General comments:

This paper introduces a modified version of a Handix POPS and demonstrates the performance improvement in measuring the aerosol number concentration, particle size and signal time responses. The improvement is promising and very important to expand the current POPS capability. In addition, the authors presented a very useful application to monitor the particle phase transition with the high-resolution light scattering amplitude data. Thus, the reviewer recommends publication after addressing the below concerns.

1. What is the modified POPS's total weight, power requirement, and cost? What is the targeted application of the modified POPS? Is it maintained the current lightweight and low-cost sensor application?
2. When the modified POPS was integrated into a dual tandem DMA, how were the multiple charges treated in the data reduction? How does the RH affect the light scattering calculation? Considering the complex light scattering responding curve, please estimate the uncertainty caused by the multiple change and RH in your results, such as in fig. 4. Additional information should be provided to quantify the sizing capability of the modified POPS.

Specific comments:

Line 63, What time and bin resolution do the DMA-OPC experiments need? How does the 10 Hz data improve the result compared with the 1 Hz data?

Line 97, What is the uncertainty caused by the sheath flow variation?

Figure 2, At what locations does the author monitor the phase transition temperature? Will the particle temperature decrease when transferring from SMPS to POPS?

Line 155, Can the author distinguish the multiple charged particles with a POPS? If so, what is the size resolution of the modified POPS?

Line 157, Do we expect the refractive index to change during this particle transition? If so, how can it affect the mode diameter determination?

Line 160, The viscosity of what chemical compound? Under what temperature?

Can the author quantify the enhancement?

Line 265, does it suggest that the MCA modification narrows the size detection range of POPS? If it is true, that will limit the application of POPS. Is there any solution for that?

Fig 8. Does the author have a plot of the temperature scans of MCA-PH response? Is it different or similar to digitizer-PH?

The nominal 166 nm is based on the electrical mobility size, right? If so, what is the corresponding optical size of the particles? Line 275, at 72 C, the mobility size is around 210 nm, similar to the doubly charged particles. How does the author separate the coalescence and the doubly-charged particles?

General comments:

Authors have modified the commercial version of POPs by adding an external MCA card and changing the flow path, which have extended the use of POPs in laboratory studies and field measurements. For example, in tandem with DMA, and real-time processing of signals allows the high time resolution measurements of aerosols under high aerosol concentration conditions. The authors have detailed characteristics of concentration response, size response, time response of the modified one and compared them with the commercial one, and also showed an application example for phase state change detection. Overall, this is a good technique paper that facilitate the use of POPs, thus only have some minor comments.  

1. Suggest authors give a picture of the modified version in Fig.1.
2. Suggest that Sect 2.2, 2.3 and 2.4 should be merged correspondingly with Sect 3.1, 3.2 3.3.
3. The commercial POPS is attractive for its relatively low cost, lightweight, low power consumption, thus very popular in characterizing vertical distributions of aerosols such as in unmanned aerial vehicle (UAV) measurements, does the light weight and power consumption of the modified version remain low? It might be better that authors add this information in discussion part, and discuss the potential usage of modified version in for example UAV measurements.

Specific comments:

L15 “The 90/10 rise and fall…..” this sentence is too technical, it is hard to follow for readers without clear clarifications, I understand clearly what authors want to deliver only when I read L120 to 124.

L182 The given typical refractive index of organics should have a larger range (Moise et al., 2015)

Moise, T., Flores, J. M., and Rudich, Y.: Optical Properties of Secondary Organic Aerosols and Their Changes by Chemical Processes, Chemical Reviews, 115, 4400-4439, 10.1021/cr5005259, 2015.

General comments:

A modified version of Commercial POPS is presented. Authors used a high-precision multichannel analyzer to improve count rate limitation. A needle valve and vacuum pump are used to provide additional flow stability. The overall goal of the manuscript is to characterize the modified POPS instrument along with the comparison with commercial one. Authors also presented a practical application of the modified POPS, which is the detection of phase transition using high-resolution pulse height data. The reviewer finds the manuscript exciting and a good read. Authors have taken care of explaining minute details and nicely present the work. Review recommends publications with the following comments/suggestions:

1. Reviewer is interested in knowing the commercial aspects of modified POPS, for example, cost, size, and portability. Can we still use the modified one for field measurement?
2. Reviewer would suggest showing a laboratory picture of modified POPS setup/instrument.
3. Reviewer thinks section 2 should contain the details of the modified POPS, and sections like 2.1 to 2.4 should be discussed in the result section (3). Additionally, section 2.5 should go in the supplementary material.  

Minor comments:

1.  Reviewer wonders why figures in the manuscript have a faded axis. Also, authors may wish to remove grid lines from figures.
2. In figure 4, reviewer thinks it is better to give the plot names instead of indicating the top, bottom, and middle. The inset of figure 7a is not clearly visible to the reviewer, and a box should be added to indicate the ROI.
3. L75, reviewer thinks author should write a line about how and why the 50 Ω terminator reduces the noise. Reviewer understands the use of a preamplifier, but what is the role of a shaping amplifier.
4. L75, maybe reviewer misunderstood why high gain is 0-1V and low gain is 0-10V, expecting the reverse. 
5. L95, what authors mean by critical flow conditions. Is it a predefined flow condition or maximum flow, or optimum flow?
6. In figure 1, the definition of HFDMA should be included in the abbreviations.  
7. L105, from the reader's point of view, a sentence should be added to explain custom DMA. Furthermore, figure 2 should be moved to page 5.
8. L115, author compared the results of other bins? If a higher bin number is assumed to give better statistics, 1000 bins would be more appropriate.
9. L140, reviewer did not understand the meaning of a high-level overview. Is it a detailed overview?
10. L140, reviewer thinks the short form of HFDMA should be introduced earlier. Maybe in figure 1 or in L105.
11. L145, Form reader point of view unit should be consistent. Author can use either inches or m.