Interactive comment on “ Towards the development of standard reference materials for soot measurements – Part 1 : Tailored graphitized soot ”

Response to All Reviewers We would like to than M. Viana and the two anonymous reviewers for their careful evaluation of the paper and their helpful comments and suggestions. All three reviewers have asked for more information about the methodology that was used for sampling the reference material with the TOA technique. We have expanded that section with a more detailed description of how the material was sampled.

Response: It is written on page 1752 of our paper: "Experience indicates that a 1-3% variation in the coating can be expected" Reviewer's comment: Page 1758: it would be useful if the authors could provide additional data on which specific SRM they suggest to be used as an international standard, out of those listed in Table 1. Do they suggest to provide all, and each lab should pick one? Or are the different SRMs suited for different purposes? From the end-user perspective, it would be useful to know exactly which standard to use.
Response: In the summary we have now recommended that the range of OC/TC ratios that we used in the initial tests would be a reasonable set of standards. All ten would not necessarily be needed as there does not seem to be any sensitivity to the the type of OC. It is necessary to choose a set of the smaller percentages of OC/TC to establish the sensitivity of a particular TOA methodology and as high a percentage as possible to look for non-linearities.
Reviewer's comment: Page 1759, line 12: the authors state that results after 6 months were "very close to nominal". Could they provide specific data, what concentrations were measured and what ratios to the originally measured concentrations (or the nominal values).

Response to Reviewer #1
Reviewer's comment: The authors present a detailed description of several instruments that allow the measurement of soot, EC, BC but without using those instruments in the validation processes. Did the authors test those techniques with the reference material? Why not including the SP2? Are those tests planned to be presented in the paper Part II? If yes, why including, in this paper, so many details about the other instruments?
Response: The authors concur and have removed the majority of the text related to the instruments that measure soot properties other than TOA that measures EC/OC. Given that at the moment the GTS SRM is best suited for excercisng TOA techniques, the Part II paper will be an extensive evaluation of these techniques using the GTS SRM and an evaluation of the SP2 and optical properties instruments will be reserved for a later time.
Reviewer's comment: What are the EC concentrations measured? It is well known that the split between EC and OC in this instrument is the major uncertainty, and from Figure 5 and 6 it can be observed that the EC concentration are far higher than those observed in the atmosphere. Is there a reference material with the level of atmospheric concentrations?
Response: EC concentration is determined as TC-OC since there is no other carbon in the produced samples. All samples of SRM were fabricated with the purpose of validating the measurement protocol for separating EC from OC, not with the idea of replicating atmospheric relevant material. In the future, the fabrication technique will allow more complex, atmospherically relevant material to be produced.
Reviewer's comment: Page 1746: It is not really clear from this paper (and from this paragraph) if there will be a common reference for all techniques or if the conclusion is already that several references will have to be used and thus the different techniques C1435

