the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2024
Phase matrix characterization of long-range transported Saharan dust using multiwavelength polarized polar imaging nephelometry
Abstract. This work investigates the scattering matrix elements during different Saharan dust outbreaks over Granada (South-East Spain) in 2022 using the Polarized Imaging Nephelometer (PI-Neph PIN100, GRASP-Earth). The PI-Neph is a unique instrument capable of measuring continuously the phase function and polarized phase function (F11 and -F12/F11) at three different wavelengths (405, 515 and 660 nm) with 1° resolution. Extreme dust events (PM10 concentration above 1000 µgm-3) occurring in March 2022 are compared with more frequent and moderate events registered in summer 2022 (PM10 concentration between 50 and 100 µgm-3). For F11 there are no remarkable differences between extreme and moderate events. However, results of -F12/F11 show large differences between extreme and moderate events, especially for the 405 nm wavelength. These differences are also observed when studying the temporal evolutions during the extreme events and reveal that -F12/F11 patterns similar to laboratory measurements occurred during the more intense periods of dust influence. Other aerosol optical properties were derived from the PI-Neph, such as the asymmetry parameter (g), the fraction of backscattered light (Bs) and the lidar ratio (LR). In general, g and Bs show typical values (g > 0.65 and Bs ~ 0.1) for both extreme and moderate Saharan dust events. However, the LR shows more variable values for the different dust events, ranging from 20 to 60 sr-1. The combination with additional in-situ instrumentation allowed to obtain scattering (SAE) and absorption (AAE) Ångström exponents and to conduct a typing classification that revealed extreme dust events as pure dust while moderate dust events were classified as a mixture of dust with urban background pollution. In addition, model simulations with the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) code reproduce well the PI-Neph measurements. Therefore, our results confirm that differences in the phase matrix elements of Saharan dust outbreaks of varying intensity can be explained by the mixing conditions of dust with the background particles, which varies from almost pure dust particles during extreme events, to a mixture of dust with local pollution during moderate events.
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CC1: 'Comment on egusphere-2024-2080', Jean-Baptiste Renard, 24 Aug 2024
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I congratulate the authors for this nice work. Nevertheless, I am a bit surprised that the authors has not considered our previous works on this subject that could help them to improve their paper and to correct some flaws.
Firstly, there are no error bars on the scattering measurements (errors bars could be large for polarization). Thus, what are the accuracy measurements? Considering them could totally change the analysis.
Line 63 : For the size distribution, the authors could consider the measurements from balloon-borne aerosol counter inside a Saharan dust plume : Renard, J.-B.; Dulac, F.; Durand, P.; Bourgeois, Q.; Denjean, C.; Vignelles, D.; Couté, B.; Jeannot, M.; Verdier, N.; Mallet, M. In situ measurements of desert dust particles above the western Mediterranean Sea with the balloon-borne Light Optical Aerosol Counter/sizer (LOAC) during the ChArMEx campaign of summer 2013. Atmos. Chem. Phys. 2018, 18, 3677-3699.
Line 74: The authors have forgotten to consider the effect of the size distribution of the particles. Scattering properties (including polarization) are sensitive to the size of the particles, even for mineral dust, as shown by our team :
Renard, J.-B., Hadamcik, R., Woms, J.-C. The laboratory PROGRA2 database to interpret the linear polarization and brightness phase curves of light scattered by solid particles in clouds and layers, Journal of Quantitative Spectroscopy and Radiatif Transfer, volume 320, July 2024, 108980
Renard, J.-B.; Françis, M.; Hadamcik, H.; Daugeron, D.; Couté, B.; Gaubicher, B.; Jeannot, Scattering properties of sand. 2. Results for sands from different origins, Applied Optics, 49, N°18, 3552-3559, 2010.
Lines 78-82: Why the large database of PROGRA2 (https://www.icare.univ-lille.fr/progra2-en/?noredirect=en_US) that contains such laboratory measurements is not cited?
Part 2.2.1. If I well understand, the instrument retrieves the light scattering parameter from a cloud of particles, without size selection. Thus, if the authors want to make comparison between different sessions of measurement in different conditions, they must be sure that the size distributions are the same. Otherwise, they will observe a combination of size distributions, refractive indexes, and even porosities of the grains and the agglomerates. The authors must explain how these different parameters influence their measurements and their analysis.
