|The authors gave satisfactory replies to almost all the reviewer questions and modified their paper accordingly. The comparison with other ice core results is now clearer in terms of the analytical methods. The reviewer strongly encourage the future use of more characterize calibration materials in order to minimize the uncertainties in the methodology, as also suggested in many recent papers.|
However, there are still a couple of questions that should be addressed before the publication of the paper:
1) In the paper it is still worryingly unclear the SP2 calibration procedure, which is essential in order to have realistic absolute values of rBC concentration. In your response it is written that the SP2 calibration was performed by DMT and that “We used a custom code written in R and CRAN R open-source software to filter broadband incandescence peaks and convert incandescence peaks to mass using the calibration relationships (supplied by Droplet Measurement Technologies)”. So the question is: The DMT supplied the calibration coefficients (as it happens usually)? If yes which calibration material was used by the DMT during the SP2 calibration? The relation between incandescence and mass is strongly dependent from the choice of the calibration material in terms of the SP2 response to that kind of material (the standard’s optical properties, i.e. its refractive index). Usually DMT performs the calibration of the SP2 by or selecting the standards particles size with a DMA and then using the density for calculating the single particle mass, or directly selecting the mass (with an APM or similar instruments), before the SP2.
If DMT supplied the calibration curve, how the calibration results reported in Figure S4 were used? Only for calculate the nebulization efficiency of the CETAC system?
Moreover, how did you make the standard? Did you assume a 100% content of BC particles in the original standard? This is a key step in order to know the actual BC concentration in a solution. No EC (Elemental carbon) measurement was performed to ensure this (as done for the Aquadag and Fullerene in other studies)? Even for Aquadag the total content of refractory BC particle is not 100%.
If you have used the “new” standard for making the calibration of the SP2 then it will be important to make and estimate of the possible degree of underestimation/overestimation of the absolute concentration values caused by the use of a different standard. In the paper “Moteki, Nobuhiro, and Yutaka Kondo "Dependence of laser-induced incandescence on physical properties of black carbon aerosols: Measurements and theoretical interpretation." Aerosol Science and Technology 44.8 (2010): 663-675” a comparison of SP2’s responses to different calibration material is given. Please use their results to establish a plausible degree of variability of your absolute concentration values if you were to use other standards (just use their slopes variability and change your calibration curve accordingly).
For the authors knowledge some sentences from the methodological paper from Lim et al. (2014) are reported, where the use of the Fullerene soot is strongly encouraged because of the similar SP2 response with atmospheric BC: “Recent studies reported the similarity between the physical properties of fullerene soot and ambient rBC for urban areas in Switzerland and Tokyo (Laborde et al., 2012a; Moteki and Kondo, 2010). The sizes of fullerene soot were selected by their electrical mobility diameter (Dmob) before entering the SP2 inlet by a differential mobility analyzer (DMA). The mass of mono-dispersed particles was empirically calculated using the relationship between the measured mobility diameter and the effective density of standard materials (Gysel et al., 2011; Moteki and Kondo, 2010).” (Lim, S., et al. "Refractory black carbon mass concentrations in snow and ice: method evaluation and inter-comparison with elemental carbon measurement." Atmospheric Measurement Techniques 7.10 (2014): 3307-3324.). The decision of using another standard that have been never properly analyzed makes the results reported in this paper less reliable in terms of absolute values.
2) Please show the plot related to the change in sensitivity also using the aging curve, so that it will be much easier to understand for the reader where there could possibly be a higher degree of uncertainty in the results.
3) Why are you using the units “rBC (10…)” in the y-axis of the Figure 4? Are you sure it is not “Incandescence Signal”? Please add the unit also in the plot of the calibration curve.
4) You should report the plot of the mass distribution in “dN/dlnDp”, not in “dN/lnDp” (use the logarithm of the diameters in the ratios).
These new requests won’t affect the interesting results of this paper but are still very important in order to be able to compare the results of this paper in a more accurate way.