Tinorua et al., 2024 “Two-year intercomparison of three methods for measuring black carbon concentration at a high-altitude research station in Europe” provides new findings on the uncertainties and specific artifacts encountered when using different techniques in determining the mass concentration of atmospheric black carbon. The manuscript is based on two years of atmospheric data and fits well with the scope of the journal. The text is well written and structured and derives rather consistent conclusions based on the analysis presented. This said, however, the analysis is rather superficial and presents mainly temporal variability of correlations and statistical uncertainty analysis. As such, the manuscript provides rather minor additions on top of the already published article by Tinorua et al., 2023 in ACP and does not evolve the analysis methodologies further towards the goals of this manuscript. The author should take advantage of the available size distribution data (existing based on Tinorua et al., 2023) and the measured aerosol optical properties (such as SSA and AAE) when evaluating the causes of the observed discrepancies in BC measurements. For example, when speculating on the ultrafine rBC particles from aviation (L342) or on the variability of the multiple scattering correction factor (P20-P21), these additional data could provide further insights for the underlying reasons behind the observations. Therefore, I would like to encourage the authors to incorporate into the analysis both the aerosol number size distribution and the aerosol single scattering albedo, as additional parameters to consider when different artifacts are evaluated. Based on Tinorua et al., 2023, the measured aerosol SSA was rather high (>0.9). How does the CxMAC value depend on the aerosol optical properties and the aerosol particle size distribution? Do the mass correlations present additional dependence on them?

In addition, I have some minor comments for the authors to consider, presented below.

Specific comments:

• Please, be specific when defining “MAC” – do you mean MAC of the aerosol or of the material black carbon? Especially in the introduction (lines 49-52) it is slightly confusing what is meant by MAC.
• L54 Note that “optical method” could include also other than filter-based absorption measurements.
• L 358-360 Consider simplifying and sharpening the key point of the sentence. For example, it seems rather intuitive that possibly “the SP2 missed the detection of a mode that is centered at lower diameter than the lower limit of detection of the SP2”.
• L 367-368 Referring to a study by Wei et al., 2020, the MAC values provided here are now a bit different than in introduction, also specifying that the MAC is for BC (material?). Please double check this reference and the correct values.
• L388 Please provide a bit more information on the model and how it was applied, e.g. for which altitude and what meteorological data were utilized.
• L394 I would not recommend calling this observed behavior a “trend”.
• L394 Explain what “measurement artifact” is suspected to explain the difference.

This study provides an intercomparison between three BC measurement techniques, using an aethalometer AE33, a thermal-optical analyzer Sunset and a single-particle soot photometer SP2. This work provides useful information as it evaluates the agreement between those three instruments with a 2-year dataset of measurements at a high-altitude research site and discusses possible reasons of biases. The text is well-structured and the results support well the conclusions.  However, the following issues need to be addressed:

1. Table 2; first row & column “measurement uncertainty”: There should be also some uncertainty in the mass calibration factor applied. Please add also a reference here.
2. Line 110: Can you please double-check that this is the correct LOT of fullerene soot and add the relationship that you used for the mass calibration?
3. Line 119: This needs to be rephrased. R_fit/meas is the fraction of the estimated ambient rBC mass that is outside of the SP2 size detection limit.
4. Line 122: The definitions of M_{rBC, fit} and M_{rBC, meas} need to be more clear.
5. Lines 134-136: Any uncertainties in BC mass calibration should be added.
6. Lines 152-155: The abbreviation used for Pyrolytic Carbon is not consistent in the text.
7. Section 3.1:

The basis presented in the paper for the trimodal fit was the better representation of the size distribution within the SP2 range: “As a first conclusion, the trimodal curve generally better follows the measurements, and in particular for rBC diameter above 150 nm.” However, such an improvement is always expected for fits with increasing numbers of parameters. To fully justify the reduced uncertainty associated with the more complex fit, it is important to have confidence that the physical basis for the additional parameters is justified. In this case, this reviewer wonders if there is a different explanation for the small structure quite consistently around 150 nm except in winter.  Could the authors provide more information about the calibration of the incandescent detectors? Was just one detector used, or could there be a gain shift around this diameter? Was a linear relationship between peak height and rBC mass used, or a more complex relationship? Finally, was there any additional information allowing separation of these assumed modes?

Note that the manner in which the largest mode is being dealt with here (i.e. fitting a poorly constrained shoulder and including the resulting inferred mass) contrasts with previous approaches, in which the larger mode with some coarse-mode contributions was not presented in the context of the accumulation-mode rBC. Here, in Fig. 1, it’s clear that the unimodal fit approximates the larger-particle mass contributions without significantly extrapolating to the coarse mode.

1. Line 207: Better change to “coarser mode” (also later in the text, e.g. line 258).
2. Line 218: Here M_{rBC, fit} is not defined as in Equation 1. Please correct either the symbol or the definition.

However, I recommend when evaluating the different fitting approaches, to present changes on the ratio of the rBC mass under the whole fitted area to the rBC mass measured over the size range covered by the SP2. This will give a straightforward comparison between the mass correction factors that you had to apply for estimating the total (accumulation?) rBC mass and will be better connected to the R_fit/meas that you discuss later in the text.

1. Line 228 and later in the text: The largest mode is around 400 nm (given the fitted peak at 377 nm).
2. Line 236: The sentence “larger differences in rBC…” is better to be removed as this statement is also given in lines 239-240 with the right reference (i.e., Fig. 1).
3. Line 281: Should be “M_EC vs M_eBC”
4. Line 400: Refer to Figure10a