The research provides direct observation and analysis of the dynamic mixing state and phase transitions of submicron particles containing inorganic and surface-active organic constituents in response to relative humidity (RH) cycling. The research also conducted on the interaction between phase transitions of aerosol particles and related hygroscopicity, and CCN activity which could make a significant contribution to the field. This approach allows for a comprehensive understanding of the dynamic evolution of inorganic-organic particles under fluctuating atmospheric conditions. This study fits within the scope of the journal Atmospheric Chemistry and Physics. However, after carefully examining and revising this article in light of the comments below, it could be considered for publishing in ACP.

Major Comments:

- In experimental section, it's important to ensure the ESEM reproducibility. It's unclear from the conclusion if the experiments were repeated and the results were consistent. Further verification in this aspect might be needed. Also, how did you determine the RH step from (a) to (c) of Figure 3? This progress occurred a really narrow range within 0.8% RH. Does it make sense to determine the decimal point of RH range because the authors used 2-3% RH/min condition for RH changes? Furthermore, in Figure 4, if you change the brightness and contrast of the images, do you still obtain same results?

- Please provide the size range of the aerosol particles investigated during the experiments and discuss/compare the size effect with previous study of Freedman et al. in the result section. A figure would be helpful.

- The authors used different temperature range of 0.1 – 25 ◦C in the experiments because temperature has negligible influence on the LLPS of AS-organic and NaCl-organic particles. Is this statement still valid for the submicron particles? Please discuss it.

- The authors showed that LLPS always occurs when the O:C ratio is 0.4 or below, and never when the O:C ratio is higher than ~0.57. This is inconsistent with previous studies (Bertram et al., 2011 ACP; You et al., 2013 ACP; Song et al., 2012 GRL). Thus, the authors should make a careful comparison with the literature and make a clear conclusion. I recommend that all experimental data points including the previous studies should be shown in Figure 5.

- The study also observed that inorganic salt-surfactant systems without LLPS undergo solid phase separation after efflorescence, demonstrating distinct separated phases (pg. 14). This is interesting and pretty new. Thus, I suggest to show the result in the main text and expand the discussion. Figure S2 should be clearer.

Minor Comments:

- The manuscript has several instances where grammar and language use could be improved for clarity. This includes sentence structure, punctuation, and the use of certain phrases.

- Lines 132 – 133: The sentence needs to be clearer. Please expand the discussion.

- Ensure that the authors used consistent terminology throughout the paper. For example, the terms "inorganic salts-surfactant systems" and "inorganic−surfactant particles" appeared to refer to the same thing, and using them interchangeably may cause confusion.

- Implication: Strengthen the conclusion by making a clear connection between the results and the larger implications for the field of atmospheric chemistry and physics. Providing a more thorough discussion on how your findings could help reduce the uncertainty of model estimation on the global radiative effect could be beneficial.

This manuscript reports an experimental study on humidity-dependent mixing states of submicron particles that contain both organic surfactants and inorganic salts. The topic is thus clearly suitable for Atmospheric Chemistry and Physics. The work covers a wide range of compositions and conditions that are potentially relevant for a range of questions on the physics and chemistry of atmospheric aerosol particles such as the impact of liquid−liquid phase separation. The chosen laboratory proxies are dicarboxylic acids, organosulfates and organosulfonates which are some of the important organic constituents in secondary organic aerosols. However, atmospheric aerosols are not as simple as the binary chemical systems studied here and given there is no model interpretation of the experimental data presented, comments on atmospheric implications have very limited applicability which is a clear weakness of the work as it is presented currently. It would thus be important to identify in the manuscript how this wider applicability could be achieved. Nevertheless, the experimental results are quite novel (incl. direct observation of Ostwald ripening, some interesting O:C dependencies and surfactant shell effects) and well described, so that they could motivate modellers to feed these results into their models to enhance the understanding of the mixing states of atmospheric aerosols and/or motivate further experimental studies on other size ranges and with complementary methods to overcome some of the shortcomings of the experimental work presented here.  This additional work should be motivated better with a much stronger atmospheric implications section outlining the groundwork needed to establish how the gap between the lab proxies used here and atmospheric interpretation could be bridged.  Once this is added, the manuscript is likely to be suitable for publication in Atmospheric Chemistry and Physics.

This study follows experimentally the mixing states of submicron particles containing inorganic salt and organic surfactants with varying organic volume fractions during humidity cycling using Environmental Scanning Electron Microscopy (ESEM); the ESEM data presented are useful and well described overall. Particle sizes studied are in an atmospheric relevant range (aerodynamic “size” – diameter I presume- of 0.7−1 μm), although the size range is very limited; another potential limitation are surface effects as the particles are not floating as e.g. possible with frequently used levitation methods – this should at least be considered and the potential implications need to be discussed (e.g. contrasting the work presented with results from levitation studies or other methods that have different limitations in terms of spatial or temporal scales).

Further main points to address:

• Short exposure times are mentioned to avoid beam damage – have the authors tested for beam damage (e.g. by moving sampling location/contrasting short vs long exposures etc.)? How was it established that 5 μs avoids beam damage and that the results are not affected by artefacts from beam damage (I can see no indication that this would be the case from the data presented, but a discussion of this potential issue is needed; the authors reference a paper (O’Brien et al, 2015) that has developed this technique, but this earlier paper does not specify 5 μs – just fast scans- so it needs to be clarified how this time limit was derived)?
• It is unclear how many experiments were performed and how reproducible results were – this is particularly important when considering the point above about beam damage; this needs to be clarified before publication to confirm suitable rigour of the experimental approach (there are a few error bars in Figs 6 and 7, but not in other plots, so this is something that needs to be addressed/clarified)
• You should contrast the applied experimental methods to other approaches (e.g. in a table) and briefly comment on advantages and shortcomings of your chosen approach; also include a discussion of the size range relevant in the atmosphere compared to the one you have studied (and generally accessible with experimental methods)
• The atmospheric implications section is very limited given this is an atmospheric science journal – it is important to bring this experimental study into context with other experimental studies covering the range of atmospheric conditions experimentally accessible, then consider relevant model studies of atmospheric aerosols and how they link to the results presented here and finally also consider if there are links that can be made to field work findings (which may or may not be possible); this should also expand into a more specific consideration of future studies to bridge the gaps between the currently studied proxy systems and the processes actually occurring in the atmosphere – an appreciation of the shortcomings of the proxies will help to understand differences and also motivate other groups to use their techniques to address the remaining challenges
• Data should -if at all possible- be made available or added in the supplement (which is very light at the moment) rather than requiring a request to the corresponding author especially since this is a purely experimental study that will need modellers to engage to allow a meaningful atmospheric interpretation

Minor points:

• Some of the figures lack clarity, e.g. need explanation of the color coding of the symbols, meaning of the shapes of the symbols used; also all scale bars should be labelled consistently, diagrams to the right of Figs 1 and 2 need further explanation and Fig. 5 needs to more clearly distinguish visually between data reported here and those from literature
• While mostly clear, the manuscript should be proof-read carefully to correct grammar/improve language/sentence structure in a few places (examples include lines 59, 133, 145 or 146)