BC mass is quite far away from the measured. Please comment on this.
18. P13, L11-15: Question regarding to the definition of the high BC episodes? Did you also use a geographic restriction? Only taking periods when the vessel´s latitude was higher than a certain value? Because looking at Figure 3 it looks like that. I do see periods, that look long enough and have high BC concentrations but were not considered as an episode at low latitudes.
19. P13, L13-14 and Table2: you state that the definition of the episodes included that the mean of the 1h BC concentration has to be above 20 ng/m3. How can it be that the overall observed mean BC is below 20 ng/m3 for E1 (first row of Table 2)?
20. Figure 4: Please change the color for the ship positions, it is very hard to see it. And the wind arrows are also hard to see. (also for all other similar figures)
21. Figure S5: You cannot see the BCbb sources, the dots are so small. Anyway: not only for S5 but also for Figure 4, S6, why don’t you show a zoomed in picture of the region which you are talking about, one could see everything much better on the plots.
22. Figure 5: same as for Figure 4, actually do it please for all similar figures.
23. P16, L20: “CO2 and O3 were not or even slightly decreased” I see there quite a decrease. Maybe use other wording.
24. P16, L23-27: I only see a decrease of CO2 when the bb plume was present. All the possible explanations would rather explain a constantly low CO2 concentration, am I not right?
25. P17, L5-6: why was the background BC not subtracted from mBC/ΔCO?
26. P23, L2: “at higher than low latitude regions” ???

Review of the manuscript titled ‘Measurement report: Shipborne observations of black carbon aerosols in the western  Arctic  Ocean  during  summer  and  autumn  2016–2020:  boreal  fire impacts’

The manuscript titled 'Shipborne Observations of Black Carbon Aerosols in the Western Arctic Ocean During Summer and Autumn 2016–2020: Boreal Fire Impacts' authored by Deng et al., presents a comprehensive analysis of black carbon levels in the Arctic Ocean during the summer and autumn seasons. Furthermore, it conducts an in-depth examination of the emission sources contributing to elevated black carbon episodes using simulations from a global chemistry-transport model. This manuscript contributes a significant and valuable dataset to the Arctic region, which is often limited in available data. After addressing the major comments outlined, I highly recommend this manuscript for publication in ACP (Atmospheric Chemistry and Physics).

The manuscript details the analysis of 10 specific episodes of black carbon (BC), yet it overlooks the explanation of additional episodes present in the dataset. There's a notable lack of comprehensive explanations for the identification of these episodes and the criteria utilized in the selection process for these events. Additionally, the manuscript employs a model with a notably coarse resolution for simulations, which might introduce biases into the results, particularly when applied to Arctic conditions. The authors must provide a more detailed account of the uncertainties inherent in the model and highlight potential biases when interpreting the simulated values within the manuscript. Addressing these concerns will significantly strengthen the manuscript's scientific rigor and ensure a more robust interpretation of the findings.

Line 20 (Page-1): I would suggest specifying the particular seasons under consideration here. Failing to do so might lead to a misconception that the findings represent an annual average. Clarifying the specific seasons, such as summer and autumn, within the analysis would prevent any misinterpretation regarding the temporal scope of the data and offer a more accurate representation of the findings.

Line 27 (Page-1): what does this mean “ with some near-surface and others in the mid-troposphere”?

Line 28-30 (Page-1): The abstract appears to contain repetitive sentences. I suggest that the authors revise the abstract to convey essential information more concisely. Condensing the content while retaining crucial details will enhance the abstract's clarity and effectiveness in summarizing the study's key findings and contributions.

Line 2 (Page-2): AMAP, 2021b ? Where is 'a'?

Line 7 (Page-2): can include? Rephrase the sentence.

Line 13(Page-2): Add relevant citation

Line 18 (Page-2): I would recommend citing Gogoi et al., 2021 (Long-term changes in aerosol radiative properties over Ny-Ålesund: Results from Indian scientific expeditions to the Arctic) for the continuous long term monitoring of aerosols in the Arctic. Further, I would recommend comparing the values in this study with Gogoi et al., 2021.

Line 32 (Page-2): What do authors want to convey here? It is obvious that the BC concentration reduces when the observed location is far from the source region.

Line 29-32 (Page-3): Rewrite the sentence.

Line 28 (Page 4): What does the ‘default parameter settings’ mean here? Please explain. Also, explain more about the loading effect corrections and other corrections applied to the data. I would also recommend the authors to explain the uncertainties in the measurements. COSMOS measures at  565 nm, and Aethalometer measurements were at 880 nm. How did the authors compare these measurements at two distinct wavelengths?

