This work describes measurements done in Beijing to understand phase transition during haze events using particle rebound and poke flow bulk viscosity measurements. The findings indicate that increased RH during haze events leads to particle phase transition from a semi-solid or solid to liquid phase state. This is shown to be due to increased inorganic fractions as well as uptake of more hygroscopic organics at elevated RH, both of which are promoted by uninhibited bulk diffusion in the liquid phase state. Overall, the level of detail and explanations of the results in this work are great and the limitations of interpretations of the results are clear. This work fits well within the scope of ACP and I would recommend it for publication once a few comments are addressed.

General comments

1. A limitation of using particle rebound fractions for ambient samples seems to be that a rebound fraction of 0.5 could indicate all your particles are semi-solid or 50% are solid and 50% are liquid and thus depends heavily on the mixing state of those ambient particles. Is there a way to validate that the particles sampled were internally mixed? Do the bulk viscosity measurements help to address this limitation?
2. It is repeatedly suggested that multiphase chemistry is responsible for the increased oxidation of organics and the increased fraction of inorganics in particles at increased RH during haze events. However, could increased partitioning of these species due to higher aerosol-associated water not also explain these observations without any actual chemistry?
3. This work shows more oxidized SOA in liquid particles with higher ALW. Can the authors comment on how this might affect the efficacy of commonly used parameterizations for OA viscosity, particularly those in Shiraiwa et al (2017) and DeRieux et al (2018) that would predict higher viscosities with higher levels of organic oxidation? Are these parameterizations still consistent with the results shown here if a composition dependent hygroscopicity parameter is used when calculating total aerosol viscosity?

Specific comments:

1. A fixed korg was used in this study and the supplement shows how a real-time korg greatly impacts the organic-associated water content. Were any sensitivity studies done on the fixed korg to see how the specific value of the fixed korg affects the results?
2. Line 364: At this point it’s been awhile since ktotal was introduced and it may be helpful to remind readers what it is here.
3. Fig 5c: Why does kinorg level off at high RH, while korg continues to increase?
4. Fig S4 does not have a legend

References:

DeRieux, W.-S. W., Li, Y., Lin, P., Laskin, J., Laskin, A., Bertram, A. K., Nizkorodov, S. A., and Shiraiwa, M.: Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition, Atmos. Chem. Phys., 18, 6331–6351, https://doi.org/10.5194/acp-18-6331-2018, 2018. 

Shiraiwa, M., Li, Y., Tsimpidi, A. P., Karydis, V. A., Berkemeier, T., Pandis, S. N., Lelieveld, J., Koop, T., and Pöschl, U.: Global distribution of particle phase state in atmospheric secondary organic aerosols, Nat. Commun., 8, 15002, https://doi.org/10.1038/ncomms15002, 2017. 

Meng et al. conducted particle rebound measurements and inferred the phase state of fine particles. They also analyzed the mass concentrations of chemical compositions in particles measured by ACSM and calculated the aerosol liquid water (ALW) content. They showed that the particle phase transition is a key factor initiating the positive feedback loops between ALW and secondary aerosol formation during haze episodes over the North China Plain. The manuscript is well written and the observation data is carefully analyzed and clearly presented. As the particle phase state measurements and analysis are still limited in East Asia, this study has significance understanding the role of particle phase state in aerosol multiphase chemistry and secondary aerosol formation in hazy days in megacities. I recommend the publication of this study after the following comments could be addressed.

General comments:

Particle phase state is related with particle chemical composition and RH, which was detailed analyzed in this study. However, besides chemical composition and RH, ambient temperature also affects the particle phase state (Koop et al., 2011). From Fig. S12 I found the temperatures between clear days and polluted episodes can be over 10 ℃ different. I suggest the authors add analysis on the relationship between temperature and particle phase state and discuss the potential effects of temperature on multiphase chemistry and gas-particle partitioning.

Specific comments:

(1) Line 19: This study focused on effects of phase transition and particulate water on secondary aerosol formation, and the particle growth was not particularly investigated. I suggest change “winter particulate growth”.

(2) Line 115-120: I am not an expert in experiments, but I am curious how long it takes for the impactor RH to be equal to the ambient RH? Did the rebounded particles reach equilibrium with the impactor RH during the measurement? This would be helpful to convince the readers that the measured phase state indeed is the phase state at the ambient RH.   

(3) Line 128: Change “organic” to “organics” or “organic aerosol”.

(4) Line 150: Delete “be” in “it should be note that” and check this all through the manuscript, e.g. Line 213, 274 and 289.

(5) Line 156-157: The calculated fixed k_org of 0.06 seems at the lowest end of the reported range in winter Beijing and lower than the predicted real-time k_org. As k_org affects the aerosol water which affects the phase state and further other results of this study, I agree with the first reviewer that sensitivity calculations should be done to evaluate the impacts of k_org on the results of this study.

(6) I agree with the General comment 3 of the first reviewer that the dependence of viscosity on oxidation state should be discussed. Dette et al. (2014), Koop et al. (2011), Li et al. (2020) and Saukko et al. (2012) are helpful for this discussion.

(7) Line 261: The authors found several points with ALW/NR-PM₁ < 5% and NR-PM₁ > 30 μg/m³ exhibited lower rebound fraction (f < 0.4) in Figure 2d and Figure S9, and they gave two possible reasons based on analyzing the ratio of ALW/NR-PM₁. Why you chose ALW/NR-PM₁ instead of ALW to interpret the results? If ALW is used for the interpretation, would the explanation be different?

(8) Line 368: I think 56 μg/m³ is for NR-PM1 instead of ALW.

(9) Line 373: Why do the mass concentrations of NR-PM1 and ALW decrease in the highest RH bin in Figure 5a?

References:

Dette, H. P., Qi, M., Schröder, D. C., Godt, A., and Koop, T.: Glass-Forming Properties of 3-Methylbutane-1,2,3-tricarboxylic Acid and Its Mixtures with Water and Pinonic Acid, The Journal of Physical Chemistry A, 118, 7024-7033, 10.1021/jp505910w, 2014.

Koop, T., Bookhold, J., Shiraiwa, M., and Poschl, U.: Glass transition and phase state of organic compounds: dependency on molecular properties and implications for secondary organic aerosols in the atmosphere, Phys. Chem. Chem. Phys., 13, 19238-19255, 10.1039/C1CP22617G, 2011.

Li, Y., Day, D. A., Stark, H., Jimenez, J. L., and Shiraiwa, M.: Predictions of the glass transition temperature and viscosity of organic aerosols from volatility distributions, Atmos. Chem. Phys., 20, 8103-8122, 10.5194/acp-20-8103-2020, 2020.

Saukko, E., Lambe, A. T., Massoli, P., Koop, T., Wright, J. P., Croasdale, D. R., Pedernera, D. A., Onasch, T. B., Laaksonen, A., Davidovits, P., Worsnop, D. R., and Virtanen, A.: Humidity-dependent phase state of SOA particles from biogenic and anthropogenic precursors, Atmos. Chem. Phys., 12, 7517-7529, 10.5194/acp-12-7517-2012, 2012.