The authors must consider some aerodynamic effects that can orient the particles during their motion in the instrument, and also the particles' speed. Such parameter can change the polarization results, as shown by our team : Daugeron, D.; Renard, J.-B.; Gaubicher, B.; Couté, B.; Hadamcik, E.; Gensdarmes, F.; Basso, G.; Fournier, C. Scattering properties of sands, 1. Comparison between different techniques of measurements, Applied Optics, 45, 8331-8337, 2006.
Figures 6, 7 and 9, and in text : The negative polarization at the large angles for the blue domain only is strange, and perhaps suspicious. Such high negative values were never observed in laboratory (and even in space) for randomly oriented compact irregular particles. More strangely, such phenomenon does not occur at the other wavelengths. The authors must explain the origin off such negative values (that in fact are almost impossible for such large dust particles). I suspect an instrumental error like a stray light contamination not accurately removed, a too log signal to noise ratio ....
Line 389: The wavelength effect was largely studied in the PROGRA2 data base and largely published by our team.
Line 465: Aging can produce more compact particles, but it is difficult to call them “spherical”.
Line 485: This analysis could be inaccurate. Other effects than the sphericity of the particles must be considered (size, refractive index, porosity). The presence of “spherical particles” do not change significantly the shape of the light scattering curves (of course, only a medium composed of perfect spheres can change the shape of the curve).
Figure 11: Yellow dots are difficult to see.
Line 620: Do they authors have also considered the super-coarse mode of particles ? Keep in mind that the largest particles are the most luminous, and thus can dominate the scattered (polarized) intensities.
Figure 12: Negative polarization down to -0.4 are unrealistic for dust sample, unless the size distribution is dominated by submicron particles without large ones.
Citation: https://doi.org/10.5194/egusphere-2024-2080-CC1 -
RC1: 'Comment on egusphere-2024-2080', Anonymous Referee #2, 26 Aug 2024
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The manuscript presents ambient measurements of Saharan dust intrusions at ground level in Southern Spain. The polarized imaging nephelometer enables the measurement of the phase function and polarized phase function and at several wavelengths at scattering angles between 5° and 175°. Especially the polarized phase function is an important quantity in passive remote sensing. They could show with their spectral measurements, that the polarized phase function behaves similar in the observed wavelength range (405 till 660 nm) for intense dust events. Whereas dust which was mixed with local pollution shows a different spectral behavior, especially at the shortest wavelength of 405 nm. Additionally, simulated (polarized) phase functions are shown. However, I see that for a publication in ACP, the research findings should be set into a broader context and the novelty of the study should be worked out stronger. After addressing my comments below, and please take my major comments serious, a publication in ACP can be recommended.
Major comments:
- The paper would benefit a lot, if you include lidar observations to your dust cases. I know that the University of Granada operates a polarization lidar (otherwise I would not ask for it) which could clearly show the Saharan dust layers above the station (Sect. 3.1).
Additionally, it may provide you with directly measured lidar ratios. Then, there is just the gap between the PI Nephelometer observations at the ground and the lidar measurements in the lofted Saharan dust layers. So please include your colleagues from the lidar group with their lidar observations at UGR. - Do you have additional size distributions for your cases? The spectral slope of your scattering properties might depend on particle size. Having an additional size distribution would add certainly a lot of value to your discussion. Could you estimate the amount of pollution in your polarization measurements? It would be an important information in quantifying your observations. By requesting additional size distributions and lidar (lidar ratio) observations underlines my request to broaden your field and take all available information into account to deliver a comprehensive picture and to place your results in a broader context.
- You have not written a lot concerning your uncertainty estimates. In Tab. 1 you provide uncertainties without mentioning what you are reporting. I guess, it is the standard deviation of the hourly mean, but it is stated nowhere. If it is the case, I am wondering about the systematic uncertainties of your measurements. Probably, it is reported elsewhere. But we need an assessment of the systematic uncertainties. Otherwise the results are not comparable to other measurements.
In Tab. 2 you don’t provide any uncertainties at all. Please add them.
Your phase matrix elements (Fig. 6,7 & 9) do not contain any uncertainties. Putting an error bar to every point would certainly overload the figure, but having at least 3 points with error bars in each plot would help to assess the range of uncertainty. - After finalizing my review, I am now reading the comments of Jean-Baptiste Renard and I want to enforce his point, that the (spectral) scattering properties depend on particle size (see my major comment #2). And there is much more literature on the size effect of the polarized scattering properties than mentioned in his comment. The size effect is almost not discussed in the manuscript. Please try seriously to get more size information than just a PM10 concentration.
