General comment

In the presented manuscript, Miller et al. showed the feasibility of initiating and observing an aerosol particle plume with uncrewed platforms that are intended for cloud seeding experiments. One of two newly developed uncrewed aerial vehicles (UAVs) handles the burning of flares used as the particle source for cloud seeds. Another UAV serves as a platform for measuring the released aerosol particles with an optical particle size spectrometer (OPSS). The major novelty of the two UAVs is their capability to be operational inside clouds and under icing conditions. An additional tethered balloon system was used as a second reference for measuring the aerosol plume created by the flares with another OPSS of the same type as the one onboard the UAV. It was successfully demonstrated that the measurement UAV and the TBS system captured the particles sourced by the seeding UAV inside and outside of clouds. The manuscript is generally well-written, has a good structure, and the figures are well-illustrated. Therefore, the work adds a valuable contribution to the atmospheric measurement community and is suitable for publication in AMT with minor revisions.

However, more emphasis should be given to the aerosol sampling methods with the OPSS onboard the UAV and the TBS. There are generalized conclusions about the negligible impact of the UAV rotors on aerosol sampling that are not entirely demonstrated and doubtful. The two aerosol properties particle number concentration (N) and particle number size distribution (PNSD) provided by the OPSS are partly mixed, leading to conclusions that are true for one but not for the other variable. The authors need to consider that the PNSD covers several orders of magnitude in N from the smallest to the largest size bin. Losses of larger particles do not have a large effect on N but on the PNSD and its derivatives volume and mass size distribution. These derivatives are highly relevant for optical closure studies with remote sensing observations or for air quality measures. Therefore, the authors need to clearly separate and specify when speaking of measurement uncertainties. In this context, an essential characterization of inlet sampling efficiencies for varying particle diameters depending on wind speed and UAV ascent/descent rate or horizontal speeds needs to be added.

Detailed comments

Title: the general recommendation is to avoid abbreviations in titles

Line 11 – 12: The negligible effect of rotor turbulence was not sufficiently demonstrated for particle number size distribution measurements and the statement is contradictory to Liu et al. 2021

Line 17 -18: please specify here and in the following as particle number concentration and particle number size distribution

Line 24- 25: airborne observations are certainly not needed to calibrate remote sensing measurements

Line 47: please remove the URL of Meteomatics and transfer it to the references

Line 52: only the year of the publications should be in brackets

Line 64: the acronym CLOUDLAB needs to be introduced

Line 83: 90 km h^-1 is the common abbreviation used at Copernicus

Line 113: 5 cm above rotor level seems low in comparison to other studies to avoid turbulence sufficiently. Please discuss your design in comparison with other studies and provide an analytical sampling efficiency estimation of the inlet for typical wind speed, ascent/descent rates, and horizontal velocity above ground

Line 173: What means instrument variability? Uncertainty in particle number concentration measurements against reference instruments or inter-unit variability? To what time resolution does it refer, 1 Hz data? If the uncertainty refers to particle number concentration, it cannot be derived from average deviations across each size bin as presented in Appendix A1. Particle numbers at small sizes are orders of magnitude higher compared to larger sizes. Thus the uncertainty per bin has to be set in relation to the number concentration in each bin. It is better to compare integrated particle numbers across the whole size range.

Line 174-175: The method presented in Appendix A2 seems not sufficient to derive a general uncertainty of 50 % for each size bin. Please discuss the difference in your findings from other studies e.g. Liu et al. 2021 or Pilz et al. 2022

Line 188-190: the turbulence created by hovering is certainly different from that created during ascent/descent or horizontal movement of the UAV, thus the conclusion seems not sufficiently supported

Line 194-195: the difference are hard to derive on a logarithmic scale, but it seems that deviations between sampling with and without rotors near the inlet are up to a factor of two for particle diameters above 800 nm. This could become relevant when deriving particle properties from the number size distribution, e.g. volume size distribution for optical closure studies e.g. Düsing et al. 2021 (https://doi.org/10.5194/acp-21-16745-2021)

Line 201: ascending/descending rates of 10 ms^-1 can significantly reduce sampling efficiency of supermicron particles depending on the inlet geometry. Please provide an analytical estimate

Line 218: rotor downwash does not appear neglectable in general, it rather depends on averaging altitude range and ascending/descending rates. Also, the variations in particle number concentration in Fig. B1 appear somewhat higher during descent than during ascent, thus indicating effects from downwash

Line 219-225: that appears as a valuable extension of the POPS usage and should therefore be part of the main manuscript

Fig. 6 (b): please specify particle number concentration on x-axis

Line 242: please briefly discuss the uncertainty of the POPS, e.g. relation of sampling frequency of 1 Hz and ascent/descent rate of 10 ms^-1 including measurement uncertainty for 1 Hz data. Is the ascent/descent rate appropriate for profiling? Would it be useful to average ascent and descent profiles of particle number concentration to one profile like for temperature and humidity (which are sampled at 10 Hz)

Line 241: can the measurement UAV really characterize a ceilometer? It is rather a complementary method

Fig. 9: please specify particle number concentration on y-axis

Fig 10 (a): the plot would benefit from an additional line showing the time-height series of the balloon with the height scale on the right y-axis

Line 319: The cloud droplets appear quite small if they are measured by the POPS. Are there measures that support this hypothesis, e.g. cloud droplet diameter derived from the cloud radar, or relative humidity measurements of the POPS sample stream

