General Comments:

In this manuscript, the contamination of long-range transported volcanic aerosol particles was captured in the local Planetary Boundary Layer (PBL) above the Ocean Science Center at Mindelo, Cabo Verde using ground-based lidar observations. More specifically, the authors use two case studies, one before (typical local PBL) and one during the 2021 eruption of the Cumbre Vieja volcano at La Palma island which is located at a distance of about 1500 km from the measurement site to showcase changes in the lidar optical properties and therefore demonstrate the importance of volcanic aerosol detection for health related purposes even further away from the volcanic activity. More specifically, the authors support on a single case study and they report the particle extinction coefficient, the particle extinction-to-backscatter ratio and the particle linear particle depolarization ratio at three wavelengths (355, 532 and 1064nm). The authors use auxiliary information to support that the changes inside the PBL are due to the volcanic activity alone using, for example, AERONET data, backward trajectories, fire presence using FIRMS (although not shown) and FLEXPART simulations. Given the need for accurate detection of volcanic particles in the atmosphere and the scarce lidar observations during volcanic eruptions the study has potential but in the current version it lacks scientific interest and clarity. Therefore, the manuscript may be considered for publication after major revisions.

Specific comments:

The title is misleading. In the manuscript, two case studies using a PollyXT lidar are shown. One before the eruption at La Palma island and a second one during the volcanic activity. Then, the authors use the two case studies to discuss the changes in the lidar optical properties over the measurement site. Therefore, I suggest choosing a more suitable title including the word case study.  Please note and correct throughout the manuscript: The name of the island is La Palma and should not be confused with Las Palmas which is the capital of Gan Canaria.

The abstract should be short, clear, and summarize the findings of the study. As written, the abstract is misleading and lacks scientific importance. It is misleading because, the authors mention the full duration of the volcanic activity, but they do not mention that the findings rise from one individual case study. It also lacks scientific importance because although it is mentioned that a special version of a PollyXT system was used which allowed the calculation of the particle extinction coefficient, the particle extinction-to-backscatter ratio and the particle linear particle depolarization ratio at three wavelengths (355, 532 and 1064nm), there is no mentioning of the 1064nm wavelength which is the added value compared to the standard high-power lidars which operate at 355 and/or 532nm. In fact, there is no comprehensive summary of the findings per wavelength per aerosol optical property. More specifically, the lidar ratio and linear particle depolarization ratio which are of great importance in aerosol classification are absent from the abstract.

The authors support the changes in the PBL optical properties during the volcanic eruption to be caused by the presence of sulphate aerosols and they exclude the presence of other aerosol sources using FLEXPART and backward trajectories together with fire location from FIRMS. It would be beneficial to include a FIRMS figure and I was also wondering whether there are in situ observations at Mindelo site to check the presence of black/organic carbon. This will solidify your conclusions regarding the higher lidar ratio observed during the volcanic activity and its origin.

Then, I really missed a long-term report of the lidar optical properties including the full period of the volcanic activity. The authors advertise in quite a few points in the manuscript that the lidar has captured the full volcanic activity. So, why did you choose to focus on one case study only and not include the full dataset? Furthermore, it will be of great importance to go a step further and estimate the mass concentration of the long-range transported sulphate aerosols.

Overall, in its current form the manuscript is a report of a single case study in which the optical properties are the result of marine and sulphate aerosol mixture of unknown contribution with limited added value to the scientific community and at places highly speculative. The presentation and usage of English should be also substantially improved.

L122-124: Can the authors comment on the stability of the calibration for this wavelength? What is the expected error in the optical products? The references in this sentence point to another lidar system and not PollyXT.  What is the error estimation for this system? Furthermore, the 1064nm depolarization capability is also a new feature. What is the uncertainty of the particle depolarization ratio for this system?

L178-179: Is there a reference to support the statement that the measurement on the 16^th of September represent the typical situation over the location?

Figures 3-5: What the error bars refer to? Do they include the systematic errors or just the variability caused by the time averaging?

Technical corrections:

Lines 67-69: Please give a reference.

Lines 75-86: Note this publication about Cumbre Vieja volcanic eruption:

Bedoya-Velásquez, A.E.; Hoyos-Restrepo, M.; Barreto, A.; García, R.D.; Romero-Campos, P.M.; García, O.; Ramos, R.; Roininen, R.; Toledano, C.; Sicard, M.; et al. Estimation of the Mass Concentration of Volcanic Ash Using Ceilometers: Study of Fresh and Transported Plumes from La Palma Volcano. Remote Sens. 2022, 14, 5680. https://doi.org/10.3390/rs14225680

Line 104: Please check that the coordinates of the measurement site are correct. I think it should be W, not E.

Figures 2-5: Use the same thickness for all lines.

Figure 3: Be consistent on the heights at which the error bars appear. For example, Figs. 5a, b and c all have the error bar in different locations.

Figure 4d: ref. lines are missing from the legend. For example, the dotted orange line which most probably refers to the bae532/1064 (RR) on the 16^th of September.

Figure C1: Include the 16^th of September 2021 as it is one of the case studies. The figure could also be a bit more zoomed to the region of interest.

