Aerosol Direct Radiative Effects under Cloud-Free Conditions over Highly-Polluted Areas in Europe and Mediterranean: A Ten-Years Analysis (2007–2016)

Round 1

Reviewer 1 Report

Manuscript: Aerosol direct radiative effects over highly-polluted areas in Europe and Mediterranean: a ten-years analysis (2007-2016)

General comments

The authors examined aerosol radiative effects on urban regions (Benelux and Po Vally) and the Sahara Desert. I have three major comments for this manuscript below:

1. Originality and Novelty of this study should be highlighted
2. More references should be added to support the research (especially in Introduction)
3. I recommend carrying out additional analysis for variation of aerosol type over the regions of interest for a decade.

Abstract

I recommend adding the main quantities of this study in the Abstract.

Line 11-17: The sentence is too long. Please separate it into several sentences rather than a single sentence.

Introduction

Please provide more references to support sentences in the Introduction.

Example:

- In this areas, the associated climate ~ with respect to other areas.

- On the other hand, the Benelux region ~ industrial district in Europe.

- can locally warm or cool ~ clouds and precipitations.

I recommend providing background or related studies for the calculation of radiative effect using AOD data.

Materials and methods

- The authors used CALIPSO aerosol classification data to calculate direct aerosol radiative effects so that the uncertainties of CALIPSO aerosol type can affect the accuracy of direct aerosol radiative effect. The authors should discuss the reliability of CALIPSO aerosol type information.

Results and Discussion

- I recommend adding trend lines in Figure 3. And please add the unit of temp. anomaly.

- In my opinion, radiative effect is different from aerosol type (absorbing/non-absorbing aerosols). The authors should discuss variation of CALIPSO aerosol types in regions of interest.  

 

Author Response

We would like to thank the reviewer for the meaningful comments that improved our manuscript. We provide a point-by-point answer below, in italic, red

General comments

The authors examined aerosol radiative effects on urban regions (Benelux and Po Vally) and the Sahara Desert. I have three major comments for this manuscript below:

1. Originality and Novelty of this study should be highlighted

Thanks, we expanded the introduction and conclusions adding more text that highlight the importance of our study.

1. More references should be added to support the research (especially in Introduction)

We added the references accordingly  (see below)

1. I recommend carrying out additional analysis for variation of aerosol type over the regions of interest for a decade.

We agree that aerosol typing is fundamental for this study. However, any kind of variation in aerosol typing over the decade is fully reflected by the heating rate and radiative effected analysis (as CALIOP already provides the aerosol typing). We added accordingly text and equations in Section 2.4 to make this point more clear. 

Abstract

I recommend adding the main quantities of this study in the Abstract.

We added the quantitative results in the abstract

Line 11-17: The sentence is too long. Please separate it into several sentences rather than a single sentence.

We split the sentence as suggested.

Introduction

Please provide more references to support sentences in the Introduction.

Example:

• In this areas, the associated climate ~ with respect to other areas.

Sophie C. Lewis, Andrew D. King, Sarah E. Perkins-Kirkpatrick, Daniel M. Mitchell, Regional hotspots of temperature extremes under 1.5 °C and 2 °C of global mean warming, Weather and Climate Extremes, Volume 26, 2019, 100233, ISSN 2212-0947, https://doi.org/10.1016/j.wace.2019.100233.

Nicholls, R., P.Wong,V. Burkett, C.Woodroffe, and J. Hay, 2008: Climate change and coastal vulnerability assessment: scenarios for integrated assessment. Sustainability Science, 3(1), 89-102.

Spiegl, T., & Langematz, U. (2020). Twenty-First-Century Climate Change Hot Spots in the Light of a Weakening Sun, Journal of Climate, 33(9), 3431-3447, DOI: 10.1175/JCLI-D-19-0059.1.

• On the other hand, the Benelux region ~ industrial district in Europe.

Harrison, P.A. and R.E. Butterfield, 1999: Modelling climate change impacts on wheat, potato and grapevine in Europe. In: Climate Change, Climate Variability and A g r i c u l t u re in Europe: An Integrated Assessment [Butterfield, R.E., P.E. Harrison, and T.E. Downing (eds.)]. Environmental Change Unit, Research Report No. 9, University of Oxford, Oxford, United Kingdom, 157 pp.

