The discrepancy between field-observed sulfate concentrations during haze episodes and the values simulated by air quality models has garnered significant attention over the past two decades. Many scientists believe the traditional mechanism for S(IV) reaction in cloud chemistry is inadequate. Therefore, the multiphase and heterogeneous chemistry of S(IV) compounds has been a particularly intriguing topic in atmospheric chemistry. However, there is a lack of models that incorporate the dominant mechanisms into air quality models for comparison, and very few simulations specifically focus on the impact of ionic strength on reaction rates. The key methodological contribution of this paper is the implementation of a model developed by the authors using CMAQ to simulate the conversion of SO₂ to sulfate and HMS, yielding accurate results in Alaska. I had a few minor reservations in my reading, but I still highly recommend this article for publication in Atmospheric Chemistry and Physics.

Here are my suggestions.

1. Line 18: The definition of “heterogeneous” needs clarification. In my understanding, Heterogeneous processes can be categorized as surface chemistry, while multiphase chemistry generally refers to reactions occurring in the liquid phase. (DOI:10.1126/science.276.5315.1058, DOI: 10.5194/acp-23-9765-2023)

2. Line 39: Please give the meaning of "2006 24-hour PM2.5 NAAQS".

3. Line 109: The introduction provides detailed information on specific reaction mechanisms in the gas phase and clouds. This paper suggests presenting the new mechanisms introduced here in detail and reconfirming the roles of heterogeneous and multiphase processes. It is recommended that the specific mechanisms introduced in this paper be listed in detail in this section and that the issues related to heterogeneous and multiphase processes be reconfirmed.

4. Line 130: How should the boundary problem of ionic strength (I) in aerosol water be addressed? Although this is mentioned later, the I values used here are based on maximum boundaries tested in laboratory tests. However, in actual aerosol during haze events, I can often reach several tens of M, which is significantly higher than the few M observed in laboratory conditions. Considering the potential exponential growth of the enhancement factor (EF) with increasing ionic strength (I), the intensity of aerosol ions may significantly impact the reaction rate. Of course, these are merely my thoughts and discussions. The authors do not need to address this issue directly, but they could consider it further in their outlook or future work.

5. Lines 320-324: It is recommended that HMS use a different color bar range than sulfate. Using a maximum value of 5, for instance, results in nearly zero HMS concentration, and the spatial distribution of HMS is not effectively captured in Figure 1c. The same issue is observed for the figures 3, 6, 8, and 10.

6. Lines 331-332: What does atmospheric acidity, particularly aerosol pH, look like in this context? It is suggested that the authors consider incorporating pH into the exploration of dominant pathways to help explain why TMI is dominant in Alaska.

7. Lines 306 and 381: The title 'Time' is not recommended. If you want to highlight the similarities between sections 3.1.1 and 3.1.2, consider combining the discussions. If the goal is to emphasize the differences, please choose a title that reflects the unique feature of each section. 

8. Line 649: I was very excited to see the HMS simulation. I'm eager to know whether the modeling of HMS and the multiphase chemistry of sulfate (including the effects of ionic strength) will be included in a future official version of CMAQ.

Many models are not able to reproduce high sulfate concentrations, and do not consider heterogeneous chemistry in aerosol droplets. This paper examines sulfate and HMS formation in aerosol droplets as a possible cause for model underestimation. This is interesting work which I recommend for publications upon completion of some minor revisions.

1. Sentence starting on line 41 is hard to read due to length and many parentheses. I suggest splitting it into two or more sentences.

2. Line 100: write out CONUS

3. Methods: It's unclear how ALW and pH were calculated. Please state explicitly where these numbers (for example the pH and ALW in line 331) come from.

4. In figures 1 and 3, the concentrations of the species are hard to see because the text partially covers it. Stating the domain size would also be helpful here.

5. In Figure 1a, it seems there's a high (~1 ug/m3) background of sulfate surrounding the Fairbanks and North Pole area, which seems strange. I would expect near-zero sulfate concentrations in these areas because there is very little anthropogenic activity.

6. Line 358: HSO3 and SO3 should have their charges written out like sulfate (SO₄^2-). Check for other mentions of HSO3 and SO3 in the paper.

7. In Figure 7, is there any explanation for the major differences on Dec 13 and 27? I think this should be discussed due to the large discrepancy between model and measurements.

8. Line 716: ALPACA should be Alaska Layered Pollution And Chemical Analysis. You may want to cite this paper as well https://doi.org/10.1021/acsestair.3c00076