Line 13 (Page 5): To avoid the influence of ship exhausts, the authors have used 1- or 5-min data records that occurred when the 1-min wind direction and speed relative to the ship's course within ±60° of the bow and >3 m s−1. Why is it done so for these specific values? What are the reasons for choosing these specific limits?

Line 26 (Page 5): Since the horizontal resolution of GEOS-chem was 2° × 2.5°, how is it valid over the Arctic regions due to the higher grid resolution?

Line 25 (Page 7): Authors have compared the measurements of BC using an Aethalometer and SP2 here. I would not recommend the direct comparison of the measurements with SP2 since the values are highly dependent on the BC size distribution. SP2 has lower and upper bounding limits for the measured BC distributions. This will result in a biased comparison.

Figure 1: I would recommend modifying the color bar to a different color option (either jet or something else). It is difficult to identify the variability using these colors.

Line 15 (Page 9): I would recommend modifying the plots with Lower whisker – 1st  percentile, upper whisker – 99th percentile.

Figure 3: I would recommend changing the left y-axis to a logarithmic scale

Line 6 (Page 11): The derived background concentrations in this study are misleading. The criteria used here need to be proven.

Line 13 (Page 11): Rewrite the sentence.

Line 27 (Page 11): Why the authors have chosen 5 days airmass trajectory for this study? I would suggest to explain the reason here.

Line 10 (Page 12): I would suggest that the authors compile the various comparisons made regarding black carbon concentrations observed in their study with those from other relevant studies in the Arctic region. This compilation could be organized into a table format detailing concentrations, seasons, instruments used, and corresponding references, etc. This tabulated presentation would provide a comprehensive and accessible comparison, aiding readers in understanding the context and variability of BC concentrations in the Arctic across different studies.

Line 17 (Page 12): I would recommend the authors show R2 values than R throughout the manuscript.

Table 2: I would recommend showing the region for these episodes in this table.

Line 10 (Page 15): I would recommend the authors to compare the median values of BC for all these episodes since the mean will not represent the actual variabilities.

Figure 6: I would recommend changing the y-axis to a logarithmic scale here for the BC mass concentration.

Figure S1: I recommend changing the color options for the color bar. It is difficult to understand.

Figure S2: I would recommend changing the axis to a logarithmic scale.

Figure S5, S7, and S10: These figure needs to be revised with detailed color contrasts.  

Figure S14: Recommend to modify the color bar

Figure S15: I would recommend showing all the episodes in this plot rather than removing the other episodes.

Measurement report: Shipborne observations of black carbon aerosols in the western Arctic Ocean during summer and autumn 2016–2020: boreal fire impacts

Summary

Yange Deng¹ and co-authors present a really valuable data set of mass concentrations of eBC within the Arctic Ocean, a region with sparse data coverage. The authors outline through the use of in situ shipborne measurements and a global chemistry transport model that the proportion of eBC mass arising from biomass burning increases with the latitude at which observations are performed.   

I recommend this paper after the following corrections are made.

Major comments:

Method section I suggest

A lack of any description of how HYSPLIT was utilised i.e. which settings were chosen, whether it was a single or ensemble run, the initialised altitude, why 5-days was chosen, whether or not air masses below the mixed-layer were selected for (assume not given the altitude plots), which metrological fields were utilised. Please add more information about how HYSPLIT was utilised. 

Please include the amount of data which is removed when doing the cleaning of the data. Also, include the amount of data which is below the limit of detection. What proportion of the measured observations is left after the various cleaning criteria are applied to the data set e.g. detection limits, averaging thresholds, and ship contamination.  

More justification is required to support why a particle MAC is applied to the absorption coefficient measurements as opposed to another i.e. a more site-specific MAC. The paper could simply report the absorption coefficient instead?

Results and discussion section

Elevated values:

The authors highlight 10 events but the underlying reason for why the events are highlighted in the first place is not so clear, this maybe due to a lack of stated research questions.

Also, the explanation for why the criteria was used to defined the episodes was not explained i.e. when the 1-h m_BC was continually greater than 10 ng m−3 for 18 h or longer and the mean of valid 1-h m_BC during the defined periods was greater than 20 ng m−3.

Was there also a latitudinal criterion imposed on the event selection to study ‘Arctic Ocean and the marginal seas’ as all events are above 65 degrees norther, and either in the ‘North of 72° N group’ or ‘between 52 and 72° N’ group.

Background:

It is not clearly explained what the authors are trying to achieve by presenting ‘background’ values. Do they mean values that are unaffected by anthropogenic emissions? Also, the criteria has no justification and seems arbitrary.

Supplement:

The supplement is of course important to explain interesting but not essential results. In this specific work, the supplementary is 18 pages long with 15 figures, are all necessary.

