Feedback comment on ACP manuscript egusphere-2024-733

General comment

Guy et al. present balloon-borne measurements of aerosol particle number concentration and size distribution in their manuscript at the Summit station in central Greenland. These observations are difficult to obtain and are a novelty in this region. On 6 days of operation, a helikite was used to carry an optical particle counter and a radiosonde to a maximum height of 830 m above ground level. The manuscript describes 6 aerosol profiles and puts them in context with atmospheric thermodynamic stability and de-coupling layers below the cloud base. Changing aerosol properties across the de-coupling layers indicate limited representativeness of surface-based aerosol measurements for cloud layers.

However, the number of cases discussed in the manuscript is rather small, and the observations were limited to certain weather conditions that allow for balloon operations. Therefore, the presented manuscript seems insufficient to evaluate the broader relevance of the measurements. The study could benefit from setting the observations in context with other surface-based aerosol observations (in situ or remote sensing). The aerosol measurements with the POPS were neither validated, calibrated, or compared with other aerosol instruments. This is a significant flaw concerning reported measurement uncertainties of the POPS (see references below). The extensive post-processing of the radiosonde data from the helikite, including a subjective selection of valid or non-valid data, indicates unusual measurement uncertainties that require further validation by a reference.

Aerosol data from the POPS

The aerosol measurements require more important detail: Which type of inlet was used? Please provide a particle size-dependent sampling efficiency estimation. Was the aerosol actively dried or did you measure wet particle diameter? Was the POPS calibrated or compared with a reference instrument? The POPS internal flow control system is well-known for increased uncertainties. Did you test the sample flow rate at the low ambient pressures at Summit?

The POPS is known for increased measurement uncertainty towards smaller diameters, particularly below 150 nm. This can significantly reduce the useful size range of the POPS, e.g.

https://doi.org/10.5194/amt-17-601-2024

https://doi.org/10.5194/amt-17-731-2024

https://doi.org/10.5194/amt-15-6889-2022

Please provide a detailed evaluation of the POPS measurement performance including measurement uncertainties.

Meteorological data from the Windsond

There seem to be too many post-processing steps for the Windsond data. How many raw data points per profile were left over before the final step 4? How did you validate the final data? How did it compare to the daily Vaisala radiosondes? Please provide a comparison of the raw data with the final post-processed data for each profile for the review process to allow for an evaluation of your post-processing routine.

Specific comments:

Table1: Which radiosonde, the windsond on the helikite or the Vaisala?

L 97: The presented uncertainties seem unrealistic compared to the extensive post-processing. Please derive a realistic measurement uncertainty for RH and T from the raw data set.

L 137:A minimum theta E increase of 0.1 K seems lower than the measurement uncertainty of the windsond for RH and T. Please provide an error propagation to justify this criterion

L 145 to 147: The total particle numbers need to be validated. Pilz et al. 2022 (https://doi.org/10.5194/amt-15-6889-2022) found that the POPS can show high noise in terms of false particle concentrations up to 10 cm^-3 during measurements of particle-free air. Your measurements could be highly biased by the POPS uncertainties.

Figure 4: Please provide further info how the standardized equivalent pot. Temperature profiles were calculated

The base of the de-coupling layer in panel (c) appears to be rather at 150 m and the upper one in panel (d) at 500 m. Please provide the raw temperature profiles from the windsond for evaluation (only for review)

Figure 5: All size distributions appear to be biased by increased noise in the bins below 150 nm, please check this with a zero filter. Also, please provide error bars or similar to indicate measurement uncertainties within the plot. Update the x-axis caption to wet diameter if the wet diameter was measured.

L213: Please provide reference measurements from the nearby station to validate this assumption

L214: Please specify what you mean by nucleation and how it contributes to a depletion in aerosol particles.

L225: Please explain in more detail how the depleted layer was potentially affected by a cloud. No cloud is visible on the ceilometer during the balloon flight. Horizontal inhomogeneity of clouds/atmospheric layers and temporal evolution is hard to disentangle with the provided information

L227-238: Which same method is applied to the radiosonde data? Please provide more information on the conducted analysis and its results within the method and results section.

Review of "Measurement report: In-situ vertical profiles of below-cloud aerosol over the central Greenland Ice Sheet" by 

Heather Guy et al.

Within the manuscript authors discuss importance of the vertical in-situ profiling versus the surface measurements. They provide dataset of nine profiles measured during five days. The measurements provide some insight into complexity of atmospheric vertical structure, however the clues what is actually observed are very limited, but that is expected for "measurement report" manuscript.  The manuscript reads well, however needs decent revision. I would suggest to authors the following revisions to improve the manuscript.

Please provide description how your setup was calibrated, the POPS has own issues as mentioned by other reviewer. Also any aerosol and meteo data from surface measurements would strengthen the manuscript, put your measurements into context of surface observations. Currently, the data is rather providing relative changes in vertical domain, any intercomparison to surface observation would help the reader to orientate what part of aerosol distribution is covered by POPS measurements. Were you observing particle growth after new particle formation, plums that are not observed at surface, long range transport, etc...?

Please provide details on aerosol sampling setup, inlet (heated/non-heated), length, diameter, expected losses.  Were there any corrections applied? What particle diameter you provide? Is it PSL equivalent or any corrections based on know refractive index of particles?

Provide the uncertainties of all sensors and their operation range.