Two-year measurements of Black Carbon properties at the high-altitude mountain site of Pic du Midi Observatory in the French Pyrenees
Abstract. Black Carbon containing particles (BC) are strong light absorbers, causing substantial radiative heating of the atmosphere. The climate-relevant properties of BC are poorly constrained in high-elevation mountain regions, where numerous complex interactions between BC, radiation, clouds and snow have important climate implications. This study presents two-year measurements of BC microphysical and optical properties at the research station of Pic du Midi (PDM), a high-altitude observatory located at 2877 m above sea level in the French Pyrenees. Among the worldwide existing long-term monitoring sites, PDM has experiences limited influence of the planetary boundary layer (PBL), making it an appropriate site for characterizing free tropospheric (FT) BC. The classification of the dominant aerosol type using the spectral optical properties of the aerosols indicates that BC was the predominant absorption component of aerosols at PDM and controlled the variation of Single Scattering Albedo (SSA) throughout the two years. Single-particle soot photometer (SP2) measurements showed a mean mass concentrations of BC (MBC) of 35 ng m−3 and a relatively constant BC core mass-equivalent diameter of around 180 nm, which are typical values for remote mountain sites. Combining the MBC with in situ absorption measurements yielded a BC mass absorption coefficient (MACBC) of 9.8 ± 2.7 m2 g−1 at 880 nm, which corresponds to an absorption enhancement (Eabs) of 2.4 compared to that of bare BC particles with equal BC core size distribution. A significant reduction of the ratio ∆BC / ∆CO when precipitation occurred along the air mass transport suggests wet removal of BC. However we found that the wet removal process did not affect the size of BC, resulting in unchanged Eabs . We observed a large seasonal contrast in BC properties with higher MBC and Eabs in summer than winter. In winter a strong diurnal variability of MBC (Eabs) with higher (lower) values in the middle of the day was linked to the injection of BC originating from the PBL. During summer in contrast, MBC showed no diurnal variation was rather constant despite more frequent PBL-conditions, implying that MBC fluctuations were rather dominated by regional and long-range transport in the FT. A body of evidence suggests that biomass burning emissions effectively altered the concentration and optical properties of BC at PDM, leading to higher Eabs in summer compared to winter. The diurnal pattern of Eabs in summer was opposite to that observed in winter with maximum values of 2.9 observed at noon. We suggest that this daily variation results from photochemical processing driving BC mixing state rather than a change in BC emission source.
Such direct two-year observations of BC properties provide quantitative constraints for both regional and global climate models and have the potential to close the gap between model predicted and observed effects of BC on regional radiation budget and climate. The results demonstrates the complex influence of BC emission sources, transport pathways, atmospheric dynamics and chemical reactivity in driving the light absorption of BC.
Sarah Tinorua et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-570', Anonymous Referee #1, 28 Apr 2023
- RC2: 'Comment on egusphere-2023-570', Anonymous Referee #2, 04 May 2023
- RC3: 'Comment on egusphere-2023-570', Anonymous Referee #3, 10 May 2023
Sarah Tinorua et al.
Sarah Tinorua et al.
Viewed (geographical distribution)
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.
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.
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 (MrBC,DrBC, 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
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