While the manuscript provides valuable insights into the influence of electromagnetic interference on lidar-derived aerosol properties, there are a few weak points that could be addressed to improve it:

Limited discussion on the potential impact of RF interference on aerosol property retrieval: The manuscript focuses primarily on the detection and suppression of RF interference but provides limited discussion on how such interference affects the accuracy and precision of aerosol property retrieval. Further exploration of the potential biases or uncertainties introduced by RF interference would enhance the manuscript.

Lack of comparison with other lidar systems: The study primarily focuses on the lidar system used in Ny-Ålesund, Svalbard, but does not provide comparisons or discussions about similar lidar systems in other locations or studies. Including such comparisons would strengthen the significance and generalizability of the findings. Maybe this point can just be indicated as future research. 

Limited exploration of noise reduction techniques: While the manuscript presents an interference suppression method, it does not explore other noise reduction techniques commonly used in lidar data analysis, such as wavelet filtering or advanced denoising algorithms. Discussing the limitations and potential improvements of the proposed interference suppression method in comparison to existing techniques would add depth to the manuscript.

Insufficient discussion on the implications for long-term data records: The manuscript briefly mentions the importance of long-term data recording and quality assurance. However, further discussion on the implications of RF interference on long-term data records, including the potential biases or uncertainties introduced over time, would provide valuable insights for researchers relying on lidar data for climatological studies.

The paper is very interesting because it deals with electromagnetic interferences that is often a problem that affects lidar systems, and the related discussion is still going on within the lidar community. Therefore, further studies on this topic are always welcome.

The paper is well written. Just some comments:

1) concerning fig.4, it would be important show also the ratio between the corrected and not corrected profiles both for the aerosol Power (i) and for the aerosol backscatter (iv), because it is not possible to evaluate the differences and draw conclusions just looking at the profiles.

2) did the authors check the influence of the filter on the backscatter profile also in in presence of a strong aerosol layer? This would be important to evaluate the effect of the filter in presence of sharp and strong changes in the measured signals.

3) did the authors try to measure the dark signal and subtract it to the measured signal to compare the results?

Review of  AMT manuscript amt-2023-79

Title: Influence of Electromagnetic Interference on the evaluation of Lidar-derived Aerosol properties from Ny-Ålesund, Svalbard

Author(s): Tim Poguntke and Christoph Ritter

The messages of the manuscript are:

1. A 5 MHz interference has been detected in the analogue signal of the lidar system KARL. 

2. It is suspected that the interference comes from the data acquisition itself, due to ageing effects.

3. The KARL measurements could be impaired since a long time, and the correction of the old measurements would be beneficial. 

I guess that future publications using the old KARL measurements need a reference to how the distortion has been corrected. 

4. A simple method for the correction of old measurements by means of digital frequency filtering is presented. 

5. Another interference signal at 5.5 MHz has been identified when two-way radios sending at 154.5MHz were used. 

6. A wide EMI spectrum has been measured close to the laser power supply.

For other lidar users it would be interesting to read about the following:

- How strong is the 5.5 MHz signal in other than 4094 laser shot averaged signals? 

    If the strength relative to the lidar signal does not change with averaging, the interference signal must be in phase with the laser repetition.

    If it is much stronger in the single shot signal, which indicates random phase with respect to the laser, it could cause more distortions than just a SNR reduction.

- How big is the influence of this interference on the lidar signal products like backscatter and extinction coefficient?

    An analysis of more than just one example would show the importance of the correction.

- Which parts of the data acquisition are suspected of ageing and as reason for the decreased suppression of the anti-aliasing filter? 

    Has the manufacturer been consulted? 

    Can it be repaired? 

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Detailed questions and proposals:

Line 26: Proposal: " ... we will show that, if EMI is present and if it manifests itself at fixed frequencies, it can be suppressed in the frequency domain."

Line 36: Proposal: KARL consists of a telescope with a mirror of 70cm diameter and a field of view...

Line 36: I couldn't find in the internet a "290/50 Spectra laser". Please detail that.

Line 38: "Hamamatsu photomultiplier (PMT)":  because the photomultipliers can also be an antenna for EMI, it would be interesting to know more details about the model.

    Please provide a reference for "Hamamatsu".

Line 38: "..with a gating from Licel."  What does this mean? Please explain. Please provide a reference for "Licel".

Line 42:  Proposal: " ... are sampled with 16bit resolution,..."

Eq. (1) and line 49:  How is the boundary condition of backscatter ratio 1.1 motivated and why does it "reduce the impact of noise in the lidar signals" ?

Line 50: Proposal: " β and β_Ray are the total and molecular (volumetric) backscatter coefficients [ m− 1 sr− 1 ]."

Line 51f: Proposal: " The backscatter ratio retrieved from the PCNT channel at 10.5 - 11.5 km is then used as a boundary condition for the AN signal there."  

Line 59:  "...a weak aerosol layer below 1km altitude...": the height below 1 km is not shown in the plot.

 

Line 60ff:  Please provide references for  "Rohde&Schwarz Spectrum Rider FPH"  and " Aaronia HyperLOG directional antenna".

Line 63:  "... device delivers comprehensible results ...": Please specify what you mean with "comprehensible results".

Line 67ff: "... one profile from UT 22:40..." and 

Line 71:   "...the duration of the whole lidar profile...": please specify (again) the duration of the lidar profile.

Line 72:  How are the transient recorders triggered if everything is switched off? Is this triggering different from the normal measurement situation?   

Line 74f: Why do you suspect "ageing effects" as a source for the interference? Which part is considered as possibly ageing? 

Line 76ff: "... which indicates that the anti-aliasing filters of the older transient recorders are somehow ineffective....": This deduction is not coercive. Please explain.

Fig. 3 caption:  i), ii) ... is not unambiguous.  

                The interpolation seems to be over more than two samples. Actually, in the text is mentioned that you "cut out" eleven "samples". In the frequency domain the spacing of frequency points depend on the resolution of the DFT. Is a filter applied?     Please explain in more detail.

Line 91:  "...  how the evaluation is improved  ...": Which evaluation?

Line 95:  Proposal: "... the presented interference suppression method improves the SNR significantly."

Line 97ff: Proposal: exchange insecurity by uncertainty. 

Line 99ff:  "...  the difference of the aerosol backscatter from one height to the other in the interval in which the AN signal was compared to the PCNT signal. The triangle brackets indicate the mean. " What does that mean?  Equation (3) and the explanations are not understandable.

Line 103f: "The improvement by the presented RF interference suppression can be observed especially for heights above 7.5km." What improvement do you observe?

Line 112:  " ...  occur at 4.5MHz ... "  Why at 4.5 MHz?    

    

Line 113:  What is the "electromagnetic environment of the laser power supply and the transient recorders" ?

 

Line 122f: "... it occurs in the lower frequency range especially for geometrically large lidar systems with long cables." 

    But in Line 72f you write: " As this disturbance occurred also when all devices but the transient recorders were switched off and the

coaxial cables were disconnected, ..."  So the cables are not the antennas that pick up the interference. Right?

Line 125f: " Especially weak signals from higher altitudes in ground-based systems benefit from interference suppression." How do they benefit?

Line 128f:  "Finally we presented measurements indicating that placing the power supply of the laser next to the transient recorders

may also lead to electromagnetic disturbances in the lidar profiles."  

Your measurements don't indicate that. While it is always possible, that RF noise can disturb measurements, it is not clear whether the RF power you measured close to the laser power supply would really disturb the signals of the transient recorders. An experimental proof or reference is missing.