Chen et al. “Influence of acidity on liquid-liquid phase transitions of mixed SOA proxy-inorganic aerosol droplets” details the measurement of particle morphology and the separation relative humidity of 3 organic compounds with ammonium sulfate as a function of bulk pH.  The results differ significantly from Losey et al. 2016 and 2018 for the two overlapping systems and Tong et al. 2022 for the one overlapping system.  The explanation of the differences from previous literature is not sufficient as currently presented in the manuscript, and the work significantly overlaps with previous literature.  Specific comments are given below.  The language of the manuscript is generally clear, but should be read through with specific attention to grammar again.  This work requires substantial revision prior to possible publication.

Specific Comments:

line 33: “Morphology can be broadly categorized into single-phase homogeneous morphology and phase separation morphology (Gorkowski et al., 2020), based on the phase state of the particle.”  Bertram et al. 2011 or earlier work (e.g. Ciobanu et al. 2009) may be more appropriate references for this statement.

line 35: “For droplets with a phase separation morphology, the two main equilibrium morphologies are a fully engulfed (core-shell) structure and a partially engulfed structure (Freedman, 2020).”  Reid et al. 2011 is a better reference for this statement.

line 41: “The reverse process, in which two liquid phases mix into a single homogenous liquid phase, is referred to as liquid-liquid phase mixing and the corresponding RH is the mixing RH (MRH; Gorkowski et al., 2017).” You et al. 2014 may be a better reference for this statement.

line 48: “More recently, Kucinski et al. (2021) found that submicrometer-sized aerosol particles had a lower SRH compared to micrometer-sized droplets.” Both this reference and the more recent Ohno et al. should be cited (see references below).

line 57: “Losey et al. (2018) measured the RH of phase transitions using optical microscopy and discovered that for low-pH aerosol particles (≤0.35), phase separation may be hindered by the addition of sulfuric acid.” Relevant for this paper is the fact that SRH was also found to be decrease as acidity increased, and a greater change was observed for systems with lower initial SRH values.  Also, both 3-methylglutaric acid and 1,2,6-hexanetriol were also studied in the cited paper.  Considering you are using both sulfuric acid and sodium hydroxide to adjust the pH of your solutions, the results of Losey et al. 2016 should also be discussed and cited.

line 62: “Nevertheless, parallel experiments in this study were not conducted to accurately determine the uncertainty of the measurements.” The uncertainty of measurements of the study presented in this manuscript or in the cited manuscript?

line 83: The addition of sodium hydroxide changes the composition of the inorganic part of the solution, and will affect the SRH values measured.

line 152: Insert “either” to make the sentence “When LLPS occurred, the droplets changed from a symmetrical homogeneous phase to either an asymmetrical…”

Fig. S3 is marked as GL/AS, which is not the system referred to on line 166.  Then on line 177, the GL/AS system is referred to.

Fig. 2: Why is it clear from the WGMs that the system is phase separating into a core-shell structure rather than a partially engulfed structure?  The WGMs in the phase separated region are very messy.

Methods section: Please clarify which systems have added H2SO4 and which systems have added NaOH.  This is important for comparison to Losey et al. 2016 and 2018.

paragraph beginning at line 186, comment 1: As the pH is dropped, the pH values differ up to ~30% from Losey et al. 2016 and ~6-7% from Losey et al. 2018.  The trend also differs from these two papers, with a constant decrease of SRH with decreasing pH instead of a maximum at a pH of ~pH=3.7.  Certainly this can not only be due to the difference in techniques.  Optical microscopy on hydrophobic substrates has been used to measure SRH in numerous papers; optical trapping has been used more rarely.  The point of using a hydrophobic substrate is to minimize interactions between the particle and the substrate.  SRH is generally thought to be accurate with optical microscopy, though morphology information is much less reliable.  Is there a known difference in the SRH measured between these two techniques?  If so, references should be provided.  Also, what system was used to calibrate the SRH of the technique used in this paper?  Data on calibration should be provided in the SI.