Response to Reviewer #2
Response: The reviewer's response was primarily associated with the opinion that then paper was not sufficient to stand on its own and should not be published without the follow-on paper. Although we have addressed this opinion in general, in our opening response to the reviewers, and have also expanded the introduction of the paper to justify the separation of the two parts, we would like to address a number of the reviewer's points that we think are critical for arguing our case.
Reviewer's comment: I don't see much value in this manuscript without the proposed follow-on work. While this manuscript does describe well-enough their approach to coating soot with organics, this alone is not of great value, in my opinion. The excitement lies, as acknowledged by the authors, in using the SRM to reduce uncertainty in measurements of OC/EC mass and absorption coefficient, and ultimately uncertainty in our understanding of air pollution health impacts and climate change. This manuscript doesn't begin to take that step, and more importantly, I am not convinced that the SRM that the authors have developed will be able to reduce measurement uncertainty.
Response: Whereas we understand the reviewer's opinion and the reasoning behind it, we respectfully disagree; however, given that other readers might share this viewpoint, we have modified the paper so as to make clear why we have only focused on the candidate SRM and not on the evaluation of the SRM beyond illustrating its capability. We believe that the application of this SRM will reduce the uncertainty in measurements of OC/EC because at the moment there is no technique for validating the protocols that are use in the currently used TOA methods.
Reviewer's comment: The main uncertainty in thermal-optical analysis of OC/EC lies in the complexity of ambient OC, which is refractory and prone to forming char when heated in the thermaloptical analyzer, as summarized by the manuscript and the stud- Discussion Paper ies cited. The OC/EC ratio of ambient particles ranges from approximately 50% to >95%. The OC is partly refractory and possibly light-absorbing, especially when the OC is secondary-organic aerosol or from biomass burning, and, therefore, it turns to EC-like char during thermaloptical analysis. Since the OC"EC in ambient particles, the EC-like char can be much greater in mass than the EC originally present in the particle sample, leading the large uncertainty in quantification of EC. That, in brief, is why it is difficult to measure EC. My trouble with this manuscript is that the authors do not make a convincing argument -they really don't explain it at all -exactly how their SRM will tackle the problems of the thermal-optical analysis method.
Response: Thanks to the reviewer's observations, we have expanded the manuscript to more clearly describe how the GTS SRM can serve as the first stage validation of the TOA protocols. As we point out, there are at least three types of TOA that are currently implemented around the world and all three use different protocols with respect to how they analyze the sample yet there has never been any published data that tests these protocols against one another using well characterized mixtures of OC/EC that where one can say what the exact proportion of OC and EC is. What has been shown is that the techniques differ in their separation of EC and OC, sometime quite a lot, when using ambient samples or when using "reference material" like urban dust. What has not been shown is why. All of the techniques uses sucrose or other organic material to calibrate total carbon but none of the techniques have validated that they correctly separates EC from OC because there is no reference that can be used to determine what the true answer is. This is where the GTS SRM acts as the first stage reference for evaluating the validity of the TOA protocol.
Reviewer's comment: Since their SRM has OC/EC ratios that are mostly <1% and has relatively simple OC composition that is similar to products of fossil fuel combustion -rather than the more chemically complex, refractory, and prone to forming char OC of secondary-organic aerosol and biomass smoke -it is true that their SRM is unlike C1439 AMTD 3, C1430-C1442, 2010 Interactive Comment Full Screen / Esc

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Interactive Discussion Discussion Paper ambient carbon-containing aerosol in composition and it is likely true that their SRM will not challenge thermal-optical analysis methods as they need to be challenged to reduce measurement uncertainty. Their simplifying step of choosing off-the-shelf OC compounds is the main problem, in my opinion (and frankly, I'm not sure there is any way to get around this problem b/c it is the complexity of ambient OC that prevents us from putting it in a bottle and using it to coat reference soots).
Response: As is already discussed, the "off the shelf" OC compounds are considered valid surrogates for the different, general, functional groups of ambient OC, as described by McFiggan et al. As we now have added to the revised manuscript, there is nothing that prevents us from creating much more complex GTS SRMs in the future, or from increasing the depth of the coating. This first paper was to introduce a unique concept that has not been previously discussed in the open literature with respect to calibrating/validating TOA techniques. What we show in the current paper, for at least the case of the Sunset TOA the response does not appear to be affected by the type of organic.
Reviewer's comment: Second, the OC coverage is more than an order of magnitude lower than in ambient aerosols. The author's have chosen an important topic, but the major flaw of this manuscript is that the benefit of the developed SRM is, in my opinion, greatly overstated and certainly unsupported by its content.
In the end, it is not clear to me that the SRM will do more than help to strictly calibrate thermal-optical analysis instruments, i.e., confirm that they quantitatively count carbon atoms. The SRM is not needed for this b/c these instruments are already suitably calibrated with sucrose or glucose samples that are easily prepared in the laboratory. powder on a filter, powder suspended in a solvent?

Response: As now included in the revised text, the SRM is always in the form of a dry powder.
Reviewer's comment: Similarly, the manuscript does not explain how the SRM was applied to filters for thermal-optical analysis or suspended for size-distribution measurement, but it should clearly explain these.
Response: As now included in the revised text, the procedure of preparing the sample has been provided in greater detail.
Reviewer's comment: The manuscript concludes with a statement that the next manuscript will discuss how the SRM will be used to calibrate the thermal-optical analysis instrument, but this has already been shown in the current manuscript, the results are shown in Figure 7.
Response: No, we have only used the preliminary evaluation with a single technique to illustrate how the SRM is used to validate the methodology for separating EC from OC.
Reviewer's comment: This current manuscript, in my opinion, should tackle the real issue -the difficult problem of separating correctly OC and EC in ambient samples (as already noted above). A much more minor concern, since the manuscript really focuses on thermal-optical analysis, is that the manuscript doesn't apply the SRM to methods of measuring absorption. The discussion of these absorption methods in the introduction could, therefore, be deleted.

Response:
We have removed the discussion on absorption measurements from the manuscript and now focus only on TOA since we feel that the SRM that has been developed can be immediately applied for these techniques but further development is needed to use it to calibrate/validate measurements of light absorption.