In this manuscript, the authors present the hygroscopic study of 100 nm ammonium sulfate and ammonium sulfate mixed with sodium nitrate/oxalic acid nanoparticles using Fourier transform infrared (FTIR). The aerosol liquid water content was obtained using the FTIR spectral, and further the hygroscopic growth factor was calculated. The GF measurements are neither better nor worse than those of previous studies. The sequential order during the deliquescence process was also discussed with the 2D-IR spectroscopic analysis. From this point, the method is quite meaningful to better understand the intermolecular interaction within the phase transition.

The work is laboratory-based but has relevance to modeling. The topic of the manuscript fits into the scope of ACP. However, there are still some small mistakes in the manuscript. More discussion could be addressed before publication. I have several comments and suggestions for the authors below.

Major comments:

1. The authors mentioned the nanoparticle with approximately 100 nm is the electrical mobility diameter (line 25). Later the diameter of 100 nm was described as volume equivalent diameter in the experiment description section. The authors better describe clearly. For spherical particles, assuming that the particle and its mobility equivalent sphere have the same charge, then Dem=Dve. For non-spherical particles, the shape factor and slip correction factor should be considered.

2. The authors stressed several times that the nanoparticle shape is one of the large uncertainties for the hygroscopicity study. Because normally the nanoparticle is assumed to have a spherical shape. Actually, the authors also supposed spherical particles when calculating the growth factor in this study, even the particles were deposited on the substrate. Since this is not a shape factor study, I would recommend do not emphasize this point.

3. About RH:

1). The authors claim the accuracy of RH measurement for the sample cell is 0.1% (Line 117). Is this 0.1% available for the whole studied RH range? Which sensor or instrument was employed? Normally the uncertainly will get larger when the RH increases (Mikhailov and Vlasenko, 2020). And 0.1% RH is extremely precise.

2). The initial RH is 45% for AS measurement and there is an OH stretching vibration peak at 3250 cm-1 for all RH below DRH in Figure 2. Can authors explain why? I am considering whether the RH downstream of diffusion dryer (or DMA) was below the ERH value of AS. If not, then the particle is a droplet at the initial 45% RH.

4. The authors mentioned the mass of nanoparticles was quantified using a simple procedure. Could authors prove more details? And it would be nice to add some discussions about the uncertainty of mass and GF. Actually, there are error bars shown in some figures. But there is no description in the main text.

5. The authors illustrated the OA does not absorb water in the RH between 40% to 90% in Figure 4. However, some previous studies demonstrated that oxalic acid dihydrate could form which will absorb water continually (Wang et al., 2017; Ma et al., 2019; Prenni et al., 2001). Since the FTIR could identify the hydration interactions, any explanations?

6. The DRH point in both AS/NA and AS/OA systems is lower than the pure AS. According to 2D-IR results, the hydrolysis reaction mechanism of AS/OA is different from AS/NA. Also considering OA does not absorb water, then why the DRH value gets smaller?

Minor comments

According to the detailed description of manuscript type in ACP (https://www.atmospheric-chemistry-and-physics.net/about/manuscript_types.html), I would recommend Research articles instead of Technical notes.

Line 38, “but only AN can change the hydrolysis reaction mechanism for AS in AS/AN and AS/OA mixtures.” Is there AS/AN and AS/OA mixture experiment?

Line 42, “… between nanoparticle and medium”, medium size particle? Please rewrite this sentence. Same Line 102

Line 47, “Nanoparticles have long atmospheric lifetimes of weeks to months”. Any references?

Line 80, “the hygroscopic growth process of a single aerosol with particle diameter of less than 100 nm” maybe become “the hygroscopic growth process of a single particle diameter less than 100 nm”. Please rewrite this sentence.

Line 155, “room temperature is assumed to be 25 °C”. What does this mean? No temperature sensor could measure the room temperature? Since the temperature has an influence on the RH, is there any temperature monitoring inside the sample cell?

Line 156, the authors described the RH varies from 50% to 95% in the present work. But in AS study, the initial RH is 45%. In the OA study, the initial RH is 40%.

Line 209, “the nanoparticle volume increases but its mass keeps constant.” Quite misleading, its means sulfate not nanoparticle, right?

Line 255, “at the RH of 79.9 ± 0.10%”, is this RH measurement from this study? And how do the authors obtain this value? Since the FTIR measurement is a real-time method. Why authors don’t provide more data points between 74% and 81% RH?

