SUMMARY

The work of Tinorua et al. provides interesting dataset on black carbon properties at a high-mountain site in Europe. These sorts of data are rare and of great interest. The aim of the manuscript is to understand the variability of black carbon properties as function of season, dynamics of the boundary layer and wet removal. Although the current dataset might allow investigating these processes, the presentation and discussion of the results prevents the authors to clearly communicate their message. The language and nomenclature are often problematic to the understanding of the text. Which require a thoughtful revision. Often, the authors jump to conclusions very fast, without a proper description of the observed parameters and with a superficial use of references. As a consequence, the processes leading to the observed changes of rBC properties are often unclear. I suggest the authors to clarify their goals, reduce to a minimum the non-essential discussion and elaborate more in details their hypothesis. I also advise caution when discussing “photochemical processes” and “hygroscopicity”, which cannot be investigated with the current dataset. In its current status, the manuscript is not suitable for publication. However, I invite Tinorua et co-authors to consider the major comments and a multitude of specific, yet not minor, comments for resubmission after major changes.

MAJOR COMMENTS

First, the manuscript would benefit from a deep revision of the language, which often results non-scientific and approximative. The authors are also invited to revise the format of citations, acronyms, and units and the grammar. See specific comments.

Nomenclature is extremely important. The authors should make sure to provide the correct information especially when using abbreviations and acronyms. 1) When dealing with optical properties, it is essential to always declare the wavelength. This is not always done, in the text and especially in the figures. In some cases, it thus results difficult to understand at what wavelengths the measurements are performed. 2)Every soot-measuring technique is based on different properties of the aerosol; hence, instrument specific nomenclature must be used. Soot measured with SP2 should be named rBC (refractory black carbon). Soot measured with filter-based photometers should be named eBC (equivalent black carbon). Soot measured with thermal-optical method should be named EC (elemental carbon). This nomenclature is not applied to the data presented here and to the results of other works. Please revise all the nomenclature and resulting abbreviation following Petzold et al. (2013).

The SP2 offers the possibility to quantify the mixing state (PSD detector and time-lag) and “composition” (colour ratio) of rBC. Unfortunately, these analyses are not performed, although it might help understanding ageing process and absorption enhancement, influence of different sources and potentially wet removal. Could the author explain why mixing-state and colour-ratio were not presented in the manuscript?

The discussion of results and its interpretation is often superficial. This is particularly true in Section 3.1 and 3.2, where a detailed variability of aerosol and BC properties is provided but not discussed with the appropriate literature context. The text reads like a list of numbers followed by a list of references, while the reasons causing the variability is often explained with short and generic sentences like “It has been attributed to the seasonal variation of the continental boundary layer height, long-range transport events (e.g. Saharan dust outbreaks, coal burning from eastern Europe) and biomass burning both from forest fires in summer and domestic heating in winter.” I suggest the author rethinking all the results section to improve their data interpretation and to set clear scientific objectives.

The figures based on time series are not particularly helpful. If the authors aim to discuss the seasonal variability it is advisable to use a longer time stamp (1 month or 2 weeks). In order to provide evidence of correlations between the various properties I also suggest using scatter plots.

SPECIFIC COMMENTS

L30: Merge the two statements, not clear what “this” refers to.

L36-37: please add a reference.

L39-40: the definition is correct, but it is not described how Mac is measured. A short description of the methodology is needed since later on (L43) the instrumental influence is mentioned.

L53: too many references, select the most relevant to deliver your message.

L58: what it is meant with “multiplied by two”?

L66-76: part of this sub-paragraph can be moved into the methodology (ABL-Topoindex). Listing of the sections is not needed. I suggest rewriting the current paragraph focussing on the goals of your work.

L90: replace “sucked” with “sampled”

L94: DMT is not based any longer in Boulder, but in Longmont

L93-113: Although being relatively tedious, nomenclature is important. BC measured via laser-induced incandesce technique is normally referred as rBC (refractory black carbon). I suggest reading Petzold et al. (2013) for more details. Considering this technicality, I also recommend the authors to replace “BC” with “rBC” in the text and in all abbreviations (M_rBC,D_rBC, etc…) when referring to their or other SP2 measurements. BC can be used for more generic discussion in the introduction.

