Dear authors, I would like to thank you for giving me the opportunity to review your work.

I invite you to read my comments in the attached file.

Regards

The manuscript describes an intercomparison of several instruments for measuring particle mass from the output of a Mini-CAST generator. The comparison includes both offline and online instruments based on various measurement principles. The results from the instruments are compared against offline gravimetric determination of the particle mass from a filter deposition. An offline OC/TC analysis is also performed to support the analysis of the results. The Mini-CAST is run with varying amounts of oxidation air flow to achieve different levels of organic content, total mass, and different particle size distributions. A potential complication here is that all the parameters are changed together, which can complicate the analysis, but the authors make convincing arguments for the dependence of the measurement results on the chancing parameters. The different methods show good agreement, to approximately 10% from the reference values, with the caveats that PPS requires a size dependent correction and with MA300, different wavelengths correlate with the total particle mass, depending on the OC/TC ratio.

I believe the topic is suitable for the journal and would be of interest to its readers. The measurements and the analysis are described thoroughly, and support the conclusions mostly, but there are a few unclear points I would ask the authors to address. I have also listed a few stylistic or typographical points at the end.

line 182: Any comments on why the PPS correctly estimates the largest particles from Mini-CAST, if the size distribution from the engines, which were used in the calibration, is mostly smaller particles (according to the geometric mean at line 119)?

figure 6 (right): I don’t quite understand the bottom axis here. Should it not match the last column of table 1? For example, the point marked 1.2 has a median diameter of roughly 60 nm in table 1, but in the figure it looks to be less than 50 nm. The point marked 1.25 also appears to be out of place in the figure. It’s “median diameter” in the figure is closer to the mode 1 diameter from table 1, to my eye. The axis title is also unclear.

line 189: The statement that the concentration ratio of PPS/gravimetric is independent of the median diameter may be a little misleading, since the diameter is very close to each other in this range of oxidant flow rate. That is, based on the table 1 data, unless the figure 6 data is correct and I have misunderstood something here.

line 238: Did you try to evaluate the MA300 data from the different wavelength channels together? For example, could you try to estimate the total mass from the BC and BrC fractions together, with less dependence on the OC/TC ratio?

Minor comments:

line 45: Although it becomes clear from context later, it is unclear here, what instrument the sentence “and considered as the reference measurement” refers to.

line 47: you should mention the OC/TC measurement done from the filters here. Right now, it sounds like SMPS could do that as well.

figure 5: I would suggest changing the legend entry for “Improve A” to “OC/TC analysis” or mentioning the protocol also in the figure caption.

figure 7 (left): there seems to be a typo in the bottom axis title.

The article compares the mass concentration of the mini-CAST soot generator based on an experiment that evaluates both online and offline measurement methods. The primary motivation for the study appears to be the determination of correction calibration parameters for mass calibration of the PPS instrument, which is based on aerosol active surface area.

Although the experiments were conducted with technical accuracy, I believe the experimental design should have been more robust. In addition, the conclusions drawn are overly general and simplistic. Therefore, I do not recommend that this manuscript be accepted for publication in Atmospheric Research.

Below, I outline several points that do not meet the standards of a research article and should instead be considered for publication as a technical report in a more appropriate journal once the following revisions are implemented.

1. Experimental setup, heated sampling line, and mass of PM from OC/EC analysis:

The experimental setup, as described in the text, states: "This setup includes the soot generation source, the mini-CAST, followed by two heated lines maintained at 180 °C. One line is used for filter sampling, while the other feeds a dilution system that distributes the diluted and cooled aerosol to various measurement instruments: the TEOM, PPS, MA300, and SMPS." However, the schematic only shows a heated line leading toward the offline sampling system. This raises an immediate questions: Was the heated line actually used in both branches (online and offline), or only in the offline branch? Why was a heated line used at all, given that it can significantly alter the chemical and physical properties of aerosols?

For comparison, if the OC/EC analysis followed the IMPROVE protocol—where the OC1 fraction is defined at 140 °C—this fraction would be entirely lost within a 180 °C heated line. Furthermore, heating the sampled air can lead to misinterpretation of pyrolyzed carbon (PC) in thermal-optical OC/EC analysis, as the initial reflectance from the filter may already include pyrolyzed material, resulting in an overestimation of elemental carbon (EC).

