The manuscript “Microphysical properties of refractory black carbon aerosols for different air masses at a central European background site” presents a study on black carbon properties from a central European measurement site in December and in August. It focuses on how the air-mass origin influences these properties, including not only BC mass concentration and size distribution but also the BC coating thickness. I think it is a very valuable study, which suits well the scope of the journal, and it is worth publishing, however, the manuscript still needs important changes to be better understandable. I think that many of the discussions are way too long and too specific, tries to explain even the smallest details of the different figures and sometimes even speculate too much what could cause those small details, that might not even be there considering the uncertainty of the measurements. I do think, that less would be more here. I often needed to read sentences twice until I really understood what the authors would like to say, and then I needed to look at the figures for a long time to see that as well. I do suggest focusing more on the important things. I hope my detailed comments help with that.

L33-35: In my opinion, the uncertainty in BC lifetime is also an important factor here, please consider adding it.

L40-41: BC is not hydrophyobic, that would mean a hygroscopicity worse than a non-soluble material, which is to my knowledge not the case. BC is simply non-hygroscopic. Please correct. And I do not really understand what the authors mean by “and independent from other atmospheric materials”, please explain better.

L41: “After emissions” please change it to “After emission”

L56: start maybe a new paragraph here

L58-62: sentence too long and hardly understandable. Please make 2 separate sentences out of it and reformulate for better understandability.

L75: please change “volatility” to “volatile”

L75-77: You have selected the months with the highest and lowest BC concentrations of the year to analyze. Why not the whole seasons or even the whole year if you do have the data? And if those two months are the ones with the highest and the lowest BC concentrations, that means to me that in the other months of the winter the BC concentrations are lower and in summer higher. With that, in my opinion, December and August cannot represent well winter and summer. Please comment on this. As a suggestion, you could say, that you only focus on the two months with the highest and lowest BC concentrations. But still my question remains, why not the other months? Are those not that interesting?

L82-83: Figure 1 does not show any measurements, maybe change it to measurement site?

L115-117: please make it clear, if you have just identified the amount of the “missing” BC or used it for correction, and reported the corrected or uncorrected mass concentration values!

L118: 4000k, K should be capital letter

L132-133: “When considering the average coating thickness, these negative values were not counted” In my opinion the negative values should be considered when calculating the average coatings. Determining Dp with the LEO fit, has a great uncertainty (by the way, I would really like to see a discussion on that and the extent of that). And as you mention this is also one of the main reasons why negative CT values are present. I believe that this uncertainty results with kind of the same probability in a too large and a too small Dp. And with this only ignoring the negative CT values, you simply ignore a fraction of values where a too small Dp was determined. And this results in artificially too high averages. The case is of course different if the negative CT originates from the refractive index values not perfectly representing the real values. However, that gives you an uncertainty that you cannot get rid of with ignoring the negative CT values, because that bias is present than for all particles. So that would also not make it useful to ignore the negative CT values. The only case where ignoring the negative CT values would make sense is maybe the case where the core is not in the middle of the particle. But I guess that is only a minor fraction?

L143-144: people outside of the SP2 community might not understand this factor of 0.75, please add one or two more sentences to explain that.

L152: “compounds(Poulain et al., 2014)” space is missing

Figure 2 caption: “Instruments setup” please change it to “Instrument setup”

Line175: “during summer and winter were shown in Fig. 3.” Please change “were” to “are”

Figure3: the labels of the AMS measurements showing the different components are very small, maybe make them a bit larger

Figure3 caption: “mean mass fraction (mfcoated)” please indicate that you show here the mass fraction of coated particles

Figure 3 caption: “The shaded areas indicate air masses.” Change it maybe to “The shaded areas indicate different air masses.”

Table 1: “mean values and variability” is the variability you show there the standard deviation? Also everywhere else in the paper? Please state.

Table 1: it is strange to show a range with a “~” sign. Please consider changing it to “-“. Does the ranges correspond to the minimal and maximal values of the 1-hour averages? Please state as well.

Table 1: “25.32~8. 28” Typo, please change ~ to +-.

L180-182: was the MMD fitting done on the 1-hour average size distributions? Please mention.

L184: remove the additional “,” at the end of the sentence.

L190-192: the mixing state of the particles is not discussed in the “next chapter” but in chapter 3.3. the one after. Please correct.