L100: out of curiosity, did the authors ever compared the results obtained with the Python code and the SP2 Toolkit?

L102: I do not see an increase of mass concentration at diameter smaller than 90 nm in Figure S1. Please reformulate or verify the top panel of Figure S1. Figure S1 shows both mass and number size distribution, but only mass is described.

L103: “detection range”, not “detection window”.

L106-113: Please define what “dg” and “σg” mean. Assuming these are the geometric mean and geometric standard deviation, how these were defined, empirically? For mode 1. The SP2 lower size quantification limit was 90 nm. Does it mean that the lognormal fit is applied to the 90-100 nm diameter range to derive mode1?

Figure S1 shows the size distribution of rBC, but on what time scale? With what temporal resolution was the MBC-correction calculated? Would it change during different conditions (PBL, FT, winter, summer, etc…)?

The relative standard deviation of the correction factor is approximately 90%, this lets me thing that non-negligible variability was observed during the measuring period. Could the authors have used a time dependent correction factor instead of constant one for the full dataset?

Considering the temporal variability of MBC-correction, I would like to see how MAC correlate with the correction factor.

L115: please provide the model, manufacturer, company, and country for the TSI instruments, as it is nicely done for the other instruments.

L121: List the measuring wavelengths.

L125: since is not yet published, the Cref value used in the present work should be described a bit better (location of the measurement, reference instrument, wavelength) and compared to previous studies. Since the manuscript is in preparation, and not submitted the year is not relevant.

L126a: I strongly do not recommend the use of “MBC” for the BC mass concentration derived from the aethalometer data. First, the correct nomenclature should be equivalent black carbon (eBC; Petzold et al., 2013). Second, the mass concentration derived from SP2 measurements is also abbreviated as MBC. As a result, it become tremendously confusing to understand how MB is derived in the rest f the paper. Update the use of nomenclature.

L126b: Were the MeBC and σap limits corrected with Cref? At what wavelenght these values were derived, this is particoularly important (especially for σap). If I take 0.0215 Mm−1 and 0.005 μg m−3 I obtaine a MAC (or a mass attenuation coefficient) of 4.3 m2/g, please revise these values. And set the limit of AE33 based on absortion coefficient rather than MeBC, since you have a more reliable instrument (SP2) to measure the mass of rBC.

L130-132: when providing the information about the instruments try do be cosistent with the rest of the paper and provide (model, manufacturer, company, and country), as done for the aerosol instruments

L133 I suggest removing ΔBC/ΔCO in this section, since it comes out of the blue without any context and it is anyway explained later in the text.

L144: The authors should explain clearly that AAE was calculated between 450-635 nm to match the wavelength range of the Nephelometer. Since the measuring wavelengths of the Aethalometer are not listed, it becomes harder for the reader to understand why σap660 was adjusted to 635 nm.

L141-151: I believe a short explanation on what these optical properties represent is needed here. SSA, What SSA, AAE and SAE represent, why they are climatically relevant?

L155-157: I do not agree with the nomenclature choice. If ΔBC/ΔCO is the ratio of MrBC over ΔCO, it should be simply called MrBC/ΔCO, as done by previous studies cited in the result section (Liu et al., 2010; McMeeking et al., 2010).

L160: MBC under (resp. over) 160 the 5th (resp. 95th) percentile? Rephrase.

L161: I suggest giving more explanation about the influence of dust on absorption. Often, the authors do not provide adequate context to very specific statements, assuming that every reader has a deep knowledge of the treated topic.

L167-170. Provide some references for each method.

L171: correct “1,95” in “1.95”. Moreover, I strongly recommend reading Liu et al. (2020), who showed that, despite being widely used, 1.95-0.79i might not be representative of realistic condition. The authors are invited to verify the sensitivity of their calculated MACbare as function of different refractive index. As a matter of fact, Figure S3 showed a maximum MACbare below 5 m2/g which considerably lower than MAC of fresh and bare Bc presented by Bond (7.5m2/g). I imagine that Eabs presented here might be overestimated.

L183-195: for non-expert readers, this subsection might result of difficult understanding. Since the analysis is important, I suggested providing more details on how the ranking is calculated (more technical aspects could go in the supplementary). As it is, FigureS4 does not really help understanding the anabatic ranking, since zero context is provided in the supplementary.

