The Authors discuss the application of the GRASP/GARRLiC synergistic algorithm to joint sun-photometer and lidar observations of the aerosol optical properties from the EARLINET/ACTRIS-19 COVID-19 campaign. While the original goal of the study was to characterize the anthropogenic component of atmospheric aerosols during and after the COVID-19 lockdown periods in Europe, the number of observations proved not sufficient for a statistically significant characterization. However, the Authors concentrate on investigating the sensitivity of the retrieval to a number of algorithmic parameters, such as the choice of the first guess and of the smoothing constraints. Whilst the study is rather focused on technical details, its results may still be of interest for people who try to perform synergistic aerosol retrievals from passive and active observations. In general, the paper is generally well written and can be published with minor revisions. Below are my (mostly minor) comments:

- L 56. In this sentence, I suggest replacing "anthropogenic" with "anthropic"

- L62-63, "is comprised by" -> consists of

- L83, remove "the" before "lidars"

- L91. "This is though" -> "However, this is"

- L161. What is the difference between "volume" and "particle" linear depolarization ratio? Can you define these quantities somewhere in the text?

- Figures 5 and 6. Consider inverting the order of the figures, and describe them more precisely in the text (e.g., "Fig. 5 show retrieved size distributions and concentration profiles for the case study etc...., and Fig. 6 shows the agreement betwen observed values of total optical depth, sky radiance and lidar signals, and the values fitted with GARRLic/GRASP).

- Does Fig. 7 add any new information to Fig. 6? If not, you may consider removing it (isn't the optimal solution already highlighted in Fig. 6?)

- L306-307. "This indicates... part of the retrieval uncertainty of the solution". Is this a general statement or does it just hold for the example you are showing?

- L330. "the information content... size distribution" -> "the information content of lidar measurements on the aerosol size distribution"

- L339. Some references are missing here. Are you maybe referring to Fig. 10?

- L340, eq. 1-2, Appendix B. Why do you indicate concentration with VD? Could you use a more intuitive symbol? For example, in eq. 3 you use c...

- L369-372. "h_a (pink)... full overlap height". What do you mean by "taking into account"? How do you actually take those values into account?

- Figs. 14.-15. How do the variables that appear in the plot correlate? Scatter plots may be useful in addition to histograms.

- L426. "depends also" -> "also depends"

- L460. "of the simulated signals against the input ones" -> "between simulated and measured signals"

- L462, "to obtain a quantitative metric" -> "to obtain such a metric", "input" -> "measured" ?