Line 282, what does the 1097 cm-1 stand for? NH4+? It seems hard to see a peak at 1097 cm-1 from Figure 2.

Line 310, what does the 1320 cm-1 stand for?

I would recommend the authors provide all FTIR spectral figures (OA, AS/NA, and AS/OA) at least in the supplement. 

Table1, I would recommend adding the reference (or data source) for density and solubility.

Technical corrections

Unified abbreviation. Line 33 & 37, Sodium nitrate (SN); Line 38 & 90, sodium nitrate (AN); In the Figure 5&6, NN.

Line 52, “is” not are

Line 68, “the” nanoscale

Line 73, “the” nanoparticle

Line 92, “in” real time, not on real time; also other places

Line 93, “the” molecular scale

Line 209, “large” size particles, not big

Line 212, “the” Kelvin effect

Line 238, “behavior” not behaviour

Line 247, “via direct measurement of the aerosol diameter”

Line 259, “increasing”

Line 263, “the” FTIR measurement

Line 349 & 359, “the” 2D-IR spectroscopic

Line 362, “a” better understanding

Ma, Q., Zhong, C., Liu, C., Liu, J., Ma, J., Wu, L., and He, H.: A Comprehensive Study about the Hygroscopic Behavior of Mixtures of Oxalic Acid and Nitrate Salts: Implication for the Occurrence of Atmospheric Metal Oxalate Complex, ACS Earth and Space Chemistry, 3, 1216-1225, 10.1021/acsearthspacechem.9b00077, 2019.

Mikhailov, E. F., and Vlasenko, S. S.: High-humidity tandem differential mobility analyzer for accurate determination of aerosol hygroscopic growth, microstructure, and activity coefficients over a wide range of relative humidity, Atmos Meas Tech, 13, 2035-2056, 10.5194/amt-13-2035-2020, 2020.

Prenni, A. J., DeMott, P. J., Kreidenweis, S. M., Sherman, D. E., Russell, L. M., and Ming, Y.: The Effects of Low Molecular Weight Dicarboxylic Acids on Cloud Formation, J. Phys. Chem. A, 105, 11240-11248, 10.1021/jp012427d, 2001.

Wang, X., Jing, B., Tan, F., Ma, J., Zhang, Y., and Ge, M.: Hygroscopic behavior and chemical composition evolution of internally mixed aerosols composed of oxalic acid and ammonium sulfate, Atmos. Chem. Phys., 17, 12797-12812, 10.5194/acp-17-12797-2017, 2017.

In this study, the FTIR spectroscopy and two-D correlation analysis were used to investigate the hygroscopic behavior of typical aerosols. It demonstrated that the method is good at qualifying and quantifying the interaction between water and aerosol. In particularly, the 2-D correlation analysis can provide more detail information about the hydration process. Thus, this method is helpful to understand the hygroscopicity of aerosols and it is suitable for publication as a technical note. However, the comments of referee #2 are also my concerns, and I have some other concerns as following.

1. Since the samples were deposited on a ZnSe plate in stacking state, can they also be considered as nanoparticles? Further experiments using samples prepared by deposited solution with further dry process were recommended. Then the results of these two different preparation methods would be compared.
2. Line 305, what’s “surface-limited process”? Since all the initial step of particle hydration could be water adsorption on surface, surface limited process should always determine the hygroscopic behavior.
3. In the previous study of wang et al. (2017), the formation of ammonium hydrogen oxalate (NH₄HC₂O₄) and ammonium hydrogen sulfate (NH₄HSO₄) from interactions between OA and AS in aerosols during the dehydration process were observed. Did you detect these species in the present study? Could the 2-D correlation analysis confirm this reaction?

This study used the FTIR spectroscopy and two-D correlation analysis to investigate the hygroscopicity of several single-component aerosols and their mixtures. The sequential order during the hydration process was discussed with the 2D-IR spectroscopic analysis. The method is new. However, as a technical note, validation of the performance of current method should be conducted in a more thorough way. I have several comments regarding current version of the manuscript and hope the authors could carefully address those before being considered for publication.