Minor comments:
- Title: “polarized polar imaging nephelometry” – in the manuscript, you always state Polarized Imaging Nephelometer (PI-Neph). Why do you use the term “polar” only in the title?
- L20 Please mention already in the introduction the angular range of the instrument.
- L56 a strange unit.
- L67 complex refractive index
- Your introduction (L70-85) is focused on passive remote sensing. I am missing the active remote sensing with e.g., CALIPSO or EarthCARE and the linked laboratory studies which focus on the backscattered light close to 180° (e.g., Sakai et al., 2010, Järvinen et al., 2016 or Miffre et al., 2023).
- Eq 5 + 6: Do you use the decadic logarithm (log) or the natural logarithm (ln) for your definition?
- According to eq. (7), the units of the LR should be sr and not sr-1. Please change it throughout the manuscript (including figures and tables).
- How do you extrapolate to the scattering angle of 180°. Please add some description and assessment of the related uncertainties.
- From the reader’s perspective, I would suggest a different section numbering to avoid the 4th level of subsections (e.g., 3.2.1.1). My suggestions are:
- Give the meteorological conditions (currently Sect. 3.1) an own section (Sect. 3) before you go to your results of the PI-Neph in Sect. 4 (Results or Aerosol phase matrix from different dust scenarios)
- Section 3.2.3 and 3.2.4 can be combined to new section: Sect. 5 – Discussion with 5.1 and 5.2 for the two respective subsections.
- The meteorological conditions (Sect. 3.1) are given in great level of detail. I am wondering, if the 4 CAMS model outputs for each case are necessary, because this information is not used in the next sections. To my opinion, it can be combined to one CAMS output per dust case. In this section some lidar observations of the Granada station would be helpful to demonstrate the vertical layering of the dust above your station. It must not be a difference in the strength of the dust outbreak between the two cases in March, but on 15 March, the dust was mixed to a larger extend towards the ground and therefore to your PI-Nephelometer. Lidar observations would reveal the vertical layering of the dust.
- L200 Libya
- L250 Please provide the values for usual dust outbreaks and not just a reference. Similarly, in line 373
- Fig. 5 Please add more detailed steps to the time axis. The scale for SAE is not optimal.
- L329-334 This text might be moved to the figure caption of Fig. 6. At least Fig. 6 needs some more explanations in the caption. The same holds for L 374-377 and Fig. 7.
- L340 “notable spectral separation” – I am not sure if it just a manner of scaling the y-axis. Fig. 6a1 has a maximum value of 10^3 whereas the other subfigures extend to 10^4. Therefore the spectral separation is better visible in Fig. 6a1.
- Figs 6,7,9 lower row: the y-scaling is quite coarse, please add some ticks in between.
You may add the value of the PM10 concentration for the different time steps because you are discussing it a lot and it is not always visible from Fig. 5+8.
The term “moment” seems not the best choice, maybe better “instances” or “time steps” or something else. Please consider changing it here and in the text. - L381-383 Sentence unclear.
- L387-389 Be more precise. The whole paragraph on page 11 needs some rephrasing to be more precise.
- Tab 1+2 “The angular range of the PI-Neph is used as the integration range of sigma_sca.” Please provide the angular range otherwise this information is not very helpful.
Why are some values at 405 nm are missing on 25 March? - L450: A newer and more comprehensive overview of lidar ratios was reported in Floutsi et al., AMT 2023.
- Fig 8. Sorry, but it is really challenging to get anything out of Fig. 8. Please use more space to show your results, half of a page minimum. Or remove the figure from the manuscript. I see only dots and can hardly infer any value from the y-axis. You probably want to show that there are more dust outbreaks during summer.
- Tab 2 and Fig. 9: Do you show the measurements for the whole day? Or for one hour? Or for the periods marked in Fig. 8?
- Fig 10 and surrounding text: BC and BrC are not defined.
- L539-541: Polarization measurements are very valuable for separating dust and non-dust contributions. This potential is used in the active remote sensing community for two decades now, starting with Shimizu et al., 2004, continuing to Tesche et al., 2009 and Mamouri and Ansmann 2017. You now adding PI-Nephelometer measurements to this separation.
- Fig 11 Do you expect that the seasonal average of -F12/F11 should go back to zero for 180° or could it stay negative?