Hinkel, J., R. Nicholls,A.Vafeidis, R.Tol, and T.Avagianou, 2010:Assessing risk of and adaptation to sea-level rise in the European Union: an application of DIVA. Mitigation and Adaptation Strategies for Global Change, 15(7), 703-719.

• can locally warm or cool ~ clouds and precipitations.

Crosier, J., Allan, J.D., Coe, H., Bower, K.N., Formenti, P. and Williams, P.I. (2007), Chemical composition of summertime aerosol in the Po Valley (Italy), northern Adriatic and Black Sea. Q.J.R. Meteorol. Soc., 133: 61-75. https://doi.org/10.1002/qj.88.

Perrino, C., Catrambone, M., Dalla Torre, S. et al. Seasonal variations in the chemical composition of particulate matter: a case study in the Po Valley. Part I: macro-components and mass closure. Environ Sci Pollut Res 21, 3999–4009 (2014). https://doi.org/10.1007/s11356-013-2067-1.

Stanhill, G. (2007), A perspective on global warming, dimming, and brightening, Eos Trans. AGU, 88( 5), 58– 58, doi:10.1029/2007EO050007

we added the references accordingly

I recommend providing background or related studies for the calculation of radiative effect using AOD data.

Added in the text (Sections 2.3 and 2.4). We used the same methodology as in:

Tosca, Mika G., et al. "Attributing accelerated summertime warming in the southeast united states to recent reductions in aerosol burden: Indications from vertically-resolved observations." Remote Sensing 9.7 (2017): 674.

Materials and methods

• The authors used CALIPSO aerosol classification data to calculate direct aerosol radiative effects so that the uncertainties of CALIPSO aerosol type can affect the accuracy of direct aerosol radiative effect. The authors should discuss the reliability of CALIPSO aerosol type information.

As explained before, we followed the methodology in Tosca et al., where also limits of the methodology are explained. However, we agree with the reviewer that the explanation was too short. For this reason we added a large paragraph in Section 2.3 to explain the used methodology.

Results and Discussion

• I recommend adding trend lines in Figure 3. And please add the unit of temp. anomaly.

The authors agree with the review, but the issue related to the temperature anomaly trends is addressed in the Fig. 2 by using a different approach for calculating the trends before and after the 1980. The Fig. 3 is an illustration of time series over different period according to the parameters (e.g. AOD and anomaly temperature) and locations (e.g. areal means and measurement sites). The unit of temperature anomaly was added.

• In my opinion, radiative effect is different from aerosol type (absorbing/non-absorbing aerosols). The authors should discuss variation of CALIPSO aerosol types in regions of interest.  

The FLG code depends on aerosol typing, as different aerosols produce different aerosol effects. As stated by the reviewer, the radiative effect is very different between absorbing and non-absorbing aerosols. To make this point clear, we added more text in the FLG description (section 2.4)

Reviewer 2 Report

The analysis in the manuscript focuses on two highly polluted regions and one region presumably immune to anthropogenic influence to quantify the trends in atmospheric turbidity to changes in bottom of the atmosphere energy flux. The paper first demonstrates an increasing trend in surface temperatures for all three regions. Then it uses CALIPSO aerosol extinction profiles and aerosol type information within the FLG radiative transfer model to compute the bottom of atmosphere (BOA) radiative forcing due to the presence of aerosol. In section 3, trends are computed as Theil-Sens slopes for CALIPSO AOD at four arbitrary altitude levels in the mixing layer and for the whole column for the years 2007-2016. The manuscript reports decreasing AOD with increasing BOA radiative flux in the polluted regions and increasing AOD with decreasing BOA radiative flux in the desert region. The analysis then compares 40-year trends in temperature anomalies to AERONET and CALIPSO AOD trends (when available) for these regions. The manuscript reports increasing surface temperatures in all regions coinciding with decreasing AOD in the polluted regions and a lack of AOD reduction in the desert region. From this the manuscript concludes that “the aerosol loading should be taken in consideration when assessing the temporal variability of the surface flux budget”.

