| 25 Sep 2025
Characterization of the annual cycle of atmospheric aerosol over Mindelo, Cabo Verde, by means of continuous multiwavelength lidar observations
Abstract. This paper presents an analysis of the annual cycle of aerosol optical, and geometrical properties based on multiwavelength-Raman-polarization lidar measurements for Mindelo, Cabo Verde, from July 2021 to August 2023. A quality-assured data set of more than 70 automatically-calibrated lidar profiles was manually evaluated. For the first time, a two-year time series of, e.g., layer-resolved aerosol optical depth (AOD), lidar ratio profiles, and particle depolarization profiles are presented for Cabo Verde to characterize the complete annual cycle of aerosol in the planetary boundary layer (PBL) and in the lofted aerosol layers. The aerosol conditions over Mindelo are complex with different mixing states of dust and non-dust components. A strong annual cycle was found in the overall aerosol layer top height and the geometrical extent, the AOD, and the dust fraction of the lofted layers. Furthermore, the data was used to explicitly define aerosol-related seasons. The dust season, characterized by geometrically and optically thick lofted layers dominated by Saharan dust above a slightly polluted marine PBL, ranges from June to September. Aerosol occurs up to 7 km height. The seasonal mean lidar ratios and particle depolarization ratios at 355, 532, and 1064 nm are 32–34 sr and 0.02–0.05 for the PBL and 39–48 sr and 0.16–0.22 for the lofted layers. The mixing season covers the months November to March and is characterized by a large variability of aerosol, including mixtures of dust and smoke. The mean lidar ratios and depolarization ratios are 33–38 sr and 0.03–0.06 and 48–60 sr and 0.09–0.16.
Review on the manuscript of Gebaur H. et al.: “Characterization of the annual cycle of atmospheric aerosol over Mindelo, Cabo Verde, by means of continuous multiwavelength lidar observations”
General comments
The manuscript presents the results on the analysis of the annual cycle of atmospheric aerosols over Mindelo, Cabo Verde using weekly PollyXT lidar observations from a 2-year dataset (July 2021 – August 2023) and collocated AERONET measurements. Moreover, the authors make use of the POLIPHON method for the derivation of the dust fraction and the DeLiAn database to support their analysis and findings. This study focuses on the investigation of the temporal development of the geometrical and optical properties among the different aerosol layers that are being identified (Planetary Boundary Layer and the lofted aerosol layers) for each measurement case aiming to define aerosol-related seasons such as the dust season, the mixing season, and the transition one. Even though I am a bit surprised about the low particle linear depolarization values at 355 nm (~0.16 ± 0.07) being reported during the dust season, I think that this study provides detailed information, that our community could benefit from, about the predominant aerosols and their geometrical and optical properties above Cabo Verde Islands, a region in the outflow of the Saharan desert.
Overall, the manuscript is well structured and well written but some parts could be further explained and/or discussed. The scientific significance makes the manuscript suitable for publication in AMT, after some minor revisions have been considered from the authors.
Specific comments
Line 13 “The mean lidar ratios … and 0.09-0.16”: I guess the authors provide the lidar ratio and depol. ratio values for the PBL and the lofted layers. Please clarify in the text.
Lines 40 – 43 “For example, SAMUM–2 … on the African continent.”: I would suggest to add an indicative citation for this statement.
Line 50 “these observations”: These observations refer only to Barreto et al. or SAMUM are included too? Please clarify.
Line 84 “ESA’s satellite Aeolus”: Here you could also cite some of the historical Aeolus mission papers (e.g. Stoffelen et al., 2005, https://doi.org/10.1175/BAMS-86-1-73; Reitebuch et al., 2012, https://doi.org/10.1007/978-3-642-30183-4_49). Moreover in the following sentences you could also add one indicative citation for the Aeolus wind products and one for the aerosol products.
Lines 94 – 95 “The PollyXT … (nr) measurements”: Please provide the full overlap height for the fr and the nr measurements.
Line 119 – 120 “Furthermore, … Tesche et al. (2009)”: Please provide the thresholds you used in the lidar ratio and the particle linear depolarization ratio for the dust and non-dust separation.
Line 145 – 150: It is not so clear many measurements have been used in the study after all. The authors mention a dataset of 73 cases (Fri/Sat dataset) which, however, to my understanding it is further reduced for the analysis presented from Figure 3 and on (i.e. sections 3.2, 3.3, and 4). More specifically, for this analysis, the authors use cases where both the nr and the fr backscatter and extinction coefficients (at 532 and 355 nm) should be available which may reduce the number of 73 cases. Please clarify and elaborate on possible change of the dataset being used in the analysis of each section.
Line 166 – 167 “lofted aerosol layers … Ångström exponent.”: Please elaborate on how the fr-particle backscatter coeff. is being used for the definition of the lofted layers. According to lines 167 – 169 only the depol. ratio and the BAE are used as metrics.
Line 170 “noticeable”: What does this mean? Please give more details or threshold values that indicate a noticeable difference.
Line 176 “… lowermost available value to be constant down to the ground.”: which altitude value is this for the backscatter coeff. and for the extinction coeff.? How do you define the height bin with the lowermost available value of beta and alpha (using full overlap height for nr)?
