the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Dec 2024
Microphysical properties of refractory black carbon aerosols for different air masses at a central European background site
Abstract. Uncertainties remain in estimating black carbon’s (BC) radiative forcing due to a limited understanding of its microphysical properties. This study investigated the physical properties of refractory black carbon (rBC) at the central European background site Melpitz during summer and winter, using a single particle soot photometer coupled with a thermodenuder. Different air masses associated with distinct rBC properties were identified in both seasons. In summer, rBC exhibited a similar mass concentration (~0.16 μg m-3) among different air masses, with the smallest mass median diameter (MMD) of rBC overserved in the long transportation from the northwest (140 nm), while in winter, the highest concentration (1.23 μg m-3) and largest MMD (216 nm) were both observed in the air mass influenced by the easterly winds. Thickly coated rBC fractions increased during the daytime in summer, indicating the photochemical processes significantly influence the rBC mixing state. In winter, a higher fraction (27 %) of rBC with thick coating in the cold air mass compared to the warm air mass (14 %) suggests the contribution of residential heating emissions to the mixing state. Most rBC retained a low-volatile coating in the thermodenuder samples (58 % mass fraction). In summer, photochemical processes also contribute to coating volatility, showing a higher fraction of rBC particles containing low-volatile coatings during the daytime. In winter, low-volatile coatings showed no significant diurnal variation and were more dependent on ambient temperature. Therefore, rBC coating volatility in winter is more influenced by emission sources, particularly residential heating, rather than atmospheric processes.
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RC1: 'Comment on egusphere-2024-3539', Anonymous Referee #1, 03 Jan 2025
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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 25th percentile is higher. If you discuss why the 75th percentile is lower, then you should discuss why the 25th 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.
Citation: https://doi.org/10.5194/egusphere-2024-3539-RC1
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