General comments: This manuscript presents results from an analysis of an urban aerosol sample collected on a substrate and analyzed using STXM/NEXAFS in a cell that enables RH control. The composition of the particles was characterized along with the changes at high RH. A large fraction of the particles displayed core/shell morphology with an inorganic core and an organic coating. Many particles also had signal for black carbon or soot. Many of the particles took up water and generally the particles became smoother at higher RH. Some information collected at the same time on the submicron aerosol population is also presented. Overall, this is an interesting measurement report on these particles. These types of studies are challenging to do, and I appreciate the care that was taken in terms of the loading on the substrates to enable analysis of ambient urban particles. However, there are many places where broad general statements are made that could be more specific. There are also some locations where more information is needed to clarify the study or the conclusions that are being drawn. After addressing these concerns, I think this manuscript will be of interest to the readers of EGU Sphere and I recommend acceptance.

Specific comments:

1. The introduction lacks information on the location for the sample collection. Please include that.
2. Both the introduction and the conclusions have general statements about a need for physicochemical properties for aerosol particles. One example is given for why this is needed to understand reactive uptake/multiphase processes (N2O5). This is a big field, and I think a stronger and more thorough background on what is known and what is not known in urban multiphase chemistry would really help this article. Right now, I don’t have a good understanding of why the type of information that this study gives is helpful except the general statement that core/shell morphology is important. What are the knowledge gaps that this study is filling?
3. For the paragraph starting on line 66, it is stated that extensive research has been conducted on physiochemical properties of bulk aerosol. I’m a little confused by this statement because these are online techniques and some of the instruments can do single particle analysis. I understand the distinction you are making between these measurements and your single-particle imaging analysis. But I would recommend rephrasing and being a little more precise about what is being measured. The methods you mentioned also have a big strength in statistics that STXM/NEXAFS lacks, and it would be good to present a more balanced comparison between all the different methods.
4. On line 80 it is noted “as it can resolve compositional contrast at the single particle level.”. I’m not sure what compositional contrast means, please rephrase.
5. You note that these samples are frozen on page 5. Do you anticipate any changes during freezing? Have any studies been done to test this for these samples? I know other groups avoid freezing before imaging work.
6. Were the spectra collected on the same particles as the ones imaged for figures 2 and 3? Was there any evidence for beam damage in these samples?
7. For the quantification of the water uptake, was any change in the baseline observed in the oxygen spectra? I’m curious if you can see a thin water layer on the substrate surface and how this changes the cut-off for the particle diameter.
8. In Figure 1 you show a pie chart for the full pollution episode. How does the pie chart in the period around your sample compare? This can be in the supplemental, but I’m having trouble eyeballing it to compare.
9. In Figure 1 the pie chart for POA vs. SOA is shown. How do these factors vary with time during the campaign? Do you expect to see more POA in your sample from traffic?
10. On line 268 it is stated that “EC/ soot (colored in red), found either near the center or the edge of the individual particles”. This is a very general statement and I’m not sure why it is being made. What are the other options for its location in an internally mixed particle except for near the center or the edge?
11. Just below that on line 269 a possible reason for soot on the particle edges is given. Is this the only possibility? 
12. On line 271 it is noted that the soot showed fractal or compact structures of various sizes. I’m a bit uncomfortable with the statement that these are fractal. I don’t think you have the resolution needed to really characterize the soot at that level. I would recommend rephrasing.
13. On line 278 it is noted that soot particles with thin coatings have smaller absorption enhancements compared to thick coatings. This is true, but does that apply here? How does the position of the soot within the particle impact this? Would you expect the same type of absorption enhancement for a soot particle on the edge compared to one in the center?
14. The O/C of the organic is estimated at the bottom of page 11. How does this compare to the O/C measured with the AMS in the same time range?
15. On line 315 it is stated “This phenomenon aligns with a previous study which indicates that the phase transition of phase-separated particles without phase mixing will not cause the redistribution of soot within individual particles..”. I’m not sure what this sentence means, and I recommend rephrasing.
16. For Figure 2 vs. Figure 3, I can see some differences in the particles that are interesting and not discussed in the manuscript. In Figure 3 there are particles in i, v, and vii, that had clear EC/soot in Figure 2 and now lack a clear EC/soot signal in Figure 3 at high RH. Why is this happening? In iii I see a particle that is fully green (org) in Figure 2 but that has some blue inclusions (inorganic) in Figure 3. What is driving these changes and does this say anything about thresholds in the cutoffs for the different components (OC, IN, EC)?
17. On line 344 a calculation of kappa from the AMS is given, please provide a reference for this.
18. On line 359 it is noted that the peak at 288.6 eV is always found at the outer shell of particles. This seems odd to me as I have seen this peak when I have looked at the center of organic particles. Please clarify.
19. It is noted that potassium could correspond to biomass burning. Was there any evidence for biomass burning in the AMS data?
20. On line 381 it is noted that a peak is characteristic of sulfate-rich particles. Please include a citation for this statement or some standards for comparison.
21. In the first sentence of the conclusions, I don’t understand what is being referred to by the word “which”. Please clarify this sentence.
22. The last couple of statements are very broad and the last sentence is not clear to me what future studies are being proposed. I recommend being more specific here.

This study investigates individual particles from an urban environment using single-particle X-ray microscopy/spectroscopy techniques (STXM/NEXAFS). The authors utilize an environmental cell to explore water uptake potential and report that a significant fraction of the particles are internally mixed with organic and inorganic components and less than half of total particles took up water at 86% relative humidity. Major fractions of these particles exhibit core-shell morphology under both dry and humidified conditions. This is an interesting study, and the use of an environmental cell for water uptake analysis provides critical data for understanding how particle composition influences hygroscopic behavior at the functional group level. I have a few suggestions and questions that may help improve the clarity and impact of the manuscript.

Specific Comments:

Why were the water uptake experiments performed only up to 86% relative humidity? Was this due to instrumental or operational limitations?

From the images, it appears that OC particles did not uptake water at 86% RH, whereas in some cases, the IN component of OCIN mixed particles exhibited morphological changes. Can the authors expand the discussion on this observation?

I suggest reorganizing Figures 2 and 3. Instead of displaying particles under dry and humid conditions in separate figures, consider presenting side-by-side comparisons of the same particles before and after humidification. This would help readers more clearly visualize the morphological changes due to water uptake.

The study would benefit from an analysis of the mass growth of individual particles. Did the authors attempt to estimate the mass growth factor using oxygen maps data?

The conclusion section could be strengthened. For example, what are the implications of observing core-shell morphology under dry conditions? Similarly, how might the presence of such morphologies at 86% RH affect atmospheric processes?