The authors Testa et al describe a series of ice nucleation measurements conducted on authentic aircraft engine soot and accompanied by chemical composition and morphology analyses for relation to INA. The extensive chemical and physical characterization of aviation soot support plausible explanations for observed INA and the results are important in the contexts of parameterizing atmospheric INA and understanding the many prior studies on soot and soot proxy INA. I have two comments/questions about the methodology and representativeness of the results and a few more minor comments below and would likely suggest publication after these comments are addressed.

The authors analyze soot produced by P&W and CFM International engines using Jet-A1 type fuel. Approximately how large is the market/usage share for these engines and fuel type? Can the authors comment on how generalizable the INA trends might be to other engines or fuel types?

The soot particles used for INA analysis undergo coagulation prior to analysis and the authors extrapolate results from coagulated particles to smaller particles that are more likely to be present in the atmosphere. Based on the TEM analysis, are there any differences in overall particle morphologies between uncoagulated and coagulated soot that may affect INA? The authors discuss that compaction of soot particle aggregates may decrease INA (lines 412-420). Based on the TEM images in Figures C1 a-e and k-o, there appear to be some differences in particle morphology (e.g., fractal dimension) between uncoagulated and coagulated particles. Can the authors comment on any potential differences in particle morphology and INA as a result of coagulation? Can soot morphology change during impaction and could this vary with particle size?

Line 40: these are cavities and voids formed by soot spherule overlap, compaction, and aggregation, correct? Clarify this, to make clear that cavities aren’t present on the surfaces of individual spherules.

Line 41: clarify that this initial freezing is step two in the list presented here.

Line 44: approximate size of the critical ice cluster?

Line 47: clarify “overlap:” the extent to which primary spherules are “pressed” together by sintering and compaction?

Line 56: is the sulfur internally mixed with soot present as sulfuric acid? This line implies that it is but is not explicitly clear.

Lines 64-65: I’m a little confused by the phrase “averaged aggregate D_PP.” From the context, it sounds like this sentence is saying that D_PP increases in size with increasing thrust, but I’m not certain what the modifier “averaged aggregate” means for D_PP.

Line 98: could the size-dependency of particle losses impact the INA measurements in this work? Might particles losses be significant for this analysis given the detection limits of the HINC and low INA of the samples?

Line 105: to what extent is rapid coagulation of aircraft aviation soot is expected under ambient conditions?

Line 157: please include a description of how equivalent spherical diameter, convexity, and circularity are calculated and/or defined.

Line 190: given the spot size, each measurement is expected to originate from an individual particle? This is referenced later but should be clarified here.

Line 267: how many experiments, exactly?

Line 323: please clarify the meaning of the first sentence of this paragraph.

Line 372: Can sulfur be present in samples in forms other than sulfuric acid, for example bound to carbon? What sulfur bonding environment presents an overlap at the 538 eV value? Are there any signals clearly attributable to sulfur or is the expected signal (based on the atomic %, line 405) below the limit of detection?

Line 405: are the H2SO4 wt % in Figure J1 relevant to the expected amount of sulfuric acid and water based on the sulfur atomic %?

Lines 515-516: is this assumption on real-world coating extent based on SOA and sulfate that condenses on particles after emission, or engine emissions condensing onto soot due to low temperatures? The ambient atmosphere is also a much more dilute environment than the chamber, which would disfavor thicker coatings.

 Figure C1: please add length scale labels that are more legible.

Testa et al. comprehensively analyze the aviation engine-emitted soot aerosol particles to understand their ice nucleation ability at cirrus cloud temperatures. Overall, the experiments were well-designed, and the results support their conclusions. Moreover, this study can fill the knowledge gap in the indirect climate effects of soot emitted from aviation engines, which might be an unignore source of soot at high altitudes. I only have some minor comments and questions that can help improve the manuscript. Thus, I recommend publishing it with minor revisions. Please see my comments below:

Minor comments:

1. Could the authors comment on how high-altitude ambient conditions might affect the results since the experiments were conducted at the ground level?
2. I am curious to see the morphology change with and without CS based on the SMPS and Tandem DMA-CPMA measurements.
3. Do you expect any physical (e.g., partition on the soot and cause compression) or chemical reaction (oxidation) to happen inside the tank?
4. Please note that C, N, and O are semiquantitative in EDX. Moreover, some C and O signal might come from substrates.

In this study Testa et al. investigates the ice nucleation activity of aviation soot that were sampled directly from the modern in-use commercial aircraft engines. The effects of engine thrust and soot particle size, mixing state, physical and chemical properties were tested in various experiments using a continuous flow diffusion chamber. Their results indicate that the overall ice nucleation abilities of real aviation soot are not so high as previously thought since in the latter case surrogate of aviation soot were used to estimate their ice nucleation activity. The study is very interesting and meaningful for further evalutating ice formation and climate effect associated with aviation. I recommend the paper to be publised in ACP with minor revisions.  What I suggest is that the authors may consider adding a short comment/discussion about the implicationand and usage of their experiment results and data for modelling work in the Conclusions.