paragraph beginning at line 186, comment 2: As the pH decreases, ammonium sulfate becomes ammonium bisulfate.  The salting out ability of sulfate vs. bisulfate should be different.  This is the argument made in Losey et al. 2018.

paragraph beginning at line 186, comment 3:  This manuscript reports that MRH differs from SRH for all pH values except 5.21.  Losey et al. 2016 finds that MRH differs from SRH only at pH 5.17 and 6.45.  MRH is the same as SRH at all other values of pH used in Losey et al. 2016 and 2018.  Why is a difference observed between these two papers?  Also, if MRH differs from SRH, one would expect a higher value (just as DRH>ERH because of the activation barrier required for ERH), but this is not the case for pH 6.53.  What is the author’s explanation of this result?

paragraph beginning at line 186, comment 4: Kucinski et al. 2019 is a study of particles < 3 microns in diameter.  It is unclear based on the literature that size effects are likely for 100 micron vs. 10 micron diameter particles, as the literature (e.g. Laskina et al. 2015) compare micron to nm diameter particles.  The Kelvin effect is not an adequate explanation as it tends to affect the properties of aerosol particles starting at a size of ~100 nm and below.

paragraph beginning at line 186, comment 5: Both 10 micron diameter droplets and 100 micron droplets are not representative of organic aerosol found in the atmosphere, which is generally < 500 nm.  These larger droplet sizes are orders of magnitude different than the real system they are trying to simulate.  The last sentence of this paragraph should be deleted.

paragraph starting at line 209, comment 1: The results for hexanetriol/ammonium sulfate differ from those of Losey et al. 2018, where a dramatic decrease in SRH with decreasing pH is observed.  The trend is not similar.  The data also differ significantly from Tong et al. 2022.  At the lowest pH values, SRH was not observed in Losey et al. 2018.  What is the author’s explanation of this difference between this manuscript, Losey et al. 2018, and Tong et al. 2022?  Why is only some of the data from Losey et al. 2018 plotted in Fig. 3b?

line 232: It is unclear how you have reached this conclusion when all of the systems in this manuscript exhibit high SRH values (> ~70% RH) at the lowest pH values studied.

line 237: What is the volatility of each of the organic compounds used?

Table 2: Should the title be “separation refractive index” and “mixing refractive index” rather than “relative index”?  These results are not discussed in the manuscript.  What is the number of repeated experiments at each phase transition at each pH?  For some systems, it appears that there are only 1 or 2 repeats according to the SI.  Three or more trials should be performed.  Also, the significant figures retained are often too many considering the magnitude of the error.

Fig. 4: This figure has been shown in the literature multiple times, and it is unclear what it adds to this paper.  Further, these ratios at which phase separation occurs can be incorrect, depending on the system (see e.g. Ott et al. 2020).  Because the main part of the paper is about aerosol pH, it is unclear why this plot is included and how it adds to the paper.  Also You et al. 2013 deals with ammonium sulfate and other salts.  Is only the ammonium sulfate data plotted?  And for the data from this manuscript, is this only plotted for systems with no additional H2SO4 or NaOH?  I recommend deleting this figure and the associated text.

line 280: How low was SRH taken in the experiments?  Did the morphology change from core shell to partially engulfed as the RH decreased, as found in Kucinski et al. 2020 or Stewart et al. 2015?

References:

Bertram, A. K.; Martin, S. T.; Hanna, S. J.; Smith, M. L.; Bodsworth, A.; Chen, Q.; Kuwata, M.; Liu, A.; You, Y.; Zorn, S. R. Predicting the Relative Humidities of Liquid-Liquid Phase Separation, Efflorescence, and Deliquescence of Mixed Particles of Ammonium Sulfate, Organic Material, and Water Using the Organic-to-Sulfate Mass Ratio of the Particle and the Oxygen-to-Carbon Ele. Atmos. Chem. Phys. 2011, 11 (21), 10995–11006.