If one sampling branch was heated and the other was not, this introduces a fundamental inconsistency that compromises the comparability of the results.

Additionally, the authors should provide a more detailed explanation of how they calculated mass concentration based on OC/EC analysis. Assuming that the total aerosol mass (PM) is equivalent to the mass of carbonaceous aerosols (CA), the following equation applies:

PM=CA= OA+EC= TC⋅(OA/OC)−EC⋅(OA/OC−1)

where:

OC = OC1 + OC2 + OC3 + OC4 + PC

EC = EC1 + EC2 + EC3 – PC

OA/OC is the organic aerosol to organic carbon ratio, which can be obtained either through complementary measurements (e.g., ACSM for OA, OC/EC analysis for OC) or sourced from literature. What OA/OC ratio was used in the mass estimation? Was this ratio measured directly or adopted from previous studies? How do the authors justify the use of a heated line, and how do they address the uncertainties associated with the loss of the OC1 fraction and the potential misclassification of pyrolyzed carbon in thermal-optical analysis?

2. Experimental Design and Showing  Method Equivalence

In demonstrating the suitability of using the PPS, two critical steps are missing. The reader is concerned by the simultaneous variation of two parameters at the aerosol source—namely, the mass concentration and the aerosol composition as a function of the oxidation flow rate in the miniCAST. This affects the OC/TC ratio and introduces uncertainty. Additionally, there is a lack of evidence demonstrating the equivalence between the reference gravimetric method and the TEOM online method.

As a result, the reader cannot apply the calibration factors for the PPS in their own experiment, since two parameters are being altered simultaneously. Therefore, the experiment should be redesigned so that, for a fixed OC/TC ratio at the source, only the mass concentration is varied. This would allow for the determination of calibration factors under controlled conditions. This procedure should be repeated for multiple OC/TC ratios. Only in this way can the reader reliably apply the calibration factors in their experimental setup.

Furthermore, prior to this, the equivalence between the TEOM and the offline gravimetric method for measuring mass concentration should be demonstrated. Establishing this equivalence would validate the TEOM as a reference method against which all other measurements (SMPS, MA300, PPS) can be compared. It would also confirm the correct operation of the diluter and eliminate concerns regarding potential heating effects in the sampling line. For the comparison between the offline gravimetric method and the TEOM, authors should apply, for example, equivalence assessment tools as outlined in the European standard EN 16450:2017, which specifies requirements for automated measuring systems for particulate matter,  including slope and offset of the orthogonal regression assessment. Only after establishing this equivalence can the results from SMPS, MA300, and PPS be interpreted with confidence and used to draw scientifically robust conclusions.

3. Lack of critical evaluation in the interpretation of filter photometer measurement results

The comparison of mass concentration measurements based on attenuation using the MA300 filter photometer is overly superficial. When using filter photometers with laboratory-generated aerosols, it is essential to carefully consider filter loading corrections, and the C parameter (multiple scattering correction factor), which depends not only on the properties of the filter substrate but also on the optical characteristics of the aerosol.

These optical properties include aerosol coating effects, which may be transparent or light-absorbing, and cannot be adequately described by a simple OC-to-TC ratio, as suggested in the manuscript.

Moreover, filter loading effects can differ between the two wavelengths used in the MA300. Therefore, the conclusion that one can simply select the wavelength that yields a result within ±10% of the gravimetric mass concentration is methodologically flawed and inappropriate for a journal such as Atmospheric Research. 

The authors should provide a detailed description of the parameters used in their analysis, including the multiple scattering parameter, loading compensation parameter, and the mass absorption cross-section. Furthermore, a thorough comparison of different mass concentrations at a constant OC/TC ratio should be conducted and clearly presented.

Absorption at 880 nm can serve as a reliable indicator of black or elemental carbon mass across varying OC/TC ratios. In contrast, the signal in the UV range is expected to increase primarily due to enhanced scattering at higher OC/TC ratios.