The experiment setup, comparing two polluted regions to one without anthropogenic influence, seems logical and the data being used is appropriate. It would have been helpful to include more details about the remote sensing data being used and their uncertainties. For instance, was the CALIOP extinction data quality screened (it should be). If so, how? The method used to aggregate these extinction profiles can influence how aerosol is distributed vertically in the radiative transfer model, so it is important to describe or cite these details. Similarly, the AERONET “quality assured” data is used, but no information is given on what exactly that means. Further, it is not clear how statistically significant the Theil-Sens slopes are relative to the variability in the time series. Table 1 lists AOD trends based on these slopes and primary conclusions of the paper are drawn from these slopes, but are they significant? Another publication by Toth et al., 2016 compute AOD trends from CALIPSO data and their discussion goes to great lengths to disprove sensor degradation as an influence on the time series and clearly when trends are significant or not. Similar discussions should be added to this manuscript to explain how reliable the trends reported are – even a brief discussion and appropriate references. It is difficult to establish confidence in the conclusions without understanding the uncertainties.

Altogether, the writing and background information provided is sufficient. The topic is appropriate for Remote Sensing. I have some specific comments below that can easily be addressed in a revised manuscript. My main concern is the lack of detail describing the remote sensing data used, their weaknesses, and uncertainty analyses of the trends reported. Because the conclusions of the manuscript rely on this information, I feel major changes are warranted before publication.

Specific comments:

Lines 24-25. Can references be added supporting these claims? “air-quality and climate related research activities identified some European regions as climate hot-spots. In this areas, the associated climate changes happen faster with respect to other areas.”

Figure 1: Consider labeling Benelux and Po Volley on the map.

Line 118: Consider adding “0.5 latitude-longitude resolution by CRU” to avoid confusion with temperature degrees.

Lines 143-144. This sentence seems incomplete. Does it mean to say, “CALIPSO measurements provide critical information…” or just measurements in general? The next sentence makes me think the first is talking about CALIPSO measurements because it starts with, “In addition, the optical depth derived from CALIPSO…”. The 

Line 147. It seems like the reference [23] is given here to show an example of how CALIPSO aerosol optical depths are used in combination with passive L2 aerosol retrievals, but reference [23] does not appear to have anything to do with CALIPSO retrievals based on the abstract. Is there a better reference?

Line 149. Sunphotometers misspelled.

Line 164. The official CALIOP name for this species in version 4 is “polluted continental/smoke” to reflect that the aerosol could be polluted continental or smoke or a combination of the two. CALIOP level 2 algorithms cannot distinguish between these types when the layer top is < 2.5 km. Only when the layer top is above 2.5 km will the layer be classified as smoke (Kim et al., 2018). This has a relevance for how the CALIOP observations are parameterized into the FLG RT model which causes some uncertainty. This should probably be disclosed in the manuscript as a potential error source. 

Line 188. The more appropriate reference [32] for lidar ratios used by CALIOP in version 4 is Kim et al., 2018. They are different than those used in Omar et al., 2009.

Line 194: “Section ??”

Table 1. How are the BOA radiative flux trend values calculated? The units are W/m2, so is this the difference in BOA radiative flux for 2016 minus 2007? This information (however this is calculated) needs to be added into the main text. It is not clear. Also, section 2.4 describes the computation of radiative forcing using the FLG RT model, but the language in the Table 1 caption is radiative flux. Is different terminology being used to describe the same thing?

Table 1 caption. Units would be helpful to add, such as “per year”.

Line 202. Missing a negative sign before “3.0x10-3”

Line 206. “The authors speculate that high concentration levels originated from combustion processes…” High concentration levels of what? Please specify.

Line 219. “Table ??”

Line 243: Here it says “The AOD reduction…is completely missing in the desert,” but in line 226 it says, “over the desert regions, our results highlight an increase of the total AOD over the years.” These statements are not entirely at odds with each other, but the statement in line 243 could be clearer by acknowledging the increase in AOD as opposed to a lack of a reduction.

Line 246. “trends are negative for both Po Valley and Benelux…” There are positive trends at levels 3 and 4 for Po Valley, so in general it cannot be said that the trends are negative. Does this sentence just refer to level 1 or the total column? It is not clear. Please add information to this sentence to clarify which level is being referred to.

Figure 3, middle panel. Why is there a large spike in CALIPSO AOD around 2009 or 2010?

Figure 3, caption. Observational misspelled.

Line 259. Regarding this result, “in urbanized areas (i.e. where anthropogenic aerosols are dominant) the columnar aerosol loading and the surface air temperature trends are inversely related (i.e., consistent with a reduction in solar dimming)” This statement sounds like a general conclusion; i.e., that it applies to all urbanized areas rather than the specific two selected for this study. Can that general conclusion be made or should the sentence be revised so that the conclusion be narrowly stated for these two specific urbanized areas?

