Aerosol optical, microphysical and radiative properties at regional
background insular sites in the western Mediterranean

Sicard, Michaël; Barragan, Rubén; Dulac, François; Alados-Arboledas, Lucas; Mallet, Marc

doi:https://doi.org/10.5194/acp-16-12177-2016

Articles | Volume 16, issue 18

https://doi.org/10.5194/acp-16-12177-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

https://doi.org/10.5194/acp-16-12177-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 18

Research article

|

28 Sep 2016

Research article |

| 28 Sep 2016

Aerosol optical, microphysical and radiative properties at regional background insular sites in the western Mediterranean

Michaël Sicard, Rubén Barragan, François Dulac, Lucas Alados-Arboledas, and Marc Mallet

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Final revised paper (published on 28 Sep 2016)
Preprint (discussion started on 19 Jan 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'acp-2015-823 Review', Anonymous Referee #2, 15 Feb 2016
RC2: 'Revision ACP-2015-823', Anonymous Referee #1, 16 Feb 2016
AC1: 'Partial answer to the referees’ comments and future plans from the authors', Michael Sicard, 11 Mar 2016
AC2: 'Final answer to the referee's comments', Michael Sicard, 09 May 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Michael Sicard on behalf of the Authors (09 May 2016) Manuscript

ED: Referee Nomination & Report Request started (11 May 2016) by Oleg Dubovik

RR by Anonymous Referee #2 (24 May 2016)

RR by Anonymous Referee #3 (09 Jun 2016)

Suggestions for revision or reasons for rejection

Review of the manuscript “Aerosol optical, microphysical and radiative properties at regional background insular sites in the western Mediterranean”, by Sicard et al. The paper deals with aerosol optical and microphysical properties derived from AERONET observations at three sites located in the western Mediterranean. It is well written and structured. However a major concern arises from the fact that many conclusions are derived from scarce data, especially the optical properties derived from inversions. I recommend the paper can be published after major revision. General and specific comments follow here.

General comments

1. The paper extracts conclusions in a climatic perspective, indicating that they use a long-term database. In principle 8 years of data are available at Ersa and 5 years at Palma de Mallorca. However the number of level 2 inversion data is too low for a climatological analysis. The AOD440>0.4 threshold removes most of the inversion data. In this frame, the authors are no longer analyzing the aerosol properties of the sites, but only the high turbidity events (dust or pollution). This should be clarified and the discussion re-focused accordingly.

2. The analysis of AOD has in principle sufficient data for long-term analysis, even if some caution should be taken because the datasets do not comprise the standard 30-years to be considered climatological in a strict sense. However in the analysis of Gobbi plots (Angstrom exponent difference versus Angstrom exponent) the data with AOD675<0.15 are removed. The authors do not specify the percentage of data that are ignored because of this threshold, but from average values in Table 1, I can presume is clearly above 60% of observations. Therefore the analysis presented in the paper is actually ignoring the predominant aerosol conditions at both sites, which will typically consist of few polluted marine aerosol. IN the title you have the work “background”, but the background conditions are not investigated. It is critical that those data are part of the analysis and the conclusions. A simple graphical method (Angstrom exponent versus AOD, as can be seen in Holben et al., JGR 2001) allows aerosol typing without the need of removing low AOD cases. Once this is established, the Gobbi plot can complement the investigation with some further insight in fine mode fraction, etc. for the cases with AOD675>0.15.

3. The sampling of AOD and inversion data within AERONET strongly depend on cloudiness and solar declination. Therefore the aggregation into monthly, yearly or seasonal averages cannot be accomplished without taking this issue into account. The normal procedure (see for instance the AERONET website) is to produce daily averages, and from them compute monthly means, seasonal means or multi-annual monthly means. If this is not done in this way, the much larger number of observations in summer (longer day duration and less cloudiness) produces a bias in the dataset averages. See for instance how similar are the year and the summer mean size distributions in Fig 5. In the case of inversion products, it is critical to produce daily means and from them produce seasonal means. The yearly mean should be the mean of the 4 seasonal averages. And so on.

4. Following comment nr 3 above, the number of data indicated in all plots should be no longer the number of single observation points but the number of days. This is valid for most of the figures. In this sense, the analyzed inversion data in figure 6 would be even less, but that’s the real situation using level 2 inversions and that’s the reason of comment 1: if you restrict to AOD440>0.4, you are investigating just few cases, but ignoring the predominant aerosol conditions.

Specific comments

Page 5, line 6: >2 years is not that long-term. Please reformulate.