Ciobanu, V. G.; Marcolli, C.; Krieger, U. K.; Weers, U.; Peter, T. Liquid-Liquid Phase Separation in Mixed Organic/Inorganic Aerosol Particles. J. Phys. Chem. A 2009, 113 (41), 10966–10978.

Kucinski, T. M.; Freedman, M. A. Flash Freeze Flow Tube to Vitrify Aerosol Particles at Fixed Relative Humidity Values. Anal. Chem. 2020, 92 (7), 5207–5213.

Losey, D. J.; Parker, R. G.; Freedman, M. A. PH Dependence of Liquid-Liquid Phase Separation in Organic Aerosol. J. Phys. Chem. Lett. 2016, 7 (19), 3861–3865.

Ohno et al. ACS Earth Space Chem. 2021, 5, 1223–1232.

Ott et al. ACS Earth Space Chem. 2020, 4, 591-601.

Reid, J. P.; Dennis-Smither, B. J.; Kwamena, N. O. A.; Miles, R. E. H.; Hanford, K. L.; Homer, C. J. The Morphology of Aerosol Particles Consisting of Hydrophobic and Hydrophilic Phases: Hydrocarbons, Alcohols and Fatty Acids as the Hydrophobic Component. Phys. Chem. Chem. Phys. 2011, 13 (34), 15559–15572.

Song, M.; Marcolli, C.; Krieger, U. K.; Zuend, A.; Peter, T. Liquid-Liquid Phase Separation in Aerosol Particles: Dependence on O:C, Organic Functionalities, and Compositional Complexity. Geophys. Res. Lett. 2012, 39 (19), 1–5. https://doi.org/10.1029/2012GL052807.

You, Y.; Smith, M. L.; Song, M.; Martin, S. T.; Bertram, A. K. Liquid-Liquid Phase Separation in Atmospherically Relevant Particles Consisting of Organic Species and Inorganic Salts. Int. Rev. Phys. Chem. 2014, 33 (1), 43–77.

This work systematically studied the influence of acidity on aerosol liquid-liquid phase separation and mixing by aerosol optical tweezers coupled with Raman spectroscopy. The results showed that the higher acidity decreased the separation relative humidity (SRH), and phase separation of organic acids was more sensitive to acidity compared to alcohols. The mixing relative humidity (MRH) was found to be higher than SRH. Additionally, the results on the influence of oxygen-to-carbon ratios (O:C) showed that, while phase separation occurred in the system with O:C of 0.33, 0.50 and 0.33, no phase separation was observed in the system with high O:C (i.e., 1). These findings are interesting and important. However, I have concerns about this work as detailed in the following.

Major comments:

1. Figure1: What are the origins of the spontaneous Raman peaks at ~3050 and 3300 cm^-1? The y axis showed the normalized intensity – please clarify to which peak the peaks were normalized to.

1. Lines 141 - 142, please explain why the area below the spontaneous Raman signal was used to normalize the Raman spectra. Normally, peak intensity is used to normalize peak intensity and peak area is used to normalize peak area.

1. Lines 186 – 189, can the different speciation of 3-MGA under different pH conditions be the underlying reason for the different SRH. For example, under highly acidic conditions, 3-MGA mainly appears in the protonated form (conjugated acid), while under high pH conditions, the deprotonated form (conjugated base) is the major species. Different species can show different phase separation properties.

1. Lines 234 -236, the statement may not be valid. Droplet pH may differ from the bulk solution pH, depending on the difference in the chemical composition between droplets and bulk solution.

1. Lines 237 -239, the statement may not be valid, as evaporation of volatile species from microdroplets have been widely observed. The authors may want to confirm that the influence in your system is neglected from the droplet size change profile. For example, a constant droplet size under a constant RH can indicate that the evaporation is neglected in your system.

Minor comments:

1. Please spell out the abbreviation of OIR.
2. Please provide the Raman spectra of droplets with different chemical composition and assign the spontaneous Raman peak in each spectrum.