L191: I find the note particularly disturbing. Please avoid statements like “make no sense”. The fluctuation after rank 282 are not negligible and more noisy than in Griffiths et al. (2014). Please try to argue what might be the natural causes leading to the radon fluctuation. Could it be that these values are false negatives? Could the radon ranking be verified as function of water vapour as done in Griffiths et al. (2014)?

L192-195: I am not sure to properly understand this final selection. The periods under the influence of PBL presented later are based on hourly selection and not daily selection (for ranking below 200 in the “anabatic-subset”), right? The opposite was done for FT influence. I expect the PBL-periods to occur preferentially during day-time, while FT-periods during night-time. Is the analysis only considering day-time or it does include also night-time?

L202: m.s−1. Remove the dot.

L223-230: SSA at what wavelength? In figure 4a there are values well below 0.93. Is a monthly minimum, a season minimum? Please explain better. The simultaneous increase of SAE and absorption does not automatically indicate that absorbing particles are small in size. It must be kept in mind that BC is co-emitted with other fine aerosol species such as sulfate. I fund however interesting that the maximum peak of absorption does not correspond with a minimum of SSA. The reasons beyond the seasonal variability are actually not explained (“It has been attributed to the seasonal variation of the continental boundary layer height, long-range transport events and biomass burning both from forest fires in summer and domestic heating in winter.” is a very generic statement).

L259: check reference format

L260: why “MBCs”?

L260: Jungfraujoch name.

L260-265: The seasonal variability of BC mass and absorption is opposite to background and polluted stations, where higher values are observed during winter compared to summer (among others: Yttri et al., 2007; Zanatta et al., 2016). The authors should explain this difference and potentially exploit it to introduce the analysis performed in the following sections of their works.

L268: “Seasonal differences between the origin of highest MBC are thrown into relief,”…not sure what it is meant here.

L273: discussion discussed. Avoid repetitions.

L268-272: From my point of view, Figure 7 shows that 1) the wind patterns are similar in winter and summer; 2) high MBC are associated with low wind speed; 3) and that there is a north scarred signal in winter and southern signal in summer. With the MBC scale and so many points, I cannot identify any clear correlation between wind direction and BC concentration, so I do not agree with the statement “highlighting different BC geographical sources” Similar reasoning can be done for Figure S5, where the overall origin of the air masses lays in the same western sector in both seasons. To improve the visualisation and interpretation of the data, I suggest organizing the wind direction in broader classes (10-20 degrees) and normalize the MBc to its maximum. This modification might help identify a correlation between wind direction and BC concentration

L270: please define summer and winter, this applies elsewhere in the text.

L275-285: So, what it is the conclusion of this analysis?

L320: All paper is based on winter and summer differences. I suggest removing non-essential information like the daily cycle in autumn and spring. Considering that little to no explanation is given about the diurnal-seasonal change, I cannot fully understand the relevance or the aim of this anylsis. As said already, the authors should try to motivate the observed variability, giving context and explanation. Section 3.3 suffers, in its entirety, of this problem.

L325:Section 4 is still part of the results, right? So it should be Section 3.x. Please correct.

L327: As it is shown in the following section, BC mass concentration and BC/CO ratio drastically change (at least in winter) due to anabatic injection from the PBL. Under the influence of PBL injection of fresh BC, wet removal has a smaller impact of BC properties compared to free tropospheric conditions. If the authors excluded periods affected by precipitation in Section 4.2, period under the influence of PBL should be excluded here. This additional filter will reduce the number of atmospheric variable and, perhaps, improve the interpretation of the results.

L356: biomass burning influence. rBC emitted by different sources might show a difference in properties. If the biomass plumes were fresh, the authors should be able to see a difference in the size distribution, and, potentially, in the colour-ratio (ratio of BB over NB channel of the SP2).

L357: Be consistent with cross-references…heather is “Fig.X” or “Figure X”

L362: this is most likely due to the lower concentration of BC observed in the PBL in the summer period.

L348-390: why the size distribution of rBC is not shown here? It might help with the data interpretation.

L369: what is “this evidence”. Reduce the use of “this”, it makes difficult to understand what the authors refer to.

L371: IAGOS…Always explain every abbreviation

L375-379: long unclear sentence, rephrase.