Line 326: please provide additional information on the inlet of the POPS _TBS . Airborne particle measurements should assure sample air relative humidity below 40 % to provide the dry particle diameter by heating or using a dryer

Line 334-335: this conclusion is not sufficiently supported

Line 339: particle number size distributions during profiling were not shown

Line 344: without proper estimations of uncertainties for particle number size distribution measurements and information about the sample air relative humidity, the system can certainly not serve as a “benchmark” to validate remote sensing retrievals

Line 361: it was not shown that microphysical changes within the plume can be assessed

Line 371: PSL suspensions (not solutions) should only be produced with distilled water

Line 377-378: The comparison to the findings by Pilz et al. 2022 is methodologically wrong. They reported comparisons of total particle number concentration measured by a POPS against a reference CPC for specific PSL sizes filtered by a DMA and not the uncertainties in single size bins measured at undefined aerosols. Pilz et al. concluded average uncertainties for particle number concentration in the range of Gao and Mynard and not up to 110%.

Line 385: how are the corresponding particle diameters are selected from the SMPS and APS? The geometric centers of each bin are certainly different across the instruments

Line 407: was the flow rate of 0.9 cm³ s^-1 used for all measurements? This very low flow has certainly an effect on the sampling efficiency of supermicron particles

General Comments:

In the presented manuscript, Miller et al. describe two Uncrewed Aerial Vehicles (UAVs) for cloud research: (i) the “measurement” UAV equipped with an optical particle counter for measuring particle size between 0.1 and 3.4 μm (in diameter) and met sensors, and (ii) the “seeding” UAV equipped with seeding flares, initiating ice particles in supercooled clouds. There is a comprehensive description of the cloud seeding procedure. Tests were made to validate the POPS measurements on-board the multi-rotor, by (i) comparing the ascent and descent profiles, and (ii) assessing the observations with and without the rotors. Characterization of a dispersion of an out-of-cloud seeding plume produced by the flares on-board the UAV was made successfully through the experiment. Results are also shown from an in-cloud seeding experiment, where the seeding UAV was inside a supercooled cloud and the TBS system was downstream. This study successfully demonstrates the capabilities of the measurement UAV and the TBS system to capture the plume produced by the seeding UAV inside and outside the clouds. This work adds a valuable contribution to the atmospheric measurement community and is suitable for publication in AMT with minor revisions.

Although, there are some specific areas in the manuscript that need further elaboration/clarification. Here are the main parts that should be revised.

• First of all, the scope of each experiment should be clearly described in the beginning of each related section. There is no clear distinction between the technical purpose and the scientific aim of each experiment.
• The authors did not use the UAV observations to estimate the PBL height, but the UAV data were only used for validating the PBL height retrieved by the ceilometer measurements. I would suggest that the authors describe the method of how the PBL height can retrieve only using the met profiles from the UAV, and then validating it with the results from the ceilometer.
• In addition, as there was a holographic imager on-board the UAV, it would be good to show some results from it.
• It would be good to show an in-depth characterization of inlet sampling efficiencies for a range of particle sizes and how the 3-D wind affects its efficiency, as well as whether this efficiency differs between ascent and descent profiles.

Specific Comments:

Lines 98, 99: It would be good to stick on the same metric system. Ms^-1, and kmh^-1 are both used.

Lines 100-105: Which met sensors are used on-board? Are they custom-made or commercial? Provide details.

Line 128: Is it isokinetic inlet? Elaborate if yes, or not.

Lines 151-155: Needs elaboration here – provide details on how you decide the ideal seeding altitude, which parameters/conditions are important for this?

Line 187: Why only those 2 “small” sizes were chosen for the calibration? How about the accuracy for larger sizes than these (i.e. for 2 μm, 3 μm)?

Line 217: Have you checked this in lower ascending/descending speeds than 10ms^-1? It would be interesting to see whether similar results are derived.

Line 234: The ascent/descent speed for the experiment were 10ms^-1?

Line 246-248: There are many more ways to calculate the vertical profiles of aerosol concentrations. The sentence needs rephrasing, to be completely valid.

Line 251: Text needs to be ahead the figure (i.e. Fig. 6).

Line 253: How the blue line was derived by the RH minimum gradient? Provide justification.

Line 302-304: Not clear aim of this experiment (Sec. 5.2). Which was the main purpose of this specific experiment? Why not using a sensor for measuring the large particles in the plume too, so as to measure the supercooled cloud droplets and ice crystals there? Was the sole scope to measure only the remaining inactivated seeding particles?

Line 305: It would be good to show some results from the holographic imager.

Line 311: Are there specific criteria to choose the seeding altitude?

Lines 337-339: Good thinking and reasoning, are there any relevant references from literature?

Line 341-344: Additional information is needed for the inlet of POPS. Also, could you also use any “drying and/or heating mechanism” to tackle the tackle the built-up moisture in the inlet?

Line 350: “comparable to other aerosol instrument measurements”: add some quantitative results here, e.g. XX% deviation, etc.

Line 355: Not correct wording. The concentration of the UAV profile was not compared to the backscatter signal from the ceilometer; as the parameter shown was not the same to be able to compare them. The sentence needs rephrasing.

Line 376: There was not robust assessment of the microphysical properties/changes.

Line 473: XX needs to be replaced with the corresponding text.

Figures:

General comment: Text introducing figures should always be ahead the figures

Figure 2: Not very clear image

Figure A1: Not very clear. It would be better to visualise the two sizes in 2 different figures, one next to the other.