P5, L17: if you are asking for long-term analysis, why including the short term data at Alboran? It changes the focus of the paper: you cannot any longer extract “climatological” conclusions of the gradients, only about autumn 2011. How do you justify this?

P6, L15: reference to Dubovik et al., 2006 (the spheroid retrieval for dust) is missing

P13, L15: this is the reason why short periods of data should not be analyzed in a climatic perspective (see general comments)

P14, L22: note the contradiction here: you are talking about marine aerosol here, that according to the cited paper by Smirnov et al. (2002) is found for AOD440<0.15. However all those low AOD data are removed in the Gobbi-type analysis with the condition AOD675>0.15!

P15, L15: there are not enough level 2 inversion data to establish seasonal characteristics, which wouldn’t be in any case representative for the typical site conditions

P15, L28: you can’t compute such average for the dust events because you are only looking at the subset with AOD400>0.4. There exist weaker dust events (actually they are more frequent) and the mean AOD during dust events is not 0.47.

P17, L31: the Mie(spherical) computations are very unlikely to be the reason because AERONET version 2 inversion uses spheroids and normally the portion of spherical particles considered in dust events is very close to 0.

P24, L14: you are supposedly using satellite data to validate AERONET fluxes but in the end you use AERONET data to screen out satellite data contaminated by clouds or glint. So it’s a circular argument. If you select only data that are in agreement, then the agreement is good (Fig 9b). I don’t think this is a valid approach.

Fig 11d,e,f,etc.: replace “month” with “2011”.

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ED: Reconsider after major revisions (17 Jun 2016) by Oleg Dubovik

AR by Michael Sicard on behalf of the Authors (21 Jul 2016) Manuscript

ED: Referee Nomination & Report Request started (21 Jul 2016) by Oleg Dubovik

RR by Anonymous Referee #2 (06 Aug 2016)

Suggestions for revision or reasons for rejection

Summary:
The authors have done a great deal of work and it has significantly improved the manuscript. The paper is much more readable and understandable. I find the material scope acceptable. A few minor details below, the only issues is related to validation of the AERONET fluxes section. I believe a little work, few additional sentences, could greatly improve understanding.

For the solar downward flux comparison, the pyranometer measurements have an uncertainty of 3 % (stated on page 22, line 17). Page 6 lines 20-25 talk about the AERONET upward/download flux retrievals, but do not give accuracy values on these. For a measurement to be validated, the comparison between the AERONET and pyranometer measurements need to agree within the uncertainties. Hence, if the author’s want to conclude validation, they need to give uncertainties for each parameter, and the overall difference of there comparison analysis has to be less than the combined uncertainties of the two methods. This may have been done, or could easily be done; however, it needs to be made explicit in the paper, otherwise, I feel that a comparison was done but not validation.

Even if validation is show, I would suggest that “Validation” in section title “6.1 Validation of AERONET radiative fluxes with ground-based and satellite data” be changed to “Comparison of AERONET ..”. The analysis is to compare the methods, if the comparison shows that the overall difference is less than the uncertainty, then the methods/measurements have been validated. By using the word validation in the Section Title, I feel you are assuming the conclusion before presenting the analysis.

Detailed Comments:
In the abstract, only use “validated” if the paper explicitly shows that the comparison is less than the overall uncertainty. Other wise use “comparison” instead of validated.
Page 2: Line 9-12- Seems to indicated that all forest fire produce anthropogenic particles which I don’t believe are true since forest fires can start naturally. Suggest rewording this sentence.
Page 2, Line 32. This is one example of a few I noticed in the paper where there is a comma missing for an introductory phase, for example sentence should be, “For that reason,the AOD ...” other examples Page 3, line 23, Page 3, line 35.
Tense usage is mostly consistent in current version; however, I would suggest looking at Page 5 Line 19, Page 54, Line 8, Page 55, Line 8, Page 62 Line 6,
Page 6 Line 29-31 While it likely does not make a difference to the numerical accuracy given in the paper, I believe you should do the “grand mean” of a season mean based on all the daily means, not the monthly means because the month have different days in them so should be weighted differently. See http://www.theanalysisfactor.com/when-unequal-sample-sizes-are-and-are-not-a-problem-in-anova/ for discussion of un-equal samples in calculating "grand means"
Figure 4-5, the font size for the labels inside the plot are a little on the small side. Nice if they could be made larger.
Page 56, the is an additional character after (a) Typically, letter labels to plots are given in the upper left, not the lower right. Not sure the style for this Journal but should check.