L384-390: Are the SMPS data filtered for FT and PBL conditions? Please specify. If this is not the case, PBL aerosol injection might potentially explain the concentration increase of smaller particles (PBL influence timing is exactly the same FigureS8). Overall, the statement is mostly speculative since the authors cannot prove the occurrence of coagulation and condensation on rBC cores. I thus would not call it “evidence” but rather “hypothesis”. Moreover, the SP2 is capable of providing coating thickness (via the position sensitive detector) and a simpler proxy for mixing-degree (scattering- incandescence time lag). Could the authors explain why these two analyses were not applied?

L399-403: In the present work no evidence is provided on interaction with snow, coating thickness, lifetime, condensation rate or gaseous precursors. Only results obtained by the present study should be discussed in the conclusion section. This part is mostly speculative and I suggest removing it.

L405: I would like to see if these results might change by removing the PBL periods.

L406-407: avoid the use of references in the conclusions. Especially 4 in a row.

L415: What is the “evidence” exactly. Please elaborate.

L423-427: ageing time scale and its impact on cloud activation and optical properties of BC is not treated in the present work. Saying that wet removal is independent from size and mixing state, and that hygroscopicity is not treated properly in models is a bold statement…I recommend caution. Same goes for the following statement.

F1: This figure might benefit some editing. Besides the low resolution, I suggest removing the picture (although beautiful) and introduce a double map with a continental and regional scale. More info could be provided within the figure such as coordinates, altitude, managing institute, ACTRIS name, station type (mountain, background…), instrument list…

F2: I do not think that Figure 2 is needed. The text in section 3.1 describes well enough the general meteorological conditions. Since day-by-day variability is not discussed (and there is no need), I suggest removing the full figure

F6: Figure 6, as Figure 2 and 4, suffers from the choice of using a daily temporal resolution. Since the authors are mostly discussing the seasonal variability, a longer time scale (month) will help visualizing the seasonal changes.

F10b-e: axis is Eabs, caption is MACbc, correct.

FS6: I suggest plotting this graph with daily or weekly temporal resolution.

FS9 Shouldn’t the points have the same colour in the top and bottom panels?

FS10: usually nucleation mode is defined as D<10 nm

REFERENCES

Griffiths, A. D., Conen, F., Weingartner, E., Zimmermann, L., Chambers, S. D., Williams, A. G., and Steinbacher, M.: Surface-to-mountaintop transport characterised by radon observations at the Jungfraujoch, Atmospheric Chem. Phys., 14, 12763–12779, https://doi.org/10.5194/acp-14-12763-2014, 2014.

Liu, D., Flynn, M., Gysel, M., Targino, A., Crawford, I., Bower, K., Choularton, T., Jurányi, Z., Steinbacher, M., Hüglin, C., Curtius, J., Kampus, M., Petzold, A., Weingartner, E., Baltensperger, U., and Coe, H.: Single particle characterization of black carbon aerosols at a tropospheric alpine site in Switzerland, Atmos Chem Phys, 10, 7389–7407, https://doi.org/10.5194/acp-10-7389-2010, 2010.

Liu, F., Yon, J., Fuentes, A., Lobo, P., Smallwood, G. J., and Corbin, J. C.: Review of recent literature on the light absorption properties of black carbon: Refractive index, mass absorption cross section, and absorption function, Aerosol Sci. Technol., 54, 33–51, https://doi.org/10.1080/02786826.2019.1676878, 2020.

McMeeking, G. R., Hamburger, T., Liu, D., Flynn, M., Morgan, W. T., Northway, M., Highwood, E. J., Krejci, R., Allan, J. D., Minikin, A., and Coe, H.: Black carbon measurements in the boundary layer over western and northern Europe, Atmos Chem Phys, 10, 9393–9414, https://doi.org/10.5194/acp-10-9393-2010, 2010.

Petzold, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S.-M., Baltensperger, U., Holzer-Popp, T., Kinne, S., Pappalardo, G., Sugimoto, N., Wehrli, C., Wiedensohler, A., and Zhang, X.-Y.: Recommendations for reporting “black carbon” measurements, Atmos Chem Phys, 13, 8365–8379, https://doi.org/10.5194/acp-13-8365-2013, 2013.