Major comments

Since submitted as a technical note, the authors should provide a solid and sound validation of the accuracy and stability of their technique. For instance, was your DMA calibrated? How much did you dry your aerosols? How many particles did you deposit onto the substrate before further measurements, would the concentration of the particles deposited influence your results? How was the RH measured, what was its uncertainty? Was the cell well-insulated that the RH could be maintained? What parameters did you directly obtain from the technique and how did you interpret them? Each single part of your instrument should be described clearly. Second, the authors used AS to calibrate the instrument. However, the figures the authors plotted are lack of critical data points, particularly between 74% RH and 81% RH.

The authors deposited 100 nm particles onto a substrate. These material could not represent nanoparticles anymore. The condensation of water molecules on a curved particle could be much different from that on a flat surface with deposited substances on. The author should clarify these differences and give proper discussion, emphasizing its own atmospheric relevance. A section discussing its atmospheric implications is suggested.

The authors stated in their abstract as well as in the intro part that: “current techniques are also difficult to identify the intermolecular chemical interactions of phase transition micro-dynamics during nanoparticle deliquescence process because their limited temporal resolutions are unable to capture the complex femtosecond-level intermediate states”. I am curious from current results you presented did not show a higher temporal resolutions and femtosecond-level intermediate states were not displayed, either. Otherwise, please clarify.

Based on my third comment, the author should emphasis the novelty of current technique. What are the advantages compared to traditional hygroscopicity measurements? I assume Sect 3.3 and 3.4 are quite important, contributing new insights to current understandings. However, the discussion in these part was kind of weak and plain. Proper comparisons with other studies may help.

Minor comments:

Line 90：AN or SN? Similar in line 96.

Line 102: Current work is is of great significance, not only for the haze control across China, but also for the whole atmospheric community. There remains huge gaps and requires significant efforts for the haze control across China using current results. Consider rephrase the sentence or extend the content.

Line 108: How much you dried your aerosols? For instance, to which RH condition?

Line 110: How many particles you deposited onto the substrate? Will they overlap on each other?

Line 151-160: Even though you described our instrumentation, I am still confused how you measure or derive Dwet.

Line 193: What do you mean by “area of OH”?

Line 211: Your DRH value for small particles is similar to that of large ones from other studies. But this is not enough to prove the accuracy of your method, as GF value of AS at a certain RH is also a necessity and it varies between small particles and large ones. Rephrase the sentence and make thorough comparison.

Line 211, also for Fig.2 and Fig. 3: Moreover, I observed that you performed the measurements at 74% RH and at 81% RH. At 81% RH, you observed deliquesence and you reported this value as DRH. This is not sound, it could be any values between 74% and 81% RH, please consider add more data points between these two RHs, specifically for your Fig.2 and Fig. 3. Before you add more data points at those RHs, I cannot be convinced.

Line 218: What do you mean by ‘nanoparticle volume increases but its mass keeps constant’? Please clarify.

Line 223: When you referred the calculation to previous section, for instance, Mwater and M0, you should specify all of these quantities in previous part. I am quite confused that how you obtained Mwater, Mwet-Mo?   

Line 228: Please add comparison with other studies, especially 100 nm particles of oxalic acid. Boreddy et al. (2018) measured the GF of oxalic acid using a HTDMA and they found that oxalic acid started to absorb water at RH < 45% and the GF of oxalic acid was 1.47 at 90% RH. Please consider giving proper discussion.

Line 236: I suggest you add the results for sodium nitrate (SN) particles before you discuss the results for mixtures. Again, it should be SN but not AN, isn’t it? Also in Fig. 5, what does NN stand for?

Line 254-259: Similarly, add more data points for GF at RH between 74% and 81% to complete your validation. And the content of this paragraph was well known by hygroscopicity community, you don’t have to repeat it, but give detailed description of your DRH value as well as corresponding GF and compare with other studies.

Line 260-265, also for Fig. 6: At RH between 70% to 85%, your results for mixtures deviate from the predictions. Give proper explanations. You cannot just state that they are in good agreement with other studies without presenting their data points.

Line 269: What does absorption peak mean? Please clarify.

Line 271: Since 2D-IR spectroscopic technique is a major instrument for your results, you should describe it in a more detailed way. For instance, in Sect2.1. The description was too simplified and should be extended. Give possible examples and emphasis its significance. I do think line 267-277 should go to the experiment section. And please further explain “Synchronous correlation maps” and “asynchronous correlation maps”, give examples and how we use these results, what do these results indicate or how we interpret them.

Line 310: So all the auto-peaks were all the same during all the RH conditions? Similar questions to the asynchronous correlation maps.

Line 341-345: Too long sentence, please rephrase it.