L201-203: it is the first time here, that back trajectories, clustering and NWP was mentioned. Please add a section to the methods, and introduce what methods were used and how. Adding only one reference and no more details is not enough in my opinion.

L203: “measured above 6 meters above the ground.” Please remove the first “above”

L205: not in Table 4, in Table 2, please change.

L215: “All trajectory clusters arrived in the northwestern or southwestern sections of Melpitz” Is A_SBaltic is not north-east?

L201-214: Add a sentence that you will later define AWE and ASE.

L240-241: Add a reference to Fig 5c

L262: “sizes(Liu et al., 2014; Zhang et al., 2020)“ space missing

L293-294: I do not understand this sentence, please rewrite, do you mean here, that the ambient samples were considered for the mixing state analysis and not the thermodenuded ones?

L295-299: why volume? Why not simply the number of BC particles?

L300-302: “The area labeled ‘Small rBC without coating information’ (rBCsmall), as shown in the bottom-left slashed region of each panel of Fig. 7, indicates rBC particles that exhibited neither a positive CT nor a negative CT.” ??? Is this not the category, where simply Dp is too small to determine with the LEO-fit, and therefore you do not have any data on their coating?

Figure 7 caption: the black dashed lines, I do not see y=300 line. Did you not mean x=300 there? And there are more lines than equations. Please check that again.

L308: “most particles exhibited negative (rBCsmall)” Is that not then the category rBCuncoated and not rBCsmall?

L311-314: This explanation should come where you define the rBCsmall, and then you can ignore my comment for L300-302.

L315: it is strange here as well to use the “~” sign to present ranges. Please consider using “-“ here (and everywhere) as well.

L316-317: please change “associated with the smaller size distribution” it to “associated with size distribution shifted towards smaller sizes”

L317-320: very good that you discovered, and describe this problem of not being able to detect the thin coating of the small BC particles, but do you also have an estimate of how much your CT values are biased due to it? If not, would that not be better just to not include any CT values in e.g. the average CT values for Dc below 150? And state that clearly?

L357-358: why are heating systems running continuously in Melpitz in August? Are you sure, that this is the source for the thinly coated BC throughout the day?

L358-360: “However, during evening rush hours after 18:00, the increasing volume in rBCthin and rBCuncoated regions can be found in the size-resolved CT plot forASE and ASNW.” I do see the increase in rBCuncoated, but not at all in rBCthin. You also state in the previous sentence that  there is not really a diurnal variability in rBCthin. I have the feeling that you try to explain even the smallest changes and speculate a bit too much about this figure. If I look at it, I simply see, that the diurnal behaviour is very similar for all airmasses in summer, and there is almost no diurnal variability, the only thing which might be there is that there are a bit higher fractions of uncoated particles during the rush hours, and the amount of moderately and thickly coated during the day. I would not overcomplicate the description of this plot.

L373-385: same for the winter description, that could also be made a bit simpler. And I also do not see everything that is stated there. E.g. “In contrast, high volumes of rBCmoderate or rBCthick were observed at night for AWE and AWNW…” I do see higher volumes of rBCmoderrate from somewhere 3:00 to 12:00, that is not exactly night for me. Please rewrite a bit this part. And another suggestion, or idea: it seems that the most important influence is the local influence on the diurnal variation of CT. Would that not be simpler, if you would not sort this data according to the different air-masses, and just show and discuss the diurnal variability for the combined data? Just an idea.

Figure 9: The times seem strange for the diurnal categories. 00-02, then 03-05, then 06-08… There is always an hour missing. Was the data really sorted like that? Why? Or is that only a typo?

Figure 10: The discussion about this figure would be better right after the discussion on figure 5. Here, I had to go back and forth between figure 5 and 10 to follow it. Please consider it. And again, the discussion seems to me here as well overcomplicated. I would discuss this right after Figure 5 and state that in the average CT there is no real difference in summer between the air-masses, whereas in winter there is. And then after this discuss the size resolved CT figures. Showing what you see there, and also what more that brings compared to the bulk CT.

L396-397: “However, the 75th percentile CT in ASE is lower than the other two air masses during summer” and the 25^th percentile is higher. If you discuss why the 75^th percentile is lower, then you should discuss why the 25^th is higher as well. Again, here, probably speculating less would be better.

L398: “In winter, the influence of the detection limit seems to be less significant.” What do you base this statement on? Please explain. I do not understand it.