Yttri, K. E., Aas, W., Bjerke, A., Cape, J. N., Cavalli, F., Ceburnis, D., Dye, C., Emblico, L., Facchini, M. C., Forster, C., Hanssen, J. E., Hansson, H. C., Jennings, S. G., Maenhaut, W., Putaud, J. P., and Tørseth, K.: Elemental and organic carbon in PM10: a one year measurement campaign within the European Monitoring and Evaluation Programme EMEP, Atmos Chem Phys, 7, 5711–5725, https://doi.org/10.5194/acp-7-5711-2007, 2007.

Zanatta, M., Gysel, M., Bukowiecki, N., Müller, T., Weingartner, E., Areskoug, H., Fiebig, M., Yttri, K. E., Mihalopoulos, N., Kouvarakis, G., Beddows, D., Harrison, R. M., Cavalli, F., Putaud, J. P., Spindler, G., Wiedensohler, A., Alastuey, A., Pandolfi, M., Sellegri, K., Swietlicki, E., Jaffrezo, J. L., Baltensperger, U., and Laj, P.: A European aerosol phenomenology-5: Climatology of black carbon optical properties at 9 regional background sites across Europe, Atmos. Environ., 145, 346–364, https://doi.org/10.1016/j.atmosenv.2016.09.035, 2016.

This study conducted two-year measurements of black carbon properties at a mountain site in the French Pyrenees to investigate BC microphysical and optical properties at high altitude and characterize BC in both PBL and FT. The study reported detailed BC mass concentration, BC mass absorption coefficient, Single Scattering Albedo, and absorption enhancement in different seasons, while ΔBC/ΔCO was introduced to quantify the emission and wet scavenging of BC. Generally, this paper is well-organized and informative, providing vital information on climate-relevant properties of BC at high altitude while aiding in constraints for climate models. However, before acceptance of this paper, the following issues need to be resolved:

1. Instrumentation: In Sec. 2.2.1, the air was heated to keep the samplings dry. Could this measure alter the properties of BC, e.g., leading to some mass loss of volatile substances in the coating materials of BC particles?
2. It will be also useful to examine the coated size of BC measured by the SP2 for the discussion about the mixing state of BC.
3. Eq. (1) and Eq. (2) are wrong, it should be log (450/635) instead of log(450)/log(635).
4. Line 276: Based on your results, what can be the main influence that caused a higher ΔBC/ΔCO in summer than in winter when considering the larger amount of precipitation in summertime?
5. Line 313-314: “It is expected that the BC particles reaching PDM would be aged and relatively thickly coated.” some evidence is needed here, a previous study which also conducted experiments on a mountain site influenced by the PBL intrusion (doi: 10.5194/acp-19-6749-2019) may support your results.
6. The authors use “the size of BC core sampled at PDM was higher than other studies” to explain “BC wet scavenging did not significantly affect the size of BC-containing particles,” which is not convincing. A previous study performed on a mountain site during wintertime in Beijing reported larger BC core than the present study but showed significantly lowered BC core size after wet scavenging due to larger BC cores were preferentially wet removed (doi: 10.1029/2019GL083171). Please give more explanations.
7. The wet scavenging process of BC due to PBL cloud during its vertical transport within PBL around midday may partly explain the relationship between the diurnal variation of Eabs and ΔBC/ΔCO in summer (doi: 10.1029/2020JD033096)

Minor:

1. Line 171: “1,95 should be 1.95”
2. Line 173: what do you mean by “artifacts”
3. BL and PBL need to be uniform.
4. Abstract, line 18: This sentence was confusing.

Major comments

Tinorua et al. provides a description of an important black carbon data set collected at a high-altitude mountain site in France – these sorts of data sets and measurements are indeed essential to improving climate models. However, the impact of this data set is not compellingly described in the manuscript – there are some sections bogged down in numbers and lists, some paragraphs that making sweeping general statements without the rigorous analysis / discussion that I would expect of an ACP paper, and perhaps most importantly, the implications of the findings on climate models (as described in the article introduction) are limited to a brief 6-line paragraph at the end of the manuscript.