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RR by Anonymous Referee #1 (11 Aug 2016)

Suggestions for revision or reasons for rejection

Revision of the manuscript titled “Aerosol optical, microphysical and radiative properties at regional background insular sites in the Western Mediterranean” by Sicard et al.
This paper presents the analysis of the optical, microphysical, and radiative aerosol properties in two insular sites in the western Mediterranean (Ersa and Palma de Mallorca). Additionally, a case study dealing with the possible Northeast and Southwest gradients in aerosol properties is addressed, including a few months of data from a third insular site in Alborán. The analysis is based in AERONET measurements and inversion products.
The authors have done a lot of work and consequently the focus and perspective of the paper have been substantially improved from the first to the latest version of the manuscript.
With the new approach and aerosol classification the authors analyse the aerosol properties for different scenarios, taking into account separately: all aerosol conditions (including background), moderate turbidity (AOD675 > 0.15) and strong episodes (AOD440 > 0.4).
The used dataset is too short to extract conclusions from the climatic point of view, as it would be desirable. Despite of that, the analysis carried out in this paper is useful to establish differences among the sites in terms of the seasonal evolution of the aerosol characteristics, and to determine differences in the contribution of the main aerosol types (pollution and mineral dust) to this seasonal variability.
Even if the authors now address their results and conclusions in those terms, I found one point that compromises the seasonal representativeness of these data:
- As the authors assert several times throughout the paper, at the Palma site the data during spring season is poorly representative (due to the short database) (e.g. P12-L18; P14-L19)

P12-L17: What N means in this sentence?, The number of points remaining? The number of points removed?, please specify.
P12-L18: If the database is unexplotaible in winter at Ersa (N=1) and Palma (N=0), it should be also unexplotaible in spring at Palma (since, N=1). Instead of that, the authors say that “is not totally representative”…. Please explain.
What criteria do you follow to define the fine and coarse clusters? The discussion related to this clustering is highly dependent on the way that you define the clusters and results confusing. Please explain how you define the fine and coarse clusters, is it only for cases with AOD440 > 0.4 or not? For example looking at the Fig. 4a, it seems that changing the AE limit from 0.5 to 0.6 for the coarse mode cluster at Ersa may considerably vary the number of points within the cluster. The discussion on P13-L15 about the differences in the number of dust cases between spring and summer is also dependent on the definition of these clusters.
P12-L23: In Fig. 4 and 5 don’t clearly see the behaviour exposed in the sentence “At both sites the AOD440 increase observed is associated to an increase of the fine mode radius and of the fine mode fraction”.
P12-L29: I don’t find clearly the relationship the authors assert in the sentence “At the same time coarse particles, likely maritime aerosols mixed with mineral 30 dust, superimpose their signal onto this fine mode. A concurrent increase in the coarse mode fraction with moderate AOD (AOD440 < 0.4) along the rf curves (between 0.08 and 0.13 μm) and for η < 70 % is observed” when I look at the Fig. 4 and 5.
P12-L32: The Coarse mode cluster in summer is referring also to AOD440 > 0.4, as in the fine mode cluster? Please specify.

P20-L25: The introduction of Sect. 6 and part of Sect. 6.1 is mostly methodology, I would suggest moving it (or at least part of these sections) to the instruments or methods sections.

P26-L30: The sentence “ This seems to indicate that ARFTOA is not as much affected by dust long-range transport ….”, is surprising. The differences observed in ARF TOA between both sites may seem negligible, in absolute value. Have you try with the relative differences? I suspect that it will be larger at TOA than at surface.

P28-L24: Please change “Di Sarra ..” by “di Sarra...”
P29-L3: Please change “Di Sarra ..” by “di Sarra...”
P36-L12: Please change “Di Sarra ..” by “di Sarra...”
P36-L15: Please change “Di Sarra ..” by “di Sarra...”

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ED: Reconsider after minor revisions (Editor review) (20 Aug 2016) by Oleg Dubovik

AR by Michael Sicard on behalf of the Authors (02 Sep 2016) Manuscript

ED: Publish subject to technical corrections (17 Sep 2016) by Oleg Dubovik

AR by Michael Sicard on behalf of the Authors (19 Sep 2016) Author's response Manuscript

Short summary

The seasonal variability of the aerosol optical, microphysical and radiative properties at three insular sites in the western Mediterranean Basin is presented. The main drivers of the observed annual cycles and NE–SW gradients are mineral dust outbreaks in summer and European continental aerosols in spring. The lack of NE–W gradients of some aerosol properties is attributed to a homogeneous spatial distribution of the fine particle load and absorption low values in the southwesternmost site.