Kim, M.-H., A. H. Omar, J. L. Tackett, M. A. Vaughan, D. M. Winker, C. R. Trepte, Y. Hu, Z. Liu, L. R. Poole, M. C. Pitts, J. Kar and B. E. Magill, 2018: “The CALIPSO Version 4 Automated Aerosol Classification and Lidar Ratio Selection Algorithm”, Atmos. Meas. Tech., 11, 6107-6135, https://doi.org/10.5194/amt-11-6107-2018.

Toth, T. D., J. Zhang, J. R. Campbell, J. S. Reid, and M. A. Vaughan, 2016: “Temporal Variability of Aerosol Optical Thickness Vertical Distribution Observed from CALIOP”, J. Geophys. Res. Atmos., 121, 9117-9139, https://doi.org/10.1002/2015JD024668.

Author Response

We would like to thank the reviewer for the meaningful comments that improved our manuscript. We provide a point-by-point answer below, in italic and in red

The experiment setup, comparing two polluted regions to one without anthropogenic influence, seems logical and the data being used is appropriate. It would have been helpful to include more details about the remote sensing data being used and their uncertainties. For instance, was the CALIOP extinction data quality screened (it should be). If so, how? The method used to aggregate these extinction profiles can influence how aerosol is distributed vertically in the radiative transfer model, so it is important to describe or cite these details. Similarly, the AERONET “quality assured” data is used, but no information is given on what exactly that means. Further, it is not clear how statistically significant the Theil-Sens slopes are relative to the variability in the time series. Table 1 lists AOD trends based on these slopes and primary conclusions of the paper are drawn from these slopes, but are they significant? Another publication by Toth et al., 2016 compute AOD trends from CALIPSO data and their discussion goes to great lengths to disprove sensor degradation as an influence on the time series and clearly when trends are significant or not. Similar discussions should be added to this manuscript to explain how reliable the trends reported are – even a brief discussion and appropriate references. It is difficult to establish confidence in the conclusions without understanding the uncertainties.

We thank the reviewer for the meaningful comment.  Nevertheless, being the considered time series noisy and not enough long – the trends are weakly significant (i.e. all the slopes reported in the Table 1 of the original version are obtained on trends calculations with p-values > 0.1; notice that alpha value is set to 0.05 for TheilSen computation). The take home message that the authors would like to point out is a qualitative signal trend to speculate about the increase/decrease behavior of AOD and BOA over the considered decade. Following the review recommendation, the authors substitute the slope values reported in the Table 1 with the percentage variation per decade. Such computation is done by using the main value (of AOD and BOA) for normalizing the slopes (reported in the original version of the paper).  Besides, we incorporated the lev4 (original version) into the level 3 (1 to 2 km in the reviewed version), in order to provide more robustness to the statement reported in the text (please see lines 219 – 228).  We have inverted the order of AOD and BOA rows in the Table 1. The errors and uncertainties discussion with relative references are also added in the text. 

Altogether, the writing and background information provided is sufficient. The topic is appropriate for Remote Sensing. I have some specific comments below that can easily be addressed in a revised manuscript. My main concern is the lack of detail describing the remote sensing data used, their weaknesses, and uncertainty analyses of the trends reported. Because the conclusions of the manuscript rely on this information, I feel major changes are warranted before publication.

Thanks. we agree that the provided information is not complete. For this reason we integrated the manuscript with new sections explaining how CALIOP data are aggregated to reduce the uncertainty. Also, main error sources are now added in the text. 

Specific comments:

Lines 24-25. Can references be added supporting these claims? “air-quality and climate related research activities identified some European regions as climate hot-spots. In this areas, the associated climate changes happen faster with respect to other areas.”

The references are added in the introduction, together with text.

Figure 1: Consider labeling Benelux and Po Volley on the map.

Done.

Line 118: Consider adding “0.5 latitude-longitude resolution by CRU” to avoid confusion with temperature degrees.

Done.

Lines 143-144. This sentence seems incomplete. Does it mean to say, “CALIPSO measurements provide critical information…” or just measurements in general? The next sentence makes me think the first is talking about CALIPSO measurements because it starts with, “In addition, the optical depth derived from CALIPSO…”. The 

We agree that the sentence was incomplete and misleading. We changed accordingly to improve clarity. 

