This manuscript presents the characterization of a high-resolution planar DMA for cluster classification. I recommend it to be published in Atmospheric Measurement Techniques considering its topic, as transmission and resolution of a DMA are important to the quantification of the measured clusters and nanoparticles. However, I  am confused by the presentation in several aspects and recommend a major revision to the manuscript.

Major comments:

1. A major concern is how the audience digest the results and use them in their future studies. The authors need to clarify whether the transmission and resolution are specific to their experimental setup or whether other P5 DMAs are expected to share similar values. Detailed comments are given below:

    a. A home-build recirculation system is mentioned several times, making the manuscript very confusing. A home-build system may indicate that the results of this study do not apply to other P5 DMA systems due to the difference in the recirculation system. More discussions are needed when reporting the values from this system: do the authors expect similar transmission and resolution in other P5 systems as a function of different working conditions?

    b. More figures and tables on the performance of the P5 DMA under different working conditions will be appreciated. The aim is to help the audience to estimate the resolution and transmission of the P5 DMA in their studies without repeating the same calibration experiment. These figures and tables can go to the SI.

    c. Transmission vs penetration. The use of the transmission is rather confusing. I spent quite some time checking section 3.2 and found the authors made no mistakes. However, I am afraid that some readers may be misled as penetration is 1 - particle loss, while transmission is also affected by the peak shape. Their relationship is

        transmission = penetration * peakHeight_of_transferFunction_withoutLoss

    For an ideal DMA for sub-micron particles at balanced flow rates, peakHeight_of_transferFunction_withoutLoss = 1. However, for clusters and nanoparticles, this value is less than 1.0 as diffusion, turbulence, and other non-idealness will decrease the peak height even though there is no particle loss. Using the transmission may be straightforward when converting the cluster signal to concentration in DMA-MS measurement with a known signal-component cluster sample, yet penetration is used when inverting the signal to cluster/aerosol size distribution. More importantly, different P5 systems may share the same penetration even though the resolution is different due to different levels of turbulence, whereas transmission is expected to vary with the system.  I recommend the authors follow the review by Stozenburg (2018) when reporting the DMA parameters, as least reporting penetration together with transmission.

    Related to this, Fig. 4 is a bit confusing as none of these DMA peaks reach exactly 1.0 due to diffusional broadening, yet it is probably ok since Fig. 4 is on resolution.

    d. More discussion is needed in Section 3.2 and some parts in 3.1, including more details on different modes of the setup and the applicability of the results to other setups. For instance, when measuring atmospheric clusters, the clusters are introduced to the DMA from the polydispersed aerosol flow inlet, right? Then the discussion related to Fig. 1 can be confusing as clusters are injected into the DMA using an electrospray. Whether electrostatic losses vary with the working modes also needs to be explained.

    e. The abstract and conclusion could be sharpened such that the audience can better understand the main contribution of this study to the research community. For instance, "we assessed the performance of a commercial planar DMA integrated with the home-build recirculation system" in the abstract is rather confusing, as it seems to emphasize that the results of this study do not apply to other P5 DMA systems due to the difference in the recirculation system.

2. Due to the lack of explanation, the comparison among different DMAs seems to be a bit arbitrary. 

    a. Taking Fig. 4 as an example, what are the sheath and aerosol flow rates of the DMAs, and why these values are used for comparison? Were the aerosol-to-sheath flow rate are the same for all the DMAs or the aerosol flow rate is set to the same value? Are the flow rates chosen to represent typical ambient measurement conditions or they are for different conditions? The underlying question behind these several questions is, if e.g. TSI 3086 DMA works with a resolution of 5 in a setup for ambient cluster/particle measurement, will the P5 DMA provide a resolution higher than 50, or does it simply not usable due to the difference in flow configurations?

    b. Related to this, it seems the "conventional cylindrical DMAs" in the abstract (line 21) do not include Hermann DMA and the half-mini DMAs. Why?

    c. The results in Fig. 5b need to be improved. TSI 3086 was not developed in 2011 so one cannot only cite Jiang et al. without explanation. It might be better to use the same bar for TSI 3085 and 3086 and cite Stolzenburg et al. (2018). The transmission of the Grimm nanoDMA has been improved and the results can be found in Stolzenburg et al. (2017).

    d. The resolution in Fig. 2a looks different from Fig. 3 in Amo-González and Pérez (2018). I would like to see a discussion on this difference and how it affects the results.

3. The novelty of the study can be better emphasized by shortening Section 3.3 (moving some parts to the SI) and leaving more space for 3.1 and 3.2. Fig. 8 can be moved to Section 3.1. The authors are encouraged to emphasize more on the characterization results instead of emphasizing the high resolution of P5 without restricting the working conditions, as it is known that P5 can reach a high resolution of > 100 at certain conditions.