Please indicate the wavelengths for the Granada Amsterdam Light Scattering data base in the caption or even in the figure. By the way, 488 nm are much closer to 515 nm than to 405 nm. Why did you choose to show it together with the 405 nm?
Yellow is probably not the best choice – could you choose a different color? And overall, the final publication should get the figure in a higher resolution. It is hard for me to distinguish the open circles from the stars.
Caption: “top” instead of “up” - Do you have a pure pollution case for comparison? It would certainly enhance the message to have a contrasting pollution case presented or repeated from Bazo et al., 2024 in this paper as well.
- Paragraph (L598-612): You already mentioned Teri et al., 2024. Here some more discussion to the polluted dust cases in the Eastern Mediterranean would be helpful. Overall, I would recommend to place your work in a broader context besides of previous measurements at Granada station.
- L654-657: Sentence unclear. Please rephrase to be more precise.
- Fig. 12 The negative values for the -F12/F11 close to 180° for the pure dust case are not seen in the observations of the extreme dust events. Is it a modelling artefact or is it missing in the observations? What is your explanation? Please discuss in your paper.
- L668: “To our knowledge, these are the first measurements of this type for ambient mineral dust transported to southern Europe” – only in Southern Europe. Where else ambient mineral dust measurements have been performed? What is the difference to your observations?
- L691 versus --> to
- L699 SSA and LR are intensive properties
- The author’s contributions section is missing.
References:
Järvinen, E.; Kemppinen, O.; Nousiainen, T.; Kociok, T.; Möhler, O.; Leisner, T. & Schnaiter, M.: Laboratory investigations of mineral dust near-backscattering depolarization ratios, Journal of Quantitative Spectroscopy and Radiative Transfer, 2016, 178, 192 – 208.
Floutsi, A. A.; Baars, H.; Engelmann, R.; Althausen, D.; Ansmann, A.; Bohlmann, S.; Heese, B.; Hofer, J.; Kanitz, T.; Haarig, M.; Ohneiser, K.; Radenz, M.; Seifert, P.; Skupin, A.; Yin, Z.; Abdullaev, S. F.; Komppula, M.; Filioglou, M.; Giannakaki, E.; Stachlewska, I. S.; Janicka, L.; Bortoli, D.; Marinou, E.; Amiridis, V.; Gialitaki, A.; Mamouri, R.-E.; Barja, B. & Wandinger, U.: DeLiAn -- a growing collection of depolarization ratio, lidar ratio and Ångström exponent for different aerosol types and mixtures from ground-based lidar observations, Atmospheric Measurement Techniques, 2023, 16, 2353-2379.
Mamouri, R.-E. & Ansmann, A.: Potential of polarization/Raman lidar to separate fine dust, coarse dust, maritime, and anthropogenic aerosol profiles, Atmospheric Measurement Techniques, 2017, 10, 3403-3427.
Miffre, A.; Cholleton, D.; Noël, C. & Rairoux, P.: Investigating the dependence of mineral dust depolarization on complex refractive index and size with a laboratory polarimeter at 180.0degree lidar backscattering angle, Atmospheric Measurement Techniques, 2023, 16, 403-417.
Sakai, T.; Nagai, T.; Zaizen, Y. & Mano, Y.: Backscattering linear depolarization ratio measurements of mineral, sea-salt, and ammonium sulfate particles simulated in a laboratory chamber, Appl. Opt., OSA, 2010, 49, 4441-4449.
Shimizu, A.; Sugimoto, N.; Matsui, I.; Arao, K.; Uno, I.; Murayama, T.; Kagawa, N.; Aoki, K.; Uchiyama, A. & Yamazaki, A.: Continuous observations of Asian dust and other aerosols by polarization lidars in China and Japan during ACE-Asia, Journal of Geophysical Research: Atmospheres, 2004, 109, D19S17.
Tesche, M.; Ansmann, A.; Müller, D.; Althausen, D.; Engelmann, R.; Freudenthaler, V. & Gross, S.: Vertically resolved separation of dust and smoke over Cape Verde using multiwavelength Raman and polarization lidars during Saharan Mineral Dust Experiment 2008, Journal of Geophysical Research: Atmospheres, 2009, 114, D13202.
Citation: https://doi.org/10.5194/egusphere-2024-2080-RC1 - The paper would benefit a lot, if you include lidar observations to your dust cases. I know that the University of Granada operates a polarization lidar (otherwise I would not ask for it) which could clearly show the Saharan dust layers above the station (Sect. 3.1).
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