Sengupta et al. investigated the impact of surface modulated organic acid dissociation (and related acidity change) in organic aerosol on aqueous-phase SO2 oxidation, cloud responses, and radiative effects using a box model. This is an interesting topic and the authors clearly showed that the impacts could be significant. However, how this result translates into a 3D aerosol-chemistry-climate model (not a box-model version) is unclear. I think that the authors should be clearer about the goal of this paper. Is it just to demonstrate that these impacts can be significant, or to improve aerosol-chemistry-climate models by providing information about this previously largely overlooked aspect? If it is closer to the latter, how to implement the results of this paper in an aerosol-chemistry-climate model is key information needed (e.g., how to set up similar calculations in aerosol-chemistry-climate models or what are the recommended values for relevant key parameters in aerosol-climate simulations). Otherwise, the authors should make it very clear that the results of this paper are not ensured to be transferable to real-world aerosol-chemistry-climate simulations. Even so, more could be done to show the implications of these box-model-based results for aerosol-chemistry-climate simulations.

Specific comments:

Section 2.3: does inorganic ion surface propensity play a role in SO2 oxidation here? What about ionic strength, as Liu et al. (2020) showed to substantially affect this chemistry.

Line 289: the sentence containing “pKabulk+1” is confusing. pKabulk+1 does not need an extrapolation to be obtained from pKabulk, but properties at these pHs do.

Figures 1-6: all these results are only for pKabulk, pKabulk+1, and pKabulk+2. The pKas of malonic and decanoic acids are different (2.8 and 4.9, respectively). At a certain pH, these 2 acids in an aerosol can have behaviors quite different than described in this paper. In an aerosol with a more realistic composition, more acids have even more pKas. The authors should explore the behavior of such acid mixture, or at least, that of individual acids in a wide range of pH, not just near their pKa.

Technical corrections:

Line 215: “F-“ and “HF” should be “A-“ and “HA”, respectively.

Many numbers in the manuscript have too many significant digits, e.g., “14.70%”, “83.14%”, “21.73%”, and “64.72%” in Line 403.

Review of Impact of acidity and surface modulated acid dissociation on cloud response to organic aerosol

Overview:

This manuscript focuses on how different chemical properties at aerosol surfaces can potentially modify cloud responses to organic aerosol. It is a detailed look that attempts to bridge from very detailed chemical processes to large scale impacts on climate. The authors are commended for attempting to bridge from the nanoscale to the macroscale, though many assumptions are needed in order to bridge these spatial gaps. The emphasis that this is empirical in the abstract and strong use of caveats in the text is key, as I worry a manuscript like this, even with the caveats, could be over generalized by others later on, but it is still important work. While there are some suggested edits and revisions, I think a revised manuscript should be considered for publication and will provide a unique and interesting contribution to the field.

Specific Comments:

-    It would be helpful in the abstract if it was stated more clearly that what the author are probing is enhanced acidity at the aerosol surface. This can be inferred, but a direct statement would be helpful.

-    A new article came out around the time the authors submitted in PNAS showing for 3 micron aerosol that enhanced acidity can be observed at the surface of particles.1 This manuscript bolsters the importance of the authors work and would make sense to include during the revision process.

-    My one major gripe is that ionic strength is really important for these types of aerosols and is not mentioned at all. This is not meant to single out the authors here, as a number of other manuscripts recently in this space also don’t discuss it even when submicron particles (and particularly ultrafine particles) will have very very high ionic strengths. Activity coefficients are mentioned briefly, but the connection to ionic strength and the implications for this work are really key. In a low water environment, which most of the submicron particles in this model will be high ionic strengths leading to activity coefficients that are << 1 (or bouncing back to >> 1 for really high ionic strengths) are central to modified acid dissociation behavior. While the discussion kind of talks around it, the manuscript would be significantly improved if it directly discussed ionic strengths and their importance under the non-dilute conditions of aerosols. 

-    Line 93: The word “the” is spelled “thhe”

-    For the simulation in Table 1, the total number concentration is quite low (700 #/cm3). A little more justification for why such a low concentration was used would be helpful.

-    Perhaps I missed it, but I didn’t see any discussion organosulfur compounds and their impacts on aerosol sulfur. I think a few lines on the extent of its formation would be helpful.2-5

-    In the direction of organosulfates their impact at the surface, particularly given recent working showing that they are primarily deprotonated,6 and potentially surface active would be worth briefly discussing, even if not accounted for in the already extensive calculations and results.

-    Another topic that I think could impact these results is aerosol mixing state. The work of Nicole Riemer showing impacts on CCN for externally versus internally mixed populations is worth noting.7,8 As is the definition in her review from 2019 of “physicochemical mixing state” to describe not only compositional differences particle-to-particle, but also within individual particles (Figure 3).9

-    Line 155 it says they follow the procedure from Liu et al. 2020, which is valid for pH > 2, but what if it is < 2, which is common in the atmosphere (per Pye et al., which they cite).

-    Line 217 a space between the two l’s in “moll-1”

-    A different topic worth mentioning, particularly around line 265, even if it is not the focus of this work, is phase (solid, semi-solid, vs. liquid) and liquid liquid phase separations, which could provide a different interface and complicated enhanced interfacial active region. Reviews by Reid,10 Freedman,11 and Bertram12 could be worth mentioning or references therein.

-    Perhaps I missed this, but what depth is assumed for this modified dissociation? I think this should be explicitly state somewhere. 