Line 147. It seems like the reference [23] is given here to show an example of how CALIPSO aerosol optical depths are used in combination with passive L2 aerosol retrievals, but reference [23] does not appear to have anything to do with CALIPSO retrievals based on the abstract. Is there a better reference?

We substituted reference [23] with a more relevant one

Line 149. Sunphotometers misspelled.

Corrected

Line 164. The official CALIOP name for this species in version 4 is “polluted continental/smoke” to reflect that the aerosol could be polluted continental or smoke or a combination of the two. CALIOP level 2 algorithms cannot distinguish between these types when the layer top is < 2.5 km. Only when the layer top is above 2.5 km will the layer be classified as smoke (Kim et al., 2018). This has a relevance for how the CALIOP observations are parameterized into the FLG RT model which causes some uncertainty. This should probably be disclosed in the manuscript as a potential error source. 

We agree that we didn't take into consideration this issue. We added the information in the manuscript accordingly together with the reference.

Line 188. The more appropriate reference [32] for lidar ratios used by CALIOP in version 4 is Kim et al., 2018. They are different than those used in Omar et al., 2009.

Changed accordingly

Line 194: “Section ??”

Fixed

Table 1. How are the BOA radiative flux trend values calculated? The units are W/m2, so is this the difference in BOA radiative flux for 2016 minus 2007? This information (however this is calculated) needs to be added into the main text. It is not clear. Also, section 2.4 describes the computation of radiative forcing using the FLG RT model, but the language in the Table 1 caption is radiative flux. Is different terminology being used to describe the same thing?

Thanks for the comment. We clarified the obscure points and we used the same terminology to avoid possible confusion. Now, following the review recommendation, the authors substitute the slope values reported in the Table 1 with the percentage variation per decade (please see reviewed version). Such computation is done by using the main value (of AOD and BOA) for normalizing the slopes (reported in the original version of the paper).

Table 1 caption. Units would be helpful to add, such as “per year”.

Done.

Line 202. Missing a negative sign before “3.0x10-3”

Fixed

Line 206. “The authors speculate that high concentration levels originated from combustion processes…” High concentration levels of what? Please specify.

We agree with the reviewer that the sentence is incomplete. We added in the text the following statement: ...that high concentrations of fine particles originated … 

Line 219. “Table ??”

Fixed

Line 243: Here it says “The AOD reduction…is completely missing in the desert,” but in line 226 it says, “over the desert regions, our results highlight an increase of the total AOD over the years.” These statements are not entirely at odds with each other, but the statement in line 243 could be clearer by acknowledging the increase in AOD as opposed to a lack of a reduction.

The reviewer is right. We added the following sentence to make it clear: 
… on the other hand, as stated above, an increase of atmospheric turbidity is observed over the desert region (see Table 1)...

Line 246. “trends are negative for both Po Valley and Benelux…” There are positive trends at levels 3 and 4 for Po Valley, so in general it cannot be said that the trends are negative. Does this sentence just refer to level 1 or the total column? It is not clear. Please add information to this sentence to clarify which level is being referred to.

The authors agree with the reviewer and we  added the following sentence in the new version of Table 1.:… As reported in Table 1,  an AOD decrement is observed both in the Po Valley (up to 1km) and Benelux (overall the column), on contrary the AOD enhances (overall the column) for Sahara...

Figure 3, middle panel. Why is there a large spike in CALIPSO AOD around 2009 or 2010?

Thanks to the reviewer for noting that. We made a research and the spike is not related to a fewer number of observations. We can speculate that in 2010, the Eyjafjallajökull volcano injected a lot of aerosols in the atmosphere and Benelux was downwind the eruption.

Figure 3, caption. Observational misspelled.

Fixed

Line 259. Regarding this result, “in urbanized areas (i.e. where anthropogenic aerosols are dominant) the columnar aerosol loading and the surface air temperature trends are inversely related (i.e., consistent with a reduction in solar dimming)” This statement sounds like a general conclusion; i.e., that it applies to all urbanized areas rather than the specific two selected for this study. Can that general conclusion be made or should the sentence be revised so that the conclusion be narrowly stated for these two specific urbanized areas?

We agree with the reviewer that this statement cannot be generalized for all the urbanized areas. We changed it accordingly

Kim, M.-H., A. H. Omar, J. L. Tackett, M. A. Vaughan, D. M. Winker, C. R. Trepte, Y. Hu, Z. Liu, L. R. Poole, M. C. Pitts, J. Kar and B. E. Magill, 2018: “The CALIPSO Version 4 Automated Aerosol Classification and Lidar Ratio Selection Algorithm”, Atmos. Meas. Tech., 11, 6107-6135, https://doi.org/10.5194/amt-11-6107-2018.