Figures S5 & S7: can the mass concentration of eBC not be grouped based on the metres AGL of the collocated back trajectories.

Figure S15: There is a mention of the fact that backward air mass trajectories (back trajectories) are initialised at 500 m above the ship.

Specific comments:

The title is a bit misleading. The manuscript present absorption data, not BC data or eBC data. Justify why you made this decision.

Title: ‘impacts of boreal fires’

Abstract:

L3: observations of BC

L15-17: Say the actual number of research expeditions

L20: replace ‘over’ with ‘greater than’

L22: perhaps due to ‘that’ more frequent wildfires ‘had occurred’  in the Arctic… or perhaps due to more frequent wildfires ‘occurring’ …

L23: biomass burning contributed the most to the observed BC in …

L23: by mass

L26: change to transported from regions with boreal fires

Introduction:

L2: ‘climate warming rate’ be more specific e.g. average surface temperature

L3: ‘decline in Arctic sea ice’ be more specific, what is the parameter? Volume? Extent? What is the season you are referring to? Annual? summertime?

L5: ‘SLCFs’ to SCLF

L6: ‘has’ to have

L6: Can you not refer to the more updated AMAP reports?

L6: Arctic aerosol chemical

L7: American spelling: ‘sulphate’ to sulfate

L7: ‘sea-salt’ to sea-spray aerosol

L9: causes direct and/or semi-direct or can cause direct and semi-direct climate forcing

L9: it is not just BC acting as cloud condensation nuclei (CCN) – it makes it seem like this by the way you single BC out.

L11: change ‘radiation’ to radiation budget and ‘besides’ to leading to an acceleration in the melting of snow and ice

L15: remove the word ‘warranted’

L16: ‘in observatories’ to at ground-based Arctic observatories 

17: ‘Barrow/Utqiagvik’ choose one or write formerly called Barrow for example

L18.  ‘whereas those data’ to These mong-term data sets provided…

L22: ‘Airborne observations, especially aircraft-based ones, earth-surface conditions’ – not completely understood what is meant by this. Please rephrase.

L22: Why ‘different seasons’ makes it soon like the reason for airborne observations are to expand on the number of seasons in which measurements can be performed

L23: ‘Shipborne observations allow for in situ measurements in the remote Arctic Ocean especially in summer and autumn when the Arctic sea ice is at the minimum’ – Shipborne observations allow for measurements in Arctic Ocean at all times of the year, it is just that in summer and autumn these measurements are easier to preform as the Arctic Ocean is more accessible.

L28: ‘over’ to ‘in’

L29: remove the word ‘preliminary’

L30: Arctic Ocean?

L30-33: Break up sentence too long

L32: ‘decreases with the growing distance from the source region’ – remove ‘the growing’ and replace with ‘increasing’

Page3:

L1: Mention the Arctic Haze phenomenon or refer to  

L1: Relatively ‘large’ what BC mass concentrations

L2: ‘those’ to ‘these studies were limited’

L4-5: the parts of the seas which are close to land or are you simply mentioning that these seas are close to land (if so not really necessary)

L9-13: mention the importance of preforming measurements in regions with sparse coverage

L16: Remove ‘whereas models have been improving in the past two decades’

L18: ‘The main obstacles include poor understanding in’ replace in with of

L24: ‘in the context of climate change, the likelihood of extreme Arctic fire weather will increase.’ – what do you mean by ‘Arctic fire weather’ this term is unknown to me.

L25: the impact on BC emissions

L27: ‘is’ replace with ‘are’ as you mention ‘studies’

L29: ‘BC monitors based on light absorption theory’ rephrase’

L1-4: No aims of the study are listed but the results are described

Shipborne observations:

L8: Makes it sound like this is the only thing that was done to clean the data from the ship exhaust, however, later on you mention other steps.

L11: Would be nice to mention clear how many expeditions were carried out in total even though it is clear from seeing the results and assuming that one cruise was carried out each year.

L13: Mention the concept of the mass absorption cross-section (MAC) here as this is an important concept

L14: Why is it referred to as ‘BC’ when what you are measuring is eBC?

L18: ‘data integration time’ what is this term and what does it refer to? Not completely clear.

L23: Why was the wavelength 880nm chosen? Why not the wavelength most similar to 565nm, can you justify your reasoning?

L26: Is this MAC value site-specific i.e. for the Arctic? Is there a more relevant MAC value?

L28: default parameter settings such as …

L28: Aethalometer

Page 5:

L6: ‘lump’ replace with lamp

L21-22: why impose this criterion of 40 minutes of valid data records. Does that mean that at least 40/5 = 8 5-min values or 40 1-minute values are needed.