-    Also, how long do the molecules spend in that volume? Is the reaction so fast near the surface that the fact most of the particle has a more “normal” pKa, in other words one that is not the modified dissociation at the surface, does not matter? 

-    Line 353, the first sentence of this paragraph is missing a word or confusingly written, please revise.

-    While the authors lay out in the text what the x-axis is in Figure 1, 2, 4. It is still non-intuitive to lots of readers. It would be helpful if the authors could either add the explanation again to each caption or annotate the figure further to help clarify what that axis means (I had to re-read a number of times, and I was reading pretty intently). The figures with the chi as organic fraction are more intuitive.

-    Figure A3 caption, “od” should be “of”

References

1.    Gong, K.; Ao, J.; Li, K.; Liu, L.; Liu, Y.; Xu, G.; Wang, T.; Cheng, H.; Wang, Z.; Zhang, X.; Wei, H.; George, C.; Mellouki, A.; Herrmann, H.; Wang, L.; Chen, J.; Ji, M.; Zhang, L.; Francisco, J. S., Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy. Proc. Natl. Acad. Sci. U. S. A. 2023, 120, (20), e2219588120.

2.    Brüggemann, M.; Xu, R.; Tilgner, A.; Kwong, K. C.; Mutzel, A.; Poon, H. Y.; Otto, T.; Schaefer, T.; Poulain, L.; Chan, M. N.; Herrmann, H., Organosulfates in Ambient Aerosol: State of Knowledge and Future Research Directions on Formation, Abundance, Fate, and Importance. Environ. Sci. Technol. 2020, 54, (7), 3767-3782.

3.    Riva, M.; Chen, Y.; Zhang, Y.; Lei, Z.; Olson, N. E.; Boyer, H. C.; Narayan, S.; Yee, L. D.; Green, H. S.; Cui, T.; Zhang, Z.; Baumann, K.; Fort, M.; Edgerton, E.; Budisulistiorini, S. H.; Rose, C. A.; Ribeiro, I. O.; e Oliveira, R. L.; dos Santos, E. O.; Machado, C. M. D.; Szopa, S.; Zhao, Y.; Alves, E. G.; de Sá, S. S.; Hu, W.; Knipping, E. M.; Shaw, S. L.; Duvoisin Junior, S.; de Souza, R. A. F.; Palm, B. B.; Jimenez, J.-L.; Glasius, M.; Goldstein, A. H.; Pye, H. O. T.; Gold, A.; Turpin, B. J.; Vizuete, W.; Martin, S. T.; Thornton, J. A.; Dutcher, C. S.; Ault, A. P.; Surratt, J. D., Increasing Isoprene Epoxydiol-to-Inorganic Sulfate Aerosol Ratio Results in Extensive Conversion of Inorganic Sulfate to Organosulfur Forms: Implications for Aerosol Physicochemical Properties. Environ. Sci. Technol. 2019, 53, (15), 8682-8694.

4.    Chen, Y.; Dombek, T.; Hand, J.; Zhang, Z.; Gold, A.; Ault, A. P.; Levine, K. E.; Surratt, J. D., Seasonal Contribution of Isoprene-Derived Organosulfates to Total Water-Soluble Fine Particulate Organic Sulfur in the United States. ACS Earth Space Chem. 2021.

5.    Hettiyadura, A. P. S.; Al-Naiema, I. M.; Hughes, D. D.; Fang, T.; Stone, E. A., Organosulfates in Atlanta, Georgia: anthropogenic influences on biogenic secondary organic aerosol formation. Atmos. Chem. Phys. 2019, 19, (5), 3191-3206.

6.    Fankhauser, A. M.; Lei, Z.; Daley, K. R.; Xiao, Y.; Zhang, Z.; Gold, A.; Ault, B. S.; Surratt, J. D.; Ault, A. P., Acidity-Dependent Atmospheric Organosulfate Structures and Spectra: Exploration of Protonation State Effects via Raman and Infrared Spectroscopies Combined with Density Functional Theory. J. Phys. Chem. A 2022, 126, (35), 5974-5984.

7.    Ching, J.; Zaveri, R. A.; Easter, R. C.; Riemer, N.; Fast, J. D., A three-dimensional sectional representation of aerosol mixing state for simulating optical properties and cloud condensation nuclei. J. Geophys. Res.: Atmos. 2016, 121, (10), 5912-5929.

8.    Razafindrambinina, P. N.; Malek, K. A.; De, A.; Gohil, K.; Riemer, N.; Asa-Awuku, A. A., Using particle-resolved aerosol model simulations to guide the interpretations of cloud condensation nuclei experimental data. Aerosol Sci. Technol. 2023, 57, (7), 608-618.

9.    Riemer, N.; Ault, A. P.; West, M.; Craig, R. L.; Curtis, J. H., Aerosol Mixing State: Measurements, Modeling, and Impacts. Rev. Geophys. 2019, 57, (2), 187-249.

10.    Reid, J. P.; Bertram, A. K.; Topping, D. O.; Laskin, A.; Martin, S. T.; Petters, M. D.; Pope, F. D.; Rovelli, G., The viscosity of atmospherically relevant organic particles. Nat. Commun. 2018, 9, (1), 956.

11.    Freedman, M. A., Phase separation in organic aerosol. Chem. Soc. Rev. 2017, 46, (24), 7694-7705.

12.    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.