L419: “studies(Xu et al., 2018; Wang et al., 2021a)” space is missing

Figure 11 (and same for Figure 8): the blue color of the pie chart does not match the blue of the label, please change.

L439-440: “although the reduced core mass fraction of rBCthin in the TD sample ranged from 8 % to 13 % compared to the ambient sample” I think you do want to write here, that the masss fraction was 8-13% less of rBCthin after the thermodenuder compared to the ambient case. As it is written now, I do not understand. Please reformulate.

L465: “In contrast to summer, the mixing state of rBC in the TD sample did not exhibit evident diurnal variations in winter” What about the diurnal variation of mfcoated? There I do see a diurnal variation.

This manuscript presents a comprehensive analysis of the microphysical properties of refractory black carbon (rBC) aerosols, focusing on their seasonal variability and correlations with air mass trajectories at a central European background site. The data collected, along with the advanced instrumentation employed (e.g., SP2 and thermodenuder systems), provide valuable insights into the physical characteristics and atmospheric interactions of rBC. Overall, the manuscript is well-structured. However, there are a few areas where additional clarification or detail could enhance the clarity of the work.

L75: Why were August and December specifically chosen to represent summer and winter? Are there other reasons for this selection, beyond the observed highest and lowest rBC concentrations?

Lines 9-10: I suggest revising to: "Uncertainties persist in estimating the radiative forcing of black carbon (BC) due to an incomplete understanding of its microphysical properties."

Lines 82-83: I recommend revising to: "Figure 1 presents measurements taken at the Melpitz research site (12°56’E, 51°32’N, 86 m a.s.l.) of the Leibniz Institute for Tropospheric Research (TROPOS), located 50 km northeast of Leipzig, Germany."

L120: Consider adding a brief explanation of the 'LEO-fit' method for readers who may be unfamiliar with this technique.

Lines 165-168: I suggest revising to: "An aerosol chemical speciation monitor (ACSM, Aerodyne Research, MA, USA; Ng et al., 2011) and an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, hereafter referred to as AMS, DeCarlo et al., 2006) were used to measure the bulk chemical composition of non-refractory PM1 aerosol species, including organic aerosols (OA), nitrate, sulfate, ammonium, and chloride."

Lines 175-178: I recommend revising to: "Aerosol components exhibited clear seasonal variations. In summer, organic aerosols (OA) dominated, with a mean mass fraction of 55 ± 13%. In contrast, during winter, the OA fraction decreased to 29 ± 14%, while the nitrate fraction significantly increased to 29 ± 15%, compared to 8 ± 5% in summer." It is important to ensure consistency in the number of decimal places throughout the manuscript.

Additionally, while organic aerosols are the predominant component in summer, nitrate plays a key role in winter. However, the manuscript does not discuss the reasons behind these seasonal variations. I would suggest expanding the discussion on the possible drivers behind these variations. For instance, could meteorological factors like temperature and humidity be influencing the observed seasonal trends, or are they more likely related to anthropogenic activities? This would provide a more comprehensive context for understanding the data."

L200: Why were 100-meter back trajectories chosen for the analysis? A brief explanation of this choice and the methodology used for back-trajectory and wind analysis would be beneficial in the methods section.

Lines 267-268: I recommend providing some evidence regarding local residential wood heating or burning activities in this context. Additionally, consider reorganizing this section for better clarity.

L245-260: The role of biomass burning in influencing rBC size is well addressed. However, do you have any hypotheses regarding why rBC size is smaller in summer, aside from biomass burning? Could other emission sources or atmospheric processes also contribute to this seasonal variation?

Figure 7: Could you provide the rationale for using particle volume in this plot? A brief explanation would be helpful for clarity.

L325: Could you provide further evidence or examples to support the connection between "thinly coated rBC" and liquid fuel combustion?

L410-415: The statement "This non-simultaneous variation between CT and mfcoated may relate to local emissions and atmospheric processes" could benefit from further clarification. Could you elaborate on how these factors might contribute to this variation?

An additional suggestion would be the manuscript could benefit from a more detailed explanation of the observed relationship between rBC size and/or coatings with atmospheric chemical processes. Could the authors elaborate on how factors like photochemistry (or indicator), relative humidity, or long-range transport of pollutants might contribute to the observed changes in rBC physical properties?