Lines 177 – 178 “The sum of the layer-AODs was compared with the columnar AERONET AOD.”: Please add which AERONET AODs are used for the comparison with the lidar-retrieved AODs from 532 and 355 nm.
Line 216 “The temporal development … illustrated in Fig. 2.”: which lidar profiles (and wavelength) have been used to create Figure 2 (PBL top height and lofted layer base/top heights from manual inspection vs from Hofer et al.)?
Lines 217 – 218 “Additionally, the manually-defined layer top heights are added in red for the
cases having no particle backscatter coefficient at 532 nm”: what about the availability of part. backscatter coef. at 355 nm? Why not to use also these profiles?
Lines 216 – 229: Why do the automatically retrieved layer top heights are being showed and discussed here, since the manual retrievals with visual inspection is considered as the best approach? Since the automatic method (Hofer et al., 2020) has its limitations for Mindelo cases (Lines 152 – 163, 192 – 197), please update figure 2 and corresponding analysis and discussion using the manually retrieved layer top heights. The current Figure 2 should be moved in an appendix and could be updated by showing both the automatic and manually retrieved layer top heights, if the authors aim for an investigation of the applicability of Hofer’s method to Mindelo.
Figures 2 – 5: A suggestion for better visualization would be to consider adding gridlines (at least for the major ticks) in the plots. Especially for the time series figures I think it would be helpful to add minor ticks in x-axis to indicate the non-labeled months (Aug, Sep, Nov, Dec, etc). Same applies for figures in the Appendix.
Line 286: Please revise the sentence to be more clear.
Line 292 “… slightly below 0,…”: please provide an average or indicative number.
Figure 5: Since the monthly mean values per year are not statistically significant, then why not using all data points?
Lines 316 – 318 “A cluster of data points… Floutsi et al., 2023)”: To my understanding this cluster is cluster 1. Please integrate the cluster numbers (cluster 1 – 6) throughout the whole paragraph to be easier to the reader to link Fig. 5 with the discussion (lines 313 – 342).
Lines 325 – 326 “According to Floutsi et al. (2023), data points with a particle linear depolarization ratio larger than 0.25 point to the occurrence of pure dust in the lofted layers”: Here the authors use the threshold value of 0.25 for defining the pure dust cluster (cluster 6) but this contradicts the use of particle linear depolarization of 0.31 for the pure dust separation in the POLIPHON products in section 3.2 which leads to the point of existence of a non-dust contribution in all lofted layers (lines 276 – 281).
Line 349 “mean particle linear depolarization ratio”: please provide the wavelength.
Lines 351 – 352 “Similarly… dust season”: I am confused. Is this an additional criterion or the result after applying the 3 criteria from lines 347 – 350 into the measurements for these months? If it is an extra criterion, does this mean that measurements from the months of the dust dominated cluster do not fulfill the 3 criteria?
Line 354 “transition months”: What are the criteria for defining a month as a transition one? Please include a short description in the text.
Lines 357 – 359: Are these conditions/criteria being used to define a mixing season? Different parameters are being used to define a dust (total AOD, contribution of lofted AOD, particle linear depol. ratio) and a mixing season (dust fraction, Ångström exponent of lidar ratio). Could the authors support why they do not use common parameters (with different threshold values) to distinguish the aerosol related seasons.
Line 359: The Ångström exponent of the lidar ratio is never mentioned before in the analysis or in the plots, but is used here as a criterion. Could the authors elaborate on why they do not use the backscatter or extinction related exponents?
Line 362 “August”: Do you mean October?
Line 368 “is 5.6±0.9 km (automatically-retrieved layer top heights).”: Again here, why do the authors use the automatic retrieved layer top heights instead of the manually retrieved ones which are considered as the best approach according to section 2.3.
Line 393 “using the two-layer approach according to Berjon et al. (2019)”: I would suggest to add a brief comment about the main differences/assumptions of the Berjon et al., compared to the direct retrieval of the lidar ratio from the lidar.
Line 413 “… and a lower particle linear depolarization ratio …”: I would like to see a discussion from the authors on the differences in the depol. ratio (mainly for 355 nm) with respect to the SAMUM-2b results (and the DeliAn database to my opinion). Currently only the lidar ratio differences are being discussed.
Line 454 “(depth of around 4 km and AOD up to 0.5 at 532 nm)”: It is not clear if these values correspond to seasonal mean values. Please clarify and use seasonal means (if not the case).
Lines 451 – 465: here the authors could also add key conclusions also for the rest intensive optical properties (e.g. lidar ratio, particle depol. ratio) for each aerosol-related season.
Technical corrections
Line 48 “multiple complete years”: I would suggest an addition like “multiple complete years above Canary Islands” or similar.
Line 80 “Aeolus was a equipped with a wind…”: Is something missing here?
Line 112 “0.7 x 105 MHz m.”: is the “m” at the end a typo?
Line 143 “In most cases, single optical properties at a certain wavelength and only one complete nighttime measurement”: Looks like something is missing here. Please revise.
Line 234 “The latter PBL top heights are often …”: Suggested change to " The highest observed PBL top heights (*maybe add here the range of PBL top heights for the cases with no lofted layers) are often…" or similar?