Review of https://doi.org/10.5194/egusphere-2023-2305

EGUSPHERE-2023-2305 | Research article

Henriette Gebauer et al., Tropospheric sulfate from Cumbre Vieja volcano at Las Palmas, transported towards Cabo Verde – lidar measurements of aerosol extinction, backscatter and depolarization at 355, 532 and 1064 nm

General comments:

The manuscript describes the measurements of a volcanic aerosol cloud from the Cumbre Vieja volcano eruption in Sept. 2021 that was transported 1500 km over the Atlantic ocean from the island La Palma to the measurement site in Mindelo on the Cape Verde Islands. Measurements were performed with a multi-wavelength Raman and depolarisation lidar and a sun- and moon-photometer. Additionally, trajectory calculations are used to verify the origin of the measured airmasses in the boundary layer and in the free troposphere. The comparison of the measured values of the optical properties with the ones from a clean reference period a few days earlier and from lidar measurements of other campaigns indicate that the measured aerosol in the boundary layer is sulfate aerosol from the volcanic eruption. 

As there are only few high quality multi-wavelength lidar measurements of aerosol stemming from volcanic eruption - especially spanning the wavelength range from 355 nm to 1064 nm, the retrieved aerosol optical properties add valuable data to the global database that can be used for aerosol characterization. The manuscript is very well organized and written.

Specific comments:

Las Palmas is a municipality and city on Gran Canaria.  

The Cumbre Vieja volcano is located on the island called La Palma.

The dual-field-of-view channels are mentioned in chapter 2.1 but unfortunately not used. 

Why? 

And if not used, why mentioning it?  

The absolute scale of the "attenuated backscatter coefficient" shown in the fig. 2 need reference values at reference heights for each plot. 

This is even more necessary as measurements of two different days are compared (figs. 2c and 2d). 

Figs. 3e and 4e: the x scales should be the same for an easier comparison of the values in the two plots.

Fig. 5b and 5c: the red 1064 nm (RR) line does not match the description in the figure caption. If an increased smoothing of 742.5m is used for the particle extinction coefficient and the lidar ratio at 1064nm, then there can be only one value in the considered height range between 0.25 km and 1.0 km, but there is a line of values between 0.75 km and 1.0 km. 

 

The error bars in figs. 3 to 5 are not explained:

1. What are the error bars showing? Should be mentioned in each figure caption.

2. Why does the error not change over large height ranges for e.g. in figs. 3c, 3d, and 3e, etc? 

3. Figs. 3e and 4e should have the same x-scale for a better comparison.

4. The error bars in fig. 5b of the 355 nm and 532 nm are unrealistically small.

5. In fig. 5c the error bar of the 1064 nm RR curve is much larger than the layer variability. What does it show?

The header of table 1 is a bit confusing:

1. In fig. 5b the values at 1 km height are already less than half of the mean values below. What does then the "edge effect" mean? 

2. It is unclear what "standard deviation" means and how it is derived. Is it the uncertainty due to signal noise or the parameter variability over height?

3. Is it possible that table 1 does not show any uncertainties?

4. Why is there no 1064 nm AOD? 

5. What do the +- values of the AOD mean? Standard deviation?

The error bars / standard deviation values in the plots and table need a better discrimination and clearer description. This is especially necessary if these values should be used in other studies for aerosol typing. 

Furthermore, are systematic uncertainties considered?

Considering the uncertainties and variability, is the increase of the lidar ratio mentioned in lines 237ff really significant?

In line 293f it is stated about the AE values:  

    During the measurement period, the values decreased to 0.7 ± 0.1 and 1.7 ± 0.3, respectively, due to hygroscopic growth of the sulfate particles.

=> It is not clear, why the AE decreases. What is the difference between the aerosol parcels at the start and at the end of the "measurement period" - with respect to particle growth? Why do the latter grow more than the former?

Some correction proposals:

Line 28: diameter lower than => diameter smaller than

Line 63: ...which is in the range of 30 ±5% for pure dust ...  => at which wavelength?

Line 65: ...lower particle linear depolarization ratio ... => typical values?

Line 66:  ; John et al., 2011).  => (John et al., 2011).

Line 145: In addition, the add(horizontal) distribution of the volcanic plume was monitored. 

Line 150:  A time series of the AOD => A time series of the CIMEL AOD at Mindelo 

Line 151:   the hourly mean AOD was about 0.4.  =>  at which wavelength?

Line 154:    AODs, with values close to 1.0,  =>  at which wavelength?

Line 193:    than the once measured => ones

Line 195:   The lidar ratio at 1064nm is in line with dust observations at 195

Leipzig, Germany  => are there no changes due to long range transport to be expected?

Line 207:  In addition, vertical smoothing was reduced, which improves

the accuracy of the near-field profiles => in which sense does the accuracy improve? The resolution is improved. But for which purpose?

Line 278:   the measured quantities => Do you mean aerosol "quantities" or aerosol "optical properties"?

Line 321:  The intense pollution caused an unusually high AOD of more than 1.0 at different spectral bands. => change "different spectral bands" to "at the smaller wavelengths" - or so.

Line 351ff: " Having measurements at all three wavelengths allows us to get new insights in lidar-based aerosol typing and to enlarge our data sets. The findings of this study provide useful insights on the lidar-derived optical properties of volcanic aerosol."

=> This are diffuse statements. What are the new and useful insights?