Minor comments:

4. Please revise the title to "Characterization of a planar differential mobility analyzer (DMA P5): resolving power, transmission efficiency and its application towards

atmospheric cluster measurements". It seems none of the measured clusters in this study are sampled from the atmosphere.

5. The P5 DMA is described as newly developed, which is a bit confusing. Is it a new model or the same as the one reported by Amo-González and Pérez (2018)? Also, the DEG-SMPS in 2011 cannot be described as "newly developed".

6.     a. line 17, page 1, abstract, "0-3.9 nm". Better to use sub-3.9 or start with a very small diameter. 0 nm does not practically make sense.

    b. line 23, page 1, abstract, "stopped linearly increase". increasing?

    c. line 23, page 1, abstract, "enter a plateau". entered

    d. line 24, page 1, abstract, "one factor of magnitude". one order of magnitude or a factor of 10

    e. "thorough" in multiple places. through.

    f. line 149, page 6, "much closer". Significantly closer. It is still far from the ideal resolution.

    g. Table 1, diameter. Please specify which diameter it is in the caption or the table header.

    h. line 278, page 14. "is 7-16 times higher". Can be. "is" is too strong and hence incorrect as it depends on the flow configurations.

    i. Please check the colon in the title

References:

Stolzenburg, D., Steiner, G., and Winkler, P. M.: A DMA-train for precision measurement of sub-10 nm aerosol dynamics, Atmospheric Measurement Techniques, 10, 1639-1651, 10.5194/amt-10-1639-2017, 2017.

Stolzenburg, M. R.: A review of transfer theory and characterization of measured performance for differential mobility analyzers, Aerosol Science and Technology, online available, 10.1080/02786826.2018.1514101, 2018.

Stolzenburg, M. R., Scheckman, J. H. T., Attoui, M., Han, H.-S., and McMurry, P. H.: Characterization of the TSI Model 3086 Differential Mobility Analyzer for Classifying Aerosols down to 1 nm, Aerosol Science and Technology, 52, 748-756, 10.1080/02786826.2018.1456649, 2018.

Amo-González, M. and Pérez S.: Planar Differential Mobility Analyzer with a Resolving Power of 110, Analytical Chemistry, 90, 6735–6741, 10.1021/acs.analchem.8b00579, 2018

This work reports characterization of a parallel plate DMA (P5). The resolving power and transmission efficiency of the system are measured at different instrument operating conditions and the reasons behind their variations are discussed. Afterwards the DMA is used to characterized sulfuric acid clusters, demonstrating its potential application to atmospheric clusters. This work falls into the scope of AMT and it may be published after major revisions.

Major comments:

1. Line 17: Can the sizing range actually reach 0? No DMA can size infinitely small particles (e.g., electrons) due to diffusion. Even for ions I believe there is some limit if the size of the ion gets very small.
2. Line 23: ‘when the VDMA was above 3554.3V’. It is more appropriate to report a flowrate here (and in other similar sentences in the manuscript) since the authors have argued it is a flow field effect that affect the system resolution.
3. Line 166: What is the reason that higher Qin leads to higher signal strength? Are more ions carried to the DMA inlet by the higher flowrate?
4. In Figure 2b, I suppose there should be two lines of the counter-flow mode curve corresponding to Qout = 1L/min and Qout = 2L/min.
5. The phrase ‘signal intensity’ is bit ambiguous in the manuscript. In line 162, it seems to refer to the ‘number concentration of the sizing aerosol’. In Line 170, it refers to the total current measured by the electrometer. Please make it clear what signal intensity means exactly throughout the manuscript.
6. Are the lines in Fig. 4 measured/calculated/taken from literature?
7. Section 3.2：The P5 was operated at fixed voltages corresponding to the THA+ monomer peak?
8. Fig 5a: It is interesting to know if there is an upper limit for the positive relation between ion transmission and Qout.
9. 5a: Another interesting comparison would be comparing the transmission of ions with different sizes at the same flowrate (using ions presented in Fig 6). It would be interesting to know if a single transmission can be applied to different ions at a given flow configuration.
10. Atmospheric clusters -> atmospherically relevant clusters. For the DMA-electrometer or DMA-MS system, one challenge to detect the atmospheric clusters is their low concentration. It has not been shown that atmospheric clusters can actually be measured by the parallel plate DMAs in this manuscript.

Technical corrections:

Line 58: parallel plate

Line 94: springer?

Eq. (4): what is delta_L0.5?