These problems of narrative, hypothesis generation and testing, and rigor can even be seen in the title, which is overly generic – I would like to see the authors more precisely focus and describe their key findings and make sure these findings are well supported in the paper, clear in the title and that the methods are described in more detail. In summary, I think the manuscript has a lot of potential after major revisions to sharpen the focus. I look forward to this paper being published in ACP after making these adjustments.

There are also some grammatical / style issues throughout that would benefit from an additional proofread, and the time series figures spanning the full 2-year campaign are generally overwhelming. I would recommend that some of these figures be moved to the SI and that focused vignettes of the data be presented in the main manuscript. Additionally, the Supporting Information lacks a lot of detail and description; it is simply a collection of figures, which limits its utility.

Specific Comments

Title: I recommend making the title more descriptive of the findings. It’s too generic as written.

Line 54: Can you define what is meant by “short periods” – currently subjective

Figure 1: Map resolution is poor – can you make this clearer?

Lines 89-91: I would like to see a bit more description of the inlet and its design – what is the total flow rate and diameter? What are the particle losses? If there is a citation that describes this inlet in more detail perhaps it could be provided.

Line 101-102: The process of developing the size distributions shown in Figure S1 in the Supplement are not described – I don’t see any descriptive text in the SI, either. Can you provide some more detail in the SI? Feel like there is a big jump here. How are you measuring size distribution? How do you know that all of the ultrafine particles are indeed BC? I think I am just missing a few steps here because I don’t have the relevant expertise / background.

Section 2.2.3: I see now that you are describing the SMPS in the 10-1000 nm size range – would recommend re-ordering the manuscript so you aren’t introducing results before you have mentioned the methods. My question on how you know the size distribution below 90 nm is BC still stands.

Lines 124-126: I do not believe these in preparation materials were provided for review – can you provide more details on the calculation of the C value in the response to reviewers?

Section 2.3: I find the description of the equations (1-3) to be confusing to follow. You mention on line 143 the calculation of the aerosol absorption coefficient at 635 nm using another AAE calculation but the justification for this approach seems to be missing. I believe this is linked to the nephelometer wavelength ranges but its difficult to follow the narrative.

Line 161: You are citing material that to my knowledge is not available for review – please provide details even if it is confidentially to reviewers only. I would like to see some of these details from the in preparation manuscript if possible.

Lines 167-171: I find the justification of choice for MAC_bare to be lacking – what are the implications of your choice? Is this still standard practice given that you are citing a paper from 2006? Given the interest in the subject matter I require more convincing that this is the appropriate method.

Figure 2: I’m not sure this figure has a lot of value in the main manuscript given that you have already described the findings in the text in Section 3.1. I would rather see this figure in the SI and instead have you provide more details on the methods section, which I found to be lacking.

Figure 4: The time series are difficult to follow because you are plotting the full two years. I am not sure this is the most effective way to communicate this information. Perhaps you can re-arrange to highlight the seasonality you describe in the text and move the full time series to the SI.

Lines 253-254: If this is a key conclusion of the work then this should be better reflected in abstract / title of the paper. Also given that it was unexpected, can you probe more deeply why you think this was observed and provide more detail that you are confident that it isn’t an artifact of your sampling methods or calculations? Would just like to see this better and more specifically justified.

Lines 297-298: I feel like you could be more precise describing how different techniques and correction factors influence the differences. This feels overly generalized as written. Additionally, if you have confidence in your very high MAC value, then what are the implications of this?

Figure 7: You should have a consistent scale for panels (a) and (b).

Line 320: Please avoid sensational language like “remarkable”. Do you mean a notable diurnal profile?

Section 4.1 and wet scavenging: Conclusions about the significance of precipitation I think would benefit from a more rigorous statistical test vs descriptive observations that you have provided. Additionally, this whole section reads as quite speculative. This is important given that this is one of your main conclusions of note.

Lines 348-350: Can you clarify why precipitation events were removed? Because of the influence on the ΔBC/ΔCO ratio? Was slightly unclear to me.

Lines 378-379: Can you describe what these additional measurements would be? And the more precise source apportionment?

Lines 420-427: To me these are the most important lines in the manuscript – the implications of your findings. I am a little disappointed that this is relegated to one brief paragraph and that your major conclusion re: wet scavenging is not as thoroughly assessed in the paper as is likely warranted given the conclusion. I would like to see the implications section for climate models more rigorously discussed.