Added

Toth, T. D., J. Zhang, J. R. Campbell, J. S. Reid, and M. A. Vaughan, 2016: “Temporal Variability of Aerosol Optical Thickness Vertical Distribution Observed from CALIOP”, J. Geophys. Res. Atmos., 121, 9117-9139, https://doi.org/10.1002/2015JD024668.

Added

Reviewer 3 Report

The goal of the article “Aerosol direct radiative effects over highly-polluted areas in Europe and Mediterranean: a ten-years analysis (2007-2016)” is to assess the direct radiative effect associated with anthropogenic aerosol emissions over a decade (2007-2016) in three regions: two highly-urbanized Euro-Mediterranean areas, i.e. the Po-Valley and the Benelux, and a remote region not affected by human activity as the Sahara Desert.

 

This study takes into account long term temperature anomalies, observations of atmospheric turbidity (from CALIOP instrument and sun photometers of the AERONET network) and the calculation of direct radiative effects using the Fu-Liou-Gu Radiative Transfer Model.

The evidences of the study indicate that:

• in urbanized areas the columnar aerosol loading and the surface air temperature trends are inversely related
• in desert areas the atmospheric turbidity does not appear to be temporally correlated with changes in surface air temperature.

 

As consequence the author reinforce the idea that that the aerosol loading should be taken in to account when assessing the temporal variability of the surface flux budget and suggest that the concomitant increment of atmospheric transparency and the enhancement of atmospheric greenhouse gases concentrations, can be likely considered as the two most significant effects that promote surface temperature increase.

 

There are several texting revision that suggest an accurate editing revision.

line 110: change “..Bodele Depression) it is recognize..” in “..Bodele Depression) is recognize..”

line 141: change “mean profiles over on a uniform”  in “mean profiles over a uniform”  

line 143: change “critical information on vertical distribution”  in “critical information on te vertical distribution”  

line 170: change “obtained considring a pristine”  in “obtained a pristine”  

line 172: change “mid-latitude standard standard atmosphere”  in “mid-latitude standard atmosphere”  

line 193 “see Section ??”

line 219 “Table ??”)

 

 

There are some points that have to be adjusted and/or modify before the paper can be published.

Line 7: Authors do not examine aerosol emissions but aerosol load, which is very different, so the word emission is misleading

Line 151: Which AERONET sites are selected for each box?

Line 175: The mid-latitude standard atmosphere seems not appropriate for the desert region, especially when simulating the longwave irradiance. Do authors take into account possible differences in integrated water vapour among the three boxes? Are RT model calculation performed on a monthly basis for the whole period of CALIOP data?

TABLE1: Why numbers in bold?

Line 202: these are the trends of AOD at level 1, am I correct?

Line 204: is this only solar or solar and infrared radiation?

Line 220: This is a speculation that is not supported. Did the authors tried and analysed the CALIOP aerosol types at level 3 over the Po Valley to support their hypothesis?

Line 228: it seems that the author do not take into account infrared radiation, which is a relevant component of the surface radiative budget. But in section 2.4 it seems that they simulated also longwave irradiance. Please, clarify this point.

Line 251: in situ make the reader believe that local meteorological measurements are used, which is not the case. Please, clarify this point.

Author Response

We would like to thank the reviewer for the meaningful comments that improved our manuscript. We provide a point-by-point answer below, in italic and in red

The goal of the article “Aerosol direct radiative effects over highly-polluted areas in Europe and Mediterranean: a ten-years analysis (2007-2016)” is to assess the direct radiative effect associated with anthropogenic aerosol emissions over a decade (2007-2016) in three regions: two highly-urbanized Euro-Mediterranean areas, i.e. the Po-Valley and the Benelux, and a remote region not affected by human activity as the Sahara Desert.

This study takes into account long term temperature anomalies, observations of atmospheric turbidity (from CALIOP instrument and sun photometers of the AERONET network) and the calculation of direct radiative effects using the Fu-Liou-Gu Radiative Transfer Model.

The evidences of the study indicate that:

• in urbanized areas the columnar aerosol loading and the surface air temperature trends are inversely related
• in desert areas the atmospheric turbidity does not appear to be temporally correlated with changes in surface air temperature.