Model simulations:

L31: When was the data accessed? Why were small fires not included?

Page 6:

L5: Expand on the criteria you used for the HYSPLIT runs. Defend and state the number of hours you ran the model for? What was the initialised height? What was the temporal resolution for the runs? How did you track the movement of the ship using the model?

Line 18-20: unclear what is achieved by the groupings and they are not used so much afterwards:

South of 52°N: (in the North Pacific Ocean),

North of 72° N: (mainly in the Canada Basin and the east part of the East Siberian Sea, which are noted as western central Arctic Ocean in the following sections of this study),

between 52 and 72° N: (mainly in the Bering, Chukchi, and Beaufort Seas).

Page 7:

L4: remove ‘pre-existing’

L34-page, L1: ‘necessity to further study the spatial-temporal variations of BC in the Arctic Ocean’ move to discussion

L3: ‘shouldn’t’ to should not

L7: ‘Barrow’ to ‘Utqiagvik’

L10: Is it possible to know whether BB reach further north or that the ratio increases because the anthropogenic influence decreases at the latitude increases?

Page 10:

Figure 2: The smaller values are not very clear. Seems as though values are cut-off – represent or detail in the caption the values that you cut-off. The font size for the y-axis can be increased. Remove ‘Year/month/day’ not needed.  You could separate the plot based on latitudes and focus on the events which are all above 65 degrees north.  Not 100% clear what time resolution is presented here.

4.2 Background BC concentration in the western central Arctic Ocean

L9: remove ‘pristine region’ not scientific or perhaps rephrase to distant from sources of anthropogenic pollutants

L10-13: ‘should be close to zero’ why? What does ‘close to zero’ mean? Use the literature to suggest typical values as opposed to some arbitrary ‘close to zero’ statement which is quite meaningless.  Who claims that the marine Arctic boundary layer should have a ‘zero background’? BC has been transported up to the Arctic prior to pre-industrial times (McConnell et al., 2007). McConnell et al., (2007) present historical BC from Greenland ice-cores showing non-zero values.

L13: ‘anthropogenic productive’ replace with ‘industry’

L14: lower latitude

L14: ‘export’ replace with transport

L15: how do commercial fisheries significantly impact Arctic BC?

L17: rephrase the sentence ‘coastal region along the Arctic climate warming’ doesn’t make sense

L18: which allows for the transport…

L19: Separate sentence/start a new sentence: ‘and the stable atmospheric conditions’  

L20: replace with higher

L21: surface layer atmosphere

L24-25: Why are summer and early autumn months most suitable to evaluate background levels? What are you trying to achieve? By background do you mean pre-industrial – the background will have a seasonality as descripted by the Arctic Haze phenomenon.

L26: The definition of ‘background periods’ needs to be properly explained it is not clear why the criteria you mentioned have been applied.  What is it you want to achieve with this notion of background (perhaps unaffected by anthropogenic emissions?). Background Arctic Ocean conditions perhaps is better wording.  The definition relies on values below the detection limit ‘below the lowest detection limit’ there is it valid? You could additional work out how often the measurements exceed these background values.

L28: remove ‘continually’

Page 12:

L2: replace ‘imported’ with long-range transport

L3-4: ‘might be twice the actual values because the default’ this is repeated

L7: There are decreasing trends in the Arctic and this has been well reported and hence is representative of this time period e.g. Hirdman et al., 2010, Sharma et al., 2004,

L9: Unsure how the background is calculated.

L17: Why not R2?

Page 13,  4.4 Sources of High BC episodes:

In 4.4. Sources of High BC episodes, the phrasing is not consistent, these episodes are referred to as ‘high BC episodes’, ‘elevated BC mass concentration periods’ and ‘high BC mass concentration episodes.

Winiger et al., 2016 might be a good study to read and reference.

L12 – 15: Why are the elevated values defined as such what is the reasoning? i.e. why the criteria ‘1-h m_BC was continually greater than 10ngm^-3 for 18 h or longer and the mean of valid 1-h m_BC during the defined periods was greater than 20 ngm^-3. Why 18 h? Why 10 ngm^-3, why 20 ngm^-3? What is the motivation?

L21-22: ‘Note that in addition to these 10 episodes, high BC was also observed in the Arctic at other times, such as on 14-18 August 2020’ – is this sentence needed?

L25: support what you mean by ‘well reproduced’?

Page 14:

L6: replaced ‘occurred’ with was measured

L11: Remove ‘Contour plots’

Page 16:

L3: rephrase ‘… burning BC and surface winds before to after Episode 8’

5 Summary and conclusions:

Page 23:

L2 ‘higher than low latitude’ – what does this mean?

L8-10: Repeated definition

L24: Arctic BC mass