Aerosol characterization in the Subtropical Eastern North Atlantic region derived from long-term AERONET measurements

The paper describes and summarizes the columnar aerosol optical characteristics of air masses passing over Tenerife island toward Europe in a long period, ranging from 2005 to 2020. The measurements, taken in the 4 sites located at different levels from the surface, allow a good characterization of the atmospheric layers. Although the paper doesn’t present novel concepts or ideas, the produced results are very important for climate studies and also to understand how optical characteristics of air masses coming from Africa modifies during their passage toward Europe.

The analysis is well structured and clearly explained and the substantial conclusions are reached. The week part, on my opinion, is the section 3.3: Long-Term trends. The supplementary material contains not the trends but the time behavior of AOD -total, fine and coarse- at IZO (between 2005 and the end of 2020, please correct the caption) and AOD -total and coarse at SCO (between 2005 and the end of 2020, please correct the caption). The time behavior of AOD -fine is shown only in Figure 10, together with its trend. It would be better showing AOD -fine in the supplementary plot too. From the analysis it seems that only SCO shows a trend in AOD-fine. Would it be possible to analyze for both SCO and IZO what happen to the absorption capabilities (Absorption Aerosol optical Depth, or SSA) and refractive index? Is there any trend in these parameters in both the sites?

I also have some minor corrections:

 Abstract:

line 4: ”this site is can …” please correct;

line 16: “and fine mode fraction <0.35” correct with “ volume contribution of the fine mode fraction Vf/Vt <0.35”

Introduction:

Line 50-51: state somewhere the site is Tenerife:

Line 56 define and provide a ref for “dust belt”

Section 3.1:

Line 152 What about the minimum visible in May?

This study from A. Barreto et al. performs a multi-annual characterization of the columnar aerosol properties (AOD, AE, SSA, etc) obtained by AERONET at four sites in the Tenerife island, two of them representative of the marine boundary layer, and the other two representative of the free troposphere. 

The location of the island and the instruments is privileged and the length of the different data series is notable (9 to 16 years) so the results are of evident interest for the scientific community. Although the techniques are not novel, they are extensively used, and the data analysis, the presentation and the use of English is good. My recommendation is to publish the article after few corrections listed below. No major general corrections are placed.  

 

Specific comments:

Abstract.

- Line 4: "This site is can be..." please correct. 

- Line 9: Please write Angstrom the same in all appearances in the text (A^o_ngström)

Text:

- Line 90: I think ground-baseD is more adequate. 

- Lines 119-120: the limited availability of data at TPO is due to bad weather conditions. But, is the instrument removed from the site? I would bet it is,  but it could be stated so in the text. 

- Line 170: What are the AE values recorded in background conditions? (only AOD in background conditions is indicated)

- Line 185: the slight increase in March merits a short explanation even if it is extracted from literature. 

- Line 206: it is said that the coarse mode in Figure 5 is slightly increased in the summer and spring months in case of LLO, but in the figure it seems the yellow line corresponding to June has the smaller coarse mode, even if summer. Not sure about all the months due to color and symbols. Some of them are hardly visible. Please consider to group them in seasons, or use only selected months, or increase the size of symbols. 

- Line 266: this line has been added in this manuscript version but it needs to be rewrited (grammar).

- Line 317: please rewrite the sentence. 

- Line 325: are the two break points from SO2 obtained with same Lanzante method?

Figures:

- Figure 1, caption: "Image" instead of "imagen"

- Figure 2, why 1020 nm AOD and AE is not shown?

- Figure 9: Y-axis is "Asymetric" parameter. Please correct. Asymmetry parameter should be correct. 

- Figure 9, caption: error bars is a more common term, instead of error brackets

This work of Barreto et al, focus on four very close AERONET stations on Tenerife island. Timeseries are long enough to provide adequate results for the area and the topography makes the place ideal for studying the stratification of Boundary Layer and the effect on the Aerosol optical properties. Additionally, the location is in a region with very frequent Saharan dust intrusions that alters the aerosols regime. The study is of high interest and in it fits the journal. The presentation is very well organized and the results will push forward the scientific knowledge on for the aerosol state above north Atlantic ocean. I suggest to accept it for publication after minor revisions.

Comments:

A lot of commas are missing. Please read carefully and correct the syntax. Eg line 13,40,43.

L66. In the previous sentence the typical stratification of  the troposphere in the region is described. This sentence begins with “ these contrasting aerosol regimes”. I think a sentence is missing linking the layers with aerosol regimes. Also, the characterization of “key site for aerosol monitoring” is repeated 10 lines earlier.

L136-7 Also, Solar Zenith Angle cut off for inversion product should be mentioned here, since it limits a large number of available data.

L178-187 It is not consistent that different AE thresholds for dust laden conditions are set at the stations.  In theory, AE should be independent of the station when saharan dust is dominating the aerosol mixture. This should be discussed in detailed. If there is some assumption related to the aerosol in the lower column , it should be thoroughly explained, since it affects the analysis of the next sections.

L290. Since the criterion for FT is AOD500nm ≤0.10 , there should be no AERONET SSA retrieval at these cases. Please , explain if this SSA values are refering to FT or rephrase.

L303 It is not very common to refer to some analysis that is not shown (at least in an appendix). Since it is important enough to be mentioned it in the manuscript, is should be presented.

3.3 It is valuable to present also the trends in frequencies of MABL, FT, Dust laden cases in those stations. Is there a change in the frequency of any of those layers, that is linked to the observed trends in optical properties?

L332 The first break point of SO2 in 2009, is not convincing, it seems more like a 3 month parenthesis to the activities. In the 2009-2013 the higher SO2 values are recorded.  I suggest to just use two periods like in AOD fine mode fraction or discuss thoroughly the reasons for selecting 3 periods