 

As consequence the author reinforce the idea that that the aerosol loading should be taken in to account when assessing the temporal variability of the surface flux budget and suggest that the concomitant increment of atmospheric transparency and the enhancement of atmospheric greenhouse gases concentrations, can be likely considered as the two most significant effects that promote surface temperature increase.

There are several texting revision that suggest an accurate editing revision.

line 110: change “..Bodele Depression) it is recognize..” in “..Bodele Depression) is recognize..”

Fixed

line 141: change “mean profiles over on a uniform”  in “mean profiles over a uniform”  

Fixed

line 143: change “critical information on vertical distribution”  in “critical information on te vertical distribution”  

Fixed

line 170: change “obtained considring a pristine”  in “obtained a pristine”  

The sentence is no more actual

line 172: change “mid-latitude standard standard atmosphere”  in “mid-latitude standard atmosphere”  

Fixed

line 193 “see Section ??”

Fixed

line 219 “Table ??”)

Fixed

There are some points that have to be adjusted and/or modify before the paper can be published.

Line 7: Authors do not examine aerosol emissions but aerosol load, which is very different, so the word emission is misleading

The authors agree with the reviewer, the word emissions is misleading and it was replaced by aerosols

Line 151: Which AERONET sites are selected for each box?

Thanks for pointing this out. The AERONET sites considered for our elaborations are reported in the legend of the Fig3 (a,b,c). Following the reviewer advise, in the Fig 3 caption were added the sites coordinates

Line 175: The mid-latitude standard atmosphere seems not appropriate for the desert region, especially when simulating the longwave irradiance. Do authors take into account possible differences in integrated water vapour among the three boxes? Are RT model calculation performed on a monthly basis for the whole period of CALIOP data?

The reviewer is right, in the manuscript we omitted that the  FLG RT simulations over the Sahara desert region are computed inputting the tropical standard atmosphere except for humidity that is taken from arctic standard atmosphere. The same profile is used for the computations in the different boxes.  The computation is done using as input the monthly averaged extinction profile for each box considering a solar zenith angle corresponding to the 15th day of the considered month at noon local time. During daytime, the longwave radiative flux is at least one order of magnitude smaller with respect to the shortwave. We specified those considerations in the manuscript.

TABLE1: Why numbers in bold?

The reviewer is right. No reason for that. In the new version of the table no bold numbers are present.

Line 202: these are the trends of AOD at level 1, am I correct?

Yes, the mentioned statement is referred to both the level 1 and the total/columnar AOD even if is in the parenthesis are reported the values of the total AOD

Line 204: is this only solar or solar and infrared radiation?

The reviewer is right,  both solar and longwave radiations are considered. However, the net longwave fluxes at surface are negligible for Po Valley and Benelux (no dust aerosols), while for dust the longwave net fluxes are one order of magnitude smaller.  

Line 220: This is a speculation that is not supported. Did the authors tried and analysed the CALIOP aerosol types at level 3 over the Po Valley to support their hypothesis?

We agree with the reviewer, but the literature provides such information:
a reference was added for supporting the  two statements about the saharan dust transport events (altitude and frequency).

Line 228: it seems that the author do not take into account infrared radiation, which is a relevant component of the surface radiative budget. But in section 2.4 it seems that they simulated also longwave irradiance. Please, clarify this point.

We thank the reviewer for the comment. Indeed the sentence is misleading as also the longwave flux should be taken into account for dust aerosol layer. We changed the sentence accordingly.

Line 251: in situ make the reader believe that local meteorological measurements are used, which is not the case. Please, clarify this point.

The authors agree with reviewer, the sentence is misleading. The following phrase was added: "Using interpolated surface temperature measurements and remote sensing observations of the Aerosol Optical Depth …"

Round 2

Reviewer 1 Report

The authors reflected and answered my comments well.  

I recommand this manuscript for publication.

 

Author Response

Thanks again for your feedback. We addressed all the minor issues raised by the reviewer. 

Reviewer 2 Report

The authors have addressed all of my concerns. There are two minor items that should be addressed.

Line 168-169: This sentence is incomplete, "In [33], among others, the authors characterize CALIOP AOD together with the accuracy"

Table 1 caption: "Mean" misspelled.

 

Author Response

Thanks again for your feedback. We addressed all the minor issues raised by the reviewer.