Thank you for this fascinating manuscript. Please make the following technical corrections.

Main Manuscript:

• Please rephrase the following lines for clarity and readability:
  
  
  
  
  • Lines 41-42
  • Lines 473-475
  • Lines 476-477
  • Lines 502-509
    
    
    
    
• Please correct the following typos:
  
  
  
  
  • Lines 281-282: Change to "Figure 4"
  • Line 394: Change to "percentile"
  • Line 474: Change to "height"

Supplement Document:

In Section S 4, please rephrase the last sentence starting with "Despite..." for better clarity.

Synthesis:

The manuscript by Jost et al. examines the implementation of thermal flow sensors on passive-flow Universal Cloud and Aerosol Sounding Systems (UCASS). The continuously measured flows can be used for more precise calculations of the particle number concentration with the UCASS. While the experiments conducted for this study were satisfactory and support the implementation of the TFS, the manuscript is difficult to read in places and the presentation of the results could be improved. After addressing my few major to minor comments, I believe that this manuscript provides valuable information for the determination of aerosol particle number concentration with balloon-borne instruments and fits the scope of AMT.

Major comments:

• The heat flow sensors (TFS) are of crucial importance for this study. Nevertheless, technical details about the applicability for measurements in the UTLS were not mentioned. Please add a description of the ability of these sensors to operate at temperatures lower than -20°C (= lowest temperature of the cold chamber test).
• It is not clear to me why 19 different (?) sensors were used for this study as Table S1 suggests. Are there any (technical) differences between the sensors? Furthermore, the nomenclature of the TFS is not consistent throughout the manuscript (TFS A/B in Line 464; TFS 7/8 in Lines 407-408; TFS°1/2 in Lines 451-452). Again, what are the differences here? Please add more information and change to a consistent nomenclature which also makes it easier for the reader.
• In the abstract it says “In-flight comparisons revealed that UCASS sample flows rarely match the balloon’s ascent rate, instead, equality (v_GPS = v_TFS) is achieved only at AOA ≠ 0°, potentially affecting the UCASS-internal flow pattern and particle transmission efficiency.” which I would consider as a major finding of this study. If I am not mistaken, this aspect is extensively discussed in Section 5.1, but not explicitly measured or investigated (AOA was not explicitly mentioned in Section 5). Furthermore, Figure 7 shows the deviations in velocities at different altitudes, but neither does it show the angle of attack. Can more information about the wind conditions during the ambient measurements be provided here? For example, can additional information on wind velocity be included in Figure 7?
• Line 332 + Line 335: The terms HRC and TPC were introduced as if they were common knowledge, which they are not for me (and probably for many other readers). What are these types of calibrations and why were they used for this specific issue? Why does the HRC only offer a “relatively” high-resolution (as written in Line 332)? Why is the TPC “robust” as stated in the conclusion? Please add the relevant information.

Minor/Technical comments:

• General: Please follow the AMT Submission-Guideline “For items other than units of time or measure, use words for cardinal numbers less than 10; use numerals for 10 and above.” throughout the manuscript (e.g. Line 232: fifteen velocity measurements).
• Line 235: I don’t see any horizontal bars in Figure 3.
• Line 281: Figure 4
• Code availability: This will be the software package used for the fits, but does not describe whether and how the code or script used for this study is/will be made available.